The fight between the teleost fish immune response and aquatic viruses

Innate immune response of snakehead fish to Indian strain of snakehead rhabdovirus (SHRV-In) infection and the infectivity potential of the virus to other freshwater fishes

A new virus known as snakehead rhabdovirus (SHRV-In) was discovered in South India in striped snakehead (Channa striata) that had hemorrhagic patches and cutaneous ulcerations. The virus is the most potentially harmful pathogen of snakehead because it could cause 100% mortality within 5 days. The goal of the current investigation was to evaluate the infectivity of rhabdovirus in freshwater fishes and to analyze the immune response in snakehead fish after challenge with SHRV-In. The infectivity study of SHRV-In against three freshwater fish such as tilapia, grass carp and loach showed that the virus could not induce mortality in any of them. Snakehead fish challenged with SHRV-In showed significant (p < 0.05) changes in haematological parameters such as red blood cell (RBC), haemoglobin (HGB), haematocrit (HCT), mean corpuscular haemoglobin concentration (MCHC), mean corpuscular volume (MCV), mean corpuscular haemoglobin (MCH), white blood cell (WBC), total platelet (PLT) counts, mean platelet volume (MPV) and immunological markers such as respiratory burst, superoxide dismutase, catalase activity and myeloperoxidase activity at 6, 12, 24 and 48 hpi. Real time PCR was executed to examine the expression profile of innate immune genes such as IRF-7, IL-8 and IL-12 in Snakehead fish at 6, 12, 24 and 48 h post SHRV-In infection. Immune gene expression of IRF-7, IL-8 and IL-12 were up-regulated in the spleen when compared to kidney at 6 and 12 hpi. However, the expression level of all the genes was down-regulated at 24 and 48 hpi. The down regulation of innate immune genes after 24 hpi in these tissues may be the result of increased multiplication of SHRV-In by interfering with the immune signaling pathway.

Role of Medaka (Oryzias latipes) Foxo3 in Resistance to Nervous Necrosis Virus Infection

Upon encountering a virus, fish initiate an innate immune response, guided by IFNs. Foxo3 plays a part in the body's immune response; however, its specific role in the IFN-guided immune response in fish is yet to be clarified. In this study, we characterized foxo3 in Japanese medaka (Oryzias latipes) and examined its role in the IFN-dependent immune response upon infection with the RGNNV. The results show that the coding region of the medaka foxo3 gene is 2007 base pairs long, encoding 668 amino acids, and possesses a typical forkhead protein family structural domain. The product of this gene shares high homology with foxo3 in other fish species and is widely expressed, especially in the brain, eyes, testes, and heart. Upon RGNNV infection, foxo3-/- mutant larvae showed a lower mortality rate, and adults exhibited a significant reduction in virus replication. Moreover, the absence of foxo3 expression led to an increase in the expression of irf3, and a decrease in the expression of other IFN-related genes such as tbk1 and mapk9, implying that foxo3 may function as a negative regulator in the antiviral signaling pathway. These findings provide crucial insights for disease-resistant breeding in the aquaculture industry.

Vegetable omega-3 and omega-6 fatty acids differentially modulate the antiviral and antibacterial immune responses of Atlantic salmon

The immunomodulatory effects of omega-3 and omega-6 fatty acids are a crucial subject of investigation for sustainable fish aquaculture, as fish oil is increasingly replaced by terrestrial vegetable oils in aquafeeds. Unlike previous research focusing on fish oil replacement with vegetable alternatives, our study explored how the omega-6 to omega-3 polyunsaturated fatty acid (PUFA) ratio in low-fish oil aquafeeds influences Atlantic salmon's antiviral and antibacterial immune responses. Atlantic salmon were fed aquafeeds rich in soy oil (high in omega-6) or linseed oil (high in omega-3) for 12 weeks and then challenged with bacterial (formalin-killed Aeromonas salmonicida) or viral-like (polyriboinosinic polyribocytidylic acid) antigens. The head kidneys of salmon fed high dietary omega-3 levels exhibited a more anti-inflammatory fatty acid profile and a restrained induction of pro-inflammatory and neutrophil-related genes during the immune challenges. The high-omega-3 diet also promoted a higher expression of genes associated with the interferon-mediated signaling pathway, potentially enhancing antiviral immunity. This research highlights the capacity of vegetable oils with different omega-6 to omega-3 PUFA ratios to modulate specific components of fish immune responses, offering insights for future research on the intricate lipid nutrition-immunity interplay and the development of novel sustainable low-fish oil clinical aquaculture feeds.

Mannose modified targeted immersion vaccine delivery system improves protective immunity against Infectious spleen and kidney necrosis virus in mandarin fish (Siniperca chuatsi)

Vaccination is an effective method to prevent viral diseases. However, the biological barrier prevents the immersion vaccine from achieving the best effect without adding adjuvants and carriers. Researches on the targeted presentation technology of vaccines with nanocarriers are helpful to develop immersion vaccines for fish that can break through biological barriers and play an effective role in fish defense. In our study, functionally modified single-walled carbon nanotubes (SWCNTs) were used as carriers to construct a targeted immersion vaccine (SWCNTs-M-MCP) with mannose modified major capsid protein (MCP) to target antigen-presenting cells (APCs), against iridovirus diseases. After bath immunization, our results showed that SWCNTs-M-MCP induced the presentation process and uptake of APCs, triggering a powerful immune response. Moreover, the highest relative percent survival (RPS) was 81.3% in SWCNTs-M-MCP group, which was only 41.5% in SWCNTs-MCP group. Altogether, this study indicates that the SWCNTs-based targeted immersion vaccine induces strong immune response and provided an effective protection against iridovirus diseases.

Structural insights of Labeo catla (catla) myxovirus resistance protein,GTP binding recognition and constitutive expression induced with Poly I:C

The Myxovirus resistance (Mx) proteins are critical effectors belonging to the super-family of guanidine triphosphatase, often stimulated by type I interferon (IFN) and mediates antiviral responses to restrict the replication of numerous viral genes in fishes. In teleosts, Mx proteins display diverse and complicated antiviral activity in different species. The present investigation seeks to characterize the Mx gene from Labeo catla upon induction by double-stranded (ds) RNA, polyinosinic-polycytidylic acid, (poly I: C). Molecular modeling and all-atoms molecular dynamics (MD) simulations were employed to understand the architecture of the GTPase domain and its plausible mode of GTP recognition in Mx protein. The full-length L. catla Mx (LcMx) gene sequence (1821 bp nucleotides) encodes an open reading frame of 606 amino acids. Domain search indicated conserved tripartite domain architecture of LcMx and forms a major cluster with the Mx from other teleosts. The positively charged Arginine and polar Glutamine residues from helix 3 and 4 of stalk region LcMx aid in homo-oligomerization. MD simulation portrayed the role of conserved critical residues aid in GTP recognition by the GTPase domain which perfectly corroborates with experimental findings and prior MD studies. After injection of poly I:C, the temporal mRNA profile showed that LcMx expression was significantly elevated in the spleen, brain, kidney, liver, muscle, heart, intestine, and gill tissues. Collectively, these results suggest that the elevated expression of the major innate immune defense gene Mx was able to inhibit the poly I: C mediated virulence in fish.Communicated by Ramaswamy H. Sarma.

