Improved FcγRIIb Targeting Functionally Translates into Enhanced Inhibition of Basophil Activation

Background: Mast cells and basophils express the high-affinity IgE receptor, FceRI, as well as the low-affinity IgG receptor, FcγRIIb. While FceRI is responsible for IgE-dependent degranulation upon coaggregation with allergens, FcγRIIb has been shown to downregulate degranulation through cross-linking with FceRI. A previously developed fusion protein consisting of an anti-IgE DARPin linked to the human IgG1-Fc part (DE53-Fc) has been shown to simultaneously target FceRI and FcγRIIb with low affinity and to thereby prevent basophil activation. The affinity of a ligand for its receptor is known to be critical for the functional consequences of the binding. So we generated two mutated DE53-Fc molecules with either an improved (DE53-Fc mut+) or a reduced (DE53-Fc mut-) binding to FcγRIIb and assessed their potential to inhibit IgE-dependent basophil activation. Methods: DE53-Fc was modified by introducing single site-directed point mutations in the Fc part. The mutated constructs were used to assess kinetic parameters as well as the inhibitory capacity on basophil activation and the production of leukotriene C4 (LTC4) and IL-13. Results: DE53-Fc mut+ showed increased affinity for FcγRIIb as well as an enhanced potential to inhibit IgG1 binding to FcγRIIb, resulting in improved efficacy in functional assays. Furthermore, DE53-Fc mut+ decreased de novo-synthesized LTC4 as well as the cytokine IL-13, suggesting that it might be an inhibitor of the allergic late-phase reaction. Conclusion: Our data suggest that improved binding to FcγRIIb at constant low-affinity binding to IgE leads to more efficient coaggregation of FceRI-FcγRIIb and results in the enhanced inhibition of basophil activation.