Mast cells and inflammation

Mast cells are well known for their role in allergic and anaphylactic reactions, as well as their involvement in acquired and innate immunity. Increasing evidence now implicates mast cells in inflammatory diseases where they are activated by non-allergic triggers, such as neuropeptides and cytokines, often exerting synergistic effects as in the case of IL-33 and neurotensin. Mast cells can also release pro-inflammatory mediators selectively without degranulation. In particular, IL-1 induces selective release of IL-6, while corticotropin-releasing hormone secreted under stress induces the release of vascular endothelial growth factor. Many inflammatory diseases involve mast cells in cross-talk with T cells, such as atopic dermatitis, psoriasis and multiple sclerosis, which all worsen by stress. How mast cell differential responses are regulated is still unresolved. Preliminary evidence suggests that mitochondrial function and dynamics control mast cell degranulation, but not selective release. Recent findings also indicate that mast cells have immunomodulatory properties. Understanding selective release of mediators could explain how mast cells participate in numerous diverse biologic processes, and how they exert both immunostimulatory and immunosuppressive actions. Unraveling selective mast cell secretion could also help develop unique mast cell inhibitors with novel therapeutic applications. This article is part of a Special Issue entitled: Mast cells in inflammation.     

Mast cells and inflammation

Mast cells are well known for their role in allergic and anaphylactic reactions, as well as their involvement in acquired and innate immunity. Increasing evidence now implicates mast cells in inflammatory diseases where they are activated by non-allergic triggers, such as neuropeptides and cytokines, often exerting synergistic effects as in the case of IL-33 and neurotensin. Mast cells can also release pro-inflammatory mediators selectively without degranulation. In particular, IL-1 induces selective release of IL-6, while corticotropin-releasing hormone secreted under stress induces the release of vascular endothelial growth factor. Many inflammatory diseases involve mast cells in cross-talk with T cells, such as atopic dermatitis, psoriasis and multiple sclerosis, which all worsen by stress. How mast cell differential responses are regulated is still unresolved. Preliminary evidence suggests that mitochondrial function and dynamics control mast cell degranulation, but not selective release. Recent findings also indicate that mast cells have immunomodulatory properties. Understanding selective release of mediators could explain how mast cells participate in numerous diverse biologic processes, and how they exert both immunostimulatory and immunosuppressive actions. Unraveling selective mast cell secretion could also help develop unique mast cell inhibitors with novel therapeutic applications. This article is part of a Special Issue entitled: Mast cells in inflammation.     

P2X7 receptors induce degranulation in human mast cells

Mast cells play important roles in host defence against pathogens, as well as being a key effector cell in diseases with an allergic basis such as asthma and an increasing list of other chronic inflammatory conditions. Mast cells initiate immune responses through the release of newly synthesised eicosanoids and the secretion of pre-formed mediators such as histamine which they store in specialised granules. Calcium plays a key role in regulating both the synthesis and secretion of mast-cell-derived mediators, with influx across the membrane, in particular, being necessary for degranulation. This raises the possibility that calcium influx through P2X receptors may lead to antigen-independent secretion of histamine and other granule-derived mediators from human mast cells. Here we show that activation of P2X7 receptors with both ATP and BzATP induces robust calcium rises in human mast cells and triggers their degranulation; both effects are blocked by the P2X7 antagonist AZ11645373, or the removal of calcium from the extracellular medium. Activation of P2X1 receptors with αβmeATP also induces calcium influx in human mast cells, which is significantly reduced by both PPADS and NF 449. P2X1 receptor activation, however, does not trigger degranulation. The results indicate that P2X7 receptors may play a significant role in contributing to the unwanted activation of mast cells in chronic inflammatory conditions where extracellular ATP levels are elevated.     

