Functional characterization of a nonmammalian IL-21: rainbow trout Oncorhynchus mykiss IL-21 upregulates the expression of the Th cell signature cytokines IFN-gamma, IL-10, and IL-22

The identification and expression of an interleukin-21 receptor in large yellow croaker (Larimichthys crocea)

BACKGROUND

We identified a homologue of IL-21R (LcIL-21R) in large yellow croaker (Larimichthys crocea, Lc). Our investigation focused on understanding the molecular structural features and immune function of LcIL-21R.

METHODS

We cloned the LcIL-21R gene from the genome of Larimichthys crocea by RT‒PCR, and the molecular and structural characteristics of LcIL-21R were analyzed by a series of protein analysis tools. We used real-time PCR to investigate the tissue distribution of LcIL-21R, and LcIL-21R gene expression regulation was also measured in head kidney leukocytes under trivalent bacterial vaccine or poly (I:C) stimulation.

RESULTS

The open reading frame (ORF) of the LcIL-21R gene is 1629 bp long and encodes a precursor protein of 542 amino acids (aa), with a 23-aa signal peptide and a 519-aa mature peptide containing four putative N-glycosylation sites. LcIL-21R has two fibronectin type III (FNIII)-like domains (D1 and D2), a transmembrane domain, and a cytoplasmic region. A conserved WSXWS motif was also found in the D2 domain. The predicted structure of the extracellular region of LcIL-21R (LcIL-21R-Ex) is highly similar to that of human IL-21R. LcIL-21R was constitutively expressed in all tissues examined, and LcIL-21R mRNA levels were increased in the head kidney and spleen upon inactivated trivalent bacterial vaccine or poly(I:C) stimulation.

CONCLUSIONS

Our results suggest that LcIL-21R shares structural and functional properties with IL-21Rs found in other vertebrates, indicating its potential involvement in the IL-21-mediated immune response to pathogenic infections. These findings contribute to our understanding of the evolutionary conservation of IL-21 signaling and its role in the immune system.

Assessment of Bacillus subtilis fermented Caulerpa microphysa byproduct as feed additive on the growth performance, immune regulation and disease resistance of white shrimp (Litopenaeus vannamei)

In this study, the immunomodulatory and antioxidant activity of fermented Caulerpa microphysa byproduct (FCMB) by Bacillus subtilis was evaluated, and its potential as a feed additive for white shrimp (Litopenaeus vannamei) was explored. In vitro experiments showed that the FCMB supernatant contained polysaccharides, polyphenols and flavonoids, and exhibited antioxidant properties as assessed by various antioxidant assays. Additionally, the FCMB supernatant was found to increase the production rate of reactive oxygen species and the activity of phenoloxidase in hemocytes in vitro. Furthermore, the results of the in vivo feeding trial showed that dietary 5 g kg-1 FCMB significantly improved the weight gain and specific growth rate of white shrimp after 56 days of feeding. Although there were no significant differences in total hemocyte count, phagocytosis, superoxide anion production rate, and phenoloxidase activity among the experimental groups, upregulation of immune-related genes was observed, particularly in the hepatopancreas and hemocytes of shrimps fed with 5 g or 50 g FCMB per kg feed, respectively. In the pathogen challenge assay, white shrimp fed with 5 % FCMB exhibited a higher survival rate compared to the control group following Vibrio parahaemolyticus challenge. Therefore, it is concluded that the fermented byproduct of C. microphysa, FCMB, holds potential as a feed additive for enhancing the growth performance and disease resistance against V. parahaemolyticus in white shrimp.

Characterization of ccl20a.3 and ccl20l as gene markers for Th17 cell in turbot

T-helper 17 lymphocytes (Th17) are the most common inflammatory cells identified in mammals. However, the identification of Th17 cells and the clarification of their function in teleost fish remain largely unknown. In this study, we took advantage of the single-cell RNA sequencing-based immune cell atlas that was identified in turbot (Scophthalmus maximus), and revealed two chemokine-related genes, ccl20a.3 and ccl20l, that were specifically expressed in Th17 cells. Moreover, through immuno-fluorescence analysis, we found that CCL20a.3 or CCL20l was co-expressed with the classical makers in Th17 cells, including IL17a/f1 and IL22. Furthermore, through a Th17 lineage-specific transcription factor RORc inhibitor GSK805 treatment, we found that the expression of ccl20a.3 and ccl20l was significantly impaired, compared with other T cell markers. Besides, we also found that ccl20a.3 and ccl20l exhibited the same dynamic response with the classical markers that were identified in Th17 cells during bacterial infection. Taken together, these results provide potential gene markers for better understanding of the dynamic immune responses of Th17 cells in teleost fish.

The Biological Functions and Intestinal Inflammation Regulation of IL-21 in Grass Carp (Ctenopharyngodon idella) during Infection with Aeromonas hydrophila

Interleukin (IL) 21 is a pleiotropic cytokine that plays an important role in regulating innate and adaptive immune responses. In fish, the biological functions and cell source of IL-21 remain largely unknown. In this study, we performed qRT-PCR, Western blotting and immunofluorescent microscopy to examine the expression of IL-21 at the mRNA and protein levels. We found that il21 expression was induced in the primary head kidney leukocytes of grass carp (Ctenopharyngodon idella) by heat-inactivated Aeromonas hydrophila (A. hydrophila) and LPS and in tissues after infection with A. hydrophila. Recombinant IL-21 protein produced in the CHO-S cells was effective in elevating the expression of antibacterial genes, including β-defensin and lysozyme, and, interestingly, inhibited the NF-κB signaling pathway. Furthermore, we investigated the response of the IL-21 expressing cells to A. hydrophila infection. Immunofluorescent assay showed that IL-21 protein was detected in the CD3γ/δ T cells and was markedly accumulated in the anterior, middle and posterior intestine. Collectively, the results indicate that IL-21 plays an important role in regulating the intestinal inflammation induced by bacterial infection in grass carp.

Catch of the Day: New Serum Amyloid A (SAA) Antibody Is a Valuable Tool to Study Fish Health in Salmonids

Serum amyloid A (SAA) proteins belong to a family of acute-phase reactants, playing an integral role in defending the organism from pathological damage. Despite a wealth of data on the regulation of SAA transcripts in teleosts, there is only limited information on these proteins' abundance in fish. The aim of this study is to characterise SAA protein levels in salmonids using a newly developed antibody specific to salmonid SAA. The salmonid SAA antibody detected SAA and accurately discriminated between stimulated and control specimens from rainbow trout macrophage cell line (RTS-11) in vitro, as well as rainbow trout challenged with Aeromonas salmonicida- or flagellin-stimulated Atlantic salmon in vivo. The presence of SAA protein was analysed in RTS-11 cell line supernatants, liver, and spleen samples using ELISA, immunoblotting, and immunohistochemistry. This study is the first to characterise SAA protein levels in salmonids in vivo and in vitro. The newly developed salmonid SAA antibody was able to discriminate between stimulated and unstimulated specimens, showing that it can be used to study the acute-phase response in salmonids with the potential to be further developed into assays to monitor and evaluate health in wild and farmed fish.

Investigation of the functional role of UNC93B1 in Nile tilapia (Oreochromis niloticus): mRNA expression, subcellular localization, and physical interaction with fish-specific TLRs

Nile tilapia (Oreochromis niloticus) is one of the major food fish worldwide. The farming business, on the other hand, has faced considerable obstacles, such as disease infestations. Toll-like receptors (TLRs) play an important function in the activation of the innate immune system in response to infections. Unc-93 homolog B1 (UNC93B1) is a key regulator of nucleic acid (NA)-sensing TLRs. Here the UNC93B1 gene, which was cloned from Nile tilapia tissue for this investigation, had the same genetic structure as a homologous gene in humans and mice. Phylogenetic analysis revealed that Nile tilapia UNC93B1 clustered with UNC93B1 from other species and separately from the UNC93A clade. The gene structure of the Nile tilapia UNC93B1 was found to be identical to that of human UNC93B1. Our gene expression studies revealed that Nile tilapia UNC93B1 was highly expressed in the spleen, followed by other immune-related tissues such as the head kidney, gills, and intestine. Moreover, Nile tilapia UNC93B1 mRNA transcripts were up-regulated in vivo in the head kidney and spleen tissues from poly I:C and Streptococcus agalactiae injected Nile tilapia, as well as in vitro in LPS stimulated Tilapia head kidney (THK) cells. The Nile tilapia UNC93B1-GFP protein signal was detected in the cytosol of THK cells and was co-localized with endoplasmic reticulum and lysosome but not with mitochondria. Moreover, the results of a co-immunoprecipitation and immunostaining analysis showed that Nile tilapia UNC93B1 can be pulled down with fish-specific TLRs such as TLR18 and TLR25 from Nile tilapia, and was found to be co-localized with these fish-specific TLRs in the THK cells. Overall, our findings highlight the potential role of UNC93B1 as an accessory protein in fish-specific TLR signaling.

