Human Herpesvirus 8 Replication Research Articles

Plasmablastic microlymphoma occurring in human herpesvirus 8 (HHV‐8)‐positive multicentric Castleman's disease and featuring a follicular growth pattern

Plasmablastic microlymphoma (PML) is defined as the accumulation of monotypic but polyclonal plasmablasts in lymphoid tissues involved in human herpes virus 8 (HHV-8)-positive multicentric Castleman's disease (MCD). So far, the nature of this very rare condition remains poorly determined. In this study, we describe a human immunodeficiency virus (HIV)-seropositive patient who developed a PML in the setting of HHV-8-positive MCD. In contrast to the cases previously reported, most of the plasmablasts in our patient were localized within the germinal center (GC) of lymphoid follicles. These plasmablasts expressed the multiple myeloma-1/interferon regulatory factor-4 (MUM1/IRF4) protein as well as IgMlambda in a monotypic fashion. They did not show any immunoreactivity with antibodies directed against Pax-5, CD20, CD79a, CD10, CD30, CD23, CD138, epithelial membrane antigen (EMA) or BCL-6. These cells exhibited a high proliferation rate, expressed the HHV-8 latent nuclear antigen-1, and secreted the HHV-8 viral homologue of human interleukin-6. Polymerase chain reaction analysis did not demonstrate any clonal rearrangement of the genes coding for the heavy chain of the immunoglobulin. Moreover, no Epstein-Barr virus (EBV) RNA transcript could be found, using in situ hybridization. The present case illustrates that PML may arise within the GC of lymphoid follicles in the absence of EBV coinfection. In our opinion, PML occurring in MCD likely represents a variant of HHV-8-positive MCD in which lytic HHV-8 replication is particularly prominent, due to a local or systemic immune imbalance.

P1434 Incidence of double positive amplifications from immunocompetent patients screened for active human herpesvirus replication

P1434 Incidence of double positive amplifications from immunocompetent patients screened for active human herpesvirus replication

HHV-6 and the immune system: mechanisms of immunomodulation and viral escape

Clinical and experimental evidence indicates that human herpesvirus 6 (HHV-6) can interfere with the function of the host immune system through a variety of mechanisms. Both HHV-6A and B can infect, either productively or nonproductively, several types of immune cells. The primary target for HHV-6 replication, both in vitro and in vivo, is the CD4 + T lymphocyte, a pivotal cell in the generation of humoral and cell-mediated adaptive immune responses. HHV-6A, but not B, also replicates in various cytotoxic effector cells, such as CD8 + T cells, γδ T cells and natural killer cells. In professional antigen-presenting cells like macrophages and dendritic cells, HHV-6 infection is typically nonproductive; yet, it induces dramatic functional abnormalities, including a selective suppression of IL-12, a critical cytokine in the generation of Th1-polarized antiviral immune responses. This and other immunomodulatory effects seem to be mediated by the engagement of the primary HHV-6 receptor, CD46. Moreover, HHV-6 infection results in a generalized loss of CD46 expression in lymphoid tissue, which may lead to an aberrant activation of autologous complement. Additional mechanisms of immunomodulation by HHV-6 include alterations in cell surface receptor expression and cytokine/chemokine production. HHV-6 can also modulate influence responses through the expression of virally-encoded homologs of chemokines and chemokine receptors. By modulating specific antiviral immune responses, HHV-6 can facilitate its own spread and persistence in vivo, as well as enhance the pathogenic effects of other agents, such as human immunodeficiency virus.

Pregnancy and Human Herpesvirus 8 Reactivation in Human Immunodeficiency Virus Type 1-Infected Women

To investigate the impact of pregnancy on human herpesvirus 8 (HHV-8) reactivation in human immunodeficiency virus type 1 (HIV-1)-infected women, the HHV-8 DNA presence and load were analyzed in peripheral blood mononuclear cells (PBMCs) and cervicovaginal secretions (CVSs) from 15 pregnant women coinfected with HIV-1 and HHV-8. HHV-8 detection was analyzed in relation to anti-HHV-8 antibodies and HIV-1-related parameters. Nucleotide sequence analysis of an ORFK1 hypervariable region of the HHV-8 strains was performed. HHV-8 was detected in maternal PBMCs (5/15 women) from the second trimester and in CVSs (5/15 women) mainly from the third trimester. The HHV-8 load significantly increased late in pregnancy in both maternal compartments and was associated with a significant increase in HIV-1 shedding in the genital tract. Antilytic antibodies were significantly more common in HHV-8 DNA-positive women. An elevated HHV-8 load was found in the PBMCs of an infant born to a mother with large amounts of HHV-8 in both compartments at delivery. Different ORFK1 subtypes were found in maternal samples, whereas the same subtype was identified in the mother-child pair. These data suggest that pregnancy may induce HHV-8 replication in HIV-1-infected women. An augmented HHV-8 load may, in turn, influence mother-to-child transmission, since one of the HIV-1-infected mothers with HHV-8 reactivation transmitted her ORFK1 subtype to the infant, who showed a high level of HHV-8 viremia indicative of a primary infection. This finding documents for the first time the perinatal transmission of a specific HHV-8 subtype. Vertical transmission may thus play a role in HHV-8 spread also in areas of subendemicity among HIV-1-infected women.

