Regulatory T cell-mediated dominant tolerance has been demonstrated to play an important role in the prevention of autoimmunity. Here, we present data arguing that the forkhead transcription factor Foxp3 acts as the regulatory T cell lineage specification factor and mediator of the genetic mechanism of dominant tolerance. We show that expression of Foxp3 is highly restricted to the subset alphabeta of T cells and, irrespective of CD25 expression, correlates with suppressor activity. Induction of Foxp3 expression in nonregulatory T cells does not occur during pathogen-driven immune responses, and Foxp3 deficiency does not impact the functional responses of nonregulatory T cells. Furthermore, T cell-specific ablation of Foxp3 is sufficient to induce the identical early onset lymphoproliferative syndrome observed in Foxp3-deficient mice. Analysis of Foxp3 expression during thymic development suggests that this mechanism is not hard-wired but is dependent on TCR/MHC ligand interactions.

Mice lacking the transcription factor Foxp3 (Foxp3(-)) lack regulatory T (T(reg)) cells and develop fatal autoimmune pathology. In Foxp3(-) mice, many activated effector T cells express self-reactive T cell receptors that are expressed in T(reg) cells in wild-type mice. Thus, in wild-type mice, most self-reactive thymocytes escaping negative selection are diverted into the T(reg) lineage, and whether T(reg) cells are critical in self-tolerance in wild-type mice remains unknown. Here, acute in vivo ablation of T(reg) cells demonstrated a vital function for T(reg) cells in neonatal and adult mice. We suggest that self-reactive T cells are continuously suppressed by T(reg) cells and that when suppression is relieved, self-reactive T cells become activated and facilitate accelerated maturation of dendritic cells.

Regulatory T cells (T(reg) cells) expressing the forkhead family transcription factor Foxp3 are critical mediators of dominant immune tolerance to self. Most T(reg) cells constitutively express the high-affinity interleukin 2 (IL-2) receptor alpha-chain (CD25); however, the precise function of IL-2 in T(reg) cell biology has remained controversial. To directly assess the effect of IL-2 signaling on T(reg) cell development and function, we analyzed mice containing the Foxp3(gfp) knock-in allele that were genetically deficient in either IL-2 (Il2(-/-)) or CD25 (Il2ra(-/-)). We found that IL-2 signaling was dispensable for the induction of Foxp3 expression in thymocytes from these mice, which indicated that IL-2 signaling does not have a nonredundant function in the development of T(reg) cells. Unexpectedly, Il2(-/-) and Il2ra(-/-) T(reg) cells were fully able to suppress T cell proliferation in vitro. In contrast, Foxp3 was not expressed in thymocytes or peripheral T cells from Il2rg(-/-) mice. Gene expression analysis showed that IL-2 signaling was required for maintenance of the expression of genes involved in the regulation of cell growth and metabolism. Thus, IL-2 signaling seems to be critically required for maintaining the homeostasis and competitive fitness of T(reg) cells in vivo.

The regulatory T (Treg) cells restrain immune responses through suppressor-function elaboration that is dependent upon expression of the transcription factor Foxp3. Despite a critical role for Treg cells in maintaining lympho-myeloid homeostasis, it remains unclear whether a single mechanism or multiple mechanisms of Treg cell-mediated suppression are operating in vivo and how redundant such mechanisms might be. Here we addressed these questions by examining the role of the immunomodulatory cytokine IL-10 in Treg cell-mediated suppression. Analyses of mice in which the Treg cell-specific ablation of a conditional IL-10 allele was induced by Cre recombinase knocked into the Foxp3 gene locus showed that although IL-10 production by Treg cells was not required for the control of systemic autoimmunity, it was essential for keeping immune responses in check at environmental interfaces such as the colon and lungs. Our study suggests that Treg cells utilize multiple means to limit immune responses. Furthermore, these mechanisms are likely to be nonredundant, in that a distinct suppressor mechanism most likely plays a prominent and identifiable role at a particular tissue and inflammatory setting.

