T cell diversity and TcR repertoires in teleost fish

Dynamic Changes in Lymphocyte Populations Establish Zebrafish as a Thymic Involution Model

The thymus is the site of T lymphocyte development and T cell education to recognize foreign, but not self, Ags. B cells also reside and develop in the thymus, although their functions are less clear. During "thymic involution," a process of lymphoid atrophy and adipose replacement linked to sexual maturation, thymocytes decline. However, thymic B cells decrease far less than T cells, such that B cells comprise ∼1% of human neonatal thymocytes but up to ∼10% in adults. All jawed vertebrates possess a thymus, and we and others have shown zebrafish (Danio rerio) also have thymic B cells. In this article, we investigated the precise identities of zebrafish thymic T and B cells and how they change with involution. We assessed the timing and specific details of zebrafish thymic involution using multiple lymphocyte-specific, fluorophore-labeled transgenic lines, quantifying the changes in thymic T- and B-lymphocytes pre- versus postinvolution. Our results prove that, as in humans, zebrafish thymic B cells increase relative to T cells postinvolution. We also performed RNA sequencing on D. rerio thymic and marrow lymphocytes of four novel double-transgenic lines, identifying distinct populations of immature T and B cells. Collectively, this is, to our knowledge, the first comprehensive analysis of zebrafish thymic involution, demonstrating its similarity to human involution and establishing the highly genetically manipulatable zebrafish model as a template for involution studies.

Subgenomic T cell receptor alpha and delta (TRA/TRD) loci in common carp

In jawed vertebrates, the T cell receptor alpha (TRA) and delta (TRD) genes, which encode the TRα and TRδ chains, respectively, are located as a nested structure on a single chromosome. To date, no animal has been reported to harbor multiple TRA/TRD loci on different chromosomes. Therefore, herein, we describe the first full annotation of the TRA/TRD genomic regions of common carp, an allo-tetraploid fish species that experiences cyprinid-specific whole-genome duplication (WGD) in evolution. Fine genomic maps of TRA/TRD genomic regions 1 and 2, on LG30 and LG22, respectively, were constructed using the annotations of complete sets of TRA and TRD genes, including TRA/TRD variable (V), TRA junction (J), and constant (C), TRD diversity (D), and the J and C genes. The structure and synteny of the TRA/TRD genomic regions were highly conserved in zebrafish, indicating that these regions are on individual chromosomes. Furthermore, analysis of the variable regions of the TRA and TRD genes in a monoclonal T cell line revealed that both subgenomic regions 1 and 2 were indeed rearranged. Although carp TRAV and TRDV genes were phylogenetically divided into different lineages, they were mixed and organized into the TRA/TRD V gene clusters on the genome, similar to that in other vertebrates. Notably, 285 potential TRA/TRD V genes were detected in the TRA/TRD genomic regions, which is the most abundant number of genes in vertebrates and approximately two-fold that in zebrafish. The recombination signal sequences (RSSs) at the end of each V gene differed between TRAV and TRDV, suggesting that RSS variations might separate each V gene into a TRα or TRδ chain. This study is the first to describe subgenomic TRA/TRD loci in animals. Our findings provide fundamental insights to elucidate the impact of WGD on the evolution of immune repertoire.

CD4+ T lymphocyte responses to viruses and virus-relevant stimuli in teleost fish

Fish diseases caused by viruses are a major threat to aquaculture. Development of disease protection strategies for sustainable fish aquaculture requires a better understanding of the immune mechanisms involved in antiviral defence. The innate and adaptive arms of the vertebrate immune system collaborate to mount an effective defence against viral pathogens. The T lymphocyte components of the adaptive immune system, comprising two major classes (helper T, Th or CD4+ and cytotoxic T lymphocytes, CTLs or CD8+ T cells), are responsible for cell-mediated immune responses. In particular, CD4+ T cells and their different subsets orchestrate the actions of various other immune cells during immune responses, making CD4+ T cells central drivers of responses to pathogens and vaccines. CD4+ T cells are also present in teleost fish. Here we review the literature that reported the use of antibodies against CD4 in a few teleost fish species and transcription profiling of Th cell-relevant genes in the context of viral infections and virus-relevant immunomodulation. Studies reveal massive CD4+ T cell proliferation and expression of key cytokines, transcription factors, and effector molecules that evoke mammalian Th cell responses. We also discuss gaps in the current understanding and evaluation of teleost CD4+ T cell responses and how development and application of novel tools and approaches to interrogate such responses could bridge these gaps. A greater understanding of fish Th cell responses will further illuminate the evolution of vertebrate adaptive immunity, inform strategies to address viral infections in aquaculture, and could further foster fish as model organisms.

Dynamic Changes in Lymphocyte Populations Establish Zebrafish as a Thymic Involution Model

The thymus is the site of T lymphocyte development and T cell education to recognize foreign, but not self, antigens. B cells also reside and develop in the thymus, although their functions are less clear. During 'thymic involution,' a process of lymphoid atrophy and adipose replacement linked to sexual maturation, thymocytes decline. However, thymic B cells decrease far less than T cells, such that B cells comprise ~1% of human neonatal thymocytes, but up to ~10% in adults. All jawed vertebrates possess a thymus, and we and others have shown zebrafish (Danio rerio) also have thymic B cells. Here, we investigated the precise identities of zebrafish thymic T and B cells and how they change with involution. We assessed the timing and specific details of zebrafish thymic involution using multiple lymphocyte-specific, fluorophore-labeled transgenic lines, quantifying the changes in thymic T- and B-lymphocytes pre- vs. post-involution. Our results prove that, as in humans, zebrafish thymic B cells increase relative to T cells post-involution. We also performed RNA sequencing (RNA-seq) on D. rerio thymic and marrow lymphocytes of four novel double-transgenic lines, identifying distinct populations of immature T and B cells. Collectively, this is the first comprehensive analysis of zebrafish thymic involution, demonstrating its similarity to human involution, and establishing the highly genetically-manipulatable zebrafish model as a template for involution studies.

The acute inflammatory response of teleost fish

Acute inflammation is crucial to the immune responses of fish. The process protects the host from infection and is central to induction of subsequent tissue repair programs. Activation of proinflammatory signals reshapes the microenvironment within an injury/infection site, initiates leukocyte recruitment, promotes antimicrobial mechanisms and contributes to the resolution of inflammation. Inflammatory cytokines and lipid mediators are primary contributors to these processes. Uncontrolled or persistent induction results in delayed tissue healing. The kinetics by which inducers and regulators of acute inflammation exert their actions is essential for understanding the pathogenesis of fish diseases and identifying potential treatments. Although, a number of these are well-conserved across, others are not, reflecting the unique physiologies and life histories of members of this unique animal group.

Characterization of T-cell receptors and immunoglobulin heavy chains loci and identification of T/B cell clusters in teleost

Bacterial disease is one of the important factors leading to economic losses in the turbot (Scophthalmus maximus) cultivation industry. T lymphocytes are major components of cellular immunity, whereas B lymphocytes produce immunoglobulins (Ig) that are key elements of humoral immune responses against infection. However, the genomic organization of genes encoding T-cell receptors (TCR) and immunoglobulin heavy chains (IgHs) in turbot remains largely unknown. In this study, abundant full-length transcripts of TCRs and IgHs were sequenced by Isoform-sequencing (Iso-seq), and we investigated and annotated the V, D, J and C gene loci of TCRα, TCRβ, IgT, IgM and IgD in turbot. Furthermore, through single-cell RNA sequencing (scRNA-seq) of blood leukocytes, we confirmed that these identified TCRs and IgHs were highly expressed in T/B cell clusters, respectively. Meanwhile, we also identified the IgM⁺IgD⁺ B and IgT⁺ B cells with differential gene expression profiles and potential functions. Taken together, our results provide a comprehensive understanding of TCRs and IgHs loci in turbot, which will contribute to evolutionary and functional characterization of T and B lymphocytes in teleost.

