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.

Anaphylaxis represents the most extreme, life-threatening form of allergic disease and is often an emergency requiring immediate intervention. It occurs following cross-linking of the high-affinity IgE receptor (FcɛRI) on the surface of mast cells by allergen-IgE complexes; however, non-IgE-mediated and idiopathic mast cell activation also contribute. Causes include environmental allergens, foods, venoms, and medications. In addition, some people with mastocytosis have repeated anaphylactic episodes during normal daily activities with no exposure to these triggers. While acute therapy consists primarily of epinephrine and supportive care, chronic therapy relies heavily on desensitization to the inciting allergen—a time-consuming and sometimes unsuccessful process. Desensitization also requires identification of the trigger which is not always possible. Here, we report the use of an exon-skipping oligonucleotide, previously termed KitStop, to safely reduce the severity of anaphylaxis via mast cell depopulation in tissues. KitStop administration results in the integration of a premature stop codon within the mRNA transcript of the Kit receptor—a receptor tyrosine kinase found primarily on mast cells and whose gain-of-function mutation can result in systemic mastocytosis. After either local or systemic KitStop treatment, mice had significantly reduced mast cell numbers in the skin, peritoneum, and lungs. In addition, KitStop-treated mice had a significantly diminished anaphylactic response in a model of passive systemic anaphylaxis. These data demonstrate that KitStop has the potential to serve as a powerful treatment option for patients that experience anaphylaxis, including idiopathic anaphylaxis. Research was supported by the National Institutes of Health (NIH), National Institute of Allergy and Infectious Diseases (NIAID), Award Number R01AI143985 (G.C.), Department of Molecular Biomedical Sciences, College of Veterinary Medicine, start-up funds (G.C.), NCSU Center for Human Health and the Environment NIH P30ES025128 (G.C.), Institutional National Research Service Award from the Office of the Director of National Institutes of Health, NIH T32OD011130 (B.H., D.S.).

Allergic diseases are triggered by signaling through the high affinity IgE receptor, FcɛRI. In mast cells (MCs) and basophils, FcɛRI is a tetrameric receptor complex comprising a ligand binding α subunit (FcɛRIα); a tetraspan β subunit (FcɛRIβ) responsible for trafficking the complex and amplifying signaling; and a signal transducing dimer of single transmembrane γ subunits (FcɛRIγ). However, FcɛRI also exist as presumed trimeric complexes that lack FcɛRIβ and are expressed on several cell types outside of the MC and basophil compartments. There are known differences in the trimeric FcɛRI complex between humans and mice and questions remain as to how this trimeric FcɛRI complex traffics and whether it signals in the absence of FcɛRIβ. We have previously reported that targeting FcɛRIβ with exon skipping oligonucleotides diminishes IgE-mediated degranulation in mouse MCs, but equivalent targeting in human MCs was not effective at reducing degranulation. Here, we report that an FcɛRIβ-like protein exists in humans and we identify that the Membrane Spanning 4A (MS4A) gene cluster member, MS4A6A compensates for FcɛRIβ in FcɛRI trafficking and signaling. The function of MS4A6A is currently unknown, but MS4A6A and FcɛRIβ are encoded by highly related genes within the same family (MS4A gene family) that cluster within the human gene loci 11q12–q13, a region linked to allergy and asthma susceptibility. Human MS4A6A promotes surface expression of FcɛRI complexes and promotes degranulation. Indeed, our data suggest the presence of either FcɛRIβ or MS4A6A is sufficient for degranulation. This study suggests that MS4A6A could be an elusive FcɛRIβ-like protein in human MCs that performs compensatory functions to FcɛRIβ. Research was supported by the National Institutes of Health (NIH), National Institute of Allergy and Infectious Diseases (NIAID), Award Number R01AI143985, Department of Molecular Biomedical Sciences, College of Veterinary Medicine, start-up funds, Institutional National Research Service Award from the Office of the Director of National Institutes of Health, NIH T32OD011130.