Mechanisms of Fish Macrophage Antimicrobial Immunity

SHP-1 negatively regulates LPS-induced M1 polarization, phagocytic activity, inflammation and oxidative stress in primary macrophages of Chinese tongue sole (Cynoglossus semilaevis)

Macrophages serve as the primary effector cells in antibacterial immunity in teleost, engaging in both innate and adaptive immune response. However, the specific role of SHP-1, a multi-functional protein tyrosine phosphatase, in teleost macrophages remains elusive. In this study, we first established a cellular immune model using lipopolysaccharide (LPS), a major pathogenic component of Gram-negative bacteria, and then we comprehensively elucidated the function of SHP-1 in primary macrophages derived from Chinese tongue sole. Our results demonstrated that overexpression of SHP-1 inhibited M1 polarization, phagocytosis, respiratory burst of primary macrophages, suppressing the generation of excessive reactive oxygen species (ROS), malondialdehyde (MDA), and proinflammatory cytokines (il-1β, il-6), but increasing the expression of superoxide dismutase (SOD) and anti-inflammatory cytokine (il-10). Whereas SHP-1 silencing (through siRNA or inhibitor) exerted completely opposite effects, further emphasizing its roles as a negative regulator. More in-depth, we revealed that SHP-1 suppressed the activation/transduction of the TLR5-MYD88-NFκB and JAK-STAT3 signal pathways, thereby mitigating the excessive immune reaction in macrophages of Chinese tongue sole. In summary, our findings systematically delineate the functions of SHP-1 and offer mechanistic insights into the management of oxidative stress/ inflammation-related diseases, which will contribute to the sustainable development of aquaculture.

Seventy years of coexistence: Parasites and Mediterranean fish aquaculture

What can be regarded as a seedling of the contemporary aquaculture in the Mediterranean began back in the 1950s. The development of the industry did not always align with the development of ichthyopathology, a veterinary discipline aimed at identifying and combating fish diseases. Therefore, and due to the lack of published data, we are not always able to pinpoint the first outbreaks that accompanied the increase in aquaculture production. Nonetheless, fish pathogens, and parasites in particular, have shown diversity related to host species, their farming conditions and geography. Two parasite species currently regarded as dominant in Mediterranean aquaculture are the histozoic myxozoan Enteromyxum leei and the haematophagous polyopisthocotylean Sparicotyle chrysophrii, both of which infect gilthead seabream (Sparus aurata). The interactions between parasite and host with regard to the immune activity of both have been well studied using conventional immunology and omics approaches. For the remaining parasite-fish systems, our understanding of host responses and parasite mitigation mechanisms is still vague and mostly transposed from what we know of other systems. This review compiles the knowledge on fish response to the most frequent and economically important parasites in Mediterranean aquaculture, highlights the gaps and suggests further directions.

Amphibian (Xenopus laevis) Macrophage Subsets Vary in Their Responses to the Chytrid Fungus Batrachochytrium dendrobatidis

The chytrid fungus, Batrachochytrium dendrobatidis (Bd), infects amphibian skin, causing chytridiomycosis, which is a contributing cause of worldwide declines and extinctions of amphibians. Relatively little is known about the roles of amphibian skin-resident immune cells, such as macrophages, in these antifungal defenses. Across vertebrates, macrophage differentiation is controlled through the activation of colony-stimulating factor-1 (CSF1) receptor by CSF1 and interleukin-34 (IL34) cytokines. While the precise roles of these respective cytokines in macrophage development remain to be fully explored, our ongoing studies indicate that frog (Xenopus laevis) macrophages differentiated by recombinant forms of CSF1 and IL34 are functionally distinct. Accordingly, we explored the roles of X. laevis CSF1- and IL34-macrophages in anti-Bd defenses. Enriching cutaneous IL34-macrophages, but not CSF1-macrophages, resulted in significant anti-Bd protection. In vitro analysis of frog macrophage-Bd interactions indicated that both macrophage subsets phagocytosed Bd. However, IL34-macrophages cocultured with Bd exhibited greater pro-inflammatory gene expression, whereas CSF1-macrophages cocultured with Bd showed greater immunosuppressive gene expression profiles. Concurrently, Bd-cocultured with CSF1-macrophages, but not IL34-macrophages, possessed elevated expression of genes associated with immune evasion. This work marks a step forward in our understanding of the roles of frog macrophage subsets in antifungal defenses.

The Leydig organ of elasmobranchs: Shed light on an active and mysterious defense center with immunological characterization of its cells

In elasmobranchs, blood cells originate from the thymus, spleen, and two organs associated with the gonads and esophagus: the epigonal and Leydig organs (LO), respectively. In the LO parenchyma, various granulocyte types, thrombocytes, and lymphocytes were observed, with notable species-specific diversity. Cells of the LO in the catshark Galeus melastomus were characterized using immunohistochemistry, immunofluorescence, and electron microscopy. Image analysis of sections treated with the anti-proliferative cell nuclear antigen antibody revealed that approximately 40 % of LO cells were in mitosis and approximately 15 % were myeloid progenitors positive for the anti-c-kit antibody. Additionally, a panel of 12 immunological markers revealed that 18.4 %, 17.8 %, 13.7 %, and 14.1 % of the LO section area was occupied by cells positive for anti-serotonin, interleukin-6, histamine, and immunoglobulin E-like receptor antibodies, respectively. Two main reactive cell types, mast cell and macrophage lineage, were revealed by co-localization image analysis using confocal microscopy. Ultrastructural observation identified granulocytes and macrophages as the main cell types. To our knowledge, this is the first report on the distinct patterns of LO cell populations and their morphological features, confirming granulopoietic role of the elasmobranch LO.

Effects of fulvic acid on zebrafish (Danio rerio) growth, immunity and antioxidant status

This experiment aimed to determine the efficacy of fulvic acid (FLA) on growth performance, innate immune system, antioxidant parameters, and expression of immune and antioxidant-related genes in zebrafish (Danio rerio). To this end, 12 tanks (3 per group), each containing 50 zebrafish (with an average weight of 85.7 ± 10.05 mg) in 72 L, were assigned to diets containing FLA at four levels: 0 (control), 0.25 (FLA1), 0.5 (FLA2), and 1 (FLA3) g/kg diet. Following an eight-week culture period, no significant differences in growth performance were observed among the treatment groups (P > 0.05). However, lysozyme activity, total immunoglobulin (Ig), and total protein concentrations in whole-body extracts were significantly enhanced in the 0.5-1 g FLA/kg diet groups compared to the other treatments (P < 0.05). No significant differences were observed among the groups in catalase (CAT), glutathione peroxidase (GPx), or superoxide dismutase (SOD) activities (P > 0.05). The supplementation of FLA significantly upregulated the gene expression of interferon-α (IFN-α) and tumor necrosis factor-alpha (TNF-α), with the highest expression observed in the 0.5 g FLA/kg diet group (P < 0.05). Additionally, interleukin 1 (IL-1) expression was markedly elevated in this group in comparison to the other treatments (P < 0.05). While there was a significant increase in GPx gene expression with dietary FLA (P < 0.05), no notable differences were observed among FLA treatments (P > 0.05). CAT gene expression remained consistent across all groups (P > 0.05). In contrast, SOD gene expression significantly increased in response to all FLA-supplemented diets, with the highest level observed in the 0.5 g FLA/kg group (P < 0.05). These findings suggest that FLA may serve as an effective dietary supplement to enhance the immune response and antioxidant capacity in zebrafish.

Contribution of interleukins in the regulation of teleost fish immunity: A review from the perspective of regulating macrophages

Interleukins (ILs) are potent secreted regulators of a wide range of cell types and cellular activities, particularly in the immune system. They are able to participate in intercellular communication in homeostasis and disease, thereby exerting immune functions. Macrophages serve as the innate immune cells of vertebrates and play a pivotal role in defending against and eliminating external pathogens. In mammals, the immune response mounted by macrophages is intricately linked to ILs. Given the fact that teleost fish have evolved an innate immune system that closely resembles those of mammals, particularly in terms of the functionality of macrophages, raises the intriguing possibility that the regulatory function of ILs in macrophage-mediated immunity might be evolutionarily conserved across both mammal and teleost fish lineages. Consequently, from the perspective of interleukin regulation of macrophages, this review outlines the relationship between ILs and macrophages in teleost fish, and elucidates the regulatory role of ILs of immune cell function in teleost fish, thereby contributing to our understanding of the key role of these cytokines in the prevention and control of aquaculture diseases.

Physiological and biochemical responses of Labeo rohita to neonicotinoids imidacloprid, clothianidin, and their mixture

Neonicotinoids, widely used insecticides, pose severe environmental risks due to their persistence in soil and water, adversely affecting non-target organisms and ecosystem integrity. The present study examined the 56 days effects of imidacloprid (66.6 mg/l), clothianidin (30 mg/l), and their combination (33.3 mg/l and 15 mg/l) on Labeo rohita, using one-third of the LC50 sub-lethal concentrations. Survival, weight gain, and the hepatosomatic index decreased insignificantly in the IMI group and significantly in the CLO and Mix groups. Haematological indicators, including erythrocyte counts, haemoglobin, and haematocrit values, were also significantly reduced. Blood glucose and serum creatinine levels increased, while serum albumin, globulin, and plasma total proteins decreased. White blood cell counts elevated, while immunoglobulin (IgM), respiratory burst, and lysozyme activities were significantly inhibited. Liver, brain and muscle lactate and malate dehydrogenases were elevated, whereas succinate and glutamate dehydrogenases were decreased. Liver aspartate aminotransferase activity was substantially higher than that of brain and muscle, which had considerably higher levels of alanine aminotransferase in muscle than in the brain and liver. Additionally, muscle alkaline phosphatase activity was significantly higher than in the liver and brain, whereas liver acid phosphatase showed a greater elevation than in the muscle and brain. The physiological, haematological, and biochemical indices peaked on day 28 and slight recovery was observed on day 56 (IMI > CLO > Mix). The study highlights that the mixture of insecticides poses greater hazards compared to a single active compound, and the indiscriminate use of these insecticides jeopardizes non-target organisms, ecosystems, and public health.

