Background

In December 2019, the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been identified as the cause of a respiratory illness coronavirus disease 2019 (COVID-19) [1]. The most common treatment for COVID-19 patients remains to be supportive care. Despite the emerging therapeutic agents have been assessed for the treatment of COVID-19, none has yet been shown to be efficacious [2, 3]. To date, no dedicated therapeutic agent has been implemented yet, nor a vaccination strategy that has been confirmed to prevent COVID-19. The case fatality rate (CFR) has been estimated by the WHO to range from 0.3 to 1%, higher than that of influenza A [4].

Immune-mediated lung injury and acute respiratory distress syndrome (ARDS) are associated with poor prognosis in COVID-19 patients [5]. Symptoms of COVID-19 usually range from mild upper respiratory tract symptoms to progressive life-threatening viral pneumonia and progressive hypoxemia requiring mechanical ventilatory support. The leading cause of mortality in COVID-19 patients is hypoxemic respiratory failure most frequently resulting in ARDS, characterized by diffuse lung damage with edema, hemorrhage, and intra-alveolar fibrin deposition [6, 7]. More interestingly, laboratory findings indicate a hyperactivated nature of the immune system, specifically high levels of circulating CD4+ and CD8+ lymphocytes. Looking at the hyperactive immune response detected in COVID-19 patients, several potential treatments relating to key immunoregulators have been proposed. Another important factor influencing the prognosis of COVID-19 patients is having a state of hyperinflammation, where several immunosuppression modalities have provided a tool to decrease the mortality in patients with severe condition [8]. Understanding the pathogenesis of SARS-CoV-2 in association with the host immune response will help elucidate some key targeted treatment options. Repurposing of previously approved medications, such as the anti-malarial drug hydroxychloroquine, anti-rheumatic drugs, such as tocilizumab (interleukin [IL]-6 receptor inhibitor), baricitinib (Janus kinase [JAK] inhibitor), and anakinra (IL-1 receptor antagonist), have been employed to treat COVID-19, largely attributed to their known pharmacokinetic and safety profiles [9].

Mesenchymal stromal/stem cells (MSCs) offer a promising emerging therapeutic approach toward modifying the adverse effects of the infection in SARS-CoV-2 patients. This therapy has been found to decrease the cytokine storm and exert anti-inflammatory, immunomodulatory, and regenerative functions by altering the expression of pro-inflammatory cytokines, and aid in repairing the damaged tissues in COVID-19 patients. Several clinical trials have already provided a proof of concept showing that intravenous (IV) infusion of MSCs is a safe option and could lead to clinical and immunological improvement in some patients with severe COVID-19 pneumonia [10]. Such findings support employing phase 2 randomized controlled trial, where other randomized trials with a control arm consisting of standard treatment, will help to elucidate the mechanistic potential of MSC-based therapeutic strategy. This review summarizes the immunopathogenesis of the SARS-CoV-2 and the therapeutic potentials of MSCs for treating lung injuries associated with COVID-19. Furthermore, we highlight the current clinical trials using MSCs for treating COVID-19 patients and discuss limitations of the existing MSC-based treatment strategies.

Pathogenesis of SARS-COV-2

The lung alveoli are lined with the alveolar epithelium consisting of a monolayer of alveolar type I (AT1) cells and alveolar type II (AT2) cells. Under normal condition, the AT2 cells secrete surfactant covering all the lining epithelium to facilitate alveolus expansion. AT1 and AT2 are tightly connected with tight junctions, which control the transfer of ions and fluid across the epithelium. The endothelial cells of the blood capillaries are connected by intercellular junctions and control the influx of inflammatory cells and fluid into the interstitial space between the aveoli. Initially, the spike glycoprotein (S protein) expressed on viral envelopes binds to the angiotensin-converting enzyme 2 (ACE2) receptor [11], a very similar structure to that of SARS; however, with a 10–20 times much higher binding affinity when compared to the SARS S protein [12]. This binding capability partially explains the high transmission of SARS-CoV-2 [12]. The main target cells for SARS-CoV-2 infection are AT2 cells and resident alveolar macrophages, because they are expressing ACE2. SARS-CoV-2 utilizes ACE2 for entry and the serine protease TMPRSS2, which is also expressed by the alveolar cells, for S protein priming [13]. This activation induces chemokine and cytokine secretion that recruits inflammatory and immune cells into the infected alveoli, followed by other waves of cytokine release. Activated macrophages have a significant role in hemophagocytic lymphohistiocytosis (HLH)-like cytokine storm during COVID-19 [14]. Secondary HLH could be precipitated by a genetic defect in cytolytic pathways or observed in during infection, malignancy, and rheumatic disease. HLH is characterized by a predominance of inflammatory cytokines and expansion of tissue macrophages displaying hemophagocytic activity [15]. Cytopenias, a state of elevated inflammatory cytokines or hypercytokinaemia, unremitting fever, elevated ferritin level, and multi-organ damage, are among the key characteristics of HLH seen in seriously ill COVID-19 patients [8]. Type I interferons (IFN) and natural killer (NK) cells result in cytolytic immune responses, following a successful recognition of pathogen-associated molecular pattern. This serves as a first line of defense against SARS-CoV-2 infection through the innate immune system. Activated cytotoxic T cells and B cells are key players of the adaptive immunity helping with viral clearance via destruction of virus-infected cells and antibody production, respectively. However, when the anti-viral immune response remains active, an aberrant and uncontrolled production of inflammatory cytokines occurs, causing what is known as the “cytokine storm”, leading to damage in the pulmonary tissue [16, 17].

Severely ill COVID-19 patients, especially the ones with pneumonia, show disproportionate immune profile, with considerably lower lymphocyte counts (lymphocytopenia) and increased concentrations of inflammatory cytokines. Among the significant inflammatory interleukins (ILs) are IL2, IL-6, IL-7, IL-10 (Th2), IL-1β and IFNγ (Th1), and tumor necrosis factor (TNF) [6]. Furthermore, in patients with severe symptoms, an elevation in granulocyte-colony stimulating factor (G-CSF), IFNγ-induced protein-10 (IP-10), macrophage inflammatory protein 1α (MIP-1α/CCL3), and macrophage chemoattractant protein-1 (MPC-1/CCL2) are noticed [18]. A recent study has performed a screen for 48 cytokines in 53 COVID-19 patients with moderate and severe symptoms recorded a dramatic increase of 14 cytokines in COVID-19 patients in comparison to healthy individuals [19]. Of those cytokines, the increased hepatocyte growth factor (HGF), MCP3, IP-10, monokine induced gamma interferon (MIG), and MIP1α are associated with the severity of the symptoms [19]. Key cells in the adaptive immunity, such as CD4+ T cells, CD8+ T cells, and NK cells are also decreased in severely ill patients [5]. On the other hand, an elevation of CD14+ CD16+ monocytes, IL-17-producing CCR4+ CCR6+ CD4+ (T-helper 17/Th17) cells, perforin and granulysin-expressing cytotoxic T cells are reported. These constitute the pro-inflammatory subsets of T cells responsible for the severe immune injury in the lungs [5].

