First Demonstration of Antigen Induced Cytokine Expression by CD4-1+ Lymphocytes in a Poikilotherm: Studies in Zebrafish (Danio rerio)

Identification and functional analyses of CD4-1+ cells in grass carp (Ctenopharyngodon idella)

In mammals, CD4 is found to be expressed on T cells and innate immune cells, however, teleost cells bearing CD4 have not been well identified and characterized. In this study, we identified two different CD4-1+ cell subsets in grass carp (Ctenopharyngodon idella): CD4-1+ lymphocytes (Lym) and CD4-1+ myeloid cells (Mye), both of which had the highest proportions in the head kidney. The mRNA expression analysis showed that CD4-1, CD4-2, TCRβ, CD3γ/δ, and LCK1 are highly expressed in CD4-1+ Lym and also expressed in CD4-1+ Mye. Furthermore, we found that CD4-1+ Lym have a Lym morphology and highly express T-cell cytokines, suggesting that they are CD4+ T cells equivalent to mammalian Th cells. On the other hand, CD4-1+ Mye were found to have a morphology of macrophage and highly express macrophage marker gene MCSFR, indicating that they are macrophages. In addition, functional analysis revealed that CD4-1+ Mye possess phagocytic ability and great antigen-processing ability. Taken together, our study sheds further light on the composition and function of CD4+ cells in teleost fish.

Characteristics of CD4-1 gene and its immune responses against Aeromonas veronii infection by activating NF-κB signaling in Qihe crucian carp Carassius auratus

CD4-1 found in bony fish contains four extracellular immunoglobulin (Ig)-like domains similar to that of mammalian CD4, which is crucial for the activation of CD4+ helper T-cell. However, there is limited knowledge regarding the molecular markers, immune functions and regulation mechanism of CD4-1 in teleosts due to their vast diversity. In this study, we cloned and characterized two isoforms of Qihe crucian carp CD4-1, designated as CaCD4-1.1 and CaCD4-1.2. We further explored their expression responses upon stimulation with Aeromonas veronii, and the regulation of their immune responses against A. veronii by NF-κB. The ORF of CaCD4-1.1 and CaCD4-1.2 cDNA encoded 477 and 466 amino acids, respectively. Both proteins contained seven conserved cysteine residues in the extracellular domain, and a CCC motif in their cytoplasm, respectively. However, CaCD4-1.1 exhibited a relatively limited similarity with CaCD4-1.2 in the ectodomain. The quantitative real-time polymerase chain reaction (qRT-PCR) analysis revealed that the mRNA expression of CaCD4-1.1 and CaCD4-1.2 exhibited differential constitutive expression across all examined tissues. Furthermore, the expression level of CD4-1.2 was higher than that of CD4-1.1 in the gills, head kidney, and spleen of Qihe crucian carp subjected to A. veronii challenge, while it was lower in the trunk kidney. Inhibition of NF-κB activity resulted in a decrease in the expression levels of CD4-1.1 and CD4-1.2 mRNA in the gill, while inducing an increase in expression levels in the spleen, in accordance with the observed ultrastructural changes in both organs. Interestingly, the impact of NF-κB on the mRNA expression level of CD4-1.1 appears to be stronger than that of CD4-1.2. Our results suggest that CaCD4-1.1 and CaCD4-1.2 could be expressed on T cells and antigen-sampling cells that exhibit similar characteristics to mammalian M cells, respectively, and differentially regulated by NF-κB in adaptive immune responses against bacterial infection. This research contributes to a better understanding of the crucial role of CD4-1 in the immune response of Qihe crucian carp and provide novel insights for the prevention and treatment of fish diseases in aquaculture.

CD4 and LAG-3 from sharks to humans: related molecules with motifs for opposing functions

CD4 and LAG-3 are related molecules that are receptors for MHC class II molecules. Their major functional differences are situated in their cytoplasmic tails, in which CD4 has an activation motif and LAG-3 an inhibitory motif. Here, we identify shark LAG-3 and show that a previously identified shark CD4-like gene has a genomic location, expression pattern, and motifs similar to CD4 in other vertebrates. In nurse shark (Ginglymostoma cirratum) and cloudy catshark (Scyliorhinus torazame), the highest CD4 expression was consistently found in the thymus whereas such was not the case for LAG-3. Throughout jawed vertebrates, the CD4 cytoplasmic tail possesses a Cx(C/H) motif for binding kinase LCK, and the LAG-3 cytoplasmic tail possesses (F/Y)xxL(D/E) including the previously determined FxxL inhibitory motif resembling an immunoreceptor tyrosine-based inhibition motif (ITIM). On the other hand, the acidic end of the mammalian LAG-3 cytoplasmic tail, which is believed to have an inhibitory function as well, was acquired later in evolution. The present study also identified CD4-1, CD4-2, and LAG-3 in the primitive ray-finned fishes bichirs, sturgeons, and gars, and experimentally determined these sequences for sterlet sturgeon (Acipenser ruthenus). Therefore, with CD4-1 and CD4-2 already known in teleosts (modern ray-finned fish), these two CD4 lineages have now been found within all major clades of ray-finned fish. Although different from each other, the cytoplasmic tails of ray-finned fish CD4-1 and chondrichthyan CD4 not only contain the Cx(C/H) motif but also an additional highly conserved motif which we expect to confer a function. Thus, although restricted to some species and gene copies, in evolution both CD4 and LAG-3 molecules appear to have acquired functional motifs besides their canonical Cx(C/H) and ITIM-like motifs, respectively. The presence of CD4 and LAG-3 molecules with seemingly opposing functions from the level of sharks, the oldest living vertebrates with a human-like adaptive immune system, underlines their importance for the jawed vertebrate immune system. It also emphasizes the general need of the immune system to always find a balance, leading to trade-offs, between activating and inhibiting processes.

Identification of a pharyngeal mucosal lymphoid organ in zebrafish and other teleosts: Tonsils in fish?

The constant exposure of the fish branchial cavity to aquatic pathogens causes local mucosal immune responses to be extremely important for their survival. Here, we used a marker for T lymphocytes/natural killer (NK) cells (ZAP70) and advanced imaging techniques to investigate the lymphoid architecture of the zebrafish branchial cavity. We identified a sub-pharyngeal lymphoid organ, which we tentatively named "Nemausean lymphoid organ" (NELO). NELO is enriched in T/NK cells, plasma/B cells, and antigen-presenting cells embedded in a network of reticulated epithelial cells. The presence of activated T cells and lymphocyte proliferation, but not V(D)J recombination or hematopoiesis, suggests that NELO is a secondary lymphoid organ. In response to infection, NELO displays structural changes including the formation of T/NK cell clusters. NELO and gill lymphoid tissues form a cohesive unit within a large mucosal lymphoid network. Collectively, we reveal an unreported mucosal lymphoid organ reminiscent of mammalian tonsils that evolved in multiple teleost fish families.

