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

Regulation of Trafficking and Signaling of the High Affinity IgE Receptor by FcεRIβ and the Potential Impact of FcεRIβ Splicing in Allergic Inflammation

Mast cells are tissue-resident immune cells that function in both innate and adaptive immunity through the release of both preformed granule-stored mediators, and newly generated proinflammatory mediators that contribute to the generation of both the early and late phases of the allergic inflammatory response. Although mast cells can be activated by a vast array of mediators to contribute to homeostasis and pathophysiology in diverse settings and contexts, in this review, we will focus on the canonical setting of IgE-mediated activation and allergic inflammation. IgE-dependent activation of mast cells occurs through the high affinity IgE receptor, FcεRI, which is a multimeric receptor complex that, once crosslinked by antigen, triggers a cascade of signaling to generate a robust response in mast cells. Here, we discuss FcεRI structure and function, and describe established and emerging roles of the β subunit of FcεRI (FcεRIβ) in regulating mast cell function and FcεRI trafficking and signaling. We discuss current approaches to target IgE and FcεRI signaling and emerging approaches that could target FcεRIβ specifically. We examine how alternative splicing of FcεRIβ alters protein function and how manipulation of splicing could be employed as a therapeutic approach. Targeting FcεRI directly and/or IgE binding to FcεRI are promising approaches to therapeutics for allergic inflammation. The characteristic role of FcεRIβ in both trafficking and signaling of the FcεRI receptor complex, the specificity to IgE-mediated activation pathways, and the preferential expression in mast cells and basophils, makes FcεRIβ an excellent, but challenging, candidate for therapeutic strategies in allergy and asthma, if targeting can be realized.

Novel Approaches in the Inhibition of IgE-Induced Mast Cell Reactivity in Food Allergy

Allergy is an IgE-dependent type-I hypersensitivity reaction that can lead to life-threatening systemic symptoms such as anaphylaxis. In the pathogenesis of the allergic response, the common upstream event is the binding of allergens to specific IgE, inducing cross-linking of the high-affinity FcεRI on mast cells, triggering cellular degranulation and the release of histamine, proteases, lipids mediators, cytokines and chemokines with inflammatory activity. A number of novel therapeutic options to curb mast cell activation are in the pipeline for the treatment of severe allergies. In addition to anti-IgE therapy and allergen-specific immunotherapy, monoclonal antibodies targeted against several key Th2/alarmin cytokines (i.e. IL-4Rα, IL-33, TSLP), active modification of allergen-specific IgE (i.e. inhibitory compounds, monoclonal antibodies, de-sialylation), engagement of inhibitory receptors on mast cells and allergen-specific adjuvant vaccines, are new promising options to inhibit the uncontrolled release of mast cell mediators upon allergen exposure. In this review, we critically discuss the novel approaches targeting mast cells limiting allergic responses and the immunological mechanisms involved, with special interest on food allergy treatment.

Expression of FcεRIα and tryptase in human lung tissue during drug-induced anaphylactic death

The diagnosis of drug-induced anaphylactic death is an important component of forensic medicine. The purpose of this study was to explore the expression of FcεRIα and tryptase in human lung tissue during drug-induced anaphylactic death and its value for forensic medicine. The expression of FcεRIα and tryptase in lung tissues of the drug-induced anaphylactic death group (n = 30) and control group (who died due to sudden cardiac death, falling from a height, or traffic accidents, n = 30) was detected by immunohistochemistry and immunofluorescence staining. The difference in FcεRIα and tryptase expression in the lung tissue between the drug-induced anaphylactic death group and the control group was statistically significant (p < 0.05). The levels of FcεRIα and tryptase expression greatly increased in the lung tissue of the drug-induced anaphylactic death group, which may provide morphological evidence and a reference for the diagnosis of drug-induced anaphylactic death in forensic medicine.

Response of peripheral blood basophils in subjects with chronic spontaneous urticaria during treatment with omalizumab

BACKGROUND

Treatment of patients with asthma or food allergy with omalizumab results in several consistent changes in circulating basophils. The multiple basophil phenotypes observed in patients with chronic spontaneous urticaria (CSU) present some unique attributes that may not respond in a similar fashion to patients with asthma or food allergy. As part of a clinical study on the therapeutic outcomes of omalizumab treatment in CSU, the basophil compartment was examined for changes in characteristics predicted by prior studies.

OBJECTIVE

This study sought to examine the changes in basophil function and its relationship to auto-antibodies in serum during treatment with omalizumab.

METHODS

At multiple time points before and during omalizumab treatment of patients with CSU, basophil surface IgE and FcεRI expression, cellular spleen tyrosine kinase (SYK) expression, IgE-mediated histamine release (HR), and the presence of auto-antibodies in serum were determined.

RESULTS

Three basophil phenotypes were enumerated in the clinical study and used to group results in this basophil study: subjects with (1) basopenia, (2) normal basophil numbers with normal IgE-mediated HR, and (3) normal basophil numbers with poor HR. Basopenia was highly associated with the presence of auto-antibodies to unoccupied FcεRI and basophil numbers did not change during treatment. Likewise, subjects who are basopenic showed no changes in SYK expression or HR during treatment. In basophils of subjects who are nonbasopenic, increases in SYK expression and HR showed the expected inverse relationship to starting SYK and HR levels. Treatment with omalizumab resulted in similar kinetics for decreases in surface FcεRI and IgE in all 3 groups.

CONCLUSIONS

A unifying interpretation of the results revolves around the presence of auto-antibodies to FcεRI in CSU. If present, basopenia and an absence of changes in basophils during omalizumab treatment are observed. If auto-antibodies are absent, the changes in the basophil compartment are consistent with prior studies of asthma and food allergy. These group differences also are related to efficacy of the treatment for clinical outcomes, as found in the parent clinical study.

The relationship between released soluble FceRI-alpha and its cell surface density on human basophils

Background

The IgE-mediated activation of mast cells and basophils results in the secretion of many substances, including the release of FceRI-alpha subunit. This released alpha subunit can bind IgE and it may act as a down-regulator of subsequent IgE-dependent reactions. However, previous studies do not observe loss of the mass of FceRI-alpha associated with the cells, at least not for human basophils. This study was designed to understand the basis for the discordant observations.

Methods

Purified human basophils were stimulated with multiple activating secretagogues and supernatants were examined for histamine and released FceRI-alpha. In addition, cell surface IgE densities (occupied and unoccupied) were measured by flow cytometry and total cellular content of mature and immature FceRI-alpha determined with Western blots.

Results

Released FceRI-alpha, on average, represented 7% of the total surface FceRI before the reaction. The molecular weight of the soluble FceRI-alpha was approximately 54 kD, larger than immature subunit and somewhat smaller than surface subunit. In addition, 1) release ceased long before internalized FceRI-alpha was processed, 2) release was insensitive to Bafilomycin A, 3) release was independent of the starting density of FceRI and 4) release occurred more effectively with non-IgE-dependent stimuli, FMLP or C5a.

Conclusions

There appears to be relatively constant amount of nearly mature FceRI-alpha that is susceptible to secretion events induced by any form of stimulation. The amount, on average, represents about 7% of the mature form of FceRI-alpha.

