Grouper (Epinephelus coioides) BCR signaling pathway was involved in response against Cryptocaryon irritans infection

The role of SYK phosphorylation in LPS-induced immunoglobulin responses of B cells in large yellow croaker (Larimichthys crocea)

Spleen tyrosine kinase (SYK), a non-receptor protein tyrosine kinase, is a key component of B cell receptor signaling and can regulate multiple physiological functions of B cells in mammals. In this study, a SYK gene was cloned and characterized from large yellow croaker (Larimichthys crocea) (LcSYK), whose open reading frame consists of 1851 base pairs and encodes 616 amino acid residues. The predicted LcSYK protein contains two N-terminal tandem Src homology 2 domains and a C-terminal tyrosine kinase catalytic domain, and shares a high amino acid sequence identity with SYK sequences in other vertebrate species. LcSYK was mainly expressed in immune tissues, such as head kidney, trunk kidney, spleen, and gill. The mRNA expression of LcSYK in primary head kidney leukocytes was not changed at 12, 24, and 48 h after lipopolysaccharide (LPS) stimulation. LPS stimulation upregulated the mRNA expression and protein production of IgM in IgM+ B cells, accompanied by an increase in the phosphorylation level, but not the total protein level, of LcSYK. Moreover, when we used PRT062607 HCl to inhibit the phosphorylation of LcSYK, both mRNA expression and protein production of IgM in IgM+ B cells were significantly suppressed. These results suggest that SYK phosphorylation may play a role in LPS-induced IgM production by IgM+ B cells, improving our understanding of the role of SYK in immunoglobulin responses of B cells in fish.

Seventy years of coexistence: Parasites and Mediterranean fish aquaculture

What can be regarded as a seedling of the contemporary aquaculture in the Mediterranean began back in the 1950s. The development of the industry did not always align with the development of ichthyopathology, a veterinary discipline aimed at identifying and combating fish diseases. Therefore, and due to the lack of published data, we are not always able to pinpoint the first outbreaks that accompanied the increase in aquaculture production. Nonetheless, fish pathogens, and parasites in particular, have shown diversity related to host species, their farming conditions and geography. Two parasite species currently regarded as dominant in Mediterranean aquaculture are the histozoic myxozoan Enteromyxum leei and the haematophagous polyopisthocotylean Sparicotyle chrysophrii, both of which infect gilthead seabream (Sparus aurata). The interactions between parasite and host with regard to the immune activity of both have been well studied using conventional immunology and omics approaches. For the remaining parasite-fish systems, our understanding of host responses and parasite mitigation mechanisms is still vague and mostly transposed from what we know of other systems. This review compiles the knowledge on fish response to the most frequent and economically important parasites in Mediterranean aquaculture, highlights the gaps and suggests further directions.

Immune response and protective efficacy of mannosylated polyethylenimine (PEI) as an antigen delivery vector, administered with a Streptococcus agalactiae DNA vaccine in Nile tilapia (Oreochromis niloticus)

This study examined the effectiveness of a DNA vaccine for S. agalactiae that was delivered by mannose-based polyethyleneimine (Man-PEI). The results showed that Man-PEI/pcDNA-Sip stimulated a higher serum antibody titer compared to control or other vaccine groups (p < 0.05). Additionally, it induced higher expression of immune-related genes, and increased activities of superoxide dismutase (SOD), acid phosphatase (ACP) and alkaline phosphatase (AKP). Furthermore, the Man-PEI/pcDNA-Sip group showed an improved relative percent survival (RPS) of 85.71%. These results demonstrate the potential value of Man-PEI as a vaccine delivery vehicle, and suggest that it can be effective in boosting the immune protective rate induced by pcDNA-Sip vaccines.

Host responses against the fish parasitizing ciliate Cryptocaryon irritans

The parasitic ciliate Cryptocaryon irritans, which infects almost all marine fish species occurring in both tropical and subtropical regions throughout the world. The disease, cryptocaryonosis, accounts for significant economic losses to the aquaculture industry. This review attempts to provide a comprehensive overview of the biology of the parasite, host-parasite interactions and both specific and non-specific host defense mechanisms are responsible for the protection of fish against challenge infections with this ciliate. Also, this article reflects the current interest in this subject area and the quest to develop an available vaccine against the disease. Due to the high frequency of clinical fish cryptocaryonosis, the study of fish immune responses to C. irritans provides an optimal experimental model for understanding immunity against extracellular protozoa.

