In vivo kinetics of cytokine expression during peritonitis in carp: evidence for innate and alternative macrophage polarization

Immune function modulation during artificial ovarian maturation in Japanese eel (Anguilla japonica): A transcriptome profiling approach

The Japanese eel (Anguilla japonica) experiences dramatic internal and external environmental changes during its transoceanic reproductive migrations. Here, we assess immune function changes in the primary and secondary immune organs (head kidney and spleen) of A. japonica during artificial ovarian maturation at the previtellogenic (PV), midvitellogenic (MV), and ovulating (OV) stages by transcriptome analyses. Stress responses were also assessed by determining the serum concentrations of lysozyme, alkaline phosphatase, acid phosphatase, total antioxidant capacity, and superoxide dismutase. Our results showed that together with increased serum 17β-estrogen and testosterone, lysozyme activity and antioxidant capacity were suppressed during artificial ovarian maturation. Comparisons across these developmental stages identified 60 (head kidney) and 36 (spleen) differentially expressed genes associated with the immune system. Genes related to the key activation markers of innate immune function, such as CXCL10, CXCL11, CCL20, HSP90B, MMP9, and MMP13, were upregulated and significantly enriched in the interleukin-17 signaling pathway. Adaptive immune function-related genes (IGM and MHC1) were upregulated in the head kidney from PV to MV, and their levels increased thereafter in the spleen. Moreover, a correlation between Pax5 expression and IGM expression in the spleen of MV (IGM+/Pax5+) and OV (IGM++/Pax5-) stage suggests that adaptive immune function was enhanced during ovarian maturation. To our knowledge, the present study is the first to describe transcriptome profiling of immune organs during ovarian maturation in teleost. Our findings suggest that the interleukin-17 pathway and IgM may play important roles in spawning.

The Acute Immune Responses of the Common Carp Cyprinus carpio to PLGA Microparticles-The Interactions of a Teleost Fish with a Foreign Material

Poly lactic-co-glycolic acid (PLGA) particles safely and effectively deliver pharmaceutical ingredients, with many applications approved for clinical use in humans. In fishes, PLGA particles are being considered as carriers of therapeutic drugs and vaccine antigens. However, existing studies focus mainly on vaccine antigens, the endpoint immune responses to these (e.g., improved antibody titres), without deeper understanding of whether fishes react to the carrier. To test whether or not PLGA are recognized by or interact at all with the immune system of a teleost fish, we prepared, characterized and injected PLGA microparticles intraperitoneally into common carp. The influx, phenotype of inflammatory leukocytes, and their capacity to produce reactive oxygen species and phagocytose PLGA microparticles were tested by flow cytometry, qPCR, and microscopy. PLGA microparticles were indeed recognized. However, they induced only transient recruitment of inflammatory leukocytes that was resolved 4 days later whereas only the smallest µm-sized particles were phagocytosed. The overall response resembled that described in mammals against foreign materials. Given the similarities between our findings and those described in mammals, PLGA particles can be adapted to play a dual role as both antigen and drug carriers in fishes, depending on the administered dose and their design.

Effects of Chronic Hypoxia on the Immune Status of Pikeperch (Sander lucioperca Linnaeus, 1758)

Simple Summary

Inadequate oxygen saturation, or hypoxia, belongs to one of the critical stress factors in intensive aquaculture. Exposure of fish to low oxygen levels over prolonged periods substantially affects their well-being and immune competence, resulting in increased disease susceptibility and consequent economic losses. In this interdisciplinary research, we aimed to provide a deeper understanding of the effect of chronic low oxygen saturation on pikeperch farmed in recirculating aquaculture systems. The obtained data offer unprecedented insights into the changes in the immunocompetence of studied fish and suggest high robustness of this new aquaculture species to the stress factors of intensive aquaculture.

Abstract

Inadequate oxygen saturation can induce stress responses in fish and further affect their immunity. Pikeperch, recently introduced in intensive aquaculture, is suggested to be reared at nearly 100% DO (dissolved oxygen), yet this recommendation can be compromised by several factors including the water temperature, stocking densities or low circulation. Herein, we aimed to investigate the effect of low oxygen saturation of 40% DO (±3.2 mg/L) over 28 days on pikeperch farmed in recirculating aquaculture systems. The obtained data suggest that—although the standard blood and health parameters did not reveal any significant differences at any timepoint—the flow cytometric analysis identified a slightly decreased proportion of lymphocytes in the HK (head kidney) of fish exposed to hypoxia. This has been complemented by marginally downregulated expression of investigated immune and stress genes in HK and liver (including FTH1, HIF1A and NR3C1). Additionally, in the model of acute peritoneal inflammation induced with inactivated Aeromonas hydrophila, we observed a striking dichotomy in the sensitivity to the low DO between innate and adaptive immunity. Thus, while the mobilization of myeloid cells from HK to blood, spleen and peritoneal cavity, underlined by changes in the expression of key proinflammatory cytokines (including MPO, IL1B and TNF) was not influenced by the low DO, hypoxia impaired the influx of lymphocytes to the peritoneal niche in the later phases of the immune reaction. Taken together, our data suggest high robustness of pikeperch towards the low oxygen saturation and further encourage its introduction to the intensive aquaculture systems.

The influence of 17β-oestradiol on lymphopoiesis and immune system ontogenesis in juvenile sea bass, Dicentrarchus labrax

The female sex steroid 17β-oestradiol (E2) is involved in the regulation of numerous physiological functions, including the immune system development and performance. The role of oestrogens during ontogenesis is, however, not well studied. In rodents and fish, thymus maturation appears to be oestrogen-dependent. Nevertheless, little is known about the function of oestrogen in immune system development. To further the understanding of the role of oestrogens in fish immune system ontogenesis, fingerlings of European sea bass (Dicentrarchus labrax) were exposed for 30 days to 20 ng E2·L^-1, at two ages tightly related to thymic maturation, i.e., 60 or 90 days post hatch (dph). The expression of nuclear and membrane oestrogen receptors was measured in the thymus and spleen, and the expression of several T cell-related gene markers was studied in both immune organs, as well as in the liver. Waterborne E2-exposure at 20.2 ± 2.1 (S.E.) ng·L^-1 was confirmed by radioimmunoassay, leading to significantly higher E2-contents in the liver of exposed fish. The majority of gene markers presented age-dependent dynamics in at least one of the organs, confirming thymus maturation, but also suggesting a critical ontogenetic window for the implementation of liver resident γδ and αβ T cells. The oestrogen receptors, however, remained unchanged over the age and treatment comparisons with the exception of esr2b, which was modulated by E2 in the younger cohort and increased its expression with age in the thymus of the older cohort, as did the membrane oestrogen receptor gpera. These results confirm that oestrogen-signalling is involved in thymus maturation in European sea bass, as it is in mammals. This suggests that esr2b and gpera play key roles during thymus ontogenesis, particularly during medulla maturation. In contrast, the spleen expressed low or non-detectable levels of oestrogen receptors. The E2-exposure decreased the expression of tcrγ in the liver in the cohort exposed from 93 to 122 dph, but not the expression of any other immune-related gene analysed. These results indicate that the proliferation/migration of these innate-like T cell populations is oestrogen-sensitive. In regard to the apparent prominent role of oestrogen-signalling in the late thymus maturation stage, the thymic differentiation of the corresponding subpopulations of T cells might be regulated by oestrogen. To the best of our knowledge, this is the first study investigating the dynamics of both nuclear and membrane oestrogen receptors in specific immune organs in a teleost fish at very early stages of immune system development as well as to examine thymic function in sea bass after an exposure to E2 during ontogenesis.

