Zoned out: functional mapping of stromal signaling microenvironments in the thymus

A clinically applicable and scalable method to regenerate T-cells from iPSCs for off-the-shelf T-cell immunotherapy

Clinical successes demonstrated by chimeric antigen receptor T-cell immunotherapy have facilitated further development of T-cell immunotherapy against wide variety of diseases. One approach is the development of “off-the-shelf” T-cell sources. Technologies to generate T-cells from pluripotent stem cells (PSCs) may offer platforms to produce “off-the-shelf” and synthetic allogeneic T-cells. However, low differentiation efficiency and poor scalability of current methods may compromise their utilities. Here we show improved differentiation efficiency of T-cells from induced PSCs (iPSCs) derived from an antigen-specific cytotoxic T-cell clone, or from T-cell receptor (TCR)-transduced iPSCs, as starting materials. We additionally describe feeder-free differentiation culture systems that span from iPSC maintenance to T-cell proliferation phases, enabling large-scale regenerated T-cell production. Moreover, simultaneous addition of SDF1α and a p38 inhibitor during T-cell differentiation enhances T-cell commitment. The regenerated T-cells show TCR-dependent functions in vitro and are capable of in vivo anti-tumor activity. This system provides a platform to generate a large number of regenerated T-cells for clinical application and investigate human T-cell differentiation and biology.

T-cell immunotherapies, such as CAR-T immunotherapy, are being developed against a wide variety of diseases. Here the authors report the feeder-free, scalable differentiation of human induced pluripotent cells (iPSCs) to T-cells with T-cell receptor dependent anti-tumour function in vitro and in vivo.

CXCL12-driven thymocyte migration is increased by thymic epithelial cells treated with prolactin in vitro

The prolactin hormone (PRL), in addition to its known effects on breast development and lactation, exerts effects on the immune system, including pleiotropic effects on the thymus. The aim of this study was to evaluate the influence of PRL on the epithelial compartment of the thymus. Thymic epithelial cells (TECs) (2BH4 cells) and fresh thymocytes were used. Immunofluorescence assay revealed that PRL treatment (10 ng/ mL) increases the deposition of laminin and expression of the chemokine CXCL12 in 2BH4 cells. However, no change was observed in the deposition of fibronectin. Moreover, PRL altered F-actin polymerisation, allowing the formation of focal adhesion complexes in treated cells. When 2BH4 cells were pre-treated with PRL, thymocyte adhesion was not altered. However, in the cell migration assay, pre-treatment with PRL potentiated the chemotactic effect of CXCL12 on the migration of total, double-positive, CD4-positive, and CD8-positive thymocytes. Together, the results of this study demonstrate the effect of PRL on thymic epithelial cells, particularly on CXCL12-driven thymocyte migration, confirming that this hormone is a regulator of thymic physiology.

Innate Immunity and Biological Therapies for the Treatment of Sjögren's Syndrome

Sjögren's syndrome (SS) is a systemic autoimmune disorder affecting approximately 3% of the population in the United States. This disease has a female predilection and affects exocrine glands, including lacrimal and salivary glands. Dry eyes and dry mouths are the most common symptoms due to the loss of salivary and lacrimal gland function. Symptoms become more severe in secondary SS, where SS is present along with other autoimmune diseases like systemic lupus erythematosus, systemic sclerosis, or rheumatoid arthritis. It is known that aberrant activation of immune cells plays an important role in disease progression, however, the mechanism for these pathological changes in the immune system remains largely unknown. This review highlights the role of different immune cells in disease development, therapeutic treatments, and future strategies that are available to target various immune cells to cure the disease.

Interaction between γδTCR signaling and the E protein-Id axis in γδ T cell development

γδ T cells acquire their functional properties in the thymus, enabling them to exert rapid innate-like responses. To understand how distinct γδ T cell subsets are generated, we have developed a Two-Stage model for γδ T cell development. This model is predicated on the finding that γδTCR signal strength impacts E protein activity through graded upregulation of Id3. Our model proposes that cells enter Stage 1 in response to a γδTCR signaling event in the cortex that activates a γδ T cell-specific gene network. Part of this program includes the upregulation of chemokine receptors that guide them to the medulla. In the medulla, Stage 1 cells receive distinct combinations of γδTCR, cytokine, and/co-stimulatory signals that induce their transit into Stage 2, either toward the γδT1 or the γδT17 lineage. The intersection between γδTCR and cytokine signals can tune Id3 expression, leading to different outcomes even in the presence of strong γδTCR signals. The thymic signaling niches required for γδT17 development are segregated in time and space, providing transient windows of opportunity during ontogeny. Understanding the regulatory context in which E proteins operate at different stages will be key in defining how their activity levels impose functional outcomes.

Type II Cytokines Fine-Tune Thymic T Cell Selection to Offset Murine Central Nervous System Autoimmunity

Early thymic progenitors (ETPs) are bone marrow-derived hematopoietic stem cells that remain multipotent and give rise to a variety of lineage-specific cells. Recently, we discovered a subset of murine ETPs that expresses the IL-4Rα/IL-13Rα1 heteroreceptor (HR) and commits only to the myeloid lineage. This is because IL-4/IL-13 signaling through the HR inhibits their T cell potential and enacts commitment of HR⁺ETPs to thymic resident CD11c⁺CD8α⁺ dendritic cells (DCs). In this study, we discovered that HR⁺-ETP-derived DCs function as APCs in the thymus and promote deletion of myelin-reactive T cells. Furthermore, this negative T cell selection function of HR⁺-ETP-derived DCs sustains protection against experimental allergic encephalomyelitis, a mouse model for human multiple sclerosis. These findings, while shedding light on the intricacies underlying ETP lineage commitment, reveal a novel, to our knowledge, function by which IL-4 and IL-13 cytokines condition thymic microenvironment to rheostat T cell selection and fine-tune central tolerance.

Resveratrol ameliorates thymus senescence changes in D-galactose induced mice

The thymic microenvironment plays an important role in the development of T cells. A decrease of thymic epithelial cells is the main cause of age-related thymic atrophy or degeneration. Resveratrol (RSV), a phytoalexin produced from plants, has been shown to inhibit the adverse effects of dietary obesity on the structure and function of the thymus. D-Galactose (D-gal) can induce accelerated aging in mice. In the present study, young mice (2 months old) were injected with D-gal (120 mg/kg/day) for 8 consecutive weeks to construct an accelerated aging model. Compared with normal control mice, the thymus epithelium of the D-gal treated mice had structural changes, the number of senescent cells increased, the number of CD4+ T cells decreased, and CD8+ T cells increased. After RSV administration by gavage for 6 weeks, it was found that RSV improved the surface phenotypes of D-gal treated mice, and recovered thymus function by maintaining the ratio of CD4+ to CD8+ cells. It also indicated that RSV enhanced the cell proliferation and inhibited cell senescence. Increased autoimmune regulator (Aire) expression was present in the RSV treated mice. The lymphotoxin-beta receptor (LTβR) expression also increased. These findings suggested that RSV intake could restore the alterations caused by D-gal treatment in the thymus via stimulation of Aire expression.

