Characterization of a NK-lysin antimicrobial peptide gene from channel catfish

Expression profiles of four Nile Tilapia innate immune genes during early stages of Aeromonas veronii infection

OBJECTIVE

During Egypt's hot summer season, Aeromonas veronii infection causes catastrophic mortality on Nile Tilapia Oreochromis niloticus farms. Egypt is ranked first in aquaculture production in Africa, sixth in aquaculture production worldwide, and third in global tilapia production. This study aimed to investigate, at the molecular level, the early innate immune responses of Nile Tilapia to experimental A. veronii infection.

METHODS

The relative gene expression, co-expression clustering, and correlation of four selected immune genes were studied by quantitative real-time polymerase chain reaction in four organs (spleen, liver, gills, and intestine) for up to 72 h after a waterborne A. veronii challenge. The four genes studied were nucleotide-binding oligomerization domain 1 (NOD1), lipopolysaccharide-binding protein (LBP), natural killer-lysin (NKL), and interleukin-1 beta (IL-1β).

RESULT

The four genes showed significant transcriptional upregulation in response to infection. At 72 h postchallenge, the highest NOD1 and IL-1β expression levels were recorded in the spleen, whereas the highest LBP and NKL expression levels were found in the gills. Pairwise distances of the data points and the hierarchical relationship showed that NOD1 clustered with IL-1β, whereas LBP clustered with NKL; both genes within each cluster showed a significant positive expression correlation. Tissue clustering indicated that the responses of only the gill and intestine exhibited a significant positive correlation.

CONCLUSION

The results suggest that NOD1, LBP, NKL, and IL-1β genes play pivotal roles in the early innate immune response of Nile Tilapia to A. veronii infection, and the postinfection expression profile trends of these genes imply tissue-/organ-specific responses and synchronized co-regulation.

New insight into the biological activity of Salmo salar NK-lysin antimicrobial peptides

NK-lysin is a potent antimicrobial peptide (AMP) with antimicrobial activity against bacteria, fungi, viruses, and parasites. NK-lysin is a type of granulysin, a member of the saposin-like proteins family first isolated from a pig's small intestine. In previous work, for the first time, we identified four variants of nk-lysin from Atlantic salmon (Salmo salar) using EST sequences. In the present study, we reported and characterized two additional transcripts of NK-lysin from S. salar. Besides, we evaluated the tissue distribution of three NK-lysins from S. salar and assessed the antimicrobial, hemolytic, and immunomodulatory activities and signaling pathways of three NK-lysin-derived peptides. The synthetic peptides displayed antimicrobial activity against Piscirickettsia salmonis (LF-89) and Flavobacterium psychrophilum. These peptides induced the expression of immune genes related to innate and adaptive immune responses in vitro and in vivo. The immunomodulatory activity of the peptides involves the mitogen-activated protein kinases-mediated signaling pathway, including p38, extracellular signal-regulated kinase 1/2, and/or c-Jun N-terminal kinases. Besides, the peptides modulated the immune response induced by pathogen-associated molecular patterns (PAMPs). Our findings show that NK-lysin could be a highly effective immunostimulant or vaccine adjuvant for use in fish aquaculture.

Antibacterial and anticancer activity of two NK-lysin-derived peptides from the Antarctic teleost Trematomus bernacchii

The NK-lysin antimicrobial peptide, first identified in mammals, possesses both antibacterial and cytotoxic activity against cancer cell lines. Homologue peptides isolated from different fish species have been examined for their functional characteristics in the last few years. In this study, a NK-lysin transcript was identified in silico from the head kidney transcriptome of the Antarctic teleost Trematomus bernacchii. The corresponding amino acid sequence, slightly longer than NK-lysins of other fish species, contains six cysteine residues that in mammalian counterparts form three disulphide bridges. Real time-PCR analysis indicated its predominant expression in T. bernacchii immune-related organs and tissues, with greatest mRNA abundance detected in gills and spleen. Instead of focusing on the full T. bernacchii derived NK-lysin mature molecule, we selected a 27 amino acid residue peptide (named NKL-WT), corresponding to the potent antibiotic NK-2 sequence found in human NK-lysin. Moreover, we designed a mutant peptide (named NKL-MUT) in which two alanine residues substitute the two cysteines found in the NKL-WT. The two peptides were obtained by solid phase organic synthesis to investigate their functional features. NKL-WT and NKL-MUT displayed antibacterial activity against the human pathogenic bacterium Enterococcus faecalis and the ESKAPE pathogen Acinetobacter baumannii, respectively. Moreover, at the determined Minimum Inhibitory Concentration and Minimum Bactericidal Concentration values against these pathogens, both peptides showed high selectivity as they did not exhibit any haemolytic activity on erythrocytes or cytotoxic activity against mammalian primary cell lines. Finally, the NKL-MUT selectively triggers the killing of the melanoma cell line B16F10 by means of a pro-apoptotic pathway at a concentration range in which no effects were found in normal mammalian cell lines. In conclusion, the two peptides could be considered as promising candidates in the fight against antibiotic resistance and tumour proliferative action, and also be used as innovative adjuvants, either to decrease chemotherapy side effects or to enhance anticancer drug activity.

Antimicrobial activity and mechanisms of a derived antimicrobial peptide TroNKL-27 from golden pompano (Trachinotus ovatus) NK-lysin

NK-lysin, a homologue of granulysin among human, is predominantly found in natural killer cells and cytotoxic T-lymphocytes, which plays a pivotal part in innate immune responses against diverse pathogenic bacteria. Nonetheless, in teleosts, the research on antimicrobial activity and mechanisms of NK-lysin are seldom reported. In this study, we determined the antimicrobial activity of the truncated peptide TroNKL-27 that derived from golden pompano (Trachinotus ovatus) NK-lysin, and investigated its antimicrobial mechanisms. The results showed that TroNKL-27 had considerable antimicrobial potency against both Gram-positive (Staphylococcus aureus, Streptococcus agalactiae) and Gram-negative bacteria (Vibrio harveyi, V. alginolyticus, Escherichia coli, Edwardsiella tarda). Cytoplasmic membrane depolarization and propidium iodide (PI) uptake assay manifested that TroNKL-27 could induce the bacterial membrane depolarization and change its membrane permeability, respectively. In the light of scanning electron microscopy (SEM) observation, TroNKL-27 was capable of altering morphological structures of bacteria and leading to leakage of cellular contents. Moreover, the results of gel retardation assay indicated TroNKL-27 had the ability to induce the degradation of bacterial genomic DNA. As regards in vivo assay, TroNKL-27 could reduce the replication of V. harveyi in tissues of golden pompano, protect the tissue from pathological changes. Moreover, TroNKL-27 in vivo could significantly increase the expression of the immune genes (such as IL1β, TNFα, IFN-γ, C3 and Mx) in presence or absence of V. harveyi infection. All of these results suggest that TroNKL-27 is a novel antimicrobial peptide possessing antibacterial and immunoregulatory function in vivo and in vitro, and the observed effects of TroNKL-27 will lay a solid foundation for the development of new antimicrobial agents used in aquaculture.

Antimicrobial and immunoregulatory activities of the derived peptide of a natural killer lysin from black rockfish (Sebastes schlegelii)

Natural killer lysin (NK-lysin) is a small molecule antimicrobial peptide secreted by natural killer cells and T lymphocytes. In this study, we characterized a cDNA sequence encoding an NK-lysin homologue (SsNKL1) from black rockfish, Sebastes schlegelii. The open reading frame (ORF) of SsNKL1 encodes a putative protein of 149 amino acids and shares 44%-87% overall sequence identities with other teleost NK-lysins. SsNKL1 possesses conserved NK-lysin family features, including a signal sequence and a surfactant-associated protein B (SapB) domain, sequence analysis revealed that SsNKL1 is most closely related to false kelpfish (Sebastiscus marmoratus) NK-lysin (with 87% sequence identity). SsNKL1 transcripts were detected in all the tested tissues, with the highest level in the kidney, followed by the spleen and gills. Upon Listonella anguillarum infection, the mRNA expression of SsNKL1 in the black rockfish was significantly up-regulated in the liver and kidney. The derived peptide SsNKLP27 from SsNKL1 was synthesized, and its biological function was studied. SsNKLP27 showed direct antibacterial activity against Gram-negative and Gram-positive bacteria, including Staphylococcus aureus, Bacillus subtilis, L. anguillarum, Vibrio parahaemolyticus, Vibrio alginolyticus and Vibrio vulnificus. SsNKLP27 treatment facilitated the bactericidal process of erythromycin by enhancing the permeability of the outer membrane. In the process of interaction with the target bacterial cells, SsNKLP27 changed the permeability and retained the morphological integrity of the cell membrane, then penetrated into the cytoplasm, and induced the degradation of genomic DNA and total RNA. In vivo studies showed that administration of SsNKLP27 before bacterial and viral infection significantly reduced the transmission and replication of pathogens in tissues. In vitro analysis showed that SsNKLP27 could enhance the respiratory burst ability and regulate the expression of some immune-related genes of macrophages. In summary, these results provided new insights into the function of NK-lysins in teleost fish and support that SsNKLP27 is a new broad-spectrum antimicrobial peptide that has a potential application prospect in aquaculture against pathogenic infection.

