Journal of virology 2017 12 13() pii 10.1128/JVI.01779-17
The subtype C HIV-1 isolate MW965.26 is a highly neutralization-sensitive tier-1a primary isolate that is widely used in vaccine studies, but the basis for the sensitive neutralization phenotype of this isolate is not known. Substituting the MW965.26 V1/V2 domain into a neutralization-sensitive SF162 Env clone resulted in high resistance to standard anti-V3 monoclonal antibodies, demonstrating that this region possessed strong masking activity in a standard Env backbone and indicating that determinants elsewhere in MW965.26 Env were responsible for its unusual neutralization sensitivity. Key determinants for this phenotype were mapped by generating chimeric Envs between MW965.26 Env and a typical resistant Env clone, ConC, and localized to two residues, Cys384 in the C3 domain and Asn502 in the C5 domain. Substituting the sensitizing mutations Y384C and K502N at these positions into several resistant primary Envs resulted in conversion to neutralization-sensitive phenotypes, demonstrating the generalizability of this effect. In contrast to the sensitizing effects of these substitutions on normally masked epitopes, these mutations reduced the sensitivity of VRC01-like epitopes overlapping the CD4-binding domain, while having no effect on several other classes of broadly neutralizing epitopes, including members of several lineages of V2-dependent quaternary epitopes and representatives of N332 glycan-dependent epitopes (PGT121) and quaternary, cleavage-dependent epitopes centered at the gp41-gp120 interface on intact HIV-1 Env trimers (PGT151). These results identified novel substitutions in gp120 that regulate the expression of alternative conformations of Env and differentially affect the exposure of different classes of epitopes, thereby influencing the neutralization phenotype of primary HIV-1 isolates.ImportanceA better understanding of the mechanisms that determine the wide range of neutralization sensitivity of circulating primary HIV-1 viruses would provide important information about the natural structural and conformational diversity of HIV-1 Env, and how this affects neutralization phenotype. A useful way of studying this is to determine the molecular basis for the unusually high neutralization sensitivities of the limited number of available tier 1a viruses. This study localized the neutralization sensitivity of MW965.26, an extremely sensitive subtype C-derived primary isolate, to two rare substitutions in the C3 and C5 domains, and demonstrated that the sequences at these positions differentially affected the presentation of epitopes recognized by different classes of standard and conformational-dependent broadly neutralizing antibodies. These results provide novel insight into how these regions regulate neutralization phenotype, and provide tools for controlling Env conformation that could have applications both for structural studies and in vaccine design.