The thymus as a neuroendocrine organ. Synthesis of vasopressin and oxytocin in human thymic epithelium

History of the Thymus: From a Vestigial Organ to the Programming of Immunological Self-Tolerance

This introductive chapter presents the most important disruptions of concepts concerning the thymus since its discovery in Antique Greece. For centuries, the thymus was considered as a vestigial organ, and its role in T-cell differentiation was proposed only in the 1960s. Most recent studies attribute to the thymus an essential and unique role in programming central immunological self-tolerance. The basic mechanism implicated in this function is the transcription in the thymic epithelium of genes encoding precursors of neuroendocrine-related and tissue-restricted self-peptides. Their processing leads to the presentation of self-antigens by the major histocompatibility complex (MHC) machinery expressed by thymic epithelial and dendritic cells. Already during foetal life, this presentation promotes negative selection of T lymphocytes harbouring a receptor with high affinity for MHC/self-peptide complexes. Mainly after birth, this presentation also drives the generation of regulatory T cells specific for these complexes. Numerous studies, as well as the identification of Aire and Fezf2 genes, have shown that a thymus defect plays a crucial role in the development of autoimmunity. The discovery of the central tolerogenic action of the thymus revolutionized the whole field of immunology, and such knowledge will pave the way for innovative tolerogenic therapies against autoimmunity, the so heavy tribute paid by mankind for the extreme diversity and efficiency of adaptive immunity.

Thymus as Incontrovertible Target of Future Immune Modulatory Therapeutics

Thymus plays a crucial role in cellular immunity by acting as a warehouse for proliferating and differentiating lymphocytes. Thymic stromal cells educate T-cells to differentiate self from non-self antigens while nurse cells and thymoproteasome play a major role in the maturation and differentiation of T-cells. The thymic conditions dictate T-cells to cope with the risk of cancer development. A study was designed to demonstrate potential mechanisms behind the failure to eliminate tumors and impaired immune surveillance as well as the impact of delay in thymus regression on cancer and autoimmune disorders. Scientific literature from Pubmed; Scopus; WOS; JSTOR; National Library of Medicine Bethesda, Maryland; The New York Academy of Medicine; Library of Speech Rehabilitation, NY; St. Thomas' Hospital Library; The Wills Library of Guys Hospital; Repository of Kings College London; and Oxford Academic repository was explored for pathological, physiological, immunological and toxicological studies of thymus. Studies have shown that systemic chemotherapy may lead to micro inflammatory environment within thymus where conventionally and dynamically metastasized dormant cells seek refuge. The malfunctioning of the thymus and defective T and Treg cells, bypassing negative selection, contributes to autoimmune disorders, while AIRE and Fezf2 play significant roles in thymic epithelial cell solidity. Different vitamins, TCM, and live cell therapy are effective therapeutics. Vitamin A, C, D, and E, selenium and zinc, cinobufagin and dietary polysaccharides, and glandular extracts and live cell injections have strong potential to restore immune system function and thymus health. Moreover, the relationship between different ages/ stages of thymus and their corresponding T-cell mediated anti-tumor immune response needs further exploration.

Oxytocin Modulates Osteogenic Commitment in Human Adipose-Derived Stem Cells

Human adipose-derived stem cells (hASCs) are commonly harvested in minimally invasive contexts with few ethical concerns, and exhibit self-renewal, multi-lineage differentiation, and trophic signaling that make them attractive candidates for cell therapy approaches. The identification of natural molecules that can modulate their biological properties is a challenge for many researchers. Oxytocin (OXT) is a neurohypophyseal hormone that plays a pivotal role in the regulation of mammalian behavior, and is involved in health and well-being processes. Here, we investigated the role of OXT on hASC proliferation, migratory ability, senescence, and autophagy after a treatment of 72 h; OXT did not affect hASC proliferation and migratory ability. Moreover, we observed an increase in SA-β-galactosidase activity, probably related to the promotion of the autophagic process. In addition, the effects of OXT were evaluated on the hASC differentiation ability; OXT promoted osteogenic differentiation in a dose-dependent manner, as demonstrated by Alizarin red staining and gene/protein expression analysis, while it did not affect or reduce adipogenic differentiation. We also observed an increase in the expression of autophagy marker genes at the beginning of the osteogenic process in OXT-treated hASCs, leading us to hypothesize that OXT could promote osteogenesis in hASCs by modulating the autophagic process.

Oxytocin promotes hepatic regeneration in elderly mice

Liver aging impairs the ability of hepatocyte regeneration. Recent studies have found that oxytocin (OT) plays an important role in promoting tissue repair and maintaining differentiation and regeneration of stem cells. Here, we reported that OT receptors, which are specifically located in hepatocytes, decrease with aging in human and mice. Interestingly, the level of serum OT also decline with age. Notably, OT promotes hepatocyte regeneration only in aged mice but not in young mice in vitro and in vivo. Further studies reveal that OT promotes autophagy in either AML12 mouse hepatocytes or aged mice after partial hepatectomy or with CCl4-induced acute liver injury. In conclusion, OT promotes liver regeneration, especially in aged mice, which may be achieved by promoting autophagy. All these results support the possibility of OT and its analog being a potent anti-aging drug and promote liver rejuvenation.

The thymus and the science of self

The conventional perception asserts that immunology is the science of ‘discrimination’ between self and non-self. This concept is however no longer tenable as effector cells of the adaptive immune system are first conditioned to be tolerant to the body’s own antigens, collectively known as self until now. Only then attain these effectors the responsiveness to non-self. The acquisition of this essential state of tolerance to self occurs for T cells in the thymus, the last major organ of our body that revealed its intricate function in health and disease. The ‘thymus’ as an anatomical notion was first notably documented in Ancient Greece although our present understanding of the organ’s functions was only deciphered commencing in the 1960s. In the late 1980s, the thymus was identified as the site where clones of cells reactive to self, termed ‘forbidden’ thymocytes, are physically depleted as the result of a process now known as negative selection. The recognition of this mechanism further contributed to the belief that the central rationale of immunology as a science lies in the distinction between self and non-self. This review will discuss the evidence that the thymus serves as a unique lymphoid organ able to instruct T cells to recognize and be tolerant to harmless self before adopting the capacity to defend the body against potentially injurious non-self-antigens presented in the context of different challenges from infections to exposure to malignant cells. The emerging insight into the thymus’ cardinal functions now also provides an opportunity to exploit this knowledge to develop novel strategies that specifically prevent or even treat organ-specific autoimmune diseases.

