IgE-mediated anaphylactic degranulation of isolated human skin mast cells.

Isolated human skin mast cells (HSMC) were prepared and cultured overnight before functional and electron microscopic studies. Mast cell suspensions were examined after stimulation with anti-IgE to produce anaphylactic degranulation or examined in buffer-incubated controls. Histamine release was measured in replicate samples. Control, isolated HSMC studied by electron microscopy were well preserved and fully granulated. Although all granule patterns reported for human mast cells were found, crystal granules were the most prevalent, as is true for HSMC in situ. Individual mast cells containing both crystal and scroll granules occurred. Lipid bodies were rare, as in HSMC in situ. Control, isolated mast cells did not express granule changes associated with either piecemeal degranulation or recovery during wound healing in situ; nor were morphologic changes of anaphylactic degranulation present. Spontaneous histamine release was 0% in control samples. Anaphylactic degranulation of isolated HSMC was accompanied by 24% maximum histamine release and characteristically showed extrusion of altered, membrane-free granules through multiple pores in the plasma membrane to the exterior of the cell. Other morphologic aspects of anaphylactic degranulation, as expressed in isolated human lung mast cells, were also present. These events included granule swelling, fusion, alteration of matrix contents, degranulation channel formation, pore formation, and shedding of granules, membranes, and surface processes. The ultrastructural morphology of isolated HSMC and their IgE-mediated degranulation shows some differences from similar studies of isolated human lung mast cells and of human lung and gut mast cells in biopsy samples. These differences include crystal granules as the predominant granule pattern, minor numbers of lipid bodies, and extrusion of granules during anaphylactic degranulation as characteristic for HSMC. By contrast, isolated human lung and gut mast cells have more scroll granules and particle granules, respectively, and more lipid bodies. In isolated human lung mast cells, anaphylactic degranulation is almost exclusively an intracellular fusion event characterized by the formation of complex degranulation channels within which altered granule matrix materials solubilize. In addition to morphologic differences between mast cells of skin, lung, or gut origin, functional differences have also been reported among mast cells of these organs. The ultrastructural morphology of isolated HSMC is identical to that of skin mast cells in biopsy samples, thereby validating the usefulness of this new source of HSMC for correlative functional and morphologic studies.     

Effect of alveolar hypoxia on pulmonary mast cells in vivo

This study was undertaken to explore the effects of alveolar hypoxia on perivascular and periairway mast cell populations. Pulmonary mast cells were exposed to unilateral alveolar hypoxia by ventilating one lung of a cat with nitrogen. Mast cells from the contralateral lung, which was simultaneously ventilated with air, were used as a control. The granule content of perivascular and airway mast cells was determined from electron micrographs using morphometric methods. In response to alveolar hypoxia there was a 12% (p less than 0.005) decrease in the granule content of perivascular mast cells but no statistically significant change in periairway mast cell content. Perivascular mast cells from the hypoxic lungs did not show any of the morphological changes seen in IgE-mediated mast cell degranulation, such as granule swelling, fusion, or exocytosis. In the hypoxic lung, morphometric analysis revealed a significant decrease in the proportion of heterogeneous-appearing granules in the perivascular mast cells. The different reactivity of perivascular and periairway mast cells may explain why alveolar hypoxia does not induce significant bronchospasm.     

Role of adenylate cyclase in immunologic release of mediators from rat mast cells: agonist and antagonist effects of purine- and ribose-modified adenosine analogs.

The initial monophasic rise in cyclic AMP beginning 5-15 sec after bridging of rat mast cell IgE-Fc receptors precedes the secretion of granule constituents, thereby implying a causal relationship. Direct evidence for a relationship between IgE-dependent transmembrane activation of adenylate cyclase and granule secretion was provided by the capacity of purine-modified (R site active) and ribose-modified (P site active) adenosine analogs, respectively, to augment and suppress mediator release while simultaneously increasing and decreasing the activity of adenylate cyclase. R site stimulation alone does not cause granule secretion but augments the rate and magnitude of IgE-Fc receptor-induced secretion, reflecting the coupled relationship of such receptors. Inhibition of adenylate cyclase at the P site attenuates the rise in cellular cyclic AMP and suppresses IgE-dependent mediator release in a parallel and superimposable dose-response fashion. Further, the relationship between the attenuation in the rise in cyclic AMP and the diminution in immunologic mediator release is linear with the regression line passing through the origin, indicating a direct relationship between the IgE-dependent activation of adenylate cyclase and preformed mediator release. Although not the only events in coupled mast cell activation--secretion, there is a sequential relationship among perturbation of IgE-Fc receptors, transmembrane activation of adenylate cyclase, elevation of cytoplasmic levels of cyclic AMP, activation of cyclic AMP-dependent protein kinase, and secretion of mast cell granules.     

