Intrathymic T cell development and selection proceeds normally in the absence of glucocorticoid receptor signaling

Thymus involution sets the clock of the aging T-cell landscape: Implications for declined immunity and tissue repair

Aging is generally characterized as a gradual increase in tissue damage, which is associated with senescence and chronic systemic inflammation and is evident in a variety of age-related diseases. The extent to which such tissue damage is a result of a gradual decline in immune regulation, which consequently compromises the capacity of the body to repair damages, has not been fully explored. Whereas CD4 T lymphocytes play a critical role in the orchestration of immunity, thymus involution initiates gradual changes in the CD4 T-cell landscape, which may significantly compromise tissue repair. In this review, we describe the lifespan accumulation of specific dysregulated CD4 T-cell subsets and their coevolution with systemic inflammation in the process of declined immunity and tissue repair capacity with age. Then, we discuss the process of thymus involution-which appears to be most pronounced around puberty-as a possible driver of the aging T-cell landscape. Finally, we identify individualized T cell-based early diagnostic biomarkers and therapeutic strategies for age-related diseases.

Glucocorticoids in T cell development, differentiation and function

Glucocorticoids (GCs) are small lipid hormones produced by the adrenals that maintain organismal homeostasis. Circadian and stress-induced changes in systemic GC levels regulate metabolism, cardiovascular and neural function, reproduction and immune activity. Our understanding of GC effects on immunity comes largely from administration of exogenous GCs to treat immune or inflammatory disorders. However, it is increasingly clear that endogenous GCs both promote and suppress T cell immunity. Examples include selecting an appropriate repertoire of T cell receptor (TCR) self-affinities in the thymus, regulating T cell trafficking between anatomical compartments, suppressing type 1 T helper (T_H1) cell responses while permitting T_H2 cell and, especially, IL-17-producing T helper cell responses, and promoting memory T cell differentiation and maintenance. Furthermore, in addition to functioning at a distance, extra-adrenal (local) production allows GCs to act as paracrine signals, specifically targeting activated T cells in various contexts in the thymus, mucosa and tumours. These pleiotropic effects on different T cell populations during development and immune responses provide a nuanced understanding of how GCs shape immunity.

Glucocorticoid Receptor-Deficient Foxp3+ Regulatory T Cells Fail to Control Experimental Inflammatory Bowel Disease

Activation of the immune system increases systemic adrenal-derived glucocorticoid (GC) levels which downregulate the immune response as part of a negative feedback loop. While CD4⁺ T cells are essential target cells affected by GC, it is not known whether these hormones exert their major effects on CD4⁺ helper T cells, CD4⁺Foxp3⁺ regulatory T cells (Treg cells), or both. Here, we generated mice with a specific deletion of the glucocorticoid receptor (GR) in Foxp3⁺ Treg cells. Remarkably, while basal Treg cell characteristics and in vitro suppression capacity were unchanged, Treg cells lacking the GR did not prevent the induction of inflammatory bowel disease in an in vivo mouse model. Under inflammatory conditions, GR-deficient Treg cells acquired Th1-like characteristics and expressed IFN-gamma, but not IL-17, and failed to inhibit pro-inflammatory CD4⁺ T cell expansion in situ. These findings reveal that the GR is critical for Foxp3⁺ Treg cell function and suggest that endogenous GC prevent Treg cell plasticity toward a Th1-like Treg cell phenotype in experimental colitis. When equally active in humans, a rationale is provided to develop GC-mimicking therapeutic strategies which specifically target Foxp3⁺ Treg cells for the treatment of inflammatory bowel disease.

Macrophages engulf apoptotic and primary necrotic thymocytes through similar phosphatidylserine-dependent mechanisms

One of the major roles of professional phagocytes is the removal of dead cells in the body. We know less about the clearance of necrotic cells than apoptotic cell phagocytosis, despite the fact that both types of dead cells need to be cleared together and necrotic cells appear often in pathological settings. In the present study, we examined phagocytosis of heat‐ or H₂O₂‐killed necrotic and apoptotic thymocytes by mouse bone marrow‐derived macrophages (BMDMs) in vitro and found that the two cell types are engulfed at equal efficiency and compete with each other when added together to BMDMs. Phagocytosis of both apoptotic and necrotic thymocytes was decreased by (a) blocking phosphatidylserine on the surface of dying cells; (b) inhibition of Mer tyrosine kinase, Tim‐4, integrin β3 receptor signaling, or Ras‐related C3 botulinum toxin substrate 1 activity; or (c) using BMDMs deficient for transglutaminase 2. Stimulation of liver X, retinoid X, retinoic acid or glucocorticoid nuclear receptors in BMDMs enhanced not only apoptotic, but also necrotic cell uptake. Electron microscopic analysis of the engulfment process revealed that the morphology of phagosomes and the phagocytic cup formed during the uptake of dying thymocytes is similar for apoptotic and necrotic cells. Our data indicate that apoptotic and necrotic cells are cleared via the same mechanisms, and removal of necrotic cells in vivo can be facilitated by molecules known to enhance the uptake of apoptotic cells.

Defining the role of glucocorticoids in inflammation

An established body of knowledge and clinical practice has argued in favor of the use of glucocorticoids in various chronic inflammatory and autoimmune diseases. However, the very well-known adverse effects associated with their treatment hampers continuation of therapy with glucocorticoids. Analyses of the molecular mechanisms underlying the actions of glucocorticoids have led to the discovery of several mediators that add complexity and diversity to the puzzling world of these hormones and anti-inflammatory drugs. Such mediators hold great promise as alternative pharmacologic tools to be used as anti-inflammatory drugs with the same properties as glucocorticoids, but avoiding their metabolic side effects. This review summarizes findings about the molecular targets and mediators of glucocorticoid function.

