A 2020 View of Thymus Stromal Cells in T Cell Development

The Proteostasis of Thymic Stromal Cells in Health and Diseases

The thymus is the key immune organ for the development of T cells. Different populations of thymic stromal cells interact with T cells, thereby controlling the dynamic development of T cells through their differentiation and function. Proteostasis represents a balance between protein expression, folding, and modification and protein clearance, and its fluctuation usually depends at least partially on related protein regulatory systems for further survival and effects. However, in terms of the substantial requirement for self-antigens and their processing burden, increasing evidence highlights that protein regulation contributes to the physiological effects of thymic stromal cells. Impaired proteostasis may expedite the progression of thymic involution and dysfunction, accompanied by the development of autoimmune diseases or thymoma. Hence, in this review, we summarize the regulation of proteostasis within different types of thymic stromal cells under physiological and pathological conditions to identify potential targets for thymic regeneration and immunotherapy.

Sulforaphane Ameliorate Cadmium-Induced Blood-Thymus Barrier Disruption by Targeting the PI3K/AKT/FOXO1 Axis

Cadmium (Cd) is a transition metal ion that is extremely harmful to human and animal biological systems. Cd is a toxic substance that can accumulate in the food chain and cause various health issues. Sulforaphane (SFN) is a natural bioactive compound with potent antioxidant properties. In our study, 80 1 day-old chicks were fed with Cd (140 mg/kg BW/day) and/or SFN (50 mg/kg BW/day) for 90 days. The blood-thymus barrier (BTB) is a selective barrier separating T-lymphocytes from blood and cortical capillaries in the thymus cortex. Our research revealed that Cd could destroy the BTB by downregulating Wnt/β-catenin signaling and induce immunodeficiency, leading to irreversible injury to the immune system. The study emphasizes the health benefits of SFN in the thymus. SFN could ameliorate Cd-triggered BTB dysfunction and pyroptosis in the thymus tissues. SFN modulated the PI3K/AKT/FOXO1 axis, improving the level of claudin-5 (CLDN5) in the thymus to alleviate BTB breakdown. Our findings indicated the toxic impact of Cd on thymus, and BTB could be the specific target of Cd toxicity. The finding also provides evidence for the role of SFN in maintaining thymic homeostasis for Cd-related health issues.

International Society for Cell and Gene Therapy Clinical Translation Committee recommendations on mesenchymal stromal cells in graft-versus-host disease: easy manufacturing is faced with standardizing and commercialization challenges

Mesenchymal stromal cells (MSCs) have been used in multiple clinical trials for steroid-refractory moderate-severe (grade II-IV) acute graft-versus-host disease (aGVHD) across the world over the last two decades. Despite very promising results in a variety of trials, it failed to get widespread approval by regulatory agencies such as the U.S. Food and Drug Administration and the European Medicines Agency. What lessons can we learn from this for future studies on MSCs and other cell therapy products? Broad heterogeneity among published trials using MSCs in aGVHD was likely the core problem. We propose a standardized approach in regards to donor-related factors, MSCs-related characteristics, as well as clinical trial design, to limit heterogeneity in trials for aGVHD and to fulfill the requirements of regulatory agencies. This approach may be expanded beyond MSCs to other Cell and Gene therapy products and trials in other diseases.

Mechanism study of ubiquitination in T cell development and autoimmune disease

T cells play critical role in multiple immune processes including antigen response, tumor immunity, inflammation, self-tolerance maintenance and autoimmune diseases et. Fetal liver or bone marrow-derived thymus-seeding progenitors (TSPs) settle in thymus and undergo T cell-lineage commitment, proliferation, T cell receptor (TCR) rearrangement, and thymic selections driven by microenvironment composed of thymic epithelial cells (TEC), dendritic cells (DC), macrophage and B cells, thus generating T cells with diverse TCR repertoire immunocompetent but not self-reactive. Additionally, some self-reactive thymocytes give rise to Treg with the help of TEC and DC, serving for immune tolerance. The sequential proliferation, cell fate decision, and selection during T cell development and self-tolerance establishment are tightly regulated to ensure the proper immune response without autoimmune reaction. There are remarkable progresses in understanding of the regulatory mechanisms regarding ubiquitination in T cell development and the establishment of self-tolerance in the past few years, which holds great potential for further therapeutic interventions in immune-related diseases.

