Specificity of human IgM M-proteins that bind to myelin-associated glycoprotein: peptide mapping, deglycosylation, and competitive binding studies

Ten patients with neuropathy and IgM M-proteins that bind to the myelin-associated glycoprotein (MAG) were studied to determine whether the M-proteins bind to common regions of MAG and whether the reactive determinants contain carbohydrate residues. The M-protein of one patient was biotinylated, and binding to human MAG was quantitated by enzyme-linked immunosorbent assay (ELISA) by using avidin-biotin-peroxidase complexes. Serum from the same patient and nine others, but not from controls, competed with the biotinylated M-protein for binding to human MAG. Bovine MAG was digested with staph protease or cleaved with cyanogen bromide, and the resultant fragments were separated by electrophoresis and were transferred onto nitrocellulose sheets. Serum from all patients immunostained the peptide fragments identically. Bovine MAG was deglycosylated by trifluoromethanesulfonic acid, and binding of the M-proteins to MAG and to deglycosylated MAG was tested by immunoblotting. None of the patient's M-proteins bound to deglycosylated MAG. Deglycosylated MAG was visualized by using a mouse monoclonal antibody, GEN-S3, directed at the polypeptide core of MAG. The effectiveness of deglycosylation was ascertained by electrophoresis and by binding of biotinylated concanavalin A. These data and the observed identical species specificity of the M-proteins suggest that the respective anti-MAG M-proteins all bind to the same region in MAG and that the reactive determinants may contain carbohydrate moieties.

Specificity of mouse and human monoclonal antibodies to myelin-associated glycoprotein

Some patients with neuropathy have IgM M-proteins that bind to myelin and to myelin-associated glycoprotein (MAG). We compared the binding properties of a human anti-MAG M-protein with three mouse monoclonal anti-MAG antibodies (GEN-S1, GEN-S3, GEN-S8) and with a mouse monoclonal antibody (HNK-1) that binds to both MAG and to human natural killer cells. The antibodies GEN-S1, GEN-S3, and GEN-S8 bound to different epitopes in the polypeptide portion of MAG as shown by peptide mapping, deglycosylation and competitive binding studies. The M-protein and HNK-1 bound to both CNS and PNS MAG and to several additional protein bands of 70K, 30K, 26K, and 23K daltons in peripheral, but not in central myelin; they did not bind to deglycosylated MAG. The M-protein and HNK-1 immunostained myelin diffusely, whereas GEN-S8 immunostained only the periaxonal and outer regions of myelin sheath, and there was no staining with GEN-S1 or GEN-S3. The human M-proteins probably bind to a carbohydrate moiety in MAG that is also present in other PNS myelin proteins. This may explain the observed differences in immunostaining and the sparing of the CNS in patients with anti-MAG M-proteins.