The human mast cell☆☆☆★

Human mast cells are a heterogeneous group of multifunctional tissue-dwelling cells with roles in conditions as diverse as allergy, parasite infestation, inflammation, angiogenesis, and tissue remodeling. The cells were initially named mastzellen (Gk mastos breast) in 1878 by Paul Ehrlich because he believed that the intracellular granules, which appeared purple in color when stained with aniline blue dyes, contained phagocytosed materials or nutrients. This change in color, or metachromasia, we now know represents the interaction of the dyes with the highly acidic heparin contained within mast cell granules. In a series of elegant drawings, Ehrlich also gave us our first clues to the diverse roles of mast cells, describing their association with inflamed tissues, blood vessels, nerves, and neoplastic foci. The study of mast cell development is notoriously difficult because mast cells cannot be readily identified until they mature in the tissues and express their characteristic granules and high-affinity receptor for IgE (FcϵRI). However, it is believed that they are hemopoietic in origin, entering the circulation from the bone marrow as mononuclear cell precursors, which may only be tentatively recognized by their cytoplasmic expression of messenger RNA for stem cell factor, their primary growth factor, and the presence of extracellular membrane receptors for stem cell factor, the so called c-kit or Steel factor receptor. From the blood, the precursors migrate into the tissues where, under the influence of local microenvironmental factors, they undergo their final phases of differentiation and maturation into recognizable mast cells. It should be pointed out at this stage that, although originally believed to be circulating mast cells, basophils are more closely related to eosinophils, developing in the bone marrow from granulocyte precursors and entering the circulation only when fully mature. Immunocytochemical studies by Schwartz et al. have shown the presence within the tissues of two mast cell phenotypes distinguishable by their neutral protease content, the MC T phenotype containing only tryptase and the MC TC phenotype containing both tryptase and chymase (Fig. 1). Initially, these respective subtypes were suggested to be the equivalents of the “mucosal” and “connective tissue” previously described in experimental animals. However, it is now realized that variable amounts of both mast cell subtypes are present within any given tissue; their relative abundance changes with disease (e.g., in allergy or fibrosis). However, some rules are becoming apparent, as shown in Fig. 1. Thus MC T phenotypes appear to be “immune system–related” mast cells with a primary role in host defense, whereas MC TC phenotypes appear to be “non-immune system–related” mast cells with functions in angiogenesis and tissue remodeling rather than immunologic protection. However, it should be remembered that both phenotypes express FcϵRI and may therefore participate fully in IgE-dependent allergic or parasitic reactions. In addition, there appears to be a functional heterogeneity between mast cells of different tissues. This appears to be largely unrelated to immunocytochemical heterogeneity. For example, it has been shown by Valent et al. that the mast cells of the skin express CD88, the receptor for the anaphylatoxin C5a, allowing them to be activated in complement-mediated disease. In addition, skin mast cells alone respond to a variety of basic nonimmunologic secretagogues, including neuropeptides and drugs such as morphine, codeine, and muscle relaxants; the response to these drugs in particular explains the flushing reactions observed in sensitive individuals in the absence of overt rhinorrhoea or bronchoconstriction. The secretory granule of the human mast cell contains a crystalline complex of preformed inflammatory mediators (Fig. 2) ionically bound to a matrix of proteoglycan. When mast cell activation occurs, the granules swell and lose their crystalline nature as the mediator complex becomes solubilized; and the individual mediators including histamine, proteases, and proteoglycans are expelled into the local extracellular environment by a process of compound exocytosis. The mediator most readily associated with the mast cell, the simple diamine histamine, is present in the granules at ~100 mmol/L, equivalent to about 1 to 4 pg/cell. Histamine exerts many effects pertinent to the immediate phase of allergic responses, including vasodilatation, increased vasopermeability, contraction of bronchial and intestinal