Evidence for the presence of immunoreactive POMC-derived peptides and cytokines in the thymus of the goldfish (Carassius c. auratus)

Stress differentially affects the systemic and leukocyte estrogen network in common carp

Both systemic and locally released steroid hormones, such as cortisol and estrogens, show immunomodulatory actions. This research gives evidence that circulating and leukocyte-derived estrogens can be involved in the regulation of the immune response in common carp, during homeostasis and upon restraining stress. It was found that stress reduced level of blood 17β-estradiol (E2) and down-regulated the gene expression of components of the "classical" estrogen system: the nuclear estrogen receptors and the aromatase CYP19, in the hypothalamus, the pituitary and in the ovaries. In contrast, higher gene expression of the nuclear estrogen receptors and cyp19a was found in the head kidney of stressed animals. Moreover, stress induced changes in the E2 level and in the estrogen sensitivity at local/leukocyte level. For the first time in fish, we showed the presence of physiologically relevant amounts of E2 and the substrates for its conversion (estrone - E1 and testosterone - T) in head kidney monocytes/macrophages and found that its production is modulated upon stress. Moreover, stress reduced the sensitivity of leukocytes towards estrogens, by down-regulation the expression of the erb and cyp19 genes in carp phagocytes. In contrast, era expression was up-regulated in the head kidney monocytes/macrophages and in PBLs derived from stressed animals. We hypothesize that, the increased expression of ERα, that was observed during stress, can be important for the regulation of leukocyte differentiation, maturation and migration. In conclusion, these results indicate that, in fish, the estrogen network can be actively involved in the regulation of the systemic and local stress response and the immune response.

Thymus: Conservation in evolution

From an evolutionary point of view, the thymus is a new organ observed for the first time in fish concomitantly with the appearance of adaptive clonotypical immunity. Hormone and neuropeptide expression was demonstrated in different species suggesting a conserved role of these molecules. An integrated evolution of immune and neuroendocrine responses appears to have been realized by means of the re-use of ancestral material, such as neuroendocrine cells and mediators, to create a thymic microenvironment for the maturation and differentiation of T cells.

Estrogen-dependent seasonal adaptations in the immune response of fish

Clinical and experimental evidence shows that estrogens affect immunity in mammals. Less is known about this interaction in the evolutionary older, non-mammalian, vertebrates. Fish form an excellent model to identify evolutionary conserved neuroendocrine-immune interactions: i) they are the earliest vertebrates with fully developed innate and adaptive immunity, ii) immune and endocrine parameters vary with season, and iii) physiology is constantly disrupted by increasing contamination of the aquatic environment. Neuro-immuno-endocrine interactions enable adaption to changing internal and external environment and are based on shared signaling molecules and receptors. The presence of specific estrogen receptors on/in fish leukocytes, implies direct estrogen-mediated immunoregulation. Fish leukocytes most probably are also capable to produce estrogens as they express the cyp19a and cyp19b - genes, encoding aromatase cytochrome P450, the enzyme critical for conversion of C19 steroids to estrogens. Immunoregulatory actions of estrogens, vary among animal species, and also with dose, target cell type, or physiological condition (e.g., infected/non-infected, reproductive status). They moreover are multifaceted. Interestingly, season-dependent changes in immune status correlate with changes in the levels of circulating sex hormones. Whereas E2 circulating in the bloodstream is perhaps the most likely candidate to be the physiological mediator of systemic immune-reproductive trade-offs, leukocyte-derived hormones are hypothesized to be mainly involved in local tuning of the immune response. Contamination of the aquatic environment with estrogenic EDCs may violate the delicate and precise allostatic interactions between the endogenous estrogen system and the immune system. This has negative effects on fish health, but will also affect the physiology of its consumers.

