Approximately 20% of humans infected with human immunodeficiency virus type 1 (HIV-1) develop a neurological disease known as HIV- associated cognitive/motor complex or AIDS dementia complex. In this application, we propose to use a pathogenic, molecular clone of simian- human immunodeficiency virus (SHIV/KU-2MC4) containing the tat,rev, vpu and env from HIV-1 in a genetic background of SIV/mac239 as a model to study aspects of HIV neuropathogenesis which will clarify the role of the brain as a viral sanctuary. SHIV/KU-2MC4 infection of rhesus macaques results in high virus burdens, depletion of the CD4+ subset of T-cells, and a neuropathology (perivascular cuffing, multinucleated giant cells) similar to that seen in HIV-1 infected humans. Using the macaques sacrificed at different sequential time points following inoculation of with SHIV/KU- 2MC4, we will compare the virus burdens in the CNS and lymphoid tissues. Virus burdens in the CNS and lymphoid tissues will be correlated with: a) the chronological development of neuropathology; b) the presence of viral p27; c) microglial and astrocyte activation and d) the expression of viral co- receptors CXCR4, CCR3, and CCR5. Using this model, we will generate a deletion mutant to examine the role of the HIV-1 specific gene vpu on the virus burdens and the neuropathogenesis of SHIV/KU-2MC4. During the derivation of pathogenic SHIV/KU-2MC4 from non-pathogenic SHIV-4, numerous amino acid substitutions occurred in the Env and Nef. We will investigate the role of the amino acid changes in the Env and Nef on the development of SHIV-associated neurological diseases by generating interviral recombinants in which the env and nef genes of SHIV/KU-2MC4 will be exchanged with those from non- pathogenic SHIV-4. In preliminary studies, we showed that SHIV/KU- 2MC4 exclusively used that CXCR4 co-receptor for entry into susceptible cells. Using virus recovered from CNS and lymphoid tissues from macaques sacrificed at different sequential time points, we will determine the co- receptor use of the env genes during the development of SHIV-associated neurological disease. It is anticipated that the proposed studies will provide valuable information which will improve our understanding of HIV-1 neuropathogenesis.