Barbel steed (Hemibarbus labeo) NK-lysin protects against Aeromonas hydrophila infection via immunomodulatory activity

NK-lysins (NKLs) are a family of multifunctional antimicrobial peptides that have activity against various microorganisms. However, the immunomodulatory activity of NKL in fish remains unclear. In this study, the cDNA sequence of barbel steed (Hemibarbus labeo) NKL gene was cloned. Barbel steed NKL amino acid sequence comprised a signal peptide and a mature peptide. The saposin B domain in the mature peptide has six conserved cysteines that form three disulfide bonds. Phylogenetic analysis showed that the barbel steed NKL was most closely related to that of the common carp (Cyprinus carpio) NKL. Differential expression analysis showed that the barbel steed NKL gene was expressed in all tested tissues, with the highest expression in the spleen. In response to Aeromonas hydrophila infection, NKL was significantly upregulated in the liver, spleen, head kidney, and gill. The barbel steed NKL showed strong antibacterial activity against Vibrio parahaemolyticus, V. alginolyticus, V. vulnificus, and Listeria monocytogenes. However, NKL had no antibacterial activity against the pathogenic bacteria A. hydrophila. Lactate dehydrogenase release assays showed that NKL damaged the V. parahaemolyticus cell membrane. NKL significantly increased barbel steed survival rate after A. hydrophila infection and upregulated IL-1β and TNF-α expression in the spleen and head kidney. NKL induced monocyte/macrophage chemotaxis and enhanced the respiratory burst and proinflammatory cytokine expression. Our study shows that fish NKL exhibits immunomodulatory effects and protects the host from pathogenic infections independent of direct bacterial clearance.

A novel NK-lysin in hybrid crucian carp can exhibit cytotoxic activity in fish cells and confer protection against Aeromonas hydrophila infection in comparison with Carassius cuvieri and Carassius auratus red var

NK-lysin, an effector of natural killer (NK) cells and cytotoxic T lymphocytes (CTLs), not only exhibits cytotoxic effect in fish cells, but also participates in the immune defense against pathogenic infection. In this study, ORF sequences of RCC-NK-lysin, WCC-NK-lysin and WR-NK-lysin were 369 bp. Tissue-specific analysis revealed that the highest expressions of RCC-NK-lysin and WCC-NK-lysin were observed in gill, while the peaked level of WR-NK-lysin mRNA was observed in spleen. A. hydrophila infection sharply increased RCC-NK-lysin, WCC-NK-lysin and WR-NK-lysin mRNA expression in liver, trunk kidney and spleen. In addition, elevated levels of NK-lysin mRNA were observed in cultured fin cell lines of red crucian carp (RCC), white crucian carp (WCC) and their hybrid offspring (WR) after Lipopolysaccharide (LPS) challenge. RCC-NK-lysin, WCC-NK-lysin and WR-NK-lysin exerted regulatory roles in inducing ROS generation, modulating mitochondrial membrane potential, decreasing fish cell viability and antagonizing survival signalings, respectively. RCC/WCC/WR-NK-lysin-overexpressing fish could up-regulate expressions of inflammatory cytokines and decrease bacterial loads in spleen. These results indicated that NK-lysin in hybrid fish contained close sequence similarity to those of its parents, possessing the capacities of cytotoxicity and immune defense against bacterial infection.

Identification and characterization of SaRpAMP, a 60S ribosomal protein L27-derived antimicrobial peptide from amur catfish, Silurus asotus

Aquatic freshwater fish like catfish, Silurus asotus, lives in microbe-rich environments, which enable this fish to develop necessary defense mechanisms. Antimicrobial peptides, along with other innate immune factors, are regarded as an important group in this defense. An antimicrobial peptide, which was isolated from the skin of S. asotus, was identified as a C-terminal fragment of 60S ribosomal protein L27 from S. asotus. The peptide was, then, designated Silurus asotus 60S ribosomal protein L27-derived antimicrobial peptide, SaRpAMP. Primary structure analyses and cDNA cloning revealed that SaRpAMP was 4185.36 Da and composed of 33 amino acids (AAs). Its precursor had a total of 136 AAs containing a pro-sequence of 103 AAs encoded by the nucleotide sequence of 512 bp that comprises a 5' untranslated region (UTR) of 32 bp, an open reading frame (ORF) of 411 bp, and a 3' UTR of 69 bp. Secondary structure analyses showed that SaRpAMP had two α-helices with turns and coils and an amphiphilic structure, a finding consistent with the 3D model of the peptide. SaRpAMP exhibited potent antibacterial activity comparable to piscidin 1, a powerful positive control. Its antimicrobial activity against fungus C. albicans was relatively weak. The antimicrobial activity of SaRpAMP was not diminished by heat treatment and changes in pH but was abolished by proteolytic enzyme digestion. Membrane permeability assays suggested that SaRpAMP interacts with both the outer and inner bacterial membranes. This was consistent with the results of lipid titration and quenching of Trp fluorescence that demonstrated SaRpAMP's interaction with acidic liposomes. Collectively, these findings suggest that the identified peptide, SaRpAMP, was the first antimicrobial peptide reported to be derived from the C-terminal region of 60S ribosomal protein L27. The findings also suggest that the action mechanism of SaRpAMP involved the interaction of the peptide with the bacterial membranes.

Defense involvement of piscidin from striped murrel Channa striatus and its peptides CsRG12 and CsLC11 involvement in an antimicrobial and antibiofilm activity

In this study, we have evaluated bioinformatics characterization and antimicrobial role of two piscidin (Pi) peptide identified from the established transcriptome of striped murrel Channa striatus (Cs). The identified CsPi cDNA contains 256 nucleotides encode a protein with 70 amino acids in length which has two antimicrobial peptides and named CsRG12 and CsLC11. The gene expression analysis with various immune stimulants indicated an induced expression pattern of CsPi. Antibiogram showed that CsRG12 and CsLC11 was active against Staphylococcus aureus ATCC 33592, a major multi-drug resistant (MDR) bacterial pathogen and Bacillus cereus ATCC 2106. The minimum inhibitory concentration (MIC) and antibiofilm assays were conducted to observe the activity of pathogenic bacteria with these derived antimicrobial peptides. Flow cytometry analysis noticed that the CsRG12 and CsLC11 disrupt the membrane formation of S. aureus and B. cereus, which was further assured by scanning electron microscopic (SEM) images that bleb formation leads to disruption around the bacterial membrane. Overall, it is reported that CsPi is involved in innate immunity as the gene expression plays a remarkable role in up and down regulation during infection. In addition, the involvement of peptides in antibiofilm formation and bacterial membrane disruption support its immune character. This study leads to a possibility for the development of therapeutics in aquaculture biotechnology.