smooth muscle, and increased mucus production. However, these effects are normally of relatively short duration because histamine is rapidly metabolized, usually within 1 to 2 minutes, by histamine-N-methyltransferase (~70%) and by diamine oxidase (histaminase) (~30%). Studies by Metcalfe et al. have shown the dominant proteoglycan in human mast cells to be heparin, which constitutes some 75% of the total, with a mixture of chondroitin sulfates making up the remainder. Human heparin is composed of a single-chain 17.6 kDa peptide core containing a region of alternating Ser-Gly residues to which the glycosaminoglycan side chains are attached. Within the granule, proteoglycan may be viewed as a storage matrix because the acidic sulfate groups of the glycosaminoglycans provide binding sites for the other preformed mediators. Once released, heparin and, to a lesser extent, chondroitin sulfate, may affect the stability or function of other mast cell mediators (e.g., heparin stabilizes the active tetramer of tryptase). Other actions include anticoagulant, anticomplement, and antikallikrein effects; the ability to sequester eosinophil major basic protein; enhancement of collagen binding to fibronectin; and numerous growth factor–enhancing activities. The major mast cell protease, present in all mast cells regardless of subtype, is tryptase, a ~130 kd tetrameric serine protease encoded on chromosome 16, which is stored in a fully active form in the granule. Recently, Schwartz et al. have shown that there are at least two distinct forms of tryptase with 90% amino acid sequence homology: α-tryptase, which appears to be enzymatically inactive, and the enzymatically active β-tryptase. Interestingly, the present Pharmacia immunoassay for tryptase is selective for β-tryptase, the form predominating in allergic reactions, and does not readily detect α-tryptase, the form usually associated with mastocytosis. When released into the extracellular environment, the neutral pH allows tryptase to become enzymatically functional. The properties of tryptase include: (1) cleavage of the bronchodilator peptides, vasoactive intestinal polypeptide, peptide histidine methionine, and the vasodilator calcitonin gene-related peptide but not the bronchoconstrictor neuropeptide substance P, giving rise to hypothesis that an imbalance of airway neuropeptides may contribute to bronchial hyperresponsiveness in asthma; (2) sensitization of bronchial smooth muscle to contractile agents; (3) a kallikrein-like activity; cleavage of matrix components including 75 kDa gelatinase/type IV collagen, fibronectin, and type VI collagen and activation of stromelysin, which may in turn cleave other matrix components; (4) mitogenic activity for fibroblasts and epithelial cells; (5) stimulation of the release of the granulocyte chemoattractant IL-8; (6) upregulation of intercellular adhesion molecule-1 expression on epithelial cells. Although there appear to be no endogenous inhibitors of tryptase, it is likely to have only very local effects because, in the absence of heparin, the biologically active tetrameric form of tryptase rapidly dissociates into inactive monomers. Chymase is a 30 kDa monomeric protease encoded on chromosome 14, which, although present only in the MC TC subset of mast cells, is stored in the same secretory granules as tryptase. Like tryptase, chymase is stored within the granule in its fully active form so that it needs no further processing before release. Chymase degrades the neuropeptide neurotensin, but not substance P or vasoactive intestinal polypeptide. It cleaves angiotensin I to angiotensin II more effectively than angiotensin-converting enzyme, an action that has led to much interest in chymase from cardiac mast cells in the control of the vasculature in coronary disease. Chymase may also contribute to the role of mast cells in tissue remodeling by cleaving type IV collagen and splitting the dermal-epidermal junction. Actions pertinent to mucosal inflammation include the conversion of IL-1β to IL-1, the degradation of IL-4, and the stimulation of secretion from submucosal gland cells. Two other proteinases, carboxypeptidase and cathepsin G, have been associated with the MC TC subset of human mast cells. Carboxypeptidase is a unique 34.5 kDa metalloproteinase, which removes the carboxyl terminal residues from a range of peptides including angiotensin, leu-enkephalin, kinetensin, neuromedin N, and neurotensin. Cathepsin ≥ is a chymotryptic enzyme with a structure seemingly identical to that of neutrophil cathepsin G. Interestingly, the genes encoding