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.

Germ cells do not progress through spermatogenesis in the infertile zebrafish testis

Vertebrate spermatogonial stem cells maintain sperm production over the lifetime of an animal but fertility declines with age. While morphological studies have greatly informed our understanding of typical spermatogenesis, the molecular and cellular mechanisms underlying spermatogenesis are not yet understood, particularly with respect to the onset of fertility. We used single-cell RNA sequencing to generate a developmental atlas of the zebrafish testis. Using 5 timepoints across the adult life of a zebrafish, we described cellular profiles in the testis during and after fertility. While all germ cell stages of spermatogenesis are detected in testes from fertile adult zebrafish, testes from older infertile males only contained spermatogonia and a reduced population of spermatocytes. These remaining germ cells are transcriptionally distinct from fertile spermatogonia. Immune cells including macrophages and lymphocytes drastically increase in abundance in infertile testes. Our developmental atlas reveals the cellular changes as the testis ages and defines a molecular roadmap for the regulation of male fertility.

CD4-1 and CD4-2 single positive are two major CD4 lymphocyte subpopulations in ginbuna crucian carp Carassius auratus langsdorfii

In this study, we established a murine cell line that expresses ginbuna crucian carp (ginbuna) CD4-2 and used it to develop an anti-CD4-2 monoclonal antibody (mAb). An established mAb, named D5, showed good reactivities to BALB/3T3 cells expressing CD4-2 and a lymphocyte population in the ginbuna leukocytes. Gene expression analysis showed that D5⁺ cells express CD4-2 and TCRβ genes but not CD4-1 and IgM genes, meanwhile May Grunwald-Giemsa staining of sorted D5⁺ cells had the typical morphology of lymphocytes. Two-color immunofluorescence analysis with anti-CD4-1 mAb (6D1) and anti-CD4-2 mAb (D5) by flow cytometry revealed that the percentages of CD4-1 single positive (SP) and CD4-2 SP lymphocytes were comparatively higher than CD4-1/CD4-2 double positive (CD4 DP) lymphocytes in all tissues examined in ginbuna. The highest percentages of CD4-2 SP cells (∼40%) were found in the thymus, while the head-kidney exhibited the highest percentages of CD4-1 SP (∼30%) and CD4 DP (∼5%) cells. These findings indicated that ginbuna CD4⁺ lymphocyte population consists of two major subpopulations (CD4-1 SP and CD4-2 SP) and a minor subset (CD4 DP).

Effects of artificial photoperiods on antigen-dependent immune responses in rainbow trout (Oncorhynchus mykiss)

In this study, we investigated the effects of the artificial photoperiods that mimic summer (16L:8D) and winter (8L:16D) solstices, equinoxes (12L:12D), and the artificial 24-h light regimen (24L:0D) on the leukocyte populations and the T helper and regulatory type responses on rainbow trout (Oncorhynchus mykiss). Using flow cytometry analysis, we found that photoperiod induces changes in head kidney leukocyte subsets. The lymphoid subset increased in the 16L:8D summer solstice regime. The analysis using antibodies against B and T cells showed the increase of CD4-1⁺ T lymphocytes and other unidentified lymphoid cells, with no changes in the B cells. To investigate the modulatory influence of the photoperiod on the fish T cell response, we quantified in the head kidney the transcript levels of genes involved in the Th1 type response (t-bet, ifn-ƴ, il-12p35, il-12p40c), Th2 type response (gata3, il-4/13a), Th17 response (ror-ƴt, il-17a/f), T regulatory response (foxp3α, il-10a, tgf-β1), and the T cell growth factor il-2. The results showed that the seasonal photoperiod alone has a limited influence on the expression of these genes, as the only difference was observed in il-14/13a and il-10a transcripts of fish kept on the 16L:8D regimen. In addition, the 24L:0D treatment used in aquaculture produces a reduction of il-14/13a and il-17a/f. We also evaluated the effect of photoperiod in the presence of an antigenic stimulus. Thus, in fish immunized with the recombinant viral protein 1 (rVP1) of infectious pancreatic necrosis virus (IPNV), the photoperiod had a striking influence on the type of adaptive immune response. Each photoperiod fosters a unique immune signature of antigenic response. A classical type 1 response is observed in fish subjected to the 16D:8L photoperiod. In contrast, fish in the 12L:12D photoperiod showed only the upregulation of il-12p40c. Furthermore, none of the cytokines were increased in fish maintained on the artificial 24L:0D regimen, and a decrease in the master transcription factors (t-bet, ror-ƴt, and foxp3α) was observed. Thus, fish on the 12L:12D and 24L:0D photoperiod appear hyporesponsive regarding the T cell response. Altogether, this study showed that photoperiods modify the magnitude and quality of the T-helper response in rainbow trout and thus impact essential mechanisms for the generation of immune memory and protection against microorganisms.

Cytokine networks provide sufficient evidence for the differentiation of CD4+ T cells in teleost fish

Cytokines, a class of small molecular proteins with a wide range of biological activities, are secreted mainly by immune cells and function by binding to the corresponding receptors to regulate cell growth, differentiation and effects. CD4⁺ T cells can be defined into different lineages based on the unique set of signature cytokines and transcription factors, including helper T cells (Th1, Th2, Th17) and regulatory T cells (Treg). In teleost, CD4⁺ T cells have been identified in a variety of fish species, thought to play roles as Th cells, and shown to be involved in the immune response following specific antigen stimulation. With the update of sequencing technologies, a variety of cytokines and transcription factors capable of characterizing CD4⁺ T cell subsets also have been described in fish, including hallmark cytokines such as IFN-γ, TNF-α, IL-4, IL-17, IL-10, TGF-β and unique transcription factors such as T-bet, GATA3, RORγt, and Foxp3. Hence, there is increasing evidence that the subpopulation of Th and Treg cells present in mammals may also exist in teleost fish. However, the differentiation, plasticity and precise roles of Th cell subsets in mammals remain controversial. Research on the identification and differentiation of fish Th cells is still in its infancy and requires more significant effort. Here we will review recent research advances in characterizing the differentiation of fish CD4⁺ T cells by cytokines and transcription factors, mainly including the identification of Th and Treg cell hallmark cytokines and transcription factors, the regulatory role of cytokines on Th cell differentiation, and the function of Th and Treg cells in the immune response. The primary purpose of this review is to deepen our understanding of cytokine networks in characterizing the differentiation of CD4⁺ T cells in teleost.

