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

Investigating the nutritional viability of marine-derived protein for sustainable future development

Marine-derived proteins are emerging as a pivotal resource with diverse applications in food, pharmaceuticals, and biotechnological industries. The marine environment offers many protein sources, including fish, shellfish, algae, and microbes, which garnered attention due to their nutritional composition. Evaluating their protein and amino acid profiles is essential in assessing their viability as substitutes for conventional protein sources. Continuously exploiting marine ecosystems for protein extraction has led to significant environmental impacts. The optimization of aquacultural practices and implementation of innovative practices are imperative for the sustainable production of marine-based protein. This review will discuss the different sources of marine proteins, their nutritional profile, and their associated environmental impact. It also reviews the relationship between aquaculture advancements and regulatory frameworks toward attaining sustainable practices, alongside exploring the challenges and potentials in utilizing marine sources for protein production.

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.

Assessment of the antimicrobial and immunomodulatory activity of QS-CATH, a promising therapeutic agent isolated from the Chinese spiny frogs (Quasipaa spinosa)

Cathelicidins are important antimicrobial peptides in various vertebrate species where they are crucial parts of the innate immune system. The current understanding of amphibian cathelicidins is limited, particularly with regard to their immunomodulatory effects. To address this knowledge gap, we produced the cDNA sequence of the cathelicidin gene from a skin transcriptome of the Chinese spiny frog Quasipaa spinosa. The amino acid sequence of the Quasipaa spinosa cathelicidin (QS-CATH) was predicted to consist of a signal peptide, a cathelin domain, and a mature peptide. Comparative analysis of the QS-CATH amino acid sequence with that of other amphibian cathelicidins revealed high variability in the functional mature peptide among amphibians, whereas the cathelin domain was conserved. The QS-CATH gene was expressed in several tissues, with the highest level of expression in the spleen. Upregulation of QS-CATH after Aeromonas hydrophila infection occurred in the kidney, gut, spleen, skin, and liver. Chemically synthesized QS-CATH exhibited pronounced antibacterial activity against Shigella flexneri, Staphylococcus warneri, Escherichia coli, Salmonella enterica, and Listeria monocytogenes. Furthermore, QS-CATH disrupted the cell membrane integrity of S. flexneri, as evidenced by a lactate dehydrogenase release assay, and it hydrolyzed the genomic DNA of S. flexneri. Additionally, QS-CATH elicited chemotaxis and modulated the expression of inflammatory cytokine genes in RAW264.7 mouse leukemic monocyte/macrophage cells. These findings confirm the antimicrobial effects of amphibian cathelicidin and its ability to influence immune cell function. This will expedite the potential utilization of amphibian antimicrobial peptides as therapeutic agents.

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.

Synthetic Antimicrobial Peptides Fail to Induce Leucocyte Innate Immune Functions but Elicit Opposing Transcriptomic Profiles in European Sea Bass and Gilthead Seabream

Antimicrobial peptides (AMPs) are promising molecules in diverse fields, including aquaculture. AMPs possess lytic effects on a wide range of pathogens, resulting in a potential replacement for traditional antimicrobials in aquaculture. In addition, they also have modulatory effects on host immune responses. Thus, the objective of this work was to evaluate the immunomodulatory capability of three known synthetic AMPs derived from European sea bass, NK-lysin (Nkl), hepcidin (Hamp), and dicentracin (Dic), in head-kidney cell suspensions from European sea bass and gilthead seabream. The tested peptides were neither cytotoxic for European sea bass nor gilthead seabream cells and failed to modulate the respiratory burst and phagocytosis activities. However, they modified the pattern of transcription of immune-related genes differently in both species. Peptides were able to promote the expression of marker genes for anti-inflammatory (il10), antiviral (mx, irf3), cell-mediated cytotoxicity (nccrp1, gzmb), and antibody responses (ighm) in European sea bass, with the Nkl peptide being the most effective. Contrary to this, the effects of those peptides on gilthead seabream mainly resulted in the suppression of immune responses. To conclude, European sea bass-derived peptides can be postulated as potential tools for immunostimulation in European sea bass fish farms, but more efforts are required for their universal use in other species.

