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Th2 responses and the role of cytokines
Clinical Biochemistry 34 (2001) 167–171
Immunopathogenesis of hepatitis C viral infection: Th1/Th2 responses and the role of cytokines Pearl M. Jacobson Brown, Manuela G. Neuman* Division of Clinical Pharmacology, E-240, Sunnybrook HSC, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada Received 8 February 2001; received in revised form 21 March 2001; accepted 27 March 2001
Abstract Hepatitis C virus is a common cause of hepatocellular injury that is associated with complex and vigorous immunologic mechanisms. Both humoral and cell-mediated immune responses participate in the host defense against hepatitis C viral infection, but there is increasing recognition of the roles played by the cell-mediated response, and in particular the cytokine system, in the immunopathogenesis of chronic hepatitis C. The cell-mediated response depends on cytotoxic and helper T-cell activity, and functions through the actions of cytokines to regulate macrophages, natural killer cells, and antiviral cellular proteins. Cytokines produced in the liver are essential in defending the host against hepatitis C invasion, but they have also been implicated in the hepatocellular injury seen in the majority of chronically infected patients. Cytokines are thought to be involved in the pathogenesis of hepatitis C under conditions where the virus can mutate effectively and evade T-cell immune defense mechanisms. Persistent infection upsets the balance between immunostimulatory and inhibitory cytokines which can prolong inflammation and lead to necrosis, fibrosis, and chronic liver disease. © 2001 The Canadian Society of Clinical Chemists. All rights reserved. Keywords: Hepatitis C; Immune response; Th1; Th2 cells; T lymphocyte; Cytokine
1. Introduction Infection with the hepatitis C virus is a major cause of chronic liver disease throughout the world, with 60 to 85% of patients developing chronic hepatitis C following infection with the virus [1]. Hepatitis C infects 170 million people worldwide and is a factor in the development of cirrhosis in at least 20% of chronically infected patients [2]. The final outcome of chronic hepatitis C infection is thought to depend on the balance between the rate of replication of the virus and the rapidity, specificity and effectiveness of the host immune response. Understanding changes in immunologic parameters such as T-cell proliferation and cytokine production may contribute to the development of new management strategies to deal with this infection and evaluate clinical outcome. The present review considers the current understanding of the immunopathogenesis of chronic hepatitis C infection, and underlines the interaction * Corresponding author. Tel.: ⫹(416)480-6100, ext. 3503; fax: ⫹(416) 480-6025. E-mail address: [email protected] (M.G. Neuman).
between different types of cell-mediated responses in the progression of hepatitis C-related liver damage.
2. Humoral vs. cellular immune responses The role of the humoral immune system in fighting hepatitis C viral infection is not yet clear, but early studies have implicated the activity of virus neutralizing antibodies in the recovery from viral infection. Chimpanzees inoculated with hepatitis C virus recovered from infection when given the same inoculum that caused original infection [3]. Sera derived from patients with chronic hepatitis C infection has been used to inhibit HCV infection in lymphoid lines and cell cultures in vitro [4]. Hyperimmune sera derived from immunization of rabbits with a synthetic analogue of the E2 site on the hepatitis C viral antigen has some efficacy in protecting chimpanzees from infection with a homologous viral strain [5]. Despite these findings, studies of humans naturally infected with hepatitis C virus show that neutralizing antibodies are produced during hepatitis C infection regardless of whether viral resolution is achieved. Persistent infection in the presence of circulating antibodies
