A targeted glucocorticoid receptor antisense transgene increases thymocyte apoptosis and alters thymocyte development

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.

Glucocorticoids Oppose Thymocyte Negative Selection by Inhibiting Helios and Nur77

Glucocorticoid (GC) signaling in thymocytes shapes the TCR repertoire by antagonizing thymocyte negative selection. The transcription factors Nur77 and Helios, which are upregulated in TCR-signaled thymocytes, have been implicated in negative selection. In this study, we found that GCs inhibited Helios and, to a lesser extent, Nur77 upregulation in TCR-stimulated mouse thymocytes. Inhibition was increased by GC preincubation, and reductions in mRNA were prevented by a protein synthesis inhibitor, suggesting that GCs suppress indirectly via an intermediary factor. Upregulation of Helios in TCR-stimulated thymocytes was unaffected by deletion of Nur77, indicating Nur77 and Helios are regulated independently. Whereas CD4⁺ thymocytes are positively selected in wild-type AND TCR-transgenic B6 mice, loss of GC receptor expression resulted in increased negative selection. Correspondingly, Helios and Nur77 levels were elevated in TCR^hiCD4⁺CD8⁺ (TCR-signaled) thymocytes. Notably, deletion of Helios fully reversed this negative selection, whereas deletion of Nur77 had no effect on CD4⁺CD8⁺ cell numbers but reversed the loss of mature CD4⁺ thymocytes. Thus, Nur77 and Helios are GC targets that play nonredundant roles in setting the signaling threshold for thymocyte negative selection.

Cutting Edge: De Novo Glucocorticoid Synthesis by Thymic Epithelial Cells Regulates Antigen-Specific Thymocyte Selection

Glucocorticoid (GC) signaling in thymocytes counters negative selection and promotes the generation of a self-tolerant yet Ag-responsive T cell repertoire. Whereas circulating GC are derived from the adrenals, GC are also synthesized de novo in the thymus. The significance of this local production is unknown. In this study we deleted 11β-hydroxylase, the enzyme that catalyzes the last step of GC biosynthesis, in thymic epithelial cells (TEC) or thymocytes. Like GC receptor-deficient T cells, T cells from mice lacking TEC-derived but not thymocyte-derived GC proliferated poorly to alloantigen, had a reduced antiviral response, and exhibited enhanced negative selection. Strikingly, basal expression of GC-responsive genes in thymocytes from mice lacking TEC-derived GC was reduced to the same degree as in GC receptor-deficient thymocytes, indicating that at steady-state the majority of biologically active GC are paracrine in origin. These findings demonstrate the importance of extra-adrenal GC even in the presence of circulating adrenal-derived GC.

Prenatal Administration of Betamethasone Causes Changes in the T Cell Receptor Repertoire Influencing Development of Autoimmunity

Prenatal glucocorticoids are routinely administered to pregnant women at risk of preterm delivery in order to improve survival of the newborn. However, in half of the cases, birth occurs outside the beneficial period for lung development. Glucocorticoids are potent immune modulators and cause apoptotic death of immature T cells, and we have previously shown that prenatal betamethasone treatment at doses eliciting lung maturation induce profound thymocyte apoptosis in the offspring. Here, we asked if there are long-term consequences on the offspring’s immunity after this treatment. In the non-obese diabetic mouse model, prenatal betamethasone clearly decreased the frequency of pathogenic T cells and the incidence of type 1 diabetes (T1D). In contrast, in the lupus-prone MRL/lpr strain, prenatal glucocorticoids induced changes in the T cell repertoire that resulted in more autoreactive cells. Even though glucocorticoids transiently enhanced regulatory T cell (Treg) development, these cells did not have a protective effect in a model for multiple sclerosis which relies on a limited repertoire of pathogenic T cells for disease induction that were not affected by prenatal betamethasone. We conclude that prenatal steroid treatment, by inducing changes in the T cell receptor repertoire, has unforeseeable consequences on development of autoimmune disease. Our data should encourage further research to fully understand the consequences of this widely used treatment.

