Background:  GB Virus C (GBV-C) viremia is associated with improved survival among HIV-infected people when compared to those without active GBV-C infection. For previously unclear reasons, HIV-infected people without active infection, but who had evidence of past GBV-C infection (E2 antibody positive) survived longer than did HIV-positive 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:  GBV-C E2 anti-E2 murine monoclonal antibodies were prepared as previously described or purchased commercially. HIV neutralization assays were performed using E2 antibodies (or appropriate control antibodies) in primary peripheral blood mononuclear cell (PBMC) cultures. HIV replication was determined by measuring culture supernatant p24 antigen, or reporter gene (CD24) expression. HIV particles were ³⁵S-labeled for immunoprecipitation experiments.

Results:  Incubation of E2 monoclonal antibodies with HIV prior to infection resulted in a dose-dependent inhibition of HIV infection compared to control antibodies. Neutralizing activity was observed against clinical and laboratory strains of HIV that utilized either the CCR5 or CXCR4 co-receptor. 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. None of the E2 antibodies reacted with denatured HIV proteins in Western blots, and only 1 monoclonal antibody (Mc6) reacted with denatured GBV-C E2. Anti-E2 monoclonal antibodies immunoprecipitated HIV particles. The Mc6 antibody was shown to react with a 17 amino acid (aa) region on E2.  The 17 aa peptide was synthesized, conjugated to keyhole limpet hemocyanin, and used to immunize rabbits. Low-level neutralizaton activity and HIV immunoprecipitation activity were observed in post-immune rabbit IgG compared to pre-immune IgG.

Conclusion:  GBV-C E2 protein contains a conformational antigenic region that induces antibodies that are neutralizing against HIV. This may explain the survival advantage of prior GBV-C infection in HIV-positive people observed in several clinical studies. Furthermore, it provides a novel, potentially disease-modifying HIV vaccine immunogen and may direct the design of novel anti-HIV therapies.

Keywords: HIV; neutralization; GB Virus C