Among the histological profiles of COVID-19 are the significant alterations in the morphology of the endothelial cells, which also express ACE2. These changes include damage of the intercellular junctions, a loss of attachment to the basement membrane, and cell swelling [20] (Fig. 1). The migrated neutrophils and monocyte-derived macrophages release toxic mediators, causing endothelial and epithelial injuries (Figs. 1 and 2). The intercellular junctions are disrupted leading to formation of spaces between the alveolar cells as well as between the endothelial cells, resulting in an increase in the permeability of the epithelial and endothelial cells (Figs. 1, 2, and 3). The increase in the permeability facilitates the migration of inflammatory cells and allows the influx of RBCs and fluid from the blood capillary. Large volume of fluid (alveolar edema) fills the airspace leading to a difficulty in the breathing. Also, the inflammatory reactions may lead to alveolar cell death, fibrin deposition, and hyaline membrane formation. These findings support an important role of endothelial cells in the vascular phase of COVID-19. Furthermore, pulmonary intussusceptive angiogenesis and other pulmonary vascular lesions have been observed in autopsy specimen of COVID-19 patients [20].

Fig. 1
figure 1

Immunopathogenesis of the SARS-CoV-2. The alveolar epithelium consists of a monolayer of alveolar type I (AT1) cells and alveolar type II (AT2) cells. Under normal condition, the AT2 cells secrete surfactant covering all the lining epithelium to facilitate alveolus expansion. AT1 and AT2 are tightly connected with tight junctions, which control the transfer of ions and fluid across the epithelium. The endothelial cells of the blood capillaries are connected by intercellular junctions and control the influx of inflammatory cells and fluid into the interstitial space. SARS-CoV-2 infects AT2 cells and resident alveolar macrophages that express ACE2. This activation induces chemokine secretion that recruits inflammatory and immune cells into the infected alveoli. The increased inflammatory cells in the lung lead to secretion of large amounts of pro-inflammatory cytokines “cytokine storm” that lead to damages in the lung. The migrated neutrophils and monocytes release toxic mediators, causing endothelial and epithelial injuries. The intercellular junctions are disrupted leading to formation of gaps between the alveolar cells as well as between the endothelial cells, resulting in an increase in the permeability of the epithelial and endothelial cells. The increase in the permeability facilitates the migration of inflammatory cells and allows the influx of RBCs and fluid from the blood capillary. Large volume of fluid (alveolar edema) fills the airspace leading to a difficulty in the breathing. Also, the inflammatory reactions may lead to alveolar cell death, fibrin deposition, and hyaline membrane formation

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Fig. 2
figure 2

Anticipated immunomodulatory actions of MSCs in the lung infected with SARS-CoV-2. MSCs perform immunomodulatory functions by multiple ways, including cell-cell contact, paracrine factor secretion, and extracellular vesicles (EVs). Upper panel shows the inhibitory effect of MSCs on immune cells, which are highly activated by the viral infection and secrete chemokines and cytokines in response to the infection. These chemokines and cytokines increase lung inflammation and cause epithelial and endothelial damage. Lower panel show the stimulatory effect of MSCs on other immune cells, which are crucial for SARS-CoV-2 clearance. AT1, alveolar type I epithelial cells; AT2, alveolar type II epithelial cells; NK cells, natural killer cells; Th17, T helper 17 cells; DCs, dendritic cells; DCregs, regulatory dendritic cells; Tregs, regulatory T cells; Bregs, regulatory B cells

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Fig. 3
figure 3

Anticipated effect of MSCs on lung cells in SARS-CoV-2-induced lung injury. High levels of pro-inflammatory cytokines (cytokine storm) associated with SARS-CoV-2 infection lead to lung cell damage and an increase in the permeability of pulmonary capillaries. The affected cells include alveolar type I epithelial cells (AT1), alveolar type II epithelial cells (AT2), and endothelial cells. MSCs secrete several paracrine factors and extracellular vesicles (EVs), which have anti-apoptotic functions. This effect enhances cell survival and improves lung functions. Ang-1, Angiopoietin 1; HGF, hepatocyte growth factor; FGF7, fibroblast growth factor-7; VEGF, vascular endothelial growth factor

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Severe respiratory illness could be a major symptom of SARS-CoV-2 infection, because the ACE2 receptor is expressed in the lung AT2 cells, alveolar macrophage, and capillary endothelial cells [11] (Fig. 1). The expression of the ACE2 has been detected in other tissues, such as the cardiovascular, hepatic, renal, pancreatic, and the gastrointestinal tissues. This expression profile partially explains why some infected patients not only develop ARDS, but also develop other complications, such as myocardial injury (MI), arrhythmia, acute kidney injury (AKI), shock, multi organ failure, diabetes, and ultimately death [21].

IL-6 has an essential part in inflammatory cytokine storm in COVID-19. IL-6-producing CD14+ CD16+ inflammatory monocytes are significantly high [22]; therefore, the rationale for using tocilizumab has been used in COVID-19 patients. Tocilizumab, which is a recombinant humanized monoclonal antibody against the IL-6 receptor, is likely to induce its antagonistic effect on IL-6-producing monocytes following activated Th1 cells in the lung. Tocilizumab is a first drug for the treatment of cytokine storm in COVID-19, especially in patients with multiple comorbidities. Despite the numerous ongoing trials assessing the safety and efficacy of tocilizumab in COVID-19 patients, IL-6 play a role in controlling the lung inflammation and is important for the clearance of viruses [23]. Therefore, inhibiting IL-6 raises the possibility of impaired viral clearance or exacerbation of lung inflammation [9].

