LYMPHOCYTE DEVELOPMENT IN IRRADIATED THYMUSES - DYNAMICS OF COLONIZATION BY PROGENITOR CELLS AND REGENERATION OF RESIDENT CELLS

Lymphocyte development in irradiated thymuses was analyzed using two c omplementary strategies: an in vitro experimental model and computer s imulations. In the in vitro model, fetal thymus lobes were irradiated and the regeneration of cells that survived irradiation were examined, with the results compared to those of reconstitution of the thymus by donor bone marrow cells and their competition with the thymic residen t cells. In vitro measurements of resident cell kinetics showed that c ell proliferation is slowed down significantly after a relatively low (10 Gy) irradiation dose. Although the number of thymocytes that survi ved irradiation remained low for several days post-irradiation, furthe r colonization by donor cells was not possible, unless performed withi n 6 h after irradiation. These experimental results, coupled with the analysis by computer simulations, suggest that bone marrow cell engraf tment in the irradiated thymus may be limited by the presence of radia tion-surviving thymic resident cells and the reduced availability of s eeding niches. (C) 1995 Academic Press Limited