In vitro inhibition of human herpesvirus-6 by phosphonoformate

Selective Activity of Various Nucleoside and Nucleotide Analogues against Human Herpesvirus 6 and 7

We have developed a new sensitive enzyme immunoassay (EIA) and MTT (tetrazolium salt) assay for screening compounds against two variants of human herpesvirus 6 (HHV-6A, HHV-6B) and human herpesvirus 7 (HHV-7) and evaluated the anti-HHV-6 and HHV-7 activity of a series of anti-herpesvirus compounds and acyclic nucleoside phosphonate analogues. The results indicate that the pattern of activity of these compounds against these betaherpesviruses is similar to that for human cytomegalovirus (HCMV). The highest potency and selectivity against the two variants of HHV-6 and HHV-7 was demonstrated by S2242 (N7-isomer of 6-deoxy-ganciclovir). Also, ganciclovir (GCV), foscarnet, (phosphonoformic acid; PFA) and the acyclic nucleoside phosphonate analogues such as cidofovir (HPMPC) exhibited selective inhibitory activity against these viruses. Thymidine kinase (TK)-dependent drugs (acyclovir, ACV; brivudin, BVDU; and sorivudine, BVaraU) showed little, if any, activity. These results suggest a structural homology of the DNA polymerase and a lack of TK gene among these three betaherpesviruses (HHV-6, HHV-7 and HCMV). The finding that HHV-7 was highly sensitive to GCV also suggests that HHV-7 may have an HCMV-UL97-homologue gene for the phosphorylation of GCV. The present EIA method is more rapid and sensitive than the previously reported procedures and could be useful for the large-scale screening of compounds against HHV-6 and HHV-7.

CT and MRI findings of human herpesvirus 6-associated encephalopathy: comparison with findings of herpes simplex virus encephalitis

OBJECTIVE

It is important to differentiate human herpesvirus 6 (HHV-6)-associated encephalopathy from herpes simplex encephalitis (HSE). Although these conditions are similar with regard to involvement of the mesial temporal lobe, HSE is sensitive to acyclovir but HHV-6 encephalopathy is not. We compared the imaging findings of the two conditions.

MATERIALS AND METHODS

We encountered eight cases of HHV-6 encephalopathy and nine cases of HSE. We divided an observation time into early, middle, and late periods defined as 0-2, 3-30, and more than 30 days from the onset of neurologic symptoms. Differences between HHV-6 encephalopathy and HSE on CT scans in the early period and in distribution and temporal changes in the affected regions on MR images in the three periods were analyzed.

RESULTS

At MRI in the early and middle periods, all eight patients with HHV-6 encephalopathy had exclusive involvement of the mesial temporal lobes, and all nine patients with HSE had involvement of both the mesial temporal lobes and the extratemporal regions (p < 0.01). Among patients who underwent head MRI, six of six with HHV-6 encephalopathy but none of six with HSE had resolution of high signal intensity on T2-weighted and FLAIR images (p < 0.01). Among patients who underwent head CT in the early period, none of the four with HHV-6 encephalopathy and six of the seven with HSE had abnormal findings, including parenchymal swelling, decreased attenuation of affected regions, and abnormal gyral enhancement (p < 0.05).

CONCLUSION

Serial MRI showed transient abnormal signal intensity in the mesial temporal lobes in patients with HHV-6 encephalopathy but persistent abnormal signal intensity in both the mesial temporal lobes and the extratemporal regions in patients with HSE. CT in the early period showed no abnormality in patients with HHV-6 encephalopathy but definite abnormal findings in patients with HSE. These differences may be useful in the differential diagnosis of the two conditions.

