CK11, a Teleost Chemokine with a Potent Antimicrobial Activity

A review of soluble factors and receptors involved in fish skin immunity: The tip of the iceberg

The immune system of fish possesses soluble factors, receptors, pathways and cells very similar to those of the other vertebrates' immune system. Throughout evolutionary history, the exocrine secretions of organisms have accumulated a large reservoir of soluble factors that serve to protect organisms from microbial pathogens that could disrupt mucosal barrier homeostasis. In parallel, a diverse set of recognition molecules have been discovered that alert the organism to the presence of pathogens. The known functions of both the soluble factors and receptors mentioned above encompass critical aspects of host defense, such as pathogen binding and neutralization, opsonization, or modulation of inflammation if present. The molecules and receptors cooperate and are able to initiate the most appropriate immune response in an attempt to eliminate pathogens before host infection can begin. Furthermore, these recognition molecules, working in coordination with soluble defence factors, collaboratively erect a robust and perfectly coordinated defence system with complementary specificity, activity and tissue distribution. This intricate network constitutes an immensely effective defence mechanism for fish. In this context, the present review focuses on some of the main soluble factors and recognition molecules studied in the last decade in the skin mucosa of teleost fish. However, knowledge of these molecules is still very limited in all teleosts. Therefore, further studies are suggested throughout the review that would help to better understand the functions in which the proteins studied are involved.

The Teleost CXCL13-CXCR5 Axis Induces Inflammatory Cytokine Expression through the Akt-NF-κB, p38-AP-1, and p38-NF-κB Pathways

The ancestors of chemokines originate in the most primitive of vertebrates, which has recently attracted great interest in the immune functions and the underlying mechanisms of fish chemokines. In the current study, we identified an evolutionarily conserved chemokine, CiCXCL13, from a teleost fish, grass carp. CiCXCL13 was characterized by a typical SCY (small cytokine CXC) domain and four cysteine residues (C34, C36, C61, C77), with the first two cysteines separated by a random amino acid residue, although it shared 24.2-54.8% identity with the counterparts from other vertebrates. CiCXCL13 was an inducible chemokine, whose expression was significantly upregulated in the immune tissues of grass carps after grass carp reovirus infection. CiCXCL13 could bind to the membrane of grass carp head kidney leukocytes and promote cell migration, NO release, and the expression of >15 inflammatory cytokines, including IL-1β, TNF-α, IL-10 and TGF-β1, thus regulating the inflammatory response. Mechanistically, CiCXCL13 interacted with its evolutionarily conserved receptor CiCXCR5 and activated the Akt-NF-κB and p38-AP-1 pathways, as well as a previously unrevealed p38-NF-κB pathway, to efficiently induce inflammatory cytokine expression, which was distinct from that reported in mammals. Zebrafish CXCL13 induced inflammatory cytokine expression through Akt, p38, NF-κB, and AP-1 as CiCXCL13. Meanwhile, the CiCXCL13-CiCXCR5 axis-mediated inflammatory activity was negatively shaped by grass carp atypical chemokine receptor 2 (CiACKR2). The present study is, to our knowledge, the first to comprehensively define the immune function of CXCL13 in inflammatory regulation and the underlying mechanism in teleosts, and it provides a valuable perspective on the evolution and biology of fish chemokines.

Two CcCCL19bs orchestrate an antibacterial immune response in Yellow River carp (Cyprinus carpio haematopterus)

Chemokines are a group of chemotactic cytokines with an essential role in homeostasis as well as immunity via specific G protein-coupled receptors and atypical receptors. In our study, two Yellow River carp (Cyprinus carpio haematopterus) CCL19b genes (CcCCL19bs), tentatively named CcCCL19b_a and CcCCL19b_b, were cloned. The open reading frames (ORFs) of CcCCL19b_a and CcCCL19b_b were both 333 bp that encoded a 12 kDa protein with 110 amino acid residues. CcCCL19bs contained a signal peptide and a SCY domain with four typical conserved cysteine residues. The two CcCCL19b proteins shared high similarities with each other in both secondary and three-dimensional structure. Phylogenetic analysis showed that CcCCL19bs and other CCL19bs from tetraploid cyprinid fish were clustered into one clade. CcCCL19bs were highly expressed in gill and intestine in healthy fish, and a significant up-regulation of gene expression after Aeromonas hydrophila infection and poly(I:C) stimulation was observed in gill, liver, and head kidney. Furthermore, chemotaxis and antibacterial activity of CcCCL19bs were studied. The results indicated that recombinant CcCCL19b_a and CcCCL19b_b protein (rCcCCL19b_a and rCcCCL19b_b) exhibited significant attraction to primary head kidney leukocytes (HKLs). Meanwhile, both of rCcCCL19bs could promote the proliferation of HKLs, and significantly up-regulate the expressions of IL-1β, CCR7, and IL-6, and down-regulate the expression of IL-10 in primary HKLs. In vitro, rCcCCL19bs could bind and aggregate A. hydrophila and Staphylococcus aureus. The rCcCCL19bs exhibited significant antibacterial activity against A. hydrophila, but not S. aureus. Moreover, they inhibited the growth of A. hydrophila and S. aureus. In vivo, overexpression of CcCCL19bs contributed to the bacterial clearance. These studies suggested that CcCCL19bs orchestrate an antibacterial immune response.

