Transcriptome profiling of lumpfish (Cyclopterus lumpus) head kidney to Renibacterium salmoninarum at early and chronic infection stages

Renibacterium salmoninarum causes Bacterial Kidney Disease (BKD) in several fish species. Atlantic lumpfish, a cleaner fish, is susceptible to R. salmoninarum. To profile the transcriptome response of lumpfish to R. salmoninarum at early and chronic infection stages, fish were intraperitoneally injected with either a high dose of R. salmoninarum (1 × 109 cells dose-1) or PBS (control). Head kidney tissue samples were collected at 28- and 98-days post-infection (dpi) for RNA sequencing. Transcriptomic profiling identified 1971 and 139 differentially expressed genes (DEGs) in infected compared with control samples at 28 and 98 dpi, respectively. At 28 dpi, R. salmoninarum-induced genes (n = 434) mainly involved in innate and adaptive immune response-related pathways, whereas R. salmoninarum-suppressed genes (n = 1537) were largely connected to amino acid metabolism and cellular processes. Cell-mediated immunity-related genes showed dysregulation at 98 dpi. Several immune-signalling pathways were dysregulated in response to R. salmoninarum, including apoptosis, alternative complement, JAK-STAT signalling, and MHC-I dependent pathways. In summary, R. salmoninarum causes immune suppression at early infection, whereas lumpfish induce a cell-mediated immune response at chronic infection. This study provides a complete depiction of diverse immune mechanisms dysregulated by R. salmoninarum in lumpfish and opens new avenues to develop immune prophylactic tools to prevent BKD.

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

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

Efficacy of saponin-based inactivated rock bream iridovirus (RBIV) vaccine in rock bream (Oplegnathus fasciatus)

Rock bream iridovirus (RBIV) causes severe mortality in rock bream (Oplegnathus fasciatus) for last two decades. In view of this constant threat of RBIV to the rock bream industry, we conducted the present study with the aim to develop a safe and efficient remedial measure against the virus. In this study, we evaluated the safety and potentiality of squalene, aluminium hydroxide and saponin adjuvants, singly or in combinations, which can be used for developing an efficient inactivated (IV) vaccine to protect rock bream from RBIV infection. The evaluation results demonstrated that saponin (Sa) has the required potential in enacting the antiviral immune response in the host and in providing protection against virus mediated lethality, without causing any adverted side-effects. The study further, showed that a single primary dose of Sa-adjuvanted IV vaccine can confer moderate protections in short (60.04% relative percent mortality (RPS) at 4 wpv) and medium (53.38% RPS at 8 wpv) term post RBIV challenge; whereas, the same vaccine when administered in a prime-boost strategy, it resulted enhanced 93.34% RPS post virus challenge at 4 and 8 wpv. The moderate to high survivability demonstrated by the Sa-adjuvanted IV vaccine, was substantiated by the significant (p < 0.05) upregulation of IL-1β, Mx and PKR gene transcript. All surviving fish from the Sa-adjuvanted IV vaccine groups were strongly protected from re-infection with RBIV (1.1 × 10⁷) at 70 days post infection (dpi). In conclusion, it can be inferred that, Sa-adjuvanted IV RBIV vaccine can be an efficient control measure to protect the rock bream aquaculture industry against the lethal RBIV virus.

hnRNPub inhibits LPS-induced NF-κB pathway by targeting TRAF6 for K48-linked ubiquitination in miiuy croaker (Miichthys miiuy)

As an important adaptor protein in innate immunity, TRAF6 is not only responsible for the transduction of signal pathways, but its E3 ligase activity to transfer ubiquitination has also been widely studied. Under LPS stimulation, TRAF6 transfers the K63-linked ubiquitination chain to TAK1, which in turn activates the transcription factor NF-κB and cell signaling factors downstream of the signaling pathway. However, how TRAF6 expression is regulated remains largely unknown, especially in teleost. In this study, we identified hnRNPub as a suppressor of TRAF6 expression. The mRNA level of hnRNPub significantly increased under LPS stimulation, and hnRNPub inhibited NF-κB signaling pathway by targeting TRAF6. Knockdown of hnRNPub potentiated inflammatory cytokines, such as TNFα,IL-1β,IL-8. Mechanistically, hnRNPub inhibited NF-κB signaling pathway through mediating K48-linked ubiquitination and proteasomal degradation of TRAF6. Thus, our findings reveal that hnRNPub limits LPS-induced innate activation by promoting K48-linked polyubiquitination and proteasomal degradation of TRAF6.

Efficacy testing of an immersion vaccine against Aeromonas salmonicida and immunocompetence in ballan wrasse (Labrus bergylta, Ascanius)

The development of effective vaccines is a critical step towards the domestication of emerging fish species for aquaculture. However, traditional vaccine delivery through intraperitoneal (i.p.) injection requires fish to reach a minimum size and age and therefore cannot provide protection at early developmental stages when infection may occur. This study investigated the effectiveness of immersion vaccination with respect to immunocompetence in a cleaner fish species (ballan wrasse, Labrus bergylta, Ascanius) used in Atlantic salmon farming as an alternative means to control sea lice. The species is susceptible to atypical strains of Aeromonas salmonicida (aAs) at early life stages (<15 g), when i.p. vaccination is not applicable. While immersion vaccination is currently used in commercial hatcheries, the optimal fish size for vaccination, and efficacy of the vaccine delivered by this route has not yet been established. Importantly, efficacy depends on the capability of the species immune system to recognise antigens and process antigens to trigger and produce an adaptive immune response, (process known as immunocompetence). In this study, the efficacy of a polyvalent autogenous vaccine administered by immersion in juvenile ballan wrasse and the subsequent immune response induced was investigated after prime and booster vaccination regimes. In addition, temporal expression (0-150 days post hatch) of adaptive immune genes including major histocompatibility complex (MHC II CD74 molecule) and immunoglobulin M (IgM) was assessed using quantitative PCR (qPCR). Prime and/or boost vaccination by immersion of juvenile ballan wrasse (0.5 g and 1.5 g corresponding to 80 and 170 days post hatch (dph), respectively) did not provide significant protection against aAs vapA V after bath challenge under experimental conditions. Despite no evident protection >80 dph, MHC II and IgM transcripts were first reported at 35 and 75 dph, respectively, suggesting a window of immunocompetence. The results provide important new information on the onset of adaptive immunity in ballan wrasse and highlight that immersion vaccination in the species for protection against aAs should be performed at later developmental stages (>1.5 g) in the hatchery.

Brain immunity response of fish Eleginops maclovinus to infection with Francisella noatunensis

The brain's immune system is selective and hermetic in most species, including fish, favoring immune responses mediated by soluble immunomodulatory factors such as serotonin and the availability of nutrients against infectious processes. Francisella noatunensis coexist with fish such as Eleginops maclovinus, which raises questions about the susceptibility and immune response of the brain of E. maclovinus against Francisella. In this study, we inoculated fish with different doses of Francisella and took samples for 28 days. We detected bacteria in the brain of fish injected with a high concentration of Francisella at all time points. qPCR analysis of immune genes indicated a response mainly in the medium-dose and early expression of genes involved in iron metabolism. Finally, brain serotonin levels were higher than in uninfected fish in all conditions, suggesting possible immunomodulatory participation in an infectious process.

Molecular characterization, expression analysis and cellular location of IL-4/13 receptors in large yellow croaker (Larimichthys crocea)

Interleukin (IL)-4 and IL-13 are closely related class I cytokines that play key roles in the T helper (Th)-2 immune response via heterodimeric receptors. IL-4 signals via both the type I (IL-4Rα/γc) and type II (IL-4Rα/IL-13Rα1) receptor complexes, while IL-13 signals only via the type II receptor complex. IL-13Rα2 is traditionally considered a "decoy" receptor for IL-13. However, the IL-4/13 system and its response to pathogenic infection are still not fully understood in fish. In this study, we identified four IL-4/13 receptor subunit genes in the large yellow croaker (Larimichthys crocea): LcIL-4Rα1, LcIL-4Rα2, LcIL-13Rα1, and LcIL-13Rα2. Sequence analysis showed that these receptors possessed typical characteristic domains, including a signal peptide, two fibronectin type III (FN III)-like domains, and a transmembrane domain, but their cytoplasmic regions were not well conserved. The mRNA and protein of the four IL-4/13 receptors were constitutively expressed in all examined tissues of large yellow croaker. Their mRNAs were also detected in primary head kidney macrophages (PKMs), primary head kidney granulocytes (PKGs), and primary head kidney lymphocytes (PKLs). Immunofluorescence assay further showed that LcIL-4Rα and LcIL-13Rα1 were expressed on the membrane of IgM ⁺ B cells. After stimulation by Vibrio alginolyticus and poly (I:C) (a viral dsRNA mimic), the mRNA levels of LcIL-4/13 receptors were significantly upregulated in the head kidney and spleen. Their mRNA levels were also upregulated in head kidney leukocytes in response to poly (I:C) and lipopolysaccharide (LPS) treatment. Moreover, both recombinant LcIL-4/13A and LcIL-4/13B upregulated LcIL-4Rα1 and LcIL-4Rα2 in primary leukocytes, but only recombinant LcIL-4/13A upregulated LcIL-13Rα1 and LcIL-13Rα2. These results indicated that LcIL-4/13 receptors, containing conserved functional domains, may be involved in the IL-4/13-mediated immune response to pathogenic infections in the large yellow croaker.