Novel reactivation and degranulation of mast cells

Mast cells (MCs) degranulation is a key process during the allergic inflammatory response. MCs release their preformed and new synthesized granules after activation. We found that granules were released partially and selectively after the activation of bone marrow-derived mouse mast cells (BMMCs). Next, we investigated the response of degranulated MCs to a new challenge. BMMCs were activated by antibody/antigen (IgE/Ag) or compound 48/80 (C48/80). The degranulated BMMCs were then reactivated by either IgE/Ag or C48/80 without time intervals. Flow cytometry was used to detect the expression of CD117, FcεRI, and intracellular granules of BMMCs, and BMMCs degranulation was detected using the β-hexosaminidase release assay. The morphology of BMMCs was observed by staining with toluidine blue. Degranulated BMMCs activated by IgE/Ag failed to respond to the same IgE/Ag challenge and released β-hexosaminidase independent of unoccupied FcεRI, but responded to C48/80. Degranulated BMMCs activated by C48/80 responded to either IgE/Ag or C48/80. These results indicated that degranulated BMMCs could be reactivated and released granule mediators again, this revealed the unique mediator releasing mechanism of degranulated MCs and their potential function in maintaining inflammation or causing hypersensitivity.

Dependence of Optimal Histamine Release on Cell Surface IgE Density on Human Basophils: Nature of the Stimulus

BACKGROUND

The characteristics of the aggregation reaction that follows allergen binding to cell surface IgE on basophils and mast cells depend on a variety of factors that include the density of IgE and the affinity of the allergen for IgE. For simple bivalent stimuli, one prediction is that the location of the optimum for aggregation is not dependent on IgE density, only the affinity for IgE. However, this behavior does not occur for stimulation with an anti-IgE antibody (Ab) during the treatment of patients with omalizumab.

METHODS

This study re-examined the stability of the optimum for histamine release, relative to cell surface IgE density, using the simple bivalent penicillin hapten (BPO2) or a bivalent monoclonal anti-IgE Ab.

RESULTS

The results validated one prediction for one bivalent hapten, BPO2. Across a range of BPO-specific IgE density of 270-23,500/cell, optimal histamine release remained constant (10 nM BPO2). In contrast, across a range of approximately 6,000-110,000/cell, optimal histamine release shifted 8- to 30-fold for anti-IgE Ab. The distinguishing characteristic between the 2 bivalent stimuli was the difference in their crosslink re-equilibration. Recent modeling of histamine release suggested that the SYK-to-receptor ratio could determine the position of histamine release optimum. The study showed that there were significant shifts in the SYK-to-receptor ratio (from 1: 6 to 5: 1) but the basophil's ability to sense this ratio was restricted to transient crosslinks, as occurred with anti-IgE Ab.

CONCLUSIONS

The results suggest that ligand crosslinking dynamics couple with SYK and receptor expression levels to determine qualitative characteristics of the dose response curve for secretion.

Autoantibodies to IgE and FcεRI and the natural variability of spleen tyrosine kinase expression in basophils

BACKGROUND

Secretion from human basophils and mast cells requires spleen tyrosine kinase (SYK) activity, but SYK expression is highly variable in the general population, and this variability predicts the magnitude of IgE-mediated secretion. One known mechanism of modulating SYK expression in human basophils is aggregation of FcεRI.

OBJECTIVE

This study examines the possibility that functional autoantibodies are present in a wide variety of subjects and, in particular, subjects whose basophils poorly express SYK. It also tests whether any found antibodies could modulate SYK expression in maturing basophils and whether interaction with FcγRIIb/CD32b modulates the effect.

METHODS

An experimental algorithm for classifying the nature of histamine release induced by serum from 3 classes of subjects was developed.

RESULTS

The frequency of functional autoantibodies that produce characteristics concordant with FcεRI-mediated secretion was zero in 34 subjects without chronic spontaneous urticaria (CSU). In patients with CSU, the frequency was lower than expected, approximately 7%. For the 5 of 68 unique sera from patients with CSU tested that contained anti-FcεRI or anti-IgE antibodies, these antibodies were found to induce downregulation of SYK in both peripheral blood basophils and basophils developed from CD34⁺ progenitors. Blocking interaction of these antibodies with CD32b did not alter their ability to downregulate SYK expression.

CONCLUSIONS

This study establishes that functional autoantibodies to IgE/FcεRI do not provide a good explanation for the variability in SYK expression in basophils in the general population. They do show that if antibodies with these characteristics are present, they are capable of modulating SYK expression in developing basophils.

FceRI density and spontaneous secretion from human basophils

Mast cells and basophils depend on aggregation of the high-affinity IgE receptor, FceRI, to initiate secretion. A variety of studies have shown that FceRI densities vary 100 fold among subjects' basophils and it has been speculated that high densities might be responsible for unusual behaviors of the cells, notably sensitivity to certain monomeric IgE antibodies or spontaneous release. These studies experimentally examined the density dependence of spontaneous release and signaling element expression in subjects' basophils with FceRI densities ranging from approximately 6000 to 600,000 per cell. Through the use of numerical simulation, this study examined the expectations for spontaneous receptor aggregation and aggregate persistence at densities of FceRI ranging from 5000 to 500,000. Experimentally, FceRI density was not associated with greater spontaneous histamine release even when secretion was enhanced by the inclusion of deuterium oxide in the buffers. There was also no association of 15 activating or de-activating signaling elements with FceRI density. The numerical simulations demonstrated that at densities of ≈500,000 receptors, 13% of receptors were involved in transient aggregates at any given moment but that these aggregates rarely persisted for greater than 10 milliseconds. In contrast, a weak linear antigen aggregator, with ligand affinities known to induce secretion, would generate aggregates persisting for an average of ≈60 milliseconds. These results suggest that although a high density of FceRI likely produces a large number of transient aggregates, these aggregates do not persist long enough to induce signaling that results in secretion and do not induce the cells to alter their expression of several signaling elements known to be important in regulating secretion from human basophils. The results set some boundaries on the aggregation requirements for inducing histamine release from human basophils.

Recovery from desensitization of IgE-dependent responses in human lung mast cells

BACKGROUND

Clinical desensitization and oral food immunotherapy are therapeutic interventions that allow individuals who react adversely to an allergen (drug or food) to be made tolerant to the allergen. However, tolerance is brief, and allergen hypersensitivity can recur within days following allergen withdrawal.

OBJECTIVE

We hypothesize that the reason these treatments are temporary reflects rapid recovery of mast cells from a desensitized state. We sought to test this.

METHODS

Desensitization of IgE-mediated histamine release from human lung mast cells was explored by methods that partially replicate the pattern of treatment during clinical desensitization. Specific and non-specific desensitization and changes in surface IgE were examined following desensitization. Recovery from desensitization was also studied.

RESULTS

Desensitization of mast cell responses was readily induced with concentrations of antigen or anti-IgE that were suboptimal for secretion. There was little or no non-specific desensitization when lung mast cells were exposed to antigens. There was no loss of cell surface IgE following desensitization. Removing the desensitizing stimulus from the media following desensitization allowed the cells to recover with half-point of recovery of ~1.5 days and complete recovery after 5 days. Both the functional response and histamine content recovered within this time frame. The recovery appeared possible because both antigens and anti-IgE dissociated rapidly from cells after washing to remove excess stimulus.

CONCLUSIONS AND CLINICAL RELEVANCE

Human lung mast cells readily recover from a desensitized state following removal of desensitizing antigen. This finding provides a potential explanation for the ephemeral nature of clinical desensitization.

