TLR7 is required for optimal immune defense against bacterial infection in tongue sole (Cynoglossus semilaevis)

Genome-wide identification, characterization and expression profiling of TLR family genes in Chromileptesaltivelis

Toll-like receptors (TLRs) represent a prominent category of pattern recognition receptors that have been extensively investigated for their pivotal role in combating pathogen incursions. Despite this, there has been a notable absence of comprehensive identification and exploration of the immune response associated with the TLR family genes in C. altivelis. This study successfully identified and named fourteen genes as Catlr1-1, Catlr1-2, Catlr2-1, Catlr2-2, Catlr3, Catlr5, Catlr7, Catlr8, Catlr9, Catlr13-1, Catlr13-2, Catlr18, Catlr21, and Catlr22. A series of bioinformatic analysis were performed, encompassing analysis of protein properties, examination of gene structures, evolutionary assessments, and prediction of protein tertiary structures. The expression patterns of Catlr genes were analyzed in five immune tissues: liver, spleen, kidney, gill, and intestine, in both healthy and bacterial stimulated-fish. The results showed that different tissue and different genes showed differed expression patterns after V. harveyi infection, indicating the involvement of all Catlr members in mounting immune responses following infection in various tissues. Additionally, histological evaluations of immune tissues unveiled varying levels of damage. In conclusion, this investigation into the TLR gene family offers novel information that contribute to a more profound comprehension of the immune response mechanisms in C. altivelis.

Vegetable omega-3 and omega-6 fatty acids differentially modulate the antiviral and antibacterial immune responses of Atlantic salmon

The immunomodulatory effects of omega-3 and omega-6 fatty acids are a crucial subject of investigation for sustainable fish aquaculture, as fish oil is increasingly replaced by terrestrial vegetable oils in aquafeeds. Unlike previous research focusing on fish oil replacement with vegetable alternatives, our study explored how the omega-6 to omega-3 polyunsaturated fatty acid (PUFA) ratio in low-fish oil aquafeeds influences Atlantic salmon's antiviral and antibacterial immune responses. Atlantic salmon were fed aquafeeds rich in soy oil (high in omega-6) or linseed oil (high in omega-3) for 12 weeks and then challenged with bacterial (formalin-killed Aeromonas salmonicida) or viral-like (polyriboinosinic polyribocytidylic acid) antigens. The head kidneys of salmon fed high dietary omega-3 levels exhibited a more anti-inflammatory fatty acid profile and a restrained induction of pro-inflammatory and neutrophil-related genes during the immune challenges. The high-omega-3 diet also promoted a higher expression of genes associated with the interferon-mediated signaling pathway, potentially enhancing antiviral immunity. This research highlights the capacity of vegetable oils with different omega-6 to omega-3 PUFA ratios to modulate specific components of fish immune responses, offering insights for future research on the intricate lipid nutrition-immunity interplay and the development of novel sustainable low-fish oil clinical aquaculture feeds.

Identification and characterization of toll-like receptor genes in silver pomfret (Pampus argenteus) and their involvement in the host immune response to Photobacterium damselae subsp. Damselae and Nocardia seriolae infection

