Abstract
Suppression of thymic lymphoma induction by low-dose (0.075 Gy) pre-irradiation in C57BL mice irradiated with 4 consecutive doses of 1.6 Gy was already reported. Also, it is known that lymphoid tumors develop in nonirradiated syngeneic thymic grafts implanted into thymectomized, irradiated mice. This indirect effect of radiation on the induction of lymphomas indicates one of the mechanisms for radiation carcinogenesis.
The indirect effect might be caused by hypoplasia of grafted thymus, which might increase genetic and epigenetic plasticity during forced proliferation of cells. Here we studied the effect of low-dose pre-irradiation on carcinogenesis in the thymus graft system. As group A, 51 B10.Thy1.2 mice were thymectomized and irradiated with 4 consecutive doses of1.6 Gy at 7-days intervals from 5 wks of age, and as group B, 51 mice were pre-exposed to low-dose irradiation of 0.075 Gy 6 hr before every 1.6 Gy irradiation. Thymuses from new born C57BL/10.Thy1.1 mice were implanted subcutaneously in B10.Thy1.2 mice within 1hr after the final irradiation. The origins of the tumor were assayed by flow cytometry after treatment with anti-Thy1.1 or anti-Thy1.2 antibody. In both groups, tumor incidence of graft origin was 31%, and the mean latencies of groups A and B were 159 and 140 days, respectively(p=0.08). Although the difference was not significant, a delay in lymphoma development suggest that pre-irradiation has a little effect on the suppression of indirect action of radiation on lymphomagenesis.