Physiological responses of pacu (Piaractus mesopotamicus) to intermittent cold exposure: A comprehensive analysis of stress, immunity, antioxidant, and metabolic adaptations

This study examined the energy-dependent physiological responses, including stress, innate immune, and antioxidant systems, as well as indicators of energy mobilization, in pacu (Piaractus mesopotamicus) exposed to intermittent cold, aiming to assess the correlations between these responses. The fish were acclimated to 28 °C, divided into two groups, a control group maintained at 28 °C, and another exposed to 16 °C for two 24 h periods with a 5-day interval between them. The fish were sampled at six time points: baseline (after acclimatization to 28 °C), 24 h after the 1st exposure to 16 °C, after 5 days of recovery at 28 °C, 24 h after the 2nd exposure to 16 °C, and after 24 and 48 h of recovery at 28 °C. The reduction in temperature activated the stress response, the innate immune system, and the antioxidative system as well as mobilized lipids from the visceral stores and preserved the circulating levels of triglycerides. Intermittent exposure of fish to cold increased plasma cortisol after both exposures, activated leukocyte respiratory activity after the 2nd exposure, and triggered the compensatory lysozyme response after temperature recovery in both cold exposures in addition to increasing the number of circulating monocytes and granulocytes. The activity of the enzymes catalase (CAT) and superoxide dismutase (SOD) increased after the 1st and 2nd cold exposures, respectively. Glutathione peroxidase (GPx) activity increased after the 2nd exposure compared to the control. The subtropical fish pacu was sensitive to intermittent cold exposure and was able to display protective physiological responses.

The role of sodium butyrate in modulating growth, intestinal health, and antimicrobial efficacy in turbot (Scophthalmus maximus L.) fed high soy diets

Butyrate is one of the most abundant short-chain fatty acids (SCFAs), which are important metabolites of dietary fiber by fermentation of gut commensals, and has been shown to be vital in maintaining host health. The present study mainly investigated how sodium butyrate (NaB) supplementation in the diet with high proportion of soybean meal (SBM) affected turbot. Four experimental diets were formulated: (1) fish meal (FM) based diet (control group), (2) SBM protein replacing 45% FM protein in the diet (high SBM group), (3) 0.2% NaB supplementation in the high SBM diet (high SBM + 0.2% NaB group), and (4) 0.5% NaB supplementation in the high SBM diet (high SBM + 0.5% NaB group). The fish were fed four different diets for 8 weeks. The results showed that the high SBM diet significantly suppressed growth performance, induced typical enteritis symptoms and decreased resistance to bacterial infection. However, inclusion of 0.2% and 0.5% NaB in the high SBM diet both effectively increased the growth performance of turbot. Meanwhile, dietary NaB protected the intestinal morphology, and regulated the gene expression of inflammatory cytokines to relieve the inflammation of turbot, such as TNFα, IL-1β, NFκB and IL-10. Moreover, supplementation with NaB in the high SBM diet activated HIF-1α/IL-22/Lysozyme signaling pathway to against Edwardsiella tarda (E. tarda) infection, especially 0.5% NaB supplementation exerted more effectively to defence bacterial infection under inflammatory state. In conclusion, dietary NaB significantly promoted growth and gut health of turbot. Besides, it enhanced the resistance of fish to bacterial infection, especially dietary 0.5% NaB supplementation.

Transcriptome analysis reveals the mechanism of black rockfish (Sebastes schlegelii) macrophages respond to Edwardsiella piscicida infection in vivo

Sebastes schlegelii is an economically significant marine fish that faces serious threats from various pathogens. Edwardsiella piscicida is a pathogenic bacterium that primarily affects fish, including S. schlegelii, leading to severe disease. Although numerous reports have documented the transcriptome sequencing of various fish tissues in response to E. piscicida infection, studies focusing on specific cells remain scarce. In this study, S. schlegelii were infected by intraperitoneal injection of E. piscicida. Severe external clinical signs were observed in E. piscicida-infected S. schlegelii and pathological examination demonstrated structural damage of the head kidney following treatment with E. piscicida. Furthermore, macrophages were isolated from the head kidneys of both the control and E. piscicida-infected groups for RNA sequencing (RNA-seq). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated that the DEGs were closely associated with immune response and oxidative stress. Additionally, Weighted Gene Co-expression Network Analysis (WGCNA) was performed based on the data from this study and RNA-seq files of macrophages infected with E. piscicida in vitro, revealing that immune responses, oxidative stress, and mitochondrial damage were involved in the macrophage response to E. piscicida infection both in vivo and in vitro. This study provides a reference for understanding the mechanisms by which teleost immune cells respond to pathogen invasion and enhances our comprehension of teleost innate immunity.

A novel long non-coding RNA linc-93.2 participates in bisphenol induced oxidative stress and macrophage polarization in red common carp (Cyprinus carpio)

Previous studies show that bisphenol A (BPA) and its analogs induce oxidative stress and promote inflammatory response. However, the key molecules in regulating this process remain unclear. Here, we report significant inductive effects of BPA and bisphenol AF (BPAF) on a newly found long non-coding RNA linc-93.2 accompanied by oxidative stress and activation of pro-inflammatory pathways in treated fish and fish primary macrophages. Silencing linc-93.2 in fish primary macrophages in vitro or fish in vivo significantly promotes the expression of anti-oxidative stress-related genes and anti-inflammatory cytokines. This inhibition of pro-inflammatory cytokine expression, showing cell status disruption towards to M2 polarization. Followed by exposure to BPA or BPAF, silencing linc-93.2 in vitro or in vivo significantly attenuates the increased production of reactive oxygen species and malondialdehyde level aroused by bisphenol treatment, possibly owing to the enhancement of total antioxidant capacity observed in cells and tissue after linc-93.2 knockdown. RNA-sequencing further revealed regulation of nuclear factor-kappa b (NF-κB) in linc-93.2's downstream network, combining with our previous observation on the upstream regulation of linc-93.2 via NF-κB, which together suggest a critical role of linc-93.2 in promoting NF-κB positive feedback loop that may be an important molecular event initiating the immunotoxicity of bisphenols.

Polystyrene nanoplastic and engine oil synergistically intensify toxicity in Nile tilapia, Oreochromis niloticus : Polystyrene nanoplastic and engine oil toxicity in Nile tilapia

Polystyrene nanoplastic (PS-NPs) and Engine oil (EO) pose multiple ecotoxic effects with increasing threat to fish ecosystems. The current study investigated the toxicity of 15 days exposure to PS-NPs and / or EO to explore their combined synergistic effects on Nile tilapia, Oreochromis niloticus (O. niloticus). Hematobiochemical parameters, proinflammatory cytokines, and oxidative stress biomarkers as well as histological alterations were evaluated. The experimental design contained 120 acclimated Nile tilapia distributed into four groups, control, PS-NPs (5 mg/L), EO (1%) and their combination (PS-NPs + EO). After 15-days of exposure, blood and tissue samples were collected from all fish experimental groups. Results indicated that Nile tilapia exposed to PS-NPs and / or EO revealed a significant decrease in almost all the measured hematological parameters in comparison to the control, whereas WBCs and lymphocyte counts were significantly increased in the combined group only. Results clarified that the combined PS-NPs + EO group showed the maximum decrease in RBCs, Hb, MCH and MCHC, and showed the maximum significant rise in interleukin-1β (IL-1β), and interleukin-6 (IL-6) in comparison to all other exposed groups. Meanwhile, total antioxidant capacity (TAC) showed a significant (p < 0.05) decline only in the combination group, whereas reduced glutathione (GSH) showed a significant decline in all exposed groups in comparison to the control. Both malondialdehyde (MDA) and aspartate aminotransferase (AST) showed a significant elevation only in the combination group. Uric acid showed the maximum elevation in the combination group than all other groups, whereas creatinine showed significant elevation in the EO and combination group when compared to the control. Furthermore, the present experiment proved that exposure to these toxicants either individually or in combination is accompanied by pronounced histomorpholgical damage characterized by severe necrosis and hemorrhage of the vital organs of Nile tilapia, additionally extensively inflammatory conditions with leucocytes infiltration. We concluded that combination exposure to both PS-NPs and EO caused severe anemia, extreme inflammatory response, oxidative stress, and lipid peroxidation effects, thus they can synergize with each other to intensify toxicity in fish.