Temperature impacts Atlantic salmon's (Salmo salar) immunological response to infectious salmon anemia virus (ISAv)

Ocean temperatures continue to rise annually due to the ever-growing consequences of global climate change. These temperature changes can have an impact on the immunological robustness of cultured fish, especially cold-water species such as Atlantic salmon. The salmon farming industry already loses hundreds of millions of dollars each year to infectious and non-infectious diseases. One particularly important and WOAH reportable disease is infectious salmon anemia caused by the orthomyxovirus ISAv. Considering the changing environment, it is necessary to find ways to mitigate the effect of diseases on the industry. For this study, 20 Atlantic salmon families were housed in each of 38 different tanks at the AVC, with half of the fish being kept at 10 °C and half being kept at 20 °C. Donor Atlantic salmon IP- injected with a highly virulent ISAv isolate (HPR4; TCID₅₀ of 1 × 10⁵/mL) were added to each tank as the source of co-habitation infection. Both temperatures were sampled at onset of mortality in co-habited fish and at resolution of mortality. Family background and temperature significantly impacted ISAv load, as assessed by qPCR, time to mortality and overall mortality. Mortality was more acute at 20 °C, but overall mortality was higher at 10 °C. Based on percent mortality calculated over the course of the study, different families demonstrated different levels of survival. The three families that demonstrated the highest percent mortality, and the three families with the lowest percent mortality were then assessed for their antiviral responses using relative gene expression. Genes significantly upregulated between the unexposed fish and ISAv exposed fish included mx1, il4/13a, il12rb2, and trim25, and these were further impacted by temperature. Understanding how ISAv resistance is impacted by temperature can help identify seasonal risks of ISAv outbreaks as well as ideal responses to be targeted through immunopotentiation.

Advancements of fish-derived peptides for mucormycosis: a novel strategy to treat diabetic compilation

Mucormycosis, an extremely fatal fungal infection, is a major hurdle in the treatment of diabetes consequences. The increasing prevalence and restricted treatment choices urge the investigation of novel therapeutic techniques. Because of their effective antimicrobial characteristics and varied modes of action, fish-derived peptides have lately emerged as viable options in the fight against mucormycosis. This review examines the potential further application of fish-derived peptides in diagnosing and managing mucormycosis in relation to diabetic complications. First, we examine the pathophysiology of mucormycosis and the difficulties in treating it in diabetics. We emphasize the critical need for alternative therapeutic methods for tackling the limitations of currently available antifungal medicines. The possibility of fish-derived peptides as an innovative approach to combat mucormycosis is then investigated. These peptides, derived from several fish species, provide wide antimicrobial properties against a variety of diseases. They also have distinct modes of action, such as rupture of cell membranes, suppression of development, and modification of the host immunological response. Furthermore, we investigate the problems and prospects connected with the clinical application of fish-derived peptides. Ultimately, future advances in fish-derived peptides, offer interesting avenues for the management of mucormycosis in the context of diabetic comorbidities. More research and clinical trials are needed to properly investigate these peptide's therapeutic potential and pave the way for their adoption into future antifungal therapies.

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.

Functional and molecular characterization of the Atlantic salmon gill epithelium cell line ASG-10; a tool for in vitro gill research

Fish gills are not only the respiratory organ, but also essential for ion-regulation, acid-base control, detoxification, waste excretion and host defense. Multifactorial gill diseases are common in farmed Atlantic salmon, and still poorly understood. Understanding gill pathophysiology is of paramount importance, but the sacrifice of large numbers of experimental animals for this purpose should be avoided. Therefore, in vitro models, such as cell lines, are urgently required to replace fish trials. An Atlantic salmon gill epithelial cell line, ASG-10, was established at the Norwegian Veterinary institute in 2018. This cell line forms a monolayer expressing cytokeratin, e-cadherin and desmosomes, hallmarks of a functional epithelial barrier. To determine the value of ASG-10 for comparative studies of gill functions, the characterization of ASG-10 was taken one step further by performing functional assays and comparing the cell proteome and transcriptome with those of gills from juvenile freshwater Atlantic salmon. The ASG-10 cell line appear to be a homogenous cell line consisting of epithelial cells, which express tight junction proteins. We demonstrated that ASG-10 forms a barrier, both alone and in co-culture with the Atlantic salmon gill fibroblast cell line ASG-13. ASG-10 cells can phagocytose and express several ATP-binding cassette transport proteins. Additionally, ASG-10 expresses genes involved in biotransformation of xenobiotics and immune responses. Taken together, this study provides an overview of functions that can be studied using ASG-10, which will be an important contribution to in vitro gill epithelial research of Atlantic salmon.

