Susceptibility of human herpesvirus 6 to antiviral compounds by flow cytometry analysis

[Resistance to antiviral drugs in human viruses from the subfamily Betaherpesvirinae]

The review provides information on the mechanisms of the emergence of resistance to antiviral drugs in human viruses from the subfamily Betaherpesvirinae. Data on the principles of action of antiviral drugs and their characteristics are given. The occurrence rates of viral resistance in various groups of patients is described and information about the possible consequences of the emergence of resistance to antiviral drugs is given. Information is provided regarding the virus genes in which mutations occur that lead to viral resistance, and a list of such mutations that have described so far is given. The significance of the study of mutations leading to the resistance of the virus to antiviral drugs for medical practice is discussed.

Determination of infectious hepatitis B virus particles by an end-point dilution assay identifies a novel class of inhibitors

The quantification of infectious virus particles is fundamental to perform in vitro virology studies. To determine the number of hepatitis B virus (HBV) genome-containing particles in vitro, the genome equivalents (GEq) are measured using quantitative PCR (qPCR). However, in addition to infectious virions, HBV DNA-containing, non-infectious HBV particles are also produced in vitro, which can lead to an over-estimation of the number of infectious HBV particles when analyzed by qPCR. Here, we establish an end-point dilution assay that can precisely determine the number of infectious HBV particles. The cell-based HBV infection assay uses a 384-well plate format and enables the calculation of the 50% tissue culture infective dose (TCID₅₀) in a semi-automated manner. Cell culture-derived HBV (HBVcc), produced by either stable HBV-replicating cells (HepAD38) or HBV-infected HepG2-NTCP cells, as well as patient-derived HBV sera were serially diluted and used to infect naïve target cells. Applying the end-point dilution assay, we infected HepG2-NTCP cells with PEG precipitated HBV derived from HepAD38-and HepG2-NTCPsec+ cell supernatants, calculated the TCID₅₀/mL, converted to plaque-forming units (PFUs), and generated the specific infectivity (ratio of PFU/GEq). As a result, a TCID₅₀/mL of 7.22 × 10⁶ and 2.16 × 10⁶, and the specific infectivity of 1/13,816 and 1/8798 were calculated for HepAD38 and HepG2-NTCPsec+ cell supernatants, respectively. The specific infectivity further increased by approximately 2-fold after removal of non-infectious "naked" particles by immunoprecipitation. Purification of HepAD38 cell supernatants by heparin columns increased the TCID₅₀/mL and specific infectivity by 18- and 15-fold, respectively. Interestingly, non-purified patient-derived HBV sera from two individuals had a specific infectivity of 1/88 and 1/3609. After converting TCID₅₀ to multiplicity of infection (MOI) values, we inoculated HepG2-NTCP cells with HBVcc based on GEq or MOI values and demonstrated that MOI-based infection leads to more reproducible infection rates. Furthermore, the assay was validated using serially diluted lamivudine, an HBV replication inhibitor, inhibiting HBV DNA secretion and infectious viral progeny by approx. 56- and 470-fold, respectively. Interestingly, we identified dexmedetomidine (DMM), an alpha-2 adrenergic agonist, inhibiting the secretion of infectious viral progeny by approx. 6-fold, without interfering in the secretion of HBV DNA. Taken together, we developed an assay that is suitable for the standard quantification of infectious HBV particles. We identified DMM as a novel inhibitor that exclusively interferes with the secretion of infectious HBV particles without affecting the secretion of HBV genomes. This end-point dilution assay enables the precise determination of the number of infectious HBV particles, assessment of the specific infectivity and stability of HBV particles, and identification of novel classes of HBV inhibitors.

[Significance of PCR detection of HHV6 in gastro biopsy on the course of diarrhea in patients with severe diarrhea after allogeneic hematopoietic stem cell transplantation]

目的

探讨肠黏膜人疱疹病毒6型（HHV6）检测在异基因造血干细胞移植（allo-HSCT）术后重度腹泻患者中的意义。

方法

回顾性分析2015年2月至2016年8月于北京大学血液病研究所行allo-HSCT后出现重度腹泻并行肠镜检查及肠黏膜活检的患者资料；HHV6、CMV、EBV检测采用RT-PCR方法；肠黏膜病理检查也包括免疫组化方法检测CMV早期抗原、CMV晚期抗原，原位杂交检测EBV。

结果

共有45例患者纳入研究，其中21例（46.7%）肠黏膜活检HHV6阳性，包括男13例，女8例，中位年龄29（14~54）岁；肠黏膜HHV6阳性与阴性组患者血CMV阳性检出率差异无统计学意义（76.2%对87.5%，P>0.05），但EBV血症发生率前者（6/21，28.6%）明显高于后者（1/24，4.2%）（P=0.028）；两组患者在腹泻发生的时间、次数及量方面差异无统计学意义（P值均>0.05）。45例患者中44例接受了膦甲酸钠和（或）更昔洛韦抗病毒治疗，抗病毒治疗并没有影响腹泻的进程。

结论

肠黏膜HHV6阳性在allo-HSCT术后重症腹泻患者中发生率高，未发现其对于治疗及预后有临床意义。肠黏膜HHV6阳性并不能证明HHV6在肠道的感染。

Human Herpesvirus-6 corneal Endotheliitis after intravitreal injection of Ranibizumab

BACKGROUND

To report the first case of human herpesvirus-6 (HHV-6) corneal endotheliitis that developed after intravitreal ranibizumab injections.

CASE PRESENTATION

A 63-year-old man with a medical history of diabetes and systemic steroid treatment for bullous pemphigoid had been receiving intravitreal injections of ranibizumab in the left eye for 2 years according to a Pro Re Nata treatment regimen for macular edema associated with branch retinal vein occlusion. Twenty days after the last injection, the patient presented with pain and decreased visual acuity in his left eye. His best corrected visual acuity in the left eye was 2/200, and intraocular pressure was 45 mmHg with edema of the central stromal cornea, mild conjunctival injection, intermediate keratic precipitates, and mild anterior chamber reaction. HHV-6 DNA was detected in the aqueous humor using multiplex strip polymerase chain reaction, and it was identified as variant A, HHV-6A. A diagnosis of HHV-6A-associated corneal endotheliitis was made. Oral valganciclovir and topical ganciclovir therapy was initiated with good resolution of all symptoms and signs.

