Prediction of the prototype of the human Toll-like receptor gene family from the pufferfish, Fugu rubripes, genome

Diversification of Toll-like receptor 1 in swamp eel (Monopterus albus)

Toll-like receptor 1 (TLR1) is a pattern recognition receptor that plays critical roles in triggering immune activation via detecting bacterial lipoproteins and lipopeptides. In this study, the genetic characteristic of TLR1 was studied for an important aquaculture fish, swamp eel Monopterus albus. The eel has been seriously threatened by infectious diseases. However, a low level of genetic heterogeneity in the fish that has resulted from a demographic bottleneck presents further challenges in breeding for disease resistance. A comparison with the homologue of closely related species M. javanensis revealed that amino acid replacement (nonsynonymous) but not silent (synonymous) differences have accumulated nonrandomly over the coding sequences of the receptors at the early stage of their phylogenetic split. The combined results from comparative analyses of nonsynonymous-to-synonymous polymorphisms showed that the receptor has undergone significant diversification in M. albus driven by adaptive selection likely after the genetic bottleneck. Some of the changes reported here have taken place in the structures mediating heterodimerization with co-receptor TLR2, ligand recognition, and/or formation of active signaling complex with adaptor, which highlighted key structural elements and strategies of TLR1 in arms race against exogenous challenges. The findings of this study will add to the knowledge base of genetic engineering and breeding for disease resistance in the eel.

Teleost-specific TLR23 in Takifugu rubripes recruits MyD88 to trigger ERK pathway and promotes antibacterial defense

Takifugu rubripes is a highly valued cultured fish in Asia, while pathogen infections can result in severe diseases and lead to substantial economic losses. Toll-like receptors (TLRs), as pattern recognition receptors, play a crucial role on recognition pathogens and initiation innate immune response. However, the immunological properties of teleost-specific TLR23 remain largely unknown. In this study, we investigated the biological functions of TLR23 (TrTLR23) from T. rubripes, found that TrTLR23 existed in various organs. Following bacterial pathogen challenge, the expression levels of TrTLR23 were significantly increased in immune related organs. TrTLR23 located on the cellular membrane and specifically recognized pathogenic microorganism. Co-immunoprecipitation and antibody blocking analysis revealed that TrTLR23 recruited myeloid differentiation primary response protein (MyD88), thereby mediating the activation of the ERK signaling pathway. Furthermore, in vivo showed that, when TrTLR23 is overexpressed in T. rubripes, bacterial replication in fish tissues is significantly inhibited. Consistently, when TrTLR23 expression in T. rubripes is knocked down, bacterial replication is significantly enhanced. In conclusion, these findings suggested that TrTLR23 played a critical role on mediation TLR23-MyD88-ERK axis against bacterial infection. This study revealed that TLR23 involved in the innate immune mechanism, and provided the foundation for development disease control strategies in teleost.

Identification and functional analysis of Toll-like receptor 2 from razor clam Sinonovacula constricta

Toll-like receptor 2 (TLR2) is a member of TLR family that plays important roles in the innate immune system, such as pathogen recognition and inflammation regulation. In this study, the TLR2 homologue was cloned from razor clam Sinonovacula constricta (denoted as ScTLR2) and its immune function was explored. The full-length cDNA of ScTLR2 comprised 2890 nucleotides with a 5'-UTR of 218 bp, an open reading frame of 2169 bp encoding 722 amino acids and a 3'-UTR of 503 bp. The deduced amino acid of ScTLR2 showed similar structure to TLR2 homologue with a conserved signal peptide, four LRR domains, one LRR-TYP domain, one LRR-CT domain, one transmembrane domain and a conserved TIR domain. ScTLR2 mRNA was detected in all examined tissues with the highest expression in the gill. After Vibrio parahaemolyticus challenge, the mRNA expression of ScTLR2 was significantly induced both in gill and haemocytes. The recombinant ScTLR2-LRR protein could bind all tested PAMPs including LPS, PGN and MAN. Bacterial agglutination assay showed that rScTLR2 could agglutinate the six tested bacteria with a calcium dependent manner. More importantly, ScTLR2 silencing by siRNA transfection could significantly depress the mRNA expression of Myd88, NF-κB, Tollip, IRF1, and IRF8. The survival rate of S. constricta was markedly decreased after V. parahaemolyticus challenge under this condition. Our current study demonstrated that ScTLR2 served as a pattern recognition receptor to induce immune response against invasive pathogen.

Transcriptomic Responses to Koi Herpesvirus in Isolated Blood Leukocytes from Infected Common Carp

Koi herpesvirus (KHV, CyHV-3) causes severe economic losses in carp farms. Its eradication is challenging due to the establishment of latency in blood leukocytes and other tissues. To understand the molecular mechanisms leading to KHV infection in leukocytes, common carp were bath-exposed to KHV at 17 °C. After confirming the presence of viral transcripts in blood leukocytes at ten days post infection, RNA-Seq was performed on peripheral blood leukocytes on the Illumina NovaSeq. KHV infection triggered a robust immune response mediated by pattern recognition receptors, mainly toll-like receptors (tlr2, tlr5, tlr7, and tlr13), urokinase plasminogen activator surface receptor-like, galectin proteins, and lipid mediators such as leukotriene B4 receptor 1. Enriched pathways showed increased mitochondria oxidative phosphorylation and the activation of signalling pathways such as mitogen-activated protein kinases (MAPKs) and vascular endothelial growth factor (VEGF). KHV-infected leukocytes showed low production of reactive oxygen species (ROS) and glutathione metabolism, high iron export and phagocytosis activity, and low autophagy. Macrophage polarization was deduced from the up-regulation of genes such as arginase non-hepatic 1-like, macrophage mannose receptor-1, crem, il-10, and il-13 receptors, while markers for cytotoxic T cells were observed to be down-regulated. Further work is required to characterise these leukocyte subsets and the molecular events leading to KHV latency in blood leukocytes.

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.

Comprehensive transcripts analysis based on single-molecule real-time sequencing and Illumina sequencing provides insights into the mining of Toll-like receptor family in Schizothorax lissolabiatus

Schizothorax lissolabiatus is an economically important cold-water fish species in southwestern China. Because of water pollution and habitat destruction, the number of wild populations has dramatically decreased. In this study, we used PacBio single-molecule real-time (SMRT) sequencing and Illumina sequencing to generate the first full-length transcriptome and transcriptome, respectively. A total of 19 310 polished consensus reads (PC) were obtained, with an average length of 1379 bp and an N50 length of 1485 bp. Meanwhile, 12 253 transcripts were successfully annotated as known homologous genes. The pathway annotation indicated that the enrichment and expression of most genes were mainly related to membrane, signal transduction and binding, and immune response. Furthermore, we identified 16 Toll-like receptors (TLRs) by mining the data from the transcripts. Phylogeny analysis showed that S. lissolabiatus TLR genes (slTLRs) supported the classification of TLRs into six families as in other vertebrates. Selection pressure analyses showed that 16 slTLRs revealed purification selection at the overall evolutionary selection. Further, positive selection signals were still detected in eight slTLRs, and most of the positive selection sites were located in the leucine-rich repeat region (LRR domain) associated with the recognition of pathogenic microorganisms, indicating that the function of these slTLR genes may be affected. Tissue specific expression analysis showed all slTLRs are present in kidney, spleen and liver but the relative expression varied among tissues. In conclusion, this study not only provided a valuable resource of transcripts for further research on S. lissolabiatus, but also contributed to improve the current understanding of the evolutionary history of immune-related genes and the TLR gene family in S. lissolabiatus.

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.

Genetic diversity of Toll-like receptor 9 in swamp eels (Monopterus albus)

The swamp eel, Monopterus albus, is an important aquaculture species in Asia (mainly China) whose production has seriously suffered from infectious diseases. In spite of the critical requirement for aquaculture practices, to date there is scant information on its immune defence. Here, the genetic characteristics of Toll-like receptor 9 (TLR9), which plays crucial roles in the initiation of host defence against microbial invasion, were analysed. It exhibits a striking lack of genetic variation resulting from a recent demographic bottleneck. A comparison with the homologue of M. javanensis revealed that replacement but not silent differences have nonrandomly accumulated in the coding sequences at the early stage following their split from a common ancestor. Furthermore, the replacements relevant to the type II functional divergence have mainly occurred in structural motifs mediating ligand recognition and receptor homodimerization. These results provide hints to understand the diversity-based strategy of TLR9 in the arms race against pathogens. Furthermore, the findings reported here give credence to the importance of basic immunology knowledge, especially for the key elements, in genetic engineering and breeding for disease resistance in the eel and other fishes.

