Molecular cloning and characterization of Toll-like receptor 3 in Japanese flounder, Paralichthys olivaceus

Nile tilapia TLR3 recruits MyD88 and TRIF as adaptors and is involved in the NF-κB pathway in the immune response

TLR3 plays a crucial role in innate immunity. In the present study, OnTLR3 was identified in the Nile tilapia Oreochromis niloticus, with a conserved LRR domain and a C-terminal TIR domain. OnTLR3 was broadly expressed in all tissues tested, with the highest expression levels in the blood and the lowest in the kidney. TLR3 mRNA could be detected from pharyngula (2.5 dpf) to late larva (8.5 dpf) during embryonic and larval development. Moreover, the expression level of OnTLR3 was clearly altered in all five tissues after Streptococcus agalactiae infection in vivo and could be induced by LPS, poly(I:C), S. agalactiae WC1535 and △CPS in Nile tilapia macrophages. When OnTLR3 was overexpressed in 293 T cells, it was distributed in the cytoplasm and could significantly increase NF-κB activation. The pulldown assays showed that OnTLR3 interacted with both OnMyD88 and OnTRIF. The binding assays revealed the specificity of OnTLR3 for pathogen-associated molecular patterns (PAMPs) and bacteria that included S. agalactiae, Aeromonas hydrophila and poly(I:C), LPS and PGN. Taken together, these findings suggest that OnTLR3, as a pattern recognition receptor (PRR), might play an important role in the immune response to pathogen invasion.

The Influence of Temperature on the Antiviral Response of mIgM+ B Lymphocytes Against Hirame Novirhabdovirus in Flounder (Paralichthys olivaceus)

Hirame novirhabdovirus (HIRRV) is an ongoing threat to the aquaculture industry. The water temperature for the onset of HIRRV is below 15°C, the peak is about 10°C, but no mortality is observed over 20°C. Previous studies found the positive signal of matrix protein of HIRRV (HIRRV-M) was detected in the peripheral blood leukocytes of viral-infected flounder. Flow cytometry and indirect immunofluorescence assay showed that HIRRV-M was detected in mIgM⁺ B lymphocytes in viral-infected flounder maintained at 10°C and 20°C, and 22% mIgM⁺ B lymphocytes are infected at 10°C while 13% are infected at 20°C, indicating that HIRRV could invade into mIgM⁺ B lymphocytes. Absolute quantitative RT-PCR showed that the viral copies in mIgM⁺ B lymphocytes were significantly increased at 24 h post infection (hpi) both at 10°C and 20°C, but the viral copies in 10°C infection group were significantly higher than that in 20°C infection group at 72 hpi and 96 hpi. Furthermore, the B lymphocytes were sorted from HIRRV-infected flounder maintained at 10°C and 20°C for RNA-seq. The results showed that the differentially expression genes in mIgM⁺ B lymphocyte of healthy flounder at 10°C and 20°C were mainly enriched in metabolic pathways. Lipid metabolism and Amino acid metabolism were enhanced at 10°C, while Glucose metabolism was enhanced at 20°C. In contrast, HIRRV infection at 10°C induced the up-regulation of the Complement and coagulation cascades, FcγR-mediated phagocytosis, Platelets activation, Leukocyte transendothelial migration and Natural killer cell mediated cytotoxicity pathways at 72 hpi. HIRRV infection at 20°C induced the up-regulation of the Antigen processing and presentation pathway at 72 hpi. Subsequently, the temporal expression patterns of 16 genes involved in Antigen processing and presentation pathway were investigated by qRT-PCR, and results showed that the pathway was significantly activated by HIRRV infection at 20°C but inhibited at 10°C. In conclusion, HIRRV could invade into mIgM⁺ B lymphocytes and elicit differential immune response under 10°C and 20°C, which provide a deep insight into the antiviral response in mIgM⁺ B lymphocytes.

Progresses on three pattern recognition receptor families (TLRs, RLRs and NLRs) in teleost

Pattern recognition receptors (PRRs) are a class of immune sensors that play crucial roles in detecting and responding to the conserved patterns of microorganisms. To date, many PRRs, such as TLRs, RLRs and NLRs, as well as their downstream molecules have been identified and characterized in teleost, while their ligands and immunoregulatory mechanisms remain largely unknown. In the present review, we described and discussed the main members of TLR/RLR/NLR families, including their expression profiles, signaling transductions and functions in teleost. And some splicing isoforms from TLR/RLR/NLR families were also addressed, which play synergistic and/or antagonistic roles in response to pathogen infections in teleost. TLRs sense different pathogens by forming homodimer and/or heterodimer. Beyond, functions of TLRs can also be affected by migrating. And some endolysosomal TLRs undergo proteolytic cleavage and in a pH-dependent mechanism to attain a mature functional form that mediate ligand recognition and downstream signaling. Until now, more than 80 members in TLR/RLR/NLR families have been identified in teleost, while only TLR5, TLR9, TLR19, TLR21, TLR22, MDA5, LGP2, NOD1 and NOD2 have direct evidence of ligand recognition in teleost. Meanwhile, new ligands as well as signaling pathways do occur during evolution of teleost. This review summarizes progresses on the TLRs/RLRs/NLRs in teleost. We attempt to insight into the ligands recognition and signaling transmission of TLRs/RLRs/NLRs in teleost.

