Pathogenesis, Natural History, Treatment, and Prevention of Hepatitis C

Dr. T. Jake Liang (Liver Diseases Section, National Institute of Diabetes and Digestive and Kidney Diseases [NIDDK], National Institutes of Health [NIH], Bethesda, Maryland): The identification of hepatitis C virus (HCV) as the cause of non-A, non-B hepatitis represents a technical tour de force of modern molecular medicine (1). Characterization of the viral genome and the structures and functions of viral gene products has led to a better understanding of the viral life cycle and the pathogenesis of HCV-associated disease. This knowledge will contribute to the development of an effective vaccine and better therapies. Hepatitis C virus is a member of the Flaviviridae family, which includes the flaviviruses and pestiviruses (2). There are at least 6 HCV genotypes and more than 50 subtypes. The virion contains a positive single-stranded RNA genome of 9.5 kilobases (Figure 1). The genome consists of 5 and 3 untranslated regions (5UTR and 3XR) that have little sequence variation among all genotypes and are important for translation of viral proteins and replication of the virus. The viral genome encodes a large single polyprotein of about 3000 amino acids; the N-terminal one third harbors the structural proteins, and the C-terminal two thirds contains the nonstructural proteins. The HCV structural proteins comprise the core protein and the two envelope glycoproteins E1 and E2. The nonstructural proteins, including proteases (NS2/3 and NS3), helicase (an enzyme that unwinds double-stranded nucleic acid) (NS3), and RNA-dependent RNA polymerase (NS5B), perform various functions essential for the viral life cycle (Table 1). Cleavage of structural proteins from the polyprotein is catalyzed by a host signal peptidase, whereas polyprotein cleavage in the nonstructural region requires HCV-encoded proteases. Figure 1. Schematic diagram of the hepatitis C virus genome. 5UTR 3XR Table 1. Functions of Genetic Elements of Hepatitis C Virus Hepatitis C virus enters a susceptible host either directly, through needle inoculation or transfusion of contaminated blood products, or inadvertently, through breakage of a percutaneous barrier (as exemplified by sexual or perinatal transmission) (3). The virus then enters hepatocytes or other susceptible cells, probably through a unique surface molecule or molecules, as the viral receptor (4). After uptake, the virus uncoats and releases the genome to begin replication. The viral genome first serves as the template for translation of the polyprotein. The processed nonstructural proteins then form a complex with the genome and initiate synthesis of the negative strand, which in turn functions as the template for positive strand synthesis. The replication complex probably resides in a membranous compartment in the cytoplasm, presumably derived from the endoplasmic reticulum. The RNA replicative intermediate matures and interacts with the core and envelope proteins to assemble into a virion. Although most of the replicative processes have not been defined, some nonstructural proteins clearly play critical roles in viral replication and productive infection; these proteins are therefore the focus of antiviral development. Immunopathogenesis of Hepatitis C Dr. Barbara Rehermann (Liver Diseases Section, NIDDK, NIH): Unlike other hepatitis viruses, the hepatitis C virus is more likely to cause clinically inapparent, chronic infection in persons who are otherwise considered immunocompetent. Thus, the virus is capable of circumventing an efficient immune response of the host. Components of Antiviral Immune Response The mechanisms whereby HCV circumvents immune response and establishes persistent infection are currently undefined. It is well known that the specific immune response to any viral infection is primed by macrophages and dendritic cells that present viral proteins to B cells, helper T cells, and cytotoxic T cells (Figure 2). In many viral infections, B cells produce antibodies that can clear circulating virus and protect from reinfection. For example, antibodies against the hepatitis B virus surface antigen are critical for viral clearance. Through specific T-cell receptors on the cell surface, helper T cells recognize viral peptides that are derived from phagocytosed and proteolytically cleaved HCV proteins and are presented in the context of class II MHC molecules. Figure 2. Components of the antiviral immune response. HCV On activation of their specific T-cell receptors, HCV-specific helper T cells assist with activation and differentiation of B cells as well as induction (5) and stimulation of virus-specific cytotoxic T cells. Most of these effects are mediated by different sets of immunoregulatory Th1 (interferon- and interleukin-2) or Th2 (interleukin-4, interleukin-5, and interleukin-10) cytokines. In the context of class I MHC molecules, CD8-positive cytotoxic T cells recognize HCV peptides that are synthesized and processed in infected