Toward Fully Synthetic Carbohydrate-Based HIV Antigen Design:  On the Critical Role of Bivalency

Abstract

Synthetic gp120331-335 glycopeptide fragments carrying hybrid and high-mannose type N-linked glycans were evaluated for binding to broadly neutralizing antibody 2G12 using surface plasmon resonance technology. None of the hybrid-type constructs demonstrated binding to 2G12. In the high-mannose series, the "Cys dimer" construct, presenting two undecasaccharide glycans, showed significantly higher binding than the Cys-protected monomer. The binding of the dimeric structure was further investigated in competition with recombinant gp120. The data suggest that gp120 and its designed synthetic epitope construct bind to the same site on 2G12.

An oligosaccharide-based HIV-1 2G12 mimotope vaccine induces carbohydrate-specific antibodies that fail to neutralize HIV-1 virions

“…Crystallographic studies of Fab 2G12 bound to a Man 9 GlcNAc 2 oligosaccharide revealed a unique interlocked V H domain-swapped dimer, which presents an extended high-affinity binding surface optimized for interaction with clustered carbohydrates (13). Although several reports have confirmed the critical role of a Man␣1-2Man disaccharide in the 2G12 epitope, the optimal form of a synthetic carbohydrate mimetic of the epitope remains undefined (14)(15)(16)(17)(18)(19).…”

A peptide inhibitor of HIV‐1 neutralizing antibody 2G12 is not a structural mimic of the natural carbohydrate epitope on gp120

“…Significant efforts are in progress to develop synthetic oligosaccharide immunogens that elicit 2G12 (14,16,(18)(19)(20)42,43). Two other groups have reported isolation of phage clones that bind 2G12, from phage-displayed random peptide libraries (44,45), but their peptides display very low affinity for 2G12.…”

“…Gly L93 , in particular, moves by ∼2 Å between the two complexes, supporting the notion that competing interactions with this residue may be part of the mechanism that prevents 2G12 from binding both peptide and Manα1-2Man simultaneously. Significant efforts are in progress to develop synthetic oligosaccharide immunogens that elicit 2G12 (14,16,(18)(19)(20)42,43 Table 3), may also be incorporated and tested in further rounds of optimization.…”

“…Gly L93 , in particular, moves by ∼2 Å between the two complexes, supporting the notion that competing interactions with this residue may be part of the mechanism that prevents 2G12 from binding both peptide and Manα1-2Man simultaneously. Significant efforts are in progress to develop synthetic oligosaccharide immunogens that elicit 2G12 (14,16,(18)(19)(20)42,43 Table 3), may also be incorporated and tested in further rounds of optimization.Peptide ligands for antibodies against carbohydrate epitopes have been mostly sought as immunogens to overcome the poor immunogenicity of carbohydrate antigens, and the difficulties associated with synthesis of complex polysaccharides. For a peptide to succeed as an immunogen that can generate a specific anticarbohydrate antibody response, carbohydratepeptide cross-reactivity must translate into immunogenic mimicry (i.e., the ability of a crossreactive peptide to elicit that same antibody, or functionally similar ones).…”

“…Since the identification of its epitope, MAb 2G12 has received significant attention as a template for HIV-1 vaccine development. Considerable effort has been directed at characterizing its oligosaccharide-binding profile (12)(13)(14)(15), and the generation of novel antigens in the form of synthetic oligomannoses (13,16,17) or short, synthetic glycopeptides (18)(19)(20). In an attempt to generate immunogens that elicit 2G12-like antibodies, we chose to isolate peptides that bind to 2G12 and inhibit antibody binding to the native oligomannose epitope on gp120, by screening a set of phage-displayed peptide libraries with MAb 2G12.…”