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

Enhanced release of IgE-dependent early phase mediators from nasal polyp tissue

Background

The mast cell is a crucial effector cell in allergic rhinitis and other inflammatory diseases. During the acute allergic reaction preformed mediators such as histamine, but also de novo produced mediators such as leukotrienes (LTC₄/D₄/E₄) and prostaglandins (PGD₂) are released. Mast cells represent targets for therapeutic intervention, and thus a human ex-vivo model to stimulate mast cells taken from mucosal sites would be instrumental for drug intervention studies. We have aimed to activate mast cells within ex-vivo human nasal tissue by IgE/anti-IgE specific (ε chain specific) stimulations and in this respect to test the usability of nasal polyps versus inferior turbinates

Methods

Biopsy samples were collected from patients with nasal polyps and inferior turbinates from patients who underwent sinus or septal surgery. Tissue fragments were primed with IgE 1 μg/ml for 60 minutes and then stimulated for 30 minutes with tissue culture medium (negative control), anti-IgE 10 μg/ml, anti-IgE 30 μg/ml and ionomycin 10 μM (positive control). Histamine, leukotrienes and PGD₂ were measured in supernatants. To help provide an understanding of the extent of the response, the number of tryptase and FcεRIα positive cells was evaluated by means of immunohistochemistry and the FcεRIα-chain was measured by means of quantitative PCR in the nasal polyp and inferior turbinate tissues. Finally, the correlation between IgE concentrations in the nasal tissue and the release of mediators was analysed.

Results

Stimulations with anti-IgE on IgE-primed nasal tissue fragments lead to a concentration-dependent release of histamine, leukotrienes and PGD₂. The release of these early phase mediators was significantly higher in nasal polyps compared to inferior turbinates, although tryptase, FcεRIα positive cells and FcεRIα-chain transcripts were equally present in both groups. No correlation was found between baseline concentrations of IgE, and the release of histamine, LTC₄/LTD₄/LTE₄ and PGD₂ after stimulation.

Conclusion

This human nasal challenge model mimics the allergic early phase reaction. The release of histamine, cys-leukotrienes and PGD₂ was significantly higher in nasal polyps versus inferior turbinates, however, this observation could not be explained by differences in mast cell or FcεRI+ cell numbers.

Rabaptin-5 regulates receptor expression and functional activation in mast cells

Rab5 is a small GTPase that regulates early endocytic events and is activated by RabGEF1/Rabex-5. Rabaptin-5, a Rab5 interacting protein, was identified as a protein critical for potentiating RabGEF1/Rabex-5's activation of Rab5. Using Rabaptin-5 shRNA knockdown, we show that Rabaptin-5 is dispensable for Rab5-dependent processes in intact mast cells, including high affinity IgE receptor (FcepsilonRI) internalization and endosome fusion. However, Rabaptin-5 deficiency markedly diminished expression of FcepsilonRI and beta1 integrin on the mast cell surface by diminishing receptor surface stability. This in turn reduced the ability of mast cells to bind IgE and significantly diminished both mast cell sensitivity to antigen (Ag)-induced mediator release and Ag-induced mast cell adhesion and migration. These findings show that, although dispensable for canonical Rab5 processes in mast cells, Rabaptin-5 importantly contributes to mast cell IgE-dependent immunologic function by enhancing mast cell receptor surface stability.

Association of three sets of high-affinity IgE receptor (FcepsilonR1) polymorphisms with aspirin-intolerant asthma

BACKGROUND AND OBJECTIVE

The high-affinity IgE receptor comprises a tetramer of the ligand-binding alpha chain, a signal-augmenting beta chain, and a signal-transducing gamma chain dimer on mast cells. We hypothesized that the three subsets of the FCER1 gene may play a role in the development of the aspirin-intolerant asthma (AIA) phenotype and analyzed these genetic polymorphisms in association with clinical parameters in AIA patients.

SUBJECTS AND METHODS

Six polymorphisms of FCER1 (FCERIA-344C>T, FCER1A-95T>C, MS4A2-109T>C, MS4A2 E237G, FCER1G-237A>G, FCER1G-54G>T) were genotyped in 126 AIA patients compared to 177 patients with aspirin-tolerant asthma (ATA) and 222 normal health controls (NC).

RESULTS

A significant difference in the genotype frequencies of FCER1G-237A>G was detected between AIA and ATA patients (p<0.05) both in co-dominant and recessive analysis models, whereas no significant relationships were identified between the frequencies of the other five single-nucleotide polymorphisms and AIA, ATA, and NC subjects. In addition, AIA patients carrying the homozygous AA genotype of FCER1G-237A>G exhibited significantly higher total serum IgE levels than did those with the GG/AG genotype (p=0.012). AIA patients expressing the CT/TT genotype at FCERIA-344C>T showed a higher prevalence of serum IgE specific to Staphylococcal enterotoxin A than did those with the CC genotype (p=0.008).

CONCLUSION

The FCER1G-237A>G and FCERIA-344C>T polymorphisms may contribute to the development of AIA in a Korean population.

Pivotal advance: IgE accelerates in vitro development of mast cells and modifies their phenotype

Antigen-dependent activation of IgE-bound mast cells is critical for immediate hypersensitivity and other allergic disorders. Recent studies have revealed the effects of monomeric IgEs on mast cell survival and activation. Furthermore, IgE molecules exhibit a wide range of heterogeneity in the ability to induce mast cell activation in the absence of antigen. Highly cytokinergic (HC) IgEs can induce a variety of activation events including cell survival, degranulation, cytokine production, and migration, whereas poorly cytokinergic (PC) IgEs can do so inefficiently. Here, we show that culture of bone marrow cells in the presence of monomeric IgEs results in an increased number of mast cells compared with cultures grown without IgE. Furthermore, time in culture required to generate > or =80% pure mast cells is decreased. IgE molecules can directly influence mast cell progenitors to differentiate into mast cells. mRNA expression of several mast cell proteases and mast cell-related transcription factors is higher in mast cells cultured with an HC IgE than those cultured with a PC IgE or without IgE. Expression of early growth response factor-1, a transcription factor that is involved in the production of TNF-alpha in mast cells, is enhanced in cultures containing high and low concentrations of HC IgE and a high concentration of PC IgE. Consistent with this, expression of TNF-alpha is higher in mast cells cultured with HC IgE than PC IgE. Therefore, our results suggest that monomeric IgEs, especially HC IgEs, not only promote mast cell development but also modulate the mast cell phenotype.

Allergen induces the migration of platelets to lung tissue in allergic asthma

RATIONALE

Platelets are essential for pulmonary leukocyte recruitment, airway hyperresponsiveness, and bronchial remodeling in animals with allergic inflammation and can be found in bronchoalveolar lavage of sensitized animals. No studies, however, have explored the direct migration of platelets to lungs.

OBJECTIVES

To assess whether platelets migrate into lung parenchyma in response to inhaled allergen in ovalbumin-sensitized mice; to assess the role of the FcepsilonRI receptor in this phenomenon; and to evaluate whether platelets from patients with asthma, or from sensitized mice, undergo chemotaxis in vitro in response to relevant antigens.

METHODS

Ovalbumin-sensitized wild-type (WT) mice, or FcRgamma(-/-) mice lacking the FcepsilonRIgamma, were challenged with aerosolized allergen and lungs analyzed by platelet-specific immunohistochemistry. In some experiments, mice were depleted of platelets and cross-transfused with either WT or FcRgamma(-/-) platelets to assess the role of platelet FcRgamma(-/-). Chemotaxis of platelets from patients with asthma or from sensitized mice was studied in vitro.

MEASUREMENTS AND MAIN RESULTS

Histology of lungs revealed isolated platelets, migrating out of vessels and localizing underneath the airways after allergen challenge in WT but not in FcRgamma(-/-) mice. Platelets from patients with asthma and from sensitized WT mice, but not from sensitized FcRgamma(-/-) mice, migrated in vitro toward the relevant allergen or an anti-IgE. Platelets from normal mice were found to express FcepsilonRIgamma and platelet-bound IgEs were increased in sensitized mice.

CONCLUSIONS

Platelets migrate extravascularly in response to a sensitizing allergen via a mechanism dependent on the interaction among allergen, allergen-specific IgE, and the FcepsilonRI, and this may allow them to participate directly in allergic tissue inflammation.