17α-ethinylestradiol and 4-tert-octylphenol concurrently disrupt the immune response of common carp

The aquatic environment is massively polluted with endocrine-disrupting compounds (EDCs) including synthetic estrogens (e.g. 17α-ethinylestradiol, EE2) and alkylphenols (e.g. 4-tert-octylphenol, 4t-OP). A major mechanism of action for estrogenic EDCs is their interaction with estrogen receptors and consequently their modulation of the action of enzymes involved in steroid conversion e.g. aromatase CYP19. We now studied the effects of EE2 and 4t-OP on the anti-bacterial immune response of common carp. We investigated effects on the number/composition of inflammatory leukocytes and on the gene expression of mediators that regulate inflammation and EDC binding. In vitro we found that high concentrations of both EE2 and 4t-OP down-regulated IFN-γ2 and IFN-γ-dependent immune responses in LPS-stimulated monocytes/macrophages. Similarly, during bacterial infection in fish, in vivo treated with EE2 and 4t-OP, decreased gene expression of il-12p35 and of ifn-γ2 was found in the focus of inflammation. Moreover, during A. salmonicida-induced infection in EE2-treated carp, but not in fish fed with 4t-OP-treated food, we found an enhanced inflammatory reaction manifested by high number of inflammatory peritoneal leukocytes, including phagocytes and higher expression of pro-inflammatory mediators (inos, il-1β, cxcl8_l2). Furthermore, in the liver, EE2 down-regulated the expression of acute phase proteins: CRPs and C3. Importantly, both in vitro and in vivo, EDCs altered the expression of estrogen receptors: nuclear (erα and erβ) and membrane (gpr30). EDCs also induced up-regulation of the cyp19b gene. Our findings reveal that contamination of the aquatic milieu with estrogenic EDCs, may considerably violate the subtle and particular allostatic interactions between the immune response and endogenous estrogens and this may have negative consequences for fish health.

Cortisol Metabolism in Carp Macrophages: A Role for Macrophage-Derived Cortisol in M1/M2 Polarization

Macrophages are crucial not only for initiation of inflammation and pathogen eradication (classically polarized M1 macrophages), but also for inflammation inhibition and tissue regeneration (alternatively polarized M2 macrophages). Their polarization toward the M1 population occurs under the influence of interferon-γ + lipopolysaccharide (IFN-γ + LPS), while alternatively polarized M2 macrophages evolve upon, e.g., interlukin 4 (IL-4) or cortisol stimulation. This in vitro study focused on a possible role for macrophage-derived cortisol in M1/M2 polarization in common carp. We studied the expression of molecules involved in cortisol synthesis/conversion from and to cortisone like 11β-hydroxysteroid dehydrogenase type 2 and 3. (11β-HSD2 and 3) and 11β-hydroxylase (CYP11b), as well as the expression of glucocorticoid receptors (GRs) and proliferator-activated receptor gamma (PPARγ) in M1 and M2 macrophages. Lastly, we analyzed how inhibition of these molecules affect macrophage polarization. In M1 cells, upregulation of gene expression of GRs and 11β-HSD3 was found, while, in M2 macrophages, expression of 11β-hsd2 was upregulated. Moreover, blocking of cortisol synthesis/conversion and GRs or PPARγ induced changes in expression of anti-inflammatory interleukin 10 (IL-10). Consequently, our data show that carp monocytes/macrophages can convert cortisol. The results strongly suggest that cortisol, via intracrine interaction with GRs, is important for IL-10-dependent control of the activity of macrophages and for the regulation of M1/M2 polarization to finally determine the outcome of an infection.

17β-Estradiol affects the innate immune response in common carp

Inflammation is the evolutionary conserved immune response to harmful stimuli such as pathogens or damaged cells. This multistep process acts by removing injurious stimuli and initiating the healing process. Therefore, it must be tightly regulated by cytokines, chemokines, and enzymes, as well as neuroendocrine mediators. In the present work, we studied the immunoregulatory properties of 17β-estradiol (E2) in common carp. We determined the in vitro effects of E2 on the activity/polarization of macrophages and the in vivo effects during Aeromonas salmonicida-induced inflammation. In vitro, E2 reduced the lipopolysaccharide (LPS)-stimulated expression of pro- and anti-inflammatory mediator genes but did not change the gene expression of the estrogen receptors and of aromatase CYP19. In contrast, in vivo in the head kidney of A. salmonicida-infected fish, E2-treated feeding induced an upregulation of gene expression of pro-inflammatory (il-12p35 and cxcb2) and anti-inflammatory (arginase 1, arginase 2, il-10, and mmp9) mediators. Moreover, in infected fish fed with E2-treated food, a higher gene expression of the estrogen receptors and of the aromatase CYP19 was found. Our results demonstrate that estrogens can modulate the carp innate immune response, though the in vitro and in vivo effects of this hormone are contrasting. This implies that estradiol not only induces a direct effect on macrophages but rather exerts immunomodulatory actions through indirect mechanisms involving other cellular targets.

Differential Effects of Drinking Water Quality on Phagocyte Responses of Broiler Chickens Against Fungal and Bacterial Challenges

Combinatorial effects of xenobiotics in water on health may occur even at levels within current acceptable guidelines for individual chemicals. Herein, we took advantage of the sensitivity of the immune system and an avian animal model to examine the impact of xenobiotic mixtures on animal health. Water was derived from an underground well in Alberta, Canada and met guidelines for consumption, but contained a number of contaminants. Changes to chicken immunity were evaluated following acute (7d) exposure to contaminated water under basal and immune challenged conditions. An increase in resident macrophages and a decrease in CD8+ lymphocytes were identified in the abdominal cavity, which served as a relevant site where immune leukocytes could be examined. Subsequent intra-abdominal immune stimulation detected differential in vivo acute inflammatory responses to fungal and bacterial challenges. Leukocyte recruitment into the challenge site and activation of phagocyte antimicrobial responses were affected. These functional responses paralleled molecular changes in the expression for pro-inflammatory and regulatory genes. In all, this study primarily highlights dysregulation of phagocyte responses following acute (7d) exposure of poultry to contaminated water. Given that production food animals hold a unique position at the interface of animal, environmental and human health, this emphasizes the need to consider the impact of xenobiotic mixtures in our assessments of water quality.

Effects of stress and cortisol on the polarization of carp macrophages

In teleost fish, myelopoiesis is maintained both in the head (HK) and trunk kidney (TK), but only the HK holds the endocrine cells that produce the stress hormone cortisol. We now compared the effects of prolonged restraint stress (in vivo) and cortisol (in vitro) on the polarization of HK and TK-derived carp macrophages. Monocytes/macrophages from both sources were treated in vitro with cortisol, lipopolysaccharide or with both factors combined. In vivo, fish were challenged by a prolonged restraint stress. Gene expression of several markers typical for classical M1 and alternative M2 macrophage polarization, as well as glucocorticoid receptors, were measured. Cells from both sources did not differ in the constitutive gene expression of glucocorticoid receptors, whereas they significantly differed in their response to cortisol and stress. In the LPS-stimulated HK monocytes/macrophages, cortisol in vitro counteracted the action of LPS while the effects of cortisol on the activity of TK monocytes/macrophages were less explicit. In vivo, restraint stress up-regulated gene expression of M2 markers in freshly isolated HK monocytes/macrophages, while at the same time it did not affect TK monocytes/macrophages. Moreover, LPS-stimulated HK monocytes/macrophages from stressed animals showed only minor differences in the gene expression of M1 and M2 markers, compared to LPS-treated monocytes/macrophages from control fish. In contrast, stress-induced changes in TK-derived LPS-treated cells were more pronounced. However, these changes did not clearly indicate whether in TK monocytes/macrophages stress will stimulate classical or alternative polarization. Altogether, our results imply that cortisol in vitro and stress in vivo direct HK, but not TK, monocytes/macrophages to the path of alternative polarization. These findings reveal that like in mammals, also in fish the glucocorticoids form important stimulators of alternative macrophage polarization.

