**3. The role of immune response in HCV infection**

**1. Introduction**

24 Recent Advances in Liver Diseases and Surgery

universal access to new treatment strategies.

**2. Structure and replicative cycle of HCV**

cirrhosis.

Hepatitis C virus (HCV) is a progressive disease that infects more than 185 million individuals worldwide and is associated with persistence of viral replication and ongoing necroinflam‐ mation and fibrosis. To date, 20% of patients chronically infected with HCV progress to

Epidemiological studies demonstrate that the incidence of HCV is not well known since acute infection is generally asymptomatic. The global prevalence is about 2.2%, and there is a large degree of geographic variability. Before the 2011, the gold standard of therapy for the treatment of chronic hepatitis C (CHC) was based on the combination of pegylated interferon (peg-IFN) and ribavirin (RBV). However, several aspects related to safety profile limited their use in clinical practice. In the recent years, thanks to basic research on HCV structure and replicative cycle, it has been possible to develop direct acting antiviral drugs that have dramatically increased the viral clearance rate. This new therapeutic strategy contemplates the use of interferon-free treatment protocols that are shorter and well tolerated, and this might improve the management of patients. These new medications for hepatitis C are effective disease modifiers and could potentially eradicate the infection in a long-term perspective. However, their costs are even high and unlikely sustainable for the National Health Systems (NHSs), and new pharmaceutical policy and a global commitment are required for achieving the

The structure of the HCV virion remains poorly characterized despite several substantial progress in biochemical and morphological studies, and most of the HCV proteins are now actively being pursued as antiviral targets. HCV, discovered in 1989, is a positive-sense, singlestranded RNA virus, approximately 9600 nt in length, which belongs to the Flaviviridae family (*Flavivirus* genus), also including many arthropod-borne human pathogens such as yellow fever virus, West Nile virus, and dengue virus. HCV has been classified by the World Health Organization (WHO) as an oncogenic virus [1]. HCV-RNA encodes a polyprotein that is cleaved by cellular and viral proteases into structural and nonstructural proteins, each with a specific function. The structural proteins include two envelope glycoproteins E1 and E2, which are targets of the host antibody response and are crucial for viral entry and fusion, and a core protein (C), which interacts with the viral genome to form the nucleocapsid. The nonstructural proteins P7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B form a complex with the RNA of the virus to initiate viral replication, which occurs by budding through intracellular membranes. Mature virions are released into the extracellular milieu by exocytosis, and nascent virions incorporate cellular lipoproteins and apolipoproteins (e.g., apoE and apoB) as lipoviral particles [2]. HCV specifically infects hepatocytes, entering the cells by receptor-mediated endocytosis. During primary infection, HCV particles are transported by the blood stream and come in contact with hepatocytes after spanning the fenestrated endothelium of the liver sinusoids. In the Disse space, virions are in direct contact with the basolateral surface of hepatocytes that interact with multiple cell surface molecules, including attachment factors

HCV has a very high replicative capacity, and a viral titer of >106 IU/mL can be measured in the serum within days after infection (averages 1–2 weeks) [9]. Innate immune response is the first line of host defense during infection, and interferons (IFNs) are the family of cytokines specialized in coordinating immunity against viruses and for the induction of an antiviral state in cells, by activation and regulation of cellular components of innate immunity, such as natural killer (NK) cells [10]. Furthermore, the induction of the endogenous IFN system in the liver can be ineffective in clearing the infection and in preventing response to therapies with peg-IFN and RBV [11,12]. Types I and II IFNs are in general the major elements of the innate immune response against viruses [10]. Type III IFN family (also known as IFNs-λ) is composed of interleukins (IL)-29, IL-28A, and IL-28B and is induced in response to several viral patho‐ gens. In the liver, type III IFN receptors are expressed at significant levels as a functional fulllength form, suggesting intact type III IFN signaling as part of the intrahepatic innate immune response [13,14]. Genetic variants of the IFN-λ3 and IFN-λ4 locus are strongly associated with spontaneous clearance of HCV and with response to therapy with peg-IFN and RBV. The molecular mechanisms that link genetic variants near the IFN-λ4 gene with constitutive activation of the endogenous IFN system in the liver are not entirely known, but it might involve an ongoing stimulation of the JAK–STAT pathway by IFN-λ4 through the IFN-λ receptors on hepatocytes. In contrast to the innate immune response, which is induced within hours to days after infection, the adaptive immune response against HCV is not detectable before 6–8 weeks and involves all components of the adaptive immune system, i.e., humoral antibodies, CD4+ T cells, and CD8+ T cells [10]. All these three components were shown to be associated with viral clearance. A well-coordinated interaction of the different immune cells might be essential for a successful immune response against HCV; however, little is known about the precise dynamic of this cross-talk [15]. HCV-specific T cells are recruited to the liver, and the viral replication is inhibited by both noncytolytic and cytolytic mechanisms. In about 20% of patients, the immune reaction during acute hepatitis C is strong enough to eliminate the infection. Immunocompetent HCV-infected individuals produce antibodies against epitopes within the structural as well as nonstructural proteins. Most of them, however, have no relevant antiviral activity, and only a small fraction of antibodies is able to inhibit virus binding, entry, or uncoating. These "neutralizing antibodies" target linear as well as confor‐ mational discontinuous epitopes mainly located within the envelope glycoproteins E1 and E2. While strong data indicate the neutralizing activity of these antibodies *in vitro*, their efficiency *in vivo* is less understood [10,15]. HCV elimination is associated with strong and sustained CD4+ and CD8+ cell responses that target multiple epitopes within the different HCV proteins and that remain detectable long after resolution of infection [10,16,17]. They act noncytolyti‐ cally, by secreting antiviral cytokines such as IFN-γ, as well as cytolytically, through perforin secretion and by engaging the FAS/FAS-L pathway [15]. Despite the intervention of both innate and adaptive immune response in CHC, the virus is able to escape from these barriers through yet unknown mechanisms.
