Identification and characterization of TLR8 and MyD88 homologs in Atlantic salmon (Salmo salar)

Transcriptomics of early responses to purified Piscine orthoreovirus-1 in Atlantic salmon (Salmo salar L.) red blood cells compared to non-susceptible cell lines

Piscine red blood cells (RBC) are nucleated and have been characterized as mediators of immune responses in addition to their role in gas exchange. Salmonid RBC are major target cells of Piscine orthoreovirus-1 (PRV-1), the etiological agent of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar). PRV-1 replicates in RBC ex vivo, but no viral amplification has been possible in available A. salmon cell lines. To compare RBC basal transcripts and transcriptional responses to PRV-1 in the early phase of infection with non-susceptible cells, we exposed A. salmon RBC, Atlantic salmon kidney cells (ASK) and Salmon head kidney cells (SHK-1) to PRV-1 for 24 h. The RNA-seq analysis of RBC supported their previous characterization as pluripotent cells, as they expressed a wide repertoire of genes encoding pattern recognition receptors (PRRs), cytokine receptors, and genes implicated in antiviral activities. The comparison of RBC to ASK and SHK-1 revealed immune cell features exclusively expressed in RBC, such as genes involved in chemotactic activity in response to inflammation. Differential expression analysis of RBC exposed to PRV-1 showed 46 significantly induced genes (≥ 2-fold upregulation) linked to the antiviral response pathway, including RNA-specific PRRs and interferon (IFN) response factors. In SHK-1, PRV induced a more potent or faster antiviral response (213 genes induced). ASK cells showed a differential response pattern (12 genes induced, 18 suppressed) less characterized by the dsRNA-induced antiviral pathway. Despite these differences, the RIG-I-like receptor 3 (RLR3) in the family of cytosolic dsRNA receptors was significantly induced in all PRV-1 exposed cells. IFN regulatory factor 1 (IRF1) was significantly induced in RBC only, in contrast to IRF3/IRF7 induced in SHK-1. Differences in IRF expression and activity may potentially affect viral propagation.

Ablation of myd88 alters the immune gene expression and immune cell recruitment during VHSV infection in zebrafish

Myeloid differentiation primary response protein-88 (MYD88) is an essential adaptor molecule in pathogen-related pattern recognition signaling pathways. Toll-like and interleukin receptors recognize numerous signals and are funneled through MyD88 to express genes responsible for the innate and adaptive immune systems. In the present study, the relevance of MyD88 in viral hemorrhagic septicemia virus (VHSV) was investigated by generating myd88-/- zebrafish. The model was challenged with VHSV, and viral propagation was quantified by evaluating clinical symptoms, mortality, and VHSV copy number. The infected fish showed abnormal morphologies, such as subcutaneous hemorrhages, abdominal swelling, and bulging eyes, which were comparatively more intense in myd88-/- fish than in the wild-type. An injury infection experiment conducted in zebrafish larvae indicated a substantial spread of VHSV in the wound site. The number of neutrophils and macrophages recruited to the wounded area were markedly reduced in myd88-/- fish. According to gene expression analysis, VHSV NP gene expression was considerably upregulated in myd88-/- fish. Substantial gene expression and immune cell marker modulation were observed in the mutant model compared to that in the wild-type. These results suggest that the lack of a significant adaptor protein for immune signal transduction results in enhanced VHSV replication.

Molecular Cloning of Toll-like Receptor 2 and 4 (SpTLR2, 4) and Expression of TLR-Related Genes from Schizothorax prenanti after Poly (I:C) Stimulation

Toll-like receptor (TLR) signaling is conserved between fish and mammals, except for TLR4, which is absent in most fish. In the present study, we aimed to evaluate whether TLR4 is expressed in Schizothorax prenanti (SpTLR4). The SpTLR2 and SpTLR4 were cloned and identified, and their tissue distribution was examined. The cDNA encoding SpTLR4 and SpTLR2 complete coding sequences (CDS) were identified and cloned. Additionally, we examined the expression levels of seven SpTLRs (SpTLR2, 3, 4, 18, 22-1, 22-2, and 22-3), as well as SpMyD88 and SpIRF3 in the liver, head kidney, hindgut, and spleen of S. prenanti, after intraperitoneal injection of polyinosinic-polycytidylic acid (poly (I:C)). The SpTLR2 and SpTLR4 shared amino acid sequence identity of 42.15-96.21% and 36.21-93.58%, respectively, with sequences from other vertebrates. SpTLR2 and SpTLR4 were expressed in all S. prenanti tissues examined, particularly in immune-related tissues. Poly (I:C) significantly upregulated most of the genes evaluated in the four immune organs compared with the PBS-control (p < 0.05); expression of these different genes was tissue-specific. Our findings demonstrate that TLR2 and TLR4 are expressed in S. prenanti and that poly (I:C) affects the expression of nine TLR-related genes, which are potentially involved in S. prenanti antiviral immunity or mediating pathological processes with differential kinetics. This will contribute to a better understanding of the roles of these TLR-related genes in antiviral immunity.

Salmonid alphavirus non-structural protein 2 is a key protein that activates the NF-κB signaling pathway to mediate inflammatory responses

Salmonid alphavirus (SAV) infection of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) causes pancreas disease (PD) with typical inflammatory responses, such as necrosis of the exocrine pancreas, cardiomyopathy and skeletal myopathy. However, the pathogenic mechanism underlying SAV infection is still unclear. Inflammation may cause damage to the body, but it is a defense response against infection by pathogenic microorganisms, of which nuclear factor-kappa B (NF-κB) is the main regulator. This study revealed that SAV can activate NF-κB, of which the viral nonstructural protein Nsp2 is the major activating protein. SAV activates the NF-κB signaling pathway by simultaneously up-regulating TLR3, 7, 8 and then the expression of the signaling molecule myeloid differentiation factor 88 (Myd88) and tumor necrosis factor receptor-associated factor 6 (TRAF6). We found that Nsp2 can induce IκB degradation and p65 phosphorylation and transnucleation, and activate NF-κB downstream inflammatory cytokines. Nsp2 may simultaneously activate NF-κB through TLR3,7,8-dependent signaling pathways. Overexpression of Nsp2 can up-regulate mitochondrial antiviral signaling protein (MAVS) and then promote the expression of IFNa1 and antiviral protein Mx, which inhibits viral replication. This study shows that Nsp2 acts as a key activator protein for the NF-κB signaling pathway, which induces inflammation post-SAV infection. This study systematically analyzes the molecular mechanism of SAV activation of the NF-κB signaling pathway, and provides a theoretical basis for revealing the mechanism of innate immune response and inflammatory injury caused by SAV.

Salmonid MyD88 is a key adapter protein that activates innate effector mechanisms through the TLR5M/TLR5S signaling pathway and protects against Piscirickettsia salmonis infection

The membrane-anchored and soluble Toll-like Receptor 5 -TLR5M and TLR5S, respectively-from teleost recognize bacterial flagellin and induce the pro-inflammatory cytokines expression in a MyD88-dependent manner such as the TLR5 mammalian orthologous receptor. However, it has not been demonstrated whether the induced signaling pathway by these receptors activate innate effector mechanisms MyD88-dependent in salmonids. Therefore, in this work we study the MyD88 dependence on the induction of TLR5M/TLR5S signaling pathway mediated by flagellin as ligand on the activation of some innate effector mechanisms. The intracellular and extracellular Reactive Oxygen Species (ROS) production and conditioned supernatants production were evaluated in RTS11 cells, while the challenge with Piscirickettsia salmonis was evaluated in SHK-1 cells. Our results demonstrate that flagellin directly stimulates ROS production and indirectly stimulates it through the production of conditioned supernatants, both in a MyD88-dependent manner. Additionally, flagellin stimulation prevents the cytotoxicity induced by infection with P. salmonis in a MyD88-dependent manner. In conclusion we demonstrate that MyD88 is an essential adapter protein in the activation of the TLR5M/TLR5S signaling pathway mediated by flagellin in salmonids, which leads downstream to the induction of innate effector mechanisms, promoting immuno-protection against a bacterial challenge with P. salmonis.

