HHV-6B infection, T-cell reconstitution, and graft-vs-host disease after hematopoietic stem cell transplantation

A systematic review and meta-analysis of HHV-6 and mortality after hematopoietic cell transplant

Human herpesvirus-6B (HHV-6B) reactivation has been associated with non-relapse mortality (NRM) and overall mortality (OM) following allogeneic hematopoietic stem cell transplant (HCT). We performed a systematic review and meta-analysis to better quantify the association. Studies were included if they systematically tested a cohort of HCT recipients for HHV-6 infection or reactivation and described mortality for patients with and without HHV-6B. Random effects models were used to assess the pooled effect of HHV-6B positivity on each outcome of interest. Bayesian aggregation was additionally performed if models included 10 or fewer studies. Eight studies were included in the NRM analysis, which demonstrated a significant association between HHV-6 detection and NRM (pooled effect: 1.84; 95% CI: 1.29-2.62) without significant heterogeneity (I2 = 0.0%, p = 0.55). A Bayesian aggregation of the raw data used to construct the NRM random effects model supported these findings (95% credible interval: 0.15-1.13). Twenty-five studies were included in OM analysis, which showed a significant positive association (pooled effect: 1.37; 95% CI: 1.07-1.76), though considerable heterogeneity was observed (I2 = 36.7%, p < 0.05). HHV-6 detection is associated with NRM and OM following HCT. Randomized trials are warranted to evaluate if preventing or treating HHV-6B reactivation improves outcomes.

Impact of "day 90" CD4+ T cells on clinical outcomes in children with relapsed/refractory acute myeloid leukemia after allogeneic hematopoietic stem cell transplantation

BACKGROUND

The severity of complications after hematopoietic stem cell transplantation (HSCT) is dictated by the degree of immune reconstitution. However, the connection between immune reconstitution and the prognosis of pediatric patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT) remains unclear. Therefore, the aim of this study was to evaluate the impact of lymphocyte subsets in children diagnosed with refractory or relapsed acute myeloid leukemia (R/R-AML) after allo-HSCT.

METHODS

We retrospectively investigated the prognosis and lymphocyte subsets at d 90 (D90) post-allo-HSCT in 130 children diagnosed with R/R-AML between September 2019 and October 2022 at the Children's Hospital of Soochow University. Lymphocyte subgroups were assessed by flow cytometric analysis on D90 and compared among human leukocyte antigen (HLA)-matched sibling donor HSCT (MSD) (n = 14), haploidentical donor HSCT (n = 94), and HLA-matched unrelated donor HSCT (n = 22) groups. The associations between the counts and frequencies of lymphocyte subgroups and prognosis were assessed.

RESULTS

In the MSD group, CD4+ T cell frequency and count were the highest (P < 0.001). Among the examined lymphocyte subsets, a lower proportion of CD4+ T cells (<14.535 %) at D90 correlated with a higher risk of cytomegalovirus infection (P = 0.002). A higher CD4+ T cell count (>121.39/μL) at D90 after HSCT was the single predictor of a lower fatality risk across all lymphocyte subgroups (univariate: P = 0.038 cut-off: 121.39/μL; multivariate: P = 0.036). No association with relapse was observed.

CONCLUSIONS

CD4+ T cell count may be used to identify pediatric patients with R/R-AML with a greater mortality risk early after HSCT.

Risk factors and long-term outcomes for human herpesvirus 6 encephalitis in the early period after allogeneic stem cell transplantation

Human herpesvirus 6 (HHV6) encephalitis is a rare but life-threatening complication for patients undergoing allogeneic hematopoietic stem cell transplantation (allo-HSCT). However, reports on susceptibility factors and clinical outcomes are limited. We enrolled HHV6 encephalitis patients following allo-HSCT between 2018 and 2022, then conducted a 1:4 nested case-control cohort study to evaluate risk factors and long-term outcomes. Among 1350 patients, 20 (1.48%) developed HHV6 encephalitis, with a median onset time of 25.5 days after HSCT. Patient age<30 (odds ratio [OR], 3.24, P = 0.016) and NK cell count<115/ul at 21 days (OR, 6.07, P = 0.018) were identified as independent risk factors in multivariate analysis. Moreover, the HHV6 encephalitis group was significantly associated with higher incidence of grade II-IV graft-versus-host disease (aGVHD) (hazard ratio [HR], 5.52, P < 0.001) and transplant-associated microangiopathy (HR,9.86, P < 0.001), and demonstrated a significantly higher non-relapse mortality (NRM) (HR, 5.28, P = 0.004) and a lower overall survival (HR, 4.34, P = 0.001) or progression-free survival (HR, 3.94, P = 0.001) compared to control group. In conclusion, patients <30 years old or with delayed NK cell recovery are more susceptible to HHV6 encephalitis after allo-HSCT, and patients with HHV6 encephalitis after transplantation have poorer clinical outcomes.

