Nodavirus encephalopathy in turbot (Scophthalmus maximus): inflammation, nitric oxide production and effect of anti-inflammatory compounds

The susceptibility of shi drum juveniles to betanodavirus increases with rearing densities in a process mediated by neuroactive ligand-receptor interaction

Nervous necrosis virus (NNV) is one of the greatest threats to Mediterranean aquaculture, infecting more than 170 fish species and causing mortalities up to 100% in larvae and juveniles of susceptible species. Intensive aquaculture implies stressed conditions that affect the welfare of fish and their ability to fight against infections. In fact, a higher susceptibility to NNV has been related to poor welfare conditions. In order to analyze the physiological link between stressed conditions and increased susceptibility to NNV, as well as its possible role in the pathogenesis of this disease, we reared shi drum (Umbrina cirrosa) juveniles (30.7 ± 3.10 g body weight), which are expected to be asymptomatic upon NNV infection, at three stocking densities (2, 15, and 30 kg/m3) for 27 days and subsequently challenged them with NNV. We firstly characterized the stressed conditions of the specimens before and after infection and recorded the mortalities, demonstrating that stressed specimens reared at 30 kg/m3 suffered mortalities. However, the viral loads in different tissues were similar in all experimental groups, allowing horizontal and vertical transmission of the virus from asymptomatic specimens. All of these data suggest that shi drum tolerates wide ranges of culture densities, although high densities might be a setback for controlling NNV outbreaks in this species. In an attempt to understand the molecular pathways orchestrating this susceptibility change in stressed conditions, we performed a transcriptomic analysis of four tissues under mock- and NNV-infected conditions. In addition to the modification of the exceptive pathways such as cell adhesion, leukocyte migration, cytokine interaction, cell proliferation and survival, and autophagy, we also observed a heavy alteration of the neuroactive ligand-receptor pathway in three of the four tissues analyzed. Our data also point to some of the receptors of this pathway as potential candidates for future pharmacological treatment to avoid the exacerbated immune response that could trigger fish mortalities upon NNV infection.

Identification and functional characterization of fish IL-17 receptors suggest important roles in the response to nodavirus infection

Th17 is a lymphocyte T helper (Th) subpopulation relevant in the control and regulation of the immune response characterized by the production of interleukin (IL)-17. This crucial cytokine family acts through their binding to the IL-17 receptors (IL-17R), having up to six members. Although the biology of fish Th17 is well-recognized, the molecular and functional characterization of IL-17 and IL-17R has been limited. Thus, our aim was to identify and characterize the IL-17R repertory and regulation in the two main Mediterranean cultured fish species, the gilthead seabream (Sparus aurata) and the European sea bass (Dicentrarchus labrax). Our in silico results showed the clear identification of six members in each fish species, from IL-17RA to IL-17RE-like, with well-conserved gene structure and protein domains with their human orthologues. All of them showed wide and constitutive transcription in naïve tissues but with highest levels in mucosal tissues, namely skin, gill or intestine. In leucocytes, T mitogens showed the strongest up-regulation in most of the il17 receptors though il17ra resulted in inhibition by most stimulants. Interestingly, in vivo nodavirus infection resulted in alterations on the transcription of il17 receptors. While nodavirus infection led to some increments in the il17ra, il17rb, il17rc and il17rd transcripts in the susceptible European sea bass, many down-regulations were observed in the resistant gilthead seabream. Our data identify the presence and conservation of six coding IL-17R in gilthead seabream and European sea bass as well as their differential regulation in vitro and upon nodavirus infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-024-00225-1.

Immune response of European sea bass (Dicentrarchus labrax L.) against combination of antigens from three different pathogens

Three of the most important diseases of Mediterranean intensive European sea bass farming are, viral nervous necrosis (VNN) caused by the red grouper nervous necrosis virus (RGNNV) genotype of b-nodavirus, photobacteriosis caused by Photobacterium damselae subsp. piscicida (Phdp) and vibriosis caused mainly by the O1 serotype of Vibrio anguillarum (VaO1). Prevention against these diseases is performed through vaccination with a monovalent vaccine against the viral disease and, usually, with bivalent vaccines against the bacterial diseases. However, it is very difficult to program two vaccinations during the same season for the same fish stock and producers are forced to either vaccinate for the viral or the bacterial diseases or to perform double vaccination with both vaccines, without any prior knowledge on any interactions that may occur due to the plethora of antigens (Ag) injected. Ideally, therefore, a trivalent vaccine should be developed against all three diseases. The objective of this work was to analyse the immune response of sea bass against combinations of Ags from all three pathogens, namely viral particles, Phdp whole cells (WC), lipopolysaccharide (LPS), capsular polysaccharide (CPS) and extracellular products (ECPs) and VaO1 WC and ECPs in respect to the identification of any phenomena of immunodominance/immunosuppression between Ags with a view to select candidate Ags for inclusion in a trivalent vaccine formulation. Eight triplicate groups of fish were immunized with different combinations of the aforementioned Ags and another triplicate group served as negative control. Blood serum was isolated at various time-points post-immunization for the measurement of specific antibodies against each Ag and, in addition, leucocytes were isolated at day 29 post-immunization for analysis of various cellular activities. Results indicated that best levels of specific a-NNV virus antibodies (Abs) were produced when VaO1 ECPs were not included in the Ag combinations, in contrast to the leucocytes proliferation assay where best stimulation against NNV Ags was measured when VaO1 ECPs were present in Ag combinations. VaO1 ECPs apparently is a strong immunogen for both humoral and cellular responses but suppresses immunological reactions against the other Ags.VaO1 WC, Phdp LPS and ECPs raised good humoral immune responses in the groups with best responses against VNN Ags, but only VaO1 WC and Phdp ECPs provided good stimulation of leucocytes, with Phdp WC and CPS effecting either similar stimulation with untrained leucocytes (control groups) or down-stimulation. Results are discussed with a view to select Ags from all three pathogens for inclusion in trivalent vaccine against all three pathogens.

