Identification, molecular characterization and functional analysis of interleukin (IL)-2 and IL-2like (IL-2L) cytokines in sea bass (Dicentrarchus labrax L.)

In mammals, interleukin (IL)-2, initially known as a T-cell grow factor, is an immunomodulatory cytokine involved in the proliferation of T cells upon antigen activation. In bony fish, some IL-2 orthologs have been identified, but, recently, an additional IL-2like (IL-2L) gene has been found. In this paper, we report the presence of these two divergent IL-2 isoforms in sea bass (Dicentrarchus labrax L.). Genomic analyses revealed that they originated from a gene duplication event, as happened in most percomorphs. These two IL-2 paralogs show differences in the amino acid sequence and in the exon 4 size, and these features could be an indication that they bind preferentially to different specific IL-2 receptors. Sea bass IL-2 paralogs are highly expressed in gut and spleen, which are tissues and organs involved in fish T cell immune functions, and the two cytokines could be up-regulated by both PHA stimulation and vaccination with a bacterial vaccine, with IL-2L being more inducible. To investigate the functional activities of sea bass IL-2 and IL-2L we produced the corresponding recombinant molecules in E. coli and used them to in vitro stimulate HK and spleen leukocytes. IL-2L is able to up-regulate the expression of markers related to different T cell subsets (Th1, Th2 and Th17) and to Treg cells in HK, whereas it has little effect in spleen. IL-2 is not active on these markers in HK, but shows an effect on Th1 markers in spleen. Finally, the stimulation with recombinant IL-2 and IL-2L is also able to induce in vitro proliferation of HK- and spleen-derived leukocytes. In conclusion, we have demonstrated that sea bass possess two IL-2 paralogs that likely have an important role in regulating T cell development in this species and that show distinct bioactivities.

Vaccination and immune responses of European sea bass (Dicentrarchus labrax L.) against betanodavirus

This review summarizes the available knowledge on the immune defences of European sea bass against antigenic preparations derived from the viral encephalopathy and retinopathy virus (betanodavirus), which represents a major threat to the health of this fish species. The nodavirus is widely present and differentiates into several strains that infect invertebrates (in insects, alphanodavirus) and teleost fish, and thus may represent a great problem for farmed fish species. Many efforts have been directed to discovering new immunizations to induce protection in sea bass, especially at young stages, and these efforts have included employing diverse betanodavirus strains, antigen preparation, vaccination routes, and the addition of adjuvants and/or immunostimulants. The obtained results showed that inactivated preparations of betanodavirus that were administered intraperitoneally may induce both immune recognition and protection. Attempts at performing mucosal immunization by immersion and/or oral administration, which is a vaccination route that is highly preferred for sea bass, have shown intriguing results, and more studies are necessary for its improvement. Overall, the objective of identifying a reliable vaccine that also cross-protects against different genotypes or reassortant viruses for use in European sea bass against betanodavirus appears to be an attainable goal in the near future.

Immuno-related gene transcription and antibody response in nodavirus (RGNNV and SJNNV)-infected European sea bass (Dicentrarchus labrax L.)

The immune response of European sea bass to RGNNV and SJNNV infections has been evaluated by quantifying the transcription of some genes involved in the IFN I system, as well as in the inflammatory and adaptive immune mechanisms. The transcription of IFN-I, ISG-12, ISG-15 and MxA genes has been analyzed in brain and head kidney, showing that RGNNV genotype induces a more intense response of the IFN I system than SJNNV in both organs. In addition, the results obtained indicate the importance of the inflammatory response in nodavirus pathogenesis, with the transcription of IL-8 and TNF-α significantly higher in brain than in head kidney, being RGNNV the strongest inductor. An important difference between the immune response induced by both genotypes refers to the IgM titre in sera, which was higher in SJNNV-inoculated fish. The acquired response is also important locally, since TR-γ transcription is higher in brain than in head kidney (especially in the RGNNV-inoculated group). To our knowledge, this is the first study addressing the sea bass anti-SJNNV immune response.

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.

Immunoglobulin T from sea bass (Dicentrarchus labrax L.): molecular characterization, tissue localization and expression after nodavirus infection

Background

Immunoglobulins (Igs) are fundamental components of the adaptive immune system of vertebrates, with the IgT/IgZ isotype specific of Teleosts. In this paper we describe the identification of an IgT heavy chain from the European sea bass (Dicentrarchus labrax L.), its molecular characterization and tissue mRNA localization by in situ hybridization.

