Allergy-associated FcRbeta is a molecular amplifier of IgE- and IgG-mediated in vivo responses

IL-10 Inhibits FcεRI Expression in Mouse Mast Cells

FcepsilonRI expression and function is a central aspect of allergic disease. Using bone marrow-derived mouse mast cell populations, we have previously shown that the Th2 cytokine IL-4 inhibits FcepsilonRI expression and function. In the current study we show that the Th2 cytokine IL-10 has similar regulatory properties, and that it augments the inhibitory effects of IL-4. FcepsilonRI down-regulation was functionally significant, as it diminished inflammatory cytokine production and IgE-mediated FcepsilonRI up-regulation. IL-10 and IL-4 reduced FcepsilonRI beta protein expression without altering the alpha or gamma subunits. The ability of IL-4 and IL-10 to alter FcepsilonRI expression by targeting the beta-chain, a critical receptor subunit known to modulate receptor expression and signaling, suggests the presence of a Th2 cytokine-mediated homeostatic network that could serve to both initiate and limit mast cell effector function.

FcεRIγ-ITAM Is Differentially Required for Mast Cell Function In Vivo

The cross-linking of IgE-bound FcepsilonRI by Ags triggers mast cell activation leading to allergic reactions. The in vivo contribution of FcepsilonRIgamma signaling to IgE/FcepsilonRI-mediated mast cell responses has not yet been elucidated. In this study FcepsilonRIgamma(-/-) mast cells were reconstituted with either wild-type or mutant FcepsilonRIgamma in transgenic mice and transfected mast cells in vitro. We demonstrate that FcepsilonRIgamma-immunoreceptor tyrosine-based activation motif is essential for degranulation, cytokine production, and PG synthesis as well as for passive systemic anaphylaxis. Recent reports have suggested that cell surface FcepsilonRI expression and mast cell survival are regulated by IgE in the absence of Ag, although the molecular mechanism is largely unknown. We also found that the promotion of mast cell survival by IgE without Ags is mediated by signals through the FcepsilonRIgamma-immunoreceptor tyrosine-based activation motif. In contrast, the IgE-mediated up-regulation of FcepsilonRI is independent of FcepsilonRIgamma signaling. These results indicate that FcepsilonRIgamma-mediated signals differentially regulate the receptor expression, activation, and survival of mast cells and systemic anaphylaxis.

Mast cells are required for experimental oral allergen-induced diarrhea

Gastrointestinal allergic disorders represent a diverse spectrum of inflammatory diseases that are occurring with increasing incidence and severity. An essential question concerning these disorders is to determine the specific cells and mediators responsible for specific clinical manifestations. With this in mind, we developed a murine model of oral allergen-induced intestinal inflammation accompanied by strong Th2-associated humoral and cellular responses and focused on the immunopathogenesis of allergic diarrhea. Exposure of OVA/alum-sensitized mice to repeated doses of intragastric OVA induced genetically restricted, dose-dependent, acute diarrhea associated with increased intestinal permeability, eosinophilia, and mastocytosis. Mice developed limited systemic manifestations of anaphylaxis, even though they developed marked intestinal mucosal mast cell degranulation. Notably, experiments involving mast cell depletion (with anti-c-kit mAb), anti-IgE treatment, and Fc epsilon RI-deficient mice indicated a critical effector role for mast cells in mediating allergic diarrhea. Furthermore, allergic diarrhea was dependent upon synergistic signaling induced by serotonin and platelet-activating factor (PAF), but not histamine. These results demonstrate that oral allergen-induced diarrhea associated with experimental Th2 intestinal inflammation is largely mast cell, IgE, serotonin, and PAF dependent.

IL-4 selectively enhances FcgammaRIII expression and signaling on mouse mast cells

Fc receptors for IgG (FcgammaR) are widely expressed in the hematopoietic system and mediate a variety of inflammatory responses. There are two functional classes of FcgammaR, activation and inhibitory receptors. Since IgG immune complexes (IgG IC) bind each class with similar affinity, co-expression of these receptors leads to their co-ligation. Thus, expression levels of this antagonistic pair play a critical role in determining the cellular response. Murine mast cells co-express the activation receptor FcgammaRIII and the inhibitory receptor FcgammaRIIb and can be activated by IgG IC. Mast cell activation contributes to allergic and other inflammatory diseases-particularly those in which IgG IC may play important roles. Using mouse bone marrow-derived mast cells, we report that IL-4 selectively increases FcgammaRIII expression without altering FcgammaRIIb. This enhanced expression could be induced by Stat6 activation alone, and appeared to be mediated in part by increased FcgammaRIIIalpha protein synthesis without significant changes in transcription. The increase in FcgammaRIII expression was functionally significant, as it was matched by enhanced FcgammaR-mediated degranulation and cytokine production. Selective regulation of mast cell FcgammaR by interleukin-4 could alter inflammatory IgG responses and subsequently disease severity and progression.

LD mapping of maternally and non-maternally derived alleles and atopy in FcepsilonRI-beta

Polymorphisms in the beta chain of the high affinity receptor for IgE (Fc epsilon RI-beta, MS4A2) are consistently associated with traits underlying asthma and atopy (immunoglobulin E-mediated allergy). However, the causal variants and haplotypes underlying disease have not yet been identified. Maternal effects, with association confined to maternally derived alleles, have been shown in some studies but not in others. We have therefore extended the known sequence and systematically detected polymorphisms across an 18.1 Kb genomic region that includes Fc epsilon RI-beta. Association testing in two panels of subjects showed the presence of single-nucleotide polymorphisms (SNPs) affecting prick skin tests and specific IgE responses in several clusters. Stepwise analyses indicated that the clusters represent independent effects. Interferon regulatory factor 2 (IRF-2) sites were altered by significantly associated SNPs in two regions. Strong association to maternally derived alleles was seen in one panel of subjects and not in the other. Maternal and non-maternally derived associations tended to share the same SNP clusters, but associations were stronger in the presence of maternal effects. Two regions of increased CpG concentration were identified in Fc epsilon RI-beta. One of these approximated a SNP cluster that showed strong association and maternal effects, providing a potential substrate for epigenetic effects.

Human serum IgE-mediated mast cell degranulation shows poor correlation to allergen-specific IgE content

BACKGROUND

Although allergen-specific IgE content in serum can be determined immunochemically, little is known about the relationship between this parameter and the strength of the degranulation response upon allergen triggering.

OBJECTIVES

Analyse the degranulation capacity of immunochemically defined purified and serum IgE after challenge with anti-IgE or allergen using a rat mast cell line (RBL) transfected with the alpha-chain of the human high-affinity IgE receptor (FcepsilonRI).