The presentation of neuroendocrine self-peptides in the thymus: an essential event for individual life and vertebrate survival

Confirming Burnet's early hypothesis, elimination of self‐reactive T cells in the thymus was demonstrated in the late 1980s, and an important question immediately arose about the nature of the self‐peptides expressed in the thymus. Many genes encoding neuroendocrine‐related and tissue‐restricted antigens (TRAs) are transcribed in thymic epithelial cells (TECs). They are then processed for presentation by proteins of the major histocompatibility complex (MHC) expressed by TECs and thymic dendritic cells. MHC presentation of self‐peptides in the thymus programs self‐tolerance by two complementary mechanisms: (1) negative selection of self‐reactive “forbidden” T cell clones starting already in fetal life, and (2) generation of self‐specific thymic regulatory T lymphocytes (tT_reg cells), mainly after birth. Many studies, including the discovery of the transcription factors autoimmune regulator (AIRE) and fasciculation and elongation protein zeta family zinc finger (FEZF2), have shown that a defect in thymus central self‐tolerance is the earliest event promoting autoimmunity. AIRE and FEZF2 control the level of transcription of many neuroendocrine self‐peptides and TRAs in the thymic epithelium. Furthermore, AIRE and FEZF2 mutations are associated with the development of autoimmunity in peripheral organs. The discovery of the intrathymic presentation of self‐peptides has revolutionized our knowledge of immunology and is opening novel avenues for prevention/treatment of autoimmunity.

Oxytocin in metabolic homeostasis: implications for obesity and diabetes management

Oxytocin was once understood solely as a neuropeptide with a central role in social bonding, reproduction, parturition, lactation and appetite regulation. Recent evidence indicates that oxytocin enhances glucose uptake and lipid utilization in adipose tissue and skeletal muscle, suggesting that dysfunction of the oxytocin system could underlie the pathogenesis of insulin resistance and dyslipidaemia. Murine studies revealed that deficiencies in oxytocin signalling and oxytocin receptor expression lead to obesity despite normal food intake, motor activity and increased leptin levels. In addition, plasma oxytocin concentration is notably lower in obese individuals with diabetes, which may suggest an involvement of the oxytocin system in the pathogenesis of cardiometabolic disease. More recently, small scale studies demonstrated that intranasal administration of oxytocin was associated with significant weight loss as well as improvements in insulin sensitivity and pancreatic β-cell responsivity in human subjects. The multi-pronged effects of oxytocin signalling on improving peripheral insulin sensitivity, pancreatic function and lipid homeostasis strongly suggest a role for this system as a therapeutic target in obesity and diabetes management. The complexity of obesity aetiology and the pathogenesis of obesity-related metabolic complications underscore the need for a systems approach to better understand the role of oxytocin in metabolic function.

Expression and developmental regulation of oxytocin (OT) and oxytocin receptors (OTR) in the enteric nervous system (ENS) and intestinal epithelium

Although oxytocin (OT) and oxytocin receptor (OTR) are known for roles in parturition and milk let-down, they are not hypothalamus-restricted. OT is important in nurturing and opposition to stress. Transcripts encoding OT and OTR have been reported in adult human gut, and OT affects intestinal motility. We tested the hypotheses that OT is endogenous to the enteric nervous system (ENS) and that OTR signaling may participate in enteric neurophysiology. Reverse transcriptase polymerase chain reaction confirmed OT and OTR transcripts in adult mouse and rat gut and in precursors of enteric neurons immunoselected from fetal rats. Enteric OT and OTR expression continued through adulthood but was developmentally regulated, peaking at postnatal day 7. Coincidence of the immunoreactivities of OTR and the neural marker Hu was 100% in the P3 and 71% in the adult myenteric plexus, when submucosal neurons were also OTR-immunoreactive. Co-localization with NeuN established that intrinsic primary afferent neurons are OTR-expressing. Because OTR transcripts and protein were detected in the nodose ganglia, OT signaling might also affect extrinsic primary afferent neurons. Although OT immunoreactivity was found only in approximately 1% of myenteric neurons, extensive OT-immunoreactive varicosities surrounded many others. Villus enterocytes were OTR-immunoreactive through postnatal day 17; however, by postnatal day 19, immunoreactivity waned to become restricted to crypts and concentrated at crypt-villus junctions. Immunoelectron microscopy revealed plasmalemmal OTR at enterocyte adherens junctions. We suggest that OT and OTR signaling might be important in ENS development and function and might play roles in visceral sensory perception and neural modulation of epithelial biology.

Thymus-Dependent T Cell Tolerance of Neuroendocrine Functions: Principles, Reflections, and Implications for Tolerogenic/Negative Self-Vaccination

Under the evolutionary pressure exerted by the emergence of adaptive immunity and its inherent risk of horror autotoxicus, the thymus appeared some 500 million years ago as a novel lymphoid structure able to prevent autoimmunity and to orchestrate self-tolerance as a cornerstone in the physiology of the immune system. Also, the thymus plays a prominent role in T cell education to neuroendocrine principles. Some self-antigens (oxytocin, neurotensin, insulin-like growth factor 2 [IGF-2]) have been selected to be predominantly expressed in thymic epithelium and to be presented to thymus T cells for educating them to tolerate other antigens related to them. In the insulin family, IGF2 is dominantly transcribed in cortical (c) and medullary (m) thymic epithelial cells (TECs), whereas the insulin gene (INS) is expressed at low level by only a few subsets of mTECs. Intrathymic transcription of both IGF2 and INS is under the control of the autoimmune regulator (Aire) gene. The highest concentrations of IGF-2 in the thymus explain why this peptide is much more tolerated than insulin, and why tolerance to IGF-2 is so difficult to break by active immunization. The high level of tolerance to IGF-2 is correlated to the development of a tolerogenic/regulatory profile when the sequence B11-25 of IGF-2 (homologous to the autoantigen insulin B9-23) is presented to DQ8+ type 1 diabetic patients. Since subcutaneous and oral insulin does not exert any tolerogenic properties, IGF-2 and other thymus self-antigens related to type 1 diabetes (T1D) should be preferred to insulin for the design of novel specific antigen-based preventive approaches against T1D.

