Immune response to HHV-6 and implications for immunotherapy

Human herpesvirus reactivation and its potential role in the pathogenesis of post-acute sequelae of SARS-CoV-2 infection

The emergence of SARS-CoV-2 has precipitated a global pandemic with substantial long-term health implications, including the condition known as post-acute sequelae of SARS-CoV-2 infection (PASC), commonly referred to as Long COVID. PASC is marked by persistent symptoms such as fatigue, neurological issues, and autonomic dysfunction that persist for months beyond the acute phase of COVID-19. This review examines the potential role of herpesvirus reactivation, specifically Epstein-Barr virus (EBV) and cytomegalovirus (CMV), in the pathogenesis of PASC. Elevated antibody titers and specific T cell responses suggest recent herpesvirus reactivation in some PASC patients, although viremia is not consistently detected. SARS-CoV-2 exhibits endothelial trophism, directly affecting the vascular endothelium and contributing to microvascular pathologies. These pathologies are significant in PASC, where microvascular dysfunction may underlie various chronic symptoms. Similarly, herpesviruses like CMV also exhibit endothelial trophism, which may exacerbate endothelial damage when reactivated. Evidence suggests that EBV and CMV reactivation could indirectly contribute to the immune dysregulation, immunosenescence, and autoimmune responses observed in PASC. Additionally, EBV may play a role in the genesis of neurological symptoms through creating mitochondrial dysfunction, though direct confirmation remains elusive. The reviewed evidence suggests that while herpesviruses may not play a direct role in the pathogenesis of PASC, their potential indirect effects, especially in the context of endothelial involvement, warrant further investigation.

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.

The Spectrum of Neurological Manifestations of Human Herpesvirus 6 Infection in Children

Human herpesvirus 6 (HHV-6) is a member of the Herpesviridae family. There are two HHV-6 species: HHV-6A and HHV-6B. HHV-6B causes the majority of documented primary infections and reactivation events. In this case series, we illustrate the varied spectrum of clinical and radiological features of HHV-6 encephalitis and its management in children.

We have described three cases of HHV-6 encephalitis in the age group between nine months and two years. All had an HHV-6 viral load detected in cerebrospinal fluid (CSF) samples. Two of which are of immunocompetent patients.

This case series highlights the importance of including HHV-6 infection as one of the differential diagnoses in a child with suspected central nervous system infection and of considering adding CSF HHV-6 polymerase chain reaction (PCR) test for detection. Increasing awareness of this condition will aid physicians in the timely diagnosis and early treatment of HHV-6 encephalitis. 

Targeting Host Cellular Factors as a Strategy of Therapeutic Intervention for Herpesvirus Infections

Herpesviruses utilize various host factors to establish latent infection, survival, and spread disease in the host. These factors include host cellular machinery, host proteins, gene expression, multiple transcription factors, cellular signal pathways, immune cell activation, transcription factors, cytokines, angiogenesis, invasion, and factors promoting metastasis. The knowledge and understanding of host genes, protein products, and biochemical pathways lead to discovering safe and effective antivirals to prevent viral reactivation and spread infection. Here, we focus on the contribution of pro-inflammatory, anti-inflammatory, and resolution lipid metabolites of the arachidonic acid (AA) pathway in the lifecycle of herpesvirus infections. We discuss how various herpesviruses utilize these lipid pathways to their advantage and how we target them to combat herpesvirus infection. We also summarize recent development in anti-herpesvirus therapeutics and new strategies proposed or under clinical trials. These anti-herpesvirus therapeutics include inhibitors blocking viral life cycle events, engineered anticancer agents, epigenome influencing factors, immunomodulators, and therapeutic compounds from natural extracts.

Human Herpesvirus-6 and -7 in the Brain Microenvironment of Persons with Neurological Pathology and Healthy People

During persistent human beta-herpesvirus (HHV) infection, clinical manifestations may not appear. However, the lifelong influence of HHV is often associated with pathological changes in the central nervous system. Herein, we evaluated possible associations between immunoexpression of HHV-6, -7, and cellular immune response across different brain regions. The study aimed to explore HHV-6, -7 infection within the cortical lobes in cases of unspecified encephalopathy (UEP) and nonpathological conditions. We confirmed the presence of viral DNA by nPCR and viral antigens by immunohistochemistry. Overall, we have shown a significant increase (p < 0.001) of HHV antigen expression, especially HHV-7 in the temporal gray matter. Although HHV-infected neurons were found notably in the case of HHV-7, our observations suggest that higher (p < 0.001) cell tropism is associated with glial and endothelial cells in both UEP group and controls. HHV-6, predominantly detected in oligodendrocytes (p < 0.001), and HHV-7, predominantly detected in both astrocytes and oligodendrocytes (p < 0.001), exhibit varying effects on neural homeostasis. This indicates a high number (p < 0.001) of activated microglia observed in the temporal lobe in the UEP group. The question remains of whether human HHV contributes to neurological diseases or are markers for some aspect of the disease process.

