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

A comprehensive review on the dynamic role of toll-like receptors (TLRs) in frontier aquaculture research and as a promising avenue for fish disease management

Toll-like receptors (TLRs) represent a conserved group of germline-encoded pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and play a crucial role in inducing the broadly acting innate immune response against pathogens. In recent years, the detection of 21 different TLR types in various fish species has sparked interest in exploring the potential of TLRs as targets for boosting immunity and disease resistance in fish. This comprehensive review offers the latest insights into the diverse facets of fish TLRs, highlighting their history, classification, architectural insights through 3D modelling, ligands recognition, signalling pathways, crosstalk, and expression patterns at various developmental stages. It provides an exhaustive account of the distinct TLRs induced during the invasion of specific pathogens in various fish species and delves into the disparities between fish TLRs and their mammalian counterparts, highlighting the specific contribution of TLRs to the immune response in fish. Although various facets of TLRs in some fish, shellfish, and molluscs have been described, the role of TLRs in several other aquatic organisms still remained as potential gaps. Overall, this article outlines frontier aquaculture research in advancing the knowledge of fish immune systems for the proper management of piscine maladies.

Understanding the molecular response of non-mammalian toll-like receptor 22 (TLR22) in amphibious air-breathing catfish, Clarias magur (Hamilton, 1822) to bacterial infection or ligand stimulation through molecular cloning and expression profiling

Toll-like receptor (TLR) 22 is a non-mammalian TLR, which is identified initially as a functional substitute of mammalian TLR3 in recognizing cell surface long dsRNA in teleosts. To understand the pathogen surveillance role played by TLR22 in an air-breathing catfish model the full-length cDNA of TLR22 was identified in Clarias magur and found to be consisted of 3597 nucleotides encoding for 966 amino acids. In the deduced amino acid sequence of C. magur TLR22 (CmTLR22) key signature domains such as one signal peptide, 13 LRRs, one transmembrane domain, one LRR_CT domain and an intracellular TIR domain could be identified. The CmTLR22 formed a separate cluster with other catfish TLR22 genes and situated within the TLR22 cluster in the phylogenetic analysis of teleost TLR groups. The CmTLR22 was constitutively expressed in all the 12 tested tissues of healthy C. magur juveniles with the highest transcript abundance in spleen followed by brain, intestine and head kidney. Following induction with the dsRNA viral analogue, poly (I:C), the level of expression of CmTLR22 was up-regulated in tissues such as kidney, spleen and gills. Whereas, in Aeromonas hydrophila-challenged C. magur, the expression levels of CmTLR22 was found to be up-regulated in gills, kidney and spleen, and down-regulated in liver. The findings of the current study suggest that the specific function of TLR22 is evolutionarily conserved in C. magur and might play a key role in mounting immune response by recognizing Gram-negative fish pathogen such as A. hydrophila and aquatic viruses in air-breathing amphibious catfishes.

A novel portable label-free electrochemical immunosensor for ultrasensitive detection of Aeromonas salmonicida in aquaculture seawater

Infectious diseases caused by Aeromonas salmonicida (A. salmonicida) have a huge impact and produce significant losses in aquaculture and fish farming. Fish pathogen early detection is a critical step for the rapid identification and prevention of these problems. This work presents a novel portable label-free ultrasensitive electrochemical immunosensor for A. salmonicida detection in seawater. It consists of a fluidic integrated electrochemical-cell-chip (ECC) with independent chambers enclosing three electrochemical cells (ECs). Anti-A. salmonicida (AbSalm) antibodies were covalently attached to the gold surface of the microfabricated electrodes and were used for the sensitive detection of A. salmonicida. The antibody-antigen immunoreaction was studied by enzyme-linked immunosorbent assay (ELISA), and the surface functionalization was characterized by using quartz crystal microbalance (QCM), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). The performance of the developed immunosensor, in terms of sensitivity, repeatability, and specificity, was also studied. The linear working range varied between 1 and 10⁷ CFU mL^-1, with a limit of detection (LOD) as low as 1 CFU mL^-1. The suitability of the immunosensor for real sample detection was successfully demonstrated via recovery studies performed in spiked seawater samples. The proposed technology supports the use of low-cost and portable instrumentation that concedes the ultrasensitive, simple, and fast quantification of the A. salmonicida. To the best of our knowledge, this is the first portable sensing system for the detection of A. salmonicida in seawater samples, which provides a promising online monitoring platform for the detection of this bacterium in aquaculture facilities.