From Viral Infection to Autoimmune Reaction: Exploring the Link between Human Herpesvirus 6 and Autoimmune Diseases

The complexity of autoimmunity initiation has been the subject of many studies. Both genetic and environmental factors are essential in autoimmunity development. Among others, environmental factors include infectious agents. HHV-6 is a ubiquitous human pathogen with a high global prevalence. It has several properties suggestive of its contribution to autoimmunity development. HHV-6 has a broad cell tropism, the ability to establish latency with subsequent reactivation and persistence, and a range of immunomodulation capabilities. Studies have implicated HHV-6 in a plethora of autoimmune diseases-endocrine, neurological, connective tissue, and others-with some studies even proposing possible autoimmunity induction mechanisms. HHV-6 can be frequently found in autoimmunity-affected tissues and lesions; it has been found to infect autoimmune-pathology-relevant cells and influence immune responses and signaling. This review highlights some of the most well-known autoimmune conditions to which HHV-6 has been linked, like multiple sclerosis and autoimmune thyroiditis, and summarizes the data on HHV-6 involvement in autoimmunity development.

Human herpesvirus 6-specific T-cell immunity in allogeneic hematopoietic stem cell transplant recipients

Human herpesvirus 6 (HHV-6) can reactivate after allogeneic hematopoietic stem cell transplant (allo-HSCT) and may lead to severe symptoms. HHV-6-specific immune responses after HSCT are largely unexplored. We conducted a prospective observational study on 208 consecutive adult patients who received allo-HSCT to investigate HHV-6 reactivations and specific immune responses. Interferon gamma-producing HHV-6-specific T cells were quantified using enzyme-linked immunospot assay (ELISpot). HHV-6 reactivation occurred in 63% of patients, at a median of 25 days from allo-HSCT. Only 40% of these presented a clinically relevant infection, defined by the presence of classical HHV-6 end-organ diseases (EODs), based on European Conference on Infections in Leukaemia (ECIL) guidelines, and other possible HHV6-related EODs. Using multivariate analysis, we identified risk factors for HHV-6 reactivation: previous allo-HSCT, posttransplant cyclophosphamide (PT-Cy), and time-dependent steroids introduction. The use of PT-Cy and steroids were associated with clinically relevant infections, whereas higher CD3+ cell counts seemed to be protective. Interestingly, circulating HHV-6-specific T cells were significantly higher in patients with reactivated virus. Moreover, HHV-6-specific T-cell responses, quantified at >4 days after the first viremia detection, predicted clinically relevant infections (P < .0001), with higher specificity (93%) and sensitivity (79%) than polyclonal CD3+ cells per μL. Overall survival and transplant-related mortality were not affected by time-dependent HHV-6 reactivation, whereas a significant association was observed between clinically relevant infections and acute graft-versus-host disease. These results shed light on the role of HHV-6 in allo-HSCT and may affect HHV-6 monitoring and treatment.

Immunopathogenic Insights on Preferential Human Herpesvirus-6 Reactivation in Drug Rash With Eosinophilia and Systemic Symptoms: A Scoping Review

INTRODUCTION

Human herpesvirus-6 (HHV-6) is a ubiquitous lymphotropic betaherpesvirus that can reactivate in drug rash with eosinophilia and systemic symptoms (DRESS). Despite recent publications advancing our understanding of HHV-6 in DRESS, the exact role of HHV-6 in disease pathogenesis remains unclear.

METHODS

A scoping review with the PubMed query "(HHV 6 AND (drug OR DRESS OR DIHS)) OR (HHV6 AND (drug OR DRESS OR DIHS))" was conducted in accordance with PRISMA guidelines. Articles containing original data on at least one DRESS patient with HHV-6 testing were included.