The peptide molecular links between the central nervous and the immune systems

The central nervous system (CNS) and the immune system were for many years considered as two autonomous systems. Now, the reciprocal connections between them are generally recognized and very well documented. The links are realized mainly by various immuno- and neuropeptides. In the review the influence of the following immunopeptides on CNS is presented: tuftsin, thymulin, thymopoietin and thymopentin, thymosins, and thymic humoral factor. On the other side, the activity in the immune system of such neuropeptides as substance P, neurotensin, some neurokinins, enkephalins, and endorphins is discussed.

Thymic transcription of neurohypophysial and insulin-related genes: impact upon T-cell differentiation and self-tolerance

The thymus is the unique lymphoid organ responsible for the generation of a diverse repertoire of T lymphocytes that are competent against non self-antigens while being tolerant to self-antigens. A vast repertoire of neuroendocrine-related genes is transcribed in the nonlymphoid cellular compartment of the thymus (thymic epithelial cells, dendritic cells and macrophages). The precursors encoded by these genes engage two types of interactions with developing T cells (thymocytes). First, they are not processed in a classical neuroendocrine way but as the source of self-antigens that are presented to pre-T cells by the major histocompatibility complex proteins of the thymus. This presentation could be responsible for the establishment of central T-cell self-tolerance to neuroendocrine functions. Second, they also deliver signal ligands that are able to bind to neuroendocrine-type receptors expressed by thymocytes. This interaction activates several types of intracellular signalling pathways implicated in the developmental process of T lymphocytes. Several experimental arguments support a role for thymic dysfunction as a crucial factor in the development of organ-specific autoimmune endocrinopathies, such as 'idiopathic' central diabetes insipidus and type 1 diabetes mellitus. The rational use of tolerogenic neuroendocrine self-antigens for the prevention/treatment of autoimmune endocrinopathies is currently under investigation.

Upregulation of receptor activator of nuclear factor-kappaB ligand expression in the thymic subcapsular, paraseptal, perivascular, and medullary epithelial cells during thymus regeneration

The receptor activator of nuclear factor (NF)-kappaB ligand (RANKL; also termed TRANCE/OPGL/ODF/TNFSF11), a new member of the tumor-necrosis factor (TNF) superfamily, was identified as a key cytokine involved in the differentiation of the immune system and the regulation of immunity as well as in bone metabolism. In particular, RANKL-deficient mice showed defects in the early differentiation of T lymphocytes, suggesting that RANKL is a novel regulator of early thymocyte development. Here, we describe the expression of RANKL during regeneration following acute involution induced by cyclophosphamide in the rat thymus. The present study demonstrates the presence and upregulated expression of the RANKL in thymic subcapsular, paraseptal, perivascular, and medullary epithelial cells during thymus regeneration. Our results suggest that the RANKL expressed in these thymic epithelial cells plays a role in the development of T cells during thymic regeneration.

The significance of vasoactive intestinal peptide in immunomodulation

First identified by Said and Mutt some 30 years ago, the vasoactive intestinal peptide (VIP) was originally isolated as a vasodilator peptide. Subsequently, its biochemistry was elucidated, and within the 1st decade, their signature features as a neuropeptide became consolidated. It did not take long for these insights to permeate the field of immunology, out of which surprising new attributes for VIP were found in the last years. VIP is rapidly transforming into something more than a mere hormone. In evolving scientifically from a hormone to a novel agent for modifying immune function and possibly a cytokine-like molecule, VIP research has engaged many physiologists, molecular biologists, biochemists, endocrinologists, and pharmacologists and it is a paradigm to explore mutual interactions between neural and neuroendocrine links in health and disease. The aim of this review is firstly to update our knowledge of the cellular and molecular events relevant to VIP function on the immune system and secondly to gather together recent data that support its role as a type 2 cytokine. Recognition of the central functions VIP plays in cellular processes is focusing our attention on this "very important peptide" as exciting new candidates for therapeutic intervention and drug development.

Corticotropin-releasing hormone immunoreactivity in human T and B cells and macrophages: colocalization with arginine vasopressin

Corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) are expressed in cells of the immune system where they exert immunomodulatory roles, but these neuropeptides are poorly characterized in human immune tissues. The aim of this study was to determine concentrations and distribution of CRH and AVP in nonactivated human peripheral blood mononuclear cells (PBMC). PBMC from normal human subjects were separated into enriched subpopulations of T and B cells and monocytes/macrophages by a magnetic bead/monoclonal antibody technique. CRH and AVP were measured in cell extracts by radioimmunoassay (RIA). CRH-immunoreactivity (ir) ranged 0.24-0.8 fmol/million cells (n = 6 subjects) in T cell extracts, 0.4-2.7 fmol/million cells (n = 4) in B cells and 0.63-2.16 fmol/million cells (n = 4) in macrophages. AVP-ir ranged 0.2-0.95 fmol/million cells in T cell extracts, <0.1-0.8 fmol/million cells in B cells and 0.14-3.19 fmol/million cells in macrophages. Reversed-phase high-performance liquid chromatography (HPLC) of T and B cell extracts revealed a peak of CRH-ir which coeluted with synthetic CRH-41; this peak was not present in macrophages. A second peak of CRH-ir which eluted in a more hydrophobic position was observed in extracts of T and B cells and macrophages. This unidentified form of CRH-ir is the predominant form of CRH-ir in nonactivated human PBMC. This is the first study to demonstrate that CRH-ir and AVP-ir are colocalized within human T cells, B cells and monocytes/macrophages. We have confirmed observations of a variant form of CRH-ir in human PBMC and show that this is the predominant form in macrophages and B cells whereas CRH-ir, which coelutes with CRH(1-41) on HPLC, is present in significant amounts only in T cells. These data also confirm that CRH-ir in human PBMC is not urocortin because the antiserum used in the CRH RIA does not bind to urocortin.

