We have investigated virulence determinants of equine infectious anemia virus (EIAV), a macrophage-tropic lentivirus, and have shown that both the surface glycoprotein (SU) and the auxiliary protein, S2, significantly influence disease expression. Based on preliminary studies we hypothesize that (1) A virulent EIAV strain, through the activities of its SU and S2 proteins, modulates host cell gene expression;(2) Modified gene expression alters host cell physiology, generating an environment optimally suited to high titer virus replication and disease expression. Specific Aims of the proposal are: AIM 1. Characterize changes in gene expression induced by acutely virulent EIAV and its SU protein. We will: (i) Measure induction of cytokine and chemokine gene expression by virulent EIAV and purified SU in relevant equine cells, (ii) Map the regions of SU responsible for modulation of cytokine/chemokine gene expression using chimeric viruses and recombinant SU proteins, (iii) Test sub-regions of SU for their contribution to the development of acute disease in a Shetland pony model. AIM 2. We will characterize the properties of the EIAV S2 protein using both in vivo and in vitro assays by site-directed mutagenesis of predicted functional domains. The results obtained from our in vivo acute pathogenesis assay with site-specific virus mutants will serve to guide our in vitro studies that are directed at (i) identifying cellular proteins that interact with the S2 protein and (ii) the characterization of in vitro biologic activities attributable to the S2 protein. Animal studies related to Aims 1 and 2 will include determination of platelet counts, plasma virus loads, provirus levels in circulating PBMC and in tissues, cytokine/chemokine gene expression in circulating PBMC and tissues, and viral gene expression in PBMC and tissues. As EIAV replication and persistence are centered on monocytes/macrophages, and disease expression is rapid (2-3 weeks), this model is ideal for specifically probing the interactions of virus and macrophages that contribute to lentiviral-induced disease.