Transcriptome analysis reveals temperature-regulated antiviral response in turbot Scophthalmus maximus

IL-18 signaling is regulated by caspase 6/8 and IL-18BP in turbot (Scophthalmus maximus)

Interleukin (IL)-18 is synthesized as a precursor that requires intracellular processing to become functionally active. In human, IL-18 is processed by caspase 1 (CASP1). In teleost, the maturation and signal transduction mechanisms of IL-18 are unknown. We identified two IL-18 variants, IL-18a and IL-18b, in turbot. IL-18a, but not IL-18b, was processed by CASP6/8 cleavage. Mature IL-18a bound specifically to IL-18 receptor (IL-18R) α-expressing cells and induced IL-18Rα-IL-18Rβ association. Bacterial infection promoted IL-18a maturation in a manner that required CASP6 activation and correlated with gasdermin E activation. The mature IL-18a induced proinflammatory cytokine expression and enhanced bacterial clearance. IL-18a-mediated immune response was suppressed by IL-18 binding protein (IL-18BP), which functioned as a decoy receptor for IL-18a. IL-18BP also functioned as a pathogen pattern recognition receptor and directly inhibited pathogen infection. Our findings revealed unique mechanism of IL-18 maturation and conserved mechanism of IL-18 signaling and regulation in turbot, and provided new insights into the regulation and function of IL-18 related immune signaling.

Transcriptomic response of lumpfish (Cyclopterus lumpus) head kidney to viral mimic, with a focus on the interferon regulatory factor family

The economic importance of lumpfish (Cyclopterus lumpus) is increasing, but several aspects of its immune responses are not well understood. To discover genes and mechanisms involved in the lumpfish antiviral response, fish were intraperitoneally injected with either the viral mimic polyinosinic:polycytidylic acid [poly(I:C)] or phosphate-buffered saline (PBS; vehicle control), and head kidneys were sampled 24 hours post-injection (hpi) for transcriptomic analyses. RNA sequencing (RNA-Seq) (adjusted p-value <0.05) identified 4,499 upregulated and 3,952 downregulated transcripts in the poly(I:C)-injected fish compared to the PBS-injected fish. Eighteen genes identified as differentially expressed by RNA-Seq were included in a qPCR study that confirmed the upregulation of genes encoding proteins with antiviral immune response functions (e.g., rsad2) and the downregulation of genes (e.g., jarid2b) with potential cellular process functions. In addition, transcript expression levels of 12 members of the interferon regulatory factor (IRF) family [seven of which were identified as poly(I:C)-responsive in this RNA-Seq study] were analyzed using qPCR. Levels of irf1a, irf1b, irf2, irf3, irf4b, irf7, irf8, irf9, and irf10 were significantly higher and levels of irf4a and irf5 were significantly lower in the poly(I:C)-injected fish compared to the PBS-injected fish. This research and associated new genomic resources enhance our understanding of the genes and molecular mechanisms underlying the lumpfish response to viral mimic stimulation and help identify possible therapeutic targets and biomarkers for viral infections in this species.

Investigation of Gene Networks in Three Components of Immune System Provides Novel Insights into Immune Response Mechanisms against Edwardsiella tarda Infection in Paralichthys olivaceus

As a quintessential marine teleost, Paralichthys olivaceus demonstrates vulnerability to a range of pathogens. Long-term infection with Edwardsiella tarda significantly inhibits fish growth and even induces death. Gills, blood, and kidneys, pivotal components of the immune system in teleosts, elicit vital regulatory roles in immune response processes including immune cell differentiation, diseased cell clearance, and other immunity-related mechanisms. This study entailed infecting P. olivaceus with E. tarda for 48 h and examining transcriptome data from the three components at 0, 8, and 48 h post-infection employing weighted gene co-expression network analysis (WGCNA) and protein-protein interaction (PPI) network analysis. Network analyses revealed a series of immune response processes after infection and identified multiple key modules and key, core, and hub genes including xpo1, src, tlr13, stat1, and mefv. By innovatively amalgamating WGCNA and PPI network methodologies, our investigation facilitated an in-depth examination of immune response mechanisms within three significant P. olivaceus components post-E. tarda infection. Our results provided valuable genetic resources for understanding immunity in P. olivaceus immune-related components and assisted us in further exploring the molecular mechanisms of E. tarda infection in teleosts.

