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

The Mediterranean sea bass Dicentrarchus labrax: A marine model species in fish immunology

The Mediterranean sea bass, Dicentrarchus labrax, is a species of great interest due to the extensive knowledge accumulated about its immune system and the application of these findings in aquaculture health management. The available data indicate that sea bass has the morphological and immunological features typical of jawed vertebrates, with minor anatomical differences compared to evolutionarily older teleosts. Namely, all the master genes coding for Tc and Th T cells have been found to be expressed, together with related cytokine families, and Tc/Th activities can be investigated using in vitro models. The B lymphocytes produce IgM/IgT/IgD antibodies in response to antigenic/vaccine stimulation and maintain an IgM-B cell memory for antigens and vaccines. Mucosal and systemic immunity with associated leukocyte populations is present and functional, and it can be modulated by substances added to water or food. Studies on the ontogenesis of immune components defined precise points of lymphocyte development during larval life. Finally, the central nervous system of sea bass has been shown to contain resident lymphocytes, whose number can be modulated by pathogenic infection. Based on the available knowledge summarized in this review, it can be certainly assumed that the Dicentrarchus labrax is a valuable marine model species for studies in immunology and physiology of vertebrates.

The manipulation of cell suspensions from zebrafish intestinal mucosa contributes to understanding enteritis

Background

Although zebrafish are commonly used to study intestinal mucosal immunity, no dedicated procedure for isolating immune cells from zebrafish intestines is currently available. A speedy and simple operating approach for preparing cell suspension from mucosa has been devised to better understanding of intestinal cellular immunity in zebrafish.

Methods and results

The mucosal villi were separated away from the muscle layer by repeated blows. The complete deprivation of mucosa was done and evidenced by HE and qPCR results. Higher expression of both innate (mpeg1, mpx, and lck) and adaptive immune genes (zap70, blnk, foxp3a, and foxp3b) was revealed compared to cells obtained by typical mesh rubbing. The cytometric results also revealed that the tested operation group had a higher concentration and viability. Further, fluorescent-labelled immune cells from 3mo Tg(lyz:DsRED2), Tg(mpeg1:EGFP), Tg(Rag2:DsRED), and Tg(lck:EGFP), were isolated and evaluated for the proportion, and immune cells' type could be inferred from the expression of marker genes. The transcriptomic data demonstrated that the intestinal immune cell suspension made using the new technique was enriched in immune-related genes and pathways, including il17a/f, il22, cd59, and zap70, as well as pattern recognition receptor signaling and cytokine-cytokine receptor interaction. In addition, the low expression of DEG for the adherent and close junctions indicated less muscular contamination. Also, lower expression of gel-forming mucus-associated genes in the mucosal cell suspension was consistent with the current less viscous cell suspension. To apply and validate the developed manipulation, enteritis was induced by soybean meal diet, and immune cell suspensions were analyzed by flow cytometry and qPCR. The finding that in enteritis samples, there was inflammatory increase of neutrophils and macrophages, was in line with upregulated cytokines (il8 and il10) and cell markers (mpeg1 and mpx).

Conclusion

As a result, the current work created a realistic technique for studying intestinal immune cells in zebrafish. The immune cells acquired may aid in further research and knowledge of intestinal illness at the cellular level.

CD4-1 and CD8α T lymphocytes subsets in spotted sea bass (Lateolabrax maculatus) and comparison on antigenicity of T lymphocytes subsets in other three marine fish species

CD4 and CD8 molecules play an important role in the identification of T lymphocytes, and diverse among fish species. In this study, CD4-1 and CD8α gene of spotted sea bass (Lateolabrax maculatus) were cloned, polyclonal antibodies against CD4-1 (CD4-1 pAbs) and CD8α (CD8α pAbs) were produced, respectively. And the variations in CD4-1⁺ and CD8α⁺ T-lymphocytes in spotted sea bass and the cross-reactivity with leukocytes in pearl gentian grouper (Epinephelus fuscoguttatus x E. lanceolatus), schlegel's black rockfish (Sebastes schlegelii) and flounder (Paralichthys olivaceus) were investigated using CD4-1 pAbs and CD8α pAbs. The results showed that CD4-1 molecule ORF was 1413 bp and CD8α was 690 bp, both molecules are transmembrane glycoproteins with high amino acid homology to grouper. The CD4-1 pAbs specifically recognized both the CD4-1 recombinant and natural proteins, as does the CD8α pAbs to CD8α molecule, and no cross-reactivity between the two antibodies. CD4-1⁺ and CD8α⁺ T lymphocytes were detected in peripheral blood, spleen and head kidney leukocytes in spotted sea bass. In cross-reactivity assay with other three fish, CD4-1 pAbs could recognize the lymphocytes from pearl gentian grouper and schlegel's black rockfish, both with highest proportions in the spleen leukocytes, 5.3 ± 0.4% and 2.6 ± 0.3%, respectively, and CD8α pAbs could only recognize the lymphocytes in pearl gentian grouper, and no cross-reactivities to lymphocytes of flounder. These data suggested that the CD4-1 and CD8α molecules varied by fish species in the genes features and antigenicity, which might result in the diversities of T lymphocytes subpopulations. This will be a key to elucidating the classification and evolution of T lymphocytes in fish.

