CsSAP, a teleost serum amyloid P component, interacts with bacteria, promotes phagocytosis, and enhances host resistance against bacterial and viral infection

Comparative proteome analysis revealed potential biomarkers and the underlying immune mechanisms in Vibrio-resistant hybrid grouper, Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂

Vibrio alginolyticus is the causative agent of vibriosis, a common bacterial infection in grouper aquaculture that is associated with the development of haemorrhagic and non-haemorrhagic ulcerations on the fish. In the present study, comparative proteome analysis was performed on serum samples from Vibrio-resistant and Vibrio-susceptible grouper. Samples were analysed using high-throughput LC-MS/MS and identified 2770 unique peptides that corresponded to 344 proteins. Subsequent analysis identified 21 proteins that were significantly up-regulated in the resistant group compared to the control and the susceptible groups. Those proteins are associated with immunostimulatory effects, signalling and binding cascade, metabolism, and maintaining tissue integrity and physiological condition. Besides, potential protein biomarkers related to the immune system were identified, which could be associated with the disease-resistant phenotype. These data provide insights into the underlying immune mechanism of hybrid groupers upon Vibrio sp. infection.

Tetraspanin CD53 regulates peripheral blood leucocytes vitality and pathogen infection in turbot (Scophthalmus maximus)

Cluster of differentiation 53 (CD53) also known as OX44 or tetraspanin 25 (TSPAN25) is a glycoprotein belonging to the tetraspanin family. Members of the tetraspanin family are characterized by four transmembrane domains, including intracellular N- and C-termini, and small and large extracellular domains. Currently, the function of CD53 in teleost is not well understood. In this study, we identified a CD53 (named SmCD53) from turbot (Scophthalmus maximus) and examined its expression and biological activity. SmCD53 contained 231 amino acid residues and was predicted to be a tetraspanin with small and large extracellular domains. SmCD53 expression was observed in different tissues, particularly in immune-related organs. Experimental infection with bacterial or viral pathogen significantly up-regulated SmCD53 expression in a time-dependent manner. Immunofluorescence microscopy analysis showed that SmCD53 was localized on the surface of PBL and was recognized by antibody against its large extracellular domain. Ligation of SmCD53 onto PBLs with antibodies suppressed the respiratory burst activity, inflammatory reaction, and enhanced cell viability. SmCD53 knockdown significantly enhanced bacterial dissemination and proliferation in turbot. Overall, these results underscore the importance of CD53 in the maintenance of the function and homeostasis of the immune system.

Innate immune and chronic heat stress responses in sturgeons: Advances and insights from studies on Russian sturgeons

Chronic stress deteriorates the immune function of fish, thereby increasing their vulnerability to infections. However, the molecular and cellular mechanisms underlying stress-mediated immunosuppression and infection susceptibility in fish remain largely unknown. Understanding these mechanisms will contribute to improving fish welfare and their farm production. Herein, we review the challenges of sturgeon aquaculture in subtropical countries, where current climate change has giving rise to significant temperature increments during summer. This leads to the exposure of fish to stressful conditions during these months. Chronic heat stress deserves attention considering the rapid warming rate of the planet. It is already affecting wild fish populations, with disastrous consequences for sturgeons, which are one of the most endangered fish species in the world. In this context, we discuss the most recent advances through the studies on the effects of chronic heat stress on the innate immune components of sturgeons. To this end, we summarise the findings of studies focusing on the aquaculture of Russian sturgeons and observations made on other Acipenser species. Special attention is given to acute-phase proteins, as they might be valuable biomarkers of heat stress and infection, with applicability in monitoring the fish health status in farms.

Pentraxins in invertebrates and vertebrates: From structure, function and evolution to clinical applications

The immune system is divided into two broad categories, consisting of innate and adaptive immunity. As recognition and effector factors of innate immunity and regulators of adaptive immune responses, lectins are considered to be important defense chemicals against microbial pathogens, cell trafficking, immune regulation, and prevention of autoimmunity. Pentraxins, important members of animal lectins, play a significant role in protecting the body from pathogen infection and regulating inflammatory reactions. They can recognize and bind to a variety of ligands, including carbohydrates, lipids, proteins, nucleic acids and their complexes, and protect the host from pathogen invasion by activating the complement cascade and Fcγ receptor pathways. Based on the primary structure of the subunit, pentraxins are divided into short and long pentraxins. The short pentraxins are comprised of C-reactive protein (CRP) and serum amyloid P (SAP), and the most important member of the long pentraxins is pentraxin 3 (PTX3). The CRP and SAP exist in both vertebrates and invertebrates, while the PTX3 may be present only in vertebrates. The major ligands and functions of CRP, SAP and PTX3 and three activation pathways involved in the complement system are summarized in this review. Their different characteristics in various animals including humans, and their evolutionary trees are analyzed. The clinical applications of CRP, SAP and PTX3 in human are reviewed. Some questions that remain to be understood are also highlighted.

WR-PTXF, a novel short pentraxin, protects gut mucosal barrier and enhances the antibacterial activity in Carassius cuvieri × Carassius auratus red var

The pentraxin family is an evolutionarily conserved group that plays an important role in innate immunity. C-reactive protein (CRP) and serum amyloid P component (SAP) are classical members of the short pentraxins and are known to be the major acute phase proteins. In this work, we have cloned a novel pentraxin fusion protein, WR-PTXF, from Carassius cuvieri × Carassius auratus red var. In fish, the biological function of PTXF is essentially unknown. For this purpose, we report the identification and analysis of WR-PTXF and elucidate its role in the antibacterial innate immunity. WR-PTXF contains 224 amino acids and shares 79.8% and 23.0% sequence identities with crucian carp CRP and SAP, respectively. Blast analysis shows that WR-PTXF and goldfish PTXF had the highest similarity (97.3%). WR-PTXF is expressed in multiple tissues and is upregulated by Aeromonas hydrophila infection. WR-PTXF contains a short pentraxin domain and recombinant WR-PTXF protein (rWR-PTXF) can bind the A. hydrophila in a concentration-dependent manner. Further, rWR-PTXF displays apparent bacteriostatic activity against A. hydrophila in vitro by enhancing the uptake of the bound bacteria by host phagocytes. When introduced in vivo, rWR-PTXF not only protects the gut mucosa but also limits the colonization of A. hydrophila in systemic immune organs. Consistently, knockdown of WR-PTXF significantly promotes bacterial dissemination in the tissues of host. These results indicate that WR-PTXF is a classic pattern recognition molecule that exerts a protective effect against bacterial infection.