CONCLUSIONS

HHV-6A can be a possible complication of intravitreal ranibizumab therapy. To the best of our knowledge, this is the first reported case of HHV-6A corneal endotheliitis following intravitreal ranibizumab injection.

Fatal outcome after reactivation of inherited chromosomally integrated HHV-6A (iciHHV-6A) transmitted through liver transplantation

HHV-6A and HHV-6B are found as inherited and chromosomally integrated forms (iciHHV-6A and -6B) into all germinal and somatic cells and vertically transmitted in a Mendelian manner in about 1% of the population. They were occasionally shown to be horizontally transmitted through hematopoietic stem cell transplantation. Here, we present a clinical case of horizontal transmission of iciHHV-6A from donor to recipient through liver transplantation. Molecular analysis performed on three viral genes (7.2 kb) in the recipient and donor samples supports transmission of iciHHV-6A from the graft. Transmission was followed by reactivation, with high viral loads in several compartments. The infection was uncontrollable, leading to severe disease and death, despite antiviral treatments and the absence of resistance mutations. This case highlights the fact that physicians should be aware of the possible horizontal transmission of iciHHV-6 and its consequences in case of reactivation in immunocompromised patients.

Effects of Prophylactic Foscarnet on Human Herpesvirus-6 Reactivation and Encephalitis in Cord Blood Transplant Recipients: A Prospective Multicenter Trial with an Historical Control Group

Cord blood transplantation (CBT) is a distinct risk factor for human herpesvirus-6 (HHV-6) reactivation and HHV-6 encephalitis. In a prospective multicenter trial we investigated the effects of prophylactic foscarnet (90 mg/kg i.v. infusion from days 7 to 27 after CBT) on the occurrence of HHV-6 reactivation, HHV-6 encephalitis, and acute graft-versus-host disease (aGVHD) in CBT recipients. Between 2014 and 2016, 57 patients were included in a foscarnet-prophylaxis group. Outcomes were compared with an historical control group who received CBT between 2010 and 2014 (standard-treatment group, n = 63). The cumulative incidence of high-level HHV-6 reactivation, defined as plasma HHV-6 DNA ≥ 10⁴ copies/mL, at 60 days after CBT was significantly lower in the foscarnet-prophylaxis group than in the standard-treatment group (18.3% versus 57.3%, P < .001). Multivariate analysis revealed that myeloablative preconditioning and standard treatment were significant risk factors for high-level HHV-6 reactivation. The cumulative incidence of HHV-6 encephalitis at 60 days after CBT was not different between the groups (foscarnet-prophylaxis group, 12.4%; standard-treatment group, 4.9%; P = .14). The cumulative incidences of grades II to IV and grades III to IV aGVHD at 60 days after CBT were not different between the groups (grades II to IV aGVHD: foscarnet-prophylaxis group, 42.0%; standard-treatment group, 40.5%; P = .96; grades III to IV aGVHD: foscarnet-prophylaxis group, 14.5%; standard-treatment group, 14.5%; P = 1.00). In the setting of this study foscarnet significantly suppressed systemic HHV-6 reactivation in CBT recipients but failed to prevent the development of HHV-6 encephalitis. Suppression of HHV-6 reactivation by foscarnet did not show any effects against the incidence of aGVHD.

HHV-6 in liver transplantation: A literature review

Human herpesvirus 6 (HHV-6A and HHV-6B) can cause primary infection or reactivate from latency in liver transplant recipients, which can result in a variety of clinical syndromes, including fever, hepatitis, encephalitis and higher rates of graft dysfunction as well as indirect effects including increased risks of mortality, CMV disease, hepatitis C progression and greater fibrosis scores. Although HHV-6 infection is currently diagnosed by quantifying viral DNA in plasma or blood, biopsy to demonstrate histopathological effects of HHV-6 remains the gold standard for diagnosis of end-organ disease. HHV-6 reactivation may be restricted to the infected organ with no evidence of active infection in the blood. HHV-6 infections in liver transplant patients are mostly asymptomatic, but clinically significant tissue-invasive infections have been treated successfully with ganciclovir, foscarnet or cidofovir. Inherited chromosomally integrated HHV-6 (ciHHV-6), in either the recipient or the donor organ, may create confusion about systemic HHV-6 infection. Recipients with inherited ciHHV-6 may have an increased risk of opportunistic infection and graft rejection. This article reviews the current scientific data on the clinical effects, risk factors, pathogenesis, diagnosis and treatment of HHV-6 infections in liver transplant recipients.

Evaluation of a method to measure HHV-6B infection in vitro based on cell size

BACKGROUND

Human herpesvirus 6 (HHV-6A and HHV-6B) infection of cell cultures can be measured by different methods, including immunofluorescence microscopy, flow cytometry, or quantification of virus DNA by qPCR. These methods are reliable and sensitive but require long processing times and can be costly. Another method used in the field relies on the identification of enlarged cells in the culture; this method requires little sample processing and is relatively fast. However, visual inspection of cell cultures can be subjective and it can be difficult to establish clear criteria to decide if a cell is enlarged. To overcome these issues, we explored a method to monitor HHV-6B infections based on the systematic and objective measurement of the size of cells using an imaging-based automated cell counter.

RESULTS

The size of cells in non-infected and HHV-6B-infected cultures was measured at different times post-infection. The relatively narrow size distribution observed for non-infected cultures contrasted with the broader distributions observed in infected cultures. The average size of cultures shifted towards higher values after infection, and the differences were significant for cultures infected with relatively high doses of virus and/or screened at longer times post-infection. Correlation analysis showed that the trend observed for average size was similar to the trend observed for two other methods to measure infection: amount of virus DNA in supernatant and the percentage of cells expressing a viral antigen. In order to determine the performance of the size-based method in differentiating non-infected and infected cells, receiver operating characteristic (ROC) curves were used to analyze the data. Analysis using size of individual cells showed a moderate performance in detecting infected cells (area under the curve (AUC) ~ 0.80-0.87), while analysis using the average size of cells showed a very good performance in detecting infected cultures (AUC ~ 0.99).

CONCLUSIONS

The size-based method proved to be useful in monitoring HHV-6B infections for cultures where a substantial fraction of cells were infected and when monitored at longer times post-infection, with the advantage of being relatively fast and easy. It is a convenient method for monitoring virus production in-vitro and bulk infection of cells.