Protective immunity against spring viremia of carp virus by mannose modified chitosan loaded DNA vaccine

Spring viremia of carp virus (SVCV) usually be considered as one of the serious in viral diseases of aquaculture, and DNA vaccine with novel delivery mechanism or adjuvant has proven to be a promising and effective strategy to control aquatic animal diseases. In this study, the mannose-modified chitosan, a carrier system for vaccine delivery, were used to developed a chitosan-encapsulated DNA vaccine (CS-M-G) against SVCV, then investigated immune response induced by the vaccine. Our results showed that CS-M-G was confirmed the spherical or elliptical with even distribution and ranging from approximately 50 to 150 nm in size, the expression of the antigen gene could still be detected after 21 d post vaccination. The CS-M-G induces the highest antibody levels in the 20 μg dose group which is about 3 times than naked plasmid group at 21 d post vaccination, and still hold a higher level than control group at 28 d post vaccination. On the side, strongest protection with relative percent survival of 62.1% in the 20 μg CS-M-G group, which could produce significantly higher enzyme activities and up-regulated expression of immune-associated genes than control group. Thus, our results indicate that DNA vaccine loaded with mannose-modified chitosan induces strong immune response and provided an effective protection against SVCV infection, may be helpful and extended for developing more aquatic animal vaccines in the future.

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.

Structural characteristics and mucosal immune response of the interbranchial lymphoid tissue in the gills of flounder (Paralichthys olivaceus)

A specialized lymphoepithelial tissue termed the interbranchial lymphoid tissue (ILT) is recently identified in several fish species. However, the structural variation and mucosal immune functions of the ILT remain largely unknown. In this study, the anti-Zap-70 MAb was firstly determined to specifically recognize ZAP-70 protein, and CD4-1⁺, CD4-2⁺ and CD8β⁺ T-cells, but not IgM⁺ B cells, in peripheral blood leucocytes of flounder (Paralichthys olivaceus). Then we found that aggregates of Zap-70⁺ cells were located in the epithelium covering the bottom of the interbranchial cleft and along the afferent and efferent edges of the filaments in a cross view, where a meshwork of epithelial cells containing diffused lymphoid cells was exhibited, confirming these structures as the ILT; In a sagittal view, Zap-70⁺ cells were situated at the base of the filaments (here named as proximal ILT, pILT) and in the interlamellar epithelium (named as distal ILT, dILT). Also, a few IgM⁺ B cells were distributed at these sites. The lymphoepithelium within pILT and dILT was very thin with a low number of Zap-70⁺ cells in premetamorphosis and postclimax larvae of flounder, and got thicker containing much more Zap-70⁺ cells in juvenile and adult individuals. The aggregates of CD4-1⁺/Zap-70⁺, CD4-2⁺/Zap-70⁺, and CD8β⁺/Zap-70⁺ T-cell subsets were identified in the ILT. Post bath vaccination with inactivated Edwardsiella tarda and then intraperitoneal injection of EdU, the amounts of EdU⁺ and Zap-70⁺ cells obviously increased at 3 d and 7 d, and co-localization of EdU⁺/Zap-70⁺ cells identified the presence of proliferative T cells; meanwhile, MHC class II-expressing cells were increased. These findings indicated that the ILT in gills of flounder was an important site for the induction of local T cell-mediated immunity, which would lead to a better understanding of mucosal immunity and defense mechanisms of teleost fish.

Transcriptome Profiling of Atlantic Salmon (Salmo salar) Parr With Higher and Lower Pathogen Loads Following Piscirickettsia salmonis Infection

Salmonid rickettsial septicemia (SRS), caused by Piscirickettsia salmonis, is one of the most devastating diseases of salmonids. However, the transcriptomic responses of Atlantic salmon (Salmon salar) in freshwater to an EM-90-like isolate have not been explored. Here, we infected Atlantic salmon parr with an EM-90-like isolate and conducted time-course qPCR analyses of pathogen load and four biomarkers (campb, hampa, il8a, tlr5a) of innate immunity on the head kidney samples. Transcript expression of three of these genes (except hampa), as well as pathogen level, peaked at 21 days post-injection (DPI). Multivariate analyses of infected individuals at 21 DPI revealed two infection phenotypes [lower (L-SRS) and higher (H-SRS) infection level]. Five fish from each group (Control, L-SRS, and H-SRS) were selected for transcriptome profiling using a 44K salmonid microarray platform. We identified 1,636 and 3,076 differentially expressed probes (DEPs) in the L-SRS and H-SRS groups compared with the control group, respectively (FDR = 1%). Gene ontology term enrichment analyses of SRS-responsive genes revealed the activation of a large number of innate (e.g. “phagocytosis”, “defense response to bacterium”, “inflammatory response”) and adaptive (e.g. “regulation of T cell activation”, “antigen processing and presentation of exogenous antigen”) immune processes, while a small number of general physiological processes (e.g. “apoptotic process”, development and metabolism relevant) was enriched. Transcriptome results were confirmed by qPCR analyses of 42 microarray-identified transcripts. Furthermore, the comparison of individuals with differing levels of infection (H-SRS vs. L-SRS) generated insights into the biological processes possibly involved in disease resistance or susceptibility. This study demonstrated a low mortality (~30%) EM-90-like infection model and broadened the current understanding of molecular pathways underlying P. salmonis-triggered responses of Atlantic salmon, identifying biomarkers that may assist to diagnose and combat this pathogen.

On the relationship between extant innate immune receptors and the evolutionary origins of jawed vertebrate adaptive immunity

For over half a century, deciphering the origins of the genomic loci that form the jawed vertebrate adaptive immune response has been a major topic in comparative immunogenetics. Vertebrate adaptive immunity relies on an extensive and highly diverse repertoire of tandem arrays of variable (V), diversity (D), and joining (J) gene segments that recombine to produce different immunoglobulin (Ig) and T cell receptor (TCR) genes. The current consensus is that a recombination-activating gene (RAG)-like transposon invaded an exon of an ancient innate immune VJ-bearing receptor, giving rise to the extant diversity of Ig and TCR loci across jawed vertebrates. However, a model for the evolutionary relationships between extant non-recombining innate immune receptors and the V(D)J receptors of the jawed vertebrate adaptive immune system has only recently begun to come into focus. In this review, we provide an overview of non-recombining VJ genes, including CD8β, CD79b, natural cytotoxicity receptor 3 (NCR3/NKp30), putative remnants of an antigen receptor precursor (PRARPs), and the multigene family of signal-regulatory proteins (SIRPs), that play a wide range of roles in immune function. We then focus in detail on the VJ-containing novel immune-type receptors (NITRs) from ray-finned fishes, as recent work has indicated that these genes are at least 50 million years older than originally thought. We conclude by providing a conceptual model of the evolutionary origins and phylogenetic distribution of known VJ-containing innate immune receptors, highlighting opportunities for future comparative research that are empowered by this emerging evolutionary perspective.

A Bioactive Extract Rich in Triterpenic Acid and Polyphenols from Olea europaea Promotes Systemic Immunity and Protects Atlantic Salmon Smolts Against Furunculosis

In the present study, the modulation of the transcriptional immune response (microarray analysis) in the head kidney (HK) of the anadromous fish Atlantic salmon (Salmo salar) fed a diet supplemented with an olive fruit extract (AQUOLIVE^®) was evaluated. At the end of the trial (133 days), in order to investigate the immunomodulatory properties of the phytogenic tested against a bacterial infection, an in vivo challenge with Aeromonas salmonicida was performed. A total number of 1,027 differentially expressed genes (DEGs) (805 up- and 222 downregulated) were found when comparing the transcriptomic profiling of the HK from fish fed the control and AQUOLIVE^® diets. The HK transcripteractome revealed an expression profile that mainly favored biological processes related to immunity. Particularly, the signaling of i-kappa B kinase/NF-kappa and the activation of leukocytes, such as granulocytes and neutrophils degranulation, were suggested to be the primary actors of the innate immune response promoted by the tested functional feed additive in the HK. Moreover, the bacterial challenge with A. salmonicida that lasted 12 days showed that the cumulative survival was higher in fish fed the AQUOLIVE^® diet (96.9 ± 6.4%) than the control group (60.7 ± 13.5%). These results indicate that the dietary supplementation of AQUOLIVE^® at the level of 0.15% enhanced the systemic immune response and reduced the A. salmonicida cumulative mortality in Atlantic salmon smolts.

