Interferon induction by viruses. VI. Reovirus: virion genome dsRNA as the interferon inducer in aged chick embryo cells

The interferon-inducing particle (IFP) activity of avian and human reoviruses in aged chick embryo cells was determined by analyzing dose (multiplicity)-response (interferon yield) curves. These curves fit best a model in which each cell infected with greater than or equal to 1 IFP produces a quantum yield of interferon. Avian reovirus stocks contained as many as 60 times more IFP than plaque-forming particles (PFP). Upon UV-irradiation the ratio of IFP:PFP became 197, suggesting that virtually every physical particle of avian reovirus could function as an interferon-inducing particle. Thus, about one-third of the non-infectious particles were intrinsically IFP and the other two-thirds could be converted to IFP status at an optimal dose of UV radiation, the equivalent of 9.4 lethal hits, i.e., 8000 ergs/mm2. UV-irradiated avian reovirus induced about twice the usual yield of interferon on a per cell basis. Wildtype human reovirus (type 3) and mutants ts201(A,RNA+) and ts447(C,RNA-) were excellent inducers of interferon, but only about 1 in 3 infectious particles functioned as an interferon-inducing particle, meaning that virtually all physical particles failed to function as IFP, UV-irradiation of human reoviruses resulted in a slight loss of IFP activity. Our data support the hypothesis that virion genome dsRNA constitutes the interferon inducer moiety of avian reoviruses and that in its permissive host cell the processing of genome dsRNA from most particles to a putative recognition site in the cytoplasm occurs naturally with a high probability. For human reovirus this is a much rarer event which may be intrinsic only to infectious virus, and may require limited transcription for expression. The sensitivity of the avian reovirus-aged chick embryo cell system recommends it for further study on the mechanism of interferon induction by virions containing pre-existing dsRNA.