CFLIP_L Interrupts IRF3-CBP-DNA interactions to inhibit IRF3-driven transcription

Type I IFN induction is critical for antiviral and anticancer defenses. Proper downregulation of type I IFN is equally important to avoid deleterious imbalances in the immune response. The cellular FLIP long isoform protein (cFLIP_L) controls type I IFN production, but opposing publications show it as either an inhibitor or inducer of type I IFN synthesis. Regardless, the mechanistic basis for cFLIP_L regulation is unknown. Because cFLIP_L is important in immune cell development and proliferation, and is a target for cancer therapies, it is important to identify how cFLIP_L regulates type I IFN production. Data in this study show that cFLIP_L inhibits IFN regulatory factor 3 (IRF3), a transcription factor central for IFN-b and IFN-stimulated gene expression. This inhibition occurs during virus infection, cellular exposure to polyinosinic-polycytidylic acid, or TBK1 overexpression. This inhibition is independent of capase-8 activity. cFLIP_L binds to IRF3 and disrupts IRF3 interaction with its IFN-b promoter and its coactivator protein (CREB-binding protein). Mutational analyses reveal that cFLIP_L nuclear localization is necessary and sufficient for inhibitory function. This suggests that nuclear cFLIP_L prevents IRF3 enhanceosome formation. Unlike other cellular IRF3 inhibitors, cFLIP_L did not degrade or dephosphorylate IRF3. Thus, cFLIP_L represents a different cellular strategy to inhibit type I IFN production. This new cFLIP_L function must be considered to accurately understand how cFLIP_L affects immune system development and regulation.