Immunopurification of T-cells from sea bass Dicentrarchus labrax (L.)

Cluster of differentiation antigens: essential roles in the identification of teleost fish T lymphocytes

Cluster of differentiation (CD) antigens are cell surface molecules expressed on leukocytes and other cells associated with the immune system. Antibodies that react with CD antigens are known to be one of the most essential tools for identifying leukocyte subpopulations. T lymphocytes, as an important population of leukocytes, play essential roles in the adaptive immune system. Many of the CD antigens expressed on T lymphocytes are used as surface markers for T lymphocyte classification, including CD3, CD4 and CD8 molecules. In this review, we summarize the recent advances in the identification of CD molecules on T lymphocytes in teleosts, with emphasis on the functions of CD markers in the classification of T lymphocyte subsets. We notice that genes encoding CD3, co-receptors CD4 and CD8 have been cloned in several fish species and antibodies have been developed to study protein expression in morphological and functional contexts. T lymphocytes can be divided into CD4⁺ and CD8⁺ cells discriminated by the expression of CD4 and CD8 molecules in teleost, which are functionally similar to mammalian helper T cells (Th) and cytotoxic T cells (Tc), respectively. Further studies are still needed on the particular characteristics of teleost T cell repertoires and adaptive responses, and results will facilitate the health management and development of vaccines for fish.

Microcystin-Induced Immunotoxicity in Fishes: A Scoping Review

Cyanobacteria (blue-green algae) have been present on Earth for over 2 billion years, and can produce a variety of bioactive molecules, such as cyanotoxins. Microcystins (MCs), the most frequently detected cyanotoxins, pose a threat to the aquatic environment and to human health. The classic toxic mechanism of MCs is the inhibition of the protein phosphatases 1 and 2A (PP1 and PP2A). Immunity is known as one of the most important physiological functions in the neuroendocrine-immune network to prevent infections and maintain internal homoeostasis in fish. The present review aimed to summarize existing papers, elaborate on the MC-induced immunotoxicity in fish, and put forward some suggestions for future research. The immunomodulatory effects of MCs in fish depend on the exposure concentrations, doses, time, and routes of exposure. Previous field and laboratory studies provided strong evidence of the associations between MC-induced immunotoxicity and fish death. In our review, we summarized that the immunotoxicity of MCs is primarily characterized by the inhibition of PP1 and PP2A, oxidative stress, immune cell damage, and inflammation, as well as apoptosis. The advances in fish immunoreaction upon encountering MCs will benefit the monitoring and prediction of fish health, helping to achieve an ecotoxicological goal and to ensure the sustainability of species. Future studies concerning MC-induced immunotoxicity should focus on adaptive immunity, the hormesis phenomenon and the synergistic effects of aquatic microbial pathogens.

Molecular, Cellular and Functional Analysis of TRγ Chain along the European Sea Bass Dicentrarchus labrax Development

In jawed vertebrates, adaptive immune responses are enabled by T cells. Two lineages were characterized based on their T cell receptor (TcR) heterodimers, namely αβ or γδ peptide chains, which display an Ig domain-type sequence that is somatically rearranged. γδ T cells have been less extensively characterized than αβ and teleost fish, in particular, suffer from a severe scarcity of data. In this paper, we worked on the well-known model, the European sea bass Dicentrarchus labrax, to broaden the understanding of teleost γδ-T cells. The T cell receptor chain (TR) γ transcript was expressed at a later developmental stage than TRβ, suggesting a layered appearance of fish immune cells, and the thymus displayed statistically-significant higher mRNA levels than any other organ or lymphoid tissue investigated. The polyclonal antibody developed against the TRγ allowed the localization of TRγ-expressing cells in lymphoid organs along the ontogeny. Cell positivity was investigated through flow cytometry and the highest percentage was found in peripheral blood leukocytes, followed by thymus, gut, gills, spleen and head kidney. Numerous TRγ-expressing cells were localized in the gut mucosa, and the immunogold labelling revealed ultrastructural features that are typical of T cells. At last, microalgae-based diet formulations significantly modulated the abundance of TRγ⁺ cells in the posterior intestine, hinting at a putative involvement in nutritional immunity. From a comparative immunological perspective, our results contribute to the comprehension of the diversity and functionalities of γδ T cells during the development of a commercially relevant marine teleost model.

Effects of Probiotic Bacteria Bacillus on Growth Performance, Digestive Enzyme Activity, and Hematological Parameters of Asian Sea Bass, Lates calcarifer (Bloch)

This study was conducted to evaluate different doses of two species of Bacillus (Bacillus licheniformis and Bacillus subtilis), on growth parameters, chemical composition of fish, activity of liver, and digestive enzymes of Asian sea bass. During 8 weeks, juvenile Asian sea bass received diets supplemented with 1 × 10³, 1 × 10⁶, and 1 × 10⁹ CFU g^-1 probiotic in addition to a control diet without added microorganisms. At the end of the trial, growth indices (total weight, total length, specific growth rate, total weight gain, food conversion ratio, and condition factor), body composition (crude protein, crude lipid, ash, and dry matter), digestive enzymes (protease, lipase, and amylase), liver enzymes [aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP)], immunologic indicators (lysozyme), and hematological parameters [hematocrit (Hct), hemoglobin (Hb), red blood cells (RBCs), white blood cells (WBCs)] were assessed. Asian sea bass receiving diets supplemented with probiotic Bacillus (Bacillus licheniformis and Bacillus subtilis) showed significantly better growth than those fed the basal diet (control). Regarding body composition, total protein levels and dry matter were higher and lipid levels were lower in fish fed the diet containing 1 × 10⁶ CFU g^-1 probiotic compared with the control group (P < 0.05). Digestive enzymes (protease, lipase, and amylase) and hematological parameters (RBC, WBC, and Hb) were all highest in fish fed diet supplemented with 1 × 10⁶ CFU g^-1 probiotic Bacillus. Also, liver enzymes (AST, ALT, ALP) were lower in fish fed diet supplemented with 1 × 10⁶ CFU g^-1 probiotic Bacillus. Being that supplementation of 1 × 10⁶ CFU g^-1 of Bacillus in the diet is the dose which delivers the best results.

