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.