Innate immunity and HCV

Hepatitis C virus (HCV) infections become chronic in the majority of infected individuals, and chronic hepatitis C (CHC) can lead to cirrhosis and hepatocellular carcinoma. The innate immune system is central to host-virus interactions during the entire natural course of the disease. The HCV NS3/4A protease efficiently cleaves and inactivates two important signaling molecules in the sensory pathways that react to HCV pathogen-associated molecular patterns (PAMPs) to induce interferons (IFNs), i.e., mitochondrial antiviral signaling protein (MAVS) and Toll-IL-1 receptor domain-containing adaptor inducing IFN-beta (TRIF). Despite this viral escape mechanism, the innate immune system strongly reacts to HCV within the first days after infection. The sensory pathways, the type(s) of IFNs involved and the cellular source of IFNs are largely unknown. After 4-8weeks, HCV specific T cells are recruited to the liver. IFN-gamma-stimulated genes get strongly expressed in the liver. In about 30% of patients, the virus is eliminated during the acute phase of the infection by T cell-mediated antiviral mechanisms. In the remaining 70% of patients, HCV persists for decades. During this phase, T cell-derived IFN-gamma cannot be detected any more in liver biopsies. Instead, in about half of the patients, hundreds of type I or III IFN-stimulated genes become again strongly expressed. However, this innate immune reaction is ineffective against HCV. Moreover, patients with constitutive IFN-stimulated gene (ISG) expression have a poor response to treatment with pegylated IFN-alpha (PegIFN-alpha) and ribavirin. The viral escape mechanisms that protect HCV from IFN-mediated innate immune reactions are not entirely understood, but might involve blockade of ISG protein translation at the ribosome, localization of viral replication to cells with refractory IFN signal transduction pathways or to cell compartments that are not accessible to antiviral IFN-stimulated effector systems. Recently, genetic variations near the IL28B (IFN-lambda3) were found to be strongly associated with spontaneous clearance of HCV and response to treatment with PegIFN-alpha and ribavirin. The finding supports a central role of the innate immune response in host-viral interactions. The signaling pathways that link genetic variants of IL28B with immune answers to HCV remain to be elucidated. The present review article attempts to summarize current knowledge of some central aspects of the interactions of HCV with the innate immune system.