In vitro assay for neutralizing antibody to hepatitis C virus: evidence for broadly conserved neutralization epitopes

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 infection. We have recently described a 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 existence 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 to one or more epitopes elsewhere in the envelope of the virus. Neutralizing 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.

Fig. 1.

Results of a single neutralization assay with coded sera (1:50) tested against two concentrations of HCV genotype 1a pp. Shown is the mean percent of cells positive for GFP (determined by FACS analysis), compared with the mean percent infected in the presence of the same dilution of the two normal sera, set to 100%. Samples 683 and 691, normal human sera; Vu, patient with chronic HCV infection; bars, standard deviation of the two determinations.

Fig. 2.

Plot of reciprocal titers of Nt Ab on abscissa vs. reciprocal ELISA titers of anti-E1 (▵), anti-E2 (□), and anti-HVR1 (•) on ordinate.

Fig. 3.

Titration of neutralizing anti-HCV titers against pp bearing HCV E1 and E2 glycoproteins of genotype 1a or 1b. Sera/plasma were tested at final dilutions of 1:20, 1:80, and 1:320. Results were adjusted for concentration effects by comparing with comparable dilutions of normal anti-HCV negative sera.

Fig. 4.

Alignment of predicted amino acid sequence of E1 and E2 of HCV strain H77 (13), E1 and E2 derived from H77 and incorporated into the HCV genotype 1a pseudotype particle (10), E1 and E2 derived from HCV strain TN (genotype 1a) (19, 20), and E1 and E2 derived from HCV strain J (genotype 1b) and incorporated into the HCV genotype 1b pseudotype particle (11). Black, C′ end of core; blue, E1; brown, E2; bold, HVR1 of E2.