Nonstructural Protein 11 of Porcine Reproductive and Respiratory Syndrome Virus Suppresses Both MAVS and RIG-I Expression as One of the Mechanisms to Antagonize Type I Interferon Production

Type I interferons (IFN-alpha/beta) play a key role in antiviral defense, and porcine reproductive and respiratory syndrome virus (PRRSV) is known to down-regulate the IFN response in virus-infected cells and pigs. In this study, we showed that the overexpression of nsp11 of PRRSV induced a strong suppression of IFN production. Nsp11 suppressed both IRF3 and NF-KB activities when stimulated with a dsRNA analogue and TNF-alpha, respectively. This suppression was RLR dependent, since the transcripts and proteins of MAVS and RIG-I, two critical factors in RLR-mediated pathway, were both found to be reduced in the presence of overexpressed nsp11. Since nsp11 is an endoribonuclease (EndoU), the structure function relationship was examined using a series of nsp11 EndoU mutant plasmids. The mutants that impaired the EndoU activity failed to suppress IFN and led to the normal expression of MAVS. Seven single amino acid substitutions (4 in subdomain A and 3 in subdomain B) plus one insertion (frame-shift in nsp11) were then introduced into PRRSV infectious cDNA clones to generate nsp11 mutant viruses. Unfortunately, all EndoU knock-out nsp11 mutant viruses appeared replication-defective and no progenies were produced. Three mutations in EndoU subdomain A expressed the N and nsp2/3 proteins but their infectivity diminished after 2 passages. Taken together, our data show that PRRSV nsp11 endoribonuclease activity is critical for both viral replication and IFN antagonism. More importantly, the endoribonuclease activity of nsp11 demonstrates the substrate specificity towards MAVS and RIG-I (transcripts and proteins) over p65 and IRF3 in the context of gene transfection and overexpression. This is likely a mechanism of nsp11 suppression of type I IFN production.