Mucosal immunoglobulins and B cells of teleost fish

Characterization and expression profiles of toll-like receptor genes (TLR2 and TLR5) in immune tissues of hybrid yellow catfish under bacterial infection

The yellow catfish (Pelteobagrus fulvidraco) is one of the most economically important freshwater species in Asia. However, pathogenic bacterial infections often cause high rates of mortality and economic losses in practical aquaculture. Previous studies in mammals have shown that Toll-like receptor 2 (TLR2) and Toll-like receptor 5 (TLR5) are involved in the recognition of cell wall components such as lipopolysaccharides and flagella of various bacteria, thereby acting as key regulators in the innate immunity response. However, TLR2 and TLR5 in yellow catfish have not been characterized. In the present study, TLR2 and TLR5 were examined through comparative genomic approaches. The gene structure, collinearity, protein spatial structure, and phylogenetic relationships were compared with those in multiple representative vertebrates. Meanwhile, quantitative real-time PCR was conducted to explore transcriptional changes in TLR2 and TLR5 in immune tissues after infection with exogenous A. hydrophila and E. tarda. The results demonstrated the presence of TLR2 and TLR5 in yellow catfish. However, a systematic analysis showed that TLR2 was not associated with the arrangement of diverse neighboring genes. The expression of hybrid yellow catfish TLR2 transcripts in multiple tissues (including liver, spleen, kidney, and intestine) was significantly up-regulated after infection with A. hydrophila and E. tarda, suggesting that hybrid yellow catfish TLR2 and TLR5 may participate in the immune process. Taken together, the results indicate that TLR2 and TLR5 are conserved in terms of evolution and possess significant antibacterial activity as well as regulatory properties in immune-related tissues and thus play key roles in host defense against pathogen invasion.

Serum proteinogram of gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax) as a new useful approach for detecting loss of haemostasis

Proteinograms, a semiquantitative analytical method that separates proteins into multiple bands, have not been explored in teleosts for diagnostic or prognostic purposes. This study aimed to establish reference values for proteinograms in the serum of gilthead seabream (Sparus aurata) and European seabass (Dicentrarchus labrax), two important farmed fish species in the Mediterranean region. Serum proteins were studied using SDS-PAGE, electropherogram, and HPLC-mass spectrometry. SDS-PAGE analysis revealed four major bands of proteins around 11, 25, 70, and 100 kDa in the serum of gilthead seabream and European seabass. Electropherogram results showed that a protein with a molecular weight of 76.8 kDa was the most abundant protein in the serum of gilthead seabream, while a peak of 75.5 kDa was the most abundant in European seabass. HPLC-mass spectrometry detected 87 proteins and 119 proteins in the serum of gilthead seabream and European seabass, respectively, including α1-globulins, α2-globulins, β-globulins, and γ-globulins. Notably, the albumin sequence was not detected in either of the two species. These results help to characterise the serum protein profile and to establish reference proteinograms for these two fish species. They also provide a basis for the development of novel approaches for the rapid detection of loss of haemostasis due to stress, health disorders or disease in farmed fish.

Possible transport routes of IgM to the gut of teleost fish

Fish rely on mucosal surfaces as their first defence barrier against pathogens. Maintaining mucosal homeostasis is therefore crucial for their overall well-being, and it is likely that secreted immunoglobulins (sIg) play a pivotal role in sustaining this balance. In mammals, the poly-Ig receptor (pIgR) is an essential component responsible for transporting polymeric Igs across mucosal epithelia. In teleost fish, a counterpart of pIgR has been identified and characterized, exhibiting structural differences and broader mRNA expression patterns compared to mammals. Despite supporting evidence for the binding of Igs to recombinant pIgR proteins, the absence of a joining chain (J-chain) in teleosts challenges the conventional understanding of Ig transport mechanisms. The transport of IgM to the intestine via the hepatobiliary route is observed in vertebrates and has been proposed in a few teleosts. Investigations on the stomachless fish, ballan wrasse, revealed a significant role of the hepatobiliary route and interesting possibilities for alternative IgM transport routes that might include pancreatic tissue. These findings highlight the importance of gaining a thorough understanding of the mechanisms behind Ig transport to the gut in various teleosts. This review aims to gather existing information on pIgR-mediated transport across epithelial cells and immunoglobulin transport pathways to the gut lumen in teleost fish. It provides comparative insights into the hepatobiliary transport of Igs to the gut, emphasizing the current understanding in teleost fish while exploring potential alternative pathways for Ig transport to the gut lumen. Despite significant progress in understanding various aspects, there is still much to uncover, especially concerning the diversity of mechanisms across different teleost species.

Ontogeny of myeloperoxidase (MPO) positive cells in flounder (Paralichthys olivaceus)

Neutrophils represent an important asset of innate immunity. Neutrophils express myeloperoxidase (MPO) which is a heme-containing peroxidase involved in microbial killing. In this study, by using real-time quantitative PCR and Western blot analysis, the flounder MPO (PoMPO) was observed to be highly expressed in the head kidney, followed by spleen, gill, and intestine during ontogeny - during developmental stages from larvae to adults. Furthermore, PoMPO positive cells were present in major immune organs of flounder at all developmental stages, and the number of neutrophils was generally higher as the fish grew to a juvenile stage. In addition, flow cytometry analysis revealed that the proportion of PoMPO positive cells relative to leukocytes, in the peritoneal cavity, head kidney, and peripheral blood of flounder juvenile stage was 18.3 %, 34.8 %, and 6.0 %, respectively, which is similar to the adult stage in flounder as previously reported. The presence and tissue distribution of PoMPO during ontogeny suggests that PoMPO positive cells are indeed a player of the innate immunity at all developmental stages of flounder.

Review of current perspectives and future outlook on bacterial disease prevention through vaccination in Asian seabass (Lates calcarifer)

Asian seabass, Lates calcarifer, is an important aquatic species in mariculture. Intensive farming of this species has faced episodes of bacterial diseases, including those due to vibriosis, scale drop, and muscle necrosis disease, big belly disease, photobacteriosis, columnaris, streptococcosis, aeromoniasis, and tenacibaculosis. Vaccination is one of the most efficient, non-antibiotic, and eco-friendly strategies for protecting fish against bacterial diseases, contributing to aquaculture expansion and ensuring food security. As of now, although numerous vaccines have undergone laboratory research, only one commercially available inactivated vaccine, suitable for both immersion and injection administration, is accessible for preventing Streptococcus iniae. Several key challenges in developing vaccines for Asian seabass must be addressed, such as the current limited understanding of immunological responses to vaccines, the costs associated with vaccine production, forms, and routes of vaccine application, and how to increase the adoption of vaccines by farmers. The future of vaccine development for the Asian seabass industry, therefore, is discussed with these key critical issues in mind. The focus is on improving our understanding of Asian seabass immunity, including maternal immunity, immunocompetence, and immune responses post-vaccination, as well as developing tools to assess vaccine effectiveness. The need for an alignment of fish vaccines with state-of-the-art vaccine technologies employed in human and terrestrial animal healthcare is also discussed. This review also discusses the necessity of providing locally-produced autogenous vaccines, especially for immersion and oral vaccines, to benefit small-scale fish farmers, and the potential benefits that might be extended through changes to current husbandry practices such as the vaccination of broodstock and earlier life stages of their off-spring.

The role of animal hosts in shaping gut microbiome variation

Millions of years of co-evolution between animals and their associated microbial communities have shaped and diversified the nature of their relationship. Studies continue to reveal new layers of complexity in host-microbe interactions, the fate of which depends on a variety of different factors, ranging from neutral processes and environmental factors to local dynamics. Research is increasingly integrating ecosystem-based approaches, metagenomics and mathematical modelling to disentangle the individual contribution of ecological factors to microbiome evolution. Within this framework, host factors are known to be among the dominant drivers of microbiome composition in different animal species. However, the extent to which they shape microbiome assembly and evolution remains unclear. In this review, we summarize our understanding of how host factors drive microbial communities and how these dynamics are conserved and vary across taxa. We conclude by outlining key avenues for research and highlight the need for implementation of and key modifications to existing theory to fully capture the dynamics of host-associated microbiomes. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Triclosan, an antimicrobial drug, induced reproductive impairment in the freshwater fish, Anabas testudineus (Bloch, 1792)

Triclosan (TCS), an antimicrobial drug, is known to occupy different compartments in aquatic ecosystems. The present study focused to evaluate the reproductive toxicity of triclosan, at environmentally relevant (0.009 and 9 μg L-1) and sublethal (176.7 μg L-1) concentrations for 90 days in the pre-spawning phase of the fish, Anabas testudineus. The reproductive biomarkers, namely, gonadal steroidogenic enzymes, expression of aromatic genes, levels of serum gonadotropins, sex hormones, and histology of gonads were analyzed. The weight of the animal, brain weights along with gonadosomatic index decreased while mucus deposition increased significantly at all concentrations of triclosan as the primary defensive mechanism to prevent the entry of toxicants. Triclosan disrupted gonadal steroidogenesis as evidenced by a reduction in the activities of gonadal steroidogenic enzymes. The expressions of cyp19a1a and cyp19a1b genes were up-regulated in the brain of both sexes and testis, while down-regulated in the ovary indicating estrogenic effects of the compound. The endocrine-disrupting effects of triclosan were confirmed. The current results suggest that chronic exposure to triclosan altered reproductive endpoints thereby impairing normal reproductive functions in fish.

