THE DIFFERENCES IN SIGNIFICANCE OF ALPHA-2,3GAL-LINKED AND ALPHA-2,6GALNAC-LINKED SIALIC-ACID RESIDUES IN BLOOD-GROUP M-RELATED AND N-RELATED EPITOPES RECOGNIZED BY VARIOUS MONOCLONAL-ANTIBODIES

The blood group M and N determinants of glycophorin A (GPA) contain O- linked oligosaccharide chains with alpha 2,3Gal-linked and alpha 2,6Ga lNAc-linked sialic acid residues which are required for the activity o f most epitopes recognized by various anti-M and anti-N antibodies. In order to check whether these two types of sialic acid residues differ in their contribution to antigenic properties, the GPA-M and GPA-N pr eparations with monosialylated oligosaccharide chains were obtained an d tested for binding of anti-M and anti-N monoclonal antibodies (MAbs) . The GPAs with sialic acid residues linked to Gal (GPA2,3) were obtai ned by selective resialylation of asialoGPAs with alpha 2,3-sialyl-tra nsferase. These preparations were tested by inhibition of binding of M Abs to enzyme-linked immunosorbent assay (ELISA) plates coated with th e respective untreated target antigens. The GPAs with sialic acid resi dues linked to GalNAc (GPA2,6) were generated by treating GPAs adsorbe d on ELISA plates with Newcastle disease virus (NDV) isolate (expressi ng sialidase specific for alpha 2,3Gal linkage), which was followed by testing the binding of MAbs to NDV-treated antigens. Different patter ns of activity were obtained among 14 MAbs specific for sialic acid-de pendent epitopes (eight anti-M and six anti-N). The results indicated that at least half of the MAbs showed distinct requirements for the pr esence of only one of two kinds of sialic acid residues (Gal or GalNAc linked) in the epitope. Only four MAbs (two anti-M and two anti-N) di d not react with any of the 'monosialylated' forms of GPA. Four of the eight anti-M MAbs (three of which reacted most strongly with GPA2,3) bound similarly to untreated and periodate-oxidized GPA-M, while the r eactivity of the remaining anti-M and anti-N MAbs was abolished by per iodate oxidation of the GPA preparations. In conclusion, the results o btained showed that Gal-linked and GalNAc-linked sialic acid residues of the tetrasaccharide present in GPA (and many other glycoproteins) p lay distinct roles in the activity of sialic acid-dependent glycopepti dic epitopes recognized by various antibodies.