Mechanism of interferon action: inhibition of vesicular stomatitis virus replication in human amnion U cells by cloned human gamma-interferon. II. Effect on viral macromolecular synthesis.

The effects of recombinant human gamma-interferon (IFN-gamma) on vesicular stomatitis virus (VSV) macromolecular synthesis in human amnion U cells were examined. Saturating concentrations of IFN-gamma caused only a 3 to 5-fold reduction of viral protein synthesis in wild-type VSV-infected cells, an extent insufficient to account for the 100-fold inhibition of viral infectivity. By use of the VSV mutant tsG41, which is competent in RNA transcription but defective in RNA replication at 40 degrees C, it was shown that the apparent IFN-induced inhibition of viral protein synthesis was likely due to a reduction in the synthesis of primary transcripts in IFN-gamma-treated U cells. Dot blot hybridization analysis revealed that saturating concentrations of IFN-gamma reduced both primary (measured with mutant tsG41-infected U cells) and total (measured with wild-type-infected U cells) viral RNA synthesis by about 4-fold, an extent of inhibition comparable to the observed reduction in viral protein synthesis. Analysis of RNA, fractionated by agarose gel electrophoresis after denaturation with glyoxal, with cDNA probes to individual VSV mRNAs did not reveal any detectable difference in the structural integrity of VSV mRNA isolated from IFN-gamma treated as compared to untreated U cells. These results suggest that IFN-gamma treatment causes a small reduction in the efficiency of transcript formation catalyzed by input parental virions. However, the results also indicate that the principal cause of the IFN-gamma-induced inhibition of VSV replication in U cells is the alteration of a step in replication other than viral macromolecular synthesis. This implies that the molecular mechanism of viral inhibition by IFN-gamma is fundamentally different from that of IFN-alpha in human amnion U cells.