Novirhabdoviruses versus fish innate immunity: A review

Novirhabdoviruses belong to the Rhabdoviridae family of RNA viruses. All of the four members are pathogenic for bony fish. Particularly, Infectious hematopoietic necrosis virus (IHNV) and Viral hemorrhagic septicemia virus (VHSV) often cause mass animal deaths and huge economic losses, representing major obstacles to fish farming industry worldwide. The interactions between fish and novirhabdoviruses are becoming better understood. In this review, we will present our current knowledge of fish innate immunity, particularly type I interferon (IFN-I) response, against novirhabdoviral infection, and the evasion strategies exploited by novirhabdoviruses. Members of Toll-like receptors (TLRs) and RIG-I-like receptors (RLRs) appear to be involved in novirhabdovirus surveillance. NF-κB activation and IFN-I induction are primarily triggered for antiviral defense. Autophagy can also be induced by viral glycoprotein (G). Although sensitive to IFN-I, novirhabdoviruses have nucleoprotein (N), matrix protein (M), and non-virion protein (NV) to interfere with host signal transduction and gene expression steps toward antiviral state establishment. Moreover, novirhabdoviruses may exploit some microRNAs for immunosuppression.

Assessment of a natural grass carp reovirus genotype II avirulent strain GD1108 shows great potential as an avirulent live vaccine

Vaccine immunization is currently the only effective way to prevent and control the grass carp haemorrhagic disease, and the primary pathogen in these infections is grass carp reovirus genotype II (GCRV-II) for which there is no commercial vaccine. In this study, we evaluated the safety of the GCRV-II avirulent strain GD1108 which isolated in the early stage of the laboratory through continuously passed in grass carp. The immunogenicity and protective effects were evaluated after immunization by injection and immersion. The avirulent strain GD1108 could infect and replicate in the fish which did not revert to virulence after continuous passage. No adverse side effects were observed and the vaccine strain did not spread horizontally among fish. Two routes of immunization induced high serum antibody titers of OD_450nm value were 0.79 and 0.76 and neutralization titers of 320 and 320 for the injection and immersion routes of inoculation, respectively. The expression of immune-related genes increased after immunization and the levels were statistically significant. Challenge of immunized fish with a virulent GCRV-II strain resulted in protection rates of 93.88% and 76.00% for the injection and immersion routes, respectively. The avirulent strain GD1108 revealed good safety and immunogenicity via two different inoculation routes. Although the injection route provided the best immune effect, two pathways provided protection against infection with virulent GCRV-II strains in various degrees. These results indicated that the avirulent strain GD1108 can be used for the development and application as live vaccine.

Kinetics of transcriptional response against poly (I:C) and infectious salmon anemia virus (ISAV) in Atlantic salmon kidney (ASK) cell line

Vaccine adjuvants induce host innate immune responses improving long-lasting adaptive immunity against vaccine antigens. In vitro models can be used to compare these responses between adjuvants and the infection targeted by the vaccine. We utilized transcriptomic profiling of an Atlantic salmon cell line to compare innate immune responses against ISAV and an experimental viral vaccine adjuvant: poly (I:C). Induction of interferon and interferon induced genes were observed after both treatments, but often with different amplitude and kinetics. Using KEGG ortholog database and available software from Bioconductor we could specify a complete bioinformatic pipeline for analysis of transcriptomic data from Atlantic salmon, a feature not previously available. We have identified important differences in the transcriptional profile of Atlantic salmon cells exposed to viral infection and a viral vaccine adjuvant candidate, poly (I:C). This report increases our knowledge of viral host-pathogen interaction in salmon and to which extent these can be mimicked by adjuvant compounds.

Metabolic and immune responses of Chinook salmon (Oncorhynchus tshawytscha) smolts to a short-term poly (I:C) challenge

Polyinosinic:polycytidylic acid [poly (I:C)] was administered in vivo to Chinook salmon (Oncorhynchus tshawytscha) post-smolts to determine the immune responses on haematological and cellular functional parameters, including spleen (SP), head kidney (HK) and red blood cell (RBC) cytokine expression, as well as serum metabolomics. Poly (I:C) in vivo (24 h exposure) did not affect fish haematological parameters, leucocyte phagocytic activity and phagocytic index, reactive oxygen species and nitric oxide production. Gas chromatography-mass spectrometry-based metabolomics revealed that poly (I:C) significantly altered the serum biochemistry profile of 25 metabolites. Metabolites involved in the branched-chain amino acid/glutathione and transsulphuration pathways and phospholipid metabolism accumulated in poly (I:C)-treated fish, whereas those involved in the glycolytic and energy metabolism pathways were downregulated. At cytokine transcript level, poly (I:C) induced a significant upregulation of antiviral ifnγ in HK and Mx1 protein in HK, SP and RBCs. This study provides evidence for poly (I:C)-induced, immune-related biomarkers at metabolic and molecular levels in farmed O. tshawytscha in vivo. These findings provide insights into short-term effects of poly (I:C) at haematological, innate and adaptive immunity and metabolic levels, setting the stage for future studies.

Molecular characterization and expression analyses of two homologues of interferon-stimulated gene ISG15 in Larimichthys crocea (Family: Sciaenidae)

In this study, we sequenced and characterized two homologues of interferon-stimulated gene ISG15, termed as LcISG15-1 and LcISG15-2, from the large yellow croaker (Larimichthys crocea). The LcISG15-1 encodes 159 amino acids and the LcISG15-2 encodes 155 amino acids, both of which contain two tandem ubiquitin-like domains and the conserved C-terminal LRGG conjugation motif. The sequence analyses showed that both the LcISG15-1 and LcISG15-2 exhibit high similarity with ISG15 from other fishes. A putative IFN-stimulatory response element (ISRE) was detected in promoter regions of both the LcISG15-1 and LcISG15-2. Phylogenetic analyses revealed a close evolutionary relationship of both the LcISG15-1 and LcISG15-2 with other teleostean ISG15. Molecular evolutionary analyses suggested a gene duplication event of ISG15 in the ancestor of the Sciaenidae, with a signature of positive selection was found in the ISG15-2 gene copy of sciaenid fishes. The Real-time PCR analyses showed that the LcISG15-1 and LcISG15-2 were both found to be ubiquitously expressed in ten examined organs in large yellow croaker, with predominant expressions both in peripheral blood. Expression analyses showed that both the LcISG15-1 and LcISG15-2 were rapidly and significantly upregulated in vivo after poly (I:C) challenge in liver and spleen organs. However, the LcISG15-1 and LcISG15-2 were both significantly induced after pathogen Vibrio parahemolyticus infection only in the liver but not in the spleen. These results indicated that there are two ISG15 homologues in the large yellow croaker, both of which are likely to be involved in host immune defense against viral and bacterial infection.