Interleukin 1 receptor type I (IL-1RI) is involved in the innate immune response of olive flounder (Paralichthys olivaceus) to resist pathogens

The pleiotropic cytokine IL -1 is involved in important immune responses such as thymocyte proliferation and B cell growth and differentiation. Activation of the IL -1 pathway requires its functional receptor IL -1RI, making IL -1RI the critical point of the IL -1 pathway. In-depth study of IL -1RI will help to understand the immune mechanism involved in IL -1. In this study, we identified the cDNA of the IL -1RI gene of olive flounder (PoIL-1RI). The total length of the PoIL-1RI cDNA is 2490 bp, the open reading frame is 1689 bp long and encodes a protein of 562 amino acids. The protein has three Ig domains and a typical TIR domain, as in other mammals and fish. We found that PoIL-1RI is widely expressed in the tissues studied and shows a significant immune response after stimulation with bacteria and pathogen-associated molecular patterns (PAMPs) both in vitro and in vivo. After PoIL-1RI was overexpressed in olive flounder embryonic cell line (FEC), pro-inflammatory cytokines (IL -1β, IL -6, IL -8, TNF-α) and interferon (IFN-α, IFN-γ) were significantly upregulated. And we found that after overexpressing PoIL-1RI in FEC, the antibacterial ability of FEC was significantly stronger than that of the control group, and we found that overexpression of PoIL-1RI gene significantly increased the activity of NF-κB signaling pathway. These results suggest that PoIL-1RI plays an important role in innate immune response.

Ctenopharyngodon idella Tollip regulates MyD88-induced NF-κB activation

Toll-interacting protein (Tollip) and MyD88 are key components of the TLR/IL-1R signaling pathway in mammals. MyD88 is known as a universal adaptor protein involving in TLR/IL-1R-induced NF-κB activation. Tollip is a crucial negative regulator of TLR-mediated innate immune responses. Previous studies have demonstrated that teleost Tollip served as a negative regulator of MyD88-dependent TLR signaling pathway. However, the mechanism is still unclear. In particular, the effect of TBD, C2, and CUE domains of Tollip on MyD88-NF-κB signaling pathway remains to be elucidated. In this study, we found that the response of grass carp Tollip (CiTollip) to LPS stimulation was faster and stronger than that of poly I:C treatment, and CiTollip diminished the expression of tnf-α induced by LPS. Further assays indicated that except for the truncated mutant of △CUE2 (1-173 aa), wild type CiTollip and other truncated mutants (△N-(52-276 aa), △C2-(173-276 aa) and △CUE1-(1-231 aa)) could associate with MyD88 and negatively regulate MyD88-induced NF-κB activation. It suggested that the C-terminal (173-276 aa), in particular the connection section between C2 and CUE domains (173-231 aa), played a pivotal role in suppressing MyD88-induced activation of NF-κB.

R848 Is Involved in the Antibacterial Immune Response of Golden Pompano (Trachinotus ovatus) Through TLR7/8-MyD88-NF-κB-Signaling Pathway

R848 is an imidazoquinoline compound that is a specific activator of toll-like receptor (TLR) 7/8 and is often used in immunological research in mammals and teleosts. However, the immune responses initiated by R848 through the TLR7/8 pathway in response to bacterial infection remain largely unexplored in teleosts. In the current study, we investigated the antibacterial response and the participating signaling pathway initiated by R848 in golden pompano (Trachinotus ovatus). We found that R848 could stimulate the proliferation of head kidney lymphocytes (HKLs) in a dose-dependent manner, enhance the survival rate of HKLs, and inhibit the replication of bacteria in vivo. However, these effects induced by R848 were significantly reduced when chloroquine (CQ) was used to blocked endosomal acidification. Additionally, an in vivo study showed that R848 strengthened the antibacterial immunity of fish through a TLR7/8 and Myd88-dependent signaling pathway. A cellular experiment showed that Pepinh-MYD (a Myd88 inhibitor) significantly reduced the R848-mediated proliferation and survival of HKLs. Luciferase activity analysis showed that R848 enhanced the nuclear factor kappa B (NF-κB) activity, whereas this activity was reduced when CQ and Pepinh-MYD were present. Additionally, when an NF-κB inhibitor was present, the R848-mediated pro-proliferative and pro-survival effects on HKLs were significantly diminished. An in vivo study showed that knockdown of TLR7, TLR8, and Myd88 expression in golden pompano via siRNA following injection of R848 resulted in increased bacterial dissemination and colonization in fish tissues compared to that of fish injection of R848 alone, suggesting that R848-induced antibacterial immunity was significantly reduced. In conclusion, these results indicate that R848 plays an essential role in the antibacterial immunity of golden pompano via the TLR7/8-Myd88-NF-κB- signaling pathway.

Infection and microbial molecular motifs modulate transcription of the interferon-inducible gene ifit5 in a teleost fish

Interferon-induced proteins with tetratricopeptide repeats (IFITs) are involved in antiviral defense. Members of this protein family contain distinctive multiple structural motifs comprising tetratricopeptides that are tandemly arrayed or dispersed along the polypeptide. IFIT-encoding genes are upregulated by type I interferons (IFNs) and other stimuli. IFIT proteins inhibit virus replication by binding to and regulating the functions of cellular and viral RNA and proteins. In teleost fish, knowledge about genes and functions of IFITs is currently limited. In the present work, we describe an IFIT5 orthologue in Atlantic salmon (SsaIFIT5) with characteristic tetratricopeptide repeat motifs. We show here that the gene encoding SsaIFIT5 (SsaIfit5) was ubiquitously expressed in various salmon tissues, while bacterial and viral challenge of live fish and in vitro stimulation of cells with recombinant IFNs and pathogen mimics triggered its transcription. The profound expression in response to various immune stimulation could be ascribed to the identified IFN response elements and binding sites for various immune-relevant transcription factors in the putative promoter of the SsaIfit5 gene. Our results establish SsaIfit5 as an IFN-stimulated gene in A. salmon and strongly suggest a phylogenetically conserved role of the IFIT5 protein in antimicrobial responses in vertebrates.

Molecular cloning and expression analysis of myd88 from oriental weatherfish (Misgurnus anguillicaudatus) in response to bacterial challenge

Myeloid differentiation factor 88 (Myd88) plays an important role in both innate and adaptive immune response. In this study, the full-length complementary DNA (cDNA) of myd88 from Misgurnus anguillicaudatus was characterized. The myd88 cDNA is 1333 bp in length and contains an 855 bp open reading frame encoding a predicted protein of 284 amino acids. The predicted protein possesses typical Myd88 domain structural features including a death domain in the N-terminus, and box 1, 2, and 3 motifs of the Toll/IL-1 receptor domain in the C-terminus. Quantitative real-time PCR (qRT-PCR) revealed that myd88 messenger RNA (mRNA) was ubiquitously expressed in all examined tissues, especially highly in brain, kidney, blood, intestines and liver. qRT-PCR and western blotting were used to determine the mRNA and protein levels of Myd88 after Aeromonas veronii challenge, respectively. The Myd88 was remarkably upregulated in response to infection of A. veronii. These results suggested that Myd88 may play a vital role during the immune response of M. anguillicaudatus against bacterial infection.

Intracellular distribution and transcriptional regulation of Atlantic salmon (Salmo salar) Rab5c, 7a and 27a homologs by immune stimuli

Rab GTPases control trafficking of intracellular vesicles and are key regulators of endocytic and secretory pathways. Due to their specific distribution, they may serve as markers for different endolysosomal compartments. Since Rab GTPases are involved in uptake and trafficking of endocytosed ligands and cell receptors, as well as secretion of immune mediators, they have been implicated in diverse immunological processes and their functions are often exploited by intracellular pathogens such as viruses. While Rab proteins have been studied extensively in mammals, their functions in vesicle trafficking in teleosts are not well known. In the present work, Atlantic salmon Rab5c, Rab7a and Rab27a homologs were studied in terms of intracellular distribution and gene expression. Structured illumination microscopy demonstrated that transgenic, GFP-tagged salmon Rab5c and Rab7a are, predominantly, located within early endosomes and late endosomes/lysosomes, respectively. In contrast, Rab27a showed a broader distribution, which indicates that it associates with diverse intracellular vesicles and organelles. Infection of salmon with Salmonid alphavirus subtype 3 (SAV3) enhanced the mRNA levels of all of the studied Rab isoforms in heart and head kidney and most of them were upregulated in spleen. This may reflect the capacity of the virus to exploit the functions of these rab proteins. It is also possible that the transcriptional regulation of Rab proteins in SAV3-infected organs may play a role in the antiviral immune response. The latter was further supported by in vitro experiments with adherent head kidney leukocytes. The expression of Rab5c and Rab27a was upregulated in these cells following stimulation with TLR ligands including CpG oligonucleotides and polyI:C. The expression of most of the analyzed Rab isoforms in the primary leukocytes was also enhanced by stimulation with type I IFN. Interestingly, IFN-gamma had a negative effect on Rab7a expression which may be linked to the priming activity of this cytokine on monocytes and macrophages. Overall, these data demonstrate that the intracellular distribution of Rab5c, Rab7a and Rab27a is phylogenetically conserved within vertebrates and that these molecules might be implicated in viral infections and the regulation of the antiviral immune response in Atlantic salmon.