Mucosal immune responses of gut IgM in common carp (Cyprinus carpio) following infection with spring viremia of carp virus (SVCV)

Immunoglobulin M (IgM) specifically recognizes various antigens and can activate complement, mediate cytotoxicity, opsonize and agglutinate pathogens to induce phagocytosis, all of which play an important role in immunity. However, the IgM response of common carp (Cyprinus carpio) in the intestinal mucosa after viral infection has not been thoroughly. Therefore, we successfully produced an anti-carp IgM monoclonal antibody and developed a model of viral infection to study the kinetics of immune responses after viral infection. Our results showed that the expression of IL1-β and Igs were dramatically increased, implying that common carp exhibited a significant innate and adaptive immune response to viral infection. Furthermore, we found that the IgM responses varied between the two infection strategies. At 14 days post-infection (DPI), a significant population of IgM+ B cells were observed in the gut, accompanied by a sharp rise in IgM levels. The immune response to secondary infection started at 7 DPI, suggesting that the IgM response is faster in the gut after re-infection. Importantly, we also explored the variability of different gut compartments to viral infection, and result revealed a stronger immune response in the hindgut than in the foregut and midgut. Overall, our findings indicate that IgM plays an important role in the intestinal immune response following primary and secondary viral infection, in which the hindgut plays a major immune function.

Infection of epithelioma papulosum cyprini (EPC) cells with spring viremia of carp virus (SVCV) induces autophagy and apoptosis through endoplasmic reticulum stress

Spring viremia of carp virus (SVCV) is a lethal freshwater pathogen of cyprinid fish that has caused significant economic losses to aquaculture. To reduce the economic losses caused by SVCV, its pathogenic mechanism needs to be studied more thoroughly. Here, we report for the first time that SVCV infection of Epithelioma papulosum cyprini (EPC) cells can induce cellular autophagy and apoptosis through endoplasmic reticulum stress. The presence of autophagic vesicles in infected EPC cells was shown by transmission electron microscopy. Quantitative fluorescence PCR and Western blot results showed that p62 mRNA expression was decreased, and the expression of Beclin1 and LC3 mRNA was increased. The p62 protein was decreased, and the Beclin1 protein and LC3 were increased in the endoplasmic reticulum stress activation state. To further clarify the mode of death of SVCV-infected EPC cells, we examined caspase3, caspase9, BCL-2, and Bax mRNA, which showed that they were all increased. Apoptosis of SVCV-infected cells increased upon activation of endoplasmic reticulum stress. Our results suggest that endoplasmic reticulum stress can regulate SVCV infection-induced autophagy and apoptosis. The results of this study provide theoretical data for the pathogenesis of SVCV and lay the foundation for future drug development and vaccine construction.

Evaluation of in vitro toxicity information for zebrafish as a promising alternative for chemical hazard and risk assessment

In vitro assays are widely proposed as a test alternative to traditional in vivo standard acute and chronic toxicity tests. However, whether toxicity information derived from in vitro assays instead of in vivo tests could provide sufficient protection (e.g., 95 % of protection) for chemical risks remain evaluated. To investigate the feasibility of zebrafish (Danio rerio) cell-based in vitro test method as a test alternative, we comprehensively compared sensitivity differences among endpoints, among test methods (in vitro, FET and in vivo), and between zebrafish and rat (Rattus norvegicus), respectively using chemical toxicity distribution (CTD) approach. For each test method involved, sublethal endpoints were more sensitive than lethal endpoints for both zebrafish and rat, respectively. Biochemistry (zebrafish in vitro), development (zebrafish in vivo and FET), physiology (rat in vitro) and development (rat in vivo) were the most sensitive endpoints for each test method. Nonetheless, zebrafish FET test was the least sensitive one compared to its in vivo and in vitro tests for either lethal or sublethal responses. Comparatively, rat in vitro tests considering cell viability and physiology endpoints were more sensitive than rat in vivo test. Zebrafish was found to be more sensitive than rat regardless of in vivo or in vitro tests for each pairwise endpoint of concern. Those findings indicate that zebrafish in vitro test is a feasible test alternative to zebrafish in vivo and FET test and traditional mammalian test. It is suggesting that zebrafish in vitro test can be optimized by choosing more sensitive endpoints, such as biochemistry to provide sufficient protection for zebrafish in vivo test and to establish applications of zebrafish in vitro test in future risk assessment. Our findings are vital for evaluating and further application of in vitro toxicity toxicity information as an alternative for chemical hazard and risk assessment.

Impact of Global Warming on the Severity of Viral Diseases: A Potentially Alarming Threat to Sustainable Aquaculture Worldwide

With an ever-increasing human population, food security remains a central issue for the coming years. The magnitude of the environmental impacts of food production has motivated the assessment of the environmental and health benefits of shifting diets, from meat to fish and seafood. One of the main concerns for the sustainable development of aquaculture is the emergence and spread of infectious animal diseases in a warming climate. We conducted a meta-analysis to investigate the influence of global warming on mortality due to viral infections in farmed aquatic animals. We found a positive trend between increasing temperature and increasing viral virulence, with an increase in water temperature of 1 °C resulting in an increase in mortality of 1.47-8.33% in OsHV-1 infected oysters, 2.55-6.98% in carps infected with CyHV-3 and 2.18-5.37% in fishes infected with NVVs. We suggest that global warming is going to pose a risk of viral disease outbreaks in aquaculture and could compromise global food security.

Toll-like Receptor 3 in the Hybrid Yellow Catfish (Pelteobagrus fulvidraco ♀ × P. vachelli ♂): Protein Structure, Evolution and Immune Response to Exogenous Aeromonas hydrophila and Poly (I:C) Stimuli

As a major mediator of cellular response to viral infection in mammals, Toll-like receptor 3 (TLR3) was proved to respond to double-stranded RNA (dsRNA). However, the molecular mechanism by which TLR3 functions in the viral infection response in teleosts remains to be investigated. In this study, the Toll-like receptor 3 gene of the hybrid yellow catfish was identified and characterized by comparative genomics. Furthermore, multiple sequence alignment, genomic synteny and phylogenetic analysis suggested that the homologous TLR3 genes were unique to teleosts. Gene structure analysis showed that five exons and four introns were common components of TLR3s in the 12 examined species, and interestingly the third exon in teleosts was the same length of 194 bp. Genomic synteny analysis indicated that TLR3s were highly conserved in various teleosts, with similar organizations of gene arrangement. De novo predictions showed that TLR3s were horseshoe-shaped in multiple taxa except for avian (with a round-shaped structure). Phylogenetic topology showed that the evolution of TLR3 was consistent with the evolution of the studied species. Selection analysis showed that the evolution rates of TLR3 proteins were usually higher than those of TLR3-TIR domains, indicating that the latter were more conserved. Tissue distribution analysis showed that TLR3s were widely distributed in the 12 tested tissues, with the highest transcriptions in liver and intestine. In addition, the transcription levels of TLR3 were significantly increased in immune-related tissues after infection of exogenous Aeromonas hydrophila and poly (I:C). Molecular docking showed that TLR3 in teleosts forms a complex with poly (I:C). In summary, our present results suggest that TLR3 is a pattern recognition receptor (PRR) gene in the immune response to pathogen infections in hybrid yellow catfish.

