Effect of interferon on exogenous murine leukemia virus infection

When interferon (IF)-treated NIH/3T3 cells were exogenously infected with the Moloney strain of murine leukemia virus (M-MLV), no viral progeny release was detected as long as IF remained in the medium. This was evidently an inhibition of the virus replication and not a consequence of interfering with the establishment of the infection since, when IF was removed either before infection or later after infection, virus release gradually approached the rate of untreated control after a temporary delay. Furthermore, a direct examination revealed that IF treatment had no effect on the formation of infectious centers. IF treatment led to a reduced viral RNA synthesis. A similar inhibition of viral RNA synthesis was observed when the potent protein synthesis inhibitor cycloheximide (CH) was added early after infection. However, when added at a late stage, the drug had no effect on viral RNA synthesis. It appears, therefore, that IF-induced inhibition of viral RNA synthesis is not a feedback consequence of an inhibition of a later step but, rather, an inhibition of some early step. This conclusion was substantiated by the finding that when IF was eliminated from pretreated cultures up to 10 hr before infection, progeny release was still affected.