HIV Gp120 Epitope Profile on Cell-Bound Virions Assessed using Various Microscopy Techniques

Abstract

Background: HIV gp120 experiences multiple molecular interactions and structural rearrangements during host cell attachment and entry. However, the antigenic correlates of these dynamic changes are essentially unknown for single HIV particles bound to target cells. Gp120 on cell-bound virions is a target for multiple humoral effector mechanisms mediated by anti-gp120 antibodies, including direct neutralizing activity and elimination by Fc-mediated effector functions such as antibody-dependent cell cytotoxicity (ADCC). It is therefore critical to understand the antigenicity of these conserved epitopes that become exposed during productive viral replication, to provide important guidance for designing antiviral strategies such as vaccines to raise protective antibody responses. Methods: We studied the antigenicity of conserved epitopes by visualizing their exposure on single HIV particles as they interact with target TZM-bl cells using confocal microscopy. Epitope exposure was probed by visualizing the binding of cognate antibodies. We then employed direct stochastic optical reconstruction microscopy (dSTORM) to determine the number of antibodies bound to cell-attached HIV, and their location relative to the virus - cell contact site. Results: CD4-induced (CD4i) epitopes thought to be sterically occluded from the virus - cell interface, were exposed on cell-bound HIV gp120. The level of A32, 17b and C11 CD4i antibody staining was similar to neutralizing antibodies b12 and 2G12. The location of these CD4i epitopes was distal to the virus - cell contact site. Conclusion: The patterns of cell-bound gp120 epitope exposure are consistent with the ADCC activities of cognate antibodies. CD4i epitopes were unexpectedly exposed distal to the HIV - cell interface, where they can be accessed by antibodies involved in ADCC. These findings indicate that HIV-1 exhibits a diversity of epitope exposure upon attachment that may provide unique insights for understanding how humoral immunity impacts HIV infection.