Gene network analyses of larvae under different egg-protecting behaviors provide novel insights into immune response mechanisms of Amphioctopus fangsiao

Amphioctopus fangsiao was a representative economic species in cephalopods, which was vulnerable to marine bacteria. Vibrio anguillarum was a highly infectious pathogen that have recently been found to infect A. fangsiao and inhibit its growth and development. There were significant differences in the immune response mechanisms between egg-protected and egg-unprotected larvae. To explore larval immunity under different egg-protecting behaviors, we infected A. fangsiao larvae with V. anguillarum for 24 h and analyzed the transcriptome data about egg-protected and egg-unprotected larvae infected with 0, 4, 12, and 24 h using weighted gene co-expression networks (WGCNA) and protein-protein interaction (PPI) networks. Network analyses revealed a series of immune response processes after infection, and identified six key modules and multiple immune-related hub genes. Meanwhile, we found that ZNF family, such as ZNF32, ZNF160, ZNF271, ZNF479, and ZNF493 might play significant roles in A. fangsiao immune response processes. We first creatively combined WGCNA and PPI network analysis to deeply explore the immune response mechanisms of A. fangsiao larvae with different egg-protecting behaviors. Our results provided further insights into the immunity of V. anguillarum infected invertebrates, and laid the foundation for exploring the immune differences among cephalopods with different egg protecting behaviors.

Splenic protection network revealed by transcriptome analysis in inactivated vaccine-immunized flounder (Paralichthys olivaceus) against Edwardsiella tarda infection

The protective immune response produced by fish after vaccination is crucial for vaccine effectiveness. Our previous studies have shown inactivated vaccine against Edwardsiella tarda can induce immune response in flounder (Paralichthys olivaceus). To elucidate the protective immune response at the genetic level, in this study, flounder was immunized with inactivated E. tarda for 5 weeks, and then they were challenged with E. tarda. The spleen was dissected at 7th day post immunization, 1st and 7th day post challenge, respectively. Transcriptome analysis showed that average of 46 million clean reads were obtained per library, while percentage of clean reads being mapped to reference genome was more than 89% in all cases, which suggested good quality of samples. As for differentially expressed genes (DEGs) identification in inactivated E. tarda groups, at 7th day post immunization, 1422 DEGs were identified and significantly enriched in innate immune-related pathways, such as Phagosome, Cell adhesion molecules and NF-kappa B signaling pathway; At 1st post challenge day, 1210 DEGs were identified and enriched to Antigen processing and presentation and Cell adhesion molecules, indicating that the pathogen was rapidly recognized and delivered; At 7th post challenge day, 1929 DEGs were identified, belonged to Toll-like receptor signaling pathway, Antigen processing and presentation, Th1 and Th2 cell differentiation and Th17 cell differentiation. Compared to 7th post immunization day, 73 immune-associated DEGs were identified at 1st post challenge day. Protein-protein interaction networks analysis revealed 11 hub genes (TLR7, TLR3, CXCR4, IFIH1, TLR8 etc), associated with recognition of pathogens and activation of innate immunity; while for 7th post challenge day, 141 immune-associated DEGs were identified. 30 hub genes (IL6, STAT1, HSP90A.1, TLR7, IL12β etc) were associated with stimulation of lymphocyte differentiation and activation of cellular immunity. Ten immune-related genes were randomly selected for RT-qPCR validation at each time point. In conclusion, data revealed protection of flounder against E. tarda infection by inactivated vaccine is mediated via immediate recognition of pathogen and subsequently activation of cellular immunity. Results give new aspect for vaccine protection cascades, is good references for vaccine evaluation.

Comparative transcriptome analysis of differentially expressed genes and pathways in Procambarus clarkii (Louisiana crawfish) at different acute temperature stress

Procambarus clarkii is an important economic species in China, and exhibit heat and cold tolerance in the main culture regions. To understand the mechanisms, we analyzed the hepatopancreas transcriptome of P. clarkii treated at 10 °C, 25 °C, and 30 °C, then 2092 DEGs and 6929 DEGs were found in 30 °C stress group and 10 °C stress group, respectively. KEGG pathway enrichment results showed that immune pathway is the main stress pathway for 10 °C treatment and metabolic pathway is the main response pathway for 30 °C treatment, which implies low temperature stress induces the damage of the immune system and increases the susceptibility of bacteria while the body response to high temperature stress through metabolic adjustment. In addition, flow cytometry proved that both high and low temperature stress caused different degrees of apoptosis of hemocytes, and dynamic transcription heat map analysis also identified the differential expression of HSPs family genes and apoptosis pathway genes under different heat stresses. This indicates that preventing damaged protein misfolding and accelerating cell apoptosis are necessary mechanisms for P. clarkii to cope with high and low temperature stress. Our research has deepened our understanding of the complex molecular mechanisms of P. clarkii in response to acute temperature stress, and provided a potential strategy for aquatic animals to relieve environmental duress.

