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

Characterization of NK-lysin A, a potent antimicrobial peptide from the zebrafish Danio rerio

Antimicrobial peptides (AMPs) are important players of the innate immune system with a major role in the defense against invading pathogens. AMPs belonging to the family of saposin-like proteins (SAPLIPs) include the porcine NK-lysin and the human granulysin. In the zebrafish Danio rerio, transcript analyses of NK-lysin encoding genes have been reported, but a biochemical characterization at the protein level are missing so far. Here, we present the recombinant expression, purification, and characterization of one of these homologs, namely of NK-lysin A (DaNKlA). To remove the affinity tag from DaNKlA, we made use of a self-splicing intein. Recombinant DaNKlA depolarized liposomes over a broad pH range and showed a preference for negatively charged lipids. DaNKlA inhibited the growth of and killed different Gram-positive and Gram-negative bacteria, including the fish pathogenic bacterium Vibrio anguillarum, by membrane permeabilization but displayed substantially lower activity against yeast cells. Structural modelling and bioinformatic comparison of DaNKlA with characterized SAPLIPs suggest membrane destabilization accompanied by strong electrostatic interactions as the mode of action.

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.

Functional characterization of obscure puffer ToNK-lysin: A novel immunomodulator possessing anti-bacterial and anti-inflammatory properties

NK-lysins are one of the most abundant antimicrobial peptides produced by cytotoxic T lymphocytes (CTLs) and natural killer cells (NKs), and identified as a new class of intrinsically disordered proteins, playing critical roles in the cell-mediated cytotoxicity response, as well as immunomodulatory and antimicrobial activities upon a significant range of pathogens. In the present study, an NK-lysin was identified from Obscure puffer Takifugu obscurus (ToNK-lysin). The open reading frame of ToNK-lysin sequence spans 423 bp, encoding a peptide with 140 amino acids which shares a moderate residue identity (18%-60%) with NK-lysin of mammals and other teleost species. Phylogenetic analysis revealed that ToNK-lysin was most closely related to NK-lysins from the Pleuronectiformes (Bastard halibut Paralichthys olivaceus and Pacific halibut Hippoglossus stenolepis). Comprehensive computational analysis revealed that ToNK-lysin have substantial level of intrinsic disorder, which might be contribute to its multifunction. The transcripts of the ToNK-lysin were detected in multiple examined tissues and most abundant in gills. After bacterial and Poly I:C challenge, the transcriptional levels of ToNK-lysin were significantly up-regulated in the head kidney, liver and spleen at different time points. The recombinant ToNK-lysin showed significant antibacterial activity against Vibrio harveyi and Escherichia coli, and the ToNK-lysin treatment not only reduced the bacterial loads in liver and head kidney, but also alleviated the pathogen-mediated upregulation of immune-related genes. In addition, the co-incubation with rToNK-lysin protein remarkably degraded bacterial genomic DNA, suggesting the potential mechanism of ToNK-lysin against microbes. These results suggest that ToNK-lysin possess antibacterial and immunoregulatory function both in vivo and in vitro, which may allow it a potential applicability to the aquaculture industry.

Cloning, subcellular localization and antibacterial functional analysis of NK-lysin in yellow drum (Nibea albiflora)

Vibrio harveyi is the primary pathogenic bacteria affecting Nibea albiflora aquaculture. In a previous phase, our laboratory intentionally exposed N. albiflora to V. harveyi and analyzed the outcomes using a combination of genome-wide association study (GWAS) and RNA-seq. The results revealed that the antimicrobial peptide NK-lysin (YdNkl-1) was a candidate gene for resistance to V. harveyi disease in N. albiflora. To investigate the role of the antimicrobial peptide NK-lysin in N. albiflora's antimicrobial immunity, we screened the YdNkl-1 gene from the transcriptome database. The full-length cDNA of YdNkl-1 gene is 508 bp, with an open reading frame (ORF) of 477 bp, encoding 158 amino acids. The deduced amino acid sequence of YdNkl-1 contains a signal peptide (1st-22nd amino acids) and a Saposin B domain (50th-124th amino acids), akin to mammalian NK-lysin. Phylogenetic tree analysis confirmed that the NK-lysin of teleost fish clustered into a single species, and YdNkl-1 was most closely related to Larimichthys crocea. Subcellular localization showed that YdNkl-1 was distributed in cytoplasm and nucleus of yellow drum kidney cells. Furthermore, YdNkl-1 mRNA transcripts were significantly up-regulated in the skin, gill, intestine, head-kidney, liver, and spleen after V. harveyi infection, suggesting a critical role in N. albiflora's defense against V. harveyi infection. Additionally, we purified and observed the YdNkl-1 protein, which exhibited a potent membrane-disrupting effect on V. harveyi, Pseudomonas plecoglossicida, Vibrio parahaemolyticus, Escherichia coli and Bacillus subtilis. These findings underscore the significance of NK-lysin in N. albiflora's resistance to V. harveyi infection and provide new insights into the crucial role of NK-lysin in the innate immunity of teleost fishes.

