Hassall’s corpuscles with cellular-senescence features maintain IFNα production through neutrophils and pDC activation in the thymus

Sex-biased human thymic architecture guides T cell development through spatially defined niches

Within the thymus, regulation of the cellular crosstalk directing T cell development depends on spatial interactions within specialized niches. To create a spatially defined map of tissue niches guiding human postnatal T cell development, we employed the multidimensional imaging platform co-detection by indexing (CODEX) as well as cellular indexing of transcriptomes and epitopes sequencing (CITE-seq) and assay for transposase accessible chromatin sequencing (ATAC-seq). We generated age-matched 4- to 5-month-old human postnatal thymus datasets for male and female donors, identifying significant sex differences in both T cell and thymus biology. We demonstrate a possible role for JAG ligands in directing thymic-like dendritic cell development, identify important functions of a population of extracellular matrix (ECM)- fibroblasts, and characterize the medullary niches surrounding Hassall's corpuscles. Together, these data represent an age-matched spatial multiomic resource to investigate how sex-based differences in thymus regulation and T cell development arise, providing an essential resource to understand the mechanisms underlying immune function and dysfunction in males and females.

Senolytic Vaccines from the Central and Peripheral Tolerance Perspective

Preventive medicine has proven its long-term effectiveness and economic feasibility. Over the last century, vaccination has saved more lives than any other medical technology. At present, preventative measures against most infectious diseases are successfully used worldwide; in addition, vaccination platforms against oncological and even autoimmune diseases are being actively developed. At the same time, the development of medicine led to an increase in both life expectancy and the proportion of age-associated diseases, which pose a heavy socio-economic burden. In this context, the development of vaccine-based approaches for the prevention or treatment of age-related diseases opens up broad prospects for extending the period of active longevity and has high economic potential. It is well known that the development of age-related diseases is associated with the accumulation of senescent cells in various organs and tissues. It has been demonstrated that the elimination of such cells leads to the restoration of functions, rejuvenation, and extension of the lives of experimental animals. However, the development of vaccines against senescent cells is complicated by their antigenic heterogeneity and the lack of a unique marker. In addition, senescent cells are the body's own cells, which may be the reason for their low immunogenicity. This mini-review discusses the mechanisms of central and peripheral tolerance that may influence the formation of an anti-senescent immune response and be responsible for the accumulation of senescent cells with age.

A spatial human thymus cell atlas mapped to a continuous tissue axis

T cells develop from circulating precursor cells, which enter the thymus and migrate through specialized subcompartments that support their maturation and selection1. In humans, this process starts in early fetal development and is highly active until thymic involution in adolescence. To map the microanatomical underpinnings of this process in pre- and early postnatal stages, we established a quantitative morphological framework for the thymus-the Cortico-Medullary Axis-and used it to perform a spatially resolved analysis. Here, by applying this framework to a curated multimodal single-cell atlas, spatial transcriptomics and high-resolution multiplex imaging data, we demonstrate establishment of the lobular cytokine network, canonical thymocyte trajectories and thymic epithelial cell distributions by the beginning of the the second trimester of fetal development. We pinpoint tissue niches of thymic epithelial cell progenitors and distinct subtypes associated with Hassall's corpuscles and identify divergence in the timing of medullary entry between CD4 and CD8 T cell lineages. These findings provide a basis for a detailed understanding of T lymphocyte development and are complemented with a holistic toolkit for cross-platform imaging data analysis, annotation and OrganAxis construction (TissueTag), which can be applied to any tissue.

Advanced three-dimensional X-ray imaging unravels structural development of the human thymus compartments

BACKGROUND

The thymus, responsible for T cell-mediated adaptive immune system, has a structural and functional complexity that is not yet fully understood. Until now, thymic anatomy has been studied using histological thin sections or confocal microscopy 3D reconstruction, necessarily for limited volumes.

METHODS

We used Phase Contrast X-Ray Computed Tomography to address the lack of whole-organ volumetric information on the microarchitecture of its structural components. We scanned 15 human thymi (9 foetal and 6 postnatal) with synchrotron radiation, and repeated scans using a conventional laboratory x-ray system. We used histology, immunofluorescence and flow cytometry to validate the x-ray findings.