Gene expression analysis of isolated salmonid GALT leucocytes in response to PAMPs and recombinant cytokines

Increased knowledge of the immune response of the intestine, a physiologically critical organ involved in absorption, secretion and homeostasis in a non-sterile environment, is needed to better understand the mechanisms involved in the induction of long-lasting immunity and, subsequently, the development of efficacious gastrointestinal immunization approaches. To this end, analysis of isolated gut cells will give an insight into the cell types present and their immune capability. Hence, in this study we first optimised a method for salmonid gut leucocyte isolation and characterised the cells on the basis of their expression of a range of selected cell markers associated with T & B cells and dendritic cells. The GALT leucocytes were then stimulated with a variety of PAMPs, recombinant cytokines and PHA, as a means to help characterise the diversity of the immune repertoire present in such cells. The stimulants tested were designed to examine the nature of the antibacterial, antiviral and T cell type responses in the cells (at the transcript level) using a panel of genes relevant to innate and adaptive immunity. The results showed distinct responses to the stimulants, with a clear delineation seen between the stimulant used (eg viral or bacterial PAMP) and the pathway elicited. The changes in the expression patterns of the immune genes in these cells indicates that the salmonid intestine contains a good repertoire of competent immune cells able to respond to different pathogen types. Such information may aid the development of efficient priming by oral vaccination in salmonids.

Antioxidant enzymes, hematology and histology of spleen in Nile tilapia fed supplemented diet with natural extracts challenged with Aeromonas hydrophila

This study investigated the effects of dietary supplementation with the extrats of propolis and Aloe barbadensis (aloe) on the antioxydant enzime activity, hematology and histology of the spleen of Nile tilapia challenged with Aeromonas hydrophila. Seventy two juvenile Nile tilapia were divided in four treatments and three replicates and fed extract mixture for 15 days: fish fed supplemented diet with 1% of the mixture of extracts of propolis and aloe (1:1) injected with phosphate-buffered saline (PBS); fish fed suplemented diet with 1% of the mixture of extracts of propolis and aloe (1:1) injected with the A. hydrophila, fish fed supplemented diet with the mixture of propolis extracts and aloe, injected with PBS and injected with A. hydrophila. The influence of the supplementation of propolis and Aloe extracts on the immunomodulation in tilapias was observed by the evaluation of the survival of the animals after challenge with A. hydrophila. Non-supplemented fish had a 44.5% survival rate and those supplemented with 1% of the mixture of extracts showed 55.6% survival 7 days after challenge. The supplemented animals also showed a significant increase in the number of lymphocytes in the evaluation of the blood parameters and, consequently, in the histopathological evaluation, presented greater presence of centers of melanomacrophages. In addition, the activity of the antioxidant enzymes glutathione reductase (GR) in the spleen presented a significant difference in fish supplemented with 1% of the extracts mixture, being superior in the animals injected with PBS when compared to those challenged with A. hydrophila.

Evolution of lymphocytes. Immunoglobulin T of the teleost sea bass (Dicentrarchus labrax): Quantitation of gene expressing and immunoreactive cells

Immunoglobulin T (IgT) is one of the key effector molecules of jawed vertebrate's adaptive immune system, and in this work we describe the quantitative distribution of IgT-expressing and IgT-producing cells in tissues of the European seabass Dicentrarchus labrax by using mRNA riboprobes and a specific anti-IgT antibody. A polyclonal antiserum (pAb) was prepared by immunizing rabbits with three synthetic peptides deduced from the full length IgT cDNA sequence and located in a surface-exposed CH3 domain of IgT constant region. The obtained antiserum, named RAIgT1, was able to recognize by ELISA immunization antigens and IgT from intestinal mucus and serum. In western blots of head kidney leukocytes lysates the antiserum recognized a 180 kDa polypeptide in non-reducing, and a 75 kDa peptide in reducing conditions. Interestingly, the RAIgT1 pAb crossreacted intensely in western blots with rainbow trout IgT purified from mucus and serum. Antisense mRNA IgT oligonucleotide sequences were employed in in situ hybridization to detect IgT-expressing cells in sections from lymphoid tissues, and positive cells were observed in head kidney, spleen, intestine and gills. By employing RAIgT1 in quantitative immunohistochemistry, the highest number of IgT-producing cells was observed in the gills (9.5 ± 0.7%), followed by intestine (8.4 ± 1.2%), head kidney (6.2 ± 1.4%), and spleen (4.1 ± 0.7%). Interestingly, the number of IgT-B cells showed a regionalization in the intestine, increasing from the proximal to the terminal part. By immunofluorescence and flow cytometry of live leukocytes, the percentages of RAIgT1 stained cells were 34 ± 11% in the intestine, 22 ± 5% in head kidney, 16 ± 7% in spleen, and 9 ± 5% in gills. At the fluorescence microscope, live cells from these tissues showed a typical membrane-associated positivity and a lymphocytic morphology, and no IgT/IgM double positive cells were detected. Immunoreactive cells have been purified from head kidney using magnetic beads, and IgT-enriched cells showed by RT-PCR an enhanced expression of the IgT gene, whereas IgT-depleted cells had an highest expression of IgM and TRβ genes. These data describe for the first time a quantitative panel of IgT-expressing and IgT-immunoreactive cells in tissues of a teleost fish species.

T cell immunity in the teleost digestive tract

Fish (along with cyclostomes) constitute the most ancient animal group in which an acquired immune system is present. As in higher vertebrates, both B and T lymphocytes cooperate in implementing an adequate response. Although there is still a debate on whether fish possess a true gut associated lymphoid tissue (GALT), the presence of diffuse B and T lymphocytes throughout all mucosal surfaces has been demonstrated in a wide variety of fish species. The lack of antibodies against T lymphocyte markers has hampered the performance of functional assays in both systemic and mucosal compartments. However, most components associated with T lymphocyte function have been identified in fish through extensive genomic research, suggesting similar functionalities for fish and mammalian T lymphocytes. Thus, the aim of this review is to briefly summarize what is known in teleost concerning the characteristics and functionalities of the different T cell subsets, to then focus on what is known to date regarding their presence and role in the gastrointestinal tract, through either direct functional assays or indirectly by conclusions drawn from transcriptomic analysis.

