Dose-effect relationship of apoptosis induced by fission-neutron in murine thymocytes

Objective　To investigate the effectiveness of high LET fission-neutron to induce apoptosis in murine thymocytes and to compare it with that of low LET 60Co γ-ray. Methods Apoptosis induction was studied qualitatively by light and transmission electron microscopy and DNA gel electrophoresis,also quantitatively by flow cytometry(FCM) and diphenylamine (DPA)methods. Results DNA ladders of murine thymocytes were detectable,the typical apoptosis of thymocytes could be observed morphologically by means of light and electron microscopy at 6 h after fission-neutron irradiation with doses ranging from 0.5 to 5.0 Gy,meanwhile the percentages of apoptosis increased with increasing doses.After exposure to γ-rays with doses ranging from 1.0 to 30 Gy,the expermiental results were similar to those from neutron radiation.The incidence of apoptosis peaked at about 20 Gy，the percentages did not increase further when doses increased. Conclusion Apoptosis of murine thymocytes can be induced when mice are exposed to either fission-neutron (0.5-5.0 Gy) or to γ-ray (1-30 Gy).Although the relationship between apoptosis and radiation doses is similar,the percentage of apoptosis induced by neutron irradiation is higher than that induced by γ-irradiation.The RBE values of fission-neutron for inducing apoptosis murine thymocytes are 2.09 (by FCM method) and 2.37 (by DPA method),respectively.These results also suggest that fission-neutron-induced murine immune tissue is more severe than that induced by γ-rays at several hours post-irradiation and this might be the basis for heavy damage to immune tissues induced by fission-neutron-irradiation in later period.