Specific function and modulation of teleost monocytes/macrophages: polarization and phagocytosis

Macrophages exist in most tissues and play a variety of functions in vertebrates. Teleost fish species are found in most aquatic environments throughout the world and are quite diverse for a group of vertebrate animals. Due to whole genome duplication and environmental adaptation, teleost monocytes/macrophages possess a variety of different functions and modulations compared with those of mammals. A deeper understanding of teleost monocytes/macrophages in the immune system will not only help develop teleost-specific methods of disease prevention but will also help improve our understanding of the various immune mechanisms in mammals. In this review, we summarize the differences in polarization and phagocytosis of teleost and mammalian macrophages to improve our understanding of the various immune mechanisms in vertebrates.

Studies Into β-Glucan Recognition in Fish Suggests a Key Role for the C-Type Lectin Pathway

Immune-modulatory effects of β-glucans are generally considered beneficial to fish health. Despite the frequent application of β-glucans in aquaculture practice, the exact receptors and downstream signalling remains to be described for fish. In mammals, Dectin-1 is a member of the C-type lectin receptor (CLR) family and the best-described receptor for β-glucans. In fish genomes, no clear homologue of Dectin-1 could be identified so far. Yet, in previous studies we could activate carp macrophages with curdlan, considered a Dectin-1-specific β-(1,3)-glucan ligand in mammals. It was therefore proposed that immune-modulatory effects of β-glucan in carp macrophages could be triggered by a member of the CLR family activating the classical CLR signalling pathway, different from Dectin-1. In the current study, we used primary macrophages of common carp to examine immune modulation by β-glucans using transcriptome analysis of RNA isolated 6 h after stimulation with two different β-glucan preparations. Pathway analysis of differentially expressed genes (DEGs) showed that both β-glucans regulate a comparable signalling pathway typical of CLR activation. Carp genome analysis identified 239 genes encoding for proteins with at least one C-type Lectin Domains (CTLD). Narrowing the search for candidate β-glucan receptors, based on the presence of a conserved glucan-binding motif, identified 13 genes encoding a WxH sugar-binding motif in their CTLD. These genes, however, were not expressed in macrophages. Instead, among the β-glucan-stimulated DEGs, a total of six CTLD-encoding genes were significantly regulated, all of which were down-regulated in carp macrophages. Several candidates had a protein architecture similar to Dectin-1, therefore potential conservation of synteny of the mammalian Dectin-1 region was investigated by mining the zebrafish genome. Partial conservation of synteny with a region on the zebrafish chromosome 16 highlighted two genes as candidate β-glucan receptor. Altogether, the regulation of a gene expression profile typical of a signalling pathway associated with CLR activation and, the identification of several candidate β-glucan receptors, suggest that immune-modulatory effects of β-glucan in carp macrophages could be a result of signalling mediated by a member of the CLR family.

Psychoneuroimmunology and immunopsychiatry of zebrafish

Despite the high prevalence of neural and immune disorders, their etiology and molecular mechanisms remain poorly understood. As the zebrafish (Danio rerio) is increasingly utilized as a powerful model organism in biomedical research, mounting evidence suggests these fish as a useful tool to study neural and immune mechanisms and their interplay. Here, we discuss zebrafish neuro-immune mechanisms and their pharmacological and genetic modulation, the effect of stress on cytokines, as well as relevant models of microbiota-brain interplay. As many human brain diseases are based on complex interplay between the neural and the immune system, here we discuss zebrafish models, as well as recent successes and challenges, in this rapidly expanding field. We particularly emphasize the growing utility of zebrafish models in translational immunopsychiatry research, as they improve our understanding of pathogenetic neuro-immune interactions, thereby fostering future discovery of potential therapeutic agents.

Comparative Immune- and Stress-Related Transcript Response Induced by Air Exposure and Vibrio anguillarum Bacterin in Rainbow Trout (Oncorhynchus mykiss) and Gilthead Seabream (Sparus aurata) Mucosal Surfaces

Fish have to face various environmental challenges that may compromise the efficacy of the immune response in mucosal surfaces. Since the effect of acute stress on mucosal barriers in fish has still not been fully elucidated, we aimed to compare the short-term mucosal stress and immune transcriptomic responses in a freshwater (rainbow trout, Oncorhynchus mykiss) and a marine fish (gilthead seabream, Sparus aurata) to bacterial immersion (Vibrio anguillarum bacterin vaccine) and air exposure stress in skin, gills, and intestine. Air exposure and combined (vaccine + air) stressors exposure were found to be inducers of the cortisol secretion in plasma and skin mucus on both species in a time-dependent manner, while V. anguillarum bacterin exposure induced cortisol release in trout skin mucus only. This was coincident with a marked differential increase in transcriptomic patterns of stress- and immune-related gene expression profiles. Particularly in seabream skin, the expression of cytokines was markedly enhanced, whereas in gills the response was mainly suppressed. In rainbow trout gut, both air exposure and vaccine stimulated the transcriptomic response, whereas in seabream, stress and immune responses were mainly induced by air exposure. Therefore, our comparative survey on the transcriptomic mucosal responses demonstrates that skin and gut were generally more reactive in both species. However, the upregulation of immune transcripts was more pronounced in gills and gut of vaccinated trout, whereas seabream appeared to be more stress-prone and less responsive to V. anguillarum bacterin in gills and gut. When fish were subjected to both treatments no definite pattern was observed. Overall, the results indicate that (1) the immune response was not homogeneous among mucosae (2), it was greatly influenced by the specific traits of each stressor in each surface and (3) was highly species-specific, probably as a result of the adaptive life story of each species to the microbial load and environmental characteristics of their respective natural habitats.

