To study prevention of AIDS, we have generated efficient DNA expression vectors, which are currently evaluated as vaccines against SIV and HIV. This work is based on our previous recognition that RNA elements (called INS) present within the gag/pol and env coding regions of HIV are responsible for nuclear retention and instability of the transcripts in the absence of Rev, and that these elements can be eliminated by changing the RNA composition without affecting the amino acid sequence. These RNA-optimized gag and env expression vectors mediate the development of protective immune responses in vaccinated macaques when used as DNA only as well as DNA prime vaccine modality. We have now shown that that a combination of DNA vaccine vectors producing native and modified antigens are able to induce immune responses able to protect from high viremia in the rhesus macaque/SIVmac251 model. In collaboration with other investigators we have studied immunogenicity of additional HIV and SIV genes either as DNAs or as part of viral vectors. We have shown that these DNAs provide an excellent prime in DNA prime-recombinant Herpes boost studies inducing protective immune responses. We have also shown that combinations of SIV DNA vaccine together with IL-12 or IL-15 as DNA adjuvants improve immunogenicity. Importantly, we also found that DNA vaccination during antiretroviral treatment (ART) induces potent immune responses that are able to control viremia after ART interruption. Thus, DNA vaccination has great potential to be used as an additional therapeutic modality. Another important new development is that we have shown that DNA delivery via in vivo electroporation elicits greatly improved levels of cellular immune responses and these responses are long-lasting. Thus, these results indicate that improved gene delivery and expression dramatically improves immunogenicity and effectiveness of DNA vaccination. To design better vaccine stratgeis, it is important to dissect correlates of protective immunity. Thus infected macaques, able to control viremia serve as model to study the underlying mechanisms. Among the 'controllers' are animals from our vaccine studes as well as animals infected with live-attenuated SIV strains. These 'controllers' provide an important model to study cellular and viral determinants that contribute to disease development and they also provide a unique resources to study mechanisms leading to protective immunity. In a recent long-term follow-up study of animals infected with the live-attenuated SIV show persistent humoral and cellular SIV-specific immune responses, consistent with chronic infection but lack of pathogenicity. Multicolor flow cytometric analysis demonstrated preservation of the Central Memory subset of T-cells in the attenuated SIV-infected animals. This study demonstrates a potent, long-lasting control of the live-attenuated SIV in macaques. Therefore, the animals infected with live-attenuated SIV strains provide us not only with a unique resource to dissect cellular and viral determinants that contribute to AIDS development, but are also useful for studying correlates of protective immunity. These studies will provide critical information about the establishment and maintenance of host immune responses during chronic retroviral infections with distinct pathogenic outcomes and will aid in our design to improve DNA vaccination studies.