Identification and expression modulation of a C-type lectin domain family 4 homologue that is highly expressed in monocytes/macrophages in rainbow trout (Oncorhynchus mykiss)

Paecilomyces variotii improves growth performance and modulates immunological biomarkers and gut microbiota in vaccinated Atlantic salmon pre-smolts

Functional feeds, as a prophylactic strategy, are a promising alternative to address stressful production-related activities (e.g., seawater transfer, delousing) and infectious diseases in salmon farming. To understand the effect of Paecilomyces variotii on short-term growth performance and health responses of vaccinated Atlantic salmon pre-smolts, a control diet (D1) and three experimental diets, where P. variotii replaced 5 % (D2), 10 % (D3) or 20 % (D4) of crude protein, were fed to fish for 28 days in freshwater. Fish fed diets containing P. variotii had a significant dose-dependent linear improvement in feed conversion ratio. Also, D4 group showed a gene expression related to signal truncation and gut homeostasis, while in head kidney (HK), P. variotii activated and controlled immune responses through c-type lectin receptor, downstream signalling molecules (myd88, tollip), cytokines (tnfα, il1β, ifnγ), and effector molecules (cath-2, c3, prx). Moreover, an upregulation of antigen presenting cell markers (mhcii, cd83) and T cell transcriptional factors (gata3, rorc, foxp3) was detected in HK, suggesting that P. variotii could coordinate the innate and adaptive mechanisms. Interestingly, D2 increased specific IgM against Vibrio anguillarum in vaccinated salmon. KEGG analysis revealed that D4 induced decreased abundance of proteins related to inflammatory pathways, e.g., like salmonella infection, apoptosis and necroptosis as well as innate and adaptive signalling pathways in the HK. On the contrary, D4 induced high abundance of proteins related to these inflammatory pathways in the skin mucus (Skm). In addition, complement proteins (i.e., C1q, C4, C7) and arginine metabolism were also in high abundance in the SKm. In relation to the gut microbiota, fish fed D2 and D3 showed low abundance of key lactic acid bacteria (e.g., Weisella, Leuconostoc, Lactobacillus) but high abundance Photobacterium and Ligilactobacillus compared with D1 in the gut. Overall, feed inclusion of P. variotii improved fish growth performance and modulated health response in Atlantic salmon pre-smolts.

Integrated analysis of proteome and transcriptome revealed changes in multiple signaling pathways involved in immunity in the northern snakehead (Channa argus) during Nocardia seriolae infection

The northern snakehead (Channa argus) is a valuable aquaculture species across certain Asian countries, contributing significantly to economic prosperity and dietary needs. However, its productivity faces significant challenges, particularly from diseases such as nocardiosis, caused by Nocardia seriolae. To date, the majority of research efforts have focused on describing the observed phenomena related to N. seriolae infection. However, there remains a notable gap in knowledge concerning the infectivity of N. seriolae and the immune response it elicits. To better understand the modulation of the immune responses to N. seriolae infection in snakeheads, we investigated the splenic proteome profiles. Specifically, we compared the profiles between uninfected northern snakehead specimens and those infected with N. seriolae at 96 h using the label-free data-independent acquisition methodology. A total of 700 differentially expressed proteins (DEPs) were obtained. Of these, 353 proteins exhibited upregulation, whereas 347 proteins displayed downregulation after the infection. The DEPs were mapped to the reference canonical pathways in Kyoto Encyclopedia of Genes and Genomes database, revealing several crucial pathways that were activated following N. seriolae infection. Noteworthy, among these were pathways such as ferroptosis, complement and coagulation cascades, chemokine signaling, tuberculosis, natural killer cell-mediated cytotoxicity, and Th17 cell differentiation. Furthermore, protein-protein interaction networks were constructed to elucidate the interplay between immune-related DEPs. These results revealed expression changes in multiple signaling pathways during the initial colonization phase of N. seriolae. This discovery offers novel insights into the infection mechanisms and host interaction dynamics associated with N. seriolae.

Differential expression of immune-related biomarkers in primary cultures from Atlantic salmon (Salmo salar) exposed to processed Paecilomyces variotii with or without inactivated Moritella viscosa