NKHs27, a sevenband grouper NK-Lysin peptide that possesses immunoregulatory and antimicrobial activity

As an effective and broad-spectrum antimicrobial peptide, NK-Lysin is attracted more and more attention at present. However, the functions and action mechanism of NK-Lysin peptides are still not comprehensive enough at present. In this study, a sevenband grouper (Hyporthodus septemfasciatus) NK-Lysin peptide, NKHs27, was identified and synthesized, and its biological functions were studied. The results indicated that NKHs27 shares 44.44%∼88.89% overall sequence identities with other teleost NK-Lysin peptides. The following antibacterial activity assay exhibited that NKHs27 was active against both Gram-negative and Gram-positive bacteria, including Staphylococcus aureus, Listonella anguillarum, Vibrio parahaemolyticus and Vibrio vulnificus. Additionally, NKHs27 showed a synergistic effect when it was combined with rifampicin or erythromycin. In the process of interaction with the L. anguillarum cells, NKHs27 changed the cell membrane permeability and retained its morphological integrity, then penetrated into the cytoplasm to act on genomic DNA or total RNA. Then, in vitro studies showed that NKHs27 could enhance the respiratory burst ability of macrophages and upregulate immune-related genes expression in it. Moreover, NKHs27 incubation improved the proliferation of peripheral blood leukocytes significantly. Finally, in vivo studies showed that administration of NKHs27 prior to bacterial infection significantly reduced pathogen dissemination and replication in tissues. In summary, these results provide new insights into the function of NK-Lysin peptides in teleost and support that NKHs27, as a novel broad-spectrum antibacterial peptide, has potential applications in aquaculture against pathogenic infections.

Molecular characterization and antimicrobial activity of NK-lysin in black scraper (Thamnaconus modestus)

NK-lysin (NKL) is a positively charged antimicrobial peptide with broad-spectrum bactericidal activities. In this study, the cDNA sequence of NKL (TmNKL) from black scraper (Thamnaconus modestus) was cloned, which encodes a predicted polypeptide of 150 amino acids that contains a surfactant protein B domain with three disulfide bonds. Phylogenetically, TmNKL was most closely related to its teleost counterpart from tiger puffer (Takifugu rubripes). Expression analysis demonstrated that TmNKL transcripts were constitutively expressed in all tested tissues, with the highest expression levels in the gills. Its expression was significantly upregulated in the gills, head kidney, and spleen after infection with Vibrio parahaemolyticus. A linear peptide (TmNKLP40L) and a disulfide-type peptide (TmNKLP40O) were further synthesized and results showed that disulfide bonds are not essential for bactericidal activities of TmNKL, and that both forms of TmNKL exhibited potent bactericidal activities against 4 gram- negative bacteria, including V. parahaemolyticus, V. alginolyticus, Edwardsiella tarda, and V. harveyi. Observed antimicrobial activities are likely due to the effects of TmNKLP40L and TmNKLP40O treatment on disrupting the integrity of both inner and outer membrane of V. parahaemolyticus, resulting in hydrolysis of bacterial genomic DNA. Damaged cell membranes and leakage of intracellular contents were further confirmed using scanning and transmission microscopy. Moreover, administration of 1.0 μg/g TmNKLP40L or TmNKLP40O significantly decreased bacterial load in tissues and thus, pronouncedly enhanced the survival of V. parahaemolyticus-infected fish. Overall, our results demonstrated that TmNKL is a potent innate effector and provides protective effects against bacterial infection.

