An estrogen-dominant state is associated with protection against HIV transmission in both non-human primate animal models and in epidemiologic studies of HIV transmission in women. Although estrogen regulates expression of many genes and modulates immune responses, especially at mucosal surfaces, the underlying mechanism for this protection is unclear. Two separate observations from our lab and others suggest that estrogen may regulate HIV transmission and/or HIV infection beyond its effects on vaginal wall remodeling: (1) We recently identified a ubiquitous signaling cascade, the Wnt/b-catenin pathway, as a potent represser of HIV replication. This pathway was induced by estrogen treatment in human peripheral blood mononuclear cells (PBMCs) and estrogen inhibited HIV replication in PBMCs and in ex vivo cervical tissue. We propose that the ability of estrogen to activate this anti-HIV pathway (Wnt/b-catenin) contributes to its ability to mediate inhibition of HIV replication in target cells (AIM 1). (2) Mounting evidence indicates that estrogen has complex and profound effects on leukocyte recruitment and activity in several physiologic compartments, including the female genital tract. In particular, estrogen at peak physiologic levels decreases inflammatory T cell and macrophage recruitment and reduces production of Th1/proinflammatory cytokines by T cells, macrophages, and dendritic cells. We propose that estrogen also attenuates HIV transmission and infection by reducing leukocyte recruitment, immune activation, and inflammation in the female genital tract (AIM 2). The above two propositions form the basis of our central hypothesis - that estrogen diminishes HIV transmission and replication by modulating two mechanisms essential for establishing HIV infection in women: 1) Wnt/b-catenin signaling and 2) mucosal immune responses. The net outcome is diminished HV transmission and replication under an estrogen-dominant state. We will use a cervical explant model and a dual chamber model of leukocyte transmigration to evaluate the two proposed mechanisms by which estrogen reduces HV replication and transmission in the female genital tract. We will also determine if this reationship occurs in vivo by determining the impact of estrogen on Wnt/b-catenin and HIV replication among HIV-infected premenapausal and post-menapousal women (cross-sectional study) and during the menstrual cycle (longitudinal study)(Aim 3). Identifying these mechanisms is critical because it will improve our understanding of female sex hormone effects on HIV infection, which will, in turn, provide new approaches to anti-HIV therapy and prophylaxis in women. RELEVANCE (See instructions): Our studies will define the mechanism(s) by which estrogen is protective against HIV transmission and replication. This knowledge is essential towards efforts to devise novel targets for HIV intervention. Such strategies would exploit the pathway that estrogen engages to diminish HIV transmission and replication.