Glutamine Metabolism Underlies the Functional Similarity of T Cells between Nile Tilapia and Tetrapod

As the lowest organisms possessing T cells, fish are instrumental for understanding T cell evolution and immune defense in early vertebrates. This study established in Nile tilapia models suggests that T cells play a critical role in resisting Edwardsiella piscicida infection via cytotoxicity and are essential for IgM⁺ B cell response. CD3 and CD28 monoclonal antibody crosslinking reveals that full activation of tilapia T cells requires the first and secondary signals, while Ca²⁺ -NFAT, MAPK/ERK, NF-κB, and mTORC1 pathways and IgM⁺ B cells collectively regulate T cell activation. Thus, despite the large evolutionary distance, tilapia and mammals such as mice and humans exhibit similar T cell functions. Furthermore, it is speculated that transcriptional networks and metabolic reprogramming, especially c-Myc-mediated glutamine metabolism triggered by mTORC1 and MAPK/ERK pathways, underlie the functional similarity of T cells between tilapia and mammals. Notably, tilapia, frogs, chickens, and mice utilize the same mechanisms to facilitate glutaminolysis-regulated T cell responses, and restoration of the glutaminolysis pathway using tilapia components rescues the immunodeficiency of human Jurkat T cells. Thus, this study provides a comprehensive picture of T cell immunity in tilapia, sheds novel perspectives for understanding T cell evolution, and offers potential avenues for intervening in human immunodeficiency.

Full-length transcriptome sequencing of lymphocytes respond to IFN-γ reveals a Th1-skewed immune response in flounder (Paralichthys olivaceus)

Interferon gamma (IFN-γ), the member of type II interferons, is a major driver and effector cytokine for Th1 cells and plays broad roles in regulating the function of immune cells. Teleost fish represents the oldest living bony vertebrates containing T-lymphocyte subsets. However, whether or how the regulatory mechanisms of IFN-γ on Th1 cells occur in teleost fish remain unknown. In this study, full-length transcriptome sequencing was performed to analyze the differentially expressed genes (DEGs) and signaling pathways in the IFN-γ stimulated lymphocytes of flounder (Paralichthys olivaceus), the data showed 811 genes were upregulated and 1107 genes were downregulated, Th1 and Th2 cell differentiation pathway was remarkably enriched from DEGs, and the genes in the Th1 cell differentiation pathway were upregulated and verified. Accordingly, variations on Th1 cell differentiation marker genes and CD4⁺ cells were investigated after IFN-γ stimulation, the results confirmed that CD4⁺ T lymphocytes proliferated significantly after IFN-γ stimulation, accompanied by eight genes significant upregulation and increased T-bet expression in lymphocytes. In conclusion, the results revealed an induction of IFN-γ on Th1-type immune response, providing novel perspectives into the differentiation of CD4⁺ T lymphocytes in teleost.

Evolutional perspective and functional characteristics of interleukin-17 in teleosts

Interleukin (IL)-17 is a proinflammatory cytokine and plays essential roles in adaptive and innate immune responses against bacterial and fungal infections. Especially in mammalian mucosal tissues, it is well known that innate immune responses via IL-17A and IL-17F, such as the production of antimicrobial peptides, are very important for microbiota control. In contrast, interesting insights into the functions of IL-17 have recently been reported in several teleost species, although little research has been conducted on teleost IL-17. In the present review, we focused on current insights on teleost IL-17 and speculated on the different or consensus parts of teleost IL-17 signaling compared to that of mammals. This review focuses on the role of teleost IL-17 in intestinal immunity. We expect that this review will encourage a further understanding of the roles and importance of IL-17 signaling in teleosts.

IL-2-mTORC1 signaling coordinates the STAT1/T-bet axis to ensure Th1 cell differentiation and anti-bacterial immune response in fish

Utilization of specialized Th1 cells to resist intracellular pathogenic infection represents an important innovation of adaptive immunity. Although transcriptional evidence indicates the potential presence of Th1-like cells in some fish species, the existence of CD3+CD4+IFN-γ+ T cells, their detailed functions, and the mechanism determining their differentiation in these early vertebrates remain unclear. In the present study, we identified a population of CD3+CD4-1+IFN-γ+ (Th1) cells in Nile tilapia upon T-cell activation in vitro or Edwardsiella piscicida infection in vivo. By depleting CD4-1+ T cells or blocking IFN-γ, Th1 cells and their produced IFN-γ were found to be essential for tilapia to activate macrophages and resist the E. piscicida infection. Mechanistically, activated T cells of tilapia produce IL-2, which enhances the STAT5 and mTORC1 signaling that in turn trigger the STAT1/T-bet axis-controlled IFN-γ transcription and Th1 cell development. Additionally, mTORC1 regulates the differentiation of these cells by promoting the proliferation of CD3+CD4-1+ T cells. Moreover, IFN-γ binds to its receptors IFNγR1 and IFNγR2 and further initiates a STAT1/T-bet axis-mediated positive feedback loop to stabilize the Th1 cell polarization in tilapia. These findings demonstrate that, prior to the emergence of tetrapods, the bony fish Nile tilapia had already evolved Th1 cells to fight intracellular bacterial infection, and support the notion that IL-2-mTORC1 signaling coordinates the STAT1/T-bet axis to determine Th1 cell fate, which is an ancient mechanism that has been programmed early during vertebrate evolution. Our study is expected to provide novel perspectives into the evolution of adaptive immunity.

CXCL9-11 chemokines and CXCR3 receptor in teleost fish species

The coordinated migration of immune cells from lymphoid organs to in or out of the bloodstream, and towards the site of infection or tissue damage is fundamental for an efficient innate and adaptive immune response. Interestingly, an essential part of this movement is mediated by chemoattractant cytokines called chemokines. Although the nature and function of chemokines and their receptors are well documented in mammals, much research is needed to accomplish a similar level of understanding of the role of chemokines in fish immunity. The first chemokine gene identified in teleosts (rainbow trout, Oncorhynchus mykiss) was CK1 in 1998. Since then, the identification of fish chemokine orthologue genes and characterization of their role has been more complex than expected, primarily because of the whole genome duplication processes occurring in fish, and because chemokines evolve faster than other immune genes. Some of the most studied chemokines are CXCL9, CXCL10, CXCL11, and the CXCR3 receptor, all involved in T cell migration and in the induction of the T helper 1 (Th1) immune response. Data from the zebrafish and rainbow trout CXCL9-11/CXCR3 axis suggest that these chemokines and the receptor arose early in evolution and must be present in most teleost fish. However, the pieces of knowledge also indicate that different numbers of gene copies can be present in different species, with distinct regulatory expression mechanisms and probably, also with different roles, as the differential expression in fish tissues suggest. Here, we revised the current knowledge of the CXCL9-11/CXCR3 axis in teleost fishes, identifying the gaps in knowledge, and raising some hypotheses for the role of CXCL9, CXCL10 CXCL11, and CXCR3 receptor axis in fish, which can encourage further studies in the field.

