Induction of interferon by temperature-sensitive mutants of newcastle disease virus

Spontaneously-selected and mutagen-induced temperature-sensitive ( ts) mutants of Newcastle disease virus (NDV) were used to study interferon induction in chick embryo (CE) cells at temperatures permissive (37°) and nonpermissive (42°) for virus replication. Both infectious and UV-irradiated virus were tested for interferon-inducing ability in cells pretreated or not pretreated with homologous interferon. At 37°, only UV-irradiated NDV was capable of inducing interferon in cells not treated with interferon before infection. In cells pretreated with interferon, on the other hand, both unirradiated and UV-irradiated virus stimulated the production of interferon. At 42°, the interferon-inducing phenotype for some UV-irradiated ts mutants was dependent on whether or not cells were pretreated with interferon. For example, out of 10 mutants examined, one UV-irradiated ts mutant induced interferon in both untreated and interferon pretreated cells; 7 mutants failed to induce in untreated cells but induced from 25–100% of the wild-type level of interferon in cells pretreated with interferon; and two mutants failed to induce interferon in both types of cells. In addition, one mutant (NDV 0 ts-100) induced low or undetectable levels of interferon at both 37° and 42°, conditions under which wild-type virus (NDV 0) produced significant levels of interferon. Co-infection of cells with UV-irradiated ts-100 and a preparation of NDV 0 exposed to prolonged irradiation resulted in considerable production of interferon. These results suggest the possibility that more than one virus function may be involved in interferon induction by NDV in CE cells.