Morphometric Analysis of the Thymic Epithelial Cell (TEC) Network Using Integrated and Orthogonal Digital Pathology Approaches

The thymus, a central primary lymphoid organ of the immune system, plays a key role in T cell development. Surprisingly, the thymus is quite neglected with regards to standardized pathology approaches and practices for assessing structure and function. Most studies use multispectral flow cytometry to define the dynamic composition of the thymus at the cell population level, but they are limited by lack of contextual insight. This knowledge gap hinders our understanding of various thymic conditions and pathologies, particularly how they affect thymic architecture, and subsequently, immune competence. Here, we introduce a digital pathology pipeline to address these challenges. Our approach can be coupled to analytical algorithms and utilizes rationalized morphometric assessments of thymic tissue, ranging from tissue-wide down to microanatomical and ultrastructural levels. This pipeline enables the quantitative assessment of putative changes and adaptations of thymic structure to stimuli, offering valuable insights into the pathophysiology of thymic disorders. This versatile pipeline can be applied to a wide range of conditions that may directly or indirectly affect thymic structure, ranging from various cytotoxic stimuli inducing acute thymic involution to autoimmune diseases, such as myasthenia gravis. Here, we demonstrate applicability of the method in a mouse model of age-dependent thymic involution, both by confirming established knowledge, and by providing novel insights on intrathymic remodeling in the aged thymus. Our orthogonal pipeline, with its high versatility and depth of analysis, promises to be a valuable and practical toolset for both basic and translational immunology laboratories investigating thymic function and disease.

TRPA1: A promising target for pulmonary fibrosis?

Pulmonary fibrosis is a disease characterized by progressive scar formation and the ultimate manifestation of numerous lung diseases. It is known as "cancer that is not cancer" and has attracted widespread attention. However, its formation process is very complex, and the mechanism of occurrence has not been fully elucidated. Current research has found that TRPA1 may be a promising target in the pathogenesis of pulmonary fibrosis. The TRPA1 channel was first successfully isolated in human lung fibroblasts, and it was found to have a relatively concentrated distribution in the lungs and respiratory tract. It is also involved in various acute and chronic inflammatory processes of lung diseases and may even play a core role in the progression and/or prevention of pulmonary fibrosis. Natural ligands targeting TRPA1 could offer a promising alternative treatment for pulmonary diseases. Therefore, this review delves into the current understanding of pulmonary fibrogenesis, analyzes TRPA1 biological properties and regulation of lung disease with a focus on pulmonary fibrosis, summarizes the TRPA1 molecular structure and its biological function, and summarizes TRPA1 natural ligand sources, anti-pulmonary fibrosis activity and potential mechanisms. The aim is to decipher the exact role of TRPA1 channels in the pathophysiology of pulmonary fibrosis and to consider their potential in the development of new therapeutic strategies.

The alarmin IL33 orchestrates type 2 immune-mediated control of thymus regeneration

As the primary site of T-cell development, the thymus dictates immune competency of the host. The rates of thymus function are not constant, and thymus regeneration is essential to restore new T-cell production following tissue damage from environmental factors and therapeutic interventions. Here, we show the alarmin interleukin (IL) 33 is a product of Sca1+ thymic mesenchyme both necessary and sufficient for thymus regeneration via a type 2 innate immune network. IL33 stimulates expansion of IL5-producing type 2 innate lymphoid cells (ILC2), which triggers a cellular switch in the intrathymic availability of IL4. This enables eosinophil production of IL4 to re-establish thymic mesenchyme prior to recovery of thymopoiesis-inducing epithelial compartments. Collectively, we identify a positive feedback mechanism of type 2 innate immunity that regulates the recovery of thymus function following tissue injury.

