Cytotoxic T cell function in fish

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

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

Use of Integrated Core Proteomics, Immuno-Informatics, and In Silico Approaches to Design a Multiepitope Vaccine against Zoonotic Pathogen Edwardsiella tarda

Multidrug-resistant Edwardsiella tarda has been reported as the main causative agent for massive fish mortality. The pathogen is well-known for causing hemorrhagic septicemia in fish and has been linked to gastrointestinal infections in humans. Formalin-inactivated Edwardsiella vaccination has previously been found to be ineffective in aquaculture species. Therefore, based on E. tarda’s integrated core complete sequenced genomes, the study aimed to design a subunit vaccine based on T and B cell epitopes employing immunoinformatics approach. Initially, the top immunodominant and antigenic epitopes were predicted from the core complete sequenced genomes of the E. tarda genome and designed the vaccine by using linkers and adjuvant. In addition, vaccine 3D structure was predicted followed by refinement, and molecular docking was performed for the analysis of interacting residues between vaccines with TLR5, MHC-I, and MHC-II, respectively. The final vaccine constructs demonstrated strong hydrogen bond interactions. Molecular dynamic simulation of vaccine-TLR5 receptor complex showed a stable structural binding and compactness. Furthermore, E. coli used as a model organism for codon optimization proved optimal GC content and CAI value, which were subsequently cloned in vector pET2+ (a). Overall, the findings of the study imply that the designed epitope vaccine might be a good option for prophylaxis for E. tarda.

Lympho-Hematopoietic Microenvironments and Fish Immune System

Simple Summary

Teleost fish, the most abundant group of vertebrates, represent an excellent tool to establish possible correlations between the histological organization of their lymphoid organs and their immunological capacities. This approach allows us to analyze embryonic and larval lymphopoiesis, the remarkable organization of the teleost thymus, the role of the kidney as a true equivalent of the lympho-hematopoietic bone marrow of higher vertebrates, the mechanisms of antigen trapping in both ellipsoids and the so-called melano-macrophage centers (MMCs) and their relation with the generation of memory and the lack of germinal centers, and the extended development of the lymphoid tissue associated to mucosae.

Abstract

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.

Designing a novel mRNA vaccine against Vibrio harveyi infection in fish: an immunoinformatics approach

Vibrio harveyi belongs to the family Vibrionaceae of class Gammaproteobacteria. Around 12 Vibrio species can cause gastroenteritis (gastrointestinal illness) in humans. A large number of bacterial particles can be found in the infected cells, which may cause death. Despite these devastating complications, there is still no cure or vaccine for the bacteria. As a result, we used an immunoinformatics approach to develop a multi-epitope vaccine against the most pathogenic hemolysin gene of V. harveyi. The immunodominant T- and B-cell epitopes were identified using the hemolysin protein. We developed a vaccine employing three possible epitopes: cytotoxic T-lymphocytes, helper T-lymphocytes, and linear B-lymphocyte epitopes, after thorough testing. The vaccine was developed to be antigenic, immunogenic, and non-allergenic, as well as have a better solubility. Molecular dynamics simulation revealed significant structural stiffness and binding stability. In addition, the immunological simulation generated by computers revealed that the vaccination might elicit immune reactions Escherichia coli K12 as a model, codon optimization yielded ideal GC content and a higher codon adaptation index value, which was then included in the cloning vector pET2+ (a). Altogether, our experiment implies that the proposed peptide vaccine might be a good option for vibriosis prophylaxis.

Production of monoclonal antibody against grouper (Epinephelus coioides) CD4-1 and the distribution of CD4-1+ cells

CD4-a transmembrane glycoprotein molecule expressed on the surface of helper T (Th) cells-plays a central role in adaptive immune protection. In the current study, we developed a monoclonal antibody (mAb) against the grouper CD4-1. Western blotting and immunohistochemistry results revealed that the CD4-1 mAb could recognize the recombinant and natural protein of grouper CD4-1 as well as the CD4-1⁺ cells in the various tissues from grouper. Tissue distribution analyses revealed that the grouper CD4-1⁺ cells were expressed in all tissues tested in the healthy grouper, with greater localization in the thymus, head kidney, and spleen tissues. In addition, we tested the changes in the proportion of CD4-1⁺ cells in the thymus, head kidney, and the gills of grouper post the infection by C. irritans. Our data suggest that the CD4-1 mAb produced against grouper in the current study can be used as a tool to characterize CD4-1⁺ cells and to investigate the functions of the grouper CD4-1⁺ cells in the host response against pathogens infection.

Identification and functional analysis of the perforin-1 like gene in disease resistance in half smooth tongue sole (Cynoglossus semilaevis)

The pore-forming protein perforin is one of the effectors of cell-mediated killing via the granule exocytosis pathway. In this study, a genome-wide association study was conducted in Vibrio harveyi disease-resistant and disease-susceptible families of half smooth tongue sole (Cynoglossus semilaevis) to determine the genes accounting for host resistance, and a perforin homologue was identified, designated perforin-1 like (CsPRF1l). The full-length cDNA of CsPRF1l is 1835 bp, and encodes 514 amino acids. The CsPRF1l gene consists of 10 exons and 9 introns, spanning approximately 7 kb. The amino acid sequence of CsPRF1l shows 60.35, 54.03, 41.92, and 34.17% identities to Morone saxatilis PRF1l, Oryzias melastigma PRF1l, Danio rerio PRF1.5 and Homo sapiens PRF, respectively. Sequence analysis revealed the presence of membrane attack complex/perforin (MACPF) and C2 domains in CsPRF1l. Quantitative real-time PCR showed that CsPRF1l presented a higher intestinal expression level in disease-resistant families than in susceptible families. Tissue expression pattern analysis showed that CsPRF1l is present in most of the tested tissues and highly expressed in the intestine, brain, stomach and gills. After challenge with V. harveyi, CsPRF1l mRNA was markedly upregulated in the liver, spleen, kidney, intestine, gills and skin. In addition, the recombinant CsPRF1l protein exhibited obvious antimicrobial activity against V. harveyi in vitro and in a zebrafish model. Collectively, these data indicate that CsPRF1l modulates host immune defense against V. harveyi invasion and provide clues about the efficacy of rCsPRF1l in fish that will give rise to useful therapeutic applications for V. harveyi infection in C. semilaevis.