The influence of anaerobic bacteria and mycoplasma isolated from the genital tract on mast cell activation

Nowadays, it is known that mast cells, numerously appearing in all organs and being a source of a wide range of mediators and cytokines, are involved both in physiological and pathological processes. The aim of our study was to examine whether vaginal bacteria, especially those participating in Bacterial vaginosis, are able to activate mast cells to mediators secretion. The study was done on rat peritoneal mast cells. The mast cells were incubated in vitro with suspensions of Bacteroides capillosus, Actinomyces naeslundii (2 strains), Peptostreptococcus spp., Lactobacillus fermentum (2 strains), Mycoplasma hominis or Ureaplasma urealyticum killed by temperature. Activation of mast cells was estimated on the basis of histamine release. It was established that M. hominis, U. urealyticum and B. capillosus strongly stimulated rat mast cells to histamine secretion (histamine release 53.0%, 17.4% and 10.0%, respectively). Histamine release induced by Peptostreptococcus spp., A. naeslundii and L. fermentum was lower (at a range of 2.4%-8.2%). The obtained results can suggest that presumably interactions between vaginal bacteria and placental mast cells could influence the course of pregnancy.     

Prostaglandin Production by Mast Cells and the Regulation of Airway Smooth Muscle Responses

The role of mast cell activation and the generation of mediator release are important factors in determining the reactivity of lung tissue during allergic reactions and asthma. In this article, the advantages of our procedure for isolating mast cells from lung tissue over other methods are discussed. Our studies have demonstrated the importance of cross talk between mast cells and other cell types that also release mediators of bronchoconstriction during activation, thus amplifying the response. In addition, it has been found that inositol phospholipid turnover in response to various mast cell mediators is enhanced in hyperresponsive lung tissue. The consequences to the contractility of the tissue of such alterations in the cellular signaling system in lung tissue are discussed.     

The role of PAR1 in protective action of activated protein C during nonimmune activation of mast cells

Activated protein C (APC) regulates the functional activity of mast cells by reducing release of β-hexosaminidase, the marker of mast cell degranulation. APC modulated not only spontaneous secretion from mast cells, but also secretion induced by the degranulators, proteinase-activated receptor agonist peptide (PAR1-AP) and compound 48/80. PAR1 desensitization by thrombin abolished the decrease of β-hexosaminidase secretion induced by low APC concentrations (≤1.5 nM). APC inactivated by phenylmethylsulfonyl fluoride (PMSF), did non mimic the enzyme action on mast cells. Duodenase (the duodenal proteinase) activated peritoneal mast cell via PAR1. APC abolished the proinflammatory effect of duodenase and PAR1-AP by reducing release of mast cell mediators. The effect of APC could be attributed to nitric oxide generation by mast cells because in the presence of L-NAME the secretory function restored. These data suggest involvement of mast cell PAR1 into regulatory mechanism responsible for the anti-inflammatory effect of APC.     

Mast Cells Release Chemokine CCL2 in Response to Parkinsonian Toxin 1-Methyl-4-Phenyl-Pyridinium (MPP<Superscript>+</Superscript>)

Microglial activation and release of inflammatory cytokines and chemokines are crucial events in neuroinflammation. Microglial cells interact and respond to other inflammatory cells such as T cells and mast cells as well as inflammatory mediators secreted from these cells. Recent studies have shown that neuroinflammation causes and accelerates neurodegenerative disease such as Parkinson’s disease (PD) pathogenesis. 1-methyl-4-phenyl-pyridinium ion (MPP⁺), the active metabolite of neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydro pyridine activates glial cells and mediate neurodegeneration through release of inflammatory mediators. We have shown that glia maturation factor (GMF) activates glia and induces neuroinflammation and neurodegeneration and that MPP⁺ activates mast cells and release proinflammatory cytokines and chemokines. The chemokine (C-C motif) ligand 2 (CCL2) levels have been shown to be elevated and play a role in PD pathogenesis. In the present study, we analyzed if MPP⁺ activates mouse and human mast cells to release chemokine CCL2. Mouse bone marrow-derived mast cells (BMMCs) and human umbilical cord blood-derived cultured mast cells (hCBMCs) were incubated with MPP⁺ (10 µM) for 24 h and CCL2 levels were measured in the supernatant media by ELISA. MPP⁺-significantly induced CCL2 release from BMMCs and hCBMCs. Additionally, GMF overexpression in BMMCs obtained from wild-type mice released significantly more CCL2, while BMMCs obtained from GMF-deficient mice showed less CCL2 release. Further, we show that MPP⁺-induced CCL2 release was greater in BMMCs–astrocyte co-culture conditions. Uncoupling protein 4 (UCP4) which is implicated in neurodegenerative diseases including PD was detected in BMMCs by immunocytochemistry. Our results suggest that mast cells may play role in PD pathogenesis.     