Genome-wide association study of host resistance to the ectoparasite Ichthyophthirius multifiliis in the Amazon fish Colossoma macropomum

BACKGROUND

Tambaqui, Colossoma macropomum, is the most important native fish species farmed in South America, particularly in Brazil, where its production is limited in the southern and southeastern regions due to disease outbreaks caused by the parasite Ichthyophthirius multifiliis. Therefore, genome level analysis to understand the genetic architecture of the host resistance against I. multifiliis is fundamental to improve this trait in tambaqui. The objective of the present study was to map QTL (quantitative trait loci) associated with resistance to I. multifiliis in tambaqui by GWAS (genome-wide association study).

METHODS AND RESULTS

Individuals belonging to seven families, which were previously submitted to an experimental challenge to assess the natural resistance to the parasite I. multifiliis, were used for genomic analysis. A total of 7717 SNPs were identified in this population by ddRAD (double digest restriction site associated DNA). GWAS revealed four SNPs significantly associated in the LGs (linkage groups) 2, 9, 11 and 20 for the traits time of death and parasite load. The SNPs explained a low proportion of the variance to I. multifiliis resistance for time of death and parasite load (about 0.622% and 0.375%, respectively). The SNPs were close to 11 genes related to the immune system: abcf3, znf830, ccr9, gli3, ackr4, tbata, ndr2, tgfbr3, nhej1, znf644b, and cldn10a.

CONCLUSIONS

In conclusion, the resistance to I. multifiliis is probably under polygenic control in tambaqui, in which different QTLs of low variance can be involved in the immune responses against this ectoparasite.

Genome-wide identification, evolutionary analysis, and antimicrobial activity prediction of CC chemokines in allotetraploid common carp, Cyprinus carpio

Chemokines are a group of secreted small molecules which are essential for cell migration in physiological and pathological conditions by binding to specific chemokine receptors. They are structurally classified into five groups, namely CXC, CC, CX3C, XC and CX. CC chemokine group is the largest one among them. In this study, we identified and characterized 61 CC chemokines from allotetraploid common carp (Cyprinus carpio). The sequence analyses showed that the majority of CC chemokines had an N-terminal signal peptide, and an SCY domain, and all CC chemokines were located in the extracellular region. Phylogenetic, evolutionary and syntenic analyses confirmed that CC chemokines were annotated as 11 different types (CCL19, CCL20, CCL25, CCL27, CCL32, CCL33, CCL34, CCL35, CCL36, CCL39, and CCL44), which exhibited unique gene arrangement pattern and chromosomal location respectively. Furthermore, genome synteny analyses between common carp and four representative teleost species indicated expansion of common carp CC chemokines resulted from the whole genome duplication (WGD) event. Additionally, the continuous evolution of gene CCL25s in teleost afforded a novel viewpoint to explain the WGD event in teleost. Then, we predicted the three-dimensional structures and probable function regions of common carp CC chemokines. All the CC chemokines core structures were constituted of an N-loop, a three-stranded β-sheet, and a C-terminal helix. Finally, 43 CC chemokines were predicted to have probable general antimicrobial activity. Their tertiary structures, cationic and amphiphilic physicochemical property supported the viewpoint. To verify the prediction, six recombinant CCL19s proteins were prepared and the antibacterial activity against Escherichia coli and Aeromonas hydrophila were verified. The results supported our prediction that rCCL19a.1s (rCCL19a.1_a, rCCL19a.1_b) and rCCL19bs (rCCL19b_a, rCCL19b_b), especially rCCL19bs, exhibited extremely significant inhibition to the growth of both E. coli and A. hydrophila. On the contrary, two rCCL19a.2s had no significant inhibitory effect. These studies suggested that CC chemokines were essential in immune system evolution and not monofunctional during pathogen infection.

