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

IL-20 is produced by CD3γδ T cells and induced in the mucosal tissues of grass carp during infection with Aeromonas hydrophila

Interleukin (IL) 20 is a multifunctional cytokine and plays a vital role in regulating autoimmune diseases, inflammation, and immune responses. IL-20 homologs have been described in fish. However, due to the lack of antibodies, cellular sources and immunological functions of fish IL-20 in response to infections have not been fully characterized. In this study, a monoclonal antibody (mAb) was generated against the recombinant grass carp (Ctenopharyngodon idella) IL-20 protein and characterized by immunoblotting, immunofluorescent microscopy and flow cytometry. It was shown that the IL-20 mAb specifically recognized recombinant IL-20 proteins expressed in the E. coli cells and HEK293 cells. Using confocal microscopy, the IL-20+ cells were identified in the head kidney, gills and intestine of grass carp, and induced after infection with Aeromonas hydrophila. Moreover, the IL-20 protein was found to be secreted mainly by CD3γδ T cells which were located predominantly in the gill filaments and intestinal mucosa. Taken together, our results suggest that IL-20 producing T cells are required for the mucosal immunity against bacterial infection in fish.

Megalobrama amblycephala IL-22 attenuates Aeromonas hydrophila induced inflammation, apoptosis and tissue injury by regulating the ROS/NLRP3 inflammasome axis

Mammalian interleukin-22 (IL-22) attenuates organismal injury by inhibiting reactive oxygen species (ROS) and impeding the NLRP3 inflammasome activation. However, the role of fish IL-22 in this process remains unclear. We characterized MaIL-22, an IL-22 homolog in blunt snout bream (Megalobrama amblycephala). Despite its low sequence identity, it shares conserved structures and close evolutionary relationships with other teleost IL-22s. Furthermore, Aeromonas hydrophila (A. hydrophila) infection leads to tissue injury in M. amblycephala immune organs and concomitantly altered Mail-22 mRNA expression, suggesting that MaIL-22 was involved in the antimicrobial immune response. To explore MaIL-22's biological functions, we produced recombinant MaIL-22 (rMaIL-22) protein and demonstrated it significantly enhanced the survival of M. amblycephala post-A. hydrophila infection. To unravel its protective mechanisms, we explored the ROS/NLRP3 inflammasome axis and its downstream signaling responses. The results showed that rMaIL-22 treatment significantly elevated antioxidant enzyme (T-SOD, CAT and GSH-PX) activities to inhibit MDA activity and scavenge ROS in visceral tissues. Meanwhile, rMaIL-22 impeded the activation of NLRP3 inflammasome by suppressing NLRP3 protein and mRNA expression. This indicated that rMaIL-22 contributed to inhibit A. hydrophila-induced activation of the ROS/NLRP3 inflammasome axis. Consistent with these findings, rMaIL-22 treatment attenuated the expression of proinflammatory cytokines (il-1β, tnf-α and il-6) and proapoptotic genes (caspase-3 and caspase-8) while promoting antiapoptotic genes (bcl-2b and mcl-1a) expression, ultimately mitigating tissue injury in visceral tissues. In conclusion, our research underscores MaIL-22's key role in microbial immune regulation, offering insights for developing IL-22-targeted therapies and breeding programs.

Identification and functional characterization of interleukin-22 (IL-22) in orange-spotted grouper (Epinephelus coioides)

In mammals, IL-22 is considered as a critical cytokine regulating of immunity and homeostasis at barrier surfaces. Although IL-22 have been functional characterization in different species of fish, the studies about distinct responses of IL-22 in different organs/tissues/cell types is rather limited. Here, we identified and cloned IL-22 gene (named as Ec-IL-22) from grouper (Epinephelus coioides). Ec-IL-22 gene was detected in all orangs/tissues examined, and was induced in intestine, gill, spleen, head kidney, and primary head kidney/intestine leukocytes following the stimulation of LPS and poly (I:C), as well as Vibrio harveyi and Singapore grouper iridovirus infection (SGIV). In addition, the stimulation of DSS could induce the expression of Ec-IL-22 in intestine and primary leukocytes from intestine. Importantly, the treatment of recombinant Ec-IL-22 induced the mRNA level of proinflammatory cytokines in primary intestine/head kidney leukocytes. The present results improve the understanding of expression patterns and functional characteristics of fish IL-22 in different organs/tissues/cell types.

