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

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 Fc epsilon RI leads to cellular activation, and the low-affinity Fc receptor Fc gamma RIIb is a key inhibitor of subsequent degranulation. Fc gamma RIIb, when coengaged with Fc epsilon RI 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 Fc epsilon RI and Fc gamma RIIb in the absence of Fc epsilon RI 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 Fc epsilon-Fc gamma biologic, we engineered its IgG1 Fc domain to bind to human Fc gamma RIIb with 100-fold enhanced affinity relative to native IgG1 Fc. Using mast cells from mice transgenic for human Fc gamma RIIb, we show that this tandem Fc binds with high affinity to murine Fc epsilon RI and human Fc gamma RIIb on mast cells, triggers phosphorylation of Fc gamma RIIb, and inhibits Fc epsilon RI-dependent calcium mobilization. Control tandem Fc biologics containing a native IgG1 Fc domain or lacking binding to Fc gamma receptors were markedly less active, demonstrating that the affinity-optimized tandem Fc can inhibit degranulation through stimulation of Fc gamma RIIb signaling as well as through competition with allergen-IgE immune complex for Fc epsilon RI binding. We propose that in the context of a fully human tandem Fc biologic, high-affinity coengagement of Fc epsilon RI and Fc gamma RIIb has potential as a novel therapy for allergy and other mast cell and basophil-mediated pathologies. (C) 2012 Elsevier B.V. All rights reserved.