Synthesis and crystal structure of methyl 4-6-dideoxy-4-(3-deoxy-L- glycero-tetronamido)-2-O-methyl-alpha-D-mannopyranoside, the methyl alpha-glycoside of the terminal unit, and presumed antigenic determinant, of the O-specific polysaccharide of Vibrio cholerae O:1, serotype Ogawa

Safety and Immunogenicity of a Shigella Bivalent Conjugate Vaccine (ZF0901) in 3-Month- to 5-Year-Old Children in China

No licensed Shigella vaccine is presently available globally. A double-blinded, randomized, placebo-controlled, age descending phase II clinical trial of a bivalent conjugate vaccine was studied in China. The vaccine ZF0901 consisted of O-specific polysaccharides purified and detoxified from lipopolysaccharide (LPS) of S. flexneri 2a and S. sonnei and covalently bonded to tetanus toxoid. A total of 224, 310, and 434 children, consented by parents or guardians, aged 3 to 6 and 6 to 12 months and 1 to 5 years old, respectively, were injected with half or full doses, with or without adjuvant or control Hib vaccine. There were no serious adverse reactions in all recipients of ZF0901 vaccine independent of age, dosage, number of injections, or the adjuvant status. Thirty days after the last injection, ZF0901 induced robust immune responses with significantly higher levels of type-specific serum antibodies (geometric mean concentrations (GMCs) of IgG anti-LPS) against both serotypes in all age groups compared with the pre-immune or the Hib control (p < 0.0001). Here, we demonstrated that ZF0901 bivalent Shigella conjugate vaccine is safe and immunogenic in infants and young children and is likely suitable for routine immunization.

Conjugate Vaccines from Bacterial Antigens by Squaric Acid Chemistry: A Closer Look

By using O-SP-core (O-SPcNH2 ) polysaccharide, isolated from Vibrio cholera O1 lipopolysaccharide (LPS) and related synthetic substances, a detailed study of factors that affect conjugation of bacterial polysaccharides to protein carriers through squaric acid chemistry to form conjugate vaccines has been carried out. Several previously unrecognized processes that take place during the squarate labeling of the O-SPcNH2 and subsequent conjugation of the formed squarate (O-SPcNH-SqOMe) have been identified. The efficiency of conjugation at pH 8.5, 9.0, and 9.5 to bovine serum albumin (BSA) and to the recombinant tetanus toxin fragment C (rTT-Hc) has been determined. The study led to a protocol for more efficient labeling of O-SPcNH2 antigen with the methyl squarate group, to yield a higher-quality, more potent squarate conjugation reagent. Its use resulted in about twofold increases in conjugation efficiency (from 23-26 % on BSA to 51 % on BSA and 55 % on rTT-Hc). The spent conjugation reagent could be recovered and regenerated by treatment with MeI in the absence of additional base. The immunological properties of the experimental vaccine made from the regenerated conjugation reagent were comparable with those of the immunogen made from the parent O-SPcNH-SqOMe.

Synthesis and molecular structure of the 5-methoxycarbonylpentyl α-glycoside of the upstream, terminal moiety of the O-specific polysaccharide of Vibrio cholerae O1, serotype Inaba

The trimethylsilyl trifluoromethanesulfonate (TMSOTf)-catalyzed reaction of methyl 6-hydroxyhexanoate with 3-O-benzyl-4-(2,4-di-O-acetyl-3-deoxy-l-glycero-tetronamido)-4,6-dideoxy-2-O-levulinoyl-α-d-mannopyranosyl trichloroacetimidate followed by a two-step deprotection (hydrogenolysis over Pd/C catalyst and Zemplén deacylation, to simultaneously remove the acetyl and levulinoyl groups) gave 5-(methoxycarbonyl)pentyl 4-(3-deoxy-l-glycero-tetronamido)-4,6-dideoxy-α-d-mannopyranoside. The structure of the latter, for which crystals were obtained in the analytically pure state for the first time, followed from its NMR and high-resolution mass spectra and was confirmed by X-ray crystallography. The molecule has two approximately linear components; a line through the aglycon intersects a line through the mannosyl and tetronylamido groups at 120°. The crystal packing separates the aglycon groups from the tetronylamido and mannosyl groups, with only C-H…O hydrogen bonding among the aglycon groups and N-H…O, O-H…O and C-H…O links among the tetronylamido and mannosyl groups. A carbonyl oxygen atom accepts the strongest O-H…O hydrogen bond and two strong C-H…O hydrogen bonds. The geometric properties were compared with those of related molecules.

