Effects of radiation on survival and recovery of T lymphocyte subsets in C3H/HeN mice.

The aims of this study were to determine the radiosensitivities of murine thymic and splenic CD4+ and CD8+ lymphocytes and to evaluate the regeneration of these cells in a model of radiation-induced hematopoietic and immune suppression. CD4+ and CD8+ cells were quantitated using two-color flow-cytometric analysis. Cells obtained from C3H/HeN mice 24 hours after exposure to 0.25-8.0 Gy (0.4 Gy/min) 60Co were used to determine D0 values. Thymic CD4+ cells contained a radiosensitive subpopulation with a D0 of 0.97 +/- 0.05 Gy and a radioresistant subpopulation that survived exposures up to 8.0 Gy. CD8+ cells also contained a radiosensitive subpopulation with a D0 of 1.24 +/- 0.05 Gy and a radioresistant subpopulation with a D0 of 3.93 +/- 2.01 Gy. Double-positive thymic CD4+/CD8+ cells were uniformly radiosensitive, with a D0 of 1.03 +/- 0.28 Gy. Multiple T lymphocyte subpopulations based on radiosensitivity and CD4/CD8 antigen expression were also observed in the spleen. When mice were exposed to a sublethal 6.5-Gy radiation dose and recovery of T lymphocyte subsets was monitored, the relative radioresistance of CD4+ cells resulted in a selective enrichment of these cells among the surviving thymocytes and splenic lymphocytes. This relative enrichment of CD4+ cells became even more prominent 7 days after irradiation, when atrophy of the organs was greatest. Similar, although less dramatic, effects were observed for CD8+ cells. These studies demonstrate that (1) multiple T lymphocyte subpopulations can be identified based on radiosensitivity and CD4/CD8 antigen expression; (2) both CD4+ and CD8+ cells contain radioresistant subpopulations, with the CD4+ subpopulation being more resistant than the CD8+ subpopulation; and (3) although the number of radioresistant CD4+ cells is quite small, they persist in increased proportions during the periods preceding and corresponding to postirradiation hematopoietic recovery.