Successful treatment and FDG-PET/CT monitoring of HHV-6 encephalitis in a non-neutropenic patient: case report and literature review

Human herpesvirus (HHV)-6 reactivation is associated with severe forms of encephalitis among patients undergoing hematopoietic stem cell transplantation. Cases in non-neutropenic patients are uncommon. The efficacy of ganciclovir and other compounds that are used for the treatment of HHV-6 encephalitis remains suboptimal and linked to toxicity. Valganciclovir, the oral prodrug of ganciclovir, could be practical to treat outpatients, but it is not commonly used for severe cases. We report a case of HHV-6 encephalitis in a non-neutropenic patient successfully treated with valganciclovir and undergoing therapeutic drug monitoring in plasma and in the cerebrospinal fluid. Resolution of infectious foci was documented by cerebral MRI and F18-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT). A review of the literature on HHV-6 encephalitis is also reported.

Clinical characteristics and outcome of human herpesvirus-6 encephalitis after allogeneic hematopoietic stem cell transplantation

In this retrospective analysis using the Transplant Registry Unified Management Program, we identified 145 patients with human herpesvirus (HHV)-6 encephalitis among 6593 recipients. The cumulative incidences of HHV-6 encephalitis at 100 days after transplantation in all patients, recipients of bone marrow or PBSCs and recipients of cord blood were 2.3%, 1.6% and 5.0%, respectively. Risk factors identified in multivariate analysis were male sex, type of transplanted cells (relative risk in cord blood transplantation, 11.09, P<0.001; relative risk in transplantation from HLA-mismatched unrelated donor, 9.48, P<0.001; vs transplantation from HLA-matched related donor) and GvHD prophylaxis by calcineurin inhibitor alone. At 100 days after transplantation, the overall survival rate was 58.3% and 80.5% among patients with and without HHV-6 encephalitis, respectively (P<0.001). Neuropsychological sequelae remained in 57% of 121 evaluated patients. With both foscarnet and ganciclovir, full-dose therapy (foscarnet ⩾180 mg/kg, ganciclovir ⩾10 mg/kg) was associated with better response rate (foscarnet, 93% vs 74%, P=0.044; ganciclovir, 84% vs 58%, P=0.047). HHV-6 encephalitis is not rare not only in cord blood transplant recipients but also in recipients of HLA-mismatched unrelated donors. In this study, development of HHV-6 encephalitis was associated with a poor survival rate, and neurological sequelae remained in many patients.

Viral load and ganciclovir (GCV) concentration in cerebrospinal fluid of patients successfully treated with GCV or valGCV for human herpesvirus 6 encephalitis/myelitis following umbilical cord blood transplantation

We describe successful treatment of 3 cases of human herpesvirus 6 (HHV-6) encephalitis/myelitis following cord blood transplantation (CBT). Ganciclovir (GCV) (10 mg/kg/day) reduced HHV-6 load to undetectable levels in cerebrospinal fluid (CSF). Early dose reduction in the presence of HHV-6 detectable in CSF resulted in an increased HHV-6 load. GCV was capably shifted to valganciclovir (VGCV) with an almost equivalent concentration. GCV/VGCV may be effective for HHV-6 encephalitis/myelitis after CBT, although HHV-6 load in CSF should be monitored.

Analysis of ganciclovir-resistant human herpesvirus 6B clinical isolates using quenching probe PCR methodology

Quenching probe PCR (QP-PCR) analysis was used to determine the frequency of ganciclovir (GCV) resistance among clinical isolates of human herpesvirus 6B (HHV-6B) obtained from patients with primary viral infection and viral reactivation. Forty-two HHV-6B clinical isolates were repeatedly recovered from 15 hematopoietic stem cell transplant (HSCT) recipients, and 20 isolates were recovered from 20 exanthem subitum (ES) patients. Of the 15 HSCT recipients, 9 received GCV during the observation period; however, none of the ES patients were treated with GCV. Two established laboratory strains, Z29 and HST, were used as standards in this study. Regions 1 and 2 of the U69 gene of all of the clinical isolates demonstrated the same melting temperature as regions 1 and 2 of the Z29 strain. For region 3, the melting temperatures of all clinical isolates fell between the melting temperature of the plasmid containing the A462D single nucleotide polymorphism (SNP) and the melting temperature of the Z29 strain, and the melting temperatures profiles of all clinical isolates were similar to the melting temperature profile of the Japanese HST strain. As expected, none of the 20 clinical isolates recovered from the ES patients and the 14 isolates recovered from the HSCT recipients who did not receive GCV treatment carried the six known SNPs associated with GCV resistance. Interestingly, these six SNPs were not detected in the 28 clinical isolates recovered from the 9 HSCT recipients who received GCV. Additional sequence analysis of the U69 gene from the 15 representative isolates from the 15 HSCT recipients identified other SNPs. These SNPs were identical to those identified in the HST strain. Therefore, the rate of emergence of GCV-resistant HHV-6B strains appears to be relatively low, even in HSCT recipients treated with GCV.

The development of new therapies for human herpesvirus 6

Human herpesvirus 6 (HHV-6) infections are typically mild and in rare cases can result in encephalitis. A common theme among all the herpesviruses, however, is the reactivation upon immune suppression. HHV-6 commonly reactivates in transplant recipients. No therapies are approved currently for the treatment of these infections, although small studies and individual case reports have reported intermittent success with drugs such as cidofovir, ganciclovir, and foscarnet. In addition to the current experimental therapies, many other compounds have been reported to inhibit HHV-6 in cell culture with varying degrees of efficacy. Recent advances in the development of new small molecule inhibitors of HHV-6 will be reviewed with regard to their efficacy and spectrum of antiviral activity. The potential for new therapies for HHV-6 infections will also be discussed, and they will likely arise from efforts to develop broad spectrum antiviral therapies for DNA viruses.