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.

Endocytosis, recycling, and degradation of unoccupied FcepsilonRI in human basophils

Previous studies about basophils and mast cells identified the ability of IgE to up-regulate FcepsilonRI expression by a process that depends on stabilization of the surface receptor by IgE. However, the mechanism of loss from the cell surface, when unoccupied, is not known. The current studies have examined whether unoccupied FcepsilonRI on basophils is lost by shedding or endocytosis. IgE was dissociated partially from purified human basophils to augment loss of the unoccupied receptor, and comparisons were made between basophils +/- IgE resensitization prior to 1-day culture. Incubation did not result in a detectable receptor in culture supernatants. However, in the presence of IL-3, although total cell surface expression decreased by 30% (relative to resensitized cells), FcepsilonRI from whole cell lysates was not statistically different between the two conditions. Incubation for 18 h without IL-3 resulted in the same loss from the cell surface but equivalent loss in whole cell lysates. This degradation process was reversible with Bafilomycin A. There was also evidence that the internalized receptor could be recycled. After the initial 18-h down-regulation, the receptor could be found partially restored to the cell surface if IgE were added back to the culture +/- cycloheximide. Loss of the unoccupied receptor, as well as accumulation of the receptor under the influence of IgE, was found to be insensitive to the presence of a src-family kinase inhibitor, PP1. These studies establish that the unoccupied receptor is lost by a process of endocytosis, partially recycled to the cell surface, and ultimately degraded by a lysosomal mechanism.

Inhibition of IgE-mediated phosphorylation of FcεRIγ protein by antiallergic drugs in rat basophilic leukemia (RBL-2H3) cells: A novel action of antiallergic drugs

We examined the effect of antiallergic drugs, azelastine and epinastine, on the expression of FcepsilonRIalpha, beta, and gamma chains and phosphorylation of the gamma chains in rat basophilic leukemia (RBL-2H3) cells. The cells were cultured for 24 h with IgE treatment in the presence of azelastine or epinastine at the concentration of 10(-5) M. The FcepsilonRIalpha mRNA expression was determined by northern blot analysis. The protein level of FcepsilonRI expressed on the plasma membrane was examined following IgE treatment by immunoprecipitation with anti-IgE light chain, followed by western blot analysis with anti-gamma chain of FcR. Azelastine and epinastine had no effect on the FcepsilonRIalpha, beta and gamma mRNA levels. Although the amount of gamma chain assembled into IgE-bound FcepsilonRI was not changed by treatment with azelastine nor epinastine, phosphorylation levels of gamma chains of IgE-bound FcepsilonRI were inhibited by azelastine. The inhibitory effect of azelastine on the IgE-mediated expression of FcepsilonRIgamma protein is not due to their inhibition of mRNA and protein expression, but due to abrogating phosphorylation of the gamma chains, which is important for initiation of FcepsilonRI signaling cascade elicited by IgE interaction.

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.

Anti-IgE antibodies for the treatment of IgE-mediated allergic diseases

The pharmacological purposes of the anti-IgE therapy are to neutralize IgE and to inhibit its production to attenuate type I hypersensitivity reactions. The therapy is based on humanized IgG1 antibodies that bind to free IgE and to membrane-bound IgE on B cells, but not to IgE bound by the high-affinity IgE.Fc receptors on basophils and mast cells or by the low-affinity IgE.Fc receptors on B cells. After nearly 20 years since inception, therapeutic anti-IgE antibodies (anti-IgE) have been studied in about 30 Phase II and III clinical trials in many allergy indications, and a lead antibody, omalizumab, has been approved for treating patients (12 years and older) with moderate-to-severe allergic asthma. Anti-IgE has confirmed the roles of IgE in the pathogenesis of asthma and helped define the concept "allergic asthma" in clinical practice. It has been shown to be safe and efficacious in treating pediatric allergic asthma and treating allergic rhinitis and is being investigated for treating peanut allergy, atopic dermatitis, latex allergy, and others. It has potential for use to combine with specific and rush immunotherapy for increased safety and efficacy. Anti-IgE thus appears to provide a prophylactic and therapeutic option for moderate to severe cases of many allergic diseases and conditions in which IgE plays a significant role. This chapter reviews the evolution of the anti-IgE concept and the clinical studies of anti-IgE on various disease indications, and presents a comprehensive analysis on the multiple intricate immunoregulatory pharmacological effects of anti-IgE. Finally, it reviews other approaches that target IgE or IgE-expressing B cells.

CD2 identifies a monocyte subpopulation with immunoglobulin E-dependent, high-level expression of Fc epsilon RI

BACKGROUND

Fc epsilon RI expression by monocytes can affect monocyte function via multiple mechanisms, thereby potentially influencing the generation of allergic inflammation. Previous studies on the in vivo regulation of monocyte Fc epsilon RI expression by ambient IgE have yielded conflicting results.

OBJECTIVE

We hypothesized that monocyte Fc epsilon RI expression is limited to a specific monocyte subset, and that within that subset Fc epsilon RI surface expression is correlated to serum IgE.

METHODS

Study 1: Blood was obtained from non-allergic subjects (n=14) and subjects with allergic asthma (n=18), hypereosinophilic syndrome (n=2), hyper-IgE syndrome (n=6), and helminth infection (n=4). Study 2: Blood was obtained from allergic subjects in a clinical trial of omalizumab before and during study drug treatment. Monocyte surface Fc epsilon RI expression was measured using flow cytometry.

RESULTS

Fc epsilon RI expression was significantly greater in the CD2(high) vs. CD2(low) monocyte subsets (31% vs. 1.9% median Fc epsilon RI+, respectively). In asthmatic and non-atopic healthy control subjects, CD2(low) monocytes expressed little or undetectable Fc epsilon RI. In study 1, Fc epsilon RI expression was highly correlated to serum IgE in the CD2(high), but not in the CD2(low) monocyte subpopulation (R values of 0.67 and 0.41, respectively). In study 2, omalizumab, but not placebo, caused a significant and sustained decline in Fc epsilon RI expression within the CD2(high) monocyte subset.

CONCLUSIONS

CD2 defines a monocyte subset with high Fc epsilon RI expression, whose magnitude is highly correlated to serum IgE. As such, this new description of CD2(high) monocytes as Fc epsilon RI-bearing cells suggests that they may be potential targets of anti-IgE immunomodulatory therapies.

Roles of the transmembrane domain and the cytoplasmic domain of Fc?RI? in immunoglobulin E-mediated up-regulation of surface Fc?RI expression

BACKGROUND

Human mast cells express both Fc epsilon RI alpha and Fc gamma RI alpha. IgE up-regulates Fc epsilon RI alpha expression, but IgG1 does not up-regulate Fc gamma RI alpha expression. The transmembrane domain (TM) of Fc gamma RI alpha determines the stability of cell surface expression of this receptor.

OBJECTIVE

The aim of this study was to clarify the roles of the TM and cytoplasmic domain (CY) of Fc epsilon RI alpha in IgE-mediated Fc epsilon RI up-regulation.

METHODS

Chimeric receptors created by domain shuffling between Fc epsilon RI alpha and Fc gamma RI alpha were transduced into human mast cell line HMC-1. Cell surface expression of the chimeric receptors and the effect of IgE or IgG1 on chimeric receptor expression were examined by FACS. The association of the chimeric receptors with FcR gamma was investigated by immunoprecipitation.

RESULTS

The results showed that the TM and CY of Fc epsilon RI alpha are not essential for IgE-mediated up-regulation of surface Fc epsilon RI.

CONCLUSION

The extracellular domain of each Fc receptor determines the diversity of Ig-regulated Fc receptor expression.

Significant association of FcepsilonRIalpha promoter polymorphisms with aspirin-intolerant chronic urticaria

BACKGROUND

Although the mechanism that underlies aspirin hypersensitivity is not completely understood, an IgE-mediated response was reported for a patient with aspirin-intolerant chronic urticaria (AICU).

OBJECTIVE

We investigated whether genetic polymorphisms on the alpha-chain of the high-affinity IgE receptor (FcepsilonRIalpha) gene were associated with the AICU phenotype.