Neuroendocrine mechanisms for immune system regulation during stress in fish

In the last years, the aquaculture crops have experienced an explosive and intensive growth, because of the high demand for protein. This growth has increased fish susceptibility to diseases and subsequent death. The constant biotic and abiotic changes experienced by fish species in culture are challenges that induce physiological, endocrine and immunological responses. These changes mitigate stress effects at the cellular level to maintain homeostasis. The effects of stress on the immune system have been studied for many years. While acute stress can have beneficial effects, chronic stress inhibits the immune response in mammals and teleost fish. In response to stress, a signaling cascade is triggered by the activation of neural circuits in the central nervous system because the hypothalamus is the central modulator of stress. This leads to the production of catecholamines, corticosteroid-releasing hormone, adrenocorticotropic hormone and glucocorticoids, which are the essential neuroendocrine mediators for this activation. Because stress situations are energetically demanding, the neuroendocrine signals are involved in metabolic support and will suppress the "less important" immune function. Understanding the cellular mechanisms of the neuroendocrine regulation of immunity in fish will allow the development of new pharmaceutical strategies and therapeutics for the prevention and treatment of diseases triggered by stress at all stages of fish cultures for commercial production.

Neuroendocrine-immune interaction in fish: differential regulation of phagocyte activity by neuroendocrine factors

Coping with physical, chemical and biological disturbances depends on an extensive repertoire of physiological, endocrinological and immunological responses. Fish provide intriguing models to study bi-directional interaction between the neuroendocrine and the immune systems. Macrophages and granulocytes are the main actors in the first and rapid innate immune response. They are resident in different organs and are moreover rapidly recruited and activated upon infection. They act in response to recognition of pathogen-associated molecular patterns (PAMPs) via a repertoire of surface and intracellular receptors by inducing a plethora of defense reactions aiming to eradicate the pathogen. Subsequent production of inflammatory mediators stimulates other leukocytes required to develop an adaptive and specific antibody response. The type of phagocyte reaction will therefore depend on their differentiation state, specific receptor repertoire and their specific location. Apart from these pathogen induced responses, immune reactivity may be modulated by neuroendocrine factors. Over the last years we extensively studied changes in carp stress axis activity and the effect of its end-products on the immune system in an acute stress paradigm. We focus on specific neuroendocrine receptors on leukocytes and their effect on crucial phagocyte activities. We performed identification and functional analyses of different glucocorticoid, opioid and adrenergic receptors on carp phagocytes. Results show that their ligands of neuroendocrine origin may have substantial impact on specific phagocyte functions in a differential way. Inflammatory and microbicidal responses fight pathogens but may be detrimental to the host tissue. Neuroendocrine modulation may regulate inflammation to reach an optimum defense while preventing excessive host cell damage.

Evidence of conserved neuroendocrine interactions in the thymus: intrathymic expression of neuropeptides in mammalian and non-mammalian vertebrates

The function of lymphoid organs and immune cells is often modulated by hormones, steroids and neuropeptides produced by the neuroendocrine and immune systems. The thymus intrinsically produces these factors and a comparative analysis of the expression of neuropeptides in the thymus of different species would highlight the evolutionary importance of neuroendocrine interaction in T cell development. In this review, we highlight the evidence which describes the intrathymic expression and function of various neuropeptides and their receptors, in particular somatostatin, substance P, vasointestinal polypeptide, calcitonin gene-related peptide and neuropeptide Y, in mammals (human, rodent) and non-mammals (avian, amphibian and teleost), and conclude that neuropeptides play a conserved role in vertebrate thymocyte development.

Morphine affects the inflammatory response in carp by impairment of leukocyte migration

Opioid peptides are evolutionary conserved and in teleost fish their specific receptor types have been identified not only on neuroendocrine cells but also on immunocytes. In the present work we have studied the effects of morphine, ligand for the mu3 opioid receptor, on innate immune responses of common carp. Both in vitro and in vivo, during zymosan-induced peritonitis, morphine reduced gene expression of pro-inflammatory cytokines/chemokines and chemokine receptors. Furthermore, in vitro morphine administration also affects nitric oxide production, chemotaxis and apoptosis of head kidney leukocytes. These results provide evidence for an anti-inflammatory function of morphine and suggest an evolutionary conserved cross-talk between chemokines and opioids.