Interestingly, an abnormal coagulation profile has been shown in COVID-19 patients during the late stage of the disease; specifically, increased concentrations of D-dimer and other fibrin degradation products are mainly associated with poor prognosis [24]. The HScore is a recommended evaluation as well as prognostic tool used in patients with secondary HLH at high risk of hyperinflammation. The score combines both critical laboratory as well as clinical parameters, assessing for an underlying of immunosuppression and cytopenias, measuring serum aspartate aminotransferase (AST), triglycerides, fibrinogen, ferritin, body temperature, organomegaly, and hemophagocytosis on bone marrow aspirate [8]. The HScores generate a probability for the presence of secondary HLH; a score more than 169 is 93% sensitive and 86% specific for HLH [8].

Finally, since the anti-viral immunity is needed to recover from COVID-19, the use of immunosuppressants on these patients should be used with caution. One strategy to avoid the inhibition of anti-viral immunity is to use targeted instead of broad immunosuppressive medications. Unfortunately, we still lack consensus on the optimal timing of treatment administration to decrease the harmful effects of immunosuppression, as well as the routes of their administration.

Mesenchymal stem cells (MSCs): characteristics and types

MSCs are a heterogeneous cell population propagating in vitro as plastic-adherent cells, have fibroblast-like morphology, and form colonies in vitro [25]. The International Society for Cellular Therapy (ISCT) defining criteria for MSCs is that they adhere to plastic, express the surface markers CD90, CD73, and CD105, are negative for the hematopoietic markers CD14, CD34, CD45, CD19, and HLA-DR, and should express a multilineage differentiation capability into adipogenic, osteogenic, and chondrogenic lineages [26].

Bone marrow MSCs (BM-MSCs) are considered the most widely used and investigated type of MSCs, which was first isolated from the bone marrow by Friedenstein and colleagues in 1974 [27]. Later, MSCs were identified and successfully produced from other sources, such as the perivasculature [28], adipose [29], dental pulp [30], muscle [31], dermis [31], and fetal tissue [32]. The abundance of adipose-derived stem cells (ASCs), their ease of isolation using a minimally invasive procedure [33], and their expansion as well as their differentiation ability into multiple lineages make ASCs a promising less-invasive alternative to BM-MSCs for therapeutic applications [34, 35]. The most commonly used adult sources for human MSCs are bone marrow [36] and the adipose tissue stromal vascular fraction [29, 34, 35]. The highly harvestable bone marrow or unwanted/waste product of adipose sources forms the foundation for most of the data in the field of MSC-based therapeutics. The umbilical cord (UC) tissue [37] and the placenta [38, 39] and their associated tissue Wharton jelly (WJ), and amniotic fluid (AF), are among the other young “adult” tissues, that are also considered good sources of human MSCs, where they are normally discarded after birth.

MSCs are generally recognized as immune evasive making them safe when used in allogeneic settings [40]. Allogeneic MSCs are able to bypass the immune system due to low expression of the major histocompatibility complex-1 (MHC-I) and -II proteins. MSCs are often referred to as being “immunoprivileged” due to lack of the T cell costimulatory molecules, CD80 and CD86 [41]. Previous studies reported that fetal MSCs, adult BM-MSCs, and ASCs express HLA-I and do not express HLA-II [42,43,44,45,46]; however, these MSCs start to express HLA-II after stimulation with IFN-γ [44, 45, 47, 48]. A recent study demonstrated that iPSC-derived MSCs do not express HLA-II and costimulatory molecules [49]. Interestingly, induced pluripotent stem cell (iPSC)-derived MSCs express a very low level of HLA-II in comparison to MSCs derived from fetuses and adult sources after their stimulation with interferon-γ (IFN-γ) [49]. These findings present iPSC-MSCs as an efficient source for allogenic transplantation without the risk of immune rejection due to the lower immunogenicity compared to adult MSCs.

Therapeutic potentials of MSCs

MSCs have been extensively studied over the past ~ 30 years for their wide clinical applications and regenerative capacity. MSCs have made their way over the past 25 years into now over 950 registered clinical trials listed with the FDA, exhibiting an excellent safety profile. With over a 10,000 patients treated with MSCs in a controlled clinical setting, and upon successful completion of phase 1 or phase 2 trials, several tens of MSCs-based studies have advanced to phase 3 clinical trials (www.clinicaltrials.gov).

A fundamental clinical decision remains to choose among whether to use autologous vs allogeneic sources of MSCs, where both have displayed successful production of large numbers of MSCs [50, 51]. MSC replacement in the large numbers is needed to treat significant tissue injury, a process that further requires orchestrated steps involving successful engraftment and cell differentiation [52]. A target dose of 100–150 million MSCs can be obtained from cell culturing and expansion of 25 ml of BM-MSC aspirate. In about 3 weeks duration, a volume of about 0.4–0.5 ml of packed cells can be generated [53]. The MSC isolation from different tissues, such as BM and adipose tissue and their re-implantation at other sites highlight their ability to repair tissues in vivo. However, this process clearly diminishes in aging population compared to younger adults [54]. In addition, MSCs could be generated in vitro in large number from human pluripotent stem cells (hPSCs) [55, 56], which showed a lower immunogenicity in comparison to adult sources [49].

MSCs have been extensively examined for their therapeutic capacity in regenerative medicine, because of their ability to home to sites of inflammation and damaged tissue, ultimately serving as a source of growth and trophic factors and regenerative molecules. The potential therapeutic effect of MSCs is based on their low immunogenicity, their immunomodulatory characteristics, and their ability to secrete growth factors, as well as anti-microbial peptides [57]. MSCs administered systemically tend to migrate to the injury region to promote functional recovery [58]. MSCs can also extravasate from the blood vessels, just like immune cells, via the expression of cell surface adhesion molecules. Migration of MSCs occur in response to chemokines binding to cognate receptors present on their cell surface [59] and result in the stimulation of matrix metalloproteinases degrading the basal membrane and allowing subsequent extravasation [60]. By displaying a coordinated rolling, MSCs contact the endothelial cells in a P-selectin- and vascular cell-adhesion molecule 1 (VCAM1)-dependent manner [61]. Guided by chemotactic signals, MSCs migrate through the interstitium to the injured area. An increase in the MSC migration capacity toward chemokines is achieved via the upregulation of their receptors, CCR2, CCR3, and CCR4. Also, interleukin (IL)-8, an inflammatory chemokine, may induce migration of MSCs to injured areas [62, 63].

