Nonstructural Proteins NS1 and NS2 of Bovine Respiratory Syncytial Virus Block Activation of Interferon Regulatory Factor 3

Abstract

We have previously shown that the nonstructural (NS) proteins NS1 and NS2 of bovine respiratory syncytial virus (BRSV) mediate resistance to the alpha/beta interferon (IFN)-mediated antiviral response. Here, we show that they, in addition, are able to prevent the induction of beta IFN (IFN-β) after virus infection or double-stranded RNA stimulation. In BRSV-infected MDBK cells upregulation of IFN-stimulated genes (ISGs) such as MxA did not occur, although IFN signaling via JAK/STAT was found intact. In contrast, infection with recombinant BRSVs lacking either or both NS genes resulted in efficient upregulation of ISGs. Biological IFN activity and IFN-β were detected only in supernatants of cells infected with the NS deletion mutants but not with wild-type (wt) BRSV. Subsequent analyses of IFN-β promoter activity showed that infection of cells with the double deletion mutant BRSV ΔNS1/2, but not with BRSV wt, resulted in a significant increase in IFN-β gene promoter activity. Induction of the IFN-β promoter depends on the activation of three distinct transcription factors, NF-κB, ATF-2/c-Jun, and IFN regulatory factor 3 (IRF-3). Whereas NF-κB and ATF-2/c-Jun activities were readily detectable and comparable in both wt BRSV- and BRSV ΔNS1/2-infected cells, phosphorylation and transcriptional activity of IRF-3, however, were observed only after BRSV ΔNS1/2 infection. NS protein-mediated inhibition of IRF-3 activation and IFN induction should have considerable impact on the pathogenesis and immunogenicity of BRSV.