Plant-derived bioactive compounds as key players in the modulation of immune-related conditions

Abstract

The immune system is a complex and fundamental network for organism protection. A minimal unbalance in the host defense system homeostasis can originate severe repercussions in human health. Fundamentally, immune-related diseases can arise from its compromise (immunodeficiency diseases), overactivation against itself (autoimmune diseases) or harmless substances (allergies), and failure of eliminating the harmful agent (chronic inflammation). The notable advances and achievements in the immune system diseases pathophysiology have been allowing for a dramatic improvement of the available treatments. Nevertheless, they present some drawbacks, including the inappropriate benefit/risk ratio. Therefore, there is a strong and urgent need to develop effective therapeutic strategies. Nature is a valuable source of bioactive compounds that can be explored for the development of new drugs. Particularly, plants produce a broad spectrum of secondary metabolites that can be potential prototypes for innovative therapeutic agents. This review describes the immune system and the inflammatory response and examines the current knowledge of eight plants traditionally used as immunomodulatory medicines (Boswellia serrata, Echinacea purpurea, Laurus nobilis, Lavandula angustifolia, Olea europaea, Salvia officinalis, Salvia rosmarinus, and Taraxacum officinale). Moreover, the issues responsible for possible biologic readout inconsistencies (plant species, age, selected organ, developmental stage, growth conditions, geographical location, drying methods, storage conditions, solvent of extraction, and extraction method) will also be discussed. Furthermore, a detailed list of the chemical composition and the immunomodulatory mechanism of action of the bioactive compounds of the selected plant extracts are presented. This review also includes future perspectives and proposes potential new avenues for further investigation.

Introduction

Immune-related diseases pose a huge global burden, affecting both male and female of all ages. Over the years, the incidence and prevalence of immune-related disorders have increased (Wu et al. 2023). For instance, chronic inflammatory diseases worldwide have shown a consistent yearly rise in both incidence and prevalence, with an estimated increase of 19.1% and 12.5%, respectively (Miller 2023). Moreover, a recent study demonstrated that autoimmune disorders can affect about one in ten individuals (Conrad et al. 2023). Indeed, millions of people suffer from these diseases with a serious impact on their quality of life (Vos et al. 2020). Moreover, immune-related diseases present a substantial socioeconomic challenge (Wu et al. 2023). Therefore, there is a pressing need to develop new, safe, and more effective therapies to restore immune system homeostasis and enhance the well-being of patients.

Currently, the pharmaceutical industry primarily focuses on developing new synthetic compounds and researching previously known molecules (Atanasov et al. 2015). Nonetheless, natural products continue being used for drug discovery. Indeed, several drugs derived from various natural sources, such as plants, marine organisms, and microorganisms, continue to successfully emerge (Torre and Albericio 2017; De la Torre and Albericio 2018; G. de la Torre and Albericio 2019; de la Torre and Albericio 2020, 2021, 2022, 2023). Particularly, plants have long been employed for the traditional treatment of various diseases, such as immune-related diseases. Indeed, plant-derived formulations can present promising immunomodulatory bioactivity. These formulations can, for example, influence immune cell function, modulating the production of pro-inflammatory mediators, and regulating the levels of antioxidant molecules (Alhazmi et al. 2021; Elkhawaga et al. 2023; Moudgil and Venkatesha 2023). Thus, plant-derived formulations can help restoring immune system homeostasis and improve treatment outcomes by targeting specific dysregulated components. Furthermore, plants, presenting a rich chemical diversity of compounds, are an excellent resource for the discovery of novel, safe, and effective immunomodulatory drugs to potentially revolutionize immune-related therapeutics. Moreover, with the advances in pharmacology and pathophysiology, the integration of historical sources in the exploration of plant-based drugs can be a highly successful approach for the discovery of effective immunomodulatory agents (Ulriksen et al. 2022).

Despite the increasing focus on exploring the immunomodulatory effects of plant extracts and its isolated compounds, a comprehensive literature on its mechanism of action is required. The previous reviews provided only general discussions on the topic. Indeed, they have addressed the wide array of bioactivities associated with plant extracts and its isolated compounds (Almeida-da-Silva et al. 2022; Burlou-Nagy et al. 2022; Rufino-Palomares et al. 2022; Alesci et al. 2022; Awada et al. 2023), lacking an in-depth overview of the immune system and the inflammatory response (Gupta et al. 2021). This can pose difficulties for readers who are not familiar with these topics, particularly those who are not experts in the field. Furthermore, most of the reviews have been limited in scope, concentrating on the description of a single plant species or family (Ramos da Silva et al. 2021; Pilkington and Pilkington 2022; Batiha et al. 2023). In addition, to the best of our knowledge, none of the existing reviews offer a comprehensive and exhaustive list of the phytochemical composition of the plants under investigation. Instead, they primarily focus on the most widely recognized compounds (Burlou-Nagy et al. 2022; Rufino-Palomares et al. 2022; Awada et al. 2023; Dobros et al. 2023; Habtemariam 2023). Additionally, the existing reviews fail to approach the crucial aspects in identifying inconsistencies associated with the bioactivity of plant extracts. Although multiple reviews can offer additional insights, supporting one another, the absence of centralized and consolidated information can result in additional difficulties for the readers searching for relevant information across different sources.

Our review compiles an exhaustive, up-to-date, and critical evaluation of literature exploring several exciting and outstanding fields that will offer the readers an integrated analysis of immunology, natural products, and drug development. This review presents an overall description of the immune system and the inflammatory response, exploring the major inflammatory signaling pathways. Examples of chronic inflammatory diseases and therapeutic options available are also provided. The role of plant-derived formulations in the pharmacological field, as well as the inconsistencies in their therapeutic effects are presented.

This review carefully examines eight plants traditionally used as immunomodulatory medicines (Boswellia serrata, Echinacea purpurea, Laurus nobilis, Lavandula angustifolia, Olea europaea, Salvia officinalis, Salvia rosmarinus, and Taraxacum officinale). It highlights their main active principle(s) and the respective mechanism of action. In addition, this review presents the most complete and comprehensive list of chemical compounds present in each selected immune-related plant ever reported to date. Those tables are complemented with additional parameters that impact the extraction process (e.g., solvent of extraction, extraction technique, and organ of the plant). It finishes with insights into future directions of further research in the field of immunomodulatory plant bioactive compounds.

Thus, this review effectively fills the gaps identified in the current literature by providing a more comprehensive and exhaustive examination of the immune-related properties of eight plants. Importantly, it goes beyond the well-known compounds, offering a more complete phytochemical composition of the plants under investigation. In summary, this review serves as a centralized and extensive resource that brings together all the pertinent and relevant data on the immune system and the selected plants.

Immune system

The immune system is in constant surveillance, recognizing and eliminating harmful stimuli that can cause diseases (Medzhitov 2008). Indeed, it has fast, specific, and effective mechanisms that are triggered by tissue injury or trauma and harmful entities (e.g., cancer cells, viruses, fungi, parasites, bacteria, allergens, and toxic compounds) (Medzhitov 2008). The immune system is highly adaptable and involves a complex and dynamic network of cells, soluble molecules, and organized pathways. Moreover, it has different recognition and destruction mechanisms to match the several shapes, sizes, and chemical structures of harmful entities. Hence, an immune response is elicited to ensure the protection of all tissues and organs and to maintain or restore homeostasis in the body (Medzhitov 2008).

Organization of the immune system

The immune system involves innate (non-specific) and adaptive (specific) immunity (Medzhitov and Janeway 2000).

The effector mechanism of innate immunity—the first line of defense—comprises features that are encoded by genes in the germline, including anatomical barriers and cellular responses (Chaplin 2010). The main physical barriers for preventing pathogens to enter in the body are the epithelial layers of the skin, the mucosal tissues (e.g., gastrointestinal, respiratory, and urogenital tracts), and the glandular tissues (e.g., salivary, lacrimal, and mammary glands) (Turvey and Broide 2010). In addition, the secretions of these tissues (e.g., mucus and urine) wash away potential invaders and can contain enzymes (e.g., lysozyme) and peptides (e.g., defensins) with antimicrobial properties. Moreover, the acidic pH in some tissues, such as stomach and urogenital tract, is important in conferring protection against bacterial and fungal pathogens (Turvey and Broide 2010). Innate immune cells include monocytes, macrophages, neutrophils, eosinophils, mast cells, basophils, and dendritic cells (Chaplin 2010). They recognize in a fast and effectively way a wide range of pathogens after entering inside the organism, restricting and controlling their replication rate within hours (Medzhitov and Janeway 2000). The innate cells express an array of germline-encoded cellular membrane and intracellular receptors. They are defined as pattern-recognition receptors (PRRs), that immediately recognize specific conserved molecular components on the surfaces of pathogens (pathogen-associated molecular patterns, PAMPs) or endogenous molecules released from damaged cells (danger-associated molecular patterns, DAMPs) (Medzhitov and Janeway 2000; Iwasaki and Medzhitov 2010). PRRs can be categorized into secreted, transmembrane, and cytosolic classes, ensuring the recognition of PAMPs and DAMPs in both extracellular and intracellular compartments of a cell (Iwasaki and Medzhitov 2010). PAMPs could be bacterial and viral nucleic acids and wall components, including lipopolysaccharides (LPS), carbohydrates (e.g., mannans and β-glucans), peptidoglycans, lipoteichoic acids, and surface proteins (Medzhitov 2008; Takeuchi and Akira 2010). Conversely, DAMPs are breakdown products of extracellular matrix, products of the proteolytic cascades activated by vascular and endothelial damage, or molecules released from debris cells (e.g., adenosine triphosphate—ATP, potassium ions, uric acid, high mobility group box—HMGB—1 protein, and S100 calcium-binding proteins family) (Murao et al. 2021).

The adaptive immune system provides a second and more focused set of reactive molecules and cellular components against pathogens. The adaptive immune response is slower (5 or 6 days after initial exposure) than the innate response but is more specific against the pathogen (Bonilla and Oettgen 2010). Adaptive immune cells are composed of the effectors of cellular immunity—T cells—and the antibody-producing cells—B cells—, whose receptors are not genetically encoded, but are generated by somatic recombination in each organism (Iwasaki and Medzhitov 2010; Bonilla and Oettgen 2010). The adaptive immune response is generated under the influence of signals provided by the innate immune system that can be either directly by circulating offending agents or indirectly by antigen-presenting cells (APCs) migrating to secondary lymphoid organs (Bonilla and Oettgen 2010). The antibodies (also called immunoglobulins) produced by B cells confer humoral immunity. The various subpopulations of the T cells secreting soluble cytokines confer cellular immunity (Bonilla and Oettgen 2010).

Although the innate immune system triggers the adaptive immune system, both systems communicate through cellular and molecular interactions, allowing for the protection of the organism (Medzhitov and Janeway 2000).

Cells of the immune system

The immune response involves an interconnected communication among diverse cell types that drive from hematopoietic stem cells (HSC) in the bone marrow (Fig. 1) (Mann et al. 2022). In homeostatic conditions, most HSCs are quiescent. In the presence of a stimulus (e.g., pathogen or cancer cells), HSCs differentiate into a common myeloid-erythroid progenitor (CMP)—myeloid lineage—or a common lymphoid progenitor (CLP)—lymphoid lineage (Mann et al. 2022).

Fig. 1

Immune cell lineages. TC, T cytotoxic cells; TFH, T follicular helper cells; TH, T helper cells; TREG, regulatory T cell

Myeloid cells are the first to respond to the invasion of a pathogen and communicate the presence of the insult to cells of the lymphoid lineage. The immune cells that arise from the myeloid lineage include granulocytes and phagocytic cells (Mann et al. 2022). Granulocytes can be classified as neutrophils, basophils, mast cells, or eosinophils, depending on the cellular morphology and the cytoplasmic characteristic granules (Chaplin 2010). Indeed, their cytoplasm is enriched in cytotoxic granules that are released after contact with different pathogens. Granulocytes present different functions. Neutrophils are implied in many types of infections and are responsible for the engulfing of the pathogen—phagocytosis (Chaplin 2010). Basophils are critical for protecting the organism against parasites, particularly helminths. Mast cells protect mucosal surfaces against pathogens. Eosinophils are key players in the defense against multicellular parasitic organisms, including worms. The group of phagocytic cells includes monocytes, macrophages, and dendritic cells, that have APCs function (Chaplin 2010). Monocytes migrate into tissues and differentiate into tissue-resident phagocytic cells, namely macrophages and dendritic cells. Depending on the nature of the activating signals, macrophages can undergo classical macrophage activation or alternative macrophage activation, yielding pro-inflammatory M1 macrophages or anti-inflammatory M2 macrophages. The first and second phenotypes participate in immune responses and tissue repair, respectively. During an immune response, M1 macrophages exhibit great phagocytic activity, improved ability to kill ingested microbes, increased secretion of inflammatory and cytotoxic mediators, and enhanced capacity to activate T cells. Dendritic cells monitor the body for signs of invasion by pathogens and capture intruding or foreign antigens, which are presented to naïve lymphocytes in the secondary lymphoid organs. All APCs express both major histocompatibility complex (MHC) class I and class II molecules, cell membrane proteins, that are essential for the activation of T cells (Chaplin 2010).

Despite the myeloid cells exhibit an unspecific immune response, the lymphoid lineage regulates a specific immune response. In this lineage, lymphocytes are the main cells. According to their functional and phenotypic differences, lymphocytes can be categorized into B cells (B lymphocytes), T cells (T lymphocytes), and natural killer cells (Mann et al. 2022). B cells, matured in the bone marrow, express B-cell receptor (BCR), a membrane-bound antibody molecule that binds to processed or unprocessed antigens (Chaplin 2010). After surface antibody link to a unique antigen, they become activated. Activated B cells differentiate into effector cells—plasma cells—and secrete antibodies. T cells, matured in the thymus, express a specific antigen-binding receptor—T-cell receptor (TCR). TCR only recognizes processed pieces of antigen bound to MHC molecules (Chaplin 2010). According to the presence of glycoproteins, namely CD4 or CD8, on their surface, T cells can be classified as T helper (TH) cells and T cytotoxic (TC) cells, respectively. TH cells (CD4⁺ T) recognize antigens in complex with MHC class II (expressed by APCs). Conversely, TC cells (CD8⁺ T) recognize antigens in complex with MHC class I (expressed by nearly all nucleated cells of vertebrate species). When naïve CD4⁺ T cells bind their TCR to the MHC-peptide complex, they become activated and proliferate and differentiate into a variety of effector T cell subsets. Depending on the type of pathogen, T cell subsets can be T helper type 1 (TH1), T helper type 2 (TH2), T helper type 17 cells (TH17), T follicular helper cells (TFH), and regulatory T cell (TREG) (Chaplin 2010). TH1 and TH2 cells regulate the immune response to intracellular and extracellular pathogens, respectively. TH17 cells play an important role in cell-mediated immunity and may help the defense against fungi infections. TFH cells play an important role in humoral immunity and regulate B-cell development in germinal centers. TREG cells have the unique capacity to inhibit an immune response. Similar to naïve CD4⁺ T cells, naïve CD8⁺ T cells also become activated after binding their TCR to the MHC-peptide complex, and then, they proliferate and differentiate into an effector cell called cytotoxic T lymphocyte (Chaplin 2010). Natural killer cells, presenting cytotoxic granules, attack and induce cell death in a variety of abnormal cells, such as tumor cells and virus-infected cells (Chaplin 2010). They recognize these cell debris by the absence of MHC class I on their surface. Natural killer cells also express immunoglobulin-like receptors and can, therefore, bind pathogens or proteins from pathogens on the surface of infected cells.

Inflammation

Inflammation is a natural and essential protective mechanism, triggered by noxious stimuli or trauma, that aims to restore the homeostasis of the body (Medzhitov 2008; Chovatiya and Medzhitov 2014). The hallmarks, traditionally called cardinal signs, of an inflammatory response are calor (heat), rubor (redness), tumor (swelling), dolor (pain), and functio laesa (loss of function) (Medzhitov 2010). These signals are consequences of vascular changes and leukocyte recruitment, influx, and activation.

Some pathogens can penetrate the body through damaged anatomical barriers (e.g., wounds, abrasions, and insect bites). Once inside the human organism, the offending agent is rapidly recognized by soluble proteins and/or PRRs on the membrane surface of tissue-resident macrophages, neutrophils, dendritic cells, and mast cells (Iwasaki and Medzhitov 2010). When activated, the immune cells rapidly fight the pathogen (Medzhitov and Janeway 2000; Takeuchi and Akira 2010). The soluble phagocytosis-enhancing proteins, namely opsonins (e.g., surfactant proteins—SP—A and D, mannose-binding lectin—MBL—, L-ficolin, C-reactive protein—CRP—, and several components of complement system), that can bind to PAMPs, initiates the opsonization of the pathogen (Fig. 2A). Once bound to the surface of the pathogen, opsonins are recognized by membrane opsonin receptors on phagocytic cells, activating and enhancing the phagocytosis (Uribe-Querol and Rosales 2020). Immediately, actin polymerization is activated in phagocytic cells to perform the phagocytosis (Fig. 2B). Briefly, phagocytic cells extent their plasma membrane around the offending agent, and internalize it into phagosomes, which then fuse with lysosomes (Xu et al. 2021). The resulting phagolysosomes contain antimicrobial proteins (e.g., lactoferrin and psoriasin) and peptides (e.g., defensins and cathelicidins), low pH, acid-activated hydrolytic enzymes (e.g., lipases and proteases), and free radicals (e.g., reactive oxygen and nitrogen species—ROS/RNS), to kill and degrade the internalized offending agent (Uribe-Querol and Rosales 2017).

Fig. 2

Phagocytosis and intracellular destruction of invading agent (e.g., bacterium). A Opsonization of invading agent by different opsonin receptors on the surface of phagocytic cells. B Phagocytosis steps: (1) recognition and attachment: invading agent binds to phagocytic receptors present in the membrane; (2) engulfment: extension of the phagocytic membrane around the invading agent; (3) ingestion: the invading agent is ingested, forming a phagosome. (4) fusion: phagosome fuses with a lysosome; (5) killing and degradation: the invading agent is killed and then digested by lysosomal enzymes and reactive oxygen and nitrogen species (ROS/RNS); (6) releasing: digestion products are released from the cell

The recognition of the offending agent by phagocytic cells also triggers different signal-transduction pathways that activate the expression of important genes in the inflammatory process. Among the proteins encoded by these genes, there are several inflammatory mediators that amplify the inflammatory response. Briefly, the neutrophils create a cytotoxic environment by releasing noxious chemicals from their cytoplasmic granules, including proteinases and antimicrobial proteins (Witko-Sarsat et al. 2000). Neutrophils and macrophages also initiate a massive respiratory burst, releasing high levels of ROS/RNS, as well as pro-inflammatory mediators, namely cytokines, chemokines, prostaglandins (PGs), and leukotrienes (LTs) (Nguyen et al. 2017; Gideon et al. 2019; Mosser et al. 2021; Canton et al. 2021). Histamine is released by mast cells. Some of these inflammatory mediators act on the vascular musculature to promote vasodilation, which increases blood flow and capillary permeability, to allow for the influx of leukocytes (Leick et al. 2014). The recruited leukocytes adhere to vascular endothelial cells through cell adhesion molecules (CAMs), at the site of inflammation, passing then through the walls of capillaries into the tissue space (extravasation). In the target tissue, recruited phagocytic cells are activated (Xu et al. 2021). The APCs, typically dendritic cells and macrophages, recognize and process the offending agent, and expose the pathogen-derived peptides (antigen) on the cell surface complexed with MHC class I and class II, as previously referred. Activated APCs also express high levels of costimulatory ligands (e.g., CD80 and CD86) to activate the lymphocytes. Then, they migrate through the lymphatic vessels to nearby secondary lymphoid tissues (e.g., lymph nodes and spleen) where they present the antigen to the T and B cells (Théry and Amigorena 2001).