Evaluation of Cottonseed Meal as an Alternative to Fish Meal in Diet for Juvenile Asian Red-Tailed Catfish Hemibagrus wyckioides

A 10-week trial was performed to investigate the effects of replacing fishmeal with cottonseed meal (CSM) on the growth rate, protein metabolism, and antioxidant response of Asian red-tailed catfish Hemibagrus wyckioides. Five isonitrogenous and isocaloric diets (C0, C8.5, C17.2, C25.7, and C34.4) were prepared to contain 0%, 8.5%, 17.2%, 25.7%, and 34.4% CSM replacing fishmeal, respectively. The weight gain, daily growth coefficient, pepsin, and intestinal amylase activities initially increased and then decreased with the raising dietary CSM levels; the highest values were observed in the C17.2 group (P < 0.05). However, feed cost exhibited the opposite trend. With the increasing dietary CSM levels, the protein efficiency ratio and intestinal trypsin activity decreased but feed conversion rate increased gradually; while no differences were observed among the C0, C8.5, and C17.2 groups (P > 0.05). Dietary CSM inclusion regardless of levels increased the plasma growth hormone level as well as hepatic aspartate aminotransferase (AST) and γ-glutamyl transpeptidase activities but decreased the plasma glutamate dehydrogenase and AST activities (P < 0.05). With the increasing dietary CSM levels, the plasma alkaline phosphatase (AKP) and hepatic superoxide dismutase activities decreased but malondialdehyde content increased gradually, while no differences were observed among the C0, C8.5, and C17.2 groups (P > 0.05). The plasma immunoglobulin M content and hepatic glutathione reductase activity initially increased but then decreased with the raising dietary CSM levels; the highest values were found in the C17.2 group. These results indicated that dietary CSM inclusion level up to 17.2% improved the growth rate, feed cost, digestive enzyme activity, and protein metabolism without compromising antioxidant capacity of H. wyckioide, whereas these parameters were depressed by further inclusion of CSM. CSM is a potentially cost-effective alternative plant protein source in diet of H. wyckioide.

Docosahexaenoic Acid Ameliorates the Toll-Like Receptor 22-Triggered Inflammation in Fish by Disrupting Lipid Raft Formation

BACKGROUND

Although dietary DHA alleviates Toll-like receptor (TLR)-associated chronic inflammation in fish, the underlying mechanism is not well understood.

OBJECTIVES

This study aimed to explore the role of Tlr22 in the innate immunity of large yellow croaker and investigate the anti-inflammatory effects of DHA on Tlr22-triggered inflammation.

METHODS

Head kidney-derived macrophages of croaker and HEK293T cells were or were not pretreated with 100 μM DHA for 10 h prior to polyinosinic-polycytidylic acid (poly I:C) stimulation. We executed qRT-PCR, immunoblotting, and lipidomic analysis to examine the impact of DHA on Tlr22-triggered inflammation and membrane lipid composition. In vivo, croakers (12.03 ± 0.05 g) were fed diets containing 0.2% [control (Ctrl)], 0.8%, and 1.6% DHA for 8 wk before injection with poly I:C. Inflammatory genes expression and rafts-related lipids and protein expression were measured in the head kidney. Data were analyzed by ANOVA or Student t test.

RESULTS

The activation of Tlr22 by poly I:C induced inflammation, and DHA diminished Tlr22-targeted inflammatory gene expression by 56-73% (P ≤ 0.05). DHA reduced membrane sphingomyelin (SM) and SFA-containing phosphatidylcholine (SFA-PC) contents, as well as lipid raft marker caveolin 1 amounts. Furthermore, lipid raft disruption suppressed Tlr22-induced Nf-κb and interferon h activation and p65 nuclear translocation. In vivo, expression of Tlr22 target inflammatory genes was 32-64% lower in the 1.6% DHA group than in the Ctrl group upon poly I:C injection (P ≤ 0.05). Also, the 1.6% DHA group showed a reduction in membrane SM and SFA-PC contents, accompanied by a decrease in caveolin 1 amounts, compared with the Ctrl group.

CONCLUSIONS

The activation of Tlr22 signaling depends on lipid rafts, and DHA ameliorates the Tlr22-triggered inflammation in both head kidney and head kidney-derived macrophages of croaker partially by altering membrane SMs and SFA-PCs that are required for lipid raft organization.

Viral-mediated activation and inhibition of programmed cell death

Viruses are ubiquitous intracellular genetic parasites that heavily rely on the infected cell to complete their replication life cycle. This dependency on the host machinery forces viruses to modulate a variety of cellular processes including cell survival and cell death. Viruses are known to activate and block almost all types of programmed cell death (PCD) known so far. Modulating PCD in infected hosts has a variety of direct and indirect effects on viral pathogenesis and antiviral immunity. The mechanisms leading to apoptosis following virus infection is widely studied, but several modalities of PCD, including necroptosis, pyroptosis, ferroptosis, and paraptosis, are relatively understudied. In this review, we cover the mechanisms by which viruses activate and inhibit PCDs and suggest perspectives on how these affect viral pathogenesis and immunity.

Interleukin-6 mediates lipopolysaccharide-inhibited irisin secretion in Nile tilapia (Oreochromis niloticus)

Irisin is a novel immunomodulatory adipomyokine released upon cleavage of the fibronectin type III domain-containing protein 5 (FNDC5). We aimed to examine interleukin-6 (IL-6) role in mediating irisin secretion in immunologically challenged animal and primary head kidney leukocytes cultured from tilapia. Intraperitoneal injection of lipopolysaccharide (LPS) increased plasma IL-6 levels and decreased irisin secretion, suggesting a causal relationship between the induction of IL-6 and irisin. To address this relationship, we further produced recombinant tilapia IL-6 and the anti-tilapia IL-6 polyclonal antiserum. Intraperitoneal injection of recombinant tilapia IL-6 inhibited plasma irisin levels. Consistent with this observation, LPS-induced inhibition of plasma irisin was significantly attenuated by neutralizing circulating IL-6 using an IL-6 antiserum. Besides, IL-6 treatment could inhibit irisin secretion and FNDC5 gene expression in primary cultures of tilapia head kidney leukocytes. In parallel experiments, both LPS and IL-6 blockade of irisin secretion could be reverted by IL-6 receptor antagonism. At the level of the leukocyte, IL-6 treatment also triggered rapid phosphorylation of Janus kinase 2 (JAK2) and signal transducer and activator of transcription 3 (STAT3), whereas IL-6-reduced irisin secretion could be negated by inhibiting the JAK2 and STAT3 signaling pathways. These results, as a whole, provide the first evidence that IL-6 is the mediator of LPS-inhibited irisin secretion via activation of the JAK2/STAT3 signaling pathway.

Engagement of gcFKBP5/TRAF2 by spring viremia of carp virus to promote host cell apoptosis for supporting viral replication in grass carp

Spring viremia of carp virus (SVCV) causes severe morbidity and mortality in grass carp (Ctenopharyngodon idellus) in Europe, America and several Asian countries. We found that FKBP5 (FK506-binding protein 5) is an SVCV infection response factor; however, its role in the innate immune mechanism caused by SVCV infection remains unknown. This study cloned gcFKBP5 (grass carp FKBP5) and made its mimic protein structure for function discussion. We found that gcFKBP5 expression in the primary innate immune organs of grass carp, including intestine, liver and spleen, was highly upregulated by SVCV in 24 h, with a similar result in fish cells by poly(I:C) treatment. gcFKBP overexpression aggravates viral damage to cells and increases viral replication. Furthermore, SVCV engages gcFKBP5 interacting with TRAF2 (tumour necrosis factor receptor-associated factor 2) to promote host cell apoptosis for supporting viral replication. The enhanced viral replication seems not to be due to the repression of IFN and other antiviral factors as expected. For the first time, these data show the pivotal role of gcFKBP5 in the innate immune response of grass carp to SVCV infection.

