Melanogenesis and evidence for melanosome transport to the plasma membrane in a CD83 teleost leukocyte cell line

Melanisation in Salmonid Skeletal Muscle: A Review

Melanisation can occur in the musculature of fish. A well-known form is the melanised focal changes, or 'black spots', in the fillet of farmed Atlantic salmon (Salmo salar). The aetiology of black spots has not been fully determined, though recent research has emphasised the role of fat necrosis in their development. The initial stages of the changes are observed as focal haemorrhages or 'red spots', and these can progress into melanised focal changes (MFCs). The focal haemorrhages are acute changes characterised by necrotic myocytes and adipocytes and diffuse haemorrhage in the tissue. These changes evolve into a chronic inflammation dominated by fibrosis, encapsulated lipid droplets or pseudocysts, presence of epithelioid cells, granulomas of varying character, giant cells and melano-macrophages, whose presence accounts for the discolouration. The inflammation ranges from mild to severe, and the severity of the lesion has been associated with localised piscine orthoreovirus 1 (PRV 1) replication in macrophages and melano-macrophages within granulomas. The possibility of a genetic impact on the condition has not been supported by available data. The lipid composition and the antioxidative properties of the feed have been shown to affect the development of the changes. Physiological and environmental factors are also believed to influence the prevalence and severity of the condition. Here, we review the current state of knowledge concerning melanisation in fish skeletal musculature, with a special emphasis on the MFCs in Atlantic salmon.

Establishment of a genetically amenable fibroblast cell line from Atlantic salmon skin

The Atlantic salmon, Salmo Salar, is a societally important species of fish, both as a food source and as a component of aquatic biosphere. Its sustainable production is hampered by a wide range of infectious diseases, which is difficult to address due to the lack of in vitro tools to study the disease-host interaction. In this paper, we describe the establishment and characterization of a homogenous Atlantic salmon skin fibroblast (ASSF) cell line. This immortalized cell line grows well in standard media formulations and is capable of migration. It is transcriptionally stable over dozens of passages, and its transcriptome is distinct from other publicly available Atlantic salmon cell lines (SHK1 and ASK). Even though ASSF cells show limited cytopathic effects when challenged with Infectious Pancreatic Necrosis Virus (IPNV) molecular evidence reveals that they are infected and support IPNV production, especially compared to other cell lines like ASK or SHK1. The potential of the ASSF cell line as a tool for Atlantic salmon research is highlighted by its permissibility to genetic manipulation with various methods including CRISPR/cas9, transfection and transduction.

Analysis of morphology, histology characteristics, and circadian clock gene expression of Onychostoma macrolepis at the overwintering period and the breeding period

Fish typically adapt to their environment through evolutionary traits, and this adaptive strategy plays a critical role in promoting species diversity. Onychostoma macrolepis is a rare and endangered wild species that exhibits a life history of overwintering in caves and breeding in mountain streams. We analyzed the morphological characteristics, histological structure, and expression of circadian clock genes in O. macrolepis to elucidate its adaptive strategies to environmental changes in this study. The results showed that the relative values of O. macrolepis eye diameter, body height, and caudal peduncle height enlarged significantly during the breeding period. The outer layer of the heart was dense; the ventricular myocardial wall was thickened; the fat was accumulated in the liver cells; the red and white pulp structures of the spleen, renal tubules, and glomeruli were increased; and the goblet cells of the intestine were decreased in the breeding period. In addition, the spermatogenic cyst contained mature sperm, and the ovaries were filled with eggs at various stages of development. Throughout the overwintering period, the melano-macrophage center is located between the spleen and kidney, and the melano-macrophage center in the cytoplasm has the ability to synthesize melanin, and is arranged in clusters to form cell clusters or white pulp scattered in it. Circadian clock genes were identified in all organs, exhibiting significant differences between the before/after overwintering period and the breeding period. These findings indicate that the environment plays an important role in shaping the behavior of O. macrolepis, helping the animals to build self-defense mechanisms during cyclical habitat changes. Studying the morphological, histological structure and circadian clock gene expression of O. macrolepis during the overwintering and breeding periods is beneficial for understanding its unique hibernation behavior in caves. Additionally, it provides an excellent biological sample for investigating the environmental adaptability of atypical cavefish species.

The melano-macrophage: The black leukocyte of fish immunity

Melanin and the process of melanin synthesis or melanogenesis have central roles in the immune system of insects, and production of melanin-synthesizing enzymes from their haemocytes may be induced following activation through danger signals. Melanin-containing macrophage-like cells have been extensively studied in amphibians and they are also present in reptiles. In fish, melano-macrophages are especially recognized with respect to melano-macrophage centres (MMCs), hypothesized to be analogues of germinal centres in secondary lymphoid organs of mammals and some birds. Melano-macrophages are in addition present in several inflammatory conditions, in particular melanised focal changes, or black spots, in the musculature of farmed Atlantic salmon, Salmo salar. Melanins are complex compounds that may be divided into different forms which all have the ability to absorb and scatter light. Other functions include the quenching of free radicals and a direct effect on the immune system. According to the common view held in the pigment cell community, vertebrate melanin synthesis with melanosome formation may only occur in cells of ectodermal origin. However, abundant information suggests that also myeloid cells of ectothermic vertebrates may be classified as melanocytes. Here, we discuss these opposing views and review relevant literature. Finally, we review the current status on the research concerning melanised focal muscle changes that represent the most severe quality problem in Norwegian salmon production, but also other diseases where melano-macrophages play important roles.