mast cell chymase and cathepsin ≥ are closely linked on chromosome 14q11.2; the mast cell transcribes both, and neutrophils only transcribe the latter. When mast cells are activated, chymase, carboxypeptidase, and cathepsin ≥ are released together in a 400 to 500 kDa complex with proteoglycan and are likely to act in concert with the other enzymes to degrade proteins. Immunologic activation of mast cells induces the liberation of membrane-derived arachidonic acid, a fatty acid, which is then primarily oxidized through either cyclooxygenase (COX) to form prostaglandin (PG)D 2 or 5-lipoxygenase to form leukotriene (LT)C 4. In the absence of definitive evidence, we had always considered COX and 5-lipoxygenase to be located on the plasma membrane and the eicosanoids generated directly into the extracellular environment. Recently, however, this assumption has been proved to be incorrect: the enzymes have been shown to be associated with the nuclear envelope instead. Thus both prostaglandins and LTs are initially generated into the cytoplasm of the cells in which they are formed, raising the possibility of intracellular actions, which have not hitherto been considered. There are now known to be two distinct COX enzymes: COX1 is expressed constitutively, and COX2 is the inducible form. In mast cells, Austen et al. have found COX1 to be responsible for the early burst of PGD 2 associated with mast cell activation, whereas COX2 is associated with the prolonged generation of the prostanoid, which occurs later in the allergic response. With respect to LT synthesis, Peters-Golden et al. have shown recently that on cell activation, 5-lipoxygenase translocates from the cytoplasm and becomes associated with its activating protein, 5-lipoxygenase–activating protein, on the nuclear envelope where it catalyzes the formation of the reactive intermediate, LTA 4. Furthermore, Austen et al. have also recently localized the gene encoding LTC 4 synthase, the enzyme responsible for the further processing of LTA 4 to LTC 4, to long arm chromosome 5q, the chromosome encoding the genes for the cytokines IL-3, IL-4, IL-5, IL-13, and granulocyte-macrophage colony-stimulating factor for which upregulation has been suggested to be associated with allergic disease. PGD 2 is a potent bronchoconstrictor agent, which is rapidly degraded to another bronchoconstrictor agent, 9α,11β-PGF 2. Both of these substances are believed to exert the majority of their bronchoconstrictor actions by the occupation of thromboxane receptors. In addition, PGD 2 is chemokinetic for human neutrophils, augments LTB 4-induced neutrophilia, and is a powerful inhibitor of platelet aggregation. LTC 4 is made by a variety of inflammatory cells in the lungs, including mast cells and eosinophils. In the extracellular environment, LTC 4 is metabolized to the active LTD 4 and then to the inactive LTE 4; the presence of the latter in the urine is an indicator of active allergic disease. The effects of the LTs include potent contraction of bronchial and vascular smooth muscle, enhanced permeability of postcapillary venules, increased bronchial mucus secretion, and eosinophil chemoattraction, most of which are mediated by LTD 4. Because of their potent effects on the airways and their possible upregulation in patients with allergy, LTs are regarded as important molecules in the pathogenesis of asthma in particular. In addition to the above, the mast cell has been suggested to produce small amounts of LTB 4, thromboxane B 2, PGE 2, and platelet activating factor, which are important factors in chemotaxis, bronchoconstriction, and vasopermeability. Cytokines, generated by both resident and freshly recruited cells, are responsible for the initiation and coordination of many local processes, including allergic inflammation and tissue remodeling. In IgE-dependent allergic inflammation, it would be logical to expect that the spectrum cytokines generated would be directed to initiating and maintaining allergic inflammation. Although all cells involved in the allergic response, and in particular T lymphocytes, contribute to the cytokine pool, studies in the United States by Galli et al. and Schulman et al. and in England by Church et al. have shown that the human mast cell generates IL-4 and IL-13, which are involved in switching the B lymphocyte to IgE production. IL-5 and granulocyte-macrophage colony-stimulating factor (which attract, prime, and prolong the life of eosinophils) and tumor necrosis factor-α, a key cytokine in allergic inflammation, are involved in initiating NF-κB upregulation of endothelial