The naïve T cells are co-stimulated by the engagement with both MHC-peptide complex by TCR (Signal 1) and costimulatory ligands (Signal 2) on APCs (Chaplin 2010). However, even with the increased functional avidity offered by coreceptors and adhesion molecules, the T cells are not fully activated. Indeed, a third set of signals, provided by local cytokines (Signal 3) are needed to completely activate the T cells (Fig. 3). Then, as previously described, activated T cells and their progeny gain unique functional abilities, becoming effector TH or TC that directly or indirectly act to clear the offending agent (Bonilla and Oettgen 2010). CD8⁺ T cells leave the secondary lymphoid tissues and migrate to sites of inflammation. Activated CD8⁺ T cells acquire the ability to induce the death of the target cell, becoming cytotoxic T lymphocyte (Bonilla and Oettgen 2010). CD4⁺ T cells produce cytokines that coordinate the activity of other immune cells (B cells, macrophages, and other T cells) (Chaplin 2010). As previously referred, depending on the nature of the pathogen and its binding to specific PRRs, APCs activate different signaling pathways and induce the secretion of specific cytokines that influence the fate of the naïve CD4⁺ T cells into different subsets of cytokine-producing phenotype (TH1, TH2, TH17, or TREG) (Saravia et al. 2019).

Fig. 3

Activation of naïve T cells requires three signals. (1) The T-cell receptor/major histocompatibility complex (TCR/MHC) antigen interaction, along with CD4 and CD8 co-receptors and adhesion molecules provides Signal 1. (2) Co-stimulation by a separate set of molecules, including CD28, provides Signal 2. (3) Both Signal 1 and Signal 2 initiate a signal transduction cascade that results in activation of transcription factors and cytokines (Signal 3) that direct T-cell proliferation and differentiation

The naïve B cells can be directly activated by the recognition of antigen-associated APCs. Moreover, they can also be activated by two different mechanisms: T-dependent (TD) response, T-independent (TI) response (Fig. 4). The TD response is generated upon recognition of an antigen and requires the participation of CD4⁺ T cells (Bonilla and Oettgen 2010). First, the antigen binds to the membrane immunoglobulin receptors of B cells (Fig. 4A). Some of the antigens are internalized into specialized vesicles within the B cells, processed, allocated in the antigen-binding MHC class II, and presented to CD4⁺ T cells. Then, a T cell previously activated by an antigen-bearing APC binds to the B cell through its MHC-peptide receptor and an interaction between CD40/CD80 or CD86 (on B cell) and CD40L/CD28 (on TH cell). To finalize the activation process, the bounded T cell release cytokines (e.g., IL-2 and IL-4), and other signals to the B cell. The TI response occurs directly toward multivalent or highly polymerized antigens and does not require T-cells involvement (Bonilla and Oettgen 2010). There are two subclasses of TI antigens. Type 1 TI (TI-1) antigens interact with B cells through both membrane immunoglobulin receptors and innate immune receptors (Fig. 4B). Type 2 TI (TI-2) antigens are frequently bound by complement components and crosslink both immunoglobulin and CD21 receptors on B cells (Fig. 4C).

Fig. 4

B cell activation. A T-dependent antigens bind to the immunoglobulin receptor of B cells (1); some of the antigens are processed and presented to helper T (T_H) cells. T cells bind to the MHC-peptide antigen and deliver further activating signals to the B cell via interaction between CD40L (on T cells) and CD40 (on B cells) (2); in addition, T cells secrete activating cytokines (e.g., IL-2 and IL-4), which are recognized by receptors on the B-cell surface (3). B Type 1 T-independent antigens bind to B cells through both immunoglobulin (1) and innate immune receptors (2). (C) Type 2 T-independent antigens have the ability to crosslink both the immunoglobulin receptor and CD21 receptors on B cells (1)

Some of the antigen-activated B cells move into specific regions, where they differentiate into clusters of activated B cells. There, they complete their differentiation into plasma cells to secrete large amounts of antibodies (Moro-García et al. 2018). Some antigen-stimulated B cells also undergo further differentiation that results in the secretion of antibodies with altered sequences in their antigen-combining sites. This process generates B cells bearing receptors and secreting antibodies whose affinity for antigen increases as the immune response progresses. The antibodies generated by activated B cells can protect the body against the offending agents using different mechanisms: (i) blocking receptors that offending agents use to enter into the cells; (ii) opsonizing the offending agents by binding to or recruiting phagocytic cells; (iii) initiate the complement cascade, puncturing cell membrane (Moro-García et al. 2018). The slower activity of the adaptive immune system is due to the development of the antigen specificity. An important population of both T and B cells are preserved as memory T and B cells in this primary response, ensuring a heightened reactivity to a subsequent challenge with the same antigen (Bonilla and Oettgen 2010; Chaplin 2010).

All these mechanisms result in a successful acute inflammatory response with the clearance of the invading pathogens, dead cells, and damaged tissue. This is followed by a resolution and repair phase, which is mediated mainly by M2 macrophages, restoring tissue homeostasis.

Inflammatory mediators

The inflammatory response is coordinated by several mediators that form complex regulatory networks (Medzhitov 2008). The inflammatory mediators include substances derived from plasma proteins or that are secreted by cells (Medzhitov 2008; Kumar et al. 2018). These molecules initiate and regulate inflammatory reactions and modify the functional states of tissues and organs. Inflammatory mediators act through specific receptors on target cells, generally having multiple effects in different cell types (King 2007). Depending on the biochemical properties, the inflammatory mediators can be vasoactive amines, vasoactive peptides, fragments of complement components, eicosanoids, cytokines, chemokines, free radicals, and proteolytic enzymes (Medzhitov 2008). Table 1 presents several inflammatory mediators, as well as the main producing cells, the target cells, and the most common functions. Some of the most important inflammatory mediators will be discussed hereafter.

Table 1 Principal mediators of inflammation (Mohammed 2003; O’Connor et al. 2004; Schulze-Topphoff et al. 2008; Jutel et al. 2009; Weidinger and Kozlov 2015; Merle et al. 2015; Herr et al. 2017; Yao and Narumiya 2019; Sasaki and Yokomizo 2019; Jaén et al. 2021; Liu et al. 2021)

The two major vasoactive amines are histamine and serotonin, which are stored in the granules of mast cells, basophils, macrophages, dendritic cells, and T cells (Kumar et al. 2018). Under certain stimuli, these cells release histamine and serotonin by degranulation (Ribeiro et al. 2015). For example, they have effects on the vasculature, leading to vasodilation and thus to increase the vascular permeability (Medzhitov 2008). Vasoactive peptides promote similar events. These mediators are stored in an active form in secretory vesicles (e.g., substance P) or are generated by proteolytic cleavage of inactive precursors in the extracellular fluid (e.g., bradykinin) (Medzhitov 2008).

The complement components are synthesized by hepatocytes and by other cell types, including blood monocytes, tissue macrophages, fibroblasts, and epithelial cells of the gastrointestinal and genitourinary tracts. The complement products (e.g., C5b and C6–C9) bind to a pathogen and kill it by creating pores on its membranes. Intermediate cleavage products (e.g., C3a, C3b, C4a, and C5a) also stimulate the release of the histamine from mast cells, recruit and activate leukocytes (neutrophils, monocytes, eosinophils, and basophils), and promote phagocytosis (Medzhitov 2008; Kumar et al. 2018).

The eicosanoids comprise PGs, thromboxanes (TXs), LTs, and lipoxins (LXs) (Medzhitov 2008). PGs (e.g., PGD2, PGE2, PGF2, PGI2, and PGJ2) and TXs (e.g., TXA2) produced by mast cells, macrophages, and endothelial cells during a stimulus, are involved in the vascular and systemic reactions of inflammation (Ribeiro et al. 2015; Kumar et al. 2018). LTs (e.g., 5-hydroxyeicosatetraenoic acid—5-HETE—, 5-hydroperoxyeicosatetraenoic acid—5-HPETE—, LTB4, LTC4, LTD4, and LTE4) produced by leukocytes and mast cells, have prominent effects on bronchial smooth muscle, vasculature, and leukocyte recruitment (Ribeiro et al. 2015; Kumar et al. 2018). Conversely, LXs (e.g., LXA4 and LXB4) are important in the resolution of inflammation, by inhibiting the recruitment of leukocytes (Kumar et al. 2018).

Cytokines are signaling molecules produced by several cell types, mainly activated lymphocytes, macrophages, and dendritic cells (Kumar et al. 2018). They can be pro-inflammatory (e.g., interleukin—IL—1, IL-2, IL-4, IL-6, IL-12, IL-17, tumor necrosis factor—TNF—α, and interferon—IFN—γ) and anti-inflammatory (e.g., IL-10 and transforming growth factor—TGF—β) cytokines. The pro-inflammatory cytokines play several roles in the inflammatory process, such as leukocyte recruitment and activation, endothelium activation, and induction of the acute-phase response. The anti-inflammatory cytokines reduce or limit the inflammation in the body. Cytokines have a pleiotropic action (induce different biological effects) and are redundant (two or more cytokines mediate similar functions). They can have synergistic (combined effect of two cytokines is greater than the additive effects of the individual cytokines) or antagonistic actions (the effect of one cytokine inhibits the effect of another). Cytokines also display a cascade induction mode (action of one cytokine-induced production of one or more additional cytokines). Chemokines (e.g., IL-8 and monocyte chemoattractant protein—MCP—1), produced by several cell types, control leukocyte extravasation and chemotaxis towards the injured tissues (Medzhitov 2008). They influence the assembly, disassembly, and contractility of cytoskeleton proteins and the expression of cell-surface adhesion molecules. Both cytokines and chemokines can act in an endocrine (act on cells some distance away from the secreting cell), paracrine (act on cells near the secreting cell), and/or autocrine (act on own cell) manner.

The activated neutrophils, macrophages, and dendritic cells produce also large amounts of ROS/RNS to that also leads to the efficient killing of the offending agent (Ribeiro et al. 2015). ROS (e.g., superoxide radical—O₂^·–—, hydroxyl radical—^·OH—, peroxyl radical—ROO^·) and RNS (e.g., nitric oxide—^·NO—and peroxynitrite—ONOO^–) strongly modify the chemical structure of some molecules of the offending agent (e.g., oxidation and nitration).

Proteolytic enzymes, such as matrix metalloproteinases (MMP; e.g., MMP-2, MMP-3, MMP-10, MMP-14, MMP-19, and MMP-28), play an important role in tissue remodeling, in regulation of leukocyte migration and activation, as well as in controlling the cytokine and chemokine expression (Medzhitov 2008).

Inflammatory signaling pathways

If an antigen (e.g., PAMPS or DAMPS) or inflammatory mediator (e.g., cytokines or chemokines) binds to an immune cell receptor (e.g., PPRs), there are intracellular changes at the molecular level that regulate and coordinate the inflammatory response. These series of biochemical reactions within the cell are known as intracellular signaling pathways, also called signal transduction cascades.

The immune signaling pathways result in the enhancement or inhibition of the expression of important genes involved in the inflammatory response (Medzhitov 2010). These genes encode several inflammatory mediators, such as proteins with antimicrobial properties, chemokines, cytokines, and enzymes, that orchestrate the elimination of the offending agent and the healing of the damaged tissue.

The immune cells share common downstream inflammatory signaling pathways, including nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) (Li and Verma 2002), mitogen-activated protein kinase (MAPK) family (Arthur and Ley 2013), inflammasome activation (Guo et al. 2015), cytokine and chemokine signaling (Turner et al. 2014), cyclooxygenase (COX) and lipoxygenase (LOX) signaling (Dennis and Norris 2015), ROS/RNS mediated signaling (Conner and Grisham 1996; Mittal et al. 2014), among others.

NF-κB signaling pathway

NF-κB pathway is rapidly activated in response to a wide range of stimuli, including PAMPs, stress agents, mitogens, and pro-inflammatory cytokines (e.g., TNF and IL-1) (Li and Verma 2002; Liu et al. 2017). NF-κB transcription factor family is composed of Rel (c-Rel), RelA (p65), RelB, NF-κB1 (p50 and its precursor p105), and NF-κB2 (p52 and its precursor p100) (Hayden and Ghosh 2008). Each member, except RelB, can form homodimers and heterodimers with each other (Li and Verma 2002). NF-κB enhances or promotes target genes transcription of several inflammatory mediators involved in different immune responses, including pro-inflammatory cytokines (e.g., TNF-α, IL-1, and IL-6), chemokines (e.g., CCL2 and IL-8), ROS/RNS, CAMs, acute phase proteins, immunoreceptors, proteins involved in antigen presentation, stress-response proteins, and enzymes (e.g., COX-2 and inducible nitric oxide synthase—iNOS). It also participates in the regulation of the activation of inflammasome, and of the activation, differentiation, and effector functions of lymphocytes (Li and Verma 2002; Hayden and Ghosh 2008; Liu et al. 2017).

NF-κB pathway may be activated by two distinct pathways, namely the classical (or canonical) and the alternative (or noncanonical) pathways (Fig. 5). In the classical pathway, the NF-κB p65/p50 heterodimer is inactive in the cytosol if associated with inhibitory NF-κB (IκB) proteins (IκBα, IκBβ, and IκBε) (Li and Verma 2002). Signaling through several receptors, such PPRs (e.g., toll-like receptors—TLR—, C-type lectin receptors—CLR—, and nucleotide oligomerization domain leucine-rich repeat-containing receptors—NLR), G-protein-coupled receptors (GPCR), TNF receptor—TNFR—1, IL-1 receptor (IL-1R), TCR and BCR, recruit and activate specific intracellular adaptor proteins. This event phosphorylates the IκB kinases (IKK) complex that consists of two kinases (IKKα and IKKβ) and a regulatory subunit NF-κB essential modulatory (NEMO) (Hayden and Ghosh 2008). Consequently, the IKK complex phosphorylates IκB proteins, leading to its ubiquitination and proteasomal degradation, releasing NF-κB dimers, namely p50/p65 and p50/Rel dimers (Li and Verma 2002; Hayden and Ghosh 2008; Liu et al. 2017). Free NF-κB dimers are further activated by post-translational modifications and translocate to the nucleus where they bind to the specific site of the target genes to regulate their transcription (Hayden and Ghosh 2008). In the alternative pathway, the NF-κB RelB/p100 complex is inactive in the cytosol. Signaling through TNFR superfamily members (e.g., lymphotoxin-β receptor—LTβR) activates the kinase NF-κB-inducing kinase (NIK), which in turn activates the IKKα subunit that phosphorylates NF-κB2 precursor protein p100 (Hayden and Ghosh 2008; Liu et al. 2017). Phosphorylation of the NF-κB2 precursor protein p100 leads to its ubiquitination and proteasomal processing to NF-κB2 p52. This creates mature NF-κB p52/RelB complexes that translocate to the nucleus and induce the gene expression referred for the classical pathway.

Fig. 5

NF-κB signaling pathway. BRC, B-cell receptor; CLR, C-type lectin receptor; DNA, deoxyribonucleic acid; GPCR, G-protein-coupled receptor; IKK, inhibitor of nuclear factor kappa B kinase; IL-1R, interleukin-1 receptor; IκB, inhibitor of nuclear factor kappa B; NEMO, nuclear factor kappa-light-chain-enhancer of activated B cells essential modulatory; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; NIK, nuclear factor kappa-light-chain-enhancer of activated B cells-inducing kinase; NLR, nucleotide-binding domain and leucine-rich repeat containing; RNS, reactive nitrogen species; ROS, reactive oxygen species; TCR, T-cell receptor; TLR, toll-like receptor; TNFR, tumor necrosis factor receptor;

MAPK signaling pathway

MAPK signaling pathway consists of the activation of three protein kinases. It initiates with the phosphorylation of MAPK kinase kinase (MPAKKK), followed by the phosphorylation and activation of MAPK kinase (MAPKK), which, in turn, activates the MAPK by dual phosphorylation (Fig. 6) (Zhang and Liu 2002; Arthur and Ley 2013). Once activated, the MAPK phosphorylates diverse substrates in the cytosol and nucleus (e.g., NF-κB, c-Jun, ELK-1, and activator protein—AP—1), changing the protein function and gene expression of several cellular processes, such as cell growth, proliferation, differentiation, the production of pro-inflammatory cytokines development, cycle, survival, and death (Zhang and Liu 2002). MAPK activation also induces the production of pro-inflammatory cytokines and chemokines.

Fig. 6

MAPK signaling pathway. ASK, apoptosis signal-regulating kinase; DNA, deoxyribonucleic acid; ERK, extracellular signal-regulated kinase; JNK, C-Jun N-terminal kinase; MAPK, mitogen-activated protein kinase; MPAKK, mitogen-activated protein kinase kinase; MPAKKK, mitogen-activated protein kinase kinase kinase; RAF, rapidly accelerated fibrosarcoma; TAK, transforming growth factor-β-activated kinase

MAPK family includes extracellular signal-regulated kinase (ERK), C-Jun N-terminal kinase/stress-activated protein kinase (JNK/SAPK), and p38 kinase signaling pathways (Fig. 6) (Arthur and Ley 2013).

ERK signaling pathway

ERK1 and ERK2 are the predominant members of the ERK family (Lu and Malemud 2019). The ERK1/2 pathway is activated by growth factors (e.g., epidermal growth factor—EGF—and platelet-derived growth factor—PDGF) and cytokines (IL-1β, IL-6, and TNF-α) through receptors tyrosine kinases (RTKs), GPCRs, integrins, and ion channels (Zarrin et al. 2021). When these receptors are bound with their ligands, the adaptor proteins recruit the MAPKKK cytoplasmic rapidly accelerated fibrosarcoma (RAF) to the cell membrane for activation (Fig. 6). Activated RAF phosphorylates the MAPKK MEK1/2, which then phosphorylates the MAPK ERK1/2. It can regulate targets in the cytosol and also translocate to the nucleus where it activates several transcription factors that induce cell growth and differentiation (Soares-Silva et al. 2016). Once inside the nucleus, ERK phosphorylates and activates a transcription factor, ELK-1, which cooperates with a second protein, serum response factor (SRF), to activate the transcription of the Fos gene. The Fos protein is also phosphorylated by ERK, and along with its partner, Jun, forms the master transcription factor, AP-1. ERK induces the production of pro-inflammatory cytokines and promotes leukocyte recruitment and phagocytosis.

JNK signaling pathway

JNK family members are constituted by JNK1, JNK2, and JNK3. The JNK pathway is activated by environmental stress (e.g., oxidative stress and DNA damage), inflammatory cytokines (e.g., TNF), and growth factors (Hammouda et al. 2020). Adaptor proteins phosphorylate and activate the MAPKKK, such as MEKK1/4, apoptosis signal-regulating kinase—ASK—1 or TGF-β-activated kinase—TAK—1, which consequently phosphorylates the MAPKK MKK4/7 (Fig. 6). Activated MKK4/7 phosphorylates the MAPK JNK1/2/3 that translocates to the nucleus and activates several genes involved in apoptosis, inflammation, cytokine production, and metabolism (Hammouda et al. 2020). JNK stimulates the production of pro-inflammatory cytokines, chemokines, ROS/RNS, and adhesion molecules. It also participates in the recruitment and activation of leukocytes.

p38 signaling pathway

The p38 family members include p38α, p38β, p38γ, and p38δ. The p38 pathway is activated by environmental stress, growth factors, and inflammatory cytokines (e.g., TNF-α and IL-1β) (Canovas and Nebreda 2021). Adaptor proteins phosphorylate and activate the MAPKKK, such as MEKK1/4, ASK1, or TAK1 (Fig. 6). These molecules phosphorylate the MAPKK MKK3/6. Consequently, MAPK p38 is phosphorylated and activated, which participates in the regulation of genes involved in cell apoptosis, differentiation, and cycle regulation (Arthur and Ley 2013). The p38 also stimulates the pro-inflammatory cytokine, chemokine, and ROS/RNS production and promotes leukocyte recruitment and phagocytosis.