Rapid inhibitory effect of corticosterone on histamine release from rat peritoneal mast cells.

Glucocorticoids are steroids endowed with powerful anti-inflammatory properties, which are routinely believed to require several hours to take effect through modulation of gene expression. Our recent report has shown that glucocorticoids could inhibit allergic reaction within 10 minutes, which the classical genomic mechanism could not explain. Histamine is thought to be one of major mediators in the allergic reaction, and IgE-mediated histamine release from mast cells plays a pivotal role in allergic diseases. Here, we have determined a rapid effect of corticosterone on histamine release from rat peritoneal mast cells, using fluorometric assay. The results showed that corticosterone could inhibit antigen-induced histamine release from rat peritoneal cells within 15 minutes (p<0.05), which could be mimicked by membrane-impermeable BSA conjugated corticosterone (p<0.05). Neither glucocorticoid nuclear receptor antagonist nor protein synthesis inhibitor could block the rapid action (p<0.05). The study provided evidence that nongenomic mechanism might be involved in rapid effect of glucocorticoids on mast cells in allergic disease.     

Cyclical mechanical stretch enhances degranulation and IL‐4 secretion in RBL‐2H3 mast cells

Mast cells are widely distributed in the body and affect their surrounding environment through degranulation and secretion of cytokines. Conversely, mast cells are influenced by environmental stimuli such as cyclical mechanical stretch (CMS), such as that induced by heartbeat and respiration. Peripherally distributed mast cells are surrounded by extracellular matrix, where they bind IgE on their surface by expressing the high‐affinity Fc receptor for IgE (FcεRI), and they release mediators after cross‐linking of surface‐bound IgE by allergen. To analyse how CMS affects mast cell responses, we examined the effect of applying CMS on the behaviour of IgE‐bound mast cells (RBL‐2H3 cell line) adhering to fibronectin as a substitute for extracellular matrix. We found that CMS enhanced FcεRI‐mediated secretion in the presence of antigen (2,4‐dinitrophenol–bovine serum albumin). CMS increased expression of IL‐4 mRNA and secretion of IL‐4 protein. Western blot analysis showed that CMS changes the signal transduction in mitogen‐activated protein kinases and AKT, which in turn alters the regulation of IL‐4 and increases the secretion of IL‐4. These results suggest that CMS modulates the effect of mast cells on inflammation and resultant tissue remodelling. Understanding how CMS affects mast cell responses is crucial for developing therapies to treat mast cell‐related diseases. Copyright © 2013 John Wiley & Sons, Ltd.     

3-Benzyl-5-((2-nitrophenoxy) methyl)-dihydrofuran-2(3H)-one suppresses FcεRI-mediated mast cell degranulation via the inhibition of mTORC2-Akt signaling