RESULTS

Our search returned a total of 373 publications, of which 89 met eligibility criteria. HHV-6 reactivation occurred in 63% of DRESS patients (n = 748), which was significantly more often than other herpesviruses. HHV-6 reactivation was associated with worse outcomes and greater severity in controlled studies. Case reports have demonstrated sometimes fatal HHV-6-related multi-organ involvement. Temporally, HHV-6 reactivation typically occurs 2 to 4 weeks after DRESS onset and has been linked to markers of immunologic signaling, such as OX40 (CD134), an HHV-6 entry receptor. Efficacy of antiviral or immunoglobulin treatment has only been demonstrated anecdotally, and steroid use may affect HHV-6 reactivation.

CONCLUSION

HHV-6 is implicated in DRESS more than in any other dermatologic condition. It is still unclear whether HHV-6 reactivation is cause or consequence of DRESS dysregulation. Similar pathogenic mechanisms precipitated by HHV-6 in other contexts may be relevant in DRESS. Future randomized controlled studies to assess effects of viral suppression on clinical outcomes is needed.

Potential molecular mechanisms of chronic fatigue in long haul COVID and other viral diseases

Historically, COVID-19 emerges as one of the most devastating diseases of humankind, which creates an unmanageable health crisis worldwide. Until now, this disease costs millions of lives and continues to paralyze human civilization's economy and social growth, leaving an enduring damage that will take an exceptionally long time to repair. While a majority of infected patients survive after mild to moderate reactions after two to six weeks, a growing population of patients suffers for months with severe and prolonged symptoms of fatigue, depression, and anxiety. These patients are no less than 10% of total COVID-19 infected individuals with distinctive chronic clinical symptomatology, collectively termed post-acute sequelae of COVID-19 (PASC) or more commonly long-haul COVID. Interestingly, Long-haul COVID and many debilitating viral diseases display a similar range of clinical symptoms of muscle fatigue, dizziness, depression, and chronic inflammation. In our current hypothesis-driven review article, we attempt to discuss the molecular mechanism of muscle fatigue in long-haul COVID, and other viral diseases as caused by HHV6, Powassan, Epstein-Barr virus (EBV), and HIV. We also discuss the pathological resemblance of virus-triggered muscle fatigue with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS).

Intracerebral opportunistic infections caused by immunosuppressants after orthotopic liver transplantation: Report of two cases and literature review

Central nervous system (CNS) infections in adults are rare because of normal immunity and the existence of the blood brain barrier, which prevents the invasion of pathogenic microorganisms. Liver transplant recipients are at an increased risk of opportunistic infections (OI) due to immunosuppressive therapy compared to those with normal immunity. Early diagnosis and timely implementation of treatment are critical for the successful treatment of these infections. We present two cases of intracerebral OI after orthotopic liver transplantation (OLT), with different clinical presentations. Patient 1 presented with epileptic seizures, mainly manifested as unresponsiveness, unconsciousness, and coma complicated with involuntary limb twitching. Patient 2 presented with a consciousness disorder, mainly manifested as unclear consciousness content, poor orientation, calculation power, and logical ability. Next-generation sequencing (NGS) examination of the cerebrospinal fluid confirmed human herpesvirus 6 B (HHV-6B) infection in patient 1 and intracranial Aspergillus infection in patient 2. Intracranial OI has insidious onset and atypical clinical manifestations. NGS can allow for the proper diagnosis and monitoring of the effects of treatment.

Epstein–Barr Virus and Human Herpesvirus-6 Reactivation in Acute COVID-19 Patients

Beyond their pulmonary disease, many COVID-19 patients experience a complex constellation of characteristics, including hyperinflammatory responses, autoimmune disorders, and coagulopathies. However, the pathogenesis of these aspects of COVID-19 is obscure. More than 90% of people are latently infected with the lymphotropic herpesviruses Epstein–Barr Virus (EBV) and/or Human Herpesvirus-6 (HHV-6). Some of the inflammatory features of COVID-19 resemble clinical syndromes seen during EBV and HHV-6 infection, and these latent viruses can be reactivated by inflammatory mediators. We hypothesized that EBV and HHV-6 reactivation might be a common feature of early COVID-19, particularly in patients with more inflammation. We tested for EBV and HHV-6 reactivation in 67 patients acutely hospitalized with COVID-19 using previously validated quantitative PCR assays on the plasma. In our cohort, we found that 15/67 (22.4%) patients had detectable EBV and 3/67 (4.5%) had detectable HHV-6. This frequency of activation is somewhat more than the frequency reported for some healthy cohorts, such as blood donors and other healthy control cohorts. There was no association between EBV or HHV-6 and markers indicative of more inflammatory disease. We conclude that EBV and HHV-6 activation at about day 7 of hospitalization occurred in a modest fraction of our cohort of COVID-19 patients and was not associated with high levels of inflammation. In the modest fraction of patients, EBV and HHV-6 reactivation could contribute to some features of acute disease and pre-disposition to post-acute sequelae in a subset of patients.