Testicular subcutaneous allografting followed by immunosuppressive treatment promotes maintenance of spermatogonial cells in rainbow trout (Oncorhynchus mykiss)

Germ cell transplantation and testis graft represent promising biotechnologies that can be applied for the reproduction of commercial or endangered species. However, mechanisms of rejection from the host immune system might remove the transplanted donor cells/tissues and limit the surrogate production of gametes. In this work, we administered emulsion containing-immunosuppressants to verify whether they are capable to prevent immune rejection and promote survival of testis allografts in rainbow trout. In the first part of this study, we demonstrated in vitro that tacrolimus and cyclosporine were able to affect viability, inhibit leucocyte proliferation, and suppress il2 expression in vitro. In in vivo experiments, both doses of tacrolimus (0.5 and 1.5 mg/kg) and the lower dose of cyclosporine (20 mg/kg) significantly inhibited the expression of il2 in head kidney, three days post-injection. A higher dose of cyclosporine (40 mg/kg) was able to inhibit il2 expression for up to seven days post-injection. In the second part, testis allografts were conducted in fish treated weekly with emulsion containing-tacrolimus. Immunohistochemical, conventional histology, and qRT-PCR (vasa) analysis demonstrated the presence of spermatogonial cells by the fifth week, in animals treated with 0.5 mg/kg of tacrolimus similar as found in autografted group. In the group treated with the highest tacrolimus dose (1.5 mg/kg) and in the non-treated group (without immunosuppressant), no germ cells or their respective markers were detected. il2 expression in head kidney was also suppressed in grafted animals treated with tacrolimus compared to non-treated group. These results suggest that tacrolimus may be a promising immunosuppressant for testis allografts or germ cell transplantation in rainbow trout. Co-administration combining tacrolimus (at lower dose) with other immunosuppressive drugs for inhibiting other activation pathways of the immune system, as performed in human organ transplantation, could be an alternative approach to optimize the immunosuppressive effects in host organisms.

IκBα phosphorylation and associated NF-κB activation are essential events in lymphocyte activation, proliferation, and anti-bacterial adaptive immune response of Nile tilapia

Inhibitory protein IκBα plays a crucial role in the inflammatory process and immune response by regulating the activity of transcription factor NF-κB. In teleost, great progress has been achieved regarding NF-κB signaling for innate immunity, but whether this pathway modulates adaptive immunity, and how, remains largely unclear. In this study, after characterizing the sequence, structure, and phylogeny of Nile tilapia Oreochromis niloticus IκBα (defined as On-IκBα), we investigated the association between IκBα-regulated NF-κB activation and the lymphocyte-mediated adaptive immune response in Nile tilapia. We found that On-IκBα was evolutionarily conserved, and its mRNA was expressed widely in various tissues, with most abundance in the trunk kidney. mRNA expression of On-IκBα was significantly upregulated in spleen at both innate and adaptive immune stages after Aeromonas hydrophila infection. Moreover, phosphorylation of On-IκBα and the downstream On-NF-κB p65 was obviously elevated in spleen leukocytes at 3, 5, or 8 days after A. hydrophila infection, indicating the activation of NF-κB signaling. Correlating with the augmented protein phosphorylation, leukocyte proliferation was enhanced during the same immune stage, suggesting the potential association of IκBα and IκBα-regulated NF-κB signaling in the primary adaptive immune response. Although lymphocyte activation by the T cell-specific mitogen PHA did not alter On-IκBα mRNA expression significantly, lymphocyte activation by the agonist PMA obviously elevated On-IκBα and OnNF-κB p65 phosphorylation in spleen leukocytes. Together, the results suggest that IκBα phosphorylation and its regulated NF-κB activation are essential events associated with lymphocyte activation, proliferation, and anti-bacterial adaptive immune response in Nile tilapia. Our study aids to understand the regulatory mechanism of adaptive immunity in teleost.

Fish Granzyme A Shows a Greater Role Than Granzyme B in Fish Innate Cell-Mediated Cytotoxicity

Granzymes (Gzm) are serine proteases, contained into the secretory granules of cytotoxic cells, responsible for the cell-mediated cytotoxicity (CMC) against tumor cells and intracellular pathogens such as virus and bacteria. In fish, they have received little attention to their existence, classification or functional characterization. Therefore, we aimed to identify and evaluate their functional and transcriptomic relevance in the innate CMC activity of two relevant teleost fish species, gilthead seabream and European sea bass. Afterwards, we wanted to focus on their regulation upon nodavirus (NNV) infection, a virus that causes great mortalities to sea bass specimens while seabream is resistant. In this study, we have identified genes encoding GzmA and GzmB in both seabream and sea bass, as well as GzmM in seabream, which showed good phylogenetic relation to their mammalian orthologs. In addition, we found enzymatic activity related to tryptase (GzmA and/or GzmK), aspartase (GzmB), metase (GzmM), or chymase (GzmH) in resting head-kidney leucocytes (HKLs), with the following order of activity: GzmA/K ~ GzmM >> GzmH >>> GzmB. In addition, during innate CMC assays consisting on HKLs exposed to either mock- or NNV-infected target cells, though all the granzyme transcripts were increased only the tryptase activity did. Thus, our data suggest a high functional activity of GzmA/K in the innate CMC and a marginal one for GzmB. Moreover, GzmB activity was detected into target cells during the CMC assays. However, the percentage of target cells with GzmB activity after the CMC assays was about 10-fold lower than the death target cells, demonstrating that GzmB is not the main inductor of cell death. Moreover, in in vivo infection with NNV, gzm transcription is differently regulated depending on the fish species, genes and tissues. However, the immunohistochemistry study revealed an increased number of GzmB stained cells and areas in the brain of seabream after NNV infection, which was mainly associated with the lesions detected. Further studies are needed to ascertain the molecular nature, biological function and implication of fish granzymes in the CMC activity, and in the antiviral defense in particular.

Involvement of H-Ras in the adaptive immunity of Nile tilapia by regulating lymphocyte activation

H-Ras is a guanosine triphosphatase (GTPase), which acts as a molecular switch and controls multiple important cellular processes including lymphocyte activation and function. However, regulatory mechanism of adaptive immune response by H-Ras remains unclear in non-mammalian animals. In the present study, we investigated the involvement of H-Ras in lymphocyte activation with a teleost model Oreochromis niloticus. H-Ras from O. niloticus (On-H-Ras) is highly conserved with those from other vertebrates. The mRNA of On-H-Ras showed a wide expression pattern in the lymphoid-tissues and with the highest level in liver. After Aeromonas hydrophila infection, transcription of On-H-Ras was significantly induced on day 8 but came back to basal level on day 16, suggesting that On-H-Ras potentially participated in primary response during the adaptive immunity. Furthermore, On-H-Ras mRNA was obviously up-regulated when leukocytes were activated by T lymphocyte mitogen PHA in vitro. Meanwhile, protein level of H-Ras was also augmented once leukocytes were stimulated with lymphocyte receptor signaling agonist PMA and ionomycin. More importantly, once Ras activity was inhibited by specific inhibitor, the up-regulation of lymphocyte activation marker CD122 was obviously impaired during lymphocyte activation process. Therefore, On-H-Ras regulated lymphocyte activation through both mRNA and protein level. Altogether, our results illustrated the involvement of H-Ras in teleost adaptive immunity via controlling lymphocyte activation, and thus provided a novel perspective to understand evolution of the lymphocyte-mediated adaptive immunity.