Local glucocorticoid production in lymphoid organs of mice and birds: Functions in lymphocyte development

Circulating glucocorticoids (GCs) are powerful regulators of immunity. Stress-induced GC secretion by the adrenal glands initially enhances and later suppresses the immune response. GC targets include lymphocytes of the adaptive immune system, which are well known for their sensitivity to GCs. Less appreciated, however, is that GCs are locally produced in lymphoid organs, such as the thymus, where GCs play a critical role in selection of the T cell antigen receptor (TCR) repertoire. Here, we review the roles of systemic and locally-produced GCs in T lymphocyte development, which has been studied primarily in laboratory mice. By antagonizing TCR signaling in developing T cells, thymus-derived GCs promote selection of T cells with stronger TCR signaling. This results in increased T cell-mediated immune responses to a range of antigens. We then compare local and systemic GC patterns in mice to those in several bird species. Taken together, these studies suggest that a combination of adrenal and lymphoid GC production might function to adaptively regulate lymphocyte development and selection, and thus antigen-specific immune reactivity, to optimize survival under different environmental conditions. Future studies should examine how lymphoid GC patterns vary across other vertebrates, how GCs function in B lymphocyte development in the bone marrow, spleen, and the avian bursa of Fabricius, and whether GCs adaptively program immunity in free-living animals.

Intermittent neonatal hypoxia elicits the upregulation of inflammatory-related genes in adult male rats through long-lasting programming effects

The long-term effects of neonatal intermittent hypoxia (IH), an accepted model of apnea-induced hypoxia, are unclear. We have previously shown lasting "programming" effects on the HPA axis in adult rats exposed to neonatal IH. We hypothesized that neonatal rat exposure to IH will subsequently result in a heightened inflammatory state in the adult. Rat pups were exposed to normoxia (control) or six cycles of 5% IH or 10% IH over one hour daily from postnatal day 2-6. Plasma samples from blood obtained at 114 days of age were analyzed by assessing the capacity to induce transcription in a healthy peripheral blood mononuclear cell (PBMC) population and read using a high-density microarray. The analysis of plasma from adult rats previously exposed to neonatal 5% IH versus 10% IH resulted in 2579 significantly regulated genes including increased expression of Cxcl1, Cxcl2, Ccl3, Il1a, and Il1b. We conclude that neonatal exposure to intermittent hypoxia elicits a long-lasting programming effect in the adult resulting in an upregulation of inflammatory-related genes.

Thymocyte responsiveness to endogenous glucocorticoids is required for immunological fitness

Generation of a self-tolerant but antigen-responsive T cell repertoire occurs in the thymus. Although glucocorticoids are usually considered immunosuppressive, there is also evidence that they play a positive role in thymocyte selection. To address the question of how endogenous glucocorticoids might influence the adaptive immune response, we generated GRlck-Cre mice, in which the glucocorticoid receptor gene (GR) is deleted in thymocytes prior to selection. These mice were immunocompromised, with reduced polyclonal T cell proliferative responses to alloantigen, defined peptide antigens, and viral infection. This was not due to an intrinsic proliferation defect, because GR-deficient T cells responded normally when the TCR was cross-linked with antibodies or when the T cell repertoire was "fixed" with αβ TCR transgenes. Varying the affinity of self ligands in αβ TCR transgenic mice showed that affinities that would normally lead to thymocyte-positive selection caused negative selection, and alterations in the TCR repertoire of polyclonal T cells were confirmed by analysis of TCR Vβ CDR3 regions. Thus, endogenous glucocorticoids are required for a robust adaptive immune response because of their promotion of the selection of T cells that have sufficient affinity for self, and the absence of thymocyte glucocorticoid signaling results in an immunocompromised state.

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.

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.

Glucocorticoid-induced apoptosis of healthy and malignant lymphocytes

Glucocorticoids exert a wide range of physiological effects, including the induction of apoptosis in lymphocytes. The progression of glucocorticoid-induced apoptosis is a multi-component process requiring contributions from both genomic and cytoplasmic signaling events. There is significant evidence indicating that the transactivation activity of the glucocorticoid receptor is required for the initiation of glucocorticoid-induced apoptosis. However, the rapid cytoplasmic effects of glucocorticoids may also contribute to the glucocorticoid-induced apoptosis-signaling pathway. Endogenous glucocorticoids shape the T-cell repertoire through both the induction of apoptosis by neglect during thymocyte maturation and the antagonism of T-cell receptor (TCR)-induced apoptosis during positive selection. Owing to their ability to induce apoptosis in lymphocytes, synthetic glucocorticoids are widely used in the treatment of haematological malignancies. Glucocorticoid chemotherapy is limited, however, by the emergence of glucocorticoid resistance. The development of novel therapies designed to overcome glucocorticoid resistance will dramatically improve the efficacy of glucocorticoid therapy in the treatment of haematological malignancies.

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.

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.

