The biosynthesis and characterization of unreleased IgM in a case of CLL

Intracellular IgM k is described in a case of CLL. The intracellular immunoglobulin occurred as amorphous spherical globules enclosed by rough endoplastic reticulum in the cytoplasms of the neoplastic lymphocytes. No abnormal IgM paraprotein could be found in the serum nor was there any Bence Jones protein detectable in concentrated urine. Biosynthetic studies confirmed the cells were able to synthesize IgM k immunoglobulin. On sizing gel analysis the intracellular IgM occurred as 7S monomer. The biosynthesis studies did not indicate secretion of IgM and was consistent with the serological findings. However, cells in culture secreted k light chain into the supernatant medium. It appears in this case that the secretory block is operating against heavy chain and not against free light chain. Some explanations for these findings are discussed.