Characterization and expression analysis of Toll-like receptor 2 gene in large yellow croaker, Larimichthys crocea

A soluble TLR5 is involved in the flagellin-MyD88-mediated immune response via regulation rather than activation in large yellow croaker (Larimichthys crocea)

Toll-like receptor 5 (TLR5) plays a crucial role in the immune response through recognizing bacterial flagellin. Some teleosts possess two forms of TLR5, including a canonical membrane TLR5 (TLR5M) ortholog and a piscine soluble TLR5 (TLR5S). In this report, the full-length cDNA sequences of Larimichthys crocea TLR5M (LcTLR5M) and TLR5S (LcTLR5S) were identified. The predicted 885-aa-LcTLR5M protein contained a 20-aa signal peptide, followed by 12 leucine-rich repeats (LRRs), a transmembrane (TM) region, and a cytoplasmic Toll/Interleukin-1 receptor homology (TIR) domain while the predicted 642-aa-LcTLR5S only contained 13 LRRs. The LcTLR5M transcripts were detected in most tissues examined, with the highest expression in heart and the lowest in stomach. The expression of LcTLR5S was high in liver whereas low in other examined tissues. Both LcTLR5M and LcTLR5S transcripts could be induced by immune challenge. Subcellular localization revealed that LcTLR5M existed on the cell membrane while LcTLR5S expressed in the cytoplasm. Furthermore, to investigate the role of LcTLR5S in downstream signaling transduction, a LcTLR5S-TIR chimera was constructed by fusing the ORF of LcTLR5S with TM and TIR domains from LcTLR5M. A dual-luciferase reporter assay revealed that the TIR domain is essential in the flagellin induced MyD88-mediated-TNFα activation but not in -NF-κB activation. However, the flagellin-LcTLR5M-MyD88-mediated NF-κB and TNFα activation was largely suppressed by LcTLR5S. These findings suggested that the flagellin-LcTLR5M/LcTLR5S mediated immune activation was MyD88-dependent, and the role of the TIR-domain might differ between NF-κB and TNFα signaling transduction.

Single-strain probiotics enhance growth, anti-pathogen immunity, and resistance to Nocardia seriolae in grey mullet (Mugil cephalus) via gut microbiota modulation

Grey mullet (Mugil cephalus) aquaculture is economically vital due to the high value of its roe. However, it faces significant risks from disease outbreaks, particularly from Nocardia seriolae. Current reliance on antibiotics has drawbacks, highlighting the potential of probiotics as a promising alternative. Despite this, no studies have focused on the effects and mechanisms of probiotics in disease prevention and treatment in grey mullet. This study, therefore, investigates the efficacy of probiotics in enhancing disease resistance and promoting growth in grey mullet. Three strains of probiotics, Lacticaseibacillus rhamnosus FS3051, Limosilactobacillus reuteri FS3052, and Bacillus subtilis natto NTU-18, were selected to evaluate their anti-N. seriolae activity and hydrolytic enzyme secretion in vitro. Then, 144 grey mullet were randomly divided into four groups: control, L. rhamnosus FS3051, L. reuteri FS3052, and B. subtilis natto NTU-18. After being fed the corresponding diet for 28 days, fish were measured for immune gene expression and short-term growth followed by challenge of N. seriolae. Survival rates were recorded for 35 days post challenge. Additionally, the gut microbiota of the control and probiotic groups with effects on both growth and protection against N. seriolae were analyzed to investigate the potential role of gut microbiota. Results demonstrated that L. rhamnosus FS3051 and L. reuteri FS3052 inhibited N. seriolae, while B. subtilis natto NTU-18 did not inhibited N. seriolae. Probiotics also had the ability to secrete hydrolytic enzymes. Probiotic-fed grey mullet showed significant improvements in weight gain ratio, feed efficiency, and specific growth rate, particularly in the B. subtilis natto NTU-18 group. Immune gene expression was enhanced by probiotics, especially L. rhamnosus, FS3051, which induced IL-8, IL-1β, TNF-α, IFN-γ, and MHCI. Survival rates post-N. seriolae challenge improved significantly for L. rhamnosus FS3051-fed fish. L. rhamnosus FS3051 also altered the gut microbiota, enriching beneficial genera like Lactobacillus, which correlated positively with immune responses and growth, while reducing Mycoplasma and Rhodobacter, which were negatively correlated with immune responses. This study underscores the potential of probiotics in enhancing disease resistance and growth via regulating gut microbiota in grey mullet.

Characterization and expression profiles of toll-like receptor genes (TLR2 and TLR5) in immune tissues of hybrid yellow catfish under bacterial infection

The yellow catfish (Pelteobagrus fulvidraco) is one of the most economically important freshwater species in Asia. However, pathogenic bacterial infections often cause high rates of mortality and economic losses in practical aquaculture. Previous studies in mammals have shown that Toll-like receptor 2 (TLR2) and Toll-like receptor 5 (TLR5) are involved in the recognition of cell wall components such as lipopolysaccharides and flagella of various bacteria, thereby acting as key regulators in the innate immunity response. However, TLR2 and TLR5 in yellow catfish have not been characterized. In the present study, TLR2 and TLR5 were examined through comparative genomic approaches. The gene structure, collinearity, protein spatial structure, and phylogenetic relationships were compared with those in multiple representative vertebrates. Meanwhile, quantitative real-time PCR was conducted to explore transcriptional changes in TLR2 and TLR5 in immune tissues after infection with exogenous A. hydrophila and E. tarda. The results demonstrated the presence of TLR2 and TLR5 in yellow catfish. However, a systematic analysis showed that TLR2 was not associated with the arrangement of diverse neighboring genes. The expression of hybrid yellow catfish TLR2 transcripts in multiple tissues (including liver, spleen, kidney, and intestine) was significantly up-regulated after infection with A. hydrophila and E. tarda, suggesting that hybrid yellow catfish TLR2 and TLR5 may participate in the immune process. Taken together, the results indicate that TLR2 and TLR5 are conserved in terms of evolution and possess significant antibacterial activity as well as regulatory properties in immune-related tissues and thus play key roles in host defense against pathogen invasion.

