Protein A from orange-spotted grouper nervous necrosis virus triggers type I interferon production in fish cell

Family Nodaviridae consists of two genera: Alphanodavirus and Betanodavirus, and the latter is classified into four genotypes, including red-spotted grouper nervous necrosis virus, tiger puffer nervous necrosis virus, striped jack nervous necrosis virus, and barfin flounder nervous necrosis virus. Type I interferons (IFNs) play a central role in the innate immune system and antiviral responses, and the interactions between IFN and NNV have been investigated in this study. We have found that the RNA-dependent RNA polymerase (RdRp) from orange-spotted nervous necrosis virus (OGNNV), named protein A, was capable of activating IFN promoter in fathead minnow (FHM) cells. Transient expression of protein A was found to induce IFN expression and secretion, endowing FHM cells with anti-tiger frog virus ability. Protein A from SJNNV can also induce IFN expression in FHM cells but that from Flock House virus (FHV), a well-studied representative species of genus Alphanodavirus, cannot. RdRp activity and mitochondrial localization were shown to be required for protein A to induce IFN expression by means of activating IRF3 but not NFκB. Furthermore, DsRNA synthesized in vitro transcription and poly I:C activated IFN promoter activity when transfected into FHM cells, and dsRNA were also detected in NNV-infected cells. We postulated that dsRNA, a PAMP, was produced by protein A, leading to activation of innate immune response. These results suggest that protein As from NNV are the agonists of innate immune response. This is the first work to demonstrate the interaction between NNV protein A and innate immune system, and may help to understand pathogenesis of NNV.