Cell atlas of the Atlantic salmon spleen reveals immune cell heterogeneity and cell-specific responses to bacterial infection

The spleen is a conserved secondary lymphoid organ that emerged in parallel to adaptive immunity in early jawed vertebrates. Recent studies have applied single cell transcriptomics to reveal the cellular composition of spleen in several species, cataloguing diverse immune cell types and subpopulations. In this study, 51,119 spleen nuclei transcriptomes were comprehensively investigated in the commercially important teleost Atlantic salmon (Salmo salar L.), contrasting control animals with those challenged with the bacterial pathogen Aeromonas salmonicida. We identified clusters of nuclei representing the expected major cell types, namely T cells, B cells, natural killer-like cells, granulocytes, mononuclear phagocytes, endothelial cells, mesenchymal cells, erythrocytes and thrombocytes. We discovered heterogeneity within several immune lineages, providing evidence for resident macrophages and melanomacrophages, infiltrating monocytes, several candidate dendritic cell subpopulations, and B cells at distinct stages of differentiation, including plasma cells and an igt + subset. We provide evidence for twelve candidate T cell subsets, including cd4+ T helper and regulatory T cells, one cd8+ subset, three γδT subsets, and populations double negative for cd4 and cd8. The number of genes showing differential expression during the early stages of Aeromonas infection was highly variable across immune cell types, with the largest changes observed in macrophages and infiltrating monocytes, followed by resting mature B cells. Our analysis provides evidence for a local inflammatory response to infection alongside B cell maturation in the spleen, and upregulation of ccr9 genes in igt + B cells, T helper and cd8+ cells, and monocytes, consistent with the recruitment of immune cell populations to the gut to deal with Aeromonas infection. Overall, this study provides a new cell-resolved perspective of the immune actions of Atlantic salmon spleen, highlighting extensive heterogeneity hidden to bulk transcriptomics. We further provide a large catalogue of cell-specific marker genes that can be leveraged to further explore the function and structural organization of the salmonid immune system.

Infection of endemic chub Squalius tenellus with the intestinal tapeworm Caryophyllaeus brachycollis (Cestoda): histopathology and ultrastructural surveys

The endemic chub Squalius tenellus (Heckel, 1843) was introduced more than 100 years ago to Lake Blidinje (Bosnia-Herzegovina). Only 1 species of enteric helminth was found in a sample of 35 chubs, the tapeworm Caryophyllaeus brachycollis (Janiszewska, 1953). The paper includes histopathological investigation with identification of innate immune cells involved in host reaction and molecular data allowed correct designation of the cestode species. Of 35 specimens of chub examined, 21 (60%) harboured individuals of C. brachycollis and a total of 1619 tapeworms were counted, the intensity of infection ranged from 1 to 390 worms per fish (46.2 ± 15.3, mean ± s.e.). Histopathological and ultrastructural investigations showed strict contact between the worm's body and the epithelia and increase in the number of mucous cells, rodlet cells among the epithelial cells. Within the tunica propria-submucosa, beneath the site of scolex attachment, numerous neutrophils and mast cells were noticed. This is the first study of the occurrence of C. brachycollis in chub from Lake Blidinje and on the response of the innate immune cells of S. tenellus to this tapeworm. Interestingly, in 3 very heavily infected chubs, perforation of the intestinal wall was documented; this is uncommon among cestodes which use fish as a definitive host.

Effect of dietary koumine on the immune and antioxidant status of carp (Cyprinus carpio) after Aeromonas hydrophila infection

Koumine (KM) has anxiolytic, anti-inflammatory and growth-promoting effects in pigs and sheep. Based on the growth-promoting and immunological effects of koumine, the present study was conducted on Cyprinus carpio (C. carpio) with four KM concentrations: 0 mg/kg, 0.2 mg/kg, 2 mg/kg, and 20 mg/kg for 10 weeks, followed by a 1-week Aeromonas hydrophila (A. hydrophila) infection experiment. The effect of KM on the immunity of A. hydrophila infected carp was analyzed by histopathology, biochemical assay, and qRT-PCR to assess the feasibility of KM in aquaculture. The results showed that the presence of KM alleviated pathogen damage to carp tissues. At 2 mg/kg and 20 mg/kg concentrations of KM successively and significantly elevated (p < 0.05) the SOD activities in the intestinal tract, hepatopancreas and kidney of carp. The expression levels of hepatopancreatic antioxidant genes Nrf2 and IGF-1 were significantly up-regulated in the same group (p < 0.05), while the expression levels of immune genes IL-8 and IL-10 were down-regulated. In summary, KM at concentrations of 2 mg/kg and 20 mg/kg could regulate the expression of antioxidant and immune genes in various tissues in an orderly and rapid manner, and significantly improve the antioxidant and immune abilities of carp, which is conducive to the improvement of the resilience of carp.

Role of HIF in fish inflammation

The hypoxia-inducing factor (HIF) is a central transcription factor in cellular oxygen sensing and regulation. It is common that the inflammation always appears in many diseases, like infectious diseases in fishes, and the inflammation is often accompanied by hypoxia, as a hallmark of inflammation. Besides coordinating cellular responses to low oxygen, HIF-mediated hypoxia signaling pathway is also crucial for immune responses such as the regulations of innate immune cell phenotype and function, as well as metabolic reprogramming under the inflammation. However, the understanding of the molecular mechanisms by which HIFs regulate the inflammatory response in fish is still very limited. Here, we review the characteristics of HIF as well as its roles in innate immune cells and the infections caused by bacteria and viruses. The regulatory effects of HIF on the metabolic reprogramming of innate immune cells are also discussed and the future research directions are outlooked. This paper will serve as a reference for elucidating the molecular mechanism of HIF regulating inflammation and identifying treatment strategies to target HIF for fish disease.

Tryptophan and Cortisol Modulate the Kynurenine and Serotonin Transcriptional Pathway in the Kidney of Oncorhynchus kisutch

Aquaculture fish are kept for long periods in sea cages or tanks. Consequently, accumulated stress causes the fish to present serious problems with critical economic losses. Fish food has been supplemented to reduce this stress, using many components as amino acids such as tryptophan. This study aims to determine the transcriptional effect of tryptophan and cortisol on primary cell cultures of salmon head and posterior kidney. Our results indicate activation of the kynurenine pathway and serotonin activity when stimulated with tryptophan and cortisol. An amount of 95% of tryptophan is degraded by the kynurenine pathway, indicating the relevance of knowing how this pathway is activated and if stress levels associated with fish culture trigger its activation. Additionally, it is essential to know the consequence of increasing kynurenic acid "KYNA" levels in the short and long term, and even during the fish ontogeny.

Teleost innate immunity, an intricate game between immune cells and parasites of fish organs: who wins, who loses

Fish, comprising over 27,000 species, represent the oldest vertebrate group and possess both innate and adaptive immune systems. The susceptibility of most wild fish to parasitic infections and related diseases is well-established. Among all vertebrates, the digestive tract creates a remarkably favorable and nutrient-rich environment, which, in turn, renders it susceptible to microparasites and macroparasites. Consequently, metazoan parasites emerge as important disease agents, impacting both wild and farmed fish and resulting in substantial economic losses. Given their status as pathogenic organisms, these parasites warrant considerable attention. Helminths, a general term encompassing worms, constitute one of the most important groups of metazoan parasites in fish. This group includes various species of platyhelminthes (digeneans, cestodes), nematodes, and acanthocephalans. In addition, myxozoans, microscopic metazoan endoparasites, are found in water-dwelling invertebrates and vertebrate hosts. It is worth noting that several innate immune cells within the fish alimentary canal and certain visceral organs (e.g., liver, spleen, and gonads) play active roles in the immune response against parasites. These immune cells include macrophages, neutrophils, rodlet cells, and mast cells also known as eosinophilic granular cells. At the site of intestinal infection, helminths often impact mucous cells number and alter mucus composition. This paper presents an overview of the state of the art on the occurrence and characteristics of innate immune cells in the digestive tract and other visceral organs in different fish-parasite systems. The data, coming especially from studies employed immunohistochemical, histopathological, and ultrastructural analyses, provide evidence supporting the involvement of teleost innate immune cells in modulating inflammatory responses to metazoan and protozoan parasitic infections.

The role of CcPTGS2a in immune response against Aeromonas hydrophila infection in common carp (Cyprinus carpio)

Prostaglandin-endoperoxide synthase 2 (PTGS2), a crucial enzyme in prostaglandin synthesis, catalyzes the conversion of arachidonic acid to prostaglandins and plays a significant role in the inflammatory response. This investigation aimed to determine the regulatory role of PTGS2a in the innate immune response to bacterial infection in fish. To achieve this objective, the CcPTGS2a gene was identified and characterized in common carp (Cyprinus carpio), and its function in immune defense was investigated. According to the sequence and structural analysis results, CcPTGS2a had an open reading frame of 1806 bp that encoded 602 amino acids. It was estimated that the protein's theoretical molecular weight was 69.0 kDa, and its isoelectric point was 8.10. The structure of CcPTGS2a was observed to be conserved, with an epidermal growth factor domain and a peroxidase domain present. Moreover, the amino acid sequence of CcPTGS2a exhibited significant homology with the amino acid sequences of several fish species. CcPTGS2a mRNA was detected in the healthy tissues of common carp, with higher expression in the head kidney, spleen, gills, and liver. Following the challenges with Aeromonas hydrophila and lipopolysaccharide, CcPTGS2a mRNA showed unique geographic and temporal expression patterns, with significant increases detected in the head kidney, gills, spleen, and liver. Additionally, the recombinant CcPTGS2a protein exhibited detectable bacterial binding to various bacteria. As determined by subcellular localization analysis, CcPTGS2a was predominantly localized in the nucleus and cytoplasm. Furthermore, it was discovered that the overexpression of CcPTGS2a stimulated the up-regulation of ferroptosis-related genes and inflammatory cytokine mRNA expression in fish and EPC (Epithelioma papulosum cyprinid) cells while concurrently reducing the bacterial load of A. hydrophila. In contrast, the interference of CcPTGS2a decreased the mRNA expression of ferroptosis-related genes and inflammatory cytokines in fish and EPC cells and increased the bacterial load of A. hydrophila. Notably, A. hydrophila stimulation resulted in the up-regulation of CcPTGS2a protein expression in EPC cells. These results suggested that CcPTGS2a was involved in the immune response to bacterial infections in common carp.