METHODS

Purified IgE specific for 4-hydroxy-3nitrophenylacetyl, purified IgE of unknown specificity, and sera from allergic patients sensitive to Dermatophagoides pteronyssinus and Dactylis glomerata were assessed. Degranulation was measured by a beta-hexosaminidase release assay after anti-IgE or allergen-specific challenge.

RESULTS

For purified monoclonal IgE a significant correlation (r = 0.97) was found between the proportion of bound allergen-specific IgE and the strength of the degranulation response. In contrast, no correlation (r = 0.27) was detected after sensitization with serum IgE.

CONCLUSION

Our studies demonstrate that mast cell activation mediated through IgE from allergic patients is a result of complex relationships that are not only dependent on allergen-specific IgE content but also relate to the capacity to efficiently sensitize and trigger the signalling responses that lead to degranulation.

Regulation of the human high affinity IgE receptor beta-chain gene expression via an intronic element

The high affinity IgE receptor, FcepsilonRI, is a key regulatory molecule in the allergic reaction. By screening for cis-acting elements over the entire region of the human FcepsilonRI beta-chain gene, a sequence located in the fourth intron was revealed to serve as a repressor element. This element was recognized by a transcription factor, myeloid zinc finger protein 1 (MZF-1). Introduction of MZF-1 antisense inhibited the suppressive effect of the element on the beta-chain promoter and increased the mRNA for the beta-chain in KU812 cells, indicating that MZF-1 repressed human FcepsilonRI beta-chain gene expression via the element in the fourth intron. Furthermore, it was suggested that a cofactor binding with MZF-1, whose expression level was different among the cell types, was required for transcriptional repression by MZF-1.

A lipid raft environment enhances Lyn kinase activity by protecting the active site tyrosine from dephosphorylation

The plasma membrane contains ordered lipid domains, commonly called lipid rafts, enriched in cholesterol, sphingolipids, and certain signaling proteins. Lipid rafts play a structural role in signal initiation by the high affinity receptor for IgE. Cross-linking of IgE-receptor complexes by antigen causes their coalescence with lipid rafts, where they are phosphorylated by the Src family tyrosine kinase, Lyn. To understand how lipid rafts participate in functional coupling between Lyn and FcepsilonRI, we investigated whether the lipid raft environment influences the specific activity of Lyn. We used differential detergent solubility and sucrose gradient fractionation to isolate Lyn from raft and nonraft regions of the plasma membrane in the presence or absence of tyrosine phosphatase inhibitors. We show that Lyn recovered from lipid rafts has a substantially higher specific activity than Lyn from nonraft environments. Furthermore, this higher specific activity correlates with increased tyrosine phosphorylation at the active site loop of the kinase domain. Based on these results, we propose that lipid rafts exclude a phosphatase that negatively regulates Lyn kinase activity by constitutive dephosphorylation of the kinase domain tyrosine residue of Lyn. In this model, cross-linking of FcepsilonRI promotes its proximity to active Lyn in a lipid raft environment.

Competing functions encoded in the allergy-associated F(c)epsilonRIbeta gene

Allergic reactions are triggered via crosslinking of the high-affinity receptor for immunoglobulin E, F(c)epsilonRI. In humans, F(c)epsilonRI is expressed as a tetramer (alphabetagamma(2)) and a trimer (alphagamma(2)). The beta subunit is an amplifier of F(c)epsilonRI surface expression and signaling. Here, we show that as a consequence of alternative splicing, the F(c)epsilonRIbeta gene encodes two proteins with opposing and competing functions. One isoform is the full-length classical beta, the other a novel truncated form, beta(T). In contrast to beta, beta(T) prevents F(c)epsilonRI surface expression by inhibiting alpha chain maturation. Moreover, beta(T) competes with beta to control F(c)epsilonRI surface expression in vitro. We propose that the relative abundance of the products of the beta gene may control the level of F(c)epsilonRI surface expression and thereby influence susceptibility to allergic diseases.

Investigation of early events in Fc epsilon RI-mediated signaling using a detailed mathematical model

Aggregation of Fc epsilon RI on mast cells and basophils leads to autophosphorylation and increased activity of the cytosolic protein tyrosine kinase Syk. We investigated the roles of the Src kinase Lyn, the immunoreceptor tyrosine-based activation motifs (ITAMs) on the beta and gamma subunits of Fc epsilon RI, and Syk itself in the activation of Syk. Our approach was to build a detailed mathematical model of reactions involving Fc epsilon RI, Lyn, Syk, and a bivalent ligand that aggregates Fc(epsilon)RI. We applied the model to experiments in which covalently cross-linked IgE dimers stimulate rat basophilic leukemia cells. The model makes it possible to test the consistency of mechanistic assumptions with data that alone provide limited mechanistic insight. For example, the model helps sort out mechanisms that jointly control dephosphorylation of receptor subunits. In addition, interpreted in the context of the model, experimentally observed differences between the beta- and gamma-chains with respect to levels of phosphorylation and rates of dephosphorylation indicate that most cellular Syk, but only a small fraction of Lyn, is available to interact with receptors. We also show that although the beta ITAM acts to amplify signaling in experimental systems where its role has been investigated, there are conditions under which the beta ITAM will act as an inhibitor.

Regulation of the human Fc epsilon RI alpha-chain distal promoter

The alpha-chain of the high affinity receptor for IgE (Fc epsilon RI) is essential for cell surface expression of Fc epsilon RI and binding of the IgE Ab. The human alpha-chain gene possesses two promoters: the proximal promoter, which is highly conserved with that of rodent; and the distal promoter, the structure and role of which are largely unknown. Transcriptional regulation of the alpha-chain distal promoter was investigated in this study. Transient reporter assay revealed critical region for transcription activity located within -27/-17. EMSA identified Elf-1, YY1, and PU.1 as transcription factors binding to this region. In contrast to the proximal promoter, which was trans-activated by YY1 and PU.1, these transcription factors exhibited repressive function on this promoter. Addition of IL-4 caused a marked increase in transcription from the distal promoter and subsequently increased the intracellular production of the alpha-chain. These results indicate that IL-4-dependent up-regulation of the human alpha-chain was due to enhancement of distal promoter activity and suggests that the two promoters have different regulatory mechanisms for alpha-chain expression.