Glucocorticoids Drive Diurnal Oscillations in T Cell Distribution and Responses by Inducing Interleukin-7 Receptor and CXCR4

Glucocorticoids are steroid hormones with strong anti-inflammatory and immunosuppressive effects that are produced in a diurnal fashion. Although glucocorticoids have the potential to induce interleukin-7 receptor (IL-7R) expression in T cells, whether they control T cell homeostasis and responses at physiological concentrations remains unclear. We found that glucocorticoid receptor signaling induces IL-7R expression in mouse T cells by binding to an enhancer of the IL-7Rα locus, with a peak at midnight and a trough at midday. This diurnal induction of IL-7R supported the survival of T cells and their redistribution between lymph nodes, spleen, and blood by controlling expression of the chemokine receptor CXCR4. In mice, T cell accumulation in the spleen at night enhanced immune responses against soluble antigens and systemic bacterial infection. Our results reveal the immunoenhancing role of glucocorticoids in adaptive immunity and provide insight into how immune function is regulated by the diurnal rhythm.

Absence of PTHrP nuclear localization and C-terminus sequences leads to abnormal development of T cells

Parathyroid hormone-related protein (PTHrP), a ubiquitously expressed protein, is composed of four functional domains including N-terminus, mid region, nuclear localization signal (NLS) and C-terminus. Under the direction of NLS, PTHrP can enter cell nucleus from cytoplasm and stimulate mitogenesis. Although PTHrP is considered to have important developmental roles, the role of PTHrP NLS and C-terminus in developmental process remains unknown, especially in T-cell development. Here, we used a knock-in mouse model, which expresses a truncated form of PTHrP missing the NLS (87-107) and C-terminus (108-139) of the protein, to examine the role of PTHrP NLS and C-terminus in T-cell development. Our results showed that the truncated PTHrP (1-84) led to abnormal subpopulations, impaired proliferation and increased apoptosis in the thymus, indicating that PTHrP is involved in the development of T cells, and the NLS and C-terminus part is necessary for the normal role of PTHrP in T-cell development.

Expression of 11beta-hydroxysteroid-dehydrogenase type 2 in human thymus

11beta-hydroxysteroid-dehydrogenase type 2 (11β-HSD2) is a high affinity dehydrogenase which rapidly inactivates physiologically-active glucocorticoids to protect key tissues. 11β-HSD2 expression has been described in peripheral cells of the innate and the adaptive immune system as well as in murine thymus. In absence of knowledge of 11β-HSD2 expression in human thymus, the study aimed to localize 11β-HSD2 in human thymic tissue. Thymic tissue was taken of six healthy, non-immunologically impaired male infants below 12months of age with congenital heart defects who had to undergo correction surgery. 11β-HSD2 protein expression was analyzed by immunohistochemistry and Western blot. Kidney tissue, peripheral blood mononuclear cells (PBMCs) and human umbilical vein endothelial cells (HUVEC) were taken as positive controls. Significant expression of 11β-HSD2 protein was found at single cell level in thymus parenchyma, at perivascular sites of capillaries and small vessels penetrating the thymus lobuli and within Hassall's bodies. The present study demonstrates that 11β-HSD2 is expressed in human thymus with predominant perivascular expression and also within Hassall's bodies. To our knowledge, this is the first report confirming 11β-HSD2 expression at the protein level in human thymic tissue underlining a potential role of this enzyme in regulating glucocorticoid function at the thymic level.

Hormonal control of T-cell development in health and disease

The physiology of the thymus, the primary lymphoid organ in which T cells are generated, is controlled by hormones. Data from animal models indicate that several peptide and nonpeptide hormones act pleiotropically within the thymus to modulate the proliferation, differentiation, migration and death by apoptosis of developing thymocytes. For example, growth hormone and prolactin can enhance thymocyte proliferation and migration, whereas glucocorticoids lead to the apoptosis of these developing cells. The thymus undergoes progressive age-dependent atrophy with a loss of cells being generated and exported, therefore, hormone-based therapies are being developed as an alternative strategy to rejuvenate the organ, as well as to augment thymocyte proliferation and the export of mature T cells to peripheral lymphoid organs. Some hormones (such as growth hormone and progonadoliberin-1) are also being used as therapeutic agents to treat immunodeficiency disorders associated with thymic atrophy, such as HIV infection. In this Review, we discuss the accumulating data that shows the thymus gland is under complex and multifaceted hormonal control that affects the process of T-cell development in health and disease.

Local glucocorticoid production in the thymus

Besides generating immunocompetent T lymphocytes, the thymus is an established site of de novo extra-adrenal glucocorticoid (GC) production. Among the compartments of the thymus, both stromal thymic epithelial cells (TECs) and thymocytes secrete biologically active GCs. Locally produced GCs secreted by the various thymic cellular compartments have been suggested to have different impact on thymic homeostasis. TEC-derived GCs may regulate thymocyte differentiation whereas thymocyte-derived GCs might regulate age-dependent involution. However the full biological significance of thymic-derived GCs is still not fully understood. In this review, we summarize and describe recent advances in the understanding of local GC production in the thymus and immunoregulatory steroid production by peripheral T cells and highlight the possible role of local GCs for thymus function.

Proteasome inhibition with bortezomib depletes plasma cells and specific autoantibody production in primary thymic cell cultures from early-onset myasthenia gravis patients

Bortezomib is a potent inhibitor of proteasomes currently used to eliminate malignant plasma cells in multiple myeloma patients. It is also effective in depleting both alloreactive plasma cells in acute Ab-mediated transplant rejection and their autoreactive counterparts in animal models of lupus and myasthenia gravis (MG). In this study, we demonstrate that bortezomib at 10 nM or higher concentrations killed long-lived plasma cells in cultured thymus cells from nine early-onset MG patients and consistently halted their spontaneous production not only of autoantibodies against the acetylcholine receptor but also of total IgG. Surprisingly, lenalidomide and dexamethasone had little effect on plasma cells. After bortezomib treatment, they showed ultrastructural changes characteristic of endoplasmic reticulum stress after 8 h and were no longer detectable at 24 h. Bortezomib therefore appears promising for treating MG and possibly other Ab-mediated autoimmune or allergic disorders, especially when given in short courses at modest doses before the standard immunosuppressive drugs have taken effect.