Characterization and expression analysis of rockfish (Sebastes schlegelii) myeloid differentiation factor-88 (SsMyD88) and evaluation of its ability to induce inflammatory cytokines through NF-ĸB

Innate immunity is characterized by nonspecific, prompt reactions toward armada of antigens. Animals funnel down a repertoire of immune stimulants to activate non-selective defense mechanisms rapidly. This study was conducted to characterize the rockfish (Sebastes schlegelii) adaptor protein MyD88 (SsMyD88), which interacts with both toll-like receptors and interleukin receptors. The tissue expression of unchallenged SsMyD88 was evaluated by quantitative real time PCR (qPCR). Fish were intraperitoneally injected with immune stimulants including poly I:C, lipopolysaccharides, and Streptococcus iniae. Then, the temporal expression of SsMyD88 was analyzed. Finally, the inflammatory gene expression and downstream promoter activation were analyzed. Strongest expressions were reported in the liver, gills and spleen in unchallenged conditions. All diverse immune stimulants were found to be capable of significantly altering SsMyD88 transcription during the challenge experiment. Evaluation of downstream promoter biases by SsMyD88 found a predominant activation of NF-ĸB transcription factors when compared with the AP-1, revealing significant and substantial upregulation of major inflammatory mediators such as IL-1-β, IL-6, iNOS, COX-2 and TNF-α. Fluorescent detection confirmed an intense production of NO and the predominant differentiation of macrophages into M1 lineage with the overexpression of SsMyD88 in vitro. These results further corroborate the role of SsMyD88 as a mediatory molecule that bridges distinct immune stimulants to induce drastic immune responses in fish.

Characterization, subcellular localization and function analysis of myeloid differentiation factor 88 (Pt-MyD88) in swimming crab, Portunus trituberculatus

Myeloid differentiation factor 88 (MyD88) is a universal and essential adaptor protein required for the Toll-like receptors (TLRs) pathway activation in invertebrates as well as in vertebrates. Herein, we characterized a MyD88 (Pt-MyD88) cDNA sequence in the swimming crab (Portunus trituberculatus). The Pt-MyD88 ORF is predicted to encode 469 peptides with an N-terminal death domain and a typical C-terminal TIR domain. Real-Time quantitative PCR analysis showed that the Pt-MyD88 transcriptions were constitutively expressed in hemocytes, gill, intestine, heart and muscle in normal crab. The expressions of Pt-MyD88 would be down-regulated by V. alginolyticus or LPS challenge, and be up-regulated by WSSV infection in hemocytes. Intracellular localization showed Pt-MyD88 was distributed mainly in the cytoplasm when it was over-expressed in human cell HEK293T or in Drosophila Schneider 2 (S2). Functionally, over-expression of Pt-MyD88 could either activate the NF-κB in HEK293T cells or activate the promoters of Drosophila antimicrobial peptide genes (AMPs) in S2 cell. In primary cultured hemocytes of swimming crab, after Pt-MyD88 was knocked-down by specific long double strand RNA, the expression of anti-lipopolysaccharide factor1 (ALF1), hyastatin3, crustin1 and crustin3 have been significantly inhibited, while the expression of other AMPs is normal compared to non-specific dsRNA treated cells.

Myeloid differentiation factor 88 (Myd88) is involved in the innate immunity of black carp (Mylopharyngodon piceus) defense against pathogen infection

Myeloid differentiation factor 88 (MyD88) is an important transduction protein in the Toll-like receptor signaling pathway. In this study, we identified the cDNA of the MpMyD88 gene in black carp. We found that MpMyD88 was widely distributed in the tissues tested and showed significant immune responses both in vitro and in vivo after stimulation with bacterial and pathogen-associated molecular patterns. After MpMyD88 overexpression/silencing, proinflame-matory cytokines (TNF-α, IFN-α, IL-6, and IL-8) also showed significant up-regulation/down-regulation. Moreover, we found that the antibacterial ability of cells over-expressing MpMyD88 was significantly stronger than that of control cells, while that of silenced MpMyD88 was significantly lower than that in control cells. Besides, we found that the overexpression of MpMyD88 significantly increased the activity of NF-κB. These results indicate that MpMyD88 plays an important role in the innate immune response.

Northeast Chinese lamprey (Lethenteron morii) MyD88: Identification, expression, and functional characterization

Myeloid differentiation factor 88 (MyD88) is a key adaptor of Toll-like receptors (TLR), an important pattern recognition receptor of the innate immune system. To study the origin and evolution of the vertebrate TLR signaling pathway in innate immune systems, we analyzed the biological characteristics and functions of the MyD88 gene in Northeast Chinese lamprey (Lethenteron morii) using PCR amplification, real-time PCR analysis, dual luciferase reporter gene assay, immunofluorescence assay, and other methods. Bioinformatics analysis showed that LmMyD88 has a modular structure consisting of Toll/IL-1R domain (TIR) and death domain (DD), which is typical of the MyD88 family. A phylogenetic tree showed that the homology of LmMyD88 was consistent with the phylogenetic status of lampreys. Tissue expression analysis indicated that the mRNA expression was expressed in some normal tissues of larval and adult L. morii. Real-time PCR analysis showed that the expression of LmMyD88 in tissues, such as gill and kidney, of the adult increased significantly after infection by Pseudomonas aeruginosa. Subcellular localization results showed that LmMyD88 was expressed in the nucleus, cytoplasm, and other parts. A dual luciferase reporter assay indicated that LmMyD88 activated nuclear factor kappa B downstream of the TLR signaling pathway. This study suggested that LmMyD88 might play an important role in the innate immune signal transduction process of L. morii.

At Least Two Genes Encode Many Variants of Irak3 in Rainbow Trout, but Neither the Full-Length Factor Nor Its Variants Interfere Directly With the TLR-Mediated Stimulation of Inflammation

The interleukin-1-receptor-associated kinase 3 (IRAK3) is known in mammals as a negative feedback regulator of NF-κB-mediated innate-immune mechanisms. Our RNA-seq experiments revealed a prototypic 1920-nt sequence encoding irak3 from rainbow trout (Oncorhynchus mykiss), as well as 20 variants that vary in length and nucleotide composition. Based on the DNA-sequence information from two closely related irak3 genes from rainbow trout and an irak3-sequence fragment from Atlantic salmon retrieved from public databases, we elucidated the underlying genetic causes for this striking irak3 diversity. Infecting rainbow trout with a lethal dose of Aeromonas salmonicida enhanced the expression of all variants in the liver, head kidney, and peripheral blood leucocytes. We analyzed the functional impact of the full-length factor and selected structural variants by overexpressing them in mammalian HEK-293 cells. The full-length factor enhanced the basal activity of NF-κB, but did not dampen the TLR2-signaling-induced levels of NF-κB activation. Increasing the basal NF-κB-activity through Irak3 apparently does not involve its C-terminal domain. However, more severely truncated factors had only a minor impact on the activity of NF-κB. The TLR2-mediated stimulation did not alter the spatial distribution of Irak3 inside the cells. In salmonid CHSE-214 cells, we observed that the Irak3-splice variant that prominently expresses the C-terminal domain significantly quenched the stimulation-dependent production of interleukin-1β and interleukin-8, but not the production of other immune regulators. We conclude that the different gene and splice variants of Irak3 from trout play distinct roles in the activation of immune-regulatory mechanisms.