IgE-mediated activation of NK cells through Fc gamma RIII

NK cells express Fc gamma RIII (CD16), which is responsible for IgG-dependent cell cytotoxicity and for production of several cytokines and chemokines. Whereas Fc gamma RIII on NK cells is composed of both Fc gamma RIII alpha and FcR gamma chains, that on mast cells is distinct from NK cells and made of Fc gamma RIII alpha, FcR beta, and FcR gamma. Mast cells show degranulation and release several mediators, which cause anaphylactic responses upon cross-linking of Fc gamma RIII as well as Fc epsilon RI with aggregated IgE. In this paper, we examined whether IgE activates NK cells through Fc gamma RIII on their cell surface. We found that NK cells produce several cytokines and chemokines related to an allergic reaction upon IgE stimulation. Furthermore, NK cells exhibited cytotoxicity against IgE-coated target cells in an Fc gamma RIII-dependent manner. These effects of IgE through Fc gamma RIII were not observed in NK cells from FcR gamma-deficient mice lacking Fc gamma RIII expression. Collectively, these results demonstrate that NK cells can be activated with IgE through Fc gamma RIII and exhibit both cytokine/chemokine production and Ab-dependent cell cytotoxicity. These data imply that not only mast cells but also NK cells may contribute to IgE-mediated allergic responses.

FcgammaR polymorphisms: Implications for function, disease susceptibility and immunotherapy

Leukocyte Fcgamma receptors (FcgammaR) confer potent cellular effector functions to the specificity of IgG. FcgammaR-induced leukocyte functions, including antibody-dependent cellular cytotoxicity, phagocytosis, superoxide generation, degranulation, cytokine production and regulation of antibody production, are essential for host defense and immune regulation. The efficacy of IgG-induced FcgammaR function displays inter-individual heterogeneity due to genetic polymorphisms of three FcgammaR subclasses, FcgammaRIIa (CD32a), FcgammaRIIIa (CD16a), and FcgammaRIIIb (CD16b). FcgammaR polymorphisms have been associated with infectious and autoimmune disease, or with disease severity. FcgammaR polymorphisms may furthermore serve as markers for therapeutic efficacy and side-effects of treatment with monoclonal antibodies. In this review, FcgammaR function and the relevance of FcgammaR polymorphisms as prognostic markers for inflammatory disease and antibody-based immunotherapy are discussed.

Application of human Fc epsilon RI alpha-chain-transfected RBL-2H3 cells for estimation of active serum IgE

We transfected the alpha-chain of human FcepsilonRI into rat basophilic leukemia cell line RBL-2H3, established several stable transfected cells, and screened them by beta-hexosaminidase release induced by sensitization with human IgE and stimulation with anti-human IgE antibody. A cloned cell line RBL-hEIa-2B12 was the strongest responder among the transfected cell clones. The concentrations of cytosolic free Ca2+ concentration in the human IgE-sensitized cells increased after stimulation with anti-human IgE antibody. Thus, it is suggested that the alpha-chain of human FcepsilonRI is associated with the beta-chain and/or gamma-chain of rat FcepsilonRI, and that they form functional high affinity IgE receptor complexes. The total IgE concentrations of the sera from allergic patients were determined by using the beta-hexosaminidase release assay, where the transfected cells were sensitized with diluted and heat-inactivated (at 56 degrees C for 30 min) serum and stimulated with anti-human IgE antibody. The IgE concentration obtained correlated with those measured by an enzyme immunoassay method. beta-Hexosaminidase release induced by stimulation with 5 times diluted serum was sometimes less than the release induced by the same serum; diluted 25 times or 125 times, suggesting that these serum contained factors that blocked IgE binding to FcepsilonRI or cross-linking by anti-human IgE antibody. The results suggested that our system will be useful for detecting FcepsilonRIalpha-bindable IgE in human serum.

Regulation of cell type-specific mouse Fc epsilon RI beta-chain gene expression by GATA-1 via four GATA motifs in the promoter

The FcR beta-chain, a subunit of two related multisubunit receptor complexes, the FcepsilonRI and FcgammaRIII, amplifies the mast cell response and is necessary for the cell surface expression of FcepsilonRI in mouse. The transient reporter assay indicated that -69/+4 region is required for cell type-specific transcriptional regulation of mouse beta-chain gene. EMSA using Abs against transcription factors or competitive oligonucleotides demonstrated that -58/-40 region (containing overlapping three GATA-1 sites, -53/-48, -46/-51, and -42/-47) and -31/-26 region (containing one GATA-1 site) are recognized by GATA-1. The promoter activity of beta-chain was decreased by nucleotide replacements of the GATA-1 sites in mouse mast cell line PT18. Furthermore, exogenously produced GATA-1 up-regulated the promoter activity in CV-1 cells, which are negative in the beta-chain production and the up-regulation was apparently suppressed by GATA-1 site mutations. These results indicate that cell type-specific transcription of mouse beta-chain gene is regulated by GATA-1.

[The high-affinity IgE receptor: lessons from structural analysis]

The high affinity receptor for IgE, FcERI, is at the core of the allergic reaction. This receptor is expressed mainly on mast cells and basophils. Interaction of an allergen with its specific IgE bound to FcERI triggers cell activation, which induces the release of numerous mediators that are responsible for allergic manifestations. The recent increase in the prevalence of allergic diseases in developed countries has resulted in renewed efforts towards the development of new drugs. One of these is a humanised antibody directed against the IgE ligand. This antibody recognises specifically free but not FcERI-bound IgE thus preventing ligand binding and subsequent cell activation. This antibody has shown some efficacy in clinical trials involving patients with asthma and allergic rhinitis. The recent elucidation of the tridimensional structure of the complex between IgE and FcERI provides unexpected information regarding the mechanism of assembly of the complex, which now can be used to design small chemical compounds capable of specifically inhibiting this interaction.

