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

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.

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.

Differential induction of IgE-mediated anaphylaxis after soluble vs. cell-bound tolerogenic peptide therapy of autoimmune encephalomyelitis

The ability of different forms of myelin peptides to induce tolerance for the treatment of preestablished murine experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis, was evaluated. i.v. administration of myelin peptide-pulsed, ethylene carbodiimide-fixed syngeneic splenocytes, but not soluble myelin peptide monomers or oligomers, proved exceedingly effective at treating preestablished EAE, resulting in amelioration of disease progression. In addition to the lack of therapeutic efficacy of soluble peptide and peptide oligomer, administering them i.v. after the onset of clinical symptoms in many but not all peptide-induced EAE models led to a rapid-onset anaphylactic reaction characterized by respiratory distress, erythema, decreased body temperature, unresponsiveness, and, often, death. By using anti-IgE antibody treatments and mice with targeted mutations of the FcgammaRIII alpha-chain or the common gamma-chain of FcepsilonRI and FcgammaRI/III, we demonstrate that IgE crosslinking of FcepsilonRI appears to be necessary and sufficient for myelin peptide-induced anaphylaxis. The implications of these findings to myelin peptide/protein tolerance strategies for the treatment of multiple sclerosis are discussed.

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.

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.

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.

p28, a novel IgE receptor-associated protein, is a sensor of receptor occupation by its ligand in mast cells

Mast cells express the high affinity receptor for IgE (FcepsilonRI). Aggregation of this receptor by IgE and antigen leads to a signaling cascade resulting in the secretion of histamine, in the synthesis of other pro-inflammatory mediators such as leukotrienes and prostaglandins, and in the production of various cytokines, all of which participate in the development of the allergic reaction. In the last years, growing evidence accumulated that binding of IgEs to FcepsilonRI in itself induces active signals leading to mast cell survival, increased expression of FcepsilonRI, transient induction of histidine decarboxylase synthesis, and increased cell adhesion. The mechanisms underlying monomeric IgE signaling in the absence of receptor aggregation are still poorly understood. Here, we show that a protein of 28 kDa (p28) is physically and constitutively associated with FcepsilonRI in mast cells. Coimmunoprecipitation studies from (125)I surface-labeled cells demonstrated that this association involves at least 50% of membrane-expressed FcepsilonRI. After the addition of monomeric IgE to the cells, the p28.FcepsilonRI complex dissociates almost completely in less than 2 min. This dissociation is temperature-sensitive and is not due to the recruitment of additional proteins to the complex. Stripping bound IgE from the cells by acidic treatment promotes a rapid reassociation between p28 and FcepsilonRI. Altogether, these data are consistent with a conformational regulation of the complex. Thus, p28 is a sensor for FcepsilonRI occupation by IgE on mast cells, and its dissociation from the receptor could represent an early step of monomeric IgE signaling.

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.

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.

IgE alone-induced actin assembly modifies calcium signaling and degranulation in RBL-2H3 mast cells

In the mast cell signaling pathways, the binding of immunoglobulin E (IgE) to FcepsilonRI, its high-affinity receptor, is generally thought to be a passive step. In this study, we examined the effect of IgE alone, that is, without antigen stimulation, on the degranulation in mast cells. Monomeric IgE (500-5,000 ng/ml) alone increased cytosolic Ca2+ level ([Ca2+]i) and induced degranulation in rat basophilic leukemia (RBL)-2H3 mast cells. Monomeric IgE (5,000 ng/ml) alone also increased [Ca2+]i and induced degranulation in bone marrow-derived mast cells. Interestingly, monomeric IgE (5-50 ng/ml) alone, in concentrations too low to induce degranulation, increased filamentous actin content in RBL-2H3 mast cells. We next examined whether actin dynamics affect the IgE alone-induced RBL-2H3 mast cell activation pathways. Cytochalasin D inhibited the ability of IgE alone (50 ng/ml) to induce de novo actin assembly. In cytochalasin D-treated cells, IgE (50 ng/ml) alone increased [Ca2+]i and induced degranulation. We have summarized the current findings into two points. First, IgE alone increases [Ca2+]i and induces degranulation in mast cells. Second, IgE, at concentrations too low to increase either [Ca2+]i or degranulation, significantly induces actin assembly, which serves as a negative feedback control in the mast cell Ca2+ signaling and degranulation.

Surface expression of Fc epsilon RI on Langerhans' cells of clinically uninvolved skin is associated with disease activity in atopic dermatitis, allergic asthma, and rhinitis

BACKGROUND

Fc epsilon RI expressed on the surface of human epidermal Langerhans' cells facilitates uptake of IgE-associated allergens and plays a pivotal role in the pathogenesis of atopic dermatitis. Seminal results from studies investigating Langerhans' cell Fc epsilon RI in skin biopsy sections or epidermal cell suspensions demonstrate the highest receptor expression in lesional skin of patients with active atopic dermatitis.