Mesenchymal stromal cells ameliorate chronic GVHD by boosting thymic regeneration in a CCR9-dependent manner in mice

Chronic graft-versus-host disease (cGVHD) is a major cause of morbidity and mortality after allogeneic hematopoietic stem cell transplantation. Mature donor T cells within the graft contribute to severe damage of thymic epithelial cells (TECs), which are known as key mediators in the continuum of acute GVHD (aGVHD) and cGVHD pathology. Mesenchymal stromal cells (MSCs) are reportedly effective in the prevention and treatment of cGVHD. In our previous pilot clinical trial in patients with refractory aGVHD, the incidence and severity of cGVHD were decreased, along with an increase in levels of blood signal joint T-cell receptor excision DNA circles after MSCs treatment, which indicated an improvement in thymus function of patients with GVHD, but the mechanisms leading to these effects remain unknown. Here, we show in a murine GVHD model that MSCs promoted the quantity and maturity of TECs as well as elevated the proportion of Aire-positive medullary TECs, improving both CD4+CD8+ double-positive thymocytes and thymic regulatory T cells, balancing the CD4:CD8 ratio in the blood. In addition, CCL25-CCR9 signaling axis was found to play an important role in guiding MSC homing to the thymus. These studies reveal mechanisms through which MSCs ameliorate cGVHD by boosting thymic regeneration and offer innovative strategies for improving thymus function in patients with GVHD.

The initial age-associated decline in early T-cell progenitors reflects fewer pre-thymic progenitors and altered signals in the bone marrow and thymus microenvironments

Age-related thymus involution results in decreased T-cell production, contributing to increased susceptibility to pathogens and reduced vaccine responsiveness. Elucidating mechanisms underlying thymus involution will inform strategies to restore thymopoiesis with age. The thymus is colonized by circulating bone marrow (BM)-derived thymus seeding progenitors (TSPs) that differentiate into early T-cell progenitors (ETPs). We find that ETP cellularity declines as early as 3 months (3MO) of age in mice. This initial ETP reduction could reflect changes in thymic stromal niches and/or pre-thymic progenitors. Using a multicongenic progenitor transfer approach, we demonstrate that the number of functional TSP/ETP niches does not diminish with age. Instead, the number of pre-thymic lymphoid progenitors in the BM and blood is substantially reduced by 3MO, although their intrinsic ability to seed and differentiate in the thymus is maintained. Additionally, Notch signaling in BM lymphoid progenitors and in ETPs diminishes by 3MO, suggesting reduced niche quality in the BM and thymus contribute to the early decline in ETPs. Together, these findings indicate that diminished BM lymphopoiesis and thymic stromal support contribute to an initial reduction in ETPs in young adulthood, setting the stage for progressive age-associated thymus involution.

Obesity-induced thymic involution and cancer risk

Declining thymic functions associated either with old age (i.e., age-related thymic involution), or with acute involution as a result of stress, infectious disease, or cytoreductive therapies (e.g., chemotherapy/radiotherapy), have been associated with cancer development. A key mechanism underlying such increased cancer risk is the thymus-dependent debilitation of adaptive immunity, which is responsible for orchestrating immunoediting mechanisms and tumor immune surveillance. In the past few years, a blooming set of evidence has intriguingly linked obesity with cancer development and progression. The majority of such studies has focused on obesity-driven chronic inflammation, steroid/sex hormone and adipokine production, and hyperinsulinemia, as principal factors affecting the tumor microenvironment and driving the development of primary malignancy. However, experimental observations about the negative impact of obesity on T cell development and maturation have existed for more than half a century. Here, we critically discuss the molecular and cellular mechanisms of obesity-driven thymic involution as a previously underrepresented intermediary pathology leading to cancer development and progression. This knowledge could be especially relevant in the context of childhood obesity, because impaired thymic function in young individuals leads to immune system abnormalities, and predisposes to various pediatric cancers. A thorough understanding behind the molecular and cellular circuitries governing obesity-induced thymic involution could therefore help towards the rationalized development of targeted thymic regeneration strategies for obese individuals at high risk of cancer development.

Intrathymic dendritic cell-biased precursors promote human T cell lineage specification through IRF8-driven transmembrane TNF

The cross-talk between thymocytes and thymic stromal cells is fundamental for T cell development. In humans, intrathymic development of dendritic cells (DCs) is evident but its physiological significance is unknown. Here we showed that DC-biased precursors depended on the expression of the transcription factor IRF8 to express the membrane-bound precursor form of the cytokine TNF (tmTNF) to promote differentiation of thymus seeding hematopoietic progenitors into T-lineage specified precursors through activation of the TNF receptor (TNFR)-2 instead of TNFR1. In vitro recapitulation of TNFR2 signaling by providing low-density tmTNF or a selective TNFR2 agonist enhanced the generation of human T cell precursors. Our study shows that, in addition to mediating thymocyte selection and maturation, DCs function as hematopoietic stromal support for the early stages of human T cell development and provide proof of concept that selective targeting of TNFR2 can enhance the in vitro generation of T cell precursors for clinical application.

