Optimization and Application of a Multiplex Bead-Based Assay To Quantify Serotype-Specific IgG against Streptococcus pneumoniae Polysaccharides: Response to the Booster Vaccine after Immunization with the Pneumococcal 7-Valent Conjugate Vaccine

We describe the optimization and application of a multiplex bead-based assay ( Luminex) to quantify antibodies against polysaccharides of 13 pneumococcal serotypes. In the optimized multiplex immunoassay (MIA), intravenous immune globulin was introduced as an in-house reference serum, and nonspecific reacting antibodies were adsorbed with the commercial product pneumococcal C polysaccharides Multi. The antibody concentrations were assessed in 188 serum samples obtained pre- and post-booster vaccination at 11 months after administration of a primary series of the pneumococcal seven-valent conjugate vaccine (PCV-7) at 2, 3, and 4 months of age. The results of the MIA were compared with those of the ELISA for the serotypes included in the seven-valent conjugated polysaccharide vaccine and for a non-vaccine serotype, serotype 6A. The geometric mean concentrations of the antibodies determined by MIA were slightly higher than those determined by ELISA. The correlations between the assays were good, with R-2 values ranging from 0.84 to 0.91 for all serotypes except serotype 19F, for which R-2 was 0.70. The concentrations of antibody against serotype 6A increased after the administration of PCV-7 due to cross-reactivity with serotype 6B. The differences between the results obtained by ELISA and MIA suggest that the internationally established protective threshold of 0.35 mu g/ml should be reevaluated for use in the MIA and may need to be amended separately for each serotype.