Regulation of the pleiotropic effects of tissue-resident mast cells
Authors: Michael Huber, Andrew C.B. Cato, George K. Ainooson, Marc Freichel, Volodymyr Tsvilovskyy, Rolf Jessberger, Eva Riedlinger, Christian P. Sommerhoff, Stephan C. Bischoff
Journal: Journal of Allergy and Clinical Immunology (2019)

Mast cells (MCs), which are best known for their detrimental role in patients with allergic diseases, act in a diverse array of physiologic and pathologic functions made possible by the plurality of MC types. Their various developmental avenues and distinct sensitivity to (micro-) environmental conditions convey extensive heterogeneity, resulting in diverse functions. We briefly summarize this heterogeneity, elaborate on molecular determinants that allow MCs to communicate with their environment to fulfill their tasks, discuss the protease repertoire stored in secretory lysosomes, and consider different aspects of MC signaling. Furthermore, we describe key MC governance mechanisms (ie, the high-affinity receptor for IgE [FcεRI]), the stem cell factor receptor KIT, the IL-4 system, and both Ca2+- and phosphatase-dependent mechanisms. Finally, we focus on distinct physiologic functions, such as chemotaxis, phagocytosis, host defense, and the regulation of MC functions at the mucosal barriers of the lung, gastrointestinal tract, and skin. A deeper knowledge of the pleiotropic functions of MC mediators, as well as the molecular processes of MC regulation and communication, should enable us to promote beneficial MC traits in physiology and suppress detrimental MC functions in patients with disease. Mast cells (MCs), which are best known for their detrimental role in patients with allergic diseases, act in a diverse array of physiologic and pathologic functions made possible by the plurality of MC types. Their various developmental avenues and distinct sensitivity to (micro-) environmental conditions convey extensive heterogeneity, resulting in diverse functions. We briefly summarize this heterogeneity, elaborate on molecular determinants that allow MCs to communicate with their environment to fulfill their tasks, discuss the protease repertoire stored in secretory lysosomes, and consider different aspects of MC signaling. Furthermore, we describe key MC governance mechanisms (ie, the high-affinity receptor for IgE [FcεRI]), the stem cell factor receptor KIT, the IL-4 system, and both Ca2+- and phosphatase-dependent mechanisms. Finally, we focus on distinct physiologic functions, such as chemotaxis, phagocytosis, host defense, and the regulation of MC functions at the mucosal barriers of the lung, gastrointestinal tract, and skin. A deeper knowledge of the pleiotropic functions of MC mediators, as well as the molecular processes of MC regulation and communication, should enable us to promote beneficial MC traits in physiology and suppress detrimental MC functions in patients with disease. The term mast cell (MC) describes a highly heterogeneous cell population. Numerous MC subtypes have now been characterized in both human subjects and animals based on differences in cell morphology, histochemical properties, protease content in granules, piecemeal or anaphylactic types of degranulation, receptor expression, and function.1Dvorak A.M. Morgan E.S. Diamine oxidase-gold enzyme-affinity ultrastructural demonstration that human gut mucosal mast cells secrete histamine by piecemeal degranulation in vivo.J Allergy Clin Immunol. 1997; 99: 812-820Abstract Full Text PDF PubMed Scopus (0) Google Scholar, 2Welle M. Development, significance, and heterogeneity of mast cells with particular regard to the mast cell-specific proteases chymase and tryptase.J Leukoc Biol. 1997; 61: 233-245Crossref PubMed Scopus (260) Google Scholar A certain subtype plasticity has been recognized depending on the tissue environment, and in vitro MCs change their phenotype depending on culture conditions.3Bischoff S.C. Sellge G. Lorentz A. Sebald W. Raab R. Manns M.P. IL-4 enhances proliferation and mediator release in mature human mast cells.Proc Natl Acad Sci U S A. 1999; 96: 8080-8085Crossref PubMed Scopus (0) Google Scholar, 4Dwyer D.F. Barrett N.A. Austen K.F. Immunological Genome Project ConsortiumExpression profiling of constitutive mast cells reveals a unique identity within the immune system.Nat Immunol. 