Results

Sea bass IgT consists of 552 aa (Accession Number KM410929) and it contains a putative 19 amino acids long signal peptide and one potential N-glycosylation site. The C-region consists of four C_H domains; each contains the cysteine and tryptophan residues required for their correct folding. Based on the recent sequencing of sea bass genome, we have identified five different genomic contigs bearing exons unequivocally pertaining to IgT (C_H2, C_H3 and C_H4), but none corresponded to a complete IgH locus as IgT sequences were found in the highly fragmented assembled genomic regions which could not be assigned to any major scaffold. The 3D structure of sea bass IgT has been modelled using the crystal structure of a mouse Ig gamma as a template, thus showing that the amino acid sequence is suitable for the expected topology referred to an immunoglobulin-like architecture. The basal expression of sea bass IgT and IgM in different organs has been analysed: gut and gills, important mucosal organs, showed high IgT transcripts levels and this was the first indication of the possible involvement of sea bass IgT in mucosal immune responses. Moreover, sea bass IgT expression increased in gills and spleen after infection with nodavirus, highlighting the importance of IgT in sea bass immune responses. In situ hybridization confirmed the presence of IgT transcripts in the gut and it revealed a differential expression along the intestinal tract, with a major expression in the posterior intestine, suggesting the hindgut as a site for the recruitment of IgT⁺ cells in this species. IgT transcripts were also found in gill filaments and parallel lamellae and, for the first time, we identified scattered IgT positive cells in the liver, with a strong signal in the hepatic parenchyma.

Conclusions

In conclusion, we performed a full molecular characterization of IgT in sea bass that points out its possible involvement in mucosal immune responses of this species.

Immune response of the Antarctic teleost Trematomus bernacchii to immunization with Psychrobacter sp. (TAD1)

Adult Trematomus bernacchii have been immunized intraperitoneally with heat-killed cells of the Antarctic marine bacterium Psychrobacter sp. (TAD1) up to 60 days. After immunizations and sampling at various times, fish sera were tested for specific IgM by ELISA, and different tissues (head kidney and spleen) were investigated for transcription of master genes of the acquired immune response (IgM, IgT, TRβ, TRγ). Results from ELISA assays showed a time-dependent induction of specific serum anti-TAD1 IgM, and western blot analysis of TAD1 lysates probed with fish sera revealed enhanced immunoreactivity in immunized animals compared to controls. Quantitative PCR analysis of transcripts coding for IgM, IgT, TRβ, TRγ was performed in T. bernacchii tissues to assess basal expression, and then on cDNAs of cells from head kidney and spleen of fish injected for 8, 24, and 72 h with inactivated TAD1. The results showed a differential basal expression of transcripts in the examined tissues, and a time-dependent strong up-regulation of IgT, TRβ, TRγ genes upon in vivo stimulation with TAD1. These results represent a first in vivo study on the mounting of a specific immune response in an Antarctic teleost species.

MHC II-β chain gene expression studies define the regional organization of the thymus in the developing bony fish Dicentrarchus labrax (L.)

MHC II-β chain gene transcripts were quantified by real-time PCR and localised by in situ hybridization in the developing thymus of the teleost Dicentrarchus labrax, regarding the specialization of the thymic compartments. MHC II-β expression significantly rose when the first lymphoid colonization of the thymus occurred, thereafter increased further when the organ progressively developed cortex and medulla regions. The evolving patterns of MHC II-β expression provided anatomical insights into some mechanisms of thymocyte selection. Among the stromal cells transcribing MHC II-β, scattered cortical epithelial cells appeared likely involved in the positive selection, while those abundant in the cortico-medullary border and medulla in the negative selection. These latter most represent dendritic cells, based on typical localization and phenotype. These findings provide further proofs that efficient mechanisms leading to maturation of naïve T cells are operative in teleosts, strongly reminiscent of the models conserved in more evolved gnathostomes.

