G-103 Masking of the epitopes in the V2 and V3 loops of the HIV-1 envelope are modulated by different mechanisms

Variable loops V1V2 and V3 present on the apex of the trimeric HIV-1 envelope (Env) spike play opposing roles in antibody recognition, as these loops not only contain conserved neutralizing epitopes but also participate in epitope masking. This study investigated the different factors that play role in modulating the exposure of V2i epitopes as compared to the better characterized V3 epitopes. V2i (V2-intergrin) are conformation-dependent epitopes that encompass the integrin α4β7-binding motif and other conserved elements in the V1V2 loop of HIV-1 Env gp120. First, we asked whether CD4-induced Env conformational changes affect V2i epitopes similar to V3. CD4 treatment of BaL and JRFL pseudoviruses increased their neutralization sensitivity to V3 but not to the V2i mAbs. Second, the contribution of N-glycans on masking V2i versus V3 epitopes was evaluated by testing neutralization of viruses produced in the presence of a glycosidase inhibitor, kifunensine. Pseudoviruses grown in kifunensine were more sensitive to neutralization by V3 but not V2i mAbs. Finally, we evaluated the time-dependent dynamics of the V2i and V3 epitopes. Extending the time of virus-mAb interaction, before adding target cells, increased virus neutralization by some V2i mAbs and all V3 mAbs tested. Importantly, with the extended incubation time, significant neutralization was detected against resistant Tier 2 HIV-1 isolates. The levels of neutralization achieved with the longer incubation time, but not the standard 1hr incubation, correlated with relative affinity of the V2i mAbs as measured by half maximal binding to gp120. These data demonstrate that distinct mechanisms contribute to the masking of V3 and V2i epitopes. Although the biologic significance of neutralization detectable with prolonged incubation time remains unknown, this study reveals the importance of the structural dynamics of V2i and V3 epitopes in determining HIV-1 neutralization by antibodies targeting these sites.