New insights into basophil heterogeneity

Basophils have become increasingly recognized as important innate immune cells that mediate antihelminth immunity and barrier inflammation. Recent discoveries have uncovered previously unrecognized heterogeneity in basophil populations. However, how diversity in basophil regulation and function impacts human disease remains poorly defined. The goal of the present review is to highlight how new insights into basophil heterogeneity can help us to better understand disease pathogenesis and inform the development of new therapeutics.

Expression profiling of human basophils: modulation by cytokines and secretagogues

Human basophils are an accessible participant of the human allergic reaction. There is natural variation in various functional endpoints and in signaling molecule expression but there has been only a limited effort to place this information in the context of mRNA expression profiles. This study examined the hypothesis that unique mRNA signatures could be identified during the response of human basophils to several known forms of stimulation. Highly purified human basophils were cultured in vitro and exposed to IL-3, IL-5, NGF, IL-33, IL-2, anti-IgE Ab, or FMLP and the mRNA profiles examined by microarrays. The response to IL-3 and anti-IgE Ab were examined on 2–3 time frames and the response to IL-3 examined at several concentrations. In addition, the mRNA signatures of 3 different potential phenotypes were examined. These included basophils with the so-called non-releaser phenotype, and basophils from atopic and non-atopic subjects. Given the role of IL-3 in basophil maturation and the known profound effects on mature basophil function, it was not surprising that IL-3 showed the greatest influence on the basophil transcriptome. However, it also became apparent that the act of isolating and culturing basophils was sufficient to induce a large number of changes in the transcriptome, despite high viability and recovery. These “culture-effect” changes dominated the changes in mRNA profiles induced by other stimuli. Unique signatures for anti-IgE antibody and IL-33 could be identified although the number of gene transcripts (6–30) that were unique to these two stimuli was very limited. There were no apparent unique profiles for IL-5, NGF, IL-2 or FMLP. Therefore, a potential tool for screening basophil phenotypes was limited to changes that could be induced by IL-3 (or no IL-3), IL-33 and anti-IgE Ab.

Rehmannia Glutinosa Pharmacopuncture Solution Regulates Functional Activation, FcεRI Expression, and Signaling Events in Mast Cells

Objectives:

Rehmannia glutinosa pharmacopuncture solution (RGPS) was investigated to determine both its anti-allergic inflammatory effects on mast cells and its detailed mechanism of actions.

Methods:

We investigated whether RGPS suppress cytokines, enzymes, FcεRI expression and FcεRImediated signaling in RBL-2H3 cells stimulated with anti-DNP IgE/DNP-HSA. The suppressive effects of RGPS on the levels of cytokines such as IL-1β, IL-6 and GM-CSF were measured using emzyme-linked immunospecific assay (ELISA). The mRNA expression levels of cytokines, enzymes (HDC2, COX-1, COX-2 and 5LO) and FcεRI αβγsubunits were measured using reverse transcription polymerase chain reaction (RTPCR) method. The activation of FcεRI-mediated signaling was examined using Western blot analyses.

Results:

RGPS suppressed production of proinflamm-atory cytokines (IL-1β, IL-6, and GM-CSF) in stimulated RBL-2H3 cells significantly (p< 0.05). RGPS also suppressed mRNA expression of inflammatory enzymes (HDC2, COX-1, COX-2, 5LO). In addition, mRNA expression levels of FcεRIα, FcεRIβand FcεRIγ were lowered by treatment with RGPS. Finally, RGPS prevented phosphrylation of Lyn, Syk, LAT, Gab2, PLC γ1/2, PI3K, Akt, cPLA₂ and IκBα

Conclusions:

RGPS effectively suppresses mast cell activations such as degranulation and inflammatory response via down-regulation of the FcεRI-mediated signaling pathways in IgE/Ag-stimulated mast cells.

FcεRI expression and dynamics on mast cells

Mast cells are key effector and immunoregulatory cells in IgE-associated immune responses, including allergic disorders. IgE antibodies bind to the high-affinity IgE receptor, FcεRI, expressed on the surface of mast cells; antigen-induced cross-linking of FcεRI-bound IgE molecules activates the mast cell to release an array of proinflammatory and immunomodulatory mediators. Because mast cells often respond to very low levels of antigen in vivo, the level of FcεRI expressed on the surface of these cells is an important factor in determining the responsiveness of these cells to antigen. FcεRI surface expression is regulated by a number of processes, including FcεRI stabilization, FcεRI recycling, and antigen-induced internalization. Although members of the Rab family of small GTPases and the ubiquitin ligase, Cbl, have recently emerged as major regulators of many of the membrane trafficking events that govern FcεRI expression levels, the mechanisms and intracellular pathways that regulate FcεRI trafficking remain poorly defined. This chapter outlines a number of flow cytometry-based assays that can be used to investigate cell surface FcεRI expression and dynamics (stabilization, recycling, and internalization) on bone marrow-derived mast cells (BMCMCs), the most commonly used model system for studying mast cells in vitro. Given the importance of FcεRI levels to mast cell responsiveness and function, the characterization of FcεRI expression and dynamics on different mast cell populations is critical when trying to compare IgE-dependent processes between different mast cell populations.

Regulation of IgE-mediated signalling in human basophils by CD32b and its role in Syk down-regulation: basic mechanisms in allergic disease

BACKGROUND

CD32b has been previously demonstrated to modulate IgE-mediated secretion from human basophils. However, exploration of the implications of this regulation has been limited. One unstudied area is whether regulation of signalling by CD32 also alters some of the phenotypic changes induced by IgE-mediated activation. The reported character of CD32-mediated signal transduction is not clear for human basophils and the two primary mechanisms considered important in this reaction predict different long-term outcomes, notably predicting different outcomes for down-regulation of syk expression.

OBJECTIVE

Syk expression was considered a unique point of phenotypic control in human basophils and the role of CD32b in its regulation is explored in this study. However, initial pilot studies discovered that IL-3 could markedly up-regulate CD32 expression and first describing the consequences of this up-regulation became an additional focus of this study.

METHODS

Human basophils were examined for the changes in IgE-mediated signalling during simultaneous engagement of CD32b.

RESULTS

Preliminary experiments noted that CD32b could be up-regulated by IL-3 (3- to 12-fold). Both natural variation and induced up-regulation of CD32b modulated the efficacy of this receptor to inhibit IgE-mediated release. Signalling induced by engagement of CD32b (lyn, syk, SHP-1, or SHIP1 phosphorylation) was more consistent with a mode of action involving SHIP1 rather than SHP-1. IgE-mediated down-regulation of syk expression was not altered by co-engagement of CD32b, a result also consistent with a SHIP1-dependent mechanism of inhibition.

CONCLUSIONS

Taken together these results suggest that the combined action of IgE and IgG could generate a natural mechanism, whereby the significant variation in syk expression in allergic subjects occurs without necessarily also inducing mediator release.

NOX2-derived ROS-mediated surface translocation of BLT1 is essential for exocytosis in human eosinophils induced by LTB4

BACKGROUND

Leukotriene B4 (LTB4) is a proinflammatory lipid mediator that elicits eosinophil exocytosis, leading to allergic inflammation. However, the detailed intracellular signaling mechanisms of eosinophil exocytosis induced by LTB4 are poorly understood. Herein, we report that NADPH oxidase (NOX)2-derived reactive oxygen species (ROS)-mediated BLT1 migration to the cell surface is required for exocytosis in human eosinophils induced by LTB4.