c-Raf participates in adaptive immune response of Nile tilapia via regulating lymphocyte activation

RAF proto-oncogene serine/threonine-protein kinase (c-Raf) is a MAP kinase kinase kinase (MAPKKK) that participates in the Erk1/2 pathway and plays an important role in lymphocyte activation. However, the study on how c-Raf regulates adaptive immunity in non-mammal is still limited. In present study, based on analysis of sequence characteristics of c-Raf from Oreochromis niloticus (On-c-Raf), we investigated its regulation roles on teleost lymphocyte activation. The On-c-Raf was highly conserved during evolution, which was composed of a Raf-like Ras-binding domain (RBD), a protein kinase C conserved region 1 (C1) domain and a serine/threonine protein kinase catalytic (S_TKc) domain. Its mRNA showed a wide distribution in tissues of O. niloticus and with the highest expression in gill. After Aeromonas hydrophila infection, during the adaptive immune stage transcription level of On-c-Raf was significantly upregulated on day 8, but came back to original level on day 16 and 30, suggesting the potential involvement of On-c-Raf in primary response but not memory formation. Furthermore, On-c-Raf mRNA in leukocytes of Nile tilapias was obviously induced by in vitro stimulation of T cell mitogen PHA. More importantly, in vitro stimulation of lymphocytes agonist PMA augmented phosphorylation level of On-c-Raf in leukocytes detected by western-blot and immunofluorescent. Thus, c-Raf regulated lymphocyte activation of Nile tilapia on both mRNA and phosphorylation level. Together, our results revealed that the c-Raf from teleost Nile tilapia engaged in adaptive immune response by regulating lymphocytes activation. Since the regulatory mechanism of lymphocyte-mediated adaptive immunity is largely unknown in teleost, our study provided important evidences to understand teleost adaptive immunity, and also shed a novel perspective for the evolution of adaptive immune system.

Tapasin's protein interactions in the rainbow trout peptide-loading complex

Major histocompatibility complex (MHC) class I receptors play a key role in the immune system by presenting non-self peptides to T cell lymphocytes. In humans, the assembly of the MHC class I with a peptide is mediated by machinery in the endoplasmic reticulum referred as the peptide loading complex (PLC). Although, the identity of the PLC has been widely explored in humans, this complex has not been characterized in fish. Co-immunoprecipitation and mass spectrometry analysis revealed that the protein-protein interactions which exist in the human PLC are conserved in the monocyte/macrophage rainbow trout cell line (RTS11), in particular the interaction of tapasin with the transporter associated with antigen processing (TAP), MHC class I and ERp57. Importantly, a 20 kDa tapasin version that contains an intact C and N terminal domains was found to associate with ERp57 and form a 75 kDa heterodimer. These results suggest a possible novel alternative spliced version of tapasin may regulate the formation of the peptide-loading complex in teleosts.

Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ+ CD83+ Antigen-Presenting Cells

The histozoic myxozoan parasite Kudoa thyrsites causes postmortem myoliquefaction and is responsible for economic losses to salmon aquaculture in the Pacific Northwest. Despite its importance, little is known about the host-parasite relationship, including the host response to infection. The present work sought to characterize the immune response in Atlantic salmon during infection, recovery, and reexposure to K. thyrsites After exposure to infective seawater, infected and uninfected smolts were sampled three times over 4,275 degree-days. Histological analysis revealed infection severity decreased over time in exposed fish, while in controls there was no evidence of infection. Following a secondary exposure of all fish, severity of infection in the controls was similar to that measured in exposed fish at the first sampling time but was significantly reduced in reexposed fish, suggesting the acquisition of protective immunity. Using immunohistochemistry, we detected a population of MHIIβ+ cells in infected muscle that followed a pattern of abundance concordant with parasite prevalence. Infiltration of these cells into infected myocytes preceded destruction of the plasmodium and dissemination of myxospores. Dual labeling indicated a majority of these cells were CD83+/MHIIβ+ Using reverse transcription-quantitative PCR, we detected significant induction of cellular effectors, including macrophage/dendritic cells (mhii/cd83/mcsf), B cells (igm/igt), and cytotoxic T cells (cd8/nkl), in the musculature of infected fish. These data support a role for cellular effectors such as antigen-presenting cells (monocyte/macrophage and dendritic cells) along with B and T cells in the acquired protective immune response of Atlantic salmon against K. thyrsites.

Early nutritional programming affects liver transcriptome in diploid and triploid Atlantic salmon, Salmo salar

BACKGROUND

To ensure sustainability of aquaculture, plant-based ingredients are being used in feeds to replace marine-derived products. However, plants contain secondary metabolites which can affect food intake and nutrient utilisation of fish. The application of nutritional stimuli during early development can induce long-term changes in animal physiology. Recently, we successfully used this approach to improve the utilisation of plant-based diets in diploid and triploid Atlantic salmon. In the present study we explored the molecular mechanisms occurring in the liver of salmon when challenged with a plant-based diet in order to determine the metabolic processes affected, and the effect of ploidy.

RESULTS

Microarray analysis revealed that nutritional history had a major impact on the expression of genes. Key pathways of intermediary metabolism were up-regulated, including oxidative phosphorylation, pyruvate metabolism, TCA cycle, glycolysis and fatty acid metabolism. Other differentially expressed pathways affected by diet included protein processing in endoplasmic reticulum, RNA transport, endocytosis and purine metabolism. The interaction between diet and ploidy also had an effect on the hepatic transcriptome of salmon. The biological pathways with the highest number of genes affected by this interaction were related to gene transcription and translation, and cell processes such as proliferation, differentiation, communication and membrane trafficking.

CONCLUSIONS

The present study revealed that nutritional programming induced changes in a large number of metabolic processes in Atlantic salmon, which may be associated with the improved fish performance and nutrient utilisation demonstrated previously. In addition, differences between diploid and triploid salmon were found, supporting recent data that indicate nutritional requirements of triploid salmon may differ from those of their diploid counterparts.

Innate Cell-Mediated Cytotoxic Activity of European Sea Bass Leucocytes Against Nodavirus-Infected Cells: A Functional and RNA-seq Study

Nervous necrosis virus (NNV) causes high mortalities in several marine species. We aimed to evaluate the innate cell-mediated cytotoxic (CMC) activity of head-kidney leucocytes (HKLs) isolated from naïve European sea bass (Dicentrarchus labrax) and gilthead seabream (Sparus aurata), a very susceptible and resistant fish species to NNV, respectively, against fish cell lines infected with NNV. Seabream HKLs showed significantly increased innate CMC activity against NNV-infected cells, compared to those uninfected, while sea bass HKLs failed to do so. Thus, we performed a RNA-seq study to identify genes related to the CMC activity of sea bass leucocytes. Thus, we found that sea bass HKLs incubated with DLB-1 cells alone (CMC_DLB1) or with NNV-infected DLB-1 cells (CMC_DLB1-NNV) showed very similar transcriptomic profiles and the GO analysis revealed that most of the up-regulated genes were related to immunity. Strikingly, when the CMC samples with and without NNV were compared, GO analysis revealed that most of the up-regulated genes in CMC_DLB1-NNV samples were related to metabolism and very few to immunity. This is also in agreement with the functional data. These data point to the escape of CMC activity by NNV infection as an important factor involved in the high susceptibility to nodavirus infections of European sea bass.

