Glucocorticoid-mediated regulation of thymic dendritic cell function

Parathyroid Hormone-related Peptide (hPTHrP) and Parathyroid Hormone-related Peptide Receptor Type 1 (PTHR1) Expression in Human Thymus

Parathyroid hormone-related peptide (hPTHrP) is expressed in human tissues and regulates cellular proliferation, differentiation, and apoptosis by an autocrine/paracrine loop. In rodent thymus, both parathormone and parathyroid hormone-related peptide (PTHrP) are expressed by thymic epithelial cells (TECs). The present study demonstrated by RT-PCR and immunohistochemistry that hPTHrP and parathyroid hormone-related peptide receptor type 1 (PTHR1) were expressed in human thymus at both RNA and protein levels. hPTHrP was expressed mainly in the thymic medulla by epithelial (cytokeratin-positive), mature dendritic (CD40+/86+) and plasmacytoid interleukin (IL)-3Rα+ cells. This protein was also present in some cells forming Hassall's bodies and a few subcapsular and cortical TECs. PTHR1 was expressed by scattered subcapsular and cortical TECs and by rare TECs in the medulla. Thymocytes did not express either hPTHrP or PTHR1. Primary cultures of human TECs revealed the presence of both hPTHrP and PTHR1 mRNAs, confirming the capacity of TECs to synthesize both peptides. Moreover, synthetic (1–39) hPTHrP peptide administered on cultured TECs induced the expression of IL-6 mRNA, suggesting that hPTHrP can regulate thymic functions by inducing in TECs the expression of IL-6, which is involved in the development and maturation of thymocytes.

Glucocorticoid receptor translational isoforms underlie maturational stage-specific glucocorticoid sensitivities of dendritic cells in mice and humans

Although glucocorticoids are a profoundly important class of anti-inflammatory and immunosuppressive agents, their actions in dendritic cells (DCs) are not well understood. We found that dexamethasone, a potent glucocorticoid, selectively induced apoptosis in mature, but not in immature, DCs in healthy mice, in mice with experimental airway inflammation, and in vitro in bone marrow–derived DCs. Distinct glucocorticoid receptor (GR) translational isoforms expressed in immature and mature DCs probably contribute to the DC maturational stage-specific glucocorticoid sensitivity. The GR-D isoforms were the predominant isoforms in immature DCs, whereas the proapoptotic GR-A isoform was the main isoform in mature DCs. Ectopic expression of the GR-A isoform in immature DCs increased glucocorticoid sensitivity and RU486, a selective GR antagonist, inhibited the glucocorticoid sensitivity of mature DCs. Furthermore, the distinct expression pattern of GR isoforms in immature and mature murine DCs was also observed in human monocyte–derived DCs. These studies suggest that glucocorticoids may spare immature DCs and suppress mature DCs and inflammation via differential expression of GR translational isoforms.

Catecholaminergic signalling through thymic nerve fibres, thymocytes and stromal cells is dependent on both circulating and locally synthesized glucocorticoids

Glucocorticoids have been shown to modulate the expression of noradrenaline metabolizing enzymes and β(2)- and α(1B)-adrenoceptors in a tissue- and cell- specific manner. In the thymus, apart from extensive sympathetic innervation, a regulatory network has been identified that encompasses catecholamine-containing non-lymphoid and lymphoid cells. We examined a putative role of adrenal- and thymus-derived glucocorticoids in modulation of rat thymic noradrenaline levels and adrenoceptor expression. Seven days postadrenalectomy, the thymic levels of mRNAs encoding tyrosine hydroxylase, dopamine β-hydroxylase, monoamine oxidase-A and, consequently, noradrenaline were decreased. Catecholamine content was diminished in autofluorescent nerve fibres (judging by the intensity of fluorescence) and thymocytes (considering HPLC measurements of noradrenaline and the frequency of tyrosine hydroxylase-positive cells), while it remained unaltered in non-lymphoid autofluorescent cells. In addition, adrenalectomy diminished the thymocyte expression of β(2)- and α(1B)-adrenoceptors at both mRNA and protein levels. Administration of ketoconazole (an inhibitor of glucocorticoid synthesis/action; 25 mg kg(-1) day(-1), s.c.) to glucocorticoid-deprived rats increased the thymic levels of tyrosine hydroxylase, dopamine β-hydroxylase and, consequently, noradrenaline. The increased intensity of the autofluorescent cell fluorescence in ketoconazole-treated rats indicated an increase in their catecholamine content, and suggested differential glucocorticoid-mediated regulation of catecholamines in thymic lymphoid and non-lymphoid cells. In addition, ketoconazole increased the thymocyte expression of α(1B)-adrenoceptors. Thus, this study indicates that in the thymus, as in some other tissues, glucocorticoids not only act in concert with cateholamines, but they may modulate catecholamine action by tuning thymic catecholamine metabolism and adrenoceptor expression in a cell-specific manner. Additionally, the study indicates a role of thymus-derived glucocorticoids in this modulation.

