Effect of ammonia-N and pathogen challenge on complement component 8α and 8β expression in the darkbarbel catfish Pelteobagrus vachellii

Evaluation of Ammonia Nitrogen Exposure in Immune Defenses Present on Spleen and Head-Kidney of Wuchang Bream (Megalobrama amblycephala)

Ammonia is one of the most important environmental factors in aquatic ecosystems. However, there are limited studies on the effects of chronic or long-term ammonia stress and its potential molecular mechanism in fish. This study aimed to investigate the immune response and molecular mechanisms in the spleen and head-kidney of fish following chronic ammonia exposure. Megalobrama amblycephala (9.98 ± 0.48 g) were exposed to different concentrations of total ammonia nitrogen (0-30 mg/L) for 30 days. Ammonia exposure caused significant increases in cortisol levels and decreases in lysozyme and complement 3/4 concentrations in the serum, indicating inhibitory effects of ammonia stress on innate immune responses. Ammonia exposure also induced concentration-dependent increases in ammonia concentrations in tissue, pathological damage and indexes of spleen and head-kidney. Additionally, the contents of immunoglobulin M (IgM), interleukin 1β (IL-1β) and tumor necrosis factor α (TNF-α) as well as mRNA levels of toll-like receptors (TLRs)/Myeloid differentiation factor 88 (MyD88)-independent signaling molecules in the spleen and head-kidney were significantly downregulated after ammonia exposure. Our findings suggested that chronic ammonia exposure caused the suppression of innate and adaptive immune responses through downregulating TLR/MyD88-independent signaling. Adverse influences of chronic ammonia stress were more severe in the spleen than in the head-kidney.

Diurnal expression of CXC receptors 4 (CXCR4) and CXC chemokine ligand 12 (CXCL12) in Pelteobagrus vachellii

The CXC chemokine ligand 12/CXC receptor 4 ligand/receptor interaction is the most ancient chemokine system in vertebrates, and it plays a pivotal role in the immune system's response against bacterial infection. In the current study, 1211 bp CXCR4 and 937 bp CXCL12 genes, which encode 364 and 99 amino acids, respectively, were isolated. Within the 24-hour light/dark cycle, the maximum of CXCR4 in the intestine, spleen, and anterior kidney of Pelteobagrus vachellii occurs at 8:00, 16:00, and 16:00, respectively. The maximum of CXCL12 in the intestine, spleen, and anterior kidney of P. vachellii occurs at 20:00, 12:00, and 20:00, respectively. CXCR4 and CXCL12 expressions showed 24-hour variation, which contributed to understanding of the immune rhythm of the teleost.

Diurnal rhythm expression of transcription factor NF-κB subunit p65 in Pelteobagrus vachellii

The pleiotropic transcription factor nuclear factor-kappa B (NF-κB) has important functions in viral resistance. In the present study, we isolated a p65 subunit of NF-κB cDNA from Pelteobagrus vachellii (termed Pvp65) and characterized it. The full-length p65 cDNA comprised 3651 bp, including a 148-bp 5'-untranslated region (UTR), a 106-bp 3'-UTR, and an open reading frame encoding a 1067-amino acid putative protein. The protein sequence comprised a DNA binding motif, a Rel-homology domain, a Rel protein signature, a putative transcription activation domain, a nuclear localization signal, and a transcription initiation factor IIA domain. The expression of Pvp65 displayed a daily rhythm, with an acrophase at approximately at 15:32 h in the liver, 11:34 h in the spleen, and 16:45 h in the head kidney. In addition, infection with Aeromonas hydrophila caused Pvp65 expression to increase significantly (P < 0.05), and peaking at 12 h post infection in the spleen, at 24 in the head kidney, and at 12 h in the liver. Thus, NF-κB expression might be under light/dark cycle control in P. vachellii, and may be involved in the immune response to A. hydrophila.

Toxic effects of waterborne nitrite exposure on antioxidant responses, acetylcholinesterase inhibition, and immune responses in olive flounders, Paralichthys olivaceus, reared in bio-floc and seawater

Paralichthys olivaceus (mean weight, 280.1 ± 10.5 g; mean length, 28.37 ± 2.3 cm) was reared in bio-floc and seawater for 6 months to determine the toxic effects of waterborne nitrite exposure (0, 25, 50, 100, and 200 mg/L) for 1 week, compared to those observed with bio-floc and seawater only. The effects on antioxidant activity, immune responses, and acetylcholinesterase activity were measured. Following nitrite exposure, superoxide dismutase activity in the liver and gills was significantly elevated and catalase activity was significantly increased, except for in the gills of P. olivaceus reared in bio-floc. Further, glutathione S-transferase activity was significantly elevated in the liver and gills, and glutathione was significantly lower. Meanwhile, acetylcholinesterase activity in the liver and gills was significantly inhibited and plasma lysozyme activity and immunoglobulin M were considerably elevated.

