Pathogenesis of chronic hepatitis C: immunological features of hepatic injury and viral persistence

Antibodies against hepatitis C virus-like particles and viral clearance in acute and chronic hepatitis C

We recently described the efficient assembly of hepatitis C virus (HCV) structural proteins into HCV-like particles (HCV-LPs) in insect cells. These noninfectious HCV-LPs have similar morphologic and biophysical properties as putative virions isolated from HCV-infected humans and can induce a broadly directed immune response in animal models. The HCV envelope proteins of HCV-LPs are presumably presented in a native, virion-like conformation and may therefore interact with antienvelope antibodies directed against conformational epitopes. In this study, HCV-LPs were used as capture antigens in an enzyme-linked immunosorbent assay (ELISA) to detect and quantify antibodies against HCV structural proteins in patients with acute and chronic hepatitis C. High titers of anti-HCV-LP antibodies were detected in patients chronically infected with HCV genotypes 1 to 6. In contrast to individuals with chronic hepatitis C, patients with acute self-limited hepatitis C displayed only a transient and weak seroreactivity against HCV-LPs. Patients with chronic HCV infection successfully treated with interferon demonstrated a gradual decline of anti-HCV-LP titers during or subsequent to viral clearance. Sustained interferon responders were characterized by significantly higher pretreatment levels of anti-HCV-LP antibodies as compared with nonresponders (P =.0001). In conclusion, HCV infection is associated with limited humoral immunity against the envelope proteins present on the HCV-LPs. An HCV-LP-based ELISA may be a useful diagnostic tool to distinguish acute hepatitis C from chronic HCV infection with exacerbation, and to predict viral clearance in response to interferon.

Protection or damage: a dual role for the virus-specific cytotoxic T lymphocyte response in hepatitis B and C infection?

During infection with hepatitis B or C viruses, cytotoxic T lymphocytes (CTLs) have been implicated as both the mediators of protection and the principal effectors of liver pathology. Recent studies have allowed an investigation of the relationship between virus-specific CTL responses, liver damage and viral replication. In the presence of an efficient virus-specific CTL response, a scenario is emerging where inhibition of viral replication can be independent of liver pathology. We discuss the possibility that an inadequate CTL response--unable to control viral replication--may contribute to liver pathology not only directly but also via the recruitment of non-virus-specific T cells.

Modulation of host immune responses by clinically relevant human DNA and RNA viruses

Numerous mechanisms allow viruses to evade host immune surveillance, and new evasion strategies continue to be identified. In addition to interference with antigen processing and presentation, direct viral modulation of host immune responses can also be achieved by altering the host cytokine milieu and the development of immunoregulatory cells. A better understanding of these viral evasion strategies will help to define critical host defense mechanisms and will lead to novel immune-based therapeutic strategies in the future.

Protection or damage: a dual role for the virus-specific cytotoxic T lymphocyte response in hepatitis B and C infection?

During infection with hepatitis B or C viruses, cytotoxic T lymphocytes (CTLs) have been implicated as both the mediators of protection and the principal effectors of liver pathology. Recent studies have allowed an investigation of the relationship between virus-specific CTL responses, liver damage and viral replication. In the presence of an efficient virus-specific CTL response, a scenario is emerging where inhibition of viral replication can be independent of liver pathology. We discuss the possibility that an inadequate CTL response--unable to control viral replication - may contribute to liver pathology not only directly but also via the recruitment of non-virus-specific T cells.

Activation of intracellular signaling by hepatitis B and C viruses: C-viral core is the most potent signal inducer

To clarify the effects of hepatitis C virus (HCV) infection on hepatocytes, we analyzed and compared the induction of intracellular signals by HCV and hepatitis B virus (HBV) proteins. We examined the influence of 7 HCV (core, NS2, NS3, NS4A, NS4B, NS5A, and NS5B) and 4 HBV (precore, core, polymerase, and X) proteins on 5 well-defined intracellular signaling pathways associated with cell proliferation, differentiation, and apoptosis by use of a reporter assay. Viral protein-expression vectors were cotransfected into mammalian cells with reporter vectors having a luciferase gene driven by the following inducible cis-enhancer elements: the cyclic adenosine monophosphate response element, the serum response element (SRE), and the binding sites for nuclear factor kappaB (NF-kappaB), activator protein 1 (AP-1), and serum response factor (SRF). In addition, the activation of signals by HCV proteins was examined in a reporter plasmid having a natural interleukin-8 (IL-8) promoter upstream of a luciferase gene. Of 11 HCV and HBV proteins, HCV core had the strongest influence on intracellular signals, especially NF-kappaB-, AP-1-, and SRE-associated pathways. HCV core's activation level exceeded that of HBV X protein, a well-characterized transactivator of these signals. Moreover, HCV core activated the IL-8 promoter through NF-kappaB and AP-1. For the other proteins, HCV NS4B showed signal activation, but signals were activated at a lesser extent. The luciferase reporter assay, a recently introduced technique, helped in the elucidation of molecular events underlying the inflammatory and proliferation process in the liver induced by HCV.

