Introduction

Mast cells (MCs) are round, about 20 μm diameter cells of the immune system containing cytoplasmic granules variably filled with many messenger substances (mediators). They originate in hematopoietic tissue; white adipose tissue has been identified as a reservoir of MC precursors, too (Poglio et al. 2010). They are resident in all vascularized organs and tissues; the majority are located at the interfaces to the outside world, such as mucous membranes and skin. At these sites, MCs are best positioned to sense when tissues are under attack by potentially harmful pathogens (parasites, bacteria, viruses, venoms) and can act accordingly. In addition, MCs likely have many more underappreciated roles in the human homeostasis of organs that undergo continuous growth and remodeling such as hair follicles and bones, wound healing, disease response, tissue repair, and angiogenesis. They are sensors of hypoxemia, air pressure, vibratory stimuli, and light. In addition, MCs are an integral component of the stress response system (Afrin et al. 2016).

MCs developed more than 500 million years ago (Crivellato et al. 2015), i.e., before the development of adaptive immunity, suggesting that MCs act as effector immune cells and as regulatory immune cells and play central roles in both innate and adaptive immunity (Gri et al. 2012). Through evolution, MCs have become optimized for their already discovered functions such as regulatory control of homeostasis of the organism, potent effector cells of the immune system, and regulation of the functional interaction of the innate and adaptive immune system (Norrby 2022). It seems likely that many other MC mediators with their associated functions remain to be discovered.

The aim of the present survey is to provide all those working in the field of MCs, scientifically and clinically, with a comprehensive compilation of human MC mediators released by exocytosis that can be used as a reference work.

Methods

The compilation of the data is essentially based on the COPE® database (Ibelgaufts 2023), which also contains references that go beyond the references given in the tables herein. This database was last accessed in March 2023. These data were supplemented with data on the expression of proteins in human MCs which had been investigated by, and were published in, Liang et al. (2018), Motakis et al. (2014), Haenisch et al. (2013), Okayama (2005), Halloran et al. (2019), and Babina et al. (2004). In addition, the PubMed database was searched with the phrase human “mast cell*” mediator*. The selective information on the potential effects of the compiled mediators were taken from the GeneCards® database (https://www.genecards.org/).

Results

On the basis of the analyzed databases, 390 substances could be identified (Online Resource 1 and 2) which are formed intracellularly by human MCs and can be secreted by exocytosis into the extracellular space by activation of the MC and can induce effects in effector cells. In studies on murine MCs, another 55 substances have been identified (data not shown) as potential mediators. However, since these substances have not yet been detected in human MCs, they are not further considered as human MC mediators in the following. Each of the 390 potential mediators is able to induce several effects on effector cells (GeneCards®). Selected manifestations of MC activation have been linked to specific mediators (Table 1) as an example of using the data from Online Resource 1.

Table 1 Selected manifestations of mast cell activation (MCA)
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Understanding the autocrine/paracrine activation of MCs (Fig. 1) is essential for understanding the development of an acute MC mediator release episode (He et al. 2012). Therefore, Table 2 lists all mediators which are likely to induce, via 30 distinct receptor classes, autocrine activation of the releasing MC, and paracrine activation of other MCs in the proximity of the releasing MC. This finding agrees well with the clinical observation of acute to subacute activation phases of MCs beyond anaphylactic reactions. These 30 activating mechanisms are opposed only by seven autocrine/paracrine receptors that can inhibit MC activation (Table 2).

Fig. 1
figure 1

Mast cell activation after mediator (red circles) exocytosis by autocrine and paracrine stimulation of mast cell receptors for this specific released mediator

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Table 2 Facilitatory and inhibitory autocrine regulation of mast cells
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Two further phenomena could be important for MC activation: first, the possibility of reuptake of mediators released by the MCs for later re-exocytosis. Such reuptake of released mediators may not be accompanied by stimulation of the corresponding receptor because (1) the receptor may still be inactivated due to previous autocrine activation, and (2) reuptake may take place via receptor-independent specific reuptake mechanisms (e.g., transporters). Second, substances originally formed and released by other cells which were taken up and stored by the MCs can potentially act as MC mediators when subsequently released from the MCs (Table 3). The possibility of reuptake or uptake of substances or groups of substances into the MCs which then can act as mediators could be identified for 15 compounds (Table 3).

Table 3 Uptake of substances as potential mediators into human mast cells
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Discussion

The central role of MCs in immunological as well as non-immunological processes is reflected by the large number of mediators by which MCs may influence other cells (Lundequist and Pejler 2011). The profile of mediators and cytokines stored or produced de novo in MCs can markedly differ between and even within organs/tissues depending upon a wide array of macro- and micro-environmental factors including antigenic and physical stimuli. Although the number of MC mediators has been assumed to be large, there has not yet been any comprehensive compilation of human MC mediators. In this article, the known human MC mediators are comprehensively compiled for the first time. And indeed, the number of mediators, at least 390, turns out to be extraordinarily high compared to the number of messenger substances known to be formed and released by other cells. However, this number still might substantially underestimate the actual number of MC mediators, once one takes into consideration broader definitions of “mediatorˮ and broader definitions of effector mechanisms than we consider for our present purposes.