Upregulation of TrkA neurotrophin receptor expression in the thymic subcapsular, paraseptal, perivascular, and cortical epithelial cells during thymus regeneration

Neuroimmune networks in the thymic microenvironment are thought to be involved in the regulation of T cell development. Here, we report upon an examination of the expression of the TrkA neurotrophin receptor, the high affinity receptor for nerve growth factor, during regeneration following acute involution induced by cyclophosphamide in the rat thymus. Light and electron microscopic immunocytochemistry demonstrated enhanced expression of the TrkA receptor in the subcapsular, paraseptal, perivascular, and cortical epithelial cells during thymus regeneration. In addition, various morphological alterations, suggestive of a hyperfunctional and dynamic state, of the subcapsular, paraseptal, and perivascular epithelial cells were also observed. The presence of TrkA protein in extracts from the control and regenerating rat thymus was confirmed by western blot. Furthermore, RT-PCR analysis supported these results by demonstrating that thymic extracts contain TrkA mRNA at higher levels during thymus regeneration. Thus, our results suggest that the TrkA receptor located on the thymic subcapsular, paraseptal, perivascular, and cortical epithelial cells could play a role in the development of new T cells to replace T cells damaged during thymus regeneration.

The oxytocin receptor system: structure, function, and regulation

The neurohypophysial peptide oxytocin (OT) and OT-like hormones facilitate reproduction in all vertebrates at several levels. The major site of OT gene expression is the magnocellular neurons of the hypothalamic paraventricular and supraoptic nuclei. In response to a variety of stimuli such as suckling, parturition, or certain kinds of stress, the processed OT peptide is released from the posterior pituitary into the systemic circulation. Such stimuli also lead to an intranuclear release of OT. Moreover, oxytocinergic neurons display widespread projections throughout the central nervous system. However, OT is also synthesized in peripheral tissues, e.g., uterus, placenta, amnion, corpus luteum, testis, and heart. The OT receptor is a typical class I G protein-coupled receptor that is primarily coupled via G(q) proteins to phospholipase C-beta. The high-affinity receptor state requires both Mg(2+) and cholesterol, which probably function as allosteric modulators. The agonist-binding region of the receptor has been characterized by mutagenesis and molecular modeling and is different from the antagonist binding site. The function and physiological regulation of the OT system is strongly steroid dependent. However, this is, unexpectedly, only partially reflected by the promoter sequences in the OT receptor gene. The classical actions of OT are stimulation of uterine smooth muscle contraction during labor and milk ejection during lactation. While the essential role of OT for the milk let-down reflex has been confirmed in OT-deficient mice, OT's role in parturition is obviously more complex. Before the onset of labor, uterine sensitivity to OT markedly increases concomitant with a strong upregulation of OT receptors in the myometrium and, to a lesser extent, in the decidua where OT stimulates the release of PGF(2 alpha). Experiments with transgenic mice suggest that OT acts as a luteotrophic hormone opposing the luteolytic action of PGF(2 alpha). Thus, to initiate labor, it might be essential to generate sufficient PGF(2 alpha) to overcome the luteotrophic action of OT in late gestation. OT also plays an important role in many other reproduction-related functions, such as control of the estrous cycle length, follicle luteinization in the ovary, and ovarian steroidogenesis. In the male, OT is a potent stimulator of spontaneous erections in rats and is involved in ejaculation. OT receptors have also been identified in other tissues, including the kidney, heart, thymus, pancreas, and adipocytes. For example, in the rat, OT is a cardiovascular hormone acting in concert with atrial natriuretic peptide to induce natriuresis and kaliuresis. The central actions of OT range from the modulation of the neuroendocrine reflexes to the establishment of complex social and bonding behaviors related to the reproduction and care of the offspring. OT exerts potent antistress effects that may facilitate pair bonds. Overall, the regulation by gonadal and adrenal steroids is one of the most remarkable features of the OT system and is, unfortunately, the least understood. One has to conclude that the physiological regulation of the OT system will remain puzzling as long as the molecular mechanisms of genomic and nongenomic actions of steroids have not been clarified.

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.

Antisense inhibition of pro-opiomelanocortin and proenkephalin A messenger RNA translation alters rat immune cell function in vitro

Pro-opiomelanocortin (POMC) and proenkephalin A (PEA) antisense oligodeoxynucleotides respectively reduced and enhanced proliferation of rat splenocytes incubated with concanavalin A in vitro. Nonsense base sequences used as controls were without effect. Coincubation with the exogenous synthetic opioid peptides, ACTH, beta-endorphin, met-enkephalin or [D-ala,D-leu]-enkephalin did not significantly alter either the POMC or PEA antisense response, indicating potential differences in bioactivity of immunocyte opioid peptides compared with synthetic equivalents. Levels of the POMC opioid products, ACTH and beta-endorphin, were significantly reduced in splenocytes incubated with POMC antisense probes. These data provide evidence for functional effects of endogenous opioid peptides on rat splenocyte proliferation in vitro.

Regulation of VIP production and secretion by murine lymphocytes

Vasoactive intestinal peptide (VIP) is a neuropeptide present in the lymphoid microenvironment with a multiplicity of actions. Two sources for VIP have been described in the immune system, the terminals present in central and peripheral lymphoid organs and the immune cells. Although VIP is synthesized by lymphocytes, there is no evidence demonstrating that VIP is released, and which stimuli are able to induce VIP production and secretion. In this study, we demonstrated for the first time, that agents that mediate important immune functions, such as proliferation and antigenic stimulation (Con A, LPS, and anti-TCR antibody), inflammation (LPS, TNFalpha, IL-6 and IL-1beta) or apoptosis (dexamethasone) induce the production and release of VIP to the lymphoid microenvironment. We conclude that VIP is produced and secreted by lymphocytes and propose that during an immune response, the timely release of VIP within the lymphoid organs and peritoneum should influence the differentiation and/or downregulation of the ongoing response.