Two Duplicated Ptpn6 Homeologs Cooperatively and Negatively Regulate RLR-Mediated IFN Response in Hexaploid Gibel Carp

Src homology region 2 domain-containing phosphatase 1 (SHP1), encoded by the protein tyrosine phosphatase nonreceptor type 6 (ptpn6) gene, belongs to the family of protein tyrosine phosphatases (PTPs) and participates in multiple signaling pathways of immune cells. However, the mechanism of SHP1 in regulating fish immunity is largely unknown. In this study, we first identified two gibel carp (Carassius gibelio) ptpn6 homeologs (Cgptpn6-A and Cgptpn6-B), each of which had three alleles with high identities. Then, relative to Cgptpn6-B, dominant expression in adult tissues and higher upregulated expression of Cgptpn6-A induced by polyinosinic-polycytidylic acid (poly I:C), poly deoxyadenylic-deoxythymidylic (dA:dT) acid and spring viremia of carp virus (SVCV) were uncovered. Finally, we demonstrated that CgSHP1-A (encoded by the Cgptpn6-A gene) and CgSHP1-B (encoded by the Cgptpn6-B gene) act as negative regulators of the RIG-I-like receptor (RLR)-mediated interferon (IFN) response via two mechanisms: the inhibition of CaTBK1-induced phosphorylation of CaMITA shared by CgSHP1-A and CgSHP1-B, and the autophagic degradation of CaMITA exclusively by CgSHP1-A. Meanwhile, the data support that CgSHP1-A and CgSHP1-B have sub-functionalized and that CgSHP1-A overwhelmingly dominates CgSHP1-B in the process of RLR-mediated IFN response. The current study not only sheds light on the regulative mechanism of SHP1 in fish immunity, but also provides a typical case of duplicated gene evolutionary fates.

Transport Stress Induces Skin Innate Immunity Response in Hybrid Yellow Catfish (Tachysurus fulvidraco♀ × P. vachellii♂) Through TLR/NLR Signaling Pathways and Regulation of Mucus Secretion

The transport of live fish is a necessary step for commercial production. The skin of teleost fish is the first non-specific immune barrier against exogenous stimuli, and it plays an important protective role under transport stress. Thus, the aim of this study was to explore the skin responses to transport stress in hybrid yellow catfish (Tachysurus fulvidraco♀ × Pseudobagrus vachellii♂) through transcriptome and biochemical analyses. Water samples were collected during a simulated transport treatment. Biochemical indexes and/or gene expression in blood, skin, and mucus in fish in control groups and transport-stress groups (0 h, 2 h, 4 h, 8 h, 16 h) were assayed. The levels of total ammonia-nitrogen and nitrite-nitrogen in the water increased with increasing transport time. Comparison of skin transcriptomes between the control group and the group subjected to 16 h of transport revealed 1547 differentially expressed genes (868 up-regulated and 679 down-regulated). The results of the transcriptome analysis were validated by analyses of the expression levels of selected genes by qRT-PCR. The results indicated that the toll-like receptors and nod-like receptors signaling pathways mediate the skin's immune response to transport stress: tlr9, mfn2, and ikbke were significantly up-regulated and nfkbia and map3k7cl were significantly down-regulated under transport stress. With increasing transport time, lysozyme activity and the immunoglobulin M content in skin mucus first increased and then decreased. The number of mucous cells peaked at 8 h of transport stress, and then decreased. The mucus cells changed from types II and IV to types I, II, III, and IV. The amounts of red and white blood cells and the levels of hemoglobin and hematocrit first increased and then decreased during 16 h of transport stress. Together, the results showed that the skin responds to transport stress by activating the immune signaling pathway and regulating mucus secretion. These findings have important biological significance for selecting strains that tolerate transport, as well as economic significance for optimizing the transport conditions for scaleless fish.

Immune Response of Senegalese Sole against Betanodavirus Mutants with Modified Virulence

Nervous necrosis virus (NNV), genus Betanodavirus, the etiological agent of the viral encephalopathy and retinopathy (VER), presents a genome with two positive-sense single-stranded RNA segments. Striped jack nervous necrosis virus (SJNNV) and red-spotted grouper nervous necrosis virus (RGNNV), together with reassortants RGNNV/SJNNV, are the betanodaviruses predominantly isolated in Southern Europe. An RGNNV/SJNNV reassortant isolated from Senegalese sole (wt160) causes high mortalities in this fish species. This virus presents differences in the sequence of the 3’ non-coding region (NCR) of both segments compared to RGNNV and SJNNV reference strains. Previously, it has been reported that the reversion of two of these differences (nucleotides 1408 and 1412) in the RNA2 3’NCR to the SJNNV-type (recombinant r1408-1412) resulted in a decrease in sole mortality. In the present study, we have applied an OpenArray^® to analyse the involvement of sole immune response in the virulence of several recombinants: the r1408-1412 and two recombinants, developed in the present study, harbouring mutations at positions 3073 and 3093 of RNA1 3’NCR to revert them to RGNNV-type. According to the correlation values and to the number of expressed genes, the infection with the RNA2-mutant provoked the most different immune response compared to the immune response triggered after the infection with the rest of the viruses, and the exclusive and high upregulation of genes related to the complement system. The infection with the RNA1-mutants also provoked a decrease in mortality and their replication was delayed at least 24 h compared to the wt160 replication, which could provoke the lag observed in the immune response. Furthermore, the infection with the RNA1-mutants provoked the exclusive expression of pkr and the downregulation of il17rc.

Epitope screening of the major capsid protein within grouper iridovirus of Taiwan and the immunoprotective effect with SWCNTs as the vaccine carrier

Iridovirus can cause a mass of death in grouper, leading to huge economic loss in recent years. At present, practical vaccine is still the best way to control the outbreak of this virus. Many researches had indicated that the major capsid protein (MCP) of grouper iridovirus of Taiwan (TGIV) is an effective antigen to induce a specific immune response in grouper. However, these traditional vaccines that based on large proteins or whole organisms are faced with challenges because of the unnecessary antigenic load. Thus, in this study, we screened the dominant linear epitope within the MCP of TGIV and then, a new peptide vaccine (P2) was developed via prokaryotic expression system. Furthermore, SWCNTs was used as a vaccine carrier to enhance the immunoprotective effect. To evaluate the immunoprotective effect of this vaccine, a total of 245 fish were vaccinated with P2 (5, 10, 20 mg L^-1) and SWCNTs-P2 (5, 10, 20 mg L^-1) via immersion before being challenged with live TGIV at 28 days post immunization (d.p.i.). Results showed that the serum antibody titer, enzymatic activity, expression level of some immune-related genes (CC chemokine, IgM and TNF-α) and survival rate were significantly increased (SWCNTs-P2, 20 mg L^-1, 100%) compared to the control group (0%). These results indicated that this peptide vaccine could effectively induce specific immune response in vaccinated groupers. Functionalized SWCNTs could serve as a carrier of the peptide vaccine to enhance the immunoprotective effect via immersion. To sum up, epitope screening might be a potential way to develop an effective vaccine nowadays, and SWCNTs might provide a practical method that can be used in large-scale vaccination, especially for juvenile fish, to fight against diseases in aquaculture industry.

Characterization of mandarin fish (Siniperca chuatsi) IL-6 and IL-6 signal transducer and the association between their SNPs and resistance to ISKNV disease

In fish, interleukin-6 (IL-6) is a very important immune-regulatory cytokine that plays a polyfunctional role in inflammation, metabolism, regeneration, and neural processes. IL-6 signal transducer (IL-6ST) is a specific receptor for IL-6 and expressed mainly in immune cells and hepatocytes. In this study, the complete cDNA and genomic DNA sequences of mandarin fish (Siniperca chuatsi) IL-6 and IL-6ST genes were identified and analyzed. Quantitative real-time PCR showed that IL-6 and IL-6ST were chiefly expressed in the immune organs. After challenge with infectious spleen and kidney necrosis virus (ISKNV), the expression levels of IL-6 were significantly up-regulated after 6 h and 24 h in the head kidney and spleen, respectively (p < 0.01), the peak value for both reached at 72 h, IL-6ST increased significantly after 120 h with a peak at 168 h in the head kidney (p < 0.01) and improved markedly at 168 h in the spleen (p < 0.01). Besides, IL-6 and IL-6ST have been identified 3 and 8 single nucleotide polymorphisms (SNPs), respectively. Statistical analysis showed that one SNP locus (1625C/T) in the coding region of IL-6 was significantly related to the resistance of mandarin fish against ISKNV. The 1625C→T locus in the coding region of IL-6 is a synonymous mutation; compared with the susceptible group, the frequency of allele T in the disease resistance group was significantly higher, which may be due to the rare codon produced by the mutation affecting translation. The involvement of IL-6 and IL-6ST in response to ISKNV infection in mandarin fish clearly indicate that the role of SNP markers in IL-6 was associated with the ISKNV resistance, which was demonstrated for the first time in our results. Thus, the current study may provide fundamental information for further breeding of mandarin fish with resistance to ISKNV infection.

