Appearance of low molecular weight IgM during course of infective endocarditis.

Low molecular weight (LMW) IgM is the monomeric subunit of pentameric IgM. It is not found in healthy adults but occurs in a number of autoimmune, lymphoproliferative and infectious conditions. It has not been described before in infective endocarditis (IE). Eighteen patients with IE were studied; 16 with subacute bacterial endocarditis (SBE) and two with acute endocarditis. LMW IgM was detected in the sera of six patients, all having SBE in association with circulating rheumatoid factor (RF). Of the remaining 12 patients without LMW IgM only three had RF in low quantities. Sequential studies revealed that LMW IgM appeared during the later stages of the illness at or following the peak RF and IgM response. LMW IgM was not detected in any of 20 control sera. Immunoblot analysis of sera containing LMW IgM revealed the presence of small quantities of dimeric and oligomeric IgM in addition to monomeric IgM. We conclude that LMW IgM occurs predominantly in those patients with IE who have associated RF. Immunoblot analysis suggests that the presence of monomeric and oligomeric LMW IgM reflects a disorder of IgM polymerization occurring in those patients.