Returning to normal? Assessing transcriptome recovery over time in male rainbow darter (Etheostoma caeruleum) liver in response to wastewater-treatment plant upgrades

The present study measured hepatic transcriptome responses in male rainbow darter (Etheostoma caeruleum) exposed to 2 municipal wastewater-treatment plants (MWWTPs; Kitchener and Waterloo) over 4 fall seasons (2011-2014) in the Grand River (Ontario, Canada). The overall goal was to determine if upgrades at the Kitchener MWWTP (in 2012) resulted in transcriptome responses indicative of improved effluent quality. The number of differentially expressed probes in fish downstream of the Kitchener outfall (904-1223) remained comparable to that downstream of Waterloo (767-3867). Noteworthy was that year and the interaction of year and site explained variability in more than twice the number of transcripts than site alone, suggesting that year and the interaction of year and site had a greater effect on the transcriptome than site alone. Gene set enrichment analysis revealed a gradual reduction in the number of gene ontologies over time at exposure sites, which corresponded with lower contaminant load. Subnetwork enrichment analysis revealed that there were noticeable shifts in the cell pathways differently expressed in the liver preupgrade and postupgrade. The dominant pathways altered preupgrade were related to genetic modifications and cell division, whereas postupgrade they were associated with the immune system, reproduction, and biochemical responses. Molecular pathways were dynamic over time, and following the upgrades, there was little evidence that gene expression profiles in fish collected from high-impact sites postupgrade were more similar to those in fish collected from reference site. Environ Toxicol Chem 2017;36:2108-2122. © 2017 SETAC.

Selection for brain size impairs innate, but not adaptive immune responses

Both the brain and the immune system are energetically demanding organs, and when natural selection favours increased investment into one, then the size or performance of the other should be reduced. While comparative analyses have attempted to test this potential evolutionary trade-off, the results remain inconclusive. To test this hypothesis, we compared the tissue graft rejection (an assay for measuring innate and acquired immune responses) in guppies (Poecilia reticulata) artificially selected for large and small relative brain size. Individual scales were transplanted between pairs of fish, creating reciprocal allografts, and the rejection reaction was scored over 8 days (before acquired immunity develops). Acquired immune responses were tested two weeks later, when the same pairs of fish received a second set of allografts and were scored again. Compared with large-brained animals, small-brained animals of both sexes mounted a significantly stronger rejection response to the first allograft. The rejection response to the second set of allografts did not differ between large- and small-brained fish. Our results show that selection for large brain size reduced innate immune responses to an allograft, which supports the hypothesis that there is a selective trade-off between investing into brain size and innate immunity.

A Leukocyte Immune-Type Receptor Subset Is a Marker of Antiviral Cytotoxic Cells in Channel Catfish, Ictalurus punctatus

Channel catfish, Ictalurus punctatus, leukocyte immune type receptors (LITRs) represent a multigene family that encodes Ig superfamily proteins that mediate activating or inhibitory signaling. In this study, we demonstrate the use of mAb CC41 to monitor viral cytotoxic responses in catfish and determine that CC41 binds to a subset of LITRs on the surface of catfish clonal CTLs. Homozygous gynogenetic catfish were immunized with channel catfish virus (CCV)-infected MHC-matched clonal T cells (G14D-CCV), and PBL were collected at various times after immunization for flow cytometric analyses. The percentage of CC41(+) cells was significantly increased 5 d after primary immunization with G14D-CCV and at 3 d after a booster immunization as compared with control fish only injected with G14D. Moreover, CC41(+) cells magnetically isolated from the PBL specifically killed CCV-infected targets as measured by (51)Cr release assays and expressed messages for CD3γδ, perforin, and at least one of the CD4-like receptors as analyzed by RNA flow cytometry. When MLC effector cells derived from a G14D-CCV-immunized fish were preincubated with CC41 mAb, killing of G14D-CCV targets was reduced by ∼40%, suggesting that at least some LITRs have a role in target cell recognition and/or cytotoxicity. The availability of a LITR-specific mAb has allowed, to our knowledge for the first time, functional characterization of LITRs in an autologous system. In addition, the identification of an LITR subset as a cytotoxic cell marker will allow for more effective monitoring of catfish immune responses to pathogens.

T Cells in Fish

Cartilaginous and bony fish are the most primitive vertebrates with a thymus, and possess T cells equivalent to those in mammals. There are a number of studies in fish demonstrating that the thymus is the essential organ for development of T lymphocytes from early thymocyte progenitors to functionally competent T cells. A high number of T cells in the intestine and gills has been reported in several fish species. Involvement of CD4⁺ and CD8α⁺ T cells in allograft rejection and graft-versus-host reaction (GVHR) has been demonstrated using monoclonal antibodies. Conservation of CD4⁺ helper T cell functions among teleost fishes has been suggested in a number studies employing mixed leukocyte culture (MLC) and hapten/carrier effect. Alloantigen- and virus-specific cytotoxicity has also been demonstrated in ginbuna and rainbow trout. Furthermore, the important role of cell-mediated immunity rather than humoral immunity has been reported in the protection against intracellular bacterial infection. Recently, the direct antibacterial activity of CD8α⁺, CD4⁺ T-cells and sIgM⁺ cells in fish has been reported. In this review, we summarize the recent progress in T cell research focusing on the tissue distribution and function of fish T cells.

Effects of 17 α-methyltestosterone on transcriptome, gonadal histology and sex steroid hormones in rare minnow Gobiocypris rarus

The 17α-methyltestosterone (MT), a synthetic androgen, is known for its interference effects on the endocrine system. Aiming to investigate the transcriptome profiling of gonads induced by MT and to understand the molecular mechanism by which MT causes adverse effects in fish, transcriptome profiling of gonads, gonadal histology and the sex steroid hormones in response to MT were analyzed in Gobiocypris rarus. Eight libraries, 4 from the ovary and 4 from the testis, were constructed and sequenced and then a total number of clean reads per sample ranging from 7.03 to 9.99 million were obtained. In females, a total of 191 transcripts were differentially regulated by MT, consisting of 102 up-regulated transcripts and 89 down-regulated transcripts. In males, 268 differentially expressed genes with 108 up-regulated and 160 down-regulated were detected upon MT exposure. Testosterone serves as the major sex steroid hormone content in G. rarus of both sexes. The concentrations of 17β-estradiol, testosterone and 11-ketotestosterone were significantly increased in females and decreased in males after MT exposure. Interestingly, MT caused a decreased number of vitellogenic oocytes in the ovary and spermatozoa in the testis. After MT exposure, four differentially expressed genes (ndufa4, slc1a3a, caskin-2 and rpt3) were found in G. rarus of both sexes. Overall, we suggest that MT seemed to affect genes involved in pathways related to physiological processes in the gonads of G. rarus. These processes include the electron transfer of Complex IV, endothelial cell activation, axon growth and guidance, and proteasome assembly and glutamate transport metabolic.

