The long-term objective of this application is to identify critical Simian Immunodeficiency Virus (SIV) epitopes required to elicit protection in macaques against pathogenic SIV. For this purpose, the well-defined rhesus monkey model system will be used to evaluate the efficacy of replication-defective strains of SIV as vaccine prototypes. Our first aim is to determine the possible genetic defect(s) of two stable mutant viruses, that will be evaluated in vaccine protection studies. Our strategy is to determine whether non replicating viruses (containing native proteins that elicited neutralizing antibodies (n.a.) in other mammals), will induce an effective immune response in macaques, against pathogenic SIV. This strategy is based on the hypothesis that conformational (i.e. native) SIV epitopes are needed to induce both, n.a. as well as specific CMI responses, which are required for protection against challenge. To this effect, groups of monkeys will be vaccinated with defective virus preparations obtained by our laboratory. Response to vaccination will be ascertained by measuring virus load, (using viremia and "nested" PCR assays), antibody production (using ELISA, immunoblot and n.a. assays) as well as CMI responses (by measuring lymphocyte subsets, cytotoxic and helper T-cell functions, and antibody dependent cellular cytotoxicity). Since these immunogenic, stable mutants failed to produce viral DNA "in vitro," it is reasonable to expect that protective immunity will be induced in the absence of "in vivo" viral replication. To determine the efficacy of these preparations, vaccinated animals will be sequentially challenged with cell-free, homologous and heterologous virus preparation grown in macaque cells. Each challenge experiment will be monitored as describe above. Results should allow investigators to determine the critical SIV epitopes needed to elicit protection, and could lead to the development of an efficacious vaccine. At the present time, the protective role of humoral and/or cell-mediated immunity in HIV and SIV infections is unclear. Through a collaborative effort with researchers at University of California, Davis, the virus epitopes involved in eliciting protection will be mapped. These critical viral determinants could be fully utilized for the development of new generation subunit vaccines. This approach could be used for the evaluation of vaccines against other lentiviruses, including HIV.