The Mechanisms Of TRIM8 Negatively Regulates TLR3/4-mediated Innate Immue Response

Innate immune responses provide the first line of host defense against microbial pathogens and,together with adaptive immunity,ensure effective clearance of invading pathogens.The initial sensing of infection is mediated by innate pattern recognition receptors(PRRs).Among these,toll-like receptors(TLRs)family plays a central role in regulating innate immunity.Signaling by TLRs is mediated by the recruitment of adaptor proteins that contain the Toll-interleukin 1(IL-1)receptor(TIR)domain,of which there are five:MyD88,Mal,TRIF,TRAM and SRAM.TLR3 has been reported to recognize viral double-stranded RNA as well as its analog poly(I:C).Recognition of these ligands by TLR3 activates signaling pathways leading to the activation of NF-?B and IRF3 and subsequent production of type ? IFNs and proinflammatory cytokines.TLR3-mediated signaling critically depends on the TIR domain-containing adapter-inducing interferon-?(TRIF,also known as TICAM1).Recognition of Gram-negative bacterial product lipopolysaccharide(LPS)by Toll-like receptor 4(TLR4)triggers activation of the transcription factors NF-?B and interferon regulated factor 3,leading to induction of type ? interferons and proinflammatory cytokines.TLR4 signals the induction of cytokines genes from the early endosome via the adaptor TRIF-related adaptor molecule(TRAM,also known as TICAM2).TRAM interacts with TLR4 and,after LPS recognition,it recruits TRIF to the receptor complex.The E3 ubiquitin ligase TRIM8 had been reported to serve as a positive regulator of TNFa and IL-1? induced NF-?B activation by mediating K63-linked polyubiquitination of TAK1 by our research group.In this study,we confirmed that Trim8-deficiency reduces TNF? and IL-1? induced NF-?B activation in MEFs and immune cells.Further study showed that Trim8 as a negative regulator in TLR3/4 signaling pathway.The production of type ? IFNs and proinflammatory cytokines in response to the TLR3 ligand poly(I:C)and the TLR4 ligand LPS was both elevated in Trim8-deficiency MEFs and immune cells.And Trim8-deficient mice also exhibited increased susceptibility to poly(I:C),LPS as well as Salmonella enterica serovar Typhimurium(S.Typhimurium)induced death than the control littermates.Mechanistically,TRIM8 impaired the recruitment of TRIF to TLR4 mediated by TRAM by mediating the polyubiquitination of TRAM.Moreover,TRIM8 impaired the association of TRIF and TBK1.Our study showed that TRIM8 serves as a double-faced regulator in innate immune responses with distinct mechanisms.