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.

Tongue sole creatine kinases function as DAMP and activate antimicrobial immunity via TLR2

Creatine kinase (CK) is an enzyme that regulates adenosine triphosphate (ATP) metabolism to maintain energy homeostasis. Although CK has been reported to be involved in pathogen infection, the immune function of CK remains elusive. In this study, we identified two muscle-type CK from the teleost tongue sole Cynoglossus semilaevis (designated CsCKM-1 and CsCKM-2). Bacterial infection modulated CsCKM-1/2 expression in tongue sole tissues and induced the release of CsCKM-1/2 into serum. Recombinant CsCKM-1/2 (rCsCKM-1/2) exhibited robust kinase activity and bound to bacterial pathogens and pathogen-associated molecular patterns. rCsCKM-1/2 also bound to tongue sole peripheral blood leukocytes (PBLs) and promoted PBLs to uptake bacterial pathogens, inhibit bacterial proliferation, and express proinflammatory cytokines. When co-expressed in HEK293T cells, CsCKM-1/2 were found to interact with the leucine rich domain of toll-like receptor 2 (TLR2). The presence of TLR2 antagonist significantly reduced CsCKM-1/2-induced immune response and antibacterial effect. Taken together, these results indicated that tongue sole creatine kinases function as damage-associated molecular pattern (DAMP) molecules and play an important role in antimicrobial immunity via TLR2.

GATA-3 in Atlantic salmon (Salmo salar): Tissue distribution and its regulation of IL-4/13a promoter

GATA3 is a transcription factor that plays an important role in T cell lineage differentiation and T-helper 2 (Th2) type immune responses. In this study, we developed two rat antibodies against Atlantic salmon GATA-3 (anti-rSsGATA-3a and anti-rSsGATA-3b, respectively). The western blotting and immunofluorescence results showed that anti-rSsGATA-3b antibodies recognized endogenous SsGATA-3 proteins, while the anti-rSsGATA-3a antibodies did not bind SsGATA-3. Immunohistochemical analysis revealed that SsGATA-3 positive cells were detected in all tissues tested, with relatively high number of immune reactive cells in the gills and spleen. Furthermore, the immunohistochemical study revealed that SsGATA-3 was expressed in pillar cells, epithelial cells, chondrocytes, perichondrium cells, and some undifferentiated basal cells. In addition, we determined 577 bp of the upstream promoter sequence of SsIL-4/13a and found four motifs that matched SsGATA-3 binding sites. The promoter regions of SsIL-4/13a were assessed by transfecting four deletion reporter constructs and SsGATA-3 overexpression plasmids. The result showed that SsGATA-3 enhanced the activity of SsIL-4/13a promoters within the region ranging from -317 to -302 bp upstream of the transcriptional start site. Antibodies against Th2 markers such as GATA-3 are valuable in addressing the diversity of T cell responses in fish.

SAP deficiency aggravates periodontitis possibly via C5a-C5aR signaling-mediated defective macrophage phagocytosis of Porphyromonas gingivalis

INTRODUCTION

Serum amyloid P component (SAP) regulates the innate immune system and microbial diseases. Periodontitis is an inflammatory oral disease developed by the host immune system's interaction with the dysbiotic oral microbiome, thereby SAP could play a role in periodontitis pathogenicity.

OBJECTIVES

To investigate the role of SAP in oral microbiome modulation and peridontitis pathogenicity.

METHODS

In this study, wildtype and SAP-knockout (KO) mice were used. Ligature-based periodontitis was developed in mice. Oral microbiome diversity was analyzed by 16 s rRNA sequencing. Macrophages and Porphyromonas gingivalis (P. gingivalis) co-culture system analyzed the effect of SAP in macrophage phagocytosis of P. gingivalis.

RESULTS

The level of SAP was upregulated in the periodontitis-affected periodontium of humans and mice but not in the liver and blood circulation. Periodontal macrophages were the key source of upregulated SAP in periodontitis. SAP-KO aggravated periodontal inflammation, periodontitis, and a higher number of M1-type inflammatory macrophage infiltration in the periodontium. The oral microbiome of SAP-KO periodontitis mice was altered with a higher abundance of Porphyromonas at the genus level. SAP-KO macrophages showed compromised phagocytosis of P. gingivalis in the co-culture system. Co-culture of SAP-KO macrophages and P. gingivalis induced the C5a expression and exogenous SAP treatment nullified this effect. Exogenous recombinant SAP treatment did not affect P. gingivalis growth and opsonization. PMX205, an antagonist of C5a, treatment robustly enhanced P. gingivalis phagocytosis by SAP-KO macrophages, indicating the involvement of the C5a-C5aR signaling in the compromised P. gingivalis phagocytosis by SAP-KO macrophages.

CONCLUSION

SAP deficiency aggravates periodontitis possibly via C5a-C5aR signaling-mediated defective macrophage phagocytosis of P. gingivalis. A higher abundance of P. gingivalis during SAP deficiency could promote M1 macrophage polarization and periodontitis. This finding suggests the possible protecting role of elevated levels of periodontal SAP against periodontitis progression.

