Hepatitis C virus (HCV) remains a significant threat to the general health of the world's population, and there is a pressing need for the development of new treatments and preventative vaccines. Here, we describe the generation of retrovirus-based pseudoparticles (HCVpp) incorporating a panel of full-length E1E2 clones representative of the major genotypes 1 through 6, and their application to assess the reactivity and neutralizing capability of antisera and monoclonal antibodies raised against portions of the HCV E2 envelope protein.Rabbit antisera raised against either the first hypervariable region or ectodomain of E2 showed limited and strain specific neutralization. By contrast, the monoclonal antibody (MAb) AP33 demonstrated potent neutralization of infectivity against HCVpp carrying E1E2 representative of all genotypes tested. The concentration of AP33 required to achieve 50% inhibition of infection by HCVpp of diverse genotypes ranged from 0.6 to 32 g/ml. The epitope recognized by MAb AP33 is linear and highly conserved across different genotypes of HCV. Thus, identification of a broadly neutralizing antibody that recognizes a linear epitope is likely to be of significant benefit to future vaccine and therapeutic antibody development.Hepatitis C virus (HCV), a positive-strand RNA virus belonging to the Flaviviridae family, is the major cause of non-A, non-B viral hepatitis. HCV has infected approximately 200 million people worldwide and current estimates suggest that as many as 3 million individuals are newly infected each year (4). Approximately 80% of those infected fail to clear the virus; a chronic infection ensues, frequently leading to severe chronic liver disease, cirrhosis, and hepatocellular carcinoma (2, 41). Current treatments for chronic infection are ineffective for approximately 50% of patients, and there is a pressing need to develop preventative and therapeutic vaccines.Due to the error-prone nature of the RNA-dependent RNA polymerase and the high replicative rate in vivo (30, 46), HCV exhibits a high degree of genetic variability. Crucially, this propensity for genetic change allows the virus to respond to and overcome a variety of selective pressures, including host immunity and antiviral therapy (18,26,37,44,53). 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 nucleotide level, respectively (59, 61). A significant challenge for the development of vaccines will lie in identifying protective epitopes that are conserved in the majority of viral genotypes and subtypes. This problem is compounded by the fact that the envelope proteins, the natural targets for the neutralizing response, are two of the most variable proteins (10).The envelope proteins E1 and E2 are responsible for cell binding and entry (5,8,16,51,57). They are N-linked glycosylated (23,31,43,62) transmembrane proteins with a Nterminal ectodomain and a C-terminal hydrophobic membrane anchor (12,21,22). In vitro ex...