Pneumococcal type 22F polysaccharide absorption improves the specificity of a pneumococcal-polysaccharide enzyme-linked immunosorbent assay

The specificity of the immune response to the 23-valent pneumococcal-polysa ccharide (PS) vaccine in healthy adults and to a pneumococcal conjugate vac cine in infants was examined by measuring immunoglobulin G (IgG) antibody t iters by enzyme-linked immunosorbent assay (ELISA) and the opsonophagocytos is assay. ELISA measures total antipneumococcal IgG titers including the ti ters of functional and nonfunctional antibodies, while the opsonophagocytos is assay measures only functional-antibody titers, Twenty-four pairs of pre - and post-pneumococcal vaccination sera from adults were evaluated (ELISA) for levels of IgG antibodies against serotypes 4, 6B, 9V, 14, 18C, 19F, an d 23F. Twelve of the pairs were also examined (opsonophagocytosis assay) fo r their functional activities. The correlation coefficients between assay r esults for most types ranged from 0.75 to 0.90, but the correlation coeffic ient was only about 0.6 for serotypes 4 and 19F. The specificities of these antibodies were further examined by the use of competitive ELISA inhibitio n, A number of heterologous polysaccharides (types 11A, 12F, 15B, 22F, and 33A) were used as inhibitors. Most of the sera tested showed cross-reacting antibodies, in addition to those removed by pneumococcal C PS absorption. Our data suggest the presence of a common epitope that is found on most pne umococcal PS but that is not absorbed by purified C PS. Use of a heterologo us pneumococcal PS (22F) to adsorb the antibodies to the common epitope inc reased the correlation between the IgG ELISA results and the opsonophagocyt osis assay results. The correlation coefficient improve from 0.66 to 0.92 f or type 4 and from 0.63 to 0.80 for type 19F. These common epitope antibodi es were largely absent in infants at 7 months of age, suggesting the carboh ydrate nature of the epitope.