Ocean Warming Leads to Increases in Aerobic Demand and Changes to Gene Expression in the Pinfish (Lagodon rhomboides)

Anthropogenic climate change is causing increases in the frequency, intensity, and duration of marine heatwaves (MHWs). These short-term warming events can last for days to weeks and can produce severe disruptions in marine ecosystems, as many aquatic species are poikilotherms that depend on the conditions of the environment for physiological processes. It is crucial to investigate the effects of these thermal fluctuations on species that play a disproportionate ecological role in marine ecosystems, such as the pinfish (Lagodon rhomboides) in the Gulf of Mexico and western Atlantic. In this study, we exposed pinfish to a simulated MHW in aquaria and examined the impacts of acute warming on two life stages (juvenile and adult), measuring oxygen consumption and gene expression in two relevant tissue types (liver and muscle). We saw significant increases in routine metabolic rate with increasing temperature in both juveniles (24.58 mgO2/kg/h increase per 1°C of warming) and adults (10.01 mgO2/kg/h increase per 1°C of warming). These results indicate that exposure to increased temperatures was more metabolically costly for juveniles than for adults, on a mass-specific basis. This was also observed in the molecular analyses, where the largest number of differentially expressed genes were observed in the juvenile pinfish. The analyses of gene expression suggest warming produces changes to immune function, cell proliferation, muscle contraction, nervous system function, and oxygen transport. These results indicate that this ecologically relevant species will be significantly impacted by projected increases in frequency and magnitude of MHWs, particularly in the juvenile stage.

Ly-6D of Japanese flounder (Paralichthys olivaceus) functions as a complement regulator and promotes host clearance of pathogen

The Lymphocyte antigen-6 (Ly-6) superfamily has been considered to play an important role in the innate immunity of mammals. The functions of Ly-6 proteins are diverse since their low sequence homology. Currently, the function of Ly-6D, a member of Ly-6 family proteins, is completely unknown in teleost. In the present study, we identified and characterized a Ly-6D homologue (named PoLy-6D) from the teleost fish Paralichthys olivaceus and examined its immune function. PoLy-6D possesses a hydrophobic signal peptide, a LU domain including a conserved "LXCXXC" motif in N-terminus and a "CCXXXXCN" motif in C-terminus. Under normal physiological condition, PoLy-6D expression distributes in all the examined tissues, the highest three tissues are successively spleen, head kidney, and blood. When infected by extracellular and intracellular bacterial pathogens and viral pathogen, PoLy-6D expression was induced and the patterns vary with different types of microbial pathogens infection and different immune tissues. In vitro experiment showed recombinant PoLy-6D (rPoLy-6D) inhibited the lysis of rabbit red blood cells by serum and selectively improved bacterial survival in serum. After serum were treated by antibody of rPoLy-6D, bacteriostatic effect of serum was obviously enhanced. These results indicate the importance of PoLy-6D as a complement regulator. rPoLy-6D possessed the binding activity to multiple bacteria but did not exhibit antimicrobial activities. The interaction between rPoLy-6D and bacteria suggests that PoLy-6D is involved in host clearance of pathogens probably by serving as a receptor for pathogens. Overexpression of PoLy-6D in vivo promoted the host defense against invading E. piscicida. These findings add new insights into the regulation mechanism of the complement system in teleost and emphasize the importance of Ly-6D products for the control of pathogen infection.

