Partial blockade of T-cell differentiation during ontogeny and marked alterations of the thymic microenvironment in transgenic mice with impaired glucocorticoid receptor function

Prenatal maternal distress associates with a blunted cortisol response in rhinovirus-positive infants

INTRODUCTION

Prenatal exposure to maternal psychological distress (PD) may have programming effects on the fetus/infant hypothalamic-pituitary-adrenal (HPA) axis and subsequently on the development of the fetus' immune function. Therefore, our aim was to study whether prenatal exposure to PD is related to early infant HPA axis reactivity in the context of a subclinical rhinovirus infection that challenges infants HPA axis postnatally.

METHODS

This study included 336 10-week-old infants from the nested case control Focus Cohort of the FinnBrain Birth Cohort Study. The outcome was infant HPA axis reactivity in a stress test. The acute stressor comprised of pediatric examination with venipuncture and nasal swabs for virus assessment. Saliva cortisol samples were collected at 5 time points: baseline, 0, 15, 25 and 35 min after the stressor. HPA axis reactivity was defined by the cumulative post-stressor cortisol concentration.

RESULTS

HPA axis reactivity was blunted in the PD/rhinovirus + group compared to the average of control/rhinovirus+, PD/rhinovirus-, and control/rhinovirus- groups (difference: 14.7 ln [nmol/L] × min, 95% confidence interval 3.8-25.6, p =  .008). HPA axis reactivity was significantly blunted only in boys with rhinovirus detected when separately tested for boys and girls (p =  .04).

CONCLUSION

Our finding of PD-exposed rhinovirus-positive infants having blunted cortisol secretion gives rise to a hypothesis that maternal PD during pregnancy influences infant HPA axis functioning and the functioning of the immune system. Future studies are needed to test whether this suppression of the HPA axis that co-occurs with rhinovirus infection associates with later disease development (e.g., asthma).

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.

Role of the hypothalamic-pituitary-adrenal axis in developmental programming of health and disease

Adverse environments during the fetal and neonatal development period may permanently program physiology and metabolism, and lead to increased risk of diseases in later life. Programming of the hypothalamic-pituitary-adrenal (HPA) axis is one of the key mechanisms that contribute to altered metabolism and response to stress. Programming of the HPA axis often involves epigenetic modification of the glucocorticoid receptor (GR) gene promoter, which influences tissue-specific GR expression patterns and response to stimuli. This review summarizes the current state of research on the HPA axis and programming of health and disease in the adult, focusing on the epigenetic regulation of GR gene expression patterns in response to fetal and neonatal stress. Aberrant GR gene expression patterns in the developing brain may have a significant negative impact on protection of the immature brain against hypoxic-ischemic encephalopathy in the critical period of development during and immediately after birth.

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.

Prenatal stress, fetal imprinting and immunity

A comprehensive number of epidemiological and animal studies suggests that prenatal and early life events are important determinants for disorders later in life. Among them, prenatal stress (i.e. stress experienced by the pregnant mother with impact on the fetal ontogeny) has programming effects on the hypothalamic-pituitary-adrenocortical axis, brain neurotransmitter systems and cognitive abilities of the offspring. This review focuses on the impact of maternal stress during gestation on the immune function in the offspring. It compares results from different animal species and highlights potential mechanisms for the immune effects of prenatal stress, including maternal glucocorticoids and placental functions. The existence of possible windows of increased vulnerability of the immune system to prenatal stress during gestation is discussed. Several gaps in the present knowledge are pointed out, especially concerning the time when prenatal stress effects are expressed during postnatal life, why this expression is delayed after birth and whether prenatal stress predisposes to immune-related pathologies later in life.