Elevated histamine and tryptase levels in smokers' bronchoalveolar lavage fluid. Do lung mast cells contribute to smokers' emphysema?

Human lung mast cells have been reported recently to contain small amounts of the elastolytic protease present in the neutrophil and implicated in the pathogenesis of alveolar wall destruction in emphysema. Since mast cells are numerous within alveolar walls, release of inflammatory mediators (and possibly elastase) by cigarette smoking could contribute to alveolar injury in this disease. We therefore examined bronchoalveolar lavage (BAL) fluid for the mast cell granule constituents histamine and tryptase. The results, while not conclusive, supported the possibility that cigarette smoking increases secretion of histamine releasing activity by alveolar macrophages with subsequent degranulation of local mast cells. Mast cell discharge of inflammatory mediators (including neutrophil chemotactic factors and perhaps the elastolytic protease) could then participate in the destruction of alveolar walls.     

Adenosine potentiates mediator release from human lung mast cells

Micromolar concentrations of adenosine were found to potentiate the release of histamine and leukotriene C4 (LTC4) from immunologically activated human lung mast cells (HLMC). Structurally modified congeners of adenosine including 5'-N-ethylcarboxamideadenosine (NECA) and R-phenylisopropyladenosine (R-PIA) also potentiated mediator release. A rank order of potency was established where NECA greater than R-PIA for the potentiation of both LTC4 production and histamine secretion. Mast cells isolated by either enzymatic or mechanical means from human lung parenchyma were both similarly responsive to the modulatory effects of adenosine and analogues, and the potency series of NECA greater than R-PIA also applied. Moreover, histamine release induced by the calcium ionophore A23187 was augmented by NECA, R-PIA, and adenosine and in that potency order. Dipyridamole, an agent thought to impede the intracellular uptake of adenosine, failed to reverse the nucleoside's enhancement of IgE-mediated secretion. The irreversible inhibitor of adenosine deaminase, deoxycoformycin, did not modify the adenosine enhancement of stimulated secretion. Low concentrations of methylxanthines, which antagonize responses mediated at cell surface adenosine receptors, were inconsistent in their effects. Theophylline modestly reversed the adenosine-induced potentiation of IgE-mediated LTC4 generation but not histamine release. Studies employing 8-phenyltheophylline were complicated by the methylxanthine possessing inhibitory properties of its own at concentrations expected to antagonize a nucleoside-mediated effect. In total, these results suggest that the response of HLMC to adenosine describes properties most consistent with an A2/Ra-like process, although an interaction via an, as yet, uncharacterized cell surface receptor cannot be excluded.     

Immunoglobulin E-mediated release of a kininogenase from purified human lung mast cells

Certain manifestations of the allergic response, such as vasodilation and edema, may be attributed to the involvement of kinins, but, as yet, little data exist to show how these peptides could be produced during IgE-mediated events. We now report that purified human lung mast cells contain a kininogenase that is released in a dose-dependent fashion by anti-IgE. Release of kinin-generating activity parallels that of histamine and, like histamine release, kininogenase release is temperature-dependent. Kininogenase release also parallels increasing histamine release when mast cells of increasing size are stimulated with an optimal level of anti-IgE. Finally, when mast cells of greater than 99% purity were optimally challenged, kininogenase activity again paralleled histamine levels in the release supernatant and residual cell pellet. The mast cell kininogenase appears to be a preformed mediator in that it occurs in lysates of unstimulated mast cells of greater than 99% purity. Optimal generation of kinin from highly purified human low molecular weight kininogen occurred at pH 5.5 and in 8 preparations was 380 +/- 237 ng kinin/h/10(6) mast cells (means +/- SD). Thus, human lung mast cells contain a kininogenase that is released during, and may participate in, IgE-mediated inflammatory disorders in humans.     

Stimulation of mast cells leads to cholesterol accumulation in macrophages in vitro by a mast cell granule-mediated uptake of low density lipoprotein.