Lympho-Hematopoietic Microenvironments and Fish Immune System

In the last 50 years information on the fish immune system has increased importantly, particularly that on species of marked commercial interest (i.e., salmonids, cods, catfish, sea breams), that occupy a key position in the vertebrate phylogenetical tree (i.e., Agnatha, Chondrichtyes, lungfish) or represent consolidated experimental models, such as zebrafish or medaka. However, most obtained information was based on genetic sequence analysis with little or no information on the cellular basis of the immune responses. Although jawed fish contain a thymus and lympho-hematopoietic organs equivalents to mammalian bone marrow, few studies have accounted for the presumptive relationships between the organization of these cell microenvironments and the known immune capabilities of the fish immune system. In the current review, we analyze this topic providing information on: (1) The origins of T and B lymphopoiesis in Agnatha and jawed fish; (2) the remarkable organization of the thymus of teleost fish; (3) the occurrence of numerous, apparently unrelated organs housing lympho-hematopoietic progenitors and, presumably, B lymphopoiesis; (4) the existence of fish immunological memory in the absence of germinal centers.

Gut Immune System and the Implications of Oral-Administered Immunoprophylaxis in Finfish Aquaculture

The gastrointestinal immune system plays an important role in immune homeostasis regulation. It regulates the symbiotic host-microbiome interactions by training and developing the host's innate and adaptive immunity. This interaction plays a vital role in host defence mechanisms and at the same time, balancing the endogenous perturbations of the host immune homeostasis. The fish gastrointestinal immune system is armed with intricate diffused gut-associated lymphoid tissues (GALTs) that establish tolerance toward the enormous commensal gut microbiome while preserving immune responses against the intrusion of enteric pathogens. A comprehensive understanding of the intestinal immune system is a prerequisite for developing an oral vaccine and immunostimulants in aquaculture, particularly in cultured fish species. In this review, we outline the remarkable features of gut immunity and the essential components of gut-associated lymphoid tissue. The mechanistic principles underlying the antigen absorption and uptake through the intestinal epithelial, and the subsequent immune activation through a series of molecular events are reviewed. The emphasis is on the significance of gut immunity in oral administration of immunoprophylactics, and the different potential adjuvants that circumvent intestinal immune tolerance. Comprehension of the intestinal immune system is pivotal for developing effective fish vaccines that can be delivered orally, which is less labour-intensive and could improve fish health and facilitate disease management in the aquaculture industry.

Immunological memory in teleost fish

Immunological memory can be regarded as the key aspect of adaptive immunity, i.e. a specific response to first contact with an antigen, which in mammals is determined by the properties of T, B and NK cells. Re-exposure to the same antigen results in a more rapid response of the activated specific cells, which have a unique property that is the immunological memory acquired upon first contact with the antigen. Such a state of immune activity is also to be understood as related to "altered behavior of the immune system" due to genetic alterations, presumably maintained independently of the antigen. It also indicates a possible alternative mechanism of maintaining the immune state at a low level of the immune response, "directed" by an antigen or dependent on an antigen, associated with repeated exposure to the same antigen from time to time, as well as the concept of innate immune memory, associated with epigenetic reprogramming of myeloid cells, i.e. macrophages and NK cells. Studies on Teleostei have provided evidence for the presence of immunological memory determined by T and B cells and a secondary response stronger than the primary response. Research has also demonstrated that in these animals macrophages and NK-like cells (similar to mammalian NK cells) are able to respond when re-exposed to the same antigen. Regardless of previous reports on immunological memory in teleost fish, many reactions and mechanisms related to this ability require further investigation. The very nature of immunological memory and the activity of cells involved in this process, in particular macrophages and NK-like cells, need to be explained. This paper presents problems associated with adaptive and innate immune memory in teleost fish and characteristics of cells associated with this ability.

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

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

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.

Molecular characterization of the T cell costimulatory receptors CD28 and CTLA4 in the European sea bass

For the activation of T cells, it is necessary the specific recognition of the peptide by the T cell receptors (TCR) in the surface of antigen-presenting cells (APCs) and additional signals delivered by costimulatory receptors. In fish, knowledge about the presence of these costimulatory signals is limited and functional evidence almost absent. Thus, in this study, we have identified the stimulatory CD28 and the inhibitory cytotoxic T-lymphocyte-associated protein 4 (CTLA4) coreceptors in the European sea bass (Dicentrarchus labrax), and evaluated their transcription. In parallel, the transcription encoding for the T cell markers CD8α and CD4 was also evaluated. Both coreceptors showed the canonical architecture including a signal peptide, an immunoglobulin domain, a transmembrane region and a cytosolic tail. Protein predictions and phylogenetic tree identify them as true mammalian orthologues of CD28 and CTLA4. We found these genes constitutively expressed in all studied organs of European sea bass with high expression in lymphoid organs (thymus, spleen and head-kidney) and liver. The molecular expression pattern of these genes was up-regulated in head-kidney leucocytes stimulated with T mitogens as concanavalin A and phytohemagglutinin (PHA), but not with the B cell mitogen lipopolysaccharide (LPS). Fish challenged with nodavirus (NNV) evidenced a differential and opposing regulation of the cd28 and ctla4 transcription levels in the brain, the target organ for viral replication, and head-kidney. While cd28 transcription tends to decrease over the infection time in both organs the expression of the ctla4 gene tends to increase. Interestingly, the coreceptor expression is highly and significantly correlated to the transcription of the T cell markers. Our results highlight the important role of CD28 and CTLA4 as costimulatory receptors of T cells in European sea bass but further studies are deserved.