Kinetics of CD4-1+ lymphocytes in brown trout after exposure to viral haemorrhagic septicaemia virus

T-helper cells express CD4 as a co-receptor that binds to major histocompatibility complex class II to synchronize the immune response against upcoming threats via mediating several cytokines. We have previously reported the presence of CD4 homologues in brown trout. The study of cellular immune responses in brown trout is limited by the availability of specific antibodies. We here describe the generation of a polyclonal antibody against CD4-1 that allows for the investigation of CD4⁺ cells. We used this novel tool to study CD4⁺ cells in different tissues during viral haemorrhagic septicaemia infection (VHSV) using flow cytometric technique. Flow cytometric analyses revealed an enhanced level of surface CD4-1 expression in the infected group in major lymphoid organs and in the intestine. These results suggest an important role for the T-helper cells within the immune response against viruses, comparable to the immune response in higher vertebrates.

Integrated analysis of mRNA and long non-coding RNA expression profiles reveals the potential roles of lncRNA-mRNA network in carp macrophage immune regulation

Long non-coding RNAs (lncRNAs) have emerged as a hot topic in research as mounting evidence has indicated their transcriptional or post-transcriptional regulatory potential in multiple biological processes. Previous studies have revealed the involvement of lncRNAs in the immunoregulation of mammalian macrophages by changing mRNA expression; however, studies on the lncRNAs in fish macrophages and their potential roles in the immune system remain unknown. Primary macrophages were isolated from the head kidney (HK) of red common carp (Cyprinus carpio) and high-throughput lncRNA-mRNA sequencing was performed using the Illumina HiSeq platform. The results revealed that the most highly expressed mRNAs in primary HK macrophages were mainly involved in immune-related signal pathways. Furthermore, the most enriched immune-related GO term and KEGG pathway of the mRNAs were "immune system development" and "chemokine signaling pathway," respectively. A total of 20,333 lncRNAs, composed of 10,512 known and 9821 novel lncRNAs, were identified, and functional enrichment analysis of the lncRNA-mRNA network indicated that the expressed lncRNAs in primary HK macrophages could be associated with the regulation of multiple immune-related signaling pathways. In addition, the expressions of several selected lncRNAs and their related mRNAs were determined in carp macrophages following a 6-h exposure to lipopolysaccharide (LPS) and Poly(I: C), the results of which confirmed the co-expression regulation of lncRNAs and target mRNAs in the immune response of carp macrophages. These results suggest the correlative of the lncRNA-mRNA network in fish macrophage immune response, which may further affect the cross-talk of various signaling pathways by interaction with other network genes. Here, we provided fundamental data about the transcriptome profiles of primary HK macrophages from red common carp by analysis of the lncRNA-mRNA network, and ultimately suggest the potential roles of lncRNA-mRNA networks in immune regulation in teleost fish.

Teleost CD4+ helper T cells: Molecular characteristics and functions and comparison with mammalian counterparts

CD4⁺ helper T cells play key and diverse roles in inducing adaptive immune responses in vertebrates. The CD4 molecule, which is found on the surfaces of CD4⁺ helper T cells, can be used to distinguish subsets of helper T cells. Teleosts are the oldest living species with bona-fide CD4 coreceptors. Although some components of immune systems of teleosts and mammals appear to be similar, many physiological differences are represented between them. Previous studies have shown that two CD4 paralogs are present in teleosts, whereas only one is present in mammals. Therefore, in this review, the CD4 molecular structure, expression profiles, subpopulations, and biological functions of teleost CD4⁺ helper T cells were summarized and compared with those of their mammalian counterparts to understand the differences in CD4 molecules between teleosts and mammals. This review provides suggestions for further studies on the CD4 molecular function and regulatory mechanism of CD4⁺ helper T cells in teleost fish and will help establish therapeutic strategies to control fish diseases in the future.

Evolution of Cellular Immunity Effector Cells; Perspective on Cytotoxic and Phagocytic Cellular Lineages

The immune system has evolved to protect organisms from infections caused by bacteria, viruses, and parasitic pathogens. In addition, it provides regenerative capacities, tissue maintenance, and self/non-self recognition of foreign tissues. Phagocytosis and cytotoxicity are two prominent cellular immune activities positioned at the base of immune effector function in mammals. Although these immune mechanisms have diversified into a wide heterogeneous repertoire of effector cells, it appears that they share some common cellular and molecular features in all animals, but also some interesting convergent mechanisms. In this review, we will explore the current knowledge about the evolution of phagocytic and cytotoxic immune lineages against pathogens, in the clearance of damaged cells, for regeneration, for histocompatibility recognition, and in killing virally infected cells. To this end, we give different immune examples of multicellular organism models, ranging from the roots of bilateral organisms to chordate invertebrates, comparing to vertebrates' lineages. In this review, we compare cellular lineage homologies at the cellular and molecular levels. We aim to highlight and discuss the diverse function plasticity within the evolved immune effector cells, and even suggest the costs and benefits that it may imply for organisms with the meaning of greater defense against pathogens but less ability to regenerate damaged tissues and organs.

Phytogenic Bioactive Compounds Shape Fish Mucosal Immunity

Aquaculture growth will unavoidably involve the implementation of innovative and sustainable production strategies, being functional feeds among the most promising ones. A wide spectrum of phytogenics, particularly those containing terpenes and organosulfur compounds, are increasingly studied in aquafeeds, due to their growth promoting, antimicrobial, immunostimulant, antioxidant, anti-inflammatory and sedative properties. This trend relies on the importance of the mucosal barrier in the fish defense. Establishing the phytogenics' mode of action in mucosal tissues is of importance for further use and safe administration. Although the impact of phytogenics upon fish mucosal immunity has been extensively approached, most of the studies fail in addressing the mechanisms underlying their pharmacological effects. Unstandardized testing as an extended practice also questions the reproducibility and safety of such studies, limiting the use of phytogenics at commercial scale. The information presented herein provides insight on the fish mucosal immune responses to phytogenics, suggesting their mode of action, and ultimately encouraging the practice of reliable and reproducible research for novel feed additives for aquafeeds. For proper screening, characterization and optimization of their mode of action, we encourage the evaluation of purified compounds using in vitro systems before moving forward to in vivo trials. The formulation of additives with combinations of compounds previously characterized is recommended to avoid bacterial resistance. To improve the delivery of phytogenics and overcome limitations associated to compounds volatility and susceptibility to degradation, the use of encapsulation is advisable. Besides, newer approaches and dedicated methodologies are needed to elucidate the phytogenics pharmacokinetics and mode of action in depth.

Tipping the Scales With Zebrafish to Understand Adaptive Tumor Immunity

The future of improved immunotherapy against cancer depends on an in-depth understanding of the dynamic interactions between the immune system and tumors. Over the past two decades, the zebrafish has served as a valuable model system to provide fresh insights into both the development of the immune system and the etiologies of many different cancers. This well-established foundation of knowledge combined with the imaging and genetic capacities of the zebrafish provides a new frontier in cancer immunology research. In this review, we provide an overview of the development of the zebrafish immune system along with a side-by-side comparison of its human counterpart. We then introduce components of the adaptive immune system with a focus on their roles in the tumor microenvironment (TME) of teleosts. In addition, we summarize zebrafish models developed for the study of cancer and adaptive immunity along with other available tools and technology afforded by this experimental system. Finally, we discuss some recent research conducted using the zebrafish to investigate adaptive immune cell-tumor interactions. Without a doubt, the zebrafish will arise as one of the driving forces to help expand the knowledge of tumor immunity and facilitate the development of improved anti-cancer immunotherapy in the foreseeable future.