Immunogenicity of synthetic saccharide fragments of Vibrio cholerae O1 (Ogawa and Inaba) bound to Exotoxin A

Recombinant exotoxin A (rEPA) from Pseudomonas aeruginosa conjugated to Vibrio cholerae O1 serotype-specific polysaccharides (mono-, di- and hexasaccharide) were immunogenic in mice. Monosaccharide conjugates boosted the humoral responses to the hexasaccharide conjugates. Prior exposure to purified Ogawa lipopolysaccharide (LPS) enabled contra-serotype hexasaccharide conjugates to boost the vibriocidal response, but Inaba LPS did not prime for an enhanced vibriocidal response by a contra-serotype conjugate. Prior exposure to the carrier, and priming B cells with the LPS of either serotype, resulted in enhanced vibriocidal titers if the Ogawa hexasaccharides were used, but a diminished response to the Inaba LPS. These studies demonstrate that the 'functional' B cell epitopes on the LPS differ from those of the neoglycoconjugates and that the order of immunization and the serotype of the boosting conjugate can influence the epitope specificity and function of the antisera.

Studies toward a conjugate vaccine for anthrax. Synthesis and characterization of anthrose [4,6-dideoxy-4-(3-hydroxy-3-methylbutanamido)-2-O-methyl-d-glucopyranose] and its methyl glycosides

The key step in the first chemical synthesis of anthrose (16) and its methyl alpha- (6) and beta-glycoside (22) was inversion of configuration at C-2 in triflates 10, 2, and 18, respectively, obtained from the common intermediate, methyl 4-azido-3-O-benzyl-4,6-dideoxy-alpha-D-mannopyranoside (1). To prepare methyl alpha-anthroside (6), methylation at O-2 of the gluco product 3, obtained from 2, was followed by hydrogenation/hydrogenolysis of the formed 2-methyl ether 4, to simultaneously remove the protecting benzyl group and reduce the azido function. Subsequent N-acylation of the formed amine 5 with 3-hydroxy-3-methylbutyric acid gave the target methyl alpha-glycoside 6. Synthesis of methyl beta-anthroside (22) comprised the same sequence of reactions, starting from the known methyl 4-azido-3-O-benzyl-4,6-dideoxy-beta-D-mannopyranoside (17), which was prepared from 1. In the synthesis of anthrose (16), 1-thio-beta-glucoside 11, obtained from 1 through 10, was methylated at O-2, and the azido function in the resulting benzylated 1-thioglycoside 12 was selectively reduced to give amine 13. After N-acylation with 3-hydroxy-3-methylbutyric acid, 1-thioglycoside 14 was hydrolyzed to give the corresponding reducing sugar, aldol 15, which was debenzylated to afford anthrose.

Lipopolysaccharides of Vibrio cholerae. I. Physical and chemical characterization

Vibrio cholerae is the causative organism of the disease cholera. The lipopolysaccharide (LPS) of V. cholerae plays an important role in eliciting the antibacterial immune response of the host and in classifying the vibrios into some 200 or more serogroups. This review presents an account of our up-to-date knowledge of the physical and chemical characteristics of the three constituents, lipid-A, core-polysaccharide (core-PS) and O-antigen polysaccharide (O-PS), of the LPS of V. cholerae of different serogroups including the disease-causing ones, O1 and O139. The structure and occurrence of the capsular polysaccharide (CPS) on V. cholerae O139 have been discussed as a relevant topic. Similarity and dissimilarity between the structures of LPS of different serogroups, and particularly between O22 and O139, have been analysed with a view to learning their role in the causation of the epidemic form of the disease by avoiding the host defence mechanism and in the evolution of the newer pathogenic strains in future. An idea of the emerging trends of research involving the use of immunogens prepared from synthetic oligosaccharides that mimic terminal epitopes of the O-PS of V. cholerae O1 in the development of a conjugate anti cholera vaccine is also discussed.

