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295 B2.1, a Peptide That Specifically Binds the Broadly HIV-1-Neutralizing, Human Antibody, b12
A. Menendez1, M. Zwick 1, L. Bonnycastle 1, M. Irving 1, M. Montero 1, C. Barbas 2, P. Parren 2, D. Burton 2, and J. Scott* 1
1Simon Fraser Univ., Burnaby, BC, Canada and 2The Scripps Res. Inst., La Jolla, CA, USA
Background: Human monoclonal antibody (MAb) b12 recognizes a conformational epitope that overlaps with the CD-4-binding site of the HIV-1 envelope. It neutralizes a broad range of HIV-1 primary isolates and protects against primary virus challenge in animal models. However, b12-like neutralizing antibodies are rarely produced in both HIV-1-infected and envelope-vaccinated subjects, which suggests that the b12 epitope is poorly immunogenic in the envelope context.
Methods and Results: The peptide B2.1 was isolated from a phage-displayed peptide library using IgG1 b12 as the selecting agent. The peptide is a homodimer whose activity depends on an intact disulfide bridge joining its polypeptide chains. The B2.1 peptide occupies the b12 antigen-binding site as shown by competition studies with gp120 as well as preliminary x-ray crystallography studies. B2.1 is highly specific for b12 since it selected only phage-bearing b12 Fab from a complex phage-displayed Fab library enriched for anti-HIV-1 antibodies. The B2.1 peptide binds the b12 antibody the best when presented in the context of a fusion to the phage coat protein with an affinity close to that of b12-gp120. In contrast, the interaction between b12 MAb and a synthetic peptide is much weaker, with a Kd in the micromolar range. This indicates that the recognition by b12 is sensitive to the context of the peptide. The synthetic peptide conjugated to carrier proteins elicited high titers of anti-peptide antibodies in mice, but only weak cross-reactivity with gp120.
Conclusions: B2.1 peptide is a specific marker for b12. Further studies are required to improve it as an immunogen capable of eliciting HIV-1-neutralizing antibodies.
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