Immunity elicited by AMP-encoding plasmids fails to increase the protection of European sea bass against nodavirus

Antimicrobial peptides (AMPs) are a potent arm of the innate immune system that can directly kill pathogens and induce immunomodulation. In the marine aquaculture, European sea bass (Dicentrarchus labrax L.) is one of the most prosperous species but is highly susceptible to nodavirus (NNV), which produces high rates of mortality in larvae and juvenile stages. Thus, we aimed to evaluate whether AMPs exert immunomodulatory and/or NNV-preventive actions in sea bass. To do this, plasmids encoding the sea bass AMPs dicentracin (pDIC), beta-defensin (pDB1), hepcidin (pHAMP2) or NK-lysin (pNKL) were generated and intramuscularly injected into sea bass juveniles to evaluate their immunomodulatory and anti-NNV roles. Sea bass muscle transcribes the AMPs and produces an increase in their circulating levels, along with an increase of the antibacterial activity. Immune-related gene analysis revealed a great activation of the inflammatory response and the recruitment of neutrophilic granulocytes at the site of injection. However, AMP-encoding plasmids, namely pHAMP2, negatively affected to NNV disease by increasing fish mortality. In conclusion, plasmids encoding AMPs show immunostimulatory effects on European sea bass but do not improve the resistance to NNV.

Zebrafish as a Vertebrate Model for Studying Nodavirus Infections

Nervous necrosis virus (NNV) is a neurotropic pathogenic virus affecting a multitude of marine and freshwater fish species that has a high economic impact on aquaculture farms worldwide. Therefore, the development of new tools and strategies aimed at reducing the mortality caused by this virus is a pivotal need. Although zebrafish is not considered a natural host for NNV, the numerous experimental advantages of this species make zebrafish an attractive model for studying different aspects of the disease caused by NNV, viral encephalopathy and retinopathy (VER). In this work, we established the best way and age to infect zebrafish larvae with NNV, obtaining significant mortalities in 3-day-postfertilization larvae when the virus was inoculated directly into the brain or by intramuscular microinjection. As occurs in naturally susceptible fish species, we confirmed that after intramuscular injection the virus was able to migrate to the central nervous system (CNS). As expected, due to the severe damage that this virus causes to the CNS, alterations in the swimming behavior of the zebrafish larvae were also observed. Taking advantage of the existence of transgenic fluorescent zebrafish lines, we were able to track the migration of different innate immune cells, mainly neutrophils, to the site of infection with NNV via the brain. However, we did not observe colocalization between the viral particles and neutrophils. RNA-Seq analysis of NNV-infected and uninfected larvae at 1, 3 and 5 days postinfection (dpi) revealed a powerful modulation of the antiviral immune response, especially at 5 dpi. We found that this response was dominated by, though not restricted to, the type I interferon system, the major defence mechanism in the innate immune response against viral pathogens. Therefore, as zebrafish larvae are able to develop the main characteristic of NNV infection and respond with an efficient immune arsenal, we confirmed the suitability of zebrafish larvae for modelling VER disease and studying different aspects of NNV pathogenesis, immune response and screening of antiviral drugs.

