Project Summary Vaccine-triggered CD4 T cell help is essential for establishing long-lived humoral and cellular immunity and is central to HIV vaccine efficacy. But, CD4 is also the major receptor for HIV which raises concerns as to whether vaccine induced CD4 T cells could subvert vaccine efficacy; a premise supported by the STEP trial and several HIV vaccine studies in non-human primates, including our own. The cellular mechanisms underlying the paradox of vaccine-induced susceptibility to HIV remain poorly articulated, and yet identifying the mechanism involved is the first step to effective vaccine design. This proposal is focused on understanding CD4 T cells in the female genital tract (FGT), the major portal of HIV entry by heterosexual transmission. In Aim 1 of this research project, we will investigate functional characteristics of highly vulnerable HIV target CD4 T cells within the genital tract of HIV uninfected humans and rhesus macaques. In Aim 2 of this proposal, we will investigate how a DNA-prime/protein boost vaccination regimen, a widely used HIV vaccine platform, impacts the phenotype of FGT associated HIV target CD4 T cells. We will determine whether vaccine-specific CD4 T cells in the FGT express key markers of HIV vulnerability and whether this is driven by vaccine induced changes in the cytokine milieu. We will also determine how preventing migration of vaccine-induced cells to the FGT impacts the vaginal inflammatory environment. Finally, we will elucidate dynamics of vaccine-induced CD4 T cells in the FGT after simian (S)IV exposure and explore cells disseminated to the rectal mucosa and lymphoid tissue. The chances of ameliorating the virus is greatest in the earliest stages of infection, at the mucosal point of entry; thus, our findings could lay the foundation for determining whether inhibiting the migration of HIV target cells to the FGT during the post-vaccination effector phase is a strategy to decrease vaccine-induced susceptibility to HIV.