Functional Characterization of Serum Amyloid P Component (SAP) in Host Defense against Bacterial Infection in a Primary Vertebrate

Serum amyloid P component (SAP), an ancient short pentraxin of the pentraxin family, plays an essential role in resistance to bacterial infection. In this study, the expression and functional characterization of SAP (OnSAP) in Nile tilapia (Oreochromis niloticus), a primary vertebrate, are investigated. The open reading frame of OnSAP is 645 bp of a nucleotide sequence encoding a polypeptide of 214 amino acids. As a calcium-binding protein, the structure and relative motif of OnSAP is highly similar to those of humans, containing amino acid residues Asn, Glu, Gln and Asp. In healthy fish, OnSAP mRNA is extensively distributed in all eleven tissues examined, with the highest level in spleen. The mRNA expression of OnSAP was significantly up-regulated after being challenged with gram-positive bacterium Streptococcus agalactiae and gram-negative bacterium Aeromonas hydrophila in vivo. In addition, recombinant OnSAP ((r)OnSAP) protein had capacities of binding S. agalactiae or A. hydrophila in the presence of Ca²⁺. Further, (r)OnSAP helped monocytes/macrophages to efficiently phagocytize bacteria. Moreover, the (r)OnSAP was able to enhance the complement-mediated lysis of the chicken red blood cells. Collectively, the evidence of SAP in tilapia, based on the results including its evolutionary conserved protein structure, bacterial binding and agglutination, opsonophagocytosis of macrophage and hemolysis enhancement, enriches a better understanding of the biological functions of the pentraxin family.

Molecular characterization and antibacterial activities of a goose-type lysozyme gene from roughskin sculpin (Trachidermus fasciatus)

Lysozymes, acting as antimicrobial molecules, play a vital role in the host's innate immune response to pathogen infections. In the present study, a g-type lysozyme gene termed Tf-LyzG from roughskin sculpin, Trachidermus fasciatus was firstly reported. The deduced amino acid sequence of Tf-LyzG contained 188 residues and possessed conserved catalytic residues (Glu⁷¹, Asp⁸⁴, and Asp⁹⁵). Gene expression analysis revealed that Tf-LyzG was widely distributed in the tested eleven tissues with the highest expression in the gill and could be significantly induced post lipopolysaccharide (LPS) challenge. The lysozyme activity of the purified recombinant protein (rTf-LyzG) was found to be most active at pH 5.5 and 37 °C. rTf-LyzG exhibited a wide spectrum of potent bacteriolytic activity against four Gram-positive bacteria and six Gram-negative bacteria. It also displayed a high affinity to polysaccharides on bacteria surfaces including LPS, lipoteichoic acid (LTA), and peptidoglycan (PGN). rTf-LyzG was capable of binding and agglutinating all nine bacteria. Flow cytometry assay further revealed that rTf-LyzG could disrupt the membrane of Micrococcus lysodeikticus which is confirmed by scanning electron microscope (SEM) analysis that reveals blebs around the bacterial cell membrane. In summary, these data indicate that Tf-LyzG is of great importance in the fish immune response against pathogens invasion.

Different response of Acipenser gueldenstaedtii CRP/SAP and SAA to bacterial challenge and chronic thermal stress sheds light on the innate immune system of sturgeons

Sturgeons are chondrostean fish critically endangered due to anthropogenic loss and degradation of natural habitat and overfishing for meat and caviar production. Consequently, sturgeon aquaculture has extensively developed lately, being Russian sturgeon (Acipenser gueldenstaedtii) the second most important species reared for caviar production. However, Russian sturgeon aquaculture in subtropical countries, such as Uruguay, confronts difficulties because fish have to endure excessive summertime warm temperatures, which weaken their innate defences facilitating opportunistic infections. To address this problem, we look for identifying putative acute phase proteins (APPs), which might be robust serum biomarkers of both infection and chronic thermal stress, applied to monitoring Russian sturgeon health status in farms. We focused on the C-Reactive Protein/Serum Amyloid P (CRP/SAP) pentraxin since the pentraxin family includes well-known APPs, better characterised in mammals than fish. We identified A.gueldenstaedtii CRP/SAP (AgCRP/SAP), as a member of the universal CRP/SAP pentraxin sub-family, and studied AgCRP/SAP involvement in sturgeon response to bacterial challenge and chronic thermal stress, in comparison with A. gueldenstaedtii Serum Amyloid A (AgSAA), a previously described positive APP. Results showed that AgCRP/SAP is a constitutive serum component that remained constant upon Aeromonas hydrophila challenge and chronic thermal stress. Contrastingly, serum AgSAA was subjected to regulation by bacterial and thermal stress challenges, showing a 50-fold increase and 3-fold decline in serum levels, respectively. Overall, results highlight the potential value of AgSAA, but not of AgCRP/SAP, as a biomarker of bacterial infection and the need to continue searching for robust chronic thermal stress biomarkers in sturgeons.

A Novel microRNA of Japanese Flounder Regulates Antimicrobial Immunity Involving a Bacteria-Binding CSF3