Mast cells express high-affinity IgE receptor (FcεRI) on their surface, cross-linking of which leads to the immediate release of proinflammatory mediators such as histamine but also late-phase cytokine secretion, which are central to the pathogenesis of allergic diseases. Despite the growing evidences that mammalian target of rapamycin (mTOR) plays important roles in the immune system, it is still unclear how mTOR signaling regulates mast cell function. In this study, we investigated the effects of 3-benzyl-5-((2-nitrophenoxy) methyl)-dihydrofuran-2(3H)-one (3BDO) as an mTOR agonist on FcεRI-mediated allergic responses of mast cells. Our data showed that administration of 3BDO decreased β-hexosaminidase, interleukin 6 (IL-6), and tumor necrosis factor-α (TNF-α) release in murine bone marrow-derived mast cells (BMMCs) after FcεRI cross-linking, which was associated with an increase in mTOR complex 1 (mTORC1) signaling but a decrease in activation of Erk1/2, Jnk, and mTORC2-Akt. In addition, we found that a specific Akt agonist, SC79, is able to fully restore the decrease of β-hexosaminidase release in 3BDO-treated BMMCs but has no effect on IL-6 release in these cells, suggesting that 3BDO negatively regulates FcεRI-mediated degranulation and cytokine release through differential mechanisms in mast cells. The present data demonstrate that proper activation of mTORC1 is crucial for mast cell effector function, suggesting the applicability of the mTORC1 activator as a useful therapeutic agent in mast cell-related diseases.     

The role of mast cell-derived secreted phospholipases A2 in respiratory allergy

Secreted phospholipases A₂ (sPLA₂s) are molecules released in plasma and biological fluids of patients with systemic inflammatory, autoimmune and allergic diseases. These molecules exert proinflammatory effects by either enzymatic-mechanisms or through binding to surface molecules expressed on inflammatory cells. sPLA₂s are released at low levels in the normal airways and tend to increase during respiratory allergies (e.g., rhinitis and bronchial asthma) as the result of local secretion. Several sPLA₂ isoforms are expressed in the human lung and some of them (e.g., group IIA and group X) are released in the airways of patients with rhinitis or asthma. Mast cells play a major role in the pathogenesis of respiratory allergies and other chronic inflammatory lung diseases. Recent evidence indicates that mast cells purified from human lung express most of the sPLA₂ isoforms so far described. IgE-mediated activation of these cells induce the release of sPLA₂s suggesting that mast cells are a main source of extracellular sPLA₂s during allergic reactions. Once released, sPLA₂s may contribute to the generation of eicosanoids (e.g., PGD₂ and LTC₄) and to the release of preformed mediators (e.g., histamine) by an autocrine loop involving the interaction of sPLA₂s with surface molecules such as heparan sulphate proteoglycans or the M-type receptor. Thus, mast cell-derived sPLA₂s may play an important role in the initiation and amplification of the inflammatory reactions in patients with allergic rhinitis and bronchial asthma.     

The role of the mast cell in allergic bronchospasm

In allergic bronchospasm inhaled allergen interacts with specific IgE antibody on the surface of mast cells, inducing the release of mediators, particularly histamine and leukotrienes, which induce bronchoconstriction. Disodium cromoglycate, previously considered to be predominantly a mast cell stabilizing agent, is effective prophylactically in inhibition of early and late phase asthmatic reactions. However, the microenvironment of the airways contains many cell types and the precise role of mast cells is not clear. Lymphocytes, alveolar macrophages, eosinophils, platelets, and neutrophils possess low affinity surface receptors for IgE and can respond to allergen, releasing mediators that have diverse functions. These observations compound the problem of which mediator(s) is most important in pathogenesis of asthma. Moreover, mast cell products modulate the functions of many cells, and thus whether mast cells act directly or indirectly on bronchial smooth muscle requires clarification. Neuropeptides activate or modulate mast cells, and together with evidence of the close association of mast cells and nerves, these observations provide exciting new directions for investigation. Evidence that mast cells from different sites are heterogeneous in their response to stimuli and antiallergic drugs and differ in mediator production and function amplifies the problems identified above. In summary, the role of mast cells in bronchoconstriction is complex and systematic analysis of interactions between mast cells and other cells of the airways is essential.     