Syk regulates the haemocyte autophagy through inducing the mRNA expressions of autophagy-related genes and the cleavage of CgLC3 in oyster antibacterial immunity

Spleen tyrosine kinase (Syk) is reported to be involved in activating the autophagy. Recently, a homologue of Syk was identified from Pacific oyster Crassostrea gigas (defined as CgSyk). In the present study, the molecular characteristics of CgSyk and its regulation mechanism in autophagy were investigated in oyster C. gigas. The full-length cDNA of CgSyk was of 4566 bp with an open reading frame (ORF) of 1989 bp. CgSyk encoded a polypeptide of 662 amino acids, containing two Src homology 2 (SH2) domains and one tyrosine kinase catalytic (TyrKc) domain. The deduced amino acid sequence of CgSyk shared low similarity with the previously identified Syks from other species. In the phylogenetic tree, CgSyk was first clustered with Crassostrea virginica CvSyk, and then classified into a branch of invertebrate Syks. In CgSyk-RNAi oysters, the mRNA expressions of CgLC3, CgP62, CgBeclin-1 and CgATG5 in haemocytes decreased significantly at 12 h after Vibrio splendidus stimulation. At the same time, the abundance of CgLC3Ⅱ in haemocytes, and the autophagy rate of haemocytes in CgSyk-RNAi oysters decreased significantly at 12 h after V. splendidus stimulation. All the results collectively suggested that CgSyk regulated the autophagy through inducing the mRNA expressions of autophagy-related genes and the cleavage of CgLC3 to defend against bacterial invasion in oysters.

Transcriptome analysis and protein-protein interaction in resistant and susceptible families of Japanese flounder (Paralichthys olivaceus) to understand the mechanism against Edwardsiella tarda

Edwardsiella tarda is one of the most harmful bacterial pathogens for aquaculture flatfish. After artificial infection of 47 Japanese flounder (Paralichthys olivaceus) families, resistant and susceptible families were identified in this study. High-throughput sequencing was performed on the liver transcriptome of uninfected groups (PoRU and PoSU) and infected groups (PoRC and PoSC). Through assembly and annotation, a total of 3012 and 1386 differentially expressed genes (DEGs) were identified in PoRU vs. PoSU and PoRC vs. PoSC. The significant enrichment pathways between PoRU and PoSU were mainly in metabolic and biosynthesis pathways. A total of thirty dominant enrichment pathways between PoRC and PoSC mainly focused on some immune-related pathways, including the hematopoietic cell lineage, cytokine-cytokine receptor interaction, complement and coagulation cascades, antigen processing and presentation, the intestinal immune network for immunoglobulin A (IgA) production and T/B cell receptor signaling pathway. Under the protein-protein interaction (PPI) analysis, hub genes, including CD molecules, complement component factors and chemokines, were identified in the network, and 16 core genes were differentially expressed in resistant and sustainable families in quantitative polymerase chain reaction (qPCR) validation. This study represents the first transcriptome analysis based on resistant and susceptible families and provides resistant genes to understand the potential molecular mechanisms of antibacterial function in marine fish. The results obtained in this study provide crucial information on gene markers for resistant breeding of Japanese flounder.

Severe Natural Outbreak of Cryptocaryon irritans in Gilthead Seabream Produces Leukocyte Mobilization and Innate Immunity at the Gill Tissue

The protozoan parasite Cryptocaryon irritans causes marine white spot disease in a wide range of fish hosts, including gilthead seabream, a very sensitive species with great economic importance in the Mediterranean area. Thus, we aimed to evaluate the immunity of gilthead seabream after a severe natural outbreak of C. irritans. Morphological alterations and immune cell appearance in the gills were studied by light microscopy and immunohistochemical staining. The expression of several immune-related genes in the gills and head kidney were studied by qPCR, including inflammatory and immune cell markers, antimicrobial peptides (AMP), and cell-mediated cytotoxicity (CMC) molecules. Serum humoral innate immune activities were also assayed. Fish mortality reached 100% 8 days after the appearance of the C. irritans episode. Gill filaments were engrossed and packed without any space between filaments and included parasites and large numbers of undifferentiated and immune cells, namely acidophilic granulocytes. Our data suggest leukocyte mobilization from the head kidney, while the gills show the up-regulated transcription of inflammatory, AMPs, and CMC-related molecules. Meanwhile, only serum bactericidal activity was increased upon infection. A potent local innate immune response in the gills, probably orchestrated by AMPs and CMC, is triggered by a severe natural outbreak of C. irritans.

Kinetics of Parasite-Specific Antibody and B-Cell-Associated Gene Expression in Brown Trout, Salmo trutta during Proliferative Kidney Disease

Tetracapsuloides bryosalmonae, a myxozoan endoparasite often causes chronic infection in brown trout. Antiparasite immunity mediated by antibodies and B cells is known as an important determinant of host survival and parasite proliferation during chronic infections. Accordingly, studying their time course during proliferative kidney disease (PKD) might be helpful in improving our understanding of its chronic nature. Therefore, we conducted this study to examine parasite specific serum antibody and B-cell-mediated response in laboratory-infected brown trout at different time points. Brown trout were exposed to the spores of T. bryosalmonae, derived from infected bryozoans. Samples were collected at different time points and processed for indirect ELISA, histopathology, and qRT-PCR. T. bryosalmonae specific antibody was detected at 4 weeks post exposure (wpe) and it persisted until 17 wpe. Additionally, the expressions of C4A, CD34, CD79A, BLNK, CD74, BCL7, and CD22 were differentially regulated in the important immune organs, kidney and spleen. To our knowledge, this is the first study addressing anti-T. bryosalmonae antibody response in brown trout at different time points. The results from this study provide valuable insights into the processes leading to changes in B cell development, inflammation and antibody production during the course of PKD in brown trout.

