Regulatory T (T reg) cells are critical regulators of immune tolerance. Most T reg cells are defined based on expression of CD4, CD25, and the transcription factor, FoxP3. However, these markers have proven problematic for uniquely defining this specialized T cell subset in humans. We found that the IL-7 receptor (CD127) is down-regulated on a subset of CD4+ T cells in peripheral blood. We demonstrate that the majority of these cells are FoxP3+, including those that express low levels or no CD25. A combination of CD4, CD25, and CD127 resulted in a highly purified population of T reg cells accounting for significantly more cells that previously identified based on other cell surface markers. These cells were highly suppressive in functional suppressor assays. In fact, cells separated based solely on CD4 and CD127 expression were anergic and, although representing at least three times the number of cells (including both CD25+CD4+ and CD25−CD4+ T cell subsets), were as suppressive as the “classic” CD4+CD25hi T reg cell subset. Finally, we show that CD127 can be used to quantitate T reg cell subsets in individuals with type 1 diabetes supporting the use of CD127 as a biomarker for human T reg cells.

Developing thymocytes are selected for recognition of molecules encoded by the major histocompatibility complex, purged of self-reactive cells and committed to either the CD4 or CD8 lineage. The 1% of thymocytes that complete these tasks emigrate and join the population of peripheral lymphocytes. Whether T cell maturation is complete at the time of thymic exit has been a subject of debate. Using mice transgenic for green fluorescent protein driven by the recombination activating gene 2 promoter to identify recent thymic emigrants, we now show that T cell differentiation continues post-thymically, with progressive maturation of both surface phenotype and immune function. In addition, the relative contribution of CD4 and CD8 recent thymic emigrants was modulated as they entered the peripheral T cell pool. Thus, T cell maturation and subset contribution are both finalized in the lymphoid periphery.

Immunological tolerance to self antigens is a tightly regulated process. Recent work has demonstrated that the forkhead family member Foxp3 is a critical element in the differentiation and function of mouse CD4 + CD25 + regulatory T cells (Treg). Recent work has suggested an important role for IL-2 in the development and maintenance of Treg. To directly assess the effect of IL-2 signaling on Treg development and function, we analyzed mice that were genetically deficient in components of the IL-2 receptor (IL-2R). Mice lacking CD25 (IL-2Ra) displayed a slight decrease in Treg within the thymus, while peripheral numbers are unchanged. In contrast, we found that mice deficient in CD122 (IL-2Rb) had a profound reduction in both thymic and peripheral Treg, coinciding with more rapid development of a fatal lymphoproliferative disease. Expression of a Foxp3 transgene restored Treg and protected against the onset of autoimmunity. Thus, a signal mediated by IL-2Rb is essential for the development and homeostasis of Foxp3 + Treg in vivo.Supporting information for this article is available at http://www.wiley-vch.de/contents/jc_2040/2007/37101_s.pdf IntroductionPeripheral tolerance to self antigens is regulated, in part, by a population of CD4 + CD25 + Foxp3 + regulatory T cells (Treg). Genetic lesions in the Foxp3 gene (the scurfy mutant mouse or human patients with immune dysfunction/polyendocrinopathy/enteropathy/X-linked syndrome) lack Treg and develop a fatal autoimmune lymphoproliferative disease [1][2][3][4][5][6][7]. A similar, but less severe, phenotype is observed in mice deficient in IL-2 and components of its receptor complex, . Thus, disrupting the IL-2R signaling pathway results in severe T cell-mediated autoimmunity rather than immune deficiency, indicating a significant role for IL-2R signaling in Treg development and function.Signaling through the IL-2R complex is initiated by ligand-induced heterodimerization of CD122 and CD132, which activates the associated tyrosine kinases JAK1 and JAK3. Since it has previously been shown that a covalently linked JAK3 molecule can functionally replace the cytoplasmic domain of CD132, the role of CD132 in IL-2R signaling appears to be restricted to the recruitment of JAK3 [13]. By contrast, CD122 provides essential docking sites for at least two major downstream signaling mediators, the adaptor protein Shc and the transcription factor STAT5. CD122 is common to both the IL-2R and IL-15R, with each receptor having a unique a chain that defines receptor specificity (IL-2Ra and IL-15Ra, respectively). Unlike CD122, the role of CD25 appears to be confined to increasing binding affinity (KD *10 -9 -10 -11 M) and it is postulated that the short cytoplasmic domain is unlikely to contribute to signaling [13][14][15][16][17][18].Several lines of evidence suggest that IL-2 is essential for the homeostatic proliferation of Treg. For example, bone marrow chimera studies showed that IL-2 production by non-Treg is required for Treg maintenance [19]. In addition, treatment of mice with...