Hypoxia Stress Modifies Na+/K+-ATPase, H+/K+-ATPase, [Formula: see text], and nkaα1 Isoform Expression in the Brain of Immune-Challenged Air-Breathing Fish

Fishes are equipped to sense stressful stimuli and are able to respond to environmental stressor such as hypoxia with varying pattern of stress response. The functional attributes of brain to hypoxia stress in relation to ion transport and its interaction during immune challenge have not yet delineated in fish. We, therefore, explored the pattern of ion transporter functions and messenger RNA (mRNA) expression of α1-subunit isoforms of Na⁺/K⁺-ATPase (NKA) in the brain segments, namely, prosencephalon (PC), mesencephalon (MC), and metencephalon (MeC) in an obligate air-breathing fish exposed either to hypoxia stress (30 minutes forced immersion in water) or challenged with zymosan treatment (25-200 ng g⁻¹ for 24 hours) or both. Zymosan that produced nonspecific immune responses evoked differential regulation of NKA, H⁺/K⁺-ATPase (HKA), and Na+/NH4+-ATPase (NNA) in the varied brain segments. On the contrary, hypoxia stress that demanded activation of NKA in PC and MeC showed a reversed NKA activity pattern in MeC of immune-challenged fish. A compromised HKA and NNA regulation during hypoxia stress was found in immune-challenged fish, indicating the role of these brain ion transporters to hypoxia stress and immune challenges. The differential mRNA expression of α1-subunit isoforms of NKA, nkaα1a, nkaα1b, and nkaα1c, in hypoxia-stressed brain showed a shift in its expression pattern during hypoxia stress-immune interaction in PC and MC. Evidence is thus presented for the first time that ion transporters such as HKA and NNA along with NKA act as functional brain markers which respond differentially to both hypoxia stress and immune challenges. Taken together, the data further provide evidence for a differential Na⁺, K⁺, H⁺, and NH4+ ion signaling that exists in brain neuronal clusters during hypoxia stress-immune interaction as a result of modified regulations of NKA, HKA, and NNA transporter functions and nkaα1 isoform regulation.

Zymosan-induced immune challenge modifies the stress response of hypoxic air-breathing fish (Anabas testudineus Bloch): Evidence for reversed patterns of cortisol and thyroid hormone interaction, differential ion transporter functions and non-specific immune response

Fishes have evolved physiological mechanisms to exhibit stress response, where hormonal signals interact with an array of ion transporters and regulate homeostasis. As major ion transport regulators in fish, cortisol and thyroid hormones have been shown to interact and fine-tune the stress response. Likewise, in fishes many interactions have been identified between stress and immune components, but the physiological basis of such interaction has not yet delineated particularly in air-breathing fish. We, therefore, investigated the responses of thyroid hormones and cortisol, ion transporter functions and non-specific immune response of an obligate air-breathing fish Anabas testudineus Bloch to zymosan treatment or hypoxia stress or both, to understand how immune challenge modifies the pattern of stress response in this fish. Induction of experimental peritonitis in these fish by zymosan treatment (200ngg^-1) for 24h produced rise in respiratory burst and lysozomal activities in head kidney phagocytes. In contrast, hypoxia stress for 30min in immune-challenged fish reversed these non-specific responses of head kidney phagocytes. The decline in plasma cortisol in zymosan-treated fish and its further suppression by hypoxia stress indicate that immune challenge suppresses the cortisol-driven stress response of this fish. Likewise, the decline in plasma T₃ and T₄ after zymosan-treatment and the rise in plasma T₄ after hypoxia stress in immune-challenged fish indicate a critical role for thyroid hormone in immune-stress response due to its differential sensitivity to both immune and stress challenges. Further, analysis of the activity pattern of ion-dependent ATPases viz. Na⁺/K⁺-ATPase, H⁺/K⁺-ATPase and Na⁺/NH₄⁺-ATPase indicates a functional interaction of ion transport system with the immune response as evident in its differential and spatial modifications after hypoxia stress in immune-challenged fish. The immune-challenge that produced differential pattern of mRNA expression of Na⁺/K⁺-ATPase α-subunit isoforms; nkaα1a, nkaα1b and nkaα1c and the shift in nkaα1a and nkaα1b isoforms expression after hypoxia stress in immune-challenged fish, presents transcriptomic evidence for a modified Na⁺/K⁺ ion transporter system in these fish. Collectively, our data thus provide evidence for an interactive immune-stress response in an air-breathing fish, where the patterns of cortisol-thyroid hormone interaction, the ion transporter functions and the non-specific immune responses are reversed by hypoxia stress in immune-challenged fish.

Cytokine modulation by stress hormones and antagonist specific hormonal inhibition in rainbow trout (Oncorhynchus mykiss) and gilthead sea bream (Sparus aurata) head kidney primary cell culture

A tight interaction between endocrine and immune systems takes place mainly due to the key role of head kidney in both hormone and cytokine secretion, particularly under stress situations in which the physiological response promotes the synthesis and release of stress hormones which may lead into immunomodulation as side effect. Although such interaction has been previously investigated, this study evaluated for the first time the effect of stress-associated hormones together with their receptor antagonists on the expression of cytokine genes in head kidney primary cell culture (HKPCC) of the freshwater rainbow trout (Oncorhynchus mykiss) and the seawater gilthead sea bream (Sparus aurata). The results showed a striking difference when comparing the response obtained in trout and seabream. Cortisol and adrenocorticotropic hormone (ACTH) decreased the expression of immune-related genes in sea bream but not in rainbow trout and this cortisol effect was reverted by the antagonist mifepristone but not spironolactone. On the other hand, while adrenaline reduced the expression of pro-inflammatory cytokines (IL-1β, IL-6) in rainbow trout, the opposite effect was observed in sea bream showing an increased expression (IL-1β, IL-6). Interestingly, this effect was reverted by antagonist propranolol but not phentolamine. Overall, our results confirm the regional interaction between endocrine and cytokine messengers and a clear difference in the sensitivity to the hormonal stimuli between the two species.

Fish skeletal muscle tissue is an important focus of immune reactions during pathogen infection

Skeletal muscle in mammals can express and secrete immune-related molecules during pathogen infection. Despite in fish is known that classical immune tissues participate in innate immunity, the role of skeletal muscle in this function is poorly understood. To determine the immunocompetence of fish skeletal muscle, juvenile fine flounder (Paralichthys adpersus) were challenged with Vibrio ordalii. Different Toll-like receptors, pro-inflammatory cytokines (TNFα, Il-1β, and IL-8), and immune-effector molecules (NKEF and the antimicrobial peptides hepcidin and LEAP-2) were analyzed. Infection initially triggered IL-1β upregulation and P38-MAPK/AP-1 pathway activation. Next, the NFĸB pathway was activated, together with an upregulation of intracellular Toll-like receptor expressions (tlr3, tlr8a tlr9, and tlr21), TNFα production, and leap-2 expression. Finally, transcriptions of il-1β, il-8, tnfα, nkef-a, and hepcidin were also upregulated. These results suggest that fish skeletal muscle is an immunologically active organ that could play an important role against pathogens.

CXCR3.1 and CXCR3.2 Differentially Contribute to Macrophage Polarization in Teleost Fish

The study of multiple copies of chemokine receptor genes in various teleosts has long appealed to investigators seeking to understand the evolution of the immune system. The CXCR CXCR3 gene has two isoforms, CXCR3.1 and CXCR3.2, which are both expressed in macrophages. The distinct roles of teleost CXCR3s have not been identified previously. In this article, we found that CXCR3.1 and CXCR3.2 differentially contributed to macrophage polarization in the teleosts: ayu (Plecoglossus altivelis), grass carp (Ctenopharyngodon idella), and spotted green pufferfish (Tetraodon nigroviridis). In ayu macrophages, the P. altivelis CXCR3.1 (PaCXCR3.1) gene was constitutively expressed, whereas the P. altivelis CXCR3.2 (PaCXCR3.2) gene was induced postinfection with Escherichia coli Upon E. coli infection, PaCXCR3.1⁺ and PaCXCR3.2⁺ macrophages showed an M1 and an M2 phenotype, respectively. CXCL9-11-like proteins mediated M1 and M2 polarization by interacting with the PaCXCR3.1 and PaCXCR3.2 proteins on macrophages, respectively. The transcription factors P. altivelis STAT1 and P. altivelis STAT3 were activated in PaCXCR3.1⁺ and PaCXCR3.2⁺ macrophages, respectively. Furthermore, the prognosis of septic ayu adoptively transferred with PaCXCR3.2⁺ macrophages was improved. Our data reveal a previously unknown mechanism for macrophage polarization, suggesting that redundant genes may regulate crucial functions in the teleost immune system.

