Osteoprotegerin-Mediated Homeostasis of Rank+ Thymic Epithelial Cells Does Not Limit Foxp3+ Regulatory T Cell Development

Generation and repair of thymic epithelial cells

In the vertebrate immune system, thymus stromal microenvironments support the generation of αβT cells from immature thymocytes. Thymic epithelial cells are of particular importance, and the generation of cortical and medullary epithelial lineages from progenitor stages controls the initiation and maintenance of thymus function. Here, we discuss the developmental pathways that regulate thymic epithelial cell diversity during both the embryonic and postnatal periods. We also examine how thymus microenvironments respond to injury, with particular focus on mechanisms that ensure regeneration of thymic epithelial cells for the restoration of thymus function.

Developmental conversion of thymocyte-attracting cells into self-antigen-displaying cells in embryonic thymus medulla epithelium

Thymus medulla epithelium establishes immune self-tolerance and comprises diverse cellular subsets. Functionally relevant medullary thymic epithelial cells (mTECs) include a self-antigen-displaying subset that exhibits genome-wide promiscuous gene expression promoted by the nuclear protein Aire and that resembles a mosaic of extrathymic cells including mucosal tuft cells. An additional mTEC subset produces the chemokine CCL21, thereby attracting positively selected thymocytes from the cortex to the medulla. Both self-antigen-displaying and thymocyte-attracting mTEC subsets are essential for self-tolerance. Here, we identify a developmental pathway by which mTECs gain their diversity in functionally distinct subsets. We show that CCL21-expressing mTECs arise early during thymus ontogeny in mice. Fate-mapping analysis reveals that self-antigen-displaying mTECs, including Aire-expressing mTECs and thymic tuft cells, are derived from CCL21-expressing cells. The differentiation capability of CCL21-expressing embryonic mTECs is verified in reaggregate thymus experiments. These results indicate that CCL21-expressing embryonic mTECs carry a developmental potential to give rise to self-antigen-displaying mTECs, revealing that the sequential conversion of thymocyte-attracting subset into self-antigen-displaying subset serves to assemble functional diversity in the thymus medulla epithelium.

Developmental conversion of thymocyte-attracting cells into self-antigen-displaying cells in embryonic thymus medulla epithelium

Thymus medulla epithelium establishes immune self-tolerance and comprises diverse cellular subsets. Functionally relevant medullary thymic epithelial cells (mTECs) include a self-antigen-displaying subset that exhibits genome-wide promiscuous gene expression promoted by the nuclear protein Aire and that resembles a mosaic of extrathymic cells including mucosal tuft cells. An additional mTEC subset produces the chemokine CCL21, thereby attracting positively selected thymocytes from the cortex to the medulla. Both self-antigen-displaying and thymocyte-attracting mTEC subsets are essential for self-tolerance. Here we identify a developmental pathway by which mTECs gain their diversity in functionally distinct subsets. We show that CCL21-expressing mTECs arise early during thymus ontogeny. Fate-mapping analysis reveals that self-antigen-displaying mTECs, including Aire-expressing mTECs and thymic tuft cells, are derived from CCL21-expressing cells. The differentiation capability of CCL21-expressing embryonic mTECs is verified in reaggregate thymus experiments. These results indicate that CCL21-expressing embryonic mTECs carry a developmental potential to give rise to self-antigen-displaying mTECs, revealing that the sequential conversion of thymocyte-attracting subset into self-antigen-displaying subset serves to assemble functional diversity in the thymus medulla epithelium.

Thymic mimetic cells function beyond self-tolerance

Development of immunocompetent T cells in the thymus is required for effective defence against all types of pathogens, including viruses, bacteria and fungi. To this end, T cells undergo a very strict educational program in the thymus, during which both non-functional and self-reactive T cell clones are eliminated by means of positive and negative selection1.Thymic epithelial cells (TECs) have an indispensable role in these processes, and previous studies have shown the notable heterogeneity of these cells2-7. Here, using multiomic analysis, we provide further insights into the functional and developmental diversity of TECs in mice, and reveal a detailed atlas of the TEC compartment according to cell transcriptional states and chromatin landscapes. Our analysis highlights unconventional TEC subsets that are similar to functionally well-defined parenchymal populations, including endocrine cells, microfold cells and myocytes. By focusing on the endocrine and microfold TEC populations, we show that endocrine TECs require Insm1 for their development and are crucial to maintaining thymus cellularity in a ghrelin-dependent manner; by contrast, microfold TECs require Spib for their development and are essential for the generation of thymic IgA+ plasma cells. Collectively, our study reveals that medullary TECs have the potential to differentiate into various types of molecularly distinct and functionally defined cells, which not only contribute to the induction of central tolerance, but also regulate the homeostasis of other thymus-resident populations.

Immune Dysfunction in Medication-Related Osteonecrosis of the Jaw

The pathogenesis of medication-related osteonecrosis of the jaw (MRONJ) is multifactorial and there is a substantial consensus on the role of antiresorptive drugs (ARDs), including bisphosphonates (BPs) and denosumab (Dmab), as one of the main determinants. The time exposure, cumulative dose and administration intensity of these drugs are critical parameters to be considered in the treatment of patients, as cancer patients show the highest incidence of MRONJ. BPs and Dmab have distinct mechanisms of action on bone, but they also exert different effects on immune subsets which interact with bone cells, thus contributing to the onset of MRONJ. Here, we summarized the main effects of ARDs on the different immune cell subsets, which consequently affect bone cells, particularly osteoclasts and osteoblasts. Data from animal models and MRONJ patients showed a deep interference of ARDs in modulating immune cells, even though a large part of the literature concerns the effects of BPs and there is a lack of data on Dmab, demonstrating the need to further studies.

