Shigellosis is a major enteric disease worldwide causing diarrhea and dysentery, an acute invasive disease of the lower intestines in many parts of the world with a very low infectious dose (approximately 100 bacteria). Because of the multiple-antibiotic resistance of many strains of Shigellae, the treatment of shigellosis is difficult and prevention by vaccination should be an alternative to control this disease. We hypothesized that extended fragments of the O-specific polysaccharide (O-SP) components of the lipopolysaccharides of Shigellae may be suitable for the induction of protective antibodies when covalently coupled to immunogenic proteins. The use of synthetic saccharides of defined structure instead of polysaccharides of highly complex architecture offers numerous advantages including enhanced uniformity of conjugates, the possibility of using saccharides with pre-determined termini on the saccharide chain, and elimination of much of the analytical difficulties associated with the polysaccharide-protein vaccines. In addition, the possible dangers inherent with materials of biological origin are also minimized. Based on this hypothesis we are developing synthetic oligosaccharide-based immunogens of well-defined molecular characteristics. We designed a synthetic strategy to prepare oligosaccharide fragments of the O-SP of S. dysenteriae type 1. The O-SP consists of a tetrasaccharide repeating unit that is composed of D-galactose, N-acetyl-D-glucosamine, and L-rhamnose. Starting from monosaccharides that carry orthogonal protecting and activating groups, a tetrasaccharide donor/acceptor molecule was assembled. Iterative combination of the building blocks afforded di-, tri- and tetramers of the repeating unit corresponding to octa-, dodeca- and hexadeca-saccharide portions of the native O-SP. These oligo/polysaccharides are equipped with an w-hydrazinocarbonylpentyl aglycon that can be used to anchor the saccharides to proteins and other macromolecules. A new heterobifunctional linker molecule was developed and used to attach the saccharides to proteins under mild conditions.
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