Roles Of The Severe Fever With Thrombocytopenia Syndrome Virus NSs Protein In Evasion Of Innate Immunity And Promotion Of Viral Replication

SFTSV (Severe Fever with Thrombocytopenia Syndrome Virus), which is firstly isolated and identified by Chines scientists in2010, is a new member of Phlebovirus genus in the family Bunyaviridae. SFTSV infection would cause a new emerging fever infectious disease, patients with which present high fever, thrombocytopenia and leukocytopenia, and, in severe cases, multi-organ failure, and the average clinical fatility is approximately10%. Up to now, little has been known about the pathogenesis of SFTS. Previous studies in other Bunyaviridae viruses have demonstrated that nonstructural proteins encoded by S segment (NSs) can inhibit the cellular IFN-beta response and act as a crucial virulence factor in the pathogenesis. Similarly, latest results indicate that NSs of SFTSV decrease the activity of IFN-beta (IFN-β) promoter and NF-κB signaling pathway, however, the related mechanisms remain unclear. Therefore we intend to focus on the biology function of SFTS virus NSs proteins in modulating the cellular innate immunity response and facilitating viral replication.We demonstrate that unique structures defined as viral inclusion bodies are formed by NSs proteins both in SFTS virus infected and plasmid-transfected cells. The number of IBs decrease and yet the size of IBs increase on the contrary in viral infected, as well as, transfected cells. TBK1, a vital protein kinase in signaling pathway of IFNs response, can be hijacked into IBs through interaction with NSs proteins both in viral infected and transfected cells. Through this interaction, NSs is able to sequester the IKK complex, including IKKε and IRF3, into IBs indirectly. Furthermore, active phosphorylated IRF3(p-IRF3) stimulated by influenza virus is translocated into the cellular nucleus. Astonishingly, in the presence of NSs, p-IRF3is trapped into cytoplasmic IBs, leading to reduced IFN-β induction after the influenza virus infection. We also indicate that SFTS virus NSs interacts with itself, which might contribute to the formation of inclusion bodies. Besides, NP as well as viral RNAs similarly have an interaction with NSs, implying that NSs might be involved in viral replication. Additionally, we observe the colocalization of NSs and lipid droplets by using Nile Red, a selective fluorescent stain for intracellular lipid droplets, and that inhibiting the biosynthesis of fatty acid results in decreased IBs formation in transfected cells as well as infected cells, and reduced fold change of viral RNAs in infected cells. Ultimately, we show that viral dsRNAs which is replicative intermediate of viral RNAs also localize in IBs, suggesting that NSs-formed IBs might be implicated in the resplication of SFTSV.Therefore, we uncover a novel mechanism for viral evasion of innate immunity that SFTS virus NSs hijack the IKK complex including TBK1, IKKε and IRF3, and active IRF3into viral inclusion bodies formed by NSs proteins, resulting in decreased induction of IFN-β which is an efficient weapon of innate immunity in response to viral infection. We also identify a new function of SFTS virus NSs proteins in acting as scaffolding component to form unique structures which might function as viral factories, a platform for viral replication. These findings will broaden our understanding of viral pathogenesis of SFTSV and update the knowledge of biological functions of IBs which might be served as new targets of anti-viral drugs in the future.