*1.3. Mast Cell Mediators*

The most eye-catching feature of MCs visualized using electron microscopy is the multitude of electron-dense vesicles/granula. These organelles are secretory lysosomes that store preformed pro-inflammatory mediators (e.g., histamine; proteoglycans, such as heparin and chondroitin-sulfate; various proteases, such as tryptase, chymase, carboxypeptidase A3, granzyme B, and active caspase-3; and certain cytokines) that are released immediately upon antigen-triggered activation of the IgE-bound FcεRI in a process called degranulation [34–37]. Another important MC-selective receptor able to induce the process of degranulation upon recognition of its ligand(s) is the Mas-related G protein-coupled receptor X2 (MRGPRX2; in mice: MRGPRB2), which can trigger the secretory response upon binding to antimicrobial peptides, neuropeptides, eosinophil peroxidase, and various peptidergic drugs, amongst others [38]. Before the identification of this receptor, many detrimental reactions in patients were referred to as "drug-induced pseudoallergies" without having the notion that it is only one receptor that recognizes this variety of different molecules [39].

A further immediate response of MCs is the generation and release of arachidonic acid metabolites, particularly the leukotrienes LTB4 and LTC4, and prostaglandin PGD2. These mediators, in a situation-dependent context, are able to initiate, amplify, or attenuate inflammatory responses. Furthermore, they can influence the magnitude, duration, and nature of subsequent immune responses [40]. In addition, MCs are capable of producing and secreting numerous cytokines, chemokines, growth factors, and angiogenic factors mediating multiple pro-inflammatory, anti-inflammatory, and/or immunoregulatory effects in a situation-dependent manner [2,41]. Amongst others, MCs are able to synthesize and secrete TNF-α, IL-1β, IL-3, IL-5, IL-6, IL-8, IL-9, IL-13, CCL5, TGF-β1, and FGF.

The composition of MC responses is largely dependent on the activated receptor(s). In response to an antigen, MCs degranulate and produce arachidonic acid metabolites, as well as cytokines and chemokines. In contrast, the stimulation of MCs via cytokines or pattern recognition receptors (e.g., toll-like receptors (TLRs)) only induces the generation of arachidonic acid metabolites and cytokines/chemokines. With respect to TLR4 activation by lipopolysaccharide (LPS), MCs are markedly different compared to macrophages concerning their receptor composition and organization of signaling pathways. MCs do not express the GPI-anchored protein mCD14, which affects the chemotype of recognized LPS molecules (R-LPS >> S-LPS) [42]. Moreover, upon LPS recognition, MCs do not activate the TRIF pathway, and thus, MCs do not produce IFN-β [43,44].

### *1.4. Murine Models to Study Mast Cell Involvement*

To study the role of MCs in different disease situations, mostly mouse models are used, though models in rats, hamsters, dogs, and rabbits have also been used for the investigation of certain

diseases. Most mouse studies have made use of animals that do not express KIT and thus are devoid, though not completely, of MCs ("*Kit* mutant MC-deficient mice"). Different mutant mice carrying mutations in the *Kit* gene/locus have been frequently used (e.g., WBB6F1-*KitW*/*W-v* and C57BL/6-*KitW-sh*/*W-sh* mice [7,45]) to study disease development in the absence of MCs. Moreover, in vitro differentiated bone-marrow-derivedMCs (BMMCs) have been used to engraft anMC population in these genetically MC-deficient mice ("MC knock-in mice") and disease development has been studied [46]. If changes in MC-deficient mice, compared to the respective wild-type mice, could be reverted via the re-establishment of MC populations, then this was taken as a proof of MC involvement in the particular disease process. However, it should be noted that KIT is also expressed on hematopoietic stem cells and almost all myeloid progenitor cells, allowing for altered adaptive and innate immune reactions in KIT-deficient mice, which cannot only be attributed to missing MCs. Not unexpected, using a *Kit*-independent mouse model of MC deficiency (Cpa3Cre/+ mice (Cre-Master mice) carrying a targeted insertion of Cre recombinase in the carboxypeptidase A3 locus), Feyerabend et al. could not verify all results from studies that used *Kit*-dependent MC-deficient mouse models [47]. Examples of such MC-independent diseases were antibody-induced autoimmune arthritis and experimental autoimmune encephalomyelitis. Additional mutant mice with constitutive MC deficiency unrelated to *Kit* abnormalities are the *Mcpt5-Cre*; *R-DTA* mice and the *Cpa3-Cre*; *Mcl1fl*/*fl* mice. For the generation of *Mcpt5-Cre*; *R-DTA* mice, *Mcpt5-Cre* transgenic mice [48] were crossed with *R-DTAfl*/*fl* mice [49] to yield a mouse strain in which CTMCs are ablated by the expression of the diphtheria toxin α chain [50]. For the generation of transgenic *Cpa3-Cre*; *Mcl1fl*/*fl* mice (also known as "Hello Kitty" mice), mice expressing *Cre* under the control of a *Cpa3* promoter fragment were crossed with *Mcl1fl*/*fl* mice [51], allowing for the deletion of the gene of the anti-apoptotic factor MCL1 [52]. The different mouse models of MC deficiency have been comprehensively reviewed by Galli et al. [53].
