Suppression of mast cell degranulation through a dual-targeting tandem IgE–IgG Fc domain biologic engineered to bind with high affinity to FcγRIIb

Mast cells and basophils play a central role in allergy, asthma, and anaphylaxis, as well as in non-allergic inflammatory, neurological and autoimmune diseases. Allergen-mediated cross-linking of IgE bound to FcɛRI leads to cellular activation, and the low-affinity Fc receptor FcγRIIb is a key inhibitor of subsequent degranulation. FcγRIIb, when coengaged with FcɛRI via allergen bound to IgE, stimulates ITIM domain-mediated inhibitory signaling that efficiently suppresses mast cell and basophil activation. To assess the therapeutic potential of directed coengagement of FcɛRI and FcγRIIb in the absence of FcɛRI crosslinking, we developed a fusion protein comprising the coupled Fc domains of murine IgE and human IgG1. As a key functional component of this tandem Fcɛ–Fcγ biologic, we engineered its IgG1 Fc domain to bind to human FcγRIIb with 100-fold enhanced affinity relative to native IgG1 Fc. Using mast cells from mice transgenic for human FcγRIIb, we show that this tandem Fc binds with high affinity to murine FcɛRI and human FcγRIIb on mast cells, triggers phosphorylation of FcγRIIb, and inhibits FcɛRI-dependent calcium mobilization. Control tandem Fc biologics containing a native IgG1 Fc domain or lacking binding to Fcγ receptors were markedly less active, demonstrating that the affinity-optimized tandem Fc can inhibit degranulation through stimulation of FcγRIIb signaling as well as through competition with allergen-IgE immune complex for FcɛRI binding. We propose that in the context of a fully human tandem Fc biologic, high-affinity coengagement of FcɛRI and FcγRIIb has potential as a novel therapy for allergy and other mast cell and basophil-mediated pathologies.

► We generated an Fcɛ–Fcγ fusion protein (tandem Fc) to suppress mast cell activation.
► Enhanced binding to FcγRIIb via Fc engineering significantly improves potency.
► Novel model uses mast cells derived from mice transgenic for human FcγRIIb.
► Tandem Fc has potential therapeutic use in basophil and mast cell pathologies.