Naturally occurring low molecular weight IgM in patients with rheumatoid arthritis, systemic lupus erythematosus and macroglobulinemia. I. Purification and immunologic studies.

A method for purification of low molecular weight IgM (LMW IgM) which utilized salt precipitation, gel filtration, and insoluble immunoadsorbents is described. The yield from pathologic sera containing LMW IgM in the range of 15 to 36 mg% was approximately 20%. Antisera prepared against LMW IgM, IgM, and reduced-alkylated IgM recognized common determinants. Purified LMW IgM from seropositive rheumatoid subjects did not have rheumatoid factor activity in agglutination tests but demonstrated binding to 125I-labeled aggregated IgG. LMW IgM from a patient with SLE and vasculitis had anti-nuclear factor activity and anti-native DNA activity. When complexed with reduced-alkylated aggregated IgG, LMW IgM and IgM with anti-IgG activity were essentially equivalent on a weight basis in complement fixation. The source and function of LMW in biologic processes remain unknown.