| Despite the importance of CD4+CD25+ regulatory T cells in the maintenance of self tolerance, many of the fundamental signals which dictate their development, function and survival remain to be elucidated. At the onset of this dissertation, we focused on two important aspects of CD4+CD25+ T cell biology: (1) What is the role of MHC class II in the generation of functional CD4+CD25+ T cells? and (2) What are the molecular mechanisms which regulate the IL-2 mediated survival and expansion of CD4+CD25+ T cells? In chapter 3, we examine the thymic development of CD4+CD25+ T cells, and use genetic models of limited MHC class II expression to place their development in the context of positive and negative selection events. We demonstrate that expression of MHC class II limited to thymic cortical epithelium is sufficient for the phenotypic and functional development of CD4+CD25+ regulatory T cells. Analysis of the specificities of CD4+CD25+ T cells in two distinct models of attenuated negative selection suggests that a subset of CD4+CD25+ T cells is subject to negative selection on hematopoietic APC. In chapter 4, we broadly characterize the cellular and molecular responses of CD4+CD25+ T cells to IL-2 signals, and find that CD4+CD25+ T cells have a distinct pattern of IL-2R signaling. Whereas engagement of the IL-2R on Tregs results in the activation of the JAK/STAT signaling pathway, we could not detect activation of downstream targets of the PI3K signaling pathway, such as Akt or p70s6kinase. Examination of the PI3K signaling pathway in CD4+CD25+ T cells reveals that the lipid phosphatase PTEN negatively regulates PI3K signaling. Lastly, analysis of CD4+CD25+ T cells from mice that are haploinsufficient for PTEN demonstrates that this phosphatase plays a critical role in regulating the proliferative response of Tregs to IL-2 in vitro. Collectively, the studies presented in chapters 3, 4 and 5 significantly extend our understanding of the biology of CD4+CD25+ immunoregulatory T cells. |