Thrombotic microangiopathy (TMA) and stroke due to human herpesvirus-6 (HHV-6) reactivation in an adult receiving high-dose melphalan with autologous peripheral stem cell transplantation

We report an adult autologous stem cell transplant (ASCT) patient who developed transplant-associated thrombotic microangiopathy (TMA) due to human herpesvirus-6 (HHV-6) reactivation. A 58-year-old female with Stage IIIA IgGkappa multiple myeloma received a melphalan (200 mg/m2) ASCT with discharge home after resolution of ASCT-related toxicities. She presented on D+20 with dyspnea, rash, and fever to 105 degrees F, followed by worsening dyspnea, hypotension, and capillary leak. Mental status (MS) changes were noted on D+23, but head CT and EEG were unremarkable. On D+29, a generalized seizure occurred with decline in platelet count and haptoglobin. TMA was noted on peripheral blood smear and therapeutic plasma exchange (TPE) was initiated on D+31. Lumbar puncture (LP) revealed CSF protein 74 mg/dL and white blood count 7,000/mm3 with 74% lymphocytosis. TPE was continued without improvement in her MS or thrombocytopenia despite improvement in microangiopathy. An MRI of the brain showed a left hippocampus abnormality, and an EEG was consistent with encephalopathy. Serum polymerase chain regimen (PCR) was negative for CMV, HSV1, and HSV2 but was strongly positive for HHV-6. Repeat LP protein was 597 mg/dL. Foscarnet was initiated, and cerebrospinal fluid (CSF) PCR for HHV-6 revealed 1,400 DNA copies/mL. Her MS greatly improved within 48 hr of antiviral therapy, serum HHV-6 became negative, and TPE was tapered without recurrence of her TMA. TMA with HHV-6 reactivation is likely an underdiagnosed entity. Given its fulminant course and favorable response to therapy, HHV-6 reactivation should be considered a potential etiology in patients with TMA after ASCT.

Newer viral encephalitides

BACKGROUND

Viral encephalitis occurs in epidemic settings or is sporadic. New encephalitis patterns reflect the roles that biologic reservoirs and vectors play in determining virus-human interactions. "New" viral encephalitis can also result from human host modifications that increase susceptibility to neuroinvasive viral infection.

REVIEW SUMMARY

Three human viruses, Nipah virus, Human Herpesvirus-6, and West Nile virus, present examples of how "new" viral encephalitides emerge in a specific geographic region or clinical setting. Nipah virus encephalitis emerged after the molecular evolution of a new zoonotic viral genus within the Paramyxovirinae family. Human herpesvirus-6 encephalitis has emerged in the immune suppressed human host harboring this ubiquitous but typically benign herpesvirus. West Nile virus encephalitis has emerged in the Western hemisphere after apparent abrupt translocation of this mosquito-borne virus to a distant geographic region with immunologically naive avian and human hosts.

CONCLUSION

While the clinical features of these viral encephalitides are somewhat distinct, they each emerged as the result of human-derived factors that altered the biologic dynamic between humans and their viral pathogens.

Human Herpesvirus-6: A Short Review of Its Biological Behavior

HHV-6 shows a widespread distribution with life-long persistence. The virus is frequently reactivated, yet remains clinically inapparent unless the patient is immunodeficient in some way. Even then, HHV-6 reactivation may simply enhance the pathogenicity of other viruses or existing autoimmune disorders rather than becoming a pathogen itself. Future clinical studies need to focus on such indirect viral influences mediated through molecular mimicry and interference with cell receptor expression, and cytokine and chemokine network regulation. Nevertheless, such disturbances may afford therapeutic intervention to disrupt herpesvirus interference and improve certain disease processes. There are only a few diseases for which an immediate causal relationship to HHV-6 infection has been suggested.

Flow cytometric evaluation of antiviral agents against human herpesvirus 6

Antiviral activities of acyclovir (9-[(2-hydroxyethoxy) methyl] guanine, ACV), penciclovir (9-[4-hydroxy-3-(hydroxymethyl) butyl] guanine, PCV), ganciclovir ([9-(1,3-dihydroxy-2-propoxy) methyl] guanine, GCV), and foscarnet (phosphonoformic acid, PFA) were determined against Human Herpesvirus 6 (HHV-6) by flow cytometric technique. The technique is based on the detection of gp116 antigen expression in virus infected cells. Susceptibility was defined in terms of drug concentration which reduced the number of cells expressing HHV-6 gp116 antigen with a mean fluorescent intensity (MFI) by 50% as compared to virus infected untreated cells. GCV was found to be most effective against HHV-6 followed by PFA, PCV and ACV. For HHV-6A, the mean 50% inhibitory concentrations (IC50) of GCV and PFA were found to be 3.4 microM and 34.7 microM respectively, whereas the IC50 of ACV and PCV were found to be 53.7 microM and 37.9 microM respectively. For HHV-6B, the IC50 of GCV and PFA were found to be 5.7 microM and 71.4 microM respectively, whereas the IC50 of ACV and PCV were found to be 119.0 microM and 77.8 microM respectively. Flow cytometry is a valuable technique for the evaluation of antiviral compounds against viruses including HHV-6.