Characterization of the TLR Family in Branchiostoma lanceolatum and Discovery of a Novel TLR22-Like Involved in dsRNA Recognition in Amphioxus

Toll-like receptors (TLRs) are important for raising innate immune responses in both invertebrates and vertebrates. Amphioxus belongs to an ancient chordate lineage which shares key features with vertebrates. The genomic research on TLR genes in Branchiostoma floridae and Branchiostoma belcheri reveals the expansion of TLRs in amphioxus. However, the repertoire of TLRs in Branchiostoma lanceolatum has not been studied and the functionality of amphioxus TLRs has not been reported. We have identified from transcriptomic data 30 new putative TLRs in B. lanceolatum and all of them are transcribed in adult amphioxus. Phylogenetic analysis showed that the repertoire of TLRs consists of both non-vertebrate and vertebrate-like TLRs. It also indicated a lineage-specific expansion in orthologous clusters of the vertebrate TLR11 family. We did not detect any representatives of the vertebrate TLR1, TLR3, TLR4, TLR5 and TLR7 families. To gain insight into these TLRs, we studied in depth a particular TLR highly similar to a B. belcheri gene annotated as bbtTLR1. The phylogenetic analysis of this novel BlTLR showed that it clusters with the vertebrate TLR11 family and it might be more related to TLR13 subfamily according to similar domain architecture. Transient and stable expression in HEK293 cells showed that the BlTLR localizes on the plasma membrane, but it did not respond to the most common mammalian TLR ligands. However, when the ectodomain of BlTLR is fused to the TIR domain of human TLR2, the chimeric protein could indeed induce NF-κB transactivation in response to the viral ligand Poly I:C, also indicating that in amphioxus, specific accessory proteins are needed for downstream activation. Based on the phylogenetic, subcellular localization and functional analysis, we propose that the novel BlTLR might be classified as an antiviral receptor sharing at least partly the functions performed by vertebrate TLR22. TLR22 is thought to be viral teleost-specific TLR but here we demonstrate that teleosts and amphioxus TLR22-like probably shared a common ancestor. Additional functional studies with other lancelet TLR genes will enrich our understanding of the immune response in amphioxus and will provide a unique perspective on the evolution of the immune system.

Effects of food availability on growth performance and immune-related gene expression of juvenile olive flounder (Paralichthys olivaceus)

Unfavorable environmental conditions and inappropriate culture practices have increased the vulnerability of cultured fish to disease infection. Up to date many studies have aimed to determine a feeding regimen to maximize productivity; however, very little information on immune responses of cultured fish in response to underfeeding or overfeeding is available. Therefore, a preliminary study was conducted to evaluate effects of graded feeding levels (i.e., food availability) on growth performance and immune-related gene expression of juvenile olive flounder (Paralichthys olivaceus). Six different feeding rates including 1, 4, 7, 10, 13, and 16% body weight per day (BW/d) were randomly assigned to three replicate tanks stocking 150 fish (average initial body weight: 0.27 ± 0.02 g; mean ± SD) per tank. A feeding trial lasted for two weeks. Based on the results of the weight gain, nutrient gain, and whole-body compositions and energy content, the feeding rate of 10%, 13%, and 16% BW/d resulted in high nutritional status, whereas the feeding rate of 1% and 4% BW/d resulted in low nutritional status. Intermediate nutritional status was observed at the feeding rate of 7% BW/d. In the given rearing conditions the optimum feeding rate resulting in the maximum growth was estimated to be 11.9% BW/d based on the quadratic broken-line regression model, chosen as the best-fit model among the tested models. Expression of immune-related genes including IL-8 and IgM was significantly down-regulated in the flounder fed at 1% BW/d in comparison to those fed at 7% BW/d. Interestingly, expression of these genes in the flounder fed at 10%, 13%, and 16% BW/d was relatively down-regulated in comparison to that of the flounder fed at 7% BW/d. Although no statistical difference was detected, overall response patterns of other immune-related genes, including TLR3, polymeric Ig receptor, lysozyme C-type, GPx, SOD, and Trx followed what IL-8 and IgM exhibited in response to the various feeding rates. Given the current challenges in aquaculture of the flounder our findings suggest to prohibit underfeeding or overfeeding (i.e., ad-libitum feeding) when culturing the young flounder.

Toll-Like Receptors, Associated Biological Roles, and Signaling Networks in Non-Mammals

The innate immune system is the first line of defense against pathogens, which is initiated by the recognition of pathogen-associated molecular patterns (PAMPs) and endogenous damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs). Among all the PRRs identified, the toll-like receptors (TLRs) are the most ancient class, with the most extensive spectrum of pathogen recognition. Since the first discovery of Toll in Drosophila melanogaster, numerous TLRs have been identified across a wide range of invertebrate and vertebrate species. It seems that TLRs, the signaling pathways that they initiate, or related adaptor proteins are essentially conserved in a wide variety of organisms, from Porifera to mammals. Molecular structure analysis indicates that most TLR homologs share similar domain patterns and that some vital participants of TLR signaling co-evolved with TLRs themselves. However, functional specification and emergence of new signaling pathways, as well as adaptors, did occur during evolution. In addition, ambiguities and gaps in knowledge still exist regarding the TLR network, especially in lower organisms. Hence, a systematic review from the comparative angle regarding this tremendous signaling system and the scenario of evolutionary pattern across Animalia is needed. In the current review, we present overview and possible evolutionary patterns of TLRs in non-mammals, hoping that this will provide clues for further investigations in this field.