Protein Phosphatase PP1 Negatively Regulates IRF3 in Response to GCRV Infection in Grass Carp (Ctenopharyngodon idella)

Protein phosphatase-1 (PP1) has an important role in many cell functions, such as cell differentiation, development, immune response and tumorigenesis. However, the specific role of PP1 in the antiviral response in fish remains to be elucidated. In this study, the PPP1R3G homolog was identified in the grass carp (Ctenopharyngodon idella) and its role in defence against the GCRV infection was investigated. Phylogenetic analysis demonstrated that CiPPP1R3G clustered with homologues from other teleosts. Temporal expression analysis in vivo revealed that the expression level of CiPPP1R3G was significantly up-regulated in response to GCRV infection in grass carps, especially in the intestine and head-kidney. Cellular distribution analysis revealed that CiPPP1R3G was located in the nucleus and cytoplasm. Overexpression of CiPPP1R3G significantly negatively regulated the expression of CiIRF3, thus inhibiting its activation. In summary, we systematically analyzed the PPP1R3G gene in grass carp and illustrated its function as a negative regulator in the anti-GCRV immune responses.

Gut immunity in European sea bass (Dicentrarchus labrax): a review

In this review, we summarize and discuss the trends and supporting findings in scientific literature on the gut mucosa immune role in European sea bass (Dicentrarchus labrax L.). Overall, the purpose is to provide an updated overview of the gastrointestinal tract functional regionalization and defence barriers. A description of the available information regarding immune cells found in two immunologically-relevant intestinal compartments, namely epithelium and lamina propria, is provided. Attention has been also paid to mucosal immunoglobulins and to the latest research investigating gut microbiota and dietary manipulation impacts. Finally, we review oral vaccination strategies, as a safe method for sea bass vaccine delivery.

Effective isolation of GALT cells: Insights into the intestine immune response of rainbow trout (Oncorhynchus mykiss) to different bacterin vaccine preparations

The teleost gut is a multifunction complex structure that plays a pivotal immunological role in homeostasis and the maintenance of health, in addition to digestion of food and/or nutrient absorption. In vitro examination of the intestine leucocyte repertoire has the potential to aid our understanding of gut immune competence and allows a rapid screen of host-microorganism interactions in different immunological contexts. To explore this possibility, in the present study we investigated the response of isolated gut leucocytes to 4 bacterins of Aeromonas salmonicida, prepared from different strains, combinations and strains grown in different environments, in comparison to a Yersinia ruckeri bacterin for which a commercial/effective oral booster vaccine has been developed. To aid this study we also optimized further our method of GALT cell isolation from rainbow trout, so as to avoid mechanical clearance of the intestine contents. This drastically increased the cell yield from ~12 × 10⁶ to ~210 × 10⁶/fish with no change in the percent cell viability over time or presence of transcripts typical of the key leucocyte types needed for the study of immune modulation (i.e. T- and B-cells, dendritic cells and macrophages). A wide array of immune transcripts were modulated by the bacterins, demonstrating the diversity of GALT cell responses to bacterial stimulation. Indeed, the GALT leucocyte responses were sensitive enough to distinguish the different bacterial species, strains and membrane proteins, as seen by distinct kinetics of immune gene expression. However, the response of the GALT cells was often relatively slow and of a low magnitude compared to those of PBL. These results enhance our knowledge of the gut biocapacity and help validate the use of this model for screening of oral vaccine candidates.

Dietary Approaches to Attain Fish Health with Special Reference to their Immune System

Fish despite their low collocation in the vertebrate phylum possess a complete immune system. In teleost fish both innate and adaptive immune responses have been described with melanomacrophage centers (MMCs) equivalent to mammalian germinal centers. Primary lymphoid organs are represented by the thymus and kidney, while spleen and mucosa-associated lymphoid tissues act as secondary lymphoid organs. Functions of either innate immune cells (e.g., macrophages and dendritic cells) or adaptive immune cells (T and B lymphocytes) will be described in detail, even including their products, such as cytokines and antibodies. In spite of a robust immune arsenal, fish are very much exposed to infectious agents (marine bacteria, parasites, fungi, and viruses) and, consequentially, mortality is very much enhanced especially in farmed fish. In fact, in aquaculture stressful events (overcrowding), microbial infections very frequently lead to a high rate of mortality. With the aim to reduce mortality of farmed fish through the reinforcement of their immune status the current trend is to administer natural products together with the conventional feed. Then, in the second part of the present review emphasis will be placed on a series of products, such as prebiotics, probiotics and synbiotics, β-glucans, vitamins, fatty acids and polyphenols all used to feed farmed fish. With special reference to polyphenols, results of our group using red grape extracts to feed farmed European sea bass will be illustrated. In particular, determination of cytokine production at intestinal and splenic levels, areas of MMCs and development of hepatopancreas will represent the main biomarkers considered. All together, our own data and those of current literature suggests that natural product administration to farmed fish for their beneficial effects may, in part, solve the problem of fish mortality in aquaculture, enhancing their immune responses.

