Mast Cell Function

Origin

Phylogenetic studies point to the appearance of a possible primitive counterpart of vertebrate mast cells in Ciona intestinalis, a 550-million-year-old urochordate regarded as ancestor of both cephalochordates and vertebrates. This primitive mast cell-like cell contains metachromatic, electron-dense granules and resembles connective tissue mast cells, and is also able to release histamine and prostaglandins upon activation. Accordingly, mast cells could have evolved long before the development of an adaptive immune response (Stevens and Adachi 2007).

Although mammalian mast cells were first described more than a century ago, their origin remained controversial for several decades. Due to their association with connective tissue, it was initially assumed that mast cells were derived from undifferentiated mesenchymal cells (Combs 1966). Lymphocytes, multipotent progenitors, and myeloid cells have also been suggested as mast cell precursors (Yong 1997; Chen et al. 2005; Arinobu et al. 2009; Franco et al. 2010). Owing to morphological and physiological similarities, basophils were also pointed to being mast cell precursors, and a bi-potent, committed progenitor for both cells was identified in the mouse spleen (Zucker-Franklin 1980; Arinobu et al. 2005).

The hematopoietic origin of adult mast cells was established by the pioneering work of Kitamura et al. in 1977. When the bone marrow from beige mice (C57Bl Bg^J/ Bg^J) was transplanted into irradiated wild type C57Bl mice, tissue mast cells with large abnormal granules from the beige mouse bone marrow appeared in the tissue of the recipient mice. This finding, which suggested that mast cells derive from bone marrow precursor cells, was reinforced when the mast cell population in deficient mice (W/W^V) could be reconstituted by bone marrow from wild type mice (Kitamura et al. 1978). The hematopoietic origin of human mast cells was also confirmed after allogeneic bone marrow transplantation in a leukemic patient, where 198 days after the transplant, mast cells isolated from the recipients’ bone marrow displayed the donor’s genotype (Födinger et al. 1994).

The existence of a mast cell committed precursor (MCcp) has been described in mouse bone marrow. Using sequential immunomagnetic isolation with two mast cell-specific antibodies (mAb AA4 and mAb BGD6), Jamur and colleagues (Jamur et al. 2005) isolated and characterized a MCcp from the bone marrow of adult Balb/c mice (Fig. 2). This precursor cell was CD34⁺CD13⁺c-Kit⁺FcϵRI^-, and contained mRNA for the α and β subunits of FcϵRI as well as for the mast cell-specific proteases mMCP-5, mMCP-7, and mouse carboxypeptidase A (CPA). Moreover, the MCcp gave rise only to mast cells in vitro and were able to reconstitute mast cells in lethally irradiated mice. Chen et al. (2005) also identified a putative MCcp in the bone marrow of C57BL/6 mice, which was Lin^-, Sca-1^-, c-Kit⁺, Ly6c^-, FcϵRIα^-, CD27^-, β7⁺, and T1/ST2⁺, and gave rise only to mast cells in culture. These cells were able to reconstitute mast cells in Kit^W-sh/ Kit^W-sh mice. The authors also proposed the existence of a common precursor for mast cells and other myeloid cells within the multipotent progenitor population in the bone marrow (Chen et al. 2005).

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Figure 2.

A committed mast cell precursor (AA4^-/BGD6+) from bone marrow of an adult Balb/c mouse bound to a magnetic bead conjugated to mAb BGD6. B, magnetic bead; N, nucleus. Transmission electron microscopy. Bar = 1 µm.

The embryonic origin of mast cells has also been a matter of great debate. Earlier, indirect studies indicated the liver and yolk sac as sites of embryonic mast cell origin but the lack of specific markers made it difficult to distinguish between mast cell precursors and pluripotent stem cells (Kitamura et al. 1979; Sonoda et al. 1983; Palis et al. 1999; Medvinsky et al. 2011). Using previously characterized, direct immunological methods, Guiraldelli et al. have recently described, for the first time, the aorta-gonad-mesonephros (AGM) region as the site of origin of rat embryonic mast cells (Jamur et al. 2005; Guiraldelli et al. 2013). The AGM is a region of embryonic mesoderm that develops from the para-aortic splanchnopleura mesoderm in chick, mouse and human embryos. During mouse development, the AGM is the site where definitive hematopoiesis initiates between E10.5 and E12 (Müller et al. 1994; Medvinsky and Dzierzak 1996; de Bruijn et al. 2000; Cumano et al. 2001; Dzierzak and Speck 2008). Therefore, the MCcps found in the AGM at E11.5 appeared concurrently with the initiation of definitive hematopoiesis in mouse embryos (Guiraldelli et al. 2013). These embryonic MCcps were very similar to the adult MCcps previously described and gave rise only to mast cells in vitro (Jamur et al. 2005; Guiraldelli et al. 2013).

Distribution

Mast cells have a widespread tissue distribution and are found predominantly at the interface between the host and the external environment (Fig. 1a) at places of potential entry of pathogens or contact with harmful substances, such as skin, respiratory mucosa, and gastrointestinal tract (Ehrlich, 1878; Metcalfe et al. 1997; Galli et al. 2005b; Jamur, 2005; Metcalfe and Boyce, 2006). Mast cells populate connective tissue, particularly in sub-epithelial regions and in the connective tissue surrounding blood vessels, nerves, smooth muscle cells, mucus glands, and hair follicles (Galli et al. 2005a). The far-reaching distribution of the mast cell population relies on mechanisms of constitutive homing, enhanced recruitment, survival, and local maturation of mast cell progenitors. Unlike other cells of hematopoietic origin, which differentiate and mature in the bone marrow before being released to the blood stream, mast cells migrate as immature progenitor cells through the blood stream to peripheral tissues where they complete their maturation (Kitamura et al. 1985; Kitamura et al. 1993; Huff et al. 1995; Hallgren and Gurish 2007). Studies using peripheral resident progenitor cells from the thymus and lymph nodes of rodents and the connective tissue sheath of mouse fibrissa hair follicles showed that progenitor mast cells are present in peripheral tissues and are able to differentiate and mature in vitro (Ginsburg 1963; Ginsburg and Sachs 1963; Ginsburg and Lagunoff 1967; Ishizaka et al. 1976; Ishizaka et al. 1977; Ito et al. 2010). Limiting dilution and colony-forming assays provided evidence that colony-forming mast cells reside in the bone marrow, spleen, peripheral blood, mesenteric lymph nodes, and in the gastrointestinal mucosa (Crapper and Schrader 1983; Guy-Grand et al. 1984; Kasugai et al. 1995). Resident mast cells are long-lived cells that can survive for up to 12 weeks in the skin of Wistar rats (Kiernan 1979). Under specific conditions, mature mast cells are able to proliferate after appropriate stimuli (Kitamura 1989; Galli et al. 2005b; Ryan et al. 2007). In rodents, the recruitment of mast cell progenitors from the bone marrow as well as the proliferation of recently recruited progenitors are responsible for repopulation of the peritoneal cavity after mast cell depletion by distilled water injection (Kanakura et al. 1988a; Jamur et al. 2010). Nakano et al. (1985) observed that reconstitution of the mast cell-deficient WBB6F1-WW^v mice with bone marrow cells from congenic WBB6F1-+/+ causes an increase in MCps in the peritoneal cavity and that these progenitors differentiate into morphologically identifiable mast cells. The intraperitoneal injection of bone marrow-cultured mast cells before reconstitution significantly inhibited recruitment to and differentiation of MCps in the peritoneal cavity (Waki et al. 1990).

Only mast cell progenitor cells, not MCcps, were found in the blood stream and were responsible for populating peripheral tissues (Jamur et al. 2010). The mechanisms for homing or recruitment of progenitor mast cells to peripheral tissues during physiological and inflammatory states are not fully elucidated. The difficulties encountered in studying this process lie with the low number of mast cell progenitors in the bone marrow or recruited to peripheral tissues as well as in the difficulty in identifying these cells. Also, the surface expression of chemoattractant receptors and adhesion molecules, which directly affect migration to target tissues, varies considerably according to maturation stage, target tissue, and cytokines and growth factors encountered in the microenvironment (Collington et al. 2011). Nevertheless, several studies from the past decade highlight the importance of some integrins, adhesion molecules, chemokines and their receptors, as well as cytokines and growth factors as important players in directed migration of mast cells to specific locations under normal and pathological circumstances (reviewed in Collington et al. 2011).

Mast cell progenitor migration seems to be controlled in a tissue-specific manner. Major progress has been achieved in clarifying mast cell progenitor migration to the small intestine and lungs. Mast cell progenitors are found in high numbers in the small intestine. The maintenance of mast cell numbers in the intestine occurs through constitutive homing that is contingent on the binding of α4β7 integrin, expressed on mast cells, with their corresponding adhesion molecules mucosal addressin cell adhesion molecule-1 (MAdCAM-1) or vascular cell adhesion molecule-1 (VCAM-1) on the endothelium (Gurish et al. 2001; Gurish and Boyce 2006). The enhanced recruitment of mast cells to the intestinal mucosa during T. spiralis infection was also dependent on the β7 integrin subunit expressed on mast cell progenitors (Artis et al. 2000; Pennock and Grencis 2004). Furthermore, CXC chemokine receptor 2 (CXCR2), expressed on mast cell progenitors, has been implicated in the directed migration of mast cells to the small intestine (Abonia et al. 2005).