Thymic Engraftment by in vitro-Derived Progenitor T Cells in Young and Aged Mice

T cells play a critical role in mediating antigen-specific and long-term immunity against viral and bacterial pathogens, and their development relies on the highly specialized thymic microenvironment. T cell immunodeficiency can be acquired in the form of inborn errors, or can result from perturbations to the thymus due to aging or irradiation/chemotherapy required for cancer treatment. Hematopoietic stem cell transplant (HSCT) from compatible donors is a cornerstone for the treatment of hematological malignancies and immunodeficiency. Although it can restore a functional immune system, profound impairments exist in recovery of the T cell compartment. T cells remain absent or low in number for many months after HSCT, depending on a variety of factors including the age of the recipient. While younger patients have a shorter refractory period, the prolonged T cell recovery observed in older patients can lead to a higher risk of opportunistic infections and increased predisposition to relapse. Thus, strategies for enhancing T cell recovery in aged individuals are needed to counter thymic damage induced by radiation and chemotherapy toxicities, in addition to naturally occurring age-related thymic involution. Preclinical results have shown that robust and rapid long-term thymic reconstitution can be achieved when progenitor T cells, generated in vitro from HSCs, are co-administered during HSCT. Progenitor T cells appear to rely on lymphostromal crosstalk via receptor activator of NF-κB (RANK) and RANK-ligand (RANKL) interactions, creating chemokine-rich niches within the cortex and medulla that likely favor the recruitment of bone marrow-derived thymus seeding progenitors. Here, we employed preclinical mouse models to demonstrate that in vitro-generated progenitor T cells can effectively engraft involuted aged thymuses, which could potentially improve T cell recovery. The utility of progenitor T cells for aged recipients positions them as a promising cellular therapy for immune recovery and intrathymic repair following irradiation and chemotherapy, even in a post-involution thymus.

From stem cell to immune effector: how adhesion, migration, and polarity shape T-cell and natural killer cell lymphocyte development in vitro and in vivo

Lymphocyte development is a complex and coordinated pathway originating from pluripotent stem cells during embryogenesis and continuing even as matured lymphocytes are primed and educated in adult tissue. Hematopoietic stem cells develop in a specialized niche that includes extracellular matrix and supporting stromal and endothelial cells that both maintain stem cell pluripotency and enable the generation of differentiated cells. Cues for lymphocyte development include changes in integrin-dependent cell motility and adhesion which ultimately help to determine cell fate. The capacity of lymphocytes to adhere and migrate is important for modulating these developmental signals both by regulating the cues that the cell receives from the local microenvironment as well as facilitating the localization of precursors to tissue niches throughout the body. Here we consider how changing migratory and adhesive phenotypes contribute to human natural killer (NK)- and T-cell development as they undergo development from precursors to mature, circulating cells and how our understanding of this process is informed by in vitro models of T- and NK cell generation.

DNT Cell-based Immunotherapy: Progress and Applications

Cancer immunotherapy has firmly established a dominant status in recent years. Adoptive cellular immunotherapy (ACI) is the main branch of immunotherapy. Recently, the immune effector cells of ACI, such as T cells, NK cells, and genetically engineered cells, have been used to achieve significant clinical benefits in the treatment of malignant tumors. However, the clinical applications have limitations, including toxicity, unexpectedly low efficiency, high costs and strict technical requirements. More exploration is needed to optimize ACI for cancer patients. CD3+CD4-CD8- double negative T cells (DNTs) have emerged as functional antitumor effector cells, according to the definition of adoptive immunotherapy. They constitute a kind of T cell subset that mediates nontumor antigen-restricted immunity and has important immune regulatory functions. Preclinical experiments showed that DNTs had a dual effect by killing tumor cells and inhibiting graft-versus-host disease. Notably, DNTs can be acquired from healthy donors and expanded in vitro; thus, allogeneic DNTs may be provided as “off-the-shelf” cellular products that can be readily available for direct clinical application. We review the progress and application of DNTs in immunotherapy. DNTs may provide some novel perspectives on cancer immunotherapy.

Comprehensive analysis of circRNA expression profiles and circRNA-associated competing endogenous RNA networks in the development of mouse thymus

The thymus plays an irreplaceable role as a primary lymphoid organ. However, the complicate processes of its development and involution are incompletely understood. Accumulating evidence indicates that non-coding RNAs play key roles in the regulation of biological development. At present, the studies of the circRNA profiles and of circRNA-associated competing endogenous RNAs (ceRNAs) in the thymus are still scarce. Here, deep-RNA sequencing was used to study the biological mechanisms underlying the development process (from 2-week-old to 6-week-old) and the recession process (from 6-week-old to 3-month-old) of the mouse thymus. It was found that 196 circRNAs, 233 miRNAs and 3807 mRNAs were significantly dysregulated. The circRNA-associated ceRNA networks were constructed in the mouse thymus, which were mainly involved in early embryonic development and the proliferation and division of T cells. Taken together, these results elucidated the regulatory roles of ceRNAs in the development and involution processes of the mouse thymus.

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.

Semaphorin-3A-Related Reduction of Thymocyte Migration in Chemically Induced Diabetic Mice

BACKGROUND

Previous work revealed the existence of a severe thymic atrophy with massive loss of immature CD4+CD8+ thymocytes in animals developing insulin-dependent diabetes, chemically induced by alloxan. Furthermore, the intrathymic expression of chemokines, such as CXCL12, is changed in these animals, suggesting that cell migration-related patterns may be altered. One molecular interaction involved in normal thymocyte migration is that mediated by soluble semaphorin-3A and its cognate receptor neuropilin-1.

OBJECTIVES

We investigated herein the expression and role of semaphorin-3A in the migratory responses of thymocytes from alloxan-induced diabetic mice. We characterized semaphorin-3A and its receptor, neuropilin-1, in thymuses from control and diabetic mice as well as semaphorin-3A-dependent migration of developing thymocytes in both control and diabetic animals.

METHODS

Diabetes was chemically induced after a single injection of alloxan in young adult BALB/c mice. Thymocytes were excised from control and diabetic individuals and subjected to cytofluorometry for simultaneous detection of semaphorin-3A or neuropilin-1 in CD4/CD8-defined subsets. Cell migration in response to semaphorin-3A was performed using cell migration transwell chambers.