Pathogen-Specific T Cells Beyond CMV, EBV and Adenovirus

PURPOSE OF REVIEW

Infectious diseases contribute significantly to morbidity and mortality in recipients of allogeneic haematopoietic stem cell transplantation (aHSCT), particularly in the era of highly immunosuppressive transplant regimens and alternate donor transplants. Delayed cellular immune recovery is a major mechanism for the increased risk in these patients. Adoptive cell therapy with ex vivo manipulated pathogen-specific T cells (PSTs) is increasingly taking its place as a treatment strategy using donor-derived or third party-banked cells.

RECENT FINDINGS

The majority of clinical trial data in the form of early-phase studies has been in the prophylaxis or treatment of cytomegalovirus (CMV), Epstein-Barr virus (EBV) and adenovirus (AdV). Advancements in methods to select and enrich PSTs offer the opportunity to target the less common viral pathogens as well as fungi with this technology. Early clinical studies of PSTs targeting polyomaviruses (BK virus and JC virus), human herpesvirus 6 (HHV6), varicella zoster virus (VZV) and Aspergillus spp. have shown promising results in small numbers of patients. Other potential targets include herpes simplex virus (HSV), respiratory viruses and other invasive fungal species. In this review, we describe the burden of disease of this wider spectrum of pathogens, the progress in the development of manufacturing capability, early clinical results and the opportunities and challenges for implementation in the clinic.

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.

Guidelines from the 2017 European Conference on Infections in Leukaemia for management of HHV-6 infection in patients with hematologic malignancies and after hematopoietic stem cell transplantation

Of the two human herpesvirus 6 (HHV-6) species, human herpesvirus 6B (HHV-6B) encephalitis is an important cause of morbidity and mortality after allogeneic hematopoietic stem cell transplant. Guidelines for the management of HHV-6 infections in patients with hematologic malignancies or post-transplant were prepared a decade ago but there have been no other guidelines since then despite significant advances in the understanding of HHV-6 encephalitis, its therapy, and other aspects of HHV-6 disease in this patient population. Revised guidelines prepared at the 2017 European Conference on Infections in Leukaemia covering diagnosis, preventative strategies and management of HHV-6 disease are now presented.

Phenotypic and Functional Differences between Human Herpesvirus 6- and Human Cytomegalovirus-Specific T Cells

Human herpesvirus 6 (HHV-6) infects >90% of the population and establishes a latent infection with asymptomatic episodes of reactivation. However, HHV-6 reactivation is associated with morbidity and sometimes mortality in immunocompromised patients. To date, control of the virus in healthy virus carriers and the failure to control it in patients with disease remain poorly understood. In particular, knowledge of HHV-6-specific T-cell responses is limited. Here, we characterized HHV-6A- and HHV-6B-specific CD4⁺ and CD8⁺ T-cell responses from peripheral blood mononuclear cells (PBMCs) of healthy donors. We studied the phenotype of effector HHV-6-specific T cells ex vivo, as well as of induced specific suppressive regulatory CD4⁺ T cells in vitro poststimulation, in comparison to human cytomegalovirus (HCMV) responses. Compared to that for HCMV, we show that ex vivo T-cell reactivity in peripheral blood is detectable but at very low frequency, both for HHV-6A and -6B viruses. Interestingly, the phenotype of the specific T cells also differs between the viruses. HHV-6A- and HHV-6B-specific CD4⁺ T lymphocytes are less differentiated than HCMV-specific T cells. Furthermore, we show a higher frequency of HHV-6-specific suppressive regulatory T cells (eTregs) than HCMV-specific eTregs in coinfected individuals. Despite the strong similarity of HHV-6 and HCMV from a virologic point of view, we observed immunological differences, particularly in relation to the frequency and phenotype of effector/memory and regulatory virus-specific T cells. This suggests that different immune factors are solicited in the control of HHV-6 infection than in that of HCMV infection.IMPORTANCE T cells are central to an effective defense against persistent viral infections that can be related to human cytomegalovirus (HCMV) or human herpesvirus 6 (HHV-6). However, knowledge of HHV-6-specific T-cell responses is limited. In order to deepen our knowledge of T-cell responses to HHV-6, we characterized HHV-6A- and HHV-6B-specific CD4⁺ and CD8⁺ T-cell responses directly ex vivo from healthy coinfected blood donors. Despite the strong similarity of HHV-6 and HCMV from a virologic point of view, we observed immunological differences, particularly in relation to the frequency and phenotype of effector/memory and regulatory virus-specific T cells. This suggests that different immune factors are solicited in the control of HHV-6 infection than in that of HCMV infection. Our findings may encourage immunomonitoring of patients with viral replication episodes to follow the emergence of effector versus regulatory T cells.