Molecular cloning and expression analysis of CD79a and CD79b in rainbow trout (Oncorhynchus mykiss) after bacterial, parasitic, and viral infection

CD79a and CD79b heterodimers are important components that consist of B cell receptor compound, which play a crucial role in transduction activation signal of the antigen binding BCR, and B cell development and antibody production. In order to investigate the characters and potential functions of CD79a and CD79b in rainbow trout (Oncorhynchus mykiss), we firstly cloned and analyzed the expression of CD79a and CD79b and found that the cDNA sequences of CD79a and CD79b both contained open reading frame of 711 and 645 bp in length for encoding the protein of 237 and 215 amino acid residues, respectively. The predicted amino acid sequences from trout were highly conserved with those of other teleost fishes in structure. Phylogenetic tree was constructed to analyze the evolutionary relationship between the trout and other known species, the result indicated that CD79a and CD79b of trout clustered at high bootstrap values with Salmo salar. Moreover, three trout infection models with F. columnare G₄, I. multifiliis and infectious hematopoietic necrosis virus (IHNV) were constructed, which resulted in morphological changes and serious lesions in skin and gills. Importantly, the high expression of CD79a and CD79b occurred in skin, gills, and followed by head kidney in response to bacterial, parasitic, and viral infection, as its expression was closely related to that of Igs. Our findings indicated that CD79a and CD79b play vital roles in both systemic and mucosal immune responses of rainbow trout during bacterial, parasitic, and viral infection, which will contribute to explore the roles of CD79 subunits in B cell signaling during ontogeny and disease.

The gill transcriptome reveals unique antimicrobial features that protect Nibea albiflora from Cryptocaryon irritans infection

Cryptocaryonosis is the greatest threat to most teleost species among all parasitic diseases, causing mass loss to the marine aquaculture industry. Epidemiological investigation of teleost susceptibility to Cryptocaryon irritans infection revealed that yellow drum (Nibea albiflora) is highly resistant. In order to further understand the activation of the immune system in the gill, which is one of the main mucosal-associated lymphoid tissues and a target of parasites, transcriptome analysis of the yellow drum gill was performed. Gill samples were collected from fish challenged after 24 hr and 72 hr with theronts at a median death rate (2050 theronts per gram fish). Gene expression profiles showed that TLR5 was the only receptor that activated the downstream immune response. The infection activated complement cascade through alternative pathway and increased the expression of C5a anaphylatoxin chemotactic receptor 1. In addition, possible antimicrobial molecules, including lipoprotein and haptoglobin, which are responsible for trypanolysis in humans, were among the top significantly upregulated genes at 24 hr. After 72 hr, the expression of secreted immunoglobulin T-related genes was induced. These results suggested a rapid innate and adaptive immune response at the mucosal level. In conclusion, the results provide new perspectives on mucosal immune resistance in yellow drum against cryptocaryonosis and provide the possibility of mining resistance genes for future therapy.

Development of Fish Parasite Vaccines in the OMICs Era: Progress and Opportunities

Globally, parasites are increasingly being recognized as catastrophic agents in both aquaculture sector and in the wild aquatic habitats leading to an estimated annual loss between 1.05 billion and 9.58 billion USD. The currently available therapeutic and control measures are accompanied by many limitations. Hence, vaccines are recommended as the "only green and effective solution" to address these concerns and protect fish from pathogens. However, vaccine development warrants a better understanding of host-parasite interaction and parasite biology. Currently, only one commercial parasite vaccine is available against the ectoparasite sea lice. Additionally, only a few trials have reported potential vaccine candidates against endoparasites. Transcriptome, genome, and proteomic data at present are available only for a limited number of aquatic parasites. Omics-based interventions can be significant in the identification of suitable vaccine candidates, finally leading to the development of multivalent vaccines for significant protection against parasitic infections in fish. The present review highlights the progress in the immunobiology of pathogenic parasites and the prospects of vaccine development. Finally, an approach for developing a multivalent vaccine for parasitic diseases is presented. Data sources to prepare this review included Pubmed, google scholar, official reports, and websites.