RESULTS

Confirming previous data, we observed a severe decrease in the total numbers of thymocytes in diabetic mice, which comprised alterations in both immature (double-negative subpopulations) and mature CD4/CD8-defined thymocyte subsets. These were accompanied by a decrease in the absolute numbers of semaphorin-3A-bearing thymocytes, comprising CD4-CD8-, CD4+CD8+, and CD4-CD8+ cells. Additionally, immature CD4-CD8- and CD4+CD8+ developing T cells exhibited a decrease in the membrane density of semaphorin-3A. The relative and absolute numbers of neuropilin-1-positive thymocytes were also decreased in diabetic mouse thymocytes compared to controls, as seen in CD4-CD8-, CD4+CD8+, and CD4-CD8+ cell subpopulations. Functionally, we observed a decrease in the chemorepulsive role of semaphorin-3A, as revealed by transwell migration chambers. Such an effect was seen in all immature and mature thymocyte subsets.

CONCLUSIONS

Taken together, our data clearly unravel a disruption in the normal cell migration pattern of developing thymocytes following chemically induced insulin-dependent diabetes, as ascertained by the altered migratory response to sempahorin-3A. In conceptual terms, it is plausible to think that such disturbances in the migration pattern of thymocytes from these diabetic animals may exert an impact in the cell-mediated immune response of these mice.

T cell regeneration: an update on progress and challenges

T cells play essential roles in antitumor therapy. Via gene engineering technique to enhance tumor-antigen specificity, patient peripheral blood-derived T cells (PBT) show encouraging clinical outcomes in treating certain blood malignancies. However, the high costs, functionality exhaustion, and disease-condition-dependent availability of PBT prompt the attempts of exploring alternative T cell sources. Theoretically, induced T cells from pluripotent stem cells (PSC) are ideal candidates that integrate plenty of advantages that primary T cells lack, including unlimited off-the-shelf cell source and precision gene editing feasibility. However, researchers are still struggling with developing a straightforward protocol to induce functional and immunocompetent human T cells from PSC. Based on stromal cell-expressing or biomaterial-presenting Notch ligands DLL1 or DLL4, natural and induced blood progenitors can differentiate further toward T lineage commitment. However, none of the reported T induction protocols has yet translated into any clinical application, signaling the existence of numerous technical barriers for regenerating T cells functionally matching their natural PBT counterparts. Alternatively, new approaches have been developed to repopulate induced T lymphopoiesis via in vivo reprogramming or transplanting induced T cell precursors. Here, we review the most recent progress in the T cell regeneration field, and the remaining challenges dragging their clinical applications.

Programming for T-lymphocyte fates: modularity and mechanisms

In this review, Rothenburg discusses the gene regulatory network and chromatin-based kinetic constraints that determine activities of transcription factors in the primary establishment of T-cell identity.

T-cell development in mammals is a model for lineage choice and differentiation from multipotent stem cells. Although T-cell fate choice is promoted by signaling in the thymus through one dominant pathway, the Notch pathway, it entails a complex set of gene regulatory network and chromatin state changes even before the cells begin to express their signature feature, the clonal-specific T-cell receptors (TCRs) for antigen. This review distinguishes three developmental modules for T-cell development, which correspond to cell type specification, TCR expression and selection, and the assignment of cells to different effector types. The first is based on transcriptional regulatory network events, the second is dominated by somatic gene rearrangement and mutation and cell selection, and the third corresponds to establishing a poised state of latent regulator priming through an unknown mechanism. Interestingly, in different lineages, the third module can be deployed at variable times relative to the completion of the first two modules. This review focuses on the gene regulatory network and chromatin-based kinetic constraints that determine activities of transcription factors TCF1, GATA3, PU.1, Bcl11b, Runx1, and E proteins in the primary establishment of T-cell identity.

RBPJ-dependent Notch signaling initiates the T cell program in a subset of thymus-seeding progenitors

T cell specification and commitment require Notch signaling. Although the requirement for Notch signaling during intrathymic T cell development is known, it is still unclear whether the onset of T cell priming can occur in a prethymic niche and whether RBPJ-dependent Notch signaling has a role during this event. Here, we established an Rbpj-inducible system that allowed temporal and tissue-specific control of the responsiveness to Notch in all hematopoietic cells. Using this system, we found that Notch signaling was required before the early T cell progenitor stage in the thymus. Lymphoid-primed multipotent progenitors in the bone marrow underwent Notch signaling with Rbpj induction, which inhibited development towards the myeloid lineage in thymus-seeding progenitors. Thus, our results indicated that the onset of T cell differentiation occurred in a prethymic setting, and that Notch played an important role during this event.

Dynamic changes in epithelial cell morphology control thymic organ size during atrophy and regeneration

T lymphocytes must be produced throughout life, yet the thymus, where T lymphocytes are made, exhibits accelerated atrophy with age. Even in advanced atrophy, however, the thymus remains plastic, and can be regenerated by appropriate stimuli. Logically, thymic atrophy is thought to reflect senescent cell death, while regeneration requires proliferation of stem or progenitor cells, although evidence is scarce. Here we use conditional reporters to show that accelerated thymic atrophy reflects contraction of complex cell projections unique to cortical epithelial cells, while regeneration requires their regrowth. Both atrophy and regeneration are independent of changes in epithelial cell number, suggesting that the size of the thymus is regulated primarily by rate-limiting morphological changes in cortical stroma, rather than by their cell death or proliferation. Our data also suggest that cortical epithelial morphology is under the control of medullary stromal signals, revealing a previously unrecognized endocrine-paracrine signaling axis in the thymus.

Adhesion between medullary thymic epithelial cells and thymocytes is regulated by miR-181b-5p and miR-30b

In this work, we demonstrate that adhesion between medullary thymic epithelial cells (mTECs) and thymocytes is controlled by miRNAs. Adhesion between mTECs and developing thymocytes is essential for triggering negative selection (NS) of autoreactive thymocytes that occurs in the thymus. Immune recognition is mediated by the MHC / TCR receptor, whereas adhesion molecules hold cell-cell interaction stability. Indeed, these processes must be finely controlled, if it is not, it may lead to aggressive autoimmunity. Conversely, the precise molecular genetic control of mTEC-thymocyte adhesion is largely unclear. Here, we asked whether miRNAs would be controlling this process through the posttranscriptional regulation of mRNAs that encode adhesion molecules. For this, we used small interfering RNA to knockdown (KD) Dicer mRNA in vitro in a murine mTEC line. A functional assay with fresh murine thymocytes co-cultured with mTECs showed that single-positive (SP) CD4 and CD8 thymocyte adhesion was increased after Dicer KD and most adherent subtype was CD8 SP cells. Analysis of broad mTEC transcriptional expression showed that Dicer KD led to the modulation of 114 miRNAs and 422 mRNAs, including those encoding cell adhesion or extracellular matrix proteins, such as Lgals9, Lgals3pb, Tnc and Cd47. Analysis of miRNA-mRNA networks followed by miRNA mimic transfection showed that these mRNAs are under the control of miR-181b-5p and miR-30b*, which may ultimately control mTEC-thymocyte adhesion. The expression of CD80 surface marker in mTECs was increased after Dicer KD following thymocyte adhesion. This indicates the existence of new mechanisms in mTECs that involve the synergistic action of thymocyte adhesion and regulatory miRNAs.

