In the thymus, diverse populations of thymic epithelial cells (TECs), including cortical and medullary TECs and their subpopulations, have distinct roles in coordinating the development and repertoire selection of functionally competent and self-tolerant T cells. Here, we review the expanding diversity in TEC subpopulations in relation to their functions in T cell development and selection as well as their origins and development.

In the adult thymus, the development of self-tolerant thymocytes requires interactions with thymic epithelial cells (TECs). Although both cortical and medullary TECs (cTECs/mTECs) are known to arise from common bipotent TEC progenitors, the phenotype of these progenitors and the timing of the emergence of these distinct lineages remain unclear. Here, we have investigated the phenotype and developmental properties of bipotent TEC progenitors during cTEC/mTEC lineage development. We show that TEC progenitors can undergo a stepwise acquisition of first cTEC and then mTEC hallmarks, resulting in the emergence of a progenitor population simultaneously expressing the cTEC marker CD205 and the mTEC regulator Receptor Activator of NF-κB (RANK). In vivo analysis reveals the capacity of CD205+ TECs to generate functionally competent cortical and medullary microenvironments containing both cTECs and Aire+ mTECs. Thus, TEC development involves a stage in which bipotent progenitors can co-express hallmarks of the cTEC and mTEC lineages through sequential acquisition, arguing against a simple binary model in which both lineages diverge simultaneously from bipotent lineage negative TEC progenitors. Rather, our data reveal an unexpected overlap in the phenotypic properties of these bipotent TECs with their lineage-restricted counterparts.

Thymic epithelial cells (TECs) provide key instructive signals for T-cell differentiation. Thymic cortical (cTECs) and medullary (mTECs) epithelial cells constitute two functionally distinct microenvironments for T-cell development, which derive from a common bipotent TEC progenitor. While seminal studies have partially elucidated events downstream of bipotent TECs in relation to the emergence of mTECs and their progenitors, the control and timing of the emergence of the cTEC lineage, particularly in relation to that of mTEC progenitors, has remained elusive. In this review, we describe distinct models that explain cTEC/mTEC lineage divergence from common bipotent progenitors. In particular, we summarize recent studies in mice providing evidence that mTECs, including the auto-immune regulator+ subset, derive from progenitors initially endowed with phenotypic properties typically associated with the cTEC lineage. These observations support a novel “serial progression” model of TEC development, in which progenitors serially acquire cTEC lineage markers, prior to their commitment to the mTEC differentiation pathway. Gaining a better understanding of the phenotypic properties of early stages in TEC progenitor development should help in determining the mechanisms regulating cTEC/mTEC lineage development, and in strategies aimed at thymus reconstitution involving TEC therapy.

T-cell tolerance in the thymus is a key step in shaping the developing T-cell repertoire. Thymic medullary epithelial cells play multiple roles in this process including negative selection of autoreactive thymocytes, influencing thymic dendritic cell positioning, and the generation of FoxP3+ Regulatory T-cells. Previous studies show that mTEC development involves haemopoietic crosstalk, and numerous Tumour Necrosis Factor Receptor Superfamily members have been implicated in this process. While CD40 and RANK represent key examples, interplay between these receptors, and the individual cell types providing their ligands at both fetal and adult stages of thymus development, remain unclear. Here, by analysis of the cellular sources of RANKL and CD40L during fetal and adult crosstalk in the mouse, we show that innate immune cells system drive initial fetal mTEC development via expression of RANKL but not CD40L. In contrast, crosstalk involving the adaptive immune system involves both RANKL and CD40L, with analysis of distinct subsets of intrathymic CD4+ T-cells revealing a differential contribution of CD40L by conventional, but not FoxP3+ regulatory, T-cells. We also provide evidence for a stepwise involvement of TNF-Receptors in mTEC development, with CD40 up-regulation induced by initial RANK signalling subsequently controlling proliferation within the mTEC compartment. Collectively, our findings show how multiple haemopoietic cell types regulate mTEC development through differential provision of RANKL/CD40L during ontogeny, revealing molecular differences in fetal and adult haemopoietic crosstalk. They also suggest a stepwise process of mTEC development, in which RANK is a master player in controlling the availability of other TNF-Receptor family members.