METHODS

We genotyped 2 promoter polymorphisms (-344C>T and -95T>C) of FcepsilonRIalpha gene in the Korean population, and the functional effect of the -344C>T polymorphism was analyzed by using a luciferase reporter assay and an electrophoretic mobility shift assay.

RESULTS

The rare allele frequency of the -344C>T polymorphism was significantly higher in the patients with AICU compared with the other subjects (P= .008 for AICU vs aspirin-tolerant chronic urticaria; P= .03 for AICU vs controls). This polymorphism was also significantly associated with total serum IgE concentrations and a higher rate of atopy in the patients with AICU (P= .01 and .05, respectively). The reporter plasmid that carried the -344T allele exhibited significantly higher promoter activity in a rat mast cell line (RBL-2H3) compared with the promoter activity of the -344C allele (P< .001). We found that transcription factor Myc-associated zinc finger protein preferentially bound the -344C promoter. Moreover, patients with AICU with the heterozygous CT genotype of the -344C>T polymorphism exhibited greater anti-IgE-mediated histamine release compared with those with the homozygous CC genotype.

CONCLUSION

These results suggest that the -344C>T polymorphism of the FcepsilonRIalpha promoter may be associated with increased expression of FcepsilonRIalpha on mast cells and enhanced release of histamine.

CLINICAL IMPLICATIONS

The FcepsilonRIalpha -344C>T polymorphism may contribute to the development of AICU.

Fcε- and Fcγ-receptor signaling in diseases

It has become increasingly clear that receptors for the immunoglobulin Fc region play pivotal roles in immune homeostasis and disease. This review describes the fine regulation of the high-affinity IgE-receptor (FcepsilonRI) signaling, especially focusing on the early events that are coordinately regulated by Src family protein tyrosine kinases (PTKs), FcepsilonRI beta-subunit, and membrane lipid rafts. Because allergen-mediated FcepsilonRI cross-linking leads to the synthesis and release of a variety of proinflammatory mediators and cytokines, the duration and amplitude of the signal need to be strictly controlled, and the counterbalancing signaling is provided by specialized inhibitory receptors and molecules. However, recent work have revealed that Src family PTKs and FcepsilonRI beta-subunit transduce both positive and negative signaling with unexpectedly complex mechanisms. FcgammaRIIB exerts a unique inhibitory function on cell activation processes after the engagement of Fcgamma, FcepsilonRI and B cell receptors. Recent work has shown that FcgammaRIIB polymorphisms are associated with systemic lupus erythematosus, and that a transmembrane polymorphism in FcgammaRIIB results in an impaired distribution to lipid rafts and a reduced inhibitory function. Studies addressing the functions of disease-associated polymorphisms in the FcepsilonRI beta-subunit and low-affinity FcgammaRs are also considered.

Immunoglobulin E: role in asthma and allergic disease: lessons from the clinic

The role of immunoglobulin E (IgE) in allergic asthmatic disease is well established. Allergen-specific IgE binds to its cognate receptors, thus triggering a series of cellular events. These events include presentation of antigen by dendritic cells and the degranulation of mast cells and basophils to release numerous factors that play an integral part in potentiating the disease symptoms. Studies in the mouse indicate that a reduction in IgE levels could lead to significant attenuation of the allergic inflammatory response associated with diseases such as asthma, making IgE a target for the development of new therapeutic agents. Omalizumab (Xolair), a recombinant humanised monoclonal anti-IgE antibody that blocks the interaction of IgE with its receptors, is the first anti-IgE agent to undergo clinical development. Several clinical studies have been performed in adults and children with moderate-to-severe allergic asthma to evaluate the efficacy and safety of this agent, but which have also enabled an insight into the role of IgE in human disease. IgE plays a significant role in a number of allergic conditions including allergic rhinitis and allergies to various substances. Recent data suggests that local IgE production may occur in mucosal tissues and that locally significant concentrations of IgE, not reflected by serum IgE concentrations, indicate that it may play a role in non-atopic as well as atopic disease.

Genome-wide gene expression profiling of human mast cells stimulated by IgE or FcepsilonRI-aggregation reveals a complex network of genes involved in inflammatory responses

Background

Mast cells are well established effectors of IgE-triggered allergic reactions and immune responses to parasitic infections. Recent studies indicate that mast cells may play roles in adaptive and innate immunity, suggesting an innovative view of the regulation of immune responses. Here, we profiled the transcriptome of human mast cells sensitized with IgE alone, or stimulated by FcεRI aggregation.

Results

Our data show that among 8,793 genes examined, 559 genes are differentially regulated in stimulated mast cells when compared with resting/unstimulated mast cells. The major functional categories of upregulated genes include cytokines, chemokines, and other genes involved in innate and adaptive immune-responses. We observed the increased expression of over 63 gene-transcripts following IgE-sensitization alone. Our data was validated using Real-Time-PCR; ELISA and western blot. We confirmed that IgE alone does not trigger mast cell-immediate responses, such as calcium signals, degranulation or protein-phosphorylation.

Conclusion

This report represents a substantial advance in our understanding of the genome wide effects triggered by "passive sensitization" or active stimulation of human mast cells, supporting mast cells' potential involvement in a wide range of inflammatory responses.

An intermediate pH unfolding transition abrogates the ability of IgE to interact with its high affinity receptor FcepsilonRIalpha

The interaction between IgE-Fc (Fcepsilon) and its high affinity receptor FcepsilonRI on the surface of mast cells and basophils is a key event in allergen-induced allergic inflammation. Recently, several therapeutic strategies have been developed based on this interaction, and some include Fcepsilon-containing moieties. Unlike well characterized IgG therapeutics, the stability and folding properties of IgE are not well understood. Here, we present comparative biophysical analyses of the pH stability and thermostability of Fcepsilon and IgG1-Fc (Fcgamma). Fcepsilon was found to be significantly less stable than Fcgamma under all pH and NaCl conditions tested. Additionally, the Cepsilon3Cepsilon4 domains of Fcepsilon were shown to become intrinsically unfolded at pH values below 5.0. The interaction between Fcepsilon and an Fcgamma-FcepsilonRIalpha fusion protein was studied between pH 4.5 and 7.4 using circular dichroism and a combination of differential scanning calorimetry and isothermal titration calorimetry. Under neutral pH conditions, the apparent affinity of Fcepsilon for the dimeric fusion protein was extremely high compared with published values for the monomeric receptor (KD < 10(-12) m). Titration to pH 6.0 did not significantly change the binding affinity, and titration to pH 5.5 only modestly attenuated affinity. At pH values below 5.0, the receptor binding domains of Fcepsilon unfolded, and interaction of Fcepsilon with the Fcgamma-FcepsilonRIalpha fusion protein was abrogated. The unusual pH sensitivity of Fcepsilon may play a role in antigen-dependent regulation of receptor-bound, non-circulating IgE.

Different stabilities of the structurally related receptors for IgE and IgG on the cell surface are determined by length of the stalk region in their alpha-chains

A variant of the high affinity IgE receptor FcepsilonRI, which is composed of alpha- and gamma-chains without the beta-chain, is expressed on human APC, such as dendritic cells, and has been suggested to facilitate Ag uptake through IgE and hence to facilitate Ag presentation to T cells. The level of FcepsilonRI on these cells is correlated with the serum IgE concentration, suggesting IgE mediates the up-regulation of the alphagamma2-type FcepsilonRI. The IgE-mediated FcepsilonRI up-regulation on mast cells and basophils has been shown to enhance the ability of these cells to release chemical mediators and cytokines that are responsible for allergic inflammatory reactions. Here, to elucidate the mechanism controlling FcepsilonRI expression, we compared two structurally related Ig receptors, human FcepsilonRI and FcgammaRIIIA, which carry different alpha-chains but the same gamma-chains. The half-life of FcepsilonRI on the cell surface was short unless it bound IgE, whereas FcgammaRIIIA was stably expressed without IgG binding. Shuffling of the non Ig-binding portions of the FcepsilonRIalpha and FcgammaRIIIAalpha chains revealed that the stalk region was critical in determining the difference in their stability and ligand-induced up-regulation. Unexpectedly, analyses with added or deleted amino acids in the stalk region strongly suggested that the length rather than the amino acid sequence of the stalk region was of major importance in determining the different stabilities of FcepsilonRI and FcgammaRIIIA on the cell surface. This finding provides new insights into the mechanism regulating surface FcepsilonRI expression.