2016; 17: 878-887Crossref PubMed Scopus (96) Google Scholar, 5Pejler G. Abrink M. Ringvall M. Wernersson S. Mast cell proteases.Adv Immunol. 2007; 95: 167-255Crossref PubMed Scopus (193) Google Scholar In addition to intraspecies heterogeneity, interspecies heterogeneity has been discovered.6Bischoff S.C. Role of mast cells in allergic and non-allergic immune responses: comparison of human and murine data.Nat Rev Immunol. 2007; 7: 93-104Crossref PubMed Scopus (420) Google Scholar This is of relevance because a large part of the scientific literature on MCs is based on animal work, in particular mouse studies. Data on human MCs are scarce because they are difficult to collect from tissue sources. Therefore many laboratories use genetically transformed human MC lines instead of primary MCs from tissue or immature MCs derived from bone marrow (BM) or peripheral blood that are partially maturated under particular culture conditions. Although it is well known that such MCs differ functionally from primary tissue MCs, they have nonetheless provided useful insights into the regulation and function of human MCs. Human MCs can be characterized according to the types of proteases they contain, such as mast cells expressing tryptase and chymase (MCTCs), mast cells expressing only tryptase (MCTs), and the rare mast cells expressing only chymase (MCCs). MCTCs are located at nonmucosal sites (in the skin, the submucosa, adjacent to blood vessels, and in the peritoneum in mice), whereas MCTs are primarily found at mucosal sites (nose, lung, and intestine). Further division of these subtypes into site-specific populations has been proposed to reflect the microenvironment of the anatomic compartments where they are localized, such as "lung MCT" to reflect prevalence in the bronchi, bronchioles, and alveolar parenchyma and "MCTC" to denote abundance in pulmonary vessels and the pleura. Within each MCTC and MCT population, there are also distinct localization-specific expression patterns of receptors, enzymes, and growth factors that can be used for further characterization. For instance, bronchial MCTs consistently express more histidine decarboxylase than alveolar MCTs, whereas for both MCTCs and MCTs, the high-affinity receptor for IgE (FcεRI) is highly expressed in conducting airways but virtually absent in alveolar parenchyma.7Andersson C.K. Mori M. Bjermer L. Lofdahl C.G. Erjefalt J.S. Novel site-specific mast cell subpopulations in the human lung.Thorax. 2009; 64: 297-305Crossref PubMed Scopus (77) Google Scholar It is not only the tissue site but also the disease status that influences MC heterogeneity. In patients with poorly controlled, severe, TH2-associated asthma, an altered subtype that could play a role in the pathophysiology of this disease8Fajt M.L. Wenzel S.E. Mast cells, their subtypes, and relation to asthma phenotypes.Ann Am Thorac Soc. 2013; 10: S158-S164Crossref PubMed Scopus (0) Google Scholar has been described to express tryptase, chymase, and/or carboxypeptidase A3 (CPA3). Genetic analyses have revealed even more heterogeneity among MCs from different sites within one tissue, although the clinical implications are yet unclear.9Frossi B. Mion F. Sibilano R. Danelli L. Pucillo C.E.M. Is it time for a new classification of mast cells? What do we know about mast cell heterogeneity?.Immunol Rev. 2018; 282: 35-46Crossref PubMed Scopus (17) Google Scholar In mice and rats MCs are traditionally subtyped into connective tissue mast cells (CTMCs) and mucosal mast cells (MMCs) based on their histochemical properties, mediator content, and functional properties.10Hallgren J. Gurish M.F. Pathways of murine mast cell development and trafficking: tracking the roots and routes of the mast cell.Immunol Rev. 2007; 217: 8-18Crossref PubMed Scopus (0) Google Scholar CTMCs can be stained with both Alcian blue and safranin, whereas MMCs can only be stained with Alcian blue. In terms of their mediator content, heparin proteoglycans predominate in CTMCs, whereas chondroitin sulfate predominates in MMCs. In their functional properties CTMCs typically contain rodent tryptases, chymases, and CPA3, whereas MMCs contain chymases only