A milestone for hepatitis C virus research: a virus generated in cell culture is fully viable in vivo.

C hronic hepatitis C virus (HCV) infection is a leading cause of end-stage liver disease in the United States, and the Centers for Disease Control has predicted that 10,000 Americans die annually from this disease. Since the discovery of HCV in 1989, basic research on this important pathogen has been difficult because of the lack of a robust cell-culture system. Thus, the recent development of a cell culture resulting in production of significant levels of infectious virus particles (1–3) will without doubt lead to rapid progress in our understanding of the HCV viral life cycle and, it is hoped, development of better therapies. The development of molecular clones of strain H77 (genotype 1a) infectious for chimpanzees in 1997 was an important initial advance, but these clones and subsequent infectious clones of other genotype strains did not produce viruses in cell culture (4–7). Another critical advance was the development of subgenomic replicons in 1999 that permitted studies of RNA replication (8). Efficient replication, however, depended on adaptive mutations (9, 10). In 2003, Kato et al. (11) observed that the WT JFH1 strain of HCV, belonging to genotype 2a, replicated to high levels in the replicon system. Subsequently, in 2005, Wakita and coworkers (2) demonstrated that RNA transcripts from the full-length JFH1 genome could produce viruses in a human liver hepatoma cell line (Huh-7 cells), and it was demonstrated by Chisari and coworkers (3) that this system could be developed to generate relatively high titers of HCV. In a different approach, Rice and coworkers (1) developed a chimeric genome in which the structural genes (C, E1, and E2), p7, and NS2 from an infectious clone of another genotype 2a strain (strain HC-J6) (6) were inserted into the subgenomic replicon of the JFH1 strain and demonstrated that RNA transcripts from this full-length chimeric genome could produce relatively high titers of HCV. In the current issue of PNAS, a study by Lindenbach et al. (12) demonstrated that chimeric J6 JFH1 viruses generated in vitro were fully viable also in vivo, as tested in chimpanzees and in SCID-uPA mice with human liver grafts, and that virus recovered from infected animals efficiently infected naı̈ve Huh-7 cells. An overview of this important study is shown in Fig. 1. Therefore, it appears that not only has a cell-culture system been developed for HCV, but this system produces viruses that are biologically relevant. The chimpanzee model is the only animal model available for HCV that permits studies of infectivity and pathogenesis (reviewed in ref. 13). The course of infection observed with the J6 JFH1 viruses derived from cell culture was similar to experimental acute infections with other HCV strains, with early appearance of viremia, peak viral titers of 104 to 105 units ml, seroconversion, and induction of intrahepatic innate and adaptive immune responses. Neither of the two chimpanzees developed clear evidence of acute viral