Background: Eliciting relevant neutralizing antibodies (nAbs) is a major challenge of HIV vaccine research. NAbs to autologous virus arise in most HIV-infected individuals, and sera from some individuals neutralize heterologous primary isolates. However, the epitopes recognized by nAbs in vivo are not well understood, particularly for heterologous neutralization. While a few broadly neutralizing human monoclonal antibodies (HuMAbs) have been isolated, we hypothesize that heterologous neutralization by patient sera most often reflects the combined activity of antibodies to several neutralization epitopes.

Methods: As part of a natural history study of HIV infection in Trinidad and Tobago, we evaluated development of nAbs in 22 individuals ascertained during acute HIV infection and followed in the absence of antiviral therapy. Viruses were isolated by co-culture of patient PBMC with PHA-activated normal donor PBMCs. Neutralization of primary isolates by patient sera was assessed in a cMAGI-based single-cycle infection/neutralization assay. Evolution of env diversity was estimated by heteroduplex tracking assay (HTA) of sequences spanning the V1-C2 and C2-V5 regions.

Results: Autologous nAbs were observed in 18/22 individuals, with subsequent development of antibodies neutralizing heterologous cohort viruses in 10 individuals. In each case, heterologous nAbs followed the development of HTA-detectable env diversity. The breadth of heterologous neutralization increased over time, and ~40-month sera from 3 individuals each neutralized early virus from 8–13 members of the cohort. Interestingly, these sera-neutralized, overlapping subsets of the cohort viruses, such that together they neutralized 20 of the 22 early virus isolates. In addition, these sera neutralized early primary virus isolates from the U.S. and Italy.

Conclusions: The timing of env diversity and heterologous nAb supports the hypothesis that broad neutralization reflects polyspecific antisera. Neutralization of primary viruses from the U.S. and Europe suggests that the epitopes recognized by antibodies in these sera are relevant to neutralization of globally disseminated HIV-1. We are currently mapping the epitopes targeted by these broadly neutralizing sera on autologous and heterologous virus using envs cloned from serial virus isolates and HuMAbs derived from cryopreserved PBMC from the Trinidad Cohort. The results of these studies may significantly impact design of candidate HIV vaccines.