Immunochemistry of streptococcal group C polysaccharide and the nature of its crossreaction with the Forssman glycolipid.

Acid hydrolysis of streptococal Group C polysaccharide yields a disaccharide, 3-O-alpha-N-acetylgalactosaminosyl-N-acetylgalactosamine (3-O-alpha-GalNAc-GalNAc) which expresses Group C antigenic activity. This disaccharide, which exists as a side chain in the intact polysaccharide, can completely inhibit the binding between Group C polysaccharide and most Group C antibodies, indicating that this unit is the immunodominant feature of the intact polysaccharide. The alpha anomeric configuration and N-acetylation are required for the expression of the antigenic activity by the haptenic disaccharide. Also obtained from the acid hydrolysis of the Group C polysaccharide are rhamnose oligosaccharides with structural identity to the Group A-variant polysaccharide and with Group A-variant antigenic activity. It is inferred from these data that the Group A-varient polysaccharide structure is the core unit of the Group C polysaccharide. The nature of the immunologic crossreactivity between the Forssman glycolipid and Group C polysaccharide, which possess identical nonreducing terminal digalactosamine units, was investigated. Rabbit anti-Group C antibodies bound the Forssman glycolipid with approximately the same affinity as 3-O-alpha-GalNAc-GalNAc and were capable of mediating lysis of sheep red blood cells (SRBC). Antibody fractions isolated from anti-sheep hemolysin were likewise able to bind Group C polysaccharide. The heterologous reactions were in most assay systems weaker than reactions with the immunizing antigen.