Skin transcriptome, tissue distribution of mucin genes and discovery of simple sequence repeats in crucian carp (Carassius auratus)

Crucian carp (Carassius auratus) is one of the major freshwater species and important food fish in China. Fish skin acts as the first line of defence against pathogens, yet its molecular and immune mechanism remains unclear. In this study, a de novo transcriptome assembly of C. auratus skin was performed with the Illumina Hiseq 2000 platform. A total of 49,154,776 clean reads were assembled, among which 60,824 (46.86%), 37,103 (28.59%), 43,269 (33.33%) unigenes were annotated against National Center for Biotechnology Information, Gene Onotology and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases, respectively. KEGG Orthology categories were significantly involved in immune system (20.50%), signal transduction (18.04%) and mucosal mucin genes (e.g., muc2, muc5AC, muc5B, muc17, muc18). The high expression of muc18 gene was observed in brain; that of muc2 in intestine; and that of muc5AC in skin, liver, spleen, intestine and muscle. Moreover, the potential 28,928 simple sequence repeats with the three most abundant dinucleotide repeat motifs (AC/GT, AG/CT, AT/AT) were detected in C. auratus. To authors' knowledge, this is the first report to describe the transcriptome analysis of C. auratus skin, and the outcome of this study contributed to the understanding of mucosal immune response of the skin and molecular markers in cyprinid species.

Identification and characterization of c-raf from orange-spotted grouper (Epinephelus coioides)

C-Raf proto-oncogene serine/threonine kinase is a mitogen-activated protein kinase (MAP) kinase kinase, which can initiate a mitogen-activated protein kinase (MAPK) cascade by phosphorylating the dual-specific MAP kinase kinases (MEK1/2), and in turn activate the extracellular signal-regulated kinases (ERK1/2). To study the function of c-Raf in teleost fish, a c-Raf cDNA sequence from orange-spotted grouper (Epinephelus coioides) was cloned. Ecc-Raf shared 81%-99% amino acid identity with other vertebrate c-Raf molecules, and shared the highest amino acid identity (99%) with Lates calcarifer c-Raf. Genomic structure analysis revealed that grouper c-Raf shared a conserved exon structure with other vertebrates. Tissue distribution showed that Ecc-Raf was mainly transcribed in systemic immune organs. Ecc-Raf was distributed throughout the cytoplasm of transfected GS cells and the overexpression of Ecc-Raf only slightly enhanced the activation of Activator protein 1. The phosphorylation levels of Ecc-Raf can be induced by PMA and H₂O₂ treatment, in contrast to DMSO or untreated HKLs. Moreover, the phosphorylation level of the Raf-MEK-ERK axis was downregulated after 24 h of SGIV infection. On the other hand, the total level and phosphorylation level of c-Raf significantly increased post C. irritans infection and showed an enhanced level post immunization. The results of this study suggested that the Raf-MEK-ERK cascade was involved in the response to viral or parasitic infections.

Lyn is involved in host defense against S. agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

Lyn, a member of Src protein kinase family, plays a crucial role in immune reactions against pathogenic infection. In this study, Lyn from Nile tilapia (Oreochromis niloticus) (OnLyn) was identified and characterized at expression pattern against bacterial infection, and regulation function in BCR signaling. The open reading frame of OnLyn contained 1536 bp of nucleotide sequence encoded a protein of 511 amino acids. The OnLyn protein was highly conversed to other species Lyn, including SH3, SH2 and a catalytic Tyr kinase (TyrKc) domain. Transcriptional expression analysis revealed that OnLyn was detected in all examined tissues and was highly expressed in the head kidney. The up-regulation OnLyn expression was observed in the head kidney and spleen following challenge with Streptococcus agalactiae (S. agalactiae) in vivo, and was also displayed in head kidney leukocytes challenge with S. agalactiae and LPS in vitro. In addition, after induction with mouse anti-OnIgM mAb in vitro, the OnLyn expression and phosphorylation of OnLyn (Y⁵⁰⁷) were significantly up-regulated in the head kidney leukocytes. Moreover, after treatment with AZD0530 and mouse anti-OnIgM monoclonal antibody, the down-regulation of cytoplasmic free-Ca²⁺ concentration was detected in the head kidney leukocytes in vitro. Taken together, the findings of this study revealed that OnLyn might play potential roles in BCR signaling and get involved in host defense against bacterial infection in Nile tilapia.

B cell receptor accessory molecule CD79 gets involved in response against Streptococcus agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

CD79, composed of two distinct chains called CD79a and CD79b, is a transmembrane protein that forms a B cell antigen receptor with membrane immunoglobulin, and generates a signal following antigen recognition by the B cell receptor. In this study, the CD79a (OnCD79a) and CD79b (OnCD79b) were cloned and identified from Nile tilapia (Oreochromis niloticus). The cDNA of ORF for OnCD79a and OnCD79b are 669 and 627 bp, coding 222 and 208 amino acids, respectively. The deduced protein analysis showed that both CD79a andCD79b contain an immunoreceptor tyrosine-based activation motif in their intracellular tails that used to propagate a signal in a B cell. Expression analysis revealed that both CD79a and CD79b expressed at high levels in immune tissues, such as anterior kidney and spleen, and in IgM⁺ B cells. Upon Streptococcus agalactiae (S. agalactiae) infection, the expressions of OnCD79a and OnCD79b were significantly up-regulated in anterior kidney and spleen. The significant up-regulations of OnCD79a and OnCD79b were also detected in leukocytes after in vitro challenge with S. agalactiae. Further, stimulations of LPS and anti-OnIgM monoclonal antibody induced significant up-regulations of OnCD79a and OnCD79b in leukocytes. Taken together, the results of this study indicated that CD79 molecule, playing roles in BCR signaling, was likely to get involved in host defense against bacterial infection in Nile tilapia.