MC actions can be targeted very precisely. Occasionally, MCs release pre-stored mediators via classic non-selective whole-MC degranulation (as in anaphylaxis), but this is the exception, not the rule, in MC activation (Theoharides et al. 2007, 2023). Otherwise, anaphylactic reaction would occur consistently in every episode of MC activation, but this is obviously not the case. Rather than wholly degranulate, MCs much more commonly selectively release specific mediators, referred to as differential release (Table 4), i.e., release of the content of individual secretory granules or individual mediators without whole-MC degranulation (Theoharides et al. 1982). This process is distinct from “piecemeal degranulation” that has additionally been reported (Dvorak 2005). MCs can also form synapses for targeted secretion (Table 4).With regard to the possibility that, under certain circumstances, almost all molecules that can be produced by a MC might be able to act as mediators, four release options are of particular interest: (1) diffusion of substances into the extracellular space; (2) release of mRNA, microRNA, and proteins expressed in the MC by secretion of exosomes and vesicles (Savage et al. 2023), some of them containing KIT (Pfeiffer et al. 2022); (3) formation of nanotubules with exchange of intracellular material which seems to be involved in inducing apoptosis in cancer cells (Ahani et al. 2022); and (4) formation of MC extracellular traps (Möllerherm et al. 2016; Table 4). These four mechanisms, by which MCs can use almost any molecule as a mediator, underline the extraordinary role of these cells in our immune system. At the same time, this creates an almost insurmountable hurdle for precisely attributing specific clinical symptoms to specific messenger substances. This problem of assigning (a) certain MC mediator(s) to symptoms is further complicated by the fact that released MC mediators can maintain and enhance MC activation in autocrine and paracrine manners (Fig. 1), and additionally by the possibility of MCs taking up substances from their immediate environment and then re-releasing them. In this context, it has to be noted that MCs are able to survive even complete degranulation followed by regranulation (Iskarpatyoti et al. 2022). Interestingly, MCs have altered granule contents and structure after regranulation, likely depending on the trigger that had induced the degranulation (Friend et al. 1996; Iskarpatyoti et al. 2022, further references therein).

Table 4 Forms of communication between mast cells and effector cells
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Clinical impact

It does not require a great imagination to envision that the very same mechanisms which enable MCs to protect the organism can wreak focused or multisystem havoc when uncontrolled, potentially causing a vast array of diseases, some of which might be quite severe. In this context, primary systemic MC disease (dominantly MC activation syndrome (MCAS)) is of particular interest for at least two reasons: (1) its prevalence of about 20% (Molderings et al. 2013; Maitland et al. 2020) represents a significant socio-economic problem; and (2) due to its epigenetic causation with transgenerational transmission (Molderings 2022), it tends to manifest in successive generations more severely and at steadily earlier ages, creating increasing treatment challenges. Systemic mast cell disease (also presently termed mast cell activation disease (MCAD)), in its assorted variants (including systemic mastocytosis and MCAS), is usually driven, at the level of the individual, by multiple stem cell germline and somatic mutations (emerging out of complex interactions between stressor-induced cytokine storms and a genome rendered insufficiently robust, by the aforementioned epigenetic variants, at repairing or eradicating induced mutations) leading directly or indirectly to inappropriate chronic constitutive and reactive activation of the affected MCs (Weinstock et al. 2021). Due to both their widespread distribution and the great heterogeneity of aberrant mediator expression patterns, symptoms may occur in all organs and tissues. Hence, the clinical presentation of MCAD disease is very diverse, with a myriad of combinations of symptoms, ranging in the severity of illness from trivial to disabling and even life-threatening (Afrin et al. 2016).

Perspective

The present survey of the potential MC mediators in the narrower sense (Online Resource 1 and 2) and broader sense (Table 4), together with the findings of autocrine and paracrine stimulation and the ability of the MC to (re)use substances it takes up as mediators, are not of interest merely to researchers. These tables can be consulted by attending physicians, too, when trying to gain clarity about MC mediators which may be involved in patients with MC disease symptoms which are often resistant to therapy, such as hyper-/hypotension, transient tachyarrhthmias, or migrating pain. Such a procedure might be extraordinarily effective if, based on the available tables and with the help of special computer programs to be developed, all the information contained in relevant databases such as GeneCards®, PubMed, EMBL’s European Bioinformatics Institute, Embase, Cochrane Library, and others could help link the symptoms in a patient to given mediator expression profiles, thereby hopefully providing personalized therapeutic insights. This might enable the selection of treatments (Molderings et al. 2016) more likely to help patients exhibiting specific MC-mediator-induced symptoms. Ultimately, though, routine performance in the clinical laboratory of MC-specific genome sequencing (using pipelines already in place in many laboratories for sequencing the tumor cells in biopsies, but re-tuned, likely based on strong CD117 expression, to select the MCs in the sample) will be needed to discover not only which mutational profiles reliably correlate with which symptom profiles but also which treatments will best address the phenotypes driven by particular mutational profiles.