Dietary β-glucans differentially modulate immune and stress-related gene expression in lymphoid organs from healthy and Aeromonas hydrophila-infected rainbow trout (Oncorhynchus mykiss)

Although β-glucans stimulating effects have already been demonstrated on the immune system of numerous animal species, available data remain relatively variable and more research should be done regarding the complexity of underlying mechanisms. In this context, the present study aimed to evaluate the stress and immune-related effects of dietary β-glucans (i.e. Macrogard^®) by considering a number of influencing factors such as the dose (0, 0.1, 0.2 and 0.5% in food), feeding duration (15 versus 30 days), tissue (blood, kidney, spleen, gills) and infection status (healthy or infected). Blood parameters (lysozyme, ACH50 activities, leucocyte populations) and mRNA expression level of several immune- and stress-related genes (TFN-α1, IL-1β, IL10, COX-2, TGF-β, MC2R, HSP70) were measured. Our results suggest that spleen may be a highly responsive organ to dietary β-glucans both in healthy or infected fish, and that this organ may therefore significantly contribute to the immune reinforcement induced by such immunostimulatory diet. Our study further reveals that overdoses of β-glucans and/or prolonged medication can lead to a non-reactive physiological status and, consequently, to a poor immune response. All in all, the current data emphasizes the need for further extensive research in the field of dietary β-glucans as a preventive method for farmed fish protection.

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.

A role for multiple estrogen receptors in immune regulation of common carp

Estrogens are important for bi-directional neuroendocrine-immune interaction. They act via nuclear estrogen receptors (ERα and ERβ) and/or G-protein coupled receptor - GPR30. We found expression of ERα, ERβ and GPR30 in carp lymphoid tissues and head kidney monocytes/macrophages, neutrophils and lymphocytes. Interestingly, ERβ is also expressed in some head kidney lymphocytes but not in naive PBLs. Immune stimulation altered the cell type specific profile of expression of these receptors, which depends on both activation and maturation stage. This implies direct leukocyte responsiveness to estrogen stimulation and therefore in vitro effects of 17β-estradiol (E2) on reactive oxygen species (ROS) production in monocytes/macrophages were determined. Short-time incubation with E2 increased ROS production in PMA-stimulated cells. Results comply with mediation by GPR30, partially functioning via phosphoinositide 3-kinase activation. These results furthermore demonstrate that neuroendocrine-immune communication via estrogen receptors is evolutionary conserved.

Characterization and expression analysis of an interferon-γ2 induced chemokine receptor CXCR3 in common carp (Cyprinus carpio L.)

Chemokine and chemokine receptor signalling pairs play a crucial role in regulation of cell migration, morphogenesis, and cell activation. Expressed in mammals on activated T and NK cells, chemokine receptor CXCR3 binds interferon-γ inducible chemokines CXCL9-11 and CCL21. Here we sequenced the carp CXCR3 chemokine receptor and showed its relationship to CXCR3a receptors found in other teleosts. We found high expression of the CXCR3 gene in most of the organs and tissues of the immune system and in immune-related tissues such as gills and gut, corroborating a predominantly immune-related function. The very high expression in gill and gut moreover indicates a role for CXCR3 in cell recruitment during infection. High in vivo expression of CXCR3 at later stages of inflammation, as well as its in vitro sensitivity to IFN-γ2 stimulation indicate that in carp, CXCR3 is involved in macrophage-mediated responses. Moreover, as expression of the CXCR3 and CXCb genes coincides in the focus of inflammation and as both the CXCb chemokines and the CXCR3 receptor are significantly up-regulated upon IFN-γ stimulation it is hypothesized that CXCb chemokines may be putative ligands for CXCR3.

Mechanisms involved in apoptosis of carp leukocytes upon in vitro and in vivo immunostimulation

During inflammation leukocyte activity must be carefully regulated, as high concentrations and/or prolonged action of pro-inflammatory mediators e.g. reactive oxygen species (ROS) can be detrimental not only for pathogens but also for host tissues. Programmed cell death - apoptosis is a most effective regulatory mechanism for down regulation of leukocyte activity, but little is known about this process in fish. We aimed to reveal the mechanisms of initiation and regulation of apoptosis in carp neutrophilic granulocytes and macrophages. During zymosan-induced peritonitis in carp, activated inflammatory neutrophilic granulocytes and monocytes/macrophages died by apoptosis. This correlated with a strong production of ROS, but pretreatment of the fish with NADPH oxidase inhibitor only slightly decreased late apoptosis. Interestingly in vitro incubation with zymosan or phorbol ester, but not lipopolisaccharide and poli I:C induced apoptosis of head kidney neutrophilic granulocytes. This coincided with loss of mitochondrial membrane potential. Moreover, in zymosan-stimulated neutrophilic granulocytes NADPH oxidase inhibitor not only reduced the production of ROS but also apoptosis. A similar effect was not observed in cells stimulated with phorbol ester, where DPI reduced ROS production, but not apoptosis. In PMA-stimulated neutrophilic granulocytes both the respiratory burst and apoptosis were reduced by protein kinase inhibitor. Furthermore, a short neutrophil stimulation either with PMA or with zymosan did induce caspase-independent apoptosis. These results show that in carp, apoptosis is an important regulatory process during in vitro and in vivo immunostimulation. In neutrophils, protein kinase, but not NADPH oxidase, is involved in PMA-induced apoptosis while apoptosis induced by zymosan is ROS-dependent.

Inflammatory responses and side effects generated by several adjuvant-containing vaccines in turbot

Several of the adjuvants used in fish vaccines cause adhesions in internal organs when they are injected intraperitoneally. We describe the damage caused by vaccines containing different adjuvants in the turbot Scophthalmus maximus and show that internal adhesions can be greatly reduced by injecting the fish in a specific way. Injection of fish with the needle directed towards the anterior part of the peritoneal cavity induced formation of a single cell-vaccine mass (CVM) that became attached to the parietal peritoneum. However, injection of the fish with the needle pointing in the opposite direction generated many small CVM that became attached to the visceral and parietal peritoneum and in some cases caused internal adhesions. We describe the structural and cellular changes in the adjuvant-induced CVMs. The CVMs mainly comprised neutrophils and macrophages, although most of the former underwent apoptosis, which was particularly evident from day 3 post-injection. The apoptotic cells were phagocytosed by macrophages, which were the dominant cell type from the first days onwards. All of the vaccines induced angiogenesis in the area of contact between the CVM and the mesothelium. Vaccines containing oil-based adjuvants or microspheres induced the formation of granulomas in the CVM; however, no granulomas were observed in the CVM induced by vaccines containing aluminium hydroxide or Matrix-Q(®) as adjuvants. All of the vaccines induced strong migration of cells to the peritoneal cavity. Although some of these cells remained unattached in the peritoneal cavity, most of them formed part of the CVM. We also observed migration of the cells from the peritoneal cavity to lymphoid organs, indicating bidirectional traffic of cells between the inflamed areas and these organs.