Correlation between eosinophils and protection against reinfection with Schistosoma mansoni and the effect of human immunodeficiency virus type 1 coinfection in humans

Longitudinal investigations of an adult male population of Kenyan car washers who have heavy and quantifiable occupational exposure to Schistosoma mansoni cercariae revealed that some individuals develop resistance to reinfection while others remain highly susceptible. We sought to characterize immune correlates associated with host protection in this population. Previous studies have demonstrated an association of peripheral eosinophilia with resistance to reinfection with schistosomes. Thus, we investigated the relationship between the percentage of circulating eosinophils and the effect of human immunodeficiency virus type 1 (HIV-1) coinfection on the susceptibility of the car washers to reinfection with schistosomes. Elevated percentages of circulating eosinophils were associated with resistance to reinfection by S. mansoni in HIV-1-seronegative persons. In the HIV-1-seropositive cohort, low CD4+-T-cell counts were associated with a less intense eosinophilia. Moreover, eosinophils from the car washers expressed high levels of FcepsilonRI beta chain, a molecule important in immunoglobulin E (IgE)-mediated immunity. Levels of FcepsilonRI beta chain expression correlated with serum levels of total and antigen-specific IgE for HIV-1-negative car washers, but this was not the case for individuals coinfected with HIV-1. Overall, these data further implicate eosinophils as having a potential role in development of protective immunity against schistosomes and suggest that changes associated with HIV-1 coinfection increase susceptibility to reinfection.

IgE and FcepsilonRI regulation

The high-affinity immunoglobulin (Ig)E receptor, FcepsilonRI, regulates the action of mast cells and basophils and therefore, regulates the expression of atopic disease. There have been several recent observations that demonstrate new behaviors for this receptor. The control of FcepsilonRI expression, control of cell function by FcepsilonRI, and expression of FcepsilonRI on other cell types are important new areas of understanding currently being explored.

Are basophil histamine release and high affinity IgE receptor expression involved in asymptomatic skin sensitization?

BACKGROUND

Immunoglobulin (Ig)E-sensitized persons with positive skin prick test, but no allergy symptoms, are classified as being asymptomatic skin sensitized (AS). The allergic type 1 disease is dependant on IgE binding to the high affinity IgE-receptor (FcepsilonRI) expressed on basophils and mast cells. However, a relationship between the AS status and FcepsilonRI has not been investigated. We aimed to characterize basophils from AS by looking at histamine release (HR) (sensitivity and reactivity) and the FcepsilonRI molecule, and compare it with nonatopic (NA) or allergic (A) persons.

METHODS

Blood was obtained from NA (n = 14), grass and/or birch A persons (n = 17) and mono-sensitized grass or birch pollen AS (n = 12). The basophil sensitivity and reactivity were examined by anti-IgE triggered HR. Surface expression of FcepsilonRI and IgE were measured by flow cytometry, FcepsilonRIalpha protein was identified using a radioimmunoassay and Western blot. mRNA coding for the classic FcepsilonRIbeta-chain and the truncated form (FcepsilonRIbetaT) were determined by real-time PCR.

RESULTS

The AS group was less reactive than NA or A persons when triggered by anti-IgE and had a significant higher number of nonresponders. However, there was no difference in sensitivity among the three groups and furthermore; the groups did not vary in FcepsilonRI- and IgE-surface expression, FcepsilonRIalpha-protein level or beta/betaT ratio.

CONCLUSION

Basophils from AS persons are less reactive and include more nonresponders than basophils from NA and A persons, but do not differ regarding the FcepsilonRI molecule.

Transport of the IgE receptor alpha-chain is controlled by a multicomponent intracellular retention signal

The human high affinity IgE receptor (FcepsilonRI) is a central component of the allergic response and is expressed as either a trimeric alphagamma2 or tetrameric alphabetagamma2 complex. It has been previously described that the cytoplasmic domain (CD) of the alpha-chain carries a dilysine motif at positions -3/-7 from the C terminus that functions in intracellular retention prior to assembly with other FcepsilonRI subunits. In this report we have further explored the role of the -3/-7 dilysine signal in controlling steady-state alpha-chain transport by mutational analysis and found little surface expression of a -3/-7 dialanine alpha-chain mutant but significant Golgi localization. We compared the transport properties of a series of alpha-chain cytoplasmic domain truncation mutants and observed that truncation mutants lacking 23 or more C-terminal residues showed a dramatic increase in steady-state transport suggesting a role for the membrane-proximal CD sequence in alpha-chain retention. By performing alanine-scanning mutagenesis we identified a dilysine sequence (Lys(212)-Lys(216)) proximal to the transmembrane domain (TMD) that is important for both alpha-chain cell-surface expression and intracellular stability. Furthermore, co-mutation of the Lys(212)-Lys(216) residues with the -3/-7 dilysine signal produced a dramatic increase in alpha-chain surface expression that was further increased by co-mutation of the lone charged residue (Asp(192)) in the TMD thereby defining three regions that function to regulate alpha-chain transport and in a highly synergistic manner.

Fc receptors as determinants of allergic reactions

Activation of the high-affinity receptor for IgE (FcepsilonRI) on allergic effector cells induces a multitude of positive signals via immunoreceptor tyrosine-based activation motifs, which leads to the rapid manifestation of allergic inflammatory reactions. As a counterbalance, the coaggregation of the IgG receptor FcgammaRIIB mediates inhibitory signals via immunoreceptor tyrosine-based inhibition motifs. Advances in the positive and negative regulation of Fc receptor expression and signaling have shed light on the role of Fc receptors in our immune system, indicating them to be bifunctional, inhibitory and activating structures. Based on these findings, exciting new therapeutic strategies have been developed, such as the use of chimeric fusion proteins, which concomitantly activate FcepsilonRI and FcgammaRIIB. These new approaches successfully take advantage of the bivalent character of Fc receptors and pave the way for innovative strategies to modulate allergic immune reactions.

Comparative analysis of nasal and oral mucosa dendritic cells

BACKGROUND

Mucosal dendritic cells (DC) play a crucial role in tolerance induction as seen in mucosal immunotherapy of atopic diseases. Nevertheless little is known about the phenotypical differences of oral and nasal mucosal DC (nmDC). Recently, we could show that oral mucosal myeloid CD1a(+) DC (omDC) differ from their skin counterparts especially by the expression of high affinity receptor for immunoglobulin E (IgE; FcepsilonRI). However, expression pattern of FcepsilonRI and phenotypical characteristics of CD1a(+) nmDC have not been elucidated in detailed yet.

METHODS

We performed detailed phenotypical comparison of nmDC and omDC of atopic and nonatopic individuals.

RESULTS

As reported for omDC, FcepsilonRI on nmDC of atopic donors was elevated and mostly occupied by IgE while FcepsilonRI was present only in low amounts on nmDC of nonatopic donors. Nevertheless, the highest FcepsilonRI expression has been observed on omDC. Furthermore, significant amounts of costimulatory molecules CD40, CD80 and CD86 could be detected on nmDC that expressed more CD80 compared with omDC. Moreover, nmDC displayed less major histocompatability complex (MHC) class I and II molecules than omDC. In addition, nmDC expressed more C-type lectins CD205, CD206 as well as myeloid marker CD11b while omDC displayed increased expression of CD207 and lipopolysaccharide (LPS) receptor CD14.

CONCLUSION

Together these data imply that nmDC phenotypical differ from omDC which might result in diverse functional properties and might be of relevance for selecting routes for immunotherapy of atopic diseases. Moreover these data provide a basis for further studies investigating immunological mechanisms underlying mucosal immunotherapy.