T cell transcripts and T cell activities in the gills of the teleost fish sea bass (Dicentrarchus labrax)

The gills of fish are a mucosal tissue that contains T cells involved in the recognition of non-self and pathogens, and in this work we describe some features of gill-associated T cells of European sea bass, a marine model species. A whole transcriptome was obtained by deep sequencing of RNA from unstimulated gills that has been analyzed for the presence of T cell-related transcripts. Of the putative expressed sequences identified in the transcriptome, around 30 were related to main functions related to T cells including Th1/Th2/Th17/Treg cell subpopulations, thus suggesting their possible presence in the branchial epithelium. The number of T cells in the gills of sea bass, measured with the specific T cell mAb DLT15 range from 10% to 20%, and IHC analysis shows their abundance and distribution in the epithelium. Leukocytes from gills are able to proliferate in the presence of lectins ConA and PHA, as measured by flow cytometry using CFSE fluorescence incorporation, and during proliferation the number of T cells counted by immunofluorescence increased. In lectin-proliferating cells the expression of T cell-related genes TRβ, TRγ, CD4, CD8α, CD45 and IL-10 increased dramatically. Our data represent a first analysis on T cell genes and on basic T cell activities of fish gills, and suggest the presence of functionally active subpopulations of T lymphocytes in this tissue.

The pyloric caeca area is a major site for IgM(+) and IgT(+) B cell recruitment in response to oral vaccination in rainbow trout

Although previous studies have characterized some aspects of the immune response of the teleost gut in response to diverse pathogens or stimuli, most studies have focused on the posterior segments exclusively. However, there are still many details of how teleost intestinal immunity is regulated that remain unsolved, including the location of IgM⁺ and IgT⁺ B cells along the digestive tract and their role during the course of a local stimulus. Thus, in the current work, we have studied the B cell response in five different segments of the rainbow trout (Oncorhynchus mykiss) digestive tract in both naïve fish and fish orally vaccinated with an alginate-encapsulated DNA vaccine against infectious pancreatic necrosis virus (IPNV). IgM⁺ and IgT⁺ cells were identified all along the tract with the exception of the stomach in naïve fish. While IgM⁺ cells were mostly located in the lamina propria (LP), IgT⁺ cells were primarily localized as intraepithelial lymphocytes (IELs). Scattered IgM⁺ IELs were only detected in the pyloric caeca. In response to oral vaccination, the pyloric caeca region was the area of the digestive tract in which a major recruitment of B cells was demonstrated through both real time PCR and immunohistochemistry, observing a significant increase in the number of both IgM⁺ and IgT⁺ IELs. Our findings demonstrate that both IgM⁺ and IgT⁺ respond to oral stimulation and challenge the paradigm that teleost IELs are exclusively T cells. Unexpectedly, we have also detected B cells in the fat tissue associated to the digestive tract that respond to vaccination, suggesting that these cells surrounded by adipocytes also play a role in mucosal defense.

17β-Estradiol induces changes in cytokine levels in head kidney and blood of juvenile sea bass (Dicentrarchus labrax, L., 1758)

The cytokine network is involved in the immune system communication. As estrogens influence the cytokine expression in mammals, this study investigated the impact of exogenous estrogenic pollutants on selected cytokines in Dicentrarchus labrax. The gene expression of Interleukin 6, Tumour Necrosis Factor α, Transforming Growth Factor β1 and Interleukin 1β was assessed and accomplished with protein measurements in the blood for the last two. Impacts through 17β-estradiol mainly occurred at the beginning of organ regionalisation, thus falling together with a developmentally induced increase of Interleukin 1β and Tumour Necrosis Factor α gene expression in 102 dph fish. 17β-estradiol depressed this modification after 35 days of exposure and the cytokine gene expression tended to be generally down-regulated independently of the 17β-estradiol concentrations after 56 days of exposure. This impact was confirmed at the protein level, showing that 17β-estradiol affects the fine control of the cytokine network in sea bass.

Enhanced intestinal epithelial barrier health status on European sea bass (Dicentrarchus labrax) fed mannan oligosaccharides

The study assesses the effects of dietary mannan oligosaccharides (MOS) in European sea bass (Dicentrarchus labrax) posterior intestinal lipid class composition and its possible relation to the potential prostaglandins production and Gut Associated Lymphoid Tissue (GALT) stimulation. Fish were fed 4 g kg(-1) MOS (Bio-Mos(®) Aquagrade, Alltech, Inc., USA) for eight weeks. Fish fed MOS presented higher (P ≤ 0.05) weight gain, total length, and specific and relative growth rates than fish fed the control diet. Stimulated posterior gut of fish fed MOS showed higher (P ≤ 0.05) prostaglandins production than fish fed the control diet. Lipid class analyses of posterior gut revealed a reduction (P ≤ 0.05) in the neutral lipid fraction in fish fed MOS compared to fish fed the control diet, particularly due to a reduction (P ≤ 0.05) in triacylglycerols content. The polar lipid fraction increased (P ≤ 0.05) in fish fed MOS compared to fish fed the control diet, mainly due to an increase (P ≤ 0.05) in phosphatidylethanolamine and phosphatidylcoline contents. Light microscopy of posterior gut revealed increased number or goblet cells as well as higher level of infiltrated eosinophilic granulocytes for fish fed MOS. Transmission electron microscopy qualitative observations revealed a better preserved cytoarchitecture of the intestinal epithelial barrier in the posterior gut of fish fed MOS. Posterior gut of fish fed MOS presented more densely packed non-damaged enterocytes, better preserved tight junctions structure, healthier and more organized microvilli, and a higher presence of infiltrated lymphocytes and granulocytes compared fish fed the control diet. The present study indicates that dietary MOS enhances European sea bass posterior gut epithelial defense by increasing membrane polar lipids content in relation to a stimulation of the eicosanoid cascade and GALT, promoting posterior gut health status.

