African swine fever virus E120R protein inhibits interferon-β production by interacting with IRF3 to block its activation.

African swine fever is a devastating disease of swine caused by African swine fever virus (ASFV). The pathogenesis of the disease remains largely unknown, leaving the uncontrolled spreading of the disease in many countries and regions. Here, we identified the E120R, a structural protein of ASFV, as a key virulent factor and late phase expression protein of the virus. E120R revealed an activity to suppress host antiviral response through blocking IFN-β production, and the 72-73 amino acid sites in the C-terminal domain were essential for this function. E120R interacted with the interferon regulatory factor 3 (IRF3) and interfered with the recruitment of IRF3 to TBK1, which in turn suppressed IRF3 phosphorylation, decreasing interferon production. The recombinant mutant ASFV was further constructed to confirm the claimed mechanism. The ASFV lacking the complete E120R region could not be rescued, whereas the virus could tolerate the deletion of the 72nd and 73rd residuals in the E120R (ASFV E120R-Δ72-73aa). ASFV E120R with the two amino acids deletion failed to interact with IRF3 during ASFV E120R-Δ72-73aa infection, and the viral infection highly activated IRF3 phosphorylation and induced more robust type I interferon production in comparison with its parental ASFV. An unbiased transcriptome-wide analysis of gene expression also confirmed that a considerably higher level of ISGs was detected in ASFV E120R-Δ72-73aa-infected porcine alveolar macrophages (PAMs) than that in the wildtype ASFV-infected PAMs. Together, our findings found a novel mechanism evolved by ASFV to inhibit host antiviral response and provide a new target for guiding the development of ASFV live-attenuated vaccine. IMPORTANCE African swine fever is a highly contagious animal disease affecting pig industry worldwide, which has brought enormous economic losses. The causative agent African swine fever virus (ASFV) infection causes severe immunosuppression during viral infection, attributing to serious clinical manifestation. Therefore, identification of the viral proteins involved in immunosuppression is critical for ASFV vaccine design and development. Here, for the first time, we demonstrated that E120R protein, a structural protein of ASFV, played an important role in suppression of interferon regulatory factor 3 (IRF3) phosphorylation and type I interferon production by binding to IRF3 and blocking the the recruitment of IRF3 to TBK1. Deletion of the crucial binding sites in E120R critically increased interferon response during ASFV infection. This study explored a novel antagonistic mechanism of ASFV, which is critical for guiding the development of ASFV live-attenuated vaccines.