Inhibitory FcγRIIb-Mediated Soluble Antigen Clearance from Plasma by a pH-Dependent Antigen-Binding Antibody and Its Enhancement by Fc Engineering

Fc engineering can modulate the Fc-Fc gamma R interaction and thus enhance the potency of Abs that target membrane-bound Ags, but it has not been applied to Abs that target soluble Ags. In this study, we revealed a previously unknown function of inhibitory Fc gamma RII in vivo and, using an Ab that binds to Ag pH dependently, demonstrated that the function can be exploited to target soluble Ag. Because pH-dependent Ab dissociates Ag in acidic endosome, its Ag clearance from circulation reflects the cellular uptake rate of Ag/Ab complexes. In vivo studies showed that Fc gamma R but not neonatal FcR contributes to Ag clearance by the pHdependent Ab, and when Fc binding to mouse Fc gamma RII and III was increased, Ag clearance was markedly accelerated in wild-type mice and FcR g-chain knockout mice, but the effect was diminished in Fc gamma RII knockout mice. This demonstrates that mouse Fc gamma RII efficiently promotes Ab uptake into the cell and its subsequent recycling back to the cell surface. Furthermore, when a human IgG1 Fc variant with selectively increased binding to human Fc gamma RIIb was tested in human Fc gamma RIIb transgenic mice, Ag clearance was accelerated without compromising the Ab half-life. Taken together, inhibitory Fc gamma RIIb was found to play a prominent role in the cellular uptake of monomeric Ag/Ab immune complexes in vivo, and when the Fc of a pH-dependent Ab was engineered to selectively enhance human Fc gamma RIIb binding, the Ab could accelerate soluble Ag clearance from circulation. We assume such a function would enhance the therapeutic potency of Abs that target soluble Ags.