Membrane disruptive antimicrobial potential of NK-lysin from yellow catfish (Pelteobagrus fulvidraco)

NK-lysins, a type of broad-spectrum antimicrobial peptide (AMP), act as an essential effector of innate defense against microbial attack in higher vertebrates and so in fish. The present study delineates the structural and functional characterization of NK-lysin from yellow catfish (Pelteobagrus fulvidrac) (Pelteobagrus fulvidraco). PfNK-lysin encodes a 153-residue peptide, which displays the hallmark features of other known NK-lysins with the ordered array of six well-conserved cysteine residues and five-exon/four-intron structure. It was found to be ubiquitous in tissues, being detected most abundantly in gill and head kidney. In vivo exposure to stimuli (LPS, PolyI:C, and Edwardsiella ictaluri) induced PfNK-lysin expression in head kidney and spleen. Synthetic PfNK-lysin-derived peptide exhibited in vitro bactericidal potency against both Gram-positive and Gram-negative bacteria, with the highest inhibitory effect on pathogen Edwardsiella ictaluri. Fluorescence microscopy and scanning electron microscopy further confirmed its capacity to cause damage to the bacterial plasma membrane. Taken together, these data suggest that PfNK-lysin might participate in antimicrobial defense of yellow catfish by membrane-disruptive action.

Identification and characterization of five Nk-lysins from Pseudocrossocheilus bamaensis and their diverse expression patterns in response to bacterial infection

Nk-lysin is an effector protein of cytotoxic T lymphocytes and natural killer cells. It is known to possess anti-bacterial, anti-fungal, anti-viral, and anti-tumor activity. Here we describe five Nk-lysin genes (PbNkla, PbNklb, PbNklc, PbNkld, and PbNkle) from Pseudocrossocheilus bamaensis, a rare indigenous species distributed in Guangxi, China. The open reading frames (ORFs) consisted of 426 (PbNkla), 435 (PbNklb), 369 (PbNklc), 366 (PbNkld), and 339 (PbNkle) bp nucleic acids. The surfactant-associated protein B (SapB) domain and six conserved cysteine residues were identified in each PbNkl gene. Phylogenetic analysis revealed similar results to homology comparison that PbNkla and PbNklb consist of five exons and four introns and grouped together, whereas PbNklc and PbNkld each contain four exons and three introns and formed a separate clade. PbNkle had three exons and two introns and formed an independent clade separate from the four other PbNkls. qPCR analysis demonstrated that PbNkla, PbNklc, PbNkld, and PbNkle were ubiquitously expressed in all tissues examined, whereas PbNklb was expressed only after bacterial infection. Aeromonas hydrophila challenge significantly up- and down-regulated PbNkls at different time points post-injection and in different immune-related tissues. These results suggested that PbNkls were conserved immune molecules that may be involved in the immune response to pathogen invasion.

Molecular characterization of the NK-lysin in a teleost fish, Boleophthalmus pectinirostris: Antimicrobial activity and immunomodulatory activity on monocytes/macrophages

NK-lysin (NKL) is a cationic host defense peptide that plays an important role in host immune responses against various pathogens. However, the immunomodulatory activity of NKL in fishes is rarely investigated. In this study, we characterized a cDNA sequence encoding an NK-lysin homolog (BpNKL) from the fish, mudskipper (Boleophthalmus pectinirostris). Sequence analysis revealed that BpNKL is most closely related to tiger puffer (Takifugu rubripes) NKL. BpNKL transcript was detected in all the tested tissues, with the highest level in the gill, followed by the spleen and kidney. Upon Edwardsiella tarda infection, the mRNA expression of BpNKL in the mudskipper was significantly upregulated in the spleen, kidney, and gill. A shortened peptide derived from BpNKL, BpNKLP40, was then chemically synthesized and its biological functions were investigated. BpNKLP40 exhibited a direct antibacterial activity against some Gram-negative bacteria, including E. tarda, Vibrio parahaemolyticus, Vibrio alginolyticus, and Vibrio harveyi, and induced hydrolysis of E. tarda genomic DNA. Intraperitoneal injection of 1.0 μg/g BpNKLP40 significantly improved the survival of mudskipper following E. tarda infection and reduced the bacterial burden in tissues and blood. Moreover, 1.0 μg/ml BpNKLP40 treatment had an enhanced effect on the intracellular killing of E. tarda by monocytes/macrophages (MO/MФ) as well as on the mRNA expression of pro-inflammatory cytokines in MO/MФ. In conclusion, our study reveals that BpNKL plays a role against E. tarda infection in the mudskipper by not only directly killing bacteria but also through an immunomodulatory activity on MO/MФ.

In vitro immunomodulatory activities of peptides derived from Salmo salar NK-lysin and cathelicidin in fish cells

Antimicrobial peptides (AMPs) are amphipathic peptides, which play an important role in innate defence. These peptides are gene-encoded and either constitutively expressed and/or upregulated during an infection. NK-lysins are AMPs with a three-dimensional globular structure. They are larger molecules, which comprise 74-78 amino acid residues and six conserved cysteine residues forming three disulphide bonds. Cathelicidins are a family of antimicrobial peptides that act as important components of the innate immune system with a broad spectrum of antimicrobial activity and immunomodulatory properties. Although they are widely studied in mammals, little is known about their immunomodulatory function. In the present study, we identified and characterized for the first time four NK-lysin-like transcripts from Atlantic salmon (Salmo salar) based on EST reported sequences. In vitro, NK-lysin derived peptides were able to induce the expression of IL-1β and IL-8 in Salmo salar head kidney leukocytes. We also tested Salmo salar cathelicidin 1 derived peptide in a similar assay, showing its ability to induce the expression of IFN-γ. These results indicate that NK-lysin and cathelicidin 1 derived peptides are able to modulated immune response, suggesting their potential use to enhance immune response in fish.

Antiviral Activity of a Turbot (Scophthalmus maximus) NK-Lysin Peptide by Inhibition of Low-pH Virus-Induced Membrane Fusion

Global health is under attack by increasingly-frequent pandemics of viral origin. Antimicrobial peptides are a valuable tool to combat pathogenic microorganisms. Previous studies from our group have shown that the membrane-lytic region of turbot (Scophthalmus maximus) NK-lysine short peptide (Nkl_71–100) exerts an anti-protozoal activity, probably due to membrane rupture. In addition, NK-lysine protein is highly expressed in zebrafish in response to viral infections. In this work several biophysical methods, such as vesicle aggregation, leakage and fluorescence anisotropy, are employed to investigate the interaction of Nkl_71–100 with different glycerophospholipid vesicles. At acidic pH, Nkl_71–100 preferably interacts with phosphatidylserine (PS), disrupts PS membranes, and allows the content leakage from vesicles. Furthermore, Nkl_71–100 exerts strong antiviral activity against spring viremia of carp virus (SVCV) by inhibiting not only the binding of viral particles to host cells, but also the fusion of virus and cell membranes, which requires a low pH context. Such antiviral activity seems to be related to the important role that PS plays in these steps of the replication cycle of SVCV, a feature that is shared by other families of virus-comprising members with health and veterinary relevance. Consequently, Nkl_71–100 is shown as a promising broad-spectrum antiviral candidate.

Identification, expression analysis, and antibacterial activity of NK-lysin from common carp Cyprinus carpio

Natural killer lysin (NK-lysin), produced by cytotoxic T lymphocytes and natural killer cells, is a cationic antimicrobial peptide that has a broad antimicrobial spectrum, including bacteria, viruses, and parasites. Nevertheless, the implication of NK-lysin in the protection against bacterial infection is not aware in common carp. In this study, six different NK-lysin genes (nkl1, nkl2, nkl3, nkl4, nkl5 and nkl6) were identified in the common carp genome. Each of the mature peptides of common carp NK-lysin has six well-conserved cysteine residues, and shares a Saposin B domain, characteristic of saposin-like protein (SALIP) family. The gene nkl1 contains 5 extrons and 4 introns, and nkl2, nkl3, nkl4 or nkl5 contains 4 extrons and 3 introns, however, the nkl6 has 3 extrons and 2 introns. By quantitative real-time PCR, nkl2 transcripts were predominantly expressed in spleen of healthy common carp, while elevated mainly in gill and spleen upon Aeromonas hydrophila infection. The recombinant NK-lysin-2 purified from Pichia pastoris shows antibacterial activity against Staphylococcus aureus (Gram-positive), and Escherichia coli M15, Aeromonas hydrophila, as well as Edwardsiella tarda (Gram-negative), the latter two are important pathogens of aquaculture. Our results indicate that NK-lysin in common carp might play an important role in fish immune response by enhancing antibacterial defense against bacterial pathogens.

Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ+ CD83+ Antigen-Presenting Cells

The histozoic myxozoan parasite Kudoa thyrsites causes postmortem myoliquefaction and is responsible for economic losses to salmon aquaculture in the Pacific Northwest. Despite its importance, little is known about the host-parasite relationship, including the host response to infection. The present work sought to characterize the immune response in Atlantic salmon during infection, recovery, and reexposure to K. thyrsites After exposure to infective seawater, infected and uninfected smolts were sampled three times over 4,275 degree-days. Histological analysis revealed infection severity decreased over time in exposed fish, while in controls there was no evidence of infection. Following a secondary exposure of all fish, severity of infection in the controls was similar to that measured in exposed fish at the first sampling time but was significantly reduced in reexposed fish, suggesting the acquisition of protective immunity. Using immunohistochemistry, we detected a population of MHIIβ+ cells in infected muscle that followed a pattern of abundance concordant with parasite prevalence. Infiltration of these cells into infected myocytes preceded destruction of the plasmodium and dissemination of myxospores. Dual labeling indicated a majority of these cells were CD83+/MHIIβ+ Using reverse transcription-quantitative PCR, we detected significant induction of cellular effectors, including macrophage/dendritic cells (mhii/cd83/mcsf), B cells (igm/igt), and cytotoxic T cells (cd8/nkl), in the musculature of infected fish. These data support a role for cellular effectors such as antigen-presenting cells (monocyte/macrophage and dendritic cells) along with B and T cells in the acquired protective immune response of Atlantic salmon against K. thyrsites.

Transcriptome analysis of yellow catfish (Pelteobagrus fulvidraco) liver challenged with polyriboinosinic polyribocytidylic acid (poly I:C)

Yellow catfish (Pelteobagrus fulvidraco) is one of the most important economic freshwater species in China. However, infection by bacterial pathogenic diseases has caused high mortality and great economic loss in aquaculture. It is necessary for disease control to know more about the P. fulvidraco immune system and its related genes in response to bacterial or viral infections. In this study, the transcriptomic profiles of liver from P. fulvidraco stimulated by polyriboinosinic polyribocytidylic acid (poly I:C) was analyzed using high-throughput sequencing method. After assembly and annotation, total 67,447 unigenes were acquired, with an average length of 1091 bp. Under the infection of poly I:C, 522 differentially expressed genes (DEGs) were identified, including 307 up-regulated genes and 215 down-regulated genes. To further investigate the immune-related DEGs, Gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment were performed. The result of GO enrichment indicated gene response to external stimulus, regulation of response to stimulus, cellular response to stimulus, immune response and immune system progress. Significant KEGG enrichment analysis identified major immune related pathways. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) analysis revealed that 13 immune response genes were identified to be up-regulated after 12 h of poly I:C stimulation compared to controls. Taken together, the results of our study are beneficial for better understanding of the immune system and defense mechanisms of yellow catfish in response to poly I:C infection.

A review on comparative mechanistic studies of antimicrobial peptides against archaea

Archaea was until recently considered as a third domain of life in addition to bacteria and eukarya but recent studies support the existence of only two superphyla (bacteria and archaea). The fundamental differences between archaeal, bacterial, and eukaryal cells are probably the main reasons for the comparatively lower susceptibility of archaeal strains to current antimicrobial agents. The possible emerging pathogenicity of archaea and the role of archaeal methanogens in methane emissions, a potent greenhouse gas, has led many researchers to examine the sensitivity patterns of archaea and make attempts to find agents that have significant anti-archaeal activity. Even though antimicrobial peptides (AMPs) are well known with several published reviews concerning their mode of action against bacteria and eukarya, to our knowledge, to date no reviews are available that focus on the action of these peptides against archaea. Herein, we present a review on all the peptides that have been tested against archaea. In addition, in an attempt to shed more light on possible future work that needs to be performed we have included a brief overview of the chemical characteristics, spectrum of activity, and the known mechanism of action of each of these peptides against bacteria and/or fungi. We also discuss the nature of and key physiological differences between Archaea, Bacteria, and Eukarya that are relevant to the development of anti-archaeal peptides. Despite our relatively limited knowledge about archaea, available data suggest that AMPs have an even broader spectrum of activity than currently recognized.

Characterization of Two Antimicrobial Peptides from Antarctic Fishes (Notothenia coriiceps and Parachaenichthys charcoti)

We identified two antimicrobial peptides (AMPs) with similarity to moronecidin in Antarctic fishes. The characteristics of both AMPs were determined using moronecidin as a control. Moronecidin, which was first isolated from hybrid striped bass, is highly salt-resistant, and possesses broad-spectrum activity against various microbes. The moronecidin-like peptide from Notothenia coriiceps exhibited a narrower spectrum of activity and a higher salt sensitivity than moronecidin. The AMP from Parachaenichthys charcoti exhibited similar antimicrobial activity to moronecidin, and similar salt sensitivity. In an experiment to identify toxic effects, both of the moronecidin-like peptides from the Antarctic fishes exhibited lower hemolytic activity than moronecidin. In spite of its low toxicity, the AMP from N. coriiceps is unlikely to be considered as a candidate for antibiotic development, owing to its narrow spectrum of activity and high salt sensitivity. In contrast, the high salt resistance and broad-spectrum activity of the AMP from P. charcoti could be more advantageous for clinical use than moronecidin, since it could kill bacteria under physiological conditions with low toxicity. A further comparison of these two AMPs from Antarctic fishes with other AMPs could help to reduce the toxicity of AMPs for clinical use.

The chemokinome superfamily in channel catfish: I. CXC subfamily and their involvement in disease defense and hypoxia responses

Chemokines are a superfamily of structurally related chemotactic cytokines exerting significant roles in regulating cell migration and activation. They are defined by the presence of four conserved cysteine residues and are divided into four subfamilies depending on the arrangement of the first two conserved cysteines residues: CXC, CC, C and CX3C. In this study, a complete set of 17 CXC chemokine ligand (CXCL) genes was systematically identified and characterized from channel catfish genome through data mining of existing genomic resources. Phylogenetic analysis allowed annotation of the 17 CXC chemokines. Extensive comparative genomic analyses supported their annotations and orthologies, revealing the existence of fish-specific CXC chemokines and the expansion of CXC chemokines in the teleost genomes. The analysis of gene expression after bacterial infection indicated the CXC chemokines were expressed in a gene-specific manner. CXCL11.3 and CXCL20.3 were expressed significantly higher in resistant fish than in susceptible fish after ESC infection, while CXCL20.2 were expressed significantly higher in resistant fish than in susceptible fish after columnaris infection. The expression of those CXC chemokines, therefore can be a useful indicator of disease resistance. A similar pattern of expression was observed between resistant and susceptible fish with biotic and abiotic stresses, ESC, columnaris and hypoxia, suggesting that high levels of expression of the majority of CXC chemokines, with exception of CXC11 and CXC20, are detrimental to the host.

cDNA cloning and expression analysis of a hepcidin gene from yellow catfish Pelteobagrus fulvidraco (Siluriformes: Bagridae)

Hepcidin is a small, cysteine-rich antimicrobial peptide with a highly conserved β-sheet structure that plays a vital role in innate host immunity against pathogenic organisms. In this study, a hepcidin gene was identified in Pelteobagrus fulvidraco, an economically important freshwater fish in China. The gene is named PfHep. The complete PfHep cDNA was 723 bp, including a 5'-untranslated region (UTR) of 102 bp, a 3'-UTR of 339 bp and an open reading frame of 282 bp encoding a polypeptide of 93 amino acids, which includes a predicted signal peptide and the Hepcidin domain. The predicted mature, cationic PfHep protein has a typical hepcidin RX (K/R)R motif and eight conserved cysteine residues. The deduced PfHep protein sequence has 70%, 54% and 39% percent identity with hepcidins from Ictalurus punctatus, Danio rerio, and Homo sapiens, respectively. The predicted tertiary structure of PfHep is very similar to that of hepcidin in other animals. Phylogenetic analysis revealed that PfHep is closely related to the hepcidins of I. punctatus and I. furcatus. Real-time quantitative reverse transcription-PCR showed that the PfHep gene was expressed most in liver of healthy P. fulvidraco, and expressed to some extent in all the tissues tested. After challenge with lipopolysaccharide and polyriboinosinic:polyribocytidylic acid (poly I:C), respectively, the expression levels of PfHep were markedly upregulated in liver, spleen, head kidney and blood at different time points. Together these results imply that PfHep may be an important component of the innate immune system and be involved in immune defense against invading pathogens.