Immunoregulatory functions of MSCs

One of the major therapeutic characteristics of MSCs is their immunomodulatory role, including a network of cytokines and cell-cell interactions. Interestingly, MSCs only exert its immunoregulatory capacity after receiving the activation signals from the inflammatory milieu; therefore, MSC’s immunoregulatory capacity is not constitutive, rather is driven by “licensing” process [64]. Previous studies showed that the macrophages play an essential role during wound healing; thus, they have emerged as key candidate targets in therapeutic tissue regeneration approaches [64, 65]. Macrophages exhibit functional repolarization as tissue repair progresses, shifting from the pro-inflammatory or M1-phenotype to an anti-inflammatory or M2-phenotype. M1 macrophages secrete high levels of pro-inflammatory cytokines, while M2 macrophages secrete lower levels of pro-inflammatory cytokines, exhibit tissue repair, and enhance the resolution of inflammation [66]. Imbalance between M1- and M2- activities can lead to continuous inflammation and hinders the normal repair process, both contributing to impaired tissue repair [67]. MSCs enhance tissue repair and regeneration by modulating the immune response, acting as sensors and switchers of inflammation, rather than by replacing damaged cells. This is largely attributed to the secretion of growth factors; among the immunoregulatory factors are prostaglandin E2 (PGE2) and IL-6 that help in transitioning macrophages toward M2 phenotype [68, 69] (Fig. 2). Further, the classical pro-inflammatory cytokines produced at the acute stage of inflammation, such as IFN-γ, TNF-α, or IL-1β enhances the paracrine effects of MSCs exerted on macrophages [70, 71].

In order to stimulate the MSC immunosuppressive effect, threshold levels of inflammatory factors are required. Insufficient MSC activation can lead to an increase in the inflammation [72]. Recently, it has been shown that IL-10 alone is insufficient to enhance MSC immunomodulation, rather enhances the priming influence of TNF-α, indicating that MSC activation by IL-10 is dependent on TNF-α [64]. MSCs further decrease TNF-α secretion via PGE2 but not IL-6, supporting the concept that MSC immunomodulatory potential is highly correlated to the release of PGE2 [64].

Among the other MSC-derived molecules shown to exert an immunoregulatory functions are transforming growth factor beta (TGF-β), hepatocyte growth factor (HGF), and indoleamine 2,3-dioxygenase (IDO) [73] (Fig. 2). TGF-β secreted by MSCs could shift lipopolysaccharide-activated macrophage polarization toward the M2-phenotype, decrease inflammatory reactions, and enhance the phagocytic activity through the Akt/FoxO1 pathway [74], while HGFs modulate IL-10 production in monocytes via the ERK1/2 pathway [75]. MSC IDO activity is involved in the differentiation of monocytes into IL-10-secreting M2 immunosuppressive macrophages (CD14+/CD206+) [71]. These processes decrease immune cell maturation and activation, in addition to enhancing the differentiation of T cells into regulatory T cells (Tregs) [52].

The immunoregulatory effects of MSCs is highlighted by the ability of BM-MSCs to suppress T cell proliferation [76, 77] and suppress the conversion of monocytes and CD34+ hematopoietic progenitor cells into dendritic cells (DCs) in vitro [78,79,80,81]. Mature DCs cultured with MSCs have reduced production of IL-12 and MHC class II molecules, CD11c, CD83, though hindering the DC antigen-presenting function [78,79,80,81].

Anti-inflammatory and antiproliferative effects of MSCs

MSCs reduce the pro-inflammatory effect of DCs by suppressing their secretion of TNF [82]. Also, plasmacytoid DCs (pDCs), a set of specific cells for the secretion of high levels of type I IFN, increase the production of IL-10 following the incubation with MSCs [82]. MSCs can further inhibit the cytotoxic activity of resting NK cells by reducing the production of natural cytotoxicity receptor 3 (NKp30) and natural-killer group 2, member D (NKG2D), involved in the activation of NK cells and target cell killing [83]. Therefore, MSCs inhibit NK cell proliferation and IFN production [84, 85]. Also, neutrophils are important cells of innate immunity, undergoing a process known as the respiratory burst when binding to an antigen. MSCs have been reported to eliminate the respiratory burst and to prevent the neutrophil cell death by an IL-6-dependent mechanism [86]. Also, MSCs play a key role in the adaptive immune system, where it inhibits the proliferation of T cells activated with antigens [76]. This leads to a reduction in the IFN production and an increase in IL-4 production by T helper 2 (T2) cells, indicating a change in T cells from a pro-inflammatory (IFN-producing) to an anti-inflammatory (IL-4-producing) state [82].

Furthermore, MSCs have been shown to downregulate CD8+ cytotoxic T lymphocytes (CTL)-mediated cytotoxicity [87] and further inhibit B cell expansion in vitro. Also, MSCs can suppress B cell differentiation and the constitutive secretion of chemokine receptors, affected by the MSC-mediated suppression of T cell functions [88]. Furthermore, MSC-derived IDO has been shown to be required in the inhibition of the expansion of IFN-secreting Th1 cells and, together with PGE2, to stop NK cell activity [89].

Anti-apoptotic and protective functions of MSCs

Several pro-inflammatory molecules modulate the immunosuppressive, trafficking, and paracrine potential of MSCs. Enhanced paracrine potential of MSCs induced by TNF-α, IL-1b, and nitric oxide (NO), ultimately increases MSC secretions of regenerative, immunomodulatory, and trafficking molecules, including the key factor, insulin-like growth factor 1 (IGF-1) [90]. Heme oxygenase-1 (HO-1) is upregulated by TNF-α, IL-1α, or NO in endothelial cells or alveolar cells, where MSCs overexpressing HO-1 showed an increase in the anti-inflammatory, anti-apoptotic, and vascular remodeling properties [91]. Upregulation of HO-1 increases production of trophic molecules, such as FGF2, and IGF-1, and VEGF [90]. Fibroblast growth factor-10 (FGF-10), keratinocyte growth factor-2 (KGF-2), has been found to regulate epithelial-mesenchymal interactions that are crucial for the development of lung [92]. FGF-10 exerts a role in lung resident-MSC propagation, mobilization, and the protective effects against acute lung injury [93]. MSCs can affect on the endothelial differentiation of endothelial progenitor cells in vitro, mainly dependent on VEGF [94]. Human leukocyte antigen-G5 (HLA-G5) is another soluble factor secreted by MSCs and its secretion is IL-10-dependent. HLA-G5 is required to suppress the function of T lymphocytes and NK cells and to activate regulatory T cells [95]. Galectin-1 and 3 (Gal-1 and Gal-3) as well as Semaphorin-3A (Sema-3A) are other secreted MSC immune regulators, known for their inhibitory activities. Gal-1 and Sema-3A are two soluble factors that can suppress T cell proliferation via neuropilin-1 (NP-1) binding [96, 97]. MSC-derived Gal-1 significantly regulates the release of TNFα, IFNγ, IL-2, and IL-10 [97].

