Chemical synthesis and immunological properties of oligosaccharides derived from the vegetative cell wall of Bacillus anthracis

Synthesis and preliminary immunologic properties of di-/trisaccharide-conjugates related to Bacillus anthracis

Herein, the di- and trisaccharide mimics of the hexasaccharide antigen related to Bacillus anthracis were synthesized and covalently coupled with carrier proteins, such as keyhole limpet hemocyanin (KLH) and bovine serum albumin (BSA), to form the corresponding glycoconjugates 1-6. 2,3,4,6-Tetra-O-benzyl thioglycoside and 2-deoxyl-2-phthalylamino-3,4,6-tri-O-benzyl thioglycoside were applied as glycosyl donors to guarantee α or β-configuration of the newly formed glycosidic bonds. Glutaraldehyde was used as a homobifunctional cross-linker for high-efficiency coupling. The synthetic KLH-glycoconjugates 2, 4 and 6 were also used to vaccinate female Balb/c mice and the preliminary results of ELISA uncovered that all three KLH-conjugates could induce immune responses and generate oligosaccharide-specific total IgG antibodies. The trisaccharide 8, the glycosyl part of glycoconjugate 4, is of great immunogenicity.

Stereoselective Synthesis of 2-Azido-2-deoxy-β-d-mannosides via Cs2CO3-Mediated Anomeric O-Alkylation with Primary Triflates: Synthesis of a Tetrasaccharide Fragment of Micrococcus luteus Teichuronic Acid

Cesium carbonate-mediated anomeric O-alkylation of various protected 2-azido-2-deoxy-d-mannoses with primary triflate electrophiles afforded corresponding 2-azido-2-deoxy-β-mannosides in good yields and excellent anomeric selectivity. In addition, 1,3-dibromo-5,5-dimethylhydantoin was found to be the optimal oxidant for preparation of those 2-azido-2-deoxy-d-mannoses from their corresponding thioglycosides. The utilization of this method was demonstrated in the synthesis of a tetrasaccharide fragment of Micrococcus luteus teichuronic acid containing N-acetyl-β-d-mannosaminuronic acid (ManNAcA).

A General Chemoenzymatic Strategy for the Synthesis of Glycosphingolipids

A concise, prototypical, and stereoselective strategy for the synthesis of therapeutically and immunologically significant glycosphingolipids has been developed. This strategy provides a universal platform for glycosphingolipid synthesis by block coupling of enzymatically prepared free oligosaccharideglycans to lipids using glycosyl N-phenyltrifluoroacetimidates as efficient activated intermediates. As demonstrated here, two different types of glycosphingolipids were obtained in excellent yields using the method.

Endolysins of Bacillus anthracis bacteriophages recognize unique carbohydrate epitopes of vegetative cell wall polysaccharides with high affinity and selectivity

Bacteriophages express endolysins which are the enzymes that hydrolyze peptidoglycan resulting in cell lysis and release of bacteriophages. Endolysins have acquired stringent substrate specificities, which have been attributed to cell wall binding domains (CBD). Although it has been realized that CBDs of bacteriophages that infect Gram-positive bacteria target cell wall carbohydrate structures, molecular mechanisms that confer selectivity are not understood. A range of oligosaccharides, derived from the secondary cell wall polysaccharides of Bacillus anthracis, has been chemically synthesized. The compounds contain an α-d-GlcNAc-(1→4)-β-d-ManNAc-(1→4)-β-d-GlcNAc backbone that is modified by various patterns of α-d-Gal and β-d-Gal branching points. The library of compounds could readily be prepared by employing a core trisaccharide modified by the orthogonal protecting groups N(α)-9-fluorenylmethyloxycarbonate (Fmoc), 2-methylnaphthyl ether (Nap), levulinoyl ester (Lev) and dimethylthexylsilyl ether (TDS) at key branching points. Dissociation constants for the binding the cell wall binding domains of the endolysins PlyL and PlyG were determined by surface plasmon resonance (SPR). It was found that the pattern of galactosylation greatly influenced binding affinities, and in particular a compound having a galactosyl moiety at C-4 of the nonreducing GlcNAc moiety bound in the low micromolar range. It is known that secondary cell wall polysaccharides of various bacilli may have both common and variable structural features and in particular differences in the pattern of galactosylation have been noted. Therefore, it is proposed that specificity of endolysins for specific bacilli is achieved by selective binding to a uniquely galactosylated core structure.

Immunologic mapping of glycomes: implications for cancer diagnosis and therapy

Cancer associated glycoconjugates are important biomarkers, as exemplified by globo-H, CA125, CA15.3 and CA27.29. However, the exact chemical structures of many such biomarkers remain unknown because of technological limitations. In this article, we propose the "immunologic mapping" of cancer glycomes based on specific immune recognition of glycan structures, which can be hypothesized theoretically, produced chemically, and examined biologically by immuno-assays. Immunologic mapping of glycans not only provides a unique perspective on cancer glycomes, but also may lead to the invention of powerful reagents for diagnosis and therapy.

Recent advances in carbohydrate-based vaccines

Vaccinations provide an efficient and cost-effective way to combat devastating human diseases. Besides pathogenic protein markers, cell surface carbohydrates from biological sources are widely used as vaccines. Recently, synthetic immunogenic carbohydrate-protein conjugates have been advanced to vaccine candidates. Progress in the chemical synthesis of oligosaccharides and conjugation methods stimulated the development of novel carbohydrate-based vaccine candidates.