Human herpesviruses-6, -7 and -8 in organ transplant recipients

The newer herpesviruses are being increasingly recognized as significant opportunistic pathogens in organ transplant recipients. Published data support the role of human herpesvirus-6 as a potential cause of encephalitis and bone marrow suppression in transplant setting. An association of human herpesvirus-6 with fungal infections and cytomegalovirus infection has also been documented. Human herpesvirus-7 also appears to be an immunomodulatory agent and may facilitate the pathogenicity of cytomegalovirus. Unlike human herpesviruses -6 and -7, human herpesvirus -8 is not ubiquitous; its seroprevalence exhibits wide geographic variation. Human herpesvirus-8 has been causally associated with post-transplant Kaposi's sarcoma. The complete spectrum of pathogenicity and ultimately the effective prophylaxis and management of these viruses has yet to be fully elucidated.

Comparison of antiviral compounds against human herpesvirus 6 and 7

Four classes of antiviral compounds were evaluated for inhibitory activity against two variants of human herpesvirus 6 (HHV-6A and -6B) and human herpesvirus 7 (HHV-7). These included: (1) a pyrophosphate analog, phosphonoformic acid (PFA); (2) beta-guanine analogs, 9-(2-hydroxyethoxymethyl)guanine (acyclovir or ACV), 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine (ganciclovir or GCV) and 9-(4-hydroxy-3-hydroxy-3-hydroxymethylbutylyl)guanine (penciclovir or PCV); (3) acyclic nucleoside phosphonates, (S)-1-[(3-hydroxy-2-phosphonylmethoxy)propyl]cytosine [cidofovir or (S)-HPMPC] and its cyclic derivative (S)-cyclic-HPMPC (cHPMPC), 9-[[2-hydroxy-1-phosphonomethoxy)ethoxy]methyl]guanine (HPMEMG) and 9-[(2-phosphonylmethoxy)ethyl]-2,6-diaminopurine (PMEDAP), and the seven other related compounds; and (4) a series of benzimidazole ribonucleosides, including 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl)benzimidazole (BDCRB). End-point inhibitory concentration (EPC) and 50% effective inhibitory concentration (EC50) values were determined by a dot-blot antigen detection method in cord blood mononuclear cells infected with HHV-6A, HHV-6B or HHV-7 at a multiplicity of infection of 0.004 CCID50/cell. (S)-HPMPC and cHPMPC had an EC50 value of approximately 0.3 microg/ml for HHV-6A, 1.2 microg/ml for HHV-6B and 3.0 microg/ml for HHV-7. These compounds were the most active of those tested against each virus. The EC50 value of GCV for HHV-6A was 0.65 microg/ml, 1.33 microg/ml for HHV-6B, and >7 microg/ml for HHV-7. The EC50 values of ACV and PCV were approximately 6-8 microg/ml for HHV-6A, 16-24 microg/ml for HHV-6B and 121-128 microg/ml for HHV-7. These drugs were the least active. The sensitivity of HHV-7 to the guanine analogs was different from HHV-6, suggesting a difference in selectivity of specific viral enzymes.

Human herpesvirus-6 infection in liver transplant recipients: documentation of pathogenicity

BACKGROUND

The new herpesvirus, human herpesvirus-6 (HHV-6), is able to cause clinical illness after transplantation; however, the pathogenic potential and the clinical features of HHV-6 have not been defined in liver transplant recipients.

