Low molecular weight IgM is the monomeric subunit of pentameric IgM and is not generally found in the blood of healthy individuals. Using a sensitive immunoblotting technique, low molecular weight IgM was detected in all 17 patients with primary biliary cirrhosis and constituted up to 5% of the total circulating IgM. This low molecular weight IgM moiety correlated significantly with total IgM (p less than 0.01) but not with the specific biliary cirrhosis mitochondrial autoantibody anti-M2. Furthermore it was not possible to show that a partially purified sample of low molecular weight IgM contained M2 binding activity. Mitogen stimulated peripheral blood mononuclear cells from two of four patients were observed to secrete low molecular weight IgM in vitro, a finding seen in only one of six healthy subjects. Immunoblot analysis of patients sera revealed the presence of other oligomers of IgM in addition to low molecular weight IgM. In conclusion this study suggests that during the enhanced IgM synthesis observed in primary biliary cirrhosis a defect occurs in the assembly of the IgM pentamer with release of monomeric and oligomeric IgM into the circulation. The pathogenic significance of these circulating low molecular weight IgM species is unknown.
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