Within seconds to minutes of IgE crosslinking, granules in the cytoplasm of the mast cell fuse with each other and with the cell surface membrane, ejecting their contents into the extracellular milieu. The contents of the granules depend on the conditions under which the mast cell has matured, but include histamine, proteoglycans (for example heparin), and a series of neutral proteases broadly grouped into tryptases, chymases, and carboxy-peptidases. Histamine promotes vascular permeability; proteoglycans provide a scaffold within the granule that allows the packaging of proteases; and the neutral proteases cleave proteins from matrix and plasma in addition to activating propeptides such as the precursors for interleukin-1β (IL-1β) and angiotensin II. The tryptase mMCP6 (murine mast cell protease 6) also contributes potently to neutrophil chemotaxis 11. Certain subsets of mast cells store tumor necrosis factor (TNF) within the granules as well, representing the body's only source of TNF available for immediate release 12.

Within minutes of IgE-mediated activation, mast cells begin to generate eicosanoids derived from cleavage of arachidonic acid from membrane phospholipids 13. Important arachidonic acid metabolites include the leukotrienes (leukotriene B₄ and the cysteinyl leukotrienes), which increase vascular permeability, induce vasoconstriction and recruit leukocytes, and prostaglandins including the neutrophil chemoattractant and vasoactive mediator prostaglandin D₂.

Within hours, a later phase of mast cell activation through IgE becomes evident with the induction of new gene transcription and translation, generating a host of cytokines and chemokines (Table 1). The mix of cytokines generated by a particular mast cell depends on its individual state of differentiation.

The importance of IgE-mediated mast cell activation to the health of the organism is still incompletely defined. The preservation of this system under evolutionary pressure, despite allergic diseases and anaphylaxis, is strong suggestive evidence that there is benefit to the host. One likely candidate function is resistance to parasitic disease, because mice deficient in IgE exhibit impaired defense against the helminths Schistosoma mansoni and Trichinella spiralis 1415.

The physiological importance of mast cells in defense against bacteria has been clearly demonstrated. Mast-cell-deficient W/W^v mice have impaired clearance of bacterial infection in the peritoneum 1718 and lung 18, accompanied by markedly higher mortality after experimental infection. This vulnerability was found to be associated with decreased infiltration of neutrophils to the site of infection and could be corrected by reconstitution with wild-type mast cells. Within an hour of peritoneal infection, lavage fluid shows a striking increase in TNF levels in the presence of mast cells. Anti-TNF treatment largely abrogates the effect of mast cell reconstitution, whereas injection of TNF concurrent with infection substantially mimics the benefits of reconstitution in mast-cell-deficient mice. Although mast cells can phagocytose and kill bacteria 19, the results imply that the critical role of mast cells in these models is not direct anti-bacterial action but the generation of TNF and other mediators (such as leukotrienes 20) that recruit neutrophils and possibly other cells to contain the infection.

Mast cells possess multiple mechanisms to detect bacterial invasion. These include Toll-like receptors (TLRs) 1, 2, 4, and 6, CD48 (a receptor for a Gram-negative fimbrial protein), and receptors for anaphylatoxins C3a and C5a and the complement opsonin iC3b 2122232425. Interestingly, mast cells triggered by means of these mechanisms seem capable of responses that are substantially more differentiated than those unleashed through IgE/FcεR1. In contrast to the wholesale 'anaphylactic' degranulation that characterizes maximal IgE-mediated stimulation, bacteria can trigger a gradual and partial (so-called 'piecemeal') degranulation proportional to the stimulus 1926. The production of lipid mediators and cytokines/chemokines seems also to be tailored to the event, and can even be entirely decoupled from the release of granule contents (reviewed in 27).

An important consequence of mast cell activation may be the mobilization of adaptive immunity. Mast cell leukotriene B4 recruits memory CD4⁺ and CD8⁺ T cells, which can then be activated locally by mast cells presenting phagocytosed peptides via both MHC class II and MHC class I molecules 28293031. Mast cells might also potentiate de novo antigen-specific responses by promoting the migration of dendritic cells to lymph nodes and recruiting circulating naive T cells to these nodes by means of TNF and macrophage inflammatory protein-1β (MIP-1β) 83233. Although the ultimate physiological importance of each of these defensive capabilities remains to be established, it seems probable that antimicrobial efficacy accounts at least in part for the remarkable evolutionary conservation of the mast cell.

