The erythropoietic effects of continuous γ-irradiation with a daily regime of 0.029, 0.083 and 0.374 Gy were studied in mice. Irradiation was performed with ¹³⁷Cs γ-rays for 22 hr/day. The length of irradiation time varied from 3 to 112 days. Erythropoiesis was investigated on the basis of clearance of ⁵⁹Fe from the circulation and of incorporation of ⁵⁹Fe into circulating erythrocytes and erythropoietic tissue. A chemical method for the separation of heme and nonheme iron-containing fractions was employed to examine the uptake of ⁵⁹Fe into both the heme and nonheme iron fractions. Daily exposure to 0.029 and 0.083 Gy caused no significant changes in erythropoiesis. Daily exposure to 0.374 Gy caused some significant changes in erythropoiesis. On day 7 of continuous irradiation, the amount of ⁵⁹Fe incorporated into erythrocytes decreased, but the values returned to normal on day 14 and 28 of continuous irradiation, indicating recovery within erythropoietic tissues at earlier time. On day 56, depressed incorporation of ⁵⁹Fe into erythrocytes with normal rate of disappearance of ⁵⁹Fe from the circulation and increased heme level of ⁵⁹Fe in the femoral marrow were observed. Results observed on day 56 may suggest the possibility of ineffective erythropoiesis during the continuous irradiation. On day 112, some mice showed almost the same changes in erythropoiesis as those mice exposed to acute X-rays radiation.

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The ability of 2-chlorodeoxyadenosine, a ribonucleotide reductase inhibitor, to inhibit the repair of potentially lethal damage was demonstrated in Chinese hamster V79 cells after X irradiation in plateau-phase cultures. This ability of the drug was completely diminished when deoxycytidine was added at the same time, though this was slightly affected by the addition of adenosine, suggesting that this drug was phosphorylated by deoxycytidine kinase to serve as an inhibitor of the repair of potentially lethal damage. Compared with hydroxyurea, another ribonucleotide reductase inhibitor, this drug appeared to contain its own activity which suppressed the repair of potentially lethal damage. A combined study of post-irradiation treatment with hypertonic salt solution and with this drug on the fixation of potentially lethal damage revealed that this drug inhibited the repair of hypertonicinsensitive potentially lethal damage.

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The effects of 60 cobalt gamma rays (⁶⁰Co), tritiated water (HTO) beta rays and 252 californium (²⁵²Cf) neutrons on stromal stem cells (CFU-F) from human fresh bone marrow were studied using an in vitro cell culture technique. Based on the survival curves of CFU-F, the D₀ values of ⁶⁰Co, HTO and ²⁵²Cf were 124 ± 30, 92 ± 12 and 60 ± 11 rad, respectively. From the D₀ values of ⁶⁰Co gamma rays, the relative biological effectiveness (RBE) of HTO beta rays and ²⁵²Cf neutrons were calculated to be 1.3 and 2.1. However, due to the difference in the survival curves with or without initial shoulder, the RBEs of HTO and ²⁵²Cf were dependent on the range of radiation dose. Both RBEs became higher than 3 in the low dose range of radiation (under 100 rad).

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A bone-seeking radioisotope, ⁸⁹Sr eliminates blood monocytes and their precursors in the bone marrow after a selective deposition in the skeletal bone. In the present study, we investigated whether or not such a prominent monocyte depletion can induce any alterations in tissue macrophage populations in ⁸⁹Sr-injected mice. The number of monocytes or leukocytes possessing a macrophage differentiation antigen, Mac-1, phagocytic or Fc receptor (FcR) activity was significantly reduced in the blood and the bone marrow for about 6 weeks after ⁸⁹Sr administration. Splenic macrophages characterized by these phenotypic or functional markers were not, however, altered in number during the post-injection period, despite the fact that the total number of spleen cells recovered significantly increased together with macrophage colony forming stem cells (M-CFC) and cells under DNA synthesis. The population of lavaged pulmonary alveolar macrophages (PAM) was invariable not only in the number recovered but also in DNA synthesis. Colony formation by PAM was consistently noted, and the total number of M-CFC in the lung was not reduced during the 8 weeks post-injection period. These results indicate that ⁸⁹Sr-induced depletion of bone marrow-derived monocytes/macrophages or their precursors has little effect at least on the number of splenic and pulmonary alveolar macrophage populations in a normal steady state.

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To clarify the long-term effects of occupational exposure to low doses of radiation, Epstein-Barr virus (EBV)-specific antibody titers in sera from 104 radiological technologists (R.T.) and 118 controls in Kumamoto prefecture were measured by the immunofluorescence method. Antibody titers to viral capsid antigen (VCA)-IgG increased with the years of experience as R.T., and the prevalence of abnormal antibody titers to both VCA-IgG and early antigen (EA)-IgG were significantly higher in R.T. with over 15 years of experience or 30 rads of cumulative radiation dose than in the controls. However, there was no correlation between exposure and the frequency of abnormal EBV-associated nuclear antigen (EBNA) antibody titers. The EBV-specific antibody titers of 24 Hiroshima atomic-bomb survivors were also measured. They were similar to those of the R.T. with over 30 years of experience. The EBV-specific antibody titers of R.T. suggest that there may be an impairment of immunologic competence after continuous long-term exposure to low doses of radiation. Also, the correlation of EBV-specific antibody titers and frequency of cells with chromosome aberrations in 53 R.T. was studied. Some correlations were found between the antibody titers to both of the VCA-IgG and EBNA and the frequency of cells with chromosome aberrations.

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