A high-affinity natural autoantibody from human cord blood defines a physiologically relevant epitope on the FcepsilonRIalpha

Natural Abs represent the indigenous immune repertoire and are thus present at birth and persist throughout life. Previously, human autoantibodies to the alpha domain of the high-affinity IgE receptor (FcepsilonRIalpha) have been isolated from Ab libraries derived from normal donors and patients with chronic urticaria. To investigate whether these anti-FcepsilonRIalpha Abs are present in the germline repertoire, we constructed a phage Fab display library from human cord blood, which represents the naive immune repertoire before exposure to exogenous Ags. All isolated clones specific to the FcepsilonRIalpha had the same sequence. This single IgM Ab, named CBMalpha8, was strictly in germline configuration and had high affinity and functional in vitro anaphylactogenic activity. Inhibition experiments indicated an overlapping epitope on the FcepsilonRIalpha recognized by both CBMalpha8 and the previously isolated anti-FcepsilonRIalpha Abs from autoimmune and healthy donors. This common epitope on FcepsilonRIalpha coincides with the binding site for IgE. Affinity measurements demonstrated the presence of Abs showing CBMalpha8-like specificity, but with a significantly lower affinity in i.v. Ig, a therapeutic multidonor IgG preparation. We propose a hypothesis of escape mutants, whereby the resulting lower affinity IgG anti-FcepsilonRIalpha Abs are rendered less likely to compete with IgE for binding to FcepsilonRIalpha.

Negative Signaling in Fc Receptor Complexes

Cell activation results from the transient displacement of an active balance between positive and negative signaling. This displacement depends in part on the engagement of cell surface receptors by extracellular ligands. Among these are receptors for the Fc portion of immunoglobulins (FcRs). FcRs are widely expressed by cells of hematopoietic origin. When binding antibodies, FcRs provide these cells with immunoreceptors capable of triggering numerous biological responses in response to a specific antigen. FcR-dependent cell activation is regulated by negative signals which are generated together with positive signals within signalosomes that form upon FcR engagement. Many molecules involved in positive signaling, including the FcRbeta subunit, the src kinase lyn, the cytosolic adapter Grb2, and the transmembrane adapters LAT and NTAL, are indeed also involved in negative signaling. A major player in negative regulation of FcR signaling is the inositol 5-phosphatase SHIP1. Several layers of negative regulation operate sequentially as FcRs are engaged by extracellular ligands with an increasing valency. A background protein tyrosine phosphatase-dependent negative regulation maintains cells in a "resting" state. SHIP1-dependent negative regulation can be detected as soon as high-affinity FcRs are occupied by antibodies in the absence of antigen. It increases when activating FcRs are engaged by multivalent ligands and, further, when FcR aggregation increases, accounting for the bell-shaped dose-response curve observed in excess of ligand. Finally, F-actin skeleton-associated high-molecular weight SHIP1, recruited to phosphorylated ITIMs, concentrates in signaling complexes when activating FcRs are coengaged with inhibitory FcRs by immune complexes. Based on these data, activating and inhibitory FcRs could be used for new therapeutic approaches to immune disorders.

Regulation of allergy by Fc receptors

The aggregation of high-affinity IgE receptors (FcepsilonRI) on mast cells and basophils has long been known as the critical event that initiates allergic reactions. Monomeric IgE was recently found to induce a variety of effects when binding to FcepsilonRI. Upregulation of FcepsilonRI only requires binding, whereas other responses require FcepsilonRI aggregation. Interestingly, FcepsilonRI aggregation has recently been understood to generate a mixture of positive and negative intracellular signals. Mast cells and basophils also express low-affinity and, under specific conditions, high-affinity IgG receptors. When co-engaging these receptors with FcepsilonRI, IgG antibodies can amplify or dampen IgE-induced mast cell activation. On the basis of these findings, it has been proposed that FcRs can be used as targets and/or tools for new therapeutic approaches to allergies.

Mast cell survival and activation by IgE in the absence of antigen: a consideration of the biologic mechanisms and relevance

Mast cells are not only major effector cells in allergy and host defense against parasites and bacteria but also important cellular components in other immune responses. Recent studies on the effects of monomeric IgE on mast cell survival and activation have made an impact on our view of the IgE binding to its high-affinity receptors, Fc epsilonRI. Traditionally, IgE binding to Fc epsilonRI has been considered as a passive action of "sensitization" before receptor aggregation by Ag. However, recent studies indicate that at high concentrations some monoclonal IgEs have effects on mast cells similar to or identical to those induced by IgE+Ag stimulation. These effects may be due to induction of Fc epsilonRI aggregation by these IgEs in the absence of Ag. This review will synthesize recent findings of the heterogeneity of IgEs in their ability to induce survival and activation events, their mechanisms, the potential in vivo significance of IgE-Fc epsilonRI interactions, and the implications of the mouse studies to human diseases.

IgE and Fc{epsilon}RI regulation

A central feature of allergic reactions is the aggregation of the high-affinity IgE receptor, FcepsilonRI, to initiate a change in the behavior of the cell expressing the receptor. It is now clear that a number of cell types can express this receptor, which broadens the biology that revolves around IgE antibody. It is also quite clear that the presence of monomeric IgE antibody alters the expression of FcepsilonRI. There remains considerable uncertainty about the importance of independent regulation of the FcepsilonRIbeta subunit or its splice variant beta(T), in terms of regulating both expression and function of FcepsilonRI. There is also only primitive understanding of the role of various polymorphisms in the subunit genes on the atopic phenotype. There are, however, many efforts being made to resolve these issues and to discover other factors that regulate expression of this receptor. Of particular interest for understanding the variation in expression in atopy among patients, the role of this receptor on non-mast cell/basophils will be important to elucidate.

The role of immunoglobulin E and immune inflammation: implications in allergic rhinitis

Immunoglobulin E (IgE) plays a critical role in the allergic inflammatory process in diseases such as allergic rhinitis. Cross-linking IgE bound to its receptor on cells by multivalent allergens initiates a chain of events resulting in allergic immune responses. Mast cells and basophils are involved in the early, immediate response, which is marked by cellular degranulation and the release of proinflammatory mediators, including histamine. Antigen-presenting cells are also activated by allergen-loaded IgE, resulting in immunomodulation of T-cell responses. The IgE molecule binds to two types of receptors, the high-affinity (Fc epsilonRI) and low-affinity (Fc epsilonRII or CD23) receptors, that have differing properties important in mediating allergen-induced responses. New therapies targeting the IgE molecule reduce allergen-stimulated immune responses and improve the clinical symptoms in subjects with allergic rhinitis. Understanding the role of the IgE molecule is necessary to appreciate the development and use of novel therapies targeting its actions.

The immunoglobulin E receptor: expression and regulation

The interaction of immunoglobulin E (IgE) and its receptors is critical for the manifestation of allergic disease. Currently, IgE receptors include the high-affinity Fc epsilonRI and the low-affinity Fc epsilonRII. Fc epsilonRI is a tetrameric or trimeric complex, and each has distinct expression patterns and unique functional consequences. In general, serum levels of IgE regulate Fc epsilonRI expression, and novel therapies that lower the concentration of IgE have provided insights into the contribution of this receptor in allergic disease. Newer therapies targeting Fc epsilonRI-signaling elements and the low-affinity IgE receptor, Fc epsilonRII (CD23), are being developed.

IgE generation and mast cell effector function in mice deficient in IL-4 and IL-13

IL-4 and IL-13 are potent cytokines that drive production of IgE, which is critical to the development of atopic disease. In this study, we directly compared IgE generation and IgE-dependent mast cell effector function in mouse strains lacking IL-4, IL-13, IL-4 + IL-13, or their common receptor component, IL-4Ralpha. Although serum IgE was undetectable under resting conditions in most animals deficient in one or both cytokines, peritoneal mast cells from mice lacking IL-4 or IL-13 had only partial reductions in surface IgE level. In contrast, peritoneal mast cells from IL-4/13(-/-) and IL-4Ralpha(-/-) animals were severely deficient in surface IgE, and showed no detectable degranulation following treatment with anti-IgE in vitro. Surprisingly, however, intradermal challenge with high concentrations of anti-IgE Ab induced an ear-swelling response in these strains, implying some capacity for IgE-mediated effector function in tissue mast cells. Furthermore, upon specific immunization with OVA, both IL-4/IL-13(-/-) and IL-4Ralpha(-/-) mice produced detectable levels of serum IgE and Ag-specific IgG1, and generated strong ear-swelling responses to intradermal administration of anti-IgE. These findings suggest that a mechanism for IgE production exists in vivo that is independent of IL-4 or IL-13.