Thymus atrophy during Trypanosoma cruzi infection is caused by an immuno-endocrine imbalance

C57BL/6 mice infected with Trypanosoma cruzi, the causal agent of Chagas' disease, develop severe thymocyte depletion paralleled by an inflammatory syndrome mediated by tumor necrosis factor-alpha (TNF-alpha). The exacerbated inflammatory reaction induces the activation of hypothalamus-pituitary-adrenal (HPA) axis with the consequent release of corticosterone (CT) into the circulation as a protective response. Thymocyte apoptosis has been related to a rise in TNF-alpha and CT levels, and both mediators are increased in T. cruzi-infected C57BL/6 mice. The depletion of immature CD4(+)CD8(+) thymocytes by apoptosis following infection with the parasite was still present in mice defective in both types of TNF-receptors (double knockout). However, thymic atrophy was prevented by adrenalectomy combined with RU486 administration, demonstrating that this is a CT-driven phenomenon. Our results put emphasis on the importance of an appropriated immuno-endocrine balance during T. cruzi infection and show that functional deviations in the immuno-endocrine equilibrium have profound effects on the thymus and disease outcome.

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.

Characterization of T-cell population in children with prolonged fetal exposure to dexamethasone for anti-Ro/SS-A antibodies associated congenital heart block

The objectives of the study were to characterize the production, function and survival of T lymphocytes of children with prolonged fetal exposure to dexamethasone for anti-Ro/SS-A antibodies associated congenital complete heart block. The analysis of thymic function, studied by measuring the level of T-cell receptor excision circles, was performed by real time PCR, the composition of T-cell subpopulation was evaluated by flow cytometry and the T-cell diversity was assayed by heteroduplex analysis. T-cell competence was gauged at two functional levels by determining the proliferation and the number of T-cell divisions and by measuring gamma-interferon production after mitogenic stimulation. We observed that the thymic output, distribution of T-cell subsets, thymidine incorporation, number of T-cell divisions, and y-interferon production were comparable to those of age-matched control. On the contrary, heteroduplex analysis demonstrated the presence of both polyclonal and oligoclonal peripheral T-cell repertoires. In conclusion, the analysis of the T-cell compartment in children with prolonged intrauterine exposure to high dose dexamethasone did not disclose any relevant abnormality, except a restriction of T-cell receptor diversity in some patients.

Tissue specific glucocorticoid receptor expression, a role for alternative first exon usage?

The CpG island upstream of the GR is highly structured and conserved at least in all the animal species that have been investigated. Sequence alignment of these CpG islands shows inter-species homology ranging from 64 to 99%. This 3.1kb CpG rich region upstream of the GR exon 2 encodes 5' untranslated mRNA regions. These CpG rich regions are organised into multiple first exons and, as we and others have postulated, each with its own promoter region. Alternative mRNA transcript variants are obtained by the splicing of these alternative first exons to a common acceptor site in the second exon of the GR. Exon 2 contains an in-frame stop codon immediately upstream of the ATG start codon to ensure that this 5' heterogeneity remains untranslated, and that the sequence and structure of the GR is unaffected. Tissue specific differential usage of exon 1s has been observed in a range of human tissues, and to a lesser extent in the rat and mouse. The GR expression level is tightly controlled within each tissue or cell type at baseline and upon stimulation. We suggest that no single promoter region may be capable of containing all the necessary promoter elements and yet preserve the necessary proximity to the transcription initiation site to produce such a plethora of responses. Thus we further suggest that alternative first exons each under the control of specific transcription factors control both the tissue specific GR expression and are involved in the tissue specific GR transcriptional response to stimulation. Spreading the necessary promoter elements over multiple promoter regions, each with an associated alternative transcription initiation site would appear to vastly increase the capacity for transcriptional control of GR.

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.

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.

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.

Expressão gênica de caspases 3 e 8 em timo e baço de ratas recém-desmamadas e imunossuprimidas por glicocorticóide