Transcriptomic analysis of immunity in rainbow trout (Oncorhynchus mykiss) gills infected by Ichthyophthirius multifiliis

The parasite Ichthyophthirius multifiliis infecting skin, fins and gills of a wide range of freshwater fish species, including rainbow trout, is known to induce a protective immune response in the host. Although a number of studies have reported activation of several immune genes in infected fish host, the immune response picture is still considered incomplete. In order to address this issue, a comparative transcriptomic analysis was performed on infected versus uninfected rainbow trout gills and it showed that a total of 3352 (7.2%) out of 46,585 identified gene sequences were significantly regulated after parasite infection. Of differentially expressed gene sequences, 1796 genes were up-regulated and 1556 genes were down-regulated. These were classified into 61 Gene Ontology (GO) terms and mapped to 282 reference canonical pathways in the Kyoto Encyclopedia of Genes and Genomes (KEGG) databases. Infection of I. multifiliis induced a clear differential expression of immune genes, related to both innate and adaptive immunity. A total of 268 (6.86%) regulated gene sequences were known to take part in 16 immune-related pathways. These involved pathways related to the innate immunity such as the Chemokine signaling pathway, Platelet activation, Toll-like receptor signaling pathway, NOD-like receptor signaling pathway, and Leukocyte transendothelial migration. Elevated transcription of genes encoding the TLR 8 gene and chemokines (CCL4, CCL19, CCL28, CXCL8, CXCL11, CXCL13, CXCL14) was recorded indicating their roles in recognition of I. multifiliis and subsequent induction of the inflammatory response, respectively. A number of upregulated genes in infected gills were associated with antigen processing/presentation and T and B cell receptor signaling (including B cell marker CD22 involved in B cell development). Overall the analysis supports the notion that I. multifiliis induces a massive and varied innate response upon which a range of adaptive immune responses are established which may contribute to the long lasting protection of immunized rainbow trout.

Characterization and immune regulation role of an immobilization antigen from Cryptocaryon irritans on groupers

Immobilization antigens (i-antigens) are surface membrane proteins that are widely recognized to be the ideal candidates as vaccines antigens for immunization against Cryptocaryon irritans. In this study, we cloned a putative i-antigen gene from C. irritans, which was expressed in all three stages of the C. irritans life-cycle, and localized primarily to the cell surface. The recombinant GDCI3 i-antigen was expressed and purified using the free-living ciliate, Tetrahymena thermophila as an expression system. The purified recombinant protein was recognized by rabbit anti-C. irritans antiserum and was capable of eliciting immobilizing antibodies in rabbits and fish suggesting that the antigen itself was correctly folded. Following immunization and parasite challenge, groupers vaccinated with, recombinant GDCI3 i-antigen had a 25% cumulative percent survival rate compared to 8.3% for controls. Both non-specific and parasite-specific IgMs were generated in fish following immunization, with the levels of both increasing following challenge. Parasite-specific IgM in mucus could only be elicited after challenge of the GDCI3 i-antigen vaccinated groupers. To our knowledge, this is the first report using the Tetrahymena expression system to generate C. irritans i-antigens and investigate their use for fish vaccination.

Characterization and functional analysis of grouper (Epinephelus coioides) MEK1 and MEK2

MEK dual-specificity protein kinases are a group of mitogen-activated protein kinase kinases, which act as an integration point by transferring extracellular signals to the nucleus. To investigate the function of MEK in teleost fish, we cloned MEK1 and MEK2 cDNA sequences from the orange-spotted grouper (Epinephelus coioides). EcMEK1 and EcMEK2 shared 80% amino acid identity with each other. EcMEK1 had 89-99% amino acid identity with teleosts or mammals, whereas EcMEK2 shared 85-97% amino acid identity. The exon structures of the grouper MEK1/2 genes were conserved with zebrafish and human MEK1/2. Tissue distribution analysis showed that EcMEK1 and EcMEK2 had a similar expression pattern in grouper tissues and was mainly transcribe in systemic immune organs. Both EcMEK1 and EcMEK2 were distributed throughout the cytoplasm of transfected GS or HEK293T cells. Overexpression of EcMEK1 or EcMEK2 activated Activator protein 1 dependent luciferase. The phosphorylation levels of EcMEK1/2 and EcERK1/2 were significantly increased in head kidney leukocytes by stimulation with PMA treatment. The grouper MEK1/2-ERK1/2 axis was activated in Cryptocaryon irritans infection and showed an enhanced phosphorylation after immunization.

