Chemical synthesis of the repeating unit of type Ia group B Streptococcus capsular polysaccharide

Zinc(ii)-mediated stereoselective construction of 1,2-cis 2-azido-2-deoxy glycosidic linkage: assembly of Acinetobacter baumannii K48 capsular pentasaccharide derivative

The capsular polysaccharide (CPS) is a major virulence factor of the pathogenic Acinetobacter baumannii and a promising target for vaccine development. However, the synthesis of the 1,2-cis-2-amino-2-deoxyglycoside core of CPS remains challenging to date. Here we develop a highly α-selective ZnI2-mediated 1,2-cis 2-azido-2-deoxy chemical glycosylation strategy using 2-azido-2-deoxy glucosyl donors equipped with various 4,6-O-tethered groups. Among them the tetraisopropyldisiloxane (TIPDS)-protected 2-azido-2-deoxy-d-glucosyl donor afforded predominantly α-glycoside (α : β = >20 : 1) in maximum yield. This novel approach applies to a wide acceptor substrate scope, including various aliphatic alcohols, sugar alcohols, and natural products. We demonstrated the versatility and effectiveness of this strategy by the synthesis of A. baumannii K48 capsular pentasaccharide repeating fragments, employing the developed reaction as the key step for constructing the 1,2-cis 2-azido-2-deoxy glycosidic linkage. The reaction mechanism was explored with combined experimental variable-temperature NMR (VT-NMR) studies and mass spectroscopy (MS) analysis, and theoretical density functional theory calculations, which suggested the formation of covalent α-C1GlcN-iodide intermediate in equilibrium with separated oxocarbenium-counter ion pair, followed by an SN1-like α-nucleophilic attack most likely from separated ion pairs by the ZnI2-activated acceptor complex under the influence of the 2-azido gauche effect.

Convergent Synthesis of a Group B Streptococcus Type III Epitope Toward a Semisynthetic Carbohydrate-Based Vaccine

In this work, we synthesized an hexasaccharide derived from the capsular polysaccharide of group B Streptococcus type III capsular polysaccharide. Our convergent 3 + 3 strategy avoided the use of benzyl protecting groups allowing the installation of an azide anchoring group and providing a high yield for the final deprotection steps. Moreover, the minimal hexasaccharidic epitope was conjugated to CRM197 and BSA via copper-catalyzed azide-alkyne cycloaddition for the preparation of a semisynthetic carbohydrate-based vaccine.

Stereoselective Synthesis of the O-antigen of A. baumannii ATCC 17961 Using Long-Range Levulinoyl Group Participation

Herein, we described the first synthesis of the pentasaccharide and decasaccharide of the A. baumannii ATCC 17961 O-antigen for developing a synthetic carbohydrate-based vaccine against A. baumannii infection. The efficient synthesis of the rare sugar 2,3-diacetamido-glucuronate was achieved using our recently introduced organocatalytic glycosylation method. We found, for the first time, that long-range levulinoyl group participation via a hydrogen bond can result in a significantly improved β-selectivity in glycosylations. This solves the stereoselectivity problem of highly branched galactose acceptors. The proposed mechanism was supported by control experiments and DFT computations. Benefiting from the long-range levulinoyl group participation strategy, the pentasaccharide donor and acceptor were obtained via an efficient [2+1+2] one-pot glycosylation method and were used for the target decasaccharide synthesis.

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.

Chemical Synthesis and Immunological Evaluation of Fragments of the Multiantennary Group-Specific Polysaccharide of Group B Streptococcus

Group B Streptococcus (GBS) is a Gram-positive bacterium and the most common cause of neonatal blood and brain infections. At least 10 different serotypes exist, that are characterized by their different capsular polysaccharides. The Group B carbohydrate (GBC) is shared by all serotypes and therefore attractive be used in a glycoconjugate vaccine. The GBC is a highly complex multiantennary structure, composed of rhamnose rich oligosaccharides interspaced with glucitol phosphates. We here report the development of a convergent approach to assemble a pentamer, octamer, and tridecamer fragment of the termini of the antennae. Phosphoramidite chemistry was used to fuse the pentamer and octamer fragments to deliver the 13-mer GBC oligosaccharide. Nuclear magnetic resonance spectroscopy of the generated fragments confirmed the structures of the naturally occurring polysaccharide. The fragments were used to generate model glycoconjugate vaccine by coupling with CRM197. Immunization of mice delivered sera that was shown to be capable of recognizing different GBS strains. The antibodies raised using the 13-mer conjugate were shown to recognize the bacteria best and the serum raised against this GBC fragment-mediated opsonophagocytic killing best, but in a capsule dependent manner. Overall, the GBC 13-mer was identified to be a highly promising antigen for incorporation into future (multicomponent) anti-GBS vaccines.