MicroRNAs (miRNAs) are small non-coding RNAs that regulate diverse biological processes including immunity. In a previous high-throughput RNA sequencing study, a novel miRNA, pol-miR-novel_642, was identified from Japanese flounder (Paralichthys olivaceus), a farmed fish species with important economic value. In this study, we investigated the regulatory mechanism and the function of pol-miR-novel_642 and its target gene. We found that pol-miR-novel_642 targeted, in a sequence-specific manner, a flounder gene encoding an uncharacterized protein that is a structural homologue of murine granulocyte colony stimulating factor 3 (CSF3). The expression of pol-miR-novel_642 and its target gene (named PoCSF3-1) was regulated, in different manners, by the bacterial pathogen Edwardsiella tarda and the viral pathogen megalocytivirus. Overexpression of pol-miR-novel_642 or interference with PoCSF3-1 expression in flounder cells strongly potentiated E. tarda infection. Consistently, in vivo knockdown of PoCSF3-1 enhanced bacterial dissemination in flounder tissues but blocked viral replication, whereas in vivo overexpression of PoCSF3-1 inhibited bacterial dissemination and facilitated viral infection. Overexpression/knockdown of PoCSF3-1 and pol-miR-novel_642 also affected the activation of autophagy. Recombinant PoCSF3-1 (rPoCSF3-1) interacted with and inhibited the growth of Gram-negative bacteria in a manner relying on a PoCSF3-1-characteristic structural motif that is absent in mouse CSF3. rPoCSF3-1 also regulated the proliferation, inflammatory response, and immune defense of flounder head kidney leukocytes in a structure-dependent fashion. Together, these results reveal the function of a novel miRNA-CSF3 regulatory system of flounder, and add new insights into the role and mechanism of fish miRNA and CSF3 in antimicrobial immunity.

A teleost interleukin-16 is implicated in peripheral blood leukocytes recruitment and anti-bacterial immunity

Interleukin-16 (IL-16), as a lymphocyte chemoattractant cytokine, plays a crucial role in regulating cellular activities and anti-pathogen immunity. In teleost, the information about the antibacterial effect of IL-16 is scarce. In our study, we examined the immune functions of an IL-16 homologue (CsIL-16) from tongue sole Cynoglossus semilaevis. The CsIL-16 precursor (proCsIL-16) is comprised of 1181 amino acid residues, sharing 21.1%-67.3% identities with IL-16 precursor from invertebrate and vertebrate. The C-terminal proCsIL-16 containing two PDZ domains was designated as mature CsIL-16 which was released into the supernatant of peripheral blood leukocytes (PBLs). CsIL-16 was expressed in various tissues and regulated by bacterial invasion. Recombinant CsIL-16 (rCsIL-16), as a homodimer, was able to bind to the membrane of PBLs and played essential roles in regulating chemotaxis and activation of PBLs, which in vitro inhibited intracellular survival of E. tarda. Under in vivo condition, rCsIL-16 could dramatically regulate the induction of inflammatory genes, and suppress the bacterial dissemination in fish tissues. Collectively, our results reveal that CsIL-16 plays positive roles in antibacterial immunity, and provide insights into the immune function of CsIL-16.

Tongue sole (Cynoglossus semilaevis) interleukin 10 receptors are involved in the immune response against bacterial infection

Interleukin (IL)-10, an immune-regulatory cytokine, exerts various biological functions through interaction with IL-10 receptors. In teleost, very limited functional studies on IL-10 receptors have been documented. In this study, we reported the expression patterns of IL-10 receptor 1 (CsIL-10R1) and receptor 2 (CsIL-10R2) of tongue sole (Cynoglossus semilaevis) and examined their biological properties. The expression of CsIL-10R1 and CsIL-10R2 occurred in multiple tissues and were regulated by bacterial challenge. In vitro binding studies showed that recombinant extracellular region of CsIL-10R1 (rCsIL-10R1ex) rather than rCsIL-10R2ex could bind with rCsIL-10. Cellular study showed that both CsIL-10R1 and CsIL-10R2 were expressed on peripheral blood leukocytes (PBLs), and blockade of CsIL-10R1 or CsIL-10R2 by antibody could reduce inhibitory effect of CsIL-10 on ROS production of PBLs. When injected in vivo, anti-rCsIL-10R1 or anti-rCsIL-10R2 antibody dramatically promoted the expression of proinflammatory cytokines and suppressed bacterial dissemination in tongue sole tissues. Consistently, the overexpression of CsIL-10R1 or CsIL-10R2 significantly enhanced bacterial dissemination, and the overexpression of CsIL-10R1M bearing STAT3 site mutation reduced bacterial dissemination. Overall, these results demonstrate for the first time teleost IL-10 receptors play a negative role in antibacterial immunity and add insight into the function of CsIL-10 receptors.

Modulation of the Tissue Expression Pattern of Zebrafish CRP-Like Molecules Suggests a Relevant Antiviral Role in Fish Skin

Simple Summary

The clinical use of the human short pentraxin C-reactive protein as a health biomarker is expanded worldwide. The acute increase of the serum levels of short pentraxins in response to bacterial infections is evolutionarily conserved, as are the main functions of pentraxins. Interestingly, fish orthologs have been found to increase similarly after bacterial and viral stimuli, thus becoming promising candidates for health biomarkers of both types of infection in this group of vertebrates. To preliminarily assess their adequacy for this application, zebrafish and a fish rhabdovirus were chosen as infection model systems for the analysis of the levels of gene expression of all short pentraxins in healthy and infected animals in a wide range of tissues. Because some significant increases were found in skin (a very suitable sampling source for testing purposes), further transcript analyses were carried out in this tissue. Due to the functional similarities between pentraxins and antibodies, it was also checked whether short pentraxins can compensate for the deficiencies in adaptive immunity by using mutant zebrafish lacking this system. In conclusion, the obtained results suggest that short pentraxins are highly reactant against viruses in skin and their overexpression seems to reflect a mechanism to compensate for the loss of adaptive immunity.

Abstract

Recent studies suggest that short pentraxins in fish might serve as biomarkers for not only bacterial infections, as in higher vertebrates including humans, but also for viral ones. These fish orthologs of mammalian short pentraxins are currently attracting interest because of their newly discovered antiviral activity. In the present work, the modulation of the gene expression of all zebrafish short pentraxins (CRP-like proteins, CRP1-7) was extensively analyzed by quantitative polymerase chain reaction. Initially, the tissue distribution of crp1-7 transcripts and how the transcripts varied in response to a bath infection with the spring viremia of carp virus, were determined. The expression of crp1-7 was widely distributed and generally increased after infection (mostly at 5 days post infection), except for crp1 (downregulated). Interestingly, several crp transcription levels significantly increased in skin. Further assays in mutant zebrafish of recombinant activation gene 1 (rag1) showed that all crps (except for crp2, downregulated) were already constitutively highly expressed in skin from rag1 knockouts and only increased moderately after viral infection. Similar results were obtained for most mx isoforms (a reporter gene of the interferon response), suggesting a general overcompensation of the innate immunity in the absence of the adaptive one.