Mast cells as "tunable" effector and immunoregulatory cells: recent advances

This review focuses on recent progress in our understanding of how mast cells can contribute to the initiation, development, expression, and regulation of acquired immune responses, both those associated with IgE and those that are apparently expressed independently of this class of Ig. We emphasize findings derived from in vivo studies in mice, particularly those employing genetic approaches to influence mast cell numbers and/or to alter or delete components of pathways that can regulate mast cell development, signaling, or function. We advance the hypothesis that mast cells not only can function as proinflammatory effector cells and drivers of tissue remodeling in established acquired immune responses, but also may contribute to the initiation and regulation of such responses. That is, we propose that mast cells can also function as immunoregulatory cells. Finally, we show that the notion that mast cells have primarily two functional configurations, off (or resting) or on (or activated for extensive mediator release), markedly oversimplifies reality. Instead, we propose that mast cells are "tunable," by both genetic and environmental factors, such that, depending on the circumstances, the cell can be positioned phenotypically to express a wide spectrum of variation in the types, kinetics, and/or magnitude of its secretory functions.

A comparative study of the FcepsilonRI molecule on human mast cell and basophil cell lines

BACKGROUND

Mast cells and basophils express the high-affinity IgE receptor FcepsilonRI. We have analysed the human mast cell line LAD2 and four subclones of the basophil cell line KU812 in order to reveal possible differences concerning the FcepsilonRI surface regulation, anti-IgE-triggered activation, FcepsilonRIalpha protein stability and the mRNA level of FcepsilonRIalpha-, beta- and the truncated beta-chain (beta(T)), and thereby determine the utility of these cell lines in investigations of the FcepsilonRI biology.

METHODS

The surface expression of FcepsilonRI was assessed by flow cytometry, using the monoclonal antibody CRA1. The FcepsilonRI-induced cellular activation (i.e. cross-linking of FcepsilonRI) was determined by changes in the intracellular level of Ca2+, which was measured by fluorescence of Fura-2. The level of the FcepsilonRIalpha protein was determined by a Western blot technique and by a radioimmunoassay. The mRNA level of FcepsilonRIalpha, beta- and beta(T)-chain was analysed using real-time PCR.

RESULTS

Two KU812 subclones and especially LAD2 had FcepsilonRI surface expression which was capable of inducing cellular activation. Both the FcepsilonRI expression and stability of the FcepsilonRIalpha protein were increased when IgE was present. All the cell lines expressed mRNA of FcepsilonRIalpha-, beta- and beta(T), with LAD2 tending to have the highest expression. However, a determination of the beta/beta(T) ratio demonstrated no difference between any of the cell clones.

CONCLUSION

These cell lines are important tools in the investigation of both the FcepsilonRI molecule and the effects induced by its activation.

Cutting Edge: Death of a Dogma or Enforcing the Artificial: Monomeric IgE Binding May Initiate Mast Cell Response by Inducing Its Receptor Aggregation

Several recent reports have suggested that binding monomeric IgE (mIgE) to its type 1 receptor, Fc epsilon RI, on mast cells induces important responses. These observations contradict the notion that it is the aggregation of this receptor that is essential for initiating mast cell response. In the present study, we suggest that the most probable causes for the reported observations are the experimental protocol used combined with the high expression levels of the Fc epsilon RI by mast cells. Specifically, we suggest using the published data and physicochemical calculations that the exceptionally high number of cell surface Fc epsilon RI-bound monoclonal IgE yields, in the two-dimensions of the cells' membranes, a situation where even a low affinity of these mIgE for epitopes on their own structure or on another cell surface component may lead to their aggregation. Hence, we hypothesize that the reported response to mIgE binding is a result of such an Fc epsilon RI-IgE induced aggregation.

Immunoglobulin E-bearing antigen-presenting cells in atopic dermatitis

Human antigen-presenting cells (APCs) bind monomeric immunoglobulin E (IgE) via the high-affinity IgE receptor, Fc epsilon RI. Surface expression of this trimeric structure is strongly associated with the atopic status of the donors, and maximal levels are observed on Langerhans cells (LC) and inflammatory dendritic epidermal cells (IDEC) in atopic dermatitis (AD). Although intracellular expression of the Fc epsilon RI alpha-chain is induced by interleukin-4 (IL-4), the upregulation of surface levels on dendritic cells (DC) from atopics is due to enhanced expression of the Fc epsilon RI gamma-chain and stabilization by binding of its ligand IgE. A characteristic function of Fc epsilon RI-bearing APCs is the specific uptake and processing of IgE-bound allergens, which is followed by T-cell stimulation. In AD, DC-mediated presentation of aeroallergens penetrating the epidermis is thought to induce an IgE-mediated, delayed-type hypersensitivity reaction. In addition, different Fc epsilon RI-bearing APC subsets in AD skin might regulate inflammatory processes through the production of Th1/Th2-polarizing signals, proinflammatory cytokines, chemokines, and factors that are involved in the induction of tolerance.

Early divergence of Fc epsilon receptor I signals for receptor up-regulation and internalization from degranulation, cytokine production, and survival

Mast cells play a critical role in IgE-dependent immediate hypersensitivity. Monomeric IgE binding to its high affinity receptor (FcepsilonRI) results in a number of biological outcomes in mouse mast cells, including increased surface expression of FcepsilonRI and enhanced survival. IgE molecules display heterogeneity in inducing cytokine production; highly cytokinergic IgEs cause extensive FcepsilonRI aggregation, leading to potent enhancement of survival and other activation events, whereas poorly cytokinergic IgEs can do so less efficiently. In this study, we demonstrate that IgE-induced receptor up-regulation is not sensitive to monovalent hapten, which can prevent receptor aggregation induced by IgE, whereas other activation events such as receptor internalization, degranulation, IL-6 production, and survival are sensitive to monovalent hapten. IgE-induced receptor up-regulation is also unique in that no Src family kinases, Syk, or Btk are required for it. By contrast, highly cytokinergic IgE-induced receptor internalization is dependent on Lyn, but not other Src family kinases, Syk, or Btk, whereas degranulation, IL-6 production, and survival require Syk. Weak to moderate stimulation with IgE plus anti-IgE or IgE plus Ag enhances survival, while stronger signals are required for degranulation and IL-6 production. Collectively, signals emanated from IgE-bound FcepsilonRI for receptor up-regulation and internalization are shown to diverge at the receptor or receptor-proximal levels from those for other biological outcomes.

Rapid and large amount of autocrine IL-3 production is responsible for mast cell survival by IgE in the absence of antigen

Cross-linking FcepsilonRI on mast cells by immunoglobulin E (IgE) and antigen (Ag) initiates cascades leading to antiparasitic or allergic responses. It was recently reported that IgE without antigen, IgE(-Ag), actively promotes mast cell survival. Although we have demonstrated that the immunoreceptor tyrosine-based activation motif within FcRgamma is essential for IgE(-Ag)-induced mast cell survival, the underlying mechanism remains still unclear. Here, we investigated the mechanism of IgE(-Ag)-induced survival using mast cells lacking several downstream molecules. Lyn and Syk were essential, whereas Fyn, Gab2, and the phosphoinositide 3-kinase-Akt pathway were not critical for survival. Failure of survival in FcRgamma-/- bone marrow mast cells (BMMCs) was rescued by coculture with IgE-treated wild-type BMMCs, suggesting that survival is induced not directly through FcepsilonRI signals. We found that the survival is predominantly mediated by high production of interleukin 3 (IL-3), evidenced by severe impairment of survival by anti-IL-3 and in IL-3-/- BMMCs. The up-regulation of Bcl-xL/Bcl-2 by IgE was abrogated in IL-3-/- BMMCs, whereas the expression of histidine decarboxylase was normally induced. These results indicate that IL-3 plays a crucial role for IgE(-Ag)-induced mast cell survival, functioning in an autocrine manner by inducing the Bcl-xL/Bcl-2 via signal transducer and activator of transduction 5. We further suggest that IgE(-Ag)-mediated gene expression in mast cells is regulated at least 2 mechanisms: autocrine IL-3 dependent and independent.