Single-cell analyses reveal early thymic progenitors and pre-B cells in zebrafish

The zebrafish has proven to be a valuable model organism for studying hematopoiesis, but relatively little is known about zebrafish immune cell development and functional diversity. Elucidating key aspects of zebrafish lymphocyte development and exploring the breadth of effector functions would provide valuable insight into the evolution of adaptive immunity. We performed single-cell RNA sequencing on ∼70,000 cells from the zebrafish marrow and thymus to establish a gene expression map of zebrafish immune cell development. We uncovered rich cellular diversity in the juvenile and adult zebrafish thymus, elucidated B- and T-cell developmental trajectories, and transcriptionally characterized subsets of hematopoietic stem and progenitor cells and early thymic progenitors. Our analysis permitted the identification of two dendritic-like cell populations and provided evidence in support of the existence of a pre-B cell state. Our results provide critical insights into the landscape of zebrafish immunology and offer a foundation for cellular and genetic studies.

Extensive involvement of CD40 and CD154 costimulators in multiple T cell-mediated responses in a perciform fish Larimichthys crocea

CD40 and CD154 are well-characterized costimulatory molecules involved in adaptive humoral immunity in humans and other mammals. These two costimulatory molecules were found to be originated from teleost fish during vertebrate evolution. However, the functionality of fish CD40 and CD154 remains to be explored. In this study, we identified the CD40 and CD154 homologs (LcCD40 and LcCD154) from large yellow croaker (Larimichthys crocea), a marine species of the perciform fish family. The LcCD40 and LcCD154 share conserved structural features to their mammalian counterparts, and are widely expressed in immune-relevant tissues and leukocytes at different transcriptional levels. Immunofluorescence staining and FCM analysis showed that LcCD40 and LcCD154 proteins are distributed on MHC-II⁺ APCs and CD4-2⁺ T cells, and are significantly upregulated in response to antigen stimulation. Co-IP assay exhibited strong association between LcCD40 and LcCD154 proteins. Blockade of LcCD154 with anti-LcCD154 antibody (Ab) or recombinant soluble LcCD40-Ig fusion protein remarkably decreased the MHC-II⁺ APC-initiated CD4⁺ T cell response upon Aeromonas hydrophila stimulation, and alloreactive T cell activation as examined by mixed lymphocyte reaction (MLR). These findings highlight the costimulatory role of LcCD40 and LcCD154 in T cell activities in Larimichthys crocea. Thus, the CD40 and CD154 costimulators may extensively participate in the regulation of multiple T cell-mediated immune responses in teleost fish. It is anticipated that this study would provide a cross-species understanding of the evolutionary history of CD40 and CD154 costimulatory signals from fish to mammals.

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.

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.

Sleeping With the Enemy? The Current Knowledge of Piscine Orthoreovirus (PRV) Immune Response Elicited to Counteract Infection

Piscine orthoreovirus (PRV) is a virus in the genus Orthoreovirus of the Reoviridae family, first described in 2010 associated with Heart and Skeletal Muscle Inflammation (HSMI) in Atlantic salmon (Salmo salar). Three phases of PRV infection have been described, the early entry and dissemination, the acute dissemination phase, and the persistence phase. Depending on the PRV genotype and the host, infection can last for life. Mechanisms of immune response to PRV infection have been just beginning to be studied and the knowledge in this matter is here revised. PRV induces a classical antiviral immune response in experimental infection of salmonid erythrocytes, including transcriptional upregulation of ifn-α, rig-i, mx, and pkr. In addition, transcript upregulation of tcra, tcrb, cd2, il-2, cd4-1, ifn-γ, il-12, and il-18 has been observed in Atlantic salmon infected with PRV, indicating that PRV elicited a Th1 type response probably as a host defense strategy. The high expression levels of cd8a, cd8b, and granzyme-A in PRV-infected fish suggest a positive modulatory effect on the CTL-mediated immune response. This is consistent with PRV-dependent upregulation of the genes involved in antigen presentation, including MHC class I, transporters, and proteasome components. We also review the potential immune mechanisms associated with the persistence phenotype of PRV-infected fish and its consequence for the development of a secondary infection. In this scenario, the application of a vaccination strategy is an urgent and challenging task due to the emergence of this viral infection that threatens salmon farming.

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.

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.

ZAP70 activation is an early event of T cell immunity that involved in the anti-bacterial adaptive immune response of Nile tilapia

ZAP70 is essential for initiating the early events of T-cell antigen receptor (TCR) signaling cascade to ensure proper T cell activation and function. However, whether this molecule takes part in the T cell immune response of early vertebrates remains unclear. In the present study, using a teleost model Nile tilapia (Oreochromis niloticus), we investigated the potential involvement of ZAP70 in the T cell activation and adaptive immunity of fish species. Both primary and tertiary structures of O. niloticus ZAP70 (On-ZAP70) are highly conserved with those from other vertebrates. On-ZAP70 protein was widely expressed in lymphoid tissues, and with the highest level in thymus. Once Nile tilapia was infected by Aeromonas hydrophila, mRNA of On-ZAP70 in spleen lymphocytes was induced on day 5 and 8 after infection; meanwhile, phosphorylation of On-ZAP70 was also enhanced, suggesting that On-ZAP70 potentially participated in primary adaptive immune response of Nile tilapia. Furthermore, the frequency of ZAP70 positive lymphocytes was increased during the anti-bacterial adaptive immune response. More importantly, when spleen lymphocytes were activated by T cell specific mitogen PHA, a dramatical augment of On-ZAP70 could be observed at transcription, phosphorylation and cellular level, indicating the involvement of this molecule in T cells activation of Nile tilapia. Altogether, our results demonstrated that ZAP70 activation is an early event of T cell immunity that involved in the anti-bacterial adaptive immune response of Nile tilapia, and thus provided a new evidence to understand the evolution of the lymphocyte-mediated adaptive immunity.

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.

Characterisation and analysis of IFN-gamma producing cells in rainbow trout Oncorhynchus mykiss

IFN-γ is one of the key cytokines involved in Th1 immune responses. It is produced mainly by T cells and NK cells, which drive both innate and adaptive responses to promote protection against infections. IFN-γ orthologues have been discovered to be functionally conserved in fish, suggesting that type I immunity is present in early vertebrates. However, few studies have looked at IFN-γ protein expression in fish and its role in cell mediated immunity due to a lack of relevant tools. In this study, four monoclonal antibodies (mAbs) V27, N2, VAB3 and V91 raised against short salmonid IFN-γ peptides were developed and characterised to monitor IFN-γ expression. The results show that the IFN-γ mAbs specifically react to their peptide immunogens, recognise E. coli produced recombinant IFN-γ protein and rainbow trout IFN-γ produced in transfected HEK 293 cells. The mAb VAB3 was used further, to detect IFN-γ at the cellular level after in vitro and in vivo stimulation. In flow cytometry, a basal level of 3-5% IFN-γ secreting cells were detected in peripheral blood leucocytes (PBL), which increased significantly when stimulated in vitro with PAMPs (Aeromonas salmonicida bacterin), a mitogen (PHA) and recombinant cytokine (IL-2). Similarly, after injection of live bacteria (Aeromonas salmonicida) or poly I:C the number of IFN-γ⁺ cells increased in the lymphoid population of PBL, as well as in the myeloid population after infection, with the myeloid cells increasing substantially after both treatments. Immunohistochemistry was used to visualise the IFN-γ⁺ cells in spleen and head kidney following vaccination, which increased in intensity of staining and number relative to tissue from saline-injected control fish. These results show that several types of cells can produce IFN-γ in trout, and that they increase following infection or vaccination, and likely contribute to immune protection. Hence monitoring IFN-γ producing cells/protein secretion may be an important means to assess the effectiveness of Th1 responses and cell mediated immunity in 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.