Poly (I:C)-Potentiated Vaccination Enhances T Cell Response in Olive Flounder (Paralichthys olivaceus) Providing Protection against Viral Hemorrhagic Septicemia Virus (VHSV)

Viral hemorrhagic septicemia (VHS), caused by viral hemorrhagic septicemia virus (VHSV), is a viral disease affecting teleosts, and is the major cause of virus-related deaths in olive flounder (Paralichthys olivaceus). Research has focused on ways to control VHS, and recently, the use of polyinosinic-polycytidylic acid poly (I:C)-potentiated vaccination has been investigated, whereby fish are injected with poly (I:C) and then with live pathogenic virus, resulting in a significant decrease in VHSV-related mortality. T cell responses were investigated in the present study after vaccinating olive flounder with poly (I:C)-potentiated vaccination to understand the ability of poly (I:C) to induce T cell immunity. Stimulation of T cell responses with the poly (I:C)-potentiated vaccination was confirmed by examining levels of CD3+ T cells, CD4-1+ T cells and CD4-2+ T cells. Higher levels of CD4-2+ T cells were found in vaccinated fish than CD4-1+ T cells, believed to result from a synergistic effect between poly (I:C) administration and pathogenic VHSV immunization. More importantly, the role of CD4-2+ T cells in the antiviral response was clearly evident. The results of this study suggest that the outstanding protection obtained with the poly (I:C)-potentiated vaccination is due to the robust immune response initiated by the CD4-2+ T cells.

Adaptive immune responses in channel catfish exposed to Edwardsiella ictaluri live attenuated vaccine and wild type strains through the specific gene expression profiles

We extend the previous findings on the differential activity of immune-related genes in the lymphoid organs of channel catfish in the 7 days post-challenge (dpc) with E. ictaluri live attenuated vaccines (LAVs) and wild type (WT) strains by assessing the expression of these genes in the 21 dpc. The expression of T and B cell-specific genes were significantly elevated in the spleen at 14 dpc and in the AK at 21 dpc in catfish treated with E. ictaluri WT and LAV strains compared to a non-treated control group. The gene expression of IFN-γ correlated with adaptive immunity genes in the lymphoid tissues of catfish. These data indicate that two novel LAVs were able to trigger the activation of T helper1 polarization cytokine IFN-γ gene and specific lymphocyte genes in the spleen followed by their activation in the AK of catfish without causing inflammation, thus providing protective immunity in E. ictaluri infection.

Elucidating the Functional Roles of Helper and Cytotoxic T Cells in the Cell-Mediated Immune Responses of Olive Flounder (Paralichthys olivaceus)

In higher vertebrates, helper and cytotoxic T cells, referred to as CD4 and CD8 T lymphocytes, respectively, are mainly associated with adaptive immunity. The adaptive immune system in teleosts involves T cells equivalent to those found in mammals. We previously generated monoclonal antibodies (mAbs) against olive flounder (Paralichthys olivaceus) CD4 T cells, CD4-1 and CD4-2, and used these to describe the olive flounder’s CD4 Tcell response during a viral infection. In the present study, we successfully produced mAbs against CD8 T lymphocytes and their specificities were confirmed using immuno-blotting, immunofluorescence staining, flow cytometry analysis andreverse transcription polymerase chain reaction (RT-PCR). The results showed that these mAbs are specific for CD8 T lymphocytes. We also investigated variations in CD4 and CD8 T cells populations, and analyzed the expression of immune-related genes expressed by these cells in fish infected with nervous necrosis virus or immunized with thymus dependent and independent antigens. We found that both CD4 and CD8 T lymphocyte populations significantly increased in these fish and Th1-related genes were up-regulated compared to the control group. Collectively, these findings suggest that the CD4 and CD8 T lymphocytes in olive flounder are similar to the helper and cytotoxic T cells found in mammals, and Th1 and cytotoxic immune responses are primarily involved in the early adaptive immune response against extracellular antigens.

Molecular and cellular characterization of European sea bass CD3ε+ T lymphocytes and their modulation by microalgal feed supplementation

The CD3 coreceptor is a master T cell surface marker, and genes encoding CD3ζ, γδ, and ε chains have been reported in several teleost fish. Here, a complete cDNA sequence of CD3ɛ chain was identified from a sea bass (Dicentrarchus labrax L.) gill transcriptome. Its basal expression was quantified in both lymphoid and non-lymphoid organs of sea bass juveniles with real-time qPCR analysis. After either in vitro stimulation of head kidney leukocytes with the T-cell mitogen phytohaemagglutinin or in vivo stimulation with an orally administered Vibrio anguillarum vaccine, CD3ε expression levels increased in head kidney leukocytes, confirming that CD3ε T cells may play important roles in fish systemic protection against pathogens. Further, three peptides were designed on the CD3ɛ cytoplasmic tail region and employed as immunogens for antibody production in rabbit. One antiserum so obtained, named RACD3/1, immunostained a band of the expected size in a western blot of a sea bass thymocyte lysate. The distribution of CD3ε⁺ lymphocyte population in the lymphoid organs and mucosal tissues was addressed in healthy fish by IHC. In decreasing percentage order, CD3ε⁺ lymphocytes were detected by flow cytometry in thymus, peripheral blood leukocytes, gills, head kidney, gut, and spleen. Finally, a significant in vivo enhancement of CD3ε⁺ T intestinal lymphocytes was found in fish fed on diets in which 100% fish meal was replaced by the microalgae Nannochloropsis sp. biomass. These results indicate that CD3ε⁺ T cells are involved in nutritional immune responses.

Immunogenicity and protective efficacy of Yersinia ruckeri lipopolysaccharide (LPS), encapsulated by alginate-chitosan micro/nanoparticles in rainbow trout (Oncorhyncus mykiss)

Considering the many advantages of oral vaccines in aquaculture, several studies have been conducted in this area recently. In this study, immunization and protective power of the oral vaccine of Yersinia ruckeri encapsulated with Alginate-Chitosan micro/nanoparticles were evaluated in rainbow trout. For this purpose, 720 juvenile rainbow trout (9 ± 1.8 g) were divided into 8 groups in three replications (30 fish each) as follows: Groups A, B and C, were immunized with Yersinia ruckeri lipopolysaccharide (LPS), LPS+Formalin Killed Cells (FKC) and FKC alone, groups D, E, and F were immunized with encapsulated LPS, LPS+FKC and FKC, respectively. The G and H groups considered as encapsulated and non-encapsulated control, respectively. Micro/nanoencapsulation with alginate-chitosan was performed by internal emulsification method and vaccination were conductrd in the first and third weeks via oral route. Sampling was performed on days 0, 30, and 60 of experiment. Anti Y. ruckeri antibody titer in serum, intestine and skin mucus were measured via ELISA method. Non-specific immune response including: serum lysozyme, complement, bactericidal and respiratory burst activity, serum protein and globulin level, as well as white blood cell count were compared among the groups. The expression of IgT gene in the intestine and TCR gene in the anterior kidney were also investigated. At the end of the study, the fish were challenged with Y. ruckeri through immerssion and intraperitoneal routs and the relative survival rate was evaluated. Result showed that the antibody level in serum, skin and intestine was significantly higher in group E and F than control groups (P < 0.05), meanwhile serum, skin and intestine antibody level in all vaccinated groups were significantly (P < 0.01) higher in day 30 and 60 compare to zero day. Non-specific immunity factors including: serum lysozyme, complement, and respiratory burst activity as well as WBC, protein and Globulin level were significantly higher in E and F groups not only in day 30 but also in day 60 of experiment (P < 0.05). Cumulative mortality following injection and bath challenge were significantly (P = 0.004) lower (35%-45%) in groups E and F compare to control group (80%). The IgT and TCR gene expression in groups D, E and F were significantly higher (P < 0.05) than control group. Highest upregulation of IgT and TCR gene expression in vaccinated groups were seen at day 30 and 60 respectively which were significantly (P < 0.001) higher than day zero. Generally, it can be concluded that nano/micronanoencapsulation of Y. ruckeri FKC+LPS with chitosan-alginate, not only increases protective efficacy of oral vaccine, but improves specific and non-specific immune responses in rainbow trout.