Aberrant expression of tissue-specific proteins in the thymus: a hypothesis for the development of central tolerance

Herein we present the case for the existence of a thymic cortical epithelial cell that possesses an unusual gene transcription. It produces tissue-specific proteins that have their usual physiological functions outside the thymus and presents them, as well as household proteins, to the differentiating thymocytes. We suggest that this specialized cell enforces tolerance to most self-proteins by causing release of a signal for programmed cell death to thymocytes that express receptors for these self-antigens.

Heterogeneity of epithelial cells in the rat thymus

The morphological heterogeneity of the thymic epithelium has been well documented both at the light and electron microscopic level. Immunohistochemistry has revealed four broad classes of epithelial cells (EC): subcapsule/perivascular, cortical, medullary EC, and medullary Hassall's corpuscles. Ultrastructural analysis has revealed further heterogeneity. In the cortex, four EC subtypes have been described ultrastructurally: subcapsular/perivascular, "pale," "intermediate," and "dark" EC. These subtypes are also present in the medulla. Two additional EC subtypes are restricted to the medulla: an undifferentiated subtype, and a subtype displaying signs of high metabolic activity. Based on the morphological features of the epithelium, it has been hypothetized that the thymic EC subtypes represent a process of differentiation.

Evolution of neuroendocrine thymus: studies on POMC-derived peptides, cytokines and apoptosis in lower and higher vertebrates

In previous papers, we showed that neuroendocrine cells reactive to anti-POMC-derived peptides and cytokines are present in the thymus of a fish and an anuran amphibian. Here we report that this phenomenon is general, as neuroendocrine cells positive to anti-POMC-derived peptides (ACTH, beta-endorphin, alpha-MSH) and cytokines (IL-1 alpha, IL-2, IL-6, TNF-alpha) are also present in the thymus of chicken and rat. However, the number and the intrathymic localization and distribution of these cells varies in the different species examined. An analysis of apoptotic cells or cells involved in apoptosis, such as interdigitating cells and macrophages, in fish, frog, chicken and rat thymus, using an immunocytochemical method and anti-DNA mAb conjugated with peroxidase (anti-DNA-POD), showed that cells positive to anti-DNA-POD mAb are present in the same thymic areas in which POMC-derived peptides and cytokines were found. In conclusion, these data on apoptotic cells in the thymus of lower and higher vertebrates are compatible with the hypothesis that neuroendocrine cells might play a role in the selection and apoptosis of thymic lymphocytes, a phenomenon which could vary slightly in different species and taxa.

Cytokine production by human thymic epithelial cells: control by the immune recognition of the neurohypophysial self-antigen

Oxytocin (OT) has been shown to be the dominant peptide of the neurohypophysial family expressed by thymic epithelial and nurse cells (TEC/TNC) in various species. Thymic OT is not secreted but, after translocation of a hybrid neurophysin/MHC class I protein, is integrated within the plasma membrane of TEC, thus allowing its presentation to pre-T cells. In order to further demonstrate that thymic OT behaves like a membrane antigen, we assessed the effect of mAbs to OT on cytokine productions by cultures enriched in human TEC. 75-85% pure TEC cultures were prepared from human thymic fragments. Using immunofluorescence and confocal microscopy, ir-OT, ir-interleukin-1 beta (IL-1 beta), ir-interleukin-6 (IL-6) and ir-leukemia inhibitory factor (LIF) could be detected in these TEC cultures. ir-OT was restricted to TEC, while some ir-IL-6 and ir-LIF were also seen in occasional fibroblasts. In basal conditions, ir-IL-6 and ir-LIF (but not ir-OT and ir-IL-1 beta) were detected in the supernatants of human TEC cultures. MAbs to OT induced a marked increase of ir-IL-6 and ir-LIF secretion in TEC cultures. No significant effect was observed using mAbs against vasopressin, mouse immunoglobulins, or control ascitic fluid controls. These data show that OT is fully processed and recognized by specific mAbs at the outer surface of TEC plasma membrane. They further support that thymic OT behaves as the self-antigen of the neurohypophysial family.

Social support and immune status during and after chemotherapy for breast cancer

Social support and immune status were assessed in women treated with adjuvant chemotherapy for breast cancer. Perception of enhanced attachment was associated with an increased number of white blood cell levels three months after, but not during, chemotherapy. After treatment, patients with high attachment ratings had higher numbers and proportions of granulocytes, and lower proportions of lymphocytes and monocytes. It is concluded that the support experienced by a cancer patient can be associated with counts and proportions of leukocytes, but that this effect, if present during chemotherapy, is overridden by the biological factor that affects the haematopoetic process.

Neuroendocrineimmunology (NEI) at the turn of the century: towards a molecular understanding of basic mechanisms and implications for reproductive physiopathology

The interactions between the nervous, endocrine and immune systems require a complex communication network. The central nervous system (CNS) affects the immune system through endocrine, paracrine and neuronal mechanisms. Evidence that this bidirectional communication plays a vital role in the regulation of physiological homeostatic mechanisms while a disfunction of the neuroendocrineimmune balance favors the susceptibility to a number of diseases is derived largely by animal models but also by an increasing number of clinical studies in different fields, including endocrinology, reproductive physiology, pediatrics, oncology, neurology and psychiatry. An increasing number of endocrine hormones, neurotransmitters and neuropeptides are expressed in immune tissues and cells and are actively involved in the physiological regulation of immunity. Conversely, the endocrine and nervous systems harbor receptors for a wide variety of immunologically-derived substances, suggesting potential regulatory feedback loops between the three major integrative bodily systems. Major implications for the reproductive endocrinology field are that psychoneuroendocrine processes may alter fertility via immunomodulation, and that events that occur as part of immune responses influence the neuroendocrine axes, which in turn counter-regulate immune function. In the present article, some features of reproductive-immune interactions will be described, and the neuroendocrineimmune dialogue via the chief reproductive hormone, luteinizing hormone-releasing hormone (LHRH), will be summarized as prototype of intersystem crosstalk. A particular emphasis will be given to the cytokine-LHRH interrelationships both at central (i.e. especially with the astroglial compartment) and peripheral levels. The surprisingly similar communication network systems used by the gonads and the thymus will be summarized, and the sexually-driven dimorphisms dictating female versus male reproductive and immunological capacities reviewed. Evidence that neural, endocrine and immune systems work together as a single unit are emphasized in animal models and human pathologies where interruption of NEI feedback loops results in long lasting pathological consequences for the nervous, endocrine and immune functions.

