Abrogation of CD40-CD154 signaling impedes the homeostasis of thymic resident regulatory T cells by altering the levels of IL-2, but does not affect regulatory T cell development

In or out of control: Modulating regulatory T cell homeostasis and function with immune checkpoint pathways

Regulatory T cells (Tregs) are the master regulators of immunity and they have been implicated in different disease states such as infection, autoimmunity and cancer. Since their discovery, many studies have focused on understanding Treg development, differentiation, and function. While there are many players in the generation and function of truly suppressive Tregs, the role of checkpoint pathways in these processes have been studied extensively. In this paper, we systematically review the role of different checkpoint pathways in Treg homeostasis and function. We describe how co-stimulatory and co-inhibitory pathways modulate Treg homeostasis and function and highlight data from mouse and human studies. Multiple checkpoint pathways are being targeted in cancer and autoimmunity; therefore, we share insights from the clinic and discuss the effect of experimental and approved therapeutics on Treg biology.

Instructive Cues of Thymic T Cell Selection

A high diversity of αβ T cell receptors (TCRs), capable of recognizing virtually any pathogen but also self-antigens, is generated during T cell development in the thymus. Nevertheless, a strict developmental program supports the selection of a self-tolerant T cell repertoire capable of responding to foreign antigens. The steps of T cell selection are controlled by cortical and medullary stromal niches, mainly composed of thymic epithelial cells and dendritic cells. The integration of important cues provided by these specialized niches, including (a) the TCR signal strength induced by the recognition of self-peptide-MHC complexes, (b) costimulatory signals, and (c) cytokine signals, critically controls T cell repertoire selection. This review discusses our current understanding of the signals that coordinate positive selection, negative selection, and agonist selection of Foxp3⁺ regulatory T cells. It also highlights recent advances that have unraveled the functional diversity of thymic antigen-presenting cell subsets implicated in T cell selection.

IL-2 and IL-15 dependent thymic development of Foxp3-expressing regulatory T lymphocytes

Immunosuppressive regulatory T lymphocytes (Treg) expressing the transcription factor Foxp3 play a vital role in the maintenance of tolerance of the immune-system to self and innocuous non-self. Most Treg that are critical for the maintenance of tolerance to self, develop as an independent T-cell lineage from common T cell precursors in the thymus. In this organ, their differentiation requires signals from the T cell receptor for antigen, from co-stimulatory molecules, as well as from cytokine-receptors. Here we focus on the cytokines implicated in thymic development of Treg, with a particular emphasis on the roles of interleukin-2 (IL-2) and IL-15. The more recently appreciated involvement of TGF-β in thymic Treg development is also briefly discussed. Finally, we discuss how cytokine-dependence of Treg development allows for temporal, quantitative, and potentially qualitative modulation of this process.

Unique properties of thymic antigen-presenting cells promote epigenetic imprinting of alloantigen-specific regulatory T cells

Regulatory T cells (Tregs) are potential immunotherapeutic candidates to induce transplantation tolerance. However, stability of Tregs still remains contentious and may potentially restrict their clinical use. Recent work suggested that epigenetic imprinting of Foxp3 and other Treg-specific signature genes is crucial for stabilization of immunosuppressive properties of Foxp3⁺ Tregs, and that these events are initiated already during early stages of thymic Treg development. However, the mechanisms governing this process remain largely unknown. Here we demonstrate that thymic antigen-presenting cells (APCs), including thymic dendritic cells (t-DCs) and medullary thymic epithelial cells (mTECs), can induce a more pronounced demethylation of Foxp3 and other Treg-specific epigenetic signature genes in developing Tregs when compared to splenic DCs (sp-DCs). Transcriptomic profiling of APCs revealed differential expression of secreted factors and costimulatory molecules, however neither addition of conditioned media nor interference with costimulatory signals affected Foxp3 induction by thymic APCs in vitro. Importantly, when tested in vivo both mTEC- and t-DC-generated alloantigen-specific Tregs displayed significantly higher efficacy in prolonging skin allograft acceptance when compared to Tregs generated by sp-DCs. Our results draw attention to unique properties of thymic APCs in initiating commitment towards stable and functional Tregs, a finding that could be highly beneficial in clinical immunotherapy.