Differential immunogene expression profile of European sea bass (Dicentrarchus labrax, L.) in response to highly and low virulent NNV

European sea bass is highly susceptible to the nervous necrosis virus, RGNNV genotype, whereas natural outbreaks caused by the SJNNV genotype have not been recorded. The onset and severity of an infectious disease depend on pathogen virulence factors and the host immune response. The importance of RGNNV capsid protein amino acids 247 and 270 as virulence factors has been previously demonstrated in European sea bass; however, sea bass immune response against nodaviruses with different levels of virulence has been poorly characterized. Knowing the differences between the immune response against both kinds of isolates may be key to get more insight into the host mechanisms responsible for NNV virulence. For this reason, this study analyses the transcription of immunogenes differentially expressed in European sea bass inoculated with nodaviruses with different virulence: a RGNNV virus obtained by reverse genetics (rDl956), highly virulent to sea bass, and a mutated virus (Mut_247+270Dl956, RGNNV virus displaying SJNNV-type amino acids at positions 247 and 270 of the capsid protein), presenting lower virulence. This study has been performed in brain and head kidney, and the main differences between the immunogene responses triggered by both viruses have been observed in brain. The immunogene response in this organ is stronger after inoculation with the most virulent virus, and the main differences involved genes related with IFN I system, inflammatory response, cell-mediated response, and apoptosis. The lower virulence of Mut_247+270Dl956 to European sea bass can be associated with a delayed IFN I response, as well as an early and transitory inflammation and cell-mediated responses, suggesting that those can be pivotal elements in controlling the viral infection, and therefore, their functional activity could be analysed in future studies. In addition, this study supports the role of capsid amino acids at positions 247 and 270 as important determinants of RGNNV virulence to European sea bass.

Vaccines and immune protection of principal Mediterranean marine fish species

This review describes and summarizes the knowledge on established and experimental vaccines developed against viral and bacterial pathologies affecting the most important farmed marine finfish species present in the Mediterranean area, namely European seabass Dicentrarchus labrax, sea bream Sparus aurata, turbot Psetta maxima and meagre Argyrosomus regius. The diseases that have been recorded in seabass, sea bream and meagre are caused by bacteria Vibrio anguillarum, Photobacterium damselae, Tenacibaculum maritimum as well as by viruses such as Viral Encephalopathy and Retinopathy/Viral Nervous Necrosis and Lymphocystic disease. The main pathologies of turbot are instead bacteriosis provoked by Tenacibaculum maritimum, Aeromonas sp. and Vibrio anguillarum, and virosis by viral hemorrhagic septicaemia virus. Some vaccines have been optimized and are now regularly available for the majority of the above-mentioned pathogens. A measurable immune protection has been conferred principally against Vibrio anguillarum, Photobacterium damselae sub. piscicida and VER/VNN.

Inhibition of Cyclophilin A on the replication of red spotted grouper nervous necrosis virus associates with multiple pro-inflammatory factors

Cyclophilin A (CypA) is a ubiquitously expressed cellular protein and involves in diverse pathological conditions, including infection and inflammation. CypA acts as a key factor in the replication of several viruses. However, little is known about the role of CypA in the replication of the red-spotted grouper nervous necrosis virus (RGNNV). In the present report, grouper CypA (GF-CypA) was cloned from the grouper fin cell line (GF-1) derived from orange-spotted grouper (Epinephelus coioides). Sequence analysis found that GF-CypA open reading frame (ORF) of 495 bp encodes a polypeptide of 164 amino acids residues with a molecular weight of 17.4 kDa. The deduced amino acid sequence shared highly conserved regions with CypA of other animal species, showing that GF-CypA is a new member of Cyclophilin A family. We observed that GF-CypA was up-regulated in the GF-1 cells infected with RGNNV. Additionally, overexpression of CypA could significantly inhibit the replication of RGNNV in GF-1 cells. By contrast, when the GF-CypA was knock-downed by siRNA in GF-1 cells, the replication of RGNNV was enhanced. Furthermore, the expressions of pro-inflammatory factors, such as TNF-2, TNF-α, IL-1b, and ISG-15, were increased in GF-CypA transfected GF-1 cells challenged with RGNNV, indicating that GF-CypA might be involved in the regulation of the host pro-inflammatory factors. Altogether, we conclude that GF-CypA plays a vital role in the inhibitory effect of RGNNV replication that might be modulating the cytokines secretion in GF-1 cells during RGNNV infection. These results will shed new light on the function of CypA in the replication of RGNNV and will pave a new way for the prevention of the infection of RGNNV in fish.

Integrating Genomic and Morphological Approaches in Fish Pathology Research: The Case of Turbot (Scophthalmus maximus) Enteromyxosis

Enteromyxosis, caused by Enteromyxum scophthalmi, is one of the most devastating diseases stemming from myxozoan parasites in turbot (Scophthalmus maximus L.), being a limiting factor for its production. The disease develops as a cachectic syndrome, associated to catarrhal enteritis and leukocytic depletion, with morbidity and mortality rates usually reaching 100%. To date, no effective treatment exists and there are different unknown issues concerning its pathogenesis. The gross and microscopic lesions associated to enteromyxosis have been thoroughly described, and several morphopathological studies have been carried out to elucidate the mechanisms of this host-parasite interaction. More recently, efforts have been focused on a multidisciplinary approach, combining histopathology and transcriptome analysis, which has provided significant advances in the understanding of the pathogenesis of this parasitosis. RNA-Seq technology was applied at early and advanced stages of the disease on fishes histologically evaluated and classified based on their lesional degree. In the same way, the transcriptomic data were analyzed in relation to the morphopathological picture and the course of the disease. In this paper, a comprehensive review of turbot enteromyxosis is presented, starting from the disease description up to the most novel information extracted by an integrated approach on the infection mechanisms and host response. Further, we discuss ongoing strategies toward a full understanding of host-pathogen interaction and the identification of suitable biomarkers for early diagnosis and disease management strategies.

