Nonstructural proteins NS1 and NS2 of bovine respiratory syncytial virus block activation of interferon regulatory factor 3.

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-beta) 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-beta were detected only in supernatants of cells infected with the NS deletion mutants but not with wild-type (wt) BRSV. Subsequent analyses of IFN-beta promoter activity showed that infection of cells with the double deletion mutant BRSV DeltaNS1/2, but not with BRSV wt, resulted in a significant increase in IFN-beta gene promoter activity. Induction of the IFN-beta promoter depends on the activation of three distinct transcription factors, NF-kappaB, ATF-2/c-Jun, and IFN regulatory factor 3 (IRF-3). Whereas NF-kappaB and ATF-2/c-Jun activities were readily detectable and comparable in both wt BRSV- and BRSV DeltaNS1/2-infected cells, phosphorylation and transcriptional activity of IRF-3, however, were observed only after BRSV DeltaNS1/2 infection. NS protein-mediated inhibition of IRF-3 activation and IFN induction should have considerable impact on the pathogenesis and immunogenicity of BRSV.