Oral immunization with recombinant L. lactis expressing GCRV-II VP4 produces protection against grass carp reovirus infection

The hemorrhagic disease causing by grass carp reovirus (GCRV) infection, is associated with major economic losses and significant impact on aquaculture worldwide. VP4 of GCRV is one of the major outer capsid proteins which can induce an immune response in the host. In this study, pNZ8148-VP4/L. lactis was constructed to express recombinant VP4 protein of GCRV, which was confirmed by the Western-Blot and enzyme-linked immunosorbent assay. Then we performed the oral immunization for rare minnow model and the challenge with GCRV-II. After oral administration, pNZ8148-VP4/L. lactis can continuously reside in the intestinal tract to achieve antigen presentation. The intestinal and spleen samples were collected at different time intervals after immunization, and the expression of immune-related genes was detected by real-time fluorescence quantitative PCR. The results showed that VP4 recombinant L. lactis could induce complete cellular and humoral immune responses in the intestinal mucosal system, and effectively regulate the immunological effect of the spleen. The immunogenicity and the protective efficacy of the oral vaccine was evaluated by determining IgM levels and viral challenge to vaccinated fish, a significant level (P < 0.01) of antigen-specific IgM with GCRV-II neutralizing activity was able to be detected, which provided a effective protection in the challenge experiment. These results indicated that an oral probiotic vaccine with VP4 expression can provide effective protection for grass carp against GCRV-II challenge, suggesting a promising vaccine strategy for fish.

Systemic and mucosal immune responses in red tilapia (Oreochromis sp.) following immersion vaccination with a chitosan polymer-based nanovaccine against Aeromonas veronii

A mucoadhesive chitosan polymer-based nanoplatform has been increasingly recognized as an effective mucosal vaccine delivery system for fish. The present study aimed to investigate the effectiveness of immersion vaccination with a chitosan polymer-based nanovaccine to elicit an immune response in serum and mucus of red tilapia and evaluate its protective efficacy after immersion challenge with a heterogenous strain of Aeromonas veronii UDRT09. Six hundred red tilapia (22 ± 1.8 g) were randomly allocated into four experimental groups: control, empty-polymeric nanoparticle (PC), formalin-killed vaccine (FKV), and chitosan polymer-based nanovaccine (CS-NV) in triplicate. The specific IgM antibody levels and their bactericidal activity were assessed in serum and mucus for 28 days after immersion vaccination and followed by immersion challenge with A. veronii. The immersion vaccine was found to be safe for red tilapia, with no mortalities occurring during the vaccination procedure. The specific IgM antibody levels and bactericidal activity against A. veronii in both serum and mucus were significantly higher in red tilapia vaccinated with CS-NV compared to the FKV and control groups at all time points. Furthermore, the serum lysozyme activity, ACH50, and total Ig levels demonstrated a significant elevation in the groups vaccinated with CS-NV compared to the FKV and control groups. Importantly, the Relative Percentage Survival (RPS) value of the CS-NV group (71 %) was significantly higher than that of the FKV (15.12 %) and PC (2.33 %) groups, respectively. This indicates that the chitosan polymer-based nanovaccine platform is an effective delivery system for the immersion vaccination of tilapia.

A comprehensive transcriptional body map of Atlantic salmon unveils the vital role of the intestine in the immune system and highlights functional specialization within its compartments

The intestine is a barrier organ that plays an important role in the immune system of Atlantic salmon. The immune functions are distributed among the diffuse gut lymphoid tissue containing diverse immune cells, and other cell types. Comparison of intestinal transcriptomes with those of other organs and tissues offers an opportunity to elucidate the specific roles of the intestine and its relationship with other parts of the body. In this work, a meta-analysis was performed on a large volume of data obtained using a genome-wide DNA oligonucleotide microarray. The intestine ranks third by the expression level of immune genes after the spleen and head kidney. The activity of antigen presentation and innate antiviral immunity is higher in the intestine than in any other tissue. By comparing transcriptome profiles, intestine shows the greatest similarity with the gill, head kidney, spleen, epidermis, and olfactory rosette (descending order), which emphasizes the integrity of the peripheral mucosal system and its strong connections with the major lymphoid organs. T cells-specific genes dominate among the genes co-expressed in these tissues. The transcription signature of CD8+ (86 genes, r > 0.9) includes a master gene of immune tolerance foxp3 and other negative regulators. Different segments of the intestine were compared in a separate experiment, in which expression gradients along the intestine were found across several functional groups of genes. The expression of luminal and intracellular (lysosome) proteases is markedly higher in pyloric caeca and distal intestine respectively. Steroid metabolism and cytochromes P450 are highly expressed in pyloric caeca and mid intestine while the distal intestine harbors genes related to vitamin and iron metabolism. The expression of genes for antigen presenting proteins and immunoglobulins shows a gradual increase towards the distal intestine.

Are fish immunocompetent enough to face climate change?

Ongoing climate change has already been associated with increased disease outbreaks in wild and farmed fish. Here, we evaluate the current knowledge of climate change-related ecoimmunology in teleosts with a focus on temperature, hypoxia, salinity and acidification before exploring interactive effects of multiple stressors. Our literature review reveals that acute and chronic changes in temperature and dissolved oxygen can compromise fish immunity which can lead to increased disease susceptibility. Moreover, temperature and hypoxia have already been shown to enhance the infectivity of certain pathogens/parasites and to accelerate disease progression. Too few studies exist that have focussed on acidification, but direct immune effects seem to be limited while salinity studies have led to contrasting results. Likewise, multi-stressor experiments essential for unravelling the interactions of simultaneously changing environmental factors are still scarce. This ultimately impedes our ability to estimate to what extent climate change will hamper fish immunity. Our review about epigenetic regulation mechanisms highlights the acclimation potential of the fish immune response to changing environments. However, due to the limited number of epigenetic studies, overarching conclusions cannot be drawn. Finally, we provide an outlook on how to better estimate the effects of realistic climate change scenarios in future immune studies in fish.

Mucosal immune responses of gut IgM in common carp (Cyprinus carpio) following infection with spring viremia of carp virus (SVCV)

Immunoglobulin M (IgM) specifically recognizes various antigens and can activate complement, mediate cytotoxicity, opsonize and agglutinate pathogens to induce phagocytosis, all of which play an important role in immunity. However, the IgM response of common carp (Cyprinus carpio) in the intestinal mucosa after viral infection has not been thoroughly. Therefore, we successfully produced an anti-carp IgM monoclonal antibody and developed a model of viral infection to study the kinetics of immune responses after viral infection. Our results showed that the expression of IL1-β and Igs were dramatically increased, implying that common carp exhibited a significant innate and adaptive immune response to viral infection. Furthermore, we found that the IgM responses varied between the two infection strategies. At 14 days post-infection (DPI), a significant population of IgM+ B cells were observed in the gut, accompanied by a sharp rise in IgM levels. The immune response to secondary infection started at 7 DPI, suggesting that the IgM response is faster in the gut after re-infection. Importantly, we also explored the variability of different gut compartments to viral infection, and result revealed a stronger immune response in the hindgut than in the foregut and midgut. Overall, our findings indicate that IgM plays an important role in the intestinal immune response following primary and secondary viral infection, in which the hindgut plays a major immune function.

Identification, Expression and Antimicrobial Functional Analysis of Interleukin-8 (IL-8) in Response to Streptococcus iniae and Flavobacterium covae in Asian Seabass (Lates calcarifer Bloch, 1790)

In this research, the proinflammatory cytokine interleukin-8 (IL-8) was shown to play a key role in inflammatory responses in fish. This study involved the cloning of the gene that encodes IL-8 in Asian seabass (Lates calcarifer) as well as analyses of its expression and function in this fish. The expression levels of LcIL-8 indicated that it was broadly expressed in most analyzed tissues, with the most predominant expression in the whole blood 6 to 24 h after infection with S. iniae at concentrations of 105 colony-forming units (CFU)/fish (p < 0.05). After fish were immersed in F. covae, the LcIL-8 transcript was upregulated in the gills, liver and intestine, and the highest expression level was observed in the gills. However, LcIL-8 was downregulated in all the tested tissues at 48 and 96 h after infection with the two pathogenic strains, indicating that Lc-IL8 has a short half-life during the early immune responses to pathogens. Moreover, the MIC of the rLcIL-8 protein against S. iniae was 10.42 ± 3.61 µg/mL. Furthermore, functional analyses clearly demonstrated that 10 and 100 µg of the rLcIL-8 protein efficiently enhanced the phagocytic activity of Asian seabass phagocytes in vitro (p < 0.05). Additionally, in vivo injection of S. iniae following the rLcIL-8 protein indicated that 50 and 100 µg of rLc-IL-8 were highly effective in protecting fish from this pathogen (p < 0.001). The obtained results demonstrate that rLcIL-8 possesses a biological function in the defense against bacterial infections in Asian seabass.

