Abstract：H7N9 Influenza A virus (IAV) is an important pathogen of zoonosis which threatens the public health of human. In the present study, to investigate the H7N9 IAV-mediated innate immune responses, recombinant plasmids expressing the polymerase basic protein 1 (PB1) gene were first constructed. However, to exclude the influence of PB1-F2 that is encoded by PB1 via a frame-shifting manner, two nucleotides at positions of 96 and 129 in PB1 gene were mutated by site-directed mutagenesis, respectively. Subsequently, the results from a dual-specific luciferase reporter assay indicated that PB1 significantly inhibited retinoic-acid-inducible geneⅠ CARD (RIG-ⅠN)-induced interferon-β (IFN-β), typeⅠinterferon (IFN) stimulated response element (ISRE) and nuclear factor kappa-B (NF-κB) activities. Consistently, the qRT-PCR experiment confirmed that the expression of IFN-β and its downstream interferon stimulated gene 56 (ISG56)and ISG15, and C-X-C motif chemokine 10 (CXCL10) were down-regulated at the transcriptional levels, suggesting PB1 inhibits the RIG-Ⅰ-mediated signaling pathway. Furthermore, the co-immunoprecipitation experiment was performed to test the interaction between PB1 with the key molecules in the RIG-Ⅰ signaling pathway. The results indicated that H7N9 PB1 specifically interacted with mitochondrial antiviral signaling (MAVS). In addition, PB1 was co-transfected dose-dependently with RIG-Ⅰ or MAVS into HEK293 cells, the Western blotting data showed that the increased expression of H7N9 PB1 inhibited MAVS expression, but not RIG-Ⅰ. Therefore, the preliminary data in the present study demonstrated that H7N9 PB1 was specifically associated with the important adaptor, MAVS in the RLRs signaling pathway and decreased the MAVS expression, leading to the inhibition of IFNs expression and the blockade of the downstream signaling transduction. Our finding revealed that the target of H7N9 IAV PB1-associating in the RLRs signaling pathway would be helpful for the H7N9 control.