Embryonic keratin19+ progenitors generate multiple functionally distinct progeny to maintain epithelial diversity in the adult thymus medulla

The thymus medulla is a key site for immunoregulation and tolerance, and its functional specialisation is achieved through the complexity of medullary thymic epithelial cells (mTEC). While the importance of the medulla for thymus function is clear, the production and maintenance of mTEC diversity remains poorly understood. Here, using ontogenetic and inducible fate-mapping approaches, we identify mTEC-restricted progenitors as a cytokeratin19+ (K19+) TEC subset that emerges in the embryonic thymus. Importantly, labelling of a single cohort of K19+ TEC during embryogenesis sustains the production of multiple mTEC subsets into adulthood, including CCL21+ mTEClo, Aire+ mTEChi and thymic tuft cells. We show K19+ progenitors arise prior to the acquisition of multiple mTEC-defining features including RANK and CCL21 and are generated independently of the key mTEC regulator, Relb. In conclusion, we identify and define a multipotent mTEC progenitor that emerges during embryogenesis to support mTEC diversity into adult life.

Diversity in Cortical Thymic Epithelial Cells Occurs through Loss of a Foxn1-Dependent Gene Signature Driven by Stage-Specific Thymocyte Cross-Talk

In the thymus, cortical thymic epithelial cells (cTECs) and medullary thymic epithelial cells support αβT cell development from lymphoid progenitors. For cTECs, expression of a specialized gene signature that includes Cxcl12, Dll4, and Psmb11 enables the cortex to support T lineage commitment and the generation and selection of CD4+CD8+ thymocytes. Although the importance of cTECs in T cell development is well defined, mechanisms that shape the cTEC compartment and regulate its functional specialization are unclear. Using a Cxcl12DsRed reporter mouse model, we show that changes in Cxcl12 expression reveal a developmentally regulated program of cTEC heterogeneity. Although cTECs are uniformly Cxcl12DsRed+ during neonatal stages, progression through postnatal life triggers the appearance of Cxcl12DsRed- cTECs that continue to reside in the cortex alongside their Cxcl12DsRed+ counterparts. This appearance of Cxcl12DsRed- cTECs is controlled by maturation of CD4-CD8-, but not CD4+CD8+, thymocytes, demonstrating that stage-specific thymocyte cross-talk controls cTEC heterogeneity. Importantly, although fate-mapping experiments show both Cxcl12DsRed+ and Cxcl12DsRed- cTECs share a common Foxn1+ cell origin, RNA sequencing analysis shows Cxcl12DsRed- cTECs no longer express Foxn1, which results in loss of the FOXN1-dependent cTEC gene signature and may explain the reduced capacity of Cxcl12DsRed- cTECs for thymocyte interactions. In summary, our study shows that shaping of the cTEC compartment during the life course occurs via stage-specific thymocyte cross-talk, which drives loss of Foxn1 expression and its key target genes, which may then determine the functional competence of the thymic cortex.

Histopathologic and transcriptomic phenotypes of a conditional RANKL transgenic mouse thymus

Although conventional knockout and transgenic mouse models have significantly advanced our understanding of Receptor Activator of NF-κB Ligand (RANKL) signaling in intra-thymic crosstalk that establishes self-tolerance and later stages of lymphopoiesis, the unique advantages of conditional mouse transgenesis have yet to be explored. A main advantage of conditional transgenesis is the ability to express a transgene in a spatiotemporal restricted manner, enabling the induction (or de-induction) of transgene expression during predetermined stages of embryogenesis or during defined postnatal developmental or physiological states, such as puberty, adulthood, and pregnancy. Here, we describe the K5: RANKL bigenic mouse, in which transgene derived RANKL expression is induced by doxycycline and targeted to cytokeratin 5 positive medullary thymic epithelial cells (mTECs). Short-term doxycycline induction reveals that RANKL transgene expression is significantly induced in the thymic medulla and only in response to doxycycline. Prolonged doxycycline induction in the K5: RANKL bigenic results in a significantly enlarged thymus in which mTECs are hyperproliferative. Flow cytometry showed that there is a marked enrichment of CD4+ and CD8+ single positive thymocytes with a concomitant depletion of CD4+ CD8+ double positives. Furthermore, there is an increase in the number of FOXP3+ T regulatory (Treg) cells and Ulex Europaeus Agglutinin 1+ (UEA1+) mTECs. Transcriptomics revealed that a remarkable array of signals-cytokines, chemokines, growth factors, transcription factors, and morphogens-are governed by RANKL and drive in part the K5: RANKL thymic phenotype. Extended doxycycline administration to 6-weeks results in a K5: RANKL thymus that begins to display distinct histopathological features, such as medullary epithelial hyperplasia, extensive immune cell infiltration, and central tissue necrosis. As there are intense efforts to develop clinical approaches to restore thymic medullary function in the adult to treat immunopathological conditions in which immune cell function is compromised following cancer therapy or toxin exposure, an improved molecular understanding of RANKL's involvement in thymic medulla enlargement will be required. We believe the versatility of the conditional K5: RANKL mouse represents a tractable model system to assist in addressing this requirement as well as many other questions related to RANKL's role in thymic normal physiology and disease processes.