Spleen tyrosine kinase from Nile tilapia (Oreochromis niloticus): Molecular characterization, expression pattern upon bacterial infection and the potential role in BCR signaling and inflammatory response

Spleen tyrosine kinase (SYK), a member of non-receptor tyrosine kinase family, plays an important role in immune responses against pathogen infection, which is capable of activating B cells signaling pathway and regulating inflammatory response. In this study, Nile tilapia (Oreochromis niloticus) ortholog (OnSYK) was identified and characterized at expression pattern against bacterial infection, function in B cells activation pathway and inflammatory response. The cDNA of OnSYK ORF contained 1851 bp of nucleotide sequence encoding polypeptides of 616 amino acids. The deduced OnSYK protein was highly homologous to other species SYK, containing two SH2 domains and a TyrKc domain. Spatial mRNA expression analysis revealed that OnSYK had wide tissue distribution and was highly expressed in the liver. After challenge of Streptococcus agalactiae (S. agalactiae) in vivo, mRNA expression of OnSYK was significantly up-regulated in the head kidney, spleen and liver. The up-regulation of OnSYK transcript was also displayed in the head kidney and spleen leukocytes stimulation with S. agalactiae and LPS in vitro, which was confirmed at protein level in the head kidney leukocytes by FACS analysis. In addition, after induction with mouse anti-OnIgM monoclonal antibody in vitro, the expressions of OnSYK and its downstream molecules (OnLYN, OnBLNK and OnAP-1) were significantly up-regulated in the head kidney leukocytes, and pharmacological inhibition of SYK activity with inhibitor (P505-15) significantly attenuated the expressions of OnLYN, OnBLNK and OnAP-1. Moreover, upon LPS challenge, the expressions of OnSYK, OnTNF-α, OnIL-6 and OnAP-1 were also up-regulated in the head kidney monocytes/macrophages. After treatment with SYK inhibitor (BAY 61-3606), the expressions of OnTNF-α, OnIL-6 and OnAP-1 were inhibited in the LPS-challenged head kidney monocytes/macrophages. Taken together, the results of this study indicated that OnSYK, playing potential roles in BCR signaling and inflammatory response, was likely to get involved in host defense against bacterial infection in Nile tilapia.

Identification and functional analysis of grouper (Epinephelus coioides) B-cell linker protein BLNK

B-cell linker protein (BLNK) is an adaptor protein that plays a crucial role in the B cell antigen receptor (BCR) signal pathway. To investigate the function of BLNK in teleost fish, we cloned a BLNK ortholog gene from the orange-spotted grouper (Epinephelus coioides). Homology analysis showed that the grouper BLNK (EcBLNK) had a 34%-77% amino acid identity in comparison to other vertebrates and shared the highest amino acid identity with BLNK from the Asian seabass Lates calcarifer. EcBLNK comprises an N-terminal SAM domain and a C-terminal B-cell linker SH2 domain. Ten tyrosine residues were well conserved between teleost fish and mammals. Tissue distribution analysis showed that EcBLNK was expressed mainly in immune organs and expression was at the highest level in head kidney. Co-localization of EcBLNK and EcCD79a was observed in transfected HEK293T cells. Overexpression of EcBLNK did not activate nuclear factor kappa-light-chain-enhancer of activated B cells. The protein level of EcBLNK in grouper head kidney leukocytes was increased by stimulation with lipopolysaccharide. In groupers infected with Cryptocaryon irritans, EcBLNK was regulated in the infected sites and the systemic organ which suggests that EcBLNK was activated in the immune response to parasite infection.

Molecular characterization and function analysis of grouper (Epinephelus coioides) Bruton's tyrosine kinase BTK

Bruton's tyrosine kinase (BTK) is a Tec-family tyrosine kinase and plays a crucial role in B cell antigen receptor (BCR) signal pathway. Mutations in humans and mice BTK gene results in X-linked agammaglobulinemia (XLA) and X-linked immunodeficiency (XLD), respectively. To study the function of BTK in teleost, we cloned a BTK gene from orange-spotted grouper. Homology analysis showed that the grouper BTK (EcBTK) had a high amino acid identity with other vertebrates (63%-92%) and shared the highest amino acid identity with ballan wrasse Labrus bergylta BTK. EcBTK comprises a Bruton's tyrosine kinase pleckstrin homology (PH) domain, a Tec homology (TH) domain, a Src homology 3 (SH3) domain, a Src homology 2 (SH2) domain and a Protein Kinases, catalytic (PKc) domain. Tissue distribution analysis showed that EcBTK was mainly expressed in immune organs. EcBTK was uniform distributed throughout the cytoplasm of transfected HEK293T cells and overexpression of EcBTK slightly down-regulates NF-κB activity. Ibrutinib treatment can reduce the phosphorylation level of grouper's BTK. In groupers infected with Cryptocaryon irritans, up-regulation of EcBTK were not seen in the early stage of infected skin and gill until days 14-21. The phosphorylation level of grouper BTK was significantly increased in infected skin and gill.