RESULTS

Application to human thymi at pre- and post-natal stages allowed reliable tracking and quantification of the evolution of parameters such as size and distribution of Hassall's Bodies and medulla-to-cortex ratio, whose changes reflect adaptation of thymic activity. We show that Hassall's bodies can occupy 25% of the medulla volume, indicating they should be considered a third thymic compartment with possible implications on their role. Moreover, we demonstrate compatible results can be obtained with standard laboratory-based x-ray equipment, making this research tool accessible to a wider community.

CONCLUSIONS

Our study allows overcoming the resolution and/or volumetric limitations of existing approaches for the study of thymic disfunction in congenital and acquired disorders affecting the adaptive immune system.

TL1A and IL-18 synergy promotes GM-CSF-dependent thymic granulopoiesis in mice

Acute systemic inflammation critically alters the function of the immune system, often promoting myelopoiesis at the expense of lymphopoiesis. In the thymus, systemic inflammation results in acute thymic atrophy and, consequently, impaired T-lymphopoiesis. The mechanism by which systemic inflammation impacts the thymus beyond suppressing T-cell development is still unclear. Here, we describe how the synergism between TL1A and IL-18 suppresses T-lymphopoiesis to promote thymic myelopoiesis. The protein levels of these two cytokines were elevated in the thymus during viral-induced thymus atrophy infection with murine cytomegalovirus (MCMV) or pneumonia virus of mice (PVM). In vivo administration of TL1A and IL-18 induced acute thymic atrophy, while thymic neutrophils expanded. Fate mapping with Ms4a3-Cre mice demonstrated that thymic neutrophils emerge from thymic granulocyte-monocyte progenitors (GMPs), while Rag1-Cre fate mapping revealed a common developmental path with lymphocytes. These effects could be modeled ex vivo using neonatal thymic organ cultures (NTOCs), where TL1A and IL-18 synergistically enhanced neutrophil production and egress. NOTCH blockade by the LY411575 inhibitor increased the number of neutrophils in the culture, indicating that NOTCH restricted steady-state thymic granulopoiesis. To promote myelopoiesis, TL1A, and IL-18 synergistically increased GM-CSF levels in the NTOC, which was mainly produced by thymic ILC1s. In support, TL1A- and IL-18-induced granulopoiesis was completely prevented in NTOCs derived from Csf2rb-/- mice and by GM-CSFR antibody blockade, revealing that GM-CSF is the essential factor driving thymic granulopoiesis. Taken together, our findings reveal that TL1A and IL-18 synergism induce acute thymus atrophy while  promoting extramedullary thymic granulopoiesis in a NOTCH and GM-CSF-controlled manner.

Preclinical Study in Mouse Thymus and Thymocytes: Effects of Treatment with a Combination of Sodium Dichloroacetate and Sodium Valproate on Infectious Inflammation Pathways

The research presents data from a preclinical study on the anti-inflammatory effects of a sodium dichloroacetate and sodium valproate combination (DCA-VPA). The 2-week treatment with a DCA 100 mg/kg/day and VPA 150 mg/kg/day combination solution in drinking water's effects on the thymus weight, its cortex/medulla ratio, Hassall's corpuscles (HCs) number in the thymus medulla, and the expression of inflammatory and immune-response-related genes in thymocytes of male Balb/c mice were studied. Two groups of mice aged 6-7 weeks were investigated: a control (n = 12) and a DCA-VPA-treated group (n = 12). The treatment did not affect the body weight gain (p > 0.05), the thymus weight (p > 0.05), the cortical/medulla ratio (p > 0.05), or the number of HCs (p > 0.05). Treatment significantly increased the Slc5a8 gene expression by 2.1-fold (p < 0.05). Gene sequence analysis revealed a significant effect on the expression of inflammation-related genes in thymocytes by significantly altering the expression of several genes related to the cytokine activity pathway, the inflammatory response pathway, and the Il17 signaling pathway in thymocytes. Data suggest that DCA-VPA exerts an anti-inflammatory effect by inhibiting the inflammatory mechanisms in the mouse thymocytes.

A spatial human thymus cell atlas mapped to a continuous tissue axis

T cells develop from circulating precursors, which enter the thymus and migrate throughout specialised sub-compartments to support maturation and selection. This process starts already in early fetal development and is highly active until the involution of the thymus in adolescence. To map the micro-anatomical underpinnings of this process in pre- vs. post-natal states, we undertook a spatially resolved analysis and established a new quantitative morphological framework for the thymus, the Cortico-Medullary Axis. Using this axis in conjunction with the curation of a multimodal single-cell, spatial transcriptomics and high-resolution multiplex imaging atlas, we show that canonical thymocyte trajectories and thymic epithelial cells are highly organised and fully established by post-conception week 12, pinpoint TEC progenitor states, find that TEC subsets and peripheral tissue genes are associated with Hassall's Corpuscles and uncover divergence in the pace and drivers of medullary entry between CD4 vs. CD8 T cell lineages. These findings are complemented with a holistic toolkit for spatial analysis and annotation, providing a basis for a detailed understanding of T lymphocyte development.

