PROJECT SUMMARY With no universal cure, development of an effective vaccine to prevent HIV-1 infection remains a primary goal. A major hurdle for the development of successful vaccination strategies has been the lack of affordable, accessible, tractable, and relevant preclinical model systems. Investigations on HIV infection, pathogenesis, and prevention using small animal models have been limiting due to the species-specific tropism of HIV. The advent of immunodeficient mice harboring a human immune system (HIS) in many ways has broadened accessibility and interest in HIV-related translational studies including viral replication, T cell depletion, methods of transmission, and evaluation of antiretroviral therapies. Nevertheless, efforts to develop vaccination strategies against HIV in current HIS mice have been thwarted due to poor adaptive immune responses and antigen- specific antibody development following immunization. Thus, there is a great need for advances in HIS mice that promote vaccine-mediated antibody development to specific epitopes that would easily translate to humans. We hypothesize that enhanced human T cell function and priming via human antigen presenting cells in HIS mice will facilitate efficient T/B cell interactions and enable evaluation of vaccination strategies against HIV-1. To this end, we have developed a novel HIS mouse strain expressing human leukocyte antigens (HLA)-DQ and HLA-A in the absence of murine major histocompatibility complex (MHC) I/II (to ensure human T cell selection on a more appropriate molecules in the mouse thymus) in combination with human CSF1 knocked into the murine Csf1 locus. Based on published data from our group and others, this novel strain, we anticipate, will support improved T cell development/function as well as the development of human myeloid cells with increased capacity to prime human T cells following immunization and facilitate HIV-1-specific antibody production. We will leverage this new translational HIS platform to: i) Determine the scope of the human adaptive immune response to infection with HIV-1 R5 virus, ii) Determine the neutralization ability of, define the Env epitopes targeted by, and delineate the sequence features for the human adaptive immune response to HIV-1 immunization, and iii) assess a novel vaccination strategy using optimized multivalent immunogens for broad neutralizing antibodies (bNAb) elicitation. Together, these results will provide critical insights into the utility of this advanced HIS model system to assess HIV-1 vaccination strategies capable of generating bNAbs to accelerate prioritization to validate in human clinical trials.