IgE-Independent Activation of Human Mast Cells Indicates their Role in the Late Phase Reaction of Allergic Inflammation

Mast cells are tissue dwelling cells that have a clear-cut pathologic role in allergy. Besides that, they participate in several chronic inflammatory conditions, helminitic parasitosis, and in some solid tumor reactions, but also in physiological situations, such as wound healing and innate immunity. Mast cells produce and release various mediators after activation induced by either IgE-dependent or IgE-independent mechanisms. Although much information has been gathered on the immunological (IgE-dependent) mast cell activation both in vivo and in vitro, not much is known about the non-immunological (IgE-independent) activation particularly in human mast cells. Mast cell IgE-independent activation is mediated through Gi3alpha which has been identified in rat mast cells as the pertussis toxin (Ptx)-sensitive heterotrimeric G protein that interacts with cationic secretagogues inducing PLC-independent mast cell exocytosis. Mast cell IgE-independent activation in allergy most likely occurs when mast cells encounter eosinophils, the main inflammatory cells that persist throughout the late and chronic phases of the allergic reaction. This review summarizes the influence of eosinophils on mast cell activation demonstrating that IgE-independent activation has a significant role in pathophysiological processes.     

Histamine from Brain Resident MAST Cells Promotes Wakefulness and Modulates Behavioral States

Mast cell activation and degranulation can result in the release of various chemical mediators, such as histamine and cytokines, which significantly affect sleep. Mast cells also exist in the central nervous system (CNS). Since up to 50% of histamine contents in the brain are from brain mast cells, mediators from brain mast cells may significantly influence sleep and other behaviors. In this study, we examined potential involvement of brain mast cells in sleep/wake regulations, focusing especially on the histaminergic system, using mast cell deficient (W/W^v) mice. No significant difference was found in the basal amount of sleep/wake between W/W^v mice and their wild-type littermates (WT), although W/W^v mice showed increased EEG delta power and attenuated rebound response after sleep deprivation. Intracerebroventricular injection of compound 48/80, a histamine releaser from mast cells, significantly increased histamine levels in the ventricular region and enhanced wakefulness in WT mice, while it had no effect in W/W^v mice. Injection of H1 antagonists (triprolidine and mepyramine) significantly increased the amounts of slow-wave sleep in WT mice, but not in W/W^v mice. Most strikingly, the food-seeking behavior observed in WT mice during food deprivation was completely abolished in W/W^v mice. W/W^v mice also exhibited higher anxiety and depression levels compared to WT mice. Our findings suggest that histamine released from brain mast cells is wake-promoting, and emphasizes the physiological and pharmacological importance of brain mast cells in the regulation of sleep and fundamental neurobehavior.     

Regulation of rat basophilic leukemia-2H3 mast cell secretion by a constitutive Lyn kinase interaction with the high affinity IgE receptor (Fc epsilon RI)

Signaling through the high affinity IgE receptor is initiated by noncovalently associated Lyn kinase, resulting in the secretion of inflammatory mediators from mast cells. A fraction of the total cellular Lyn is associated via its N-terminal unique domain with the cytoplasmic domain of the Fc epsilonRI beta subunit before receptor aggregation. In the current study, we stably transfected the unique domain of Lyn into rat basophilic leukemia-2H3 mast cells and examined the consequences on Fc epsilonRI-induced signal transduction and mediator secretion to further define the role of the unique domain of Lyn in mast cell secretion. Tyrosine phosphorylation of Fc epsilonRI beta and gamma subunits was partially inhibited in the Lyn unique domain transfectants after Ag stimulation. Ag stimulation of Lyn unique domain transfectants was accompanied by enhanced phosphorylation of MEK and ERK-2, which are required for leukotriene C4 (LTC4) release, and production of LTC4 was increased 3- to 5-fold, compared with cells transfected with vector alone. Conversely, tyrosine phosphorylation of the adaptor protein Gab2, which is essential for mast cell degranulation, was inhibited after Ag stimulation of Lyn unique domain transfectants, and Ag-induced release of histamine was inhibited up to 48%. In rat basophilic leukemia-2H3 cells, Lyn thus plays a dual role by positively regulating Fc epsilonRI phosphorylation and degranulation while negatively regulating LTC4 production. This study provides further evidence that the constitutive interaction between the unique domain of Lyn and the Fc epsilonRI beta subunit is a crucial step in the initiation of Fc epsilonRI signaling and that Lyn is limiting for Fc epsilonRI-induced secretion of inflammatory mediators.     