Determinou-se a expressão gênica das caspases 3 e 8 mediante transcrição reversa de mRNA total e reação em cadeia da polimerase (RT-PCR) para avaliar a apoptose em timo e baço de ratas imunossuprimidas por glicocorticóides. Utilizou-se dexametasona para indução da apoptose e atrofia linfóide. Quarenta e cinco fêmeas Wistar recém-desmamadas foram separadas em três grupos: as ratas de A (n=18) e B (n=18) foram tratadas com 250 e 500mg de glicocorticóide, via intramuscular, respectivamente, e as do C (n=9) não foram tratadas. Após 24, 48 e 72 horas, seis animais de cada grupo tratado e três do controle foram anestesiados, pesados e sacrificados. O baço e o timo foram coletados e pesados. Fragmentos dos órgãos foram fixados em formol tamponado a 10% e processados segundo técnica para inclusão em parafina. Os blocos foram seccionados em 5µm, e os cortes corados em hematoxilina e eosina. A análise histopatológica aliada ao peso dos órgãos nas diferentes doses e tempos demonstrou que a dexametasona induziu hipotrofia linfóide, que ocorreu com maior intensidade no tempo de 72 horas em animais do grupo B. Fragmentos de timo e de baço foram imediatamente congelados em nitrogênio líquido para extração de mRNA e DNA. Para a padronização da técnica de RT-PCR, utilizaram-se pool de amostras de mRNA dos animais-controle e pool de mRNA de animais tratados em cada tempo de experimento. A técnica de RT-PCR foi sensível o suficiente para a detecção dos mRNAs que codificam as caspases 3 e 8, e ambas participaram do processo de apoptose induzido por dexametasona.

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.

A Role for SATB1, a Nuclear Matrix Association Region-Binding Protein, in the Development of CD8SP Thymocytes and Peripheral T Lymphocytes

Studies have suggested that binding of the SATB1 protein to L2a, a matrix association region located 4.5 kb 5' to the mouse CD8alpha gene, positively affects CD8 expression in T cells. Therefore, experiments were performed to determine the effect on T cell development of reduced expression of SATB1. Because homozygous SATB1-null mice do not survive to adulthood due to non-thymus autonomous defects, mice were produced that were homozygous for a T cell-specific SATB1-antisense transgene and heterozygous for a SATB1-null allele. Thymic SATB1 protein was reduced significantly in these mice, and the major cellular phenotype observed was a significant reduction in the percentage of CD8SP T cells in thymus, spleen, and lymph nodes. Mice were smaller than wild type but generally healthy, and besides a general reduction in cellularity and a slight increase in surface CD3 expression on CD8SP thymocytes, the composition of the thymus was similar to wild type. The reduction in thymic SATB1 does not lead to the variegated expression of CD8-negative single positive thymocytes seen upon deletion of several regulatory elements and suggested by others to reflect failure to activate the CD8 locus. Thus, the present results point to an essential role for SATB1 late in the development and maturation of CD8SP T cells.

The Expression of 11β-Hydroxysteroid Dehydrogenase Type I by Lymphocytes Provides a Novel Means for Intracrine Regulation of Glucocorticoid Activities

The 11beta-hydroxysteroid dehydrogenase (11beta-HSD) enzymes control the interconversion of active glucocorticoids (GCS) and their inactive 11-keto metabolites, a process commonly referred to as the cortisone/cortisol shuttle. Although the prereceptor metabolism of GCS by 11beta-HSD is well documented in a variety of cells and tissues, it has not yet been carefully investigated in the major cell types of the immune system. In this study, we demonstrate that 11beta-HSD1 transcripts, protein, and enzyme activities are actively expressed in murine CD4(+), CD8(+), and B220(+) lymphocytes, as well as CD11c(+) dendritic cells. Only reductase activity was observed in living cells, evidenced by the restricted conversion of cortisone to cortisol. Activation of CD4(+) T cells increased their 11beta-HSD1 activity, as did their polarization into Th1 or Th2 cells. CD4(+) T cells isolated from aged donors (>16 mo) had increased 11beta-HSD1 protein and an elevated capacity to convert cortisone to cortisol. The GCS generated in murine CD4(+) T cells from their inactive 11-keto metabolites could activate the GCS receptor, demonstrated by an up-regulation of IL-7Ralpha and GCS-induced leucine zipper gene expression. The presence of a functional 11beta-HSD1 provides lymphocytes with a novel intracrine regulatory mechanism that could influence such processes as lymphocyte development, effector function, and susceptibility to apoptosis. Thus, the presence of 11beta-HSD1 provides an additional means to facilitate GCS influences over lymphocyte activities, uncoupled from the plasma concentration of GCS.

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.