Naturally processed HLA-DR3-restricted HHV-6B peptides are recognized broadly with polyfunctional and cytotoxic CD4 T-cell responses

Human herpes virus 6B (HHV-6B) is a widespread virus that infects most people early in infancy and establishes a chronic life-long infection with periodic reactivation. CD4 T cells have been implicated in control of HHV-6B, but antigenic targets and functional characteristics of the CD4 T-cell response are poorly understood. We identified 25 naturally processed MHC-II peptides, derived from six different HHV-6B proteins, and showed that they were recognized by CD4 T-cell responses in HLA-matched donors. The peptides were identified by mass spectrometry after elution from HLA-DR molecules isolated from HHV-6B-infected T cells. The peptides showed strong binding to matched HLA alleles and elicited recall T-cell responses in vitro. T-cell lines expanded in vitro were used for functional characterization of the response. Responding cells were mainly CD3⁺ CD4⁺ , produced IFN-γ, TNF-α, and low levels of IL-2, alone or in combination, highlighting the presence of polyfunctional T cells in the overall response. Many of the responding cells mobilized CD107a, stored granzyme B, and mediated specific killing of peptide-pulsed target cells. These results highlight a potential role for polyfunctional cytotoxic CD4 T cells in the long-term control of HHV-6B infection.

Identification of CD4 and H-2Kd-restricted cytotoxic T lymphocyte epitopes on the human herpesvirus 6B glycoprotein Q1 protein

The identification of Human herpesvirus 6B (HHV-6B) epitopes that are recognized by T-cells could contribute to the development of potential vaccines and immunotherapies. Here, we identified CD4⁺ and H-2K^d-restricted CD8⁺ T-cell epitopes on the glycoprotein Q1 of HHV-6B (BgQ1), which is a unique glycoprotein and essential for HHV-6B viral entry, by using in vivo electroporation with a plasmid DNA encoding BgQ1, overlapping peptides spanning the BgQ1 sequence, ELISPOT assay for quantification of gamma interferon (IFN-γ), and computer-based T-cell epitope prediction programs. The CD4⁺ and CD8⁺ T-cell epitopes identified in BALB/c mice in this study could be a good animal model system for use in the development of T-cell responses, inducing HHV-6B vaccines or immunotherapies.

Advances in the Characterization of the T-Cell Response to Human Herpesvirus-6

Human herpesvirus (HHV) 6 is thought to remain clinically latent in most individuals after primary infection and to reactivate to cause disease in persons with severe immunosuppression. In allogeneic hematopoietic stem cell transplant recipients, reactivation of HHV-6 species B is a considerable cause of morbidity and mortality. HHV-6B reactivation is the most frequent cause of infectious meningoencephalitis in this setting and has been associated with a variety of other complications such as graft rejection and acute graft versus host disease. This has inspired efforts to develop HHV-6-targeted immunotherapies. Basic knowledge of HHV-6-specific adaptive immunity is crucial for these endeavors, but remains incomplete. Many studies have focused on specific HHV-6 antigens extrapolated from research on human cytomegalovirus, a genetically related betaherpesvirus. Challenges to the study of HHV-6-specific T-cell immunity include the very low frequency of HHV-6-specific memory T cells in chronically infected humans, the large genome size of HHV-6, and the lack of an animal model. This review will focus on emerging techniques and methodological improvements that are beginning to overcome these barriers. Population-prevalent antigens are now becoming clear for the CD4+ T-cell response, while definition and ranking of CD8+ T-cell antigens and epitopes is at an earlier stage. This review will discuss current knowledge of the T-cell response to HHV-6, new research approaches, and translation to clinical practice.