Animal models for human herpesvirus 6 infection

Human herpesvirus (HHV)-6A and HHV-6B are two enveloped DNA viruses of β-herpesvirus family, infecting over 90% of the population and associated with several diseases, including exanthema subitum (for HHV-6B), multiple sclerosis and encephalitis, particularly in immunosuppressed patients. Animal models are highly important to better understand the pathogenesis of viral infections. Naturally developed neutralizing antibodies to HHV-6 or a related virus were found in different species of monkeys, suggesting their susceptibility to HHV-6 infection. Both HHV-6 DNA and infectious virus were detected in experimentally infected Cynomolgus and African green monkeys, although most animals remained clinically asymptomatic. Furthermore, HHV-6A infection was shown to accelerate the progression of AIDS (acquired immunodeficiency syndrome) in macaques and to lead to the development of neurological symptoms in the marmoset model. Humanized SCID (severe combined immunodeficiency) mice efficiently replicated HHV-6 and were also susceptible to coinfection with HHV-6 and HIV-1 (human immunodeficiency virus 1). As CD46 was identified as a receptor for HHV-6, transgenic mice expressing human CD46 may present a potentially interesting model for study certain aspects of HHV-6 infection and neuroinflammation.

Human herpesvirus type 6 DNAemia and infection following allogeneic stem cell transplantation with a focus on long-term outcome

Cases of human herpesvirus type 6 (HHV-6) infection and disease were retrospectively analysed in a cohort of 97 allogeneic haematopoietic stem cell transplantation (allo-SCT) patients in Gothenburg, Sweden (1997-2001). Altogether 54 of 97 (56%) patients were tested for HHV-6. HHV-6 DNAemia was detected in 15 of the tested patients at a median of 76 (range 24-387) days after SCT. Nine of these patients were treated against HHV-6 infection and disease for a total of 11 treatment episodes. The morbidity associated with HHV-6 DNAemia following allo-SCT was in most cases moderate. The overall 1-y survival among the patients with HHV-6 DNAemia was 11/15 (73%) and the 5-y survival was 10/15 (67%), which was not significantly different from the whole cohort.

Evaluation of Epstein-Barr virus, human herpesvirus 6 (HHV-6), and HHV-8 antiviral drug susceptibilities by use of real-time-PCR-based assays

Evaluation of candidate antiviral drugs against Epstein-Barr virus (EBV), human herpesvirus 6 (HHV-6), and HHV-8 is hampered by the lack of convenient laboratory assays. We developed real-time quantitative PCR assays performed on supernatants of lymphoma cell lines and determined the 50% inhibitory concentrations (IC50s) of nucleoside, nucleotide, and pyrophosphate analogues against these herpesviruses.

Human herpesvirus 6 in biopsies from patients with gastrointestinal symptoms after allogeneic stem cell transplantation

Gastrointestinal complications are frequent after allogeneic stem cell transplantation (allo-SCT). Main differential diagnoses are graft-versus-host disease (GvHD) and viral infections. In this retrospective analysis, we included 50 patients with severe vomiting or diarrhea in the first year after allo-SCT. One hundred two biopsies obtained by colonoscopy or endoscopy of the upper gastrointestinal tract were analysed by conventional histology for signs of GvHD and by qualitative polymerase chain reaction (PCR) for viral DNA of human herpesvirus 6 (HHV-6) and other virus of the herpes family. DNA of HHV-6 was detected in 38 of 75 initial samples (51%) and in 19 of 27 follow-up biopsies (70%). In the initial samples (n = 75), HHV-6 DNA was detected in 20/37 (54%) biopsies in the presence of GvHD compared to 18/38 (47%) biopsies without signs of GvHD. At the time of the first endoscopic investigation, most patients received antiviral prophylaxis with aciclovir. None of the follow-up biopsies was HHV-6 DNA negative after antiviral treatment with aciclovir, foscarnet or ganciclovir. By univariate analysis, no risk factor for HHV-6 detection could be demonstrated. In this cohort of patients with severe gastrointestinal complications, there was no significant difference in the overall survival between patients with or without HHV-6 DNA detection in the gastrointestinal tract. In summary, the detection of HHV-6 DNA had no impact on overall survival. Moreover, antiviral therapy against HHV-6 was without effect. Thus, positive PCR results in GI tract samples do not necessarily reflect reactivation of HHV-6. Further studies are needed to define the significance of HHV-6 for GI tract symptoms after allo-SCT.

Ganciclovir-resistant human herpesvirus-6 encephalitis in a liver transplant patient: a case report

Human herpesvirus-6 (HHV-6) was initially isolated in 1986 from patients with lymphoproliferative disorders (Ablashi et al. 1987). Since that time, two genetically distinct variants were sequenced, HHV-6A and HHV-6B (Ablashi and Balachandran 1991). Both variants have been linked with neurologic disease (Crawford et al. 2007). HHV-6 encephalitis has been well described in literature, typically presenting with confusion, coma, seizure, and headache. The majority of HHV-6 encephalitis has been limited to post-transplant recipients (Singh and Paterson 2000). Encephalitis due to HHV-6 infection has been reported in two liver transplant recipients (Massih and Razonable 2009 and Montejo et al. 2002). Although there has been in vitro studies regarding the potential resistance patterns for HHV-6 virus, there has been only one clinic case report supporting these findings (Isegawa et al. 2009). We describe the first case of ganciclovir-resistant HHV-6 encephalitis in a post-liver transplant patient.

Testing the susceptibility of human herpesviruses to antivirals

Herpesviruses cause chronic lifelong infections in humans and may cause life-threatening diseases in immunosuppressed patients. Antiviral drugs targeted to viral DNA polymerase, such as acyclovir, penciclovir, ganciclovir, foscarnet and cidofovir, are currently available and have been proven to be efficient against clinical symptoms of herpesvirus infections. The resistance of herpesviruses to these drugs is associated with specific mutations of viral genes encoding either DNA polymerase or enzymes phosphorylating nucleoside analogs. Resistance is detected and characterized by means of specific susceptibility assays, which can be classified as phenotypic, genetic and functional. These tests are used both to investigate novel antiviral compounds and look for the emergence of resistant viruses in treated patients in case of clinical failure. Although susceptibility assays are often time consuming and present some limitations regarding the interpretation of their results, their use in the monitoring of antiherpetic treatments should be promoted and improved, in parallel to the development of novel efficient drugs.