Long non-coding RNAs are involved in immune resistance to Aeromonas hydrophila in black carp (Mylopharyngodon piceus)

A growing number of studies identified long non-coding RNAs (lncRNAs) to be closely associated with immune function through the regulation of immune cell differentiation and immune cell effector function. Here we tested whether lncRNAs are involved in immune function in black carp (Mylopharyngodon piceus) through the exposure to Aeromonas hydrophila and analysis of the spleen gene expression response using RNA-seq. A total of 9036 lncRNAs were identified with high confidence. Differential expression analysis identified a total of 3558 DElncRNAs (Differential expression lncRNA) involved in A. hydrophila infection and 4526 target genes corresponding to DElncRNAs. After screening 4526 target genes in the InnateDB database, a total of 150 immunity genes were identified. After GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis of the obtained immunity genes, the Toll-like receptor (TLR) signaling pathway, TLR2, TLR3, TLR5, and TLR8 were identified as particularly significant in A. hydrophyla-resistant black carp. At the same time, the Ras signaling pathway was particularly enriched in the spleen of susceptible black carp. Analysis of PPI (protein-protein interaction) networks of the obtained immune genes identified SRC (SRC Proto-Oncogene), MYD88 (Myeloid differentiation primary response 88), MAPK3 (Mitogen-Activated Protein Kinase 3), MYC (MYC Proto-Oncogene) as main hub genes regulated by lncRNA and possibly mediating a mechanism of susceptibility to bacteria. These results establish a functional role of lncRNAs and a mechanistic base for the immune response in black carp resistant to A. hydrophila.

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.

Alpinone: A positive regulator molecule of immune antiviral response in Atlantic salmon kidney cells

Alpinone is a flavonoid obtained from the resinous exudate of Heliotropium huascoense. This flavonoid shows antiviral activity against the infectious salmon anemia virus (ISAV), which causes severe disease in farmed Atlantic salmon. Here, we aim to elucidate mechanisms underlying the antiviral effects of the flavonoid. In this regard, we evaluated whether Alpinone can act upregulating the pattern-recognition receptor genes, i.e., the RIG-I-like, TLR3, and TLR9 genes, and the genes of the downstream signaling pathways. Transcriptional expression of the genes was analyzed using real-time PCR after 8, 24, and 48 h treatment of salmon kidney adherent cells with 15 μg/mL of Alpinone. First, we showed that Alpinone induced IFNa expression in the kidney adherent cells, indicating that this type of salmon cells is in part responsible for the effects previously reported in vivo. Upregulation of the IFN-induced myxovirus resistance (Mx) gene was also observed in the head kidney cells in response to the treatment. Overexpression reached a maximum level at 24 h post-treatment. Interestingly, Alpinone also induced upregulation of the cytosolic receptors of ssRNA, named Retinoic acid-inducible gene I (RIG-I) and Melanoma Differentiation-Associated protein 5 (MDA5), but there were no effects on the transcriptional expression of the TLR3 and TLR9 endosomal receptors. In addition, Alpinone upregulated the expression of genes encoding the main components of the RIG-I/MDA5 signaling pathways, such as the mitochondrial antiviral-signaling protein (MAVS), TNF Receptor Associated Factor 3 (TRAF3), TANK-binding kinase 1 (TBK1), I-kappaB kinase ε (IKKε), the transcription factors IRF-3, and IRF7. The increased expression of all these genes is consistent with the upregulation of IFNa and Mx mRNAs. Because BX795 completely prevents Alpinone-dependent upregulation of IFNa and IRF3, the flavonoid targets seem to be upstream of the kinases TBK1 and IKKε. Altogether, this study contributes to elucidating the mechanisms involved in Alpinone antiviral activity in fish. Alpinone can be used to counteract virus mechanisms of evasion where the onset of interferon-mediated response is prevented or delayed.

Chitosan nanoparticle formulation attenuates poly (I:C) induced innate immune responses against inactivated virus vaccine in Atlantic salmon (Salmo salar)

Many vaccine formulations, in particular vaccines based on inactivated virus, needs adjuvants to boost immunogenicity. In aquaculture, mineral and plant oil are used as adjuvant in commercial vaccines, and the advent of oil-adjuvanted vaccines was crucial to aquaculture development. Nevertheless, some of these approved vaccines display suboptimal performance in the field compared to experimental conditions. Therefore, there is a need to improve adjuvants and delivery methods for fish vaccines against viruses. We used RNA sequencing of Atlantic salmon head kidney to analyse the difference in gene expression 24 h after injection of different experimental vaccine formulations. We compared five different formulations in addition to a PBS control: inactivated virus alone (group V), soluble poly (I:C) (group P), nanoparticles containing poly (I:C) (group N), soluble poly (I:C) + inactivated virus (group PV) and finally nanoparticles containing poly (I:C) + inactivated virus (group NV). Our results showed poly (I:C)'s ability as adjuvant and its capacity influence innate immune genes expression in Atlantic salmon. Soluble poly (I:C) upregulated multiple immune related genes and was more effective compared to poly (I:C) formulated into chitosan nanoparticles (more than 10 fold increase in differentially expressed genes, DEGs). However, inclusion of inactivated ISA virus in the nanoparticle vaccine, increased the number of DEGs fivefold suggesting a synergistic effect of adjuvant and antigen. Our results indicate that the way poly (I:C) is formulated and the presence of antigen is important for the magnitude of the innate immune response in Atlantic salmon.

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.

Influence of Dietary Supplementation of Probiotic Pediococcus acidilactici MA18/5M During the Transition From Freshwater to Seawater on Intestinal Health and Microbiota of Atlantic Salmon (Salmo salar L.)

