Regulation of mast cells? secretory response by co-clustering the Type 1 Fc? receptor with the mast cell function-associated antigen

Strategies for Mast Cell Inhibition in Food Allergy

Mast cells are tissue resident allergic effector cells that drive IgE-mediated food allergies. There are several steps leading to mast cell activation in the context of allergic disease that can be targeted to prevent mast cell activation and degranulation. These include blocking IgE-FcεRI crosslinking and type 2 cytokine receptor activation; modulating cell-surface neural chemical receptors; stabilizing mast cell membranes to prevent co-localization of activating receptors; impeding intracellular signaling; and engaging cell surface inhibitory receptors. This review highlights several ITIM-containing inhibitory mast cell surface receptors that could serve as pharmaceutical targets to prevent mast cell activation and degranulation in the context of food allergy. When activated, these ITIM-containing inhibitory receptors recruit the phosphatases SHP-1, SHP-2, and/or SHIP to dephosphorylate the tyrosine kinases responsible for activation signals downstream of the IgE-FcεRI complex. We describe several members of the Ig and Ig-like inhibitory receptor and C-type lectin inhibitory receptor superfamilies. Fundamental studies exploring the behavior of these receptors within the context of experimental food allergy models are needed. A deeper understanding of how these receptors modulate mast cell-driven food allergic responses will shape future strategies to harness these inhibitory receptors to treat food allergy.

Empty mast cell syndrome: fallacy or fact?

Post-anaphylaxis mast cell anergy (PAMA), commonly referred to as 'empty mast cell (MC) syndrome', is a state of temporary loss of cutaneous MC reactivity in the immediate aftermath of anaphylaxis. Data relating to this condition are sparse and the incidence rate is currently unknown. PAMA has been described only in a few published case reports in the context of hymenoptera venom allergy and perioperative anaphylaxis. Best practice guidelines regarding optimal timing for performing skin tests postanaphylaxis are largely based on expert opinion, and allergy work-up has been recommended after 4-6 weeks postanaphylaxis to avoid false-negative results.This article provides a review of clinical literature surrounding PAMA, critically evaluates intracellular events in MCs from in vitro data and hypothesises regarding plausible immune mechanisms. There are no published data to directly explain molecular mechanisms underlying this phenomenon. Although not evidence based, PAMA has been attributed to depletion of MC granules following anaphylaxis. It is also plausible that exposure to high allergen concentrations in anaphylaxis can induce a temporary shift in MCs towards dominance of inhibitory signalling pathways, thus contributing to a state of transient hyporesponsiveness observed in some patients. Other potential contributory factors for reduced MC reactivity include downregulation of FcεRI expression, cross-linking of FcεRI to the inhibitory, low-affinity IgG receptors and administration of pharmacotherapeutic agents for anaphylaxis treatment. It is likely that this interesting phenomenon can be explained by a combination of these proposed mechanisms in addition to other genetic/host factors that have not yet been identified.

Inhibitory C-type lectin receptors in myeloid cells

C-type lectin receptors encoded by the natural killer gene complex play critical roles in enabling NK cell discrimination between self and non-self. In recent years, additional genes at this locus have been identified with patterns of expression that extend to cells of the myeloid lineage where many of the encoded inhibitory receptors have equally important functions as regulators of immune homeostasis. In the present review we highlight the roles of some of these receptors including recent insights gained with regard to the identification of exogenous and endogenous ligands, mechanisms of cellular inhibition and activation, regulated expression within different cellular and immune contexts, as well as functions that include the regulation of bone homeostasis and involvement in autoimmunity.

Advances in mast cell biology: new understanding of heterogeneity and function

Mast cells are classically viewed as effector cells of IgE-mediated allergic diseases. However, over the last decade our understanding has been enriched about their roles in host defense, innate and adaptive immune responses, and in homeostatic responses, angiogenesis, wound healing, tissue remodeling, and immunoregulation. Despite impressive progress, there are large gaps in our understanding of their phenotypic heterogeneity, regulatory mechanisms involved, and functional significance. This review summarizes our knowledge of mast cells in innate and acquired immunity, allergic inflammation and tissue homeostasis, as well as some of the regulatory mechanisms that control mast cell development, phenotypic determination, and function, particularly in the context of mucosal surfaces.

