Identification by electron microscopy of the maturation steps in vaccinia virus morphogenesis inhibited by the interferon-induced enzymes, protein kinase (PKR), 2-5A synthetase, and nitric oxide synthase (iNOS).

Interferons (IFN) play a major role as a first-line host defense mechanism against viral infections. As treatment of animal cells with IFN induces a large number of genes, it has been difficult to assign the role of these genes in the antiviral action of IFN. Vaccinia virus (VV) is an ideally suited system to study IFN action because all steps in viral morphogenesis can be followed easily by electron microscopy (EM) of ultrathin sections from infected cells. To define the role of IFN-induced genes in viral morphogenesis, we have independently expressed from VV recombinants in primary chicken embryo fibroblast (CEF) cells each of the three IFN-induced genes encoding protein kinase (PKR), 2-5A synthetase, and inducible nitric oxide synthase (iNOS). By EM analysis, we have identified the steps in VV morphogenesis that are affected by each of the IFN-induced enzymes in comparison with untreated and IFN-treated cells. We found that in cells pretreated with IFN and infected with VV, immature virus (IV) is formed, but further stages of maturation are blocked. In cells infected with a VV recombinant expressing PKR (VV-PKR), there is severe inhibition on virus factories, and only few IV are formed. In cells infected with a VV recombinant expressing 2-5A synthetase (VV-2-5A), VV assembly is inhibited at or after IV formation. In cells infected with a VV recombinant expressing iNOS (VV-iNOS), all stages in VV morphogenesis are observed but with aberrant forms. In addition to the effects on viral assembly, in cells infected with either VV-PKR, VV-2-5AS, or VV-iNOS, there is nucleus condensation characteristic of apoptosis. Our findings have identified the steps in VV morphogenesis inhibited by PKR, 2-5A, and iNOS, provided a distinction between these effects, and highlighted a functional redundancy of the IFN system to block viral infection and to induce apoptosis.