Characterization analysis of TLR5a and TLR5b immune response after different bacterial infection in grass carp (Ctenopharyngodon idella)

Toll-like receptor (TLR) is an important pattern recognition receptor, which specifically recognizes microbial components, and TLR5 recognizes bacterial flagellin in vertebrates and invertebrates. In this study, two forms of TLR5 (TLR5a and TLR5b) were identified in grass carp (Ctenopharyngodon idella). Aeromonas hydrophila and Staphylococcus aureus were used to investigate the role of grass carp TLR5a and TLR5b against bacteria (flagellate and non-flagellate) in innate immunity, and the expression of TLR5a and TLR5b genes and proteins were detected in immune-related tissues. Quantitative real-time polymerase chain reaction results showed that TLR5a and TLR5b genes of grass carp were highly expressed in the liver, spleen, and head kidney, and their expression patterns were similar in tissues. Meanwhile, the TLR5b gene expression was higher than TLR5a in most tissues. Following exposure to A. hydrophila and S. aureus, the expression levels of TLR5a and TLR5b genes in the liver, spleen, and head kidney were up-regulated significantly. Moreover, the downstream gene, NF-κB, was up-regulated significantly. After A. hydrophila infection, the expression of TLR5a gene was up-regulated in the liver and spleen at 24 h, while TLR5b was up-regulated at 6 h. In the head kidney, TLR5a was up-regulated at 6 h, while TLR5b was up-regulated at 6 h and 12 h. After S. aureus infection, TLR5a and TLR5b were up-regulated at 6 h in the liver and 12 h in the spleen. However, in the head kidney, TLR5a was down-regulated, while TLR5b was up-regulated. Compared with TLR5a, TLR5b had a higher expression level and stronger response to pathogen stimulation. The immunofluorescence results showed that TLR5a and TLR5b proteins in the liver of grass carp infected with A. hydrophila and S. aureus were similar but different in the spleen and head kidney. The results indicated that TLR5a and TLR5b play a critical role in resisting bacterial infection, and TLR5a and TLR5b had obvious tissue and pathogen specificity. TLR5b may play a major role in immune tissues, while TLR5a may play an auxiliary regulatory role in early infection. In addition, TLR5a and TLR5b have an irreplaceable regulatory role in response to flagellate and non-flagellate bacteria. This lays a foundation to explore further the role of TLR5 in resisting flagellate and non-flagellate infections in fish and provides a reference for the innate immunity research of grass carp.

Expression and function of toll-like receptor 2 in vertebrate

Toll-like receptors (TLRs) are essential for identifying and detecting pathogen-associated molecular patterns (PAMPs) produced by a variety of pathogens, including viruses and bacteria. Since TLR2 is the only TLR capable of creating functional heterodimers with more than two other TLR types, it is very important for vertebrate immunity. TLR2 not only broadens the variety of PAMPs that it can recognize but has also the potential to diversify the subsequent signaling cascades. TLR2 is ubiquitous, which is consistent with the wide variety of tasks and functions it serves. Immune cells, endothelial cells, and epithelial cells have all been found to express TLR2. This review aims to gather currently available information about the preservation of this intriguing immunological molecule in the phylum of vertebrates.

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 soluble TLR5 is involved in PBLs activation and antibacterial immunity via TLR5M-MyD88-signaling pathway in tongue sole Cynoglossus semilaevis

In higher vertebrates, there is only a membranal TLR5 (TLR5M), which is crucial for host defense against microbes via MyD88 signaling pathway. In teleost, both TLR5M and soluble TLR5 (TLR5S) are identified, whereas the antibacterial mechanism of TLR5S is largely unknown. In this study, we studied the immune antibacterial mechanism of Cynoglossus semilaevis TLR5S homologue (named CsTLR5S). CsTLR5S, a 71.1 kDa protein, consists of 649 amino acid residues and shares 41.7 %-57.8 % overall sequence identities with teleost TLR5S homologues. CsTLR5S contains a single extracellular domain (ECD) composed of 12 leucine-rich repeats. CsTLR5S expression was constitutively identified and upregulated by bacterial infection in tissues. In vitro recombinant CsTLR5S (rCsTLR5S) could interact with bacteria and tongue sole rTLR5M (rCsTLR5M). Furthermore, rCsTLR5S could bind to the membranal CsTLR5M of peripheral blood leukocytes (PBLs), which led to enhancing the activity and the antibacterial role of PBLs via Myd88-NF-κB pathway. In vivo rCsTLR5S could activate the Myd88-NF-κB pathway, facilitate the release of proinflammatory cytokines, and enhance the host antibacterial response against Vibrio harveyi. Moreover, the knockdown of CsTLR5M or the Myd88 inhibitor could significantly suppress the antibacterial effect of rCsTLR5S. Collectively, our findings added important insights into the TLR5S immune antibacterial property in a TLR5M-MyD88-dependent manner.

Recombinant Vibrio harveyi flagellin A protein and partial deletions of middle variable region and D0 domain induce immune related genes in Epinephelus coioides and Cyprinus carpio

Vibrio harveyi is a Gram-negative bacterium that causes vibriosis in various aquaculture species, including the orange-spotted grouper (Epinephelus coioides). Bacterial flagellin is a potent pathogen-associated molecule that stimulates the innate and adaptive immune systems through toll-like receptor 5 (TLR5) signaling. In this study, we isolated V. harveyi flagellin A (VhFliA) gene from V. harveyi (originated from orange-spotted grouper) and investigated the in vivo activities of recombinant VhFliA protein. Multiple sequence alignment showed that the amino acid sequence of VhFliA has conserved domains of N- and C-terminals (D0 and D1) and a middle variable (MV) region. We produced the VhFliA recombinant protein (wild type (WT)-VhFliA) by Escherichia coli and investigated its in vivo biological activity. Additionally, we prepared the VhFliA recombinant proteins with deletion of domains (ΔMV-VhFliA and ΔD0MV-VhFliA) to identify the domain for biological activity in the orange-spotted grouper. WT and ΔMV-VhFliA induced the expression of inflammatory cytokines (IFNγ, IL-1β, and IL-8) in groupers. However, ΔD0MV-VhFliA did not induce the expression of inflammatory cytokines. Additionally, to demonstrate the applicability of recombinant VhFliA to teleost species, we performed an in vivo assay of the recombinant proteins in koi carp (Cyprinus carpio). WT-VhFliA stimulates the expression of inflammatory cytokines (IL-1β, IL-6, and IL-8) in carp. ΔMV-VhFliA did not upregulate IL-1β and IL-6, whereas ΔD0MV-VhFliA induced expression in carp. These findings showed the potential of VhFliA as an effective immune stimulant adjuvant and comparative studies of flagellin - TLR5 signaling in teleosts.

Genome-wide identification and characterization of toll-like receptor 5 (TLR5) in fishes

Toll-like receptors 5 (TLR5), a member of the toll-like receptors (TLRs) family, is a class of pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs). It responds to vertebrate recognition of bacterial flagellin and participates in innate immune responses. However, genome-wide identification and characterization of TLR5 in fishes have not been investigated. Here, three TLR5M isotypes (TLR5Ma, TLR5Mb1, and TLR5Mb2) and a TLR5S are all extracted from fish genomes on the basis of phylogenetic and synteny analyses. We confirmed that the non-teleost fishes have one TLR5M gene, as well as additional TLR5 genes (TLR5M and TLR5S) in teleost fishes. In addition, some special teleost fishes possess two to three TLR5 genes, which have undergone the fourth whole-genome duplication (WGD). According to our results, we inferred that the diversity of TLR5 genes in fishes seems to be the result of combinations of WGD and gene loss. Furthermore, TLR5 isoforms displayed differences at the flagellin interaction sites and viral binding sites, and showed lineage-specific, which indicated that TLR5 duplicates may generate functional divergence. Bacterial experiments also supported the idea that CiTLR5Ma and CiTLR5Mb are subfunctionalized to sense bacterial flagellin. In summary, our present comparative genomic survey will benefit for further functional investigations of TLR5 genes in fish.

