Modulation of interferon production and signaling by SFTS virus NSs protein and Zika virus NS5 protein

Abstract

Innate immunity plays dual roles in virus-host interaction. Concurrent with its inhibition of viral replication, pro-inflammatory response is also induced. To combat innate immunity, viruses have evolved various counter-mechanisms to block interferon (IFN) production and signaling, allowing viral replication to proceed. Meanwhile, viruses may also induce pro-inflammatory response to trigger pathological inflammation leading to severe diseases. Type I and III IFN signaling mainly activates IFN-stimulated genes (ISGs) that inhibit either viral DNA/RNA or protein synthesis, whereas ISGs such as IP10 and MCP1 are part of the pro-inflammatory response activated by IFN-γ signaling. SFTS virus (SFTSV) is an emerging arbovirus identified in China and it belongs to the Bunyaviridae family. NSs protein from SFTSV is known to antagonize type I IFN induction and signaling. In the first part of my study, SFTSV NSs protein is shown to ablate both production and signaling of type I and type III IFNs but potentiate type II IFN signaling. SFTSV infection or expression of its NSs protein potently inhibited not only IFN-β production induced by double-stranded RNA but also ISG activation by IFN-β. Meanwhile, NSs expression augmented IFN-γ-induced expression of IP10, MCP1 and IRF1 genes. Association of NSs with STAT1 and STAT2 sequestered them in the cytoplasm, blocking their recruitment to ISRE promoters of ISGs. In addition, self-ubiquitination of NSs facilitated STAT2 degradation. NSs interaction with STAT1 inhibited IFN-β-induced STAT1 phosphorylation at serine 727 but increased IFN-γ-induced STAT1 phosphorylation.

Zika virus (ZIKV) is another arbovirus causing recent outbreaks and belonging to the Flaviviridae family. ZIKV induces pro-inflammatory cytokines that could facilitate infection. In the second part of my study, it was demonstrated that NS5 protein of ZIKV exerts opposite regulatory effects on IFN signaling. ZIKV and its NS5 protein suppressed type I and type III IFN signaling but activated IFN-γ signaling. Suprisingly, activated IFN-γ signaling augmented ZIKV replication. Interaction of NS5 with STAT2 targeted it for ubiquitination and degradation without affecting STAT1 stability or nuclear translocation. NS5 increased the formation of STAT1-STAT1 homodimers in the absence of STAT2. NS5 expression ablated STAT1-STAT2-IRF9 recruitment to IFN-β-stimulated genes such as MxA, ISG15 and OSA1, but augmented STAT1-STAT1 recruitment to IFN-γ-stimulated genes such as pro-inflammatory cytokine CXCL10. JAK2 inhibitor suppressed viral replication and viral induction of IFN-γ-stimulated genes in ZIKV-infected cells.

Taken together, these findings provide the first evidence for concurrent suppression of type I IFN signaling and activation of type II IFN signalling by two arboviruses- ZIKV and SFTSV, both of which are important human pathogens. This differential modulation of IFN signaling facilitate viral replication and cause diseases through different branches of IFN response. This activity might be shared by a group of viral IFN modulators.