P130 IRF9 binds to the interferon-B promoter and mediates interferon production in the absence of IRF3 in response to double-stranded RNA

IntroductionGiven its importance in IFNβ and IFN-stimulated gene (ISG) production, IFN regulatory factor 3 (IRF3) is rapidly degraded or inactivated by most viruses. As cells retain production of IFNβ in the absence of IRF3, we investigated which IRF compensates for IRF3.MethodsWe focused on fibroblasts, as these are potent inducers of IFNβ and thus play a key role in pathogen defense. Primary fibroblasts from IRF1−/−IRF3−/−, IRF7−/−IRF3−/− and IRF9−/−IRF3−/− double knock-out mice were assayed for their ability to produce IFNβ and ISGs in response to dsRNA. In addition, we monitored the nuclear translocation of IRF1, IRF7 and IRF9 in response to dsRNA in wild type and IRF3−/− fibroblasts. Finally, we performed pull down assays to identify IRFs bound to the IFNβ promoter under these conditions.ResultsOnly IRF9 was found to be essential for IFNβ production in IRF3 deficient fibroblasts. Consistent with this finding, IRF9 was the only IRF to localize to the nucleus and bind the IFNβ promoter in response to dsRNA. Surprisingly, this activity of IRF9 was only observed in the absence of IRF3. We have preliminary evidence that IRF9 plays a similar role in conventional dendritic cells.ConclusionAlthough IRF7 contributes to IFNβ production, it requires IFN signaling for its production in cells such as fibroblasts. Although IRF9 is known for its role downstream of the type I IFN receptor as a component of the ISGF3 complex, here we provide evidence that IRF9 is critical for production of IFNβ in the absence of IRF3. Thus, under conditions where IRF3 is degraded or inactivated, IRF9 is the key factor that initiates IFNβ production. Once secreted, IFNβ can elicit the production of additional key regulators, such as IRF7, to ensure host protection against pathogen invasion.