Dexamethasone-induced immunosuppression: a rabbit model

Rabbits are often used as animal models for experimental purposes; in many cases steroid-induced immunosuppression is necessary. The aim of this study was to characterise a model of immunosuppression in rabbits, based on changes in the lymphocyte subset distribution, changes in proliferative capacity of lymphocytes and activity of neutrophils 1, 3 and 7 days after the administration of 2mg/kg dexamethasone phosphate (DXP) three times at 6-h intervals. In peripheral blood, neutrophilia and lymphopenia together with eosinopenia, monocytopenia and basopenia in the absence of leukocytosis was detected. One day after DXP administration the absolute numbers of all lymphocyte subsets decreased in the blood, whereas in bone marrow, absolute numbers of all lymphocyte subsets increased significantly, except CD79alpha(+) cells that increased only in relative numbers. The effect of DXP on lymphocytes from the spleen, mesenteric and popliteal lymph nodes was less pronounced. In the thymus, DXP led to a marked reduction of the relative and absolute numbers of CD4(+)CD8(+) thymocytes. The proliferative capacity of lymphocytes after concanavalin A stimulation was lower in the peripheral blood and spleen only on day 1, no changes were detected in lymph nodes or in bone marrow. A marked increase in proliferative capacity was detected in the thymus. Spontaneous production of reactive oxygen metabolites by neutrophils was reduced on days 1 and 3 after DXP administration. The present results demonstrate clearly that this DXP application protocol is useful for the experimental induction of relatively short-lasting immunosuppression in rabbits.

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.

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.

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.

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.

Intraepithelial but not lamina propria lymphocytes in the porcine gut are affected by dexamethasone treatment

It is well established that glucocorticoids are key regulators of the immune system and act as immunosuppressive agents in high concentrations. In the pig, effects on the gut immune system and trafficking of lymphocytes between tissues and blood plasma were not investigated so far. Twelve pigs of 70 kg were fed 0.4 mg portions of dexamethasone (Dexa) twice daily for 9 days or remained untreated (controls) and were sacrificed for tissue collection at the end of Dexa treatment. Another six pigs with jugular vein catheters were left untreated for 7 days (control period) and then received Dexa for 9 days. Blood was drawn twice during the control period and at days 3, 6 and 9 of the Dexa period for characterization of peripheral blood leukocytes. Cells were obtained from thymus, mesenteric lymph nodes, jejunal mucosa and Peyer's patches. Lymphoid cells from gut tissue were isolated from two fractions: the EDTA-fraction, containing the intraepithelial lymphocytes (IEL), and the Collagenase-fraction, containing the lamina propria lymphocytes (LPL). In all samples, cell counts and phenotypic characterization of cells by flow cytometry (FCM) were performed. In thymus, Dexa led to a more than 90% reduction of the absolute cell number, which was mainly found in the CD4+CD8+ subpopulation. Dexa effects on lymphocytes from mesenteric lymph nodes were less severe (50%) and led mainly to a decrease (71%) of B-lymphocytes. The number of lymphocytes in the EDTA-fraction (IEL) of the jejunal mucosa decreased significantly by 56% in the Dexa-treated animals compared to the controls, whereas the number of lymphocytes in the Collagenase-fraction (LPL) decreased only moderately. In the Peyer's patches, a decreasing tendency in the number of lymphocytes in the EDTA-fraction was observed which, however, was not significant. In blood, monocytes and granulocytes were significantly increased in an order of 60%. The data show that supraphysiological amounts of Dexa remarkably reduce cell numbers in thymus and also in the intraepithelial compartment of the jejunal mucosa and ileal Peyer's patches. In blood, a notable homeostasis was observed for several leukocyte populations whereas both monocytes and granulocytes increased.

Glucocorticoid receptor–nitric oxide crosstalk and vulnerability to experimental parkinsonism: pivotal role for glia–neuron interactions

Inflammation and oxidative stress have been closely associated with the pathogenesis of neurodegenerative disorders, including Parkinson's disease (PD). The expression of inducible nitric oxide synthase (iNOS) in astrocytes and microglia and the production of large amounts of nitric oxide (NO) are thought to contribute to dopaminergic neuron demise. Increasing evidence, however, indicates that activated astroglial cells play key roles in neuroprotection and can promote recovery of CNS functions. Endogenous glucocorticoids (GCs) via glucocorticoid receptors (GRs) exert potent anti-inflammatory and immunosuppressive effects and are key players in protecting the brain against stimulation of innate immunity. Here we review our work showing that exposure to a dysfunctional GR from early embryonic life in transgenic (Tg) mice expressing GR antisense RNA represents a key vulnerability factor in the response of nigrostriatal dopaminergic neurons to the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), and further report that exacerbation of dopaminergic neurotoxicity with no recovery is determined by failure of astroglia to exert neuroprotective effects. Aberrant iNOS gene expression and increased glia vulnerability to cell death characterized the response of GR-deficient mice to stimulation of innate immunity. More importantly, GR-deficient glial cells failed to protect fetal dopaminergic neurons against oxidative stress-induces cell death, whereas wild-type glia afforded neuroprotection. Thus, lack of iNOS/NO regulation by GCs can program an aberrant GR-NO crosstalk in turn responsible for loss of astroglia neuroprotective function in response to stimulation of innate immunity, pointing to glia and efficient GR-NO dialogue as pivotal factors orchestrating neuroprotection in experimental parkinsonism.

