Modifications to an Fcgamma-Fcvarepsilon fusion protein alter its effectiveness in the inhibition of FcvarepsilonRI-mediated functions.

Abstract

GE2, a human bifunctional Fcgamma-Fcvarepsilon fusion protein cross-links FcgammaRIIb and FcvarepsilonRI on human mast cells and basophils and results in inhibition of FcvarepsilonRI-mediated functions. Three modified Fcgamma-Fcvarepsilon (GE) proteins were compared with GE2 for their effect on inhibition of FcvarepsilonRI-mediated cellular responses. GE2 was modified to potentially improve its therapeutic efficacy by increasing binding to FcgammaRIIb (GE S mutant) and decreasing binding to FcgammaRIII (GE H mutant) or reversing the Fcgamma and Fcvarepsilon domains and removing nonhuman linker sequences (E2G). These proteins were tested for their ability to bind a basophil-like cell line, block FcvarepsilonRI-mediated degranulation in human basophils, and inhibit passive cutaneous anaphylaxis in human FcvarepsilonRIalpha-transgenic mice. All 4 GE proteins bound cells that express FcvarepsilonRI and FcgammaRIIb, although the original GE2 retained the strongest ability to bind to these cells. E2G was as effective as GE2 in its ability to inhibit anti-Fel d 1 IgE-mediated histamine release from human basophils and block passive cutaneous anaphylaxis reactions. The GE S and GE H mutants were less effective. Optimization of GE2 as an inhibitor of FcvarepsilonRI-mediated functions showed that effectiveness was maintained when potentially immunogenic linker sequences were removed and Ig domain positions were reversed, but specific residue changes within the IgG C(H)2 domain aimed at enhancing GE2's inhibitory function by increasing FcgammaRII binding or additionally decreasing FcgammaRIII binding were not beneficial. GE2 and E2G molecules are effective inhibitors of FcvarepsilonRI-mediated degranulation and are of interest as potential therapeutics for IgE-mediated allergic reactions.