Inflammasome activation

NLRs are a type of PRRs that can assemble in the cytosol with other proteins forming a complex inflammasome (Guo et al. 2015). The inflammasome controls the maturation and release of cytokines (Schroder and Tschopp 2010). The variety of the NLR protein-forming scaffold enables the formation of several inflammasomes, with distinct components and activation mechanism (Schroder and Tschopp 2010). The best-characterized inflammasome is the NLR family pyrin domain containing 3 (NLRP3), expressed in monocytes, macrophages, neutrophils, dendritic cells, and some lymphocytes (Schroder and Tschopp 2010; Gaffo 2014).

The NLRP3 inflammasome is activated with several PAMPs (e.g., pore-forming toxin, and fungal, bacterial, or viral pathogens) or DAMPs (e.g., ATP and nucleic acids) (Schroder and Tschopp 2010; Guo et al. 2015). However, NLRP3 must be primed before activation. The priming initiates when the NF-κB signaling pathway is activated and the NLRP3 protein is upregulated (Fig. 7) (Guo et al. 2015). Then, NLRP3 and adaptor protein apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) suffer conformational changes to assemble into an NLRP3 inflammasome. The activation of NLRP3 is related to intracellular signals, such as potassium ion efflux, ROS, and/or leakage of lysosomal contents (Guo et al. 2015). This induces oligomerization of the pro-caspase-1, ASC, and NLRP3, leading to their proximity-induced activation (Schroder and Tschopp 2010; Guo et al. 2015). Oligomerization of pro-caspase-1 proteins induces their autoproteolytic cleavage into active caspase-1. Active caspase-1 cleaves the precursor cytokines pro-IL-1β and pro-IL-18 into mature cytokines IL-1β and IL-18, respectively (Schroder and Tschopp 2010). Active caspase-1 is also able to induce pyroptosis, an inflammatory form of cell death (Guo et al. 2015).

Fig. 7

NLRP3 inflammasome signaling pathway. ASC, apoptosis-associated speck-like protein containing a caspase recruitment domain; DNA, deoxyribonucleic acid; IL, interleukin; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; NLRP3, nucleotide oligomerization domain leucine-rich repeat-containing receptors family pyrin domain containing 3; TLR, toll-like receptor

Cytokines signaling

Cytokines are key inflammatory molecules that allows the communication between the cells of the immune system, regulating the intensity and duration of the immune response (Turner et al. 2014). Despite there are several pro-inflammatory cytokines, the ones with a function more prominent during an inflammatory response are IL-1β, IL-6, and TNF-α (Turner et al. 2014). Their expression is induced by the binding of PAMPs or DAMPs to the PRRs, through the activation of the inflammatory signaling pathways, resulting in a change in enzyme activity and gene expression.

IL-1β signaling

The IL-1β is synthesized as a precursor peptide (pro-IL-1β) and secreted in its mature form (IL-1β) by activated monocytes, macrophages, neutrophils, and dendritic cells (Gabay et al. 2010; Turner et al. 2014). IL-1β is a potent pleiotropic cytokine that stimulates both local and systemic responses. IL-1β induces the expression of adhesion molecules on endothelial cells, as well as induces the release of chemokines by stromal cells, promoting the recruitment of inflammatory cells at the site of inflammation (Gabay et al. 2010). IL-1β also induces the release of inflammatory mediators, such as PGE2 and ^·NO. Additionally, it promotes the differentiation of T_H cells into T_H17 cells and the production of IL-17. Additionally, IL-1β induces the liver to produce acute-phase proteins, which further promotes the generation of fever. IL-1β also amplifies its production, since it induces the expression of its own genes (Weber et al. 2010).

To exert its effects, IL-1β binds to IL-1R on the surface of several immune cells, such as monocytes, macrophages, and dendritic cells (Dinarello 2018). Then, the IL-1R accessory protein (IL-1RAcP) is recruited, forming a trimolecular signaling complex (Fig. 8) (Gabay et al. 2010; Weber et al. 2010). The complex immediately assembles with intracellular adaptor molecules, including myeloid differentiation primary response 88 (MyD88) and interleukin-1 receptor-associated kinase (IRAK). After phosphorylation, IRAK recruits TNF receptor-associated factor—TRAF—6 that activates NF-κB and MAPK family signaling pathways and leads to the effects previously referred.

Fig. 8

IL-1β signaling pathway. DNA, deoxyribonucleic acid; IL, interleukin; IL-1RAcP, interleukin-1 receptor 1 accessory protein; IL-R1, interleukin-1 receptor 1; IRAK, interleukin-1 receptor-associated kinase; MAPK, mitogen-activated protein kinase; MyD88, myeloid differentiation primary response 88; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; TRAF, tumor necrosis factor receptor-associated factor

IL-6 signaling

IL-6 is a pleiotropic cytokine secreted by phagocytes, T cells, and B cells (Turner et al. 2014). IL-6 induce hematopoiesis, promotes the expansion and activation of T cells, the survival of B cells and their differentiation into antibody-secreting plasma cells, the regulation and differentiation of TH2 and TREG phenotypes, and the regulation of the acute phase response (Turner et al. 2014; Hunter and Jones 2015). IL-6 regulates the neutrophil-activating chemokines and neutrophil apoptosis, promoting and modulating the cellular adhesion molecules. IL-6 has also the ability to differentiate proinflammatory macrophages into anti-inflammatory macrophage phenotypes, promoting wound healing in the terminal step of inflammation (Schaper and Rose-John 2015).

IL-6 binds to the IL-6 receptor (IL-6R), associated with a signal transducer gp130 (Hunter and Jones 2015; Schaper and Rose-John 2015). IL-6R is restricted to neutrophils, monocytes, CD4⁺ T cells, and hepatocytes, while gp130 is expressed on all cells. The receptor subunits are associated only loosely with each other in the membrane, and the cytoplasmic region of each of the receptor subunits is associated with inactive tyrosine Janus kinases (JAKs) (Shuai and Liu 2003; Villarino et al. 2017). The binding of IL-6 to its cell-surface receptor induces dimerization of the gp130. This leads to the activation of the receptor-associated JAKs family (Fig. 9) (Shuai and Liu 2003; Hunter and Jones 2015). Consequently, the activated JAKs promotes the MAPK signaling pathway (Hunter and Jones 2015; Schaper and Rose-John 2015) or phosphorylate various specific tyrosine residues of the receptor, resulting in the creation of docking sites for the inactive transcription factor signal transducers and activators of transcription—STAT—3 (Schaper and Rose-John 2015). The inactive STAT3 is then phosphorylated by JAKs, which dimerize and change the conformation, leaving the receptor. The STAT3 dimer translocates into the nucleus, which binds specific regulatory sequences to activate or inactivate the transcription of target genes, inducing cell proliferation, differentiation, migration, apoptosis, and survival (Shuai and Liu 2003; Villarino et al. 2017).

Fig. 9

IL-6 signaling pathway. DNA, deoxyribonucleic acid; IL, interleukin; IL-6R, interleukin-6 receptor; JAK, Janus kinases; MAPK, mitogen-activated protein kinase; STAT, signal transducers and activators of transcription

The adamalysin proteases ADAM17 and ADAM1 in response to apoptosis or bacterial toxins cleave a site in the IL-6R that is proximal to the plasma membrane of monocytes and activated T cells, resulting in the release of a soluble form of IL-6R (sIL-6R) (Hunter and Jones 2015). IL-6 can also bind to sIL-6R to form a complex that increases the circulating half-life of IL-6 and, consequently, its bioavailability (Hunter and Jones 2015; Schaper and Rose-John 2015).

TNF-α signaling

TNF-α is a central pleiotropic inflammatory mediator in the immune response, secreted by activated macrophages, monocytes, T cells, natural killer cells, and non-immune cells, such as endothelial cells and fibroblasts (Turner et al. 2014). TNF-α induces the transcription of several genes encoding inflammation (Faustman and Davis 2013). TNF-α facilitates the proliferation of immune cell clones, stimulates differentiation and recruitment of naïve immune cells, and coordinates the destruction of immune cell clones to reduce inflammation. TNF-α also induces the production of eicosanoids (Sedger and McDermott 2014).

TNF-α is synthesized as a transmembrane precursor protein (mTNF-α) expressed on the surface of cells (Turner et al. 2014). Metalloproteinase TNF-converting enzyme (TACE) cleaves mTNF-α, releasing TNF-α in the extracellular environment. Thus, TNF-α is able to bind and activate TNFRs (Brenner et al. 2015). Signaling through TNFRs can lead to survival or cell death, as previously referred, depending on the nature of the signal and the cellular context. For both pathways, TNF-α elicits its biological effects by binding to its receptor TNFR1 present in immune cells (Faustman and Davis 2013; Brenner et al. 2015). Binding of TNF-α to TNFR1 induces trimerization of the receptor and conformational alteration in its cytoplasmic domain (Turner et al. 2014; Brenner et al. 2015). This results in the recruitment of the TNFR1-associated death domain protein (TRADD, Fig. 10). Then, TRADD binds to receptor-interacting protein—RIP—1 kinase and the TRAF2 (Turner et al. 2014; Brenner et al. 2015). The assembling of these molecules (TRADD, RIP1, and TRAF2) is known as complex I. The survival signals are generated by the binding of cellular inhibitor of apoptosis protein—cIAP—1 and cIAP2 (Brenner et al. 2015). Consequently, the proteins of the linear ubiquitin chain assembly complex (LUBAC) proteins bind to it. Then, these molecules trigger MAPK and NF-κB signaling pathways (Brenner et al. 2015). This results in the inhibition of the caspase-8 action, favoring the survival and proliferation of the cells. Additionally, more pro-inflammatory genes are expressed (Fig. 10). Alternatively, apoptotic signals are produced by the release of silencer of death domain (SODD) protein inside the cell. Then, the complex I dissociates from the receptor and migrates to the cytoplasm where it binds to the adapter protein Fas-associated death domain-containing protein (FADD) (Sedger and McDermott 2014). These proteins recruit the initiator caspase, pro-caspase-8, forming complex II. Complex II activates the caspase-8 leads to cell apoptosis through DNA degradation (Fig. 10).

Fig. 10

TNF-α signaling pathway. cIAP, cellular inhibitor of apoptosis protein; DNA, deoxyribonucleic acid; FADD, Fas-associated death domain-containing protein; LUBAC, linear ubiquitin chain assembly complex; MAPK, mitogen-activated protein kinase; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; RIP, receptor interacting protein; TNF, tumor necrosis factor; TNFR1, tumor necrosis factor receptor 1; TRADD, tumor necrosis factor receptor 1associated death domain protein; TRAF, tumor necrosis factor receptor-associated factor

Chemokine signaling

Chemokines are small chemoattractants cytokines that bind to cell-surface receptors and induce the movement of leukocytes towards the site of inflammation (Turner et al. 2014). Chemokines are recognized by specific cell surface GPCRs with seven transmembrane domains (Rot and von Andrian 2004). Among the different chemokines, IL-8 is produced rapidly and abundantly in response to a variety of stimuli, including bacterial and viral pathogens, inflammatory cytokines, and other signaling molecules.

IL-8 signaling

IL-8, also known as C-X-C motif chemokine ligand—CXCL—8, is secreted by activated phagocytes and has a critical role in the recruitment and activation of myeloid cells to the site of inflammation (Turner et al. 2014). IL-8 activates the contractile system and enables leukocytes to migrate and adhere to endothelial cells (Baggiolini and Clark-Lewis 1992). IL-8 also promotes degranulation and elicits a rapid respiratory burst by activating the nicotinamide adenine dinucleotide phosphate (NADPH)-oxidase, leading to a fast generation of O₂^·− and hydrogen peroxide (H₂O₂).

IL-8 binds with a great affinity to two GPCRs, namely CXC motif chemokine receptor—CXCR—1 and 2. This leads to their activation, with consequent dissociation of the G protein from the receptor and separation into Gα and Gβγ subunits (Baggiolini and Clark-Lewis 1992; Rot and von Andrian 2004). The Gα subunit activates the membrane-bound adenylate cyclase (AC), which generates cyclic AMP (cAMP). Consequently, cAMP activates protein kinase A (PKA), leading to the activation of the MAPK ERK1/2 signaling pathway (Turner et al. 2014). The Gβγ subunit activates the enzyme phosphoinositide-3 kinase (PI3K) to phosphorylate the phospholipid phosphatidyl inositol bis-phosphate (PIP2) into phosphotidylinositol-3,4,5-trisphosphate (PIP3) (Rot and von Andrian 2004). PIP3 activates Ras-related C3 botulinum toxin substrate (Rac), which in turn activates more molecules that stimulate actin-related protein—Arp—2/3. This induces actin polymerization, responsible for the development and forward extension of the pseudopod. Gβγ subunit also activates a small cytoplasmic G protein, Rho, responsible for the movement of the cell through a gradient. Moreover, Gβγ also activates phospholipase β (PLCβ) activity, which hydrolyzes PIP2, cleaving the sugar inositol trisphosphate (IP3) from the diacylglycerol (DAG) backbone (Turner et al. 2014). IP3 is released into the cytoplasm, where it interacts with specific IP3 receptors on the surface of endoplasmic reticulum vesicles, inducing the release of stored calcium ions (Ca²⁺) into the cytoplasm, leading to the degranulation of neutrophils. DAG recruits and activates the enzyme protein kinase C (PKC), initiating a cascade that ultimately recruits TRAF6, leading to the activation of NF-κB and MAPK signaling pathways.

Eicosanoid signaling

Eicosanoids are lipid signaling molecules produced by monocytes, macrophages, neutrophils, eosinophils, mast cells, basophils, dendritic cells, and B cells (Henderson 1994; Harizi et al. 2008; Dennis and Norris 2015). Eicosanoids arise from the oxidation of arachidonic acid (AA) present in the cells membrane. An inflammatory stimulus increases the intracellular influx of Ca²⁺, which recruits phospholipase A2 (PLA2) enzymes, namely cytosolic PLA2 (cPLA2), to the cell membrane to release AA (Henderson 1994; Harizi et al. 2008). After, free AA is oxidized by COX and LOX, generating eicosanoids, such as PGs, TXs, LTs, and LXs (Harizi et al. 2008; Dennis and Norris 2015). Eicosanoids promote the induction of edema from postcapillary venules, mediating the leukocyte recruitment to the site of inflammation (Henderson 1994).

COX-2 signaling

COX-2 is an inducible enzyme primarily expressed in cells involved in inflammation, such as macrophages (Aoki and Narumiya 2012). COX-2, activated by several inflammatory stimuli (e.g., LPS, IL-1β, IL-6, TNF-α), converts AA into unstable PGG2, which is further converted to PGH2. Afterward, cell-specific prostaglandin and thromboxane synthases (e.g., PGES, PGDS, PGFS, PGIS, TBXAS1) converts PGH2 into PGs (e.g., PGE2, PGD2, PGF2a, PGI2, and PGJ2) and TXs (e.g., TXA2), respectively (Dennis and Norris 2015). PGD2 and PGE2 are also metabolized to 15-deoxy-PGJ2 and 15-keto-PGE2, respectively. After generation, PGs and TXs are rapidly released from cells and bind to GPCRs on membranes, namely PGE receptor (e.g., EP1, EP2, EP3, EP4), PGD receptor (e.g., DP1 and DP2), PGF receptor (e.g., FP), PGI receptor (e.g., IP) and TXA receptor (e.g., TP) (Harizi et al. 2008; Aoki and Narumiya 2012). PGs enhance the expression of inflammation-related genes, amplifying cytokine expression (Aoki and Narumiya 2012). Particularly, PGE2 binds to EP1, EP2, EP3, and EP4 and promotes vasodilation, vascular leakage, hyperalgesia, and fever (Dennis and Norris 2015). PGD2 binds to DP1 to promote mast cell maturation and vasodilation, while its binding to DP2 promotes eosinophil recruitment. Conversely, PGE2 also plays an anti-inflammatory role, increasing IL-10 levels and reducing TNF-α levels. TXA2 increases platelet aggregation and promotes vasoconstriction when bound to TP.

LOX signaling

LOX enzymes, including 5-LOX, 12-LOX, and 15-LOX, generates LTs, HPETE, and HETE (Dennis and Norris 2015).

5-LOX, activated by Ca²⁺, translocates from the cytosol to the cell membrane, where it converts AA, in the presence of 5-lipoxygenase activating protein (FLAP), to 5-HPETE. 5-HPETE is further reduced to a more stable form, 5-HETE, by a peroxidase (Ribeiro et al. 2015). On the other hand, 5-HPETE can be consecutively converted into the precursor LTA4 (Henderson 1994). LTA4 can be synthesized into LTB4 through the action of the cytosolic LTA4 hydrolase in neutrophils and macrophages. LTA4 can also be converted by LTC4 synthase into cysteinyl leukotrienes in eosinophils, mast cells, macrophages, and endothelial cells. The LTC4 can be metabolized extracellularly to LTD4 by the action of transpeptidases (Ribeiro et al. 2015). LTD4 is further converted to LTE4. LTs exert their effects via surface receptors. LTB4 binds to LTB4 receptor—BLT—1 and 2, members of GPCRs, coordinating and amplifying the inflammatory response (Henderson 1994; Harizi et al. 2008; Dennis and Norris 2015). LTB4 promotes neutrophil chemotaxis, adhesion of neutrophils to the endothelium, neutrophil degranulation and lysosomal enzyme release, O₂^·– generation, pain, myelopoiesis, increase IL-6 production, and T lymphocyte proliferation. It also mediates the activation of the NF-κB transcription factor. LTC4, LTD4, and LTE4 bind to cysteinyl leukotriene receptors—CYSLT—1 and 2 and induce airway, gastrointestinal, and bronchial smooth muscle contraction (Henderson 1994; Dennis and Norris 2015). It mediates the activation of the MAPK transcription factor (Ribeiro et al. 2015). LTC4, LTD4, and LTE4 also increase the venular endothelium that allows for proinflammatory cells migration to the site of inflammation, contributing to the development of edema. The leukotrienes enhance the mucous secretion in the airways and stimulate the production of IL-1β.

LXs can be generated at mucosal surfaces via leukocyte–epithelial cell interactions and within the vascular lumen during platelet–leukocyte interactions (Ribeiro et al. 2015). LXs, including LXA4 and LXB4, can be produced by the conversion of LTA4 by 5-LOX or 12-LOX (Henderson 1994; Dennis and Norris 2015). Additionally, 5-LOX and 15-LOX can metabolize AA into 15-HEPTE, which is subsequently converted into LXA4 and LXB4. LXs bind to G protein-coupled lipoxin A4 receptor/formyl peptide receptor (ALX/FPR2), playing a role in the resolution of inflammation (Chandrasekharan and Sharma-Walia 2015). LXs inhibit the immune growth of the cells, suppress cytokine synthesis, and release, and modulate the levels of immune cells to induce the resolution phase of inflammation. LXs mediate its action through the inhibition of the NF-κB, MAPK, and STAT signaling pathways.