Characterization, evolution, and expression analysis of TLR7 gene subfamily members in Mastacembelus armatus (Synbranchiformes: Mastacembelidae)

TLR7 subfamily members are important pattern recognition receptors participating in the recognition of pathogen-associated molecular patterns. In this study, we successfully identified 3 members of TLR7 subfamily from the spiny eel Mastacembelus armatus (MaTLR7, MaTLR8 and MaTLR9). The amino acid sequence identities of MaTLR7 and MaTLR8 with Monopterus albus TLR7 were 87.2% and 76.5%, respectively and the identity of MaTLR9 with Seriola lalandi TLR9 was 74.7%. The phylogenetic analysis revealed MaTLRs showed close relationship to other species in Synbranchiformes or Perciformes. Quantitative real-time PCR analysis revealed that they were expressed in all tested tissues and higher expression was found in spleen or gill. After infection with Aeromonas veronii, expression of MaTLR7, MaTLR8 and MaTLR9 were all significantly downregulated in spleen and kidney. Evolutionary analysis suggested that the ancestral lineages of teleost TLR8 and TLR9 had been subject to positive selection pressures and multiple Maximum likelihood methods recovered 3 positively selected sites in teleost TLR7, 4 in TLR8 and 8 in TLR9. Domain distribution revealed most positively selected sites were located in leucine-rich repeat domain. Our results will contribute to better understanding the antibacterial mechanism of TLRs and their co-evolution with pathogens.

Spotted knifejaw (Oplegnathus punctatus) MyD88: Intracellular localization, signal transduction function and immune responses to bacterial infection

Myeloid differentiation factor 88 (MyD88) links members of the toll-like receptor (TLR) and interleukin-1 receptor (IL-1R) superfamily to the downstream activation of NF-κB as a "bridge" molecular in response to exogenous pathogen, but the function in spotted knifejaw (Oplegnathus. punctatus), a commercial fish in China, is still unknown. We present a functional analysis of spotted knifejaw MyD88 (OppMyD88) with a typical death domain (DD) at the N-terminus and a conservative Toll/IL-1R (TIR) domain at the C-terminus and suggest that MyD88 is important for the activation of TLR-mediated NF-κB with the synergy between domains. Subcellular localization showed that OppMyD88 was distributed in the cytoplasm in a condensed form. Tissues expression profiling analysis showed that OppMyD88 ubiquitously expressed in all tested tissues with the highest expression in the liver, as determined by real-time PCR. The expression of OppMyD88 significantly upregulated in the liver, spleen, kidney and gills within 120 h post Vibrio anguillarum infection. Moreover, we further confirmed that over-expressed OppMyD88 could also induce apoptosis. These results indicate that OppMyD88 might possess important roles in defense against microbial infection and other biological processes in spotted knifejaw similar to those in mammals, which will deepen our understandings in innate immunity of spotted knifejaw.

Antiviral defense in salmonids - Mission made possible?

Viral diseases represent one of the major threats for salmonid aquaculture. Survival from viral infections are highly dependent on host innate antiviral immune defense, where interferons are of crucial importance. Neutralizing antibodies and T cell effector mechanisms mediate long-term antiviral protection. Despite an immune cell repertoire comparable to higher vertebrates, farmed fish often fail to mount optimal antiviral protection. In the quest to multiply and spread, viruses utilize a variety of strategies to evade or escape the host immune system. Understanding the specific interplay between viruses and host immunity at depth is crucial for developing successful vaccination and treatment strategies in mammals. However, this knowledge base is still limited for pathogenic fish viruses. Here, we have focused on five RNA viruses with major impact on salmonid aquaculture: Salmonid alphavirus, Infectious salmon anemia virus, Infectious pancreatic necrosis virus, Piscine orthoreovirus and Piscine myocarditis virus. This review explore the protective immune responses that salmonids mount to these viruses and the existing knowledge on how the viruses counteract and/or bypass the immune response, including their IFN antagonizing effects and their mechanisms to establish persisting infections.

Carp Toll-like receptor 8 (Tlr8): An intracellular Tlr that recruits TIRAP as adaptor and activates AP-1 pathway in immune response

Toll-like receptor 8 (Tlr8) is a member of intracellular TLRs family and play a critical role in the innate immunity. In the present study, we aimed to identify tlr8 from common carp (Cyprinus carpio L.), and explored its expression profile, localization, adaptor, and signaling pathways. A novel tlr8 cDNA sequence (Cctlr8) was identified from the carp, containing a signal peptide, a LRR-N-terminal (LRR-NT), 14 leucine-rich repeats, a LRR-C-terminal (LRR-CT), a transmembrane region and a TIR domain. Phylogenetic analysis revealed that CcTlr8 exhibited closest relationship to that of Ctenopharyngodon idella and Danio. rerio. Subcellular localization analysis indicated that CcTlr8 was localized to the endoplasmic reticulum in both HeLa cells and EPC cells. Quantitative Real-Time PCR analysis demonstrated that Cctlr8 was constitutively expressed in all the examined tissues, with the highest expression observed in the spleen. After poly (I:C) injection, the expression of Cctlr8 was significantly up-regulated in all the tested tissues. In addition, the expression of Cctlr8 was up-regulated in both PBLs and HKLs following poly (I:C) stimulation. The results of immuofluorescence and coimmunoprecipitation analysis indicated that CcTlr8 might recruit TIRAP as the adaptor. Furthermore, Luciferase reporter assays revealed that CcTlr8 could activate AP-1 in 293 T cells. Taken altogether, these findings lay the foundations for future research to investigate the mechanisms underlying fish tlr8.

A myeloid differentiation factor 88 gene from yellow catfish Pelteobagrus fulvidraco and its molecular characterization in response to polyriboinosinic polyribocytidylic acid and lipopolysaccharide challenge

Myeloid differentiation factor 88 (MyD88) is an adaptor protein of Toll-like receptor (TLR) signalling pathways that activates the innate immune system. Herein, MyD88 was identified in the economically important freshwater fish Pelteobagrus fulvidraco. The complete 2156 bp PfMyD88 cDNA includes a 147 bp 5'-untranslated region (UTR), a 1133 bp 3'-UTR, and an open reading frame (ORF) of 876 bp encoding a 291 residue protein containing Death and Toll/interleukin-1 receptor (TIR) domains. The deduced protein sequence shares 88.8%, 73.8% and 59.3% identity with orthologs in Ictalurus punctatus, Danio rerio and Homo sapiens, respectively. qRT-PCR revealed expression in all tested tissues, highest in trunk kidney, followed by spleen, and lowest in muscle. After challenge with lipopolysaccharide (LPS) or polyriboinosinic polyribocytidylic acid (Poly I:C), PfMyD88 expression was up-regulated in blood, liver, head kidney and spleen. Thus, PfMyD88 acts in innate immunity in P. fulvidraco.

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.

Comparison of polymeric immunoglobulin receptor between fish and mammals

Polymeric immunoglobulin receptor (pIgR) functions in transporting polymeric immunoglobulin across epithelial cells into external secretion in animals. During animal evolution, fish was situated at a transition point on the phylogenetic spectrum between species possessing only innate immunity (i.e., invertebrates) and species depending heavily on adaptive immunity (i.e., mammals). Previous studies reported that fish and mammals significantly differ in pIgR. This review summarized the differences in pIgR structure, function, and transcriptional regulation between fish and mammals. A model of the transcriptional regulation of the pIgR gene was suggested. In this model, microbes could activate Toll-like receptor, trigger the cascade reactions in the signaling pathway, and then activate transcription factors that regulate pIgR expression through combining with the pIgR promoter. This review provides some suggestions for further studies on the function and regulatory mechanism of pIgR in fish and other animals.

The identification of growth, immune related genes and marker discovery through transcriptome in the yellow drum (Nibea albiflora)

Yellow drum (Nibea albiflora) is a commercially important marine fish, which is widely distributed in the coastal waters of China, Japan and Korea. Wild yellow drum resources have dramatically declined due to overfishing and ocean pollution. Genetic data can contribute to biodiversity conservation and protection. And molecular markers can play important roles in genetic breeding and aid in germplasm preservation in fish. In this study, 11 tissues (brain, heart, liver, kidney, muscle, head kidney, skin, fin, spleen, gonad and air bladder) were collected for pooled RNA sequencing. The unigenes were assembled using Trinity and EvidentialGene, and were then aligned to nr, nt, Swiss-Prot GO, KEGG, and KOG for annotation. Molecular markers (e.g. simple sequence repeat, SSR and single nucleotide polymorphism, SNP) were detected using MIcroSAtellite identification tool (MISA) and Genome Analysis Tool Kit (GATK). All clean reads were assembled into 109,209 transcripts, and 31,183 unigenes were generated after pruning and classifying, ranging from 201 to 19,857 bp in length (1230 bp in average), and 26,728 (85.7%) assembled unigenes had significant hits in public databases. Total of 27 and 103 unigenes were respectively identified as involved in growth- and immune-related pathways in the N. albiflora transcriptome. In addition, we identified a considerable quantity of molecular markers, including 11,484 SSRs and 56,186 SNPs. The growth- and immune-relevant genes and the molecular markers identified here provided a meaningful reference gene set and laid a foundation for future genetic selection and breeding for this species.

