Expression profiles of toll-like receptors in channel catfish (Ictalurus punctatus) after infection with Ichthyophthirius multifiliis

Assessment of dietary yeast-based additives for cultured catfish and tilapia health

Channel catfish (Ictalurus punctatus) and Nile tilapia (Oreochromis niloticus) are two aquaculture species of great importance. Intensive production is often hindered by poor growth performance and disease mortality. The aim of this study was to evaluate the potential of a commercial fermented yeast product, DVAQUA, on channel catfish and Nile tilapia growth performance metrics and disease resistance. Channel catfish and Nile tilapia were fed practical diets supplemented with 0%, 0.1% or 0.4% of DVAQUA over approximately 2-month feeding periods in recirculation aquaculture systems. To assess the potential of the postbiotic against common aquaculture pathogens, juvenile catfish were subsequently challenged by immersion with Edwardsiella ictaluri S97-773 or virulent Aeromonas hydrophila ML09-119. Nile tilapia juveniles were challenged by injection with Streptococcus iniae ARS-98-60. Serum lysozyme activity, blood chemistry and growth metrics were measured at the end of the feeding period, but no differences were observed across the different metrics, except for survival. For the pathogen challenges, there were no differences in endpoint mortality for channel catfish with either pathogen (p > .05). In contrast, Nile tilapia survivability to S. iniae infection increased proportionally to the inclusion of DVAQUA (p = .005). Changes to sera lysozyme activity were also noted in the tilapia trial, with a reduction of activity in the fish fed the 0.4% DVAQUA diet compared to the control diet (p = .031). Expression profiles of proinflammatory genes and antibodies were also found to be modulated in channel catfish fed the postbiotic, indicating some degree of protective response. These results suggest that this postbiotic may be beneficial in protecting Nile tilapia against S. iniae infection by influencing immune parameters and additional research is needed to evaluate the potential of this DVAQUA for improving catfish health and disease control.

Immune response of Rhinogobio ventralis to Ichthyophthirius multifiliis infection: Insights from histopathological and real-time gene expression analyses

Ichthyophthirius multifiliis is a parasite that poses a considerable threat to aquaculture and the ornamental fish industry, but with limited effective treatment options available. This study employed RT-qPCR to detect and analyze the expression changes of partial toll-like receptor (TLR) genes (TLR1 and TLR21), adapter protein and signal transduction molecule genes (MyD88, TRIF, NF-κB, IRAK4, and IRF3), and cytokines (IL-6, IL-8, IL-13, CXC-α and CXCR1), as well as complement C3, in the skin, gill, fin, liver, head kidney and spleen of Rhinogobio ventralis under different infection conditions. Additionally, tissue sections and scanning electron microscopy were utilized to observe the pathological changes in the gills and fins of R. ventralis after infection with I. multifiliis. The expression patterns of TLR-related DEGs (differentially expressed genes) in diseased wild fish were analyzed, revealing upregulation of TLR1, TLR21, MyD88, NF-κB, IRAK4, TRIF, IRF3, IL-6, IL-8, IL-13, CXC-α, CXCR1, and C3 genes in various tissues, indicating that these genes may be involved in the immune response of R. ventralis to I. multifiliis infection. To further analyze the gene expression of sampled from the field, an artificial infection model of R. ventralis was established under laboratory conditions, with additional sampling from the skin and fins. These genes continued to show varying degrees of upregulation, but the results were not entirely consistent with those from Wudongde samples, which may be due to the more complex environment in the wild or differences in the degree of I. multifiliis infection in wild fish. The infection of I. multifiliis caused severe damage to the gills and fins of R. ventralis, characterized by extensive secretions on the gill and fin surfaces, with the presence of attached I. multifiliis trophonts, including damage and loss of gill filaments, swollen gill lamellae, and deformed gill plates, as well as cell proliferation and necrosis of gill epithelial cells. This study sheds light on the role of the TLR signaling pathway in resisting I. multifiliis infection and its associated histopathological changes in R. ventralis, providing valuable insights for the prevention and treatment of I. multifiliis infection in R. ventralis.

TLR21 is involved in the NF-κB and IFN-β pathways in largemouth bass (Micropterus salmoides) and interacts with TRIF but not with the Myd88 adaptor

Toll-like receptors (TLRs) are pattern recognition receptors that trigger host immune responses against various pathogens by detecting evolutionarily conserved pathogen-associated molecular patterns (PAMPs). TLR21 is a member of the Toll-like receptor family, and emerging data suggest that it recognises unmethylated CpG DNA and is considered a functional homologue of mammalian TLR9. However, little is known regarding the role of TLR21 in the fish immune response. In the present study, we isolated the cDNA sequence of TLR21 from the largemouth bass (Micropterus salmoides) and termed it MsTLR21. The MsTLR21 gene contained an open reading frame (ORF) of 2931 bp and encodes a polypeptide of 976 amino acids. The predicted MsTLR21 protein has two conserved domains, a conserved leucine-rich repeats (LRR) domain and a C-terminal Toll-interleukin (IL) receptor (TIR) domain, similar to those of other fish and mammals. In healthy largemouth bass, the TLR21 transcript was broadly expressed in all the examined tissues, with the highest expression levels in the gills. After challenge with Nocardia seriolae and polyinosinic polycytidylic acid (Poly[I:C]), the expression of TLR21 mRNA was upregulated or downregulated in all tissues tested. Overexpression of TLR21 in 293T cells showed that it has a positive regulatory effect on nuclear factor-kappaB (NF-κB) and interferons-β (IFN-β) activity. Subcellular localisation analysis showed that TLR21 was expressed in the cytoplasm. We performed pull-down assays and determined that TLR21 did not interact with myeloid differentiation primary response gene 88 (Myd88); however, it interacted with TIR domain-containing adaptor inducing interferon-β (TRIF). Taken together, these findings suggest that MsTLR21 plays important roles in TLR/IL-1R signalling pathways and the immune response to pathogen invasion.

What can we learn about fish neutrophil and macrophage response to immune challenge from studies in zebrafish

Fish rely, to a high degree, on the innate immune system to protect them against the constant exposure to potential pathogenic invasion from the surrounding water during homeostasis and injury. Zebrafish larvae have emerged as an outstanding model organism for immunity. The cellular component of zebrafish innate immunity is similar to the mammalian innate immune system and has a high degree of sophistication due to the needs of living in an aquatic environment from early embryonic stages of life. Innate immune cells (leukocytes), including neutrophils and macrophages, have major roles in protecting zebrafish against pathogens, as well as being essential for proper wound healing and regeneration. Zebrafish larvae are visually transparent, with unprecedented in vivo microscopy opportunities that, in combination with transgenic immune reporter lines, have permitted visualisation of the functions of these cells when zebrafish are exposed to bacterial, viral and parasitic infections, as well as during injury and healing. Recent findings indicate that leukocytes are even more complex than previously anticipated and are essential for inflammation, infection control, and subsequent wound healing and regeneration.