Dietary Supplementation with Eucommia ulmoides Leaf Extract Improved the Intestinal Antioxidant Capacity, Immune Response, and Disease Resistance against Streptococcus agalactiae in Genetically Improved Farmed Tilapia (GIFT; Oreochromis niloticus)

A 7-week rearing trial was designed to investigate the effects of Eucommia ulmoides leaf extract (ELE) on growth performance, body composition, antioxidant capacity, immune response, and disease susceptibility of diet-fed GIFT. The results showed that dietary ELE did not affect growth performance or whole-body composition (p > 0.05). Compared with the control group, plasma ALB contents increased in the 0.06% dietary ELE group (p < 0.05), and plasma ALT and AST activities decreased in the 0.08% dietary ELE group (p < 0.05). In terms of antioxidants, compared with GIFT fed the control diet, 0.06% dietary ELE upregulated the mRNA expression levels of Nrf2 pathway-related antioxidant genes, including CAT and SOD (p < 0.05), and 0.06% and 0.08% dietary ELE upregulated the mRNA levels of Hsp70 (p < 0.05). In terms of immunity, 0.06% dietary ELE suppressed intestinal TLR2, MyD88, and NF-κB mRNA levels (p < 0.05). Moreover, the mRNA levels of the anti-inflammatory cytokines TGF-β and IL-10 were upregulated by supplementation with 0.04% and 0.06% dietary ELE (p < 0.05). In terms of apoptosis, 0.06% and 0.08% ELE significantly downregulated the expression levels of FADD mRNA (p < 0.05). Finally, the challenge experiment with S. agalactiae showed that 0.06% dietary ELE could inhibit bacterial infection, and significantly improve the survival rate of GIFT (p < 0.05). This study demonstrated that the supplementation of 0.04−0.06% ELE in diet could promote intestinal antioxidant capacity, enhance the immune response and ultimately improve the disease resistance of GIFT against Streptococcus agalactiae.

Expressions of Toll Like Receptor (TLR) Genes in Paralichthys olivaceus After Induced by Different Extracts of Edwardsiella tarda

Toll like receptors (TLRs) are the main innate immune 'pattern recognition receptors' of animals, which play a central role in host cell recognition and responses to invasive pathogens, particularly common structures of microbial pathogens. In this study, the gene expression profiles of TLRs in the spleen, head kidney, gill, small intestine, liver, muscle, and heart of healthy Paralichthys olivaceus were detected by real-time quantitative PCR (qPCR). The TLR family members were widely expressed in different tissues with different basic expression profiles. The highest expressions of TLR1, 5m, 7, 8, 9, 14, and 21 were found in the spleen; the highest expressions of TLR3 and TLR21 were found in the gill; the highest expressions of TLR2 and 5s were found in the small intestine. The second highest expressions of TLR3, 7, and 8 were found in small intestine. The gene expression profiles of TLRs stimulated with Edwardsiella tarda DNA, RNA, and lipopolysaccharide (LPS) were also detected in spleen, head kidney and gill. TLR9 and TLR21 were sensitive to E. tarda DNA; TLR 8 and TLR21 were sensitive to E. tarda RNA; and TLR1 and TLR14 were sensitive to E. tarda LPS. The expressions of the other TLR genes showed no significant changes. The results imply that the expressions of these TLR genes in P. olivaceus are differently regulated in the whole body and play important roles in the immune response against E. tarda infection.

Enhancement of growth, antioxidative status, nonspecific immunity, and disease resistance in gibel carp (Carassius auratus) in response to dietary Flos populi extract

This study investigated the effects of dietary Flos populi extract (FPE) on the growth, antioxidation capability, innate immune response, and disease resistance in gibel carp. A total of 480 fish were fed with five different diets containing 0, 0.5, 1.0, 1.5, or 2.0 g kg^-1 FPE (designated as control, D0.5, D1.0, D1.5, or D2.0 groups) for 45 days. The fish were challenged with A. hydrophila after the feeding trial. Compared with the control, the feed efficiency (FE), weight gain (WG), final body weight (FBW), and specific growth rate (SGR) were significantly improved in groups D1.0 and D1.5. Dietary FPE significantly increased serum superoxide dismutase (SOD), glutathione peroxidase (GPx), and catalase (CAT) activities, as well as glutathione (GSH) content. The contents of protein carbonyl (PCC) and malondialdehyde (MDA) in serum decreased significantly. Additionally, FPE supplementation in diets resulted in significant improvement in serum lysozyme (LZM) and myeloperoxidase (MPO) activities, as well as immunoglobulin M (IgM) and complement 3 (C3) concentrations. The hepatic antioxidant enzymes (CAT and SOD) activities increased, whereas content of MDA decreased in fish treated with dietary FPE than those of control both pre- and post-challenged. After 12 h-challenge, an obvious downregulation of hepatic Kelch-like-ECH-associated protein 1 (Keap1), splenic tumor necrosis factor-α (TNF-α), interleukin (IL)-8, IL-1β, and toll-like receptor 2 (TLR2) mRNA levels was observed in fish treated with dietary FPE, whereas hepatic Nrf2 transcription level was upregulated compared to the control. Furthermore, compared to group D0.5, higher relative percent survival (RPS) was observed in gibel carp fed dietary 1.0-2.0 g/kg FPE. Our results reveal that FPE supplemented diet has a stimulatory effect on antioxidant capacity and nonspecific immune response, along with improved growth performance and enhanced resistance against A. hydrophila infection in juvenile gibel carp.