Virulence and Infectivity of UC, MD, and L Strains of Infectious Hematopoietic Necrosis Virus (IHNV) in Four Populations of Columbia River Basin Chinook Salmon

Infectious Hematopoietic Necrosis Virus (IHNV) infects juvenile salmonid fish in conservation hatcheries and aquaculture facilities, and in some cases, causes lethal disease. This study assesses intra-specific variation in the IHNV susceptibility of Chinook salmon (Oncorhynchus tshawytscha) in the Columbia River Basin (CRB), in the northwestern United States. The virulence and infectivity of IHNV strains from three divergent virus genogroups are measured in four Chinook salmon populations, including spring-run and fall-run fish from the lower or upper regions of the CRB. Following controlled laboratory exposures, our results show that the positive control L strain had significantly higher virulence, and the UC and MD strains that predominate in the CRB had equivalently low virulence, consistent with field observations. By several experimental measures, there was little variation in host susceptibility to infection or disease. However, a small number of exceptions suggested that the lower CRB spring-run Chinook salmon population may be less susceptible than other populations tested. The UC and MD viruses did not differ in infectivity, indicating that the observed asymmetric field prevalence in which IHNV detected in CRB Chinook salmon is 83% UC and 17% MD is not due to the UC virus being more infectious. Overall, we report little intra-species variation in CRB Chinook salmon susceptibility to UC or MD IHNV infection or disease, and suggest that other factors may instead influence the ecology of IHNV in the CRB.

Elucidating the Functional Roles of Helper and Cytotoxic T Cells in the Cell-Mediated Immune Responses of Olive Flounder (Paralichthys olivaceus)

In higher vertebrates, helper and cytotoxic T cells, referred to as CD4 and CD8 T lymphocytes, respectively, are mainly associated with adaptive immunity. The adaptive immune system in teleosts involves T cells equivalent to those found in mammals. We previously generated monoclonal antibodies (mAbs) against olive flounder (Paralichthys olivaceus) CD4 T cells, CD4-1 and CD4-2, and used these to describe the olive flounder’s CD4 Tcell response during a viral infection. In the present study, we successfully produced mAbs against CD8 T lymphocytes and their specificities were confirmed using immuno-blotting, immunofluorescence staining, flow cytometry analysis andreverse transcription polymerase chain reaction (RT-PCR). The results showed that these mAbs are specific for CD8 T lymphocytes. We also investigated variations in CD4 and CD8 T cells populations, and analyzed the expression of immune-related genes expressed by these cells in fish infected with nervous necrosis virus or immunized with thymus dependent and independent antigens. We found that both CD4 and CD8 T lymphocyte populations significantly increased in these fish and Th1-related genes were up-regulated compared to the control group. Collectively, these findings suggest that the CD4 and CD8 T lymphocytes in olive flounder are similar to the helper and cytotoxic T cells found in mammals, and Th1 and cytotoxic immune responses are primarily involved in the early adaptive immune response against extracellular antigens.

Immunogene expression analysis in betanodavirus infected-Senegalese sole using an OpenArray® platform

The transcriptomic response of Senegalese sole (Solea senegalensis) triggered by two betanodaviruses with different virulence to that fish species has been assessed using an OpenArray® platform based on TaqMan™ quantitative PCR. The transcription of 112 genes per sample has been evaluated at two sampling times in two organs (head kidney and eye/brain-pooled samples). Those genes were involved in several roles or pathways, such as viral recognition, regulation of type I (IFN-1)-dependent immune responses, JAK-STAT cascade, interferon stimulated genes, protein ubiquitination, virus responsive genes, complement system, inflammatory response, other immune system effectors, regulation of T-cell proliferation, and proteolysis and apoptosis. The highly virulent isolate, wSs160.3, a wild type reassortant containing a RGNNV-type RNA1 and a SJNNV-type RNA2 segments, induced the expression of a higher number of genes in both tested organs than the moderately virulent strain, a recombinant harbouring mutations in the protruding domain of the capsid protein. The number of differentially expressed genes was higher 2 days after the infection with the wild type isolate than at 3 days post-inoculation. The wild type isolate also elicited an exacerbated interferon 1 response, which, instead of protecting sole against the infection, increases the disease severity by the induction of apoptosis and inflammation-derived immunopathology, although inflammation seems to be modulated by the complement system. Furthermore, results derived from this study suggest a potential important role for some genes with high expression after infection with the highly virulent virus, such as rtp3, sacs and isg15. On the other hand, the infection with the mutant does not induce immune response, probably due to an altered recognition by the host, which is supported by a different viral recognition pathway, involving myd88 and tbkbp1.

Hydroxycoumarin efficiently inhibits spring viraemia of carp virus infection in vitro and in vivo

Spring viremia of carp virus (SVCV) causes devastating losses in aquaculture. Coumarin has an advantageous structure for the design of novel antiviral agents with high affinity and specificity. In this study, we evaluated a hydroxycoumarin medicine, i.e., 7-(6-benzimidazole) coumarin (C10), regarding its anti-SVCV effects in vitro and in vivo. Results showed that up to 12.5 mg/L C10 significantly inhibited SVCV replication in the epithelioma papulosum cyprini (EPC) cell line, with a maximum inhibitory rate of >97%. Furthermore, C10 significantly reduced cell death and relieved cellular morphological damage in SVCV-infected cells. Decreased mitochondrial membrane potential (ΔΨm) also suggested that C10 not only protected mitochondria, but also reduced apoptosis in SVCV-infected cells. For in vivo studies, intraperitoneal injection of C10 resulted in an anti-SVCV effect and substantially enhanced the survival rate of virus-infected zebrafish. Furthermore, C10 significantly enhanced antioxidant enzyme activities and decreased reactive oxygen species (ROS) to maintain antioxidant-oxidant balance within the host, thereby contributing to inhibition of SVCV replication. The up-regulation of six interferon (IFN)-related genes also demonstrated that C10 indirectly activated IFNs for the clearance of SVCV in zebrafish. This was beneficial for the continuous maintenance of antiviral effects because of the low viral loads in fish. Thus, C10 is suggested as a therapeutic agent with great potential against SVCV infection in aquaculture.

