Background:  GB virus C (GBV-C) viremia is associated with improved survival in HIV-infected people than in those without GBV-C viremia. In some studies, HIV-infected people with evidence of past GBV-C infection (E2 antibody⁺) survived longer than HIV⁺ people with neither active nor prior GBV-C infection. We characterized monoclonal antibodies directed against the GBV-C E2 protein to determine if they have an antiviral effect against HIV.

Methods:  Anti-GBV-C E2 murine monoclonal antibodies were tested in HIV neutralization assays. Specifically, E2 antibody-mediated neutralization of HIV was tested in primary peripheral blood mononuclear cell (PBMC) cultures, MT2 cells, or in a standardized TZM-bl cell assay. In PBMC and MT2 cells, HIV neutralization was determined by measuring the reduction in p24 antigen in culture supernatants; or in TMZ-bl cells, reduction in relative luminescence units. A panel of HIV isolates was studied.

Results:  A panel of monoclonal antibodies reacting with conformational, overlapping epitopes on GBV-C E2 (determined by competition immunoassay) neutralized infectivity of both R5 and X4 HIV isolates in PBMC, but not in an MT-2 cell assay. In 1 experiment, 1 E2 monoclonal antibody neutralized 10 of 11 isolates of HIV in TZM-bl cells at high concentrations (ranging from 23 to 45 mg/mL), but did not in a subsequent experiment. However, when the E2 monoclonal antibodies were tested in the TMZ-bl cell assay for neutralization of SF162.LS and the R5 and X4 isolates used in the PBMC assays, 1 monoclonal antibody (BD) inhibited all 3 isolates (IC₅₀ 2.6 to 4.0 mg/mL). By comparison, a combination of 3 anti-HIV monoclonal antibodies (positive control) only inhibited the SF162.LS and X4 isolate (IC₅₀ = 0.04 and 0.58, respectively) and did not inhibit the R5 isolate in TMZ-bl cells. In the PBMC assay, incubation of cells with antibody prior to HIV infection did not inhibit HIV infectivity; thus this effect was not due to reactivity with cellular surface antigens.

Conclusions:  Monoclonal antibodies against GBV-C E2 protein against a conformational antigenic region contain broad, though weak, neutralizing activity against HIV in PBMC. Most of these antibodies do not appear to inhibit HIV in cell lines, although 1 (BD) appeared to have activity at concentrations <5 mg/mL in the TMZ-bl cell system. Determination of the structural features of this GBV-C E2 antigenic site may provide a novel immunogen for HIV vaccine development, and may explain the prolonged survival of prior GBV-C infection in HIV⁺ people observed in several clinical studies.