The Npro product of classical swine fever virus and bovine viral diarrhea virus uses a conserved mechanism to target interferon regulatory factor-3

Classical swine fever virus (CSFV) is a member of the genus Pestivirus in the family Flaviviridae. The Npro product of CSFV targets the host's innate immune response and can prevent the production of type I interferon (IFN). The mechanism by which CSFV orchestrates this inhibition was investigated and it is shown that, like the related pestivirus bovine viral diarrhea virus (BVDV), this involves the Npro protein targeting interferon regulatory factor-3 (IRF-3) for degradation by proteasomes and thus preventing IRF-3 from activating transcription from the IFN-β promoter. Like BVDV, the steady-state levels of IRF-3 mRNA are not reduced markedly by CSFV infection or Npro overexpression. Moreover, IFN-α stimulation of CSFV-infected cells induces the antiviral protein MxA, indicating that, as in BVDV-infected cells, the JAK/STAT pathway is not targeted for inhibition.