Genomic Variant in NK-Lysin Gene Is Associated with T Lymphocyte Subpopulations in Pigs

As an antimicrobial peptide, NK-lysin (NKL) plays an important role in the innate immune system of organisms. In this study, 300 piglets (68 Landrace pigs, 158 Large White pigs and 74 Songliao Black pigs) were used to further explore the function of NLK gene in porcine immune system. The quantitative real-time PCR analysis detected the NKL gene's expression, and the result demonstrated that NKL mRNA was expressed in lung, spleen, stomach, kidney, liver and heart, and the expression level decreased sequentially. A single-nucleotide polymorphism (SNP, g.59070355 G > A) in intron 3 of the NKL gene was detected by PCR amplification and sequencing. The results of the Chi-square (χ2) test showed that the genotype of the SNP was consistent with the Hardy-Weinberg equilibrium. What's more, association analysis results showed the SNP in NKL gene was significantly associated with T lymphocyte subpopulations. Different genotypes had significant effects on the proportion of CD4-CD8-, CD4-CD8+, CD4+CD8+, CD8+, CD4+/CD8+ in peripheral blood (p < 0.05). These results further suggested that NKL could be recognized as a promising immune gene for swine disease resistance breeding.

A Non-Canonical Teleost NK-Lysin: Antimicrobial Activity via Multiple Mechanisms

NK-lysin (NKL) is a family of antimicrobial proteins with an important role in innate and adaptive immunity. In this study, a non-canonical NK-lysin (NKLnc) was identified in the Japanese flounder (Paralichthys olivaceus), which shares low sequence identities (15.8-20.6%) with previously reported fish NKLs and was phylogenetically separated from the canonical NKLs in teleost. NKLnc expression was upregulated in flounder tissues during bacterial infection, and interference with NKLnc expression impaired the ability of flounder cells to eliminate invading bacteria. When expressed in Escherichia coli, NKLnc was detrimental to the host cells. P35, a peptide derived from the saposin B domain (SapB) of NKLnc, bound major bacterial surface molecules and killed both Gram-negative and Gram-positive bacteria by inflicting damage to bacterial cell structure and genomic DNA. The bactericidal activity, but not the bacteria-binding capacity, of P35 required the structural integrity of the alpha 2/3 helices in SapB. Furthermore, P35 induced the migration of flounder peripheral blood leukocytes, inhibited bacterial dissemination in fish tissues, and facilitated fish survival after bacterial challenge. Together our study reveals that NKLnc plays an important part in flounder immune defense, and that NKLnc peptide exerts an antimicrobial effect via multiple mechanisms by targeting both bacteria and fish cells.

The antibacterial activity of a novel NK-lysin homolog and its molecular characterization and expression in the striped catfish, Pangasianodon hypophthalmus

Aeromonas hydrophila was a common bacterial pathogen in aquaculture resulting in considerable losses to the striped catfish aquaculture industry. As an emergent antimicrobial peptide (AMP), NK-lysin (NKL) had activity against various microorganisms. However, the antibacterial activity of NKL from striped catfish (Pangasianodon hypophthalmus) both in vitro and vivo remains unclear. In this study, the cDNA sequence of P. hypophthalmus NK-lysin gene (PhNK-lysin) was cloned and characterized. The amino acid sequence of PhNK-lysin contains a signal peptide sequence of 17 amino acid (aa) residues and a mature peptide composed of 130 aa. The saposin B domain of mature peptide comprised six conserved cysteines forming three putative disulfide bonds. Phylogenetic analysis revealed that the PhNK-lysin was most closely related to that of the channel catfish (Ictalurus punctatus) NK-lysin. The transcriptional levels of the PhNK-lysin were significantly upregulated in response to A. hydrophila infection in various tissues including heart, liver, spleen, head kidney, trunk kidney and gill. The synthetic PhNK-lysin-derived peptide consisting of 38aa showed antibacterial activity against Vibrio harveii, Aeromonas hydrophila and Escherichia coli. The MIC for V. harveii, A. hydrophila and E. coli were 15.625 μM, 250 μM and 31.25 μM respectively. Besides, the synthetic PhNK-lysin decreased the bacterial load of liver and trunk kidney in vivo as well as increased the survival rate of A. hydrophila infected striped catfish. Hence, these data suggest that PhNK-lysin had antimicrobial effect and protects the host from pathogenic infection.

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.