Future impact of thymoquinone-loaded nanoemulsion in rabbits: prospects for enhancing growth, immunity, antioxidant potential and resistance against Pasteurella multocida

Phytochemical nanoemulsions, such as thymoquinone nanoemulsions (TQN), are regarded as innovative alternatives to antimicrobials that significantly improve the performance, digestion, antioxidant potential and immunity of rabbits. Thus, the potential effects of TQN on growth, digestibility, antioxidant potential, immunity and resistance against Pasteurella multocida (P. multocida) in rabbits were assessed. Herein, 240 rabbits were offered either a basal diet or diets fortified with three TQN-graded concentrations. At 60 days of age, rabbits were challenged with multidrug-resistant (MDR) virulent P. multocida strain. Our outcomes described that dietary inclusion of TQN, especially at higher concentrations, significantly enhanced the growth performance of rabbits, which was supported by increasing the levels of jejunal lipase, amylase and trypsin enzymes. Of note, the levels of muscle and jejunal antioxidant enzymes [superoxide dismutase (SOD), glutathione peroxidase (GPX), catalase (CAT) and total antioxidant capacity (T-AOC)], serum immunological markers (IgG, IgG, IgM and total Igs) and blood phagocytic percentage were significantly provoked after TQN fortification; meanwhile, the levels of muscle and jejunal MDA, serum biochemical parameters (total cholesterol, TG and LDL), abdominal fat percentage, breast and thigh cholesterol were significantly decreased following TQN supplementations. Our findings showed that TQN protected rabbits against P. multocida experimental challenge as evidenced by reducing P. multocida counts in rabbits' lungs, downregulating the transcription levels of P. multocida virulence-related genes (ptfA, toxA and nanB) at 48 and 96 h post-infection and ameliorating the expression levels of cytokines-related genes (IL-1β, IL-10, IL-8, IL-6, DEFB1, TNF-α, TLR-4 and TLR-2) at 96 h post-infection. Our findings suggest the utilization of TQN in rabbits' diets due to their stimulating effects on digestibility as well as their growth-promoting, anti-inflammatory, antioxidant, antibacterial, anti-virulence and immunostimulant properties, which enhance the rabbits' P. multocida resistance.

Comparative analysis of the immune responses of CcIgZ3 in mucosal tissues and the co-expression of CcIgZ3 and PCNA in the gills of common carp (Cyprinus carpio L.) in response to TNP-LPS

Fish live in an aquatic environment rich in various microorganisms and pathogens. Fish mucosal-associated lymphoid tissue (MALT) plays a very important role in immune defence. This study was conducted to characterize the immune response mediated by CcIgZ3 in common carp (Cyprinus carpio.) and investigate the proliferating CcIgZ3+ B lymphocytes in gill. We determined the expression of CcIgZ3 in many different tissues of common carp following stimulation by intraperitoneal injection of TNP-LPS (2,4,6-Trinitrophenyl hapten conjugated to lipopolysaccharide) or TNP-KLH (2,4,6-Trinitrophenyl hapten conjugated to Keyhole Limpet Hemocyanin). Compared with TNP-KLH, TNP-LPS can induce greater CcIgZ3 expression in the head kidney, gill and hindgut, especially in the gill. The results indicate that the gill is one of the main sites involved in the immune response mediated by CcIgZ3. To examine the distribution of CcIgZ3+ B lymphocytes, immunohistochemistry (IHC) experiments were performed using a polyclonal antibody against CcIgZ3. The results indicated that CcIgZ3 was detected in the head kidney, hindgut and gill. To further examine whether CcIgZ3+ B lymphocytes proliferate in the gills, proliferating CcIgZ3+ B cells were analysed by immunofluorescence staining using an anti-CcIgZ3 polyclonal antibody and an anti-PCNA monoclonal antibody. CcIgZ3 and PCNA (Proliferating Cell Nuclear Antigen) double-labelled cells in the gills were located within the epithelial cells of the gill filaments of common carp stimulated with TNP-LPS at 3 dps and 7 dps, and relatively more proliferating CcIgZ3+ B cells appeared in the gills of common carp at 7 dps. These data imply that CcIgZ3+ B cells in the gills might be produced by local proliferation following TNP-LPS stimulation. In summary, compared with those in TNP-KLH, CcIgZ3 preferentially affects the gills of common carp following challenge with TNP-LPS. CcIgZ3+ B cells proliferate in the gills to quickly produce the CcIgZ3 antibody. In addition, CcIgZ3+ B cells can be activated to induce a strong immune response very early locally in the gill and produce the antibody CcIgZ3, which helps exert an immune-protective effect. These results suggest that an effective vaccine can be designed to promote production of the mucosal antibody CcIgZ3.

Immersion prime and oral boost vaccination with an inactivated Vibrio harveyi vaccine confers a specific immune response and protection in Asian seabass (Lates calcarifer)

Asian seabass (Lates calcarifer) holds significant economic value in fish farming in the Asia-Pacific region. Vibriosis caused by Vibrio harveyi (Vh) is a severe infectious disease affecting intensive farming of this species, for which prevention strategies by vaccination have been developed. This study investigated an alternative approach to injectable vaccination to prevent vibriosis in Asian seabass juveniles. The strategy begins with an immersion prime vaccination with a heat-inactivated Vh vaccine, followed by two oral booster doses administered at 14- and 28-days post-vaccination (dpv). Expression of five immune genes TNFα, IL1β, CD4, CD8, and IgM in the head kidney and spleen, along with investigation of anti-Vh antibody response (IgM) in both systemic and mucosal systems, was conducted on a weekly basis. The efficacy of the vaccines was assessed by a laboratory challenge test at 43 dpv. The results showed that the immunized fish displayed higher levels of mRNA transcripts of the immune genes after the immersion prime and the first oral booster dose compared to the control group. The expression levels peaked at 14 and 28 dpv and then declined to baseline at 35 and 42 dpv. Serum specific IgM antibodies were detected as early as 7 dpv (the first time point investigated) and exhibited a steady increase, reaching the first peak at 21 dpv, and a second peak at 35 dpv. Although the antibody levels gradually declined over subsequent weeks, they remained significantly higher than the control group throughout the experiment. A similar antibody response pattern was also observed in the mucosal compartment. The laboratory challenge test demonstrated high protection by injection with 1.65 × 104 CFU/fish, with a relative percent of survival (RPS) of 72.22 ± 7.86 %. In conclusion, our findings highlight the potential of an immersion prime-oral booster vaccination strategy as a promising approach for preventing vibriosis in Asian seabass.

The distribution and function of teleost IgT

Given the uniquely close relationship between fish and aquatic environments, fish mucosal tissues are constantly exposed to a wide array of pathogenic microorganisms in the surrounding water. To maintain mucosal homeostasis, fish have evolved a distinct mucosal immune system known as mucosal-associated lymphoid tissues (MALTs). These MALTs consist of key effector cells and molecules from the adaptive immune system, such as B cells and immunoglobulins (Igs), which play crucial roles in maintaining mucosal homeostasis and defending against external pathogen infections. Until recently, three primary Ig isotypes, IgM, IgD, and IgT, have been identified in varying proportions within the mucosal secretions of teleost fish. Similar to the role of mucosal IgA in mammals and birds, teleost IgT plays a predominant role in mucosal immunity. Following the identification of the IgT gene in 2005, significant advances have been made in researching the origin, evolution, structure, and function of teleost IgT. Multiple IgT variants have been identified in various species of teleost fish, underscoring the remarkable complexity of IgT in fish. Therefore, this study provides a comprehensive review of the recent advances in various aspects of teleost IgT, including its genomic and structural features, the diverse distribution patterns within various fish mucosal tissues (the skin, gills, gut, nasal, buccal, pharyngeal, and swim bladder mucosa), its interaction with mucosal symbiotic microorganisms, and its immune responses towards diverse pathogens, including bacteria, viruses, and parasites. We also highlight the existing research gaps in the study of teleost IgT, suggesting the need for further investigation into the functional aspects of IgT and IgT+ B cells. This research is aimed at providing valuable insights into the immune functions of IgT and the mechanisms underlying the immune responses of fish against infections.

The pathway of Edwardsiella piscicida infecting Lateolabrax maculatus via the immersion bath

Edwardsiella piscicida, an infectious bacterium, causes great economic losses to the aquaculture industry. Immersion bath which is the closest way to how the fish infect bacterial pathogens in the natural environment is an effective route of artificial infection. In this study, the dynamic process of E. piscicida infection, in the spotted sea bass (Lateolabrax maculatus) was evaluated via the immersion bath. The results showed that soaking the spotted sea bass with 3 × 106  CFU mL-1 E. piscicida for 30 min could artificially induce edwardsiellosis. The higher culture temperature (28.5 ± 0.5°C) or the longer bath time (30 min) would lead to higher mortality of fish. E.piscicida first invaded the gill, then entered the blood circulation to infect the spleen and kidney, where it is colonized, and gradually multiplied in the liver and brain. Meanwhile, the fluorescence in situ hybridization showed that the localization of E. piscicida in the gill and foregut after the immersion challenge proceeded from the exterior to the interior. The invasion of pathogens triggers the immune response of fish and causes tissue damage to the host. The quantitative real-time PCR results displayed an increase in the relative expression level of immune genes (NK-lysin, LZM, IgM and IgD). Otherwise, the most notable histopathological changes of the infected spotted sea bass were multifocal necrosis. Findings in this study broaden our understanding of the infection conditions of E. piscicida and its pathogenicity to the spotted sea bass.

Fishing Innate Immune System Properties through the Transcriptomic Single-Cell Data of Teleostei

The innate immune system is the first line of defense in multicellular organisms. Danio rerio is widely considered a promising model for IIS-related research, with the most amount of scRNAseq data available among Teleostei. We summarized the scRNAseq and spatial transcriptomics experiments related to the IIS for zebrafish and other Teleostei from the GEO NCBI and the Single-Cell Expression Atlas. We found a considerable number of scRNAseq experiments at different stages of zebrafish development in organs such as the kidney, liver, stomach, heart, and brain. These datasets could be further used to conduct large-scale meta-analyses and to compare the IIS of zebrafish with the mammalian one. However, only a small number of scRNAseq datasets are available for other fish (turbot, salmon, cavefish, and dark sleeper). Since fish biology is very diverse, it would be a major mistake to use zebrafish alone in fish immunology studies. In particular, there is a special need for new scRNAseq experiments involving nonmodel Teleostei, e.g., long-lived species, cancer-resistant fish, and various fish ecotypes.