Extra-adrenal glucocorticoid synthesis: immune regulation and aspects on local organ homeostasis

Systemic glucocorticoids (GCs) mainly originate from de novo synthesis in the adrenal cortex under the control of the hypothalamus-pituitary-adrenal (HPA)-axis. However, research during the last 1-2 decades has revealed that additional organs express the necessary enzymes and have the capacity for de novo synthesis of biologically active GCs. This includes the thymus, intestine, skin and the brain. Recent research has also revealed that locally synthesized GCs most likely act in a paracrine or autocrine manner and have significant physiological roles in local homeostasis, cell development and immune cell activation. In this review, we summarize the nature, regulation and known physiological roles of extra-adrenal GC synthesis. We specifically focus on the thymus in which GC production (by both developing thymocytes and epithelial cells) has a role in the maintenance of proper immunological function.

Nfil3 is a glucocorticoid-regulated gene required for glucocorticoid-induced apoptosis in male murine T cells

Glucocorticoids (GCs) have essential roles in the regulation of development, integrated metabolism, and immune and neurological responses, and act primarily via the glucocorticoid receptor (GR). In most cells, GC treatment results in down-regulation of GR mRNA and protein levels via negative feedback mechanisms. However, in GC-treated thymocytes, GR protein levels are maintained at a high level, increasing sensitivity of thymocytes to GCs, resulting in apoptosis termed glucocorticoid-induced cell death (GICD). CD4(+)CD8(+) double-positive thymocytes and thymic natural killer T cells in particular are highly sensitive to GICD. Although GICD is exploited via the use of synthetic GC analogues in the treatment of hematopoietic malignancies, the intracellular molecular pathway of GICD is not well understood. To explore GICD in thymocytes, the authors performed whole genome expression microarray analysis in mouse GR exon 2 null vs wild-type thymus RNA 3 hours after dexamethasone treatment. Identified and validated direct GR targets included P21 and Bim, in addition to an important transcriptional regulator Nfil3, which previously has been associated with GICD and is essential for natural killer cell development in vivo. Immunostaining of NFIL3 in whole thymus localized NFIL3 primarily to the medullary region, and double labeling colocalized NFIL3 to apoptotic cells. In silico analysis revealed a putative GC response element 5 kb upstream of the Nfil3 promoter that is strongly conserved in the rat genome and was confirmed to bind GR by chromatin immunoprecipitation. The knockdown of Nfil3 mRNA levels to 20% of normal using specific small interfering RNAs abrogated GICD, indicating that NFIL3 is required for normal GICD in CTLL-2 T cells.

CD8 lineage-specific regulation of interleukin-7 receptor expression by the transcriptional repressor Gfi1

Interleukin-7 receptor α (IL-7Rα) is essential for T cell survival and differentiation. Glucocorticoids are potent enhancers of IL-7Rα expression with diverse roles in T cell biology. Here we identify the transcriptional repressor, growth factor independent-1 (Gfi1), as a novel intermediary in glucocorticoid-induced IL-7Rα up-regulation. We found Gfi1 to be a major inhibitory target of dexamethasone by microarray expression profiling of 3B4.15 T-hybridoma cells. Concordantly, retroviral transduction of Gfi1 significantly blunted IL-7Rα up-regulation by dexamethasone. To further assess the role of Gfi1 in vivo, we generated bacterial artificial chromosome (BAC) transgenic mice, in which a modified Il7r locus expresses GFP to report Il7r gene transcription. By introducing this BAC reporter transgene into either Gfi1-deficient or Gfi1-transgenic mice, we document in vivo that IL-7Rα transcription is up-regulated in the absence of Gfi1 and down-regulated when Gfi1 is overexpressed. Strikingly, the in vivo regulatory role of Gfi1 was specific for CD8(+), and not CD4(+) T cells or immature thymocytes. These results identify Gfi1 as a specific transcriptional repressor of the Il7r gene in CD8 T lymphocytes in vivo.

Molecular control over thymic involution: from cytokines and microRNA to aging and adipose tissue

The thymus is the primary organ for T-cell differentiation and maturation. Unlike other major organs, the thymus is highly dynamic, capable of undergoing multiple rounds of almost complete atrophy followed by rapid restoration. The process of thymic atrophy, or involution, results in decreased thymopoiesis and emigration of naïve T cells to the periphery. Multiple processes can trigger transient thymic involution, including bacterial and viral infection(s), aging, pregnancy and stress. Intense investigations into the mechanisms that underlie thymic involution have revealed diverse cellular and molecular mediators, with elaborate control mechanisms. This review outlines the disparate pathways through which involution can be mediated, from the transient infection-mediated pathway, tightly controlled by microRNA, to the chronic changes that occur through aging.

Extra-adrenal glucocorticoids and mineralocorticoids: evidence for local synthesis, regulation, and function

Glucocorticoids and mineralocorticoids are steroid hormones classically thought to be secreted exclusively by the adrenal glands. However, recent evidence has shown that corticosteroids can also be locally synthesized in various other tissues, including primary lymphoid organs, intestine, skin, brain, and possibly heart. Evidence for local synthesis includes detection of steroidogenic enzymes and high local corticosteroid levels, even after adrenalectomy. Local synthesis creates high corticosteroid concentrations in extra-adrenal organs, sometimes much higher than circulating concentrations. Interestingly, local corticosteroid synthesis can be regulated via locally expressed mediators of the hypothalamic-pituitary-adrenal (HPA) axis or renin-angiotensin system (RAS). In some tissues (e.g., skin), these local control pathways might form miniature analogs of the pathways that regulate adrenal corticosteroid production. Locally synthesized glucocorticoids regulate activation of immune cells, while locally synthesized mineralocorticoids regulate blood volume and pressure. The physiological importance of extra-adrenal glucocorticoids and mineralocorticoids has been shown, because inhibition of local synthesis has major effects even in adrenal-intact subjects. In sum, while adrenal secretion of glucocorticoids and mineralocorticoids into the blood coordinates multiple organ systems, local synthesis of corticosteroids results in high spatial specificity of steroid action. Taken together, studies of these five major organ systems challenge the conventional understanding of corticosteroid biosynthesis and function.