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is associated with mutant epitopes of the hepatitis C virus which are able to escape recognition, thereby limiting the efficiency of the antibody-mediated response. The high heterogeneity of the hypervariable (HVR1) envelope region of the hepatitis C virus and the detection of quasispecies in chronic infection could contribute to an understanding of why neutralizing antibodies are associated with unresolved viremia [6]. In contrast to the humoral response, which is incapable of eliminating a virus from an infected cell, cell-mediated defenses are called into action after the virus has gained entry into the host cell. The cell-mediated immune response results from the activity of CD4⫹ helper T and CD8⫹ cytotoxic T lymphocytes (CTLs), which recognize peptides that have been synthesized in the cytosolic compartment of the host cell through the processing and degradation of viral proteins. These endogenously synthesized peptides are transported across the membrane of the host cell where they are presented to T cell receptors through class I or II HLA molecules. CD8⫹ and CD4⫹ cells interact with T cell receptors form a major histocompatibility complex (MHC). Formation of the MHC leads to activation of T cells. Activated CD8⫹ cells may directly lyse virus-infected host cells through the release of cytotoxic enzymes. CD4⫹ induce Kuppfer cells to activate natural killer cells, leading to nonspecific lysis of infected cells. They also produce cytokines, regulate B lymphocytes and enhance the activity of CD8⫹ T cells [7]. The role of cell-mediated immunity in hepatitis C infection has not yet been defined, but studies suggest that helper T and CTL responses differ among patients who recover compared to those who develop chronic infection. Several studies have shown a strong CD4⫹ response in patients with long-lasting chronic hepatitis C [8,9]. Consistently, several studies have demonstrated that a strong and maintained CD4⫹ response is correlated with viral clearance in acute hepatitis C viral infection [10,11]. There is evidence that CTL activity, while less vigorous in chronically infected patients, may be inversely related to viral load [12]. These findings suggest that CTLs function primarily in controlling viral infection in the early or acute stages of the disease.
3. The pathogenic role of helper and cytotoxic T cells The hepatitis C virus is not directly cytopathic, but its adverse hepatitic effects are thought to be mediated through the overproduction of initially protective immunologic responses. Liver-infiltrating lymphocytes from hepatitis Cinfected individuals not only lyse infected liver cells, they can start a cascade of events resulting in hepatocellular damage. Hepatitis C virus-specific CD4⫹ activity is correlated with histologic fibrosis and portal tract inflammation, and, in chronic hepatitis C-infected patients with intrahepatic CTL activity, lower levels of viremia are associated
with more active hepatocellular damage [13,14]. Relatively high levels of hepatitis C virus-specific helper and cytotoxic T cells have been detected in inflamed liver infiltrates from chronic hepatitis C-infected patients [15,16]. The T cells present in the liver are more numerous and can be functionally different from HCV-specific T cells present in peripheral blood, suggesting that some T lymphocytes may be sequestered at the site of infection [17,18]. CTL activity is correlated with reduced viremia, implying that CTLs assist in controlling hepatitis C infection. It is postulated that in the absence of complete virologic recovery, CTLs may exert harmful effects on noninfected bystander hepatocytes, leading to inflammation, necrosis, and fulminant hepatitis.
4. Immunomodulatory properties of cytokines CD4⫹ helper T cells produce small regulatory proteins called cytokines which are important in controlling viral replication in hepatitis C infection. T-helper type 1 (Th1) cytokines are required for the generation of cytotoxic T cells and the activation of natural killer cells, while T-helper type 2 (Th2) cytokines generally inhibit the development of Th1 activity [19]. In hepatitis C, certain cytokines contribute to the host defense against infection by coordinating effector mechanisms that interfere with the hepatitis C virus life cycle. Th1 cytokines interleukin-2 (IL-2), tumor necrosis factor-a´ (TNF-a´), and interferon-a˜ (IFN-a˜) produce a selflimited, acute response to hepatitis C which plays a central role in the host defense against intrahepatic pathogens like the hepatitis C virus. If Th1 cytokines are not kept under control, however, they have the capacity to cause liver injury by producing inflammatory activity and contributing to necrotic cell death [20]. Th2 cytokines inhibit Th1 activity following acute viral infection and, where hepatitis C infection persists, they may have a role in protecting the host from the potentially damaging consequences of Th1 cells.
