Human herpesvirus 6A (HHV‑6A) is a double‑stranded DNA virus belonging to the Roseolovirus genus of the Betaherpesvirinae subfamily. It is one of two closely related HHV‑6 species and can infect both neuronal and glial cells.
Biology and Pathogenesis
HHV‑6A shares the typical herpesvirus architecture: a linear double‑stranded DNA genome encased in an icosahedral capsid, surrounded by a tegument and lipid envelope with glycoproteins. Unlike HHV‑6B, which is the known agent of exanthem subitum (roseola), HHV‑6A has not been firmly linked to a specific disease but is frequently detected in the central nervous system and placenta. Studies show that HHV‑6A and HHV‑6B can infect glutamatergic and dopaminergic neurons and glial cells in vitro, whereas GABAergic neurons are resistant. HHV‑6A infection tends to produce marked cytopathic effects such as cell aggregation, syncytium formation and cell death, while HHV‑6B causes less dramatic changes. Infection with HHV‑6A triggers Toll‑like receptor 9 signalling and induces interleukin 10 production in infected cells. It also increases levels of vascular endothelial growth factor C and insulin‑like growth factor binding protein 6. The virus establishes latency in monocytes, macrophages and possibly brain tissue. Primary infection likely occurs early in life through saliva or vertical transmission, and serologic surveys indicate that by five years of age most individuals have antibodies to HHV‑6. Reactivation may contribute to neurological disorders, multiple sclerosis and complications after organ transplantation, although causal relationships remain under investigation.
Research Insights
Experimental studies comparing the two HHV‑6 species reveal that both viruses can infect neurons and glial cells but display distinct patterns of immune modulation and cytopathic effects. HHV‑6A‑infected cultures exhibit pronounced cell clustering and syncytium formation, whereas HHV‑6B infections are more localized. HHV‑6A stimulates TLR9‑dependent cytokine responses, including elevated interleukin 10, which may contribute to immune evasion. Increased production of VEGF‑C and IGFBP6 by HHV‑6A‑infected glial cells suggests that the virus can alter angiogenic and growth factor pathways. These observations support the hypothesis that HHV‑6A may have a greater neurotropic potential and unique immunologic effects compared with HHV‑6B. However, clear clinical correlations are still lacking. In transplant medicine, reactivation of HHV‑6A or HHV‑6B has been associated with graft rejection and encephalitis, highlighting the need for monitoring and research. HHV‑6A is a neurotropic betaherpesvirus that shares a close relationship with HHV‑6B but differs in cell tropism and immune modulation. Although definitive disease associations are still being explored, its ability to infect neural cells and modulate cytokine responses suggests a role in certain neurologic and immunologic conditions. Related Terms: Human Herpesvirus 6B, Human Herpesvirus 7, Cytomegalovirus, Epstein–Barr Virus, Varicella‑Zoster Virus