Toll-like receptors (TLRs) and RIG-I-like receptors (RLRs) constitute distinct families of pattern-recognition receptors that sense nucleic acids derived from viruses and trigger antiviral innate immune responses. TLR3, TLR7, and TLR9 are membrane proteins localized to the endosome that recognize viral double-stranded RNA, single-stranded RNA, and DNA, respectively, while RLRs, including RIG-I, Mda5, and LGP2, are cytoplasmic proteins that recognize viral RNA. Upon recognition of these nucleic acid species, TLRs and RLRs recruit specific intracellular adaptor proteins to initiate signaling pathways culminating in activation of NF-κB, MAP kinases, and IRFs that control the transcription of genes encoding type I interferon and other inflammatory cytokines, which are important for eliminating viruses. Here, we review recent insights into the signaling pathways initiated by TLR and RLR and their roles in innate and adaptive immune responses.

Toll-like Receptor and RIG-1-like Receptor Signaling

The expression of pattern-recognition receptors (PRRs) by immune and tissue cells provides the host with the ability to detect and respond to infection by viruses and other microorganisms. Significant progress has been made from studying this area, including the identification of PRRs, such as Toll-like receptors and RIG-I-like receptors, and the description of the molecular basis of their signalling pathways, which lead to the production of interferons and other cytokines. In parallel, common mechanisms used by viruses to evade PRR-mediated responses or to actively subvert these pathways for their own benefit are emerging. Accumulating evidence on how viral infection and PRR signalling pathways intersect is providing further insights into the function of the pathways involved, their constituent proteins and ways in which they could be manipulated therapeutically.

Viral evasion and subversion of pattern-recognition receptor signalling

https://aasldpubs.onlinelibrary.wiley.com/doi/pdf/10.1002/hep.22306

Toll-like receptors and adaptor molecules in liver disease: update.

Plasmacytoid dendritic cells: sensing nucleic acids in viral infection and autoimmune diseases

In the initiation of innate immune responses against pathogens, pattern-recognition receptors (PRRs) have an essential role in recognizing specific components of microorganisms and triggering responses that eliminate the invading microorganisms. However, inappropriate activation of PRRs can lead to prolonged inflammation and even to autoimmune and inflammatory diseases. Thus, PRR-triggered responses are regulated through the degradation or translocation of the innate receptors themselves and through the involvement of intracellular regulators or amplifiers. In addition, a complex interplay between PRRs and/or other immune pathways finely tunes the outcome of host immune defence responses. In this Review, I describe many of the numerous distinct mechanisms for the self-regulation and cross-regulation of innate immune receptor signalling.

Self-regulation and cross-regulation of pattern-recognition receptor signalling in health and disease

Hepatitis C virus (HCV) infection induces a wide range of chronic liver injuries; however, the mechanism through which HCV evades the immune surveillance system remains obscure. Blood dendritic cells (DCs) play a pivotal role in the recognition of viral infection and the induction of innate and adaptive immune responses. Several reports suggest that HCV infection induces the dysfunction of DCs in patients with chronic hepatitis C. Toll-like receptor (TLR) has been shown to play various roles in many viral infections; however, the involvement of HCV proteins in the TLR signaling pathway has not yet been precisely elucidated. In this study, we established mouse macrophage cell lines stably expressing HCV proteins and determined the effect of HCV proteins on the TLR signaling pathways. Immune cells expressing NS3, NS3/4A, NS4B, or NS5A were found to inhibit the activation of the TLR2, TLR4, TLR7, and TLR9 signaling pathways. Various genotypes of NS5A bound to MyD88, a major adaptor molecule in TLR, inhibited the recruitment of interleukin-1 receptor-associated kinase 1 to MyD88, and impaired cytokine production in response to TLR ligands. Amino acid residues 240 to 280, previously identified as the interferon sensitivity-determining region (ISDR) in NS5A, interacted with the death domain of MyD88, and the expression of a mutant NS5A lacking the ISDR partially restored cytokine production. These results suggest that the expression of HCV proteins modulates the TLR signaling pathway in immune cells.

Hepatitis C virus nonstructural protein 5A modulates the toll-like receptor-MyD88-dependent signaling pathway in macrophage cell lines.

Autographa californica nuclear polyhedrosis virus (AcNPV) is a double-stranded-DNA virus that is pathogenic to insects. AcNPV was shown to induce an innate immune response in mammalian immune cells and to confer protection of mice from lethal viral infection. In this study, we have shown that production of type I interferon (IFN) by AcNPV in murine plasmacytoid dendritic cells (pDCs) and non-pDCs, such as peritoneal macrophages and splenic CD11c+ DCs, was mediated by Toll-like receptor (TLR)-dependent and -independent pathways, respectively. IFN regulatory factor 7 (IRF7) was shown to play a crucial role in the production of type I IFN by AcNPV not only in immune cells in vitro but also in vivo. In mouse embryonic fibroblasts (MEFs), AcNPV produced IFN-beta and IFN-inducible chemokines through TLR-independent and IRF3-dependent pathways, in contrast to the TLR-dependent and IRF3/IRF7-independent production of proinflammatory cytokines. Although production of IFN-beta and IFN-inducible chemokines was severely impaired in IFN promoter-stimulator 1 (IPS-1)-deficient MEFs upon infection with vesicular stomatitis virus, AcNPV produced substantial amounts of the cytokines in IPS-1-deficient MEFs. These results suggest that a novel signaling pathway(s) other than TLR- and IPS-1-dependent pathways participates in the production of type I IFN in response to AcNPV infection.

Baculovirus Induces Type I Interferon Production through Toll-Like Receptor-Dependent and -Independent Pathways in a Cell-Type-Specific Manner

To eliminate hepatitis C virus (HCV) from infected hepatocytes, we generated two therapeutic molecules specifically activated in cells infected with HCV. A dominant active mutant of interferon (IFN) regulatory factor 7 (IRF7) and a negative regulator of HCV replication, VAP-C (Vesicle-associated membrane protein-associated protein subtype C), were fused with the C-terminal region of IPS-1 (IFNβ promoter stimulator-1), which includes an HCV protease cleavage site that was modified to be localized on the ER membrane, and designated cIRF7 and cVAP-C, respectively. In cells expressing the HCV protease, cIRF7 was cleaved and the processed fragment was migrated into the nucleus, where it activated various IFN promoters, including promoters of IFNα6, IFNβ, and IFN stimulated response element. Activation of the IFN promoters and suppression of viral RNA replication were observed in the HCV replicon cells and in cells infected with the JFH1 strain of HCV (HCVcc) by expression of cIRF7. Suppression of viral RNA replication was observed even in the IFN-resistant replicon cells by the expression of cIRF7. Expression of the cVAP-C also resulted in suppression of HCV replication in both the replicon and HCVcc infected cells. These results suggest that delivery of the therapeutic molecules into the liver of hepatitis C patients, followed by selective activation of the molecules in HCV-infected hepatocytes, is a feasible method for eliminating HCV.

Xiaoyu Wen

Papers

Hepatitis C virus nonstructural protein 5A modulates the toll-like receptor-MyD88-dependent signaling pathway in macrophage cell lines.

Baculovirus Induces Type I Interferon Production through Toll-Like Receptor-Dependent and -Independent Pathways in a Cell-Type-Specific Manner

Elimination of Hepatitis C Virus from Hepatocytes by a Selective Activation of Therapeutic Molecules