Sex biased human thymic architecture guides T cell development through spatially defined niches

Within the thymus, regulation of the cellular cross-talk directing T cell development is dependent on spatial interactions within specialized niches. To create a holistic, spatially defined map of tissue niches guiding postnatal T cell development we employed the multidimensional imaging platform CO-detection by indEXing (CODEX), as well as CITE-seq and ATAC-seq. We generated age-matched 4-5-month-old postnatal thymus datasets for male and female donors, and identify significant sex differences in both T cell and thymus biology. We demonstrate a crucial role for JAG ligands in directing thymic-like dendritic cell development, reveal important functions of a novel population of ECM- fibroblasts, and characterize the medullary niches surrounding Hassall's corpuscles. Together, these data represent a unique age-matched spatial multiomic resource to investigate how sex-based differences in thymus regulation and T cell development arise, and provide an essential resource to understand the mechanisms underlying immune function and dysfunction in males and females.

Germline T cell receptor exchange results in physiological T cell development and function

T cell receptor (TCR) transgenic mice represent an invaluable tool to study antigen-specific immune responses. In the pre-existing models, a monoclonal TCR is driven by a non-physiologic promoter and randomly integrated into the genome. Here, we create a highly efficient methodology to develop T cell receptor exchange (TRex) mice, in which TCRs, specific to the self/tumor antigen mesothelin (Msln), are integrated into the Trac locus, with concomitant Msln disruption to circumvent T cell tolerance. We show that high affinity TRex thymocytes undergo all sequential stages of maturation, express the exogenous TCR at DN4, require MHC class I for positive selection and undergo negative selection only when both Msln alleles are present. By comparison of TCRs with the same specificity but varying affinity, we show that Trac targeting improves functional sensitivity of a lower affinity TCR and confers resistance to T cell functional loss. By generating P14 TRex mice with the same specificity as the widely used LCMV-P14 TCR transgenic mouse, we demonstrate increased avidity of Trac-targeted TCRs over transgenic TCRs, while preserving physiologic T cell development. Together, our results support that the TRex methodology is an advanced tool to study physiological antigen-specific T cell behavior.

ACE2 Affects the Expression and Function of IFITM3 During SARS-CoV-2 Pseudovirus Infection in Vero E6 Cells

COVID-19 is a globally infectious viral epidemic of great public health concern caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Angiotensin-converting enzyme 2 (ACE2) plays its role as the receptor for SARS-CoV-2 through binding with S protein and the binding results in ACE2 expression decrease. The change of ACE2 is supposed to elicit a series of cellular and molecular events. Other than as the receptor, ACE2's roles on infection by regulating other molecules need to be further studied during SARS-CoV-2 infection. In the present study, we established the ACE2 knockdown model using Vero E6 cells to study how ACE2 influenced the downstream signaling molecules. Analysis of transcriptome sequencing discovered that ACE2 alteration per se caused the alteration of immune factors, including some related to the viral infection-related signaling pathways. We found that ACE2 silencing induced the reduced interferon-induced transmembrane protein 3 (IFITM3) expression. Overexpression of IFITM3 promoted the SARS-CoV-2 pseudovirus infection of Vero E6 cells lacking the ACE2. It indicates that ACE2 can affect IFITM3 expression and function to affect the SARS-CoV-2 infection. Our results reveal possible mechanisms influencing SARS-CoV-2 infectivity and contribute to explaining the rapid spread and pathogenesis especially in the case of ACE2 low expression.