How Thymocyte Deletion in the Cortex May Curtail Antigen-Specific T-Regulatory Cell Development in the Medulla

CD4⁺ T cell responses to self-antigens are pivotal for immunological self-tolerance. Activation of Foxp3^– T-conventional (T-conv) cells can precipitate autoimmune disease, whereas activation of Foxp3⁺ T-regulatory (T-reg) cells is essential to prevent autoimmune disease. This distinction indicates the importance of the thymus in controlling the differentiation of self-reactive CD4⁺ T cells. Thymocytes and thymic antigen-presenting cells (APC) depend on each other for normal maturation and differentiation. In this Hypothesis and Theory article, we propose this mutual dependence dictates which self-antigens induce T-reg cell development in the thymic medulla. We postulate self-reactive CD4⁺ CD8^– thymocytes deliver signals that stabilize and amplify the presentation of their cognate self-antigen by APC in the thymic medulla, thereby seeding a niche for the development of T-reg cells specific for the same self-antigen. By limiting the number of antigen-specific CD4⁺ thymocytes in the medulla, thymocyte deletion in the cortex may impede the formation of medullary T-reg niches containing certain self-antigens. Susceptibility to autoimmune disease may arise from cortical deletion creating a “hole” in the self-antigen repertoire recognized by T-reg cells.

Signaling Crosstalks Drive Generation and Regeneration of the Thymus

Optimal recovery of immune competence after periods of hematopoietic insults or stress is crucial to re-establish patient response to vaccines, pathogens and tumor antigens. This is particularly relevant for patients receiving high doses of chemotherapy or radiotherapy, who experience prolonged periods of lymphopenia, which can be associated with an increased risk of infections, malignant relapse, and adverse clinical outcome. While the thymus represents the primary organ responsible for the generation of a diverse pool of T cells, its function is profoundly impaired by a range of acute insults (including those caused by cytoreductive chemo/radiation therapy, infections and graft-versus-host disease) and by the chronic physiological deterioration associated with aging. Impaired thymic function increases the risk of infections and tumor antigen escape due to a restriction in T-cell receptor diversity and suboptimal immune response. Therapeutic approaches that can promote the renewal of the thymus have the potential to restore immune competence in patients. Previous work has documented the importance of the crosstalk between thymocytes and thymic epithelial cells in establishing correct architecture and function of thymic epithelium. This crosstalk is relevant not only during thymus organogenesis, but also to promote the recovery of its function after injuries. In this review, we will analyze the signals involved in the crosstalk between TECs and hematopoietic cells. We will focus in particular on how signals from T-cells can regulate TEC function and discuss the relevance of these pathways in restoring thymic function and T-cell immunity in experimental models, as well as in the clinical setting.

RANKL biology

Since the receptor activator of nuclear factor-kappa B ligand (RANKL), its cognate receptor activator of nuclear factor-kappa B (RANK), and the decoy receptor osteoprotegerin (OPG) were discovered, a number of studies have uncovered the crucial role of the RANKL-RANK-OPG pathway in controlling the key aspect of bone homeostasis, the immune system, inflammation, cancer, and other systems under pathophysiological condition. These findings have expanded the understanding of the multifunctional biology of the RANKL-RANK-OPG pathway and led to the development of therapeutic potential targeting this pathway. The successful development and application of anti-RANKL antibody in treating diseases causing bone loss validates the utility of therapeutic approaches based on the modulation of this pathway. Moreover, recent studies have demonstrated the involvement of the RANKL-RANK pathway in osteoblast differentiation and bone formation, shedding light on the RANKL-RANK dual signaling in coupling bone resorption and bone formation. In this review, we will summarize the current understanding of the RANKL-RANK-OPG system in the context of the bone and the immune system as well as the impact of this pathway in disease conditions, including cancer development and metastasis.

Regulation of thymic T regulatory cell differentiation by TECs in health and disease

The thymus produces self-limiting and self-tolerant T cells through the interaction between thymocytes and thymus epithelial cells (TECs), thereby generating central immune tolerance. The TECs are composed of cortical and medullary thymic epithelial cells, which regulate the positive and negative selection of T cells, respectively. During the process of negative selection, thymocytes with self-reactive ability are deleted or differentiated into regulatory T cells (Tregs). Tregs are a subset of suppressor T cells that are important for maintaining immune homeostasis. The differentiation and development of Tregs depend on the development of TECs and other underlying molecular mechanisms. Tregs regulated by thymic epithelial cells are closely related to human health and are significant in autoimmune diseases, thymoma and pregnancy. In this review, we summarize the current molecular and transcriptional regulatory mechanisms by which TECs affect the development and function of thymic Tregs. We also review the pathophysiological models of thymic epithelial cells regulating thymic Tregs in human diseases and specific physiological conditions.

mTORC2 negatively controls the maturation process of medullary thymic epithelial cells by inhibiting the LTβR/RANK-NF-κB axis