Immunotoxic effects of metal-based nanoparticles in fish and bivalves

There is a global research interest in metal nanoparticles (MNPs) due to their diverse applications, rapidly increasing use, and increased presence in the aquatic environment. Currently, most MNPs in the environment are at levels unlikely to cause overt toxicity. Sub-lethal effects that MNPs may induce, notable immunotoxicity, could however have significant health implications. Thus, deciphering the immunological interactions of MNPs with aquatic organisms constitutes a much-needed area of research. In this article, we critically assess the evidence for immunotoxic effects of MNPs in bivalves and fish, as key wildlife sentinels with widely differing ecological niches that are used as models in ecotoxicology. The first part of this review details the properties, fate, and fundamental physicochemical behavior of MNPs in the aquatic ecosystem. We then consider the toxicokinetics of MNP uptake, accumulation, and deposition in fish and bivalves. The main body of the review then focuses on immune reactions in response to MNPs exposure in bivalves and fish illustrating their immunotoxic potential. Finally, we identify major knowledge gaps in our current understanding of the implications of MNPs exposure for immunological functions and the associated health consequences for bivalves and fish, as well as the general lessons learned on the immunotoxic properties of the emerging class of nanoparticulate contaminants in fish and bivalves.

Anatomy of teleost fish immune structures and organs

The function of a tissue is determined by its construction and cellular composition. The action of different genes can thus only be understood properly when seen in the context of the environment in which they are expressed and function. We now experience a renaissance in morphological research in fish, not only because, surprisingly enough, large structures have remained un-described until recently, but also because improved methods for studying morphological characteristics in combination with expression analysis are at hand. In this review, we address anatomical features of teleost immune tissues. There are approximately 30,000 known teleost fish species and only a minor portion of them have been studied. We aim our review at the Atlantic salmon (Salmo salar) and other salmonids, but when applicable, we also present information from other species. Our focus is the anatomy of the kidney, thymus, spleen, the interbranchial lymphoid tissue (ILT), the newly discovered salmonid cloacal bursa and the naso-pharynx associated lymphoid tissue (NALT).

PRV-1 Infected Macrophages in Melanized Focal Changes in White Muscle of Atlantic Salmon (Salmo salar) Correlates With a Pro-Inflammatory Environment

Melanized focal changes in white skeletal muscle of farmed Atlantic salmon, "black spots", is a quality problem affecting on average 20% of slaughtered fish. The spots appear initially as "red spots" characterized by hemorrhages and acute inflammation and progress into black spots characterized by chronic inflammation and abundant pigmented cells. Piscine orthoreovirus 1 (PRV-1) was previously found to be associated with macrophages and melano-macrophages in red and black spots. Here we have addressed the inflammatory microenvironment of red and black spots by studying the polarization status of the macrophages and cell mediated immune responses in spots, in both PRV-1 infected and non-infected fish. Samples that had been collected at regular intervals through the seawater production phase in a commercial farm were analyzed by multiplex fluorescent in situ hybridization (FISH) and RT-qPCR methods. Detection of abundant inducible nitric oxide synthase (iNOS2) expressing M1-polarized macrophages in red spots demonstrated a pro-inflammatory microenvironment. There was an almost perfect co-localization with the iNOS2 expression and PRV-1 infection. Black spots, on the other side, had few iNOS2 expressing cells, but a relatively high number of arginase-2 expressing anti-inflammatory M2-polarized macrophages containing melanin. The numerous M2-polarized melano-macrophages in black spots indicate an ongoing healing phase. Co-localization of PRV-1 and cells expressing CD8⁺ and MHC-I suggests a targeted immune response taking place in the spots. Altogether, this study indicates that PRV-1 induces a pro-inflammatory environment that is important for the pathogenesis of the spots. We do not have indication that infection of PRV-1 is the initial causative agent of this condition.

Marking vertebrates langerhans cells, from fish to mammals

Langerhans cells (LCs) are specialized dendritic cells (DCs) that play a defense role in recognizing foreign antigens, in tissue where antigenic exposures occur, as in the skin and mucous membranes. LCs are able to continuously move within the tissues thanks to dendritic contraction and distension performing their surveillance and/or phagocytosis role. These cells are characterized by the presence of Birbeck granules in their cytoplasm, involved in endocytosis. LCs have been characterized in several classes of vertebrates, from fish to mammals using different histological and molecular techniques. The aim of the present review is to define the state of art and the need of information about immunohistochemical markers of LCs in different classes of vertebrates. The most used immunohistochemical (IHC) markers are Langerin/CD207, CD1a, S-100 and TLR. These IHC markers are described in relation to their finding in different vertebrate classes with phylogenetical considerations. Among the four markers, Langerin/CD207 and TLR have the widest spectrum of cross reactivity in LCs.

The effects of chronic cadmium exposure on Bufo gargarizans larvae: Histopathological impairment, gene expression alteration and fatty acid metabolism disorder in the liver

Cadmium (Cd) a highly toxic metal to human and wildlife health and it is hazardous to both terrestrial and aquatic life. In this study, we used RNA sequencing analysis to examine the effects of chronic cadmium exposure on liver lipid metabolism of Bufo gargarizans larvae. Tadpoles were exposed to cadmium concentrations at 0, 5, 10, 50, 100 and 200 μg L^-1 from Gosner stage 26-42 of metamorphic climax. The results showed high dose cadmium (50, 100 and 200 μg L^-1) caused obvious histological changes characterized by hepatocytes deformation, nuclear pyknosis, increasing melanomacrophage centers (MMCs) and aggregated lipid droplets. Moreover, transcriptome analysis showed that liver function was seriously affected by cadmium exposure. Furthermore, high dose cadmium significantly upregulated the mRNA expression of elongation of very-long-chain fatty acids 1 (ELOVL1), Mitochondrial trans-2-enoyl-CoA reductase (MECR), Trans-2, 3-enoyl-CoA reductase (TER) and Hydroxysteroid (17β) dehydrogenase type 12 (HSD17B12) which are related with fatty acid synthesis. Meanwhile, mRNA levels of genes related with fat acid oxidation such as acetyl-CoA acyltransferase 2 (ACAA2) and enoyl-coenzyme A (CoA) hydratase short chain 1 (ECHS1) were significantly upregulated while the expression of Acyl-coA thioesterase 1 (ACOT1), 3-hydroxyacyl-CoA dehydrogenase (HADH), Palmitoyl-protein thioesterase 1(PPT1) and Acetyl-CoA acyltransferase 1(ACAA1) was significantly downregulated by high dose cadmium exposure. Furthermore, the mRNA level of ATP-binding cassette subfamily B member 11 (ABCB11) related with bile secretion was significantly decreased exposed to high dose cadmium. Our results suggested cadmium can cause liver dysfunction by inducing histopathological damages, genetic expression alterations and fatty acid metabolism disorder.

