A novel human immunoglobulin Fcγ-Fcε bifunctional fusion protein inhibits FcεRI-mediated degranulation

Human mast cells and basophils that express the high-affinity immunoglobulin E (IgE) receptor, Fcepsilon receptor 1 (FcepsilonRI), have key roles in allergic diseases. FcepsilonRI cross-linking stimulates the release of allergic mediators(1). Mast cells and basophils co-express FcgammaRIIb, a low affinity receptor containing an immunoreceptor tyrosine-based inhibitory motif and whose co-aggregation with FcepsilonRI can block FcepsilonRI-mediated reactivity(2-4). Here we designed, expressed and tested the human basophil and mast-cell inhibitory function of a novel chimeric fusion protein, whose structure is gammaHinge-CHgamma2-CHgamma3-15aa linker-CHepsilon2-CHepsilon3-CHepsilon4. This Fcgamma-Fcepsilon fusion protein was expressed as the predicted 140-kD dimer that reacted with anti-human epsilon- and gamma-chain specific antibodies. Fcgamma-Fcepsilon bound to both human FcepsilonRI and FcgammaRII. It also showed dose- and time-dependent inhibition of antigen-driven IgE-mediated histamine release from fresh human basophils sensitized with IgE directed against NIP (4-hydroxy-3-iodo-5-nitrophenylacetyl). This was associated with altered Syk signaling. The fusion protein also showed increased inhibition of human anti-NP (4-hydroxy-3-nitrophenylacetyl) and anti-dansyl IgE-mediated passive cutaneous anaphylaxis in transgenic mice expressing human FcepsilonRIalpha. Our results show that this chimeric protein is able to form complexes with both FcepsilonRI and FcgammaRII, and inhibit mast-cell and basophil function. This approach, using a Fcgamma-Fcepsilon fusion protein to co-aggregate FcepsilonRI with a receptor containing an immunoreceptor tyrosine-based inhibition motif, has therapeutic potential in IgE- and FcepsilonRI-mediated diseases.