Combination treatment for chronic hepatitis C: what is the role of ribavirin?

Ribavirin in combination with interferon-alpha2b is the new standard for chronic hepatitis C (CHC) treatment. Although usually considered as an antiviral compound, this guanosine analogue shows some additional effects on the immune system that could largely contribute to its clinical efficacy in CHC. Numerous in vitro experiments demonstrate that ribavirin has a selective down-regulatory effect on TH2 cytokine release with, in some cases, a concomitant TH1 cytokine up-regulation. In vivo, combination treatment of CHC patients was shown to induce a predominant TH1 response in isolated PBMCs, but also a reduction of peripheral TH2 response. Considering that: 1) a strong CD(4)+ helper T-cell response is associated with viral clearance in acutely infected patients; 2) a weak T-cell response to the viral antigens is common in chronic infected patients; 3) responding patients to combination treatment (but not non-responding patients) altered their cytokine profile under treatment, either to express IFN-gamma or to reduce pro-inflammatory mediators; it is highly presumed that ribavirine participates to restore an efficient T-cell response and to reduce the non-specific inflammatory cytolytic activity during CHC combination treatment.

Virus, liver and autoimmunity

The immune system's ability to distinguish self from nonself is essential for both host defence against microbial antigens and protection of self-antigens from autoimmune destruction. Such discrimination is complicated by extensive structural homology shared between micro-organisms and self-antigens, a condition known as molecular mimicry. Molecular mimicry provides the foundation for an immune response directed against an exogenous agent such as a virus to cross-react with mimicked host self sequences, leading to autoimmunity, and in some cases, tissue injury and autoimmune disease. In this review we analyse studies investigating the role of molecular mimicry and cross-reactive immunity in liver-related autoimmunity.

A virological perspective on the need for vaccination

Superinfecton of chronic carriers of hepatitis B virus (HBV) or hepatitis C virus (HCV) with hepatitis A virus (HAV) is often associated with more severe liver disease than infection with HAV alone. Superinfection commonly causes markers of HBV and HCV replication to fall to significantly lower levels. The pathogenesis of acute liver damage characteristic of viral hepatitis is thought to be mediated by host cytotoxic T-lymphocytes (CTLs) directed against virus-infected hepatocytes. It has been proposed that the more aggressive liver disease observed in individuals infected with HAV in addition to chronic HBV/HCV is a result of the induction of interferon (IFN)-alpha during acute HAV infection. This accounts for the antiviral effect on the active markers of HBV/HCV replication, and the enhanced CTL response against HBV/HCV-infected hepatocytes. Alternatively, HAV may indirectly stimulate the T helper 1 (Th1)-type cytokine responses, such as interleukin (IL)-2, IFN-gamma and tumour necrosis factor (TNF)-alpha, which directly promote the antiviral CTL response. Clearance of HBV infection, and possible HAV and HCV, is associated with a specific CTL response, while viral persistence in chronic HBV and HCV infection has been attributed to an imbalance in the Th1-Th2 arms of the immune response. Vaccination against hepatitis A should be considered for patients with chronic HBV/HCV infection, to minimize the risk of exacerbating underlying liver disease.

Intracellular signalling and antiviral effects of interferons

More than 4 decades after their discovery, interferons are used now in daily clinical practice for the treatment of chronic viral hepatitis, multiple sclerosis, chronic granulomatous disease, and malignant disease such as hairy cell leukaemia, chronic myeloid leukaemia, Kaposi's sarcoma, multiple myeloma and malignant melanoma. In general, treatment with interferons is successful in only a fraction of the patients suffering from these diseases. The reasons for treatment failures in many patients are not understood a present. The discovery of the Jak-Stat pathway as the principal signalling pathway for interferons opens new research options for a better understanding of interferon resistance in various diseases. Defective Jak-Stat signal transduction has now been described in cells expressing HBV proteins, in cells expressing HCV proteins, and in cell lines derived from malignant melanomas. A better understanding of these signalling defects might lead to new therapeutic strategies making interferons more effective in a larger percentage of patients.