METHODS

Peripheral blood eosinophils were purified and stimulated for up to 60 min with LTB4. The signaling role of NOX2-derived ROS in BLT1-dependent exocytosis in LTB4-stimulated eosinophils was investigated.

RESULTS

Stimulating eosinophils with LTB4 induced intracellular ROS production and surface upregulation of the exocytosis marker protein CD63 via BLT1-mediated signaling. LTB4 induced p47(phox) phosphorylation and 91(phox) expression required for NOX2 activation in a BLT1-dependent manner. Pretreatment with NOX2 inhibitors, but not mitochondria inhibitor, prevented LTB4-induced ROS generation and exocytosis. At 30 min after stimulation with LTB4, BLT1 expression at the cell surface was upregulated. LTB4-triggered surface upregulation of BLT1 was also blocked by inhibition of ROS generation with NOX2 inhibitors. Moreover, stimulation for 30 min with LTB4 resulted in the interaction of BLT1 with NOX2 by immunoprecipitation. LTB4-induced ROS generation, surface upregulation of BLT1 and exocytosis was also inhibited by pretreatment with a lipid raft disruptor, protein kinase C inhibitor, or Src kinase inhibitor.

CONCLUSION

These results suggest that NOX2-derived ROS-mediated BLT1 trafficking to the cell surface plays a key role in the exocytosis of human eosinophils induced by LTB4.

Allergen screening bioassays: recent developments in lab-on-a-chip and lab-on-a-disc systems

Allergies occur when a person's immune system mounts an abnormal response with or without IgE to a normally harmless substance called an allergen. The standard skin-prick test introduces suspected allergens into the skin with lancets in order to trigger allergic reactions. This test is annoying and sometimes life threatening. New tools such as lab-on-a-chip and lab-on-a-disc, which rely on microfabrication, are designed for allergy testing. These systems provide benefits such as short analysis times, enhanced sensitivity, simplified procedures, minimal consumption of sample and reagents and low cost. This article gives a summary of these systems. In particular, a cell-based assay detecting both the IgE- and non-IgE-type triggers through the study of degranulation in a centrifugal microfluidic system is highlighted.

Accelerated dissociation of IgE-FcεRI complexes by disruptive inhibitors actively desensitizes allergic effector cells

BACKGROUND

The remarkably stable interaction of IgE with its high-affinity receptor FcεRI on basophils and mast cells is critical for the induction of allergic hypersensitivity reactions. Because of the exceptionally slow dissociation rate of IgE-FcεRI complexes, such allergic effector cells permanently display allergen-specific IgE on their surface and immediately respond to allergen challenge by releasing inflammatory mediators. We have recently described a novel macromolecular inhibitor that actively promotes the dissociation of IgE from FcεRI through a molecular mechanism termed facilitated dissociation.

OBJECTIVE

Here we assessed the therapeutic potential of this non-immunoglobulin-based IgE inhibitor E2_79, a designed ankyrin repeat protein (DARPin), as well as a novel engineered biparatopic DARPin bi53_79, and directly compared them with the established anti-IgE antibody omalizumab.

METHODS

IgE-FcεRI complex dissociation was analyzed in vitro by using recombinant proteins in ELISA and surface plasmon resonance, ex vivo by using human primary basophils with flow cytometry, and in vivo by using human FcεRI α-chain transgenic mice in a functional passive cutaneous anaphylaxis test.

RESULTS

We show that E2_79-mediated removal of IgE from primary human basophils fully abrogates IgE-dependent cell activation and release of proinflammatory mediators ex vivo. Furthermore, we report that omalizumab also accelerates the dissociation of IgE from FcεRI, although much less efficiently than E2_79. Using the biparatopic IgE targeting approach, we further improved the disruptive potency of E2_79 by approximately 100-fold and show that disruptive IgE inhibitors efficiently prevent passive cutaneous anaphylaxis in mice expressing the human FcεRI α-chain.

CONCLUSION

Our findings highlight the potential of such novel IgE inhibitors as important diagnostic and therapeutic tools for management of allergic diseases.

IL-33 is a mediator rather than a trigger of the acute allergic response in humans

BACKGROUND

IL-33 enhances FcεRI-induced mediator release in human basophils without inducing degranulation itself. In contrast, studies in mice suggested that in the presence of high IgE levels, IL-33 triggers degranulation and anaphylaxis of similar severity as specific allergen. Consistent with this view, sera of atopic patients contain elevated levels of IL-33 after anaphylaxis. In this study, we determined whether IL-33 is potentially anaphylactogenic in humans with high IgE levels by regulating exocytosis independent of FcεRI cross-linking. Furthermore, we investigated whether IL-33 is released upon allergen provocation in vivo.

METHODS

In subjects with high serum IgE levels, we measured IL-33-induced histamine/LTC4 in vitro, CD63 translocation ex vivo, and responsiveness of mast cells in vivo by skin prick test (SPT). In asthma patients, release of IL-33 and its correlation with early (tryptase)- and late-phase markers (IL-13 levels, eosinophil numbers) of the allergic response were assessed in bronchoalveolar lavage fluids (BALFs) after allergen challenge.

RESULTS

IL-33 itself does not trigger basophil degranulation in vitro and ex vivo, even in subjects with high serum IgE levels, and negative SPTs demonstrate that skin mast cells do not degranulate in response to IL-33. However, in response to allergen challenge, IL-33 is rapidly released into BALFs at levels that do not correlate with other immediate- and late-phase parameters.

CONCLUSION

IL-33 is unlikely an independent trigger of anaphylaxis even in subjects with high IgE levels. However, the rapid release of IL-33 upon allergen provocation in vivo supports its role as a mediator of immediate allergic responses.

Current concepts of IgE regulation and impact of genetic determinants

Immunoglobulin E (IgE) mediated immune responses seem to be directed against parasites and neoplasms, but are best known for their involvement in allergies. The IgE network is tightly controlled at different levels as outlined in this review. Genetic determinants were suspected to influence IgE regulation and IgE levels considerably for many years. Linkage and candidate gene studies suggested a number of loci and genes to correlate with total serum IgE levels, and recently genome-wide association studies (GWAS) provided the power to identify genetic determinants for total serum IgE levels: 1q23 (FCER1A), 5q31 (RAD50, IL13, IL4), 12q13 (STAT6), 6p21.3 (HLA-DRB1) and 16p12 (IL4R, IL21R). In this review, we analyse the potential role of these GWAS hits in the IgE network and suggest mechanisms of how genes and genetic variants in these loci may influence IgE regulation.

Subthreshold desensitization of human basophils re-capitulates the loss of Syk and FcεRI expression characterized by other methods of desensitization

BACKGROUND

Clinical desensitization of patients to drugs involves progressive exposure to escalating doses of drug over a period of 24 h. In prior studies, this method was re-capitulated in vitro to also demonstrate loss of mast cell or basophil responsiveness. However, most signalling studies of human basophils have identified changes in signalling by using other methods of inducing cellular desensitization.

OBJECTIVE

This study examined two well-described endpoints of basophil desensitization, loss of syk or FcεRI expression, under conditions of subthreshold desensitization.

METHODS

The loss of FcεRI and syk was examined in human basophils.

