Increased degranulation and phospholipase A2, C, and D activity in RBL cells stimulated through FcepsilonR1 is due to spreading and not simply adhesion

Neuro-immune crosstalk and food allergy: Focus on enteric neurons and mucosal mast cells

The nervous system and the immune system individually play important roles in regulating the processes necessary to maintain physiological homeostasis, respond to acute stress and protect against external threats. These two regulating systems for maintaining the living body had often been assumed to function independently. Allergies develop as a result of an overreaction of the immune system to substances that are relatively harmless to the body, such as food, pollen and dust mites. Therefore, it has been generally supposed that the development and pathogenesis of allergies can be explained through an immunological interpretation. Recently, however, neuro-immune crosstalk has attracted increasing attention. Consequently, it is becoming clear that there is close morphological proximity and physiological and pathophysiological interactions between neurons and immune cells in various peripheral tissues. Thus, researchers are now beginning to appreciate that neuro-immune interactions may play a role in tissue homeostasis and the pathophysiology of immune-mediated disease, but very little information is available on the molecular basis of these interactions. Mast cells are a part of the innate immune system implicated in allergic reactions and the regulation of host-pathogen interactions. Mast cells are ubiquitous in the body, and these cells are often found in close proximity to nerve fibers in various tissues, including the lamina propria of the intestine. Mast cells and neurons are thought to communicate bidirectionally to modulate neurophysiological effects and mast cell functions, which suggests that neuro-immune interactions may be involved in the pathology of allergic diseases.

The Alleviating Effect of Lagerstroemia indica Flower Extract on Stretch Marks through Regulation of Mast Cells

Striae distensae (SD) or stretch marks are common linear scars of atrophic skin with disintegrating extracellular matrix (ECM) structures. Although fibroblasts contribute to the construction of ECM structure in SD, some studies have reported that mast cell degranulation causes the disruption of ECM in early SD lesions. Lagerstroemia indica flower (LIF) has traditionally been used in India as a diuretic. However, little is known about the effect and molecular action of Lagerstroemia indica flower extract (LIFE) on alleviating SD. This study evaluated the effects of LIFE on mast cell degranulation and the synthesis of ECM components in fibroblasts. LIFE inhibits the adhesion of rat basophilic leukemia (RBL) cells, RBL-2H3 on fibronectin (FN) and the expression of integrin, a receptor for FN, thereby reducing focal adhesion kinase (FAK) phosphorylation. In addition, LIFE attenuated the allergen-induced granules and cytokine interleukin 3 (IL-3) through the adhesion with FN. Moreover, the conditioned medium (CM) of activated mast cells decreases the synthesis of ECM components, and LIFE restores the abnormal expressions induced by activated mast cells. These results demonstrate that LIFE suppresses FN-induced mast cell activation and promotes the synthesis of ECM components in fibroblast, which indicates that LIFE may be a useful cosmetic agent for SD treatment.

CD45 exclusion- and cross-linking-based receptor signaling together broaden FcεRI reactivity

For many years, the high-affinity receptor for immunoglobulin E (IgE) FcεRI, which is expressed by mast cells and basophils, has been widely held to be the exemplar of cross-linking (that is, aggregation dependent) signaling receptors. We found, however, that FcεRI signaling could occur in the presence or absence of receptor cross-linking. Using both cell and cell-free systems, we showed that FcεRI signaling was stimulated by surface-associated monovalent ligands through the passive, size-dependent exclusion of the receptor-type tyrosine phosphatase CD45 from plasma membrane regions of FcεRI-ligand engagement. Similarly to the T cell receptor, FcεRI signaling could also be initiated in a ligand-independent manner. These data suggest that a simple mechanism of CD45 exclusion-based receptor triggering could function together with cross-linking-based FcεRI signaling, broadening mast cell and basophil reactivity by enabling these cells to respond to both multivalent and surface-presented monovalent antigens. These findings also strengthen the case that a size-dependent, phosphatase exclusion-based receptor triggering mechanism might serve generally to facilitate signaling by noncatalytic immune receptors.