Anaphylaxis: lessons from mouse models

Studies with mouse models demonstrate 2 pathways of systemic anaphylaxis: a classic pathway mediated by IgE, FcepsilonRI, mast cells, histamine, and platelet-activating factor (PAF) and an alternative pathway mediated by IgG, FcgammaRIII, macrophages, and PAF. The former pathway requires much less antigen and antibody than the latter. This is modified, however, by IgG antibodies that prevent IgE-mediated anaphylaxis by intercepting antigen before it binds to mast cell-associated IgE. Consequently, IgG antibodies block systemic anaphylaxis induced by small quantities of antigen but mediate systemic anaphylaxis induced by larger quantities. The importance of the alternative pathway in human subjects is unknown, but human IgG, IgG receptors, macrophages, mediators, and mediator receptors have appropriate properties to support this pathway if sufficient IgG and antigen are present. The severity of systemic anaphylaxis is increased by nitric oxide produced by the enzyme endothelial nitric oxide synthase and by the cytokines IL-4 and IL-13 and decreased by endogenous beta-adrenergic stimulation and receptors that contain ITIM that bind tyrosine phosphatases. Anaphylaxis is also suppressed by other receptors and ion channels that function through distinct mechanisms. Unlike systemic anaphylaxis, intestinal anaphylaxis (allergic diarrhea) is almost totally IgE and mast cell dependent and is mediated predominantly by PAF and serotonin. Some potent food allergens, including peanuts and tree nuts, can directly enhance anaphylaxis by stimulating an anaphylactoid response through the innate immune system. Results of these studies suggest novel prophylactic agents, including nonstimulatory anti-IgE mAbs, IL-4 receptor antagonists, PAF antagonists, and agents that cross-link FcepsilonRI or FcgammaRIII to an ITIM-containing inhibitory receptor.

Regulation of the mast cell response to the type 1 Fc epsilon receptor

The type I Fc epsilon receptor (Fc epsilon RI) is one of the better understood members of its class and is central to the immunological activation of mast cells and basophils, the key players in immunoglobulin E (IgE)-dependent immediate hypersensitivity. This review provides background information on several distinct regulatory mechanisms controlling this receptor's stimulus-response coupling network. First, we review the current understanding of this network's operation, and then we focus on the inhibitory regulatory mechanisms. In particular, we discuss the different known cytosolic molecules (e.g. kinases, phosphatases, and adapters) as well as cell membrane proteins involved in negatively regulating the Fc epsilon RI-induced secretory responses. Knowledge of this field is developing at a fast rate, as new proteins endowed with regulatory functions are still being discovered. Our understanding of the complex networks by which these proteins exert regulation is limited. Although the scope of this review does not include addressing several important biochemical and biophysical aspects of the regulatory mechanisms, it does provide general insights into a central field in immunology.

New developments in FcepsilonRI regulation, function and inhibition

The high-affinity Fc receptor for IgE (FcepsilonRI), a multimeric immune receptor, is a crucial structure for IgE-mediated allergic reactions. In recent years, advances have been made in several important areas of the study of FcepsilonRI. The first area relates to FcepsilonRI-mediated biological responses that are antigen independent. The second area encompasses the biological relevance of the distinct signalling pathways that are activated by FcepsilonRI; and the third area relates to the accumulated evidence for the tight control of FcepsilonRI signalling through a broad array of inhibitory mechanisms, which are being developed into promising therapeutic approaches.

Cis binding between inhibitory receptors and MHC class I can regulate mast cell activation

Allergy is caused by immune effector cells, including mast cells and basophils. Cellular signaling that activates these effector cells is regulated by different inhibitory receptors on their surface. We show that human leukocyte immunoglobulin (Ig)-like receptor (LILR) B2 and its mouse orthologue, paired Ig-like receptor (PIR)–B, constitutively associate to major histocompatibility complex (MHC) class I on the same cell surface (in cis). The IgE-mediated effector responses were augmented in β₂-microglobulin (β₂m) and PIR-B–deficient mast cells. In addition, the increased cytokine production of β₂m-deficient mast cells was not affected by the co-culture with MHC class I–positive mast cells, showing that less cis interaction between PIR-B and MHC class I on mast cells led to the increased cytokine release. Thus, the constitutive cis binding between LILRB2 or PIR-B and MHC class I has an essential role in regulating allergic responses.