Identification and characterization of Toll-like receptor 14d in Northeast Chinese lamprey (Lethenteron morii)

Toll-like receptors (TLRs) play an important role in innate immunity of defense against bacterial or viral pathogens. To study the biological characteristics and functions of the TLR genes, TLR14d was identified from Northeast Chinese lamprey (Lethenteron morii) and named LmTLR14d. LmTLR14d coding sequence (cds) is 3285 bp in length and encodes 1094 amino acids (aa). The results showed that LmTLR14d has the typical structure of TLR molecule, which contains the extracellular domain of leucine-rich repeats (LRR), transmembrane domain, and intracellular domain of Toll/interleukin-1 receptor (TIR). The phylogenetic tree showed that LmTLR14d is a homologous gene of TLR14/18 in bony fish. Quantitative real-time PCR (qPCR) revealed that LmTLR14d was expressed in various healthy tissues, including immune and non-immune tissues. Pseudomonas aeruginosa infection up-regulated LmTLR14d in the supraneural body (SB), gill, and kidney tissues of infected Northeast Chinese lamprey. Immunofluorescence results showed that LmTLR14d was located in the cytoplasm of HEK 293T cells in clusters, and its subcellular localization was determined by the TIR domain. The immunoprecipitation results showed that LmTLR14d could recruit L.morii MyD88 (LmMyD88) but not L.morii TRIF (LmTRIF). Dual luciferase reporter results showed that LmTLR14d significantly enhanced the activity of L.morii NF-κβ (LmNF-κβ) promoter. Furthermore, co-transfection of LmTLR14d with MyD88 significantly enhanced the L.morii NF-κβ (LmNF-κβ) promoter activity. LmTLR14d can induce the expression of inflammatory cytokine genes il-6 and tnf-α downstream of NF-κB signal. This study suggested that LmTLR14d might play an important role in the innate immune signal transduction process of lamprey and revealed the origin and function of teleost-specific TLR14.

Identification and functional characteristics of two TLR5 subtypes in S. grahami

TLR5, as a member of Toll-like receptors (TLRs) family in mammals, is responsible for recognizing bacterial flagellin and initiating innate immunity, but its function is still unclear in fish species. In this study, two family members of TLR5 were cloned and identified from Sinocyclocheilus grahami (S. grahami), named sgTLR5a and sgTLR5b. The length of coding sequence of sgTLR5a and sgTLR5b is 2,622 bp and 2,658 bp, encoding 873 and 885 amino acids, respectively. Molecular phylogenetic analysis indicates that sgTLR5a and sgTLR5b have the closest genetic relationship with TLR5M (membrane-type) of Cyprinus carpio and Schizothorax prenanti, respectively. sgTLR5a and sgTLR5b were widely expressed in various tested tissues, of which the expression levels were the highest in skin tissue. After stimulations of Aeromonas hydrophila (A. hydrophila) and flagellin, the expression levels of sgTLR5a and sgTLR5b in liver, spleen and head kidney tissues were strongly up-regulated, but LPS stimulation only increased the expression of sgTLR5b in these tissues. The luciferase reporter assay displayed that sgTLR5a and sgTLR5b could specifically recognize bacterial flagellin and A. hydrophila and activate the downstream NF-κB signaling pathway in HEK293T cells. Moreover, the overexpression of sgTLR5a and sgTLR5b in EPC cells up-regulated the expression levels of IL-8 and TNF. sgTLR5a and sgTLR5b were observed to locate in the intracellular region by confocal microscope. Interestingly, we found that the NF-κB signaling pathway was positively regulated by co-transfecting sgTLR5a or sgTLR5b with TLR trafficking chaperone sgUNC93B1. In conclusion, our results reveal sgTLR5a and sgTLR5b may play an important role in antibacterial response by activating the NF-κB signaling pathway.

Computational analysis and functional characterisation of Tor putitora toll-like receptor 4 with the elucidation of its binding sites for microbial mimicking ligands

In the current study, full-length Toll-like receptor 4 (TLR4) cDNA was cloned and characterised in Tor putitora, an important fish inhibiting Himalayan rivers. The complete coding sequence of TpTLR4 is 2457 bp with nine key structural domains, including six leucine-rich repeats (LRRs). The phylogenetic tree revealed that TpTLR4 showed the closest relationship with TLR4 of Cyprinus carpio (96%), Labeo rohita (91%) and Megalobrama amblycephala (88%), all belonging to the Cyprinidae family. CELLO2GO tool revealed that TpTLR4 protein is highly localised in the plasma (67.7%), and the protein has a strong association with myeloid differentiation primary response 88 (MYD88) followed by Tumor necrosis factor receptor-associated factor (TRAF) family. In the toll-interleukin-1 receptor (TIR) domain of TpTLR4, the proline is replaced by the alanine amino acid, thus may give plasticity to the receptor to recognise both bacterial and viral ligands. Molecular docking has revealed that TpTLR4 showed the strongest affinity towards poly (I:C) with the binding energy of -6.1 kcal/mol and five hydrogen bonds among all ligands. Based on our molecular docking results, it can be presumed that TpTLR4 can sense bacterial, fungal and viral molecular patterns with binding sites mainly present in the TpTLR4 LRR9 motif, which spans between 515 and 602 amino acids. Tor putiora TLR4 transcript was ubiquitously expressed in all the tested fish tissues. Although, transcript level was found to be highest in blood and spleen followed by the kidney. The TpTLR4 transcripts showed peak expression in spleen and kidney at 12 h post-injection (hpi) (p < 0.05) of poly (I:C). The constitutive expression of TpTLR4 in various tissues, up-regulation in different tissues and strong binding affinities with poly (I:C) indicate that TpTLR4 may play an essential role in sensing pathogen-associated molecular patterns (PAMPs), particularly of viral origin.

Genome-Wide Identification, Characterization and Expression Analysis of Toll-like Receptors in Marbled Rockfish (Sebastiscus marmoratus)

Toll-like receptor (TLR) is a cluster of type I transmembrane proteins that plays a role in innate immunity. Based on the marbled rockfish (Sebastiscus marmoratus) genome database, this study used bioinformatics methods to identify and analyze its TLR gene family members. The results showed that there were 11 TLR gene family members in Sebastiscus marmoratus (SmaTLR), which could be divided into five different subfamilies. The number of amino acids encoded by the Smatlr genes ranged from 637 to 1206. The physicochemical properties of the encoded proteins of different members were also computed. The results of protein structure prediction, phylogenetic relation, and motif analysis showed that the structure and function of the SmaTLRs were relatively conserved. Quantitative Real-Time PCR (qRT-PCR) analysis revealed the expression patterns of SmaTLRs in the gill, liver, spleen, head kidney, kidney, and intestine. SmaTLRs were widely detected in the tested tissues, and they tended to be expressed higher in immune-related tissues. After polyriboinosinic polyribocytidylic acid (poly(I:C)) challenge, SmaTLR14, SmaTLR3, SmaTLR5S, SmaTLR7, and SmaTLR22 were significantly upregulated in the spleen or liver. The results of this study will help to understand the status of TLR gene family members of marbled rockfish and provide a basis for further study of the functional analysis of this gene family.