Identification of differentially expressed genes in the spleens of polyriboinosinic polyribocytidylic acid (poly I:C)-stimulated yellow catfish Pelteobagrus fulvidraco

The yellow catfish, Pelteobagrus fulvidraco (Siluriformes: Bagridae) is an economically important fish in China. However, genomic research and resources on this species are largely unavailable and still in infancy. In the present study, we constructed a cDNA library following poly I:C injection to screen for immune response genes in the spleens of P. fulvidraco using suppression subtractive hybridization (SSH). A total of 420 putative expressed sequence tag (EST) clones were identified at 24 h post-injection, which contain 103 genes consisting of 25 immune response genes, 12 cytoskeleton genes, 7 cell cycle and apoptosis genes, 7 respiration and energy metabolism genes, 7 transport genes, 26 metabolism genes, 10 stress response genes, 9 translational regulation genes, and 71 unknown genes. Real-time quantitative reverse transcription-PCR (qRT-PCR) results revealed that a set of randomly selected immune response genes were identified to be up-regulated after 24 h of poly I:C stimulation compared to controls. Our study provides an annotation of immune genes in detail and insight into fish immunity.

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, expression and antibacterial activities of an antimicrobial peptide NK-lysin from a marine fish Larimichthys crocea

As fundamental immunologic mechanism, the innate immunity system is more important than the specific immunity system in teleost fishes during pathogens infection. Antimicrobial peptides are integral parts of the innate immune system, and play significant roles against pathogens infection. NK-lysin, the compounds of the natural killer cells and cytotoxic T cells, are potent and effective antimicrobial peptides widely distributed in animals. In this study, we reported the sequence characteristics, expression profiles and antibacterial activities of a NK-lysin gene (Lc-NK-lysin) from a commercially important marine fish, the large yellow croaker (Larimichthys crocea). The open reading frame of Lc-NK-lysin cDNA sequence was 447 bp in length, coding 148 amino acids. The genomic DNA of Lc-NK-lysin has the common features of NK-lysin family, consisting of five exons and four introns, and in its deduced mature peptide, there are six well-conserved cysteine residues and a Saposin B domain. Lc-NK-lysin was expressed in all tested tissues (skin, muscle, gill, brain, head kidney, heart, liver, spleen, stomach and intestine) with different expression patterns. In pathogens infection the expression profiles of Lc-NK-lysin varied significantly in gill, head kidney, spleen and liver, indicating its role in immune response. Two peptides (Lc-NK-lysin-1 and Lc-NK-lysin-2) divided from the core region of the Lc-NK-lysin mature polypeptide were chemically synthesized and their antibacterial activities were examined; the potential function on the inhibition of bacteria propagation was revealed. Our results suggested that Lc-NK-lysin is a typical member of the NK-lysin family and as an immune-related gene it involves in the immune response when pathogens invasion.

Septin genes in channel catfish (Ictalurus punctatus) and their involvement in disease defense responses

Septins are an evolutionarily conserved family of GTP-binding proteins. They are involved in diverse processes including cytokinesis, apoptosis, infection, neurodegeneration and neoplasia. In this study, through thorough data mining of existed channel catfish genomic resources, we identified a complete set of 15 septin genes. Septins were classified into four subgroups according to phylogenetic analysis. Extensive comparative genomic analysis, including domain and syntenic analysis, supported their annotation and orthologies. The expression patterns of septins in channel catfish were examined in healthy tissues and after infection with two major bacterial pathogens, Edwardsiella ictaluri and Flavobacterium columnare. In healthy channel catfish, most septin genes were ubiquitously expressed and presented diversity patterns in various tissues, especially mucosal tissues, proposing the significant roles septin genes may play in maintaining homeostasis and host immune response activities. After bacterial infections, most septin genes were regulated, but opposite direction in expression profiles were found with the two bacterial pathogens: the differentially expressed septin genes were down-regulated in the intestine after E. ictaluri infection while generally up-regulated in the gill after F. columnare infection, suggesting a pathogen-specific and tissue-specific pattern of regulation. Taken together, these results suggested that septin genes may play complex and important roles in the host immune responses to bacterial pathogens in channel catfish.

Bovine NK-lysin: Copy number variation and functional diversification

NK-lysin is an antimicrobial peptide and effector protein in the host innate immune system. It is coded by a single gene in humans and most other mammalian species. In this study, we provide evidence for the existence of four NK-lysin genes in a repetitive region on cattle chromosome 11. The NK2A, NK2B, and NK2C genes are tandemly arrayed as three copies in ∼30-35-kb segments, located 41.8 kb upstream of NK1. All four genes are functional, albeit with differential tissue expression. NK1, NK2A, and NK2B exhibited the highest expression in intestine Peyer's patch, whereas NK2C was expressed almost exclusively in lung. The four peptide products were synthesized ex vivo, and their antimicrobial effects against both Gram-positive and Gram-negative bacteria were confirmed with a bacteria-killing assay. Transmission electron microcopy indicated that bovine NK-lysins exhibited their antimicrobial activities by lytic action in the cell membranes. In summary, the single NK-lysin gene in other mammals has expanded to a four-member gene family by tandem duplications in cattle; all four genes are transcribed, and the synthetic peptides corresponding to the core regions are biologically active and likely contribute to innate immunity in ruminants.

Complement regulatory protein genes in channel catfish and their involvement in disease defense response

Complement system is one of the most important defense systems of innate immunity, which plays a crucial role in disease defense responses in channel catfish. However, inappropriate and excessive complement activation could lead to potential damage to the host cells. Therefore the complement system is controlled by a set of complement regulatory proteins to allow normal defensive functions, but prevent hazardous complement activation to host tissues. In this study, we identified nine complement regulatory protein genes from the channel catfish genome. Phylogenetic and syntenic analyses were conducted to determine their orthology relationships, supporting their correct annotation and potential functional inferences. The expression profiles of the complement regulatory protein genes were determined in channel catfish healthy tissues and after infection with the two main bacterial pathogens, Edwardsiella ictaluri and Flavobacterium columnare. The vast majority of complement regulatory protein genes were significantly regulated after bacterial infections, but interestingly were generally up-regulated after E. ictaluri infection while mostly down-regulated after F. columnare infection, suggesting a pathogen-specific pattern of regulation. Collectively, these findings suggested that complement regulatory protein genes may play complex roles in the host immune responses to bacterial pathogens in channel catfish.

Pathogen recognition receptors in channel catfish: IV. Identification, phylogeny and expression analysis of peptidoglycan recognition proteins

Peptidoglycan recognition proteins (PGRPs) can recognize bacterial cell wall (peptidoglycan) and activate innate immune system. In addition to its function as pathogen recognition receptors (PRRs), PGRPs are also involved in directly killing bacteria, and regulating multiple signaling pathways. Recently, we have reported catfish PRRs including nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs), retinoic acid inducible gene I (RIG-I) like receptors (RLRs), and Toll-like receptors (TLRs). In this study, we identified and characterized the PGRP gene family in channel catfish which included two members, PGLYRP-5 and PGLYRP-6. Phylogenetic analysis, syntenic analysis and protein structural analysis were conducted to determine their identities and evolutionary relationships. In order to gain insight into the roles of PGRPs in catfish innate immune responses, quantitative real-time PCR was used to investigate the expression profiles in catfish healthy tissues and after bacterial infection. Both PGLYRP-5 and PGLYRP-6 were ubiquitously expressed in all 12 healthy tissues, and most highly expressed in gill and spleen, respectively. Distinct expression patterns were observed for PGRPs after infection with Edwardsiella ictaluri and Flavobacterium columnare, both Gram-negative bacteria. After infection with E. ictaluri, both PGLYRP-5 and PGLYRP-6 were significantly down-regulated at a certain time-point, while both genes were generally up-regulated in the gill after infection with F. columnare. Collectively, these findings suggested that PGRPs may play complex roles in the host immune response to bacterial pathogens in catfish.

