The teleost head kidney: Integrating thyroid and immune signalling

Organ-specific repertoires of IgNAR gene in a cartilaginous fish

Cartilaginous fish possess one of the most ancient adaptive immune systems, and they uniquely produce the heavy chain-only antibody, immunoglobulin novel antigen receptor (IgNAR). In this study, we explored the mRNA transcription of genes related to antibody production and IgNAR diversity in various organs in banded houndsharks. IgNAR and antibody production-related genes exhibited similar relative transcription levels, with the highest expression detected in the spleen. Subsequently, we examined the diversity of IgNAR using next-generation sequencing. The most frequent clones were dominant (25 %-40 %) in the epigonal organ and liver but less common in the spleen. Large individual variation was noted in the kidney and pancreas. The length of complementarity-determining region 3 ranged 2-39 amino acids. The region tended to have a narrow length distribution of approximately 13 amino acids in the epigonal organ and liver, whereas wider length variation was noted in the kidney, pancreas, and spleen. Type II IgNAR variable regions (VNARs) were predominant (60 %-96 %) in all organs, whereas Type IV and "other" not conventionally defined VNARs were present at low frequencies and in different proportion between organs. Type I VNARs were present in multiple organs. The VNAR sequences were commonly shared among the epigonal organ, liver, and/or pancreas, but few were shared in the kidney or spleen.

Transcriptome analysis of head kidney and liver in grass carp (Ctenopharyngodon idella) symptomatically or asymptomatically infected with Flavobacterium columnare

Flavobacterium columnare is an important pathogen causing columnaris disease, which can result in high mortality in freshwater fish worldwide. Understanding the immune response in infection status of fish may be essential for developing effective prevention and treatment strategies. In this study, transcriptomes of liver and head kidney tissues in grass carp (Ctenopharyngodon idella) were compared under symptomatic and asymptomatic statuses following the immersion infection of F. columnare. Significant differences in expression of genes were observed between fish showing disease symptoms and those without symptoms. The number of differentially expressed genes (DEGs) between infected and control groups ranged from 4752 to 8,277, while the DEGs between exposed and control groups ranged from 272 to 1,751, suggesting a strong acute inflammatory response in infected groups. KEGG pathway enrichment analysis of infected groups revealed that among the top 30 enriched pathways, liver and head kidney shared 22 and 16 common pathways, respectively. These common enriched pathways are involved in various functions such as metabolism, diseases, cellular processes, biological systems, and information processing, indicating their roles in the immune response to F. columnare. Notably, we investigated in detail the gene expression profiles associated with complement molecules and three classes of cytokines (interleukin, tumor necrosis factor, and interferon) in different organs/tissues and disease states during the pathogenesis of columnaris disease. The findings highlight the importance of inflammatory responses and complement pathways in the pathogenesis of columnaris disease and suggest potential targets for future research and disease management strategies. The present study thus provides valuable insights into the transcriptomic changes and immune responses in grass carp infected with F. columnare, and sheds light on how highly virulent strains of F. columnare cause morbidity and mortality in the host.

Immune Response and Protective Efficacy of β-Glucan and Alkoxyglycerol as Adjuvant in Streptococcus agalactiae Formalin-Inactivated Vaccine in Nile Tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) farms face streptococcus agalactiae infections causing mass mortality following economic losses. To date, no commercial fish vaccine is available in India to protect against streptococcosis due to a lack of proper adjuvants and vaccine targets. Multiple studies justify that immunomodulators like β-glucan and Alkoxy glycerol (AKG) have improved immunity in fish. β-glucan and AKG are natural homopolymers and lipids, respectively, which have proven immunostimulant capabilities in fish and can be effectively used as a prominent adjuvant to improve vaccine efficacy. The present study aims to demonstrate the improved adjuvant efficacy of the formalin-inactivated S. agalactiae vaccine. Nile tilapia fingerlings were immunised with formalin-inactivated S. agalactiae vaccines mixed with β-glucan and AKG. Each week postvaccination, the serum nonspecific responses were measured by lysozyme, myeloperoxidase, SOD and bactericidal activity. Following the challenge after 28 days postvaccination, the survival rate achieved was 32.14%, 57.14%, 53.57% and 71.43% in the respective experimental groups. The immune gene expressions of IgM, TCRβ and MHCII were evoked postvaccination. The specific serum IgM titre values were significantly upregulated in 14- and 28-day postvaccinated experimental groups. The study highlights the enhanced activity of β-glucan and AKG incorporated vaccine, which has the potency towards commercialisation for sustainable aquaculture.

Molecular characterization and expression analysis of interleukin-1 beta in Japanese eel (Anguilla japonica)

Interleukin 1-beta (IL-1B) is a key proinflammatory cytokine involved in disease resistance. In recent years, il-1β genes from several teleosts have been cloned. The Japanese eel (Anguilla japonica) is an evolutionarily primitive fish widely farmed in East Asia. The il-1β gene has not yet been cloned from Japanese eel. In this study, the complete cDNA of il-1β was successfully sequenced from peripheral leukocytes through the rapid amplification of cDNA ends. The il-1β gene consists of five exons and four introns, and the full-length cDNA spans 1204 bp, comprising a 99-bp 5' untranslated region, a 750-bp coding sequence, and a 355-bp 3' untranslated region. The deduced amino acid sequence includes a consensus IL-1 family signature but lacks both a signal peptide and an IL-1 converting enzyme cleavage site, similar to other teleost IL-1B proteins. Homology analysis revealed that Japanese eel IL-1B is highly conserved within the order Anguilliformes, sharing the highest similarity with American eel (Anguilla rostrata), followed by conger eel (Conger myriaster). Tissue expression analysis showed that il-1β is constitutively expressed in multiple tissues, with high expression in peripheral leukocytes and the spleen; moderate expression in the gill, liver, head kidney, trunk kidney, and intestine; and low expression in the heart, stomach, skin, and muscle. In vitro stimulation with zymosan, polyinosinic-polycytidylic acid, and lipopolysaccharide upregulated il-1β expression in peripheral leukocytes.

Autophagy dysfunction links palmitic acid with macrophage inflammatory responses in large yellow croaker (Larimichthys crocea)

Autophagy is a cellular degradation process reliant on lysosome, crucial for preserving intracellular homeostasis. The key saturated fatty acid palmitic acid (PA) has been demonstrated to exert regulatory effects on autophagic activity in mammals. However, the precise impact of PA on autophagy and its role in fish remains incompletely understood. Thus, this study aimed to investigate the regulation of PA on autophagy and explore the role of autophagy in inflammatory responses triggered by PA in the head kidney macrophages of large yellow croaker. This study indicates that PA exposure can inhibit macrophage autophagy by reducing the expression of genes related to autophagy (e.g., beclin1, ulk1, and lc3), activating the negative regulator mTORC1 signaling pathway (p70S6K and S6), and hindering autophagic flux. This effect was observed to be amplified with increasing exposure time and concentration of PA. Similarly to the in vitro results, the palm oil (PO) diet significantly reduced autophagic activity in the head kidney of the croaker in vivo. Subsequent studies demonstrated that restoring autophagy led to a notable reduction in the expression of PA and PO-induced pro-inflammatory genes (il-1β, il-6, tnf-α, and cox-2), the activation of the MAPK signaling pathway (p38 and JNK), and the NLRP3 inflammasome levels, both in vitro and in vivo. In contrast, further inhibition of autophagy produced the opposite effect in vitro. In conclusion, this study demonstrates that PA exerts a dynamic inhibitory effect on autophagy in the head kidney macrophage, which in turn promotes PA-induced inflammatory responses. These findings provide valuable insights into how PA influences autophagy and inflammatory responses in fish immune cells, contributing to the theoretical framework for improving the use of vegetable oils in aquaculture.

Evaluation of protective immune response of live-attenuated candidate vaccines ΔcpxA and ΔcpxR against Vibrio alginolyticus in pearl gentian grouper

The grouper farming industry was severely influenced by vibriosis. In this study, we developed two live-attenuated vaccine (LAV) candidates against Vibrio alginolyticus infection in pearl gentian groupers using cpxA or cpxR mutant strains of V. alginolyticus (ΔcpxA and ΔcpxR). Groupers were administrated with ΔcpxA and ΔcpxR at the dose of 1.0 × 104 CFU/fish (safety dose) to evaluate the immune protect effect of LAV. The increasing median lethal dose (LD50) of ΔcpxA and ΔcpxR indicated the decreased virulence of bacteria to groupers. Our results suggested that two LAVs achieved over 70 % relative percent survival (RPS) after groupers were challenged by V. alginolyticus on 14 days post-immunization. The immune protection was mainly attributed to the up-regulation of immune-related gene expression (IL-6, IL-12, TNF-α, TLR2, TLR5S, CD4, MHC-Iα, IFN-γ2 and NF-κB), the higher activities of catalase (CAT), lysozyme (LZM), superoxide dismutase (SOD), and the increasing production of total protein (TP) in serum. The research indicated that the vaccination of fish with ΔcpxA and ΔcpxR can induce the innate and acquired immunity and survival rate of groupers after bacterial infection, so they can be considered as the promising candidates of vaccine for grouper industry.