iTRAQ analysis of liver immune-related proteins from darkbarbel catfish (Pelteobagrus vachelli) infected with Edwardsiella ictaluri

Edwardsiella ictaluri causes enteric septicemia of catfish (ESC), a major disease occurring in these siluriform fish. As the liver is an important organ for defending against bacterial pathogens in fish, this study aimed to determine the liver immune response at the protein level. The differential proteomes of the darkbarbel catfish liver in response to E. ictaluri infection were identified with isobaric tags for relative and absolute quantitation (iTRAQ) labeling followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Using a 1.2-fold change in expression as a physiologically significant benchmark, a total of 819 differentially expressed proteins were reliably quantified using iTRAQ analysis, including 6 up-regulated proteins and 813 down-regulated proteins. GO enrichment analysis indicated that the "complement activation, alternative pathway" and "complement activation, classical pathway" were significantly enriched. KEGG enrichment analysis indicated the "antigen processing and presentation" and "bacterial secretion system" were significantly enriched. We selected the 6 up-regulated proteins and 10 immune-related down-regulated proteins for validation using real-time PCR. The 10 immune-related proteins included complement component C1r, C3, C5, C7, and C9 and plasma protease C1 inhibitor (C1-INH), signal recognition particle 54 kDa protein (SRP54), SRP receptor, proteasome activator complex subunit 1 (PSME1) and major histocompatibility complex class I (MHC class I) were selected from the GO clusters and KEGG pathways. The variations in mRNA expression for these genes were similar to the results of iTRAQ. This is the first report detailing the proteome response in the darkbarbel catfish liver during E. ictaluri infection and markedly contributes to our understanding of the defense mechanisms in the livers of darkbarbel catfish.

Diurnal rhythm and pathogens induced expression of toll-like receptor 9 (TLR9) in Pelteobagrus vachellii

Toll-like receptor 9 (TLR9) is activated by bacterial DNA and induces the production of inflammatory cytokines. In this study, the darkbarbel catfish Pelteobagrus vachellii TLR9 cDNA was cloned and sequenced. The daily expression pattern of TLR9 mRNA was investigated in various tissues. Furthermore, its expression was analyzed following exposure to the pathogen Aeromonas hydrophila. The 4249 bp cDNA includes a 3201 bp open reading frame (ORF) encoding 1067 amino acids. The predicted amino acid sequence comprises a leucine-rich domain (LRD), a toll/interleukin-1 receptor (TIR), and a transmembrane domain. P. vachellii TLR9 showed 42-87% amino acid sequence identity with TLR9 sequences of Ictalurus punctatus, Rhincodon typus, and Miichthys miiuy. The P. vachellii TLR9 mRNA was highly expressed in intestines, head kidney, and spleen in an apparently healthy fish. Following pathogen challenge, TLR9 expression increased significantly (P < 0.05) and peaked at 48 h post-exposure in the liver, at 24 in the head kidney, and at 12 h in the spleen. In addition, the pattern of TLR9 expression over a 24-h period showed a circadian rhythm in the head kidney, spleen, and intestine, with the acrophase at 20:34, 18:45, and 3:50, respectively. This result provided the basis for further study of the rhythm of innate immunity against bacteria in catfish.

Membrane attack complex-associated molecules from redlip mullet (Liza haematocheila): Molecular characterization and transcriptional evidence of C6, C7, C8β, and C9 in innate immunity

The redlip mullet (Liza haematocheila) is one of the most economically important fish in Korea and other East Asian countries; it is susceptible to infections by pathogens such as Lactococcus garvieae, Argulus spp., Trichodina spp., and Vibrio spp. Learning about the mechanisms of the complement system of the innate immunity of redlip mullet is important for efforts towards eradicating pathogens. Here, we report a comprehensive study of the terminal complement complex (TCC) components that form the membrane attack complex (MAC) through in-silico characterization and comparative spatial and temporal expression profiling. Five conserved domains (TSP1, LDLa, MACPF, CCP, and FIMAC) were detected in the TCC components, but the CCP and FIMAC domains were absent in MuC8β and MuC9. Expression analysis of four TCC genes from healthy redlip mullets showed the highest expression levels in the liver, whereas limited expression was observed in other tissues; immune-induced expression in the head kidney and spleen revealed significant responses against Lactococcus garvieae and poly I:C injection, suggesting their involvement in MAC formation in response to harmful pathogenic infections. Furthermore, the response to poly I:C may suggest the role of TCC components in the breakdown of the membrane of enveloped viruses. These findings may help to elucidate the mechanisms behind the complement system of the teleosts innate immunity.