Regulatory roles of cytokines in T and B lymphocytes-mediated immunity in teleost fish

T and B lymphocytes (T and B cells) are immune effector cells that play critical roles in adaptive immunity and defend against external pathogens in most vertebrates, including teleost fish. In mammals, the development and immune response of T and B cells is associated with cytokines including chemokines, interferons, interleukins, lymphokines, and tumor necrosis factors during pathogenic invasion or immunization. Given that teleost fish have evolved a similar adaptive immune system to mammals with T and B cells bearing unique receptors (B-cell receptors (BCRs) and T-cell receptors (TCRs)) and that cytokines in general have been identified, whether the regulatory roles of cytokines in T and B cell-mediated immunity are evolutionarily conserved between mammalians and teleost fish is a fascinating question. Thus, the purpose of this review is to summarize the current knowledge of teleost cytokines and T and B cells as well as the regulatory roles of cytokines on these two types of lymphocytes. This may provide important information on the parallelisms and dissimilarities of the functions of cytokines in bony fish versus higher vertebrates, which may aid in the evaluation and development of adaptive immunity-based vaccines or immunostimulants.

Novel insights into the cytokine network of rainbow trout Oncorhynchus mykiss using cell lines and primary leukocyte populations

Cytokines are small proteins that regulate innate and adaptive immune responses and are released by both immune and non-immune cell types. In the current study, the constitutive and induced gene expression profiles of a suite of proinflammatory and regulatory cytokines was examined comparatively in eight rainbow trout (Oncorhynchus mykiss) cell lines, in order to establish the cytokine repertoires of these different cell types, especially the understudied non-immune cells. They included three epithelial cell lines (RTgut, RTgill, and RTL), one endothelial cell line (RTH), one fibroblast cell line (RTG-2), two stromal cell lines (TSS and TPS-2) and one monocyte/macrophage-like cell line (RTS-11). Three types of primary leukocytes (derived from blood, spleen and head kidney) of trout were also included in the analysis, to allow comparison to the repertoires expressed in T cells, as a major source of cytokines in immune responses. The major findings are: 1) IL-2A, IL-2B, IL-4/13B1, IL-4/13B2, IL-10b, P40B1, P28B, IL-17A/F1b, TNF-α3, TNF-α4, IFNγ1, CCL20L2b and CCL20L3a are expressed mainly in leukocytes but IL-17 N, IL-17D, IL-20 and CCL20L1b2 are not expressed in these cells. Hence future studies in these cell lines will help establish their function in fish; 2) Some of the cytokines were differentially expressed in the cell lines, revealing the potential role of these cell types in aspects of trout mucosal and inflammatory immune responses, 3) Similar cell types grouped together in the cell cluster analysis, including the leukocyte cluster, stromal cell cluster, and epithelial and endothelial cell cluster. Taken together, this investigation of these trout cell lines forms a good database for studying the function of cytokines not expressed in isolated leukocytes or that are preferentially expressed in the cell lines. Furthermore, the cytokine expression analysis undertaken confirmed the phenotypic relationship of these cell types at the molecular level.

Cytokine networks provide sufficient evidence for the differentiation of CD4+ T cells in teleost fish

Cytokines, a class of small molecular proteins with a wide range of biological activities, are secreted mainly by immune cells and function by binding to the corresponding receptors to regulate cell growth, differentiation and effects. CD4⁺ T cells can be defined into different lineages based on the unique set of signature cytokines and transcription factors, including helper T cells (Th1, Th2, Th17) and regulatory T cells (Treg). In teleost, CD4⁺ T cells have been identified in a variety of fish species, thought to play roles as Th cells, and shown to be involved in the immune response following specific antigen stimulation. With the update of sequencing technologies, a variety of cytokines and transcription factors capable of characterizing CD4⁺ T cell subsets also have been described in fish, including hallmark cytokines such as IFN-γ, TNF-α, IL-4, IL-17, IL-10, TGF-β and unique transcription factors such as T-bet, GATA3, RORγt, and Foxp3. Hence, there is increasing evidence that the subpopulation of Th and Treg cells present in mammals may also exist in teleost fish. However, the differentiation, plasticity and precise roles of Th cell subsets in mammals remain controversial. Research on the identification and differentiation of fish Th cells is still in its infancy and requires more significant effort. Here we will review recent research advances in characterizing the differentiation of fish CD4⁺ T cells by cytokines and transcription factors, mainly including the identification of Th and Treg cell hallmark cytokines and transcription factors, the regulatory role of cytokines on Th cell differentiation, and the function of Th and Treg cells in the immune response. The primary purpose of this review is to deepen our understanding of cytokine networks in characterizing the differentiation of CD4⁺ T cells in teleost.

Interleukin-2 family cytokines: An overview of genes, expression, signaling and functional roles in teleost

The interleukin-2 (IL-2) family cytokines include IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21, which share γ chain (γc) subunit in receptors. The IL-2 family cytokines have unique biological effects that regulate differentiation, survival and activation of multiple lymphocyte lineages. Deficiency of IL-2 family signaling pathway in mammals prevents CD4⁺ T cells from developing effector functions and CD8⁺ T cells from developing immunological memory. In the present review, we addressed available information from teleost IL-2 family cytokines and discussed implications in teleost immunity. Also, we described and discussed their expression profiles, receptors, signaling transductions and functions. In teleost, IL-2 family has 5 members (IL-2, IL-4/13, IL-7, IL-15, IL-21) without IL-9, and their receptors share a common γc subunit and include other 6 subunits (IL-2Rβ1/2, IL-4Rα1/2, IL-13Rα1/2, IL-7Rα, IL-15Rα, and IL-21Rα1/2). Some paralogues have changes in domain structure and show differential expression, modulation, functions. IL-2 family cytokines constitutively express in many immune associated tissues and are largely induced after pathogenic microbial stimulation. In general, there are relatively conserved functions in the IL-2 family throughout vertebrates, and many of the key IL-2 family members are important in lymphocyte proliferation and differentiation, development, inflammation from fishes to mammals. This review will give an update on the effective information of teleost IL-2 family cytokines. Thus, it will provide a source of reference for other researchers/readers and inspire further interest.

Transcriptome analysis of the spleen provides insight into the immunoregulation of Scortum barcoo under Streptococcus agalactiae infection

Jade perch (Scortum barcoo) is a freshwater fish with substantial economic value, which has been widely cultivated all over the world. However, with the intensification and expansion of farming, several bacterial and viral diseases have occurred in jade perch. To understand the immune response of jade perch against Streptococcus agalactiae (Group B Streptococcus, GBS), we performed a histopathological examination and transcriptome sequencing of jade perch spleen after artificial bacterial infection. GBS infection can cause structural changes and even necrosis of the jade perch spleen, which may affect the survival of infected individuals. A total of 144,458 unigenes were obtained through de novo assembly of spleen transcriptome. Among them, 1821 unigenes were identified as DEGs, including 1415 up-regulated and 406 down-regulated unigenes in the infection group. Moreover, the analysis of GO and KEGG revealed that many GO terms and pathways were involved in the host immune response, such as Toll-like receptor signaling pathway, Cytokine-cytokine receptor interaction, and TNF signaling pathway. In addition, according to transcriptome data and qRT-PCR analysis, the expression levels of many cytokines that participate in the inflammatory response changed a lot after GBS infection. Overall, this transcriptomic analysis provided valuable information for studying the immune response of jade perch against bacterial infection.

Evolution of γ chain cytokines: mechanisms, methods and applications

The common γ chain family of cytokines and their receptors play fundamental roles in the immune system. Evolutionary studies of γ chain cytokines have elegantly illustrated how the immune system adapts to ever-changing environmental conditions. Indeed, these studies have revealed the uniqueness of cytokine evolution, which exhibits strong positive selection pressure needed to adapt to rapidly evolving threats whilst still conserving their receptor binding capabilities. In this review, we summarise the evolutionary mechanisms that gave rise to the characteristically diverse family of γ chain cytokines. We also speculate on the benefits of studying cytokine evolution, which may provide alternative ways to design novel cytokine therapeutic strategies. Additionally, we discuss current evolutionary models that elucidate the emergence of distinct cytokines (IL-4 and IL-13) and cytokine receptors (IL-2Rα and IL-15Rα). Finally, we address and reflect on the difficulties associated with evolutionary studies of rapidly evolving genes and describe a variety of computational methods that have revealed numerous aspects of cytokine evolution.