Control of the thymic medulla and its influence on αβT-cell development

The thymus is a primary lymphoid tissue that supports the generation of αβT cells. In this review, we describe the processes that give rise to the thymus medulla, a site that nurtures self-tolerant T-cell generation following positive selection events that take place in the cortex. To summarize the developmental pathways that generate medullary thymic epithelial cells (mTEC) from their immature progenitors, we describe work on both the initial emergence of the medulla during embryogenesis, and the maintenance of the medulla during postnatal stages. We also investigate the varying roles that receptors belonging to the tumor necrosis factor receptor superfamily have on thymus medulla development and formation, and highlight the impact that T-cell development has on thymus medulla formation. Finally, we examine the evidence that the thymic medulla plays an important role during the intrathymic generation of distinct αβT-cell subtypes. Collectively, these studies provide new insight into the development and functional importance of medullary microenvironments during self-tolerant T-cell production in the thymus.

Development of T-cell tolerance utilizes both cell-autonomous and cooperative presentation of self-antigen

The development of T-cell self-tolerance in the thymus is important for establishing immune homeostasis and preventing autoimmunity. Here, we review the components of T-cell tolerance, which includes T-cell receptor (TCR) self-reactivity, costimulation, cytokines, and antigen presentation by a variety of antigen-presenting cells (APCs) subsets. We discuss the current evidence on the process of regulatory T (Treg) cell and negative selection and the importance of TCR signaling. We then examine recent evidence showing unique roles for bone marrow (BM)-derived APCs and medullary thymic epithelial cells (mTECs) on the conventional and Treg TCR repertoire, as well as emerging data on the role of B cells in tolerance. Finally, we review the accumulating data that suggest that cooperative antigen presentation is a prominent component of T -ell tolerance. With the development of tools to interrogate the function of individual APC subsets in the medulla, we have gained greater understanding of the complex cellular and molecular events that determine T-cell tolerance.

Osteoprotegerin-Mediated Homeostasis of Rank+ Thymic Epithelial Cells Does Not Limit Foxp3+ Regulatory 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 mTEC(hi) and mTEC(lo) 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.

CD28-CD80/86 and CD40-CD40L Interactions Promote Thymic Tolerance by Regulating Medullary Epithelial Cell and Thymocyte Development

Development and central tolerance of T lymphocytes in the thymus requires both TCR signals and collaboration with signals generated through costimulatory molecule interactions. In this review, we discuss the importance of CD28-CD80/86 and CD40-CD40L costimulatory interactions in promoting normal thymic development. This discussion includes roles in the generation of a normal thymic medulla, in the development of specific T-cells subsets, including iNKT and T regulatory cells, and in the generation of a tolerant mature T-cell repertoire. We discuss recent contributions to the understanding of CD28-CD80/86 and CD40-CD40L costimulatory interactions in thymic development, and we highlight the ways in which the many important roles mediated by these interactions collaborate to promote normal thymic development.

Thymus medulla fosters generation of natural Treg cells, invariant γδ T cells, and invariant NKT cells: what we learn from intrathymic migration

The organization of the thymus into distinct cortical and medullary regions enables it to control the step-wise migration and development of immature T-cell precursors. Such a process provides access to specialized cortical and medullary thymic epithelial cells at defined stages of maturation, ensuring the generation of self-tolerant and MHC-restricted conventional CD4⁺ and CD8⁺ αβ T cells. The migratory cues and stromal cell requirements that regulate the development of conventional αβ T cells have been well studied. However, the thymus also fosters the generation of several immunoregulatory T-cell populations that form key components of both innate and adaptive immune responses. These include Foxp3⁺ natural regulatory T cells, invariant γδ T cells, and CD1d-restricted invariant natural killer T cells (iNKT cells). While less is known about the intrathymic requirements of these nonconventional T cells, recent studies have highlighted the importance of the thymus medulla in their development. Here, we review recent findings on the mechanisms controlling the intrathymic migration of distinct T-cell subsets, and relate this to knowledge of the microenvironmental requirements of these cells.