Synthetic Glycans to Improve Current Glycoconjugate Vaccines and Fight Antimicrobial Resistance

Antimicrobial resistance (AMR) is emerging as the next potential pandemic. Different microorganisms, including the bacteria Acinetobacter baumannii, Clostridioides difficile, Escherichia coli, Enterococcus faecium, Klebsiella pneumoniae, Neisseria gonorrhoeae, Pseudomonas aeruginosa, non-typhoidal Salmonella, and Staphylococcus aureus, and the fungus Candida auris, have been identified by the WHO and CDC as urgent or serious AMR threats. Others, such as group A and B Streptococci, are classified as concerning threats. Glycoconjugate vaccines have been demonstrated to be an efficacious and cost-effective measure to combat infections against Haemophilus influenzae, Neisseria meningitis, Streptococcus pneumoniae, and, more recently, Salmonella typhi. Recent times have seen enormous progress in methodologies for the assembly of complex glycans and glycoconjugates, with developments in synthetic, chemoenzymatic, and glycoengineering methodologies. This review analyzes the advancement of glycoconjugate vaccines based on synthetic carbohydrates to improve existing vaccines and identify novel candidates to combat AMR. Through this literature survey we built an overview of structure-immunogenicity relationships from available data and identify gaps and areas for further research to better exploit the peculiar role of carbohydrates as vaccine targets and create the next generation of synthetic carbohydrate-based vaccines.

Total Synthesis of the Tetrasaccharide Haptens of Vibrio vulnificus MO6-24 and BO62316 and Immunological Evaluation of Their Protein Conjugates

Vibrio vulnificus is a human pathogen that can cause fatal septicemia and necrotizing infections with a high lethal rate exceeding 50%. V. vulnificus MO6-24 and BO62316 are two predominant virulent strains associated with approximately one-third of the clinical infections. The capsular polysaccharides (CPSs) of V. vulnificus consist of several structurally unique sugars and are excellent targets for developing effective glycoconjugate vaccines. This article describes the first total synthesis of the challenging tetrasaccharide repeating units of V. vulnificus MO6-24 and BO62316 CPSs. A key feature of this synthesis was the assembly of the tetrasaccharide skeleton using a 3,4-branched trisaccharide as the glycosyl donor. A modified TEMPO/BAIB oxidation protocol was developed to directly convert α-d-GalN into α-d-GalAN in not only disaccharides but also tri- and tetrasaccharides. The synthetic haptens were covalently coupled with CRM197 carrier protein via a bifunctional linker. Preliminary immunological studies of the resultant glycoconjugates in mice revealed their high efficacy to induce robust T-cell-dependent immune responses, and the IgG antibodies elicited by each glycoconjugate showed weak cross-reactivity with the other synthetic tetrasaccharide.

Recent chemical syntheses of bacteria related oligosaccharides using modern expeditious approaches

Apart from some essential and crucial roles in life processes carbohydrates also are involved in a few detrimental courses of action related to human health, like infections by pathogenic microbes, cancer metastasis, transplanted tissue rejection, etc. Regarding management of pathogenesis by microbes, keeping in mind of multi drug-resistant bacteria and epidemic or endemic incidents, preventive measure by vaccination is the best pathway as also recommended by the WHO; by vaccination, eradication of bacterial diseases is also possible. Although some valid vaccines based on attenuated bacterial cells or isolated pure polysaccharide-antigens or the corresponding conjugates thereof are available in the market for prevention of several bacterial diseases, but these are not devoid of some disadvantages also. In order to develop improved conjugate T-cell dependent vaccines oligosaccharides related to bacterial antigens are synthesized and converted to the corresponding carrier protein conjugates. Marketed Cuban Quimi-Hib is such a vaccine being used since 2004 to resist Haemophilus influenza b infections. During nearly the past two decades research is going on worldwide for improved synthesis of bacteria related oligosaccharides or polysaccharides towards development of such semisynthetic or synthetic glycoconjugate vaccines. The present dissertation is an endeavour to encompass the recent syntheses of several pathogenic bacterial oligosaccharides or polysaccharides, made during the past ten-eleven years with special reference to modern expeditious syntheses.

