Project Summary Despite AIDS being a preventable disease, UNAIDS reports millions of newly HIV-1 infected people every year. An efficacious vaccine for HIV-1 is therefore highly sought after. Despite many efforts, attempts to create immunization regimens that reproducibly elicit broadly neutralizing antibodies (bNAbs) against HIV-1 have failed thus far. The field has focused on developing a sequential immunization strategies that through priming with immunogens that activate germline bNAb precursor B cells, followed by boosts with increasingly more native Env based-immunogens, shepherds these germline precursors through somatic hypermutation in the germinal center to the develop into mature bNAbs-expessing cells. Whereas priming immunogens now exist that can reliably activate the correct bNAb precursor B cells, the immune response over the course of sequential immunization regimens becomes increasingly less focused on target epitopes and non-neutralizing off-target responses start to dominate the antibody response over time. To aid the design of better boosting immunogens by the Bjorkman and Hahn/Shaw labs, the Nussenzweig lab will conduct experiments in wt and transgenic mice to identify better boosting immunogens and dissect mechanisms that lead to off-target epitope responses in sequential immunization. Recent experiments from the Nussenzweig lab indicate that antibody feedback and the quality and quantity of the T follicular helper cell response affect recruitment and affinity maturation of germinal center (GC) B cell. Passively infused monoclonal antibodies in immunologically intact mice and humans indicated that antibody feedback resulted in the emergence of more off-target responses by masking target epitopes. Moreover, memory B cells emerging from such GCs are selected by a mechanism favoring antibody repertoire diversity over affinity8,11. HIV-1 immunogens designed by the Bjorkman lab will be used by the Nussenzweig lab to explore how polyclonal antibody responses alter the outcome of sequential immunization with the aim to improve on-target responses during boosting. The concepts that will be delineated from these experiments will then be tested in RM models by the Hahn/Shaw laboratories.