NF-κB-inducing kinase in thymic stroma establishes central tolerance by orchestrating cross-talk with not only thymocytes but also dendritic cells

Essential roles of NF-κB-inducing kinase (NIK) for the development of medullary thymic epithelial cells (mTECs) and regulatory T cells have been highlighted by studies using a strain of mouse bearing a natural mutation of the NIK gene (aly mice). However, the exact mechanisms underlying the defect in thymic cross-talk leading to the breakdown of self-tolerance in aly mice remain elusive. In this study, we demonstrated that production of regulatory T cells and the final maturation process of positively selected conventional αβ T cells are impaired in aly mice, partly because of a lack of mature mTECs. Of note, numbers of thymic dendritic cells and their expression of costimulatory molecules were also affected in aly mice in a thymic stroma-dependent manner. The results suggest a pivotal role of NIK in the thymic stroma in establishing self-tolerance by orchestrating cross-talk between mTECs and dendritic cells as well as thymocytes. In addition, we showed that negative selection was impaired in aly mice as a result of the stromal defect, which accounts for the development of organ-specific autoimmunity through a lack of normal NIK.

Differential requirement for IL-2 and IL-15 during bifurcated development of thymic regulatory T cells

The developmental pathways of regulatory T cells (T(reg)) generation in the thymus are not fully understood. In this study, we reconstituted thymic development of Zap70-deficient thymocytes with a tetracycline-inducible Zap70 transgene to allow temporal dissection of T(reg) development. We find that T(reg) develop with distinctive kinetics, first appearing by day 4 among CD4 single-positive (SP) thymocytes. Accepted models of CD25(+)Foxp3(+) T(reg) selection suggest development via CD25(+)Foxp3(-) CD4 SP precursors. In contrast, our kinetic analysis revealed the presence of abundant CD25(-)Foxp3(+) cells that are highly efficient at maturing to CD25(+)Foxp3(+) cells in response to IL-2. CD25(-)Foxp3(+) cells more closely resembled mature T(reg) both with respect to kinetics of development and avidity for self-peptide MHC. These population also exhibited distinct requirements for cytokines during their development. CD25(-)Foxp3(+) cells were IL-15 dependent, whereas generation of CD25(+)Foxp3(+) specifically required IL-2. Finally, we found that IL-2 and IL-15 arose from distinct sources in vivo. IL-15 was of stromal origin, whereas IL-2 was of exclusively from hemopoetic cells that depended on intact CD4 lineage development but not either Ag-experienced or NKT cells.

Both retention and recirculation contribute to long-lived regulatory T-cell accumulation in the thymus

Natural Treg cells acquire their lineage-determining transcription factor Foxp3 during development in the thymus and are important in maintaining immunologic tolerance. Here, we analyzed the composition of the thymic Treg-cell pool using RAG2-GFP/FoxP3-RFP dual reporter mice and found that a population of long-lived GFP(-) Treg cells exists in the thymus. These long-lived Treg cells substantially increased with age, to a point where they represent >90% of the total thymic Treg-cell pool at 6 months of age. In contrast, long-lived conventional T cells remained at ∼ 15% of the total thymic pool at 6 months of age. Consistent with these studies, we noticed that host-derived Treg cells represented a large fraction (∼ 10%) of the total thymic Treg-cell pool in bone marrow chimeras, suggesting that long-lived Treg cells also reside in the thymus of these mice. The pool of long-lived Treg cells in the thymus was sustained by retention of Treg cells in the thymus and by recirculation of peripheral Treg cells back into the thymus. These long-lived thymic Treg cells suppressed T-cell proliferation to an equivalent extent to splenic Treg cells. Together, these data demonstrate that long-lived Treg cells accumulate in the thymus by both retention and recirculation.

