Distribution of terminal deoxynucleotidyl transferase activity between peaks I and II in host and donor thymic cells of bone-marrow-restored mice after X-irradiation.

The terminal deoxynucleotidyl transferase (TdT) activity and its distribution between peaks I and II after chromatographic elution were studied on days 15 and 17 after X-irradiation, in host- and donor-derived thymic cells of lethally irradiated (9 Gy) mice restored with BM cells. It was found that the population derived from the surviving host thymocytes differed markedly from the donor-derived population. The cells of host origin has a low TdT activity especially in peak II and the ratio peak I/peak II remained close to 1 instead of 0.1 in controls. These alterations reflect a reduced replication rate and possibly a modification of the cellular metabolic activity (phosphorylation-dephosphorylation). In contrast, the donor-derived elements displayed a very high TdT activity related to their elevated rate of replication, and the ratio peak I/peak II which was close to 1 on day 15 returned rapidly to normal. The impaired replication ability and the metabolic alteration of the host cells might be attributed either to a specific property of the radiation-resistant thymocytes or to residual cellular injury or to a combination of both.