Wnt-4 protects thymic epithelial cells against dexamethasone-induced senescence

Glucocorticoids are widely used immunosuppressive drugs in treatment of autoimmune diseases and hematological malignancies. Glucocorticoids are particularly effective immune suppressants, because they induce rapid peripheral T cell and thymocyte apoptosis resulting in impaired T cell-dependent immune responses. Although glucocorticoids can induce apoptotic cell death directly in developing thymocytes, how exogenous glucocorticoids affect the thymic epithelial network that provides the microenvironment for T cell development is still largely unknown. In the present work, we show that primary thymic epithelial cells (TECs) express glucocorticoid receptors and that high-dosage dexamethasone induces degeneration of the thymic epithelium within 24 h of treatment. Changes in organ morphology are accompanied by a decrease in the TEC transcription factor FoxN1 and its regulator Wnt-4 parallel with upregulation of lamina-associated polypeptide 2α and peroxisome proliferator activator receptor γ, two characteristic molecular markers for adipose thymic involution. Overexpression of Wnt-4, however, can prevent upregulation of adipose differentiation-related aging markers, suggesting an important role of Wnt-4 in thymic senescence.

Longitudinal alteration of circulating dendritic cell subsets and its correlation with steroid treatment in patients with severe acute respiratory syndrome

In this study, we found that 74 patients with severe acute respiratory syndrome (SARS) exhibited a rapid, dramatic decrease in numbers of circulating myeloid and plasmacytoid dendritic cells (mDCs and pDCs) during the first 2 weeks of illness (5.3- and 28.4-fold reductions for mDCs and pDCs compared with 25 healthy individuals, respectively), with slow return to normal cell numbers during convalescence (weeks 5–7 of illness on average). In addition, numbers of circulating CD4 and CD8 T cells exhibited milder reductions (2.1- and 1.8-fold at week 1) and earlier return to normal at a mean of weeks 3 and 4, respectively. A significant inverse correlation was found between numbers of DC and T-cell subsets and high-dose steroid treatment. Our novel findings thus suggest that the acute SARS-coronavirus infection probably contributes to the initial reduction of DC and T-cell subsets in blood, and that high-dose steroid administration may subsequently exacerbate and prolong low expression of the cell subsets. These findings will aid the framing of further studies of the immunopathogenesis of SARS.

Dexamethasone inhibits maturation and alters function of monocyte-derived dendritic cells from cord blood

Critically ill infants are treated with dexamethasone (Dx) and other glucocorticoids to reduce inflammation and to promote lung and cardiac function. The neonatal immune system is immature, so neonatal dendritic cells (DCs) might be especially sensitive to glucocorticoid-mediated immunosuppression. To test this, we compared Dx treatment of monocyte-derived DCs from cord (CB) and adult blood (AB). Dx decreased CD1a levels on both AB and CB DCs. CB-treated cells also exhibited decreased expression of CD83 and increased expression of CD14, alterations not observed in AB DCs. Characteristic immature endocytic activity was sustained and enhanced in Dx-treated CB DCs, whereas AB DCs matured normally. Maintenance of endocytosis corresponded with CD14 expression. Dx markedly increased CB DC IL-10, a T cell helper 2 (Th2)-preferential cytokine, while reducing IL-12, a counterbalancing Th1 cytokine. AB DCs were also affected, but increases in IL-10 and decreases in IL-12 were more modest. Dx treatment also inhibited DC-induced T cell proliferation, but CB DCs were inhibited more. In short, neonatal DCs seemed to be especially sensitive to the immunosuppressive effects of Dx as indicated by altered phenotype, endocytic function, ability to stimulate T cells, and cytokine shift favoring Th2. These alterations in DC function are consistent with an increased risk for certain infections and atopic diseases.

Prolactin stimulates maturation and function of rat thymic dendritic cells

The current study analyses the effect of PRL, a hormone involved in numerous physiological processes, on dendritic cells (DC) of rat thymus. Most thymic DC express prolactin receptors (PRL-R) as demonstrated by both immunohistochemistry and flow cytometry. PRL administration during 2 or 6 days to fetal thymus organ cultures (FTOC) does not increase the proportions of DC in cultures but stimulates their differentiation. Furthermore, PRL-treated thymic DC exhibit increased allostimulatory capacity in mixed leukocyte reaction (MLR) assays in association with increased surface expression of both MHC antigens and the co-stimulatory molecule CD80. PRL-treated DC also produce increased amounts of pro-inflammatory cytokines, such as IL-12, TNFalpha and IL-1beta, but not of IL6 or IL-10. Our data suggest a key role for IL-12 in the observed changes in the allostimulatory capacity of PRL-treated DC. Also, they permit us to hypothesize about the physiological role played by PRL in thymus ontogeny.