Membrane orthologs of TLR5 of tongue sole Cynoglossus semilaevis: Expression patterns, signaling pathway and antibacterial property

Mammalian toll-like receptor 5 (TLR5) is crucial for recognizing bacterial flagellin and initiating the inflammatory signaling cascades via myeloid differentiation factor 88 (MyD88) signaling pathway, which plays vital roles in innate immune against pathogenic bacteria. Herein, we reported the signaling pathway and antibacterial property of tongue sole (Cynoglossus semilaevis) membrane forms of TLR5 (i.e. CsTLR5M1and CsTLR5M2). CsTLR5M1/M2 contain 936 and 885 amino acid residues respectively. CsTLR5M1 shares 86.7% overall sequence identities with CsTLR5M2. CsTLR5M1/M2 possess the same extracellular domain (ECD) and transmembrane domain (TMD), but the different toll-interleukin-1 receptor (TIR) domain. CsTLR5M1/M2 expression occurred constitutively in multiple tissues and regulated by bacterial stimulation. Recombinant CsTLR5M1/M2 (rCsTLR5M) could bind to flagellin and Gram-negative/positive bacteria, which could suppress bacterial growth. Stimulation of the CsTLR5M pathway by flagellin resulted in increased expression of MyD88-dependent signaling molecules and inflammatory cytokines. Blocking rCsTLR5M by antibody markedly reduced the phagocytosis and ROS production of peripheral blood leukocytes (PBLs), which in turn in vivo promoted the dissemination of bacteria. Overall, these observations add new insights into the signaling pathway and immune function of teleost TLR5M.

Characterization, expression and function analysis of pfTLR5S and pfTLR5M in yellow catfish (Pelteobagrus fulvidraco) responding to bacterial challenge

Toll-Like Receptors (TLRs) are important pattern recognition receptors, playing critical roles in the early innate immune response to defensing against pathogen invasion. In this study, we found both soluble form TLR5 (pfTLR5S) and membrane form TLR5 (pfTLR5M) in yellow catfish Pelteobagrus fulvidraco. The open reading frames (ORFs) of pfTLR5M and pfTLR5S genes were 2655 bp and 1947 bp in length, encoding 884 and 648 amino acids, respectively. pfTLR5M was composed of thirteen LRR domains, one TIR domain and one transmembrane domain. However, pfTLR5S have only fifteen LRR domains, without any TIR domain and transmembrane domain. Both pfTLR5M and pfTLR5S genes had the highest expression in liver, especially for pfTLR5S, which showed a noticeable high expression in liver. We also compared the relative mRNA expression levels of pfTLR5M and pfTLR5S in digestive and immune-related tissues after challenge of three different bacteria. In addition, we also found that pfTLR5S can interact with pfTLR5M, and inhibit the expression of pfTLR5M protein, while induced the expression of downstream proinflammatory factors, such as TNFα and IL8. These results revealed that both pfTLR5M and pfTLR5S play important and different roles in defensing against the invasion of flagellated bacteria, and they may function by binding to each other.

Comparative Transcriptomic Immune Responses of Mullet (Mugil cephalus) Infected by Planktonic and Biofilm Lactococcus Garvieae

Lactococcus garvieae is an important pathogen of fish, associated with high rates of mortality and infection recurrence in summer or stressful conditions. Chronic infection and disease recurrence have also been reported to be associated with biofilms. However, the impact of biofilm and planktonic bacterial infection on fish immune responses remains unclear. In this study, de novo sequencing was used to compare differences of the spleen transcriptome in planktonic- and biofilm-infected mullets. Among the 181,024 unigenes obtained, 3,392 unigenes were associated with immune response genes. Comparative analysis of the gene expression between infection with the L. garvieae planktonic type and biofilm type identified a total of 3,120 and 3,489 differentially expressed genes in response to planktonic and biofilm infection, respectively, of which 1,366 and 1,458 genes were upregulated, and 1,754 and 1,458 genes were downregulated, respectively. Gene ontology enrichment analysis of immune genes identified genes involved in the complement system, toll-like receptor signaling, and antigen processing, which were further verified by qPCR. Additionally, genes encoding TLR2, IL-1β, TNF-α, C7, and MHC class II peptides were downregulated in response to biofilm infection. Importantly, the results show that biofilm infection induces a different immune pathway response compared with planktonic bacterial infection and, furthermore, illustrates that the prevention of biofilm formation may be a necessary and new strategy for controlling bacterial infection in aquaculture.

Rodlet cells in kidney of goldfish (Carassius auratus, Linnaeus 1758): A light and confocal microscopy study

Rodlet cells (RCs) have always been an enigma for scientists. RCs have been given a variety of activities over the years, including ion transport, osmoregulation, and sensory function. These cells, presumably as members of the granulocyte line, are present only in teleosts and play a role in the innate immune response. RCs are migratory cells found in a variety of organs, including skin, vascular, digestive, uropoietic, reproductive, and respiratory systems, and present distinct physical properties that make them easily recognizable in tissues and organs. The development of RCs can be divided into four stages: granular, transitional, mature, and ruptured, having different morphological characteristics. Our study aims to characterize the different stages of these cells by histomorphological and histochemical techniques. Furthermore, we characterized these cells at all stages with peroxidase and fluorescence immunohistochemical techniques using different antibodies: S100, tubulin, α-SMA, piscidin, and for the first time TLR-2. From our results, the immunoreactivity of these cells to the antibodies performed may confirm that RCs play a role in fish defense mechanisms, helping to expand the state of the art on immunology and immune cells of teleosts.

Association study between relative expression levels of eight genes and growth rate in Hungarian common carp (Cyprinus carpio)

One of the most important issues in improving the competitiveness of the fish production sector is to improve the growth rate of fish. The genetic background to this trait is at present poorly understood. In this study, we compared the relative gene expression levels of the Akt1s1, FGF, GH, IGF1, MSTN, TLR2, TLR4 and TLR5 genes in blood in groups of common carps (Cyprinus carpio), which belonged to different growth types and phenotypes. Fish were divided into groups based on growth rate (normal group: n = 6; slow group: n = 6) and phenotype (scaled group: n = 6; mirror group: n = 6). In the first 18 weeks, we measured significant differences (p < 0.05) between groups in terms of body weight and body length. Over the next 18 weeks, the fish in the slow group showed more intense development. In the same period, the slow group was characterized by lower expression levels for most genes, whereas GH and IGF1 mRNA levels were higher compared to the normal group. We found that phenotype was not a determining factor in differences of relative expression levels of the genes studied.

Evaluation of blood cell viability rate, gene expression, and O-GlcNAcylation profiles as indicative signatures for fungal stimulation of salmonid cell models

Fungal diseases of fish are a significant economic problem in aquaculture. Using high-throughput expression analysis, we identified potential transcript markers in primary head kidney and secondary embryonic cells from salmonid fish after stimulation with the inactivated fungi Mucor hiemalis and Fusarium aveneacium and with purified fungal molecular patterns. The transcript levels of most of the 45 selected genes were altered in head-kidney cells after 24 h of stimulation with fungal antigens. Stimulation with the inactivated fungus M. hiemalis induced the most pronounced transcriptional changes, including the pathogen receptor-encoding genes CLEC18A and TLR22, the cytokine-encoding genes IL6 and TNF, and the gene encoding the antimicrobial peptide LEAP2. In parallel, we analyzed the total GlcNAcylation status of embryonic salmonid cells with or without stimulation with inactivated fungi. O-GlcNAcylation modulates gene expression, intracellular protein, and signal activity, but we detected no significant differences after a 3-h stimulation. A pathway analysis tool identified the "apoptosis of leukocytes" based on the expression profile 24 h after fungal stimulation. Fluorescence microscopy combined with flow cytometry revealed apoptosis in 50 % of head-kidney leukocytes after 3 h stimulation with M. hiemalis, but this level decreased by > 5% after 24 h of stimulation. The number of apoptotic cells significantly increased in all blood cells after a 3-h stimulation with fungal molecular patterns compared to unstimulated controls. This in vitro approach identified transcript-based parameters that were strongly modulated by fungal infections of salmonid fish.