Toll-like receptors (TLRs) are vital pattern recognition receptors that play a critical role in the innate immune response against pathogenic attack. Among the bacteria commonly found in the culture process of silver pomfret, Photobacterium damselae subsp. Damselae (PDD, gram-negative) and Nocardia seriolae (NS, gram-positive), can cause large-scale mortality in this fish species. However, there is currently no research on the role of TLRs in mediating the immune response of silver pomfret to these two bacterial infections. Therefore, in this study, we identified nine PaTLRs family members, including several fish-specific TLRs (TLR14 and TLR21). Phylogenetic analysis revealed that these PaTLRs genes could be classified into five subfamilies, namely TLR1, TLR3, TLR5, TLR7, and TLR11, indicating their evolutionary conservation. To further explore the interactions of TLR genes with immune-related mediators, protein and protein interaction network (PPI) results were generated to explain the association of TLR genes with TNF receptor-associated factor 6 (TRAF6) and other relevant genes in the MyD88-dependent pathway and NF-κb signaling pathway. Subsequently, RT-qPCR was conducted to verify the expression patterns of the nine TLR genes in the gills, skin, kidney, liver, and spleen of healthy fish, with most of the TLRs showing high expression levels in the spleen. Following infection with PDD and NS, these PaTLRs exhibited different expression patterns in the spleen, with PaTLR2, PaTLR3, PaTLR5, PaTLR7, PaTLR9, and PaTLR14 being significantly up-regulated. Furthermore, when spleen cells were treated with bacterial compositions, the majority of PaTLRs expression was up-regulated in response to Lipopolysaccharide (LPS) and lipophosphorylcholic acid (LTA) treatment, except for PaTLR21. Finally, changes in the expression levels of TLR-interacting genes were also observed under the stimulation of bacteria and bacterial compositions. The results of this study provide a preliminary reference for further understanding the mechanism of the innate immune response of the TLR gene family in silver pomfret and offer theoretical support for addressing the disease problems encountered during large-scale fish breeding.

Splenic protection network revealed by transcriptome analysis in inactivated vaccine-immunized flounder (Paralichthys olivaceus) against Edwardsiella tarda infection

The protective immune response produced by fish after vaccination is crucial for vaccine effectiveness. Our previous studies have shown inactivated vaccine against Edwardsiella tarda can induce immune response in flounder (Paralichthys olivaceus). To elucidate the protective immune response at the genetic level, in this study, flounder was immunized with inactivated E. tarda for 5 weeks, and then they were challenged with E. tarda. The spleen was dissected at 7th day post immunization, 1st and 7th day post challenge, respectively. Transcriptome analysis showed that average of 46 million clean reads were obtained per library, while percentage of clean reads being mapped to reference genome was more than 89% in all cases, which suggested good quality of samples. As for differentially expressed genes (DEGs) identification in inactivated E. tarda groups, at 7th day post immunization, 1422 DEGs were identified and significantly enriched in innate immune-related pathways, such as Phagosome, Cell adhesion molecules and NF-kappa B signaling pathway; At 1st post challenge day, 1210 DEGs were identified and enriched to Antigen processing and presentation and Cell adhesion molecules, indicating that the pathogen was rapidly recognized and delivered; At 7th post challenge day, 1929 DEGs were identified, belonged to Toll-like receptor signaling pathway, Antigen processing and presentation, Th1 and Th2 cell differentiation and Th17 cell differentiation. Compared to 7th post immunization day, 73 immune-associated DEGs were identified at 1st post challenge day. Protein-protein interaction networks analysis revealed 11 hub genes (TLR7, TLR3, CXCR4, IFIH1, TLR8 etc), associated with recognition of pathogens and activation of innate immunity; while for 7th post challenge day, 141 immune-associated DEGs were identified. 30 hub genes (IL6, STAT1, HSP90A.1, TLR7, IL12β etc) were associated with stimulation of lymphocyte differentiation and activation of cellular immunity. Ten immune-related genes were randomly selected for RT-qPCR validation at each time point. In conclusion, data revealed protection of flounder against E. tarda infection by inactivated vaccine is mediated via immediate recognition of pathogen and subsequently activation of cellular immunity. Results give new aspect for vaccine protection cascades, is good references for vaccine evaluation.

Immune responses to Vibrio vulnificus formalin-killed vaccine and ghost vaccine in Scophthalmus maximus

