Evidence that the adaptive response of human lymphocytes to ionizing radiation acts on lethal damage in nonaberrant cells.

In a previous study we found that a cytogenetic adaptive response could lead to increases in survival if there was a sufficient increase in nonaberrant cells (Shadley and Dai, 1992). Since the high challenge doses used produced mainly multiply aberrant cells, we suggested using challenge doses that gave mainly singly aberrant cells in order to improve detection of a survival adaptive response. To test this, human lymphocytes from 6 donors were exposed in the first G1 phase to 5 cGy of X-rays, followed by 100 cGy 6 h later. Nearly all of the aberrant cells bore only one chromosome aberration with this challenge dose, and in agreement with our proposal, survival adaptive responses were seen in 4 of 6 donors. A near 1:1 relationship between the % nonaberrant cells and % survival was found with 100 cGy, suggesting that the lymphocyte populations scored in the survival and aberration assays were representative of each other. However, the increase in nonaberrant cells was not sufficient to account for the increase in survival. Thus, a large fraction of the increase in survival was due to a decrease in lethal damage in cytologically nonaberrant cells. Such damage could range from sub-microscopic lesions, to larger alterations not visible in Giemsa-stained cells. In conjunction with adaptive response studies of others, these results intimate that the adaptive response affects damage at different levels of chromosomal hierarchy (i.e. from the chromosome to DNA). The process(es) responsible for the effects observed in this study may act on lethal, rather than mutagenic lesions.