Aeromonas hydrophila infection induces Toll-like receptor 2 (tlr2) and associated downstream signaling in Indian catfish, Clarias magur (Hamilton, 1822)

Motile Aeromonas septicaemia (MAS), caused by Aeromonas hydrophila, is one of the most significant bacterial disease responsible for mortality in Indian catfish, Clarias magur, a potential aquaculture species in the Indian subcontinent. In fish, innate immunity elicited by pathogen recognition receptors (PRRs) plays an important role in providing protection against bacterial infection. Information on PRRs including Toll-like receptors (tlrs) and their response to bacterial pathogens remains unexplored in magur. Toll-like receptor 2 (tlr2), a phylogenetically conserved germ-line encoded PRR recognizes specific microbial structure and trigger MyD88-dependent signaling pathway to induce release of various cytokines responsible for innate immune response. In the present study, tlr2 gene of magur was characterized and downstream signaling was studied following challenge with A. hydrophila. The full-length cDNA of magur tlr2 (mtlr2) comprised of 3,066 bp with a single open reading frame of 2,373 bp encoding 790 amino acids having a theoretical pI value of 6.11 and molecular weight of 90 kDa. Structurally, it comprised of signal peptide (1–42aa), one leucine-rich repeat region (LRR) at N-terminal (LRR1-NT: 50–73 aa) and C-terminal (LRR-CT: 588–608 aa), twenty LRRs in between, one trans-membrane (Tm) domain (609–631aa) followed by cytoplasmic TIR domain (670–783aa). Phylogenetically, mtlr2 is closely related to pangasius and channel catfish. Highest basal expression of mtlr2, myd88 and il-1β in spleen, nf-kb in anterior kidney was observed. Lowest basal expression of mtlr2 in skin and myd88, nf-kb and il-1β in muscle was detected. Significant up-regulation of mtlr2 and downstream expression occurred at 3, 8, 24 h post infection to A. hydrophila in important immune organs such as liver, spleen, intestine and kidney. These findings highlight the vital role of tlr2 in eliciting innate immune defence against A. hydrophila infection.

Promoter analysis of TLR5M and TLR5S revealed NF-κB might be a conserved cis-element in TLR5S promoter of large yellow croaker

Toll like receptor 5 (TLR5) plays a crucial role in the innate immune response by recognizing bacterial flagellin proteins. In the present study, the genomic and 5'-flanking sequences of LcTLR5M (membrane) and LcTLR5S (soluble) were cloned from the large yellow croaker, Larimichthys crocea. Then, their promoter activities were determined in human embryonic kidney (HEK293T) cells. LcTLR5M contained 4 exons and 3 introns, and LcTLR5S contained 2 exons and 1 intron. Bioinformatic prediction of transcription factor binding sites (TFBSs) indicated that the promoter structures were different between LcTLR5M and LcTLR5S. A dual luciferase assay showed that the deletion mutant -471 to +189 of LcTLR5M promoter possessed the greatest activity with 36 times activity of the control (P < 0.05). For LcTLR5S, two deletion mutants, -1687 to +106 and - 720 to +106, showed high promoter activity. Furthermore, site-directed mutation demonstrated that a -392 to -391 AT/GG substitution in Oct-1 binding site, and a -104 to -103 GG/TT and a -98 to -97 CC/AA substitution in the NF-κB binding site of TLR5S caused a significant decline of luciferase activity (P < 0.05). Moreover, the co-transfection of an NF-κB/p65 over-expression plasmid with wild type TLR5S (-720 to +106) resulted in an extremely significant increase of promoter activity by more than 9 times compared with the NF-kB mutant (P < 0.01). Our findings suggest that the genomic organization and promoter structure may differ between LcTLR5M and LcTLR5S. Oct1 and NF-κB binding sites might be cis-regulatory elements in the LcTLR5S promoter. NF-κB/p65 plays an important role in LcTLR5S promoter activation through binding with the NF-κB binding site.

The Potential of Toll-Like Receptors to Modulate Avian Immune System: Exploring the Effects of Genetic Variants and Phytonutrients

Toll-like receptors (TLRs) are pathogen recognition receptors, and primitive sources of innate immune response that also play key roles in the defense mechanism against infectious diseases. About 10 different TLRs have been discovered in chicken that recognize ligands and participate in TLR signaling pathways. Research findings related to TLRs revealed new approaches to understand the fundamental mechanisms of the immune system, patterns of resistance against diseases, and the role of TLR-specific pathways in nutrient metabolism in chicken. In particular, the uses of specific feed ingredients encourage molecular biologists to exploit the relationship between nutrients (including different phytochemicals) and TLRs to modulate immunity in chicken. Phytonutrients and prebiotics are noteworthy dietary components to promote immunity and the production of disease-resistant chicken. Supplementations of yeast-derived products have also been extensively studied to enhance innate immunity during the last decade. Such interventions pave the way to explore nutrigenomic approaches for healthy and profitable chicken production. Additionally, single-nucleotide polymorphisms in TLRs have shown potential association with few disease outbreaks in chickens. This review aimed to provide insights into the key roles of TLRs in the immune response and discuss the potential applications of these TLRs for genomic and nutritional interventions to improve health, and resistance against different fatal diseases in chicken.

Identification and expression analysis of sixteen Toll-like receptor genes, TLR1, TLR2a, TLR2b, TLR3, TLR5M, TLR5S, TLR7-9, TLR13a-c, TLR14, TLR21-23 in mandarin fish Siniperca chuatsi

Toll-like receptors (TLRs), as a family of pattern recognition receptors (PRRs), possess specific pathogen-related molecular pattern (PAMP) recognition spectrum in inducing immune responses. In this study, sixteen TLRs were identified and characterized in mandarin fish (Siniperca chuatsi). All these TLRs consist of leucine-rich repeats (LRRs), a transmembrane domain and a Toll/interleukin-I receptor (TIR) domain, with the exception of TLR5S which lacks TIR domain, and they can be clustered into five branches, i.e. TLR1 subfamily, TLR3 subfamily, TLR5 subfamily, TLR7 subfamily and TLR11 subfamily in phylogenetic tree. These TLR genes were expressed in all tested tissues and had high expression levels in immune-related tissues such as head-kidney and spleen or mucosa-related tissues such as intestine and pyloric caecum. The transcripts of TLR2a, TLR2b, TLR3, TLR13a, TLR14, TLR22 and TLR23 were all significantly up-regulated after stimulation with poly(I:C); TLR1, TLR2a, TLR2b, TLR3, TLR5M, TLR5S, TLR13a and TLR13b transcripts were all significantly up-regulated after stimulation with PGN; and TLR2a, TLR2b, TLR5M, TLR5S, TLR7, TLR8, TLR9, TLR13c, TLR14 and TLR22 transcripts were all significantly up-regulated after stimulation with LPS in isolated head kidney lymphocytes of mandarin fish. The findings in this study may provide a valuable basis for functional study on TLR genes in mandarin fish.

23S rRNA from Vibrio parahaemolyticus regulates the innate immune response via recognition by TLR13 in orange-spotted grouper (Epinephelus coioides)

Toll-like receptors (TLRs) are major pattern recognition receptors (PRRs) that recognize multiple pathogen-associated molecular patterns (PAMPs) through the leucine-rich repeat (LRR) domain and mount effective immune responses. Vibrio parahaemolyticus is the main pathogen that causes vibriosis in aquatic animals, yet the mechanisms of its recognition by innate immune system in teleost fish remain unknown. Here, the results reveal that TLR13 in orange-spotted grouper (Epinephelus coioides) (EcTLR13) recognizes a conserved 23S ribosomal RNA (23S rRNA) sequence in V. parahaemolyticus, and the 13-nucleotide motif near the 23S rRNA ribozyme activation site (VP13) acts as a PAMP. After challenge with RNA and 23S rRNA from V. parahaemolyticus and with the synthetic oligoribonucleotide VP13, the expression of EcTLR13 in grouper spleen cells (GS cells) was significantly increased. EcTLR13-knockdowned GS cells were stimulated with the same stimulants as listed above, the expression of IL-6, IL-12, IL-1β and TNFα was significantly reduced. RNA-protein immunoprecipitation revealed that VP13 could directly bind to EcTLR13. The dual-luciferase reporter assay also showed that EcTLR13 enhanced the fluorescence activity of IFNβ rather than that of NF-κB when the cells were challenged with RNA from V. parahaemolyticus or with synthetic VP13. Our study established the mechanism of fish TLR13-mediated recognition of microbial products during V. parahaemolyticus infection.