Differential expression of 11beta-hydroxysteroid dehydrogenase types 1 and 2 mRNA and glucocorticoid receptor protein during mouse embryonic development

Accumulating evidence suggests that the actions of glucocorticoids in target tissues are critically determined by the expression of not only the glucocorticoid receptor (GR) but also the glucocorticoid-metabolizing enzymes, known as 11beta-hydroxysteroid dehydrogenase types 1 and 2 (11beta-HSD1 and 11beta-HSD2). To gain insight into the role of glucocorticoids in fetal development, the expression patterns of the two distinct 11beta-HSD isozymes and GR were studied in the mouse embryo from embryonic day 12.5 (E12.5, term = E19) to postnatal day 0.5 (P0.5) by in situ hybridization and immunohistochemistry, respectively. 11beta-HSD1 mRNA was detected in the heart as early as E12.5 and maintained thereafter. In the lung and liver, 11beta-HSD1 mRNA was first detected between E14.5 and E16.5, increased to high levels towards term and maintained after birth. Relatively low levels of 11beta-HSD1 mRNA were also detected in the kidney, adrenal glands and gastrointestinal tract at E18.5. However, the mRNA for 11beta-HSD1 was undetectable in all other embryonic tissues including the brain. In contrast, kidney was the only organ that expressed appreciable levels of 11beta-HSD2 mRNA during embryonic life. The level of 11beta-HSD2 mRNA in the kidney increased dramatically in the newborn, which coincided with expression of 11beta-HSD2 mRNA in the whisker follicle, tooth and salivary gland. Distinct from the profiles of 11beta-HSD1 and 11beta-HSD2 mRNA, GR protein was detectable in all tissues at all ages studied except for the thymus, salivary gland, and bone. Taken together, the present study demonstrates that tissue- and developmentally-stage specific expression of 11beta-HSD1 and 11beta-HSD2 as well as GR occurs in the developing mouse embryo, thus highlighting the importance of these two enzymes and GR in regulating glucocorticoid-mediated maturational events in specific tissues during murine embryonic development.

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.

Glucocorticoid receptor deficiency increases vulnerability of the nigrostriatal dopaminergic system: critical role of glial nitric oxide

Glucocorticoids (GCs) exert via glucocorticoid receptors (GRs) potent anti-inflammatory and immunosuppressive effects. Emerging evidence indicates that an inflammatory process is involved in dopaminergic nigro-striatal neuronal loss in Parkinson's disease. We here report that the GR deficiency of transgenic (Tg) mice expressing GR antisense RNA from early embryonic life has a dramatic impact in "programming" the vulnerability of dopaminergic neurons to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). The GR deficiency of Tg mice exacerbates MPTP-induced toxicity to dopaminergic neurons, as revealed by both severe loss of tyrosine hydroxylase positive nigral neurons and sharp decreases in striatal levels of dopamine and its metabolites. In addition, the late increase in dopamine oxidative metabolism and ascorbic acid oxidative status in GR-deficient mice was far greater than in wild-type (Wt) mice. Inducible nitric oxide synthase (iNOS) was sharply increased in activated astrocytes, macrophages/microglia of GR-deficient as compared with Wt mice. Moreover, GR-deficient microglia produced three- to fourfold higher nitrite levels than Wt mice; these increases preceded the loss of dopaminergic function and were resistant to GR the inhibitory effect of GC, pointing to peroxynitrites as candidate neurotoxic effectors. The iNOS inhibitor N6-(1-iminoethyl)-L-lysine normalized vulnerability of Tg mice, thus establishing a novel link between genetic impairment of GR function and vulnerability to MPTP.

