In vivo induction of apoptosis in human lymphocytes by therapeutic fractionated total body irradiation

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Abstract
Ionizing radiations have been reported as an in vitro apoptosis initiating stimulus in human lymphocytes. As the cytotoxicity of ionizing radiations and chemotherapeutic agents appears to be dependent on the efficacy of cell death induction, the manipulation of apoptosis initiation might be used as a means to supress some pathological process. In the present study the in vivo induction of γ-ray mediated programmed cell death in humans is reported. The in vivo induction of apoptosis in peripheral blood lymphocytes (PBL) by ionizing radiations was investigated in 33 patients after each of two sessions (2 Gy and 4 Gy) of fractionated total body irradiation (FTBI) as part of their conditioning regimen before bone marrow transplantation. PBL committed to apoptosis were scored before irradiation (SI), 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 after 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 samples whereas S3 disclosed 21.9±11.7% of circulating lymphocytes with an apoptotic nuclear morphology. After 24 h in culture, S1 samples (before irradiation) generally contained less than 20% of apoptotic lymphocytes. A higher percentage of apoptotic cells was noted in some cases in relation with recent chemotherapy and possibly with pathology. After 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 lymphocytes and that the commitment to apoptosis can be determined after low doses (2 Gy) of therapeutic whole body irradiation. The results suggest that susceptibility to apoptosis induction by ionizing radiations could be related to previous therapy by cytotoxic drugs and possibly to the type of haematological malignancy.