The aim of this study was to assess the effect of the transfer from freshwater to seawater on the distal intestinal bacterial communities of Atlantic salmon (Salmo salar L.) and to evaluate the effect of dietary inclusion of Pediococcus acidilactici MA18/5M (at 1.19 × 10⁶ CFU/g). In this context, fish health and antiviral response were also investigated. A 12-week feeding trial was conducted in a flow-through rearing system involving 6 weeks in freshwater and 6 weeks in seawater. Fish received a control and probiotic diet. The composition of the salmon gut bacterial communities was determined by high-throughput sequencing of digesta and mucosa samples from both the freshwater and seawater stage. The main phyla detected during both freshwater and seawater stages were Firmicutes, Proteobacteria, Fusobacteria, and Actinobacteria. Significant differences were observed between the intestinal microbiota in the digesta and the mucosa. Both probiotic supplementation and the seawater transfer (SWT) had a substantial impact on the microbial communities, with most pronounced changes detected in the mucosal communities after SWT. This last finding together with a significantly higher antiviral response (mx-1 and tlr3 gene expression) in the distal intestine of fish fed the probiotic diet suggest a causal link between the microbiota modulation and activation of antiviral response. Feeding probiotics during the freshwater stage did not significantly increase survival after infectious pancreatic necrosis virus (IPNV) challenge after SWT, although higher survival was observed in one out of two replicate challenge tanks. In conclusion, this study demonstrated that both dietary probiotic supplementation and transfer from freshwater to seawater have an important role in modulating the bacterial communities in the distal intestine of Atlantic salmon. Furthermore, supplementation of the diet with P. acidilactici MA18/5M can modulate antiviral response.

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.

Surplus arginine reduced lipopolysaccharide induced transcription of proinflammatory genes in Atlantic salmon head kidney cells

In aquaculture production, studies of salmon health and interaction between pathogens and nutrition are of high importance. This study aimed to compare genes and pathways involved in salmon head kidney cells and liver cells, isolated from the same fish, towards polyinosinic acid: polycytidylic acid (poly I:C) and lipopolysaccharide (LPS), with and without addition of surplus arginine. Selected transcriptional responses of genes involved in inflammation, polyamine synthesis, oxidation and apoptosis were elucidated. For the genes related to inflammation, viperin, Mx and Toll like receptor 3 (TLR3), transcription were significantly upregulated by poly I:C in head kidney cells, while viperin was upregulated in liver cells. Surplus arginine did not affect poly I:C induced responses with the exception of reducing poly I:C induced Mx transcription in head kidney cells. Gene transcription of Interleukin 1β (IL-1β), Interleukin-8 (IL-8) and cyclooxygenase 2 (Cox2) were elevated during LPS treatment in all liver and head kidney cell cultures. In addition, LPS induced significantly, CD83 transcription in liver cells and TNF-α transcription in head kidney cells. Surplus arginine significantly reduced IL-8, Cox2 and TNF-α transcription in head kidney cells. LPS upregulated arginase in head kidney cells while poly I:C upregulated S-adenosyl methionine decarboxylase (SAMdc) transcription in liver cells. This suggests that LPS and poly I:C modulates genes involved in polyamine synthesis. In addition, in head kidney cells, surplus arginine, when cultured together with LPS, increased the transcription of ornithine decarboxylase (ODC) the limiting enzyme of polyamine synthesis. The genes involved with oxidation and apoptosis were not affect by any of the treatments in liver cells, while LPS decreased caspase 3 transcription in head kidney cells. In liver cells, protein expression of catalase was reduced by surplus arginine alone and when challenged with poly I:C. Both liver cells and head kidney cells isolated from the same individual fish responded to LPS and poly I:C, depending on the gene analyzed. Additionally, arginine could modulate transcription of pro-inflammatory genes induced by LPS in salmon immune cells, thus affecting salmon immunity.

Molecular characterization and expression of TLR7 and TLR8 in barbel chub (Squaliobarbus curriculus): Responses to stimulation of grass carp reovirus and lipopolysaccharide

The barbel chub (Squaliobarbus curriculus) is a kind of small size commercial fish species that is widely spread in Asia and has shown significant resistance to disease. In this study, the full-length cDNA sequences of Toll-like receptor (TLR) 7 and 8 from S. curriculus, designated as ScTLR7 and ScTLR8, were cloned, and their differences in the structure and the responses to the grass carp (GCRV) infection and lipopolysaccharide stimulation were investigated. The full-length 3715 base pair (bp) cDNA of ScTLR7 contained a complete open reading frame of 3162 bp and encoded a putative polypeptide of 1053 amino acid residues. The full-length 4624 base pair (bp) cDNA of ScTLR8 contained a complete open reading frame of 3072 bp and encoded a putative polypeptide of 1023 amino acid residues. ScTLR7 and ScTLR8 consisted of N-terminal signal peptide, leucine-rich repeats (LRRs), and Toll/IL-1 Receptors domain. LRR motifs of ScTLR7 and ScTLR8 bend into horseshoe-like solenoid structure, while the number of LRRs between the two genes is different. Phylogenetic analysis showed that both the ScTLR7 and ScTLR8 were closely clustered with Ctenopharyngodon idellus and Megalobrama amblycephala. Quantitative real-time polymerase chain reaction analysis showed that the expression levels of ScTLR7 in S. curriculus were most abundant in the brain followed by the spleen and heart, and the lowest in the intestine. The highest expression level of ScTLR8 was observed in spleen and the lowest in the liver. After LPS stimulation, the relative expression levels of both ScTLR7 and ScTLR8 exhibited an overall trend of up-regulation. The expression levels of type I-IFN showed an overall trend of down-regulation at time points of 12, 24, 72 and 168 h compared to that of 6 h after LPS stimulation. Compared to 6 h post GCRV infection, the transcription level of ScTLR7 was up-regulated from 12 to 168 h, and transcription levels of ScTLR8, MyD88, and type I-IFN were firstly up-regulated from 12 to 72 h, and then down-regulated from 72 to 168 h. Correlation analysis showed that expression level of ScTLR7 in the spleen was significantly positively correlated with that of MyD88 (Pearson correlation coefficient: 0.909, P: 0.033), and a significantly positive correlation was also observed between expression levels of MyD88 and type I IFN (Pearson correlation coefficient: 0.962, P: 0.009), after GCRV stimulation. These results indicate that ScTLR7 and ScTLR8 may play important roles in the responses to the grass carp (GCRV) infection and lipopolysaccharide stimulation and trigger different downstream immune signal pathways.