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.

Rat peripheral CD4+CD8+ T lymphocytes are partially immunocompetent thymus-derived cells that undergo post-thymic maturation to become functionally mature CD4+ T lymphocytes

CD4+CD8+ double-positive (DP) T cells represent a minor subpopulation of T lymphocytes found in the periphery of adult rats. In this study, we show that peripheral DP T cells appear among the first T cells that colonize the peripheral lymphoid organs during fetal life, and represent approximately 40% of peripheral T cells during the perinatal period. Later their proportion decreases to reach the low values seen in adulthood. Most DP T cells are small size lymphocytes that do not exhibit an activated phenotype, and their proliferative rate is similar to that of the other peripheral T cell subpopulations. Only 30-40% of DP T cells expresses CD8beta chain, the remaining cells expressing CD8alphaalpha homodimers. However, both DP T cell subsets have an intrathymic origin since they appear in the recent thymic emigrant population after injection of FITC intrathymically. Functionally, although DP T cells are resistant to undergo apoptosis in response to glucocorticoids, they show poor proliferative responses upon CD3/TCR stimulation due to their inability to produce IL-2. A fraction of DP T cells are not actively synthesizing the CD8 coreceptor, and they gradually differentiate to the CD4 cell lineage in reaggregation cultures. Transfer of DP T lymphocytes into thymectomized SCID mice demonstrates that these cells undergo post-thymic maturation in the peripheral lymphoid organs and that their CD4 cell progeny is fully immunocompetent, as judged by its ability to survive and expand in peripheral lymphoid organs, to proliferate in response to CD3 ligation, and to produce IL-2 upon stimulation.

Stress, the immune system and vulnerability to degenerative disorders of the central nervous system in transgenic mice expressing glucocorticoid receptor antisense RNA

Current research evidence suggests that interactions between genetic and environmental factors contribute to modulate the susceptibility to degenerative disorders, including inflammatory and autoimmune diseases of the central nervous system (CNS). In this context, bidirectional communication between the neuroendocrine and immune systems during ontogeny plays a pivotal role in programming the development of neuroendocrine and immune responses in adult life, thereby influencing the predisposition to several disease entities. Glucocorticoids (GCs), the end products of the hypothalamic-pituitary-adrenocortical (HPA) axis, gender and signals generated by hypothalamic-pituitary-gonadal (HPG) axis are major players coordinating the development of immune system function and exerting powerful effects in the susceptibility to autoimmune disorders, including experimental autoimmune encephalomyelitis (EAE), the experimental model for multiple sclerosis (MS). In particular, GCs exert their beneficial immunosuppressive and anti-inflammatory effects in inflammatory disorders of the CNS, after binding to their cytoplasmic receptors (GRs). Here we review our work using transgenic (Tg) mice with a dysfunctional GR from early embryonic life on programming vulnerability to EAE. The GR-deficiency of these Tg mice confers resistance to active EAE induction. The interplay between GCs, proinflammatory mediators, gender and EAE is summarized. On the basis of our data, it does appear that exposure to a defective GR through development programs major changes in endogenous neuroendocrine and immune mechanisms controlling the vulnerability to EAE. These studies highlight the plasticity of the HPA-immune axis and its pharmacological manipulation in autoimmune diseases of the CNS.

Neuroendocrine-immune (NEI) circuitry from neuron-glial interactions to function: Focus on gender and HPA-HPG interactions on early programming of the NEI system

Bidirectional communication between the neuroendocrine and immune systems during ontogeny plays a pivotal role in programming the development of neuroendocrine and immune responses in adult life. Signals generated by the hypothalamic-pituitary-gonadal axis (i.e. luteinizing hormone-releasing hormone, LHRH, and sex steroids), and by the hypothalamic-pituitary-adrenocortical axis (glucocorticoids (GC)), are major players coordinating the development of immune system function. Conversely, products generated by immune system activation exert a powerful and long-lasting regulation on neuroendocrine axes activity. The neuroendocrine-immune system is very sensitive to preperinatal experiences, including hormonal manipulations and immune challenges, which may influence the future predisposition to several disease entities. We review our work on the ongoing mutual regulation of neuroendocrine and immune cell activities, both at a cellular and molecular level. In the central nervous system, one chief compartment is represented by the astroglial cell and its mediators. Hence, neuron-glial signalling cascades dictate major changes in response to hormonal manipulations and pro-inflammatory triggers. The interplay between LHRH, sex steroids, GC and pro-inflammatory mediators in some physiological and pathological states, together with the potential clinical implications of these findings, are summarized. The overall study highlights the plasticity of this intersystem cross-talk for pharmacological targeting with drugs acting at the neuroendocrine-immune interface.