The differentiation of mature medullary thymic epithelial cells (mTECs) is critical for the induction of central immune tolerance. Although the critical effect of mechanistic target of rapamycin complex 1 (mTORC1) in shaping mTEC differentiation has been studied, the regulatory role of mTORC2 in the differentiation and maturation of mTECs is poorly understood. We herein reported that TEC-specific ablation of a rapamycin-insensitive companion of mTOR (RICTOR), a key component of mTORC2, significantly decreased the thymus size and weight, the total cell number of TECs, and the cell number of mTECs with a smaller degree of reduced cortical thymic epithelial cells. Interestingly, RICTOR deficiency significantly accelerated the mTEC maturation process, as indicated by the increased ratios of mature mTECs (MHCIIhi , CD80+ , and Aire+ ) to immature mTECs (MHCIIlo , CD80- , and Aire- ) in Rictor-deficient mice. The RNA-sequencing assays showed that the upregulated nuclear factor-κB (NF-κB) signaling pathway in Rictor-deficient mTECs was one of the obviously altered pathways compared with wild-type mTECs. Our studies further showed that Rictor-deficient mTECs exhibited upregulated expression of receptor activator of NF-κB (RANK) and lymphotoxin β receptor (LTβR), as well as increased activity of canonical and noncanonical NF-κB signaling pathways as determined by ImageStream and Simple Western. Finally, our results showed that inhibition of NF-κB signaling pathways could partially reverse the accelerated maturation of mTECs in Rictor conditional KO mice. Thus, mTORC2 negatively controls the kinetics of the mTEC maturation process by inhibiting the LTβR/RANK-NF-κB signal axis.

Dynamics of thymus function and T cell receptor repertoire breadth in health and disease

T cell recognition of unknown antigens relies on the tremendous diversity of the T cell receptor (TCR) repertoire; generation of which can only occur in the thymus. TCR repertoire breadth is thus critical for not only coordinating the adaptive response against pathogens but also for mounting a response against malignancies. However, thymic function is exquisitely sensitive to negative stimuli, which can come in the form of acute insult, such as that caused by stress, infection, or common cancer therapies; or chronic damage such as the progressive decline in thymic function with age. Whether it be prolonged T cell deficiency after hematopoietic cell transplantation (HCT) or constriction in the breadth of the peripheral TCR repertoire with age; these insults result in poor adaptive immune responses. In this review, we will discuss the importance of thymic function for generation of the TCR repertoire and how acute and chronic thymic damage influences immune health. We will also discuss methods that are used to measure thymic function in patients and strategies that have been developed to boost thymic function.

Regulatory T Cell Heterogeneity in the Thymus: Impact on Their Functional Activities

Foxp3⁺ regulatory T cells (Treg) maintain the integrity of the organism by preventing excessive immune responses. These cells protect against autoimmune diseases but are also important regulators of other immune responses including inflammation, allergy, infection, and tumors. Furthermore, they exert non-immune functions such as tissue repair and regeneration. In the periphery, Foxp3⁺ Treg have emerged as a highly heterogeneous cell population with distinct molecular and functional properties. Foxp3⁺ Treg mainly develop within the thymus where they receive instructive signals for their differentiation. Recent studies have revealed that thymic Treg are also heterogeneous with two distinct precursors that give rise to mature Foxp3⁺ Treg exhibiting non-overlapping regulatory activities characterized by a differential ability to control different types of autoimmune reactions. Furthermore, the thymic Treg cell pool is not only composed of newly developing Treg, but also contain a large fraction of recirculating peripheral cells. Here, we review the two pathways of thymic Treg cell differentiation and their potential impact on Treg activity in the periphery. We also summarize our current knowledge on recirculating peripheral Treg in the thymus.

RANK Signaling in the Differentiation and Regeneration of Thymic Epithelial Cells

Thymic epithelial cells (TECs) provide essential clues for the proliferation, survival, migration, and differentiation of thymocytes. Recent advances in mouse and human have revealed that TECs constitute a highly heterogeneous cell population with distinct functional properties. Importantly, TECs are sensitive to thymic damages engendered by myeloablative conditioning regimen used for bone marrow transplantation. These detrimental effects on TECs delay de novo T-cell production, which can increase the risk of morbidity and mortality in many patients. Alike that TECs guide the development of thymocytes, reciprocally thymocytes control the differentiation and organization of TECs. These bidirectional interactions are referred to as thymic crosstalk. The tumor necrosis factor receptor superfamily (TNFRSF) member, receptor activator of nuclear factor kappa-B (RANK) and its cognate ligand RANKL have emerged as key players of the crosstalk between TECs and thymocytes. RANKL, mainly provided by positively selected CD4⁺ thymocytes and a subset of group 3 innate lymphoid cells, controls mTEC proliferation/differentiation and TEC regeneration. In this review, I discuss recent advances that have unraveled the high heterogeneity of TECs and the implication of the RANK-RANKL signaling axis in TEC differentiation and regeneration. Targeting this cell-signaling pathway opens novel therapeutic perspectives to recover TEC function and T-cell production.

Regulation of T cell-associated tissues and T cell activation by RANKL-RANK-OPG

The receptor activator of nuclear factor kappa-B ligand (RANKL)-RANK-osteoprotegerin (OPG) system is critical to bone homeostasis, but genetically deficient mouse models have revealed important roles in the immune system as well. RANKL-RANK-OPG is particularly important to T cell biology because of its organogenic control of thymic development and secondary lymphoid tissues influence central T cell tolerance and peripheral T cell function. RANKL-RANK-OPG cytokine-receptor interactions are often controlled by regulation of expression of RANKL on developing T cells, which interacts with RANK expressed on some lymphoid tissue cells to stimulate key downstream signaling pathways that affect critical tuning functions of the T cell compartment, like cell survival and antigen presentation. Activation of peripheral T cells is regulated by RANKL-enhanced dendritic cell survival, and dysregulation of the RANKL-RANK-OPG system in this context is associated with loss of T cell tolerance and autoimmune disease. Given its broader implications for immune homeostasis and osteoimmunology, it is critical to further understand how the RANKL-RANK-OPG system operates in T cell biology.