Polyactin A and CpG enhance inactivated Pseudomonas plecoglossicida vaccine potency in large yellow croaker (Larimichthys crocea)

Pseudomonas plecoglossicida is the causative agent of visceral granulomas disease (VGD) in large yellow croaker (LYC, Larimichthys crocea) farming. However, multi-antibiotic resistant of P. plecoglossicida creates an urgent need of an efficient vaccine to combat this pathogen. In this study, an inactivated vaccine added polyactin (PA), CpG-riched plasmid (pCpG) and aluminum adjuvant (Al) was developed. As a result, its relative percentage survival (RPS) against P. plecoglossicida were up to 64%. Comparatively, RPS of groups that vaccinated with vaccines adjuvanted with PA and Al or CpG and Al were 49% and 39%. However, an interesting result that the vaccine combined with PA, CpG and Al did not show the strongest activation of total serum protein and antibody levels in serum among three vaccinated groups. According to expressions of some cellular immune related genes, we found that the inactivated vaccine combined with PA, CpG and Al was more likely to induce a cellular immune response rather than humoral immune response. Totally, our study demonstrated that the mixture of PA, CpG and aluminum adjuvant is a potential adjuvant system for LYC vaccine development.

Immune response of silver pomfret (Pampus argenteus) to Amyloodinium ocellatum infection

Amyloodinium ocellatum (AO) infection in silver pomfret (Pampus argenteus) causes extensive mortality. Insufficient information exists on the molecular immune response of silver pomfret to AO infestation, so herein we simulated the process of silver pomfret being infected by AO. Translucent trophosomes were observed on the gills of AO-infected fish. Transcriptome profiling was performed to investigate the effects of AO infection on the gill, kidney complex and spleen. Overall, 404,412,298 clean reads were obtained, assembling into 96,341 unigenes, which were annotated against public databases. In total, 2730 differentially expressed genes were detected, and few energy- and immune-related genes were further assessed using RT-qPCR. Moreover, activities of three immune-related (SOD, AKP and ACP) and three energy-related (PKM, LDH and GCK) enzymes were determined. AO infection activated the immune system and increased interleukin-1 beta and immunoglobulin M heavy chain levels. Besides, the PPAR signalling pathway was highly enriched, which played a role in improving immunity and maintaining homeostasis. AO infection also caused dyspnoea, leading to extensive lactic acid accumulation, potentially contributing towards a strong immune response in the host. Our data improved our understanding regarding the immune response mechanisms through which fish coped with parasitic infections and may help prevent high fish mortality in aquaculture.

miR-148-1-5p modulates NF-κB signaling pathway by targeting IRAK1 in miiuy croaker (Miichthys miiuy)

microRNAs (miRNAs), a crucial class of small non-coding RNA species, have been extensively studied as key molecular in immune regulation in the past decades. Here, we discover a new miRNA miR-148-1-5p and we elaborate that miR-148-1-5p functions as a negative regulator to participate in innate immune responses. In this article, it has been researched that the regulation effect of miR-148-1-5p to the nuclear factor kappaB (NF-κB) signaling pathway by targeting IRAK1 in miiuy croaker. First, through bioinformatics software to predict the potential targets of miR-148-1-5p, we found that IRAK1 had a base complementary region with indicated miRNA. Next, the dual-luciferase assays revealed that overexpression of miR-148-1-5p mimics and pre-miR-148 plasmid could significantly inhibit the luciferase activity of wild-type IRAK1-3'UTR. However, miR-148-1-5p inhibitors attenuated the inhibition caused by miR-148-1-5p. In addition, we also confirmed that miR-148-1-5p could suppress the expression of IRAK1 at mRNA level. Collectively, the regulations of miR-148-1-5p to NF-κB signaling pathways via targeting the IRAK1 gene was studied in miiuy croaker, which provided new information to enrich the immune regulation network of miRNA in teleost fish.

Molecular characterisation and biological activity of an antiparasitic peptide from Sciaenops ocellatus and its immune response to Cryptocaryon irritans

Cryptocaryon irritans, a holotrichous ciliate parasitic protozoan, can trigger marine white spot disease and cause substantial economic losses in mariculture. However, methods of preventing and curing the disease have negatively affect fish, human, other organisms, and the natural environment. The antiparasitic activity of some antimicrobial peptides (AMPs) has garnered extensive attention of scholars. In this study, we identified and characterised a novel antiparasitic peptide, named So-pis, from Sciaenops ocellatus. The sequence analysis, structural features, and tissue distribution suggested that So-pis is genetically related to the piscidins family. However, So-pis showed a relatively low overall conservation compared with other known piscidins. So-pis is abound in glycine residues (22.7 %) and it has a neutral isoelectric point, weak amphipathicity, relatively long α-helix, and high hydrophobicity. These key elements are responsible for its biological activity. Quantitative real-time polymerase chain reaction (qRT-PCR) data indicated that So-pis is a typically gill-expressed peptide. The expression of So-pis in the gill, skin, spleen, and head kidney could be regulated during C. irritans infection, thereby implicating a role of So-pis in immune defence against C. irritans. The synthetic So-pis had limited or no antimicrobial activity against bacterial and yeasts but exhibited potent antiparasitic activity against C. irritans in vitro. The activity of synthetic So-pis against erythrocytes was less potent than its antiparasitic activity against C. irritans. These results indicated that So-pis might be one of the crucial defence cytokines against C. irritans in the red drum. Cumulatively, our data suggested that So-pis might be a potential candidate for developing a novel, effective, and safe therapeutic agent against marine white spot disease.

microRNA-148 is involved in NF-κB signaling pathway regulation after LPS stimulation by targeting IL-1β in miiuy croaker

The inflammatory response is a protective process to clear detrimental stimuli, constitutes the defense against infectious pathogens. Clearing pathogen infection requires appropriate immune and inflammatory response, but excessive inflammatory response can lead to uncontrolled inflammation, autoimmune disease, or pathogen transmission. Accumulating evidences show that miRNAs are important and multifunctional regulators of innate immunity and inflammation. However, in the inflammatory response of lower vertebrates, the miRNAs regulatory networks are largely unknown. In this study, a combination of bioinformatics and experimental techniques were used to investigate the functions of miR-148. IL-1β is a hypothetical target gene of miR-148 predicted by bioinformatics. In addition, dual-luciferase reporter gene experiment was used to verify the targeting effect of miR-148 on IL-1β-3'UTR. miR-148 inhibits IL-1β expression in a dose-dependent manner at protein and mRNA levels. It is important that miR-148 participates in regulation of LPS-induced the NF-κB signaling pathway by inhibiting IL-1β. These results will improve our understanding of the regulation of miRNAs in fish on the immune response.

Molecular characterization and expressional analysis of two poly (ADP-ribose) polymerase (PARP) domain-containing Gig2 isoforms in rockfish (Sebastes schlegelii) and their antiviral activity against viral hemorrhagic septicemia virus

Remedies toward sustainable aquaculture rely upon research that unveils the molecular mechanisms behind host immunity and their interactions with pathogens. Antiviral defense is a major innate immune response in fish. The antiviral protein GCHV-induced gene-2 (Gig2), a member of the interferon-stimulated gene (ISG), was identified and characterized from rockfish (Sebastes schlegelii). Gig2 exists in two isoforms, namely, SsGig2-I1 and SsGig2-I2, in rockfish with lengths of 163 and 223 bp, respectively. Bioinformatic analysis indicated the availability of poly (ADP-ribose) polymerase domain in both proteins, and 51.3% identity and 71.3% similarity between both isoforms were observed. The basal expression pattern revealed the highest tissue-specific expression in rockfish gills for both isoforms. The immune challenge experiment disclosed a distinctive and strong expression of each transcript in the presence of poly I:C. Both isoforms are localized in the endoplasmic reticulum. Interferon (IFN) pathway gene analysis revealed no significant upregulation of IFN related genes. Viral hemorrhagic septicemia virus (VHSV) gene expression analysis revealed strong downregulation of viral transcripts after 48 h of infection in the presence of Gig2 isoforms. Collectively, these results indicate the protective role of Gig2 in rockfish against VHSV infection and help broaden our understanding of the innate immunity of fish.