5. T-helper cell response to infection During acute hepatitis C infection, the majority of liverinfiltrating cells are pathogen-destroying cells with Th1 activity [17,18]. Th1 cells release TNF-a´, IFN-a˜, and IL-2, cytokines which can activate a wide variety of cellular genes leading to antiproliferative and antiviral effects [7]. One well-characterized antiviral mechanism in hepatitis C is the activation by IFN-␣ of protein kinase (PKR) [21]. Once activated by type-1 IFNs, PKR can block protein synthesis through the eIF-2 factor, thereby resulting in inhibition of viral replication. IL-2 has been shown to have antiviral effects in hepatitis B infection, although the possible antiviral action of IL-2 in hepatitis C infection has yet to be established [22]. TNF-␣ is released from hepatocytes and macrophages in response to inflammatory stimulus and is an
P.M. Jacobson Brown, M.G. Neuman / Clinical Biochemistry 34 (2001) 167–171
important first-line defense against hepatitis C infection. TNF-␣ exhibits direct antiviral activity against the hepatitis C virus while enhancing proliferation of lymphocytes and stimulating a targeted CTL response to combat the virus within the liver [23,24].
6. Cytokines and hepatocellular damage It has been well-documented that chronic hepatitis Cinfected patients who show resolution of viremia have a strong Th1 response, but demonstrate mild Th2 activity [25,26,27]. Individuals who have a more vigorous Th1 response to the hepatitis C viral antigen seem to be able to clear the virus better and are able to recover from acute infection. Unfortunately, hepatitis C virus persists in most infected patients despite a strong Th1 response because the high mutation rate of hepatitis C makes it possible for the virus to escape immune surveillance [28]. Hepatitis C viral strains develop resistance to the effects of cytokines produced by hepatitis C virus-specific CTLs, thus continuing to induce T-cell activity. The viral strains themselves are not cytopathic, but, in the presence of persistent infection, Th1 cells continue to release TNF-␣, IFN-␥, and IL-2, causing self-inflicted inflammation and necrosis [7]. The more the balance shifts toward the Th1 response, the more aggressive the host viral defense, but the more potential exists for the tissue-damaging effects of the Th1 cytokines. Th1 cytokines may be seen as culprits in cases where the hepatitis C virus has managed to create conditions that result in viral persistence. There are several possible ways in which Th1 cytokines may exert their harmful effects, but at present the mechanisms are undefined. TNF-␣ production, for instance, may be involved in macrophage-independent necrosis and is thought to be one means by which CTLs amplify the damage to nearby, noninfected cells [29,30]. TNF-␣ stimulates macrophages to produce transforming growth factor- (TGF-), a cytokine which activates stellate cells to increase production of extracellular matrix proteins, leading to hepatic fibrosis [2]. IFN-a˜ has been studied in animal models in hepatitis B where it has been shown to recruit inflammatory cells, cause apoptosis of hepatocytes and produce Kupffer cell hyperplasia [31]. IFN-a˜ has also been implicated in hepatitis C as contributing to self-inflicted tissue damage by activating intrahepatic macrophages [32]. Th2 cytokines such as IL-4 and IL-10 modulate selfinflicted injury by suppressing the Th1 response and counteracting the fibrogenic effects of TNF-␣, IFN-␥ and IL-2 [33]. IL-10 has prominent antifibrotic activity in that it downregulates collagen 1 expression while upregulating interstitial collagenase [34]. IL-10 can decrease expression of proinflammatory cytokines such as TNF-␣, IFN-␥, and IL-2 from T cells and it can upregulate the expression of the cytokine inhibitor TNF-R [32]. Patients with milder forms of hepatitis C have a higher expression of IL-10 compared
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to those with overt liver disease, and progressive liver injury in chronic hepatitis C is significantly correlated with the downregulation of IL-10 [35,36]. In the presence of persistent hepatitis C infection, T cell activity is continually stimulated and may be responsible for the development of liver lesions. Experimentally, CD8⫹ CTLs have been detected in the peripheral blood and the intrahepatic lymphocyte infiltrate in patients with chronic hepatitis C, suggesting that an active immune response continues to be triggered even in the absence of acute viral infection [37]. Persistent T cell response may be made possible through the initial production of cytokines by non-T cells. These cytokines, notably IL-12 and IL-15, may drive undifferentiated Th0 progenitor T cells to become CD4⫹ lymphocytes or may trigger cytolytic activity. IL-12, a proinflammatory cytokine that is secreted by macrophages and NK cells, has been shown to stimulate the proliferation of Th1 cells [38]. IL-15, a cytokine which is produced by a variety of cell types including monocytes and macrophages, can bind to NK cells, activating them to produce TNF-␣ and IFN-␥ [39]. NK cells are primitive cells that lead to nonspecific lysis of infected cells, and their impairment has been implicated in the development of chronic hepatitis C viral infection [40].