CC and CXC chemokines in turbot (Scophthalmus maximus L.): Identification, evolutionary analyses, and expression profiling after Aeromonas salmonicida infection

Chemokines are a superfamily of structurally related cytokines, which exert essential roles in guiding cell migration in development, homeostasis, and immunity. CC and CXC chemokines are the two major subfamilies in teleost species. In this study, a total of seventeen CC and CXC chemokines, with inclusion of twelve CC and five CXC chemokines, were systematically identified from the turbot genome, making turbot the teleost harboring the least number of CC and CXC chemokines among all teleost species ever reported. Phylogeny, synteny, and genomic organization analyses were performed to annotate these genes, and multiple chemokine genes were identified in the turbot genome, due to the tandem duplications (CCL19 and CCL20), the whole genome duplications (CCL20, CCL25, and CXCL12), and the teleost-specific members (CCL34-36, CCL44, and CXCL18). In addition, chemokines were ubiquitously expressed in nine examined healthy tissues, with high expression levels observed in liver, gill, and spleen. Moreover, most chemokines were significantly differentially expressed in gill and spleen after Aeromonas salmonicida infection, and exhibited tissue-specific and time-dependent manner. Finally, protein-protein interaction network (PPI) analysis indicated that turbot chemokines interacted with a few immune-related genes such as interleukins, cathepsins, stats, and TLRs. These results should be valuable for comparative immunological studies and provide insights for further functional characterization of chemokines in teleost.

A Teleost CXCL10 Is Both an Immunoregulator and an Antimicrobial

Chemokines are a group of cytokines that play important roles in cell migration, inflammation, and immune defense. In this study, we identified a CXC chemokine, CXCL10, from Japanese flounder Paralichthys olivaceus (named PoCXCL10) and investigated its immune function. Structurally, PoCXCL10 possesses an N-terminal coil, three β-strands, and a C-terminal α-helix with cationic and amphipathic properties. PoCXCL10 expression occurred in multiple tissues and was upregulated by bacterial pathogens. Recombinant PoCXCL10 (rPoCXCL10) promoted the migration, cytokine expression, and phagocytosis of flounder peripheral blood leukocytes (PBLs). rPoCXCL10 bound to and inhibited the growth of a variety of common Gram-negative and Gram-positive fish pathogens. rPoCXCL10 killed the pathogens by causing bacterial membrane permeabilization and structure destruction. When introduced in vivo, rPoCXCL10 significantly inhibited bacterial dissemination in fish tissues. A peptide derived from the C-terminal α-helix exhibited bactericidal activity and competed with rPoCXCL10 for bacterial binding. Deletion of the α-helix affected the in vitro bactericidal activity but not the chemotaxis or in vivo antimicrobial activity of PoCXCL10. Together, these results indicate that PoCXCL10 exerts the role of both an immunoregulator and a bactericide/bacteriostatic via different structural domains. These findings provide new insights into the immune function and working mechanism of fish CXC chemokines.

LcCCL28-25, Derived from Piscine Chemokine, Exhibits Antimicrobial Activity against Gram-Negative and Gram-Positive Bacteria In Vitro and In Vivo

Antimicrobial peptides (AMPs) are currently recognized as potentially promising antibiotic substitutes. Fish are an important seawater/freshwater medicinal biological resource, and the antimicrobial peptides and proteins that are key components of their innate immune systems are potential candidates for the development of novel antibacterial agents. The rainbow trout Oncorhynchus mykiss chemokine CK11 (omCK11), classified in the C-C motif chemokine ligand 27/28 (CCL27/28) family, is the only CC-type chemokine reported to play a direct antibacterial role in the immune response; however, its antibacterial domain remains unknown. In this study, we analyzed the structure-activity relationship of omCK11 and identified the antibacterial C-terminal domain. Additionally, we performed structure-function analyses of CCL27/28 proteins from different, representative freshwater and seawater fishes, revealing their shared C-terminal antibacterial domains. Surprisingly, a synthesized cationic peptide (named lcCCL28-25), derived from the large yellow croaker Larimichthys crocea CCL28, exhibited broad-spectrum and the most acceptable bactericidal activity, as well as antibiofilm activity and negligible hemolytic and cytotoxic activity in vitro. Additionally, lcCCL28-25 conferred a protective effect in the thighs of neutropenic mice infected with Staphylococcus aureus. SYTOX green fluorescence and electron microscopy experiments revealed that lcCCL28-25 was capable of rapidly destroying the integrity and permeability of the bacterial cell membrane. Overall, this study aided in the advancement of antibacterial CC-type chemokine research and also suggested a new strategy for exploring novel AMPs. Additionally, the efficacy of lcCCL28-25 in in vivo antibacterial activity in a mammalian model revealed that this compound could be a promising agent for the development of peptide-based antibacterial therapeutics.

IMPORTANCE The primary function of chemokines has been described as recruiting and activating leukocytes to participate in the immune response. Some chemokines are also broad-spectrum antibacterial proteins in mammals. The Oncorhynchus mykiss chemokine CK11 (omCK11) is the first reported and currently the only CC-type antibacterial chemokine. The present study identified the antibacterial domain of omCK11. Structure-function analysis of various fish CCL27/28 proteins identified a novel antibacterial peptide (lcCCL28-25) from Larimichthys crocea CCL28 that exhibited broad-spectrum and the most acceptable bactericidal activity in vitro, as well as a protective effect in a Staphylococcus aureus infection mouse model. The antibacterial mechanisms included membrane disruption and permeation. This study advanced the field of antibacterial chemokine research in fish and also suggested a new strategy for exploring novel AMPs. The novel peptide lcCCL28-25 may prove to be an effective antibacterial agent.

