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Abstract

The immune response to viral antigens is thought to be responsible for viral clearance and disease pathogenesis during hepatitis C virus (HCV) infection. In chronically infected patients, the T-cell response to the HCV is polyclonal and multispecific, although it is not as strong as the response in acutely infected patients who display a more vigorous T-cell response. Importantly, viral clearance in acutely infected patients is associated with a strong CD4 ؉ helper T-cell response. Thus, the dominant cause of viral persistence during HCV infection may be the development of a weak antiviral immune response to the viral antigens, with corresponding inability to eradicate infected cells. The most striking feature of hepatitis C is its tendency toward chronicity. The hepatitis C virus (HCV) contains a positive-stranded RNA genome of about 10 kilobases, which contains a single uninterrupted open reading frame that encodes a protein of 3010-3011 amino acids. 1,2 The virus is classified as a Flavivirus. HCV infects an estimated 400 million people, corresponding to more than 3% of the world population. 3 Numerous partial and complete nucleotide sequences have been derived from geographically widespread HCV isolates. The data indicate substantial sequence diversity within, and especially between, each geographic area. The European and American isolates appear to be more closely related to each other than to the Japanese isolates. Based on a polymerase chain reaction-based algorithm, a classification system has been proposed by Okamoto 4 and revised by Simmonds. 5 Different viral sequences (quasispecies) have been isolated from the same patient upon sequential sampling, and multiple isolates have been reported in a single sample. 6 The degree of similarity between isolates at the amino acid level varies from genome region to region. The nucleocapsid protein (core) has the highest sequence homology among all isolates sequenced to date. In contrast, the E1 and E2/NS1 regions present the highest degree of sequence variability among different isolates; in particular, the N terminal portion of E2/NS1 contains a hypervariable region of 30 amino acids, which shows extensive variation between virtually all known isolates. 7 The major extracellular loop of CD81, a cell-surface protein widely expressed on B and T lymphocytes and other cells including hepatocytes, has been shown to bind E2. 8 The recent success of isolating a cDNA clone capable of generating infectious RNA transcripts may be an important step toward the development of a tissue culture system. 9,10 The only animal model, the chimpanzee, mirrors several features of human HCV infection. Most importantly, the frequency of persistent infection is high in both species, and virus replication occurs despite evidence of cellular and humoral immune responses. 11,12 A key difference is that necroinflammatory lesions in chronically infected chimpanzees are almost always mild, whereas in humans, the disease spectrum is very wide, ranging from mild to severe hepatitis and end-sta...