Synthetic explorations towards 3-deoxy-3-fluoro derivatives of D-perosamine

Based on a literature precedent, preparation of methyl 4-azido-3,4,6-trideoxy-3-fluoro-alpha-D-mannopyranoside (18) was attempted via fluorination of methyl 4-azido-2-O-benzyl-4,6-dideoxy-alpha-D-altropyranoside with diethylaminosulfur trifluoride (DAST). Contrary to expectations, the reaction took place with retention of configuration at the site of the fluorination yielding methyl 4-azido-2-O-benzyl-3,4,6-trideoxy-3-fluoro-alpha-D-altropyranoside. Treatment with DAST of methyl 4-azido-2-O-benzyl-4,6-dideoxy-alpha-D-allopyranoside (8), or its 2-(p-methoxybenzyl) analog 9 resulted in fluorination with inversion of configuration at position 3, to give the corresponding 3-deoxy-3-fluoro glucopyranosides 10 and 11, respectively. Accordingly, compound 18 was prepared from 11, by de-p-methoxybenzylation at O-2, followed by inversion of configuration at C-2 in the resulting methyl 4-azido-3,4,6-trideoxy-3-fluoro-alpha-D-glucopyranoside. The 2-O-methyl analog of 18 (19) was prepared by methylation of 18. Compounds 18 and 19 were converted, conventionally, into the 3-fluoro analogs of the terminal determinants of the O-PS of Vibrio cholerae O:1, serotype Inaba and Ogawa, respectively.

Syntheses of the L-manno and some other analogs of the terminal determinants of the O-PS of Vibrio cholerae O:1

Analogs of the methyl alpha-glycosides of the terminal residues of the O-specific polysaccharides (O-PS) of Vibrio cholerae O:1, serotype Inaba and Ogawa, have been prepared as probes to study their interaction with anti V. cholerae O:1 antibodies. They differ from the termini of the respective O-PSs in anomeric or absolute configuration of perosamine, position of the O-methyl group in D-perosamine, and nature of the N-acyl side chain.

Crystal structure of an anti-carbohydrate antibody directed against Vibrio cholerae O1 in complex with antigen: molecular basis for serotype specificity

The crystal structure of the murine Fab S-20-4 from a protective anti-cholera Ab specific for the lipopolysaccharide Ag of the Ogawa serotype has been determined in its unliganded form and in complex with synthetic fragments of the Ogawa O-specific polysaccharide (O-SP). The upstream terminal O-SP monosaccharide is shown to be the primary antigenic determinant. Additional perosamine residues protrude outwards from the Ab surface and contribute only marginally to the binding affinity and specificity. A complementary water-excluding hydrophobic interface and five Ab-Ag hydrogen bonds are crucial for carbohydrate recognition. The structure reported here explains the serotype specificity of anti-Ogawa Abs and provides a rational basis toward the development of a synthetic carbohydrate-based anti-cholera vaccine.

Conformational differences among mono- and oligosaccharide fragments of O-specific polysaccharides of Vibrio cholerae O1 revealed by circular dichroism

The circular dichroism (CD) of synthetic mono- and oligosaccharides that represent the terminal, non-reducing group of O-antigens of Vibrio cholerae O1 from the subtypes Ogawa and Inaba was measured in various solvents. We found differences in the CD of the monosaccharides of these subtypes that decrease with increasing chain lengths of the oligosaccharides. The differences can be explained by different orientations of the N-acyl side chain of the terminal monosaccharides. The linear relationship of ellipticity versus the number of residues in an oligosaccharide chain follows the principle of optical superposition. This, together with a similar contribution by internal units to the overall ellipticity, suggests an identical, regular conformation of oligosaccharide fragments of both Ogawa and Inaba series.

Studies towards neoglycoconjugates from the monosaccharide determinant of Vibrio cholerae O:1, serotype Ogawa using the diethyl squarate reagent

The effect of reaction time, concentration and molar excess of hapten upon the efficiency of the conjugation of carbohydrates to proteins using the diethyl squarate reagent has been studied using chicken serum albumin (CSA) as the carrier protein and a linker-equipped D-glucose derivative as the hapten. A high degree of incorporation of the latter into CSA was achieved with high efficiency, and the use of a large excess of the ligand was not necessary. Conjugation of the immunodominant monosaccharide determinant of Vibrio cholerae O:1, serotype Ogawa, bearing the same spacer, followed a similar pattern, showing that the nature of the carbohydrate does not substantially affect the outcome of the conjugation and that a predicted degree of antigen-loading onto carrier protein is possible to achieve.