Repeated exposure affects susceptibility and responses of Atlantic salmon (Salmo salar) towards the ectoparasitic salmon lice (Lepeophtheirus salmonis)

Atlantic salmon (Salmo salar) is repeatedly exposed to and infected with ectoparasitic salmon lice (Lepeophtheirus salmonis) both in farms and in nature. However, this is not reflected in laboratory experiments where fish typically are infected only once. To investigate if a previous lice infection affects host response to subsequent infections, fish received 4 different experimental treatments; including 2 groups of fish that had previously been infected either with adult or infective salmon lice larvae (copepodids). Thereafter, fish in all treatment groups were infected with either a double or a single dose of copepodids originating from the same cohort. Fish were sampled when lice had developed into the chalimus, the pre-adult and the adult stage, respectively. Both the specific growth rate and cortisol levels (i.e. a proxy for stress) of the fish differed between treatments. Lice success (i.e. ability to infect and survive on the host) was higher in naïve than in previously infected fish (pre-adult stage). The expression of immune and wound healing transcripts in the skin also differed between treatments, and most noticeable was a higher upregulation early in the infection in the group previously infected with copepodids. However, later in the infection, the least upregulation was observed in this group, suggesting that previous exposure to salmon lice affects the response of Atlantic salmon towards subsequent lice infections.

Effect of dietary inclusion of microalgae (Nannochloropsis gaditana and Schizochytrium spp) on non-specific immunity and erythrocyte maturity in Atlantic salmon fingerlings

The parr-smolt transformation in salmonids involves a critical period characterized by systemic changes associated with the fish's immune response. In this context, as a dietary ingredient in functional diets, microalgae offer an alternative due to their nutritional and bioactive compounds that could strengthen the immune status. This study evaluated the effect of a diet supplemented with Schizochytrium spp and Nannochloropsis gaditana on the expression of genes associated with the antibacterial response. Additionally, the study assessed the effect on the leukocyte population and erythrocyte maturity in Salmo salar blood. Fish were fed for 30 days with a microalgal mixture (1:1) at a 10% inclusion. Each diet was randomly assigned to a tank using a completely randomized design (CRD) with four replications. Each tank was stocked with 70 Atlantic salmon fingerlings with an initial mean weight of 78.87 ± 0.84. Transcription levels were quantified and analyzed by qRT-PCR from cell isolates and mucus tissue. Furthermore, cell count and identification of leukocytes and classification of cellular maturity of erythrocytes using a neural network with a multilayer perceptron (MLP) were performed. Our results showed a significant (p < 0.05) increase in fold change expression of C3 (2.54 ± 0.65) and NK-Lysine (6.84 ± 0.94) in erythrocytes of microalgae-supplemented fish. Moreover, a significant increase of 1.59 and 2.35 times in monocytes and immature erythrocytes, respectively, was observed in the same group of fish (p < 0.05). This study's results indicate that dual microalgae (Schizochytrium spp and N. gaditana) supplementation can increase innate humoral antibacterial components, particularly in erythrocyte tissue, and increase phagocytic cells and immature erythrocytes in S. salar blood.

Toxicological mechanism of ammonia-N on haematopoiesis and apoptosis of haemocytes in Litopenaeus vannamei

Ammonia, as an important pollutant, contributed to the reduction of immunity, disruption of physiology in animals. RNA interference (RNAi) was performed to understand the function of astakine (AST) in haematopoiesis and apoptosis in Litopenaeus vannamei under ammonia-N exposure. Shrimps were exposed to 20 mg/L ammonia-N from 0 to 48 h with injection of 20 μg AST dsRNA. Further, shrimps were exposed to 0, 2, 10 and 20 mg/L ammonia-N also from 0 to 48 h. The results showed that the total haemocytes count (THC) decreased under ammonia-N stress and the knockdown of AST resulted in a further decrease of THC, suggesting that 1) the proliferation was decreased through the reduction of AST and Hedgehog, the differentiation was interfered by Wnt4, Wnt5 and Notch, and the migration was inhibited by the decrease of VEGF; 2) oxidative stress was induced under ammonia-N stress, leading to the increase of DNA damage with the up-regulated gene expression of death receptor, mitochondrial and endoplasmic reticulum stress pathways; 3) the changes of THC resulted from the decrease of proliferation, differentiation and migration of haematopoiesis cells and the increase of apoptosis of haemocytes. This study helps to deepen our understanding of risk management in shrimp aquaculture.

The Keap1-Nrf2 signaling pathway regulates antioxidant defenses of Ctenopharyngodon idella induced by bacterial infection

Respiratory burst is a process involving rapid production of reactive oxygen species (ROS) for eliminating invading pathogens. However, excessive ROS production can be fatal to the host organism. The Keap1-Nrf2-ARE (Kelch-like ECH-associated protein 1 [Keap1]; Nuclear factor erythroid-derived 2-like 2 [Nrf2]; Antioxidant responsive element [ARE]) signaling pathway plays an important role in alleviating oxidative stress and preserving cellular homeostasis. However, the role of Keap1 during bacterial infection in fish remains unclear. In this study, we cloned and characterized the Keap1 gene of grass carp (CiKeap1) for the first time. CiKeap1 encodes a 593-amino acid protein of the Keap1b type. The tissue distribution analysis data revealed that the brain contains the highest transcription level of Keap1, followed by the heart and liver. The infection of Aeromonas hydrophila and Staphylococcus aureus obviously modulated the gene transcription and protein expression levels of Keap1, which suggested that the CiKeap1 participates in antibacterial immune responses. Furthermore, in vitro overexpression assays clarified the defensive and regular roles of CiKeap1 in maintaining host redox homeostasis in response to bacterial infection through the Keap1-Nrf2-ARE signaling pathway. In conclusion, the present results provide an expanded perspective on the role of Keap1 in teleost immunology that can guide healthy farming cultivation of grass carp.

Psidium guajava L.- dichloromethane and ethyl acetate fractions ameliorate striped catfish (Pangasianodon hypophthalmus) status via immune response, inflammatory, and apoptosis pathways

Psidium guajava L. is known to possess immune-modulatory properties in humans and other mammals. Although the positive effects of P. guajava-based diets on the immunological status have been shown for some fish species, the underlying molecular mechanisms of its protective effects remain to be investigated. The aims of this study were to evaluate the immune-modulatory effects of two guava fractions from dichloromethane (CC) and ethyl acetate (EA) on striped catfish with in vitro and in vivo experiments. Striped catfish head kidney leukocytes were stimulated with 40, 20, 10 and 0 μg/ml of each extract fraction, and the immune parameters (ROS, NOS, and lysozyme) were examined at 6 and 24h post stimulation. A final concentration of each fraction at 40, 10 and 0 μg/fish was then intraperitoneally injected into the fish. After 6, 24, and 72h of administration, immune parameters as well as the expression of some cytokines related to innate and adaptive immune responses, inflammation, and apoptosis were measured in the head kidney. Results indicated that the humoral (lysozyme) and cellular (ROS and NOS) immune endpoints were regulated differently by CC and EA fractions depending on dose and time in both, in vitro and in vivo experiments. With regards to the in vivo experiment, the CC fraction of the guava extract could significantly enhance the TLRs-MyD88-NF-κB signaling pathway by upregulating its cytokine genes (tlr1, tlr4, myd88, and traf6), following the upregulation of inflammatory (nfκb, tnf, il1β, and il6) and apoptosis (tp53 and casp8) genes 6 h after injection. Moreover, fish treated with both CC and EA fractions significantly enhanced cytokine gene expression including lys and inos at the later time points - 24h or 72h. Our observations suggest that P. guajava fractions modulate the immune, inflammatory, and apoptotic pathways.

