Classical swine fever virus interferes with cellular antiviral defense:: Evidence for a novel function of Npro

Classical swine fever virus (CSFV) replicates efficiently in cell lines and monocytic cells, including macrophages (MPhi), without causing a cytopathic effect or inducing interferon (IFN) secretion. In the present study, the capacity of CSFV to interfere with cellular antiviral activity was investigated. When the porcine kidney cell line SK-6 was infected with CSFV, there was a 100-fold increased capacity to resist to apoptosis induced by polyinosinic-polycytidylic acid [poly(IC)], a synthetic double-stranded RNA. In M(D, the virus infection inhibited poly(IC)-induced alpha/beta IFN (type I IFN) synthesis. This interference with cellular antiviral defense correlated with the presence of the viral N-pro gene. Mutants lacking the N-pro gene (DeltaN(pro) CSFV) did not protect SK-6 cells from poly (IC) -induced apoptosis, despite growth properties and protein expression levels similar to those of the wild-type virus. Furthermore, DeltaN(pro) CSFV did not prevent poly(IC) -induced type I IFN production in MPhi) but rather induced type I IFN in the absence of poly(IC) in both MPhi and the porcine kidney cell line PK-15, but not in SK-6 cells. With MPhi and PK-15, an impaired replication of the DeltaN(pro) CSFV compared with wild-type virus was noted. In addition, DeltaN(pro) CSFV, but not wild-type CSFV, could interfere with vesicular stomatitis virus replication in PK-15 cells. Taken together, these results provide evidence for a novel function associated with CSFV N-pro with respect to the inhibition of the cellular innate immune system.