IN-VIVO INDUCTION OF APOPTOSIS IN HUMAN-LYMPHOCYTES BY THERAPEUTIC FRACTIONATED TOTAL-BODY IRRADIATION

Ionizing radiations have been reported as an in vitro apoptosis initia ting stimulus in human lymphocytes. As the cytotoxicity of ionizing ra diations and chemotherapeutic agents appears to be dependent on the ef ficacy of cell death induction, the manipulation of apoptosis initiati on might be used as a means to supress some pathological process. In t he present study the in vivo induction of gamma-ray mediated programme d cell death in humans is reported. The in vivo induction of apoptosis in peripheral blood lymphocytes (PBL) by ionizing radiations was inve stigated in 33 patients after each of two sessions (2 Gy and 4 Gy), of fractionated total body irradiation (FTBI) as part of their condition ing regimen before bone marrow transplantation. PBL committed to apopt osis were scored before irradiation (S1), 4 h (S2) and 24 h after 2 Gy (S3, 14-17 h after the second 2 Gy fraction). Nuclear morphology and chromatin-DNA were analysed by fluorescence microscopy immediately aft er blood sample withdrawal (I) and after 24 h in cell culture medium ( II). When scored immediately after withdrawal, no circulating PBL with the apoptotic nuclear morphology were observed in S1 and S2 blood sam ples whereas S3 disclosed 21.9+/-11.7% of circulating lymphocytes with an apoptotic nuclear morphology. After 24 h in culture, S1 samples (b efore irradiation) generally contained less than 20% of apoptotic lymp hocytes. A higher percentage of apoptotic cells was noted in some case s in relation with recent chemotherapy and possibly with pathology. Af ter 24 h in culture, S2 and S3 samples contained 51.7+/-17.9% and 60.4 +/-16.4% of apoptotic lymphocytes, respectively. These results confirm that ionizing radiations induce apoptosis in vivo in human lymphocyte s and that the commitment to apoptosis can be determined after low dos es (2 Gy) of therapeutic whole body irradiation. The results suggest t hat susceptibility to apoptosis induction by ionizing radiations could be related to previous therapy by cytotoxic drugs and possibly to the type of haematological malignancy.