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.

Fish Lymphocytes: An Evolutionary Equivalent of Mammalian Innate-Like Lymphocytes?

Lymphocytes are the responsible of adaptive responses, as they are classically described, but evidence shows that subpopulations of mammalian lymphocytes may behave as innate-like cells, engaging non-self rapidly and without antigen presentation. The innate-like lymphocytes of mammals have been mainly identified as γδT cells and B1-B cells, exert their activities principally in mucosal tissues, may be involved in human pathologies and their functions and tissue(s) of origin are not fully understood. Due to similarities in the morphology and immunobiology of immune system between fish and mammals, and to the uniqueness of having free-living larval stages where the development can be precisely monitored and engineered, teleost fish are proposed as an experimental model to investigate human immunity. However, the homology between fish lymphocytes and mammalian innate-like lymphocytes is an issue poorly considered in comparative immunology. Increasing experimental evidence suggests that fish lymphocytes could have developmental, morphological, and functional features in common with innate-like lymphocytes of mammals. Despite such similarities, information on possible links between conventional fish lymphocytes and mammalian innate-like lymphocytes is missing. The aim of this review is to summarize and describe available findings about the similarities between fish lymphocytes and mammalian innate-like lymphocytes, supporting the hypothesis that mammalian γδT cells and B1-B cells could be evolutionarily related to fish lymphocytes.

Comprehensive validation of T- and B-cell deficiency in rag1-null zebrafish: Implication for the robust innate defense mechanisms of teleosts

rag1^−/− zebrafish have been employed in immunological research as a useful immunodeficient vertebrate model, but with only fragmentary evidence for the lack of functional adaptive immunity. rag1-null zebrafish exhibit differences from their human and murine counterparts in that they can be maintained without any specific pathogen-free conditions. To define the immunodeficient status of rag1^−/− zebrafish, we obtained further functional evidence on T- and B-cell deficiency in the fish at the protein, cellular, and organism levels. Our developed microscale assays provided evidence that rag1^−/− fish do not possess serum IgM protein, that they do not achieve specific protection even after vaccination, and that they cannot induce antigen-specific CTL activity. The mortality rate in non-vaccinated fish suggests that rag1^−/− fish possess innate protection equivalent to that of rag1^+/− fish. Furthermore, poly(I:C)-induced immune responses revealed that the organ that controls anti-viral immunity is shifted from the spleen to the hepatopancreas due to the absence of T- and B-cell function, implying that immune homeostasis may change to an underside mode in rag-null fish. These findings suggest that the teleost relies heavily on innate immunity. Thus, this model could better highlight innate immunity in animals that lack adaptive immunity than mouse models.

Density-dependent effects of Caligus rogercresseyi infestation on the immune responses of Salmo salar

Sea lice infestations are a particular concern in the salmonid aquaculture industry due to damaging effects on fish growth, disease/infection susceptibility, and survival. Despite the impacts of sea lice parasitism, few studies have determined corresponding physiological thresholds, or the quantity of sea lice that can trigger measurable effects in the host immune response. The present study evaluated the mRNA expressions of immune-related genes in Salmo salar (Atlantic salmon) under infestation challenges with contrasting loads of the sea louse Caligus rogercresseyi. Specifically, two groups of S. salar were infected with either 35 (i.e. low parasitic load) or 100 (i.e. high parasitic load) copepodids per fish. At 14 days post-infestation, the mRNA levels of immune-related genes (e.g. related to oxidative stress, pro- and inflammatory responses, and the adaptive T_H1/T_H2 pathways) were assessed through RT-qPCR. Significant differences were found in relation to parasitic load, suggesting density-dependent effects that activated the S. salar immune system. Higher parasitic load promoted strong inflammatory and oxidative stress responses that were correlated with the T_H1 immune response. This study highlights the molecular signatures for distinct parasitic loads, providing new perspectives towards fully understanding parasite-host interactions.

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.

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.

Protective and pro-inflammatory roles of intestinal bacteria

The intestinal mucosal surface in all vertebrates is exposed to enormous numbers of microorganisms that include bacteria, archaea, fungi and viruses. Coexistence of the host with the gut microbiota represents an active and mutually beneficial relationship that helps to shape the mucosal and systemic immune systems of both mammals and teleosts (ray-finned fish). Due to the potential for enteric microorganisms to invade intestinal tissue and induce local and/or systemic inflammation, the mucosal immune system has developed a number of protective mechanisms that allow the host to mount an appropriate immune response to invading bacteria, while limiting bystander tissue injury associated with these immune responses. Failure to properly regulate mucosal immunity is thought to be responsible for the development of chronic intestinal inflammation. The objective of this review is to present our current understanding of the role that intestinal bacteria play in vertebrate health and disease. While our primary focus will be humans and mice, we also present the new and exciting comparative studies being performed in zebrafish to model host-microbe interactions.