Preliminary investigations on the role of Drp-1 dependent mitochondrial fission in attenuating RLR downstream signaling during nervous necrosis virus infection

Member of the dynamin family of large GTPases, dynamin-related protein 1 (Drp1) dependent mitochondrial fission is an intricate process regulating both cellular and organ dynamics. Present study shows that NNV perturbs mitochondrial dynamics by promoting Drp-1 dependent mitochondrial fission, which attenuates MAVS mediated downstream signaling. NNV infected SISS cells revealed induction in Drp1 expression and subsequent translocation into mitochondria. The level of MAVS expression was up-regulated over a period of 24 hpi and declined with the progression of NNV infection at 48 and 72 hpi confirmed by western blot and mRNA transcript analysis. Drp-1 displayed its association with fragmented mitochondria and the transcript abundance was significant post infection along with Mff. Expression levels of IRF-3 IFN-1 and Mx followed a similar pattern with abundant expression at 48 hpi and diminished expression during the further period. Importantly, silencing of Drp1 caused significant elevation in the RLR downstream molecules and reduction in viral RNA expression. These results suggest that NNV-induced mitochondrial fission serve to attenuate host RLR signaling. This provides an illustration of host-pathogen interaction in which the virus evades innate immunity by enhancing mitochondrial fission and perturbs MAVS, and the downstream molecules.

Immuno-related gene transcription and antibody response in nodavirus (RGNNV and SJNNV)-infected European sea bass (Dicentrarchus labrax L.)

The immune response of European sea bass to RGNNV and SJNNV infections has been evaluated by quantifying the transcription of some genes involved in the IFN I system, as well as in the inflammatory and adaptive immune mechanisms. The transcription of IFN-I, ISG-12, ISG-15 and MxA genes has been analyzed in brain and head kidney, showing that RGNNV genotype induces a more intense response of the IFN I system than SJNNV in both organs. In addition, the results obtained indicate the importance of the inflammatory response in nodavirus pathogenesis, with the transcription of IL-8 and TNF-α significantly higher in brain than in head kidney, being RGNNV the strongest inductor. An important difference between the immune response induced by both genotypes refers to the IgM titre in sera, which was higher in SJNNV-inoculated fish. The acquired response is also important locally, since TR-γ transcription is higher in brain than in head kidney (especially in the RGNNV-inoculated group). To our knowledge, this is the first study addressing the sea bass anti-SJNNV immune response.

Interleukin-1β secreted from betanodavirus-infected microglia caused the death of neurons in giant grouper brains

High interleukin (IL)-1β gene expression was observed in dead giant grouper brains after nervous necrosis virus (NNV) infection. To investigate the neuronal death caused by NNV infection, primary tissue culture of giant grouper brains (pGB) was performed. In NNV-infected pGB cells, the viral capsid protein was detected in both neurons and microglia; furthermore, microglial proliferation and neuronal death were observed. The culture supernatant (CS) of NNV-infected pGB cells contained IL-1β and tumor necrosis factor-α, which were mainly released from the microglia. A new batch of pGB cells was treated with CS, resulting in neuronal death, which could be prevented by blocking the IL-1β in the CS by using anti-IL-1β polyclonal antibodies. Moreover, pGB cells treated with recombinant IL-1β showed microglial proliferation and neuronal death. Thus, NNV infection may activate microglial proliferation and stimulate microglial secretion of IL-1β, which is a critical cytokine responsible for neuronal death in NNV-infected grouper brains.

Humoral and cytokine responses in giant groupers after vaccination and challenge with betanodavirus

Giant groupers were immunized with two dosages (V_high and V_low) of inactivated nervous necrosis virus (NNV) and subsequently challenged with NNV at 4 weeks post vaccination (wpv). Several indicators were used to analyze the protective effects of the NNV vaccine. The neutralizing antibody titer of fish serum mostly corresponded to the survival rate of immunized fish in the NNV challenge test. Extravascular IgM⁺ cells were detected in the brains of both NNV-infected and noninfected groupers. After NNV infection, CD8α and IgM gene expression increased in the brains, indicating CD8α⁺ and IgM⁺ lymphocyte infiltration. Moreover, the NNV load was not the highest in dead grouper brains, suggesting that this load in the brain was not the key factor for the death of groupers. However, the brains of dead fish showed the highest expression of the interleukin (IL)-1β gene, a neurotoxic factor in the brain. Therefore, IL-1β overexpression is likely to be associated with the death of NNV-infected groupers.