7. Cytokine measurement Cytokine levels are difficult to measure for a number of reasons. Profiles in liver compartments may vary depending on the stage of hepatitis C of a particular patient, with high amounts of antigen biasing toward a larger Th1 response [41]. Measurement of cytokines at different sites–for example, in peripheral blood cells vs. in the liver–may lead to different observations about the predominant T-cell response. Further, cytokines do not always fit neatly into the Th1 and Th2 subsets. Cytokines are produced by multiple cell types, including macrophages, CD4⫹ and CD8⫹ T cells, and a single helper-T cell clone may produce both Th1 and Th2 cytokines [42,17]. Cytokines are like neurotransmitters in that they act locally and possess very short halflives. Measurement of mRNA and protein levels may be discordant due to the fact that many strains of hepatitis C virus encode cytokine analogues, cytokine receptor genes, or proteins which bind cytokines [43].
8. Conclusion Hepatitis C is usually diagnosed when the infection is already chronic and the virus seems to have developed successful strategies for escaping immune surveillance. The pathogenesis of viral persistence is believed to involve a primarily cell-mediated immune mechanism, during which Th1/Th2 cells are unable to ward off viral invasion. Compromised host immune factors and the high
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variability of the hepatitis C virus may contribute to the virus’s capacity to elude host defenses and persist in an apparently unfavourable environment. The continual response to the virus by cytokines within the liver induces CTL proliferation and the release of necroinflammatory factors which cause irreversible hepatic tissue damage. It is a developing hypothesis that the T cell immune response is somehow compromised in patients who are chronically infected with hepatitis C, but studies are needed to define precisely how viral escape mechanisms and cytokine activity interact to produce the pathogenesis of chronic hepatitis C viral infection.
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Immunopathogenesis of hepatitis C viral infection: Th1/Th2 responses and the role of cytokines Pearl M. Jacobson Brown, Manuela G. Neuman* Division of Clinical Pharmacology, E-240, Sunnybrook HSC, 2075 Bayview Avenue, Toronto, Ontario M4N 3M5, Canada Received 8 February 2001; received in revised form 21 March 2001; accepted 27 March 2001
Abstract Hepatitis C virus is a common cause of hepatocellular injury that is associated with complex and vigorous immunologic mechanisms. Both humoral and cell-mediated immune responses participate in the host defense against hepatitis C viral infection, but there is increasing recognition of the roles played by the cell-mediated response, and in particular the cytokine system, in the immunopathogenesis of chronic hepatitis C. The cell-mediated response depends on cytotoxic and helper T-cell activity, and functions through the actions of cytokines to regulate macrophages, natural killer cells, and antiviral cellular proteins. Cytokines produced in the liver are essential in defending the host against hepatitis C invasion, but they have also been implicated in the hepatocellular injury seen in the majority of chronically infected patients. Cytokines are thought to be involved in the pathogenesis of hepatitis C under conditions where the virus can mutate effectively and evade T-cell immune defense mechanisms. Persistent infection upsets the balance between immunostimulatory and inhibitory cytokines which can prolong inflammation and lead to necrosis, fibrosis, and chronic liver disease. © 2001 The Canadian Society of Clinical Chemists. All rights reserved. Keywords: Hepatitis C; Immune response; Th1; Th2 cells; T lymphocyte; Cytokine
1. Introduction Infection with the hepatitis C virus is a major cause of chronic liver disease throughout the world, with 60 to 85% of patients developing chronic hepatitis C following infection with the virus [1]. Hepatitis C infects 170 million people worldwide and is a factor in the development of cirrhosis in at least 20% of chronically infected patients [2]. The final outcome of chronic hepatitis C infection is thought to depend on the balance between the rate of replication of the virus and the rapidity, specificity and effectiveness of the host immune response. Understanding changes in immunologic parameters such as T-cell proliferation and cytokine production may contribute to the development of new management strategies to deal with this infection and evaluate clinical outcome. The present review considers the current understanding of the immunopathogenesis of chronic hepatitis C infection, and underlines the interaction * Corresponding author. Tel.: ⫹(416)480-6100, ext. 3503; fax: ⫹(416) 480-6025. E-mail address: [email protected] (M.G. Neuman).
between different types of cell-mediated responses in the progression of hepatitis C-related liver damage.