Influence of titanium dioxide nanoparticles on 2,3,7,8-tetrachlorodibenzo-p-dioxin bioconcentration and toxicity in the marine fish European sea bass (Dicentrarchus labrax)

The present study investigated the influence of nano-TiO(2) (1 mg L(-1)) on 2,3,7,8-tetrachlorodibenzo-p-dioxin(2,3,7,8-TCDD) (46 pg L(-1)) bioconcentration and toxicity in the European sea bass (Dicentrarchus labrax) during 7 days in vivo exposure. A multimarkers approach was applied in different organs: detoxification in liver; innate immunity and pro-inflammatory response and adaptive immunity in gills and spleen; genotoxicity in peripheral erythrocytes and muscle. Bioconcentration of 2,3,7,8-TCDD in presence of nano-TiO2 was investigated in liver, skin and muscle as well as interaction between nano-TiO2 and organic pollutants in artificial sea water (ASW). Nano-TiO2 negatively influenced immune response induced by 2,3,7,8-TCDD in spleen but not in gills and reduced the DNA damage induced by 2,3,7,8-TCDD in erythrocytes. nano-TiO2 did not interfere with 2,3,7,8-TCDD detoxification and bioconcentration according to the observed no interaction of the nano-TiO2 with organic pollutants in ASW.

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.

Molecular cloning and characterization of sea bass (Dicentrarchus labrax, L.) MHC class I heavy chain and β2-microglobulin

In this work, the gene and cDNA of sea bass (Dicentrarchus labrax) β2-microglobulin (Dila-β2m) and several cDNAs of MHC class I heavy chain (Dila-UA) were characterized. While Dila-β2m is single-copy, numerous Dila-UA transcripts were identified per individual with variability at the peptide-binding domain (PBD), but also with unexpected diversity from the connective peptide (CP) through the 3' untranslated region (UTR). Phylogenetic analysis segregates Dila-β2m and Dila-UA into each subfamily cluster, placing them in the fish class and branching Dila-MHC-I with lineage U. The α1 domains resemble those of the recently proposed L1 trans-species lineage. Although no Dila-specific α1, α2 or α3 sub-lineages could be observed, two highly distinct sub-lineages were identified at the CP/TM/CYT regions. The three-dimensional homology model of sea bass MHC-I complex is consistent with other characterized vertebrate structures. Furthermore, basal tissue-specific expression profiles were determined for both molecules, and expression of β2m was evaluated after poly I:C stimulus. Results suggest these molecules are orthologues of other β2m and teleost classical MHC-I and their basic structure is evolutionarily conserved, providing relevant information for further studies on antigen presentation in this fish species.

A piscidin-like antimicrobial peptide from the icefish Chionodraco hamatus (Perciformes: Channichthyidae): molecular characterization, localization and bactericidal activity

Antimicrobial peptides (AMPs) are considered one of the most ancient components of the innate immune system. They are able to exert their protection activity against a variety of microorganisms, and are widely distributed in both vertebrates and invertebrates. In this paper we focused on an AMP identified in the Antarctic teleost Chionodraco hamatus, an icefish species. The cDNA sequence of the AMP, named chionodracine, is comprised of 515 bp and translates for a putative protein precursor of 80 amino acids, with a signal peptide of 22 amino acids. The structural features evidenced in the primary sequence of chionodracine lead to the inclusion of the peptide in the antimicrobial family of piscidins. The analysis by real-time PCR of the basal gene transcripts of chionodracine in different icefish tissues showed that the highest expression was found in gills, followed by head kidney. The chionodracine expression levels in head kidney leukocytes were up-regulated in vitro both by LPS and poly I:C, and in vivo by LPS. A putative chionodracine mature peptide was synthesized and employed to obtain a polyclonal antiserum, which was used in immunohistochemistry of gills sections and revealed a significant positivity associated with mast cells. The bactericidal activity of the peptide was investigated and found significant against Antarctic psychrophilic bacteria strains (Psychrobacter sp. TAD1 and TA144), the Gram-positive Bacillus cereus, and at a lesser extent against the Gram-negative Escherichia coli. Interestingly, the haemolytic activity of chionodracine was tested in vitro on human erythrocytes and no significant lysis occurred until peptide concentration of 50 μM.

