Immunochemical Characterization of IgM in Human Intestinal Fluids

Abstract We have investigated the immunochemical characteristics of IgM in intestinal fluids from healthy human subjects. The concentration of IgM was measured by double-antibody radioimmunoassay. The molecular size of IgM was evaluated by rate zonal sedimentation on sucrose gradients. The presence of SC on IgM was determined by solid-phase immunosorption of intestinal fluid samples with anti-µ chain-Sepharose followed by reaction of the immunosorbent with radioiodinated anti-SC antibodies. Confirmation of this method was obtained by differential immunosorption of pooled density gradient IgM fractions with anti-SC or nonimmune rabbit gamma globulin-Sepharose. The stability of 131I-IgM-125I-SC complexes was evaluated by incubation in intestinal fluid followed by density gradient ultracentrifugation. The mean concentration of IgM in small intestinal fluid was 1.53 mg/dl. IgM molecules sedimenting at 19S, 10S, 7S, and less than 3.5S were identified. SC was present on IgM in a minority of specimens and was found to be associated solely with 19S species of IgM. SC was rapidly cleaved from IgM-SC complexes by incubation in intestinal fluid at 37°C. The data are consistent with a model for IgM secretion in the human intestine in which IgM is secreted complexed with SC but is readily degraded within the intestinal lumen.