Differential Effects of Dietary Supplementation of Krill Meal, Soybean Meal, Butyrate, and Bactocell® on the Gene Expression of Atlantic Salmon Head Kidney

The head kidney is a key organ that plays a fundamental role in the regulation of the fish immune response and in the maintenance of endocrine homeostasis. Previous studies indicate that the supplementation of exogenous dietary components, such as krill meal (KM), soybean meal (SM), Bactocell® (BA), and butyrate (BU), can have a significant effect on the immune function of the head kidney. The aim of this study was to investigate the differential effect of these four dietary ingredients on the transcriptional profiles of the head kidney of the Atlantic salmon. This study revealed that just a small number of genes were responsive to the feeding regime after a long-term (12 weeks) treatment, and evidenced that the most significant alterations, both in terms of the number of affected genes and magnitude of changes in gene expression, were detectable in the BU- and KM-fed groups compared with controls, while the SM diet had a nearly negligible effect, and BA had no significant effects at all. Most of the differentially expressed genes were involved in the immune response and, in line with data previously obtained from pyloric caeca, major components of the complement system were significantly affected. These alterations were accompanied by an increase in the density of melanomacrophage centers in the KM- and SM-fed group and their reduction in the BU-fed group. While three types of dietary supplements (BU, KM, and SM) were able to produce a significant modulation of some molecular players of the immune system, the butyrate-rich diet was revealed as the one with the most relevant immune-stimulating properties in the head kidney. These preliminary results suggest that further investigations should be aimed towards the elucidation of the potential beneficial effects of butyrate and krill meal supplementation on farmed salmon health and growth performance.

Administration of a polyphenol-enriched feed to farmed sea bass (Dicentrarchus labrax L.): Kidney melanomacrophages response

The reinforcement of the defense mechanism of fish, through the administration of immunostimulants, is considered as a promising alternative to vaccines. Natural immunostimulants such as polyphenols, flavanoids, pigments and essential oils can modulate the innate immune response. In lower vertebrates, melano-macrophage centres, i.e. clusters of pigment-containing cells forming the extracutaneous pigment system, are wide-spread in the stroma of the haemopoietic tissue, mainly in kidney and spleen. In fishes, melano-macrophage centres play an important role in the immune response against antigenic stimulants and pathogens. In the present study, we evaluated the effect of a polyphenol-enriched diet on the health status of European sea bass (Dicentrarchus labrax L.). Farmed sea bass were administered a feed containing a phytocomplex, rich in catechins and epigallocatechins, which was obtained from the seeds of Canosina Nero di Troia Vitis vinifera and mixed with conventional feed at two different concentrations. The effects of such a diet were investigated in juvenile and commercial size samples, i.e. undergoing a short- and long-term period of diet, respectively, focusing on their extracutaneous pigmentary system and, in more detail, on the enzymatic activities leading to melanin biosynthesis. Our results show that prolonged dietary treatments with higher concentration of polyphenols might modulate tyrosinase activity and gene expression in commercial size fishes. An increase of melano-macrophage activity is correlated to a stimulation of cytoprotective functions against antigenic stimulants and pathogens, as an expression of a robust and protective adaptive immune response.

Alternatives to mineral oil adjuvants in vaccines against Aeromonas salmonicida subsp. salmonicida in rainbow trout offer reductions in adverse effects

In an effort to reduce the frequency and severity of adverse reactions seen from the use of mineral oil adjuvants in salmonid fish, the effects of two alternative adjuvants were assessed, focusing on the induction of adverse effects as well as protection. Using rainbow trout (Oncorhynchus mykiss) as recipients, injection vaccines based on formalin-inactivated Aeromonas salmonicida subspecies salmonicida were formulated with CpG oligodeoxynucleotides, the liposomal cationic adjuvant formulation 01 (CAF01) or with Freund’s incomplete adjuvant and administered intraperitoneally. Control groups of unvaccinated, Tris-buffered saline-injected or bacterin-injected individuals were included, and each group included in the study held a total number of 240 individuals. Subsequently, individuals from each group were examined for differences in Fulton’s condition factor, macro- and microscopic pathological changes, as well as protection against experimental infection with A. salmonicida. While adverse effects were not eliminated, reductions in microscopic and macroscopic adverse effects, in particular, were seen for both the nucleotide- and liposome-based vaccine formulations. Furthermore, the induced protection appears similar to that of the benchmark formulation, thus introducing viable, potential alternative types of adjuvants for use in future fish vaccines.

Histological changes, lipid metabolism and oxidative stress in the liver of Bufo gargarizans exposed to cadmium concentrations

Chinese toad (Bufo gargarizans) were exposed to different concentrations of cadmium (5, 50, 100, 200 and 500 μg Cd L^-1) from Gosner stage 3-42. Metamorphosis rate, body weight, total length and body length were measured. Histological alterations in thyroid gland and liver were examined. Changes in hepatocyte were also examined using Transmission electron microscopic. In addition, the mRNA expression of several genes involved in lipid metabolism, oxidative stress and thyroid hormones signaling pathways were also measured. Our results showed that 200 and 500 μg Cd L^-1 decreased the metamorphosis rate and inhibited the body size of B. gargarizans larvae at G42. Moreover, histological examinations have clearly exhibited that cadmium caused liver damage. Ultrastructural examination revealed lipid accumulation and abnormal mitochondria. Exposure to 200 and 500 μg Cd L^-1 significantly up-regulated mRNA expression of D2, SOD, GPx, ACC and FAE, but down-regulated mRNA expression of TRα, TRβ, PPARα, ACOX, CPT and SCP. However, low Cd concentration (5, 50 and 100) exposure did not cause any effect in genes expression. Thus, we conclude that high Cd concentrations could affect the normal processes of lipid metabolism though increasing lipid synthesis and reducing the ability of fatty acid β-oxidation, and disturb thyroid hormone pathways in liver, and induced oxidative stress. In addition, lipid metabolism might be regulated by THs. To our knowledge, the present study is the first to report the influence of cadmium on hepatic lipid metabolism in B. gargarizans and will greatly provide new insights into cadmium hepatotoxicity in amphibian.

