Interaction of the C-polysaccharide of Streptococcus pneumoniae with the receptor asialo-GM1.

C-polysaccharide (PnC) is the major surface component of pneumococci containing phosphoryl choline residues. In order to investigate the possibility that PnC can bind to glycolipid receptors present on epithelial cells we extracted carbohydrate material from a nonencapsulated strain of pneumococci. The components of the extract were separated by gel permeation chromatography. An ELISA was used for detection of fractions binding to the pneumococcal glycolipid receptor asialo GM1. These fractions were pooled and analysed by nuclear magnetic resonance spectroscopy (NMR). The 1H NMR spectrum showed good agreement with a reference spectrum of pure PnC showing that this substance was the major component. Binding of the purified PnC to asialo-GM1 was unaffected by protease K treatment. Immunoblots of the purified PnC after separation by SDS-PAGE resulted in a characteristic banding pattern. PnC could be released from pneumococci by heat treatment of whole bacteria in buffer as shown by reaction with a monoclonal antibody specific for the phosphoryl choline determinant. After separation by SDS-PAGE of the components of the heat extract, immunoblots showed the presence of bands characteristic for PnC. Eluates from the characteristic bands in the gel were shown to contain material binding to asialo-GM1. This binding was not reduced upon treatment with protease K. Pneumococci deprived of choline by cultivation in a medium containing ethanolamine as the only amino alcohol source did not bind to asialo-GM1, indicating that the phosphoryl choline residue of PnC is essential for the interaction between PnC and the glycolipid receptor. These data provide evidence that PnC containing an intact phosphoryl choline residue is a ligand responsible for binding of pneumococci to the receptor asialo-GM1.

Specific binding of Streptococcus pneumoniae to two receptor saccharide structures.

Specific binding of Streptococcus pneumoniae to two proposed receptor structures was studied in a solid-phase assay. The assay was based on immunodetection of the pneumococci adhering to the receptors coated to microtiter plates. Non-specific binding owing to hydrophobic forces to uncoated wells could be abolished by treatment of the plates with a blocking buffer. Binding of the pneumococcal cells was demonstrated with the glycolipid asialo-GM1 as receptor with a previously suggested specificity for the disaccharide GalNAc beta 1-4Gal. A non-capsulated mutant bound with high efficiency to this receptor. Two capsulated strains also bound well, but with lower efficiency. Binding of the non-capsulated strain was also demonstrated with lactotriaosylceramide as receptor with a suggested specificity for the disaccharide GlcNAc beta 1-3Gal. The binding assay enables the comparison of the adherence of different strains to purified receptor molecules.