PIF, a highly sensitive plaque assay for the induction of interferon

Abstract

Plaques produced by our P − mutants of vesicular stomatitis virus (VSV), which are defective in the inhibition of total protein synthesis in infected cells, stop increasing in size after several days of incubation under conditions where those produced by P + mutants increase linearly in size. The basis for the arrest in size of P − plaques has been shown to be due to the induction of interferon, and the phenotype is termed PIF + for “plaque interferon positive.” Thus P − plaques can inhibit the increase in size of adjacent P + plaques and the factor responsible has the biological and physical properties of interferon. Also P − mutants, when plaqued on VERO cells which cannot be induced to produce interferon, produce plaques which increase linearly in size like P + plaques. Finally, the inclusion of anti-interferon antibody in the overlay medium also causes P − mutants to produce plaques like P + mutants. VERO cells were found to be useful to separate the effects of is mutations on plaque size from the interferon effect. Using other cell types the latter effect (PIF assay) can be used as an assay for the ability of viruses to induce interferon, for the isolation of PIF + mutants from PIF − virus, and as a test for the ability of cells to respond to interferon induction by PIF + viruses. The assay can be increased in sensitivity through the use of specific cell types and of cell cultures preincubated for several days in the stationary phase of growth. In its most sensitive form, the assay could detect PIF + behavior in certain ts mutants of VSV at permissive temperatures and in VSV mutants emerging from persistent infection. The assay has also been used to isolate novel mutants of VSV which show alterations in the viral P function.