The moonlighting protein fructose 1,6-bisphosphate aldolase as a potential vaccine candidate against Photobacterium damselae subsp. piscicida in Asian sea bass (Lates calcarifer)

Vaccination is the most effective, safe, and environmentally friendly method to prevent the outbreak of Photobacterium damselae subsp. piscicida (Phdp), a dangerous pathogen in aquaculture worldwide. Here, recombinant proteins of catalase, superoxide dismutase, isocitrate dehydrogenase, fructose 1,6-bisphosphate aldolase (Fba), and a mixture of all four proteins were investigated for their immunoprotective effects against photobacteriosis in Asian sea bass (Lates calcarifer). After immunization, experimental fish showed an increase in specific antibody levels and lysozyme activities, especially the Fba group. After a lethal challenge with Phdp strain AOD105021, the Fba group achieved the highest relative percentage of survival rate (70.21%) and a significantly lower bacterial load in the spleens than other groups 3 days after infection. The results suggest that Fba is a good candidate for subunit vaccine development against photobacteriosis in fish.

Molecular identification of peptidoglycan recognition protein 5 and its functional characterization in innate immunity of large yellow croaker, Larimichthys crocea

Fish peptidoglycan recognition proteins (PGRPs) play important roles in microbial recognition, and bacterial elimination. In the present study, a short-type PGRP from large yellow croaker, LcPGRP5 was cloned and its functions were characterized. LcPGRP5 gene encodes a protein containing conserved PGRP domain, but no signal peptide. Phylogenetic analysis shows that LcPGRP5 is clustered with other short PGRPs identified in other teleosts. LcPGRP5 is constitutively expressed in all tissues examined, with the highest expression being detected in the head kidney. Recombinant LcPGRP5 protein features amidase activity and bactericidal activity. Notably, LcPGRP5 could enhance the phagocytosis of the bacteria by large yellow croaker macrophage, with higher phagocytic capacity being observed in Staphylococcus aureus compared to Escherichia coli. Moreover, overexpression of LcPGRP5 suppresses pro-inflammatory effects elicited by bacterial exposure in the macrophage cell line. Overall, the present results clearly indicate the important roles of LcPGRP5 played in the innate immune responses against bacterial infection.

The circular RNA circBCL2L1 regulates innate immune responses via microRNA-mediated downregulation of TRAF6 in teleost fish

Growing numbers of studies have shown that circular RNAs (circRNAs) can function as regulatory factors to regulate the innate immune response, cell proliferation, cell migration, and other important processes in mammals. However, the function and regulatory mechanism of circRNAs in lower vertebrates are still unclear. Here, we discovered a novel circRNA derived from the gene encoding Bcl-2-like protein 1 (BCL2L1) gene, named circBCL2L1, which was related to the innate immune responses in teleost fish. Results indicated that circBCL2L1 played essential roles in host antiviral immunity and antibacterial immunity. Our study also identified a microRNA, miR-30c-3-3p, which could inhibit the innate immune response by targeting inflammatory mediator TRAF6. And TRAF6 is a key signal transduction factor in innate immune response mediated by TLRs. Moreover, we also found that the antiviral and antibacterial effects inhibited by miR-30c-3-3p could be reversed with the expression of circBCL2L1. Our data revealed that circBCL2L1 functioned as a competing endogenous RNA (ceRNA) of TRAF6 by competing for binding with miR-30c-3-3p, leading to activation of the NF-κB/IRF3 inflammatory pathway and then enhancing the innate immune responses. Our results suggest that circRNAs can play an important role in the innate immune response of teleost fish.

Epitope screening of the major capsid protein within grouper iridovirus of Taiwan and the immunoprotective effect with SWCNTs as the vaccine carrier

Iridovirus can cause a mass of death in grouper, leading to huge economic loss in recent years. At present, practical vaccine is still the best way to control the outbreak of this virus. Many researches had indicated that the major capsid protein (MCP) of grouper iridovirus of Taiwan (TGIV) is an effective antigen to induce a specific immune response in grouper. However, these traditional vaccines that based on large proteins or whole organisms are faced with challenges because of the unnecessary antigenic load. Thus, in this study, we screened the dominant linear epitope within the MCP of TGIV and then, a new peptide vaccine (P2) was developed via prokaryotic expression system. Furthermore, SWCNTs was used as a vaccine carrier to enhance the immunoprotective effect. To evaluate the immunoprotective effect of this vaccine, a total of 245 fish were vaccinated with P2 (5, 10, 20 mg L^-1) and SWCNTs-P2 (5, 10, 20 mg L^-1) via immersion before being challenged with live TGIV at 28 days post immunization (d.p.i.). Results showed that the serum antibody titer, enzymatic activity, expression level of some immune-related genes (CC chemokine, IgM and TNF-α) and survival rate were significantly increased (SWCNTs-P2, 20 mg L^-1, 100%) compared to the control group (0%). These results indicated that this peptide vaccine could effectively induce specific immune response in vaccinated groupers. Functionalized SWCNTs could serve as a carrier of the peptide vaccine to enhance the immunoprotective effect via immersion. To sum up, epitope screening might be a potential way to develop an effective vaccine nowadays, and SWCNTs might provide a practical method that can be used in large-scale vaccination, especially for juvenile fish, to fight against diseases in aquaculture industry.

Dietary supplementation of fermented lemon peel enhances lysozyme activity and susceptibility to Photobacterium damselae for orange-spotted grouper, Epinephelus coioides

The waste recycling of lemon peel, as a functional feed additive in aquafeed was evaluated by estimating the effects of fermented lemon peel (FLP) supplementation in diet on growth performance, innate immune responses, and susceptibility to Photobacterium damselae of grouper, Epinephelus coioides. A basal diet was added FLP at 0%, 1%, 3%, and 5%. Four tested diets were each fed to juvenile grouper (initial weight: 15.89 ± 0.10 g, triplicate groups) in a recirculation rearing system for eight weeks. Fish fed diets with 0%-3% FLP exhibited higher (p < 0.05) final weight, weight gain, and feed efficiency than fish fed the 5% FLP-diet. After challenge test, fish fed the 3% FLP-diet appeared the lowest mortality, followed by fish fed the 1% FLP-diet, and lowest in fish fed 0% and 5% FLP-diets. Plasma lysozyme activities were higher in fish fed diets with FLP than in fish fed the FLP-free control diet before challenge test. After challenge, fish fed diets with 1% and 3% FLP showed highest lysozyme activities, followed by fish fed the diet with 5% FLP, and lowest in fish fed the control diet. Hepatic malondialdehyde content was higher in fish fed the control diet than in fish fed diets with 1%-3% FLP. Results found that diets supplemented with 1%-3% fermented lemon peel can enhance lysozyme activity and resistance to pathogen P. damselae of grouper.

microRNA-132 as a negative regulator in NF-κB signaling pathway via targeting IL-1β in miiuy croaker

The innate immune system is the first line of defense against the invasion of pathogens. It can make a rapid immune response to the invading pathogenic microorganisms, thereby eliminating the invading pathogens and protecting the body from harm. microRNAs are a family of small non-coding ribonucleic acid molecules, which are important and multifunctional regulator of immune response. In this study, we studied the role of miR-132 as a key regulatory factor of IL-1β-mediated inflammation. The seed region of miR-132 can regulate gene expression by binding to the 3'UTR of IL-1β, and inhibit the expression of IL-1β at the post-transcriptional level. More importantly, miR-132 inhibits LPS-induced NF-κB signaling pathway by targeting IL-1β, thereby preventing excessive inflammatory response from causing autoimmune diseases. These results will help to better understand the complex regulatory mechanisms of teleost fishes.

Molecular characterization and role in virus infection of Beclin-1 in large yellow croaker (Larimichthys crocea)

Beclin-1, the ortholog of yeast autophagy-related gene 6 (Atg6), has a central role in autophagy, which has been linked to diverse biological processes including immunity, development, tumor suppression, and lifespan extension. However, understanding of function of fish Beclin-1 is limited now. In this study, the complete Beclin-1 cDNA of large yellow croaker Larimichthys crocea (LcBeclin-1) was cloned, whose open reading frame (ORF) is 1344 bp long and encodes a protein of 447 amino acids (aa). The deduced LcBeclin-1 possesses a typical Bcl-2 homology domain 3(BH3) and an APG6 domain that contains a central coiled-coil domain (CCD, residues 174 to 231) and a C-terminal evolutionarily conserved domain (ECD, residues 241 to 334). LcBeclin-1 shared a high amino acid identity of 81.66-98.66% with reported Beclin-1 molecules from other vertebrate species. LcBeclin-1 gene was constitutively expressed in all tissues tested, with the highest levels in heart. LcBeclin-1 transcripts were also detected in primary head kidney granulocytes (PKGs), primary head kidney macrophages (PKMs), primary head kidney leukocytes (PKLs), and large yellow croaker head kidney cell line (LYCK), and were significantly upregulated by poly (I:C) in PKMs and LYCK cells. Subcellular localization showed that LcBeclin-1 was evenly distributed in the cytoplasm and nucleus of LYCK cells. Overexpression of LcBeclin-1 significantly increased the replication of SVCV, as evidenced by increased severity of the cytopathic effects, enhanced viral titre, and upregulated transcriptional levels of viral genes. Further studies showed that LcBeclin-1 induced the occurrence of autophagy in LYCK cells. Additionally, LcBeclin-1 also decreased the expression levels of large yellow croaker interferons (IFNs; IFNc, IFNd, and IFNh), interferon regulatory factor 3 (IRF3) and IRF7, IFN-stimulated genes (ISGs; Mx, PKR, and Viperin) in LYCK cells. All these data suggest that LcBeclin-1 promoted the viral replication possibly by inducing autophagy or negatively modulating IFN response, which will help us to further understand the function of fish Beclin-1.