Cross-sectional analysis of CD8 T cell immunity to human herpesvirus 6B

Human herpesvirus 6 (HHV-6) is prevalent in healthy persons, causes disease in immunosuppressed carriers, and may be involved in autoimmune disease. Cytotoxic CD8 T cells are probably important for effective control of infection. However, the HHV-6-specific CD8 T cell repertoire is largely uncharacterized. Therefore, we undertook a virus-wide analysis of CD8 T cell responses to HHV-6. We used a simple anchor motif-based algorithm (SAMBA) to identify 299 epitope candidates potentially presented by the HLA class I molecule B*08:01. Candidates were found in 77 of 98 unique HHV-6B proteins. From peptide-expanded T cell lines, we obtained CD8 T cell clones against 20 candidates. We tested whether T cell clones recognized HHV-6-infected cells. This was the case for 16 epitopes derived from 12 proteins from all phases of the viral replication cycle. Epitopes were enriched in certain amino acids flanking the peptide. Ex vivo analysis of eight healthy donors with HLA-peptide multimers showed that the strongest responses were directed against an epitope from IE-2, with a median frequency of 0.09% of CD8 T cells. Reconstitution of T cells specific for this and other HHV-6 epitopes was also observed after allogeneic hematopoietic stem cell transplantation. We conclude that HHV-6 induces CD8 T cell responses against multiple antigens of diverse functional classes. Most antigens against which CD8 T cells can be raised are presented by infected cells. Ex vivo multimer staining can directly identify HHV-6-specific T cells. These results will advance development of immune monitoring, adoptive T cell therapy, and vaccines.

This paper deals with the immune response to a very common virus, called human herpesvirus 6 (HHV-6). Most people catch HHV-6 in early childhood, which often leads to a disease known as three-day fever. Later in life, the virus stays in the body, and an active immune response is needed to prevent the virus from multiplying and causing damage. It is suspected that HHV-6 contributes to autoimmune diseases and chronic fatigue. Moreover, patients with severely weakened immune responses, for example after some forms of transplantation, clearly have difficulties controlling HHV-6, which puts them at risk of severe disease and shortens their survival. This can potentially be prevented by giving them HHV-6-specific "killer" CD8 T cells, which are cells of the immune system that destroy body cells harboring the virus. However, little is known so far about such T cells. Here, we describe 16 new structures that CD8 T cells can use to recognize and kill HHV-6-infected cells. We show that very different viral proteins can furnish such structures. We also observe that such T cells are regularly present in healthy people and in transplant patients who control the virus. Our results will help develop therapies of disease due to HHV-6.

CD8+ T Cells Prevent Lethality from Neonatal Murine Roseolovirus Infection

A recently described mouse homolog of the human roseoloviruses, murine roseolovirus (MRV), causes loss of peripheral and thymic CD4+ cells during neonatal infection of BALB/c mice. Despite significant disruptions to the normal adaptive immune response, infected BALB/c mice reproducibly recover from infection, consistent with prior studies on a related virus, mouse thymic virus. In this article, we show that, in contrast to published studies on mouse thymic virus, MRV appears to robustly infect neonatal C57BL/6 (B6) mice, causing severe depletion of thymocytes and peripheral T cells. Moreover, B6 mice recovered from infection. We investigated the mechanism of thymocyte and T cell loss, determining that the major thymocyte subsets were infected with MRV; however, CD4+ and CD4+CD8- T cells showed increased apoptosis during infection. We found that CD8+ T cells populated MRV-infected thymi. These CD8+ T cells expressed markers of activation, had restricted TCR repertoire, and accumulated intracellular effector proteins, consistent with a cytotoxic lymphocyte phenotype and suggesting their involvement in viral clearance. Indeed, absence of CD8+ T cells prevented recovery from MRV infection and led to lethality in infected animals, whereas B cell-deficient mice showed CD4+ T cell loss but recovered from infection without lethality. Thus, these results demonstrate that CD8+ T cells are required for protective immunity against a naturally occurring murine pathogen that infects the thymus and establish a novel infection model for MRV in B6 mice, providing the foundation for detailed future studies on MRV with the availability of innumerable mutant mice on the B6 background.