Recent Discoveries on Marine Organism Immunomodulatory Activities

Marine organisms have been shown to be a valuable source for biologically active compounds for the prevention and treatment of cancer, inflammation, immune system diseases, and other pathologies. The advantage of studying organisms collected in the marine environment lies in their great biodiversity and in the variety of chemical structures of marine natural products. Various studies have focused on marine organism compounds with potential pharmaceutical applications, for instance, as immunomodulators, to treat cancer and immune-mediated diseases. Modulation of the immune system is defined as any change in the immune response that can result in the induction, expression, amplification, or inhibition of any phase of the immune response. Studies very often focus on the effects of marine-derived compounds on macrophages, as well as lymphocytes, by analyzing the release of mediators (cytokines) by using the immunological assay enzyme-linked immunosorbent assay (ELISA), Western blot, immunofluorescence, and real-time PCR. The main sources are fungi, bacteria, microalgae, macroalgae, sponges, mollusks, corals, and fishes. This review is focused on the marine-derived molecules discovered in the last three years as potential immunomodulatory drugs.

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.

NK-lysin peptides ameliorate viral encephalopathy and retinopathy disease signs and provide partial protection against nodavirus infection in European sea bass

Antimicrobial peptides (AMP) comprise a wide range of small molecules with direct antibacterial activity and immunostimulatory role and are proposed as promising substitutes of the antibiotics. Additionally, they also exert a role against other pathogens such as viruses and fungi less evaluated. NK-lysin, a human granulysin orthologue, possess a double function, taking part in the innate immunity as AMP and also as direct effector in the cell-mediated cytotoxic (CMC) response. This molecule is suggested as a pivotal molecule involved in the defence upon nervous necrosis virus (NNV), an epizootic virus provoking serious problems in welfare and health status in Asian and Mediterranean fish destined to human consumption. Having proved that NK-lysin derived peptides (NKLPs) have a direct antiviral activity against NNV in vitro, we aimed to evaluate their potential use as a prophylactic treatment for European sea bass (Dicentrarchus labrax), one of the most susceptible cultured-fish species. Thus, intramuscular injection of synthetic NKLPs resulted in a very low transcriptional response of some innate and adaptive immune markers. However, the injection of NKLPs ameliorated disease signs and increased fish survival upon challenge with pathogenic NNV. Although NKLPs showed promising results in treatments against NNV, more efforts are needed to understand their mechanisms of action and their applicability to the aquaculture industry.

Structure and regulation of the NK-lysin (1-4) and NK-lysin like (a and b) antimicrobial genes in rainbow trout (Oncorhynchus mykiss)

Nk-lysin (Nkl), an antimicrobial peptide (AMP) product of natural killer cells and cytotoxic T cells in mammals, has recently been characterized in a number of finfish species. In this study, we identified six genes with sequence homology to Nkl and characterized their patterns of mRNA expression and abundances in rainbow trout (Oncorhynchus mykiss). The cDNA sequences for the six Nkls encoded precursor peptides of 128-133 aa in length, and mature peptides of 109-111 aa in length. Genomic DNA of the nkl1-4 genes consisted of five exons and four introns, whereas the nkl-like a & b genes consisted of four exons and three introns. Chromosomal locations of these peptides show that nkl1 was located on chromosome arm 25q, whereas the other five nkl genes were clustered on chromosome arm 19q. Phylogenetic analysis revealed a conserved structure of Nkls among the teleosts and further protein sequence analyses suggests that all six nkl genes fall within the Nkl sub-family of the Saposin family of proteins. Patterns of tissue-specific mRNA expression were asymmetric among the six trout Nkl homologues, with nkl1, nkl3, and nkl-like a & b occurring in immune competent organs such as spleen, gill, intestine and kidney, as well as pineal gland, brain and oocytes. However, nkl2 and nkl4, showed primary abundances in brain, pineal gland and oocyte tissues. Using mRNA sequencing, in whole-body pools of juvenile trout fry (1 g bw) exposed to Flavobacterium psychrophilum infection, we observed modest up-regulation (2-3 fold) of five (nkl 2-4 and nkl-like a & b) of the six nkl mRNAs over the five-day post-challenge time-course. However, no upregulation could be recorded in spleen tissue measured by qPCR in juvenile trout (270 g bw). Using mRNA sequencing again, mRNA abundances were determined in gill of juvenile trout (~57.7 g bw) exposed to various aquaculture stressors. The results indicated that all six nkls (nkl1-4 and nkl-like a and nkl-like b) were downregulated when exposed to high temperature, and that nkl1 was significantly downregulated following salinity challenge. Overall, these newly characterized AMPs may contribute to host innate immunity as they are modulated following pathogen challenge and by physiological stressors.