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.

Chitosan Microparticles Enhance the Intestinal Release and Immune Response of an Immune Stimulant Peptide in Oncorhynchus mykiss

The aquaculture industry is constantly increasing its fish production to provide enough products to maintain fish consumption worldwide. However, the increased production generates susceptibility to infectious diseases that cause losses of millions of dollars to the industry. Conventional treatments are based on antibiotics and antivirals to reduce the incidence of pathogens, but they have disadvantages, such as antibiotic resistance generation, antibiotic residues in fish, and environmental damage. Instead, functional foods with active compounds, especially antimicrobial peptides that allow the generation of prophylaxis against infections, provide an interesting alternative, but protection against gastric degradation is challenging. In this study, we evaluated a new immunomodulatory recombinant peptide, CATH-FLA, which is encapsulated in chitosan microparticles to avoid gastric degradation. The microparticles were prepared using a spray drying method. The peptide release from the microparticles was evaluated at gastric and intestinal pH, both in vitro and in vivo. Finally, the biological activity of the formulation was evaluated by measuring the expression of il-1β, il-8, ifn-γ, Ifn-α, and mx1 in the head kidney and intestinal tissues of rainbow trout (Oncorhynchus mykiss). The results showed that the chitosan microparticles protect the CATH-FLA recombinant peptide from gastric degradation, allowing its release in the intestinal portion of rainbow trout. The microparticle-protected CATH-FLA recombinant peptide increased the expression of il-1β, il-8, ifn-γ, ifn-α, and mx1 in the head kidney and intestine and improved the antiprotease activity in rainbow trout. These results suggest that the chitosan microparticle/CATH-FLA recombinant peptide could be a potential prophylactic alternative to conventional antibiotics for the treatment of infectious diseases in aquaculture.

Identification of antibacterial activity of LEAP2 from Antarctic icefish Chionodraco hamatus

Liver-expressed antimicrobial peptide 2 (LEAP2) is a small peptide, which is consisted of signal peptide, pro-peptide and the bioactive mature peptide. Mature LEAP2 is an antibacterial peptide with four highly conserved cysteines forming two intramolecular disulfide bonds. Chionodraco hamatus, an Antarctic notothenioid fish that lives in the coldest water, has white blood unlike most fish of the world. In this study, the LEAP2 coding sequence was cloned from C. hamatus, including a 29 amino acids signal peptide and mature peptide of 46 amino acids. High levels of LEAP2 mRNA were detected in the skin and liver. Mature peptide was obtained by chemical synthesis in vitro, displayed selective antimicrobial activities against Escherichia coli, Aeromonas hydrophila, Staphylococcus aureus and Streptococcus agalactiae. Liver-expressed antimicrobial peptide 2 showed bactericidal activity by destroying the cell membrane integrity and robustly combined with bacterial genomic DNA. In addition, overexpression of the Tol-LEAP2-EGFP in zebrafish larva showed stronger antimicrobial activity in C. hamatus than in zebrafish, accompanied by lower bacterial load and expression of pro-inflammatory factors. This is the first demonstration of the antimicrobial activity of LEAP2 from C. hamatus, which is of useful value in improving resistance to pathogens.

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.