Functional characterization of four TIR domain-containing adaptors, MyD88, TRIF, MAL, and SARM in mandarin fish Siniperca chuatsi

Toll/interleukin-1 receptor (TIR) domain-containing adaptors, serve as pivotal signal transduction molecules in Toll-like receptor (TLR) signalling pathway to mediate downstream signalling cascades. In this study, four TIR-domain containing adaptors, MyD88, TRIF, MAL and SARM, were identified in mandarin fish Siniperca chuatsi, and they all contain TIR domains, of which MyD88 and SARM had high sequence homology with their vertebrate homologues. The expression analysis at mRNA level indicated that these genes were ubiquitously distributed in different tissues, being high in immune- and mucosa-related tissues such as head-kidney and intestine. The transcripts of these adaptor genes were up-regulated by poly(I:C) and LPS stimulation in isolated head-kidney lymphocytes (HKLs) of mandarin fish. Fluorescence microscopy revealed that all these molecules were localized in cytoplasm, and further investigations showed that the over-expression of MyD88, TRIF and MAL activated the NF-κB, ISRE or type Ι IFN promoters and inhibited SVCV replication, whereas their antiviral effects were significantly impaired when co-transfected with SARM. It was also confirmed by co-immunoprecipitation (Co-IP) that SARM interacts separately with MyD88, TRIF and MAL, and MAL interacts with MyD88. However, the regulatory mechanisms of these adaptors involved in signalling pathways of different TLRs should be of interest for further research.

MAVS splicing variants associated with TRAF3 and TRAF6 in NF-κB and IRF3 signaling pathway in large yellow croaker Larimichthys crocea

Mitochondrial antiviral signaling protein (MAVS) acts as an essential adaptor in host RIG-I-like receptors (RLRs) mediated antiviral signaling pathway. In the present study, two MAVS transcript variants, the typical form and a splicing variant, namely Lc-MAVS_tv1 and Lc-MAVS_tv2 were characterized in large yellow croaker (Larimichthys crocea). The putative Lc-MAVS_tv1 protein contains 512 aa, with an N-terminal CARD domain, a central proline-rich region, and a C-terminal transmembrane (TM) domain, whereas Lc-MAVS_tv2 contains 302 aa and lacks the C-terminal TM domain due to a premature stop in the 102 bp intron fragment insertion. Lc-MAVS_tv1 was identified as a mitochondrion localized protein whereas Lc-MAVS_tv2 exhibited an entire cytosolic distribution. Quantitative real-time PCR revealed that Lc-MAVS_tv1 mRNA was broadly expressed in examined organs/tissues and showed extremely higher level than that of Lc-MAVS_tv2, and both of them could be up-regulated under poly I:C, LPS, PGN, and Pseudomonas plecoglossicida stimulation in vivo. Interestingly, overexpression of Lc-MAVS_tv2 could induce the activation of NF-κB but not IRF3, and Lc-MAVS_tv2 co-transfected with Lc-MAVS_tv1 induced a significantly higher level of NF-κB and IRF3 promoter activity. In addition, Lc-MAVS_tv2 overexpression could enhance TRAF3 and TRAF6 mediated NF-κB activation, but suppress TRAF3 and TRAF6 mediated IRF3 activation, implying that the splicing variant Lc-MAVS_tv2 may function as an important regulator in MAVS mediated signaling pathway.

Grouper TRAF5 exerts negative regulation on antiviral immune response against iridovirus

Tumor necrosis factor receptor-associated factor 5 (TRAF5) is an intracellular protein that binds to the cytoplasmic portion of tumor necrosis factor receptors and mediates the activation of downstream nuclear factor-kappa B (NF-κB), interferon regulatory factor 3, and mitogen activated protein kinase signaling pathways. Compared with other TRAF proteins, TRAF5 is largely unknown in teleosts. In the present study, a TRAF5 homologue (HgTRAF5) from the hybrid grouper (Epinephelus fuscoguttatus♂ × Epinephelus lanceolatus♀) was cloned and characterized. The open reading frame of HgTRAF5 consists of 1743 nucleotides encoding a 581 amino acid protein with a predicted molecular mass of 64.90 kDa. Similar to its mammalian counterpart, HgTRAF5 contains an N-terminal RING finger domain, a zinc finger domain, and a C-terminal TRAF domain, including a coiled-coil domain and a MATH domain. HgTRAF5 shares 99.83% identity with giant grouper (Epinephelus lanceolatus) TRAF5. Quantitative real-time PCR analysis indicated that HgTRAF5 mRNA was broadly expressed in all examined tissues. The expression of HgTRAF5 increased after Singapore grouper iridovirus (SGIV) infection in grouper spleen (GS) cells. Intracellular localization analysis demonstrated that the full-length HgTRAF5 protein mainly distributed in the cytoplasm. HgTRAF5 overexpression also promoted SGIV replication during viral infection in vitro. HgTRAF5 significantly promoted the activities of interferon-β, interferon-sensitive response element, and NF-κB. Taken together, these results are important for a better understanding of the function of TRAF5 in fish and reveal its involvement in the host response to immune challenge by SGIV.

Molecular characterization, expression and function analysis of Epinephelus coioides MKK4 response to SGIV and Vibrio alginolyticus infection

Mitogen-activated protein kinase 4 (MKK4), a member of the MAP kinase family, play important roles in response to many environmental and cellular stresses in mammals. In this study, three MKK4 subtypes, EcMKK4-1, EcMKK4-2 and EcMKK4-3, were obtained from grouper Epinephelus coioides. The open reading frame (ORF) of EcMKK4s are obtained and the EcMKK4s proteins contain highly conserved domains: a S_TKc domain, a canonical diphosphorylation group and two conserved MKKK ATP binding motifs, Asp-Phe-Gly (DFG) and Ala-Pro-Glu (APE). EcMKK4s could be found both in the cytoplasmic and nuclear. The EcMKK4s mRNA were detected in all E. coioides tissues examined with the different expression levels, and the expression were up-regulated during SGIV (Singapore grouper iridescent virus) or Vibrio alginolyticus infection. EcMKK4 could significantly reduce the activation of AP-1 reporter gene. The results suggested that EcMKK4s might play important roles in pathogen-caused inflammation.

Development of monoclonal antibody against IgT of a perciform fish, large yellow croaker (Larimichthys crocea) and characterization of IgT+ B cells

Teleost immunoglobulin T (IgT) is considered to be a primitive immunoglobulin class specialized in mucosal immunity. In the present study, a recombinant protein containing the CH2 region of large yellow croaker (Larimichthys crocea) IgT heavy chain was expressed, purified, and used as an immunogen to produce a monoclonal antibody (mAb) against large yellow croaker IgT. Western blotting results indicated that the obtained mouse anti-IgT mAb could specifically recognize a 45 kDa protein in the skin mucus of large yellow croaker, which was identified as the IgT heavy chain by mass spectrometric analysis. Immunofluorescence assay (IFA) analysis further demonstrated that this mouse anti-IgT mAb could recognize membrane-bound IgT (mIgT) molecules on large yellow croaker IgT⁺ leukocytes. This mAb also could be used for sorting of large yellow croaker IgT⁺ B cells by flow cytometry sorting technology. Then, flow cytometric immunofluorescence analysis (FCIA) results showed that the percentages of IgT⁺ B cells in skin, gills, gut, spleen, head kidney and peripheral blood lymphocytes were 27.553% ± 3.312%, 12.588% ± 3.538%, 12.355% ± 3.352%, 13.075 ± 2.258%, 5.552 ± 3.275%, and 2.600 ± 0.521%, respectively, indicating that mucosal tissues (skin, gills, and gut) contained a high ratio of IgT⁺ B cells. Accordingly, the high protein levels of IgT were also detected in these mucosal tissues, suggesting that IgT may play a role in mucosal immunity in large yellow croaker. Taken together, our data demonstrated that the mouse anti-IgT mAb developed in this study could be used for characterizing IgT⁺ B cells and studying the functions of IgT in large yellow croaker.

Production and characterization of a mouse monoclonal antibody against smallmouth bass (Micropterus dolomieu) IgM

A murine monoclonal antibody (mAb, IgG2a) was produced for the detection of smallmouth bass (Micropterus dolomieu) immunoglobulin (IgM). The antibody is specific for IgM heavy chain and was shown to also recognize the Ig heavy chain of largemouth bass (Micropterus salmoides) and bluegill (Lepomis macrochirus) using Western Blot analysis of plasma from 9 teleost taxa. When applied to the analysis of smallmouth bass total plasma IgM using ELISA, the mAb was found to be effective when used in an inhibition kinetic assay.