Nitric Oxide Inhibits Glucocorticoid-induced Apoptosis of Thymocytes by Repressing the SRG3 Expression

Nitric oxide (NO) plays many roles in the immune system. It has been known that NO rescues thymocytes from glucocorticoid (GC)-induced apoptosis. However, the downstream target of NO in the protection from GC-induced thymocyte apoptosis has yet to be identified. We previously reported that GC sensitivity of developing thymocytes is dependent on the expression level of SRG3. In the present report, we found that NO repressed the SRG3 expression in both primary thymocytes and 16610D9 thymoma cells. Specifically, NO down-regulated the transcription of SRG3 via the inactivation of the transcription factor Sp1 DNA-binding activity to the SRG3 promoter. In addition, overexpression of SRG3 by a heterologous promoter reduced NO-mediated rescue of thymocytes from GC-induced apoptosis. These observations strongly suggest that NO may be involved in protecting immature thymocytes from GC-induced apoptosis by repressing the SRG3 expression in thymus.

T Cell Receptor Signaling Inhibits Glucocorticoid-induced Apoptosis by Repressing the SRG3 Expression via Ras Activation

Activation of T cell antigen receptor (TCR) signaling inhibits glucocorticoid (GC)-induced apoptosis of T cells. However, the detailed mechanism regarding how activated T cells are protected from GC-induced apoptosis is unclear. Previously, we have shown that the expression level of SRG3, a murine homolog of BAF155 in humans, correlated well with the GC sensitivity of T cells either in vitro or in vivo. Intriguingly, the expression of SRG3 decreased upon positive selection in the thymus. Here we have shown that TCR signaling inhibits the SRG3 expression via Ras activation and thereby renders primary thymocytes and some thymoma cells resistant to GC-mediated apoptosis. By using pharmacological inhibitors, we have shown that Ras-mediated down-regulation of the SRG3 gene expression is mediated by MEK/ERK and phosphatidylinositol 3-kinase pathways. Moreover, TCR signals repressed the SRG3 transcription through the putative binding sites for E proteins and Ets family transcription factors in the proximal region of the SRG3 promoter. Introduction of mutations in these elements rendered the SRG3 promoter immune to the Ras or TCR signals. Taken together, these observations suggest that TCR signals result in GC desensitization in immature T cells by repressing SRG3 gene expression via Ras activation.

The glucocorticoid receptor mediates the thymic epithelial cell-induced apoptosis of CD4+8+ thymic lymphoma cells

"Negative selection" and "death by neglect" are governed by apoptotic processes occurring in the thymus that shape the repertoire of maturing T cells. We have previously developed an in vitro model that recapitulates "death by neglect": Co-cultivation of double positive (DP) thymocytes or thymic lymphoma cells (PD1.6) with thymic epithelial cells (TEC) caused TcR-independent apoptosis of the former. We further demonstrated that this apoptosis could be attenuated by aminoglutethimide, an inhibitor of steroid synthesis, suggesting a role of TEC-derived glucocorticoids (GC) in this death process. We have now substantiated the role of the GC-glucocorticoid receptor (GR) axis by using a GC-resistant subline (PD1.6Dex(-)) obtained from the GC-sensitive PD1.6 cells by repeated exposures to increasing doses of dexamethasone (Dex). The PD1.6Dex(-) cells barely express GR and are much less sensitive to TEC-induced apoptosis. Re-expression of GR in PD1.6Dex(-) cells restored their sensitivity to both Dex and TEC, highlighting the central role of GR in these apoptotic processes. Likewise, repeated exposures of PD1.6 cells to TEC led to the selection of TEC-resistant cells (PD1.6TEC(-)) that are insensitive to corticosterone and less sensitive to Dex, though their GR level was only moderately reduced. This is in line with the low levels of corticosterone secreted by TEC. Altogether, our data show that TEC eliminates DP thymic lymphoma cells in a GR-dependent manner and modulates the GC sensitivity of the surviving cells.

Apoptosis in primary lymphoid organs with aging

Age-associated changes in the immune system are responsible for an increased likelihood of infection, autoimmune diseases, and cancer in the elderly. Immunosenescence is characterized by reduced levels of the peripheral naive T cell pool derived from thymus and the loss of immature B lineage cells in the bone marrow. Primary lymphoid organs, i.e., bone marrow and thymus, exhibit a loss of cellularity with age, which is especially dramatic in the thymus. A summary of major changes associated with aging in primary lymphoid organs is described in this article. The participation of apoptosis in cell loss in the immune system, a change associated with age, as well as a description of molecular machinery involved, is presented. Finally, the involvement of different hormonal and non-hormonal agents in counteracting apoptosis in thymus and bone marrow during aging is explained. Here, we underlie the important role of glucocorticoids as immunodepressors and melatonin as an immunostimulatory agent.