Nuclear receptor signaling in macrophages

Macrophages play diverse roles in host defense and in maintenance of homeostasis. Based on their ability to promote inflammatory responses, inappropriate macrophage function also contributes to numerous pathological processes, including atherosclerosis, rheumatoid arthritis and inflammatory bowel disease. Members of the nuclear receptor superfamily of ligand-dependent transcriptions factors have emerged as key regulators of inflammation and lipid homeostasis in macrophages. These include the glucocorticoid receptor (GR), which inhibits inflammatory programs of gene expression in response to natural corticosteroids and synthetic anti-inflammatory ligands such as dexamethasone. Also, in response to endogenous eicosanoids and oxysterols, respectively, peroxisome proliferator-activated receptors (PPARs) and liver X receptors (LXRs) regulate transcriptional programs involved in inflammatory responses and lipid homeostasis. Identification of their mechanisms of action should help guide the development of new therapeutic agents useful in the treatment of diseases in which macrophages play critical pathogenic roles.

Influence of immunosuppressive drugs on dendritic cells

Immunosuppressive drugs used to control allograft rejection and in efforts to promote transplant tolerance are well recognized for their abilities to inhibit lymphocyte activation and proliferation. In recent years, evidence has accumulated that these diversely acting agents (anti-proliferative drugs, calcineurin inhibitors, rapamycin, deoxyspergualin and glucocorticoids) also affect the development and functional immunobiology of dendritic cells, in vitro and in vivo. Here we review the influence of immunosuppressive drugs on the differentiation and function of these important antigen-presenting cells. We also consider how these effects influence immune reactivity and tolerance induction, implications for furthermore understanding of dendritic cell biology and prospects for improving the outcome of organ transplantation and therapy of other immune-mediated disorders by impacting dendritic cell function.

Perinatal blockade of b7-1 and b7-2 inhibits clonal deletion of highly pathogenic autoreactive T cells

A number of in vitro studies have suggested that costimulatory molecules B7-1 and B7-2 and their receptor CD28 can promote clonal deletion, and limited in vivo studies have indicated that CD28 is involved in the clonal deletion of some T cells. However, the significance of B7-mediated clonal deletion in preventing autoimmune diseases has not been studied systematically. Here we report that the perinatal blockade of B7-1 and B7-2 substantially inhibits the clonal deletion of T cells in the thymus and leads to an accumulation of T cells capable of inducing fatal multiorgan inflammation. These results reveal a critical role for costimulatory molecules B7-1 and B7-2 in deleting pathogenic autoreactive T cells in the thymus. The critical role of B7-1 and B7-2 in T cell clonal deletion may explain, at least in part, the paradoxical increase of autoimmune disease in mice deficient for this family of costimulatory molecules, such as cytotoxic T lymphocyte associated molecule 4, CD28, and B7-2. The strong pathogenicity of the self-reactive T cells supports a central hypothesis in immunology, which is that clonal deletion plays an important role in preventing autoimmune diseases.

Neuroendocrine control of thymus physiology

The thymus gland is a central lymphoid organ in which bone marrow-derived T cell precursors undergo differentiation, eventually leading to migration of positively selected thymocytes to the peripheral lymphoid organs. This differentiation occurs along with cell migration in the context of the thymic microenvironment, formed of epithelial cells, macrophages, dendritic cells, fibroblasts, and extracellular matrix components. Various interactions occurring between microenvironmental cells and differentiating thymocytes are under neuroendocrine control. In this review, we summarize data showing that thymus physiology is pleiotropically influenced by hormones and neuropeptides. These molecules modulate the expression of major histocompatibility complex gene products by microenvironmental cells and the extracellular matrix-mediated interactions, leading to enhanced thymocyte adhesion to thymic epithelial cells. Cytokine production and thymic endocrine function (herein exemplified by thymulin production) are also hormonally controlled, and, interestingly in this latter case, a bidirectional circuitry seems to exist since thymic-derived peptides also modulate hormonal production. In addition to their role in thymic cell proliferation and apoptosis, hormones and neuropeptides also modulate intrathymic T cell differentiation, influencing the generation of the T cell repertoire. Finally, neuroendocrine control of the thymus appears extremely complex, with possible influence of biological circuitry involving the intrathymic production of a variety of hormones and neuropeptides and the expression of their respective receptors by thymic cells.