Host immune responses including cytoxic T lymphocytes (CTLs) and Natural killer (NK) cells play a role in the elimination of virus-infected cells. To avoid these immune responses, herpesviruses have evolved elaborate mechanisms that target and modulate different aspects of the host's immune system. The proposed research is directed toward investigating a novel immune evasion strategy employed by KSHV K3 and K5 proteins. We have shown that both K3 and K5 membrane proteins downregulate MHC class I molecules and that K5 additionally downregulates B7-2 and ICAM-1. We hypothesis that KSHV uses two genes similar but distinct activities to ensure comprehensive protection from host immune effectors. Specifically, K3 plays a major role in the inhibition of CTL lysis whereas K5 is primarily involved in suppression of NK cell-mediated cytotoxicity and T helper (Th) cell-mediated immune response. In this proposal, we will draw upon our experience in the biochemical analysis of viral gene function and in the development and utilization of nonhuman primate models to define the detailed mechanisms of immune evasion of K3 and K5. Our biochemical and immunological studies will define in greater detail the mechanisms used by K3 and K5 to inhibit CTL lysis, NK cell-mediated cytotoxicity, and Th cell-mediated immune response. To define the role of K3 and K5 in viral immune evasion in vivo, we will test whether K3 and K5 are capable of contributing to the establishment of persistent infection of herpesvirus saimiri (HVS) in the natural host, squirrel monkeys, and to the induction of lymphoma by HVS in the non-natural hosts, common marmosets. Because of the lack of cell culture systems and animal models for KSHV, this approach provides a unique opportunity to investigate the contribution of the K3 and K5 genes to the escape of KSHV from hostile host immune responses. The proposed studies will provide an understanding of a novel immune evasion strategy of KSHV to survive destruction by host immune effector cells and to achieve persistent infection.