Producing proT cells to promote immunotherapies

T lymphocytes are critical mediators of the adaptive immune system and they can be harnessed as therapeutic agents against pathogens and in cancer immunotherapy. T cells can be isolated and expanded from patients and potentially generated in vitro using clinically relevant systems. An ultimate goal for T-cell immunotherapy is to establish a safe, universal effector cell type capable of transcending allogeneic and histocompatibility barriers. To this end, human pluripotent stem cells offer an advantage in generating a boundless supply of T cells that can be readily genetically engineered. Here, we review emerging T-cell therapeutics, including tumor-infiltrating lymphocytes, chimeric antigen receptors and progenitor T cells (proT cells) as well as feeder cell-free in vitro systems for their generation. Furthermore, we explore their potential for adoption in the clinic and highlight the challenges that must be addressed to increase the therapeutic success of a universal immunotherapy.

Thymic Microenvironment Is Modified by Malnutrition and Leishmania infantum Infection

Detrimental effects of malnutrition on immune responses to pathogens have long been recognized and it is considered a main risk factor for various infectious diseases, including visceral leishmaniasis (VL). Thymus is a target of both malnutrition and infection, but its role in the immune response to Leishmania infantum in malnourished individuals is barely studied. Because we previously observed thymic atrophy and significant reduction in cellularity and chemokine levels in malnourished mice infected with L. infantum, we postulated that the thymic microenvironment is severely compromised in those animals. To test this, we analyzed the microarchitecture of the organ and measured the protein abundance in its interstitial space in malnourished BALB/c mice infected or not with L. infantum. Malnourished-infected animals exhibited a significant reduction of the thymic cortex:medulla ratio and altered abundance of proteins secreted in the thymic interstitial fluid. Eighty-one percent of identified proteins are secreted by exosomes and malnourished-infected mice showed significant decrease in exosomal proteins, suggesting that exosomal carrier system, and therefore intrathymic communication, is dysregulated in those animals. Malnourished-infected mice also exhibited a significant increase in the abundance of proteins involved in lipid metabolism and tricarboxylic acid cycle, suggestive of a non-proliferative microenvironment. Accordingly, flow cytometry analysis revealed decreased proliferation of single positive and double positive T cells in those animals. Together, the reduced cortical area, decreased proliferation, and altered protein abundance suggest a dysfunctional thymic microenvironment where T cell migration, proliferation, and maturation are compromised, contributing for the thymic atrophy observed in malnourished animals. All these alterations could affect the control of the local and systemic infection, resulting in an impaired response to L. infantum infection.

Coexpression of YY1 Is Required to Elaborate the Effector Functions Controlled by PLZF in NKT Cells

The promyelocytic leukemia zinc-finger transcription factor (PLZF) is essential for nearly all of the unique, innate-like functions and characteristics of NKT cells. It is not known, however, if the activity of PLZF is regulated by other factors. In this article, we show that the function of PLZF is completely dependent on the transcription factor Yin Yang 1 (YY1). Mouse NKT cells expressing wild-type levels of PLZF, but deficient for YY1, had developmental defects, lost their characteristic "preformed" mRNA for cytokines, and failed to produce cytokine protein upon primary activation. Immunoprecipitation experiments showed that YY1 and PLZF were coassociated. Taken together, these biochemical and genetic data show that the broadly expressed transcription factor, YY1, is required for the cell-specific "master regulator" functions of PLZF.

E-protein-regulated expression of CXCR4 adheres preselection thymocytes to the thymic cortex

The E-protein transcription factors E2A and HEB regulate thymocyte expression of the chemokine receptor CXCR4 to retain preselection thymocytes in the thymic cortex. TCR-mediated positive selection signals extinguish CXCR4 expression to allow positively selected thymocytes to migrate from the cortex into the thymic medulla.

Preselection thymocytes are normally retained in the thymic cortex, but the mechanisms responsible remain incompletely understood. We now report that deletion of genes encoding the E-protein transcription factors E2A and HEB disorders chemokine receptor expression on developing thymocytes to allow escape of preselection TCR⁻CD8⁺ thymocytes into the periphery. We document that CXCR4 expression normally anchors preselection thymocytes to the thymic cortex via interaction with its ligand CXCL12 on cortical thymic epithelial cells, and that disruption of CXCR4–CXCL12 engagements release preselection thymocytes from the thymic cortex. We further document that CXCR4 expression must be extinguished by TCR-mediated positive selection signals to allow migration of TCR-signaled thymocytes out of the thymic cortex into the medulla. Thus, E-protein transcription factors regulate the ordered expression pattern of chemokine receptors on developing thymocytes, and the interaction of the chemokine receptor CXCR4 with its ligand adheres TCR-unsignaled preselection thymocytes to the thymic cortex.

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.

T cell receptor signaling for γδT cell development

T cells are central to the vertebrate immune system. Two distinct types of T cells, αβT and γδT cells, express different types of T cell antigen receptors (TCRs), αβTCR and γδTCR, respectively, that are composed of different sets of somatically rearranged TCR chains and CD3 subunits. γδT cells have recently attracted considerable attention due to their ability to produce abundant cytokines and versatile roles in host defense, tissue regeneration, inflammation, and autoimmune diseases. Both αβT and γδT cells develop in the thymus. Unlike the development of αβT cells, which depends on αβTCR-mediated positive and negative selection, the development of γδT cells, including the requirement of γδTCR, has been less well understood. αβT cells differentiate into effector cells in the peripheral tissues, whereas γδT cells acquire effector functions during their development in the thymus. In this review, we will discuss the current state of knowledge of the molecular mechanism of TCR signal transduction and its role in the thymic development of γδT cells, particularly highlighting a newly discovered mechanism that controls proinflammatory γδT cell development.

Mechanisms of Action of Hematopoietic Transcription Factor PU.1 in Initiation of T-Cell Development

PU.1 is an ETS-family transcription factor that plays a broad range of roles in hematopoiesis. A direct regulator of myeloid, dendritic-cell, and B cell functional programs, and a well-known antagonist of terminal erythroid cell differentiation, it is also expressed in the earliest stages of T-cell development of each cohort of intrathymic pro-T cells. Its expression in this context appears to give T-cell precursors initial, transient access to myeloid and dendritic cell developmental competence and therefore to represent a source of antagonism or delay of T-cell lineage commitment. However, it has remained uncertain until recently why T-cell development is also intensely dependent upon PU.1. Here, we review recent work that sheds light on the molecular biology of PU.1 action across the genome in pro-T cells and identifies the genes that depend on PU.1 for their correct regulation. This work indicates modes of chromatin engagement, pioneering, and cofactor recruitment (“coregulator theft”) by PU.1 as well as gene network interactions that not only affect specific target genes but also have system-wide regulatory consequences, amplifying the impact of PU.1 beyond its own direct binding targets. The genes directly regulated by PU.1 also suggest a far-reaching transformation of cell biology and signaling potential between the early stages of T-cell development when PU.1 is expressed and when it is silenced. These cell-biological functions can be important to distinguish fetal from adult T-cell development and have the potential to illuminate aspects of thymic function that have so far remained the most mysterious.

