The Nonstructural Protein NSs of Severe Fever with Thrombocytopenia Syndrome Virus Causes a Cytokine Storm through the Hyperactivation of NF-κB

ABSTRACT

Severe fever with thrombocytopenia syndrome (SFTS) virus (SFTSV) is an emerging highly pathogenic phlebovirus. The syndrome is characterized by the substantial production of inflammatory cytokines and chemokines, described as a cytokine storm, which correlates with multiorgan failure and high mortality. SFSTV nonstructural (NSs) protein was suggested to mediate the pathogenesis by inhibiting antiviral interferon signaling in the host. However, whether SFTSV NSs protein mediates the induction of a fatal cytokine storm remains unaddressed. We demonstrated that SFTSV NSs promotes the hyperinduction of cytokine/chemokine genes in vitro, reminiscent of a cytokine storm. Using gene deletion and pharmacological intervention, we found that the induced cytokine storm is driven by the transcription factor NF-κB. Our investigation revealed that TANK-binding kinase 1 (TBK1) suppresses NF-κB signaling and cytokine/chemokine induction in a kinase activity-dependent manner and that NSs sequesters TBK1 to prevent it from suppressing NF-κB, thereby promoting the activation of NF-κB and its target cytokine/chemokine genes. Of note, NF-κB inhibition suppressed the induction of proinflammatory cytokines in SFTSV-infected type I interferon (IFN-I) receptor 1-deficient (Ifnar1^−/−) mice. These findings establish the essential role of NSs in SFTS pathogenesis and suggest NF-κB as a possible therapeutic target.

KEYWORDS:

• SFTSV NSs
• NF-κB
• cytokine storm
• TBK1
• viral pathogenesis
• NF-κB

ACKNOWLEDGMENTS

This study was supported by research grants from the Japan Agency for Medical Research and Development (Research Program on Emerging and Re-emerging Infectious Diseases [jp19fk0108081h1001 and jp20fk0108081h1202]) and from the Japan Society for the Promotion of Science, Fund for the Promotion of Joint International Research, Fostering Joint International Research [B] (18KK0232), and a Grant-in-Aid for Scientific Research B (18H02344).

J. Khalil performed the experiments, data analysis, and statistical analysis and wrote the initial manuscript. S. Yamada performed the SFTSV infection experiment in HEK293T cells. Y. Tsukamoto generated the NSs mutant plasmids. H. Abe generated the knockout cell lines. M. Shimojima performed the cytokine multiplex assay and infected the mouse model with SFTSV. H. Kato supervised the project and assisted in the study design. T. Fujita conceived the project, supervised it, analyzed the data, and wrote and revised the completed manuscript.

We declare no conflicts of interest.