Structure and serology of the native polysaccharide antigen of type Ia group B Streptococcus

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Abstract
The native polysaccharide antigen isolated from type Ia group B Streptococcus by using pH-controlled growth conditions and extraction procedures contains D-galactose, D-glucose, 2-acetamido-2-deoxy-D-glucose and sialic acid in the molar ratio of 2:1:1:1. The structure of the native type Ia antigen was elucidated. Removal of all sialic acid groups yields the incomplete type Ia polysaccharide antigen with exposed terminal .beta.-D-galactopyranose residues. Antisera to type Ia organisms produced in rabbits according to the Lancefield procedures contain antibodies specific for both the native and incomplete antigens. Although sialic acid is not itself a determinant in the formation of antibodies to the native polysaccharide, it is an essential part of a larger determinant. To maintain the high degree of immunologic specificity of the native antigen, this determinant must be at least a trisaccharide unit, because the native polysaccharide as isolated has terminal disaccharide units [.alpha.-D-NeuAcp-(2 .fwdarw. 3)-.beta.-D-Galp] identical to those found in the human M and N blood group substances and fetuin. Formation of antibodies to the incomplete antigen is due to determinants terminating in .beta.-D-galactopyranose residues. These determinants are probably generated by the removal of the masking sialic acid residues from the cell-associated native polysaccharide by degradative processes that occur in organisms grown without pH control.