Finally, interleukin-1 receptor antagonist (IL-1Ra) and programmed death-1 (PDL1) are among the other secreted regulators of MSCs. IL-1Ra is among the anti-inflammatory cytokine produced by MSCs, which can inhibit Th17 polarization. IL-1Ra expression tends to increase in MSCs exposed to IL-1β, TNF-α, and IFN-γ. Th17 cells induce the upregulation of PDL1, playing a major role in activating the MSC immunosuppressive effect [98]. PDL1 further support the cell-cell contact through MSC-mediated inhibition on Th17 cells [98]. MSC enhanced PDL1 ligand secretion suppress the activation of CD4+ T cells and downregulate IL-2 secretion [99].

MSC-derived exosomes

Extracellular vesicle (EV) is a term including both exosomes and microvesicles (MVs). The exosome diameter is less than 200 nm, while MV diameter can reach up to 1000 nm. The secretomes of MSCs and their vesicles offer a powerful tool for cell-free therapy due to their paracrine and/or endocrine effects [100]. This strategy bypasses most of the safety concerns related to cell-based therapy, such as contamination with oncogenic cells and continuous cell proliferation [101]. The key features of MSC-derived EVs are (1) non-proliferative, which reduce the risk of tumor formation; (2) negative for HLA-I and HLA-II, which can be induced, and therefore, they can be used from other individuals without any risk of immune response; (3) small in size allowing them to pass from the small blood capillaries; and (4) stored without using DMSO, which may change their characteristics [102]. EVs bind to a receptor on the cell membrane of the targeted cells, where they merge with the membrane to secrete the EV contents inside the cell or enter into the cytoplasm in the form of endocytic vesicles [103].

EVs have proposed as an effective vehicle for delivering miRNAs, which control above 60% of the mRNAs; therefore, transferring them in EVs is of clinical significance [102]. MSC engineering is one way miRNA could be loaded into EVs and still exert its therapeutic effects [104,105,106,107,108,109,110,111,112,113]. There is a lack of a consensus on miRNA signature among MSC-EVs from various sources [114]. However, the targeted pathways include Wnt signaling, antifibrotic, mitochondrial fission, cell proliferation, cell survival, and apoptosis [115]. Reports showed that the MSC-EV-mediated delivery of miRNAs in animal models have defined several key target proteins like TGF-β receptor 1, Dynamin-related protein 1(DRP1), Methyl-CpG-binding protein 2 (Mecp2), PTEN, semaphorin 3A (sema3A), stat3, Cyclin G1, IGF1R, and P4HA1, NLRP3, and Bcl-2 [9, 104,105,106,107,108,109,110,111,112,113] (Table 1).

Table 1 Studies demonstrating the MSC-EV-mediated transfer of miRNAs in animal models
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Among the targeted proteins, Sema3A has been found to induce sepsis-triggered cytokine storm through an interaction with Plexin-A4 and Toll-like receptors (TLRs) [116]. Stat3 is another targeted protein, a key upstream stimulator of inflammatory pathways during sepsis [117]. Finally, EVs act as biological regulators that can promote changes in their targets through targeted pathways. The cargo of the EVs is enriched with miRNAs and other transcripts that act as regulators of the immune system [118, 119]. Therefore, EVs are attractive tools for clinical applications as immunosuppressants, vaccines, or activators of differentiation and repair processes [120].

MSCs and their exosomes as potential therapies for COVID-19

MSCs have been well described in ALI and ARDS. It exerts its function via targeting both infectious, inflammatory, and endothelial factors. MSCs can release KGF2, PGE2, GM-CSF, IL-6, and IL-13 to facilitate phagocytosis (Figs. 2 and 3). In addition, multiple clinical studies [121,122,123,124,125] investigated the effect and mechanism of MSCs and MSC-EVs on lung injuries caused by different reasons (Table 2). MSCs and their secreted secretome exert an immunomodulatory, anti-inflammatory, anti-apoptotic, and anti-fibrotic functions in ALI and ARDS. PGE2 changes the macrophage polarization from M1 to M2 [144], IL10 decreases the recruitment of the neutrophils into the lung [145], and IDO enhances pulmonary antimicrobial activity [146]. Furthermore, the propagation, differentiation, and chemotactic features of B cells are hindered by MSCs as well [147] (Fig. 2). MSCs can further enhance restoration of capillary barrier, restore alveolar ATP [141], where the secreted growth factors KGF, VEGF, and HGF, can exert a protective effect on the alveolar cells [148]. In ALI models, the KGF mRNA has been involved in the immunomodulation noticed with MSC-EV treatment [126, 129]. MSC anti-bacterial effect is further demonstrated in inhibition of bacterial growth [57]. Several preclinical studies examined the therapeutic effects of MSCs and MSC-derived EVs in animal models of ALI, ARDS, and other lung inflammatory conditions [126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143, 149,150,151] (Table 2). These studies showed a significant decrease in the inflammatory reactions, improved edema clearance, and restored epithelial damage (Table 2). A preclinical study reported that the intratracheal administration of MSCs increases the accessibility of MSCs to both the alveolar epithelium and the pulmonary endothelium [152], where MSCs demonstrate reduction in endotoxin-induced injury to explanted human lungs [153].