MicroRNA regulation of Toll-like receptor signaling pathways in teleost fish

The innate immune system is the first line defense mechanism that recognizes, responds to, controls or eliminates invading pathogens. Toll-like receptors (TLRs) are a critical family of pattern recognition receptors (PRRs) tightly regulated by complex mechanisms involving many molecules to ensure a beneficial outcome in response to foreign invaders. MicroRNAs (miRNAs), a transcriptional and posttranscriptional regulator family in a wide range of biological processes, have been identified as new molecules related to the regulation of TLR-signaling pathways in immune responses. To date, at least 22 TLR types have been identified in more than a dozen different fish species. However, the functions and underlying mechanisms of miRNAs in the regulation of inflammatory responses related to the TLR-signaling pathway in fish is lacking. In this review, we summarize the regulation of miRNA expression profiles in the presence of TLR ligands or pathogen infections in teleost fish. We focus on the effects of miRNAs in regulating TLR-signaling pathways by targeting multiple molecules, including TLRs themselves, TLR-associated signaling proteins, and TLR-induced cytokines. An understanding of the relationship between the TLR-signaling pathways and miRNAs may provide new insights for drug intervention to manipulate immune responses in fish.

Atlantic salmon adapted to seawater for 9 weeks develop a robust immune response to salmonid alphavirus upon bath challenge

Pancreas disease (PD) caused by salmonid alphavirus (SAV) is the most serious viral disease in Norwegian aquaculture. Study of the immune response to SAV will aid preventative measures including vaccine development. The innate immune response was studied in Atlantic salmon infected by either bath immersion (BI) or by intra-muscular (i.m.) injection (IM) with SAV subtype 3, two and nine weeks after seawater transfer (Phases A and B respectively). Phase A results have been previously published (Moore et al., 2017) and Phase B results are presented here together with a comparison of results achieved in Phase A. There was a rapid accumulation of infected fish in the IM-B (IM Phase B) group and all fish sampled were SAV RNA positive by 7 dpi (days post infection). In contrast, only a few SAV RNA positive (infected) fish were identified at 14, 21 and 28 dpi in the BI-B (BI Phase B) group. Differences in the transcription of several immune genes were apparent when compared between the infected fish in the IM-B and BI-B groups. Transcription of the analysed genes peaked at 7 dpi in the IM-B group and at 14 dpi in the BI-B group. However, this latter finding was difficult to interpret due to the low prevalence of SAV positive fish in this group. Additionally, fish positive for SAV RNA in the BI-B group showed higher transcription of IL-1β, IFNγ and CXCL11_L1, all genes associated with the inflammatory response, compared to the IM-B group. Histopathological changes in the heart were restricted to the IM-B group, while (immune) cell filtration into the pancreas was observed in both groups. Compared to the Phase A fish that were exposed to SAV3 two weeks after seawater transfer, the Phase B fish in the current paper, showed a higher and more sustained innate immune gene transcription in response to the SAV3 infection. In addition, the basal transcription of several innate immune genes in non-infected control fish in Phase B (CT-B) was also significantly different when compared to Phase A control fish (CT-A).

The role of type I interferons in innate and adaptive immunity against viruses in Atlantic salmon

Type I IFNs (IFN-I) are cytokines, which play a crucial role in innate and adaptive immunity against viruses of vertebrates. In essence, IFN-I are induced and secreted upon host cell recognition of viral nucleic acids and protect other cells against infection by inducing antiviral proteins. Atlantic salmon possesses an extraordinary repertoire of IFN-I genes encompassing at least six different classes (IFNa, IFNb, IFNc, IFNd, IFNe and IFNf) most of which are encoded by several genes. This review describes recent research on the functions of salmon IFNa, IFNb, IFNc and IFNd. As in mammals, expression of different salmon IFN-I in response to virus infection is dependent on their promoters, properties of the virus and the cell's expression of nucleic acid receptors and interferon regulatory factors (IRFs). While IFNa mainly display local antiviral activity, IFNb and IFNc show systemic antiviral activity. In addition, salmon appears to possess several IFN-I receptors, which show selectivity in binding different IFN-I. This complexity in IFN-I and receptors allows for a large variation in functions of the salmon IFN-I. Studies with intramuscular injection of IFN expression plasmids have recently provided surprising results, which may be of relevance for application of IFN-I in prophylaxis against virus infection. Firstly, injection of IFNc plasmid protected salmon presmolts against virus infection for at least 10 weeks. Secondly, IFN plasmids showed potent adjuvant activity when injected together with a DNA vaccine against infectious salmon anemia virus (ISAV).

Molecular identification and functional analysis of two variants of myeloid differentiation factor 88 (MyD88) from disk abalone (Haliotis discus discus)

Myeloid differentiation factor 88 (MyD88) is a crucial adaptor protein of the Toll-like receptor (TLR)- and interleukin 1 receptor-mediated signaling pathways and is involved in a diverse array of inflammatory responses via NF-κB activation. In the present study, two MyD88 variants were identified from disk abalone (Haliotis discus discus) and designated AbMyD88-2 and AbMyD88-X. The deduced AbMyD88-2 and AbMyD88-X comprised 433 and 354 amino acids with predicted molecular masses of 48.85 kDa and 40.17 kDa, respectively. AbMyD88-2 and AbMyD88-X possessed typical MyD88 domain structural features including an N-terminal death domain (DD) and C-terminal toll interleukin 1 receptor (TIR) domain similar to those in mammals. Expression analysis of AbMyD88-2 and AbMyD88-X mRNA at different early embryonic developmental stages of abalone by qPCR revealed that their constitutive expression at all developmental stages analyzed with the considerably higher values at the 16-cell (AbMyD88-2) and morula stages (AbMyD88-X). In unchallenged disk abalones, AbMyD88-2 was highly expressed in muscles, while AbMyD88-X mRNA was predominantly transcribed in hemocytes. Moreover, AbMyD88-2 and AbMyD88-X mRNA were differentially modulated in abalone hemocytes after a challenge with live bacteria (Vibrio parahaemolyticus, Listeria monocytogenes), virus (viral hemorrhagic septicemia virus), and pathogen-associated molecular patterns (lipopolysaccharides and Poly I:C). Overexpression of AbMyD88-2 and AbMyD88-X in HEK293T cells induced the activation of the NF-κB promoter. AbMyD88-2 and AbMyD88-X involvement in inflammatory responses was characterized by their overexpression in RAW264.7 murine macrophage cells. These results revealed comparatively higher NO (Nitric oxide) production, induction of inflammatory mediator genes (iNOS and COX2), and proinflammatory genes (IL1β, IL6 and TNFα) expression in abalone MyD88s-overexpressing cells than in mock control in the presence or absence of LPS stimulation. Altogether, these results suggest that existence of a MyD88-dependent like signaling pathway in disk abalone and that both AbMyD88-2 and AbMyD88-X might be involved in innate immune and inflammatory responses.

The first invertebrate NFIL3 transcription factor with role in immune defense identified from the Hong Kong oyster, Crassostrea hongkongensis

NFIL3 (nuclear factor interleukin 3-regulated) is a basic leucine zipper type transcription factor that mediates a variety of immune responses in vertebrates. However, the sequence information and function of NFIL3 homologs in invertebrates, especially mollusks, remains unknown. In the present study, the first NFIL3 homolog was identified in a marine mollusk, Crassostrea hongkongensis (designated as ChNFIL3), followed by its functional characterization. The full-length cDNA of ChNFIL3 is 2221 bp and consists of an open reading frame (ORF) of 1536 bp that encodes a polypeptide of 551 amino acids. Simple Modular Architecture Research Tool (SMART) analysis indicated that ChNFIL3 has two basic leucin zipper domains, similar to the other known NFIL3 family proteins. Tissue distribution analysis of NFIL3 in this mollusk revealed high expression in digestive glands and hemocytes. A significant induction in the mRNA level of ChNFIL3 was observed following bacterial stimulation. ChNFIL3 was found to be localized in the nucleus and over expression of ChNIFL3 led to upregulation of transcriptional activity of an NF-κB reporter gene in HEK 293T cells, indicating its role in innate immunity. Furthermore, addition of exogenous recombinant ChNFIL3 proteins resulted in enhanced mRNA level of hemocyte interleukin 17 in vitro. In conclusion, our findings revealed that NFIL3 in molluscs, plays a conserved role in host defense, similar to its mammalian homolog.