Impacts of dietary α-tocopherol acetate on physiological response, antioxidant activity, innate immunity, and histopathological status of Nile tilapia, Oreochromis niloticus under heat and salinity stress

Aquaculture fish face stresses include temperature, ammonia levels, water salinity, and dissolved oxygen. In order to sustain finfish output, it is necessary to assess the impact of unexpected weather on their performance. Thus, this study examined whether Nile tilapia's vitamin E intake reduces heat and salinity stress. After 56 days of feeding a diet enriched with vitamin E at 0, 150, 300, 600, and 1200 mg/kg, Nile tilapia were split into two groups: one was given a 28 g/L salinity increase, and the other to a temperature increase of 36 °C. Heat and salinity stress increased cortisol, glycose AST, ALT, triglycerides, and total cholesterol while decreasing albumin, globulin, and protein. Fish diets with vitamin E significantly improved the parameters above before and after heat and salinity stress. However, heat and salinity stress inhibited superoxide dismutase, catalase, glutathione peroxidase, nitrous oxide, lysozyme, phagocytosis, and immunoglobulin, reducing antioxidant activity and immunological responses. The Nile tilapia diet's vitamin E content increased antioxidant activity and immunological response before and after stress. Heat and salinity stress increased lipid peroxidation (malondialdehyde), but vitamin E-fed fish had lower values than controls. In addition to mild hepatocyte degeneration, pyknosis, and hepatic central vein congestion, heat and salinity stress cause severe vascular congestion with gill lamellar epithelium degeneration, sloughing, and primary filament congestion. In Vit E-treated groups, histomorphology returned to normal. Results showed that vitamin E at 1200 mg/kg in the Nile tilapia diet may be an effective antioxidant immunostimulant against environmental stressors like heat and salinity.

Thyroid Disease of Fishes

The paraphyletic group referred to as fishes represents several extant and extinct classes that demonstrate the greatest diversity and abundance of any of the vertebrates on the Earth. Anatomically and physiologically, the systems of fish are comparable to those of other vertebrates and the thyroid and hypothalamic-pituitary-thyroid (HPT) axis are no exceptions. This article reviews the current literature on thyroid endocrinology of elasmobranch and teleost fishes with an emphasis on relevance to clinical management and highlights some of the anatomic and physiologic differences of the HPT axis in fishes.

A general strategy for generating expert-guided, simplified views of ontologies

Annotation with widely used, well-structured ontologies, combined with the use of ontology-aware software tools, ensures data and analyses are Findable, Accessible, Interoperable and Reusable (FAIR). Standardized terms with synonyms support lexical search. Ontology structure supports biologically meaningful grouping of annotations (typically by location and type). However, there are significant barriers to the adoption and use of ontologies by researchers and resource developers. One barrier is complexity. Ontologies serving diverse communities are often more complex than needed for individual applications. It is common for atlases to attempt their own simplifications by manually constructing hierarchies of terms linked to ontologies, but these typically include relationship types that are not suitable for grouping annotations. Here, we present a suite of tools for validating user hierarchies against ontology structure, using them to generate graphical reports for discussion and ontology views tailored to the needs of the HuBMAP Human Reference Atlas, and the Human Developmental Cell Atlas. In both cases, validation is a source of corrections and content for both ontologies and user hierarchies.

Arginine alleviates LPS-induced leukocytes inflammation and apoptosis via adjusted NODs signaling

Arginine plays a key role in regulating the immune function of fish. To evaluate the effect of arginine on the immune response of largemouth bass (Micropterus salmoides), the effects of arginine on cell viability, NADPH oxidase activity, respiratory burst activity, and NO production of leukocytes were analyzed both in vitro and in vivo. In this study, we found that arginine could promote the respiratory burst activity of leucocytes both in vivo and in vitro. By incubating leukocytes with the combination of LPS and arginine, we found that arginine supplementation inhibited the expression of inflammatory genes (tumor necrosis factor-alpha, tnfα; interleukin(il) 8 and il10) and apoptotic genes (caspase 3, caspase 8, and caspase 9) induced by LPS, as well as promoted the arginine metabolism. Arginine supplementation significantly induced (cd4-like) cd4 gene expression after LPS challenge. Further studies showed that LPS could significantly increase nucleotide-binding oligomerization domain containing 1 (nod1) gene expression, but decreased the nod2 gene. The arginine supplementation increased nuclear factor kappa-B (NF-κB) protein level. In conclusion, arginine can alleviate LPS-induced inflammatory response and apoptosis as well as induce cd4 gene expression against LPS challenge via adjusting the expression of NODs signaling.

A comparative study on the immune response in the head and trunk kidney of yellow catfish infected with Edwardsiella ictaluri

The teleost kidneys are anatomically divided into head kidney and trunk kidney, each performing distinct physiological functions. Although previous research has elucidated the role of the head kidney in immune responses, there is a paucity of literature on the comparative studies of the head and trunk kidney response to bacterial infection. Therefore, an Edwardsiella ictaluri infection model of yellow catfish was constructed to investigate and compare the immune responses between the two kidney types. The findings indicated that E. ictaluri infection induced significant pathological changes in both the head and trunk kidney. Despite variances in structure, both the head and trunk kidney of yellow catfish exhibit robust immune responses following E. ictaluri infection. Unexpectedly, the up-regulation level of IgM was found to be higher in the trunk kidney compared to the head kidney. Additionally, both the IgM+ and IgD+ B cells were increased after bacterial infection. This research elucidates the parallels and distinctions in immune functions between both the head and trunk kidney in fish, enriching the immune theory of the fish kidney, and also providing a theoretical basis for the immune response of teleost kidney against bacterial infections.

Grouper OTUB1 and OTUB2 promote red-spotted grouper nervous necrosis virus (RGNNV) replication by inhibiting the host innate immune response

Red-spotted grouper nervous necrosis virus (RGNNV) is a major viral pathogen of grouper and is able to antagonize interferon responses through multiple strategies, particularly evading host immune responses by inhibiting interferon responses. Ovarian tumor (OTU) family proteins are an important class of DUBs and the underlying mechanisms used to inhibit interferon pathway activation are unknown. In the present study, primers were designed based on the transcriptome data, and the ovarian tumor (OTU) domain-containing ubiquitin aldehyde-binding protein 1 (OTUB1) and OTUB2 genes of Epinephelus coioides (EcOTUB1 and EcOTUB2) were cloned and characterized. The homology alignment showed that both EcOTUB1 and EcOTUB2 were most closely related to E. lanceolatus with 98 % identity. Both EcOTUB1 and EcOTUB2 were distributed to varying degrees in grouper tissues, and the transcript levels were significantly up-regulated following RGNNV stimulation. Both EcOTUB1 and EcOTUB2 promoted replication of RGNNV in vitro, and inhibited the promoter activities of interferon stimulated response element (ISRE), nuclear transcription factors kappaB (NF-κB) and IFN3, and the expression levels of interferon related genes and proinflammatory factors. Co-immunoprecipitation experiments showed that both EcOTUB1 and EcOTUB2 could interact with TRAF3 and TRAF6, indicating that EcOTUB1 and EcOTUB2 may play important roles in interferon signaling pathway. The results will provide a theoretical reference for the development of novel disease prevention and control techniques.

Molecular characterization, cytoprotective, DNA protective, and immunological assessment of peroxiredoxin-1 (Prdx1) from yellowtail clownfish (Amphiprion clarkii)

Peroxiredoxin-1 (Prdx1) is a thiol-specific antioxidant enzyme that detoxifies reactive oxygen species (ROS) and regulates the redox status of cells. In this study, the Prdx1 cDNA sequence was isolated from the pre-established Amphiprion clarkii (A. clarkii) (AcPrdx1) transcriptome database and characterized structurally and functionally. The AcPrdx1 coding sequence comprises 597 bp and encodes 198 amino acids with a molecular weight of 22.1 kDa and a predicted theoretical isoelectric point of 6.3. AcPrdx1 is localized and functionally available in the cytoplasm and nucleus of cells. The TXN domain of AcPrdx1 comprises two peroxiredoxin signature VCP motifs, which contain catalytic peroxidatic (Cp-C52) and resolving cysteine (CR-C173) residues. The constructed phylogenetic tree and sequence alignment revealed that AcPrdx1 is evolutionarily conserved, and its most closely related counterpart is Amphiprion ocellaris. Under normal physiological conditions, AcPrdx1 was ubiquitously detected in all tissues examined, with the most robust expression in the spleen. Furthermore, AcPrdx1 transcripts were significantly upregulated in the spleen, head kidney, and blood after immune stimulation by polyinosinic:polycytidylic acid (poly (I:C)), lipopolysaccharide (LPS), and Vibrio harveyi injection. Recombinant AcPrdx1 (rAcPrdx1) demonstrated antioxidant and DNA protective properties in a concentration-dependent manner, as evidenced by insulin disulfide reduction, peroxidase activity, and metal-catalyzed oxidation (MCO) assays, whereas cells transfected with pcDNA3.1(+)/AcPrdx1 showed significant cytoprotective function under oxidative and nitrosative stress. Overexpression of AcPrdx1 in fathead minnow (FHM) cells led to a lower viral copy number following viral hemorrhagic septicemia virus (VHSV) infection, along with upregulation of several antiviral genes. Collectively, this study provides insights into the function of AcPrdx1 in defense against oxidative stressors and its role in the immune response against pathogenic infections in A. clarkii.

