Suppression of type I and type III IFN signalling by NSs protein of severe fever with thrombocytopenia syndrome virus through inhibition of STAT1 phosphorylation and activation

Abstract

Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne pathogen causing significant morbidity and mortality in Asia. NSs protein of SFTSV is known to perturb type I IFN induction and signalling, but the mechanism remains to be fully understood. Here, we showed the suppression of both type I and type III IFN signalling by SFTSV NSs protein is mediated through inhibition of STAT1 phosphorylation and activation. Infection with live SFTSV or expression of NSs potently suppressed IFN-stimulated genes but not NFkB activation. NSs was capable of counteracting the activity of IFN-a1, IFN-b, IFN-l1 and IFN-l2. Mechanistically, NSs associated with STAT1 and STAT2, mitigated IFN-b-induced phosphorylation of STAT1 at S727, and reduced the expression and activity of STAT1 protein in IFN-b-treated cells, resulting in the inhibition of STAT1 and STAT2 recruitment to IFNstimulated promoters. Taken together, SFTSV NSs protein is an IFN antagonist that suppresses phosphorylation and activation of STAT1.

Differential Antagonism of Human Innate Immune Responses by Tick-Borne Phlebovirus Nonstructural Proteins

“…HEK293T cells transiently expressing untagged or V5-tagged UUKV, HRTV, or SFTSV NSs proteins were treated with recombinant IFN-β for 30 min, and the cell lysates were harvested for Western blotting 24 h posttransfection. Unlike Chaudhary et al (33), we observed no difference in the levels of STAT1 or phosphorylated STAT1 (at position Ser727 or position Tyr701) in the presence of SFTSV NSs compared to the mock control. Similar results were observed for UUKV and HRTV NSs proteins (Fig.…”

“…Other published data show that SFTSV NSs directly interacts with and sequesters STAT1 and STAT2 into inclusion bodies to block type I IFN signaling (33, 34). To investigate whether HRTV NSs was also capable of these interactions, we performed immunoprecipitation of transfected V5-tagged NSs proteins (Fig.…”

“…While one study has shown that only STAT2 (but not STAT1) phosphorylation is inhibited by SFTSV NSs (34), another study has shown that the phosphorylation of STAT2 as well as of STAT1 at position Ser727 is blocked by SFTSV NSs (33). We examined the ability of HRTV or SFTSV NSs to inhibit STAT1 or STAT2 phosphorylation using UUKV NSs as a negative control, as UUKV NSs did not block type I IFN signaling (Fig.…”

“…With the emergence of highly pathogenic SFTSV, research interests have focused on understanding its underlying molecular mechanisms of pathogenicity. As a result, quickly after its emergence, SFTSV NSs was characterized as a potent antagonist of IFN induction and signaling, through the spatial isolation of TRIM25 and RIG-I (30), TBK1 (31, 32), IRF-3 (31, 32), and STAT1 and STAT2 (33, 34) in NSs-containing, round, cytoplasmic inclusion bodies (IB) or viroplasms. Although the interaction between SFTSV NSs and IKKε or IRF-3 was reported to be indirect, facilitated through the interaction between SFTSV NSs and TBK1 (32), one study also showed a direct interaction of SFTSV NSs with IKKε (31).…”

A role for glycolipid biosynthesis in severe fever with thrombocytopenia syndrome virus entry

“…The tripartite genome is comprised of the S, M, and L segments, which encode the nucleocapsid, glycoproteins, and RNA-dependent RNA polymerase, respectively. In addition to the structural proteins, some bunyaviruses, including SFTSV, also encode non-structural proteins that antagonize host innate immune mechanisms [16–19]. The viral glycoproteins G N and G C form a heterodimer on the surface of virions, and are both necessary and sufficient for viral entry [15].…”