Identification, structural characterization, and expression analysis of toll-like receptors 2 and 3 from gibel carp (Carassius auratus gibelio)

Toll-like receptors (TLRs) are important components of innate immunity. TLRs recognize pathogen-associated molecular patterns (PAMPs) and initiate downstream signaling pathways in response. In present study, we report the identification of two TLRs from gibel carp (Carassius auratus gibelio), TLR2 and TLR3 (designated CagTLR2 and CagTLR3, respectively). We report on the genomic structures and mRNA expression patterns of CagTLR2 and CagTLR3. Five exons and four introns were identified from the genomic DNA sequence of CagTLR3 (4749 bp in total length); this genomic organization is similar to that of TLR3 in zebrafish and human. However, only one intron was identified from the CagTLR2 genomic locus (3166 bp in total length); this unique genomic organization of CagTLR2 is different from that of TLR2 in fish and humans. The cDNAs of CagTLR2 and CagTLR3 encoded 791 and 904 amino acid residues, respectively. CagTLR2 and CagTLR3 contained two distinct structural/functional motifs of the TLR family: a leucine-rich repeat (LRR) domain in the extracellular portion and a toll/interleukin-1 receptor (TIR) domain in the intracellular portion. The positions of critical amino acid residues involed in PAMP recognition and signaling pathway transduction in mammalian TLRs were conserved in CagTLR2 and CagTLR3. Phylogenetic analysis revealed a closer clustering of CagTLR2 and CagTLR3 with TLRs from freshwater fish than with marine fish species. In healthy gibel carp, transcripts of these genes were detected in all examined tissues, and high expression levels of CagTLR2 and CagTLR3 were observed in liver and brain, respectively. Following injection with CyHV-2, expression levels of CagTLR2 and CagTLR3 were significantly upregulated in the spleens of gibel carp after three days, and CagTLR3 transcript levels were rapidly increased in head kidney after 12 h. These results suggest that CagTLR2 and CagTLR3 are functionally involved in the induction of antiviral immune response.

Immune response of olive flounder (Paralichthys olivaceus) infected with the myxosporean parasite Kudoa septempunctata

This study evaluated the pathophysiological, biochemical, and immunological status of olive flounder (Paralichthys olivaceus) infected with the myxosporean parasite Kudoa septempunctata. Flounder fish collected from Kudoa-infected and uninfected farms were confirmed by microscopic and TaqMan probe-based quantitative PCR screening. Morphological, biochemical, histological, and immune gene expression analyses were performed on uninfected and infected hosts to assess the effect of K. septempunctata. Histological studies confirmed the presence of Kudoa myxospores in the trunk muscles of infected flounder fish. Serum biochemical parameters, including the levels of myeloperoxidase activity, superoxide dismutase activity, alanine aminotransferase, alkaline phosphatase, amylase, bilirubin, total protein, cholesterol, calcium, potassium, sodium, phosphorus, glucose, and galactose, were found to exhibit no significant variations (p > 0.05) between uninfected and infected flounder fish. However, immune-related genes such as Mx, lysozyme, signal transducer and activator of transcription 1, interferon-γ, interferon regulatory factor, and tumour necrosis factor showed significantly elevated expression (p < 0.05) in the trunk muscles of infected flounder fish while no significant differences were noted in uninfected fish trunk muscle and head-kidney of infected and uninfected flounder fish.

Characterization of Toll-like receptor 22 in turbot (Scophthalmus maximus)

Toll-like receptors (TLRs) are essential for activation of the innate immune system in response to invading pathogens. In this paper, expression profiles of the turbot (Scophthalmus maximus) TLR22 gene (tbTLR22) were analyzed with RT-PCR and in situ hybridization. Then its expression patterns simulated with ligands or pathogens were investigated. Streptococcus iniae, Edwardsiella tarda, Hirame rhabdovirus virus (HIRRV), polyinosinic: polycytidylic acid (Poly I:C), peptidoglycan (PGN), or lipopolysaccharides (LPS) was injected to turbot; poly I:C, PGN, or LPS was added into cultured peripheral blood leukocytes (PBL); and then the tbTLR22 in liver, spleen, gill, kidney and cultured PBL was measured using Quantitative PCR. The recombinant protein of tbTLR22 (rp-tbTLR22) and its antibody were produced, then the reactions of antibody to tissues were detected by Western-blotting, and the binding of rp-tbTLR22 to all the stimulants was detected using ELISA. The results showed tbTLR22 expression was significantly up-regulated by PolyI: C, but no significant change in PGN and LPS groups; tbTLR22 significantly increased in liver and spleen after S. iniae infection with the maximum of 3.6 times and 3.3 times; in liver and kidney after E. tarda infection with the maximum of 3.4 times and 4.1 times; and then in gill and kidney after HIRRV infection by 4.8 and 4.1 times. Rp-tbTLR22 antibody could recognize the total protein from liver, kidney, gill and spleen at 40 kDa, 90 kDa and 120 kDa, respectively. The rp-tbTLR22 could bind to three ligands and pathogens in vitro. The expression and reaction data gave a clear recognization model of tbTLR22.

In Silico Analysis of nsSNPs of Carp TLR22 Gene Affecting its Binding Ability with Poly I:C

Immune response mediated by toll-like receptor 22 (TLR22), only found in teleost/amphibians, is triggered by double-stranded RNA binding to its LRR (leucine-rich repeats) ecto-domain. Accumulated evidences suggested that missense mutations in TLR genes affect its function. However, information on mutation linked pathogen recognition for TLR22 was lacking. The present study was commenced for predicting the effect of non-synonymous single-nucleotide polymorphisms (nsSNPs) on the pathogen recognizable LRR domain of TLR22 of farmed carp, Labeo rohita. The sequence-based algorithms (SIFT, PROVEAN and I-Mutant2.0) indicated that three SNPs (out of 27) such as p.L159F (rs76759876) and p.L529P (rs749355507) of LRR, and p.I836M (rs750758397) of intracellular motifs could potentially disrupt protein function. The 3D structure was generated using MODELLER 9.13 and further validated by SAVEs server. The simulated molecular docking of native TLR22 and mutants with poly I:C ligand indicated that mutations positioned at p.L159F and p.L529P of the LRR region affects the binding affinity significantly. This is the first kind of study of predicting nsSNPs of teleost TLR22 with disturbed ligand binding affinity with its extra-cellular LRR domain and thereby likely hindrance in subsequent signal transduction. This study serves as a guide for in vivo evaluation of impact of mutation on immune response mediated by teleost TLR22 gene.

