Efficient Marek's disease virus (MDV) and herpesvirus of turkey infection of the QM7 cell line that does not contain latent MDV genome

Marek's disease virus (MDV; also known as Gallid herpesvirus 2, MDV-1) causes oncogenic disease in chickens producing clinical signs that include lymphomas, visceral tumours, nerve lesions, and immunosuppression. MDV vaccines are widely used and mostly produced using primary cells: chicken embryo fibroblast cells, duck embryo fibroblast cells, chicken embryo kidney cells or chicken kidney cells. An immortalized cell line that can be used to manufacture the virus has long been desired. In this report, we demonstrate that QM7 cells were susceptible to infection with either MDV or herpesvirus of turkey (HVT; also known as Meleagrid herpesvirus 1, MDV-3). Polymerase chain reaction analysis with primers amplifying selected MDV genes revealed that QM7 cells did not contain these sequences. However, MDV genes were detected in QT35 cells, which have been reported to harbour latent MDV virus. Transfection of naked MDV DNA initiated efficient infection of QM7 cells. In addition, QM7 cell lysate, clarified supernatant, and QM7 cell pellet infected with MDV were negative for reverse transcriptase activity, indicating absence of endogenous retrovirus. QM7 cells were also found to be free of other avian pathogens in a chick embryo inoculation test. In vivo studies of MDV growing in QM7 cells showed the virus retained its pathogenicity and virulence. In ovo experiments demonstrated that both HVT and MDV propagated in QM7 cells did not interfere with hatchability of injected eggs, and viruses could be re-isolated from hatched chicks. The results suggest that QM7 could be a good host cell line for growing both MDV and HVT.