Thymic Regeneration After Lethal Irradiation Evidence for an Intra-thymic Radioresistant T Cell Precursor

The data presented indicate the existence of a significant pool of radioresistant stem cells which are capable of partially restoring the thymus of heavily irradiated mice. 3-H-TdR incorporation by the thymus of lethally irradiated mice begins 48 to 72 hr after irradiation and increases throughout the next 8 days. By the 9th day after 760 rads, typical corticomedullary architecture has been restored. 890 rads markedly suppressed, but did not totally eliminate this regeneration. Injection of large numbers of syngeneic bone marrow cells immediately after irradiation was without effect on the rate or extent of regeneration. Mice whose bone marrow and spleen were shielded from irradiation showed an identical amount of thymic regeneration as those receiving total body irradiation indicating that the precursor cell pool responsible for the early post irradiation phase of thymic regeneration is most likely an intrathymic population. The cells repopulating the thymus were morphologically indistinguishable from normal thymocytes and were susceptible to cytotoxic antisera against the thymic differentiation antigens Thy-1, TL, LyA2 and LyC2.