Control of early stages in invariant natural killer T-cell development

Natural killer T (NKT) cells develop in the thymus from the same precursors as conventional CD4(+) and CD8(+) αβ T cells, CD4(+)  CD8(+) double-positive cells. In contrast to conventional αβT cells, which are selected by MHC-peptide complexes presented by thymic epithelial cells, invariant NKT cells are selected by lipid antigens presented by the non-polymorphic, MHC I-like molecule CD1d, present on the surface of other double-positive thymocytes, and require additional signals from the signalling lymphocytic-activation molecule (SLAM) family of receptors. In this review, we provide a discussion of recent findings that have modified our understanding of the NKT cell developmental programme, with an emphasis on events that affect the early stages of this process. This includes factors that control double-positive thymocyte lifespan, and therefore the ability to generate the canonical Vα rearrangements that characterize this lineage, as well as the signal transduction pathways engaged downstream of the T-cell receptor and SLAM molecules.

The anti-inflammatory and immunosuppressive effects of glucocorticoids, recent developments and mechanistic insights

Since the discovery of glucocorticoids in the 1940s and the recognition of their anti-inflammatory effects, they have been amongst the most widely used and effective treatments to control inflammatory and autoimmune diseases. However, their clinical efficacy is compromised by the metabolic effects of long-term treatment, which include osteoporosis, hypertension, dyslipidaemia and insulin resistance/type 2 diabetes mellitus. In recent years, a great deal of effort has been invested in identifying compounds that separate the beneficial anti-inflammatory effects from the adverse metabolic effects of glucocorticoids, with limited effect. It is clear that for these efforts to be effective, a greater understanding is required of the mechanisms by which glucocorticoids exert their anti-inflammatory and immunosuppressive actions. Recent research is shedding new light on some of these mechanisms and has produced some surprising new findings. Some of these recent developments are reviewed here.

The role of the glucocorticoid receptor in inflammation and immunity

Glucocorticoids are potent immunosuppressive agents with complex actions on immune cells evoking the following effects: inducing apoptosis, changing differentiation fate, inhibition of cytokine release, inhibition of migration and other features. Distinct molecular mechanisms of the glucocorticoid receptor (GR) contribute to different anti-inflammatory effects. Recently inflammatory models have been investigated using conditional knockout and function selective mice shedding light on critical cell types and molecular mechanisms of endogenous and therapeutic GC actions. Here we review the multiple effects of GCs on major immune cells, dendritic cells, myeloid cells and B- and T-lymphocytes and give a summary of studies using conditional GR knockout mice.

Nitric oxide cooperates with glucocorticoids in thymic epithelial cell-mediated apoptosis of double positive thymocytes

T cell development in the thymus is controlled by thymic epithelial cells (TE). While it is accepted that TE interact with maturing T cells, the mechanisms by which they trigger 'death by neglect' of double-positive (DP) thymocytes are poorly understood. We and others have demonstrated a role for TE-derived glucocorticoids (GCs) in this process. We have studied TE-induced apoptosis using an in vitro system based on co-culturing a thymic epithelial cell line (TEC) with DP thymic lymphoma cells or thymocytes (DP thymic cells). Here, we demonstrate that nitric oxide (NO*) is also involved in this death process. The inducible nitric oxide synthase (iNOS) inhibitors N(G)-methyl-L-arginine and 1,4-PBIT attenuated TEC-induced apoptosis of DP thymic cells. Co-cultivation of TEC with DP thymic cells increased the expression of iNOS in TEC. A concomitant increase in NO* was detected by staining with DAF-FM diacetate. Moreover, the iNOS-regulating cytokines IL-1alpha, IL-1beta and IFNgamma were up-regulated upon interaction of TEC with DP thymic cells. Neutralizing IL-1R or IFNgamma reduced TEC-induced apoptosis of DP thymic cells. Cardinally, NO* synergizes with GCs in eliciting apoptosis of DP thymic cells. Our data indicate that a cross-talk between DP thymic cells and TEC is required for proper induction of iNOS-up-regulating cytokines with a subsequent increase in iNOS expression and NO* production in TEC. NO*, in turn, cooperates with GCs in promoting death by neglect. We suggest that NO* together with GCs fine-tune the T cell selection process.

Dysregulation of T-cell development in adrenal glucocorticoid-deprived rats

A number of different experimental approaches have been used to elucidate the impact of basal levels of adrenal gland-derived glucocorticoids (GCs) on T cell development, and thereby T cell-mediated immune responses. However, the relevance of the adrenal GCs to T cell development is still far from clear. This study was undertaken to explore the relevance of basal levels of GCs to T cell differentiation/maturation. Eight days post-adrenalectomy in adult male rats the thymocyte yield, apoptotic and proliferative rate and the relationship amongst major thymocyte subsets, as defined by TCRalphabeta/CD4/CD8 expression, were examined using flow cytometry. Adrenal GC deprivation decreased thymocyte apoptosis and altered the kinetics of T cell differentiation/maturation. In the adrenalectomized rats there was increased thymic hypercellularity and an over-representation of the CD4+CD8+ double positive (DP) TCRalphabeta(low) cells entering selection, as well as increased numbers of their DP TCRalphabeta(-) immediate precursors. These changes were accompanied with under-representation of the postselected DP TCRalphabeta(high) and the most mature CD4-CD8+ and, particularly, CD4+CD8- single positive (SP) TCRalphabeta(high) cells. This data suggests that withdrawal of adrenal GCs produces alterations in the thymocyte selection processes, possibly affecting the diversity of functional T cell repertoire and generation of potentially self-reactive cells as indicated by the reduced proportion and number of CD4-CD8- double negative TCRalphabeta(high) cells. In addition, it indicates that GCs influence the post-selection maturation of thymocytes and plays a regulatory role in controlling the ratio of mature CD4+CD8-/CD4-CD8+ SP TCRalphabeta(high) cells.