Serum amyloid P-component/C-reactive proteins in fugu (Takifugu rubripes) egg with binding ability to disease-causing bacteria by carbohydrate-recognition

Two galactose-binding proteins were purified from the eggs of Takifugu rubripes by affinity chromatography. These proteins were detected at 26 and 23 kDa under reducing and at 40 and 45 kDa under non-reducing conditions at SDS-PAGE. The peptide sequences from both proteins matched to short-type pentraxin. The 26-kDa lectin was glycosylated, while the other one was not, indicating that these could be glycoforms of pentraxin. Messenger RNA of pentraxin was detected in eggs and embryos at 1-cell stage, was undetectable till blastula, and finally detected again after gastrula, suggesting that the mRNAs in eggs and 1-cell embryos were maternal in origin, and autologous transcription of the gene occurred after blastula. Since they bind to pathogenic bacteria, egg pentraxins may have immunological functions during embryogenesis. This is the first study to show the presence of short-type pentraxin in fish eggs and the diversity of fish egg lectins.

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.

The novel fish miRNA pol-miR-novel_171 and its target gene FAM49B play a critical role in apoptosis and bacterial infection

MicroRNAs (miRNAs) are a type of small, non-coding RNAs that participate in many cellular and biological processes by regulating mRNA stability. In a previous study, we identified 96 Japanese flounder (Paralichthys olivaceus) miRNAs responsive to the infection of Edwardsiella tarda, a bacterial pathogen to fish as well as humans. In the current study, we examined the regulation and function of one novel miRNA, i.e., pol-miR-novel_171, from the above 96 miRNA pool. We found that pol-miR-novel_171 expression was regulated by E. tarda and megalocytivirus in a pathogen-specific manner, and that pol-miR-novel_171 targeted the gene of FAM49B (family with sequence similarity 49 member B) of flounder (named PoFAM49B) by negative interaction with the 3'-UTR of PoFAM49B. To date, the function fish FAM49B is unknown. We found that PoFAM49B expressed in multiple tissues of flounder, and recombinant PoFAM49B interacted with and inhibited the growth of Gram-negative bacterial pathogens. Interference with PoFAM49B expression in flounder cells promoted E. tarda infection. Similar effects on E. tarda infection were observed with pol-miR-novel_171 overexpression. Consistently, in vivo knockdown of PoFAM49B in flounder enhanced E. tarda dissemination in fish tissues. Furthermore, interference with PoFAM49B expression, or overexpression of pol-miR-novel_171, promoted apoptosis of flounder cells, while in vitro and in vivo knockdown of PoFAM49B augmented the expressions of key apoptosis-associated genes. These results revealed for the first time the immune function of fish FAM49B and the regulatory mechanism of a novel fish miRNA by demonstrating that pol-miR-novel_171, via PoFAM49B, played a critical role in apoptosis and anti-bacterial immunity.

Japanese flounder Paralichthys olivaceus interleukin 21 induces inflammatory response and plays a vital role in the immune defense against bacterial pathogen

Interleukin (IL)-21 is a pleiotropic cytokine and plays a vital role in immunity. In the current study, we examined the immune function of Japanese flounder Paralichthys olivaceus IL-21 (PoIL-21). PoIL-21 shares moderate (25.17%-46.25%) sequence identities with other teleost IL-21. PoIL-21 expression occurred in multiple tissues, especially intestine, and was regulated by bacterial infection in a time dependent manner. PoIL-21 was secreted by peripheral blood leukocytes (PBL) upon LPS stimulation. Recombinant PoIL-21 (rPoIL-21) bound to a wide range of Gram-negative and Gram-positive bacteria and inhibited the growth of the fish bacterial pathogen Streptococcus iniae. rPoIL-21 also interacted with PBL, resulting in enhanced cell proliferation, ROS production, and expression of IL-1β, TNF-α, CD8β, T-bet, PoIL-21, PoIL-21 receptor, and STAT. Consequently, the presence of rPoIL-21 significantly reduced bacterial infection in PBL. In vivo study showed that rPoIL-21 upregulated the expression of inflammatory cytokines and PoIL-21. Taken together, these results indicate that PoIL-21 is an inducible, secreted cytokine with a broad range of binding capacities and plays an important role in the regulation of anti-bacterial immunity.

Tongue sole (Cynoglossus semilaevis) interleukin 10 plays a negative role in the immune response against bacterial infection

Interleukin-10 (IL-10) is a pleiotropic cytokine and plays a crucial role in immunity. In the current study, we examined the expression patterns and biological functions of tongue sole Cynoglossus semilaevis IL-10 (CsIL-10). CsIL-10 is composed of 186 amino acid residues and shares 46.3%-71.7% identities with other teleost IL-10. Csil-10 expression occurred in multiple tissues and was regulated by bacterial infection. Recombinant CsIL-10 (rCsIL-10) in the form of a dimer bound to a wide range of bacterial species but did not affect bacterial growth. rCsIL-10 could interact with peripheral blood leukocytes (PBL) and significantly reduce the phagocytic activity, ROS production, and apoptosis of PBL. When injected in vivo, rCsIL-10 significantly suppressed the expression of proinflammatory cytokines and promoted bacterial dissemination in tongue sole tissues. Consistently, knockdown of Csil-10 significantly inhibited bacterial infection in tongue sole. Taken together, these results indicate that CsIL-10 plays a negative regulatory role in the immune response against bacterial infection.

