PROJECT SUMMARY/ ABSTRACT VRC01-class broadly neutralizing antibodies (bNAbs) bind the CD4-binding site of the HIV-1 Env and are among the most potent and broadly neutralizing antibodies known. They protect animals from experimental infection and when administered passively to chronically infected patients they reduce plasma viremia. VRC01- like antibodies are therefore a type of antibodies one would want to elicit by vaccination. We, and others, reported that the inferred germline forms of such antibodies do not recognize recombinant Env and do not neutralize HIV-1. This information led us and others to hypothesize that recombinant Envs fail to stimulate naïve B cells expressing germline VRC01-class B cell receptors (BCRs); which in part explains why previously evaluated recombinant Envs did not elicit VRC01-like antibody responses. We designed a clade C-derived Env protein (426c Core) that activates B cells engineered to express germline VRC01 BCRs, in vitro and in vivo. This ‘germline-targeting’ immunogen can be used as a prime to initiate the expansion of naïve B cells expressing germline VRC01-like BCRs, but is insufficient in and of itself to mature these B cells towards their neutralizing forms. Here, we will employ complementary methodologies, an interactive experimental approach, appropriate animal models and novel reagents to identify the optimal ‘booster’ immunogens that will guide the maturation of germline VRC01 BCRs towards their neutralizing antibody forms. We hope that our studies will advance the HIV vaccine field in a meaningful way and will contribute to the development of an effective HIV vaccine.