title: Vaccinia virus protein C6 is a virulence factor that binds TBK-1 adaptor proteins and inhibits activation of IRF3 and IRF7.
abstract: Recognition of viruses by pattern recognition receptors (PRRs) causes interferon-β (IFN-β) induction, a key event in the anti-viral innate immune response, and also a target of viral immune evasion. Here the vaccinia virus (VACV) protein C6 is identified as an inhibitor of PRR-induced IFN-β expression by a functional screen of select VACV open reading frames expressed individually in mammalian cells. C6 is a member of a family of Bcl-2-like poxvirus proteins, many of which have been shown to inhibit innate immune signalling pathways. PRRs activate both NF-κB and IFN regulatory factors (IRFs) to activate the IFN-β promoter induction. Data presented here show that C6 inhibits IRF3 activation and translocation into the nucleus, but does not inhibit NF-κB activation. C6 inhibits IRF3 and IRF7 activation downstream of the kinases TANK binding kinase 1 (TBK1) and IκB kinase-ε (IKKε), which phosphorylate and activate these IRFs. However, C6 does not inhibit TBK1- and IKKε-independent IRF7 activation or the induction of promoters by constitutively active forms of IRF3 or IRF7, indicating that C6 acts at the level of the TBK1/IKKε complex. Consistent with this notion, C6 immunoprecipitated with the TBK1 complex scaffold proteins TANK, SINTBAD and NAP1. C6 is expressed early during infection and is present in both nucleus and cytoplasm. Mutant viruses in which the C6L gene is deleted, or mutated so that the C6 protein is not expressed, replicated normally in cell culture but were attenuated in two in vivo models of infection compared to wild type and revertant controls. Thus C6 contributes to VACV virulence and might do so via the inhibition of PRR-induced activation of IRF3 and IRF7.
authors.affiliation: School of Biochemistry and Immunology, Trinity College Dublin, Dublin, Ireland.
authors.forename: Leonie
authors.initials: L
authors.lastname: Unterholzner
authors.forename: Rebecca P
authors.initials: RP
authors.lastname: Sumner
authors.forename: Marcin
authors.initials: M
authors.lastname: Baran
authors.forename: Hongwei
authors.initials: H
authors.lastname: Ren
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Mansur
authors.forename: Nollaig M
authors.initials: NM
authors.lastname: Bourke
authors.forename: Felix
authors.initials: F
authors.lastname: Randow
authors.forename: Geoffrey L
authors.initials: GL
authors.lastname: Smith
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Bowie
mesh_terms: D015967:Gene Expression Regulation, Viral; D005809:Genes, Regulator; D057809:HEK293 Cells; D006801:Humans; D051550:I-kappa B Kinase / Q000235:genetics / Q000378:metabolism; D057131:Immune Evasion; D007113:Immunity, Innate; D050838:Interferon Regulatory Factor-3 / Q000235:genetics / Q000378:metabolism*; D050839:Interferon Regulatory Factor-7 / Q000235:genetics / Q000378:metabolism*; D016899:Interferon-beta / Q000378:metabolism; D016328:NF-kappa B / Q000235:genetics / Q000378:metabolism; D016366:Open Reading Frames; D010766:Phosphorylation; D010957:Plasmids; D011485:Protein Binding / Q000235:genetics; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D051192:Receptors, Pattern Recognition / Q000378:metabolism; D015398:Signal Transduction; D014158:Transcription, Genetic; D014616:Vaccinia virus / Q000235:genetics* / Q000378:metabolism / Q000502:physiology; D014764:Viral Proteins / Q000235:genetics* / Q000378:metabolism; D037521:Virulence Factors / Q000235:genetics / Q000378:metabolism; D014779:Virus Replication
chemical_list: C494232:IRF3 protein, human; C494234:IRF7 protein, human; D050838:Interferon Regulatory Factor-3; D050839:Interferon Regulatory Factor-7; D016328:NF-kappa B; D051192:Receptors, Pattern Recognition; D014764:Viral Proteins; D037521:Virulence Factors; D016899:Interferon-beta; D017346:Protein Serine-Threonine Kinases; C403191:TBK1 protein, human; D051550:I-kappa B Kinase