METHODS

We report the first cases of invasive and symptomatic infection due to HHV-6 in liver transplant recipients.

RESULTS

HHV-6 infection occurred in four liver transplant recipients at a median of 50 days after transplant (range 17-90 days). Severe cytopenia was observed in all patients; leukopenia (with median leukocyte count of 1400/mm3) was the most commonly effected bone marrow lineage. One of the four patients had interstitial pneumonitis due to HHV-6. No other virus (e.g., cytomegalovirus) or another pathogen was detected in the lungs, blood, or bone marrow in any of the above patients.

CONCLUSIONS

Our data suggest that HHV-6 can be a pathogen in liver transplant recipients; idiopathic bone marrow suppression is its predominant clinical sequelae. Recognition of HHV-6 infection is clinically pertinent because HHV-6 is potentially treatable with the currently available antiviral agents.

Fulminant demyelinating encephalomyelitis associated with productive HHV-6 infection in an immunocompetent adult

Human herpesvirus 6 (HHV-6), the etiologic agent of roseola in young children, has been reported to be detectable in the brain of many neurologically normal adults, although regional localization to plaques of multiple sclerosis has also been demonstrated. Large amounts of this virus were present in multifocal demyelinating white matter lesions of fulminant encephalomyelitis with seizures in a 21-year-old woman with normal immune parameters. Brain biopsy after 3 weeks of neurologic deterioration revealed a viral etiology by light and electron microscopy; the virus was identified as HHV-6 by immunohistochemistry and by polymerase chain reaction (PCR) amplification in biopsy and autopsy specimens.

Human herpesvirus 6

Human herpesvirus 6 variant A (HHV-6A) and human herpesvirus 6 variant B (HHV-6B) are two closely related yet distinct viruses. These visuses belong to the Roseolovirus genus of the betaherpesvirus subfamily; they are most closely related to human herpesvirus 7 and then to human cytomegalovirus. Over 95% of people older than 2 years of age are seropositive for either or both HHV-6 variants, and current serologic methods are incapable of discriminating infection with one variant from infection with the other. HHV-6A has not been etiologically linked to any human disease, but such an association will probably be found soon. HHV-6B is the etiologic agent of the common childhood illness exanthem subitum (roseola infantum or sixth disease) and related febrile illnesses. These viruses are frequently active and associated with illness in immunocompromised patients and may play a role in the etiology of Hodgkin's disease and other malignancies. HHV-6 is a commensal inhabitant of brains; various neurologic manifestations, including convulsions and encephalitis, can occur during primary HHV-6 infection or in immunocompromised patients. HHV-6 and distribution in the central nervous system are altered in patients with multiple sclerosis; the significance of this is under investigation.

A method for detection of HHV-6 antigens and its use for evaluating antiviral drugs

A simple and reproducible method for detection of human herpesvirus 6 (HHV-6) antigens was developed using a dot blot assay in order to assess virus titer and to evaluate the effect of antiviral drugs against HHV-6. The titer of virus stocks obtained by the dot blot assay was the same as that determined by an immunofluorescence assay (IFA). This method was then applied to evaluate the effect of several antiviral drugs against HHV-6, including phosphonoformic acid (PFA), 9-(2-hydroxyethoxymethyl)guanine (ACV), 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine (DHPG) and (S)-1-[(3-hydroxy-2-phosphonylmethoxy)propyl]cytosine [(S)-HPMPC]. The end-point concentrations (EPC, which was determined visually) of DHPG and (S)-HPMPC were approximately 1 microgram/ml. These drugs were more effective than the others which had EPCs of approximately 16 micrograms/ml each. The EPC values of four drugs were almost similar to EC90 values determined by measuring density of each dot blot. Thus, the EPC values can be utilized to determine the efficacy of these drugs in the inhibition of HHV-6 replication. The block in virus replication was not due to toxic effect of these drugs on cord blood mononuclear cells (CBMCs). These results suggest that a dot blot method which detects HHV-6 antigens can be useful for titrating virus yield and evaluating antiviral drugs against HHV-6 replication.