Human herpes simplex virus-6 (HHV-6) detection and seroprevalence among Qatari nationals and immigrants residing in Qatar

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The prevalence of anti-human herpes virus-6 immunoglobulin G (IgG) was 71.7% among healthy donors in Qatar.

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One-quarter (24.3%) of healthy donors in Qatar had detectable viraemia.

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No strong association was found between viraemia and IgG positivity.

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A significant association was found between viraemia and the nationality of healthy donors.

Background

Human herpes simplex virus-6 (HHV-6) is the causative agent of exanthema subitum. Transmission mainly occurs through salivary secretions, yet blood transfusions and organ transplantations have also been reported as routes of transmission. Studies of seroprevalence of HHV-6 in the Middle East and North Africa (MENA) region and other parts of Asia are scarce. As such, this study aimed to estimate the seroprevalence of HHV-6 among healthy blood donors in Qatar.

Methods

In total, 620 healthy blood donors from different nationalities residing in Qatar, mainly from the MENA region and Southeast Asia, were tested using a commercial anti-HHV-6 immunoglobulin G (IgG) enzyme-linked immunosorbent assay kit. In addition, HHV-6 DNA from randomly selected samples was tested and quantified using quantitative reverse transcriptase polymerase chain reaction.

Results

Anti-HHV-6 IgG was detected in 71.7% (445/620) [95% confidence interval (CI) 68.2–75.3%] of the tested samples, while 24.3% (61/251) (95% CI 20.0–29.6%) had detectable HHV-6 viraemia. Only 22.5% of individuals with positive IgG status had detectable HHV-6 DNA in their blood, indicating a weak association between viraemia and IgG positivity (P=0.08). Furthermore, no significant difference was associated between HHV-6 viraemia and demographic characteristics, except for nationality.

Conclusion

The seroprevalence of HHV-6 in Qatar was found to be similar to rates reported in other parts of the world.

Analysis of biological models to predict clinical outcomes based on HLA-DPB1 disparities in unrelated transplantation

HLA compatibility is a key factor for survival after unrelated hematopoietic stem cell transplantation (HSCT). HLA-A, -B, -C, -DRB1, and -DQB1 are usually matched between donor and recipient. By contrast, HLA-DPB1 mismatches are frequent, although it is feasible to optimize donor selection and DPB1 matching with prospective typing. Because classical DPB1 allele mismatches are often unavoidable, however, several biological models have been developed to predict the optimal DPB1 mismatch combination for less graft-versus-host disease (GVHD) and better overall survival. In 909 recipient/donor pairs, we analyzed the role of 3 biological models: T-cell epitopes (TCEs) based on the immunogenicity of DPB1, cell surface expression of DPB1 molecules based on a single-nucleotide polymorphism located in the 3' untranslated region, and the Predicted Indirectly ReCognizable HLA Epitopes (PIRCHE) model based on the presentation of allogeneic peptides derived from mismatched HLA, compared with the classical allele mismatch. Matching for both DPB1 alleles remains the best option to prevent acute GVHD. In the situation of one DPB1 allele mismatch, the donor associated with the lowest acute GVHD risks is mismatched for an allele with a low expression profile in the recipient, followed by a permissive TCE3/4 mismatch and/or the absence of PIRCHE II potential against the recipient. In the context of 2 DPB1 mismatches, the same considerations apply for a permissive TCE3/4 mismatch and no PIRCHE II. By combining the biological models, the most favorable DPB1 constellation can be defined. This approach will help optimize donor selection and improve post-HSCT complications and patient prognosis.