A Review of the Immunological Mechanisms Following Mucosal Vaccination of Finfish

Mucosal organs are principle portals of entry for microbial invasion and as such developing protective vaccines against these pathogens can serve as a first line of defense against infections. In general, all mucosal organs in finfish are covered by a layer of mucus whose main function is not only to prevent pathogen attachment by being continuously secreted and sloughing-off but it serves as a vehicle for antimicrobial compounds, complement, and immunoglobulins that degrade, opsonize, and neutralize invading pathogens on mucosal surfaces. In addition, all mucosal organs in finfish possess antigen-presenting cells (APCs) that activate cells of the adaptive immune system to generate long-lasting protective immune responses. The functional activities of APCs are orchestrated by a vast array of proinflammatory cytokines and chemokines found in all mucosal organs. The adaptive immune system in mucosal organs is made of humoral immune responses that are able to neutralize invading pathogens as well as cellular-mediated immune responses whose kinetics are comparable to those induced by parenteral vaccines. In general, finfish mucosal immune system has the capacity to serve as the first-line defense mechanism against microbial invasion as well as being responsive to vaccination.

Transcription Factor T-Bet in Atlantic Salmon: Characterization and Gene Expression in Mucosal Tissues during Aeromonas Salmonicida Infection

The T-box transcription factor T-bet is expressed in a number of hematopoietic cell types in mammals and plays an essential role in the lineage determination of Th1 T-helper cells and is considered as an essential feature for both innate and adaptive immune responses in higher vertebrates. In the present study, we have identified and characterized the full-length Atlantic salmon T-bet cDNA (3502 bp). The putative primary structure of the polypeptide deduced from the cDNA sequence contained 612 aa, which possessed a T-box DNA binding domain. Phylogenetic study and gene synteny revealed it is as a homolog to mammalian T-bet. Quantitative PCR analysis of different tissues in healthy fish showed that salmon T-bet gene was highly expressed in spleen, followed by head kidney, and was expressed in intestine, skin, and liver at lower levels. Moreover, the time-dependent expression profile of T-bet, interferon gamma (IFNγ), interleukin-22 (IL-22), and natural killer enhancement factor in mucosal tissues during water-borne infection with live Aeromonas salmonicida, indicated the involvement of T-bet in mucosal immune response in Atlantic salmon.

Exogenous 17β-oestradiol (E2) modifies thymus growth and regionalization in European sea bass Dicentrarchus labrax

The effect of 17β-oestradiol (E2) on the growth of the thymus and its regionalization into cortex and medulla was investigated in juvenile European sea bass Dicentrarchus labrax as they find themselves close to sources of oestrogenic pollution whilst residing in their estuarine nursery areas. While the exposure to 2, 20 and 200 ng l(-1) in 60 days post-hatch (dph) fish tended to cause a non-monotonous dose-response curve with a significant difference of the cortex size between lowest and highest exposures, the exposure to 20 ng l(-1) E2 from 90 dph onwards resulted in a distinct enlargement of the cortex. It is probable that the alteration of the cortex size also affects the T-cell differentiation and proliferation.

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.

Adaptive immune responses at mucosal surfaces of teleost fish

This review describes the extant knowledge on the teleostean mucosal adaptive immune mechanisms, which is relevant for the development of oral or mucosal vaccines. In the last decade, a number of studies have shed light on the presence of new key components of mucosal immunity: a distinct immunoglobulin class (IgT or IgZ) and the polymeric Ig receptor (pIgR). In addition, intestinal T cells and their putative functions, antigen uptake mechanisms at mucosal surfaces and new mucosal vaccination strategies have been reported. New information on pIgR of Atlantic cod and common carp and comparison of natural and specific cell-mediated cytotoxicity in the gut of common carp and European seabass, is also included in this review. Based on the known facts about intestinal immunology and mucosal vaccination, suggestions are made for the advancement of fish vaccines.

Transcriptional characterization of the T cell population within the salmonid interbranchial lymphoid tissue

Previously, our group has shown that the interbranchial lymphoid tissue (ILT) is a distinct structure largely consisting of T cells embedded in a meshwork of epithelial cells, with no direct resemblance to previously described lymphoid tissues. In this study, we aim to focus on the T cell population and the possibility of the ILT being a thymus analog. By characterizing structural responsiveness to Ag challenge, the presence of recombination activating genes, and different T cell-related transcripts, we attempt to further approach the immunological function of the ILT in salmonid gills. In addition to eight healthy individuals, a group of eight infectious salmon anemia virus-challenged fish were included to observe T cell responses related to infection. The results showed reduced size of ILT in the infected group, no expression of RAG-1 and -2, and a high degree of T cell diversity within the ILT. Taking into account that the ILT can be regarded as a strategically located T cell reservoir and possibly an evolutionary forerunner of mammalian MALTs right at the border to the external environment, the alteration in transcription observed may likely represent a shift in the T cell population to optimize local gill defense mechanisms.

