Enhancing HIV-1 neutralization by increasing the local concentration of MPER directed bnAbs

Broadly neutralizing antibodies (bnAbs) against the membrane-proximal external region (MPER) of the gp41 component of the HIV-1 envelope (Env) are characterized by long hydrophobic heavy-chain complementarity-determining regions (HCDR3s) that interact with MPER and some viral membrane lipids, to achieve increased local concentrations. Here, we show that increasing the local concentration of MPER-directed bnAbs at the cell surface via binding to the high affinity Fc receptor (Fc{gamma}RI) potentiates their ability to prevent viral entry in a manner analogous to the previously reported observation whereby the lipid binding activity of MPER bnAbs increases their concentration at the viral surface membrane. However, binding of MPER-directed bnAb 10E8 to Fc{gamma}RI abolishes the neutralization synergy that is seen with the N-heptad repeat (NHR)-targeting antibody D5_AR and NHR-targeting small molecule enfuvirtide (T20), possibly due to decreased accessibility of the NHR in the Fc{gamma}RI-10E8-MPER complex. Taken together, our results suggest that lipid-binding activity and Fc{gamma}RI-mediated potentiation function in concert to improve the potency of MPER-directed bnAbs by increasing their local concentration near the site of viral fusion. Therefore, lipid-binding may not be a strict requirement for potent neutralization by MPER-targeting bnAbs, as alternative methods can achieve similar increase in local concentration while avoiding potential liabilities associated with immunologic host tolerance. Author summaryThe trimeric glycoprotein Env is the only viral protein expressed on the surface of HIV-1, is the target of broadly neutralizing antibodies, and is the focus of most vaccine development efforts. Broadly neutralizing antibodies targeting the membrane proximal external region (MPER) of Env show lipid-binding characteristics and modulating this interaction affects neutralization. In this study, we tested the neutralization potencies of variants of the MPER-targeting antibody 10E8 with different viral membrane-binding and host Fc{gamma}RI-binding capabilities. Our results suggest that binding to both lipid and Fc{gamma}RI improves the neutralization potency of MPER-directed antibodies by concentrating the antibody at sites of viral fusion. As such, lipid-binding may not be uniquely required for MPER-targeting broadly neutralizing antibodies, as alternative methods to increase local concentration can achieve similar improvements in potency.