and epithelial adhesion molecules and chemokine secretion, priming leukocytes for mediator secretion, and increasing bronchial responsiveness to inhaled provocants. Synthesis and secretion of all of these cytokines are upregulated after FcϵRI activation. In addition, mast cells generate IL-3 and IL-6 (both proinflammatory cytokines), and IL-8, RANTES, and IL-5, which are involved in granulocyte chemotaxis and activation. The ability of every sensitized mast cell to respond to stimulation with any individual allergen, as opposed to T cells in which only the small percentage of cells specific to that allergen respond, makes mast cells potentially important cytokine-generating cells in allergy. Furthermore, the presence within mast cells of preformed cytokines, which is not the case with T cells, suggests that they are available for rapid secretion on cell stimulation. Mast cells are the recognized key cells of type I hypersensitivity reactions. However, their ubiquitous distribution throughout both serosal and mucosal tissues, and, in particular, their close proximity to blood vessels makes the “pharmacopoeia” of their mediators available to a large variety of cell types including vascular endothelial cells, smooth muscle, nerves, glands, cells of the immune system, and fibroblasts. Consequently, mast cells have been suggested to play roles, either major or minor, not only in allergy but in conditions as diverse as rheumatoid arthritis, inflammatory bowel disease, interstitial cystitis, progressive systemic sclerosis, chronic graft-versus-host disease, pulmonary and hepatic fibrosis, sarcoidosis, asbestosis, silicosis, coronary artery spasm, scarring and keloids, scleroderma, neuromata and cancer (particularly angiogenesis in solid tumors). In allergy, the release of mast cell mediators by allergen has long been acknowledged to be the initiating step of the early-phase response. This is supported by the many studies showing increased levels of histamine and, more recently, tryptase in bronchoalveolar lavage fluid in allergic asthma, in nasal washings in rhinitis, in tears in conjunctivitis, and in the circulation in systemic anaphylaxis. In addition, mast cells—particularly through their generation of LTs, proteases, and cytokines—are critical in the initiation and control of allergic inflammation. The role of mast cells in host defense, particularly in preventing parasite re-infestation, has been the subject of much detailed research during the last decade. More recently, two letters to Nature by Echtenacher et al. and Malaviya et al. have suggested a critical protective role for mast cells in bacterial infection. These studies, which were performed in mice, suggest that mast cell–derived tumor necrosis factor-α is responsible for both the recruitment of neutrophils and bacterial clearance. A bidirectional interrelationship between mast cells, particularly MC TC phenotypes, and fibroblasts has been postulated in both tissue remodeling and fibrosis. In healthy tissue remodeling there is a balance between fibrolytic and fibrogenic activity; whereas fibrosis is characterized by an excessive production of collagen, and eventually other extracellular matrix (ECM) components, caused by either increased fibroblast activity, fibroplasia, or both. Histamine enhances both fibroblast proliferation and collagen synthesis; tryptase and chymase can digest ECM components, activate latent collagenase, and stimulate fibroblast proliferation; and mast cell–derived cytokines such as tumor necrosis factor-α, IL-4, and basic fibroblast growth factor have effects on fibroblast growth and digestion of the ECM. In addition, mast cells may produce type VIII collagen, a nonfibrillar short-chain collagen, which forms a molecular scaffolding in the ECM. In the opposite direction, it is now becoming apparent that fibroblasts may regulate the replication, adherence, viability, phenotype, and functional activity of mast cells by the production of stem cell factor and other cytokines. The possible upregulation of this bidirectional positive feedback has obvious implications for chronic inflammation. In this article we have provided a small insight into the biology of the human mast cell, which is pertinent to allergic disease, and illustrated some of the diverse roles of this multifunctional cell in other conditions. However, the mechanism of mast cell activation and, therefore, the quantity and spectrum of mediators released may differ in different situations, thus making it possible for this single cell to subserve a wide variety of biologic functions.