Regulatory CD4 1 T cells (T R cells), the development of which is critically dependent on X-linked transcription factor Foxp3 (forkhead box P3), prevent self-destructive immune responses 1 . Despite its important role, molecular and functional features conferred by Foxp3 to T R precursor cells remain unknown. It has been suggested that Foxp3 expression is required for both survival of T R precursors as well as their inability to produce interleukin (IL)-2 and independently proliferate after T-cell-receptor engagement, raising the possibility that such 'anergy' and T R suppressive capacity are intimately linked 2-4 . Here we show, by dissociating Foxp3-dependent features from those induced by the signals preceding and promoting its expression in mice, that the latter signals include several functional and transcriptional hallmarks of T R cells. Although its function is required for T R cell suppressor activity, Foxp3 to a large extent amplifies and fixes pre-established molecular features of T R cells, including anergy and dependence on paracrine IL-2. Furthermore, Foxp3 solidifies T R cell lineage stability through modification of cell surface and signalling molecules, resulting in adaptation to the signals required to induce and maintain T R cells. This adaptation includes Foxp3-dependent repression of cyclic nucleotide phosphodiesterase 3B, affecting genes responsible for T R cell homeostasis.In males, Foxp3 deficiency results in fatal early-onset systemic autoimmune disease 5 . In heterozygote Foxp3 wt/null females only one-half of T cells harbours the mutant Foxp3 allele due to random X-chromosome inactivation, whereas autoimmunity is controlled by a normal T R population expressing the Foxp3 wild-type allele. Thus, we were able to genetically mark cells actively transcribing a Foxp3 null allele, yet lacking Foxp3 protein (hereafter called T FN for Foxp3 nullexpressing T cells), through an in-frame insertion of GFP into a stop-codon-disrupted Foxp3 locus (Foxp3 gfpko ) and investigate their features in mice ( Fig. 1a; see also Supplementary Figs 1 and 2a). Female Foxp3 gfpko/wt mice were healthy, whereas male Foxp3 gfpko mice developed the same severity of autoimmunity as Foxp3 knockout (Foxp3 null ) mice 6 , resulting in death at ,4 weeks of age. Thymocyte and peripheral lymphoid organ cellularity did not differ between Foxp3 gfpko/wt and Foxp3 gfp/gfp mice, nor did the proportion of Foxp3 1 T R cells and Foxp3 2 CD4 1 T cells (data not shown). As our main focus was to characterize T FN cells in healthy Foxp3 gfpko/wt mice, analysis of autoimmune male Foxp3 gfpko mice is included as Supplementary Fig. 2.T FN cells constituted ,1-3% of mature CD4 1 thymocytes and peripheral CD4 1 T cells, indicating that Foxp3 is not required to rescue T R precursors from negative selection (Fig. 1b, c). This is consistent with a reported abundance of T-cell receptors (TCRs) characteristic of T R cells in Foxp3 null mice 7 . As ectopic expression of Foxp3 has been shown to induce a state of hyporesponsiveness in CD4 1 T cell...

MicroRNAs (miRNAs) are implicated in the differentiation and function of many cell types. We provide genetic and in vivo evidence that the two RNaseIII enzymes, Drosha and Dicer, do indeed function in the same pathway. These have previously been shown to mediate the stepwise maturation of miRNAs (Lee, Y., C. Ahn, J. Han, H. Choi, J. Kim, J. Yim, J. Lee, P. Provost, O. Radmark, S. Kim, and V.N. Kim. 2003. Nature. 425:415–419), and genetic ablation of either within the T cell compartment, or specifically within Foxp3+ regulatory T (T reg) cells, results in identical phenotypes. We found that miRNA biogenesis is indispensable for the function of T reg cells. Specific deletion of either Drosha or Dicer phenocopies mice lacking a functional Foxp3 gene or Foxp3+ cells, whereas deletion throughout the T cell compartment also results in spontaneous inflammatory disease, but later in life. Thus, miRNA-dependent regulation is critical for preventing spontaneous inflammation and autoimmunity.

Regulatory Foxp3 ؉ T cells (TR) are indispensable for preventing autoimmune pathology in multiple organs and tissues. During thymic differentiation T cell receptor (TCR)-ligand interactions within a certain increased affinity range, in conjunction with ␥c-containing cytokine receptor signals, induce Foxp3 expression and thereby commit developing thymocytes to the T R lineage. The contribution of distinct MHC class II-expressing accessory cell types to the differentiation process of Foxp3 ؉ thymocytes remains controversial, because a unique role in this process has been ascribed to either thymic dendritic cells (tDC) or to medullary thymic epithelial cells (mTEC). Furthermore, it was suggested that the thymic medulla, where the bulk of the negative selection of self-reactive thymocytes takes place, provides a specialized microenvironment supporting T R differentiation. Here, we report that the cortex, as defined by cortical thymic epithelial cells (cTEC), is sufficient for supporting T R differentiation. MHC class II expression restricted to both cTEC and mTEC or to cTEC alone did not significantly affect the numbers of Foxp3 ؉ thymocytes. Furthermore, genetic or pharmacologic blockade of thymocyte migration resulted in a prominent accumulation of Foxp3 ؉ thymocytes in the cortex, demonstrating that secondary signals required for Foxp3 up-regulation exist in the cortex. Our results suggest that mTEC or tDC do not serve as a cell type singularly responsible for T R differentiation and that neither the cortex nor the medulla exclusively provides an environment suitable for Foxp3 induction. Instead, multiple accessory cell types probably contribute to the thymic generation of regulatory Foxp3 ؉ T cells.immune tolerance ͉ selection ͉ thymus

Mouse small intestine intraepithelial lymphocytes (IEL) that express αβTCR and CD8αα homodimers are an enigmatic T cell subset, as their specificity and in vivo function remain to be defined. To gain insight into the nature of these cells, we performed global gene expression profiling using microarray analysis combined with real-time quantitative PCR and flow cytometry. Using these methods, TCRαβ+CD8αα IEL were compared with their TCRαβ+CD8β+ and TCRγδ+ counterparts. Interestingly, TCRαβ+CD8αα IEL were found to preferentially express genes that would be expected to down-modulate their reactivity. They have a unique expression pattern of members of the Ly49 family of NK receptors and tend to express inhibitory receptors, along with some activating receptors. The signaling machinery of both TCRαβ+CD8αα and TCRγδ+ IEL is constructed differently than other IEL and peripheral T cells, as evidenced by their low-level expression of the linker for activation of T cells and high expression of the non-T cell activation linker, which suppresses T cell activation. The TCRαβ+CD8αα IEL subset also has increased expression of genes that could be involved in immune regulation, including TGF-β3 and lymphocyte activation gene-3. Collectively, these data underscore the fact that, while TCRαβ+CD8αα IEL resemble TCRγδ+ IEL, they are a unique population of cells with regulated Ag reactivity that could have regulatory function.