Type I and III interferon responses restrict infection by tick-borne orthoflaviviruses through IFI6.

Tick-borne orthoflaviviruses (TBOVs) are a growing global health concern. Several members of this viral group cause fatal disease in humans with increasing case numbers throughout the last few decades. The innate immune response, especially interferon (IFN)-dependent signaling, is an essential part of the vertebrate defense system that counteracts infection with TBOVs and other viruses. Although they activate the same signaling cascade, IFNs belonging to the type I and III families trigger differing gene expression patterns. Which genes the two IFN families induce to restrict infection with TBOVs remains poorly characterized. Here, we show that type I and III IFNs are both capable of restricting TBOV infection of human cell lines in a cell type-specific manner. Infection of C57BL/6J mice with knockouts for either IFN type I or type I and III receptors further underscored the critical role of IFN signaling in controlling TBOV replication IMPORTANCE: Tick-borne orthoflaviviruses (TBOVs) are spreading in various parts of the world, like Europe, Asia, and North America, making it essential to understand how they cause disease and how the immune system responds to infection. In vertebrates, the interferon (IFN) response is a key early defense against viruses, triggering the expression of numerous IFN-stimulated genes (ISGs) with antiviral activities. Using mouse models, we demonstrated the central role of IFNs in controlling TBOV replication. To explore this further, we screened for the activity of about 2,000 individual ISGs against tick-borne encephalitis virus (TBEV) in human cells and identified IFI6 as a potent antiviral factor. Through functional studies, virological assays, biochemical analyses, and microscopic approaches, we confirmed that IFI6 limits the replication of TBOVs. These findings enhance our understanding of innate immunity against TBOV infections.