Absence of detectable IgM in enzymatically or biosynthetically labeled thymus-derived lymphocytes.

Surface proteins of mouse thymus and spleen cells were radioiodinated with lactoperoxidase. After solubilization, the labeled proteins were precipitated by antibodies directed against mouse immunoglobulin chains; the precipitates were analyzed by radioautography after Na dodecyl sulfate-gel electrophoresis. Radioactive mu and L chains were absent from thymocyte extracts and conspicuous in spleen-cell extracts. The following cells were biosynthetically labeled for 4 hr [(35)S]methionine or 24 hr with [(14)C]leucine: (1) Thymocytes, (2) cortisoneresistant thymocytes [both treated with rabbit antisera cytotoxic to bone marrow-derived (B) lymphocytes and IgM-containing plasma cells, to kill possible contaminating nonthymus-derived cells], (3) "activated thymocytes" (allogeneic cell cultures of cortisone-resistant thymocytes), (4) human Daudi cells (a B lymphoblastic cell line), and (5) purified mouse B spleen lymphocytes devoid of plasma cells. Again no mu and L chains could be detected in thymocyte or thymus-derived cell extracts by immune precipitation and gel electrophoresis, while these chains were conspicuous in B-cell extracts. "Educated thymocytes," obtained from spleens of lethally irradiated mice injected with syngeneic thymocytes and antigen, synthesized mu and L chains under similar conditions; this synthesis resulted from contamination of these cells by IgM-containing plasma cells.