An Introduction to Relevant Immunology Principles with Respect to Oral Vaccines in Aquaculture

Vaccines continue to play an enormous role in the progression of aquaculture industries worldwide. Though preventable diseases cause massive economic losses, injection-based vaccine delivery is cost-prohibitive or otherwise impractical for many producers. Most oral vaccines, which are much cheaper to administer, do not provide adequate protection relative to traditional injection or even immersion formulas. Research has focused on determining why there appears to be a lack of protection afforded by oral vaccines. Here, we review the basic immunological principles associated with oral vaccination before discussing the recent progress and current status of oral vaccine research. This knowledge is critical for the development and advancement of efficacious oral vaccines for the aquaculture industry.

Monoclonal antibody generated against Nile tilapia (Oreochromis niloticus) IgT heavy chain using a peptide-based strategy

Teleost IgT/Z plays a principal role in the defense mechanisms against infectious agents in the mucosal compartments and in systemic immunity. Previously, Nile tilapia (Oreochromis niloticus) IgT was discovered and characterized at transcription level. In this work, we generated a monoclonal antibody (mAb) that specifically recognized the Nile tilapia IgT. BALB/c mice were immunized with three synthetic peptides conjugated to KLH. The sequences of these peptides derived from the constant region of the Nile tilapia IgT heavy chain. ELISA and Western blotting confirmed the specificity of the polyclonal sera and the culture supernatant from a positive hybridoma clone. We observed immunoreactivity against a recombinant IgT fragment and native IgT in skin mucus. The anti-IgT mAb did not cross-react with purified tilapia IgM. Direct ELISA analysis allowed the quantification of skin mucus IgM and IgT concentrations. Flow cytometry analysis revealed differences in the percentage of IgT⁺ B cell populations between juveniles and adults in peripheral blood, head kidney and spleen lymphocytes and among the tissues analyzed. For further validation of the anti-IgT mAb utility, a recombinant vaccine candidate against sea lice (TT-P0 Ls) was injected into juvenile tilapia. Direct ELISA results revealed a differential secretion of skin mucus IgT and IgM after immunostimulation. In addition, the percentages of IgT⁺ B cells were determined at 7 days after booster and ex-vivo stimulation by flow cytometry. This mAb constitutes an important immunological tool to study the biological function and structural characteristics of tilapia IgT.

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

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

The oral antigen-adjuvant fusion vaccine P-MCP-FlaC provides effective protective effect against largemouth bass ranavirus infection

Largemouth bass ranavirus (LMBV) is highly contagious and lethal to largemouth bass, causing significant economic losses to the aquaculture industry. Oral vaccination is generally considered the most ideal strategy for protecting fish from viral infection. In this study, the fusion protein MCP-FlaC, consisting of the main capsid protein (MCP) as the antigen and flagellin C (FlaC) as the adjuvant, was intracellularly expressed in Pichia pastoris. Subsequently, the recombinant P. pastoris was freeze-dried to prepare the oral vaccine P-MCP-FlaC. Transmission electron microscopy and scanning electron microscopy analysis showed that the morphology and structure of the freeze-dried recombinant P. pastoris vaccine remained intact. The experiment fish (n = 100) was divided into five groups (P-MCP-FlaC, P-MCP, P-FlaC, P-pPIC3.5K, control) to evaluate the protective efficacy of the recombinant vaccine. Oral P-MCP-FlaC vaccine effectively up-regulated the serum enzymes activity (total superoxide dismutase, lysozyme, total antioxidant capacity, and complement component 3). The survival rate of P-MCP-FlaC group was significantly higher than that of the other groups. The mRNA expression of crucial immune genes (IL-1β, TNF-α, MHC-II, IFN-γ, Mx, IgM, IgT) was also signally elevated in P-MCP-FlaC group. Vaccine P-MCP-FlaC markedly inhibited the replication of LMBV in the spleen, head kidney, and intestine, while reducing the degree of lesion in the spleen. These results suggest that the oral P-MCP-FlaC vaccine could effectively control LMBV infection, proving an effective strategy for viral diseases prevention in aquaculture.

Mucosal immunity and microbiota change in the rainbow trout (Oncorhynchus mykiss) gills after being challenged with infectious hematopoietic necrosis virus

Respiratory structures are crucial for vertebrate survival, as they serve not only to perform gas-exchange processes but also as entry points for opportunistic pathogens. Previous studies have demonstrated that fish contain gill mucosal-associated lymphoid tissue, and harbor a large number of commensal bacteria on their surface and contribute to maintaining fish health. However, by far, very limited information is known regarding the effects of viral infection on gill mucosal immunity and microbiota homeostasis. In this study, we conducted an infection model by bath with infectious hematopoietic necrosis virus (IHNV) and revealed a 27 % mortality rate among rainbow trout in the first two weeks after infection. Moreover, we found that diseased fish with the highest IHNV loads in gills exhibiting severe damage, as well as increased goblet cell counts in both primary lamellae (PL) and secondary lamellae (SL). Additionally, RT-qPCR and RNA-seq analyses revealed that IHNV infection induced a strong innate and adaptive antiviral immune responses. Interestingly, an antibacterial immune response was also observed, suggesting that a secondary bacterial infection occurred in trout gills after viral infection. Furthermore, 16S rRNA analysis of trout gills revealed a profound dysbiosis marked by a loss of beneficial taxa and expansion of pathobionts following IHNV infection. Overall, our finding demonstrates that IHNV infection induces significant changes of the microbial community in the fish respiratory surface, thus triggering local antiviral and bacterial mucosal immunity.

The Distribution of IgT mRNA+ Cells in the Gut of the Atlantic Salmon (Salmo salar L.)

The newly discovered IgT+ B cell is thought to play a dominant role in mucosal immunity, but limited studies have examined its distribution in fish species, hindering our understanding of its function. This study investigated IgT and poly Ig receptor (pIgR) mRNA+ cell distribution in Atlantic salmon (Salmo salar) gut using RNAscope in situ hybridization (ISH) and assessed the effects of vaccination. The pyloric caeca, mid-intestine (first and second parts), and posterior segment in two weight stages (Group 1: avg. 153 g, Group 2: avg. 1717 g) were examined in both vaccinated and unvaccinated fish. ISH revealed more IgT mRNA+ cells in the second part of the midgut compared to other intestinal segments, as well as a higher number of positive cells in Group 2 (older fish). In line with previous findings, intraperitoneal vaccination had no significant impact on the number of IgT+ transcripts. IgT mRNA+ cells were found mostly in the lamina propria and near capillaries, while pIgR was registered in both the lamina propria and mucosa. Interestingly, vaccinated fish presented adhesions and granulomatous tissue in the peritoneum, with both IgT and pIgR mRNA+ cells. Taken together, these results suggest that the distribution of IgT mRNA+ cells in the intestine of Atlantic salmon is region-specific and is not affected by intraperitoneal vaccination but varies with fish age.

Physiological and immune profiling of tilapia Oreochromis niloticus gills by high-throughput single-cell transcriptome sequencing

The physiological and immune functions of fish gills are largely recognized, but their following functional heterogeneity at the single cell scale has been rarely reported. Here, we performed single cell RNA sequencing (scRNA-seq) on the gills of tilapia fish Oreochromis niloticus. We identified a total of 12 cell populations and analyzed their functional heterogeneity. To investigate the physiological function of O. niloticus gills, expression patterns of genes encoding ion transporters were selected from the identified H+-ATPase-rich cells (HR cells), Na+/K+-ATPase-rich cells (NaR cells), and pavement cells. Specific enrichment of ca4a, slc9a1a, and LOC100692482 in the HR cells of O. niloticus gills explained their functions in acid-base regulation. Genes encoding Ca2+ transporters, including atp2b1, LOC100696627, and LOC 100706765, were specifically expressed in the NaR cells. Pavement cells were presumably the main sites responsible for ammonia and urea transports in O. niloticus gills with specific enrichment of Rhbg and LOC100693008, respectively. The expression patterns of the four immune cell subtypes varied greatly, with B cells being enriched with the most immune-related GO terms. KEGG enrichment analysis showed that MAPK signaling pathway was the most enriched pathway among the four types of immune cells in O. niloticus gills. Our results are important in understanding the physiological and immune responses of fish gills at the cellular resolution.

Short-term swimming up-regulates pro-inflammatory mediators and cytokines in gilthead seabream (Sparus aurata)

Aerobic swimming exercise in fish has been shown to improve robustness of some species. However, the optimal conditions to be applied and the mechanisms underlying remain unknown. We investigated the effects of 6 h of induced swimming on the immune response of gilthead seabream (Sparus aurata), by analysing markers related to immune status in plasma, skin mucus, gills, heart and head-kidney. Forty fish were individually exercised in swim tunnels by applying different water currents: steady low (SL, 0.8 body lengths (BL) s-1), steady high (SH, 2.3 BL s-1), oscillating low (OL, 0.2/0.8 BL s-1) and oscillating high (OH, 0.8/2.3 BL s-1) velocities, including a non-exercised group with minimal water flow (MF, <0.1 BL s-1). Swimming conditions did not trigger a stress response or anaerobic metabolism, suggested by similar levels of cortisol, lactate, and glucose in plasma among groups. Blood haemoglobin and innate immune parameters in plasma and skin mucus also remained unaltered. However, decreased blood haematocrit was observed in fish swimming on the OL condition. Interestingly, gene expression analysis revealed that the OL condition led to the up-regulation of pro-inflammatory mediators (nfκb1 and mapk3) and cytokines (tnfα, il1β and il6) in gills. A similar response occurred in heart, with an up-regulation of nfκb1, tnfα, il6 and cox2 in the OL condition. Gene expression of these cytokines was unaltered in the head-kidney. The inflammatory response in gills and heart of gilthead seabream triggered by the OL condition highlights the importance of establishing suitable rearing conditions to improve welfare of cultured fish.

