Eomesodermin of atlantic salmon: an important regulator of cytolytic gene and interferon gamma expression in spleen lymphocytes

In Situ Detection of Salmonid Alphavirus 3 (SAV3) in Tissues of Atlantic Salmon in a Cohabitation Challenge Model with a Special Focus on the Immune Response to the Virus in the Pseudobranch

Salmonid alphavirus strain 3 is responsible for outbreaks of pancreas disease in salmon and rainbow trout in Norway. Although the extensive amount of research on SAV3 focused mainly on the heart and pancreas (of clinical importance), tropism and pathogenesis studies of the virus in other salmon tissues are limited. Here, we used a combination of RT-qPCR (Q_nsp1 gene) and in situ hybridization (RNAscope®) to demonstrate the tropism of SAV3 in situ in tissues of Atlantic salmon, employing a challenge model (by cohabitation). In addition, as previous results suggested that the pseudobranch may harbor the virus, the change in the expression of different immune genes upon SAV3 infection (RT-qPCR) was focused on the pseudobranch in this study. In situ hybridization detected SAV3 in different tissues of Atlantic salmon during the acute phase of the infection, with the heart ventricle showing the most extensive infection. Furthermore, the detection of the virus in different adipose tissues associated with the internal organs of the salmon suggests a specific affinity of SAV3 to adipocyte components. The inconsistent immune response to SAV3 in the pseudobranch after infection did not mitigate the infection in that tissue and is probably responsible for the persistent low infection at 4 weeks post-challenge. The early detection of SAV3 in the pseudobranch after infection, along with the persistent low infection over the experimental infection course, suggests a pivotal role of the pseudobranch in SAV3 pathogenesis in Atlantic salmon.

Summary and comparison of the perforin in teleosts and mammals: A review

Perforin, a pore-forming glycoprotein, has been demonstrated to play key roles in clearing virus-infected cells and tumour cells due to its ability of forming 'pores' on the cell membranes. Additionally, perforin is also found to be associated with human diseases such as tumours, virus infections, immune rejection and some autoimmune diseases. Until now, plenty of perforin genes have been identified in vertebrates, especially the mammals and teleost fish. Conversely, vertebrate homologue of perforin gene was not identified in the invertebrates. Although recently there have been several reviews focusing on perforin and granzymes in mammals, no one highlighted the current advances of perforin in the other vertebrates. Here, in addition to mammalian perforin, the structure, evolution, tissue distribution and function of perforin in bony fish are summarized, respectively, which will allow us to gain more insights into the perforin in lower animals and the evolution of this important pore-forming protein across vertebrates.

GATA1/SP1 and miR-874 mediate enterovirus-71-induced apoptosis in a granzyme-B-dependent manner in Jurkat cells

Enterovirus 71 (EV71)-induced T lymphocyte apoptosis plays an important role in hand, foot, and mouth disease (HFMD), and granzyme B (GZMB) has been shown to be critical for this process. However, the mechanisms underlying GZMB-mediated apoptosis of T lymphocytes remain unknown. In this study, we investigated whether transcription factors and microRNAs (miRNAs) are involved in GZMB-mediated apoptosis of T lymphocytes in response to EV71 infection. Our findings indicated that EV71 infection significantly induced apoptosis in Jurkat cells, a human T lymphocytes cell line, as revealed in flow cytometric analysis. Furthermore, EV71 increased the expression of pro-apoptosis Bcl-2-associated X (Bax) and cleaved caspase 3 but decreased the expression of anti-apoptosis B-cell lymphoma protein 2 (Bcl2). GZMB knockdown decreased cell apoptosis and prevented EV71-induced changes in the expression of Bax, cleaved caspase 3, and Bcl2 in Jurkat cells, highlighting the role of GZMB as a key factor in EV71-induced apoptosis. Our study also indicated that overexpression of the transcription factors GATA binding factor 1 (GATA1) and specificity protein 1 (SP1) significantly increased luciferase activity when this gene was inserted in the GZMB 3' untranslated region (3'UTR). GATA1/SP1 overexpression induced cell apoptosis, increased the expression of Bax and cleaved caspase 3, and decreased the expression of Bcl2. Finally, our results suggested that miR-874 plays an essential role in GZMB-mediated cell apoptosis, since an miR-874 mimic decreases the expression of GZMB by targeting its 3'UTR. Collectively, these data indicated that GATA1/SP1 and miR-874 mediate EV71-induced apoptosis in a granzyme B-dependent manner. This signaling pathway may provide a new pharmacological target for the prevention and treatment of HFMD.