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

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

Alterations of Plasma Biochemical and Immunological Parameters and Spatiotemporal Expression of TLR2 and TLR9 in Gibel Carp (Carassius auratus gibelio) after CyHV-2 Infection

Cyprinid herpesvirus II (CyHV-2), a highly contagious pathogen of gibel carp (Carassius auratus gibelio), causes herpesviral hematopoietic necrosis disease (HVHND) and enormous financial losses. However, there is limited information available regarding the changes in plasma biochemical and immunological parameters and the response characteristics of Toll-like receptor 2 (TLR2) and Toll-like receptor 9 (TLR9) in gibel carp after CyHV-2 infection. To address this knowledge gap, a sub-lethal CyHV-2 infection was conducted in gibel carp, and the sample was collected daily from 1 to 7 days post infection. The plasma biochemical analyses showed significant decreases in the content of glucose, total cholesterol (TCHO), and total protein (TP), along with marked increases in the level of uric acid, urea, creatinine (CREA), Complement 3 (C3), immunoglobulin D (IgD), and immunoglobulin M (IgM) as well as in the activity of alanine aminotransferase (ALT), aspartate transaminase (AST), alkaline phosphatase (ALP), and lactate dehydrogenase (LDH) in the infected group. Compared with the control group, the concentration of cortisol, triglyceride (TG), and Complement 4 (C4) had no noticeable alterations in the infected group. Real-time quantitative PCR analysis showed significant upregulation of TLR2 and TLR9 mRNA expression in the spleen, kidney, brain, liver, intestine, and gill post CyHV-2 infection. Interestingly, a time- and tissue-dependent expression profile has been comparatively observed for TLR2 and TLR9 in the above tissues of gibel carp after CyHV-2 infection, suggesting distinct roles between TLR2 and TLR9 in antiviral response to CyHV-2 infection. Overall, our results demonstrated that CyHV-2 infection led to the disruption of the physiological metabolic process and damage to the liver and kidney, and induced different spatiotemporal expression patterns of TLR2 and TLR9, ultimately stimulating antiviral response via innate and adaptive immune system. These findings may provide a deeper understanding of the host immunity response to CyHV-2 infection and offer novel perspectives for the prevention and treatment and therapeutic drug development against CyHV-2.

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

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

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

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

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.

Construction and Evaluation of Recombinant Attenuated Edwardsiella piscicida Vaccine (RAEV) Vector System Encoding Ichthyophthirius multifiliis (Ich) Antigen IAG52B

We have successfully designed and constructed a RAEV vector system with regulated-delayed attenuation in vivo attributes that synthesizes Ichthyophthirius multifiliis (Ich) protective antigen IAG52B to enable vaccination of fish susceptible to edwardsiellosis and white spot disease. The first feature of this vaccine delivery system is an Edwardsiella piscicida strain carrying genomic deletions of asdA. AsdA is an enzyme necessary for the synthesis of diaminopimelic acid (DAP), which is an essential component of the peptidoglycan layer of the cell wall of Gram-negative bacteria. asdA mutant strains have obligate growth requirements for DAP in the medium or a plasmid vector with the wild-type asdA gene enabling synthesis of DAP. This balanced-lethal plasmid vector-host system in E. piscicida enables as a second feature the synthesis of recombinant antigens to induce protective immunity against fish pathogens. Recombinant protective antigen IAG52B from the fish pathogen I. multifiliis was synthesized by RAEV strains harboring the AsdA⁺ plasmid pG8R8029. The third feature of this vaccine strain is a regulated-delayed attenuation in vivo phenotype that is based on the replacement of an arabinose-regulated araC P_araBAD cassette for the promoters of the fur and crp genes of E. piscicida such that the expression of these genes is dependent on arabinose provided during growth. Thus, following colonization, the Fur and Crp proteins stop being synthesized due to the lack of arabinose and attenuation is progressively achieved in vivo to prevent generation of diseases symptoms. Our vaccine strain χ16022 with the genotype ΔasdA10 ΔP_fur170::TT araC P_araBAD fur ΔP_crp68::TT araC P_araBAD crp contains the AsdA⁺ plasmid, pG8R8029, which encodes the IAG52B antigen. Vaccine strain χ16022(pG8R8029) is attenuated and induces systemic and mucosal IgM titer against E. piscicida and Ich in zebrafish. In addition, transcript levels of tnf-α, il-1β, il-6 and il-8 were significantly increased in different tissues of vaccinated zebrafish compared to unimmunized fish. Zebrafish vaccinated with χ16022(pG8R8029) showed 60% survival upon intracoelomic (i.c.) challenge with a lethal dose of virulent E. piscicida strain J118. Our RAEV system could be used as a generalized vaccine-vector system to protect teleost fish against multiple bacterial, viral and parasitic infectious diseases.

TLR1 in Nile tilapia: The conserved receptor cannot interact with MyD88 and TIRAP but can activate NF-κB in vitro

Toll-like receptors (TLRs) play a critical role in the innate immune response of fish. In this study, we isolated the cDNA sequence of Nile tilapia TLR1 (OnTLR1). The deduced OnTLR1 protein contains a signal peptide, 7 leucine-rich repeats (LRRs), a C-terminal LRR (LRR-CT), a transmembrane region and a highly conserved TIR domain. In healthy Nile tilapia, the OnTLR1 transcript was broadly expressed in all examined tissues, with the highest expression levels in the spleen. After infection with Streptococcus agalactiae, the OnTLR1 transcripts were upregulated in the gill and kidney. After stimulation with polyinosinic-polycytidylic acid (poly(I:C)), the expression levels of OnTLR1 were significantly downregulated in the intestine, whereas OnTLR1 transcripts were significantly upregulated in the kidney. After challenge with lipopolysaccharide (LPS), the expression levels of OnTLR1 were significantly upregulated in the spleen and kidney. The subcellular localization showed that OnTLR1 was expressed in the cytoplasm. TLR1 significantly increased MyD88-dependent NF-κB activity. However, the results of a pull-down assay showed that OnTLR1 did not interact with MyD88 or TIRAP. Binding assays revealed the specificity of OnTLR1 for pathogen-associated molecular patterns (PAMPs) and bacteria that included S. agalactiae, Aeromonas hydrophila and poly(I:C) and LPS. Taken together, these findings suggest that OnTLR1, as a pattern recognition receptor (PRR), might play an important role in the immune response to pathogen invasion.

Transcriptome analysis of grass carp (Ctenopharyngodon idella) and Holland's spinibarbel (Spinibarbus hollandi) infected with Ichthyophthirius multifiliis

Ichthyophthirius multifiliis is a protozoan ciliate that causes white spot disease (also known as ichthyophthiriasis) in freshwater fish. Holland's spinibarbel (Spinibarbus hollandi) was less susceptible to white spot disease than grass carp (Ctenopharyngodon Idella). In this study, grass carp and Holland's spinibarbel are infected by I. multifiliis and the amount of infection is 10,000 theronts per fish. All grass carp died within 12 days after infection, and the survival rate of Holland's spinibarbel was more than 80%. In order to study the difference in sensitivity of these two fish species to I. multifiliis, transcriptome analysis was conducted using gill, skin, liver, spleen and head kidney of Holland's spinibarbel and grass carp at 48 h post-infection with I. multifiliis. A total of 489,296,696 clean reads were obtained by sequencing. A total of 105 significantly up-regulated immune-related genes were obtained by Gene Ontology (GO) classification and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis in grass carp. Cluster of differentiation 40 (CD40), cluster of differentiation 80 (CD 80), tumor necrosis factor-alpha (TNF-α), toll-like receptor 4 (TLR-4), interleukin 1 beta (IL-1β) and other inflammatory-related genes in grass carp were enriched in the cytokine-cytokine receptor interaction pathway and toll-like receptor pathway. In Holland's spinibarbel, a total of 46 significantly up-regulated immune-related genes were obtained by GO classification and KEGG pathway enrichment analysis. Immune-related genes, such as Immunoglobin heavy chain (IgH), cathepsin S (CTSS), complement C1q A chain (C1qA), complement component 3 (C3) and complement component (C9) were enriched in phagosome pathway, lysosome pathway and complement and coagulation concatenation pathway. C3 was significantly up-regulated in gill and head kidney. Fluorescence in situ hybridization (FISH) showed that the C3 gene was highly expressed in gill tissue of Holland's spinibarbel infected with I. multifiliis. A small amount of C3 gene was expressed in the gill arch of grass carp after infected with I. multifiliis. In conclusion, the severe inflammatory response in vivo after infecting grass carp with I. multifiliis might be the main cause of the death of grass carp. The extrahepatic expression of the gene of Holland's spinibarbel might play an important role in the immune defense against I. multifiliis.