The FcεRI Signaling Cascade and Integrin Trafficking Converge at Patterned Ligand Surfaces

We examined the spatial targeting of early and downstream signaling mediated by the IgE receptor (FcεRI) in RBL mast cells utilizing surface-patterned 2,4 dinitrophenyl (DNP) ligands. Micron-sized features of DNP are presented as densely immobilized conjugates of bovine serum albumin (DNP-BSA) or mobile in a supported lipid bilayer (DNP-SLB). Although soluble anti-DNP IgE binds uniformly across features for both pattern types, IgE bound to FcεRI on cells shows distinctive distributions: uniform for DNP-SLB and edge-concentrated for DNP-BSA. These distributions of IgE-FcεRI propagate to the spatial recruitment of early signaling proteins, including spleen tyrosine kinase (Syk), linker for activation of T cells (LAT), and activated phospholipase C gamma 1 (PLCγ1), which all localize with engaged receptors. We found stimulated polymerization of F-actin is not required for Syk recruitment but is progressively involved in the recruitment of LAT and PLCγ1. We further found β1- and β3-integrins colocalize with IgE-FcεRI at patterned ligand surfaces as cells spread. This recruitment corresponds to directed exocytosis of recycling endosomes (REs) containing these integrins and their fibronectin ligand. Together, our results show targeting of signaling components, including integrins, to regions of clustered IgE-FcεRI in processes that depend on stimulated actin polymerization and outward trafficking of REs.

CGRP, a neurotransmitter of enteric sensory neurons, contributes to the development of food allergy due to the augmentation of microtubule reorganization in mucosal mast cells

Neuro-immune interaction in the gut is substantially involved in the maintenance of intestinal immune homeostasis and the pathology of intestinal immune diseases. We have previously demonstrated that mucosal mast cells and nerve fibers containing CGRP, a neurotransmitter of intrinsic enteric sensory neurons, are markedly increased and exist in close proximity to each other in the colon of food allergy (FA) mice. In the present study, a CGRP-receptor antagonist BIBN4096BS significantly alleviated allergic symptoms in the murine FA model. In addition, the elevated numbers of mucosal mast cells in the proximal colon of FA mice were significantly decreased in that of BIBN4096BS-treated FA mice. Thus, we investigated the effects of CGRP on calcium-independent process in degranulation of mucosal mast cells since CGRP increases intracellular cAMP levels, but not Ca(2+) concentration. CGRP did not alter a calcium ionophore A23187-increased cytosolic Ca(2+) concentration in mucosal-type bone marrow-derived mast cells (mBMMCs), but did augment microtubule reorganization in resting and A23187-activated mBMMCs. Furthermore, CGRP alone failed to cause the degranulation of mBMMCs, but CGRP significantly enhanced the degranulation of mBMMCs induced by A23187. Together, these data indicate that CGRP- enhanced microtubule reorganization augments IgE-independent/non-antigenic stimuli-induced mucosal mast cell degranulation, thereby contributing to the development of FA.

Spatiotemporal resolution of mast cell granule exocytosis reveals correlation with Ca2+ wave initiation

Mast cell activation initiated by antigen-mediated crosslinking of IgE receptors results in stimulated exocytosis of secretory lysosomes in the process known as degranulation. Much has been learned about the molecular mechanisms important for this process, including the crucial role of Ca(2+) mobilization, but spatio-temporal relationships between stimulated Ca(2+) mobilization and granule exocytosis are incompletely understood. Here we use a novel imaging-based method that uses fluorescein isothiocyanate (FITC)-dextran as a reporter for granule exocytosis in RBL mast cells and takes advantage of the pH sensitivity of FITC. We demonstrate the selectivity of FITC-dextran, accumulated by fluid-phase uptake, as a marker for secretory lysosomes, and we characterize its capacity to delineate different exocytotic events, including full fusion, kiss-and-run transient fusion and compound exocytosis. Using this method, we find strong dependence of degranulation kinetics on the duration of cell to substrate attachment. We combine imaging of degranulation and Ca(2+) dynamics to demonstrate a spatial relationship between the sites of Ca(2+) wave initiation in extended cell protrusions and exocytosis under conditions of limited antigen stimulation. In addition, we find that the spatially proximal Ca(2+) signaling and secretory events correlate with participation of TRPC1 channels in Ca(2+) mobilization.

