Low molecular weight IgM in juvenile chronic arthritis.

Low molecular weight IgM, the monomeric subunit of pentameric IgM, was clearly detected by immunoblotting and filtration chromatographic techniques in six patients with juvenile chronic arthritis and in trace quantities in a further eight of 24 patients studied. This low molecular weight IgM moiety contributed up to 33% of the total circulating IgM and was strongly associated with raised serum concentrations of IgM and the presence of antinuclear antibodies, extractable antinuclear antibodies, and rheumatoid factor. Immunoblot analysis of positive serum samples showed small quantities of other low molecular weight oligomers of IgM in addition to monomeric IgM. It is postulated that the presence of low molecular weight IgM in the serum of patients with juvenile chronic arthritis reflects a disorder of the intracellular assembly of IgM subunits during a stimulated IgM immune response. The pathogenetic role of low molecular weight IgM remains uncertain.