Molecular aspects of allergy

Atopic diseases such as asthma, rhinitis, eczema and food allergies have increased in most industrialised countries of the world during the last 20 years. The reasons for this increase are not known and different hypotheses have been assessed including increased exposure to sensitising allergens or decreased stimulation of the immune system during critical periods of development. In allergic diseases there is a polarisation of the Th2 response and an increase in the production of type 2 cytokines which are involved in the production of immunoglobulin E and the development of mast cells, basophils and eosinophils leading to inflammation and disease. The effector phase of atopy is initiated by interaction with Fc epsilon RI expressed on effector cells such as mast cells and basophils but also found on an ever increasing list of cells. Binding of a polyvalent allergen to the variable part of IgE leads to a cross-link of the receptor that triggers the cell to release histamine and pharmacological mediators of the symptomatic allergic response. Cross-linking of Fc epsilon RI by autoantibodies against the alpha-chain of the Fc epsilon RI, causing subsequent histamine release is thought to be involved in the pathogenesis of other diseases such as chronic idiopathic urticaria (CIU). To date, most therapeutic strategies are aimed at inhibiting and controlling components of the inflammatory response. Recently, new treatment strategies have emerged that focus on the development of preventive and even curative treatments. The most promising therapeutic approaches are aimed at inhibiting the IgE-Fc epsilon RI interaction with the use of non-anaphylactogenic anti-IgE or anti-Fc epsilon RIalpha autoantibodies. Clinical trials in humans using an humanised anti-IgE antibody showed that this antibody was well tolerated and reduced both symptoms and use of medication in asthma and allergic rhinitis. Thus interruption of the atopic cascade at the level of the IgE-Fc epsilon RI interaction with the use of non-anaphylactogenic antibodies is effective and represents an attractive therapy for the treatment of atopic disease.

Transcriptional regulation of the human high affinity IgE receptor alpha-chain gene

Transcriptional regulation of the gene encoding human high affinity IgE receptor (Fc epsilon RI) alpha-chain was analyzed. Previously, we reported that GATA-1 and Elf-1 recognition sites were necessary for cell type-specific activation of the alpha-chain gene promoter. More detailed analysis revealed that other transcription factors bound the regions close to the Elf-1 recognition site and there was a more complex mechanism for the regulation of the promoter activity. On the other hand, during a course of studies to find cis-elements over this gene, CAGCTG sequence in the first intron was revealed to serve as an enhancer. A complex composed of USF1 and USF2 activated the human alpha-chain gene expression via this intronic element. Furthermore, we found two novel exons at 18.4 and 12.6kb upstream from the reported first exon and discovered an additional distal promoter.

Macromolecular protein signaling complexes and mast cell responses: a view of the organization of IgE-dependent mast cell signaling

The generation of signals following engagement of cell surface receptors is an ordered process that requires tight regulation as spurious signals could result in unwanted, and possibly deleterious, cellular responses. Like other cell surface receptors, stimulation of a mast cell via the high affinity IgE receptor (FcepsilonRI) causes multiple biochemical events that ultimately result in cell activation and effector responses. Recently, our knowledge of how these events are ordered has increased. We now have identified some of the molecules involved, how they are organized into macromolecular complexes by FcepsilonRI stimulation, and the role of some of the constituents of these macromolecular signaling complexes in mast cell effector responses. In brief, we review the knowledge on macromolecular signaling complexes used by FcepsilonRI in mast cell activation and provide our view on the regulation of signal generation and its effect on mast cell activation.

IgE(+), Kit(-), I-A/I-E(-) myeloid cells are the initial source of Il-4 after antigen challenge in a mouse model of allergic pulmonary inflammation

BACKGROUND

IL-4 is generated within hours after antigen lung challenge and influences events that take place early in the induction of pulmonary inflammation. However, the cells responsible for this early IL-4 production in the lung are unknown.

OBJECTIVES

We sought to characterize the initial inflammatory events in the lung after antigen challenge and to identify cells responsible for producing IL-4 at early time points.

METHODS

Mice were sensitized with ovalbumin or passive IgE and challenged intranasally. Histologic measures of inflammation were used, and lung tissue cytokine production was analyzed by means of RT-PCR. Cells producing IL-4 were characterized by means of in situ hybridization and flow cytometry.

RESULTS

IL-4 mRNA was detectable 100 minutes after challenge in sensitized animals. Blockade of this early IL-4 downregulated vascular cell adhesion molecule 1 mRNA expression and attenuated the early recruitment of eosinophils to the lung. CD4-depleted and mast cell-deficient mice both expressed early IL-4. Cellular analysis revealed the presence of IL-4 protein at 100 minutes exclusively in IgE(+) myeloid cells that did not express CD3, Kit, or I-A/I-E. Moreover, IL-4 production induced by means of passive IgE sensitization and abrogated in FcR gamma-chain-deficient mice supports the conclusion that this IL-4 production is dependent on IgE/gamma-chain interaction.

CONCLUSION

IL-4 production by an IgE/gamma-chain-dependent mechanism occurs rapidly after allergen challenge. At these early time points, IL-4 is produced by a myeloid cell with the characteristics of a mouse basophil (IgE(+), Kit(-), I-A/I-E(-)). These data thus suggest that strategies targeting basophils should be considered in the treatment of early lung inflammation.

Structural insights into the interactions between human IgE and its high affinity receptor FcepsilonRI

The interaction of IgE antibodies with the high affinity IgE receptor, FcepsilonRI, is a key step in the initiation of anti-parasitic immunity and allergic reactions. Recent structural studies of the receptor, the IgE-Fc and the IgE-Fc:FcepsilonRI complex have revealed how these two proteins interact to prime mast cell responses to antigen. The structures have revealed a novel arrangement for the FcepsilonRI ectodomains that is also observed in homologous members of this antibody receptor family. The crystal structure of the IgE-Fc:FcepsilonRI complex clarified how a 1:1 complex between the antibody and receptor is formed, with the receptor binding each chain of the antibody Fc dimer. The IgE-Fc structure in the absence of the receptor revealed the potential for large conformational rearrangements within the IgE that may affect receptor binding. These studies provide the basis for further investigation of the specificity of antibody:receptor binding and for the development of new treatments for allergic hypersensitivities.

In vivo kinetics of the immunoglobulin E response to allergen: bystander effect of coimmunization and relationship with anaphylaxis

BACKGROUND

Murine models of hypersensitivity to allergens are useful tools for the evaluation of preclinical strategies to down-regulate the IgE response.

OBJECTIVE

To monitor the long-term kinetics of T and B cell responses to allergen as a function of allergen dosage and to investigate the effect of parallel immunization with a second antigen; to correlate B cell response with anaphylaxis.

METHODS

CBA/J mice were sensitized every other week by subcutaneous injections of phospholipase A2 (PLA2) and/or ovalbumin (OVA) adsorbed to alum. Specific antibody isotype responses, T cell proliferation, T cell cytokine production and anaphylaxis were assessed throughout the sensitization phase.

RESULTS

Low-dose immunization with PLA2 (0.1 microg) favoured a long-term, specific T helper (Th)2 response with high IgE and IL-4 production in contrast to high-dose PLA2 (10 microg) immunization, which biased the immune response towards a Th1 response with high IgG2a and low IL-4 production. Parallel immunization with an unrelated antigen (ovalbumin) had a significant bystander effect on the immunization with PLA2, which was also dose-dependent. Finally, although anaphylaxis as measured by rectal temperature drop was allergen-specific, it could be induced in the high- and low-dose immunization groups, and was not solely dependent on IgE levels.

