Gp41-targeted antibodies restore infectivity of a fusion-deficient HIV-1 envelope glycoprotein.

The HIV-1 envelope glycoprotein (Env) mediates viral entry via conformational changes associated with binding the cell surface receptor (CD4) and coreceptor (CCR5/CXCR4), resulting in subsequent fusion of the viral and cellular membranes. While the gp120 Env surface subunit has been extensively studied for its role in viral entry and evasion of the host immune response, the gp41 transmembrane glycoprotein and its role in natural infection are less well characterized. Here, we identified a primary HIV-1 Env variant that consistently supports >300% increased viral infectivity in the presence of autologous or heterologous HIV-positive plasma. However, in the absence of HIV-positive plasma, viruses with this Env exhibited reduced infectivity that was not due to decreased CD4 binding. Using Env chimeras and sequence analysis, we mapped this phenotype to a change Q563R, in the gp41 heptad repeat 1 (HR1) region. We demonstrate that Q563R reduces viral infection by disrupting formation of the gp41 six-helix bundle required for virus-cell membrane fusion. Intriguingly, antibodies that bind cluster I epitopes on gp41 overcome this inhibitory effect, restoring infectivity to wild-type levels. We further demonstrate that the Q563R change increases HIV-1 sensitivity to broadly neutralizing antibodies (bNAbs) targeting the gp41 membrane-proximal external region (MPER). In summary, we identify an HIV-1 Env variant with impaired infectivity whose Env functionality is restored through the binding of host antibodies. These data contribute to our understanding of gp41 residues involved in membrane fusion and identify a mechanism by which host factors can alleviate a viral defect. Author summary HIV-1 Env consists of the surface subunit (gp120) and the transmembrane subunit (gp41). Receptor and coreceptor binding of the Env trimer triggers structural rearrangements within gp41 leading to the formation of a six-helix bundle. These gp41 conformational changes are critical to membrane fusion and viral entry. We have identified a change in gp41, Q563R, that disrupts this six-helix bundle formation, negatively affecting viral entry and infection. Surprisingly, the humoral immune response to HIV-1 counterintuitively overcomes the conformational disruption, rescuing a viral defect to allow for increased viral infection. We also demonstrate that the Q563R change confers increased sensitivity to broadly neutralizing antibodies targeting the gp41 MPER. Thus, the Q563R change impedes six-helix bundle formation and virus entry, but these Env functions can be restored by naturally occurring antibodies. Our data further suggests that R563 might enhance the binding of MPER-specific bNAbs, and therefore could be useful in an Env immunogen.