Excessive dietary soluble arabinoxylan impairs the intestinal physical and immunological barriers via activating MAPK/NF-κB signaling pathway in rainbow trout (Oncorhynchus mykiss)

Arabinoxylan (AX) has been deemed as an antinutritional factor, but limited information has addressed the effects of dietary AX on intestinal health of fish. The present study investigated the effects of dietary AX on intestinal mucosal physical and immunological barriers of rainbow trout (Oncorhynchus mykiss). Five isoproteic and isolipidic experimental diets (AXE, AX0, AX2.5, AX5 and AX10) were formulated to contain 0.03% arabinoxylanase as well as 0%, 2.5%, 5% and 10% AX, respectively. Each diet was randomly distributed to triplicate groups of 35 juvenile (average weight 3.14 ± 0.02 g) per tank in a rearing system maintained at 17 ± 1 °C for 9 weeks. Dietary AX supplementation regardless of inclusion levels significantly (P < 0.05) depressed the growth performance and feed utilization. The plasma endothelin-1 and d-lactic acid contents as well as diamino oxidase activity were significantly higher in fish fed diet AX10 compared to fish fed diet AX0. Dietary inclusion of 5-10% AX resulted in decreased intestinal villus height, goblet cell number and desmosome density, increased crypt depth, short and irregular microvilli, widened intercellular space; down-regulated the mRNA levels of occludin in hindgut, claudin3 and ZO-1 in foregut and midgut, but up-regulated the mRNA levels of claudin12 and claudin15 in midgut as well as claudin23 in foregut, midgut and hindgut. Furthermore, dietary 5-10% AX supplementation decreased the midgut and hindgut complement 3, complement 4 and sIgT contents as well as the midgut IgM and hindgut IL-10 contents. Conversely, the hindgut TNF-α and IL-6 contents increased with the rising dietary AX level. RT-qPCR demonstrated that the pro-inflammatory cytokines (IL-1β, IL-6, IL-8, IL-12β, IFN-γ, and TNF-α) and pIgR mRNA levels in midgut and hindgut were up-regulated by dietary AX inclusion of 5-10% AX. Meanwhile, the mRNA levels of p38 MAPK, IκBα, and NF-κB p65 in midgut and hindgut raised gradually with the increasing dietary AX content. The Western blot results showed that the protein expression levels of p38 MAPK and NF-κB generally increased with the rising dietary AX content. Dietary treatment with 0.03% arabinoxylanase did not affect the growth performance and intestinal health of rainbow trout (P > 0.05). In conclusion, excessive dietary AX inclusion (5-10%) increased the intestinal permeability and induced the intestinal inflammatory response via activating MAPK/NF-κB signaling pathway, and ultimately damaged the intestinal barrier function of rainbow trout.

Cellular evidence of mucus cell immunological and differentiation characteristics in allogeneic crucian carp intestinal lamina propria

The allogeneic crucian carp is an important fish farm animal with a very different digestive system structure from that of mammals. The lamina propria of the fish intestine is also considered to be an important site of intestinal immunity in fish, but functional histological studies of the lamina propria of the allogeneic crucian carp intestine are still lacking. In this study, Identification of the ubiquitous lamina propria mucus cells in the lamina propria of the intestine by hematoxylin-eosin staining, and determination of the mucocytic properties, class, and distribution of these cells in each intestinal segment by Alcian Blue-Periodic Acid-Schiff (AB-PAS) staining. The results show that type III mucus cells were abundant in the lamina propria of the foregut and midgut, while type II and type IV mucus cells predominate in the hindgut, possibly reflecting the distinct functions of these intestinal segments. Transmission electron microscopy dissected the differentiation of mucus cells in the lamina propria of the intestine at the ultrastructural level and investigated their morphology and distribution patterns in different intestinal segments, the findings revealed that lamina propria mucus cells perform rudimentary functions such as mucous secretion, phagocytosis, and degradation functions. Moreover, immunohistochemistry labeling with CD68 and LAMP1 revealed that numerous cells in the anterior, middle, and posterior intestines were positive for both proteins. Immunofluorescence double-labeling demonstrated that these cells highly co-expressed CD68 and LAMP1. Besides, the distribution and morphology of CD68+ and LAMP1+ cells were similar to those of AB-PAS positive cells and they accounted for the majority of parenchyma cells. Considering the above results, there were abundant cells with both mucous secretion and phagocytosis in the intestinal lamina propria of allogeneic crucian carp, which are a essential component of the intestinal immune process of allogeneic crucian carp.

Isolation and pathogenicity evaluation of Escherichia coli O157:H7 from common carp, Cyprinus carpio

Escherichia coli O157:H7 is the primary serotype of enterohaemorrhagic E. coli (EHEC), which can cause diarrhea, hemorrhagic colitis, and hemolytic uremic syndrome. It is considered as a major health concern due to it being a zoonotic disease that is transmitted through food. In this study, a pathogenic bacterium was isolated from infected carp, which identified as E. coli O157:H7 named X21 through genetic sequencing, phylogenetic analysis, physiological and biochemical tests. In the experiment, crucian carp was used as a model to study the pathogenicity of the isolate, the pathological histological observations and cytokines expression of fish tissues were determined after bacterial challenge. The results showed that severe pathological damage observed in the liver, spleen, headkidney of fish infected with isolate X21. Besides, we found that accumulation of IgT+ B cells in the lamina propria of intestine, and up-regulation of SUCH-r, IL-1β, IL-10, IL-11, MyD88, and TNF-α gene in various tissues. After challenged, the survivability of crucian carp infected with isolate X21 stands at a mere 14.27%. To our knowledge, this is the first report that E. coli O157:H7 infected the freshwater fish C. carpio, which indicates that this bacterium is a potential threat to public health and freshwater fish aquaculture.

Yersinia ruckeri infection activates local skin and gill B cell responses in rainbow trout

Teleost fish lack organized structures in mucosal tissues such as those of mammals, but instead contain dispersed B and T cells with the capacity to respond to external stimuli. Nonetheless, there is still a great lack of knowledge regarding how B cells differentiate to plasmablasts/plasma cells in these mucosal surfaces. To contribute to a further understanding of the mechanisms through which fish mucosal B cells are activated, in the current study, we have studied the B cell responses in the skin and gills of rainbow trout (Oncorhynchus mykiss) exposed to Yersinia ruckeri. We have first analyzed the transcription levels of genes related to B cell function in both mucosal surfaces, and in spleen and kidney for comparative purposes. In a second experiment, we have evaluated how the infection affects the presence and size of B cells in both skin and gills, as well as the presence of plasmablasts secreting total or specific IgMs. The results obtained in both experiments support the local differentiation of B cells to plasmablasts/plasma cells in the skin and gills of rainbow trout in response to Y. ruckeri. Interestingly, these plasmablasts/plasma cells were shown to secrete specific IgMs as soon as 5 days after the exposure. These findings contribute to a further understanding of how B cells in the periphery respond to immune stimulation in teleost fish.

Dietary Debaryomyces hansenii promotes skin and skin mucus defensive capacities in a marine fish model

The present study explores the effects of two supplementation levels of Debaryomyces hansenii (1.1% and 2.2%) as a probiotic in a reference low fish meal-based diet on the skin mucosal tissue in Sparus aurata. This study includes the evaluation of fish performance coupled with a holistic study of the skin mucosa: i) a transcriptomic study of the skin tissue, and ii) the evaluation of its secreted mucus both in terms of skin mucosal-associated biomarkers and its defensive capacity by means of co-culture analysis with two pathogenic bacteria. Results showed that after 70 days of diet administration, fish fed the diet supplemented with D. hansenii at 1.1% presented increased somatic growth and a better feed conversion ratio, compared to fish fed the control diet. In contrast, fish fed the diet including 2.2% of the probiotic presented intermediate values. Regarding gene regulation, the probiotic administration at 1.1% resulted in 712 differentially expressed genes (DEGs), among which 53.4% and 46.6% were up- and down-regulated, respectively. In particular, D. hansenii modulated some skin biological processes related to immunity and metabolism. Specifically, D. hansenii administration induced a strong modulation of some immune biological-related processes (61 DEGs), mainly involved in B- and T-cell regulatory pathways. Furthermore, dietary D. hansenii promoted the skin barrier function by the upregulation of anchoring junction genes (23 DEGs), which reinforces the physical defense against potential skin damage. In contrast, the skin showed modulated genes related to extracellular exosome and membrane organization (50 DEGs). This modulated functioning is of great interest, particularly in relation to the increased skin mucus defensive capacity observed in the bacterial co-culture in vitro trials, which could be related to the increased modulation and exudation of the innate immune components from the skin cells into the mucus. In summary, the modulation of innate immune parameters coupled with increased skin barrier function and cell trafficking potentiates the skin's physical barrier and mucus defensive capacity, while maintaining the skin mucosa's homeostatic immune and metabolic status. These findings confirmed the advantages of D. hansenii supplementation in low fish meal-based diets, demonstrating the probiotic benefits on cultured marine species.