Metabolic, neurotoxic and immunotoxic effects of PFAAs and their mixtures on the proteome of the head kidney and plasma from rainbow trout (Oncorhynchus mykiss)

PFAAs (Perfluoroalkyl acids) are a class of bioaccumulative, persistent and ubiquitous environmental contaminants which primarily occupy the hydrosphere and its sediments. Currently, a paucity of toxicological information exists for short chain PFAAs and complex mixtures. In order to address these knowledge gaps, we performed a 3-week, aqueous exposure of rainbow trout to 3 different concentrations of a PFAA mixture (50, 100 and 500 ng/L) modeled after the composition determined in Lake Ontario. We conducted an additional set of exposures to individual PFAAs (25 nM each of PFOS (12,500 ng/L), PFOA (10,300 ng/L), PFBS (7500 ng/L) or PFBA (5300 ng/L) to evaluate differences in biological response across PFAA congeners. Untargeted proteomics and phosphorylated metabolomics were conducted on the blood plasma and head kidney tissue to evaluate biological response. Plasma proteomic responses to the mixtures revealed several unexpected outcomes including Similar proteomic profiles and biological processes as the PFOS exposure regime while being orders of magnitude lower in concentration and an atypical dose response in terms of the number of significantly altered proteins (FDR < 0.1). Biological pathway analysis revealed the low mixture, medium mixture and PFOS to significantly alter (FDR < 0.05) a number of processes including those involved in lipid metabolism, oxidative stress and the nervous system. We implicate plasma increases in PPARD and PPARG as being directly related to these biological processes as they are known to be important regulators in all 3 processes. In contrast to the blood plasma, the high mixture and PFOA exposure regimes caused the greatest change to the head kidney proteome, altering many proteins being involved in lipid metabolism, oxidative stress and inflammation. Our findings support the pleiotropic effect PFAAs have on aquatic organisms at environmentally relevant doses including those on PPAR signaling, metabolic dysregulation, immunotoxicity and neurotoxicity.

Increased water temperature contributes to a chondrogenesis response in the eyes of spotted wolffish

Adult vertebrate cartilage is usually quiescent. Some vertebrates possess ocular scleral skeletons composed of cartilage or bone. The morphological characteristics of the spotted wolffish (Anarhichas minor) scleral skeleton have not been described. Here we assessed the scleral skeletons of cultured spotted wolffish, a globally threatened marine species. The healthy spotted wolffish we assessed had scleral skeletons with a low percentage of cells staining for the chondrogenesis marker sex-determining region Y-box (Sox) 9, but harboured a population of intraocular cells that co-express immunoglobulin M (IgM) and Sox9. Scleral skeletons of spotted wolffish with grossly observable eye abnormalities displayed a high degree of perochondrial activation as evidenced by cellular morphology and expression of proliferating cell nuclear antigen (PCNA) and phosphotyrosine. Cells staining for cluster of differentiation (CD) 45 and IgM accumulated around sites of active chondrogenesis, which contained cells that strongly expressed Sox9. The level of scleral chondrogenesis and the numbers of scleral cartilage PCNA positive cells increased with the temperature of the water in which spotted wolffish were cultured. Our results provide new knowledge of differing Sox9 spatial tissue expression patterns during chondrogenesis in normal control and ocular insult paradigms. Our work also provides evidence that spotted wolffish possess an inherent scleral chondrogenesis response that may be sensitive to temperature. This work also advances the fundamental knowledge of teleost ocular skeletal systems.

Navigating sex and sex roles: deciphering sex-biased gene expression in a species with sex-role reversal (Syngnathus typhle)

Sexual dimorphism, the divergence in morphological traits between males and females of the same species, is often accompanied by sex-biased gene expression. However, the majority of research has focused on species with conventional sex roles, where females have the highest energy burden with both egg production and parental care, neglecting the diversity of reproductive roles found in nature. We investigated sex-biased gene expression in Syngnathus typhle, a sex-role reversed species with male pregnancy, allowing us to separate two female traits: egg production and parental care. Using RNA sequencing, we examined gene expression across organs (brain, head kidney and gonads) at various life stages, encompassing differences in age, sex and reproductive status. While some gene groups were more strongly associated with sex roles, such as stress resistance and immune defence, others were driven by biological sex, such as energy and lipid storage regulation in an organ- and age-specific manner. By investigating how genes regulate and are regulated by changing reproductive roles and resource allocation in a model system with an unconventional life-history strategy, we aim to better understand the importance of sex and sex role in regulating gene expression patterns, broadening the scope of this discussion to encompass a wide range of organisms.

Season affects the estrogen system and the immune response of common carp

The physiology of ectothermic animals, including fish, is strictly regulated by season-related external factors such as temperature or photoperiod. The immune response and the production of hormones, such as estrogens, are therefore also subject to seasonal changes. This study in common carp aimed to determine how the season affects the estrogen system and the immune response, including the antibacterial response during Aeromonas salmonicida infection. We compared the immune reaction in spring and autumn in the head kidney and liver and found that carp have higher levels of blood 17β-estradiol in autumn, while in the liver of these fish there is a higher constitutive expression of genes encoding vitellogenin, estrogen receptors and Cyp19 aromatase than in spring. Fish sampled in autumn also exhibited higher expression of immune-related genes in the liver. In contrast, in the head kidney from fish sampled in the autumn, the expression of genes encoding estrogen receptors and aromatase was lower than in spring, and a similar profile of expression was also measured in the head kidney for inos, arginases and il-10. In turn, during bacterial infection, we observed higher upregulation of the expression of inos, il-12p35, ifnγ-2, arginase 2 and il-10 in the liver of carp sampled in spring. In the liver of carp infected in spring a higher upregulation of the expression of the genes encoding CRPs was observed compared to fish infected during autumn. The opposite trend occurred in the head kidney, where the upregulation of the expression of the genes involved in the immune response was higher in fish infected in autumn than in those infected in spring. During the infection, also season-dependent changes occurred in the estrogen system. In conclusion, we demonstrated that season differentially affects the estrogenic and immune activity of the head kidney and liver. These results reinforce our previous findings that the endocrine and immune systems cooperate in maintaining homeostasis and fighting infection.

Interwoven processes in fish development: microbial community succession and immune maturation

Fishes are hosts for many microorganisms that provide them with beneficial effects on growth, immune system development, nutrition and protection against pathogens. In order to avoid spreading of infectious diseases in aquaculture, prevention includes vaccinations and routine disinfection of eggs and equipment, while curative treatments consist in the administration of antibiotics. Vaccination processes can stress the fish and require substantial farmer's investment. Additionally, disinfection and antibiotics are not specific, and while they may be effective in the short term, they have major drawbacks in the long term. Indeed, they eliminate beneficial bacteria which are useful for the host and promote the raising of antibiotic resistance in beneficial, commensal but also in pathogenic bacterial strains. Numerous publications highlight the importance that plays the diversified microbial community colonizing fish (i.e., microbiota) in the development, health and ultimately survival of their host. This review targets the current knowledge on the bidirectional communication between the microbiota and the fish immune system during fish development. It explores the extent of this mutualistic relationship: on one hand, the effect that microbes exert on the immune system ontogeny of fishes, and on the other hand, the impact of critical steps in immune system development on the microbial recruitment and succession throughout their life. We will first describe the immune system and its ontogeny and gene expression steps in the immune system development of fishes. Secondly, the plurality of the microbiotas (depending on host organism, organ, and development stage) will be reviewed. Then, a description of the constant interactions between microbiota and immune system throughout the fish's life stages will be discussed. Healthy microbiotas allow immune system maturation and modulation of inflammation, both of which contribute to immune homeostasis. Thus, immune equilibrium is closely linked to microbiota stability and to the stages of microbial community succession during the host development. We will provide examples from several fish species and describe more extensively the mechanisms occurring in zebrafish model because immune system ontogeny is much more finely described for this species, thanks to the many existing zebrafish mutants which allow more precise investigations. We will conclude on how the conceptual framework associated to the research on the immune system will benefit from considering the relations between microbiota and immune system maturation. More precisely, the development of active tolerance of the microbiota from the earliest stages of life enables the sustainable establishment of a complex healthy microbial community in the adult host. Establishing a balanced host-microbiota interaction avoids triggering deleterious inflammation, and maintains immunological and microbiological homeostasis.

Bisphenol A Induces Reactive Oxygen Species Production and Apoptosis-Related Gene Expression in Pacific Red Snapper Lutjanus peru Leukocytes

Bisphenol A is one of the most used components of the polycarbonate plastic industry in the word. This contaminant has disrupting effect in cells in in vitro and in vivo in fish. This study evaluated for the first time the cytotoxicity, oxidative stress and apoptosis induced by bisphenol A (BPA) in head-kidney and spleen leukocytes isolated from Pacific red snapper Lutjanus peru. Head-kidney and spleen leukocytes were exposed to 100, 1000 and 10,000 µg/mL of BPA at 2 and 24 h. Results showed cytotoxicity of BPA at 1000 and 10,000 µg/mL. Cell viability > 80% was observed in leukocytes exposed to 100 µg/mL for 2 h; thus, this concentration was selected for the remainder of the study. Reactive oxygen species (ROS) production, analyzed by DCF-DA and NBT assays, significantly increased in those leukocytes exposed to BPA compared to controls after 2 or 24 h. Superoxide dismutase and catalase activities increased in head-kidney leukocytes after 24 h of BPA exposure. Apoptosis was inferred from caspase (casp-1 and casp-3), granzyme A (granz-A) and perforin 1 (perf-1) gene expression, which was significantly up-regulated, at 2 h BPA exposure in head-kidney leukocytes, and from granz-A and perf-1, which were up-regulated, after 24 h BPA exposure in spleen leukocytes. Short cytoplasmic prolongations and membrane blebs, suggestive of apoptosis, were observed by scanning electron microscopy. These data suggest that BPA at 100 µg/mL induces cytotoxicity, oxidative stress, apoptosis in Pacific red snapper head-kidney and spleen leukocytes.