Maternal and early life stress effects on immune function: relevance to immunotoxicology

Stress is triggered by a variety of unexpected environmental stimuli, such as aggressive behavior, fear, forced physical activity, sudden environmental changes, social isolation or pathological conditions. Stressful experiences during very early life (particularly, maternal stress during fetal ontogeny) can permanently alter the responsiveness of the nervous system, an effect called programming or imprinting. Programming affects the hypothalamic-pituitary-adrenocortical (HPA) axis, brain neurotransmitter systems, sympathetic nervous system (SNS), and the cognitive abilities of the offspring, which can alter neural regulation of immune function. Prenatal or early life stress may contribute to the maladaptive immune responses to stress that occur later in life. This review focuses on the effect of maternal and early life stress on immune function in the offspring across life span. It highlights potential mechanisms by which prenatal stress impacts immune functions over life span. The literature discussed in this review suggests that psychosocial stress during pre- and early postnatal life may increase the vulnerability of infants to the effects of immunotoxicants or immune-mediated diseases, with long-term consequences. Neural-immune interactions may provide an indirect route through which immunotoxicants affect the developing immune system. A developmental approach to understanding how immunotoxicants interact with maternal and early life stress-induced changes in immunity is needed, because as the body changes physiologically across life span so do the effects of stress and immunotoxicants. In early and late life, the immune system is more vulnerable to the effects of stress. Stress can mimic the effects of aging and exacerbate age-related changes in immune function. This is important because immune dysregulation in the elderly is more frequently and seriously associated with clinical impairment and death. Aging, exposure to teratogens, and psychological stress interact to increase vulnerability and put the elderly at the greatest risk for disease.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

Regulation of glucocorticoid sensitivity in thymocytes from burn-injured mice

Glucocorticoids (GC) are important steroid hormones that regulate metabolism, development, and the immune system. GC are produced continuously, and maximal levels are reached following stress-related stimuli. Previous studies have demonstrated that increased GC production following thermal injury was responsible for thymic involution. Although GC are mainly synthesized by the adrenal glands, there is increasing evidence that GC may also be produced in nonadrenal tissues. The thymus was reported to express steroidogenic enzymes and to release GC. 11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD1) is predominantly a reductase in cells and is essential for the local reactivation of GC. Here, we report that increased GC-induced apoptosis in thymocytes from burn-injured mice is related to increased glucocorticoid receptor (GR) expression and 11beta-HSD1 expression in thymocytes at day 1 postburn injury. In vitro, thymocytes were able to convert 11-dehydrocorticosterone (DHC) to corticosterone (CORT), which induced their apoptosis, and this was pharmacologically inhibited by 18beta-glycyrrhetinic acid, a specific 11beta-HSD inhibitor. Moreover, 11beta-HSD1 expression was confirmed in the 267S3 thymoma-derived cell line, and its activity was responsible for greater sensitivity of these cells to CORT-induced apoptosis. Finally, proinflammatory cytokines [tumor necrosis factor-alpha, interleukin (IL)-1beta, and IL-6] increased thymocyte sensitivity to DHC-induced apoptosis through a mechanism involving 11beta-HSD1. Overall, we have shown that burn injury induced 11beta-HSD1 expression in thymocytes, which led to a greater sensitivity of these cells to CORT-induced apoptosis. Increased expression of 11beta-HSD1 and GR may play a role in intrathymic T cell development and can be major determinants of GC sensitivity after a trauma.

Glucocorticoid action and the development of selective glucocorticoid receptor ligands

Glucocorticoids are important endocrine regulators of a wide range of physiological systems ranging from respiratory development, immune function to responses to stress. Glucocorticoids in cells activate the cytoplasmic glucocorticoid receptor (GR) that dimerizes, translocates to the nucleus and functions as a ligand-dependent transcriptional regulator. Synthetic glucocorticoids such as dexamethasone and prednisolone have for decades been the cornerstone for the clinical treatment of inflammatory diseases, such as rheumatoid arthritis and asthma, and in some lymphoid cancers, yet its prolonged use has undesirable side effects such as obesity, diabetes, immune suppression and osteoporosis. Detailed knowledge on the mechanism of GR action has led to the development of novel selective glucocorticoid receptor modulators (SGRMs) that show promise of being efficacious for specific treatments of disease but with fewer side effects. SGRMs promote specific recruitment of transcriptional co-regulators that elicit specific gene responses and show promise of greater efficacy and specificity in treatment of inflammatory diseases and type-2 diabetes.

Enhanced glucocorticoid receptor signaling in T cells impacts thymocyte apoptosis and adaptive immune responses

To study the effect of enhanced glucocorticoid signaling on T cells, we generated transgenic rats overexpressing a mutant glucocorticoid receptor with increased ligand affinity in the thymus. We found that this caused massive thymocyte apoptosis at physiological hormone levels, which could be reversed by adrenalectomy. Due to homeostatic proliferation, a considerable number of mature T lymphocytes accumulated in the periphery, responding normally to costimulation but exhibiting a perturbed T-cell repertoire. Furthermore, the transgenic rats showed increased resistance to experimental autoimmune encephalomyelitis, which manifests in a delayed onset and milder disease course, impaired leukocyte infiltration into the central nervous system and a distinct cytokine profile. In contrast, the ability of the transgenic rats to mount an allergic airway response to ovalbumin was not compromised, although isotype switching of antigen-specific immunoglobulins was altered. Collectively, our findings suggest that endogenous glucocorticoids impact T-cell development and favor the selection of Th2- over Th1-dominated adaptive immune responses.