Efficacy of live NNV immersion vaccine immunized at low temperature in sevenband grouper, Epinephelus septemfasciatus

The objective of this study was to investigate safety and efficacy using a low-temperature immunization protocol with NNV in sevenband grouper, Epinephelus septemfasciatus. Further, NNV specific antibody post immunization and intramuscularly challenge was also evaluated. Immunization at low temperature resulted in a low titer virus infection in brain tissues without any clinical symptoms of infection such as sluggish behavior and/or spinning, rotating swimming being observed, and no mortality was observed. Post challenge, NNV titer NNV giving an RPS of 100 %, increased in brain tissues of naïve (non-immunized) sevenband grouper NNV giving an RPS of 100 %, with a cumulative mortality of 100 % at 25 days post-infection. No mortality or disease symptoms NNV giving an RPS of 100 %, as NNV giving and of 100 %, observed in the groups immunized at low temperature with live NNV giving an RPS of 100 %. NNV giving an RPS of 100 %. NNV specific antibody was not detected in live NNV vaccinated sevenband grouper. This is the first study that confirms that field-scale NNV immersion vaccine can protect sevenband grouper against lethal infection with NNV at natural seawater temperature under the gradually increased from 14.3-24.8 °C.

Comparative transcriptome analysis of pilchard orthomyxovirus (POMV) and infectious salmon anaemia virus (ISAV)

The Tasmanian Atlantic salmon (Salmo salar) aquaculture industry had remained relatively free of major viral diseases until the recent emergence of pilchard orthomyxovirus (POMV). The virus originally isolated from wild pilchards in Southern Australia is of great concern to the industry as it can cause high mortality. Despite its classification in the Orthomyxoviridae family, POMV is genetically divergent from infectious salmon anaemia virus (ISAV) and potentially represents a new genus within the family. Previous research has produced a formal case definition for clinical POMV, but the molecular events that underpin viral infection have not been characterized. Here we have undertaken a comparative transcriptome analysis of the response of Atlantic salmon kidney cells (ASK) in vitro to both POMV and ISAV using RNA sequencing, by harvesting cells at 6 and 24 h post infection (hpi). Despite their genomic differences, both orthomyxoviruses induced significant, and in some cases similar, innate antiviral responses. Early up-regulation of pathogen recognition receptor genes, RIG-I and TLR3, was observed in response to both viruses and triggered downstream interferon (IFN) responses. Interferon transcripts (IFN-alpha1 and INF-alpha2) were only induced in POMV infected cells at 24 hpi, but IFN-alpha3 was up-regulated in all time points and with both viruses. In addition, a strong induction of antiviral response genes (Mx and ISG15) was observed during the early infection with both viruses. Analysis of transcription factor binding sites in the up-regulated gene sets indicated that the host response to both viruses was largely driven by interferon regulatory factors (IRF) 1 and 2. Only three genes (slc35f2, odf2, LOC106608698) were differentially expressed in opposite directions, up-regulated with POMV and strongly down-regulated with ISAV at 24 hpi. Differential expression of these transcripts is possibly a consequence of virus divergence, but could also be associated to higher viral loads observed in the infection with POMV. Results from this study improve our understanding of the innate immune responses and host-pathogen interactions between POMV and Atlantic salmon. Early host response genes could potentially be exploited as subclinical biomarkers specific to POMV, and improved the development of tools for disease surveillance.

Oral immunization of trout fry with recombinant Lactococcus lactis NZ3900 expressing G gene of viral hemorrhagic septicaemia virus (VHSV)

This study has investigated the ability of Lactococcus lactic (NZ3900) carried G gene of viral haemorrhagic septicaemia virus (VHSV) under nisin-controlled gene expression (NICE) system in rainbow trout (O.Mykiss). Two groups of trout fry (7 ± 0.65 g) were immunized with 1 × 10¹⁰ cfu/g and 1 × 10⁸ cfu/g recombinant L. lactis NZ3900, two groups of fish were fed 1 × 10¹⁰ cfu/g and 1 × 10⁸ cfu/g L. lactis vector free, and one group was fed by the basal diet as a control. Oral immunization was done on days 1-7 and boosting was performed on days 15-21. The relative expression of IFN-1 and MX-1 genes significantly increased in head kidney of vaccinated fish depend on vaccine dosage compared to the control group. Fish in vaccinated group also showed elevated VHSV-specific antibody levels compared to the control groups. Relative percent survival (RPS), under virulent isolate VHSV challenge were estimated 62%, 78% for 10⁸ cfu/g 10¹⁰ cfu/g feed vaccinated groups 21 days post-vaccination, while groups fed similar doses of L. lactis vector free illustrated 22% and 27% RPSs, respectively. The significant reduction of viral loads (transcript levels of N gene) were detected in the immunized groups. Increased weight gain and decreased feed consumption in vaccinated group attributed to the probiotic effect were also observed. In conclusion, our results demonstrate the ability of recombinant L. lactis as oral vaccine against VHS in rainbow trout, which can be considered as effective method against different fish pathogens.

Evaluation on the antiviral effect of a hydroxycoumarin against infectious hematopoietic necrosis virus infection in vitro and in vivo

Infectious hematopoietic necrosis (IHN) caused by the viral pathogen infectious hematopoietic necrosis virus (IHNV) is a highly contagious disease of salmonid species, resulting in significant economic impact. The previous study showed a hydroxycoumarin derivative 7-[6-(2-methylimidazole) hexyloxy] coumarin (D5) significantly inhibited spring viraemia of carp virus (SVCV) infection, suggesting that D5 may be useful as a potential anti-IHNV agent. In this study, D5 at the concentration of up to 10 mg/L significantly inhibited IHNV replication in epithelioma papulosum cyprini (EPC) cells with a maximum inhibitory rate of >90%, maintained mitochondrial membrane potential (ΔΨm) levels, and decreased IHNV-induced apoptosis in virus-infected cells. As the consequence of protection on mitochondria, D5 enhanced antioxidant enzyme activities and decreased reactive oxygen species (ROS) to maintain the antioxidant-oxidant balance of IHNV-infected EPC cells. For in vivo study, D5 via intraperitoneal injection exhibited an anti-IHNV effect in the virus-infected fish by substantially enhancing the survival rate. Meanwhile, up-regulation of six interferon (IFN) related gene expressions demonstrated that D5 may activate IFN-related expressions for inhibiting IHNV replication during the early stage of viral infection, which is beneficial for the continuous antiviral action on controlling low viral loads in rainbow trout juvenile. Thus, D5 effective regulated IHNV-induced undesirable conditions to be an excellent potential therapeutic agent against IHNV infection.

Characterization of CD4-Positive Lymphocytes in the Antiviral Response of Olive Flounder (Paralichthys oliveceus) to Nervous Necrosis Virus

The presence of CD4 T lymphocytes has been described for several teleost species, while many of the main T cell subsets have not been characterized at a cellular level, because of a lack of suitable tools for their identification, e.g., monoclonal antibodies (mAbs) against cell markers. We previously described the tissue distribution and immune response related to CD3ε and CD4-1 T cells in olive flounder (Paralichthys oliveceus) in response to a viral infection. In the present study, we successfully produce an mAb against CD4-2 T lymphocytes from olive flounder and confirmed its specificity using immuno-blotting, immunofluorescence staining, flow cytometry analysis and reverse transcription polymerase chain reaction (RT-PCR). Using these mAbs, we were able to demonstrate that the CD3ε T cell populations contain both types of CD4⁺ cells, with the majority of the CD4 T cell subpopulations being CD4-1⁺/CD4-2⁺ cells, determined using two-color flow cytometry analysis. We also examined the functional activity of the CD4-1 and CD4-2 cells in vivo in response to a viral infection, with the numbers of both types of CD4 T cells increasing significantly during the virus infection. Collectively, these findings suggest that the CD4 T lymphocytes in olive flounder are equivalent to the helper T cells in mammals in terms of their properties and function, and it is the CD4-2 T lymphocytes rather than the CD4-1 T cells that play an important role in the Th1 immune response against viral infections in olive flounder.

