Adaptive response of thymocyte cycle progression induced by low dose ionizing radiation in mice

In this paper, dose-, dose rate- and time-effect of adaptive response of thymocyte cycle progression induced by low dose radiation were observed. Kunming mice were irradiated with different inductive doses (D1) of 25, 50, 100 and 200 mGy at a fixed dose rate of 12.5 mGy/min, or a fixed inductive dose (D1) of 75 mGy with different dose rates of 6.25, 12.5, 25, 50, 100 and 200 mGy/min. The mice were then irradiated with different challenging doses (D2) of 1.0, 1.5 and 2.0 Gy at a fixed dose rate of 287 mGy/min. The time intervals between D1 and D2 were 3, 6, 12, 24 and 60 h. Thymocyte cycle progression was measured with flow cytometer. It has been found that with the mice which had received D1 of 25, 50 and 100 mGy at 12.5 mGy/min dose rate and the D2 doses 6h after the D1, or D1 of 75 mGy at dose rates of 6.25, 12.5 and 25 mGy/min and the D2 doses 3, 6 and 12 h after the D1 or D2 of 1.0, 1.5 and 2.0 Gy, the percentage ratio of S phase thymocytes decreased significantly (p<0.05 or p<0.01), and the percentage of G0/G1 and G2+M phase thymocytes increased significantly (p<0.05 or p<0.01) too, by comparing the D2 groups to the sham-irradiation group. Comparing the D1+D2 groups to the D2 groups, however, the percentage ratio of S phase thymocytes increased significantly (p<0.05 or p<0.01), and the percentage of G0/G1 and G2+M phase thymocytes decreased in varying degrees. The results suggest that irradiating the mice to 25-100 mGy at dose rates of 6.25-25 mGy/min 3-12 h before exposing them to 1.0-2.0 Gy irradiation at 287 mGy/min dose rate, the adaptive response of thymocyte cycle progression could be induced under the condition of whole-body irradiation