Sterile alpha and TIR motif-containing protein 1 is a negative regulator in the anti-bacterial immune responses in nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is one of the most important food fish in the world. However, the farming industry has encountered significant challenges, such as pathogen infections. Toll-like receptors (TLRs) play an essential role in the initiation of the innate immune system against pathogens. Sterile alpha and TIR motif-containing protein 1 (SARM1) is one of the most evolutionarily conserved TLR adaptors, and its orthologs are present in various species from worms to humans. SARM1 plays an important role in negatively regulating TIR domain-containing adaptor proteins inducing IFNβ (TRIF)-dependent TLR signaling in mammals, but its immune function remains poorly understood in fish. In this study, O. niloticus SARM1 (OnSARM1) was cloned and its evolutionary status was verified using bioinformatic analyses. mRNA expression of OnSARM1 was found at a higher level in the trunk kidney and muscle in healthy fish. The examination of its subcellular location showed that the OnSARM1 was detected only in the cytoplasm of THK cells, and colocalized with OnMyD88, OnTRIF and OnTRIF in small speckle-like condensed granules. The transcript levels of OnMyD88, OnTIRAP, OnTRIF, and downstream effectors, including interleukin (IL)-1β, IL-8, IL-12b and type I interferon (IFN)d2.13, were regulated conversely to the expression of OnSARM1 in the head kidney from Aeromonas hydrophila and Streptococcus agalactiae infected fish. Moreover, the treatment of THK cells with lysates from A. hydrophila and S. agalactiae enhanced the activity of the NF-κB promoter, but the effects were inhibited in the OnSARM1 overexpressed THK cells. Overexpression of OnSARM1 alone did not activate the NF-κB-luciferase reporter, but it suppressed OnMyD88- and OnTIRAP-mediated NF-κB promoter activity. Additionally, OnSARM1 inhibited the mRNA expression of proinflammatory cytokines and hepcidin in A. hydrophila lysate stimulated THK cells. Taken together, these findings suggest that OnSARM1 serves as a negative regulator by inhibiting NF-κB activity, thereby influencing the transcript level of proinflammatory cytokines and antimicrobial peptides in the antibacterial responses.

TLR23, a fish-specific TLR, recruits MyD88 and TRIF to activate expression of a range of effectors in melanomacrophages in Nile tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is an important food fish species that is mainly cultivated in tropical and subtropical countries. However, microbial diseases have created various difficulties for this industry. The fundamental prerequisite for tackling disease outbreak prevention and disease resistance is to know how hosts' immune responses against invading microbes are initiated. Toll-like receptors (TLRs) are vital pattern recognition receptors and play pivotal roles in the cellular innate immunity defense that is able to recognize pathogen-associated molecular patterns (PAMPs). In this study, Oreochromis niloticus TLR23 (OnTLR23) was cloned and bioinformatic analyses revealed that OnTLR23 is not an ortholog of mammalian TLR13 as previously suggested. The basal transcript level of OnTLR23 was found to be higher in the immune-related organs and was upregulated in the spleen and/or head kidney following Aeromonas hydrophila, Streptococcus agalactiae or poly I:C injections, and increased in the melanomacrophage-like tilapia head kidney (THK) cell line after LPS and zymosan stimulation. Furthermore, we demonstrated for the first time that OnTLR23 locates mainly in the intracellular region in fish cells and the constitutively active form of OnTLR23 promotes the expression of molecules related to antigen presentation, proinflammatory cytokines, antimicrobial peptides and type I interferon in THK cells. A co-immunoprecipitation assay revealed that OnTLR23 can interact with both OnMyD88 and OnTRIF, but not with OnTIRAP. A luciferase assay showed that the NF-κB activity was not elevated in the OnTLR23 overexpressed THK cells after treatment with ligand for TLR13 as well as other known purified bacterial-derived ligands of TLRs. Taken together, OnTLR23 is likely to recruit OnMyD88 and OnTRIF as adaptors to induce the expression of various effectors in melanomacrophages, but its corresponding ligand is an issue awaiting further investigation.

Characterization of snakehead (Channa argus) interleukin-21: Involvement in immune defense against two pathogenic bacteria, in leukocyte proliferation, and in activation of JAK-STAT signaling pathway

Interleukin-21 (IL-21), a crucial immune regulatory molecule, belongs to the common γ-chain family of type I cytokines, and exerts pleiotropic effects on multiple immune cell types in mammals. However, the characteristics and functions of fish IL-21 remain unclear. To further investigate the molecular mechanism of IL-21 in teleosts, we first cloned and identified the IL-21 gene (designated shIL-21) of the snakehead (Channa argus). The full-length open reading frame of shIL-21 is 438 bp in length, and encodes a predicted protein of 145 amino acid residues. A sequence analysis showed that shIL-21 has the typical structural characteristics of other IL-21 proteins, containing four α-helices and four conserved cysteine residues. In a phylogenetic analysis, shIL-21 clustered within a subgroup of IL-21 proteins from other teleost species and shared its closest evolutionary relationship with that of Lates calcarifer. The expression analysis showed that shIL-21 was ubiquitously expressed in all the healthy snakehead tissues tested, albeit at different levels. After infection with Nocardia seriolae or Aeromonas schubertii, the relative expression of shIL-21 was mainly upregulated in the head kidney and spleen in vivo. Similarly, after stimulation with the three pathogen analogues lipoteichoic acid, lipopolysaccharides, and polyinosinic-polycytidylic acid, the expression of shIL-21 was also induced in head kidney leukocytes in vitro. A recombinant shIL-21 protein was expressed and purified, and promoted the proliferation of head kidney leukocytes, induced the expression of genes encoding critical signaling molecules in the Janus kinase (JAK) and signal transducer and activator of transcription (STAT) pathway, including JAK1, JAK3, STAT1, and STAT3, and induced the expression of endogenous shIL-21 and genes encoding several key proinflammatory cytokines (tumor necrosis factor-α, interferon-γ, and IL-1β). Taken together, these preliminary findings suggest that shIL-21 is involved in the immune defense against bacterial infection, in leukocyte proliferation, and in the activation of the JAK-STAT pathway. They thus extend the functional studies of IL-21 in teleosts.

Sarcodia suiae Water Extract Promotes the Expression of Proinflammatory and Th1-Type Cytokines and Delay the Onset of Mortality in Cobia (Rachycentron canadum) During Photobacterium damselae subsp. damselae Infection

Cobia (Rachycentron canadum) is a marine fish of high economic value that grows at a fast rate. However, intensive fish farming has led to disease outbreaks in cobia cultures, which is highly costly to the industry. The impact of infectious diseases on cobia production has led to the inappropriate and increased use of chemicals and antibiotics, which negatively affects the environment and human health and promotes the spread of drug-resistant pathogens. Hence, prophylactic measurements, such as the use of immunomodulators, are required to improve the health of cultured animals against pathogens. In this study, we examined the effects of Sarcodia suiae water extract (SSWE) in cobia in vitro and in vivo. We found that treatment with SSWE could significantly increase the expression of cytokines (e.g., IL-1β, IL-6, IL-10, IL-12, and TNF-α) and chemokines (e.g., IL-8) in primary cultured head kidney leukocytes. Intraperitoneal injection of SSWE (20 μg/g body weight) promoted higher expression of IL-6, IL-8, IL-10, IL-12, chemokines (e.g., CC1), and antibodies (e.g., IgT) in head kidney and spleen tissues of the fish compared with other dose levels. Additionally, we describe for the second time (only after India) of the isolation of Photobacterium damselae subsp. damselae (Phdd) from a deadly epizootic in cage-farmed cobia. An intraperitoneal inoculation of SSWE before Phdd challenge showed that SSWE treatment could delay the onset of mortality of cobia. Finally, fish that received SSWE intraperitoneally before infection with Phdd exhibited elevated expression of Th1-type cytokines, namely, IL-8, IL-12, TNF-α, and IFN-γ. At the same time, the expression of Th2-related factors (such as IL-10 in the head kidney, and IgM and IgT in the spleen) were lower for the fish that received SSWE instead of PBS before the Phdd challenge. The results indicate that SSWE treatment facilitates the induction of Th1-type cytokines in cobia to fight against Phdd infection and has the potential to be used as an immunostimulant and vaccine adjuvant for fish.

Dietary Agaricus blazei Spent Substrate Improves Disease Resistance of Nile Tilapia (Oreochromis niloticus) against Streptococcus agalactiae In Vivo

This study evaluated the effects of the feeding of spent mushroom substrate from Agaricus blazei on Nile tilapia (Oreochromis niloticus). The safety of 0–1000 μg/mL A. blazei spent substrate water extract (ABSSE) was demonstrated in the primary hepatic and splenic macrophages and the THK cell line (a cell line with characteristics of melanomacrophages) using a cytotoxicity assay. Here, 10 μg/mL of crude ABSSE promoted the phagocytic activity of macrophages and THK cells. Stimulating ABSSE-primed THK cells with lipopolysaccharides or peptidoglycan resulted in higher expression levels of four cytokine genes (e.g., interleukinz (IL)-1β, IL-12b, IL-8 and tumor necrosis factor α (TNFα)) and one cytokine gene (TNFα), respectively. An in vitro bacterial growth inhibition assay demonstrated that ABSSE could inhibit the growth of Streptococcus agalactiae. In the first feeding trial, Nile tilapia were fed with experimental feed containing 0, 1, or 5% of A. blazei spent substrate (ABSS) for seven and fourteen days followed by bacterial challenge assay. The best result was obtained when Nile tilapia were continuously fed for seven days on a diet containing 1% ABSS, with the survival rate being higher than in groups with 0% and 5% ABSS after challenge with S. agalactiae. In the second trial, fish were fed diets supplemented with 0% or 1% ABSS for seven days, and then all the groups were given the control feed for several days prior to bacterial challenge in order to investigate the duration of the protective effect provided by ABSS. The results showed that the protective effects were sustained at day 7 after the feed was switched. Overall, spent mushroom substrate from A. blazei is a cost-effective feed additive for Nile tilapia that protects fish from S. agalactiae infection.