Human herpesvirus 6 infections after liver transplantation

Human herpesvirus 6 (HHV-6) infections occur in > 95% of humans. Primary infection, which occurs in early childhood as an asymptomatic illness or manifested clinically as roseola infantum, leads to a state of subclinical viral persistence and latency. Reactivation of latent HHV-6 is common after liver transplantation, possibly induced and facilitated by allograft rejection and immunosuppressive therapy. Since the vast majority of humans harbor the virus in a latent state, HHV-6 infections after liver transplantation are believed to be mostly due to endogenous reactivation or superinfection (reactivation in the transplanted organ). In a minority of cases, however, primary HHV-6 infection may occur when an HHV-6 negative individual receives a liver allograft from an HHV-6 positive donor. The vast majority of documented HHV-6 infections after liver transplantation are asymptomatic. In a minority of cases, HHV-6 has been implicated as a cause of febrile illness with rash and myelosuppression, hepatitis, pneumonitis, and encephalitis after liver transplantation. In addition, HHV-6 has been associated with a variety of indirect effects such as allograft rejection, and increased predisposition and severity of other infections including cytomegalovirus (CMV), hepatitis C virus, and opportunistic fungi. Because of the uncommon nature of the clinical illnesses directly attributed to HHV-6, there is currently no recommended HHV-6-specific approach to prevention. However, ganciclovir and valganciclovir, which are primarily intended for the prevention of CMV disease, are also active against HHV-6 and may prevent its reactivation after transplantation. The treatment of established HHV-6 disease is usually with intravenous ganciclovir, cidofovir, or foscarnet, complemented by reduction in the degree of immunosuppression. This article reviews the current advances in the pathogenesis, clinical diagnosis, and therapeutic modalities against HHV6 in the setting of liver transplantation.

Length variability of telomeric repeat sequences of human herpesvirus 6 DNA

The telomeric repeat sequences (TRS) located near both ends of human herpesvirus 6 (HHV-6) genome are unique structures of unknown function among human herpesviruses. The goal of the present study was to investigate the variability of TRS copy number among different laboratory strains and HHV-6-infected clinical specimens regarding the two variants A and B of HHV-6. DNA obtained from infected cells was submitted to a PCR assay designed to amplify the part of genome containing TRS specifically either for HHV-6A or HHV-6B. Amplicons were analyzed by electrophoresis on agarose gel with ethidium bromide staining and nucleotide sequencing. The number of TRS copies was highly variable among the distinct laboratory strains and clinical specimens studied, ranging from 15 up to more than 180. However, this number was constant for a given strain after serial propagation in cell cultures as well as in different samples from the same subject. This permitted to detect a mixed infection with two distinct strains of HHV-6A within the same patient. The PCR-based analysis of HHV-6 TRS has a limited sensitivity but is highly specific, which provides the opportunity to include it in the set of molecular tools dedicated to the study of HHV-6 epidemiology.

Human herpesvirus-6 infections and infection-preventative measures in transplant recipients

Like the other herpesviruses, human herpesvirus (HHV)-6 is capable of persisting in the host after primary infection and can reactivate under immunosuppressed conditions. In stem cell transplantation recipients, HHV-6 reactivation is observed in 40–50% of patients and is linked to various clinical manifestations, including rash, delayed platelet engraftment and encephalopathy. HHV-6 reactivation is also relatively common in solid organ transplantation recipients, and is associated with graft dysfunction and subsequent development of human cytomegalovirus disease. In particular, HHV-6 has been recognized as a life-threatening pathogen for the development of encephalopathy after stem cell transplantation. Ganciclovir, foscarnet and cidofovir are effective against HHV-6 in vitro. However, success rates are not high for patients who have developed encephalopathy. Plasma HHV-6 DNA-guided pre-emptive approaches have not been successful owing to the dynamic kinetics of HHV-6 reactivation. Establishment of optimal strategies is needed to prevent HHV-6-associated complications, in particular encephalopathy.

Different Mutations in the HHV-6 DNA Polymerase Gene Accounting for Resistance to Foscarnet

Background

Human herpesvirus 6 (HHV-6), which is closely related to human cytomegalovirus, is sensitive to foscarnet (PFA). Up to now, the resistance of HHV-6 to PFA has not been investigated.

Objectives

The goal of the study was to isolate and characterize PFA-resistant HHV-6 mutants in order to determine the mechanisms of resistance to the drug.

Methods

PFA-resistant viruses, isolated in MT4 cell culture under increasing concentrations of PFA, were characterized phenotypically and genotypically. The mutations identified in the HHV-6 DNA polymerase gene were evaluated in a functional assay using recombinant mutated forms of the enzyme, and their effect on protein structure was analysed in a three-dimensional model derived from available structures of DNA polymerases.

Results

Two mutants were selected and were 8- and 15-fold more resistant to PFA than the wild-type strain. Four amino acid changes were detected in the HHV-6 DNA polymerase in association with PFA resistance: T435R, H507Y, C525S, located in the δC conserved domain, and F292S. Either alone or in combination, these substitutions significantly decreased the inhibitory effect of PFA at the level of the polymerase, as measured by the incorporation of radiolabeled nucleotides in a DNA elongation assay. In the three-dimensional model of HHV-6 DNA polymerase structure the four changes were not located within the putative catalytic site, but they might induce either a disturbance of local conformation or a restricted access of PFA to its target site.

Conclusion

This first characterization of HHV-6 resistance to PFA highlights the role of distinct DNA polymerase gene mutations.

Efficacy of antiviral compounds in human herpesvirus-6-infected glial cells

The beta-herpesvirus human herpesvirus-6 (HHV-6) is becoming increasingly recognized as an important pathogen in immunocompromised patients, particularly in post bone marrow transplant (BMT). Reactivation of latent HHV-6 resulting in encephalitis has been reported in BMT and stem cell transplant (SCT) patients. The development of HHV-6 encephalitis can be a fatal complication, the frequency of which is increasing likely due to improved diagnosis with quantitative polymerase chain reaction (PCR) of cerebrospinal fluid. There are currently no antiviral compounds approved for HHV-6, nor have any controlled clinical trials been conducted. The frequency and severity of HHV-6 encephalitis in both immunocompetent and immunocompromised patients necessitates studies on the usefulness of currently available anti-viral compounds. The authors compared the antiviral efficacy of four drugs currently used for cytomegalovirus (CMV) infection, a beta-herpesvirus sharing homology with HHV-6. In HHV-6A- and HHV-6B-infected T cells, acyclovir, ganciclovir, foscarnet, and cidofovir exhibited antiviral activity consistent with that published in other studies. In HHV-6-infected human astrocytes (U251), however, only foscarnet and cidofovir exhibited antiviral activity and this effect was restricted to infection with HHV-6 variant A. In pathological brain sections from patients with neurological disorders such as multiple sclerosis and epilepsy, HHV-6 has been localized to glial cells. Determination of antiviral activity in human glial fibrillary acidic protein (GFAP)-positive astrocytes of currently used antiviral compounds is essential for potential treatment of HHV-6 and neurological disorders. Our data highlight the necessity for further study of antiviral compound in HHV-6-infected glial cells as well as the development of more selective compounds for HHV-6.