A monoclonal antibody for the CD45 receptor in the teleost fish Dicentrarchus labrax

The CD45 tyrosine phosphatase plays an important role in regulating T lymphocyte activation in vertebrate species. In this study we describe some molecular and functional features of the CD45 receptor molecule from the European sea bass Dicentrarchus labrax. Following immunization with fixed sea bass thymocytes, we obtained a murine monoclonal antibody (mAb) able to stain fish leucocytes both alive, by immunofluorescence of thymus and mucosal tissues, and fixed, by in situ immunohistochemistry of tissue sections. The selected IgG(2) mAb (DLT22) was able to recognise by western blots polypeptides mainly at 180 kDa and 130 kDa in thymus, spleen, intestine and gill leucocyte. Accordingly, a 130 kDa polypeptide immunoprecipitated with DLT22 from thymocytes and analysed by nano-RP-HPLC-ESI-MS/MS, gave peptide sequences homologous to Fugu CD45, that were employed for the homology cloning of a partial sea bass CD45 cDNA sequence. This cDNA sequence was employed to measure by quantitative PCR the transcription of the CD45 gene both in unstimulated and in in vitro stimulated leucocytes, showing that the gene transcription was specifically modulated by LPS, ConA, PHA, IL-1, and poly I:C. When splenocytes were stimulated in vitro with ConA and PHA, a cell proliferation paralleled by an increase of DLT22-positive leucocytes was also observed. These data indicate that the DLT22 mAb recognizes a putative CD45 molecule in sea bass, documenting the presence of CD45-like developing lymphocytes in thymus and CD45-associated functional stages of lymphocytes in this species, thus dating back to teleost fish the functional activities of these cell populations in vertebrates.

Molecular characterization and increased expression of the Nile tilapia, Oreochromis niloticus (L.), T-cell receptor beta chain in response to Streptococcus agalactiae infection

The complete cDNA sequence of the Nile tilapia T-cell receptor (TCR) β chain was cloned using 5' RACE. The full-length, 1263-bp cDNA contained a 942-bp open reading frame (ORF) encoding a 314-amino-acid protein. Sequence analyses revealed that the Nile tilapia TCR β chain contains four conserved cysteine residues involved in the formation of disulphide bridges and a conserved amino acid motif believed to be important for assembly and signalling of the TCR αβ/CD3 complex, both of which are normally found in the TCR β chain of other vertebrates. As detected using semi-quantitative and quantitative RT-PCR, the highest expression level of TCR β was detected in the thymus. Interestingly, Streptococcus agalactiae significantly induced the up-regulation of the TCR β chain, and the strongest up-regulation was detected in the brain and peripheral blood leucocytes (PBLs). In in vitro experiments, concanavalin A and Aeromonas hydrophila were found to significantly increase the expression of the TCR β chain in PBLs after 48 h (P < 0.01) and 72 h (P < 0.05), respectively. Furthermore, real-time PCR analysis showed that intraperitoneal injection (IP) of 10(7) cfu mL(-1) of S. agalactiae could induce TCR β expression that was greater than the expression observed following administration of 10(9) cfu mL(-1). The presence of the TCR β chain in fish detected in this study suggests the presence of T-cell populations that have been found in higher vertebrates, which may play a crucial functional role in the response to fish pathogens.

Fish T cells: recent advances through genomics

This brief review is intended to provide a concise overview of the current literature concerning T cells, advances in identifying distinct T cell functional subsets, and in distinguishing effector cells from memory cells. We compare and contrast a wealth of recent progress made in T cell immunology of teleost, elasmobranch, and agnathan fish, to knowledge derived from mammalian T cell studies. From genome studies, fish clearly have most components associated with T cell function and we can speculate on the presence of putative T cell subsets, and the ability to detect their differentiation to form memory cells. Some recombinant proteins for T cell associated cytokines and antibodies for T cell surface receptors have been generated that will facilitate studying the functional roles of teleost T cells during immune responses. Although there is still a long way to go, major advances have occurred in recent years for investigating T cell responses, thus phenotypic and functional characterization is on the near horizon.

Transcription of T cell-related genes in teleost fish, and the European sea bass (Dicentrarchus labrax) as a model

In recent years the cloning of genes coding for immuno-regulatory peptides, as well as the sequencing of genomes, provided fish immunologists with a growing amount of information on nucleotide sequences. Research is now also addressed in investigating the functional immunology counterpart of nucleotide sequence transcripts in various fish species. In this respect, studies on functional immunology of T cell activities are still at their beginning, and much work is needed to investigate T cell responses in teleost fish species. In this review we summarise the current knowledge on the group of genes coding for main T cell-related peptides in fish, and the expression levels of these genes in organs and tissues. Particular attention is paid to European sea bass (Dicentrarchus labrax), a marine species in which some information on functional immunology has been obtained, and we reassume here the expression of some T cell-related genes in basal conditions. In addition, we provide original data showing that T cells purified from the intestinal mucosa of sea bass with a specific mAb, express transcripts for TRβ, TRγ, CD8α, and RAG-1, thus showing similarities with intra-epithelial leucocytes of mammals.

Antigen-dependent T lymphocytes (TcRβ+) are primarily differentiated in the thymus rather than in other lymphoid tissues in sea bass (Dicentrarchus labrax, L.)

All jawed vertebrates share lymphocyte receptors that allow the recognition of pathogens and the discrimination between self and non-self antigens. The T cell transmembrane receptor (TcR) has a central role in the maturation and function of T lymphocytes in vertebrates via an important role in positive selection of the variable region of TcR αβ/γδ chains. In this study, the TcRβ transcript expression and TcRβ(+) cell distribution during the ontogeny of the immune system of sea bass (Dicentrarchus labrax, L.) were analysed. RT-PCR analysis of larvae during early development demonstrated that the β chain transcript is expressed by 19 days post-fertilisation (p.f.). RNA probes specific for the β chain were synthesised and used for in situ hybridisation experiments on 30 day p.f. to 180 day old juvenile larvae. A parallel immunohistochemical study was performed using the anti-T cell monoclonal antibody DLT15 developed in our laboratory [Scapigliati et al., Fish Shellfish Immunol 1996; 6:383-401]. The first thymus anlage was detectable at 32-33 days p.f. (Corresponding to about 27 days post-hatch). DLT15(+) cells were detected at day 35 p.f. in the thymus whereas TcRβ(+) cells were recognisable at day 38 p.f. in the thymus and at day 41 p.f. in the gut. TcRβ(+) cells were observed in capillaries from 41 to 80 days p.f. At day 46 p.f., TcRβ(+) cells were identified in the head kidney and were detected in the spleen 4 days later. The present results demonstrate that TcRβ(+) cells can be differentiated first in the thymus and then in other organs/tissues, suggesting potential TcRβ(+) cell colonisation from the thymus to the middle gut. Once the epithelial architecture of the thymus is completed with the formation of the cortical-medullary border (around 70-75 days p.f.), DLT15(+) cells or TcRβ(+) cells are confined mainly to the cortex and cortical-medullary border. In particular, a large influx of TcRβ(+) cells was observed at the cortical-medullary border from 72 to 90 days p.f., suggesting a role in positive selection for this thymic region during the ontogeny of the fish immune system. This study provides novel information about the primary differentiation and distribution of TcRβ(+) cells in sea bass larvae and juveniles.