Table 2 Biological effect and molecular mechanisms of MSCs and MSC-EVs in preclinical and clinical studies looking into lung injury
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MSC-derived exosomes are a multitargeted biologic agent, which can reduce the cytokine storm and reverse the inhibition of host anti-viral defenses associated with COVID-19 [154]. The functions of the MSC-derived exosomes have been studied in in vitro and in vivo models. EVs collected from the conditioned medium of BM-MSCs have been used to treat ARDS in a mouse model. The results showed that EVs have a similar effect as MSCs in reducing the inflammation and edema in the lung [126]. The effect of MSCs on macrophage modulation in ARDS mouse models has been mainly found due to the effect of EVs [127]. Treating mouse alveolar macrophages with MSC-EVs has a protective effect in the mouse model of lipopolysaccharide (LPS)-induced lung injury [127]. It has been found that the transferred active mitochondria through EVs leads to a change in the polarization of the macrophage from M1 (pro-inflammatory) to M2 (anti-inflammatory) phenotype as a result of an increase in the oxidative phosphorylation [127]. Also, it has been reported that functional mitochondria transferred through MSC-EVs enhances mitochondrial function in primary human alveolar cells and enhances their ability to repair lung injuries [155]. In addition, the effect of MSC-EVs has been examined in pneumonia mouse model induced by E. coli [129]. The data showed that EVs could decrease lung inflammation by reducing neutrophil and macrophage recruitment as well as MIP-2 level [129]. It has been found that the EVs reduce lung edema and endothelial permeability and the expression of CD44 on the target cells are required for the binding and uptake of EVs into cells [129]. BM-MSC-EVs have been used in another ARDS mouse model induced by LPS from Pseudomonas aeruginosa [151]. Tang et al. reported that the EV-mediated transfer of angiopoietin-1 (Ang1) mRNA is important for inflammation reduction, endothelial cell protection, and barrier repair through decreasing neutrophil influx and MIP-2 level [151]. Furthermore, EVs exert an immunomodulatory function in the macrophage by inhibiting the secretion of TNF-α and enhancing the secretion of IL-10 [151]. In a pig model, the influence of MSC-EVs on influenza virus-induced ARDS has been investigated [155]. Administration of EVs has been found to decrease the influenza virus replication, pro-inflammatory cytokines, and alveolar cell death in pigs through the transfer of RNA [155]. UC-MSC-EVs have been also used in a rodent model. The study found that the UC-EVs are effective in reducing ALI and the EVs primed with INF-γ are more efficient than normal EVs in improving ALI [156]. This indicates that EVs isolated from different source could be used for lung injury. Interestingly, the primed EVs have been found to be larger in size than normal EVs; however, the mechanism of this size increase remains unclear [156]. Previous studies used in vitro human injured lungs to investigate the effect of MSC-EVs. MSC-EVs restore fluid clearance and reduce edema in human injured lungs in vitro [143, 157]. Another study examined the effect of EVs on human lungs with pneumonia induced by E. coli found that EVs reduce the permeability of lung protein and enhance alveolar fluid clearance [158]. Barrier properties of the of human lung endothelial cells injured with TNF-α, IFN-γ, and IL-1β are restored with EVs. This improvement is due to an increase in the levels of Ang-1 in the injured endothelium, treated with EVs [159]. Although there are promising results obtained from using MSC-EVs in lung injury, more mechanistic studies are needed to improve our understanding on the molecular mechanisms involved in EV effect.

MSCs could act upon two ways in the novel COVID19 treatment, namely via its immunomodulatory effects and differentiation ability. MSCs display numerous advantages of relevance to ALI and ARDS. Although progress in the management of ALI/ARDS depends on improvements in supportive measures, ultimately decreasing the mortality rates [160], the failure of pharmacologic treatments indicate the need to consider new strategies for ALI/ARDS. MSC possible therapeutic potential is attributed to their accessible derivation from several adult tissues, their low immunogenicity, indicating that they could be given allogeneically [161], and their relative ease of isolation and expansion ability in culture. In case of COVID-19 patients, autologous and allogenic MSC transplantation could be applied, because MSCs do not express ACE2 and TMPRSS2; therefore, patient’s own MSCs cannot be infected by SARS-CoV2 [162]. However, the negative effects caused by the SARS-CoV2 infection on the blood cells and different organs may influence the ability to isolate autologous MSCs of high quality and sufficient number to treat the same patient. Taken together with the low immunogenicity of MSCs and the complications associated with the SARS-CoV2 infection, using allogenic MSCs is the method of choice for COVID-19 patients.

Resolution of ALI/ARDS in COVID19 is hindered by the disruption of the epithelial barrier that suppresses alveolar fluid clearance and depletes surfactant [163]. MSC capacity to aid in restoring epithelial and endothelial function by differentiating MSCs into these cell types or by secreting paracrine and trophic factors to increase restoration of the lung tissue offers a promise for treatment of ALI/ARDS in COVID19. MSCs have been widely studies in other inflammatory conditions, where they demonstrated a reduction in injury and/or enhanced restoration of function in the kidney [164, 165], liver [166], and heart [167]. MSC immunomodulatory properties exhibit a promise for treating ALI/ARDS in COVID19 via their ability to ‘reprogramme’ the immune response to decrease the destructive inflammatory components, while maintaining the host response to infections, in addition to enhancing the repair and resolution of lung injury by acting an effector for tissue regeneration.

MSC clinical trials for COVID-19 patients

With nearly 67 registered clinical trials in ClinicalTrials.gov looking into the use of MSC/MSC-derived EVs in ARS-CoV-2 associated disease, only few have been published (Tables 3 and 4). To date, four articles have reported results of COVID-19 pneumonia treatment with MSCs. Liang et al. have demonstrated the safety and efficacy of human UC-MSCs in modulating the immune response and recovered the disrupted tissue of a 65-year-old female severally sick COVID-19 patient [168]. The patient received IV infusion of MSCs three doses (5 × 107 cells/dose, every 3 days), in which following the second dose, clinical improvement has been observed. Furthermore, the number of the neutrophils and inflammatory cells in the patient reduced to a normal level, while the number of lymphocytes elevated to their normal levels [168]. Also, a recent study reported that IV injection of clinical-grade MSCs (1 × 106/kg) into seven patients with ARS-CoV-2 leads to an improvement in the functional outcomes and a recovery enhancement [162]. Among the beneficial outcomes of MSC treatment are an observed increase in the number of peripheral lymphocytes, a decline in the C-reactive protein (CRP), a decrease of overactivated cytokine-producing immune cells (CXCR3+CD8+ T cells, CXCR3+CD4+ T cells, and CXCR3+ NK cells) and TNF-α, and increase in IL-10, mainly attributed to the anti-inflammatory and immunomodulatory functions of MSCs [162]. Furthermore, an increase in CD14+CD11c+CD11bmid regulatory DC population has been observed [162]. Leng et al. demonstrated that MSCs modulate the lung microenvironment by protecting or rejuvenating alveolar epithelial cells, reducing fibrosis, and enhancing pulmonary function [162]. RNA-seq analysis for the transplanted MSCs in COVID-19 patients showed that the transplanted MSCs do not express ACE2 or TMPRSS2, indicating that MSCs cannot be infected with COVID-19; however, they express high levels of anti-inflammatory and paracrine factors, such as HGF, FGF, EGF, TGF-β, GAL, LIF, NOA1, VEGF, NGF, and BDNF. Also, the transplanted MSCs express high levels of AT2-specific surfactant proteins, SPA and SPC, suggesting that the MSCs may differentiate into AT2 cells [162].