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 and Peripheral T-cell Polarization in an Experimental Model of Russell's Viper Venom-induced Acute Kidney Injury

Venom pathology is not restricted to the direct toxic effects of venom. Immunoinflammatory alteration as the etiology of snake venom-induced acute kidney injury (SAKI) is a less trodden path toward the development of alternative therapeutic approach. In the present study, we have associated the crest of renal damage stage to the immunological alteration, as reflected in thymic and peripheral T cell polarization in the murine model of SAKI. Renal injury in mice was confirmed from significant dysuresis and adversely altered biochemical renal markers. Histopathological alterations, as revealed by marked tubular and glomerular damage, reaffirmed kidney injury. SAKI is accompanied by significant inflammatory changes as indicated by neutrophilic leucocytosis, increased neutrophil to lymphocyte ratio and plasma CRP levels. Thymic immunophenotyping revealed significantly increased CD8+ cytotoxic T cell, and CD25+ both single positive population (p = .017-0.010) and CD44-CD25+ double negative population (DN3) (p = .002) accompanied by an insignificantly reduced CD4+ helper T cells (p = .451). Peripheral immunophenotyping revealed similar pattern as indicated by reduced helper T cells (p = .002) associated with significantly elevated cytotoxic T cells (p = .009) and CD25+ subset of both helper (p = .002) and cytotoxic (p = .024) T cells. The IL-10+ subset of both CD25+ and CD25- T cells were also found to be significantly elevated in the SAKI group (p ≤ 0.020) suggesting an immunosuppressive phenotype in SAKI. It can be concluded that T cells responds to venom-induced renal injury particularly through IL-10+ reparative phenotypes which are known for their immunosuppressive and anti-inflammatory activity.

Developmental Exposure to Endocrine Disrupter DDT Interferes with Age-Related Involution of Thymus

The impact of endocrine-disrupting chemicals on the development and involution of the immune system is a possible reason for the increased incidence of disorders associated with inappropriate immune function. The thymus is a lymphoid and also an endocrine organ, and, accordingly, its development and functioning may be impaired by endocrine disruptors. The aim was to evaluate age-related thymus involution in mature rats exposed to the endocrine disruptor DDT during prenatal and postnatal ontogeny. Methodology included in vivo experiment on male Wistar rats exposed to low doses of DDT during prenatal and postnatal development and morphological assessment of thymic involution, including the immunohistochemical detection of proliferating thymocytes. The study was carried out at the early stage of involution. Results: DDT-exposed rats exhibited a normal anatomy, and the relative weight of the thymus was within the control ranges. Histological and immunohistochemical examinations revealed increased cellularity of the cortex and the medulla, higher content of lymphoblasts, and more intensive proliferation rate of thymocytes compared to the control. Evaluation of thymic epithelial cells revealed a higher rate of thymic corpuscles formation. Conclusion: The data obtained indicate that endocrine disrupter DDT disturbs postnatal development of the thymus. Low-dose exposure to DDT during ontogeny does not suppress growth rate but violates the developmental program of the thymus by slowing down the onset of age-related involution and maintaining high cell proliferation rate. It may result in excessive formation of thymus-dependent areas in peripheral lymphoid organs and altered immune response.

Mechanisms of Direct and Indirect Presentation of Self-Antigens in the Thymus

The inevitability of evolution of the adaptive immune system with its mechanism of randomly rearranging segments of the T cell receptor (TCR) gene is the generation of self-reactive clones. For the sake of prevention of autoimmunity, these clones must be eliminated from the pool of circulating T cells. This process occurs largely in the thymic medulla where the strength of affinity between TCR and self-peptide MHC complexes is the factor determining thymocyte fate. Thus, the display of self-antigens in the thymus by thymic antigen presenting cells, which are comprised of medullary thymic epithelial (mTECs) and dendritic cells (DCs), is fundamental for the establishment of T cell central tolerance. Whereas mTECs produce and present antigens in a direct, self-autonomous manner, thymic DCs can acquire these mTEC-derived antigens by cooperative antigen transfer (CAT), and thus present them indirectly. While the basic characteristics for both direct and indirect presentation of self-antigens are currently known, recent reports that describe the heterogeneity of mTEC and DC subsets, their presentation capacity, and the potentially non-redundant roles in T cell selection processes represents another level of complexity which we are attempting to unravel. In this review, we underscore the seminal studies relevant to these topics with an emphasis on new observations pertinent to the mechanism of CAT and its cellular trajectories underpinning the preferential distribution of thymic epithelial cell-derived self-antigens to specific subsets of DC. Identification of molecular determinants which control CAT would significantly advance our understanding of how the cellularly targeted presentation of thymic self-antigens is functionally coupled to the T cell selection process.

