Table of Contents:
IL-27 inhibits regulatory T-cells via STAT3
In this thesis, the effect of the cytokine Interleukin-27 (IL-27) was investigated on the in vitro generation of murine inducible regulatory Th-cells (iTregs). IL-27 is a heterodimeric protein that belongs to the IL-12 family of cytokines and pro- and anti-inflammatory properties are attributed to it. IL-27 activates mainly the transcription factors signal transducer and activator of transcription 1 (STAT1) and STAT3 after binding to its receptor. Regulatory Th-cells (Treg) are important for immune homeostasis and are implied in the protection against autoimmune diseases. iTregs can be induced from naîve CD4+ cells by differentiation with IL-2 and TGF-β. Forkhead box P3 (Foxp3) is a transcription factor, the presence of which is necessary and sufficient for the main function of Tregs, namely to inhibit the proliferation and effector function of other CD4+ cells.
In my studies the interaction of IL-27 and TGF-β for the generation of iTreg was investigated. In naîve CD4+ cells that were differentiated in the presence of IL-27 and TGF-β, the expression of Foxp3 was effectively inhibited, compared to cells differentiated in the presence of TGF-β alone. The inhibition of Foxp3 by IL-27 could be partially reversed by nucleofection with small interfering RNA (siRNA) targeted against STAT3, so that one can conclude that the inhibiton of the expression of Foxp3 is at least partially dependent on STAT3. When STAT3 was down regulated via siRNA, the cells produced more IFN-γ, indicating the possible prevalence of the STAT1 signaling pathway in this situation. The production of the cytokines IL-2, TNF-α and, using our conditions, IFN-γ as well, increased considerably when naive Th-cells were differentiated with TGF-β and IL-27 compared to iTreg. The expression levels of the Treg markers CTLA-4 and CD25 were greater in iTreg than in cells differentiated in the additional presence of IL-27. The ability of naîve CD4+ cells differentiated with IL-27 and TGF-β to block proliferation of CD4+CD25+ responder cells was decreased compared to iTreg, although some significant suppression was still mediated by cells differentiated with TGF-β and IL-27. According to recent publications, the latter cells could represent Tr1 cells, which produce the immunosuppressive cytokine IL-10. It seems possible that STAT3 is, on the one hand, able to down regulate the expression of Foxp3 and, on the other hand, able to initiate some unknown mechanisms, which might mediate immunosuppressive effects independently of Foxp3.