IRF7 Is Involved in Both STING and MAVS Mediating IFN-β Signaling in IRF3-Lacking Chickens

Abstract

IFN regulatory factor (IRF) 3 has been identified as the most critical regulator of both RNA and DNA virus–induced IFN production in mammals. However, ambiguity exists in research on chicken IRFs; in particular IRF3 seems to be missing in chickens, making IFN regulation in chickens unclear. In this study, we comprehensively investigated the potential IFN-related IRFs in chickens and showed that IRF7 is the most critical IFN-β regulator in chickens. With a chicken IRF7 (chIRF7) knockout DF-1 cell line, we conducted a series of experiments to demonstrate that chIRF7 is involved in both chicken STING (chSTING)- and chicken MAVS (chMAVS)-mediated IFN-β regulation in response to DNA and RNA viral infections, respectively. We further examined the mechanisms of chIRF7 activation by chSTING. We found that chicken TBK1 (chTBK1) is indispensable for chIRF7 activation by chSTING as well as that chSTING interacts with both chIRF7 and chTBK1 to function as a scaffold in chIRF7 activation by chTBK1. More interestingly, we discovered that chSTING mediates the activation of chIRF7 through a conserved SLQxSyS motif. In short, we confirmed that although IRF3 is missing in chickens, they employ IRF7 to reconstitute corresponding IFN signaling to respond to both DNA and RNA viral infections. Additionally, we uncovered a mechanism of chIRF7 activation by chSTING. The results will enrich and deepen our understanding of the regulatory mechanisms of the chicken IFN system.

Knockout of IRF7 Highlights its Modulator Function of Host Response Against Avian Influenza Virus and the Involvement of MAPK and TOR Signaling Pathways in Chicken

“…This leads to a subsequent signaling cascade via adaptor molecules and protein kinases that activate the key transcription factors (AP1, NF-κB and IRF7). The MAPK and PI3K/Akt pathways are potentially engaged in this signaling module in addition to the classical type I IFN induction pathway activating IRF7 by IkB kinases (IKK) TBK1 (TANK binding kinase 1), IKKε (IKBKE) or IKKα (CHUK) [13,57,58]. Overall, our data argue that IRF7 is a major modulator of the host antiviral response, and that the transcription factors NF-κB and AP1 are also capable of regulating both IFNA and INFB in addition to the inflammatory cytokines.…”

“…The coordination of Interferon Regulatory Factor 7 (IRF7) and IRF3 on the initiation of the type I interferon (IFN) response to establish the antiviral state is well characterized in mammals; however, chickens may utilize a different modulation system due to a smaller repertoire of immune-related genes in avian species [9,10]. Particularly, chickens are missing IRF3, and only have IRF7, which has less than 40% of amino acid sequence identity to its mammalian orthologues [11][12][13]. Therefore, studying the chicken IRF7 could be a great starting point to elucidate the underlying cellular mechanisms of host immune regulation against AIV in chickens.…”

Avian Pattern Recognition Receptor Sensing and Signaling

“…Notably, C-terminal tails of chicken, duck, and goose STING bore pLxIS motif, pointing to conserved actions of TBK1-mediated phosphorylation and binding of IRF7 (IRF3 is absent in avians) to promote downstream signaling ( Figure S16) [175]. In line with this, deletion of pLxIS motif (SLQxSyS motif) in the C-terminal tail of chicken STING led to failure to activate downstream IRF7 and IFN-β promoter [175].…”

Duck Enteritis Virus VP16 Antagonizes IFN-β-Mediated Antiviral Innate Immunity

“…Upon phosphorylation from TBK1 activated by STING, IRF7 dimerizes and translocates to the nuclei, acting as a transcription factor. The dipolymer complex of IRF7 can then bind to the IRF7 receptor sites within the IFN-β promoter region, eventually activating the transcription of the IFN-β gene [69]. Therefore, I verified whether DEV could inhibit IRF7-stimulated IFN-β promoter activity.…”

“…The transcriptional expression of IFN-β is regulated by the transcriptional enhancer bound to its promoter, which includes the regulatory region of IRF3/IRF7, AP-1 (activating protein 1), and the regulatory region of NF-κB. Intriguingly, in spite of IRF3 deficiency in chicken, poultries may employ IRF7 to reconstitute corresponding IFN signaling to respond to both DNA and RNA viral infections [69]. Therefore, in this study, for the first time we identified that VP16 could directly interact with duck IRF7 whereby the VP16 of DEV plays a vital role in antagonizing the innate immune system, assisting DEV virus to counteract this antiviral activity.…”