Marek's disease virus-mediated enhancement of avian leukosis virus gene expression and virus production.

Direct interaction between two viruses in coinfected cells may promote replication and pathogenesis of one or both virus types. Synergism between herpesviruses and retroviruses is an important factor in diagnosis, treatment, and prevention of animal and human diseases. In birds, Marek's disease virus (MDV) may be an important cofactor in avian leukosis virus induced disease. Infection of susceptible cells with non-oncogenic serotype 2 MDV, an avian herpesvirus, and Rous-associated virus type 2 (RAV-2 ALV), a leukemogenic avian retrovirus, results in enhanced (greater than 3-fold) transcription of retroviral genes, relative to infection with ALV alone. A direct relationship between concentrations of retroviral gene expression and amount of input MDV suggests that MDV-encoded or -induced factors are responsible for enhanced ALV gene expression, ultimately leading to increased accumulation of ALV-specific RNA (greater than 5-fold) and protein (greater than 10-fold). At lower doses of input MDV, ALV virus production increased over 3-fold, relative to cells infected with ALV alone. Interactive laser cytometry was used to detect accumulation of both MDV and ALV antigens within single cells from coinfected cultures. These results suggest a direct role for MDV-encoded or -induced factors in enhancement of ALV gene expression and demonstrate the importance of herpesviruses as cofactors in retrovirus replication and pathogenesis in coinfected cells.