The first transmembrane region of the beta-chain stabilizes the tetrameric Fc epsilon RI complex

Structural insights into the high-affinity IgE receptor FcεRI complex

Immunoglobulin E (IgE) plays a pivotal role in allergic responses1,2. The high-affinity IgE receptor, FcεRI, found on mast cells and basophils, is central to the effector functions of IgE. FcεRI is a tetrameric complex, comprising FcεRIα, FcεRIβ and a homodimer of FcRγ (originally known as FcεRIγ), with FcεRIα recognizing the Fc region of IgE (Fcε) and FcεRIβ-FcRγ facilitating signal transduction3. Additionally, FcRγ is a crucial component of other immunoglobulin receptors, including those for IgG (FcγRI and FcγRIIIA) and IgA (FcαRI)4-8. However, the molecular basis of FcεRI assembly and the structure of FcRγ have remained elusive. Here we elucidate the cryogenic electron microscopy structure of the Fcε-FcεRI complex. FcεRIα has an essential role in the receptor's assembly, interacting with FcεRIβ and both FcRγ subunits. FcεRIβ is structured as a compact four-helix bundle, similar to the B cell antigen CD20. The FcRγ dimer exhibits an asymmetric architecture, and coils with the transmembrane region of FcεRIα to form a three-helix bundle. A cholesterol-like molecule enhances the interaction between FcεRIβ and the FcεRIα-FcRγ complex. Our mutagenesis analyses further indicate similarities between the interaction of FcRγ with FcεRIα and FcγRIIIA, but differences in that with FcαRI. These findings deepen our understanding of the signalling mechanisms of FcεRI and offer insights into the functionality of other immune receptors dependent on FcRγ.

Identification of redundancy between human FcεRIβ and MS4A6A proteins points toward additional complex mechanisms for FcεRI trafficking and signaling

BACKGROUND

Allergic diseases are triggered by signaling through the high-affinity IgE receptor, FcεRI. In both mast cells (MCs) and basophils, FcεRI is a tetrameric receptor complex comprising a ligand-binding α subunit (FcεRIα), a tetraspan β subunit (FcεRIβ, MS4A2) responsible for trafficking and signal amplification, and a signal transducing dimer of single transmembrane γ subunits (FcεRIγ). However, FcεRI also exists as presumed trimeric complexes that lack FcεRIβ and are expressed on several cell types outside the MC and basophil lineages. Despite known differences between humans and mice in the presence of the trimeric FcεRI complex, questions remain as to how it traffics and whether it signals in the absence of FcεRIβ. We have previously reported that targeting FcεRIβ with exon-skipping oligonucleotides eliminates IgE-mediated degranulation in mouse MCs, but equivalent targeting in human MCs was not effective at reducing degranulation.

RESULTS

Here, we report that the FcεRIβ-like protein MS4A6A exists in human MCs and compensates for FcεRIβ in FcεRI trafficking and signaling. Human MS4A6A promotes surface expression of FcεRI complexes and facilitates degranulation. MS4A6A and FcεRIβ are encoded by highly related genes within the MS4A gene family that cluster within the human gene loci 11q12-q13, a region linked to allergy and asthma susceptibility.

CONCLUSIONS

Our data suggest the presence of either FcεRIβ or MS4A6A is sufficient for degranulation, indicating that MS4A6A could be an elusive FcεRIβ-like protein in human MCs that performs compensatory functions in allergic disease.

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.

Exon Skipping of FcεRIβ for Allergic Diseases

Mast cells are key effector cells in allergic inflammation and consequently are ideal targets for new therapeutics. The high-affinity IgE receptor complex, FcεRI, plays a critical role in mast cell and basophil activation by allergens to drive the immediate allergic inflammatory response. The β subunit of FcεRI is critical for trafficking the FcεRI complex to the cell membrane and amplifies the FcεRI signaling cascade. We have utilized splice switching antisense oligonucleotides to force expression of a truncated isoform of FcεRIβ, which we have shown does not associate with the FcεRI complex. This approach eliminates surface FcεRI expression in mast cells by targeting protein-protein interactions. Exon skipping has several therapeutic applications, and our findings demonstrate a novel application to alter receptor trafficking and dampen allergic inflammation. Here, we describe the methods of exon skipping in mast cells and the assays used to examine the responses of mast cells in vitro and in vivo.

