Project Summary An HIV vaccine remains a critical and as yet unrealized asset in the 40-year fight against the HIV/AIDS pandemic. New insights towards achieving vaccine protection from HIV acquisition may be gained by a comparative case study of COVID-19 vaccines, which achieved nearly 100% efficacy against a similarly enveloped, RNA virus with a similarly architected, trimeric, Class I fusion viral spike mechanism. Neutralization B-cell epitopes exposed at the apex of the SARS-CoV-2 trimeric viral spike correlated clearly and strongly with protection from viral acquisition, both in humans in the real world/circulating virus setting and in non-human primate (NHP) preclinical models. We hypothesized that vaccine immunogens could be improved by focusing antibody responses to two equivalent B-cell epitopes at the HIV/SIV viral spike apex, one an epitope targeted by neutralizing antibodies (V2b) and one a purely non-neutralization epitope (V2c). In preliminary results, we showed that removal of the viral spike apical V1 loop segment (DV1-Env) masking the V2c epitope enhanced protection against viral challenge in both a highly stringent SIV and matched SHIV challenge model, achieving >90% vaccine efficacy. In further preliminary results, we designed an immunogen displaying only this V2c epitope in isolation and proved that it was highly immunogenic as an isolated epitope and indeed elicited purely non-neutralizing antibodies in vivo. The study revealed that V2c contributed to, but was not sufficient on its own, for protection. In this exploratory study, we pursue the new hypothesis that the combination of the two HIV, viral-spike apical, B-cell epitopes in a single vaccine can reconstitute an increased level of protection as observed with COVID-19 vaccines, by synergizing V2c with the V2b neutralization epitope. We will 1) design and validate a V2b-focused immunogen, and 2) test the precise combination of neutralizing, vaccine-elicited anti-V2b antibodies with non-neutralizing, cytotoxic, vaccine-elicited V2c antibodies, along with coordinated cellular immune responses in vivo for their ability to delay viral acquisition as compared to the V2c epitope alone. Successful results in these two aims could justify a research project on the design and translational development of a novel, viral-spike-apex-focused, efficacious HIV vaccine.