Transforming Growth Factor-β1 Regulates the Expression of the High-Affinity Receptor for IgE on CD34+ Stem Cell-Derived CD1a+ Dendritic Cells In Vitro

It has been reported that monocytes, Langerhans cells (LC) and other dendritic cells (DC) express the high-affinity receptor for IgE (FcepsilonRI) in patients with atopic diseases. These cells may be instrumental in the control of the immune response and the allergic inflammation. In this context, transforming growth factor beta 1 (TGF-beta1) has been highlighted as a key cytokine involved in the mechanisms aimed to orchestrate tolerance and has been suggested as a candidate gene in atopic diseases. In this report, we investigate the putative role of TGF-beta1 in the regulation of FcepsilonRI on cord blood CD34+ stem cell-derived CD1a+ DC (CD34-derived CD1a+ DC). Kinetic experiments show that FcepsilonRI spontaneously appears on the surface of CD1a+ DC, but decreases when exogenous TGF-beta1 is added at high doses (10 ng per mL) or when endogenous TGF-beta1 is neutralized in the culture conditions. In contrast, low-dose TGF-beta1 (0.5 ng per mL) stabilizes surface FcepsilonRI expression on DC. Increasing TGF-beta1 concentrations leads to the generation of LC-like DC showing an augmentation in stimulatory capacity towards allogeneic T cells. In view of these data, a picture emerges that FcepsilonRI+ on DC is finely modified by the TGF-beta1 concentration in the microenvironment and could be of primary relevance in the context of atopic diseases.

Characterization of FcepsilonRI-bearing CD123 blood dendritic cell antigen-2 plasmacytoid dendritic cells in atopic dermatitis

BACKGROUND

The high-affinity receptor for IgE (FcepsilonRI) on myeloid dendritic cells has been shown to play a major role in atopic dermatitis (AD). Plasmacytoid dendritic cells (pDCs), which are instrumental in the defense of viral infections, are present in reduced amounts in the skin of patients with AD, which is characterized by a high susceptibility to viral infections.

OBJECTIVE

We explored phenotypical and functional characteristics of pDC in the peripheral blood of patients with AD and healthy individuals.

METHODS

Blood dendritic cell antigen-2+CD123+ pDCs were enriched from the peripheral blood of patients with AD and studied in functional assays.

RESULTS

Skin-homing molecules such as cutaneous lymphocyte antigen and L-selectin CD62L were expressed in lower levels on pDCs of patients with AD. pDCs expressed high amounts of IgE-occupied FcepsilonRI. Further, FcepsilonRI aggregation on pDCs impaired the surface expression of MHC I and II, induced the production of IL-10, and enhanced the apoptosis of pDCs. Importantly, FcepsilonRI preactivated pDC produced less IFN-alpha and IFN-beta after stimulation with CpG motifs and enhanced the outcome of immune responses of the TH2 type.

CONCLUSION

From these data, we conclude that FcepsilonRI-bearing pDCs from patients with AD (1) are different from pDCs of healthy individuals, (2) might be important in the pathophysiology of AD, and (3) contribute to the enhanced susceptibility of patients with AD to viral infections.

Monomeric IgE stimulates NFAT translocation into the nucleus, a rise in cytosol Ca2+, degranulation, and membrane ruffling in the cultured rat basophilic leukemia-2H3 mast cell line

Mast cells are key regulators in allergy and inflammation, and release histamine, cytokines, and other proinflammatory mediators. In the classical view, IgE acts merely to prime mast cells, attaching to FcepsilonRs but not evoking any cell signaling response until cross-linked by the presence of a multivalent allergen. However, several recent studies have reported that IgE alone can promote cell survival and cytokine production in the absence of cross-linking by allergen. In this study we demonstrate that acute addition of monomeric IgE elicits a wide spectrum of responses in the rat basophilic leukemia-2H3 mast cell line, including activation of phospholipases Cgamma and D, a rise in cytosol Ca(2+), NFAT translocation, degranulation, and membrane ruffling within minutes. Calcium transients persist for hours as long as IgE is present resulting in the maintained translocation of the transcription factor NFAT to the nucleus. Removal of IgE reverses the signaling processes. Our results indicate that, far from simply preparing the cells for a response to allergen, monomeric IgE can stimulate signaling pathways that lead to degranulation, membrane ruffling, and NFAT translocation. The mechanism of activation is likely to be via aggregation of the FcepsilonR1 because activation by IgE can be inhibited with monovalent hapten.

Functional expression of high-affinity receptor for immunoglobulin E on mast cells precedes that of tryptase during differentiation from human bone marrow-derived CD34 progenitors cultured in the presence of stem cell factor and interleukin-6

BACKGROUND

CD34(+) progenitor cells develop into tryptase(+), CD117(+) mast cells when cultured in the presence of recombinant human stem cell factor (rhSCF). However, spontaneous IgE receptor (FcepsilonRI) expression during human mast cell development is not well examined.

OBJECTIVE

Here, the expression and function of FcepsilonRI in and on human bone marrow-derived mast cells (HBMMCs) during development were investigated.

METHODS AND RESULTS

At 4 weeks of culture, predominant cells expressed high-affinity IgE receptor alpha chain (FcepsilonRIalpha) on the cell surface determined by flow cytometry, but CD117 was less expressed. Immunocytochemistry with antitryptase mAb and anti-FcepsilonRIalpha mAb revealed intracellular and surface expression of FcepsilonRIalpha at 2 weeks of culture, but tryptase was less expressed. FcepsilonRIalpha mRNA transcript preceded that of tryptase mRNA at 2 weeks of culture determined by real-time RT-PCR, and FcepsilonRIalpha, FcepsilonRIbeta, FcepsilonRIgamma, and tryptase mRNA increased along with differentiation. FcepsilonRIalpha cross-link on HBMMC and 4-week-old mast cells/mast cell precursors induced the release of IL-5 and granulocyte macrophage-colony stimulating factor, which was enhanced by rhSCF.

CONCLUSION

These data indicated that HBMMC constitutively and spontaneously expressed functional FcepsilonRI subunits at the early stage of differentiation, probably because of the differences in the ability and functional property of progenitors.

The quantity and duration of FcRγ signals determine mast cell degranulation and survival

Immunoglobulin E (IgE) bound to multivalent antigen (Ag) elicits mast cell degranulation but not survival; on the contrary, IgE in the absence of Ag (IgE(-Ag)) induces survival only but not degranulation. Although these distinct responses are mediated through the same receptor, FcϵRI, the molecular mechanism generating the divergence is largely unknown. We recently showed that the signals through FcRγ chain are essential for IgE(-Ag)–induced mast cell survival as well as IgE(+Ag)–induced degranulation. To determine whether the cellular output is regulated by the quantity of FcRγ signal, we expressed CD8/FcRγ chimeras (CD8/γ) in bone marrow–derived mast cells (BMMCs) from FcRγ-/- mice to manipulate the strength of FcRγ signals by anti-CD8 cross-linking. Cross-linking of CD8/γ induced mast cell survival and degranulation. Survival was induced by weaker stimulation than needed for degranulation in terms of anti-CD8 concentration and the valency of chimera. However, sustained extracellular signal-regulated kinase (Erk) activation seems to regulate survival even when the activation signal was strong enough to elicit degranulation. Generation of sustained Erk activation by active mitogen-activated protein kinase kinase (MEK) induced BMMC survival. These results suggest that the duration and the magnitude of FcRγ signals may determine mast cell survival and degranulation, respectively. (Blood. 2004;103:3093-3101)

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.

Human dendritic cell 1 and dendritic cell 2 subsets express FcepsilonRI: correlation with serum IgE and allergic asthma

BACKGROUND

Type 1 dendritic cells (DC1) express the high-affinity IgE receptor (FcepsilonRI); however, the regulation of FcepsilonRI expression by DCs is not well understood. Type 2 DC (DC2) expression of FcepsilonRI has not been demonstrated.

OBJECTIVE

We hypothesized that DC2 cellsalso express FcepsilonRI and that expression of FcepsilonRI by the DC1 and DC2 subsets correlates with serum IgE and allergic asthma disease status.

METHODS

To test these hypotheses, we quantitated FcepsilonRI alpha chain expression by the peripheral blood precursor DC1 (pDC1) and pDC2 subsets by using flow cytometry.