Complement-Activating Anti-HLA Antibodies in Kidney Transplantation: Allograft Gene Expression Profiling and Response to Treatment

Complement-activating anti-HLA donor-specific antibodies (DSAs) are associated with impaired kidney transplant outcome; however, whether these antibodies induce a specific rejection phenotype and influence response to therapy remains undetermined. We prospectively screened 931 kidney recipients for complement-activating DSAs and used histopathology, immunostaining, and allograft gene expression to assess rejection phenotypes. Effector cells were evaluated using in vitro human cell cultures. Additionally, we assessed the effect of complement inhibition on kidney allograft rejection phenotype and the clinical response to complement inhibition in 116 independent kidney recipients with DSAs at transplant receiving rejection prophylaxis with eculizumab or standard of care (plasma exchange and intravenous Ig) at ten international centers. The histomolecular rejection phenotype associated with complement-activating DSA was characterized by complement deposition and accumulation of natural killer cells and monocytes/macrophages in capillaries and increased expression of five biologically relevant genes (CXCL11, CCL4, MS4A7, MS4A6A, and FCGR3A) indicative of endothelial activation, IFNγ response, CD16-mediated natural killer cell activation, and monocyte/macrophage activation. Compared with standard of care, eculizumab specifically abrogated this histomolecular rejection phenotype and associated with a decreased 3-month rejection incidence rate in patients with complement-activating DSAs (56%; 95% confidence interval [95% CI], 38% to 74% versus 19%; 95% CI, 8% to 35%; P=0.001) but not in those with noncomplement-activating DSAs (9%; 95% CI, 2% to 25% versus 13%; 95% CI, 2% to 40%; P=0.65). In conclusion, circulating complement-activating anti-HLA DSAs are associated with a specific histomolecular kidney allograft rejection phenotype that can be abrogated by complement inhibition.

Exon skipping of FcεRIβ eliminates expression of the high-affinity IgE receptor in mast cells with therapeutic potential for allergy

Allergic diseases are driven by activation of mast cells and release of mediators in response to IgE-directed antigens. However, there are no drugs currently available that can specifically down-regulate mast cell function in vivo when chronically administered. Here, we describe an innovative approach for targeting mast cells in vitro and in vivo using antisense oligonucleotide-mediated exon skipping of the β-subunit of the high-affinity IgE receptor (FcεRIβ) to eliminate surface high-affinity IgE receptor (FcεRI) expression and function, rendering mast cells unresponsive to IgE-mediated activation. As FcεRIβ expression is restricted to mast cells and basophils, this approach would selectively target these cell types. Given the success of exon skipping in clinical trials to treat genetic diseases such as Duchenne muscular dystrophy, we propose that exon skipping of FcεRIβ is a potential approach for mast cell-specific treatment of allergic diseases.

Mast cells in airway diseases and interstitial lung disease

Mast cells are major effector cells of inflammation and there is strong evidence that mast cells play a significant role in asthma pathophysiology. There is also a growing body of evidence that mast cells contribute to other inflammatory and fibrotic lung diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. This review discusses the role that mast cells play in airway diseases and highlights how mast cell microlocalisation within specific lung compartments and their cellular interactions are likely to be critical for their effector function in disease.