Inhibition of IgE-induced mast cell activation by ethyl tertiary-butyl ether, a bioethanol-derived fuel oxygenate

Objectives

The effect of ethyl tertiary-butyl ether (ETBE), which is widely used as a fuel oxygenate commonly produced from bioethanol, on immunoglobulin (Ig)E-dependent mast cell activation was investigated.

Methods

The rat mast cell line RBL2H3 sensitised with monoclonal anti-ovalbumin IgE was challenged with ovalbumin in the presence or absence of ETBE, tert-butanol (TBA), which is the main metabolite of ETBE in humans, and ethanol. Degranulation of RBL2H3 was examined by the release of β-hexosaminidase. To understand the mechanisms responsible for regulating mast cell function, the effects of ETBE, TBA and ethanol on the levels of intracellular calcium, phosphorylation of Akt (as a marker of phosphatidylinositol 3-kinase) and global tyrosine phosphorylation were also measured as indicators of mast cell activation.

Key findings

In the presence of ETBE, TBA or ethanol, IgE-induced release of β-hexosaminidase was decreased. These compounds also attenuated the IgE-mediated increase in the levels of intracellular Ca2+, phosphorylation of Akt and global tyrosine phosphorylation in RBL2H3 cells.

Conclusions

ETBE, TBA and ethanol inhibited mast cell degranulation by inhibiting the increase in intracellular calcium ion concentration and activation of phosphatidylinositol 3-kinase and protein tyrosine kinase activation, suggesting that exposure to ETBE might affect immune responses, particularly in allergic diseases.

Group IVA cytosolic phospholipase A2 (cPLA2alpha) and integrin alphaIIbbeta3 reinforce each other's functions during alphaIIbbeta3 signaling in platelets

Group IVA cytosolic phospholipase A(2) (cPLA(2)alpha) catalyzes release of arachidonic acid from glycerophospholipids, leading to thromboxane A(2) (TxA(2)) production. Some platelet agonists stimulate cPLA(2)alpha, but others require fibrinogen binding to alphaIIbbeta3 to elicit TxA(2). Therefore, relationships between cPLA(2)alpha and alphaIIbbeta3 were examined. cPLA(2)alpha and a cPLA(2)alpha binding partner, vimentin, coimmunoprecipitated with alphaIIbbeta3 from platelets, independent of fibrinogen binding. Studies with purified proteins and with recombinant proteins expressed in CHO cells determined that the interaction between cPLA(2)alpha and alphaIIbbeta3 was indirect and was dependent on the alphaIIb and beta3 cytoplasmic tails. Fibrinogen binding to alphaIIbbeta3 caused an increase in integrin-associated cPLA(2)alpha activity in normal platelets, but not in cPLA(2)alpha-deficient mouse platelets or in human platelets treated with pyrrophenone, a cPLA(2)alpha inhibitor. cPLA(2)alpha activation downstream of alphaIIbbeta3 had functional consequences for platelets in that it was required for fibrinogen-dependent recruitment of activated protein kinase Cbeta to the alphaIIbbeta3 complex and for platelet spreading. Thus, cPLA(2)alpha and alphaIIbbeta3 interact to reinforce each other's functions during alphaIIbbeta3 signaling. This provides a plausible explanation for the role of alphaIIbbeta3 in TxA(2) formation and in the defective hemostatic function of mouse or human platelets deficient in cPLA(2)alpha.

Anti-CD63 antibodies suppress IgE-dependent allergic reactions in vitro and in vivo

High-affinity IgE receptor (FcɛRI) cross-linking on mast cells (MCs) induces secretion of preformed allergy mediators (degranulation) and synthesis of lipid mediators and cytokines. Degranulation produces many symptoms of immediate-type allergic reactions and is modulated by adhesion to surfaces coated with specific extracellular matrix (ECM) proteins. The signals involved in this modulation are mostly unknown and their contribution to allergic reactions in vivo is unclear. Here we report the generation of monoclonal antibodies that potently suppress FcɛRI-induced degranulation, but not leukotriene synthesis. We identified the antibody target as the tetraspanin CD63. Tetraspanins are membrane molecules that form multimolecular complexes with a broad array of molecules including ECM protein-binding β integrins. We found that anti-CD63 inhibits MC adhesion to fibronectin and vitronectin. Furthermore, anti-CD63 inhibits FcɛRI-mediated degranulation in cells adherent to those ECM proteins but not in nonadherent cells. Thus the inhibition of degranulation by anti-CD63 correlates with its effect on adhesion. In support of a mechanistic linkage between the two types of inhibition, anti-CD63 had no effect on FcɛRI-induced global tyrosine phosphorylation and calcium mobilization but impaired the Gab2–PI3K pathway that is known to be essential for both degranulation and adhesion. Finally, we showed that these antibodies inhibited FcɛRI-mediated allergic reactions in vivo. These properties raise the possibility that anti-CD63 could be used as therapeutic agents in MC-dependent diseases.