Paecilomyces variotii (a filamentous fungus), is a promising novel protein source in fish feeds due to its high nutritional value. Also, P. variotii has Microbial-Associated Molecular Patterns (MAMPs) such as glucans and nucleic acids that could modulate the host's immune response. To understand the potential bioactive properties of this fungus in Atlantic salmon (Salmo salar), our study was conducted to evaluate the gene expression of immune-related biomarkers (e.g., cytokines, effector molecules and receptors) on primary cultures from salmon head kidney (HKLs) and spleen leukocytes (SLs) exposed to either UV inactivated or fractions from P. variotii with or without inactivated Moritella viscosa (a skin pathogen in salmonids). Moreover, the effect of the fermentation conditions and down-stream processing on the physical ultrastructure and cell wall glucan content of P. variotii was characterized. The results showed that drying had a significant effect on the cell wall ultrastructure of the fungi and the choice of fermentation has a significant effect on the quantity of β-glucans in P. variotii. Furthermore, stimulating Atlantic salmon HKLs and SLs with P. variotii and its fractions induced gene expression related to pro-inflammatory (tnfα, il1β) and antimicrobial response (cath2) in HKLs, while response in SLs was related to both pro-inflammatory and regulatory response (tnfα, il6 and il10). Similarly, the stimulation with inactivated M. viscosa alone led to an up-regulation of genes related to pro-inflammatory (tnfα, il1β, il6) antimicrobial response (cath2), intra-cellular signalling and recognition of M. viscosa (sclra, sclrb) and a suppression of regulatory response (il10) in both HKLs and SLs. Interestingly, the co-stimulation of cells with P. variotii and M. viscosa induced immune homeostasis (il6, tgfβ) and antimicrobial response (cath2) in SLs at 48h. Thus, P. variotii induces immune activation and cellular communication in Atlantic salmon HKLs and SLs and modulates M. viscosa induced pro-inflammatory responses in SLs. Taken together, the results from physical and chemical characterization of the fungi, along with the differential gene expression of key immune biomarkers, provides a theoretical basis for designing feeding trials and optimize diets with P. variotii as a functional novel feed ingredient for Atlantic salmon.

Deciphering the Immunostimulatory Effects of β-Glucan on a Rainbow Trout (Oncorhynchus mykiss) Macrophage-like Cell Line (RTS11) by Whole Transcriptome Analysis

β-glucans are a commonly used immunostimulant/prebiotic in many aquaculture applications for boosting the immune status in fish. However, the method of action as an immunostimulant has not been fully deciphered. To determine the immunomodulatory effects of β-glucans on the innate immune response, we stimulated the rainbow trout spleen macrophage-like cell line (RTS11) with β-1,3/1,6-glucans for 4 h. This study uses a whole transcriptomic approach to analyse the immunomodulatory properties of β-glucans. Several proinflammatory pathways were found to be enriched after stimulation, demonstrating the immunomodulatory effects of β-glucan supplementation. Several pathways relating to responses to bacteria were also found to be enriched. This study clearly demonstrates the immunomodulatory effects of the supplementation of β-glucans within an aquaculture setting and further validates the use of cell lines as predictive models to interpret the responses caused by dietary intervention.

Molecular characterization and functional analysis of CD209E from Nile Tilapia (Oreochromis Niloticus) involved in immune response to bacterial infection

CD209 plays significant roles in pathogen recognition, innate and adaptive immunity, and cell-cell interactions. In the present study, a CD209 antigen-like protein E from Nile tilapia (Oreochromis niloticus) (designated as OnCD209E) was identified and characterized. OnCD209E contains an open reading frame (ORF) of 771 bp encoding a 257 amino acid protein, as well as the carbohydrate recognition domain (CRD). Multiple sequence analysis exhibits that the amino acid sequence of OnCD209E was relatively high homologous to that of partial fish, especially the highly conserved CRD, in which four conserved disulfide-bonded cysteine residues, WIGL conserved motif and two Ca²⁺/carbohydrate-binding sites (EPD and WFD motifs) were founded. Quantitative real-time PCR and Western Blot revealed that OnCD209E mRNA/protein is generally expressed in all tissues examined, but with wealth in head kidney and spleen tissues. The mRNA expression of OnCD209E was significantly increased in brain, head kidney, intestine, liver, and spleen tissues in response to the stimulation with polyinosinic-polycytidylic acid, Streptococcus agalactiae and Aeromonas hydrophila in vitro. Recombinant OnCD209E protein exhibited detectable bacterial binding and agglutination activity against different bacteria as well as inhibited the proliferation of tested bacteria. Subcellular localization analysis revealed that OnCD209E was mostly localized in the cell membrane. Moreover, overexpression of OnCD209E could activate nuclear factor-kappa B reporter genes in HEK-293T cells. Collectively, these results demonstrated that CD209E may potentially involve in immune response of Nile tilapia against bacterial infection.

Gut-liver immune and redox response in hybrid fish (Carassius cuvieri ♀ × Carassius auratus red var. ♂) after gut infection with Aeromonas hydrophila

Aeromonas hydrophila can pose a great threat to fish survival. In this study, we investigated the differential immune and redox response in gut-liver axis of hybrid fish (WR) undergoing gut infection. WR anally intubated with A. hydrophila showed severe midgut injury with decreased length-to-width ratios of villi along with GC hyperplasia and enhanced antioxidant activities, but expression profiles of cytokines, chemokines, antibacterial molecules, redox sensors and tight junction proteins decreased dramatically. In contrast, immune-related gene expressions and antioxidant activities increased significantly in liver of WR following gut infection with A. hydrophila. These results highlighted the differential immune regulation and redox balance in gut-liver axis response to bacterial infection.