LECT2 Is a Novel Antibacterial Protein in Vertebrates

In vertebrates, leukocyte-derived chemotaxin-2 (LECT2) is an important immunoregulator with conserved chemotactic and phagocytosis-stimulating activities to leukocytes during bacterial infection. However, whether LECT2 possesses direct antibacterial activity remains unknown. In this article, we show that, unlike tetrapods with a single LECT2 gene, two LECT2 genes exist in teleost fish, named LECT2-a and LECT2-b Using grass carp as a research model, we found that the expression pattern of grass carp LECT2-a (gcLECT2-a) is more similar to that of LECT2 in tetrapods, while gcLECT2-b has evolved to be highly expressed in mucosal immune organs, including the intestine and skin. Interestingly, we found that gcLECT2-b, with conserved chemotactic and phagocytosis-stimulating activities, can also kill Gram-negative and Gram-positive bacteria directly in a membrane-dependent and a non-membrane-dependent manner, respectively. Moreover, gcLECT2-b could prevent the adherence of bacteria to epithelial cells through agglutination by targeting peptidoglycan and lipoteichoic acid. Further study revealed that gcLECT2-b can protect grass carp from Aeromonas hydrophila infection in vivo, because it significantly reduces intestinal necrosis and tissue bacterial load. More importantly, we found that LECT2 from representative tetrapods, except human, also possesses direct antibacterial activities, indicating that the direct antibacterial property of LECT2 is generally conserved in vertebrates. Taken together, to our knowledge, our study discovered a novel function of LECT2 in the antibacterial immunity of vertebrates, especially teleost fish, greatly enhancing our knowledge of this important molecule.

Immunodetection of rainbow trout IL-8 cleaved-peptide: Tissue bioavailability and potential antibacterial activity in a bacterial infection context

Chemokines such as IL-8 are part of an important group of proinflammatory response molecules, as well as cell recruitment. However, it has been described in both higher vertebrates and fish that IL-8 has an additional functional role by acting as an antimicrobial effector, either directly or by cleavage of a peptide derived from its C-terminal end. Nevertheless, it is still unknown whether this fragment is released in the context of infection by bacterial pathogens and if it could be immunodetected in tissues of infected salmonids. Therefore, the objective of this research was to demonstrate that the C-terminal end of IL-8 from Oncorhynchus mykiss is cleaved, retaining its antibacterial properties, and that is detectable in tissues of infected rainbow trout. SDS-PAGE and mass spectrometry demonstrated the cleavage of a fragment of about 2 kDa when the recombinant IL-8 was subjected to acidic conditions. By chemical synthesis, it was possible to synthesize this fragment called omIL-8α_80-97 peptide, which has antibacterial activity against Gram-negative and Gram-positive bacteria at concentrations over 10 μM. Besides, by fluorescence microscopy, it was possible to locate the omIL-8α_80-97 peptide both on the cell surface and in the cytoplasm of the bacteria, as well as inside the monocyte/macrophage-like cell. Finally, by indirect ELISA, Western blot, and mass spectrometry, the presence of the fragment derived from the C-terminal end of IL-8 was detected in the spleen of trout infected with Piscirickettsia salmonis. The results reported in this work present the first evidence about the immunodetection of an antibacterial, and probably cell-penetrating peptide cleaved from the C-terminal end of IL-8 in monocyte/macrophage-like cell and tissue of infected rainbow trout.

CXCL20a, a Teleost-Specific Chemokine That Orchestrates Direct Bactericidal, Chemotactic, and Phagocytosis-Killing-Promoting Functions, Contributes to Clearance of Bacterial Infections

The major role of chemokines is to act as a chemoattractant to guide the migration of immune cells to the infectious sites. In the current study, we found that CiCXCL20a, a teleost-specific chemokine from grass carp (Ctenopharyngodon idella), demonstrates broad-spectrum, potent, direct bactericidal activity and immunomodulatory functions to bacterial infections, apart from the chemotaxis. CiCXCL20a kills bacteria by binding, mainly targeting acid lipids, perforating bacterial membrane, resulting in bacterial cytoplasm leakage and death. CiCXCL20a aggregates and neutralizes LPS, agglutinates Gram-negative bacteria, and binds to peptidoglycan and Gram-positive bacteria, but not agglutinate them. All the complexes may be phagocytized and cleared away. CiCXCL20a chemoattracts leukocytes, facilitates phagocytosis of myeloid leukocytes, not lymphoid leukocytes, and enhances the bacteria-killing ability in leukocytes. We further identified its receptor CiCXCR3.1b1. Furthermore, we investigated the physiological roles of CiCXCL20a against Aeromonas hydrophila infection in vivo. The recombinant CiCXCL20a increases the survival rate and decreases the tissue bacterial loads, edema, and lesions. Then, we verified this function by purified CiCXCL20a Ab blockade, and the survival rate decreases, and the tissue bacterial burdens increase. In addition, zebrafish (Danio rerio) DrCXCL20, an ortholog of CiCXCL20a, was employed to verify the bactericidal function and mechanism. The results indicated that DrCXCL20 also possesses wide-spectrum, direct bactericidal activity through membrane rupture mechanism. The present study, to our knowledge, provides the first evidence that early vertebrate chemokine prevents from bacterial infections by direct bactericidal and phagocytosis-killing-promoting manners. The results also demonstrate the close functional relationship between chemokines and antimicrobial peptides.