Mapping and genotypic analysis of the NK-lysin gene in chicken

Background

Antimicrobial peptides (AMP) are important elements of the first line of defence against pathogens in animals. NK-lysin is a cationic AMP that plays a critical role in innate immunity. The chicken NK-lysin gene has been cloned and its antimicrobial and anticancer activity has been described but its location in the chicken genome remains unknown. Here, we mapped the NK-lysin gene and examined the distribution of a functionally significant single nucleotide polymorphism (SNP) among different chicken inbred lines and heritage breeds.

Results

A 6000 rad radiation hybrid panel (ChickRH6) was used to map the NK-lysin gene to the distal end of chromosome 22. Two additional genes, the adipocyte enhancer-binding protein 1-like gene (AEBP1) and the DNA polymerase delta subunit 2-like (POLD2) gene, are located in the same NW_003779909 contig as NK-lysin, and were thus indirectly mapped to chromosome 22 as well. Previously, we reported a functionally significant SNP at position 271 of the NK-lysin coding sequence in two different chicken breeds. Here, we examined this SNP and found that the A allele appears to be more common than the G allele in these heritage breeds and inbred lines.

Conclusions

The chicken NK-lysin gene mapped to the distal end of chromosome 22. Two additional genes, AEBP1 and POLD2, were indirectly mapped to chromosome 22 also. SNP analyses revealed that the A allele, which encodes a peptide with a higher antimicrobial activity, is more common than the G allele in our tested inbred lines and heritage breeds.

L-Rhamnose-binding lectins (RBLs) in channel catfish, Ictalurus punctatus: Characterization and expression profiling in mucosal tissues

Rhamnose-binding lectins (RBLs) have recently emerged as important molecules in the context of innate immunity in teleost fishes. Previously, using RNA-seq technology, we observed marked up-regulation of a RBL in channel catfish (Ictalurus punctatus) gill following a challenge with the bacterial pathogen Flavobacterium columnare. Furthermore, the magnitude of RBL up-regulation positively correlated with disease susceptibility. Moving forward from these findings, we wished to more broadly understand RBL function, diversity, and expression kinetics in channel catfish. Therefore, in the present study we characterized the RBL gene family present in select channel catfish tissues and profiled family member expression after challenge with two different Gram-negative bacterial pathogens. Here, six RBLs were identified from channel catfish and were designated IpRBL1a, IpRBL1b, IpRBL1c, IpRBL3a, IpRBL3b, and IpRBL5a. These RBLs contained carbohydrate recognition domains (CRD) ranging from one to three domains and each CRD contained the conserved motifs of -YGR- and -DPC-. Despite a level of structural conservation, the catfish RBLs showed low full-length identity with RBLs from outside the order Siluriformes. IpRBL expression after bacterial infection varied depending on both pathogen and tissue type, suggesting that IpRBLs may exert disparate functions or exhibit distinct tissue-selective roles in the host immune response to bacterial pathogens.

Chicken NK-lysin is an alpha-helical cationic peptide that exerts its antibacterial activity through damage of bacterial cell membranes

The antimicrobial peptides (AMP) are important elements of the first line of defense against pathogens in animals, and an important constituent of innate immunity. Antimicrobial peptides act on a broad spectrum of microbial organisms. NK-Lysin is a cationic antibacterial peptide that was originally isolated from porcine intestinal tissue based on its antibacterial activity. We synthesized peptides corresponding to each helical region of chicken NK-lysin and analyzed their secondary structures in addition to their antimicrobial activity. Circular dichroism spectroscopy of the synthetic chicken NK-lysin (cNK-78) and 4 small peptides in negatively charged liposomes demonstrated transition in the conformation of α-helical peptides relative to the charged environment. Chicken NK-lysin inhibits the growth of a representative gram-negative bacterium, Escherichia coli. The antimicrobial activity of 2 peptides designated H23 and H34 was similar to that of mature NK-lysin, cNK-78. Microscopic analyses revealed the death of bacterium with disrupted membranes after peptide treatment, suggesting that chicken NK-lysin, an alpha-helical cationic peptide, exerts its antimicrobial activity by damaging the bacterial cell membrane.

Molecular characterization of LEAP-2 cDNA in common carp (Cyprinus carpio L.) and the differential expression upon a Vibrio anguillarum stimulus; indications for a significant immune role in skin

LEAP-2 is a cysteine-rich cationic antimicrobial peptide (AMP) playing an important role in host innate immune system. LEAP-2 genes have been identified from higher vertebrates and several fish species. Here we report the cloning and identification of two LEAP-2 cDNA sequences from the liver of common carp (Cyprinus carpio L.). The LEAP-2A cDNA was 1325 bp long and contained an ORF of 279 bp encoding a protein of 92 amino acids. The LEAP-2B cDNA was 608 bp long and contained an ORF of 276 bp encoding a protein of 91 amino acids. Both LEAP-2 proteins consisted of 41 amino acid residues and shared four cysteines at the conserved positions in the predicted mature peptides, highly similar to LEAP-2 of other species. Sequence alignment showed that LEAP-2 amino acid sequences were well conserved in different species, and the phylogenetic relation of LEAP-2 was coincident with evolution of biological species. Expression analysis data revealed that LEAP-2A and LEAP-2B mRNAs were expressed in a wide range of common carp tissues including liver, spleen, head kidney, skin, gills, hindgut and foregut. When injected intraperitoneally with Vibrio anguillarum, the expression level of common carp LEAP-2A was quickly up-regulated in liver, spleen, head kidney, skin, gills, foregut and hindgut, however, the expression level of LEAP-2B was similarly up-regulated in spleen, skin, gills and hindgut but not in liver, head kidney and foregut. Our results showed that the LEAP-2A had a markedly high constitutive expression in skin, and the LEAP-2A and the LEAP-2B had a significantly high up-regulated expression after stimulus in skin. This differential expression of LEAP-2 in common carp suggests that it may play a key role in immune responses against invading pathogens and both LEAP-2 molecules may be involved in mucosal immunity.

Molecular characterization of hepcidin gene in common carp (Cyprinus carpio L.) and its expression pattern responding to bacterial challenge

Hepcidin is a cysteine-rich cationic antimicrobial peptide (AMP), which plays an important role in host innate immune system and iron regulation. A great many of hepcidin genes have been identified from vertebrates, including various fish species. Here we report the cloning and identification of a hepcidin cDNA from the liver of common carp (Cyprinus carpio L.). The full-length cDNA of the common carp hepcidin was 647 bp, which contained an ORF of 276 bp encoding a prepropeptide of 91 amino acid residues. The predicted preprohepcidin consisted of three domains: a signal peptide of 24 amino acids, a prodomain of 42 amino acids and a mature peptide of 25 amino acids, which containd eight cysteine residues at the identical conserved position. The genomic organization of common carp hepcidin was identified, which contained three exons and two introns, similarly to corresponding genes in mammals and other fish species. Sequence alignment and phylogenetic analysis showed that hepcidins were conserved in different species, and common carp hepcidin was type 1 hepcidin according to the phylogenetic tree, which had the highest identity with mud loach and zebrafish. Real-time PCR assay showed that the hepcidin gene was mostly expressed in liver, and expressed in all the other tested tissues of common carp in different levels. When challenged with pathogenic bacterium, Vibrio anguillarum, the expression level of common carp hepcidin was quickly up-regulated in liver, spleen, head kidney and hindgut, implying that hepcidin may be an important component of the innate immune system of common carp and involved in mucosal immune response against invading pathogens.