Macrophage colony stimulating factor (MCSF) of Japanese flounder (Paralichthys olivaceus): Immunoregulatory property, anti-infectious function, and interaction with MCSF receptor

Macrophage colony-stimulating factor (MCSF) is an essential growth factor to control the proliferation, differentiation and survival of the macrophage lineage in vertebrates. Sequences of MCSF have been identified in multiple teleost species, however, the functional investigations of MCSF were documented in only a few species. In this study, we examined the biological activity and the immunomodulatory property of a MCSF homologue, PoMCSF, from Japanese flounder (Paralichthys olivaceus). Structural analysis showed that PoMCSF possesses conserved structural characteristics of MCSF proteins, including a signal peptide, a CSF-1 domain, and a transmembrane region closed to the C-terminal. Under normal physiological condition, PoMCSF expression distributes in all the examined tissues, the highest three tissues are blood, muscle, and head kidney. When infected by extracellular and intracellular bacterial pathogens and viral pathogen, the PoMCSF expression patterns vary with different types of microbial pathogens infection and different immune tissues. In vitro experiment showed recombinant PoMCSF promoted the activity of macrophage. In vivo experiment indicated that PoMCSF overexpression boosted the defensive ability of flounder against Edwardsiella piscicida, a severe fish pathogen that infects multiple species of economically important fish, and regulated the expression of multiple immune-related genes. To explore the relationship between PoMCSF and its receptor PoMCSFR, anti-PoMCSFR antibody was prepared and PoMCSFR knockdown was conducted. The neutralization assay showed that when PoMCSFR was neutralized by its antibody, the role of PoMCSF on host defense against E. piscicida was weakened. Knockdown of PoMCSFR impaired the phagocytic capacity of macrophages. Collectively, these findings suggest that PoMCSF plays a crucial role in the immune defense system of Japanese flounder and the effect of PoMCSF is dependent on PoMCSFR. This study provides new insights into the biological activity of MCSF and the relationship between MCSF and MCSFR in teleost.

Two CD9 tetraspanin family members of Japanese flounder (Paralichthys olivaceus): characterization and comparative analysis of the anti-infectious immune function

CD9 is a glycoprotein of the transmembrane 4 superfamily that is involved in various cellular processes. Studies related to the immune functions and activities of CD9 in teleost fish are limited. In this study, we characterized two CD9 homologs, PoCD9.1 and PoCD9.3, from Japanese flounder (Paralichthys olivaceus). Sequence analysis showed that PoCD9.1 and PoCD9.3 possess characteristic transmembrane 4 superfamily (TM4SF) structures. PoCD9.1 shares 70.61% sequence identity with PoCD9.3. The expression of PoCD9.1 and PoCD9.3 in the three main immune tissues was significantly induced in a time-dependent manner by extracellular and intracellular pathogen infection, which indicates that the two CD9 homologs play an important role in the response to pathogenic infection. Following infection with the extracellular pathogen Vibrio anguillarum, the expression profiles of both PoCD9.1 and PoCD9.3 were similar. After infection with the intracellular pathogen Edwardsiella piscicida, the expression levels of PoCD9.1 and PoCD9.3 were different at different stages of infection, especially in the spleen. The spleen was the most important tissue for the PoCD9.1 and PoCD9.3 responses to pathogen infection among the three examined immune tissues. Knockdown of PoCD9.1 and PoCD9.3 attenuated the ability of host cells to eliminate pathogenic bacteria, and PoCD9.1 knockdown was more lethal than PoCD9.3 knockdown for host cells with E. piscicida infection. Overexpression of PoCD9.1 and PoCD9.3 promoted host or host cell defence against E. piscicida infection. These findings suggest that PoCD9.1 and PoCD9.3 serve as immune-related factors, play an important role in the immune defence system of Japanese flounder, and display different functions in response to different pathogens at different stages of infection.

Transcriptome analysis reveals temperature-dependent early immune response in flounder (Paralichthys olivaceus) after Hirame novirhabdovirus (HIRRV) infection

