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

Molecular cloning, expression analysis, and functional characterization of an interleukin-15 like gene in common carp (Cyprinus carpio L.)

Interleukin-15 (IL-15) is a crucial cytokine involved in immune system regulation, which is produced by various cell types, including dendritic cells, monocytes, and macrophages. IL-15 plays a key role in the proliferation and activation of natural killer (NK) cells, CD8+ T cells, and memory CD8+ T cells, supporting their survival and enhancing their effector functions. Although IL-15 homologues in fish have been identified, their functions remain poorly understood. In this study, we cloned and investigated the bioactivities of an IL-15 homologue, referred to as IL-15 like (CcIL-15L), in common carp (Cyprinus carpio L.). An expression pattern analysis revealed that CcIL-15L was constitutively expressed in all examined tissues of healthy common carp, with the highest expression level observed in the intestine. Additionally, CcIL-15L expression was significantly up-regulated in the head kidney, spleen, gills, and intestine following Aeromonas hydrophila infection. In vitro, the recombinant protein CcIL-15L can significantly up-regulated the gene expression levels of pro-inflammatory cytokines (IL-1β, IL-6, IFN-γ, and TNF-α) and NK cell activation (perforin and Eomesa). We constructed a 3×FLAG eukaryotic expression vector and successfully expressed it in common carp by intramuscular injection. Additionally, the heterologous CcIL-15L protein was successfully overexpressed in vivo, and immune-related genes including CD4-1, CD8β2, TNF-α, and IgM showed significant induction in the head kidney and spleen. Furthermore, CcIL-15L overexpression reduced the bacterial loads after 24 h post-A. hydrophila infection in the liver, spleen, and kidney. Phagocytic and chemotaxis assays showed that rCcIL-15L could promoted the phagocytosis and chemotactic abilities of common carp HKLs. Our study provides a new perspective on the role for CcIL-15L in immunological functions in common carp.

Distinct antiviral activities of IFNφ1 and IFNφ4 in zebrafish

Interferons (IFNs) are a group of secreted cytokines that play a crucial role in antiviral immunity. Type I IFNs display functional disparities. In teleosts, type I IFNs are categorized into two subgroups containing one or two pairs of disulfide bonds. However, their functional differences have not been fully elucidated. In this study, we comparatively characterized the antiviral activities of zebrafish IFNφ1 and IFNφ4 belonging to the group I type I IFNs. It was found that ifnφ1 and ifnφ4 were differentially modulated during viral infection. Although both IFNφ1 and IFNφ4 activated JAK-STAT signaling pathway via CRFB1/CRFB5 receptor complex, IFNφ4 was less potent in inducing phosphorylation of STAT1a, STAT1b and STAT2 and the expression of antiviral genes than IFNφ1, thereby conferring weaker antiviral resistance of target cells. Taken together, our results provide insights into the functional divergence of type I IFNs in lower vertebrates.

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.

Activated P2X receptors can up-regulate the expressions of inflammation-related genes via NF-κB pathway in spotted sea bass (Lateolabrax maculatus)

P2X receptors, including seven subtypes, i.e., P2X1-7, are the ligand-gated ion channels activated by the extracellular ATP playing the critical roles in inflammation and immune response. Even though the immune functions of P2X receptors have been characterized extensively in mammals, their functions in fish remain largely unknown. In this study, four P2X receptor homologues were characterized in spotted sea bass (Lateolabrax maculatus), which were named LmP2X2, LmP2X4, LmP2X5, and LmP2X7. Their tissue distributions and expression patterns were then investigated by real-time quantitative PCR (qPCR). Furthermore, their functions in regulating the expressions of inflammation-associated genes and possible signaling pathway were examined by qPCR and luciferase assay. The results showed that they share similar topological structures, conserved genomic organization, and gene synteny with their counterparts in other species previously investigated. And the four P2X receptors were expressed constitutively in the tested tissues. In addition, the expression of each of the four receptor genes was significantly induced by stimulation of Edwardsiella tarda and/or pathogen-associated molecular patterns (PAMPs) in vivo. Also, in primary head kidney leukocytes of spotted sea bass, LmP2X2 and LmP2X5 were induced by using PAMPs and/or ATP. Notably, the expressions of CCL2, IL-8, and TNF-α recognized as the pro-inflammatory cytokines, and of the four apoptosis-related genes, i.e., caspase3, caspase6, caspase7, and P53, were differentially upregulated in the HEK 293T cells with over-expressed LmP2X2 and/or LmP2X7 following ATP stimulation. Also, the over-expression of LmP2X4 can upregulate the expressions of IL-8, caspase6, caspase7, and P53, and LmP2X5 upregulates of IL-8, TNF-α, caspase7, and P53. Then in the present study it was demonstrated that the activation of any one of the four receptors significantly upregulated the activity of NF-κB promoter, suggesting that the activated LmP2Xs may regulate the expressions of pro-inflammatory cytokines via the NF-κB pathway. Taken together, the four P2X receptors were identified firstly from fish species in Perciformes, and they participate in innate immune response of spotted sea bass possibly by regulating the expressions of the inflammation-related genes. Our study provides the new evidences for the P2X receptors' involvement in fish immunity.

