HCV Virology

Abstract

HCV was discovered and its sequence cloned in 1989, the acme of the pioneering work of Michael Houghton’s group (Choo et al., Science. 244: 359–362, 1989). Since then, studies of the HCV replication cycle have long been hampered by the following facts: (a) HCV is a strictly human pathogen that causes a chronic infection; (b) the target cells of HCV replication are solely hepatocytes; (c) virions isolated from the serum of HCV-positive patients could not infect cell culture systems in a productive manner; and (d) viral titration (assay of viral proteins and/or genome) could not be related to infectivity. Chimpanzees are permissive to HCV, but their use comes at prohibitive costs, and features of the disease are not similar to human clinical signs. Three major breakthroughs allowed to overcome these restrictions: the first one came with the discovery and implementation of the HCV replicon system in cell culture based on a full-length consensus genome from a viral RNA isolated from an infected human liver. This genome was then used to construct subgenomic replicons, which, upon transfection into a human hepatoma cell line, replicated to high levels (Lohmann et al., Science. 285: 110–113, 1999). With this invaluable tool, scientists in the field of hepatitis C were able to address fundamental questions related to viral replication in vitro, and this also fueled intensive research on antiviral molecules. The second breakthrough came with the molecular design of viral particles able to mimic the first stages of the interaction of HCV with its target cells: virion attachment to the cell surface, recognition of specific receptor molecules, internalization in intracellular compartments, and viral fusion. This tool, so-called HCV pseudoparticles (HCVpp), was based on retroviral capsids harboring the glycoproteins E1 and E2 of HCV at the virion surface. HCVpp were subsequently used to study viral entry, fusion, and humoral immune responses and to dissect the molecular details of the steps by which HCV initiated infection (Bartosch et al., J Exp Med. 197: 633–642, 2003). The third breakthrough emerged from a clinical case report in Japan of a fulminant hepatitis caused by HCV. This unusual feature for HCV led to the isolation and discovery of a new HCV clone, displaying a full replication cycle in cell culture settings. This clone was called JFH-1 (for Japanese fulminant hepatitis) and produced infectious viral particles capable of several rounds of in vitro infection, called HCVcc (HCV grown in Cell Culture) (Wakita et al., Nat Med. 11: 791–796, 2005, Lindenbach et al., Science. 309: 623–626, 2005). These particles are used for structural and functional studies and were successfully applied to the discovery of antiviral therapies now licensed and capable of HCV eradication.