Characterization of mandarin fish (Siniperca chuatsi) IL-6 and IL-6 signal transducer and the association between their SNPs and resistance to ISKNV disease

In fish, interleukin-6 (IL-6) is a very important immune-regulatory cytokine that plays a polyfunctional role in inflammation, metabolism, regeneration, and neural processes. IL-6 signal transducer (IL-6ST) is a specific receptor for IL-6 and expressed mainly in immune cells and hepatocytes. In this study, the complete cDNA and genomic DNA sequences of mandarin fish (Siniperca chuatsi) IL-6 and IL-6ST genes were identified and analyzed. Quantitative real-time PCR showed that IL-6 and IL-6ST were chiefly expressed in the immune organs. After challenge with infectious spleen and kidney necrosis virus (ISKNV), the expression levels of IL-6 were significantly up-regulated after 6 h and 24 h in the head kidney and spleen, respectively (p < 0.01), the peak value for both reached at 72 h, IL-6ST increased significantly after 120 h with a peak at 168 h in the head kidney (p < 0.01) and improved markedly at 168 h in the spleen (p < 0.01). Besides, IL-6 and IL-6ST have been identified 3 and 8 single nucleotide polymorphisms (SNPs), respectively. Statistical analysis showed that one SNP locus (1625C/T) in the coding region of IL-6 was significantly related to the resistance of mandarin fish against ISKNV. The 1625C→T locus in the coding region of IL-6 is a synonymous mutation; compared with the susceptible group, the frequency of allele T in the disease resistance group was significantly higher, which may be due to the rare codon produced by the mutation affecting translation. The involvement of IL-6 and IL-6ST in response to ISKNV infection in mandarin fish clearly indicate that the role of SNP markers in IL-6 was associated with the ISKNV resistance, which was demonstrated for the first time in our results. Thus, the current study may provide fundamental information for further breeding of mandarin fish with resistance to ISKNV infection.

Transcriptome analysis of immune response against Siniperca chuatsi rhabdovirus infection in mandarin fish Siniperca chuatsi

As one of the piscine rhabdoviruses, Siniperca chuatsi rhabdovirus (SCRV) has caused considerable losses to mandarin fish aquaculture industry. RNA-seq, as efficient transcriptome research method, has been widely used to study the immune response of fish to pathogens. This study reported the effect of SCRV infection at 0, 24 and 60 hr on S. chuatsi at the transcriptome level. A total of 61,527 unigenes with high quality were obtained, and 3,095, 1,854 and 227 differentially expressed genes (DEGs) were labelled between the Sc24 and Sc0 groups, the Sc60 and Sc0 groups and the Sc60 and Sc24 groups, respectively. Genes involved in innate and adaptive immunity were highlighted. In Gene Ontology analysis, the DEGs that participated in immune response, innate immune response and the regulation of apoptotic process were identified as enriched classes. Kyoto Encyclopedia of Genes and Genomes pathway results indicated that most DEGs caused by SCRV infection were identified in the immune system (retinoic acid-inducible gene-I-like receptor/Toll-like receptor/nucleotide-binding oligomerization domain-like receptor/C-type lectin receptor signalling pathway), cellular processes, cell growth and death (p53 signalling pathway, cellular senescence, apoptosis and phagosome), and metabolism. Quantitative real-time PCR was used to further verify the expression levels of 15 immune-related DEGs. The transcriptome database obtained in this study provided further in-depth insight into the immune response of S. chuatsi against SCRV.

microRNA-489 negatively modulates RIG-I signaling pathway via targeting TRAF6 in miiuy croaker after poly(I:C) stimulation

The innate immune response is first line of host defense against pathogen invasion. However, excessive activation of immune responses may cause autoimmune diseases and excessive inflammation. Retinoic acid-inducible gene I (RIG-I) is an important cytoplasmic pathogen recognition receptor that is activated on virus infection. TNF-receptor-associated factor 6 (TRAF6) plays an essential role in the RIG-I-mediated signaling pathway. MicroRNAs (miRNAs) are noncoding RNAs that are emerging as important regulators of immune responses. In this study, we found that the overexpression of miR-489 mimics and pre-miR-489 significantly suppressed the luciferase activity of the wild-type TRAF6 3'UTR, whereas mutant-type led to a complete abrogation of the negative effect. In addition, we also observed that miR-489 can negatively regulate TRAF6 at the level of translation. More importantly, we demonstrated that miR-489 is a negative regulator of TRAF6 involved in the immune response to poly(I:C) stimulation. These common findings indicated that miR-489 plays a regulatory role in host-virus interactions by targeting TRAF6. Overall, all of the present results provide direct evidence that miR-489 is involved in the regulation of TRAF6 expression in miiuy croaker, which will help to better understand the complex regulatory networks of teleost fish.

β-Defensins from common goby (Pomatoschistus microps) and silver trevally (Pseudocaranx georgianus): Molecular characterization and phylogenetic analysis

Antimicrobial peptides (AMPs) are biologically active molecules involved in host defense present in a variety of organisms. They are an integral component of innate immunity, forming a front line of defense against potential pathogens, including antibiotic-resistant ones. Fishes are proven to be a prospective source of AMPs as they are constantly being challenged by a variety of pathogens and the AMPs are reported to play an inevitable role in fish immunity. Among them, β-defensins form one of the most studied multifunctional peptides with early evolutionary history and recently being considered as host defense peptides. The present study highlights the first-ever report on β-defensin AMP sequences from common goby (Pomatoschistus microps) and silver trevally (Pseudocaranx georgianus). A 192 bp cDNA fragment with an open reading frame encoding 63 amino acids (aa) comprising a 20 aa signal peptide region at the N-terminal was obtained from the mRNA of gill tissue of both P. microps and P. georgianus by RT-PCR. These peptide sequences when characterized in silico at the molecular level revealed a 43 aa cationic mature peptide with the signature intra-molecular disulphide bonded cysteine residue pattern ascertaining its β-defensin identity, further confirmed by phylogenetic analysis. The data collected will pave the way for further research on varied facets of the peptide-like, tissue level expressions, antimicrobial activities on commonly encountered pathogens, and its feasibility as a therapeutant in the aquaculture scenario.

Seaweed dietary supplements enhance the innate immune response of the mottled rabbitfish, Siganus fuscescens

Disease is one of the major bottlenecks for aquaculture development, costing the industry in excess of US $6 billion each year. The increase in pressure to phase out some traditional approaches to disease control (e.g. antibiotics) is pushing farmers to search for alternatives to treat and prevent disease outbreaks, which do not have detrimental consequences (e.g. antibiotic resistance). We tested the effects of eleven seaweed species and four established fish immunostimulants on the innate immune response (cellular and humoral immunity) of the rabbitfish Siganus fuscescens. All supplements including different seaweeds from the three groups (Chlorophyta, Phaeophyta and Rhodophyta) were included in the fish pellet at 3% (by weight) and had variably positive effects across the four innate immune parameters we measured compared to control fish. Diets supplemented with the red seaweed Asparagopsis taxiformis and the brown seaweed Dictyota intermedia led to the largest boosts in humoral and cellular innate immune defences, including particularly significant increases in haemolytic activity. Diets supplemented with Ulva fasciata also led to promising positive effects on the fish innate immune responses. We conclude that dietary seaweed supplements can boost the immune response of S. fuscescens and thus the top three species highlighted in this study should be further investigated for this emerging aquaculture species and other fish species.

Molecular cloning, inducible expression with SGIV and Vibrio alginolyticus challenge, and function analysis of Epinephelus coioides PDCD4

Programmed cell death 4 (PDCD4) in mammals, a gene closely associated with apoptosis, is involved in many biological processes, such as cell aging, differentiation, regulation of cell cycle, and inflammatory response. In this study, grouper Epinephelus coioides PDCD4, EcPDCD4-1 and EcPDCD4-2, were obtained. The open reading frame (ORF) of EcPDCD4-1 is 1413 bp encoding 470 amino acids with a molecular mass of 52.39 kDa and a theoretical pI of 5.33. The ORF of EcPDCD4-2 is 1410 bp encoding 469 amino acids with a molecular mass of 52.29 kDa and a theoretical pI of 5.29. Both EcPDCD4-1 and EcPDCD4-2 proteins contain two conserved MA3 domains, and their mRNA were detected in all eight tissues of E. coioides by quantitative real-time PCR (qRT-PCR) with the highest expression in liver. The expressions of two EcPDCD4s were significantly up-regulated after Singapore grouper iridovirus (SGIV) or Vibrio alginolyticus infection. In addition, over-expression of EcPDCD4-1 or EcPDCD4-2 can inhibit the activity of the nuclear factor-κB (NF-κB) and activator protein-1 (AP-1), and regulate SGIV-induced apoptosis. The results demonstrated that EcPDCD4s might play important roles in E. coioides tissues during pathogen-caused inflammation.

microRNA-1388-5p inhibits NF-κB signaling pathway in miiuy croaker through targeting IRAK1

Innate immune response is an important response mechanism for the host to achieve self-protection, and it plays an important role in identifying pathogens and resisting pathogen invasion. Growing evidences have shown that microRNA functions as a crucial regulator involved in the host innate immune response. In this study, the regulations of miR-1388-5p to regulate NF-κB signaling pathways via targeting the IRAK1 gene was studied in miiuy croaker. First, through bioinformatics software prediction, we found that IRAK1 is the direct target of miR-1388-5p, and then the prediction results were verified by using dual-luciferase assays. Next, we found that both miR-1388-5p mimics and pre-miR-1388 plasmids inhibit IRAK1 expression by complementing the seed sequence in the 3'-untranslated region (3'-UTR) of IRAK1. Finally, we observed that miR-1388-5p could negatively regulate NF-κB pathways through targeting IRAK1. These results provide new insights into the function of miR-1388-5p in fish innate immunity, meanwhile enriching miRNA-mediated regulatory networks.