Evolutionary conservation of divergent pro-inflammatory and homeostatic responses in Lamprey phagocytes

In higher vertebrates, phagocytosis plays a critical role in development and immunity, based on the internalization and removal of apoptotic cells and invading pathogens, respectively. Previous studies describe the effective uptake of these particles by lower vertebrate and invertebrate phagocytes, and identify important molecular players that contribute to this internalization. However, it remains unclear if individual phagocytes mediate internalization processes in these ancient organisms, and how this impacts the balance of pro-inflammatory and homeostatic events within their infection sites. Herein we show that individual phagocytes of the jawless vertebrate Petromyzon marinus (sea lamprey), like those of teleost fish and mice, display the capacity for divergent pro-inflammatory and homeostatic responses following internalization of zymosan and apoptotic cells, respectively. Professional phagocytes (macrophages, monocytes, neutrophils) were the primary contributors to the internalization of pro-inflammatory particles among goldfish (C. auratus) and lamprey (P. marinus) hematopoietic leukocytes. However, goldfish showed a greater ability for zymosan phagocytosis when compared to their jawless counterparts. Coupled to this increase was a significantly lower sensitivity of goldfish phagocytes to homeostatic signals derived from apoptotic cell internalization. Together, this translated into a significantly greater capacity for induction of antimicrobial respiratory burst responses compared to lamprey phagocytes, but also a decreased efficacy in apoptotic cell-driven leukocyte homeostatic mechanisms that attenuate this pro-inflammatory process. Overall, our results show the long-standing evolutionary contribution of intrinsic phagocyte mechanisms for the control of inflammation, and illustrate one effective evolutionary strategy for increased responsiveness against invading pathogens. In addition, they highlight the need for development of complementary regulatory mechanisms of inflammation to ensure continued maintenance of host integrity amidst increasing challenges from invading pathogens.

Novel insights into the peritoneal inflammation of rainbow trout (Oncorhynchus mykiss)

The peritoneal cavity has been extensively used as a laboratory model of inflammation in many species, including the teleost fish. Although, the peritoneal cavity of rainbow trout (Oncorhynchus mykiss) was previously shown to contain a resident population of leukocytes, closer information about their exact composition and their functional response to pathogens is still missing. In the presented work, flow cytometric analysis using monoclonal antibodies was performed to characterize this cell population and evaluate its traffic during the first 72 h after antigenic stimulation and infection with Aeromonas salmonicida. Obtained results indicate that the unstimulated peritoneal cavity represents rather a lymphoid niche, dominated by the IgM(+) B cells. Expectedly, the composition changed rapidly after stimulation, which resulted in two complete changes of dominant cell type within first 72 h post injection. While the first stage of inflammation was dominated by myeloid cells, lymphocytes predominated at the later time points, with IgM(+) B cells representing more than two thirds of all cells. Later, the infection experiment elucidated the peritoneal infection and identified the key differences to the antigenic stimulation. Additionally, the data indicate that the resolution of the inflammation depends more on the bacterial clearance by myeloid cells than on regulation by lymphocytes. Taken together, obtained results represent the first complete description of the immune reaction protecting the peritoneal cavity of the fish and shed some light on the conservation of these processes during the evolution.

Oxidative burst and nitric oxide responses in carp macrophages induced by zymosan, MacroGard(®) and selective dectin-1 agonists suggest recognition by multiple pattern recognition receptors

β-Glucans are glucose polymers that are found in the cell walls of plants, bacteria, certain fungi, mushrooms and the cell wall of baker's yeast. In mammals, myeloid cells express several receptors capable of recognizing β-glucans, with the C-type lectin receptor dectin-1 in conjunction with Toll-like receptor 2 (TLR2), considered key receptors for recognition of β-glucan. In our studies to determine the possible involvement of these receptors on carp macrophages a range of sources of β-glucans were utilized including particulate β-glucan preparations of baker's yeast such as zymosan, which is composed of insoluble β-glucan and mannan, and MacroGard(®), a β-glucan-based feed ingredient for farmed animals including several fish species. Both preparations were confirmed TLR2 ligands by measuring activation of HEK293 cells transfected with human TLR2 and CD14, co-transfected with a secreted embryonic alkaline phosphatase (SEAP) reporter gene. In addition, dectin-1-specific ligands in mammals i.e. zymosan treated to deplete the TLR-stimulating properties and curdlan, were monitored for their effects on carp macrophages by measuring reactive oxygen and nitrogen radicals production, as well as cytokine gene expression by real-time PCR. Results clearly show the ability of carp macrophages to strongly react to particulate β-glucans with an increase in the production of reactive oxygen and nitrogen radicals and an increase in cytokine gene expression, in particular il-1β, il-6 and il-11. We identified carp il-6, that was previously unknown. In addition, carp macrophages are less, but not unresponsive to selective dectin-1 agonists, suggesting recognition of β-glucans by multiple pattern recognition receptors that could include TLR but also non-TLR receptors. Candidate receptors for recognition of β-glucans are discussed.

Neuroendocrine-immune interaction: regulation of inflammation via G-protein coupled receptors

Neuroendocrine- and immune systems interact in a bi-directional fashion to communicate the status of pathogen recognition to the brain and the immune response is influenced by physiological changes. The network of ligands and their receptors involved includes cytokines and chemokines, corticosteroids, classical pituitary hormones, catecholamines and neuropeptides (e.g. opioids), as well as neural pathways. We studied the role of opioid, adrenergic and melatonin G-protein coupled receptors (GPCR) on carp (Cyprinus carpio) leucocytes. Ligand interaction by morphine and adrenaline both in vitro and in vivo resulted in considerable decrease of chemotaxis and expression of CXC chemokines and chemokine CXC receptors. These effects may have substantial influence on the process of inflammation, the efficacy of which is crucial for an effective immune response. Both opioid receptors and chemokine receptors are G-protein coupled receptors (GPCRs), and were classically assumed to function as monomers. This paradigm is now challenged by the emerging concept of homo- and hetero dimerization which may represent the native form of many receptors. G-protein coupling, downstream signaling and regulatory processes such as receptor internalization are largely influenced by the dimeric nature. The true functional importance of GPCR interactions remains enigmatic, but it certainly has implications with respect to the specificity of currently used medications. This review focuses on the important function of chemokine GPCRs during inflammation and the potential neuroendocrine modulation of this process through "neuroendocrine" GPCRs.

Neuroendocrine modulation of the inflammatory response in common carp: adrenaline regulates leukocyte profile and activity

Inflammatory responses have to be carefully controlled, as high concentrations and/or prolonged action of inflammation-related molecules (e.g. reactive oxygen species, nitric oxide and pro-inflammatory cytokines) can be detrimental to host tissue and organs. One of the potential regulators of the inflammatory process are stress mediators including adrenaline. In vivo effects of adrenaline were studied during zymosan-induced (Z) peritoneal inflammation in the common carp Cyprinus carpio L. Adrenaline injected together with zymosan (ZA) did not change the number of inflammatory leukocytes in the peritoneal cavity, however at 24h post-injection it significantly reduced the percentage of monocytes/macrophages. Moreover, compared to cells retrieved from fish treated with PBS or zymosan only, adrenaline increased the percentage of apoptotic leukocytes in the focus of inflammation. Furthermore, adrenaline significantly reduced the expression of chemokine CXCL8_L1 (a functional homolog of mammalian IL-8) and its receptors (CXCR1 and CXCR2), indicating changes in leukocyte recruitment after stress. We conclude that adrenaline may contribute to a coordinated reaction by influencing the inflammatory response via direct regulation of leukocyte migration and/or apoptosis.