Evolutionarily conserved IL-22 participates in gut mucosal barrier through its receptors IL-22BP, IL-10R2 and IL-22RA1 during bacterial infection in teleost

IL-22 is a critical cytokine of epithelial mucosal barrier. In humans, IL-22 signals through a heteroduplex receptor consisting of IL-22R and IL-10Rβ. In fish, IL-22 and its receptors homologues have been cloned in a number of species, however, no studies have been reported how the receptors are involved in IL-22 transduction. For this purpose, in this study we identified IL-22 and its soluble receptor IL-22BP and transmembrane receptors IL-22RA1 and IL-10R2 in Carassius cuvieri × Carassius auratus red var. (named WR-IL-22, WR-IL-22BP, WR-IL10R2 and WR-IL22RA1, respectively). WR-IL-22, WR-IL-22BP, WR-IL10R2 and WR-IL22RA1 were relatively conserved in the evolutionary process, sharing the same conserved domains as their higher vertebrate homologues. When the fish were infected with the Aeromonas hydrophila, the expression of WR-IL-22, WR-IL-22BP, WR-IL10R2 and WR-IL22RA1 were significantly induced in the gut. The co-IP assay showed that WR-IL-22 not only interacted with WR-IL-22BP, but also with WR-IL10R2 and WR-IL22RA1. When introduced in vivo, WR-IL-22 activated the JAK1-STAT3 axis and protected the gut mucosa from A. hydrophila infection. However, overexpression of WR-IL-22BP or knockdown of transmembrane receptors WR-IL10R2 and WR-IL22RA1 significantly inhibited the activation of WR-IL-22-mediated JAK1-STAT3 axis and promoted bacterial colonization in the gut. These results provided new insights into the role of IL-22 and its receptors in the gut mucosa barrier and immune response in teleost.

Interleukin-22 induces immune-related gene expression in the gills of Japanese medaka Oryzias latipes

The cytokine interleukin (IL)-22 has been identified in several fish species; however, its functional significance in the gills of these fish species remains unclear. In this study, we analyzed the expression of proinflammatory cytokines, antimicrobial peptides, and IL-22 binding protein in the gills of wild-type and IL-22-knockout (IL-22 KO) medaka under dextran sulfate sodium-induced inflammation. We also produced medaka recombinant IL-22 (rIL-22) and analyzed the expression of immune-related genes in rIL-22-stimulated primary cell cultures from gills. The il1b, il6, tnfa, and hamp genes were significantly upregulated in wild-type gills upon dextran sulfate sodium stimulation compared with the naïve state but not in IL-22 KO gills. il22bp transcripts were barely detectable in the IL-22 KO medaka gills. However, the expression of il1b, il6, hamp, and il22bp was upregulated in rIL-22-stimulated gill cell culture. These results suggest IL-22 could be involved in immune responses through inflammatory cytokine and antimicrobial peptide production in fish gills.

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.

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.

IL-4/13 expressing CD3γ/δ+ T cells regulate mucosal immunity in response to Flavobacterium columnare infection in grass carp