Investigation of dietary exogenous protease and humic substance on growth, disease resistance to Flavobacterium covae and immune responses in juvenile channel catfish (Ictalurus punctatus)

Many bacterial pathogens impact the US catfish industry, and disease control can be challenging for producers. Columnaris disease in channel catfish, Ictalurus punctatus, is primarily caused by Flavobacterium covae (formerly F. columnare). Immunostimulants may enhance nonspecific immune responses and offer an alternative to antibiotic treatments in catfish. Furthermore, dietary protein sources and inclusions are also essential to fish health and nutrition and may enhance overall fish performance in pond culture. The current project evaluated two immunostimulants: a protease complex (PC) and a humic substance (HS) derived from a reed-sedge peat product. A 60-day trial examined the effects of five diets on growth performance, immune response and resistance to experimental F. covae infection in channel catfish. Diets included a high-quality fishmeal diet (32%; CF32), a high-protein soy-based diet (32%; C32), a low-protein soy-based diet (28%; C28; predominately used in industry), a low-protein soy diet supplemented with C28 + PC at 175 g metric ton^-1 and C28 + HS in a low-protein diet at 23 g metric ton^-1 . Following feeding for 60 d, juvenile channel catfish were sampled for growth performance and baseline health indicators (n = 3; body mucus, blood for sera, kidney and spleen). A subset of fish was then subjected to an immersion-based in vivo challenge trial with F. covae (ALG-00-530; 10⁶  CFU mL^-1 exposure). At 60d post-initiation, there were no dietary differences in the relative growth rate (p = .063) or thermal growth coefficient (p = .055), but the 32% diets generally appeared to perform best. Post-challenge, the C32 group's mortality was higher than the C28 + PC (p = .006) and C28 + HS diets (p = .005). Although not significant, the C28 and CF32 groups also demonstrated higher mortality compared to both PC and HS diets. Sera lysozyme concentration was found to increase following pathogen challenge (p < .001) and in comparison with mock-challenged catfish (p < .001). Elevated expression levels of proinflammatory cytokines (il-1β, il-8, tnf-α and tgf-β) were observed at trial midpoint and post-infection when compared to 60d. The C28 treatment was found to have lower tnf-α expression than the C28 + PC (p = .042) and C28 + HS (p = .042) groups following exposure to F. covae. These challenge data suggest that the immunostimulants (PC and HS) in plant-based protein may be beneficial in protecting against F. covae when offered in low-protein channel catfish diets.

Immune responses in carp strains with different susceptibility to carp edema virus disease

Carp edema virus disease (CEVD), also known as koi sleepy disease (KSD), represents a serious threat to the carp industry. The expression of immune-related genes to CEV infections could lead to the selection of crucial biomarkers of the development of the disease. The expression of a total of eleven immune-related genes encoding cytokines (IL-1β, IL-10, IL-6a, and TNF-α2), antiviral response (Mx2), cellular receptors (CD4, CD8b1, and GzmA), immunoglobulin (IgM), and genes encoding-mucins was monitored in gills of four differently KSD-susceptible strains of carp (Amur wild carp, Amur Sasan, AS; Ropsha scaly carp, Rop; Prerov scaly carp, PS; and koi) on days 6 and 11 post-infection. Carp strains were infected through two cohabitation infection trials with CEV genogroups I or IIa. The results showed that during the infection with both CEV genogroups, KSD-susceptible koi induced an innate immune response with significant up-regulation (p < 0.05) of IL-1β, IL-10, IL-6a, and TNF-α2 genes on both 6 and 11 days post-infection (dpi) compared to the fish sampled on day 0. Compared to koi, AS and Rop strains showed up-regulation of IL-6a and TNF-α2 but no other cytokine genes. During the infection with CEV genogroup IIa, Mx2 was significantly up-regulated in all strains and peaked on 6 dpi in AS, PS, and Rop. In koi, it remained high until 11 dpi. With genogroup I infection, Mx2 was up-expressed in koi on 6 dpi and in PS on both 6 and 11 dpi. No significant differences were noticed in selected mucin genes expression measured in gills of any carp strains exposed to both CEV genogroups. During both CEV genogroups infections, the expression levels of most of the genes for T cell response, including CD4, CD8b1, and GzmA were down-regulated in AS and koi at all time points compared to day 0 control. The expression data for the above experimental trials suggest that both CEV genogroups infections in common carp strains lead to activation of the same expression pattern regardless of the fish's susceptibility towards the virus. The expression of the same genes in AS and koi responding to CEV genogroup IIa infection in mucosal tissues such as gill, gut, and skin showed the significant up-regulation of all the cytokine genes in gill and gut tissues from koi carp at 5 dpi. Significant down-regulation of CD4 and GzmA levels were only detected in koi gill on 5 dpi but not in other tissues. AS carp displayed significant up-expression of Mx2 gene in all mucosal tissues on 5 dpi, whereas in koi, it was up-regulated in gill and gut only. In both carp strains, gill harbored a higher virus load on 5 dpi compared to the other tissues. The results showed that resistance to CEV could not be linked with the selected immune responses measured. The up-regulation of mRNA expression of most of the selected immune-related genes in koi gill and gut suggests that CEV induces a more systemic mucosal immune response not restricted to the target tissue of gills.

Regulatory roles of cytokines in T and B lymphocytes-mediated immunity in teleost fish

T and B lymphocytes (T and B cells) are immune effector cells that play critical roles in adaptive immunity and defend against external pathogens in most vertebrates, including teleost fish. In mammals, the development and immune response of T and B cells is associated with cytokines including chemokines, interferons, interleukins, lymphokines, and tumor necrosis factors during pathogenic invasion or immunization. Given that teleost fish have evolved a similar adaptive immune system to mammals with T and B cells bearing unique receptors (B-cell receptors (BCRs) and T-cell receptors (TCRs)) and that cytokines in general have been identified, whether the regulatory roles of cytokines in T and B cell-mediated immunity are evolutionarily conserved between mammalians and teleost fish is a fascinating question. Thus, the purpose of this review is to summarize the current knowledge of teleost cytokines and T and B cells as well as the regulatory roles of cytokines on these two types of lymphocytes. This may provide important information on the parallelisms and dissimilarities of the functions of cytokines in bony fish versus higher vertebrates, which may aid in the evaluation and development of adaptive immunity-based vaccines or immunostimulants.

Immune Transcriptional Response in Head Kidney Primary Cell Cultures Isolated from the Three Most Important Species in Chilean Salmonids Aquaculture

Fish cell culture is a common in vitro tool for studies in different fields such as virology, toxicology, pathology and immunology of fish. Fish cell cultures are a promising help to study how to diagnose and control relevant viral and intracellular bacterial infections in aquaculture. They can also be used for developing vaccines and immunostimulants, especially with the ethical demand aiming to reduce and replace the number of fish used in research. This study aimed to isolate head kidney primary cell cultures from three Chilean salmonids: Salmo salar, Oncorhynchus kisutch, and Oncorhynchus mykiss, and characterize the response to bacterial and viral stimuli by evaluating various markers of the innate and adaptive immune response. Specifically, the primary cell cultures of the head kidney from the three salmonids studied were cultured and exposed to two substances that mimic molecular patterns of different pathogens, i.e., Lipopolysaccharide (LPS) (bacterial) and Polyinosinic: polycytidylic acid (POLY I:C). Subsequently, we determined the mRNA expression profiles of the TLR-1, TLR-8, IgM, TLR-5, and MHC II genes. Head kidney primary cell cultures from the three species grown in vitro responded differently to POLY I:C and LPS. This is the first study to demonstrate and characterize the expression of immune genes in head kidney primary cell culture isolated from three salmonid species. It also indicates their potential role in developing immune responses as defense response agents and targets of immunoregulatory factors.

Oral immunizations with Bacillus subtilis spores displaying VP19 protein provide protection against Singapore grouper iridovirus (SGIV) infection in grouper

Disease caused by Singapore grouper iridovirus (SGIV) results in major economic losses in the global grouper aquaculture industry. Vaccination is considered to be the most effective way to protect grouper from SGIV. In this study, the spores of Bacillus subtilis (B.subtilis) WB600 were utilized as the vehicle that the VP19 protein was displayed on the spores surface. To further investigate the effect of oral vaccination, the grouper were orally immunized with B.s-CotC-19 spores. After challenged, the survival rate of grouper orally vaccinated with B.s-CotC-19 spores was 34.5% and the relative percent survival (RPS) was 28.7% compared to the PBS group. Moreover, the viral load in the tissues of the B.s-CotC-19 group was significantly lower than that of the PBS group. The histopathological sections of head kidney and liver tissue from the B.s-CotC-19 group showed significantly less histopathology compared to the PBS group. In addition, the specific IgM levels in serum in the B.s-CotC-19 group was higher than those in the PBS group. In the hindgut tissue, the immune-related gene expression detected by quantitative real-time PCR (qRT-PCR) exhibited an increasing trend in different degrees in the B.s-CotC-19 group, suggesting that the innate and adaptive immune responses were activated. These results indicated that the oral administration of recombinant B.subtilis spores was effective for preventing SGIV infection. This study provided a feasible strategy for the controlling of fish virus diseases.