SARS-CoV-2 infection of thymus induces loss of function that correlates with disease severity

BACKGROUND

Lymphopenia, particularly when restricted to the T-cell compartment, has been described as one of the major clinical hallmarks in patients with coronavirus disease 2019 (COVID-19) and proposed as an indicator of disease severity. Although several mechanisms fostering COVID-19-related lymphopenia have been described, including cell apoptosis and tissue homing, the underlying causes of the decline in T-cell count and function are still not completely understood.

OBJECTIVE

Given that viral infections can directly target thymic microenvironment and impair the process of T-cell generation, we sought to investigate the impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on thymic function.

METHODS

We performed molecular quantification of T-cell receptor excision circles and κ-deleting recombination excision circles to assess, respectively, T- and B-cell neogenesis in SARS-CoV-2-infected patients. We developed a system for in vitro culture of primary human thymic epithelial cells (TECs) to mechanistically investigate the impact of SARS-CoV-2 on TEC function.

RESULTS

We showed that patients with COVID-19 had reduced thymic function that was inversely associated with the severity of the disease. We found that angiotensin-converting enzyme 2, through which SARS-CoV-2 enters the host cells, was expressed by thymic epithelium, and in particular by medullary TECs. We also demonstrated that SARS-CoV-2 can target TECs and downregulate critical genes and pathways associated with epithelial cell adhesion and survival.

CONCLUSIONS

Our data demonstrate that the human thymus is a target of SARS-CoV-2 and thymic function is altered following infection. These findings expand our current knowledge of the effects of SARS-CoV-2 infection on T-cell homeostasis and suggest that monitoring thymic activity may be a useful marker to predict disease severity and progression.

Multi-modular structure of the gene regulatory network for specification and commitment of murine T cells

T cells develop from multipotent progenitors by a gradual process dependent on intrathymic Notch signaling and coupled with extensive proliferation. The stages leading them to T-cell lineage commitment are well characterized by single-cell and bulk RNA analyses of sorted populations and by direct measurements of precursor-product relationships. This process depends not only on Notch signaling but also on multiple transcription factors, some associated with stemness and multipotency, some with alternative lineages, and others associated with T-cell fate. These factors interact in opposing or semi-independent T cell gene regulatory network (GRN) subcircuits that are increasingly well defined. A newly comprehensive picture of this network has emerged. Importantly, because key factors in the GRN can bind to markedly different genomic sites at one stage than they do at other stages, the genes they significantly regulate are also stage-specific. Global transcriptome analyses of perturbations have revealed an underlying modular structure to the T-cell commitment GRN, separating decisions to lose "stem-ness" from decisions to block alternative fates. Finally, the updated network sheds light on the intimate relationship between the T-cell program, which depends on the thymus, and the innate lymphoid cell (ILC) program, which does not.

SARS-CoV-2 variants induce distinct disease and impact in the bone marrow and thymus of mice

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has evolved to variants associated with milder disease. We employed the k18-hACE2 mouse model to study how differences in the course of infection by SARS-CoV-2 variants alpha, delta, and omicron relate to tissue pathology and the immune response triggered. We documented a variant-specific pattern of infection severity, inducing discrete lung and blood immune responses and differentially impacting primary lymphoid organs. Infections with variants alpha and delta promoted bone marrow (BM) emergency myelopoiesis, with blood and lung neutrophilia. The defects in the BM hematopoietic compartment extended to the thymus, with the infection by the alpha variant provoking a marked thymic atrophy. Importantly, the changes in the immune responses correlated with the severity of infection. Our study provides a comprehensive platform to investigate the modulation of disease by SARS-CoV-2 variants and underscores the impact of this infection on the function of primary lymphoid organs.