Integrated analysis of transcriptome and metabolome reveals the regulatory mechanism of largemouth bass (Micropterus salmoides) in response to Nocardia seriolae infection

Nocardia seriolae is a severe bacterial pathogen that has seriously affected the development of aquaculture industry. Largemouth bass (Micropterus salmoides) is a commercially significant freshwater fish that suffers a variety of environmental threats, including bacterial pathogens. However, the immune responses and metabolic alterations of largemouth bass to N. seriolae infection remain largely unclear. We discovered that N. seriolae caused pathological alterations in largemouth bass and shifted the transcript of immune-related and apoptotic genes in head kidney after infection. To answer the aforementioned question, a combined transcriptome and metabolome analysis was employed to explore the alterations in genes, metabolites, and metabolic pathways in largemouth bass following bacterial infection. A total of 3579 genes and 1929 metabolites are significant differentially changed in the head kidney post infection. In response to N. seriolae infection, host modifies the PI3K-Akt signaling pathway, TCA cycle, glycolysis, and amino acid metabolism. The integrated analysis of transcriptome and metabolome suggested that with the arginine metabolism pathway as the core, multiple biomarkers (arg gene, arginine) are involved in the antibacterial and immune functions of largemouth bass. Thus, we hypothesized that arginine plays a crucial role in the immune responses of largemouth bass against N. seriolae infection, and increasing arginine levels suitably is beneficial for the host against bacterial infection. Our results shed light on the regulatory mechanism of largemouth bass resistance to N. seriolae infection and contributed to the development of more effective N. seriolae resistance strategies.

Modulatory role ulexit against thiamethoxam-induced hematotoxicity/hepatotoxicity oxidative stress and immunotoxicity in Oncorhynchusmykiss

Contamination of the aquatic environment with different insecticides is a major concern in the aquatic ecosystem today. For this reason, in the designed study, Thiamethoxam (TMX) for which there is limited information on its negative effects on Oncorhynchus mykiss was investigated, its effects on hematotoxicity, oxidative status, cytotoxicity, DNA damage and apoptotic status indicators in blood/liver tissue. However, the antitoxic potential of ulexite (UX) supplementation in the elimination of TMX-mediated toxicity has been determined. LC50-96h value determined for TMX 0.73 mg/L has been determined. As a result of hematology profile, TMX application, RBC, Hgb and Hct values showed a temporal decrease compared to the control group, while increases were determined in MCV, MCH and MCHC values. It was determined that the inhibition/induction of hematological parameters was slowed down by adding UX to the medium. During the trial (48th and 96th hours), it was noted that TMX induced cortisol level, while UX supplementation slowed this induction at 48th hour. Antioxidant enzyme activities were significantly inhibited by TMX application, and MDA and MPO values increased as a result of the stimulation of ROS. It was determined that UX added to the medium showed activity in favor of antioxidants and tried to inhibit MDA and MPO levels. When Nrf-2, one of the inflammation parameters, was compared with the administration and control groups, it was determined that it inhibited depending on time, TNF-α, IL-6, DNA damage and apoptosis were induced, and UX suppressed this situation. The results obtained were evaluated as statistically meaningful. Briefly, it was determined that TMX induced oxidative damage in all tissues at 48th - 96th hours, whereas UX mitigated this situation. The results provide possible in vivo evidence that UX supplements can reduce TMX-mediated oxidative stress and tissues damage in O. mykiss blood and liver tissues.

Ulva prolifera Stress in the Yellow Sea of China: Suppressed Antioxidant Capacity and Induced Inflammatory Response of the Japanese Flounder (Paralichthys olivaceus)

As the largest green macroalgal bloom in the Yellow Sea of China, the overgrowth and degradation of Ulva prolifera (U. prolifera) have a harmful effect on marine organisms and the aquaculture industry. However, the regulation mechanism of U. prolifera stress on the antioxidant capacity and inflammatory response of marine fish is still not completely understood. A 15-day exposure experiment was conducted to evaluate the effects of U. prolifera stress on the antioxidant capacity and inflammatory response of the Japanese flounder (Paralichthys olivaceus) (283.11 ± 6.45 g). The results showed that U. prolifera stress significantly decreased their survival rate. Serum total antioxidant capacity (T-AOC) and non-specific immune-related enzyme activities were significantly impacted under U. prolifera conditions. Moreover, U. prolifera stress significantly decreased T-AOC, superoxide dismutase (SOD) activity, and catalase (CAT) activities in the liver, while malondialdehyde (MDA) contents were significantly increased. Similarly, antioxidant-related gene (cat, nrf2, and keap1) expressions were synchronously downregulated in the liver under U. prolifera stress. Furthermore, U. prolifera stress significantly upregulated pro-inflammatory gene (tnf-α, il-1β, ifn-γ, and p65) expressions and the phosphorylation levels of the p38 and JNK MAPK pathways in the head kidney. In addition, endoplasmic reticulum (ER) stress-related gene and protein expressions were also upregulated in the head kidney. Overall, these results revealed that U. prolifera stress suppressed the antioxidant capacity and induced an inflammatory response in the Japanese flounder. This study could advance the understanding of the adverse effects of U. prolifera stress on marine benthic fish and promote the sustainable development of aquaculture.

Impact of stress phenotype, elevated temperature, and bacterin exposure on male Atlantic salmon (Salmo salar) growth, stress, and immune biomarker gene expression

In this study, postsmolt male Atlantic salmon, previously identified as low responders (LRs) or high responders (HRs) based on poststress cortisol levels, had their head kidney and liver sampled at 12°C and 20°C before injection (time 0) and after injection (i.e., at 12- and 24-h postinjection, respectively) with either Forte Micro (a multivalent vaccine containing bacterin, to capture peak antibacterial responses) or an equal volume of PBS. Quantitative real-time PCR (qPCR) was then used to measure the expression of 15 biomarker genes in the head kidney and 12 genes in the liver at each temperature/sampling point. Target transcripts were chosen that were related to growth, stress, and innate antibacterial immune responses. Many temperature, phenotype, and injection effects were found for individual genes within these three broad categories, and multivariate statistical analyses (i.e., principal component analysis and permutational multivariate analysis of variance) were used to look for overall patterns in transcript expression. These analyses revealed that HR salmon at 20°C mounted a more robust response (P < 0.05) for the 10 head kidney immune-related transcripts when injected with Forte Micro than LR salmon. In contrast, the seven liver stress-related transcripts displayed a greater response (P = 0.057) in LR versus HR fish with Forte Micro at 12°C. Overall, although this research did find some differences between LR and HR fish, it does not provide strong (conclusive) evidence that the selection of a particular phenotype would have major implications for the health of salmon over the temperature range examined.NEW & NOTEWORTHY This is the first paper to describe the impact of both temperature and bacterial stimulation on head kidney and liver transcript expression in Atlantic salmon characterized as LRs versus HRs. Notably, we found that HR salmon at 20°C mounted a more robust innate antibacterial immune response than LR salmon. In addition, LR fish at 12°C may (P = 0.057) exhibit higher expression of stress-related transcripts in response to vaccine injection relative to HR fish.

Transcriptome analysis revealed immune responses in the kidney of Atlantic salmon (Salmo salar) co-infected with sea lice (Lepeophtheirus salmonis) and infectious salmon anemia virus

Sea lice (Lepeophtheirus salmonis) and infectious salmon anemia virus (ISAv) are two of the most important pathogens in Atlantic salmon (Salmo salar) farming and typically cause substantial economic losses to the industry. However, the immune interactions between hosts and these pathogens are still unclear, especially in the scenario of co-infection. In this study, we artificially infected Atlantic salmon with sea lice and ISAv, and investigated the gene expression patterns of Atlantic salmon head kidneys in response to both lice only and co-infection with lice and ISAv by transcriptomic analysis. The challenge experiment indicated that co-infection resulted in a cumulative mortality rate of 47.8 %, while no mortality was observed in the lice alone infection. We identified 240 differentially expressed genes (DEGs) under the lice alone infection, of which 185 were down-regulated and 55 were up-regulated, while a total of 994 DEGs were identified in the co-infection, of which 206 were down-regulated and 788 were significantly up-regulated. The pathway enrichment analysis revealed that single-infection significantly suppressed the innate immune system (e.g., the complement system), whereas co-infection induced a strong immune response, leading to the activation of immune-related signaling pathways such as Toll-like receptors and NOD-like receptors pathways, as well as significant upregulation of genes related to the activation of interferon and MH class I protein complex. Our results provide the first global transcriptomic study of gene expression in the Atlantic salmon head kidney in response to co-infection with sea lice and ISAv, and provided the baseline knowledge for understanding the immune responses during co-infection.

