Adaptive response of human lymphocytes to low-level radiation from radioisotopes or X-rays.

Human peripheral blood lymphocytes stimulated in vitro were exposed to low level irradiation ('adaptive dose') from radioisotopes ([3H]dThd, [14C]dThd, HTO and 32P). After 50 h in culture, they were irradiated with 50 rad of X-rays ('challenge dose') and fixed 3 h later. In another series, the lymphocytes received an adaptive dose of 5 rad of X-rays at 32 h after stimulation and a challenge dose of 150 rad at 48 h; the cells were fixed at 54 h. In cells that received both the adaptive and challenge doses, the frequencies of chromosomal aberrations (chromatid and isochromatid deletions) were lower than expected on the basis of additivity of the effects of the individual treatments. These results support those published from Wolff's laboratory in showing that human lymphocytes can become 'adapted' by prior exposure to low level irradiation so that they become less sensitive to the chromosome-breaking effects of X-rays delivered subsequently. The magnitude of reduction in frequencies in the 'adapted' cells, however, varied between the blood samples from different donors.