Toward a Carbohydrate-Based HIV-1 Vaccine:  Synthesis and Immunological Studies of Oligomannose-Containing Glycoconjugates

Abstract

Human antibody 2G12 is a broadly neutralizing antibody that exerts its anti-HIV activity by targeting a novel oligomannose cluster on HIV-1 gp120. It was previously demonstrated that synthetic oligomannose clusters could mimic the carbohydrate epitope of 2G12 and showed enhanced antigenicity (Wang L. X. et al. (2004) Chem.Biol. 11, 127). This paper describes the synthesis of oligomannose-containing glycoconjugates that include either a carrier protein or a universal T-helper epitope peptide to provide an effective immunogen. It was shown that the synthetic neoglycoconjugates containing oligomannose clusters could be recognized by the human antibody 2G12. Rabbit immunization studies revealed that only a small fraction of antibodies raised by the glycoconjugates was directed to the carbohydrate antigens, with the majority of the IgG type antibodies being directed to the linkers in the conjugates. The anti-sera showed weak cross-reactivity to HIV-1 gp120.

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

“…Inability to induce a response to the oligomannan is not the reason for this failure because we clearly show that the conjugate elicited a robust carbohydrate-specific antibody response. Our results corroborate recently published work (19) in which synthetic oligomannans were coupled to a variety of carriers and tested as potential vaccine candidates. Although in this study no attempt was made to mimic the proposed epitope structure, the conjugates showed some ability to bind 2G12.…”

“…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).…”

Robust immune responses elicited by a fully synthetic three-component vaccine

“…Furthermore, the conjugation chemistry is often difficult to control resulting in conjugates with ambiguities in composition. Also, the linker moiety can elicit strong B-cell responses 5,6 . Not surprisingly, preclinical and clinical studies with carbohydrate-protein conjugates have led to results of mixed merit.…”

“…Tumor-associated saccharides are, however, of low antigenicity, because they are self-antigens and consequently tolerated by the immune system. In addition, foreign carrier proteins such as keyhole limpet hemocyanin (KLH) and bovine serum albumin (BSA) and the linker that attach the saccharides to the carrier protein can elicit strong B-cell responses, which may lead to the suppression of antibody responses against the carbohydrate epitope 5,6 . It is clear that the successful development of carbohydrate-based cancer vaccines requires novel strategies for the more efficient presentation of tumor-associated carbohydrate epitopes to the immune system, resulting in a more efficient class switch to IgG antibodies [7][8][9][10][11][12][13][14][15][16][17] .…”

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

“…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.…”