Human Herpesviruses 6A, 6B, and 7

Human roseoloviruses include three different species, human herpesviruses 6A, 6B, and 7 (HHV-6A, HHV-6B, HHV-7), genetically related to human cytomegalovirus. They exhibit a wide cell tropism in vivo and, like other herpesviruses, induce a lifelong latent infection in humans. In about 1% of the general population, HHV-6 DNA is covalently integrated into the subtelomeric region of cell chromosomes (ciHHV-6). Many active infections, corresponding to primary infections, reactivations, or exogenous reinfections, are asymptomatic. They also may cause serious diseases, particularly in immunocompromised individuals, including hematopoietic stem-cell transplant (HSCT) and solid-organ transplant recipients, and acquired immunodeficiency syndrome (AIDS) patients. This opportunistic pathogenic role is formally established for HHV-6 infection and less clear for HHV-7. It mainly concerns the central-nervous system, bone marrow, lungs, gastrointestinal tract, skin, and liver. As the best example, HHV-6 causes both exanthema subitum, a benign disease associated with primary infection, and severe encephalitis associated with virus reactivations in HSCT recipients. Diagnosis using serologic and direct antigen-detection methods currently exhibits limitations. The most prominent technique is the quantification of viral DNA in blood, other body fluids, and organs by means of real-time polymerase-chain reaction (PCR). The antiviral compounds ganciclovir, foscarnet, and cidofovir are effective against active infections, but there is currently no consensus regarding the indications of treatment or specifics of drug administration. Numerous questions about HHV-6A, HHV-6B, HHV-7 are still pending, concerning in particular clinical impact and therapeutic options in immunocompromised patients.

Update on infections with human herpesviruses 6A, 6B, and 7

Human herpesviruses 6A, 6B, and 7 (HHV-6A, HHV-6B, HHV-7) are genetically related to cytomegalovirus. They belong to the Roseolovirus genus and to the Betaherpesvirinae subfamily. They infect T cells, monocytes-macrophages, epithelial cells, and central nervous system cells. These viruses are ubiquitous and are responsible for lifelong chronic infections, most often asymptomatic, in the vast majority of the general adult population. HHV-6B is responsible for exanthema subitum, which is a benign disease of infants. HHV-6A and HHV-6B also cause opportunistic infections in immunocompromised patients: encephalitis, hepatitis, bone marrow suppression, colitis, and pneumonitis. Their etiological role in chronic diseases such as multiple sclerosis, cardiomyopathy, and thyroiditis is still controversial. The pathogenicity of HHV-7 is less clear and seems to be much more restricted. Chromosomal integration of HHV-6A and HHV-6B is transmissible from parents to offspring and observed in about 1% of the general population. This integration raises the question of potential associated diseases and can be a confounding factor for the diagnosis of active infections by both viruses. The diagnosis of HHV-6A, HHV-6B, and HHV-7 infections is rather based on gene amplification (PCR), which allows for the detection and quantification of the viral genome, than on serology, which is mainly indicated in case of primary infection. Ganciclovir, foscarnet, and cidofovir inhibit the replication of HHV-6A, HHV-6B, and HHV-7. Severe infections may thus be treated but these therapeutic indications are still poorly defined.

Serological evidence that activation of ubiquitous human herpesvirus-6 (HHV-6) plays a role in chronic idiopathic/spontaneous urticaria (CIU)

Acute infection with viral pathogens in the herpesviridae family can trigger acute urticaria, and reactivation of herpesviridae is associated with cutaneous urticarial-like syndromes such as drug-induced hypersensitivity syndrome/drug reaction with eosinophilia and systemic symptoms (DRESS). Reactivation of latent herpesviridae has not been studied systematically in chronic idiopathic/spontaneous urticaria (CIU). This review proposes that CIU is an inflammatory disorder with autoimmune features (termed 'CVU' for chronic viral urticaria), based on serology consistent with the hypothesis that reactivation of a latent herpesvirus or -viruses may play a role in CIU. Serology obtained from a cohort of omalizumab (Xolair)-dependent patients with severe CIU was consistent with previous HHV-6 infection, persistent viral gene expression and replication. CIU patients also exhibited serological evidence of increased immune response to HHV-4 (Epstein-Barr virus, or EBV) but not all CIU patients were infected with EBV. These observations, combined with case reports of CIU response to anti-viral therapy, suggest that HHV-6, possibly interacting with HHV-4 in cutaneous tissues, is a candidate for further prospective study as a co-factor in CIU.