Human herpesvirus-6 encephalitis after unrelated cord blood transplantation

Here we describe 2 patients with acute leukemia in whom human herpesvirus-6 (HHV-6) encephalitis developed after cord blood transplantation. In patients 1 and 2, generalized seizure and coma developed on day 62 and day 15, respectively, after cord blood transplantation, which failed to engraft in patient 1. Magnetic resonance imaging (MRI) of patient 1's brain showed low-intensity signals at the gyri of the bilateral lateral lobes on T1-weighted images and high-intensity signals on T2-weighted images. MRI of patient 2's brain showed high-intensity signals in bilateral white matter on T2-weighted images and on fluid-attenuated inversion recovery (FLAIR) images. Cerebrospinal fluid examination revealed an increased protein level with pleocytosis in patient 1 and a normal protein level without pleocytosis in patient 2. Polymerase chain reaction analysis detected HHV-6 DNA in the cerebrospinal fluid of both patients. Patient 1 recovered after administration of gancyclovir for 3 weeks. However, she again suffered from encephalitis after discontinuation of gancyclovir, and died of sepsis. Patient 2 died from an anoxic brain caused by generalized seizure. When neurological symptoms and signs appear in hematopoietic stem cell transplantation recipients, we should consider HHV-6 encephalitis and promptly and empirically treat them with gancyclovir or foscarnet.

Clinical Outcomes of Human Herpesvirus 6 Reactivation after Hematopoietic Stem Cell Transplantation

BACKGROUND

Although human herpesvirus 6 (HHV-6) is known to reactivate during hematopoietic stem cell transplantation (HSCT), the clinical significance of this finding is controversial.

METHODS

We used a quantitative PCR test for HHV-6 to assay plasma samples prospectively collected from a cohort of 110 allogeneic HSCT recipients to evaluate the clinical effects of HHV-6 infection. A retrospective review of medical records was performed to determine clinical end points.

RESULTS

HHV-6 reactivation occurred in 52 (47%) of the 110 subjects. Factors that increased the risk of subsequent HHV-6 reactivation were hematologic malignancy that occurred at a time other than the first remission (adjusted P = .002), a mismatch in the sexes of donor and recipient (adjusted P=.05), younger age (adjusted P = .01), and the receipt of glucocorticoids (adjusted P = .06). HHV-6 reactivation was associated with subsequent all-cause mortality (adjusted hazard ration [HR], 2.9; 95% confidence interval [CI], 1.1-7.5), grade 3-4 graft-versus-host disease (GVHD) (adjusted HR, 4.9; 95% CI, 1.5-16), a lower probability of monocyte engraftment (adjusted HR, 0.42; 95% CI; 0.22-0.80), a lower probability of platelet engraftment (adjusted HR, 0.47; 95% CI, 0.21-1.1; P = .05) and a higher platelet transfusion requirement (adjusted P = .02). A higher level of HHV-6 DNA was associated with subsequent central nervous system (CNS) dysfunction (HR, 21; 95% CI, 1.8-249).

CONCLUSIONS

HHV-6 reactivation is common after allogeneic HSCT and is associated with subsequent delayed monocyte and platelet engraftment, increased platelet transfusion requirements, all-cause mortality, grade 3-4 GVHD, and CNS dysfunction.

Persistence of DNA in cell cultures may jeopardize the analysis of human herpesvirus 6 dynamics by means of real-time PCR

The use of real-time PCR has been described previously for analysing both the replication kinetics and antiviral susceptibility of human herpesvirus 6 in MT4 cells. It is now reported that viral DNA persists in infected cell culture long after the end of lytic virus replication. Consequently, high levels of DNA may correspond to an absence of infectivity and late readout occurring after the exponential phase of virus growth may lead to misinterpretation of the results of susceptibility assays. These limitations must be borne in mind when using real-time PCR.

Update on human herpesvirus 6 biology, clinical features, and therapy

Human herpesvirus 6 (HHV-6) is a betaherpesvirus that is closely related to human cytomegalovirus. It was discovered in 1986, and HHV-6 literature has expanded considerably in the past 10 years. We here present an up-to-date and complete overview of the recent developments concerning HHV-6 biological features, clinical associations, and therapeutic approaches. HHV-6 gene expression regulation and gene products have been systematically characterized, and the multiple interactions between HHV-6 and the host immune system have been explored. Moreover, the discovery of the cellular receptor for HHV-6, CD46, has shed a new light on HHV-6 cell tropism. Furthermore, the in vitro interactions between HHV-6 and other viruses, particularly human immunodeficiency virus, and their relevance for the in vivo situation are discussed, as well as the transactivating capacities of several HHV-6 proteins. The insight into the clinical spectrum of HHV-6 is still evolving and, apart from being recognized as a major pathogen in transplant recipients (as exemplified by the rising number of prospective clinical studies), its role in central nervous system disease has become increasingly apparent. Finally, we present an overview of therapeutic options for HHV-6 therapy (including modes of action and resistance mechanisms).

Efficacy of methylenecyclopropane analogs of nucleosides against herpesvirus replication in vitro

We have reported previously that purine methylenecyclopropane analogs are potent agents against cytomegaloviruses. In an attempt to extend the activity of these compounds, the 2-amino-6-cyclopropylaminopurine analog, QYL-1064, was selected for further study by modifying the purine 6 substituent. A total of 22 analogs were tested against herpes simplex virus types 1 and 2 (HSV-1, HSV-2), varicella zoster virus (VZV), human cytomegalovirus (HCMV), murine cytomegalovirus (MCMV), Epstein-Barr virus (EBV), human herpesvirus type 6 (HHV-6) and human herpesvirus type 8 (HHV-8). Ten of the analogs had activity against at least one of the viruses tested. One compound had moderate activity against HSV-1 and six had activity against VZV. All but one compound was active against HCMV with a mean EC50 of 2.1 +/- 0.6 microM, compared with a mean EC50 of 3.9 +/- 0.8 microM for ganciclovir. Of special interest was the fact that eight of the ten compounds were active against both HHV-6A and HHV-6B with mean EC50 values of 6.0 +/- 5.2 mciroM and <2.4 +/- 1.5 microM, respectively. Only two compounds had activity against EBV, whereas all but one compound was active against HHV-8 with a mean EC50 of 3.1 +/- 1.7 microM. These results indicate that members of this series of methylenecyclopropane analogs are highly active against HCMV, HHV-6, and HHV-8 but are less active against HSV, VZV, and EBV.