A Review of Intra- and Extracellular Antigen Delivery Systems for Virus Vaccines of Finfish

Vaccine efficacy in aquaculture has for a long time depended on evaluating relative percent survival and antibody responses after vaccination. However, current advances in vaccine immunology show that the route in which antigens are delivered into cells is deterministic of the type of adaptive immune response evoked by vaccination. Antigens delivered by the intracellular route induce MHC-I restricted CD8+ responses while antigens presented through the extracellular route activate MHC-II restricted CD4+ responses implying that the route of antigen delivery is a conduit to induction of B- or T-cell immune responses. In finfish, different antigen delivery systems have been explored that include live, DNA, inactivated whole virus, fusion protein, virus-like particles, and subunit vaccines although mechanisms linking these delivery systems to protective immunity have not been studied in detail. Hence, in this review we provide a synopsis of different strategies used to administer viral antigens via the intra- or extracellular compartments. Further, we highlight the differences in immune responses induced by antigens processed by the endogenous route compared to exogenously processed antigens. Overall, we anticipate that the synopsis put together in this review will shed insights into limitations and successes of the current vaccination strategies used in finfish vaccinology.

Development and characterization of a monoclonal antibody against the putative T cells of Labeo rohita

In this study, we have described the development and characterization of monoclonal antibodies (MAbs) directed against thymocytes of rohu, Labeo rohita. MAbs were obtained by immunizing BALB/c mice with freshly isolated and nylon wool column enriched mononuclear cells of thymus. Positive clones against thymocytes were screened by cellular ELISA. The hybridoma showing strong reactivity with nylon wool enriched mononuclear cells, and non-reactivity with a rohu thymus macrophage cell line and rohu serum was selected and subjected to single cell cloning by limiting dilution. The MAbs secreted by a positive clone were designated as E6 MAb. Western blotting of reduced protein from enriched thymocytes showed that E6 reacted with a 166.2 kDa polypeptide and belongs to the IgG1 subclass. Flow cytometric analysis of gated lymphocytes, revealed that the percentage of E6 positive (E6+) cells in thymus (n = 5, 720.4 ± 79.70 g) was 89.7 %. Similarly, the percentage of E6+ cells in kidney, spleen and blood (n = 5) was 6.71, 1.71 and 1.88 %, respectively. In indirect immunoperoxidase test, E6+ cells appeared to be lymphoid cells with a high nucleus to cytoplasmic ratio and were densely packed in the central region of thymus whereas, a few cells were found to be positive in kidney and spleen sections. E6 MAb also reacted with a small population of lymphocytes in blood smear. This MAb appears to be a suitable marker for T lymphocytes and can be a valuable tool in studying immune response and ontogeny of L. rohita immune system.

Footprints of directional selection in wild Atlantic salmon populations: evidence for parasite-driven evolution?

Mechanisms of host-parasite co-adaptation have long been of interest in evolutionary biology; however, determining the genetic basis of parasite resistance has been challenging. Current advances in genome technologies provide new opportunities for obtaining a genome-scale view of the action of parasite-driven natural selection in wild populations and thus facilitate the search for specific genomic regions underlying inter-population differences in pathogen response. European populations of Atlantic salmon (Salmo salar L.) exhibit natural variance in susceptibility levels to the ectoparasite Gyrodactylus salaris Malmberg 1957, ranging from resistance to extreme susceptibility, and are therefore a good model for studying the evolution of virulence and resistance. However, distinguishing the molecular signatures of genetic drift and environment-associated selection in small populations such as land-locked Atlantic salmon populations presents a challenge, specifically in the search for pathogen-driven selection. We used a novel genome-scan analysis approach that enabled us to i) identify signals of selection in salmon populations affected by varying levels of genetic drift and ii) separate potentially selected loci into the categories of pathogen (G. salaris)-driven selection and selection acting upon other environmental characteristics. A total of 4631 single nucleotide polymorphisms (SNPs) were screened in Atlantic salmon from 12 different northern European populations. We identified three genomic regions potentially affected by parasite-driven selection, as well as three regions presumably affected by salinity-driven directional selection. Functional annotation of candidate SNPs is consistent with the role of the detected genomic regions in immune defence and, implicitly, in osmoregulation. These results provide new insights into the genetic basis of pathogen susceptibility in Atlantic salmon and will enable future searches for the specific genes involved.

Vaccines for fish in aquaculture

Vaccination plays an important role in large-scale commercial fish farming and has been a key reason for the success of salmon cultivation. In addition to salmon and trout, commercial vaccines are available for channel catfish, European seabass and seabream, Japanese amberjack and yellowtail, tilapia and Atlantic cod. In general, empirically developed vaccines based on inactivated bacterial pathogens have proven to be very efficacious in fish. Fewer commercially available viral vaccines and no parasite vaccines exist. Substantial efficacy data are available for new fish vaccines and advanced technology has been implemented. However, before such vaccines can be successfully commercialized, several hurdles have to be overcome regarding the production of cheap but effective antigens and adjuvants, while bearing in mind environmental and associated regulatory concerns (e.g., those that limit the use of live vaccines). Pharmaceutical companies have performed a considerable amount of research on fish vaccines, however, limited information is available in scientific publications. In addition, salmonids dominate both the literature and commercial focus, despite their relatively small contribution to the total volume of farmed fish in the world. This review provides an overview of the fish vaccines that are currently commercially available and some viewpoints on how the field is likely to evolve in the near future.

