Differences i tropism and viral assembly pathways of human herpesvirus 6A and 6B (HHV-6A and 6B) and association of host cell proteins in HHV-6A virions

Abstract: Human herpesvirus 6A and 6B (HHV-6A and 6B) are considered to be T-lymphotropic viruses. The overall nucleotide sequence identity between the two variants is 90% and each differs by only nine variant-specific open reading frames. HHV-6A and 6B have often been recognized as one virus and simply termed HHV-6. HHV-6 is considered to be an emerging pathogen, involved in seizures, epilepsy, encephalitis, multiple sclerosis (MS), and morbidity in immunocompromised patients. However, the respective role of HHV-6A and 6B in neurological disorders is poorly understood. There is also a scarcity of information from direct comparisons of infections of neural cells with HHV-6A and 6B. The first aim was to analyze the growth properties of HHV-6A and 6B in glial cells and in T cells in vitro. The second aim was to investigate whether cellular proteins are associated with HHV-6 as a potential mechanism to induce autoimmune responses, as suggested in MS. Our results indicated that HHV-6A actively infected human astrocytes and an oligodendroglial cell line, as shown by increase in viral load in cells, detection of viral RNA and viral proteins. HHV-6B, on the other hand, had a non-productive infection in these cell types. These studies suggest that HHV-6A and 6B have differential tropisms and patterns of infection for astrocytes and oligdendroglial cells in vitro. To further explore the differences between HHV-6A and 6B, an ultrastructural study was conducted in SupT-1 T cells and astrocytes. Both variants replicated in SupT-1 T cells and formed viral particles. The tegument is the least understood structure of the viral particle and both variants were able to form intranuclear membrane structures called tegusomes, where tegumentation occurred. Though the majority of nucleocapsids of HHV-6A and 6B acquired their tegument while budding through the nuclear membrane. In a low proportion of HHV-6A-infected cells, annulate lamellae (AL) were present in the cytoplasm. These were not common structures, indicating that AL are not essential for the viral egress of HHV-6A in these cells. In astrocytes, only HHV-6A formed viral particles. The life cycle of HHV-6A in astrocytes resembled the life cycle in SupT-1 T cells. The role of HHV-6 in MS is not well understood. HHV-6 may participate in MS by incorporating cellular proteins into viral particles, which may elicit autoimmune responses. Purification of HHV-6A released from JJHAN T cells resulted in intact, infectious virions. Western blot analyses suggest that clathrin, ezrin, Tsg101, CD46 and actin are associated with HHV-6A virions. In conclusion, the two variants have different growth characteristics in all cell types investigated. HHV-6A, compared to HHV-6B, is more neurotropic in vitro. HHV-6A also induces early enlargement of cells and neo-forms structures, such as AL, in contrast to 6B. Furthermore, these variants have different intracellular requirements for production of viral particles. Last, cellular proteins are associated with HHV-6A virions, which suggests that cellular proteins might be associated with HHV-6A during egress in oligodendrocytes, the main cell type affected in MS.