SUMMARY: Chromosomally integrated human herpesvirus 6 (ciHHV‐6) is a condition in which the complete HHV‐6 genome is integrated into the host germ line genome and is vertically transmitted in a Mendelian manner. The condition is found in less than 1% of controls in the USA and UK, but has been found at a somewhat higher prevalence in transplant recipients and other patient populations in several small studies. HHV‐6 levels in whole blood that exceed 5.5 log10 copies/ml are strongly suggestive of ciHHV‐6. Monitoring DNA load in plasma and serum is unreliable, both for identifying and for monitoring subjects with ciHHV‐6 due to cell lysis and release of cellular DNA. High HHV‐6 DNA loads associated with ciHHV‐6 can lead to erroneous diagnosis of active infection. Transplant recipients with ciHHV‐6 may be at increased risk for bacterial infection and graft rejection. ciHHV‐6 can be induced to a state of active viral replication in vitro. It is not known whether ciHHV‐6 individuals are put at clinical risk by the use of drugs that have been associated with HHV‐6 reactivation in vivo or in vitro. Nonetheless, we urge careful observation when use of such drugs is indicated in individuals known to have ciHHV‐6. Little is known about whether individuals with ciHHV‐6 develop immune tolerance for viral proteins. Further research is needed to determine the role of ciHHV‐6 in disease. Copyright © 2011 John Wiley & Sons, Ltd.

Shortly after the discovery of human herpesvirus 6 (HHV-6), two distinct variants, HHV-6A and HHV-6B, were identified. In 2012, the International Committee on Taxonomy of Viruses (ICTV) classified HHV-6A and HHV-6B as separate viruses. This review outlines several of the documented epidemiological, biological, and immunological distinctions between HHV-6A and HHV-6B, which support the ICTV classification. The utilization of virus-specific clinical and laboratory assays for distinguishing HHV-6A and HHV-6B is now required for further classification. For clarity in biological and clinical distinctions between HHV-6A and HHV-6B, scientists and physicians are herein urged, where possible, to differentiate carefully between HHV-6A and HHV-6B in all future publications.

Previous research has suggested that human herpesvirus-6 (HHV-6) may integrate into host cell chromosomes and be vertically transmitted in the germ line, but the evidence—primarily fluorescence in situ hybridization (FISH)—is indirect. We sought, first, to definitively test these two hypotheses. Peripheral blood mononuclear cells (PBMCs) were isolated from families in which several members, including at least one parent and child, had unusually high copy numbers of HHV-6 DNA per milliliter of blood. FISH confirmed that HHV-6 DNA colocalized with telomeric regions of one allele on chromosomes 17p13.3, 18q23, and 22q13.3, and that the integration site was identical among members of the same family. Integration of the HHV-6 genome into TTAGGG telomere repeats was confirmed by additional methods and sequencing of the integration site. Partial sequencing of the viral genome identified the same integrated HHV-6A strain within members of families, confirming vertical transmission of the viral genome. We next asked whether HHV-6A infection of naïve cell lines could lead to integration. Following infection of naïve Jjhan and HEK-293 cell lines by HHV-6, the virus integrated into telomeres. Reactivation of integrated HHV-6A virus from individuals’ PBMCs as well as cell lines was successfully accomplished by compounds known to induce latent herpesvirus replication. Finally, no circular episomal forms were detected even by PCR. Taken together, the data suggest that HHV-6 is unique among human herpesviruses: it specifically and efficiently integrates into telomeres of chromosomes during latency rather than forming episomes, and the integrated viral genome is capable of producing virions.