Fish-specific TLR18 in Nile tilapia (Oreochromis niloticus) recruits MyD88 and TRIF to induce expression of effectors in NF-κB and IFN pathways in melanomacrophages

Toll-like receptors (TLRs) are evolutionarily conserved proteins of pattern recognition receptors (PRRs) and play a crucial role in innate immune systems recognition of conserved pathogen-related molecular samples (PAMPs). We identified and characterized TLR18 from Nile tilapia (Oreochromis niloticus), OnTLR18, to elucidate its role in tissue expression patterns, modulation of gene expression after microbial challenge and TLR ligands, subcellular localization in fish and human cells, and the possible effectors TLR18 induces in a melanomacrophage-like cell line (tilapia head kidney (THK) cells). OnTLR18 expression was detected in all tissues examined, with the highest levels in the intestine and the lowest in the liver. OnTLR18 transcript was up-regulated in immune-related organs after bacterial and polyinosinic-polycytidylic acid (poly I:C) challenges and in the THK cells after lipopolysaccharide (LPS) stimulation. In transfected THK and human embryonic kidney (HEK) 293 cells, OnTLR18 localizes in the intracellular compartment. OnMyD88 and OnTRIF, but not OnTIRAP, were co-immunoprecipitated with OnTLR18, suggesting that the former two molecules are recruited by OnTLR18 as adaptors. The constitutively active form of OnTLR18 induced the production of pro-inflammatory cytokines, type I interferon (IFN), and antimicrobial peptides such as tumor necrosis factor α, interferon (IFN) d2.13, tilapia piscidin (TP)2, TP3, TP4, and hepcidin in THK cells. Our results suggest that OnTLR18 plays an important role in innate immunity through initiating nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and IFN signaling pathways via OnMyD88 and OnTRIF and induces the production of various effectors in melanomacrophages.

Characterisation and analysis of IFN-gamma producing cells in rainbow trout Oncorhynchus mykiss

IFN-γ is one of the key cytokines involved in Th1 immune responses. It is produced mainly by T cells and NK cells, which drive both innate and adaptive responses to promote protection against infections. IFN-γ orthologues have been discovered to be functionally conserved in fish, suggesting that type I immunity is present in early vertebrates. However, few studies have looked at IFN-γ protein expression in fish and its role in cell mediated immunity due to a lack of relevant tools. In this study, four monoclonal antibodies (mAbs) V27, N2, VAB3 and V91 raised against short salmonid IFN-γ peptides were developed and characterised to monitor IFN-γ expression. The results show that the IFN-γ mAbs specifically react to their peptide immunogens, recognise E. coli produced recombinant IFN-γ protein and rainbow trout IFN-γ produced in transfected HEK 293 cells. The mAb VAB3 was used further, to detect IFN-γ at the cellular level after in vitro and in vivo stimulation. In flow cytometry, a basal level of 3-5% IFN-γ secreting cells were detected in peripheral blood leucocytes (PBL), which increased significantly when stimulated in vitro with PAMPs (Aeromonas salmonicida bacterin), a mitogen (PHA) and recombinant cytokine (IL-2). Similarly, after injection of live bacteria (Aeromonas salmonicida) or poly I:C the number of IFN-γ⁺ cells increased in the lymphoid population of PBL, as well as in the myeloid population after infection, with the myeloid cells increasing substantially after both treatments. Immunohistochemistry was used to visualise the IFN-γ⁺ cells in spleen and head kidney following vaccination, which increased in intensity of staining and number relative to tissue from saline-injected control fish. These results show that several types of cells can produce IFN-γ in trout, and that they increase following infection or vaccination, and likely contribute to immune protection. Hence monitoring IFN-γ producing cells/protein secretion may be an important means to assess the effectiveness of Th1 responses and cell mediated immunity in fish.

Phycoerythrin from Colaconema sp. Has Immunostimulatory Effects on the Whiteleg Shrimp Litopenaeus vannamei and Increases Resistance to Vibrio parahaemolyticus and White Spot Syndrome Virus

Simple Summary

In this study, we found that phycoerythrin from Colaconema sp. can differentially stimulate the immune response of whiteleg shrimp in vitro and in vivo and could potentially be used as an immunomodulator in shrimp culture.

Abstract

We investigated whether phycoerythrin (PE), a pigment sourced from marine algae, could act as an immunomodulatory agent in whiteleg shrimp (Litopenaeus vannamei). To this end, PE was extracted and purified from a PE-rich macroalgae, Colaconema sp. Our in vitro analysis demonstrated that PE enhanced prophenoloxidase and phagocytosis activity but inhibited the production of reactive oxygen species in hemocytes. Additionally, the PE signal could be detected using an in vivo imaging system after its injection into the ventral sinus of the cephalothorax of whiteleg shrimp. The expression profiles of fourteen immune-related genes were monitored in hemocytes from whiteleg shrimp injected with 0.30 μg of PE per gram of body weight, and crustin, lysozyme, penaiedin 4, and anti-lipopolysaccharide factor showed up-regulated post-stimulation. The induction of immune genes and enhancement of innate immune parameters by PE may explain the higher survival rates for shrimp that received different doses of PE prior to being challenged with Vibrio parahaemolyticus or white spot syndrome virus compared to controls. Combined, these results show that PE from Colaconema sp. can differentially stimulate the immune response of whiteleg shrimp in vitro and in vivo and could potentially be used as an immunomodulator in shrimp culture.

Montanide™ ISA 763A VG and ISA 761 VG induce different immune pathway responses in rainbow trout (Oncorhynchus mykiss) when used as adjuvant for an Aeromonas salmonicida bacterin

Adjuvants are the helper substances that increase vaccine efficacy by enhancing the potency and longevity of specific immune responses to antigens. Most existing fish vaccines are presented in the form of oil-based emulsions delivered by intraperitoneal injection. The characterization of their mode of action is a valuable aid to future vaccine development, particularly for the potential identification and stimulation of specific immunological pathways related to the desired protective response. This study characterized the expression of selected immune-related genes in the peritoneal cavity, head kidney and spleen following the administration of two adjuvanted-bacterial vaccines thought to induce humoral (Montanide™ ISA 763A VG) or humoral and cell mediated (Montanide™ ISA 761 VG) immune responses, to determine if differences in responsiveness are readily apparent. The most informative site was the spleen, where Montanide™ ISA 763A VG + bacterin gave rise to upregulation of genes driving T-cell/lymphoid responses, namely IL-2, IL-15 and IL-21. This combined with upregulation of IFNγ1 and IFNγ2, IL-4/13B2, p35A1 and p40 (B1 and C) indicated that the induction of Th1 and possibly Th2 immunity was occurring in fish vaccinated with this adjuvant. Perhaps the most intriguing finding was the lack of a detectable Th1 response in fish given Montanide™ ISA 761 VG + bacterin, suggesting some other arm of the immune system is activated to give protection. Whatever the reason for the different responses detected, it is clear from the present study that the adjuvant used has a major impact on the responses elicited. Since these differences are readily detectable it allows, in principle, their use to help select the most appropriate adjuvants for inclusion into fish vaccines, where the type of response elicited may need to be tailored to a particular pathogen to confer protection.

In vitro evaluation of novel (nanoparticle) oral delivery systems allow selection of gut immunomodulatory formulations

Oral delivery is the most convenient way to vaccinate cultured fish, however it is still problematic, primarily due to a lack of a commercially valid vaccine vehicle to protect the antigen against gastric degradation and ensure its uptake from the intestine. With the goal of advancing the potential to vaccinate orally, this study evaluates a novel silicon nanoparticle-based vehicle (VacSaf carrier). Aeromonas salmonicida antigens were formulated with the VacSaf carrier using different preparation methods to generate dry powder and liquid formulations. Twelve formulations were first subjected to an in vitro evaluation where the A. salmonicida bacterin conjugated to VacSaf carriers were found superior at inducing pro-inflammatory cytokine expression in primary leucocyte cultures and the macrophage/monocyte cell line RTS-11 compared with A. salmonicida bacterin alone. This was especially apparent after exposure to acid conditions to mimic stomach processing. One formulation (FD1) was taken forward to oral delivery using two doses and two administration schedules (5 days vs 10 days, the latter 5 days on, 5 days off, 5 days on), and the transcript changes of immune genes in the intestine (pyloric caeca, midgut and hindgut) and spleen were evaluated by qPCR and serum IgM was measured by ELISA. The VacSaf carrier alone was shown to be safe for use in vivo, in that no side-effects were seen, but it did induce expression of some cytokines, and may have value as an oral adjuvant candidate. The FD1 bacterin formulation was effective at inducing a range of cytokines associated with innate and adaptive immunity, mainly in the pyloric caeca, compared to A. salmonicida bacterin alone (which had almost no effect), and confirms the immune competence of this gut region following appropriate oral vaccination. These results reveal that in vitro screening of formulations for oral delivery has value and can be used to assess the most promising formulations to test further.