Intestinal T cells of Dicentrarchus labrax (L.): gene expression and functional studies

Cellular and molecular data have evidenced a gut-associated lymphoid tissue in a variety of teleost species, abundantly containing T cells, whose origin, selection and functions are still unclear. This study reports CD4, CD8-α, MHCI-α, MHCII-β, rag-1 and TCR-β gene transcription along the intestine (anterior, middle and posterior segments) and in the thymus of one year-old Dicentrarchus labrax (L.). Real-time PCR findings depicted a main role of the thymus in T-cell development, but also rag-1 and CD8-α transcripts are detected in the intestine, having significant expression in the posterior segment. In the whole intestine TCR-β and CD8-α exceeded CD4 transcripts. RNA ISH confirmed these data and detailed that mucosal CD8-α+ cells were especially numerous in the epithelium and in aggregates in the lamina propria. Regional differences in T-cell-specific gene expressions are first described in the intestine of a bony fish. High non-specific cytotoxic activity against xenogeneic and allogeneic cells was found in lymphocytes purified from the intestinal mucosa, providing further insight into their local defence roles.

Probiotics and immunity: a fish perspective

Probiotics are usually live microorganisms which when administered in adequate amounts confer a health benefits on host. Nowadays, probiotics are also becoming an integral part of the aquaculture practices to obtain high production. The common probiotics that are used for aquaculture practices include Lactobacillus, Lactococcus, Leuconostoc, Enterococcus, Carnobacterium, Shewanella, Bacillus, Aeromonas, Vibrio, Enterobacter, Pseudomonas, Clostridium, and Saccharomyces species. The involvement of probiotics in nutrition, disease resistance and other beneficial activities in fish has proven beyond any doubt. Among the numerous health benefits attributed to probiotics, modulation of immune system is one of the most commonly purported benefits of the probiotics and their potency to stimulate the systemic and local immunity under in vitro and in vivo conditions is noteworthy. Different probiotics either monospecies or multispecies supplementation can eventually elevate phagocytic, lysozyme, complement, respiratory burst activity as well as expression of various cytokines in fish. Similarly, probiotics can stimulate the gut immune system of fish with marked increase in the number of Ig(+) cells and acidophilic granulocytes. Furthermore, mono-bacterial association studies (with non-probiotic bacterial strains) in gnotobiotic fish also indicate the up-regulation of various immune related genes. Though the exact mode of action of probiotics is yet to be established in any animal including fish, probiotics often exert host specific and strain specific differences in their activities. Various factors like source, type, dose and duration of supplementation of probiotics can significantly affect the immunomodulatory activity of probiotics. The review is therefore, aiming to highlight the immunomodulatory activity of probiotics and also to evaluate the factors that regulate for the optimum induction of immune responses in fish.

Cellular and molecular immune responses of the sea bass (Dicentrarchus labrax) experimentally infected with betanodavirus

Naïve sea bass juveniles (38.4 + or - 4.5 g) were intramuscularly infected with a sublethal dose of betanodavirus isolate 378/I03, followed after 43 days by a similar boosting. This infection resulted in an overall mortality of 7.6%. At various intervals, sampling of fish tissues was performed to investigate: i) B and T lymphocyte content in organs and tissues; ii), proliferation of leucocytes re-stimulated in vitro with inactivated virus; iii) presence of serum antibody specific for betanodavirus; iv) expression of genes coding for the following immunoregulatory molecules involved in innate and acquired responses: type I IFN, Mx, IL-1, Cox-2; IL-10, TGF-beta, TCRbeta, CD4, CD8alpha, IgM, by using a quantitative PCR array system developed for sea bass. The obtained results showed a detectable increase of T cells and B cells in PBL during betanodavirus infection. Furthermore, leucocytes obtained from blood, head kidney, and gills showed a detectable "in vitro" increase in viability upon addition of inactivated viral particles, as determined by measuring intracellular ATP concentration. ELISA analysis of sera showed that exposure to nodavirus induced a low, but specific antibody titer measured 43 days after infection, despite the presence of measurable levels of natural antibody. Finally, a strong upregulation of genes coding for type I IFN, Mx, and IgM was identified after both infection and boosting. Interestingly, an upregulation of Cox-2 until boosting, and of TGF-beta and IL-10 after boosting was also observed, while the other tested genes did not show any significant variations with respect to mock-treated fish. Overall, our work represents a first comprehensive analysis of cellular and molecular immune parameters in a fish species exposed to a pathogenic virus.

Suppression of lymphocyte proliferation by ovarian cavity fluid from the viviparous fish Neoditrema ransonnetii (Perciformes; Embiotocidae)

As the fetus expresses paternal major histocompatibility complex molecules, viviparous vertebrates require sophisticated mechanisms to modulate maternal immunology to ensure successful pregnancy. We anticipated that ovarian cavity fluid (OCF) is likely to feature significantly in the modulation of ovarian cavity immunology. Consequently, we examined the effects of OCF upon leukocyte function in Neoditrema ransonnetii. OCF did not affect phagocytosis or superoxide production by phagocytes. However, OCF suppressed lymphocyte proliferation induced by ConA almost completely. As OCF contained PGE(2) at high levels during late pregnancy, we also investigated the effect of PGE(2) upon lymphocyte expansion. PGE(2) exhibited negative effects upon lymphocyte mitogenesis in a dose-dependent manner (10-1000 ng/ml). PGE(2) significantly suppressed lymphocyte proliferation when present at levels equivalent to that seen in OCF (30.2 +/- 16.1 approximately 185.4 +/- 107.4 ng/ml). Data indicate that PGE(2) is one of the key modulatory molecules of the maternal immune system ensuring successful pregnancy in this viviparous species.

