Homogeneous sensitivity of human peripheral blood lymphocytes to radiation-induced chromosome damage

Abstract

THE routine method for detecting genetic damage induced by environmental agents in man in vivo is to examine mitotic cells in short-term peripheral blood lymphocyte cultures for evidence of structural chromosome changes^1,2. Chromosome radiosensitivity in lymphocytes irradiated in vivo and in vitro is similar³, and so dose–response relationships have been established for different qualities and quantities of ionising radiations after in vitro exposure of blood samples⁴. These curves are used to estimate doses received in cases of accidental overexposure to radiation^1,2,5 and in prediction of the sensitivity of human germ cells to heritable radiation damage⁶. Their shape is used to predict the effect of dose rate on chromosome aberration frequencies^7,8 and to elucidate the mechanisms of induction of such aberrations⁸. However, accurate dose–response curves can be obtained only if cells are examined for chromosome damage at the first post-irradiation mitosis, because aberrations decline with successive divisions⁹. Crossen and Morgan¹⁰ used the harlequin-staining technique^10,12 on phytohaemagglutinin (PHA)-stimulated lymphocytes exposed to bromodeoxyuridine (BUdR) to distinguish cells at first and later mitoses in vitro (Fig. 1). They have shown that at the conventional sampling time, about 48 h after stimulation, up to 53% of cells may be in second mitosis. Another problem is that the cells examined in first division at 48 h may not be representative of the entire cell population. Several workers^13–15 have claimed that lymphocytes with different rates of response to PHA stimulation, undergoing first mitosis at different times after stimulation, carry different frequencies of aberrations. They have interpreted this as a sign that lymphocytes in the peripheral blood constitute a heterogeneous population in terms of chromosome radiosensitivity. If this were true, little significance could be attached to the shape of the dose–response curves. We report here that we have used the harlequin technique to show (1) that in spite of radiation-induced mitotic delay¹⁶ high frequencies of cells in second mitosis may still be found at 48 h, and (2) that there is no difference in aberration frequency in first-division cells sampled at various intervals after irradiation. Correct dose–response curves can therefore now be obtained with this method and any sampling time can be used provided that observations are confined to non-harlequin-stained (that is, first division) cells.