Antiviral activity of selected acyclic nucleoside analogues against human herpesvirus 6

Human herpesvirus 6 (HHV-6) was examined in vitro for its sensitivity to a broad range of nucleoside analogues, including acyclovir (ACV), ganciclovir (GCV), penciclovir (PCV), buciclovir (BCV), brivudin (BVDU), the N7-isomer of 6-deoxyganciclovir (S2242), foscarnet (phosphonoformic acid, PFA), and several acyclic nucleoside phosphonate (ANP) analogues such as (S)-HPMPA, (S)-HPMPC, PMEA and PMEDAP. Antiviral efficacy was monitored microscopically by the inhibitory effect of the compounds on HHV-6-induced cytopathic effect in human T-lymphoblastoid HSB-2 cells. In addition, a newly developed immunofluorescence/flow cytometric assay (FACS) was used to determine HHV-6-specific antigen expression. A close correlation was observed between the antiviral data obtained by the microscopic assay and the flow cytometric assay. Marked antiviral efficacy was noted for S2242, PFA and the ANP analogues (S)-HPMPA, (S)-HPMPC, (S)-cHPMPC, (S)-3-deaza-HPMPA, (S)-3-deaza-cHPMPA, (S)-HPMPG and (R)-HPMPG. Also, PMEA and PMEDAP proved highly active against HHV-6 infection, whereas (S)-FPMPA and (R)-PMPDAP were inactive. ACV was only slightly protective against HHV-6, and no activity was found for GCV, PCV, BCV and BVDU. Overall, the efficacy of the nucleoside analogues against HHV-6 appeared to correlate with their efficacy against human cytomegalovirus (HCMV).

Quantitative PCR for human herpesviruses 6 and 7

A quantitative PCR assay for the detection of human herpesvirus 6 (HHV-6) (variants A and B) and HHV-7 DNAs in clinical samples was developed. The assay uses a nonhomologous internal standard (IS) for each virus that is coamplified with the wild-type target sequence in the same vial and with the same pair of primers. This method allows for a correction of the variability of efficiency of the PCR technique. A standard curve is constructed for each experiment by coamplification of known quantities of the cloned HHV-6 or HHV-7 target templates with the respective IS. Absolute quantitation of the test samples is then achieved by determining the viral target/IS ratio of the hybridization signals of the amplification products and plotting this value against the standard curve. Using this assay, we quantitated the amount of HHV-6 or HHV-7 DNA in infected cell cultures and demonstrated an inhibitory effect of phosphonoformic acid on the replication of HHV-6 and HHV-7 in vitro. As the first clinical application of this procedure, we performed preliminary measurements of the loads of HHV-6 and HHV-7 in lymph nodes from patients with Hodgkin's disease and AIDS. Application of this quantitative PCR method should be helpful for elucidating the pathogenic roles of HHV-6 and HHV-7.

Human herpesvirus 6

HHV-6, the first T-lymphotropic human herpesvirus, is an important novel human pathogen. It is the cause of exanthem subitum in infants and may act as an opportunistic agent in immunocompromised patients. Moreover, several lines of clinical and experimental evidence suggest that HHV-6 may accelerate the progression of HIV infection. Progress in the study of HHV-6 has been rapid, in part as a consequence of the strong current interest in human lymphotropic viruses and their relationship with the immune system. Nonetheless, the full spectrum of diseases linked to this agent is still unknown (Table 2) and animal models of infection have not yet been exploited. The next few years will be crucial for a complete understanding of the potential role of HHV-6 in human disease.

Human herpesvirus 6 in AIDS

Multiple lines of clinical and experimental evidence suggest that human herpesvirus 6 (HHV-6) may act as an accelerating factor in the natural history of human immunodeficiency virus (HIV) infection. Although, in common with HIV, HHV-6 has a primary tropism for CD4+ T cells, its potential effects on the immune system are broader. For instance, HHV-6 can also infect and kill CD8+ T cells, natural killer cells and mononuclear phagocytes. Here, Paolo Lusso and Robert Gallo suggest that understanding the immunopathogenic role of HHV-6 in the course of HIV infection may shed new light on the complex mechanisms of disease progression in AIDS.

