KSHV requires vCyclin to overcome replicative senescence in primary human lymphatic endothelial cells

Kaposi's Sarcoma Herpesvirus (KSHV) is present in the main tumor cells of Kaposi's Sarcoma (KS), the spindle cells, which are of endothelial origin. KSHV is also associated with two B-cell lymphomas, Primary Effusion Lymphoma (PEL) and Multicentric Castleman's Disease. In KS and PEL, KSHV is primarily latent in the infected cells, expressing only a few genes. Although KSHV infection is required for KS and PEL, it is unclear how latent gene expression contributes to their formation. Proliferation of cancer cells occurs despite multiple checkpoints intended to prevent dysregulated cell growth. The first of these checkpoints, caused by shortening of telomeres, results in replicative senescence, where cells are metabolically active, but no longer divide. We found that human dermal lymphatic endothelial cells (LECs) are more susceptible to KSHV infection than their blood-specific endothelial cell counterparts and maintain KSHV latency to higher levels during passage. Importantly, KSHV infection of human LECs but not human BECs promotes their continued proliferation beyond this first checkpoint of replicative senescence. The latently expressed viral cyclin homolog is essential for KSHV-induced bypass of senescence in LECs. These data suggest that LECs may be an important reservoir for KSHV infection and may play a role during KS tumor development and that the viral cyclin is a critical oncogene for this process. Author summary Kaposi's Sarcoma (KS) is the most common tumor of AIDS patients world-wide and a significant tumor in sub-Saharan Africa. Kaposi's Sarcoma herpesvirus (KSHV), the etiologic agent of KS, is found predominantly in the latent state in tumor cells where the virus expresses a limited number of genes. To clarify how KSHV latency induces KS tumor formation we sought to determine how KSHV infection alters blood and lymphatic endothelial cells. KSHV infection of human LECs promotes their proliferation beyond replicative senescence, the first step of oncogenesis. Using this system we are able to identify bona fide viral oncogenes in the context of viral infection of relevant human dermal endothelial cells. These studies provide insight into how KSHV induces KS tumor formation.