Diversity, molecular characterization and expression of T cell receptor γ in a teleost fish, the sea bass (Dicentrarchus labrax, L)

Two lineages of T cells, expressing either the αβ T cell receptor (TR) or the γδ TR, exist in Gnathostomes. The latter type of T cells account for 1–10 % of T cells in blood and up to 30 % in the small intestine. They may recognize unconventional antigens (phosphorylated microbial metabolites, lipid antigens) without the need of major histocompatibility class I (MH1) or class II (MH2) presentation. In this work we have described cloning and structural characterization of TR -chain (TRG) from the teleost Dicentrarchus labrax. Further, by means of quantitative PCR analysis, we analyzed TRG expression levels both in poly I:C stimulated leukocytes in vitro, and following infection with betanodavirus in vivo. Two full length cDNAs relative to TRG, with the highest peptide and nucleotide identity with Japanese flounder, were identified. A multiple alignment analysis showed the conservation of peptides fundamental for TRG biological functions, and of the FGXG motif in the FR4 region, typical of most TR and immunoglobulin light chains. A 3D structure consisting of two domains mainly folded as beta strands with a sandwich architecture for each domain was also reported. TRG CDR3 of 8–18 AA in length and diversity in the TRG rearrangements expressed in thymus and intestine for a given V/C combination were evidenced by junction length spectratyping. TRG mRNA expression levels were high in basal conditions both in thymus and intestine, while in kidney and gut leukocytes they were up-regulated after in vitro stimulation by poly I:C. Finally, in juveniles the TRG expression levels were up-regulated in the head kidney and down-regulated in intestine after in vivo infection with betanodavirus. Overall, in this study the involvement of TRG-bearing T cells during viral stimulation was described for the first time, leading to new insights for the identification of T cell subsets in fish.

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.

A CD83-like molecule in sea bass (Dicentrarchus labrax): molecular characterization and modulation by viral and bacterial infection

The CD83 cell surface marker is an important and intriguing component of immune system. It is considered the best marker for mature human dendritic cells, but it is also important for thymic development of T cells, and it also plays a role as a regulator of peripheral B-cell function and homeostasis. A CD83-like molecule was identified in sea bass (Dicentrarchus labrax) by EST sequencing of a thymus cDNA library; the CD83 cDNA is composed of 816 bp and the mature CD83 peptide consists of 195 amino acids, with a putative signal peptide of 18 amino acids and two possible N-glycosylation sites. The comparison of sea bass CD83 sequence with its homologues in other fish species and mammals shows some differences, with two cysteine residues conserved from fish to mammals and a high variability both in the total number of cysteines and in mature CD83 sequence polypeptide length. Basal transcripts levels of CD83 mRNA are highest in liver, followed by thymus. The in vitro treatment of head kidney leukocytes with LPS resulted in a down-regulation on CD83 mRNA leves both after 4 and 24 h, whereas with poly I:C an up-regulation after 4h followed by a down-regulation at 24 h was observed. An in vivo infection of sea bass juveniles with nodavirus induced an increase of CD83 expression on head kidney leukocytes both after 6 and 24 h and a decrease after 72 h. On the other hand, an in vivo infection with Photobacterium damselae bacteria induced a decrease of CD83 transcript levels after 6 and 24 h and an increase after 72 h. These findings suggest in sea bass CD83 expression could be modulated by viral and bacterial immune response.

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.

Effects of the polycyclic ketone tonalide (AHTN) on some cell viability parameters and transcription of P450 and immunoregulatory genes in rainbow trout RTG-2 cells

To assess the potential effects of environmental pollutants belonging to the musk fragrances group in the physiology of aquatic animal species, in this work we treated rainbow trout RTG-2 cells with the polycyclic ketone tonalide (AHTN) at dilutions ranging from 3.5 to 500 ng/ml. The following parameters were monitored: intracellular ATP concentration (energy production), mitochondrial membrane potential (early apoptosis marker), cell viability (vital staining with DFP), quantitative expression of genes coding for the cytochrome P450 detoxifying enzymes CYP1A1 and CYP3A27, and of genes coding for the immunoregulatory peptides IL-1β, IL-8, TNFα, Cox-2 and TGF-β. Obtained results showed that incubation with tonalide induced in RTG-2 cells no effects on cell viability, a slight increase of mitochondrial membrane potential activity, and a significant increase in intracellular ATP concentration. However, dramatic effects were observed in transcription levels of some tested genes, with upregulation levels of 300 and 600 times measured for TGF-β and TNFα, respectively and of 150 times for the CYP3A27 gene. Our results show for the first time the potent effects exerted by tonalide on immunoregulatory genes of RTG-2 cells and also indicate that the measured sensitivity of RTG-2 towards tonalide was in the same range of that currently available using chemical methods. A possible use of the panel of genes we employed as a tool for the monitoring of musk fragrances in biological samples is discussed.