Prevalence of human herpesvirus 6 variants A and B in patients with chronic fatigue syndrome

Peripheral blood mononuclear cells collected from 13 patients with chronic fatigue syndrome and 13 healthy controls were analyzed for the presence of human herpesvirus 6 (HHV-6) DNA by variant-specific polymerase chain reaction and dot blot hybridization. HHV-6 DNA was detected in 7 of 13 (53%) patients, and of those 7 patients, 4 were positive for HHV-6 variant A DNA and 3 were for variant B. No HHV-6 DNA was detected in the controls. Serum antibody titers to the late antigen and antibody prevalence to the early antigen of HHV-6 were significantly higher in the patient group. These results suggest active replication of HHV-6 in patients with chronic fatigue syndrome.

In vitro susceptibility of Macaca nemestrina to human herpesvirus 6: a potential animal model of coinfection with primate immunodeficiency viruses

Human herpesvirus 6 (HHV-6), a lymphotropic herpesvirus, has been suggested as a potential cofactor in the acquired immunodeficiency syndrome (AIDS). Previous studies indicate that HHV-6 has a restricted range of susceptible species. In this study, we tested the in vitro susceptibility to HHV-6 of Macaca nemestrina (pig-tailed macaque), a species that has been found to be infectable by human immunodeficiency virus type I in vivo and that develops an AIDS-like syndrome following simian immunodeficiency virus (SIV) infection. Two different HHV-6 isolates (HHV-6GS and HHV-6BA), belonging to the two major HHV-6 variants (A and B, respectively), were employed. Both viruses induced a productive and cytopathic infection in phytohemagglutinin-stimulated peripheral blood T lymphocytes from M. nemestrina. In contrast, only HHV-6BA (variant B) was able to replicate in lymphocytes from Macaca mulatta (rhesus macaque). Moreover, HHV-6GS and SIVsmE660 productively coinfected individual M. nemestrina lymphocytes, resulting in increased levels of SIV replication. Genetic sequences of HHV-6 were not amplified by polymerase chain reaction from peripheral blood mononuclear cells of several adult M. nemestrina, suggesting that these animals, unlike humans, are not commonly infected by HHV-6, or a related virus. Thus, M. nemestrina may represent an optimal animal model system to investigate the in vivo interactions between HHV-6 and the primate immunodeficiency viruses.

Sero-epidemiological investigations on human herpesvirus 6 (HHV-6) infections using a newly developed early antigen assay

Monoclonal antibodies (MAbs) were developed against immunodominant HHV-6 (GS isolate) late and early proteins. The major late protein was identified as a probable glycoprotein with a molecular weight of approximately 110 kDa (gp 110). Immunoblotting of the early antigen yielded proteins of 41 and 38 kDa (p41/38). The MAb to the early protein reacted with cells infected with 14 different HHV-6 isolates. In contrast, the MAb against the late protein reacted with only 10 of these isolates, indicating that there was strain variation in this glycoprotein. The percentage of antibody-positive sera reactive with gp110 in the ELISA ranged from 56% to 96% among the different serum donor categories. In contrast, only 10-30% of the sera were positive for antibodies to p41/38 with the exception of sera from patients with African Burkitt's lymphoma (ABL) and Hodgkin's disease (HD). These antibody patterns denote the presence of active HHV-6 replication in patients with ABL and HD.

Inhibition of human herpesvirus 6 replication by 9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine (2HM-HBG) and other antiviral compounds

The in vitro susceptibilities of human herpesvirus 6 to foscarnet; the guanosine analogs acyclovir, ganciclovir, and two isomers of 9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine; and the thymidine analogs 3'-azido-3'-deoxythymidine and 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil were investigated. All compounds except 3'-azido-3'-deoxythymidine and 1-beta-D-arabinofuranosyl-E-5-(2-bromovinyl)uracil inhibited human herpesvirus 6 replication. The highest in vitro selectivity was obtained for 9-[4-hydroxy-2-(hydroxymethyl)butyl]guanine.

