XAF1 Prevents Hyperproduction Of Type Ⅰ Interferon Upon Viral Infection By Targeting IRF7

Interferon regulatory factors IRF3 and IRF7 are important transcription factors that regulate the antiviral innate immune response dependent on type I interferon（IFN-I）.At the early stage of viral infection,constitutionally expressed IRF3 is activated to induce IFN-I expression to achieve rapid response,and IFN-I produced at this stage binds to its receptor IFNAR to induce Irf7 transcription.In the late stage of viral infection,induced IRF7 further promotes IFN-I production,resulting in a positive feedback.This process is more rapid and effective than the earlier IRF3-mediated expression of IFN-I,so it is generally believed that IRF7 has a stronger ability to induce IFN-I expression than IRF3.Although IFN-I plays a key role in the innate antiviral immune response,its continued expression leads to a variety of autoimmune diseases.Therefore,regulating the activity of IRF3 and IRF7 so that they are not over-activated is essential to avoid the excessive production of IFN-I which can lead to body damage.Posttranslational modification（PTM）can significantly alter protein activity or regulatory protein stability,and it is more efficient than transcriptional regulation.A number of studies have elaborated the mechanism of ubiquitin-mediated targeted degradation of IRF3,but the ubiquitination regulation of IRF7,the stronger IFN-I inducer transcription factor,is still in the preliminary stage.In this study,we identified a key molecule,XIAP associated factor 1（XAF1）,that interacts with IRF7 and whose m RNA expression level is upregulated synchronously with that of Irf7 induced by viral stimuli in macrophages by transcriptome sequencing and mass spectrometry.By constructing myeloid-cells-specific XAF1 knockout mice(XAF1^MKO),we found that the expression of IFN-I of XAF1^MKOmouse-derived macrophages was significantly enhanced after virus stimulation,and their antiviral infection ability was also significantly improved.Mechanism studies have shown that XAF1 can promote the K48-linked polyubiquitination of IRF7 and lead to its degradation through the proteasome pathway,thereby negatively regulating the protein stability of IRF7,which was dependent on the XAF1-XIAP-KLHL22 axis.In this axis,KLHL22 was the E3 ubiquitin ligase that directly mediates the degradation of IRF7protein after ubiquitination.XAF1 can inhibit the E3 ubiquitin ligase activity of XIAP by antagonizing XIAP,so that KLHL22 itself can avoid to be ubiquitinated and degradated mediated by XIAP,and finally promote the ubiquitination of IRF7 protein by stable KLHL22 protein.IRF7 was then degraded through the proteasome pathway to inhibit the production of IFN-I.Our work revealed a new"brake"that regulated interferon production and provided a good theoretical foundation for the treatment of chronic inflammatory diseases caused by interferon overexpression,such as systemic lupus erythematosus.