Virus-specific CD8+ T cells in the liver: armed and ready to kill

Influenza A virus infection of C57BL/6 mice is a well-characterized model for studying CD8+ T cell-mediated immunity. Analysis of primary and secondary responses showed that the liver is highly enriched for CD8+ T cells specific for the immunodominant H2D(b)NP(366-374) (D(b)NP(366)) epitope. Functional analysis established that these liver-derived virus-specific CD8+ T cells are fully competent cytotoxic effectors and IFN-gamma secretors. In addition, flow cytometric analysis of early apoptotic cells showed that these influenza-specific CD8+ T cells from liver are as viable as those in the spleen, bronchoalveolar lavage, mediastinal lymph nodes, or lung. Moreover, cytokine profiles of the influenza-specific CD8+ T cells recovered from different sites were consistent with the bronchoalveolar lavage, rather than liver population, being the most susceptible to activation-induced cell death. Importantly, adoptively transferred influenza virus-specific CD8+ T cells from the liver survived and were readily recalled after virus challenge. Together, these results show clearly that the liver is not a "graveyard" for influenza virus-specific CD8+ T cells.

Low glucocorticoid receptor (GR), high Dig2 and low Bcl-2 expression in double positive thymocytes of BALB/c mice indicates their endogenous glucocorticoid hormone exposure

Several studies have shown that of the four major thymocyte subsets, the CD4/CD8 double positive (DP) thymocytes are the most sensitive to in vivo glucocorticoid hormone (GC)-induced apoptosis. Our aim was to analyse fine molecular differences among thymocyte subgroups that could underlie this phenomenon. Therefore, we characterised the glucocorticoid hormone receptor (GR) expression of thymocyte subgroups both at the mRNA and protein levels by real-time PCR and flow cytometry, and correlated these features to their apoptotic sensitivity. We also investigated the time-dependent effects of the GC agonist dexamethasone (DX) with or without GC antagonist (RU486) treatments on GR mRNA/protein expression. We also analysed the expression of two apoptosis-related gene products: dexamethasone-induced gene 2 (Dig2) mRNA and Bcl-2 protein. We found that DN thymocytes had the highest GR expression, followed by CD8 single positive (SP), CD4 SP and DP thymocytes in 4-week-old BALB/c mice, both at the mRNA and protein levels, respectively. In DP cells, the Dig2 expression was significantly higher, while the Bcl-2 expression was significantly lower than in DN, CD4 SP and CD8 SP thymocytes. Single high dose DX treatment caused time-dependent depletion of DP thymocytes due to their higher apoptosis rate, which could not be abolished with RU486 pretreatment. After a single high dose DX treatment, there was a transient, significant increase of the GR mRNA and protein level of unsorted thymocytes after 8 and 16 h, followed by a significant decrease at 24 h, respectively. The time-dependent GR expression changes after DX administration could not be inhibited by the GC antagonist RU486. Twenty-four hours after exposure to high dose DX the DN, CD4 SP and CD8 SP cells showed a significant decrease of GR mRNA and protein expression, whereas the DP thymocytes, showed no significant alteration of GR mRNA or protein expression. The kinetical analysis of GR expression and apoptotic marker changes upon single high dose GC analogue administration revealed a two-phase process in thymocytes: early events, within 4-8 h, include GR upregulation and early apoptosis induction, while the late events appear most prominently at 16-20 h, when the GR is already downregulated and apoptotic cell ratio reaches its peak, with marked DP cell depletion. The low GR, high Dig2 and low Bcl-2 expression, coupled with the absence of homologous downregulation of GR after exogenous GC analogue treatment, could contribute to the high GC sensitivity of DP thymocytes. The downregulated GR and Bcl-2 together with the upregulated Dig2 level in DP cells indicates the significance of intrathymic GC effects at this differentiation stage. Since GR expression changes and apoptotic events could not be completely inhibited by GC antagonist, we propose the involvement of non-genomic GR mechanisms in these processes.

Monocytic leukemia zinc finger protein is essential for the development of long-term reconstituting hematopoietic stem cells

Monocytic leukemia zinc finger protein (MOZ), a transcriptional coactivator and member of the MYST family of histone acetyltransferases, is the target of recurrent translocations in acute myeloid leukemia. Since genes associated with translocations in leukemia are typically important regulators of blood formation, we investigated if Moz has a role in normal hematopoiesis. We generated mice carrying a mutation in the Moz gene. Homozygous Moz mutant mice died at birth. Moz mutant fetal liver hematopoietic cells were incapable of contributing to the hematopoietic system of recipients after transplantation. We observed profound defects in the stem cell compartment of Moz-deficient mice. Progenitors of all lineages were reduced in number. However, blood cell lineage commitment was unaffected. Together, these results show that Moz is essential for a fundamental property of hematopoietic stem cells, the ability to reconstitute the hematopoietic system of a recipient after transplantation and that Moz is specifically required in the stem cell compartment.

Quantitation and cellular localization of 11beta-HSD1 expression in murine thymus

11beta-hydroxysteroid dehydrogenase type I (11beta-HSD1), an NADPH-dependent reductase, functions in intact cells to convert inactive 11-keto metabolites of glucocorticoids into biologically active glucocorticoids. The enzyme is thus capable of amplifying glucocorticoid action in tissues in which it is expressed. In the experiments presented here, we show that 11beta-HSD1 is expressed in the murine thymus and that expression increases from late fetal development to maximal levels in the adult thymus. Quantitative real time-PCR, immunoblots, and assays of enzymatic activity reveal adult thymic expression of 11beta-HSD1 mRNA and protein at levels approximately 6-7% of those observed in liver. Immunofluorescence experiments show that the enzyme is expressed in the medullary thymocytes and thymocytes present at the corticomedullary junction. These experiments extend our recognition of 11beta-HSD1 expression in cells of the immune system and lend support to the notion that glucocorticoid signaling and amplification of those signals by regeneration of active glucocorticoids from inactive 11-keto metabolites might impact intrathymic T cell development and the establishment of the immune repertoire.

