Annual and Interim Progress Report Summaries
Principal Investigator: Susan Zolla-Pazner
Project: The V3 Loop: A Conserved Structure of gp 120 that can Induce Broadly Neutralizing Antibodies Against HIV-1
March 5, 2011 (Interim Report)
Overall activities: During the past five years of our project, we have demonstrated that an immune response focused on V3 results in the induction of antibodies (Abs) that neutralize viruses from several HIV-1 subtypes. Our immune focusing strategy is based on DNA gp120 priming and epitope-protein scaffold boosting. The underlying hypothesis of the work is that the use of gp120 DNA as the prime and epitope-scaffold immunogens as the boost, the immune response will be focused on conformational epitopes of interest resulting in induction of cross-clade neutralizing Abs against HIV. This constitutes a first step in developing recombinant immunogens that focus the immune response on targeted epitopes in the envelope of HIV, resulting in an effective protective vaccine.
Recent findings: Our project has used the V3 loop of HIV envelope gp120 as a model epitope, since V3 is one of the known targets for neutralizing Ab responses. In conjunction with the VIMC, we have identified the six anti-V3 human monoclonal Abs (mAbs) that display the broadest cross-clade neutralizing activities. Based on the 3D shapes of the epitopes, the neutralizing activity of these mAbs vs. a large panel of pseudoviruses, and their immunochemical reactivity with various V3 peptides and gp120 proteins, we developed a method to derive sensitive and specific “signature motifs” within the V3 region. The motifs were then used to estimate the conservation of these epitopes across circulating worldwide HIV-1 strains. The epitopes targeted by the anti-V3 loop neutralizing mAbs are present in 5% to 87% of HIV-1 circulating strains, with the epitope of mAb 3074 being the most completely conserved among multiple HIV-1 subtypes. These data demonstrate the presence of an invariant structure hidden in the dynamic and sequence-variable V3 loop in gp120. These findings are consistent with published crystallographic data showing several conserved elements in the V3 crown. Several papers covering these findings have been published recently.[1-3]. We further utilized the signature epitope motif method to assess the pattern and extent of masking of specific V3 epitopes in a diverse panel of HIV-1 isolates. Thus, we calculated a specific masking intensity score for each virus and combined these data with estimates of the conservation of each epitope in circulating HIV-1 strains to estimate the effective neutralization potential (E(N)) for each mAb. The data show that the V3 epitope targeted by mAb 2219 is one of the least masked mAbs and it has the highest E(N). Interestingly, although the V3 loop epitope targeted by mAb 3074 is present in 87% of all viruses, it is 82.2% masked, so its E(N) is lower than that for mAb 2219. Thus, neutralization epitopes have differential patterns of masking and also display distinct patterns of distribution among circulating HIV-1 viruses. Both factors combine to contribute to the practical vaccine value of any single epitope/mAb. These results have important implications for rational design of vaccines designed to induce neutralizing Abs by revealing epitopes that are minimally masked and maximally reactive with neutralizing Abs. The paper describing this study is published .
Submitted November 2, 2010