IgM in bone marrow‐derived lymphocytes. Synthesis, surface deposition, turnover and carbohydrate composition in unstimulated mouse B cells

Abstract Small B lymphocytes can be depleted by velocity sedimentation on serum albumin gradients of a minor fraction (1 %) of large cells contained in spleen cell populations from normal nu/nu mice. Small cells are the mitogen‐sensitive cells. Large cells contain the “background” plaque‐forming cells. The populations of large cells synthesize 10 to 100 times more IgM than the populations of small cells. Large and small cells contain IgM as 7–8 S subunits and as smaller precursors of IgM. Carbohydrate moieties of IgM molecules inside small cells contain the “core” sugars glucosamine and mannose attached to the μ‐chains, but not the penultimate galactose and the terminal fucose residues common to secreted 19 S IgM. Large cells secrete IgM as 19 S pentamers, small cells shed their IgM as 7–8 S subunits. Large cells turn over IgM with a half time around 4 h, small cells turn over IgM with a half time between 20 and 80 h IgM synthesis is actinomycin D‐resistant in large cells and actinomycin‐sensitive in small cells. Surface‐bound IgM molecules can be distinguished within the total cellular pool from IgM molecules situated in intracellular compartments not exposed to the outside by their serological reaction with polyvalent anti‐Ig antibodies on intact cells. With this reaction it can be show that small cells contain over 90 % of their IgM in the surface membrane.