Curcumin loaded liposome formulation: Enhanced efficacy on performance, flesh quality, immune response with defense against Streptococcus agalactiae in Nile tilapia (Orechromis niloticus)

Application of novel trend comprising antioxidant phytogenics is aiming to minimize the stress related factors and associated diseases in intensive fish culturing. Today, the concept of exploiting and protecting natural antioxidants represents a paradigm shift for the aqua feed industry. Therefore, our principal goal targeting liposome as a novel nanocarrier for curcumin is directed to attain superior performance, fillet antioxidant stability and bacterial resistance in Nile tilapia. A total of 500 Nile tilapia fingerlings (average body weight, 10.27 ± 0.10 g) assigned into five experimental groups in 25 glass aquaria of 120 L capacity at the density 20 fish/aquaria. The experimental groups were supplemented with varying doses of liposomal curcumin-NPs, LipoCur-NPs (0, 5, 15, 25 and 35 mg/kg diet) were reared for 12 weeks and later Streptococcus agalactiae (S. agalactiae) challenged model was performed. Inclusion of LipoCur-NPs (25 and 35 mg/kg diet) had the most prominent impact on Nile tilapia growth rate and feed conversion ratio. The immune boosting outcomes post supplementing 35 mg/kg diet of LipoCur-NPs were evidenced by higher myeloperoxidase, lysozyme and total immunoglobulin levels. Even after 4 weeks frozen storage, LipoCur-NPs at the dose of 35 mg/kg diet prominently increased (P < 0.05) the fillet scavenging capability for free radicals (1,1-diphenyl-2-picrylhydrazyl and 2,2'-Azino-bis (3-ethylbenzothiazoline-6-sulfonic acid) with an inverse reduction in lipid peroxidation biomarker (malondialdehyde). Notably, upregulation of GSH-Px, CAT, and SOD genes in fillet of 35 mg/kg LipoCur-NPs fed fish coordinated with higher T-AOC and lower oxidative markers (ROS and H₂O₂). Post S. agalactiae challenge, higher supplementation levels of LipoCur-NPs (35 mg/kg diet) greatly attenuated the expression of its vital virulence genes (cfb, fbsA and cpsA) with higher expression of Igm, CXC-chemokine and MHC genes. Concordantly, downregulation of inflammatory markers (IL-1β, TNF-α and IL-8) and upregulation of anti-inflammatory ones (IL-10 and TGF-β) were remarkably documented. Based on these findings, the innovative curcumin loaded liposome was considered a novel multitargeting alternative not only playing an imperative role in Nile tilapia growth promotion and fillet stability upon storage, but also protecting efficiently against S. agalactiae.

Transcriptomic insights into the immune response of the intestine to Aeromonas veronii infection in northern snakehead (Channa argus)

Intestinal inflammation is a protective response that is implicated in bacterial enteritis triggered by gastrointestinal infection. The immune mechanisms elicited in teleost against the infection of Aeromonas veronii are largely unknown. In this study, we performed a de novo northern snakehead (Channa argus) transcriptome assembly using Illumina sequencing platform. On this basis we performed a comparative transcriptomic analysis of northern snakehead intestine from A. veronii-challenge and phosphate buffer solution (PBS)-challenge fish, and 2076 genes were up-regulated and 1598 genes were down-regulated in the intestines infected with A. veronii. The Gene Ontology (GO) enrichment analysis indicated that the differentially expressed genes (DEGs) were enriched to 27, 21 and 20 GO terms in biological process, cellular component, and molecular function, respectively. A Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis showed that 420 DEGs were involved in 194 pathways. Moreover, 33 DEGs were selected for quantitative real-time PCR analysis to validate the RNA-seq data. The results reflected the consistency of the expression levels between qRT-PCR and RNA-seq data. In addition, a time-course analysis of the mRNA expression of 33 immune-related genes further indicated that the intestinal inflammation to A. veronii infection simultaneously regulated gene expression alterations. The present study provides transcriptome data of the teleost intestine, allowing us to understand the mechanisms of intestinal inflammation triggered by bacterial pathogens. DATA AVAILABILITY STATEMENT: All data supporting the findings of this study are available within the article and Supplementary files. The RNA-seq raw sequence data are available in NCBI short read archive (SRA) database under accession number PRJNA615958.

Host-microbiome interaction in fish and shellfish: An overview

The importance of the gut microbiome in the management of various physiological activities including healthy growth and performance of fish and shellfish is now widely considered and being studied in detail for potential applications in aquaculture farming and the future growth of the fish industry. The gut microbiome in all animals including fish is associated with a number of beneficial functions for the host, such as stimulating optimal gastrointestinal development, producing and supplying vitamins to the host, and improving the host's nutrient uptake by providing additional enzymatic activities. Besides nutrient uptake, the gut microbiome is involved in strengthening the immune system and maintaining mucosal tolerance, enhancing the host's resilience against infectious diseases, and the production of anticarcinogenic and anti-inflammatory compounds. Because of its significant role, the gut microbiome is very often considered an "extra organ," as it plays a key role in intestinal development and regulation of other physiological functions. Recent studies suggest that the gut microbiome is involved in energy homeostasis by regulating feeding, digestive and metabolic processes, as well as the immune response. Consequently, deciphering gut microbiome dynamics in cultured fish and shellfish species will play an indispensable role in promoting animal health and aquaculture productivity. It is mentioned that the microbiome community available in the gut tract, particularly in the intestine acts as an innovative source of natural product discovery. The microbial communities that are associated with several marine organisms are the source of natural products with a diverse array of biological activities and as of today, more than 1000 new compounds have been reported from such microbial species. Exploration of such new ingredients from microbial species would create more opportunities for the development of the bio-pharma/aquaculture industries. Considering the important role of the microbiome in the whole life span of fish and shellfish, it is necessary to understand the interaction process between the host and microbial community. However, information pertaining to host-microbiome interaction, particularly at the cellular level, gene expression, metabolic pathways, and immunomodulation mechanisms, the available literature is scanty. It has been reported that there are three ways of interaction involving the host-microbe-environment operates to maintain homeostasis in the fish and shellfish gut i.e. host intrinsic factors, the environment that shapes the gut microbiome composition, and the core microbial community present in the gut system itself has equal influence on the host biology. In the present review, efforts have been made to collect comprehensive information on various aspects of host-microbiome interaction, particularly on the immune system and health maintenance, management of diseases, nutrient uptake, digestion and absorption, gene expression, and metabolism in fish and shellfish.

Molecular characterization and expression analysis of immunoglobulins (IgM and IgT) heavy chains in black rockfish (Sebastes schlegelii) that response to bacterial challenge

Sebastes schlegelii is a kind of fish with great economic values. Recently, with the continuous expansion of aquaculture scale and the continuous improvement of aquaculture density, outbreak of various diseases has caused huge economic losses to its aquaculture industry. Study of fish immune system can help to understand the mechanism of immune response to external pathogens and can promote the development of immune prevention and control methods. Immunoglobulins (Igs) are complex glycoproteins that appear to be unique to the vertebrates that can recognize a wide variety of pathogens and recruit immune cells and molecules to destroy pathogens, which are generated by a series of rearrangement and somatic mutations. We therefore studied the immunoglobulin genes of S. schlegelii in view of their important roles in resisting to external pathogen infections. In this study, the immunoglobulin heavy chain genes (sIgM, mIgM, sIgT, and mIgT) of S. schlegelii were successfully identified and cloned. Phylogenetic analysis showed that the IgM and IgT genes of S. schlegelii were clustered together with homologous genes of other species, indicating that they were highly conserved during the evolutionary process. Collinearity analysis showed that the immunoglobulin genes and their adjacent genes were aligned with zebrafish, Atlantic salmon and tilapia, which further confirmed the conserved immunoglobulin gene of teleost. Expression analysis of healthy tissues showed that the expression levels of sIgM, sIgT and mIgT were the highest in the skin, while mIgM was the highest in spleen. After different bacterial infection, IgM and IgT were significantly expressed in skin and gill, which may be because skin and gill are the first line of defense against the infection pathogens. Subcellular localization showed that the mIgT protein was expressed in both the cell membrane and cytoplasm. Meanwhile, recombinant protein of mIgT was obtained in vitro, which laid a foundation for subsequent protein function studies. These results provide a theoretical basis for understanding the immunity role of immunoglobulin in S. schlegelii.

Immunohistochemistry of the Gut-Associated Lymphoid Tissue (GALT) in African Bonytongue (Heterotis niloticus, Cuvier 1829)

Heterotis niloticus is a basal teleost, belonging to the Osteoglossidae family, which is widespread in many parts of Africa. The digestive tract of H. niloticus presents similar characteristics to those of higher vertebrates, exhibiting a gizzard-like stomach and lymphoid aggregates in the intestinal lamina propria. The adaptive immune system of teleost fish is linked with each of their mucosal body surfaces. In fish, the gut-associated lymphoid tissue (GALT) is generally a diffuse immune system that represents an important line of defense against those pathogens inhabiting the external environment that can enter through food. The GALT comprises intraepithelial lymphocytes, which reside in the epithelial layer, and lamina propria leukocytes, which consist of lymphocytes, macrophages, granulocytes, and dendritic-like cells. This study aims to characterize, for the first time, the leukocytes present in the GALT of H. niloticus, by confocal immuno- fluorescence techniques, using specific antibodies: toll-like receptor 2, major histocompatibility complex class II, S100 protein, serotonin, CD4, langerin, and inducible nitric oxide synthetase. Our results show massive aggregates of immune cells in the thickness of the submucosa, arranged in circumscribed oval-shaped structures that are morphologically similar to the isolated lymphoid follicles present in birds and mammals, thus expanding our knowledge about the intestinal immunity shown by this fish.

