Foxp3⁺regulatory T cells promiscuously accept thymic signals critical for their development

Abstract

Foxp3 ؉ regulatory T cells develop in the thymus and are essential for maintaining peripheral tolerance to self tissues. We report the critical requirement for CD154 up-regulation specifically on, and during the thymic development of, Foxp3 ؉ regulatory T cells for the induction of their clonal expansion within the medulla. In the absence of this signal, there was a severe reduction in their thymic generation and output, leading to decreased peripheral numbers. Importantly, CD40 expression on either thymic dendritic or epithelial cells was sufficient to promote the development of normal numbers of Foxp3 ؉ regulatory T cells. This work suggests that CD154-transduced signals promote Foxp3 ؉ regulatory T cell development and highlights the plasticity of the thymic stroma for supporting their generation. Crucially, this study demonstrates that Foxp3 ؉ regulatory T cells can promiscuously accept a single critical signal necessary for their thymic development from different cellular sources, redefining our understanding of their generation.CD154 ͉ thymus ͉ thymic epithelial cells ͉ CD40 R egulatory T cells (Tregs) actively suppress autoreactive T cells, thereby limiting aberrant immune responses and maintaining tolerance toward self tissues (1). Deficiencies in this T cell subset in humans and mice can lead to severe autoimmunity. Initially, these suppressor cells were identified as CD4 ϩ CD25 ϩ T cells (2), a phenotype that also included activated CD4 ϩ T cells, but, recently, Foxp3 has been characterized as the master regulator of their development and function (3-5) and is used as a highly specific marker for Tregs (6). Foxp3 ϩ Tregs develop in the thymus as part of the normal T cell ontogeny processes (7), and Foxp3 expression is largely restricted to the thymic medulla and CD4 ϩ CD8 Ϫ (CD4SP) thymocyte subset; the end stage of CD4 ϩ T cell development. Thymic output of Foxp3 ϩ Tregs is temporally delayed (8), and their development requires a highaffinity self-reactive T cell receptor (9, 10), the costimulatory molecule CD28 (11), and possibly IL-2 (12). It has been suggested that medullary thymic epithelial cells (mTECs), which express and present tissue-specific antigens (TSAs) on MHC class II (MHC II) molecules, drive Foxp3 ϩ Treg selection (13). Similarly, dendritic cells (DCs) are thought to induce CD4 ϩ CD25 ϩ Tregs in the human thymus through a T cell antigen receptor (TCR)-peptide-MHC II interaction (14). However, the relative contributions of thymic dendritic and epithelial cells in promoting and supporting the development of a polyclonal Foxp3 ϩ Treg repertoire are not clear. In addition, the cellular origin and temporal importance of the signals necessary for Foxp3 ϩ Treg development have not been determined, and, therefore, the rules governing how developing Foxp3 ϩ Tregs receive and process these signals are poorly understood.We report that the CD40-CD154 pathway is critical for promoting the thymic development of Foxp3 ϩ Tregs. CD154 up-regulation specifically on developing Foxp3 ϩ Tregs promoted the...

A Role for Intrathymic B Cells in the Generation of Natural Regulatory T Cells

“…The thymus harbors a network of APCs including cortical TECs, mTECs, and DCs. Evidence indicates these cells could act coordinately to control T cell development, as deletion of MHC II from either B.M.-derived cells or thymic epithelial cells still allows Treg development (41,48,49). The question remains whether the diverse range of intrathymic APCs make individual and specific contributions to the development of Tregs with unique specificities (47).…”

Autonomous role of medullary thymic epithelial cells in central CD4+ T cell tolerance

“…In both transgenic systems analyzed, DC ablation had minimal if any bearing on the fate of CD4 + T cells specific for mTEC-derived antigens, substantiating the notion that antigen recognition on mTECs can promote both deletion and T reg development 19 . Other stromal APCs have been implicated in shaping of the T reg repertoire [38][39][40][41][42] , and it is conceivable that Finally, there are several explanations for the relatively minor increase of the polyclonal T reg pool in C2TAkd mice. In view of the 'avidity hypothesis', it is conceivable that the T reg conversion of TCR specificities that would normally be negatively selected might be compensated by the 'escape' of bona fide T reg cells as naïve cells.…”

Plasmacytoid dendritic cells resident in human thymus drive natural Treg cell development

“…48 Strengthening this possibility, defective CD40-CD40L signaling leads to autoimmunity in both humans and mice. [46][47][48] In this scenario, we propose that recognition of self-peptide-major histocompatibility complexes by high-affinity TCRs expressed on positively selected DP thymocytes may be the mechanism that triggers CD40L expression on nTreg progenitors and leads to feedback maturation of thymic pDCs in vivo, thereby facilitating a functional cross-talk between pDCs and developing thymocytes, resulting in terminal nTreg differentiation. Still, further in vivo studies are required to establish the physiologic relevance of CD40-CD40L in such cross-talk.…”

“…2 Therefore, as shown for peripheral Tregs, 46,47 the CD40-CD40L pathway may play a relevant role in nTreg development in the human thymus, as proposed in mouse. 48 Strengthening this possibility, defective CD40-CD40L signaling leads to autoimmunity in both humans and mice. [46][47][48] In this scenario, we propose that recognition of self-peptide-major histocompatibility complexes by high-affinity TCRs expressed on positively selected DP thymocytes may be the mechanism that triggers CD40L expression on nTreg progenitors and leads to feedback maturation of thymic pDCs in vivo, thereby facilitating a functional cross-talk between pDCs and developing thymocytes, resulting in terminal nTreg differentiation.…”