Toll-like receptor (TLR) 22, a non-mammalian TLR in Asian seabass, Lates calcarifer: Characterisation, ontogeny and inductive expression upon exposure with bacteria and ligands

Toll-like receptor (TLR) 22 is a non-mammalian TLR found mostly in teleosts and characterized initially as a cell surface surveillance receptor for detecting extracellular long dsRNA. In the current study, the full-length cDNA sequence consisting of 3312 nucleotides encoding for 960 amino acids in Asian seabass (Lates calcarifer) TLR22 (AsTLR22) was identified. From the putative protein sequence, signature TLR domains such as 18 LRR domains, two transmembrane domains, a single LRR_CT domain and an intracellular TIR domain could be predicted. Phylogenetic analysis showed that AsTLR22 is clustered with other teleost TLR22 and is distinctly different from the other TLR groups. The transcript of AsTLR22 was ubiquitously expressed in all the tissues tested of healthy juveniles with the highest expression in gill followed by hindgut, spleen and skin. The AsTLR22 mRNA transcript was also detected in all the developmental stages as early as unfertilized eggs with higher expression in later stages such as neurula and early embryo. The dsRNA viral analogue, poly (I:C) and Gram-negative bacterium, Vibrio alginolyticus, were found to modulate the AsTLR22 expression in different tissues with the highest expression in kidney and liver. Gram-positive bacterium, Staphylococcus aureus, was also found to regulate the AsTLR22 expression at certain time-points with the highest expression in gill. Similarly, noticeable change in AsTLR22 expression was detected in SISK cell line induced with different ligands such as poly (I:C), LPS and PGN. The findings indicate that AsTLR22 responds in transcript level towards bacteria-borne PAMPs and extracellular dsRNA in the euryhaline teleost Asian seabass. Further, this might act as an important pathogen surveillance receptor during early developmental stages.

Profiling Atlantic salmon B cell populations: CpG-mediated TLR-ligation enhances IgM secretion and modulates immune gene expression

While TLR-activated pathways are key regulators of B cell responses in mammals, their impact on teleost B cells are scarcely addressed. Here, the potential of Atlantic salmon B cells to respond to TLR ligands was shown by demonstrating a constitutive expression of nucleic-acid sensing TLRs in magnetic sorted IgM⁺ cells. Of the two receptors recognizing CpG in teleosts, tlr9 was the dominating receptor with over ten-fold higher expression than tlr21. Upon CpG-stimulation, IgM secretion increased for head kidney (HK) and splenic IgM⁺ cells, while blood B cells were marginally affected. The results suggest that CpG directly affects salmon B cells to differentiate into antibody secreting cells (ASCs). IgM secretion was also detected in the non-treated controls, again with the highest levels in the HK derived population, signifying that persisting ASCs are present in this tissue. In all tissues, the IgM⁺ cells expressed high MHCII levels, suggesting antigen-presenting functions. Upon CpG-treatment the co-stimulatory molecules cd83 and cd40 were upregulated, while cd86 was down-regulated under the same conditions. Finally, ifna1 was upregulated upon CpG-stimulation in all tissues, while a restricted upregulation was evident for ifnb, proposing that salmon IgM⁺ B cells exhibit a type I IFN-response.

