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.