Characterization and expression patterns of ERK1 and ERK2 from Epinephelus coioides against Cryptocaryon irritans infection

Mitogen-activated protein kinases (MAPKs), a group of serine-threonine protein kinases, play a crucial role in immunoreaction response to extra environmental stresses. In this study, two novel MAPKs, Ec-ERK1 and Ec-ERK2, were identified from Epinephelus coioides. Both Ec-ERK1 and Ec-ERK2 sequences contain a highly conserved Thr-Glu-Tyr (TEY) motif, an HRD domain, and an ATP binding loop containing GXGXXG. An analysis of phylogenetic relationships demonstrated that ERK amino acid sequences were conserved between different species indicating that the functions may be similar. Ec-ERK1 and Ec-ERK2 mRNA can be detected in all thirteen tissues examined, but the expression level is different in these tissues. The expression patterns of these two genes in E. coioides were also detected against Cryptocaryon irritans infection, which is capable of killing large numbers of fish in a short time and has a serious impact on aquaculture. The expression was up-regulated in most of the tissues examined, with the highest expressions of Ec-ERK1 (3.9 times) occurring in the head kidney and Ec-ERK2 (3.5 times) occurring in the spleen. There was no significant correlation between the expression of Ec-ERK1/Ec-ERK2 and the expression of nuclear factor kappaB (NF-kB). The results indicated the sequences and the characters of Ec-ERK1/ERK2 were conserved, Ec-ERK1/ERK2 showed tissue-specific expression patterns in healthy grouper, and their expressions were significantly varied post C. irritans infection, suggesting Ec-ERK1/ERK2 may play important roles in these tissues during pathogen-caused inflammation.

Characterization and expression analysis of grouper (Epinephelus coioides) co-stimulatory molecules CD83 and CD80/86 post Cryptocaryon irritans infection

Co-stimulatory molecules (CD83, CD80 and CD86), belong to immunoglobulin superfamily, are type I membrane glycoprotein, which express on antigen presenting cells and provide the second signal for the activation of T lymphocytes. In the present study, we cloned the grouper's CD83 (675 bp) and CD80/86 (876 bp). Homology analysis showed that both EcCD83 and EcCD80/86 shares the highest amino acid similarity (51% and 47%) for the overall sequence with puffer fish (Takifugu rubripes). Some conserved features and important functional residues in mammalian CD83, CD80 and CD86 were also identified from these molecules of teleosts including grouper, suggesting the function of both molecules may be conserved among vertebrates. In transfected HEK293T cells, both molecules localized on the membrane surface. Tissue distribution analysis showed both EcCD83 and EcCD80/86 mRNAs were mainly expressed in immune organs, and EcCD80/86 was extremely higher expressed in mucosal immune tissues including skin and gill than systematic immune organs, which indicates these co-stimulatory molecules may prime T cell activation in local mucosal tissues. In Cryptocaryon irritans infected groupers, the expression level of EcCD83 and EcCD80/86 were both seen significant up-regulation in the skin at most tested time points.

Two novel p38 MAPKs identified from Epinephelus coioides and their expression pattern in response to Cryptocaryon irritans infection

P38 mitogen-activated protein kinases (MAPKs) are one of the most important central regulatory proteins response to extra environmental stresses. In this study, two novel p38 MAPKs, Ec-P38γ and Ec-P38δ, were identified from Epinephelus coioides, an economically important cultured fish in China and Southeast Asian counties. Both of Ec-p38γ and Ec-p38δ sequences contain a serine/threonine protein kinase (S_TKc) domain and a highly conserved Thr-Gly-Tyr (TGY) motif. Analysis of phylogenetic relationships illustrated that p38 amino acid sequences were conserved between different species indicating that the functions may be similar. The four subtypes of p38 (α, β, γ, and δ) mRNA can be detected in all thirteen tissues examined, but the expression level is different in these tissues. The expression patterns of the four Ec-p38 subtypes in E. coioides were also detected response to Cryptocaryon irritans infection, one of the most important protozoan pathogens of marine fish. The expression of four p38 subtypes was up-regulated in the tissues examined, with the highest expressions of Ec-p38α (5.2 times) and Ec-p38δ (4.2 times) occurring in the skin, while Ec-p38β (24.8 times) and γ (16.6 times) occurred in the spleen. There was no significantly correlation between the expression of Ec-p38γ/Ec-p38δ and the expression of nuclear factor kappaB (NF-kB). The results indicated the sequences and the characters of Ec-p38γ and Ec-p38δ were conserved, the p38 subtypes showed tissue-specific expression patterns in healthy grouper, and their expressions were significantly up-regulated post C. irritans infection, suggesting these p38 MAPKs may play important roles in these tissues during pathogen-caused inflammation.