Transcription analysis of two Eomesodermin genes in lymphocyte subsets of two teleost species

Eomesodermin (Eomes), a T-box transcription factor, is a key molecule associated with function and differentiation of CD8(+) T cells and NK cells. Previously, two teleost Eomes genes (Eomes-a and -b), which are located on different chromosomes, were identified and shown to be expressed in zebrafish lymphocytes. For the present study, we identified these genes in rainbow trout and ginbuna crucian carp. Deduced Eomes-a and -b amino acid sequences in both fish species contain a highly conserved T-box DNA binding domain. In RT-PCR, both Eomes transcripts were readily detectable in a variety of tissues in rainbow trout and ginbuna. The high expression of Eomes-a and -b in brain and ovary suggests involvement in neurogenesis and oogenesis, respectively, while their expression in lymphoid tissues presumably is associated with immune functions. Investigation of separated lymphocyte populations from pronephros indicated that both Eomes-a and -b transcripts were few or absent in IgM(+) lymphocytes, while relatively abundant in IgM(-)/CD8α(+) and IgM(-)/CD8α(-) populations. Moreover, we sorted trout CD8α(+) lymphocytes from mucosal and non-mucosal lymphoid tissues and compared the expression profiles of Eomes-a and -b with those of other T cell-related transcription factor genes (GATA-3, T-bet and Runx3), a Th1 cytokine gene (IFN-γ) and a Th2 cytokine gene (IL-4/13A). Interestingly, the tissue distribution of Eomes-a/b, T-bet, and Runx3 versus IFN-γ transcripts did not reveal simple correlations, suggesting tissue-specific properties of CD8α(+) lymphocytes and/or multiple modes that drive IFN-γ expressions.

Role of CD4(+) and CD8α(+) T cells in protective immunity against Edwardsiella tarda infection of ginbuna crucian carp, Carassius auratus langsdorfii

Edwardsiella tarda is an intracellular pathogen that causes edwardsiellosis in fish. Our previous study suggests that cell-mediated immunity (CMI) plays an essential role in protection against E. tarda infection. In the present study, we adoptively transferred T-cell subsets sensitized with E. tarda to isogenic naïve ginbuna crucian carp to determination the T-cell subsets involved in protecting fish from E. tarda infection. Recipients of CD4(+) and CD8α(+) cells acquired significant resistance to infection with E. tarda 8 days after sensitization, indicating that helper T cells and cytotoxic T lymphocytes plays crucial roles in protective immunity to E. tarda. Moreover, transfer of sensitized CD8α(+) cells up-regulated the expression of genes encoding interferon-γ (IFN-γ) and perforin, suggesting that protective immunity to E. tarda involves cell-mediated cytotoxicity and interferon-γ-mediated induction of CMI. The results establish that CMI plays a crucial role in immunity against E. tarda. These findings provide novel insights into understanding the role of CMI to intracellular pathogens of fish.

Effects of excretory/secretory products from Anisakis simplex (Nematoda) on immune gene expression in rainbow trout (Oncorhynchus mykiss)

Excretory/secretory (ES) products are molecules produced by parasitic nematodes, including larval Anisakis simplex, a parasite occurring in numerous marine fish hosts. The effects of these substances on host physiology have not been fully described. The present work elucidates the influence of ES substances on the fish immune system by measuring immune gene expression in spleen and liver of rainbow trout (Oncorhynchus mykiss) injected intraperitoneally with ES products isolated from A. simplex third stage larvae. The overall gene expression profile of exposed fish showed a generalized down-regulation of the immune genes tested, suggesting a role of ES proteins in immunomodulation. We also tested the enzymatic activity of the ES proteins and found that lipase, esterase/lipase, valine and cysteine arylamidases, naphthol-AS-BI-phosphohydrolase and α-galactosidase activities were present in the ES solution. This type of hydrolytic enzyme activity may play a role in nematode penetration of host tissue. In addition, based on the notion that A. simplex ES products may have an immune-depressive effect (by minimizing immune gene expression) it could also be suggested that worm enzymes directly target host immune molecules which would add to a decreased host immune response and increased worm survival.

Characterization and expression analysis of Th-POK from the Japanese pufferfish, Takifugu rubripes

In fish, T cell lineage commitment has not been studied, although there are reports related to CD4 and CD8 positive cells. This study describes the cloning and analysis of a master regulator involved in this process, the Th-POK gene in Japanese pufferfish, Takifugu rubripes. The Fugu Th-POK cDNA was composed of 1901 bp, with a 75 bp 5'-UTR, a 131 bp 3'-UTR, and a 1692 bp open reading frame which translates into a peptide of 564 amino acid residues. The deduced Fugu Th-POK protein contained a BTB/POZ domain, Krüppel motif (H/C linker) and Krüppel-like zinc finger DNA binding domain with C2H2 structure. The homology analysis of Fugu Th-POK (ZBTB7B) with other known ZBTB7 members (ZBTB7A, 7C) showed low identity, and the phylogenetic tree analysis showed the Fugu Th-POK clustered with the mammalian Th-POK, away from other ZBTB7 members. The analysis of transcriptional control region of Th-POK gene suggested that the 5'-flanking region and intron 1 include numerous canonical binding motifs for transcription factors regulating T cell development. The genomic organization of the Fugu Th-POK gene was composed of three exons and two introns, and its structure was identical to that of its human counterpart. Comparison of the Fugu and human genomes showed that high levels of conserved synteny existed around the Th-POK gene. The high expression of the Fugu Th-POK gene in unstimulated tissues was seen in head kidney, muscle, skin and gills. Moreover, the expression of the Fugu Th-POK gene in thymic cells was increased by LPS, polyI:C and PHA stimulation.

Nodavirus infection induces a great innate cell-mediated cytotoxic activity in resistant, gilthead seabream, and susceptible, European sea bass, teleost fish

Viral nervous necrosis (VNN) virus produces great mortalities in fish having susceptible and reservoir species between the most important marine aquaculture species. Cell-mediated cytotoxicity (CMC) is considered, towards the interferon (IFN), the most important mechanism of the immune response to fight against viral infections but it has been very scarcely evaluated. We aimed to evaluate the effects of VNNV infection in the reservoir gilthead seabream (Sparus aurata) and susceptible European sea bass (Dicentrarchus labrax). Firstly, after experimental infection we found mortalities in the sea bass (55%) but no in the seabream. Moreover, VNN virus replicates in the brain of both species as it was reflected by the high up-regulation of the Mx gene expression. Interestingly, the head-kidney leucocyte cell-mediated cytotoxic activity was significantly increased in both species reaching highest activity at 7 days: 3.65- and 2.7-fold increase in seabream and sea bass, respectively. This is supported by the significant up-regulation of the non-specific cytotoxic cell receptor (NCCRP-1) in the two fish species. By contrast, phagocytosis was unaffected in both species. The respiratory burst was increased in seabream 7 days post-infection whilst in sea bass this activity was significantly decreased at days 7 and 15. Our results demonstrate the significance of the CMC activity in both gilthead seabream and European sea bass against nodavirus infections but further studies are still needed to understand the role of cytotoxic cells in the antiviral immune response and the mechanisms involved in either reservoir or susceptible fish species.

