The cellular reservoirs of Kaposi's sarcoma-associated herpesvirus (KSHV) and the exact nature of the putative KSHV-infected circulating precursor of spindle cells of Kaposi's sarcoma (KS) still remain poorly defined. Because KS spindle cells are thought to be of endothelial origin, and because mature endothelial cells do not sustain persistent KSHV-infection, our attention was focalized on circulating hematopoietic precursors able to differentiate into endothelial lineage.

Late endothelial progenitor cells (late-EPCs) were cultured from the peripheral blood mononuclear cells of 16 patients with classic KS. The presence and load of KSHV genomes were analyzed by real-time polymerase chain reaction in DNA extracted from cells and supernatants of late-EPC cultures obtained from 7 patients. Endothelial colonies cultured from the peripheral blood of KS patients were found to satisfy all requisites to be defined late-EPCs: they appeared from the CD14-negative fraction of adherent cells after 11–26 days of culture, could be serially expanded in vitro, expressed high levels of endothelial antigens but lacked leukocyte markers. Late-EPC cultures were found to harbor KSHV-DNA at variable levels and to retain the virus after multiple passages in cells as well as in supernatants, suggesting that a quote of KSHV lytic infection may spontaneously occur. Lytic phase induction or hypoxia could amplify virus release in supernatants.

Our results suggest that circulating endothelial progenitors from KS patients are KSHV-infected and support viral productive replication and may therefore represent potential virus reservoirs and putative precursors of KS spindle cells.