ProCRH in the teleost Ameiurus nebulosus: gene cloning and role in LPS-induced stress response

The procorticotrophin-releasing hormone (proCRH) gene from the teleost Ameiurus nebulosus was cloned by direct and inverse PCR-based technologies and characterized. The structure of the proCRH gene shows the presence of four exons and three introns giving a total length of 1416bp. Sequence similarity with the corresponding proCRH coding sequences in Tilapia mossambica and Homo sapiens is 97.7 and 78%, respectively. Western blot experiments performed with an anti-human CRH (1-41) antibody revealed the presence of an immunoreactive molecule with an approximate MW of 18kDa, a value comparable to that of the putative catfish proCRH peptide. These data suggest that proCRH could be active in A. nebulosus without any cleavage. ProCRH immunoreactive molecules were found in the central nervous system (CNS) and were immunocytochemically detected in the head kidney and in the pancreatic gland. Western blot and immunocytochemical experiments showed an increase in proCRH expression in the CNS after 15min but not after 120min exposure to LPS. In contrast, the increased immunopositivity was detectable in the pancreas only after 120min of treatment, but in the head kidney throughout the entire period of exposure. Our findings indicate that the CNS responds to the altered conditions for a shorter period of time than the peripheral organs, suggesting a hierarchical and time-regulated stress response. However, an independent response in the peripheral organs cannot be excluded in this scenario.

Lethal and sub-lethal yessotoxin dose-induced morpho-functional alterations in intraperitoneal injected Swiss CD1 mice

Histological and immunocytochemical investigations were performed on different organs (brain, duodenum and thymus) of mice following lethal (420 microg/kg) or sublethal (10 microg/kg) intraperitoneal injection of yessotoxin (YTX). No morpho-functional modifications were observed in large neurons of the cerebral and cerebellar cortex with the sub-lethal dose, nor in the cerebral cortex with the lethal dose. The duodenum also did not show significant alterations. However, there was an inflammation response to the toxin, in which blood cells and cytokines were involved. This was more evident with the lethal YTX dose. The thymus and, in general, the immune system are the main targets of YTX at both the concentrations used. Furthermore, the alterations present in the thymus may support tumorigenic implications.

Acute toxic effect of the algal yessotoxin on Purkinje cells from the cerebellum of Swiss CD1 mice

Swiss CD1 mice died less than 2 h after intraperitoneal injection of 420 microg/kg of algal yessotoxin (YTX). The morphological, histochemical and immunocytochemical studies performed on the cerebellar cortex revealed damage to the Purkinje cells. The main cytological alterations were observed in the cytoplasm, while less sufferance was detected in the nucleus. The immunocytochemical experiments showed an increased positivity to S100 protein while there was a decreased response to calbindin D-28K, beta-tubulin and neurofilaments. These changes in intracellular Ca(2+)-binding proteins and the modifications in the cytoskeletal components of Purkinje cells suggest that YTX may be involved in neurological disorders.

Interaction between endocrine and immune systems in fish

Diseases in fish are serious problems for the development of aquaculture. The outbreak of fish disease is largely dependent on environmental and endogenous factors resulting in opportunistic infection. Recent studies, particularly on stress response, have revealed that bidirectional communication between the endocrine and immune systems via hormones and cytokines exists at the level of teleost fish. Recently information on such messengers and receptors has accumulated in fish research particularly at the molecular level. Furthermore, it has become apparent in fish that cells of the immune system produce or express hormones and their receptors and vice versa to exchange information between the two systems. This review summarizes and updates the knowledge on endocrine-immune interactions in fish with special emphasis on the roles of such mediators or receptors for their interactions.

Modulation of the fish immune system by hormones

Immune-neuroendocrine interactions in fish, as in mammals, have become a focus of considerable interest, with the modulation of immune responses by hormones receiving particular attention. Cortisol, growth hormone (GH), prolactin (PRL), reproductive hormones, melanin-concentrating hormone (MCH) and proopiomelanocortin (POMC)-derived peptides have all been shown to influence immune functions in a number of fish species. This review summarises the known effects of these hormones on the fish immune system, as well as the often complex interactions between different hormones. The possible implications for fish health, with respect to aquaculture and the changes in immunocompetence that take place during different stages in the fish life cycle are also discussed.