Impact of donor and recipient Epstein-Barr Virus serostatus on outcomes of allogeneic hematopoietic cell transplantation: a systematic review and meta-analysis

Allogeneic hematopoietic cell transplant (allo-HCT) is a potentially curative therapeutic strategy that showed encouraging long-term outcomes in hematological diseases. A number of factors can influence post-transplant clinical outcomes. While Epstein-Barr virus (EBV) constitutes a trigger for development of various adverse conditions, no clinical study yet has been powered to assess the effect of EBV serostatus on the clinical outcomes in allo-HCT population. To systematically summarize and analyze the impact of donor and recipient EBV serostatus on transplant outcomes in allo-HCT recipients, meta-analyses were conducted. Selected endpoints were overall survival (OS), relapse-free survival (RFS), relapse incidence (RI), non-relapse mortality (NRM), acute graft-versus-host disease (aGVHD), chronic graft-versus-host disease (cGVHD), and de novo cGVHD. Three studies with 26,650 patients, transplanted for acute leukemias, lymphomas, chronic hematological malignancies, or non-malignant hematological diseases were included in the meta-analysis. In the whole population, with a total of 53,300 donors and recipients, the rate of EBV seropositivity was 85.1%, including 86.6% and 83.6% among transplant recipients and healthy donors, respectively. Donor EBV seropositivity increased the risk of cGVHD by 17%, de novo cGVHD by 14%, and aGHVD by 5%. Recipient EBV seropositivity increased the risk of cGVHD by 12%, de novo cGVHD by 17%; increased NRM by 11%, increased RI by 11%, decreased OS by 14%, and decreased RFS by 11%. In performed meta-analyses, donor and recipient EBV seropositivity was found to have a significant impact on transplant outcomes in patients after allo-HCT.

Comparative Analysis of Roseoloviruses in Humans, Pigs, Mice, and Other Species

Viruses of the genus Roseolovirus belong to the subfamily Betaherpesvirinae, family Herpesviridae. Roseoloviruses have been studied in humans, mice and pigs, but they are likely also present in other species. This is the first comparative analysis of roseoloviruses in humans and animals. The human roseoloviruses human herpesvirus 6A (HHV-6A), 6B (HHV-6B), and 7 (HHV-7) are relatively well characterized. In contrast, little is known about the murine roseolovirus (MRV), also known as murine thymic virus (MTV) or murine thymic lymphotrophic virus (MTLV), and the porcine roseolovirus (PRV), initially incorrectly named porcine cytomegalovirus (PCMV). Human roseoloviruses have gained attention because they can cause severe diseases including encephalitis in immunocompromised transplant and AIDS patients and febrile seizures in infants. They have been linked to a number of neurological diseases in the immunocompetent including multiple sclerosis (MS) and Alzheimer's. However, to prove the causality in the latter disease associations is challenging due to the high prevalence of these viruses in the human population. PCMV/PRV has attracted attention because it may be transmitted and pose a risk in xenotransplantation, e.g., the transplantation of pig organs into humans. Most importantly, all roseoloviruses are immunosuppressive, the humoral and cellular immune responses against these viruses are not well studied and vaccines as well as effective antivirals are not available.

Clinical Relevance of Immunobiology in Umbilical Cord Blood Transplantation

Umbilical cord blood transplantation (UCBT) has been an important donor source for allogeneic hematopoietic stem cell transplantation, especially for patients who lack suitable matched donors. UCBT provides unique practical advantages, such as lower risks of graft-versus-host-disease (GVHD), permissive HLA mismatch, and ease of procurement. However, there are clinical challenges in UCBT, including high infection rates and treatment-related mortality in selected patient groups. These clinical advantages and challenges are tightly linked with cell-type specific immune reconstitution (IR). Here, we will review IR, focusing on T and NK cells, and the impact of IR on clinical outcomes. Better understanding of the immune biology in UCBT will allow us to further advance this field with improved clinical practice.

Inflammatory Cytokine Profile in Individuals with Inherited Chromosomally Integrated Human Herpesvirus 6