2. Humoral vs. cellular immune responses The role of the humoral immune system in fighting hepatitis C viral infection is not yet clear, but early studies have implicated the activity of virus neutralizing antibodies in the recovery from viral infection. Chimpanzees inoculated with hepatitis C virus recovered from infection when given the same inoculum that caused original infection [3]. Sera derived from patients with chronic hepatitis C infection has been used to inhibit HCV infection in lymphoid lines and cell cultures in vitro [4]. Hyperimmune sera derived from immunization of rabbits with a synthetic analogue of the E2 site on the hepatitis C viral antigen has some efficacy in protecting chimpanzees from infection with a homologous viral strain [5]. Despite these findings, studies of humans naturally infected with hepatitis C virus show that neutralizing antibodies are produced during hepatitis C infection regardless of whether viral resolution is achieved. Persistent infection in the presence of circulating antibodies
0009-9120/01/$ – see front matter © 2001 The Canadian Society of Clinical Chemists. All rights reserved. PII: S 0 0 0 9 - 9 1 2 0 ( 0 1 ) 0 0 2 1 0 - 7
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is associated with mutant epitopes of the hepatitis C virus which are able to escape recognition, thereby limiting the efficiency of the antibody-mediated response. The high heterogeneity of the hypervariable (HVR1) envelope region of the hepatitis C virus and the detection of quasispecies in chronic infection could contribute to an understanding of why neutralizing antibodies are associated with unresolved viremia [6]. In contrast to the humoral response, which is incapable of eliminating a virus from an infected cell, cell-mediated defenses are called into action after the virus has gained entry into the host cell. The cell-mediated immune response results from the activity of CD4⫹ helper T and CD8⫹ cytotoxic T lymphocytes (CTLs), which recognize peptides that have been synthesized in the cytosolic compartment of the host cell through the processing and degradation of viral proteins. These endogenously synthesized peptides are transported across the membrane of the host cell where they are presented to T cell receptors through class I or II HLA molecules. CD8⫹ and CD4⫹ cells interact with T cell receptors form a major histocompatibility complex (MHC). Formation of the MHC leads to activation of T cells. Activated CD8⫹ cells may directly lyse virus-infected host cells through the release of cytotoxic enzymes. CD4⫹ induce Kuppfer cells to activate natural killer cells, leading to nonspecific lysis of infected cells. They also produce cytokines, regulate B lymphocytes and enhance the activity of CD8⫹ T cells [7]. The role of cell-mediated immunity in hepatitis C infection has not yet been defined, but studies suggest that helper T and CTL responses differ among patients who recover compared to those who develop chronic infection. Several studies have shown a strong CD4⫹ response in patients with long-lasting chronic hepatitis C [8,9]. Consistently, several studies have demonstrated that a strong and maintained CD4⫹ response is correlated with viral clearance in acute hepatitis C viral infection [10,11]. There is evidence that CTL activity, while less vigorous in chronically infected patients, may be inversely related to viral load [12]. These findings suggest that CTLs function primarily in controlling viral infection in the early or acute stages of the disease.
