The type I and III interferon responses restrict infection with tick-borne orthoflaviviruses through IFI6

Tick-borne orthoflaviviruses (TBOVs) are a growing global health concern. Several representatives of this viral family cause fatal disease in humans with increasing case numbers throughout the last decades. The innate immune response, especially interferon (IFN)-dependent signaling, is an essential part of the human defense system that counteracts infection with TBOVs and other viruses. Even though 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 III receptors further underscored the critical role of IFN signaling in controlling TBOV replication in vivo. To assess the contribution of single genes to controlling TBOV infection in human cells, we used a CRISPR/Cas9-KO-based screening approach. This strategy identified IFI6 as a central player for IFN type I- and III-driven responses against TBOVs. We further defined IFI6 as an ER-resident protein that restricts TBOV replication at a post-entry step. Our work thus opens new perspectives for targeting weak points in the life cycle of TBOVs and other orthoflaviviruses, potentially paving the way for the development of new antiviral therapeutics. One Sentence SummaryType I and III interferons are crucial for protection against tick-borne orthoflavivirus infection in vitro and in vivo, both relying on IFI6 as a main antiviral effector.