CONCLUSION

Though low-dose allergen immunization appears to induce an efficient IgE response, the intensity and quality of this response may be modulated by bystander effects of parallel immunization and does not correlate strictly with anaphylaxis. This observation has relevance to the design of clinical immunotherapy protocols using murine model-based data.

Anomalous leukopoiesis in two patients with Crohn's disease

Crohn's disease is a major form of chronic inflammatory bowel disease in the western world. The molecular genetic basis of Crohn's disease is unknown. In this study, we present evidence for anomalous leukopoiesis-namely, the generation of a leukocyte subset characterized by aberrant expression of gammadelta T cell receptor (gammadeltaTCR) with or without CD19 on a myeloid background-in two patients with Crohn's disease. The aberrant cells of patient 1 have the surface phenotype gammadeltaTCR + CD19 - CD14 + CD64 +. The aberrant cells of patient 2 have the surface phenotype gammadeltaTCR + CD19 + CD14 - CD64 + CD16 + CD13 + CD33 +. The results presented here are significant both in light of recent speculation that a critical defect in Crohn's disease may be at the level of hematopoiesis and because the CD19 gene lies within the region on chromosome 16 that corresponds with the Crohn's disease susceptibility locus IBD1.

Drastic up-regulation of Fcepsilonri on mast cells is induced by IgE binding through stabilization and accumulation of Fcepsilonri on the cell surface

It has been shown that IgE binding to FcepsilonRI on mast cells results in increased FcepsilonRI expression, which in turn enhances IgE-dependent chemical mediator release from mast cells. Therefore, prevention of the IgE-mediated FcepsilonRI up-regulation would be a promising strategy for management of allergic disorders. However, the mechanism of IgE-mediated FcepsilonRI up-regulation has not been fully elucidated. In this study, we analyzed kinetics of FcepsilonRI on peritoneal mast cells and bone marrow-derived mast cells. In the presence of brefeldin A, which prevented transport of new FcepsilonRI molecules to the cell surface, levels of IgE-free FcepsilonRI on mast cells decreased drastically during culture, whereas those of IgE-bound FcepsilonRI were stable. In contrast, levels of FcgammaRIII on the same cells were stable even in the absence of its ligand, indicating that FcepsilonRI alpha-chain, but not beta- and gamma-chains, was responsible for the instability of IgE-free FcepsilonRI. As far as we analyzed, there was no evidence to support the idea that IgE binding to FcepsilonRI facilitated synthesis and/or transport of FcepsilonRI to the cell surface. Therefore, the stabilization and accumulation of FcepsilonRI on the cell surface through IgE binding appears to be the major mechanism of IgE-mediated FcepsilonRI up-regulation.

Cloning and characterization of a mouse homologue of the human haematopoietic cell-specific four-transmembrane gene HTm4

Haematopoietic cell-specific transmembrane-4 (HTm4) is a four-transmembrane protein most closely related to CD20 and the beta subunit of the high affinity receptor for IgE (Fc(epsilon)RIbeta). To date, it has only been described in humans, where it is expressed in haematopoietic cells of both myeloid and lymphoid lineages. The function of HTm4 is unknown; however, as for CD20 and Fc(epsilon)RI-beta, it is likely to play a role in signal transduction as part of a multi-subunit cell surface receptor complex. In this study, we report the cDNA cloning and expression distribution of mouse HTm4. The deduced mouse HTm4 protein is of 213 amino acids, and contains four putative transmembrane domains. Mouse HTm4 shows 62% overall amino acid identity with human HTm4; the transmembrane regions are highly conserved between both species (75% identity), whereas the N- and C-terminal and inter-transmembrane loop regions are more divergent (52%). Interestingly, the N-terminal domain of mouse HTm4 is predicted to be 23 amino acids shorter, and the C-terminal domain 23 amino acids longer, than that of human HTm4. Northern blot and reverse transcriptase (RT)-PCR analysis suggest that mouse HTm4 mRNA is expressed at low levels only in spleen, bone marrow and peripheral blood leucocytes. This is the first report of the cloning of HTm4 from a species other than human, and provides important sequence information towards the understanding of the function of this poorly characterized four-transmembrane molecule.

Minimal requirements for IgE-mediated regulation of surface Fc epsilon RI

The IgE-FcepsilonRI network plays a central role in allergic inflammation. IgE levels control cell surface levels of FcepsilonRI and, in turn, FcepsilonRI levels modulate the intensity of effector responses. Treatment of allergic patients with anti-IgE Abs has been shown to induce a decrease in FcepsilonRI expression on basophils and a decrease in Ag-triggered histamine release. However, the mechanisms underlying IgE-mediated regulation of FcepsilonRI expression remain unclear. Here, we designed an in vitro model system to establish the minimal cellular requirements for regulation of FcepsilonRI by IgE. Using this system, we demonstrate that transcriptional regulation, hemopoietic-specific factors, and signaling are not required for IgE-mediated increases in FcepsilonRI expression. IgE binding to the alpha-chain is the minimal requirement for the induction of FcepsilonRI up-regulation. The rate of up-regulation is independent of the baseline level of expression. The mechanism of this up-regulation is the result of a combination of three factors: 1) stabilization of the receptor at the cell surface, which prevents receptor internalization and degradation; 2) use of a preformed pool of receptor comprising recycled and recently synthesized receptors; and 3) continued basal level of protein synthesis. It is possible that in vivo additional factors contribute to modulate the basic regulatory mechanism described here.

Human eosinophils and human high affinity IgE receptor transgenic mouse eosinophils express low levels of high affinity IgE receptor, but release IL-10 upon receptor activation

FcepsilonRI expressed by human eosinophils is involved in IgE-mediated cytotoxicity reactions toward the parasite Schistosoma mansoni in vitro. However, because receptor expression is low on these cells, its functional role is still controversial. In this study, we have measured surface and intracellular expression of FcepsilonRI by blood eosinophils from hypereosinophilic patients and normal donors. The number of unoccupied receptors corresponded to approximately 4,500 Ab binding sites per cell, whereas 50,000 Ab binding sites per cell were detected intracellularly. Eosinophils from patients displayed significantly more unoccupied receptors than cells from normal donors. This number correlated to both serum IgE concentrations and to membrane-bound IgE. The lack of FcepsilonRI expression by mouse eosinophils has hampered further studies. To overcome this fact and experimentally confirm our findings on human eosinophils, we engineered IL-5 x hFcepsilonRIalpha double-transgenic mice, whose bone marrow, blood, spleen, and peritoneal eosinophils expressed FcepsilonRI levels similar to levels of human eosinophils, after 4 days culture with IgE in the presence of IL-5. Both human and mouse eosinophils were able to secrete IL-10 upon FcepsilonRI engagement. Thus, comparative analysis of cells from patients and from a relevant animal model allowed us to clearly demonstrate that FcepsilonRI-mediated eosinophil activation leads to IL-10 secretion. Through FcepsilonRI expression, these cells are able to contribute to both the regulation of the immune response and to its effector mechanisms.