Hirame novirhabdovirus (HIRRV), as a highly pathogenic fish virus, is frequently prevalent in a variety of aquaculture fish in recent years, which seriously threatens the healthy development of aquaculture industry. Epidemiological studies show that the outbreak of HIRRV is obviously temperature dependent. Virus proliferation experiments in vitro and in vivo at different temperatures indicate the antiviral response of flounder is a main reason affect the replication of HIRRV. The RNA-Seq was used to analyze the different antiviral response in flounder which infected with HIRRV at different temperatures, the experiment set two temperatures of 10 °C and 20 °C. The flounder infected with HIRRV by artificial injection, and the spleens were collected at 24 h after infection. Meanwhile, the fish injected with EPC supernatant at different temperatures were set as control groups. It can obtain four pairwise comparison groups if determine a single variable, and the differentially expressed genes were further selected. The results showed that after infection with HIRRV at 10 °C and 20 °C, the differentially expressed genes in spleen were significantly enriched in inflammatory and immune-related pathways like Arachidonic acid metabolism, Cytokine-cytokine receptor interaction, Toll-like receptor (TLR) signaling pathway, RIG-I-like receptor (RLRs) signaling pathway, NOD-like receptor (NLR) signaling pathway and Cytosolic DNA-sensing pathway etc. In addition, the expression of phagocytes, lysosomes, endocytosis related genes were significantly upregulated at high temperature whether HIRRV positive or not. But compared to the infected flounder at 10 °C, some genes of RLRs signaling pathway were significantly upregulated at 20 °C, it can be speculated that RLRs pathway may be related to the anti-HIRRV response of flounder. Therefore, key genes of RLRs signaling pathway including mda5, lgp2, mita, mavs, irf3, irf7, ifn I-3 and ifn-γ were selected, and the temporal expression patterns of these genes in infected flounder at different temperatures were further detected by qRT-PCR. The results showed that HIRRV infection can significantly stimulate and activate the RLRs pathway of flounder, and the response level of this pathway was significantly higher at 20 °C than 10 °C. In general, this study provides important data for the further study about the pathogenesis of HIRRV infection in flounder.

Transcriptome analysis provides insights into the effects of myo-inositol on the turbot Scophthalmus maximus

Myo-inositol is an essential vitamin for most animals, and it can modulate multiple physiological functions. In this study, we performed transcriptome gene expression profiling of gill tissue from turbot Scophthalmus maximus fed different concentrations of myo-inositol (0, 300, 600, 900, 1200 mg/kg). Results of expression tendency analysis, Weighted Gene Co-Expression Network Analysis (WGCNA), integrated transcriptome analyses, and KEGG annotation analysis of all differentially expressed genes (DEGs) demonstrated that the cytokine-cytokine receptor interaction played a core role in effects of myo-inositol on turbot, which was followed by the Jak-STAT signaling pathway. The results of qRT-PCR also showed myo-inositol mediated the gene expression of the cytokine-cytokine receptor interaction and the Jak-STAT signaling pathway in turbot. The ELISA assay indicated that myo-inositol affected the concentration change of interleukins (IL-2 and IL-10). Consequently, the interleukins associated with immune functions in the cytokine-cytokine receptor interaction played a core role in the effects of myo-inositol on turbot, which was followed by the Jak-STAT signaling pathway. Additionally, 10 hub genes associated with myo-inositol-traits were identified via WGCNA.

Transcriptional programs of infectious spleen and kidney necrosis virus (ISKNV) in vitro and in vivo

Infectious spleen and kidney necrosis virus (ISKNV), causing serious infectious diseases to marine and freshwater fishes, is the type species of the genus Megalocytivirus, family Iridoviridae. In this study, the transcriptional programs of ISKNV in vitro (MFF-1 cells) and in vivo (spleens from mandarin fish) were investigated using real-time PCR. Transcription of all the putative open reading frames (ORFs) of ISKNV was verified. The temporal expression patterns of ISKNV ORFs in vitro and in vivo, including peak expression times (PETs) and relative maximal expression levels, were determined and compared. The K-means clustering with Spearman rank correlation was generated in heat maps constructed based on ISKNV ORF expression profiles in vivo and in vitro. The current study may provide a global picture of ISKNV infection at the transcription level and help better understand the molecular pathogenic mechanism of megalocytiviruses.

Transcriptome analysis reveals seven key immune pathways of Japanese flounder (Paralichthys olivaceus) involved in megalocytivirus infection

Megalocytivirus is a serious viral pathogen to many farmed fish including Japanese flounder (Paralichthys olivaceus). In this study, in order to systematically identify host immune genes induced by megalocytivirus infection, we examined the transcription profiles of flounder infected by megalocytivirus for 2, 6, and 8 days. Compared with uninfected fish, virus-infected fish exhibited 1242 differentially expressed genes (DEGs), with 225, 275, and 877 DEGs occurring at 2, 6, and 8 days post infection, respectively. Of these DEGs, 728 were upregulated and 659 were downregulated. The majority of DEGs were time-specific and formed four distinct expression profiles well correlated with the time of infection. The DEGs were classified into diverse Gene Ontology (GO) functional terms and enriched in 27 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, approximately one third of which were related to immunity. Weighted co-expression network analysis (WGCNA) was used to identify 16 key immune DEGs belonging to seven immune pathways (RIG-I-like receptor signaling pathway, JAK-STAT signaling pathway, TLR signaling pathway, cytokine-cytokine receptor interaction, phagosome, apoptosis, and p53 signaling pathway). These pathways interacted extensively and formed complicated networks. This study provided a global picture of megalocytivirus-induced gene expression profiles of flounder at the transcriptome level and uncovered a set of key immune genes and pathways closely linked to megalocytivirus infection. These results provided a set of targets for future delineation of the key factors implicated in the anti-megalocytivirus immunity of flounder.

