Antibody recognition of the type 14 pneumococcal capsule. Evidence for a conformational epitope in a neutral polysaccharide.

Oligosaccharides consisting of one or more tetrasaccharide repeating units were derived from the capsular polysaccharide of type 14 pneumococcus (Pn14) by endo-beta-galactosidase digestion. The relative affinity of anticapsular antibody binding to derivative oligosaccharides of different chain lengths was measured in a Pn 14 ELISA inhibition assay. The concentration of inhibiting antigen required to achieve 50% inhibition of IgG binding increased progressively from 5.6 x 10(-4) M to 7.0 x 10(-11) M as the inhibiting saccharide chain length increased from 1 tetrasaccharide repeating unit to 2,500 repeating units. These data indicate that antibodies directed against the Pn14 polysaccharide recognize a conformational epitope fully expressed only in high molecular weight forms of the antigen. Similar results were found for inhibition of Fab fragment binding, suggesting that recognition of the conformational epitope is largely dependent on the intrinsic affinity of the Fab combining region. Unlike previously reported polysaccharides for which conformational epitopes have been described, the Pn14 polysaccharide does not contain negatively charged residues, indicating that expression of conformational determinants is not limited to acidic polysaccharides. Antibody recognition of conformational epitopes may be a common mechanism by which the host immune response discriminates between bacterial polysaccharides and host oligosaccharides of similar structure.