Overexpression of T-bet, GATA-3 and TGF-ß Induces IFN-γ, IL-4/13A, and IL-17A Expression in Atlantic Salmon

The overexpression of GATA-3, T-bet and TGF-ß may theoretically induce IL-4/A, IFN-γ and IL-17A expression, respectively. Whether this also applies to fish is not yet known. The plasmid vectors encoding reporter gene (RFP)-tagged T-bet, GATA-3 and TGF-ß were used as overexpression tools, transfected into cells or injected intramuscularly to monitor the expression of IFN-γ, IL-4/13A and IL-17A. In addition, the fish were either experimentally challenged with Vibrio anguillarum (VA group) or Piscirickettsia salmonis (PS group). The reporter gene (RFP) inserted upstream of the GATA-3, T-bet and TGF-ß genes, was observed in muscle cell nuclei and in inflammatory cells after intramuscular (i.m.) injection. PS group: following the injection of GATA-3 and T-bet-encoding plasmids, the expression of GATA-3 and T-bet was high at the injection site. The spleen expression of IFN-γ, following the injection of a T-bet-encoding plasmid, was significantly higher on day 2. VA group: The T-bet and GATA-3-overexpressing fish expressed high T-bet and GATA-3 mRNA levels in the muscles and on day 4 post-challenge. The expression of TGF-ß in the muscles of fish injected with TGF-ß-encoding plasmids was significantly higher on days 7 (8 days pre-challenge) and 19 (4 days after challenge). The protective effects of the overexpression of T-bet, GATA-3 and TGF-ß on both bacterial infections were negligible.

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.

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

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

T-box transcription factor eomesodermin/Tbr2 in Atlantic cod (Gadus morhua L.): Molecular characterization, promoter structure and function analysis

Eomesodermin (Eomes) is a member of T-box transcription factor family and plays an important role in the regulation of a wide variety of developmental processes and immune response in animals. Here we report cloning and characterization of the full-length cDNA of Atlantic cod Eomes (GmEomes), which possesses a TBOX_3 domain similar to its counterpart in mammals. The regulated expression was observed in head kidney and spleen in response to live Vibrio anguillarum infection in vivo, and spleen leukocytes in vitro after PMA and poly I:C stimulation. Furthermore, we determined a 694 bp sequence, upstream of the transcriptional start site (TSS), to contain a number of sequence motifs that matched known transcription factor-binding sites. Activities of the presumptive regulatory gene were assessed by transfecting different 5'-deletion constructs in CHSE-214 cells. The results showed that the basal promoters and positive transcriptional regulator activities of GmEomes were dependent by sequences located from -694 to -376 bp upstream of TSS. Furthermore, we found that some Eomes binding sites were present in the 5'-flanking regions of the cod IFNγ gene predicted by bioinformatics. However, Co-transfection of eomesodermin overexpression plasmids with INFγ reporter vector into CHSE-214 cells determined that Atlantic cod eomesodermin played a minor role in activation of the INFγ promoter.

