Ionizing radiation modulates the phenotype and function of human CD4+ induced regulatory T cells

BACKGROUND: The use of immunotherapy strategies for the treatment of advanced cancer is rapidly increasing. Most immunotherapies rely on induction of CD8+ tumor-specific cytotoxic T cells that are capable of directly killing cancer cells. Tumors, however, utilize a variety of mechanisms that can suppress anti-tumor immunity. CD4+ regulatory T cells can directly inhibit cytotoxic T cell activity and these cells can be recruited, or induced, by cancer cells allowing escape from immune attack. The use of ionizing radiation as a treatment for cancer has been shown to enhance anti-tumor immunity by several mechanisms including immunogenic tumor cell death and phenotypic modulation of tumor cells. Less is known about the impact of radiation directly on suppressive regulatory T cells. In this study we investigate the direct effect of radiation on human T(REG) viability, phenotype, and suppressive activity. RESULTS: Both natural and TGF-β1-induced CD4+ T(REG) cells exhibited increased resistance to radiation (10 Gy) as compared to CD4+ conventional T cells. Treatment, however, decreased Foxp3 expression in natural and induced T(REG) cells and the reduction was more robust in induced T(REGS). Radiation also modulated the expression of signature iT(REG) molecules, inducing increased expression of LAG-3 and decreased expression of CD25 and CTLA-4. Despite the disconcordant modulation of suppressive molecules, irradiated iT(REGS) exhibited a reduced capacity to suppress the proliferation of CD8+ T cells. CONCLUSIONS: Our findings demonstrate that while human T(REG) cells are more resistant to radiation-induced death, treatment causes downregulation of Foxp3 expression, as well as modulation in the expression of T(REG) signature molecules associated with suppressive activity. Functionally, irradiated TGF-β1-induced T(REGS) were less effective at inhibiting CD8+ T cell proliferation. These data suggest that doses of radiotherapy in the hypofractionated range could be utilized to effectively target and reduce T(REG) activity, particularly when used in combination with cancer immunotherapies.