In-vivo analysis of Protec™ and β-glucan supplementation on innate immune performance and intestinal health of rainbow trout

Functional diets are often given to fish during key stages to improve health through the interaction of the feed components with the host intestine. The additional factors added in these diets are known to modulate the immune response and as such may also offer protection against pathogenic challenges. The present study was undertaken to evaluate whether β-glucan supplementation for 6 weeks can alter the magnitude of immune response to immunological challenges and subsequently offer an improved innate immune response to bacterial challenge in rainbow trout. Two experimental diets were used to study these effects: a basic commercial diet supplemented with β-glucan and a commercially available functional diet (Protec™) that has β-glucan as a functional component in addition to other components were compared to a basic commercial control diet. No significant differences were observed in biometric data. Histological analysis revealed a significantly greater number of goblet cells in the fish fed Protec™ and β-glucan diets compared to those fed a control diet. Cell marker gene expression of distal intestine leucocytes indicated higher expression of T- and B-cells marker genes to both the β-glucan containing diets in comparison to control. The Protec™ diet demonstrated modulation of innate immune markers after 6 weeks of feeding with key antimicrobial genes (SAA, HAMP, IL-1β and TNFα) showing significant increases compared to the other diets. After stimulation with both PAMPs and an immune challenge with A. salmonicida fish fed the β-glucan diet and the Protec™ exhibited modulation of the innate immune response. An immune challenge with A. salmonicida was carried out to identify if dietary composition led to differences in the innate immune response of rainbow trout. Modulation of the magnitude of response in some immune genes (SAA, IL-1β and HAMP) was observed in both the distal intestine and head kidney in the Protec™ and β-glucan fed fish compared to those fed the control diet.

Polymeric Nanoparticles for Inhaled Vaccines

Many recent studies focus on the pulmonary delivery of vaccines as it is needle-free, safe, and effective. Inhaled vaccines enhance systemic and mucosal immunization but still faces many limitations that can be resolved using polymeric nanoparticles (PNPs). This review focuses on the use of properties of PNPs, specifically chitosan and PLGA to be used in the delivery of vaccines by inhalation. It also aims to highlight that PNPs have adjuvant properties by themselves that induce cellular and humeral immunogenicity. Further, different factors influence the behavior of PNP in vivo such as size, morphology, and charge are discussed. Finally, some of the primary challenges facing PNPs are reviewed including formulation instability, reproducibility, device-related factors, patient-related factors, and industrial-level scale-up. Herein, the most important variables of PNPs that shall be defined in any PNPs to be used for pulmonary delivery are defined. Further, this study focuses on the most popular polymers used for this purpose.

Time-course transcriptome analyses of spleen in rainbow trout (Oncorhynchus mykiss) post-Flavobacterium psychrophilum infection

Flavobacterium psychrophilum, the etiological agent of bacterial coldwater disease and rainbow trout fry syndrome, causes considerable losses in salmonid aquaculture globally. Systemic F. psychrophilum infections in rainbow trout (Oncorhynchus mykiss) lead to a range of clinical signs, including ulcerative lesions in the skin and muscle and splenitis. Previous studies offered an integrative analysis of the skeletal muscle response to F. psychrophilum infection in rainbow trout. However, little is known about the molecular mechanism of immune response in the spleen, which is an important immune organ of rainbow trout. Here, we investigated the time-course splenic transcriptome profiles in uninfected rainbow trout (CK) and F. psychrophilum–infected rainbow trout at day 3 and day 7 (D3, D7) by RNA-seq analyses. Among the 7,170 differentially expressed genes (DEGs) in the three comparisons (D3 vs. CK, D7 vs. CK, D3 vs. D7), 1,286 DEGs showed consistent upregulation or downregulation at D3 and D7 and were associated with pattern recognition, acute-phase response, complement cascade, chemokine and cytokine signaling, and apoptosis. The Real time quantitative PCR (RT-qPCR) analysis of eight DEGs confirmed the accuracy of the RNA-Sequencing (RNA-seq) data. Our results reflected a general process from pathogen recognition to inflammatory cytokine generation and delineated a putative Toll-like receptor signaling pathway in rainbow trout spleen, following F. psychrophilum infection. Taken together, these results provide new insights into the molecular mechanism of the immune response to F. psychrophilum infection and are a valuable resource for future research on the prevention and control of bacterial coldwater disease during salmon culture.

Differential Transcriptomic Response of Rainbow Trout to Infection with Two Strains of IPNV

The IPN virus (IPNV) causes a highly contagious disease that affects farmed salmonids. IPNV isolates have been phylogenetically classified into seven genogroups, of which two are present in Chile, genogroups 1 and 5. This study aimed to compare the transcriptomic response of rainbow trout fry challenged with two Chilean isolates of IPNV, RTTX (genogroup 1), and ALKA (genogroup 5). Tissue samples from challenged individuals and controls were taken at 1, 7, and 20 days post-challenge and analyzed by RNA-Seq. The results revealed that infection with RTTX elicited a greater modulation of the trout transcriptome compared to ALKA infection, generating a greater number of highly differentially expressed genes in relation to the control fish. Gene Ontology enrichment indicated that functions related to the inflammatory and immune responses were modulated in fish challenged with both isolates throughout the trial, but with different regulation patterns. On day 1 post challenge, these functions were activated in those challenged with ALKA, but suppressed in RTTX-challenged fish. These results suggest that rainbow trout exhibit a differential transcriptomic response to infection with the two genetically distinct IPNV isolates, especially at early times post-infection.

