Interferon potentiation occurs at a pretranscriptive stage.

Combination of interferon-gamma (IFN-gamma) with a mixture of IFN-alpha and IFN-beta (IFN-alpha/beta) produce a potentiated antiviral state. The onset of induction of the potentiated antiviral state was observed to be more rapid than the onset of induction of either IFN-gamma or IFN-alpha/beta separately. The onset of viral resistance to combined IFN-gamma and IFN-alpha/beta (25 U/ml each) began after 2 h, compared with 4 h for IFN-alpha/beta (25 U/ml) alone and 8 h for IFN-gamma (25 U/ml) alone. By adding actinomycin D at various times after interferon treatment, this more rapid onset of induction of the potentiated antiviral state was shown to be the result of a more rapid onset of cellular transcription. To determine whether increased concentrations of IFN-alpha/beta acted through a similar mechanism, we performed kinetic experiments with several concentrations of IFN-alpha/beta and with combined preparations of IFN-gamma and IFN-alpha/beta. The findings showed that treatment of cells with the combined interferons and with a high concentration of IFN-alpha/beta resulted in more rapid onset of cellular transcription and more rapid development of the antiviral state. Thus, the cellular response to potentiation appeared to involve perception by the cell of the combined interferons as a higher-than-expected effective concentration of interferon, resulting in more rapid onset of cellular transcription of mRNA for antiviral proteins.