Table 3 Ongoing clinical trials using MSCs and MSC-derived exosomes to treat COVID-19 patients
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Table 4 The investigated outcomes of the ongoing clinical trials using MSCs and MSC-derived exosomes to treat COVID-19 patients
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Another recently published study has tested the safety and efficacy of allogeneic AT-MSCs in 13 COVID-19 adult patients under invasive mechanical ventilation [169]. The patients had received previous anti-inflammatory and/or antiviral drugs, including lopinavir/ritonavir, steroids, tocilizumab and/or hydroxychloroquine, among others. Thirteen patients have been enrolled in the trial and have received two IV doses of AT-MSC 0.98 × 106 per kg of body weight, 3 days apart. The treatment has been followed by a reduction in inflammatory parameters (CRP, ferritin, LDH, IL-6) as well as increase in B-lymphocytes (67%) and CD4+ and CD8+ (100%) T lymphocytes). Remarkably, a reduction of D-dimer and fibrinogen 5 days after the first dose of AT-MSCs has been observed in most patients, and the patients do not develop a thromboembolic event [169, 170].

Furthermore, a recent clinical trial has been employed using MSC-derived exosomes. A prospective nonrandomized open-label cohort study by Sengupta et al. has evaluated the safety and efficacy of exosomes (ExoFlo™) obtained from allogeneic BM-MSCs as treatment for severe COVID-19. A total of 24 patients have received 15 ml of ExoFlo™. The treatment resulted in significant improvement in absolute neutrophil count and lymphopenia, with a decline in CRP, ferritin, and D-dimer. To our knowledge, this is the first published clinical study to use IV administration of BM-MSC-derived exosomes as treatment for COVID-19 [154]. These findings indicate that MSCs and their exosomes are promising options for treating ARDS associated with respiratory viral infections.

Points to consider in designing MSC clinical trials for COVID-19

Only regulated and compliant clinical trials can demonstrate and provide mechanistic and translational insights on the role of MSCs in ALI and ARDS in COVID-19. Designing randomized controlled trials (RCT) with setting clear inclusion and exclusion criteria will aid in laying the foundation for a safe and effective stem cell-based therapy in COVID19. Carrying a multicenter RCT (MRCT) versus a single center study is a decision that could further enhance advancing phases of previously successful phase 1/2 clinical trials. MRCT allows for capturing adequate sample size to reach significance and eliminates selection bias and confounding factors [171]. However, one has to define a consensus on cell-characterization, inclusion/exclusion criteria, and outcome measures. Although there is no consensus on the criteria of MSCs clinical trials in COVID-19 as of yet, the most common inclusion criteria are confirmed SARS-COV-2 by RT-PCR from respiratory sample, respiratory failure requiring intubation and ventilator, and meeting criteria of ARDS (PaO2/FiO2 ratio < 200 mmHg). Among the most common exclusion criteria are other causes of ARDS not attributed to COVID-19, negative RT-PCR for SARS-COV-2, pregnancy, recent history of thromboembolism, active malignancy, or previous immunosuppressive treatment. Primary outcome measures should aim at identifying safety and efficacy, by measuring adverse event rate and survival rate, respectively. Furthermore, setting a clear time frame for capturing the primary outcomes should be identified. Secondary outcome measures are also important in assessing the success of MSCs/MSC-EVs in COVID-19, mainly looking at long-term effects and measured therapeutic input. Secondary outcomes measures could include validated clinical assessment scoring like the sequential organ failure assessment (SOFA) which looks at multiple organ system functions (respiration, coagulation, liver, cardiovascular, central nervous system, and renal). Specifically, secondary outcome measure should focus on pulmonary function (PaO2/FiO2 > 200); therefore, setting a clear lung injury assessment score at baseline and at defined time frames following therapy. Finally, to be able to provide mechanistic and translational insights on the role of MSCs in ALI and ARDS in COVID-19, obtaining a basic complete blood count (CBC), comprehensive metabolic panel (CMP), and key inflammatory markers, as well as measurement of key cytokines (IL1, IL6, IL8, TNF-alpha, IL10, TGF-beta, sRAGE, Ang-2) at baseline and at defined time frames following therapy is crucial.

One of the key points in designing the clinical trial is the MSC dose and route of administration. IV route is the most commonly used method for systemic delivery of MSCs in the majority of clinical trials, with much fewer trials using intra-arterial (IA) injection [172]. The most frequently utilized route of MSC administration in ARDS and COVID-19 is also via IV infusion. Of interest, IV route remains the most well studied route used for MSC delivery in pulmonary diseases [172]. Regardless of the site of inflammation and tissue injury, and opposing to the old concept that MSCs only migrate to the site of injury following IV administration, MSCs are mostly trapped in lungs and undergo phagocytosis within 24 h [173]. In the majority of clinical indications, human MSCs are frequently transfused IV at doses ranging from 1 to 2 million cells/kg and never exceeding a dose of 12 million cells/kg [172, 174]. The median dose for IV route is 1 × 108 MSCs per patient per total dose. Analysis of MSC trials using IV route indicated minimal effective doses (MEDs). Efficacy dose-response outcome data indicated a narrower MED range of MSCs ranging from 100 to 150 million, where either higher or lower has been less efficient [172].

Challenges in treating COVID-19 using MSCs and their exosomes

One of the most significant challenges for MSC therapies is to optimize MSC homing efficiency. IV administration of MSCs shows low homing efficiency where cells get trapped in the pulmonary capillaries [175], a process that has been partially explained by insufficient production of homing factors, such as CXCR4, on MSCs [176, 177]. It has been reported that the in vitro propagation of MSCs gradually leads to dramatic reduction in the expression of homing factors [178, 179]. Several strategies have been used to improve MSC homing capacity, including targeted administration, genetic modification, magnetic guidance, in vitro priming, cell surface modification, and radiotherapeutic techniques [180, 181].