Compartmentalization of the Type I Fcε receptor and MAFA on mast cell membranes

The Mast cell Function-associated Antigen (MAFA) is a membrane glycoprotein on rat mast cells (RBL-2H3) expressed at a ratio of approximately 1:30 with respect to the Type I Fc epsilon receptor (Fc epsilon RI). Despite this stoichiometry, clustering MAFA by its specific mAb G63 substantially inhibits secretion of both granular and de novo synthesized mediators induced upon Fc epsilon RI aggregation. Since the Fc epsilon RIs apparently signal from within raft micro-environments, we investigated possible co-localization of MAFA within these membrane compartments containing aggregated Fc epsilon RI. We used cholera toxin B subunit (CTB) to cluster the raft component ganglioside GM1 and studied the effects of this perturbation on rotation of Fc epsilon RI and MAFA by time-resolved phosphorescence anisotropy of erythrosin-conjugated probes. CTB treatment would be expected to substantially inhibit rotation of raft-associated molecules. Experimentally, CTB has no effect on rotational parameters such as the long-time anisotropy (r(infinity)) of unperturbed Fc epsilon RI or MAFA. However, on cells where Fc epsilon RI-IgE has previously been clustered by antigen (DNP(14)-BSA), CTB treatment increases the Fc epsilon RI-IgE's r(infinity) by 0.010 and MAFA's by 0.014. Similarly, CTB treatment of cells where MAFA had been clustered by mAb G63 increases MAFA's r(infinity) by 0.010 but leaves Fc epsilon RI's unaffected. Evaluation of raft localization of Fc epsilon RI and MAFA using sucrose gradient ultracentrifugation of Triton X-100 treated membrane fragments demonstrates that a significant fraction of MAFA molecules sediments with rafts when Fc epsilon RI is clustered by antigen or when MAFA itself is clustered by mAb G63. The large excess of Fc epsilon RI over MAFA explains why clustering MAFA does not substantively affect Fc epsilon RI dynamics. Moreover, in single-particle tracking studies of individual Fc epsilon RI-IgE or MAFA molecules, these proteins, upon clustering by antigen, move into small membrane compartments of reduced, but similar, dimensions. This provides additional indication of constitutive interactions between Fc epsilon RI and MAFA. Taken together, these results of distinct methodologies suggest that MAFA functions within raft microdomains of the RBL-2H3 cell membrane and thus in close proximity to the Fc epsilon RI which themselves signal from within the raft environment.

Selective inhibition of the Fc epsilon RI-induced de novo synthesis of mediators by an inhibitory receptor

Aggregation of the type 1 Fc-epsilon receptors (Fc-epsilon-RI) on mast cells initiates a network of biochemical processes culminating in secretion of both granule-stored and de novo-synthesized inflammatory mediators. A strict control of this response is obviously a necessity; nevertheless, this regulation is hardly characterized. Here we report that a prototype inhibitory receptor, the mast cell function-associated antigen (MAFA), selectively regulates the Fc-epsilon-RI stimulus-response coupling network and the subsequent de novo production and secretion of inflammatory mediators. Specifically, MAFA suppresses the PLC-gamma2-[Ca2+]i, Raf-1-Erk1/2, and PKC-p38 coupling pathways, while the Fyn-Gab2-mediated activation of PKB and Jnk is essentially unaffected. Hence, the activities of several transcription/nuclear factors for inflammatory mediators (NF-kappaB, NFAT) are markedly reduced, while those of others (Jun, Fos, Fra, p90rsk) are unaltered. This results in a selective inhibition of gene transcription of cytokines including IL-1beta, IL-4, IL-8, and IL-10, while that of TNF-alpha, MCP-1, IL-3, IL-5, or IL-13 remains unaffected. Taken together, these results illustrate the capacity of an immunoreceptor tyrosine-based inhibitory motif-containing receptor to cause tight and specific control of the production and secretion of inflammatory mediators by mast cells.

Co-clustering activating and inhibitory receptors: impact at varying expression levels of the latter

Clustering the mast cell function-associated antigen (MAFA) has earlier been shown to inhibit mast cells' secretory response to the type 1 Fcepsilon receptor (FcepsilonRI) stimulus. MAFA is a type II membrane glycoprotein first identified on rat mast cells and contains an immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytosolic domain. This inhibition is induced already upon clustering MAFA alone. Still, the inhibitory capacity of MAFA-FcepsilonRI co-clustering has recently been characterized and co-clustered MAFA molecules were found to exhibit a markedly higher inhibition capacity than MAFA-clusters alone. We have now compared the inhibitory capacity of FcepsilonRI co-clustered MAFA on the secretory response of rat mucosal-type mast cells (RBL-2H3 line) expressing different levels of this inhibitory protein. Reacting these cells carrying an IgE class, 2,4 dinitrophenyl (DNP)-specific monoclonal antibody with DNP-conjugated F(ab')2 fragments of non-specific polyclonal mouse IgG causes clustering of the FcepsilonRI-IgE. Reaction of these cells with DNP-conjugated F(ab')2 fragments of the MAFA-specific, monoclonal antibody G63 co-aggregates MAFA together with the FcepsilonRI-IgE thereby producing FcepsilonRI-IgE-MAFA co-clusters. Results of measurements of the secretory responses of RBL-2H3 cells expressing higher or lower MAFA levels than those of unmodified cells provided further support to the notion that co-clustered MAFA molecules exhibit a markedly higher inhibition capacity than MAFA-clusters alone. The molecular basis for this enhanced inhibition is most probably the increased concentration of the inhibitory cell components in the immediate proximity of the co-clustered FcepsilonRI-MAFA.