Expression, signal transduction, and function analysis of TIRAP and TRIF in Nile tilapia (Oreochromis niloticus)

Toll/interleukin 1 receptor domain-containing adaptor protein (TIRAP) and toll/interleukin 1 receptor-domain-containing adapter-inducing interferon-β (TRIF) are crucial adaptors of signal transduction for the signaling pathways of toll-like receptors (TLRs). TIRAP and TRIF perform an essential function in an antimicrobial immune response; however, their function in Nile tilapia remains unknown. Herein, TIRAP and TRIF from Nile tilapia were identified and functionally characterized. Phylogenetic analysis showed that OnTIRAP and OnTRIF clustered with corresponding homologs from other fish species, with comparable gene structures to those of select vertebrate TIRAP and TRIF genes, respectively. The expression profiles of OnTIRAP and OnTRIF were broadly distributed in the ten tissues investigated, with high transcript levels noticed in immune organs. The transcription levels of OnTIRAP and OnTRIF were upregulated in response to bacterial and poly (I:C) challenges. GFP signals were only detected in the cytoplasmic region of fish cells transfected with OnTIRAP-GFP and OnTRIF-GFP expression plasmids. Moreover, overexpression of OnTIRAP and OnTRIF activated interferon-β (IFN-β) and activator protein 1 (AP1) reporters in HEK 293 cells. Activation of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) reporter was only observed in OnTRIF-overexpressing HEK 293 cells. Furthermore, the results of the co-immunoprecipitation analysis showed that OnTRIF, but not OnTIRAP, was recruited as an adaptor protein by OnTLR25. This study provides the first evidence on the functions of OnTIRAP and OnTRIF in the immune system of Nile tilapia against pathogens and may serve as the basis for further investigations on TLR signaling in fish.

Comparative transcriptomics and host-specific parasite gene expression profiles inform on drivers of proliferative kidney disease

The myxozoan parasite, Tetracapsuloidesbryosalmonae has a two-host life cycle alternating between freshwater bryozoans and salmonid fish. Infected fish can develop Proliferative Kidney Disease, characterised by a gross lymphoid-driven kidney pathology in wild and farmed salmonids. To facilitate an in-depth understanding of T.bryosalmonae-host interactions, we have used a two-host parasite transcriptome sequencing approach in generating two parasite transcriptome assemblies; the first derived from parasite spore sacs isolated from infected bryozoans and the second from infected fish kidney tissues. This approach was adopted to minimize host contamination in the absence of a complete T.bryosalmonae genome. Parasite contigs common to both infected hosts (the intersect transcriptome; 7362 contigs) were typically AT-rich (60–75% AT). 5432 contigs within the intersect were annotated. 1930 unannotated contigs encoded for unknown transcripts. We have focused on transcripts encoding proteins involved in; nutrient acquisition, host–parasite interactions, development, cell-to-cell communication and proteins of unknown function, establishing their potential importance in each host by RT-qPCR. Host-specific expression profiles were evident, particularly in transcripts encoding proteases and proteins involved in lipid metabolism, cell adhesion, and development. We confirm for the first time the presence of homeobox proteins and a frizzled homologue in myxozoan parasites. The novel insights into myxozoan biology that this study reveals will help to focus research in developing future disease control strategies.

Atlantic salmon kidney (ASK) cells are an effective model to characterise interferon (IFN) and IFN-induced gene expression following salmonid alphavirus infection

Salmonid alphavirus (SAV), the causative agent of pancreas disease, is a serious pathogen of farmed Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss). Given the economic impact of SAV outbreaks, much effort is focussed upon understanding the fish immune response following infection and the exploitation of this knowledge to reduce disease impact. Herein we examine the utility of the long-term Atlantic salmon kidney (ASK) cell line as a tool to study antiviral responses upon infection with SAV. Following infection with SAV subtype 1 (isolate V4640) we examined the kinetics and magnitude of induction of IFNa, IFN-regulatory factor (IRF) genes IRF1, IRF3, and IRF7b, as well as the antiviral effector Mx by RT-qPCR. SAV-1 non-structural protein (nsp1) transcript levels increased continuously over the experimental period, indicating viral replication, but cytopathic effect (CPE) was not observed. All the immune genes studied showed an increase in transcript levels over the 96-h study period following SAV infection, with strongest induction of Mx. Our data confirm that ASK cells are a suitable model to study the virus-associated immune responses of salmonids and may be a useful tool when assaying the effectiveness of potential prophylactic or antiviral treatments.

Ancient Cytokine Interleukin 15-Like (IL-15L) Induces a Type 2 Immune Response

Related interleukin-2, -15, and -15-like (IL-2, -15, and -15L) are ancient cytokines, with all three genes surviving in extant fish and some mammals. The present study is the first to identify IL-15L functions, namely in rainbow trout. In isolated trout splenocytes, and in vivo, purified recombinant IL-15L+IL-15Rα molecules induced expression of IL-4 and IL-13 homologs, which are markers of type 2 immunity. In contrast, trout IL-15 stimulated type 1 immunity markers, thus IL-15 and IL-15L can have opposing functions. Trout IL-15L was more dependent on "in trans" presentation by the receptor chain IL-15Rα than IL-15, and stimulated CD4^-CD8^-(IgM^-) lymphocytes from thymus and spleen. We propose an important role for IL-15L early in the type 2 immunity cytokine cascade. Trout IL-2 and IL-15 exhibited features reminiscent of their mechanistic and functional dichotomy observed in mammals; for example, IL-15 but not IL-2 required a receptor alpha chain (only IL-15Rα in the case of fish) for its stability, and only IL-15 was efficient in stimulating lymphocytes from mucosal tissues. Data suggest that IL-15L and IL-15 may be particularly effective in stimulating innate lymphocyte type 2 cells (ILC2) and natural killer (NK) cells, respectively, but further identification of the cell types is needed. An interesting finding different from in mammals was the efficient stimulation of CD4⁺CD8⁺ thymocytes by IL-2. In short, this study presents fundamental information on the evolution of the IL-2/15/15L cytokine family.

Zebrafish as a model for inflammation and drug discovery

Zebrafish is a small teleost (bony) fish used in many areas of pharmacology and toxicology. This animal model has advantages for the discovery of anti-inflammatory drugs, such as the potential for real-time assessment of cell migration mechanisms. Additionally, zebrafish display a repertoire of inflammatory cells, mediators, and receptors that are similar to those in mammals, including humans. Inflammatory disease modeling in either larvae or adult zebrafish represents a promising tool for the screening of new anti-inflammatory compounds, contributing to our understanding of the mechanisms involved in chronic inflammatory conditions. In this review, we provide an overview of the characterization of inflammatory responses in zebrafish, emphasizing its relevance for drug discovery in this research area.

Effective isolation of GALT cells: Insights into the intestine immune response of rainbow trout (Oncorhynchus mykiss) to different bacterin vaccine preparations

The teleost gut is a multifunction complex structure that plays a pivotal immunological role in homeostasis and the maintenance of health, in addition to digestion of food and/or nutrient absorption. In vitro examination of the intestine leucocyte repertoire has the potential to aid our understanding of gut immune competence and allows a rapid screen of host-microorganism interactions in different immunological contexts. To explore this possibility, in the present study we investigated the response of isolated gut leucocytes to 4 bacterins of Aeromonas salmonicida, prepared from different strains, combinations and strains grown in different environments, in comparison to a Yersinia ruckeri bacterin for which a commercial/effective oral booster vaccine has been developed. To aid this study we also optimized further our method of GALT cell isolation from rainbow trout, so as to avoid mechanical clearance of the intestine contents. This drastically increased the cell yield from ~12 × 10⁶ to ~210 × 10⁶/fish with no change in the percent cell viability over time or presence of transcripts typical of the key leucocyte types needed for the study of immune modulation (i.e. T- and B-cells, dendritic cells and macrophages). A wide array of immune transcripts were modulated by the bacterins, demonstrating the diversity of GALT cell responses to bacterial stimulation. Indeed, the GALT leucocyte responses were sensitive enough to distinguish the different bacterial species, strains and membrane proteins, as seen by distinct kinetics of immune gene expression. However, the response of the GALT cells was often relatively slow and of a low magnitude compared to those of PBL. These results enhance our knowledge of the gut biocapacity and help validate the use of this model for screening of oral vaccine candidates.

Distinct modes of action of CD40L and adaptive cytokines IL-2, IL-4/13, IL-10 and IL-21 on rainbow trout IgM+ B cells

In mammals, conventional B (B2) cells are activated within lymphoid follicles through a close relationship with T follicular helper (Tfh) cells. The interaction between CD40 expressed on B cells and its ligand (CD40L) expressed on Tfh cells is a key signal that regulates the formation of germinal centers (GCs), B cell survival, proliferation and differentiation to plasma cells (PCs) or memory cells. Additionally, certain soluble cytokines produced by T cells also strongly condition the outcome of this interaction. Despite the many differences found between fish B cells and mammalian B2 cells, and the lack of conventional GCs, rainbow trout IgM⁺ B cells have been shown to be stimulated by CD40L, however, whether cytokines commonly produced by T cells can further modulate this response has never been addressed to date. Thus, in this study, we determined the effects of recombinant rainbow trout adaptive cytokines interleukin 2B (IL-2B), IL-4/13A, IL-4/13B, IL-10 and IL-21 (cytokines known to activate B cells in mammals) on splenic IgM⁺ B cells alone or in combination with CD40L. We studied how these cytokines and CD40L cooperated to promote IgM⁺ B cell survival, proliferation and IgM secretion. The results obtained provide valuable information for the first time in teleost fish on how different T cell signals cooperate to activate B cells in the absence of GCs.