Under physiological conditions, the lung does not have a significant number of mast cell progenitors, but their numbers increase considerably during chronic allergen-induced pulmonary inflammation when mast cell progenitors are actively recruited to the site of inflammation (Ikeda et al. 2003). This recruitment occurs through the interaction between α4β7 and α4β1 integrins expressed on mast cell progenitors with VCAM-1 and CXCR2 present on the endothelium. An amplification loop, regulated by CXCR2, can cause increased expression of VCAM-1 on the endothelium, which results in an increased integrin-mediated recruitment to the lung (Abonia et al. 2006; Hallgren et al. 2007). Additionally, it has been demonstrated that the chemokine (C-C motif) receptor 2 (CCR2)/chemokine (C-C motif) ligand 2 (CCL2) axis is active during recruitment of mast cell progenitors to inflamed lungs (Collington et al. 2010).

The involvement of integrins in the targeting of mast cells to the peritoneal cavity has also been described. Mac-1, a β2 integrin important for leukocyte migration, has been shown to be required for maintenance of mast cell levels in the peritoneal cavity, peritoneal wall, and certain regions of the skin. Mast cell recruitment to the peritoneal cavity in response to rat recombinant (rr)IL-3 was significantly inhibited by a prior intraperitoneal injection of antibodies against the integrin subunits α4 and β7 (de Cássia Campos et al. 2014). The αIIbβ3 integrin has a role in the adhesion of BMMCs to different substrates and influences the homing of mast cell progenitors to the peritoneal cavity (Rosenkranz et al. 1998; Berlanga et al. 2005).

Because mast cells express several chemokine receptors, they are chemotactically responsive to various chemokines. In vitro studies have shown that mouse unstimulated BMMCs were chemoattracted to the chemokines monocyte chemotactic protein-1 (MCP-1 or CCL2) and Regulated upon Activation, Normal T Cell Expressed and Secreted (RANTES, also known as CCL5). In contrast, antigen-stimulated BMMCs migrated in response to MCP-1, RANTES, macrophage inflammatory protein-1alpha (MIP-1α or CCL3), and platelet factor-4 (PF4 or CXCL4) (Taub et al. 1995). Subcutaneous injection of RANTES induced an increase in the number of metachromatic mast cells in the dermis and the spleen of Wistar rats (de Cássia Campos et al. 2014). Although, mast cells express several chemokine receptors and many chemokines have been shown to be chemoattractants for mast cells in vitro, no mast cell-specific chemokine has been described (Taub et al. 1995; Collington et al. 2011).

Mature mast cells and their released mediators were also believed to promote increased recruitment of progenitors to an inflammatory site, thus contributing to mast cell hyperplasia as is often seen in the airways of allergic patients. In patients with severe asthma, chymase-positive mast cells were increased in small airway regions and correlated positively with lung function (Balzar et al. 2005). The lipid mediator leukotriene B4 (LTB4) was shown to be important in the recruitment of mast cell progenitors to inflamed tissues and the subcutaneous injection of LTB4 induced an increase in the number of metachromatic mast cells in both the dermis and the spleen of Wistar rats (Weller et al. 2005; de Cássia Campos et al. 2014). In addition, Transforming Growth Factor-β (TGF-β), also released by mast cells, was shown to be a potent chemoattractant for mast cells in vitro (Gruber et al. 1994; Olsson et al. 2000; Lindstedt et al. 2001; Olsson et al. 2001).

Given the importance of the widespread distribution and recruitment of mast cells, the elucidation of the mechanisms that tightly regulate organ-specific targeting of mast cell progenitors remains a crucial goal.

Development and Maturation

Maturing mast cells can be divided into three distinct stages based on their size and number of granules. Using the mast cell-specific antibody mAb AA4, which recognizes two derivatives of the ganglioside GD1b, it was shown that connective tissue-type mast cells can be identified in all stages of maturation in rat bone marrow (Jamur et al. 2001). This study also demonstrated that mast cell maturation in rat bone marrow was similar to that previously seen in the peritoneal cavity (Jamur et al. 1986; Mendonca et al. 1986).

Much of the current knowledge on the factors affecting mast cell development and maturation has been gained by the culture of mouse BMMCs in vitro in the presence of growth factors. Initial studies used several types of conditioned media that contained unidentified growth factors that supported mast cell growth and development (Hasthorpe 1980; Nabel et al. 1981; Nagao et al. 1981; Razin et al. 1981; Schrader 1981; Tertian et al. 1981). Interleukin-3 (IL-3) was found to be one of the factors in the conditioned media that was responsible for mast cell survival, development, and maturation (Ihle et al. 1981; Lee et al. 1982; Ihle et al. 1983; Razin et al. 1984; Metcalf 1986). In addition, IL-3 favors the development of a mucosal mast cell phenotype in vitro (Nakahata et al. 1986). Although critical for the development of murine BBMCs in vitro, IL-3 is not essential for mast cell development in vivo. IL-3-deficient mice are not deficient in mast cells, but the development of mast cell hyperplasia in response to nematode infection is impaired (Lantz et al. 1998). The culture of murine bone marrow cells with IL-3 alone for 1 week yielded mast cells that expressed transcripts for FcϵRI subunits, bound IgE, but had few, if any, granules. With time in culture, this population increased progressively in parallel with the expression of FcϵRI and its transcripts (Thompson et al. 1990). In humans, IL-3 does not affect mast cell differentiation of bone marrow CD34⁺ progenitors (Shimizu et al. 2008). Nonetheless, IL-3 is valuable in growing human mast cells from cord blood progenitors, as these mast cells are known to express the IL-3 receptor during all developmental stages (Dahl et al. 2004).

SCF, the ligand for the CD117/c-Kit receptor, is essential for mast cell survival and development in vitro. SCF alone is able to support the development of mast cells from mouse bone marrow (Gurish et al. 1992). The culture of mouse bone marrow enriched for hematopoietic progenitors with SCF in combination with IL-3 resulted in the surface expression of FcϵRI on mast cells and the initiation of secretory granule formation after 3 days of culture (Lantz and Huff 1995). In vivo, mouse strains bearing mutations in the genes for the c-Kit receptor (Kit^W/W-v and Kit^W-sh) or its ligand SCF (Sl/Sl^d), which are deficient in mast cells, corroborate the significance of SCF for mast cell survival and development (Huang et al. 1990; Kitamura 2000; Grimbaldeston et al. 2005). It has been shown in primates that SCF injection causes a reversible expansion of mast cells at many sites (Galli et al. 1993a). Research on SCF demonstrated that it promotes mast cell adhesion, migration, proliferation and survival (Irani et al. 1992; Iemura et al. 1994; Okayama and Kawakami 2006). Gain-of-function mutations of c-Kit, which lead to the constitutive activation of the c-Kit receptor, are associated with mastocytosis, a neoplastic disorder characterized by mast cell expansion and accumulation in humans (Orfao et al. 2007).

Phenotypic Heterogeneity and Regulation

Other growth factors and cytokines also influence mast cell development and maturation and consequently contribute to the mast cell phenotype. Thus, it is the microenvironment encountered by mast cells that ultimately determines their mature phenotype (Jamur and Oliver 2011). Accordingly, mast cells exhibit a high degree of heterogeneity and plasticity, as a direct consequence of their widespread location and the mediators or the pathogens with which they interact. Changes in phenotype can take place during virtually all stages of mast cell existence. Different subsets of mature mast cells have been described on the basis of their location and functional, structural, and biochemical characteristics. Two subtypes of mature mast cells have been described in rodents: mucosal mast cells (MMCs) and connective tissue mast cells (CTMCs) (Enerbäck 1966a; 1966b). In mouse, MMCs reside in the mucosal epithelium of the lung and gastrointestinal tract, and their protease content is characterized by the chymases mouse Mast Cell Proteases, mMCP-1 and mMCP-2, which are bound to chondroitin sulfate chains of serglycin proteoglycans, whereas CTMCs are found in the intestinal submucosa, peritoneum, and skin and contain the chymase mMCP-4, the tryptases mMCP-5 and mMCP-6, and carboxypeptidase A (mCPA) bound to heparin chains of serglycin proteoglycans (Yurt et al. 1977; Enerbäck et al. 1986; Metcalfe et al. 1997; Welle 1997; Miller and Pemberton 2002; Pejler et al. 2010). MMCs and CTMCs also differ in their ability to secret histamine and lipid mediators. Upon activation, MMCs release small amounts of histamine and large quantities of cysteinyl leukotrienes, whereas CTMCs release higher levels of histamine and prostaglandin D2 (Heavey et al. 1988). Additionally, athymic nude mice are devoid of MMCs; hence, these cells were designated as T-cell-dependent mast cells (Ruitenberg and Elgersma 1976). It is important to note that the mouse protease phenotypes described above can vary considerably among mast cells found in different tissues, in different locations of the same tissue, in the same tissue of different animal strains and also in inflamed tissues (Stevens et al. 1994; Gurish et al. 1995; Friend et al. 1996; Xing et al. 2011).

