Serotype-specific quantification of residual free polysaccharide in multivalent pneumococcal conjugate vaccines

The Streptococcus pneumoniae bacteria has over 100 known serotypes that display a continuous change in prevalence by patients' age and geographical location and therefore necessitate continued efforts toward development of new vaccines with broader protection. Glycoconjugate vaccines have been instrumental in reducing global morbidity and mortality caused by Streptococcus pneumoniae infections. In these vaccines, the bacterial polysaccharide is conjugated to a carrier protein to enhance immunogenicity. To ensure well defined immunogenicity and stability of conjugated vaccines, reliable quantification of non-conjugated (free) polysaccharide is a critical, albeit challenging step during vaccine clinical dosing, release and stability monitoring. Multivalent preparations of Cross-reactive material 197 (CRM197)- conjugated pneumococcal polysaccharide materials often contain only nanogram levels of each individual free polysaccharide at final container concentrations. We have developed a novel method for the separation of free polysaccharides from conjugated material that requires no sample derivatization, employing instead an approach of quantitative immunoprecipitation of CRM197 with 3 different monoclonal antibodies and magnetic beads. A mix of antibodies against both linear and conformational epitopes enables successful removal of conjugates regardless of the protein folded state. The remaining free polysaccharide is subsequently measured in a serotype-specific ELISA.