RESULTS

FcepsilonRI was expressed by the pDC1 and pDC2 subsets, as well as tissue DCs from tonsils. Relative FcepsilonRI expression by basophil, pDC1, and pDC2 subsets was 12:6.5:1, respectively. In both pDC subsets, FcepsilonRI expression was significantly greater in allergic asthmatic subjects than in nonatopic control subjects. pDC1 and pDC2 expression of FcepsilonRI was highly correlated to serum IgE concentration. The pDC1, pDC2, and basophil subsets demonstrated a similar magnitude of increase in FcepsilonRI expression relative to changes in serum IgE.

CONCLUSIONS

FcepsilonRI expression is characteristic of both the DC1 and DC2 subsets. Furthermore, FcepsilonRI expression by these cells is highly correlated to serum IgE and to basophil FcepsilonRI expression and is greater in subjects with allergic asthma. These data support the concept that novel therapeutic approaches directly targeted at FcepsilonRI expression would affect both the sensitization and the effector phases of the allergen-specific immune response.

Utilizing Fcε-Bak chimeric protein for studying IgE–FcεRI interactions

We previously constructed a pro-apoptotic Fcepsilon-Bak chimeric protein, targeted against cells expressing the IgE high affinity receptor (FcepsilonRI). We demonstrated that the chimeric protein is internalized by target mast cells and kills them. These results, which constitute a promising basis for applying this approach to antiallergic therapy, raise some theoretical questions with respect to two major issues: (a) is the monomeric Fcepsilon-Bak-FcepsilonRI complex able to undergo endocytosis, and (b) does the receptor binding domain of human IgE (Fcepsilon) react with rodent FcepsilonRI? In an attempt to answer these questions, we have now thoroughly investigate the interaction of human (h) and mouse (m) Fcepsilon-Bak with FcepsilonRI-positive cells. Using established cultures of rodent and human origin, as well as a primary mouse mast cell culture, we demonstrate that binding of the chimeric protein to the membrane is followed by quick endocytosis, leading to the apoptosis of specific cells. We also confirm that this interaction depends on FcepsilonRI and not on other IgE receptors. We found that the effect of Fcepsilon-Bak on the cells depends on the level of surface FcepsilonRI expression, but not on the origin of the target cells or of the Fcepsilon moiety. We suggest that endocytosis of the monomeric Fcepsilon-Bak-FcepsilonRI complex results from the inability of Fcepsilon-Bak to transduce signals, characteristic of the monomeric IgE-FcepsilonRI complex due to the absence of the variable portion of the IgE molecule. Our results also indicate that at least the Fc fragment of human IgE is able to interact with both human and rodent FcRI.

Omalizumab treatment downregulates dendritic cell FcεRI expression

BACKGROUND

Dendritic cells (DCs) are potent antigen-presenting cells that express FcepsilonRI, the high-affinity IgE receptor. Although the downregulation of basophil FcepsilonRI during anti-IgE therapy with omalizumab is well documented, its effect on FcepsilonRI expression by DCs has not been reported.

OBJECTIVE

We hypothesized that IgE regulates surface FcepsilonRI expression by DCs in vivo and that, consequently, anti-IgE therapy decreases FcepsilonRI expression by DCs.

METHODS

In a randomized, double-blind, placebo-controlled clinical trial 24 subjects (16 receiving omalizumab and 8 receiving placebo) with seasonal allergic rhinitis received the study drug on days 0 and 28. Serial blood samples drawn on days 0, 7, 14, 28, and 42 were analyzed for precursor DC1 (pDC1) and pDC2 surface expression of FcepsilonRIalpha by using flow cytometry.

RESULTS

Omalizumab caused a significant decrease in surface FcepsilonRI expression at all time points examined in both the pDC1 and pDC2 subsets. No significant change was seen with placebo. The maximum decrease in FcepsilonRI expression in the omalizumab group was 52% and 83%, respectively, for the pDC1 and pDC2 subsets. The decrease in FcepsilonRI expression by both pDC subsets correlated with the decrease in serum-free IgE and was of a similar magnitude to that found in basophils. A 10-fold decrease in IgE corresponded to a 42% and 54% decrease in surface FcepsilonRI expression by the pDC1 and pDC2 subsets, respectively.

CONCLUSION

These results demonstrate that anti-IgE therapy causes a rapid decrease in DC surface FcepsilonRI expression and establish that IgE is an important regulator of FcepsilonRI expression by DCs.

Evidence that IgE molecules mediate a spectrum of effects on mast cell survival and activation via aggregation of the FcepsilonRI

We demonstrate that binding of different IgE molecules (IgEs) to their receptor, FcepsilonRI, induces a spectrum of activation events in the absence of a specific antigen and provide evidence that such activation reflects aggregation of FcepsilonRI. Highly cytokinergic IgEs can efficiently induce production of cytokines and render mast cells resistant to apoptosis in an autocrine fashion, whereas poorly cytokinergic IgEs induce these effects inefficiently. Highly cytokinergic IgEs seem to induce more extensive FcepsilonRI aggregation than do poorly cytokinergic IgEs, which leads to stronger mast cell activation and survival effects. These effects of both types of IgEs require Syk tyrosine kinase and can be inhibited by FcepsilonRI disaggregation with monovalent hapten. In hybridoma-transplanted mice, mucosal mast cell numbers correlate with serum IgE levels. Therefore, survival effects of IgE could contribute to the pathogenesis of allergic disease.

The biology of IGE and the basis of allergic disease

Allergic individuals exposed to minute quantities of allergen experience an immediate response. Immediate hypersensitivity reflects the permanent sensitization of mucosal mast cells by allergen-specific IgE antibodies bound to their high-affinity receptors (FcepsilonRI). A combination of factors contributes to such long-lasting sensitization of the mast cells. They include the homing of mast cells to mucosal tissues, the local synthesis of IgE, the induction of FcepsilonRI expression on mast cells by IgE, the consequent downregulation of FcgammaR (through an insufficiency of the common gamma-chains), and the exceptionally slow dissociation of IgE from FcepsilonRI. To understand the mechanism of the immediate hypersensitivity phenomenon, we need explanations of why IgE antibodies are synthesized in preference to IgG in mucosal tissues and why the IgE is so tenaciously retained on mast cell-surface receptors. There is now compelling evidence that the microenvironment of mucosal tissues of allergic disease favors class switching to IgE; and the exceptionally high affinity of IgE for FcepsilonRI can now be interpreted in terms of the recently determined crystal structures of IgE-FcepsilonRI and IgG-FcgammaR complexes. The rate of local IgE synthesis can easily compensate for the rate of the antibody dissociation from its receptors on mucosal mast cells. Effective mechanisms ensure that allergic reactions are confined to mucosal tissues, thereby minimizing the risk of systemic anaphylaxis.

IgA Fc receptors

The IgA receptor family comprises a number of surface receptors including the polymeric Ig receptor involved in epithelial transport of IgA/IgM, the myeloid specific IgA Fc receptor (FcalphaRI or CD89), the Fcalpha/muR, and at least two alternative IgA receptors. These are the asialoglycoprotein receptor and the transferrin receptor, which have been implicated in IgA catabolism, and tissue IgA deposition. In this review we focus on the biology of FcalphaRI (CD89). FcalphaRI is expressed on neutrophils, eosinophils, monocytes/macrophages, dendritic cells, and Kupffer cells. This receptor represents a heterogeneously glycosylated transmembrane protein that binds both IgA subclasses with low affinity. A single gene encoding FcalphaRI has been isolated, which is located within the leukocyte receptor cluster on chromosome 19. The FcalphaRI alpha chain lacks canonical signal transduction domains but can associate with the FcR gamma-chain that bears an activation motif (ITAM) in the cytoplasmic domain, allowing activatory functions. FcalphaRI expressed alone mediates endocytosis and recyling of IgA. No FcalphaRI homologue has been defined in the mouse, and progress in defining the in vivo role of FcalphaRI has been made using human FcalphaRI transgenic (Tg) mice. FcalphaRI-Tg mice demonstrated FcalphaRI expression on Kupffer cells and so defined a key role for the receptor in mucosal defense. The receptor functions as a second line of antibacterial defense involving serum IgA rather than secretory IgA. Studies in FcalphaRI-Tg mice, furthermore, defined an essential role for soluble FcalphaRI in the development of IgA nephropathy by formation of circulating IgA-FcalphaRI complexes. Finally, recent work points out a role for human IgA in treatment of infectious and neoplastic diseases.