Deconstructing Immune Microenvironments of Lymphoid Tissues for Reverse Engineering

The immune microenvironment presents a diverse panel of cues that impacts immune cell migration, organization, differentiation, and the immune response. Uniquely, both the liquid and solid phases of every specific immune niche within the body play an important role in defining cellular functions in immunity at that particular location. The in vivo immune microenvironment consists of biomechanical and biochemical signals including their gradients, surface topography, dimensionality, modes of ligand presentation, and cell-cell interactions, and the ability to recreate these immune biointerfaces in vitro can provide valuable insights into the immune system. This manuscript reviews the critical roles played by different immune cells and surveys the current progress of model systems for reverse engineering of immune microenvironments with a focus on lymphoid tissues.

A new role for Notch in the control of polarity and asymmetric cell division of developing T cells

A fundamental question in biology is how single cells can reliably produce progeny of different cell types. Notch signalling frequently facilitates fate determination. Asymmetric cell division (ACD) often controls segregation of Notch signalling by imposing unequal inheritance of regulators of Notch. Here, we assessed the functional relationship between Notch and ACD in mouse T cell development. To attain immunological specificity, developing T cells must pass through a pivotal stage termed β-selection, which involves Notch signalling and ACD. We assessed functional interactions between Notch1 and ACD during β-selection using direct presentation of Notch ligands, DL1 and DL4, and pharmacological inhibition of Notch signalling. Contrary to prevailing models, we demonstrate that Notch controls the distribution of Notch1 itself and cell fate determinants, α-Adaptin and Numb. Further, Notch and CXCR4 signalling cooperated to drive polarity during division. Thus, Notch signalling directly orchestrates ACD, and Notch1 is differentially inherited by sibling cells.

IL-4 and IL-13 Guide Early Thymic Progenitors To Mature toward Dendritic Cells

Recently we reported that IL-4 and IL-13 signaling in murine early thymic progenitors (ETPs) expressing the heteroreceptor (HR) comprising IL-4 receptor α (IL-4Rα) and IL-13 receptor α 1 (IL-13Rα1) activate STAT6 and inhibit ETP maturation potential toward T cells. In this study, we asked whether IL-4 and IL-13 signaling through the HR mobilizes other STAT molecules to shape ETP fate decision. The findings indicate that HR⁺ ETPs undergoing cytokine signaling display increased STAT1, but not STAT3, phosphorylation in addition to STAT6 activation. In parallel, the ETPs had a STAT1-dependent heightened expression of IRF-8, a transcription factor essential for development of CD8α⁺ dendritic cells (DCs). Interestingly, STAT1 phosphorylation and IRF-8 upregulation, which are independent of STAT6 activation, guided ETP maturation toward myeloid cells with a CD8α⁺ DC phenotype. Furthermore, these CD8α⁺ DCs display a thymic resident phenotype, as they did not express SIRPα, a molecule presumed to be involved in cell migration. These findings suggest that IL-4 and IL-13 cytokine-induced HR signaling provides a double-edged sword that simultaneously blocks T cell lineage potential but advances myeloid maturation that could impact T cell selection and central tolerance.

Real Time In Vivo Tracking of Thymocytes in the Anterior Chamber of the Eye by Laser Scanning Microscopy

The purpose of the method being presented is to show, for the first time, the transplant of newborn thymi into the anterior eye chamber of isogenic adult mice for in vivo longitudinal real-time monitoring of thymocytes´ dynamics within a vascularized thymus segment. Following the transplantation, laser scanning microscopy (LSM) through the cornea allows in vivo noninvasive repeated imaging at cellular resolution level. Importantly, the approach adds to previous intravital T-cell maturation imaging models the possibility for continuous progenitor cell recruitment and mature T-cell egress recordings in the same animal. Additional advantages of the system are the transparency of the grafted area, permitting macroscopic rapid monitoring of the implanted tissue, and the accessibility to the implant allowing for localized in addition to systemic treatments. The main limitation being the volume of the tissue that fits in the reduced space of the eye chamber which demands for lobe trimming. Organ integrity is maximized by dissecting thymus lobes in patterns previously shown to be functional for mature T-cell production. The technique is potentially suited to interrogate a milieu of medically relevant questions related to thymus function that include autoimmunity, immunodeficiency and central tolerance; processes which remain mechanistically poorly defined. The fine dissection of mechanisms guiding thymocyte migration, differentiation and selection should lead to novel therapeutic strategies targeting developing T cells.

Improvement of glucocorticoid-impaired thymus function by dihydromyricetin via up-regulation of PPARγ-associated fatty acid metabolism

T lymphocytes produced by the thymus are essential mediators of immunity. Accelerated thymic atrophy appears in the patients with administration of glucocorticoids (GCs) which are commonly-used drugs to treat autoimmune and infectious diseases, leading to dysregulation of immunity with manifestation of progressive diminution of new T cell production. However, there is no ideal method to overcome such side effects of GCs. In the current study, we proposed a composition of dexamethasone (DEX) and dihydromyricetin (DMY) derived from a medicinal plant, which could protect from DEX-induced thymus damage and simultaneously enhance the anti-inflammatory effect of DEX. In the current study, we found that DEX-damaged thymic cellularity and architecture, reduced thymocyte numbers, induced thymocyte apoptosis and dropped CD4+ and CD8+ double positive T cell numbers in thymus which was effectively improved by co-treatment with DMY. Quantification of signal joint TCR delta excision circles (TRECs) and Vβ TCR spectratyping analysis were employed to determine the thymus function with indicated treatments. The results showed that DEX-impaired thymus output and decreased TCR cell diversity which was ameliorated by co-treatment with DMY. iTRAQ 2D LC-MS/MS was applied to analyze the proteomic profiling of thymus of mice treated with or without indicated agents, followed by informatics analysis to identify the correlated signaling pathway. After validated by Western blotting and Real-time PCR, we found that PPARγ-associated fatty acid metabolism was increased in the thymic tissues of the animals treated with DMY plus DEX than the animals treated with DEX alone. The agonist and antagonist of PPARγ were further employed to verify the role of PPARγ in the present study. Furthermore, DMY demonstrated a synergistic effect with co-administration of DEX on suppressing inflammation in vivo. Collectively, DMY relieved thymus function damaged by DEX via regulation of PPARγ-associated fatty acid metabolism. Our findings may provide a new strategy on protection of thymus from damage caused by GCs by using appropriate adjuvant natural agents through up-regulation of PPARγ-associated fatty acid metabolism.