Intrathymic Selection and Defects in the Thymic Epithelial Cell Development

Intimate interactions between thymic epithelial cells (TECs) and thymocytes (T) have been repeatedly reported as essential for performing intrathymic T-cell education. Nevertheless, it has been described that animals exhibiting defects in these interactions were capable of a proper positive and negative T-cell selection. In the current review, we first examined distinct types of TECs and their possible role in the immune surveillance. However, EphB-deficient thymi that exhibit profound thymic epithelial (TE) alterations do not exhibit important immunological defects. Eph and their ligands, the ephrins, are implicated in cell attachment/detachment and govern, therefore, TEC–T interactions. On this basis, we hypothesized that a few normal TE areas could be enough for a proper phenotypical and functional maturation of T lymphocytes. Then, we evaluated in vivo how many TECs would be necessary for supporting a normal T-cell differentiation, concluding that a significantly low number of TEC are still capable of supporting normal T lymphocyte maturation, whereas with fewer numbers, T-cell maturation is not possible.

When the Damage Is Done: Injury and Repair in Thymus Function

Even though the thymus is exquisitely sensitive to acute insults like infection, shock, or common cancer therapies such as cytoreductive chemo- or radiation-therapy, it also has a remarkable capacity for repair. This phenomenon of endogenous thymic regeneration has been known for longer even than its primary function to generate T cells, however, the underlying mechanisms controlling the process have been largely unstudied. Although there is likely continual thymic involution and regeneration in response to stress and infection in otherwise healthy people, acute and profound thymic damage such as that caused by common cancer cytoreductive therapies or the conditioning regimes as part of hematopoietic cell transplantation (HCT), leads to prolonged T cell deficiency; precipitating high morbidity and mortality from opportunistic infections and may even facilitate cancer relapse. Furthermore, this capacity for regeneration declines with age as a function of thymic involution; which even at steady state leads to reduced capacity to respond to new pathogens, vaccines, and immunotherapy. Consequently, there is a real clinical need for strategies that can boost thymic function and enhance T cell immunity. One approach to the development of such therapies is to exploit the processes of endogenous thymic regeneration into novel pharmacologic strategies to boost T cell reconstitution in clinical settings of immune depletion such as HCT. In this review, we will highlight recent work that has revealed the mechanisms by which the thymus is capable of repairing itself and how this knowledge is being used to develop novel therapies to boost immune function.

Generation and Regeneration of Thymic Epithelial Cells

The thymus is unique in its ability to support the maturation of phenotypically and functionally distinct T cell sub-lineages. Through its combined production of MHC-restricted conventional CD4⁺ and CD8⁺, and Foxp3⁺ regulatory T cells, as well as non-conventional CD1d-restricted iNKT cells and invariant γδT cells, the thymus represents an important orchestrator of immune system development and control. It is now clear that thymus function is largely determined by the availability of stromal microenvironments. These specialized areas emerge during thymus organogenesis and are maintained throughout life. They are formed from both epithelial and mesenchymal components, and collectively they support a stepwise program of thymocyte development. Of these stromal cells, cortical, and medullary thymic epithelial cells represent functional components of thymic microenvironments in both the cortex and medulla. Importantly, a key feature of thymus function is that levels of T cell production are not constant throughout life. Here, multiple physiological factors including aging, stress and pregnancy can have either short- or long-term detrimental impact on rates of thymus function. Here, we summarize our current understanding of the development and function of thymic epithelial cells, and relate this to strategies to protect and/or restore thymic epithelial cell function for therapeutic benefit.

High frequency vaccination-induced immune stress reduces bone strength with the involvement of activated osteoclastogenesis in layer pullets

In poultry production, vaccination is an effective measure to protect chickens from diseases. Vaccination, however, is a stressor that may induce stress responses that interfere with the growth and development of chickens. The interaction between the skeletal and immune systems on bone quality has gained more attention. In the present study, the influence of high frequency vaccinations on the bone development of layer pullets was investigated. Thirty 35-day-old SPF White Leghorn layer pullets were obtained and randomly subjected to the following treatments: vaccinated against Newcastle disease (ND) with LoSota vaccine once at 35-day-old (V1, control); 4 times at 35, 49, 63, and 77 d of age (V4); and 7 times at 35, 42, 49, 56, 63, 70, and 77 d of age (V7). The body weight and organ index of the spleen, thymus, and tibia were recorded. The antibody titer and serum and the tibia calcium and phosphorus concentrations were measured. The transcription levels of the IL-6, IL-17, TNF-α, receptor activator of NF-κB ligand (RANKL), and osteoprotegerin (OPG) genes were determined in spleen, thymus, and the tibia. The results showed that V7 decreased body weight and increased the ND antibody titer, compared to V1-chickens. The expression levels of IL-6, IL-17, and TNF-α were upregulated in spleen, thymus, and the tibia of V7 chickens. In the tibia, RANKL was upregulated, while OPG was downregulated by V7 treatment. The results indicate that high frequency vaccination induces immune stress and impairs bone development. The results suggest that the augmented cytokine expression in immune organs and the tibia is associated with activation of the OPG/RANKL pathway, which, in turn, enhances osteoclastogenesis. The appropriate frequency of vaccination should support optimal bone development and full immunoprotection in layer pullets.