Comparison of gene expression in post-smolt Atlantic salmon challenged by LF-89-like and EM-90-like Piscirickettsia salmonis isolates reveals differences in the immune response associated with pathogenicity

Piscirickettsiosis is the main bacterial disease affecting the Chilean salmon farming industry and is responsible for high economic losses. The aim of this study was to describe and comparatively quantify the immune response of post-smolt Atlantic salmon infected by cohabitation with fish bearing LF-89-like and EM-90-like Piscirickettsia salmonis. The expression of 17 genes related to the immune response was studied in head kidney from cohabitant fish by RT-qPCR. Our results at the transcriptomic level suggest that P. salmonis is able to manipulate the kinetics of cytokine production in a way that might constitute a virulence mechanism that promotes intracellular bacterial replication in cells of Atlantic salmon. This strategy involves the creation of an ideal environment for the microorganism based on induction of the inflammatory and IFN-mediated response, modulation of Th1 polarization, reduced antigen processing and presentation, modulation of the evasion of the immune response mediated by CD8⁺ T cells and promotion of the CD4⁺ T-cell response during the late stage of infection as a mechanism to escape host defences. This response was significantly exacerbated in fish infected by PS-EM-90 compared with fish infected by PS-LF-89, a finding that is probably associated with the higher pathogenicity of PS-EM-90.

A Review of the Immunological Mechanisms Following Mucosal Vaccination of Finfish

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

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

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

Acquired immunity and vaccination against infectious pancreatic necrosis virus of salmon

Acquired immunity plays an important role in the protection of salmonids vaccinated against infectious pancreatic necrosis virus (IPNV) infections. In recent years, vaccine research has taken a functional approach to find the correlates of protective immunity against IPNV infections. Accumulating evidence suggests that the humoral response, specifically IgM is a correlate of vaccine protection against IPNV infections. The role of IgT on the other hand, especially at the sites of virus entry into the host is yet to be established. The kinetics of CD4+ and CD8+ T-cell gene expression have also been shown to correlate with protection in salmonids, suggesting that other arms of the adaptive immune response e.g. cytotoxic T cell responses and Th1 may also be important. Overall, the mechanisms of vaccine protection observed in salmonids are comparable to those seen in other vertebrates suggesting that the immunological basis of vaccine protection has been conserved across vertebrate taxa.

Comparison of small interfering RNA (siRNA) delivery into bovine monocyte-derived macrophages by transfection and electroporation

The manipulation of the RNA interference pathway using small interfering RNA (siRNA) has become the most frequently used gene silencing method. However, siRNA delivery into primary cells, especially primary macrophages, is often considered challenging. Here we report the investigation of the suitability of two methodologies: transient transfection and electroporation, to deliver siRNA targeted against the putative immunomodulatory gene Mediterranean fever (MEFV) into primary bovine monocyte-derived macrophages (bMDM). Eleven commercial transfection reagents were investigated with variable results with respect to siRNA uptake, target gene knock-down, cell toxicity and type I interferon (IFN) response induction. Three transfection reagents: Lipofectamine 2000, Lipofectamine RNAiMAX and DharmaFECT 3, were found to consistently give the best results. However, all the transfection reagents tested induced an IFN response in the absence of siRNA, which could be minimized by reducing the transfection reagent incubation period. In addition, optimized siRNA delivery into bMDM by electroporation achieved comparable levels of target gene knock-down as transient transfection, without a detectable IFN response, but with higher levels of cell toxicity. The optimized transient transfection and electroporation methodologies may provide a starting point for optimizing siRNA delivery into macrophages derived from other species or other cells considered difficult to investigate with siRNA.

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

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

Aeromonas salmonicida subsp. salmonicida in the light of its type-three secretion system

Aeromonas salmonicida subsp. salmonicida is an important pathogen in salmonid aquaculture and is responsible for the typical furunculosis. The type-three secretion system (T3SS) is a major virulence system. In this work, we review structure and function of this highly sophisticated nanosyringe in A. salmonicida. Based on the literature as well as personal experimental observations, we document the genetic (re)organization, expression regulation, anatomy, putative functional origin and roles in the infectious process of this T3SS. We propose a model of pathogenesis where A. salmonicida induces a temporary immunosuppression state in fish in order to acquire free access to host tissues. Finally, we highlight putative important therapeutic and vaccine strategies to prevent furunculosis of salmonid fish.