Transcriptome profiling of immune-responsive genes in the intestine of Cynoglossus semilaevis Günther challenged with Shewanella algae

To better understand gene expression in the intestine after Shewanella algae infection and provide insights into its immune roles in the tongue sole, Cynoglossus semilaevis, sequencing-based high-throughput RNA analysis (RNA-Seq) for the intestines between the control group and 12 h post-injection group was performed. After assembly, there was an average of 23,957,159 raw sequencing reads, and 23,943,491 clean reads were obtained after filtering out low-quality reads. Then, 383 differentially expressed genes (DEGs) in the intestines in response to S. algae infection were identified. Subsequently, gene ontology (GO) enrichment and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses of the DEGs were conducted to further explore their functions. Among all of the pathways involved, sixteen pathways were related to the immune system, among which the complement and coagulation cascades pathway was the most prominent for immunity-related DEGs, followed by the leukocyte transendothelial migration pathway. Furthermore, the expression levels of twelve selected DEGs in the immune-related pathways were identified by quantitative real-time polymerase chain reaction, substantiating the reliability and reproducibility of the RNA-Seq results. In summary, this study represents an important genomic resource for understanding the potential immune role of the tongue sole intestine from the perspective of gene expression.

Molecular characterization and expression of toll-like receptor 5 genes from Pelteobagrus vachellii

Toll-like receptor 5 (TLR5) is an important pathogen recognition receptor (PRR) that recognizes the flagellin protein of pathogenic bacteria and plays a fundamental role in activating the innate immune response. In this study, full-length pvTLR5m (membrane) and pvTLR5s (soluble) genes were cloned from darkbarbel catfish Pelteobagrus vachellii, and their expression and that of downstream genes were analyzed following exposure to the Aeromonas hydrophila pathogen. The 3009 bp pvTLR5m cDNA includes a 2652 bp open reading frame (ORF) encoding 884 amino acids. The 2422 bp pvTLR5s cDNA includes a 1944 bp ORF encoding a predicted protein of 648 amino acids. The genes are most closely related to TLR5m (75%) and TLR5s (69%) from Ictalurus punctatus, respectively, and both have a typical TLR structure. Both genes were constitutively expressed in all examined tissues, and most abundantly in the head kidney and spleen. Following pathogen challenge, pvTLR5m and pvTLR5s expression was increased significantly (P <0.05) and peaked at 24 and 12 h post-exposure in the liver, 24 and 12 h in the head kidney, and 48 and 24 h in the spleen, respectively. The downstream genes interleukin-1β (IL-1β), IL-12 and tumor necrosis factor-alpha (TNF-α) were significantly up-regulated following pathogen exposure in spleen, and the NF-kB inhibitor (IκB) was down-regulated. These findings indicated that pvTLR5 may play an important role in the immune responses to A. hydrophila. These results provide new insight to elucidate the immune signalling pathways of fish TLR.

Transcriptome analysis of the spleen of the darkbarbel catfish Pelteobagrus vachellii in response to Aeromonas hydrophila infection

Intensive aquaculture has increased the susceptibility of fish to Aeromonas hydrophila, and this has led to severe economic damage. There has been little study of the host defense mechanism against A. hydrophila infection in scaleless fish. Therefore, in the present study, the transcriptome profiles of the spleen of Pelteobagrus vachellii were examined after infection with A. hydrophila. In total, 37,730 unigenes from 322 KEGG pathways were identified. Following A. hydrophila infection, 27,803 differentially expressed genes were identified, including 13,934 upregulated and 13,869 downregulated genes. Significant enrichment analysis of these differentially expressed unigenes showed that the major immune pathways were involved, including toll-like receptor pathways, B-cell receptor signaling pathways, Fcγ receptor-mediated phagocytosis, complement and coagulation cascades, and natural killer cell-mediated cytotoxicity pathways. From these pathways, 59 key immune-related differentially expressed genes were selected: 53 genes that were upregulated, including those coding for complement components, interferons, and interleukins, and six DEGs that were downregulated, including inhibitor of nuclear factor kappa-B kinase. Finally, nine DEGs, which were randomly selected, were confirmed by qRT-PCR to be differentially expressed. The results indicated that complement components, interferons and Fcγ receptor-mediated phagocytosis played key role in the response to A. hydrophila infection in the spleen of P. vachellii, which may prove useful in the future for the development of therapeutic regimens.

