The mechanism by which radiation induces human peripheral T cell apoptosis is not known. We examined sequential changes in post-irradiated peripheral blood mononuclear cells (PBMC(S)) taken from normal volunteers, by using flow-cytometer and an anti-CD3 monoclonal antibody, annexin V, propidium iodide, anti-Fas antibody, and anti-Fas ligand antibody. After 5 or 10 Gy of irradiation with a 60Co radiation therapy unit, most of the human peripheral T cells showed positivity against annexin V in 15 h, and positivity against propidium iodide in 23 h after irradiation. On a microscopy-video system, approximately 80% of mononuclear cells revealed apoptotic changes in 24 h after irradiation. Because of its proposed role in activation-induced cytotoxicity, we also examined the Fas (CD95/Apo-1) pathway in killing T cells by irradiation. Irradiated PBMC, displayed no increase in surface Fas expression and caspase-3 activity relative to non-irradiated cells. In addition, the anti-Fas ligand failed to eliminate the apoptotic death of PBMC, after irradiation. These results suggest that irradiation induces direct apoptosis of T cells by a Fas-independent mechanism.

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