Mature human mast cells were similarly divided in two large subsets based on their protease content. The mast cell tryptase/chymase (MC_TC) subset of cells store tryptases, chymases, and carboxypeptidases in their granules, whereas MC_T contain only tryptases (Irani et al. 1986; Schwartz 2006; Pejler et al. 2010). In human mast cells, serglycin proteoglycans contain both heparin and chondroitin sulfate in a 2:1 ratio (Metcalfe et al. 1979; Thompson et al. 1988). MC_T prevail in the intestinal and pulmonary mucosa, near T cells, whereas MC_TC are found in the skin and lymph nodes, in addition to the lung and the gut submucosa (Goldstein et al. 1987; Irani et al. 1987). A third phenotype of mast cells expressing tryptase and carboxipeptidase A3, but not chymase, was recently described in the airway epithelium in asthmatic subjects and esophageal samples of patients with eosinophilic esophagitis (Abonia et al. 2010; Dougherty et al. 2010). Human mast cells also differ with respect to the expression of the receptor C5aR for the complement C5a. Mature MC_TC from skin and lung, but not in mature MC_T from lung, express C5aR (Oskeritzian et al. 2005).

Mast cell phenotypic heterogeneity, reflected in their extensive range of sensitivity to activation, and the variations in stored and released mediators, underlies the array of responses mast cells are able to generate (Metcalfe et al. 1997; Galli et al. 2005b). During the lifetime of a mast cell, numerous factors can alter its phenotype and a combination of these changes can determine mast cell homeostatic or pathophysiological responses (Moon et al. 2010). Mature mast cells can quickly alter their staining characteristics as a result of changes in proteoglycan expression both in vitro and in vivo (Razin et al. 1982; Sonoda et al. 1984; Nakano et al. 1985; Levi-Schaffer et al. 1986; Sonoda et al. 1986; Otsu et al. 1987; Kanakura et al. 1988b). Trans-differentiation between mucosal and connective tissue phenotypes has also been demonstrated (Kitamura 1989). Mouse mast cells are able to reversibly alter not only their serglycin proteoglycans but also their protease profile in vivo (Friend et al. 1998). The mast cell phenotypic profile can be shaped by the cytokine and growth factor milieu they encounter (Table 1). In rodents, Granulocyte Macrophage Colony-Stimulating Factor (GM-CSF) and IL-3 induce histidine decarboxilase synthesis, which in turn leads to an increased histamine production (Schneider et al. 1987). IL-4 acts in concert with IL-3 to promote mast cell growth and survival (Tsuji et al. 1990; Rennick et al. 1995). IL-4 also inhibits the expression of CD117 and FcϵRI in mouse BMMCs (Ryan et al. 1998; Mirmonsef et al. 1999). On the other hand, IL-4 treatment of human cultured mast cells enhances cell maturation and survival, promotes the expression of FcϵRI and chymase in MC_T, and downregulates CD117 expression (Sillaber et al. 1991; Yanagida et al. 1995; Toru et al. 1996; Toru et al. 1998). In mouse BMMCs, IL-4 in combination with SCF induces differentiation of CTMCs (Karimi et al. 1999). The addition of IL-4 to SCF-treated, isolated human intestinal mast cells promotes mast cell proliferation and alters the pro-inflammatory profile of cytokines through the induction of Th2 cytokines (IL-3, IL-5, and IL-13) and the downregulation of IL-6 (Lorentz and Bischoff 2001). Using a mouse model of allergy, Oettgen et al. (Burton et al. 2013) have also established that IL-4 signaling is required for the mast cell expansion observed in the gastrointestinal mucosa in response to allergen ingestion. IL-9 is another important growth factor both for mice and human mast cells (Hültner and Moeller 1990; Godfraind et al. 1998; Matsuzawa et al. 2003). Mice that overexpress this cytokine display increased infiltration of CTMCs and MMCs into the gut, trachea, and kidney (Godfraind et al. 1998). In rodent mast cells, IL-10 exposure inhibits the expression of FcϵRI, IL-6, and CD117 (Marshall et al. 1996; Mirmonsef et al. 1999; Gillespie et al. 2004; Kennedy Norton et al. 2008) but induces the expression of mMCP-1, a serine protease preferentially expressed in mucosal mast cells of Trichinella spiralis-infected mice (Ghildyal et al. 1992b). rIL-10 can induce the expression of mMCP-2 in connective tissue-type BMMCs, whereas rIL-3 attenuates rIL-10-induced expression of this gene in vitro (Ghildyal et al. 1992a). IL-10 in combination with SCF was also shown to increase mast cell proliferation both in vivo and in vitro (Thompson-Snipes et al. 1991; Rennick et al. 1995; Kennedy Norton et al. 2008). The combination of IL-3, IL-4 and IL-10 leads to apoptosis of mouse peritoneal mast cells and BMMCs (Yeatman et al. 2000). IL-6 promotes mast cell growth and survival in the presence of SCF (Galli 1990; Yanagida et al. 1995; Saito et al. 1996; Ochi et al. 1999; Gyotoku et al. 2001). However, IL-6 negatively modulates SCF development of cord blood-derived (CD34⁺) human mast cells (Kinoshita et al. 1999). Moreover, IL-6 has been shown to support the development of splenic mast cells, protect mast cells from IL-4-induced apoptosis, and increase chymase and histamine expression in cord blood-derived human mast cells (Hu et al. 1997; Kinoshita et al. 1999; Oskeritzian et al. 1999). The addition of IL-33 to cultures of CD34⁺ human mast cell progenitors induced the earlier expression of tryptase whereas rIL-33 addition to mBMMCs increased tryptase expression both at the mRNA and protein levels (Allakhverdi et al. 2007a; Kaieda et al. 2010). IL-13, IL-15, and IL-16 all induce mast cell proliferation when combined with other cytokines (Masuda et al. 2000; Masuda et al. 2001; Qi et al. 2002; Kaur et al. 2006; Hu et al. 2007). TGF-β induces the expression of the αE integrin subunit and mast cell proteases mMCP-1, -6, and -7 in BMMCs (Miller et al. 1999; Wright et al. 2002; Funaba et al. 2005; Funaba et al. 2006). Other molecules involved in mast cell maturation include Nerve Growth Factor (NGF) and Neurotrophin-3 (NT-3). NGF increases the number of IL-3-derived BMMCs and induces a CTMC phenotype marked by increased histamine content and the expression of heparin (Matsuda et al. 1991). NGF also prevents apoptosis of murine peritoneal mast cells (Kawamoto et al. 1995). NT-3 promotes maturation of fetal mouse skin mast cells and human intestinal mast cells (Metz et al. 2004; Lorentz et al. 2007). The factors and phenotypic consequences affecting mast cells presented above are only a summary of the research published in this area.

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Table 1.

Mast Cell Phenotypic Regulation.

Cytokines/Growth Factors	Mast Cell Type/Origin	Induced Mast Cell Phenotype	References
GM-CSF, IL-3	Isolated mouse BMMC progenitors	↑ Histamine production	(Schneider et al. 1987)
IL-4+IL-3	CTMCs purified from mouse peritoneal MCs	↑ Growth and survival	(Tsuji et al. 1990)
IL-4, IL-10	Mouse mesenteric lymph node derived MCs	↑ SCF dependent mast cell growth and differentiation / ↑ Histamine production	(Rennick et al. 1995)
IL-4	Mouse BMMCs	↓ Expression of CD117 and FcϵRI	(Ryan et al. 1998; Mirmonsef et al. 1999)
IL-4	Human cultured MCs	↑ Maturation, survival and expression of chymase and FcϵRI / ↓ Expression of CD117	(Sillaber et al. 1991; Yanagida et al. 1995; Toru et al. 1996; Toru et al. 1998)
IL4+SCF	Mouse BMMCs	↑ Connective tissue phenotype	(Karimi et al. 1999)
IL4+SCF	Intestinal human MCs	↑ Proliferation and Th2 cytokine production (IL-3, IL-5, and IL-13) / ↓ IL-6	(Lorentz and Bischoff 2001)
IL-9	Mouse BMMCs, Human CD34(+) cord blood- and peripheral blood-derived MCs	↑ Proliferation and responsiveness to activation	(Hültner and Moeller 1990; Matsuzawa et al. 2003)
IL-9	Mouse CTMCs and MMCs	↑ Intraepithelial infiltration of CTMCs and MMCs in the gut, trachea, and kidneys	(Godfraind et al. 1998)
IL-10	Mouse BMMCs, rat peritoneal MCs, and human skin-derived MCs	↓ Expression of CD117, IL-6, and FcϵRI / ↑ Expression of mMCP1 and mMCP2 / ↑ SCF dependent proliferation	(Thompson-Snipes et al. 1991; Ghildyal et al. 1992a; Ghildyal et al. 1992b; Rennick et al. 1995; Marshall et al. 1996; Gillespie et al. 2004; Kennedy Norton et al. 2008)
IL-3+IL-4+IL-10	Mouse peritoneal and BMMCs	↑ Apoptosis	(Yeatman et al. 2000)
IL-6+SCF	Human cultured MCs, mouse BMMCs	↑ Growth and survival	(Yanagida et al. 1995; Saito et al. 1996; Ochi et al. 1999; Gyotoku et al. 2001)
IL-6	Human CD34(+) cord blood derived MCs	↓ SCF-dependent development / ↑ Expression of chymase and histamine production	(Kinoshita et al. 1999)
IL-33	Human peripheral blood- or cord blood-derived CD34(+) progenitor cells and mouse BMMCs	Earlier expression of tryptase / ↑ Expression of mMCP-6	(Allakhverdi et al. 2007a; Kaieda et al. 2010)
TGF-β	Mouse BMMCs	↑ Expression of αE integrin subunit, MCP-1, MCP-6, and MCP-7	(Miller et al. 1999; Wright et al. 2002; Funaba et al. 2005; Funaba et al. 2006)
NGF	Mouse BMMCs and peritoneal MCs	↑ Maturation, histamine content and heparin expression / ↓ Apoptosis	(Matsuda et al. 1991; Kawamoto et al. 1995)
NT-3	Fetal mouse skin MCs and human intestinal MCs	↑ Maturation	(Metz et al. 2004; Lorentz et al. 2007)

Abbreviations: BMMC, bone marrow mast cell; CTMC, connective tissue mast cell; GM-CSF, granulocyte macrophage colony-stimulating factor; IL, interleukin; MCs, mast cells; MMC, mucosal mast cell; mMCP, mouse mast cell protease; NGF, nerve growth factor; NT-3, neurotrophin-3; SCF, stem cell factor; TGF-β, transforming growth factor-β.