The evolution and functional characterization of CXC chemokines and receptors in lamprey

Chemokines are a large family of soluble peptides guiding cell migration in development and immune defense. They interact with chemokine receptors and are essential for the coordination of cell migration in diverse physiological processes. The CXC subfamily is one of the largest groups in the chemokine family and consists of multiple members. In this study, we identified homologues of three chemokine ligands (CXCL8, CXCL_F5 and CXCL12) and two CXC receptor like molecules (CXCR_L1 and CXCR_L2) in lamprey. Sequence analysis revealed that they share the same genomic organization with their counterparts in jawed vertebrates but synteny was not conserved. Lamprey CXCL8 and CXCL12 have four conserved cysteine residues whilst the CXCL_F5 has two additional cysteine residues. In addition, CXCL_F5 is evolutionarily related to the fish specific CXC chemokine groups previously identified and contains multiple cationic aa residues in the extended C- terminal region. The two CXCRs possess seven transmembrane domains and conserved structural elements for receptor activation and signaling, including the DRYXXI(V)Y motif in TM2, the disulphide bond connecting ECL2 and TM3, the WXP motif in TM6 and NPXXY motif in TM7. The identified CXC chemokines and receptors were constitutively expressed in tissues including the liver, kidney, intestine, heart, gills, supraneural body and primary leukocytes, but exhibited distinct expression patterns. Relatively high expression was detected in the gills for CXCL8, CXCL_F5 and CXCR_L1 and in the supraneural body for CXCL12 and CXCR_L2. All the genes except CXCL12 were upregulated by stimulation with LPS, pokeweed and bacterial infection, and the CXCL8 and CXCL_F5 was induced by poly (I:C). Functional analysis showed that the CXCL8 and CXCL_F5 specifically interacted with CXCR_L1 and CXCR_L2, respectively. Our results demonstrate that the CXC chemokine system had diversified in jawless fish.

Study on Immune Response of Organs of Epinephelus coioides and Carassius auratus After Immersion Vaccination With Inactivated Vibrio harveyi Vaccine

Immersion vaccination relies on the response of fish mucosa-associated lymphoid tissues, the Crucian carp (Carassius auratus) and Grouper (Epinephelus coioides) were researched in this paper to examine local mucosal immune responses and associated humoral system responses following immersion vaccination. We administered 1.5 × 10⁷ CFU/ml formalin-inactivated Vibrio harveyi cells and measured mucus and serum antibody titers as well as IgM, MHC II mRNA levels in immune organs. The mucosal antibody response preceded the serum response indicating a role for local mucosal immunity in immersion vaccination. IgM and MHC II mRNA levels were relatively greater for the spleen and head kidney indicating the importance and central position of systemic immunity. Expression levels were also high for the gills while skin levels were the lowest. IgM and MHC II mRNA levels were altered over time following vaccination and the hindgut, liver and spleen were similar indicating a close relationship, so the absolute value of r is used to analyze the correlation among different organs immunized. It can be inferred the existence of an internal immune molecular mechanism for Immune synergy hindgut-liver-spleen, from the peak time (14^th day), the relative ratio of genes expression in the same tissues between the immunized grouper and the control group (26 times), and Pearson correlation coefficient (0.8<|r|<1). Injection challenges with live V. harveyi indicated that the relative protection rates for the crucian carp and Grouper was basically the same at 44.4% and 47.4%, respectively. It is believe that crucian carp may be used as a substitute for the valuable grouper in immunity experiment, just from aspect of the relative percent survival (RPS) and how it changes with time. But they were not consistent about the IgM mRNA expression between that of crucian carp and grouper after immersion the Vibrio vaccine.