Hematological, immuno-antioxidant disruptions, and genes down-regulation induced by Aeromonas veronii challenge in Clarias gariepinus: The ameliorative role of silica nanoparticles

Aeromonas veronii is a pathogenic bacterium associated with various diseases in aquaculture. However, few studies address the antibacterial activity using nanoparticles (NPs). Hence, the current study is innovative to evaluate the antibacterial efficacy of silica nanoparticles (SiNPs) against A. veronii infection in-vitro with a trial for treatment in-vivo. Primarily, we assessed the in-vitro antibacterial activity against A. veronii. Further, we investigated the hematological profile, immune-antioxidant response, and gene expression of African catfish (Clarias gariepinus) in response to SiNPs exposure and the A. veronii challenge. Fish (N = 120; weight: 90 ± 6.19 g) were distributed into four groups (30 fish/group) for a ten-days-treatment trial. The first (control) and second (SiNPs) groups were treated with 0 mg/L and 20 mg/L SiNPs in water, respectively. The third (A. veronii) and fourth (SiNPs + A. veronii) groups were treated with 0 mg/L and 20 mg/L SiNPs in water, respectively, and infected with A. veronii (1.5×10⁷ CFU/mL). Results demonstrated that SiNPs displayed an in-vitro antibacterial activity against A. veronii with a 21 mm inhibitory zone. A. veronii infection caused a high mortality rate (56.67%) and substantial reductions in hematological indices and immune indicators [nitric oxide (NO) and immunoglobulin M (IgM)]. Additionally, marked decline in the level of antioxidants [superoxide dismutase (SOD), catalase (CAT), and reduced glutathione content (GSH)] as well as down-regulation in the immune-related genes [interleukins (IL-1β and IL-8) and tumor necrosis factor-alpha (TNF-α)] and antioxidant-related genes [SOD1, glutathione peroxidase (GPx), and glutathione-S-transferase (GST)] were the consequences of A. veronii infection. Surprisingly, treatment of A. veronii-infected fish with SiNPs lessened the mortality rate, enhanced the blood picture, modulated the immune-antioxidant parameters, and resulted in gene up-regulation. Overall, this study encompasses the significant role of SiNPs, a new versatile tool for combating hematological, immuno-antioxidant alterations, and gene down-regulation induced by A. veronii infection and sustainable aquaculture production.

Transcriptome profiling and differential expression analysis of altered immune-related genes in goldfish (Carassius auratus) infected with Aeromonas hydrophila

Goldfish (Carassius auratus) have been employed as a model organism to investigate the innate immune system and host-pathogen interactions. A Gram-negative bacterium called Aeromonas hydrophila has been found to cause mass mortality due to infection in a wide variety of fish species in the aquatic system. In this study, damages in Bowman's capsule, inflammatory tubular (proximal and distilled convoluted) structure, and glomerular necrosis were observed in A. hydrophila-infected head kidney of goldfish. To increase the better understanding of immune mechanisms of host defense against A. hydrophila, we performed a transcriptome analysis in head kidney of goldfish at 3 and 7 days of post-infection (dpi). Comparing to the control group, 4638 and 2580 differentially expressed genes (DEGs) were observed at 3 and 7 dpi, respectively. The DEGs were subsequently enriched in multiple immune-related pathways including Protein processing in endoplasmic reticulum, Insulin signaling pathway, and NOD-like receptor signaling pathway. The expression profile of immune-related genes such as TRAIL, CCL19, VDJ recombination-activating protein 1-like, Rag-1, and STING was validated by qRT-PCR. Furthermore, the levels of immune-related enzyme (LZM, AKP, SOD, and CAT) activities were examined at 3 and 7 dpi. The knowledge gained from the current study will be helpful for better understanding of early immune response in goldfish after A. hydrophila challenge, which will aid in future research on prevention strategies in teleost.

Identification and characterization of two novel antimicrobial peptides from Japanese sea bass (Lateolabrax japonicus) with antimicrobial activity and MO/MФ activation capability

Piscidins participate in the innate immune response of fish, which aims to eliminate recognized foreign microbes and restore the homeostasis of immune system. We characterized two piscidin-like antimicrobial peptides (LjPL-3 and LjPL-2) isolated from Japanese sea bass (Lateolabrax japonicus). LjPL-3 and LjPL-2 showed different expression patterns in tissues. After Vibrio harveyi infection, the mRNA expression of LjPL-3 and LjPL-2 was upregulated in the liver, spleen, head kidney, and trunk kidney. The synthetic mature peptides LjPL-3 and LjPL-2 exhibited different antimicrobial spectra. Furthermore, LjPL-3 and LjPL-2 treatments decreased inflammatory cytokine production while promoting chemotaxis and phagocytosis in monocytes/macrophages (MO/MФ). LjPL-2, but not LjPL-3, displayed bacterial killing capability in MO/MФ. LjPL-3 and LjPL-2 administration increased Japanese sea bass survival after V. harveyi challenge, which was accompanied by a decline in bacterial burden. These data suggested that LjPL-3 and LjPL-2 participate in immune response through direct bacterial killing and MO/MФ activation.

Differential Expression Genes of the Head Kidney and Spleen in Streptococcus iniae-Infected East Asian Fourfinger Threadfin Fish (Eleutheronema tetradactylum)

Streptococcus iniae is a Gram-positive bacterium and is considered a harmful aquaculture pathogen worldwide. In this study, S. iniae strains were isolated from East Asian fourfinger threadfin fish (Eleutheronema tetradactylum) reared on a farm in Taiwan. A transcriptome analysis of the head kidney and spleen was performed in the fourfinger threadfin fish 1 day after infection using the Illumina HiSeq™ 4000 platform for RNA-seq to demonstrate the host immune mechanism against S. iniae. A total of 7333 genes based on the KEGG database were obtained after the de novo assembly of transcripts and functional annotations. Differentially expressed genes (DEGs) (2-fold difference) were calculated by comparing the S. iniae infection and phosphate-buffered saline control group gene expression levels in each tissue sample. We identified 1584 and 1981 differentially expressed genes in the head kidney and spleen, respectively. Based on Venn diagrams, 769 DEGs were commonly identified in both the head kidney and spleen, and 815 and 1212 DEGs were specific to the head kidney and spleen, respectively. The head-kidney-specific DEGs were enriched in ribosome biogenesis. The spleen-specific and common DEGs were found to be significantly enriched in immune-related pathways such as phagosome, Th1, and Th2 cell differentiation; complement and coagulation cascades; hematopoietic cell lineage; antigen processing and presentation; and cytokine-cytokine receptor interactions, based on the KEGG database. These pathways contribute to immune responses against S. iniae infection. Inflammatory cytokines (IL-1β, IL-6, IL-11, IL-12, IL-35, and TNF) and chemokines (CXCL8 and CXCL13) were upregulated in the head kidney and spleen. Neutrophil-related genes, including phagosomes, were upregulated post-infection in the spleen. Our results could offer a strategy for the treatment and prevention of S. iniae infection in fourfinger threadfin fish.

Immune-enhancing effects of Astragalus polysaccharides and Ganoderma lucidum polysaccharides on Vibrio harveyi flgJ DNA vaccine in grouper

Astragalus polysaccharides (APS) and Ganoderma lucidum polysaccharides (GLP) have been shown to possess strong immunoregulatory properties in aquatic animals. In this study, the fragment containing Vibrio harveyi flgJ gene was ligated into pcDNA3.1(+) vector and pcDNA3.1(+)-flgJ was constructed as DNA vaccine. APS and GLP were used as DNA vaccine adjuvants to evaluate the immunoregulatory effect by intramuscular injection to pearl gentian grouper (♀Epinephelus fuscoguttatus × ♂E. lanceolatus). The results showed that pcDNA3.1(+)-flgJ combined with APS or GLP could significantly up-regulate the innate and adaptive immune response in fish, including serum-specific antibody titres, catalase and lysozyme activities. At the same time, DNA vaccine combined with APS or GLP significantly up-regulated the expression levels of CD8α, IgM, IL-1β, MHC-Iα, MyD88 and TLR3 genes in thymus, head kidney, spleen and liver of pearl gentian grouper in comparison with those of the pFlgJ group. After 42 days post-vaccination, V. harveyi was used to challenge pearl gentian grouper by intraperitoneal injection. The relative percentage of survival (RPS) of pFlgJ, pFlgJ +APS, pFlgJ +GLP and pFlgJ+APS+GLP groups were 69%, 81%, 77% and 88%, respectively. These results suggested APS and GLP were potential adjuvants for DNA vaccine against V. harveyi infection in pearl gentian grouper.

Amphibians as a model to study the role of immune cell heterogeneity in host and mycobacterial interactions

Mycobacterial infections represent major concerns for aquatic and terrestrial vertebrates including humans. Although our current knowledge is mostly restricted to Mycobacterium tuberculosis and mammalian host interactions, increasing evidence suggests common features in endo- and ectothermic animals infected with non-tuberculous mycobacteria (NTMs) like those described for M. tuberculosis. Importantly, most of the pathogenic and non-pathogenic NTMs detected in amphibians from wild, farmed, and research facilities represent, in addition to the potential economic loss, a rising concern for human health. Upon mycobacterial infection in mammals, the protective immune responses involving the innate and adaptive immune systems are highly complex and therefore not fully understood. This complexity results from the versatility and resilience of mycobacteria to hostile conditions as well as from the immune cell heterogeneity arising from the distinct developmental origins according with the concept of layered immunity. Similar to the differing responses of neonates versus adults during tuberculosis development, the pathogenesis and inflammatory responses are stage-specific in Xenopus laevis during infection by the NTM M. marinum. That is, both in human fetal and neonatal development and in tadpole development, responses are characterized by hypo-responsiveness and a lower capacity to contain mycobacterial infections. Similar to a mammalian fetus and neonates, T cells and myeloid cells in Xenopus tadpoles and axolotls are different from the adult immune cells. Fetal and amphibian larval T cells, which are characterized by a lower T cell receptor (TCR) repertoire diversity, are biased toward regulatory function, and they have distinct progenitor origins from those of the adult immune cells. Some early developing T cells and likely macrophage subpopulations are conserved in adult anurans and mammals, and therefore, they likely play an important role in the host-pathogen interactions from early stages of development to adulthood. Thus, we propose the use of developing amphibians, which have the advantage of being free-living early in their development, as an alternative and complementary model to study the role of immune cell heterogeneity in host-mycobacteria interactions.