Human herpesvirus 6 (HHV-6) U94/REP protein inhibits betaherpesvirus replication

Human herpesvirus 6 (HHV-6) is the only human herpesvirus encoding U94/rep, homologue to the parvovirus non-structural gene rep68/78. Results to date suggest that HHV-6 U94/rep might regulate viral gene expression and have a role in viral latency. To determine the effect of U94/REP upon viral replication, the protein was produced. The purified U94/REP retained the characteristic immunological features. It was internalized and localized in the nucleus of human cells, showing marked inhibitory activity on the replication of HHV-6 (both variants A and B). The effect of U94/REP was dose-dependent and sensitive to treatment with single-stranded but not double-stranded DNA. U94/REP inhibited the replication of other betaherpesviruses, HHV-7 and human cytomegalovirus, but had no effect on herpes simplex virus. These results confirm the action of U94/rep latency gene in the regulation of HHV-6 replication with implications for co-reactivations and latency of human betaherpesviruses.

The targeting of primary effusion lymphoma cells for apoptosis by inducing lytic replication of human herpesvirus 8 while blocking virus production

AbstractPrimary effusion lymphoma (PEL) is a B-cell lymphoma in which human herpesvirus-8 (HHV-8) is found within all tumor cells and represents a target for selectively destroying tumor cells. HHV-8 is latent in most PEL cells and, hence, resistant to antiviral agents that inhibit lytic replication. We demonstrate that PEL cell lines containing HHV-8 without and with coinfection with Epstein-Barr virus responded to the antiseizure medication valproate with entry into the lytic cascade and production of infectious virus. Minimal cell death occurred when noninfected BL-41 cells were incubated with valproate, whereas apoptosis occurred in response to valproate in PELs that supported lytic replication of HHV-8. The anti-viral agents ganciclovir and phosphonoformic acid (PFA) blocked valproate-induced production of infectious virus without blocking entry into the lytic cascade, and apoptosis occurred at levels that were as high as when virus production was not blocked. Ganciclovir and PFA also prevented most valproate-induced expression of the late lytic viral transcript open reading frame 26 (ORF-26), but they did not block the induction of either viral interleukin-6 (vIL-6) or viral G protein-coupled receptor (vGPCR). These studies provide evidence that incubation of PELs with valproate in the presence of ganciclovir or PFA can selectively target tumor cells for apoptosis without increasing viral load.

Inhibition of infectious human herpesvirus 8 production by gamma interferon and alpha interferon in BCBL-1 cells.

Human herpesvirus-8 (HHV-8) is aetiologically linked to Kaposi's sarcoma and primary effusion lymphoma. Although interferon-alpha (IFN-alpha) and interferon-gamma (IFN-gamma) are both antiviral cytokines, IFN-alpha blocks entry of HHV-8 into the lytic phase, whereas IFN-gamma induces an increase in the percentage of cells undergoing lytic replication. Multiple events in the lytic cascade must be completed to produce infectious virus. The ability of both types of IFN to affect the production of infectious virus was explored. Both IFN-alpha and IFN-gamma induced expression of the antiviral proteins double-stranded RNA-activated protein kinase (PKR) and 2'5'-oligoadenylate synthetase (2'5'-OAS) in HHV-8-infected BCBL-1 cells. Higher levels resulted from incubation with IFN-alpha than with IFN-gamma, whereas IFN-gamma induced higher levels of IRF-1 than did IFN-alpha. IFN-gamma induced a minor increase in lytic viral gene expression, which was not accompanied by a detectable increase in infectious virus. When lytic replication of HHV-8 was induced using TPA, high levels of infectious virus appeared in the conditioned medium. When IFN-gamma was present during TPA stimulation, the production of infectious virus was reduced by at least a 60 %, and IFN-alpha fully blocked TPA-induced production of infectious virus. The greater reduction of viral production that occurred with IFN-alpha is consistent with the higher levels of the antiviral proteins PKR and 2'5'-OAS induced by IFN-alpha than by IFN-gamma. These studies indicate that the augmentation of cellular antiviral defences by IFN-gamma was sufficient to prevent production of infectious virus despite IFN-gamma-induced entry of some cells into the lytic phase of HHV-8 replication.