Cloning of full-length genomic DNA encoding human FcepsilonRI alpha-chain and its transcriptional regulation

Two novel exons, named exon 1A and exon 2A, were found at 18.4 and 12.6 kb upstream from the exon known as the first exon of human FcepsilonRI alpha-chain gene. Transcription from the promoter present in the upstream of exon 1A was decreased by mutations introduced into the "first intron" between exon 1A and exon 2A, suggesting the presence of an intronic regulatory element in the intron. Consistent with this, electrophoretic mobility shift assay revealed the presence of a nuclear factor which bound the region in FcepsilonRI alpha-chain positive cells.

Expression and modulation of FcepsilonRIalpha and FcepsilonRIbeta in human blood basophils

BACKGROUND

The IgE receptor (FcepsilonRI) may exist as a tetramer (alphabetagamma2) or a trimer (alphagamma2) because FcepsilonRIbeta is dispensable for membrane expression of FcepsilonRIalpha. FcepsilonRIbeta amplifies signaling of FcepsilonRI so that regulation of FcepsilonRIalpha:beta stoichiometry would affect cellular responsiveness.

OBJECTIVE

We examined basophils from a variety of donors for differences in their expression of FcepsilonRIalpha and FcepsilonRIbeta protein.

METHODS

Enriched blood basophils were assessed at baseline and after IL-3 culture for FcepsilonRIalpha and FcepsilonRIbeta protein by Western blotting, surface FcepsilonRIalpha by flow cytometry, and FcepsilonRIbeta mRNA by real-time PCR. Basophil functional response was measured by allergen-triggered histamine release.

RESULTS

For the FcepsilonRIalpha subunit, 2 protein bands with molecular weights of 50 kd and 60 kd were identified by Western blots. The 60-kd band correlated to surface-expressed FcepsilonRIalpha detected by flow cytometry (Spearman R = 0.78, P <.01). Surface FcepsilonRIalpha also correlated with FcepsilonRIbeta protein (Spearman R = 0.92, P <.01). FcepsilonRIbeta protein levels increased disproportionately with higher surface FcepsilonRIalpha expression. The ratio of FcepsilonRIbeta to FcepsilonRIalpha varied 10-fold among donors and correlated with surface FcepsilonRIalpha. Basophil 50-kd alpha protein levels were similar despite a 10-fold range in surface FcepsilonRIalpha expression, implying stores of this protein such as those found in eosinophils. Unlike eosinophils, the basophil 50-kd protein was lost with culture and was absent from supernatants. Levels of beta protein and mRNA were enhanced by IL-3 culture, whereas FcepsilonRIalpha expression (by flow cytometry and 60 kd) was not.

CONCLUSION

These findings demonstrate variable stoichiometry of FcepsilonRIalpha:beta in whole cells and that this stoichiometry can be altered by IL-3 culture. With the assumption that all detected beta protein is surface expressed, these findings suggest a variable stoichiometry for FcepsilonRIalpha:beta that is also related to FcepsilonRIalpha surface expression.

Fc(epsilon)RI as a paradigm for a lipid raft-dependent receptor in hematopoietic cells

Lipid domains or rafts are currently embraced by immunologists as critical participants in receptor-mediated signaling events occurring at the plasma membrane. This view of membrane heterogeneity and its functional importance is supported by many years of different experimental approaches. We can now refine our investigations, moving beyond the simple models to ask more detailed questions about structural properties and mechanistic interactions. As highlighted for the IgE receptor (Fc(epsilon)RI), new information about initial engagement with src family kinases, cytoskeletal regulation, and coupling with downstream signaling is beginning to emerge.

Human neutrophils express the high-affinity receptor for immunoglobulin E (Fc epsilon RI): role in asthma

Polymorphonuclear neutrophils (PMNs) are important effector cells in host defense and the inflammatory response to antigen. The involvement of PMNs in inflammation is mediated mainly by the Fc receptor family, including IgE receptors. Recently, PMNs were shown to express two IgE receptors (CD23/Fc epsilon RII and galectin-3). In allergic diseases, the dominant role of IgE has been mainly ascribed to its high-affinity receptor, Fc epsilon RI. We have examined the expression of Fc epsilon RI by PMNS: mRNA and cell surface expression of Fc epsilon RI alpha chain was identified on PMNs from asthmatic subjects. Furthermore, preincubation with human IgE Fc fragment blocks completely the binding of anti-Fc epsilon RI alpha chain (mAb15--1) to human PMNS: Conversely, preincubation of PMNs with mAb15--1 inhibits significantly the binding of IgE Fc fragment to PMNs, indicating that IgE bound to the cell surface of PMNs mainly via the Fc epsilon RI. Peripheral blood and bronchoalveolar lavage (BAL) PMNs from asthmatic subjects also express intracellular Fc epsilon RI alpha and beta chain immunoreactivity. Engagement of Fc epsilon RI induces the release of IL-8 by PMNS: Collectively, these observations provide new evidence that PMNs express the Fc epsilon RI and suggest that these cells may play a role in allergic inflammation through an IgE-dependent activation mechanism.

Identification of a CD20-, FcepsilonRIbeta-, and HTm4-related gene family: sixteen new MS4A family members expressed in human and mouse

CD20, high-affinity IgE receptor beta chain (FcepsilonRIbeta), and HTm4 are structurally related cell-surface proteins expressed by hematopoietic cells. In the current study, 16 novel human and mouse genes that encode new members of this nascent protein family were identified. All family members had at least four potential membrane-spanning domains, with N- and C-terminal cytoplasmic domains. This family was therefore named the membrane-spanning 4A gene family, with at least 12 subgroups (MS4A1 through MS4A12) currently representing at least 21 distinct human and mouse proteins. Each family member had unique patterns of expression among hematopoietic cells and nonlymphoid tissues. Four of the 6 human MS4A genes identified in this study mapped to chromosome 11q12-q13.1 along with CD20, FcepsilonRIbeta, and HTm4. Thus, like CD20 and FcepsilonRIbeta, the other MS4A family members are likely to be components of oligomeric cell surface complexes that serve diverse signal transduction functions.

