IntroductionKaposi sarcoma-associated herpesvirus (KSHV; also human herpesvirus-8) is implicated in all clinical forms of KS 1,2 as well as the lymphoproliferative disorders primary effusion lymphoma (PEL) 3,4 and multicentric Castleman disease (MCD). 5 Endothelial cells harbor the KSHV genome in vivo, 1,6,7 are permissive for virus infection in vitro, [8][9][10][11] and are thought to be the precursors of spindle cells. [12][13][14][15] We and others have shown that in vitro infection of human dermal microvascular endothelial cells (DMVECs) with KSHV induces a spindle cell morphology and characteristics of a transformed phenotype, including loss of contact inhibition and growth in soft agar. [8][9][10][11] Because explanted KS cells fail to maintain the KSHV genome following serial passage, 16,17 in vitro infection of DMVECs has proved to be an invaluable tool for the study of KSHV pathogenic mechanisms. 18,19 KSHV, like other herpesviruses, has a substantial coding capacity that includes viral genes unique to this human pathogen, genes that share homology with other members of the herpesviridae, as well as genes that appear to have originated in the host genome. 20,21 Of note, at least 3 of these gene products, vFLIP (viral Fas-associating protein with death domain-like interleukin-1 [IL-1]-converting enzyme [FLICE]-inhibitory protein), viral cyclin (vCYC), and latency-associated nuclear antigen (LANA), are believed to comprise the viral latency expression program and are consistently expressed in all virally infected cells in KS, PEL, and MCD. [22][23][24] The gene product of open reading frame 71 (ORF 71), vFLIP, is thought to play an important role in prevention of apoptotic cell death. 25 The activity of a homolog of cellular cyclin D, vCYC (ORF 72), 26 together with the modulation of cellular transcription wrought by LANA (ORF 73) 27 is thought to be involved in KSHV-induced host cell transformation. Additionally, LANA is indispensable for maintenance of the viral genome. 28 Other gene products, such as viral G-protein-coupled receptor (vGCR) 29,31 are classified as lytic gene products and are expressed in only a minority of cells in vivo. [32][33][34] Although reactivation of KSHV from the latent state and subsequent completion of the viral lytic expression cascade is incompatible with survival of a host cell, lytic gene products are thought to have paracrine influences on both latently infected and uninfected cells and are thus considered vital for lesion development. 35 We have used our previously described in vitro KS model to dissect viral mechanisms of pathogenesis. 18,19 The salient features of our model include recapitulation of KS cell physiology, including spindle cell formation, loss of contact inhibition, and anchoragedependent growth restriction; long-term propagation of predominantly latently infected cells; and the ability to generate ageand passage-matched KSHV-infected and uninfected cultures. 11 This system has been amenable to gene expression profiling by cDNA microarrays and has ...