Four lysozymes (one c-type and three g-type) in catfish are drastically but differentially induced after bacterial infection

Lysozyme is an important component of the innate immune system. In this study, four lysozyme genes including one c-type lysozyme and three g-type lysozymes were identified from channel catfish (Ictalurus punctatus). The lysozyme genes are highly conserved in their structural features as compared to those from other species. Phylogenetic analyses were conducted allowing annotation of these genes. Additional analyses using conserved syntenies allowed determination of orthologies for the c-type lysozyme. Phylogenetic analysis indicated that the g-type lysozyme may have gone through species-specific gene duplications leading to multiple copies in some teleost species. Channel catfish possessed three copies of the g-type lysozyme genes. Expression analysis revealed that the catfish lysozyme genes were expressed in a broad range of tissues. The highest levels of expression were found in head kidney, liver, spleen, and trunk kidney, compatible with the immune functions of these tissues/organs. The c-type and g-type lysozymes were drastically induced after bacterial infection, but exhibited large differences in the extent of induction and the tissue with the highest level of induction, with the g-type lysozyme being most highly induced in the head kidney whereas the other three lysozymes being most highly induced in the liver, suggesting their cooperative actions in the immune responses but difference in their detailed functions.

A NK-lysin from Cynoglossus semilaevis enhances antimicrobial defense against bacterial and viral pathogens

NK-lysin is an effector protein of cytotoxic T lymphocytes and natural killer cells. Mammalian NK-lysin is known to possess antibacterial property and antitumor activity. Homologues of NK-lysin have been identified in several teleost species, but the natural function of fish NK-lysin remains essentially unknown. In this study, we identified a NK-lysin, CsNKL1, from half-smooth tongue sole (Cynoglossus semilaevis) and analyzed its expression, genetic organization, and biological effect on pathogen infection. CsNKL1 is composed of 135 residues and shares 33.1-56.5% overall sequence identities with other teleost NK-lysin. CsNKL1 possesses a Saposin B domain and six conserved cysteine residues that in mammals are known to form three intrachain disulfide bonds essential to antimicrobial activity. The genomic sequence of the ORF region of CsNKL1 is 1240bp in length and, like human NK-lysin, contains five exons and four introns. Expression of CsNKL1 occurred in multiple tissues and was upregulated by bacterial and viral infection in a time dependent manner. When CsNKL1 was overexpressed in tongue sole, significant upregulation of interleukin-1 and chemokines was observed in spleen and head kidney. Following bacterial and viral infection, the pathogen loads in the tissues of CsNKL1-overexpressing fish were significantly lower than those in control fish. These results indicate that CsNKL1 possesses the novel capacities of immunomodulation and enhancing antimicrobial defense against pathogens of both bacterial and viral nature.

Pathogen recognition receptors in channel catfish: III phylogeny and expression analysis of Toll-like receptors

Toll-like receptors (TLRs) were the earliest characterized and the most extensively studied pathogen recognition receptors (PRRs). The majority of tetrapod TLR orthologs have been found in teleost fish. In addition, a group of "fish-specific" TLRs have been identified. In catfish, a number of TLR-related sequences have been reported, but systematic phylogenetic analyses have not been conducted. In this study, we conducted phylogenetic and comparative analysis of 20 catfish TLR genes against their counterparts from various species. TLR25 and TLR26 are TLRs identified only in channel catfish. Phylogenetic analyses suggested that four catfish TLR genes have duplicated copies in the genome, i.e., TLR4, TLR5, TLR8, and TLR20. Six fish-specific TLRs were identified, and the vast majority of these belong to the TLR11 subfamily. In healthy catfish tissues, most of the tested TLR genes were ubiquitously expressed although expression levels varied among the 11 tested tissues. We tested nine TLRs for their expression in response to Edwardsiella ictaluri infection. They were significantly up-regulated in the spleen and liver, but down-regulated in the head kidney, suggesting their involvement in the immune responses against the intracellular bacterial pathogen in a tissue-specific manner in catfish, perhaps through rapid migration of phagocytes to infection sites.

Advances in research of fish immune-relevant genes: a comparative overview of innate and adaptive immunity in teleosts

Fish is considered to be an important model in comparative immunology studies because it is a representative population of lower vertebrates serving as an essential link to early vertebrate evolution. Fish immune-relevant genes have received considerable attention due to its role in improving understanding of both fish immunology and the evolution of immune systems. In this review, we discuss the current understanding of teleost immune-relevant genes for both innate and adaptive immunity, including pattern recognition receptors, antimicrobial peptides, complement molecules, lectins, interferons and signaling factors, inflammatory cytokines, chemokines, adaptive immunity relevant cytokines and negative regulators, major histocompatibility complexes, immunoglobulins, and costimulatory molecules. The implications of these factors on the evolutionary history of immune systems were discussed and a perspective outline of innate and adaptive immunity of teleost fish was described. This review may provide clues on the evolution of the essential defense system in vertebrates.

The innate immune-related genes in catfish

Catfish is one of the most important aquaculture species in America (as well as in Asia and Africa). In recent years, the production of catfish has suffered massive financial losses due to pathogen spread and breakouts. Innate immunity plays a crucial role in increasing resistance to pathogenic organisms and has generated increasing interest in the past few years. This review summarizes the current understanding of innate immune-related genes in catfish, including pattern recognition receptors, antimicrobial peptides, complements, lectins, cytokines, transferrin and gene expression profiling using microarrays and next generation sequencing technologies. This review will benefit the understanding of innate immune system in catfish and further efforts in studying the innate immune-related genes in fish.

Pathogen recognition receptors in channel catfish: I. Identification, phylogeny and expression of NOD-like receptors

Innate immune system plays a significant role in all multicellular organisms. The key feature of the system is its ability to recognize and respond to invading microorganisms. Vertebrates including teleost fish have evolved an array of pathogen recognition receptors (PRRs) for detecting and responding to various pathogen-associated molecular patterns (PAMPs), including Toll-like receptors (TLRs), nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs), and the retinoic acid inducible gene I (RIG-I) like receptors (RLRs). In this study, we identified 22 NLRs including six members of the NLR-A subfamily (NODs), two members of the NLR-B subfamily, 11 members of the NLR-C subfamily, and three genes that do not belong to any of these three subfamilies: Apaf1, CIITA, and NACHT-P1. Phylogenetic analysis indicated that orthologs of the mammalian NOD1, NOD2, NOD3, NOD4, and NOD5 were all identified in catfish. In addition, an additional truncated NOD3-like gene was also identified in catfish. While the identities of subfamily A NLRs could be established, the identities of the NLR-B and NLR-C subfamilies were inconclusive at present. Expression of representative NLR genes was analyzed using RT-PCR and qRT-PCR. In healthy catfish tissues, all the tested NLR genes were found to be ubiquitously expressed in all 11 tested catfish tissues. Analysis of expression of these representative NLR genes after bacterial infection with Edwardsiella ictaluri revealed a significant up-regulation of all tested genes in the spleen and liver, but a significant down-regulation in the intestine and head kidney, suggesting their involvement in the immune responses of catfish against the intracellular bacterial pathogen in a tissue-specific manner. The up-regulation and down-regulation of the tested genes exhibited an amazing similarity of expression profiles after infection, suggesting the co-regulation of these genes.

Molecular characterization of three L-type lectin genes from channel catfish, Ictalurus punctatus and their responses to Edwardsiella ictaluri challenge

L-type lectins have a leguminous lectin domain and can bind to high-mannose type oligosaccharides. In the secretory pathway, L-type lectins play crucial roles in selective protein trafficking, sorting and targeting. Three L-type lectins were cloned in the channel catfish, Ictalurus punctatus, the 53 kDa endoplasmic reticulum ER-Golgi intermediate compartment protein (ERGIC-53), the vesicular integral protein of 36 kDa (VIP36) and VIP36-like. Phylogenetic analysis indicated that the catfish genes are orthologous to their counterparts in other species. Southern blot analysis demonstrated that all three L-type lectin genes are likely single-copy genes in the catfish genome. Analysis of expression in healthy tissues using quantitative real time RT-PCR indicated that all three genes are expressed widely in all tested tissues, but with strong tissue preference of expression: ERGIC-53 was found to be abundantly expressed in the liver, VIP36 was found to be abundantly expressed in the head-kidney, whereas VIP36-like was found to be abundantly expressed in the brain. Upon infection with Edwardsiella ictaluri, expressions of the three genes all had significant up-regulation in the head-kidney, but had distinct expression patterns: ERGIC-53 was gradually induced with the highest expression 7 days after challenge in the head-kidney, but was down-regulated in the liver, spleen, and brain. VIP36 was highly induced in the head-kidney, and 3 days after challenge in the brain, but was not up-regulated in any other tissues or timepoints after challenge. Expression levels of the catfish VIP36-like gene appeared to also respond to infection, albeit with differing patterns among the tested tissues. Taken together, our results indicate that all three L-type lectin genes may be involved in the immune responses of catfish after infection with E. ictaluri.