Immunological memory in teleost fish

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

Montanide™ ISA 763A VG and ISA 761 VG induce different immune pathway responses in rainbow trout (Oncorhynchus mykiss) when used as adjuvant for an Aeromonas salmonicida bacterin

Adjuvants are the helper substances that increase vaccine efficacy by enhancing the potency and longevity of specific immune responses to antigens. Most existing fish vaccines are presented in the form of oil-based emulsions delivered by intraperitoneal injection. The characterization of their mode of action is a valuable aid to future vaccine development, particularly for the potential identification and stimulation of specific immunological pathways related to the desired protective response. This study characterized the expression of selected immune-related genes in the peritoneal cavity, head kidney and spleen following the administration of two adjuvanted-bacterial vaccines thought to induce humoral (Montanide™ ISA 763A VG) or humoral and cell mediated (Montanide™ ISA 761 VG) immune responses, to determine if differences in responsiveness are readily apparent. The most informative site was the spleen, where Montanide™ ISA 763A VG + bacterin gave rise to upregulation of genes driving T-cell/lymphoid responses, namely IL-2, IL-15 and IL-21. This combined with upregulation of IFNγ1 and IFNγ2, IL-4/13B2, p35A1 and p40 (B1 and C) indicated that the induction of Th1 and possibly Th2 immunity was occurring in fish vaccinated with this adjuvant. Perhaps the most intriguing finding was the lack of a detectable Th1 response in fish given Montanide™ ISA 761 VG + bacterin, suggesting some other arm of the immune system is activated to give protection. Whatever the reason for the different responses detected, it is clear from the present study that the adjuvant used has a major impact on the responses elicited. Since these differences are readily detectable it allows, in principle, their use to help select the most appropriate adjuvants for inclusion into fish vaccines, where the type of response elicited may need to be tailored to a particular pathogen to confer protection.

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.

Monoclonal antibody against Nile tilapia (Oreochromis niloticus) IgM heavy chain: A valuable tool for detection and quantification of IgM and IgM+ cells

Nile tilapia (Oreochromis niloticus) is a freshwater fish, which is extensively cultivated worldwide and constitutes one of the model species for the study of fish immunology. Monoclonal antibodies are very advantageous molecular tools for studying teleost immune system. Specifically, monoclonal antibodies that react with immunoglobulins are used successfully in the study of the humoral immune response of several fish species. In the present study, we produced and characterized a monoclonal antibody against tilapia IgM heavy chain using a peptide-based strategy. The peptide sequence was selected from the surface-exposed region between CH3-CH4 domains. The specificity of the polyclonal serum and the hybridoma culture supernatant obtained by immunization with the peptide conjugated to keyhole limpet hemocyanin were evaluated by western blotting, both showing reactivity against tilapia serum IgM. The purified mAb was able to recognize secreted IgM by western blotting and ELISA and membrane IgM by flow cytometry. We also demonstrated that the antibody doesn't cross-react with a recombinant IgT fragment. This tool allowed us to study for the first time the stimulation of mucosal immunity after Pituitary Adenylate Cyclase Activating Polypeptide administration. Overall, the results demonstrated the utility of this mAb to characterize humoral immune response in O. niloticus.

Revisiting the Teleost Thymus: Current Knowledge and Future Perspectives

Simple Summary

The thymus is the immune organ producing T lymphocytes that are essential to create immunity after encountering pathogens or vaccination. This review summarizes the thymus localization and histological studies, cell composition, and function in teleost fishes. We also describe how seasonal changes, photoperiod, water temperature fluctuations, and hormones can affect thymus development in fish species. Overall, the information helps identify future studies needed to understand thymus function in fish species and the immune system’s evolutionary origins. Since fish are exposed to pathogens, especially under aquaculture conditions, knowledge about the fish thymus and T lymphocyte can also help improve fish farming protocols, considering intrinsic and environmental conditions that can contribute to achieving the best vaccine responsiveness for disease resistance.

Abstract

The thymus in vertebrates plays a critical role in producing functionally competent T-lymphocytes. Phylogenetically, the thymus emerges early during evolution in jawed cartilaginous fish, and it is usually a bilateral organ placed subcutaneously at the dorsal commissure of the operculum. In this review, we summarize the current understanding of the thymus localization, histology studies, cell composition, and function in teleost fishes. Furthermore, we consider environmental factors that affect thymus development, such as seasonal changes, photoperiod, water temperature fluctuations and hormones. Further analysis of the thymus cell distribution and function will help us understand how key stages for developing functional T cells occur in fish, and how thymus dynamics can be modulated by external factors like photoperiod. Overall, the information presented here helps identify the knowledge gaps and future steps needed for a better understanding of the immunobiology of fish thymus.

Thymus development in the zebrafish (Danio rerio) from an ecoimmunology perspective

The thymus is present in all gnathostome vertebrates and is an essential organ for the adaptive immune system via the generation of functional mature T-cells. Over the life span of mammals, the thymus undergoes morphological and functional alterations, including an age-related involution, which in humans starts in early life. Life history tradeoffs have been suggested as possible reasons for thymus involution. While in teleost fish, only a few studies have investigated alterations of thymus structure and function over different life stages, resulting in a fragmented database. Here, we investigated the thymus growth of zebrafish (Danio rerio) from early life, throughout puberty and reproductive stage, up to 1-year-old. We assessed thymus growth by histological and morphometric analyses and thymocyte numbers. Thymus function was assessed by measuring the transcripts of the thymocyte marker genes, ikaros, tcrα, and tcrδ. Additionally, we analyzed gonad maturity and tail homogenate vitellogenin concentrations to align thymus status with the status of the reproductive system. Our results showed that the zebrafish thymus, in contrast to the human thymus, grew strongly during early life and puberty but started to undergo involution when the fish reached the reproductive age. The involution was characterized by reduced thymus area and thymocyte number, altered histoarchitecture, and decreasing thymocyte marker gene transcript levels. Our findings suggest that age-related changes of the zebrafish thymus do exist and could be partly explained in terms of resource tradeoffs, but also in terms of the ontogenetically late development of a functional adaptive immune system in teleosts.