Understanding the interaction between Betanodavirus and its host for the development of prophylactic measures for viral encephalopathy and retinopathy

Over the last three decades, the causative agent of viral encephalopathy and retinopathy (VER) disease has become a serious problem of marine finfish aquaculture, and more recently the disease has also been associated with farmed freshwater fish. The virus has been classified as a Betanodavirus within the family Nodaviridae, and the fact that Betanodaviruses are known to affect more than 120 different farmed and wild fish and invertebrate species, highlights the risk that Betanodaviruses pose to global aquaculture production. Betanodaviruses have been clustered into four genotypes, based on the RNA sequence of the T4 variable region of their capsid protein, and are named after the fish species from which they were first derived i.e. Striped Jack nervous necrosis virus (SJNNV), Tiger puffer nervous necrosis virus (TPNNV), Barfin flounder nervous necrosis virus (BFNNV) and Red-spotted grouper nervous necrosis virus (RGNNV), while an additional genotype turbot betanodavirus strain (TNV) has also been proposed. However, these genotypes tend to be associated with a particular water temperature range rather than being species-specific. Larvae and juvenile fish are especially susceptible to VER, with up to 100% mortality resulting in these age groups during disease episodes, with vertical transmission of the virus increasing the disease problem in smaller fish. A number of vaccine preparations have been tested in the laboratory and in the field e.g. inactivated virus, recombinant proteins, virus-like particles and DNA based vaccines, and their efficacy, based on relative percentage survival, has ranged from medium to high levels of protection to little or no protection. Ultimately a combination of effective prophylactic measures, including vaccination, is needed to control VER, and should also target larvae and broodstock stages of production to help the industry deal with the problem of vertical transmission. As yet there are no commercial vaccines for VER and the aquaculture industry eagerly awaits such a product. In this review we provide an overview on the current state of knowledge of the disease, the pathogen, and interactions between betanodavirus and its host, to provide a greater understanding of the multiple factors involved in the disease process. Such knowledge is needed to develop effective methods for controlling VER in the field, to protect the various aquaculture species farmed globally from the different Betanodavirus genotypes to which they are susceptible.

Immunohistochemical detection and gene expression of TNFα in turbot (Scophthalmus maximus) enteromyxosis

Enteromyxum scophthalmi (Myxozoa) constitutes one of the most devastating pathogens for turbot (Scophthalmus maximus, L.) aquaculture. This parasite causes a severe intestinal parasitosis that leads to a cachectic syndrome with high morbidity and mortality rates for which no therapeutic options are available. Presence of inflammatory infiltrates, increased apoptotic rates and epithelial detaching have been described at intestinal level, as well as leukocyte depletion in lymphohaematopoietic organs. Previous investigations on enteromyxosis in turbot showed the high susceptibility of this species to the parasite and reported the existence of a dysregulated immune response against the parasite. The pleiotropic cytokine tumour necrosis factor alpha (TNFα) plays a major role in immune response and is involved in a wide range of biological activities. In teleost, the gene expression of this cytokine has been found regulated under several pathological conditions. Teleost TNFα shows some analogous functions with its mammalian counterparts, but the extent of its activities is still poorly understood. Cytokines are generally considered as a double-edge sword and TNFα has been implicated in the pathogenesis of different inflammatory diseases as well as in wasting syndromes described in mammals. The aim of this work was to analyse the expression of TNFα during enteromyxosis with molecular (Q-PCR) and morphological (immunohistochemistry) tools. Kidney, spleen and pyloric caeca from turbot with moderate and severe infections were analysed and compared to healthy naïve fish. TNFα expression was increased in both spleen and kidney in the earlier stages of the disease, whereas in severely infected fish, the expression decreased, especially in kidney. At the intestinal level, an increase in the number of TNFα-positive cells was noticed, which was proportional to the infiltration of inflammatory cells. The results demonstrate the involvement of TNFα in the immune response to E. scophthalmi in turbot, which could be related to the development of the clinic signs and lesions.