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.

Protective Efficacy of Novel Oral Biofilm Vaccines against Lactococcus garvieae Infection in Mullet, Mugil cephalus

Lactococcus garvieae (L. garvieae) is an important pathogen that causes enormous economic losses in both marine and freshwater aquaculture. At present, antibiotics are the only option for farmers to reduce the losses caused by L. garvieae. However, the usage of antibiotics leads to environmental pollution and the production of drug-resistant strains of bacteria. Therefore, vaccination is preferred as an alternative method to prevent infectious diseases. In this study, we describe an effective approach to the production of an oral biofilm vaccine, using bacteria grown on chitosan particles to form biofilms, and thus providing an inactive pathogen that enhances the immune response in fish. We observed the formation of a biofilm on chitosan particles and administered the novel oral biofilm vaccine to fish. We analyzed the immune responses, including antibody production, phagocytic ability, albumin/globulin ratio and immune-related genes, of vaccinated and control groups of black mullet. Our results show that the phagocytic ability of the biofilm vaccine group was 84%, which is significantly higher than that of the control group, and the antibody production in this group was significantly higher compared with the other group. The mRNA expression levels of immune-related genes (TLR2, IL-1β, TNF-α) were significantly upregulated in the spleen after vaccination. In challenge experiments, the relative percent survival (RPS) was 77% in the biofilm vaccine group, 18% in the whole-cell vaccine group, and 0% in the chitosan particle group at 32 days post-vaccination. In addition, we also found that the relative percent survival (RPS) at 1 day post-vaccination was 74% in the biofilm vaccine group, 42% in the whole-cell vaccine group, and 26% in the chitosan particle group. In both long-term and short-term challenge experiments, the viability of the biofilm vaccine group was significantly higher than that of the whole-cell, chitosan particle and PBS groups. We conclude that based on its protective effect, the L. garvieae biofilm vaccine is better than the whole-cell vaccine when challenged several weeks after vaccination. In addition, the biofilm vaccine also has a greater protective effect than the whole-cell vaccine when challenged immediately after vaccination. Therefore, the biofilm vaccine might represent a novel method for the prevention and treatment of L. garvieae infection.

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.

Elucidating the Efficacy of Vaccination against Vibriosis in Lates calcarifer Using Two Recombinant Protein Vaccines Containing the Outer Membrane Protein K (r-OmpK) of Vibrio alginolyticus and the DNA Chaperone J (r-DnaJ) of Vibrio harveyi

Recombinant cell vaccines expressing the OmpK and DnaJ of Vibrio were developed and subsequently, a vaccination efficacy trial was carried out on juvenile seabass (~5 cm; ~20 g). The fish were divided into 5 groups of 50 fish per group, kept in triplicate. Groups 1 and 2 were injected with 10⁷ CFU/mL of the inactivated recombinant cells vaccines, the pET-32/LIC-OmpK and pET-32/LIC-DnaJ, respectively. Group 3 was similarly injected with 10⁷ CFU/mL of inactivated E. coli BL21 (DE3), Group 4 with 10⁷ CFU/mL of formalin killed whole cells V. harveyi, and Group 5 with PBS solution. Serum, mucus, and gut lavage were used to determine the antibody levels before all fish were challenged with V. harveyi, V. alginolyticus, and V. parahemolyticus, respectively on day 15 post-vaccination. There was significant increase in the serum and gut lavage antibody titers in the juvenile seabass vaccinated with r-OmpK vaccine. In addition, there was an up-regulation for TLR2, MyD88, and MHCI genes in the kidney and intestinal tissues of r-OmpK vaccinated fish. At the same time, r-OmpK triggered higher expression level of interleukin IL-10, IL-8, IL-1ß in the spleen, intestine, and kidney compared to r-DnaJ. Overall, r-OmpK and r-DnaJ triggered protection by curbing inflammation and strengthening the adaptive immune response. Vaccinated fish also demonstrated strong cross protection against heterologous of Vibrio isolates, the V. harveyi, V. alginolyticus, and V. parahaemolyticus. The fish vaccinated with r-OmpK protein were completely protected with a relative per cent of survival (RPS) of 90 percent against V. harveyi and 100 percent against V. alginolyticus and V. parahaemolyticus. A semi-quantitative PCR detection of Vibrio spp. from the seawater containing the seabass also revealed that vaccination resulted in reduction of pathogen shedding. In conclusion, our results suggest r-OmpK as a candidate vaccine molecule against multiple Vibrio strain to prevent vibriosis in marine fish.