Pattern recognition receptors in grass carp Ctenopharyngodon idella: I. Organization and expression analysis of TLRs and RLRs

Pattern recognition receptors (PRRs) play indispensable roles in the immune responses against invading pathogens. In the present study, we systematically identified and characterized Toll-like receptors (TLRs), retinoic acid-inducible gene I (RIG-I)-like receptors (RLRs) as well as their adaptors in grass carp (Ctenopharyngodon idella). A comprehensive analysis of BLAST and other bioinformatics methods showed that C. idella TLR family consist of 21 members and their adaptors contain four members. Phylogenetic analyses confirmed the existence of six TLR subfamilies (TLR1, 3, 4, 5, 7 and 11 subfamily) in C. idella and revealed their homologous relationships with other species. Most C. idella TLRs possess three typical structural features of TLR protein family: LRR, TM and TIR domains. Meanwhile, RLR family consist of three conserved members (RIG-I, MDA5 and LGP2) as well as two adaptors (IPS-1 and STING) in C. idella. mRNA expression analyses of TLRs, RLRs and their adaptors indicated that most members are sustainably expressed in multiple tissues before and after grass carp reovirus (GCRV) or Aeromonas hydrophila infection, while TLR9, TLR20a/b, TLR25, TIRAP, SARM1 and STING are transiently expressed in specific tissues. TLRs are transmembrane receptors with few introns, while RLRs are cytoplasmic receptors with plenty of introns. TLRs and RLRs interact with adaptors to perform their functions via various signaling pathways. In conclusion, this study systematically explores the characteristics of TLRs and RLRs in C. idella and provides evidence for the response patterns after viral and/or bacterial infection in vivo. These results contribute to studying the regulation mechanisms of TLR and RLR signaling pathways, and deeply understanding fish immune responses against pathogen infection.

Molecular cloning of MDA5, phylogenetic analysis of RIG-I-like receptors (RLRs) and differential gene expression of RLRs, interferons and proinflammatory cytokines after in vitro challenge with IPNV, ISAV and SAV in the salmonid cell line TO

The RIG-I receptors RIG-I, MDA5 and LGP2 are involved in viral recognition, and they have different ligand specificity and recognize different viruses. Activation of RIG-I-like receptors (RLRs) leads to production of cytokines essential for antiviral immunity. In fish, most research has focused on interferons, and less is known about the production of proinflammatory cytokines during viral infections. In this study, we have cloned the full-length MDA5 sequence in Atlantic salmon, and compared it with RIG-I and LGP2. Further, the salmonid cell line TO was infected with three fish pathogenic viruses, infectious pancreatic necrosis virus (IPNV), infectious salmon anaemia virus (ISAV) and salmonid alphavirus (SAV), and differential gene expression (DEG) analyses of RLRs, interferons (IFNa-d) and proinflammatory cytokines (TNF-α1, TNF-α2, IL-1β, IL-6, IL-12 p40s) were performed. The DEG analyses showed that the responses of proinflammatory cytokines in TO cells infected with IPNV and ISAV were profoundly different from SAV-infected cells. In the two aforementioned, TNF-α1 and TNF-α2 were highly upregulated, while in SAV-infected cells these cytokines were downregulated. Knowledge of virus recognition by the host and the immune responses during infection may help elucidate why and how some viruses can escape the immune system. Such knowledge is useful for the development of immune prophylactic measures.

Effects of dietary n-3 fatty acids on Toll-like receptor activation in primary leucocytes from Atlantic salmon (Salmo salar)

The shortage of the n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the international markets has led to increasing substitution of fish oil by plant oils in Atlantic salmon (Salmo salar) feed and thereby reducing the EPA and DHA content in salmon. However, the minimum required levels of these fatty acids in fish diets for securing fish health are unknown. Fish were fed with 0, 1 or 2% EPA or DHA alone or in combination of both over a period, growing from 50 to 400 g. Primary head kidney leucocytes were isolated and stimulated with Toll-like receptor (TLR) ligands to determine if EPA and DHA deficiency can affect expression of important immune genes and eicosanoid production. Several genes related to viral immune response did not vary between groups. However, there was a tendency that the high-level EPA and DHA groups expressed lower levels of IL-1β in non-stimulated leucocytes. These leucocytes were also more responsive to the TLR ligands, inducing higher expression levels of IL-1β and Mx1 after stimulation. The levels of prostaglandin E2 and leukotriene B4 in serum and media from stimulated leucocytes were lower in both low and high EPA and DHA groups. In conclusion, leucocytes from low EPA and DHA groups seemed to be less responsive towards immunostimulants, like TLR ligands, indicating that low levels or absence of dietary EPA and DHA may have immunosuppressive effects.

Involvement of LuxS in Aeromonas salmonicida metabolism, virulence and infection in Atlantic salmon (Salmo salar L)

Quorum sensing is a bacterial density dependent communication system, which regarded to regulate co-operative behaviors of community and mediated by extracellular signal molecules named autoinducers (AI). Among various signals, autoinducer-2 (AI-2) is believed to be the messengers inter species and produced by LuxS. For Aeromonas salmonicida (A. salmonicida), an opportunistic pathogen to many cold-water teleost, little information has been known about the function of AI-2 and LuxS. Therefore, our aim was to preliminarily clarify the function of LuxS in A. salmonicida. The consequences demonstrated that wild type A. salmonicida exhibited AI-2 activity and luxS defective mutant strain fail to produce AI-2 signals. Furthermore, it was suggested that luxS deficiency could impact bacterial morphology, surface properties and virulence dramatically. Challenge experiment showed a tendency that immune factors expressed earlier when Atlantic salmon was infected with ΔluxS strain. Overall, we hypothesis that AI-2 quorum sensing could regulate the expression of A-layer protein coding gene vapA, and then influence bacterial survival ability when suffered from attack of the host immune system. Though additional studies are warranted, our study will supply a new thinking to control the damage caused by A. salmonicida.