Identification of a new multigene four-transmembrane family (MS4A) related to CD20, HTm4 and beta subunit of the high-affinity IgE receptor

We report here the cloning of eight new cDNAs that encode a family of proteins related to the B-cell-specific antigen CD20, a hematopoietic-cell-specific protein HTm4, and high affinity IgE receptor beta chain (FcvarepsilonRIbeta). They include four clones from human, and another four clones from mouse. They share similar structure (four transmembrane domains) with amino acid identities of 25-40%. Therefore, they represent distinct genes and comprise a gene superfamily. This superfamily is now named membrane-spanning four-domains, subfamily A (the approved symbol is MS4A) to distinguish them from tetraspanins with similar structure. The highest homologies among these proteins are found in the transmembrane domains, especially in the first and second transmembrane domains, and conserved residues are also recognized in the inter-transmembrane domains. In northern blot, they were mostly expressed in lymphoid tissues: thymus and spleen. However, some were expressed in nonlymphoid tissues including brain, heart, kidney, liver, testis, lung, GI tracts, and pancreas. They may represent proteins functioning either directly as ligand-gated ion channels or as essential components of such channels. The identification of this relatively large gene family in various tissues will allow the further elucidation of physiological significance of this gene family, that is currently unclear.

Stimulatory and inhibitory signals originating from the macrophage Fcgamma receptors

The macrophage receptors for the Fc portion of immunoglobulin G (FcgammaR) have long been known to mediate a variety of effector functions that are vital to the adaptive immune response. Recent studies, however, have begun to stress potential regulatory roles that these receptors can play in modulating immune and inflammatory responses. In this article we discuss the activating and inhibitory properties of the individual macrophage FcgammaR and the conditions under which these heterologous responses can occur.

Vaccine strategies against schistosomiasis: from concepts to clinical trials

Schistosomiasis, the second major parasitic disease in the world after malaria, affects 200 million people. Vaccine strategies represent an essential component of the control of this chronic debilitating disease where the deposition of millions of eggs in the tissues is the main cause of pathology. Research developed in our laboratory over the last 20 years has led to the identification of novel effector mechanisms, pointing for the first time to the protective role of Th2 responses and of IgE antibodies now supported by seven studies in human populations. The identification and molecular cloning of a target antigen, a glutathione S-transferase (GST), has made it possible to demonstrate its vaccine potential in several animal species (rodents, cattle, primates) and to establish consistently the capacity of vaccination to reduce female worm fecundity and egg viability through the production of neutralizing antibodies (IgA and IgG). Following promising preclinical studies, clinical trials (phase I and II) have been undertaken using Schistosoma haematobium GST, Sh28GST. High titers of neutralizing antibodies were produced (IgG3 and IgA) together with Th2 cytokines, consistently with the concepts developed from experimental models. With these results we are on the way towards a feasible approach of vaccine development against a major human parasitic disease.

Role of the high affinity immunoglobulin E receptor in bacterial translocation and intestinal inflammation

A role for immunoglobulin E and its high affinity receptor (Fc epsilon RI) in the control of bacterial pathogenicity and intestinal inflammation has been suggested, but relevant animal models are lacking. Here we compare transgenic mice expressing a humanized Fc epsilon RI (hFc epsilon RI), with a cell distribution similar to that in humans, to Fc epsilon RI-deficient animals. In hFc epsilon RI transgenic mice, levels of colonic interleukin 4 were higher, the composition of fecal flora was greatly modified, and bacterial translocation towards mesenteric lymph nodes was increased. In hFc epsilon RI transgenic mice, 2,4,6-tri-nitrobenzenesulfonic acid (TNBS)-induced colitis was also more pronounced, whereas Fc epsilon RI-deficient animals were protected from colitis, demonstrating that Fc epsilon RI can affect the onset of intestinal inflammation.

Distinct aggregation of beta- and gamma-chains of the high-affinity IgE receptor on cross-linking

The high-affinity IgE receptor (FcepsilonRI) on mast cells and basophils consists of a ligand-binding alpha-chain and two kinds of signaling chains, a beta-chain and disulfide-linked homodimeric gamma-chains. Crosslinking by multivalent antigen results in the aggregation of the bound IgE/alpha-chain complexes at the cell surface, triggering cell activation, and subsequent internalization through coated pits. However, the precise topographical alterations of the signaling beta- and gamma-chains during stimulation remain unclarified despite their importance in ligand binding/signaling coupling. Here we describe the dynamics of FcepsilonRI subunit distribution in rat basophilic leukemia cells during stimulation as revealed by immunofluorescence and immunogold electron microscopy. Immunolocalization of beta- and gamma-chains was homogeneously distributed on the cell surfaces before stimulation, while crosslinking with multivalent antigen, which elicited optimal degranulation, caused a distinct aggregation of these signaling chains on the cell membrane. Moreover, only gamma- but not beta-chains were aggregated during the stimulation that evoked suboptimal secretion. These findings suggest that high-affinity IgE receptor beta- and gamma-chains do not co-aggregate but for the most part form homogenous aggregates of beta-chains or gamma-chains after crosslinking.

Allergy-associated polymorphisms of the Fc epsilon RI beta subunit do not impact its two amplification functions

Two variants of the beta-chain of the high affinity IgE receptor Fc epsilon RI, I181L-V183L and E237G, have been found associated with allergy. We have previously shown that the beta-chain plays at least two distinct amplifier functions. It amplifies Fc epsilon RI surface expression and signaling, resulting in an estimated 12- to 30-fold amplification of downstream events. To test the hypothesis that the I181L-V183L and E237G beta variants may be functionally relevant and could directly contribute to an allergic phenotype, we have evaluated the functional impact of the beta variants on the two amplifier functions of beta. We found that these variants have no direct effect on the beta amplifier functions. However, the possibility remains that these variants are in linkage disequilibrium with other more relevant polymorphisms or are affecting unknown beta-chain functions.