Molecular characterization and functional study of a tandem-repeat Galectin-9 from Japanese flounder (Paralichthys olivaceus)

Galectin-9 is a β-galactoside-binding lectin which could modulate a variety of biological functions including recognition, aggregation and clearance of pathogen. In this study, one Galectin-9 (named PoGalectin-9) was identified from Japanese flounder Paralichthys olivaceus. PoGalectin-9 belongs to the tandem-repeat type, containing one 127-amino acids CRD domain within N terminal and one 122-amino acids CRD domain within C-terminal. The open reading frame of PoGalectin-9 cDNA was 921 bp encoding 306 amino acids. Sequence similarity comparison confirmed that PoGalectin-9 shared high homology with other Galectin-9. The tissue distribution and expression profiles after bacterial infection were also investigated. PoGalectin-9 was widely distributed in all of the examined tissues of Japanese flounder but was predominantly expressed in the spleen, kidney and intestine. After Edwardsiella tarda challenge, the expression of PoGalectin-9 was up-regulated in spleen and down regulated in kidney. ELISA experiment showed that recombinant PoGalectin-9 (rPoGalectin-9) exhibit binding capacity to lipopolysaccharide (LPS) and peptidoglycan (PGN), which is significantly correlated with the concentration of rPoGalectin-9. Meanwhile, the rPoGalectin-9 protein showed strong agglutinating activities against both Gram-negative bacteria and Gram-positive bacteria. Bacterial binding experiments showed that rPoGalectin-9 could bind all examined bacteria. In conclusion, the present study indicate that PoGalectin-9 might play important roles during the immune responses of Japanese flounder against bacterial pathogens.

Identification, expression and pro-inflammatory effect of interleukin-17 N in common carp (Cyprinus carpio L.)

Two interleukin (IL)-17 N genes (CcIL-17Na and b) present on different linkage groups were identified in the common carp (Cyprinus carpio) genome and confirmed by polymerase chain reaction (PCR) and real time (RT)-PCR in this experiment. Synteny analysis revealed that IL-17 N is transcribed by the complement sequence of TOP3B's intron 2. It is flanked by SDF2L and PPM1F in all fish studied to date, except fugu (Takifugu rubripes). The open reading frames of the two CcIL-17Ns are 411 base pairs long and encode 136 amino acids. The amino acid identity/similarity between CcIL-17Na and b is 91.2%/97.1%. The CcIL-17Ns share identity (46.8-90.4%) with their orthologs from other teleosts. Identities/similarities to other members of the IL-17 family in common carp were low at 21.4-30.2%/31.4-51.4%. In the phylogenetic tree, IL-17Ns from spotted gar (Lepisosteus oculatus, the ancestor of teleosts) and coelacanth (Latimeria chalumnae, the ancestor of tetrapods) were grouped within the same branch with a high bootstrap value of 97%, which indicates that IL-17 N is an ancient and conserved gene. Quantitative RT-PCR results showed that CcIL-17Ns were most highly expressed in the brain of healthy individuals. The expression in brain was significantly induced at 6 h post Aeromonas hydrophila infection; at 1 day post infection, expression in liver, muscle, skin, spleen, and head kidney was up-regulated. In addition, the upregulated expression of proinflammatory cytokines IL-1β, IFN-γ, IL-6, chemokine CCL20, NF - κ B and TRAF6 in kidney tissue by ccIL-17 N recombinant protein also indicate that IL-17 N can promote inflammation through NF-κB pathway and induce the expression of chemokines and inflammatory factors.

microRNA-128 inhibits the inflammatory responses by targeting TAB2 in miiuy croaker, Miichthysmiiuy

The inflammatory response is a self-defense process that fights the pathogen invasion by eliminating harmful stimuli. However, excessive inflammation may disrupt immune homeostasis, even causing chronic inflammation or autoimmune diseases. MicroRNAs (miRNAs) are a crucial regulator that can negatively regulate gene expression and participate in multiple biological processes of growth, development, and immune response in organisms. However, the miRNA-mediated modulation networks of inflammatory responses remain largely unclear in lower vertebrates. In this study, miR-128 was identified as a negative regulator to participate in the NF-κB signaling pathway by targeting TAB2 in miiuy croaker. First, we predicted target genes of miR-128 through the bioinformatics software programs and found that TAB2 is a direct target of miR-128. We also found that miR-128 can inhibit TAB2 expression at the mRNA and protein levels. Besides, upon LPS stimulation, miR-128 inhibits the expression of inflammatory cytokines by targeting TAB2 to avoid excessive inflammation. Particularly, we found that miR-128 can regulate TAB2-mediated NF-κB signaling pathways. In summary, our results indicate that miR-128 plays a critical role in suppressing inflammatory responses by regulating the TAB2-mediated NF-κB signaling pathway in miiuy croaker.

Molecular and functional identification of a β-defensin homolog in large yellow croaker (Larimichthys crocea)

β-defensin (BD) is a cysteine-rich cationic antibacterial peptide that is active against a wide range of bacteria. Here, a β-defensin homolog (LcBD2) was identified in large yellow croaker (Larimichthys crocea). The open reading frame of LcBD2 contains 195 nucleotides, encoding a protein of 64 amino acids that possesses a typical arrangement of six conserved cysteine residues (C³¹ , C³⁷ , C⁴¹ , C⁵³ , C⁵⁹ and C⁶⁰ ). LcBD2 transcripts were constitutively expressed in all examined tissues and significantly increased in head kidney, spleen and gills by Vibrio alginolyticus. The synthetic LcBD2 peptide imparted antimicrobial effects on both Gram-negative bacteria (V. campbellii, V. parahaemolyticus, V. alginolyticus, V. harveyi and Pseudomonas plecoglossicida) and Gram-positive bacteria (Bacillus subtilis). We also observed that after treatment with synthetic LcBD2 peptide, numerous blisters appeared on the membrane of P. plecoglossicida, which in turn may result in cell membrane breakage and bacterial death. Moreover, the synthetic LcBD2 peptide significantly upregulated the expression levels of TNF-α2, IL-1β and CXCL8_L1 in monocytes/macrophages, while downregulated expression level of IL-10. The LcBD2 peptide also remarkedly enhanced the phagocytosis of monocytes/macrophages. These results indicate that LcBD2 not only protects large yellow croaker against multiple bacterial pathogens but also plays a role in activation of monocytes/macrophages.

Exogenous inositol ameliorates the effects of acute ammonia toxicity on intestinal oxidative status, immune response, apoptosis, and tight junction barriers of great blue-spotted mudskippers (Boleophthalmus pectinirostris)

Ammonia toxicity can disrupt the intestinal health of aquatic animals. It is important to find substances that alleviate these adverse effects. The present study explored the possible protective role of myo-inositol (MI) in ammonia-induced toxicity in the fish intestine. Great blue-spotted mudskippers (Boleophthalmus pectinirostris) accumulated in artificial seawater (15‰ salinity, n = 600) were randomly selected and intraperitoneally injected with NaCl (0.68%) or MI (2.5 mg/g fish in 0.68% NaCl) then exposed to artificial seawater alone (NaCl and MI group) or seawater containing 57.025 mmol/L ammonium chloride (NH₃ and NH₃ + MI group). After a 24-h experiment, it showed that ammonia exposure down-regulated the mRNA expression levels of intestinal barrier function proteins (Zo-1, Ocln, Cldn-5, Cldn-12, and Cldn-15) and anti-inflammatory cytokines (Tgf-β and Il-10) while the acute ammonia stress up-regulated the apoptosis genes (p53, Bax, Caspase-3, and Caspase-9) and pro-inflammatory cytokines (Tnf-α and Il-1β). Furthermore, ammonia challenge also induced oxidative stress, as the malondialdehyde and the protein carbonyl contents were increased. In addition, ammonia stress down-regulated the antioxidant enzymes (Cu/Zn-Sod, Cat, Gpx, and Gst) activities as well as their gene transcription levels. The administration of the exogenous myo-inositol greatly ameliorated the ammonia-induced changes in redox capacity, immune response, apoptosis, inflammation, and tight junction barrier function to levels similar to those of the NaCl group. Furthermore, fish injected with MI alone showed no significant changes compared with the NaCl group. Taken together, pretreatment with myo-inositol had no obvious side-effects and effectively protected the mudskippers' intestine from the toxicity caused by acute ammonia stress.