In addition to changes affecting the microenvironment, mast cell phenotype is also dictated by animal species and genetic background (Galli et al. 2011). The relevance of most of the in vitro findings on the influence of cytokines and growth factors on mast cell phenotype has yet to be confirmed in vivo. However, it is evident that mast cell heterogeneity in peripheral tissues encompasses a far more diverse and dynamic profile than the two mast cell subsets traditionally cited; for instance, both tracheal constitutive CTMCs and induced MMCs from sensitized mice analyzed by immunohistochemistry presented with all six mast cell proteases (Xing et al. 2011).

Mast Cell Mediators

Mast cell functions reflect their ability to secrete a diverse array of biologically active compounds (Table 2). Mast cell activation culminates with the release of a wide range of inflammatory mediators (Metcalfe et al. 1997). Activation can lead to release of three distinct classes of mediators: preformed mediators, which are stored in mast cell cytoplasmic granules; neoformed or lipid mediators, which are derived from membrane lipids; and neosynthesized mediators, which are produced following transcriptional activation and whose regulation depends on the type of stimuli and receptor involved (Galli and Lantz 1999).

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Table 2.

Mast Cell Mediators.

MEDIATORS		REFERENCES
PREFORMED		(Lundequist and Pejler 2011)
Biogenic Amines	Histamine, Serotonin (5-HT),Dopamine, Polyamines	(Ohtsu 2008; Kanerva et al. 2009; García-Faroldi et al. 2010; Rönnberg et al. 2011)
Lysosomal Enzymes	β-hexosaminidase, β-glucuronidase,β-D-galactosidase, Arylsulphatase A,Cathepsins C, B, L, D, and E	(Schwartz and Austen 1980; Schwartz et al. 1981; Dragonetti et al. 2000; Wolters et al. 2000; Henningsson et al. 2005)
Proteases	Chymase, Tryptase,Carboxypeptidase A, Cathepsin G,Granzyme B, Matrix metalloproteinases, and Renin	(Parikh et al. 2003; Silver et al. 2004; Reid et al. 2007; Maxová et al. 2010; Pejler et al. 2010; Trivedi and Caughey 2010; Caughey 2011)
Other Enzymes	Kinogenases, Heparanase, Angiogenin and Active Caspase-3	(Bashkin et al. 1990; Kulka et al. 2009; Lilla et al. 2009; García-Faroldi et al. 2013)
Proteoglycans	Serglycin (Heparin and Chondroitin sulphate)	(Yurt et al. 1977; Metcalfe et al. 1979; Enerbäck et al. 1985; Thompson et al. 1988; Abrink et al. 2004; Pejler et al. 2009; Melo et al. 2011; Rönnberg and Pejler 2012)
Cytokines	TNF-α, IL-4, IL-15	(Beil et al. 1994; Horsmanheimo et al. 1994; Gibbs et al. 1997; Orinska et al. 2007)
Chemokines	RANTES (CCL5), eotaxin (CCL11), IL-8 (CXCL8), MCP-1 (CCL2), MCP-3 (CCL7), MCP-4	(Gibbs et al. 2001; Collington et al. 2010; Collington et al. 2011)
Growth Factors	TGF-β, bFGF-2, VEGF, NGF, SCF	(Gordon and Galli 1990; Leon et al. 1994; Boesiger et al. 1998; Grützkau et al. 1998; Qu et al. 1998; Dvorak et al. 2001; Lindstedt et al. 2001; Allakhverdi et al. 2007a)
Peptides	Corticotropin-Releasing Hormone, Endorphin, Endothelin-1,LL-37/Cathelicidin, Substance P, Vasoactive Intestinal Peptide	(DiAugustine et al. 1980; Ehrenreich et al. 1992; Gulubova and Vodenicharov 2001; Di Nardo et al. 2003; Kempuraj et al. 2004; Hültner and Ehrenreich, 2005; Di Nardo et al. 2008; Pongor et al. 2011)
Others	Eosinophil Major Basic Protein (MBP)	(Butterfield et al. 1990)
NEOFORMED		(Boyce 2005)
Phospolipid Metabolites	Prostaglandin D2, E2, Leukotrienes B4, C4, and Platelet Activating Factor	(Boyce 2007)
NEOSYNTHESIZED
Cytokines	IL-33, IL-10, IL-12, IL-17, IL-5, IL-13, IL-1, IL-2, IL-3, IL-4, IL-6, IL-8, IL-9, IL-16, Type I and Type II IFN, TNF-α, MIP-2β	(Gordon and Galli 1990; Rubinchik et al. 1995; Williams and Coleman 1995; Rumsaeng et al. 1997; Ackermann et al. 1999; Masuda et al. 2000; Supajatura et al. 2001; Masuda et al. 2002; Okayama et al. 2003; Gessner et al. 2005; Kohno et al. 2005; Nakano et al. 2007; Nigrovic et al. 2007; Stassen et al. 2007; Buckland 2010; Dietrich et al. 2010; Hsu et al. 2010; Oldford et al. 2010; Lin et al. 2011; Nam et al. 2011)
Growth Factors	SCF, GM-CSF, β-FGF, NGF, PDGF, TGF-β, VEGF	(Wodnar-Filipowicz et al. 1989; Leon et al. 1994; Reed et al. 1995; Grützkau et al. 1998; Zhang et al. 1998; Aceves et al. 2010; van Steensel et al. 2012)
Reactive Oxygen Species	Nitric Oxide	(Swindle and Metcalfe 2007; Endo et al. 2011)
Others	Complement Factor C3 and C5	(Fukuoka et al. 2013)

Abbreviations: FGF, fibroblast growth factor; GM-CSF, granulocyte macrophage colony-stimulating factor; IL, interleukin; MCP, monocyte chemotactic protein; NGF, nerve growth factor; PDGF, platelet-derived growth factor; RANTES, regulated upon activation, normal T vell expressed and secreted; SCF, stem cell factor; TGF-β, transforming growth factor-β; TNF-α, tumor necrosis factor-alpha; VEGF, vascular endothelial growth factor; MIP-2β, macrophage inflammatory protein-1alpha;

Mast Cell Activation

Mast cells may be activated by several distinct stimuli acting on numerous receptors on the mast cell surface. The range and nature of mast cell responses to different stimuli can be influenced by intrinsic and microenvironmental factors that affect the expression or functionality of surface receptors and/or signaling molecules that contribute to these responses (Galli et al. 2005b; Metcalfe et al. 2009).

The most studied method of mast cell activation is the allergic reaction, an adaptive immune response mediated by the high affinity IgE receptor on the mast cell surface (Galli et al. 2005a). Currently, the innate immune regulation of mast cell activation has become center stage. As an innate immune cell, mast cells are equipped for early and rapid sensing of invading microorganisms such as bacteria, parasites, fungi, and viruses. These pathogens display conserved molecular structures called pathogen-associated molecular patterns (PAMPs), which are recognized by pattern recognition receptors (PRRs), such as Toll-like receptors (TLRs), on the mast cell surface. The direct interaction between specific PAMPs and PRRs induces mast cell activation and selective mediator release (Marshall 2004). In addition, mast cells can be activated by several other stimuli such as neuropeptides, cytokines, growth factors, toxins, basic compounds, complement, immune complexes, certain drugs, as well as physical stimuli (Tkaczyk et al. 2004a; Gilfillan et al. 2009). Certain lectins are able to activate mast cells and promote mediator release, largely through crosslinking IgE or FcϵRI on the mast cell surface (Wyczolkowska et al. 1992; Moreno et al. 2003; de Almeida Buranello et al. 2010).

Recent research has showed that receptors for numerous ligands, including adenosine, C3A, immune complexes, chemokines, cytokines, PAMPs, sphingosine-1-phosphate (S1P), and SCF, are involved in mast cell activation. These receptors are able to potentiate FcϵRI-mediated activation or to directly stimulate mediator release in an FcϵRI-independent manner. The modulation of the signaling pathways mediated by these receptors accounts for the fact that different stimuli can lead to diverse combinations of mediators being released through the differential induction of degranulation, eicosanoid and cytokine production and release (Gilfillan and Tkaczyk 2006). Although, the early events of the signaling cascades initiated by these receptors are different, they converge downstream in order to provide the necessary signals for mediator release (Gilfillan and Tkaczyk 2006).

