Bacterial polysaccharides as major surface antigens: interest in O-acetyl substitutions

International audiencePolysaccharides represent essential, although highly structurally diverse, components on microbial cell surfaces. They are the primary interface with the host and play critical roles in survival strategies. Acting as shields against environmental assaults, they are actively investigated as attractive vaccine components. Contributing to a tremendous structural diversity, a subtle but nonetheless essential microbial polysaccharide modification is O-acetylation. Focusing on bacterial capsular polysaccharides (CPS), this chapter provides some highlights on this widespread substitution. CPS O-acetylation is discussed first from a genetic and biochemical perspective, then in view of its implication in the host-pathogen crosstalk and ability to modulate CPS biological properties in a context-dependent manner. Lastly, the chapter addresses CPS O-acetylation in the context of antibacterial vaccine development

Exploratory N‐Protecting Group Manipulation for the Total Synthesis of Zwitterionic <i>Shigella sonnei</i> Oligosaccharides

International audienceShigella sonnei surface polysaccharides are well-established protective antigens against this major causeo fd iarrhoeal disease.T hey also qualify as unique zwitterionic polysaccharides (ZPSs)f eaturing ad isaccharide repeating unit made of two 1,2-trans linked rare aminodeoxy sugars, a 2-acetamido-2-deoxy-l-altruronic acid (l-AltpNAcA)a nd a2acetamido-4-amino-2,4,6-trideoxy-d-galactopyranose (AAT). Herein,t he stereoselective synthesis of S. sonnei oligosaccharides comprising two, three and four repeating units is reported for the first time. Severals ets of up to seven pro-tectingg roups wereexplored,s hedding light on the singular conformational behavior of protected altrosamine and altruronic residues. Ad isaccharide buildingb lock equipped with three distinct N-protectingg roups and featuring the uronate moiety already in place was designed to accomplish the iterative high yieldingg lycosylation at the axial 4-OH of the altruronate componenta nd achieve the challenging full deprotection step. Key to the successfulr oute was the use of a diacetyls trategy whereby the N-acetamido group of the l-AltpNAcAism asked in the form of an imide

Synthèse d'oligosaccharides bactériens et étude de leur antigénicité (l'exemple de Shigella flexneri sérotypes 3a et X)

La cible majeure de la réponse immunitaire protectrice contre Shigella flexneri, une entérobactérie Gram négatif, cause de la dysenterie bacillaire, est la composante antigène O (Ag-O) du lipopolysaccharide. Ce polymère est défini par un pentasaccharide ramifié mono-O-acétylé dans le cas de S. flexneri 3a, alors que l analogue non acétylé caractérise le sérotype X. Afin d identifier les déterminants saccharidiques cibles des anticorps protecteurs contre l infection par S. flexneri 3a, quatorze di- à hexasaccharides, mono-O-acétylés ou non, ont été synthétisés. Des stratégies de synthèse multi-étapes, linéaires et/ou convergentes, chimiques et/ou chimio-enzymatiques, optimisant compatibilité entre groupes protecteurs orthogonaux et conditions de glycosylation ont été développées. La contribution essentielle de la ramification a-D-glucose et de la fonction acétate à la reconnaissance par les anticorps a été mise en évidencePARIS-BIUP (751062107) / SudocSudocFranceF

En route vers des glycoconjugués à potentiel vaccinal contre la dysenterie bacillaire (synthèse d'oligosaccharides représentatifs de l'antigène O de Shigella flexneri sérotype 6)

Résumé français confidentielRésumé anglais confidentielPARIS5-Bibliotheque electronique (751069902) / SudocPARIS-BIUM-Bib. électronique (751069903) / SudocSudocFranceF

Toward a Multivalent Synthetic Oligosaccharide-Based Conjugate Vaccine against Shigella: State-of-the-Art for a Monovalent Prototype and Challenges

International audienceThis review focuses on the molecular glycovaccine concept, a promising option to develop a Shigella glycoconjugate vaccine. Subsequent to original developments involving, as main vaccine component, the detoxified Shigella lipopolysaccharide randomly conjugated at multiple sites to a carrier protein, novelty stems from the use of rationally designed, well-defined chemically synthesized oligosaccharide haptens conceived as functional surrogates of the main surface antigen, linked via single-point attachment onto a carrier. The concept and design of such a fine-tuned Shigella glycovaccine are presented by way of SF2a-TT15, a neoglycoprotein featuring a synthetic 15-mer oligosaccharide, which constitutes an original vaccine prototype targeting Shigella flexneri 2a, one of the predominant circulating strains in endemic settings. The clinical testing of SF2a-TT15 is summarized with the first-in-human phase I trial in young healthy adults showing a good safety profile and tolerability, while inducing bactericidal antibodies towards S. flexneri 2a bacteria. The proof-of-concept of this novel approach being established, an ongoing phase IIa clinical study in the nine-month-old infant target population in endemic area was launched, which is also outlined. Lastly, some challenges to move forward this original approach toward a multivalent cost-effective Shigella synthetic glycan conjugate vaccine are introduced

Convergent synthesis of the methyl glycosides of a tetra- and a pentasaccharide fragment of the Shigella flexneri serotype 2a O-specific polysaccharide

