[Apoptotic characteristics of spleen lymphocyte in mice irradiated by lethal dose and its relationship to the expression of Bax and Bcl-XL proteins].

OBJECTIVE: To observe the apoptotic characteristics of mouse spleen lymphocyte after lethal dose gamma-irradiation and its relationship to the expression of Bax and Bcl-XL proteins.

METHODS: Two hundred and twenty-five second-grade C57 mice were randomly divided into six groups of 0, 6, 9, 12, 15 and 20 Gy. They were sacrificed by dislocation and samples were taken on 1-28 days after whole body single gamma-irradiation. Lymphocyte apoptosis and necrosis were analyzed by TdT-mediated dUTP nick end labeling (TUNEL) and flow cytometry (FCM) techniques. The expression of Bax and Bcl-XL proteins were estimated by immunohistochemical method.

RESULTS: (1) The number of peripheral white blood cells of mice increased temporarily at 6 hours after radiation, thereafter, began to decrease rapidly, which reached the minimum on day 7 and recovered normal level basically one month after 6 Gy gamma-irradiation. (2) Apoptotic rate of spleen lymphocytes increased significantly, peaking at 6 hours after radiation, which was found to have a dose-response relationship during 6-24 hours after < or =12 Gy irradiation, but decreased after > or =15 Gy irradiation. (3) It was confirmed by FCM that the apoptotic rate of spleen lymphocytes increased along with the elevation of radiation dose. However, the apoptotic rate began to decrease and the necrotic rate rose distinctively after > or =15 Gy irradiation. The analysis of DNA gel electrophoresis supported above-mentioned results. (4) The expression of Bax protein in spleen lymphocyte enhanced at 6 hours after 6 Gy gamma-irradiation and peaked by 12 Gy-irradiation, showing a dose-dependent pattern, but which was not be found after > or =15 Gy gamma-irradiation. On the other hand, the expression of Bcl-XL protein reduced persistently with the increase of radiation dose, and also presented a better dose-dependent effect after < or =12 Gy irradiation.

CONCLUSION: After 6-12 Gy gamma-irradiation, the apoptosis is the major death way of spleen lymphocyte, while both necrosis and apoptosis are important death pathways after > or =15 Gy irradiation. Pro-apoptotic Bax and anti-apoptotic Bcl-XL play an important role in the apoptotic regulation of spleen lymphocytes induced by lethal dose radiation.