A truncated splice-variant of the FcεRIβ receptor subunit is critical for microtubule formation and degranulation in mast cells

Human linkage analyses have implicated the MS4A2-containing gene locus (encoding FcεRIβ) as a candidate for allergy susceptibility. We have identified a truncation of FcεRIβ (t-FcεRIβ) in humans that contains a putative calmodulin-binding domain and thus, we sought to identify the role of this variant in mast cell function. We determined that t-FcεRIβ is critical for microtubule formation and degranulation and that it may perform this function by trafficking adaptor molecules and kinases to the pericentrosomal and Golgi region in response to Ca2+ signals. Mutagenesis studies suggest that calmodulin binding to t-FcεRIβ in the presence of Ca2+ could be critical for t-FcεRIβ function. In addition, gene targeting of t-FcεRIβ attenuated microtubule formation, degranulation, and IL-8 production downstream of Ca2+ signals. Therefore, t-FcεRIβ mediates Ca2+ -dependent microtubule formation, which promotes degranulation and cytokine release. Because t-FcεRIβ has this critical function, it represents a therapeutic target for the downregulation of allergic inflammation.

Fc receptor-γ subunits with both polar or non-polar amino acids at position of T22 are capable of restoring surface expression of the high-affinity IgE receptor and degranulation in γ subunit-deficient rat basophilic leukemia cells

The high-affinity IgE receptor (FcɛRI) is formed by the IgE-binding α subunit, β subunit and γ subunits homodimer. All three subunits are required for proper expression of the receptor on the plasma membrane of mast cells and basophils. However, the exact molecular mechanism of inter-subunit interactions required for correct expression and function of the FcɛRI complex remains to be identified. A recent study suggested that polar aspartate at position 194 within the transmembrane domain of the α subunit could interact by hydrogen bonding with polar threonine at position 22 in the transmembrane domains of the γ subunits. To verify this, we used previously isolated rat basophilic leukemia (RBL)-2H3 variant cells deficient in the expression of the FcɛRI-γ subunit (FcR-γ), and transfected them with DNA vectors coding for FcR-γ of the wild-type or mutants in which T22 was substituted for nonpolar alanine (T22A mutant) or polar serine (T22S mutant). Analysis of the transfectants showed that both T22A and T22S mutants were capable to restore surface expression of the FcɛRI similar to wild-type FcR-γ. Furthermore, cells transfected with wild-type, T22A or T22S FcR-γ showed comparably enhanced FcɛRI-mediated degranulation. Our data indicate that substitution of FcR-γ T22 with non-polar amino acid does not interfere with surface expression of the FcɛRI and its signaling capacity.

Role of ITAM signaling module in signal integration

Diverse cell types use a small number of evolutionarily conserved signaling modules to integrate external cues and elicit distinct functions. A question thus arises as to how does a receptor, which contains a single signaling module, produce distinct outcomes to diverse signals, particularly if such module is shared amongst a family of receptors? Emerging data suggest that many immunoreceptors, all of which use a conserved ITAM-module for their signaling, can couple with members of additional classes of membrane receptors to deliver unique signal(s) to the cell. We discuss the possible biological purposes and mechanisms behind these interactions at the plasma membrane. We offer a conceptual framework to understand information processing within the immune system and discuss the new biology of old receptors involving their structural and functional collaborations that evolved to deliver unique signal(s) to the cell using a limited set of conserved signaling modules.

Development and validation of a standardized ELISA for the detection of soluble Fc-epsilon-RI in human serum

The aim of this study was to develop a standardized enzyme-linked immunosorbent assay (ELISA) for detection of human soluble Fc-epsilon-RI (sFcεRI), a serum isoform of the high affinity IgE receptor. A recombinant version of sFcεRI was produced in baculovirus and used as standard. ELISA plates were coated with anti-mouse IgG followed by incubation with the monoclonal capture antibody CRA1. This FcεRI-alpha-specific antibody binds to the stalk region of the protein and does not inhibit IgE-binding. After incubation with standards or serum samples, plates were incubated with chimeric IgE followed by detection with horseradish peroxidase conjugated anti-human IgE. Enzymatic activity was visualized with (3,3',5,5')-tetramethylbenzidine. Specificity was demonstrated by omission of capture or detection reagents. Units (U) of detection were established and the dynamic range of the assay was defined as 10-640 U/ml for a 1/5 serum dilution. Parameters of linearity (R(2)>0.999), matrix interference test (recovery of 70-110%), intra-assay variability (coefficient of variation (CV) <20%) and inter-assay variability (CV <20%) met acceptance criteria for immunoassay validation. Correlation analysis of serum units of sFcεRI measured with the new ELISA and serum IgE levels confirmed earlier published data describing a weak correlation of the two parameters in patients with elevated serum IgE while no correlation in patients with normal serum IgE or the total patient group was found. In summary, we established and validated a standardized ELISA for the detection of sFcεRI. This novel method now allows for comparative analysis of sFcεRI levels in health and disease.

