Npro of Classical Swine Fever Virus Suppresses Type III Interferon Production by Inhibiting IRF1 Expression and Its Nuclear Translocation

Abstract

Classical swine fever virus (CSFV) causes a contagious disease of pigs. The virus can break the mucosal barrier to establish its infection. Type III interferons (IFN-λs) play a crucial role in maintaining the antiviral state in epithelial cells. Limited information is available on whether or how CSFV modulates IFN-λs production. We found that IFN-λ3 showed dose-dependent suppression of CSFV replication in IPEC-J2 cells. Npro-deleted CSFV mutant (∆Npro) induced significantly higher IFN-λs transcription from 24 h post-infection (hpi) than its parental strain (wtCSFV). The strain wtCSFV strongly inhibited IFN-λs transcription and IFN-λ3 promoter activity in poly(I:C)-stimulated IPEC-J2 cells, whereas ∆Npro did not show such inhibition. Npro overexpression caused significant reduction of IFN-λs transcription and IFN-λ3 promoter activity. Both wtCSFV and ∆Npro infection induced time-dependent IRF1 expression in IPEC-J2 cells, with ΔNpro showing more significant induction, particularly at 24 hpi. However, infection with wtCSFV or Npro overexpression led not only to significant reduction of IRF1 expression and its promoter activity in poly(I:C)-treated IPEC-J2 cells but also to blockage of IRF1 nuclear translocation. This study provides clear evidence that CSFV Npro suppresses IRF1-mediated type III IFNs production by inhibiting IRF1 expression and its nuclear translocation.

TNF-Mediated Inhibition of Classical Swine Fever Virus Replication Is IRF1-, NF-κB- and JAK/STAT Signaling-Dependent

“…These results are not surprising when considering that CSFV targets specifically IRF3 for proteasomal degradation by means of N pro [8] while the TNF-induced anti-CSFV activity we observed in the PEDSV.15 cells was completely independent of IRF3 (Figure 3d). However, the lack of interference of CSFV with the IRF1-dependent pathway described here is in apparent contradiction with recent data showing that N pro of CSFV inhibits IRF1 expression and nuclear translocation in porcine intestinal epithelial IPEC-J2 cells, thereby suppressing type III IFN production [24]. This latter finding may, however, be a specific feature of IFN-λ producing cells.…”

“…Importantly, the downstream antiviral effects of TNF were independent of the type III IFN antiviral pathway in the PEDSV.15 cells since there was a strict IFNAR1 requirement (Figure 3b). This may be different in other cell types such as intestinal epithelial cells that induce type III IFNs in an IRF1-dependent manner [24]. In the PEDSV.15 cells, we demonstrated that the TNF-triggered induction of IFN-β transcripts and the resulting antiviral effect rely on intact IRF1 (Figure 4b,d), whereas IRF3 was dispensable (Figure 3d).…”

“…CSFV antagonizes the induction of type I IFN by means of N pro through IRF3 targeting [8]. In addition, a recent report showed that N pro inhibits the expression and nuclear translocation of IRF1, thereby suppressing the production of type III IFN [24]. Therefore, we hypothesized that CSFV may antagonize the TNF-induced IRF1-dependent and the LPS-induced IRF1-/IRF3-dependent IFN-β mRNA induction in PEDSV.15 cells.…”

“…Overexpression of IRF1 in PK-15 cells triggers antiviral responses against different porcine viruses, although IRF1 is dispensable for IFN-β induction by RNA viruses [23]. Finally, a recent study showed that CSFV N pro antagonizes IRF1-mediated type III IFN production by downregulating IRF1 expression and inhibiting its nuclear translocation in a porcine intestinal epithelial cell line [24].…”

The Upregulation of a Novel Long Noncoding RNA AK097647 Promotes Enterovirus 71 Replication and Decreases IFN-λ1 Secretion

“…Zika virus nonstructural protein NS5 was found to inhibit the RIG-I pathway and IFN-λ1 promoter activation by targeting IKKε [28]. Npro of classical Swine fever virus suppressed IFN-λ production by inhibiting IRF1 expression and its nuclear translocation [29]. In this study, we investigated whether lncRNA-AK097647 can play a role in host innate immunity during EV71 infection.…”