Homeostasis and Tissue Repair

Mast cells are considered crucial for the maintenance of tissue function and integrity (Maurer et al. 2003). Many mast cell mediators including NGF, platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF), and fibroblast growth factor-2 (FGF-2 or bFGF), as well as histamine and tryptase, induce epithelial cell and fibroblast proliferation (Abe et al. 2000). Furthermore, mast cells are involved in all steps of tissue repair, from the initial inflammatory reaction to extracellular matrix (ECM) remodeling (Noli and Miolo 2001). Upon injury, skin mast cells act at the very beginning to regulate primary hemostasis to seal the injured surface. Through the release of platelet activating factor (PAF), leukotrienes, and the cytokines IL-1 and IL-8, mast cells contribute to platelet activation and aggregation as well as extravascular deposition of fibrin (Mekori and Galli 1990; Kauhanen et al. 1998). Conversely, mast cells also secrete heparin, tryptase and t-plasminogen activator (tPA) thereby regulating fibrinolytic mechanisms providing the appropriate perfusion and nutrition necessary for repair (Huang et al. 1997; Gottwald et al. 1998; Thomas et al. 1998). As the inflammation proceeds, mast cells promote the recruitment of circulating leukocytes, which contribute to microbial clearance and debris removal (Rock et al. 1990; Kanwar and Kubes 1994). In the proliferation phase, mast cell mediators stimulate growth, migration and proliferation of endothelial cells, fibroblasts and keratinocytes thereby contributing to angiogenesis, collagen deposition, granulation tissue formation, epithelialization, and wound contraction (Levi-Schaffer and Kupietzky 1990; Katayama et al. 1992; Meininger and Zetter 1992; Moulin et al. 1998). The release of vasoactive amines, tryptase, IL-4, and NGF contribute to the regeneration of damaged nerve fibers (Matsuda et al. 1998; Schäffer et al. 1998). Proteolytic mediators released during mast cell degranulation influence not only the deposition of temporary connective matrix but also coordinate its replacement by a definitive connective tissue (Nishikori et al. 1998). In the late phases of repair, mast cell cytokines (IL-1, IL-4, and IL-6) and growth factors (FGF and TGF) influence the phenotype of activated fibroblasts inducing the appearance of myofibroblasts, which are important for contraction and wound healing (Hebda et al. 1993; Moulin 1995; Moulin et al. 1998).

Mast cells are also important in preserving the homeostasis of tissues and organs, which is characterized by continuous growth and remodeling, such as in hair follicles and bones. The mast cell mediators histamine, TNF, and substance P participate in tissue remodeling and help regulate the hair follicle growth cycle; mast cell-deficient mice have defects in this process (Maurer et al. 1995). Histamine promotes the recruitment and differentiation of osteoclast precursors during the initial stages of bone resorption (Lesclous et al. 2004; Fouilloux et al. 2006; Lesclous et al. 2006). Mast cell tryptase can specifically activate the protease-activated receptor-2 (PAR-2), which inhibits osteoclast differentiation (Smith et al. 2004). Similar to histamine, it is believed that IL-1, TGF-β, IL-6, and PDGF influence osteoclast recruitment and development, which in turn contribute to bone remodeling. Osteopontin (OPN), also released by mast cells, functions in the balance and maintenance of mineralization, thus contributing to the control of bone metabolism (Chiappetta and Gruber 2006; Bulfone-Paus and Paus 2008). OPN was found to stimulate degranulation and migration of mast cells in vitro and OPN (-/-) mice displayed reduced IgE-mediated passive cutaneous anaphylaxis (Nagasaka et al. 2008).

Nervous System

The distribution of mast cells around nerve endings in various tissues including skin, intestinal mucosa, lung, and the central nervous system has been described since Ehrlich. This mast cell localization and, more importantly, the mediators released by both mast cells and neurons collaborate in the establishment of a neuroimmune interaction between these cells. It has been shown that communication between mast cells and neurons can occur through synaptic-like structures sustained by adhesion molecules such as N-cadherin or synaptic cell adhesion molecule (SynCAM) (Suzuki et al. 2004; Furuno et al. 2005). Mast cell-derived serotonin contributes to neurogenesis and to the behavioral and physiological function of the hippocampus (Nautiyal et al. 2012). Mast cell proteases, such as tryptase, signal nerves through PARs; PAR2 activation has been implicated in increased intestinal permeability and visceral hypersensitivity in rodents (Déry et al. 1998; Vergnolle et al. 2001; Coelho et al. 2002; Cenac et al. 2003). On the other hand, mast cells can be activated by substance P and endothelin-1 (ET-1) (Ogawa et al. 1999; Suzuki et al. 1999). Mast cell activation by ET-1, which is an endogenous peptide of considerable toxicity, causes mast cell degranulation with a consequent release of the mast cell-specific proteases such as chymases and CPA, which promote ET-1 degradation thus limiting its toxicity (Maurer et al. 2003; Galli and Tsai 2008). Mast cell-neuron interactions also contribute to the maintenance of intestinal homeostasis by regulating ion transport, vascular permeability, secretory activity of mucus producing cells, and gastrointestinal motility (Van Nassauw et al. 2007).

Angiogenesis

Angiogenesis is a dynamic process characterized by the development and growth of blood vessels from pre-existing vessels. Angiogenesis occurs during physiological processes such as embryonic development and corpus luteum formation, as well as in pathological circumstances such as tumorigenesis and chronic inflammation. The angiogenic process depends on the action of several molecules including angiogenic factors, ECM proteins, adhesion molecules and their receptors, and proteolytic enzymes (Ribatti and Crivellato, 2012). Several factors, including VEGF, FGF, TGF-β, PDGF, IL-8, and angiopoietin 1, are known to stimulate angiogenesis (Sawatsubashi et al. 2000; Talreja et al. 2004). The proximity of mast cells to blood vessels in tissues associated with angiogenesis has long suggested a relationship between mast cells and angiogenesis. Moreover, the role of mast cells in this process is most certainly related to the release of a large spectrum of angiogenic mediators, which include angiopoietin-1, FGF-2, VEGF, IL-8, TGF-β, TNF-α, histamine, heparin, tryptase and chymase, among others (Crivellato et al. 2004). These mast cell mediators can act at various stages of angiogenesis including degradation of the ECM, migration and proliferation of endothelial cells, formation and distribution of new vessels, synthesis of ECM and pericyte mobilization (D’Amore and Thompson 1987; Juczewska and Chyczewski 1997). It has been shown that during the initiation of angiogenesis, mast cell tryptase promotes ECM degradation through the activation of MMPs and plasminogen activator (Stack and Johnson 1994). In vitro and in vivo studies have shown that mMCP-4 has a role in the processing of pro-MMP-9 and pro-MMP-2 into their active forms (MMP-2 and MMP-9), both of which are released in parallel with mMCP4 and have a role in ECM remodeling and angiogenesis (Fang et al. 1996; Fang et al. 1997; Baram et al. 2001; Tchougounova et al. 2005). Other mast cell granule contents, such as cathepsin G, elastase, and collagenase, also contribute to the degradation of ECM components (Stack and Johnson 1994). In addition, VEGF, FGF-2, and the combination of tryptase and heparin induces migration and proliferation of vascular endothelial cells (Azizkhan et al. 1980; Montesano et al. 1983; Bikfalvi et al. 1991; Blair et al. 1997; Jośko and Mazurek 2004). Histamine and heparin have also been shown to stimulate the proliferation of vascular endothelial cells and to induce the formation of new blood vessels in a rat mesenteric window assay (Sörbo et al. 1994). Tryptase was able to promote vascular tube formation in vitro in a chorioallantoic membrane (CAM) assay (Ribatti et al. 1987; Blair et al. 1997). Additionally, in vitro studies have shown that the angiogenic factors VEGF, PDGF, and FGF-2 are chemotactic for mast cells (Gruber et al. 1995). Although most angiogenic mediators released are not exclusive to mast cells, the role of mast cell-specific proteases (chymases and tryptases) in angiogenesis has gained increased prominence. In particular, data indicate that mast cells are a significant source of angiogenic and tissue remodeling factors in the tumor environment. Mast cells constitute a major inflammatory cell population with a critical role in the regulation of inflammation and immune response which will be further discussed.

Innate Immunity

Similar to DCs, mast cells are among the first cells of the immune system to interact with antigens, toxins, and pathogens. In addition to their strategic distribution, mast cells express on their surface various receptors that are able to detect potentially harmful signals and enable the cells to respond rapidly and appropriately through the release of pre-stored and neo-synthesized mediators. Mast cells can recognize pathogens through different mechanisms including direct binding of pathogens or their components to PAMP receptors on the mast cell surface, binding of antibody or complement-coated bacteria to complement or immunoglobulin receptors, or recognition of endogenous peptides produced by infected or injured cells (Hofmann and Abraham 2009). The pattern of expression of these receptors varies considerably among different mast cell subtypes. TLRs (1-7 and 9), NLRs, RLRs, and receptors for complement are accountable for most mast cell innate responses (Marshall 2004; Metz et al. 2008; Fukuda et al. 2013; Graham et al. 2013). Activation of these receptors by pathogens leads to the release of inflammatory mediators, which contribute to the containment and clearance of the infection, and also support adaptive immune responses when necessary. The pattern of mediator release through TLRs depends on the ligand and the receptor to which it binds (Leal-Berumen et al. 1994; Dvorak 2005). TLR-2 recognizes peptidoglycans from gram-positive bacteria, gram-negative bacteria, and mycobacteria, with subsequent promotion of cytokine production and degranulation. On the other hand, TLR-4 binds LPS from gram-negative bacteria, lipid A, fibrinogen, and Mycobacterium tuberculosis, with consequential cytokine production without induction of degranulation (Supajatura et al. 2002; Varadaradjalou et al. 2003).