Immunolocalization of tumor necrosis factor alpha in turbot (Scophthalmus maximus, L.) tissues

Tumor necrosis factor alpha (TNFα) is a cytokine involved in a broad spectrum of cellular and organismal responses. Its main function, as a potent pro-inflammatory mediator, has been demonstrated in numerous teleost species and there are many reports on the modulation of TNFα gene expression under pathological conditions. Nevertheless, there is still scarce knowledge about the tissue distribution and type of cells that express this cytokine in fish species, which would help to further investigate its biological activities. These studies are hampered by the lack of molecular markers for teleost that hinder the development of morphological techniques, like immunohistochemistry. The aim of this work was to develop an immunohistochemical technique for the detection of TNFα in paraffin-embedded organs from healthy turbot (Scophthalmus maximus), an economically-important marine fish species. A commercial anti-human TNFα antibody, whose specificity was confirmed by western blot analysis, was used. Immunoreactive cells were observed in higher numbers in the lymphohematopoietic organs, kidney, spleen and thymus, although TNFα-positive cells were also present in the digestive tract, liver, heart, gills and skin. Similarly to non-fish species, monocytes/macrophages appeared to be the main producers of this cytokine; nevertheless, the presence of immunoreactive rodlet cells in different tissues was also reported. The nature and distribution of the labeled cells appeared to be related with a strategic localization for defense response to antigenic challenge. The relative abundance of TNFα-positive cells in the lymphohematopoietic organs also suggests that this cytokine may have a broader role in the normal physiology of those organs. The immunohistochemical technique allowed the in-situ characterization of TNFα expression, representing a valid tool to investigate the immune response of turbot.

Immunity to betanodavirus infections of marine fish

Betanodaviruses cause viral nervous necrosis in numerous fish species, but some species are resistant to infection by these viruses. It is essential to fully characterize the immune responses that underlie this protective response. Complete characterization of the immune responses against nodaviruses may allow the development of methods that stimulate fish immunity and of an effective betanodavirus vaccine. Such strategies could include stimulation of specific immune system responses or blockage of factors that decrease the immune response. The innate immune system clearly provides a front-line defense, and this includes the production of interferons and other cytokines. Interferons that are released inside infected cells and that suppress viral replication may be the most ancient form of innate immunity. This review focuses on the immune responses of fish to betanodavirus infection.

IL-22 is a key player in the regulation of inflammation in fish and involves innate immune cells and PI3K signaling

IL-22 plays a role in various disorders in mammals, including mucosal-associated infections and inflammatory diseases. No functional IL-22 studies have been conducted on non-mammals to date. In this study, recombinant IL-22 (rIL-22) from turbot was produced to investigate its effects as a bioactive molecule. The expression of several pro-inflammatory cytokines was increased after rIL-22 treatment and reduced by pre-treatment with a JAK/STAT inhibitor. The involvement of the PI3K pathway in IL-22 induction was demonstrated. rIL-22 reduced the mortality in Aeromonas salmonicida-infected turbot, while higher Aeromonas hydrophila- or LPS-induced mortality was observed when IL-22 was blocked in zebrafish embryos. IL-22 knockdown increased pro-inflammatory cytokine expression in bacteria-stimulated fish. In zebrafish, IL-22 expression was detected primarily in the myeloid innate linage. It was found during early developmental stages when the adaptive immune response is not yet functional and in rag1(-)/(-) fish that lack an adaptive immune system. Our results clarify the conserved role of IL-22 in lower vertebrates. We suggest for the first time that IL-22 constitutes a key regulator of inflammatory homeostasis even in distant species such as teleosts, which diverged from mammals more than 350 million years ago.

The antiviral innate immune response in fish: evolution and conservation of the IFN system

Innate immunity constitutes the first line of the host defense after pathogen invasion. Viruses trigger the expression of interferons (IFNs). These master antiviral cytokines induce in turn a large number of interferon-stimulated genes, which possess diverse effector and regulatory functions. The IFN system is conserved in all tetrapods as well as in fishes, but not in tunicates or in the lancelet, suggesting that it originated in early vertebrates. Viral diseases are an important concern of fish aquaculture, which is why fish viruses and antiviral responses have been studied mostly in species of commercial value, such as salmonids. More recently, there has been an interest in the use of more tractable model fish species, notably the zebrafish. Progress in genomics now makes it possible to get a relatively complete image of the genes involved in innate antiviral responses in fish. In this review, by comparing the IFN system between teleosts and mammals, we will focus on its evolution in vertebrates.

Viral diseases in zebrafish: what is known and unknown

Naturally occurring viral infections have the potential to introduce confounding variability that leads to invalid and misinterpreted data. Whereas the viral diseases of research rodents are well characterized and closely monitored, no naturally occurring viral infections have been characterized for the laboratory zebrafish (Danio rerio), an increasingly important biomedical research model. Despite the ignorance about naturally occurring zebrafish viruses, zebrafish models are rapidly expanding in areas of biomedical research where the confounding effects of unknown infectious agents present a serious concern. In addition, many zebrafish research colonies remain linked to the ornamental (pet) zebrafish trade, which can contribute to the introduction of new pathogens into research colonies, whereas mice used for research are purpose bred, with no introduction of new mice from the pet industry. Identification, characterization, and monitoring of naturally occurring viruses in zebrafish are crucial to the improvement of zebrafish health, the reduction of unwanted variability, and the continued development of the zebrafish as a model organism. This article addresses the importance of identifying and characterizing the viral diseases of zebrafish as the scope of zebrafish models expands into new research areas and also briefly addresses zebrafish susceptibility to experimental viral infection and the utility of the zebrafish as an infection and immunology model.