FcεR(1)-Mediated Mast Cell Reactivity Is Amplified through Prolonged Toll-Like Receptor-Ligand Treatment

Background

Mast cell-derived mediators mediate several of the pathological features of asthma. Microbial infections induce asthma exacerbations in which the contribution of mast cells remains incomprehensible.

Principal Findings

In this study we have investigated the characteristic expression pattern of Toll-like receptors (TLRs) 1–9 and the effect of TLR ligand treatment on IgE-receptor mediated mast cell reactivity. For the studies we employed in vitro differentiated connective tissue like mast cells (CTLMC) and mucosal like mast cells (MLMC) from mice. Both phenotypes were treated for 24 h or 96 h with ligands for TLR1/2, TLR2/6, TLR3 and TLR4, before activation with IgE and antigen. Prolonged exposure (96 h) with TLR-ligands promoted mast cell reactivity following IgE-receptor activation. TLR4 activation with LPS generated the most pronounced effect, with an enhanced degranulation and secretion of leukotrienes, cytokines and chemokines, in both CTLMC and MLMC. The effect of LPS was mediated through a Myd88-dependent pathway and the increased effect involved JNK-dependent pathway.

Conclusion

We find that prolonged exposure of mast cells to pathogens/TLR-ligands modulates their effector responses by priming them for increased release of several inflammatory mediators when subsequently activated by IgE-receptors. These data suggest that infections might exaggerate the severity of allergic reactions such as in asthma, by enhancing mediator release from mast cells.     

Structural requirements for the inhibition of calcium mobilization and mast cell activation by the pyrazole derivative BTP2

Mast cells play a critical role in the development of the allergic response. Upon activation by allergens and IgE via the high affinity receptor for IgE (Fc?RI), these cells release histamine and other functional mediators that initiate and propagate immediate hypersensitivity reactions. Mast cells also secrete cytokines that can regulate immune activity. These processes are controlled, in whole or part, by increases in intracellular Ca(2+) induced by the Fc?RI. We show here that N-(4-(3,5-bis(trifluoromethyl)-1H-pyrazol-1-yl)phenyl)-4-methyl-1,2,3-thiadiazole-5-carboxamide (BTP2), a pyrazole derivative, inhibits activation-induced Ca(2+) influx in the rat basophil cell line RBL-2H3 and in bone marrow-derived mast cells (BMMCs), without affecting global tyrosine phosphorylation of cellular proteins or phosphorylation of the mitogen-activated protein kinases Erk1/2, JNK and p38. BTP2 also inhibits activation-induced degranulation and secretion of interleukin (IL)-2, IL-3, IL-4, IL-6, IL-13, tumor necrosis factor (TNF)-α, and granulocyte macrophage-colony stimulating factor (GM-CSF) by BMMCs, which correlates with the inhibition of Nuclear Factor of Activated T cells (NFAT) translocation. In vivo, BTP2 inhibits antigen-induced histamine release. Structure-activity relationship analysis indicates that substitution at the C3 or C5 position of the pyrazole moiety on BTP2 (5-trifluoromethyl-3-methyl-pyrazole or 3-trifluoromethyl-5-methyl-pyrazole, respectively) affected its activity, with the trifluoromethyl group at the C3 position being critical to its activity. We conclude that BTP2 and related compounds may be potent modulators of mast cell responses and potentially useful for the treatment of symptoms of allergic inflammation.     

Immunologically activated chloride channels involved in degranulation of rat mucosal mast cells.

