Cytokines and liver injury in chronic HCV genotype 4 infection
Amany Zekry, St George Hospital Clinical School, Belgrave St, Kogarah, Sydney, NSW 2217, Australia. Email: firstname.lastname@example.org
Approximately 120 million people are chronically infected with the hepatitis C virus (HCV) worldwide. Further, chronic hepatitis C (CHC), the liver disease resulting from chronic HCV infection, is, after chronic hepatitis B, the second most common cause of chronic liver disease, cirrhosis and hepatocellular carcinoma (HCC). Egypt, by far, has the highest prevalence of HCV infection in the world, with estimates ranging from 20 to 30%. In parallel, the burden of chronic HCV-related liver disease in Egypt continues to rise.1
The HCV epidemic in Egypt has been largely attributed to mass parenteral antischistosomal treatment programs and other iatrogenic exposures. The HCV genotype 4 is the most prevalent genotype in Egypt. In this issue of the Journal, Zekri et al. report the relative expression of several cytokines and apoptotic genes in 74 Egyptian subjects infected with HCV genotype 4.2 The authors found a strong association between the expression of certain pro-inflammatory cytokines and apoptotic genes with the progression of liver injury and the development of HCC.2 Although Zekri et al. only studied a relatively small group of subjects, the results highlight accumulating data that both cytokine responses and apoptosis are important steps in the pathogenesis of liver injury in chronic HCV infection. This editorial therefore focuses on a discussion of cytokine responses to HCV, with particular relevance to genotype 4 infections.
Cytokines and liver injury in chronic HCV infection
Various components of the host immune system are involved in the pathogenesis and outcome of HCV infection. HCV-specific CD4+ and CD8+ T cell responses are detectable in the liver of chronically-infected patients.3 These specific T cell responses are enhanced in the liver compared to the peripheral blood and have a predominant T helper (Th)-1 cytokine profile. Thus, despite the failure to achieve viral elimination in chronic HCV infection during the acute phase of the illness, HCV-specific T cells mediate ongoing viral clearance and liver injury by either directly killing infected hepatocytes or releasing pro-inflammatory cytokines, such as interferon (IFN)-γ and tumour necrosis factor (TNF)-α. This process triggers further significant bystander killing via different apoptotic pathways and through the recruitment of non-specific inflammatory cells. Thus, increased mRNA levels of IFN-γ and TNF-α are found in the livers of chronic HCV-infected patients.4 Importantly, the expression of these pro-inflammatory cytokines correlates with both the extent of hepatic inflammation and the development of fibrosis.4 Further, through the combined techniques of immunohistochemical staining and in situ hybridization, IFN-γ has been localized to CD3+ lymphocytes infiltrating hepatic portal tracts and lobules; this confirms the key role of IFN-γ in the chronic HCV inflammatory response. Therefore, the development of chronic liver disease in HCV is associated with the failure to clear the virus largely due to inadequate Th-1 immunity and weak HCV-specific T cell inflammatory responses.
Chemokines, can also promote hepatic inflammation in chronic HCV infection through the recruitment of lymphocytes to the liver parenchyma. Furthermore, chemokines may also be involved in liver regeneration, fibrosis and malignant transformation, which are unwanted biological responses induced by the persistence of inflammation. Accumulating data indicate that distinct chemokines and chemokine receptors may be associated with different stages of liver disease in Chronic Hepatitis C (CHC). In particular, CXCR3-associated chemokines are emerging as important in the development of hepatic necroinflammation and fibrosis in chronic HCV infection. Several investigators have found intrahepatic CXCL10/IP-10 mRNA expression is associated with increased lobular necroinflammation.5,6 Similarly, intrahepatic mRNA levels of IFN-inducible T cell alpha chemoattractant (I-TAC), another CXCR3 chemokine ligand, correlate with both portal and lobular inflammation.7
Cytokines and liver injury in chronic HCV genotype 4
There is little evidence that HCV genotype per se can invoke cytokine alterations or influence the severity of liver disease. In contrast, host factors such as male gender, duration of infection, and immunosuppression can increase the risk of liver disease progression. Of importance in relation to HCV genotype 4, both host metabolic factors, including insulin resistance, obesity and steatosis, as well as co-infection with Shistosoma mansoni play key roles in the progression of liver injury in chronic HCV infection (Table 1).