Immunity elicited by AMP-encoding plasmids fails to increase the protection of European sea bass against nodavirus

Antimicrobial peptides (AMPs) are a potent arm of the innate immune system that can directly kill pathogens and induce immunomodulation. In the marine aquaculture, European sea bass (Dicentrarchus labrax L.) is one of the most prosperous species but is highly susceptible to nodavirus (NNV), which produces high rates of mortality in larvae and juvenile stages. Thus, we aimed to evaluate whether AMPs exert immunomodulatory and/or NNV-preventive actions in sea bass. To do this, plasmids encoding the sea bass AMPs dicentracin (pDIC), beta-defensin (pDB1), hepcidin (pHAMP2) or NK-lysin (pNKL) were generated and intramuscularly injected into sea bass juveniles to evaluate their immunomodulatory and anti-NNV roles. Sea bass muscle transcribes the AMPs and produces an increase in their circulating levels, along with an increase of the antibacterial activity. Immune-related gene analysis revealed a great activation of the inflammatory response and the recruitment of neutrophilic granulocytes at the site of injection. However, AMP-encoding plasmids, namely pHAMP2, negatively affected to NNV disease by increasing fish mortality. In conclusion, plasmids encoding AMPs show immunostimulatory effects on European sea bass but do not improve the resistance to NNV.

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.

Immunomodulatory effects of chicken cathelicidin-2 on a primary hepatic cell co-culture model

Cathelicidin-2 is an antimicrobial peptide (AMP) produced as part of the innate immune system of chickens and might be a new candidate to combat infection and inflammation within the gut-liver axis. Studying the hepatic immune response is of high importance as the liver is primarily exposed to gut-derived pathogen-associated molecular patterns. The aim of the present study was to assess the effects of chicken cathelicidin-2 alone or combined with lipoteichoic acid (LTA) or phorbol myristate acetate (PMA) on cell viability, immune response and redox homeostasis in a primary hepatocyte-non-parenchymal cell co-culture of chicken origin. Both concentrations of cathelicidin-2 decreased the cellular metabolic activity and increased the extracellular lactate dehydrogenase (LDH) activity reflecting reduced membrane integrity. Neither LTA nor PMA affected these parameters, and when combined with LTA, cathelicidin-2 could not influence the LDH activity. Cathelicidin-2 had an increasing effect on the concentration of the proinflammatory CXCLi2 and interferon- (IFN-)γ, and on that of the anti-inflammatory IL-10. Meanwhile, macrophage colony stimulating factor (M-CSF), playing a complex role in inflammation, was diminished by the AMP. LTA elevated IFN-γ and decreased M-CSF levels, while PMA only increased the concentration of M-CSF. Both concentrations of cathelicidin-2 increased the H2O2 release of the cells, but the concentration of malondialdehyde as a lipid peroxidation marker was not affected. Our findings give evidence that cathelicidin-2 can also possess anti-inflammatory effects, reflected by the alleviation of the LTA-triggered IFN-γ elevation, and by reducing the M-CSF production induced by PMA. Based on the present results, cathelicidin-2 plays a substantial role in modulating the hepatic immune response with a multifaceted mode of action. It was found to have dose-dependent effects on metabolic activity, membrane integrity, and reactive oxygen species production, indicating that using it in excessively high concentrations can contribute to cell damage. In conclusion, cathelicidin-2 seems to be a promising candidate for future immunomodulating drug development with an attempt to reduce the application of antibiotics.

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.

Modulation of Host Immune Response during Leishmania infantum Natural Infection: A Whole-Transcriptome Analysis of the Popliteal Lymph Nodes in Dogs