Successful treatment of human herpesvirus 6 encephalomyelitis in immunocompetent patient

We report the case of human herpesvirus 6 (HHV-6) encephalomyelitis in an immunocompetent patient, which was confirmed by viral amplification from cerebrospinal fluid. Cidofovir was used followed by ganciclovir because of an adverse effect to probenecid. The patient recovered. HHV-6 should be recognized as one of the causes of encephalomyelitis.

Potent, selective and cell-mediated inhibition of human herpesvirus 6 at an early stage of viral replication by the non-nucleoside compound CMV423

CMV423 (2-chloro-3-pyridin-3-yl-5,6,7,8-tetrahydroindolizine-1-carboxamide) is a new antiviral agent with potent and selective in vitro activity against the beta-herpesvirus human cytomegalovirus (HCMV), but not against alpha- or gamma-herpesviruses. Here we report that its activity also extends to human herpesvirus 6 (HHV-6) and 7 (HHV-7). When compared in vitro to ganciclovir and foscarnet (the standard drugs recommended for treatment of HHV-6 infections), CMV423 showed a superior selectivity, due to its high activity (antiviral IC(50): 53nM) and low cytotoxicity (CC(50): 144microM), both in continuous cell lines and in CBLCs infected with HHV-6. From mechanistic experiments at the level of viral mRNA and protein expression, we learned that CMV423 targets an event following viral entry but preceding viral DNA replication. Its antiviral action was dependent on the cell line used, implying involvement of a cellular component. When compared to a panel of known protein kinase inhibitors, CMV423 was found to share anti-HHV-6 characteristics with herbimycin A, which affects tyrosine kinase activity through heat shock protein 90 (Hsp90) inhibition. We demonstrated that high concentrations of CMV423 have an inhibitory effect on the total cellular protein tyrosine kinase activity, and that CMV423 and herbimycin A, when combined, act synergistically against HHV-6. The activities of cyclin-dependent kinases, protein kinases A and C, and the HHV-6-encoded pU69 kinase were not affected. We, therefore, conclude that CMV423 exerts its activity against HHV-6 through inhibition of a cellular process that is critical at early stages of viral replication and that may affect protein tyrosine kinase activity.

Real-time PCR as a versatile tool for investigating the susceptibility of human herpesvirus 6 to antiviral agents

A quantitative real-time PCR assay was developed for the determination of antiviral drug susceptibility and growth kinetics of human herpesvirus 6. The susceptibility and fitness of a sensitive strain, HST, and its ganciclovir-resistant derivative, GCVR1, were then characterized, leading us to conclude that the mutations of this latter virus did not alter its fitness significantly.

Determination of antiviral efficacy against lymphotropic herpesviruses utilizing flow cytometry

Epstein-Barr virus (EBV), human herpesvirus type 6 (HHV-6), and human herpesvirus type 8 (HHV-8) comprise a group of lymphotropic herpesviruses which are responsible for a wide range of diseases, including lymphoproliferative disorders and tumors. We have developed several flow cytometric assay (FACS) systems to evaluate antiviral efficacy against EBV, HHV-6 and HHV-8. Assays using either EBV or HHV-8, members of the gammaherpesvirus subfamily, have shown that while EBV responds well to acyclovir (ACV), HHV-8 was most sensitive to cidofovir (CDV). Since HHV-6 strains are divided into two sub-groups, A and B, we evaluated antiviral efficacy for strains from each group. The group A strain, HHV-6(GS), was inhibited by foscarnet (PFA), CDV and ganciclovir (GCV) in both Sup-T1 and HSB-2 cell lines. HHV-6(Z-29), a representative group B virus, was inhibited by GCV and CDV but not by PFA. Our findings indicate that flow cytometry can be utilized to efficiently evaluate new antiviral agents against lymphotropic herpesviruses and that the results are comparable to those obtained by other methods such as immunofluorescence.

A one-step RT-PCR and a flow cytometry method as two specific tools for direct evaluation of human herpesvirus-6 replication

In order to confirm the occurrence of active Human herpesvirus-6 (HHV-6) infection, two optimal procedures were developed to detect directly replicating virus. MT4 cells and peripheral blood mononuclear cells (PBMCs) infected with two different strains (HST and a patient strain GUI) were used. The first method consisted of a one-step reverse transcription PCR amplifying a part of the late alternatively spliced U100 gene which encode the gp 82-105 viral glycoprotein. Two extraction methods and two RT-PCR kits were evaluated, leading to the selection of TaKaRa mRNA selective PCR kit. The second procedure consisted in a flow cytometry method to analyze the expression of two late viral HHV-6 antigens using 7C7 and 10G6 monoclonal antibodies. Four fixation permeabilization procedures were compared and the preparation of cells with paraformaldehyde (PFA) 4% was found to be optimal. Evaluation of these methods was then realized during a sequential culture of HST strain on MT4 cells. This kinetic study confirmed that Mabs recognized late antigens and demonstrate that the U100 gene splicing starts at a late stage of multiplication whereas unspliced forms are detectable earlier in the cycle.