Development and characterization of a cell line from tilapia head kidney with melanomacrophage characteristics

A novel cell line THK, derived from the tilapia head kidney, was developed and characterized. The THK cell line comprised fibroblastoid cells that markedly proliferated in Leibovitz L-15 medium containing 2%-15% fetal bovine serum (FBS) at 20 °C-35 °C. Cell proliferation was dependent on the FBS concentration, and the optimal temperature for proliferation ranged between 25 °C and 30 °C. THK cells were characterized for the presence of phagocytic activity, acid phosphatase, alkaline phosphatase, α-naphthyl acetate esterase, lipofuscin, and tyrosinase. Transcripts of CD33, CD53, CD82, CD205, macrophage colony stimulating factor receptor, GATA2, and GATA3 that are specific for leucocytes or monocytes/macrophages or both were detected in the THK cells through PCR. However, THK cells lacked for CD83, a specific marker for dendritic cells. The results indicated that the fibroblastoid THK cells were melanomacrophage-related progenitors. PCR revealed that the THK cells exhibited the transcripts of toll-like receptor 1 (TLR1), TLR2, TLR3, and CD200, of which concern with immunity as well as the transcripts of vascular endothelial growth factor receptor 3, angiomotin, and angiopoietin-like protein 2 that associate with angiogenesis regulation and macrophage proliferation. THK cells were subcultured more than 90 times and can be useful for investigating the development and functioning of the teleostean innate immune system.

Piscine orthoreovirus (PRV) in red and melanised foci in white muscle of Atlantic salmon (Salmo salar)

Melanised focal changes (black spots) are common findings in the white skeletal muscle of seawater-farmed Atlantic salmon (Salmo salar). Fillets with melanised focal changes are considered as lower quality and cause large economic losses. It has been suggested that red focal changes (red spots) precede the melanised focal changes. In the present work, we examined different populations of captive and wild salmon for the occurrence of both types of changes, which were investigated for the presence of different viruses by immunohistochemistry and RT-qPCR. The occurrence of red or melanised foci varied significantly between the populations, from none in wild fish control group, low prevalence of small foci in fish kept in in-house tanks, to high prevalence of large foci in farm-raised salmon. Large amounts of Piscine orthoreovirus (PRV) antigen were detected in all foci. No other viruses were detected. Red focal changes contained significantly higher levels of PRV RNA than apparently non-affected areas in white muscle of the same individuals. Some changes displayed a transient form between a red and melanised pathotype, indicating a progression from an acute to a chronic manifestation. We conclude that PRV is associated with the focal pathological changes in the white muscle of farmed Atlantic salmon and is a premise for the development of focal melanised changes.

CSE1L Links cAMP/PKA and Ras/ERK pathways and regulates the expressions and phosphorylations of ERK1/2, CREB, and MITF in melanoma cells

The Ras/ERK (extracellular signal-regulated protein kinase) and cAMP/PKA (protein kinase A) pathways are essential for the transcriptional activities of CREB (cAMP response element binding protein) and MITF (microphthalmia-associated transcription factor) in melanogenesis and the progression of melanoma. However, the interaction between Ras/ERK and cAMP/PKA pathways in the melanogenesis and progression of melanoma is not fully known. Here, we report that CSE1L (chromosome segregation 1-like protein) regulates cAMP/PKA-induced CREB and MITF expressions as well as Ras-induced ERK1/2 phosphorylation. IBMX, a cAMP/PKA activator, treatment induced CSE1L phosphorylation and augmented Ras-induced ERK1/2 phosphorylation. CSE1L knockdown by CSE1L shRNA expression vectors inhibited Ras-induced ERK1/2 phosphorylation and melanogenesis in melanoma cells. CSE1L overexpression increased phospho-CREB expression; CSE1L knockdown also inhibited Ras-induced phospho-CREB, MITF, and tyrosinase expressions, regardless of the presence of IBMX. This study identifies CSE1L links and controls the Ras/ERK and cAMP/PKA pathways in the melanogenesis of melanoma cells. Melanomas frequently develop drug resistance via paradoxical activation of Ras/Raf/MEK/ERK or alternatively activated Ras/ERK and cAMP/PKA pathways. Thus CSE1L may be a potential target for treating melanomas that harbor Ras mutations or are resistant to drugs targeting Raf/MEK/ERK. © 2015 Wiley Periodicals, Inc.

Melanins as biomarkers of ovarian follicular atresia in the catfish Heteropneustes fossilis: biochemical and histochemical characterization, seasonal variation and hormone effects

Follicular atresia is a common feature of the vertebrate ovary that occurs at different stages of folliculogenesis and ovarian regression. It has physiological significance to maintain homeostasis and control fecundity, and ensure removal of post-ovulatory follicular remnants for preparing the ovary for the next cycle. Pigments appear late in the atretic process as indigestible waste formed out of the degradation of the oocytes, follicle wall and granulocytes. In the present study, pigment accumulation was demonstrated by Schmorl's and Perls' staining methods in the atretic ovarian follicles of Heteropneustes fossilis during follicular development and regression. Melanins were characterized spectrophotometrically for the first time in fish ovary. The predominant form is eumelanin, followed by pheomelanin and alkali-soluble melanin. Melanins showed significant seasonal variations with levels low in gonad resting phase, increasing to the peak in the post-spawning phase. The concentration of melanins increased time-dependently in post-ovulated ovary after human chorionic gonadotropin treatment. In the spawning phase, in vitro incubation of ovary slices with estradiol-17β or dexamethasone for 8 or 16 h decreased both eumelanin and pheomelanin levels time-dependently. The alkali-soluble melanin showed a significant decrease only in the dexamethasone group at 16 h. The results show that melanin assay can be used as a biomarker of follicular atresia in fish ovary, natural or induced by environmental toxicants.