Functional characterization of NK-lysin in golden pompano Trachinotus ovatus (Linnaeus 1758)

NK-lysin is an important part of the innate immune defence system and plays an important role in resisting the invasion of pathogenic microorganisms. In this study, NK-lysin from golden pompano (Trachinotus ovatus) was characterized and its expression in response to Photobacterium damselae was investigated. The full-length NK-lysin cDNA was 731 bp, which comprised a 5'-UTR of 63 bp, an ORF of 444 bp, and a 3'-UTR of 224 bp, and encoded 147 amino acids; NK-lysin consisted of a conserved saposin B domain and six conserved cysteines that formed three pairs of disulfide bonds. The genomic organization of NK-lysin was also determined and the gene consisted of four introns and five exons. The predicted promoter region of ToNK-lysin contained several putative transcription factor binding sites. Quantitative real-time (qRT-PCR) analysis indicated that ToNK-lysin was ubiquitously expressed in all examined tissues; the highest mRNA levels were observed in the skin, kidney and intestine, while the lowest expression level was detected in the stomach. After P. damselae stimulation, the expression level of NK-lysin mRNA was significantly upregulated in various tissues of golden pompano. In addition, SDS-PAGE showed that the molecular mass of recombinant NK-lysin expressed in pGEX-6P-1 was approximately 37 kDa. The purified recombinant protein showed antibacterial activity against gram-positive and gram-negative bacteria. The results indicate that golden pompano NK-lysin has potential antimicrobial roles in fish innate immunity.

NK-lysin is highly conserved in European sea bass and gilthead seabream but differentially modulated during the immune response

Fish NK-lysin (NKL), an orthologous to human granulysin, exerts a dual role as an antimicrobial peptide (AMP) and as a direct executor of T cytotoxic and natural killer cells during the cell-mediated cytotoxic (CMC) response. Although its best-known function is as AMP against bacteria, recent studies point to a special role of NKL in antiviral responses. Nodavirus (NNV) is a spreading threat in Mediterranean aquaculture. In this study, we have identified and compared the expression pattern of European sea bass and gilthead seabream NKL and evaluated its transcription in different tissues and its regulation in head-kidney leucocyte (HKLs) stimulated in vitro with different immunostimulants, under CMC response and upon an in vivo infection with NNV. Our results showed that nkl transcription is highly expressed in spleen, thymus and skin with species-specific differences. Interestingly, the expression pattern in both species was very different upon treatment. While sea bass nkl transcription was increased in HKLs by the T mitogen phytohemagglutinin all the stimulators inhibited it in seabream HKLs. Similar results occurred in NNV-infected fish where the transcription was increased in sea bass tissues and down-regulated in seabream. Curiously, during CMC assays, nkl transcription was significantly increased in seabream HKLs against NNV-infected fish cell lines but this was not observed in sea bass leucocytes. The potential role of NKL as CMC effector molecule or as AMP in fish will be discussed.

Exploring small cationic peptides of different origin as potential antimicrobial agents in aquaculture

Antimicrobial peptides (AMPs) form part of the innate immune response, which is of vital importance in fish, especially in eggs and early larval stages. Compared to antibiotics, AMPs show action against a wider spectrum of pathogens, including viruses, fungi and parasites, are more friendly to the environment, and do not seem to generate resistance in bacteria. Thus, we have tested in vitro the potential use of several synthetic peptides as antimicrobial agents in aquaculture: frog Caerin1.1, European sea bass Dicentracin (Dic) and NK-lysin peptides (NKLPs) and sole NKLP27. Our results demonstrate that the highest bactericidal activity against both human and fish pathogens was obtained with Caerin1.1 followed by sea bass Dic and NKLPs, having the sea bass NKLP20.2 none to negligible activity. Interestingly, Aeromonas salmonicida was refractory to all the fish peptides tested. Regarding the antiviral activity, synthetic peptides were able to inhibit the viral infection of nodavirus (NNV), viral septicaemia haemorrhagic virus (VHSV), infectious pancreatic necrosis virus (IPNV) and spring viremia carp virus (SVCV), which are some of the most devastating virus for aquaculture. However, their effectiveness was highly dependent on the type of virus. Strikingly, IPNV resulted the most resistant virus since Caeerin1.1 and sea bass NKLP20.2 were unable to reduce its titre and the other peptides tested only reduced it to values in the 43-78% range. These data demonstrate that synthetic peptides have great antibacterial and antiviral in vitro activity against important fish pathogens and point to their use as potential therapeutic agents in aquaculture.