As noted, mast cells express receptors for IgG as well as IgE. These include FcγR2b and FcγR3a, low-affinity IgG receptors involved principally in the response to immune complexes and other constellations of colocalized IgG molecules. Under certain conditions, mast cells can also express the high-affinity receptor FcγR1 34. These receptors permit mast cells to participate in humoral defense, but they also enable a role for mast cells in antibody-induced pathology. Thus, in a mouse model of peritonitis induced by intraperitoneal injection of antibody against an antigen injected intravenously (the reverse passive Arthus reaction), peritoneal mast cells exposed to immune complexes release a burst of preformed TNF and recruit neutrophils 35. Similarly, in an analogous skin model, mast cells have been shown to potentiate the response to antibody administered subcutaneously against an antigen delivered systemically 36. Optimal mast cell participation in this reaction requires a functional complement system, suggesting that complement fixation by immune complexes provides an important auxiliary signal to mast cells, in particular via C5a 37. A related phenomenon is observed in a model of bullous pemphigoid: subcutaneous administration of an antibody against the hemidesmosomal antigen BP180 induces inflammatory attack, resulting in lysis of the dermal–epidermal junction. In the absence of mast cells or complement, inflammation is markedly attenuated 3839. As in bacterial peritonitis, the key function of mast cells in these models of antibody-mediated pathology seems to be the mobilization of neutrophils, because the wild-type phenotype can largely be rescued in mast-cell-deficient animals with injection of neutrophils or neutrophil chemotactic factors.

In the models discussed so far, the principal function of mast cells seems to be to 'jump start' the immune response, in particular to initiate the rapid recruitment of inflammatory cells. Structurally, the mast cell is uniquely equipped for this task, with its capacity for the immediate release of preformed mediators and the rapid elaboration of lipid mediators. However, the mast cell's activity does not end with this initial response. Mast cells continue to elaborate cytokines for hours after a single stimulus, and a degranulated mast cell can recharge and fire again 4041. Some mast cell mediators have effects such as the promotion of angiogenesis, whose relevance is more evident after the acute inflammatory response 42. Further, mast cells accumulate at sites of chronic inflammation, prima facie evidence that their role is not restricted to the initiation of immune responses; examples include the gut in inflammatory bowel disease or helminthic infection, the asthmatic airway, sclerodermatous skin, and lung in interstitial pulmonary fibrosis 43444546. Though no pathogenic role has yet been definitively assigned to the mast cell in these conditions, potential functions include ongoing recruitment of inflammatory cells, stimulatory effects on stromal cells resulting in fibrosis, and the development of new blood vessels. It is also conceivable that mast cells might in some cases limit or otherwise modulate local inflammation, although no data to this effect are available. Particular proinflammatory mechanisms are discussed below in detail as they pertain to the potential role of the synovial mast cell in arthritis.

The rheumatoid synovium is thick with infiltrating leukocytes. These include T lymphocytes, B lymphocytes, macrophages, mast cells and scattered neutrophils. Ongoing recruitment of these cells results from the upregulation of selectins and integrins on synovial endothelium, allowing migration up chemotactic gradients into the joint. The composition of inflammatory cells recruited in a continuing fashion by mast cells, including the degree of skewing of lymphocytes toward Th1 versus Th2 responses, might be an important determinant of the ultimate outcome of inflammation. The production of anti-inflammatory mediators by mast cells remains uncharacterized 81.

Prominent within the rheumatoid synovium is a greatly expanded population of synovial macrophages. These cells do not proliferate locally but instead are recruited from circulating monocytes 82. Mast cells are potent sources of chemokines that mediate this recruitment, including IL-8, monocyte chemoattractant protein-1, MIP-1α, and RANTES 3. Mast cells might also contribute to the activation of these macrophages through the production of interferon-γ and IL-6. Because macrophages are major sources of the proinflammatory cytokines TNF and IL-1 within the joint, mast cell effects on the size and activation state of the synovial macrophage population might functionally modulate the course of inflammatory arthritis.