3. The pathogenic role of helper and cytotoxic T cells The hepatitis C virus is not directly cytopathic, but its adverse hepatitic effects are thought to be mediated through the overproduction of initially protective immunologic responses. Liver-infiltrating lymphocytes from hepatitis Cinfected individuals not only lyse infected liver cells, they can start a cascade of events resulting in hepatocellular damage. Hepatitis C virus-specific CD4⫹ activity is correlated with histologic fibrosis and portal tract inflammation, and, in chronic hepatitis C-infected patients with intrahepatic CTL activity, lower levels of viremia are associated
with more active hepatocellular damage [13,14]. Relatively high levels of hepatitis C virus-specific helper and cytotoxic T cells have been detected in inflamed liver infiltrates from chronic hepatitis C-infected patients [15,16]. The T cells present in the liver are more numerous and can be functionally different from HCV-specific T cells present in peripheral blood, suggesting that some T lymphocytes may be sequestered at the site of infection [17,18]. CTL activity is correlated with reduced viremia, implying that CTLs assist in controlling hepatitis C infection. It is postulated that in the absence of complete virologic recovery, CTLs may exert harmful effects on noninfected bystander hepatocytes, leading to inflammation, necrosis, and fulminant hepatitis.
4. Immunomodulatory properties of cytokines CD4⫹ helper T cells produce small regulatory proteins called cytokines which are important in controlling viral replication in hepatitis C infection. T-helper type 1 (Th1) cytokines are required for the generation of cytotoxic T cells and the activation of natural killer cells, while T-helper type 2 (Th2) cytokines generally inhibit the development of Th1 activity [19]. In hepatitis C, certain cytokines contribute to the host defense against infection by coordinating effector mechanisms that interfere with the hepatitis C virus life cycle. Th1 cytokines interleukin-2 (IL-2), tumor necrosis factor-a´ (TNF-a´), and interferon-a˜ (IFN-a˜) produce a selflimited, acute response to hepatitis C which plays a central role in the host defense against intrahepatic pathogens like the hepatitis C virus. If Th1 cytokines are not kept under control, however, they have the capacity to cause liver injury by producing inflammatory activity and contributing to necrotic cell death [20]. Th2 cytokines inhibit Th1 activity following acute viral infection and, where hepatitis C infection persists, they may have a role in protecting the host from the potentially damaging consequences of Th1 cells.
5. T-helper cell response to infection During acute hepatitis C infection, the majority of liverinfiltrating cells are pathogen-destroying cells with Th1 activity [17,18]. Th1 cells release TNF-a´, IFN-a˜, and IL-2, cytokines which can activate a wide variety of cellular genes leading to antiproliferative and antiviral effects [7]. One well-characterized antiviral mechanism in hepatitis C is the activation by IFN-␣ of protein kinase (PKR) [21]. Once activated by type-1 IFNs, PKR can block protein synthesis through the eIF-2 factor, thereby resulting in inhibition of viral replication. IL-2 has been shown to have antiviral effects in hepatitis B infection, although the possible antiviral action of IL-2 in hepatitis C infection has yet to be established [22]. TNF-␣ is released from hepatocytes and macrophages in response to inflammatory stimulus and is an
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important first-line defense against hepatitis C infection. TNF-␣ exhibits direct antiviral activity against the hepatitis C virus while enhancing proliferation of lymphocytes and stimulating a targeted CTL response to combat the virus within the liver [23,24].
6. Cytokines and hepatocellular damage It has been well-documented that chronic hepatitis Cinfected patients who show resolution of viremia have a strong Th1 response, but demonstrate mild Th2 activity [25,26,27]. Individuals who have a more vigorous Th1 response to the hepatitis C viral antigen seem to be able to clear the virus better and are able to recover from acute infection. Unfortunately, hepatitis C virus persists in most infected patients despite a strong Th1 response because the high mutation rate of hepatitis C makes it possible for the virus to escape immune surveillance [28]. Hepatitis C viral strains develop resistance to the effects of cytokines produced by hepatitis C virus-specific CTLs, thus continuing to induce T-cell activity. The viral strains themselves are not cytopathic, but, in the presence of persistent infection, Th1 cells continue to release TNF-␣, IFN-␥, and IL-2, causing self-inflicted inflammation and necrosis [7]. The more the balance shifts toward the Th1 response, the more aggressive the host viral defense, but the more potential exists for the tissue-damaging effects of the Th1 cytokines. Th1 cytokines may be seen as culprits in cases where the hepatitis