Abstract
In A-bomb survivors, there are significant radiation dose-dependent decreases in the percentages of naïve T cells, suggesting a reduction in the ability of thymus to produce T cells. In the present study, we made an attempt to evaluate individual thymic ability to produce T cells, by measuring the number of T cells bearing T-cell receptor-rearrangement excision circles (TRECs) as an endpoint for the number of naive T cells that have not undergone cell division after thymic emigration. The number of TRECs in each CD4 and CD8 T-cell fraction separated from peripheral blood lymphocytes using a cell sorter was measured by real-time PCR. In the 445 survivors so far been examined, multiple regression analysis indicated that the number of TRECs in the CD4 T-cell fraction was significantly higher in females than in males and decreased significantly with age in both males and females. This analysis also suggested a possible dose-dependent decrease in the number of TRECs in the CD4 T-cell fraction of the survivors who were less than 20 years of age at the time of bombing (P = 0.09). A similar statistically significant trend for gender difference or age was observed in the CD8 T-cell fraction of the survivors. However, there was no effect of radiation exposure on the number of TRECs in the CD8-T cell fraction. The results indicate the possibility that A-bomb radiation exposure may have induced a long-term impairment in thymic CD4 T-cell production. Further investigations in a larger study population are planned to test this hypothesis.