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.

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.

IL-27 suppresses spring viremia of carp virus replication in zebrafish

Interleukin (IL) 27 is a member of the IL-12 family and is a heterodimeric cytokine composed of IL-27A and Epstein-Barr virus-induced 3 (EBI3). It plays an important role in regulating inflammation and cancer progression. IL-27A not only functions by dimerizing with EBI3 but also acts alone. Here, we report that IL-27A and EBI3 suppress spring viremia of carp virus (SVCV) replication in zebrafish. Expression analysis reveals that il-27a and ebi3 were significantly upregulated in the ZF4 cells by SVCV and poly(I:C), and in the zebrafish caudal fin (ZFIN) cells overexpressed with SVCV genes. Interestingly, il-27a and ebi3 were not modulated by IFNφ1, indicating that they are not IFN stimulated genes (ISGs). Furthermore, overexpression of IL-27A and EBI3 alone inhibited SVCV replication in the EPC cells, but less potent than co-expression of IL-27A and EBI3. Intriguingly, IL-27A could not induce the expression of irf3, ifn, isg15 and mx1. Taken together, our results demonstrate that IL-27A and EBI3 activate innate antiviral response in an IFN independent manner in zebrafish.

Molecular characteristics of interleukin (IL)-17A/F3 and its immune response on the pathogen and functional regulation on cytokines in common carp Cyprinus carpio L

Fish interleukin (IL)-17A/F is homologous with mammalian IL-17A and IL-17F, which plays a key role in regulating inflammatory responses and autoimmune diseases. In fish, IL-17A/F1, 2, and 3 have been identified and described. However, IL-17A/F3 has received little attention in fish. In this study, a homolog of IL-17A/F3 was identified in common carp (Cyprinus carpio L.), which was termed as Cc_IL-17A/F3. The deduced amino acid sequence of Cc_IL-17A/F3 has four conserved cysteine residues, which could form two intrachain disulfide bonds. Homology comparison showed that the Cc_IL-17A/F3 was in the range of 31.7-71.9% of sequence similarity with these of other fishes. The Cc_IL-17A/F3 gene was constitutively expressed in various tissues, with higher expression levels in the skin and gills. After common carp were infected by Aeromonas. hydrophila, the mRNA expression levels of Cc_IL-17A/F3 were significantly up-regulated in the spleen, head kidney, gills, and intestine. Based on the indirect immunofluorescence assay, Cc_IL-17A/F3 proteins were found to be obviously increased in the intestine and spleen upon A. hydrophila infection at 24 h post-infection. The recombinant protein rCc_IL-17A/F3 could enhance the gene expression levels of pro-inflammatory cytokines (IL-1β, IL-6, IFN-γ, and TNF-α) as well as chemokines (CXCL8 and CXCL20) in primary head kidney leukocytes. In vivo and in vitro experiments have similar stimulatory effects. When Cc_IL-17A/F3 was overexpressed in common carp, the expressions of pro-inflammatory cytokines and chemokines were significantly up-regulated in head kidney and spleen. In summary, the results derived from the present study suggested that the Cc_IL-17A/F3 plays an important role in defending against bacterial infections, and probably participates in mucosal immunity of the host.

Interleukin (IL)-22 in common carp (Cyprinus carpio L.): Immune modulation, antibacterial defense, and activation of the JAK-STAT signaling pathway

