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

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

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

Synthesis of the Zwitterionic Repeating Unit of the O‑Antigen from <i>Shigella sonnei</i> and Chain Elongation at Both Ends

<i>Shigella sonnei</i> O-antigen features a zwitterionic disaccharide repeat encompassing two rare monosaccharides. The synthesis of the <b>AB</b> repeat and of trisaccharides <b>ABA</b>′ and <b>B</b>′<b>AB</b>, which validates chain elongation at either end, is reported. All targets were synthesized using a postglycosylation oxidation strategy in combination with imidate chemistry. Precursors to residue <b>A</b> were obtained from l-glucose. The AAT (<b>B</b>) donor and acceptor were obtained from d-glucosamine. A one-step Pd­(OH)₂/C-mediated deprotection provided the propyl glycoside targets

Preparation of synthetic glycoconjugates as potential vaccines against Shigella flexneri serotype 2a disease.

International audienceThe synthesis of three neoglycopeptides incorporating carbohydrate haptens, differing in length, covalently linked to a non natural universal T helper peptide is disclosed. They were synthesized according to a blockwise strategy based on the condensation of appropriate di-, tri-, and tetrasaccharide trichloroacetimidate donors onto an azidoethyl 2-acetamido-2-deoxybeta-D-glucopyranoside acceptor. Use of thiol-maleimide coupling chemistry allowed site-selective efficient conjugation

Synthesis and NMR study of a linear pentasaccharide fragment of the Shigella Flexneri 5a O-specific polysaccharide

International audienceA convergent chemical synthesis of the methyl glycoside of the linear epitope α-d-Glcp-(1→3)-α-l-Rhap-(1→3)-α-l-Rhap-(1→3)-β-d-GlcNAcp-(1→2)-α-l-Rhap (EBCDA) corresponding to the ramification of the O-antigen of Shigella flexneri serotype 5a is described. The strategy relies on the preparation of a key EB trichloroacetimidate donor and that of an appropriate CDA trisaccharide acceptor. Trichloroacetimidate chemistry was used for the construction of all glycosidic linkages except that of DA, where a bromide donor was preferred. In depth analysis of the pentasaccharide EBCDA 1H and 13C NMR spectra shows that its conformation approaches that of the corresponding fragment in the native polysaccharide.A convergent synthesis of pentasaccharide 1 is described. NMR analysis shows that its conformation approaches that of the related fragment in the native polysaccharide

Investigation towards bivalent chemically defined glycoconjugate immunogens prepared from acid-detoxified lipopolysaccharide of Vibrio cholerae O1, serotype Inaba.

International audienceA free amino group present on the acid-detoxified lipopolysaccharide (pmLPS) of V. cholerae O1 serotype Inaba was investigated for site-specific conjugation. Chemoselective pmLPS biotinylation afforded the corresponding mono-functionalized derivative, which retained antigenicity. Thus, pmLPS was bound to carrier proteins using thioether conjugation chemistry. Induction of an anti-LPS antibody (Ab) response in BALB/c mice was observed for all conjugates. Interestingly, the sera had vibriocidal activity against both Ogawa and Inaba strains opening the way to a possible bivalent vaccine. However, the level of this Ab response was strongly affected by both the nature of the linker and of the carrier. Furthermore, no switch from IgM to IgG, i.e. from a T cell-independent to a T cell-dependent immune response was detected, a result tentatively explained by the possible presence of free polysaccharide in the formulation. Taken together, these results encourage further investigation towards the development of potent pmLPS-based neoglycoconjugate immunogens, fully aware of the challenge faced in the development of a cholera vaccine that will provide efficient serogroup coverage

Programmed chemo-enzymatic synthesis of the oligosaccharide component of a carbohydrate-based antibacterial vaccine candidate

The powerful chemo-enzymatic synthesis of the pentadecasaccharide hapten involved in the first synthetic carbohydrate-based vaccine candidate against endemic shigellosis is reported. The high yielding site-selective alpha-D-glucosylation of a lightly protected disaccharide by an engineered transglucosylase-sucrose system gave a trisaccharide, which was chemically elongated by an efficient [5+5] process