Nile tilapia CXCR4, the receptor of chemokine CXCL12, is involved in host defense against bacterial infection and chemotactic activity

CXC chemokine receptor 4 (CXCR4), a member of seven-transmembrane (7-TM) G-protein-coupled receptor superfamily, is the receptor of the CXC chemokine ligand 12 (CXCL12), and plays important roles in host defense and inflammation. In the current study, we cloned and identified a homolog of CXCR4 from Nile tilapia (Oreochromis niloticus), designated as OnCXCR4. The open reading frame of OnCXCR4 is 1149 bp encoding a peptide of 382 amino acids, and the predicted molecular weight is 42.65 kDa OnCXCR4 shares common features of CXCR4 family, including a 7-TM domain and a characteristic CXC motif (containing CYC). Expression analysis showed that OnCXCR4 constitutively expresses in various tested tissues of Nile tilapia, with the highest level in the anterior kidney. When stimulated with Streptococcus agalactiae, Aeromonas hydrophila, Poly(I:C), or LPS in vivo and in vitro, the expression of OnCXCR4 was significantly regulated. AMD3100, a CXCR4 antagonist, could not only inhibit the chemotactic activity of the recombinant OnCXCL12 protein on the leukocytes from anterior kidney, but also reduce the expression of OnCXCR4 significantly. Taken together, these results of our study above indicate that OnCXCR4 may play important roles in host defense against bacterial infectionin in Nile tilapia, and being a receptor of OnCXCL12 to exert functions.

A Major QTL for Resistance to Vibrio anguillarum in Rainbow Trout

Genetic selection of disease resistant fish is a major strategy to improve health, welfare and sustainability in aquaculture. Mapping of single nucleotide polymorphisms (SNP) in the fish genome may be a fruitful tool to define relevant quantitative trait loci (QTL) and we here show its use for characterization of Vibrio anguillarum resistant rainbow trout (Oncorhynchus mykiss). Fingerlings were exposed to the pathogen V. anguillarum serotype O1 in a solution of 1.5 × 10⁷ cfu/ml and observed for 14 days. Disease signs appeared 3 days post exposure (dpe) whereafter mortality progressed exponentially until 6 dpe reaching a total mortality of 55% within 11 days. DNA was sampled from all fish – including survivors – and analyzed on a 57 k Affymetrix SNP platform whereby it was shown that disease resistance was associated with a major QTL on chromosome 21 (Omy 21). Gene expression analyses showed that diseased fish activated genes associated with innate and adaptive immune responses. The possible genes associated with resistance are discussed.

Quantitative trait loci (QTL) associated with resistance of rainbow trout Oncorhynchus mykiss against the parasitic ciliate Ichthyophthirius multifiliis

The parasitic ciliate Ichthyophthirius multifiliis has a low host specificity eliciting white spot disease (WSD) in a wide range of freshwater fishes worldwide. The parasite multiplies rapidly whereby the infection may reach problematic levels in a host population within a few days. The parasite targets both wild and cultured fish but the huge economic impact of the protozoan is associated with mortality, morbidity and treatment in aquacultural enterprises. We have investigated the potential for genetic selection of WSD-resistant strains of rainbow trout. Applying the DNA typing system Affymetrix® and characterizing the genome of the individual fish by use of 57,501 single nucleotide polymorphisms (SNP) and their location on the rainbow trout chromosomes, we have genetically characterized rainbow trout with different levels of natural resistance towards WSD. Quantitative trait loci (QTL) used for the selection of breeders with specific markers for resistance are reported. We found a significant association between resistance towards I. multifiliis infection and SNP markers located on the two specific rainbow trout chromosomes Omy 16 and Omy 17. Comparing the expression of immune-related genes in fish-with and without clinical signs-we recorded no significant difference. However, trout surviving the infection showed high expression levels of genes encoding IgT, T-cell receptor TCRβ, C3, cathelicidins 1 and 2 and SAA, suggesting these genes to be associated with protection.

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.

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

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

Immune response to Ichthyophthirius multifiliis and role of IgT

The parasitic ciliate Ichthyophthirius multifiliis causes white spot disease in freshwater fish worldwide. The theront penetrates external surfaces of the naïve fish where it develops into the feeding trophont stage and elicits a protective immune response both at the affected site as well as at the systemic level. The present work compiles data and presents an overall model of the protective reactions induced. A wide spectrum of inflammatory reactions are established upon invasion but the specific protection is provided by adaptive factors. Immunoglobulin IgT is involved in protection of surfaces in several fish species and is thereby one of the first adaptive immune molecules reacting with the penetrating theront. IgT producing lymphocytes occur in epithelia, dispersed or associated with lymphoid cell aggregations (skin epidermis, fins, gills, nostrils and buccal cavities) but they are also present in central immune organs such as the head kidney, spleen and liver. When theronts invade immunized fish skin, they are encountered by host factors which opsonize the parasite and may result in complement activation, phagocytosis or cell-mediated killing. However, antibody (IgT, IgM and IgD) binding to parasite cilia has been suggested to alter parasite behaviour and induce an escape reaction, whereby specific IgT (or other classes of immunoglobulin in fish surfaces) takes a central role in protection against the parasite.

