Detection of enriched Thomsen-Friedenrich antigen on IgA1 from IgA nephropathy patients

Human serum IgA1 has a mucin-like structure on its hinge portion which is composed of a mucin-type sugar chain and amino acid sequence rich in proline, serine and threonine. There are incompletely glycosylated O-linked oligosaccharide(s) on the IgA1 hinge region in some nephropathy patients.

We made a detailed analysis of the incompleteness of the sugar chain by digesting IgA1 with various glycosidases. To verify the incompleteness of the sugar chains, the galactosamine/glucosamine ratio (O/N ratio) was introduced as a specific value for each IgA1 preparation.

When IgA1 from serum was treated with alpha-N-acetylgalactosaminidase and/or neuraminidase or endo-beta-N-acetylglucosaminidase H (Endo-H), the O/N ratio did not change. However, endo-alpha-N-acetylgalactosaminidase (Endo-A) reduced the O/N ratio of IgA1 from the IgA nephropathy patient whereas before treatment, the O/N ratio had been similar in the normal control and IgA nephropathy.

This result means there is a small amount of the unsubstituted and the sialylated N-acetylgalactosamine residues (Tn and sialyl Tn antigen, respectively), and abundant asialo Galbeta1,3GalNAc (TF antigen) in the IgA1 molecule. In view of the incompleteness of the IgA1 sugar chain, the decrease in the sialic acid content of the mucin-type sugar chain on IgA1 from an IgA nephropathy patient became obvious in this experiment.