Characterization, expression, and antimicrobial activity of histones from Japanese flounder Paralichthys olivaceus

Histone proteins are not only structurally important for chromosomal DNA packaging but also involved in the regulation of gene expression and the immune response of host against pathogens. Japanese flounder (Paralichthys olivaceus) as one of the most important marine flatfish, suffered from widespread outbreaks of diseases, and its immunological functioning remained to be elucidated. In the present study, we reported the expression patterns of four histones (H1, H2A, H3, and H3.3) and functional characterization of the histone H3.3 from flounder. Quantitative real time RT-PCR (RT-qPCR) analysis showed that expression of the four histones occurred in multiple tissues, but their levels of expression were relatively high in immune organs, and inducible in response to pathogens infection. Infection with extracellular and intracellular bacterial pathogens and viral pathogen regulated the expression of histones in a manner that depended on tissue type, pathogen, and infection stage. Specifically, H1 expression was highly induced by intracellular viral pathogens; H2AX and H3 expressions were highly induced by intracellular bacterial pathogen; dissimilarly, H3.3 expression was slightly induced by extracellular bacterial pathogen, but was inhibited by intracellular bacterial and viral pathogens. To further investigate H3.3 function, recombinant H3.3 (rH3.3) was obtained, and in vitro experiments showed rH3.3 possessed the capability of binding to both Gram-negative and Gram-positive bacteria and inhibiting the growth of some target bacteria. Consistently, In vivo results showed that overexpression of H3.3 promoted the host defense against invading pathogenic microorganism and regulated the expressions of several cytokines. These results suggested that flounder histones exhibit different expression patterns in response to the infection of different microbial pathogens, and H3.3 serves as an immune-related protein and plays an important role in antimicrobial immunity of Japanese flounder. Taken together, this study is the first report about the expression profile of different histones upon different kind of pathogens and anti-infectious immunity of H3.3 in teleost, which offered new insights into the immunological function of histones in teleost.

Major histocompatibility complex class I (MHC Iα) of Japanese flounder (Paralichthys olivaceus) plays a critical role in defense against intracellular pathogen infection

The major histocompatibility complex (MHC) is a highly polymorphic region of the vertebrate genome that plays a critical role in initiating immune responses towards invading pathogens. It is well known that MHC I molecules play a central role in the immune response to viruses. However, rare literatures were reported the role of MHC I in the resistance to intracellular bacteria. Sequences of MHC Iα were identified in multiple teleost species, including Japanese flounder (Paralichthys olivaceus), however, the immunological function of MHC Iα remain largely unknown. In this study, we examined the expression profile and biological activity of an MHC Iα homologue, PoMHC Iα, from P. olivaceus. Structural analysis showed that PoMHC Iα possesses conserved structural characteristics of MHC Iα proteins, including MHC_I domain, IGc1 domain, transmembrane region. Expression of PoMHC Iα was upregulated in a time-dependent manner by extracellular and intracellular bacterial pathogens and viral pathogen infection. Different expression patterns were exhibited in response to the infection of different types of microbial pathogens in different immune tissues. Recombinant PoMHC Iα increased the capability of host cells to defense against intracellular pathogen Edwardsiella tarda infection and enhanced the expression of immune related genes. The knockdown of PoMHC Iα attenuated the ability of cells to eliminate E. tarda, which was sustained by the in vivo results that overexpression of PoMHC Iα promoted the host defense against invading E. tarda. Antigen uptake assay indicated PoMHC Iα participated in cells antigen presentation. Collectively, this study is the first report that MHC Iα plays an important role in immune defense against intracellular bacterial pathogen in teleost. Taken together, these findings add new insights into the biological function of teleost MHC Iα and emphasize the importance of MHC I gene products for the control of E. tarda infection.