In models of bacterial infection, it is currently accepted that bacterial clearance is aided by the recruitment of immune cells to sites of infection. This process is facilitated by the tissue location of mast cells, their pathogen recognition ability, and release of mediators that contribute to increased vascular permeability and chemoattraction of innate immune cells, such as: (1) eosinophils by CC-chemokine ligand 11 (CCL11, or eotaxin), (2) natural killer (NK) cells by CXCL8, or IL-8, and (3) neutrophils by CXCL1/CXCL2, TNF-α, LTB4, LTC4, and MCP-6 (Gordon and Galli 1990; Huang et al. 1998; Biedermann et al. 2000; Malaviya and Abraham 2000; Marshall 2004; Burke et al. 2008; Sutherland et al. 2008; De Filippo et al. 2013). Mast cell activation by LPS from gram-negative bacteria through TLR-4 results in the production of the proinflammatory cytokines TNF-α, IL-1β, and IL-6, as well as anti-inflammatory IL-13, without eliciting degranulation. In a model of Cecal Ligation and Puncture (CLP)-induced acute septic peritonitis, this mast cell inflammatory response led to the initiation of a protective immune response by rapid infiltration of neutrophils into the peritoneal cavity which resulted in bacterial clearance (Supajatura et al. 2001). It was recently shown that mast cell signaling through TLR-2 increases IL-4 production and is critical for the effective control of replication and killing of pulmonary Francisella tularensis (Rodriguez et al. 2011; Rodriguez et al. 2012). Mast cell products also include antibacterial peptides such as cathelicidins, defensinas, and psidins, which have direct bactericidal effects upon degranulation and support bacterial clearance (Féger et al. 2002; Di Nardo et al. 2003; Wei et al. 2005; Campagna et al. 2007). In addition, mast cells can phagocytose bacteria and produce reactive oxygen species, which aid bacterial killing after phagocytosis (Malaviya et al. 1994). Recent studies have shown that activated mast cells also have antimicrobial functions through the production of structures called mast cell extracellular traps (MCETs), formed by DNA, histones, and granular proteins such as tryptase and cathelicidin LL-37 (von Köckritz-Blickwede et al. 2008). The release of mast cell proteases during degranulation also helps limit the toxicity of endogenous peptides and poisonous venoms of reptiles and arthropods by degrading these products (Maurer et al. 2004; Metz et al. 2006; Schneider et al. 2007; Piliponsky et al. 2008; Akahoshi et al. 2011).

The role of mast cells in viral infections is less well characterized. Mast cells can be infected by several viruses including HIV, dengue virus, cytomegalovirus, adenoviruses, and Influenza A virus (IAV). Mast cell activation by viral products induces the production of a characteristic pattern of cytokines and chemokines that includes IL-1β, IL-6, CCL3, CCL4, CCL5, and CCL8 (Marshall et al. 2003; Dawicki and Marshall 2007; Burke et al. 2008). The ability of mast cells to promote recruitment of CD8⁺ T lymphocytes to the site of infection and to produce IFN-1 during viral challenge indicates that viral recognition by mast cells incites cellular responses directed towards viral clearance (Kulka et al. 2004; Orinska et al. 2005). An increased viral burden within the draining lymph nodes was observed in dengue virus-infected mast cell-deficient mice and this increase was shown to be due to deficient NK and NK T cell recruitment to the site of infection (St John et al. 2011). Another in vivo study showed a protective role for mast cells using a mouse model of skin viral infection, where vaccinia virus infection caused mast cell degranulation, which in turn led to antimicrobial peptide discharge and virus inactivation (Wang et al. 2012). Aoki et al. (2013) found that intradermal injection with herpes simplex virus 2 (HSV-2) into MC-deficient Kit^W/Wv mice led to increased clinical severity and mortality with elevated virus titers in HSV-infected skins. This outcome was reversed by intradermal reconstitution with BMMCs from wild-type, but not TNF^−/− or IL-6^−/−, mice, indicating a protective role for these cytokines in HSV-induced mortality. In addition to a role in viral clearance and immune surveillance, recent work from several groups has also suggested a detrimental role for mast cells in viral infections. For instance, HIV has been shown to infect human mast cell progenitors, which can mature and develop as long-lived viral reservoirs during latent infection (Sundstrom et al. 2007). Moreover, Graham et al. (2013) observed that mast cells contributed to the establishment of IAV-induced inflammatory response and lung damage.

In parasitic infections, the production and release of growth factors (IL-3, SCF, and IL-9) by mast cells were shown to be responsible for the commonly observed mast cell hyperplasia (Newlands et al. 1995; Faulkner et al. 1998; Lantz et al. 1998). Mast cell mediator release during parasitic infection promotes immune cell recruitment and regulation of gastrointestinal permeability. Moreover, the microenvironment generated in response to mast cell mediators produces favorable conditions for the expulsion of the parasite and containment of a chronic infection (Knight et al. 2000; Gurish et al. 2004; Abraham and St John 2010). It was recently reported that mast cell degranulation regulates tissue-derived cytokines IL-25, IL-33, and thymic stromal lymphopoietin (TSLP) in the early stages of helminthic infection (Hepworth et al. 2012). These cytokines, produced mainly by epithelial and endothelial cells, have been reported to be critical for optimal Th2 responses and worm expulsion in helminth infections (Owyang et al. 2006; Allakhverdi et al. 2007b; Humphreys et al. 2008; Taylor et al. 2009). Hepworth et al. (2012) reported that innate IgE-independent regulation of tissue-derived cytokines was important for the appropriate development of an adaptive Th-2 response and the expansion of innate Th-2 cytokine-producing cells during helminthic infections.

Adaptive Immunity

Dendritic cells (DCs) are specialized antigen-presenting cells (APCs) and are indispensable for the induction of adaptive immune responses (Lambrecht et al. 2000; Vermaelen et al. 2001). Recent in vitro studies have shown that mast cells are also capable of processing and presenting antigens via MHCI and MHCII complexes (Malaviya et al. 1996; Poncet et al. 1999; Stelekati et al. 2009). Moreover, mast cells and their mediators also directly modulate activation and migration of DCs to lymph nodes (Reuter et al. 2010). Activation through TLR7 leads to the release of IL-1β and TNF, which promote migration of DCs from the skin to local lymph nodes and induce cytotoxic responses by T lymphocytes (Suto et al. 2006; Heib et al. 2007). Histamine, PGE₂, and PGD₂ modulate DCs to develop Th₂ responses (McIlroy et al. 2006; Theiner et al. 2006). During exocytosis, mast cells release exosomes, vesicles of heterogeneous size and shape derived from the lumen of multivesicular bodies and the plasma membrane (Harding et al. 1983; Raposo et al. 1997; Shefler et al. 2011). These mast cell-derived exosomes contain co-stimulatory molecules and antigens that promote functional and phenotypical maturation of DCs (Skokos et al. 2003). Moreover, mast cells can directly activate T lymphocytes through the release of TNF (Nakae et al. 2005; Nakae et al. 2006). In addition to promoting the initiation and development of adaptive immune responses, mast cells also act to limit the duration and magnitude of immune responses and are capable of suppressing immune responses through the release of anti-inflammatory cytokines such as IL-10 and TGF-β (Hart et al. 1998; Hart et al. 2002; Grimbaldeston et al. 2007; Rao and Brown 2008).

Immune Tolerance

The concept that mast cells function in the mediation of tolerance is relatively new. This view was based on the observation that mast cells were required to maintain immune tolerance (Lu et al. 2006). Further studies, most with skin mast cells, have demonstrated that, through their array of mediators, surface molecules, and co-stimulatory molecules, mast cells are able to modulate immune response both by contact-dependent and -independent mechanisms (de Vries and Noelle 2010). Mast cells are required for immune tolerance in allografts, and mast cell degranulation breaks this tolerance to established allografts (de Vries et al. 2009). Mast cell-secreted cytokines, in particular the neosynthesized cytokines IL-10 and TGF-β, are recognized for their immune-suppressive effects. Mast cell IL-10 secretion can reduce the duration and magnitude of immune responses. IL-10 and TGF-β downregulate the expression of FcϵRI-limiting IgE-mediated degranulation (Gillespie et al. 2004; Gomez et al. 2005; Kennedy Norton et al. 2008). Mast cell-derived IL-10 promotes the containment and resolution of skin reactions caused by chronic UVB irradiation or contact dermatitis by limiting leukocyte infiltration, inflammation, epidermal hyperplasia, necrosis, and ulceration (Grimbaldeston et al. 2007). Moreover, interrupting the migration of mast cells to draining lymph nodes after UV damage abolishes the UV-induced immune suppression (Byrne et al. 2008). Along with other innate immune cells, mast cells are recruited by MIP-1α in the early stages of inflammation and binding of MIP-1α to the CCR1 receptor on the mast cell surface promotes the production of IL-6, TNF-α, and TGF-β (Fifadara et al. 2009). TGF-β, in addition to IL-2, is crucial for the development of DC-induced antigen-specific regulatory T cells (T^reg) (Davidson et al. 2007; Luo et al. 2007). The T^reg is CD4⁺, CD25⁺and Fox3P⁺, develops in the thymus, and is crucial for self-tolerance. T^reg cells suppress the proliferation of effector T cells through direct contact or release of anti-inflammatory cytokines such as TGF-β and IL-10, or other molecules like PGE2 (Zheng et al. 2004; Mahic et al. 2006). Interaction of T^reg with mast cells is essential for T^reg-dependent peripheral tolerance in skin allografts. Mast cells play an important role in peripheral tolerance in a manner that is dependent on CD4⁺, CD25⁺, Fox3P⁺ T^reg in skin and heart transplants (Lu et al. 2006). Using a model of hapten-induced atopic dermatitis (AD), Hershko et al. (2011) showed that mast cell-derived IL-2 is necessary to support an adequate ratio of activated-to-regulatory T cells at the site of inflammation during the chronic phase of disease. Moreover, OX40L (CD252)-expressing mast cells interact directly with OX40-expressing T^reg. This interaction inhibits mast cell degranulation and calcium mobilization without affecting cytokine secretion, thus reducing the amplitude of immediate hypersensitivity responses (Gri et al. 2008; Piconese et al. 2009; Frossi et al. 2011).