Dietary Thymol Improved Growth, Body Composition, Digestive Enzyme Activities, Hematology, Immunity, Antioxidant Defense, and Resistance to Streptococcus iniae in the Rainbow Trout (Oncorhynchus mykiss)

In this study, thymol (TYM) at dietary levels of 0, 1, 1.5, 2, and 2.5 g/kg diet was used to evaluate its effects on growth, digestive performance, immunity, and resistances to the infection induced by Streptococcus iniae in the rainbow trout, Oncorhynchus mykiss. A number of 450 fish (35.8 ± 4.4 g; Mean ± SD) were distributed to 15 tanks (30 fish/tank) in three replicates and fed TYM for 60 days. After feeding period, Fish fed 1.5-2.5 g TYM showed better growth, higher digestive enzyme activity, and body protein content compared to other diets (P < 0.05). Regression analysis indicated a polynomial relationship between growth parameters and dietary TYM levels. Based upon the varied growth parameters, the optimum dietary TYM level was 1.89% for FCR. TYM at dietary levels of 1.5-2.5 g significantly enhanced liver antioxidant enzyme activity [superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT)], immune components in blood [alternative complement activity (C3), total immunoglobulin (Ig), lysozyme activity, bactericidal activity, and total protein], and in mucus [alkaline phosphatase (ALP), protease activity, lysozyme activity, bactericidal activity, and total protein] compared to other diets (P < 0.05). TYM at dietary levels of 2-2.5 g significantly decreased malondialdehyde (MDA) levels compared to other experimental groups (P < 0.05). In addition, use of TYM at dietary levels of 1.5-2.5 g upregulated the expression of the immune-related genes (C3, Lyz, and Ig) (P < 0.05). In contrast, the expression of inflammatory genes, tumor necrosis factor (TNF-α) and Interleukin-8 (IL-8) significantly were downregulated in response to 2-2.5 g TYM (P < 0.05). The hematology of the fish also altered in response to dietary TYM, where the values of corpuscular hemoglobin concentration (MCHC), hemoglobin (Hb), red blood cell (RBC), hematocrit (Hct), and white blood cell (WBC) significantly increased in fish fed 2-2.5 g TYM compared to other diets (P < 0.05). In addition, MCV significantly decreased in response to 2-2.5 g TYM (P < 0.05). After challenge with Streptococcus iniae, the survival rate was significantly higher in fish fed 2-2.5 g TYM compared to other diets (P < 0.05). The results of the present study concluded that TYM in the diet of rainbow trout can improve the fish growth and immunity and increase the resistance of the fish to Streptococcus iniae infection. The results of this study recommend an optimized dietary level of 2-2.5 g TYM for the fish.

Expression analysis of Igs and mucosal immune responses upon SVCV infection in common carp (Cyprinus carpio L.)

The immunoglobulin (Ig) is a crucial component of adaptive immune system in vertebrates including teleost fish. Here complete cDNA sequence of IgD heavy chain gene from common carp (Cyprinus carpio) was cloned and analyzed. The full-length cDNA of IgD heavy chain gene contained an open reading frame (ORF) of 2460 bp encoding 813 amino acids. According to amino acids sequence, multiple alignment and phylogenetic analysis showed that carp Igs are closely related to those of Cyprinidae fish. Transcriptional expression of IgD as well as IgM, IgZ1 and IgZ2 showed similar expression patterns in different organs, this is, high expression level in systemic immune tissues (ie, head kidney, heart and spleen) and low expression in mucosal tissues (ie, gill, skin and gut). Following viral infection with spring viraemia of carp virus (SVCV), obvious pathological changes in skin, gill and gut mucosa and up-regulated expression of antiviral related genes in skin, gill, gut and spleen were observed, indicating that SVCV successfully infected common carp and activated the systemic and mucosal immune system. Interestingly, IgM showed a significant up-regulation only in systemic tissue (spleen), but not in mucosal tissues (gut, gills and skin), while increased expression of IgZ1 and IgZ2 was found in gut. In contrast, the expression of IgD increased significantly in spleen, gills and skin. These strongly suggest that fish Ig isotypes play different roles in mucosal and systemic immunity during viral infection.

Common carp (Cyprinus carpio); Igs; Spring viraemia of carp virus (SVCV)

Alterations of the Mucosal Immune Response and Microbial Community of the Skin upon Viral Infection in Rainbow Trout (Oncorhynchus mykiss)

The skin is the largest organ on the surface of vertebrates, which not only acts as the first line of defense against pathogens but also harbors diverse symbiotic microorganisms. The complex interaction between skin immunity, pathogens, and commensal bacteria has been extensively studied in mammals. However, little is known regarding the effects of viral infection on the skin immune response and microbial composition in teleost fish. In this study, we exposed rainbow trout (Oncorhynchus mykiss) to infectious hematopoietic necrosis virus (IHNV) by immersion infection. Through pathogen load detection and pathological evaluation, we confirmed that IHNV successfully invaded the rainbow trout, causing severe damage to the epidermis of the skin. qPCR analyses revealed that IHNV invasion significantly upregulated antiviral genes and elicited strong innate immune responses. Transcriptome analyses indicated that IHNV challenge induced strong antiviral responses mediated by pattern recognition receptor (PRR) signaling pathways in the early stage of the infection (4 days post-infection (dpi)), and an extremely strong antibacterial immune response occurred at 14 dpi. Our 16S rRNA sequencing results indicated that the skin microbial community of IHNV-infected fish was significantly richer and more diverse. Particularly, the infected fish exhibited a decrease in Proteobacteria accompanied by an increase in Actinobacteria. Furthermore, IHNV invasion favored the colonization of opportunistic pathogens such as Rhodococcus and Vibrio on the skin, especially in the later stage of infection, leading to dysbiosis. Our findings suggest that IHNV invasion is associated with skin microbiota dysbiosis and could thus lead to secondary bacterial infection.

Current status and development prospects of aquatic vaccines

Diseases are a significant impediment to aquaculture's sustainable and healthy growth. The aquaculture industry is suffering significant financial losses as a result of the worsening water quality and increasing frequency of aquatic disease outbreaks caused by the expansion of aquaculture. Drug control, immunoprophylaxis, ecologically integrated control, etc. are the principal control strategies for fish infections. For a long time, the prevention and control of aquatic diseases have mainly relied on the use of various antibiotics and chemical drugs. However, long-term use of chemical inputs not only increases pathogenic bacteria resistance but also damages the fish and aquaculture environments, resulting in drug residues in aquatic products, severely impeding the development of the aquaculture industry. The development and use of aquatic vaccines are the safest and most effective ways to prevent aquatic animal diseases and preserve the health and sustainability of aquaculture. To give references for the development and implementation of aquatic vaccines, this study reviews the development history, types, inoculation techniques, mechanisms of action, development prospects, and challenges encountered with aquatic vaccines.

Production of a monoclonal antibody specific to sablefish (Anoplopoma fimbria) IgM and its application in ELISA, western blotting, and immunofluorescent staining

Sablefish (Anoplopoma fimbria) are an emerging aquaculture species native to the continental shelf of the northern Pacific Ocean. There is limited information on both innate and adaptive immunity for this species and new tools are needed to determine antibody response following vaccination or disease outbreaks. In this paper, a monoclonal antibody, UI-25A, specific to sablefish IgM was produced in mice. Western blotting confirmed UI-25A recognizes the heavy chain of IgM and does not cross react to proteins or carbohydrates in serum of four other teleost species. An ELISA was developed to measure Aeromonas salmonicida specific IgM in the plasma of sablefish from a previous experiment where fish were immunized with a proprietary A. salmonicida vaccine. UI-25A was used in Western blot analyses to identify immunogenic regions of A. salmonicida recognized by this specific IgM from vaccinated sablefish. Immunofluorescent staining also demonstrated the ability of UI-25A to recognize membrane-bound IgM and identify IgM + cells in the head kidney. These results demonstrate the usefulness of UI-25A as a tool to improve the understanding of antibody-mediated immunity in sablefish as well as to provide valuable information for vaccine development and expansion of aquaculture efforts for this fish species.

Transcriptome analysis identifies LGP2 as an MDA5-mediated signaling activator following spring viremia of carp virus infection in common carp (Cyprinus carpio L.)

The common carp (Cyprinus carpio L.) is an important farmed species worldwide. Mucosal-associated lymphoid tissues play an essential role in the fight against pathogen infection. Spring viremia of carp virus (SVCV) poses a serious threat to the common carp aquaculture industry. Understanding the molecular mechanisms driving mucosal immune responses to SVCV infection is critical. In this study, the mucosal tissues (gills, foregut and hindgut) were collected from normal and infected fishes for transcriptome analysis. A total of 932,378,600 clean reads were obtained, of which approximately 80% were successfully mapped to the common carp genome. 577, 1,054 and 1,014 differential expressed genes (DEGs) were identified in the gills, foregut and hindgut, respectively. A quantitative polymerase chain reaction assay indicated that the DEGs expression in the foregut following SVCV infection was consistent with the transcriptome results. Among them, two key genes of the retinoic acid-inducible gene I (RIG-I)-like receptor family, melanoma-differentiation-associated gene 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2) (i.e., CcMDA5 and CcLGP2), underwent further analysis. Overexpression of CcMDA5 or CcLGP2 increased phosphorylation of TANK-binding kinase 1 and interferon regulatory factor 3 and the expression of interferon-1 (ifn-1), myxovirus resistance (mx), viperin and interferon-stimulated gene 15 (isg15), and inhibited SVCV replication in epithelioma papulosum cyprini cells. Furthermore, CcLGP2 significantly upregulated the CcMDA5-induced ifn-1 mRNA expression and the activation of the ifn-1 promoter. Finally, confocal microscopy and coimmunoprecipitation experiments revealed that CcLGP2 colocalizes and interacts with CcMDA5 via the C-terminal regulatory domain. This study provides essential gene resources for understanding the fish immune response to SVCV infection and sheds light on the potential role of fish LGP2 in the MDA5 regulation.