The replication of viral and cellular DNA in human herpesvirus 6-infected cells

Human herpesvirus 6 (HHV-6) is a newly identified lymphotropic herpesvirus. We have analyzed viral and host DNA replication in peripheral blood lymphocytes infected in the absence of drugs or infected in the presence of phosphonoacetic acid (PAA) or acyclovir (ACV). The results revealed the following: (i) Infection with HHV-6 resulted in the shutoff of host DNA replication. (ii) PAA at concentrations of 100 and 300 micrograms/ml significantly reduced virus replication. The drug inhibited viral DNA replication, whereas host cell DNA replication was not affected. This strongly suggests that HHV-6 encodes a PAA sensitive viral DNA polymerase. (iii) ACV at 20 microM did not interfere with virus production and virus spread. ACV at 100 microM only partly interfered with virus replication, whereas at 400 microM the block was more complete. Viral DNA replication was not affected by ACV at 20 microM. However, approximately 60 and 85% inhibition in viral DNA replication was observed in the presence of 100 and 400 microM of ACV. (iv) Assays for viral thymidine kinase (TK) revealed no significant increase in TK activity, whereas increased TK activity was noted following infection of the same peripheral blood lymphocytes with herpes simplex virus. Thus, either HHV-6 does not encode a tk enzyme which can phosphorylate ACV or the inefficient block may reflect lower sensitivity of the HHV-6 DNA polymerase to the drug.

Effect of primary Epstein-Barr virus infection on human herpesvirus 6, cytomegalovirus, and measles virus immunoglobulin G titers

Immunoglobulin G antibody titers to human herpesvirus 6 (HHV-6), measles virus, and cytomegalovirus (CMV) were examined in serum samples from 31 patients with Epstein-Barr virus (EBV)-induced infectious mononucleosis (IM). Sera were drawn sequentially from the same patients less than or equal to 7 days until 3 years after onset of IM. In seropositive patients, there was a significant decrease with time after IM of the immunoglobulin G titers to the three viruses in the majority of patients; HHV-6 IgG titers decreased in 80%, measles virus IgG titers decreased in 75%, and CMV IgG titers decreased in 67%. Four patients contracted CMV infection during the observation period after IM. In these, HHV-6 IgG titers increased, while EBV and measles virus IgG titers remained essentially stationary. Polyclonal B-cell stimulation during IM is suggested to augment antiviral titers in general, but the increases of HHV-6 IgG titers during EBV and CMV infections may also be due to selective stimulation of memory B cells by related antigens or to reactivation of HHV-6 during infection with these herpesviruses.

Demonstration of the human herpesvirus 6-induced DNA polymerase and DNase

Infection of HSB-2 cells with human herpesvirus 6 (HHV6) results in an approximately 51-fold increase in the level of DNA polymerase activity and a 4.44-fold increase in the level of DNase activity when compared to mock-infected cells. There was no increase in thymidine kinase, uracil-DNA glycosylase, or deoxyuridine triphosphate nucleotidohydrolase activities in the infected cells. The HHV6-induced DNase and DNA polymerase activities could be distinguished from their normal cellular counterparts on the basis of immunological specificities and in the case of DNA polymerase based upon differences in electrophoretic migration. Serological studies also demonstrated reactivity of the antisera not only for HHV6 but also for Epstein-Barr virus.

In vitro sensitivity of human herpesvirus-6 to antiviral drugs

We studied the sensitivity of human herpesvirus-6 (HHV-6) to 4 antiviral drugs known to be effective in the treatment of infections with other human herpesviruses and human immunodeficiency virus. HHV-6 was grown in peripheral blood lymphocytes, and virus multiplication was quantified by evaluation of the cytopathic effect by molecular hybridization and indirect immunofluorescence assay. The 50% and 90% inhibitory concentrations (IC50 and IC90) were determined for each drug. The results obtained by the 3 different quantification techniques were found to correlate, and enabled us to conclude that HHV-6 replication was readily inhibited by foscarnet or ganciclovir. In contrast, inhibition of HHV-6 replication was observed only at high concentrations of acyclovir, and was not detected at the tested concentrations of zidovudine.