Heterogeneous chromosomal radiosensitivity of phytohaemagglutinin-stimulated human blood lymphocytes in culture

Human blood lymphocytes were irradiated with hard X-rays, stimulated with phytohaemagglutinin (PHA), and grown in presence of amethopterin to accumulate the responding cells at the GI/S boundary of the first cell cycle in vitro. After reversal of the GI/S block with exogenous thymidine, the frequencies of asymmetric chromosome exchanges in relation to the position of metaphases within the first generation mitotic wave were compared. Significant differences of aberration yields within replicate culture series were found in several experiments. A gradual increase of aberration frequencies with increasing duration of S + G2 phases was the most constant feature encountered. In addition, in two parallel series of cultures from one donor, the highest frequency of aberrations was found in samples corresponding to the shortest S+G2 phase duration. A significant contribution of selective mitotic delay of aberration-carrying cells to the distribution of aberration frequencies was excluded. Therefore, it was inferred that the results reflect a true variability of radiosensitivity among the PHA-responsive cells, probably of a discontinuos character, ranging over the ratio of two.