SGIV Induced and Exploited Cellular De Novo Fatty Acid Synthesis for Virus Entry and Replication

Considerable attention has been paid to the roles of lipid metabolism in virus infection due to its regulatory effects on virus replication and host antiviral immune response. However, few literature has focused on whether lipid metabolism is involved in the life cycle of lower vertebrate viruses. Singapore grouper iridovirus (SGIV) is the causative aquatic virus that extensively causes fry and adult groupers death. Here, the potential roles of cellular de novo fatty acid synthesis in SGIV infection was investigated. SGIV infection not only increased the expression levels of key enzymes in fatty acid synthesis in vivo/vitro, including acetyl-Coenzyme A carboxylase alpha (ACC1), fatty acid synthase (FASN), medium-chain acyl-CoA dehydrogenase (MCAD), adipose triglyceride lipase (ATGL), lipoprotein lipase (LPL) and sterol regulatory element-binding protein-1 (SREBP1), but it also induced the formation of lipid droplets (LDs), suggesting that SGIV altered de novo fatty acid synthesis in host cells. Using the inhibitor and specific siRNA of ACC1 and FASN, we found that fatty acid synthesis was essential for SGIV replication, evidenced by their inhibitory effects on CPE progression, viral gene transcription, protein expression and virus production. Moreover, the inhibitor of fatty acid β-oxidation could also reduce SGIV replication. Inhibition of fatty acid synthesis but not β-oxidation markedly blocked virus entry during the life cycle of SGIV infection. In addition, we also found that inhibition of ACC1 and FASN increased the IFN immune and inflammatory response during SGIV infection. Together, our data demonstrated that SGIV infection in vitro regulated host lipid metabolism and, in that process, cellular fatty acid synthesis might exert crucial roles during SGIV infection via regulating virus entry and host immune response.

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.

The gill transcriptome reveals unique antimicrobial features that protect Nibea albiflora from Cryptocaryon irritans infection

Cryptocaryonosis is the greatest threat to most teleost species among all parasitic diseases, causing mass loss to the marine aquaculture industry. Epidemiological investigation of teleost susceptibility to Cryptocaryon irritans infection revealed that yellow drum (Nibea albiflora) is highly resistant. In order to further understand the activation of the immune system in the gill, which is one of the main mucosal-associated lymphoid tissues and a target of parasites, transcriptome analysis of the yellow drum gill was performed. Gill samples were collected from fish challenged after 24 hr and 72 hr with theronts at a median death rate (2050 theronts per gram fish). Gene expression profiles showed that TLR5 was the only receptor that activated the downstream immune response. The infection activated complement cascade through alternative pathway and increased the expression of C5a anaphylatoxin chemotactic receptor 1. In addition, possible antimicrobial molecules, including lipoprotein and haptoglobin, which are responsible for trypanolysis in humans, were among the top significantly upregulated genes at 24 hr. After 72 hr, the expression of secreted immunoglobulin T-related genes was induced. These results suggested a rapid innate and adaptive immune response at the mucosal level. In conclusion, the results provide new perspectives on mucosal immune resistance in yellow drum against cryptocaryonosis and provide the possibility of mining resistance genes for future therapy.

MEKK3 in hybrid snakehead (Channa maculate ♀ ×Channa argus ♂): Molecular characterization and immune response to infection with Nocardia seriolae and Aeromonas schubertii

Mitogen-activated protein kinase/extracellular signal-regulated kinase kinase kinase 3 (MEKK3) is a serine/threonine protein kinase that acts as a key regulator and is widely involved in various innate and acquired immune signaling pathways. In this study, we first cloned the complete open reading frame (ORF) of the MEKK3 gene (named CcMEKK3) in a hybrid snakehead (Channa maculate ♀ × Channa argus ♂). The full-length ORF of CcMEKK3 is 1851 bp, and encodes a putative protein of 616 amino acids containing a serine/threonine kinase catalytic (S-TKc) domain and a Phox and Bem1p (PB1) domain. A sequence alignment and phylogenetic tree analysis showed that CcMEKK3 is highly conserved relative to the MEKK3 proteins of other teleost species. CcMEKK3 was constitutively expressed in all the healthy hybrid snakehead tissues tested, with greatest expression in the immune tissues, such as the head kidney and spleen. The expression of CcMEKK3 was usually upregulated in the head kidney, spleen, and liver at different time points after infection with Nocardia seriolae or Aeromonas schubertii. Similarly, the dynamic expression levels of CcMEKK3 in head kidney leukocytes after stimulation revealed that CcMEKK3 was induced by LTA, LPS, and poly(I:C). In the subcellular localization analysis, CcMEKK3 was evenly distributed in the cytoplasm of HEK293T cells, and its overexpression significantly promoted the activities of NF-κB and AP-1. These results suggest that CcMEKK3 is involved in the immune defense against these two pathogens, and plays a crucial role in activating the NF-κB and MAPK signaling pathways.

Identification and functional characterization of a fish-specific tlr19 in common carp (Cyprinus carpio L.) that recruits TRIF as an adaptor and induces ifn expression during the immune response

Toll-like receptor 19 (Tlr19) is a fish-specific TLR that plays a critical role in innate immunity. In the present study, we aimed to identify tlr19 from common carp (Cyprinus carpio L.) and explored its expression profile, localization, adaptor, and signaling pathways. A novel tlr19 cDNA sequence (Cctlr19) was identified in common carp. Phylogenetic analysis revealed that CcTlr19 was most closely related to Danio rerio Tlr19. Subcellular localization analysis indicates that CcTlr19 was synthesized in the free ribosome and then transported to early endosomes. Cctlr19 was constitutively expressed in all the examined tissues, with the highest expression in the brain. After poly(I:C) and Aeromonas hydrophila injection, the expression of Cctlr19 was significantly upregulated in immune-related organs. In addition, the expression of Cctlr19 was upregulated in head kidney leukocytes (HKL) upon stimulation with different ligands. Immunofluorescence and luciferase analyses indicate that CcTlr19 recruited TRIF as an adaptor. Furthermore, CcTlr19 can activate the expression of ifn-1 and viperin. Taken together, these findings lay the foundation for future research to investigate the mechanisms underlying fish tlr19.

Characterization, expression and function analysis of the TLR3 gene in golden pompano (Trachinotus ovatus)

Toll-like receptors (TLRs)are pattern recognition receptors (PRRs) that are important in invertebrate innate immunity for the recognition and elimination of pathogens. Although they were reported in many fishes, Toll-like receptors subfamily contain a large number of members with different functions that need to research in deep. In the present study, the full-length cDNA of TLR3 from the golden pompano, Trachinotus ovatus, was cloned and characterized. The full length of ToTLR3 cDNA was 3710 bp including an open reading frame of 2760 bp encoding a peptide of 919 amino acids. The derived amino acids sequence comprised of 14 leucine-rich repeats (LRR), capped with LRRCT followed by transmembrane domain and cytoplasmic Toll/IL-1R domain (TIR). Multiple sequence alignment and phylogenetic analysis revealed that ToTLR3 shared the highest similarity to the teleost fish and suggested ToTLR3 is fairly conservative in evolution process. Tissues distribution analysis indicated that ToTLR3 showed a tissue-specific variation with high expression in blood and liver. After the fish were stimulated by poly(I:C), flagellin and LPS, ToTLR3 expression in the liver, intestine, blood, kidney, skin and muscle was significantly upregulated in a time-depended manner, especially in immune related tissues such as liver, blood and kidney. Binding assay revealed the specificity of rToTLR3 for pathogen-associated molecular patterns (PAMPs) and bacteria that included Vibrio harveyi, V. vulnificus, V. anguillarum, Photobacterium damselae, Escherichia coli, Aeromonas hydrophila, Staphylococcus aureus and PolyI:C, LPS, Flagellin, and PGN. In addition, a luciferase reporter assay showed that overexpression ToTLR3 significantly increased NF-κB activity. Collectively, our results suggested that ToTLR3 might play an important role as a pattern recognition receptor (PRR) in the immune response towards pathogen infections, and transmiss the danger signal to downstream signaling pathways.