Synthesis of N-Acetyllactosamine and N-Acetyllactosamine-Based Bioactives

N-Acetyllactosamine (LacNAc) or more specifically β-d-galactopyranosyl-1,4-N-acetyl-d-glucosamine is a unique acyl-amino sugar and a key structural unit in human milk oligosaccharides, an antigen component of many glycoproteins, and an antiviral active component for the development of effective drugs against viruses. LacNAc is useful itself and as a basic building block for producing various bioactive oligosaccharides, notably because this synthesis may be used to add value to dairy lactose. Despite a significant amount of information in the literature on the benefits, structures, and types of different LacNAc-derived oligosaccharides, knowledge about their effective synthesis for large-scale production is still in its infancy. This work provides a comprehensive analysis of existing production strategies for LacNAc and important LacNAc-based structures, including sialylated LacNAc as well as poly- and oligo-LacNAc. We conclude that direct extraction from milk is too complex, while chemical synthesis is also impractical at an industrial scale. Microbial routes have application when multiple step reactions are needed, but the major route to large-scale biochemical production will likely lie with enzymatic routes, particularly those using β-galactosidases (for LacNAc synthesis), sialidases (for sialylated LacNAc synthesis), and β-N-acetylhexosaminidases (for oligo-LacNAc synthesis). Glycosyltransferases, especially for the biosynthesis of extended complex LacNAc structures, could also play a major role in the future. In these cases, immobilization of the enzyme can increase stability and reduce cost. Processing parameters, such as substrate concentration and purity, acceptor/donor ratio, water activity, and temperature, can affect product selectivity and yield. More work is needed to optimize these reaction parameters and in the development of robust, thermally stable enzymes to facilitate commercial production of these important bioactive substances.

Glycoconjugate vaccines: some observations on carrier and production methods

Glycoconjugate vaccines use protein carriers to improve the immune response to polysaccharide antigens. The protein component allows the vaccine to interact with T cells, providing a stronger and longer-lasting immune response than a polysaccharide interacting with B cells alone. Whilst in theory the mere presence of a protein component in a vaccine should be sufficient to improve vaccine efficacy, the extent of improvement varies. In the present review, a comparison of the performances of vaccines developed with and without a protein carrier are presented. The usefulness of analytical tools for macromolecular integrity assays, in particular nuclear magnetic resonance, circular dichroism, analytical ultracentrifugation and SEC coupled to multi-angle light scattering (MALS) is indicated. Although we focus mainly on bacterial capsular polysaccharide-protein vaccines, some consideration is also given to research on experimental cancer vaccines using zwitterionic polysaccharides which, unusually for polysaccharides, are able to invoke T-cell responses and have been used in the development of potential all-polysaccharide-based cancer vaccines.A general trend of improved immunogenicity for glycoconjugate vaccines is described. Since the immunogenicity of a vaccine will also depend on carrier protein type and the way in which it has been linked to polysaccharide, the effects of different carrier proteins and production methods are also reviewed. We suggest that, in general, there is no single best carrier for use in glycoconjugate vaccines. This indicates that the choice of carrier protein is optimally made on a case-by-case basis, based on what generates the best immune response and can be produced safely in each individual case.Abbreviations: AUC: analytical ultracentrifugation; BSA: bovine serum albumin; CD: circular dichroism spectroscopy; CPS: capsular polysaccharide; CRM197: Cross Reactive Material 197; DT: diphtheria toxoid; Hib: Haemophilius influenzae type b; MALS: multi-angle light scattering; Men: Neisseria menigitidis; MHC-II: major histocompatibility complex class II; NMR: nuclear magnetic resonance spectroscopy; OMP: outer membrane protein; PRP: polyribosyl ribitol phosphate; PSA: Polysaccharide A1; Sa: Salmonella; St.: Streptococcus; SEC: size exclusion chromatography; Sta: Staphylococcus; TT: tetanus toxoid; ZPS: zwitterionic polysaccharide(s).