The RANKL-RANK Axis: A Bone to Thymus Round Trip

The identification of Receptor activator of nuclear factor kappa B ligand (RANKL) and its cognate receptor Receptor activator of nuclear factor kappa B (RANK) during a search for novel tumor necrosis factor receptor (TNFR) superfamily members has dramatically changed the scenario of bone biology by providing the functional and biochemical proof that RANKL signaling via RANK is the master factor for osteoclastogenesis. In parallel, two independent studies reported the identification of mouse RANKL on activated T cells and of a ligand for osteoprotegerin on a murine bone marrow-derived stromal cell line. After these seminal findings, accumulating data indicated RANKL and RANK not only as essential players for the development and activation of osteoclasts, but also for the correct differentiation of medullary thymic epithelial cells (mTECs) that act as mediators of the central tolerance process by which self-reactive T cells are eliminated while regulatory T cells are generated. In light of the RANKL-RANK multi-task function, an antibody targeting this pathway, denosumab, is now commonly used in the therapy of bone loss diseases including chronic inflammatory bone disorders and osteolytic bone metastases; furthermore, preclinical data support the therapeutic application of denosumab in the framework of a broader spectrum of tumors. Here, we discuss advances in cellular and molecular mechanisms elicited by RANKL-RANK pathway in the bone and thymus, and the extent to which its inhibition or augmentation can be translated in the clinical arena.

Medullary thymic epithelial cells: Deciphering the functional diversity beyond promiscuous gene expression

Within the thymus, cortical and medullary thymic epithelial cells (cTECs and mTECs, respectively) provide unique microenvironments for the development of T cells that are responsive to diverse foreign antigens while self-tolerant. Essential for tolerance induction, mTECs play a critical role in negative selection and T regulatory cell differentiation. In this article, we review the current knowledge on the functional diversity within mTECs and discuss how these novel subsets contribute to tolerance induction and are integrated in the complex blueprint of mTEC differentiation.

Rethinking Thymic Tolerance: Lessons from Mice

In the thymus, distinct cortex and medulla areas emphasize the division of labor in selection events shaping the αβT cell receptor repertoire. For example, MHC restriction via positive selection is a unique property of epithelial cells in the thymic cortex. Far less clear are the events controlling tolerance induction in the medulla. By acting in concert through multiple roles, including antigen production/presentation and chemokine-mediated control of migration, we propose that medullary epithelium and dendritic cells collectively enable the medulla to balance T cell production with negative selection and Foxp3⁺ regulatory T cell (Treg) development. We examine here the features of these medullary resident cells and their roles in T cell tolerance, and discuss how imbalance in the thymus can result in loss of T cell tolerance.

RANKL Triggers Treg-Mediated Immunoregulation in Inflammatory Osteolysis

The chronic inflammatory immune response triggered by the infection of the tooth root canal system results in the local upregulation of RANKL, resulting in periapical bone loss. While RANKL has a well-characterized role in the control of bone homeostasis/pathology, it can play important roles in the regulation of the immune system, although its possible immunoregulatory role in infectious inflammatory osteolytic conditions remains largely unknown. Here, we used a mouse model of infectious inflammatory periapical lesions subjected to continuous or transitory anti-RANKL inhibition, followed by the analysis of lesion outcome and multiple host response parameters. Anti-RANKL administration resulted in arrest of bone loss but interfered in the natural immunoregulation of the lesions observed in the untreated group. RANKL inhibition resulted in an unremitting proinflammatory response, persistent high proinflammatory and effector CD4 response, decreased regulatory T-cell (Treg) migration, and lower levels of Treg-related cytokines IL-10 and TGFb. Anti-RANKL blockade impaired the immunoregulatory process only in early disease stages, while the late administration of anti-RANKL did not interfere with the stablished immunoregulation. The impaired immunoregulation due to RANKL inhibition is characterized by increased delayed-type hypersensitivity in vivo and T-cell proliferation in vitro to the infecting bacteria, which mimic the effects of Treg inhibition, reinforcing a possible influence of RANKL on Treg-mediated suppressive response. The adoptive transfer of CD4+FOXp3+ Tregs to mice receiving anti-RANKL therapy restored the immunoregulatory capacity, attenuating the inflammatory response in the lesions, reestablishing normal T-cell response in vivo and in vitro, and preventing lesion relapse upon anti-RANKL therapy cessation. Therefore, while RANKL inhibition efficiently limited the periapical bone loss, it promoted an unremitting host inflammatory response by interfering with Treg activity, suggesting that this classic osteoclastogenic mediator plays a role in immunoregulation.

Altered Maturation of Medullary TEC in EphB-Deficient Thymi Is Recovered by RANK Signaling Stimulation

In the present study, the relevance of EphB2 and EphB3 tyrosine kinase receptors for the maturation of medullary thymic epithelial cells (TECs) is analyzed. The absence of both molecules, but particularly that of EphB2, courses with altered maturation of medullary Cld3,4^hiSSEA1⁺ epithelial progenitor cells, mature medulla epithelial cells, defined by the expression of specific cell markers, including UEA1, MHCII, CD40, CD80, and AIRE, and reduced expansion of medullary islets. In vivo assays demonstrate that these changes are a consequence of the absence of EphBs in both TECs and thymocytes. On the other hand, the changes, that remains in the adult thymus, correlated well with reduced proportions of E15.5 Vγ5⁺RANKL⁺ cells in EphB-deficient thymi that could result in decreased stimulation of RANK⁺ medullary TECs to mature, a fact that was confirmed by recovering of proportions of both CD40^hiCD80⁺ and MHCII^hiUEA1⁺ mature medullary TECs of mutant E14.5 alymphoid thymic lobes by agonist anti-RANK antibody treatment. Accordingly, the effects of EphB deficiency on medullary TECs maturation are recovered by RANK stimulation.