Depletion of chop suppresses procedural apoptosis and enhances innate immunity in loach Misgurnus anguillicaudatus under ammonia nitrogen stress

Ammonia nitrogen is highly toxic to fish, and it can easily cause fish poisoning or even high mortality. So far, many studies have been conducted on the damages to fish under ammonia nitrogen stress. However, there are few studies of ammonia tolerance improvement in fish. In this study, the effects of ammonia nitrogen exposure on apoptosis, endoplasmic reticulum (ER) stress, and immune cells in loach Misgurnus anguillicaudatus were investigated. Loaches (60 d post fertilization) were exposed to different concentrations of NH4Cl, and their survival rates were examined every 6 h. The results showed that high-concentration and long-time NH4Cl exposure (20 mM + 18 h; 15 mM + 36 h) induced apoptosis and gill tissue damages, finally causing a decline in survival. chop plays an important role in ER stress-induced apoptosis, and thus we constructed a model of chop-depleted loach by using CRISPR/Cas9 technology to investigate its response to ammonia nitrogen stress. The results showed that ammonia nitrogen stress down-regulated the expressions of apoptosis-related genes in chop+/- loach gills, while wildtype (WT) exhibited an opposite gene expression regulation pattern, suggesting that the depletion of chop suppressed apoptosis level. In addition, chop+/- loach showed a larger number of immunity-related cells and higher survival rate than WT under the NH4Cl exposure, indicating that the inhibition of chop function strengthened the innate immune barrier in general, thus increasing survival. Our findings provide the theoretical basis for developing high ammonia nitrogen-tolerant germplasm with aquaculture potential.

Immunomodulatory effects of bacteriocinogenic and non-bacteriocinogenic Lactococcus cremoris of aquatic origin on rainbow trout (Oncorhynchus mykiss, Walbaum)

Lactic Acid Bacteria (LAB) are a group of bacteria frequently proposed as probiotics in aquaculture, as their administration has shown to confer positive effects on the growth, survival rate to pathogens and immunological status of the fish. In this respect, the production of antimicrobial peptides (referred to as bacteriocins) by LAB is a common trait thoroughly documented, being regarded as a key probiotic antimicrobial strategy. Although some studies have pointed to the direct immunomodulatory effects of these bacteriocins in mammals, this has been largely unexplored in fish. To this aim, in the current study, we have investigated the immunomodulatory effects of bacteriocins, by comparing the effects of a wild type nisin Z-expressing Lactococcus cremoris strain of aquatic origin to those exerted by a non-bacteriocinogenic isogenic mutant and a recombinant nisin Z, garvicin A and Q-producer multi-bacteriocinogenic strain. The transcriptional response elicited by the different strains in the rainbow trout intestinal epithelial cell line (RTgutGC) and in splenic leukocytes showed significant differences. Yet the adherence capacity to RTgutGC was similar for all strains. In splenocyte cultures, we also determined the effects of the different strains on the proliferation and survival of IgM⁺ B cells. Finally, while the different LAB elicited respiratory burst activity similarly, the bacteriocinogenic strains showed an increased ability to induce the production of nitric oxide (NO). The results obtained reveal a superior capacity of the bacteriocinogenic strains to modulate different immune functions, pointing to a direct immunomodulatory role of the bacteriocins, mainly nisin Z.

The Early Peritoneal Cavity Immune Response to Vibrio Anguillarum Infection and to Inactivated Bacterium in Olive Flounder (Paralichthys olivaceus)

The peritoneal cavity plays an important role in the immune response, and intraperitoneal administration is an ideal vaccination route in fish. However, immune responses in the peritoneal cavity of teleost fish are still not completely characterized. This study characterized the morphology of peritoneal cavity cells (PerC cells) and their composition in flounder (Paralichthys olivaceus). Flow cytometric analysis of the resident PerC cells revealed two populations varying in granularity and size. One population, approximately 15.43% ± 1.8%, was smaller with a lower granularity, designated as lymphocytes. The other population of the cells, about 78.17% ± 3.52%, was larger with higher granularity and was designated as myeloid cells. The results of cytochemical staining and transmission electron microscopy indicated that peritoneal cavity in flounder normally contains a resident population of leukocytes dominated by granulocytes, macrophages, dendritic cells, and lymphocytes. The percentages of IgM+, CD4+, G-CSFR+, MHCII+, and CD83+ leukocytes among PerC cells determined by flow cytometry were 3.13% ± 0.4%, 2.83% ± 0.53%, 21.12% ± 1.44%, 27.11% ± 3.30%, and 19.64% ± 0.31%, respectively. Further, the changes in IgM+, CD4+, G-CSFR+, MHCII+, and CD83+ leukocytes in flounder after Vibrio anguillarum infection and immunization were compared. The composition changed rapidly after the infection or vaccination treatment and included two stages, a non-specific stage dominated by phagocytes and a specific immune stage dominated by lymphocytes. Due to the virulence effectors of bacteria, the infected group exhibited a more intense and complicated PerC cells immune response than that of the immunization group. Following our previous study, this is the first report on the morphology and composition of PerC cells and the early activation of PerC cells in flounder response to V. anguillarum infection and vaccination.

Temperature effects on teleost immunity in the light of climate change

Temperature is an important environmental modulator of teleost immune activity. Susceptibility of teleosts to temperature variation depends on the species-specific adaptive temperature range, and the activity of the teleost immune system is generally temperature-dependent. Similar to many physiological and metabolic traits of ectotherms, temperature modulates the activity of immune traits. At low temperatures, acquired immunity of many teleost species is down-modulated, and their immuno-competence mainly depends on innate immunity. At intermediate temperatures, both innate and acquired immunity are fully active and provide optimal protection, including long-lasting immunological memory. When temperatures increase and reach the upper permissive range, teleost immunity is compromised. Moreover, temperature shifts may have negative effects on teleost immune functions, in particular if shifts occur rapidly with high amplitudes. On the contrary, short-term temperature increase may help teleost immunity to fight against pathogens transiently. A major challenge to teleosts therefore is to maintain immuno-competence throughout the temperature range they are exposed to. Climate change coincides with rising temperatures, and more frequent and more extreme temperature shifts. Both are likely to influence the immuno-competence of teleosts. Nonetheless, teleosts exist in habitats that differ substantially in temperature, ranging from below zero in the Arctic's to above 40°C in warm springs, illustrating their enormous potential to adapt to different temperature regimes. The present review seeks to discuss how changes in temperature variation, induced by climate change, might influence teleost immunity.

The Effects of Sodium Propionate Supplementation in the Diet with High Soybean Meal on Growth Performance, Intestinal Health, and Immune Resistance to Bacterial Infection in Turbot (Scophthalmus maximus L.)

Short-chain fatty acids (SCFAs) are the products of the microbial fermentation of dietary fiber in the intestine. Acetate, propionate, and butyrate are the most abundant SCFA metabolites and play an important role in maintaining host health. This study was aimed at investigating the effects of sodium propionate (NaP) supplementation in the diet with a high proportion of soybean meal (SBM) on the growth, inflammatory status, and anti-infectious ability in juvenile turbot. Four experimental diets were designed: (1) fish meal- (FM-) based diet (control group), (2) SBM protein replacing 45% FM protein in the diet (high SBM group), (3) 0.5% NaP supplementation in the high SBM diet (high SBM+0.5% NaP group), and (4) 1.0% NaP supplementation in the high SBM diet (high SBM+1.0% NaP group). The results confirmed that the fish fed the high SBM diet for 8 weeks showed the decreased growth performance, the typical enteritis symptoms, and the increased mortality responding to Edwardsiella tarda (E. tarda) infection. However, 0.5% NaP supplementation in the high SBM diet promoted the growth performance of turbot and restored the activities of digestive enzymes in the intestine. Moreover, dietary NaP ameliorated the intestinal morphology, enhanced the expression of intestinal tight junction proteins, improved the antioxidant capacity, and suppressed the inflammatory status in turbot. Finally, the expression of antibacterial components and the resistance to bacterial infection were increased in NaP-fed turbot, especially in high SBM+1.0% NaP group. In conclusion, the supplementation of NaP in high SBM diet promotes the growth and health in turbot and provides a theoretical basis for the development of NaP as a functional additive in fish feed.

Immune Function of Endothelial Cells: Evolutionary Aspects, Molecular Biology and Role in Atherogenesis

Atherosclerosis is one of the key problems of modern medicine, which is due to the high prevalence of atherosclerotic cardiovascular diseases and their significant share in the structure of morbidity and mortality in many countries. Atherogenesis is a complex chain of events that proceeds over many years in the vascular wall with the participation of various cells. Endothelial cells are key participants in vascular function. They demonstrate involvement in the regulation of vascular hemodynamics, metabolism, and innate immunity, which act as leading links in the pathogenesis of atherosclerosis. These endothelial functions have close connections and deep evolutionary roots, a better understanding of which will improve the prospects of early diagnosis and effective treatment.

Immune responses to prebiotics in farmed salmonid fish: How transcriptomic approaches help interpret responses

Within aquaculture, prebiotics are composed of complex carbohydrate molecules that cannot be digested by the fish directly but are metabolised by the microbial communities within the host gut, with the desire that "healthy" bacterial species are promoted with subsequently improved performance of the fish, there are likely some direct responses of intestinal cells to these dietary components. The sources and processing of prebiotics, which fall under the overarching theme of "functional feeds" are highly varied between species and types of prebiotics administered. How these feeds exert their effect, and the host responses are hard to determine, but new technologies and the development of high-throughput technologies (omics) are enabling the mechanisms and methods of action to be further understood. The recent advances in the availability of 'omics' technologies with the transition from single gene assays to microarray and RNA-seq in fish health have enabled novel functional ingredients to be analysed. This review will focus on recent studies on targeted gene expression and 'omics' technologies to characterize immune responses. Comparisons between the immunomodulatory effect of different prebiotics have been made and specific examples of how transcriptomics techniques have been used to identify immune responses to prebiotics are given.