Transgene and immune gene expression following intramuscular injection of Atlantic salmon (Salmo salar L.) with DNA-releasing PLGA nano- and microparticles

The use of poly-(D,L-lactic-co-glycolic) acid (PLGA) particles as carriers for DNA delivery has received considerable attention in mammalian studies. DNA vaccination of fish has been shown to elicit durable transgene expression, but no reports exist on intramuscular administration of PLGA-encapsulated plasmid DNA (pDNA). We injected Atlantic salmon (Salmo salar L.) intramuscularly with a plasmid vector containing a luciferase (Photinus pyralis) reporter gene as a) naked pDNA, b) encapsulated into PLGA nano- (~320 nm) (NP) or microparticles (~4 μm) (MP), c) in an oil-based formulation, or with empty particles of both sizes. The ability of the different pDNA-treatments to induce transgene expression was analyzed through a 70-day experimental period. Anatomical distribution patterns and depot effects were determined by tracking isotope labeled pDNA. Muscle, head kidney and spleen from all treatment groups were analyzed for proinflammatory cytokines (TNF-α, IL-1β), antiviral genes (IFN-α, Mx) and cytotoxic T-cell markers (CD8, Eomes) at mRNA transcription levels at days 1, 2, 4 and 7. Histopathological examinations were performed on injection site samples from days 2, 7 and 30. Injection of either naked pDNA or the oil-formulation was superior to particle treatments for inducing transgene expression at early time-points. Empty particles of both sizes were able to induce proinflammatory immune responses as well as degenerative and inflammatory pathology at the injection site. Microparticles demonstrated injection site depots and an inflammatory pathology comparable to the oil-based formulation. In comparison, the distribution of NP-encapsulated pDNA resembled that of naked pDNA, although encapsulation into NPs significantly elevated the expression of antiviral genes in all tissues. Together the results indicate that while naked pDNA is most efficient for inducing transgene expression, the encapsulation of pDNA into NPs up-regulates antiviral responses that could be of benefit to DNA vaccination.

Role of redox metabolism for adaptation of aquatic animals to drastic changes in oxygen availability

Large changes in oxygen availability in aquatic environments, ranging from anoxia through to hyperoxia, can lead to corresponding wide variation in the production of reactive oxygen species (ROS) by animals with aquatic respiration. Therefore, animals living in marine, estuarine and freshwater environments have developed efficient antioxidant defenses to minimize oxidative stress and to regulate the cellular actions of ROS. Changes in oxygen levels may lead to bursts of ROS generation that can be particularly harmful. This situation is commonly experienced by aquatic animals during abrupt transitions from periods of hypoxia/anoxia back to oxygenated conditions (e.g. intertidal cycles). The strategies developed differ significantly among aquatic species and are (i) improvement of their endogenous antioxidant system under hyperoxia (that leads to increased ROS formation) or other similar ROS-related stresses, (ii) increase in antioxidant levels when displaying higher metabolic rates, (iii) presence of constitutively high levels of antioxidants, that attenuates oxidative stress derived from fluctuations in oxygen availability, or (iv) increase in the activity of antioxidant enzymes (and/or the levels of their mRNAs) during hypometabolic states associated with anoxia/hypoxia. This enhancement of the antioxidant system - coined over a decade ago as "preparation for oxidative stress" - controls the possible harmful effects of increased ROS formation during hypoxia/reoxygenation. The present article proposes a novel explanation for the biochemical and molecular mechanisms involved in this phenomenon that could be triggered by hypoxia-induced ROS formation. We also discuss the connections among oxygen sensing, oxidative damage and regulation of the endogenous antioxidant defense apparatus in animals adapted to many natural or man-made challenges of the aquatic environment.