Development of Fish Parasite Vaccines in the OMICs Era: Progress and Opportunities

Globally, parasites are increasingly being recognized as catastrophic agents in both aquaculture sector and in the wild aquatic habitats leading to an estimated annual loss between 1.05 billion and 9.58 billion USD. The currently available therapeutic and control measures are accompanied by many limitations. Hence, vaccines are recommended as the "only green and effective solution" to address these concerns and protect fish from pathogens. However, vaccine development warrants a better understanding of host-parasite interaction and parasite biology. Currently, only one commercial parasite vaccine is available against the ectoparasite sea lice. Additionally, only a few trials have reported potential vaccine candidates against endoparasites. Transcriptome, genome, and proteomic data at present are available only for a limited number of aquatic parasites. Omics-based interventions can be significant in the identification of suitable vaccine candidates, finally leading to the development of multivalent vaccines for significant protection against parasitic infections in fish. The present review highlights the progress in the immunobiology of pathogenic parasites and the prospects of vaccine development. Finally, an approach for developing a multivalent vaccine for parasitic diseases is presented. Data sources to prepare this review included Pubmed, google scholar, official reports, and websites.

Variation in selection constraints on teleost TLRs with emphasis on their repertoire in the Walking catfish, Clarias batrachus

The high degree of conservation of toll-like receptors (TLRs), and yet their subtle variations for better adaptation of species in the host–pathogen arms race make them worthy candidates for understanding evolution. We have attempted to track the trend of TLR evolution in the most diverse vertebrate group—teleosts, where Clarias batrachus was given emphasis, considering its traits for terrestrial adaptation. Eleven C. batrachus TLRs (TLR1, 2, 3, 5, 7, 8 9, 13, 22, 25, 26) were identified in this study which clustered in proximity to its Siluriformes relative orthologues in the phylogenetic analysis of 228 TLRs from 25 teleosts. Ten TLRs (TLR1, 2, 3, 5, 7, 8 9, 13, 21, 22) with at least 15 member orthologues for each alignment were processed for selection pressure and coevolutionary analysis. TLR1, 7, 8 and 9 were found to be under positive selection in the alignment-wide test. TLR1 also showed maximum episodic diversification in its clades while the teleost group Eupercaria showed the maximum divergence in their TLR repertoire. Episodic diversification was evident in C. batrachus TLR1 and 7 alignments. These results present a strong evidence of a divergent TLR repertoire in teleosts which may be contributing towards species-specific variation in TLR functions.

Help or hindrance? The evolutionary impact of whole-genome duplication on immunogenetic diversity and parasite load

Whole-genome duplication (WGD) events occur in all kingdoms and have been hypothesized to promote adaptability. WGDs identified in the early history of vertebrates, teleosts, and angiosperms have been linked to the large-scale diversification of these lineages. However, the mechanics and full outcomes of WGD regarding potential evolutionary impacts remain a topic of debate. The Corydoradinae are a diverse subfamily of Neotropical catfishes with over 170 species described and a history of WGDs. They are divided into nine mtDNA lineages, with species coexisting in sympatric-and often mimetic-communities containing representatives of two or more of the nine lineages. Given their similar life histories, coexisting species of Corydoras might be exposed to similar parasite loads and because of their different histories of WGD and genome size they provide a powerful system for investigating the impacts of WGD on immune diversity and function in an animal system. Here, we compared parasite counts and the diversity of the immune-related toll-like receptors (TLR) in two coexisting species of Corydoras catfish (C. maculifer and C. araguaiaensis), one diploid and one putative tetraploid. In the putative tetraploid C. araguaiaensis, we found significantly lower numbers of parasites and significantly higher diversity (measured by both synonymous and nonsynonymous SNP counts) in two TLR genes than in the diploid C. maculifer. These results provide insight into how WGD may impact evolution, in this case by providing greater immunogenetic diversity.

Interactions between the Parasite Philasterides dicentrarchi and the Immune System of the Turbot Scophthalmus maximus. A Transcriptomic Analysis

The present study analyses the interactions between Philasterides dicentrarchi (a ciliate parasite that causes high mortalities in cultured flatfish) and the peritoneal cells of the turbot Scophthalmus maximus during an experimental infection. The transcriptomic response was evaluated in the parasites and in the fish peritoneal cells, at 1, 2 and 4 h post-infection (hpi) in turbot injected intraperitoneally (ip) with 10⁷ ciliates and at 12 and 48 hpi in turbot injected ip with 10⁵ ciliates. Numerous genes were differentially expressed (DE) in P. dicentrarchi, relative to their expression in control ciliates (0 hpi): 407 (369 were up-regulated) at 1 hpi, 769 (415 were up-regulated) at 2 hpi and 507 (119 were up-regulated) at 4 hpi. Gene ontology (GO) analysis of the DE genes showed that the most representative categories of biological processes affected at 1, 2 and 4 hpi were biosynthetic processes, catabolic processes, biogenesis, proteolysis and transmembrane transport. Twelve genes of the ABC transporter family and eight genes of the leishmanolysin family were DE at 1, 2 and 4 hpi. Most of these genes were strongly up-regulated (UR), suggesting that they are involved in P. dicentrarchi infection. A third group of UR genes included several genes related to ribosome biogenesis, DNA transcription and RNA translation. However, expression of tubulins and tubulin associated proteins, such as kinesins or dyneins, which play key roles in ciliate division and movement, was down-regulated (DR). Similarly, genes that coded for lysosomal proteins or that participate in the cell cycle mitotic control, glycolysis, the Krebs cycle and/or in the electron transport chain were also DR. The transcriptomic analysis also revealed that in contrast to many parasites, which passively evade the host immune system, P. dicentrarchi strongly stimulated turbot peritoneal cells. Many genes related to inflammation were DE in peritoneal cells at 1, 2 and 4 hpi. However, the response was much lower at 12 hpi and almost disappeared completely at 48 hpi in fish that were able to kill P. dicentrarchi during the first few hpi. The genes that were DE at 1, 2 and 4 hpi were mainly related to the apoptotic process, the immune response, the Fc-epsilon receptor signalling pathway, the innate immune response, cell adhesion, cell surface receptors, the NF-kappaB signalling pathway and the MAPK cascade. Expression of toll-like receptors 2, 5 and 13 and of several components of NF-κB, MAPK and JAK/STAT signalling pathways was UR in the turbot peritoneal cells. Genes expressing chemokines and chemokine receptors, genes involved in prostaglandin and leukotriene synthesis, prostaglandins, leukotriene receptors, proinflammatory cytokines and genes involved in apoptosis were strongly UR during the first four hours of infection. However, expression of anti-inflammatory cytokines such as Il-10 and lipoxygenases with anti-inflammatory activity (i.e., arachidonate 15-lipoxygenase) were only UR at 12 and/or 48 hpi, indicating an anti-inflammatory state in these groups of fish. In conclusion, the present study shows the regulation of several genes in P. dicentrarchi during the early stages of infection, some of which probably play important roles in this process. The infection induced a potent acute inflammatory response, and many inflammatory genes were regulated in peritoneal cells, showing that the turbot uses all the protective mechanisms it has available to prevent the entry of the parasite.