White et al. describe a new mechanism of thymus emigration that is controlled by expression of the type 2 IL-4 receptor by thymic stroma and production of IL-4 and IL-13 by thymic-resident invariant NKT cells.

In the thymus, medullary thymic epithelial cells (mTEC) determine the fate of newly selected CD4+ and CD8+ single positive (SP) thymocytes. For example, mTEC expression of Aire controls intrathymic self‐antigen availability for negative selection. Interestingly, alterations in both Foxp3+ Regulatory T‐cells (T‐Reg) and conventional SP thymocytes in Aire−/− mice suggest additional, yet poorly understood, roles for Aire during intrathymic T‐cell development. To examine this, we analysed thymocytes from Aire −/− mice using Rag2GFP and Foxp3 expression, and a recently described CD69/MHCI subset definition of post‐selection CD4+ conventional thymocytes. We show that while Aire is dispensable for de novo generation of conventional αβT‐cells, it plays a key role in controlling the intrathymic T‐Reg pool. Surprisingly, a decline in intrathymic T‐Reg in Aire−/− mice maps to a reduction in mature recirculating Rag2GFP− T‐Reg that express CCR6 and re‐enter the thymus from the periphery. Furthermore, we show mTEC expression of the CCR6 ligand CCL20 is reduced in Aire−/− mice, and that CCR6 is required for T‐Reg recirculation back to the thymus. Collectively, our study re‐defines requirements for late stage intrathymic αβT‐cell development, and demonstrates that Aire controls a CCR6‐CCL20 axis that determines the developmental makeup of the intrathymic T‐Reg pool.

Thymic epithelial cells (TECs) provide essential signals for αβT‐cell development, and medullary TECs (mTECs) control T‐cell tolerance through both negative selection and Foxp3+ regulatory T (Treg) cell development. Although heterogeneity within the mTEC compartment is well studied, the molecular regulators of specific stages of mTEC development are still poorly understood. Given the importance of the RANK‐RANKL axis in thymus medulla formation, we have used RANK Venus reporter mice to analyze the ontogeny of RANK+ TECs during development and correlated RANK expression with mTEC stem cells defined by SSEA‐1. In addition, we have investigated how requirements for the key regulators Foxn1 and Relb map to specific stages of mTEC development. Here, we show SSEA‐1+ mTEC stem cells emerge prior to RANK expression and are present in both nude and Relb −/− mice, providing direct evidence that mTEC lineage specification occurs independently of Foxn1 and Relb. In contrast, we show that Relb is necessary for the effective production of downstream RANK+ mTEC progenitors. Collectively, our work defines stage‐specific requirements for critical TEC regulators during medulla development, including the timing of Relb dependency, and provides new information on mechanisms controlling mTEC specification.

In the thymus, medullary thymic epithelial cells (mTEC) regulate T cell tolerance via negative selection and Foxp3+ regulatory T cell (Treg) development, and alterations in the mTEC compartment can lead to tolerance breakdown and autoimmunity. Both the receptor activator for NF-κB (RANK)/RANK ligand (RANKL)/osteoprotegerin (OPG) axis and expression of the transcriptional regulator Aire are involved in the regulation of thymus medullary microenvironments. However, their impact on the mechanisms controlling mTEC homeostasis is poorly understood, as are the processes that enable the thymus medulla to support the balanced production of mTEC-dependent Foxp3+ Treg. In this study, we have investigated the control of mTEC homeostasis and examined how this process impacts the efficacy of Foxp3+ Treg development. Using newly generated RANK Venus reporter mice, we identify distinct RANK+ subsets that reside within both the mTEChi and mTEClo compartments and that represent direct targets of OPG-mediated control. Moreover, by mapping OPG expression to a subset of Aire+ mTEC, our data show how cis- and trans-acting mechanisms are able to control the thymus medulla by operating on multiple mTEC targets. Finally, we show that whereas the increase in mTEC availability in OPG-deficient (Tnfrsf11b−/−) mice impacts the intrathymic Foxp3+ Treg pool by enhancing peripheral Treg recirculation back to the thymus, it does not alter the number of de novo Rag2pGFP+Foxp3+ Treg that are generated. Collectively, our study defines patterns of RANK expression within the thymus medulla, and it shows that mTEC homeostasis is not a rate-limiting step in intrathymic Foxp3+ Treg production.