Type 1 IFN-independent activation of a subset of interferon stimulated genes in West Nile virus Eg101-infected mouse cells

Although infection of mouse embryofibroblasts (MEFs) with WNV Eg101 induced interferon (IFN) beta production and STAT1 and STAT2 phosphorylation, these transcription factors (TFs) were not detected in the nucleus or on the promoters of four IRF-3-independent interferon stimulated genes (ISGs): Oas1a and Irf7 (previously characterized as IFN/ISGF3-dependent), Oas1b and Irf1. These ISGs were upregulated in WNV Eg101-infected STAT1−/−, STAT2−/−, and IFN alpha/beta receptor −/− MEFs. Although either IRF-3 or IRF-7 could amplify/sustain Oas1a and Oas1b upregulation at later times after infection, these factors were not required for the initial gene activation. The lack of upregulation of these ISGs in WNV Eg101-infected IRF-3/9−/− MEFs suggested the involvement of IRF-9. Activation of Irf1 in infected MEFs did not depend on any of these IRFs. The data indicate that additional alternative activation mechanisms exist for subsets of ISGs when a virus infection has blocked ISG activation by the canonical IFN-mediated pathway.

► STAT1/2 phosphorylation but not nuclear translocation occurs in WNV infected MEFs.
► Oas1a, Oas1b, Irf7 and Irf1 are activated in the absence of STAT promoter occupancy.
► IPS-1, IRF-9 and -3 or -7 are involved in the activation of the first three ISGs.
► However, Irf1 upregulation in WNV-infected MEFs is IRF-9, -3 and -7-independent.