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.

Searching for immunomodulatory sequences in sea bass (Dicentrarchus labrax L.): transcripts analysis from thymus

The thymus is a key organ of the immune system in most vertebrates and, for this reason, it has been used in this paper for the generation of a normalized cDNA library from sea bass (Dicentrarchus labrax), one of the most extensively cultured species in South Mediterranean aquaculture. A total of 1632 ESTs from this library were initially analysed for sequence quality and vector sequences and, after this control, 1264 (77% of total clones sequenced) high-quality ESTs were further processed. The total collection of D. labrax thymus ESTs has been deposited in the EBI-GenBank-DBJ database (GenBank accession numbers from FN565576 to FN566839). The functional classification of ESTs was performed by Gene Ontology and KEGG annotation and, successively, the sequences were analysed using the ImmunomeBase software to identify potentially immuno-related genes. Using this approach, we found about 100 putative genes involved in immune system responses, most new in sea bass, that were analysed more in detail. Some of the potentially interesting genes identified by these in silico analyses were studied by real-time PCR to verify their expression both at basal level and after in vitro stimulation of sea bass head kidney leukocytes. The used strategy has been confirmed as a good approach to discover new immuno-related genes and to improve the knowledge of specific markers that could help the discrimination of T-cell subpopulations in sea bass and, in general, in Teleosts.

Molecular and structural characterisation of a macrophage migration inhibitory factor from sea bass (Dicentrarchus labrax L.)

The macrophage migration inhibitory factor (MIF) is a cytokine produced in numerous cell types, mainly T lymphocytes and macrophages, in response to inflammatory stimuli. In this paper we report the identification of a cDNA encoding a MIF molecule from sea bass (Dicentrarchus labrax L.), its expression analysis and its 3D structure obtained by template-based modelling. The sea bass MIF cDNA consists of 609bp that translates in one reading frame to give the entire molecule containing 115 amino acids. The sequence contains three cysteine residues in conserved positions compared to human MIF and most Teleost fishes, with the exception of zebrafish and carp. The Cys(57)-Ala(58)-Leu(59)-Cys(60) motif, present inside the stretch important for JAB1-interaction and mediator of the thiol-protein oxidoreductase activity of MIF, is conserved in sea bass, together with the Pro(2) residue that is crucial for the tautomerase catalytic activity. Real-time PCR analyses revealed that MIF is constitutively expressed in all selected tissues and organs, with the highest mRNA level observed in thymus. MIF expression was induced after 4h in vitro stimulation of head kidney leukocytes with LPS and decreased after 24h. The predicted 3D model of sea bass MIF has been used to verify the presence of structural requirements for its known biological activities.

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.

Lymphocyte differentiation in sea bass thymus: CD4 and CD8-alpha gene expression studies

Different developmental stages (from eggs to 1-year-old juveniles) of the teleost fish Dicentrarchus labrax (L.) were assayed for CD4 gene expression. RT-PCR revealed the appearance of CD4 transcripts in post-larvae from 51 days post-hatching (dph). This finding overlaps the first detection of CD8-alpha mRNA. Real-time PCR with specific primers quantified CD4, CD8-alpha and TCR-beta transcripts in larvae and post-larvae (25, 51, 75 and 92 dph) and 1-year-old thymus. At 92 dph, TcR-beta and CD8-alpha transcripts were significantly higher (P < 0.001) than in previous stages, as CD4 transcripts compared with 51 dph (P < 0.01). High levels of TCR-beta and CD8-alpha transcripts were found in the thymus, while CD4 transcripts were lower (P < 0.05 vs. TCR-beta). In situ hybridization identified CD4 mRNAs at 51 dph, localized in thymocytes of the outer and lateral zones of the thymic glands. From 75 dph on the signal was mainly detected in the outer region, drawing a cortex-medulla demarcation. Developmental expression of CD4 and CD8-alpha almost coincided. In each adult thymic lobe CD4(+) and CD8-alpha(+) thymocytes filled the cortex. The expression patterns of CD4 and CD8-alpha largely overlap, except in the medulla, where CD4(+) thymocytes were isolated, while CD8-alpha(+) ones mainly arranged in cords. These results provide new information about the thymic compartmentalization and lymphocyte differentiation pathways in a teleost, almost demonstrating that double negative thymocytes fill the cortex giving rise to further selection in the medulla.