Identification of a gene encoding a membrane-anchored toll-like receptor 5 (TLR5M) in Oplegnathus fasciatus that responds to flagellin challenge and activates NF-κB

Toll-like receptor 5 (TLR5) recognizes bacterial flagellin and induces the downstream signaling through the myeloid differentiation primary response gene 88 (MyD88) protein to produce proinflammatory cytokines. In this study, we describe a TLR5 membrane form (OfTLR5M) and its adaptor protein MyD88 (OfMyD88) in rock bream, Oplegnathus fasciatus. Both Oftlr5m (6.7 kb) and Ofmyd88 (3.7 kb) genes displayed a quinquepartite structure with five exons and four introns. Protein structure of OfTLR5M revealed the conventional architecture of TLRs featured by an extracellular domain with 22 leucine rich repeats (LRR), a transmembrane domain and an endodomain with TIR motif. Primary OfTLR5M sequence shared a higher homology with teleost TLR5M. The evolutional analysis confirmed that TLR5 identified in the current study is a membrane receptor and the data further suggested the co-evolution of the membrane-anchored and soluble forms of TLR5 in teleosts. Inter-lineage comparison of gene structures in vertebrates indicated that the tlr5m gene has evolved with extensive rearrangement; whereas, the myd88 gene has maintained a stable structure throughout the evolution. Inspection of 5' flanking region of these genes disclosed the presence of several transcription factor binding sites including NF-κB. Quantitative real-time PCR (qPCR) detected Oftlr5m mRNA in eleven tissues with the highest abundance in liver. In vivo flagellin administration strongly induced the transcripts of both Oftlr5m and Ofmyd88 in gills and head kidney tissues suggesting their ligand-mediated upregulation. In a luciferase assay, HEK293T cells transiently transfected with Oftlr5m and Ofmyd88 demonstrated a higher NF-κB activity than the mock control, and the luciferase activity was intensified when cells were stimulated with flagellin. Collectively, our study represents the genomic, evolutional, expressional and functional insights into a receptor and adaptor molecules of teleost origin that are involved in flagellin sensing.

Immune gene profiles in Atlantic salmon (salmo salar L.) post-smolts infected with SAV3 by bath-challenge show a delayed response and lower levels of gene transcription compared to injected fish

Salmonid alphavirus (SAV) causes pancreatic disease (PD) in salmonids in Northern Europe which results in large economic losses within the aquaculture industry. In order to better understand the underlying immune mechanisms during a SAV3 infection Atlantic salmon post-smolts were infected by either i.m.-injection or bath immersion and their immune responses compared. Analysis of viral loads showed that by 14 dpi i.m.-injected and bath immersion groups had 95.6% and 100% prevalence respectively and that both groups had developed the severe pathology typical of PD. The immune response was evaluated by using RT-qPCR to measure the transcription of innate immune genes involved in the interferon (IFN) response as well as genes associated with inflammation. Our results showed that IFNa transcription was only weakly upregulated, especially in the bath immersion group. Despite this, high levels of the IFN-stimulated genes (ISGs) such as Mx and viperin were observed. The immune response in the i.m.-injected group as measured by immune gene transcription was generally faster, and more pronounced than the response in the bath immersion group, especially at earlier time-points. The response in the bath immersion group started later as expected and appeared to last longer often exceeding the response in the i.m-injected fish at later time-points. High levels of transcription of many genes indicative of an active innate immune response were present in both groups.

Fish IRF6 is a positive regulator of IFN expression and involved in both of the MyD88 and TBK1 pathways

Interferon (IFN) regulatory factors (IRF) are the crucial transcription factors for IFN expression, leading host cell response to viral infection. In mammals, only IRF6 is unaffected by IFN expression in the IRF family; however, in fish, a lower vertebrate, whether IRF6 is related to IFN regulation is unclear. In this study, we identified that zebrafish IRF6 was a positive regulator of IFN transcription and could be phosphorylated by both MyD88 and TBK1. First, the transcript level of cellular irf6 was upregulated by treatment with poly I:C (a mimic of viral RNAs), indicating IRF6 might be involved in the process of host cell response to viruses. Overexpression of IRF6 could upregulate IFN promoter activity significantly, meaning IRF6 is a positive regulator of IFN transcription. Subsequently, at the protein regulation level and in the interaction relationship, IRF6 was phosphorylated by and associated with both MyD88 and TBK1. In addition, overexpression of IRF6 activated the transcription of isg15, rig-i and mavs of host cells; meanwhile, the transcripts of p, m and n genes of SVCV were significantly declined in IRF6-overexpressing cells. Taken together, our data demonstrate that fish IRF6 is distinguished from the homolog of mammals by being a positive regulator of IFN transcription and phosphorylated by MyD88 and TBK1, suggesting that differences in the IRF6 regulation pattern exist between lower and higher vertebrates.

Identification and characterization of the toll-like receptor 8 gene in the Chinese raccoon dog (Nyctereutes procyonoides)

TLR8 is an important sensor of single-stranded RNA (ssRNA) from the viral genome and plays an essential role in innate antiviral responses via the recognition of conserved viral molecular patterns. In this report, TLR8 in the Chinese raccoon dog was characterized and analyzed for the first time. The full-length sequence of raccoon dog TLR8 (RdTLR8) cDNA was cloned by rapid amplification of cDNA ends (RACE) and is 3191bp with a 3117-bp open reading frame (ORF) encoding 1038 amino acids. The putative protein exhibits typical features of the TLR families, with 19 leucine-rich repeats (LRRs) in the extracellular domain and a cytoplasmic TIR domain. Comparative analyses of the RdTLR8 amino acid sequence indicated a 73.6-99.4% sequence identity with dog, horse, pig, sheep, cattle, human and mouse TLR8. Phylogenetic analysis grouped 71 mammalian TLR proteins into five sub-families, wherein RdTLR8 was clustered into a monophyletic TLR8 clade in the TLR9 family, which was completely coincident with the evolutionary relationship among mammals. Quantitative real-time PCR analysis revealed extensive expression of RdTLR8 in tissues from healthy Chinese raccoon dogs with the highest expression in the peripheral blood mononuclear cells (PBMCs) and the lowest expression in the skeletal muscle. HEK293 cells cotransfected with a RdTLR8 expression plasmid and an NF-κB-luciferase reporter plasmid significantly responded to the agonist 3M-002, indicating a functional TLR8 homolog. In addition, raccoon dog PBMCs exposed to the canine distemper virus (CDV) wild strain CDV-PS and the TLR8 agonist 3M-002 showed significant upregulation of RdTLR8 mRNA and proinflammatory cytokines TNF-α and IFN-α, suggesting that RdTLR8 might play an important role in the immune response to viral infections in the Chinese raccoon dog.

Toll-like receptors in maraena whitefish: Evolutionary relationship among salmonid fishes and patterns of response to Aeromonas salmonicida

Toll-like receptors (TLRs) interact directly with particular pathogenic structures and are thus highly important to innate immunity. The present manuscript characterises a suite of 14 TLRs in maraena whitefish (Coregonus maraena), a salmonid species with increasing importance for aquaculture. Whitefish TLRs were structurally and evolutionary analysed. The results revealed a close relationship with TLRs from salmonid fish species rainbow trout and Atlantic salmon. Profiling the baseline expression of TLR genes in whitefish indicated that mainly members of the TLR11 family were highly expressed across all investigated tissues. A stimulation model with inactivated Aeromonas salmonicida was used to induce inflammation in the peritoneal cavity of whitefish. This bacterial challenge induced the expression of pro-inflammatory cytokine genes and evoked a strong influx of granulated cells of myeloid origin into the peritoneal cavity. As a likely consequence, the abundance of TLR-encoding transcripts increased moderately in peritoneal cells, with the highest levels of transcripts encoding non-mammalian TLR22a and a soluble TLR5 variant. In the course of inflammation, the proportion of granulated cells increased in peripheral blood accompanied by elevated TLR copy numbers in spleen and simultaneously reduced TLR copy numbers in head kidney at day 3 post-stimulation. Altogether, the present study provides in-vivo evidence for relatively modest TLR response patterns, but marked trafficking of myeloid cells as an immunophysiological consequence of A. salmonicida inflammation in whitefish. The present results contribute to improved understanding of the host-pathogen interaction in salmonid fish.

