Porcine Reproductive and Respiratory Syndrome Virus Nonstructural Protein 4 Antagonizes Beta Interferon Expression by Targeting the NF-κB Essential Modulator

Porcine reproductive and respiratory syndrome virus (PRRSV) is a highly infectious pathogen that causes severe diseases in pigs and great economic losses to the swine industry worldwide. Type I interferons (IFNs) play a crucial role in antiviral immunity. In the present study, we demonstrated that infection with the highly pathogenic PRRSV strain JXwn06 antagonized type I IFN expression induced by poly(I.C) in both porcine alveolar macrophages (PAMs) and blood monocyte-derived macrophages (BMo). Subsequently, we showed that the inhibition of poly(I.C)-induced IFN-beta production by PRRSV was dependent on the blocking of NF-kappa B signaling pathways. By screening PRRSV nonstructural and structural proteins, we demonstrated that nonstructural protein 4 (nsp4), a viral 3C-like serine protease, significantly suppressed IFN-beta expression. Moreover, we verified that nsp4 inhibited NF-kappa B activation induced by signaling molecules, including RIG-I, VISA, TRIF, and IKK beta. nsp4 was shown to target the NF-kappa B essential modulator (NEMO) at the E349-S350 site to mediate its cleavage. Importantly, nsp4 mutants with defective protease activity abolished its ability to cleave NEMO and inhibit IFN-beta production. These findings might have implications for our understanding of PRRSV pathogenesis and its mechanisms for evading the host immune response. IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is a major agent of respiratory diseases in pigs. Like many other viruses, PRRSV has evolved a variety of strategies to evade host antiviral innate immunity for survival and propagation. In this study, we show that PRRSV nsp4 is a novel antagonist of the NF-kappa B signaling pathway, which is responsible for regulating the expression of type I interferons and other crucial cytokines. We then investigated the underlying mechanism used by nsp4 to suppress NF-kappa B-mediated IFN-beta production. We found that nsp4 interfered with the NF-kappa B signaling pathway through the cleavage of NEMO (a key regulator of NF-kappa B signaling) at the E349-S350 site, leading to the downregulation of IFN-beta production induced by poly(I.C). The data presented here may help us to better understand PRRSV pathogenesis.