Early treatment with Lactobacillus delbrueckii strain induces an increase in intestinal T-cells and granulocytes and modulates immune-related genes of larval Dicentrarchus labrax (L.)

Lactobacillus delbrueckii ssp. delbrueckii (AS13B), isolated from the gut of adult Dicentrarchus labrax, was administered live to developing sea bass using rotifers and Artemia as live carriers. Immune-related gene transcripts were quantified in post-larvae at day 70 post-hatch (ph) and histology, electron microscopy and immunocytochemistry of the intestinal tissue were performed at day 74 ph. Since the probiotic was orally administered the studies were focused on intestinal immunity. In treated fish gut integrity was unaffected, while the density of T-cells and acidophilic granulocytes in the intestinal mucosa was significantly higher than in controls. Probiotic-induced increases in intestinal T-cells and total body TcR-beta transcripts are first reported in fish. Significantly lower IL-1beta transcripts and a trend towards lower IL-10, Cox-2 and TGF-beta transcription were found in the treated group. Evidence is provided that early feeding with probiotic-supplemented diet stimulated the larval gut immune system and lowered transcription of key pro-inflammatory genes.

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.

A CD4 homologue in sea bass (Dicentrarchus labrax): molecular characterization and structural analysis

CD4 is a transmembrane glycoprotein fundamental for cell-mediated immunity. Its action as a T cell co-receptor increases the avidity of association between a T cell and an antigen-presenting cell by interacting with portions of the complex between MHC class II and TR molecules. In this paper we report the cDNA cloning, expression and structural analysis of a CD4 homologue from sea bass (Dicentrarchus labrax). The sea bass CD4 cDNA consists of 2071 bp that translates in one reading frame to give the entire molecule containing 480 amino acids. The analysis of the sequence shows the presence of four putative Ig-like domains and that some fundamental structural features, like a disulphide bond in domain D2 and the CXC signalling motif in the cytoplasmic tail, are conserved from sea bass to mammals. Real-time PCR analysis showed that very high levels of CD4 mRNA transcripts are present in thymus, followed by gut and gills. In vitro stimulation of head kidney leukocytes with LPS and PHA-L gave an increase of CD4 mRNA levels after 4h and a decrease after 24h. Homology modelling has been applied to create a 3D model of sea bass CD4 and to investigate its interaction with sea bass MHC-II. The analysis of the 3D complex between sea bass CD4 and sea bass MHC-II suggests that the absence of a disulfide bond in the CD4 D1 domain could make this molecule more flexible, inducing a different conformation and affecting the binding and the way of interaction between CD4 and MHC-II. Our results will add new insights into the sea bass T cell immune responses and will help in the identification of T cell subsets in teleost fishes to better understand the evolution of cell-mediated immunity from fish to mammals.

Molecular cloning, differential expression and 3D structural analysis of the MHC class-II beta chain from sea bass (Dicentrarchus labrax L.)

The major histocompatibility complex class I and II molecules (MHC-I and MHC-II) play a pivotal role in vertebrate immune response to antigenic peptides. In this paper we report the cloning and sequencing of the MHC class II beta chain from sea bass (Dicentrarchus labrax L.). The six obtained cDNA sequences (designated as Dila-DAB) code for 250 amino acids, with a predicted 21 amino acid signal peptide and contain a 28bp 5'-UTR and a 478bp 3'-UTR. A multiple alignment of the predicted translation of the Dila-DAB sequences was assembled together with other fish and mammalian sequences and it showed the conservation of most amino acid residues characteristic of the MHC class II beta chain structure. The highest basal Dila-DAB expression was found in gills, followed by gut and thymus, lower mRNA levels were found in spleen, peripheral blood leucocytes (PBL) and liver. Stimulation of head kidney leukocytes with LPS for 4h showed very little difference in the Dila-DAB expression, but after 24h the Dila-DAB level decreased to a large extent and the difference was statistically significant. Stimulation of head kidney leukocytes with different concentrations of rIL-1beta (ranging from 0 to 100ng/ml) resulted in a dose-dependent reduction of the Dila-DAB expression. Moreover, two 3D Dila-DAB*0101 homology models were obtained based on crystallographic mouse MHC-II structures complexed with D10 T-cell antigen receptor or human CD4; features and differences between the models were evaluated and discussed. Taken together these results are of interest as MHC-II structure and function, molecular polymorphism and differential gene expression are in correlation with disease resistance to virus and bacteria in teleost fish.

