IgM in bone marrow‐derived lymphocytes. Changes in synthesis, turnover and secretion, and in numbers of molecules on the surface of B cells after mitogenic stimulation

Abstract Stimulation of small, resting B cells by the mitogen lipopolysaccharide from E. coli induces changes in synthesis, turnover, carbohydrate composition, surface deposition and active secretion of IgM. With increasing time stimulation, the rate of synthesis of the glycoprotein IgM increases over the rates of syntheses of other proteins and carbohydrate‐containing macromolecules. IgM synthesis, being actinomycin D‐sensitive in small, resting lymphocytes, becomes more and more actinomycin D‐resistant. IgM molecules with a median disappearance time around 4 h are synthesized in increasing amounts. These rapidly turning over IgM molecules are no longer shed from the cells as 7–8 S subunits, but are actively secreted as 19 S pentamers. In the secreted 19 S form, they contain the branch sugars galactose and fucose attached to the Hμ‐chains. Within 20 min after addition of the mitogen to unstimulated B cells, surface‐bound Ig determinants disappear and can no longer be approached from the outside of intact cells by anti‐Ig antibodies. “ De novo ” synthesis refurnishes the original amount of surface‐bound IgM molecules within 12 h. Beyond 12 h after exposure to the mitogen, more and more IgM molecules appear on the surface. At 40 to 60 h after stimulation, between 35 and 100 times more surface‐bound IgM molecules are on the stimulated B cells. The majority of these surface‐bound IgM molecules turn over rapidly with a median disappearance time around 5 h. The surface membrane appears to be an intermediate station in the migration of IgM molecules out of the cell during active secretion. IgM synthesis before and after mitogenic stimulation is discussed as a part of the overall biogenesis of membranes in B cells.