Evidence of genotoxicity, neurotoxicity, and antioxidant imbalance in silver catfish Rhamdia quelen after subchronic exposure to diisopentyl phthalate

Diisopentyl phthalate (DiPeP) is a plasticizer with significant offer and application in Brazilian industries. This is attributed to its origin, which is closely linked to the refining process of sugarcane for ethanol production in the country. In this work, we developed a model for trophic exposure to environmentally relevant doses (5, 25, and 125 ng/g of DiPeP) to identify possible target tissues and toxic effects promoted by subchronic exposure to DiPeP in a Neotropical catfish species (Rhamdia quelen). After thirty days of exposure, blood, liver, kidney, brain, and muscle were collected and studied regarding DNA damage in blood cells and biochemical analyses. The kidney was the most affected organ, as in the head kidney, genotoxicity was evidenced in all groups exposed to DiPeP. Besides, the caudal kidney showed a reduction in the superoxide dismutase and glutathione peroxidase activities as well as a reduced glutathione concentration. In the liver, exposure to 125 ng/g of DiPeP increased glutathione S-transferase activity and reduced glutathione levels. In muscle, acetylcholinesterase (AChE) was reduced. However, in the brain, an increase in AChE activity was observed after the exposure to lowest doses. In contrast, a significant reduction of brain AChE activity after exposure to the highest dose was detected. The pronounced genotoxicity observed in head kidney cells is of concern, as it may compromise different functions performed by this organ (e.g., hematopoiesis, immune and endocrine functions). In our study, DiPeP proved to be a compound of environmental concern since we have evidenced its nephrotoxic and neurotoxic potential even in low doses.

Transcriptome analysis reveals the neuroactive receptor genes response to Streptococcus agalactiae infection in tilapia, Oreochromis niloticus

The detailed crosstalk between the neuroendocrine and immune systems in Oreochromis niloticus, an economically important fish, in response to pathogenic infections, remains unclear. This study revealed the head kidney transcriptional profiles of O. niloticus upon infections with Streptococcus agalactiae, a prevalent pathogen known to cause severe meningitis. Twelve cDNA libraries of O. niloticus head kidney, representing four treatment time points (0, 6, 24, and 48 h), were constructed and a total of 2,528 differentially expressed genes were identified based on pairwise comparisons. KEGG pathway analysis revealed a significant enrichment of the 'neuroactive ligand-receptor interaction' pathway (ko04080), with 13 genes exhibiting differential expression during S. agalactiae infection. Among these, six neuroactive receptor genes (lepr, nr3c1, ptger4, thrb, tspo, and β2-ar) were selected, cloned, and characterized. Although these genes are ubiquitously expressed, and in head kidney leukocytes, their expression was mainly observed in T cells, Mo/Mφ, and NCCs, which are characterized by antimicrobial responses. Furthermore, we examined the response patterns of these six neuroactive receptor genes to gram-positive (S. agalactiae) and gram-negative (Aeromonas hydrophila) bacteria in four different tissues. Notably, lepr, ptger4, tspo, and β2-ar were upregulated in all selected tissues in response to S. agalactiae and A. hydrophila infections. However, nr3c1 and thrb were downregulated in response to S. agalactiae infection in the head kidney and spleen, whereas nr3c1 was upregulated, and thrb was unresponsive to A. hydrophila infection. Our findings provide a theoretical foundation for understanding new links between the neuroendocrine and immune systems during bacterial infection in teleost fish.

Perfluorooctanoic Acid Promotes Recruitment and Exocytosis of Rodlet Cells in the Renal Hematopoietic Tissue of Common Carp

Per- and polyfluoroalkyl substances (PFAS) are ubiquitous environmental contaminants, with perfluorooctanoic acid (PFOA) being a prominent member. PFOA poses a risk to aquatic ecosystems and human health due to its presence in water, environmental persistence, and bioaccumulation. Since rodlet cells (RCs) have emerged as potential biomarkers for chemical stressors, this study aimed to investigate the effects of sub-chronic PFOA exposure on RCs in the renal hematopoietic tissue of common carp. Three groups of fish were used: an unexposed control group and two groups exposed to environmentally relevant (200 ng L-1) and elevated (2 mg L-1) PFOA concentrations. Light and transmission electron microscopy were employed to assess RCs' distribution patterns and exocytosis, while biometry quantified RCs in the hematopoietic tissue. The results showed that, even at environmentally relevant concentrations, PFOA significantly influenced RCs' distribution patterns, leading to increased occurrence and cluster formation, as well as heightened exocytosis activity. This research highlights PFOA's immunotoxicity in fish and suggests the potential of RCs as sentinel cells in the immunological response to environmental contaminants. These findings enhance our understanding of PFAS toxicity and emphasise the importance of monitoring their impact on fish as representative vertebrates and reliable animal models.

Effects of light and circadian clock on the antiviral immune response in zebrafish

The circadian clock mechanism, which is evolutionarily conserved across various organisms, plays a crucial role in synchronizing physiological responses to external conditions, primarily in response to light availability. By maintaining homeostasis of biological processes and behavior, the circadian clock serves as a key regulator. This biological mechanism also coordinates diurnal oscillations of the immune response during infections. However there is limited information available regarding the influence of circadian oscillation on immune regulation, especially in lower vertebrates like teleost fish. Therefore, the present study aimed to investigate the effects of light and the timing of infection induction on the antiviral immune response in zebrafish. To explore the relationship between the timing of infection and the response activated by viral pathogens, we used a zebrafish model infected with tilapia lake virus (TiLV). Our findings demonstrated that light availability significantly affects the antiviral immune response and the functioning of the molecular clock mechanism during TiLV infection. This is evident through alterations in the expression of major core clock genes and the regulation of TiLV replication and type I IFN pathway genes in the kidney of fish maintained under LD (light-dark) conditions compared to constant darkness (DD) conditions. Moreover, infection induced during the light phase of the LD cycle, in contrast to nocturnal infection, also exhibited similar effects on the expression of genes associated with the antiviral response. This study indicates a more effective mechanism of the zebrafish antiviral response during light exposure, which inherently involves modification of the expression of key components of the molecular circadian clock. It suggests that the zebrafish antiviral response to infection is regulated by both light and the circadian clock.

Single-cell transcriptome, phagocytic activity and immunohistochemical analysis of crucian carp (Carassius auratus) in response to Rahnella aquatilis infection

In teleost fish, kidney is an important immune and hematopoietic organ with multiple physiological functions. However, the immune cells and cellular markers of kidney require further elucidation in crucian carp (C. auratus). Here we report on the single-cell transcriptional landscape in posterior kidney, immunohistochemical and phagocytic features of C. auratus with R. aquatilis infection. The results showed that a total of 18 cell populations were identified for the main immune cells such as monocytes/macrophages (Mo/Mφ), dendritic cells (DCs), B cells, T cells, granulocytes and hematopoietic progenitor cells (HPCs). Pseudo-time trajectory analysis was reconstructed for the immune cells using Monocle2 to obtain additional insights into their developmental lineage relationships. In the detected tissues (liver, spleen, kidney, intestine, skin, and gills) of infected fish exhibited positive immunohistochemical staining with prepared for antibody to R. aquatilis. Apoptotic cells were fluorescently demonstrated by TUNEL assay, and bacterial phagocytic activity were observed for neutrophils and Mo/Mφ cells, respectively. Moreover, a similar up-ward/down-ward expression trend of the selected immune and inflammatory genes was found in the kidney against R. aquatilis infection, which were significantly involved in TLR/NLR, ECM adhesion, phago-lysosome, apoptosis, complement and coagulation pathways. To our knowledge, this is the first report on the detailed characterization of immune cells and host-R. aquatilis interaction, which will contribute to understanding on the biology of renal immune cells and repertoire of potential markers in cyprinid fish species.

Dietary restriction to optimize T cell immunity is an ancient survival strategy conserved in vertebrate evolution

Recent advances highlight a key role of transient fasting in optimizing immunity of human and mouse. However, it remains unknown whether this strategy is independently acquired by mammals during evolution or instead represents gradually evolved functions common to vertebrates. Using a tilapia model, we report that T cells are the main executors of the response of the immune system to fasting and that dietary restriction bidirectionally modulates T cell immunity. Long-term fasting impaired T cell immunity by inducing intense autophagy, apoptosis, and aberrant inflammation. However, transient dietary restriction triggered moderate autophagy to optimize T cell response by maintaining homeostasis, alleviating inflammation and tissue damage, as well as enhancing T cell activation, proliferation and function. Furthermore, AMPK is the central hub linking fasting and autophagy-controlled T cell immunity in tilapia. Our findings demonstrate that dietary restriction to optimize immunity is an ancient strategy conserved in vertebrate evolution, providing novel perspectives for understanding the adaptive evolution of T cell response.

Ghrelin Modulates Differential Expression of Genes Relevant to Immune Activities and Antimicrobial Peptides in Primary Head Kidney Cells of Rainbow Trout (Oncorhynchus mykiss)

Ghrelin is a peptide hormone/cytokine that regulates metabolic processes and plays essential roles in the immune system. To evaluate the immunomodulatory actions of ghrelin isoforms in rainbow trout (RT), an in vitro model was utilized with primary cells isolated from fish head kidney (HKD). These RT-HKD cells were treated with synthetic rainbow trout ghrelin and its truncated isoform, desVRQ-ghrelin, over time (0, 2, 4, and 24 h). Reverse transcriptase-coupled qPCR was used to measure the differential expression patterns of genes relevant to various immune processes and genes of antimicrobial peptides. Ghrelin isoform treatments resulted in functional perturbations that displayed overlapping and divergent patterns of gene expression. The differing actions between the two ghrelin isoforms on various assessed genes, and at differing time points, suggested that the two analogs may activate unique pathways, thereby eliciting distinct responses in fish immunity.

