Molecular Mechanism Of Porcine Reproductive And Respiratory Syndrome Virus Infection Targets IKKβ To Inhibit NF-κB Signaling Pathway

Porcine reproductive and respiratory syndrome(PRRS),caused by porcine reproductive and respiratory syndrome virus(PRRSV),is a highly contagious disease characterized by reproductive disorders in pregnant sows and respiratory diseases in fattening pigs.Because of the clinic phenomenon,the ears of infected swines become blue during infection,PRRS also called as “blue-ear” disease.Type I interferon(IFN-Ⅰ)is an antiviral cytokine,which produced by multiple signaling pathways.Numerous studies have shown that several proteins encoded by PRRSV are multi-functional,which are involved in negatively regulate the host innate immune responses by inhibiting the production of IFN-Ⅰ during PRRSV infection.The canonical NF-κB pathway plays a crucial role in regulating innate immune responses.As a key molecule in the classical NF-κB pathway,IKKβ plays crucial roles in regulating the expression of IFN-Ⅰ and inflammatory cytokines by which selectively recognizes and phosphorylates Ikappa B(IκB)family members.Previous studies reported that IFN-β is markedly inhibited by numerous PRRSV nonstructural proteins(Nsps).However,the role of IKKβ,a key regulator in the production of IFN-Ⅰ and the activation of NF-κB,in PRRSV infection is unclear.In this study,we detected the IFN-β promoter and NF-κB promoter activation induced by IKKβ,and found that Nsp4 has the most obvious inhibitory effect among the PRRSV Nsps.Additionally,we also found that PRRSV Nsp4 significantly inhibited the phosphorylation and nuclear translocation of p65 induced by IKKβ.Mechanistically,we clearly demonstrated that PRRSV Nsp4 impairs NF-κB signaling by cleaving IKKβ at E378 site,which depends on the 3C-like serine protease activity of NSP4.Unlike intact IKKβ,the two cleavage fragments of IKKβ lose their function to phosphorylate IκBα and activate NF-κB signaling pathway.Of note,the two cleavage fragments of IKKβ lose their function to phosphorylate IκBα and activate NF-κB signaling pathway.Furthermore,we found that hydrophobic patch at the KD-ULD junction of IKKβ could be disrupted by PRRSV Nsp4 via the cleavage of the E378 site,resulting in disruption of NF-κB activity.Overall,we revealed a new mechanism by which PRRSV infection evades the host innate immune responses,which help us to further understand the relationship between PRRSV and the host immune system.