Fish and mammalian phagocytes differentially regulate pro-inflammatory and homeostatic responses in vivo

Phagocytosis is a cellular mechanism that is important to the early induction of antimicrobial responses and the regulation of adaptive immunity. At an inflammatory site, phagocytes serve as central regulators for both pro-inflammatory and homeostatic anti-inflammatory processes. However, it remains unclear if this is a recent evolutionary development or whether the capacity to balance between these two seemingly contradictory processes is a feature already displayed in lower vertebrates. In this study, we used murine (C57BL/6) and teleost fish (C. auratus) in vitro and in vivo models to assess the evolutionary conservation of this dichotomy at a site of inflammation. At the level of the macrophage, we found that teleost fish already displayed divergent pro-inflammatory and homeostatic responses following internalization of zymosan or apoptotic bodies, respectively, and that these were consistent with those of mice. However, fish and mice displayed significant differences in vivo with regards to the level of responsiveness to zymosan and apoptotic bodies, the identity of infiltrating leukocytes, their rate of infiltration, and the kinetics and strength of resulting antimicrobial responses. Unlike macrophages, significant differences were identified between teleost and murine neutrophilic responses. We report for the first time that activated murine, but not teleost neutrophils, possess the capacity to internalize apoptotic bodies. This internalization translates into reduction of neutrophil ROS production. This may play an important part in the recently identified anti-inflammatory activity that mammalian neutrophils display during the resolution phase of inflammation. Our observations are consistent with continued honing of inflammatory control mechanisms from fish to mammals, and provide added insights into the evolutionary path that has resulted in the integrated, multilayered responses that are characteristic of higher vertebrates.

Diversification of IFNγ-inducible CXCb chemokines in cyprinid fish

We earlier identified two CXCL8-like lineages in cyprinid fish, which are functional homologues of the mammalian CXCL8, but with diverged functions. We here investigated whether the carp IFN-γ-inducible CXCb gene, related to the mammalian CXCL9, -10 and -11 chemokines, was subject to a similar diversification. On the zebrafish genome, a cluster of seven CXCb genes was found on chromosome five. Analysis of the promoter of the zebrafish CXCb genes suggests a partially shared, but differential induction. A second CXCb gene, CXCb2, was identified in common carp by homology cloning. CXCb2 is constitutively expressed in immune-related tissues, predominantly in head kidney lymphocytes/monocytes. Interestingly, an induction of CXCb2 gene expression with recombinant carp IFN-γ2 and LPS was observed in macrophages and granulocytes. Finally, difference in sensitivity to LPS, and kinetics of CXCb1 and CXCb2 gene expression during zymosan-induced peritonitis, was observed. These results indicate a functional diversification for cyprinid CXCb chemokines, with functional homology to mammalian CXCL9-11.

Reduced inflammatory response to Aeromonas salmonicida infection in common carp (Cyprinus carpio L.) fed with β-glucan supplements

The objective of the present study was to determine the action of β-glucans as feed additives on the gene expression profile of some inflammatory-related cytokines from common carp (Cyprinus carpio L.) during the early stages of a non-lethal bacterial infection with Aeromonas salmonicida. β-glucan (MacroGard(®)), was administered daily to carp (6 mg per kg body weight) in the form of supplemented commercial food pellets for 14 days prior to infection. Control and treated fish were then intraperitoneally injected with PBS or 4×10(8) bacteria per fish and were sampled at time 0 and 6h, 12h, 1 day, 3 days and 5 days post-injection. Head kidney and gut were collected and the gene expression patterns for tnfα1, tnfα2, il1β, il6 and il10 were analyzed by quantitative PCR. Results obtained showed that treatment with β-glucans generally down-regulated the expression of all measured genes when compared to their corresponding controls. After injection, highest changes in the gene expression levels were obtained at 6h; particularly, in head kidney there was higher up-regulation of tnfa1 and tnfa2 in infected fish fed β-glucans in comparison to control feed; however, in gut there was a significant down-regulation of tnfα1, tnfα2, il1β and il6 in infected fish fed β-glucans. Analysis of carp specific antibodies against A. salmonicida 30 days after injection revealed their levels were reduced in the infected β-glucan group. In conclusion, a diet supplemented with β-glucan (MacroGard(®)) reduced the gene expression levels of some inflammation-related cytokines in common carp. Such a response appears to be dependent of organ studied and therefore the immunostimulant may be preventing an acute and potential dangerous response in gut, whilst enhancing the inflammatory response in head kidney when exposed to A. salmonicida.

Pro-inflammatory functions of carp CXCL8-like and CXCb chemokines

Numerous CXC chemokines have been identified in fish, however, their role in inflammation is not well established. Here, CXC chemokines of the CXCL8-like (CXCa_L1 and CXCL8_L2) and CXCL9/10/11-like (CXCb) subset were investigated in carp. Recombinant CXCa_L1, CXCL8_L2 and CXCb all stimulated chemotaxis of macrophages and granulocytes in vitro. CXCb also attracted lymphocytes. Distinct effects on phagocyte activation were observed: the CXCL8-like chemokines increase respiratory burst activity, but not nitrite production. The three chemokines differentially induced a moderate increase in IL-1β, CXCa_L1 and CXCL8_L2 gene expression. Intracellular calcium mobilization in granulocytes upon CXCa_L1 stimulation implies signal transduction through G-protein coupled CXC receptors. Notably, upon intraperitoneal administration, carp CXCL8-like chemokines strongly induced in vivo leukocyte recruitment, including neutrophils and monocytes/macrophages, in contrast to CXCb, for which the number of recruited leukocytes was low. The results indicate functional homology for carp CXCL8-like and CXCb chemokines with mammalian CXCL8 and CXCL9-11, respectively.

Adrenergic regulation of the innate immune response in common carp (Cyprinus carpio L.)

Catecholamines exert their physiological actions through α and β adrenergic receptors (ARs). As ARs are not exclusively expressed on neuroendocrine cells, but also on leukocytes, they may facilitate neuroendocrine modulation of immune responses. We sequenced the β(2a)-AR in common carp, and studied its expression profile and involvement in the regulation of teleost innate immune responses. β(2a)-AR messenger RNA was found to be constitutively expressed in brain areas, especially in the preoptic nucleus (NPO, homologous to the mammalian hypothalamus), and in immune organs. During the active phase of an in vivo inflammatory response, induced by i.p. zymosan treatment, β(2a)-AR gene expression was up-regulated in the peritoneal leukocytes. Additionally, adrenaline in vitro reduced the synthesis of oxygen radical species and nitric oxide, while it enhanced arginase activity in fish phagocytes. Furthermore, in vitro adrenaline administration inhibited expression of pro-inflammatory cytokines, chemokines and their receptors. It is therefore hypothesized that adrenaline will down-regulate phagocyte skewing toward classical/innate polarization.