Short-term effect of bisphenol-a on oxidative stress responses in Atlantic salmon kidney cell line: a transcriptional study

Bisphenol A (BPA) is regularly detected in aquatic ecosystems due to increased use of products based on polycarbonate plastics and epoxy resins. It migrates from these products directly into rivers and marine waters or indirectly through effluents from wastewater treatment plants and landfilled sites. BPA can affect aquatic organisms both chronically and acutely at sensitive live stages. Despite reports indicating harmful effects of BPA, little is known about its role in oxidative stress responses in fish. In this study, we investigated the transcriptional effect of BPA (0, 1, 10, 100 μM) on an Atlantic salmon kidney (ASK) cell line for 6 h and 24 h by monitoring expression of 11 genes: elongation factor 1-alpha (ef1a), 18S ribosomal RNA (18s), gluthation (gsh), superoxide dismutase (sod), thioredoxin (txd), Salmo salar oxidative stress-responsive serine-rich 1 (oxr), glucose-regulated protein 78 (grp78), heat shock protein 70 (hsp70), sequestosome1 (p62), interleukin-1 beta (il-1beta) and toll-like receptor 8 (tlr8). In general, only the 100 μM concentration treatment altered the mRNA expression. BPA down-regulated the expression of gsh and sod genes for both exposure-times while txd gene was the only down-regulated after 6-h exposure. The up-regulation of genes in the ASK cell line exposed for 6 h was only observed in il-1beta, while the 24-h exposure resulted in the up-regulation of oxr, tlr8, hsp70, p62 and il-1beta genes. The last three genes increased several fold compared to the others. The results showed that BPA exposure at 100 μM imposed oxidative stress on the ASK cell line and longer exposure time involved transcriptional responses of immune-related genes. This may indicate the possible role of BPA-associated oxidative stress in induction of inflammatory response in this macrophage-like cell type.

De Novo Transcriptome Analysis Shows That SAV-3 Infection Upregulates Pattern Recognition Receptors of the Endosomal Toll-Like and RIG-I-Like Receptor Signaling Pathways in Macrophage/Dendritic Like TO-Cells

A fundamental step in cellular defense mechanisms is the recognition of “danger signals” made of conserved pathogen associated molecular patterns (PAMPs) expressed by invading pathogens, by host cell germ line coded pattern recognition receptors (PRRs). In this study, we used RNA-seq and the Kyoto encyclopedia of genes and genomes (KEGG) to identify PRRs together with the network pathway of differentially expressed genes (DEGs) that recognize salmonid alphavirus subtype 3 (SAV-3) infection in macrophage/dendritic like TO-cells derived from Atlantic salmon (Salmo salar L) headkidney leukocytes. Our findings show that recognition of SAV-3 in TO-cells was restricted to endosomal Toll-like receptors (TLRs) 3 and 8 together with RIG-I-like receptors (RLRs) and not the nucleotide-binding oligomerization domain-like receptors NOD-like receptor (NLRs) genes. Among the RLRs, upregulated genes included the retinoic acid inducible gene I (RIG-I), melanoma differentiation association 5 (MDA5) and laboratory of genetics and physiology 2 (LGP2). The study points to possible involvement of the tripartite motif containing 25 (TRIM25) and mitochondrial antiviral signaling protein (MAVS) in modulating RIG-I signaling being the first report that links these genes to the RLR pathway in SAV-3 infection in TO-cells. Downstream signaling suggests that both the TLR and RLR pathways use interferon (IFN) regulatory factors (IRFs) 3 and 7 to produce IFN-a2. The validity of RNA-seq data generated in this study was confirmed by quantitative real time qRT-PCR showing that genes up- or downregulated by RNA-seq were also up- or downregulated by RT-PCR. Overall, this study shows that de novo transcriptome assembly identify key receptors of the TLR and RLR sensors engaged in host pathogen interaction at cellular level. We envisage that data presented here can open a road map for future intervention strategies in SAV infection of salmon.

Structurally diverse genes encode Tlr2 in rainbow trout: The conserved receptor cannot be stimulated by classical ligands to activate NF-κB in vitro

The mammalian toll-like receptor 2 (TLR2) is a dominant receptor for the recognition of Gram-positive bacteria. Its structure and functional properties were unknown in salmonid fish. In RT-PCR and RACE experiments, we obtained the full-length cDNA sequence encoding Tlr2 from rainbow trout (Oncorhynchus mykiss) as well as a copy of an unspliced nonsense message from a highly segmented gene. The primary structure of the encoded receptor complies with the domain structure and ligand-binding sites known from mammals and other fish species and sorts well into the evolutionary tree of teleostean Tlr2s. We retrieved a gene version encoding the receptor on a single exon (tlr2a) and also a partial sequence of a second gene variant being segmented into multiple exons (tlr2b). Surprisingly, the abundances of both transcript variants accounted only for ∼10% of all Tlr2-encoding transcripts in various tissues and cell types of healthy fish. This suggests the expression of several distinct tlr2 gene variants in rainbow trout. We expressed tlr2a in HEK-293 cells, but were unable to demonstrate its functionality through NF-κB activation. Neither synthetic lipopeptides known to stimulate mammalian TLR2 nor different bacterial challenges induced OmTLR2-mediated NF-κB activation, not in HEK-293 or in salmon CHSE-214 cells. Positive demonstration of TLR2-MYD88 interaction excluded that its functional impairment caused the failure of NF-κB activation. We discuss impaired heterodimerization with a necessary Tlr partner as one from among several alternatives to explain the dysfunction of Tlr2a in the interspecies reconstitution system of TLR signaling.

Molecular cloning and expression studies of the adapter molecule myeloid differentiation factor 88 (MyD88) in turbot (Scophthalmus maximus)

Myeloid differentiation factor 88 (MyD88) is an adapter protein involved in the interleukin-1 receptor (IL-1R) and Toll-like receptor (TLR)-mediated activation of nuclear factor-kappaB (NF-κB). In this study, a full length cDNA of MyD88 was cloned from turbot, Scophthalmus maximus. It is 1619 bp in length and contains an 858-bp open reading frame that encodes a peptide of 285 amino acid residues. The putative turbot (Sm)MyD88 protein possesses a N-terminal death domain and a C-terminal Toll/IL-1 receptor (TIR) domain known to be important for the functions of MyD88 in mammals. Phylogenetic analysis grouped SmMyD88 with other fish MyD88s. SmMyD88 mRNA was ubiquitously expressed in all examined tissues of healthy turbots, with higher levels observed in immune-relevant organs. To explore the role of SmMyD88, its gene expression profile in response to stimulation of lipopolysaccharide (LPS), CpG oligodeoxynucleotide (CpG-ODN) or turbot reddish body iridovirus (TRBIV) was studied in the head kidney, spleen, gills and muscle over a 7-day time course. The results showed an up-regulation of SmMyD88 transcript levels by the three immunostimulants in all four examined tissues, with the induction by CpG-ODN strongest and initiated earliest and inducibility in the muscle very weak. Additionally, TRBIV challenge resulted in a quite high level of SmMyD88 expression in the spleen, whereas the two synthetic immunostimulants induced the higher levels in the head kidney. These data provide insights into the roles of SmMyD88 in the TLR/IL-1R signaling pathway of the innate immune system in turbot.

Comparison of transcriptomic responses to pancreas disease (PD) and heart and skeletal muscle inflammation (HSMI) in heart of Atlantic salmon (Salmo salar L)

Pancreas disease (PD) and heart and skeletal muscle inflammation (HSMI) are viral diseases associated with SAV (salmonid alphavirus) and PRV (piscine reovirus), which induce systemic infections and pathologies in cardiac and skeletal muscle tissue of farmed Atlantic salmon (Salmo salar L), resulting in severe morbidity and mortality. While general features of the clinical symptoms and pathogenesis of salmonid viral diseases are relatively well studied, much less is known about molecular mechanisms associated with immunity and disease-specific changes. In this study, transcriptomic analyses of heart tissue from PD and HSMI challenged Atlantic salmon were done, focusing on the mature phases of both diseases at respectively 28-35 and 42-77 days post infection. A large number of immune genes was activated in both trials with prevalence of genes associated with early innate antiviral responses, their expression levels being slightly higher in PD challenged fish. Activation of the IFN axis was in parallel with inflammatory changes that involved diverse humoral and cellular factors. Adaptive immune response genes were more pronounced in fish with HSMI, as suggested by increased expression of a large number of genes associated with differentiation and maturation of B lymphocytes and cytotoxic T cells. A similar down-regulation of non-immune genes such as myofiber and mitochondrial proteins between diseases was most likely reflecting myocardial pathology. A suite of genes important for cardiac function including B-type natriuretic peptide and four neuropeptides displayed differential expression between PD and HSMI. Comparison of results revealed common and distinct features and added to the understanding of both diseases at their mature phases with typical clinical pictures. A number of genes that showed disease-specific changes can be of interest for diagnostics.

