IFIT5 Negatively Regulates the Type I IFN Pathway by Disrupting TBK1-IKKε-IRF3 Signalosome and Degrading IRF3 and IKKε.

IFN-induced protein with tetratricopeptide repeats (IFITs), known as canonical IFN-stimulated genes (ISGs), play critical roles in regulating immune responses against pathogens and maintaining homeostasis. How the IFIT5 regulates innate immune responses is rarely reported and remains enigmatic. In this study, we discover that human IFIT5 (hIFIT5) functions as a negative regulator of the type I IFN (IFN) pathway in HEK293T cell lines. Our data illustrated that hIFIT5 inhibited the promotor activities of IFN-β induced by IRF3 and its upstream factors but not by IRF3-5D (activated form of IRF3), suggesting that IRF3 might be a target of hIFIT5. Further investigations revealed that hIFIT5 downregulated the phosphorylation of IRF3 and IKKε and blocked the IRF3 nuclear translocation. Moreover, hIFIT5 impaired the IRF3-TBK1-IKKε complex, accompanied by IRF3 and IKKε degradation. In conclusion, these findings indicate that hIFIT5 is a negative modulator in the type I IFN signaling pathway, opening additional avenues for preventing hyperactivation and maintaining immunity homeostasis.