[The role of IL-22 in T cell reconstitution after thymus damage induced by ionizing radiation]

Objective: To explore the levels of IL-22 in thymus damaged by γ-ray total body irradiation (TBI), and to study the role of IL-22 in T cell reconstitution after thymic injury induced by TBI. Methods: To induce thymic injury, mice were treated by sub-lethal TBI. Levels of intra-thymic and circulatory IL-22 were detected by using ELISA assay. Untreated mice were used as control. After receiving sub-lethal TBI, mice were intraperitoneally injected with PBS or recombinant mouse IL-22, which were marked as TBI+PBS or TBI+IL-22, respectively. Mice were monitored for counts of total thymic cells and circulatory white blood cells. Flow cytometry was applied to analyze percentages of thymic epithelial cells (TEC), thymocyte subsets and circulatory T cells. Real-time PCR assay was applied to analyze the mRNA expression levels of Foxn1, Ccl25, Aire and Dll4 in thymus. Results: ①Sub-lethal TBI treated mice expressed higher levels of intra-thymic and circulatory IL-22, compared with untreated ones (all P<0.05). ②After injection of recombinant IL-22, TBI+IL-22 mice had higher levels of intra-thymic IL-22 than TBI+PBS mice (all P<0.05). ③On day 14 after irradiation, real-time PCR assay showed that TBI+IL-22 mice had higher mRNA levels of Foxn1, Ccl25, Aire and Dll4 in thymus compared with TBI+PBS ones. Meanwhile, the TBI+IL-22 mice had higher counts of total thymic cells[(5.93±3.19)×10(6)/ml vs (1.42±0.46)×10(6)/ml, t=3.128, P=0.033] and circulatory white blood cells[(3.08±0.94)×10(6)/ml vs (1.43±0.30)×10(6)/ml, t=3.730, P=0.015] than those of TBI+PBS mice. Flow cytometry analysis indicated that TBI+IL-22 mice had higher counts of TEC and thymocytes than TBI+PBS mice on day 14 after irradiation (all P<0.05). On days 7 and 14 after irradiation, TBI+IL-22 mice had higher counts of circulatory white blood cells and T cells than TBI+PBS mice (all P<0.05). Conclusion: Sub-lethal TBI induces upregulation of intra-thymic IL-22, and injecting of recombinant IL-22 increases level of IL-22 in thymus. Injecting of recombinant IL-22 improves recovery of TEC and increases numbers of thymocyte subsets and circulatory T cell after thymic injury.

Dynamic regulation of NOTCH1 activation and Notch ligand expression in human thymus development

T-cell development is a complex dynamic process that relies on ordered stromal signals delivered to thymus-seeding progenitors that migrate throughout different thymus microenvironments (TMEs). Particularly, Notch signaling provided by thymic epithelial cells (TECs) is crucial for T-cell fate specification and generation of mature T cells. Four canonical Notch ligands (Dll1, Dll4, Jag1 and Jag2) are expressed in the thymus, but their spatial distribution in functional TMEs is largely unknown, especially in humans, and their impact on Notch1 activation during T-lymphopoiesis remains undefined. Based on immunohistochemistry and quantitative confocal microscopy of fetal, postnatal and adult human and mouse thymus samples, we show that spatial regulation of Notch ligand expression defines discrete Notch signaling niches and dynamic species-specific TMEs. We further show that Notch ligand expression, particularly DLL4, is tightly regulated in cortical TECs during human thymus ontogeny and involution. Also, we provide the first evidence that NOTCH1 activation is induced in vivo in CD34⁺ progenitors and developing thymocytes at particular cortical niches of the human fetal and postnatal thymus. Collectively, our results show that human thymopoiesis involves complex spatiotemporal regulation of Notch ligand expression, which ensures the coordinated delivery of niche-specific NOTCH1 signals required for dynamic T-cell development.

Intrathymic Notch3 and CXCR4 combinatorial interplay facilitates T-cell leukemia propagation

Notch hyperactivation dominates T-cell acute lymphoblastic leukemia development, but the mechanisms underlying “pre-leukemic” cell dissemination are still unclear. Here we describe how deregulated Notch3 signaling enhances CXCR4 cell-surface expression and migratory ability of CD4⁺CD8⁺ thymocytes, possibly contributing to “pre-leukemic” cell propagation, early in disease progression. In transgenic mice overexpressing the constitutively active Notch3 intracellular domain, we detect the progressive increase in circulating blood and bone marrow of CD4⁺CD8⁺ cells, characterized by high and combined surface expression of Notch3 and CXCR4. We report for the first time that transplantation of such CD4⁺CD8⁺ cells reveals their competence in infiltrating spleen and bone marrow of immunocompromised recipient mice. We also show that CXCR4 surface expression is central to the migratory ability of CD4⁺CD8⁺ cells and such an expression is regulated by Notch3 through β-arrestin in human leukemia cells. De novo, we propose that hyperactive Notch3 signaling by boosting CXCR4-dependent migration promotes anomalous egression of CD4⁺CD8⁺ cells from the thymus in early leukemia stages. In fact, in vivo CXCR4 antagonism prevents bone marrow colonization by such CD4⁺CD8⁺ cells in young Notch3 transgenic mice. Therefore, our data suggest that combined therapies precociously counteracting intrathymic Notch3/CXCR4 crosstalk may prevent dissemination of “pre-leukemic” CD4⁺CD8⁺ cells, by a “thymus-autonomous” mechanism.

CCR8 is expressed by post-positive selection CD4-lineage thymocytes but is dispensable for central tolerance induction

Following positive selection, thymocytes migrate into the medulla where they encounter diverse self-antigens that induce central tolerance. Thymocytes expressing T cell receptors (TCRs) with high affinity for self-antigens displayed by medullary antigen presenting cells (APCs) undergo either negative selection or diversion to the regulatory T cell (Treg) lineage, thus ensuring maturation of non-autoreactive T cells. Because many self-antigens are expressed by only a small percentage of medullary thymic epithelial cells, thymocytes must enter the medulla and efficiently scan APCs therein to encounter the full array of self-antigens that induce central tolerance. Chemokine receptors play a critical role in promoting medullary entry and rapid motility of post-positive selection thymocytes. We found that the chemokine receptor CCR8 is expressed by post-positive selection CD4⁺ single positive (SP) thymocytes in mice, while the corresponding chemokine ligands are expressed by medullary APCs, and thus hypothesized that CCR8 would promote thymocyte medullary entry and/or rapid motility to induce negative selection. However, despite a subtle decline in thymocyte medullary accumulation and the presence of autoantibodies in aged CCR8-deficient mice, CCR8 was not required for thymocyte differentiation, rapid motility, or negative selection.