Interleukin (IL) 4 and 13 are signature cytokines orchestrating Th2 immune response. Teleost fish have two homologs, termed IL-4/13A and IL-4/13B, and have been functionally characterized. However, what cells express IL-4/13A and IL-4/13B has not been investigated in fish. In this work, the recombinant IL-4/13A and IL-4/13B proteins of grass carp (Ctenopharyngodon idella) were produced in the Escherichia coli (E. coli) cells and purified. Monoclonal antibodies (mAbs) against the recombinant CiIL-4/13A and CiIL-4/13B proteins were prepared and characterized. Western blotting analysis showed that the CiIL-4/13A and CiIL-4/13B mAbs could specifically recognize the recombinant proteins expressed in the E. coli cells and HEK293T cells and did not cross-react with each other. Confocal microscopy revealed that the CiIL-4/13A⁺ and CiIL-4/13B⁺ cells were present in the gills, intestine and spleen and could be upregulated in fish infected with Flavobacterium columnare (F. columnare). Interestingly, the cells expressing CiIL-4/13A and CiIL-4/13B were mostly CD3γ/δ⁺ cells. The CD3γ/δ⁺/IL-4/13A⁺ and CD3γ/δ⁺/IL-4/13B⁺ cells were significantly upregulated in the gill filaments and the intestinal mucosa after F. columnare infection. Our results imply that the CD3γ/δ⁺/IL-4/13A⁺ and CD3γ/δ⁺/IL-4/13B⁺ cells are important for homeostasis and the regulation of mucosal immunity.

Butyrate-induced IL-22 expression in fish macrophages contributes to bacterial clearance

IL-22 has been characterized as a critical cytokine in maintaining barrier integrity and host immunity. So far, it has been known that IL-22 is mainly produced by lymphoid lineage cells. In the present study, we have thoroughly investigated butyrate-induced production and function of IL-22 in fish macrophages. Our results demonstrated that short-chain fatty acids (SCFAs), major microbiota-derived metabolites, promoted the expression of IL-22 in head kidney macrophages (HKMs) of turbot (Scophthalmus maximus L.). Interestingly, butyrate-mediated intracellular bacterial killing in HKMs diminished when IL-22 expression was interfered. Furthermore, the turbot fed the diet containing sodium butyrate (NaB) exhibited significantly lower mortality after bacterial infection, compared to the fish fed a basal diet. At the meantime, a higher level of IL-22 expression and bactericidal activity was detected in HKMs from the turbot fed NaB-supplemented diet. In addition, NaB treatment promoted the expression of antimicrobial peptides (AMPs) β-defensins in zebrafish (Danio rerio). However, butyrate-induced expression of AMPs was reduced in IL-22 mutant zebrafish compared to wild-type (WT) fish. Meanwhile, NaB treatment was incapable to protect IL-22 mutant fish from bacterial infection as it did in WT zebrafish. Importantly, our results demonstrated that IL-22 expression was remarkably suppressed in macrophage-depleted zebrafish, indicating that macrophage might be a cell source of IL-22 production in vivo. In conclusion, all these findings collectively revealed that SCFAs regulated the production and function of IL-22 in fish macrophages, which facilitated host resistance to bacterial invasion.

Vitamin D influences gut microbiota and acetate production in zebrafish (Danio rerio) to promote intestinal immunity against invading pathogens

Although evidence has shown that vitamin D (VD) influences gut homeostasis, limited knowledge is available how VD regulates intestinal immunity against bacterial infection. In the present study, cyp2r1 mutant zebrafish, lacking the capacity to metabolize VD, and zebrafish fed a diet devoid of VD, were utilized as VD-deficient animal models. Our results confirmed that the expression of antimicrobial peptides (AMPs) and IL-22 was restrained and the susceptibility to bacterial infection was increased in VD-deficient zebrafish. Furthermore, VD induced AMP expression in zebrafish intestine by activating IL-22 signaling, which was dependent on the microbiota. Further analysis uncovered that the abundance of the acetate-producer Cetobacterium in VD-deficient zebrafish was reduced compared to WT fish. Unexpectedly, VD promoted the growth and acetate production of Cetobacterium somerae under culture in vitro. Importantly, acetate treatment rescued the suppressed expression of β-defensins in VD-deficient zebrafish. Finally, neutrophils contributed to VD-induced AMP expression in zebrafish. In conclusion, our study elucidated that VD modulated gut microbiota composition and production of short-chain fatty acids (SCFAs) in zebrafish intestine, leading to enhanced immunity.

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.