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.

Functional characterisation of a TLR accessory protein, UNC93B1, in Atlantic salmon (Salmo salar)

Toll-like receptors (TLRs) are indispensable components of the innate immune system, which recognise conserved pathogen associated molecular patterns (PAMPs) and induce a series of defensive immune responses to protect the host. Biosynthesis, localisation and activation of TLRs are dependent on TLR accessory proteins. In this study, we identified the accessory protein, UNC93B1, from Atlantic salmon (Salmo salar) whole-genome shotgun (WGS) contigs aided by the conserved gene synteny of genes flanking UNC93B1 in fish, birds and mammals. Phylogenetic analysis showed that salmon UNC93B1 grouped with other vertebrate UNC93B1 molecules, and had highest amino acid identity and similarity to zebrafish UNC93B1. The salmon UNC93B1 gene organisation was also similar in structure to mammalian UNC93B1. Our gene expression studies revealed that salmon UNC93B1 was more highly expressed in spleen, liver and gill tissues but was expressed at a lower level in head kidney tissue in post-smolts relative to parr. Moreover, salmon UNC93B1 mRNA transcripts were up-regulated in vivo in spleen tissue from polyI:C treated salmon and in vitro in polyI:C or IFNγ stimulated Salmon Head Kidney-1 (SHK-1) cells. Initial studies into the functional role of salmon UNC93B1 in fish TLR signalling found that both wild type salmon UNC93B1 and a molecule with a site-directed mutation (H424R) co-immunoprecipitated with salmon TLR19, TLR20a and TLR20d. Overall, these data illustrate the potential importance of UNC93B1 as an accessory protein in fish TLR signalling.

Immune effects of R848: evidences that suggest an essential role of TLR7/8-induced, Myd88- and NF-κB-dependent signaling in the antiviral immunity of Japanese flounder (Paralichthys olivaceus)

The imidazoquinoline compound R848 is a specific agonist of toll-like receptor (TLR) 7/TLR8 that has been used as an immunostimulant in humans against viral diseases. Although R848-induced immune response has been reported in teleost fish, the relevant mechanism is not clear. In this study, we investigated the antiviral potential and the signaling pathway of R848 in a model of Japanese flounder (Paralichthys olivaceus). We found that R848 was able to inhibit the replication of megalocytivirus, stimulated the proliferation of peripheral blood leukocytes (PBL), enhanced the expression of immune genes, and reduced apoptosis of PBL. When endosomal acidification was blocked by chloroquine (CQ), R848-mediated antiviral activity and immune response were significantly reduced. Likewise, inhibition of Myd88 activation markedly impaired the pro-proliferation and anti-apoptosis effect of R848. Cellular study showed that cultured founder cells treated with R848 exhibited augmented NF-κB activity, which, however, was dramatically reduced in the presence of CQ and Myd88 inhibitor. Furthermore, when NF-κB was inactivated, the effect of R848 on cell proliferation and apoptosis was significantly decreased. Taken together, these results indicate that R848 is an immunostimulant with antiviral property in a teleost species, and that the immune response of R848 is mediated by, most likely, TLR7/TLR8 signaling pathway, in which Myd88 and NK-κB play an essential role.

Cloning and characterization of the proximal promoter region of rainbow trout (Oncorhynchus mykiss) interleukin-6 gene

Interleukin-6 (IL6) is a pleiotropic cytokine with important immunoregulatory functions. Its expression is inducible in immune cells and tissues of several fish species. We also found that IL6 mRNA abundance was significantly increased in spleen, liver, and gill of rainbow trout after experimental infection with Aeromonas salmonicida. Genomic DNA sequences of IL6 orthologs from three salmonid species revealed a conserved exon/intron structure and a high overall nucleotide identity of >88%. To uncover key mechanisms regulating IL6 expression in salmonid fish, we amplified a fragment of the proximal IL6 promoter from rainbow trout and identified in-silico conserved binding sites for NF-κB and CEBP. The activity of this IL6 promoter fragment was analyzed in the established human embryonic kidney line HEK-293. Luciferase- and GFP-based reporter systems revealed that the proximal IL6 promoter is activated by Escherichia coli. Essentially, both reporter systems proved that NF-κB p50, but not NF-κB p65 or CEBP, activates the IL6 promoter fragment. Truncation of this fragment caused a significant decrease in IL6 promoter activation. This characterization of the proximal promoter of the IL6-encoding gene provides basic knowledge about the IL6 gene expression in rainbow trout.

Grouper (Epinephelus coioides) MyD88 and Tollip: intracellular localization and signal transduction function

Myeloid differentiation factor 88 (MyD88) and Toll-interacting protein (Tollip) are two important regulatory proteins of the Toll-like receptor (TLR) signaling pathways. In this paper, a Tollip sequence of grouper (Epinephelus coioides) was identified and the signal transduction functions of Tollip and MyD88 were studied. The full length of E. coioides Tollip (EcTollip) cDNA with an open reading frame (ORF) of 1734 nucleotides encoded a putative protein of 274 amino acid residues. The EcTollip protein had conservative domains with mammalian homologous proteins, and high identity (78%-95%) with other vertebrates. MyD88 and Tollip were distributed in the HeLa cytoplasm in a highly condensed form. Over-expression of MyD88 could activate nuclear factor-κB (NF-κB) and its function was dependent on the death domain and ID domain on the N-terminal. Some important functional sites of mammalian MyD88 also affected fish MyD88 signal transduction. Tollip impaired NF-κB signals activated by MyD88, and its activity was dependent on the coupling of ubiquitin to the endoplasmic reticulum degradation (CUE) domain on the C-terminal. These results suggest that MyD88 and Tollip of fish and mammals are conservative on function during evolution.

Reciprocal interaction between fish TGF-β1 and IL-1β is responsible for restraining IL-1β signaling activity in grass carp head kidney leukocytes

In the present study, we found that recombinant grass carp IL-1β (rgcIL-1β) simultaneously up-regulated grass carp IL-1β (gcIL-1β) and TGF-β1 (gcTGF-β1) expression via NF-κB and MAPK signaling in grass carp head kidney leukocytes (HKLs), promoting us to clarify whether TGF-β1 is an effective antagonist in IL-1β expression and activity. Our results showed that a stimulation of gcIL-1β on its own expression was noted within 6 h, but gcTGF-β1 neutralizing antibody prolonged gcIL-1β autostimulation up to 12 h, indicating a possible inhibitory role of gcTGF-β1 in regulating gcIL-1β effect. This notion was reinforced by the fact that recombinant grass carp TGF-β1 (rgcTGF-β1) could impede rgcIL-1β-induced gcIL-1β gene expression and secretion in a reciprocal manner. Further studies revealed that rgcTGF-β1 was able to attenuate rgcIL-1β-induced mRNA expression of its own receptor signaling molecules and the activation of NF-κB. By contrast, rgcIL-1β significantly amplified rgcTGF-β1-mediated gcTGF-β1 type I receptor (ALK5) expression and Smad2 phosphorylation in the same cell model. Taken together, these data shed light on an intrinsic mechanism for controlling inflammatory response by the reciprocal interaction between TGF-β1 and IL-1β in teleost.

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.

Effects of TLR agonists and viral infection on cytokine and TLR expression in Atlantic salmon (Salmo salar)

The development of efficient and cheap vaccines against several aquatic viruses is necessary for a sustainable fish farming industry. Toll-like receptor (TLR) ligands have already been used as good adjuvants in human vaccines. With more understanding of TLR expression, function, and ligand specificity in fish, more efficient adjuvants for fish viral vaccines can be developed. In this paper, we examine all known TLRs in Atlantic salmon (Salmo salar) and demonstrate that head kidney and spleen are the main organs expressing TLRs in salmon. We also show that adherent head kidney leucocytes from salmon are able to respond to many of the known agonists for human TLRs, and that viral infection can induce up-regulation of several TLRs. These findings substantiate these receptors' role in immune responses to pathogens in salmonids making their ligands attractive as vaccine adjuvant candidates.