In Vitro Elicitation of Intestinal Immune Responses in Teleost Fish: Evidence for a Type IV Hypersensitivity Reaction in Rainbow Trout

In fish the gut immune system has been the subject of few investigations until now. Here, we provide novel morphological and immunological data on the gut isolated from rainbow trout Salmo gairdneri. The pyloric (P) and terminal (T) segments of trout gut, when morphologically examined, evidenced lymphocytes and macrophages (MØ) loosely dispersed in the intestinal mucosa and in the lamina propria in the absence of typical Peyer's patches-like structures. Furthermore, incubation of P and T sections with Candida albicans (Ca) and functional analysis of supernatants generated some interesting results. In fact, active supernatants, when compared with controls, exhibited cytokine-like activities attributable to the presence of interferon (IFN)-gamma and migration inhibiting factor (MIF), respectively. In particular, IFN-gamma-like activity gave rise to an enhancement of Ca phagocytosis by MØ, whereas MIF inhibited MØ migration in agarose. Taken together, these in vitro data suggest that the gut-associated lymphoreticular tissue in fish possesses the appropriate armamentarium to mount a type IV hypersensitivity response when challenged by microbial antigens.

Cell markers and determinants in fish immunology

Despite the impressive increase in the cloning and expression of genes encoding fish immunoregulatory molecules, the knowledge on "in vivo" and "in vitro" functional immunology of the corresponding peptide products is still at an initial stage. This is partly due to the lacking of specific markers for immunoregulatory peptides, that represent an indispensible tool to dissect immune reactions and to trace the fate of cellular events downstream of the activation. In this review we summarise the available information on functional immune activities of some teleost species and discuss the obtained data in an evolutionary and applied context.

What happens to the DNA vaccine in fish? A review of current knowledge

The primary function of DNA vaccines, a bacterial plasmid DNA containing a construct for a given protective antigen, is to establish specific and long-lasting protective immunity against diseases where conventional vaccines fail to induce protection. It is acknowledged that less effort has been made to study the fate, in terms of cellular uptake, persistence and degradation, of DNA vaccines after in vivo administration. However, during the last year some papers have given new insights into the fate of DNA vaccines in fish. By comparing the newly acquired information in fish with similar knowledge from studies in mammals, similarities with regard to transport, blood clearance, cellular uptake and degradation of DNA vaccines have been found. But the amount of DNA vaccine redistributed from the administration site after intramuscular administration seems to differ between fish and mammals. This review presents up-to-date and in-depth knowledge concerning the fate of DNA vaccines with emphasis on tissue distribution, cellular uptake and uptake mechanism(s) before finally describing the intracellular hurdles that DNA vaccines need to overcome in order to produce their gene product.

Molecular cloning and characterization of common carp (Cyprinus carpio L.) TCRgamma and CD3gamma/delta chains

Partial cDNA sequences of TCRgamma and CD3gamma/delta were isolated from the thymus of common carp (Cyprinus carpio L.) by the method of suppression subtractive hybridization (SSH). Subsequently the full length cDNAs of carp TCRgamma and CD3gamma/delta were obtained by means of 3' RACE and 5' RACE, respectively. The full length of carp TCRgamma chain is 1368bp and encodes 326 amino acids including a signal peptide region of 19 amino acids and a transmembrane region of 23 amino acids at the C-terminal region from aa 291 to 313. The V region of carp TCRgamma contains 109 amino acids, the core motif FGXG in J segment was also found in carp TCRgamma. The C region of carp TCRgamma contains the characteristic CX6PX6WX45C motif. The CP region of carp TCR Cgamma contains 37 amino acids. The full length of carp CD3gamma/delta is 790bp and encodes 175 amino acids including a signal peptide region of 17 amino acids and a transmembrane region of 23 amino acids from aa 93 to 115. Similar to other known CD3gamma/deltas, four cysteine residues in the extracellular domain and an immunoreceptor tyrosine-based activation motif ITAM (YxxL/Ix6-8YxxL/I) in the intracellular domain are also included in carp CD3gamma/delta. Differing from other known CD3gamma/deltas, carp CD3gamma/delta lacks the CXXCXE motif in the extracellular domain. RT-PCR analysis demonstrated that the expression of TCRgamma gene was mainly in the thymus and gill of 6-month carp, but in 18-month carp, TCRgamma gene was detected in all the examined tissues. The expression of CD3gamma/delta gene was detected in all examined tissues of 6 and 18-month carp; among them, the highest expression level was in the thymus of 6-month carp. In situ hybridization showed that CD3gamma/delta-expressing cells were widely distributed in the head kidney, spleen and kidney of carp, whereas in the thymus, they were densely distributed in the lymphoid outer zone and scattered in the epithelioid inner zone.

Compartmentalisation of T cells expressing CD8alpha and TCRbeta in developing thymus of sea bass Dicentrarchus labrax (L.)

Eggs, larvae, post-larvae and sexually immature juveniles of the teleost Dicentrarchus labrax (L.) were assayed for the expression of genes encoding the T cell receptor beta and CD8alpha. RT-PCR of RNA extracted from larvae revealed TCRbeta transcripts from day 25 post-hatching (ph) and CD8alpha transcripts from 26 days later. At day 51 ph, CD8alpha and TCRbeta mRNAs were localised by in situ hybridisation in thymocytes of the outer and lateral zones of the thymic paired glands. From day 75 ph onwards the signal was mainly detected in the outer region, drawing a cortex-medulla demarcation. In 1-year-old fish, CD8alpha+ and TCRbeta+ thymocytes almost filled the cortex and extended in large cords in the medulla. A CD8alpha(-)TCRbeta+ subcapsular lymphoid zone was evident near the septa coming from the inner connective capsule that delimited the thymus. The localisation of CD8alpha and TCRbeta transcripts demonstrated a compartmentalisation of the juvenile thymus due to distinct localisation of thymocytes at different developmental stages.