Age-associated remodeling of T cell immunity and metabolism

Aging results in remodeling of T cell immunity and is associated with poor clinical outcomes in age-related diseases such as cancer. Among the hallmarks of aging, changes in host and cellular metabolism critically affect the development, maintenance, and function of T cells. Although metabolic perturbations impact anti-tumor T cell responses, the link between age-associated metabolic dysfunction and anti-tumor immunity remains unclear. In this review, we summarize recent advances in our understanding of aged T cell metabolism, with a focus on the bioenergetic and immunologic features of T cell subsets unique to the aging process. We also survey insights into mechanisms of metabolic T cell dysfunction in aging and discuss the impacts of aging on the efficacy of cancer immunotherapy. As the average life expectancy continues to increase, understanding the interplay between age-related metabolic reprogramming and maladaptive T cell immunity will be instrumental for the development of therapeutic strategies for older patients.

Klf4 protects thymus integrity during late pregnancy

Pregnancy causes abrupt thymic atrophy. This atrophy is characterized by a severe decrease in the number of all thymocyte subsets and qualitative (but not quantitative) changes in thymic epithelial cells (TECs). Pregnancy-related thymic involution is triggered by progesterone-induced functional changes affecting mainly cortical TECs (cTECs). Remarkably, this severe involution is rapidly corrected following parturition. We postulated that understanding the mechanisms of pregnancy-related thymic changes could provide novel insights into signaling pathways regulating TEC function. When we analyzed genes whose expression in TECs was modified during late pregnancy, we found a strong enrichment in genes bearing KLF4 transcription factor binding motifs. We, therefore, engineered a Psmb11-iCre : Klf4^lox/lox mouse model to study the impact of TEC-specific Klf4 deletion in steady-state conditions and during late pregnancy. Under steady-state conditions, Klf4 deletion had a minimal effect on TEC subsets and did not affect thymic architecture. However, pregnancy-induced thymic involution was much more pronounced in pregnant females lacking Klf4 expression in TECs. These mice displayed a substantial ablation of TECs with a more pronounced loss of thymocytes. Transcriptomic and phenotypic analyses of Klf4 ^-/- TECs revealed that Klf4 maintains cTEC numbers by supporting cell survival and preventing epithelial-to-mesenchymal plasticity during late pregnancy. We conclude that Klf4 is essential for preserving TEC's integrity and mitigating thymic involution during late pregnancy.

HISTOULTRASTRUCTURAL FEATURES OF THYMOCYTES DUE TO THE IMPACT OF THE EXPERIMENTAL GENERAL DEHYDRATION OF A MILD DEGREE

OBJECTIVE

The aim: The objective of the current study was to reveal ultrastructural changes in rats' thymocytes in experimental data in conditions of mild general dehydration.

PATIENTS AND METHODS

Materials and methods: The study was conducted on 20 non-linear adult male laboratory rats weighing 150-170 g. Histological and semi-thin slides of the thymus were prepared according to the required guidelines.

RESULTS

Results: On average, in the cortical zone of the thymus, there was decreased cellularity by 13.4% (p<0.001), while in the medulla zone this indicator turned out to be unreliable - 5.5% (р=0.19), compared to the indicators in animals of the control group. The study showed that a slight degree of general dehydration of the body causes ultrastructural changes in the thymus and is accompanied by a cell-mediated response of the central link of immunogenesis and results in morphological changes in the thymus, which are atrophic in nature with a typical pattern of remodeling of the organ's microstructure, which corresponds to cellular aging and the associated sign of accelerated involution.

CONCLUSION

Conclusions: General dehydration of a mild degree in the experiment is accompanied by a cell-mediated response of the central link of immunogenesis and results in morphological changes in the thymus, which are atrophic in nature with a typical pattern of remodeling of the organ's microstructure, which corresponds to cellular aging and the associated sign of accelerated involution.

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.

Mesenchymal stromal cells in the thymus

The microenvironment of the thymus is composed of a group of stromal cells that include endoderm-derived thymic epithelial cells (TECs) and mesenchymal stromal cells such as fibroblasts and serves as a site for the development of T cells. TECs are known to play an essential role in T cell differentiation and selection. Mesenchymal stromal cells have been less studied in terms of their immunological significance compared to TECs. Recently, new technologies have made it possible to identify and characterize mesenchymal stromal cells in the thymus, revealing their unique functions in thymic organogenesis and T cell development. This review outlines the current views on mesenchymal stromal cells in the thymus, particularly highlighting the newly discovered function of thymic fibroblasts in T cell repertoire selection.