In this research, Vibrio vulnificus formalin-killed (FKCs) vaccine and ghost (VVGs) vaccine were successfully developed, and shown to prevent vibriosis of Scophthalmus maximus resulting from V. vulnificus. The antibody titre of FKCs and VVGs vaccine was 1: 2⁸ and 1: 2¹¹ . The RPS of FKCs and VVGs vaccine was 60% and 80%. In order to improve the understanding of vaccine protection mechanism, transcriptome data was used to analyse the immune response of S. maximus infected with V. vulnificus after vaccination with FKCs and VVGs vaccine. In the SmCon and SmIV groups, a series of innate immune-related genes were upregulated (such as, TLR5, Tp12, AP-1 and IL-1β) or downregulated (such as, CASP6 and CASP8), which suggested that the immune protection mechanism induced by inactivated vaccine was similar to that of autoimmune response. In the SmIV and SmGho group, a number of innate and adaptive immune-related genes (such as, STAT1, IFN-γ and MHC Ia) were activated, in which the expression of these genes was higher in SmGho, and VVGs vaccine induced stronger innate and acquired immune responses. In conclusion, the results lay a foundation for further study on the molecular mechanisms of immune protection induced by VVGs vaccine and FKCs vaccine.

Characterization, evolution and expression analysis of Toll-like receptor 7 (TLR7) in turbot (Scophthalmus maximus L.)

The pattern recognition receptors (PRRs) can recognize the conserved molecular structures of pathogens to active the innate immune responses, and subsequently induce the antigen-specific adaptive immune responses for the clearance of infected pathogen. Among the PRRs, Toll-like receptors (TLRs) are the first and best characterized PRRs across all the species. Among the TLR members, TLR7 showed significant conservation across the vertebrates, with the lowest rate of evolution for its LRR domains from primates to fishes. In the current study, one TLR7 (SmTLR7) gene was captured in turbot, with a 3144 bp open reading frame (ORF), that encoding 1047 amino acid residues. Following multiple sequence comparison, SmTLR7 was found to have the highest similarity and identity both to Paralichthys olivaceus with 91.9% and 85.9%, respectively. In phylogenetic analysis, SmTLR7 was firstly clustered with Japanese flounder, and then clustered with fugu, rainbow trout, and zebrafish. In addition, SmTLR7 was widely expressed in all the examined tissues with the highest expression level in spleen, followed by skin, while the lowest expression level was detected in blood. Following both Edwardsiella tarda and Vibrio anguillarum challenge, SmTLR7 was significantly down-regulated in gill and intestine, and up-regulated in skin. Moreover, SmTLR7 was significantly up-regulated in head kidney macrophages following LPS, LTA, PGN and polyI:C stimulation, as well as showed the strongest binding ability to LPS, followed by PGN, LTA, and polyI:C in a dose-dependent manner. Finally, following RNAi of SmTLR7, MyD88 and IL-1β were slightly up-regulated, while TRAF6 and IL-8 were significantly down-regulated. The characterization of TLR7 can expand our understanding of the PRRs in teleost fishes, and eventually aid the exploration of interactions between host and pathogen.

Functional differentiation of two lhx8 paralogs and possible regulatory role of lhx8a in Japanese flounder (Paralichthys olivaceus)

Lhx8, belonging to the LIM-Homebox family, is involved in the tooth, nervous system, and primordial follicles development in mammals. However, little is known about the regulatory roles of lhx8 in teleosts. In this study, two lhx8 duplicates were identified in Paralichthys olivaceus, termed Polhx8a and Polhx8b, respectively. Bioinformatic analysis showed that Polhx8a was more likely to be a teleost-specific paralog. According to expression analysis, Polhx8a transcripts were almost exclusively concentrated in the oocytes, while Polhx8b was weakly expressed in the spleen, gill, and some facial organs, indicating sub-functionalization of this gene pair during evolution. Furthermore, Polhx8a mRNA level elevated from perinucleolar oocyte (PNO) stage to vitellogenic oocyte (VO) stage transition and changed after exogenous hormone stimulation, proving that Polhx8a was involved in the oocyte development and could be regulated by sex hormones. Yeast two-hybrid, bimolecular fluorescence complementation (BiFC) and co-immunoprecipitation (co-IP) experiments captured the positive protein interactions between PoLhx8a and the other two oocyte-specific transcription factors: PoFigla and PoNobox. After knocking down lhx8a in embryos or adult ovaries in vivo, the expression of oocyte-associated genes was significantly down-regulated (P < 0.05). Our findings suggest the evolution and functional differentiation of lhx8 genes, and shed light on the potential role of lhx8a in protein interactions and gene regulation in teleosts.

