Regulatory T cells in health and disease : putting the pieces together

Abstract

CD4+FOXP3+ Regulatory T cells (Treg) are indispensable for immune balance. In patients with autoimmunity, Treg are either present in lower numbers or not functioning properly and, therefore, inflammation is not suppressed. In murine models and in human studies increase of Treg frequency or function suppresses autoimmunity. Therefore, Treg are important candidates for therapeutic intervention in inflammatory diseases. In order to develop a safe therapy utilizing Treg, it is important to know how human Treg work, to identify markers and to investigate how Treg are regulated. In this thesis, we investigated human regulatory T cells as targets for therapeutic intervention in autoimmune diseases. We have extended findings from murine to human Treg, from protein modifications to in vitro Treg function, but also from in vitro to in vivo Treg properties. Our aim has been to establish a clear overview of the human Treg phenotype, mechanism of suppression, and suppressive function in vivo. Furthermore, we evaluated the clinical significance of Treg in peripheral blood and muscle tissue of Juvenile dermatomyositis (JDM) patients. Moreover, we investigated whether self-antigen stimulation of T cells or post-translational modifications of FOXP3 increase human Treg frequency or function. We clearly showed that there are differences in human and murine Treg function, since, unlike murine Treg, human Treg mediated suppression is independent of apoptosis induction in effector T cells. Furthermore, we showed that human Heat Shock Protein 60 (HSP60) was able to induce human Treg, with specificity for HSP60 epitopes. Furthermore, expression of the cell surface receptor CD30 distinguished Treg with suppressive function from non-suppressive T cells. In order to investigate the safety of in vitro cultured Treg for therapeutic application, we explored the differences between in vitro expanded Treg (nTreg) and induced Treg (iTreg) in both in vitro assays and to evaluate the in vitro results, we also analyzed Treg function in an in vivo model for xenogeneic Graft versus Host Disease (x-GvHD). In vitro assays demonstrated that both populations of Treg exhibited suppressive capacity. However, shortly after in vivo administration iTreg lost FOXP3 expression and, thereby, suppressive function. Since continuous FOXP3 expression is important for Treg function, we investigated whether post-translational modifications regulate FOXP3 expression. We showed that acetylation of FOXP3 prevented FOXP3 degradation. Moreover, HDAC (histone deacetylase) inhibitors promoted FOXP3 expression and suppressive function of iTreg. Last, we showed that patients with Juvenile dermatomyositis (JDM) had normal Treg percentages in peripheral blood compared to age-matched controls, independent of disease activity, while in patients treated with high doses of corticosteroids increased Treg percentages were observed. Furthermore, Treg from JDM patients with remitting disease exhibited suppressive function in vitro, while Treg from patients with active disease were not consistently suppressive. In inflamed muscle tissue of JDM patients with active disease many FOXP3+ Treg were present. In conclusion, human Treg mediated suppression is independent of apoptosis induction in effector T cells. HSP60 induces human Treg, CD30 distinguishes Treg with suppressive function, and HDAC inhibitors promote FOXP3 expression and Treg function. Furthermore, in vitro assays are insufficient to determine human Treg suppressive function and, therefore, suppressive capacity should be analyzed in vivo. We showed that Treg may be involved in JDM pathogenesis, but seem unable to suppress inflammation. Therefore, promotion of Treg function and frequency could improve JDM disease outcome