No abstract

This review describes the developments in the synthesis of teichoic acids (TA) - glycosylated poly(alditolphosphates) - and the application of these fragments in immunological studies. These structurally diverse biopolymers are omnipresent constituents of the Gram-positive bacterial cell wall where they fulfill a variety of vital functions. They have been and continue to be attractive synthetic targets because of their challenging structures and the fact that their microheterogeneity precludes their isolation in single and pure enough form from natural sources. Progress in glycosylation chemistry and the development of effective phosphorylation chemistry has driven TA synthesis over the years, and highly complex and large TA structures can now reliably be targeted. Starting from the first TA synthesis in 1981, this review highlights the progress made in the field over the years. The synthesized TA fragments have been used to unravel their role in immunology and it is described how focused libraries of TAs have been used to discover the active principles of the TA polymers that interact with the innate immune system. Recently, synthetic TA fragments have also found applications as well-defined synthetic antigens for the generation of novel vaccine modalities to combat Gram-positive bacterial infections. It is foreseen that synthetic TA fragments will be valuable tools in the future to unravel the mode of action of these biomolecules at the molecular level. They will be instrumental in discovering and characterizing their designated biological binding partners, be it pattern recognition receptors or carbohydrate binding lectins or biomachinery enzymes. This review thus serves to showcase the potential of organic synthesis for (chemical) biology and immunology.

Lipoteichoic acids (LTA) are amphiphilic polymers that are important constituents of the cell wall of many Gram-positive bacteria. The chemical structures of LTA vary among organisms, albeit in the majority of Gram-positive bacteria the LTAs feature a common poly-1,3-(glycerolphosphate) backbone. Previously, the specificity of opsonic antibodies for this backbone present in some Gram-positive bacteria has been demonstrated, suggesting that this minimal structure may be sufficient for vaccine development. In the present work, we studied a well-defined synthetic LTA-fragment, which is able to inhibit opsonic killing of polyclonal rabbit sera raised against native LTA from Enterococcus faecalis 12030. This promising compound was conjugated with BSA and used to raise rabbit polyclonal antibodies. Subsequently, the opsonic activity of this serum was tested in an opsonophagocytic assay and specificity was confirmed by an opsonophagocytic inhibition assay. The conjugated LTA-fragment was able to induce specific opsonic antibodies that mediate killing of the clinical strains E. faecalis 12030, Enterococcus faecium E1162, and community-acquired Staphylococcus aureus strain MW2 (USA400). Prophylactic immunization with the teichoic acid conjugate and with the rabbit serum raised against this compound was evaluated in active and passive immunization studies in mice, and in an enterococcal endocarditis rat model. In all animal models, a statistically significant reduction of colony counts was observed indicating that the novel synthetic LTA-fragment conjugate is a promising vaccine candidate for active or passive immunotherapy against E. faecalis and other Gram-positive bacteria.

The glycosylation properties of gulopyranosides have been mapped out, and it is shown that gulose has an intrinsic preference for the formation of 1,2-cis-glycosidic bonds. It is postulated that this glycosylation behaviour originates from nucleophilic attack at the oxacarbenium ion, which adopts the most favourable 3H4 conformation. Building on the stereoselectivity of gulose, a guluronic acid alginate trisaccharide was assembled for the first time by using gulopyranosyl building blocks.

This communication describes the first automated solid phase synthesis of teichoic acids (TAs) and the preparation by this method of a number of well-defined TA structures, which were probed for their antigenicity. An opsonophagocytic killing assay revealed a clear TA-length-activity relationship and indicated a promising candidate for future vaccine development.