A C-type lectin with a single CRD from Onychostoma macrolepis mediates immune recognition against bacterial challenge

C-type lectins (CTL) are a large group of pattern-recognition proteins and to play important roles in glycoprotein metabolism, multicellular integration, and immunity. Based on their overall domain structure, they can be classified as different groups that possess different physiological functions. A typical C-type lectin (named as OmLec1) was identified from the fish, Onychostoma macrolepis, an important cultured fish in China. Open reading frame of OmLec1 contains a 570 bp, encoding a protein of 189 amino acids that includes a signal peptide and a single carbohydrate-recognition domain. The phylogenetic analysis showed that OmLec1 could be grouped with C-type lectin from other fish. OmLec1 was expressed in all the tissues in our study, and the expression level was highest in liver. And its relative expression levels were significantly upregulated following infection with Aeromonas hydrophila. The recombinant OmLec1 protein (rOmLec1) could agglutinate some Gram-negative bacteria and Gram-positive bacteria in vitro in the presence of Ca²⁺, showing a typical Ca²⁺-dependent carbohydrate-binding protein. Furthermore, rOmLec1 purified from E. coli BL21 (DE3), strongly bound to LPS and PGN, as well as all tested bacteria in a Ca²⁺-dependent manner. These results indicate that OmLec1 plays a central role in the innate immune response and as a pattern recognition receptor that recognizes diverse pathogens among O. macrolepis.

Identification and characterization of a C-type lectin in turbot (Scophthalmus maximus) which functioning as a pattern recognition receptor that binds and agglutinates various bacteria

C-type lectins (CTLs) are important pathogen pattern recognition receptors that recognize carbohydrate structures. In present study, a C-type lectin domain family 4 member E-like gene from turbot, which tentatively named SmCLEC4E-like (SmCLEC4EL), was identified, and the expressional and functional analyses were performed. In our results, SmCLEC4EL showed conserved synteny with CLEC4E-like genes from several fish species in genome, and possessed a typical type II transmembrane CTL architecture: an N-terminal intracellular region, a transmembrane domain and a C-terminal extracellular region which contained a predicted carbohydrate recognition domain (CRD). In addition, SmCLEC4EL exhibited the highest expression level in spleen in healthy fish, and showed significantly induced expression in mucosal tissues, intestine and skin, under bacteria challenge. Finally, the recombinant SmCLEC4EL protein combined with LPS, PGN, LTA and five different kinds of bacteria in a dose-dependent manner, and agglutinated these bacteria strains in the presence of calcium. These findings collectively demonstrated that SmCLEC4EL, a calcium-dependent CTL, could function as a pattern recognition receptor in pathogen recognition and participate in host anti-bacteria immunity.

Molecular characterization and biological effect of a C-type lectin receptor in Qihe crucian carp, Carassius auratus

C-type lectin receptors, as the important members of pattern-recognition receptors, play the crucial roles in the innate immune system, which discriminate self and non-self by recognizing and binding the carbohydrates on the surface of microorganism. In this study, we identified a C-type lectin receptor gene in Qihe crucian carp Carassius auratus (named as CaCLR). The full-length cDNA of CaCLR was composed of 1130 bp, with a 226 bp 5'-untranslated region (UTR), a 792 bp ORF encoding a 263aa protein, and a 112 bp 3'-UTR with a polyadenylation signal sequence AATAAA and a poly (A) tail. The predicted amino acid sequence of CaCLR is a single transmembrane receptor with a typical carbohydrate recognition domain (CRD) at its C-terminus. With regard to the mRNA transcript of CaCLR, it was ubiquitously detected in the tested tissues, among which it was the most abundant in head kidney. The temporal expressions of CaCLR were obviously up-regulated in liver, spleen, kidney, and head kidney after Aeromonas hydrophila and poly I: C challenge, respectively, and the patterns of expression changes were in a time-depended manner. The recombinant CaCLR (rCaCLR) purified from Escherichia coli BL21 (DE3), exhibited strong binding ability with lipopolysaccharide (LPS), peptidoglycan (PGN), β-Glucan, and Mannan, as well as five microorganisms including fungus (Saccharomyces cerevisiae), Gram-negative bacteria (A. hydrophila, E. coli and Vibrio anguillarum), and Gram-positive bacteria (Micrococcus lysodeikticus). In the presence of rCaCLR, the eliminating capacity against A. hydrophila could be enhanced in C. auratus. Taken together, CaCLR is involved in the antibacterial defense in C. auratus.