Effect of lipopolysaccharides from Vibrio alginolyticus on the Mx gene expression and virus recovery from gilthead sea bream (Sparus aurata L.) experimentally infected with Nodavirus

Infections with nodavirus affect a wild and farmed fish species throughout the world, mostly from the marine environment. The aim of this work was to determine the immune status of gilthead sea bream that comes as a result of a Nodavirus infection, induced by activation of the interferon response pathway by lipopolysaccharides from Vibrio alginolyticus and the expression of interferoninduced Mx protein in liver samples. The enhancement of Mx protein gene expression was detected in liver samples of experimentally nodavirus infected fish and, furthermore, the immunostimulant LPS of V. alginolyticus decreased almost three times the virus titration with respect to no-immunized or infected with nodavirus group of fish.

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

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

Viral nervous necrosis virus persistently replicates in the central nervous system of asymptomatic gilthead seabream and promotes a transient inflammatory response followed by the infiltration of IgM+ B lymphocytes

The viral nervous necrosis virus (VNNV) is the causal agent of viral encephalopathy and retinopathy (VER), a worldwide fish disease that is responsible for high mortality in both marine and freshwater species. Infected fish suffer from encephalitis, which leads to abnormal swimming behavior and extensive cellular vacuolation and neuronal degeneration in the central nervous system (CNS) and retina. The marine fish gilthead seabream (Sparus aurata) does not develop VER but it is an asymptomatic carrier of VNNV. In this study, we report that VNNV was able to replicate and persist for up to 3 months in the CNS of the gilthead seabream without causing any neural damage. In addition, we found an early inflammatory response in the CNS that was characterized by the induction of genes encoding pro-inflammatory cytokines, a delayed but persistent induction of anti-inflammatory cytokines, and the infiltration of IgM(+) B lymphocytes, suggesting that local adaptive immunity played a major role in the control of VNNV in the CNS of this species.

Atlantic halibut experimentally infected with nodavirus shows increased levels of T-cell marker and IFNγ transcripts

The transcript levels of viral RNAs, selected T-cell marker and cytokine genes, toll like receptor (TLR) 7, and two interferon stimulated genes (ISG) were analysed in sexually immature adult Atlantic halibut (Hippoglossus hippoglossus L.) experimentally infected with nodavirus. The expression of the T-cell markers, TLR7 and the cytokine genes was further explored in in vitro stimulated anterior kidney leucocytes (AK leucocytes) isolated from the experiment fish and from additional untreated non-injected fish. The levels of viral RNA1 and RNA2 were increasing in brain and eye at around 4 and 8weeks post injection (wpi), respectively, and still increasing at the end of the experiment, especially in eye. Immuno-positive cells and signs of vacuolisation in both brain and eye were seen at 14wpi. Increased transcript levels of TCRβ, CD4-2, CD4, CD8α, and Lck in brain and eye of the experimentally infected halibut suggested an involvement of halibut T-cells in the immune response against nodavirus. Interestingly, a similar expression pattern of TCRβ, CD4 and Lck was seen in both brain and eye. However, compared to brain that showed elevated transcript levels of TCRβ, CD4 and Lck mainly at 10 and 14wpi, the increase appeared earlier between 3 and 4wpi in the eye. Yet, an increase in the transcript level of IFNγ was seen at 10 and 14wpi in both organs. Moreover, elevated levels of TLR7, IL-1β, IL-6, ISG15 and Mx were detected in vivo. The in vitro experiments, stimulating AK leucocytes with ConA-PMA, imiquimod or nodavirus, further supported an involvement of IL-6 and IFNγ in the immune response against nodavirus and the involvement of CD8β(+) cells. Results from the present study thus indicate an importance of T-cells, IFNγ and the analysed ISGs in the immune response against nodavirus in Atlantic halibut, and would be of great help in future vaccination trials giving the possibility to monitor the immune response rather than mortality during post-vaccination challenge experiments.

Differential expression profiling of orange-spotted grouper larvae, Epinephelus coioides (Hamilton), that survived a betanodavirus outbreak

Nervous necrosis virus (NNV), a piscine nodavirus, has caused serious viral nervous necrosis and viral encephalopathy and retinopathy in hatchery-reared larvae and juveniles of a wide range of marine teleost species worldwide in the last two decades. Although the mortality of NNV-infected larvae is nearly 100%, there are still some larvae that survive this catastrophe. To comprehensively understand the variations of these survivors at the molecular level, we collected orange-spotted grouper larvae that survived an NNV outbreak in an indoor hatchery in southern Taiwan to study differential gene expression. Healthy larvae with high, medium and low levels of detected NNV were compared with morbid larvae using a 9600-clone-containing grouper larva cDNA microarray, and differential gene expression was further confirmed by a quantitative real-time polymerase chain reaction. Significant variation exists in healthy larvae. The following genes were upregulated: adenylate kinase 1-2, myosin binding protein H-like, myosin light chain 2, myosin light chain 3, tropomyosin, fast/white muscle troponin T embryonic isoform, and parvalbumin 1 and 2 genes. The following genes were downregulated: apolipoprotein A-I, trypsinogen, pyruvate kinase and astacin-like metalloprotease. Moreover, immunoglobulin M heavy chain gene transcription was significantly higher in healthy larvae that had high virus levels, indicating that humoral immunity might protect organisms from viral infection. These results suggest that some non-immune-related genes may have played important roles in survival during the larval metamorphosis stage, after betanodavirus infection.