Polyunsaturated Fatty Acids Influence LPS-Induced Inflammation of Fish Macrophages Through Differential Modulation of Pathogen Recognition and p38 MAPK/NF-κB Signaling

Polyunsaturated fatty acids (PUFAs) not only serve as essential nutrients but also function as modulators of the immune response in marine fish. However, their immunomodulatory mechanism is poorly understood given that the underlying regulation of the innate immune response in fish has not been fully elucidated. Hence, study of the innate immunity of fish could help elucidate the mechanism by which PUFAs affect the fish immune response. Here, we used combined transcriptome analysis and in vitro experimentation to study the mechanism of LPS-induced inflammation. Transcriptome profiling indicated that LPS elicited strong pro-inflammatory responses featuring high expression levels of pathogen recognition receptors (PRRs) and cytokines along with the activation of NF-κB and MAPK signaling pathways. The transcription factor p65 alone could increase the transcription of IL1β by binding to the promoter of IL1β, and this promoting effect disappeared after mutation or deletion of its binding sites. We then examined the effects of PUFAs on the levels of gene expression and the abundance of proteins of critical kinases associated with LPS-induced inflammation. We found that LA exerts pro-inflammatory response while ALA, EPA, and DHA induced anti-inflammatory effects by modulating the expression of PRRs, phosphorylation of IKK and p38, and the nuclear translocation of p65. Overall, this study advances our understanding of the regulatory mechanisms by which PUFAs regulate LPS-induced inflammation in a non-model fish species.

Molecular and immunological characterizations of interleukin-11 in large yellow croaker (Larimichthys crocea)

Interleukin (IL)-11 is a multifunctional cytokine that exerts a series of important immunomodulatory effects and exists in many tissues and cells. A 1106-bp nucleotide sequence representing the complete cDNA of IL-11 was obtained from large yellow croaker (Larimichthys crocea), containing an open reading frame (ORF) of 603 bp encoding for 200 amino acids (aa). The predicted LcIL-11 protein included a 12aa signal peptide and a conserved IL-11 domain. The polypeptide sequence identities between LcIL-11 and its counterparts in mammals and other fish are from 84% to 92% with known fish IL-11a and 22%-27% with fish IL-11b. LcIL-11 mRNA existed in most tissues with the most predominant expression in the gill. After immune challenge, the expression levels of LcIL-11 were induced largely in vivo and in vitro, with the peak-value of 32 times as much as the control in the liver at 24 h after Vibrio parahaemolyticus injection (p < 0.05) and the greatest value of 13.9 times as much as the control in LCK cells at 12 h after poly I:C stimulation (p < 0.05). Furthermore, the overexpression vector pcDNA3.1-LcIL-11 was constructed and transfected to LCK cells. Our results showed that the transcriptional expression levels of tumor necrosis factor (TNF)-α and myxovirus resistant protein (Mx) significantly up-regulated in LCK cells after LcIL-11 overexpression (p < 0.05). However, no significant changes of IL-1β, janus kinase (JAK)2 and signal transducers and activators of transcription (STAT)5 was detected. Our finding indicated that LcIL-11 might enhance TNF-α and antiviral protein Mx expression in large yellow croaker.

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.

Dietary yeast culture modulates immune response related to TLR2-MyD88-NF-kβ signaling pathway, antioxidant capability and disease resistance against Aeromonas hydrophila for Ussuri catfish (Pseudobagrus ussuriensis)

The aim of the present study was to investigate effects of dietary yeast culture on immune response related to TLR2-MyD88-NF-kβ signaling pathway, antioxidant capability and disease resistance against Aeromonas hydrophila for Ussuri catfish (Pseudobagrus ussuriensis). A total of 240 Ussuri catfish (mean weight of 7.39 ± 0.32 g) were randomly distributed into four groups that fed diets containing 0 (Y0), 10 (Y1), 20 (Y2) and 30 (Y3) g kg^-1 yeast culture for 8 weeks. The results indicated that dietary 10 g kg^-1 yeast culture supplementation significantly down-regulated mRNA levels of TLR2, MyD88, NF-kβ p65, IL-1β and IL-8 in the liver tissue compared with the control group (P < 0.05). Simultaneously, serum lysozyme (LZM) activity, respiratory burst activity (RBA) of phagocytes, plasma alkaline phosphatase (AKP) activity and immunoglobulin M (IgM) content were significantly improved in fish fed Y1 diet (P < 0.05). Fish fed Y1 diet had significantly higher serum alternative complement pathway activity (ACH50) and plasma complement 3 (C3) content than the Y3 group (P < 0.05). However, no significant differences were observed in plasma acid phosphatase (ACP) activity and complement 4 (C4) content among the groups (P > 0.05). Fish cumulative mortality rate (CMR) in the Y1 and Y2 groups were significantly lower than that in Y0 and Y3 groups (P < 0.05), and the lowest CMR was observed in the Y1 group after challenge by A. hydrophila. The highest hepatic superoxide dismutase and glutathione peroxidase activities, total antioxidant capacity and the lowest malondialdehyde content were found in Y1 group, but no significant difference was found in hepatic catalase activity among the groups (P > 0.05). These results demonstrate that dietary 10 g kg^-1 yeast culture could effectively improve the immunity, antioxidant capability and disease resistance against A. hydrophila for Ussuri catfish and could down-regulate the mRNA expression levels of pro-inflammatory cytokines modulated by TLR2-MyD88-NF-kβ signaling pathway.

