The immune response to group B streptococcus type III capsular polysaccharide is directed to the -Glc-GlcNAc-Gal- backbone epitope

The structures of the branched capsular polysaccharides of group B streptococcus type III (GBSIIIPS) and Streptococcus pneumoniae type 14 (Pn14PS) are identical apart from the (α2→3)-linked sialic acid in the side chains of GBSIIIPS. The present study tries to determine the minimal epitope in GBSIIIPS, using both a panel of anti-Pn14PS mouse sera and sera of humans vaccinated with either Pn14PS or GBSIIIPS. Type-specific Pn14PS antibodies that recognize the branched structure of Pn14PS have a low affinity for the native GBSIIIPS. Desialylation of GBSIIIPS results in dramatically higher affinity of anti-Pn14PS antibodies. Epitope specific anti-Pn14PS mouse antibodies and human sera of PCV7 vaccinees only recognized structures with the branching element -Glc-(Gal-)GlcNAc-, in particular -Gal-Glc-(Gal-)GlcNAc- in Pn14PS. On the other hand anti-GBSIIIPS human antibodies recognize predominantly the linear structure in the backbone of Pn14PS or GBSIIIPS, i.e., -Glc-GlcNAc-Gal-. This difference in antigenicity of Pn14PS and GBSIIIPS is in agreement with the difference in flexibility of the two polysaccharides caused by the presence or absence of sialic acid