Applications of antimicrobial peptides from fish and perspectives for the future

Fish are a major component of the aquatic fauna. Like other organisms, fish secrete different kinds of antimicrobial peptides (AMPs), which are positively charged short amino-acid-chain molecules involved in host defense mechanisms. Environmental hazards and the greenhouse effect have led to increased evolution of drug- and vaccine-resistant pathogenic strains, and it is necessary to find new drugs with structural uniqueness to fight them. Aquatic sources contain thousands of fish species, and each secretes AMPs with structural differences which can be used by the pharmaceutical industry in its search for novel drugs to treat drug-resistant pathogens. Not only limited to antimicrobial functions, AMPs possess other desirable characteristics which may be exploited in the near future. In this review, we list fish AMPs available from published reports, and discuss application-oriented functions of these AMPs. Notably, the possibilities of using fish AMPs as antimicrobial agents, vaccine adjuvants, inactivated vaccines, and antitumor agents are discussed in this review.

Expression profiles of toll-like receptors in anterior kidney of channel catfish, Ictalurus punctatus (Rafinesque), acutely infected by Edwardsiella ictaluri

Using quantitative PCR (QPCR), the relative transcriptional levels of five toll-like receptors (TLR2, TLR3, TLR5, TLR20a and TLR21) were studied in the channel catfish, Ictalurus punctatus (Rafinesque), under uninfected and acutely infected conditions [1-, 2-, 4-, 6-, 12-, 24-, 36- and 48-h post-injection (hpi)]. Under uninfected conditions, the transcriptional levels of the five TLRs were significantly lower than that of 18S rRNA (P < 0.001). QPCR results also revealed that the transcriptional levels of TLR20a and TLR5 were higher than those of TLR2, TLR3 or TLR21. The transcriptional level of TLR3 was significantly lower than that of the other four TLRs (P < 0.001). However, when channel catfish were acutely infected by Edwardsiella ictaluri through intraperitoneal injection, the transcriptional levels of TLRs increased significantly (P < 0.005) at 6 hpi. Among the five TLRs studied, the transcriptional levels of TLR3, TLR5 and TLR21 were never significantly lower than under uninfected conditions (P = 0.16, 0.27 and 0.19, respectively), suggesting these three TLRs might play important roles in host defence against infection by E. ictaluri. The amount of E. ictaluri in the anterior kidney increased at 12 and 24 hpi but decreased at 36 and 48 hpi. Our results suggest that TLRs are important components in the immune system in the channel catfish, and their rapid transcriptional upregulation (within 6 hpi) in response to acute E. ictaluri infection might be important for survival from enteric septicaemia of catfish.

Development of genomic resources in support of sequencing, assembly, and annotation of the catfish genome

Major progress has been made in catfish genomics including construction of high-density genetic linkage maps, BAC-based physical maps, and integration of genetic linkage and physical maps. Large numbers of ESTs have been generated from both channel catfish and blue catfish. Microarray platforms have been developed for the analysis of genome expression. Genome repeat structures are studied, laying grounds for whole genome sequencing. USDA recently approved funding of the whole genome sequencing project of catfish using the next generation sequencing technologies. Generation of the whole genome sequence is a historical landmark of catfish research as it opens the real first step of the long march toward genetic enhancement. The research community needs to be focused on aquaculture performance and production traits, take advantage of the unprecedented genome information and technology, and make real progress toward genetic improvements of aquaculture brood stocks.

Detection of antimicrobial peptides related to piscidin 4 in important aquacultured fish

Epithelial surfaces of fish, such as the gut, skin and gills, comprise a large surface area for possible pathogen invasion. Antimicrobial peptides (AMPs), innate immunity components, play a significant role in protecting fish. Piscidins are a family of AMPs. In this study, we detected the presence of the recently discovered piscidin 4 via bug blot, Western blot, ELISA and/or immunohistochemistry in striped bass (Morone saxatilis), white bass (M. chrysops), European seabass (Dicentrarchus labrax), gilthead seabream (Sparus aurata), red drum (Sciaenops ocellatus), and barramundi (Lates calcarifer). Via bug blot, gill extracts from all species had antibacterial activity corresponding to the migration rate of piscidin 4. Western blotting showed that piscidin 4 immunoreactivity was greatest in striped bass gill extract. The concentrations of piscidin 4 detected by the ELISA in striped bass gill (approximately 20 microg/ml) were well within the levels that are inhibitory to important fish bacterial pathogens. Piscidin 4 was also detected via immunohistochemistry in all fish except barramundi. Piscidin 4-positive cells were identified as mast cells (MC), but not all MC were piscidin 4-positive. Species, age, size and physiological condition at sampling were some factors that might affect piscidin expression in different species. Our data provide strong evidence that piscidin 4 isoforms are present in all these commercially important species.

NOD-like subfamily of the nucleotide-binding domain and leucine-rich repeat containing family receptors and their expression in channel catfish

The NLRs (nucleotide-binding domain and leucine-rich repeat containing family receptors) are a recently identified family of pattern recognition receptors in vertebrates. Several subfamilies of NLRs have been characterized in human, mouse, and zebrafish, but studies of NLRs in other species, especially teleost species, have been lacking. Here we report characterization of five NLRs from channel catfish: NOD1, NOD2, NLRC3, NLRC5, and NLRX1. Structural analysis indicated that the genes were organized in a similar fashion as in the mammals and in zebrafish. Phylogenetic analysis suggested that they were orthologous to the NOD-like subfamily of NLRs. All five NOD-like genes exist as a single copy gene in the catfish genome. Hybridization of gene-specific probes allowed mapping of three NLR genes to the catfish physical map, laying a foundation for genome characterization and for establishing orthologies with NLR genes from other species. These genes are widely expressed in various tissues and leukocyte cell lines. While the majority of the NLR genes appeared to be constitutively expressed, NOD1 was induced after infection with a bacterial pathogen, Edwardsiella ictaluri, the causative agent of enteric septicemia of catfish (ESC), suggesting its involvement in immunity against the intracellular pathogen.

Development of an ELISA for quantification of the antimicrobial peptide piscidin 4 and its application to assess stress in fish

Antimicrobial peptides (AMP) are an integral component of innate immunity. One of the most widespread AMP in fish are the piscidins, which have potent, broad-spectrum activity against viruses, bacteria, fungi, and parasites. The widespread phylogenetic distribution of piscidins suggests that they might play an important host defense role in many fish. Quantifying their expression is important in understanding how and where they function. Using a novel piscidin (piscidin 4) that we recently isolated from commercially cultured hybrid striped bass (white bass, Morone chrysops Rafinesque, female symbolxstriped bass, Morone saxatilis Walbaum male symbol), we optimized the conditions for measuring this piscidin via sandwich ELISA. We used an antibody to the highly conserved amino terminus of all piscidins as the capture antibody and a peroxidase-labeled antibody specific for the carboxy terminus of piscidin 4 as the detecting antibody. Specificity of the detecting antibody was confirmed by lack of cross-reactivity with other piscidins in ELISA, as well as specificity for piscidin 4 in tissue extracts via Western blotting. The accuracy of the test, defined as piscidin 4 recovery, was 96-103%. Precision, measured by the coefficient of variation, was 13-19%, and parallelism, determined by linearity of the response, had an r(2)>0.99. The ELISA paralleled the results obtained via Western blotting. Piscidin 4 levels expressed in gill tissue of healthy hybrid striped bass were well within concentrations that are lethal to important fish pathogens. Mean gill piscidin 4 in healthy hybrid striped bass was significantly greater than in either nutritionally stressed fish or in diseased (ectoparasite-infested) fish, suggesting that piscidin 4 can be significantly downregulated with stress or disease. These data suggest that the piscidin 4 ELISA might be a useful indicator of disease susceptibility, providing a new, sensitive tool for rapid screening of population health.