Genomic structure and molecular characterization of Toll-like receptors 1 and 2 from golden pompano Trachinotus ovatus (Linnaeus, 1758) and their expression response to three types of pathogen-associated molecular patterns

Toll-like receptors (TLRs) play an essential role in the immune response. Here two Toll-like receptors from golden pompano (Trachinotus ovatus), ToTLR1 and ToTLR2, were characterized, the full-length cDNAs were 3126 bp and 7430 bp, and the deduced proteins consisted of 801 and 825 amino acids, respectively. ToTLR1 and ToTLR2 both contained the typical TLR domain architecture including signal peptide, leucine rich repeat (LRR), C-terminal LRR domain at the extracellular region and Toll/interleukin (IL)-1 receptor (TIR) domain in the cytoplasmic region. ToTLR1 only had one intron and two exons, but ToTLR2 consisted of twelve introns and thirteen exons. The promoters of ToTLR1 and ToTLR2 contained several putative transcription factor binding sites. Phylogenetic analysis showed that ToTLR1 and ToTLR2 were clustered into the clade of TLR1 and TLR2, respectively. Tissues distribution analysis indicated that both genes were ubiquitously expressed in all examined tissues, with higher expression levels observed in blood, head-kidney and spleen. After injection with poly inosinic:cytidylic [poly(I:C)], flagellin and lipopolysaccharides (LPS), ToTLR1 and ToTLR2 mRNAs were significantly up-regulated in the immune related tissues, indicating the possible the role of ToTLR1 and ToTLR2 in defense against pathogenic microbes. Further research should be carried out to identify ligands of fish TLR1 and TLR2 in order to understand the function of these receptors.

Toll-Like Receptors, Associated Biological Roles, and Signaling Networks in Non-Mammals

The innate immune system is the first line of defense against pathogens, which is initiated by the recognition of pathogen-associated molecular patterns (PAMPs) and endogenous damage-associated molecular patterns (DAMPs) by pattern recognition receptors (PRRs). Among all the PRRs identified, the toll-like receptors (TLRs) are the most ancient class, with the most extensive spectrum of pathogen recognition. Since the first discovery of Toll in Drosophila melanogaster, numerous TLRs have been identified across a wide range of invertebrate and vertebrate species. It seems that TLRs, the signaling pathways that they initiate, or related adaptor proteins are essentially conserved in a wide variety of organisms, from Porifera to mammals. Molecular structure analysis indicates that most TLR homologs share similar domain patterns and that some vital participants of TLR signaling co-evolved with TLRs themselves. However, functional specification and emergence of new signaling pathways, as well as adaptors, did occur during evolution. In addition, ambiguities and gaps in knowledge still exist regarding the TLR network, especially in lower organisms. Hence, a systematic review from the comparative angle regarding this tremendous signaling system and the scenario of evolutionary pattern across Animalia is needed. In the current review, we present overview and possible evolutionary patterns of TLRs in non-mammals, hoping that this will provide clues for further investigations in this field.

The interaction of TAK1 and TAB1 enhances LPS-induced cytokine release via modulating NF-κB activation (Larimichthys crocea)

Transforming growth factor-β-activating kinase 1 (TAK1) is triggered by foreign pathogenic infection and involves in proinflammatory response through the activation of nuclear factor-κB (NF-κB), which is specifically regulated by TAK1-binding protein 1 (TAB1). However, the expression and regulatory characterizations of TAK1 and TAB1 in fish immune response remain largely unknown. In the present study, the cDNA sequences of TAK1 (LcTAK1) and TAB1 (LcTAB1) were identified from large yellow croaker, Larimichthys crocea. The open reading frame (ORF) of LcTAK1 was 1725 bp in length, encoding 574 amino acids. The putative LcTAK1 protein contained a protein kinase domain and a C-terminal coiled-coil region. The ORF of LcTAB1 was 1518 bp encoding 505 amino acids. And a typical PP2Cc domain and a conserved sequence motif (PYVDFSQFYLLWGSDH) at C-terminal were identified in the predicted LcTAB1 protein. Multiple alignments showed that LcTAK1 shared 74.0-97.9% and LcTAB1 shared 37.4-95.8% sequence identities with TAK1 and TAB1 proteins from other species, respectively. Quantitative PCR analysis indicated that both LcTAK1 and LcTAB1 were broadly expressed in all examined tissues, with the most predominant expression in brain and the weakest expression in muscle, respectively. Subcellular localization revealed that both LcTAK1 and LcTAB1 expressed in the cytoplasm. In addition, LcTAK1 transcripts increased significantly in LCK cells after flagellin, LPS and poly I:C stimulation while LcTAB1 enhanced greatly after LPS and poly I:C challenge. Furthermore, the roles of them in NF-κB activation were investigated by overexpression of LcTAK1 and LcTAB1 in HEK293T cells. Our results revealed that NF-κB luciferase promoter expression could not be induced by overexpression of LcTAK1 or LcTAB1 alone, however, it could be induced by co-expression of LcTAK1 and LcTAB1 together. Moreover, the roles of LcTAK1 and LcTAB1 in immune response analysis showed that NF-κB activation enhanced significantly in co-overexpressed HEK293T cells following LPS and poly I:C stimulation. However, the expression levels of tumor necrosis factor (TNF)-α, Interleukin-6 (IL-6) and IL-8 were induced only after LPS challenge (p < .05). These findings suggested that the TAK1-TAB1 complex of large yellow croaker might play an important role in pro-inflammatory cytokines and chemokine release after LPS stimulation via inducing NF-κB activation.