Majority of TcRβ+ T-lymphocytes located in thymus and midgut of the bony fish, Dicentrarchus labrax (L.)

Real-time polymerase chain reaction (PCR) and in situ hybridization analyses were performed to investigate the occurrence and distribution of T-lymphocytes expressing TcRbeta in intestine and lymphoid tissues of the bony fish, Dicentrarchus labrax (sea bass). Immunohistochemistry with the monoclonal antibody DLT15 (pan-T-cell marker) was carried out to compare the cytology, distribution and number of T-cells and TcRbeta+ cells in the various sampled lymphoid organs. The highest TcRbeta expression was revealed by real-time PCR in the thymus, with high levels also being found in the gut. In the thymus, DLT15+ and TcRbeta+ cell populations were concentrated in the cortex and TcRbeta+ cells were notably reactive at the cortical-medullary border, suggesting a specialized role of this region in thymocyte selection. The density of DLT15+ T-cells increased from the anterior to posterior intestine, whereas TcRbeta+ lymphocytes were more numerous in the middle intestine compared with other segments. The existence, in fish thymus, of a medulla and a cortex comparable with those of mammals is revealed by this study. The concentration of TcRbeta+ cells in the sea bass midgut also strongly suggests a special role of this intestinal segment in antigen-specific cellular immunity. The large population of TcRbeta(-)/DLT15+ T-cells in the posterior gut can probably be ascribed to the TcRgammadelta phenotype fraction.

Costimulatory Receptors in a Teleost Fish: Typical CD28, Elusive CTLA4

T cell activation requires both specific recognition of the peptide-MHC complex by the TCR and additional signals delivered by costimulatory receptors. We have identified rainbow trout sequences similar to CD28 (rbtCD28) and CTLA4 (rbtCTLA4). rbtCD28 and rbtCTLA4 are composed of an extracellular Ig-superfamily V domain, a transmembrane region, and a cytoplasmic tail. The presence of a conserved ligand binding site within the V domain of both molecules suggests that these receptors likely recognize the fish homologues of the B7 family. The mRNA expression pattern of rbtCD28 and rbtCTLA4 in naive trout is reminiscent to that reported in humans and mice, because rbtCTLA4 expression within trout leukocytes was quickly up-regulated following PHA stimulation and virus infection. The cytoplasmic tail of rbtCD28 possesses a typical motif that is conserved in mammalian costimulatory receptors for signaling purposes. A chimeric receptor made of the extracellular domain of human CD28 fused to the cytoplasmic tail of rbtCD28 promoted TCR-induced IL-2 production in a human T cell line, indicating that rbtCD28 is indeed a positive costimulator. The cytoplasmic tail of rbtCTLA4 lacked obvious signaling motifs and accordingly failed to signal when fused to the huCD28 extracellular domain. Interestingly, rbtCTLA4 and rbtCD28 are not positioned on the same chromosome and thus do not belong to a unique costimulatory cluster as in mammals. Finally, our results raise questions about the origin and evolution of positive and negative costimulation in vertebrate immune systems.

Phenotypic and Functional Similarity of Gut Intraepithelial and Systemic T Cells in a Teleost Fish

Gut-associated lymphocytes were described in fish, but their involvement in immune responses is still unknown. In rainbow trout, intraepithelial lymphocytes (IELs) are scattered between gut epithelial cells, but neither Peyer's patches nor mesenteric lymph nodes were identified. Rainbow trout IELs contain mainly T cells, because they expressed transcripts of T cell marker homologs of CD8, CD4, CD28, CD3epsilon, TCRzeta, TCRgamma, and TCRbeta and lacked IgM. However, trout IELs did not show specific homing to the gut mucosa, which in mammals defines IELs as a distinctive mucosal population. A detailed analysis of the TCRbeta repertoire of rainbow trout IELs was performed in both naive and virus-infected animals. TCRbeta transcripts of rainbow trout IELs were highly diverse and polyclonal in adult naive individuals, in sharp contrast with the restricted diversity of IEL oligoclonal repertoires described in birds and mammals. Significant modifications of the trout IEL TCRbeta repertoire were observed after a systemic infection with a fish rhabdovirus and were especially marked for Vbeta4-bearing receptors as previously reported for spleen cells. Thus, we could not find any specific properties of the trout IEL TCRbeta repertoire compared with the spleen and pronephros TCRbeta repertoire, which questions the reality of a distinct IEL compartment in teleosts. Our findings suggest that a highly diversified alphabeta TauCR repertoire is maintained in fish IELs in the absence of Peyer's patches and mesenteric lymph nodes, whereas the restricted diversity of mouse alphabeta IELs is attributed to multiple cycles of activation and recirculation, allowing a progressive narrowing of the repertoire.

Cytotoxic activities of fish leucocytes

Like mammalian leucocytes, white blood cells of fish are able to kill altered (e.g. virus-infected) and foreign (allogeneic or xenogeneic) cells. The existence of natural killer (NK)-like and specific cytotoxic cells in fish was first shown using allogeneic and xenogeneic effector/target cell systems. In addition to in vivo and ex vivo studies, very important contributions were made by in vitro analysis using a number of different long-term cytotoxic cell lines established from channel catfish. In mammals, specific cell-mediated cytotoxicity (CMC) as part of the adaptive immune response requires a number of key molecules expressed on effector leucocytes and target cells. CD8+ T lymphocytes kill infected cells only, if their antigen receptor (TCR) matches the MHC class I with bound peptide of the target cell. Expression patterns of the fish gene homologues for TCR, CD8 and MHC class I, as well as related genes, are in agreement with similar function. Convenient systems for the analysis of specific CMC have only recently become available for fish with the combination of clonal fish with syngeneic or allogeneic but MHC class I matching cell lines. It was demonstrated that both, NK- and cytotoxic T (Tc) cells are involved in the killing of virus infected MHC class I matching and mismatching target cells. Analysis of these lymphocyte subsets is only starting for fish. There is also evidence that the different viral proteins trigger different subsets of killer cells. This review further discusses findings on fish CMC with regard to temperature/seasons and ontogeny.

