Factors affecting the sensitivity of different viruses to interferon.

When the sensitivities to interferon of Newcastle disease virus (NDV) and vesicular stomatitis virus (VSV) were compared by the plaque reduction method in chick embryo cell cultures, NDV was found to be 45-fold more resistant than VSV. This difference was exaggerated when a multiple-cycle yield inhibition method was employed. In marked contrast, when the same viruses were tested by a single-cycle yield inhibition method, the difference in sensitivity to interferon of the two viruses was virtually eliminated. Further investigation showed that, in chick embryo cells exposed to interferon, the resistance to NDV decayed more rapidly than resistance to VSV. This finding explained the divergent results obtained with the two viruses when single- or multiple-cycle replication techniques were employed. Experiments carried out with L cells showed that cellular antiviral resistance decayed much more slowly in these cells than in chick embryo cells. Consequently, when measured by the plaque reduction method in L cells, no difference was observed in the sensitivity to interferon of VSV and NDV(pi), a mutant of NDV which replicates efficiently in L cells. A procedure is suggested for determining the relative sensitivities to interferon of different viruses under conditions which minimize the role of decay of antiviral resistance in the host cells.