The Mechanisms Of Virus Induced Type ? Interferon Activation By IFITM3

RNA virus is the largest groups in the virus.During RNA virus infection,it could produce replicative intermediate double-stranded RNA(dsRNA).Innate immune system recognizes and responds to viral dsRNA by endosomal Toll-like receptors 3(TLR3)or cytosolic viral sensors RIG-I like receptors(RLRs).RLRs recognize cytosolic viral dsRNA via helicase domain,and interacted with adaptor protein VISA(virus-induced signaling adaptor)in the mitochondria through the CARD domain.Activated VISA recruits different downstream signaling proteins TRAFs,TRAFs recruit TBK1 or IKK complex respectively to activate the transcription factors IRF3 and NF-?B.The two kinds of transcription factors cooperate to induce the expression of type I interferon and inflammatory cytokine.Type I interferons play an important role in antiviral natural immunity,it could induce a large number of interferon induced genes(ISGs)expression to inhibit viral replication or eradicate virus-infected cells;it could also promote the maturation of dendritic cells to strengthen the activation of antiviral adaptive immunity.Excessive production of IFN generates pathological tissue damage or autoimmune diseases,so the precise regulation of IFN has been one of research hotspots in the field.Interferon induced transmembrane protein 3(IFITM3)belongs to ISGs,and it can be induced by IFNs and viral infection.It has been reported that IFITM3 inhibits the entry process of various viruses,such as Influenza A virus,Dengue virus and West Nile Virus.Most of viruses inhibited by IFITM3 are enveloped virus and entry host cell cytoplasm through endosomal pathway.IFITM3 performs anti-virus function by expanding the volume of acid organelles such as endosome and lysosome,altering the endosomal pH or membrane characteristics to suppress the membrane fusion between virus and endosome.So far,the study of IFITM3 mainly focused on its influence on entry process and replication of viruses,but whether it impacts the virus-induced type ? interferon expression signaling pathways has not been reported.Sendai virus(SeV),which fuses with the cell surface but not endosome,is reportedly not restricted by IFITM3.We use artificial dsRNA analog Poly(I:C)and SeV as stimulus,to explore whether IFITM3 regulates RLRs-mediated type ? interferon expression signaling pathway.In our study,we demonstrate that overexpression of IFITM3 inhibited the SeV-or transfected Poly(I:C)-triggered transcription of IFN-? or downstream ISGs and inhibited IFN-?protein production,whereas knockdown of IFITM3 had the opposite effects.By co-transfection experiments,we identified IFITM3 regulated RLRs signaling pathways on IRF3 level.Overexpression of IFITM3 significantly reduced SeV-induced IRF3 phosphorylation levels and also reduced the total amount of IRF3 protein,while knockdown of IFITM3 enhanced the IRF3 phosphorylation,dimerization and nuclear translocation induced by SeV and significantly enhanced the total protein IRF3.But further studies showed that IFITM3 had no effect on IRF3 transcription level.IFITM3 interacted with IRF3 institutively,and they co-located in cells.IFITM3 can promote the process of autophagy and induce the migration of IRF3 from dispersed distribution in cytoplasm to autophagy.By further research,we found that the autophagy pathway inhibitor 3MA can inhibit the degradation of IRF3 mediated by IFITM3,suggesting that IFITM3-mediated degradation of IRF3 was autophagy-dependent.The expression of IFITM3 was induced while the expression of IRF3 was decreased following virus infection,suggesting that the induced IFITM3 mediated IRF3 degradation.Type ?interferon induced the expression of IFITM3,which in turn negative regulated type ?interferon production,formed negative feedback regulation of type I interferon prodution signaling pathways.Our study found antiviral protein IFITM3 can not only suppress the entry of the viruses,but also promote autophagic degradation of transcription factor IRF3 to negatively regulate RLRs mediated type ? interferon production signal transduction pathways.We reveals a new negative-feedback regulatory mechanism of antiviral innate immune,and is also an important supplement to regulation network of antiviral signaling pathway.