Thymic epithelial cells require lipid kinase Vps34 for CD4 but not CD8 T cell selection

The generation of a functional, self-tolerant T cell receptor (TCR) repertoire depends on interactions between developing thymocytes and antigen-presenting thymic epithelial cells (TECs). Cortical TECs (cTECs) rely on unique antigen-processing machinery to generate self-peptides specialized for T cell positive selection. In our current study, we focus on the lipid kinase Vps34, which has been implicated in autophagy and endocytic vesicle trafficking. We show that loss of Vps34 in TECs causes profound defects in the positive selection of the CD4 T cell lineage but not the CD8 T cell lineage. Utilizing TCR sequencing, we show that T cell selection in conditional mutants causes altered repertoire properties including reduced clonal sharing. cTECs from mutant mice display an increased abundance of invariant chain intermediates bound to surface MHC class II molecules, indicating altered antigen processing. Collectively, these studies identify lipid kinase Vps34 as an important contributor to the repertoire of selecting ligands processed and presented by TECs to developing CD4 T 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.

Thymic Microenvironment: Interactions Between Innate Immune Cells and Developing Thymocytes

The thymus is a crucial organ for the development of T cells. T cell progenitors first migrate from the bone marrow into the thymus. During the journey to become a mature T cell, progenitors require interactions with many different cell types within the thymic microenvironment, such as stromal cells, which include epithelial, mesenchymal and other non-T-lineage immune cells. There are two crucial decision steps that are required for generating mature T cells: positive and negative selection. Each of these two processes needs to be performed efficiently to produce functional MHC-restricted T cells, while simultaneously restricting the production of auto-reactive T cells. In each step, there are various cell types that are required for the process to be carried out suitably, such as scavengers to clean up apoptotic thymocytes that fail positive or negative selection, and antigen presenting cells to display self-antigens during positive and negative selection. In this review, we will focus on thymic non-T-lineage immune cells, particularly dendritic cells and macrophages, and the role they play in positive and negative selection. We will also examine recent advances in the understanding of their participation in thymus homeostasis and T cell development. This review will provide a perspective on how the thymic microenvironment contributes to thymocyte differentiation and T cell maturation.

SRSF1 Deficiency Impairs the Late Thymocyte Maturation and the CD8 Single-Positive Lineage Fate Decision

The underlying mechanisms of thymocyte development and lineage determination remain incompletely understood, and the emerging evidences demonstrated that RNA binding proteins (RBPs) are deeply involved in governing T cell fate in thymus. Serine/arginine-rich splicing factor 1 (SRSF1), as a classical splicing factor, is a pivotal RBP for gene expression in various biological processes. Our recent study demonstrated that SRSF1 plays essential roles in the development of late thymocytes by modulating the T cell regulatory gene networks post-transcriptionally, which are critical in response to type I interferon signaling for supporting thymocyte maturation. Here, we report SRSF1 also contributes to the determination of the CD8⁺ T cell fate. By specific ablation of SRSF1 in CD4⁺CD8⁺ double positive (DP) thymocytes, we found that SRSF1 deficiency impaired the maturation of late thymocytes and diminished the output of both CD4⁺ and CD8⁺ single positive T cells. Interestingly, the ratio of mature CD4⁺ to CD8⁺ cells was notably altered and more severe defects were exhibited in CD8⁺ lineage than those in CD4⁺ lineage, reflecting the specific function of SRSF1 in CD8⁺ T cell fate decision. Mechanistically, SRSF1-deficient cells downregulate their expression of Runx3, which is a crucial transcriptional regulator in sustaining CD8⁺ single positive (SP) thymocyte development and lineage choice. Moreover, forced expression of Runx3 partially rectified the defects in SRSF1-deficient CD8⁺ thymocyte maturation. Thus, our data uncovered the previous unknown role of SRSF1 in establishment of CD8⁺ cell identity.