ROS/RNS-mediated cellular signaling

ROS and RNS, as previously described, are produced by phagocytic cells to kill the pathogen. They promote direct toxicity in the pathogen through molecule oxidation, hydroxylation, chlorination, nitration, and S-nitrosylation, along with the formation of sulfonic acids and the destruction of iron-sulfur clusters in proteins (Conner and Grisham 1996; Mittal et al. 2014). During the inflammatory response and in the presence of high concentrations of oxygen, the NADPH oxidase enzyme, present in the membrane of the cells, generates large amounts of O₂^·– (Conner and Grisham 1996). Activated neutrophils and macrophages also secrete the enzyme myeloperoxidase (MPO), which uses the H₂O₂, produced by dismutation of O₂^·–, to oxidize chloride ions (Cl^–) into hypochlorous acid (HOCl), a powerful anti-bacterial agent (Halliwell 1991; Conner and Grisham 1996). Moreover, free radicals can interact between them, resulting in the generation of new radical species. For instance, ^·NO can react rapidly with O₂^·– producing a reactive ONOO^– (Gutteridge 1994; Conner and Grisham 1996). Importantly, the decomposition of ONOOH involves the intermediate generation of the highly reactive ^·OH and nitrogen dioxide (^·NO₂) radicals. Moreover, H₂O₂ has the ability to diffuse across the cell membrane, having a high capacity to form ROS in the presence of transition metals (e.g., iron and cupper), also generating ^·OH (Gutteridge 1994). Additionally, O₂^·– promotes the release of redox-active iron from iron storage proteins, such as ferritin, thereby providing the catalyst for ^·OH generation (Halliwell 1991). ROS can also activate signaling pathways (NF-κB and p38 MAPK) and upregulate several genes involved in inflammation (Mittal et al. 2014). ROS promotes the synthesis of proinflammatory cytokines (e.g., IL-1β, IL-6, and TNF-α), as well as participates in the activation of the inflammasome. Additionally, ROS controls the extravasation of leukocytes through the promotion of endothelial integrity disassembly and the expression and activation of CAMs (e.g., E-selectin, intercellular adhesion molecule—ICAM—1—, vascular cell adhesion molecule—VCAM—1). This results in the higher endothelial permeability and adhesion of leukocytes to the site of inflammation, respectively. Under an inflammatory scenario, the transcription of the iNOS gene is also activated and large amounts of ^·NO are produced by the oxidation of L-arginine to L-citrulline (Martinez and Andriantsitohaina 2009). ^·NO is a potent antimicrobial and vasodilator agent, also contributing for the elimination of the harmful agent.

Resolution of inflammation

The final event of an acute inflammatory response is its successful resolution, restoring the homeostasis of tissue (Medzhitov 2008). The resolution of inflammation is characterized by the decrease of leukocyte infiltration, clearance of inflammatory cells, and restore of macrophages and lymphocytes to their normal pre-inflammatory phenotypes (Serhan and Savill 2005). Leukocytes have the ability to trigger a self-limiting response to acute inflammation, switching the generation of pro-inflammatory (e.g., PGE2 and PGD2) to anti-inflammatory eicosanoids (e.g., LXA4 and LXB4). As previously referred, LXs and HETES can suppress neutrophil chemotaxis, vascular dilatation, permeability, fibrosis, and pain (Nathan and Ding 2010). They also promote the phagocytosis of apoptotic neutrophils, initiating tissue remodeling (Nathan and Ding 2010). Additionally, the phagocytosis of apoptotic cells can stimulate macrophages to release anti-inflammatory mediators, such as TGF-β1 and IL-10. These molecules are suppressors of classical pro-inflammatory macrophage activation and mediators of tissue repair (Serhan and Savill 2005; Nathan and Ding 2010). Mast cells, basophils, and TH2 cells also release IL-4 to enhance the switch of pro-inflammatory to anti-inflammatory macrophage phenotype to suppress inflammation and favor wound healing (Nathan and Ding 2010). Myeloid-derived suppressor cells, a macrophage population, have the ability to suppress T cell responses. Moreover, immune cells, such as macrophages and neutrophils, independently convert omega-3 polyunsaturated fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) to resolvins and protectins, which are anti-inflammatory lipid mediators (Serhan and Savill 2005).

Antioxidant defenses or scavenging systems are used to remove the excess of ROS and RNS to prevent cellular damage. Antioxidants are also chemical compounds obtained from food, such as α-tocopherol (vitamin E), ascorbic acid (vitamin C), flavonoids, and polyphenols (Burton et al. 1985). In addition, the induction of several antioxidant and cryoprotective genes is mainly regulated by the redox-sensitive transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) (He et al. 2020). Under physiological conditions, Nrf2 is inactivated in the cytosol via ubiquitination by Kelch-like ECH-associated protein 1 (KEAP1). When the cell is exposed to mild oxidative stress, ROS/RNS can disrupt the Nrf2-KEAP1 complex and Nrf2 is activated (Fig. 11). Free Nrf2 translocates into the nucleus and dimerizes with specific molecules. Then, it binds to antioxidant response elements (ARE), inducing the activation of antioxidant and cytoprotective genes, encoding quinone oxidoreductase-1 (NQO-1), glutathione peroxidase (GPx), heme oxygenase-1 (HO-1), glutathione peroxidase (GSH-Px), glutathione-S-transferase (GST), catalase (CAT), and superoxide dismutase (SOD). Therefore, the Nfr2-KEAP1 pathway confers protection against cellular oxidative stress by boosting the expression of antioxidant enzymes, being essential for the resolution of inflammation and repair of tissue damage (Nathan and Ding 2010).

Fig. 11

Nrf2-KEAP1 signaling pathway. DNA, deoxyribonucleic acid; KEAP1, Kelch-like ECH-associated protein 1; Nrf2, nuclear factor erythroid 2-related factor 2; RNS, reactive nitrogen species; ROS, reactive oxygen species

Dysresolution of the immune system

The immune system response may not always lead to the expected outcomes. Indeed, if the immune systems fail to effectively eliminate the offending agent, a state of chronic inflammation can ensue. Conversely, the immune response can also be compromised, leading to the development of immunodeficiency. Further elucidation of these events will be provided in this section.

Chronic inflammation

If the resolution phase fails, the inflammatory response can become exacerbated in magnitude and time, leading to chronic inflammation. For example, the inflammatory response can be prolonged by a failure of neutrophils to undergo apoptosis, of macrophages to clear apoptotic neutrophils, and/or by deficiency of other factors that promote ingestion of apoptotic cells by macrophages (Nathan and Ding 2010). Additionally, a failure in the suppression of T cells prolongs the expansion and activation of myeloid-derived suppressor cells. A failed phenotypic switch in macrophage and T cell populations and an inadequate production of resolution mediators also lead to the non-resolution of inflammation. Chronic inflammation can also arise in the presence of some microorganisms, such as mycobacteria, that are difficult to eradicate, culminating in persistent infections and after prolonged exposure to potentially toxic agents (e.g., exogenous particulate silica and endogenous cholesterol). Finally, an excessive and inappropriate activation of the immune system against self-antigens (e.g., autoimmune diseases) and common environmental substances (e.g., allergic diseases) can also lead to chronic inflammation (Nakayama 2017; McInnes and Gravallese 2021).

Chronic inflammation is characterized by a long duration (weeks, months, or even a lifetime) (Oronsky et al. 2022). The hallmarks of chronic inflammation are the constant infiltration and presence of macrophages and lymphocytes, angiogenesis, tissue destruction, and fibrosis (Nathan and Ding 2010). Macrophages are the dominant cells in most chronic inflammatory reactions. Activated macrophages contribute to the prolonged inflammatory state by secreting ROS/RNS, cytokines, chemokines, and eicosanoids that activate several cells, notably T cells (Nathan and Ding 2010). An excess of ROS/RNS can be accumulated in cells and tissues, a process called oxidative and nitrosative stress (Martinez and Andriantsitohaina 2009; Mittal et al. 2014). These high amounts of ROS/RNS in the tissues alter the normal redox state within the cell environment, which can irreversibly damage several biomolecules, resulting in the loss of their function. For instance, ROS/RNS can lead to proteins unfolding and fragmentation, changing their conformation, interactions with other biological molecules, and turnover, as well as can damage the purine and pyrimidine bases of DNA, resulting in deoxyribose oxidation, strand breakage, mutations, and alterations in the encoded proteins and enzymes (Mönig et al. 1987; Aruoma et al. 1989). Moreover, ROS/RNS can induce lipid peroxidation, a free radical chain reaction, in both cellular and organellar membranes, yielding ROO^· (Halliwell 1991). The accumulation of lipid peroxides in the membrane disrupts its function, inactivating, e.g., receptors and membrane-bound enzymes, which can culminate in its collapse. The activated T cells, as well as B cells, amplify and propagate chronic inflammation by the production of cytokines and recruitment and activation of more immune cells. These interactions strongly contribute for the cycle—positive feedback—of chronic inflammation. Because of this dysregulated scenario, prolonged and continuous tissue damage can be translated into a chronic inflammatory disease, such as arthritic diseases (e.g., osteoarthritis and rheumatoid arthritis), autoimmune diseases (e.g., psoriasis, inflammatory bowel disease, and type 1 diabetes), neurodegenerative diseases (e.g., multiple sclerosis, Alzheimer’s disease, and Parkinson’s disease), hypersensitivity reactions (e.g., allergies), asthma, and cancer. Indeed, more than half of global deaths are correlated with chronic inflammatory diseases (Furman et al. 2019) and, consequently, World Health Organization ranked them as the most significant cause of death in the world nowadays (WHO (World Health Organization) 2020a, b). Moreover, its prevalence is estimated to increase persistently for the next decades (WHO (World Health Organization) 2022).

Immunodeficiency

Immunodeficiency results from the failure or absence of elements of the immune system (e.g., phagocytes, lymphocytes, and complement system) to fight against infectious diseases (Pac and Bernatowska 2016). This immunodeficiency can be either primary or secondary (acquired). The first results from an inherited genetic or developmental defect in the immune system. Examples are the Bruton disease (B-cell deficiency) (Hernandez-Trujillo et al. 2014), DiGeorge syndrome (T-cell immunodeficiency) (Lambert et al. 2018), Wiskott-Aldrich syndrome (T-cell and B-cell deficiencies) (Byrne et al. 2018), Bare leukocyte syndrome (MHC deficiency) (Aluri et al. 2018), and chronic granulomatous disease (phagocyte deficiencies) (Zhou et al. 2018). The secondary immunodeficiency occurs when the immune system is compromised by an external agent, such as a treatment or illness. The most well-known secondary immunodeficiency is the acquired immunodeficiency syndrome (AIDS), which results from infection with the human immunodeficiency virus (HIV) (Ndjoyi-Mbiguino et al. 2020). Thus, these individuals have an increased risk of opportunistic infections. Usually, these microorganisms are easily eradicated in healthy organisms, but they can cause disease and even death in those with significantly impaired immune function.

Anti-inflammatory drugs in the market

At the present time, there is no cure for chronic inflammatory diseases, being the main goals of current treatments to reduce pain and improve tissue function. Symptoms are usually treated with a combination of therapies, including, for instance, co-administration of painkillers (e.g., acetaminophen) and/or nonsteroidal anti-inflammatory drugs (NSAIDs, e.g., salicylic acid, diclofenac, and celecoxib) (Bullock et al. 2018). More potent drugs, such as corticosteroids (e.g., dexamethasone and betamethasone), conventional disease-modifying antirheumatic drugs (cDMARDs; e.g., methotrexate), biologic (b)DMARDs (e.g., TNF or IL-6 inhibitors), and targeted synthetic (ts)DMARDs (e.g., JAK inhibitors) can also be administered (Aletaha and Smolen 2018; Oo et al. 2018). Nonetheless, these anti-inflammatory drugs can be associated with severe side effects mainly if administered for long periods (e.g., gastrointestinal, cardiovascular, blood, renal, and adrenal injuries), being their use limited (Bindu et al. 2020; Noetzlin et al. 2022). In addition, immune-suppressant agents present an increased risk for infection and/or cancer (Ben Mrid et al. 2022). Therefore, newer and safer drugs are urgently needed to efficiently treat chronic inflammatory diseases.

Nature as a source of new drugs

Nature is a broad source of bioactive compounds. In fact, the approval of natural products and their derivatives for the clinic has been increasing over the years (Cragg et al. 1997; Newman et al. 2003; Newman and Cragg 2007, 2012, 2016, 2020). From 1981 to 2019, more than half of approved drug entities (a total of 1881) are derived from natural sources (Newman and Cragg 2020). From 2019 to 2022, Food and Drug Administration (FDA) authorized 31 new drugs inspired by natural products (de la Torre and Albericio 2020, 2021, 2022, 2023). These new chemical entities include natural, semisynthetic, and synthetic natural products. The major disease areas with the highest number of drug approvals are infectious diseases (402 drugs), cancer (247 drugs), hypertension (82 drugs), antidiabetic (63 drugs), and inflammation (53 drugs) (Newman and Cragg 2020). More specifically, from the 53 approved anti-inflammatory drugs, 13 were natural product derivatives and 1 was a synthetic drug obtained after identification of the natural product (Newman and Cragg 2020). A small number of immunostimulatory natural-derived drugs were also approved. Of the 14 immunostimulants, 8 have origin in botanical drugs (defined mixture), 3 were unaltered natural products, and 2 were natural product derivatives (Newman and Cragg 2020). Particularly, new molecular entities derived from non-mammalian natural products are mainly obtained from plants (45%), followed by bacteria (29%) and fungi (22%) (Patridge et al. 2016). These data highlight the important role of plants in drug discovery. Indeed, they present compounds with unique structural diversity. Thus, this opens several opportunities for the research of novel drugs.

Plant-derived bioactive compounds in the market

Plants synthesize several chemical compounds, known as secondary metabolites, to protect themselves against surrounding environment adversities, such as herbivores, pathogens and abiotic environmental stresses (Zaynab et al. 2018; Holopainen et al. 2018; Isah 2019; Divekar et al. 2022). Interestingly, many of those compounds have positive effects in the treatment of certain human diseases. In fact, plants have been the basis of the traditional medicine in many cultures for thousands of years to treat several conditions. Even nowadays, plants are widely used by the world population, especially in developing countries, as an important part of the primary healthcare (WHO (World Health Organization) 2011). Thus, the search of plant-derived compounds as potential drugs increased, leading to the isolation of many therapeutically relevant natural products. Indeed, widely used pharmaceuticals, including analgesics, and anti-inflammatory and anti-cancers drugs, among others, arise from compounds present in plants. Table 2 comprises some of the plant-derived bioactive compounds in clinical use. The first plant-derived drug included in the clinical use was morphine, a strong analgesic isolated from the Papaver somniferum (Sertuerner 1817). Salicylic acid, obtained from the bark of the willow tree Salix alba L., was used to produce acetylsalicylic acid, known as aspirin in the market to treat inflammatory conditions (Buchner 1828). The most widely used anti-cancer drugs are also derived from plants, namely paclitaxel (Taxus brevifolia), as well as vincristine and vinblastine (Catharanthus roseus) (Noble et al. 1958; Svoboda et al. 1961; Wani et al. 1971). All these mentioned compounds are considered the safest and most effective medicines required in a health system by the World Health Organization’s List of Essential Medicines (World Health Organization 2019).

Table 2 Examples of plant-derived bioactive compounds in clinical use (Cragg et al. 1997; Newman et al. 2003; Newman and Cragg 2007, 2012, 2016, 2020; G. de la Torre and Albericio 2019; de la Torre and Albericio 2021, 2022)

Plant-derived bioactive compounds under clinical trials

Many plant-derived compounds are currently under clinical trials for the treatment of several diseases. Besides individual chemical entities, formulations of whole plant extracts are also investigated due to the synergistic and additive effects that can arise from the different bioactive compounds. Indeed, these more complex mixtures usually involve one or more bioactive compounds with known biological activities at high concentrations. Moreover, as several chemical compounds are present in these plant mixtures, the standardization process regarding the content of the main active principle(s) must be employed (Govindaraghavan and Sucher 2015). Therefore, it is ensured the safety, efficacy, and quality of the plant extract formulation.

For the treatment of immune system-related diseases there are several examples of plant-based drugs or formulations under clinical trials. For instance, a plant-based medication composed of concentrated extracts of Dioscorea cirrhosa, Impatiens balsamina, Eclipta prostrata, Phyllanthus urinaria, Adenosma glutinosum, mixed with ascorbic acid was used to elevate T and B lymphocytes levels and decrease the damage of HIV on CD4⁺ T cells (phase 4, completed, NCT04770701). Indeed, the mixture involve compounds with complementary activities. The bioactive compound diosgenin, presented in Dioscorea cirrhosa, is a precursor of cortisol and when combined with ascorbic acid helps to balance the cortisol levels, reducing cell inflammation. Flavonoids present in Impatiens balsamina (e.g., kaempferol, quercetin, rutin, and astragalin) reduce the impact of the virus on T cells, leads to the production of antihistamine compounds, and protect the cell from inflammation. The flavonoids present in Eclipta prostrata, Phyllanthus urinaria, and Adenosma glutinosum also protect and increase the CD4⁺ T cell population.

Two natural oils obtained from borage and echium seed were employed in asthma treatment (phase 3, completed, NCT01560988). This formulation aimed to decrease asthma symptoms through the reduction of LTC4 production by granulocytes. Another clinical trial analyzed the benefits of consuming ginger by individuals with asthma (early phase 1, completed, NCT03705832). Ginger blocks one of the critical inflammatory pathways in asthma, reducing the airway inflammation and relaxation of airway smooth muscle, as well as the serum levels of asthma-related inflammatory markers.

Nabiximols (trade name Sativex), an oromucosal spray formulation containing a 1:1 fixed ratio of delta-9-tetrahydrocannabinol (THC) and cannabidiol (CBD) derived from cloned Cannabis sativa L., was proposed for multiple sclerosis (completed, NCT03186664). The main active substance, THC, acts as a partial agonist of human cannabinoid receptors (CB1 and CB2) and may modulate the effects of excitatory (glutamate—GLU) and inhibitory (gamma-aminobutyric acid—GABA) neurotransmitters, leading to muscle relaxation, which in turn is responsible for spasticity improvement. In addition, the anti-inflammatory properties of the cannabinoids THC and CBD in individuals across the weight spectrum will also be assessed (recruiting, NCT04114903). The effects of nanocurcumin, the main active principle of turmeric, in multiple sclerosis was also evaluated (phase 2, completed, NCT03150966). A reduction of the number of TREG and TH17 cells, of the expression levels of their associated transcription factors, and of the secretion levels of cytokines is expected.

Inconsistences in pharmacological effects of plant-based formulations

The description in the literature of different therapeutic outcomes using “similar” plant-derived formulations occurs with some frequency. These incongruences among the published studies is intimately related to the use of formulations with different composition and purity, as well as the employment of different extraction techniques, in vitro assays, animal models, and administration routes. Indeed, the characteristics of the plant used, as well as the experimental procedure employed for the recovery of bioactive compounds demonstrate a strong impact in the biological activity. Thus, factors, such as (i) specie, (ii) age, (iii) selected organ, (iv) developmental stage, (v) growth conditions, (vi) geographical location, (vii) drying methods, (viii) storage conditions, (ix) solvent of extraction, and (x) extraction method are important in defining the final composition of the plant extract. Therefore, the extracts with the strongest bioactivity are obtained by the selection of the optimal conditions of these factors.

Plant specie

Plants are catalogued on different levels according to their morphological, histological, genetic, and molecular characteristics (Maskour et al. 2022). At the species level, the plants share certain common characteristics and are closely related genetically. However, different plant species can produce extracts with distinct compositions. For instance, Farhat et al. studied the extracts content obtained from different Salvia species and detected high variable composition between them (Farhat et al. 2013). In this study, the phenolic content was higher in S. officinalis, followed by S. aegyptiaca, S. verbenaca, and S. argentea. Moreover, large amounts of the bioactive compounds caffeic acid, rosmarinic acid, carnosic acid, and carnosol were found in S. officinalis. S. argentea exhibited amounts of caffeic acid and rosmarinic acid approximately four times lower. The other species presented carnosic acid and carnosol at levels around a hundred times lower. Consequently, S. officinalis extracts exhibited the highest antioxidant activity. Therefore, plant species can exhibit significant variations in their chemical composition, resulting in different levels of bioactivity.

