Two large scale nonhuman primate (NHP) efficacy studies have convincingly demonstrated that CMV/SIV vectors can 1) re-infect CMV-seropositive rhesus macaques (RM), 2) during re- infection, elicit potent and persistent SIV-specific CD4+ and CD8+ T cell responses with a strong effector memory bias, and 3) protect ~50% of vaccinated RM from progressive SIV infection after limiting dose rectal challenge with the highly pathogenic, CCR5-tropic SIVmac239 virus. The protection manifested in these RM is distinct from previous vaccines in its abruptness and extent, with protected RM manifesting a viral burst in plasma of varying size upon initial infection, followed by immediate control to undetectable levels. Protection correlates with the extent of total SIV-specific CD8+ T cells generation during the vaccine phase, and is stable in the vast majority of protected RM for at least 6 months. These data indicate a novel pattern of protection consistent with very early control, likely taking place in the site of viral entry and/or early sites of viral replication and amplification, and mediated by tissue-resident TEM. To date, our efficacy trials have studied CMV vectors that were systemically administered via subcutaneous inoculation. We have, however, demonstrated that oral inoculation of these vectors can also initiate re-infection of CMV-seropositive RM, and elicit robust SIV-specific T cell immunity. We hypothesize that oral inoculation of CMV vectors, and initial induction of vaccine- generated SIV-specific T cells in a mucosal milieu, might change the character of the resultant SIV-specific TEM, perhaps increasing their representation and function in mucosal sites, and potentially increase the efficacy of these vaccines in protecting against mucosal challenge. In the proposed project we will compare in detail the phenotypic, migratory, functional and gene expression properties of SIV-specific T cells elicited by CMV vectors via systemic vs oral route, and determine whether oral inoculation provides enhanced protection against limiting dose, intra-rectal SIVmac239 challenge. PUBLIC HEALTH RELEVANCE: Worldwide, there were ~2.5 million new HIV infections in 2007, and it is generally agreed that an effective prophylactic vaccine, especially one that could be given once and provide a lifetime of protection, is the only practical means by which the HIV/AIDS epidemic can be controlled. We have demonstrated that CMV/SIV vectors can harness tissue resident effector memory T cells to combat the virus very early in infection, and can protect rhesus monkeys from progressive infection after mucosal highly pathogenic SIV challenge. The work in this proposal will provide crucial information for the further clinical development of this novel vaccine approach by defining the protective mechanism(s) of CMV vectors and optimizing their efficacy.