Role of the human herpesvirus 6 u69-encoded kinase in the phosphorylation of ganciclovir

The human herpesvirus 6 (HHV-6) U69 gene product (pU69) is the presumed functional homolog of the human cytomegalovirus (HCMV) UL97-encoded kinase (pUL97), which converts ganciclovir to its monophosphate metabolite in HCMV-infected cells. It has been reported that insertion of U69 into baculovirus confers sensitivity to ganciclovir in insect cells (J Virol 73:3284-3291, 1999). Our metabolic studies in HHV-6-infected human T-lymphoblast cells indicated that the efficiency of ganciclovir phosphorylation induced by HHV-6 was relatively poor. Recombinant vaccinia viruses (rVVs), expressing high levels of pU69 from two HHV-6 strains (representing the A and B variant), were constructed and used to compare the ganciclovir-phosphorylating capacity of pU69 and pUL97 in human cells. Metabolic studies with [8-(3)H]ganciclovir showed that ganciclovir was phosphorylated in human cells infected with pU69-expressing rVVs, although the levels of phosphorylated ganciclovir metabolites were approximately 10-fold lower than those observed with pUL97. We also demonstrated that pU69, like pUL97, is expressed as a nuclear protein. Our results indicate that the limited phosphorylation of ganciclovir by pU69 may contribute to its modest antiviral activity against HHV-6 in certain cell systems.

Development of a real-time polymerase chain reaction assay for the diagnosis of human herpesvirus-6 infection and application to bone marrow transplant patients

A quantitative real-time PCR assay was developed for human herpesvirus-6 (HHV-6) genome based on TaqMan technology. After choosing a region of interest into the U65-U66 genes of HHV-6 genome, its nucleotide sequence was determined among four HHV-6 strains (one variant A and three variants B) to exclude a variability of sensitivity due to interstrain sequence differences. A plasmid containing HHV-6 target sequences identical to those of reference type viruses was constructed with the aim of standardisation. This HHV-6 genomic quantitation assay has a threshold sensitivity of ten copy equivalents (EqCop) per reaction. In order to test the feasibility of this assay directly on human samples, the technique was applied to the quantitation of HHV-6 genome in 30 blood samples from healthy subjects as well as 31 blood samples and three samples of cerebrospinal fluid (CSF) from 21 bone marrow transplant (BMT) recipients and four patients with a haematological disease but not treated by bone marrow transplantation. HHV-6 load ranged between 0.00015 and 0.0008 equivalent DNA copy number (EqCop) per 100 peripheral blood mononuclear cells (PBMCs) in healthy subjects whereas it ranged from <10 to 7500 EqCop per 100 PBMCs, and from <10 to 415,820 EqCop per 100 microl of whole CSF in patients. The efficacy of treatment with antiherpetic drug was associated with a decrease of the viral load in the CSF of one patient. This method leads to relevant results in term of range of quantitation, sensitivity, and safety against contamination by amplicons, and might constitute a useful tool for the follow-up of BMT recipients particularly in the presence of antiherpetic therapy.

Effect of antivirals on human herpesvirus 6 replication in hematopoietic stem cell transplant recipients

Human herpesvirus 6 (HHV-6) appears to cause central nervous system (CNS) syndromes, especially in hematopoietic stem cell transplant (HSCT) recipients. We reviewed our experience with HHV-6-associated CNS disease to evaluate both the clinical and virological presentation and response to antiviral therapy. A search of our virology database from January 1998 through June 2000 identified 11 HSCT recipients who had HHV-6 DNA detected by polymerase chain reaction in cerebrospinal fluid (CSF); 8 of whom had CNS dysfunction without another clear etiology identified. HHV-6 levels in serum and CSF were evaluated before and after ganciclovir and/or foscarnet therapy. Median log HHV-6 CSF levels appeared to decrease over time concurrent with antiviral therapy (serum level, 2.0 vs. 0 copies/mL [P=.38]; CSF level, 4.4 vs. 2.0 copies/mL [P=.13], sign test). Our data suggests that HHV-6 may cause moderate to severe CNS disease after HSC transplantation. Prospective studies are needed to define the spectrum of HHV-6-associated disease and to determine whether antiviral therapy offers clinical benefit.

Antiviral agents active against human herpesviruses HHV-6, HHV-7 and HHV-8

A series of antiviral compounds were examined for their activity against human herpesvirus type 6 (HHV-6), type 7 (HHV-7) and type 8 (HHV-8). They were selected either because they are already approved for clinical use in the treatment of herpesvirus infections (acyclovir, valaciclovir, penciclovir, famciclovir, ganciclovir, brivudin, foscarnet and cidofovir) or have demonstrated marked activity against herpesviruses (lobucavir, H2G, A-5021, D/L-cyclohexenyl G and S2242). In view of their host cell specificity, different cells and assays had to be used for determining antiviral activity against these three viruses. The most potent compounds with the highest antiviral selectivity index were: (i) for HHV-6; foscarnet, S2242, A-5021 and cidofovir; (ii) for HHV-7; S2242, cidofovir and foscarnet; and (iii) for HHV-8; S2242, cidofovir and ganciclovir. As mycophenolic acid has been shown to enhance significantly the activity of acyclic guanosine analogues (such as acyclovir, penciclovir and ganciclovir) in vitro against HSV-1, HSV-2, VZV and HCMV, it would seem worth evaluating whether mycophenolic acid also potentiates the activity of these acyclic guanosine analogues against HHV-6, -7 and -8.

Selection of the same mutation in the U69 protein kinase gene of human herpesvirus-6 after prolonged exposure to ganciclovir in vitro and in vivo

After serial passage in the presence of increasing concentrations of ganciclovir (GCV) in vitro, a human herpesvirus-6 (HHV-6) mutant exhibiting a decreased sensitivity to the drug was isolated. Analysis of drug susceptibility showed that the IC(50) of this mutant was 24-, 52- and 3-fold higher than that of the wild-type (wt) IC(50) in the case of GCV, cidofovir and foscarnet, respectively. Genotypic analysis showed two single nucleotide changes as compared to the wild-type: an A-->G substitution of the U69 protein kinase (PK) gene resulted in an M(318)V amino acid substitution and the other change, located in the C-terminal part of the U38 gene, resulted in an A(961)V amino acid substitution within the DNA polymerase. The M(318)V change was located within the consensus sequence DISPMN of the putative catalytic domain VI of the PK. This change was homologous to the M(460)V and M(460)I changes that had been reported previously within the consensus sequence DITPMN of the human cytomegalovirus (HCMV) UL97 PK and associated with the resistance of HCMV to GCV. The M(318)V change was also detected by PCR in HHV-6-infected PBMCs from an AIDS patient who had been treated with GCV for a long period of time and exhibited a clinically GCV-resistant HCMV infection. These findings provide strong circumstantial evidence that the M(318)V change of the PK gene is associated with resistance to GCV and raise the question of cross resistance to this drug among different betaherpesviruses.