Signal transducers and activators of transcription (STATs) are latent cytoplasmic transcription factors that transduce signals from the cell membrane to the nucleus upon activation by tyrosine phosphorylation. Several protein-tyrosine kinases can induce phosphorylation of STATs in cells, including Janus kinase (JAK) and Src family kinases. One STAT family member, Stat3, is constitutively activated in Src-transformed NIH3T3 cells and is required for cell transformation. However, it is not entirely clear whether Src kinase can phosphorylate Stat3 directly or through another pathway, such as JAK family kinases. To address this question, we investigated the phosphorylation of STATs in baculovirus-infected Sf-9 insect cells in the presence of Src. Our results show that Src can tyrosine-phosphorylate Stat1 and Stat3 but not Stat5 in this system. The phosphorylated Stat1 and Stat3 proteins are functionally activated, as measured by their abilities to specifically bind DNA oligonucleotide probes. In addition, the JAK family member Jak1 efficiently phosphorylates Stat1 but not Stat3 in Sf-9 cells. By contrast, we observe that AG490, a JAK family-selective inhibitor, and dominant negative Jak1 protein can significantly inhibit Stat3-induced DNA binding activity as well as Stat3-mediated gene activation in NIH3T3 cells. Furthermore, wild-type or kinase-inactive platelet-derived growth factor receptor enhances Stat3 activation by v-Src, consistent with the receptor serving a scaffolding function for recruitment and activation of Stat3. Our results demonstrate that Src kinase is capable of activating STATs in Sf-9 insect cells without expression of JAK family members; however, Jak1 and platelet-derived growth factor receptor are required for maximal Stat3 activation by Src kinase in mammalian cells. Based on these findings, we propose a model in which Jak1 serves to recruit Stat3 to a receptor complex with Src kinase, which in turn directly phosphorylates and activates Stat3 in Src-transformed fibroblasts.

Human herpesvirus-6A (HHV-6A) and human herpesvirus-6B (HHV-6B) are two closely related viruses that infect T-cells. Both HHV-6A and HHV-6B possess telomere-like repeats at the terminal regions of their genomes that facilitate latency by integration into the host telomeres, rather than by episome formation. In about 1% of the human population, human herpes virus-6 (HHV-6) integration into germline cells allows the viral genome to be passed down from one generation to the other; this condition is called inherited chromosomally integrated HHV-6 (iciHHV-6). This review will cover the history of HHV-6 and recent works that define the biological differences between HHV-6A and HHV-6B. Additionally, HHV-6 integration and inheritance, the capacity for reactivation and superinfection of iciHHV-6 individuals with a second strain of HHV-6, and the role of hypomethylation of human chromosomes during integration are discussed. Overall, the data suggest that integration of HHV-6 in telomeres represent a unique mechanism of viral latency and offers a novel tool to study not only HHV-6 pathogenesis, but also telomere biology. Paradoxically, the integrated viral genome is often defective especially as seen in iciHHV-6 harboring individuals. Finally, gaps in the field of HHV-6 research are presented and future studies are proposed.

Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8, has been implicated as the causative agent of Kaposi's sarcoma. Retrospective studies show that the risk of development of Kaposi's sarcoma is significantly lower in AIDS patients who received ganciclovir or phosphonoformic acid (PFA) therapy. Therefore, in vitro antiviral drug sensitivity of KSHV was studied. The KSHV genome is a latent episome in lymphoma cells such as the BCBL-1 cell line. Lytic KSHV DNA synthesis is induced by the phorbol ester 12-O-tetradecanoyl phorbol-13-acetate in BCBL-1 cells; this system was used to evaluate the effects of antiviral drugs on KSHV DNA synthesis. Linear (lytic) KSHV DNA synthesis and virus secretion was inhibited in BCBL-1 cell cultures by cidofovir (median inhibitory concentration, 0.05 microM), ganciclovir (5.1 microM) and PFA (97 microM), and by aciclovir (75 microM). Prolonged incubation of BCBL-1 cells with antiviral drugs had no effect on episomal KSHV DNA synthesis. The antiviral drug assay developed shows that KSHV is very sensitive to cidofovir, moderately sensitive to ganciclovir and PFA, and weakly sensitive to aciclovir. Therefore, low doses of cidofovir, or high doses of PFA or ganciclovir could suppress clinical reactivation of KSHV. Antiviral drugs did not inhibit episomal virus DNA synthesis, suggesting that the latent form of viral DNA is replicated by host DNA polymerases. Consequently, no benefit can be expected from antiviral drugs in KSHV-positive B-cell lymphomas or during latency.