Identification and expression analysis of group II C-type lectin domain containing receptors in grass carp Ctenopharyngodon idella

Group II C-type lectin domain (CTLD) containing receptors belong to a large family of pattern recognition receptors which mainly act on the innate immunity. They are structurally related and consist of a cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM) and a single extracellular CTLD. Although they have been described in teleost fish, their involvement in immune responses is not well understood. In this study, four immune-related lectin-like receptors (termed CiILLR1 and CiILLR5-7), belonging to the group II CTLD receptors, were identified in grass carp (Ctenopharyngodon idella). They contain a short cytoplasmic tail and a single CTLD in the extracellular region. The CiILLR1 has a WxHxxxxxY motif similar to the WxHxxxxY motif which is required for the recognition of β-glucans by some of the group II CTLD containing lectins in mammals. Further, a modified QPD motif (EPD) known to be involved in binding to carbohydrate ligands is present in the CiILLR1, 5 and 6. However, CiILLR7 lacks these motifs. Expression analysis revealed that they were constitutively expressed in the head kidney and spleen. Moreover, CiILLR1, 5 and 6 could be up-regulated in the head kidney and spleen of fish after infection with Flavobacterium columnare and in the primary head kidney leukocytes by LPS and PHA. Expression of CiILLR1, CiILLR5 and CiILLR6 were mainly detected in the enriched lymphocytes whilst CiILLR7 was expressed in the enriched monocytes/macrophages. The results expand existing knowledge on the immune responses of the C-type lectin receptors in teleost fish.

Cellular Immune Responses in Rainbow Trout (Onchorhynchus mykiss) Following Vaccination and Challenge Against Salmonid Alphavirus (SAV)

Viral disease outbreaks remain a significant limiting factor for aquaculture. The majority of licensed vaccines used in the industry are administered as oil-adjuvanted formulations carrying inactivated whole pathogens. Cell-mediated immune responses, in particular those based on virus-specific cytotoxic T-cells (CTLs) to conventional inactivated oil-based vaccines, are largely unexplored. As vaccines cannot be optimized against viral pathogens if knowledge of host cellular immune mechanisms remains unknown, in this study we examined fundamental cell-mediated immune responses after vaccination of rainbow trout with an oil-adjuvanted inactivated vaccine against salmonid alphavirus (SAV) and after infection with SAV. A unique in vitro model system was developed to examine MHC class I restricted CTL responses in a clonal line of rainbow trout. The levels of cell-mediated cytotoxicity were compared to pathology, virus load, specific antibody response, changes in immune cell populations, and mRNA expression. Our results hint that different protective mechanisms are being triggered by infection compared to vaccination. While vaccination itself did not cause a strong cytotoxic or humoral response, subsequent challenge of vaccinated fish resulted in significantly stronger and faster specific cytotoxicity, alongside reduced viral titers and pathology. Hence, testing a vaccine on the capacity to induce cell-mediated cytotoxicity will still require a challenge test. Examination of cellular markers additionally indicates that the initial innate response induced by the vaccine could play an important role in steering adaptive mechanisms.

C-Type Lectins in Veterinary Species: Recent Advancements and Applications

C-type lectins (CTLs), a superfamily of glycan-binding receptors, play a pivotal role in the host defense against pathogens and the maintenance of immune homeostasis of higher animals and humans. CTLs in innate immunity serve as pattern recognition receptors and often bind to glycan structures in damage- and pathogen-associated molecular patterns. While CTLs are found throughout the whole animal kingdom, their ligand specificities and downstream signaling have mainly been studied in humans and in model organisms such as mice. In this review, recent advancements in CTL research in veterinary species as well as potential applications of CTL targeting in veterinary medicine are outlined.

Back From the Brink: Alterations in B and T Cell Responses Modulate Recovery of Rainbow Trout From Chronic Immunopathological Tetracapsuloides bryosalmonae Infection

Proliferative kidney disease (PKD) caused by the myxozoan parasite Tetracapsuloides bryosalmonae is one of the most serious infectious diseases negatively impacting farmed and wild salmonids throughout Europe and North America. PKD pathogenesis results in a massive B cell proliferation and dysregulation with aberrant immunoglobulin production and plasma cell differentiation along with a decrease in myeloid cells and inhibition of innate pathways. Despite the huge immunopathological reaction in the kidney during infection, under specific conditions, fish can survive and return to full fitness. Fish are unique in this ability to recover renal structure and functionality from extensive tissue damage in contrast to mammals. However, only limited knowledge exists regarding the host immune response coinciding with PKD recovery. Moreover, almost no studies of the immune response during disease recovery exist in fish. We utilized the rainbow trout-T. bryosalmonae system as an immunological model of disease recovery. Our results demonstrated that recovery is preceded by an intense immune response at the transcript level, decreasing parasite burden, and an increased degree of kidney inflammation. Later in the recovery phase, the immune response transpired with a significant decrease in lymphocytes and an increase in myeloid cells. These lymphocytes populations contained lower levels of B cells comparative to the control in the anterior and posterior kidney. Additionally, there was downregulation of several transcripts used as markers for plasma cells (blimp1, igt sec, igm sec, igd sec, and cd38) and T cell subsets (cd4, cd8α, cd8β, and tcrβ). The decrease in these T cell transcripts significantly correlated with decreasing parasite intensity. Alternatively, there was strong upregulation of pax-5 and igt mem. This suggests a change in B cell processes during the recovery phase relative to clinical PKD may be necessary for the host to re-establish homeostasis in terms of an arrest in the dominant antibody like response transitioning to a transcriptional profile associated with resting B cells. The knowledge generated here in combination with earlier studies illuminates the full power of analyzing the entire trajectory of disease from the normal healthy state to recovery enabling the measurement of an immune response to pinpoint a specific disease stage.

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.

