Prothymocytes in postirradiation regenerating rat thymuses: a model for studying early stages in T cell...

Prothymocytes were obtained from regenerating thymuses of intrathymic-irradiated, bone marrow-shielded rats. In contrast to cortical thymocytes, which are small nondividing cells containing nuclear TdT, prothymocytes are characterized by their large size, high mitotic activity, lack of natural attachment, absence of PNA-binding capacity, nonexpression of membranal thymic specific antigens, and absence of nuclear TdT. In addition, these cells are capable of responding to the mitogens Con-A and PHA, and are sensitive to in vitro lysis by physiologic concentrations of corticosterone and cortisol. Prothymocytes incubated for 3 days on thymic monolayers differentiated into small lymphocytes expressing cortical thymocyte characteristics. Light and electron microscopy studies demonstrated the infiltration of prothymocytes from the circulation via the thymic blood vessel wall into the perivascular sinuses. Prothymocytes isolated from the thymuses, however, did not exhibit specific "homing" to the thymus when transfused back into the animals. In view of the observed accelerated thymic repopulation in adrenalectomized rats, and the high in vitro glucocorticoid sensitivity of the prothymocytes, it is suggested that thymic homeostasis is regulated by specific effect of adrenocortical hormones on the prothymocyte subset.