Extreme genomic volatility characterizes the evolution of the immunoglobulin heavy chain locus in cyprinodontiform fishes

The evolution of the adaptive immune system has provided vertebrates with a uniquely sophisticated immune toolkit, enabling them to mount precise immune responses against a staggeringly diverse range of antigens. Like other vertebrates, teleost fishes possess a complex and functional adaptive immune system; however, our knowledge of the complex antigen-receptor genes underlying its functionality has been restricted to a small number of experimental and agricultural species, preventing systematic investigation into how these crucial gene loci evolve. Here, we analyse the genomic structure of the immunoglobulin heavy chain (IGH) gene loci in the cyprinodontiforms, a diverse and important group of teleosts present in many different habitats across the world. We reconstruct the complete IGH loci of the turquoise killifish (Nothobranchius furzeri) and the southern platyfish (Xiphophorus maculatus) and analyse their in vivo gene expression, revealing the presence of species-specific splice isoforms of transmembrane IGHM. We further characterize the IGH constant regions of 10 additional cyprinodontiform species, including guppy, Amazon molly, mummichog and mangrove killifish. Phylogenetic analysis of these constant regions suggests multiple independent rounds of duplication and deletion of the teleost-specific antibody class IGHZ in the cyprinodontiform lineage, demonstrating the extreme volatility of IGH evolution. Focusing on the cyprinodontiforms as a model taxon for comparative evolutionary immunology, this work provides novel genomic resources for studying adaptive immunity and sheds light on the evolutionary history of the adaptive immune system.

Effect of the complete replacement of dietary fish meal by soybean meal on histopathology and immune response of the hindgut in grass carp (Ctenopharyngodon idellus)

A 14-day experiment was conducted to explore the pathological process and immune response of soybean meal (SBM) induced enteritis (SBMIE) in grass carp (Ctenopharyngodon idellus). The complete replacement of dietary fish meal (FM) with SBM resulted in a remarkable reduction in final body weight, weight gain ratio, and feed conversion efficiency (p < 0.05). The typical histopathological changes of SBMIE appeared starting at day 4, and progressively increased in severity until day 8, then gradually subsided after day 11. The course of SBMIE could be divided into incubation period (days 1-2), prodromal period (days 3-6), symptomatic period (days 7-10), and convalescent period (days 11-14). Transcription levels of pro-inflammatory cytokines, including IL-1β, TNF-α, IL-6, IL-8, IL-17A/F1 and IFN-γ2, were up-regulated during the prodromal period, and then down-regulated during the convalescent period. Transcript levels of anti-inflammatory cytokines (IL-10 and TGFβ1) and their receptors (IL-10R1 and TβRII), were up-regulated during the prodromal and convalescent periods. Transcript levels of MHCIIβ, Igμ, Igτ, TCRδ, TCRβ, CD4, and CD8α were altered in SBMIE. Furthermore, expression levels of T-bet, IFN-γ2, RORγ2 and IL-17A/F1 were significantly increased in the initiation of enteritis, whereas the transcript levels of Foxp3 and IL-2/15Ra were significantly up-regulated in the repair of enteritis. In conclusion, grass carp SBMIE is regulated by the adjustment of SBM-based diet intake, and the changes of the above-mentioned genes expression suggest that these genes may be involved in SBMIE.

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.

T and B lymphocytes immune responses in flounder (Paralichthys olivaceus) induced by two forms of outer membrane protein K from Vibrio anguillarum: Subunit vaccine and DNA vaccine

To further elucidate the roles of T and B lymphocytes in fish subunit and DNA candidate vaccines for immunisation, the immune responses of T and B lymphocytes to recombinant protein (rOmpK) and plasmid OmpK (pOmpK) from Vibrio anguillarum plus cyclosporine A (CsA) were investigated in flounder (Paralichthys olivaceus). The results showed that in the rOmpK-immunised groups, the percentages of CD4-1⁺ and CD4-2⁺ T (PCD4-1⁺ and PCD4-2⁺ T) lymphocytes significantly increased to a peak on days 5 or 7. The percentages of IgM⁺ B (PIgM⁺ B) lymphocytes and specific antibodies markedly increased to a peak at weeks 4 or 5. The nine immune-related genes were significantly up-regulated and the expression levels of CD4-1, CD4-2 and MHC II genes were higher than that of CD8α, CD8β and MHC I genes. The CD4⁺ T lymphocytes, IgM⁺ B lymphocytes, and specific antibodies were significantly inhibited by CsA. Therefore, the responses of CD4⁺ T lymphocytes influenced the responses of the B lymphocytes and antibodies. In the pOmpK-immunised groups, the PCD4-1⁺, PCD4-2⁺, and PCD8β⁺ T lymphocytes significantly increased to a peak on days 11 or 14, days 9 or 11, and days 7 or 9, respectively. The PIgM⁺ B lymphocytes and specific antibodies significantly increased to a peak at weeks 5 or 6. Immune related genes upregulated, and CD4⁺ and CD8⁺ T lymphocytes, IgM⁺ B lymphocytes and specific antibodies all suppressed by CsA, suggesting that the responses of T lymphocytes subpopulations influenced B lymphocytes and antibodies responses. Therefore, the subpopulations of T lymphocytes played an important role in the immune responses induced by subunit and DNA candidate vaccines of OmpK and regulated the immune responses of B lymphocytes in flounder.

Teleost cytotoxic T cells

Cell-mediated cytotoxicity is one of the major mechanisms by which vertebrates control intracellular pathogens. Two cell types are the main players in this immune response, natural killer (NK) cells and cytotoxic T lymphocytes (CTL). While NK cells recognize altered target cells in a relatively unspecific manner CTLs use their T cell receptor to identify pathogen-specific peptides that are presented by major histocompatibility (MHC) class I molecules on the surface of infected cells. However, several other signals are needed to regulate cell-mediated cytotoxicity involving a complex network of cytokine- and ligand-receptor interactions. Since the first description of MHC class I molecules in teleosts during the early 90s of the last century a remarkable amount of information on teleost immune responses has been published. The corresponding studies describe teleost cells and molecules that are involved in CTL responses of higher vertebrates. These studies are backed by functional investigations on the killing activity of CTLs in a few teleost species. The present knowledge on teleost CTLs still leaves considerable room for further investigations on the mechanisms by which CTLs act. Nevertheless the information on teleost CTLs and their regulation might already be useful for the control of fish diseases by designing efficient vaccines against such diseases where CTL responses are known to be decisive for the elimination of the corresponding pathogen. This review summarizes the present knowledge on CTL regulation and functions in teleosts. In a special chapter, the role of CTLs in vaccination is discussed.