Interleukin (IL)-22 is an IL-10 family cytokine secreted by CD4⁺ T cells and plays an important role in regulating inflammation and infection elimination. IL-22 homologues have been reported in the teleost, but the functions of IL-22 are still unclear. In this study, we identified two duplicated IL-22 genes in common carp (Cyprinus carpio L.), termed Cc_IL-22A and Cc_IL-22B. Sequence analysis showed that Cc_IL-22A and Cc_IL-22B had four conserved cysteine residues, which could form two intra-chain disulfide bridges. The Cc_IL-22A and Cc_IL-22B were constitutively expressed in various tissues, with the highest expression in the gill. The mRNA expression levels of Cc_IL-22A and Cc_IL-22B were significantly up-regulated in the gill, intestine, head kidney, and spleen of common carp challenged with Aeromonas. hydrophila. In vivo study showed that the expression levels of pro-inflammatory cytokines were significantly up-regulated in the head kidney and spleen when Cc_IL-22A or Cc_IL-22B were over-expressed. Furthermore, the over-expression of Cc_IL-22A and Cc_IL-22B indicated a protective effect on tissues, with only lymphocytic infiltration observed in comparison to the control and pcN3 groups, without obvious change in tissue morphology. Similar stimulatory effects of rIL-22A and rIL-22B were observed in vitro. When HKLs were stimulated with rIL-22A or rIL-22B, the expression levels of critical signaling molecules in the Janus kinase (JAK) and signal transducer and activator of transcription (STAT) pathway were significantly induced, including JAK1, JAK3, STAT1, and STAT3, as well as pro-inflammatory cytokines (IL-1β and TNF-α). Together, these results suggest that Cc_IL-22A and Cc_IL-22B may regulate inflammatory responses through the JAK-STAT signaling pathway and have a significant impact on the immune defense of common carp against bacterial infection. Therefore, our study provides a new perspective on the functions of Cc_IL-22A and Cc_IL-22B in the immune defense mechanism of fish.

Molecular characterizations, immune modulation, and antibacterial activity of interleukin-17A/F1a and interleukin-17A/F1b in common carp Cyprinus carpio

Interleukin-17 (IL-17), as a pro-inflammatory cytokine family, mediates different pro-inflammatory mediators in various cell types (e.g., epithelial cells, macrophages, endothelial cells, and fibroblasts), which play an important role in defending against pathogens. The IL-17A/F1 genes have recently been reported in fish. However, the functions of these genes are still unclear. In this study, we identified two duplicated IL-17A/F1 genes in common carp (Cyprinus carpio L.), namely, CcIL-17A/F1a and CcIL-17A/F1b. Sequence analysis showed that CcIL-17A/F1a and CcIL-17A/F1b proteins had four conserved cysteine residues, which could form two intra-chain disulfide bridges. Homology comparison displayed that the deduced amino acid sequences of CcIL-17A/F1a and CcIL-17A/F1b shared 31.1%-77.3% and 32.5%-75.7% of sequence similarity to IL-17A/F1 homologues from other fish species, respectively. The mRNA expression levels of CcIL-17A/F1a and CcIL-17A/F1b were obviously increased in gill and head-kidney of fish challenged with A. hydrophila. The recombinant protein rCcIL-17A/F1a and rCcIL-17A/F1b could enhance the expression levels of pro-inflammatory cytokines (IL-1β, IL-6, IFN-γ, and TNF-α) and chemokines (CXCL8 and CXCL20). The 3 × Flag eukaryotic expression vectors to express protein rCcIL-17A/F1a (or rCcIL-17A/F1b) were constructed and intramuscularly injected in common carp. The rCcIL-17A/F1a (or rCcIL-17A/F1b) could be successfully expressed in vivo. Four immune-related genes, namely, CD4, CD8, TNF-α, and IgM, were also significantly induced to be expressed at higher mRNA levels compared with the control. The pretreatment with CcIL-17A/F1a or CcIL-17A/F1b could markedly increase the survival rate of common carp challenged with A. hydrophila. Our results demonstrated that CcIL-17A/F1a or CcIL-17A/F1b plays an important role in immune responses and immune defense against bacteria. CcIL-17A/F1a or CcIL-17A/F1b could also be potentially used as an immunopotentiator to prevent diseases in common carp.

Characterization of Pannexin1, Connexin32, and Connexin43 in Spotted Sea Bass (Lateolabrax maculatus): They Are Important Neuro-Related Immune Response Genes Involved in Inflammation-Induced ATP Release