Opposite changes in the content of oxytocin- and vasopressin-like immunoreactive peptides in the rat thymus during aging

The content of oxytocin- and vasopressin-like immunoreactive (IR) peptides was measured in the thymic extract of 2, 5, 10, 15 and 20 month-old rats by radioimmunoassay before or after fractionation by high-pressure liquid chromatography. In both cases the content of the oxytocin-like IR peptide, which behaved like authentic oxytocin in the chromatography column, increased during aging. Compared to 2 month-old rats a significant 30% increase was observed in 5 month-old rats, whereas the maximal increase (200%) was found in 20 month-old rats. In contrast, the content of the vasopressin-like IR peptide, which behaved like authentic arg8-vasopressin in the chromatography column, decreased during aging. The decrease (30%) was evident in 5 month-old rats, and was maximal (80%) in 15 month-old rats. The present results suggest that the mechanisms regulating the content of oxytocin- and vasopressin-like IR peptides in the rat thymus undergo differential changes during aging. These processes might be linked to thymic involution.

Thymic vasopressin (AVP) transgene expression in rats: a model for the study of thymic AVP hyper-expression in T cell differentiation

The peptide arginine vasopressin (AVP) is present within tissues of the immune system and has been implicated in T cell differentiation. We have investigated the expression and production of AVP in the thymus of rats which carry a rat AVP transgene. A 100% increase in thymic AVP immunoreactivity (ir) was detected in transgenic (TG) animals compared to age-matched wild-type (WT) controls. When tissues from TG and WT thymuses were subjected to reversed-phase high-performance liquid chromatography, ir-AVP eluted as a single peak which co-eluted with the standard. Immunocytochemical staining identified the presence of AVP in large epithelial cells within the thymic cortex in both WT and TG animals. The AVP precursor product neurophysin was also detected in epithelial cells in WT and TG thymuses. In situ hybridisation histochemistry using a probe specific for transgenic AVP mRNA revealed that the AVP transgene was expressed in TG thymic cells with a similar morphology and distribution to those which expressed endogenous AVP peptide in WT animals. These results demonstrate that the cellular location and immunoreactive form of AVP expressed in TG animals are similar to that found in WT controls. Thus the TG rat appears to be a model of true physiological, rather than ectopic, over-expression of AVP in the thymus. The hyper-expression of AVP in the thymic epithelial cells of TG animals provides a model in which can be studied the influence of AVP on T cell development and differentiation within the thymus.

Interleukin-1 immunoreactive nerves in heterotopic bone induced by DBM

The occurrence of interleukin-1-positive nerves was investigated by immunohistochemistry in developing heterotopic bone, induced by demineralized allogeneic bone matrix (DBM) in the rat. Interleukin-1 immunoreactivity was observed 1 week after implantation and remained until the end of the experiment at 12 weeks. Immunoreactive material was first identified in mononuclear cells at day 7. Interleukin-1 immunoreactive nerve fibers were first observed in the fibrous tissue at 2 weeks after implantation. A maximum density of fibers was reached at 8 weeks. Abundant immunofluorescent fibers were observed in the marrow tissue of the ossicles, and also in the surrounding fibrous tissue. A substantial number were vascular, but in the bone marrow most of the nerve fibers appeared as irregularly arranged, non-vascular terminals with ramifications and varicosities, intermingled between the marrow cells. No fibers could be detected in the proper bone tissue. The distribution of interleukin-1-positive nerves in the ossicles strongly resembled that previously observed in rat long bones. Moreover, the shape and distribution of the fibers exhibited a striking similarity to that of noradrenergic fibers identified previously both in ossicles and normal rat long bones. The late occurrence and predominant distribution in marrow tissue would seem to imply that neuronal interleukin-1 does not participate in the early differentiation of bone cells. The most important finding seems to be the presence of interleukin-1-positive nerve terminals in blood vessel walls and amidst marrow cells.

Changes in ACTH and beta-endorphin immunoreactivity in immune tissues during a chronic inflammatory stress are not correlated with changes in corticotropin-releasing hormone and arginine vasopressin

We have previously demonstrated that the chronic inflammatory stress of adjuvant-induced arthritis in rats can alter levels of the neuropeptides adrenocorticotropin (ACTH), beta-endorphin, arginine vasopressin (AVP) and corticotropin-releasing hormone (CRH) in tissues of the immune system. We now present data showing that the patterns of these changes in the spleens and thymuses of Piebald-Viral-Glaxo rats are quite dissimilar throughout the course of the disease. Immunoreactive (ir)-CRH, AVP, ACTH and beta-endorphin were measured by radioimmunoassays in spleen and thymic extracts taken at days 3, 7, 11 and 14 following injection of adjuvant. AVP was increased in the spleen at day 14 compared to the controls (79.4 +/- 4.4 and 60.0 +/- 9.0 fmol/g tissue respectively), but no change occurred in the thymus. CRH contents were increased in the spleen at day 14 (33.4 +/- 3.5) compared to controls (22.1 +/- 2.4 fmol/g tissue), and in the thymus at day 11 (24.0 +/- 2.3) compared to controls (14.1 +/- 2.5 fmol/g tissue). Increases in ACTH content were observed in spleens from arthritic rats at days 3 (365 +/- 23), 11 (359 +/- 32) and 14 (355 +/- 45 fmol/g tissue) compared to controls (198 +/- 37 fmol/g tissue). In the thymus, however, ACTH was elevated only at day 14. Beta-Endorphin levels in the spleen were elevated only at day 14 (289 +/- 41) compared to controls (97 /+- 22 fmol/g tissue).(ABSTRACT TRUNCATED AT 250 WORDS)

Oxytocin-producing and vasopressin-producing eosinophils in the mouse spleen: immunohistochemical, immuno-electron-microscopic and in situ hybridization studies

