Circulating functional T cells specific to human herpes virus 6 (HHV6) antigens in individuals with chromosomally integrated HHV6

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.

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.

HHV-6 Specific T-Cell Immunity in Healthy Children and Adolescents

Objective: Primary infection with human herpes virus 6 (mainly HHV-6B) commonly occurs in the first 2 years of life leading to persistence and the possibility of virus reactivation later in life. Consequently, a specific cellular immune response is essential for effective control of virus reactivation. We have studied cell-mediated immune response to HHV-6 (U54) in healthy children and adolescents. Materials and Methods: By flow cytometry, the amount of cytokine (interferon gamma-IFN- γ, interleukin 2-IL-2, tumor necrosis factor alpha-TNF-α) secreting T-cells were measured after 10 days of pre-sensitization and 6 h of re-stimulation with mixtures of pooled overlapping peptides from U54, staphylococcal enterotoxin B (SEB, positive control), or Actin (negative control) in healthy children and adolescents without any underlying immune disorder or infectious disease. Results: All individuals showed a virus-specific response for at least one cytokine in either CD4+ or CD8+ cells. Percentages of individuals with HHV-6-specific TNF-α response in CD4+ (48% of individuals) as well as CD8+ (56% of individuals) were always the highest. Our data show significantly higher frequencies of HHV-6-specific TNF-α producing CD8+ T-cells in individuals older than 10 years of life (p = 0.033). Additionally, the frequency of HHV-6 specific TNF-α producing CD8+ T-cells positively correlated with the age of the individuals. Linear regression analysis showed a positive relation between age and frequency of HHV-6-specific TNF-α producing CD8+ T-cells. Conclusion: Results indicate that T-cell immune response against HHV-6 is commonly detectable in healthy children and adolescents with higher frequencies of antigen-specific T-cells in older children and adolescents possibly reflecting repeated stimulation by viral persistence and subclinical reactivation.

Latency, Integration, and Reactivation of Human Herpesvirus-6

Human herpesvirus-6A (HHV-6A) and human herpesvirus-6B (HHV-6B) are two closely related viruses that infect T-cells. Both HHV-6A and HHV-6B possess telomere-like repeats at the terminal regions of their genomes that facilitate latency by integration into the host telomeres, rather than by episome formation. In about 1% of the human population, human herpes virus-6 (HHV-6) integration into germline cells allows the viral genome to be passed down from one generation to the other; this condition is called inherited chromosomally integrated HHV-6 (iciHHV-6). This review will cover the history of HHV-6 and recent works that define the biological differences between HHV-6A and HHV-6B. Additionally, HHV-6 integration and inheritance, the capacity for reactivation and superinfection of iciHHV-6 individuals with a second strain of HHV-6, and the role of hypomethylation of human chromosomes during integration are discussed. Overall, the data suggest that integration of HHV-6 in telomeres represent a unique mechanism of viral latency and offers a novel tool to study not only HHV-6 pathogenesis, but also telomere biology. Paradoxically, the integrated viral genome is often defective especially as seen in iciHHV-6 harboring individuals. Finally, gaps in the field of HHV-6 research are presented and future studies are proposed.

HHV-6A/B Integration and the Pathogenesis Associated with the Reactivation of Chromosomally Integrated HHV-6A/B

Unlike other human herpesviruses, human herpesvirus 6A and 6B (HHV-6A/B) infection can lead to integration of the viral genome in human chromosomes. When integration occurs in germinal cells, the integrated HHV-6A/B genome can be transmitted to 50% of descendants. Such individuals, carrying one copy of the HHV-6A/B genome in every cell, are referred to as having inherited chromosomally-integrated HHV-6A/B (iciHHV-6) and represent approximately 1% of the world's population. Interestingly, HHV-6A/B integrate their genomes in a specific region of the chromosomes known as telomeres. Telomeres are located at chromosomes' ends and play essential roles in chromosomal stability and the long-term proliferative potential of cells. Considering that the integrated HHV-6A/B genome is mostly intact without any gross rearrangements or deletions, integration is likely used for viral maintenance into host cells. Knowing the roles played by telomeres in cellular homeostasis, viral integration in such structure is not likely to be without consequences. At present, the mechanisms and factors involved in HHV-6A/B integration remain poorly defined. In this review, we detail the potential biological and medical impacts of HHV-6A/B integration as well as the possible chromosomal integration and viral excision processes.