MRL-lpr/lpr mice are used as a model of systemic lupus erythematosus. We previously reported attenuation of autoimmune disease in MKL-lpr/lpr mice by repeated γ irradiation (0.5 Gy each time). In this study, we investigated the mechanisms of this attenuation by measuring the weight of the spleen and the population of highly activated CD3+CD4- CD8 -B220+ T cells, which are characteristically involved in autoimmune pathology in these mice. Splenomegaly and an increase in the percentage of CD3+CD4-CD8-B220+ T cells, which occur with aging in nonirradiated mice, were suppressed in irradiated mice. The high proliferation rate of CD3+CD4 -CD8-B220+ T cells was suppressed in the irradiated animals. The production of autoantibodies and the level of IL6, which activates B cells, were also lowered by radiation exposure. These results indicate that progression of pathology is suppressed by repeated 0.5-Gy γ irradiation. To uncover the mechanism of the immune suppression, we measured the regulatory T cells, which suppress activated T cells and excessive autoimmune responses. We found that regulatory T cells were significantly increased in irradiated mice. We therefore conclude that repeated 0.5-Gy γ irradiation suppresses the proliferation rate of CD3+CD4-CD8 -B220+ T cells and the production of IL6 and autoantibodies and up-regulates regulatory T cells.
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