Identification, structural characterization, and expression analysis of toll-like receptors 2 and 3 from gibel carp (Carassius auratus gibelio)

Toll-like receptors (TLRs) are important components of innate immunity. TLRs recognize pathogen-associated molecular patterns (PAMPs) and initiate downstream signaling pathways in response. In present study, we report the identification of two TLRs from gibel carp (Carassius auratus gibelio), TLR2 and TLR3 (designated CagTLR2 and CagTLR3, respectively). We report on the genomic structures and mRNA expression patterns of CagTLR2 and CagTLR3. Five exons and four introns were identified from the genomic DNA sequence of CagTLR3 (4749 bp in total length); this genomic organization is similar to that of TLR3 in zebrafish and human. However, only one intron was identified from the CagTLR2 genomic locus (3166 bp in total length); this unique genomic organization of CagTLR2 is different from that of TLR2 in fish and humans. The cDNAs of CagTLR2 and CagTLR3 encoded 791 and 904 amino acid residues, respectively. CagTLR2 and CagTLR3 contained two distinct structural/functional motifs of the TLR family: a leucine-rich repeat (LRR) domain in the extracellular portion and a toll/interleukin-1 receptor (TIR) domain in the intracellular portion. The positions of critical amino acid residues involed in PAMP recognition and signaling pathway transduction in mammalian TLRs were conserved in CagTLR2 and CagTLR3. Phylogenetic analysis revealed a closer clustering of CagTLR2 and CagTLR3 with TLRs from freshwater fish than with marine fish species. In healthy gibel carp, transcripts of these genes were detected in all examined tissues, and high expression levels of CagTLR2 and CagTLR3 were observed in liver and brain, respectively. Following injection with CyHV-2, expression levels of CagTLR2 and CagTLR3 were significantly upregulated in the spleens of gibel carp after three days, and CagTLR3 transcript levels were rapidly increased in head kidney after 12 h. These results suggest that CagTLR2 and CagTLR3 are functionally involved in the induction of antiviral immune response.

Molecular characterization and expression analysis of three TLR genes in yellow catfish (Pelteobagrus fulvidraco): Responses to stimulation of Aeromonas hydrophila and TLR ligands

Toll-like receptors (TLRs) are one of the most extensively researched pattern recognition receptors (PRRs) and play an important role in the innate immune system. In this study, partial cDNA sequences of the Pf_TLR18 and Pf_TLR19 genes and complete cDNA sequence of the Pf_TLR21 gene were cloned from yellow catfish (Pelteobagrus fulvidraco). The open reading frames (ORFs) of the Pf_TLR18, Pf_TLR19 and Pf_TLR21 genes were 1956 bp, 2262 bp and 2949 bp in length, encoding 651, 753 and 982 amino acids, respectively. The Pf_TLR18 and Pf_TLR19 consist of leucine-rich repeats (LRRs), a transmembrane domain and a Toll/interleukin-I receptor domain, and the Pf_TLR21 only has LRRs and TIR domain. Homologous identity revealed that the Pf_TLR18, Pf_TLR19 and Pf_TLR21 genes have high nucleotide and protein sequence similarity with channel catfish, especially the TIR domains that exhibited the greatest conservation compared to channel catfish. Ontogenetic expression analyses indicated that the mRNA expressions of the Pf_TLR18, Pf_TLR19 and Pf_TLR21 genes could be detected from fertilized eggs to 30 day post-hatching and they exhibited different variation trends after hatching. The three TLR genes were expressed in various tissues, but they were mostly highly expressed in the spleen. The mRNA expression levels of the three genes were up-regulated in the spleen, head kidney, trunk kidney, liver and blood after challenge of killed Aeromonas hydrophila. In addition, the expressions of the three TLR genes were induced to up-regulate in isolated peripheral blood lymphocytes of yellow catfish after stimulation with lipopolysaccharides (LPS), peptidoglycan (PGN) and polyinosinic-polycytidylic acid (Poly I:C). Our findings indicate that the three TLR genes may play a potential role in the host defense against pathogenic microbes. These results will provide valuable information to better understand the function of TLR genes in the innate immune system of yellow catfish.

Cloning and expression study of a Toll-like receptor 2 (tlr2) gene from turbot, Scophthalmus maximus

Toll-like receptor 2 (TLR2) in mammals is a member of the ancient Toll-like family of receptors that predominantly recognizes conserved components of Gram-positive bacteria. In the present study, a tlr2 gene and its 5'-flanking sequence were cloned from turbot, Scophthalmus maximus, its responsive expressions to various immunostimulants were subsequently studied in vivo. The turbot (sm)tlr2 gene spans over 9.0 kb with a structure of 12 exon-11 intron and encodes 816 amino acids. The deduced protein shows the highest sequence identity (76.1%) to Japanese flounder Tlr2 and possesses a signal peptide sequence, a leucine-rich repeat (LRR) domain composed of 19 LRR motifs, a transmembrane region and a Toll/interleukin-1 receptor (TIR) domain. Phylogenetic analysis grouped it with other neoteleostei Tlr2as. A number of transcription factor binding sites known to be important for the basal transcriptional activity of TLR3 and response of TLR2 to lipopolysaccharide (LPS) signalling in mammals were predicted in the 5'-flanking sequence of smtlr2. Quantitative real-time PCR (qPCR) analysis demonstrated the constitutive expression of smtlr2 mRNA in all twelve examined tissues with higher levels in the lymphomyeloid-rich tissues and liver. Further, smtlr2 expression was up-regulated following stimulation with LPS, peptidoglycan (PGN) or polyinosinic: polycytidylic acid [poly(I:C)] in the gills, head kidney, spleen and muscle. Finally, for all three immunostimulants, a two-wave induced smtlr2 expression was observed in the head kidney and spleen in a 7-day time course and the strongest inducibility in the head kidney. These findings suggest a possible role of Smtlr2 in the immune responses to the infections of a broad range of pathogens that include Gram-positive and Gram-negative bacteria and RNA virus.