Oxytocin-like and vasopressin-like immunoreactive cells, and the cells expressing mRNAs for these peptides in the spleen of the C57BL/6 mouse were studied by immunohistochemistry, immuno-electron microscopy and in situ hybridization. Immunoreactive cells were distributed mainly in the splenic cord and marginal zone, whereas there were few in the lymphocyte-packed periarteriolar-lymphoid sheath, lymphoid follicle and germinal center. More numerous vasopressin-positive cells were seen in the splenic cord. The colocalization of oxytocin-like and vasopressin-like immunoreactivity in the same cells was identified by the investigation of mirror sections. By the pre-embedding immuno-electron-microscopic method using antisera against oxytocin and vasopressin, immunopositive reaction products were localized in the matrix around the specific granules, small clear vesicles and mitochondrial membrane of the eosinophils. No immunoreactivity to these peptides was found within the specific granules of the eosinophils. In situ hybridization with synthetic oligonucleotide probes labeled with 32P revealed the presence of mRNAs for oxytocin and vasopressin in the cells of the spleen, the distribution of the mRNAs for these peptides being the same as that of immunopositive cells. These observations suggest that eosinophils synthesize both oxytocin and vasopressin and store them in the matrix. Possible differences in the mechanism of synthesis and storage of these peptides between peripheral eosinophils and hypothalamic neurons are discussed.

Presence of immunoreactive pro-opiomelanocortin-derived peptides and cytokines in the thymus of an anuran amphibian (Rana esculenta)

Pro-opiomelanocortin (POMC)-derived peptide [adrenocorticotropic hormone (ACTH), beta-endorphin, alpha-melanocyte-stimulating hormone (MSH)]- and cytokine (IL-1 alpha, IL-1 beta, IL-2, IL-6, TNF-alpha)-like molecules were demonstrated in PAS positive epithelial cells of the thymus of the anuran amphibian Rana esculenta by an immunocytochemical procedure. Three groups of PAS positive epithelial cells were identified in subcapsular cortex, inner cortex and medulla, respectively. The cells containing ACTH-, alpha-MSH- and cytokine-like molecules were distributed in the cortex and those containing beta-endorphin-like molecules in the medulla and inner cortex. Thymic lymphocytes were always negative for POMC-derived peptides and cytokines. These results suggest that the neuroendocrine function of the thymus can be traced back to lower vertebrates.

The interaction of (1-4)-fragment of thymosin beta 4 with calmodulin-sensitive cAMP phosphodiesterase from hypothalamus

Evidence was accumulated indicating that cyclic nucleotides are involved in regulation of growth, differentiation and function of lymphoid cells. It was previously shown that the N-fragment (1-4) of thymosin beta 4 (Ac-Ser-Asp-Lys-Pro-OH) inhibits in vivo the entry of cell populations into S-phase. In the course of the study of the interrelationship between the immune and neuroendocrine systems we have found that the tetrapeptide caused incomplete competitive inhibition of hypothalamic calmodulin (CaM)-dependent phosphodiesterase (PDE) stimulated by CaM. In the presence of the peptide, the 20-fold increase of the constant for PDE activation by CaM was accompanied by an insignificant rise in the maximum rate of cAMP hydrolysis. The value of the inhibition constant (Ki) amounted to 600 nM. In the absence of CaM, the peptide at saturating concentrations reduced the basal activity of PDE nearly 2- to 3-fold. The effect of the peptide on PDE was noncompetitive with respect to cAMP. The results support our suggestion that the tetrapeptide realizes its effects in the immuno-neuroendocrine system by the mechanism of cyclic nucleotide metabolism.

Cyclosporin A antagonizes phenylephrine, oxytocin and angiotensin effects on glucose metabolism in rat thymus lymphocytes

Effects of phenylephrine, oxytocin and angiotensin on fructose 2,6-bisphosphate (Fru 2,6-P2) content and glycolytic parameters were studied in incubated thymus lymphocytes. These hormones modified Fru 2,6-P2 content dependent upon the energetic status of the cells. In non-preincubated thymus lymphocytes (with relatively high levels of glycogen and ATP), phenylephrine, oxytocin and angiotensin depressed Fru 2,6-P2 content in a dose-dependent manner. The opposite was found when the cells were preincubated for 2 h without substrates (low levels of ATP and glycogen). Changes in lactate release were less evident, but significant. Phenylephrine did not modify the maximal activities of phosphofructokinase (PFK)-1 or PFK-2. However, both submaximal PFK-1 and PFK-2 activities were inhibited by phenylephrine, and the response to exogenous Fru 2,6-P2 on PFK-1 was also altered. The activities of Fru 1,6-P2 and pyruvate kinase were not modified by phenylephrine or A23187 treatment. Simultaneous presence of Cyclosporin A (CsA), an immunosuppressive drug, antagonizes the alpha-adrenergic effect on Fru 2,6-P2 content. CsA alone did not alter basal levels of ATP, hexose phosphate or Fru 2,6-P2, and its opposing effect to alpha-agonist was dose-dependent. CsA cannot change the positive action of PMA or the negative action of A23187 on Fru 2,6-P2 content. The present data suggest that CsA acts prior to calcium liberation and protein kinase C activation. Different possible molecular models are discussed.

Corticotropin-releasing factor mRNA in rat thymus and spleen

Corticotropin-releasing factor (CRF) initiates stress-induced immunosuppression via the hypothalamic-pituitary-adrenal axis. CRF has also been shown to have direct stimulatory and suppressive effects on immune cells. We have previously detected immunoreactive and bioactive CRF in the rat spleen and thymus. To determine if CRF is synthesized in these tissues, we analyzed rat spleen and thymus for the presence of CRF mRNA. RNA was reverse transcribed, and the resulting cDNA was amplified by the polymerase chain reaction with CRF gene-specific oligonucleotide primers. After Southern blotting and hybridization with an internal CRF gene probe, a product of the expected size was detected in the spleen, thymus, and hypothalamus (positive control) but not in liver or kidney (negative controls), indicating that CRF is synthesized in the spleen and thymus. Furthermore, CRF could be secreted from splenic and thymic adherent cells in culture. Secretion increased severalfold in response to nordihydroguaiaretic acid (NDGA), a lipoxygenase pathway inhibitor, whereas interleukin 1 had no effect, suggesting that regulation of CRF secretion may differ from that in the hypothalamus. CRF mRNA was detected in NDGA-stimulated thymic adherent cells and in both control and NDGA-stimulated splenic nonadherent cells. The finding that CRF is synthesized in the spleen and thymus suggests that locally synthesized "immune" CRF, acting as an autocrine or paracrine cytokine, may have direct regulatory effects on immune function.

