Because of the lack of a robust cell culture system, relatively little is known about the molecular details of the cell entry mechanism for hepatitis C virus (HCV). Recently, we described infectious HCV pseudo-particles (HCVpp) that were generated by incorporating unmodified HCV E1E2 glycoproteins into the membrane of retroviral core particles. These initial studies, performed with E1E2 glycoproteins of genotype 1, noted that HCVpp closely mimic the cell entry and neutralization properties of parental HCV. Because sequence variations in E1 and E2 may account for differences in tropism, replication properties, neutralization, and response to treatment in patients infected with different genotypes, we investigated the functional properties of HCV envelope glycoproteins from different genotypes/subtypes. Our studies indicate that hepatocytes were preferential targets of infection in vitro, although HCV replication in extrahepatic sites has been reported in vivo. Receptor competition assays using antibodies against the CD81 ectodomain as well as ectopic expression of CD81 in CD81-deficient HepG2 cells indicated that CD81 is used by all the different genotypes/subtypes analyzed to enter the cells. However, by silencing RNA (siRNA) interference assays, our results show that the level of Scavenger Receptor Class-B Type-I (SR-BI) needed for efficient infection varies between genotypes and subtypes. Finally, sera from chronic HCV carriers were found to exhibit broadly reactive activities that inhibited HCVpp cell entry, but failed to neutralize all the different genotypes. In conclusion, we characterize common steps in the cell entry pathways of the major HCV genotypes that should provide clues for the development of cell entry inhibitors and vaccines. (HEPATOLOGY 2005;41:265-274.) H epatitis C virus (HCV) is an RNA-enveloped virus belonging to the Flaviviridae family. HCV exhibits a high degree of genetic heterogeneity. The propensity for genetic change is associated primarily with the error-prone nature of its RNA-dependent RNA polymerase together with the high HCV replicative rate in vivo. 1,2 This results in infected individuals harboring a diverse population of viral variants known as a quasispecies, which evolve in response to a variety of selective pressures. 3 Although HCVspecific immunity develops after primary infection, it frequently fails to eliminate the virus. [4][5][6] HCV has infected approximately 170 million people worldwide, and approximately 80% of those infected will develop chronic infection. The outcome of chronic infection varies widely between individuals, but large proportions of patients develop serious liver diseases such as cirrhosis and hepatocellular carcinoma. 7 Current therapies are inadequate, and development of appropriate therapeutic and prophylactic vaccines remains a significant challenge.
HCV can be classified into six genetically distinct genotypes and further subdivided into at least 70 subtypes, which differ by approximately 30% and 15% at the nu-