Murine Terminal Deoxynucleotidyl Transferase

Terminal deoxynucleotidyl transferase (TdT) 1 is an enzyme which has the unique property of polymerizing deoxyribonucleotides onto a primer in the absence of a template (1-4) . Chang (4) first reported that TdT is present only in the thymus among various avian and mammalian tissues examined . Her finding was extended to humans by McCaffrey et al . (5), who found that the enzyme was restricted to the thymus in normal tissues but was found in peripheral blood lymphocytes from a patient with acute lymphoblastic leukemia. The specific association of TdT with the thymus has led to the suggestion that TdT might act as a somatic mutator andcould be implicated in the generation of immunological diversity (6) . Since TdT was thought to be restricted to the thymus, its distribution among thymocytes has been a critical problem . At least two major thymocyte subpopulations have been identified in mice and other animals (7-13) . The major subpopulation is immunoincompetent, cortisone-sensitive, rich in Thy, TL, Gix, and Ly antigens, but bears little or no H-2 antigen . The minor subpopulation, on the other hand, is immunocompetent, cortisone-resistant, rich in H-2 but low in Thy and Ly antigens, and carries no detectable TL or G, x. Whether the minor subpopulation is derived from the major one is still debatable (12) . In this communication we present evidence for two forms of TdT in murine thymocytes and for their distribution between two thymocyte subpopulations . Also, the occurrence of TdT in murine bone marrow cells is reported .