Innate immune responses in hepatitis C virus infection

Abstract

Hepatitis C virus (HCV) is a major causative agent of chronic hepatitis and hepatocellular carcinoma worldwide and thus poses a significant public health threat. A hallmark of HCV infection is the extraordinary ability of the virus to persist in a majority of infected people. Innate immune responses represent the front line of defense of the human body against HCV immediately after infection. They also play a crucial role in orchestrating subsequent HCV-specific adaptive immunity that is pivotal for viral clearance. Accumulating evidence suggests that the host has evolved multifaceted innate immune mechanisms to sense HCV infection and elicit defense responses, while HCV has developed elaborate strategies to circumvent many of these. Defining the interplay of HCV with host innate immunity reveals mechanistic insights into hepatitis C pathogenesis and informs approaches to therapy. In this review, we summarize recent advances in understanding innate immune responses to HCV infection, focusing on induction and effector mechanisms of the interferon antiviral response as well as the evasion strategies of HCV.

Figure 1. Organization of the HCV genome and the proteins HCV encodes

The ∼10-kb positive-sense, single-stranded RNA genome encodes a large polyprotein that is processed into 10 individual proteins by a combination of viral and cellular proteases. Flanking the HCV polyprotein are the highly structured, 5′- and 3′-nontranslated regions (NTRs) important for viral RNA replication and/or protein translation. A frameshift (F, a.k.a., ARF) protein may be expressed from the core-coding region as a result of translation from an alternative reading frame. The junctions between the structural proteins, C, E1 and E2 (shaded in light blue) and those between E2 and p7, p7 and NS2 are cleaved by cellular signal peptidase (filled triangles). An additional processing proximal to the C-terminus of core is mediated by cellular signal peptide peptidase (empty triangle). NS2, along with the N-terminal portion of NS3, constitute the NS2-3 autoprotease that cleaves the NS2-3 junction. The downstream NS proteins are processed by the NS3 serine protease with the aid of the cofactor NS4A. Among the NS proteins (shaded in light yellow), only NS3 to NS5B are required for HCV RNA replication. In addition to its role in viral replication, NS5A contains an IFN sensitivity-determining region (ISDR, spanning codons 2209 to 2248), which has been associated with efficacy of IFN therapy in genotype 1b HCV-infected Japanese patients.

Figure 2. Innate intracellular signaling pathways that sense RNA virus infections and lead to expression of type I and III IFNs, cytokines and chemokines

Viral dsRNAs and 5′-triphosphate-bearing, poly-U rich viral RNAs are recognized by the RLRs (RIG-I, MDA5 and LGP2) that are constitutively expressed in the cytoplasm and that initiate MAVS-dependent signaling leading to activation of IRF3/7 and NF-κB, and ultimately, synthesis of type I and III IFNs and inflammatory cytokines/chemokines. Viral dsRNAs can also be sensed by TLR3 in late endosomes and trigger a TRIF-dependent pathway activating innate immune responses. Both the RIG-I and TLR3 pathways are capable of sensing HCV infection in hepatocytes. Viral ssRNAs can be sensed by TLR7 or TLR8 in endosomes and activate MyD88-dependent innate immune responses. TLR7 operates exclusively in pDCs and mainly results in IRF7 activation and subsequent induction of IFN-α and IFN-λs, while TLR8 operates in mDCs and mainly leads to expression of NF-κB-dependent cytokines. pDCs use the TLR7 pathway to sense co-cultured HCV-infected hepatocytes, resulting in IFN-α production.

Figure 3. Induction of ISGs by type I and III IFNs through the JAK-STAT signaling pathway and its inhibition by HCV

Type I and III IFNs bind to distinct, heterodimeric cell surface receptors, but then activate a similar intracellular signaling pathway involving recruitment and activation of the Janus kinases, JAK1 and TYK2, phosphorylation of STAT1 and STAT2, and the formation of the heterotrimeric transcription factor complex, IFN-stimulated gene factor-3 (ISGF3) comprising phosphorylated STAT1 and STAT2 and IRF9. ISGF3 migrates into the nucleus and induces the transcription of hundreds of ISGs by binding to the IFN-stimulated response element (ISRE) in ISG promoters. HCV core and NS5A associate with STAT1, suppressing its phosphorylation and/or nuclear translocation. Core can also induce SOCS3 expression, which negatively regulates JAK. HCV-induced endoplasmic reticulum stress up-regulates expression of protein phosphatase 2A (PP2A), which inhibits the transcriptional activity of STAT1 by suppressing protein arginine methyltransferase 1 (PRMT1), thereby promoting STAT1 hypomethylation and STAT1 association with the protein inhibitor of activated STAT1 (PIAS1). Up-regulation of ubiquitin-like specific protease 18 (USP18) in HCV infection also attenuates JAK-STAT signaling at the receptor level through direct binding to IFNAR2.