Integrating fish models in tuberculosis vaccine development

Tuberculosis is a chronic infection by Mycobacterium tuberculosis that results in over 1.5 million deaths worldwide each year. Currently, there is only one vaccine against tuberculosis, the Bacillus Calmette–Guérin (BCG) vaccine. Despite widespread vaccination programmes, over 10 million new M. tuberculosis infections are diagnosed yearly, with almost half a million cases caused by antibiotic-resistant strains. Novel vaccination strategies concentrate mainly on replacing BCG or boosting its efficacy and depend on animal models that accurately recapitulate the human disease. However, efforts to produce new vaccines against an M. tuberculosis infection have encountered several challenges, including the complexity of M. tuberculosis pathogenesis and limited knowledge of the protective immune responses. The preclinical evaluation of novel tuberculosis vaccine candidates is also hampered by the lack of an appropriate animal model that could accurately predict the protective effect of vaccines in humans. Here, we review the role of zebrafish (Danio rerio) and other fish models in the development of novel vaccines against tuberculosis and discuss how these models complement the more traditional mammalian models of tuberculosis.

Summary: In this Review, we discuss how zebrafish (Danio rerio) and other fish models can complement the more traditional mammalian models in the development of novel vaccines against tuberculosis.

Kinetics of T lymphocyte subsets and B lymphocytes in response to immunostimulants in flounder (Paralichthys olivaceus): implications for CD4+ T lymphocyte differentiation

CD4⁺ T lymphocytes play crucial roles in the adaptive immune system. CD4, as the most effective marker to delineate the T-helper subsets, was identified in many fish species. Two CD4 homologs, CD4-1 and CD4-2, have been reported in flounder (Paralichthys olivaceus). In this study, monoclonal antibodies (mAbs) against CD4-1 and CD4-2 of flounder were produced, CD4⁺ T lymphocytes were isolated and identified, and the variations in CD4⁺ and CD8⁺ T lymphocytes and IgM⁺ B lymphocytes after Poly I:C, PMA or β-glucan stimulation were investigated. Then, the expression of transcription factors and cytokines in sorted CD4⁺ T lymphocytes was analyzed. The results showed that the mAbs were specific to flounder CD4-1⁺ and CD4-2⁺ T cells. CD4-1⁺ and CD4-2⁺ cells responded to all three stimulants, while CD8⁺ T lymphocytes only give a strong response to Poly I:C, and the percentages of IgM⁺ B lymphocytes showed a tendency to increase. After stimulation, the expression of transcription factors and cytokines of Th1, Th2 and Th17 cells varied in CD4⁺ T cells. These results will provide crucial foundations for the differentiation and function of teleost CD4⁺ T lymphocytes.

Rainbow trout mount a robust specific immune response upon anal administration of thymus-independent antigens

Despite the strong demand for orally-delivered fish vaccines and the deficient response of those currently available in the market, little is known about how teleost B cells differentiate to antibody secreting cells (ASCs) in response to antigens delivered to the intestinal mucosa. To fill this gap, in the current study, we have studied the dynamics of B cell differentiation in spleen and kidney of rainbow trout (Oncorhynchus mykiss) anally immunized with antigens catalogued in mammals as thymus dependent (TD) or thymus-independent (TI). Our results show that, in the absence of additional adjuvants, rainbow trout preferentially responded to a model TI antigen such as TNP-LPS (2,4,6-trinitrophenyl hapten conjugated to lipopolysaccharide). The anal administration of TNP-LPS elicited TNP-specific serum antibodies, and a significant increase in the number of total and TNP-specific ASCs in both spleen and kidney, being the kidney the site where most ASCs are found at later time points. In the spleen, a proliferative response of both IgM⁺ B and T cells was also clearly visible, while the proliferative response was weaker in the kidney. Finally, TNP-LPS also provoked a transcriptional regulation of some immune genes in the spleen and the intestine, including a decreased transcription of foxp3a and foxp3b in intestine that suggests a breach in tolerogenic responses in response to TI stimulation. These results contribute to a better understanding of how intestinal immunity is regulated in teleost and will aid in the future design of effective oral strategies for aquaculture.

Early-life infection with a bacterial pathogen increases expression levels of innate immunity related genes during adulthood in zebrafish

Early-life exposure to different stressors can lead to various consequences on fish health status in later life development. To evaluate the effects of Aeromonas salmonicida achromogenes infection in the early-life on immunity in adulthood, zebrafish were either early-infected at 18 days post-fertilization (dpf), chronically infected from 18 to 35 dpf, or late infected at 35 dpf and then grown up to 61 dpf to be re-infected with the pathogen. The age of first infection was shown to influence both, level and timing of the immune gene expressions, especially for inflammation-related genes. In addition, evidence for an innate immune memory in zebrafish primarily infected with the pathogen at 35 dpf and re-infected at 61dpf provide new insights to consolidate the concept of a "trained" innate immunity in fish.

Lymphoid Tissue in Teleost Gills: Variations on a Theme

In bony fish, the gill filaments are essential for gas exchanges, but also are vulnerable to infection by water-borne microorganisms. Omnipresent across fish, gill-associated lymphoid tissues (GIALT) regulate interactions with local microbiota and halt infection by pathogens. A special GIALT structure has recently been found in Salmonids, the interbranchial lymphoid tissue (ILT). However, the structural variation of GIALT across bony fish remains largely unknown. Here, we show how this critical zone of interaction evolved across fishes. By labeling a conserved T-cell epitope on tissue sections, we find that several basal groups of teleosts possess typical ILT, while modern teleosts have lymphoepithelium of different shape and size at the base of primary gill filaments. Within Cypriniformes, neither body size variation between two related species, zebrafish and common carp, nor morphotype variation, did have a drastic effect on the structure of ILT. Thereby this study is the first to describe the presence of ILT in zebrafish. The ILT variability across fish orders seems to represent different evolutionary solutions to balancing trade-offs between multiple adaptations of jaws and pharyngeal region, and immune responses. Our data point to a wide structural variation in gill immunity between basal groups and modern teleosts.

