Until recently, T cell-inducing vaccines based on replication-defective adenovirus (Ad) vectors of the human serotype 5 (AdHuS) were viewed as the most premising vaccine platform for HIV-1. In a large-scale phase lib trial (STEP trial) an AdHuS vaccine express!ig antigens of HIV-1 was highly immunogenic in humans but failed to have efficacy and instead showecJ a trend towards higher HIV-1 acquisition in vaccinated males with preexisting neutralizing antibod es to the vaccine carrier. The failure of the STEP trial necessitates more pre-clinical research to determine correlates of protection against heterologous challenge with SIV or SIV/HIV chimeras (SHIV) in experimental c nimals, i.e., nonhuman primates (NHPs). It is the primary objective of this project to elucidate correlates of immune-mediated protection against HIV-1/SIV infection in preclinical animal models and to determine if they can be induced by subunit vaccines. To this end, Project 1 will compare humoral and cell-mediated immune responses to an attenuated SIV, termed SIVAGY, known to induce protective immijnity to subsequent challenge with a heterologous SIV to those induced by different prime-boost vaccine regirnens using combinations of DMA vaccines, E1-deleted adenovirus (Ad) vectors and poxvirus vectors. E1 -deleted Ad vectors are highly immunogenic Notwithstanding some of their characteristics may prevent induction of T cells, which have the qualities nBeded to optimally protect against HIV-1. Firstly, E1- deleted Ad vectors persist in a transcriptionally active form at low levels, mainly in activated T cells, Persistence maintains high frequencies of T cells specific to the transgene product and presumably to antigens of the vaccine carrier and delays transition of effector T cells into central memory T cells. Although effector T cells presumably provide an important first \ijyer of defense at the port of viral entry, central memory T cells are more capable of expansion upon r 3-encounter of the antigen and may therefore be needed to control a rapidly replicating pathogen. Secondly, Ad vectors in spite of the E1-deletion produce low levels of late viral gene products. As most adult humans had previous infections with Ad viruses, one could envision that stimulation of Ad-specific memory "" cells by an Ad vaccine vector could impair the magnitude and quality, such as the breadth, of primary T cell responses to vaccine antigens. It is the secondary objective of this project to generate andteist two types of 2nd generation E1 -deleted Ad vectors; one will be designed to allow for conditional termination of transgene product expression to prevent continuous stimulation of immune responses to theva<xine antigen, the other will have an additional deletion of E2a to reduce synthesis of the highly immunogenic structural antigens of Ad vectors. RELEVANCE (See instructions): The goal of this project is to further our knowledge of correlates of protection against HIV-1 using pre-clinical animal models. It is another goal of this project to optimize Ad vectors so that induced T cell responses may be better equipped to control HIV-1 infections. Efforts to develop a vaccine against HIV-1 either by increasing our knowledge on correlates of protection or by optimizing vaccine vectors are of interest to public health considering the global impact of HIV-1 infections.