Disseminated infections by human cytomegalovirus (HCMV), such as pneumonia and retinitis, account for one of the most common AIDS-associated opportunistic complications. Studies of CMV systemic infection should provide insight into the pathogenesis and virulence of disseminated CMV diseases. During primary acute infection, the viruses produced in the spleen disseminate to other organs and represent one of the major sources for subsequent infection of many organs, including the salivary gland, kidney, and bone marrow, where CMV ultimately establishes persistent and latent infections. Moreover, the spleen is also a site for viral persistent and latent infections. CMV replication in the spleen is a major determinant of viral virulence and pathogenesis. Understanding the mechanism of CMV infections in the spleen is important for developing strategies to block viral replication in the organ and will provide insight into the treatment and prevention of CMV-associated diseases. Using murine cytomegalovirus (MCMV) infection in immunodeficient animals as a model system, the proposed study is to identify the viral genes required for CMV replication in the spleen and to study the functions of these viral determinants in supporting CMV infections in the organ. We have recently generated a pool of MCMV mutants that contained a transposon sequence and have isolated a viral mutant that was not virulent at all in killing immunodeficient animals and was defective in replication in the spleen. In the proposed research, animals will be infected with viral mutants though direct administering the virus to the spleens and those mutants that are defective in replicating in the spleens will be isolated. The virulence of these mutants will be studied and the genes that are mutated will be identified. Moreover, the mechanism of how the identified viral determinants function in supporting MCMV infections in the spleen will be investigated. These studies will lead to the identification of viral determinants for CMV replication in the spleen and the investigation of the functions of these genes in systemic CMV infections. Identification of viral virulence factors and understanding the mechanism of CMV virulence and pathogenesis will facilitate the development of novel strategies for treatment and prevention of disseminated CMV infections.