Twenty Years of AIRE

About two decades ago, cloning of the autoimmune regulator (AIRE) gene materialized one of the most important actors on the scene of self-tolerance. Thymic transcription of genes encoding tissue-specific antigens (ts-ags) is activated by AIRE protein and embodies the essence of thymic self-representation. Pathogenic AIRE variants cause the autoimmune polyglandular syndrome type 1, which is a rare and complex disease that is gaining attention in research on autoimmunity. The animal models of disease, although not identically reproducing the human picture, supply fundamental information on mechanisms and extent of AIRE action: thanks to its multidomain structure, AIRE localizes to chromatin enclosing the target genes, binds to histones, and offers an anchorage to multimolecular complexes involved in initiation and post-initiation events of gene transcription. In addition, AIRE enhances mRNA diversity by favoring alternative mRNA splicing. Once synthesized, ts-ags are presented to, and cause deletion of the self-reactive thymocyte clones. However, AIRE function is not restricted to the activation of gene transcription. AIRE would control presentation and transfer of self-antigens for thymic cellular interplay: such mechanism is aimed at increasing the likelihood of engagement of the thymocytes that carry the corresponding T-cell receptors. Another fundamental role of AIRE in promoting self-tolerance is related to the development of thymocyte anergy, as thymic self-representation shapes at the same time the repertoire of regulatory T cells. Finally, AIRE seems to replicate its action in the secondary lymphoid organs, albeit the cell lineage detaining such property has not been fully characterized. Delineation of AIRE functions adds interesting data to the knowledge of the mechanisms of self-tolerance and introduces exciting perspectives of therapeutic interventions against the related diseases.

Thymic homing of activated CD4+ T cells induces degeneration of the thymic epithelium through excessive RANK signaling

Activated T cells have been shown to be able to recirculate into the thymus from the periphery. The present study was aimed to elucidate the functional consequences of thymic homing of activated T cells upon developing thymocytes and thymic epithelial cells (TEC). In the presence of activated T cells, especially CD4⁺ T cells, T cell development was found to be inhibited in thymic organ cultures with markedly reduced cellularity. Thymic transplantation demonstrated that the inhibitory effect was most likely due to a defective microenvironment. As the major component of the thymic stroma, the TEC compartment was severely disturbed after prolonged exposure to the activated T cells. In addition to reduced cell proliferation, TEC differentiation was heavily skewed to the mTEC lineage. Furthermore, we demonstrated that RANKL highly expressed by activated CD4⁺ T cells was primarily responsible for the detrimental effects. Presumably, excessive RANK signaling drove overproduction of mTECs and possibly exhaustion of epithelial progenitors, thereby facilitating the deterioration of the epithelial structures. These findings not only reveal a novel activity of activated T cells re-entering the thymus, but also provide a new perspective for understanding the mechanism underlying thymic involution.

Generation of diversity in thymic epithelial cells

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

Intrathymic T cell development is a complex process that depends upon continuous guidance from thymus stromal cell microenvironments. The thymic epithelium within the thymic stroma comprises highly specialized cells with a high degree of anatomic, phenotypic, and functional heterogeneity. These properties are collectively required to bias thymocyte development toward production of self-tolerant and functionally competent T cells. The importance of thymic epithelial cells (TECs) is evidenced by clear links between their dysfunction and multiple diseases where autoimmunity and immunodeficiency are major components. Consequently, TECs are an attractive target for cell therapies to restore effective immune system function. The pathways and molecular regulators that control TEC development are becoming clearer, as are their influences on particular stages of T cell development. Here, we review both historical and the most recent advances in our understanding of the cellular and molecular mechanisms controlling TEC development, function, dysfunction, and regeneration.

How antigen specificity directs regulatory T-cell function: self, foreign and engineered specificity

Regulatory T cells (Tregs) are a suppressive subset of T cells that have important roles in maintaining self-tolerance and preventing immunopathology. The T-cell receptor (TCR) and its antigen specificity play a dominant role in the differentiation of cells to a Treg fate, either in the thymus or in the periphery. This review focuses on the effects of the TCR and its antigen specificity on Treg biology. The role of Tregs with specificity for self-antigen has primarily been studied in the context of autoimmune disease, although recent studies have focused on their role in steady-state conditions. The role of Tregs that are specific for pathogens, dietary antigens and allergens is much less studied, although recent data suggest a significant and previously underappreciated role for Tregs during memory responses to a wide range of foreign antigens. The development of TCR- or chimeric antigen receptor (CAR)-transduced T cells means we are now able to engineer Tregs with disease-relevant antigen specificities, paving the way for ensuring specificity with Treg-based therapies. Understanding the role that antigens play in driving the generation and function of Tregs is critical for defining the pathophysiology of many immune-mediated diseases, and developing new therapeutic interventions.

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.