A Comparison of the Innate and Adaptive Immune Systems in Cartilaginous Fish, Ray-Finned Fish, and Lobe-Finned Fish

The immune system is composed of two subsystems-the innate immune system and the adaptive immune system. The innate immune system is the first to respond to pathogens and does not retain memory of previous responses. Innate immune responses are evolutionarily older than adaptive responses and elements of innate immunity can be found in all multicellular organisms. If a pathogen persists, the adaptive immune system will engage the pathogen with specificity and memory. Several components of the adaptive system including immunoglobulins (Igs), T cell receptors (TCR), and major histocompatibility complex (MHC), are assumed to have arisen in the first jawed vertebrates-the Gnathostomata. This review will discuss and compare components of both the innate and adaptive immune systems in Gnathostomes, particularly in Chondrichthyes (cartilaginous fish) and in Osteichthyes [bony fish: the Actinopterygii (ray-finned fish) and the Sarcopterygii (lobe-finned fish)]. While many elements of both the innate and adaptive immune systems are conserved within these species and with higher level vertebrates, some elements have marked differences. Components of the innate immune system covered here include physical barriers, such as the skin and gastrointestinal tract, cellular components, such as pattern recognition receptors and immune cells including macrophages and neutrophils, and humoral components, such as the complement system. Components of the adaptive system covered include the fundamental cells and molecules of adaptive immunity: B lymphocytes (B cells), T lymphocytes (T cells), immunoglobulins (Igs), and major histocompatibility complex (MHC). Comparative studies in fish such as those discussed here are essential for developing a comprehensive understanding of the evolution of the immune system.

Immune gene expression in gilthead seabream (Sparus aurata) after Lymphocystis disease virus (LCDV-Sa) challenge resulting in asymptomatic infection

AIMS

To determine the immune gene expression response of gilthead seabream (Sparus aurata) that is experimentally infected with the lymphocystivirus LCDV-Sa.

METHODS AND RESULTS

Viral DNA and transcripts were detected by qPCR in all samples from fish injected with LCDV-Sa, demonstrating that the virus establish a systemic and asymptomatic infection. The expression of 23 immune-related genes was also analysed by RT-qPCR in the head kidney (HK) and intestine at several times post-infection (dpi). In HK, the expression of five type I interferon (IFN)-related genes (ifn, irf3, mx2, mx3 and isg15), il10 and ck10 was upregulated at 1-3 dpi, while genes related to the inflammation process (tnfα, il1ß, il6, casp1) were not differentially expressed or even downregulated. The expression profile in the intestine was different regarding type I INF-related genes. An upregulated c3 and ighm expression was observed in both HK and intestine at 3-8 dpi. Finally, the transcription of nccrp1 and mhcIIα was induced in HK, whereas tcrβ expression was downregulated in both organs.

CONCLUSIONS

LCDV-Sa seems to trigger an immune response in gilthead seabream characterized by a partial activation of type I IFN system and a lack of systemic inflammatory response which may be related to viral persistence.

SIGNIFICANCE AND IMPACT OF THE STUDY

The immune response observed in gilthead seabream infected by LCDV-Sa could be implicated in the establishment of an asymptomatic persistent infection.

Analysis of immunoglobulin and T cell receptor gene expression in ballan wrasse (Labrus bergylta) revealed an extraordinarily high IgM expression in the gut

The serum IgM concentration of ballan wrasse is relatively high, estimated to approximately 13 mg/ml in adult wild fish of 800 g. The present study revealed an unusual high abundance of IgM mRNA in the gut of ballan wrasse. Initially, transcripts encoding IgM, IgT, IgD, TCRα, TCRδ and CD3ε were quantified by RT-qPCR in several tissues of wild caught fish (approx. 800 g), indicating an elevated immune activity in hindgut and an extraordinarily high expression of IgM. Subsequently, a new RT-qPCR analysis was performed on the entire intestine, cut into four different segments, of reared fish (32-100 g). The analysis indicated immune activity along the entire intestine, but not as strong as in the hindgut. Furthermore, similar to the larger fish, the relative abundance of IgM transcripts was higher in the hindgut than in kidney and spleen, although the absolute level of IgM was in general higher in the larger fish. The secreted form of IgM was completely dominant in comparison to the membrane bound form of IgM and the other analysed genes. IgM was purified from gut mucus and external mucosal surfaces by magnetic beads coated with protein A. Mucus IgM reacted with rabbit antisera raised against serum IgM and contained subunits of the same size. Regarding the elevated immune activity in the intestine it is tempting to speculate on a possible compensatory strategy in this lineage of stomach-less fish, and that natural antibodies have an important role in the first line defence.

T cell receptor (TCR) α and β genes of loach (Misgurnus anguillicaudatus): Molecular cloning and expression analysis in response to bacterial, parasitic and fungal challenges

In vertebrates, the T cell receptor (TCR) plays a crucial role in immune system. To date, the roles of fish TCRs in response to pathogen infection are still poorly understood. In the present study, we firstly cloned and identified the TCRα and TCRβ from dojo loach (Misgurnus anguillicaudatus) by RACE approaches. The full-length cDNAs of Ma-TCRα and Ma-TCRβ include an open reading frame (ORF) of 723 and 879 bp encoding a polypeptide of 241 and 293 amino acids, respectively. Structural analysis indicated that Ma-TCRα and Ma-TCRβ had a signal peptide, IgV domain, IgC domain, a connecting peptide (CPS), a transmembrane region (TM) and a cytoplasmic (CYT), which are similar to their counterparts described in other teleost. Phylogenetic analysis supported that Ma-TCR Cα and Ma-TCR Cβ were closely related to the Cα and Cβ region of Cyprinidae family, respectively. Transcriptional expression analysis indicated that Ma-TCRα and Ma-TCRβ mRNAs were ubiquitously expressed in a wide array of tissues and most abundantly found in skin, brain, kidney, gill and spleen. The expression patterns of Ma-TCRα and Ma-TCRβ after bacteria (F. columnare G₄), parasite (Ichthyophthirius multifiliis) and fungus (Saprolegnia) infection were detected by qRT-PCR. Additionally, the morphological changes of gill and skin following the three infection models were investigated. The results clearly indicated that Ma-TCRα and Ma-TCRβ was significant up-regulated not only in spleen and kidney, but also in skin and gill. In summary, our present findings suggested that Ma-TCRα and Ma-TCRβ might play significantly roles in the modulation of immune response and protect loach from different pathogens infection.

Characterization of sickness behavior in zebrafish

In a previous study we showed a clear relationship between immune system and behavior in zebrafish and we hypothesized that the immune system is capable of inducing behavioral changes. To further investigate this subject and to address our main question, here we induced an inflammatory response in one group of fish by the inoculation of formalin-inactivated Aeromonoas hydrophila bacterin and compared their social and exploratory behavior with control groups. After the behavioral tests, we also analyzed the expression of cytokines genes and markers of neuronal activity in fish brain. In the bacterin-inoculated fish, the locomotor activity, social preference and exploratory behavior towards a new object were reduced compared to the control fish while the expression of proinflammatory cytokines in the brain was upregulated. With this study we demonstrated for the first time that the immune system is capable of causing behavioral changes that are consistent with the sickness behavior observed in mammals.

Hints on T cell responses in a fish-parasite model: Enteromyxum leei induces differential expression of T cell signature molecules depending on the organ and the infection status

BACKGROUD

Enteromyxum leei is a myxozoan parasite that produces a slow-progressing intestinal disease. This parasite invades the paracellular space of the intestinal epithelium and progresses from the posterior to the anterior intestine. The aim of the present study was to gain insights into fish T cell responses in the gilthead sea bream-E. leei infection model using a PCR-array with 30 signature molecules for different leukocyte responses in head kidney, spleen, anterior and posterior intestine.

RESULTS

The PCR-array results suggest that E. leei induced migration of T cells from head kidney to intestines where T_H1, CTL and T_H17 profiles were activated and kept in balance by the upregulation of regulatory cytokines. These results were partially validated by the use of cross-reacting antibodies and BrdU immunostaining to monitor proliferation. Zap70 immunostaining supported the increased number of T cells in the anterior intestine detected by gene expression, but double staining with BrdU did not show active proliferation of this cell type at a local level, supporting the migration from lymphohaematopoietic tissues to the site of infection. Global analyses of the expression profiles revealed a clear separation between infected and exposed, but non-infected fish, more evident in the target organ. Exposed, non-infected animals showed an intermediate phenotype closer to the control fish.