RESULTS

It was shown that both loss of syk and FcεRI/IgE occurred during an escalating series of stimulation (anti-IgE Ab) and that expression loss occurred despite the presence of little histamine release. If basophils were first cultured for 3 days in 10 ng/mL IL-3, the concentration-dependence of histamine release shifted to 100-fold lower concentrations of stimulus. However, loss of syk did not show any change in its EC50 while loss of FcεRI also shifted 100-fold. From the perspective of early signal element activation, the marked shift in the EC50 for histamine release was not accompanied by similar shifts in the EC50s for several signalling elements. The EC50s for phospho-Src, phospho-SHIP1, phospho-Syk, or phospho-Cbl did not change while the EC50s for phospho-Erk and the cytosolic calcium response did shift 100-fold.

CONCLUSIONS

These studies show that under normal conditions, subthreshold desensitization leads to loss of two critical signalling molecules (FcεRI and syk) but under at least one condition, treatment with IL-3, it is possible to markedly blunt the loss of syk, but not FcεRI, while executing a proper subthreshold titration. These data also suggest that IL-3 modifies only the sensitivity of signalling elements that are downstream of syk activation.

Suppression of the basophil response to allergen during treatment with omalizumab is dependent on 2 competing factors

BACKGROUND

A recent study of subjects with peanut allergy treated with omalizumab generated some results that were concordant with a study of subjects with cat allergy treated with omalizumab. However, there were differences that provided additional insight into the nature of the cellular responses in allergic subjects.

OBJECTIVE

We sought to determine the cause for failure to suppress the allergen-induced basophil response during treatment with omalizumab.

METHODS

Patients with peanut allergy were treated with omalizumab. Clinical, serologic, and cellular indices relevant to the response of the subjects and their peripheral blood basophil values (specific/total IgE ratio, cell-surface FcεRI expression, and histamine release responses to anti-IgE antibody or peanut allergen) were obtained at 3 times.

RESULTS

After treatment, approximately 60% of the subjects' basophil responses to peanut allergen did not significantly decrease. In 40% of cases, the in vitro basophil response to peanut allergen increased 2- to 7-fold. The increases were associated with 2 primary factors: a high (>10%) specific/total IgE ratio and an increase in the intrinsic response of the basophil to IgE-mediated stimulation. The extent to which the basophil response to peanut allergen increased was inversely correlated with improvement in the patient's ability to tolerate ingestion of peanut.

CONCLUSION

The basophil response during treatment with omalizumab is a consequence of 2 competing factors: suppression of allergen-specific IgE on the cell surface versus increased intrinsic sensitivity to IgE-mediated stimulation. In subjects with peanut allergy, the basophil response appears to mitigate against the ability of omalizumab to improve the patient's tolerance of oral allergen.

Live Cells as Dynamic Laboratories: Time Lapse Raman Spectral Microscopy of Nanoparticles with Both IgE Targeting and pH-Sensing Functions

This review captures the use of live cells as dynamic microlaboratories through implementation of labeled nanoparticles (nanosensors) that have both sensing and targeting functions. The addition of 2,4-ε-dinitrophenol-L-lysine (DNP) as a FcεRI targeting ligand and 4-mercaptopyridine (4-MPy) as a pH-sensing ligand enables spatial and temporal monitoring of FcεRI receptors and their pH environment within the endocytic pathway. To ensure reliability, the sensor is calibrated in vivo using the ionophore nigericin and standard buffer solutions to equilibrate the external [H⁺] concentration with that of the cell compartments. This review highlights the nanosensors, ability to traffic and respond to pH of receptor-bound nanosensors (1) at physiological temperature (37°C) versus room temperature (25°C), (2) after pharmacological treatment with bafilomycin, an H⁺ ATPase pump inhibitor, or amiloride, an inhibitor of Na⁺/H⁺ exchange, and (3) in response to both temperature and pharmacological treatment. Whole-cell, time lapse images are demonstrated to show the ability to transform live cells into dynamic laboratories to monitor temporal and spatial endosomal pH. The versatility of these probes shows promise for future applications relevant to intracellular trafficking and intelligent drug design.

House dust mite-specific immunotherapy alters the basal expression of T regulatory and FcεRI pathway genes

BACKGROUND

Regulatory T (Treg) cells and IgE-mediated signaling pathways could play important roles in the induction of allergen tolerance during house dust mite-specific subcutaneous immunotherapy (HDM-SCIT). Our aim was to compare the basal expression levels of Treg, T helper 1 (Th1) and Th2 transcription factors and components involved in IgE-mediated signaling in healthy subjects with those in HDM-allergic patients both untreated and successfully treated with HDM-SCIT.

METHODS

Thirty-nine HDM-allergic patients who completed a 3- to 5-year course of mite extract SCIT, 20 mite-allergic controls and 25 healthy controls participated in this study. The efficacy of SCIT was monitored using skin-prick tests (SPTs), total immunoglobulin E (tIgE), specific IgE (sIgE), sIgG(4), nasal challenge and visual analog scale (VAS) scores at several time points. The mRNA levels of forkhead box protein 3 (FOXP3), T-BET, GATA-3, FcεRI, spleen tyrosine kinase (Syk), phosphatidylinositol 3 kinase (PI3K) and SH2 domain-containing inositol phosphatase (SHIP) were quantified by real-time RT-PCR using nonstimulated whole blood samples.

RESULTS

Decreased wheal sizes and VAS scores, negative challenges and increased sIgG(4) levels indicated that SCIT was effective in the treated patients. Basal expression levels of FOXP3 and GATA-3 decreased and T-BET levels increased in both treated patients and in healthy controls compared to untreated patients. The IgE-mediated pathway kinases Syk and PI3K exhibited reduced expression, whereas SHIP phosphatase levels were elevated in both treated patients and healthy controls relative to untreated patients. The expression levels of FcεRI were not significantly altered.

CONCLUSIONS

Immunotherapy using HDM extracts results in a modification of the basal expression levels of several IgE-related signaling factors and induces a highly significant upregulation of Th1-response and downregulation of Th2-response transcription factors. Interestingly, this therapy also appears to reduce the basal expression of FOXP3.

Omalizumab inhibits acceleration of FcεRI-mediated responsiveness of immature human mast cells by immunoglobulin E

BACKGROUND

A large body of evidence has demonstrated that treatment with omalizumab is clinically effective for the management of moderate to severe allergic asthma, emphasizing the importance of IgE in the pathogenesis of allergic asthma. We hypothesized that IgE accelerates FcεRI-mediated responsiveness of "immature" human mast cells (MCs) and that omalizumab downregulates the acceleration.

OBJECTIVES

To examine when MC progenitors acquired the ability to degranulate following FcεRI aggregation, whether IgE accelerates the responsiveness of immature MCs following FcεRI aggregation, and whether omalizumab regulates such an acceleration.

METHODS

Gene expression was examined using a microarray and quantitative reverse transcription polymerase chain reaction. Protein expression was investigated using FACS. Histamine release was examined using an EIA.

