Characterization of a linear epitope in the nonstructural region 4 of hepatitis C virus with reactivity to seroconversion antibodies.

Mapping and possible diagnostic meaning of a highly conserved, linear NS4 epitope (NS4/3), located outside the C100-3 antigen within the carboxyl terminal proportion of the NS4 region, with major immunoreactivity with specimens of patients with HCV infection from various geographic origins is described. Transient, acute-phase IgM anti-HCV NS4/3 was detected coincidentally or earlier than active IgG anti-HCV NS4/3 response with four well-characterized seroconversion panels. GenBank alignment studies identified patch homologies between the NS4/3 sequence and a number of non-HCV proteins, which may explain part of the cross-reactivity of the NS4/3 epitope. Some of the "false positive reactivities" of the NS4/3 epitope with asymptomatic blood donors, not being confirmed with FDA-approved anti-HCV assays without the NS4/3 epitope, may be explained by recognition of very early seroconversion antibodies.