Structural and Functional Thymic Biomarkers Are Involved in the Pathogenesis of Thymic Epithelial Tumors: An Overview

The normal human thymus originates from the third branchial cleft as two paired anlages that descend into the thorax and fuse on the midline of the anterior–superior mediastinum. Alongside the epithelial and lymphoid components, different types of lymphoid accessory cells, stromal mesenchymal and endothelial cells migrate to, or develop in, the thymus. After reaching maximum development during early postnatal life, the human thymus decreases in size and lymphocyte output drops with age. However, thymic immunological functions persist, although they deteriorate progressively. Several major techniques were fundamental to increasing the knowledge of thymic development and function during embryogenesis, postnatal and adult life; these include immunohistochemistry, immunofluorescence, flow cytometry, in vitro colony assays, transplantation in mice models, fetal organ cultures (FTOC), re-aggregated thymic organ cultures (RTOC), and whole-organ thymic scaffolds. The thymic morphological and functional characterization, first performed in the mouse, was then extended to humans. The purpose of this overview is to provide a report on selected structural and functional biomarkers of thymic epithelial cells (TEC) involved in thymus development and lymphoid cell maturation, and on the historical aspects of their characterization, with particular attention being paid to biomarkers also involved in Thymic Epithelial Tumor (TET) pathogenesis. Moreover, a short overview of targeted therapies in TET, based on currently available experimental and clinical data and on potential future advances will be proposed.

A model of preferential pairing between epithelial and dendritic cells in thymic antigen transfer

Medullary thymic epithelial cells (mTECs), which produce and present self-antigens, are essential for the establishment of central tolerance. Since mTEC numbers are limited, their function is complemented by thymic dendritic cells (DCs), which transfer mTEC-produced self-antigens via cooperative antigen transfer (CAT). While CAT is required for effective T cell selection, many aspects remain enigmatic. Given the recently described heterogeneity of mTECs and DCs, it is unclear whether the antigen acquisition from a particular TEC subset is mediated by preferential pairing with a specific subset of DCs. Using several relevant Cre-based mouse models that control for the expression of fluorescent proteins, we have found that, in regards to CAT, each subset of thymic DCs preferentially targets a distinct mTEC subset(s). Importantly, XCR1⁺-activated DC subset represented the most potent subset in CAT. Interestingly, thymic DCs can also acquire antigens from more than one mTEC, and of these, monocyte-derived dendritic cells (moDCs) were determined to be the most efficient. moDCs also represented the most potent DC subset in the acquisition of antigen from other DCs. These findings suggest a preferential pairing model for the distribution of mTEC-derived antigens among distinct populations of thymic DCs.

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.

A 2020 View of Thymus Stromal Cells in T Cell Development

The thymus is an intricate primary lymphoid organ, wherein bone marrow-derived lymphoid progenitor cells are induced to develop into functionally competent T cells that express a diverse TCR repertoire, which is selected to allow for the recognition of foreign Ags while avoiding self-reactivity or autoimmunity. Thymus stromal cells, which can include all non-T lineage cells, such as thymic epithelial cells, endothelial cells, mesenchymal/fibroblast cells, dendritic cells, and B cells, provide signals that are essential for thymocyte development as well as for the homeostasis of the thymic stroma itself. In this brief review, we focus on the key roles played by thymic stromal cells during early stages of T cell development, such as promoting the homing of thymic-seeding progenitors, inducing T lineage differentiation, and supporting thymocyte survival and proliferation. We also discuss recent advances on the transcriptional regulation that govern thymic epithelial cell function as well as the cellular and molecular changes that are associated with thymic involution and regeneration.

Post-Aire Medullary Thymic Epithelial Cells and Hassall's Corpuscles as Inducers of Tonic Pro-Inflammatory Microenvironment

While there is convincing evidence on the role of Aire-positive medullary thymic epithelial cells (mTEC) in the induction of central tolerance, the nature and function of post-Aire mTECs and Hassall's corpuscles have remained enigmatic. Here we summarize the existing data on these late stages of mTEC differentiation with special focus on their potential to contribute to central tolerance induction by triggering the unique pro-inflammatory microenvironment in the thymus. In order to complement the existing evidence that has been obtained from mouse models, we performed proteomic analysis on microdissected samples from human thymic medullary areas at different differentiation stages. The analysis confirms that at the post-Aire stages, the mTECs lose their nuclei but maintain machinery required for translation and exocytosis and also upregulate proteins specific to keratinocyte differentiation and cornification. In addition, at the late stages of differentiation, the human mTECs display a distinct pro-inflammatory signature, including upregulation of the potent endogenous TLR4 agonist S100A8/S100A9. Collectively, the study suggests a novel mechanism by which the post-Aire mTECs and Hassall's corpuscles contribute to the thymic microenvironment with potential cues on the induction of central tolerance.

