Pleiotropism of human herpes virus-8 in Kaposi's Sarcoma : infection of endothelial progenitor cell and altered maturation of B lymphocytes

Human herpesvirus 8 (HHV8), is an essential factor in the pathogenesis of Kaposi's sarcoma (KS). In addition to KS, HHV8 is also involved in the pathogenesis of two lymphoproliferative disorders: primary effusion lymphoma (PEL) and the plasma cell variant of multicentric Castleman's disease (MCD). It has also been linked to other, non-malignant, clinical manifestations such as bone marrow failure in transplant recipients and haemophagocytic syndrome. Therefore, HHV-8 infection has a “pleiotropic effects” because the virus can promote different pathological conditions; however, the precise events leading to the develop of the different diseases still remain poorly defined. One fundamental unresolved question regards the cellular reservoirs of HHV8 and the exact nature of the putative HHV8-infected circulating precursor of spindle cells the hallmark cells of KS lesions. Because KS spindle cells are thought to be of endothelial origin our attention was focalized on circulating hematopoietic precursors able to differentiate into endothelial lineage. Furthermore, B cells are now defined as one of the principal circulating HHV8-reservoir able to sustain the lytic reactivation and dissemination of the virus. However, the consequences of HHV-8 infection of B cells in KS patients have been poorly characterized so far. Therefore, in this study we investigated also whether the frequency and/or the immunophenotypic profile of different peripheral B cell subsets were affected in patients with cKS. Methods and Findings: Endothelial progenitor cells (EPCs) were quantified by flow cytometry and were found increased in the peripheral blood of patients with cKS. Late endothelial progenitor cells (late-EPCs) were cultured from the peripheral blood mononuclear cells of patients and the presence and load of KSHV genomes were analyzed by real-time PCR in DNA extracted from cells and supernatants of late-EPC cultures. 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. Moreovere, late-EPCs could be infected in vitro by HHV8, supporting the hypothesis that these cells are one of the more important virus target. Then, we used flow-cytometry to evaluate the frequency of circulating B cells and their subpopulations in cKS patients. Analysis of the peripheral blood of patients compared to age and sex matched controls, showed that in cKS patients the frequency of circulating CD19, naïve, pre-naïve and transitional B cells were increased in patients with cKS. Conversely, the frequency of memory B cells (class-switched, IgM Memory and CD27- memory B cells) was decreased in cKS patients compared to healthy controls. Finally, Marginal Zone-like B cells did not change between groups. Conclusion: Our results suggest that circulating endothelial progenitors from KS patients are increased in the peripheral blood of cKS patients and are HHV8-infected. Moreover, they support viral productive replication and may therefore represent potential virus reservoirs and putative precursors of KS spindle cells. In addiction, our results clearly indicate that circulating B cells are altered in patients with cKS, with an expansion of B cells in the early stages and a concomitant reduction of memory B cells.