A novel acute phase reactant, serum amyloid A-like 1, from Oplegnathus fasciatus: genomic and molecular characterization and transcriptional expression analysis

Acute phase response is a significant component of innate immunity, playing a vital role in the signaling processes and elimination of invading pathogens. Acute phase proteins are synthesized in liver and secreted into the blood for transportation to an infection site, where the defense function is exerted. Serum amyloid A (SAA) and C-reactive proteins are the major positive acute phase proteins. In this study, we have identified and characterized a novel SAA related gene from rock bream (Oplegnathus fasciatus), designated OfSAAL1. Genomic characterization revealed the presence of 13 exons and 12 introns, similar to SAAL1 in zebrafish. Multiple protein sequence alignment revealed high conservation with other SAAL1 homologues. Phylogenetic analysis showed that OfSAAL1 clustered with another fish homologue, and pairwise alignment revealed highest identity and similarity at the amino acid level with zebrafish SAAL1. Promoter region analysis revealed the presence of immunologically significant transcription factor binding sites. Tissue distribution profiling to indicate physiological relevance showed the highest levels occur in blood, followed by liver, suggesting a positive immune role in rock bream. Transcriptional analysis by reverse transcription polymerase chain reaction to understand OfSAAL1 responsiveness to immune challenge with poly I:C, Edwardsiella tarda, Streptococcus iniae and rock bream iridovirus, revealed a significant level of elevation from 12h to 48 h post-infection in blood, spleen, head kidney, and liver. To our knowledge, OfSAAL1 is the first characterized SAAL1 homologue from teleosts. We anticipate that its identification will prove inspiring for further studies of SAAL1 homologues as biomarkers of the acute phase response.

Gene expression analysis of common carp (Cyprinus carpio L.) lines during Cyprinid herpesvirus 3 infection yields insights into differential immune responses

Cyprinid herpesvirus 3 (CyHV-3), also known as koi herpesvirus (KHV), is the etiological agent of a virulent and lethal disease in common and koi carp. This study aimed to determine the genetic basis underlying the common carp immune response to the CyHV-3 virus. Two common carp lines (R3 and K) were infected with CyHV-3 by immersion. The R3 line presented a 20% higher survival rate compared to the K line and significantly lower viral loads as measured at day 3 post infection (p.i.). Microarray analysis using a common carp slides containing a number of 10,822 60-mer probes, revealed that 581 genes in line K (330 up-regulated, 251 down-regulated) and 107 genes in line R3 (77 up-regulated, 30 down-regulated), showed at least a 2-fold difference in expression at day 3 p.i. compared to day 0. Genes which showed at least a 4-fold difference in expression in both lines were selected as potential markers of a CyHV-3 infection in common carp. Additionally, 76 genes showed at least 2-fold differentially expression between K and R3 lines at day 3 p.i. Significantly higher expression of several immune-related genes including number of those which are involve in pathogen recognition, complement activation, MHC class I-restricted antigen presentation and development of adaptive mucosal immunity was noted in more resistant R3 line. Further real-time PCR based analysis provided evidence for higher activation of CD8(+) T cells in R3 line. This study uncovered wide array of immune-related genes involved into antiviral response of common carp toward CyHV-3. It is also demonstrated that the outcome of this severe disease in large extent could be controlled by genetic factors of the host.

Cytotoxic T cells in teleost fish

The presence of antigen-specific cytotoxic T cells has been suggested in a number of in vivo and in vitro studies in fish. Acute allograft rejection with an accelerated response on second-set grafts and the presence of graft-versus-host reaction (GVHR) has been reported in teleost. Alloantigen- and virus-specific cytotoxicity has also been demonstrated in ex vivo studies in ginbuna and rainbow trout. In addition, alloantigen-specific cytotoxic T cell clones have been produced in cultures initiated with peripheral blood leukocytes (PBL) from an alloantigen-immunized channel catfish. Over the last decade several fish genomes have been sequenced and genetic information is rapidly accumulating. Thanks to these genome data bases and EST analysis, mRNA expression of T cell surface marker genes in alloantigen- or virus-specific effector cells has been reported in some fish species, e.g. TCR α or β and CD8α in ginbuna and rainbow trout, and TCR α, β or γ in channel catfish. These findings suggest the presence of CD8(+) cytotoxic T lymphocyte (CTL) in fish similar to those of higher vertebrates. Recently, monoclonal antibodies against CD8α and CD4 antigens have been produced in some fish species. Investigation on the characteristics of CTL and cell-mediated immune mechanisms is now possible at defined T cell subsets, although identification of T cell subset is limited in a few fish species at present. In this review, we describe the recent progress in this field focusing on cells involved in antigen specific cytotoxicity.

Fish T cells: recent advances through genomics

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

Chronic transplantation immunity in newts: temperature susceptibility of an effector phase in allo-skin graft rejection

Urodele amphibians are unique due to their greatly reduced immune responsiveness compared to bony fishes, which show acute immune responsiveness. In newts, the mean survival time of allogenic skin grafts in the transplantation immunity was 48.8 ± 8.3 days at 25°C, suggesting that it occurs in a chronic manner. The graft rejection process was categorized into three stages: a latent stage with frequent blood circulation, or the immune induction phase; a vascular stoppage stage with dominant infiltrating cells of T cells; and a rejection stage showing the change of the dominant cells to monocytes/macrophages, probably as effector cells, tetntatively referred to as the immune effector phase. The immune induction phase is susceptible to the cyclophosphamide (CY) mitosis inhibitor, but not to a temperature shift from 18 to 27°C, while the immune effector phase is susceptible to temperature shifts, but not CY-treatment, although the temperature shift failed to shorten the graft survival time to less than 25 days, which nearly equals that of the secondary set of grafts where the lack of complete blood circulation is remarkable and graft rejection is resistant to CY-treatment. In contrast, a very low temperature (5-10°C) completely prevented effector generation in newts; in frogs, however, it is reported that such low temperatures did not prevent the generation of effectors. Taken together, these data suggest that chronic responses in newts are due to effector cells other than cytotoxic T cells; possible effector cells are discussed.

Viral hemorrhagic septicaemia virus (VHSV) up-regulates the cytotoxic activity and the perforin/granzyme pathway in the rainbow trout RTS11 cell line

A survey of immune-relevant genes that might be up-regulated in response to viral hemorrhagic septicaemia virus (VHSV) in the rainbow trout monocyte-macrophage cell line, RTS11, unexpectedly revealed an increased expression of perforin (PRF) and granzyme (GRZ) genes, which represent components of the major cytotoxic pathway. The natural killer-enhancing factor (NKEF), also known to modulate cytotoxic activity, was up-regulated at the gene but strikingly down-regulated at protein level. The expression of these genes was not affected in head kidney leukocytes (HKLs) infected with VHSV, leading us to evaluate the potential cytotoxic activity of RTS11 and HKLs. For the first time, the cytotoxic activity of RTS11 against xenogeneic targets has been demonstrated, although this was modest relative to HKLs. Yet the activity in RTS11 was significantly increased by VHSV, as in HKLs. This cytotoxic activity elicited by viral infection appeared to require viral gene expression because inactivated VHSV failed to increase RTS11 cytotoxic activity. As for other immune functions, RTS11 cells provide a model for further studying cytotoxic activities of fish monocyte-macrophages.