Impacts of rising temperatures and water acidification on the oxidative status and immune system of aquatic ectothermic vertebrates: A meta-analysis

Species persistence in the Anthropocene is dramatically threatened by global climate change. Large emissions of carbon dioxide (CO₂) from human activities are driving increases in mean temperature, intensity of heatwaves, and acidification of oceans and freshwater bodies. Ectotherms are particularly sensitive to CO₂-induced stressors, because the rate of their metabolic reactions, as well as their immunological performance, are affected by environmental temperatures and water pH. We reviewed and performed a meta-analysis of 56 studies, involving 1259 effect sizes, that compared oxidative status or immune function metrics between 42 species of ectothermic vertebrates exposed to long-term increased temperatures or water acidification (≥48 h), and those exposed to control parameters resembling natural conditions. We found that CO₂-induced stressors enhance levels of molecular oxidative damages in ectotherms, while the activity of antioxidant enzymes was upregulated only at higher temperatures, possibly due to an increased rate of biochemical reactions dependent on the higher ambient temperature. Differently, both temperature and water acidification showed weak impacts on immune function, indicating different direction (increase or decrease) of responses among immune traits. Further, we found that the intensity of temperature treatments (Δ°C) and their duration, enhance the physiological response of ectotherms, pointing to stronger effects of prolonged extreme warming events (i.e., heatwaves) on the oxidative status. Finally, adult individuals showed weaker antioxidant enzymatic responses to an increase in water temperature compared to early life stages, suggesting lower acclimation capacity. Antarctic species showed weaker antioxidant response compared to temperate and tropical species, but level of uncertainty in the antioxidant enzymatic response of Antarctic species was high, thus pairwise comparisons were statistically non-significant. Overall, the results of this meta-analysis indicate that the regulation of oxidative status might be one key mechanism underlying thermal plasticity in aquatic ectothermic vertebrates.

Expression and functional characterization of three β-defensins in grass carp (Ctenopharyngodon idella)

β-defensins (BDs) are a group of cysteine-rich cationic antimicrobial peptides and play important roles in the first line of defense against infection. In this study, the expression and antibacterial activities of three grass carp (Ctenopharyngodon idella) (Ci) β-defensin (BD) peptides were comparatively investigated. Expression analysis reveals that CiBD1-3 were constitutively expressed in tissues, with the highest expression detected in the skin. The CiBD-1 transcripts were more abundant than CiBD-2 and CiBD-3. In the primary head kidney leukocytes, CiBDs were induced by PHA, LPS, poly(I:C) and cytokines such as IL-1β and IFN-γ. In vivo challenge of fish with Aeromonas hydrophila resulted in the up-regulation of CiBDs in the head kidney and hindgut. To determine the biological activities, recombinant CiBD proteins were produced in the HEK293-F cells and purified for the minimum inhibitory concentration assay. It was found that all three recombinant CiBD proteins were effective to inhibit the growth of Gram-negative fish bacterial pathogens including Aeromonas hydrophila, Edwardsiella tarda, Flavobacterium columnare and Klebsiella pneumoniae and Gram-positive Staphylococcus aureus. CiBD-2 and CiBD-3 were more effective than CiBD-1. Our results demonstrate that all the three CiBDs have broad antibacterial activity against fish bacterial pathogens.

Differentially expressed genes in head kidney of Pelteobagrus fulvidraco following Vibrio cholerae challenge

The yellow catfish (Pelteobagrus fulvidraco) is a freshwater fish with high economic value in eastern China. Nevertheless, pathogens causing bacterial diseases in P. fulvidraco have brought about huge economic loss and high mortality in artificial aquaculture. For disease control, it is critical to further understand the immune system of yellow catfish and immune-related genes with which they respond to pathogenic infections. In this study, high-throughput sequencing methods were used to analyze the transcriptomic spectrum of the head kidney from P. fulvidraco challenged by Vibrio cholera. A total of 45,544 unique transcript fragments (unigenes) were acquired after assembly and annotation, with an average length of 1,373 bp. Additionally, 674 differentially expressed genes (DEGs) were identified after stimulation with V. cholerae, 353 and 321 genes were identified as remarkably up- or downregulated, respectively. To further study the immune-related DEGs, we performed KEGG enrichment and GO enrichment. The results showed gene regulation of response to stimulus, immune response, immune system progress, response to external stimuli and cellular response to stimuli. Analysis of KEGG enrichment is important to identify chief immune related pathways. Real-time quantitative reverse transcription-PCR (qRT-PCR) results indicated 10 immune response genes that were found to be upregulated compared to a control group after 6 h of V. cholerae challenging. In summary, the results of our study are helpful to determine the defense mechanisms and immune system responses of yellow catfish in reaction to bacterial challenges.

Identification and functional characterization of complement regulatory protein CD59 in golden pompano (Trachinotus ovatus)

CD59, one of the essential inhibitors of the complement membrane attack complex (MAC), plays a crucial role in regulation of complement activation. In this study, we cloned and identified the CD59 gene (named ToCD59) of golden pompano (Trachinotus ovatus). The ORF sequence of ToCD59 is 357 bp long encoding 118 amino acids with a molecular weight of 13.09 kDa. Prediction of protein domains showed that ToCD59 contained an Lu domain and a C-terminal glycosylphosphatidylinositol (GPI) partial anchor. Homology comparisons indicated that ToCD59 shared the high sequence similarity with other fish CD59. RT-qPCR analysis showed that ToCD59 was expressed in all tested healthy tissues of golden pompano, with the highest level of expression in the brain. After stimulation with bacteria, ToCD59 expression levels were significantly up-regulated in head kidney, liver, gill and brain, but down-regulated in spleen. Subcellular localization results showed that ToCD59 localized to the cytoplasm of A549 cells. The hemolytic activity analysis showed that rToCD59 might have complement inhibitory activity through the alternative complement pathway. In addition, antibacterial test showed that rToCD59 had antibacterial ability against S. agalactiae and V. alginolyticus in vitro. These results suggest that ToCD59 might play an important role in the immune response against pathogens, which would provide basic information for elucidating the functional evolutionary history of complement system in teleost.

Evaluation of establishment and maintenance of primary cell cultures from several strains of common carp (Cyprinus carpio L.)

As a surrogate for the whole organism, primary cultures and cell lines serve as valuable tools for investigating exogenous and endogenous cytopathy. Studying cell responsiveness to diseases and contaminants is considered a less demanding and more readily accessible research approach that minimizes animal distress and provides more specific data. In the current work, the authors established primary cultures from several different organs and tissues of common carp (Cyprinus carpio L.) for subsequent use in other applications. They investigated the technical challenges in obtaining successful and durable carp-derived tissue cultures. The trials indicate that the type of tissue grown, carp strain and fish age impact equally upon culturing success, as do the cultivating conditions. Cells from gill epithelia, head and trunk kidneys, spleen, skin, gonads and ocular tissue were successfully established and maintained for further use in in-vitro testing. The primary cultures were, therefore, used to investigate and assess pathogens and pollutants emerging in carp's environment.

Molecular Characterization, Expression, Evolutionary Selection, and Biological Activity Analysis of CD68 Gene from Megalobrama amblycephala

CD68 is a highly glycosylated transmembrane glycoprotein that belongs to the lysosome-associated membrane glycoprotein family and is involved in various immune processes. In this study, Megalobrama amblycephala CD68 (MaCD68) was cloned and characterized, and its expression patterns and evolutionary characteristics were analyzed. The coding region of MaCD68 was 987 bp, encoding 328 amino acids, and the predicted protein molecular weight was 34.9 kDa. MaCD68 contained two transmembrane helical structures and 18 predicted N-glycosylation sites. Multiple sequence alignments showed that the MaCD68 protein had high homology with other fish, and their functional sites were also highly conserved. Phylogenetic analysis revealed that MaCD68 and other cypriniformes fish clustered into one branch. Adaptive evolution analysis identified several positively selected sites of teleost CD68 using site and branch-site models, indicating that it was under positive selection pressure during evolution. Quantitative real-time reverse transcription polymerase chain reaction analysis showed that MaCD68 was highly expressed in the head kidney, spleen, and heart. After Aeromonas hydrophila infection, MaCD68 was significantly upregulated in all tested tissues, peaking at 12 h post-infection (hpi) in the kidney and head kidney and at 120 hpi in the liver and spleen, suggesting that MaCD68 participated in the innate immune response of the host against bacterial infection. Immunohistochemical and immunofluorescence analyses also showed that positive signals derived from the MaCD68 protein were further enhanced after bacterial and lipopolysaccharide treatment, which suggested that MaCD68 is involved in the immune response and could be used as a macrophage marker. Biological activity analysis indicated that recombinant MaCD68 (rMaCD68) protein had no agglutination or bactericidal effects on A. hydrophila but did have these effects on Escherichia coli. In conclusion, these results suggest that MaCD68 plays a vital role in the immune response against pathogens, which is helpful in understanding the immune responses and mechanisms of M. amblycephala.