Mast cell–mediated inflammatory responses require the α2β1 integrin

Although the α2β1 integrin is widely expressed and has been extensively studied, it has not been previously implicated in mast cell biology. We observed that α2 integrin subunit-deficient mice exhibited markedly diminished neutrophil and interleukin-6 responses during Listeria monocytogenes– and zymosan-induced peritonitis. Since exudative neutrophils of wild-type mice expressed little α2β1 integrin, it seemed unlikely that this integrin mediated neutrophil migration directly. Here, we demonstrate constitutive α2β1 integrin expression on peritoneal mast cells. Although α2-null mice contain normal numbers of peritoneal mast cells, these α2-null cells do not support in vivo mast cell–dependent inflammatory responses. We conclude that α2β1 integrin provides a costimulatory function required for mast cell activation and cytokine production in response to infection.

Mutant RBL mast cells defective in Fc epsilon RI signaling and lipid raft biosynthesis are reconstituted by activated Rho-family GTPases

Characterization of defects in a variant subline of RBL mast cells has revealed a biochemical event proximal to IgE receptor (Fc epsilon RI)-stimulated tyrosine phosphorylation that is required for multiple functional responses. This cell line, designated B6A4C1, is deficient in both Fc epsilon RI-mediated degranulation and biosynthesis of several lipid raft components. Agents that bypass receptor-mediated Ca(2+) influx stimulate strong degranulation responses in these variant cells. Cross-linking of IgE-Fc epsilon RI on these cells stimulates robust tyrosine phosphorylation but fails to mobilize a sustained Ca(2+) response. Fc epsilon RI-mediated inositol phosphate production is not detectable in these cells, and failure of adenosine receptors to mobilize Ca(2+) suggests a general deficiency in stimulated phospholipase C activity. Antigen stimulation of phospholipases A(2) and D is also defective. Infection of B6A4C1 cells with vaccinia virus constructs expressing constitutively active Rho family members Cdc42 and Rac restores antigen-stimulated degranulation, and active Cdc42 (but not active Rac) restores ganglioside and GPI expression. The results support the hypothesis that activation of Cdc42 and/or Rac is critical for Fc epsilon RI-mediated signaling that leads to Ca(2+) mobilization and degranulation. Furthermore, they suggest that Cdc42 plays an important role in the biosynthesis and expression of certain components of lipid rafts.

Antibody cross-linking of human CD9 and the high-affinity immunoglobulin E receptor stimulates secretion from transfected rat basophilic leukaemia cells

Previous studies have shown that antibody cross-linking of the tetraspanin protein CD9 stimulates the degranulation of platelets and eosinophils, although the mechanism of activation is unclear. In this work we transfected human CD9 into the rat basophilic leukaemia (RBL-2H3) cell line and studied the stimulation of secretion from these cells in response to a panel of anti-CD9 antibodies. Intact immunoglobulin G1 (IgG1) antibodies activated transfected cells whereas F(ab')2 fragments of antibody and an intact IgG2a did not. Stimulation of secretion was inhibited by co-incubation with monomer murine immunoglobulin E (IgE) but not with an IgG1 isotype control, indicating that the response involves the endogenous high-affinity IgE receptor (FcepsilonRI). The anti-CD9 antibody activation curve was biphasic, and supraoptimal antibody concentrations stimulated little or no degranulation, indicating that multivalent binding of human CD9 molecules is necessary for the formation of an active complex with rat FcepsilonRI. Immunoprecipitation of FcepsilonRI under mild detergent conditions co-precipitated CD9, suggesting the presence of pre-existing complexes of CD9 and FcepsilonRI that could be activated by antibody cross-linking. These data are further evidence that tetraspanins are involved in FcepsilonRI signalling and may reflect the participation of tetraspanins in the formation of complexes with other membrane proteins that use components of Fc receptors for signal transduction.