NK-lysin, dicentracin and hepcidin antimicrobial peptides in European sea bass. Ontogenetic development and modulation in juveniles by nodavirus

Antimicrobial peptides (AMPs) are considered to be amongst the most powerful tools for the fight against pathogens in fish, since they form part of the innate immune response, which is especially vital in eggs and early larval stages, when the immune system is developing. The fish responsible for a large part of the profits in Mediterranean aquaculture is European sea bass (Dicentrarchus labrax), a species greatly susceptible to nodavirus (NNV), especially in the larval and juvenile stages. In this work, polyclonal antibodies were developed and used to detect and quantify NK-lysin, dicentracin and hepcidin AMPs in European sea bass eggs and during larval development, as well as to evaluate their regulation in juvenile specimens upon NNV infection. Basal and detectable levels of all the AMPs studied were present in eggs, confirming the maternal transfer of peptides, which increased in one or two waves during larval development up to 69 days post-fertilization. After NNV infection, the mRNA of all the AMPs analysed was up-regulated five days after infection in most of the tissues, whilst peptide quantification of all three AMPs decreased in the brain, the target tissue for NNV, but increased in the head-kidney 5 days after infection. Further research should be carried out to ascertain the role of AMPs in fish innate immunity and to understand how NNV evades the immune response to be disseminated.

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.

Teleost cytotoxic T cells

Cell-mediated cytotoxicity is one of the major mechanisms by which vertebrates control intracellular pathogens. Two cell types are the main players in this immune response, natural killer (NK) cells and cytotoxic T lymphocytes (CTL). While NK cells recognize altered target cells in a relatively unspecific manner CTLs use their T cell receptor to identify pathogen-specific peptides that are presented by major histocompatibility (MHC) class I molecules on the surface of infected cells. However, several other signals are needed to regulate cell-mediated cytotoxicity involving a complex network of cytokine- and ligand-receptor interactions. Since the first description of MHC class I molecules in teleosts during the early 90s of the last century a remarkable amount of information on teleost immune responses has been published. The corresponding studies describe teleost cells and molecules that are involved in CTL responses of higher vertebrates. These studies are backed by functional investigations on the killing activity of CTLs in a few teleost species. The present knowledge on teleost CTLs still leaves considerable room for further investigations on the mechanisms by which CTLs act. Nevertheless the information on teleost CTLs and their regulation might already be useful for the control of fish diseases by designing efficient vaccines against such diseases where CTL responses are known to be decisive for the elimination of the corresponding pathogen. This review summarizes the present knowledge on CTL regulation and functions in teleosts. In a special chapter, the role of CTLs in vaccination is discussed.

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.

Turbot (Scophthalmus maximus) Nk-lysin induces protection against the pathogenic parasite Philasterides dicentrarchi via membrane disruption

P. dicentrarchi is one of the most threatening pathogens for turbot aquaculture. This protozoan ciliate is a causative agent of scuticociliatosis, which is a disease with important economic consequences for the sector. Neither vaccines nor therapeutic treatments are commercially available to combat this infection. Numerous antimicrobial peptides (AMPs) have demonstrated broad-spectrum activity against bacteria, viruses, fungi, parasites and even tumor cells; an example is Nk-lysin (Nkl), which is an AMP belonging to the saposin-like protein (SAPLIP) family with an ability to interact with biological membranes. Following the recent characterization of turbot Nkl, an expression plasmid encoding Nkl was constructed and an anti-Nkl polyclonal antibody was successfully tested. Using these tools, we demonstrated that although infection did not clearly affect nkl mRNA expression, it induced changes at the protein level. Turbot Nkl had the ability to inhibit proliferation of the P. dicentrarchi parasite both in vivo and in vitro. Moreover, a shortened peptide containing the active core of turbot Nkl (Nkl71-100) was synthesized and showed high antiparasitic activity with a direct effect on parasite viability that probably occurred via membrane disruption. Therefore, the nkl gene may be a good candidate for genetic breeding selection of fish, and either the encoded peptide or its shortened analog is a promising antiparasitic treatment in aquaculture.