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.

Cellular activities during a mixed leucocyte reaction in the teleost sea bass Dicentrarchus labrax

In this investigation a number of "in vitro" activities of sea bass peripheral blood leucocytes (PBL) against allogeneic PBL inactivated by irradiation were studied. Stimulator PBL were cultured with inactivated allogeneic PBL, and direct counting of lymphocytes was done after 2 weeks by immunofluorescence and flow cytometry using mAbs DLT15 and DLIg3 specific for T-cells and B-cells, respectively. In a one-way mixed leucocyte reaction (MLR), results showed an increase of T lymphocytes, whereas B lymphocytes had values similar to those in control PBL. The increase of T-cells in MLR cultures was also confirmed using RT-PCR by analyzing the expression of the T-cell receptor (beta-subunit) mRNA. The addition of 5 microg/ml of cyclosporin A (CsA) to the MLR caused a significant decrease in T-cell proliferation. Leucocytes from MLR cultures displayed an enhanced cytotoxic activity against xenogeneic target cells with respect to control PBL, raising the possibility of the presence of cytotoxic-like T lymphocytes. Cellular activation of PBL was confirmed in 2 weeks MLR by measuring antibody-induced intracellular Ca(++) mobilization with Fura-2 AM. This work represents the first direct quantitative determination of an "in vitro" T-cell activity in a teleost species.

Evolution of cytokine responses: IL-1β directly affects intracellular Ca2+ concentration of teleost fish leukocytes through a receptor-mediated mechanism

In this work we studied the biological activities of recombinant IL-1beta from the teleosts sea bass (Dicentrarchus labrax) and rainbow trout (Oncorhynchus mykiss) by investigating the effects induced on intracellular Ca2+ concentrations ([Ca2+]i) of spleen leucocytes. Splenocytes were loaded with the Ca2+-permeant Fura-2AM, and then stimulated with rIL-1beta. The emitted fluorescence was read for 5 min at 1 min intervals on a dual excitation fluorescence fluorimeter. Results showed that rIL-1beta induced in both species a rise in [Ca2+]i, and a subsequent decrease until 5 min after stimulation. The stimulating effect was dose-dependent in both species reaching a plateau at 200 ng/ml of rIL-1beta, was abolished by heat-treatment of rIL-1beta, and affected in a dose-dependent fashion by treatment of leucocytes with trypsin. These features suggested a functional IL-1 receptor was involved in the binding. The observed rise in [Ca2+]i was not detected in human PBMC and was species-specific, since rIL-1beta from sea bass, trout, and human were unable to interfere each other in the assay. Moreover, incubation of splenocytes with rIL-1beta induced a rapid tyrosine phosphorylation of a 24 kDa polypeptide in both species. This work represents the first evidence of a direct effect on [Ca2+]i induced by IL-1beta and suggests that in the evolution of IL-1 activities, teleost fishes display a peculiar IL-1-associated behaviour that is lacking in mammals.

The CD8alpha from sea bass (Dicentrarchus labrax L.): Cloning, expression and 3D modelling