Inhibited cell death, NF-kappaB activity and increased IL-10 in TCR-triggered thymocytes of transgenic mice overexpressing the glucocorticoid-induced protein GILZ

Glucocorticoids promote thymocyte apoptosis and modulate transcription of several genes including GILZ, which is strongly up-regulated in the thymus. We used transgenic mice overexpressing GILZ in the T-cell lineage to investigate TCR-triggered functions of GILZ-overexpressing thymocytes. TCR-triggered apoptosis, but not glucocorticoid-induced apoptosis, was inhibited in transgenic mice compared to their controls. In vivo anti-CD3 administration did not reduce CD4(+)CD8(+) thymocyte number. Analysis of TCR-triggered molecular changes indicated that p65 NF-kappaB nuclear translocation and DNA binding activity was inhibited in transgenic mice, which might be linked with apoptosis inhibition. IL-10 release increased whereas release of IL-2, IFN-gamma, IL-13 and IL-4 remained unchanged. These results support the hypothesis that GILZ regulates, at least in part, T-cell development by influencing thymus function at cellular and molecular levels.

Genomic and non-genomic effects of different glucocorticoids on mouse thymocyte apoptosis

Glucocorticoids, widely used therapeutic agents for several pathologies, act upon diverse cells and tissues, including the lympho-haemopoietic system. Glucocorticoid-mediated apoptosis has been described as one of the mechanisms underlying their pharmacological and physiological effects. Glucocorticoids induce apoptosis in thymocytes through genomic and non-genomic signals. We tested thymocyte apoptosis rates as induced by a panel of glucocorticoids. Using four glucocorticoids that are widely adopted in clinical practice we compared their induction of thymocyte apoptosis and activation of non-genomic and genomic signals, including phosphatidylinositol-specific phospholipase C (PI-PLC), caspase-8, -9 and -3, and Glucocorticoid-Induced Leucine Zipper (GILZ). GILZ is a protein that is rapidly induced by glucocorticoids treatment and involved in apoptosis modulation. Results indicate different glucocorticoids have different apoptotic activity which is related to their ability to induce both genomic, evaluated as caspases activation and GILZ expression, and non-genomic effects, evaluated as PI-PLC phosphorylation.

Glucocorticoids in T cell apoptosis and function

Glucocorticoids (GCs) are a class of steroid hormones which regulate a variety of essential biological functions. The profound anti-inflammatory and immunosuppressive activity of synthetic GCs, combined with their power to induce lymphocyte apoptosis place them among the most commonly prescribed drugs worldwide. Endogenous GCs also exert a wide range of immunomodulatory activities, including the control of T cell homeostasis. Most, if not all of these effects are mediated through the glucocorticoid receptor, a member of the nuclear receptor superfamily. However, the signaling pathways and their cell type specificity remain poorly defined. In this review, we summarize our present knowledge on GC action, the mechanisms employed to induce apoptosis and the currently discussed models of how they may participate in thymocyte development. Although our knowledge in this field has substantially increased during recent years, we are still far from a comprehensive picture of the role that GCs play in T lymphocytes.

Rescuing developing thymocytes from death by neglect

The major function of the thymus is to eliminate developing thymocytes that are potentially useless or autoreactive, and select only those that bear functional T cell antigen receptors (TCRs) through fastidious screening. It is believed that glucocorticoids (GCs) are at least in part responsible for cell death during death by neglect. In this review, we will mainly cover the topic of the GC-induced apoptosis of developing thymocytes. We will also discuss how thymocytes that are fated to die by GCs can be rescued from GC-induced apoptosis in response to a variety of signals with antagonizing properties for GC receptor (GR) signaling. Currently, a lot of evidence supports the notion that the decision is made as a result of the integration of the multiple signal transduction networks that are triggered by GR, TCR, and Notch. A few candidate molecules at the converging point of these multiple signaling pathyways will be discussed. We will particularly describe the role of the SRG3 protein as a potent modulator of GC-induced apoptosis in the crosstalk.

TCR signaling inhibits glucocorticoid-induced apoptosis in murine thymocytes depending on the stage of development

Signaling by either the TCR or glucocorticoid receptor (GR) induces apoptosis in thymocytes. Interestingly, it has been shown previously that hybridoma T cells escape apoptosis induced by either TCR or GR when both of these receptors signal simultaneously. Whether such mutual antagonism is present in primary thymocytes was the subject of the present study. Both glucocorticoids (GC) and anti-TCR/CD28 (or anti-CD3/CD28) mAb induced apoptosis in total thymocytes. When these signals were present at the same time, GC-induced apoptosis was partially inhibited by TCR/CD3 signaling. Costimulation by anti-CD28 enhanced the inhibitory effects of anti-CD3 on GC-induced apoptosis about 30-fold. However, subset analysis revealed that most cells rescued from GC-induced apoptosis were mature CD4+ and CD8+ thymocytes, and these cells were resistant to TCR/CD3-induced apoptosis in the absence of GC. Similar results were obtained with mature splenic CD4+ and CD8+ T cells. TCR/CD3 signaling alone, while inducing apoptosis in CD4+(CD8+)TCRlow thymocytes, rescued a small subset of CD4+(CD8+)TCRlow thymocytes from GC-induced apoptosis. Thus, TCR signaling increasingly reverses GC-induced apoptosis as thymocyte development progresses. As GC are infinitely present in vivo, these findings support a model wherein TCR signaling may be required to prevent GC-induced apoptosis both under basal and immune challenging conditions.