and thus differ fundamentally from their human counterparts in mucosal tissues that contain tryptase only. The subtyping of murine MCs into CTMCs and MMCs might be an oversimplification for the mouse intestine and peritoneum but not for the lung.11Xing W. Austen K.F. Gurish M.F. Jones T.G. Protease phenotype of constitutive connective tissue and of induced mucosal mast cells in mice is regulated by the tissue.Proc Natl Acad Sci U S A. 2011; 108: 14210-14215Crossref PubMed Scopus (59) Google Scholar Development of MCs can be subdivided into 3 major steps: differentiation of hematopoietic stem cells (HSCs) to mast cell precursors (MCps) in the BM, distribution of MCps through blood and transendothelial migration into target tissues, and eventually phenotypic maturation in different tissues. Although there are still certain discrepancies concerning our understanding of MCp development in the BM, it is justified to suggest that committed MCps stem from bipotent progenitors endowed with the capacity to differentiate into both MCs and basophils, as has been studied in mice.12Dahlin J.S. Hallgren J. Mast cell progenitors: origin, development and migration to tissues.Mol Immunol. 2015; 63: 9-17Crossref PubMed Scopus (104) Google Scholar Expression of the receptor tyrosine kinase KIT for MC development is mandatory. Likewise, expression of the KIT ligand stem cell factor (SCF) is obligatory for MC development in vivo. Mature MCs are usually characterized by their expression of KIT and FcεRI, as well as distinct metachromatic cytoplasmic granules. MCps do not necessarily express FcεRI and are typically not identifiable by means of metachromatic staining, indicating that commitment to the MC lineage occurs before FcεRI expression. Indeed, early committed MCps lack FcεRI expression but show KIT expression.13Rodewald H.R. Dessing M. Dvorak A.M. Galli S.J. Identification of a committed precursor for the mast cell lineage.Science. 1996; 271: 818-822Crossref PubMed Google Scholar Hence MCps ready to leave the BM can be either FcεRI positive or FcεRI negative. Less is known about committed MCps in human subjects, although an immature human MCp population, similar to mouse MCps in terms of amount and developmental state, has recently been identified in human blood. These have been reported to be lineage-negative CD34hiKITint/hiFcεRI+ cells and to give rise only to granulated tryptase-positive KIT+FcεRI+ MCs.14Dahlin J.S. Malinovschi A. Ohrvik H. Sandelin M. Janson C. Alving K. et al.Lin- CD34hi CD117int/hi FcεRI+ cells in human blood constitute a rare population of mast cell progenitors.Blood. 2016; 127: 383-391Crossref PubMed Scopus (0) Google Scholar Interestingly, the frequency of development of such MCs starting from a lineage-negative CD34hiKITint/hiFcεRI− cell subset was reduced by more than a factor of 10, indicating the importance of FcεRI expression for MC commitment. After leaving the BM, MCps are distributed through the blood to their target tissues. This requires productive interaction between surface molecules of MCps and endothelial cells for transendothelial migration. Homing of MCps to the small intestine requires expression of the α4β7 integrin on MCps15Gurish M.F. Tao H. Abonia J.P. Arya A. Friend D.S. Parker C.M. et al.Intestinal mast cell progenitors require CD49db7 (a4b7 integrin) for tissue-specific homing.J Exp Med. 2001; 194: 1243-1252Crossref PubMed Scopus (0) Google Scholar and mucosal vascular addressin cell adhesion molecule 1 and vascular cell adhesion molecule 1 as counterligands on intestinal endothelial cells for interaction.15Gurish M.F. Tao H. Abonia J.P. Arya A. Friend D.S. Parker C.M. et al.Intestinal mast cell progenitors require CD49db7 (a4b7 integrin) for tissue-specific homing.J Exp Med. 2001; 194: 1243-1252Crossref PubMed Scopus (0) Google Scholar, 16Abonia J.P. Austen K.F. Rollins B.J. Joshi S.K. Flavell R.A. Kuziel W.A. et al.Constitutive homing of mast cell progenitors to the intestine depends on autologous expression of the chemokine receptor CXCR2.Blood. 2005; 105: 4308-4313Crossref PubMed Scopus (74) Google Scholar Integrin activation requires chemokine receptor–mediated inside-out signaling by CXCR2 expressed on MCps.16Abonia J.P. Austen K.F. Rollins B.J. Joshi S.K. Flavell R.A. Kuziel W.A. et al.Constitutive homing of mast cell progenitors to the intestine depends on autologous expression of the chemokine receptor CXCR2.Blood. 