The mucosal immune system of fish: the evolution of tolerating commensals while fighting pathogens

The field of mucosal immunology research has grown fast over the past few years, and our understanding on how mucosal surfaces respond to complex antigenic cocktails is expanding tremendously. With the advent of new molecular sequencing techniques, it is easier to understand how the immune system of vertebrates is, to a great extent, orchestrated by the complex microbial communities that live in symbiosis with their hosts. The commensal microbiota is now seen as the "extended self" by many scientists. Similarly, fish immunologist are devoting important research efforts to the field of mucosal immunity and commensals. Recent breakthroughs on our understanding of mucosal immune responses in teleost fish open up the potential of teleosts as animal research models for the study of human mucosal diseases. Additionally, this new knowledge places immunologists in a better position to specifically target the fish mucosal immune system while rationally designing mucosal vaccines and other immunotherapies. In this review, an updated view on how teleost skin, gills and gut immune cells and molecules, function in response to pathogens and commensals is provided. Finally, some of the future avenues that the field of fish mucosal immunity may follow in the next years are highlighted.

Functional aspects of fish lymphocytes

After almost 40 years of studies in comparative immunology, some light has been shed on the evolutive immunobiology of vertebrates, and experimental evidences have shown that acquired immunity, defined by somatic recombination of antigen-binding molecules and memory, is an achievement as ancient as jawless vertebrates. However, the molecular processes generating antigen receptors evolved independently between jawless and jawed fishes, and produced lymphocytic cells with similar functions but employing different sets of genes. In recent years, data have been provided describing some in vitro and in vivo functional responses of fish lymphocytes. After a long gap, the number of specific markers for fish lymphocytes is increasing, thus allowing a first characterisation of lymphocyte subsets. Overall, in the near future it will be possible to open a new chapter in fish immunology and investigate functional immunity of lymphocyte responses by combining the extensive knowledge on immune gene products with markers for molecules and cells. The present review summarizes current knowledge on functional features of fish lymphocytes.

Recombinant TNFα as oral vaccine adjuvant protects European sea bass against vibriosis: insights into the role of the CCL25/CCR9 axis

Vibrio anguillarum is the main causative agent of vibriosis in cultured sea bass. Unfortunately, available vaccines against this disease do not achieve the desired protection. In this study, to accomplish uptake, processing, and presentation of luminal antigens, a commercial sea bass oral vaccine against V. anguillarum was improved with the addition of recombinant fish-self tumor necrosis factor α (rTNFα), as adjuvant. To explore mechanisms, systemic and local responses were analyzed through serum specific IgM titers, gene expression, lymphocytes spatial distribution in the gut, and in vitro functional assays. We found along the trial, over expressed transcripts of genes encoding cytokines and antimicrobial molecules at the gut of rTNFα supplied group. Orally immunized fish with vaccine alone confer protection against V. anguillarum challenge throughout a short time period. In contrast, adjuvant-treated group significantly extended the response. In both cases, achieved protection was independent of serum IgM. Yet, IgT transcripts were found to increase in the gut of rTNFα-treated fish. More importantly, fish treated with rTNFα showed a dramatic change of their T lymphocytes distribution and localization in gut mucosal tissue, suggesting specific antigen recognition and further intraepithelial T lymphocytes (IEL) activation. To determine the mechanism behind IEL infiltration, we characterized the constitutive and activated pattern of chemokines in sea bass hematopoietic tissues, identifying for the first time in fish gut, an intimate relation between the chemokine ligand/receptor CCL25/CCR9. Ex-vivo, chemotaxis analyses confirmed these findings. Together, our results demonstrate that improved oral vaccines targeting key cytokines may provide a means to selectively modulate fish immune defence.

Cytokine Gene Expression in CD4 Positive Cells of the Japanese Pufferfish, Takifugu rubripes

CD4⁺ T (Th) cells are a central component of the adaptive immune response and are divided into distinct sets based on their specific cytokine production pattern. Several reports have suggested that fish possess Th subset activity similar to that of mammals. The aim of the present study was to isolate CD4⁺ T cells from the blood of Japanese pufferfish, Fugu rubripes, and to characterize their cytokine expression profile. We produced a specific antibody against Fugu CD4 and performed cell sorting with the magnetic activated cell sorting system. Sorted Fugu CD4⁺ cells were characterized by morphology and expression analysis of cell marker genes. Fugu CD4⁺ cells expressed T-cell marker genes but not macrophage or B-cell marker genes. In addition, peripheral blood lymphocytes were stimulated with lipopolysaccharide (LPS), polycytidylic acid (polyI:C), concanavalin A (ConA) prior to sorting, and then Multiplex RT-PCR was used to examine the expression of Th cytokines by the stimulated Fugu CD4⁺ cells. LPS and polyI:C stimulation upregulated the expression of Th1, Th17 and Treg cytokines and downregulated the expression of Th2 cytokines. ConA stimulation upregulated the expression of all Th cytokines. These results suggest that fish exhibit the same upregulation of Th-specific cytokine expression as in mammals.