Pattern-recognition receptors in duck (Anas platyrhynchos): identification, expression and function analysis of toll-like receptor 3

1. Innate immunity provides the first line of defence against pathogenic organisms through a myriad of germline encoded receptors called pattern-recognition receptors (PRRs). Toll-like receptor (TLR) 3, as an important member of PRRs, is indispensable for host defence against viral infection by recognising virus-derived RNAs. However, little is known about the structure and function of TLR3 in ducks (Anas platyrhynchos), a natural host for the avian influenza virus.2. This study cloned the full-length cDNA of duck TLR3 using reverse transcription polymerase chain reaction (RT-PCR) with rapid amplification of cDNA ends (RACE). The cDNA sequence of duck TLR3 was 4046 bp in length and encoded 895 amino acids. Multiple sequence alignment showed that duck TLR3 shared high similarity with that from other vertebrates.3. Quantitative real-time PCR (qRT-PCR) analysis suggested that TLR3 mRNA was constitutively expressed in all tissues tested, having higher levels in the kidney, liver, breast muscle, ovary and heart. After stimulation with viral- or bacterial-mimics, including LPS, poly(I:C), pam3CSK4, FLS-1, FLA-ST and R848, the TLR3 transcript was significantly upregulated. Meanwhile, overexpression of duck TLR3 significantly promoted the transcription of IFN-β, IRF7, TRIF, Mx, STAT1 and STAT2 mRNA after stimulation with poly(I:C).4. These results suggested that TLR3 play an important role in resistance against viral and bacterial infections in ducks.

NF-κB signaling induces inductive expression of the downstream molecules and IgD gene in the freshwater carp, Catla catla

Toll-like receptors (TLRs) in innate immune system act as primary sensors in detecting the microbial components and activate their signaling cascades to induce NF-κB (nuclear factor NF-κB) towards the augmentation of immunoglobulin (Ig) synthesis. To gain insights into the efficacy of NF-κB pathway in immunoglobulin D (IgD) synthesis in the Indian Major Carp Catla catla, cloning and sequencing of TLR-signaling downstream molecules [TRAF3 (TNF receptor-associated factor 3), NEMO (nuclear factor-kappa B essential modulator), NF-κB and BAFF (B cell activating factor)] were performed by infecting the fish with pathogens. mRNA expression analysis of the downstream molecules and IgD showed significant up-regulation of these genes in kidney (P ≤ 0.001) as compared to spleen (P ≤ 0.05). To ascertain the role of NF-κB pathway in IgD synthesis, the primary cell culture of kidney and spleen in monolayer cell suspension was treated with NF-κB inhibitor (BAY 11-7082) and down-regulation of BAFF, NEMO, NF-κB, and IgD gene was observed. These results highlight the importance of NF-κB signaling pathway in augmenting the IgD gene expression in the freshwater carp, Catla catla.

Marking vertebrates langerhans cells, from fish to mammals

Langerhans cells (LCs) are specialized dendritic cells (DCs) that play a defense role in recognizing foreign antigens, in tissue where antigenic exposures occur, as in the skin and mucous membranes. LCs are able to continuously move within the tissues thanks to dendritic contraction and distension performing their surveillance and/or phagocytosis role. These cells are characterized by the presence of Birbeck granules in their cytoplasm, involved in endocytosis. LCs have been characterized in several classes of vertebrates, from fish to mammals using different histological and molecular techniques. The aim of the present review is to define the state of art and the need of information about immunohistochemical markers of LCs in different classes of vertebrates. The most used immunohistochemical (IHC) markers are Langerin/CD207, CD1a, S-100 and TLR. These IHC markers are described in relation to their finding in different vertebrate classes with phylogenetical considerations. Among the four markers, Langerin/CD207 and TLR have the widest spectrum of cross reactivity in LCs.

Toll-Like Receptor 5 of Golden Pompano Trachinotus ovatus (Linnaeus 1758): Characterization, Promoter Activity and Functional Analysis

Toll-like receptors (TLRs), as important pattern recognition receptors, represent a significant component of fish immune systems and play an important role in resisting the invasion of pathogenic microorganisms. The TLR5 subfamily contains two types of TLR5, the membrane form of TLR5 (TLR5M) and the soluble form of TLR5 (TLR5S), whose detailed functions have not been completely elucidated. In the present study, we first identified two genes, TLR5M (ToTLR5M) and TLR5S (ToTLR5S), from golden pompano (Trachinotus ovatus). The full-length ToTLR5M and ToTLR5S cDNA are 3644 bp and 2329 bp, respectively, comprising an open reading frame (ORF) of 2673 bp, encoding 890 amino acids, and an ORF of 1935 bp, encoding 644 amino acids. Both the ToTLR5s possess representative TLR domains; however, only ToTLR5M has transmembrane and intracellular TIR domains. Moreover, the transcription of two ToTLR5s was significantly upregulated after stimulation by polyinosinic:polycytidylic acid (poly (I:C)), lipopolysaccharide (LPS), and flagellin in both immune-related tissues (liver, intestine, blood, kidney, and skin) and nonimmune-related tissue (muscle). Furthermore, the results of bioinformatic and promoter analysis show that the transcription factors GATA-1 (GATA Binding Protein 1), C/EBPalpha (CCAAT Enhancer Binding Protein Alpha), and ICSBP (Interferon (IFN) consensus sequence binding protein) may play a positive role in moderating the expression of two ToTLR5s. Overexpression of ToTLR5M and ToTLR5S notably increases NF-κB (nuclear factor kappa-B) activity. Additionally, the binding assay revealed that two rToTLR5s can bind specifically to bacteria and pathogen-associated molecular patterns (PAMPs) containing Vibrio harveyi, Vibrio anguillarum, Vibrio vulnificus, Escherichia coli, Photobacterium damselae, Staphylococcus aureus, Aeromonas hydrophila, LPS, poly(I:C), flagellin, and peptidoglycan (PGN). In conclusion, the present study may help to elucidate the function of ToTLR5M/S and clarify their possible roles in the fish immune response to bacterial infection.

Genome-wide characterization of Toll-like receptors in black rockfish Sebastes schlegelii: Evolution and response mechanisms following Edwardsiella tarda infection

As one of the key components of pattern recognition receptors, Toll-like receptors (TLRs) play pivotal roles in the innate immune system. However, little information is available about the TLR genes in Sebastes schlegelii. In the present study, 17 TLR genes were identified and classified based on the whole genome database. Tandem duplication events in TLR1, TLR2, TLR5 and TLR13 played major role in the expansion of S. schlegelii TLR genes; both TLR2-3 and TLR2-4 had the same largest number of introns/exons, 11 exons and 10 introns. The syntenic analysis showed neighboring genes of TLR genes were most conserved in S. schlegelii and in L. crocea. Phylogenetic and evolutionary analysis showed that these TLR genes were divided into five subfamilies and exhibited different selection pressures. Meanwhile, the expression patterns of TLR genes in the intestine after E. tarda infection were investigated by qRT-PCR. Finally, protein and protein interaction (PPI) network analysis indicated that TLR genes interacted with IFN-inducible genes, inflammatory cytokines, and participated in MyD88-dependent pathway. In summary, this study provided valuable information for further functional characterization of TLR genes in S. schlegelii.

Molecular cloning, tissue distribution and function analysis of duck TLR7

Toll-like receptors (TLRs) play an important role in detecting pathogen-associated molecular patterns (PAMPs). Among the TLRs, TLR7 is involved in the recognition of antiviral compounds and single-stranded RNA. This study was designed to explore the structure and function of TLR7 in duck (Anas platyrhynchos), a natural host for avian influenza virus. Firstly, the full-length cDNA of Shaoxing egg-laying duck TLR7 (duTLR7) was obtained using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). It consisted of 38 base pairs (bp) 5'-untranslated region (UTR), 187 bp 3'-UTR, and 3270 bp open reading frame that encodes a single protein of 1089 amino acid residues. DuTLR7 shares high identity with TLR7 genes from other vertebrates. In healthy ducks, duTLR7 transcripts were broadly expressed in different tissues, with higher expression levels in the liver, kidney, and thymus. The highest relative transcript level of duTLR7 could be induced with R848 stimulation. In addition, overexpression of duTLR7 by stimulating with poly(I:C) significantly promoted IFN-β, NF-κB, IRF7, TRIF, Mx, STAT1 and STAT2 expressions. Taken together, these results suggest that TLR7 may play an important role in the innate immune response of ducks.