Age of the plant

The biological age stage (e.g., juvenile, young, mature, and old) significantly affects the characteristics of the plant (Gatsuk et al. 1980). These characteristics are correlated with anatomical, physiological, and biochemical changes, such as the capture of nutrients, the development of the pattern of branching of the root and shoots, the form of the leaves, among others. Thus, it is not surprising that, for example, an adult plant presents different features compared to a young plant (Henn and Damschen 2021). Particularly in compounds composition, Lavandula angustifolia essential oils, for example, evidenced a decrease in monoterpenes and sesquiterpenes concentration in two consecutive years (the second and third year after planting) (Pistelli et al. 2017). The young leaves of Olea europaea also presented a high amount of secoiridoids (e.g., oleuropein, oleoside, demethyloleuropein, 2″-methoxyoleuropein, ligstroside, and nuzhenide) in comparison with the aged leaves (Abaza et al. 2017). Therefore, the biological age of the plants influences its biochemical changes, leading to variations in compound composition and, consequently, potential implications in their properties and benefits.

Organ of the plant

A plant is composed of different organs, such as roots, stems, leaves, buds, flowers, and seeds (Simpson 2019). Each organ is responsible for different functions in the plant organism. Consequently, distinct chemical compositions will be observed in each of them. For instance, an aqueous extract prepared from flowers, leaves, or roots of Echinacea purpurea showed a particular chemical fingerprint (Vieira et al. 2022). Alkylamides were more abundant in roots extracts, while phenolic/carboxylic acids were richer in leaves extracts. Flowers extracts showed a balanced mix of both chemical compounds. These extracts exhibited the strongest anti-inflammatory activity in the reduction of IL-1β production in LPS-stimulated macrophages, demonstrating that the mixture of compounds can be beneficial. Therefore, when specific bioactivity is required, careful consideration should be given to the selection of the appropriate plant organ.

The development stage of the plant

The plant life-cycle has cascading effects on multiple levels of its biological organization (Stucky et al. 2018). These phenological phases include processes, such as leaves emergence, flowering, fruiting, seeding, and leaves senescence, that lead to severe alterations of plant metabolites composition (Gray and Ewers 2021). For example, the concentration and composition of secondary metabolites changed significantly with the flower developmental stages (e.g., bud, full blooming, and wilting) of E. purpurea (Thomsen et al. 2018). The highest concentration of alkylamides in aerial parts was achieved when those parts were harvested in the wilting stage, whereas the highest concentration of caffeic acid derivatives was achieved when those parts were collected in the bud stage. Particularly, the content of the alkylamides dodeca-2E,4E,8Z,10E/Z-tetraenoic acid isobutylamide, undeca-2E,4Z-diene-8,10-diynoic acid isobutylamide, and trideca-2E,7Z-diene-10,12-diynoic acid isobutylamide in aerial parts significantly increased with the harvesting of the flowers at the development stages from but to wilting. Conversely, the content of dodeca-2Z,4E-diene-8,10-diynoic acid isobutylamide, caftaric acid, echinacoside, and chicoric acid decreased from flower bud stage to wilting stage. It was also observed that the oleuropein levels increased within the flower maturation (from floral buds to open flowers) of the O. europaea (Abaza et al. 2017). The amounts of rutin, apigenin, and luteolin glucoside followed the inverse tendency. Therefore, the development stage of the plant has a strong impact on the plant composition, highlighting the importance of considering the optimal harvesting stage for desired compound profiles.

Growth conditions

To obtain a healthy growth of a plant it is required light, water, nutrients (e.g., nitrogen, phosphorus, and potassium), and an adequate soil (Sun et al. 2022). However, when the growth conditions are not ideal—stress growth conditions—, the plants can adapt to the surrounding environment and switch their mechanism to an increased production of secondary metabolites (Isah 2019). Nowadays, different industrial and pharmaceutical crops have been using this approach to increase the production of the bioactive compounds of interest in plants. For example, the salinity stress increased the production of caftaric acid, cynarin, and chicoric acid in E. purpurea roots (Sabra et al. 2012). The drought stress intensification also increased the amount of chicoric acid and caftaric acid in E. purpurea roots (Attarzadeh et al. 2020). The combination of phosphorous fertilizer with biological resources, such as mycorrhizal arbuscular fungi and/or the bacteria Pseudomonas fluorescens, also markedly improved the concentration of chlorogenic acid, caftaric acid, chicoric acid, cynarin, and echinacoside in E. purpurea roots (Attarzadeh et al. 2020). For L. angustifolia, it was observed that high levels of phosphorous and nitrogen strongly affected its growth and root development, respectively (Chrysargyris et al. 2016). These nutrients also led to an improvement in the synthesis of some constituents (e.g., 1,8-cineole, α-terpineol, trans-pinocarveol, camphor, borneol, myrtenal, α-bisabolol, and pinocarvone) and, therefore, enhanced antioxidant activity of the aqueous methanolic extracts. Interestingly, plant propagation also has a critical effect on secondary metabolites (Dušková et al. 2016). The total amount of essential oil recovered from L. angustifolia flowers was higher in cultivars propagated vegetatively than in cultivars propagated by seeds (Dušková et al. 2016).The seasonal conditions, in particular weather conditions (e.g., temperature and precipitation), are also extremely important in plant development (Hatfield and Prueger 2015). Plants also adapt to significant seasonal variations, suspending and becoming physiologically inactive, a state-denominated dormancy (Hu et al. 2022). Particularly, high or low temperatures strongly affect the metabolic process of plants. For instance, the phenolic acids, flavonoids, and tocopherols content in Taraxacum officinale was higher in the winter-early spring, in comparison with mid-late spring (Petropoulos et al. 2019). The phenolic acids and alkylamides content in E. purpurea roots also changes during seasonal conditions (early winter, early spring, late spring, summertime, and mid-autumn) (Thomsen et al. 2012). Undeca-2E,4Z-diene-8,10-diynoic acid isobutylamide, undeca-2Z,4E-diene-8,10-diynoic acid isobutylamide, undeca-2E,4Z-diene-8,10-diynoic acid 2-methylbutylamide, dodeca-2E,4E,10E-triene-8-ynoic acid isobutylamide, and trideca-2E,7Z-diene-10,12-diynoic acid isobutylamide reached the highest concentration during summer. The highest concentration of dodeca-2E,4Z-diene-8,10-diynoic acid isobutylamide, dodeca-2Z,4E-diene-8,10-diynoic acid isobutylamide, dodeca-2E,4E-diene-8,10-diynoic acid 2-methylbutylamide, dodeca-2Z,4E-diene-8,10-diynoic acid 2-methylbutylamide, and dodeca-2E,4Z,10E-triene-8-ynoic acid 2-methylbutylamide was detected during early spring. Conversely, dodeca-2E,4E,8Z,10E/Z-tetraenoic acid isobutylamide, dodeca-2E,4E,8Z-trienoic acid isobutylamide and dodeca-2E,4E-dienoic acid isobutylamide did not have a significant change of their levels throughout the year. Considering phenolic compounds, their highest concentration was reached in spring. More particularly, echinacoside showed the highest content in early spring and both chicoric acid and caftaric acid showed the greatest amount in late spring. For L. angustifolia, a decrease in the average temperature resulted in a gradually reduction of the essential oil percentage (Hassiotis et al. 2014). The relative amount of linalool was negatively affected after the rainfall and several days of low temperatures, while the synthesis of the α-terpineol, borneol, lavandulyl acetate, 1,8-cineole, and terpinene-4-ol was enhanced under these environmental conditions. Indeed, the environmental factors play a crucial role in shaping the production and composition of secondary metabolites in plants. Factors, such as growth conditions, nutrient availability, propagation methods, and seasonal variations have a significant impact on the final plant extract. Understanding and effectively manipulating these factors become crucial to optimize the production of desired compounds for industrial and pharmaceutical applications.

Geographical location

The geographical location—latitude, longitude, and altitude—of plants can provide different conditions of, e.g., atmospheric pressure, precipitation, solar radiation, and soil nutrients (Daco et al. 2021). Plants can adapt to these factors by changing their genetic fingerprint, growth rate and metabolites production (Li et al. 1998). For instance, S. officinalis cultivated at high altitudes favored the monoterpene synthesis, while the sesquiterpene synthesis was mainly privileged at low altitudes (Ben Farhat et al. 2009). Sage extracts obtained from low altitudes demonstrated the most powerful antioxidant activity. Therefore, the geographical location of plants strongly influences their genetic traits, growth patterns, and metabolite production, leading to distinct compositions and potential health benefits.

Drying methods

Drying is the most employed post-harvesting process to store the plants over long periods (Hazrati et al. 2021). The drying process of plant material inhibits microorganism growth and prevents certain biochemical changes in metabolites. However, the drying method (e.g., sun, shade, oven, freeze, microwave, vacuum, infrared) can alter the initial plant material composition in different ways (Nurhaslina et al. 2022). The loss of compounds can be related to their dragging to the surface of plant when water is evaporating or to degradation (e.g., oxidation and hydrolysis) (Sellami et al. 2012). Moreover, if high temperatures are used, the cell membrane integrity can be disrupted, releasing bioactive compounds, as well as some undesired enzymes (e.g., polyphenol oxidase) that can degrade the compounds of interest (Zhang et al. 2011). Nonetheless, the degradation products generated can also demonstrate potential biological activity (Zhang et al. 2012; Medicherla et al. 2016). For example, the extraction yield of essential oils was higher when aerial parts of S. officinalis were dried with an infrared moisture analyzer, followed by the air-drying method, oven, and microwave (Sellami et al. 2012). Indeed, the concentration of major compounds present in the fresh plant increased (e.g., 1,8-cineole, camphor, β-thujone, and viridiflor) or decreased (e.g., α-thujone and terpinen-4-ol) with the drying method. Some compounds were lost (e.g., sabinene, α-terpinene, valencene, and methyl eugenol), while others only appear when the plant was dried (e.g., β-pinene, octanal, β-caryophyllene, α-humulene and α-selinene). The same tendency was observed for the phenolic and flavonoid content in S. officinalis methanolic extracts (Hamrouni-Sellami et al. 2013). In this case, ferulic acid, rosmarinic acid, and carnosic acid are presenting higher amounts in dried aerial parts in comparison with fresh ones. On the other hand, caffeic acid concentration decreased and luteolin was lost in the drying method. Similarly, the increase of the temperature to dry aerial parts of S. officinalis significantly reduced (e.g., caffeic acid and rosmarinic acid) or enhanced (e.g., carnosol and carnosic acid) the amount of some compounds in the final extract (Hamrouni-Sellami et al. 2013). The high drying temperature also significantly reduced the content of chicoric acid and caftaric acid in E. purpurea extracts (Kim et al. 2000a; Lin et al. 2011). As well, alkylamides are susceptible to degradation at high drying temperatures (Kim et al. 2000b). Thus, the specific drying method employed can significantly influence the composition of bioactive compounds, with some compounds being lost, and others generated. This highlights the importance of selecting appropriate drying methods to maintain the desired composition and quality of plant materials.

Storage conditions

Appropriate storage conditions contribute to the preservation of the plant material. Factors, such as water content, oxygen concentration, temperature, humidity, and light strongly influence the quality of the plant over time (Laher et al. 2013; Coradi et al. 2020). For example, low temperatures (10–20 °C), low relative humidity (40–60%), and dark conditions were preferred to store E. purpurea samples in sealed polyethylene terephthalate/aluminum foil/polyethylene or nylon/polyethylene bags, since their content of chicoric acid and caftaric acid did not change over 180 days (Lin et al. 2011). Indeed, light accelerates the loss of chicoric acid and caftaric acid in dried samples stored at 20 °C for longer than 90 days. Also, the re-uptake of moisture from air with a high relative humidity (80%) favors the chicoric acid degradation due to the enhanced enzymatic activity. The proper storage conditions are essential for preserving the quality and stability of plant materials, preventing degradation of the bioactive compounds.

Solvent of extraction

The solvent strongly influences the type of bioactive compounds recovered from the plant material (Mandal et al. 2015). The ability of different solvents to solubilize dissimilar chemical classes of molecules is determined by their polarity (Zarrinmehr et al. 2022). Polar solvents (e.g., water, methanol, and ethanol) have the ability to extract more polar compounds (e.g., alkaloids, flavonoids, glycosides, tannins, and phenolic compounds) (Bonventre 2014). Nonpolar solvents (e.g., hexane and dichloromethane) are not miscible in water and remove lipophilic compounds (e.g., fatty acids and alkanes). For example, the solid–liquid extraction efficacy of O. europaea was significantly different for several solvents, namely hexane—0.9%; ethanol—4.1%; dichloromethane—6.2%; acetone—6.8%; water—5.7%; and methanol—7.2% (Borges et al. 2020). However, no correlation was obtained between the extraction yield and scavenging activity. The acetonic extracts exhibited the highest biological activity and methanolic and ethanolic extracts presented the lowest. Also, aqueous and ethanolic S. officinalis extracts exhibited different extraction yields, chemical composition, and anti-inflammatory activity (Vieira et al. 2020). Rosmarinic acid was found in aqueous and ethanolic extracts, while carnosol and carnosic acid were only present in ethanolic extracts. Interestingly, ethanolic extracts, with the smallest extraction yield (12.2%), were more efficient in the reduction of IL-6 production, than the aqueous extracts, with approximately twice the extraction yield (29.9%). Different chemical compositions and consequent bioactivity were also observed for E. purpurea aqueous and ethanolic extracts obtained from leaves (Vieira et al. 2022). Only phenolic/carboxylic acids were found in aqueous extracts, while a mixture of phenol/carboxylic acid with alkylamides was detected in ethanolic extracts. Consequently, ethanolic extracts reduced more efficiently the IL-6 production in LPS-stimulated macrophages than the aqueous extracts.

The ratio between plant material and volume of solvent also directly influences the recovery of specific chemical compounds. The extraction of dodeca-2E,4E,8Z,10E/Z-tetraenoic acid isobutylamide and dodeca-2E,4E,8E,10Z-tetraenoic acid isobutylamide from E. purpurea roots was more efficient with a plant to solvent ratio of 1:5 (W:V) (Spelman et al. 2009). The ratio of 1:11 (W:V) was more effective for the removal of dodeca-2E-ene-8,10-diynoic acid isobutylamide.

As demonstrated in the selected studies, the meticulous selection of solvents of extraction and the ratio between plant material and volume of solvent are crucial in the extraction process, as they strongly influence the extraction efficiency, chemical composition, and subsequent bioactivity of the resulting formulations.

Extraction method

There are different extraction methods, which can be divided into traditional and advanced techniques (Zhang et al. 2018; Jha and Sit 2022). The traditional methods can mimic the conventional use of plant preparations (Zhang et al. 2018). Examples of traditional methods are infusion, decoction, maceration, stirring, shaking, Soxhlet apparatus, and reflux. Steam distillation and hydrodistillation by Clevenger apparatus are also employed. Other equipment are also applied to increase the efficacy of the extraction process (e.g., UltraTurrax, enamel boiler, Unger apparatus, and ball mill). Nowadays, new advanced techniques have arisen to improve the recovery of bioactive compounds and reduce, for instance, the time of extraction (Belwal et al. 2018). Examples of new advanced techniques are ultrasounds, supercritical fluid extraction, accelerated solvent extraction, and microwaves. Furthermore, the employment of temperature can improve the efficiency of the extraction by changing the physicochemical properties of the solvent (Deans et al. 2020). Nonetheless, as heat can have a significant impact on structure integrity of the bioactive compound, a balance between the level of temperature and the preservation of compound must be achieved. Methods that include an enzymatic, mechanical, or chemical pre-treatment of the plant material can also increase the concentration of some chemical compounds since they degrade the cell wall structure, releasing the intracellular content. For example, the recovery of several compounds from L. angustifolia was improved with its pre-treatment with cellulase (e.g., cis-linalool oxide, trans-linalool oxide, lavandulol, 1-octen-3-ol acetate, and borneol), hemicellulase (e.g., 1,8-cineole), or a mixture of both (e.g., linalool, lavandulyl acetate, α-terpinol, geranyl acetate, geraniol, and neryl acetate) (Rashed et al. 2017). The same tendency was observed in the extraction of essential oils from L. angustifolia (Rashed et al. 2017) and Laurus nobilis (Boulila et al. 2015) if preceded by an enzymatic pre-treatment. The mechanochemical pre-treatment of L. nobilis leaves with lithium carbonate in a ball mill also yielded comparable phenolic content to the one obtained for Soxhlet extraction (Rincón et al. 2019).

Each extraction method has its own particularities, obtaining, therefore, different chemical compounds and/or concentrations. For instance, significant differences between the oil composition of S. officinalis obtained by infusion (25 compounds) and hydrodistillation (60 compounds) were reported (Lima et al. 2004). More particularly, high-volatile compounds (e.g., ocimene, camphene, and 1-octen-3-ol) were present in the extracts obtained by hydrodistillation, but not when infusion was used (Radulescu et al. 2004). In fact, hydrodistillation is based on distillation of the volatile compounds at a lower temperature than their boiling points. Conversely, infusion uses the boiling temperature to extract the compounds. In addition, the infusion is performed in an open extraction system, during which the most volatile compounds are lost due to direct volatilization and co-vaporization with water vapors. In another study, a high percentage of compounds with higher polarity (e.g., linalool) were obtained by hydrodistillation of L. angustifolia flowers, while increased amounts of compounds with lower polarity (e.g., linalyl acetate and neryl acetate) were observed after microwave-assisted extraction (Dănilă et al. 2018). Conversely, the main compounds of the L. nobilis leaves extracts were obtained using different extraction methods—steam distillation, microwave-assisted hydrodistillation, and ohmic-assisted hydrodistillation, hydrodistillation—but their amount was dramatically different (Taban et al. 2018). Microwave-assisted hydrodistillation and ohmic-assisted hydrodistillation showed high yield for oxygenated monoterpenes, whereas sesquiterpenes recovery was enhanced by hydrodistillation and steam distillation.

Given the potential variations in the obtained chemical compounds composition and their concentrations, it is imperative to thoroughly consider the selection of the extraction method for biological assays.

Plant-derived immunomodulatory compounds and their mechanism of action

A variety of preparations obtained from several species of plants have been reported to have ability to interact with immune cells, either stimulating or inhibiting their function. The immunomodulatory activity is closely related to the chemical composition of the plant extract. Various extraction methods and solvents have been used to prepare different plant extracts that can modulate both in vitro and in vivo inflammatory signaling pathways. Particularly, Boswellia serrata, Echinacea purpurea, Laurus nobilis, Lavandula angustifolia, Olea europaea, Salvia officinalis, Salvia Rosmarinus, and Taraxacum officinale formulations have been traditionally used as immunomodulatory medicines, whose bioactivity is being under investigation. Therefore, in the next sections, the bioactive compounds present in formulations obtained from these plants, as well as their mechanism of action are reviewed and cataloged in Tables 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and 15. Some of the more complete studies presented in these tables were selected to be discussed in detail. Furthermore, the phytochemical composition of these plants is presented, together with, when provided, the solvent of extraction, the extraction method, and the organ of the plant used in Supplementary Tables S1–S8.