A novel method to identify Post-Aire stages of medullary thymic epithelial cell differentiation

Autoimmune regulator+ (Aire) medullary thymic epithelial cells (mTECs) play a critical role in tolerance induction. Several studies demonstrated that Aire+ mTECs differentiate further into Post-Aire cells. Yet, the identification of terminal stages of mTEC maturation depends on unique fate-mapping mouse models. Herein, we resolve this limitation by segmenting the mTEChi (MHCIIhi CD80hi ) compartment into mTECA/hi (CD24- Sca1- ), mTECB/hi (CD24+ Sca1- ), and mTECC/hi (CD24+ Sca1+ ). While mTECA/hi included mostly Aire-expressing cells, mTECB/hi contained Aire+ and Aire- cells and mTECC/hi were mainly composed of cells lacking Aire. The differential expression pattern of Aire led us to investigate the precursor-product relationship between these subsets. Strikingly, transcriptomic analysis of mTECA/hi , mTECB/hi , and mTECC/hi sequentially mirrored the specific genetic program of Early-, Late- and Post-Aire mTECs. Corroborating their Post-Aire nature, mTECC/hi downregulated the expression of tissue-restricted antigens, acquired traits of differentiated keratinocytes, and were absent in Aire-deficient mice. Collectively, our findings reveal a new and simple blueprint to survey late stages of mTEC differentiation.

Non-Epithelial Thymic Stromal Cells: Unsung Heroes in Thymus Organogenesis and T Cell Development

The stromal microenvironment in the thymus is essential for generating a functional T cell repertoire. Thymic epithelial cells (TECs) are numerically and phenotypically one of the most prominent stromal cell types in the thymus, and have been recognized as one of most unusual cell types in the body by virtue of their unique functions in the course of the positive and negative selection of developing T cells. In addition to TECs, there are other stromal cell types of mesenchymal origin, such as fibroblasts and endothelial cells. These mesenchymal stromal cells are not only components of the parenchymal and vascular architecture, but also have a pivotal role in controlling TEC development, although their functions have been less extensively explored than TECs. Here, we review both the historical studies on and recent advances in our understanding of the contribution of such non-TEC stromal cells to thymic organogenesis and T cell development. In particular, we highlight the recently discovered functional effect of thymic fibroblasts on T cell repertoire selection.

miRNA-146a-5p mitigates stress-induced premature senescence of D-galactose-induced primary thymic stromal cells

Senescent thymic stromal cells (TSCs) producing senescence-associated secretory phenotype (SASP) may play a role at later phases of thymic involution. However, the etiology and mechanisms responsible for TSC senescence remain to be elucidated. In the present study, the effects of oxidative stress on TSCs and role of miRNA-146a-5p in stress-induced premature senescence (SIPS) were identified. D-galactose (D-gal) induced oxidative stress in primary TSCs and a limited cumulative oxidative stress induced premature senescence but not apoptosis of TSCs. miRNA-146a-5p overexpression can mitigate the SIPS by targeting tumor necrosis factor receptor-associated factor 6 (TRAF6) instead of increasing autophagy clearance. Furthermore, exogenous miRNA-146a-5p reversed the upregulation of chemokines including Cxcl5, pro-inflammatory cytokines, and antimicrobial peptides in TSCs with SIPS. In conclusion, the accumulated oxidative stress may be partially responsible for senescence in TSCs and modulation of miRNA-146a-5p may attenuate this process.

The thymus medulla and its control of αβT cell development

αβT cells are an essential component of effective immune responses. The heterogeneity that lies within them includes subsets that express diverse self-MHC-restricted αβT cell receptors, which can be further subdivided into CD4⁺ helper, CD8⁺ cytotoxic, and Foxp3⁺ regulatory T cells. In addition, αβT cells also include invariant natural killer T cells that are very limited in αβT cell receptor repertoire diversity and recognise non-polymorphic CD1d molecules that present lipid antigens. Importantly, all αβT cell sublineages are dependent upon the thymus as a shared site of their development. Ongoing research has examined how the thymus balances the intrathymic production of multiple αβT cell subsets to ensure correct formation and functioning of the peripheral immune system. Experiments in both wild-type and genetically modified mice have been essential in revealing complex cellular and molecular mechanisms that regulate thymus function. In particular, studies have demonstrated the diverse and critical role that the thymus medulla plays in shaping the peripheral T cell pool. In this review, we summarise current knowledge on functional properties of the thymus medulla that enable the thymus to support the production of diverse αβT cell types.