The uptake of low density lipoprotein (LDL) by cultured mouse macrophages was markedly promoted by isolated rat mast cell granules present in the culture medium. The granule-mediated uptake of LDL enhanced the rate of cholesteryl ester synthesis in the macrophages, the result being accumulation of cholesteryl esters in these cells. Binding of LDL to the granules was essential for the granule-mediated uptake of LDL by macrophages, for the uptake process was prevented by treating the granules with avidin or protamine chloride or by treating LDL with 1,2-cyclohexanedione, all of which inhibit the binding of LDL to the granules. Inhibition of granule phagocytosis by the macrophages with cytochalasin B also abolished the granule-mediated uptake of LDL. Finally, mouse macrophage monolayers and LDL were incubated in the presence of isolated rat serosal mast cells. Stimulation of the mast cells with compound 48/80, a degranulating agent, resulted in dose-dependent release of secretory granules from the mast cells and a parallel increase in cholesteryl ester synthesis in the macrophages. The results show that, in this in vitro model, the sequence of events leading to accumulation of cholesteryl esters in macrophages involves initial stimulation of mast cells, subsequent release of their secretory granules, binding of LDL to the exocytosed granules, and, finally, phagocytosis of the LDL-containing granules by macrophages.     

The role of mast cells in asthma

Mast cells store or generate a number of substances, such as histamine, leukotrienes, specific chemotactic factors, and proteases, which have potent inflammatory effects. Mast cells are present in mammalian lung in the bronchial wall, in the epithelium itself, and in airway lumens. Evidence implicating mast cells in the induction of airway muscle constriction and mucosal inflammation includes direct studies of mast cell secretion, measurement of histamine release, and inhibition of asthmatic reactions with antihistamines and with the inhibitor of mast cell degranulation, sodium cromoglycate. Definitive evaluation of the role of the mast cell in asthma remains elusive, in part because of the general hyperactivity of airway smooth muscle in asthmatics.     

Vesicular transport of histamine in stimulated human basophils

Human basophils participating in experimentally produced contact allergy display progressive secretion of electron-dense secretory granule contents and retention of cytoplasmic granule containers in the absence of entire granule extrusion, a process termed piecemeal degranulation (PMD) and postulated to be effected by vesicular transport (Dvorak HF, Dvorak AM: Clin Hematol 4:651, 1975). Proof of this hypothesis was sought using models of human basophil-stimulated secretion, partially purified human peripheral blood basophils, and a morphometric analysis of the fraction of total cellular cytoplasmic vesicles loaded with histamine, a major proinflammatory mediator present in basophil secretory granules. The subcellular localization of histamine was accomplished using a new ultrastructural enzyme-affinity- gold method based on the affinity of diamine oxidase for its substrate, histamine (Dvorak et al: J Histochem Cytochem 41:787, 1993). Two models were selected for a kinetic analysis of stimulated vesicle transport of histamine based on known biochemical and ultrastructural characteristics (MacGlashan et al: J Immunol 136:2231, 1986; Warner et al: J Leukoc Biol 45:558, 1989; Dvorak et al: Am J Pathol 141:1309, 1992; Dvorak et al: Lab Invest 64:234, 1991). These models were selected to include the rapid release reaction stimulated by the bacterial peptide, FMLP, and the slow release reaction stimulated by the phorbol diester tumor promoter, TPA. The results of this study showed that the fraction of histamine-loaded cytoplasmic vesicles (%VG/TV/micron2) in TPA-stimulated basophils significantly exceeded the fraction in unstimulated cells, a process that persisted for 45 minutes after TPA stimulation and was associated with extensive PMD and no morphologic evidence of recovery. Similarly, the fraction of histamine- loaded cytoplasmic vesicles after FMLP stimulation significantly exceeded the fraction in unstimulated cells, a process that persisted for 10 minutes after FMLP stimulation and was associated with the morphologic continuum of PMD-->anaphylactic degranulation (characterized by extrusion of granules)-->recovery, a process largely complete in the 10-minute samples. These studies establish for the first time that an important proinflammatory mediator, histamine, traffic from secretory granules to the extracellular milieu in small cytoplasmic vesicles in stimulated human basophils. The association of this process with the ultrastructural release reaction defined as PMD produced primarily by TPA and in part by FMLP establishes vesicular transport as the mechanism for effecting this type of regulated secretion. Vesicular transport of histamine was also significant in the more complex stimulated secretory and recovery model produced by exposure of human basophils to the bacterial peptide FMLP.     