Thymus medulla under construction: Time and space oddities

The development of effective T-cell-based immunotherapies to treat infection, cancer, and autoimmunity should incorporate the ground rules that control differentiation of T cells in the thymus. Within the thymus, thymic epithelial cells (TECs) provide microenvironments supportive of the generation and selection of T cells that are responsive to pathogen-derived antigens, and yet tolerant to self-determinants. Defects in TEC differentiation cause syndromes that range from immunodeficiency to autoimmunity, which makes the study of TECs of fundamental and clinical importance to comprehend how immunity and tolerance are balanced. Critical to tolerance induction are medullary thymic epithelial cells (mTECs), which purge autoreactive T cells, or redirect them to a regulatory T-cell lineage. In this issue of the European Journal of Immunology, studies by Baik et al. and Mayer et al. [Eur. J. Immunol. 2016. 46: XXXX-XXXX and 46: XXXX-XXXX]) document novel spatial-temporal singularities in the lineage specification and maintenance of mTECs. While Baik et al. define a developmental checkpoint during mTEC specification in the embryo, Mayer et al. reveal that the generation and maintenance of the adult mTEC compartment is temporally controlled in vivo. The two reports described new developmentally related, but temporally distinct principles that underlie the homeostasis of the thymic medulla across life.

Thymic stromal cell subsets for T cell development

The thymus provides a specialized microenvironment in which a variety of stromal cells of both hematopoietic and non-hematopoietic origin regulate development and repertoire selection of T cells. Recent studies have been unraveling the inter- and intracellular signals and transcriptional networks for spatiotemporal regulation of development of thymic stromal cells, mainly thymic epithelial cells (TECs), and the molecular mechanisms of how different TEC subsets control T cell development and selection. TECs are classified into two functionally different subsets: cortical TECs (cTECs) and medullary TECs (mTECs). cTECs induce positive selection of diverse and functionally distinct T cells by virtue of unique antigen-processing systems, while mTECs are essential for establishing T cell tolerance via ectopic expression of peripheral tissue-restricted antigens and cooperation with dendritic cells. In addition to reviewing the role of the thymic stroma in conventional T cell development, we will discuss recently discovered novel functions of TECs in the development of unconventional T cells, such as natural killer T cells and γδT cells.

Increased generation of Foxp3(+) regulatory T cells by manipulating antigen presentation in the thymus

Regulatory T-cell (Treg) selection in the thymus is essential to prevent autoimmune diseases. Although important rules for Treg selection have been established, there is controversy regarding the degree of self-reactivity displayed by T-cell receptors expressed by Treg cells. In this study we have developed a model of autoimmune skin inflammation, to determine key parameters in the generation of skin-reactive Treg cells in the thymus (tTreg). tTreg development is predominantly AIRE dependent, with an AIRE-independent component. Without the knowledge of antigen recognized by skin-reactive Treg cells, we are able to enhance skin-specific tTreg cell generation using three approaches. First, we increase medullary thymic epithelial cells by using mice lacking osteoprotegerin or by adding TRANCE (RANKL, Tnfsf11). Second, we inject intrathymically peripheral dendritic cells from skin-draining sites. Finally, we inject skin tissue lysates intrathymically. These findings have implications for enhancing the generation of organ-specific Treg cells in autoimmune diseases.

Relb acts downstream of medullary thymic epithelial stem cells and is essential for the emergence of RANK(+) medullary epithelial progenitors

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.

CCR7 Controls Thymus Recirculation, but Not Production and Emigration, of Foxp3(+) T Cells

Current models of Foxp3⁺ regulatory T cell (Treg) development involve CCR7-mediated migration of thymocytes into the thymus medulla to enable essential interactions with medullary epithelium. However, increased Foxp3⁺ thymic Treg numbers in Ccr7^−/− mice challenge this view, and the role of CCR7 in Treg development, emigration, and/or recirculation is unknown. Here, we have examined CCR7 and Rag2pGFP levels during Treg development and generated Rag2pGFPCcr7^−/− mice to study its impact on the intrathymic Treg pool. We reveal surprising developmental heterogeneity in thymocytes described as Treg precursors, showing that they contain recirculating CCR6⁺CCR7⁻Rag2pGFP⁻ T cells. Although CCR7 defines bona fide Rag2GFP⁺ Treg precursors, it is not required for Treg production and emigration. Rather, we show that lack of CCR7 renders the thymus more receptive to Treg thymus homing. Our study reveals a role for CCR7 in limiting Treg recirculation back to the thymus and enables separation of the mechanisms controlling Treg production and thymic recirculation.

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Chemokine receptors define recirculating and newly selected Tregs

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CCR7 is dispensable for Treg development and thymus egress

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CCR7 is required to rate-limit the thymus recirculation of peripheral CCR6⁺ Tregs

Switching on the Aire conditioner

Aire has been cloned as the gene responsible for a hereditary type of organ-specific autoimmune disease. Aire controls the expression of a wide array of tissue-restricted Ags by medullary thymic epithelial cells (mTECs), thereby leading to clonal deletion and Treg-cell production, and ultimately to the establishment of self-tolerance. However, relatively little is known about the mechanism responsible for the control of Aire expression itself. In this issue of the European Journal of Immunology, Haljasorg et al. [Eur. J. Immunol. 2015. 45: 3246-3256] have reported the presence of an enhancer element for Aire that binds with NF-κB components downstream of the TNF receptor family member, RANK (receptor activator of NF-κB). The results suggest that RANK has a dual mode of action in Aire expression: one involving the promotion of mTEC differentiation and the other involving activation of the molecular switch for Aire within mature mTECs.