| Hepatitis C virus (HCV) is a small, enveloped RNA virus belonging to the Flaviviridae family, geus Hepacivirus, that cause acute and chronic liver disease in humans, especially including chronic hepatitis, cirrhosis, and hepatocellular carcinoma. During HCV infection, innate immune response would be actived.Innate immunity is a central component of the immunity system where its function as blocking the growth and dissemination of pathogens at early stages of infection. Cells of innate immune system express a number of pattern recognition receptors such as TLRs, RLRs and NLRs.TLRs are type I transmembrane proteins, which recognize pathogen-associated molecular patterns (PAMPs). TLRs selectively recognize unique components of pathogens. For example, TLR3 recognize double-stranded RNA from virus, TLR4 recognize lipopolysaccharide of gram-negative bacteria, TLR7 and TLR8 recognize single-stranded RNA from virus. The TLRs detective PAMPs and trigger the intracellular networks signaling pathways which result in the development or acceleration of inflammatory and autoimmune diseases.TLR7, expressing in dendritic cells and human B cell, is located in endosomal compartments. The ligands of TLR7 were originally identified as imiquimod, resquimod (R848) and guanine analogs such as loxoribine. TLR7 also recognize single strand RNA which originate from viruses such as HIV, RSV and influenza virus. Studies indicated that TLR7 has the ability to recognize the synthetic poly(U) RNA and some small interfering RNAs as well. TLR7 are composed of extracellular domains, transmembrane domains and cytoplasmic domains. The extracellular domain are mainly made up of 19-25 leucine-rich repeats (LRRs), which mediate the specific interaction of ligand-receptor. The cytoplasmic domain are mainly composed of Toll/Interleukin-1 receptor (TIR), which mediate the intracellular signaling. Although TLR7 could recognize many sigle strand RNA virus, the function of TLR7 during HCV infection is unclear.After the activation of TLR7 by its ligands, TLR7 recruit MyD88 by its TIR domain. By recruiting IRAK4 and IRAK6, MyD88 finally lead to the activation of NF-κB dependent signaling pathway which result in the production of cytokines and type I INF. Although the signaling pathway has been substantiated by many studies, the mechanism by which the TIR domain recruits MyD88 is unknown. In this study, we identify the fuction of TLR7 during HCV infection firstly.We found HCV genome RNA and ssRNA oligonucleotides (ORNs) derivered from HCV genome could trigger innate immune response. Further study indicated that HCV-ORNs activate innate immune reponse via TLR7 signal pathway and TLR7 is required for immune activation.Then we studied the mechanisms for the signal transduction site in TLR7 cytoplasmic domain.We performed truncating mutagenesis and site-direct mutagenesis analyses in order to illustrate the mechanisms of TLR7 intracelluar signal transduction. We identified a region, consisted of 16 amino acids, locating between TLR7Δ40 and TLR7Δ60 that could be crucial part for TLR7 signal transmission. We showed that this region was essential for combination between TLR7 and MyD88. The further study indicated that the two arginines at 1004 and 1006 in this region could impact the activation of NF-κB, and only arginine at 1004 is essential for conjunction of MyD88.In conclusion, TLR7 is an essential receptor during HCV infection, deficient in TLR7 signal would lead to susceptible HCV infection. TLR7 TIR domain has one crucial amino acids which could impact TLR7 signal transmission. Those finding provide a new target for prevention or therapy of clinical disease.
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