Porcine Reproductive and Respiratory Syndrome Virus Nonstructural Protein 1β Modulates Host Innate Immune Response by Antagonizing IRF3 Activation

Porcine reproductive and respiratory syndrome virus (PRRSV) infection of swine leads to a serious disease characterized by a delayed and defective adaptive immune response. It is hypothesized that a suboptimal innate immune response is responsible for the disease pathogenesis. In the study presented here we tested this hypothesis and identified several nonstructural proteins (NSPs) with innate immune evasion properties encoded by the PRRS viral genome. Four of the total ten PRRSV NSPs tested were found to have strong to moderate inhibitory effects on beta interferon (IFN-beta) promoter activation. The strongest inhibitory effect was exhibited by NSP1 followed by, NSP2, NSP11, and NSP4. We focused on NSP1 alpha and NSP1 beta (self-cleavage products of NSP1 during virus infection) and NSP11, three NSPs with strong inhibitory activity. All of three proteins, when expressed stably in cell lines, strongly inhibited double-stranded RNA (dsRNA) signaling pathways. NSP1 beta was found to inhibit both IFN regulatory factor 3 (IRF3)- and NF-kappa B-dependent gene induction by dsRNA and Sendai virus. Mechanistically, the dsRNA-induced phosphorylation and nuclear translocation of IRF3 were strongly inhibited by NSP1 beta. Moreover, when tested in a porcine myelomonocytic cell line, NSP1 beta inhibited Sendai virus-mediated activation of porcine IFN-beta promoter activity. We propose that this NSP1 beta-mediated subversion of the host innate immune response plays an important role in PRRSV pathogenesis.