RESULTS

The time-course analysis of the mRNA expression of MC-related genes, including FcεRI, in Kit(+) sorted cells during the differentiation and histamine experiments revealed that the expression level of FcεRI in 5 week (w)-cultured MCs was not sufficient to induce degranulation following FcεRI aggregation but that 5 w-cultured MCs were fully responsive to calcium ionophore. By addition of IgE in culture medium FcεRI expression level and FcεRI-mediated histamine release of 5 w-cultured MCs were significantly increased compared with those without addition of IgE, whereas the expression level of tryptase and number of MCs was not affected. Omalizumab significantly inhibited IgE-dependent enhancement of FcεRI expression level and FcεRI-mediated histamine release.

CONCLUSIONS

High levels of IgE in the microenvironment in vivo may upregulate the responsiveness of immature MCs to allergens. Omalizumab may inhibit the IgE-mediated responsiveness of not only mature MCs, but also immature MCs.

Immunology in the Clinic Review Series; focus on allergies: basophils as biomarkers for assessing immune modulation

Allergen-specific immunotherapy is an effective clinical treatment for hypersensitivity to many allergens. Studies of basophils during immunotherapy have provided insight into underlying immune mechanisms and support the potential use of basophil activation as a biomarker of clinical outcomes. This review examines the evidence for different pathways of basophil modulation associated with various forms of immunotherapy. Better understanding the molecular mechanisms of basophil activation and desensitization and the relationship between suppression of these effector cells to clinical outcomes holds promise for further development and improvement in potential therapies for allergic diseases.

Association of polymorphisms in the promoter region of FCER1A gene with atopic dermatitis, chronic uticaria, asthma, and serum immunoglobulin E levels in a Han Chinese population

The high-affinity receptor for immunoglobulin E (IgE) plays a central role in allergy diseases. Previous studies have reported the association of variants in the proximal promoter of FCER1A with IgE levels as well as allergy disorders. Another promoter gene polymorphism that is located upstream of exon 1 has not been investigated. We investigated the association of variants in the promoter located upstream of FCER1A exon 1 with serum IgE levels and allergy diseases in a Han Chinese population. A total of 97 patients with atopic dermatitis (AD), 123 patients with chronic urticaria (CU), 286 children with asthma, and control groups were screened for polymorphisms in the promoter region located upstream of FCER1A exon 1 by the polymerase chain reaction-ligation detection reaction method. Total serum IgE levels were tested in groups. The rare allele A of the rs2427837 A/G polymorphism was significantly different in the AD group compared with the controls. No association with the polymorphism was observed in the CU group. In asthmatic patients, IgE levels were higher in the mutation genotypes GA of rs2427837 and TC of rs2251746 compared with normal genotype individuals. The minor allele of rs2427837 and rs2251746 in FCER1A is a genetic risk factor of high IgE levels.

High-affinity immunoglobulin E receptor expression is increased in large and small airways in fatal asthma

BACKGROUND

IgE and its high-affinity receptor FcɛRI play an important role in allergy and asthma. The distribution of FcɛRI expression in the airways and within the airway wall, however, is largely unknown.

OBJECTIVE

In this study, we aimed to map the distribution of FcɛRI in different layers of large airways (LA) and small airways (SA) in lung tissue from non-smoking and smoking patients who died of asthma [fatal asthma (FA)] and non-smoking controls (CTR).

METHODS

Postmortem lung tissue from 24 cases of non-smoking FA, 13 smoking FA patients and from 19 subjects who died of non-pulmonary causes (CTR) was immunohistochemically stained for FcɛRI and AA1 (mast cell tryptase marker). The expression of these markers was analysed in inner, muscle, and outer layers of both LA and SA by image analysis.

RESULTS

FcɛRI expression was higher in non-smoking and smoking FA compared with CTR in the inner and outer layer of SA. In the outer layer of LA, FcɛRI expression was higher in non-smoking FA compared with CTR. AA1 was higher in non-smoking FA compared with smoking FA and CTR in the outer layer of the SA, which was correlated with FcɛRI in this layer.

CONCLUSION

Our results show that the expression of FcɛRI is higher in both LA and SA in FA compared with CTR. These differences are predominantly found in the outer layer where they can be attributed in part to the increased mast cell numbers. These results indicate an increased capacity to mount IgE-mediated reactions in FA, both in LA and SA.

Characterization of syk expression in human lung mast cells: relationship with function

BACKGROUND

Previous studies indicate that the protein tyrosine kinase, syk, is critical in transducing FcɛRI-mediated signals. In human basophils, 'releasability' has been linked to the extent of syk expression. Human lung mast cells, like basophils, are also found to be variably responsive to IgE-dependent activation.

OBJECTIVE

The aim of the present study was to determine whether the wide variability in human lung mast cell responses, following IgE-dependent activation, has a relationship with syk expression.

METHODS

Mast cells were isolated from human lung tissue and 'releasability' was determined by activating the cells with a maximal releasing concentration of anti-IgE. Syk levels in mast cells were determined by immunoblotting and flow cytometry.

RESULTS

Histamine release from mast cells, challenged with a maximal releasing concentration of anti-IgE, ranged from 0% to 69% (mean±SEM, 24±2%, n=53). A proportion of these preparations (nine out of 53) released very low levels of histamine (5%) in response to anti-IgE. Flow cytometry of a subset of preparations indicated that a weak response to anti-IgE was not related to a lack of surface IgE. Immunoblotting and flow cytometry studies demonstrated that, compared with mononuclear cells, human lung mast cells express low and variable levels of syk. However, there was no correlation between syk expression and mast cell releasability. Nonetheless, a number of putative inhibitors of syk including NVP-QAB205 (EC₅₀, 0.2 μm) effectively attenuated the IgE-dependent release of histamine from mast cells.

CONCLUSION AND CLINICAL RELEVANCE

These studies indicate that although syk may play an important role in mediating degranulation, the relative level of syk expression does not govern human lung mast cell releasability. Identification of the mechanisms that govern IgE-dependent activation of human lung mast cells is likely to be of wider clinical significance, given the central role that mast cells play in the development of allergic asthma.

SERS nanosensors that report pH of endocytic compartments during FcεRI transit

Recently, the development of an IgE receptor (FcεRI)-targeted, pH-sensitive, surface-enhanced Raman spectroscopy (SERS) nanosensor has been demonstrated by Nowak-Lovato and Rector (Appl Spectrosc 63:387-395, 2009). The targeted nanosensor enables spatial and temporal pH measurements as internalized receptors progress through endosomal compartments in live cells. Trafficking of receptor-bound nanosensors was compared at physiological temperature (37 °C) versus room temperature (25 °C). As expected, we observed markedly slower progression of receptors through low-pH endocytic compartments at the lower temperature. We also demonstrate the utility of the nanosensors to measure directly changes in the pH of intracellular compartments after treatment with bafilomycin or amiloride. We report an increase in endosome compartment pH after treatment with bafilomycin, an H(+) ATPase pump inhibitor. Decreased endosomal luminal pH was measured in cells treated with amiloride, an inhibitor of Na(+)/H(+) exchange. The decrease in amiloride-treated cells was transient, followed by a recovery period of approximately 15-20 min to restore endosomal pH. These experiments demonstrate the novel application of Raman spectroscopy to monitor local pH environment in live cells with the use of targeted SERS nanosensors.

Therapeutic antibodies for autoimmunity and inflammation

The development of therapeutic antibodies has evolved over the past decade into a mainstay of therapeutic options for patients with autoimmune and inflammatory diseases. Substantial advances in understanding the biology of human diseases have been made and tremendous benefit to patients has been gained with the first generation of therapeutic antibodies. The lessons learnt from these antibodies have provided the foundation for the discovery and development of future therapeutic antibodies. Here we review how key insights obtained from the development of therapeutic antibodies complemented by newer antibody engineering technologies are delivering a second generation of therapeutic antibodies with promise for greater clinical efficacy and safety.

