T cells are classically recognized as distinct subsets that express αβ or γδ TCRs. We identify a novel population of T cells that coexpress αβ and γδ TCRs in mice and humans. These hybrid αβ-γδ T cells arose in the murine fetal thymus by day 16 of ontogeny, underwent αβ TCR–mediated positive selection into CD4+ or CD8+ thymocytes, and constituted up to 10% of TCRδ+ cells in lymphoid organs. They expressed high levels of IL-1R1 and IL-23R and secreted IFN-γ, IL-17, and GM-CSF in response to canonically restricted peptide antigens or stimulation with IL-1β and IL-23. Hybrid αβ-γδ T cells were transcriptomically distinct from conventional γδ T cells and displayed a hyperinflammatory phenotype enriched for chemokine receptors and homing molecules that facilitate migration to sites of inflammation. These proinflammatory T cells promoted bacterial clearance after infection with Staphylococcus aureus and, by licensing encephalitogenic Th17 cells, played a key role in the development of autoimmune disease in the central nervous system.

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 alpha beta T-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 alpha beta T-cell development, and demonstrates that Aire controls a CCR6-CCL20 axis that determines the developmental makeup of the intrathymic T-Reg pool.

In the thymus, stromal microenvironments support a developmental program that generates mature T cells ready for thymic exit. The cellular and molecular specialization within thymic stromal cells that enables their regulation of specific stages of thymocyte development is poorly understood. Here, we show the thymic microenvironment expresses the type 2 IL-4R complex and is functionally responsive to its known ligands, IL-4 and IL-13. Absence of IL-4Rα limits thymocyte emigration, leading to an intrathymic accumulation of mature thymocytes within medullary perivascular spaces and reduced numbers of recent thymic emigrants. Thymus transplantation shows this requirement maps to IL-4Rα expression by stromal cells, and we provide evidence that it regulates thymic exit via a process distinct from S1P-mediated migration. Finally, we reveal a cellular mechanism by which IL-4+IL-13+ invariant NKT cells are necessary for IL-4Rα signaling that regulates thymic exit. Collectively, we define a new axis for thymic emigration involving stimulation of the thymic microenvironment via type 2 cytokines from innate T cells.

The development of CD4 helper and CD8 cytotoxic T-cells expressing the ab form of the T-cell receptor (abTCR) takes place in the thymus, a primary lymphoid organ containing distinct cortical and medullary microenvironments. While the cortex represents a site of early T-cell precursor development, and the positive selection of CD48 thymocytes, the thymic medulla plays a key role in tolerance induction, ensuring that thymic emigrants are purged of autoreactive abTCR specificities. In recent years, advances have been made in understanding the development and function of thymic medullary epithelial cells, most notably the subset defined by expression of the Autoimmune Regulator (Aire) gene. Here, we summarize current knowledge of the developmental mechanisms regulating thymus medulla development, and examine the role of the thymus medulla in recessive (negative selection) and dominant (T-regulatory cell) tolerance.

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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.

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.

The thymus is a primary lymphoid tissue that is unique in its ability to support a full program of T cell development. Thymic microenvironments consist of anatomically distinct cortical and medullary areas that shape the developing T cell repertoire in a step-wise fashion to become both self-tolerant and responsive to foreign antigens. Specialized epithelial cells in the cortex and medulla, together with mesenchyme populations that are becoming increasingly well defined, are key in providing the signals that control T cell development. Here, we outline our current understanding of the developmental pathways that give rise to distinct cortical and medullary epithelial cell lineages in the thymus. We also summarize knowledge of the functional heterogeneity of thymic stromal cells in relation to their roles in alpha beta T cell development.

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.

The development of CD4(+) helper and CD8(+) cytotoxic T-cells expressing the αβ form of the T-cell receptor (αβTCR) takes place in the thymus, a primary lymphoid organ containing distinct cortical and medullary microenvironments. While the cortex represents a site of early T-cell precursor development, and the positive selection of CD4(+)8(+) thymocytes, the thymic medulla plays a key role in tolerance induction, ensuring that thymic emigrants are purged of autoreactive αβTCR specificities. In recent years, advances have been made in understanding the development and function of thymic medullary epithelial cells, most notably the subset defined by expression of the Autoimmune Regulator (Aire) gene. Here, we summarize current knowledge of the developmental mechanisms regulating thymus medulla development, and examine the role of the thymus medulla in recessive (negative selection) and dominant (T-regulatory cell) tolerance.

Lymphocytes normally develop within anatomically distinct tissues. In Cell Reports, Swann et al. (2014) reconstruct the primordial thymus and suggest that it was a site of combined T and B lymphopoiesis before evolving into an organ specialized for T cell production.

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.

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.

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 medullary thymic epithelial cell (mTEC) development involves hemopoietic cross-talk, and numerous TNFR superfamily members have been implicated in this process. Whereas 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. In this study, by analysis of the cellular sources of receptor activator for NF-κB ligand (RANKL) and CD40L during fetal and adult cross-talk in the mouse, we show that the innate immune cell system drives initial fetal mTEC development via expression of RANKL, but not CD40L. In contrast, cross-talk 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 TNFRs in mTEC development, with CD40 upregulation induced by initial RANK signaling subsequently controlling proliferation within the mTEC compartment. Collectively, our findings show how multiple hemopoietic cell types regulate mTEC development through differential provision of RANKL/CD40L during ontogeny, revealing molecular differences in fetal and adult hemopoietic cross-talk. They also suggest a stepwise process of mTEC development, in which RANK is a master player in controlling the availability of other TNFR family members.

Toll-like receptors (TLRs), which recognize structurally conserved components among pathogens, are mainly expressed by antigen-presenting cells such as dendritic cells (DCs), B cells, and macrophages. Recognition through TLRs triggers innate immune responses and influences antigen-specific adaptive immune responses. Although studies on the expression and functions of TLRs in antigen-presenting cells have been extensively reported, studies in lymphoid tissue inducer (LTi) cells have been limited. In this study, we observed that LTi cells expressed TLR2 and TLR4 mRNA as well as TLR2 protein and upregulated OX40L, CD30L, and TRANCE expression after stimulation with the TLR2 ligand zymosan or TLR4 ligand LPS. The expression of tumor necrosis factor superfamily (TNFSF) members was significantly upregulated when cells were cocultured with DCs, suggesting that upregulated TNFSF expression may contribute to antigen-specific adaptive immune responses.

Here, we identify cells within human adult secondary lymphoid tissues that are comparable in phenotype and location to the lymphoid tissue inducer (LTi) cells that persist in the adult mouse. Identified as CD117 + CD3 − CD56 − cells, like murine LTi cells, they lack expression of many common lineage markers and express CD127, OX40L and TRANCE. These cells were detected at the interface between the B‐ and T‐ zones, as well as at the subcapsular sinus in LNs, the location where LTi cells reside in murine spleen and LNs. Furthermore, like murine LTi cells, these cells expressed high levels of IL‐22 and upregulated IL‐22 expression upon IL‐23 stimulation. Importantly, these cells were not an NK cell subset since they showed no expression of IFN‐γ and perforin. Interestingly, a subset of the CD117 + CD3 − CD56 − OX40L + population expressed NKp46, again similar to recent findings in mice. Finally, these cells supported memory CD4 + T‐cell survival in an OX40L‐dependent manner. Combined, these data indicate that the CD117 + CD3 − CD56 − OX40L + cells in human secondary lymphoid tissues are comparable in phenotype, location and function to the LTi cells that persist within adult murine secondary lymphoid tissues.