Identification and functional analysis of the toll-like receptor 20.2 gene in grass carp, Ctenopharyngodon idella

We characterized and identified the cDNA sequence of Toll-like receptor 20.2 in Ctenopharyngodon idella (gctlr20.2); it consisted of 3197 bp, with an open reading frame of 2835 bp that encoded a 944 amino acid polypeptide. Relatively, high expression levels of gctlr20.2 were observed in the spleen, head kidney, liver and brain tissues, with lower expression levels in the trunk kidney, intestine and heart tissues. In vivo and in vitro, after being challenged with Aeromonas hydrophila or grass carp reovirus (GCRV), gctlr20.2 expression was induced in C. idella kidney cells stimulated with lipopolysaccharide, flagellin or polyinosinic-polycytidylic acid. Overexpression of gctlr20.2 increased the expression of il1β, il8 and tnf-α, but not ifn, and also increased the activity of the nf-κB signal pathway. Silencing, via siRNA-tlr20.2, inhibited gctlr20.2 transcription by 65.7% and down-regulated the expression of inflammatory cytokine genes, but not tnf-α. This study increases understanding of the immune system in C. idella.

Characterization of Toll-like receptor gene expression in goldfish (Carassius auratus) during Dactylogyrus intermedius infection

Toll-like receptors (TLRs), the first and best understood innate immune receptors, play a notable role in the innate immune system by sensing pathogenic agents and initiating appropriate immune responses. However, studies about the roles of fish TLRs in response to the infection of the ectoparasitic monogenean Dactylogyrus intermedius have been surprisingly vacant. In the present study, cDNA fragments of five members of TLRs family in goldfish (Carassius auratus) were cloned and the expression patterns of nine TLRs in five tissues at different time points during D. intermedius infection were subsequently investigated. We found that the expressions of TLR4, TLR5, TLR20 and TLR22 were significantly elevated after infection at some time points, of which the transcription of TLR5 was progressively increased nearly in all tissues, whereas the mRNA levels of other TLRs (TLR2, 3, 7, 9 and 21) were down-regulated or showed no significant change compared with the control at most time points. Additionally, this paper was also conducted to explore the expression of above TLRs after re-infected with D. intermedius. The results showed a significant upregulation of TLR4, TLR5 and TLR22 in all tested tissues at these two time points, especially the levels of TLR4 and TLR22 expression, were even higher comparing with the first infection. Besides, tissue-specific expression analysis revealed that spleen featured the highest expressions of almost all the TLR-encoding genes among detected tissues. The informations obtained here could be helpful towards understanding the functions of TLRs in response to parasitic infection in goldfish and provide new insights for the development of preventive and therapeutic approaches against D. intermedius infection.

TLR21's agonists in combination with Aeromonas antigens synergistically up-regulate functional TLR21 and cytokine gene expression in yellowtail leucocytes

The purpose of this study was to characterize the TLR21 gene from yellowtail (Seriola lalandi) and its functional activity using TLR agonist stimulation and Aeromonas antigens. The TLR21 nucleotide sequence from yellowtail was obtained using the whole-genome shotgun sequencing method and bioinformatics tools. Basal TLR21 gene expression was analyzed in several tissues. Subsequently, the gene expression of TLR21 and cytokines IL-1β and TNF-α was evaluated in TLR agonist (CpG-ODN2006, LPS, and Poly I:C) exposing head kidney leucocytes, which were then subjected to Aeromonas antigen stimulation. The yellowtail full-length cDNA sequence of SlTLR21 was 3615 bp (980 aa) showing a high degree of similarity with the counterparts of other fish species and sharing the common structural architecture of the TLR family, including LRR domains, one C-terminal LRR region, and a TIR domain. Gene expression studies revealed the constitutive expression of TLR21 mRNA in all the analyzed tissues; the highest levels were observed in spleen and head kidney where they play an important role in the fish immune system. Transcripts of TLR21 and the downstream IL-1β and TNF-α cytokine genes were most strongly up-regulated after exposure to the TLR agonists following Aeromonas antigen stimulation, suggesting they are involved in immune response. The results indicated that TLR agonists, in combination with Aeromonas antigens in head kidney leucocytes, synergistically enhance TLR21 and cytokines in yellowtail.

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.

Protective effects of leucine against lipopolysaccharide-induced inflammatory response in Labeo rohita fingerlings