On the antigenic determinants of the lipopolysaccharides of Vibrio cholerae O:1, serotypes Ogawa and Inaba

Monoclonal, murine IgG1s S-20-4, A-20-6, and IgA 2D6, directed against Vibrio cholerae O:1 Ogawa-lipopolysaccharide exhibited the same fine specificities and similar affinities for the synthetic methyl alpha-glycosides of the (oligo)saccharide fragments mimicking the Ogawa O-polysaccharide (O-PS). They did not react with the corresponding synthetic fragments of Inaba O-PS. IgG1s S-20-4 and A-20-6 have absolute affinity constants for synthetic Ogawa mono- to hexasaccharides of from approximately 10(5) to approximately 10(6) M-1. For IgG1s S-20-4, A-20-6, and IgA 2D6, the nonreducing terminal residue of Ogawa O-PS is the dominant determinant, accounting for approximately 90% of the maximal binding energy shown by these antibodies. Binding studies of derivatives of the Ogawa monosaccharide and IgGs S-20-4 and A-20-6 revealed that the C-2 O-methyl group fits into a somewhat flexible antibody cavity and that hydrogen bonds involving the oxygen and, respectively, the OH at the 2- and 3-position of the sugar moiety as well as the 2'-position in the amide side chain are required. Monoclonal IgA ZAC-3 and IgG3 I-24-2 are specific for V. cholerae O:1 serotypes Ogawa/Inaba-LPS.1 The former did not show binding with members of either series of the synthetic ligands related to the O-antigens of the Ogawa or Inaba serotypes, in agreement with its reported specificity for the lipid/core region (1). Inhibition studies revealed that the binding of purified IgG3 I-24-2 to Ogawa-LPS might be mediated by a region in the junction of the OPS to the lipid-core region of the LPS. cDNA cloning and analysis of the anti-Ogawa antibodies S-20-4, A-20-6, and 2D6 revealed a very high degree of homology among the heavy chains. Among the light chains, no such homology between S-20-4 and A-20-6 on the one hand, and 2D6 on the other hand, exists. For the anti-Inaba/Ogawa antibodies I-24-2 and ZAC-3, their heavy chains are completely different, with some homology among the light chains.

Synthesis of methyl alpha-glycosides of some higher oligosaccharide fragments of the O-antigen of Vibrio cholerae O1, serotype Inaba and Ogawa

The title oligosaccharides, the tri- through the hexasaccharide in the Inaba series and the penta- and the hexasaccharide in the Ogawa series, have been synthesized using 1-thioglycosides of precursors to 3-O-benzyl-perosamine (4-amino-4,6-dideoxy-D-mannose) as building blocks and N-iodosuccinimide/silver triflate as a promoter. The azido groups in the assembled oligosaccharides were reduced to amino groups, which were then acylated using 2,4-O-benzylidene-3-deoxy-L-glycero-tetronic acid as the derivatizing reagent. Catalytic hydrogenolysis, simultaneously of the benzyl and benzylidene groups, gave the desired products that were characterized by 1H and 13C NMR spectroscopy.

Synthesis of eight glycosides of hexasaccharide fragments representing the terminus of the O-polysaccharide of Vibrio cholerae O:1, serotype Inaba and Ogawa, bearing aglycons suitable for linking to proteins

The title substances were prepared from intermediate, fully acetylated alpha-trimethylsilylethyl (SE) glycosides. The latter were assembled in a blockwise manner, using as the glycosyl donor the alpha-glycosyl chloride of a disaccharide bearing two 4-azido-4-deoxy functions. Next, the azido groups in the assembled hexasaccharides were converted to the corresponding amines, and these were acylated with 4-O-benzyl-3-deoxy-L-glycero-tetronic acid in the presence of a water-soluble carbodiimide. The SE glycosides were then transformed to glycosyl imidates, and these were coupled with methyl 6-hydroxyhexanoate or methyl 2-(2-hydroxyethylthio) propionate. The aglycons in the glycosides thus obtained were then converted to the corresponding carboxylic acids or acyl hydrazides. Such compounds are suitable for linking to proteins to obtain neoglycoproteins.