OBJECTIVE

We sought to investigate and localize Fc epsilon RI expression on Langerhans' cells within a minimally disturbed tissue environment in clinically uninvolved skin and to compare receptor expression between healthy donors and patients with atopic dermatitis or other allergic diseases.

METHODS

Intact epidermal sheets from skin suction blisters, immunofluorescently stained with Langerhans' cell markers and anti-Fc epsilon RI alpha (mAbs 15E5 and 22E7) or anti-IgE, were examined by means of confocal microscopy. Samples incubated with anti-Fc epsilon RI alpha before or after cell fixation-permeabilization were compared to discriminate between cytoplasmic and membrane localization.

RESULTS

Cytoplasmic Fc epsilon RI alpha chain was found in Langerhans' cells from all donors, irrespective of atopic status. Surface Fc epsilon RI-bound IgE was detected in the skin of individuals with active atopic dermatitis and in the skin of those with active asthma or rhinitis. No surface Fc epsilon RI was expressed in the skin of patients with a clinical history of atopic dermatitis, asthma, or rhinitis whose disease was in remission or in the skin of nonatopic individuals.

CONCLUSION

In clinically uninvolved skin, Langerhans' cell-surface Fc epsilon RI expression is not only linked to atopic dermatitis but is also generally associated with allergic disease. This supports the concept of a systemic regulatory mechanism associated with active allergic disease, which is further aggravated by local inflammation in atopic skin lesions.

Long term maintenance of IgE-mediated memory in mast cells in the absence of detectable serum IgE

Mast cells and basophils involved in allergic responses do not have clonotypic Ag receptors. However, they can acquire Ag specificity through binding of Ag-specific IgE to FcepsilonRI expressed on their surface. Previous studies demonstrated that IgE binding induced the stabilization and accumulation of FcepsilonRI on the cell surface and resulted in up-regulation of FcepsilonRI. In this study we have further analyzed the maintenance of IgE-mediated memory in mast cells and basophils in vivo by comparing kinetics of serum IgE levels, FcepsilonRI expression, and ability to induce systemic anaphylaxis. A single i.v. injection of trinitrophenyl-specific IgE induced 8-fold up-regulation of FcepsilonRI expression on peritoneal mast cells in B cell-deficient (micro m(-/-)) mice. Serum IgE levels became undetectable by day 6, but the treatment of mice with anti-IgE mAb induced a significant drop in body temperature on days 14, 28, and 42. The administration of trinitrophenyl -BSA, but not BSA, in place of anti-IgE mAb gave similar results, indicating the Ag specificity of the allergic response. This long term maintenance of Ag-specific reactivity in the allergic response was also observed in normal mice passively sensitized with IgE even though the duration was shorter than that in B cell-deficient mice. The appearance of IgE with a different specificity did not interfere with the maintenance of IgE-mediated memory of mast cells and basophils. These results suggest that IgE-mediated stabilization and up-regulation of FcepsilonRI enables mast cells and basophils not only to acquire Ag specificity, but also to maintain memory in vivo for lengthy periods of time.

Regulation of mast-cell and basophil function and survival by IgE

Mast cells and basophils are important effector cells in T helper 2 (T(H)2)-cell-dependent, immunoglobulin-E-associated allergic disorders and immune responses to parasites. The crosslinking of IgE that is bound to the high-affinity receptor Fc epsilon RI with multivalent antigen results in the aggregation of Fc epsilon RI and the secretion of products that can have effector, immunoregulatory or autocrine effects. This response can be enhanced markedly in cells that have been exposed to high levels of IgE, which results in the increased surface expression of Fc epsilon RI. Moreover, recent work indicates that monomeric IgE (in the absence of crosslinking) can render mast cells resistant to apoptosis induced by growth-factor deprivation in vitro and, under certain circumstances, can induce the release of cytokines. So, the binding of IgE to Fc epsilon RI might influence mast-cell and basophil survival directly or indirectly, and can also regulate cellular function.

The future of antigen-specific immunotherapy of allergy

More than 25% of the population in industrialized countries suffers from immunoglobulin-E-mediated allergies. The antigen-specific immunotherapy that is in use at present involves the administration of allergen extracts to patients with the aim to cure allergic symptoms. However, the risk of therapy-induced side effects limits its broad application. Recent work indicates that the epitope complexity of natural allergen extracts can be recreated using recombinant allergens, and hypoallergenic derivatives of these can be engineered to increase treatment safety. It is proposed that these modified molecules will improve the current practice of specific immunotherapy and form a basis for prophylactic vaccination.