Identification and Expression Profiling of Toll-Like Receptors of Brown Trout (Salmo trutta) during Proliferative Kidney Disease

Proliferative kidney disease is an emerging disease among salmonids in Europe and North America caused by the myxozoan parasite Tetracapsuloides bryosalmonae. The decline of endemic brown trout (Salmo trutta) in the Alpine streams of Europe is fostered by T. bryosalmonae infection. Toll-like receptors (TLRs) are a family of pattern recognition receptors that acts as sentinels of the immune system against the invading pathogens. However, little is known about the TLRs' response in salmonids against the myxozoan infection. In the present study, we identified and evaluated TLR1, TLR19, and TLR13-like genes of brown trout using data-mining and phylogenetic analysis. The expression pattern of TLRs was examined in the posterior kidney of brown trout infected with T. bryosalmonae at various time points. Typical Toll/interleukin-1 receptor protein domain was found in all tested TLRs. However, TLR13-like chr2 had a short amino acid sequence with no LRR domain. Phylogenetic analysis illustrated that TLR orthologs are conserved across vertebrates. Similarly, a conserved synteny gene block arrangement was observed in the case of TLR1 and TLR19 across fish species. Interestingly, all tested TLRs showed their maximal relative expression from 6 to 10 weeks post-exposure to the parasite. Our results suggest that these TLRs may play an important role in the innate defense mechanism of brown trout against the invading T. bryosalmonae.

Toll-like receptors (TLRs) respond to tributyltin chloride (TBT-Cl) exposure in the river pufferfish (Takifugu obscurus): Evidences for its toxic injury function

Tributyltin chloride (TBT-Cl) residual in water body had become a noticeable ecological problem for aquatic ecosystems. Toll-like receptors (TLRs) are an ancient family of pattern recognition receptors that play key roles in detecting nonself antigens and immune system activation. In this study, we explored the effect of TBT-Cl exposure on four TLRs expression in river pufferfish, Takifugu obscurus. The four T. obscurus Toll-like receptors (To-TLRs) contained different types of domains such as leucine-rich repeats (LRRs), leucine-rich repeats, typical subfamily (LRR_TYP) and other special domains. The To-TLRs mRNA transcripts expressed in all tissues, also To-TLR2 was investigated with higher level in kidney, as well as To-TLR3 in kidney, while To-TLR18 in liver and To-TLR22 in intestine. After the acute and chronic exposure of TBT-Cl, To-TLR2 and To-TLR3 mRNA transcripts were significantly down-regulated in gill. However, To-TLR18 and To-TLR22 were significantly up-regulated in gill and liver. Moreover, the histology and immunohistochemistry (IHC) results showed the different injury degrees of TBT-Cl in liver and gill and implied the cytoplasm reorganization after TBT-Cl stress and the function of immunoregulation for To-TLRs to TBT-Cl exposure. All the results indicated that To-TLRs might involve in sensing and mediating innate immune responses caused by TBT-Cl for keeping detoxification homeostasis.

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.

The potential sensing molecules and signal cascades for protecting teleost fishes against lipopolysaccharide

Lipopolysaccharide (LPS) is a classical pathogen-associated molecular pattern that can trigger strong inflammatory response mainly by TLR4-mediated signaling pathway in mammals, but the molecular mechanism of anti-LPS immunity is unclear in teleost fishes. In this study, we analyzed the gene expression features based on transcriptome analysis in Schizothorax prenanti (S. prenanti), after stimulation with two sources of LPS from Aeromonas hydrophila and Escherichia coli (Ah. LPS and Ecoli. LPS). 921 different expression genes (DEGs) after Ah. LPS stimulation and 975 DEGs after Ecoli.LPS stimulation were acquired, but only 706 and 750 DEGs were successfully annotated into the databases, respectively. Both of two groups of DGEs were significantly enriched into immune-related pathways by KEGG enrichment analysis, such as "Toll-like receptor signaling pathway", "Cytokine-cytokine receptor interaction" and "JAK-STAT signaling pathway". The annotated DEGs from Ah. LPS and Ecoli. LPS stimulation shared 470 DEGs, including 88 immune-related DEGs (IRGs) identified mainly by KEGG enrichment to immune-related signaling pathways. Among the shared IRGs, four pattern-recognition genes (TLR5, TLR25, PTX3 and C1q) were induced with high expression foldchange, and IFN-γ and relative genes also showed higher expression levels than control. Meanwhile, inflammatory signals were highlighted by upregulating the expression of inflammatory cytokines (IL-1β, IL-10 and IL-8). Moreover, some non-shared IRGs (including TLR2 and TLR4) were identified, suggesting that different sources of LPS own different potentials for the induction of immune gene expression. In conclusion, TLR5, TLR25, PTX3 and C1q may function as the sensing molecules to catch the invasion signal of LPS. The anti-LPS immune response may be involved into TLR25/TLR5-mediated inflammatory signals that regulate subsequently the activation of PTX3/C1q-modulated complement pathway upon the induction of PTX3 expression, and the crosstalk between IFN-γ and TLR signaling pathways in teleost fishes. This study will contribute to further explore the molecular mechanism of LPS-induced immunity in teleost fishes.

Duplication of toll-like receptor 22 in teleost fishes

The TLRs of teleost fishes have distinct features and are highly diverse, but the duplication characteristics and expression patterns of the tlr22 gene remain unclear. Here, we identified paralogous tlr22 genes in 13 teleost fishes by screening available fish genomic resources and using molecular cloning. We then conducted comprehensive bioinformatics analyses and investigated spatiotemporal differences in the expression patterns of the tlr22 genes in G. eckloni. The results indicated that more than three paralogous tlr22 genes were possessed by some teleost fishes. Of these, tlr22c is specific to some subfamilies of the Cyprinidae (e.g., Barbinae, Cyprininae, Schizothoracinae, and Leuciscinae). Phylogenetic and syntenic analyses showed that the paralogous tlr22 genes originated from two single-gene duplication events. Molecular clock calculations dated the two gene duplication events at 49.5 and 39.3 MYA, which is before the common carp-specific genome duplication event and well after the fish-specific genome duplication. Gene duplication of tlr22 was followed by gene loss or pseudogene events in certain lineages. Spatiotemporal expression differences between the three duplicated tlr22 genes from G. eckloni suggested that these genes diverged functionally after gene duplication.

Chicken toll-like receptors and their significance in immune response and disease resistance

Infectious diseases are a major challenge for the poultry industry that causes widespread production losses. Thus, management and control of poultry health and diseases are essential for the viability of the industry. Toll-like receptors are best characterized as membrane-bound receptors that perform a central role in immune homeostasis and disease resistance by recognition of pathogen-associated molecular patterns. In response to pathogen recognition, TLRs initiate both innate and adaptive immune responses which may help to develop immunomodulatory therapeutics for TLR associated diseases. Vaccination produces specific immunity in the animal's body towards pathogens. However, due to certain disadvantages of vaccines, (inactivation of attenuated pathogens into the virulent strains and weak immunogenicity of inactivated vaccines) there is a crucial need to develop the safe and effective therapeutic intervention. TLR ligands have been classified as a potential adjuvant against the infectious diseases in farm animals. TLR adjuvants induce both specific and nonspecific immune responses in chickens to combat several bacterial, viral and parasitic diseases. Therefore, the aim of this review was to explore the chicken TLR4 and their role in immune responses and disease resistance to develop disease resistance poultry breeds in future.