CD45 in ocular tissues during larval and juvenile stages and early stages of V. anguillarum infection in young lumpfish (Cyclopterus lumpus)

Immune responses to infectious diseases impacting lumpfish (Cyclopterus lumpus) eye tissue are only starting to be studied at a molecular and histopathological level. In this study, we extend our understanding of lumpfish sensory organ anatomy, of components of the lumpfish nasal and ocular immune system and the nature of the intraocular response to Vibrio anguillarum infection. We have evaluated the expression of cluster of differentiation (CD) 45 protein, a tyrosine phosphatase, in larval and juvenile lumpfish tissues in order to spatially survey ocular and related head structures that may participate in early stages of intraocular immune responses. We provide here a histological mapping of the larval lumpfish nasal chamber system since its connectively with the eye though mucosal epithelia have not been explored. These results build upon our growing understanding of the lumpfish intraocular immune response to pathogens, exemplified herein by experimental nasally delivered V. anguillarum infection. CD45 is developmentally regulated in lumpfish eyes and periocular anatomy with early expression appearing in larvae in corneal epithelium and in nasal structures adjacent to the eye. Normal juvenile and adult lumpfish eyes express CD45 in the corneal epithelium, in leukocyte cells within blood vessel lumens of the rete mirabile, choroid body and choriocapillaris vasculatures. Experimental nasally delivered V. anguillarum infection led to qualitative and quantitative changes in CD45 expression in head kidney renal tubule tissues by 7 days post infection (dpi). The same animals showed redistribution and upregulation of corneal epithelial CD45 expression, corneal epithelial dysplasia and an increased frequency of CD45⁺ cells in ocular vasculature. Interestingly, while CD45 upregulation and/or CD45⁺ cell infiltration into inner ocular and retinal tissues was not observed under this experimental scenario, subtle neural retinal changes were observed in infected fish. This work provides new fundamental knowledge on North Atlantic teleost visual systems and vision biology in general.

Serotonin system is partially involved in immunomodulation of Nile tilapia (Oreochromis niloticus) immune cells

Serotonin (5-hydroxytryptamine) is a well-known neurotransmitter affecting emotion, behavior, and cognition. Additionally, numerous immunomodulatory functions of serotonin have been discovered in mammals. However, the regulatory role of the serotonin system in fish immunity remains unclear. In this study, various serotonergic markers in Nile tilapia (Oreochromis niloticus) were identified and characterized. The involvement of the serotonin system during bacterial infection was investigated. Moreover, the expression characteristics and specific functions of serotonergic markers within Nile tilapia immune cells were also assessed. Overall, 22 evolutionarily conserved serotonergic marker genes in Nile tilapia were cloned and characterized. Transcriptional levels of these molecules were most abundant in the brain, and their transcripts were induced during Streptococcus agalactiae infection. Nevertheless, few serotonergic markers exist on Nile tilapia immune cells, and no distinct immunomodulation effect was observed during an immune response. The present study lays a theoretical foundation for further investigation of the immunological mechanisms in fish as well as the evolution of the serotonin system in animals.

Perfluorooctanoic Acid Affects Thyroid Follicles in Common Carp (Cyprinus carpio)

Carp kidney is comprised of nephrons, hemopoietic tissue, and also hormonally-active thyroid follicles. Given this anatomical trait, it has been used to assess the thyroid disrupting potential of perfluorooctanoic acid (PFOA), a widespread and feared per- poly-fluoroalkyl substance and a persistent organic pollutant capable of interfering with the endocrine system in animals and humans. The occurrence and morphology of thyroid follicles in kidneys of carp experimentally exposed to 200 ng L⁻¹ or 2 mg L⁻¹ waterborne PFOA for 56 days were studied. The abundance of thyroid follicles was significantly higher and vesiculation increased in exposed fish as compared to controls. The number of vesiculated follicles/total number of follicles was positively correlated with PFOA blood concentration in fish exposed to the highest dose (2 mg L⁻¹). The structure and ultrastructure of thyroid follicles were affected by PFOA also at the lower, environmentally relevant, concentration (200 ng L⁻¹). Increased cellular projections, enhanced colloid endocytosis, rough endoplasmic reticulum enlargement and fragmentation and cytoplasm vacuolation were the main features displayed by PFOA-exposed carp. These results show that PFOA affects the occurrence and status of follicles and suggest the utility of fish kidney as a multipurpose biomarker organ in environmental pathology research, according to the One Health approach.

TLR23, a fish-specific TLR, recruits MyD88 and TRIF to activate expression of a range of effectors in melanomacrophages in Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is an important food fish species that is mainly cultivated in tropical and subtropical countries. However, microbial diseases have created various difficulties for this industry. The fundamental prerequisite for tackling disease outbreak prevention and disease resistance is to know how hosts' immune responses against invading microbes are initiated. Toll-like receptors (TLRs) are vital pattern recognition receptors and play pivotal roles in the cellular innate immunity defense that is able to recognize pathogen-associated molecular patterns (PAMPs). In this study, Oreochromis niloticus TLR23 (OnTLR23) was cloned and bioinformatic analyses revealed that OnTLR23 is not an ortholog of mammalian TLR13 as previously suggested. The basal transcript level of OnTLR23 was found to be higher in the immune-related organs and was upregulated in the spleen and/or head kidney following Aeromonas hydrophila, Streptococcus agalactiae or poly I:C injections, and increased in the melanomacrophage-like tilapia head kidney (THK) cell line after LPS and zymosan stimulation. Furthermore, we demonstrated for the first time that OnTLR23 locates mainly in the intracellular region in fish cells and the constitutively active form of OnTLR23 promotes the expression of molecules related to antigen presentation, proinflammatory cytokines, antimicrobial peptides and type I interferon in THK cells. A co-immunoprecipitation assay revealed that OnTLR23 can interact with both OnMyD88 and OnTRIF, but not with OnTIRAP. A luciferase assay showed that the NF-κB activity was not elevated in the OnTLR23 overexpressed THK cells after treatment with ligand for TLR13 as well as other known purified bacterial-derived ligands of TLRs. Taken together, OnTLR23 is likely to recruit OnMyD88 and OnTRIF as adaptors to induce the expression of various effectors in melanomacrophages, but its corresponding ligand is an issue awaiting further investigation.

Transcriptome analysis of turbot (Scophthalmus maximus) head kidney and liver reveals immune mechanism in response to Vibrio anguillarum infection

The diseases triggered by Vibrio anguillarum infection have created huge economic losses to the turbot (Scophthalmus maximus) farming industry. However, the immune mechanism of turbot to V. anguillarum infection has not been deeply investigated. To better understand the immune response of turbot to V. anguillarum infection, transcriptome analysis of the head kidney and liver of turbot was performed. A total of 15,948 and 11,494 differentially expressed genes (DEGs) were obtained from the turbot head kidney and liver, respectively. Transcriptome analysis revealed that the head kidney and liver of turbot have some differences in the gene ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analysis of the DEGs for the different functions of these two organs. Although there are many uncertain factors in this immune process, such as the occurrence of alternative splicing (AS) events and the differences in the protein structure of the DEGs, the NFκB signalling pathway, MKK-dependent AP-1 activation, JAK-STAT signalling pathway, the signal transmission of MHC Ⅰ and a series of DEGs including HSP90 driving NLRP3 to produce inflammatory factors (IL-1β, IL-8, TNFα, etc.) were possible important immune response pathways for turbot to V. anguillarum infection. Overall, our research has conducted a preliminary exploration of the immune mechanism of turbot in response to V. anguillarum infection.

Transcriptomic profiles of consistent risk-taking behaviour across time and contexts in European sea bass

Bolder individuals have greater access to food sources and reproductive partners but are also at increased risk of predation. Boldness is believed to be consistent across time and contexts, but few studies have investigated the stability of this trait across variable environments, such as varying stress loads or long periods of time. Moreover, the underlying molecular components of boldness are poorly studied. Here, we report that boldness of 1154 European sea bass, evaluated using group risk-taking tests, is consistent over seven months and for individuals subjected to multiple environments, including a chronically stressful environment. Differences in risk-taking behaviour were further supported by differences observed in the responses to a novel environment test: shy individuals displayed more group dispersion, more thigmotaxic behaviour and lower activity levels. Transcriptomic analyses performed on extreme phenotypes revealed that bold individuals display greater expression for genes involved in social and exploration behaviours, and memory in the pituitary, and genes involved in immunity and responses to stimuli in the head kidney. This study demonstrates that personality traits come with an underpinning molecular signature, especially in organs involved in the endocrine and immune systems. As such, our results help to depict state-behaviour feedback mechanisms, previously proposed as key in shaping animal personality.

Experimental Handling Challenges Result in Minor Changes in the Phagocytic Capacity and Transcriptome of Head-Kidney Cells of the Salmonid Fish Coregonus maraena

Aquaculture management involves regular handling procedures, but these can evoke stress responses in farmed fish. We compiled an extensive list of published parameters that indicate the most likely handling-induced physiological deviations from the norm. However, since these parameters are based almost exclusively on studies of rainbow trout and Atlantic salmon, we conducted a handling-challenge experiment with maraena whitefish (Coregonus maraena). This salmonid fish was sampled at either 3 or 24 h after a single 1-min handling or after 10 days of daily repeated 1-min handling. The cortisol levels were strongly elevated in some individuals at 3 h after the single handling challenge, but these elevations were not significantly different between the challenged and control cohorts. The phagocytic capacity of myeloid head-kidney cells stimulated with fluorophore-labeled, inactivated Aeromonas salmonicida was significantly decreased in maraena whitefish at 3 h after the handling challenge compared to control fish. Microarray analysis of head-kidney samples from the challenged and control fish revealed 12 differentially expressed genes at 3 h and 70 at 24 h after the single handling episode, but only 5 differentially expressed genes after 10 days of repeated daily handling. The identified genes were assigned to numerous stress- and immune-relevant functional pathways, including "glucocorticoid receptor signaling" (3 h post-challenge), "HIF1A signaling" (24 h post-challenge), or "complement system" (10 days of repeated challenge). Our data reveal the tight interconnection of immune and stress pathways in the head kidney of maraena whitefish and corroborate several parameters previously found regulated in other tissues of handling-stressed rainbow trout. These findings indicate that handling may compromise the health and welfare of maraena whitefish in aquaculture.