2005; 105: 4308-4313Crossref PubMed Scopus (74) Google Scholar Although β7 integrin–deficient mice showed impaired MCp homing to the intestine, homing to the lung was still possible, although reduced. By using a model of ovalbumin-induced allergic airway inflammation, pulmonary recruitment of MCps was shown to depend on both α4β7 and α4β1 integrins on the MCps and on vascular cell adhesion molecule 1 on the endothelial side.17Abonia J.P. Hallgren J. Jones T. Shi T. Xu Y. Koni P. et al.a-4 integrins and VCAM-1, but not MAdCAM-1, are essential for recruitment of mast cell progenitors to the inflamed lung.Blood. 2006; 108: 1588-1594Crossref PubMed Scopus (0) Google Scholar The same transmigration mechanism was recently demonstrated in influenza A–infected mice.18Zarnegar B. Mendez-Enriquez E. Westin A. Soderberg C. Dahlin J.S. Gronvik K.O. et al.Influenza infection in mice induces accumulation of lung mast cells through the recruitment and maturation of mast cell progenitors.Front Immunol. 2017; 8: 310Crossref PubMed Scopus (10) Google Scholar Homing of MCps to the small intestine occurs in a constitutive manner in naive mice,15Gurish M.F. Tao H. Abonia J.P. Arya A. Friend D.S. Parker C.M. et al.Intestinal mast cell progenitors require CD49db7 (a4b7 integrin) for tissue-specific homing.J Exp Med. 2001; 194: 1243-1252Crossref PubMed Scopus (0) Google Scholar although it can be amplified by inflammatory conditions. In contrast, in the lung the numbers of MCps that home constitutively is strongly exceeded by numbers recruited on inflammation in a T cell–dependent manner.19Jones T.G. Hallgren J. Humbles A. Burwell T. Finkelman F.D. Alcaide P. et al.Antigen-induced increases in pulmonary mast cell progenitor numbers depend on IL-9 and CD1d-restricted NKT cells.J Immunol. 2009; 183: 5251-5260Crossref PubMed Scopus (0) Google Scholar In addition to the intestine and the lung, the skin is an important MC-containing tissue. However, less is known about MCps homing to the skin, although a promoting role for the fractalkine receptor CX3CR1 on MCps has been suggested.20Papadopoulos E.J. Fitzhugh D.J. Tkaczyk C. Gilfillan A.M. Sassetti C. Metcalfe D.D. et al.Mast cells migrate, but do not degranulate, in response to fractalkine, a membrane-bound chemokine expressed constitutively in diverse cells of the skin.Eur J Immunol. 2000; 30: 2355-2361Crossref PubMed Scopus (0) Google Scholar After migration to their respective tissues, MCps differentiate into 2 major subclasses of mature tissue MCs: MCTCs and MCTs (see the section "Heterogeneity of mast cells"). Finally, maturation of MCs, both in vitro and in different tissues, is associated with downregulation of cell-surface adhesion molecules and chemokine receptors. As an example, primary peritoneal and pulmonary MCps from naive mice express considerably higher levels of β7 integrin than the respective mature MCs.21Dahlin J.S. Ding Z. Hallgren J. Distinguishing mast cell progenitors from mature mast cells in mice.Stem Cells Dev. 2015; 24: 1703-1711Crossref PubMed Scopus (16) Google Scholar Moreover, analysis of MCps and mature MCs differentiated in vitro from human cord blood revealed expression of 4 different chemokine receptors (CXCR2, CCR3, CXCR4, and CCR5) on MCps, whereas only CCR3 was retained on mature MCs.22Ochi H. Hirani W.M. Yuan Q. Friend D.S. Austen K.F. Boyce J.A. T helper cell type 2 cytokine-mediated comitogenic responses and CCR3 expression during differentiation of human mast cells in vitro.J Exp Med. 1999; 190: 267-280Crossref PubMed Scopus (284) Google Scholar Generally, expression of plasma membrane proteins important for migration changes during MC maturation. Very recently, with the aid of a new hematopoietic fate mapping model, murine MCs were demonstrated to have dual developmental origins arising through both primitive (yolk sac–derived) and definitive (HSC-derived) hematopoiesis.23Gentek R. Ghigo C. Hoeffel G. Bulle M.J. Msallam R. Gautier G. et al.Hemogenic endothelial fate mapping reveals dual developmental origin of mast cells.Immunity. 