CONCLUSIONS

These results evidence a clear modulation of the T cell response of gilthead sea bream upon E. leei infection. The effects occurred both at local and systemic levels, but the response was stronger and more specific at the site of infection, the intestine. Altogether, this research poses a promising basis to understand the response against this important parasite and establish effective preventive or palliative measures.

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.

Omics in fish mucosal immunity

The mucosal immune system of fish is a complex network of immune cells and molecules that are constantly surveilling the environment and protecting the host from infection. A number of "omics" tools are now available and utilized to understand the complexity of mucosal immune systems in non-traditional animal models. This review summarizes recent advances in the implementation of "omics" tools pertaining to the four mucosa-associated lymphoid tissues in teleosts. Genomics, transcriptomics, proteomics, and "omics" in microbiome research require interdisciplinary collaboration and careful experimental design. The data-rich datasets generated are proving really useful at discovering new innate immune players in fish mucosal secretions, identifying novel markers of specific mucosal immune responses, unraveling the diversity of the B and T cell repertoires and characterizing the diversity of the microbial communities present in teleost mucosal surfaces. Bioinformatics, data analysis and storage platforms should be developed to facilitate rapid processing of large datasets, especially when mammalian tools such as bioinformatics analysis software are not available in fishes.

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.

Characterizations of CD4-1, CD4-2 and CD8β T cell subpopulations in peripheral blood leucocytes, spleen and head kidney of Japanese flounder (Paralichthys olivaceus)

In the previous study, antibodies against CD3 molecule have been produced and were used in labeling T cells in Japanese flounder (Paralichthys olivaceus). In this paper, CD4⁺ and CD8⁺ lymphocytes subpopulations in peripheral blood leucocytes (PBL), spleen and head kidney of flounder were investigated. The flounder CD4-1, CD4-2 and CD8β recombinant proteins and their antibodies (Abs) were produced, then the cross-reactivity of the Abs to CD4-1, CD4-2 and CD8β was detected by Western blotting, respectively, and the reactions of Abs to PBL were analyzed by immunofluorescence staining (IFS) and flow cytometry (FCM). CD4-1⁺/CD3⁺, CD4-2⁺/CD3⁺, and CD8β⁺/CD3⁺ lymphocytes in PBL, spleen and head kidney were observed by double IFS, then their proportions were analyzed using two-color FCM, respectively. Further, CD4-1/CD8β, CD4-2/CD8β, or CD4-1/CD4-2 lymphocytes were analyzed using double-IFS and two-color FCM. Finally, CD4-1⁺, CD4-2⁺, and CD8β⁺ lymphocytes in spleen and head kidney were observed by immunohistochemistry. The results showed that the Abs were specific for CD4-1, CD4-2 and CD8β molecules, respectively. The proportions of CD4-1⁺/CD3⁺, CD4-2⁺/CD3⁺, and CD8β⁺/CD3⁺ lymphocytes were 6.7±2.0%, 8.6±2.8%, 2.1±1.3% in PBL; 13.6±3.6%, 15.6±5.2%, 2.8±1.4% in spleen; 20.0±4.6%, 20.5±4.6%, 3.2±1.5% in head kidney, respectively. Most CD4⁺ and CD8⁺ cell subpopulations belonged to CD3⁺ cells; there were no cross-reactivity between CD4⁺ and CD8⁺ cells. CD4-1⁺/CD4-2^-, CD4-1^-/CD4-2⁺, and CD4-1⁺/CD4-2⁺ cells presented different proportions in PBL, spleen and head kidney, among them, CD4-1⁺/CD4-2⁺ cell is the majority of CD4T cell subpopulation.

The expressed TCRβ CDR3 repertoire is dominated by conserved DNA sequences in channel catfish

We analyzed by high-throughput sequencing T cell receptor beta CDR3 repertoires expressed by αβ T cells in outbred channel catfish before and after an immunizing infection with the parasitic protozoan Ichthyophthirius multifiliis. We compared CDR3 repertoires in caudal fin before infection and at three weeks after infection, and in skin, PBL, spleen and head kidney at seven and twenty-one weeks after infection. Public clonotypes with the same CDR3 amino acid sequence were expressed by αβ T cells that underwent clonal expansion following development of immunity. These clonally expanded αβ T cells were primarily located in spleen and skin, which is a site of infection. Although multiple DNA sequences were expected to code for each public clonotype, each public clonotype was predominately coded by an identical CDR3 DNA sequence in combination with the same J gene in all fish. The processes underlying this shared use of CDR3 DNA sequences are not clear.

CD4-Transgenic Zebrafish Reveal Tissue-Resident Th2- and Regulatory T Cell-like Populations and Diverse Mononuclear Phagocytes

CD4+ T cells are at the nexus of the innate and adaptive arms of the immune system. However, little is known about the evolutionary history of CD4+ T cells, and it is unclear whether their differentiation into specialized subsets is conserved in early vertebrates. In this study, we have created transgenic zebrafish with vibrantly labeled CD4+ cells allowing us to scrutinize the development and specialization of teleost CD4+ leukocytes in vivo. We provide further evidence that CD4+ macrophages have an ancient origin and had already emerged in bony fish. We demonstrate the utility of this zebrafish resource for interrogating the complex behavior of immune cells at cellular resolution by the imaging of intimate contacts between teleost CD4+ T cells and mononuclear phagocytes. Most importantly, we reveal the conserved subspecialization of teleost CD4+ T cells in vivo. We demonstrate that the ancient and specialized tissues of the gills contain a resident population of il-4/13b-expressing Th2-like cells, which do not coexpress il-4/13a Additionally, we identify a contrasting population of regulatory T cell-like cells resident in the zebrafish gut mucosa, in marked similarity to that found in the intestine of mammals. Finally, we show that, as in mammals, zebrafish CD4+ T cells will infiltrate melanoma tumors and obtain a phenotype consistent with a type 2 immune microenvironment. We anticipate that this unique resource will prove invaluable for future investigation of T cell function in biomedical research, the development of vaccination and health management in aquaculture, and for further research into the evolution of adaptive immunity.

Ontogenetic onset of immune-relevant genes in the common sole (Solea solea)

Fish are free-living organisms since initial stages of development and are exposed to numerous pathogens before their lymphoid organs have matured and adaptive immunity has developed. Susceptibility to diseases and juvenile mortality represent key critical factors for aquaculture. In this context, the characterization of the appearance kinetics of the immune system key members will be useful in understanding the ability of a particular species in generating immune protection against invading pathogens at different developmental stages. The present study characterized, for the first time, the transcriptional onset of un-explored relevant genes of both innate and adaptive immune system during the Solea solea ontogenesis. Gene expression profiles of immune relevant genes was investigated, by means of DNA microarray, in ten developmental stages, from hatching (1 day post-hatching, dph) to accomplishment of the juvenile form (33 dph). The obtained results revealed that transcripts encoding relevant members of innate immune repertoire, such as lysozyme, AMPs (hepcidin, β-defensin), PPRs and complement components are generally characterized by high expression levels at first stages (i.e. hatch and first feeding) indicating protection from environmental pathogens even at early development. Transcription of adaptive immune genes (i.e. Class I and class II MHC, TCRs) differs from that of the innate immune system. Their onset coincides with metamorphosis and larvae-to-juvenile transition, and likely overlaps with the appearance and maturation of the main lymphoid organs. Finally, data collected suggest that at the end of metamorphosis S. solea cell-mediated immune system hasn't still undergone full maturation.