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.

Pathogen recognition and mechanisms in Atlantic cod (Gadus morhua) head kidney cells: bacteria (LPS) and virus (poly I:C) signals through different pathways and affect distinct genes

Understanding pathogen recognition and mechanisms in Atlantic cod are of significant importance for both basic research on wild populations and health management in aquaculture. A microarray approach was utilized to search for effects of viral (polyinosinic acid:polycytidylic acid), bacterial (lipopolysaccharide) and polyclonal activator (phytohaemoagglutinin) stress in Atlantic cod head kidney cells. LPS cell activation increased mRNA expression of interleukin 8; interleukin-1β; cyclooxygenase 2; leukocyte derived chemotaxin 2; carboxyl-esterase 2 and environmental biomarker cytochrome P450 1A. Mitogen activated protein kinase p38 and cathepsin F were down regulated by LPS. The antiviral responses induced by double stranded RNA clearly increased transcription of Toll like receptor 3 and interferon stimulating gene 15. The phytohaemoagglutinin response seemed to be more non-specific. Special for the phytohaemoagglutinin induction was the increase in major histocompatibility complex class I. CC chemokine type 2 mRNA expression was increased by phytohaemoagglutinin, lipopolysaccharide and polyinosinic acid:polycytidylic acid, while mitogen activated protein kinase p38 and leukocyte derived chemotaxin 2 were down regulated by phytohaemoagglutinin. Oxidative stress related genes like catalase and glutaredoxin and the anti-apoptotic gene Bcl-2 showed no transcriptional changes compared to control in any of the treatments. Eicosanoids like prostaglandin 2, leukotriene B4 and B5 were constitutively produced by cod head kidney cells in vitro. The most remarkable feature of eicosanoid secretion is the higher production of leukotrienes against prostaglandins, indicating that the lipooxygenase pathway is preferred over the cyclooxygenase pathway. Although there were no significant differences in eicosanoid secretion between the groups, polyinosinic acid:polycytidylic acid showed a clear tendency to increase the levels of leukotriene B4 and B5. This study reveals distinct signatures of bacteria and virus transcriptional responses in cod head kidney cells. In addition, the novel finding that cytochrome P450 1A was upregulated during the antibacterial response indicates a connection between immunity and aryl hydrocarbon receptor activation in Atlantic cod.

Immunomodulatory effects of dietary β-1,3-glucan from Euglena gracilis in rainbow trout (Oncorhynchus mykiss) immersion vaccinated against Yersinia ruckeri

Potential immunostimulatory effects of orally administered β-glucan were investigated in combination with immersion vaccination against enteric redmouth disease caused by Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss). A linear, unbranched and pure (purity ≥98%) β-1,3-glucan (syn. paramylon) from the alga Euglena gracilis was applied at an inclusion level of 1% β-glucan in feed administered at a rate of 1% biomass day(-1) for 84 consecutive days. Fish were vaccinated after two weeks of experimental feeding and bath challenged with live Y. ruckeri six weeks post-vaccination. Blood and head kidney were sampled at day 0, 13 (1 day pre-vaccination), 15, 55, 59 (day 3 post-challenge (p.c.)), 70 and 84. Vaccination induced significantly increased survival p.c., whereas the β-glucan had no effect on survival in either unvaccinated or vaccinated fish. Expression in head kidney of genes related to the acute phase response, i.e. interleukin-1β (IL-1β), serum amyloid A (SAA), precerebellin, and hepcidin, was significantly different in vaccinated fish receiving β-glucan compared to vaccinated controls at day 3 p.c., while no effect of β-glucan was observed among unvaccinated fish. Significant interaction between β-glucan and vaccination was found for the regulation of IL-1β, tumour necrosis factor-α, interferon-γ, SAA, precerebellin and hepcidin p.c. For SAA, the significant effect of β-glucan in vaccinated fish persisted at day 14 p.c. and 28 p.c. The difference in gene expression among vaccinated fish was mainly observed as down-regulations in vaccinated, β-glucan fed fish compared to up-regulations or no regulation in vaccinated controls. Slightly increased levels of plasma lysozyme activity were found in fish (both unvaccinated and vaccinated) receiving β-glucan at day 3 p.c. compared to control fed groups. This was associated with a faster clearance of Y. ruckeri in unvaccinated fish receiving β-glucan. In contrast to the trend towards a beneficial effect of β-glucan on plasma lysozyme activity, a trend towards suppression of plasma antibodies was seen in both unvaccinated and vaccinated fish receiving β-glucan. However, the effects of β-glucan were not reflected in the survival curves, and the differences seen in plasma lysozyme activity and antibody levels may have counteracted and set off each other as well as counteracted any potential effect represented by the differences in gene expression found.