Table 3 Boswellia serrata bioactive compounds, methods of their extraction, biological assays, and anti-inflammatory effects

Table 4 Boswellia serrata bioactive compounds, methods of their extraction, biological assays, and pro-inflammatory effects

Table 5 Echinacea purpurea bioactive compounds, methods of their extraction, biological assays, and anti-inflammatory effects

Table 6 Echinacea purpurea bioactive compounds, methods of their extraction, biological assays, and pro-inflammatory effects

Table 7 Laurus nobilis bioactive compounds, methods of their extraction, biological assays, and anti-inflammatory effects

Table 8 Lavandula angustifolia bioactive compounds, methods of their extraction, biological assays, and anti-inflammatory effects

Table 9 Lavandula angustifolia bioactive compounds, methods of their extraction, biological assays, and pro-inflammatory effects

Table 10 Olea europaea L bioactive compounds, methods of their extraction, biological assays, and anti-inflammatory effects

Table 11 Salvia officinalis bioactive compounds, methods of their extraction, biological assays, and anti-inflammatory effects

Table 12 Salvia officinalis bioactive compounds, methods of their extraction, biological assays, and pro-inflammatory effects

Table 13 Salvia rosmarinus bioactive compounds, methods of their extraction, biological assays, and anti-inflammatory effects

Table 14 Salvia rosmarinus bioactive compounds, methods of their extraction, biological assays, and pro-inflammatory effects

Table 15 Taraxacum officinale bioactive compounds, methods of their extraction, biological assays, and anti-inflammatory effects

Boswellia serrata

B. serrata (Burseraceae family), commonly known as Indian Frankincense, olibanum or Salai guggal, is a small branching tree native to North Africa and Asia. It is used in Ayurvedic medicine for centuries. Its oleo gum resin, collected through the exudes from an incision in the bark of the trunks, has been traditionally used as a decoction or tincture to reduce the pain and to treat asthma, rheumatoid arthritis, osteoarthritis, and inflammatory bowel disease.

B. serrata extracts (Boswe-E) are mainly constituted by monoterpenes, triterpenes, sesquiterpenes, diterpenes, monosaccharides, and fatty acids (Supplementary Table S1). The triterpene boswellic acids (e.g., β-boswellic acid, acetyl-β-boswellic acid, 11-keto-β-boswellic acid and acetyl-11-keto-β-boswellic acid) were found to be the main responsible compounds for the biological activity of the Boswe-E (Tables 3 and 4).

The anti-inflammatory activity of Boswe-E is related to (i) the inhibition of the complement system; (ii) the decrease in immune cell proliferation; (iii) the diminution of immune cells migration; (iv) the reduction of pro-inflammatory cytokines, chemokines, prostaglandins, and leukotrienes expression and production; (v) the decrease of respiratory burst and molecular oxidation levels; (vi) the reduction of antibodies production; (vii) the amelioration of edema, pain, and tissue injury; and (viii) the increase in antioxidant enzymes levels and anti-inflammatory cytokines (Ammon et al. 1993; Knaus and Wagner 1996; Sharma et al. 1996; Safayhi et al. 2000; Kimmatkar et al. 2003; Gayathri et al. 2007; Sengupta et al. 2008, 2011; Kokkiripati et al. 2011; Shehata et al. 2011; Hartmann et al. 2012, 2014; Umar et al. 2014; Wang et al. 2014; Forouzanfar et al. 2016; Governa et al. 2018; Gomaa et al. 2019; Majeed et al. 2019, 2021; Banji et al. 2022; Elhaddad et al. 2023). The details of the anti-inflammatory studies are described in Table 3. Some of the most relevant studies will be presented in more detail.

The β-boswellic acid, isolated from B. serrata, exhibited strong activity in the inhibition of the complement system (Knaus and Wagner 1996). The β-boswellic acid reduced the hemolytic activity of guinea pig and human serum towards antibody-coated erythrocytes, showing its potential in the inhibition of the classical pathway of the complement system. The β-boswellic acid also demonstrated a strong activity on the inhibition of the alternative pathway of the complement. The analysis of the single complement component showed a conversion of Factor B, but no fragments of the complement compound C3 were detectable. Based on this, the authors suggested that there was an inhibition of the C3-convertase. Aqueous ethanolic and aqueous Boswe-E, prepared from gum resin, were reported to have antioxidant properties under an oxidative stress scenario (Kokkiripati et al. 2011). Both Boswe-E were capable to reduce cellular ROS production in hydrogen peroxide-stimulated human THP-1 monocytes, thought the upregulation of cellular CAT activity and the sustaining of cellular reducing power. Curiously, the aqueous ethanolic extracts presented higher antioxidant activity than the aqueous extracts. However, the authors did not attribute this difference to the presence of 3-acetyl-11-keto-β-boswellic acid, which was higher in the first one.

Boswe-E was also proposed as a promising herbal drug for inflammatory bowel diseases (Governa et al. 2018). The pre-treatment of Boswe-E in human peripheral blood mononuclear cells (PBMCs) attenuated the production of IL-8, while the pre-treatment in LPS-stimulated HMC-1.1 mast cells significantly reduced the production of IL-6. Moreover, this extract also demonstrated a strong protective effect on the epithelial barrier by reducing immune cells infiltration. In addition,– a methanolic Boswe-E, isolated from the gum resin, showed potent anti-inflammatory activity in LPS-stimulated human PBMCs (Gayathri et al. 2007). The treatment with this Boswe-E led to a high down-regulation of the TNF-α and IL-1β production. Moreover, this extract significantly reduced the gene expression of T_H1 cytokines (IL-12 and IFN-γ) and enhanced the gene expression T_H2 cytokines (IL-10 and IL-4). In fact, the treatment with methanolic Boswe-E in LPS-stimulated PBMCs had a significant inhibitory effect on the phosphorylation of JNK and p38 in MAPK pathway. Then, a decrease in the c-jun transcription factors was observed, which could be correlated with the reduction of the production of the pro-inflammatory cytokines. A decreased in the ^·NO production in LPS-stimulated murine RAW 264.7 macrophages was also observed, which could result from the downregulation of the iNOS gene expression. The strong anti-inflammatory activity can be intimately related with the presence of 12-ursene 2-diketone in the Boswe-E.

In another study, ethanolic Boswe-E obtained from the gum resin was purposed for the treatment of diabetes (Shehata et al. 2011). The administration of ethanolic Boswe-E in multiple low-dose streptozotocin-induced diabetic mice attenuated the inflammatory state, through the reduction of the T-lymphocyte infiltration into pancreatic islets—identified by CD3 receptors of T-cells and thymocytes—and of production of several inflammatory cytokines in the serum—G-CSF, GM-CSF, IL-1α, IL-1β, IL-2, IL-6, IFN-γ, and TNF-α. Additionally, the decrease of caspase-3 activation was observed, indicting a reduction of the tissue injury. The authors suggested two mechanisms of action for the anti-inflammatory effect of this Boswe-E. Thus, the observed activity can be due to the reduction of the production of cytokines by immune cells, or by the decrease in the releasing of NF-κB from these cells, which further diminished its transcription action for production of cytokines. The ethanolic Boswe-E also reduced the progression of rheumatoid arthritis in collagen-induced arthritis rats (Umar et al. 2014). The oral administration of this ethanolic Boswe-E markedly decreased the articular elastase and MMP levels. Ethanolic Boswe-E also decreased the infiltration and activation of the neutrophils in the synovial tissue of the joints, reducing the joint swelling. The treatment with ethanolic Boswe-E showed a protective role mediated via its antioxidant effect through the reduction of lipid peroxidation and ^·NO levels and the boosting of antioxidant enzymes activity (GSH, SOD, and CAT). The authors related the reduction of disease severity with the downregulation of the production of the inflammatory mediator (IL-1β, IL-6, TNF-α, IFN-γ, and PGE2) and with the upregulation of IL-10 in the joint. Therefore, ethanolic Boswe-E reduced the cellular flux and rats’ joint erosion with minimum necrotic lesions.

Oral and topical preparations of Boswe-E, enriched in boswellic acid, also showed a robust effect in the treatment of osteoarthritis (Wang et al. 2014). The both oral and topical administration of this Boswe-E in destabilization of the medial meniscus-induced osteoarthritis in C57BL/6 J mice markedly decreased the cartilage erosion, osteophyte formation, and knee synovitis. Moreover, a reduction of several inflammatory cytokines (IL-1β, IL-12p40, MCP-1, RANTES, and TNF-α) in the synovium were observed. The authors also suggested that the presence of other undefined molecules, in addition to boswellic acid, could contribute to the therapeutic effect in the reduction of joint damage in osteoarthritis. Another study corroborated the strong efficacy of Boswe-E in the management of osteoarthritis in the knee of newly diagnosed or untreated patients (Majeed et al. 2019). The oral supplementation of Boswe-E significantly improved physical function ability by reducing join pain, stiffness, osteophytes (spur) formation and serum levels of CRP in those patients. The authors suggested that the major compounds (3‐acetyl‐11‐keto‐β‐boswellic acid, 11‐keto‐β‐boswellic acid, β‐boswellic acid, and 3‐acetyl‐β‐boswellic acid) acted synergistically to exert the anti-arthritic activities.

Finally, aqueous and ethanolic Boswe-E obtained from oleo gum resin showed a neuroprotective effect on ischemic cerebral injury induced by middle cerebral artery occlusion in rats (Forouzanfar et al. 2016). The treatment with aqueous and ethanolic Boswe-E significantly improved neurological deficits and decreased the tissue injury. Moreover, Boswe-E prevented the increase of MDA levels and increased the GSH content and SOD activity in the right cortical region. The authors suggested that the biological activity of Boswe-E could be attributed to the 3-acetyl-11-keto-β-boswellic acid, present in both extracts.

Some studies also reported a pro-inflammatory effect of Boswe-E (Table 4). This bioactivity is related to (i) the activation of the complement system; (ii) the activation of the immune cells; (iii) the stimulation of phagocytosis; (iv) the increase of immune cell proliferation; (v) the increment of clonal expansion of immune cells; (vi) the promotion of the pro-inflammatory cytokine, antibody, and immunoglobulin production; and (vii) the increase of respiratory burst (Sharma et al. 1996; Khajuria et al. 2007, 2008; Gupta et al. 2011). For example, a Boswe-E fraction, composed of arabinose, glucose, and galactose, was proposed as an enhancer of antigen-specific humoral and cell-mediated immune response in ovalbumin-challenged Balb/C mice (Gupta et al. 2011). The IgG1 and IgG2a antibody levels were significantly enhanced in the serum in the presence of Boswe-E fraction. Moreover, it was also observed the splenocyte proliferation and the upregulation of the cell surface marker CD4/CD8 and co-stimulatory molecules CD80/CD86. In addition, the production of IL-2 and IFN-γ in the splenic lymphocytes was induced.

Echinacea purpurea

E. purpurea (Asteraceae family), also known as the purple coneflower, is a perennial flower native to eastern and central North America. Tea, freshly pressed juice, and tinctures, containing around 60% ethanol, are traditional E. purpurea preparations obtained from flowers, leaves, and/or roots. They are often therapeutically employed to prevent and relief a variety of different inflammatory conditions, including swollen gums, sore throats, skin inflammation, and gastrointestinal disorders. Additionally, Echinacea preparations have been used to prevent or treat the common cold, flu, and infections in the upper respiratory tract. E. purpurea preparations have also been traditionally and highly used as an immune booster during common cold or infection.

E. purpurea extracts (Echi-E) are rich in alkylamides, phenolic compounds, sesquiterpenes, monoterpenes, monosaccharides, polysaccharides, flavonoids, and fatty acids (Supplementary Table S2). Chicoric acid, caftaric acid, a mixture of alkylamides—particularly dodeca-2E,4E,8Z,10E/Z-tetraenoic acid isobutylamide—, and polysaccharides are the main responsible for the anti-inflammatory activity of Echi-E. This bioactivity of Echi-E is related to (i) the inhibition of the complement system; (ii) the decrease of intracellular calcium ion; (iii) the reduction of pro-inflammatory cytokines expression and production; (iv) the decrease of respiratory burst and molecular oxidation levels; (v) the reduction of immune cells infiltration; (vi) the reduction of the degranulation process; (vii) the decrease of clonal expansion of immune cells; (viii) the blocking of receptors on immune cells; (ix) the increase in antioxidant enzymes and anti-inflammatory cytokines levels; and (x) the amelioration of edema, pain, and tissue injury (Sasagawa et al. 2006; Wang et al. 2006; Dong et al. 2006, 2009; Cech et al. 2006; Matthias et al. 2007; McCann et al. 2007; Zhai et al. 2007, 2009; Chicca et al. 2009; Benson et al. 2010; Chen et al. 2010; Hou et al. 2010, 2020; Šutovská et al. 2015; Capek et al. 2015; Todd et al. 2015; Aarland et al. 2017; Nyalambisa et al. 2017; Gulledge et al. 2018; Abd El-Twab et al. 2019; Li et al. 2020; Zhang et al. 2020a; Jiang et al. 2021; Vieira et al. 2022, 2023; Gu et al. 2023; Dosoky et al. 2023). The details of the anti-inflammatory studies are described in Table 5. Some of the most relevant studies will be presented in more detail. For instance, an aqueous ethanolic Echi-E obtained from roots efficiently reduced the TNF-α and ^·NO production in LPS-stimulated murine RAW 264.7 macrophages (Zhai et al. 2009). Dichloromethanolic Echi-E obtained from roots significantly reduced the production of two main proinflammatory cytokines—IL-6 and TNF-α—and ROS/RNS in LPS-stimulated primary human monocyte-derived macrophages. This Echi-E exerted their anti-inflammatory activity through the downregulation of the phosphorylation of p38, ERK 1/2, STAT 3, and NF-κB signaling pathways. In addition, the downregulation of cyclooxygenase 2 expression was also observed. The authors proposed that alkylamides were the main class of compounds responsible for the this strong bioactivity of extracts. Another aqueous ethanolic Echi-E prepared from aerial parts (steams, leaves, and flowers) modulated the fate of the murine bone marrow-derived dendritic cells (BMDCs) (Benson et al. 2010). This Echi-E, primarily composed of chicoric acid and dodeca-2E,4E,8Z,10E/Z-tetraenoic acid isobutylamide, decreased the frequency and relative expression of molecules involved in antigen presentation (MHC class II) and co-stimulation (CD86 and CD54), as well as the activity of COX-2. The pre-treatment of ovalbumin-sensitized murine BMDCs with this Echi-E also reduced the frequency of dendritic cells engulfing the antigen. At this point, the APC functions of dendritic cells are compromised, and the activation of T cells, consequently, can be affected. The co-culture of ovalbumin-sensitized murine BMDCs pre-treated with extracts and T cells demonstrated that Echi-E significantly decreased the clonal expansion of CD4⁺ T cells. Therefore, alkylamides might inhibit T cell-mediated immune response by potentially affecting both innate (antigen uptake) and adaptive (CD4⁺ T cells) immunity. Other studies demonstrated that cynarin isolated from Echi-E was a strong immunosuppressive agent. It can block the CD28 receptor of human Jukart T-cells, preventing the binding of other ligands, such as CD80 of human Raji B lymphocyte cells (Dong et al. 2006, 2009). Thus, cynarin disturbed the CD28-dependent T-cell activation and reduced the production of IL-2 through the inhibition of the signal 2 pathway, acting as an antagonist of the CD28/CD80 interaction. The Echi-E essential oils were also recently reported to reduce the innate immunity (Dosoky et al. 2023). The pre-treatment of human neutrophils with flower essential oils—mainly composed of germacrene D, α-phellandrene, β-caryophyllene, γ-curcumene, α/β-pinene, and δ-cadinene—inhibited the intracellular Ca²⁺ mobilization and the chemotaxis.In vivo studies also demonstrated the potential of Echi-E as anti-inflammatory formulations. The treatment with an aqueous ethanolic Echi-E obtained from the whole plant in 2,4,6 trinitrobenzene sulfonic acid (TNBS)-induced acute intestinal inflammation in Sprague–Dawley rats demonstrated strong protective effects (Gu et al. 2023). This Echi-E improved the symptoms of the ulcerative colitis, as well as the integrity of intestinal epithelial barrier, and, consequently, attenuated colon injury. The production of the IL-6 and TNF-α was significantly reduced. In addition, the expression of complement factor B (CFB), CD55, TLR4, and NLRP3 were diminished. Interestingly, the Echi-E was also able to achieve therapeutic effect by inhibiting C3a/C3aR signal pathway. Furthermore, the antioxidant capacity was significantly enhanced by the increase in the expression of GSH-Px levels, resulting in the reduction of ROS production.

The administration of Echi-E prepared from flowers in ovalbumin sensitized-guinea pigs to analyze the effects on asthma treatment was also reported (Šutovská et al. 2015). The long-term treatment with Echi-E significantly decreased bronchoconstriction, basal hyperreactivity, and specific airway resistance. Echi-E significantly decreased the levels of exhaled ^·NO and key cytokine mediating asthma (IL-4, IL-5, IL-13, and TNF-α), both in bronchoalveolar lavage fluid (BALF) and plasma. The contractile response of tracheal and pulmonary smooth muscle was also significantly lower in the presence of Echi-E. Furthermore, long-term treatment of animals with Echi-E did not change the frequency of cilia beating, maintaining its basal levels, which allowed mucociliary clearance.

The immunostimulatory activity of Echi-E is related to (i) the activation of different immune cells; (ii) the promotion of cytokine, chemokine, and prostaglandins expression and production; (iii) the stimulation of phagocytosis; (iv) the increase of respiratory burst and molecular oxidation levels; (v) the increase of immune cell and clonal proliferation; and (vi) the stimulation of immune cells migration and mobility (See et al. 1997; Goel et al. 2002a, b; Gertsch et al. 2004; Brovelli et al. 2005; Wang et al. 2006, 2008; Raduner et al. 2006; Benson et al. 2010; Cech et al. 2010; Yin et al. 2010; Ramasahayam et al. 2011; Fonseca et al. 2014; Li et al. 2017; Fu et al. 2017; Vieira et al. 2022; Sudeep et al. 2023). The details of the pro-inflammatory studies are described in Table 6. Some of the most relevant studies will be presented in more detail.

A commercial Echi-E (chicoric acid, caftaric acid, chlorogenic acid, and undeca-2Z,4E-diene-8,10-diynoic acid isobutylamide) exhibited the ability to modulate the polarization of BMDCs (Li et al. 2017) and murine bone marrow-derived macrophages (BMDMs) (Fu et al. 2017). The CD80 and CD86 were markedly increased in both cell types. The expression of key molecules CCR7 and MHC class II was also significantly increased in macrophages, while CD40 and CD83 were promoted in dendritic cells. Moreover, this Echi-E significantly induced the gene expression in M1 macrophages (IL-6, TNF-α, IL-12p70, and iNOS). This was validated by the increase in the production of ^·NO and several pro-inflammatory cytokines (IL-1β, IL-6, IL-12p70, TNF-α, IFN-γ). Echi-E also enhanced the phagocytic and bactericidal capacity of murine macrophages. These effects were obtained due to Echi-E ability to significantly activate the MAPK signaling pathway, through the phosphorylation of ERK, JNK, and p38. NF-κB signaling pathway was also triggered with an increase of p65 protein in the nucleus, with a correspondent decrease of IκB protein in the cytoplasm. Interestingly, the stimulation of the murine dendritic cells was promoted via the activation of the same inflammatory signaling pathways, resulting in higher production of IFN-γ and IL-12 production. A butanol fraction of Echi-E also exhibited a strong capacity to mature human monocyte-derived immature dendritic cells (Wang et al. 2006). The Echi-E fraction enhanced the expression of the CD83 and induced the gene expression of over different genes involved in various key immune modulatory activities (e.g., CCL2, CCL5, IL-8, JAK2, TANK, and CXCL2). Additionally, dendritic cells exposed to the butanol fraction of Echi-E significantly up-regulated the antioxidant defense enzymes, including manganese superoxide dismutase (MnSOD), CAT, and peroxiredoxin 6, which are essential to induce T cell activation. Moreover, cytoskeletal and actin-binding proteins were up-regulated, enabling the formation of its characteristic dendrites and veils, important for the motility and migration to lymph nodes. Other classes of proteins were also enhanced, including cell growth- or maintenance-related proteins, energy pathway-related proteins, metabolic enzymes, proteins involved in protein metabolism or degradation, ion channel/transport proteins, signal transduction, and cell communication. A bioinformatic approach hypothesized that the butanol fraction of Echi-E may activate the cAMP and PKC pathways, leading to the regulation of adenylate cyclase 8 (AC8) thought a Ca²⁺ receptor calmodulin. The same Echi-E also increased the responsive genes related to cell adhesion (e.g., CDH10, ITGA6, CDH1, GJA1, and MMP8), chemokines (e.g., CXCL2 and CXCL7), and signaling molecules (e.g., NRXN1, PKCE, and ACSS1) in murine BMDCs (Yin et al. 2010). Also, the expression of the metabolic, cytoskeleton, and singling-related proteins, led to the enhancement of mobility, migration, and adhesion of the cells in the presence of Echi-E. The maturation of dendritic cells by an aqueous Echi-E obtained from roots, mainly composed of polysaccharides, was also studied (Benson et al. 2010). Moreover, polysaccharides-enriched Echi-E have been proposed as adjuvant effectors on Jukart T cells (Fonseca et al. 2014). The pre-treatment of T cells with this Echi-E enhanced the response to PMA and ionomycin, significantly increasing the production of IL-2 and IFN-γ. The authors proposed that Echi-E could influence Ca²⁺ mobilization to increase the intensity of IL-2 production since a dose-related effect was observed when the ionomycin was used alone. Particularly, dodeca-2E,4E-dienoic acid isobutylamide, isolated from a hexane Echi-E, presented ability to bind to the human CB2 receptor, increasing the total intracellular free Ca²⁺ concentration via a CB2-mediated G-protein-coupled mechanism, leading to stimulation of PLC, in human CB2-positive promyelocytic HL60 cells (Raduner et al. 2006).

