Amino acid residues in the non-structural protein 1 of porcine reproductive and respiratory syndrome virus involved in down-regulation of TNF-cx expression in vitro and attenuation in vivo

Porcine reproductive and respiratory syndrome virus (PRRSV) suppresses tumor necrosis factor-alpha (TNF-α) production at both transcriptional and post-transcriptional levels by its non-structural proteins 1α and 1β (Nsp1α and Nsp1β). To identifY the amino acid residues responsible for this activity, we generated several alanine substitution mutants of Nsp1α and Nsp1β. Examination of the mutant proteins revealed that Nsp1α residues Gly90, Asn91 , Arg97, Argl 00 and Arg124 were necessary for TNF-α promoter suppression, whereas several amino acids spanning the entire Nsp1β ~ were found to be required for this activity. Two mutant viruses, with mutations at Nsp1α Gly90 or Nsp1β residues 70-74, generated from infectious cDNA clones, exhibited attenuated viral replication in vitro and TNF-α was found to be up regulated in infected macrophages. In infected pigs, the Nsp1β mutant virus was attenuated in growth. These studies provide insights into how PRRSV evades the effector mechanisms of innate immunity dUling infection

Identification of amino acid residues important for anti-IFN activity of porcine reproductive and respiratory syndrome virus non-structural protein 1

The non-structural protein 1 (nsp1) of porcine reproductive and respiratory syndrome virus is partly responsible for inhibition of type I interferon (IFN) response by the infected host. By performing alanine-scanning mutagenesis, we have identified amino acid residues in nsp1α and nsp1β~ (the proteolytic products of nsp1) that when substituted with alanine(s) exhibited significant relief of IFNsuppression. A mutant virus (16-SA, in which residues 16-20 of nsp1β were substituted with alanines) encoding mutant nsp1β recovered from infectious cDNA clone was shown to be attenuated for growth in vitro and induced significantly higher amount of type I IFN transcripts in infected macrophages. In infected pigs, the 16-SA virus exhibited reduced growth at early times after infection but quickly regained wild type growth properties as a result of substitutions within the mutated sequences. The results indicate a strong selection pressure towards maintaining the IFN-inhibitory property of the virus for successful propagation in pigs

Amino acid residues in the non-structural protein 1 of porcine reproductive and respiratory syndrome virus involved in down-regulation of TNF-cx expression in vitro and attenuation in vivo

Porcine reproductive and respiratory syndrome virus (PRRSV) suppresses tumor necrosis factor-alpha (TNF-α) production at both transcriptional and post-transcriptional levels by its non-structural proteins 1α and 1β (Nsp1α and Nsp1β). To identifY the amino acid residues responsible for this activity, we generated several alanine substitution mutants of Nsp1α and Nsp1β. Examination of the mutant proteins revealed that Nsp1α residues Gly90, Asn91 , Arg97, Argl 00 and Arg124 were necessary for TNF-α promoter suppression, whereas several amino acids spanning the entire Nsp1β ~ were found to be required for this activity. Two mutant viruses, with mutations at Nsp1α Gly90 or Nsp1β residues 70-74, generated from infectious cDNA clones, exhibited attenuated viral replication in vitro and TNF-α was found to be up regulated in infected macrophages. In infected pigs, the Nsp1β mutant virus was attenuated in growth. These studies provide insights into how PRRSV evades the effector mechanisms of innate immunity dUling infection

Induction of Interferon and Interferon Signaling Pathways by Replication of Defective Interfering Particle RNA in Cells Constitutively Expressing Vesicular Stomatitis Virus Replication Proteins▿

We show here that replication of defective interfering (DI) particle RNA in HEK293 cells stably expressing vesicular stomatitis virus (VSV) replication proteins potently activates interferon (IFN) and IFN signaling pathways through upregulation of IFN-β promoter, IFN-stimulated response element (ISRE) promoter, and NF-κB promoter activities. Replication of DI particle RNA, not mere expression of the viral replication proteins, was found to be critical for induction of IFN and IFN signaling. The stable cells supporting replication of DI RNA described in this report will be useful in further examining the innate immune signaling pathways and the host cell functions in viral genome replication

Induction of Stress Granule-Like Structures in Vesicular Stomatitis Virus-Infected Cells

Previous studies from our laboratory revealed that cellular poly(C) binding protein 2 (PCBP2) downregulates vesicular stomatitis virus (VSV) gene expression. We show here that VSV infection induces the formation of granular structures in the cytoplasm containing cellular RNA-binding proteins, including PCBP2, T-cell-restricted intracellular antigen 1 (TIA1), and TIA1-related protein (TIAR). Depletion of TIA1 via small interfering RNAs (siRNAs), but not depletion of TIAR, results in enhanced VSV growth and gene expression. The VSV-induced granules appear to be similar to the stress granules (SGs) generated in cells triggered by heat shock or oxidative stress but do not contain some of the bona fide SG markers, such as eukaryotic initiation factor 3 (eIF3) or eIF4A, or the processing body (PB) markers, such as mRNA-decapping enzyme 1A (DCP1a), and thus may not represent canonical SGs or PBs. Our results revealed that the VSV-induced granules, called SG-like structures here, contain the viral replicative proteins and RNAs. The formation and maintenance of the SG-like structures required viral replication and ongoing protein synthesis, but an intact cytoskeletal network was not necessary. These results suggest that cells respond to VSV infection by aggregating the antiviral proteins, such as PCBP2 and TIA1, to form SG-like structures. The functional significance of these SG-like structures in VSV-infected cells is currently under investigation

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-β) promoter activation. The strongest inhibitory effect was exhibited by NSP1 followed by, NSP2, NSP11, and NSP4. We focused on NSP1α and NSP1β (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. NSP1β was found to inhibit both IFN regulatory factor 3 (IRF3)- and NF-κB-dependent gene induction by dsRNA and Sendai virus. Mechanistically, the dsRNA-induced phosphorylation and nuclear translocation of IRF3 were strongly inhibited by NSP1β. Moreover, when tested in a porcine myelomonocytic cell line, NSP1β inhibited Sendai virus-mediated activation of porcine IFN-β promoter activity. We propose that this NSP1β-mediated subversion of the host innate immune response plays an important role in PRRSV pathogenesis