Transient resistance of influenza virus to interferon action attributed torandom multiple packaging and activity of NS genes

Interferon (IFN) action survival curves for an avian influenza virus (AIV) in chicken or quail cells showed that 40-60% of the virions in a stock of v irus were highly sensitive to the inhibitory effects of chicken IFN-alpha ( ChIFN-alpha), whereas the rest were up to 100 times less sensitive. This gr eater resistance to IFN was transient, that is, was not a stable characteri stic, in that virus stocks grown from plaques that formed in the presence o f 50-800 U/ml IFN gave rise to virus populations that contained both sensit ive and resistant virions. If AIV was serially passaged several times in th e presence of IFN, the proportion of transiently IFN-resistant virus was gr eater. We propose a model to account for this transient resistance of AIV t o IFN action based on the reported inactivation of the dsRNA-dependent prot ein kinase (PKR) and its activator dsRNA by the NS1 protein of influenza vi rus and also on the increase in the survival of AIV in IFN-treated cells ex posed to 2-aminopurine, a known inhibitor of PKR. We suggest that IFN-resis tant AIV is generated from a random packaging event that results in virions that contain two or more copies of RNA segment 8, the gene segment that en codes the NS1 protein of AIV, and that these virions will produce correspon dingly elevated levels of NS1. The experimental data fit well to theoretica l curves based on this model and constructed from the fraction of virus in the population expected by chance to contain one, two, or three copies of t he NS gene when packaging an average of 12 influenza gene segments that inc lude the 8 segments essential for infectivity.