Inflammatory Responses during Tumour Initiation: From Zebrafish Transgenic Models of Cancer to Evidence from Mouse and Man

The zebrafish is now an important model organism for cancer biology studies and provides unique and complementary opportunities in comparison to the mammalian equivalent. The translucency of zebrafish has allowed in vivo live imaging studies of tumour initiation and progression at the cellular level, providing novel insights into our understanding of cancer. Here we summarise the available transgenic zebrafish tumour models and discuss what we have gleaned from them with respect to cancer inflammation. In particular, we focus on the host inflammatory response towards transformed cells during the pre-neoplastic stage of tumour development. We discuss features of tumour-associated macrophages and neutrophils in mammalian models and present evidence that supports the idea that these inflammatory cells promote early stage tumour development and progression. Direct live imaging of tumour initiation in zebrafish models has shown that the intrinsic inflammation induced by pre-neoplastic cells is tumour promoting. Signals mediating leukocyte recruitment to pre-neoplastic cells in zebrafish correspond to the signals that mediate leukocyte recruitment in mammalian tumours. The activation state of macrophages and neutrophils recruited to pre-neoplastic cells in zebrafish appears to be heterogenous, as seen in mammalian models, which provides an opportunity to study the plasticity of innate immune cells during tumour initiation. Although several potential mechanisms are described that might mediate the trophic function of innate immune cells during tumour initiation in zebrafish, there are several unknowns that are yet to be resolved. Rapid advancement of genetic tools and imaging technologies for zebrafish will facilitate research into the mechanisms that modulate leukocyte function during tumour initiation and identify targets for cancer prevention.

Involvement of CD4-1 T cells in the cellular immune response of olive flounder (Paralichthys olivaceus) against viral hemorrhagic septicemia virus (VHSV) and nervous necrosis virus (NNV) infection

The occurrence of CD4 helper T cells has already been established for a number of teleost species, though, it has not been possible to analyze these responses at a cellular level due to a large lack of appropriate monoclonal antibodies (mAbs). In the present study, we produced a mAb against olive flounder (Paralichthys olivaceus) CD4-1 lymphocyte to investigate the functional activity of the cells to improve our understanding of the T cell response in this species. This mAb is specifically able to detect CD4-1 lymphocytes in olive flounder proved by immunofluorescence staining and RT-PCR analysis. In flow cytometry analysis, the number of CD4-1-positive lymphocytes was observed to gradually increase from 3 days post infection (dpi) and then reach peak at 7 dpi against two viruses challenge. As a conclusion, both the basic properties of CD4-1 T cells and its response to viral infections in olive flounder are very similar to the helper T cells in terrestrial animals.

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.

Teleost contributions to the understanding of mycobacterial diseases

Few pathogens have shaped human medicine as the mycobacteria. From understanding biological phenomena driving disease spread, to mechanisms of host-pathogen interactions and antibiotic resistance, the Mycobacterium genus continues to challenge and offer insights into the basis of health and disease. Teleost fish models of mycobacterial infections have progressed significantly over the past three decades, now supplying a range of unique tools and new opportunities to define the strategies employed by these Gram-positive bacteria to overcome host defenses, as well as those host antimicrobial pathways that can be used to limit its growth and spread. Herein, we take a comparative perspective and provide an update on the contributions of teleost models to our understanding of mycobacterial diseases.

Functional Conservation of a Developmental Switch in Mammals since the Jurassic Age

ThPOK is a "master regulator" of T lymphocyte lineage choice, whose presence or absence is sufficient to dictate development to the CD4 or CD8 lineages, respectively. Induction of ThPOK is transcriptionally regulated, via a lineage-specific silencer element, SilThPOK. Here, we take advantage of the available genome sequence data as well as site-specific gene targeting technology, to evaluate the functional conservation of ThPOK regulation across mammalian evolution, and assess the importance of motif grammar (order and orientation of TF binding sites) on SilThPOK function in vivo. We make three important points: First, the SilThPOK is present in marsupial and placental mammals, but is not found in available genome assemblies of nonmammalian vertebrates, indicating that it arose after divergence of mammals from other vertebrates. Secondly, by replacing the murine SilThPOK in situ with its marsupial equivalent using a knockin approach, we demonstrate that the marsupial SilThPOK supports correct CD4 T lymphocyte lineage-specification in mice. To our knowledge, this is the first in vivo demonstration of functional equivalency for a silencer element between marsupial and placental mammals using a definitive knockin approach. Finally, we show that alteration of the position/orientation of a highly conserved region within the murine SilThPOK is sufficient to destroy silencer activity in vivo, demonstrating that motif grammar of this "solid" synteny block is critical for silencer function. Dependence of SilThPOK function on motif grammar conserved since the mid-Jurassic age, 165 Ma, suggests that the SilThPOK operates as a silenceosome, by analogy with the previously proposed enhanceosome model.

CD4: a vital player in the teleost fish immune system

CD4 is a nonpolymorphic transmembrane glycoprotein molecule that is expressed on the surface of T-helper cells and plays an essential role in the immune response. It functions as a coreceptor with the T-cell receptor by binding to major histocompatibility complex class II on the surface of dendritic cells that present antigens. CD4⁺ T cells hold a key position in coordinating the immune system through production of several cytokines after activation and differentiation. The CD4⁺ T helper subtypes (T-helper 1, T-helper 2, T-helper 17, T-helper 9, and regulatory-T cells) perform different immune functions subsequent to their differentiation from the naive T cells. Different types of CD4⁺ T cells require different cytokines such as drivers and effectors, as well as master transcription factors for their activation. Fish cells that express CD4-related genes are activated in the presence of a pathogen and release cytokines against the pathogen. This review highlights the types of CD4⁺ T cells in fish and describes their direct role in cell-mediated and humoral immunity for protection against the intracellular bacterial as well as viral infections in fish.

Influence of CD4-1+, CD4-2+ and CD8+ T lymphocytes subpopulations on the immune response of B lymphocytes in flounder (Paralichthys olivaceus) immunized with thymus-dependent or thymus-independent antigen

In order to elucidate the influence of T lymphocytes subpopulations on B lymphocytes immune response, in this paper, CD4-1⁺, CD4-2⁺, CD8⁺ T lymphocytes and B lymphocytes responses to thymus-independent (TI) or thymus-dependent (TD) antigen plus immunosuppressant were investigated in flounder (Paralichthys olivaceus). The results showed that in LPS-immunized group, the percentages of CD4-1⁺, CD4-2⁺, CD8β⁺ T (PCD4-1⁺ T, PCD4-2⁺ T and PCD8β⁺ T) lymphocytes in peripheral blood leucocytes (PBLs) had no significant variations, the percentages of IgM⁺ B (PIgM⁺ B) lymphocytes and LPS-specific antibodies (LA) significantly increased and peaked at 3^rd or 4^th week post-injection; CsA had no inhibition on both T/B lymphocytes and LA; RaPa only suppressed the PIgM⁺ B lymphocytes and LA, and the inhibition maximum (Imax) were about 35% and 20%, respectively. In KLH-immunized group, the PCD4-1⁺, PCD4-2⁺ and PCD8β⁺ T lymphocytes significantly increased and peaked at 3^rd or 5^th day, successively the PIgM⁺ B lymphocytes and KLH-specific antibodies (KA) significantly increased to the peak at 5^th week; the PCD4-1⁺, PCD4-2⁺ T and PIgM⁺ B lymphocytes and LA were inhibited significantly by both CsA and RaPa, and the Imax on them were 13%-33%, 11%-25%, 19%-34%, 22%-26%, respectively, while the PCD8β⁺ T lymphocytes showed no significant suppression. The results indicated that the suppression of PIgM⁺ B lymphocytes in KLH + CsA group was not directly derived from CsA, but due to the suppression of T lymphocytes, especially CD4⁺ T lymphocytes subpopulations. The results showed for the first time that, similar to higher vertebrates, T lymphocytes didn't respond to TI antigen, moreover, T lymphocyte subpopulations had a regulation on the immune response of B lymphocyte for TD antigen in flounder.

