6. Project SummaryAbstract There is an urgent need for development of a safe and effective HIV vaccine. Vaccine vectors based on attenuated vesicular stomatitis virus (VSV) are potent inducers of both cellular and humoral immunity. They can provide long-term protection against AIDS in rhesus macaques in an SIV-HIV hybrid challenge model, but not in the more stringent SIVmac251 challenge model. Wyeth Vaccines Inc. is moving attenuated, VSV-based HIV vaccine vectors into clinical trials in 2008. Although VSV vectors have shown no pathogenicity in more than 100 non-human primates when given by nasal, oral, or intramuscular routes, approval for clinical trials has been slow because of concerns about potential pathogenicity of live VSV vectors in humans. To address such safety concerns, new highly attenuated or single-cycle vectors based on VSV and a hybrid VSV-Semliki Forest Virus (SFV) propagating replicon, have been developed and tested in mice with excellent results. These vectors also have the significant advantage that there is no pre-existing immunity to them in the human population. The major goal this project is to test the effectiveness of these new vectors in a stringent non-human primate model in which neutralizing antibody can potentially play a role in protection from AIDS. In the first aim of the project, single-cycle, VSV-based priming vectors expressing SIVsmE660 Env and Gag proteins with and without the cytokine GM-CSF will be prepared and characterized. Expression of this cytokine during priming is known to enhance memory T-cell recall in mice upon boosting. In addition, VSV-SFV hybrid replicon particles expressing the same Env and Gag proteins will be prepared as boosting vectors. In the second aim, these vectors will be evaluated in rhesus macaques. SIV neutralizing antibody responses and SIV-specific T-cell responses will be evaluated in detail following prime and boost. Vaccinated macaques and controls will be challenged with the pathogenic SIVsmE660 strain and detailed virological and immunological analyses will be performed to determine the correlates of protection if it is obtained. Use of the SIVsmE660 challenge model has the advantage that significant neutralizing antibodies to the challenge virus are generated during infection of rhesus macaques. If these antibodies can be generated or primed for during vaccination, there is the potential to advance an SIV model of AIDS in which neutralizing antibody may have an effective role