C virus has managed to create conditions that result in viral persistence. There are several possible ways in which Th1 cytokines may exert their harmful effects, but at present the mechanisms are undefined. TNF-␣ production, for instance, may be involved in macrophage-independent necrosis and is thought to be one means by which CTLs amplify the damage to nearby, noninfected cells [29,30]. TNF-␣ stimulates macrophages to produce transforming growth factor- (TGF-), a cytokine which activates stellate cells to increase production of extracellular matrix proteins, leading to hepatic fibrosis [2]. IFN-a˜ has been studied in animal models in hepatitis B where it has been shown to recruit inflammatory cells, cause apoptosis of hepatocytes and produce Kupffer cell hyperplasia [31]. IFN-a˜ has also been implicated in hepatitis C as contributing to self-inflicted tissue damage by activating intrahepatic macrophages [32]. Th2 cytokines such as IL-4 and IL-10 modulate selfinflicted injury by suppressing the Th1 response and counteracting the fibrogenic effects of TNF-␣, IFN-␥ and IL-2 [33]. IL-10 has prominent antifibrotic activity in that it downregulates collagen 1 expression while upregulating interstitial collagenase [34]. IL-10 can decrease expression of proinflammatory cytokines such as TNF-␣, IFN-␥, and IL-2 from T cells and it can upregulate the expression of the cytokine inhibitor TNF-R [32]. Patients with milder forms of hepatitis C have a higher expression of IL-10 compared
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to those with overt liver disease, and progressive liver injury in chronic hepatitis C is significantly correlated with the downregulation of IL-10 [35,36]. In the presence of persistent hepatitis C infection, T cell activity is continually stimulated and may be responsible for the development of liver lesions. Experimentally, CD8⫹ CTLs have been detected in the peripheral blood and the intrahepatic lymphocyte infiltrate in patients with chronic hepatitis C, suggesting that an active immune response continues to be triggered even in the absence of acute viral infection [37]. Persistent T cell response may be made possible through the initial production of cytokines by non-T cells. These cytokines, notably IL-12 and IL-15, may drive undifferentiated Th0 progenitor T cells to become CD4⫹ lymphocytes or may trigger cytolytic activity. IL-12, a proinflammatory cytokine that is secreted by macrophages and NK cells, has been shown to stimulate the proliferation of Th1 cells [38]. IL-15, a cytokine which is produced by a variety of cell types including monocytes and macrophages, can bind to NK cells, activating them to produce TNF-␣ and IFN-␥ [39]. NK cells are primitive cells that lead to nonspecific lysis of infected cells, and their impairment has been implicated in the development of chronic hepatitis C viral infection [40].
7. Cytokine measurement Cytokine levels are difficult to measure for a number of reasons. Profiles in liver compartments may vary depending on the stage of hepatitis C of a particular patient, with high amounts of antigen biasing toward a larger Th1 response [41]. Measurement of cytokines at different sites–for example, in peripheral blood cells vs. in the liver–may lead to different observations about the predominant T-cell response. Further, cytokines do not always fit neatly into the Th1 and Th2 subsets. Cytokines are produced by multiple cell types, including macrophages, CD4⫹ and CD8⫹ T cells, and a single helper-T cell clone may produce both Th1 and Th2 cytokines [42,17]. Cytokines are like neurotransmitters in that they act locally and possess very short halflives. Measurement of mRNA and protein levels may be discordant due to the fact that many strains of hepatitis C virus encode cytokine analogues, cytokine receptor genes, or proteins which bind cytokines [43].
8. Conclusion Hepatitis C is usually diagnosed when the infection is already chronic and the virus seems to have developed successful strategies for escaping immune surveillance. The pathogenesis of viral persistence is believed to involve a primarily cell-mediated immune mechanism, during which Th1/Th2 cells are unable to ward off viral invasion. Compromised host immune factors and the high
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variability of the hepatitis C virus may contribute to the virus’s capacity to elude host defenses and persist in an apparently unfavourable environment. The continual response to the virus by cytokines within the liver induces CTL proliferation and the release of necroinflammatory factors which cause irreversible hepatic tissue damage. It is a developing hypothesis that the T cell immune response is somehow compromised in patients who are chronically infected with hepatitis C, but studies are needed to define precisely how viral escape mechanisms and cytokine activity interact to produce the pathogenesis of chronic hepatitis C viral infection.
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