Effect of ionizing radiation on thymic epithelial cell function. I. Radiation-spared thymic epithelial grafts expedite the recovery of T cell function in lethally irradiated and fetal liver reconstituted mice

A murine model system was developed to determine whether ionizing radiation has a detrimental influence on thymic epithelium, cell function. Normal mice were lethally irradiated, grafted intracamerally with normal fetal thymic epithelium, and then reconstituted with fetal liver cells. These animals were compared with a group of animals who received their thymic grafts before the irradiation protocol. Analysis of the reconstitution of T cell function in peripheral lymph nodes and spleens at various times post transplantation demonstrated that animals with radiation-spared thymic grafts had superior proliferative responses to T cell mitogens and alloantigens. It was also determined that the capacity of these animals to elicit contact hypersensitivity responses was significantly greater when compared with animals whose thymic grafts had been radiated. The observed difference in T cell function could not be ascribed to a difference in the rate of export of mature T cells from the thymic grafts since the absolute number of Thy-1+, L3T4+, or Lyt-2+ lymphocytes present in the peripheral lymphoid compartment of our two groups of animals was equivalent. Immunohistologic analysis of the thymic grafts demonstrated a marked reduction in the medullary compartment of the repopulated grafts that had been exposed to ionizing radiation. The results of this study suggest: 1) that irradiation of the thymic microenvironment during marrow ablative preparative regimens may be in part responsible for some of the immune alterations observed in marrow transplant recipients, and 2) that our model system may provide a valuable tool for delineating the roles played by medullary and cortical epithelial cells of the thymus on the T cell maturation and education processes.