Intranasal sensitization of Japanese cedar pollen by the co-administration of low doses of cholera toxin but not its recombinant B subunit to mice

We evaluated the effects of cholera toxin (CT) and the B subunit of cholera toxin (CTB) on the intranasal sensitization of Japanese cedar pollen (JCP) in mice. JCP suspended in phosphate-buffered saline was administered into the nostrils of mice in combination with varying doses of CT or recombinant CTB(r-CTB) once a week for 5 weeks. Antibody responses specific to sugi basic protein (SBP) were monitored by ELISA for seven weeks. The sensitization of JCP alone did not induce IgG1, IgG2b, IgG2a, IgE or IgA. In contrast, sensitization of JCP in combination with CT (JCP/CT) elicited the prominent production of SBP-specific IgG1 and low levels of IgG2b and IgG2a on Day 49. IgE production was detected only in the serum of mice which were treated with JCP/CT, and not under any other protocol. Using spleen cells from these mice, cytokine production was examined by ELISA in culture supernatants after they had been stimulated in vitro with major cedar pollen allergens, Cry j 1, Cry j 2 or SBP. Notable responses were an increase of IFN-gamma as well as IL-4 in JCP/CT-sensitized cells stimulated with Cry j 2, but not in those stimulated with Cry j 1. No significant differences were detected in IL-5 production among the experimental groups. Histopathological examination, however, showed that eosinophil infiltration was evident in the nasal mucosa of the JCP/CT-sensitized mice following challenge with JCP/CT, but weak with BSA/CT or CT alone. Thus, the immunological and histological analyses indicated that the co-administration of a low dose of CT in combination with JCP allows the induction of pollen-allergic states in mice.

Expression of a functional Fc epsilon RI on rat eosinophils and macrophages

Besides its crucial role in type I hypersensitivity reactions, IgE is involved in anti-parasite immunity. This role has been clearly demonstrated in both human and rat schistosomiasis, but remains controversial in the mouse. Since the cellular distribution of the high affinity IgE receptor, Fc epsilon RI, differs in humans and mice, it might explain the differences in effector function of IgE between the two species. In humans, eosinophils and macrophages induce IgE-dependent cytotoxicity toward Schistosoma mansoni larvae, which involves Fc epsilon RI in the case of eosinophils. In the present study, we have investigated the expression and function of Fc epsilon RI in rat eosinophils and macrophages. We demonstrate, by flow cytometry, fluorescence microscopy, and western blot analysis, that in rats, as in humans, a functional alpha gamma 2 trimeric Fc epsilon RI is expressed on eosinophils and macrophages. We also show that these two cell types can induce IgE-mediated, Fc epsilon RI-dependent cellular cytotoxicity toward schistosomula. These results thus provide a molecular basis for the differences observed between rat and mouse regarding IgE-mediated anti-parasite immunity.

Polymorphisms in FcepsilonRI beta chain do not affect IgE-mediated mast cell activation

Genetic polyymorphisms that result in three amino acid changes in FcepsilonRI beta chain (Ile(181)-->Leu, Val(183)-->Leu, and Glu(237)-->Gly) have been identified as candidates that associate with allergic disorders such as atopy and asthma. To elucidate the biological significance of these polymorphisms in regulating the expression and function of FcepsilonRI, we generated four types of transfectants that express wild-type or mutant mouse beta chains corresponding to these human variants by retrovirus-mediated gene transfer into beta chain-deficient mouse-derived mast cells. No significant functional differences between the wild-type beta chain transfectant and any of the mutant beta chain transfectants were observed in beta-hexosaminidase release, intracellular calcium mobilization, or cytokine and leukotriene C(4) production in response to FcepsilonRI crosslinking. Our results suggest that these polymorphisms in FcepsilonRI beta chain do not affect FcepsilonRI-mediated mast cell activation at least in our mouse in vitro system.

Contributing factors to the pathobiology. The genetics of asthma

Markers in 19 chromosomal regions have shown some evidence of linkage to asthma, atopy, or related phenotypes in multiple independent genome-wide searches. Linkages to five of these regions (5q, 6p, 11q, 12q, and 13q) have also been reported in non-genome-wide screens. In addition, at least two independent studies have reported linkages to markers on 16p. Numerous candidate genes in these regions have shown varying levels of association to asthma or atopic phenotypes, potentially implicating them as disease susceptibility loci. These include the IL4, CD14, and B2ADR genes on 5q, the HLA-DRB1 and TNF genes on 6p, the FCERB1 and CC16 genes on 11q, and the IL4RA gene on 16p. It still remains to be determined whether polymorphisms in these genes account for the reported linkages in these regions. Studies are underway in laboratories around the world to identify the disease-causing variations in these genes that account for the linkages just discussed. Identifying specific genetic polymorphisms that influence asthma and atopic phenotypes will shed light on the molecular pathways involved in these complex disorders and provide a better understanding of the pathophysiology of asthma and atopy.

The diverse potential effector and immunoregulatory roles of mast cells in allergic disease

Mast cells are of hematopoietic origin but typically complete their maturation in peripheral connective tissues, especially those near epithelial surfaces. Mast cells express receptors that bind IgE antibodies with high affinity (FcepsilonRI), and aggregation of these FcepsilonRI by the reaction of cell-bound IgE with specific antigens induces mast cells to secrete a broad spectrum of biologically active preformed or lipid mediators, as well as many cytokines. Mast cells are widely thought to be essential for the expression of acute allergic reactions, but the importance of mast cells in late-phase reactions and chronic allergic inflammation has remained controversial. Although it is clear that many cell types may be involved in the expression of late-phase reactions and chronic allergic inflammation, studies in genetically mast cell-deficient and congenic normal mice indicate that mast cells may be critical for the full expression of certain features of late-phase reactions and may also contribute importantly to clinically relevant aspects of chronic allergic inflammation. Moreover, the pattern of cytokines that can be produced by mast cell populations, and the enhancement of such cytokine production in mast cells that have undergone IgE-dependent up-regulation of their surface expression of FcepsilonRI, suggests that mast cells may contribute to allergic diseases (and host defense) by acting as immunoregulatory cells, as well as by providing effector cell function.

A second amplifier function for the allergy-associated Fc(epsilon)RI-beta subunit

Genetics studies have identified the gene for the high-affinity IgE receptor (FC(epsilon)RI) beta subunit as a candidate gene for atopy. We have shown that beta is an intrinsic signaling amplifier leading to enhanced allergic responses in vivo. Here we report that beta has a second amplification function: the amplification of Fc(epsilon)RI cell surface expression. This function is due to an early association of beta with alpha, resulting in improved trafficking and maturation of alpha and receptor complexes. These data provide a possible molecular explanation for the large difference in Fc(epsilon)RI density between beta-cells such as monocytes, dendritic cells, and beta+ effector cells (mast cells, basophils). In beta+ cells, the combined signaling and expression amplification results in an estimated 12- to 30-fold amplification of downstream events.