2018; 48: 1160-1171.e5Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar MC maintenance in adult tissues was found to occur largely independent of the BM, probably through proliferation of tissue-resident MCps differentiated from HSCs during embryogenesis.23Gentek R. Ghigo C. Hoeffel G. Bulle M.J. Msallam R. Gautier G. et al.Hemogenic endothelial fate mapping reveals dual developmental origin of mast cells.Immunity. 2018; 48: 1160-1171.e5Abstract Full Text Full Text PDF PubMed Scopus (35) Google Scholar This intriguing study proves that much more remains to be discovered in MC development and migration to target tissues, and it would be exciting to learn about the respective processes in human subjects. Proteases, particularly tryptases, chymases, and CPA3, expression of which is largely restricted to MCs, contribute importantly to the pleiotropic effector functions of these cells. Proteases are the major proteins of MC secretory granules, which also contain histamine, serotonin, and proteoglycans. After release by means of degranulation, these proteases primarily exert proinflammatory functions and have been implicated in the pathogenesis of MC-related disorders, but they also contribute to tissue homeostasis and host defense.24Caughey G.H. Mast cell proteases as pharmacological targets.Eur J Pharmacol. 2016; 778: 44-55Crossref PubMed Scopus (54) Google Scholar, 25Wernersson S. Pejler G. Mast cell secretory granules: armed for battle.Nat Rev Immunol. 2014; 14: 478-494Crossref PubMed Scopus (337) Google Scholar, 26Sommerhoff C.P. Schaschke N. Mast cell tryptase b as a target in allergic inflammation: an evolving story.Curr Pharm Des. 2007; 13: 313-332Crossref PubMed Scopus (0) Google Scholar Human MCs express 4 tryptases (α, β, γ, and δ), 2 proteases with chymotrypsin-like activity (chymase and cathepsin G [also expressed in neutrophils]), and CPA3. Tryptase β, which forms noncovalent tetramers with limited trypsin-like activity, is considered the prototypical and biological active "tryptase."27Pereira P.J. Bergner A. Macedo-Ribeiro S. Huber R. Matschiner G. Fritz H. et al.Human b-tryptase is a ring-like tetramer with active sites facing a central pore.Nature. 1998; 392: 306-311Crossref PubMed Scopus (0) Google Scholar, 28Sommerhoff C.P. Bode W. Pereira P.J. Stubbs M.T. Sturzebecher J. Piechottka G.P. et al.The structure of the human bII-tryptase tetramer: fo(u)r better or worse.Proc Natl Acad Sci U S A. 1999; 96: 10984-10991Crossref PubMed Scopus (0) Google Scholar Tryptase α can assemble to virtually identical tetramers but is inactive.29Marquardt U. Zettl F. Huber R. Bode W. Sommerhoff C. The crystal structure of human a1-tryptase reveals a blocked substrate-binding region.J Mol Biol. 2002; 321: 491-502Crossref PubMed Scopus (0) Google Scholar Tryptase δ lacks the C-terminal 40-amino-acid residues,30Wang H.W. McNeil H.P. Husain A. Liu K. Tedla N. Thomas P.S. et al.δ tryptase is expressed in multiple human tissues, and a recombinant form has proteolytic activity.J Immunol. 2002; 169: 5145-5152Crossref PubMed Google Scholar and from a structural point of view, its folding and activity are enigmatic. Tryptase γ is less closely related to the other tryptases, and it is likely membrane bound in a monomeric form.31Wong G.W. Foster P.S. Yasuda S. Qi J.C. Mahalingam S. Mellor E.A. et al.Biochemical and functional characterization of human transmembrane tryptase (TMT)/tryptase γ. TMT is an exocytosed mast cell protease that induces airway hyperresponsiveness in vivo via an interleukin-13/interleukin-4 receptor a/signal transducer and activator of transcription (STAT) 6-dependent pathway.J Biol Chem. 2002; 277: 41906-41915Crossref PubMed Scopus (0) Google Scholar Although chymase and CPA3 are exclusive to MCs, chymase, cathepsin G, and CPA3 are usually expressed only in the MCTC subset of MCs. MCs with distinct protease expression patterns, such as chymase-only MCs32Weidner N. Austen K.F. Heterogeneity of mast cells at multiple body sites. Fluorescent determination of avidin binding and immunofluorescent determination of chymase, tryptase, and carboxypeptidase content.Pathol Res Pract. 