Age dependence of T-lymphocyte apoptosis induced by high-energy proton exposure.

Peripheral blood samples from three donors of different ages were exposed to 300 kVp x-rays or 138 MeV protons (0.2, and 9 Gy dose). After 48 h incubation, CD4 and CD8 T-lymphocytes were labelled with specific monoclonal antibodies and cellular DNA was stained by propidium iodine. Radiation-induced apoptosis was followed by flow cytometry and the data were processed by LYSIS II software. The data analysis revealed an age-dependent sensitivity to radiation-induced apoptosis by 300 kVp x-rays and 138 MeV protons, for both CD4 and CD8 T-lymphocytes. Radiation-induced apoptosis was about 4 times greater in CD4 lymphocytes from the youngest donor than the oldest donor and was about 2 times greater in CD8 T-lymphocytes, both after x-ray and proton exposures. RBE values for CD4 T-lymphocytes ranged from 0.9 to 1.4 and for CD8-positive cells from 0.7 to 0.9. It is concluded that radiation-induced apoptosis of CD4 and CD8 T-lymphocytes, which is already exploited to predict patient response in conventional radiotherapy, may also be used to predict response in proton treatment planning.