Expression profile, subcellular localization and signaling pathway analysis of fish-specific TLR25 in Nile tilapia (Oreochromis niloticus)

The Nile tilapia (Oreochromis niloticus) is one of the major food fish species produced in tropical and subtropical regions. However, this industry has been facing significant challenges from microbial infections. Understanding how hosts initiate immune responses against invading microbes is the first requirement for addressing disease outbreak prevention and disease resistance. Toll-like receptors (TLRs) are a family of evolutionarily conserved proteins that can recognize pathogen-associated molecular patterns (PAMPs). They thus play an essential role in innate immunity. TLR25 is a newly identified fish-specific member of the TLR1 subfamily. In this study, we investigate the molecular and functional characteristics of O. niloticus TLR25 (OnTLR25) via tissue expression patterns, gene expression modulation after challenge with bacteria and TLR ligands, subcellular localization in human and fish cells, and the signaling pathways TLR25 may induce. Transcriptional levels of OnTLR25 are high in immune-related organs such as the spleen and head kidney, and are increased following bacterial challenges. In addition, we show that OnTLR25 preferentially localizes to the intracellular compartment in transfected tilapia head kidney (THK) cell line. Furthermore, overexpression of the truncated form of OnTLR25 in THK cell line induced the expression of proinflammatory cytokines, such as tumor necrosis factor α, interleukin (IL)-1β, IL-8, IL-12a, and interferon-d2.13. Combined, our results suggest that TLR25 is likely to play an important role in the antimicrobial responses of the innate immune system of Nile tilapia.

A portrait of the immune response to proliferative kidney disease (PKD) in rainbow trout

Proliferative kidney disease (PKD), caused by the myxozoan Tetracapsuloides bryosalmonae, is one of the most serious parasitic diseases of salmonids in which outbreaks cause severe economic constraints for the aquaculture industry and declines of wild species throughout Europe and North America. Given that rainbow trout (Oncorhynchus mykiss) is one of the most widely farmed freshwater fish and an important model species for fish immunology, most of the knowledge on how the fish immune response is affected during PKD is from this organism. Once rainbow trout are infected, PKD pathogenesis results in a chronic kidney immunopathology mediated by decreasing myeloid cells and increasing lymphocytes. Transcriptional studies have revealed the regulation of essential genes related to T-helper (Th)-like functions and a dysregulated B-cell antibody type response. Recent reports have discovered unique details of teleost B-cell differentiation and functionality and characterized the differential immunoglobulin (Ig)-mediated response. These studies have solidified the rainbow trout T. bryosalmonae system as a sophisticated disease model capable of feeding key advances into mainstream immunology and have contributed essential information to design novel parasite disease prevention strategies. In our following perspective, we summarize these efforts to evaluate the immune mechanisms of rainbow trout during PKD pathogenesis.

Changes in the immune function of rainbow trout (Oncorhynchus mykiss) provide insights into strategies against BDE-47 stress

Polybrominated diphenyl ethers (PBDEs) are ubiquitous in marine ecosystems and have been suggested to bioaccumulate in aquatic food webs, with potentially negative impacts on marine organism. In this study, a 21-day experiment was performed under controlled laboratory conditions, in which 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), the most biotoxic PBDE in the marine environment, was fed to rainbow trout (Oncorhynchus mykiss) at concentrations of 50 and 500 ng g^-1 in the diet. BDE-47 significantly decreased the specific growth rate of O. mykiss and was highly concentrated in the liver and head kidney, as evidenced by increased bioaccumulation factor (BAF) values. Tissue observation revealed impairment of the microstructure of the head kidney. Important immune factors in the skin, blood and head kidney were significantly inhibited by BDE-47 treatment (p < 0.05), whereas the respiratory burst activity of macrophages was enhanced. Additionally, immune-related genes were strongly downregulated following BDE-47 exposure (p < 0.05). In a bacterial challenge, the treatment groups had much higher mortality than did the control group (p < 0.05). BDE-47 accumulated and impaired immune organs, and the hierarchy of immune responses was impaired, consequently reducing O. mykiss resistance to pathogen invasion.

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

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

Comprehensive transcriptional changes in the liver of Kanglang white minnow (Anabarilius grahami) in response to the infection of parasite Ichthyophthirius multifiliis

Simple Summary

Kanglang white minnow (KWM, Anabarilius grahami), is a typical “3E” (Endangered, Endemic and Economic) fish species in Yunnan-Guizhou Plateau. As one of the traditional “Four Famous Fishes” in Yunnan province, it has become the major local aquaculture species with increasing demand after the success of artificial breeding. However, this economically important fish is highly susceptible to the infection of a parasite ciliate, Ichthyophthirius multifiliis (Ich), during the practical procedure of artificial breeding. To examine the host immune responses to Ich, we divided the experimental fishes into three groups (including control, early-infected stage, and late-infected stage) for transcriptome sequencing to analyze the differentially expressed genes (DEGs) and immune response mechanisms.

Abstract

The notorious parasite Ichthyophthirius multifiliis (Ich) has been recorded worldwide in fish species and causes white spot disease, posing major threats and resulting in severe losses to international fish production. Extensively effective strategies for treating Ich are not available yet, and genetic mechanisms of hosts in response to the parasite are still largely unknown. In this study, we selected Kanglang white minnow (KWM, Anabarilius grahami) to examine its liver transcriptional changes after Ich infection, as white spot disease is one bottleneck problem in exploring this economically important species. We divided the experimental fishes into three groups (control, early-infected, and late-infected) to examine differentially expressed genes (DEGs). A total of 831 DEGs were identified and classified into 128 significantly enriched GO (Gene Ontology) terms and 71 significantly enriched KEGG (Kyoto Encyclopedia of Genes and Genomes) pathways. Most of these terms or pathways were functionally enriched in immunity, inflammatory response, and apoptosis, such as nucleotide-binding oligomerization domain-like (NOD-like) receptor signaling, tumor necrosis factor (TNF) signaling, interleukin-17 (IL-17) signaling, and apoptosis pathways. We also identified 178 putative antimicrobial peptides (AMPs) and AMP precursors based on our previously reported genome assembly of KWM, and revealed that the expressional patterns varied according to different types. In summary, our work reported the first comprehensive transcriptional changes in KWM in response to the exogenous infection of Ich, which would lay a solid foundation for in-depth studies on disease defense or resistant strains selection in this valuable fish.

Immune response of channel catfish (Ictalurus punctatus) against Ichthyophthirius multifiliis post vaccination using DNA vaccines encoding immobilization antigens

The channel catfish (Ictalurus punctatus) immune response against Ichthyophthirius multifiliis (Ich) after vaccination using plasmid DNA vaccines pcDNA3.1-IAg52a and pcDNA3.1-IAg52b, encoding Ich immobilization antigen genes was studied. Parasite infection level, serum anti-Ich antibodies level, fish mortality after theront challenge, and immune-related gene expression were measured. After in vitro transfection of walking catfish gill cells (G1b) with both pcDNA3.1-IAg52a and pcDNA3.1-IAg52b, antigens IAG52A and IAG52B were detected. During the vaccination trial, 76-fold increase in the Iag52b gene expression was observed in the vaccinated fish group h4 post vaccination. Administration of DNA vaccines by IM injection induced significant gene up-regulation in the head kidney, including immunoglobulin M (IgM), cluster of differentiation 4 (CD4), major histocompatibility I (MHC I), and T cell receptor α (TcR-α) from h4 to d5 post immunization. Fish vaccinated with DNA vaccines or theronts showed increased gene expression of the cytokine interferon (IFN-γ), complement component 3 (C3), and toll-like receptor-1 (TLR-1). Anti-Ich antibodies were detected in fish received pcDNA3.1-IAg52a, pcDNA3.1-IAg52b and the combination of both vaccines d10 post vaccination. Fish vaccinated with pcDNA3.1-IAg52b showed mild parasite infection level, partial survival (20%) and longer mean day to death (MDD) after theront challenge. By contrast, a heavy parasite load, 0% survival and short MDD were observed in the sham vaccinated control fish that received pcDNA3.1 (plasmid without genes encoding Ich immobilization antigen). Further research is needed to improve DNA vaccines for Ich that can induce strong protective immunity in fish. Suggested studies include improved transfection efficiency, use of appropriate adjuvants and including additional parasite antigen genes in the plasmid.