Eosinophils are an essential element of a type 2 immune axis that controls thymus regeneration

Therapeutic interventions used for cancer treatment provoke thymus damage and limit the recovery of protective immunity. Here, we show that eosinophils are an essential part of an intrathymic type 2 immune network that enables thymus recovery after ablative therapy. Within hours of damage, the thymus undergoes CCR3-dependent colonization by peripheral eosinophils, which reestablishes the epithelial microenvironments that control thymopoiesis. Eosinophil regulation of thymus regeneration occurs via the concerted action of NKT cells that trigger CCL11 production via IL4 receptor signaling in thymic stroma, and ILC2 that represent an intrathymic source of IL5, a cytokine that therapeutically boosts thymus regeneration after damage. Collectively, our findings identify an intrathymic network composed of multiple innate immune cells that restores thymus function during reestablishment of the adaptive immune system.

Short histological kaleidoscope - recent findings in histology. Part II

This article focuses on the latest histological knowledge in the field regarding the peripheral lymphoid system [mucosa-associated lymphoid tissue (MALT), bronchus-associated lymphoid tissue (BALT), gut-associated lymphoid tissue (GALT)], the thymus stroma, some of the various corpuscles of the human body (Hassall's corpuscles in thymus, arenaceous corpuscles in pineal gland, corpora amylacea in prostate and other locations) and Fañanas glial cells in the cerebellum.

Transcriptional and epigenetic regulation in thymic epithelial cells

The thymus is required for the development of both adaptive and innate-like T cell subsets. There is keen interest in manipulating thymic function for therapeutic purposes in circumstances of autoimmunity, immunodeficiency, and for purposes of immunotherapy. Within the thymus, thymic epithelial cells play essential roles in directing T cell development. Several transcription factors are known to be essential for thymic epithelial cell development and function, and a few transcription factors have been studied in considerable detail. However, the role of many other transcription factors is less well understood. Further, it is likely that roles exist for other transcription factors not yet known to be important in thymic epithelial cells. Recent progress in understanding of thymic epithelial cell heterogeneity has provided some new insight into transcriptional requirements in subtypes of thymic epithelial cells. However, it is unknown whether progenitors of thymic epithelial cells exist in the adult thymus, and consequently, developmental relationships linking putative precursors with differentiated cell types are poorly understood. While we do not presently possess a clear understanding of stage-specific requirements for transcription factors in thymic epithelial cells, new single-cell transcriptomic and epigenomic technologies should enable rapid progress in this field. Here, we review our current knowledge of transcription factors involved in the development, maintenance, and function of thymic epithelial cells, and the mechanisms by which they act.

The thymic microenvironment gradually modulates the phenotype of thymus-homing peripheral conventional dendritic cells

Background & Aims

Thymic conventional dendritic cells (t‑DCs) are crucial for the development of T cells. A substantial fraction of t‑DCs originates extrathymically and migrates to the thymus. Here, these cells contribute to key processes of central tolerance like the clonal deletion of self‑reactive thymocytes and the generation of regulatory T (Treg) cells. So far, it is only incompletely understood which impact the thymic microenvironment has on thymus‑homing conventional DCs (cDCs), which phenotypic changes occur after the entry of peripheral cDCs into the thymus and which functional properties these modulated cells acquire.

Materials & Methods

In the present study, we mimicked the thymus‑homing of peripheral cDCs by introducing ex vivo isolated splenic cDCs (sp‑DCs) into reaggregated thymic organ cultures (RTOCs).

Results

Already after two days of culture, the transcriptomic profile of sp‑DCs was modulated and had acquired certain key signatures of t‑DCs. The regulated genes included immunomodulatory cytokines and chemokines as well as costimulatory molecules. After four days of culture, sp‑DCs appeared to have at least partially acquired the peculiar Treg cell‐inducing capacity characteristic of t‑DCs.

Discussion & Conclusion

Taken together, our findings indicate that peripheral cDCs possess a high degree of plasticity enabling them to quickly adapt to the thymus‐specific microenvironment. We further provide indirect evidence that thymus‐specific properties such as the efficient induction of Treg cells under homeostatic conditions can be partially transferred to thymus‑homing peripheral cDC subsets.

