Low molecular weight IgM in primary Sjögren's syndrome

Low molecular weight IgM (LMW IgM), the monomeric subunit of pentameric IgM, was measured in the serum of 27 patients with primary Sjögren's syndrome. LMW IgM was also measured in a control group consisting of 24 patients with psoriatic arthritis (PA) and 8 patients with active rheumatoid arthritis (RA). LMW IgM was found in the majority of patients with primary SS (63%) and those patients had a longer disease duration than those without SS (80.7 +/- 43.9 months vs 37 +/- 18.3, p = 0.01). Although the differences were not significant, SS patients with LMW IgM showed higher rates of: seropositive disease (71% vs 50%), anti-Ro (59% vs 30%) and anti-La antibodies (12% vs 0%), extraglandular involvement (76% vs 60%) and raised gammaglobulins levels (47% vs 30%) compared to those without LMW IgM. Our only two patients with active RA and vasculitis had LMW IgM. None of the patients with PA showed LMW IgM, regardless of their Kammer subgroup classification, disease activity or radiological evidence of erosions. The presence of LMW IgM in a high percentage of patients with primary SS appears to be the expression of a dysregulation of B cell state that may predispose these patients to developing malignant lymphoproliferation.