International audienceThe branched pentasaccharide, α-l-Rhap-(1→3)-[α-d-Glcp-(1→4)]-α-l-Rhap-(1→3)-β-d-GlcNAcp-(1→2)-α-l-Rhap (B(E)CDA) and the corresponding linear tetrasaccharide (ECDA), which are part of the Shigella flexneri serotype 2a O-antigen, were synthesized as their methyl glycosides according to a convergent strategy. The syntheses rely on the use of suitable B(E)C and EC trichloroacetimidate donors, respectively, and involve an appropriate DA acceptor which bears an isopropylidene acetal to block OH-4 and OH-6 of residue D. The preparation of the related linear CDA-OMe trisaccharide, which was used as a model, is also described.A convergent synthesis of the pentasaccharide B(E)CDA-OMe is described, together with those of the corresponding tri- and tetrasaccharide, CDA-OMe and ECDA-OMe, respectively

Synthesis of branched tri-to pentasaccharides representative of fragments of Shigella flexneri serotypes 3a and/or X O-antigens

International audienceFragments of the {2)-[α-d-Glcp-(1→3)]-α-l-Rhap-(1→2)-α-l-Rhap-(1→3)-[Ac→2]-α-l-Rhap-(1→3)-β-d-GlcpNAc-(1→}n ((E)ABAcCD)n polymer were synthesized. D(E)A, CD(E)A, AcCD(E)A were obtained according to a linear strategy, whereas BCD(E)A and BAcCD(E)A were derived from the condensation of appropriate BC and D(E)A building blocks. Oligosaccharides were synthesized as their propyl glycoside, relying on (i) the efficient trichloroacetimidate chemistry, (ii) a common EA allyl glycoside, and (iii) a 2-trichloroacetamido-d-glucopyranose precursor to residue D. Final Pd/C-mediated deprotection, run under a high pressure of hydrogen, ensured O-acetyl stability. All targets are parts of the O-antigen of Shigella flexneri 3a, a prevalent serotype. Non-O-acetylated oligosaccharides are shared by the S. flexneri serotype X O-antigen

Vaccination against shigellosis: is it the path that is difficult or is it the difficult that is the path?

International audienceFollowing several decades of research, there is not yet a convincing vaccine against shigellosis. It is still difficult, in spite of the breadth of strategies (i.e. live attenuated oral, killed oral, subunit parenteral) to select an optimal option. Two approaches are clearly emerging: (i) live attenuated deletion mutants based on rational selection of genes that are key in the pathogenic process, and (ii) conjugated detoxified polysaccharide parenteral vaccines, or more recently conjugated synthetic carbohydrates. Some of these approaches have already undergone phase I and II clinical trials with promising results, but important issues have also emerged, particularly the discrepancy between colonization and immunogenic potential of live attenuated vaccine candidates depending upon the population concerned (i.e. non endemic vs. endemic areas). Efforts are needed to definitely establish the proof of concept of these approaches, and thus the need for clinical trials which should also soon explore the possibility to associate different serotypes, in response to serotype specific protection against shigellosis. More basic research is also required to improve what we can still consider as first-generation vaccines, and to explore possible new paradigms including the search for cross-protective antigens

Efficient synthesis of six tri- to hexasaccharide fragments of Shigella flexneri serotypes 3a and/or X O-antigen, including a study on acceptors containing N-trichloroacetylglucosamine versus N-acetylglucosamine.

International audienceSix tri- to hexasaccharide fragments of the {2)-[alpha-D-Glcp-(1-->3)]-alpha-L-Rhap-(1-->2)-alpha-L-Rhap-(1-->3)-[Ac-->2]-alpha-L-Rhap-(1-->3)-beta-D-GlcpNAc-(1-->}(n) polymer ([(E)AB(Ac)CD](n)) were synthesized as their propyl glycosides. All targets share the (E)AB sequence. Following a thorough investigation on the use of N-trichloroacetylglucosamine- versus N-acetylglucosamine-containing tri- and tetrasaccharide acceptors, the successful strategy was based on an efficient combination of the trichloroacetimidate chemistry, a trichloroacetyl used as permanent N-protection, and an allyl aglycon as temporary and/or permanent anomeric protection of selected building blocks. Use of an EAB intermediate orthogonally protected at 2(A) provided both the trisaccharide target and acceptor 12, the condensation of which with a chain terminator D followed by full deprotection, gave tetrasaccharide D(E)AB. Alternatively, stepwise glycosylation of 12 with a D donor compatible with a selective deblocking at position 3(D) and a 2-O-acetyl C donor following exposure of OH-3(D) led to a pentasaccharide, which was partially and fully deprotected into free (Ac)CD(E)AB and CD(E)AB, respectively. Furthermore, chain elongation of the common D(E)AB acceptor with a 2(B)-O-levulinoyl rhamnobiose donor BC and subsequent partial or total deprotection of the resulting hexasaccharide provided B(Ac)CD(E)AB and BCD(E)AB, respectively. All of the synthesized oligosaccharides are parts of the O-antigen of Shigella flexneri 3a, a prevalent serotype. Moreover, the non-O-acetylated fragments are also parts of the S. flexneri serotype X O-antigen