Regulation of Syk kinase and FcRbeta expression in human basophils during treatment with omalizumab

BACKGROUND

In human basophils from different subjects, maximum IgE-mediated histamine release and the level of Syk protein expression correlate well. Recent studies suggest that in some patients treated with omalizumab, the response to stimulation with anti-IgE antibody increases. In unrelated studies there is also evidence that the composition of FcepsilonRI in basophils differs among subjects. This observation raised the possibility that the stoichiometry of FcRbeta/FcepsilonRIalpha is not fixed to a 1:1 ratio and might be modifiable during changes in the basophil's environment.

OBJECTIVE

We sought to determine whether treatment with omalizumab results in increases in Syk expression and anti-IgE-mediated histamine release and disproportionately alters the relative presence of FcRbeta and FcepsilonRIalpha.

METHOD

Syk, FcepsilonRIalpha, and FcRbeta expression was monitored during the treatment of subjects with omalizumab.

RESULTS

Treatment with omalizumab reduced histamine release from peripheral blood leukocytes stimulated with cat allergen in vitro, but histamine release stimulated with anti-IgE antibody increased 2-fold. Expression of Syk increased 1.86-fold. There was no change in the expression of c-Cbl, a signaling element that is sensitive to the presence of IL-3, and no increase in response to formyl-met-leu-phe (tripeptide), a response that also increases in the presence of IL-3. There was a 60% decrease in the FcRbeta/FcepsilonRIalpha ratio in patients treated with omalizumab.

CONCLUSIONS

In the context of previous studies, these studies provide support for a proposal that Syk expression is modulated in vivo through an IgE-dependent mechanism and that the ratio of FcepsilonRI alpha and beta subunits in basophils is influenced by factors extrinsic to the cell.

Regulation of the IgE response

IgE was the last of the five immunoglobulin classes to be discovered and is the antibody that is responsible for much of human type I allergic disease. This review summarizes recent developments with respect to control of IgE synthesis with an emphasis on Th2 (T helper 2) control and regulation using IgE Fc receptors.

Regulation of Fc epsilon RI expression during murine basophil maturation: the interplay between IgE, cell division, and Fc epsilon RI synthetic rate

Expression of Fc epsilonRI on basophils and mast cells is modulated by IgE Ab. Previous studies have noted in vivo receptor expression dynamics that are discordant with expectations derived from in vitro studies. The current study presents a formal hypothesis to explain the discordant observations and tests two assumptions that underlie a proposed model of receptor dynamics. After first showing that a murine model of receptor expression on basophils recapitulates observations made using human basophils, the effect of changes in IgE on basophil egress rates was examined. In the proposed model, egress rates from bone marrow (BM) were assumed to be unaffected by changes in IgE concentration. Egress was tested by examining the labeling of BM and peripheral blood (BL) basophils at various times after injection of BrdU with and without injection with IgE. The IgE Ab did not alter the appearance of BrdU label in peripheral BL basophils. In addition, BM and BL basophils were responsive to the elevations in IgE, with receptor expression increasing on BM basophils before BL basophils. It was also noted that BL basophils expressed approximately 50% of the receptor density of BM basophils. There was a 3-fold greater synthetic rate of Fc epsilonRI on BM basophils that readily explained the difference. These results provide support for the proposed hypothesis of rapid changes in receptor expression being controlled by cell replacement. The studies also support a model whereby receptor expression is limited by cell division and that basophils, once mature, slow their rate of receptor synthesis.

Effects of omalizumab on basophil and mast cell responses using an intranasal cat allergen challenge

BACKGROUND

Omalizumab treatment suppresses FcepsilonRI expression faster on blood basophils than skin mast cells.

OBJECTIVE

We used omalizumab to elucidate the relative contributions of basophil versus mast cell FcepsilonRI activation in a nasal allergen challenge (NAC) model.

METHODS

Eighteen subjects with cat allergy were enrolled in a 3.5-month, double-blind, randomized (3.5:1), placebo-controlled trial of omalizumab using standard dosing. At baseline, subjects underwent NAC with lavage for prostaglandin D(2) measurement, skin prick test titration (SPTT), and blood sampling for basophil histamine release (BHR) and basophil IgE/FcepsilonRI measurements. Basophil studies were repeated at day 3 and then weekly until cat allergen-induced BHR was <20% of baseline or until day 45. Baseline visit procedures were repeated after the BHR reduction (midstudy NAC) and at the treatment period's completion (final NAC).

RESULTS

Subjects treated with omalizumab who completed all NACs (n = 12) demonstrated significant mean reduction in BHR to an optimal dose of cat allergen by midstudy NAC compared with baseline (74% decrease; P = .001). In addition, these subjects demonstrated significant decreases in mean combined nasal symptom scores (50% decrease; P = .007) and total sneeze counts (59% decrease; P = .01) by midstudy NAC relative to baseline NAC. In contrast, measures of mast cell response (SPTT and nasal lavage prostaglandin D(2)) were only significantly reduced by the final NAC. Subjects on placebo (n = 4) did not experience a shift in basophil, NAC symptom, or mast cell measures.

CONCLUSION

Reduction in nasal symptom scores occurred when the basophil, but not mast cell, response was reduced on omalizumab, implicating a role for basophils in the acute NAC response.

Syk expression in peripheral blood leukocytes, CD34+ progenitors, and CD34-derived basophils

In human basophils from different subjects, maximum IgE-mediated histamine release and the level of syk protein expression correlate well. It is not clear when in the basophil's lifetime the set-point for syk expression is reached or how expression levels are determined for a given individual. An examination of syk expression in peripheral blood eosinophils, neutrophils, monocytes, B and T cells, DCs, and NK cells showed that with the exception of T cells, basophils were unique in expressing low levels of syk. No correlations were observed between syk expression in basophils and other types of leukocytes, suggesting a unique mechanism of regulation for basophils. The expression level of syk in CD34+ progenitors was approximately 11-fold higher than in peripheral blood basophils, and it remained at this level during maturation of the cells in IL-3 to a cell with characteristics of peripheral blood basophils. Down-regulation of syk expression in the culture-derived basophils was induced by culturing under conditions of chronic aggregation of FcepsilonRI. Syk was down-regulated to peripheral blood basophil levels in 50% of the cells. Despite the chronic aggregation of FcepsilonRI, the cells retained the same expression of FcepsilonRI, histamine content, and morphological staining of granules as cells not experiencing chronic aggregation. These results suggest that chronic stimulation through FcepsilonRI during basophil maturation might be a mechanism for down-regulating syk expression, while retaining other characteristics associated with mature peripheral blood basophils.

Proinflammatory and Th2 cytokines regulate the high affinity IgE receptor (FcepsilonRI) and IgE-dependant activation of human airway smooth muscle cells

Background

The high affinity IgE receptor (FcεRI) is a crucial structure for IgE-mediated allergic reactions. We have previously demonstrated that human airway smooth muscle (ASM) cells express the tetrameric (αβγ2) FcεRI, and its activation leads to marked transient increases in intracellular Ca²⁺ concentration, release of Th-2 cytokines and eotaxin-1/CCL11. Therefore, it was of utmost importance to delineate the factors regulating the expression of FcεRI in human (ASM) cells.