Role of glucocorticoids in early T-cell differentiation

The results of the T-cell differentiation in the progeny of adrenalectomized pregnant rats (Adx fetuses), an experimental model that ensures the absence of glucocorticoids (GCs) during the first stages of development, are summarized. In Adx thymuses there is an accelerated maturation of thymocytes that is reversed by in vivo GC replacement. In addition, Adx thymuses show decreased cell content, which correlates with both the increased numbers of apoptotic cells and an early migration of DP (CD4+CD8+) and SP (both CD4+CD8- and CD4-CD8+) thymocytes to the spleen. As shown by in vitro recolonization assays, accelerated T-cell differentiation is a consequence of changes in the biology of lymphoid precursors occurring in the fetal liver of Adx fetuses. They arrive at the thymic primordium earlier and mature faster than the fetal liver lymphoid progenitors from Sham control fetuses. After the establishment of a fetal hypothalamus-pituitary-gland-adrenal-gland (HPA) axis, there is a gradual normalization of the T-cell development Adx fetuses.

Gender, neuroendocrine-immune interactions and neuron-glial plasticity. Role of luteinizing hormone-releasing hormone (LHRH)

Signals generated by the hypothalamic-pitutary-gonadal (HPG) axis powerfully modulate immune system function. This article summarizes some aspects of the impact of gender in neuroendocrine immunomodulation. Emphasis is given to the astroglial cell compartment, defined as a key actor in neuroendocrine immune communications. In the brain, the principal hormones of the HPG axis directly interact with astroglial cells. Thus, luteinizing hormone releasing hormone, LHRH, influences hypothalamic astrocyte development and growth, and hypothalamic astrocytes direct LHRH neuron differentiation. Hormonally induced changes in neuron-glial plasticity may dictate major changes in CNS output, and thus actively participate in sex dimorphic immune responses. The impact of gender in neuroimmunomodulation is further underlined by the sex dimorphism in the expression of genes encoding for neuroendocrine hormones and their receptors within the thymus, and by the potent modulation exerted by circulating sex steroids during development and immunization. The central role of glucocorticoids in the interactive communication between neuroendocrine and immune systems, and the impact of gender on hypothalamic-pituitary-adrenocortical (HPA) axis modulation is underscored in transgenic mice expressing a glucocorticoid receptor antisense RNA.

Stress-free T-cell development: glucocorticoids are not obligatory

A role for glucocorticoids in thymopoiesis has been suggested by studies using glucocorticoid receptor (GR) anti-sense transgenic mice, glucocorticoid synthesis inhibitors and GR antagonists. Unfortunately, no consensus has been reached on exactly how glucocorticoids influence T-cell development. The most recent approach, using GR knockout (GR(-/-)) mice, indicates that GR signaling is, in fact, dispensable in this entire process.

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.

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

Glucocorticoids are believed to play a role in T cell development and selection, although their precise function is controversial. Glucocorticoid receptor (GR)-deficient mice were used to directly investigate this problem. GR-deficient thymocytes were resistant to dexamethasone-mediated apoptosis, confirming the absence of glucocorticoid responsiveness. An absence of GR signaling had no impact on thymocyte development either in vivo or in vitro. T cell differentiation, including positive selection, was normal as assessed by normal development of CD4+CD8+, alphabetaTCR+CD4+, and alphabetaTCR+CD8+ thymocytes. Negative selection, mediated by the superantigen staphylococcal enterotoxin B (SEB), or anti-CD3/CD28, was also normal in the absence of GR signaling. In contrast to earlier reports, these data demonstrate that GR signaling is not essential for intrathymic T cell development or selection.