Immunohistochemical examination of immune cells in adipose tissue of rainbow trout (Oncorhynchus mykiss) following intraperitoneal vaccination

Mammalian perivisceral adipose has been shown to play an important role in the regulation of the peritoneal immune responses. Recently it has been demonstrated that peritoneal antigens are collected by leukocytes within the visceral adipose mass, and a broad range of immunomodulatory genes are differentially expressed in adipose tissue after intraperitoneal vaccination in rainbow trout. To assess the immune cell component in adipose, immunohistochemical analysis was used to examine B-cell, T-cell and antigen presenting cell (APC) numbers and distribution in rainbow trout adipose tissue 24 and 72 h post vaccination in comparison to control fish. The results of this study support previous work on mammals with omental milky spots in naïve fish found to contain APCs and T-cells which then increased in size, number and complexity following vaccination. It suggests that following peritoneal stimulation the visceral adipose mass in fish likely plays an important role in vaccine antigen uptake and presentation by APCs, as well as subsequent T-cell activation and differentiation.

Identification and expression analysis of IL-4/13 receptors in grass carp Ctenopharyngodon idella

Interleukin (IL)-4 and IL-13 are T helper 2 (Th2) cytokines with pleiotropic functions. IL-4 interacts with two receptors consisting of IL-4Rα/γ chain receptor (γC) and IL-4Rα/IL-13Rα1. In contrast, IL-13 binds to IL-13Rα2 but also shares the receptor complex containing IL-4Rα/IL-13Rα1. In fish, two IL-4/13 homologs have been identified but their phylogenetic relationships with IL-4 and IL-13 are ambiguous. In this study, we identified six putative IL-4/13 receptor homologs in grass carp, including γC1, γC2, IL-4Rα1, IL-13Rα1, IL-13Rα2 and a soluble form of IL-4Rα2. Comparative sequence analyses revealed that these receptors possess conserved characteristic domains and the genes encoding them share conserved gene synteny with their human counterparts. All six receptors contain a cytokine binding homology domain (CHD) and two fibronectin type Ⅲ (FNⅢ) like domains, with IL-13Rα1 and IL-13Rα2 harbouring an extra Ig-like domain preceding the CHD domain. Interestingly, grass carp IL-13Rα1 and IL-13Rα2 lack the characteristic WSXWS motif, a typical feature of mammalian type I cytokine receptors. The IL-4/13 receptor genes are differentially expressed in tissues and primary leukocytes of head kidney and can be modulated by Flavobacterium cloumnare (F. cloumnare), suggesting they are involved in immune response against F. cloumnare infection.

A Novel Lipopolysaccharide Recognition Mechanism Mediated by Internalization in Teleost Macrophages

Macrophages in teleosts are less sensitive to lipopolysaccharide (LPS) compared to mammals. The functional equivalent of the mammalian LPS surface receptor in teleost macrophages for the pro-inflammatory response is either non-existent or replaced by negative regulation. LPS signaling in teleost macrophages remains unclear. Here, we found a scavenger receptor class B 2a (PaSRB2a) that played a crucial role in LPS signaling in teleost macrophages. The internalization of LPS and subsequent pro-inflammatory responses in macrophages were mediated by PaSRB2a, which is a novel isoform of the mammalian SRB2 gene. LPS internalization by PaSRB2a is dependent on its C-terminal intracellular domain. Following LPS internalization, it interacts with the ayu intracellular receptors nucleotide-binding oligomerization domain protein 1 (PaNOD1) and PaNOD2. Moreover, LPS pre-stimulation with sub-threshold concentrations reduced the effect of secondary LPS treatment on pro-inflammatory responses that were mediated by PaSRB2a. The pro-inflammatory responses in LPS-treated ayu were down-regulated upon PaSRB2a knockdown by lentivirus siRNA delivery. In grass carp and spotted green pufferfish, SRB2a also mediated LPS internalization and pro-inflammatory responses. Our work identifies a novel LPS signaling pathway in teleosts that differs from those in mammals, and contributes to our understanding of the evolution of pathogen recognition in vertebrates.

Full-length and a smaller globular fragment of adiponectin have opposite roles in regulating monocyte/macrophage functions in ayu, Plecoglossus altivelis

Adiponectin (ADP), a regulator of the innate immune system, plays a role in the progression of inflammation and metabolic disorders in mammals. However, the role of ADP in fish is poorly understood. Here, we cloned the cDNA sequence of a ADP homolog (PaADP) gene from ayu. Multiple sequence alignment revealed that PaADP presented typical characteristics of ADPs. Phylogenetic tree analysis showed that PaADP was most closely related to that of rainbow trout. In healthy ayu, the transcripts of PaADP were detected in most of the tested tissues and cells, with the highest level in the adipose tissue. Upon V. anguillarum infection, the mRNA expression of PaADP was significantly up-regulated in the tissues and cells except adipose tissue. Subsequently, the full-length mature PaADP (fPaADP) and the globular domain fragment (gPaADP) were prokaryotically expressed in bacteria and purified, and anti-PaADP antibodies were produced. Western blot analysis revealed that three fragments including fPaADP and gPaADP were existed in ayu serum. The recombinant fPaADP (rfPaADP) had an anti-inflammatory effect on ayu MO/MФ by upregulating anti-inflammatory cytokine expressions, downregulating pro-inflammatory cytokine expressions, inhibiting the phagocytosis and subsequent bacterial killing. In contrast, the recombinant gPaADP (rgPaADP) presented a pro-inflammatory effect on ayu MO/MФ by upregulating pro-inflammatory cytokine expression, downregulating anti-inflammatory cytokine expressions, enhancing the phagocytosis and subsequent bacterial killing. These results suggested that fPaADP and gPaADP have opposite roles in the regulation of MO/MФ functions in ayu.

