The long term-goal of our laboratory is to investigate novel approaches to prevent HIV transmission by the use of antivirals and microbicides. For this purpose we have developed and implemented a novel small animal model where human stem cells are used to reconstitute the hematopoietic system of immunodeficient mice. In these humanized mice (designated BLT to represent the fact they are generated from a bone marrow transplant of mice previously implanted with a piece of autologous human fetal liver and thymic tissue) there is systemic reconstitution with human hematopoietic cells in all hematopoietic and non-hematopoietic tissues tested. As shown in the Preliminary Results section of this grant, these humanized mice are susceptible to intrarectal and intravaginal infection with X4- and R5-Tropic HIV-1. Infection results in plasma antigenemia, progressive depletion of human CD4+ cells from the peripheral blood and the development of HIV-1 specific human antibodies. In addition, our data show that human CD4 T cell depletion is systemic and most dramatic in the human thymic organoid. Using in situ hybridization we demonstrate systemic infection by showing the presence of infected cells in the large and small intestine, mesenteric lymph nodes, spleen, and thymic organoid. In this grant we propose to expand on these remarkable results and to establish the suitability of this system to evaluate pre-exposure prophylaxis of HIV transmission and by antivirals and/or microbicides. With this in mind we propose the following Specific Aims: Specific Aim 1) To characterize the role of co-receptor tropism in the infection of BLT mice after intrarectal inoculation. Specific Aim 2) To evaluate the efficacy of topical and systemic administration of antivirals to prevent intrarectal HIV-1 infection of BLT mice. Specific Aim 3) To evaluate the susceptibility BLT mice to intravaginal infection with HIV-1. Specific Aim 4). To determine the efficiency of topical and systemic antivirals to prevent intravaginal HIV transmission. By establishing these basic parameters we will determine the potential utility of this novel system to provide important pre-clinical data to serve as the possible basis for the future implementation of clinical trials.