Effects of environmental salinity on the immune response of the coastal fish Scatophagus argus during bacterial infection

The coastal aquaculture is characterized with environmental salinity fluctuation, and the effects of salinity stress on the immunity of cultured fish are needed to be further explored. Scatophagus argus is an important species in the wild fisheries and aquaculture industry, it would be of great value to reveal the impact of salinity change on the immune response in this species. Understanding the effects of salinity stress on immune response can provide valuable insights into salinity management in the aquacultural process. The head kidney, which is an organ unique for teleost fish, functions not only as a central immune organ but also as a crucial role in the stress response during which the secretion of immunoregulatory molecules i.e. cytokines is facilitated. In the present study, Individuals of S. argus acclimated to 3 different salinities [0‰ (FW), 10‰ (BW), and 25‰ (SW)] were injected intraperitoneally with A. hydrophila, and then monitored throughout one week. The effects of environmental salinity on the immune response in S. argus stimulated by A. hydrophila infection were investigated. mRNA expression profiles of cytokine genes IL-1β, IL-6, IL-10 and TNF-α in different salinity groups was quite different. mRNA expression of cytokine genes in BW group and SW group rose more quickly and significantly higher than FW group (p < 0.05) at early stages (6-24 hpi) after bacterial injection, and before 96 hpi, the highest value of cytokine expression at each time point was recorded in SW group. Immune parameters such as lysozyme level, complement C3 activity and IgM content in BW and FW groups were lower than SW group at each time point from 24 to 144 hpi after bacterial injection. In addition, leukocyte profiles in the head kidney and blood were also investigated. Although hypoosmotic acclimation could temporarily stimulate monocyte and neutrophil proliferation, it was observed that the number of monocytes, neutrophils and lymphocytes of the head kidney and blood in SW group increased more quickly than BW and FW groups after bacterial infection. Our results indicate that hypoosmotic stress due to the decrease of environmental salinity has suppressive immunoregulatory effects on the immune response of S. argus.

Transcriptome Analysis Indicates Immune Responses against Vibrio harveyi in Chinese Tongue Sole (Cynoglossus semilaevis)

Simple Summary

Limited understanding of molecular mechanisms of immune response constrains marine fish farming. Analyzing the dynamic gene expression profile of fish in response to pathogen infection is gaining interest. We analyzed the expression changes of the Chinese tongue sole kidney after Vibrio harveyi infection with a series of transcriptome data. Notably, we observed rapid up-regulation of IL-17, TNF and TLR signaling pathways, indicating treatment measures should be taken in the early stage after infection. We also found a close connection between the immune system and neuroendocrine system, which may be the new strategy to improve immune function. Our research provides insights into disease prevention and treatment in fish farming.

Abstract

Pathogenic infection of fishes is an important constraining factor affecting marine aquaculture. Insufficient understanding of the molecular mechanisms has affected the diagnosis and corresponding treatment. Here, we reported the dynamic changes of gene expression patterns in the Chinese tongue sole kidney at 16 h, 48 h, 72 h and 96 h after Vibrio harveyi infection. In total, 366, 214, 115 and 238 differentially expressed genes were obtained from the 16 h−vs. −C, 48 h−vs. −C, 72 h−vs. −C and 96 h−vs. −C group comparisons, respectively. KEGG enrichment analysis revealed rapid up-regulation of several immune-related pathways, including IL-17, TNF and TLR signaling pathway. More importantly, time-series analyses of transcriptome showed that immune genes were specifically up-regulated in a short period of time and then decreased. The expression levels of chemokines increased after infection and reached a peak at 16 h. Specifically, Jak-STAT signaling pathway played a crucial role in the regulation during Vibrio harveyi infection. In the later stages of infection, genes in the neuroendocrine pathway, such as glucocorticoid-related genes, were activated in the kidney, indicating a close connection between the immune system and neuroendocrine system. Our dynamic transcriptome analyses provided profound insight into the gene expression profile and investigation of immunogenetic mechanisms of Chinese tongue sole.

Anisakidae parasitism activated immune response and induced liver fibrosis in wild anadromous Coilia nasus

Anisakidae nematode larvae is one of the most common parasites in wild anadromous Coilia nasus. This study aims to explore the mechanism of the C. nasus immune response to the parasitism of Anisakid nematode larvae. Results found that Anisakid nematode larvae parasitism caused liver injury as evidenced by histomorphology results as well as high levels of aminotransferase and aspertate aminotransferase. Furthermore, Anisakid nematode larvae parasitism induced an immune response in the host, which was characterized by the elevated populations of macrophages and neutrophils in the liver and head-kidney in the Anisakidae-infected group compared to the noninfected group. The expression of immunoglobulin IgM and IgD in the liver and head-kidney was also increased in the Anisakidae-infected group. The Anisakidae-infected group showed higher activity of antioxidant enzymes catalase and superoxide dismutase, which indicates severe oxidative stress, and increased production of pro-inflammatory cytokines, TNF-α, IL-6 as well as MCP-1 in the liver compared with the noninfected group. As a result of inflammation, livers of hosts in the Anisakidae-infected group showed fibrosis, and elevated expression of associated proteins including α-smooth muscle actin, fibronectin, collagen type I and type III compared with the noninfected group. We demonstrated that Anisakid nematode larvae parasitism results in injury and fibrosis in the liver, and triggers immune cell infiltration and inflammation in the liver and head-kidney of C. nasus. Altogether, the results provide a foundation for building an interaction between parasite and host, and will contribute to C. nasus population and fishery resource protection.

Comparative chronic toxicities of PFOS and its novel alternatives on the immune system associated with intestinal microbiota dysbiosis in adult zebrafish

6:2 Chlorinated polyfluorinated ether sulfonate (F-53B) and sodium p-perfluorous nonenoxybenzene sulfonate (OBS) are widely used as perfluorooctane sulfonate (PFOS) alternatives in the Chinese market. Here, adult zebrafish were chronically exposed to 1 μM PFOS, F-53B, and OBS for 21 days to investigate the comparative immunotoxicity of these three per- and polyfluoroalkyl substances (PFAS). PFOS induced more severe oxidative stress in the liver than F-53B and OBS, and these three PFAS induced similar anti-inflammatory effects by repressing the expression of pro-inflammatory cytokines. The intestinal microbiota analysis showed that the relative abundance of Plesiomonas, Aeromonas, Cetobacterium, Shewanella, and Vibrio changed with the same trend in the three PFAS treatment groups. Furthermore, the PFAS increased the expression of hepcidin, muc, the immune-related genes mpo and saa, and decreased the expression of the tight junction-related gene occ in the intestine; moreover, villus height of the intestine was reduced after PFAS exposure, which indicated the functional disruption of the intestine. In particular, the significant correlation between the changed intestinal microbiota and liver and intestinal indicators also suggested the interaction between the immune system and intestinal microbiota. Taken together, our results indicate that exposure to PFOS and its alternatives F-53B and OBS can induce hepatic immunotoxicity associated with intestinal microbiota dysbiosis in adult zebrafish.

Screening and Validation of p38 MAPK Involved in Ovarian Development of Brachymystax lenok

Brachymystax lenok (lenok) is a rare cold-water fish native to China that is of high meat quality. Its wild population has declined sharply in recent years, and therefore, exploring the molecular mechanisms underlying the development and reproduction of lenoks for the purposes of artificial breeding and genetic improvement is necessary. The lenok comparative transcriptome was analyzed by combining single molecule, real-time, and next generation sequencing (NGS) technology. Differentially expressed genes (DEGs) were identified in five tissues (head kidney, spleen, liver, muscle, and gonad) between immature [300 days post-hatching (dph)] and mature [three years post-hatching (ph)] lenoks. In total, 234,124 and 229,008 full-length non-chimeric reads were obtained from the immature and mature sequencing data, respectively. After NGS correction, 61,405 and 59,372 non-redundant transcripts were obtained for the expression level and pathway enrichment analyses, respectively. Compared with the mature group, 719 genes with significantly increased expression and 1,727 genes with significantly decreased expression in all five tissues were found in the immature group. Furthermore, DEGs and pathways involved in the endocrine system and gonadal development were identified, and p38 mitogen-activated protein kinases (MAPKs) were identified as potentially regulating gonadal development in lenok. Inhibiting the activity of p38 MAPKs resulted in abnormal levels of gonadotropin-releasing hormone, follicle-stimulating hormone, and estradiol, and affected follicular development. The full-length transcriptome data obtained in this study may provide a valuable reference for the study of gene function, gene expression, and evolutionary relationships in B. lenok and may illustrate the basic regulatory mechanism of ovarian development in teleosts.