Repeated measures study of human herpesvirus 8 (HHV-8) DNA and antibodies in men seropositive for both HHV-8 and HIV.

To study the natural history and pathogenesis of human herpesvirus 8 (HHV-8) infection in HHV-8-seropositive, immunosuppressed men. Longitudinal study of 87 HHV-8- and HIV-seropositive men [42 with Kaposi's sarcoma (KS)] during four visits over a 2 month period. : Patients provided oral fluid and blood. HHV-8 antibody titers were measured with peptide-based enzyme-linked immunosorbent assays (ELISA) for ORF65 and K8.1; HHV-8 DNA was detected with polymerase chain reaction ELISA. HHV-8 DNA was present in oral fluid or peripheral blood mononuclear cells (PBMC) at one or more of the four visits in 71% of men with KS and 56% of men without KS. The strongest correlate of HHV-8 DNA in PBMC was the presence of KS [odds ratio (OR), 8.7; 95% confidence interval (CI), 3.4-22]. Detection of HHV-8 DNA in oral fluid or PBMC was often intermittent, but individuals who shed virus at one time point were more likely to shed at other times. Some men had incomplete epitope recognition in their anti-HHV-8 antibody response. High antibody titers were associated with the absence of circulating HHV-8, particularly for the ORF65 seroassay (OR, 0.16; 95% CI, 0.05-0.51). Among HHV-8 seropositive men, circulating virus is common even in the absence of disease. The link between KS and HHV-8 DNA in PBMC suggests that anti-herpes drugs may impede KS development or progression. Seroassays should target multiple epitopes to achieve maximal sensitivity. HHV-8 replication may be limited by high antibody titers or other immune function for which antibodies are a marker.

Inhibition of infection and replication of human herpesvirus 8 in microvascular endothelial cells by alpha interferon and phosphonoformic acid.

Infection of endothelial cells with human herpesvirus 8 (HHV-8) is an essential event in the development of Kaposi's sarcoma. When primary microvascular endothelial cells (MECs) were infected with HHV-8 at a low multiplicity of infection, considerable latent replication of HHV-8 occurred, leading to a time-dependent increase in the percentage of virus-infected cells that was accompanied by cellular spindling and growth to a high density with loss of contact inhibition. Only a low percentage of MECs supported lytic replication of HHV-8 and produced infectious virus. Phosphonoformic acid blocked production of infectious virus but did not inhibit the rapid expansion of latently infected MECs. Pretreatment of MECs with alpha interferon (IFN-alpha) prior to infection effectively reduced HHV-8 viral gene expression, latent replication, and production of infectious virus. High levels of the double-stranded RNA activated protein kinase (PKR) were expressed in HHV-8-infected cells, and incubation with IFN-alpha increased PKR expression more in virus-infected cells than in uninfected cells. MECs that were immortalized with simian virus 40 large-T antigen differed from nonimmortalized MECs in their response to infection with HHV-8 and demonstrated that cells with elevated levels of expression of antiviral transcripts expressed viral transcripts at reduced levels. These studies demonstrate that MECs respond to HHV-8 with enhanced expression of cellular antiviral genes and that augmentation of innate antiviral defenses with IFN-alpha is a more effective strategy than inhibition of viral lytic replication to protect MECs from infection with HHV-8 and to restrict proliferation of virus-infected MECs.

Short duration of elevated vIRF-1 expression during lytic replication of human herpesvirus 8 limits its ability to block antiviral responses induced by alpha interferon in BCBL-1 cells.