Immune-modulation of two BATF3 paralogues in rainbow trout Oncorhynchus mykiss

Basic leucine zipper transcription factor ATF-like (BATF) -3 is a member of the activator protein 1 (AP‑1) family of transcription factors and is known to play a vital role in regulating differentiation of antigen-presenting cells in mammals. In this study, two BATF3 homologues (termed BATF3a and BATF3b) have been identified in rainbow trout (Oncorhynchus mykiss). Both genes were constitutively expressed in tissues, with particularly high levels of BATF3a in spleen, liver, pyloric caecae and head kidney. BATF3a was also more highly induced by PAMPs and cytokines in cultured cells, with type II IFN a particularly potent inducer. In rIL-4/13 pre-stimulated cells, the viral PAMPS polyI:C and R848 had the most pronounced effect on BATF3 expression. BATF3 expression could also be modulated in vivo, following infection with Yersinia ruckeri, a bacterial pathogen causing redmouth disease in salmonids, or with the rhabdovirus IHNV. The results suggest that BATF3 may be functionally conserved in regulating the differentiation and activation of immune cells in lower vertebrates and could be explored as a potential marker for comparative investigation of leucocyte lineage commitment across the vertebrate phyla.

Basal polarization of the immune responses to Streptococcus agalactiae susceptible and resistant tilapia (Oreochromis niloticus)

One of the highest priority areas for improvement is the development of effective strategies for decreasing disease mortality levels in aquaculture production, a better understanding of the components of the fish immune system and their functions in the context of pathogen invasion is needed. Tilapia is the most common fish in South China, and Streptococcus agalactiae has become the most serious disease problem for tilapia industry in China. Here, we profiled gene expression differences between tilapia differing in their susceptibility to S. agalactiae both basally (before infection) and at three early timepoints post-infection (5 h, 50 h, and 7 d). Between group comparisons revealed 5756 unique genes differentially expressed greater than 2-fold at one or more timepoints. And the resistant fish showed much more strong ability in pathogen recognition, antigen presentation, immune activation, while the susceptible fish showed fast activation of apoptosis. Taken together, the immune profiles expand our knowledge for molecular mechanisms for disease resistance, as well as provide solid molecular resources for further identification of the candidate markers for disease-resistant selection and evaluation of disease prevention and treatment options for tilapia industry.

Characterisation of ZBTB46 and DC-SCRIPT/ZNF366 in rainbow trout, transcription factors potentially involved in dendritic cell maturation and activation in fish

ZBTB46 and DC-SCRIPT/ZNF366 are two zinc finger transcription factors that play important roles in regulating differentiation of dendritic cells in mammals. In this study, the ZBTB46 and DC-SCRIPT/ZNF366 homologues were identified in rainbow trout Oncorhynchus mykiss and their expression analysed in vivo and in vitro. As transcription factors, they are well conserved in sequence, genomic organisation and gene synteny. Their expression was differentially modulated by bacterial and viral PAMPs in the monocyte/macrophage-like cell line RTS-11, in primary head kidney (HK) macrophages, and in HK macrophages cultured with IL-4/13A. In the RTS-11 cells and primary HK macrophages, all the ZBTB46 and DC-SCRIPT/ZNF366 homologues were down-regulated by interferon gamma (type II IFN) but unaffected by IFN2 (type I IFN), administered as recombinant proteins to cell cultures. In fish gills, infection with amoebae (Paramoebae perurans) resulted in reduction of ZBTB46 and DC-SCRIPT/ZNF366 expression in Atlantic salmon Salmo salar, whilst infection with Yersinia ruckeri induced gene expression in rainbow trout.

Identification and molecular profiling of DC-SIGN-like from big belly seahorse (Hippocampus abdominalis) inferring its potential relevancy in host immunity

Dendritic-cell-specific ICAM-3-grabbing non-integrin (DC-SIGN) is a C-type lectin that functions as a pattern recognition receptor by recognizing pathogen-associated molecular patterns (PAMPs). It is also involved in various events of the dendritic cell (DC) life cycle, such as DC migration, antigen capture and presentation, and T cell priming. In this study, a DC-SIGN-like gene from the big belly seahorse Hippocampus abdominalis (designated as ShDCS-like) was identified and molecularly characterized. The putative, complete ORF was found to be 1368 bp in length, encoding a protein of 462 amino acids with a molecular mass of 52.6 kDa and a theoretical isoelectric point of 8.26. The deduced amino acid sequence contains a single carbohydrate recognition domain (CRD), in which six conserved cysteine residues and two Ca²⁺-binding site motifs (QPN, WND) were identified. Based on pairwise sequence analysis, ShDCS-like exhibits the highest amino acid identity (94.6%) and similarity (97.4%) with DC-SIGN-like counterpart from tiger tail seahorse Hippocampus comes. Quantitative real-time PCR revealed that ShDCS-like mRNA is transcribed universally in all tissues examined, but with abundance in kidney and gill tissues. The basal mRNA expression of ShDCS-like was modulated in blood cell, kidney, gill and liver tissues in response to the stimulation of healthy fish with lipopolysaccharides (LPS), Edwardsiella tarda, or Streptococcus iniae. Moreover, recombinant ShDCS-like-CRD domain exhibited detectable agglutination activity against different bacteria. Collectively, these results suggest that ShDCS-like may potentially involve in immune function in big belly seahorses.