Subcellular localization of immunoreactive oxytocin within thymic epithelial cells of the male mouse

Immunoreactive oxytocin is expressed by thymic epithelial cells, which share properties with neuroendocrine cells. In order to investigate the assumed paracrine secretion of oxytocin, we studied the subcellular localization of immunoreactive oxytocin within thymic tissue and cultured thymic epithelial cells of the male mouse. Three types of immunoreactive cells were distinguished with the electron microscope. Immunoreactive oxytocin was found to be restricted to the cytoplasm by the use of pre- and postembedding methods. Some epithelial cells, especially in the cortex, showed a pronounced labelling of vesicular membranes and membrane tubules of the endoplasmic reticulum. In some cells, keratin filaments were associated with the electron-dense stain. Under culture conditions immunoreactive cells of different shapes were found, all displaying similar patterns of labelling. The contents of different types of vacuoles were only rarely labelled. A special class of immunoreactive exocytotic vesicles could not be identified. Thus, our results do not support neuroendocrine secretion of oxytocin via vesicles of thymic epithelial cells but offer alternative modes of secretion.

The influence of interleukin-2 on vasopressin and oxytocin gene expression in the rodent hypothalamus

There is growing evidence of interactions between the central nervous system and the immune system. We present evidence that the cytokine interleukin-2 (IL-2) influences expression of the genes encoding the neuropeptides vasopressin (VP) and oxytocin (OT) in the hypothalamus of the nude mouse. A single injection of recombinant mouse IL-2 (rmIL-2) caused a significant increase in VP and OT mRNA levels in the hypothalamus of nude mice. This effect was specific to the nude mouse. These observations stress the potential value of the nude mouse for studying interactions between the central nervous system (CNS) and the immune system.

Sexual activity decreases oxytocin receptor densities in the thymus

Sexual interactions have multiple effects on oxytocin systems in the brain. In the present study we observed that allowing estrogen-progesterone treated ovariectomized rats to be mounted ten times by a male significantly decreased the density (beta max) of thymic oxytocin receptors. Animals were ovariectomized and after recovery injected once daily for three consecutive days with 0.5 microgram estradiol benzoate (EB) followed by an injection of 500 micrograms progesterone on the fourth day 4-5 hr before testing. They were either placed in a cage with a sexually-active male until mounted ten times (mounted) or were briefly placed in the cage and removed before being mounted (unmounted). Both groups were then killed and their thymuses aseptically removed. Computerized analysis of saturation binding data showed that the densities of oxytocin receptors from mounted animals were significantly (p < 0.02) lower than those of unmounted controls (beta max for unmounted animals = 8.45 +/- 0.84 fmol/mg protein; and for mounted rats = 5.5 +/- 0.33 fmol/mg protein; t6 = 3.23). The possibility is discussed that sexual activity reduces thymic oxytocin receptors which may alter immune responsiveness to sexually-transmitted diseases.

Neuropeptide Y immunoreactivity in the spleen and thymus of normal rats and following adjuvant-induced arthritis

Immunoreactive neuropeptide Y (irNPY) was detected by radioimmunoassay within the rat thymus and spleen. Total spleen and thymus irNPY contents in control animals were 77 +/- 3 ng and 23 +/- 1 ng respectively (means +/- S.E.M., n = 10). Total tissue contents of irNPY 14 days following bilateral adrenalectomy or induction of inflammatory arthritis were not significantly altered compared to controls. Most spleen irNPY coeluted with synthetic NPY after reversed-phase high-performance liquid chromatography, but two peaks of irNPY were detected in thymic extracts. This suggests that NPY may be differentially expressed in tissues of the immune system.

A hypothalamic activator of calmodulin-dependent enzymes is thymosin beta 4 (1-39)

A new class of stimulators of basal activity of a number of calmodulin-dependent enzymes have been previously isolated from bovine hypothalamus. One of these stimulators, denoted as C3, has been purified to homogeneity by reverse phase HPLC and tentatively identified as thymosin beta 4 (1-39) by mass spectrometry and Edman microsequence analysis. The stimulating effect of C3 on rabbit skeletal muscle MLCK basal activity was compared with that of thymosin alpha 1 and thymosin beta 4 (16-38). Evidence is presented that all the indicated compounds are Ca(2+)-independent high-affinity MLCK stimulators. The potency of the stimulators in activating the enzyme was: C3 greater than beta 4 greater than (CaM+Ca2+) greater than alpha 1.

Innervation of lymphoid organs and implications in development, aging, and autoimmunity

We now have substantial evidence demonstrating noradrenergic sympathetic and peptidergic innervation of both primary and secondary lymphoid organs. We have established criteria for norepinephrine, and some of the neuropeptides, as neurotransmitters, and have found changes in immune responsiveness following pharmacological manipulation of noradrenergic sympathetic or peptidergic nerves. Classic receptor binding studies have demonstrated a wide variety of target cells that possess beta-adrenoceptors and receptors for neuropeptides on cells of the immune system, including lymphocyte subsets, macrophages, accessory cells, or stromal elements. In this chapter we describe noradrenergic and peptidergic innervation of primary and secondary lymphoid organs in development, at maturation and during the normal aging process, and discuss possible functional implications of direct neural signals onto cells of the immune system at critical time points in the lifespan of an animal. Further, we examine for involvement of noradrenergic sympathetic and peptidergic innervation in the development and progression of several autoimmune disorders, including adjuvant-induced arthritis, New Zealand mice strains as a model for hemolytic anemia and lupus-like syndrome, and the experimental allergic encephalomyelitis model for multiple sclerosis.