Novel Teleost CD4-Bearing Cell Populations Provide Insights into the Evolutionary Origins and Primordial Roles of CD4+ Lymphocytes and CD4+ Macrophages

Tetrapods contain a single CD4 coreceptor with four Ig domains that likely arose from a primordial two-domain ancestor. Notably, teleost fish contain two CD4 genes. Like tetrapod CD4, CD4-1 of rainbow trout includes four Ig domains, whereas CD4-2 contains only two. Because CD4-2 is reminiscent of the prototypic two-domain CD4 coreceptor, we hypothesized that by characterizing the cell types bearing CD4-1 and CD4-2, we would shed light into the evolution and primordial roles of CD4-bearing cells. Using newly established mAbs against CD4-1 and CD4-2, we identified two bona-fide CD4(+) T cell populations: a predominant lymphocyte population coexpressing surface CD4-1 and CD4-2 (CD4 double-positive [DP]), and a minor subset expressing only CD4-2 (CD4-2 single-positive [SP]). Although both subsets produced equivalent levels of Th1, Th17, and regulatory T cell cytokines upon bacterial infection, CD4-2 SP lymphocytes were less proliferative and displayed a more restricted TCRβ repertoire. These data suggest that CD4-2 SP cells represent a functionally distinct population and may embody a vestigial CD4(+) T cell subset, the roles of which reflect those of primeval CD4(+) T cells. Importantly, we also describe the first CD4(+) monocyte/macrophage population in a nonmammalian species. Of all myeloid subsets, we found the CD4(+) population to be the most phagocytic, whereas CD4(+) lymphocytes lacked this capacity. This study fills in an important gap in the knowledge of teleost CD4-bearing leukocytes, thus revealing critical insights into the evolutionary origins and primordial roles of CD4(+) lymphocytes and CD4(+) monocytes/macrophages.

Th17 master transcription factors RORα and RORγ regulate the expression of IL-17C, IL-17D and IL-17F in Cynoglossus semilaevis

The RAR-related orphan receptors (RORs) are members of the nuclear receptor family of intracellular transcription factors. In this study, we examined the regulatory properties of RORα (CsRORα) and RORγ (CsRORγ) in tongue sole (Cynoglossus semilaevis). CsRORα and CsRORγ expression was detected in major lymphoid organs and altered to significant extents after bacterial and viral infection. CsRORα enhanced the activities of CsIL-17C, CsIL-17D, and CsIL-17F promoters, which contain CsRORα and CsRORγ binding sites. CsRORγ also upregulated the promoter activities of CsIL-17D and CsIL-17F but not CsIL-17C. CsRORα and CsRORγ proteins were detected in the nucleus, and overexpression of CsRORα in tongue sole significantly increased the expression of CsIL-17C, CsIL-17D, and CsIL-17F, whereas overexpression of CsRORγ significantly increased the expression of CsIL-17C and CsIL-17F but no CsIL-17D. These results indicate that RORα and RORγ in teleost regulate the expression of IL-17 members in different manners.

The Mucosal Immune System of Teleost Fish

Teleost fish possess an adaptive immune system associated with each of their mucosal body surfaces. Evidence obtained from mucosal vaccination and mucosal infection studies reveal that adaptive immune responses take place at the different mucosal surfaces of teleost. The main mucosa-associated lymphoid tissues (MALT) of teleosts are the gut-associated lymphoid tissue (GALT), skin-associated lymphoid tissue (SALT), the gill-associated lymphoid tissue (GIALT) and the recently discovered nasopharynx-associated lymphoid tissue (NALT). Teleost MALT includes diffuse B cells and T cells with specific phenotypes different from their systemic counterparts that have co-evolved to defend the microbe-rich mucosal environment. Both B and T cells respond to mucosal infection or vaccination. Specific antibody responses can be measured in the gills, gut and skin mucosal secretions of teleost fish following mucosal infection or vaccination. Rainbow trout studies have shown that IgT antibodies and IgT(+) B cells are the predominant B cell subset in all MALT and respond in a compartmentalized manner to mucosal infection. Our current knowledge on adaptive immunity in teleosts is limited compared to the mammalian literature. New research tools and in vivo models are currently being developed in order to help reveal the great intricacy of teleost mucosal adaptive immunity and help improve mucosal vaccination protocols for use in aquaculture.

Modulation of immunity and gut microbiota after dietary administration of alginate encapsulated Shewanella putrefaciens Pdp11 to gilthead seabream (Sparus aurata L.)

The potential benefits of probiotics when administering to fish could improve aquaculture production. The objective of this study was to examine the modulation of immune status and gut microbiota of gilthead seabream (Sparus aurata L.) specimens by a probiotic when administered encapsulated. Commercial diet was enriched with Shewanella putrefaciens Pdp11 (SpPdp11, at a concentration of 10(8) cfu g(-1)) before being encapsulated in calcium alginate beads. Fish were fed non-supplemented (control) or supplemented diet for 4 weeks. After 1, 2 and 4 weeks the main humoral and cellular immune parameters were determined. Furthermore, gene expression profile of five immune relevant genes (il1β, bd, mhcIIα, ighm and tcrβ) was studied by qPCR in head kidney. On the other hand, intestinal microbiota of fish was analysed at 7 and 30 days by DGGE. Results demonstrated that administration of alginate encapsulated SpPdp11 has immunostimulant properties on humoral parameters (IgM level and serum peroxidase activity). Although no immunostimulant effects were detected on leucocyte activities, significant increases were detected in the level of mRNA of head-kidney leucocytes for mhcIIα and tcrβ after 4 weeks of feeding the encapsulated-probiotic diet. The administration of SpPdp11 encapsulated in alginate beads produced important changes in the DGGE patterns corresponding to the intestinal microbiota. Predominant bands related to lactic acid bacteria, such as Lactococcus and Lactobacillus strains, were sequenced from the DGGE patterns of fish fed the probiotic diet, whereas they were not sequenced from fish receiving the control diet. The convenience or not of probiotic encapsulation is discussed.

The cellular protein expression of Foxp3 in lymphoid and non-lymphoid organs of Nile tilapia

In the present study, an antibody highly specific to the Nile tilapia (Oreochromis niloticus) Foxp3 was produced and characterized. Immunohistochemistry analysis indicates that Foxp3 was expressed in peripheral blood mononuclear cells (PBMC) and certain packed lymphocytes in particular, what's more, the percentage of Foxp3(+) cells among PBMC was 5.7 ± 2.0% (n = 5) in healthy adults and could be significantly up-regulated after phytohemagglutinin (50 μg/ml) stimulation in vitro at 6, 12 and 24 h, respectively. In the lymphoid tissues, such as the thymus, spleen and head kidney, Foxp3 expression was observed mainly in lymphocyte-like cells. Surprisingly, in the non-lymphoid organ stomach, Foxp3 was detected in epithelial-like cells within the mucosa. Our study demonstrates for the first time that Foxp3 protein expression occurs not only in hematopoietic cells of lymphoid organ systems but also non-hematopoietic cells of non-lymphoid organ in lower vertebrates such as the fish tilapia. The conserved expression pattern of Foxp3 at the protein and cellular levels implies that it might have conserved functions from fish to mammals.

Immune parameters in the intestine of wild and reared unvaccinated and vaccinated Atlantic salmon (Salmo salar L.)

Forming a barrier to the outside world, the gut mucosa faces the challenge of absorbing nutrients and fluids while initiating immune reactions towards potential pathogens. As a continuation to our previous publication focusing on the regional intestinal morphology in wild caught post smolt and spawning Atlantic salmon, we here investigate selected immune parameters and compare wild, reared unvaccinated and vaccinated post smolts. We observed highest transcript levels for most immune-related genes in vaccinated post smolts followed by reared unvaccinated and finally wild post smolts, indicating that farming conditions like commercial feed and vaccination might contribute to a more alerted immune system in the gut. In all groups, higher levels of immune transcripts were observed in the second segment of mid-intestine and in the posterior segment. In the life stages and conditions investigated here, we found no indication of a previously suggested population of intestinal T cells expressing MHC class II nor RAG1 expression.

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.