Cloning of crystallin from orange-spotted grouper and characterization of its activity as potential protective agent

Oxidative stress associated with nodavirus infection is poorly understood, especially pertaining to infection-mediated brain injury. Indirect evidence indicates that infection increases cellular abundance of reactive oxygen species (ROS) with consequent increase in cellular dityrosine production. The detection of dityrosine in nodavirus-infected grouper was demonstrated using immunohistochemical (IHC) staining. Proteomic analyses with eye tissues of healthy grouper revealed more abundant expression of crystallin protein in the eye than in various tissues, which was confirmed by real-time polymerase chain reaction and IHC analyses. Grouper crystallin belongs to a small heat shock protein family with chaperone-like function that prevents heat-induced and oxidative stress-induced protein aggregation. Recombinant crystallin induced nitric oxide (NO) production in RAW 264.7 cells after treatment. The results provide new insight into the pathogenesis of nodavirus and demonstrate an experimental rationale for antioxidant therapy research.

Molecular cloning and functional analysis of an orange-spotted grouper (Epinephelus coioides) secreted protein acidic and rich in cysteine (SPARC) and characterization of its expression response to nodavirus

Mammalian secreted protein acidic and rich in cysteine (SPARC) is the primary regulator of cell shape and cell adhesion to fibronectin. We, for the first time, report the complete sequencing of SPARC cDNA from orange-spotted grouper. Despite the difference in the lengths of the SPARC transcripts, all of the SPARC molecules encoded a signal peptide, follistain-like copper binding sequence (KGHK) domain, and extracellular domain. The grouper SPARC gene was differentially expressed in vivo and contributed differently to high-level expression of SPARC in muscle. Immunohistochemical staining demonstrated a decreased level of SPARC in nodavirus-infected grouper compared with healthy grouper. Comparative real-time polymerase chain reaction analyses of eye tissues of viral nervous necrosis grouper and healthy grouper were performed. Recombinant SPARC produced changes in grouper cell shape 24 h after treatment. The results provide new insight into the pathogenesis of nodavirus, and demonstrate an experimental rationale for SPARC characterization in nodavirus-infected grouper.

The expression of two novel orange-spotted grouper (Epinephelus coioides) TNF genes in peripheral blood leukocytes, various organs, and fish larvae

The tumour necrosis factor (TNF) super-family is a group of important cytokines involved in inflammation, apoptosis, cell proliferation, and the general stimulation of the immune system. The TNF gene has been cloned in some bony fish; however, its counterparts are still unidentified in the majority of fish species. In this study, we cloned gTNF-1 and gTNF-2 from the orange-spotted grouper (Epinephelus coioides), an economically important farmed fish. Both genes include 4 exons and 3 introns and encoded 253 and 241 amino acid proteins with a molecular weight of approximately 27 and 26 kDa, respectively. The identity of the putative amino acid sequences between gTNF-1 and gTNF-2 was only 38%. The positions of cysteine residues, a protease cleavage site, and a transmembrane domain sequence derived from gTNF-1 and gTNF-2 were similar to those in other fish and mammalian TNF-α. The mRNA expression levels of the 2 gTNF molecules were evaluated in unstimulated/stimulated peripheral blood leukocytes, various organs, and fish larvae. Following lipopolysaccharide (LPS) treatment, gTNF-2 was expressed at higher levels, was up-regulated more quickly, and was more sensitive to the immune response than gTNF-1. gTNF-1 was constitutively expressed in the thymus, brain, and spleen, but it was also expressed in the heart, head kidney, and trunk kidney after LPS stimulation. gTNF-2 was constitutively expressed in the thymus, head kidney, trunk kidney, spleen, and intestine; further, gTNF-2 was highly expressed in all organs post-LPS stimulation. Finally, the gTNF expression levels were evaluated at various developmental stages in grouper larvae. A higher variation of gTNF expression levels was observed in fish larvae from a contaminated hatchery. This study revealed the different expression patterns of gTNF-1 and gTNF-2. In addition, gTNF-2 was more sensitive to pathogens than gTNF-1; therefore, it may be an appropriate marker for pathogen invasion and the evaluation of the larval rearing environment.