Kaposi's sarcoma-associated herpesvirus (KSHV) is associated with Kaposi's sarcoma (KS), primary effusion lymphoma (PEL), and multicentric Castleman's disease. Most KS tumor cells are latently infected with KSHV and are of endothelial origin. While PEL-derived cell lines maintain KSHV indefinitely, all KS tumor-derived cells to date have lost viral genomes upon ex vivo cultivation. To study KSHV latency and tumorigenesis in endothelial cells, we generated telomerase-immortalized human umbilical vein endothelial (TIVE) cells. TIVE cells express all KSHV latent genes 48 h postinfection, and productive lytic replication could be induced by RTA/Orf50. Similar to prior models, infected cultures gradually lost viral episomes. However, we also obtained, for the first time, two endothelial cell lines in which KSHV episomes were maintained indefinitely in the absence of selection. Long-term KSHV maintenance correlated with loss of reactivation in response to RTA/Orf50 and complete oncogenic transformation. Long-term-infected TIVE cells (LTC) grew in soft agar and proliferated under reduced-serum conditions. LTC, but not parental TIVE cells, formed tumors in nude mice. These tumors expressed high levels of the latency-associated nuclear antigen (LANA) and expressed lymphatic endothelial specific antigens as found in KS (LYVE-1). Furthermore, host genes, like those encoding interleukin 6, vascular endothelial growth factor, and basic fibroblast growth factor, known to be highly expressed in KS lesions were also induced in LTC-derived tumors. KSHV-infected LTCs represent the first xenograft model for KS and should be of use to study KS pathogenesis and for the validation of anti-KS drug candidates.Kaposi's sarcoma-associated herpesvirus (KSHV), also called human herpesvirus 8 (HHV-8), is believed to be the causative agent for Kaposi's sarcoma (KS) (for a review, see references 1 and 27). Within KS tumor lesions, the majority of cells express endothelial markers and are latently infected with KSHV, as defined by the presence of the circular viral genome and limited viral-gene expression. Studying KSHV's role in KS is complicated by the fact that cells explanted from KS lesions lose the KSHV genome after several cell divisions in culture (2, 43). In addition to KS, KSHV is associated with two lymphoproliferative diseases: primary effusion lymphomas (PEL) and multicentric Castleman's disease (MCD) (11, 59). In contrast to KS lesions, PEL-derived cell lines that are latently infected with KSHV are readily established in culture. These cells maintain viral episomes indefinitely and remain dependent on KSHV for survival (31,33). Therefore, many aspects of KSHV biology have been studied in PEL-derived cell lines rather than in endothelial cells (56; for a review, see reference 1).Several endothelial-cell-derived tissue culture models have been described. Common to these models, which are based on dermal microvascular endothelial cells (DMVEC), are their susceptibility to cell-free infection with PEL-derived KSHV and their capabili...