RNA-Sequencing Analysis of the Spleen and Gill of Takifugu rubripes in Response to Vibrio harveyi Infection

Takifugu rubripes is commonly subjected to the disease-causing bacterium, Vibrio harveyi. However, the mechanism involved in the immune response of T. rubripes to V. harveyi infection is unclear. We conducted a transcriptomic analysis of the spleen and gill from T. rubripes infected with V. harveyi. We obtained 60,981,357 and 60,760,550 clean reads from the control and infected spleens, and 57,407,586 and 57,536,651 clean reads from the control and infected gills, respectively. We also identified 1,560 and 1,213 differentially expressed genes in the spleen and gill, respectively. Gene ontology analysis revealed that the most enriched biological process in both the spleen and gill was "immune response". The most enriched Kyoto Encyclopedia of Genes and Genomes immune response-related pathways were the NOD-like receptor signaling pathway in the spleen and cytokine-cytokine receptor interaction in the gill. We found 10 candidate immune-related genes in the spleen and gill. These putative immune pathways and candidate genes will provide insight into the immune response mechanisms of T. rubripes against V. harveyi.

A genome-wide survey of NOD-like receptors in Chinese tongue sole (Cynoglossus semilaevis): Identification, characterization and expression analysis in response to bacterial infection

As intracellular pathogen recognition receptors (PRRs), nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs, NOD-like receptors) are involved in innate immune responses in vertebrates. However, there is no systemic study on NLRs in Chinese tongue sole (Cynoglossus semilaevis), a popular maricultured fish in China. In the present study, a genome-wide survey of NLRs was performed in C. semilaevis, with the identification of 29 NLRs, including five genes from the NLR-A subfamily (referred to as CsNOD1-5), two genes from the NLR-B subfamily, 18 genes from the NLR-C subfamily (referred to as CsNLR-C1 to 18) and four other NLR genes. Phylogenetic analysis implied that CsNOD1-5 contained conserved functional domains and had orthologous relationships with human NOD1-5. Moreover, CsNLR-C genes all possessed the FISHNA domain, which is a fish-specific NACHT subdomain. Expression analysis showed that CsNOD1-5 and CsNLR-C1/2 were ubiquitously expressed in various normal tissues. Bacterial infection with Vibro harveyi revealed distinct expression patterns of all the tested CsNLRs in gill, intestine, trunk kidney, liver and spleen. In particular, CsNOD1-4 and CsNLR-C2 were significantly upregulated in gills at 48 h post bacterial infection. In addition, CsNOD3 and CsNOD4 were significantly elevated in infectious intestine, trunk kidney, liver and spleen, revealing that their expressions were more sensitive to bacterial infection than other CsNLRs. Together with the computational protein-protein interaction network of CsNLRs, it was suggested that individual NLR genes had different roles in the innate immune cascades of C. semilaevi against bacterial infection. This study provides valuable information for further studies on CsNLR immune function.

Nile tilapia Toll-like receptor 7 subfamily: Intracellular TLRs that recruit MyD88 as an adaptor and activate the NF-κB pathway in the immune response