The effect of different intensities of the ectoparasitic salmon lice (Lepeophtheirus salmonis) on Atlantic salmon (Salmo salar)

The effect of different intensities of the ectoparasitic salmon lice (Lepeophtheirus salmonis) on stress, growth and the expression of immune and wound healing transcripts in the skin of Atlantic salmon (Salmo salar) was investigated. Lice infection success and survival were similar at the chalimus and preadult stage in the low and high dose group, but infection success and survival were significantly lower in the high than in the low dose group at the adult stage. The expression of investigated transcripts was not correlated to lice intensities, but several of them were significantly differently expressed locally in the skin at the site of lice attachment in infected fish compared to controls. This included an up-regulation of pro-inflammatory markers at the site of lice attachment (e.g., interleukin 1-beta, interleukin 8 and the acute phase protein serum amyloid A), a reduction of markers of adaptive immunity (cluster of differentiation 8-alpha and immunoglobulin M) and decreased expression of the anti-inflammatory cytokine interleukin 10.

Comparative Transcriptomic Immune Responses of Mullet (Mugil cephalus) Infected by Planktonic and Biofilm Lactococcus Garvieae

Lactococcus garvieae is an important pathogen of fish, associated with high rates of mortality and infection recurrence in summer or stressful conditions. Chronic infection and disease recurrence have also been reported to be associated with biofilms. However, the impact of biofilm and planktonic bacterial infection on fish immune responses remains unclear. In this study, de novo sequencing was used to compare differences of the spleen transcriptome in planktonic- and biofilm-infected mullets. Among the 181,024 unigenes obtained, 3,392 unigenes were associated with immune response genes. Comparative analysis of the gene expression between infection with the L. garvieae planktonic type and biofilm type identified a total of 3,120 and 3,489 differentially expressed genes in response to planktonic and biofilm infection, respectively, of which 1,366 and 1,458 genes were upregulated, and 1,754 and 1,458 genes were downregulated, respectively. Gene ontology enrichment analysis of immune genes identified genes involved in the complement system, toll-like receptor signaling, and antigen processing, which were further verified by qPCR. Additionally, genes encoding TLR2, IL-1β, TNF-α, C7, and MHC class II peptides were downregulated in response to biofilm infection. Importantly, the results show that biofilm infection induces a different immune pathway response compared with planktonic bacterial infection and, furthermore, illustrates that the prevention of biofilm formation may be a necessary and new strategy for controlling bacterial infection in aquaculture.

Chronic Inflammation Modulates Opioid Receptor Gene Expression and Triggers Respiratory Burst in a Teleost Model

Stress-inducing husbandry and rearing conditions, bacterial infections or parasitic diseases may all lead to chronic inflammation. The immune response will then channel energy away from growth, reproduction and other important physiological processes, to fuel immune-related metabolic responses. The present study aims to unravel the mechanisms and contribute with new information on the molecular, cellular and humoral parameters of European seabass (Dicentrarchus labrax) undergoing chronic inflammation that can be used as health indicators for application in fish health management. European seabass individuals were intra-peritoneally injected with either Freund’s Incomplete Adjuvant (FIA) to induce inflammation or Hanks Balanced Salt Solution (HBSS) to serve as sham. Fish were sampled at 24 h, 7, 14 and 21 days post-injection and blood, plasma and head-kidney were collected. The results found were clear indicators of an inflamed peritoneal cavity and an ongoing systemic immune response that persisted for at least 21 days. Locally, inflammation was characterized by an intense recruitment of immune cells that was still evident 21 days after injection, thus illustrating the chronic character of the immune response. Cellular response was also noticed peripherally with leukocyte numbers rising in the blood of FIA-injected fish. Furthermore, the cellular-mediated respiratory burst peaked at 21 days post-FIA injection, suggesting that phagocytes were still actively fighting the phlogistic agent. Regarding the head-kidney molecular analysis, cxcr4 and il34 appear to be good markers of a chronic inflammation response due to their importance for pathways with high relevance in chronic inflammation settings. In addition, opioid receptor nopr seems to be a good marker of a chronic inflammation response due to its role in detecting noxious stimuli. The present study can serve as a baseline to assess long-term immune-related responses in future studies. For that, more research is nonetheless required to select more responsive and specific molecular markers.

Nitric Oxide Production and Regulation in the Teleost Cardiovascular System

Nitric Oxide (NO) is a free radical with numerous critical signaling roles in vertebrate physiology. Similar to mammals, in the teleost system the generation of sufficient amounts of NO is critical for the physiological function of the cardiovascular system. At the same time, NO amounts are strictly controlled and kept within basal levels to protect cells from NO toxicity. Changes in oxygen tension highly influence NO bioavailability and can modulate the mechanisms involved in maintaining the NO balance. While NO production and signaling appears to have general similarities with mammalian systems, the wide range of environmental adaptations made by fish, particularly with regards to differing oxygen availabilities in aquatic habitats, creates a foundation for a variety of in vivo models characterized by different implications of NO production and signaling. In this review, we present the biology of NO in the teleost cardiovascular system and summarize the mechanisms of NO production and signaling with a special emphasis on the role of globin proteins in NO metabolism.

Why Does Piscirickettsia salmonis Break the Immunological Paradigm in Farmed Salmon? Biological Context to Understand the Relative Control of Piscirickettsiosis

Piscirickettsiosis (SRS) has been the most important infectious disease in Chilean salmon farming since the 1980s. It was one of the first to be described, and to date, it continues to be the main infectious cause of mortality. How can we better understand the epidemiological situation of SRS? The catch-all answer is that the Chilean salmon farming industry must fight year after year against a multifactorial disease, and apparently only the environment in Chile seems to favor the presence and persistence of Piscirickettsia salmonis. This is a fastidious, facultative intracellular bacterium that replicates in the host’s own immune cells and antigen-presenting cells and evades the adaptive cell-mediated immune response, which is why the existing vaccines are not effective in controlling it. Therefore, the Chilean salmon farming industry uses a lot of antibiotics—to control SRS—because otherwise, fish health and welfare would be significantly impaired, and a significantly higher volume of biomass would be lost per year. How can the ever-present risk of negative consequences of antibiotic use in salmon farming be balanced with the productive and economic viability of an animal production industry, as well as with the care of the aquatic environment and public health and with the sustainability of the industry? The answer that is easy, but no less true, is that we must know the enemy and how it interacts with its host. Much knowledge has been generated using this line of inquiry, however it remains insufficient. Considering the state-of-the-art summarized in this review, it can be stated that, from the point of view of fish immunology and vaccinology, we are quite far from reaching an effective and long-term solution for the control of SRS. For this reason, the aim of this critical review is to comprehensively discuss the current knowledge on the interaction between the bacteria and the host to promote the generation of more and better measures for the prevention and control of SRS.

Matrix metalloproteinase-25 from Japanese sea bass (Lateolabrax japonicus) is involved in pro-inflammatory responses

Matrix metalloproteinases (MMPs) are highly expressed in leukocytes and macrophages, which play a role in the innate immune response. Here, the cDNA sequence of MMP25 from Japanese sea bass (Lateolabrax japonicus) (LjMMP25) was identified. Phylogenetic analysis revealed that LjMMP25 was most closely related to large yellow croaker MMP25. Multiple sequence alignment of LjMMP25 with MMP25 sequences from other teleosts revealed that regions of known functional importance were highly conserved. Expression analysis revealed that LjMMP25 was highly expressed in the head kidney and widely expressed in other tissues including gill, spleen, and liver. LjMMP25 was found to regulate inflammatory cytokine production and promote phagocytosis and bacterial killing in monocytes/macrophages (MO/MФ). Furthermore, LjMMP25 regulated the inflammatory response by modulating NF-κB signaling. These findings reveal new information about the role of LjMMP25 in regulating pro-inflammatory responses in this species.

mtROS Induced via TLR-2-SOCE Signaling Plays Proapoptotic and Bactericidal Role in Mycobacterium fortuitum-Infected Head Kidney Macrophages of Clarias gariepinus

The mechanisms underlying Mycobacterium fortuitum-induced mycobacteriosis remain unexplored. Using head kidney macrophages (HKM) from catfish (Clarias gariepinus), we report that Ca²⁺ surge across mitochondrial-Ca²⁺ uniporter (MICU), and consequent mitochondrial ROS (mtROS) production, is imperative for mycobactericidal activity. Inhibition of mtROS alleviated HKM apoptosis and enhanced bacterial survival. Based on RNA interference (RNAi) and inhibitor studies, we demonstrate that the Toll-like receptor (TLR)-2–endoplasmic reticulum (ER) stress–store-operated calcium entry (SOCE) axis is instrumental for activating the mt-Ca²⁺/mtROS cascade in M. fortuitum-infected HKM. Additionally, pharmacological inhibition of mtROS attenuated the expression of CHOP, STIM1, and Orai1, which suggests a positive feedback loop between ER-stress-induced SOCE and mtROS production. Elevated tumor necrosis factor alpha (TNF-α) levels and caspase-8 activity were observed in HKM consequent to M. fortuitum infection, and our results implicate that mtROS is crucial in activating the TNF-mediated caspase-8 activation. Our results for the first time demonstrate mitochondria as an innate immune signaling center regulating mycobacteriosis in fish. We conclude that M. fortuitum-induced persistent SOCE signaling leads to mtROS production, which in turn activates the TNF-α/caspase-8 axis culminating in HKM apoptosis and bacterial clearance.