C-reactive protein/serum amyloid P promotes pro-inflammatory function and induces M1-type polarization of monocytes/macrophages in mudskipper, Boleophthalmus pectinirostris

C-reactive protein (CRP) and serum amyloid P (SAP) play essential roles in the phagocytic cell-mediated innate immune response of mammals. In-depth studies into CRP and SAP have been completed in mammals; however, such studies, particularly those relating to the functions of CRP and SAP, are rare in fish species. In this study, a homolog of CRP/SAP (BpCRP/SAP) was identified in mudskipper (Boleophthalmus pectinirostris), which had the typical characteristics of a fish short pentraxin protein. Phylogenetic tree analysis revealed that BpCRP/SAP was most closely related to mudskipper CRP/SAP-l3. BpCRP/SAP transcripts were detected in all tested tissues, with the highest level observed in the liver; transcripts in the immune tissues and protein expression in the serum were induced in response to Edwardsiella tarda infection. The active recombinant BpCRP/SAP (rBpCRP/SAP) was able to augment the mRNA expression of pro-inflammatory cytokines and attenuate the mRNA expression of anti-inflammatory cytokines in monocytes/macrophages (MO/MΦ). In addition, phagocytosis and bacterial killing of E. tarda by mudskipper MO/MΦ were boosted by rBpCRP/SAP stimulation. rBpCRP/SAP also promoted M1-type MO/MΦ polarization, but inhibited M2-type polarization. In conclusion, the present research describes the pro-inflammatory function of BpCRP/SAP in mudskipper against E. tarda infection.

Acute phase protein response to viral infection and vaccination

Organisms respond in multiple ways to microbial infections. Pathogen invasion tipically triggers an inflammatory response where acute phase proteins (APP) have a key role. Pentraxins (PTX) are a family of highly conserved APP that play a part in the host defense against infection. The larger proteins of the family are simply named pentraxins, while c-reactive proteins (CRP) and serum amyloid proteins (SAA, SAP) are known as short pentraxins. Although high APP levels have been broadly associated with bacterial infections, there is a growing body of evidence revealing increased PTX, CRP and SAP expression upon viral infection. Furthermore, CRP, PTX and SAP have shown their potential as diagnostic markers and predictors of disease outcome. Likewise, the measurement of APP levels can be valuable to determine the efficacy of antiviral therapies and vaccines. From the practical point of view, the ability of APP to reduce viral infectivity has been observed in several virus-host models. This has prompted investigation efforts to assess the role of acute phase response proteins as immunoregulatory molecules and their potential as therapeutic reagents. This work aims to present an overview of the APP response to viral infections reviewing the current knowledge in the field.

First characterization of two C-type lectins of the tubeworm Alaysia sp. from a deep-sea hydrothermal vent

C-type lectins (CTLs) play an important role in innate immune defense. In this study, we identified and characterized two CTLs (Lec1 and Lec2) from the tubeworm Alaysia sp. collected from a hydrothermal vent in Pacmanus. Lec1 and Lec2 possess the typical CTL domain but share low sequence identities (10.8%-20.4%) with known CTLs. Recombinant (r) of Lec1 and Lec2 bound to various PAMPs and a wide arrange of bacteria from neritic and deep-sea environments in a Ca²⁺-independent manner, but only rLec1 caused agglutination of the bound bacteria. The activities of rLec1 and rLec2 were most stable and highest at 4 °C, the ambient temperature of the hydrothermal vent, and decreased at higher temperatures. Both lectins inhibited bacterial growth in a highly selective manner and agglutinated the erythrocytes of fish, rabbit, and chicken in a Ca²⁺-dependent manner. These results provided the first insights into the functional properties of CTLs in deep-sea Alaysia sp.

Ayu C-reactive protein/serum amyloid P agglutinates bacteria and inhibits complement-mediated opsonophagocytosis by monocytes/macrophages

The short-chain pentraxins (PTXs), including C-reactive protein (CRP) and serum amyloid P (SAP), are soluble pattern recognition molecules (PRMs) that exhibit calcium-dependent binding to bacterial surface molecules. They opsonize pathogens or other particles by phagocytic clearance. However, the detailed functions of short-chain PTXs in teleosts remained unclear. In this study, we identified a short-chain PTX gene from ayu, Plecoglossus altivelis, and tentatively named as PaCRP/SAP. Sequence analysis revealed that PaCRP/SAP has typical characteristics of fish CRP/SAP and is mostly closely related to rainbow smelt (Osmerus mordax) SAP. PaCRP/SAP transcripts were detected in all tested tissues, with the highest level in the liver, and its expression significantly upregulated following Vibrio anguillarum infection. The active recombinant mature PaCRP/SAP (rPaCRP/SAPm) agglutinated Gram-negative bacteria (Escherichia coli, V. anguillarum, Aeromonas hydrophila, and Vibrio parahaemolyticus) and Gram-positive bacteria (Staphylococcus aureus and Listeria monocytogenes) in a calcium-dependent manner in vitro, and it correspondingly bound peptidoglycan and lipopolysaccharide in a dose-dependent manner. The binding of rPaCRP/SAPm to E. coli and S. aureus resulted in a clear inhibition of the deposition of ayu complement 3 (PaC3) on the bacteria. Furthermore, rPaCRP/SAPm decreased phagocytosis of rPaCRP/SAPm-bound E. coli and S. aureus cells by ayu monocytes/macrophages (MO/MΦ) in a complement-dependent way. However, rPaCRP/SAPm alone had no significant influence on phagocytosis. These results provided the first evidence that PaCRP/SAP might function in ayu immune responses via agglutinating bacteria and inhibiting complement-mediated opsonophagocytosis by MO/MΦ.