Transcriptome analysis and discovery of genes involved in immune pathways in large yellow croaker (Larimichthys crocea) under high stocking density stress

The large yellow croaker, Larimichthys crocea, is an economically important maricultured species in southeast China. Owing to the importance of stocking densities in commercial fish production, it is crucial to establish the physiological responses and molecular mechanisms that govern adaptation to crowding in order to optimize welfare and health. In the present study, an extensive immunity-related analysis was performed at the transcriptome level in L. crocea in response to crowding stress. Over 145 million high-quality reads were generated and de novo assembled into a final set of 40,123 unigenes. Gene Ontology and genome analyses revealed that molecular function, biological process, intracellular, ion binding, and cell process were the most highly enriched pathways among genes that were differentially expressed under stress. Among all of the pathways involved, 16 pathways were related to the immune system, among which the complement and coagulation cascades pathway was the most enriched for differentially expressed immunity-related genes, followed by the chemokine signaling pathway, toll-like receptor signaling pathway, and leukocyte transendothelial migration pathway. The consistently high expression of immune-related genes in the complement and coagulation cascades pathway (from 24 to 96 h after being subjected to stress) suggested its importance in both response to stress and resistance against bacterial invasion at an early stage. These results also demonstrated that crowding can significantly induce immunological responses in fish. However, long-term exposure to stress eventually impairs the defense capability in fish.

Transcriptomic variation of locally-infected skin of Epinephelus coioides reveals the mucosal immune mechanism against Cryptocaryon irritans

Fish skin is the largest immunologically active mucosal organ, providing first-line defense against external pathogens. However, the skin-associated immune mechanisms of fish are still unclear. Cryptocaryon irritans is an obligate ectoparasitic ciliated protozoan that infects almost all marine fish, and is believed to be an excellent pathogen model to study fish mucosal immunity. In this study, a de novo transcriptome assembly of Epinephelus coioides skin post C. irritans tail-infection was performed for the first time using the Illumina HiSeq™ 2500 system. Comparative analyses of infected skin (group Isk) and uninfected skin (group Nsk) from the same challenged fish and control skin (group C) from uninfected control fish were conducted. As a result, a total of 91,082 unigenes with an average length of 2880 base pairs were obtained and among them, 38,704 and 48,617 unigenes were annotated based on homology with matches in the non-redundant and zebrafish database, respectively. Pairwise comparison resulted in 10,115 differentially-expressed genes (DEGs) in the Isk/C group comparison (4,983 up-regulated and 5,132 down-regulated), 2,275 DEGs in the Isk/Nsk group comparison (1,319 up-regulated and 956 down-regulated) and 4,566 DEGs in the Nsk/C group comparison (1,534 up-regulated and 3,032 down-regulated). Seven immune-related categories including 91 differentially-expressed immune genes (86 up-regulated and 5 down-regulated) were scrutinized. Both DEGs and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis and immune-related gene expression analysis were used, and both analyses showed that the genes were more significantly altered in the locally-infected skin than in the uninfected skin of the same challenged fish. This suggests the skin's local immune response is important for host defense against this ectoparasite infection. Innate immune molecules, including hepcidin, C-type lectin, transferrin, transferrin receptor protein, serum amyloid A, cathepsin and complement components were significantly up-regulated (fold-change ranged from 3.3 to 12,944) in infected skin compared with control skin. The up-regulation of chemokines and chemokine receptors and activation of the leukocyte transendothelial migration pathway suggested that leucocytes intensively migrated to the local infected sites to mount a local immune defense. Toll-like receptors (TLRs) 1, 2, 5 and 5S were most significantly up-regulated in the infected skin, suggesting that these TLRs may be involved in parasite pathogen-associated molecular pattern (PAMPs) recognition. Up-regulation of the dendritic cell markers CD209 and CD83 and other antigen presentation pathway molecules provided evidence for skin local antigen presentation. Up-regulation of the T cell markers CD4 and CD48, B cell markers CD22 and CD81 and B cell receptor signaling kinase Lyn, showed the presence and population expansion of T/B cells at locally-infected sites, which suggested possible activation of a local specific immune response in the skin. Our results will facilitate in-depth understanding of local immune defense mechanisms in fish skin against ectoparasite infection.