Human herpesvirus 8 (HHV-8) encodes multiple proteins that disrupt the host antiviral response, including viral interferon (IFN) regulatory factor 1 (vIRF-1). The product of the vIRF-1 gene blocks responses to IFN when overexpressed by transfection, but the functional consequence of vIRF-1 that is expressed during infection with HHV-8 is not known. These studies demonstrate that BCBL-1 cells that were latently infected with HHV-8 expressed low levels of vIRF-1 that were associated with PML bodies, whereas much higher levels of vIRF-1 were transiently expressed during the lytic phase of HHV-8 replication. The low levels of vIRF-1 that were associated with PML bodies were insufficient to block alpha IFN (IFN-alpha)-induced alterations in gene expression, whereas cells that expressed high levels of vIRF-1 were resistant to some changes induced by IFN-alpha, including the expression of the double-stranded-RNA-activated protein kinase. High levels of vIRF-1 were expressed for only a short period during the lytic cascade, so many cells with HHV-8 in the lytic phase responded to IFN-alpha with increased expression of antiviral genes and enhanced apoptosis. Furthermore, the production of infectious virus was severely compromised when IFN-alpha was present early during the lytic cascade. These studies indicate that the transient expression of high levels of vIRF-1 is inadequate to subvert many of the antiviral effects of IFN-alpha so that IFN-alpha can effectively induce apoptosis and block production of infectious virus when present early in the lytic cascade of HHV-8.

Remission of HHV-8 and HIV-associated multicentric Castleman disease with ganciclovir treatment

AbstractMulticentric Castleman disease (MCD) is a lymphoproliferative disorder associated with human herpesvirus 8 (HHV-8) infection among persons with human immunodeficiency virus (HIV) infection. Treatment often includes chemotherapy, and progression to non-Hodgkin lymphoma frequently occurs. MCD is characterized in part by active HHV-8 replication, and many of the symptoms of MCD may be attributable to viral gene products. We describe the effect of ganciclovir on the clinical and virologic course of MCD in a series of 3 case reports. Two patients experienced a reduction in the frequency of episodic flares of MCD and detectable HHV-8 DNA with intravenous or oral ganciclovir, whereas the third patient recovered from an acute episode of renal and respiratory failure with intravenous ganciclovir therapy. These data provide in vivo evidence for the utility of antiviral agents against HHV-8 in the management of MCD.

Salivary production of IgA and IgG to human herpes virus 8 latent and lytic antigens by patients in whom Kaposi's sarcoma has regressed.

IgG and IgA antibodies with specificities to a latent and a lytic antigen of human herpes virus 8 (HHV-8) were detectable in the saliva and serum of eight patients whose Kaposi's sarcoma had regressed, seven of whom were HIV-1 infected. The measurement of antibody-specific activity and secretion rate, and the detection of secretory IgA all indicate anti-HHV-8 antibody activity in saliva. The specific humoral responses possibly influence mucosal replication of HHV-8, and in turn, that of HIV.

Structure of replicating intermediates of human herpesvirus type 6

We have studied the structure of the replicative intermediates of human herpesvirus 6 (HHV-6) using pulsed-field gel electrophoresis, partial digestion, two-dimensional gel electrophoresis, and sedimentation centrifugation. The results show that DNA replication of HHV-6 produces head-to-tail concatemeric intermediates as well as approximately equal amounts of circular monomers or oligomers. Unlike the situation in herpes simplex virus, the intermediates of human herpesvirus 6 replication are not highly branched, suggesting a difference in the mechanism of replication or a lower frequency of homologous recombination in human herpesvirus 6 compared to herpes simplex virus.

In vitro activities of benzimidazole D- and L-ribonucleosides against herpesviruses.

Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2), varicella-zoster virus (VZV), cytomegalovirus (CMV), Epstein-Barr virus (EBV), human herpesvirus 6 (HHV-6), and human herpesvirus 8 (HHV-8) are responsible for a number of clinical manifestations in both normal and immunocompromised individuals. The parent benzimidazole ribonucleosides evaluated in this series, 2-bromo-5,6-dichloro-1-(beta-D-ribofuranosyl)benzimidazole (BDCRB) and maribavir (1263W94), are potent and selective inhibitors of human CMV replication. These nucleosides act by two different mechanisms. BDCRB blocks the processing and maturation of viral DNA, whereas 1263W94 inhibits the viral enzyme pUL97 and interferes with DNA synthesis. In the present study, we have evaluated the in vitro antiviral activity of BDCRB, an analog, GW275175X (175X), and 1263W94 against the replication of HSV-1, HSV-2, VZV, CMV, EBV, HHV-6, and HHV-8. By using various methodologies, significant activity was observed against human CMV and EBV but not against HSV-1, HSV-2, VZV, HHV-6, or HHV-8. Plaque reduction assays performed on a variety of laboratory and clinical isolates of human CMV indicated that all strains, including those resistant to ganciclovir (GCV) and foscarnet, were sensitive to all three benzimidazole ribonucleosides, with mean 50% effective concentration values of about 1 to 5 microM compared to that of GCV at 6 microM. The toxicity of these compounds in tissue culture cells appeared to be similar to that observed with GCV. These results demonstrate that the benzimidazole ribonucleosides are active against human CMV and EBV and suggest that they may be useful for the treatment of infections caused by these herpesviruses.

