Synthetic polysaccharide type 3-related di-, tri-, and tetrasaccharide-CRM197 conjugates induce protection against Streptococcus pneumoniae type 3 in mice

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 mug 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 pneumoniae type 3, Therefore, synthetic oligosaccharide-protein conjugates might have potential as vaccines.