Antiviral response of adult zebrafish (Danio rerio) during tilapia lake virus (TiLV) infection

Tilapia lake virus (TiLV) is a novel enveloped orthomyxo-like virus with a genome of 10 segments of linear negative-sense single-stranded RNA. It causes massive mortality of wild and farmed tilapia species and because of its spread in Asia, Africa, South and North America, it is considered a threat to tilapia aquaculture. Here, we have evaluated the possible use of zebrafish (Danio rerio) to study immune response and host-pathogen interactions during an infection with TiLV. Adult zebrafish were infected with TiLV by intraperitoneal (i.p) injection or by cohabitation. Increased viral load was observed in liver, spleen and kidney of i.p. injected fish at 1, 3, 6, and 14 days post infection (dpi) but not in fish from the cohabitation group (only liver was tested). We also demonstrated that in spleen and kidney i.p. injection of TiLV induced up-regulation of the expression of the immune-related genes encoding pathogen recognition receptors involved in sensing of viral dsRNA (rig-I, tlr3, tlr22), transcription factors (irf3, irf7), type I interferon (infϕ1), antiviral protein (mxa), pro-inflammatory (il-1β, tnf-α, il-8, ifnγ1-2) and anti-inflammatory (il-10) cytokines, CD4 markers (cd4-1, cd4-2), and IgM (igm). Moreover, tissue tropism of TiLV and histopathological changes were analyzed in selected organs of i.p. injected zebrafish. Our results indicate that zebrafish is a good model to study mechanisms of the TiLV infection and to follow antiviral responses.

Involvement of CD4-1 T cells in the cellular immune response of olive flounder (Paralichthys olivaceus) against viral hemorrhagic septicemia virus (VHSV) and nervous necrosis virus (NNV) infection

The occurrence of CD4 helper T cells has already been established for a number of teleost species, though, it has not been possible to analyze these responses at a cellular level due to a large lack of appropriate monoclonal antibodies (mAbs). In the present study, we produced a mAb against olive flounder (Paralichthys olivaceus) CD4-1 lymphocyte to investigate the functional activity of the cells to improve our understanding of the T cell response in this species. This mAb is specifically able to detect CD4-1 lymphocytes in olive flounder proved by immunofluorescence staining and RT-PCR analysis. In flow cytometry analysis, the number of CD4-1-positive lymphocytes was observed to gradually increase from 3 days post infection (dpi) and then reach peak at 7 dpi against two viruses challenge. As a conclusion, both the basic properties of CD4-1 T cells and its response to viral infections in olive flounder are very similar to the helper T cells in terrestrial animals.

The immunoprotective effect of whole-cell lysed inactivated vaccine with SWCNT as a carrier against Aeromonas hydrophila infection in grass carp

Aeromonas hydrophila is a strong gram-negative bacterium that can cause a mass death of grass carp, and result in the huge economic loss. Development of practical vaccines is the best way to control the outbreak of this bacterial disease. In this study, a whole-cell inactivated vaccine was obtained via sonication, and then single-walled carbon nanotubes (SWCNTs) was used to link to the bacterial lysate (BL) for a novel vaccine (SWCNTs-BL). A total of 400 fish were vaccinated with BL and SWCNTs-BL via immersion (5, 10 mg L^-1) or injection (5, 10 μg/fish) before challenge with live A. hydrophila at the 28 days post immunization (d.p.i.). The results showed that the antibody titer, enzymatic activity, expression of some immune-related genes (especially IgM and TNF-α) and RPS of fish in the injection groups were significantly increased compared to the control group after 28 d.p.i. For the immersion groups, immunological parameters were increased compared to the control group. Furthermore, the immuno-protective effects of SWCNTs-BL were better than BL. The above results indicated that BL of A. hydrophila can effectively induce specific immune response of grass carp, and BL linked with functionalized SWCNTs could enhance the protective effect of immersion immunization. Our results may provide a practical vaccine, with a simple production, to fight against bacterial diseases in aquaculture industry.

Inhibition of a novel coumarin on an aquatic rhabdovirus by targeting the early stage of viral infection demonstrates potential application in aquaculture

Spring viremia of carp virus (SVCV) is one of the most serious pathogens in aquaculture, resulting in devastating damage in cyprinid. In this study, we designed and synthesized a novel coumarin derivative (C3007) for evaluating its in vitro and in vivo anti-SVCV effects. Here, we determined that up to 25 mg/L C3007 significantly decreased SVCV protein gene expression levels in EPC cells by a maximum inhibitory rate of >95%. When C3007 was preincubated with SVCV, infectivity was significantly inhibited in vitro in a time-dependent manner, with complete inhibition at 25 mg/L. For in vivo studies, C3007 exhibited an anti-SVCV effect by substantially enhancing the survival rate of virus-infected fish via intraperitoneal injection. Although the horizontal transmission of SVCV was hindered by C3007 in a static cohabitation challenge model, it was not completely blocked, showing that the viral loads in recipient fish were obviously reduced. Thus, C3007 could potentially be used as a therapeutic agent with great potential in aquatic systems and may also be suitable for applications in pond aquaculture settings against viral transmission. Additionally, the C3007-preincubated virus induced an antiviral immune response with high levels of IFN expression, suggesting that C3007 pre-treatment could be used in vaccine development.

Transcriptome comparative analysis of immune tissues from asymptomatic and diseased Epinephelus moara naturally infected with nervous necrosis virus

Epinephelus moara is an economically important fish in Southeast Asian countries but is suffering from nervous necrosis virus (NNV) infection. A deeper understanding of the host-NNV interaction mechanisms makes sense for disease control, however, at present, the pathogenesis of natural NNV infection and the resistance mechanism in host remains poorly understood. In this study, asymptomatic and diseased E. moara with clinical symptoms of viral nervous necrosis (VNN) from a grouper farm were both detected with a positive RT-PCR signal of NNV, then transcriptome sequencing of their immune tissues (liver, spleen and kidney) were performed for comparation analysis. The de novo assemblies yielded 53,789 unigenes which had a length varied from 201 to 19,675 bp and a N50 length of 2115 bp, and 29,451 unigenes were functionally annotated, with 83, 250 and 5632 unigenes being differentially expressed in liver, spleen and kidney respectively. KEGG pathway enrichment analysis of the DEGs showed many DEGs were enriched in immune related pathways. Although the expression of class I major histocompatibility complex (MHC) was significantly higher in three immune tissues of the diseased grouper, many immune related genes, including humoral immune molecules (such as antibodies), the cellular mediated cytotoxic molecules (such as perforin) and some adhesion related genes were down regulated in the diseased grouper. Our results provided many unigenes that might play important roles in NNV resistance for further research. Furthermore, a total of 8666 unigenes containing 11,623 simple sequence repeats (SSRs) were identified, which provided useful information for screening molecular markers associated with NNV resistance in E. moara.

Immersion vaccination of Mandarin fish Siniperca chuatsi against infectious spleen and kidney necrosis virus with a SWCNTs-based subunit vaccine

Infectious spleen and kidney necrosis virus (ISKNV) cause a high mortality disease which lead to significant economic loss on mandarin fish in China. There is no effective drug or vaccine against this fatal disease at present. Meanwhile, many drugs and vaccines had no effect in many cases account of several impenetrable barriers (cell, skin and gastrointestinal tract). Here we reported an immersion subunit vaccine system (SWCNTs-MCP) encoding MCP gene of ISKNV based on single-walled carbon nanotubes (SWCNTs). To evaluate its efficacy against ISKNV, we found a stronger and longer duration immune response (serum antibody production, enzyme activities and immune-related genes expression) can be induced in fish vaccinated with SWCNTs-MCP in comparison with those vaccinated with MCP alone. Importantly, SWCNTs can increase the immune protective effect of naked subunit vaccine by ca. 23.8%. Thereby, this study demonstrates that SWCNTs as a promising carrier for subunit vaccine might be used to vaccinate large-scale juvenile mandarin fish by bath administration approach.