Many immunological diseases can be treated by regulating neurobehavior, in which extracellular ATP is a vital member of endogenous danger-associated molecular pattern signaling molecule that plays a crucial part in innate neuro-related immunity. It is actively released through pannexin (Panx) and connexin (Cx) hemichannels from activated or stressed cells during inflammation, injury, or apoptosis. In addition to participating in ATP release, Panxs and Cxs also have crucial immune functions. In this study, pannexin1, three connexin32 isoforms and connexin43 were identified and characterized in spotted sea bass (Lateolabrax maculatus), which were named LmPanx1, LmCx32.2, LmCx32.3, LmCx32.7, and LmCx43. Their similar topological structures were discovered by sequence analysis: a relatively unconserved C-terminal region and four highly conserved transmembrane (TM) domains, and so on. Each extracellular (ECL) region of Panx1 has two conserved cysteine residues. Unlike Panx1, each ECL region of Cx32 and Cx43 contains three conserved cysteine residues, forming two conserved motifs: CX6CX3C motif in ECL1 and CX4CX5C motif in ECL2. Furthermore, Panx1 and Cx43 share similar genomic organization and synteny with their counterparts in selected vertebrates. Cx32 and CX43 were located in the same locus in fish, but diverged into two loci from amphibian. Moreover, despite varying expression levels, the identified genes were constitutively expressed in all examined tissues. All genes were upregulated by PAMP [lipopolysaccharide and poly(I:C)] stimulation or bacterial infection in vivo and in vitro, but they were downregulated in the brain at 6 or 12 h after stimulation. Especially, the three LmCx32 isoforms and LmCx43 were upregulated by ATP stimulation in primary head kidney leukocytes; however, downregulation of LmCx32.3 and LmCx43 expression were noted at 12 h. Conversely, ATP treatment inhibited the expression of LmPanx1. Importantly, we showed that the spotted sea bass Panx1, Cx43, and Cx32 were localized on the cellular membrane and involved in inflammation-induced ATP release. Taken together, our results demonstrated that Panx1, Cx32, and Cx43 are important neuro-related immune response genes involved in inflammation-induced ATP release.

Molecular characterization, expression analysis and cellular location of IL-4/13 receptors in large yellow croaker (Larimichthys crocea)

Interleukin (IL)-4 and IL-13 are closely related class I cytokines that play key roles in the T helper (Th)-2 immune response via heterodimeric receptors. IL-4 signals via both the type I (IL-4Rα/γc) and type II (IL-4Rα/IL-13Rα1) receptor complexes, while IL-13 signals only via the type II receptor complex. IL-13Rα2 is traditionally considered a "decoy" receptor for IL-13. However, the IL-4/13 system and its response to pathogenic infection are still not fully understood in fish. In this study, we identified four IL-4/13 receptor subunit genes in the large yellow croaker (Larimichthys crocea): LcIL-4Rα1, LcIL-4Rα2, LcIL-13Rα1, and LcIL-13Rα2. Sequence analysis showed that these receptors possessed typical characteristic domains, including a signal peptide, two fibronectin type III (FN III)-like domains, and a transmembrane domain, but their cytoplasmic regions were not well conserved. The mRNA and protein of the four IL-4/13 receptors were constitutively expressed in all examined tissues of large yellow croaker. Their mRNAs were also detected in primary head kidney macrophages (PKMs), primary head kidney granulocytes (PKGs), and primary head kidney lymphocytes (PKLs). Immunofluorescence assay further showed that LcIL-4Rα and LcIL-13Rα1 were expressed on the membrane of IgM ⁺ B cells. After stimulation by Vibrio alginolyticus and poly (I:C) (a viral dsRNA mimic), the mRNA levels of LcIL-4/13 receptors were significantly upregulated in the head kidney and spleen. Their mRNA levels were also upregulated in head kidney leukocytes in response to poly (I:C) and lipopolysaccharide (LPS) treatment. Moreover, both recombinant LcIL-4/13A and LcIL-4/13B upregulated LcIL-4Rα1 and LcIL-4Rα2 in primary leukocytes, but only recombinant LcIL-4/13A upregulated LcIL-13Rα1 and LcIL-13Rα2. These results indicated that LcIL-4/13 receptors, containing conserved functional domains, may be involved in the IL-4/13-mediated immune response to pathogenic infections in the large yellow croaker.

Lipopolysaccharide-induced TNFα factor (LITAF) promotes inflammatory responses and activates apoptosis in zebrafish Danio rerio

Lipopolysaccharide-induced TNFα factor (LITAF) is an important transcription factor which activates the transcription of TNFα and regulates cell apoptosis and inflammatory response. In the present study, a LITAF gene homologue was identified in zebrafish (Danio rerio) and was shown to be well conserved in the protein sequence, genomic organization and synteny with human LITAF. DrLITAF was constitutively expressed in tissues, with the highest expression detected in the gills. Its expression could be modulated by LPS, poly(I:C), and infection with Edwardsiella tarda, Aeromonus hydrophila and septicemia viremia of carp virus (SVCV). DrLITAF, when overexpressed, was shown to be located on the cellular membrane and nuclear membrane of HEK293T and ZF4 cells and was associated with the endoplasmic reticulum. Stimulation with LPS resulted in rapid translocation of DrLITAF into the nucleus. In addition, DrLITAF was able to induce cell apoptosis and the expression of caspase 3. The results demonstrate that DrLITAF is involved in the immune defence against bacterial and viral infection and plays a role in regulating inflammation and apoptosis.

