Synthetic Polysaccharide Type 3-Related Di-, Tri-, and Tetrasaccharide–CRM 197 Conjugates Induce Protection against Streptococcus pneumoniae Type 3 in Mice

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Abstract
Di-, tri-, and tetrasaccharides, synthesized according to the chemical structure of pneumococcal polysaccharide type 3 (PS3), were coupled to the cross-reactive material (CRM197) of modified diphtheria toxin in different molar carbohydrate/protein ratios using the squarate coupling method. To study protective immunity, female BALB/c mice were subcutaneously immunized twice (with a 3-week interval) using the amount of conjugates corresponding to 2.5 μg of oligosaccharide per mouse. The conjugates evoked PS3 binding immunoglobulin G antibodies that lasted for at least 7 weeks after the booster. Immunogenicity was not influenced by the carbohydrate/protein ratio. All mice with PS3-specific antibodies survived the intraperitoneal challenge with Streptococcus pneumoniaetype 3. Therefore, synthetic oligosaccharide-protein conjugates might have potential as vaccines.

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