Molecular characterization and expression analysis of toll-like receptor 2 in response to bacteria in silvery pomfret intestinal epithelial cells

Toll-like receptor 2 (TLR2) has been shown to play a crucial role in the host defense of pathogenic microbes in innate immunity. In this study, the full-length cDNA of TLR2 in silvery pomfret (Pampus argenteus) was cloned by homology cloning and the rapid amplification of cDNA ends (RACE) technique. The complete cDNA sequence of TLR2 was 2932 bp, containing an open reading frame (ORF) of 2469 bp encoding 822 amino acids. A multiple alignment analysis of the silvery pomfret TLR2 protein-coding sequence with other known TLR2 sequences from Oplegnathus fasciatus, Epinephelus coioides, Larimichthys crocea, Miichthys miiuy, Oreochromis niloticus, Paralichthys olivaceus, Trematomus bernacchii, Sparus aurata, and Chionodraco hamatus exhibited a high degree of homology of 78.83%, 75.91%, 74.21%, 74.94%, 71.95%, 72.57%, 73.68%, 75%, and 72.52 respectively, between these fish. Analysis of the TLR2 domain structures indicated that TLR2 from the silvery pomfret has the typical structural features of proteins that belong to the TLR family, including one transmembrane domain, eleven leucine-rich repeats (LRRs), and one Toll/IL-1 receptor homology domain (TIR). In vitro immunostimulation experiments revealed that Lactobacillus plantarum and Clostridium butyricum induce high levels of TLR2 mRNA and protein expression, but they induce only moderate levels of IL-8 and TNF-α production compared to Vibrio anguillarum. This suggests that TLR2 might play a vital role in the L. plantarum and C. butyricum-mediated immune response. In contrast, V. anguillarum significantly increased the secretion of IL-8 and TNF-α and induced cell apoptosis and necrosis. Due to the lower expression of TLR2 and higher levels of IL-8 and TNF-α induced by V. anguillarum, we hypothesize that a V. anguillarum infection is independent of the TLR2-induced production of pro-inflammatory cytokines. These results indicate that TLR2 may be involved in molecular interactions between the host and commensal bacteria, that exist in the silvery pomfret intestinal tract.

Molecular and functional characterization of Toll-like receptor (Tlr)1 and Tlr2 in common carp (Cyprinus carpio)

Toll-like receptors (TLRs) are fundamental components of innate immunity that play significant roles in the defence against pathogen invasion. In this study, we present the molecular characterization of the full-length coding sequence of tlr1, tlr2a and tlr2b from common carp (Cyprinus carpio). Each is encoded within a single exon and contains a conserved number of leucine-rich repeats, a transmembrane region and an intracellular TIR domain for signalling. Indeed, sequence, phylogenetic and synteny analysis of carp tlr1, tlr2a and tlr2b support that these genes are orthologues of mammalian TLR1 and TLR2. The tlr genes are expressed in various immune organs and cell types. Furthermore, the carp sequences exhibited a good three-dimensional fit with the heterodimer structure of human TLR1-TLR2, including the potential to bind to the ligand Pam3CSK4. This supports the possible formation of carp Tlr1-Tlr2 heterodimers. However, we were unable to demonstrate Tlr1/Tlr2-mediated ligand binding in transfected cell lines through NF-κB activation, despite showing the expression and co-localization of Tlr1 and Tlr2. We discuss possible limitations when studying ligand-specific activation of NF-κB after expression of Tlr1 and/or Tlr2 in human but also fish cell lines and we propose alternative future strategies for studying ligand-binding properties of fish Tlrs.

Identification and characterization of three TLR1 subfamily members from the orange-spotted grouper, Epinephelus coioides

Toll-like receptors (TLRs), which play important roles in host defense against pathogen infection, are the most intensively studied pattern recognition receptors (PRRs). In this study, we identified three novel TLR1 subfamily members, including TLR1 (EcTLR1b), TLR2 (EcTLR2b) and TLR14 (EcTLR14), from the orange-spotted grouper (Epinephelus coioides). EcTLR1b and EcTLR2b displayed low sequence identity with the previously reported grouper TLR1 (EcTLR1a) and TLR2 (EcTLR2a), respectively. The open reading frames (ORFs) of EcTLR1b, EcTLR2b and EcTLR14 contain 2484 bp, 2394 bp and 2640 bp, which encode the corresponding 827 amino acids (aa), 797 aa and 879 aa, respectively. All three TLRs have leucine-rich repeat (LRR) domains (including an LRR-NT (except for EcTLR1b), several LRR motifs and an LRR-CT), a trans-membrane region and a Toll/interleukin-1 receptor (TIR) domain. The TIR domains of the three TLRs exhibited conserved boxes, namely box1, box2 and box3, and their 3D models were similar to those of human TLR1 or TLR2. Sequence alignment demonstrated that the TIR domains of the three TLRs shared higher sequence identity with those of other species than the full-length receptors. Phylogenetic analysis indicated that EcTLR1s and EcTLR2s are characterized by their differing evolutionary status, whereas EcTLR14 was found to be in the same group as other piscine TLR14/18s. The three TLRs were ubiquitously expressed in seven tested tissues of healthy groupers, although their expression profiles were different. Post Cryptocaryon irritans infection, TLR1s expression was up-regulated in the gills. The expression of TLR2b was mainly increased in the spleen, but decreased in the gills, which was similar to the expression pattern of TLR2a post C. irritans infection. Unlike EcTLR1b and EcTLR2b, however, the grouper TLR14 transcript was substantially induced in both tissues post challenge. These findings may be helpful in understanding the innate immune mechanism of host anti-parasite infection.