Molecular characterization of the T cell costimulatory receptors CD28 and CTLA4 in the European sea bass

For the activation of T cells, it is necessary the specific recognition of the peptide by the T cell receptors (TCR) in the surface of antigen-presenting cells (APCs) and additional signals delivered by costimulatory receptors. In fish, knowledge about the presence of these costimulatory signals is limited and functional evidence almost absent. Thus, in this study, we have identified the stimulatory CD28 and the inhibitory cytotoxic T-lymphocyte-associated protein 4 (CTLA4) coreceptors in the European sea bass (Dicentrarchus labrax), and evaluated their transcription. In parallel, the transcription encoding for the T cell markers CD8α and CD4 was also evaluated. Both coreceptors showed the canonical architecture including a signal peptide, an immunoglobulin domain, a transmembrane region and a cytosolic tail. Protein predictions and phylogenetic tree identify them as true mammalian orthologues of CD28 and CTLA4. We found these genes constitutively expressed in all studied organs of European sea bass with high expression in lymphoid organs (thymus, spleen and head-kidney) and liver. The molecular expression pattern of these genes was up-regulated in head-kidney leucocytes stimulated with T mitogens as concanavalin A and phytohemagglutinin (PHA), but not with the B cell mitogen lipopolysaccharide (LPS). Fish challenged with nodavirus (NNV) evidenced a differential and opposing regulation of the cd28 and ctla4 transcription levels in the brain, the target organ for viral replication, and head-kidney. While cd28 transcription tends to decrease over the infection time in both organs the expression of the ctla4 gene tends to increase. Interestingly, the coreceptor expression is highly and significantly correlated to the transcription of the T cell markers. Our results highlight the important role of CD28 and CTLA4 as costimulatory receptors of T cells in European sea bass but further studies are deserved.

Characterization of a common carp intelectin gene with bacterial binding and agglutination activity

Intelectin (ITLN) is a type of glycan-binding lectin involved in many physiological processes and some human diseases. Here we report a common carp intelectin (cITLN). Like other orthologs, cITLN also contains a conserved fibrinogen-related domain (FReD) and a unique intelectin domain, expresses in all the tissues tested with the highest level in the hindgut, and responds to bacterial challenge in the acute phase. We also expressed cITLN in Escherichia coli (E. coli) system, and the purified recombinant cITLN could neither affect the surface of bacteria nor inhibit the growth of bacteria, but it can agglutinate both gram-positive and gram-negative bacteria in a calcium-dependent manner. The cITLN's ability of agglutination of gram-positive bacteria is stronger than that of gram-negative bacteria. This is probably because recombinant cITLN could binding peptidoglycan (PGN) with a higher degree to lipopolysaccharide (LPS). Our results of cITLN provided new insight into the function of intelectin in the intestinal mucosal immunity.

IRF11 regulates positively type I IFN transcription and antiviral response in mandarin fish, Siniperca chuatsi

In vertebrates, a total of eleven interferon (IFN) regulatory factors (IRFs), IRF1 to IRF11 are reported, with the conserved presence of IRF1 to IRF9 in all classes of vertebrates. However, IRF10 has been reported only in fish and birds, and IRF11 seems to be a fish specific IRF member. In this study, IRF11 in mandarin fish Siniperca chuatsi was found upregulated following virus infection, and IRF11 was localized constitutively in nucleus as revealed through immunofluorescence test. The overexpression and/or luciferase reporter assays showed that IRF11 can induce transcriptionally the ISRE activity, and the expression of type I IFNs, IFNc and IFNh, as well as the IFN-stimulated gene, Mx, thus inhibiting the Siniperca chuatsi rhabdovirus (SCRV) replication as indicated in the reduced expression of virus protein genes. It is thus suggested that IRF11 in mandarin fish and probably in other teleost fish can exert its antiviral effect through the upregulation of type I IFNs and ISGs.

Expression and antibacterial analysis of galectin-8 and -9 genes in mandarin fish, Siniperca chuatsi

Galectin-8 and galectin-9 belong to tandem repeat-type galectins, and in the present study, these two genes were cloned in mandarin fish Siniperca chuatsi. The open reading frame (ORF) of the mandarin fish galectin-8 and galectin-9 contains 942, and 1008 bp, encoding 313 and 335 amino acids, respectively. As a conserved feature, an N-terminal carbohydrate recognition domain (CRD), and a C-terminal CRD were observed in each of the two galectins in mandarin fish. In healthy fish, galectin-8 and -9 were constitutively expressed in all organs/tissues examined, and their expression can be induced following the stimulation of LPS and poly(I:C). It is obvious that galectin-8 had a higher increase at mRNA level following the stimulation of poly(I:C). It is further demonstrated that mandarin fish galectin-8 inhibited the growth of Flavobacterium columnare and Streptococcus agalactiae, and in addition to the two species of bacteria, galectin-9 inhibited also the growth of Edwardsiella piscicida, which provides the basis for further understanding their antibacterial role in immune response of mandarin fish.

Characteristics delineation of piscidin 5 like from Larimichthys crocea with evidence for the potent antiparasitic activity

Several researches reported that piscidin members of teleosts owned strong antiparasitic activity. Cryptocaryon irritans, a type of ectoparasite, could infect most of the marine teleosts. Larimichthys crocea could severely suffer from marine white spot disease caused by C. irritans, and their mortality rate was significantly high. Concentrating on this problem, we have done many related works. Piscidin 5 like (termed Lc-P5L) was another piscidin member isolated from a comparative transcriptome of C. irritans-immuned L. crocea. In the paper, quantitative Real-time PCR (qRT-PCR) showed Lc-P5L was upregulated in examined tissues, including gill, head kidney, muscle, liver, spleen and intestine after challenged by C. irritans, the significant upregulation time was in accordance to key developmental stages of C. irritans, which implied different infection stages could result in host immune response. Furthermore, using microscope techniques, we observed theronts or trophonts became weakly motile, cilia became detached, cells were out of shape, membranes eventually lysed in different cell positions and cytoplasmic contents leaked. Laser confocal scanning microscope (LCSM) observed theronts macronucleus grew swell and depolymerized after treated by recombinant Lc-P5L (rLc-P5L). Data suggested rLc-P5L was significantly lethal to C. irritans, and the death state of the parasite incubated with rLc-P5L was remarkably similar to other piscidin members or other antiparasitic peptides (APPs). Thus, these data provided new insights into L. crocea immunity against C. irritans and potential of rLc-P5L as a therapeutic agent against pathogen invasion.

IRF11 enhances the inhibitory effect of IκBα on NF-κB activation in miiuy croaker

NF-κB is a typical transcription factor that regulates expression of various genes involved in inflammatory and immune responses. Therefore, it is essential that NF-κB signaling tightly regulated to maintain immune balance. Compared with those of mammals, the regulatory mechanisms of NF-κB signaling is rarely reported in teleost fish. IκBα is a prominent negative feedback regulator in the NF-κB signaling system. In this study, we determined that IRF11 enhances the inhibitory effect of IκBα on NF-κB activation in teleost fish. Overexpression of IRF11 can inhibit IκBα degradation, whereas its knockdown has the opposite effect of IκBα. Our study further indicates that IκBα was regulated via ubiquitin-proteasome degradation pathway, IRF11 inhibits IκBα in ubiquitin-proteasome degradation. This study provides a novel evidence on the regulation of innate immune signaling pathways in teleost fish and thus provides new insights into the regulatory mechanisms in mammals.

Genomic and functional characterization of the lect2 gene from Siniperca chuatsi

Mandarin fish (Siniperca chuatsi) is an important economic fish in China. Viral and bacterial diseases seriously affect the artificial culture of S. chuatsi. As a carnivorous fish, artificial feed domestication is also an important means to improve the scale of S. chuatsi culture. Therefore, the study of immunology and digestive physiology is very important to the industrial development of S. chuatsi. In this work, we analyzed the expression and function of the S. chuatsi leukocyte cell-derived chemotaxin 2 (Sc-lect2) gene on a basis of next generation, single-molecule long-read sequencing. Sc-lect2 was mainly expressed in the liver but barely expressed in the gill, skin, muscle, kidney, head kidney, brain, stomach, and intestine. When the fish were infected with infectious spleen and kidney necrosis virus and challenged with lipopolysaccharide and polyinosinic-polycytidylic acid, Sc-lect2 expression significantly increased by about 40, 17, and 7-fold, respectively, compared with unstimulated samples. We also found that Sc-lect2 increases by approximately 8-fold after the fish are fed an artificial diet. These results show that mandarin fish liver can not only digest food but also express specific immune genes. Changes in the diet can cause the differential expression of Sc-lect2 genes. Four Sc-lect2 interaction genes were differentially expressed in the skin or blood. Interestingly, miR-145-3p could inhibit Sc-lect2 gene expression by targeting its coding sequence region. One CpG island in the promoter region showed a high level of methylation, suggesting that high methylation does not affect Sc-lect2 gene expression in the liver.