Molecular Identification and Dual Functions of Two Different CXC Chemokines in Nile Tilapia (Oreochromis niloticus) against Streptococcus agalactiae and Flavobacterium columnare

Two CXC chemokines in Nile tilapia (On-CXC1 and On-CXC2) were identified at both the genomic and proteomic levels. A southern blot analysis and comparison searching in Ensembl confirmed the typical structure of the CXC chemokine genes and provided evidence for unusual mechanisms used to generate the two different CXC chemokine transcripts that have not been reported in other vertebrate species so far. The expression levels of On-CXC1 and On-CXC2 were analyzed by quantitative real-time PCR. These two mRNAs were detected in various tissues of normal Nile tilapia, especially in the spleen, heart, and head kidney, indicating a homeostatic function in immunosurveillance. A time-course experiment clearly demonstrated that these two transcripts were effectively enhanced in the head kidney, spleen and trunk kidney of Nile tilapia 6, 12 and 24 h after injection with Streptococcus agalactiae but were down-regulated in all tested tissues at 48 h, reflecting the fact that they have short half-lives during the crucial response to pathogens that is characteristic of CXC chemokine genes in other vertebrates. Functional analyses obviously exhibited that these two CXC chemokines at concentrations of 1–10 μg strongly inactivated S. agalactiae and Flavobacterium columnare and effectively induced phagocytosis of leukocytes in vitro.

Recent Advances on Phagocytic B Cells in Teleost Fish

The momentous discovery of phagocytic activity in teleost B cells has caused a dramatic paradigm shift from the belief that phagocytosis is performed mainly by professional phagocytes derived from common myeloid progenitor cells, such as macrophages/monocytes, neutrophils, and dendritic cells. Recent advances on phagocytic B cells and their microbicidal ability in teleost fish position B cells at the crossroads, bridging innate with adaptive immunity. Most importantly, an increasing body of experimental evidence demonstrates that, in both teleosts and mammals, phagocytic B cells can recognize, take up, and destroy particulate antigens and then present those processed antigens to CD4⁺ T cells to elicit adaptive immune responses and that the phagocytosis is mediated by pattern recognition receptors and involves multiple cytokines. Thus, current findings collectively indicate that teleost phagocytic B cells, as well as their counterpart mammalian B1-B cells, can be considered one kind of professional phagocyte. The aim of this review is to summarize recent advances regarding teleost phagocytic B cells, with a particular focus on the recognizing receptors and modulating mechanisms of phagocytic B cells and the process of antigen presentation for T-cell activation. We also attempt to provide new insights into the adaptive evolution of the teleost fish phagocytic B cell on the basis of its innate and adaptive roles.

A CCL25 chemokine functions as a chemoattractant and an immunomodulator in black rockfish, Sebastes schlegelii

Chemokines are small cytokines that are classified into four groups, one of which is called CC chemokines. In the present study, the full-length cDNA of a CCL25 chemokine was identified from black rockfish, Sebastes schlegelii (named as SsCCL25) by EST (expressed sequence tag) analysis. The cDNA of SsCCL25 consisted of a 5-terminal untranslated region (UTR) of 74 bp, a 3-UTR of 882 bp with a poly (A) tail, and an open reading frame (ORF) of 303 bp encoding a polypeptide of 100 amino acids with the putative molecular mass of 11.5 kDa. There was a SCY domain in the deduced amino acid sequence of SsCCL25. The phylogenetic relationships and syntenic analyses provided evidences for the identities of SsCCL25 with CCL25 group. The mRNA transcripts of SsCCL25 were expressed in all detected tissues and dominantly in liver, muscle and gill. Moreover, after Vibrio anguillarum stimulation, the mRNA expression levels of SsCCL25 were significantly up-regulated at 24 h (p < 0.05) in the liver and during 4-8 h (p < 0.05) in the spleen. Recombinant SsCCL25 protein induced chemotaxis of both control and LPS-stimulated peripheral blood leukocytes (PBL) and enhanced their resistance to bacterial infection in a dose-dependent manner. Furthermore, rSsCCL25 showed significant inhibitory effect on V. anguillarum and Edwardsiella tarda growth. All these results collectively indicated that SsCCL25 might contribute to the defense against microbe infection and function as a chemoattractant in black rockfish.

Characterization and Antimicrobial Activity of the Teleost Chemokine CXCL20b

Fish are a potential source of diverse organic compounds with a broad spectrum of biological activities. Many fish-derived antimicrobial peptides and proteins are key components of the fish innate immune system. They are also potential candidates for development of new antimicrobial agents. CXCL20b is a grass carp (Ctenopharyngodon idella) CXC chemokine strongly transcribed at the early stage of bacterial infections, for which the immune role had not been reported to date. In the present study, we found that CXCL20b is a cationic amphipathic protein that displays potent antimicrobial activity against both Gram-positive and Gram-negative bacteria. The results of DiOC₂(3) and atomic force microscopy (AFM) assays indicated that CXCL20b could induce bacterial membrane depolarization and disruption in a short time. By performing further structure-activity studies, we found that the antimicrobial activity of CXCL20b was mainly relative to the N-terminal random coil region. The central part of this cytokine representing β-sheet region was insoluble in water and the C-terminal α-helical region did not show an antimicrobial effect. The results presented in this article support the poorly understood function of CXCL20b, which fulfills an important role in bony fish antimicrobial immunity.