Montanide IMS 1312 VG adjuvant enhances the efficacy of immersion vaccine of inactivated viral hemorrhagic septicemia virus (VHSV) in olive flounder, Paralichthys olivaceus

Vaccination by immersion is suitable for mass vaccination of small size fish. However, no viral vaccine has been developed for immersion applications, because of low efficacy. In this study, we evaluated the efficacy and safety of immersion vaccine against viral hemorrhagic septicemia (VHS) containing Montanide IMS 1312 VG adjuvant in olive flounder (Paralichthys olivaceus). Healthy fish were vaccinated by an immersion method with a heat-inactivated FP-VHS2010-1 strain of VHS virus (VHSV) in combination with Montanide IMS 1312 VG for 5 min at 20 ± 2 °C. The control group was vaccinated with sterile PBS. No toxicity of immersion vaccine with Montanide IMS 1312 VG adjuvant was observed by hematological and histopathological analysis. Immersion vaccine with adjuvant enhanced gene expression of immune-associated genes, i.e., genes encoding interleukin (IL)-1β, IL-6, IL-8, and Toll-like receptor (TLR) 3. Relative percent survival (RPS) of fish was measured on weeks 4 and 8 post vaccination. In fish vaccinated with adjuvant, RPS was significantly higher than that of fish vaccinated without adjuvant. The results of the present study provide evidence that the VHSV immersion vaccine with Montanide IMS 1312 VG induces protective immunity in olive flounder against VHS.

Insights into Trx1, TRP14, and Prx1 homologs of Paralichthys olivaceus: molecular profiles and transcriptional responses to immune stimulations

Thioredoxin (Trx) proteins are involved in several cellular processes, such as anti-oxidative stress and cellular redox homeostasis. In this study, we isolated the full-length cDNAs of PoTrx1 and PoTRP14 from Japanese flounder (Paralichthys olivaceus). PoTrx1 is 723 bp in length, with a 366-bp open reading frame (ORF) that encodes for 121 amino acids. PoTRP14 is 909 bp in length, with a 372-bp ORF that encodes for 123 amino acids. PoTrx1 and PoTRP14 are highly conserved in Cys-Gly-Pro-Cys and Cys-Pro-Asp-Cys forms, respectively. Tissue distribution analysis revealed that the transcripts of PoTrx1 and PoTRP14 were ubiquitously expressed in all tested tissues and particularly abundant in immunity-related organs, such as the liver, intestine, gill, and spleen. Development expression profiles indicated that PoTrx1 transcript was expressed from the neurula stage to the 1 day post-hatching stage; the maximum transcript levels were recorded at the somatic stage. The mRNA level of PoTRP14 was constantly expressed at all examined developmental stages, reaching the peak at the before-hatching stage. Prx1 is a peroxiredoxin family member that serves similar functions to PoTrx1 and PoTRP14. A primary hepatocyte culture system was established to examine the immunoregulatory properties of PoTrx1, PoTRP14, and Prx1 in response to lipopolysaccharide, CuSO4, and H2O2 stimulation. Results revealed that the transcript levels of PoTrx1, PoTRP14, and Prx1 were significantly up-regulated in a time-dependent manner after the immunostimulant challenge. These data suggest that PoTrx1, PoTRP14, and Prx1 play critical roles in anti-oxidation and immunoregulation.

Sensors of Infection: Viral Nucleic Acid PRRs in Fish

Viruses produce nucleic acids during their replication, either during genomic replication or transcription. These nucleic acids are present in the cytoplasm or endosome of an infected cell, or in the extracellular space to be sensed by neighboring cells during lytic infections. Cells have mechanisms of sensing virus-generated nucleic acids; these nucleic acids act as flags to the cell, indicating an infection requiring defense mechanisms. The viral nucleic acids are called pathogen-associated molecular patterns (PAMPs) and the sensors that bind them are called pattern recognition receptors (PRRs). This review article focuses on the most recent findings regarding nucleic acids PRRs in fish, including: Toll-like receptors (TLRs), RIG-I-like receptors (RLRs), cytoplasmic DNA sensors (CDSs) and class A scavenger receptors (SR-As). It also discusses what is currently known of the downstream signaling molecules for each PRR family and the resulting antiviral response, either type I interferons (IFNs) or pro-inflammatory cytokine production. The review highlights what is known but also defines what still requires elucidation in this economically important animal. Understanding innate immune systems to virus infections will aid in the development of better antiviral therapies and vaccines for the future.

Insights into the antiviral immunity against grass carp (Ctenopharyngodon idella) reovirus (GCRV) in grass carp

Global fish production from aquaculture has rapidly grown over the past decades, and grass carp shares the largest portion. However, hemorrhagic disease caused by grass carp reovirus (GCRV) results in tremendous loss of grass carp (Ctenopharyngodon idella) industry. During the past years, development of molecular biology and cellular biology technologies has promoted significant advances in the understanding of the pathogen and the immune system. Immunoprophylaxis based on stimulation of the immune system of fish has also got some achievements. In this review, authors summarize the recent progresses in basic researches on GCRV; viral nucleic acid sensors, high-mobility group box proteins (HMGBs); pattern recognition receptors (PRRs), Toll-like receptors (TLRs) and retinoic acid inducible gene I- (RIG-I-) like receptors (RLRs); antiviral immune responses induced by PRRs-mediated signaling cascades of type I interferon (IFN-I) and IFN-stimulated genes (ISGs) activation. The present review also notices the potential applications of molecule genetic markers. Additionally, authors discuss the current preventive and therapeutic strategies (vaccines, RNAi, and prevention medicine) and highlight the importance of innate immunity in long term control for grass carp hemorrhagic disease.

