Translation Of Single Polyprotein
The HCV virion is made up of 9.6 kb single stranded positive RNA genome with highly invariant 5 and 3 untranslated regions (Rehermann and Nascimbeni, 2005; Wieland and Chisari, 2005). It is contained in a capsid and is enveloped by a lipid bilayer, within which two different glycoproteins are anchored (Asselah et al., 2009).When the host cell is infected with virus, the viral genome is uncoated and functions as a template for the translation of single polyprotein that is processed by host and viral proteases. The synthesis of negative-strand RNA is initiated by the non-structural viral proteins and then negative-strand RNA acts as a replication template for the propagation of new positive-strand viral genomes (Rehermann and Nascimbeni, 2005; Wieland and Chisari, 2005).
Viral replication is exceedingly fast; more than ten trillion viral particles are estimated to be produced per day, even in the chronic phase of infection (Neumann et al., 1998).Viral replication is carried out through an RNA-dependent RNA polymerase that lacks proof-reading capability, contributes to the generation of sequence diversity but related quasispecies within an infected individual, presenting a major challenge with respect to host immune response and chronic infection (Bowen and Walker, 2005; Rehermann and Nascimbeni, 2005; Dustin and Rice, 2007).Hepatocytes are the major target of HCV, but the other targets of virus may include monocytes, lymphocytes and endothelial cells (Lai et al., 2006; Pham et al., 2008). HCV is considered to be the common cause of chronic hepatitis and is one of the most crucial aetiologic agents of postransfusional hepatitis (Balanescu et al., 2012).
Studies on HCV infection had been carried out in infected patients (Lechner et al., 2000; Takaki et al., 2000) and chimpanzees (Thimme et al., 2002; Shoukry et al., 2004).Cytokines; molecular weight of less than 30 kDa; are defined as regulatory proteins or glycoproteins; secreted by white blood cells and various other cells in the body in response to various stimuli. More than 200 cytokines have been identified (Goldsby, 2003). Cytokines play an indispensable role in regulating the host responses to infection, immune responses, inflammation and trauma (Dinarello, 2000).Structural studies have indicated that cytokines are classified into four groups: the hematopoietin family, the interferon family, the chemokine family and the tumor necrosis factor family (Goldsby, 2003).Cytokines can be categorized as pro-inflammatory and anti-inflammatory cytokines.
Proinflammatory cytokines are involved in making the disease worse by promoting inflammation whereas anti-inflammatory cytokines function to reduce inflammation and promote healing process (Dinarello, 2000). According to some researchers; cytokines are responsible for both immunoregulation and immune impairment (Woitas et al., 1999; Cianci et al., 2005; Wright et al., 2005).Hepatitis C virus (HCV) is considered to be the main cause of hepatocellular injury that is related with the complicated immunologic systems (Zekri et al., 2005).
In order to regulate immune responses, cytokines have major influence in controlling the underlying pathogenesis and the resulting effect of HCV infection (Amini and Poustchi, 2012).Host immune defense against HCV infection is regulated by both humoral and cell-mediated immune responses but it is indicated that cell-mediated immune response to the cytokine system is involved in the immunopathogenesis of chronic hepatitis C (Jacobson et al., 2001).Cytokines consist of a complicated network of molecules that are responsible for regulating the inflammatory response and the homeostasis of organ functions. Furthermore, cytokines participate in many physiological and pathological actions occurring in the liver; which include regulation of liver growth, regeneration and other inflammatory processes such as viral liver disease, liver fibrosis and cirrhosis (Zekri et al., 2005).
During chronic HCV infection, cytokines play a central role in the regulation of hepatic inflammation and fibrogenesis (Zhang et al., 2012).It is indicated that cytokines themselves are manipulated by polymorphisms in their genes. However most of the genetic variants that confer a significant risk for chronic HCV infection have been localized in genes responsible for cytokine synthesis and the ultimate immune response (Amini and Poustchi, 2012).Interferons belong to the multigene family of inducible cytokines (Blatt et al., 1996; Diaz et al., 1996; Young, 1996; Roberts et al., 1998; Stark et al., 1998). Interferon was first discovered as an antiviral agent by Isaacs and Lindenmann more than 50 years ago (Isaacs and Lindenmann, 1957).
More than 10 mammalian IFN species and several subspecies have been reported (Pestka, 2007) and all IFNs possess antiviral activity (Pestka et al., 1987; Samuel, 1988; Stark et al., 1998).IFNs are categorized into three main groups: type I, type II and type III IFNs (Heim, 2012). The type I IFNs comprises all IFNαs, IFNβ, IFNε, IFNκ, IFNω and IFNν (Pestka, 2007). Type II IFNs include only one class i.e., IFNγ (Heim, 2012), and type III IFNs have been identified as IFNλ1, IFNλ2 and IFNλ3 and these are also known as IL29, IL28A and IL28B respectively (Kotenko et al., 2003; Sheppard et al., 2003).