Further studies are needed to define the optimal source and dose of MSCs, administration route, the time window of MSC administration, and dose frequency (single vs. multiple-dose regimen). Due to MSC expression of tissue factor (TF/CD142), which triggers the coagulation, a pro-coagulation can be triggered ultimately leading to thromboembolic events following infusion. Thus, the use of anti-coagulant during MSC administration can be considered during administration guidelines and protocols [182]. Due to the trapping effect of MSCs occurring in the lung, a justification for the high doses of MSCs has been proposed [183]. Therefore, MSCs could be genetically modified to overexpress selected genes in order to increase in their engraftment. Pre-treatment with a series of preconditioning approaches could also promote MSCs therapeutic effects and enhance their survival in the lung. Of note, challenges with autologous MSC transplantation in ARDS are demonstrated by their impaired potential due to the immunomodulatory effects of bone marrow MSCs [184].

As extracorporeal membrane oxygenation (ECMO) is among the salvage therapy for refractory respiratory failure in the context of acute respiratory compromise associated with SARS-CoV-2, it is critical to learn how MSCs would act in such setting [185]. IV administration of MSCs has been found to attach to membrane oxygenator fibers during ECMO in an in vitro ARDS model, leading to a significant decrease in the flow through the circuit [186]. Intratracheal and IV infusions of MSCs before ECMO or during a pause in the flow are some suggested strategies to overcome this limitation. Interestingly, a more recent study demonstrated an enhanced endogenous MSC mobilization in patients with ARDS undergoing ECMO [187]. Therefore, intratracheal administration of MSCs might be an option in ARDS requiring continuous high-flow ECMO.

The fate of MSCs after infusion also remains to be investigated. Cell migration and distribution studies have shown that the majority of MSCs localize to the lungs after IV infusion [188, 189]. Intravascular arrest of MSCs is due to MSC’s diameter ranging from 10 to 20 μm, bigger than the width of the pulmonary micro-capillaries [190]. However, following IV administration, MSCs still tend to migrate to sites of injury and move from the lungs to other organs, such as the liver and spleen [191, 192]. Nevertheless, challenges remain in interpreting this data as this tracking could be detecting phagocytosed MSCs. This is in line with the evidence showed that most of MSCs become apoptotic after administration [193]. To avoid poor cell survival following MSC transplantation, several preconditioning strategies have been proposed. Ang1-preconditioned cell survival was significantly increased via increased Akt phosphorylation. This has further reduced the apoptotic rate in vitro via increased expression of B cell lymphoma protein 2 (Bcl-2) and the ratio of Bcl-2/Bcl-associated X (Bax) [193]. Several priming strategies with pharmacological agents, inflammatory cytokines or mediators, hypoxia, and biomaterial have been shown to enhance the therapeutic efficacy of MSC transplantation. MSCs enhanced trafficking and homing to sites of injury is demonstrated in the high expression of chemokine receptors, such as CXCR4, CXCR7, and CX3CR1 [194]. Finally, although several in vitro culturing strategies have been developed to mimic the natural MSC niche, preserving the function and quality of a scalable clinical-grade cell expansion remain a challenge.

The adult sources of MSCs include painful and invasive procedures with possible donor site morbidity [195]. Regardless the origin, the MSCs display heterogeneity in their abilities to propagate and differentiate [196]. The differences in the properties of MSCs are associated with the variations in the age of the donor, method of MSC isolation, and in vitro culturing approaches. As an alternative source, hPSCs could differentiate into unlimited number of MSCs, displaying MSC characteristics [55, 197, 198]. Further studies are needed to extensively examine the differences between MSCs isolated from different tissues and those derived from hPSCs.

A systemic procoagulant state has been observed in severely ill COVID-19 patients, which tends to result in poor outcome. Such patients are at high risk of disseminated intravascular coagulation (DIC) and thromboembolism. Looking at the risk of hypercoaguable state in COVID-19 patients, the safety profile of MSCs could be further challenged [199]. DIC and thromboembolism taking place after the administration of TF/CD142-expressing MSC products have been reported [182]. The production of the highly procoagulant tissue factor TF/CD142 between products could vary. BM-MSCs have the lowest TF/CD142 expression, whereas ASC display the highest expression profile [182]. Finally, the dynamics of the current pandemic and the rising global demand highlight the need for scalable manufacturing required to provide enough doses of MSC product of high quality in a reproducible and timely manner.

Conclusion and future perspectives

MSCs have a potential therapeutic function in COVID-19, which is displayed in their ability to enhance alveolar fluid clearance and promote epithelial and endothelial recovery through transfer of EV components together with the cell-cell contact as well as their secreted soluble factors. As ACE2 is widely expressed in other tissue types in addition to lungs, it is intuitive to consider MSC effect on the other organs as well. MSC treatment may reduce the progression of ARDS in severely ill COVID-19 patients with multiple organ failure.

Although several clinical trials have been recently registered to examine the safety and efficacy of MSCs as an emerging therapeutic option for COVID-19-induced disease, fewer studies have been published. Learning from these clinical trials, MSCs could exert its immunomodulatory and regenerative capacity in COVID-19 patients. Although there is no approved treatment for COVID-19 as of yet, MSC therapies continue to show improvement in the treatment of some of the leading causes of mortality in COVID-19 patients, namely acute ARDS, pneumonia, inflammation, and sepsis. The majority of these clinical trials are based on IV infusion of MSCs and their derived exosomes. Despite the clinical improvement witnessed, the safest and most effective route of MSC delivery into COVID-19 patients remains unclear, especially in the context of the heterogeneity of MSC-based products, intravascular arrest, and poor cell survival. IV infusions of poorly characterized MSC products remain one of the most significant drawbacks of MSC cell-based therapy, which could theoretically promote the risk for thromboembolism. The best delivery route for MSCs giving the highest positive effects with minimum toxic effects remains to be resolved. Whether any difference between IV, intratracheal, and intraperitoneal administration routes exists also remains unclear. Furthermore, elucidating the molecular mechanisms of MSCs during lung injury is crucial to understand their role in ARDS. The premature marketing of unproven stem cell therapy to the public resulted in the unfortunate increase of unregulated stem cell clinics; therefore, we cannot recommend any MSC-based treatment, which does not use characterized cell product, perform functional mechanisms, define variability in donor and tissue source, measure intermediate parameters, and define final patient endpoints(s), key steps that are common practice in FDA registered trials. Lack of consensus underlines major challenges to the clinical translation of MSC-based therapy.

Future studies should focus on developing genetically modified MSCs, generating significantly large number of EVs that could safely transfer different potent and effective therapeutic factors [114]. Finally, optimizing the clinical-grade production of MSCs as well as establishing a consensus on registered clinical trials based on cell-product characterization and mode of delivery would aid in laying the foundation for a safe and effective MSC-based therapy in COVID19.