Abstract. It is known that, besides its direct cytotoxic effect as an alkylating chemotherapeutic agent, cyclophosphamide also has immuno-modulatory effects, such as depletion of CD4 + CD25 + regulatory T cells. However, its optimal concentration has not yet been fully elucidated. Therefore, we first compared the effects of different doses of cyclophosphamide on T cell subsets including CD4 + CD25 + T cells in mice. Cyclophosphamide (20 mg/kg) decreased the numbers of splenocytes, CD4 + and CD8 + T cells by ~50%, while a decline in CD4+ CD25 + T cell number was more profound, leading to the remarkably lower ratios of CD4 + CD25 + T cells to CD4 + T cells. In contrast, 200 mg/kg cyclophosphamide severely decreased the numbers of all the T cell subsets by >90% although the decreased ratios of CD4 + CD25 + T cells to CD4 + T cells were still observed. Next, low-dose cyclophosphamide significantly inhibited in vivo growth of murine hepatoma MH129 tumor in immuno-competent but not immuno-deficient mice. This anti-tumor effect was abolished by CD4 + CD25 + T cell repletion. In contrast, high-dose cyclophosphamide exhibited similar anti-tumor effects in both mice. In addition, contrary to antibody-mediated CD4 + CD25 + T cell depletion, administration of low-dose cyclophosphamide after tumor inoculation was more efficacious than the prior administration. Our data show that low-dose cyclophosphamide selectively depletes CD4 + CD25 + T cells, leading to enhanced anti-tumor effects against pre-existing tumors, while the anti-tumor effect of high-dose cyclophosphamide is solely attributed to its direct cytotoxicity. These findings appear to be highly crucial in a clinical setting of combined chemotherapy and immunotherapy for cancer treatment.

Purpose: To evaluate how irradiation affects thyroid autoimmunity in mouse models of Hashimoto's thyroiditis and Graves' hyperthyroidism. Gy once a week from one week before NaI or Ad-TSHR immunization, or a single regional irradiation to the thyroid gland with 0.5 Gy one week before NaI. The effect of a single irradiation with 0.05, 0.5 or 3 Gy on splenocytes was also evaluated. Materials and methods:Results: A single whole-body irradiation with 0.5 Gy one week before NaI exacerbated thyroiditis and increased anti-Tg antibody titers in NOD-H2 h4 mice. In contrast, any irradiation protocols employed did not affect incidence of hyperthyroidism or anti-TSHR antibody titers in BALB/c mice. High-dose irradiation increased the relative ratios of effector T cells to regulatory T cells (an indication of enhanced immune status) but kills most of T cells. Conclusions:These results indicate that a single whole-body low-dose irradiation with 3 0.5 Gy exacerbates thyroiditis in NOD-H2 h4 mice, data consistent with some clinical evidence for increased incidence of thyroid autoimmunity by environmental irradiation.4

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