Mast cell proteases also contribute to immune tolerance in that they reduce antigenicity and leukocyte recruitment through cleavage of antigens, toxic peptides, cytokines, and chemotactic factors (Mellon et al. 2002; Pang et al. 2006; Rauter et al. 2006; Thakurdas et al. 2007; Rauter et al. 2008; Waern et al. 2009). Even though histamine is generally viewed as a proinflammatory mediator, its binding to histamine receptor 2 (H2R) mediates immune suppression as seen in mast cell-dependent, H2R-dependent immune suppression in response to UVB radiation (McGlade et al. 2007). In addition, there was a gradual loss of H2R expression on mast cells in lupus-like lesions (Furukawa et al. 2009). The lipid mediator PGE2 also seems to have a role in immune suppression. Jet fuel immune suppression was impaired upon deletion of PGE2 from mast cells (Furukawa et al. 2009). Moreover, PGE2 induces IL-10 production in DCs and appears to inhibit DC maturation. These indirect effects can be blocked by cyclooxygenase (COX)-2 inhibitors and NSAIDs (non-steroidal anti-inflammatory drugs), which inhibit the synthesis of prostaglandins (Harizi and Gualde 2006; Sinha et al. 2007; Sá-Nunes et al. 2007; Lee et al. 2009).

Allergy

Allergies arise when components of the immune system, particularly mast cells, respond in an inappropriate manner to innocuous antigens. Mast cells are recognized as the main effector cell responsible for IgE-mediated allergic reactions. Sensitization is the primary immune response in which allergens are recognized, processed, and presented by APCs to naive T lymphocytes that recognize the allergen as foreign and differentiate into Th2 lymphocytes. Th2 lymphocytes produce cytokines that then induce antigen-specific IgE production by B lymphocytes. Mast cells also have the capacity to process and present antigens through MHCI and MHCII complexes. Therefore, mast cells themselves have a role in sensitization. Furthermore, there is evidence that they coordinate and direct Th2 responses toward innocuous antigens (Eisenbarth et al. 2002; Nigo et al. 2006).

Initial studies on the relationship between mast cells and allergic reactions, including asthma, have focused on the acute phase of these reactions. FcϵRI activation by a polyvalent allergen that is recognized by the receptor-bound IgE, leads to an immediate hypersensitivity reaction characterized by the instantaneous release of pre-formed and neoformed mast cell mediators. These mediators are responsible for allergic symptoms such as erythema, edema, increased vascular permeability, smooth muscle contraction, and augmented mucus secretion (Hofmann and Abraham 2009). The immediate release of histamine, PGD₂, and LTC₄ contributes to the symptoms of asthma, causing bronchoconstriction, mucus secretion, and respiratory mucosal edema. However, allergic reactions are complex and multiphasic. In addition to the immediate acute phase, there is also a late phase. In the later phases, proinflammatory cytokines released by mast cells are responsible for the recruitment of inflammatory cells such as eosinophils, basophils, and T cells to the site of inflammation and also contribute to the development of the chronic phase (Bradding 1999; Hofmann and Abraham 2009). The late phase is characterized by leukocyte infiltration at the site of inflammation and the initiation of an acquired immune response. This is followed by a chronic phase associated with persistent inflammation, tissue remodeling and fibrosis. These phases are observed in several allergic disorders including asthma, allergic rhinitis and atopic dermatitis, among others (Williams and Galli 2000; Grimbaldeston et al. 2006; Brown et al. 2008).

Crohn’s Disease

Crohn’s disease is a chronic inflammatory intestinal disease, with the involvement of immune cells. However, there is no confirmation of an autoimmune etiology. This condition can affect any part of the gastrointestinal tract and causes diverse symptoms. Histologically, a perivascular distribution of lymphocytic infiltrates and the presence of occasional granulomas can be observed (Whithead 1980). Stricture and fibrosis of gut, often resulting in partial or complete obstruction, is a common finding in Crohn’s disease. In Crohn’s disease, the mast cells are redistributed and found in the muscle layers of the stricture, which has led to the suggestion that mast cells and their mediators may play a role in stricture formation (Dvorak et al. 1980a; Dvorak et al. 1980b; Gelbmann et al. 1999). There is also evidence of increased expression of IL-16 in Crohn’s disease. This cytokine can be produced and released by mast cells, indicating a possible association between mast cell activation and CD4⁺ T lymphocyte recruitment during the inflammatory response. Moreover, mast cells are located near blood vessels and mast cell-released IL-16 can recruit circulating lymphocytes from the blood stream (Middel et al. 2001).

Autoimmune Diseases

When the immune system fails to recognize self from non-self molecules, self-reactive lymphocytes can be activated by innate immune cells and mount an autoimmune response. It is widely accepted that mast cells can promote increased inflammation in several disease states. In accord with this view, mast cells are able to stimulate the priming of autoreactive T cells and recruit immune cells to the site of inflammation. The inflammatory environment favored by mast cells can induce T cell activation. In this context, mast cells can act in concert with T cells to cause tissue damage (Christy and Brown 2007).

There are several examples of mast cell association with autoimmune diseases including: Type I diabetes, Guillain-Barré syndrome, bollous pemphigoid, Sjogren syndrome, chronic idiopathic urticaria and experimental vasculitis (Wintroub et al. 1978; Yamamoto et al. 1995; Dines and Powell 1997; Geoffrey et al. 2006; Ishii et al. 2009; Saini et al. 2009). Much of the interest on the role of mast cells in the initiation and propagation of autoimmune diseases comes from studies on multiple sclerosis (MS) and its experimental model, allergic encephalitis (EAE) (Steinman 2001; Nigrovic and Lee 2007).

MS is a chronic inflammatory disease of the nervous system of unknown etiology, characterized by a deterioration of the blood-brain barrier, with consequent mononuclear cell infiltration into the white matter and eventual demyelination of axons. EAE depends on inflammatory CD4⁺ Th17 cells, B cells, and antibodies produced by these cells (Weaver et al. 2006). Many studies suggest a positive correlation of mast cell numbers and distribution with the development of MS or EAE (Orr 1988; Brenner et al. 1994; Ibrahim et al. 1996; Dines and Powell 1997; Brown et al. 2002). The observation of increased degranulation and the presence of tryptase in the cerebrospinal fluid provides evidence for an increase in mast cell activation during the course of the disease (Brenner et al. 1994; Rozniecki et al. 1995). Drugs known to stabilize mast cells, such as sodium cromoglycate, seem to relieve the severity of EAEs (Seeldrayers et al. 1989). Mast cell function in this context appears to be dependent on the surface binding of IgGs, because disease progression relies on the expression of FcγR by mast cells (Brown et al. 2002). Kit^W/W-v and Kit^W-sh were reported to be more resistant than wild type mice to the myelin oligodendrocyte glycoprotein (MOG) peptide-induced EAE (Secor et al. 2000).

Another autoimmune disease involving mast cells is rheumatoid arthritis, a chronic inflammatory disease of the joints. The cause of this disease may be associated with the enzyme glucose-6-phosphate isomerase (GPI) (Matsumoto et al. 1999; Zhang et al. 2011). A widely used model for rheumatoid arthritis is the K/BxN mouse. These mice express both the T cell receptor transgene KRN and the MHCII molecule A(g7). They produce auto-antibodies recognizing GPI and also develop a severe inflammatory arthritis. Serum from these mice causes a similar arthritis in a wide range of mouse strains. The antibodies form aggregates with GPI leading to the deposition of immune complexes on the surface of the articular cavity. These complexes initiate a signaling cascade that involves neutrophils as well as mast cells. The complement pathway, FcRs, and cytokines such as IL-1 and TNF are all involved (Ravetch and Bolland 2001; Ji et al. 2002b; Ji et al. 2002a; Hueber et al. 2010). Notably, FcγRIII activation by immune complexes has been implicated as an important event for the development of RA in this model (Corr and Crain 2002; Ji et al. 2002b; Nigrovic et al. 2007). Kit^W/W-v mice deficient in mast cells are resistant to autoimmune inflammatory arthritis induced by injection of sera from K/BxN mice and the mast cell reconstitution of these animals restores their sensitivity. However, mast cell-reconstituted Kit^W-sh mice are still susceptible to arthritis induced by sera from K/BxN mice (Lee et al. 2002a; Zhou et al. 2007).

Mast cells accumulate in the synovial tissues and fluids of patients with rheumatoid arthritis and produce inflammatory mediators. In fact, mast cell degranulation in the articular cavity is one of the first events observed after antibody administration (Lee et al. 2002a). Results using this model also show that activation of mast cells through the IgG immune complex receptor FcγRIII can precipitate the initiation of inflammation within the joint through the production and release of IL-1 (Nigrovic et al. 2007). In addition, mast cell-derived TNF-α, can induce fibroblasts to produce SCF, which increases the recruitment of mast cells and creates an amplification loop (Woolley and Tetlow 2000; Benoist and Mathis 2002).

