Semisynthetic Glycoconjugate Vaccine Candidates against Escherichia coli O25B Induce Functional IgG Antibodies in Mice.

Extraintestinal pathogenic Escherichia coli (ExPEC) is a major health concern due to emerging antibiotic resistance. Along with O1A, O2, and O6A, E. coli O25B is a major serotype within the ExPEC group, which expresses a unique O-antigen. Clinical studies with a glycoconjugate vaccine of the above-mentioned O-types revealed O25B as the least immunogenic component, inducing relatively weak IgG titers. To evaluate the immunological properties of semisynthetic glycoconjugate vaccine candidates against E. coli O25B, we here report the chemical synthesis of an initial set of five O25B glycan antigens differing in length, from one to three repeat units, and frameshifts of the repeat unit. The oligosaccharide antigens were conjugated to the carrier protein CRM197. The resulting semisynthetic glycoconjugates induced functional IgG antibodies in mice with opsonophagocytic activity against E. coli O25B. Three of the oligosaccharide-CRM197 conjugates elicited functional IgGs in the same order of magnitude as a conventional CRM197 glycoconjugate prepared with native O25B O-antigen and therefore represent promising vaccine candidates for further investigation. Binding studies with two monoclonal antibodies (mAbs) revealed nanomolar anti-O25B IgG responses with nanomolar K D values and with varying binding epitopes. The immunogenicity and mAb binding data now allow for the rational design of additional synthetic antigens for future preclinical studies, with expected further improvements in the functional antibody responses. Moreover, acetylation of a rhamnose residue was shown to be likely dispensable for immunogenicity, as a deacylated antigen was able to elicit strong functional IgG responses. Our findings strongly support the feasibility of a semisynthetic glycoconjugate vaccine against E. coli O25B.

Serendipitous one-pot synthesis of chiral dienes from pyranosidic 2,4-bistriflates.

Attempted nucleophilic displacements of l-rhamnosyl 2,4-bistriflates led to serendipitous formation of a chiral diene via competing cascade eliminations. The reaction also followed the same pathway with d-rhamnosyl and d-mannosyl 2,4-bistriflates substrates providing access to dienes with opposite stereochemistry. The reaction presumably proceeds through E2 elimination of C2 triflate followed by allylic rearrangement. The easily accessible chiral dienes would be useful in the synthesis of natural products.

Chimeric oligosaccharide conjugate induces opsonic antibodies against Streptococcus pneumoniae serotypes 19A and 19F.

Streptococcus pneumoniae 19A (ST19A) and 19F (ST19F) are among the prevalent serotypes causing pneumococcal disease worldwide even after introduction of a 13-valent pneumococcal conjugate vaccine (PCV13). Synthetic glycoconjugate vaccines have defined chemical structures rather than isolated polysaccharide mixtures utilized in marketed vaccines. Ideally, a minimal number of synthetic antigens would cover as many bacterial serotypes to lower cost of goods and minimize the response to carrier proteins. To demonstrate that a chimeric oligosaccharide antigen can induce a protective immune response against multiple serotypes, we synthesized a chimeric antigen (ST19AF) that is comprised of a repeating unit of ST19A and ST19F capsular polysaccharide each. Synthetic glycan epitopes representing only ST19A, and ST19F were prepared for comparison. Semisynthetic glycoconjugates containing chimeric antigen ST19AF induced high antibody titers able to recognize native CPS from ST19A and ST19F in rabbits. The antibodies were able to kill both strains of pneumococci. Chimeric antigens are an attractive means to induce an immune response against multiple bacterial serotypes.

From l-Rhamnose to Rare 6-Deoxy-l-Hexoses.

Efficient and rapid transformation of cheaply available l-rhamnose into all the isomeric 6-deoxy-l-hexoses via regio- and stereoselective nucleophilic displacements of triflates is reported. The synthesis entails regioselective protections, one-pot double displacements of triflates, and cascade inversions. The methodology allows facile access to all the rare 6-deoxy-l-hexoses as stable thioglycoside building blocks.

Expedient Route To Access Rare Deoxy Amino l-Sugar Building Blocks for the Assembly of Bacterial Glycoconjugates.

Bacterial glycoproteins and oligosaccharides contain several rare deoxy amino l-sugars which are virtually absent in the human cells. This structural difference between the bacterial and host cell surface glycans can be exploited for the development of carbohydrate based vaccines and target specific drugs. However, the unusual deoxy amino l-sugars present in the bacterial glycoconjugates are not available from natural sources. Thus, procurement of orthogonally protected rare l-sugar building blocks through efficient chemical synthesis is a crucial step toward the synthesis of structurally well-defined and homogeneous complex glycans. Herein, we report a general and expedient methodology to access a variety of unusual deoxy amino l-sugars starting from readily available l-rhamnose and l-fucose via highly regioselective, one-pot double serial and double parallel displacements of the corresponding 2,4-bistriflates using azide and nitrite anions as nucleophiles. Alternatively, regioselective monotriflation at O2, O3, and O4 of l-rhamnose/l-fucose allowed selective inversions at respective positions leading to diverse rare sugars. The orthogonally protected deoxy amino l-sugar building blocks could be stereoselectively assembled to obtain biologically relevant bacterial O-glycans, as exemplified by the first total synthesis of the amino linker-attached, conjugation-ready tetrasaccharide of O-PS of Yersinia enterocolitica O:50 strain 3229 and the trisaccharide of Pseudomonas chlororaphis subsp. aureofaciens strain M71.

Chemical synthesis of asparagine-linked archaeal N-glycan from Methanothermus fervidus.

Several N-linked glycoproteins have been identified in archaea and there is growing evidence that the N-glycan is involved in survival and functioning of archaea in extreme conditions. Chemical synthesis of the archaeal N-glycans represents a crucial step towards understanding the putative function of protein glycosylation in archaea. Herein the first total synthesis of the archaeal L-asparagine linked hexasaccharide from Methanothermus fervidus is reported using a highly convergent [3+3] glycosylation approach in high overall yields. The synthesis relies on efficient preparation of regioselectively protected thioglycoside building blocks for orthogonal glycosylations and late stage N-aspartylation.