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.

Notch1a can widely mediate innate immune responses in zebrafish larvae infected with Vibrio parahaemolyticus

The Notch signaling pathway is known to regulate innate immunity by influencing macrophage function and interacting with the Toll-like receptor (TLR) signaling pathway. However, the comprehensive role of the Notch signaling pathway in the innate immune response remains unknown. To assess the function of Notch1a in immunity, we examined the innate immune responses to Vibrio parahaemolyticus strain Vp13 of wild-type (WT) and notch1a^-/- zebrafish larvae generated using the clustered regularly interspaced short palindromic repeats/CRISPR associated protein 9 (CRISPR/Cas9) system. The median lethal dose (LD₅₀) of V. parahaemolyticus was significantly lower in notch1a^-/- larvae than in WT larvae 3 days post fertilization (dpf). Transcriptome data analysis revealed 359 significantly differentially expressed genes (DEGs), including 246 significantly down-regulated genes and 113 significantly up-regulated genes, in WT infected groups compared with WT control groups. In contrast, 986 significantly DEGs were found in notch1a^-/- infected groups compared with notch1a^-/- control groups, of which 82 genes were significantly down-regulated and 904 genes were significantly up-regulated. These DEGs belonged to the tumor necrosis factor (TNF), complement, nuclear factor kappa B (NF-κB), cathepsin, interleukin (IL), chemokine, serpin peptidase inhibitor, matrix metallopeptidase, innate immune cells, pattern recognition receptor (PRR), and other cytokine families. Our results indicate that Notch1a plays roles in inhibiting many immunity-related genes and could comprehensively mediate the innate immune response by regulating TLRs, nucleotide-binding-oligomerization-domain-like receptors (NLRs), lectins, complement, ILs, chemokines, TNF, cathepsin, and serpin. Further studies are required to understand the specific mechanism of Notch1a in innate immunity.

Immune responses of fish to Ichthyophthirius multifiliis (Ich): A model for understanding immunity against protozoan parasites

The parasitic ciliate Ichthyophthirius multifiliis (Ich), which infects almost all freshwater fish species, provides an optimal model for the study of immunity against extracellular protozoa. Ich invades the epithelia of mucosal tissues, forms white spots covering the whole body, and induces high mortality, while survivor fish develop both innate and adaptive immunity against Ich attack in systemic and mucosal tissues. Besides the protective roles of the Toll-like receptor (TLR)-mediated innate immune response, the critical immune functions of novel IgT in the skin, gut, gill, and olfactory organ of teleosts have been demonstrated in recent years, and all this information contributes to the ontogeny of the mucosal immune response in vertebrates. Especially in rainbow trout, Ich-infected fish exhibited higher IgT concentrations and titers in the mucosa and increased IgT⁺ B-lymphocyte proliferation in mucosal tissues. IgM mainly functions in the adaptive immune response in the systemic tissues of rainbow trout, accompanied with increased IgM⁺ B-lymphocyte proliferation in the head kidney of Ich-infected trout. However, little is known about the interaction between these mucosal tissues and systemic immune organs and the interaction between the inductive immune organs and functional immune organs. Immobilization antigens (Iags), located on the parasite cell and ciliary membranes, have been characterized to be targeted by specific antibodies produced in the host. The crosslinking of antigens mediated by antibodies triggers either an escape response or the immobilization of Ich. With more knowledge about the Iags of Ich and the immunity of teleosts, a more targeted vaccine, even a DNA vaccine, can be developed for the immune control strategy of Ich. Due to the high frequency of clinical fish ichthyophthiriasis, the study of fish immune responses to Ich provides an optimal experimental model for understanding immunity against extracellular protozoa.

Transcriptomic analysis of immunity in rainbow trout (Oncorhynchus mykiss) gills infected by Ichthyophthirius multifiliis

The parasite Ichthyophthirius multifiliis infecting skin, fins and gills of a wide range of freshwater fish species, including rainbow trout, is known to induce a protective immune response in the host. Although a number of studies have reported activation of several immune genes in infected fish host, the immune response picture is still considered incomplete. In order to address this issue, a comparative transcriptomic analysis was performed on infected versus uninfected rainbow trout gills and it showed that a total of 3352 (7.2%) out of 46,585 identified gene sequences were significantly regulated after parasite infection. Of differentially expressed gene sequences, 1796 genes were up-regulated and 1556 genes were down-regulated. These were classified into 61 Gene Ontology (GO) terms and mapped to 282 reference canonical pathways in the Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Infection of I. multifiliis induced a clear differential expression of immune genes, related to both innate and adaptive immunity. A total of 268 (6.86%) regulated gene sequences were known to take part in 16 immune-related pathways. These involved pathways related to the innate immunity such as the Chemokine signaling pathway, Platelet activation, Toll-like receptor signaling pathway, NOD-like receptor signaling pathway, and Leukocyte transendothelial migration. Elevated transcription of genes encoding the TLR 8 gene and chemokines (CCL4, CCL19, CCL28, CXCL8, CXCL11, CXCL13, CXCL14) was recorded indicating their roles in recognition of I. multifiliis and subsequent induction of the inflammatory response, respectively. A number of upregulated genes in infected gills were associated with antigen processing/presentation and T and B cell receptor signaling (including B cell marker CD22 involved in B cell development). Overall the analysis supports the notion that I. multifiliis induces a massive and varied innate response upon which a range of adaptive immune responses are established which may contribute to the long lasting protection of immunized rainbow trout.

Transcriptome profiles in the spleen of African catfish (Clarias gariepinus) challenged with Aeromonas veronii

The African catfish, Clarias gariepinus, an important cultured freshwater species in many countries, possess the characteristic of high disease resistance. However, little genomic information for this character of the fish is available up to now. To address the shortfall and to better understand C. gariepinus immune response to pathogen infection at molecular level, C. gariepinus were challenged with potent A. veronii and the high-throughput RNA sequencing (RNA-seq) technology were employed to produce transcriptomes from spleen. In total, an average of 46,073,372 clean reads obtained were de novo assembled into 156,955 unigenes with an average length of 1082 bp. All of unigenes were annotated to seven public databases. Three comparisons were separately conducted between the infected groups at 3 h, 24 h, 48 h post-challenge and control group. A total of 2482 differentially expressed unigenes (DEGs) were identified. Among these, 114 immune-related DEGs were captured, including 88, 42, and 31 genes at 3 h, 24 h and 48 h after infection respectively, for analysis of expression pattern and enrichment. The 114 DEGs displayed four expression patterns by cluster analysis and they were significantly enriched in 38 pathways (q < 0.01) related to the immune or disease, five of which were NF-kappa B, TNF, NLR, TLR and RLR pathways. Finally, the expression levels of twelve selected immune-related DEGs involved in above five pathways were scrutinized. Seven of which were up-regulated at 3 h after infection, afterward, their expression dropped to control level. In summary, this study provides valuable transcriptome resource for understanding the defense mechanisms of C. gariepinus in resistance to pathogens from the gene expression viewpoint, which also open up the possibility to study the immune complexity and to better comprehend the interrelationships between some immune pathways in C. gariepinus.