Leishmania infantum, the etiological agent of canine leishmaniosis (CanL) in Europe, was responsible of the largest outbreak of human leishmaniosis in Spain. The parasite infects and survives within myeloid lineage cells, causing a potentially fatal disease if left untreated. The only treatment option relies on chemotherapy, although immunotherapy strategies are being considered as novel approaches to prevent progression of the disease. To this aim, a deeper characterization of the molecular mechanisms behind the immunopathogenesis of leishmaniosis is necessary. Thus, we evaluated, for the first time, the host immune response during L. infantum infection through transcriptome sequencing of the popliteal lymph nodes aspirates of dogs with CanL. Differential expression and weighted gene co-expression network analyses were performed, resulting in the identification of 5,461 differentially expressed genes (DEGs) and four key modules in sick dogs, compared to controls. As expected, defense response was the highest enriched biological process in the DEGs, with six genes related to immune response against pathogens (CHI3L1, SLPI, ACOD1, CCL5, MPO, BPI) included among the ten most expressed genes; and two of the key co-expression modules were associated with regulation of immune response, which also positively correlated with clinical stage and blood monocyte concentration. In particular, sick dogs displayed significant changes in the expression of Th1, Th2, Th17 and Tr1 cytokines (e. g. TNF-α, IFN-γ, IL-21, IL-17, IL-15), markers of T cell and NK cell exhaustion (e. g. LAG3, CD244, Blimp-1, JUN), and B cell, monocyte and macrophage disrupted functionality (e. g. CD40LG, MAPK4, IL-1R, NLRP3, BCMA). In addition, we found an overexpression of XBP1 and some other genes involved in endoplasmic reticulum stress and the IRE1 branch of the unfolded protein response, as well as one co-expression module associated with these processes, which could be induced by L. infantum to prevent host cell apoptosis and modulate inflammation-induced lymphangiogenesis at lymph nodes. Moreover, 21 lncRNAs were differentially expressed in sick dogs, and one key co-expression module was associated with chromatin organization, suggesting that epigenetic mechanisms could also contribute to dampening host immune response during natural L. infantum infection in the lymph nodes of dogs suffering from clinical leishmaniosis.

Molecular Cloning, Functional and Biophysical Characterization of an Antimicrobial Peptide from Rhizosphere Soil

AIM

This study was designed to screen and identify an antimicrobial peptide from rhizosphere soil. The study was further focused towards overexpression, purification and characterization of this antimicrobial peptide, and to functionally validate its efficiency and efficacy as an antimicrobial agent. Yet the study was further aimed at corroborating structural and functional studies using biophysical tools.

BACKGROUND

Antimicrobial resistance is emerging as one of the top 10 global health crisis, it is multifaceted and is the second largest cause of mortality. According to the World Health Organization (WHO), around the world, an estimated 700,000 people die each year from infection caused by antibiotic-resistant microbes. Antimicrobial peptides offers best alternative to combat and overcome this crisis. In this manuscript, we report cloning, expression, purification and characterization of an antimicrobial peptide discovered from rhizosphere soil.

OBJECTIVE

Objectives of this study includes construction, screening and identification of antimicrobial peptide from metagenome followed by its expression, purification and functional and biophysical investigation. Yet another objective of the study was to determine antimicrobial efficacy and efficiency as an antimicrobial peptide towards MRSA strains.

METHODS

In this study, we used array of molecular biology tools that include genetic engineering, PCR amplification, construction of an expression construct and NI-NTA based purification of the recombinant peptide. We have also carried out antimicrobial activity assay to determine MIC and IC50 values of antimicrobial peptide. To establish structural and functional relationship, circular dichroism, and both extrinsic and intrinsic fluorescence spectroscopy studies were carried out.

RESULTS

Screening of metagenomic library resulted in identification of gene (~500bp) harbouring an open reading frame (ORF) consisting of 282 bp. Open reading frame identified in gene encodes an antimicrobial peptide which had shared ~95% sequence similarity with the antimicrobial peptide of Bacillus origin. Purification of recombinant protein using Ni-NTA column chromatography demonstrated a purified protein band of ~11 kDa on 14% SDS-PAGE which is well corroborated to theoretical deduced molecular weight of peptide from its amino acids sequence. Interestingly, the peptide exhibited antimicrobial activity in broad range of pH and temperature. MIC (minimum inhibitory concentration) determined against gram positive Bacillus sp. was found to be 0.015mg/ml, whereas in case of gram negative E. coli, it was calculated to be 0.062mg/ml. The peptide exhibited IC50 values corresponding to ~0.25mg/ml against Bacillus and ~0.5 mg/ml against E. coli. Antimicrobial susceptibility assay performed against methicillin resistant Staphylococcus aureus strain ATCC 3412 and standard strain of Staphylococcus aureus ATCC 9144 revealed its strong inhibitory activity against MRSA, whereby we observed a ~16mm clearance zone at higher peptide concentrations ~2mg/ml (~181.8µM). Biophysical investigation carried out using Trp fluorescence, ANS fluorescence and circular dichroism spectroscopy further revealed conformational stability in its secondary and tertiary structure at wide range of temperature and pH.