T-cell glucocorticoid receptor is required to suppress COX-2-mediated lethal immune activation

Glucocorticoids, acting through the glucocorticoid receptor, potently modulate immune function and are a mainstay of therapy for treatment of inflammatory conditions, autoimmune diseases, leukemias and lymphomas. Moreover, removal of systemic glucocorticoids, by adrenalectomy in animal models or adrenal insufficiency in humans, has shown that endogenous glucocorticoid production is required for regulation of physiologic immune responses. These effects have been attributed to suppression of cytokines, although the crucial cellular and molecular targets remain unknown. In addition, considerable controversy remains as to whether glucocorticoids are required for thymocyte development. To assess the role of the glucocorticoid receptor in immune system development and function, we generated T-cell-specific glucocorticoid receptor knockout mice. Here we show that the T-cell is a critical cellular target of glucocorticoid receptor signaling, as immune activation in these mice resulted in significant mortality. This lethal activation is rescued by cyclooxygenase-2 (COX-2) inhibition but not steroid administration or cytokine neutralization. These studies indicate that glucocorticoid receptor suppression of COX-2 is crucial for curtailing lethal immune activation, and suggest new therapeutic approaches for regulation of T-cell-mediated inflammatory diseases.

Diabetes protection and restoration of thymocyte apoptosis in NOD Idd6 congenic strains

Type 1 diabetes in the nonobese diabetic (NOD) mouse is a multifactorial and polygenic disease. The NOD-derived genetic factors that contribute to type 1 diabetes are named Idd (insulin-dependent diabetes) loci. To date, the biological functions of the majority of the Idd loci remain unknown. We have previously reported that resistance of NOD immature thymocytes to depletion by dexamethazone (Dxm) maps to the Idd6 locus. Herein, we refine this phenotype using a time-course experiment of apoptosis induction upon Dxm treatment. We confirm that the Idd6 region controls apoptosis resistance in immature thymocytes. Moreover, we establish reciprocal Idd6 congenic NOD and B6 strains to formally demonstrate that the Idd6 congenic region mediates restoration of the apoptosis resistance phenotype. Analysis of the Idd6 congenic strains indicates that a 3-cM chromosomal region located within the distal part of the Idd6 region controls apoptosis resistance in NOD immature thymocytes. Together, these data support the hypothesis that resistance to Dxm-induced apoptosis in NOD immature thymocytes is controlled by a genetic factor within the region that also contributes to type 1 diabetes pathogenesis. We propose that the diabetogenic effect of the Idd6 locus is exerted at the level of the thymic selection process.

Corticosteroid-induced apoptosis of airway epithelium: a potential mechanism for chronic airway epithelial damage in asthma

Damage to the airway epithelium is one prominent feature of chronic asthma. Mucosal damage includes gap openings, partial denudation, and loss of ciliated cells. Apoptosis of the airway epithelium is increasingly recognized as a potential mechanism by which damage may occur. Corticosteroids (CSs) induce apoptosis in inflammatory cells, which in part explains their ability to suppress airway inflammation. However, CS therapy does not necessarily reverse epithelial damage. We examined whether CS therapy actually could induce airway epithelial apoptosis using culture models of primary airway epithelial cells and cell lines. The administration of CSs in low-micromolar concentrations induces apoptosis that involves the disruption of mitochondrial polarity, the activation of caspases, and the involvement of Bcl-2. Clear differences exist between CS-induced apoptosis in the cultured epithelium vs cultured hematopoietic cells in regard to time course and resistance to apoptosis. Our data suggest that the use of CSs, in concentrations that could be attained in vivo with the inhalation of potent preparations or with systemic administration, may be one factor in the airways remodeling and epithelial damage that is seen in many patients with chronic, persistent asthma.

Glucocorticoid receptor deficient thymic and peripheral T cells develop normally in adult mice

The involvement of glucocorticoid receptor (GR) signaling in T cell development is highly controversial, with several studies for and against. We have previously demonstrated that GR(-/-) mice, which usually die at birth because of impaired lung development, exhibit normal T cell development, at least in embryonic mice and in fetal thymus organ cultures. To directly investigate the role of GR signaling in adult T cell development, we analyzed the few GR(-/-) mice that occasionally survive birth, and irradiated mice reconstituted with GR(-/-) fetal liver precursors. All thymic and peripheral T cells, as well as other leukocyte lineages, developed and were maintained at normal levels. Anti-CD3-induced cell death of thymocytes in vitro, T cell repertoire heterogeneity and T cell proliferation in response to anti-CD3 stimulation were normal in the absence of GR signaling. Finally, we show that metyrapone, an inhibitor of glucocorticoid synthesis (commonly used to demonstrate a role for glucocorticoids in T cell development), impaired thymocyte development regardless of GR genotype indicating that this reagent inhibits thymocyte development in a glucocorticoid-independent fashion. These data demonstrate that GR signaling is not required for either normal T cell development or peripheral maintenance in embryonic or adult mice.

