Accessory Protein 5a Is a Major Antagonist of the Antiviral Action of Interferon against Murine Coronavirus

Abstract

The type I interferon (IFN) response plays an essential role in the control of in vivo infection by the coronavirus mouse hepatitis virus (MHV). However, in vitro, most strains of MHV are largely resistant to the action of this cytokine, suggesting that MHV encodes one or more functions that antagonize or evade the IFN system. A particular strain of MHV, MHV-S, exhibited orders-of-magnitude higher sensitivity to IFN than prototype strain MHV-A59. Through construction of interstrain chimeric recombinants, the basis for the enhanced IFN sensitivity of MHV-S was found to map entirely to the region downstream of the spike gene, at the 3 end of the genome. Sequence analysis revealed that the major difference between the two strains in this region is the absence of gene 5a from MHV-S. Creation of a gene 5a knockout mutant of MHV-A59 demonstrated that a major component of IFN resistance maps to gene 5a. Conversely, insertion of gene 5a, or its homologs from related group 2 coronaviruses, at an upstream genomic position in an MHV-A59/S chimera restored IFN resistance. This is the first demonstration of a coronavirus gene product that can protect that same virus from the antiviral state induced by IFN. Neither protein kinase R, which phosphorylates eukaryotic initiation factor 2, nor oligoadenylate synthetase, which activates RNase L, was differentially activated in IFN-treated cells infected with MHV-A59 or MHV-S. Thus, the major IFN-induced antiviral activities that are specifically inhibited by MHV, and possibly by other coronaviruses, remain to be identified. Type I interferons (alpha interferon [IFN-␣]and IFN-␤) are essential for host protection against the coronavirus mouse hepatitis virus (MHV) (3,4,16,21,36). Knockout mice lacking the IFN-␣/␤ receptor rapidly succumb to MHV infection at doses that are controlled and cleared by wild-type mice (4, 16). On the other hand, most strains of MHV, including commonly studied laboratory strain A59, are poor inducers of IFN (13,37,49,54,61,62) and are relatively insensitive to IFN in tissue culture (37,58,61). Such a pattern suggests that MHV has evolved mechanisms to evade or antagonize the action of this cytokine, even though IFN induction and responses by specialized classes of cells manage to ultimately overcome infection in vivo (3,4,(34)(35)(36)(37).The type I IFN system functions at three levels to combat viral infections (19,33,40,43). Initially, extracellular and intracellular pattern recognition receptors (PRRs) respond to the presence of individual classes of pathogen-associated molecular signatures displayed by viruses and other microorganisms. PRR stimulation triggers signaling events that give rise to the activation or recruitment of multiple transcription factors necessary for induction of the IFN-␤ gene. Most notable among these components are cytoplasmic IFN regulatory factor 3 (IRF-3) and NF-B, both of which must translocate to the nucleus to upregulate transcription. At the second level, IFN-␤ binds to the IFN-␣/␤ receptor, leading to a second wave of sig...

Molecular Evolution of Human Coronavirus Genomes

“…Conversely, the HCoV-OC43 protein (encoded by ORF5, originally denoted NS12.9) is unrelated to the other CoV viroporins, both in terms of sequence and of domain topology [34] ( Figure 2C). A protein homologous to the HCoV-OC43 viroporin is instead encoded by MHV (accessory protein NS5a) and functions as an antagonist of IFNinduced antiviral responses [34,36]. Whether the HCoV-OC43 viroporin has the same IFNantagonizing activity remains to be investigated; however, mutant viruses lacking ORF5 display growth defects in vitro and in vivo, as well as reduced virulence in mice [34].…”

The ns12.9 Accessory Protein of Human Coronavirus OC43 Is a Viroporin Involved in Virion Morphogenesis and Pathogenesis

“…The type I interferons (IFNs) play an essential role in protection against CoV infection (64,65). MHV 5a, a homologous accessory protein of HCoV-OC43 ns12.9, recently has been reported to be an antagonist of IFN-induced antiviral action (66). Because HCoV-OC43-⌬ns12.9 presents propagation reduction and virulence attenuation in mice, ns12.9 may be a viral factor to antagonize IFN antiviral responses during HCoV-OC43 infection, which should be investigated further.…”

Human coronaviruses: Viral and cellular factors involved in neuroinvasiveness and neuropathogenesis

“…Moreover, its acetyl-esterase activity strongly enhances the production of infectious virions, which can disseminate in murine mixed primary CNS cultures (Desforges et al, 2013a). Coronaviruses also possess non-structural (ns) or accessory proteins that appear to mainly play a role in pathogenesis and in the virushost interactions (Narayanan et al, 2008) as well as in virulence and tropism associated with the capacity to replicate in different cell types and organs (Cruz et al, 2011;Dedeurwaerder et al, 2013;Koetzner et al, 2010;Zhao et al, 2013Zhao et al, , 2012Zhao et al, , 2011.…”