Granulocyte colony stimulating factor (GCSF) of Japanese flounder (Paralichthys olivaceus): Immunoregulatory property and anti-infectious function

Granulocyte colony stimulating factor (GCSF) is a key regulator of neutrophil production, and plays a vital role in immune response of mammals and teleost against pathogen. Sequences of GCSF were identified in several teleost species, however, the function and activity of GCSF in teleost remain largely unknown. In this study, we examined the biological activity and the immunomodulatory property of a GCSF homologue, PoGCSF, from Japanese flounder (Paralichthys olivaceus). Structural analysis showed that PoGCSF possesses conserved structural characteristics of GCSF proteins, including a signal peptide and a typical IL-6 domain. The expression of PoGCSF was upregulated in a time-dependent manner by extracellular and intracellular bacterial pathogens and viral pathogen. Different expression patterns were exhibited in response to the infection of different types of microbial pathogens in different immune tissues. Recombinant PoGCSF increased the capability of host cells to defense against pathogen infection and enhanced the expression of immune related genes. The knockdown of PoGCSF attenuated the ability of host cells to eliminate pathogenic bacteria. In vivo results showed that overexpression of PoGCSF promoted the host defense against invading pathogenic microorganism. Collectively, this study is the first report about the immunoregulatory property and anti-infectious immunity of GCSF in teleost. These findings suggested that PoGCSF serves as an immune-related cytokine and plays an important role in the immune defense system of Japanese flounder.

Transcriptomic analysis of changes in gene expression of immune proteins of gill tissue in response to low environmental temperature in fathead minnows (Pimephales promelas)

In the face of ongoing climate change, it is imperative to understand better the effects of temperature on immune function in freshwater teleosts. It is unclear whether previously observed changes were caused by temperature per se. We studied changes in the gill transcriptome of fathead minnows (Pimephales promelas) at low temperature to understand better the effects of temperature on immune function. De novo assembly of the transcriptome using Trinity software resulted in 73,378 assembled contigs. Annotation using the Trinotate package yielded 58,952 Blastx hits (accessions). Expression of 194 unique mRNA transcripts changed in gill tissue of fathead minnows acclimatized to 5° compared to controls at 22 °C. At 5 °C mRNAs coding for proteins involved in innate immune responses were up-regulated. Those included proteins that block early-stage viral replication and macrophage activation. Expression of mRNAs coding for pro-inflammatory molecules and mucus secretion were also enhanced. Messenger RNAs coding for proteins associated with adaptive immune responses were down-regulated at 5 °C. Those included antigen-presenting proteins and proteins involved in immunoglobin production. Messenger RNAs coding for proteins that stimulate the cell cycle were also down-regulated at 5 °C. Histological comparison revealed that gills of cold acclimated fish had fewer mucus cells but cells contained larger mucus droplets. We conclude that decreased temperature modifies the immune systems of freshwater teleosts, leading to genome-wide upregulation of innate immunity and down regulation of adaptive immunity. Such acclimation likely evolved as an adaptive strategy against seasonal changes in infectious insults.

Transcriptome Analysis Based on RNA-Seq in Understanding Pathogenic Mechanisms of Diseases and the Immune System of Fish: A Comprehensive Review

In recent years, with the advent of next-generation sequencing along with the development of various bioinformatics tools, RNA sequencing (RNA-Seq)-based transcriptome analysis has become much more affordable in the field of biological research. This technique has even opened up avenues to explore the transcriptome of non-model organisms for which a reference genome is not available. This has made fish health researchers march towards this technology to understand pathogenic processes and immune reactions in fish during the event of infection. Recent studies using this technology have altered and updated the previous understanding of many diseases in fish. RNA-Seq has been employed in the understanding of fish pathogens like bacteria, virus, parasites, and oomycetes. Also, it has been helpful in unraveling the immune mechanisms in fish. Additionally, RNA-Seq technology has made its way for future works, such as genetic linkage mapping, quantitative trait analysis, disease-resistant strain or broodstock selection, and the development of effective vaccines and therapies. Until now, there are no reviews that comprehensively summarize the studies which made use of RNA-Seq to explore the mechanisms of infection of pathogens and the defense strategies of fish hosts. This review aims to summarize the contemporary understanding and findings with regard to infectious pathogens and the immune system of fish that have been achieved through RNA-Seq technology.