A Comparison of the Innate and Adaptive Immune Systems in Cartilaginous Fish, Ray-Finned Fish, and Lobe-Finned Fish

The immune system is composed of two subsystems-the innate immune system and the adaptive immune system. The innate immune system is the first to respond to pathogens and does not retain memory of previous responses. Innate immune responses are evolutionarily older than adaptive responses and elements of innate immunity can be found in all multicellular organisms. If a pathogen persists, the adaptive immune system will engage the pathogen with specificity and memory. Several components of the adaptive system including immunoglobulins (Igs), T cell receptors (TCR), and major histocompatibility complex (MHC), are assumed to have arisen in the first jawed vertebrates-the Gnathostomata. This review will discuss and compare components of both the innate and adaptive immune systems in Gnathostomes, particularly in Chondrichthyes (cartilaginous fish) and in Osteichthyes [bony fish: the Actinopterygii (ray-finned fish) and the Sarcopterygii (lobe-finned fish)]. While many elements of both the innate and adaptive immune systems are conserved within these species and with higher level vertebrates, some elements have marked differences. Components of the innate immune system covered here include physical barriers, such as the skin and gastrointestinal tract, cellular components, such as pattern recognition receptors and immune cells including macrophages and neutrophils, and humoral components, such as the complement system. Components of the adaptive system covered include the fundamental cells and molecules of adaptive immunity: B lymphocytes (B cells), T lymphocytes (T cells), immunoglobulins (Igs), and major histocompatibility complex (MHC). Comparative studies in fish such as those discussed here are essential for developing a comprehensive understanding of the evolution of the immune system.

Isolation and expression of four Megalobrama amblycephala toll-like receptor genes in response to a bacterial infection

Toll-like receptors (TLRs) are a category of pattern recognition receptors (PRRs), which recognize pathogen associated molecular patterns (PAMPs) and participate in the immune responses. We identified tlr5a, tlr5b, tlr9 and tlr21 from the genome of blunt snout bream (Megalobrama amblycephala). All four tlrs were constitutively expressed in all examined tissues. After an immune bacterial challenge with Aeromonas hydrophila, their expressionwas up-regulated in lymphoid organs and tissues. Recombinant eukaryotic plasmid pEGFP-N1 was transfected into the common carp (Cyprinus carpio) EPC (epithelioma papulosum cyprini) cells for the purpose of subcellular localization. pcDNA3.1(+) recombinant eukaryotic plasmid was used to investigate the effects of overexpression of tlrs on the expression of downstream interferon-associated immune factors. The four Tlrs were distributed in the cytoplasm of transfected cells and appeared as filamentous or reticular. The expression of irf3, irf7, isg15, mx1, pkr and viperin at 0, 6, 12, 18, 24, 36, 48 and 72 h post-transfection in transfected EPC cells was quantified by qPCR. Overexpression of tlrs upregulated the expression of viperin, isg15, irf3, irf7, mx1 and pkr (in that order of magnitude). We also cloned the following promoters of irfs: Irf1-p, irf2-p, irf6-p, irf7-p, irf8-p and irf9-p. Results of the dual luciferase reporter assay suggested that tlr5a, tlr5b and tlr9 enhanced the activities of irf7-p, while tlr5b enhanced the activities of irf1-p and irf7-p. This suggests that they all play a role in the innate immunity. The experiments also indicated that TLRs activate irf3 or irf7 signaling to induce IFN secretion and subsequent upregulation of IFN-stimulated genes. These results indicate that tlrs and irfs play an important immune role in response to A. hydrophila infection in blunt snout bream, and pave the way for further studies of immune mechanisms mediated by TLRs in fish.

Genome-wide identification and characterization of toll-like receptor genes in spotted sea bass (Lateolabrax maculatus) and their involvement in the host immune response to Vibrio harveyi infection

Toll-like receptor (TLR) genes are the earliest reported pathogen recognition receptors (PRRs) and have been extensively studied. These genes play pivotal roles in the innate immune defense against pathogen invasion. In this study, a total of 16 tlr genes were identified and characterized in spotted sea bass (Lateolabrax maculatus). The tlr genes of spotted sea bass were classified into five subfamilies (tlr1-subfamily, tlr3-subfamily, tlr5-subfamily, tlr7-subfamily, and tlr11-subfamily) according to the phylogenetic analysis, and their annotations were confirmed by a syntenic analysis. The protein domain analysis indicated that most tlr genes had the following three major TLR protein domains: a leucine-rich repeat (LRR) domain, a transmembrane region (TM) and a Toll/interleukin-1 receptor (TIR) domain. The tlr genes in spotted sea bass were distributed in 11 of 24 chromosomes. The mRNA expression levels of 16 tlr genes in response to Vibrio harveyi infection were quantified in the head kidney. Most genes were downregulated following V. harveyi infection, while only 5 tlr genes, including tlr1-1, tlr1-2, tlr2-2, tlr5, and tlr7, were significantly upregulated. Collectively, these results help elucidate the crucial roles of tlr genes in the immune response of spotted sea bass and may supply valuable genomic resources for future studies investigating fish disease management.

Molecular cloning, characterization and expression analysis of MHCI and chemokines CXCR3 and CXCR4 gene from freshwater carp, Catla catla

The immune system with large number of molecules protects the host against a plethora of continuously evolving microbes. Major histocompatibility complex (MHC) molecules serve as cardinal elements of the adaptive immune system responsible for the activation of the adaptive immunity in the host. The present study reports MHCI molecule in freshwater carp, Catla catla, and its differential expression in immunologically relevant tissues post-infection with Gram-negative and Gram-positive bacteria. The MHCI sequence of C. catla had 502 bp nucleotides encoding putative 146 amino acids. The phylogenetic analysis exhibited its evolutionary conservation within the Cyprinidae family and formed a different clade with the higher vertebrates. Simultaneously, CXCR3 and CXCR4 chemokines were cloned and characterized for their expression in infected tissues. Analysis of immunologically relevant tissues of the infected fish exhibited an increase of MHCI gene expression and the down-regulation of CXCR3 and CXCR4 chemokines, indicating a tricky interaction between the innate and adaptive immune system. It was found that intestine, skin and spleen played a crucial role in the contribution of the defense activity which instigated the self-immunity. These immune activities can provide useful information to understand the interaction of self and non-self- immune system in freshwater fish, Catla catla.

Red Queen revisited: Immune gene diversity and parasite load in the asexual Poecilia formosa versus its sexual host species P. mexicana

In accordance with the Red Queen hypothesis, the lower genotypic diversity in clonally reproducing species should make them easier targets for pathogen infection, especially when closely related sexually reproducing species occur in close proximity. We analyzed two populations of clonal P. formosa and their sexual parental species P. mexicana by correlating individual parasite infection with overall and immune genotype. Our study revealed lower levels of overall genotypic diversity and marginally fewer MHC class I alleles in P. formosa individuals compared to sexually reproducing P. mexicana. Parasite load, however, differed only between field sites but not between species. We hypothesize that this might be due to slightly higher genotypic diversity in P. formosa at the innate immune system (toll like receptor 8) which is likely due to the species’ hybrid origin. In consequence, it appears that clonal individuals do not necessarily suffer a disadvantage compared to sexual individuals when fighting parasite infection.

A Toll-like receptor 3 (TLR3) agonist ARNAX for therapeutic immunotherapy

Vaccine immunotherapy consisting of tumor antigens combined with an immune-enhancing adjuvant fosters cytotoxic T cell (CTL) proliferation. Clinically, polyI:C has been used as an adjuvant to enhance cancer vaccine protocols. However, according to its long history, polyI:C promotes inflammation that causes cytokine toxicity. Although checkpoint inhibitor immunotherapy has improved the prognoses of patients with progressive cancer, over 75% of patients continue to experience resistance to antibody (Ab) against anti-programmed cell death-protein 1 (PD-1) or its ligand, PD-L1 therapy. In most cases, patients suffer from adverse events resulting from inflammation during anti-PD-1/L1 Ab therapy, which is a serious obstacle to patients' quality of life. We have studied the functional properties of double-stranded (ds)RNA and polyI:C, and developed a nucleic acid adjuvant that barely induces a significant increase in the level of serum inflammatory cytokines in mouse models. This adjuvant, termed ARNAX, consists of DNA-capped dsRNA that specifies the endosomal target for Toll-like receptor 3 (TLR3) in dendritic cells (DCs). We expect that this adjuvant is safe for administration in elderly patients with cancer receiving immunotherapy. Here, we summarize the properties of ARNAX for immunotherapy in mice. We suggest that DC-priming is essential to induce anti-tumor immunity; neither exogenous inflammation nor the administration of tumor antigens is always a prerequisite for DC-mediated CTL proliferation. If our mouse data can be extrapolated to humans, ARNAX and the liberated endogenous tumor antigens may facilitate effect of current therapies on patients with therapy-resistant tumors.