First in-depth analysis of the novel Th2-type cytokines in salmonid fish reveals distinct patterns of expression and modulation but overlapping bioactivities

IL-4 and IL-13 are closely related canonical type-2 cytokines in mammals and have overlapping bioactivities via shared receptors. They are frequently activated together as part of the same immune response and are the signature cytokines produced by T-helper (Th)2 cells and type-2 innate lymphoid cells (ILC2), mediating immunity against extracellular pathogens. Little is known about the origin of type-2 responses, and whether they were an essential component of the early adaptive immune system that gave a fitness advantage by limiting collateral damage caused by metazoan parasites. Two evolutionary related type-2 cytokines, IL-4/13A and IL-4/13B, have been identified recently in several teleost fish that likely arose by duplication of an ancestral IL-4/13 gene as a consequence of a whole genome duplication event that occurred at the base of this lineage. However, studies of their comparative expression levels are largely missing and bioactivity analysis has been limited to IL-4/13A in zebrafish. Through interrogation of the recently released salmonid genomes, species in which an additional whole genome duplication event has occurred, four genomic IL-4/13 loci have been identified leading to the cloning of three active genes, IL-4/13A, IL-4/13B1 and IL-4/13B2, in both rainbow trout and Atlantic salmon. Comparative expression analysis by real-time PCR in rainbow trout revealed that the IL-4/13A expression is broad and high constitutively but less responsive to pathogen-associated molecular patterns (PAMPs) and pathogen challenge. In contrast, the expression of IL-4/13B1 and IL-4/13B2 is low constitutively but is highly induced by viral haemorrhagic septicaemia virus (VHSH) infection and during proliferative kidney disease (PKD) in vivo, and by formalin-killed bacteria, PAMPs, the T cell mitogen PHA, and the T-cell cytokines IL-2 and IL-21 in vitro. Moreover, bioactive recombinant cytokines of both IL-4/13A and B were produced and found to have shared but also distinct bioactivities. Both cytokines rapidly induce the gene expression of antimicrobial peptides and acute phase proteins, providing an effector mechanism of fish type-2 cytokines in immunity. They are anti-inflammatory via up-regulation of IL-10 and down-regulation of IL-1β and IFN-γ. They modulate the expression of cellular markers of T cells, macrophages and B cells, the receptors of IFN-γ, the IL-6 cytokine family and their own potential receptors, suggesting multiple target cells and important roles of fish type-2 cytokines in the piscine cytokine network. Furthermore both cytokines increased the number of IgM secreting B cells but had no effects on the proliferation of IgM⁺ B cells in vitro. Taken as a whole, fish IL-4/13A may provide a basal level of type-2 immunity whilst IL-4/13B, when activated, provides an enhanced type-2 immunity, which may have an important role in specific cell-mediated immunity. To our knowledge this is the first in-depth analysis of the expression, modulation and bioactivities of type-2 cytokines in the same fish species, and in any early vertebrate. It contributes to a broader understanding of the evolution of type-2 immunity in vertebrates, and establishes a framework for further studies and manipulation of type-2 cytokines in fish.

Molecular identification and functional analysis of KLF2 in Plecoglossus altivelis (ayu): It's regulatory role in monocyte/macrophage activation

Monocytes/macrophages (MO/MФ) play an important role in the response to infection in Plecoglossus altivelis (ayu). However, the role of transcription factors in the function of ayu MO/MФ is poorly understood. Here, we cloned the cDNA sequence of the Kruppel-like factor 2 (PaKLF2) gene from ayu. Phylogenetic analysis indicated that PaKLF2 was closest to that of Atlantic salmon (Salmo salar). Real time quantitative PCR (RT-qPCR) revealed that the PaKLF2 mRNA level was highest in the peripheral blood mononuclear cells among all tested tissues. The mRNA expression of PaKLF2 was upregulated in the head kidney, liver, spleen, and brain after Listonella anguillarum infection. Subsequently, PaKLF2 was expressed and purified to prepare anti-PaKLF2 antibodies. After L. anguillarum challenge, the PaKLF2 mRNA and protein levels were significantly upregulated in ayu MO/MФ. Moreover, PaKLF2 knockdown in MO/MФ resulted in the enhancement of cytokine production as well as phagocytotic and bactericidal capability. Therefore, PaKLF2 may modulate the immune response in ayu by suppressing the function of MO/MФ.