Vaccinia Virus Protein C6 Inhibits Type I IFN Signalling in the Nucleus and Binds to the Transactivation Domain of STAT2

Abstract

The type I interferon (IFN) response is a crucial innate immune signalling pathway required for defense against viral infection. Accordingly, the great majority of mammalian viruses possess means to inhibit this important host immune response. Here we show that vaccinia virus (VACV) strain Western Reserve protein C6, is a dual function protein that inhibits the cellular response to type I IFNs in addition to its published function as an inhibitor of IRF-3 activation, thereby restricting type I IFN production from infected cells. Ectopic expression of C6 inhibits the induction of interferon stimulated genes (ISGs) in response to IFNα treatment at both the mRNA and protein level. C6 inhibits the IFNα-induced Janus kinase/signal transducer and activator of transcription (JAK/STAT) signalling pathway at a late stage, downstream of STAT1 and STAT2 phosphorylation, nuclear translocation and binding of the interferon stimulated gene factor 3 (ISGF3) complex to the interferon stimulated response element (ISRE). Mechanistically, C6 associates with the transactivation domain of STAT2 and this might explain how C6 inhibits the type I IFN signalling very late in the pathway. During virus infection C6 reduces ISRE-dependent gene expression despite the presence of the viral protein phosphatase VH1 that dephosphorylates STAT1 and STAT2. The ability of a cytoplasmic replicating virus to dampen the immune response within the nucleus, and the ability of viral immunomodulators such as C6 to inhibit multiple stages of the innate immune response by distinct mechanisms, emphasizes the intricacies of host-pathogen interactions and viral immune evasion.

The actin nucleator Spir-1 is a virus restriction factor that promotes IRF3 activation

“…In contrast, Myc-tagged GFP had no effect, and VACV protein C6 was inhibitory, as described (Unterholzner et al, 2011). Spir-1 did not affect activation of ISRE-dependent gene expression downstream of type I IFN, whereas C6, but not N1, was inhibitory (Stuart et al, 2016) (Fig. 2B).…”

Ten Strategies of Interferon Evasion by Viruses

“…This paper describes how the NS5 protein of yellow fever virus binds to STAT2 upon exposure to IFN, preventing binding of this transcription factor to the IFN-responsive promoter elements of the ISGs, and inhibiting the antiviral action of IFN ( Figure 2 ). The binding-dependent inhibition of STAT1 and STAT2 have also been demonstrated for other viral proteins encoded by a diverse range of viruses, such as the V and W proteins of paramyxoviruses ( Rodriguez et al., 2002 , Shaw et al., 2004 ), the P protein of rabies virus ( Vidy et al., 2005 ), or the C6 protein of vaccinia virus ( Stuart et al., 2016 ), among others.…”

Removal of the C6 vaccinia virus interferon-β inhibitor in the hepatitis C vaccine candidate MVA-HCV elicited in mice high immunogenicity in spite of reduced host gene expression

“…We have previously described that deletion of the immunomodulatory VACV C6L gene, encoding an inhibitor of IFN-β (21, 23), in the vector backbone of an MVA-based HIV/AIDS vaccine candidate induced the production of IFN-β and type I IFN inducible genes in infected innate immune cells and elicited an enhancement in the HIV-specific immunogenicity in immunized mice (22, 24). Thus, to examine whether VACV C6L gene could also influence the type I IFN production and the immunogenicity profile of HCV antigens delivered from a poxvirus vector, we deleted C6L from the vector backbone of the HCV vaccine candidate MVA-HCV (expressing the nearly full-length genome from HCV genotype 1a) (26), generating the recombinant MVA-HCV ΔC6L deletion mutant (Fig.…”