Synthesis of the methyl alpha-glycosides of a di-, tri-, and a tetra-saccharide fragment mimicking the terminus of the O-polysaccharide of Vibrio cholerae O:1, serotype Ogawa

Methyl 4-(3-deoxy-L-glycero-tetronamido)-4,6-dideoxy-2-O-methyl-alpha-D- mannopyranoside was acetylated, and the fully protected methyl glycoside was treated with dichloromethyl methyl ether-ZnCl2 (DCMME-ZnCl2) reagent to give 3-O-acetyl-4-(2,4-di-O-acetyl-3- deoxy-L-glycero-tetronamido)-4,6-dideoxy-2-O-methyl-alpha-D-mannop yranosyl chloride (3). Condensation of 3 with methyl 3-O-acetyl-4-(2,4-di-O-acetyl-3-deoxy-L-glycero-tetronamido)-4,6- dideoxy-alpha-D-mannopyranoside (4) gave the fully acetylated disaccharide 5, which was deacetylated yielding the methyl alpha-glycoside of title disaccharide. The disaccharide glycosyl donor required for the blockwise synthesis of the title tri- and the tetra-saccharide, 3-O-acetyl-4-(2,4-di-O-acetyl-3-deoxy-L-glycero-tetronamido)-4,6-d ideoxy-2-O- methyl-alpha-D-mannopyranosyl-(1-->2)-3-O-acetyl-4- (2,4-di-O-acetyl-3-deoxy-L-glycero-tetronamido)-4,6-dideoxy-alpha- D- mannopyranosyl chloride (12), was obtained by condensation of 3 with the 1-O-acetyl analog of 4, followed by treatment of the disaccharide formed with DCMME-ZnCl2. The synthesis of the methyl alpha-glycoside of the title trisaccharide involved a condensation of 12 with 4, followed by deacetylation. Similarly, the condensation of 12 with 15, the latter being the analog of 5 having a free HO-2, followed by deacetylation, gave the methyl alpha-glycoside of the title tetrasaccharide. All glycosylation reactions were mediated by silver trifluoromethanesulfonate in the presence of 2,4,6-trimethylpyridine. 4-(3-Deoxy-L-glycero-tetronamido)-4,6-dideoxy-2-O-methyl-alpha,bet a-D- mannopyranose was prepared for the first time. It was characterized by NMR spectroscopy, and via its crystalline per-O-acetyl derivative. It is the saccharide whose alpha-form constitutes the terminal, non-reducing end-group of the O-PS of V. cholerea O:1, serotype Ogawa.

Synthesis of the methyl alpha-glycoside of a trisaccharide mimicking the terminus of the O antigen of Vibrio cholerae O:1, serotype Inaba

Coupling of methyl 4-amino-4,6-dideoxy-2-O-4-methoxybenzyl-alpha-D-mannopyranoside, obtained from the corresponding 4-azido derivative by treatment with H2S, with 3-deoxy-L-glycero-tetronolactone gave the crystalline methyl 4-(3-deoxy-L-glycero-tetronamido)-4,6-dideoxy-2-O-4-methoxybenzyl- alpha-D- mannopyranoside (7). Subsequent acetylation of 7, followed by O-demethoxybenzylation of the 8 formed gave the crystalline methyl 3-O-acetyl-4,6-dideoxy-4-(2,4-di-O-acetyl-3-deoxy-L-glycero-tetronami do)-alpha- D-mannopyranoside (9), which was used as the key intermediate in the construction of the title trisaccharide. To make a glycosyl donor allowing the extension of the oligosaccharide chain at O-2, compound 9 was converted, via conventional transformations, into 3-O-acetyl-2-O-bromoacetyl-4,6-dideoxy-4-(2,4-di-O-acetyl-3-deoxy-L-glyc ero- tetronamido)-alpha-D-mannopyranosyl chloride (12). Condensation of 12 with 9 afforded the disaccharide 20 having a selectively removable protecting group at O-2(2). The latter was O-debromoacetylated, and the disaccharide nucleophile thus obtained was treated with 2,3-di-O-acetyl-4,6-dideoxy-4-(2,4-di-O-acetyl-3-deoxy-L-glycero-tetr onamido)- alpha-D-mannopyranosyl chloride to give, after O-deacetylation, the target, title trisaccharide. The constituent monosaccharide of the O-specific polysaccharide antigen of Vibrio cholerae serotype Inaba, 4-(3-deoxy-L-glycero-tetronamido)-4,6-dideoxy-D-mannopyranose (18), was obtained from the peracetate of its methyl alpha-glycoside by acetolysis, followed by O-deacetylation. The amorphous compound 18 was characterized by 1H and 13C NMR spectroscopy and through its crystalline alpha-per-O-acetyl derivative.