cells (Figure 2). This encounter can lead to lysis of virus-infected cells. Together with helper T cells, cytotoxic T lymphocytes may also secrete cytokines, such as interferon- and tumor necrosis factor-, that inhibit replication and gene expression of several viruses, such as hepatitis B virus, cytomegalovirus, and rotavirus (6-8). Humoral Immune Response Hepatitis C virus can establish persistent infection despite an active humoral and cellular immune response that is generally targeted against all viral proteins. The virus may escape from the humoral immune response if the kinetics of infection and viral replication do not allow complete neutralization of the virus by HCV-specific antibodies after primary infection. Although virus-specific antibodies may interfere with viral entry into host cells and opsonize the virus for elimination by macrophages, they cannot eliminate HCV from infected cells. In addition, HCV has a high mutation rate, especially in the hypervariable region of the envelope proteins that can be recognized by neutralizing antibodies (antibodies that can bind and eliminate virus) (9, 10). Several studies have demonstrated that the humoral immune response can select HCV variants with sequence changes that allow escape from antibody recognition (11-14). However, recent studies in chimpanzees have suggested that HCV can cause persistent infection in the absence of mutations in the hypervariable region (15, 16). Thus, progression to persistent HCV infection is most likely a multifactorial process that depends on multiple aspects of virus-host interaction. Cellular Immune Response The cellular immune response probably plays an important role in the outcome of HCV infection because of its ability to recognize and eliminate virus from infected cells. Most studies have concentrated on the antigen-specific immune response that is mediated by CD4-positive helper T cells and CD8-positive cytotoxic T cells. Because chronic rather than acute infection is diagnosed in most patients, immunologic studies have been performed on patients with persistent infection who could not clear HCV. Only a few studies have analyzed the cellular immune response during the acute phase of infection. These studies suggest that the strength and quality of both helper T-cell (17, 18) and cytotoxic T-cell responses (19) differ between patients who recover and those who develop chronic infection. More important, the viral sequences that are recognized most frequently and vigorously by HCV-specific T cells vary little among all the HCV genotypes. Furthermore, several of these frequently recognized viral peptides bind with high affinity to many different class II MHC molecules, suggesting that they can be efficiently presented and recognized by patients with different MHC haplotypes (17, 20). Thus, these viral sequences could be explored for development of preventive or therapeutic vaccines against HCV. The cellular response against HCV could be interfered with in several ways. First, HCV elicits only a weak T-cell response in patients who develop chronic infection (17, 21). In the blood of patients with chronic hepatitis C, the frequency of cytotoxic T-cell precursors that are specific for individual HCV peptides is much lower than the frequency of T cells that recognize an influenza virus peptide as a recall antigen (22) or peptides of other viruses that can be cleared, such as cytomegalovirus (23). The reasons for this relative weakness of the cellular immune response are not known. Certainly, general immune tolerance or immunosuppression is not the cause of persistent HCV infection, because most chronically infected patients display normal immune responses against other viral agents (22). The emergence of viral mutants or quasi-species with sequence variations in T-cell epitopes may contribute to the apparent ineffectiveness of cell-mediated immune response (24-26). There is also increasing evidence that several HCV proteins, such as core (27), E2 (28), and NS5A (29), interfere with the immune response. Furthermore, infected hepatocytes, which lack co-stimulatory molecules, may be relatively inefficient in priming the immune system, and the liver has been proposed as the major site where activated T cells are destroyed (30). Finally, the cellular immune response is a double-edged sword. An immune response that is ineffective in clearing HCV infection may be more harmful to the liver, causing chronic inflammation, hepatocellular injury, and, over several decades, liver fibrosis and cirrhosis. Progression to persistent infection and the immunologic mechanisms of liver injury are the consequence of complicated interactions between the virus and host. Identification of immunologic correlates of viral clearance may contribute to the development of an effective vaccine and better therapy for HCV infection. Natural History of Hepatitis C Dr. Leonard B. Seeff (Division of Digestive Diseases and Nutrition, NIDDK, NIH): The natural history of hepatitis C continues to be