Interferon induced by UV-irradiated murine cytomegalovirus decreases type C virus expression in BALB/c cells treated by 5-iodo-deoxyuridine or cycloheximide

UV-irradiated mouse cytomegalovirus (MCMV) activates at low incidence (2.0 X 10(-4) - 8.0 X 10(-4)) a xenotropic type C virus in Kirsten sarcoma virus-transformed nonproducer BALB 3T3 (KBALB) cells. When KBALB cells are treated with UV-MCMV and subsequently with potent chemical inducers, such as 5-Iododeoxyuridine (IdUrd) or cycloheximide, the activation frequencies of type C virus are significantly lower than in controls exposed to the chemical inducers alone: 60% for IdUrd and 44% for cycloheximide. This decrease of activation appears to be mediated by endogenous interferon (IFN) induced by UV-MCMV in KBALB cells, as could be proven by neutralization of the inhibitory activity. Indeed, after exposure of UV-MCMV-infected cells to anti-IFN gamma globulin, the level of C type virus increases, reaching values obtained with IdUrd or cycloheximide in the absence of UV-MCMV. The inhibitory activity associated with UV-MCMV is dose-dependent and the xenotropic type C virus appears to be more sensitive than the ecotropic (N-tropic) retrovirus of BALB/c cells.