Immunostimulatory Activities of CpG-Oligodeoxynucleotides in Teleosts: Toll-Like Receptors 9 and 21

Toll-like receptors (TLRs) are pattern-recognition receptors that detect a wide variety of microbial pathogens for the initiation of host defense immunological responses. Thirteen TLRs have been identified in mammals, and teleosts contain 22 mammalian or non-mammalian TLRs. Of these, TLR9 and TLR21 are the cytosine-phosphate-guanosine-oligodeoxynucleotides (CpG-ODNs) recognition TLRs in teleosts. TLR9 is a mammalian TLR expressed in teleost but not in the avian species. TLR21 is a non-mammalian TLR expressed in both teleost and the avian species. Synthetic CpG-ODNs are potent immunostimulants that are being studied for their application against tumors, allergies, and infectious diseases, and as a vaccine adjuvant in humans. The immunostimulatory effects of CpG-ODNs as vaccine adjuvants and their antimicrobial function in domestic animals and teleosts are also being investigated. Most of our current knowledge about the molecular basis for the immunostimulatory activity of CpG-ODNs comes from earlier studies of the interaction between CpG-ODN and TLR9. More recent studies indicate that in addition to TLR9, TLR21 is another receptor for CpG-ODN recognition in teleosts to initiate immune responses. Whether these two receptors have differential functions in mediating the immunostimulatory activity of CpG-ODN in teleost has not been well-studied. Nevertheless, the existence of two recognition TLRs suggests that the molecular basis for the immunostimulatory activity of CpG-ODN in teleosts is different and more complex than in mammals. This article reviews the current knowledge of TLR9 and TLR21 activation by CpG-ODNs. The key points that need to be considered for CpG-ODNs as immunostimulants with maximum effectiveness in activation of immune responses in teleosts are discussed. This includes the structure/activity relationship of CpG-ODN activities for TLR9 and TLR21, the structure/functional relationship of these two TLRs, and differential expression levels and tissue distributions for these two TLRs.

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.

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

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

Non-mammalian Toll-like receptor 18 (Tlr18) recognizes bacterial pathogens in common carp (Cyprinus carpio L.): Indications for a role of participation in the NF-κB signaling pathway

Toll-like receptors are important pattern recognition receptors that can recognize pathogen-associated molecular patterns (PAMPs) and play a critical role in innate immunity. In the present study, tlr18 was identified from common carp (Cyprinus carpio L.) (named Cctlr18). The deduced amino acid sequence contained only a signal peptide, eight LRR (leucine-rich repeat) motifs, a transmembrane region and a TIR (Toll/IL-1 receptor) domain. Phylogenetic analysis showed that CcTlr18 was most closely related to Ctenopharyngodon idella Tlr18. Quantitative real-time PCR analysis showed that Cctlr18 was constitutively expressed in all investigated tissues with the highest expression level in the skin and lowest expression in the gonad. After injection with inactivated Aeromonas hydrophila, Cctlr18 expression was significantly up-regulated in the head kidney, foregut, hindgut and skin. Moreover, significant up-regulation of Cctlr8 was observed in the spleen, head kidney, hindgut and skin after immersion with live A. hydrophila. In addition, the expression of Cctlr18 was up-regulated in PGN or flagellin-stimulated HKLs. Luciferase reporter assays showed that Cctlr18 activated NF-κB in 293 T cells and that NF-κB activity was enhanced in Cctlr18 and Ccmyd88 co-transfected cells. Furthermore, Cctlr18 could induce the expression of cytokines genes, including ifn, il-1β and il-10, in EPC cells. The results suggested that Cctlr18 plays an important role in the immune response and provides basic information for investigating the mechanisms of fish tlr18.

Transcriptomic profiling of Tibetan highland fish (Gymnocypris przewalskii) in response to the infection of parasite ciliate Ichthyophthirius multifiliis

Gymnocypris przewalskii is a native cyprinid in the Lake Qinghai of the Qinghai-Tibetan Plateau. G. przewalskii is highly susceptible to the infection of a parasite, Ichthyophthirius multifiliis, in the artificial propagation and breeding. To better understand the host immune reaction to I. multifiliis infection, we characterize the gene expression profiles in the spleen of healthy and I. multifiliis infected G. przewalskii by RNA-seq. Totally, the transcriptomic analysis produces 463,031,110 high quality reads, which are assembled to 213,538 genes with N50 of 1918 bp and the average length of 1205 bp. Of assembled genes, 90.52% are annotated by public databases. The expression analysis shows 744 genes are significantly changed by the infection of I. multifiliis, which are validated by qRT-PCR with the correlation coefficient of 0.896. The differentially expressed genes are classified into 689 GO terms and 230 KEGG pathways, highlighting the promoted innate immunity in I. multifiliis infected G. przewalskii at 2 days post infection. Our results pinpoint that the up-regulated genes are enriched in TLR signaling pathway, inflammatory response and activation of immune cell migration. On the contrary, complement genes are down-regulated, indicating the evasion of host complement cascades by I. multifiliis. The repressed genes are also enriched in the pathways related to metabolism and endocrine, suggesting the metabolic disturbance in I. multifiliis treated G. przewalskii. In summary, the present study profiles the gene expression signature of G. przewalskii in the responses to I. multifiliis infection, and improves our understanding on molecular mechanisms of host-parasite interaction in G. przewalskii, which focuses the crucial function of TLRs, cytokines and complement components in the host defense against I. multifiliis.

Aeromonas hydrophila utilizes TLR4 topology for synchronous activation of MyD88 and TRIF to orchestrate anti-inflammatory responses in zebrafish

Toll-like receptor 4 (TLR4) plays a critical role in host immunity against Gram-negative bacteria. It transduces signals through two distinct TIR-domain-containing adaptors, MyD88 and TRIF, which function at the plasma membrane and endosomes, respectively. Using zebrafish Aeromonas hydrophila infection model, we demonstrate that synchronization of MyD88 and TRIF dependent pathways is critical for determining the fate of infection. Zebrafish were infected with A. hydrophila, and bacterial recovery studies suggested its effective persistence inside the host. Histopathological assessment elucidates that A. hydrophila did not provoke inflammatory responses in the spleen. Immunofluorescence revealed the presence of TLR4-bound A. hydrophila on the plasma membrane at 3 h post-infection (p.i.), and inside endosomes 1 day p.i. Quantitative PCR studies suggest that TLR4 activates the downstream pathway of MyD88–IRAK4 axis at early stages followed by a shift to TRIF–TRAF6 axis at late stages of infection coupled with fold increase in NFκB. Our results implicated the involvement of p110δ isoform of PI(3)Kinase in this transition. Coupled to this, we noted that the TLR4–TRIF–NFκB axis prompted burgeoned secretion of anti-inflammatory cytokines. We observed that A. hydrophila inhibits endosome maturation and escapes to cytoplasm. Significant downregulation of cytosolic-NLR receptors further suggested that A. hydrophila represses pro-inflammatory responses in cytosol aiding its persistence. Our findings suggest a novel role of ‘TLR4 topology’ in A. hydrophila-induced pathogenesis. We propose that A. hydrophila manipulates translocation of TLR4 and migrates to endosome, where it triggers TRIF-dependent anti-inflammatory responses, interferes with endosomal maturation and escapes to cytosol. Inside the cytosol, A. hydrophila avoids detection by suppressing NLRs, facilitating its survival and ensuing pathogenesis.