Transcriptional regulation of the mouse IL-7 receptor alpha promoter by glucocorticoid receptor

Expression of the IL-7R alpha-chain (IL-7Ralpha) is strictly regulated during the development and maturation of lymphocytes. Glucocorticoids (GC) have pleiotypic effects on the growth and function of lymphocytes. Although GC have been reported to induce the transcription of IL-7Ralpha gene in human T cells, its molecular mechanism is largely unknown. In this study, we show that GC up-regulate the levels of IL-7Ralpha mRNA and protein in mouse T cells. This effect does not require protein synthesis de novo, because protein synthesis inhibitors do not block the process. Mouse IL-7Ralpha promoter has striking homology with human and rat, containing consensus motifs of Ikaros, PU.1, and Runx1 transcription factors. In addition, a conserved noncoding sequence (CNS) of approximately 270 bp was found 3.6-kb upstream of the promoter, which was designated as CNS-1. A GC receptor (GR) motif is present in the CNS-1 region. Importantly, we show by reporter assay that the IL-7Ralpha promoter has specific transcription activity in T cells. This activity highly depends on the PU.1 motif. Furthermore, GC treatment augments the transcriptional activity through the GR motif in the CNS-1 region. We also demonstrate that GR binds to the GR motif by EMSA. In addition, by chromatin immunoprecipitation assay, we show that GR is rapidly recruited to endogenous CNS-1 chromatin after GC stimulation. These results demonstrate that GR binds to the GR motif in the CNS-1 region after GC stimulation and then activates the transcription of the IL-7Ralpha promoter. Thus, this study identifies the IL-7Ralpha CNS-1 region as a GC-responsive element.

Different roles for glucocorticoids in thymocyte homeostasis?

Glucocorticoids (GCs) have important immunoregulatory effects on thymocytes and T cells. Ectopic production of GCs has been demonstrated in thymic epithelial cells (TECs) but the role of GCs in thymocyte homeostasis is controversial. Studies in several different mouse models, genetically modified for the GC receptor (GR) expression or function, have demonstrated conflicting results in terms of the effect of the hormone on thymocytes. Here, we summarize these data and suggest that GCs can mediate both positive and negative effects in the organ depending on the local hormonal concentration. Basal GC levels might promote growth of early thymocytes in young mice, and increased levels, generated through a stress reaction, apoptosis in these cells. A gradual loss of GC synthesis in TECs during aging might contribute to thymic involution, a process so far unexplained.

Glucocorticoids inhibit activation-induced cell death (AICD) via direct DNA-dependent repression of the CD95 ligand gene by a glucocorticoid receptor dimer

Glucocorticoids (GCs) play an important role in the regulation of peripheral T-cell survival. Their molecular mechanism of action and the question of whether they have the ability to inhibit apoptosis in vivo, however, are not fully elucidated. Signal transduction through the glucocorticoid receptor (GR) is complex and involves different pathways. Therefore, we used mice with T-cell-specific inactivation of the GR as well as mice with a function-selective mutation in the GR to determine the signaling mechanism. Evidence is presented for a functional role of direct binding of the GR to 2 negative glucocorticoid regulatory elements (nGREs) in the CD95 (APO-1/Fas) ligand (L) promoter. Binding of GRs to these nGREs reduces activation-induced CD95L expression in T cells. These in vitro results are fully supported by data obtained in vivo. Administration of GCs to mice leads to inhibition of activation-induced cell death (AICD). Thus, GC-mediated inhibition of CD95L expression of activated T cells might contribute to the anti-inflammatory function of steroid drugs. (Blood. 2005;106:617-625)

Molecular mechanisms of glucocorticoids in the control of inflammation and lymphocyte apoptosis

The immune system must be tightly controlled not only to guarantee efficient protection from invading pathogens and oncogenic cells but also to avoid exaggerated immune responses and autoimmunity. This is achieved through interactions amongst leukocytes themselves, by signals from stromal cells and also by various hormones, including glucocorticoids. The glucocorticoids are a class of steroid hormones that exert a wide range of anti-inflammatory and immunosuppressive activities after binding to the glucocorticoid receptor. The power of these hormones was acknowledged many decades ago, and today synthetic derivatives are widely used in the treatment of inflammatory disorders, autoimmunity and cancer. In this review, we summarize our present knowledge of the molecular mechanisms of glucocorticoid action, their influence on specific leukocytes and the induction of thymocyte apoptosis, with an emphasis on how molecular genetics has contributed to our growing, although still incomplete, understanding of these processes.

Thymus-derived glucocorticoids are insufficient for normal thymus homeostasis in the adult mouse

BACKGROUND

It is unclear if thymus-derived glucocorticoids reach sufficient local concentrations to support normal thymus homeostasis, or if adrenal-derived glucocorticoids from the circulation are required. Modern approaches to this issue (transgenic mice that under or over express glucocorticoid receptor in the thymus) have yielded irreconcilably contradictory results, suggesting fundamental problems with one or more the transgenic mouse strains used. In the present study, a more direct approach was used, in which mice were adrenalectomized with or without restoration of circulating corticosterone using timed release pellets. Reversal of the increased number of thymocytes caused by adrenalectomy following restoration of physiological corticosterone concentrations would indicate that corticosterone is the major adrenal product involved in thymic homeostasis.

RESULTS

A clear relationship was observed between systemic corticosterone concentration, thymus cell number, and percentage of apoptotic thymocytes. Physiological concentrations of corticosterone in adrenalectomized mice restored thymus cell number to normal values and revealed differential sensitivity of thymocyte subpopulations to physiological and stress-inducible corticosterone concentrations.

CONCLUSION

This indicates that thymus-derived glucocorticoids are not sufficient to maintain normal levels of death by neglect in the thymus, but that apoptosis and possibly other mechanisms induced by physiological, non stress-induced levels of adrenal-derived corticosterone are responsible for keeping the total number of thymocytes within the normal range.