Immunoglobulin class switch recombination deficiency type 1 or CD40 ligand deficiency: from bedside to bench and back again

The immunoglobulin class switch recombination deficiency or hyper-IgM syndrome is characterized by normal or elevated serum IgM and low serum levels of other immunoglobulins. Since the first reported patient with hyper-IgM, more than 200 patients with this phenotype resulted from CD40 ligand deficiency have been reported. However, in addition to this common finding, they presented with different manifestations like opportunistic infections, autoimmunity and malignancies each of them are worth a detailed look. In this review, we will focus on different underlying mechanisms of these presentations to review what we have learned from our patients. In the end, we will discuss different treatment options available for these patients using this knowledge.

PTPN22 silencing in the NOD model indicates the type 1 diabetes-associated allele is not a loss-of-function variant

PTPN22 encodes the lymphoid tyrosine phosphatase (LYP) and is the second strongest non-HLA genetic risk factor for type 1 diabetes. The PTPN22 susceptibility allele generates an LYP variant with an arginine-to-tryptophan substitution at position 620 (R620W) that has been reported by several studies to impart a gain of function. However, a recent report investigating both human cells and a knockin mouse model containing the R620W homolog suggested that this variation causes faster protein degradation. Whether LYP R620W is a gain- or loss-of-function variant, therefore, remains controversial. To address this issue, we generated transgenic NOD mice (nonobese diabetic) in which Ptpn22 can be inducibly silenced by RNA interference. We found that Ptpn22 silencing in the NOD model replicated many of the phenotypes observed in C57BL/6 Ptpn22 knockout mice, including an increase in regulatory T cells. Notably, loss of Ptpn22 led to phenotypic changes in B cells opposite to those reported for the human susceptibility allele. Furthermore, Ptpn22 knockdown did not increase the risk of autoimmune diabetes but, rather, conferred protection from disease. Overall, to our knowledge, this is the first functional study of Ptpn22 within a model of type 1 diabetes, and the data do not support a loss of function for the PTPN22 disease variant.

TNFR2 is critical for the stabilization of the CD4+Foxp3+ regulatory T. cell phenotype in the inflammatory environment

Several lines of evidence indicate the instability of CD4(+)Foxp3(+) regulatory T cells (Tregs). We have therefore investigated means of promoting the stability of Tregs. In this study, we found that the proportion of Tregs in mouse strains deficient in TNFR2 or its ligands was reduced in the thymus and peripheral lymphoid tissues, suggesting a potential role of TNFR2 in promoting the sustained expression of Foxp3. We observed that upon in vitro activation with plate-bound anti-CD3 Ab and soluble anti-CD28 Ab, Foxp3 expression by highly purified mouse Tregs was markedly downregulated. Importantly, TNF partially abrogated this effect of TCR stimulation and stabilized Foxp3 expression. This effect of TNF was blocked by anti-TNFR2 Ab, but not by anti-TNFR1 Ab. Furthermore, TNF was not able to maintain Foxp3 expression by TNFR2-deficient Tregs. In a mouse colitis model induced by transfer of naive CD4 cells into Rag1(-/-) mice, the disease could be inhibited by cotransfer of wild-type Tregs, but not by cotransfer of TNFR2-deficient Tregs. Furthermore, in the lamina propria of the colitis model, most wild-type Tregs maintained Foxp3 expression. In contrast, an increased number of TNFR2-deficient Tregs lost Foxp3 expression. Thus, our data clearly show that TNFR2 is critical for the phenotypic and functional stability of Tregs in the inflammatory environment. This effect of TNF should be taken into account when designing future therapy of autoimmunity and graft-versus-host disease by using TNF inhibitors.