A soluble form of the high affinity IgE receptor, Fc-epsilon-RI, circulates in human serum

Soluble IgE receptors are potential in vivo modulators of IgE-mediated immune responses and are thus important for our basic understanding of allergic responses. We here characterize a novel soluble version of the IgE-binding alpha-chain of Fc-epsilon-RI (sFcεRI), the high affinity receptor for IgE. sFcεRI immunoprecipitates as a protein of ∼40 kDa and contains an intact IgE-binding site. In human serum, sFcεRI is found as a soluble free IgE receptor as well as a complex with IgE. Using a newly established ELISA, we show that serum sFcεRI levels correlate with serum IgE in patients with elevated IgE. We also show that serum of individuals with normal IgE levels can be found to contain high levels of sFcεRI. After IgE-antigen-mediated crosslinking of surface FcεRI, we detect sFcεRI in the exosome-depleted, soluble fraction of cell culture supernatants. We further show that sFcεRI can block binding of IgE to FcεRI expressed at the cell surface. In summary, we here describe the alpha-chain of FcεRI as a circulating soluble IgE receptor isoform in human serum.

A novel FcεRIβ-chain truncation regulates human mast cell proliferation and survival

Mast cells contribute to allergy through IgE-dependent activation via the high-affinity IgE receptor FcεRI. The role of the FcεRIβ chain (MS4A2) in mast cell function is not understood fully, although it serves to amplify FcεRI-dependent signaling. We demonstrate the expression of a novel MS4A2 truncation lacking exon 3 in human mast cells termed MS4A2(trunc). MS4A2(trunc) gene expression was regulated negatively by the mast cell growth factor stem cell factor (SCF), and its expression was not detected in the SCF receptor gain-of-function human mast cell line HMC-1. Unlike MS4A2, MS4A2(trunc) did not traffic to the cytoplasmic membrane but instead was associated with the nuclear membrane. Overexpression of MS4A2(trunc) induced human lung mast cell death and profoundly inhibited HMC-1 cell proliferation by inducing G(2)-phase cell cycle arrest and apoptosis. Thus, we have identified a novel splice variant of MS4A2 that might be important in the regulation of human mast cell proliferation and survival. This finding demonstrates that the MS4A2 gene has multiple roles, extending beyond the regulation of acute allergic responses. By understanding the mechanisms regulating its function, it might be possible to induce its expression in mast cells in vivo, which could lead to better treatments for diseases such as mastocytosis and asthma.

The SCHOOL of nature: I. Transmembrane signaling

Receptor-mediated transmembrane signaling plays an important role in health and disease. Recent significant advances in our understanding of the molecular mechanisms linking ligand binding to receptor activation revealed previously unrecognized striking similarities in the basic structural principles of function of numerous cell surface receptors. In this work, I demonstrate that the Signaling Chain Homooligomerization (SCHOOL)-based mechanism represents a general biological mechanism of transmembrane signal transduction mediated by a variety of functionally unrelated single- and multichain activating receptors. within the SCHOOL platform, ligand binding-induced receptor clustering is translated across the membrane into protein oligomerization in cytoplasmic milieu. This platform resolves a long-standing puzzle in transmembrane signal transduction and reveals the major driving forces coupling recognition and activation functions at the level of protein-protein interactions-biochemical processes that can be influenced and controlled. The basic principles of transmembrane signaling learned from the SCHOOL model can be used in different fields of immunology, virology, molecular and cell biology and others to describe, explain and predict various phenomena and processes mediated by a variety of functionally diverse and unrelated receptors. Beyond providing novel perspectives for fundamental research, the platform opens new avenues for drug discovery and development.