The present study investigated the protective effects of leucine against lipopolysaccharide (LPS)-induced inflammatory responses in Labeo rohita (rohu) in vivo and in vitro. Primary hepatocytes, isolated from the hepatopancreas, were exposed to different concentrations of LPS for 24 h to induce an inflammatory response, and the protective effects of leucine against LPS-induced inflammation were studied. Finally, we investigated the efficiency of dietary leucine supplementation in attenuating an immune challenge induced by LPS in vivo. Exposure of cells to 10-25 μg mL(-1) of LPS for 24 h resulted in a significant production of nitric oxide and release of lactate dehydrogenase to the medium, whereas cell viability and protein content were reduced (p < 0.05). LPS exposure (10 μg mL(-1)) increased mRNA levels of the pro-inflammatory cytokines TNF-α, IL-1β and IL-8 in vitro (p < 0.05). However, pretreatment with leucine prevented the LPS-induced upregulation of TNF-α, IL-1β and IL-8 mRNAs by downregulating TLR4, MyD88, NF-κBp65, and MAPKp38 mRNA expression. Interestingly, mRNA expression of the anti-inflammatory cytokine, IL-10, which was increased by LPS treatment, was further enhanced (p < 0.05) by leucine pretreatment. The enhanced expression of IL-10 might inhibit the production of other pro-inflammatory cytokines. It was found that leucine pretreatment attenuated the excessive activation of LPS-induced TLR4-MyD88 signaling as manifested by lower level of TLR4, MyD88, MAPKp38, NF-κBp65 and increased level of IκB-α protein in leucine pre-treatment group. In vivo experiments demonstrated that leucine pre-supplementation could protect fish against LPS-induced inflammation through an attenuation of TLR4-MyD88 signaling pathway. Taken together, we propose that leucine pre-supplementation decreases LPS-induced immune damage in rohu by enhancing the expression of IL-10 and by regulating the TLR4-MyD88 signaling pathways.

Immunostimulatory effect of salmon prolactin on expression of Toll-like receptors in Oncorhynchus mykiss infected with Piscirickettsia salmonis

In aquaculture, antibiotics are the traditional treatment used against bacterial infections. However, their use has increasingly come into question given their effects on fish and, possibly, on human health. Consequently, there is interest in developing alternative treatments aimed at stimulating the innate immune response of fish, which is the first line of defense against pathogens. In relation to this, the Toll-like receptors (TLR) aid in the selective identification of pathogens. The present study evaluated immunostimulatory activity of prolactin (PRL) hormone on expression levels of TLR1, 9, and 22, MyD88, and IL-1β during in vitro infection with the fish pathogen Piscirickettsia salmonis, in primary cultures of Oncorhynchus mykiss head kidney cells. Results indicated that PRL increased expression of TLRs and MyD88 during the first hours of bacterial infection, while a constant increase in expression was found for IL-1β. These findings suggest that PRL indirectly modulates expression of TLRs by activating expression of suppressors of cytokine signaling, thereby regulating immune response over long periods of time during bacterial infection.

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.

Use of Poly(I:C) Stabilized with Chitosan As a Vaccine-Adjuvant Against Viral Hemorrhagic Septicemia Virus Infection in Zebrafish

There is an urgent need for more efficient viral vaccines in finfish aquaculture worldwide. Here, we report the use of poly(I:C) stabilized with chitosan as an adjuvant for development of better finfish vaccines. The adjuvant was co-injected with inactivated viral hemorrhagic septicemia virus (VHSV) (CSpIC+iV vaccine) in adult zebrafish and its efficiency in protection against VHSV infection was compared to a live, attenuated VHS virus vaccine (aV). Both free and stabilized poly(I:C) were strong inducers of an antiviral state, measured by transcriptional activation of the genes of viral sensors: toll-like receptors, interferons, and interferon-stimulated genes, such as MXa within 48 h after injection. Both the CSpIC+iV and the aV formulations provided a significant protection against VHSV-induced mortality. However, when plasma from survivors was tested for neutralizing antibodies in an in vitro protection assay, we could not demonstrate any protective effect. On the contrary, plasma from aV vaccinated fish enhanced cytopathic effects, indicating that antibody-dependent entry may play a role in this system. Our results show that poly(I:C) is a promising candidate as an adjuvant for fish vaccination against viral pathogens, and that the zebrafish is a promising model for aquaculture-relevant vaccination studies.

Aeromonas salmonicida Infection Only Moderately Regulates Expression of Factors Contributing to Toll-Like Receptor Signaling but Massively Activates the Cellular and Humoral Branches of Innate Immunity in Rainbow Trout (Oncorhynchus mykiss)

Toll-like receptors (TLRs) are known to detect a defined spectrum of microbial structures. However, the knowledge about the specificity of teleost Tlr factors for distinct pathogens is limited so far. We measured baseline expression profiles of 18 tlr genes and associated signaling factors in four immune-relevant tissues of rainbow trout Oncorhynchus mykiss. Intraperitoneal injection of a lethal dose of Aeromonas salmonicida subsp. salmonicida induced highly increased levels of cytokine mRNAs during a 72-hour postinfection (hpi) period. In contrast, only the fish-specific tlr22a2 and the downstream factor irak1 featured clearly increased transcript levels, while the mRNA concentrations of many other tlr genes decreased. Flow cytometry quantified cell trafficking after infection indicating a dramatic influx of myeloid cells into the peritoneum and a belated low level immigration of lymphoid cells. T and B lymphocytes were differentiated with RT-qPCR revealing that B lymphocytes emigrated from and T lymphocytes immigrated into head kidney. In conclusion, no specific TLR can be singled out as a dominant receptor for A. salmonicida. The recruitment of cellular factors of innate immunity rather than induced expression of pathogen receptors is hence of key importance for mounting a first immune defense against invading A. salmonicida.

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.

