Lentiviruses consistently cause CNS disease in their hosts. The chronic progressive diseases characteristic of all lentivirus infections has an unusually long period between infection and the manifestation of the systemic and neurological diseases. The pathogenetic mechanisms involved in lentivirus invasion of the CNS and the resulting disease process is incompletely understood. The infection of monocyte/macrophage lineage cells is important in the infection of the CNS and recent evidence has also identified the brain microvessel endothelial cell as an important cell in the infection of the CNS by the lentiviruses-HIV-1, SIV and visna virus. Visna virus and caprine arthritis-encephalitis virus (CAEV) were identified because of CNS disease in their hosts and provide models of lentivirus CNS disease that are relevant to humans infected with HIV-1. Studies on the molecular basis of the pathogenesis of these lentiviral infections provides an understanding of the disease process initiated by infection and replication of lentiviruses in monocytes/macrophages and endothelial cells. Regulation of gene expression of lentiviruses involves both cellular and viral proteins. In the macrophage, visna virus gene expression is dependent on maturation of the cell for transcriptional activity. The visna virus transactivator gene, tat, is also involved in the activation of viral and cellular gene expression. Activation of cellular genes in macrophages, brain endothelial cells and other target cell in vivo contributes to the pathogenetic processes during infection. The aim of this proposal is to understand the molecular mechanism of visna Tat activation and its role in CNS disease, and to determine the role of the tat gene in the pathogenesis of visna virus using transgenic sheep containing the tat gene. In addition, the role of cellular transcription factors important in activation of virus gene expression in vivo and in primary cells in vitro will be studied. Visna virus gene expression is regulated in macrophages by the cellular factors Jun and Fos acting at an AP-l binding site. The viral Tat protein has also been shown to mediate its transcriptional effect through this AP- l site. Recent studies in our laboratory have shown that the visna Tat protein has a potent acidic activation domain that is regulated by a small leucine-rich inhibitory domain. Aim 1 of the proposal will examine the mechanism of transcriptional activation by the functional domains of Tat. Aim 2 will identify the cellular proteins that interact with Tat and mediate its action. Aim 3 will examine the role of the Tat protein in the alteration of normal cell function using tat transgenic sheep. Aim 4 will utilize transgenic sheep that express the visna virus env and rev genes from the viral LTR to study the control of gene expression in vivo and in primary cells from these animals in vitro by in vivo footprinting and ligation-mediated PCR. These studies should lead to an understanding of the molecular events that lead to the pathogenesis of lentiviral disease in the CNS.