In vitro modulation of gilthead seabream (Sparus aurata L.) leukocytes by Bacillus spp. extracellular molecules upon bacterial challenge

Stimulation of the fish immune system using immunostimulants is an environmentally friendly strategy to minimize bacterial outbreaks in aquaculture. Different biological and synthetic immunostimulants can enhance non-specific innate immune responses by directly activating immune cells. An example are Bacillus spp., known for their immunostimulatory effects, although the exact mechanisms by which Bacillus spp. offer protection against diseases remains to be elucidated. Furthermore, most studies have focused on Bacillus spp. cells, while the immunostimulant effect of their extracellular metabolome, known to harbour biologically important metabolites, including antimicrobial molecules, has been scarcely evaluated. Here, we evaluated the in vitro immune-modulatory properties of extracellular extracts of three Bacillus spp. strains (B. subtilis FI314, B. vezelensis FI436 and B. pumilus FI464), previously isolated from fish-guts and characterized for their in vitro and in vivo antimicrobial activity against a wide range of fish pathogens. Bacillus spp. extracellular extracts did not affect immune cells viability, but remarkably increased pathogens' phagocytosis when seabream head-kidney leukocytes were challenged with Vibrio anguillarum and Edwardsiella tarda. All extracts significantly increased the engulfment of bacterial pathogens 1 h post-infection. Cells stimulated with the extracellular extracts showed an up-regulation of the expression of immune-relevant genes associated with inflammation, including IL-1β, IL-6, and COX-2. In cells challenged with E. tarda, FI314 extracellular extract significantly increased the expression of IL-1β, IL-6, and COX-2, while FI436 and FI464 significantly increased IL-6 expression. The results of this study revealed that the extracellular molecules from Bacillus spp. fish isolates improved the in vitro response of gilthead seabream immune cells and are thus promising candidates to act as immunostimulants, helping fish fight diseases.

Kynurenine to tryptophan ratio as a biomarker of acute stress in fish

The aim of this study was to determine the kynurenine (KYN) to tryptophan (TRP) ratio (KTR) in fish tissue to assess its usefulness as a biomarker of acute stress. Laboratory held rainbow trout (Oncorhynchus mykiss) were subjected to an acute stressor and KYN, TRP and cortisol were measured in liver and brain tissues at 4- and 48-h post-stress. The analytical method used to determine our analytes was based on lyophilization, and liquid-solid extraction followed by isotope dilution high-performance liquid chromatography positive ion electrospray tandem mass spectrometry. The [KYN]/[TRP] ratio (KTR) was greater in fish liver and brain in the 48-h post-stress exposure group (n = 8) relative to controls (n = 8, p < 0.05); a similar increase was not observed in fish in the 4-h post-stress exposure group. Hepatic and brain cortisol levels were also elevated in fish from both stress-induced groups relative to their respective controls implying that cortisol responded more quickly to the stressful stimulus than KYN and TRP. Our results suggest that the KTR is a promising acute stress diagnostic biomarker in fish. Efforts are ongoing to assess whether the KTR can be used as a biomarker for chronic stress in fish exposed to aquatic contaminants and other environmental stressors and if similar assessments can be made on tissues collected via non-lethal approaches.

Sarcodia suiae Water Extract Promotes the Expression of Proinflammatory and Th1-Type Cytokines and Delay the Onset of Mortality in Cobia (Rachycentron canadum) During Photobacterium damselae subsp. damselae Infection

Cobia (Rachycentron canadum) is a marine fish of high economic value that grows at a fast rate. However, intensive fish farming has led to disease outbreaks in cobia cultures, which is highly costly to the industry. The impact of infectious diseases on cobia production has led to the inappropriate and increased use of chemicals and antibiotics, which negatively affects the environment and human health and promotes the spread of drug-resistant pathogens. Hence, prophylactic measurements, such as the use of immunomodulators, are required to improve the health of cultured animals against pathogens. In this study, we examined the effects of Sarcodia suiae water extract (SSWE) in cobia in vitro and in vivo. We found that treatment with SSWE could significantly increase the expression of cytokines (e.g., IL-1β, IL-6, IL-10, IL-12, and TNF-α) and chemokines (e.g., IL-8) in primary cultured head kidney leukocytes. Intraperitoneal injection of SSWE (20 μg/g body weight) promoted higher expression of IL-6, IL-8, IL-10, IL-12, chemokines (e.g., CC1), and antibodies (e.g., IgT) in head kidney and spleen tissues of the fish compared with other dose levels. Additionally, we describe for the second time (only after India) of the isolation of Photobacterium damselae subsp. damselae (Phdd) from a deadly epizootic in cage-farmed cobia. An intraperitoneal inoculation of SSWE before Phdd challenge showed that SSWE treatment could delay the onset of mortality of cobia. Finally, fish that received SSWE intraperitoneally before infection with Phdd exhibited elevated expression of Th1-type cytokines, namely, IL-8, IL-12, TNF-α, and IFN-γ. At the same time, the expression of Th2-related factors (such as IL-10 in the head kidney, and IgM and IgT in the spleen) were lower for the fish that received SSWE instead of PBS before the Phdd challenge. The results indicate that SSWE treatment facilitates the induction of Th1-type cytokines in cobia to fight against Phdd infection and has the potential to be used as an immunostimulant and vaccine adjuvant for fish.

Functional analysis and regulation mechanism of interferon gamma in macrophages of large yellow croaker (Larimichthys crocea)

Interferon gamma (IFN-γ) is a widely expressed cytokine that has potent antiviral and immunomodulatory effects. The expression and bioactivity of IFN-γ have been reported in several fish species. However, the molecular mechanism mediated by IFN-γ in fish macrophages has not been completely elucidated. This study used the macrophage cell line to investigate the functional activities and regulation mechanism of large yellow croaker IFN-γ (LcIFN-γ). Herein, the mRNA expression of Lcifn-γ was significantly upregulated in macrophages after LPS and poly(I:C) treatment. Recombinant LcIFN-γ protein (rLcIFN-γ) significantly enhanced the phagocytic ability and respiratory burst activity of macrophages. Meanwhile, rLcIFN-γ induced M1 phenotype polarization of macrophages with the upregulated expressions of pro-inflammatory gene. Moreover, rLcIFN-γ upregulated the IFN-stimulated genes (ISGs) expression and activated JAK (Janus tyrosine kinases)-STAT (signal transducer and activator of transcription) signaling pathway by causing the phosphorylation of JAK1 and STAT1^Tyr701. Furthermore, the promoter activity of IFN-regulatory factor 1 (IRF1) was significantly upregulated by the phosphorylated transcription factor STAT1 through binding to its promoter region. In addition to the classical JAK-STAT pathway, rLcIFN-γ also activated multiple distinct signaling cascades such as mitogen-activated protein kinase (MAPK) and protein kinase B (AKT) pathways. Overall, this study indicated the powerful effects of LcIFN-γ on macrophage activation of large yellow croaker and its molecular mechanism.

A Single-Cell Transcriptome Profiling of Anterior Kidney Leukocytes From Nile Tilapia (Oreochromis niloticus)

Teleost fish anterior kidney (AK) is an important hematopoietic organ with multifarious immune cells, which have immune functions comparable to mammalian bone marrow. Myeloid and lymphoid cells locate in the AK, but the lack of useful specific gene markers and antibody-based reagents for the cell subsets makes the identification of the different cell types difficult. Single-cell transcriptome sequencing enables single-cell capture and individual library construction, making the study on the immune cell heterogeneity of teleost fish AK possible. In this study, we examined the transcriptional patterns of 11,388 AK leukocytes using 10× Genomics single-cell RNA sequencing (scRNA-seq). A total of 22 clusters corresponding to five distinct immune cell subsets were identified, which included B cells, T cells, granulocytes, macrophages, and dendritic cells (DCs). However, the subsets of myeloid cells (granulocytes, macrophages, and DCs) were not identified in more detail according to the known specific markers, even though significant differences existed among the clusters. Thereafter, we highlighted the B-cell subsets and identified them as pro/pre B cells, immature/mature B cells, activated B/plasmablasts, or plasma cells based on the different expressions of the transcription factors (TFs) and cytokines. Clustering of the differentially modulated genes by pseudo-temporal trajectory analysis of the B-cell subsets showed the distinct kinetics of the responses of TFs to cell conversion. Moreover, we classified the T cells and discovered that CD3+CD4-CD8-, CD3+CD4+CD8+, CD4+CD8-, and CD4-CD8+ T cells existed in AK, but neither CD4+CD8- nor CD4-CD8+ T cells can be further classified into subsets based on the known TFs and cytokines. Pseudotemporal analysis demonstrated that CD4+CD8- and CD4-CD8+ T cells belonged to different states with various TFs that might control their differentiation. The data obtained above provide a valuable and detailed resource for uncovering the leukocyte subsets in Nile tilapia AK, as well as more potential markers for identifying the myeloid and lymphoid cell types.