Targeting Janus kinase 3 in mast cells prevents immediate hypersensitivity reactions and anaphylaxis

Janus kinase 3 (JAK3), a member of the Janus family protein-tyrosine kinases, is expressed in mast cells, and its enzymatic activity is enhanced by IgE receptor/FcepsilonRI cross-linking. Selective inhibition of JAK3 in mast cells with 4-(4'-hydroxylphenyl)-amino-6, 7-dimethoxyquinazoline) (WHI-P131) blocked the phospholipase C activation, calcium mobilization, and activation of microtubule-associated protein kinase after lgE receptor/FcepsilonRI cross-linking. Treatment of IgE-sensitized rodent as well as human mast cells with WHI-P131 effectively inhibited the activation-associated morphological changes, degranulation, and proinflammatory mediator release after specific antigen challenge without affecting the functional integrity of the distal secretory machinery. In vivo administration of the JAK3 inhibitor WHI-P131 prevented mast cell degranulation and development of cutaneous as well as systemic fatal anaphylaxis in mice at nontoxic dose levels. Thus, JAK3 plays a pivotal role in IgE receptor/FcepsilonRI-mediated mast cell responses, and targeting JAK3 with a specific inhibitor, such as WHI-P131, may provide the basis for new and effective treatment as well as prevention programs for mast cell-mediated allergic reactions.

Genetic and biochemical evidence for a critical role of Janus kinase (JAK)-3 in mast cell-mediated type I hypersensitivity reactions

We investigated the role of JAK3 in IgE receptor/FcepsilonRI-mediated mast cell responses. IgE/antigen induced degranulation and mediator release were substantially reduced with Jak3-/- mast cells from JAK3-null mice that were generated by targeted disruption of Jak3 gene in embryonic stem cells. Further, treatment of mast cells with 3'bromo-4'-hydroxylphenyl)-amino-6,7-dimethoxyquinazoline (WHI-P154), a potent inhibitor of JAK3, inhibited degranulation and proinflammatory mediator release after IgE receptor/ FcepsilonRI crosslinking. Thus, JAK3 plays a pivotal role in IgE receptor/ FcepsilonRI-mediated mast cell responses and targeting JAK3 may provide the basis for new and effective treatment as well as prevention programs for mast cell-mediated allergic reactions.

The Role of Actin Microfilaments in the Down-Regulation of the Degranulation Response in RBL-2H3 Mast Cells

Cross-linking of FcεRI on rat basophilic leukemia (RBL) cells initiates a signaling cascade leading to degranulation of the cells and the release of inflammatory mediators. Inhibitors that disrupt microfilaments, such as latrunculin and cytochalasin D, do not cause any degranulation on their own, but they do enhance FcεRI-mediated degranulation. Dose-response studies show a good correlation between inhibition of actin polymerization and increased degranulation. In RBL cells, latrunculin causes a decrease in basal levels of filamentous actin (F-actin), while cytochalasin D does not. This is particularly evident in the Triton-insoluble pool of F-actin which is highly cross-linked and associated with the plasma membrane. A concentration of 500 nM latrunculin decreases the basal level of Triton-insoluble F-actin by 60–70% and total F-actin levels by 25%. Latrunculin increases both the rate and extent of Ag-induced degranulation while having no effect on pervanadate-induced degranulation. Pervanadate activates the signaling pathways directly and bypasses the cross-linking of the receptor. RBL cells, activated through FcεRI in the presence of latrunculin, show increased phospholipase activity as well as increased tyrosine phosphorylation of Syk and increased tyrosine phosphorylation of the receptor itself by the tyrosine kinase Lyn. This indicates that the very earliest signaling events after receptor cross-linking are enhanced. These results suggest that actin microfilaments may interact, either directly or indirectly, with the receptor itself and that they may regulate the signaling process at the level of receptor phosphorylation. Microfilaments may possibly act by uncoupling Lyn from the cross-linked receptor.