Immunoglobulin isotypes in Atlantic salmon, Salmo salar

There are three major immunoglobulin (Ig) isotypes in salmonid fish: IgM, IgD and IgT, defined by the heavy chains μ, δ and τ, respectively. As a result of whole genome duplication in the ancestor of the salmonid fish family, Atlantic salmon (Salmo salar) possess two highly similar Ig heavy chain gene complexes (A and B), comprising two μ genes, two δ genes, three intact τ genes and five τ pseudogenes. The μA and μB genes correspond to two distinct sub-populations of serum IgM. The IgM-B sub-variant has a characteristic extra cysteine near the C-terminal part of the heavy chain and exhibits a higher degree of polymer disulfide cross-linking compared to IgM-A. The IgM-B:IgM-A ratio in serum is typically 60:40, but skewed ratios are also observed. The IgT isotype appears to be specialized to mucosal immune responses in salmonid fish. The concentration of IgT in serum is 100 to 1000 times lower than IgM. Secreted forms of IgD have been detected in rainbow trout, but not yet in Atlantic salmon.

A multidisciplinary study of the extracutaneous pigment system of European sea bass (Dicentrarchus labrax L.). A possible relationship between kidney disease and dopa oxidase activity level

Infectious diseases and breeding conditions can influence fish health status. Furthermore it is well known that human and animal health are strongly correlated. In lower vertebrates melano-macrophage centres, clusters of pigment-containing cells forming the extracutaneous pigment system, are widespread in the stroma of the haemopoietic tissue, mainly in kidney and spleen. In fishes, melano-macrophage centres play an important role in the immune response against antigenic stimulants and pathogens. Hence, they are employed as biomarker of fish health status. We have investigated this cell system in the European sea bass (Dicentrarchus labrax L.) following the enzyme activities involved in melanin biosynthesis. We have found a possible relationship between kidney disease of farmed fishes and dopa oxidase activity level, suggesting it as an indicator of kidney disease. Moreover variations of dopa oxidase activity in extracutaneous pigment system have been observed with respect to environmental temperature. At last, for the first time, using femtosecond transient absorption spectroscopy (Femto-TA), we pointed out that pigment-containing cells of fish kidney tissue present melanin pigments.

Antiviral functions of CD8(+) cytotoxic T cells in teleost fish

Cytotoxic T-cells (CTLs) play a pivotal role in eliminating viruses in mammalian adaptive immune system. Many recent studies on T-cell immunity of fish have suggested that teleost CTLs are also important for antiviral immunity. Cellular functional studies using clonal ginbuan crucian carp and rainbow trout have provided in vivo and in vitro evidence that in many respects, virus-specific CTLs of fish have functions similar to those of mammalian CTLs. In addition, mRNA expression profiles of CTL-related molecules, such as CD8, TCR and MHC class I, have shown that in a wide range of fish species, CTLs are involved in antiviral adaptive immunity. These findings are a basis to formulate possible vaccination strategies to trigger effective antiviral CTL responses in teleost fish. This review describes recent advances in our understanding of antiviral CTL functions in teleost fish and discusses vaccination strategies for efficiently inducing CTL activities.

B cells in teleost fish act as pivotal initiating APCs in priming adaptive immunity: an evolutionary perspective on the origin of the B-1 cell subset and B7 molecules

The long-held paradigm that B cells cannot uptake nonspecific particulate Ags for the initiation of primary adaptive immunity has been challenged by the recent discovery that teleost B cells have potent phagocytic and microbicidal abilities. This discovery provides preliminary clues that primitive B cells might act as initiating APCs in priming adaptive immunity. In this study, zebrafish B cells clearly showed a potent Ag-presenting ability to both soluble Ags and bacterial particles to prime naive CD4(+) T cell activation. This finding demonstrates the innate-like nature of teleost B cells in the interface of innate and adaptive immunity, indicating that they might consist of a major population of initiating APCs whose performance is similar to that of dendritic cells. Given the functional similarities between teleost B cells and the mammalian B-1 subset, we hypothesize that B-1 lineage and teleost B cells might originate from a common ancestor with potent phagocytic and initiating APC capacities. In addition, CD80/86 and CD83 costimulatory signals were identified as being essential for B cell-initiated adaptive immunity. This result suggests that the costimulatory mechanism originated as early as the origin of adaptive immunity and is conserved throughout vertebrate evolution. In fish, only a single CD80/86 copy exists, which is similar to mammalian CD86 rather than to CD80. Thus, CD86 might be a more primordial B7 family member that originated from fish. This study provides valuable insights into the evolutionary history of professional APCs, B cell lineages, and the costimulatory mechanism underlying adaptive immunity as a whole.

Transcription of the tyrosinase gene family in an Atlantic salmon leukocyte cell line (SHK-1) is influenced by temperature, but not by virus infection or bacterin stimulation

The present study was performed to address putative links between the immune and pigmentary systems. A pigment-producing leukocyte-like cell-line (SHK-1 cells) of Atlantic salmon (Salmo salar L.) was exposed to different temperatures, poly I:C, bacterin or infected with virus (infectious pancreatic necrosis virus or infectious salmon anaemia virus). The effect of this stimulation regarding the transcription-pattern of the tyrosinase gene family (melanin genes) and the immune-related genes MHC class II and IFN-1 was analysed using real-time RT-qPCR. At 10°C cultivation, tyrosinase and dopachrome tautomerase remained unregulated. At 15°C, a moderate up-regulation was induced, while at 20°C, these genes were up-regulated in an exponential manner over time. Temperature did not affect the transcription of the immune-related genes. Virus infections, poly I:C or bacterin had no influence on the transcription of the melanogenesis-related genes, but triggered the immune-related genes. Our findings revealed no connections between the pigmentary and immune systems, but demonstrated a hereto undiscovered temperature-effect on the tyrosinase gene family.