Histones and chymotrypsin-like elastases play significant roles in the antimicrobial activity of tongue sole neutrophil extracellular traps

Neutrophil extracellular traps (NETs) are a form of extracellular antimicrobial structure of neutrophils observed in higher and lower vertebrates, the latter including the teleost fish tongue sole Cynoglossus semilaevis. However, the antimicrobial mechanism of fish NETs is unknown. In the present study, we examined the potential contribution of histones and elastases to the antibacterial effect of tongue sole NETs. For this purpose, two histones (CsH2B and CsH4) and two elastases (CsEla1 and CsEla2) of tongue sole were investigated. The histones and elastases possess the conserved domain structures characteristic of that of histones H2B/H4 and trypsin-like serine protease, respectively. Recombinant CsH2B, CsH4, CsEla1, and CsEla2 bound a wide range of Gram-negative and Gram-positive bacteria, and some of the bound bacteria were inhibited in growth by the bound histones/elastases. CsH2B, CsH4, CsEla1, and CsEla2 were all localized in NETs induced by various stimuli including bacterial pathogen. Treatment of NETs with antibodies targeting CsH2B, CsH4, CsEla1, and CsEla2 significantly reduced the antimicrobial effect of NETs. These results indicate that histones and chymotrypsin-like elastases are fundamental components of teleost NETs that play important roles in the antimicrobial activity of NETs.

Structure and functionalities of the human c-reactive protein compared to the zebrafish multigene family of c-reactive-like proteins

Because of the recent discovery of multiple c-reactive protein (crp)-like genes in zebrafish (Danio rerio) with predicted heterogeneous phospholipid-binding amino acid sequences and heterogeneous transcript expression levels in viral survivors and adaptive-deficient mutants, zebrafish constitute an attractive new model for exploring the evolution of these protein's functions, including their possible participation in fish trained immunity. Circulating human CRP belongs to the short pentraxin family of oligomeric proteins that are characteristic of early acute-phase innate responses and is widely used as a clinical inflammation marker. In contrast to pentameric human CRP (pCRP), zebrafish CRPs are trimeric (tCRP); however monomeric CRP (mCRP) conformations may also be generated when associated with cellular membranes as occurs in humans. Compared to human CRP, zebrafish CRP-like proteins show homologous amino acid sequence stretches that are consistent with, although not yet demonstrated, cysteine-dependent redox switches, calcium-binding spots, phosphocholine-binding pockets, C1q-binding domains, regions interacting with immunoglobulin Fc receptors (FcR), unique mCRP epitopes, mCRP binding peptides to cholesterol-enriched rafts, protease target sites, and/or binding sites to monocyte, macrophage, neutrophils, platelets and/or endothelial cells. Amino acid variations among the zebrafish CRP-like multiprotein family and derived isoforms in these stretches suggest that functional heterogeneity best fits the wide variety of aquatic pathogens. As occurs in humans, phospholipid-tagged tCRP-like multiproteins might also influence local inflammation and induce innate immune responses; however, in addition, different zebrafish tCRP-like proteins and/or isoforms might fine tune new still unknown functions. The information reviewed here could be of value for future studies not only to comparative but also medical immunologists and/or fisheries sectors. This review also introduces some novel speculations for future studies.

CD94 of tongue sole Cynoglossus semilaevis binds a wide arrange of bacteria and possesses antibacterial activity

In this study, we examined the expression patterns and the functions of the tongue sole Cynoglossus semilaevis CD94, CsCD94. CsCD94 is composed of 209 amino acid residues and shares 43.0-50.2% overall identities with known teleost CD94 sequence. CsCD94 has a C-type lectin-like domain. Expression of CsCD94 occurred in multiple tissues and was upregulated during bacterial infection. Recombinant CsCD94 (rCsCD94) exhibited apparent binding and agglutinating activities against both Gram-positive and Gram-negative bacteria in a Ca²⁺-dependent manner. Treatment of bacteria with rCsCD94 enhanced phagocytosis of the bacteria by peripheral blood leukocytes. Furthermore, incubation of rCsCD94 with bacteria reduced the survival of the bacteria in vitro. Taken together, these results indicate that rCsCD94 is a key factor in the bactericidal and phagocytic effects of tongue sole, and reveal for the first time an essential role of fish CD94 in antibacterial immunity, thereby adding insight into the function of CD94.

Tongue sole (Cynoglossus semilaevis) CD59: A complement inhibitor that binds bacterial cells and promotes bacterial escape from the killing of fish serum

CD59 is a complement regulatory protein that inhibits the formation of membrane attack complex of complement. In this study, we examined the expression and activity of tongue sole (Cynoglossus semilaevis) CD59 (CsCD59). CsCD59 possesses the conserved structural features of CD59 and shares 33%-46% sequence identities with other fish CD59. Expression of CsCD59 was high in liver, spleen, and muscle, and was stimulated by infection of bacterial pathogens. Recombinant CsCD59 (rCsCD59) exhibited an apparent inhibition effect on the activation of tongue sole serum complement. ELISA and microscopy detected binding of rCsCD59 to a number of Gram-negative and Gram-positive bacteria. Interaction with rCsCD59 did not affect bacterial viability but significantly enhanced bacterial resistance against the killing effect of fish serum. Together these results indicate that fish CD59 may to some degrees facilitate a general escape of bacteria from complement-mediated immunity.

