Bone marrow or lymph node cells from 10 patients whose sera contained large amounts of monoclonal IgM proteins were incubated with radioactive amino acids in short-term tissue culture. Samples of soluble cytoplasmic extracts and secreted material were examined by immunologic precipitation with specific antisera, acrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, and sucrose gradient centrifugation. In all samples studied, 8S IgM was the major intracellular precursor of the fully assembled 19S protein. Cells obtained from some patients contained little or no fully assembled 19S protein intracellularly; however, cells from most patients seemed to accumulate fully assembled 19S molecules intracellularly before secretion. Secreted material from both groups contained large amounts of 19S IgM. The differentiation between accumulating and nonaccumulating cells did not correlate with heavy or light chain antigenic type. Synthesis and assembly appeared to be identical in cells obtained from different anatomic sites in the same patient Studies carried out in one patient before and after therapy revealed no qualitative differences in the pathway of assembly and reflected only a decrease in the total number of immunoglobulin-producing cells.

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