Regioselective Glycosylation Strategies for the Synthesis of Group Ia and Ib Streptococcus Related Glycans Enable Elucidating Unique Conformations of the Capsular Polysaccharides

Group B Streptococcus serotypes Ia and Ib capsular polysaccharides are key targets for vaccine development. In spite of their immunospecifity these polysaccharides share high structural similarity. Both are composed of the same monosaccharide residues and differ only in the connection of the Neu5Acα2-3Gal side chain to the GlcNAc unit, which is a β1-4 linkage in serotype Ia and a β1-3 linkage in serotype Ib. The development of efficient regioselective routes for GlcNAcβ1-3[Glcβ1-4]Gal synthons is described, which give access to different group B Streptococcus (GBS) Ia and Ib repeating unit frameshifts. These glycans were used to probe the conformation and molecular dynamics of the two polysaccharides, highlighting the different presentation of the protruding Neu5Acα2-3Gal moieties on the polysaccharide backbones and a higher flexibility of Ib polymer relative to Ia, which can impact epitope exposure.

Chemical Synthesis of Human Milk Oligosaccharides: Lacto-N-hexaose Galβ1→3GlcNAcβ1→3 [Galβ1→4GlcNAcβ1→6] Galβ1→4Glc

The first synthesis of lacto-N-hexaose (LNH) has been completed using a convergent strategy. The donor-acceptor protecting-leaving group combinations were found to be of paramount significance for achieving successful glycosylations and minimizing side reactions. Lacto-N-tetraose, another common human milk oligosaccharide, was also obtained en route to the target LNH.

The chemical synthesis of human milk oligosaccharides: Lacto-N-neotetraose (Galβ1→4GlcNAcβ1→3Galβ1→4Glc)

The discovery of innovative methods that offer new capabilities for obtaining individual oligosaccharides from human milk will help to improve understanding their roles and boost practical applications. The total chemical synthesis of lacto-N-neotetraose (LNnT) has been completed using both linear and convergent strategies. The donor and acceptor protecting and leaving group combinations were found to be of paramount significance to successful couplings.

Synthesis of a dimer of the repeating unit of type Ia Group B Streptococcus extracellular capsular polysaccharide and immunological evaluations of related protein conjugates

Type Ia group B Streptococcus (GBS) is one of the major causes of fatal infections in neonates. Its extracellular capsular polysaccharide (CPS) is a useful target for the development of anti-type Ia GBS vaccines. To explore the structure-activity relationships of type Ia GBS CPS and design more effective vaccines, a dimer of the branched pentasaccharide repeating unit of this CPS was synthesized by a highly convergent strategy highlighted by constructing the key intermediate via one-pot iterative glycosylation and imposing two side chains in one step via dual glycosylation. This represented the first total synthesis of a dimer of the repeating unit of any GBS CPS reported so far and the strategy should be applicable to higher oligomers of this repeating unit. The synthetic dimer and its monomeric analog were coupled with CRM197 carrier protein to generate conjugates that were evaluated in mice. Immunological results revealed that both carbohydrate antigens could induce robust total and IgG antibody responses and the elicited antibodies were cross-reactive with both carbohydrate antigens. It was concluded that both the monomeric and the dimeric repeating units may be employed as haptens for anti-type Ia GBS vaccine development.

Chemical synthesis of the dimeric repeating unit of type Ia group BStreptococcuscapsular polysaccharide

The first synthesis of the dimeric repeating unit of type Ia GBS CPS containing two sialotrisaccharide side chains and adjacent 3,4-di-branched Gal motifs was achieved.