The synovial mesenchyme, consisting principally of synovial fibroblasts, is prominently involved in joint inflammation. Fibroblasts increase greatly in numbers and assume a histological appearance suggestive of increased synthetic activity, with expansion of the endoplasmic reticulum and increased numbers of granules in the cytoplasm 83. Indeed, synovial fibroblasts make up the shroud-like pannus characteristic of the rheumatoid joint and are an important source of multiple mediators implicated in arthritis. These include degradative enzymes such as collagenase and stromelysin and proinflammatory molecules including IL-1, IL-6, and prostaglandin E₂ (reviewed in 84). They contribute to the differentiation and activation of osteoclasts, the effector cell responsible for bone erosions, through the production of macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB ligand (RANKL) 8586.

Mast cells may potently influence synovial fibroblast biology in RA. Consistent with a proposed role in wound healing and in multiple fibrotic disease states, mast cells produce a range of mediators with powerful effects on fibroblasts (Table 1) 87. Further, synovial mast cells are often noted in close physical proximity to synovial fibroblasts 50. Mast cell tryptase promotes chemotaxis and collagen synthesis in fibroblasts, and histamine stimulates fibroblast proliferation 888990. Other fibroblast mitogens produced by mast cells include nerve growth factor, basic fibroblast growth factor, platelet-derived growth factor, vascular endothelial growth factor (VEGF), and transforming growth factor-β (TGF-β) 91. The cytokine IL-4, produced predominantly by mast cells of a tryptase–chymase phenotype, induces proliferation and collagen production by fibroblasts 92, and indeed, as noted above, MC_TC cells tend to reside in more fibrotic areas of the inflamed joint. Because leukotriene C₄ seems to have antifibrotic effects, it remains possible that mast cells can limit as well as promote fibrosis, although scattered foci of fibrosis associated with mast cell infiltrates in systemic mastocytosis suggest a net profibrotic effect 919394.

Mast cells may also potentiate mediator production by synovial fibroblasts through the elaboration of cytokines such as TNF and IL-1. IL-1 induces the elaboration of collagenase and prostaglandin E₂, and TNF elicits similar responses while also inducing synovial fibroblasts to generate IL-1 959697. Indeed, the production of collagenase and other inflammatory products of fibroblasts has been noted to localize to the immediate environment of activated mast cells 98.

This communication between mast cells and synovial fibroblasts is bidirectional. Mast cells require stimulation by SCF for differentiation in situ as well as activation 6. Fibroblasts in inflamed or healing tissues express higher levels of SCF, and upregulation of SCF expression has been noted in synovial specimens exposed to TNF 99100101. Indeed, such surface expression seems to be of particular importance to mast cell development, because Sl/Sl^d mice unable to display surface-bound SCF lack tissue mast cells despite an intact production of soluble SCF 102103. Further, transwell experiments demonstrate that physical contact is required for certain stimulatory effects of fibroblasts on mast cells 104105. Fibroblasts might also promote the survival of mast cells by means of SCF-independent pathways yet to be fully defined 106.

In addition to fibroblasts, the synovial mesenchyme also contains blood vessels. As would be expected, the expanded cellular population in the inflamed synovium requires an enhanced blood supply, and neoangiogenesis has an important pathophysiological function in RA. Mast cell mediators implicated in the promotion of angiogenesis include heparin, vascular endothelial growth factor, TGF-β, TNF, IL-1, and IL-18 42107. Further, TNF can induce synovial fibroblast production of another pro-angiogenic factor, angiopoietin-1 108. Though the ultimate importance of mast cells in synovial angiogenesis remains unclear, the association of mast cells with blood vessels, including newly developing blood vessels, makes the promotion of angiogenesis a plausible role for mast cells in vivo (reviewed in 109).

Finally, some data suggest that mast cell mediators might exert a direct effect on cartilage and bone. Thus, whereas the coculture of chondrocytes with inactive mast cells tends to promote the synthesis of proteoglycans, the activation of mast cells in this context favors proteoglycan degradation 110. Further, the activation of chondrocytes via IL-1, TNF, and histamine might induce the production of matrix metalloproteinases and prostaglandins 111112. Finally, mast cell mediators including histamine and MIP-1α might directly promote the differentiation and activation of osteoclasts, the final common pathway of bone destruction in inflammatory arthritis 113114115. Corroboration in vivo will be required to establish the importance of these in vitro findings.