Despite the near complete suppression of detectable virus in many HIV infected patients undergoing highly active antiretroviral therapy, viremia reemerges rapidly after interruption of treatment. Postintegration latency refers to latently infected resting memory CD4 + T cells containing transcriptionally silent integrated HIV-1 genomes. Postintegration latency contributes to the persistence of the virus under HAART and represents a known barrier to eradication of HIV infection. In this proposal, four groups of investigators with extensive experience in the molecular virology and biology of HIV and SIV propose to address two major knowledge gaps in our understanding of postintegration latency: 1.Development of in vitro and in vivo models for HIV latency. Dr. Verdin's group has used a GFP-tagged virus to demonstrate that HIV infection reproducibly results in a state of postintegration latency after infection of a CD4 + cell line in vitro and propose to use the same stategy in primary human CD4 T cells. Drs. North and Luciw have recently demonstrated that infection of rhesus macaques with a recombinant SIV virus (RT-SHIV) can be suppressed to undetectable levels using treatment with Efavirenz+3TC+PMPA and that virus rapidly rebounds after cessation of therapy. They propose to use this SIV model system to define the natural reservoirs of SIV during infection in vivo; 2. Study of the molecular mechanism(s) of latent HIV. Drs. Verdin, Greene, Peterlin, North and Luciw propose to use the new experimental models described above, to dissect in vitro and in vivo the molecular mechanism of postintegration latency. They will use their combined experience in the molecular biology of HIV and extensive collaborations between their groups to assess the role of chromatin at the site of integration of the virus in the genome (Verdin), the role of the NF-KB transcription factor (Greene) and the role of Tat and its associated cofactors (Peterlin) in the establishment, the maintenance and the reactivation of latent HIV. Core B, directed by Dr. Luciw will use the SIV macaque system to define to full extent of viral reservoir in infected and treated macaques and will test in the SIV/macaque system predictions made from the basic studies. This collaborative effort will culminate in the testing of biological agents aimed at reactivating latent SIV expression in infected macaques in an attempt to flush the latent pools. We anticipate that this effort will lead to increased understanding of molecular mechanisms responsible for postintegration latency and to the opening of new therapeutic opportunities aimed at eradicating HIV infection.