1993; 189: 156-162Crossref PubMed Google Scholar and, more recently, CPA3+ MCTs have been described in patients with eosinophilic allergic inflammatory disorders.33Dougherty R.H. Sidhu S.S. Raman K. Solon M. Solberg O.D. Caughey G.H. et al.Accumulation of intraepithelial mast cells with a unique protease phenotype in T(H)2-high asthma.J Allergy Clin Immunol. 2010; 125: 1046-1053.e8Abstract Full Text Full Text PDF PubMed Scopus (145) Google Scholar, 34Takabayashi T. Kato A. Peters A.T. Suh L.A. Carter R. Norton J. et al.Glandular mast cells with distinct phenotype are highly elevated in chronic rhinosinusitis with nasal polyps.J Allergy Clin Immunol. 2012; 130: 410-420.e5Abstract Full Text Full Text PDF PubMed Scopus (70) Google Scholar Comparative genomic analysis has shown that tryptases and chymases are poorly conserved between human subjects and rodents and even between human subjects and chimpanzees.35Perez-Silva J.G. Espanol Y. Velasco G. Quesada V. The Degradome database: expanding roles of mammalian proteases in life and disease.Nucleic Acids Res. 2016; 44: D351-D355Crossref PubMed Scopus (12) Google Scholar The mouse genome encodes 4 tryptases: mouse mast cell protease (mMCP) 6, mMCP-7, mMCP-11, and tryptase γ/transmembrane tryptase. mMCP-6 is most likely the functional counterpart of human tryptase β, whereas mMCP-7 apparently is dispensable and not expressed in some mouse strains, such as C57BL/6.36Hunt J.E. Stevens R.L. Austen K.F. Zhang J. Xia Z. Ghildyal N. Natural disruption of the mouse mast cell protease 7 gene in the C57BL/6 mouse.J Biol Chem. 1996; 271: 2851-2855Crossref PubMed Scopus (73) Google Scholar The chymase locus has expanded considerably and encodes 7 chymases: mMCP-1, mMCP-2, mMCP-4, mMCP-5, mMCP-8, mMCP-9, and mMCP-10. mMCP-4 appears to be the functional counterpart of human chymase, whereas its rodent orthologs mMCP-5 and rMCP-5 have altered elastase-like activity.37Karlson U. Pejler G. Tomasini-Johansson B. Hellman L. Extended substrate specificity of rat mast cell protease 5, a rodent α-chymase with elastase-like primary specificity.J Biol Chem. 2003; 278: 39625-39631Crossref PubMed Scopus (44) Google Scholar, 38Kunori Y. Koizumi M. Masegi T. Kasai H. Kawabata H. Yamazaki Y. et al.Rodent α-chymases are elastase-like proteases.Eur J Biochem. 2002; 269: 5921-5930Crossref PubMed Scopus (0) Google Scholar Instead of being expressed in MCs, mMCP-11 and mMCP-8 are preferentially expressed in basophils,39Ugajin T. Kojima T. Mukai K. Obata K. Kawano Y. Minegishi Y. et al.Basophils preferentially express mouse mast cell protease 11 among the mast cell tryptase family in contrast to mast cells.J Leukoc Biol. 2009; 86: 1417-1425Crossref PubMed Scopus (76) Google Scholar, 40Tsutsui H. Yamanishi Y. Ohtsuka H. Sato S. Yoshikawa S. Karasuyama H. The basophil-specific protease mMCP-8 provokes an inflammatory response in the skin with microvascular hyperpermeability and leukocyte infiltration.J Biol Chem. 2017; 292: 1061-1067Crossref PubMed Scopus (11) Google Scholar which are virtually devoid of tryptases and chymases in human subjects. In contrast to the heterogeneity of tryptases and chymases, a single CPA3 gene is expressed in human subjects and rodents. Considering their role in host defense versus their proinflammatory potential, it is not surprising that the expression and activity of MC proteases is extensively regulated. On the genomic level, the variation of α- and β-tryptases is complex: of the 4 tryptase genes (TPSAB1, TPSB2, TPSG1, and TPSD1), TPSAB1 and TPSB2 can harbor alleles encoding enzymatically active β, inactive α, or deficiency alleles.41Trivedi N.N. Tamraz B. Chu C. Kwok P.Y. Caughey G.H. Human subjects are protected from mast cell tryptase deficiency despite frequent inheritance of loss-of-function mutations.J Allergy Clin Immunol. 2009; 124 (e1-4): 1099-1105Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar All subjects initially surveyed have 2 to 4 β alleles, suggesting that the β allele has essential functions, whereas the enzymatically inactive α allele is dispensable.41Trivedi N.N. Tamraz B. Chu C. Kwok P.Y. Caughey G.H. Human subjects are protected from mast cell tryptase deficiency despite frequent inheritance of loss-of-function mutations.J Allergy Clin Immunol. 