Very little is known about the virologic and host factors responsible for HIV- I transmission, because of the lack of a suitable viral transmission model that reflects the in vivo situation. We have recently developed a cervical tissue derived organ culture model that has been used to show transmission of infectious HIV- I from infected cells across the mucosal layer. Such viral transmission required infectious virus. In situ hybridization has detected HIV-1-infected cells to be CD45RO+ T cells at the submucosal layer. In this proposal we hypothesize that sexual transmission of HIV- I depends on the phenotypic and genotypic properties of HIV- I and host factors. To test this hypothesis we will use the organ culture model to: a) examine whether viral transmission is dependent on the phenotypic properties of HIV- 1; b) evaluate the role of envelope gene in transmission of HIV-1; c) determine the role of host factors, such as semen, progesterone and cytokines, on the transmission of HIV. In addition, the organ culture system will also be used to identify the cell types that become first infected during sexual transmission of HIV. Transmission of HIV- I across the tissue will be monitored by the infectivity assay in cMAGI cells, and simultaneous immunophenotyping and in situ hybridization for HIV- I RNA Information from this project will be important for our understanding of pathogenesis of HIV- I and the development of a protective vaccine against HIV-1.