Inborn Errors of Adaptive Immunity in Down Syndrome

Down syndrome fits an immunophenotype of combined immunodeficiency with immunodysregulation, manifesting with increased susceptibility to infections, autoimmunity, autoinflammatory diseases, and hematologic malignancies. Qualitative and quantitative alterations in innate and adaptive immunity are found in most individuals with Down syndrome. However, there is substantial heterogeneity and no correlation between immunophenotype and clinical presentation. Previously, it was thought that the immunological changes in Down syndrome were caused by precocious aging. We emphasize in this review that the immune system in Down syndrome is intrinsically different from the very beginning. The overexpression of specific genes located on chromosome 21 contributes to immunodeficiency and immunodysregulation, but gene expression differs between genes located on chromosome 21 and depends on tissue and cell type. In addition, trisomy 21 results in gene dysregulation of the whole genome, reflecting the complex nature of this syndrome in comparison to well-known inborn errors of immunity that result from monogenic germline mutations. In this review, we provide an updated overview focusing on inborn errors of adaptive immunity in Down syndrome.

Toll-like receptor signaling in thymic epithelium controls monocyte-derived dendritic cell recruitment and Treg generation

The development of thymic regulatory T cells (Treg) is mediated by Aire-regulated self-antigen presentation on medullary thymic epithelial cells (mTECs) and dendritic cells (DCs), but the cooperation between these cells is still poorly understood. Here we show that signaling through Toll-like receptors (TLR) expressed on mTECs regulates the production of specific chemokines and other genes associated with post-Aire mTEC development. Using single-cell RNA-sequencing, we identify a new thymic CD14⁺Sirpα⁺ population of monocyte-derived dendritic cells (CD14⁺moDC) that are enriched in the thymic medulla and effectively acquire mTEC-derived antigens in response to the above chemokines. Consistently, the cellularity of CD14⁺moDC is diminished in mice with MyD88-deficient TECs, in which the frequency and functionality of thymic CD25⁺Foxp3⁺ Tregs are decreased, leading to aggravated mouse experimental colitis. Thus, our findings describe a TLR-dependent function of mTECs for the recruitment of CD14⁺moDC, the generation of Tregs, and thereby the establishment of central tolerance.

Immune tolerance is mediated by the deletion of autoreactive T cells via medullary thymic epithelial cells (mTEC) and dendritic cells (DC), and by the induction of regulatory T cells (Treg). Here the authors show that mTEC receiving toll-like receptor signaling control the recruitment of CD14⁺Sirpα⁺ DC population that is capable of inducing Treg for establishing tolerance.

Qualitative Changes in Cortical Thymic Epithelial Cells Drive Postpartum Thymic Regeneration

During gestation, sex hormones cause a significant thymic involution which enhances fertility. This thymic involution is rapidly corrected following parturition. As thymic epithelial cells (TECs) are responsible for the regulation of thymopoiesis, we analyzed the sequential phenotypic and transcriptomic changes in TECs during the postpartum period in order to identify mechanisms triggering postpartum thymic regeneration. In particular, we performed flow cytometry analyses and deep RNA-sequencing on purified TEC subsets at several time points before and after parturition. We report that pregnancy-induced involution is not caused by loss of TECs since their number does not change during or after pregnancy. However, during pregnancy, we observed a significant depletion of all thymocyte subsets downstream of the double-negative 1 (DN1) differentiation stage. Variations in thymocyte numbers correlated with conspicuous changes in the transcriptome of cortical TECs (cTECs). The transcriptomic changes affected predominantly cTEC expression of Foxn1, its targets and several genes that are essential for thymopoiesis. By contrast, medullary TECs (mTECs) showed very little transcriptomic changes in the early postpartum regenerative phase, but seemed to respond to the expansion of single-positive (SP) thymocytes in the late phase of regeneration. Together, these results show that postpartum thymic regeneration is orchestrated by variations in expression of a well-defined subset of cTEC genes, that occur very early after parturition.