Thymocyte apoptosis induced by T cell activation is mediated by glucocorticoids in vivo

Glucocorticoids, administered in pharmacological doses, potently modulate immune system function and are a mainstay therapy for many common human diseases. Physiologic production of glucocorticoids may play a role in optimization of the immune repertoire both centrally and peripherally. Possible effects include alteration of lymphocyte development and down-regulation of cytokine responses, but essential roles remain unclear. To determine the part that endogenous glucocorticoids play in thymocyte development, we used fetal liver from mice lacking the glucocorticoid receptor GRko for immunological reconstitution of lethally irradiated wild-type (WT) mice. We find normal numbers and subset distribution of GRko thymocytes. GRko thymocytes also exhibit similar sensitivity to apoptosis induced by activating anti-CD3epsilon Ab as WT thymocytes in vitro. Surprisingly, GRko thymocytes are significantly more resistant than WT thymocytes to anti-CD3epsilon-mediated thymocyte apoptosis in vivo. Consistent with this finding, in vivo TCR complex activation induces sustained high levels of glucocorticoids that correlate strongly with thymocyte apoptosis in WT mice. We find that while direct engagement of the TCR complex may cause death of a subset of thymocytes, glucocorticoids are required for deletion of the majority of thymocytes. Thus, TCR stimulation by Ab administration may more accurately reflect polyclonal T cell activation than negative selection in vivo.

Green fluorescent protein-glucocorticoid receptor knockin mice reveal dynamic receptor modulation during thymocyte development

To delineate the cellular targets and mechanisms by which glucocorticoids (GCs) exert their actions, we generated mice in which a green fluorescent protein (GFP)-GC receptor (GR) fusion gene is knocked into the GR locus. In these mice, the GFP-GR protein, which is functionally indistinguishable from endogenous GR, allows the tracking and quantitation of GR expression in single living cells. In GFP-GR thymus, GR expression is uniform among embryonic thymocyte subpopulations but gradually matures over a 3-wk period after birth. In the adult, GR is specifically induced to high levels in CD25(+)CD4(-)CD8(-) thymocytes and returns to basal levels in CD4(+)CD8(+) thymocytes of wild-type and positively selecting female HY TCR-transgenic mice, but not negatively selecting male HY TCR-transgenic mice. In GFP-GR/recombinase-activating gene 2(-/-) thymocytes, GR expression is down-regulated by pre-TCR complex stimulation. Additionally, relative GR expression is dissociated from GC-induced apoptosis in vivo. Results from these studies define differential GR expression throughout ontogeny, suggest pre-TCR activation as a specific mechanism of GR down-regulation, define immature CD8(+) thymocytes as novel apoptosis-sensitive GC targets, and separate receptor abundance from susceptibility to apoptosis across thymocyte populations.

Increased p300 expression inhibits glucocorticoid receptor-T-cell receptor antagonism but does not affect thymocyte positive selection

Positive selection of T cells is postulated to be dependent on the counterinteraction between glucocorticoid receptor (GR)- and T-cell-receptor (TCR)-induced death signals. In this study we used T-cell-specific expression of p300 to investigate whether GR-TCR cross talk between thymocytes was affected. Activation of the p300-transgenic T cells led to enhanced thymocyte proliferation and increased interleukin 2 production. Thymocyte death, induced by TCR engagement, was no longer prevented by dexamethasone in p300-transgenic mice, indicating an absence of GR-TCR cross-inhibition. This was accompanied by a 50% reduction in the number of thymocytes in p300-transgenic mice. However, the CD4/CD8 profile of thymocytes remained unchanged in p300-transgenic mice. There was no effect on positive selection of the bulk thymocytes or thymocytes with transgenic TCR in p300-transgenic mice. In addition, there was no apparent TCR repertoire "hole" in the selected antigens examined. Our results illustrate a critical role of CBP/p300 in thymic GR-TCR counterinteraction yet do not support the involvement of GR-TCR antagonism in thymocyte positive selection.