In vivo allograft rejection in a bony fish Dicentrarchus labrax (L.): characterisation of effector lymphocytes

Lymphoid cell subpopulations involved in allograft rejection in the teleost Dicentrarchus labrax were characterised at the ultrastructural level and quantified by using monoclonal antibodies against T- and B-lymphocytes. T-cells positive for T-cell receptor beta-chain (TcR beta) were detected by reverse transcription/polymerase chain reaction (RT-PCR) and in situ hybridisation by using RNA probes for TcR beta. Flow cytometry detected a similar percentage of T- and B-lymphocytes (around 17%) in the leucocyte-enriched fraction from allografts. Two different types of T-lymphocytes (DLT 15-immunoreactive) infiltrating the allografts were identified by cytomorphology: small cells with high nuclear/cytoplasmic ratio and cells with a higher cytoplasmic content. RT-PCR revealed a single band (513 bp) corresponding to the TcR beta. In situ hybridisation showed that TcR beta-positive cells in the grafted muscle fibres were less numerous compared with DLT 15-positive cells, as evidenced in parallel sections, suggesting that cytotoxic cells might express different TcR phenotypes. DLIg 3-immunoreactive Ig-producing lymphocytes had: 1) a high nuclear/cytoplasmic ratio or 2) a larger size similar to that of pre-plasma cells (plasma cells lacked any membrane labelling).

Characterisation and expression analysis of interleukin 2 (IL-2) and IL-21 homologues in the Japanese pufferfish, Fugu rubripes, following their discovery by synteny

This investigation provides the first conclusive evidence for the existence of the interleukin 2 (IL-2) and IL-21 genes in bony fish. The IL-2 and IL-21 sequences have been determined in Fugu rubripes by exploiting the conservation of synteny that is found between regions of the human and Fugu genomes. The predicted 149-amino acid IL-2 homologue contains the IL-2 family signature, has a predicted secondary structure of three alpha helixes and has the two cysteines important in disulphide-bond formation. It shows low amino acid identities (24-34%) with other known IL-2 sequences. The predicted 155-amino acid IL-21 homologue has a predicted secondary structure of four alpha helixes and has the four cysteines important in disulphide-bond formation. It shows low amino acid identities (29-31%) with other known IL-21 sequences. The gene organisation of Fugu IL-2 and IL-21 and the level of synteny between the human and Fugu genomes has been well conserved during evolution, with the order and orientation of the genes matching exactly to human Chromosome 4. Phytohaemagglutinin stimulation of Fugu kidney cells resulted in a large increase in the Fugu IL-2 and IL-21 transcripts. In vivo stimulation of Fugu with LPS and poly I:C showed IL-21 expression to be localised within mucosal tissues. The discovery of IL-2 and IL-21 in fish will now allow more detailed investigations into T-helper cell responses.

Rhabdovirus infection induces public and private T cell responses in teleost fish

Many viruses induce a strong T cell response that contributes to the elimination of infected cells presenting viral peptides by MHC molecules. The structure and expression of genes encoding molecules homologous to mammalian alphabeta TCRs have been recently characterized in rainbow trout and in several teleost species, but the alphabeta T cell response against pathogens has not been directly demonstrated. To study the modifications of the T cell repertoire during an acute viral infection in rainbow trout, we adapted the immunoscope methodology, which consists of spectratyping the complementarity-determining region 3 length of the TCRbeta chain. We showed that the naive T cell repertoire is polyclonal and highly diverse in the naive rainbow trout. Using viral hemorrhagic septicemia virus (VHSV), which provokes an acute infection in rainbow trout, we identified skewed complementarity-determining region 3 size profiles for several VbetaJbeta combinations, corresponding to T cell clonal expansions during primary and secondary response to VHSV. Both public and private T cell expansions were shown by immunoscope analysis of spleen cells from several infected individuals of a rainbow trout clone sharing the same genetic background. The public response to VHSV consisted of expansion of Vbeta4Jbeta1 T cell, which appeared early during the primary response and was strongly boosted during the secondary response.

Phylogeny of cytokines: molecular cloning and expression analysis of sea bass Dicentrarchus labrax interleukin-1beta

In this paper the cloning of interleukin-1beta (IL-1beta) from the fish Dicentrarchus labrax (sea bass) is described. Using degenerate primers designed from known IL-1beta sequences, a cDNA fragment was amplified by PCR and elongated by 3' and 5' RACE to give the full-length coding sequence for sea bass IL-1beta. The cDNA is 1292 bp, lacks a putative ICE cut site, and codes for a deduced peptide of 29.4 kDa with a pI of 5.1. Sequence analysis showed highest amino acid similarity with rainbow trout (62%), Xenopus (46%), and carp (45.5%) IL-1beta sequences. Expression studies show that sea bass IL-1beta can be upregulated by bacterial lipopolysaccharide both in vitro and in vivo in leucocytes from blood, head-kidney, spleen, gills and liver, whereas the IL-1beta transcript was not detectable in thymus and gut-associated lymphoid tissue. Northern blot analysis with head-kidney leucocyte RNA showed a main LPS-upregulated band at 1.3 kb, and two minor bands at 0.9 and 3.0 kb, respectively. Phylogenetic comparisons with IL-1beta from other vertebrates is presented.

Ontogeny of B and T cells in sea bass (Dicentrarchus labrax, L.)

Monoclonal antibodies specific to sea bass Ig heavy (WDI 1) and light (WDI 3) chains and T cells (DLT15) were used in an ontogenetic study of sea bass by flow cytometry and immunocytochemistry. The influence of weight and age, as well as season, on B cell development was studied in the fastest and slowest growing offspring from the same spawn (5-305 days post hatch: dph). Additionally, B and T cell development was followed in samples of different offspring (5-137 dph). The results suggest that DLT15 recognises very early (pre-?) T cells as well as mature T cells and that these very early T cells might have their origin in a different compartment and subsequently mature in the thymus. They also appeared much earlier in ontogeny (between 5-12 dph onwards) than pre-B cells having cytoplasmic Ig (from 52 dph onwards). With the monoclonal antibodies used, adult levels of T and B cells were both reached between 137-145 dph, suggesting that sea bass is immunologically mature from at least that age onwards. As in other teleosts, the thymus appears to be the primary organ for T lymphocytes and head kidney the primary organ for B lymphocytes. For sea bass, age seems to be more important in determining B cell maturation than body weight.