Compositional similarity between immunoglobulins binding to asialo-, agalacto-IgA1-Sepharose and those deposited in glomeruli in IgA nephropathy.

BACKGROUND: It has been found that the immunoglobulin A1-binding protein (IgA1-BP) can be separated from human serum using an asialo-, agalacto-IgA1 (aglyco-IgA1)-Sepharose column (13). As the IgA content in IgA1-BP was significantly higher in IgA nephropathy patients than that in other nephropathy patients, the relationship between IgA in IgA1-BP and glomerular deposited IgA was predicted.
METHODS: IgA1-BP was separated from serum using the aglyco-IgA1-Sepharose column and the aglyco-IgA1-HPLC column. A jacalin-agarose column fractionated the IgA. The immunoglobulins were analyzed by the enzyme-linked immunosorbent assay (ELISA).
RESULTS: A rapid separation method of IgA1-BP from serum by the aglyco-IgA1-HPLC column was developed and this column confirmed the reproduction of the IgA1-BP separation. The following similarities between IgA in IgA1-BP and glomerular deposited IgA were detected. A major portion of IgA in IgA1-BP was the IgA1 subclass. The IgA was rich in medium-size IgA and in the jacalin-high-affinity IgA1 fraction. The IgA showed a statistically significant lower kappa/lambda ratio in its light-chain composition than that of serum IgA, i.e. abundance in IgA bearing the lambda light chain. Other immunoglobulin classes (IgG and IgM) in IgA1-BP also exhibited a significantly low kappa/lambda ratio.
CONCLUSIONS: In this experiment, preferential bindings of the IgA1 subclass, the medium-size IgA and the IgA with the lambda light chain to the aglyco-IgA1-column were observed. Based on previous reports concerning aglyco-IgA1 self-aggregation, the interaction of aglyco-IgA1 with matrix proteins and the rat glomerular deposition of artificially deglycosylated IgA1, IgA in IgA1-BP is thought to be partially the same molecule as the IgA deposited on the mesangial matrix in the IgA nephropathy patient.