Interleukin-22 Deficiency Contributes to Dextran Sulfate Sodium-Induced Inflammation in Japanese Medaka, Oryzias latipes

Mucosal tissue forms the first line of defense against pathogenic microorganisms. Cellular damage in the mucosal epithelium may induce the interleukin (IL)-22-related activation of many immune cells, which are essential for maintaining the mucosal epithelial barrier. A previous study on mucosal immunity elucidated that mammalian IL-22 contributes to mucus and antimicrobial peptides (AMPs) production and anti-apoptotic function. IL-22 has been identified in several teleost species and is also induced in response to bacterial infections. However, the roles of IL-22 in teleost immunity and mucus homeostasis are poorly understood. In this study, Japanese medaka (Oryzias latipes) was used as a model fish. The medaka il22, il22 receptor A1 (il22ra1), and il22 binding protein (il22bp) were cloned and characterized. The expression of medaka il22, il22ra1, and il22bp in various tissues was measured using qPCR. These genes were expressed at high levels in the mucosal tissues of the intestines, gills, and skin. The localization of il22 and il22bp mRNA in the gills and intestines was confirmed by in situ hybridizations. Herein, we established IL-22-knockout (KO) medaka using the CRISPR/Cas9 system. In the IL-22-KO medaka, a 4-bp deletion caused a frameshift in il22. To investigate the genes subject to IL-22-dependent regulation, we compared the transcripts of larval medaka between wild-type (WT) and IL-22-KO medaka using RNA-seq and qPCR analyses. The comparison was performed not only in the naïve state but also in the dextran sulfate sodium (DSS)-exposed state. At the transcriptional level, 368 genes, including immune genes, such as those encoding AMPs and cytokines, were significantly downregulated in IL-22-KO medaka compared that in WT medaka in naïve states. Gene ontology analysis revealed that upon DSS stimulation, genes associated with cell death, acute inflammatory response, cell proliferation, and others were upregulated in WT medaka. Furthermore, in DSS-stimulated IL-22-KO medaka, wound healing was delayed, the number of apoptotic cells increased, and the number of goblet cells in the intestinal epithelium decreased. These results suggested that in medaka, IL-22 is important for maintaining intestinal homeostasis, and the disruption of the IL-22 pathway is associated with the exacerbation of inflammatory pathology, as observed for mammalian IL-22.

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.

Specific bioactivity of IL-22 in intestinal cells as revealed by the expression of IL-22RA1 in Mandarin fish, Siniperca chuatsi

IL-22, a multifunctional cytokine, acts as an important regulator in host immunity in mammals. IL-22 homologues have been characterized in several species of fish, with its expression found in multiple tissues/cells in fish, but its target cells have not been fully analyzed. In the present research, different organ/tissue isolated cells were examined for the expression of IL-22 and the induced IL-22 responses in mandarin fish. The mandarin fish IL-22 was found to be expressed in all these tested cells with high basal expression in intestinal cells. The HKLs showed low basal expression but significant increase in expression of IL-22 after LPS treatment or bacterial infection. Only intestinal cells showed response to IL-22 by enhanced expression of hepcidin, LEAP2 and IL-22BP, with unresponsiveness observed in other tested cells, which indicated the cell-specificity of IL-22 bioactivity in mandarin fish. One of the heterodimeric receptor components for IL-22, the IL-22RA1, was cloned in mandarin fish, with four tandem fibronectin type III (FNIII) domains identified in its extracellular part. IL-22RA1 exhibited an intestinal cell-specific expression pattern, although another receptor component of IL-22, IL-10R2, displayed constitutive expressions in all these tested cells. The present study reveals that the mandarin fish IL-22 exhibits its bioactivity in a cell-specific manner in intestinal cells, which is reflected in the restrictive expression of its receptor unit, IL-22RA1.

