Mechanism of adaptive response: Estimation of human lymphocyte adaptive response to radiation using various criteria

Blood lymphocytes of 15 healthy donors have been investigated for their ability to reduce radiosensitivity after low-dose irradiation-radio-induced adaptive response (AR). The frequency of unstable chromosome aberrations was used to evaluate cell radiosensitivity after the irradiation of cells in low adaptive (5 cGy) and high challenge (1 Gy) doses in comparison with the effect of challenge irradiation only. Three indexes have been used, i.e., (A) the frequency of cells with aberrations per total analyzed cell, (B) the number of chromosome aberrations per one cell, (C) and the number of chromosome aberrations per one aberrant cell. It was found that the donors can be divided in the four following groups: 1. AR was not estimated any of the indexes used; 2. AR was estimated with indexes A and B, but not C; 3. AR was shown by indexes B and C; 4. AR was evident with all three indexes. The generally accepted AR repair model only explains the appearance of group-3 and-4 donors, but not group-2. For the purpose of understanding the AR mechanisms and the difference in AR estimations with various criteria, the metaphase distribution by the number of chromosome aberrations has been analyzed for each donor. It was shown that, in group-2 donors, the number of cells without aberrations after adaptive and challenge irradiations was significantly higher than after irradiation with a challenge dose only. Thus, in this group, AR is formed as a result of the changed frequency of cells in the 0 class (population shift). A similar shift is observed in the metaphase distribution in the donors of group 4, but not in group 3. The data obtained show that AR is probably a result of several processes, including the activation of the reparation of premutational genome damages, population shifts evident in the frequency of undamaged cells, and, possibly, the activation of apoptotic cell death. The complex character of AR is reflected to different degrees in each criterion of radiosensitivity.