Identification and characterization of three CXC chemokines in Asian swamp eel (Monopterus albus) uncovers a third CXCL11_like group in fish

Chemokines direct cell migration in development and immune defense, and bridge between innate and adaptive immune responses. The chemokine gene family has been rapidly evolving and has undergone species/lineage-specific expansion. Mammals possess inflammatory CXC chemokines CXCL1-8/15 and CXCL9-11 sub-groups, and homeostatic CXCL12-14, 16-17. Orthologues of mammalian CXCL12-14, three chemokines related to CXCL1-8/15 (CXCL8_L1-3), two chemokines related to CXC9-11 (CXCL11_L1-2), and five fish-specific chemokines (CXCL_F1-5) have been described in teleosts. In this study, we reported three novel CXC chemokines in Asian swamp eel Monopterus albus, a commercially important freshwater fish species in China. Two of them belong to the fish-specific CXCL_F2 group, named CXCL_F2a/b, that share 89.5% amino acid identity. The other (CXCL11_L3) belongs to a third CXCL11_L related to the mammalian CXCL9-11 subfamily found only in percomorph fish species, and is the only CXCL9-11 related molecules in this lineage. Mammalian CXCL9-11 attract Th1 cells, and block the migration of Th2 cells in an immune response. This study suggests that all major lineages of teleosts have a CXCL9-11 related chemokine that will aid future functional investigation of CXCL11_L in fish. Cxcl_f2a is highly expressed constitutively in the skin of swamp eels that may attract immune cells to protect the skin in the absence of scales. Cxcl11_l3 and cxcl_f2b are highly expressed in immune tissues/organs and are up-regulated by the viral mimic poly I:C, but not bacterial infection in vivo, suggesting their role in anti-viral defense. The two cxcl_f2 paralogues are differentially expressed and modulated, indicating sub- and/or neo-functionalization.

Review on Immersion Vaccines for Fish: An Update 2019

Immersion vaccines are used for a variety of aquacultured fish to protect against infectious diseases caused by bacteria and viruses. During immersion vaccination the antigens are taken up by the skin, gills or gut and processed by the immune system, where the resulting response may lead to protection. The lack of classical secondary responses following repeated immersion vaccination may partly be explained by the limited uptake of antigens by immersion compared to injection. Administration of vaccines depends on the size of the fish. In most cases, immersion vaccination is inferior to injection vaccination with regard to achieved protection. However, injection is problematic in small fish, and fry as small as 0.5 gram may be immersion vaccinated when they are considered adaptively immunocompetent. Inactivated vaccines are, in many cases, weakly immunogenic, resulting in low protection after immersion vaccination. Therefore, during recent years, several studies have focused on different ways to augment the efficacy of these vaccines. Examples are booster vaccination, administration of immunostimulants/adjuvants, pretreatment with low frequency ultrasound, use of live attenuated and DNA vaccines, preincubation in hyperosmotic solutions, percutaneous application of a multiple puncture instrument and application of more suitable inactivation chemicals. Electrostatic coating with positively charged chitosan to obtain mucoadhesive vaccines and a more efficient delivery of inactivated vaccines has also been successful.

Differential immune gene response in gills, skin, and spleen of rainbow trout Oncorhynchus mykiss infected by Ichthyophthirius multifiliis

Infection of rainbow trout with the parasitic ciliate Ichthyopthirius multifiliis induces differential responses in gills, skin and spleen. A controlled experimental infection was performed and expression of immune-relevant genes in skin, gills, and spleen were recorded by qPCR at day 1 and 8 after parasite exposure. Infection induced a marked reaction involving regulation of innate and adaptive immune genes in rainbow trout at day 8 post-infection. The expression level of a total of 22 out of 24 investigated genes was significantly higher in gills compared to skin reflecting the more sensitive and delicate structure of gills. Especially pro-inflammatory cytokines IL-6, IL-17 C1, regulatory cytokines IL-4/13A, IL-10, TGFβ, complement factor C5, chemokines CK10, CK12, acute phase proteins (precerebellin, hepcidin) and immunoglobulins (IgM, IgT) displayed differential expression levels. The spleen, a central immune organ with no trace of the parasite, showed elevated expression of IgM, IgT, complement factor C5 and chemokine CK10 (compared to skin and gills directly exposed to the parasite), indicating an interaction between the infected surface sites and central immune organs. This communication could be mediated by chemokines CK10 and CK12 and cytokine IL-4/13A and may at least partly explain the establishment of a systemic response in rainbow trout against the parasite.