Characterization of IL-22 Bioactivity and IL-22-Positive Cells in Grass Carp Ctenopharyngodon idella

Interleukin (IL)-22 plays an important role in regulating inflammation and clearance of infectious pathogens. IL-22 homologs have been discovered in fish, but the functions and sources of IL-22 have not been fully characterized. In this study, an IL-22 homolog was identified in grass carp and its bioactivities were investigated. The grass carp IL-22 was constitutively expressed in tissues, with the highest expression detected in the gills and hindgut. It was upregulated in the spleen after infection with Flavobacterium columnare and grass carp reovirus and in the primary head kidney and spleen leukocytes stimulated with LPS and IL-34. Conversely, it was downregulated by Th2 cytokines such as IL-4/13B and IL-10. The recombinant IL-22 produced in bacteria showed a stimulatory effect on the expression of inflammatory cytokines and STAT3 in the primary head kidney leukocytes and CIK cells. Moreover, the IL-22-positive cells were found to be induced in the hindgut and head kidney 24 h after infection by F. columnare. Our data suggest that IL-22 plays an important role in regulating mucosal and systemic immunity against bacterial and viral infection.

The Atlantic Salmon Gill Transcriptome Response in a Natural Outbreak of Salmon Gill Pox Virus Infection Reveals New Biomarkers of Gill Pathology and Suppression of Mucosal Defense

The salmon gill poxvirus (SGPV) is a large DNA virus that infects gill epithelial cells in Atlantic salmon and is associated with acute high mortality disease outbreaks in aquaculture. The pathological effects of SGPV infection include gill epithelial apoptosis in the acute phase of the disease and hyperplasia of gill epithelial cells in surviving fish, causing damage to the gill respiratory surface. In this study, we sampled gills from Atlantic salmon presmolts during a natural outbreak of SGPV disease (SGPVD). Samples covered the early phase of infection, the acute mortality phase, the resolving phase of the disease and control fish from the same group and facility. Mortality, the presence and level of SGPV and gill epithelial apoptosis were clearly associated. The gene expression pattern in the acute phase of SGPVD was in tune with the pathological findings and revealed novel transcript-based disease biomarkers, including pro-apoptotic and proliferative genes, along with changes in expression of ion channels and mucins. The innate antiviral response was strongly upregulated in infected gills and chemokine expression was altered. The regenerating phase did not reveal adaptive immune activity within the study period, but several immune effector genes involved in mucosal protection were downregulated into the late phase, indicating that SGPV infection could compromise mucosal defense. These data provide novel insight into the infection mechanisms and host interaction of SGPV.

Advances in salmonid fish immunology: A review of methods and techniques for lymphoid tissue and peripheral blood leucocyte isolation and application

Evaluating studies over the past almost 40 years, this review outlines the current knowledge and research gaps in the use of isolated leucocytes in salmonid immunology understanding. This contribution focuses on the techniques used to isolate salmonid immune cells and popular immunological assays. The paper also analyses the use of leucocytes to demonstrate immunomodulation following dietary manipulation, exposure to physical and chemical stressors, effects of pathogens and parasites, vaccine design and application strategies assessment. We also present findings on development of fish immune cell lines and their potential uses in aquaculture immunology. The review recovered 114 studies, where discontinuous density gradient centrifugation (DDGC) with Percoll density gradient was the most popular leucocyte isolation method. Fish head kidney (HK) and peripheral blood (PB) were the main sources of leucocytes, from rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). Phagocytosis and respiratory burst were the most popular immunological assays. Studies used isolated leucocytes to demonstrate that dietary manipulations enhance fish immunity, while chemical and physical stressors suppress immunity. In addition, parasites, and microbial pathogens depress fish innate immunity and induce pro-inflammatory cytokine gene transcripts production, while vaccines enhance immunity. This review found 10 developed salmonid cell lines, mainly from S. salar and O. mykiss HK tissue, which require fish euthanisation to isolate. In the face of high costs involved with density gradient reagents, the application of hypotonic lysis in conjunction with mico-volume blood methods can potentially reduce research costs, time, and using nonlethal and ethically flexible approaches. Since the targeted literature review for this study retrieved no metabolomics study of leucocytes, indicates that this approach, together with traditional technics and novel flow cytometry could help open new opportunities for in vitro studies in aquaculture immunology and vaccinology.