Characterization of myocardial lesions associated with cardiomyopathy syndrome in Atlantic salmon, Salmo salar L., using laser capture microdissection

Cardiomyopathy syndrome (CMS) in Atlantic salmon, Salmo salar L., is characterized by focal infiltration in the spongy myocardium and endocardium of the heart. The origin of the mononuclear infiltrate is unknown. Using experimentally infected fish, we investigated localization of the causative agent, piscine myocarditis virus (PMCV), within the heart and characterized the cell population associated with myocardial lesions. Cellular and transcriptional characteristics in the lesions were compared with adjacent non-infiltrated tissues using laser capture microdissection, RT-qPCR and immunohistochemistry. Our results reveal that PMCV is almost exclusively present in myocardial lesions. The inflammatory infiltrate comprises a variety of leucocyte populations, including T cells, B cells, MHC class II(+) and CD83(+) cells, most likely of the macrophage line. Correlation analyses demonstrated co-ordinated leucocyte activity at the site of the virus infection. Cellular proliferation and/or DNA repair was demonstrated within the myocardial lesions. Different cell populations, mainly myocytes, stained positive for proliferating cell nuclear antigen (PCNA). Densities of endothelial cells and fibroblasts were not significantly increased. The simultaneous presence of PMCV and various inflammatory cells in all myocardial lesions analysed may indicate that both viral lytic and immunopathological effects may contribute to the pathogenesis of CMS.

Melanogenesis in visceral tissues ofSalmo salar. A link between immunity and pigment production?

Melanogenesis is mostly studied in melanocytes and melanoma cells, but much less is known about other pigment cell systems. Liver, spleen, kidney, and other organs of lower vertebrates harbour a visceral pigment cell system with an embryonic origin that differs from that of melanocytes. In teleosts, melanin-containing cells occur in the reticulo-endothelial system and are mainly in the kidney and spleen. The Atlantic salmon ( Salmo salar L.) is an ichthyic breeding species of considerable economic importance. The accumulation of pigments in salmon visceral organs and musculature adversely affects the quality of fish products and is a problem for the aquaculture industry. With the aim to reveal novel functions and behaviour of the salmonid extracutaneous pigment system, we investigated aspects of the melanogenic systems in the tissues of Atlantic salmon, as well as in SHK-1 cells, which is a long-term cell line derived from macrophages of the Atlantic salmon head-kidney. We demonstrate that a melanogenic system is present in SHK-1 cells, head-kidney, and spleen tissues. As teleosts lack lymph nodes and Peyer’s patches, the head-kidney and spleen are regarded as the most important secondary lymphoid organs. The detection of tyrosinase activity in lymphoid organs indicates that a link exists between the extracutaneous pigmentary system and the immune system in salmon.

Characterization of small, mononuclear blood cells from salmon having high phagocytic capacity and ability to differentiate into dendritic like cells

Phagocytes are the principal component of the innate immune system, playing a key role in the clearance of foreign particles that include potential pathogens. In vertebrates, both neutrophils and mononuclear cells like monocytes, macrophages and dendritic cells are all professional phagocytes. In teleosts, B-lymphocytes also have potent phagocytic ability. We have isolated a population of small (<5 µm), mononuclear blood cells from Atlantic salmon (Salmo salar L.) not previously characterized. In order to identify them, we have performed morphological, gene expression, flow cytometry, cytochemical, ultrastructural and functional analyses. Interestingly, they highly express the gene encoding CD83, the most characteristic cell surface marker for dendritic cells in mammals, and MHC class II limited to professional antigen presenting cells. They did not express genes nor did they have cell markers for B-cells, T-cells, monocytes/macrophages or neutrophils as shown by qRT-PCR, flow cytometry and immunoblotting. A remarkable feature of these cells is their potent phagocytic capacity. Their oxygen-independent killing mechanism, as shown by intense acid phosphatase staining, is supported by lack of respiratory burst and myeloperoxidase activity and the acid phosphatase's sensitivity to tartrate. They show a high level of morphological plasticity, as, upon stimulation with mitogens, they change morphology and obtain branching protrusions similarly to dendritic cells. We suggest, based on our findings, that the small, round cells described here are progenitor cells with potential to differentiate into dendritic like cells, although we can not exclude the possibility that they represent a novel cell type.

Pigment-producing granulomatous myopathy in Atlantic salmon: a novel inflammatory response

Melanin comprises a complex group of pigmented polymers whose primary function is ascribed to dermal solar protection, but may also have an interesting role in innate immunity. In ectothermic vertebrates, melanogenesis is reported in leukocyte populations, but it is not known if this occurs in connection with inflammatory reactions. Melanin accumulations in ectopic locations, in particular muscle, represent a serious quality problem in salmon production. Here, we investigated such changes for the expression of dopachrome tautomerase and tyrosinase as well as some important immune genes and pathogens. Furthermore, the nature of the pathological changes was addressed by morphological methods. Gene transcripts encoding key enzymes in melanogenesis, suggesting a de novo melanin synthesis in pigmented muscle, were found. MHC class II transcripts were up-regulated and there was no indication of bacterial or viral infection. The histological examination revealed granulomatous inflammation with distribution of MHC class II positive cells and T cells, analogous to the pattern found in mammals. Importantly, in contrast to mammals pigmented cells were contributing in the inflammation. We demonstrate that melanin production occurs in granulomatous inflammation in salmon, revealing a close and hitherto unreported link between the pigmentary and immune systems.

Functional identification of dendritic cells in the teleost model, rainbow trout (Oncorhynchus mykiss)

Dendritic cells are specialized antigen presenting cells that bridge innate and adaptive immunity in mammals. This link between the ancient innate immune system and the more evolutionarily recent adaptive immune system is of particular interest in fish, the oldest vertebrates to have both innate and adaptive immunity. It is unknown whether dendritic cells co-evolved with the adaptive response, or if the connection between innate and adaptive immunity relied on a fundamentally different cell type early in evolution. We approached this question using the teleost model organism, rainbow trout (Oncorhynchus mykiss), with the aim of identifying dendritic cells based on their ability to stimulate naïve T cells. Adapting mammalian protocols for the generation of dendritic cells, we established a method of culturing highly motile, non-adherent cells from trout hematopoietic tissue that had irregular membrane processes and expressed surface MHCII. When side-by-side mixed leukocyte reactions were performed, these cells stimulated greater proliferation than B cells or macrophages, demonstrating their specialized ability to present antigen and therefore their functional homology to mammalian dendritic cells. Trout dendritic cells were then further analyzed to determine if they exhibited other features of mammalian dendritic cells. Trout dendritic cells were found to have many of the hallmarks of mammalian DCs including tree-like morphology, the expression of dendritic cell markers, the ability to phagocytose small particles, activation by toll-like receptor-ligands, and the ability to migrate in vivo. As in mammals, trout dendritic cells could be isolated directly from the spleen, or larger numbers could be derived from hematopoietic tissue and peripheral blood mononuclear cells in vitro.

