In vitro apoptosis in peripheral blood mononuclear cells induced by low-dose radiotherapy displays a discontinuous dose-dependence

Purpose: Cells undergoing apoptosis contribute to the regulation of activat ed mononuclear cells (Voll ed al. 1997). Low-dose radiotherapy (LD-RT) is k nown to improve inflammatory symptoms, but the mechanism of action is still unclear. The aim of this study was to investigate the rate of apoptosis of peripheral blood mononuclear cells (PBMC) induced by LD-RT within the ther apeutic dose range of anti-inflammatory RT. Materials and methods: PBMC were isolated from venous blood of ten healthy volunteers and were irradiated with single doses between 0.1 and 3.0 Gy. Ap optotic nuclei were detected by flow cytometry after propidium iodide (PI) triton staining, and apoptotic cells were detected by annexin V/PI staining and cell scatter analysis. Since apoptotic cells display increased cytopla smatic granularity and concomitant reduced cell size, they can be distingui shed from viable cells in forward/side scatter (FSC/SSC) histograms. Apopto tic PBMC were further subtyped by double staining with annexin V and direct ly labelled monoclonal antibodies recognizing the lineage-specific surface markers CD4, CDS, and CD19, respectively. The apoptosis rate of irradiated cells was analysed in a time and dose dependent fashion and was compared to a sham-irradiated control. Results: After irradiation, a dose-dependent increase in apoptosis was obse rved, with a discontinuity (plateau or peak) between 0.3 Gy and 0.7 Gy in 9 /10 donors (90%) and 59/80 samples (74%). 8/10 donors (80%) and 38/80 sampl es (47%) showed not only a discontinuous increase with a plateau but a rela tive maximum of apoptosis peaking within the dose range of 0.3 Gy and up to 0.7 Gy. Conclusion: LD-RT induces a relative maximum of apoptosis in PBMC in the do es range between 0.3 Gy and 0.7 Gy. This may contribute to its anti-inflamm atory effect observed clinically.