Functional characterization and gene expression profile of perforin-2 in starry flounder (Platichthys stellatus)

The membrane attack complex/perforin (MACPF) superfamily consists of multifunctional proteins that form pores on the membrane surface of microorganisms to induce their death and have various immune-related functions. PFN2 is a perforin-like protein with an MACPF domain, and humans with deficient PFN2 levels have increased susceptibility to bacterial infection, which can lead to fatal consequences for some patients. Therefore, in this study, we confirmed the antimicrobial function of PFN2 in starry flounder (Platichthys stellatus). The molecular properties were confirmed based on the verified amino acid sequence of PsPFN2. In addition, the expression characteristics of tissue-specific and pathogen-specific PsPFN2 mRNA were also confirmed. The recombinant protein was produced using Escherichia coli, and the antimicrobial activity was then confirmed. The coding sequence of PFN2 (PsPFN2) in P. stellatus consists of 710 residues. The MACPF domain was conserved throughout evolution, as shown by multiple sequence alignment and phylogenetic analysis. PsPFN2 mRNA is abundantly distributed in immune-related organs such as the spleen and gills of healthy starry flounder, and significant expression changes were confirmed after artificial infection by bacteria or viruses. We cloned the MACPF domain region of PFN2 to produce a recombinant protein (rPFN2) and confirmed its antibacterial effect against a wide range of bacterial species and the parasite (Miamiensis avidus).

Xbp1-u and Xbp1-s from Nile tilapia (Oreochromis niloticus): Transcriptional profiling upon Streptococcus agalactiae infection and the potential role in B cell activation and differentiation

X-box protein 1 (Xbp1), an essential transcription factor including an unstable form (Xbp1-u) and a stable form (Xbp1-s), plays an vital role in B cell activation and differentiation to plasma cells. In this study, we cloned and identified Xbp1-u gene from Nile tilapia (Oreochromis niloticus), containing 783 bp of nucleotide sequence encoding 260 amino acids. The deduced protein possesses a basic region leucine zipper domain (bZIP) and 26 ribonucleotides of OnXbp1-u transcript. Transcription analysis revealed OnXbp1-u and OnXbp1-s were widely distributed in all examined tissues, with a high expression in immune-related tissues. When stimulated with Streptococcus agalactiae in vivo, the expressions of OnXbp1-u and OnXbp1-s were significantly up-regulated in liver, spleen, head kidney, blood, skin and intestine. After in vitro challenge upon S.agalactiae, the similar up-regulations of OnXbp1-u and OnXbp1-s were also demonstrated in head kidney leukocytes. Moreover, the OnXbp1-u and OnXbp1-s could get involved in LPS-inducible B cell activation and (r)OnIL6-inducible B cell differentiation. Taken together, the results indicated that OnXbp1-u and OnXbp1-s might not only involved in the immune response against S. agalactiae challenge, but also in the B cell activation and differentiation in Nile tilapia.

Molecular characterization of the interferon regulatory factor (IRF) family and functional analysis of IRF11 in the large yellow croaker (Larimichthys crocea)

Interferon regulatory factors (IRFs) are a family of transcription factors involved in regulating interferon (IFN) responses and immune cell development. A total of 11 IRFs have been identified in teleost fish. Here, a complete repertoire of 11 IRFs (LcIRFs) in the large yellow croaker (Larimichthys crocea) was characterized with the addition of five newly identified members, LcIRF2, LcIRF5, LcIRF6, LcIRF10, and LcIRF11. These five LcIRFs possess a DNA-binding domain (DBD) at the N-terminal that contains five to six conserved tryptophan residues and an IRF-association domain (IAD) or IAD2 at the C-terminal that is responsible for interaction with other IRFs or co-modulators. Phylogenetic analysis showed that the 11 LcIRFs were divided into four clades including the IRF1 subfamily, IRF3 subfamily, IRF4 subfamily, and IRF5 subfamily. These are evolutionarily related to their respective counterparts in other fish species. The 11 LcIRFs were constitutively expressed in all examined tissues, although at different expression levels. Upon polyinosinic: polycytidylic acid (poly (I:C)) stimulation, the expression of all 11 LcIRFs was significantly induced in the head kidney and reached the highest levels at 6 h post-stimulation (except LcIRF4). LcIRF1, LcIRF3, LcIRF7, LcIRF8, and LcIRF10 were more strongly induced by poly (I:C) than the other LcIRFs. Significant induction of all LcIRFs was observed in the spleen, with LcIRF2, LcIRF5, LcIRF6, LcIRF7, LcIRF9, and LcIRF11 reaching their highest levels at 48 h LcIRF3 and LcIRF11 showed a stronger response to poly (I:C) in the spleen than the other LcIRFs. In addition, LcIRF1, LcIRF3, LcIRF7, LcIRF9, LcIRF10, and LcIRF11 were significantly induced by Vibro alginolyticus in both the spleen and the head kidney, with LcIRF1 strongly induced. Thus, LcIRFs exhibited differential inducible expression patterns in response to different stimuli in different tissues, suggesting that LcIRFs have different functions in the regulation of immune responses. Furthermore, overexpression of LcIRF11 activated the promoters of LcIFNc, LcIFNd, and LcIFNh, and differentially induced the expression levels of LcIFNs and IFN-stimulated genes (ISGs). Overexpression of LcIRF11 in epithelioma papulosum cyprinid (EPC) cells inhibited the replication of viral genes after infection of spring viremia of carp virus (SVCV). These data suggested that LcIRF11 may function as a positive regulator in regulating the cellular antiviral response through induction of type I IFN expression. Taken together, the present study reported molecular characterization and expression analysis of 11 IRFs in the large yellow croaker, and investigated the role of LcIRF11 in the antiviral response, which laid a good foundation for further study on the evolution and functional characterization of fish IRFs.

Unique duplication of IFNh genes in Nile tilapia (Oreochromis niloticus) reveals lineage-specific evolution of IFNh in perciform fishes

Fish appear to harbour a complex type I IFN repertoire containing subgroups a, b, c, d, e, f, and h, and IFNh is only reported in perciform fishes. However, no multiple copies of IFNh gene has been identified in fish to date. In this study, two IFNh genes named On-IFNh1 and On-IFNh2 were cloned from Nile tilapia, Oreochromis niloticus. The predicted proteins of On-IFNh1 and On-IFNh2 contain several structural features known in type I IFNs, and estimation of divergence time revealed that these two genes may have arisen from a much recent local duplication event. On-IFNh genes were constitutively expressed in all tissues examined, with the highest expression level observed in gill, and were rapidly induced in all organs/tissues tested following the stimulation of poly(I:C). In addition, both recombinant On-IFNh1 and On-IFNh2 trigger a relative delayed but sustained induction of interferon-stimulated genes (ISGs), whereas recombinant On-IFNc elicits a rapid and transient expression of ISGs in vivo. The present study thus contributes to a better understanding of the functional properties of tilapia interferons, and also provides a new insight into the evolution of IFNh in fish.

Characterisation of IL-15 and IL-2Rβ in grass carp: IL-15 upregulates cytokines and transcription factors of type 1 immune response and NK cell activation

Interleukin (IL) -15 belongs to the common cytokine receptor γ chain (γC) family and has diverse functions in regulating the development, proliferation and activation of NK and T cells. It activates a hetero-trimeric receptor complex consisting of IL-2Rα, IL-2Rβ and a common γ chain (γC). In this study, the full-length cDNA sequences of IL-15 and IL-2Rβ were identified in grass carp (Ctenopharyngodon idella, Ci) and their expression profiles analysed. The CiIL-15 and CiIL-2Rβ were shown to be broadly expressed in tissues, with the highest levels detected in the spleen. Moreover, the CiIL-15 and CiIL-2Rβ were modulated in primary head kidney leucocytes (HKLs) and splenocytes by immunostimulants and cytokines, and in the head kidney and spleen of fish after infection of Flavobacterium columnare and grass carp reovirus. The bioactivity of bacteria derived recombinant CiIL-15 protein was evaluated in the primary leucocytes. The CiIL-15 was shown to induce signature genes of type 1 immune response (IFN-γ and T-bet) and NK cell activation (perforin and Eomesa), whilst exhibiting inhibitory effects on the genes involved in the type 2 immune response (IL-4/13, IL-10 and Gata3). Our data suggest that IL-15 is a key regulator in promoting the type 1 immune response and NK cell activation in fish.