Despite mounting evidence of the involvement of mast cells in these autoimmune disease models, Feyerabend et al. (2011), using the mouse strain Cpa3^Cre/+, which is deficient exclusively in mast cells, found no evidence for an active role of mast cells both in the K/BxN serum transfer model of RA and the EAE model of MS. In fact, the precise contribution of mast cells to the pathophysiology of autoimmune diseases remains a matter of great debate (reviewed in Brown and Hatfield 2012).

Mastocytosis

Mastocytosis are disorders characterized by the clonal accumulation of mast cells and their products in organs such as skin, gastrointestinal tract, bone marrow, liver, spleen, and lymph nodes (Horny et al. 2008). They are usually caused by activating mutations of the c-Kit receptor (Metcalfe 2008; Deho’ and Monticelli 2010). Mastocytosis presents many variants, which display a range of symptoms and prognoses. The two main variants are cutaneous mastocytosis (CM) and systemic mastocytosis (SM), which are based on disease distribution and clinical manifestation. Clinical manifestations of this pathology include pruritus, flushing, nausea, vomiting, diarrhea, and vascular instability (Metcalfe 2008). CM is most common in children and presents in three forms: (i) urticaria pigmentosa or maculopapular mastocytosis; (ii) diffuse cutaneous mastocytosis, and (iii) mastocytomas. SM is characterized by the involvement of at least one extracutaneous organ, even in the absence of skin lesions. There are many variants of SM, including indolent systemic mastocytosis, bone marrow mastocytosis, mastocytic leukemia, and mastocytic sarcoma (WHO 2008). A diagnosis of mastocytosis is based on histological confirmation of mast cell accumulation, whereas classification of systemic mastocytosis is contingent on the correlation between clinical and laboratory evaluations (Valent et al. 2001). Mast cell metachromatic granules can be observed with Giemsa and toluidine blue staining. However, tissue processing can diminish mast cell granule staining, which is typically less prominent in abnormal neoplastic mast cells. Due to these difficulties immunophenotypic studies become a more suitable choice in the diagnosis of mastocytosis (Li 2001). Neoplastic mast cells express CD33, CD43, CD68, CD117 and tryptase, of which tryptase is the only marker exclusive to mast cells (Valent et al. 1992; Yang et al. 2000; Miettinen and Lasota 2005; Chiu and Orazi 2012). Moreover, neoplastic mast cells usually express CD2 and/or CD25, which are not expressed in normal mast cells (Jordan et al. 2001; Escribano et al. 2002; Sotlar et al. 2004; Krokowski et al. 2005).

The prognosis depends on the type of mastocytosis. Although childhood and adult-onset mastocytosis are both associated with activating mutations, the course of the disease is very different. Children often present a skin-limited disease that regresses with age, whereas adults generally present with persistent multi-organ involvement that is often accompanied by a second non-mast cell hematologic neoplasm (Pardanani 2012).

Cardiovascular Disease

Increasing evidence implicates cardiac mast cells in coronary disease. Cardiac mast cells participate in the development of atherosclerosis, coronary inflammation, and cardiac ischemia (Patella et al. 1995). These types of mast cells are more evident in the adventitial tunic of coronary arteries during spasm (Forman et al. 1985) and accumulate in the angular region of atherosclerotic plaques (Kaartinen et al. 1994; Constantinides 1995). In the heart, chymase is the main source of the converting enzyme that produces the coronary constrictor angiotensin II (Jenne and Tschopp 1991). Both chymase and tryptase released by mast cells induce proteolytic changes in high-density lipoprotein (HDL) particles, which interfere with cholesterol efflux by macrophages leading to the formation of foam cells that constitute the atheroma (Lindstedt et al. 1996; Lee et al. 2002b; Lee et al. 2003).

Cancer

The tumor microenvironment is comprised of fibroblasts, myofibroblasts, ECM, existing and newly formed blood vessels, and inflammatory cells. The relationship between mast cells, inflammation, and cancer is contradictory and consists of both promotion of and protection against tumor progression. Mast cell accumulation is typically observed around rodent and human tumors. This accumulation is associated with a poor prognosis in various cancers and suggests an involvement of mast cells in tumor progression (Takanami et al. 2000; Conti et al. 2007). However, the opposite has been observed in some breast cancers (Dabiri et al. 2004). Mast cells are recruited by tumor-derived factors. One of these factors, SCF, induces mast cell infiltration and activation, with the consequential release of inflammatory mediators that participate in tissue remodeling and immune suppression (Huang et al. 2008). The role of mast cells in cancer promotion includes immunosuppression, the release of pro-angiogenic and mitogenic factors, and degradation of the ECM (Ch’ng et al. 2006). Tumor histamine content correlates positively with mast cell numbers in breast carcinomas (Bowrey et al. 2000). Histamine can simultaneously stimulate tumor proliferation through its interaction with histamine H1 receptors and suppress the immune system through H2 receptors, thus contributing to carcinogenesis (Conti et al. 2007). Mast cell modulation of the immune response through the release of histamine, IL-10, and TNF-α, contributes to tumor growth. In colorectal carcinoma, mast cells may counteract the anti-inflammatory function of regulatory T cells, and mast cell-mediated immunosuppression may contribute to the development of basal cell carcinoma (Hart et al. 2001; Blatner et al. 2010). It is believed that mast cells participate in tumorigenesis through the release of pro-angiogenic factors such as heparanase, angiopoetin-1, TNF, FGF-2, VEGF, and IL-18 in addition to mast cell-specific proteases that assist in ECM degradation and subsequent tumor invasion (Ribatti et al. 2001; Maltby et al. 2009). Stimulation of angiogenesis is probably the most important function of mast cells in the promotion of tumor growth (Dyduch et al. 2012). VEGF, FGF-2, TGF-β, TNF-α, and IL-18 are all potent pro-angiogenic factors. A role for mast cell-specific proteases has also been proposed (Muramatsu et al. 2000b; Muramatsu et al. 2000a; Tóth-Jakatics et al. 2000; Norrby,2002; Feoktistov et al. 2003; Yoshii et al. 2005). It has recently been observed that mast cells, through the action of their specific proteases, are involved in the initial phases of tumor growth and also in modulating vascular growth in the later stages of tumor progression (de Souza et al. 2012). Mast cells also secrete proteases such as MMP-2 and MMP-9, which digest ECM and, together with heparin, stimulate heparin-binding pro-angiogenic factors in the tumor microenvironment, thus influencing tumor progression and metastasis (Coussens et al. 1999; Baram et al. 2001; Norrby 2002). In mast cell-deficient mice, tumor induction was accompanied by reduced angiogenesis and metastatic capacity (Ribatti et al. 2001).

The anti-neoplastic effects of mast cells include inhibition of cell growth, an augmented inflammatory anti-tumor reaction, induction of apoptosis, and decreased cell mobility (Dyduch et al. 2012). TNF-α, IL-1, and IL-6 were reported to suppress melanoma growth, and prostacyclin, which is produced by endothelial cells in response to histamine, is a potent anti-metastatic factor (Dyduch et al. 2012). IL-6 production and release in response to TLR-2 activation was shown to inhibit tumor growth both in vivo and in vitro (Oldford et al. 2010). Furthermore, eosinophil recruitment and survival, promoted by mast cell tryptase and IL-5, respectively, leads to tumor regression (Maltby et al. 2009).

The clinical relevance of the mast cell/tumor relationship remains to be discovered. Nonetheless, mast cells have been shown to be involved in tumor progression and neoangiogenesis in several cancer types (Takanami et al. 2000; Benítez-Bribiesca et al. 2001; Grimbaldeston et al. 2004; Ribatti et al. 2005; Yoshii et al. 2005; Ch’ng et al. 2006; Diaconu et al. 2007; Nonomura et al. 2007; Fleischmann et al. 2009; Carlini et al. 2010; Johansson et al. 2010; Ribatti et al. 2010).

In conclusion, mast cells are ancient cells whose ancestor is a urochordate mast cell-like cell 550-million years old. Although mammalian mast cells were first described more than a century ago, their detailed functions still remain to be elucidated. Today, mast cells are considered to be multifunctional immune cells implicated in several physiological and disease states. As a consequence of their widespread location and the mediators or the pathogens they interact with, mast cells exhibit a high degree of heterogeneity and plasticity. It is increasingly evident that mast cell maturation, phenotype, and function are dictated by the local microenvironment which has a significant influence on ability of mast cells to recognize and respond to stimuli.

The widespread tissue distribution of mast cells and their versatility allow them to respond to harmful situations as a first-response and respond to environmental changes through the interactions with other cells implicated in physiological and immunological responses. Their ubiquitous distribution places mast cells in a privileged position to act not only as guardians of the immune system, but to also participate in many biological processes and in the maintenance of homeostasis. Mast cells have both immunomodulatory as well as physiological functions. It is currently acknowledged that mast cells modulate innate and adaptive immune responses, both directly and indirectly, through communication with other immune cells. Moreover, mast cells are able to modulate immune responses through their array of mediators, surface molecules, and co-stimulatory molecules.

During the lifetime of a mast cell, numerous factors can alter its phenotype and a combination of these changes can determine mast cell homeostatic or pathophysiological responses. Those features that provide mast cells with the ability to interact with the microenvironment are the same ones that, when inadequately regulated, can have serious consequences to the organism. Mast cell contributions for many disease states are thus the focus of continuous assessment.