Terminal deoxynucleotidyl transferase (TdT) is a specialized DNA polymerase that is potentially mutagenic. It is expressed only in the thymus and in some bone marrow cells, suggesting that it acts selectively in immature lymphoid cells, especially T cells. We investigated the extent to which de novo synthesis of TdT is correlated with the clonal expansion that T cell precursors undergo in the thymus. Using biosynthetic pulse-labeling and immune precipitation, we found that thymocytes from late-fetal and neonatal mice make TdT at considerably lower levels than weanling or adult mouse thymocytes. Adult levels of TdT synthesis are only reached a week after birth. Thus, the high proportion of proliferating cells in the perinatal thymus does not entail a correspondingly high production' of TdT synthesis is blocked by cytolytic treatment with anti-Lyt-2 and complement, suggesting that the inducible cells are already Lyt-2+ like most adult TdT+ thymocytes. These observations imply that the stroma of the perinatal thymus either suppresses or fails to trigger high-level TdT synthesis. At the same time, it is in this microenvironment that maximum proliferation and export of functional T cell precursors take place. In contrast, the conditions that stimulate high TdT synthesis in vitro are not associated with mitogenesis but rather with growth arrest. High-level TdT synthesis in general is not regulated so as to precede or coincide with clonal expansion, either in perinatal or in adult thymocyte populations. The possibility remains that in many cells it is linked with a commitment to die.

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