In this paper we describe the cloning, expression and structural study by modelling techniques of the CD8alpha from sea bass (Dicentrarchus labrax L.). The sea bass CD8alpha cDNA is comprised of 1490 bp and is translated in one reading frame to give a protein of 217 amino acids, with a predicted 26 amino acids signal peptide, a 88 bp 5'-UTR and a 748 bp 3'-UTR. A multiple alignment of CD8alpha from sea bass with other known CD8alpha sequences shows the conservation of most amino acid residues involved in the peculiar structural domains found within CD8alpha's. Cysteine residues that are involved in disulfide bonding to form the V domain are conserved. In contrast, an extra cysteine residue found in most mammals in this region is not present in sea bass. The transmembrane and cytoplasmic regions are the most conserved regions within the molecule in the alignment analysis. However, the motif (CXCP) that is thought to be responsible for binding p56lck is missing in the sea bass sequence. Phylogenetic analysis conducted using amino acid sequences showed that sea bass CD8alpha grouped with other known teleost sequences and that three different clusters were formed by the mammalian, avian and fish CD8alpha sequences. The thymus was the tissue with the highest CD8alpha expression, followed by gut, gills, peripheral blood leukocytes and spleen. Lower CD8alpha mRNA levels were found in head kidney, liver and brain. It was possible to create a partial 3D model using the human and mouse structures as template. The CD8alpha 11-120 amino acid region was taken into consideration and the best obtained 3D model shows the presence of ten beta-strands, involving about 50% of the sequence. The global structure was defined as an immunoglobulin-like beta-sandwich made of two anti-parallel sheets. Two cysteines were present in this region and they were at a suitable distance to form an S-S bond as seen in the template human and mouse structures.

Immunoglobulin protein and gene transcripts in sea bream (Sparus aurata L.) oocytes

Cellular mechanisms of Ig transfer to Sparus aurata developing oocytes were analysed. Homologous serum Ig was purified and used to raise the rabbit polyclonal antiserum ORa. Immunohistochemistry revealed an active role of both follicular cells (already at a pre-vitellogenetic stage) and oocytes in the Ig uptake. Early vitellogenetic oocytes (lipid vesicle stages) had ORa staining of outer cortex and oolemma as well as of their follicular cells. In protein yolk granule oocytes, ORa staining was notably found in the pore canals crossing the egg envelope and at the periphery of yolk platelets. A transfer of Ig from the blood to the follicles appears likely. In addition, RT-PCR using specific primers for the constant region of sea bream Ig L chain detected Ig mRNA in released eggs and no signal in post-hatching larvae. These findings show that a significant level of Ig gene transcription in the oocyte and/or a transfer of transcripts may also occur.

Biological activity of sea bass (Dicentrarchus labrax L.) recombinant interleukin-1beta

Biological activities of a putative mature sea bass interleukin-1beta peptide, produced as a recombinant protein (rIL-1beta) in Escherichia coli, have been investigated. The rIL-1beta contains a 6-histidine tag at the N-terminus, and protein purification has been achieved through this tag by affinity chromatography. Biological activities have been investigated both at the cellular and gene expression levels. In in vitro assays sea bass rIL-1beta induced the proliferation of murine D10.G4.1 cells and increased yeast phagocytosis by sea bass head kidney leukocytes. The purified cytokine was also tested in a lymphocyte-activation factor assay, where it induced the proliferation of sea bass thymocytes. Finally, in an in vivo assay, rIL-1beta administered intraperitoneally increased expression levels of the IL-1beta gene and activated macrophages to produce a cyclooxygenase 2 homologue (COX-2) gene in the head kidney.

Expression in Escherchia coli and purification of sea bass (Dicentrarchus labrax) interleukin 1beta, a possible immunoadjuvant in aquaculture

Interleukin 1beta (IL-1beta) is a pleiotropic cytokine that plays a pivotal role in regulating immune responses. Our group has recently cloned IL-1beta from sea bass (Dicentrarchus labrax), one of the main Mediterranean aquacultured fish species. The cDNA is 1292 bp and codes for a deduced peptide of 29.4 kDa with a pI of 5.1. As for trout and carp IL-1beta precursor sequence, no candidate cut site for ICE (IL-1beta converting enzyme) enzyme was apparent in the alignments of sea bass IL-1beta with other mammalian IL-1betas. Nevertheless, a possible mature peptide could start at Ala86, giving a protein of 176 amino acids. The nucleotide sequence coding for this polypeptide was cloned into a pQE-30 expression vector. The plasmid was then transformed in Escherichia coli, and the recombinant protein was purified. Finally, we demonstrated that this purified recombinant IL-1beta was able to induce IL-1beta gene expression in a dose-dependent manner on cells purified from sea bass head kidney and could have immunoadjuvant effects in sea bass vaccination experiments.