Exposure to a dysfunctional glucocorticoid receptor from early embryonic life programs the resistance to experimental autoimmune encephalomyelitis via nitric oxide-induced immunosuppression

Glucocorticoid (GC) hormones play a central role in the bidirectional communication between the neuroendocrine and the immune systems and exert, via GC receptors (GR), potent immunosuppressive and anti-inflammatory effects. In this study, we report that GR deficiency of transgenic mice expressing GR antisense RNA from early embryonic life has a dramatic impact in programming the susceptibility to experimental autoimmune encephalomyelitis (EAE), an animal model for multiple sclerosis. GR deficiency renders mice resistant to myelin oligodendrocyte glycoprotein-induced EAE, and such mice do not develop clinical or histological signs of disease compared with EAE-susceptible wild-type mice. Resistance to EAE in GR-deficient mice is associated not with endogenous GC levels, but with a significant reduction in spleen and lymph node cell proliferation. The use of NO inhibitors in vitro indicates that NO is the candidate immunosuppressor molecule. GR-deficient mice develop 3- to 6-fold higher nitrite levels in the periphery and are resistant to NO inhibition by GCs. Specific inhibition of NO production in vivo by treatment with the inducible NO synthase inhibitor, L-N(6)-(1-iminoethyl)-lysine, suppressed circulating nitrites, increased myelin oligodendrocyte glycoprotein-specific cell proliferation, and rendered GR-deficient mice susceptible to EAE. Thus, life-long GR deficiency triggers inducible NO synthase induction and NO generation with consequent down-regulation of effector cell proliferation. These findings identify a novel link among GR, NO, and EAE susceptibility and highlight NO as critical signaling molecule in bidirectional communication between the hypothalamic-pituitary-adrenocortical axis and the immune system.

Glucocorticoid receptor variants: clinical implications

Following exposure to stress, cortisol is secreted from the adrenal cortex under the control of the hypothalamic-pituitary-adrenal axis (HPA-axis). Central in the regulation of the HPA-axis is a two tied corticosteroid-receptor system, comprised of high and low affinity receptors, the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR), respectively. In addition, these corticosteroid receptors mediate the effects of cortisol during stress on both central and peripheral targets. Cortisol modulates gene-expression of corticosteroid-responsive genes, with the effect lasting from hours to days. Mutations in the GR-gene are being associated with corticosteroid resistance and haematological malignancies, although these mutations are relatively rare and probably not a common cause of these diseases. However, several GR-gene variants and single nucleotide polymorphisms (SNP) in the GR-gene have been identified which are relatively common in the human population. The GRbeta-variant, for example, has been proposed to influence corticosteroid-sensitivity and most evidence has been derived from the immune system and in particular asthma. With respect to polymorphisms, a BclI restriction fragment polymorphism and a Asp363Ser have been described, which not only influence the regulation of the HPA-axis, but are also associated with changes in metabolism and cardiovascular control. These associations of a GR-gene polymorphism with metabolism and cardivascular control, and also with the regulation of the HPA-axis, indicates an important underlying role of cortisol in the etiology of these complex disorders. Therefore, we propose that a common underlying defect in these complex disorders is a disregulation of the HPA-axis, especially during stress. The clinical implication is that the regulation of the HPA-axis should be envisioned as a primary target of new drugs for the treatment of stress-related disorders.

Effects of altered glucocorticoid sensitivity in the T cell lineage on thymocyte and T cell homeostasis

The homeostatic regulation that controls total thymocyte and peripheral T-cell numbers is not clearly understood. We describe here a direct hormonal influence of endogenous levels of glucocorticoids (GCs) on thymocyte and peripheral T-cell homeostasis independent of indirect systemic effects of GCs. The results were obtained by generating transgenic mice with an altered GC sensitivity targeted to thymocytes and peripheral T cells by increasing or decreasing glucocorticoid receptor (GR) expression specifically in thymocytes and peripheral T cells. A twofold increase in GC sensitivity resulted in a major decrease in thymocyte number, affecting all subpopulations, although single-positive CD8+ cells were less influenced. In the thymus, this was due to increased apoptosis in the organ, whereas proliferation of thymocyte populations was unaffected. In the periphery, a pronounced reduction in T-cell number was seen, demonstrating an effect of endogenous GCs also on T-cell homeostasis. The effects were confirmed in transgenic mice with reduced GR expression, which showed increased thymocyte and T-cell numbers. Thus, our data demonstrate that physiological GC levels are directly involved in controlling the size of both thymocyte and T-cell pools.