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.

Conservation of characteristics and functions of CD4 positive lymphocytes in a teleost fish

The presence of helper and cytotoxic T cells in fish has been suggested, although T cell subsets have yet to be identified at the cellular level. In order to investigate the functions of CD4 and CD8α positive T cells we attempted to produce and characterize monoclonal antibodies (mAbs) against teleost CD4 and CD8α. Here we report the successful production of mAbs against CD4 and CD8α in clonal ginbuna crucian carp Carassius auratus langsdorfii and the function of CD4 positive T cells. In this study we demonstrate the presence of teleost CD4- and CD8α-positive T cell subsets with morphology, tissue distribution and gene expression similar to those of mammalian CD4- and CD8-positive T lymphocytes. Using mAbs we found that CD4/CD8 double positive T cells are only present in the thymus, suggesting that it is the site of T cell development. We further demonstrated in vitro proliferation of CD4 positive T cells by allogeneic combination of mixed leukocyte culture and antigen-specific proliferation of CD4 positive T cells after in vitro sensitization with OVA. In our previous study we showed that CD8α-positive lymphocytes are the primary cell type showing specific cytotoxicity against allogeneic targets. Collectively, these findings suggest that CD4 and CD8α positive T cells in ginbuna are equivalent to helper and cytotoxic T lymphocytes (CTL) in mammals, respectively. This is the first report to show the characteristics and functions of CD4 positive T cells in fish and these findings shed light into the evolutionary origins and primordial functions of helper T cells.

Cytotoxic CD8α+ leucocytes have heterogeneous features in antigen recognition and class I MHC restriction in grouper

CD8 is a membrane glycoprotein found primarily on the surface of T lymphocytes such as cytotoxic T lymphocytes (CTL), natural killer cells (NK) and γδ T lymphocytes. It helps T lymphocytes to kill the infected cells that presents microbial antigen on the cell surface. However, analysis of fish cellular immunity has been limited because of the lack of CD8 antibodies in grouper. In this present study, we cloned full-length CD8α cDNAs from orange-spotted grouper (Epinephelus coioides), an important fish species economically. The deduced protein of CD8α contained 227 amino acid residues in length and included one signal peptide, Ig superfamily V domain, hinge region, transmembrane domain, cytoplasmic tail and conserved binding motif associated with tyrosine kinase p56(lck). The molecular weight of the mature protein was estimated at 22.5 kDa and pI at 9.55. Phylogenetically, the predicted grouper CD8α protein was similar to CD8α from other marine fish species in which the identity was 50-60%. Real-time PCR revealed that CD8α transcript was constitutively expressed in thymus, head kidney, gill, spleen, gut and peripheral blood leucocyte (PBL); and the highest expression in thymus. CD8α transcript in the spleen of fish injected with nervous necrosis virus (NNV) was significantly up-regulated at 4 days post-injection compared to the untreated fish. Rabbit antiserum prepared against recombinant CD8α protein was able to recognize specifically the subset lymphocytes which have a diameter of 7 μm, a high nucleus/cytoplasm ratio and a ring-shaped cytoplasm. The cytotoxicity of CD8α(+) lymphocytes at one-week post-NNV infection was enhanced significantly against NNV-infected autologous fin cells in comparison with NNV-infected allogeneic or RSIV-infected autologous fin cells. Flow cytometry analysis revealed that both the number and mean fluorescence intensity (MFI) of CD8α(+) PBL were significantly increased at 7 days post-NNV infection. The specific cytotoxicity and MHC class I restriction of the lymphocytes sorted by rCD8α antibody are properties that can be attributed to CTL. In addition, low level of cytotoxicity was found in PBL against allogeneic targets as well as CD8α(+) effectors killed autologous targets nonspecifically, implicated presence of cytotoxic T subsets, possibly nonspecific cytotoxic cells (NCC) and γδ T lymphocytes, without MHC class I restriction. In conclusion, grouper cytotoxic CD8α(+) PBL have heterogeneous features in specific antigen recognition and class I MHC restriction.

Description of an elasmobranch TCR coreceptor: CD8α from Rhinobatos productus

Cell-mediated immunity plays an essential role for the control and eradication of intracellular pathogens. To learn more about the evolutionary origins of the first signal (Signal 1) for T-cell activation, we cloned CD8α from an elasmobranch, Rhinobatos productus. Similar to full-length CD8α cDNAs from other vertebrates, Rhpr-CD8α (1800bp) encodes a 219 amino acid open reading frame composed of a signal peptide, an extracellular IgSF V domain and a stalk/hinge region followed by a well-conserved transmembrane domain and cytoplasmic tail. Overall, the mature Rhpr-CD8α protein (201 aa) displays ∼ 30% amino acid identity with mammalian CD8α including absolute conservation of cysteine residues involved in the IgSf V domain fold and dimerization of CD8αα and CD8αβ. One prominent feature is the absence of the LCK association motif (CXC) that is needed for achieving signal 1 in tetrapods. Both elasmobranch and teleost CD8α protein sequences possess a similar but distinctly different motif (CXH) in the cytoplasmic tail. The overall genomic structure of CD8α has been conserved during the course of vertebrate evolution both for the number of exons and phase of splicing. Finally, quantitative RTPCR demonstrated that elasmobranch CD8α is expressed in lymphoid-rich tissues similar to CD8 in other vertebrates. The results from this study indicate the existence of CD8 prior to the emergence of the gnathostomes (>450 MYA) while providing evidence that the canonical LCK association motif in mammals is likely a derived characteristic of tetrapod CD8α, suggesting potential differences for T-cell education and activation in the various gnathostomes.

BCG vaccine confers adaptive immunity against Mycobacterium sp. infection in fish

Mycobacteriosis, caused by Mycobacterium sp., results in severe loss of fish production in Japan's aquaculture industry. In this study, the effects of two vaccine candidates, Bacillus Calmette and Guèrin (BCG, an attenuated strain of Mycobacterium bovis) and formalin-killed cells of Mycobacterium sp. were evaluated in Japanese flounder, Paralichthys olivaceus. In the immediate response and tuberculin response, BCG-vaccinated fish showed higher gene expression levels of inflammatory cytokines such as IL-1beta, IL-6, IFN-gamma and TNFalpha. Furthermore, BCG vaccination conferred protective efficacy against Mycobacterium sp. infection in Japanese flounder. Transcriptome analysis using a Japanese flounder cDNA microarray revealed that BCG vaccination induced not only adaptive immunity against Mycobacterium sp. antigen but also the expression of non-specific bactericidal proteins such as lysozyme. These data suggest that BCG confers immunity to Mycobacterium sp. infection and is a potent vaccine candidate for fish mycobacteriosis.

Teleost B7 expressed on monocytes regulates T cell responses

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