2009; 124 (e1-4): 1099-1105Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar Recently, subjects with duplications or triplications of the α-encoding TPSAB1 gene, increased basal serum tryptase levels, and multisystem complaints have been identified, a trait now classified as hereditary α-tryptasemia syndrome.42Lyons J.J. Yu X. Hughes J.D. Le Q.T. Jamil A. Bai Y. et al.Elevated basal serum tryptase identifies a multisystem disorder associated with increased TPSAB1 copy number.Nat Genet. 2016; 48: 1564-1569Crossref PubMed Scopus (72) Google Scholar On the level of gene regulation, expression of human tryptase, its mouse counterpart mMCP-6, and several mouse chymases is driven by the microphthalmia-associated transcription factor, which is also central to the development of MCs.43Lee S.H. Lee J.H. Kim D.K. Involvement of MITF-A, an alternative isoform of mi transcription factor, on the expression of tryptase gene in human mast cells.Exp Mol Med. 2010; 42: 366-375Crossref PubMed Scopus (0) Google Scholar, 44Morii E. Tsujimura T. Jippo T. Hashimoto K. Takebayashi K. Tsujino K. et al.Regulation of mouse mast cell protease 6 gene expression by transcription factor encoded by the mi locus.Blood. 1996; 88: 2488-2494PubMed Google Scholar Posttranscriptional and posttranslational mechanisms contribute to this regulation and result in an increased number of tryptase species. For example, alternative splicing of human α- and β-tryptases has been reported, which likely affects the tetrameric architecture, stability, and enzymatic activity of these proteins.45Jackson N.E. Wang H.W. Bryant K.J. McNeil H.P. Husain A. Liu K. et al.Alternate mRNA splicing in multiple human tryptase genes is predicted to regulate tetramer formation.J Biol Chem. 2008; 283: 34178-34187Crossref PubMed Scopus (11) Google Scholar Posttranslational processing further results in several forms of the proteins with slight differences in glycosylation, charge, and enzymatic activity.46Peng Q. McEuen A.R. Benyon R.C. Walls A.F. The heterogeneity of mast cell tryptase from human lung and skin.Eur J Biochem. 2003; 270: 270-283Crossref PubMed Scopus (0) Google Scholar In contrast to most other proteases, tryptases and chymases are not regulated at the level of zymogen activation. Rather, they are activated by cathepsin C and potentially by other cathepsins before storage in granules, a feature shared with other "granule-associated serine proteases of immune defense."47Caughey G.H. A pulmonary perspective on GASPIDs: granule-associated serine peptidases of immune defense.Curr Respir Med Rev. 2006; 2: 263-277Crossref PubMed Scopus (11) Google Scholar, 48Ahmad J. Bird P.I. Kaiserman D. Analysis of the evolution of granule associated serine proteases of immune defence (GASPIDs) suggests a revised nomenclature.Biol Chem. 2014; 395: 1253-1262Crossref PubMed Scopus (9) Google Scholar Within the granules, the activity of tryptases and chymases is likely controlled by compartmentalization, its acidic pH, and its close packaging. After degranulation, binding to high-molecular-weight proteoglycans continues to modulate diffusion, activity, and substrate specificity. In particular, the active β-tryptase tetramer requires stabilization by bound proteoglycans that protect it from dissociation into (almost) inactive monomers. Enzymatically active monomers with altered substrates and inhibition profiles can be formed in vitro,49Hallgren J. Spillmann D. Pejler G. Structural requirements and mechanism for heparin-induced activation of a recombinant mouse mast cell tryptase, mouse mast cell protease-6: formation of active tryptase monomers in the presence of low molecular weight heparin.J Biol Chem. 2001; 276: 42774-42781Crossref PubMed Scopus (0) Google Scholar but these remain to be demonstrated in vivo. Tetramerization is likely the main mechanism regulating tryptase β activity and selectivity in vivo because it restricts the access of large substrates to the active sites located within the central pore and sterically blocks inhibition by most protease inhibitors.26Sommerhoff C.P. Schaschke N. Mast cell tryptase b as a target in allergic inflammation: an evolving story.Curr Pharm Des. 2007; 13: 313-332Crossref PubMed Scopus (0) Google Scholar, 28Sommerhoff C.P. Bode W. Pereira P.J. Stubbs M.T. Sturzebecher J. 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