Toll-like receptor 7 (TLR7) subfamily members are important pattern recognition receptors that participate in the recognition of pathogen-associated molecular patterns. In the present study, three TLR family members, OnTLR7, OnTLR8 and OnTLR9, were identified in the Nile tilapia Oreochromis niloticus. TLR7-, TLR8-and TLR9-deduced proteins have typical structural characteristics of TLRs, including Toll/interleukin-1 receptor (TIR), leucine-rich repeat (LRR) and transmembrane region (TM). OnTLR7, OnTLR8 and OnTLR9 were broadly expressed in all of the tissues tested, with the highest expression levels in the brain (TLR7) and spleen (TLR8 and TLR9). Moreover, the expression levels of OnTLR7, OnTLR8 and OnTLR9 were significantly increased in most tested tissues after Streptococcus agalactiae infection in vivo. After LPS stimulation, OnTLR7 and OnTLR9 mRNA expression levels were downregulated in the intestine and upregulated in the liver, spleen and kidney; however, OnTLR8 mRNA expression levels were upregulated in the kidney only after LPS stimulation for 5 d. After Poly I:C stimulation, OnTLR7 and OnTLR9 mRNA expression levels were upregulated in the intestine, liver, spleen and kidney, and the highest expression was found in the liver, while OnTLR8 mRNA expression levels were upregulated in the intestine, liver and kidney and downregulated in the spleen. Subcellular localization of OnTLR7, OnTLR8, and OnTLR9 in 293T cells showed that OnTLR9 was distributed in both the cytoplasm and nucleus while OnTLR8 and OnTLR7 were distributed mainly in the cytoplasm. Overexpression of OnTLR7, OnTLR8 and OnTLR9 in 293T cells had no significant effect on the activity of NF-κB, but they could significantly enhance MyD88-mediated NF-κB activity after cotransfection with MyD88. Pulldown assays showed that OnTLR7, OnTLR8, and OnTLR9 could interact with OnMyD88. Taken together, these results indicate that TLR7 subfamily genes play a role in the immune response to pathogen invasion of Nile tilapia.

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

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

Transcriptome Analysis of Paralichthys olivaceus Erythrocytes Reveals Profound Immune Responses Induced by Edwardsiella tarda Infection

Unlike mammalian red blood cells (RBCs), fish RBCs are nucleated and thus capable of gene expression. Japanese flounder (Paralichthys olivaceus) is a species of marine fish with important economic values. Flounder are susceptible to Edwardsiella tarda, a severe bacterial pathogen that is able to infect and survive in flounder phagocytes. However, the infectivity of and the immune response induced by E. tarda in flounder RBCs are unclear. In the present research, we found that E. tarda was able to invade and replicate inside flounder RBCs in both in vitro and in vivo infections. To investigate the immune response induced by E. tarda in RBCs, transcriptome analysis of the spleen RBCs of flounder challenged with E. tarda was performed. Six sequencing libraries were constructed, and an average of 43 million clean reads per library were obtained, with 85% of the reads being successfully mapped to the genome of flounder. A total of 1720 differentially expressed genes (DEGs) were identified in E. tarda-infected fish. The DEGs were significantly enriched in diverse Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, especially those associated with immunity, disease, and infection. Ninety-one key DEGs involved in 12 immune-related pathways were found to form extensive interaction networks. Twenty-one genes that constituted the hub of the networks were further identified, which were highly regulated by E. tarda and involved in a number of immune processes, notably pathogen recognition and signal transduction, antigen processing and presentation, inflammation, and splicing. These results provide new insights into the immune role of flounder RBCs during bacterial infection.

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.

Global profiling and characterization of Japanese flounder (Paralichthys olivaceus) kidney microRNAs regulated by Edwardsiella tarda infection in a time-dependent fashion

Japanese flounder (Paralichthys olivaceus) is an important economic fish species farmed in China and other countries. It is susceptible to infection by Edwardsiella tarda, a severe fish pathogen with a broad host range. In this study, we employed high-throughput deep sequencing technology to identify, in a global scale, flounder kidney microRNAs (miRNAs) induced by E. tarda at different stages of infection. Differentially expressed miRNAs (DEmiRNAs) and mRNAs (DEmRNAs) exhibiting significantly altered expression levels before and after E. tarda infection were examined. A total of 96 DEmiRNAs were identified, for which 2779 target genes were predicted. Eighty-seven miRNA-mRNA pairs, involving 29 DEmiRNAs and 86 DEmRNAs, showed negative correlations in their expression patterns. GO and KEGG enrichment analysis revealed that the putative target genes of the DEmiRNAs were associated with diverse biological processes, cellular components, and molecular functions. One of the DEmiRNAs, pol-miR-182-5p, was demonstrated to regulate sphingosine-1-phosphate receptor 1 (PoS1PR1) negatively in a manner that depended on the specific interaction between the seed sequence of pol-miR-182-5p and the 3'-UTR of PoS1PR1. Overexpression of pol-miR-182-5p in flounder cells promoted apoptosis and inhibited cellular viability. Knockdown of PoS1PR1 in flounder enhanced E. tarda invasion and dissemination in fish tissues. These results provide new insights into miRNA-mediated anti-bacterial immunity in flounder.