A toll-like receptor 3 homologue that is up-regulated by poly I:C and DNA virus in turbot Scophthalmus maximus

In this study, the gene and promoter sequences of turbot Scophthalmus maximus (Sm) toll-like receptor 3 (Tlr3) were cloned and its mRNA tissue distribution and gene expression in response to polyinosinic:polycytidylic acid (poly I:C) and turbot reddish body iridovirus (TRBIV) challenges were studied in vivo. The smtlr3 gene spans over 4·4 kb with a structure of five exons-four introns and encodes a peptide of 916 amino acids. The putative protein shares the highest sequence identity of 52·8-78·5% with fish Tlr3 and contains a signal peptide sequence, 13 leucine-rich repeat (LRR) motifs, a transmembrane region and a toll/interleukin-1 receptor (TIR) domain. Phylogenetic analysis grouped it with other teleost Tlr3s. A number of transcription factor binding sites were identified in the 1538 bp 5' flanking region of smtlr3, including interferon-stimulated response element (ISRE) and those for interferon regulatory factors (IRF) and signal transducer and activator of transcriptions (STATs) smtlr3 transcripts were expressed ubiquitously with higher levels in the head kidney, heart and digestion organs. They were up-regulated by both poly I:C and TRBIV in immune and non-immune organs, but most strongly in the head kidney. Finally, the smtlr3 exhibited a two-wave induced expression during a five day time course when exposure of S. maximus to poly I:C. These findings provide insights into the role of SmTlr3 in antiviral response.

Toll-like receptor recognition of bacteria in fish: ligand specificity and signal pathways

Pattern recognition receptors (PRRs) recognize the conserved molecular structure of pathogens and trigger the signaling pathways that activate immune cells in response to pathogen infection. Toll-like receptors (TLRs) are the first and best characterized innate immune receptors. To date, at least 20 TLR types (TLR1, 2, 3, 4, 5M, 5S, 7, 8, 9, 13, 14, 18, 19, 20, 21, 22, 23, 24, 25, and 26) have been found in more than a dozen of fish species. However, of the TLRs identified in fish, direct evidence of ligand specificity has only been shown for TLR2, TLR3, TLR5M, TLR5S, TLR9, TLR21, and TLR22. Some studies have suggested that TLR2, TLR5M, TLR5S, TLR9, and TLR21 could specifically recognize PAMPs from bacteria. In addition, other TLRs including TLR1, TLR4, TLR14, TLR18, and TLR25 may also be sensors of bacteria. TLR signaling pathways in fish exhibit some particular features different from that in mammals. In this review, the ligand specificity and signal pathways of TLRs that recognize bacteria in fish are summarized. References for further studies on the specificity for recognizing bacteria using TLRs and the following reactions triggered are discussed. In-depth studies should be continuously performed to identify the ligand specificity of all TLRs in fish, particularly non-mammalian TLRs, and their signaling pathways. The discovery of TLRs and their functions will contribute to the understanding of disease resistance mechanisms in fish and provide new insights for drug intervention to manipulate immune responses.

Poly(I:C) induces antiviral immune responses in Japanese flounder (Paralichthys olivaceus) that require TLR3 and MDA5 and is negatively regulated by Myd88

Polyinosinic:polycytidylic acid (poly(I:C)) is a ligand of toll-like receptor (TLR) 3 that has been used as an immunostimulant in humans and mice against viral diseases based on its ability to enhance innate and adapt immunity. Antiviral effect of poly(I:C) has also been observed in teleost, however, the underling mechanism is not clear. In this study, we investigated the potential and signaling mechanism of poly(I:C) as an antiviral agent in a model of Japanese flounder (Paralichthys olivaceus) infected with megalocytivirus. We found that poly(I:C) exhibited strong antiviral activity and enhanced activation of head kidney macrophages and peripheral blood leukocytes. In vivo studies showed that (i) TLR3 as well as MDA5 knockdown reduced poly(I:C)-mediated immune response and antiviral activity to significant extents; (ii) when Myd88 was overexpressed in flounder, poly(I:C)-mediated antiviral activity was significantly decreased; (iii) when Myd88 was inactivated, the antiviral effect of poly(I:C) was significantly increased. Cellular study showed that (i) the NF-κB activity induced by poly(I:C) was upregulated in Myd88-overexpressing cells and unaffected in Myd88-inactivated cells; (ii) Myd88 overexpression inhibited and upregulated the expression of poly(I:C)-induced antiviral genes and inflammatory genes respectively; (iii) Myd88 inactivation enhanced the expression of the antiviral genes induced by poly(I:C). Taken together, these results indicate that poly(I:C) is an immunostimulant with antiviral potential, and that the immune response of poly(I:C) requires TLR3 and MDA5 and is negatively regulated by Myd88 in a manner not involving NK-κB. These results provide insights to the working mechanism of poly(I:C), TLR3, and Myd88 in fish.