Expression of immune genes in skin of channel catfish immunized with live theronts of Ichthyophthirius multifiliis

The objective of this study was to evaluate differential expression of innate and adaptive immune genes, including immunoglobulin, immune cell receptor, cytokine, inflammatory protein, toll-like receptors (TLR) and recombination-activating gene (RAG) in skin from channel catfish, Ictalurus punctatus after immunization with live theronts of Ichthyophthirius multifiliis (Ich) by intraperitoneal injection. The immunized catfish showed significantly higher survival rate (95%) than those of mock-immunized control fish (0% survival) after the theront challenge. The gene expression of innate immune system, such as cytokines (IL-1β type a, IL-1β type b, IFN-γ, TGF1-β and TNF-α) and inflammatory proteins (NF-kB and iNOS 2), showed significant upregulation at day 1 (D1) post-immunization. Expression of TLR genes exhibited a rapid increase from hour 4 (h4) to D10 post-immunization. Genes of the adaptive response, such as the cell receptor MHC I, CD8⁺ , CD4⁺ and TCR-α, showed upregulation at D1, D6 and D10. The TCR-β expression increased rapidly at h4 and remained upregulated until D10. Immunoglobulin IgM upregulation was detected from h4 until D2 while IgD expression was increased from D1 until D10. Rapid upregulation of innate and adaptive immune genes in skin of catfish following live theront vaccination was demonstrated in this study ultimately resulting in significant protection against Ich infection.

Identification, molecular evolution of toll-like receptors in a Tibetan schizothoracine fish (Gymnocypris eckloni) and their expression profiles in response to acute hypoxia

Hypoxia plays an important role in regulating a variety of physiological responses as well as in pathological situations, but to date the roles of Toll-like receptors (TLRs) in fish in response to hypoxia are still poorly understood. Here, we sequenced the transcriptome of G. eckloni and identified the members of TLR family by scanning transcriptome, and then investigated the expression profiles of a complete set of TLRs in G. eckloni in response to acute hypoxia (4 h at DO = 0.3 ± 0.1 mg/L). The de novo-assembled transcriptome consisted of a total of 162,235 transcripts, further clustered into 110,231 unigenes. Based on the transcriptome, a total of 18 TLRs were identified in G. eckloni, and of them three TLRs (TLR5, TLR8 and TLR22) possessed two distinct paralogous genes. The duplicated genes of TLR22 were discovered for the first time in cyprinid fish, but did not origin from a recent duplication event. Of them TLR22b may be specific for schizothoracine fish, at least for G. eckloni. Phylogenetic analysis supported the classification of TLRs into six families as in other vertebrates but was partly different from the previous study. The sliding window analysis showed strong signals of positive selection in TLR2, TLR 4, TLR 5a, TLR 7, TLR 19, TLR 20, TLR 21, TLR 22a and TLR 22b, but most codons under positive selection were located in the putative LRR regions. The mRNA expression of most TLRs in head kidney, spleen and gill decreased significantly or remained unchanged under acute hypoxia, whereas acute hypoxia increased expressions of TLR2 and TLR3 in head kidney, of TLR8a, TLR12 and TLR19 in spleen, and of TLR1 in gill, suggesting tissues-specific expressions of TLRs play important roles in mediating innate immune responses for host defense against tissue damages or physiological changes induced by hypoxia.

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.

Expression of immune genes in systemic and mucosal immune tissues of channel catfish vaccinated with live theronts of Ichthyophthirius multifiliis

Ichthyophthiriasis caused by Ichthyophthirius multifiliis (Ich) has a worldwide distribution and affects most freshwater fishes. Fish surviving natural infection and/or immunized with Ich develop strong innate and adaptive immune responses. However, there is a lack of the knowledge regarding immune gene expression patterns in systemic and mucosal immune tissues, and how immune genes interact and lead to innate and adaptive immune protection against Ich infection in fish. The objective of this study was to investigate the expression of innate and adaptive immune-related genes in systemic (liver, spleen) and mucosal (gill, intestine) tissues of channel catfish over time following vaccination with live Ich theronts. The vaccinated fish showed significantly higher antibody titers and survival (95%) than those of mock immunized fish. Expression of IgM and IgD heavy chain genes exhibited a rapid increase from 4 h (h4) to 2 days (d2) post-vaccination in systemic immune tissues. Immune cell receptor genes (CD4, CD8-α, MHC I, MHC II β, TcR-α, and TcR-β) were more highly upregulated and remained upregulated for longer duration in systemic tissues than in mucosal tissues of the vaccinated fish. The cytokine genes IL-1βa and IFN-γ were rapidly upregulated in both systemic and mucosal tissues of vaccinated fish, with peak expression from h4 to d1 post-vaccination. Toll-like receptor genes TLR-1 and TLR-9 showed relatively stable upregulation in the gill of immunized fish following vaccination. Results of this study revealed the molecular immune responses in mucosal and systemic tissues of vaccinated fish and demonstrated that Ich vaccination resulted in innate and adaptive immune responses against Ich infection.

The extent to which immunity, apoptosis and detoxification gene expression interact with 17 alpha-methyltestosterone

Innate immunity is the first line of defence against invasion by foreign pathogens. One widely used synthetic androgen for the production of all-male fish, particularly commercially valuable Nile tilapia, Oreochromis niloticus, is 17 alpha-methyltestosterone (MT). The present study investigates the effect of MT on innate immunity, cellular apoptosis and detoxification and the mortality rate, during and after the feeding of fry with 0-, 40-and 60-mg MT/kg. Expression analysis was completed on interleukin 1 beta (il1β), interleukin 8 (il8), tumour necrosis factor alpha (tnfα), CXC2- and CC-chemokines, interferon (ifn), myxovirus resistance (mx), toll-like receptor 7 (tlr7), immunoglobulin M heavy chain (IgM heavy chain), vitellogenin (vtg), cellular apoptosis susceptibility (cas) and glutathione S-transferase α1 (gstα1). Expression analysis revealed that MT had a significant impact on these genes, and this impact varied from induction to repression during and after the treatment. Linear regression analysis showed a significant association between the majority of the tested gene transcript levels and mortality rates on the 7^th and 21^st days of hormonal treatment and 2 weeks following hormonal cessation. The results are thoroughly discussed in this article. This is the first report concerning the hazardous effect of MT on a series of genes involved in immunity, apoptosis and detoxification in the Nile tilapia fry.

Pattern recognition receptors in zebrafish provide functional and evolutionary insight into innate immune signaling pathways

Pattern recognition receptors (PRRs) and their signaling pathways have essential roles in recognizing various components of pathogens as well as damaged cells and triggering inflammatory responses that eliminate invading microorganisms and damaged cells. The zebrafish relies heavily on these primary defense mechanisms against pathogens. Here, we review the major PRR signaling pathways in the zebrafish innate immune system and compare these signaling pathways in zebrafish and humans to reveal their evolutionary relationship and better understand their innate immune defense mechanisms.

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

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

Characterization of immune-related genes in the yellow catfish Pelteobagrus fulvidraco in response to LPS challenge

Fish are considered an excellent model for studies in comparative immunology as they are a representative population of lower vertebrates linked to invertebrate evolution. To gain a better understanding of the immune response in fish, we constructed a subtractive cDNA library from the head kidney of lipopolysaccharide-stimulated yellow catfish (Pelteobagrus fulvidraco) using suppression subtractive hybridization (SSH). A total of 300 putative EST clones were identified which contained 95 genes, including 27 immune-related genes, 7 cytoskeleton-related genes, 3 genes involved in the cell cycle and apoptosis, 9 respiration and energy metabolism-related genes, 7 genes related to transport, 24 metabolism-related genes, 10 genes involved in stress responses, seven genes involved in regulation of transcription and translation and 59 unknown genes. Using real-time quantitative reverse transcription PCR, a subset of randomly selected genes involved in the immune response to lipopolysaccharide challenge were investigated to verify the reliability of the SSH data which identified 16 up-regulated genes. The genes identified in this study provide novel insight into the immune response in fish.

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.