Anti-GM1 antibodies as a model of the immune response to self-glycans

Glycans are a class of molecules with high structural variability, frequently found in the plasma membrane facing the extracellular space. Because of these characteristics, glycans are often considered as recognition molecules involved in cell social functions, and as targets of pathogenic factors. Induction of anti-glycan antibodies is one of the early events in immunological defense against bacteria that colonize the body. Because of this natural infection, antibodies recognizing a variety of bacterial glycans are found in sera of adult humans and animals. The immune response to glycans is restricted by self-tolerance, and no antibodies to self-glycans should exist in normal subjects. However, antibodies recognizing structures closely related to self-glycans do exist, and can lead to production of harmful anti-self antibodies. Normal human sera contain low-affinity anti-GM1 IgM-antibodies. Similar antibodies with higher affinity or different isotype are found in some neuropathy patients. Two hypotheses have been developed to explain the origin of disease-associated anti-GM1 antibodies. According to the "molecular mimicry" hypothesis, similarity between GM1 and Campylobacter jejuni lipopolysaccharide carrying a GM1-like glycan is the cause of Guillain-Barré syndrome associated with anti-GM1 IgG-antibodies. According to the "binding site drift" hypothesis, IgM-antibodies associated with disease originate through changes in the binding site of normally occurring anti-GM1 antibodies. We now present an "integrated" hypothesis, combining the "mimicry" and "drift" concepts, which satisfactorily explains most of the published data on anti-GM1 antibodies.

Unusual presence of anti-GM1 IgG-antibodies in a healthy individual, and their possible involvement in the origin of disease-associated anti-GM1 antibodies

Anti-GM1 antibodies of the IgG isotype are found in serum from patients with Guillain-Barré syndrome. In normal human sera, anti-GM1 IgM-antibodies are commonly present, but their IgG counterpart has not been previously demonstrated. During routine screening, we found a normal human serum with a high titer of anti-GM1 IgG-antibodies (IgG1 subclass). Affinity estimation by soluble antigen-binding inhibition indicated that they are low-affinity antibodies with IC50 values between one and two orders of magnitude higher than those of anti-GM1 IgG-antibodies from Guillain-Barré patients. Various antibody parameters remained fairly constant for 1 year, in additional serum samples taken at 4-month intervals. Such anti-GM1 IgG1-antibodies were not detected in > 100 other normal serum samples tested, indicating a very low frequency in the general population. The low affinity of these unusually present antibodies could explain the absence of disease, despite their relatively high titer. The significance of this finding in the origin of disease-associated anti-GM1 antibodies is discussed.

Normally occurring human anti-GM1 immunoglobulin M antibodies and the immune response to bacteria

Anti-GM(1) antibodies of the immunoglobulin M (IgM) isotype are normal components of the antibody repertoire of adult human serum. Using a sensitive high-performance thin-layer chromatography (HPTLC) immunostaining assay, we found that these antibodies were absent in the umbilical vein and children <1 month of age but could be detected after 1 month of age. Although most of the children older than 6 months of age were positive, there were still a few negative children. The appearance of anti-GM(1) IgM antibodies showed a perfect concordance with two well-characterized antibacterial antibodies, anti-Forssman and anti-blood group A, which indicates a similar origin. We also studied IgM reactivity with lipopolysaccharides (LPSs) from gram-negative bacteria isolated from stool samples from healthy babies and from Escherichia coli HB101 in serum from individuals of different ages. We found a positive reaction with both LPSs in all the children more than 1 month of age analyzed, even in those that were negative for anti-GM(1) antibodies. Anti-GM(1) IgM antibodies were purified from adult serum by affinity chromatography and tested for the ability to bind LPSs from different bacteria. This highly specific preparation showed reactivity only with LPS from a strain of Campylobacter jejuni isolated from a patient with diarrhea. We conclude that normally occurring IgM antibodies are generated after birth, probably during the immune defense against specific bacterial strains.