CONCLUSION

Altogether, the peptide discovered from rhizosphere metagenome hold potential in inhibiting the growth of both the gram positive and gram negative bacteria, and was equally effective in inhibiting the multidrug resistant pathogenic strains (MRSA).

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.

Molecular characterization of cathelicidin in tiger frog (Hoplobatrachus rugulosus): Antimicrobial activity and immunomodulatory activity

Cathelicidins are an important antimicrobial peptide family and are expressed in many different vertebrates. They play an important role in the innate immune system of the host. However, amphibian cathelicidins are poorly understood. In this study, the cDNA of the cathelicidin gene was obtained from the skin transcriptome of tiger frog (Hoplobatrachus rugulosus). The predicted amino acid sequence of tiger frog cathelicidin (HR-CATH) comprises a signal peptide, a cathelin domain, and a mature peptide. The HR-CATH amino acid sequence alignment with other frog cathelicidins showed that the functional mature peptide is highly variable in amphibians, whereas the cathelin domain is conserved. A phylogenetic tree analysis showed that HR-CATH is most closely related to cathelicidin-NV from Nanorana ventripunctata. HR-CATH was chemically synthesized and its in vitro activity was determined. It had high antibacterial activity against Vibrio parahaemolyticus, Staphylococcus aureus, and the pathogenic bacterium Aeromonas hydrophila. HR-CATH damaged the cell membrane integrity of A. hydrophila according to a lactate dehydrogenase release assay and was able to hydrolyze the genomic DNA from A. hydrophila in a dose-dependent manner. Furthermore, in RAW264.7 cells (mouse leukemic monocyte/macrophage cell line), HR-CATH induced chemotaxis and enhanced respiratory burst. Our study shows that amphibian cathelicidin has antimicrobial activity and an immunomodulatory effect on immune cells.

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.

Antimicrobial Peptides of Salmonid Fish: From Form to Function

Antimicrobial peptides (AMPs) are small, usually cationic, and amphiphilic molecules that play a crucial role in molecular and cellular host defense against pathogens, tissue damage, and infection. AMPs are present in all metazoans and several have been discovered in teleosts. Some teleosts, such as salmonids, have undergone whole genome duplication events and retained a diverse AMP repertoire. Salmonid AMPs have also been shown to possess diverse and potent antibacterial, antiviral, and antiparasitic activity and are induced by a variety of factors, including dietary components and specific molecules also known as pathogen-associated molecular patterns (PAMPs), which may activate downstream signals to initiate transcription of AMP genes. Moreover, a multitude of cell lines have been established from various salmonid species, making it possible to study host-pathogen interactions in vitro, and several of these cell lines have been shown to express various AMPs. In this review, the structure, function, transcriptional regulation, and immunomodulatory role of salmonid AMPs are highlighted in health and disease. It is important to characterize and understand how salmonid AMPs function as this may lead to a better understanding of host-pathogen interactions with implications for aquaculture and medicine.

Antimicrobial efficacy of Cecropin A (1-7)- Melittin and Lactoferricin (17-30) against multi-drug resistant Salmonella Enteritidis

