The overall goal of this proposal is to examine the relative fitness of SIV variants cloned from early and late stages of infection and to characterize the factors regulating infection and replication. Studies using the SIV macaque model have shown that viral burden and pathogenicity are influenced by the phenotype of the infecting virus and variants that evolve from it, and that increased pathogenicity correlates with greater viral replication capacity in T-cells. However, it is unclear whether increased pathogenicity necessarily equals greater overall fitness or if changes in pathogenicity may result in a fitness cost in terms of sexual transmission. We propose to test the hypothesis that the phenotoypic properties of SIV will influence its fitness, and therefore, infection and persistence in the host. We predict that upon infection, the capacity of a virus to infect T-cells and modulate and utilize cellular activation signals will influence replicative fitness, facilitating dissemination. However, increased replication in T-cells may not confer a fitness advantage during the establishment of infection after mucosal transmission. To test the hypothesis, we will further characterize the cellular and viral factors involved in regulating infection and replication of cloned SIVmne variants of known pathogenicity, and we will examine the relative fitness of the variants in the macaque host. Three specific aims are proposed for these studies. AIM1: To examine the competitive replication fitness of SIVmne variants with distinct phenotypes. To examine the impact of macrophages and dendritic cells on competitive replication fitness. AIM2: To characterize the viral determinants that promote SIVmne replication in T-cells. Specifically, we plan to define the mechanism by which RT mutations increase viral infectivity, and to determine whether the RT infectivity determinant enhances nef expression in resting T-cells. AIMS: To define the relative in vivo fitness of SIVmne variants. To examine the amplification and dissemination of SIVmne variants during dual-infection of pig-tailed macaques. The results of these studies may provide greater insight into the types of viruses that infect and predominate at early and late stages of infection, and the mechanisms that they use to infect and persist in the host. Furthermore, they may have important implications for the design of vaccine and therapeutic strategies against HIV.