Table 1. Pathogenic responses relevant to liver injury in chronic hepatitis C genotype 4 infection
|Insulin resistance||Enhance NF-kappa-β mediated transcription of pro-inflammatory cytokines|
|Stimulate hepatic stellate cell proliferation|
|Increased expression of connective tissue growth factor|
|Steatosis||Exaggerated Th-1 immune response with increased expression of IFN-γ and TNF-α|
|Increased expression of the cheomokines CXCL10/IP10 and CCL2/MCP-1|
|Increased hepatic lymphocyte recruitment|
|HCV/S. mansoni co-infection||Shift of T-cell response to a Th-2 profile Limited HCV-specific cellular immune response|
Cytokine profiles in HCV and metabolic disorders
In CHC, insulin resistance, obesity and hepatic steatosis are major determinants of liver disease progression and response to antiviral therapy.8,9 Insulin resistance and type 2 diabetes have been observed to be increasingly prevalent in subjects infected with HCV genotype 4, independent of the presence of obesity or liver fibrosis.10,11 This observation is relevant since insulin potentiates pro-infammatory and pro-fibrotic responses within the liver. Thus, in vitro treatment of hepatocyte cell lines with insulin enhances nuclear factor-kappa-B (NF-κB)-mediated transcription of pro-inflammatory cytokines such as TNF-α.12 This effect is further potentiated in the presence of other pro-inflammatory cytokines including interleukin (IL)-6 and IL-1β. Hence, hepatic inflammation produces insulin resistance with the resultant hyerinsulinaemia promoting the release of further pro-infammatory cytokines. Further, glycemia and hyperinsulinemia directly stimulate hepatic stellate cell proliferation and increase expression of connective tissue growth factor, a key regulator of fibrosis progression.13
Obesity and steatosis have also been shown to potentiate liver injury. Animal studies indicate that steatosis can exacerbate T-cell-mediated hepatic injury, partly by polarizing T-cells towards a Th-1 cytokine response, with excess production of IFN-γ and IL-2. In addition, steatosis is associated with enhanced lymphocyte responsiveness to hepatic chemokines, resulting in increased lymphocyte homing to the liver in response to inflammatory insults.14 In support of these findings, obesity and steatosis in chronic HCV is associated with increased hepatic mRNA expression of IFN-γ and TNF-α, which correlate with the extent of hepatic inflammation.15,16 Further, the expression of both CXCL10/IP-10 and CCL-2/MCP-1 is significantly enhanced in the liver of obese HCV-infected subjects with steatosis.16 This chemokine profile in obese HCV subjects with steatosis was associated with both increased hepatic inflammation and enhanced T cell infiltration.16
Cytokine profiles in HCV and S mansoni co-infection
Concomitant infection with HCV and S. mansoni is common in Egypt. Co-infected patients have higher HCV RNA titers, increased necroinfammatory scores, a higher incidence of cirrhosis and the development of HCC.17 Not surprisingly, the mortality rate of the co-infected group is higher than those infected with HCV alone. There are several lines of evidence that S. mansoni perturbs the immune response in chronic HCV infection. A study evaluating the intrahepatic kinetics of HCV-specific CD4+ T cell responses in paired liver biopsies 8 years apart, reported the frequency, magnitude and breadth of the HCV specific CD4+ T cell responses to be lower in co-infected subjects compared to those with HCV infection alone.17 Importantly, an inverse association was found between intrahepatic CD4+ T cell responses and the progression of fibrosis.17,18 Apart from influencing the HCV cellular immune response, S. mansoni has the ability to perturb the T helper cytokine milieu. In this setting, in the presence or absence of HCV co-infection, S. mansoni induces a shift of T helper responses towards a Th-2 cytokine profile.17,19 Thus, serum levels of the Th-2 cytokines, IL-4 and IL-10 are increased in co-infected subjects compared to those with HCV monoinfection. Further, patients with concomitant schistosomiasis demonstrated decreased Th-1-specific IFN-γ production by peripheral blood mononuclear cells following stimulation with HCV antigens.18 Collectively the data suggest that schistosomiasis co-infection may shift the HCV immune response to favor viral persistence with resultant increased hepatic damage.
Cytokine profiles in HCV related-HCC
Conflicting data exists concerning the cytokine profile in associated with the development of HCC in chronic HCV infection. Some investigators report that the development of HCC in the cirrhotic liver is associated with a predominant Th-2 cytokine profile with increased IL-10 expression, while others could not find such an association. Similarly, Th-1 cytokines were found by some, but not others, to be increased in the setting of HCC.20,21 HCV-genotype 4 infected subjects who developed HCC were found to have significantly higher hepatic expression of IL-2 receptor (R), TNF-RII, and TNF-RI, but lower TNF-α and IL-6 levels.2 It is noteworthy, however, that the majority of these patients were co-infected with S mansoni, which, as described earlier, could also have influenced the cytokine profile of co-infected patients with HCC.
A recent study has highlighted the relationship between the activation of genes involved in the IL-6 signaling pathway, including IL-6 and STAT-3 and the development of HCC.22 Thus, suppression of IL-6 signaling, through the generation of mouse knockouts, resulted in a reduction in HCC development.22
Immune-mediated inflammatory damage is a key process in the pathogenesis of chronic HCV-related liver injury, characterized by a Th-1 cytokine response. In HCV genotype 4 infection, metabolic host factors such as obesity, insulin resistance and steatosis may be associated with an exaggerated Th-1 response with increased pro-inflammatory and pro-fibrogenic cytokines. In contrast, co-infection with S. mansoni shifts the cytokine responses towards a Th-2 profile, favouring viral persistence and further liver damage. Importantly, with both of these HCV genotype 4 infection scenarios, there is increased liver injury. Articles by Zekri and colleagues in this issue of JGH are important in enhancing our understanding of the pathogenic process involved in liver disease progression in HCV genotype 4 infection.