The present study evaluated intracellular antibacterial efficacy of two short-chain cationic antimicrobial peptides (AMPs) namely, Cecropin A (1-7)-Melittin and lactoferricin (17-30) against three field strains of multi-drug resistant Salmonella Enteritidis. Initially, antimicrobial ability of both the AMPs was evaluated for their minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) against multi-drug resistant S. Enteritidis strains. Besides, the AMPs were evaluated for its in vitro stability (high-end temperatures, proteases, physiological concentrations of cationic salts and pH) and safety (haemolytic assay in sheep erythrocytes; cytotoxicity assay in murine macrophage RAW 264.7 cell line and human epithelioma HEp-2 cell line and beneficial gut lactobacilli). Later, a time-kill assay was performed to assess the intracellular antibacterial activity of Cecropin A (1-7)-Melittin and lactoferricin (17-30) against multi-drug resistant S. Enteritidis in RAW 264.7 and HEp-2 cells. The observed MBC values of Cecropin A (1-7)-Melittin and lactoferricin (17-30) against multi-drug resistant S. Enteritidis (128 μM; 256 μM) were generally twice or four-fold greater than the MIC values (64 μM). Further, both the AMPs were found variably stable after exposure at high-end temperatures (70 °C and 90 °C), protease treatment (trypsin, proteinase K, lysozyme), higher concentration of physiological salts (150 mM NaCl and 2 mM MgCl₂) and hydrogen ion concentrations (pH 4.0 to 8.0). Both the AMPs were found non-haemolytic on sheep erythrocytes, revealed minimal cytotoxicity in RAW 264.7 and HEp-2 cells, and was tested safe against beneficial gut lactobacilli (L. acidophilus and L. rhamnosus). Intracellular bacteriostatic effect with both cationic AMPs against multi-drug resistant S. Enteritidis was evident in RAW 264.7 cells; however, in both the cell lines, the significant bactericidal effect was not observed (P > 0.05) with both cationic AMPs understudy against multi-drug resistant S. Enteritidis. Based on the results of the present study, both the cationic AMPs understudy may not be useful for the intracellular elimination of multi-drug resistant S. Enteritidis; hence, further studies such as conjugation of these AMPs with either cell-penetrating peptides (CPP) and/or nanoparticles (NPs) are warranted.

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.

The host defense peptide β-defensin confers protection against Vibrio anguillarum in ayu, Plecoglossus altivelis

β-defensin is a cationic host defense peptide actively participating in host innate immune response against pathogens. In teleost fish, β-defensin exhibits a diversity in genotypes and functions. Herein, a β-defensin homolog (PaBD) was identified from ayu, Plecoglossus altivelis, showing multiple tissues' upregulation against Vibrio anguillarum challenge. In vivo experiments revealed that intraperitoneal injection of chemically synthesized mature PaBD (mPaBD) increased the survival rate of V. anguillarum-infected ayu, accompanied by reduced bacterial load and decreased tissue mRNA levels of tumor necrosis factor α (PaTNF-α) and interleukin 1β (PaIL-1β). However, in vitro, mPaBD showed weak bactericidal activity against V. anguillarum. Interestingly, mPaBD enhanced phagocytosis, intracellular bacterial killing, and respiratory burst of ayu monocytes/macrophages (MO/MΦ). Moreover, it inhibited mRNA levels of PaIL-1β and PaTNF-α in MO/MФ upon V. anguillarum infection. In conclusion, PaBD protects ayu against V. anguillarum challenge not only through its direct antibacterial ability, but also through its immunomodulation in MO/MΦ.

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.

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.

Antimicrobial and Immunomodulatory Properties and Applications of Marine-Derived Proteins and Peptides

Marine organisms provide an abundant source of potential medicines. Many of the marine-derived biomaterials have been shown to act as different mechanisms in immune responses, and in each case they can significantly control the immune system to produce effective reactions. Marine-derived proteins, peptides, and protein hydrolysates exhibit various physiologic functions, such as antimicrobial, anticancer, antioxidant, antihypertensive, and anti-inflammatory activities. Recently, the immunomodulatory properties of several antimicrobial peptides have been demonstrated. Some of these peptides directly kill bacteria and exhibit a variety of immunomodulatory activities that improve the host innate immune response and effectively eliminate infection. The properties of immunomodulatory proteins and peptides correlate with their amino acid composition, sequence, and length. Proteins and peptides with immunomodulatory properties have been tested in vitro and in vivo, and some of them have undergone different clinical and preclinical trials. This review provides a comprehensive overview of marine immunomodulatory proteins, peptides, and protein hydrolysates as well as their production, mechanisms of action, and applications in human therapy.