Information and phylogenetic groupings of sequences used in this study. 1, 2, 3, and X denotes groups 1, 2, 3, and unclassified (early viruses; see main text for explanation). Empty entries indicate the unavailability (e.g., no sequence found, too short, too many ambiguous codes, and too many gaps) of the sequence.  Endothelial Cells Support Persistent Gammaherpesvirus 68 Infection A variety of human diseases are associated with gammaherpesviruses, including neoplasms of lymphocytes (e.g. Burkitt's lymphoma) and endothelial cells (e.g. Kaposi's sarcoma). Gammaherpesvirus infections usually result in either a productive lytic infection, characterized by expression of all viral genes and rapid cell lysis, or latent infection, characterized by limited viral gene expression and no cell lysis. Here, we report characterization of endothelial cell infection with murine gammaherpesvirus 68 (γHV68), a virus phylogenetically related and biologically similar to the human gammaherpesviruses. Endothelial cells supported γHV68 replication in vitro, but were unique in that a significant proportion of the cells escaped lysis, proliferated, and remained viable in culture for an extended time. Upon infection, endothelial cells became non-adherent and altered in size, complexity, and cell-surface protein expression. These cells were uniformly infected and expressed the lytic transcription program based on detection of abundant viral gene transcripts, GFP fluorescence from the viral genome, and viral surface protein expression. Additionally, endothelial cells continued to produce new infectious virions as late as 30 days post-infection. The outcome of this long-term infection was promoted by the γHV68 v-cyclin, because in the absence of the v-cyclin, viability was significantly reduced following infection. Importantly, infected primary endothelial cells also demonstrated increased viability relative to infected primary fibroblasts, and this increased viability was dependent on the v-cyclin. Finally, we provide evidence for infection of endothelial cells in vivo in immune-deficient mice. The extended viability and virus production of infected endothelial cells indicated that endothelial cells provided a source of prolonged virus production and identify a cell-type specific adaptation of gammaherpesvirus replication. While infected endothelial cells would likely be cleared in a healthy individual, persistently infected endothelial cells could provide a source of continued virus replication in immune-compromised individuals, a context in which gammaherpesvirus-associated pathology frequently occurs. Endothelial cells create a physical barrier on the luminal surface of blood and lymphatic vessels. This barrier must be traversed by blood-borne pathogens and immune cells trafficking between tissues and the bloodstream. Many herpesviruses require systemic spread for persistence within a host, and therefore must cross such an endothelial cell barrier. To date, herpesviruses have been implicated as potential initiators of arterial injury, endothelial dysfunction, and local inflammation, possibly contributing to the pathogenesis of atherosclerosis [1] [2] [3] [4] . Human cytomegalovirus (HCMV), a betaherpesvirus, infects endothelial cells in vivo. Studies have shown that infected endothelial cells play a role in HCMV dissemination and pathogenesis [5] . Endothelial cells exhibit regional specialization in gene expression and morphology depending on the local physiologic demands of their respective organs and tissues [6] . In light of this diversity, it is not surprising that endothelial cells from different tissues differ in their susceptibility to HCMV infection [7] . Kaposi's sarcoma-associated herpesvirus (KSHV), a human gammaherpesvirus, also infects endothelial cells in vivo. More importantly, KSHV is the causative agent of the endothelial cell neoplasm, Kaposi's sarcoma (KS). The murine gammaherpesvirus, cHV68, has been detected in aortic endothelium after infection of apoE deficient mice, as well as on the luminal surface of explanted aortas infected in vitro [8] . While a relationship between gammaherpesviruses and endothelial cells has been noted, the role of endothelial cells in chronic gammaherpesvirus infection and pathogenesis is ill-defined.