The in vivo immunostimulatory activity was recently corroborated with a standardized aqueous ethanolic Echi-E containing higher chicoric acid content (Sudeep et al. 2023). The cyclophosphamide-induced immunosuppressed BALB/c mice treated with this Echi-E significantly improved the phagocytosis, the B and T lymphocyte proliferation, and the NK cell activity. Moreover, the levels of TNF-α, IL-6, IL-1β, and IFN-γ in the serum was observed. Consequently, the administration of Echi-E promoted the normal architecture of the thymus and spleen.

Laurus nobilis

L. nobilis (Lauraceae family), commonly known as bay, is native to the southern Mediterranean region. Its leaves have been used as a traditional medicine in the Mediterranean and Europe to treat several immunological diseases. Essential oils of bay leaves are mostly used as a topical formulation to relax contracture, to ameliorate muscle pain, joint inflammations, and rheumatism. Bay leaf oil extracts are also used to treat paronychia. Additionally, cough and bronchitis can be treated with leaves cataplasm. Infusion from the bay leaves is traditionally used to alleviate headaches and asthma. Leaves decoctions as a compress are also used to heal skin inflammation.

L. nobilis extracts (Laur-E) are rich in sesquiterpenes, monoterpenes, flavonoids, phenolic acids, fatty acids, and megastigmanes (Supplementary Table S3). Essential oils (e.g., 1,8-cineole, sabinene, linalool, α-terpinyl acetate, α-pinene, β-elemene, camphene, and α-terpineol), lindoldhamine, magnolialide, and costunolidares were found to be the main responsible compounds for the biological activity of the Laur-E (Table 7).

The anti-inflammatory activity of Laur-E is related to (i) the inhibition of complement system; (ii) the reduction of phagocytosis; (iii) decrease of degranulation process; (iv) the reduction of pro-inflammatory cytokines expression and production; (v) the decrease of clonal expansion of immune cells; (vi) the decrease of respiratory; (vii) the increase in antioxidant enzymes levels; and (viii) the amelioration of edema, pain, and tissue injury (Kim et al. 2011; Lee et al. 2013; Pérez-Rosés et al. 2015, 2016; Osmakov et al. 2019; Brinza et al. 2021; Al-Mijalli et al. 2022; Odeh et al. 2022; Smach et al. 2024). The details of the anti-inflammatory studies are described in Table 7. Some of the most relevant studies will be presented in more detail.

The Laur-E essential oils demonstrated a strong anti-inflammatory efficacy. For example, the Laur-E essential oils significantly reduce the phagocytosis in human neutrophils Particle-stimulated human neutrophils. It also participated in the reduction of the classical pathway of the complement system (Pérez-Rosés et al. 2015). Moreover, the pre-treatment of the same Laur-E essential oils in LPS-stimulated human leukocytes resulted in the decrease of MPO activity and ROS production (Pérez-Rosés et al. 2016).

Particularly, costunolide, obtained from methanolic Laur-E, can down-regulate the early and late phase of allergic disorders, not only through the reduction of RBL-2H3 mast cells activation, but also throught the decrease of subsequent IgE-amplification by the IL-5-dependent Y16 B cell proliferation (Kim et al. 2011). Magnolialide, isolated from a combined dichloromethanolic and methanolic Laur-E, had also a therapeutic potential for a type-I IgE-mediated allergic inflammatory disorder, such as asthma and atopic dermatitis (Lee et al. 2013). Magnolialide reduced the early phases of mast cell activation, through the drastic down-regulation of IgE-mediated degranulation from RBL-2H3 mast cells. Its bioactivity was higher than cromoglycate and ketotifen, stabilizers of the mast cell traditionally used in the clinic. Moreover, the gene expression and production of IL-4 were significantly diminished. Additionally, the treatment with magnolialide significantly reduced the proliferation of the IL-5-dependent Y16 early B cells. Finally, lindoldhamine, isolated from aqueous acetic acid Laur-E, had a significant anti-inflammatory effect, through the reduction of both paw edema and thermal hyperalgesia in complete Freund’s adjuvant-induced male CD-1 mice (Osmakov et al. 2019). Laur-E essential oils also demonstrated in vivo efficacy. Its administration in scopolamine-induced memory deficits in Swiss albino mice significantly reduced the tissue damage, thought the reduction of MDA levels and the induction of the GHS antioxidant enzymes (Smach et al. 2024).

Lavandula angustifolia

L. angustifolia (formerly L. officinalis or L. vera), belonging to Lamiaceae family and being native to Mediterranean coast, is popularly known as lavender, English lavender or True lavender (Kıvrak 2018). Its flowers and/or leaves are used in the preparation of tea, juice, tinctures, or essential oils, to treat rheumatic, kidney and gastrointestinal disorders, sinusitis, and asthma. Additionally, lavender is widely used to relieve anxiety, depression, stress, migraines, fatigue, panic attacks, and heart problems, as well as to avoid insomnia.

L. angustifolia extracts (Lava-E) are rich in monoterpenes, sesquiterpenes, flavonoids, fatty acids, phenolic acids, and monossacharides (Supplementary Table S4). Bioactive compounds, including linalool, linalyl acetate, caryophyllene, terpineol, borneol, 1,8-cineole, pinene, and camphor are described as the major bioactive compounds in the Lava-E (Tables 8 and 9).

The anti-inflammatory activity of Lava-E is associated with (i) the reduction of pro-inflammatory cytokines and prostaglandins expression and production; (ii) the decrease in immune cell proliferation; (iii) the decrease of respiratory burst and molecular oxidation levels; (iv) the increase in antioxidant enzymes levels; and (v) the amelioration of edema, pain, and tissue injury (Hajhashemi et al. 2003; Rahmati et al. 2013; Hancianu et al. 2013; Ueno-Iio et al. 2014; Xu et al. 2016; Luo et al. 2019; Sanna et al. 2019; Chen et al. 2020; Donatello et al. 2020; Rai et al. 2020; Seo et al. 2021; Slighoua et al. 2022). The details of the anti-inflammatory studies are described in Table 8. Some of the most relevant studies will be presented in more detail.

A Lava-E mainly composed by essential oils was applied in PMA-induced ear inflammation in mice (Chen et al. 2020). This Lava-E efficiently reduced the ear edema, in a dose-dependent manner. Importantly, it led to better therapeutic outcomes than the clinical used anti-inflammatory drug, ibuprofen, at the same dosage. Lava-E significantly down-regulated the expression of NF-κB transcription factor, COX-2, and TNF-α. The authors suggested that linalyl acetate, linalool, borneol and eucalyptol were responsible for the anti-inflammatory activity. Another study corroborates the anti-inflammatory activity of Lava-E essential oils applied in PMA-induced ear inflammation in mice. A markedly reduction in ear edema and in the expression of NF-κB were also observed, which in turn reduced the IL-6 and TNF-α levels, being again more promise than ibuprofen at the same dose (Luo et al. 2019). Three different Lava-E obtained from leaves also presented anti-nociceptive, analgesic and anti-inflammatory activities (Hajhashemi et al. 2003). In this study, hydroethanolic Lava-E, polyphenolic Lava-E and essential oils Lava-E were applied on formalin- and acetic acid-induced writhes in mice, as well as carrageenan-induced paw edema in rats. Lava-E essential oils showed to be the best anti-nociceptive, analgesic, and anti-inflammatory extract, inhibiting 90% of paw licking, 27% of abdominal twitches and 48% of paw edema, respectively. As these Lava-E essential oils were mainly composed by 1,8-cineole, borneol, and camphor, the authors suggested that these compounds were the principal responsible of the pharmacological activities.

Lava-E have also been applied in the treatment of asthma, a bronchial allergic inflammation (Ueno-Iio et al. 2014). A bronchial asthma-induced mice model, sensitized by ovalbumin and further challenged with nebulized ovalbumin, were treated through the inhalation of suspended essential oil components. The anti-inflammatory effect of this Lava-E were confirmed by the significant decrease of inflammatory cells accumulation, mainly eosinophils, in the bronchoalveolar fluids and in the peribronchial and perivascular tissues. The airway hyperresponsiveness and the mucous cell hyperplasia were also significantly reduced after inhalation of the essential oils. Additionally, the treatment with this Lava-E drastically decreased the levels of TH2 cytokines, IL-5, and IL-13, in bronchoalveolar fluids, reaching basal levels of the non-asthma control, as well as markedly decreased the gene expression of IL-4 and IL-5 in the lung. Moreover, Lava-E had the ability to reduce Muc5b gene expression in the lungs, but no changes were observed in Muc5ac gene expression. However, Lava-E was not able to decrease the IgE levels in serum.

A single administration of Lava-E was also tested in spared nerve injury-induced neuropathic pain in mice (Sanna et al. 2019). The treatment with Lava-E significantly down-regulated the MAPK signaling pathway, preventing the phosphorylation of the ERK1/2 and JNK1, as well as decreased the iNOS expression. The authors suggested that these effects can be related with the antinociceptive effects. Interestingly, the inhibition of p38 phosphorylation was not observed. Likewise, the treatment with Lava-E did not modify the IκBα levels, suggesting that it was unable to prevent the activation of NF-κB signaling pathway. The authors hypothesized that linalool and linalyl acetate, as major constituents of this Lava-E, had the main contribute to the molecular mechanism of the reduction of sensitivity to pain.

Finally, Lava-E showed neuroprotective activity in an Alzheimer’s disease model, by enhancing the antioxidant defenses (SOD, GPx, and CAT) and decreasing the lipid peroxidation in the hippocampus and temporal cortex of scopolamine treated mice (Hancianu et al. 2013).

Some studies also reported the strengthen of the immune system by Lava-E. They can (i) induce the complement system; (ii) stimulate some immune cells; (iii) enhance the respiratory burst; (iv) promote the cytokine production; and (v) increase the clonal expansion of immune cells (Georgiev et al. 2017a, b) (Table 9).

Lava-E pectic polysaccharides fractions, chemically characterized with predominant homogalacturonan fragments that are acetylated and highly methoxylated, activated innate and adaptive immune response (Georgiev et al. 2017b). The chPS-L1 fraction expressed a high complement fixation activity, through both classical and alternative pathways. This could be related, not only with the antibody-dependent, but also with the presence of AGII content that would interact directly with C3 component (C3b). The production of ROS by human whole blood phagocytes and neutrophils was increased in the presence of the two fractions, but chPS-L2 strongly increased the CD18 expression on neutrophils. The low molecular weight compounds and the presence of monosaccharides characteristic for RGII in chPS-L2 fraction could be the base of this immunomodulatory activity. In the presence of Lava-E pectins, murine macrophages were also activated and able to express higher levels of iNOS protein and to produce large levels of NO. Moreover, these activities contributed to the murine Peyer’s patch-mediated bone marrow cell proliferation through the modulation of CD4⁺/CD25⁺ and CD8⁺/CD25⁺ T-cells and phagocytes (Georgiev et al. 2017a). Additionally, the pectic polysaccharides induced the IL-6 production from Peyer’s patch cells and human white blood cells.

Olea europaea

O. europaea L. (Oleaceae), commonly known as olive tree, grows in the Mediterranean region. Since the Ancient Egypt, tea made from the dried leaves of the olive tree has been traditionally used for the treatment of different inflammatory conditions, such as diabetes, asthma, and rheumatism. Tincture of olive leaves is also used to reduce the fever.

O. europaea extracts (Olea-E) are enriched in flavonoids, iridoids, secoiridoids, monoterpenes, phenolic acids, fatty acids, sesquiterpenes, and monossacharides (Supplementary Table S5). Oleuropein, hydroxytyrosol, tyrosol, and oleacein are reported as the most active compounds in the Olea-E (Table 10). Interestingly, olive cultivation has resulted in the development of hundreds of different cultivars within the O. europaea specie. These cultivars have been selectively bred over time to possess specific traits, such as fruit size, oil content, flavor profile, disease resistance, and growth habit (Barazani et al. 2023). Some of the most widely recognized and cultivated olive cultivars include Arbequina, Picual, Frantoio, and Manzanilla. As expected, each cultivar has unique characteristics that can affect the quality of the resulting Olea-E. Therefore, in Supplementary Table S5, the cultivars used in the study are described when reported by the authors.

The anti-inflammatory activity of Olea-E is associated with (i) the reduction of pro-inflammatory cytokines, chemokines, and prostaglandins expression and production; (ii) the decrease of respiratory burst and molecular oxidation levels; (iii) the reduction of immune cells differentiation and proliferation; (iv) the diminution of infiltration of inflammatory cells; (v) preservation of genetic integrity; (vi) the increase in antioxidant enzymes and anti-inflammatory cytokines levels; and (vii) the amelioration of edema, pain, and tissue injury (Jemai et al. 2008, 2020; Gong et al. 2009, 2012; Miljković et al. 2009; Cvjetićanin et al. 2010; Esmaeili-Mahani et al. 2010; Dekanski et al. 2011; Kaeidi et al. 2011; Rabiei et al. 2012; Takeda et al. 2013; Cabarkapa et al. 2014; Andreadou et al. 2014; Fakhraei et al. 2014; Lockyer et al. 2015; Žukovec Topalović et al. 2015; Talhaoui et al. 2016; Al-Quraishy et al. 2017; Vezza et al. 2017, 2019; Abdel-Kader et al. 2019; Guex et al. 2019; Soliman et al. 2019; Song et al. 2019, 2021; Rouibah et al. 2019; De Cicco et al. 2020; Zhang et al. 2020b; González-Hedström et al. 2021; Hong et al. 2021). The details of the anti-inflammatory studies are described in Table 10. Some of the most relevant studies will be presented in more detail. For example, a methanolic Olea-E demonstrated strong anti-inflammatory activity in the presence of LPS-stimulated RAW 264.7 macrophages (Song et al. 2019). The pre-treatment of M1 macrophages with this methanolic Olea-E significantly decreased the ^·NO and PGE2 production, through the reduction of the iNOS and COX-2 expression and production, respectively. The methanolic Olea-E also reduced the protein expression of pro-inflammatory cytokines (IL-6, IL-1β, and TNF-α), via down-regulation of the NF-κB p65 and p50 subunits nuclear translocation. The phosphorylation of the upstream targets—IκBα and IKKα/β—was also observed. This methanolic Olea-E was composed by flavonoids, including quercetin, luteolin and kaempferol. The authors suggested that kaempferol was the main phytochemical compound responsible for the bioactivity due to its high content. Another study reported an ethanolic Olea-E, obtained from leaves, with strong anti-inflammatory effects in a rat model of rheumatoid arthritis (Hong et al. 2021). The Freund’s complete adjuvant-induced arthritis rats treated with Olea-E demonstrated great reduction of swelling, redness, and pain of paws. Additionally, Olea-E prevented the cartilage degeneration and reduced the tissue injury. The oleuropein and flavonoids (e.g., quercetin, apigenin, and luteolin) were proposed as the main bioactive compounds, downregulating the NF-κB p65 signaling pathway, AP-1 transcription factor (c-Jun and c-Fos nuclear protein levels). Furthermore, the IL-1β and iNOS gene expression were dramatically decreased. For multiple sclerosis treatment, a standardized Olea-E EFLA 943 was suggested (Miljković et al. 2009). The treatment of spinal cord homogenate/Freund’s adjuvant-induced autoimmune encephalomyelitis in Dark Agouti rats with Olea-E promoted the reduction of various parameters of autoimmune severity. Indeed, it decreased the activation and proliferation of draining lymph nodes, the cells infiltration in the spinal cord, as well as their production of IFN-γ and IL-17. The authors suggested that the bioactivity was related to the polyphenolic fraction, together with the other compounds present in the mixture, including tannins and flavonoids. Olea-E were also employed for asthma treatment. The ovalbumin-sensitized Wistar rats treated with aqueous Olea-E obtained from leaves enhanced the levels of antioxidant enzymes, namely GSH, GPx, CAT, and SOD (Rouibah et al. 2019). Consequently, reduced MDA levels in the lungs were observed. Moreover, aqueous Olea-E promoted the decrease of lung injury, through the reduction of inflammatory cells infiltration and the induction of mucus production in bronchial airways. The authors attributed these anti-asthmatic effects to oleuropein, oleuropein aglycone, and hydroxytyrosol. Their ability to increase SOD, CAT, GPx, GRx, and GSH levels, strongly reduced the lipid peroxidation and the tissues injury. An aqueous methanolic Olea-E obtained from leaves was also proposed for inflammatory bowel disease treatment (Vezza et al. 2017). Dinitrobenzene sulfonic acid-induced colitis in CD1 mice and dextran sulfate sodium-induced colitis in C57BL/6 J mice pre- and co-treated, respectively, with Olea-E improved the inflammatory status of the colon and promoted the tissue function and integrity. Olea-E reduced the expression of pro-inflammatory mediators (IL-1β, TNF-α, IL-6, IL-17, MIP-2, iNOS, and COX-2), allowing for a decrease in inflammatory cells infiltration and edema in the mucosa. Moreover, Olea-E was able to decrease the IL-1β, IL-6, IL-8, and TNF-α levels in LPS- or IL-1β-stimulated PBMCs obtained from human inflammatory bowel disease. These bioactive effects can be attributed to the presence of different phenolic compounds. Particularly, the authors suggested that the oleuropein content (almost 80% of the whole extract) exerted the main anti-inflammatory activity.

Salvia officinalis

S. officinalis (Lamiaceae family), also known as garden sage, Dalmatian sage, or common sage, is cultured worldwide but mainly in the Mediterranean region. Sage tea has been traditionally used to treat mouth, throat, and bronchial inflammations, coughs, asthma, fever, aphthas, ulcers, amygdalitis, and stomach pain. Topical applications of sage preparations (tinctures) are also used after sun exposure to prevent sunburn and for the treatment of wounds.

