The sample with the greatest infectivity for the cells was an acute phase plasma sample (H77) obtained from Pt H in 1977, 7 weeks after transfusion and just as he was developing liver enzyme elevations. GUID:?B36509EC-C86F-471D-8D3A-0CDD82210A1C pnas_100_24_14199__pnashead.gif (1.4K) GUID:?0191A8A7-F1DF-4574-8718-E7BCCF2EE429 pnas_100_24_14199__pnasbar.gif CX-4945 (Silmitasertib) (1.9K) GUID:?4F70B2AD-C3E6-4E37-B451-B95874E078F5 pnas_100_24_14199__current_head.gif (501 bytes) GUID:?29B2B374-46D2-45C4-B0CC-34B417F58877 pnas_100_24_14199__spacer.gif (43 bytes) GUID:?B5C86591-E92D-48BC-AE8B-CD34AA851753 pnas_100_24_14199__archives_head.gif (411 bytes) GUID:?BBF7BF87-6ADC-4EA7-A2B3-203FDD4DB6C3 pnas_100_24_14199__spacer.gif (43 bytes) GUID:?B5C86591-E92D-48BC-AE8B-CD34AA851753 pnas_100_24_14199__online_head.gif (622 bytes) GUID:?F01305E7-8B45-4699-AB7D-C9582B17ED8D pnas_100_24_14199__spacer.gif (43 bytes) GUID:?B5C86591-E92D-48BC-AE8B-CD34AA851753 pnas_100_24_14199__advsrch_head.gif (481 bytes) GUID:?AC9B2F6E-E3AD-4F28-82A0-6DFCEC61CF61 pnas_100_24_14199__spacer.gif (43 bytes) GUID:?B5C86591-E92D-48BC-AE8B-CD34AA851753 pnas_100_24_14199__arrowTtrim.gif (51 bytes) GUID:?68968319-B2B3-4D3F-B326-485BBCB9DE01 pnas_100_24_14199__arrowTtrim.gif (51 bytes) GUID:?68968319-B2B3-4D3F-B326-485BBCB9DE01 pnas_100_24_14199__spacer.gif (43 bytes) GUID:?B5C86591-E92D-48BC-AE8B-CD34AA851753 pnas_100_24_14199__spacer.gif (43 bytes) GUID:?B5C86591-E92D-48BC-AE8B-CD34AA851753 pnas_100_24_14199__arrowTtrim.gif (51 bytes) GUID:?68968319-B2B3-4D3F-B326-485BBCB9DE01 pnas_100_24_14199__arrowTtrim.gif (51 bytes) GUID:?68968319-B2B3-4D3F-B326-485BBCB9DE01 Abstract Our understanding of the humoral immune response to hepatitis C virus (HCV) is limited because the virus can be studied only in humans and chimpanzees and because previously described neutralization assays have not been robust or simple to perform. Nevertheless, epidemiologic and laboratory studies suggested that neutralizing Ab to HCV might be important in preventing contamination. We have recently described a CX-4945 (Silmitasertib) neutralization assay based on the neutralization of pseudotyped murine retrovirus constructs bearing HCV envelope glycoproteins on their surface. We have applied the assay to well characterized clinical samples from HCV-infected patients and chimpanzees, confirmed the presence of neutralizing Ab to HCV, and validated most previously reported neutralizations of the virus. We did not find neutralizing anti-HCV in resolving infections but did find relatively high titers ( 1:320) of such Ab in chronic infections. Neutralizing Ab was directed not only to epitope(s) in the hypervariable region of the E2 envelope protein but also CX-4945 (Silmitasertib) to one or more epitopes elsewhere in the envelope of the virus. Neutralizing ENTPD1 Ab was broadly reactive and could neutralize pseudotype particles bearing the envelope glycoproteins of two different subgenotypes (1a and 1b). The ability to assay neutralizing anti-HCV should permit an assessment of the prospects for successful Ab-mediated passive and active immunoprophylaxis against hepatitis C. Hepatitis C virus (HCV) is a small enveloped virus made up of single-stranded positive sense RNA. It infects up to 170 million people worldwide and is a major cause of chronic hepatitis, cirrhosis, and hepatocellular carcinoma. Although HCV accounts for only 12% of acute hepatitis in the United States, its high rate of persistence (70C80%) makes it responsible CX-4945 (Silmitasertib) for almost half of the economic burden of this disease. A better understanding of the pathogenesis of hepatitis C and its control is usually hampered by certain characteristics of the virus. (method developed for detecting neutralizing antibodies (Nt Ab) to HCV in this system is so difficult to perform that it has not been widely used. Consequently, although the two envelope proteins of HCV, E1 and E2, have been expressed individually and as heterodimers, and Ab to them has been measured, there is no confirmation that such Ab accurately reflects the response to the virus or that it is Nt Ab. Previously, we identified Nt Ab to HCV by their ability to prevent replication of the virus in a lymphoid cell line (1, 2) or to prevent hepatitis C CX-4945 (Silmitasertib) in chimpanzees (3, 4). These and a small number of similar studies (5) have provided, for almost a decade, the only direct evidence for Ab-mediated neutralization of HCV. Consequently, although considerable new knowledge has been gained about the cellular immune response to HCV, little is known about the role of the humoral immune response. However, there is historical evidence that normal immune globulin manufactured before the screening of donor plasma for HCV contamination guarded against hepatitis C, whereas globulin manufactured subsequently does not (6). Recently we described the construction of infectious HCV pseudotype particles (pp) that were assembled in cell culture from three plasmids bearing parts of a truncated murine retrovirus genome encoding genes for the retroviral.