Absence of Detectable IgM in Enzymatically or Biosynthetically Labeled Thymus-Derived Lymphocytes

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Abstract
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 μ 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 μ 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 μ and L chains under similar conditions; this synthesis resulted from contamination of these cells by IgM-containing plasma cells.