Npro of Classical Swine Fever Virus Prevents Type I Interferon-Mediated Priming of Conventional Dendritic Cells for Enhanced Interferon-α Response

A hallmark of acute classical swine fever is the high interferon (IFN)-alpha levels found in the serumearly after infection, followed by an inflammatory cytokine storm. Plasmacytoid dendritic cells (pDCs) represent the only known cell type that produces IFN-alpha upon classical swine fever virus (CSFV) infection in vitro. In primary target cells of the virus the viral protein N-pro inhibits the induction of type I IFN via the degradation of IRF3. We hypothesized that the early systemic pDC-derived IFN-alpha response sensitizes immune cells for enhanced responsiveness and augment cytokine responses after CSFV infection through the upregulation of IRF7. Therefore, bone marrow-derived granulocyte macrophage-colony stimulating factor (GM-CSF)-induced DCs, were pretreated with IFN-beta or conditionedmedium fromCSFV-activated enriched pDC, and expression of the pro-inflammatory cytokines interleukin (IL)-1 beta, IL-6, and IFN-alpha was assessed after infection with wild-type CSFV and with an N-pro mutant [N-pro(D136N)] unable to interact with IRF3 and IRF7. While type I IFN treatment sensitized the DCs for enhanced IFN and cytokine responses after stimulation with influenza virus, lipopolysaccharide or poly(I):poly(C), this was not observed for CSFV. In contrast, the N-pro (D136N) mutant CSFVinduced elevated IFN-alpha responses in type I IFN-pretreatedGM-CSFDCs. These results indicate that CSFV has evolved to prevent type I IFN sensitization in infected cells through the action of the N-pro.