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 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.

Transcriptome profiling of immune-responsive genes in the intestine of Cynoglossus semilaevis Günther challenged with Shewanella algae

To better understand gene expression in the intestine after Shewanella algae infection and provide insights into its immune roles in the tongue sole, Cynoglossus semilaevis, sequencing-based high-throughput RNA analysis (RNA-Seq) for the intestines between the control group and 12 h post-injection group was performed. After assembly, there was an average of 23,957,159 raw sequencing reads, and 23,943,491 clean reads were obtained after filtering out low-quality reads. Then, 383 differentially expressed genes (DEGs) in the intestines in response to S. algae infection were identified. Subsequently, gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses of the DEGs were conducted to further explore their functions. Among all of the pathways involved, sixteen pathways were related to the immune system, among which the complement and coagulation cascades pathway was the most prominent for immunity-related DEGs, followed by the leukocyte transendothelial migration pathway. Furthermore, the expression levels of twelve selected DEGs in the immune-related pathways were identified by quantitative real-time polymerase chain reaction, substantiating the reliability and reproducibility of the RNA-Seq results. In summary, this study represents an important genomic resource for understanding the potential immune role of the tongue sole intestine from the perspective of gene expression.

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

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

A teleost complement factor Ba possesses antimicrobial activity and inhibits bacterial infection in fish

Complement factor B (Bf) is a component of the complement system. Following activation of the alternative pathway of the complement system, factor B is cleaved into Ba and Bb fragments. In fish, the Bf of rainbow trout is known to act as a C3 convertase, but the function of the Ba fragment is essentially unknown. In this study, we examined the expression patterns of tongue sole Cynoglossus semilaevis Bf (named CsBf) and the biological activity of the Ba fragment of CsBf (named CsBa). CsBf possesses the conserved domains of Bf and shares 39.9%-56.4% sequence identities with other fish Bf. CsBf expression was high in liver, muscle, and heart, and low in intestine, blood, and kidney. Bacterial infection significantly induced CsBf expression in kidney, spleen, and liver in a time-dependent manner. Recombinant CsBa (rCsBa) exhibited apparent binding capacities to bacteria and tongue sole peripheral blood leukocytes, and binding of rCsBa to bacteria inhibited bacterial growth. When overexpressed in tongue sole, CsBa significantly reduced bacterial dissemination in fish tissues. Together these results indicate for the first time that a fish Ba possesses antibacterial effect as well as immune cell-binding capacity, and thus probably plays a role in host immune defense against bacterial infection.

Toll-like receptor 2 of tongue sole Cynoglossus semilaevis: Signaling pathway and involvement in bacterial infection

Toll-like receptor (TLR) 2 is a member of the TLR family that plays a pivotal role in innate immunity. In mammals, TLR2 is known to recognize specific microbial structures and trigger MyD88-dependent signaling to induce various cytokine responses. In this study, we examined the expression and function of the tongue sole Cynoglossus semilaevis TLR2, CsTLR2. CsTLR2 is composed of 898 amino acid residues and shares 25.6%-27.3% overall sequence identities with known teleost TLR2. CsTLR2 is a transmembrane protein with a toll/interleukin-1 receptor domain and eight leucine-rich repeats. Expression of CsTLR2 occurred in multiple tissues and was upregulated during bacterial infection. Stimulation of the CsTLR2 pathway led to enhanced expression of MyD88-dependent signaling molecules. Recombinant CsTLR2 (rCsTLR2) corresponding to the extracellular region was able to bind to a wide range of bacteria. Under both in vitro and in vivo conditions, rCsTLR2 significantly reduced bacterial infection. These observations add new insights into the signaling and function of teleost TLR2.