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

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

Splicing variants of grass carp (Ctenopharyngodon idellus) IL-21: Functions in IgM+ B cell proliferation and IgM secretion

In mammals, interleukin 21 (IL-21) is a type I four-helical bundle cytokine produced by activated T cells that has pleiotropic functions on immune cells. Although IL-21 has been discovered in fish, the splicing variants of this cytokine and their functions on B cells are unclear. In this study, based on the original transcript of grass carp IL-21 (named gcIL-21sv1 in this study), two alternative splicing variants, named gcIL-21sv2 and gcIL-21sv3, were cloned and characterized. The protein sequences of gcIL-21sv1 and gcIL-21sv2 consist of four α-helixes, and only the six amino acid residues at the C-terminal are different. Unlike gcIL-21sv1 and gcIL-21sv2, gcIL-21sv3 lacks the C-terminal region. The expression analysis showed that gcIL-21sv1, gcIL-21sv2, and gcIL-21sv3 were constitutively expressed in all the tested tissues, and their expression could be significantly up-regulated by LPS and Poly (I:C) in head kidney leukocytes (HKLs), with the fold change of gcIL-21sv1 being higher than that of gcIL-21sv2 and gcIL-21sv3. Recombinant gcIL-21sv1 and gcIL-21sv2, but not gcIL-21sv3, could induce the proliferation of IgM⁺ B cells and the secretion of IgM, with the activity of gcIL-21sv1 being stronger than that of gcIL-21sv2, indicating that the C-terminal region plays important roles in the function of gcIL-21. Taken together, this study found that, like IL-21 in human and mouse, IL-21 splicing variants also exist in fish, and the regulatory activities of these variants in humoral immunity are differ, suggesting that grass carp may balance the immune response mediated by IL-21 through alternative splicing.

Immunostimulatory effects of Sarcodia suiae water extracts on Nile tilapia Oreochromis niloticus and its resistance against Streptococcus agalactiae

In this study, water extracts of the red seaweed Sarcodia suiae were obtained using solid-liquid extraction (SLE) or pressurized liquid extraction (PLE) methods. The extracts were used to investigate immunostimulatory activity by measuring the phagocytic activity of Nile tilapia (Oreochromis niloticus) hepatic and splenic macrophages and the tilapia head kidney (THK) cell line, and modulation of immune-related genes in primary head kidney (HK) cells and THK cells. At 10 μg/ml, both extracts promoted the proliferation of hepatic and splenic macrophages. Expression levels of proinflammatory cytokines (IL-1β and IL-8), antimicrobial peptides (TP2 and TP4), and pattern recognition receptors (TLR5) were elevated in SLE extracts-treated primary HK leukocytes. Similarly, IL-1β, IL-8, and TNFα expression was also induced by SLE extract in THK cells. Phagocytic activity in primary HK cells and THK cells was induced by SLE extract 12 h and 24 h post-stimulation, while PLE extract only induced phagocytic activity in THK cells at early time points. SLE extract (100 μg/g body weight) increased the expression of IL-1β, IL-8, TNFα, TP2, TP4, TLR2 and TLR5 in the spleen and immunoprotective efficiency against Streptococcus agalactiae infection. Taken together, these results show that S. suiae can differentially stimulate the immune response of tilapia in vitro and in vivo and could potentially be used as an immunomodulator in tilapia culture.

Type I Interferon Regulates the Survival and Functionality of B Cells in Rainbow Trout

Interferons (IFNs) orchestrate antiviral responses in jawed vertebrates and can be classified into three types based on different aspects of their genomic organization, structure and receptors through which they signal and function. Generally, type I and type III IFNs include cytokines that directly induce an antiviral response, whereas type II IFNs are well-known for their immunomodulatory role during viral infections. In mammals, type I IFNs have been shown to also regulate many aspects of B cell development and differentiation. Yet, these functions have been only faintly investigated for teleost IFNs. Thus, in the current study, we have examined the effects of a model type I rainbow trout IFN molecule (IFNa) on blood naïve (IgM⁺IgD⁺) B cells, comparing them to those exerted by type II IFN (IFNγ). Our results demonstrate that IFNa increases the survival of naïve rainbow trout B cells, in the absence of lymphoproliferative effects, by rescuing them from spontaneous apoptosis. Additionally, IFNa increased the phagocytic capacity of blood IgM⁺IgD⁺ B cells and augmented the number of IgM-secreting cells in blood leukocyte cultures. IFNγ, on the other hand, had only minor effects up-regulating IgM secretion, whereas it increased the phagocytic capacity of IgM⁻ cells in the cultures. Finally, given the recent identification of 9 mx genes in rainbow trout, we have also established which of these genes were transcriptionally regulated in blood naïve B cells in response to IFNa. This study points to a previously undescribed role for teleost type I IFNs in the regulation of B cell responses.

Sargassumhorneri extracts stimulate innate immunity, enhance growth performance, and upregulate immune genes in the white shrimp Litopenaeus vannamei

The white shrimp Litopenaeus vannamei is one of the most commercially important aquatic species. However, intensive farming to meet economic demands has been linked to animal stress and subsequent disease outbreaks. In this study, we explore the immunomodulatory effects of Sargassum horneri hot-water extract (SHE) on the immune parameters of L. vannamei. Hemocytes incubated in vitro with 10 mg/ml of SHE showed the highest response in phenoloxidase (PO) activity and reactive oxygen species production. In the in vivo trials, shrimp were fed diets containing 0 (control), 2.5 (SHE 2.5), 5.0 g (SHE 5.0), and 10 g (SHE10.0) of lyophilized SHE per kg feed for four weeks, after which immune parameters were measured. Group SHE10.0 had the highest total hemocyte count, reactive oxygen species production, and phagocytic rate during the 4-week feeding period, followed by groups SHE5.0 and SHE2.5. However, group SHE5.0 showed a better growth performance among all examined groups after four weeks of rearing. Furthermore, gene expression analysis revealed that L. vannamei fed diets containing 0.5% SHE over 28 days showed significant modulation of 11 immune-related genes, including prophenoloxidase I, prophenoloxidase II, peroxinectin, α2macroglobulin, clotting protein, lysozyme, superoxide dismutase, glutathione peroxidase, penaiedin2-4, and crustin. Combined, these results show that SHE is a functional feed additive that can be applied orally to enhance innate immunity and growth performance of white shrimps.

Membrane IgM+ plasma cells in grass carp (Ctenopharyngodon idella): Insights into the conserved evolution of IgM+ plasma cells in vertebrates

Teleost fish are the most primitive bony vertebrates that contain B cells; thus, comparative analysis of teleost naïve/mature B cells and plasma cells can provide helpful evidence for understanding the evolution paradigms of these two B-cell subpopulations in vertebrates. In this study, we developed monoclonal antibody against grass carp IgM and identified two different IgM⁺ cell subsets: IgM⁺ lymphocytes (Lym), resembling naïve/mature B cells, and IgM⁺ myeloid cells (Mye), resembling plasma cells. Like plasma cells in mammals, the size of IgM⁺ Mye is significantly larger than that of IgM⁺ Lym, as revealed by flow cytometric analysis and transmission electron microscopy. The IgM⁺ Mye were further verified as plasma cells because they showed gene expression patterns similar with those of human plasma cells and a great capacity to secrete IgM. Like mammalian IgM⁺ and IgA⁺ plasma cells, not IgG⁺ plasma cells, grass carp IgM⁺ Mye also expressed membrane immunoglobulins, a feature conserved in IgM⁺ plasma cells in vertebrates. Furthermore, recombinant CD40L or IL-21 alone could induce the plasma cell generation and IgM secretion, while the combination of CD40L and IL-21 had greater effect on IgM secretion, but not on plasma cell generation. This study fills an important gap in the knowledge of plasma cells in teleost fish and provides critical insights into the conserved evolution of IgM⁺ plasma cells in vertebrates.

