Expression of the GB virus C E2 glycoprotein using the Semliki Forest virus vector system and its utility as a serologic marker.

A 336-amino-acid segment of the GB virus C second envelope protein (E2) has been produced in BHK-21 cells using the Semliki Forest virus vector system. Secretion of this protein was facilitated by deletion of a hydrophobic region at the C-terminus that may represent the membrane anchoring domain. The E2 protein recovered from the culture supernatant exhibited a molecular mass of approximately 52 kDa, with the increase in size relative to the polyprotein backbone being contributed by N-linked glycosylation. A radioimmunoprecipitation assay using GBV-C E2 was developed to test for the presence of antibodies against this protein in human sera. The prevalence of antibodies to E2 was high among injection drug users and other individuals at risk for acquiring parenterally transmitted agents. There was a much higher percentage of anti-E2 seropositivity in GBV-C RT-PCR negative compared to GBV-C RT-PCR positive samples from these populations. In addition, serial samples from patients transfused with blood containing GBV-C showed seroconversion to anti-E2 positivity and loss of GBV-C viremia as measured by RT-PCR within 11 months of transfusion in five of seven individuals. Thus, this system provided a rapid means to identify GBV-C E2 as a useful antigen for the study of GBV-C exposure.