Heterogeneity of macrophage activation in fish

In this review, we focus on four different activation states of fish macrophages. In vitro, stimulation with microbial ligands induces the development of innate activated macrophages whereas classically activated macrophages can be induced by stimulation with LPS in combination with (recombinant) IFNγ. Both types of macrophages show elevated phagocytic activity, expression of pro-inflammatory cytokine genes and radical production. Alternatively activated macrophages require the cytokines IL-4/IL-13 for induction of, among others, arginase activity. Until in vitro studies identify the effects of putative IL-4 and IL-13 homologues on fish macrophages, arginase enzyme activity remains the most reliable marker for the presence of alternatively activated macrophages in fish. The best evidence for the existence of regulatory macrophages, associated with the presence of IL-10, comes from in vivo studies, for example during parasitic infections of carp. Altogether, differentially activated macrophages in fish largely resemble the phenotypes of mammalian macrophages. However, the presence of fish-specific ligand recognition by TLRs and of duplicated genes coding for proteins with particular activities, poses additional challenges for the characterization of phenotype-specific gene signatures and cell surface markers.

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.

CXCL8 chemokines in teleost fish: two lineages with distinct expression profiles during early phases of inflammation

BACKGROUND

During the inflammatory process, chemokine CXCL8 plays a pivotal role in recruitment of human neutrophilic granulocytes. A diversity of sequences similar to CXCL8 was reported in fish, but their evolutionary relationships and functional homology with their human homolog remain unclear.

PRINCIPAL FINDINGS

We screened fish genomes to seek for sequences related to CXCL8. A first lineage was retrieved in all teleosts, while a second CXCL8 lineage was found in zebrafish and carp only. An early inflammatory function for both lineages was indicated by several lines of evidence. The induction of carp CXCL8s, CXCb, and CXC receptor-1 and -2 was analyzed after in vitro stimulation of leukocyte subpopulations and in two in vivo inflammation models. Recombinant proteins of carp CXCL8 proteins were produced and showed significant chemotactic activity for carp leukocytes.

CONCLUSIONS

While both carp CXCL8s appear to be functional homologs of mammalian CXCL8, their different induction requirements and kinetics evoke a gene-specific sub-functionalization.

The immune response differentially regulates Hsp70 and glucocorticoid receptor expression in vitro and in vivo in common carp (Cyprinus carpio L.)

Heat shock or stress proteins and glucocorticoids (cortisol) regulate a sequential pro-inflammatory and anti-inflammatory cytokine expression profile to effectively kill pathogens, whilst minimizing damage to the host. Cortisol elicits its effects through the glucocorticoid receptor (GR) for which Hsp70 and Hsp90 are required as chaperones. In common carp, (Cyprinus carpio) duplicated glucocorticoid receptor genes and splice variants with different cortisol sensitivities exist. We investigated the expression profiles of heat shock proteins Hsp70, Hsc70, Hsp90alpha and Hsp90beta and the three different variants of GR in vitro in and in vivo to define their role in immune modulation. A rapid transient induction of GR1 (a and b) and Hsp70 was seen after LPS treatment in vitro in head kidney phagocytes, whereas cortisol treatment did not affect constitutive or LPS-induced expression of Hsp70 or GR1 expression. In vivo zymosan-induced peritonitis upregulated GR and Hsp70 expression which appears to increase sensitivity for cortisol-induced immune modulation. Indeed, the increased GR and Hsp70 expression correlates with inhibition of both LPS- and zymosan-induced expression of pro-inflammatory cytokines. Infection with the blood parasite T. borreli decreases GR1a expression in thymus, but increases GR2 expression in spleen. Differentially regulated expression of Hsp70 and of glucocorticoid receptor variants with different cortisol sensitivities, underlines their physiological importance in a balanced immune response.

Cloning of opioid receptors in common carp (Cyprinus carpio L.) and their involvement in regulation of stress and immune response

In mammals opiate alkaloids and endogenous opioid peptides exert their physiological and pharmacological actions through opioid receptors (MOR, DOR and KOR) expressed not only on neuroendocrine cells but also on leukocytes. Therefore, opioids can modulate the immune response. We cloned and sequenced all three classical opioid receptors (MOR, DOR and KOR) in common carp, and studied changes in their expression during stress and immune responses. Messenger RNA of opioid receptors was constitutively expressed in brain areas, specially in the preoptic nucleus NPO (homologous to mammalian hypothalamus). After exposure to prolonged restraint stress, mRNA levels of MOR and DOR decreased in the NPO and in the head kidney. Increased expression of all studied opioid receptors was observed in the pituitary pars distalis (containing ACTH-producing cells). In immune organs, constitutive but lower expression of opioid receptor genes was observed. During in vivo zymosan-induced peritonitis or after in vitro LPS-induced stimulation, when pro-inflammatory functions are activated, expression of the OR genes in leukocytes was concomitantly up-regulated. Additionally, specific agonists of opioid receptors especially reduced leukocyte migratory properties, manifested by reduced chemotaxis and down-regulated expression of chemokine receptors. Our data indicate an evolutionary conserved role for the opioid system in maintaining a dynamic equilibrium while coping with stress and/or infection.

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.

Differential expression of two interferon-gamma genes in common carp (Cyprinus carpio L.)

Two interferon gamma (IFN-gamma) genes are expressed in immune cells of teleost fish and are potentially implicated in B- and T-lymphocyte responses. IFN-gamma-2 shows structural and functional characteristics to other vertebrate IFN-gamma genes and is associated with T-lymphocyte function. Expression profiling shows IFN-gamma-2 upregulation in T-lymphocytes after phytohemagglutinin (PHA) stimulation in vitro. Unexpectedly, we found IFN-gamma-1, which is structurally different from IFN-gamma-2, to be expressed in lipopolysacharide (LPS)-stimulated IgM+ (B- lymphocyte enriched) fractions. Expression of T-box transcription factor T-bet, but not of GATA-binding protein 3 (GATA3), correlated with expression of both IFN-gamma genes. In-vivo parasite infection, but as predicted not zymosan-induced inflammation, resulted in concomitant upregulation of T-bet and IFN-gamma-2. This corroborates a genuine T-lymphocyte associated role for IFN-gamma-2.

Expression profiles of matrix metalloproteinase 9 in teleost fish provide evidence for its active role in initiation and resolution of inflammation

Matrix metalloproteinase 9 (MMP-9) belongs to a family of zinc-dependent endopeptidases. As a consequence of its ability to cleave structural extracellular matrix molecules, mammalian MMP-9 is associated with vital inflammatory processes such as leucocyte migration and tissue remodelling and regeneration. Interestingly, MMP-9 genes have been identified in fish, but functional data are still limited and focus on the involvement of MMP-9 in embryonic development, reproduction and post-mortem tenderization. Here, we describe the involvement of MMP-9 in the innate immunity of carp. In carp, MMP-9 was most notably expressed in classical fish immune organs and in peritoneal and peripheral blood leucocytes, indicating a role of MMP-9 in immune responses. In our well-characterized zymosan-induced peritonitis model for carp, we analysed expression of the MMP-9 gene and the gelatinolytic levels of both pro- and activated forms of MMP-9. The biphasic profile of MMP-9 mRNA expression indicated involvement during the initial phase of inflammation and during the later phase of tissue remodelling. Also, in vitro stimulation of carp phagocytes with lipopolysaccharide or concanavalin A increased MMP-9 gene expression, with a peak at 24 hr. The increase of MMP-9 mRNA correlated with the peak of MMP-9 gelatinolytic level in culture supernatants. These results provide evidence for an evolutionarily conserved and relevant role of MMP-9 in the innate immune response.