Characterization and bioactivity of grass carp (Ctenopharyngodon idella) interleukin-21: Inducible production and involvement in inflammatory regulation

In mammals, interleukin 21 (IL-21) is a broad pleiotropic cytokine that plays critical roles in the development of several inflammatory and autoimmune diseases. In fish, functional information of Il-21 is limited, and its role in immune response is largely unknown. In the present study, we cloned a coding sequence of grass carp (Ctenopharyngodon idella) il21 gene (gcil21). To characterize the release patterns and biological activity of gcIl-21, we prepared recombinant gcIl-21 (rgcIl-21) and obtained the polyclonal antibody with gcIl-21 specificity. Western blotting analysis showed that in grass carp head kidney leukocytes (HKLs), gcIl-21 was undetected in culture supernatant of untreated cells but drastically induced by heat-killed Aeromonas hydrophila (A. hydrophila), uncovering the release features of gcIl-21 and its possible involvement in immune response. Subsequent functional experiments revealed that rgcIl-21 did not affect the mRNA expression of grass carp il1b and tgfb, but induced a strong expression of grass carp il10, and to a lesser extent of grass carp tnfa in HKLs, suggesting a dominant effect of gcIl-21 in modulating Il-10 signaling as seen in rainbow trout and mammals. Furthermore, in vivo studies showed that intraperitoneal injection of rgcIl-21 was able to increase the survival rate of grass carp infected with live A. hydrophila, and reduce the pathological responses caused by the same pathogenic bacteria in head kidney and intestine. Taken together, these results for the first time revealed the close relationship of fish Il-21 production and function with inflammatory responses, and highlighted its anti-bacterial and anti-inflammatory ability, thereby providing a new insight into host defense mechanisms in fish.

Five subfamilies of β-defensin genes are present in salmonids: Evolutionary insights and expression analysis in Atlantic salmon Salmo salar

β-defensins (BD) are the largest family of vertebrate defensins with potent antimicrobial, chemotactic and immune-regulatory activities. Four BD genes (BD1-4) have been cloned previously in rainbow trout but none have been reported in other salmonids. In this study seven BD genes (BD1a-b, 2-4, 5a-b) are characterised in Atlantic salmon and additional BD genes (BD1b and BD5) in rainbow trout. Bioinformatic analysis revealed up to seven BD genes in the genomes of other salmonids that belong to five subfamilies (BD1-5) due to whole genome duplications. BD1-2 and BD4-5 are also present in basal teleosts but only BD1 and/or BD5 are present in advanced teleosts due to loss of one chromosomal locus. BD3 is salmonid specific. Fish BD have a unique three-coding exon structure. Fish BD are highly divergent between subfamilies but conserved within each subfamily. Atlantic salmon BD genes are differentially expressed in tissues, often with low level expression in systemic immune organs (head kidney and spleen) yet with at least one BD gene highly expressed in mucosal tissues, heart, blood and liver. This suggests an important role of these BD genes in innate immunity in mucosa, liver and blood in Atlantic salmon.

Japanese flounder Paralichthys olivaceus interleukin 21 induces inflammatory response and plays a vital role in the immune defense against bacterial pathogen

Interleukin (IL)-21 is a pleiotropic cytokine and plays a vital role in immunity. In the current study, we examined the immune function of Japanese flounder Paralichthys olivaceus IL-21 (PoIL-21). PoIL-21 shares moderate (25.17%-46.25%) sequence identities with other teleost IL-21. PoIL-21 expression occurred in multiple tissues, especially intestine, and was regulated by bacterial infection in a time dependent manner. PoIL-21 was secreted by peripheral blood leukocytes (PBL) upon LPS stimulation. Recombinant PoIL-21 (rPoIL-21) bound to a wide range of Gram-negative and Gram-positive bacteria and inhibited the growth of the fish bacterial pathogen Streptococcus iniae. rPoIL-21 also interacted with PBL, resulting in enhanced cell proliferation, ROS production, and expression of IL-1β, TNF-α, CD8β, T-bet, PoIL-21, PoIL-21 receptor, and STAT. Consequently, the presence of rPoIL-21 significantly reduced bacterial infection in PBL. In vivo study showed that rPoIL-21 upregulated the expression of inflammatory cytokines and PoIL-21. Taken together, these results indicate that PoIL-21 is an inducible, secreted cytokine with a broad range of binding capacities and plays an important role in the regulation of anti-bacterial immunity.

Novel functions of grass carp three p40 isoforms as modulators of Th17 signature cytokine expression in head kidney leukocytes

Interleukin (IL)-12p40, a component of IL-12 and IL-23, can be secreted as monomer and homodimer in mammals. Our previous study has proved the existence of natural three p40 isoforms and their proinflammatory properties in grass carp. In the present study, we unexpectedly found that recombinant grass carp p40a/b/c (rgcp40a, rgcp40b and rgcp40c) were able to enhance the mRNA levels of grass carp il-17a/f1 (gcil-17a/f1) in a dose- and time-dependent manner in head kidney leukocytes (HKLs). In agreement with these findings, the enzyme-linked immunosorbent assay (ELISA) showed that rgcp40a, rgcp40b and rgcp40c markedly stimulated gcIl-17a/f1 secretion from the HKLs. Together with their stimulatory effects on grass carp gcil-22 and gcil-26 expression, our data suggested their potential to mediate Th17-like response in grass carp. To support this notion, we investigated the underlying mechanisms for the regulation of rgcp40 isoforms on gcil-17a/f1 expression, and found that three rgcp40 isoforms significantly induced the activation of Erk, Jnk and Stat3 pathways in a time-dependent oscillation in the same cell model. Moreover, three rgcp40 isoforms-induced gcil-17a/f1 mRNA expression was suppressed by the inhibition on Erk, Jnk and Stat3 pathways, suggesting the signaling pathways in the p40 isoforms-mediating il-17a/f1 transcription. These studies for the first time proved the involvement of three gcp40 isoforms in mediating Th17 signature cytokine expression in fish immune cells, therefore providing new insights into the roles of p40 in teleost immunity.

Expansion of fish CCL20_like chemokines by genome and local gene duplication: Characterisation and expression analysis of 10 CCL20_like chemokines in rainbow trout (Oncorhynchus mykiss)

Mammalian CCL20, or macrophage inflammatory protein-3α, can function as a homeostatic and inflammatory chemokine. In relation to the latter, it is responsible for the chemoattraction of lymphocytes and dendritic cells to mucosal immune sites under inflammatory and pathological conditions. CK1, CK8A and CK8B are rainbow trout (Oncorhynchus mykiss) CC chemokines that were reported previously to be phylogenetically related to mammalian CCL20. In the current study, an additional seven CCL20_L paralogues in rainbow trout are reported, that are divided into three subgroups and have been designated here as: CCL20_L1a (also referred to as CK1), CCL20_L1b1-2, CCL20_L2a (CK8A), CCL20_L2b (CK8B), CCL20_L3a, and CCL20_L3b1-4. Multiple CCL20_L genes were also identified in other salmonids that arose from both whole genome duplication and local gene duplication. Phylogenetic tree, homology and synteny analysis support that CCL20_L1-3 found in salmonids are also present in most teleosts arose from the 3 R whole genome duplication and in some species, local gene duplication. Like mammalian CCL20, rainbow trout CCL20_L molecules possess a high positive net charge with a pI of 9.34-10.16, that is reported to be important for antimicrobial activity. Rainbow trout CCL20_L paralogues are differentially expressed and in general highly expressed in mucosal tissues, such as gills, thymus and intestine. The expression levels of rainbow trout CCL20_L paralogues are increased during development and following PAMP/cytokine stimulation. For example, in RTS-11 cells CCL20_L3b1 and CCL20_L3b2 are highly up-regulated by LPS, Poly I:C, recombinant(r) IFNa and rIL-1β. Trout CCL20_L paralogues are also increased after Yersinia ruckeri infection or Poly I:C stimulation in vivo, with CCL20_L3b1 and CCL20_L3b2 again highly up-regulated. Overall, this is the first report of the complete CCL20 chemokine subfamily in rainbow trout, and the analysis of their expression and modulation in vitro and in vivo. These results suggest that teleosts possess divergent CCL20_L molecules that may have important roles in anti-viral/anti-bacterial defence and in mucosal immunity.

Induction of IL-22 protein and IL-22-producing cells in rainbow trout Oncorhynchus mykiss

IL-22 is a critical cytokine which is involved in modulating tissue responses during inflammation, and is produced mainly by T cells and innate leucocytes. In mammals, IL-22 is a key component in mucosal defences, tissue repair, epithelial cell survival and proliferation. In teleosts, IL-22 has been cloned and studied in several species, and the transcript is highly expressed in mucosal tissues and induced by pathogen associated molecular patterns (PAMPs), suggesting IL-22 also functions as an important component of the innate immune response in fish. To investigate these immune responses further, we have validated and characterised two monoclonal antibodies (mAbs) which were raised against two different peptide immunogens of salmonid IL-22. Our results show that both mAbs specifically react to their own peptide immunogens and recombinant IL-22, and are able to detect the induction of native protein expression after stimulation. In flow cytometry, an increase in IL-22 positive cells was detected after stimulation in vitro with cytokines and PAMPs and in vivo after bacterial challenge. The immunohistochemistry results showed that IL-22 is highly upregulated in the gills after challenge, both in cells within the gill filaments and in the interbranchial lymphoid tissue. Such results suggest IL-22 may have a role in triggering local antimicrobial defences in fish that may facilitate efficient microbial clearance. Hence monitoring IL-22 producing cells/protein secretion may provide an alternative mean to assess the effectiveness of mucosal vaccines.