The effect of sub-culturing on the basal level of type I interferon (IFN) gene expression in the Salmon Head Kidney (SHK-1) cell line

Over sub-culturing a cell line generates a selective pressure which can result in key cellular functions being altered such as gene and protein expression. The present study set out to determine whether serial sub-culturing affects the antiviral state of the Salmon Head Kidney (SHK-1) cell line. Cells were cultured under constant conditions and real-time PCR was performed to measure the level of interferon (IFN) and Mx gene expression over different passage numbers. A significant increase in the basal level of IFN and Mx gene expression was recorded at passage number 58 (3 and 14-fold increase versus passage number 53), suggesting a sub-culturing effect on the type I IFN response in SHK-1 cells. Passage dependent variations in morphology and cell sub-populations have been previously observed in SHK-1 cells. Such variations in cell sub-types were suspected to be responsible for the fluctuations in IFN and Mx gene expression recorded in this study.

Teleost B7 expressed on monocytes regulates T cell responses

In mammals, professional APCs induce adaptive immunity via the activation of T cells. During this process, B7 family molecules present upon APCs are known to play crucial roles in optimal T cell stimulation. In contrast, the confirmation of APCs in a nonmammalian vertebrate has yet to be achieved. To obtain further insights into the evolutionary origin of APCs, we have identified three members of the B7 family in the teleost Takifugu rubripes (fugu): B7-H1/DC, B7-H3, and B7-H4. The three fugu B7s were expressed on the surface of blood monocytes. The B7(+) monocytes, which are composed of at least two distinct populations, expressed the MHC class II component gene. The fugu B7 molecules bound to activated T cells, indicating that putative B7 receptors were expressed upon T cells. Fugu B7-H1/DC inhibited T cell proliferation concomitant with increasing levels of both IL-10 and IFN-gamma expression, whereas both B7-H3 and B7-H4 promoted T cell growth following IL-2 induction and the suppression of IL-10. These observations indicate that fugu B7s regulate T cell responses via receptors upon T cells. We suggest that fish B7(+) monocytes are APCs and that a costimulatory system has already developed in fish via the evolutionary process.

A highly phagocytic cell line TO from Atlantic salmon is CD83 positive and M-CSFR negative, indicating a dendritic-like cell type

Leucocyte cell lines are valuable tools for immunological studies. In this study the TO cell line, originating from Atlantic salmon head kidney leucocytes, is described with respect to enzyme cytochemistry, functional studies, reactivity with leucocyte specific antibodies and immune gene expression. Pronounced characteristics of the TO cell line are the rapid adherence to the plastic growth surface, high phagocytic capacity and bactericidal functions. No respiratory burst activity, and little or no NO production were detected under the experimental conditions tested, and thus the TO cells appear to have other effective killing mechanisms. The cells are reactive with a leucocyte specific monoclonal antibody (MAb), but does not bind a neutrophil specific MAb or stain for myeloperoxidase. Real-time RT-PCR showed the expression in TO cells of several immune genes, some of which were significantly regulated following LPS stimulation. The expression of CD83 might indicate a dendritic cell (DC) origin of the TO cells, as this marker is considered a hallmark for DC. Expression of TCR-alpha or the macrophage marker M-CSFR was not detected. Based on the present analyses the TO cells display a mixture of known characteristics for macrophages and DCs. At the same time the TO cells lack some central functions of phagocytic/myeloid cells. As the TO cells are developed to a long-term culture one cannot exclude that some functions might have been lost in this process. Nevertheless, the features of the TO cells indicate their potential as a model system for immunological studies of salmon phagocytic cells.

Comparison of gene expression profiles of Fenneropenaeus chinensis challenged with WSSV and Vibrio

Microarray technique was used to analyze the gene expression profiles of shrimp when they were challenged by WSSV and heat-inactivated Vibrio anguillarum, respectively. At 6 h post challenge (HPC), a total of 806 clones showed differential expression profile in WSSV-challenged samples, but not in Vibrio-challenged samples. The genes coding energy metabolism enzyme and structure protein were the most downregulated elements in 6 h post WSSV-challenged (HPC-WSSV) tissues. However, a total of 155 clones showed differential expression in the Vibrio-challenged samples, but not in WSSV-challenged samples. Serine-type endopeptidase and lysosome-related genes were the most upregulated elements in tissues 6 h post Vibrio challenge (HPC-Vibrio). Totally, 188 clones showed differential expression in both 6 and 12 HPC-WSSV and HPC-Vibrio samples. Most of the differentially expressed genes (185/188) were downregulated in the samples of 12 HPC-WSSV, whereas upregulated in the samples at 6 and 12 HPC-Vibrio and 6 HPC-WSSV. The expression profiles of three differentially expressed genes identified in microarray hybridization were analyzed in hemocytes, lymphoid organ, and hepatopancreas of shrimp challenged by WSSV or Vibrio through real-time PCR. The results further confirmed the microarray hybridization results. The data will provide great help for us in understanding the immune mechanism of shrimp responding to WSSV or Vibrio.