Dietary arginine and citrulline supplementation modulates the immune condition and inflammatory response of European seabass

The present study was designed to determine the modulatory effects of arginine and citrulline dietary supplementation on the immune condition and inflammatory response of European seabass, Dicentrarchus labrax. Four diets were manufactured: a control diet (CTRL) was formulated to meet the indispensable amino acids profile established for seabass. Based on this formulation, three other diets were supplemented with l-arginine at two different levels (0.5% and 1%, ARG1 and ARG2, respectively) and l-citrulline at 0.5% (CIT). Fish were fed these diets for 2 or 4 weeks under controlled conditions. At the end of 4 weeks, fish from all dietary treatments were intraperitoneally-injected with Photobacterium damselae piscicida and sampled after 4, 24 our 48 h. Immune status was characterized by a lymphocyte time-dependent decrease regardless of dietary treatment, whereas peroxidase values dropped in time in fish fed ARG1 and ARG2 and was lower at 4 weeks in fish fed ARG1 than in fish fed CTRL. Up-regulation of several genes was more evident in ARG1-and CIT-fed fish, though pro-inflammatory cytokines were down-regulated by CIT dietary treatment. Following immune stimulation, seabass fed ARG1 showed a decrease in neutrophils and monocytes circulating numbers. On the other hand, expression of 17 selected immune and inflammatory responses genes was barely affected by dietary treatments. Based on the analyzed parameters, results suggest an active role of dietary arginine/citrulline supplementation in modulating immune defences that seem to translate into a suppressed immune repertoire, mostly at the cell response level. The observed changes due to citrulline dietary supplementation were in part similar to those caused by arginine, suggesting that citrulline might have been used by macrophages as an arginine precursor and then engaged in similar immune-impairment leading mechanisms.

The Atlantic salmon interleukin 4/13 receptor family: Structure, tissue distribution and modulation of gene expression

Interleukin (IL)-4 and IL-13 play a central role in T helper 2 immune response in mammals. The cell signalling is mediated by the type I heterodimeric receptor containing the IL-4Rα and γC chains, and the type II receptors formed by IL-4Rα and IL-13Rα1. In salmonid species, three paralogues of the IL-4 and IL-13 cytokines have been reported, il-4/13a, il-4/13b1 and il-4/13b2. In regard to receptors, two paralogues of each IL-4/13 receptor chains have been identified in rainbow trout while five genes named γc1, il-4rα, il-13rα1a, il-13rα1b, and il-13rα2 have identified in Atlantic salmon. Since Atlantic salmon is an important farmed fish species, the aim of this work was to get new insights into distribution, structure and expression regulation of the IL-4/13 receptors in salmon. By using qRT-PCR, it was shown that all γc1, il-4rα, il-13rα1a, il-13rα1b, and il-13rα2 receptor chains were expressed in lymphoid and non-lymphoid tissues of healthy salmon, nonetheless γC expression was higher in lymphoid than non-lymphoid tissues. The in silico structural analysis and homology modelling of the predicted receptor proteins showed that domains and most motifs present in the superior vertebrate chains are conserved in salmon suggesting a conserved role for these receptor chains. Only IL-13Rα1B is a receptor chain with a unique structure that seem not to be present in higher vertebrates but in fish species. In order to determine the regulatory role of IL-4/13 on the expression of receptor chains, Atlantic salmon il-4/13A gene was synthetized and cloned in pET15b. The recombinant IL-4/13A was produced in E. coli and the activity of the purified cytokine was confirmed in vitro. The regulatory role of IL-4/13A on the expression of their potential receptors was tested in salmon receiving the recombinant cytokine and effects were compared with those of the control group. The results showed that IL-4/13A induced the expression of its own gene and GATA-3, in the head kidney of fish but not in the spleen, while IL-10 increased in both lymphoid organs indicating a regulatory role of this cytokine on the induction of Th2 responses in salmon. IFN-γ and MHC class II were also later induced in head kidney. In regard to the expression of the receptor chains, IL-4/13A upregulated the expression of γC, IL-13Rα1A and IL-13Rα2A in the spleen but not in the head kidney of salmon, indicating a role on the modulation of cell signalling for the Th2 response. Furthermore, Piscirickettsia salmonis infection of Atlantic salmon occurred with an increase of γC and IL-13Rα1A suggesting a potential role of the IL-4/13 system in bacterial immunity or pathogenesis. This study contributes to a better understanding of the IL-4/13A system in salmon, which as a key axis for Th2 response may be involved not only in pathogen elimination but also in adaptive immune repair that seems critical tolerance to infectious diseases.