Characterization, expression analysis and localization pattern of toll-like receptor 1 (tlr1) and toll-like receptor 2 (tlr2) genes in grass carp Ctenopharyngodon idella

In this study, the toll-like receptor 1 (tlr1) and toll-like receptor 2 (tlr2) genes of grass carp Ctenopharyngodon idella were cloned and characterized. tlr1 and tlr2 were found to be highly expressed in immune system organs such as spleen, middle kidney and heart kidney. The expression level of tlr1 and tlr2 was found to be up-regulated at the later stage of viral challenge process. Moreover, subcellular localization indicated that Tlr1 and Tlr2 shared similar localization pattern and both of them may locate in the plasma membrane of transfected cells.

Molecular and immune response characterizations of IL-6 in large yellow croaker (Larimichthys crocea)

Interleukin-6 (IL-6) is a multifunctional inflammatory cytokine which exists in multiple tissues and cell lines. In the present study, the full-length cDNA and the genomic sequence of IL-6 (LcIL-6) were cloned from large yellow croaker, Larimichthys crocea. The full-length cDNA of LcIL-6 was 1066 base pairs (bp), containing an open reading frame (ORF) of 678 bp encoding for 225 amino acids, a 5' untranslated region (UTR) of 71 bp and a 3' UTR of 317 bp. The predicted LcIL-6 protein included a 24 amino acids (aa) signal peptide and a conserved IL-6 domain. However, the polypeptide sequence identities between LcIL-6 and its counterparts in mammals and other fish are from 12% to 45%. The genome sequence of LcIL-6 gene was composed of 2126 bp, including five exons and four introns. Phylogenetic analysis revealed that LcIL-6 showed a close relationship with the IL-6 from other bony fish. Quantitative real-time PCR (qRT-PCR) analysis revealed that LcIL-6 mRNA was expressed in most examined tissues, with the most predominant expression in stomach, followed by blood and very weak expression in other tissues. The expression levels of LcIL-6 after challenged with LPS, poly I:C and Vibrio parahaemolyticus were investigated in spleen, head-kidney and liver. LcIL-6 transcripts were induced significantly after immune challenge, with the peak-value of 33.5 times as much as the control in the head-kidney at 3 h after LPS injection (p < 0.05). Overexpression of LcIL-6 enhanced tumor necrosis factor (TNF)-α transcripts significantly (p < 0.05) in L. crocea kidney (LCK) cells. Additionally, recombinant LcIL-6 mature peptide was obtained in the supernatant of Escherichia coli BL21 (DE3). The purified recombinant LcIL-6 fusion protein was also demonstrated to improve the transcriptional expression levels of TNF-α significantly in LCK cells (p < 0.05). However, no significant changes of Mx (myxovirus resistant protein), IL-1β, janus kinase (JAK)2, signal transducers and activators of transcription (STAT)3 and STAT5 in LCK cells was detected after LcIL-6 overexpression or recombinant LcIL-6 protein stimulation. Our results indicated that LcIL-6 might be important in large yellow croaker immune response and improve the inflammatory response by through activation TNF-α expression.

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

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

Identification and characterization of a novel Toll-like receptor 2 homologue in the large yellow croaker Larimichthys crocea

Toll-like receptors (TLRs) are key components of innate immunity that play significant roles in immune defence against pathogen invasion. In the present study, we identified a novel TLR2 homologue (LycTLR2b) in large yellow croaker (Larimichthys crocea) that shared low sequence identity with the previously reported large yellow croaker TLR2 (tentatively named LycTLR2a). The full-length cDNA of LycTLR2b was 2926 nucleotides (nt) long and encoded a protein consisting of 797 amino acids (aa). The deduced LycTLR2b protein exhibited a typical TLR domain architecture including a signal peptide, seven leucine-rich repeats (LRRs) in the extracellular region, a transmembrane domain, and a Toll-Interleukin 1 receptor (TIR) domain in the cytoplasmic region. Phylogenetic analysis showed that both LycTLR2a and LycTLR2b fall into a major clade formed by all TLR2 sequences, and are divided into two distinct branches. Genomic organization revealed that the LycTLR2b gene lacks intron, which is similar to zebrafish and human TLR2 genes, whereas the LycTLR2a gene contains multiple introns, as found in damselfish TLR2a and Fugu TLR2 genes. Syntenic analysis suggested that the occurrence of LycTLR2a and LycTLR2b may result from a relatively recent genome duplication event. LycTLR2b mRNA was constitutively expressed in all tissues examined although at different levels. Following bacterial vaccine challenge, LycTLR2b expression levels were significantly up-regulated in both spleen and head kidney tissues. Taken together, these results indicated that two different TLR2 homologues, which may play roles in antibacterial immunity, exist in large yellow croaker.