Upon viral infection, cells produce IFNαs, IFNβ and IFNλs (Heim, 2012).Two major pathways have been reported that are involved in the detection of the viral genomes and in the induction of type I and type II interferons: the toll-like receptor (TLR) dependent pathway (Iwasaki and Medzhitov, 2004; Akira et al., 2006) and the cytosolic pathway involving the binding of viral genome to the RNA helicases retinoic acid inducible gene-I (RIG-I) and melanoma differentiation antigen 5 (MDA5) (Yoneyama, 2004; Yoneyama and Fujita, 2007).Interferons are not only involved in the initial host response to HCV, but also play crucial role in an immune response to chronic phase of hepatitis C which lasts for many years and subsequently results in the cirrhosis and hepatocellular carcinoma (Heim, 2012). The constantly activated endogenous IFN system has been observed in the liver of many infected patients with chronic hepatitis C and the expression of hundreds of IFN stimulated genes (ISGs) in hepatocytes have been indicated (MacQuillan et al.,2003; Asselah et al.,2003; Chen et al., 2005; Asselah et al., 2008; Sarasin et al., 2008).Role of type I interferons including IFN-α, -β and -ω against viral infection are related with innate immune responses (Ding et al., 2012).
The products of interferon-stimulated genes (ISGs) include the cellular factors that mediate this defense mechanism. However little information is available about antiviral potential, target specificity and the mechanism of action of most ISG products (Schoggins et al., 2011). Direct signaling pathways by viral infection are involved in the IFN gene transcription and IFN activates the innate cellular antiviral response which serves to inhibit the viral replication (Yang et al., 2011).Suppression in production of HCV virion by affecting the viral RNA and protein synthesis, encouragement of immune lysis of HCV infected cells, inhibitory actions on hepatic fibrosis and negative effect on HCV induced carcinogenesis; are some of the prominent functions of type-II interferon i.e. IFN- γ (Cecere et al., 2004).
In addition to these, spontaneous viral elimination is also related with IFN-γ production in HCV infection (Major et al., 2002; Thimme et al., 2002).Type-III interferon (IFN- λ, including IL-29, IL-28A and IL-28B); a new subfamily of interferon; are associated with the inhibition of viral replication in vitro and in vivo (Ding et al., 2012). However genes encoding these type-III interferon members are found to be clustered on human chromosome 19q13.
Structurally, they are related to IL-10 superfamily of cytokines but their functions are similar to type-I interferons as they are also activated by viral infections and possess anti-viral activity in vitro (Kotenko et al., 2003; Sheppard et al., 2003).Recent studies have shown that there is an association of genetic variants near the IL28B gene and ISG expression in the liver (Honda et al., 2010; Urban et al., 2010; Dill et al., 2011) but the mechanism of induction of ISGs linked with IL28B genotype is still obscure (Heim,2012). Moreover, viral eliminating processes that are responsible for HCV to persist, in spite of such powerful activation of hepatic IFN system, are not well understood.
It is indicated that activated endogenous IFN system is not only involved in HCV clearance, but also suppresses the response to IFN- based therapies (Chen et al., 2005; Asselah et al., 2008; Sarasin et al., 2008).Immunoregulatory cytokines and T lymphocytes are of vital importance in the host defense against HCV infection (Zekri et al., 2005). On the basis of differences in cytokine secretion, activated CD4+ T cells can be classified into two subsets: Th1 and Th2 subsets (Gioia et al., 2005; Lanzilli et al., 2005).Th1 and Th2 cytokines are important in viral infections and disturbances in the regulation of these cytokines result in viral persistence and emergence of chronic disease.
Recent studies have shown that Th1 cytokines are predominant in chronic hepatitis C and are associated with liver immunopathology (Gigi et al., 2008).The unsuccessful event of spontaneous clearance of virus and chronic persistence of virus is associated with an inadequate Th1 immunity as well as weak HCV-infected T cell response at an inflammatory site. In order to induce the Th1 immunity, production of interleukin-12 (IL-12) is required, leading to the elimination of pathogens and viruses (Cecere et al., 2004).T-helper type 1 (Th1) cytokines (IL-2, IFN-γ) are needed for host anti-viral responses, while T-helper type 2 (Th2) cytokines (IL-4, IL-10) can suppress the development of these effectors. Cacciarelli et al. observed increased levels of IL-2, IL-4, IL-10, and IFN-γ in patients infected with chronic HCV.
It was reported that after treating HCV-infected patients with interferon therapy, levels of IL-4 and IL-10 had decreased and resulted in diminished activity of HCV RNA (Cacciarelli et al., 1996).Another study has indicated that patients infected with chronic HCV infection exhibited augmented levels of Th1 cytokines, including IL-2, IL-2R, and IFN-γ; whereas levels of Th2 cytokines IL-4 and IL-6 were lower in the patients as compared to control subjects (Cribier et al., 1998).Experimental studies have indicated that antigen-specific Th1 immunity and pro-inflammatory cytokines are involved in liver injury related to HCV and viral clearance (Schvoerer et al., 2003; Wright et al., 2005; Katia et al., 2006), however little is known about pathogenesis of chronic HCV infection (Wang et al., 2012).
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