OxLDL-IgG immune complexes induce expression and secretion of proatherogenic cytokines by cultured human mast cells

ObjectiveHuman atherosclerotic lesions contain mast cells and immunoglobulin G immune complexes containing oxidized low-density lipoproteins (oxLDL-IgG ICs). Here we studied whether such oxLDL-IgG ICs can activate human mast cells and induce them to express and secrete pro-inflammatory cytokines that are potentially capable of inducing and amplifying atherogenic processes.Methods and resultsIncubation of cultured human mast cells in the presence of oxLDL-IgG ICs led to a significant dose-dependent upregulation of the expression and secretion of tumor necrosis factor-alpha (TNF-a) and interleukin-8 (IL-8), and the chemotactic cytokine monocyte chemoattractant protein-1 (MCP-1). The secretory responses were dose-dependent and associated with moderate release of histamine and tryptase, which are preformed mast cell mediators contained in the cytoplasmic secretory granules of the cells. Also native LDL-IgG ICs induced similar pro-inflammatory cytokine response, suggesting that ICs per se are important for the IgG IC-induced mast cell activation.ConclusionMast cells in atherosclerotic lesions which also contain oxLDL-IgG ICs may become activated by the ICs and secrete many pro-inflammatory cytokines. Our results suggest that intimal mast cells act as a cellular link between oxLDL-IgG ICs and the inflammatory response in atherosclerosis.     

Stem cell factor and interleukin-4 induce murine bone marrow cells to develop into mast cells with connective tissue type characteristics in vitro

In this study, we have developed a method to obtain mast cells with connective tissue type mast cell (CTMC) characteristics directly from mouse bone marrow (BM) cells. BM cells were grown for 3 weeks in presence of interleukin-4 (IL-4) plus stem cell factor (SCF). SCF alone poorly supported growth and development of mast cells. IL-4 dose-dependently enhanced the expression of c-kit and high-affinity receptor for IgE (Fc(epsilon)RI) on the cell surface of SCF-cultured BM cells. Furthermore, cytoplasmic granulation and histamine synthesis of BM-derived mast cells were increased in presence of IL-4 and SCF. Histochemical staining demonstrated that granules were safranin positive. BM-derived mast cells could be activated for granule exocytosis (beta-hexosaminidase release) and lipid mediator generation (LTC4 production) via Fc(epsilon)RI after sensitization with IgE and subsequent crosslinking with multivalent antigen. In addition, mast cells derived from BM cells cultured with SCF plus IL-4 could be activated by substance P, a nonimmunologic stimulus, to release beta-hexosaminidase. The results presented indicate that IL-4 and SCF both have a prominent role in the development of mast cells from murine BM cells in vitro. Mast cells can directly be derived from BM cells in presence of SCF and IL-4 and the cultured cells show typical hallmarks of CTMC, indicating that precursor cells for CTMC may be present in BM. The described culture procedure may be useful to investigate the molecular aspects of the development of committed mast cell lineages.     

Studies on the exocytosis of cultured mast cells derived from mouse bone marrow

Mast cells were obtained from mouse bone marrow cells cultured for 14 days in medium derived from Concanavalin A (Con A) stimulated mouse spleen cells. Upon passive sensitization of the cultured cells with immunoglobulin E (IgE), histamine release from mast cells was approximately 200% above control within 1 min of incubation with anti-IgE. The calcium inonophore A 23187 also evoked a concentration-dependent (10(-8) M to 6 x 10(-7) M) histamine release following a 6 min incubation. Transmission electron microscopy (TEM) demonstrated that the secretory granules of the cultured cells have a peripheral crystalline pattern, like that previously demonstrated for mast cells. Scanning electron microscopy (SEM) illustrated spherical cells with surfaces traversed by many ridge-like folds. Intragranular fusion, following exposure of the IgE-bearing cells to anti-IgE, led to accumulation of the granules into channels which, on study with both transmission and scanning electron microscopy, appeared to be associated with the cell surface.     