Methodology/Principal Findings

Incubation of human bronchial and tracheal smooth muscle (B/TSM) cells with TNF-α, IL-1β or IL-4 resulted in a significant increase in FcεRI-α chain mRNA expression (p<0.05); and TNF-α, IL-4 enhanced the FcεRI-α protein expression compared to the unstimulated control at 24, 72 hrs after stimulation. Interestingly, among all other cytokines, only TNF-α upregulated the FcεRI-γ mRNA expression. FcεRI-γ protein expression remained unchanged despite the nature of stimulation. Of note, as a functional consequence of FcεRI upregulation, TNF-α pre-sensitization of B/TSM potentially augmented the CC (eotaxin-1/CCL11 and RANTES/CCL5, but not TARC/CCL17) and CXC (IL-8/CXCL8, IP-10/CXCL10) chemokines release following IgE stimulation (p<0.05, n = 3). Furthermore, IgE sensitization of B/TSM cells significantly enhanced the transcription of selective CC and CXC chemokines at promoter level compared to control, which was abolished by Lentivirus-mediated silencing of Syk expression.

Conclusions/Significance

Our data depict a critical role of B/TSM in allergic airway inflammation via potentially novel mechanisms involving proinflammatory, Th2 cytokines and IgE/FcεRI complex.

Expression of the high affinity IgE receptor by neutrophils of individuals with allergic asthma is both minimal and insensitive to regulation by serum IgE

We evaluated the hypothesis that serum IgE regulates neutrophil FcepsilonRI expression in the same manner as described for other FcepsilonRI+ cells. FcepsilonRI expression by neutrophils of 40 asthma subjects and 20 control subjects did not correlate with serum IgE levels, whereas FcepsilonRI expression by basophils of the same subjects showed a highly significant correlation. The level of FcepsilonRI expression by neutrophils of both asthma and control subjects was approximately 1% of that for basophil FcepsilonRI expression. IgE+ neutrophils were minimally detectable, and FcepsilonRI alpha-subunit was not detected in Western blots of neutrophil membranes and cytosol. The neutrophil FcepsilonRI did not support anti-IgE stimulated superoxide release or IgE-induced increase in neutrophil survival. We conclude that FcepsilonRI expression by neutrophils of both asthma patients and control individuals is minimal at best and that, if present, neutrophil FcepsilonRI expression, unlike that of other human FcepsilonRI+ cells, is not regulated by serum IgE.

Polymerization of actin does not regulate desensitization in human basophils

Previous studies have suggested that maintenance of IgE-mediated signaling results from regulation of the activity of signaling complexes by actin polymerization. This process is also hypothesized to be related to desensitization of basophils and mast cells. Recent studies demonstrated that any signaling process dependent on syk or PI-3K activity cannot be a mechanism of desensitization, and in this context, syk and PI-3K inhibitors were found to inhibit actin polymerization. Inhibitors of actin polymerization were tested for their effect on desensitization of human peripheral blood basophils. Latrunculin A, in particular, removed all resting and stimulated f-actin but did not inhibit desensitization. Cytochalasin D and latrunculin A also did not reverse the loss of syk phosphorylation that accompanies desensitization. These results demonstrate that desensitization mechanisms are not dependent on actin polymerization. In this context, it was also shown that progressive immobilization of Fc epsilon RI during aggregation was sensitive to syk or actin polymerization inhibition. Therefore, desensitization is also not dependent on receptor immobilization. These studies demonstrate that desensitization is not the result of two signaling pathways once considered relevant to down-regulation of IgE-mediated signaling.

IgE receptor and signal transduction in mast cells and basophils

There are many aspects of mast cell and basophil biology that are being explored today. Notably, there is a wide variety of studies of the roles these two cell types play in the development of a multitude of diseases and the role they play in the general immune response. But the original reasons for studying these two cells types--because they are considered crucial to immediate hypersensitivity reactions--remain a driving force for research. These two cell types express the complete high affinity IgE receptor and aggregation of this receptor results in the secretion of multiple potent mediators that cause many of the signs and symptoms of an allergic reaction. Understanding more about the biology of the receptor and the signaling that it initiates remains important to the development of new therapeutic approaches to inhibit this reaction.

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.

Inducing an anergic state in mast cells and basophils without secretion

BACKGROUND

IgE-mediated secretion from mast cells or basophils depends on the activity of both spleen tyrosine kinase (syk) and phosphatidyl inositol 3' kinase (PI3K), but several specific downregulatory pathways (eg, loss of syk expression) do not.

OBJECTIVE

We tested whether stimulation with antigen in the presence of a syk inhibitor (NVP-QAB205) would ablate secretion while simultaneously allowing anergy.

METHODS

The anergic or desensitized state in human basophils, cultured-derived mast cells, and in situ stimulated airway mast cells (in organ baths) was assessed after stimulation with antigen in the presence of syk inhibitor.

RESULTS

Antigen caused 35 +/- 7% and 62 +/- 10% histamine release from basophils and mast cells, respectively, and it caused an 87 +/- 5% histamine/leukotriene D(4)-dependent contraction of human isolated bronchi. All of these responses were blocked >95% by the syk inhibitor. Rechallenging the preparations with antigen, after first washing out the syk inhibitor and antigen, revealed that near complete anergy (92% to 100%) occurred in each case. A similar result was found when using a PI3K inhibitor, LY294002, in studies of basophils.

CONCLUSION

Although the syk inhibitor nearly abolished the antigen-induced secretion from mast cells and basophils, it had little effect on the pathways involved in anergy. These results suggest that syk and PI3K are not involved in downregulation leading to anergy.

Assessment of three human FcepsilonRI-transfected RBL cell-lines for identifying IgE induced degranulation utilizing peanut-allergic patient sera and peanut protein extract

Specific IgE sera screening studies are employed to investigate protein cross-reactivity. Such nonfunctional immunochemical methods cannot measure the biological activity of proteins. Therefore, an assay using RBL cells transfected with human FcepsilonRI was developed. Our objective was to evaluate the degranulation of three cell-lines expressing either the alpha-(RBL-hEI(a)-2B12 and RBL-30/25cells) or alpha-, beta-, and gamma-subunits (RBL SX-38) of the human FcepsilonRI by beta-hexosaminidase release. Purified human IgE and serum-derived polyclonal IgE from peanut-allergic subjects following challenge with anti-IgE or peanut protein extract, respectively, were utilized. Robust degranulation was induced in all three: RBL-30/25 (84%), -hEI(a)-2B12 (54%), SX-38 (94%), respectively, using purified IgE+anti-human IgE. Good release (18%, 40-45%, and 65%, respectively) occurred for one peanut-allergic subject+peanut extract with all cell-lines. With serum from three other peanut-allergic subjects, no beta-hexosaminidase release occurred with RBL-hEI(a)-2B12 cells+peanut extract, while only serum from one subject induced good degranulation, 30% and 60%, respectively, with RBL-30/25 and RBL SX 38 cells. Consistent degranulation with a potent food allergen (peanuts) was not observed. The assay's utility in safety assessment, predictive value and reproducibility for evaluating the cross-reactivity of proteins with allergens needs further investigation with additional proteins and well-characterized sera.