Quantitative assessment of masking of neutralization epitopes in HIV-1.

Despite the frequent observation of masking of HIV-1 neutralization epitopes, its extent has not been previously systematically assessed either for multiple epitopes presented by individual viruses or for individual epitopes across multiple viral strains. Using a recently developed method to identify amino acid sequence motifs required for recognition by HIV-1-neutralizing monoclonal antibodies (mAbs), we visualized the patterns of masking of specific epitopes targeted by mAbs in a diverse panel of HIV-1 isolates. We also calculated a specific masking intensity score for each virus based on the observed neutralization activity of mAbs against the epitopes in the virus. Finally, we combined these data with estimates of the conservation of each mAb-targeted epitope in circulating HIV-1 strains to estimate the effective neutralization potential (E(N)) for each mAb. Focusing on the V3 loop of gp120 as a prototype neutralization domain, we found that the V3 loop 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 over 87% of all viruses, it is 82.2% masked, so its E(N) is lower than that for mAb 2219. Notably, 50% of the viruses that mAb 3074 is able to neutralize are classified as subtype C viruses, while 70% or more of the viruses neutralized by mAbs 2219, 2557 or 447-52D are classified as subtype B. Thus, neutralization epitopes (in this case, in the V3 loop) 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. Here we have developed a quantitative score for this value. 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.