Antibody reactivity to latent and lytic antigens to human herpesvirus-8 in longitudinally followed homosexual men.

Tests for human herpesvirus-8 (HHV-8) often disagree on which individuals are infected. To evaluate HHV-8 antibody changes over many years, 245 homosexual men enrolled in a prospective study between 1982 and 1999 were studied. Samples collected annually were evaluated by immunofluorescence to HHV-8 latent nuclear antigen and by an ELISA for antibody against the HHV-8 lytic antigen K8.1. When positive, samples from most subjects were persistently reactive in both assays. Titers to both lytic and latent antibodies increased for many years, usually in parallel, and were higher in human immunodeficiency virus-infected men. The increasing titers and expanding epitope recognition suggest ongoing, low-grade replication of HHV-8. Use of multiple assays is important in determining prevalence in asymptomatic populations. Because persons with more advanced infections have higher antibody titers, they are easier to detect, which may skew our understanding of the epidemiology of HHV-8.

Human interleukin-6 induces human herpesvirus-8 replication in a body cavity-based lymphoma cell line.

Human herpesvirus-8 (HHV-8) is etiologically associated with Kaposi's sarcoma (KS), body cavity-based lymphoma (BCBL), and multicentric Castleman's disease (MCD). These HHV-8-associated diseases arise predominantly in acquired immunodeficiency syndrome (AIDS) patients. Human interleukin-6 (huIL-6) elevated in the serum of AIDS patients is suggested to stimulate the growth of KS and BCBL and to augment the symptoms of MCD. To determine whether huIL-6 stimulates HHV-8 replication directly, expression of the HHV-8 ORF-50 immediate-early gene (transcription activator) and ORF-26 late lytic gene (a capsid protein) was assessed in a BCBL-1 cell line stimulated by huIL-6 by means of real-time reverse transcription-polymerase chain reaction. huIL-6 induced both ORF-50 and ORF-26 expression, and the maximal ORF-50 expression appeared earlier than that of ORF-26. The data indicate that huIL-6 reactivates HHV-8 in BCBL-1 cells through inducing ORF-50. We also confirmed the previously reported activities of HHV-8-encoded huIL-6 homologue (viral interleukin-6 [vIL-6]) on human immunodeficiency virus (HIV) replication in U1 cell line and huIL-6 production by MT-4 T cells, and utilizing monoclonal antibodies to the huIL-6 receptor components, we elucidated that gp130 is the signaling molecule necessary for these vIL-6 activities. These data suggest the possible existence of interaction between HIV and HHV-8 via IL-6, and that the blockade of IL-6 signal by anti-IL-6R antibody or anti-gp130 antibody can constitute a strategy to treat HIV/HHV-8 dually infected patients.

Retrospective analysis of HHV-8 viremia and cellular viral load in HIV-seropositive patients receiving interleukin 2 in combination with antiretroviral therapy

The combination of interleukin 2 (IL-2) and antiretroviral therapy (ART) represents an emerging strategy in the treatment of patients infected with HIV. Aside from its immunomodulatory role, however, IL-2 may induce replication of human herpesvirus 8 (HHV-8)/Kaposi sarcoma (KS)-associated herpesvirus. We retrospectively evaluated HHV-8 plasma viremia and cellular load, as well as anti-HHV-8 antibody titers, in sequential samples from 84 patients receiving ART alone or in combination with IL-2. At baseline, HHV-8 plasma viremia was present only in 2 HHV-8-seropositive patients in whom KS subsequently developed during or immediately after termination of IL-2 therapy. The level of viremia increased during follow-up and peaked at the time of the clinical manifestation of KS. Moreover, transient peaks of HHV-8 viremia were temporally associated with administration of IL-2. HHV-8 plasma viremia was never detected in the other 47 patients receiving IL-2 nor in 35 controls treated only with ART. Thus, IL-2 therapy seems safe in most patients infected with both HIV and HHV-8, except for those with detectable HHV-8 viremia, who may not be eligible for IL-2 treatment.

Human herpesvirus 8 reactivation and human immunodeficiency virus type 1 gp120.