Aire Disruption Influences the Medullary Thymic Epithelial Cell Transcriptome and Interaction With Thymocytes

The function of medullary thymic epithelial cells (mTECs) is associated with thymocyte adhesion, which is crucial for the negative selection of autoreactive thymocytes in the thymus. This process represents the root of central tolerance of self-components and prevents the onset of autoimmune diseases. Since thymic epithelia correspond to an important target of donor T cells during the onset of chronic graft-vs-host-disease, mTEC-thymocyte adhesion may have implications for alloimmunity. The Aire and Fezf2 genes function as transcriptome controllers in mTECs. The central question of this study is whether there is a mutual relationship between mTEC-thymocyte adhesion and the control of the mTEC transcriptome and whether Aire is involved in this process. Here, we show that in vitro mTEC-thymocyte adhesion causes transcriptome changes in mTECs and upregulates the transcriptional expression of Aire and Fezf2, as well as cell adhesion-related genes such as Cd80 or Tcf7, among others. Crispr-Cas9-mediated Aire gene disruption demonstrated that this gene plays a role in the process of mTEC-thymocyte adhesion. Consistent with the nuclear localization signal (NLS) encoded by Aire exon 3, which was targeted, we demonstrate that Aire KO-/- mTECs impair AIRE protein localization in the nucleus. Consequently, the loss of function of Aire reduced the ability of these cells to adhere to thymocytes. Their transcriptomes differed from their wild-type Aire+/+ counterparts, even during thymocyte adhesion. A set of mRNA isoforms that encode proteins involved in cell adhesion were also modulated during this process. This demonstrates that both thymocyte interactions and Aire influence transcriptome profiling of mTEC cells.

Regulation of Hematopoietic Cell Development and Function Through Phosphoinositides

One of the most paramount receptor-induced signal transduction mechanisms in hematopoietic cells is production of the lipid second messenger phosphatidylinositol(3,4,5)trisphosphate (PIP₃) by class I phosphoinositide 3 kinases (PI3K). Defective PIP₃ signaling impairs almost every aspect of hematopoiesis, including T cell development and function. Limiting PIP₃ signaling is particularly important, because excessive PIP₃ function in lymphocytes can transform them and cause blood cancers. Here, we review the key functions of PIP₃ and related phosphoinositides in hematopoietic cells, with a special focus on those mechanisms dampening PIP₃ production, turnover, or function. Recent studies have shown that beyond “canonical” turnover by the PIP₃ phosphatases and tumor suppressors phosphatase and tensin homolog (PTEN) and SH2 domain-containing inositol-5-phosphatase-1 (SHIP-1/2), PIP₃ function in hematopoietic cells can also be dampened through antagonism with the soluble PIP₃ analogs inositol(1,3,4,5)tetrakisphosphate (IP₄) and inositol-heptakisphosphate (IP₇). Other evidence suggests that IP₄ can promote PIP₃ function in thymocytes. Moreover, IP₄ or the kinases producing it limit store-operated Ca²⁺ entry through Orai channels in B cells, T cells, and neutrophils to control cell survival and function. We discuss current models for how soluble inositol phosphates can have such diverse functions and can govern as distinct processes as hematopoietic stem cell homeostasis, neutrophil macrophage and NK cell function, and development and function of B cells and T cells. Finally, we will review the pathological consequences of dysregulated IP₄ activity in immune cells and highlight contributions of impaired inositol phosphate functions in disorders such as Kawasaki disease, common variable immunodeficiency, or blood cancer.

Thymic stromal cells: Roles in atrophy and age-associated dysfunction of the thymus

Atrophy of the thymus, the primary site of T lymphocyte generation, is a hallmark of the aging immune system. Age-associated thymic atrophy results in diminished output of new, naïve T cells, with immune sequelae that include diminished responses to novel pathogenic challenge and vaccines, as well as diminished tumor surveillance. Although a variety of stimuli are known to regulate transient thymic atrophy, mechanisms governing progressive age-associated atrophy have been difficult to resolve. This has been due in part to the fact that one of the primary targets of age-associated thymic atrophy is a relatively rare population, thymic stromal cells. This review focuses on changes in thymic stromal cells during aging and on the contributions of periodic, stochastic, and progressive causes of thymic atrophy.

Cytokines, Transcription Factors, and the Initiation of T-Cell Development

Multipotent blood progenitor cells migrate into the thymus and initiate the T-cell differentiation program. T-cell progenitor cells gradually acquire T-cell characteristics while shedding their multipotentiality for alternative fates. This process is supported by extracellular signaling molecules, including Notch ligands and cytokines, provided by the thymic microenvironment. T-cell development is associated with dynamic change of gene regulatory networks of transcription factors, which interact with these environmental signals. Together with Notch or pre-T-cell-receptor (TCR) signaling, cytokines always control proliferation, survival, and differentiation of early T cells, but little is known regarding their cross talk with transcription factors. However, recent results suggest ways that cytokines expressed in distinct intrathymic niches can specifically modulate key transcription factors. This review discusses how stage-specific roles of cytokines and transcription factors can jointly guide development of early T cells.

Niches for Hematopoietic Stem Cells and Their Progeny

Steady-state hematopoietic stem cells' (HSCs) self-renewal and differentiation toward their mature progeny in the adult bone marrow is tightly regulated by cues from the microenvironment. Recent insights into the cellular and molecular constituents have uncovered a high level of complexity. Here, we review emerging evidence showing how HSCs and their progeny are regulated by an interdependent network of mesenchymal stromal cells, nerve fibers, the vasculature, and also other hematopoietic cells. Understanding the interaction mechanisms in these intricate niches will provide great opportunities for HSC-related therapies and immune modulation.

Small interference ITGA6 gene targeting in the human thymic epithelium differentially regulates the expression of immunological synapse-related genes

The thymus supports differentiation of T cell precursors. This process requires relocation of developing thymocytes throughout multiple microenvironments of the organ, mainly with thymic epithelial cells (TEC), which control intrathymic T cell differentiation influencing the formation and maintenance of the immunological synapse. In addition to the proteins of the major histocompatibility complex (MHC), this structure is supported by several adhesion molecules. During the process of thymopoiesis, we previously showed that laminin-mediated interactions are involved in the entrance of T-cell precursors into the thymus, as well as migration of differentiating thymocytes within the organ. Using small interference RNA strategy, we knocked-down the ITGA6 gene (which encodes the CD49f integrin α-chain) in cultured human TEC, generating a decrease in the expression of the corresponding CD49f subunit, in addition to modulation in several other genes related to cell adhesion and migration. Thymocyte adhesion to TEC was significantly impaired, comprising both immature and mature thymocyte subsets. Moreover, we found a modulation of the MHC, with a decrease in membrane expression of HLA-ABC, in contrast with increase in the expression of HLA-DR. Interestingly, the knockdown of the B2M gene (encoding the β-2 microglobulin of the HLA-ABC complex) increased CD49f expression levels, thus unraveling the existence of a cross-talk event in the reciprocal control of CD49f and HLA-ABC. Our data suggest that the expression levels of CD49f may be relevant in the general control of MHC expression by TEC and consequently the corresponding synapse with developing thymocytes mediated by the T-cell receptor.