Molecular characterization reveals involvement of four caspases in the antibacterial immunity of tongue sole (Cynoglossus semilaevis)

Caspases are a family of proteases involved in many important biological processes including apoptosis and inflammation. In this study, we analyzed in a comparative manner the expression patterns and immune effects of four different types of caspases, i.e. caspase-1 (CsCas1), caspase-2 (CsCas2), caspase-3 (CsCas3), and caspase-9 (CsCas9), from the teleost fish tongue sole (Cynoglossus semilaevis). CsCas1, CsCas2, CsCas3, and CsCas9 share 35.4%-58.5%, 61.2%-75.3%, 52.3%-65.6%, and 63.0%-76.2% overall sequence identities, respectively, with their counterparts in teleost species. CsCas1, CsCas2, CsCas3, and CsCas9 possess the caspase domain and catalytic site conserved in known caspases. The expressions of the four caspases were detected in a wide range of tissues, however, the expression levels varied between different tissues and caspases. Following bacterial infection, the expressions of the four caspases were upregulated or downregulated to significant extents in a time- and tissue-dependent manner. In vivo analysis showed that overexpression of each of the caspases in tongue sole significantly enhanced the ability of the fish to resist bacterial dissemination in and colonization of tissues. These results indicate an involvement of fish caspases in bacteria-induced immune response and demonstrate for the first time that caspase-1, 2, 3, and 9 are essential to the optimal defense against bacterial infection in fish.

CsPTX1, a pentraxin of Cynoglossus semilaevis, is an innate immunity factor with antibacterial effects

Pentraxin 1 (PTX1) is a member of the pentraxin protein family, which plays important roles in the innate immunity of vertebrates. In fish, the biological function of PTX1 is essentially unknown. In this study, we examined the expression and function of a PTX homologue (CsPTX1) from the tongue sole, Cynoglossus semilaevis. CsPTX1 contains 223 amino acids and shares 49.3%-38.8% overall sequence identity with other known fish pentraxins. CsPTX1 is expressed in multiple tissues and is upregulated by bacterial and viral infection. CsPTX1 contains a pentraxin domain, which is known to bind extracellular antigens, and recombinant CsPTX1 (rCsPTX1) bound a wide range of Gram-positive and Gram-negative bacteria. rCsPTX1 also agglutinated all the bacteria tested in a Ca(2+)-dependent manner and the agglutinating capacity of rCsPTX1 was abolished in the absence of calcium. As well as its ability to agglutinate bacterial cells, rCsPTX1 displayed apparent bacteriostatic activity against Pseudomonas fluorescens in vitro by influencing the permeability of the microbial envelope. When introduced in vivo, rCsPTX1 enhanced the host's resistance to bacterial infection. These results indicate that CsPTX1 is a classic pattern recognition molecule that defends C. semilaevis against bacterial infection.

Pseudomonas fluorescens Filamentous Hemagglutinin, an Iron-Regulated Protein, Is an Important Virulence Factor that Modulates Bacterial Pathogenicity

Pseudomonas fluorescens is a common bacterial pathogen to a wide range of aquaculture animals including various species of fish. In this study, we employed proteomic analysis and identified filamentous hemagglutinin (FHA) as an iron-responsive protein secreted by TSS, a pathogenic P. fluorescens isolate. In vitro study showed that compared to the wild type, the fha mutant TSSfha (i) exhibited a largely similar vegetative growth profile but significantly retarded in the ability of biofilm growth and producing extracellular matrix, (ii) displayed no apparent flagella and motility, (iii) was defective in the attachment to host cells and unable to form self-aggregation, (iv) displayed markedly reduced capacity of hemagglutination and surviving in host serum. In vivo infection analysis revealed that TSSfha was significantly attenuated in the ability of dissemination in fish tissues and inducing host mortality, and that antibody blocking of the natural FHA produced by the wild type TSS impaired the infectivity of the pathogen. Furthermore, when introduced into turbot as a subunit vaccine, recombinant FHA elicited a significant protection against lethal TSS challenge. Taken together, these results indicate for the first time that P. fluorescens FHA is a key virulence factor essential to multiple biological processes associated with pathogenicity.

Comparative analysis of the expression patterns of eight suppressors of cytokine signaling in tongue sole, Cynoglossus semilaevis

Suppressor of cytokine signaling (SOCS) family members are inhibitors of cytokine signaling pathways and key regulators of immunological homeostasis. They have been extensively studied in mammalian models, but systematic analyses of SOCS in fish are limited. In the current study, a total of eight SOCS genes from tongue sole (Cynoglossus semilaevis) were characterized. All eight CsSOCS exhibit conserved structures of SOCS and were phylogenetically grouped together with the respective SCOS members known in mammalian and teleost species. Under normal physiological conditions, the expressions of the eight CsSOCS genes were detected at varied levels in nine major tissues, with most CsSOCS highly expressed in kidney. Following challenge with intracellular and extracellular bacterial pathogens, the majority of CsSOCS genes exhibited distinctly different expression profiles in a time-, tissue-, and pathogen-dependent manner. In general, intracellular pathogen caused wider and higher levels of CsSOCS expressions than extracellular pathogen. These results suggest that different members of SOCSs in teleost may play different roles in the infection processes of different bacterial pathogens.

A Teleost Bactericidal Permeability-Increasing Protein Kills Gram-Negative Bacteria, Modulates Innate Immune Response, and Enhances Resistance against Bacterial and Viral Infection

Bactericidal/permeability-increasing protein (BPI) is an important factor of innate immunity that in mammals is known to take part in the clearance of invading Gram-negative bacteria. In teleost, the function of BPI is unknown. In the present work, we studied the function of tongue sole (Cynoglossus semilaevis) BPI, CsBPI. We found that CsBPI was produced extracellularly by peripheral blood leukocytes (PBL). Recombinant CsBPI (rCsBPI) was able to bind to a number of Gram-negative bacteria but not Gram-positive bacteria. Binding to bacteria led to bacterial death through membrane permeabilization and structural destruction, and the bound bacteria were more readily taken up by PBL. In vivo, rCsBPI augmented the expression of a wide arrange of genes involved in antibacterial and antiviral immunity. Furthermore, rCsBPI enhanced the resistance of tongue sole against bacterial as well as viral infection. These results indicate for the first time that a teleost BPI possesses immunoregulatory effect and plays a significant role in antibacterial and antiviral defense.

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.