Crosslinking of type I Fc epsilon receptors (Fc epsilon RI) on the surface of basophils or mast cells initiates a cascade of processes leading to the secretion of inflammatory mediators. We report here a correlation between mediator secretion and the activation of Cl- channels in rat mucosal-type mast cells (line RBL-2H3). Stimulation of RBL cells by either IgE and antigen or by a monoclonal antibody specific for the Fc epsilon RI, resulted in the activation of Cl- ion channels as detected by the patch-clamp technique. Channel activation occurred slowly, within minutes after stimulation. The channel has a slope conductance of 32 pS at potentials between 0 and -100 mV, and an increasing open-state probability with increasing depolarization. Activation of apparently the same Cl- channels could be mimicked without stimulation by isolating inside-out membrane patches in tyrode solution. Parallel inhibition of both Cl- channel activity and mediator secretion, as monitored by serotonin release, was observed by two compounds, the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and the anti-allergic drug cromolyn. NPPB inhibited both the antigen-induced Cl- current and the serotonin release, where half-maximal inhibition occurred at similar doses, at 52 microM and 77 microM, respectively. The drug cromolyn, recently found to inhibit immunologically induced mediator secretion from RBL cells upon intracellular application, also blocks Cl- channels (IC50 = 15 microM) when applied to the cytoplasmic side of an inside-out membrane patch. The observed Cl- channel activation upon immunological stimulation and the parallel inhibition of channel current and of serotonin release suggests a functional role for this Cl- channel in mediator secretion from the mast cells studied.     

Does intracellular histamine mediate mast cell histamine release?

Previously, we demonstrated that through binding a novel intracellular receptor of microM affinity (HIC), histamine mediates, and the HIC antagonist N,N-diethyl-2-[4-(phenylmethyl)phenoxy]ethanamine. HCl (DPPE) inhibits, platelet aggregation and serotonin granule secretion; the latter response is dependent upon the same processes that mediate histamine release from mast cell granules. We now show that, as for platelet serotonin release, DPPE blocks concanavalin A-stimulated mast cell histamine release with a potency (IC50 = 30 microM) greater than the H1-antagonist, pyrilamine (IC50 = 150 microM) or the H2-antagonist cimetidine (IC50 = 5 mM), correlating with rank order of potency to inhibit 3H-histamine binding in rat brain membranes and liver microsomes. We postulate that histamine release from mast cells is mediated at HIC by second messenger intracellular histamine. However, unlike platelets, mast cells do not appear to rely on newly synthesized histamine. Rather, as for calcium, histamine may be mobilized from bound stores to mediate histamine secretion.     

A comparison of mediators released or generated by IFN-gamma-treated human mast cells following aggregation of Fc gamma RI or Fc epsilon RI

The high affinity receptor for IgG (Fc gamma RI, CD64) is expressed on human mast cells, where it is up-regulated by IFN-gamma and, thus, may allow mast cells to be recruited through IgG-dependent mechanisms in IFN-gamma-rich tissue inflammation. However, the mediators produced by human mast cells after aggregation of Fc gamma RI are incompletely described, and it is unknown whether these mediators are distinct from those produced after activation of human mast cells via Fc epsilon RI. Thus, we investigated the release of histamine and arachidonic acid metabolites and examined the chemokine and cytokine mRNA profiles of IFN-gamma-treated cultured human mast cells after Fc gamma RI or Fc epsilon RI aggregation. Aggregation of Fc gamma RI resulted in histamine release and PGD(2) and LTC(4) generation. These responses were qualitatively indistinguishable from responses stimulated via Fc epsilon RI. Aggregation of Fc epsilon RI or Fc gamma RI led to an induction or accumulation of 22 cytokine and chemokine mRNAs. Among them, seven cytokines (TNF-alpha, IL-1beta, IL-5, IL-6, IL-13, IL-1R antagonist, and GM-CSF) were significantly up-regulated via aggregation of Fc gamma RI compared with Fc epsilon RI. TNF-alpha mRNA data were confirmed by quantitative RT-PCR and ELISA. Furthermore, we confirmed histamine and TNF-alpha data using IFN-gamma-treated purified human lung mast cells. Thus, aggregation of Fc gamma RI on mast cells led to up-regulation and/or release of three important classes of mediators: biogenic amines, lipid mediators, and cytokines. Some cytokines, such as TNF-alpha, were released and generated to a greater degree after Fc gamma RI aggregation, suggesting that selected biologic responses of mast cells may be preferentially generated through Fc gamma RI in an IFN-gamma-rich environment.