Making Thymus Visible: Understanding T-Cell Development from a New Perspective

T-cell development is coupled with a highly ordered migratory pattern. Lymphoid progenitors must follow a precise journey; starting from the hematopoietic tissue, they move toward the thymus and then migrate into and out of distinct thymic microenvironments, where they receive signals and cues required for their differentiation into naïve T-cells. Knowing where, when, and how these cells make directional “decisions” is key to understanding T-cell development. Such insights can be gained by directly observing developing T-cells within their environment under various conditions and following specific experimental manipulations. In the last decade, several model systems have been developed to address temporal and spatial aspects of T-cell development using imaging approaches. In this perspective article, we discuss the advantages and limitations of these systems and highlight a particularly powerful in vivo model that has been recently established. This model system enables the migratory behavior of all thymocytes to be studied simultaneously in a noninvasive and quantitative manner, making it possible to perform systems-level studies that reveal fundamental principles governing T-cell dynamics during development and in disease.

Differential Expression of PU.1 and Key T Lineage Transcription Factors Distinguishes Fetal and Adult T Cell Development

The PU.1 transcription factor plays a critical role in the regulation of T cell development, so a report that it is dispensable for fetal thymopoiesis is puzzling. To understand this paradox, we examined the requirement for PU.1, encoded by Spi1, during fetal, neonatal, and adult thymopoiesis in a PU.1 hypomorphic mouse generated by deletion of the Spi1 14-kb upstream regulatory element and by analysis of patterns of gene expression in fetal and adult T cell progenitors. Our data demonstrate that the initiation of thymopoiesis during early gestation is less dependent on PU.1 compared with T cell differentiation in adults and that fetal T cell progenitors express lower levels of Spi1 compared with their adult counterparts. We also show that expression of the core network of T lineage transcription factors regulated by PU.1 differs in fetal and adult T cell progenitors. In particular, PU.1-regulated genes that promote T cell differentiation are differentially expressed in fetal versus adult early T lineage progenitors. These results indicate that the transcriptional differences between the fetal and adult T cell developmental programs are driven in part by differential levels of PU.1 expression and that this likely underlies the differences in the properties of fetal and adult T cell progenitors.

Adenosine in the Thymus

Adenosine is an ancient extracellular signaling molecule that regulates various biological functions via activating four G-protein-coupled receptors, A₁, A_2A, A_2B, and A₃ adenosine receptors. As such, several studies have highlighted a role for adenosine signaling in affecting the T cell development in the thymus. Recent studies indicate that adenosine is produced in the context of apoptotic thymocyte clearance. This review critically discusses the involvement of adenosine and its receptors in the complex interplay that exists between the developing thymocytes and the thymic macrophages which engulf the apoptotic cells. This crosstalk contributes to the effective and immunologically silent removal of apoptotic thymocytes, as well as affects the TCR-driven T-cell selection processes.

Cellularity of Thymic Epithelial Cells in the Postnatal Mouse

The molecular and cellular biology of thymic epithelial cells (TECs) often relies on the analysis of TECs isolated in enzymatically digested single-cell suspensions derived from mouse thymus. Many independent studies have reported that the estimated cellularity of total TECs isolated from one adult mouse is on the order of up to 10⁵ However, these numbers appear extremely small given that the cellularity of total thymocytes exceeds 10⁸ and that TECs play multiple roles in thymocyte development and repertoire formation. In the present study, we aimed to measure the numbers of β5t-expressing cortical TECs and Aire-expressing medullary TECs in postnatal mouse thymus in situ without enzymatic digestion. The numbers of these TECs were manually counted in individual thymic sections and were three-dimensionally summed throughout the entire thymic lobes. The results show that the cellularity of total TECs in one 5-wk-old female mouse exceeds 10⁶, containing ∼9 × 10⁵ β5t⁺ cortical TECs and ∼1.1 × 10⁶ Aire⁺ medullary TECs. These results suggest that the use of conventional enzymatic digestion methods for the isolation of TECs may have resulted in the underestimation of the cellularity, and possibly the biology, of TECs.

microRNA-449a modulates medullary thymic epithelial cell differentiation

Medullary thymic epithelial cells (mTECs) ectopically express a diversity of peripheral tissue-restricted antigens (PTAs) and provide unique cues for the expansion, maturation and selection of a repertoire of functionally diverse T lymphocytes. Genetic deletion of all mature microRNAs in thymic epithelial cells (TECs) results in premature thymic involution, progressive disorganisation of the thymic epithelium, and alteration in thymic T cell lineage commitment, consequently eliciting autoimmune disorders. In the present study, we identified that microRNA-449a (miR-449a), a member of miR-449 cluster, regulated mTEC differentiation. Expression of miR-449a was induced by RANK ligand in mouse fetal thymus. In in vitro studies, overexpression of miR-449a induced thymic epithelial progenitor cells (TEPCs) differentiation into mature mTECs. Despite abundant expression of miR-449a in developing thymus, miR-449a-mutant mice exhibited normal thymic development. This might be partially due to in miR-449a-mutant thymus the up-regulation of miR-34a which shared similar seed sequence with miR-449a. However, thymic expression of miR-449/34 sponge which was able to neutralize the function of miR-449/34 family members significantly reduced the number of mature Ly51^-MHCII^hi mTECs. Taken together, our data suggested that miR-449a modulated mTEC differentiation, and members of miR-34 cluster functioned redundantly to rescue miR-449a deficiency in thymus development.

Sublethal Total Body Irradiation Causes Long-Term Deficits in Thymus Function by Reducing Lymphoid Progenitors

Total body irradiation (TBI) damages hematopoietic cells in the bone marrow and thymus; however, the long-term effects of irradiation with aging remain unclear. In this study, we found that the impact of radiation on thymopoiesis in mice varied by sex and dose but, overall, thymopoiesis remained suppressed for ≥12 mo after a single exposure. Male and female mice showed a long-term dose-dependent reduction in thymic cKit⁺ lymphoid progenitors that was maintained throughout life. Damage to hematopoietic stem cells (HSCs) in the bone marrow was dose dependent, with as little as 0.5 Gy causing a significant long-term reduction. In addition, the potential for T lineage commitment was radiation sensitive with aging. Overall, the impact of irradiation on the hematopoietic lineage was more severe in females. In contrast, the rate of decline in thymic epithelial cell numbers with age was radiation-sensitive only in males, and other characteristics including Ccl25 transcription were unaffected. Taken together, these data suggest that long-term suppression of thymopoiesis after sublethal irradiation was primarily due to fewer progenitors in the BM combined with reduced potential for T lineage commitment. A single irradiation dose also caused synchronization of thymopoiesis, with a periodic thymocyte differentiation profile persisting for at least 12 mo postirradiation. This study suggests that the number and capability of HSCs for T cell production can be dramatically and permanently damaged after a single relatively low TBI dose, accelerating aging-associated thymic involution. Our findings may impact evaluation and therapeutic intervention of human TBI events.