Origin and development of rat thymus dependent (T) lymphocytes

Bone marrow (BM) cells of LBNF₁ donor rats, from which detectable thymus dependent (T) lymphocytes were removed by anti T lymphocyte antisera and C, were infused into lethally irradiated (IrBM) and into thymectomized lethally irradiated (TxIrBM) BN recipients. The lymphoid tissues of these animals were examined from 6 to 24 days after reconstitution for donor cells (AgB), donor T cells (AgB T), and T cells (host and donor). Donor cells and donor T cells progressively increased in number in all lymphoid tissues of IrBM rats. In TxIrBM rats donor cells increased to a lesser extent and donor T cells decreased 24 days after reconstitution. Spleen and lymph node cells of IrBM and TxIrBM responded little or not at all to PHA and Con A, in vitro. The response was less than expected of the number of T cells present in culture. T lymphocytes can differentiate from bone marrow precursor elements and thymus seems to be a necessary organ for maintenance of T cell levels. The newly differentiated T cells differ from normal T cells in their response to mitogens.