Double-stranded RNA- and CpG DNA-induced immune responses in Atlantic salmon: comparison and synergies

Several TLR agonists are shown to activate piscine immunity and are interesting adjuvant candidates in vaccine development. To test the outcome of stimulating Atlantic salmon with CpG DNA and poly I:C, ligands for TLR9 and 3, respectively, we have measured the in vivo expression of different immune molecules in spleen and head kidney. The expression profiles for individual treatments with CpGs or poly I:C not only showed similarities but they also displayed unique profiles. When combining them a synergistic up-regulation of the genes interferon (IFN)-alpha1/alpha2, Mx, CXCL10, IL-1beta, IFN-gamma and CD83 was detected. Interestingly, synergies between two different CpG ODNs classes also resulted in pronounced IFN-alpha1/alpha2 and IFN-gamma transcripts levels. To our knowledge this is the first study showing synergy by combining two different TLR9 ligands. In conclusion, detection of dsRNA and CpG DNA in fish may mimic viral recognition, resulting in an enhanced innate immune response that could be used for vaccination.

Characterization of CD8+ leukocytes in fugu (Takifugu rubripes) with antiserum against fugu CD8alpha

We have investigated the characteristics of CD8+ leukocytes by using an anti-CD8alpha antiserum raised in mouse by DNA-immunization. The magnetically sorted CD8alpha+ peripheral blood leukocyte (PBL) population comprised lymphocytes/thrombocytes and monocytes, whereas CD8alpha- PBLs consisted of lymphocytes/thrombocytes, monocytes, and neutrophils. Expression analysis demonstrated that both groups of cells expressed the CD3epsilon and TCRalpha genes. The CD8alpha and CD8beta genes were detected only in CD8alpha+ cells, whereas expression of CD4 and immunoglobulin light chain (IgL) was observed only in CD8alpha- cells. These results suggest that fugu CD8alpha+ leukocytes contain CD8+ T cells, but not CD4+ T cells or B cells. Furthermore, mitogenesis of the CD8+ lymphocyte/thrombocyte population was induced by phytohemaglutinin stimulation, suggesting that fish CD8+ lymphocytes/thrombocytes (probably CD8+ T cells) have characteristics similar to mammalian CD8+ T cells. Neutrophils and monocytes/macrophages infiltrating a subcutaneous inflammatory site expressed only CD8alpha, but not CD8beta, CD4, TCRalpha, or IgL. This result suggests that similar to mammalian dendritic cells, fugu monocytes/macrophages express CD8alpha.

The innate immune response of finfish--a review of current knowledge

The decline in the fisheries of traditional marine species has been an incentive for the diversification of today's aquaculture sector into the intensive rearing of many finfish species. The increasing interest in commercial farming of different finfish species is expected to result in similar environmental and husbandry-related problems as have been experienced in the development of the salmonid farming industry. An understanding of the biology of the fish species being cultured, in particular the immune response is important for improved husbandry and health management of the species. The innate immune system of fish has generated increasing interest in recent years and is now thought to be of key importance in primary defence and in driving adaptive immunity. This review focuses on key components (cellular and humoral) of the innate immune responses of different fish species of commercial importance.

CD83 expression in sea bream macrophages is a marker for the LPS-induced inflammatory response

CD83, a cell surface membrane glycoprotein member of the Ig superfamily which is commonly used as standard surface marker for dendritic cells, was cloned from gilthead sea bream macrophages using degenerate primers against conserved motifs of known CD83 sequences. The obtained cDNA contains an open reading frame of 669 nucleotides that translate into a 222 amino acid putative peptide. The deduced protein sequence shows conservation of features shared by vertebrate CD83 and multiple alignment with fish CD83 sequences reveals high homology. In cultured sea bream macrophages CD83 mRNA expression was significantly enhanced in a dose- and time-dependent fashion after stimulation with Escherichia coli LPS. These results indicate that in fish, macrophages express high levels of CD83 mRNA after LPS exposure and CD83 is therefore a good marker for activated mature myeloid cells in fish.

Distribution of the professional phagocytic granulocytes of the bony fish gilthead seabream (Sparus aurata L.) during the ontogeny of lymphomyeloid organs and pathogen entry sites

Although it is believed that fish fry depend fundamentally on their innate defence mechanisms, the ontogeny of fish innate immune cells is poorly understood. In the present study, we have used a specific monoclonal antibody against acidophilic granulocytes (AGs), the main professional phagocytic cell type of the bony fish gilthead seabream, to study their localization during the development of the main lymphomyeloid organs, namely the head kidney, spleen and thymus, and of the two major portals for pathogen entry, namely the gills and intestine. AGs were observed in the posterior intestine and in the blood earlier than in the haematopoietic kidney (21 vs. 27 days post-hatching, dph). AGs were observed scattered between other cells of the haematopoietic lineage in the head kidney of larvae, but were grouped around the blood vessels of this organ in juveniles and adults, where they were also much more numerous. In the spleen and in the thymus, AGs were observed much later (62 dph) and appeared scattered. AGs were also observed in the gill lamella and the posterior intestine near the anus throughout development.

Isolation of the Atlantic salmon tyrosinase gene family reveals heterogenous transcripts in a leukocyte cell line

In ectothermic vertebrates, visceral organs harbor melanin-containing cells. Their ability as pigment producers is nevertheless disputed. To address expression of the key genes for melanogenesis in Atlantic salmon (Salmo salar), a tyrosinase-positive leukocyte cell line (SHK-1) and skin were used to obtain full-length tyrosinase (Tyr), tyrosinase-like protein-1 (Tyrp1), and dopachrome tautomerase (Dct) mRNA transcripts. In the SHK-1 cells, two different Tyrp1 transcripts were identified, one lacking exon 1. However, only the full-length version of Tyrp1 was identified in the skin. Sequencing of Tyrp1 genomic region revealed that the two Tyrp1 transcripts might originate from two different loci, possibly a result of pseudo-tetraploidity of the Atlantic salmon genome. Expression of Tyr, Tyrp1 and Dct was investigated by quantitative real-time reverse transcriptase polymerase cain reaction showing highest expression in the SHK-1 cell line and skin, intermediate in pronephros, and negligible or absent in liver and muscle. Histological approaches were used to demonstrate melanin and revealed presence of melanized cells in skin, kidney and liver, and absence of such cells in muscle. In addition to verify melanin synthesis abilities of visceral-located cells, our results indicate loci-specific transcription differences between populations of melanin-producing cells in Atlantic salmon.