A Simple Proteomics-Based Approach to Identification of Immunodominant Antigens from a Complex Pathogen: Application to the CD4 T Cell Response against Human Herpesvirus 6B

Most of humanity is chronically infected with human herpesvirus 6 (HHV-6), with viral replication controlled at least in part by a poorly characterized CD4 T cell response. Identification of viral epitopes recognized by CD4 T cells is complicated by the large size of the herpesvirus genome and a low frequency of circulating T cells responding to the virus. Here, we present an alternative to classical epitope mapping approaches used to identify major targets of the T cell response to a complex pathogen like HHV-6B. In the approach presented here, extracellular virus preparations or virus-infected cells are fractionated by SDS-PAGE, and eluted fractions are used as source of antigens to study cytokine responses in direct ex vivo T cell activation studies. Fractions inducing significant cytokine responses are analyzed by mass spectrometry to identify viral proteins, and a subset of peptides from these proteins corresponding to predicted HLA-DR binders is tested for IFN-γ production in seropositive donors with diverse HLA haplotypes. Ten HHV-6B viral proteins were identified as immunodominant antigens. The epitope-specific response to HHV-6B virus was complex and variable between individuals. We identified 107 peptides, each recognized by at least one donor, with each donor having a distinctive footprint. Fourteen peptides showed responses in the majority of donors. Responses to these epitopes were validated using in vitro expanded cells and naturally expressed viral proteins. Predicted peptide binding affinities for the eight HLA-DRB1 alleles investigated here correlated only modestly with the observed CD4 T cell responses. Overall, the response to the virus was dominated by peptides from the major capsid protein U57 and major antigenic protein U11, but responses to other proteins including glycoprotein H (U48) and tegument proteins U54 and U14 also were observed. These results provide a means to follow and potentially modulate the CD4 T-cell immune response to HHV-6B.

Interplay between β herpesviruses and fungal infections in transplant patients: from the bench to the bedside

ABSTRACT 

The studies on the interplay between β-herpesviruses (CMV, human herpes viruses 6 and 7) and fungi in immunocompromised hosts, have demonstrated that a detailed knowledge of the interaction between the host and the above infectious agents may have a significant clinical relevance. β-herpesviruses may directly be associated to different pathological conditions and may indirectly be involved in the development of opportunistic infections (e.g., fungal infections), allograft rejection and decreased patient survival. Recent in vitro and in vivo studies have pointed out the importance of the microbiome, exposure to microbes and the innate immune system in determining the risk of developing infections; such microbial interactions may modulate the expression of the infection, change the microbial pathogenicity, or increase the immunosuppression.

Laboratory and clinical aspects of human herpesvirus 6 infections

Human herpesvirus 6 (HHV-6) is a widespread betaherpesvirus which is genetically related to human cytomegalovirus (HCMV) and now encompasses two different species: HHV-6A and HHV-6B. HHV-6 exhibits a wide cell tropism in vivo and, like other herpesviruses, induces a lifelong latent infection in humans. As a noticeable difference with respect to other human herpesviruses, genomic HHV-6 DNA is covalently integrated into the subtelomeric region of cell chromosomes (ciHHV-6) in about 1% of the general population. Although it is infrequent, this may be a confounding factor for the diagnosis of active viral infection. The diagnosis of HHV-6 infection is performed by both serologic and direct methods. The most prominent technique is the quantification of viral DNA in blood, other body fluids, and organs by means of real-time PCR. Many active HHV-6 infections, corresponding to primary infections, reactivations, or exogenous reinfections, are asymptomatic. However, the virus may be the cause of serious diseases, particularly in immunocompromised individuals. As emblematic examples of HHV-6 pathogenicity, exanthema subitum, a benign disease of infancy, is associated with primary infection, whereas further virus reactivations can induce severe encephalitis cases, particularly in hematopoietic stem cell transplant recipients. Generally speaking, the formal demonstration of the causative role of HHV-6 in many acute and chronic human diseases is difficult due to the ubiquitous nature of the virus, chronicity of infection, existence of two distinct species, and limitations of current investigational tools. The antiviral compounds ganciclovir, foscarnet, and cidofovir are effective against active HHV-6 infections, but the indications for treatment, as well as the conditions of drug administration, are not formally approved to date. There are still numerous pending questions about HHV-6 which should stimulate future research works on the pathophysiology, diagnosis, and therapy of this remarkable human virus.