Grouper (Epinephelus coioides) TCR signaling pathway was involved in response against Cryptocaryon irritans infection

T cell activation is a complicated process accompanying with the activation of T cell receptor (TCR) signaling pathway, which is not well described in teleost fish. The initiation of this pathway depends on the interaction of membrane TCR co-receptors (e.g. CD4/8, CD3 and CD45) and a series of cytoplasmic protein tyrosine kinases (e.g. Lck, Fyn and ZAP70). Cyptocaryon irritans is a ciliate pathogen of marine fish white spot disease causing huge economic lost in marine aquaculture. This parasite can infect fish gill and skin and is considered to be a good pathogen model for fish gill and skin mucosal immunity. Our previous studies showed the locally mucosal antibody response was important for fish defense against this parasite. While how TCR signaling pathway involved in T cell activation to help B cell activation in C. irritans infected fish is still not known. In the present study, we cloned a grouper TCR co-receptor gene EcCD3ε (537 bp) and its three kinase genes, including EcLck (1512 bp), EcFyn (1605 bp) and EcZAP70 (1893 bp). Homology analysis showed that they all shared the highest identity with corresponding genes from Takifugu rubripes (EcCD3ε 41%, EcLck 88%, EcFyn 98% and EcZAP70 93%), and their conserved motifs involved in the signaling transduction were analyzed. The tissue distribution analysis showed these four genes were high expressed in thymus, and it is interesting to find their comparative high expression in skin, gill and midgut mucosal immune tissues. In C. irritans infected grouper, the expression of three TCR co-receptors (EcCD4-1, EcCD3ε and EcCD45) and three kinases (EcLck, EcFyn and EcZAP70) was tested in skin, gill, head kidney and spleen at 0, 12 h, 24 h, 2 d, 3 d, 5 d and 7 d. All six genes were significantly up-regulated in skin at most tested time points, which indicate the possibility of skin local T cell activation to support the local antibody response. Compared to three TCR co-receptors, significantly up-regulation of three kinases were seen in the spleen, and the spleen fold changes of these three kinases were much higher than head kidney, which indicates spleen maybe the major systematic immune organs for T cell activation in C. irritans infected fish.

Comparative transcriptional profile of the fish parasite Cryptocaryon irritans

BACKGROUND

Cryptocaryon irritans is an obligate ectoparasitic ciliate pathogen of marine fishes. It can infect most marine teleosts and cause heavy economic losses in aquaculture. There is currently no effective method of controlling this disease, and little information is available regarding the genes involved in its development and virulence. We aimed to investigate the distinct features of the three major life-cycle stages of C. irritans in terms of gene transcription level, and identify candidate vaccines/drug targets. We established a reference transcriptome of C. irritans by RNA-seq.

METHODS

Three cDNA libraries using total poly(A)⁺ mRNA isolated from trophonts, tomonts, and theronts was constructed and sequenced, respectively. Clean reads from the three stages were de novo assembled to generated unigene. Annotation of unigenes and transcriptomic comparison of three stages was performed.

RESULTS

Totals of 73.15, 62.23, and 109.57 million clean reads were generated from trophont, tomont, and theront libraries, respectively. After de novo assembly, 49,104 unigenes were obtained, including 9,253 unigenes with significant similarities to proteins from other ciliates. Transcriptomic comparisons revealed that 2,470 genes were differentially expressed among the three stages, including 2,011, 1,404, and 1,797 genes that were significantly differentially expressed in tomont/theront, tomont/trophont, and theront/trophont pairwise comparisons, respectively. Based on the results of hierarchical clustering, all differentially expressed genes (DEGs) were located in five major clusters. DEGs in clusters 1 and 2 were more highly expressed in tomonts than in other stages, DEGs in cluster 3 were dominant in the tomont and trophont stages, whereas clusters 4 and 5 included genes upregulated in the theront stage. In addition, Immobilization antigens (I-antigens) and proteases have long been considered major targets for vaccine development and potential drug targets in parasites, respectively. In the present study, nine putative I-antigens transcripts and 161 protease transcripts were found in the transcriptome of C. irritans.

CONCLUSION

It was concluded that DEGs enriched in tomonts were involved in cell division, to increase the number of theronts and ensure parasite continuity. DEGs enriched in theronts were associated with response to stimuli, whereas genes enriched in trophonts were related to nutrient accumulation and cell growth. In addition, the I-antigen and protease transcripts in our transcriptome could contribute to the development of vaccines or targeted drugs. Together, the results of the present study provide novel insights into the physiological processes of a marine parasitic ciliate.