Human Immunodeficiency Virus Type 1 Tat Acts Synergistically With Human Herpesvirus 6 To Enhance Reactivation Of Human Herpesvirus 8

To study the effect of human herpesvirus 6 (HHV-6) and human immunodeficiency virus type 1 (HIV-1) Tat in combination on lytic cycle replication of human herpesvirus 8 (HHV-8), we evaluated HHV-8 ORF26 mRNA in BCBL-1 cells fused with HHV-6-infected cells and cocultured with HHV-6-infected cells by direct cell-cell contact or in Transwell system. RT-PCR and real-time quantitative PCR were performed to detect ORF26 mRNA expression in heterokaryons that were created by BCBL-1 cells fused with HHV-6-infected JJhan cells (also known as HV6-J). The results demonstrated that HHV-8 was significantly activated by HHV-6 in heterokaryons (t test, P < 0.05). Meanwhile, real-time quantitative PCR were carried out to detect ORF26 mRNA expression in BCBL-1 cells cocultured HV6-J by direct cell-cell contact and cultured in the supernatants from HV6-J. And IFN-y and IL-10 expression in their supernatants were monitored by ELISA. The results indicated that ORF26 mRNA in BCBL-1 cells cocultured HV6-J by direct cell-cell contact and cultured in the supernatants from HV6-J were significantly increased compared to that of control (t test, P < 0.05).However, there were no differences in expression of IFN-y and 1L-10 between test and control groups, but the expression of IFN- y in BCBL-1 cultured in the supernatants from HV6-J was increased compared to that of control (t test, P > 0.05). After infection with HHV-6 particles and heat-inactivated or LJV-irradiated viral particles, ORF26 mRNA expression in BCBL-1 cells were elevated on different time point. Furthermore, gene transfection assay and Transwell system were performed to study the effect of extracellular Tat cooperated with HHV-6 and intracellular Tat alone on the expression of ORF26 in BCBL-1 cells. The results from real-time quantitative PCR analysis for ORF26 mRNA showed that HHV-6 and HIV-1 Tat in combination are responsible for HHV-8 replication and induction of lytic cycle proteins (K8.1), but, neither HHV-6 nor extracelluar Tat alone can activate KSHV in Transwell system. Meanwhile, intracellular Tat also failed to induce KSHV lytic phase replication. These findings suggest that both HHV-6 and HIV-1 Tat participate in KS by promoting KSHV replication and, hence, increasing KSHV viral load.