Conformational epitope of the type III group B Streptococcus capsular polysaccharide

The protective epitope of the type III group B streptococcal polysaccharide (GBSPIII) is length dependent and conformational. To obtain a more accurat e characterization of the conformational epitope, ELISA inhibition and surf ace plasmon resonance studies were conducted on two GBSPIII-specific mAbs u sing a large panel of oligosaccharide probes. The results of the studies co nfirmed that 2 repeating units (RU) is the minimum binding unit and that, w hile increases in chain length from 2 RU to 7 RU caused further optimizatio n of the epitope, it remained monovalent, A 3-fold increase in affinity was observed between 7 RU and 20 RU, which, by surface plasmon resonance studi es on a Fab, was shown to be due to both further optimization of the indivi dual epitope and the occurrence of multivalency of epitope. The data suppor t our hypothesis that the conformational epitope is an extended helical seg ment of the GBSPIII. GBSPIII exists mainly in the random coil form, which s tructurally mimics short oligosaccharide self Ags, but it fan infrequently and spontaneously form extended helices. Although not prevalent in GBSPIII, the immune system preferentially selects these helical epitopes because th ey are unique to the polysaccharide, Contrary to a previously proposed mode l of GBSPIII binding in which the binding of the first Ab propagates a cont inuum of helical epitopes, our binding kinetics are consistent only with th e helical epitope's being discontinuous and infrequent.