The impact of Aeromonas salmonicida infection on innate immune parameters of Atlantic salmon (Salmo salar L)

Enzyme activities and gene expression of a number of innate immune parameters in the serum, mucus and skin of Atlantic salmon (Salmo salar) were investigated after challenge with a pathogenic strain of Aeromonas salmonicida (A. salmonicida). Fish were injected in the dorsal muscle with either 100 μl bacterium solution, about 3.05 × 10(7) CFU/ml A. salmonicida, or 100 μl 0.9% NaCl (as control group) and tissue samples were collected at days 0, 2, 4 and 6 post-injection. Lysozyme (LSZ) and alkaline phosphatase (AKP) activities in serum, mucus and skin, and LSZ and AKP mRNA expression in skin of the challenged fish were higher than those of the control at most of the experimental time, with significant differences at several time points (P < 0.05), indicating the involvement of LSZ and AKP in the innate immunity of Atlantic salmon to A. salmonicida. Superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) activities in mucus and skin, along with the SOD, POD and CAT mRNA expression in skin significantly decreased at day 4 and 6, indicating the decreased antioxidant capacity of the challenged fish. Glutamate pyruvate transaminase (GPT) and glutamic oxalacetic transaminase (GOT) activities in serum, mucus and skin of the challenged group were all higher than those of the control after the injection, and at several time points significant differences were found between the two groups, suggesting organs of fish were impaired after the pathogen infection. The changes of the GPT and GOT activities could be used as potential biomarkers for the impairment of physiological functions caused by the pathogen infection. Identified biomarkers of the immune responses will contribute to the early-warning system of the disease. So this study will not only provide a theoretical basis for vaccine development, but also provide basic data for the establishment of early warning systems for diseases caused by A. salmonicida in Atlantic salmon rearing.

In vitro and in vivo protective effect of arginine against lipopolysaccharide induced inflammatory response in the intestine of juvenile Jian carp (Cyprinus carpio var. Jian)

The present study was designed to assess the possible protective effects of arginine (Arg) against lipopolysaccharide (LPS) induced inflammatory response in juvenile Jian carp (Cyprinus carpio var. Jian) in vivo and in enterocytes in vitro. Firstly, inflammatory response was established by exposing enterocytes to different concentrations of LPS for 24 h. Secondly, the protective effects of Arg against subsequent LPS exposure were studied in enterocytes. Finally, we investigated whether dietary Arg supplementation could attenuate immune challenge induced by LPS in vivo. The result indicated that 10 mg/L LPS could induced inflammatory response in enterocytes. Cells exposed to LPS (10-30 mg/L) alone for 24 h resulted in a significant increase in lactate dehydrogenase release (LDH) (P < 0.05). The cell viability, protein content, alkaline phosphatase activity were decreased by LPS (P < 0.05). Moreover, LPS exposure significantly increased TNF-α, IL-1β, and IL-6 mRNA expression in vitro (P < 0.05). However, pre-treatment with Arg remarkably prevented the increase of TNF-α, IL-1β, and IL-6 by inhibiting the excessive activation of TLR4-Myd88 signaling pathway through down-regulating TLR4, Myd88, NFκB p65, and MAPK p38 mRNA expression (P < 0.05). Interestingly, the experiment in vivo showed that Arg pre-supplementation could attenuate immune challenge induced by LPS via TLR4-Myd88 signaling pathway, and thus protect fish against LPS-induced inflammatory response. In conclusion, all of these results indicated pre-supplementation with Arg decreased LPS induced immune damage and regulated TLR4-Myd88 signaling pathway in juvenile Jian carp in vivo and in enterocytes in vitro.

Identification and characterisation of TLR18-21 genes in Atlantic salmon (Salmo salar)

Teleost fish possess many types of toll-like receptor (TLR) some of which exist in other vertebrate groups and some that do not (ie so-called "fish-specific" TLRs). In this study, we identified in Atlantic salmon (Salmo salar) whole-genome shotgun (WGS) contigs seven TLRs that are not found in mammals, including six types of fish-specific TLRs (one TLR18, one TLR19, and four TLR20 members (two of which are putative soluble forms (s)) and one TLR21. Phylogenetic analysis revealed that teleost TLR19-21 are closely related with murine TLR11-TLR13, whilst teleost TLR18 groups with mammalian TLR1, 2, 6 and 10. A typical TLR protein domain structure was found in all these TLRs with the exception of TLR20b(s) and TLR20c(s). TLR-GFP expression plasmids transfected into SHK-1 cells showed that salmon TLR19, TLR20a and TLR20d were preferentially localised to the intracellular compartment. Real time PCR analysis suggested that salmon TLR19-TLR21 are mainly expressed in immune related organs, such as spleen, head kidney and gills, while TLR18 transcripts are more abundant in muscle. In vitro stimulation of primary head kidney cells with type I IFN, IFNγ and IL-1β had no impact on TLR expression. Infectious salmon anaemia virus (ISAV) infection, in vivo, down-regulated TLR20a, TLR20b(s), TLR20d and TLR21 in infected salmon kidney tissue. In contrast, up-regulation of TLR19 and TLR20a expression was found in posterior kidney in rainbow trout with clinical proliferative kidney disease (PKD).