Immunoreactive POMC-derived peptides and cytokines in the chicken thymus and bursa of Fabricius microenvironments: age-related changes

Changes from hatching to the involutive stage in the thymus and bursa of Fabricius of Gallus domesticus were studied. Pro-opiomelanocortin (POMC)-derived peptides and cytokines were also tested by immunocytochemical procedures. Thymic histological modifications appeared at 3 months and involved an increase in connectival argyrophilic reticular fibres, a proliferation of non-epithelial cells in the reticulum network, the presence of mucous cells and small mucous cysts, and an increase in the number of eosinophilic and myoid cells. A clear distinction between the cortex and the medulla was lost. Immunoreactive POMC-related molecules and cytokines were demonstrated in thymic cells from 4-day-old chicken, and their number increased with ageing. These molecules were expressed in a few single epithelial cells and in interdigitating cells. With ageing, the number of immunoreactive interdigitating cells also increased, and these appeared in an activated phase. Histological modifications in the bursa of Fabricius appeared at 2 months and concerned a folding of the interfollicular surface epithelium covering the bursal plicae and a reduction in lymphoid follicle-associated epithelium. Fibrous tissue gradually increased, and large mucoid cysts were evident. The expression of POMC-derived peptide-and cytokine-like molecules differed during the development and involution phases of the organ. Cells of follicle associated epithelium and dendritic reticular cells of lymphoid follicles were immunoreactive to beta-endorphin between 4 days and 2 months, while ACTH-, alpha-MSH- and cytokine-like molecules were observed in follicles after 2 months. The findings indicate a physiological role of these molecules during the growth and involution of the two organs.

Molecular cloning of interleukin 1beta from rainbow trout Oncorhynchus mykiss reveals no evidence of an ice cut site

The complete coding sequence of rainbow trout IL-1beta has been obtained. The gene contains a short 5' UTR (97 bp), a 780 bp open reading frame and a 466 bp 3' UTR, which includes a polyadenylation signal, 7 ATTTA motifs and an 18 bp poly A tail. The predicted amino acid sequence (260 amino acids) contains 3 potential glycosylation sites, with a predicted molecular weight of 29 kDa, and shows between 49 and 56% amino acid similarity to mammalian IL-1betas and 57% similarity to carp IL-1beta. Greatest homology was apparent within the secondary structure of the gene, with few of the amino acids known to bind to the IL-1 receptor being conserved. No ICE cut site was apparent but multiple alignment with mammalian sequences allowed a putative mature peptide of 166 amino acids to be identified, in which Ala(95)would be the amino terminus. Northern blot analysis showed that whilst no IL-1beta expression was detectable in head kidney leukocytes immediately after isolation, expression could be induced by stimulation with LPS for 4 h in culture. Similarly, with isolated head kidney macrophages expression was significantly increased following stimulation with LPS.

Cloning and expression of two proopiomelanocortin mRNAs in the common carp (Cyprinus carpio L.)

Proopiomelanocortin (POMC) is the precursor for a number of biologically active peptides such as adrenocorticotropic hormone (ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. It is well known that these peptides are involved in the stress response in fish as well as in mammals. We have cloned two different carp POMC cDNAs called, POMC-I and POMC-II. The nucleotide sequences of 955 bp for POMC-I and 959 bp for POMC-II share 93.5% identity in their cDNAs, and the deduced amino acid sequences (both 222 amino acids) are 91.4% identical. In the ACTH and beta-MSH domain, two amino acid substitutions are found, whereas alpha-MSH and beta-endorphin are identical. For beta-MSH, the serine replacement (in POMC-I) by a glycine (in POMC-II) results in a putative amidation site Pro-X-Gly for POMC-II. We used RT-PCR to show that both POMC mRNAs are expressed in the hypophysis, hypothalamus and other parts of the brain of a single fish. Furthermore, in a phylogenetic tree based on POMC sequences the divergence of carp POMC-I and -II from tetraploid animals (salmon, trout and xenopus) is demonstrated.

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.

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.