The role of mast cells in allergic inflammation

The histochemical characteristics of human basophils and tissue mast cells were described over a century ago by Paul Ehrlich. When mast cells are activated by an allergen that binds to serum IgE attached to their Fc?RI receptors, they release cytokines, eicosanoids and their secretory granules. Mast cells are now thought to exert critical proinflammatory functions, as well as potential immunoregulatory roles, in various immune disorders through the release of mediators such as histamine, leukotrienes, cytokines chemokines, and neutral proteases (chymase and tryptase). The aim of this review is to describe the role of mast cells in allergic inflammation. Mast cells interact directly with bacteria and appear to play a vital role in host defense against pathogens. Drugs, such as glucocorticoids, cyclosporine and cromolyn have been shown to have inhibitory effects on mast cell degranulation and mediator release. This review shows that mast cells play an active role in such diverse diseases as asthma, rhinitis, middle ear infection, and pulmonary fibrosis. In conclusion, mast cells may not only contribute to the chronic airway inflammatory response, remodeling and symptomatology, but they may also have a central role in the initiation of the allergic immune response, that is providing signals inducing IgE synthesis by B-lymphocytes and inducing Th2 lymphocyte differentiation.     

Mast cells possess distinct secretory granule subsets whose exocytosis is regulated by different SNARE isoforms

Mast cells degranulate and release the contents of intracellular secretory granules in response to the cross-linking of FcepsilonRI by multivalent antigens. These granules contain a variety of biologically active inflammatory mediators; however, it is not clear whether these granules are homogenous or whether there is heterogeneity within the secretory granule population in mast cells. By using genetically altered mice lacking specific vesicle-associated SNARE membrane fusion proteins, we found that VAMP-8-deficient mast cells exhibited defects in FcepsilonRI-regulated exocytosis, whereas synaptobrevin 2- or VAMP-3-deficient mast cells did not. Surprisingly, the defect in secretion in VAMP-8-deficient mice was limited to the subpopulation of mast cell secretory granules containing serotonin and cathepsin D, whereas regulated exocytosis of secretory granules containing histamine and TNF-alpha was normal. Confocal microscopy confirmed that serotonin and histamine were present in distinct intracellular granules and that most serotonin-containing granules were VAMP-8-positive. Thus, this study demonstrates that mast cells do indeed possess distinct subsets of secretory granules and that these subsets use different SNARE isoforms for exocytosis.     

Characterization of human synovial mast cells

Human synovium obtained at arthroplasty from patients with rheumatoid arthritis (RA) and osteoarthritis (OA) were characterized by assessing mast cell morphology, content and function. Histological studies confirmed significant numbers of mast cells in both RA and OA synovium. Electron microscopic data support the morphologic similarity between human synovial mast cells and human mast cells in lung and intestine. Likewise, synovial mast cells do not appear to be functionally different from pulmonary or intestinal mucosal mast cells. Mast cell suspensions with a cellular histamine content of 4.3 +/- 0.5 pg/cell (mean +/- SEM) released histamine following provocation with anti-IgE and calcium ionophore but not compound 48/80, f-met peptide or bradykinin. Prostaglandin D2 (PGD2) and leukotriene C4 (LTC4) were also released in response to anti-IgE. Auranofin inhibited anti-IgE provoked histamine, PGD2 and LTC4 release while gold sodium thiomalate, cromolyn and indomethacin had no effect on histamine release. Theophylline inhibited anti-IgE induced histamine release only at concentrations greater than or equal to 10(-3) M. Our study argues against functional or morphologic mast cell heterogeneity of human intestinal, lung and synovial origin and suggests that mast cells may have a pathogenic role in both RA and OA.     

Activation of human lung mast cells by monomeric immunoglobulin E.

The mechanism of chronic mast cell activation in asthma is unclear. Monomeric immunoglobulin (Ig)E in the absence of allergen induces mediator release from rodent mast cells, indicating a possible role for IgE in the continued activation of mast cells within the asthmatic bronchial mucosa. In this study it was investigated whether monomeric IgE induces Ca2+ influx and mediator release from human lung mast cells (HLMC). Purified HLMC were cultured for 4 weeks and then exposed to monomeric human myeloma IgE. Ratiometric Ca2+ imaging was performed on single fura-2-loaded cells. Histamine release was measured by radioenzymatic assay; leukotriene C4 (LTC4) and interleukin (IL)-8 were measured by ELISA. At concentrations experienced in vivo, monomeric IgE induced dose-dependent histamine release, LTC4 production and IL-8 synthesis. This was associated with a rise in cytosolic free Ca2+. Enhanced histamine release was still evident 1 week after initial exposure to IgE suggesting that continued exposure maintains enhanced secretion. Monomeric immunoglobulin E alone activates cultured human lung mast cells initiating Ca2+ influx, degranulation, arachidonic acid metabolism and cytokine synthesis. These findings support the hypothesis that immunoglobulin E loading of mast cells within the asthmatic airway contributes to the disordered airway physiology of this disease.     

