ORIGINAL RESEARCH PAPER The effect of ionizing radiation on the homeostasis and functional integrity of murine splenic regulatory T cells Andrea Balogh • Eszter Persa • Enik} o Noe ´mi Bogda ´ndi • Anett Benedek • Hargita Hegyesi • Ge ´za Sa ´fra ´ny • Katalin Lumniczky Received: 13 August 2012 / Revised: 28 September 2012 / Accepted: 9 October 2012 Ó Springer Basel 2012 Abstract Objective Radiotherapy affects antitumor immune res- ponses; therefore, it is important to study radiation effects on various compartments of the immune system. Here we report radiation effects on the homeostasis and function of regulatory T (Treg) cells, which are important in down- regulating antitumor immune responses. Methods C57Bl/6 mice were irradiated with 2 Gy and alterations in splenic lymphocyte fractions analyzed at different intervals. Results Total CD4? numbers showed stronger decrease after irradiation than CD4?Foxp3? Tregs. Tregs were less prone to radiation-induced apoptosis than CD4?Foxp3- T cells. The ratio of CD4?Foxp3- and CD4?Foxp3? fractions within the proliferating CD4? pool progressively changed from 74:26 in control animals to 59:41 eleven days after irradiation, demonstrating a more dynamic increase in the proliferation and regeneration of the Treg pool. The CD4?Foxp3? fraction expressing cell-surface CTLA4, an antigen associated with Treg cell activation increased from 5.3 % in unirradiated mice to 10.5 % three days after irra- diation. The expression of IL-10 mRNA was moderately upregulated, while TGF-b expression was not affected. On the other hand, irradiation reduced Treg capacity to sup- press effector T cell proliferation by 2.5-fold. Conclusion Tregs are more radioresistant, less prone to radiation-induced apoptosis, and have faster repopulation kinetics than CD4?Foxp3- cells, but irradiated Tregs are functionally compromised, having a reduced suppressive capacity. Keywords Regulatory T cells Á Irradiation Á Apoptosis Á Ki67 Á CTLA4 Introduction Radiation therapy is one of the main treatment modalities of malignant tumors. Radiation-induced tumor regression is mainly due to the direct cytotoxic effect of radiation on malignant cells. A growing amount of evidence shows that radiation, apart from its local effects, can influence systemic antitumor responses as well [1–4]. The induction of a sys- temic antitumor immune response involves the coordinated participation of several cell types of the immune system. It is well established that cytotoxic CD8? T lymphocytes and natural killer (NK) cells are usually responsible for the direct tumor cell kill. However, the CD4? T lymphocyte sub- population is also required for the induction of an efficient antitumor immune response [5–7]. Within the CD4? T cell population, the classical T helper cells have effector functions which, depending on their secreted cytokine profile, can promote either the cellular or the humoral immune system. A small fraction of the CD4? cells, known as regulatory T cells (Treg), are CD25? and Foxp3?, and are considered Responsible Editor: Andras Falus. A. Balogh and E. Persa contributed equally to this work. A. Balogh Á E. Persa Á E. N. Bogda ´ndi Á A. Benedek Á K. Lumniczky (&) Division of Cellular and Immune Radiobiology, Fre ´de ´ric Joliot-Curie National Research Institute for Radiobiology and Radiohygiene, Anna u. 5, Budapest 1221, Hungary e-mail: lumniczky.katalin@osski.hu H. Hegyesi Á G. Sa ´fra ´ny Division of Molecular and Tumor Radiobiology, Fre ´de ´ric Joliot-Curie National Research Institute for Radiobiology and Radiohygiene, Budapest, Hungary Inflamm. Res. DOI 10.1007/s00011-012-0567-y Inflammation Research 123