S. officinalis extracts (Sage-E) are composed of a wide range of chemical compounds, including flavonoids, monoterpenes, sesquiterpenes, diterpenes, phenolic acids, fatty acids, triterpenes, and polysaccharides (Supplementary Table S6). Carnosol, carnosic acid, rosmarinic acid, ursolic acid, oleanolic acid, apigenin, and luteolin have been considered the main responsible for the anti-inflammatory activity of Sage-E. This bioactivity of Sage-E is associated with (i) the reduction of pro-inflammatory cytokines and prostaglandins expression and production; (ii) the decrease of respiratory burst and molecular oxidation levels; (iii) the reduction of immune cells proliferation and infiltration; (iv) the increase in antioxidant enzymes and anti-inflammatory cytokines levels; and (v) the amelioration of edema, pain, and tissue injury (Baricevic et al. 2001; Rodrigues et al. 2012; Arranz et al. 2014; Li et al. 2019a, b; Vieira et al. 2020; Jedidi et al. 2020, 2021; Brindisi et al. 2021; Ayoub et al. 2022; Silva et al. 2023). The details of the anti-inflammatory studies are described in Table 11. Some of the most relevant studies will be presented in more detail.

An ethanolic Sage-E obtained from leaves inhibited the ^·NO production in macrophages LPS-stimulated RAW 264.7 macrophages (Li et al. 2019a, b). Indeed, further fractionations revealed that C20-norabietane diterpenoid salofficinoids were responsible for the anti-inflammatory activity All the salofficinoids also decreased the ^·NO production in LPS-stimulated macrophages, being salofficinoid G the most potent compound (Li et al. 2019b). Additionally, salofficinoid G significantly inhibited the phosphorylation of JNK in the MAPK signaling pathway, down-regulating the expression of iNOS and COX-2. The authors proposed that the presence of the intact C-ring, the hydroxyl at C-12, the furan-ring, and the double bound in the A-ring can increase the ^·NO inhibitory effect. Moreover, officinalin A was also important in this bioactivity (Li et al. 2019a). Officinalin A markedly decreased the iNOS and COX-2 production, via downregulating the phosphorylation of p38 in the MAPK signaling pathway in LPS-stimulated RAW 264.7 macrophages. However, officinalin A did not prevent the phosphorylation of ERK and JNK. Additionally, the potential of two Sage-E obtained by supercritical extraction (S1 and S2) were investigated in the prevention of atherosclerosis with oxidized low-density lipoproteins (ox-LDL)-stimulated THP-1-derived macrophages (Arranz et al. 2014). The co-treatment with supercritical Sage-E S1 and S2 significantly decreased the production of TNF-α, IL-1β, and IL-6, which was related to the marked decrease of the gene expression of these cytokines. Particularly, the highest tested concentration of S1 led to lower amounts of TNF-α in comparison with basal conditions in non-stimulated macrophages. Moreover, S1 also exhibited higher ability to reduce IL-1β production compared to indomethacin. Supercritical extracts did not interfere in the production of IL-10, but their gene expression was significantly increased. The main compounds presented in supercritical Sage-E—camphor, borneol, and 1,8-cineole—constituted 62.4% of S1 and 48.1% of S2, indicating a potential correlation between their presence and the observed high anti-inflammatory activity.

The efficacy of Sage-E extracts was also confirmed in vivo. For instance, the pre-treatment with hydroethanolic Sage-E—mainly composed ofcarnosol,ursolic acid, and oleanolic acid,—in visceral inflammation and nociceptive response induced in mice exhibited anti-inflammatory and analgesic properties (Rodrigues et al. 2012). The administration of Sage-E significantly decreased the number of writhes, the number of total leukocytes, and the plasmatic extravasation in the inflamed tissue. Sage-E also promoted the inhibition of the neurogenic and inflammatory phases and nociception and decreased the edema. The authors proposed that hydroethanolic Sage-E potentially inhibits the transient receptor potential ankyrin 1 (TRPA1) and transient receptor potential cation channel subfamily V member 1 (TRPV1) pain and heat receptors. This in turn reduced the neurogenic inflammation, being the carnosol, ursolic acid, and oleanoic acid possibly responsible for the activity of the Sage-E. Moreover, ursolic acid emerged as the primary active compound of the chloroformic Sage-E in the reduction of ear edema in croton oil-induced ear edema in mice (Baricevic et al. 2001). Finally, Sage-E presented the ability to decrease oxidative stress in vivo. The administration of an ethanolic Sage-E (mainly composed of methyl carnosate, carnosic acid, carnosol, rosmanol and salvianolic acid) in scopolamine-induced memory impairment albino rats significantly increase the CAT and GSH enzyme activity and decrease the MDA levels, and, consequently, the tissue injury was ameliorated (Ayoub et al. 2022).

Despite Sage-E have only been traditionally used for anti-inflammatory diseases, some studies show their pro-stimulatory activity. The immunostimulatory activity of Sage-E is related to (i) the increase of respiratory burst; (ii) the promotion of cytokine production; and (iii) the increment of the mitogenic and co-mitogenic cell activity (Capek et al. 2003; Capek and Hříbalová 2004; Kontogianni et al. 2013). The main active compounds associated with this activity are polysaccharides, including for example arabinogalactan, rhamnogalacturonan, pectin, homogalacturonan, glucuronoxylan, and xylan. Phenols and flavonoids also contributed to the strength of the immune system. The details of the pro-inflammatory studies are described in Table 12. Some of the most relevant studies will be presented in more detail. For instance, an ethyl acetate Sage-E had the ability to activate resident peritoneal macrophages, increasing the production of ^·NO and TNF-α (Kontogianni et al. 2013). Moreover, different polysaccharide fractions induced rat thymocyte proliferation (Capek et al. 2003). The particular composition of each fraction contributed to the different extent of the immunostimulatory activity. In summary, potassium hydroxide-extractable polysaccharides (composed of xylan-related polysaccharides and glucuronoxylan-related polymers) exhibited the most potent immunostimulatory activity, followed by ammonium oxalate- (composed by arabinan-rich pectin component), and water-extractable polysaccharides (composed by arabinogalactans associated with a galacturonan and/or rhamnogalacturonan core). The water-extractable polysaccharides subfractions (A1–A6) also demonstrated mitogenic and co-mitogenic activity (Capek and Hříbalová 2004). The highest immunostimulatory activity was observed in the acidic subfractions A2–A4 but all subfractions were significantly more potent than the original fraction. The highest content of 3-O-methylgalactose residues in subfraction A2 and the arabinan- or arabinogalactan-rich side chains in subfractions A2–A4 could be important for the enhancement of the immunostimulatory activity. Indeed, although the polysaccharide fractions and subfractions presented similar monosaccharide composition, they considerably differ in the primary structure of their backbones, generating a significant increase in their immunostimulatory activity.

Salvia rosmarinus

S. rosmarinus (syn. Rosmarinus officinalis), known as rosemary, is an evergreen shrub native from Mediterranean region that belongs to the Lamiaceae family. Rosemary tea, obtained from dried or fresh flowering tops and leaves, has been traditionally used to treat and alleviate asthma, arthritis, tonsillitis, sore throats, inflammation, headaches, ulcers, chronic bronchitis, swelling and pain.

S. rosmarinus extracts (Rosm-E) are rich in flavonoids, monoterpenes, diterpenes, sesquiterpenes, triterpenes, phenolic acids, and fatty acids (Supplementary Table S7). Rosmarinic acid, rosmanol, carnosic acid and carnosol are the main chemical compounds proposed as active principles in the Rosm-E (Tables 13 and 14).

The anti-inflammatory activity of Rosm-E is associated to (i) the inhibition of complement system; (ii) the reduction of phagocytosis; (iii) the reduction of the degranulation process; (iv) the reduction of pro-inflammatory cytokines, chemokines, prostaglandins, and leukotrienes expression and production; (v) the decrease of respiratory burst and molecular oxidation levels; (vi) the reduction of the immune cell differentiation; (vii) the reduction of immune cells infiltration; (viii) the increase in antioxidant enzymes and anti-inflammatory cytokines levels; and (ix) the amelioration of edema, pain, and tissue injury (Sotelo-Félix et al. 2002; Lo et al. 2002; Hosseinzadeh and Nourbakhsh 2003; González-Trujano et al. 2007; Benincá et al. 2011; Kuo et al. 2011; Afonso et al. 2013; Amaral et al. 2013, 2018; Yu et al. 2013; da Rosa et al. 2013; Zhang et al. 2015; Silva et al. 2015; Pérez-Rosés et al. 2015; Rocha et al. 2015; Medicherla et al. 2016; Ghasemzadeh et al. 2016; Ghasemzadeh Rahbardar et al. 2017; de Almeida Gonçalves et al. 2018; Borges et al. 2018; Luo et al. 2019; Wang et al. 2020; Yousef et al. 2020; Sasaki et al. 2021; Mansouri Torghabeh et al. 2022; Li et al. 2023; Francolino et al. 2023). The details of the anti-inflammatory studies are described in Table 13. Some of the most relevant studies will be presented in more detail. For example, essential oils obtained from S. rosmarinus strongly inhibited the classical pathway of the complement system, but not the alternative pathway (Pérez-Rosés et al. 2015). These essential oils were also able to reduce the phagocytosis in human neutrophils. Besides, in vitro studies, the Rosm-E bioactivity was also validated in in vivo models. For example, methanolic Rosm-E were proposed as a protective formulation for intestinal inflammation (Medicherla et al. 2016). The administration of methanolic Rosm-E in dextran sulphate sodium-induced ulcerative colitis mice model significantly ameliorated the disease severity. The infiltration of inflammatory cells was significantly reduced, which was corroborated by the decrease in the MPO activity. In another study, rosmarinic acid, isolated from aqueous Rosm-E, exhibited powerful antioxidant effects on aging mice (Zhang et al. 2015). The administration of this compound significantly increased the antioxidant enzymes levels (SOD, CAT, and GSH-Px) in the liver and kidney of aging mice, being the values comparable with the normal control group. Consequently, the MDA levels decreased, suggesting the prevention of lipid peroxidation by free radicals scavenging. Rosmarinic acid also reduced the tissue injury. The antioxidant activity of Rosm-E was corroborated in the following study. Arthritic rats treated with aqueous Rosm-E presented a significant decrease of the oxidative damage (de Almeida Gonçalves et al. 2018). A significant decrease in ROS and protein carbonyls levels was observed. Conversely, a markedly enhancing of the activities of GSH, CAT, GRS, SOD, and GPx was detected. Moreover, the treatment with aqueous Rosm-E also delayed and reduced the edema, the number of recruited polymorphonuclear leukocytes in the knee, and the weight of the lymph nodes and adrenal glands. Interestingly, this extract showed similar activity to the NSAID widely prescribed ibuprofen. In another study, aqueous Rosm-E significantly decreased neutrophils migration, and the secretion of pro-inflammatory mediators (LTB4, PGE2, IL-6, TNF-α, and ^·NO) in a carrageenan-induced inflammation in Wistar rats (Silva et al. 2015). Interestingly, the results were similar with the clinical prescribed indomethacin treatment. The blockade of neutrophil migration by this aqueous Rosm-E was associated with the modulation of the neutrophil chemotaxis and the inhibition of the adhesion molecules expression. Additionally, treatment with aqueous Rosm-E markedly increased SOD levels, decreasing the lipid peroxidation. Another aqueous ethanolic Rosm-E exerted potent anti-inflammatory activity in sciatic nerve chronic constriction injury-induced neuropathic pain in Wistar rats (Ghasemzadeh et al. 2016; Ghasemzadeh Rahbardar et al. 2017). The treatment with aqueous ethanolic Rosm-E significantly reduced the inflammatory response through the down-regulation of the inflammatory mediator’s expression (IL-1β, TNF-α, PGE2, ^·NO, iNOS, COX-2, and MMP2) and the reduction of the tissue injury.

Rosm-E also displayed strong bioactivity in allergies. The Rosm-E, obtained by steeped in dichloromethane-methanol, modulated mast cell functional responses, attenuating IgE-mediated mast cell activation (Yousef et al. 2020). The application of this Rosm-E to anti-TNP IgE-sensitized and cognate allergen-stimulated primary murine mast cells rapidly impaired the activation of MAPK and NF-κB signaling pathways. The phosphorylation of p38 and JNK, but not ERK, was prevented. Moreover, a decrease in the NF-κB transcriptional activity was observed. In addition, Rosm-E treatment significantly decreased the gene expression and further production and release of several pro-inflammatory cytokines and chemokines (IL-6, IL-13, TNF, CCL1, and CCL3), contributing for the resolution of late-phase of mast cell activation. Rosm-E also significantly prevented the rapid degranulation of mast cells, highlighting its capability in the reduction of the early-phase of mast cell activation.

Rosm-E have also immunostimulatory activity related to the promotion of cytokines production; and (ii) the increase of the respiratory burst (Kontogianni et al. 2013). Stimulatory study details are reported in Table 14. For example, the treatment of naïve peritoneal macrophages with ethyl acetate Rosm-E enhanced the production of ^·NO and TNF-α (Kontogianni et al. 2013).

Taraxacum officinale

T. officinale (Asteraceae), also known as common dandelion, is native to Europe and Asia. Despite being considered a weed in many crops around world, dandelion has a long history as a traditional herbal remedy to treat skin inflammation, upper respiratory tract infections, bronchitis, pneumonia, arthritic and rheumatic conditions. Usually, the dandelion infusion is made of its dried leaves, flowers and/or roots.

T. officinale extracts (Tara-E) are composed by a wide range of chemical compounds, such as flavonoids, phenolic acids, fatty acids, triterpenes, sesquiterpenes, polysaccharides, coumarins, and diterpenes (Supplementary Table S8). The main active compounds in Tara-E are taraxasterol, luteolin, luteolin-7-glucoside, caffeic acid, and chlorogenic acid (Table 15).

The anti-inflammatory activity of Tara-E is associated with (i) the reduction of pro-inflammatory cytokines, prostaglandins, and immunoglobulins expression and production; (ii) the decrease of respiratory burst and molecular oxidation levels; (iii) the reduction of immune cells infiltration and activation; (iv) the increase in antioxidant enzymes levels; and (v) the amelioration of edema, pain, and tissue injury (Hu and Kitts 2004, 2005; Park et al. 2010, 2011, 2014; Liu et al. 2013; Xiong et al. 2014; Zhang et al. 2014; Wang et al. 2016; Cai et al. 2017; Kala et al. 2017; Razak et al. 2020; Majewski et al. 2021; Pfingstgraf et al. 2021; Epure et al. 2023; Zhou et al. 2023). The details of the anti-inflammatory studies are described in Table 15. Some of the most relevant studies will be presented in more detail. For instance, aqueous and methanolic Tara-E, obtained from leaves, ameliorated the oxidative stress and the inflammatory response in LPS-stimulated RAW 264.7 macrophages (Park et al. 2011). The pre-treatment with both extracts reduced the ^·NO and MDA levels through the downregulation of the iNOS expression and production. This occurred via inactivation of the NF-κB and the upregulation of the antioxidant enzymes levels (CAT, SOD, GPx, GR, and GSH). Interestingly, the antioxidative activity of the methanolic Tara-E was significantly higher than the aqueous Tara-E, which could be related with the higher content in phenols, including luteolin and chicoric acid. Anti-inflammatory effects of polysaccharides isolated from T. officinale (TOP1 and TOP2) on LPS-stimulated murine macrophages RAW 264.7 were also resported (Park et al. 2014). The pre-treatment with TOP markedly increased HO-1 expression due to the Nrf2 nuclear accumulation, through the activation of all the MAPK (ERK, JNK, p38) and PI3K/Akt signaling pathways. Moreover, TOP enhanced the cytoprotective ability, through Nrf2-mediated HO-1 upregulation, in t-BHP-induced oxidative stress in murine macrophages. Additionally, NF-κB and Akt signaling pathways were inhibited since the phosphorylation of IκBα, p65 and Akt were attenuated by TOP treatment. However, they had no effect on the phosphorylation of ERK, JNK and p38. Consequently, the ^·NO production, and iNOS and TNF-α production were reduced, however not the production of the PGE2 and COX-2. Taraxasterol was also implied in the anti-inflammatory activity. It led to the downregulation of the iNOS and COX-2 expression and production, mediated through the inhibition of ERK1/2 and p38 phosphorylation (Xiong et al. 2014). The anti-inflammatory activity of this compound was also demonstrated in vivo. Indeed, it showed a promising protective effect against Freund's complete adjuvant-induced arthritis in Wistar rats (Wang et al. 2016). The treatment with this isolated compound significantly reduced the paw swelling and arthritis index. In fact, an amelioration of the degree of synovial hyperplasia, cartilage and bone destruction, as well as the inflammatory cell infiltration into the synovium joint-space narrowing were observed. Moreover, the spleen and thymus indexes were strongly reduced, suggesting that taraxasterol may help in the recovery of the hyperfunctioning of immune organs without causing damage. Also, the serum levels of some pivotal inflammatory mediators that contribute to the clinical manifestations of rheumatoid arthritis, namely TNF-α, IL-1β and PGE2, were significantly decreased. Therefore, taraxasterol was able to inhibit bone destruction in an animal model of rheumatoid arthritis.

Recently, a Tara-E obtained from aerial parts demonstrated its antioxidant activity in vivo (Epure et al. 2023). The treatment of turpentine-induced acute inflammation in rats with this Tara-E strongly reduced the oxidative stress, by the decrease of MDA, thiols, and nitrites/nitrates levels. Moreover, the administration of the Tara-E also reduced the expression of NF-κB signaling pathway.

Conclusions and future perspectives

Recently, the interest in the discovery of new plant-based drugs have been re-arising as promising treatments for the health care management of immune-related diseases. In fact, several plants have high-value bioactive compounds that can interact with immune cells, enhancing or reducing the main inflammatory mediators. Thus, they can be extremely important in the biopharmaceutical and biomedical fields. Usually, the few side-effects and the low cost-effective development than the ones required for developing a new drug increase the demand in plant-based drugs for diseases where the immune system is compromised or hyperactive.

Plant extracts are a complex mixture of phytochemicals whose composition can change due to multiple variables herein discussed. Thus, incongruences between the composition of the plant extracts derived from the same plant species and, consequently, the different biological outcomes, are a well-recognized concern. Moreover, although the main bioactive compounds of plant extracts have been characterized, new entities are still arising due to the complexity of the natural mixture. Another problem in the chemical characterization of the plant extract is the various possibilities of naming a chemical compound.

The isolation and purification of a specific active principle is a major goal in the plant-based drug discovery. However, the identification of the bioactive molecules in a complex mixture could be extremely difficult. Therefore, a bio-guided fractionated method must be employed to determine the bioactive principles. Moreover, the coupling of extraction techniques with chromatographic methods could be an added-value resource in the identification and quantification of the bioactive compounds. Additionally, the advances in metabolomics and bioinformatics can help in the determination of the extract composition, molecular structure, and the mechanism of action of the main(s) bioactive molecule(s) before their corroboration with in vitro and in vivo experiments. This can reduce the time of experiment and ensures the best selection of the bioactive compounds. Additionally, if an additive or synergistic activity arise from the complex mixture of chemical compounds, the development of a standardized plant extract should be employed. The standardization of the plant extract concentration of the active principles will certainly attenuate and/or overcome the divergence between biological activities.

The therapeutic potential of plant-based drugs can also be significantly improved. The future of plant-based drugs will be shaped by the latest advancements in cultivation practices, particularly those that enable the stimulation of a target chemical(s) compound(s) production within the plant organism. These advancements hold the potential to enhance the bioactive value of studied plants by optimizing the presence of desired bioactive molecules.

The development of a green, sustainable, and industrially scalable extraction technique is a future requirement. A faster extraction method that enables the reduction of the use of organic solvents, the decrease of the energy consumed, and the reduction of waste products must be in the basis of the plant-based drug discovery. This approach aligns with the goal of creating environmentally friendly and sustainable processes in this field.

In conclusion, phytochemicals can have huge medical, economic, and environmental impacts. Indeed, considering the urgent need of effective drugs to safely treat immune-related conditions, this is a valid strategy. The future of the plant-based immunomodulatory drugs will (re)arise from transforming low-value crops into potent and high-value formulations.