Interaction between hyperosmolar and IgE-mediated histamine release from basophils and mast cells

Because both hyperosmolar and IgE-dependent stimuli may be encountered in the airway, and because hyperosmolarity causes histamine release in basophils, we examined the effects of the 2 stimuli on human lung mast cells. Mast cells prepared by enzymatic digestion of human lung were suspended in buffers made hyperosmolar with mannitol. Significant histamine release was seen above 360 mOsm/kg H2O, increasing to 11.9 +/- 1.0% at 770 mOsm/kg H2O, and release was synergistically enhanced by anti-IgE. Cells that had been rendered unresponsive to IgE-dependent stimuli by exposure to anti-IgE in the absence of Ca++ became markedly more responsive to hyperosmolar stimulation, and released as much as 32 +/- 2% histamine in hyperosmolar buffers alone. Antigen-induced histamine release from the basophils of allergic donors was also synergistically enhanced in buffers above 460 mOsm/kg H2O. These data show that immunologic and nonimmunologic stimuli may interact, and that human lung mast cells are capable of mediator release when exposed to osmolarities that may occur in the airway, especially during hyperventilation. Hyperosmolar mediator release is a plausible mechanism by which exercise-induced hyperventilation might induce asthma.     

New insights into the immunoregulatory functions of mast cells

Mast cells, which are preferentially located in connective tissues and epithelial layers, are now recognized as effector cells that participate in innate and acquired immunity. Upon activation with various secretagogues, mast cells release a large number of mediators stored in their secretory granules which consist of inflammatory mediators, cytokines, proteoglycans and neutral proteases. In addition to soluble mediators, mast cell granules have recently been shown to harbour small vesicles with immunoregulatory properties. Isolated exosomes have been shown to activate B and T lymphocytes and act as potent adjuvants for specific antibody responses in vivo. In this article I will discuss the mechanisms by which mast cells fulfill immunoregulatory functions that may be beneficial for the host.     

Human synovial mast cells. I. Ultrastructural in situ and in vitro immunologic characterization

Objective. To examine the ultrastructure of human synovial mast cells in situ, to identify immunologic and nonimmunologic stimuli that activate these cells in vitro, and to quantify a number of preformed and de novo–synthesized mediators. Methods. We conducted an ultrastructural study of synovial mast cells in situ and performed immunoelectron microscopy localization of tryptase and chymase. Isolated synovial mast cells were analyzed biochemically, immunologically, and functionally in vitro and compared with cells from human lung, heart, and skin. Results. Ultrastructural study of synovial tissue revealed mast cells with homogeneously dense, scrolled, crystal, and mixed granules, and lipid bodies in the cytoplasm. A small percentage of mast cells showed evidence of degranulation. Immunoelectron microscopy demonstrated the subcellular localization of tryptase and chymase over granules of >90% of the mast cells, which were of the MC_TC subtype. Isolated synovial mast cells released histamine in response to immunologic (anti-IgE and anti-Fcε receptor I [anti-FcεRI]) and nonimmunologic (substance P, recombinant human stem cell factor, and 48/80) stimuli, but did not respond to recombinant human C5a in vitro. Synovial mast cells differed from those isolated from other human tissues, in a variety of immunologic and biochemical features. There was a linear correlation between the percentage of histamine secretion and tryptase release (r = 0.79, P < 0.001) induced by cross-linking of FcεRI. Cross-linking of IgE with anti-IgE on synovial mast cells induced de novo synthesis of prostaglandin D₂ (mean ± SEM 87.5 ± 4.9 ng/10⁶ cells) and of leukotriene C₄ (57.6 ± 17.8 ng/10⁶ cells). Conclusion. Mast cells ultrastructurally characterized in situ in synovial tissue were seen to differ from mast cells previously isolated from other human tissues. This raises the possibility that the local microenviroment influences their phenotype. Isolation of mast cells from human synovia can be useful for studying their role and their mediatrors in patients with arthritis.