Interleukin-2 receptor downstream events in regulatory T cells: implications for the choice of immunosuppressive drug therapy.

Naturally occurring CD4+CD25(high)FOXP3+ regulatory T cells (Tregs) constitute a powerful mechanism of immune regulation and therefore, have important therapeutic potential for disorders such as autoimmune diseases, allograft rejection and graft-versus-host disease. Disruption of the IL-2R signalling pathway by genetic defects of the interleukin (IL)-2 gene or components of the IL-2 receptor (R) complex results in severe T cell-mediated autoimmunity rather than immunodeficiency, indicating a crucial role for IL-2R signalling for Treg development and function. Signalling downstream of the IL-2R can act through the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR pathway, the Janus kinase (JAK)/Signal transducers and Activators of Transcription (STAT) pathway and the mitogen-activated protein kinase (MAPK) pathway. In this report we focus on the relevance of these pathways as well as the impact of immunosuppressive drugs that may affect or enhance Treg function by targeting IL-2R signalling.