Variable patterns of anti-GM₁ IgM-antibody populations defined by affinity and fine specificity in patients with motor syndromes: Evidence for their random origin

Elevated titers of serum antibodies against GM₁-ganglioside are associated with a variety of autoimmune neuropathies. Although much evidence indicates that these autoantibodies play a primary role in the disease processes, the mechanism of their appearance is unclear. Low-affinity anti-GM₁ antibodies of the IgM isotype are part of the normal human immunological repertoire. In patients with motor syndromes, we found that in addition to the usual anti-GM₁ antibodies, the sera contain IgM-antibodies that recognize GM₁ with higher affinity and/or different specificity. This latter type of antibodies was not detected in other autoimmune diseases. We studied the fine specificity of both normal and motor disease-associated antibodies using HPTLC-immunostaining of GM₁ and structurally related glycolipids, soluble antigen binding inhibition, and GM₁ affinity columns. Normal low-affinity anti-GM₁ antibodies cross-react with GA₁ and/or GD_1b. In the motor syndrome patients, different populations of antibodies characterized by their affinity and cross-reactivity were detected. Although one population is relatively common (low affinity, not cross-reacting with GA₁ and GD_1b), there are remarkably few sera having the same set of populations. These results suggest that the appearance of the new antibody populations is a random process. When the different antibody populations were analyzed in relation to the three-dimensional structure of GM₁, a restricted area of the GM₁ oligosaccharide (the terminal Galβ1-3GalNAc) was found to be involved in binding of normal anti-GM₁ antibodies. Patient antibodies recognize slightly different areas, including additional regions of the GM₁ molecule such as the NeuNAc residue. We hypothesize that disease-associated antibodies may originate by spontaneous mutation of normal occurring antibodies.