Human herpesvirus 8 (HHV-8) is the presumed etiologic agent of Kaposi sarcoma (KS), the most common neoplasm in patients with acquired immunodeficiency syndrome. Current evidence indicates HHV-8 is necessary, but not sufficient, for KS development without the involvement of other cofactors. One potentially important cofactor is human immunodeficiency virus type 1 (HIV-1). Although HIV-1 is not essential for development of KS, studies have shown factors released from HIV-1-infected cells, including HIV-1 proteins and cytokines, promote the growth of KS cells in vitro. Recently, studies have shown that coculture of HIV-1-infected T cells with HHV-8-infected primary effusion lymphoma cell lines results in HHV-8 reactivation. This response was due, in part, to cytokines. However, only a portion of induced HHV-8 replication could be accounted for by cytokine stimulation, indicating that other factors, including HIV-1-associated proteins, may also be involved. To investigate a possible role for HIV-1 gp120 in HHV-8 reactivation. Using an in vitro model system, we examined the effect of recombinant HIV-1 gp120 protein on HHV-8 replication in latently infected primary effusion lymphoma cell lines. Reactivation of HHV-8 was analyzed using Northern blot analysis and quantitative polymerase chain reaction for ORF26 messenger RNA expression, a gene encoding for the HHV-8 minor capsid protein produced only during reactivation. The results were extended and confirmed using a luciferase reporter construct driven by the HHV-8 ORF50 promoter, the first promoter activated during HHV-8 replication. No evidence of enhanced HHV-8 replication was found following treatment with HIV-1 gp120. In addition, HIV-1 gp120 was unable to act synergistically with interferon-gamma or hepatocyte growth factor/scatter factor to enhance reactivation of the virus in infected primary effusion lymphoma cell lines. HIV-1 gp120 does not appear to be responsible for the reactivation of HHV-8 demonstrated in our previous studies. Further studies are necessary to determine if other HIV-associated proteins, particularly Tat, gp160, and/or gp41, which are also released from infected cells, may be important in inducing HHV-8 reactivation.

Human herpesvirus 8 reactivation and human immunodeficiency virus type 1 gp120.

Abstract Context.—Human herpesvirus 8 (HHV-8) is the presumed etiologic agent of Kaposi sarcoma (KS), the most common neoplasm in patients with acquired immunodeficiency syndrome. Current evidence indicates HHV-8 is necessary, but not sufficient, for KS development without the involvement of other cofactors. One potentially important cofactor is human immunodeficiency virus type 1 (HIV-1). Although HIV-1 is not essential for development of KS, studies have shown factors released from HIV-1–infected cells, including HIV-1 proteins and cytokines, promote the growth of KS cells in vitro. Recently, studies have shown that coculture of HIV-1–infected T cells with HHV-8–infected primary effusion lymphoma cell lines results in HHV-8 reactivation. This response was due, in part, to cytokines. However, only a portion of induced HHV-8 replication could be accounted for by cytokine stimulation, indicating that other factors, including HIV-1–associated proteins, may also be involved. Objective.—To investigate a possi...

Anti-viral properties of interferon beta treatment in patients with multiple sclerosis.

Viral infections are potentially associated with the etiology and pathogenesis of multiple sclerosis (MS). It has been speculated that the treatment efficacy of interferon beta (IFN beta) in MS may relate to its anti-viral properties. The study was undertaken to evaluate the in vivo anti-viral effects of IFN beta-1a in patients with MS. Human herpesvirus-6 (HHV-6) was studied as an example for being a latent neurotropic virus. IFN beta used at concentrations of approximately 0.5 microg/ml was shown to significantly reduce in vitro HHV-6 replication in a susceptible T-cell line. Sera derived from 23 MS patients treated with IFN beta-1a were examined for serum cell-free DNA of HHV-6 as an indicator for viral replication and the reactivity of IgM antibodies to a recombinant HHV-6 virion protein containing a known immunoreactive region. The results were compared with those of control sera obtained from untreated MS (n=29) and healthy individuals (n=21). The findings indicated that IFN beta treatment significantly reduced HHV-6 replication as evident by decreased cell-free DNA in treated MS specimens. The results correlated with decreased IgM reactivity to the HHV-6 antigen in treated MS patients compared to untreated controls, suggesting reduced exposure to HHV-6. The findings were confirmed in paired sera obtained from seven MS patients before and after the treatment The study provides new evidence indicating that IFN beta has potent in vivo anti-viral effects that may contribute to the treatment efficacy in MS.