TGF-β1 exerts opposing effects on grass carp leukocytes: implication in teleost immunity, receptor signaling and potential self-regulatory mechanisms

In fish immunity, the regulatory role of transforming growth factor-β1 (TGF-β1) has not been fully characterized. Here we examined the immunoregulatory effects of TGF-β1 in grass carp peripheral blood leukocytes (PBL) and head kidney leukocytes (HKL). It is interesting that TGF-β1 consistently stimulated the cell viability and the mRNA levels of pro-inflammatory cytokines (Tnfα and Ifnγ) and T/B cell markers [Cd4-like (Cd4l), Cd8α, Cd8β and Igμ] in PBL, which contrasted with its inhibitory tone in HKL. Further studies showed that grass carp TGF-β1 type I receptor, activin receptor-like kinase 5 (ALK5), was indispensable for the immunoregulatory effects of TGF-β1 in PBL and HKL. Notably, TGF-β1 persistently attenuated ALK5 expression, whereas immunoneutralization of endogenous grass carp TGF-β1 could increase ALK5 mRNA and protein levels. It is consistent with the observation that TGF-β1 decreased the number of ALK5(+) leukocytes in PBL and HKL, revealing a negative regulation of TGF-β1 signaling at the receptor level. Moreover, transient treatment with TGF-β1 for 24 h was sufficient to induce similar cellular responses compared with the continuous treatment. This indicated a possible mechanism by which TGF-β1 triggered the down-regulation of ALK5 mRNA and protein, leading to the desensitization of grass carp leukocytes toward TGF-β1. Accordingly, our data revealed a dual role of TGF-β1 in teleost immunity in which it can serve as a positive or negative control device and provided additional mechanistic insights as to how TGF-β1 controls its signaling in vertebrate leukocytes.

The expression of CD8α discriminates distinct T cell subsets in teleost fish

CD8, belonging to the TCR complex, is the main marker molecule of CTLs. Although CD8 genes have been detected in many fish species, the analysis of teleost CD8+ cells has been limited because of the lack of antibodies. Using newly established mAbs against rainbow trout CD8α, we found high ratios of CD8α+ cells in trout thymus, gill and intestine, but relatively low abundance in pronephros, spleen and blood. Accordingly, tissue sections revealed many CD8α+ cells in thymus, numerous intra- and subepithelial CD8α+ cells in intestine and gill and few scattered CD8α+ cells in spleen and pronephros. In secondary lymphoid tissues, CD8α+ lymphocytes, which did not react with anti-thrombocyte or anti-IgM mAbs, expressed CD8α, CD8β and TCRα, while Ig and CD4 transcripts were found in CD8α⁻ lymphocytes. In contrast, considerable CD4 expression in CD8α+ thymocytes suggests the presence of double-positive early T cells. Highly expressed TCRγ, LAG3 and CTLA4 in CD8α+ lymphocytes imply that they constitute a heterogeneous population different from found in non-mucosal tissues. PHA stimulation resulted in an up-regulation of CTL effector genes (perforin, granulysin and IFN-γ) in CD8α+ pronephrocytes, while both Th1 (IFN-γ) and Th2 (IL-4/13A) cytokines were up-regulated in CD8α⁻ pronephrocytes. Although the basic characteristics of CD8α+ lymphocytes seem similar in teleost and mammals, features such as the low proportion of teleost CD8α+ lymphocytes in blood and their high abundance in respiratory tissue reveal a unique dynamics and distribution.

Characterisation and expression analysis of the Atlantic halibut (Hippoglossus hippoglossus L.) cytokines: IL-1β, IL-6, IL-11, IL-12β and IFNγ

Genes encoding the five Atlantic halibut (Hippoglossus hippoglossus L.) cytokines; interleukin (IL)-1β, IL-6, IL-11b, IL-12βc, and interferon (IFN) γ, were cloned and characterised at a molecular level. The genomic organisation of the halibut cytokine genes was similar to that seen in mammals and/or other fish species. Several mRNA instability motifs were found within the 3′-untranslated region (UTR) of all cytokine cDNA sequences. The putative cytokine protein sequences showed a low sequence identity with the corresponding homologues in mammals, avian and other fish species. Nevertheless, important structural features were presumably conserved such as the presence, or absence in the case of IL-1β, of a signal peptide, secondary structure and family signature motifs. The relative expression pattern of the cytokine genes was analyzed in several halibut organs, revealing a constitutive expression in both lymphoid and non-lymphoid organs. Interestingly, the gills showed a relatively high expression of IL-1β, IL-12βc and IFNγ. The real time RT-PCR data also showed that the mRNA level of IL-1β, IL-6, IL-12βc and IFNγ was high in the thymus, while IL-11b was relatively highly expressed in the posterior kidney and posterior gut. Moreover, the halibut brain showed a relatively high level of IL-6 transcripts. Anterior kidney leucocytes in vitro stimulated with imiquimod showed a significant increase in mRNA level of the five halibut cytokine genes. The sequence and characterisation data presented here will be useful for further investigation of both innate and adaptive immune responses in halibut, and be helpful in the design of vaccines for the control of various infectious diseases.

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.