Acute graft-versus-host-disease (aGVHD) is a major complication following hematopoietic cell transplantations (HCTs). We have shown that HCT recipients in whom either the donor or patient had inherited chromosomally integrated human herpesvirus 6 (iciHHV-6) have a higher incidence of developing more severe aGVHD. Previous studies established that increased proinflammatory cytokines are associated with increased risk for aGVHD and nonrelapse mortality post-HCT. We hypothesized that HCT recipients with donor or recipient iciHHV-6 (iciHHV-6^pos HCT cases) will have higher cytokine levels compared with HCT recipients without iciHHV-6 (iciHHV-6^neg HCT controls). We identified 64 iciHHV-6^pos HCT cases with plasma from days 7, 14, and/or 21 post-HCT and before aGVHD onset in patients who developed aGVHD. We identified 64 iciHHV-6^neg HCT controls matched for aGVHD risk factors. We also identified 28 donors with iciHHV-6 and 56 matched donors without iciHHV-6. We measured plasma cytokine concentrations for IL-6, suppression of tumorigenicity 2, T cell immunoglobulin and mucin-domain containing 3, TNFα, soluble TNF receptor 1 (TNFRp55), and C-reactive protein (CRP). We used Mann-Whitney tests and repeated-measures models to compare cytokine levels. iciHHV-6^pos HCT cases had higher CRP levels on day 7 and day 21 and higher TNFRp55 levels on day 14 and day 21 compared with iciHHV-6^neg HCT controls. These findings were recapitulated in a repeated-measures model. The differences were most evident among patients who subsequently developed aGVHD grades 2 to 4. Additionally, iciHHV-6^pos HCT cases had earlier-onset aGVHD (median, 20 versus 27 days post-HCT; P = .02). There were no differences in cytokine levels among healthy donors with or without iciHHV-6. This study demonstrates that HCT recipients with iciHHV-6 have higher proinflammatory cytokines that may be associated with increased risk for aGVHD.

Genome-Wide Approach to the CD4 T-Cell Response to Human Herpesvirus 6B

Human herpesvirus 6 (HHV-6) and cytomegalovirus (CMV) are population-prevalent betaherpesviruses with intermittent lytic replication that can be pathogenic in immunocompromised hosts. Elucidation of the adaptive immune response is valuable for understanding pathogenesis and designing novel treatments. Knowledge of T-cell antigens has reached the genome-wide level for CMV and other human herpesviruses, but study of HHV-6 is at an earlier stage. Using rare-cell enrichment combined with an HLA-agnostic, proteome-wide approach, we queried HHV-6B-specific CD4 T cells from 18 healthy donors with each known HHV-6B protein. We detected a low abundance of HHV-6-specific CD4 T cells in blood; however, the within-person CD4 T-cell response is quite broad: the median number of open reading frame (ORF) products recognized was nine per person. Overall, the data expand the number of documented HHV-6B CD4 T-cell antigens from approximately 11 to 60. Epitopes in the proteins encoded by U14, U90, and U95 were mapped with synthetic peptides, and HLA restriction was defined for some responses. Intriguingly, CD4 T-cell antigens newly described in this report are among the most population prevalent, including U73, U72, U95, and U30. Our results indicate that selection of HHV-6B ORFs for immunotherapy should consider this expanded panel of HHV-6B antigens.IMPORTANCE Human herpesvirus 6 is highly prevalent and maintains chronic infection in immunocompetent individuals, with the potential to replicate widely in settings of immunosuppression, leading to clinical disease. Antiviral compounds may be ineffective and/or pose dose-limiting toxicity, and therefore, immune-based therapies have garnered increased interest in recent years. Attempts at addressing this unmet medical need begin with understanding the cellular response to HHV-6 at the individual and population levels. The present study provides a comprehensive assessment of HHV-6-specific T-cell responses that may inform the development of cell-based therapies directed at this virus.

Human Herpesvirus 6 and Malignancy: A Review

In order to determine the role of human herpesvirus 6 (HHV-6) in human disease, several confounding factors, including methods of detection, types of controls, and the ubiquitous nature of the virus, must be considered. This is particularly problematic in the case of cancer, in which rates of detection vary greatly among studies. To determine what part, if any, HHV-6 plays in oncogenesis, a review of the literature was performed. There is evidence that HHV-6 is present in certain types of cancer; however, detection of the virus within tumor cells is insufficient for assigning a direct role of HHV-6 in tumorigenesis. Findings supportive of a causal role for a virus in cancer include presence of the virus in a large proportion of cases, presence of the virus in most tumor cells, and virus-induced in-vitro cell transformation. HHV-6, if not directly oncogenic, may act as a contributory factor that indirectly enhances tumor cell growth, in some cases by cooperation with other viruses. Another possibility is that HHV-6 may merely be an opportunistic virus that thrives in the immunodeficient tumor microenvironment. Although many studies have been carried out, it is still premature to definitively implicate HHV-6 in several human cancers. In some instances, evidence suggests that HHV-6 may cooperate with other viruses, including EBV, HPV, and HHV-8, in the development of cancer, and HHV-6 may have a role in such conditions as nodular sclerosis Hodgkin lymphoma, gastrointestinal cancer, glial tumors, and oral cancers. However, further studies will be required to determine the exact contributions of HHV-6 to tumorigenesis.