SUMMARY HIV affects more women than any other life-threatening infectious agent. It is most often sexually transmitted, where virus must evade the genital mucosal barrier to cause systemic infection. Clinical studies suggest HIV more easily penetrates this defense among women using the injectable progestin depot-medroxyprogesterone acetate (DMPA). Offering biological plausibility for this possibility, our research group showed that DMPA promoted mouse susceptibility to HSV-2 infection by reducing expression of the cell-cell adhesion molecule desmoglein-1 (DSG1) and increasing genital mucosal permeability. DMPA similarly boosted susceptibility of humanized mice to genital HIV infection. Identifying a potential mechanism for these results, we found DMPA treatment of mice lowered vaginal levels of ephrin A3 (EFNA3); an estrogen (E) receptor target gene shown to promote DSG1 expression in epithelial tissue. Providing clinical relevance for our findings, we showed women initiating DMPA use display changes in ectocervical DSG1 expression and mucosal permeability identical to those seen in mice. Likewise, pharmacologically relevant DMPA doses comparably enhanced genital mucosal permeability in rhesus macaques (RMs). These data newly reveal DMPA impairs mucosal barrier protection. However, using a humanized mouse model of HIV infection, we also uncovered that combined treatment with DMPA and intravaginal (ivag) E blocks virus acquisition by enhancing genital mucosal integrity. These results thus identified an unexpected advantage of hormonal contraceptive strategies that combine use of exogenous progestin and E (i.e., they avert loss of barrier protection caused by progestin use alone). As necessary steps in establishing this approach, this proposal will elucidate mechanisms of E-mediated enhancement of genital mucosal barrier function and define capacity of an E-releasing ivag ring (E-IVR) to protect RMs from genital SIV transmission. Expressly, we will use mice to define E-mediated regulation of EFNA3 pathways that induce DSG1 expression and boost genital mucosal barrier function (Aim 1). We will formulate a RM-sized E-IVR to deliver pharmacologically relevant drug doses, and use these rings to compare EFNA3 and DSG1 expression and mucosal barrier function in RM treated with DMPA, DMPA and placebo IVR, or DMPA and E-IVR (Aim 2). Finally, we will compare genital SIV transmission in RMs administered DMPA, DMPA and placebo IVR, or DMPA and E-IVR with repetitive genital challenges with escalating inoculums of the virus (Aim 3). This work will identify mechanisms by which E induces EFNA3 signaling pathways promoting genital epithelial integrity, and demonstrate that E-IVRs block SIV transmission by abrogating DMPA-mediated weakening of genital mucosal barrier function. These studies will thus deliver important new information in a highly relevant clinical model, and justify exploration of similar contraceptive strategies in populations at high risk for HIV acquisition.