HHV-6 and HHV-7 as exogenous agents in human lymphocytes

Human herpesviruses 6 and 7 (HHV-6 and HHV-7), the newly recognized lymphotropic members of the herpesvirus family, were isolated from peripheral blood mononuclear cells (PBMC) of healthy individuals. HHV-7 was recovered from PBMC after exposing the cells to conditions leading to T cell activation. No virus could be recovered from quiescent non-activated cultures, suggesting that the virus resided latently in these cells and that it could be induced from latency by T cell activation. HHV-6 could not be recovered from PBMC using similar approaches. However, it could be repeatedly reactivated from PBMC after infection of the cells with HHV-7. Upon further passaging in culture, HHV-6 took over the population and virus stocks appeared to be clear of the original HHV-7 helper virus. The results of this study suggest that genomes of the lymphotropic herpesviruses are resident in PBMC of healthy adults. HHV-7 can supply the functions required for its reactivation from latency. It can also act as a helper virus for the reactivation of HHV-6 from latency. Following the initial reactivation step, HHV-6 can replicate autonomously in the cells. These results may be relevant to the findings that HHV-6 infection is induced in immunosuppressed patients and in patients undergoing organ transplantation. Furthermore, these studies may exemplify situations whereby propagation of known agent(s) in vitro results in the activation of human pathogens resident latently in the cells.