Evolution of IFN subgroups in bony fish - 1:Group I-III IFN exist in early ray-finned fish, with group II IFN subgroups present in the Holostean spotted gar, Lepisosteus oculatus

The present study helps clarify when the fish type I IFN groups/subgroups evolved, by examination of the IFN genes present in the Holostean spotted gar, Lepisosteus oculatus, in relation to the IFN genes present in the Chondrostea (sturgeon). It confirms that all three IFN groups (I-III), and group II subgroups, existed prior to the appearance of teleost fish. Preliminary expression analysis in a gar cell line (GARL) suggests these IFN genes will have a role in antiviral defence in Holostean fish, in that they are induced by poly(I:C). A refined model of IFN evolution within the actinopterygian fish is proposed.

Teleost cytotoxic T cells

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

A potential probiotic Chromobacterium aquaticum with bacteriocin-like activity enhances the expression of indicator genes associated with nutrient metabolism, growth performance and innate immunity against pathogen infections in zebrafish (Danio rerio)

The use of probiotics as alternatives to antibiotics for disease control is a relatively eco-friendly approach in aquaculture; hence, studies isolating and assessing the benefit of potential probiotics to fish farming are common. The zebrafish is an excellent model system for validating beneficial functions of potential probiotics before their practical application in aquaculture. Here, a potentially probiotic Chromobacterium aquaticum was isolated from lake water samples and characterized by biochemical analysis and 16S rDNA sequencing. The probiotic produced extracellular enzymes (protease and xylanase) and a bacteriocin-like substance, which exhibited tolerance to extreme pH and high-temperature conditions and broad-spectrum bactericidal activity against diverse pathogens, including aquatic, foodborne, clinical and plant pathogens. The effects of C. aquaticum on zebrafish nutrient metabolism, growth performance and innate immunity were evaluated by measuring the expression of indicator genes after C. aquaticum feeding for 8 weeks. Fish administered the probiotic exhibited significantly increased hepatic mRNA expression of carbohydrate metabolism-related genes, including glucokinase (GK), hexokinase (HK), glucose-6-phosphatase (G6Pase), and pyruvate kinase (PK-L), and growth-related genes, including the growth hormone receptor (GHR) and insulin-like growth factor-1 (IGF-1). Innate immune-related genes (IL-1β, IL-6, TNF-α, IL-10, IL-21, NF-κb, lysozyme and complement C3b) were induced in fish with probiotic supplementation. Probiotic-treated fish exhibited a higher survival rate than control fish after challenge with Aeromonas hydrophila and Streptococcus iniae. Together, these data suggest that C. aquaticum, as a probiotic feed supplement, could enhance nutrient metabolism and growth performance and could modulate innate immunity against A. hydrophila and S. iniae in zebrafish.

Different origins of paralogues of salmonid TNR1 and TNFR2: Characterisation and expression analysis of four TNF receptor genes in rainbow trout Oncorhynchus mykiss

Mammalian TNFR1 and TNFR2 bind TNFα and TNFβ, and provide key communication signals to a variety of cell types during development and immune responses that are crucial for cell survival, proliferation and apoptosis. In teleost fish TNFβ is absent but TNFα has been expanded by the third whole genome duplication (3R WGD) and again by a 4R WGD in some lineages, leading to the four TNFα paralogues known in salmonids. Two paralogues for each of TNFR1 and TNFR2 have been cloned in rainbow trout in this study and are present in other salmonid genomes. Whilst the TNFR2 paralogues were generated via the 4R salmonid WGD, the TNFR1 paralogues arose from a local en bloc duplication. Functional diversification of TNFR paralogues was evidenced by differential gene expression and modulation, upstream ATGs affecting translation, ATTTA motifs in the 3'-UTR regulating mRNA stability, and post-translational modification by N-glycosylation. Trout TNFR are highly expressed in immune tissues/organs, and other tissues, in a gene- and tissue-specific manner. Furthermore, their expression is differentially modulated by PAMPs and cytokines in a cell type- and stimulant-specific manner. Such findings suggest an important role of the TNF/TNFR axis in the immune response and other physiological processes in fish.

Distinct response of immune gene expression in peripheral blood leucocytes modulated by bacterin vaccine candidates in rainbow trout Oncorhynchus mykiss: A potential in vitro screening and batch testing system for vaccine development in aquaculture

Fish aquaculture is the world's fastest growing food production industry and infectious diseases are a major limiting factor. Vaccination is the most appropriate method for controlling infectious diseases and a key reason for the success of salmonid cultivation and has reduced the use of antibiotics. The development of fish vaccines requires the use of a great number of experimental animals that are challenged with virulent pathogens. In vitro cell culture systems have the potential to replace in vivo pathogen exposure for initial screening and testing of novel vaccine candidates/preparations, and for batch potency and safety tests. PBL contain major immune cells that enable the detection of both innate and adaptive immune responses in vitro. Fish PBL can be easily prepared using a hypotonic method and is the only way to obtain large numbers of immune cells non-lethally. Distinct gene expression profiles of innate and adaptive immunity have been observed between bacterins prepared from different bacterial species, as well as from different strains or culturing conditions of the same bacterial species. Distinct immune pathways are activated by pathogens or vaccines in vivo that can be detected in PBL in vitro. Immune gene expression in PBL after stimulation with vaccine candidates may shed light on the immune pathways involved that lead to vaccine-mediated protection. This study suggests that PBL are a suitable platform for initial screening of vaccine candidates, for evaluation of vaccine-induced immune responses, and a cheap alternative for potency testing to reduce animal use in aquaculture vaccine development.

An insight into piscidins: The discovery, modulation and bioactivity of greater amberjack, Seriola dumerili, piscidin

Antimicrobial peptides (AMPs) play an important role in the innate immune response of vertebrates by creating a hostile environment for any invading pathogens. Piscidins are potent teleost specific AMPs, which have a broad spectrum activity. We have identified a novel piscidin active peptide, in the greater amberjack, Seriola dumerili, that consists of 25 aa, which forms an amphipathic helix with distinct hydrophobic and positively charged regions. Following homology and phylogenetic analysis the greater amberjack piscidin was deemed to belong to the group 3 family of piscidins. Piscidin was expressed constitutively at immune sites, with transcript level highest in the spleen and gut, at an intermediate level in the gills and lowest in the head kidney. Following in vivo stimulation with PAMPs (poly I:C, LPS and flagellin) piscidin transcript level increased in gills in response to flagellin, in gut and spleen in response to poly I:C, and in head kidney in response to poly I:C, LPS and flagellin. Head kidney and spleen cells were then isolated from greater amberjack and incubated with each of the PAMPs for 4, 12 and 24 h. Piscidin expression was unchanged at 4 and 12 h post PAMP stimulation in head kidney cells but at 24 h transcript level was markedly upregulated compared to control (unstimulated) cells, especially with the bacterial PAMPs. In contrast, spleen cells upregulated piscidin expression by 4 h post stimulation with poly I:C and flagellin, and remained upregulated to 24 h with flagellin exposure, but had returned to baseline levels by 12 h using poly I:C. To determine if piscidin expression could be modulated by diet, greater amberjack were fed diets supplemented with MOS and cMOS for 30 days when transcript level was determined. It was found that MOS supplemented diets increased expression in the spleen, cMOS supplemented diets upregulated transcript levels in the gills and head kidney, whilst a diet containing both MOS and cMOS upregulated transcript in the gut, when compared to fish fed the control diet. Finally, a synthetic greater amberjack piscidin was produced and showed bacteriostatic activity against a number of bacterial strains, including both Gram positive and Gram negative fish pathogens.

Transcriptomic analysis of the impacts of ethinylestradiol (EE2) and its consequences for proliferative kidney disease outcome in rainbow trout (Oncorhynchus mykiss)

Freshwater fish are threatened by the cumulative impact of multiple stressors. The purpose of this study was to unravel the molecular and organism level reactions of rainbow trout, Oncorhynchus mykiss, to the combined impact of two such stressors that occur in the natural habitat of salmonids. Fish were infected with either the myxozoan parasite, Tetracapsuloides bryosalmonae, which causes proliferative kidney disease (PKD), or exposed to ethinylestradiol (EE2) an estrogenic endocrine disrupting compound, or to a combination of both (PKD × EE2). PKD is a slow progressive chronic disease here we focused on a later time point (130-day post-infection (d.p.i.)) when parasite intensity in the fish kidney has already started to decrease. At 130 d.p.i., RNA-seq technology was applied to the posterior kidney, the main target organ for parasite development. This resulted with 280 (PKD), 14 (EE2) and 444 (PKD × EE2) differentially expressed genes (DEGs) observed in the experimental groups. In fish exposed to the combination of stressors (PKD × EE2), a number of pathways were regulated that were neither observed in the single stressor groups. Parasite infection, alone and in combination with EE2, only resulted in a low intensity immune response that negatively correlated with an upregulation of genes involved in a variety of metabolic and inflammation resolution processes. This could indicate a trade-off whereby the host increases investment in recovery/resolution processes over immune responses at a later stage of disease. When PKD infection took place under simultaneous exposure to EE2 (PKD × EE2), parasite intensity decreased and pathological alterations in the posterior kidney were reduced in comparison to the PKD only condition. These findings suggest that EE2 modulated these response profiles in PKD infected fish, attenuating the disease impact on the fish.