Ligand specificities of Toll-like receptors in fish: indications from infection studies

Toll like receptors (TLRs) are present in many different fish families from several different orders, including cyprinid, salmonid, perciform, pleuronectiform and gadiform representatives, with at least some conserved properties among these species. However, low conservation of the leucine-rich repeat ectodomain hinders predictions of ligand specificities of fish TLRs based on sequence information only. We review the presence of a TLR genes, and changes in their gene expression profiles as result of infection, in the context of different fish orders and fish families. The application of RT-qPCR and availability of increasing numbers of fish genomes has led to numerous gene expression studies, including studies on TLR gene expression, providing the most complete dataset to date. Induced changes of gene expression may provide (in)direct evidence for the involvement of a particular TLR in the reaction to a pathogen. Especially when findings are consistent across different studies on the same fish species or consistent across different fish species, up-regulation of TLR gene expression could be a first indication of functional relevance. We discuss TLR1, TLR2, TLR4, TLR5 and TLR9 as presumed sensors of bacterial ligands and discuss as presumed sensors of viral ligands TLR3 and TLR22, TLR7 and TLR8. More functional studies are needed before conclusions on ligands specific to (groups of) fish TLRs can be drawn, certainly true for studies on non-mammalian TLRs. Future studies on the conservation of function of accessory molecules, in conjunction with TLR molecules, may bring new insight into the function of fish TLRs.

Innate immunity of finfish: primordial conservation and function of viral RNA sensors in teleosts

During the past decade, huge progress has been made in research into teleost PAMPs (pathogen-associated molecule patterns) recognition receptors (PRRs). Numerous fish PRR genes have been identified, and the primordial functions of PRRs involved in the innate immune response to viral infection (especially those responsible for sensing viral RNA) have been increasingly clarified in teleosts. Particular progress has been made in our understanding of Toll-like receptors (TLRs) and retinoic acid inducible gene I (RIG-I)-like receptors (RLRs). However, there are important evolutionary differences between teleosts and mammals; for instance, seven TLR repertoires (TLR5S, -14, -19, -20, -21, -22 and -23) are present in teleosts but not in mammals, indicating that some TLRs likely possess different functions. Thus, comparison of PRRs in teleosts and mammals may help us understand the immune responses triggered by host-pathogen interactions in teleosts. In this article, the evolutionary conservations and divergences in the PRR mechanisms of teleosts and mammals are examined, with a focus on their molecular features and the recognition of viral RNA by fish TLRs and RLRs. In addition, the mechanism of type I interferon gene expression in teleosts, which is enhanced after the recognition of viral RNA by fish TLRs and RLRs, is also introduced.

Molecular characterization and expression analysis of Toll-like receptor 21 cDNA from Paralichthys olivaceus

Toll-like receptor (TLR) is believed to play crucial role in host defense of pathogenic microbes in innate immune system. In the present study, the full-length cDNA of Paralichthys olivaceus Toll-like receptor 21 (Po-TLR21) was cloned by homology cloning and rapid amplification of cDNA ends (RACE) technique. The Po-TLR21 cDNA sequence was 3687 bp, containing an open reading frame of 2922 bp encoding 973 amino acids. TMHMM and SMART program analysis indicated that protein contained one transmembrane domain, eighteen leucine-rich repeats (LRRs), and one Toll/IL-1 receptor homology domain (TIR). Multiple alignment analysis of the Po-TLR21 protein-coding sequence with other known TLR21 from grouper, pufferfish, zebrafish, cod, catfish, carp and chicken showed the homology of 67%, 63%, 54%, 52%, 51%, 49%, and 39%, respectively. The Po-TLR21 mRNA expression patterns were measured by real-time PCR. The results revealed that TLR21 is widely expressed in various tested healthy tissues, and highly expressed in spleen and gill. In vivo immunostimulation experiments revealed that expression of TLR21 is modulated by Vibrio anguillarum (V. anguillarum), CpG oligodeoxynucleotides (CpG ODN) and poly I:C. Moreover, the inhibitor of homodimerization of myeloid differentiation factor 88 (MyD88) could significantly reduce the up-regulation of TLR21, MyD88, and tumor necrosis factor (TNF) expression in CpG ODN or poly I:C-treated head kidney cells in vitro. These results indicate that TLR21 may be involved in the pathogen recognition in the early innate immune.

Molecular characterization and functional analysis of Toll-like receptor 3 gene in orange-spotted grouper (Epinephelus coioides)

Toll-like receptor 3 (TLR3) plays an important role in activating innate immune responses during viral infection. In this report, TLR3 (EcTLR3) was characterized and analyzed for the first time in Epinephelus coioides. The full-length EcTLR3 cDNA is predicted to encode a 909 amino acid polypeptide that contains a signal peptide sequence, 18 leucine-rich repeat (LRR) motifs, a transmembrane region and a Toll/interleukin-1 receptor (TIR) domain. Quantitative real-time PCR revealed that the EcTLR3 mRNA was much more abundant in the liver than in other immune organs, and that the expression levels were very low in hemocyte and muscle. During development of the grouper, the levels of EcTLR3 transcripts increased with age, with very low expression levels at the early stages of development. EcTLR3 mRNA levels were examined in the liver at different times after treatment with polyriboinosinic polyribocytidylic acid (Poly I:C), and in nervous necrosis virus (NNV)-infected larval groupers. The results suggested that EcTLR3 plays an important role in a fish's defense against viral infection.

Trunk kidney of grass carp (Ctenopharyngodon idella) mediates immune responses against GCRV and viral/bacterial PAMPs in vivo and in vitro

