Lack of information on the immunologic mechanisms responsible for protection against HIV infection remains one of the major obstacles to the development of a safe and effective AIDS vaccine. Vaccination of macaques with attenuated SIV strains has consistently proven to be the most effective means to induce protection against pathogenic SIV challenge in macaques. Intensive study of macaques vaccinated with attenuated SIV strains therefore represents one of the best experimental models available for the determination of mechanisms of protective immunity against lentivirus infection. The overall goal of this Program Project application is to undertake a comprehensive, multidisciplinary effort to define mechanisms of immune protection mediated by live attenuated SIV strains. Complementary experiments conducted by three principal investigators with distinct areas of expertise will examine: 1. Mechanisms of mucosal protection induced by attenuated SIV. These experiments will undertake a detailed examination of the evolution of adaptive and innate immune responses induced by SIV?nef and correlate these responses with protection, examine the effect of prolonged B cell depletion on protective immunity, and study viral replication and immune responses in the female reproductive tract of SIV?nef-vaccinated animals after vaginal challenge. 2. The contribution of anti-envelope immune responses to protection mediated by live attenuated SIV. Specific questions include: Does a mismatch of envelope sequences in the challenge virus decrease the degree of protection? Does challenge with a closely-matched SIV strain that differs dramatically in coreceptor usage influence the degree of protection? Does variation in the strength of the anti-envelope antibody response induced using modified single-cycle SIV influence the degree of protection? 3. Mucosal immunity and heterologous protection induced by single-cycle SIV (scSIV). These experiments will examine if the site of immunization with scSIV determines the mucosal homing properties of T cell responses and resistance to an intrarectal challenge with SIVmac239;whether the site of priming influences the ability of virus-specific T cell responses to protect against a vaginal challenge with SIVmac239;and whether immunization with a mixture of antigenically diverse strains of scSIV can broaden virus-specific immune responses and enhance protection against a heterologous challenge with SIVmac239. Results from these studies should shed light on the nature of immune responses able to protect against HIV/SIV infection, which remains one of the outstanding unanswered questions of AIDS vaccine research, and thus will have important implications for the design of clinically applicable AIDS vaccines. PROJECT 1: Mechanisms of mucosal protection induced by attenuated SIV (Johnson, R. Paul) PROJECT 1 DESCRIPTION (provided by applicant): Lack of information on mechanisms of protection against HIV/SIV infection remains one of the leading obstacles to the development of a safe and effective AIDS vaccine. Vaccination of macaques with attenuated SIV strains has consistently proved to be the most effective means of inducing protection against pathogenic SIV challenge and offers the best available experimental model to define specific mechanisms responsible for protection. Previous studies from our group have provided evidence that SIV-specific CD8+ T cell and humoral responses both contribute to protective immunity induced by SIV?nef but have not been able to assess their relative importance or the potential contributions of novel adaptive (e.g. CD4+ T effector cells) or innate immune responses to protection. The goal of the current application is to utilize a number of novel techniques to carry out a comprehensive analysis of the role of adaptive and innate immune responses in mediating protection induced by SIV?nef against vaginal challenge, one of the most important modes of HIV transmission. Specific aims include: 1: To examine the evolution of adaptive and innate immune responses induced by SIV?nef and to correlate these responses with protection against homologous and heterologous challenge. 2: To examine the effect of prolonged B cell depletion on protective immunity induced by SIV?nef. 3: To examine viral replication, innate and adaptive immune responses in the female reproductive tract of SIV?nef-vaccinated animals after vaginal challenge.