Immune-Related Transcriptional Responses to Parasitic Infection in a Naturally Inbred Fish: Roles of Genotype and Individual Variation

Parasites are strong drivers of evolutionary change and the genetic variation of both host and parasite populations can co-evolve as a function of parasite virulence and host resistance. The role of transcriptome variation in specific interactions between host and parasite genotypes has been less studied and can be confounded by differences in genetic variation. We employed two naturally inbred lines of a self-fertilizing fish to estimate the role of host genotype in the transcriptome response to parasite infection using RNA-seq. In addition, we targeted several differentially expressed immune-related genes to further investigate the relative role of individual variation in the immune response using RT-qPCR, taking advantage of the genomic uniformity of the self-fertilizing lines. We found significant differences in gene expression between lines in response to infection both in the transcriptome and in individual gene RT-qPCR analyses. Individual RT-qPCR analyses of gene expression identified significant variance differences between lines for six genes but only for three genes between infected and control fish. Our results indicate that although the genetic background plays an important role in the transcriptome response to parasites, it cannot fully explain individual differences within genetically homogeneous lines, which can be important for determining the response to parasites.

Interleukin 10 mutant zebrafish have an enhanced interferon gamma response and improved survival against a Mycobacterium marinum infection

Tuberculosis ranks as one of the world’s deadliest infectious diseases causing more than a million casualties annually. IL10 inhibits the function of Th1 type cells, and IL10 deficiency has been associated with an improved resistance against Mycobacterium tuberculosis infection in a mouse model. Here, we utilized M. marinum infection in the zebrafish (Danio rerio) as a model for studying Il10 in the host response against mycobacteria. Unchallenged, nonsense il10^e46/e46 mutant zebrafish were fertile and phenotypically normal. Following a chronic mycobacterial infection, il10^e46/e46 mutants showed enhanced survival compared to the controls. This was associated with an increased expression of the Th cell marker cd4-1 and a shift towards a Th1 type immune response, which was demonstrated by the upregulated expression of tbx21 and ifng1, as well as the down-regulation of gata3. In addition, at 8 weeks post infection il10^e46/e46 mutant zebrafish had reduced expression levels of proinflammatory cytokines tnfb and il1b, presumably indicating slower progress of the infection. Altogether, our data show that Il10 can weaken the immune defense against M. marinum infection in zebrafish by restricting ifng1 response. Importantly, our findings support the relevance of M. marinum infection in zebrafish as a model for tuberculosis.

Psychoneuroimmunology and immunopsychiatry of zebrafish

Despite the high prevalence of neural and immune disorders, their etiology and molecular mechanisms remain poorly understood. As the zebrafish (Danio rerio) is increasingly utilized as a powerful model organism in biomedical research, mounting evidence suggests these fish as a useful tool to study neural and immune mechanisms and their interplay. Here, we discuss zebrafish neuro-immune mechanisms and their pharmacological and genetic modulation, the effect of stress on cytokines, as well as relevant models of microbiota-brain interplay. As many human brain diseases are based on complex interplay between the neural and the immune system, here we discuss zebrafish models, as well as recent successes and challenges, in this rapidly expanding field. We particularly emphasize the growing utility of zebrafish models in translational immunopsychiatry research, as they improve our understanding of pathogenetic neuro-immune interactions, thereby fostering future discovery of potential therapeutic agents.

Identification of protective postexposure mycobacterial vaccine antigens using an immunosuppression-based reactivation model in the zebrafish

Roughly one third of the human population carries a latent Mycobacterium tuberculosis infection, with a 5-10% lifetime risk of reactivation to active tuberculosis and further spreading the disease. The mechanisms leading to the reactivation of a latent Mycobacterium tuberculosis infection are insufficiently understood. Here, we used a natural fish pathogen, Mycobacterium marinum, to model the reactivation of a mycobacterial infection in the adult zebrafish (Danio rerio). A low-dose intraperitoneal injection (∼40 colony-forming units) led to a latent infection, with mycobacteria found in well-organized granulomas surrounded by a thick layer of fibrous tissue. A latent infection could be reactivated by oral dexamethasone treatment, which led to disruption of the granuloma structures and dissemination of bacteria. This was associated with the depletion of lymphocytes, especially CD4⁺ T cells. Using this model, we verified that ethambutol is effective against an active disease but not a latent infection. In addition, we screened 15 mycobacterial antigens as postexposure DNA vaccines, of which RpfB and MMAR_4207 reduced bacterial burdens upon reactivation, as did the Ag85-ESAT-6 combination. In conclusion, the adult zebrafish-M. marinum infection model provides a feasible tool for examining the mechanisms of reactivation in mycobacterial infections, and for screening vaccine and drug candidates.This article has an associated First Person interview with the first author of the paper.

Identification and characterization of T reg-like cells in zebrafish

Kasheta et al. report the identification of T reg–like cells in zebrafish, a means to track and live-image these cells, and foxp3a-deficient mutants that display lymphoproliferation, severe inflammation, and other hallmarks of T reg deficiency syndromes.

Regulatory T (T reg) cells are a specialized sublineage of T lymphocytes that suppress autoreactive T cells. Functional studies of T reg cells in vitro have defined multiple suppression mechanisms, and studies of T reg–deficient humans and mice have made clear the important role that these cells play in preventing autoimmunity. However, many questions remain about how T reg cells act in vivo. Specifically, it is not clear which suppression mechanisms are most important, where T reg cells act, and how they get there. To begin to address these issues, we sought to identify T reg cells in zebrafish, a model system that provides unparalleled advantages in live-cell imaging and high-throughput genetic analyses. Using a FOXP3 orthologue as a marker, we identified CD4-enriched, mature T lymphocytes with properties of T reg cells. Zebrafish mutant for foxp3a displayed excess T lymphocytes, splenomegaly, and a profound inflammatory phenotype that was suppressed by genetic ablation of lymphocytes. This study identifies T reg–like cells in zebrafish, providing both a model to study the normal functions of these cells in vivo and mutants to explore the consequences of their loss.