Trunk kidney is a vital organ for excretion in teleosts. There have been sporadic reports of processing pathogens for the immune function in trunk kidney. However, molecular processes of pathogen recognition receptors (PRRs) responding to virus and viral/bacterial pathogen-associated molecular patterns (PAMPs) are poorly elucidated in trunk kidney. In the present study, we investigated transcriptional profiles of twelve representative immune-related genes (TLRs (TLR3, TLR7 and TLR22); RLRs (RIG-I, MDA5 and LGP2); NLRs (NOD1 and NOD2); adapter molecules (MyD88 and IPS-1); effector molecule type I interferon (IFN-I) and immunoglobulin M (IgM)) in trunk kidney tissue of grass carp (Ctenopharyngodon idella) (designated as Ci) injection of grass carp reovirus (GCRV) utilizing quantitative real-time RT-PCR (qRT-PCR). Furthermore, mRNA expression patterns of these genes (IgM excepted) were examined post GCRV infection and polyinosine-polycytidylic acid (poly(I:C)), lipopolysaccharide (LPS) or peptidoglycan (PGN) stimulation in primary trunk kidney cells of grass carp. The relative values of CiTLR3, CiTLR22 and CiMyD88 were increased post GCRV challenge and viral/bacterial PAMPs stimulation. The mRNA transcriptions of CiTLR7 were obviously activated with GCRV challenge. Remarkably, the mRNA expressions of CiRIG-I, CiMDA5, CiLGP2 and CiIPS-1 were largely up-regulated with GCRV challenge and viral/bacterial PAMPs stimulation. Interestingly, the expression tendencies of CiNOD1 and CiNOD2 were differential not only in GCRV challenge and poly(I:C) stimulation, but also in LPS and PGN stimulation. It was demonstrated that CiIFN-I induced powerful anti-viral and anti-bacterial effects in trunk kidney. In addition, the expression of CiIgM was induced at 72 h post GCRV injection in vivo. Collectively, these results suggest that trunk kidney of grass carp serves as an important immune organ, and plays crucial roles in triggering anti-viral and anti-bacterial immune responses both in vivo and in vitro.

Identification and expression analysis of two Toll-like receptor genes from sea cucumber (Apostichopus japonicus)

Toll-like receptors (TLRs) are a family of type I integral membrane glycoproteins which play pivotal roles in innate immunity. In this study, two TLRs named AjTLR3 and AjToll were cloned from sea cucumber (Apostichopus japonicus). The full-length cDNA sequences of AjTLR3 and AjToll are 3484 bp and 4211 bp, with an open reading frame (ORF) of 2679 bp and 2853 bp, encoding 892 and 950 amino acids, respectively. Both AjTLR3 and AjToll are composed of a leucine-rich repeat (LRR) domain, a transmembrane (TM) domain and an intracellular Toll/interleukin-1 receptor (TIR) domain. Evolution analysis revealed that AjTLR3 and AjToll were clustered with the vertebrate-like TLRs (V-TLRs) and the protostome-like TLRs (P-TLRs), respectively. These two genes were widely expressed in all five tested tissues (body wall, coelomocytes, tube feet, intestine and respiratory tree), but showed different expression patterns. The significantly up-regulated expressions of AjTLR3 and AjToll after peptidoglycan (PGN), lipopolysaccharides (LPS), Zymosan A and polyinosinic-polycytidylic acid (PolyI:C) challenges suggested that they were functionally involved in the immune responses to the Cram-positive bacteria, Gram-negative bacteria, fungi and double-stranded RNA (dsRNA) viruses, respectively.

Molecular cloning and characterization of Toll-like receptor 3, and inductive expression analysis of type I IFN, Mx and pro-inflammatory cytokines in the Indian carp, rohu (Labeo rohita)

Toll-like receptors (TLRs) are one of the key components of innate or non-specific immunity. Among various types of TLRs, TLR3 recognizes dsRNA, the genetic material or replicative intermediate of many RNA viruses and triggers TIR-domain-containing adapter-inducing interferon-β dependent signalling pathway to induce type I interferon (IFN) and pro-inflammatory cytokines. In this study, we cloned and characterized full-length TLR3 cDNA in rohu (Labeo rohita), that comprised of 2,619 bp nucleotides encoding a putative protein of 873 amino acid with the estimated molecular mass of 98.57 kDa. The constitutive expression of TLR3 gene was detected in all embryonic developmental stages and in various organs/tissues of rohu fingerlings. In vivo tissue specific modulation of TLR3, type I IFN, Mx (myxovirus-resistant protein) and pro-inflammatory cytokines (TNF-α and IL-1β) gene expression were analysed by quantitative real-time PCR following intravenous injection of polyinosinic-polycytidylic acid (poly I:C), a synthetic analogue of viral dsRNA. A significant relationship of TLR3 induction, and type I IFN, Mx, IL-1β and TNF-α gene expression were observed in majority of the treated fish tissues, as compared to their control. Together, these data highlight the important role of TLR3 in recognizing dsRNA, and in augmenting the innate immunity in fish in response to viral infections.

Molecular regulation of interferon antiviral response in fish

Interferon (IFN) response is the first line of host defense against virus infection. The recent years have witnessed tremendous progress in understanding of fish IFN antiviral response. Varied number of IFN genes has been identified in different fish species but obviously, they do not show a one-to-one orthologous relationship with mammalian IFN homologs. These genes are divided into two groups with different abilities to induce downstream gene expression through binding to different receptor complexes. Consistently, some fish IFN-stimulated genes such as Mx and PKR have been confirmed for their antiviral effects. In this review, we focus on how fish cells respond to IFNs and how fish IFNs are triggered through TLR pathway and RLR pathway. We highlight the roles of IRF3 and IRF7 in activation of fish IFN response. In addition, the unique mechanisms underlying IRF3/7-dependent fish IFN response and auto-regulation of fish IFN gene expression are discussed.

Structural insights of rohu TLR3, its binding site analysis with fish reovirus dsRNA, poly I:C and zebrafish TRIF

In response to double stranded RNA (dsRNA) viruses, toll-like receptor 3 (TLR3) in fish activates signaling like human, and induces innate immunity. This suggested the existence of dsRNA binding domains in fish TLR3 as reported in higher vertebrates. In in silico analysis, leucine rich repeat (LRR) regions (4-6, 13-14, 20-22), and LRR (8-15, 17-24) were identified as key domains in rohu TLR3 as poly I:C and dsRNA of fish reovirus (AGCRV,VHSV and IHNV) binding regions. 3D-models of rohu TLR3-TIR and zebrafish TRIF were generated by homology and ab initio modeling respectively, and their interacting domains were predicted. This is the first report of TLR3 modeling in fish.