Oncogene-Induced Reprogramming in Acute Lymphoblastic Leukemia: Towards Targeted Therapy of Leukemia-Initiating Cells

Simple Summary

Acute lymphoblastic leukemia is a heterogeneous disease characterized by a diversity of genetic alterations, following a sophisticated and controversial organization. In this review, we present and discuss the concepts exploring the cellular, molecular and functional heterogeneity of leukemic cells. We also review the emerging evidence indicating that cell plasticity and oncogene-induced reprogramming should be considered at the biological and clinical levels as critical mechanisms for identifying and targeting leukemia-initiating cells.

Abstract

Our understanding of the hierarchical structure of acute leukemia has yet to be fully translated into therapeutic approaches. Indeed, chemotherapy still has to take into account the possibility that leukemia-initiating cells may have a distinct chemosensitivity profile compared to the bulk of the tumor, and therefore are spared by the current treatment, causing the relapse of the disease. Therefore, the identification of the cell-of-origin of leukemia remains a longstanding question and an exciting challenge in cancer research of the last few decades. With a particular focus on acute lymphoblastic leukemia, we present in this review the previous and current concepts exploring the phenotypic, genetic and functional heterogeneity in patients. We also discuss the benefits of using engineered mouse models to explore the early steps of leukemia development and to identify the biological mechanisms driving the emergence of leukemia-initiating cells. Finally, we describe the major prospects for the discovery of new therapeutic strategies that specifically target their aberrant stem cell-like functions.

Novel Combination of Surface Markers for the Reliable and Comprehensive Identification of Human Thymic Epithelial Cells by Flow Cytometry: Quantitation and Transcriptional Characterization of Thymic Stroma in a Pediatric Cohort

Thymic epithelial cells (TECs) are essential in supporting the development of mature T cells from hematopoietic progenitor cells and facilitate their lineage-commitment, proliferation, T-cell receptor repertoire selection and maturation. While animal model systems have greatly aided in elucidating the contribution of stromal cells to these intricate processes, human tissue has been more difficult to study, partly due to a lack of suitable surface markers comprehensively defining human TECs. Here, we conducted a flow cytometry based surface marker screen to reliably identify and quantify human TECs and delineate medullary from cortical subsets. These findings were validated by transcriptomic and histologic means. The combination of EpCAM, podoplanin (pdpn), CD49f and CD200 comprehensively identified human TECs and not only allowed their reliable distinction in medullary and cortical subsets but also their detailed quantitation. Transcriptomic profiling of each subset in comparison to fibroblasts and endothelial cells confirmed the identity of the different stromal cell subsets sorted according to the proposed strategy. Our dataset not only demonstrated transcriptional similarities between TEC and cells of mesenchymal origin but furthermore revealed a subset-specific distribution of a specific set of extracellular matrix-related genes in TECs. This indicates that TECs significantly contribute to the distinct compartmentalization - and thus function - of the human thymus. We applied the strategy to quantify TEC subsets in 31 immunologically healthy children, which revealed sex-specific differences of TEC composition early in life. As the distribution of mature CD4- or CD8-single-positive thymocytes was correspondingly altered, the composition of the thymic epithelial compartment may directly impact on the CD4-CD8-lineage choice of thymocytes. We prove that the plain, reliable strategy proposed here to comprehensively identify human TEC subpopulations by flow cytometry based on surface marker expression is suitable to determine their frequency and phenotype in health and disease and allows sorting of live cells for downstream analysis. Its use reaches from a reliable diagnostic tool for thymic biopsies to improved phenotypic characterization of thymic grafts intended for therapeutic use.

The fibroblast: An emerging key player in thymic T cell selection

Fibroblasts have recently attracted attention as a key stromal component that controls the immune responses in lymphoid tissues. The thymus has a unique microenvironment comprised of a variety of stromal cells, including fibroblasts and thymic epithelial cells (TECs), the latter of which is known to be important for T cell development because of their ability to express self‐antigens. Thymic fibroblasts contribute to thymus organogenesis during embryogenesis and form the capsule and medullary reticular network in the adult thymus. However, the immunological significance of thymic fibroblasts has thus far only been poorly elucidated. In this review, we will summarize the current views on the development and functions of thymic fibroblasts as revealed by new technologies such as multicolor flow cytometry and single cell–based transcriptome profiling. Furthermore, the recently discovered role of medullary fibroblasts in the establishment of T cell tolerance by producing a unique set of self‐antigens will be highlighted.