The use of the novel glycosyl acceptor and supramer analysis in the synthesis of sialyl-α(2-3)-galactose building block

A new glycosyl acceptor to be used in sialylation was designed as a 3-hydroxy derivative of 4-methoxyphenyl β-d-galactopyranoside with 2-O-acetyl group and O-4 and O-6 protected as benzylidene acetal. Two alternative syntheses of this compound were compared. Sialylation of 3-OH group of the glycosyl acceptor with O-chloroacetylated N-trifluoroacetylneuraminic acid phenyl thioglycoside (NIS, TfOH, MeCN, MS 3 Å, -40 °C) was studied in a wide concentration range (5-150 mmol L^-1). The outcome of sialylation generally followed the predictions of supramer analysis of solutions of sialyl donor in MeCN, which was performed by polarimetry and static light scattering and revealed two concentration ranges differing in solution structure and the structures of supramers of glycosyl donor. The optimized conditions of sialylation (C = 50 mmol L^-1) were used to synthesize protected Neu-α(2-3)-Gal disaccharide (78%, α:β = 13:1), which was then converted to sialyl-α(2-3)-galactose imidate building block useful for the synthesis of complex sialo-oligosaccharides.

Recent Advances in the Synthesis of Glycoconjugates for Vaccine Development

During the last decade there has been a growing interest in glycoimmunology, a relatively new research field dealing with the specific interactions of carbohydrates with the immune system. Pathogens’ cell surfaces are covered by a thick layer of oligo- and polysaccharides that are crucial virulence factors, as they mediate receptors binding on host cells for initial adhesion and organism invasion. Since in most cases these saccharide structures are uniquely exposed on the pathogen surface, they represent attractive targets for vaccine design. Polysaccharides isolated from cell walls of microorganisms and chemically conjugated to immunogenic proteins have been used as antigens for vaccine development for a range of infectious diseases. However, several challenges are associated with carbohydrate antigens purified from natural sources, such as their difficult characterization and heterogeneous composition. Consequently, glycoconjugates with chemically well-defined structures, that are able to confer highly reproducible biological properties and a better safety profile, are at the forefront of vaccine development. Following on from our previous review on the subject, in the present account we specifically focus on the most recent advances in the synthesis and preliminary immunological evaluation of next generation glycoconjugate vaccines designed to target bacterial and fungal infections that have been reported in the literature since 2011.

Chemical Synthesis of Glycosides of N-Acetylneuraminic Acid

Investigations of methodologies aimed on improving the stereoselective synthesis of sialosides and the efficient assembly of sialic acid glycoconjugates has been the mission of dedicated research groups from the late 1960s. This review presents major accomplishments in the field, with the emphasis on significant breakthroughs and influential synthetic strategies of the last decade.

Synthesis of the Cancer-Associated KH-1 Antigen by Block Assembly of Its Backbone Structure Followed by One-Step Grafting of Three Fucose Residues

A robust, convergent, and efficient strategy was developed for the synthesis of the nonasaccharide cancer antigen KH-1. This strategy featured a one-pot block assembly of the linear hexasaccharide backbone using three disaccharides followed by grafting of three fucose residues onto the backbone in one step.

Synthesis and Evaluation of GM2-Monophosphoryl Lipid A Conjugate as a Fully Synthetic Self-Adjuvant Cancer Vaccine

An efficient method was developed for the synthesis of a GM2 derivative suitable for the conjugation with various biomolecules. This GM2 derivative was covalently linked to keyhole limpet hemocyanin (KLH) and monophosphoryl lipid A (MPLA) to form novel therapeutic cancer vaccines. Immunological evaluations of the resultant conjugates in mice revealed that they elicited robust GM2-specific overall and IgG antibody responses. Moreover, the GM2-MPLA conjugate was disclosed to elicit strong immune responses without the use of an adjuvant, proving its self-adjuvant property. The antisera of both conjugates showed strong binding and mediated similarly effective complement-dependent cytotoxicity to GM2-expressing cancer cell line MCF-7. Based on these results, it was concluded that both GM2-MPLA and GM2-KLH are promising candidates as therapeutic cancer vaccines, whereas fully synthetic GM2-MPLA, which has homogeneous and well-defined structure and self-adjuvant property, deserves more attention and studies.

Chemical Synthesis of the Repeating Unit of Type V Group B Streptococcus Capsular Polysaccharide

The first chemical synthesis of the heptasaccharide repeating unit of type V group B Streptococcus capsular polysaccharide (CPS) by a preactivation-based one-pot [2 + 1 + 4] glycosylation strategy is described. The synthetic efficiency was further improved by using diol glycosyl acceptors for regio-/stereoselective glycosylation reactions. The synthetic target contains a free amino group at the reducing end, facilitating additional regioselective elaboration. The results should be useful for synthetic and biological studies of related CPSs.