Increased tumorigenicity of cells carrying recombinant human herpesvirus 8.

Human herpesvirus 8 (HHV-8) infection is associated with the development of Kaposi's sarcoma and primary effusion lymphoma. The cloning of the HHV-8 genome into a bacterial artificial chromosome (BAC) allows researchers to mutate and identify the relative importance of HHV-8 genes essential for growth and replication in tissue culture systems. However, in vivo models to study the impact of such mutations are very limited. Consequently, the objective of this study was to determine whether cells carrying the HHV-8 BAC would form tumors when injected into mice, enabling the use of this model to assess the influence of viral gene mutation on tumorigenesis. To do so, 293T and 293T-E1 cells carrying recombinant HHV-8 were injected into SCID mice and tumor growth was analyzed. Our results clearly show that mice injected with 293T-E1 cells had a significantly higher tumor incidence level as well as increased tumor volumes and weights compared to mice injected with 293T control cells. Cells carrying the HHV-8 genome grew faster and more aggressively in SCID mice than control 293T cells, highlighting the oncogenic properties of HHV-8. The model presented could therefore be used for the identification of HHV-8 genes contributing to tumorigenesis in the context of the entire viral genome.

Phenotypic alterations and survival of monocytes following infection by human herpesvirus-6.

Freshly isolated monocytes rapidly undergo physiological changes in vitro, resulting in programmed cell death (apoptosis). Activation of monocytes, which promotes differentiation into macrophages, is known to inhibit apoptotic processes. In the present study, we report that human herpesvirus-6 (HHV-6) prevents monocytes from undergoing spontaneous apoptosis during the first 72 hours of culture. Furthermore, significant alterations in cell-surface phenotype were observed after 72 hours of infection with HHV-6. HHV-6-infected monocyte cultures have considerably reduced levels of CD14, CD64 (FcgammaRI) and HLA-DR antigen on their surface, while CD32 (FcgammaRII) expression is unaffected. On the basis of these results, we hypothesize that HHV-6 promotes monocytes survival and causes phenotypic modifications that could favor immune evasion and ensure its persistence within the infected host.

Human herpesvirus 8 viral FLICE-inhibitory protein inhibits Fas-mediated apoptosis through binding and prevention of procaspase-8 maturation.

Viral FLICE-inhibitory proteins (v-FLIPs) encoded by several herpesviruses and poxviruses share the ability to inhibit apoptosis after engagement of death receptors. In the current article, we provide insights into the mechanisms by which the v-FLIP of human herpesvirus 8 (HHV-8) (also referred to as Kaposi's sarcoma-associated virus) protects cells from apoptosis after Fas-induced signaling. Using v-FLIP expression vectors, our results clearly show that HHV-8 v-FLIP reduces the cleavage of procaspase-8 into its active p18 and p10 protease subunits upon Fas-induced cell death. These results were confirmed by lower caspase-8 and caspase-3 protease activities in extracts of HeLa cells expressing HHV-8 v-FLIP. Coimmunoprecipitation studies further indicate that HHV-8 v-FLIP physically interacts with procaspase-8, but not with Fas-associated protein with death domain in the cellular cytoplasm. These results suggest that binding of HHV-8 v-FLIP to procaspase-8 affects the recruitment and the activation of the latter at the death-induced signaling complex, resulting in diminished apoptotic cascade initiation. Because cellular FLIP was recently reported to modulate promoter containing NF-kappaB motifs and that both HHV-8 and human immunodeficiency virus type 1 (HWV-1) can infect monocytes, we studied the effects of v-FLIP on HIV-1 gene expression. Cotransfection experiments indicated that v-FLIP expression is associated with activation of HIV long terminal repeats: events that were strictly dependent on the presence of NF-kappaB consensus elements. In conclusion, HHV-8 v-FLIP can possibly contribute to the pathogenesis of both HHV-8 and HIV-1 through impaired Fas-dependent killing of infected cells by cytotoxic T cells and through activation of HIV gene expression.