Differential expression of innate and adaptive immune genes in the survivors of three gibel carp gynogenetic clones after herpesvirus challenge

BACKGROUND

Accompanied with rapid growth and high density aquaculture, gibel carp has been seriously threatened by Carassius auratus herpesvirus (CaHV) since 2012. In previous study, distinct CaHV resistances and immune responses were revealed in the diseased individuals of three gibel carp gynogenetic clones (A+, F and H). However, little is known about the gene expression changes in the survivors after CaHV challenge, particularly their differences of innate and adaptive immune system between susceptible clone and resistant clone.

RESULTS

We firstly confirmed the CaHV carrier state in the survivors of three gibel carp clones after CaHV challenge by evaluating the abundances of five CaHV genes. The assay of viral loads indicated the resistant clone H possessed not only stronger resistance but also higher tolerance to CaHV. Then, 2818, 4047 and 3323 differentially expressed unigenes (DEUs) were screened from the head-kidney transcriptome profiles of survivors compared with controls from clone A+, F and H. GO and KEGG analysis suggested that a persistent immune response might sustain in resistant clone H and F, while susceptible clone A+ had a long-term impact on the circulatory system which was consistent with the major symptoms of bleeding caused by CaHV. Among the top 30 enriched pathways of specifically up-regulated DEUs in respective clones, 26, 7 and 15 pathways in clone H, F and A+ were associated with infections, diseases, or immune-related pathways respectively. In addition, 20 pathways in clone F belonged to "metabolism" or "biogenesis", and 7 pathways involved in "circulatory system" were enriched in clone A+. Significantly, we revealed the differential expression changes of IFN system genes and immunoglobulin (Ig) genes among the survivors of three clones. Finally, myosins and Igs were identified as co-expression modules which were positively or negatively correlated to CaHV viral loads respectively.

CONCLUSIONS

Our results revealed the common and distinct gene expression changes in immune and circulatory system in the survivors of three gibel carp gynogenetic clones with different CaHV resistances. The current study represents a paradigm of differential innate and adaptive immune reactions in teleost, and will be beneficial to the disease-resistance breeding of gibel carp.

Antiviral efficiency of a coumarin derivative on spring viremia of carp virus in vivo

Spring viraemia of carp (SVC) in aquaculture is challenging because there are few preventative measures and/or treatments. The previous study demonstrated that an antiviral coumarin derivative, 7-(4-(4-methyl-imidazole))-coumarin (C2), inhibits spring viremia of carp virus (SVCV) infection by targeting Nrf2-ARE signaling pathway in fish cells. Thus, we hypothesized whether C2 may be used as a potential therapeutic agent for controlling SVCV infection in aquaculture. In this study, SVCV infectivity was significantly inhibited in vitro in a dose-dependent manner by preincubation with C2. C2 was verified against SVCV in zebrafish, in which the mortality and viral titer in fish body were decreased. Like other coumarins, C2 was stable with a prolonged inhibitory half-life (3.5 days) at 15 °C in the early stage of SVCV infection. The results show that horizontal transmission of SVCV was reduced by C2 in a static cohabitation challenge model, especially for recipient fish in injection treatment, which suggested that C2 may be suitable as a possible therapeutic agent for SVCV in aquaculture. Overall, this study provides the new insight that a small molecule antiviral drug can be used to control rhabdovirus infection in fish aquacultures.

Characterization and Transcript Expression Analyses of Atlantic Cod Viperin

Viperin is a key antiviral effector in immune responses of vertebrates including the Atlantic cod (Gadus morhua). Using cloning, sequencing and gene expression analyses, we characterized the Atlantic cod viperin at the nucleotide and hypothetical amino acid levels, and its regulating factors were investigated. Atlantic cod viperin cDNA is 1,342 bp long, and its predicted protein contains 347 amino acids. Using in silico analyses, we showed that Atlantic cod viperin is composed of 5 exons, as in other vertebrate orthologs. In addition, the radical SAM domain and C-terminal sequences of the predicted Viperin protein are highly conserved among various species. As expected, Atlantic cod Viperin was most closely related to other teleost orthologs. Using computational modeling, we show that the Atlantic cod Viperin forms similar overall protein architecture compared to mammalian Viperins. qPCR revealed that viperin is a weakly expressed transcript during embryonic development of Atlantic cod. In adults, the highest constitutive expression of viperin transcript was found in blood compared with 18 other tissues. Using isolated macrophages and synthetic dsRNA (pIC) stimulation, we tested various immune inhibitors to determine the possible regulating pathways of Atlantic cod viperin. Atlantic cod viperin showed a comparable pIC induction to other well-known antiviral genes (e.g., interferon gamma and interferon-stimulated gene 15-1) in response to various immune inhibitors. The pIC induction of Atlantic cod viperin was significantly inhibited with 2-Aminopurine, Chloroquine, SB202190, and Ruxolitinib. Therefore, endosomal-TLR-mediated pIC recognition and signal transducers (i.e., PKR and p38 MAPK) downstream of the TLR-dependent pathway may activate the gene expression response of Atlantic cod viperin. Also, these results suggest that antiviral responses of Atlantic cod viperin may be transcriptionally regulated through the interferon-activated pathway.

The immune evasion strategies of fish viruses

Viral infection of a host rapidly triggers intracellular signaling events that induce interferon production and a cellular antiviral state. Viral diseases are important concerns in fish aquaculture. The major mechanisms of the fish antiviral immune response are suggested to be similar to those of mammals, although the specific details of the process require further studies. Throughout the process of pathogen-host coevolution, fish viruses have developed a battery of distinct strategies to overcome the biochemical and immunological defenses of the host. Such strategies include signaling interference, effector modulation, and manipulation of host apoptosis. This review provide an overview of the different mechanisms that fish viruses use to evade host immune responses. The basic mechanisms of immune evasion of fish virus are discussed, and some examples are provided to illustrate particular points.

Applications of transcriptomics and proteomics in understanding fish immunity

With the development of intensive aquaculture, economic losses increasingly result from fish mortality due to pathogen infection. In recent years, a growing number of researchers have used transcriptomic and proteomic analyses to study fish immune responses to exogenous pathogen infection. Integrating transcriptomic and proteomic analyses provides a better understanding of the fish immune system including gene expression, regulation, and the intricate biological processes underlying immune responses against infection. This review focuses on the recent advances in the fields of transcriptomics and proteomics, which have contributed to our understanding of fish immunity to exogenous pathogens.

SNP-based susceptibility-resistance association and mRNA expression regulation analyses of tlr7 to grass carp Ctenopharyngodon idella reovirus

Eleven single nucleotide polymorphisms (SNP) in Ctenopharyngodon idella toll-like receptor 7 (citlr7) gene, containing two in the 5'-flanking region, three within the single intron and six distributed in the coding sequence (CDS), were identified. A case-control study of 73 susceptible individuals and 67 resistant individuals was conducted to test the SNPs-based susceptibility-resistance association and mRNA expression of citlr7 to grass carp reovirus (GCRV), showing that both 820 A/G and 1726 A/G were significantly correlative sites in genotype (P < 0·05). Multifactor dimensionality reduction (MDR) analysis suggested the exertion of antiviral effects of 820 A/G might rely on SNPs interactions of citlr7 and C. idella toll-like receptor 8 (citlr8). Combining the mortality rate and citlr7 mRNA expression, it was suggested that 1726 GG-genotyped individuals might be more resistant than 1726 A/G genotyped individuals, indicating the selection on synonymous mutations in 1726 A/G might be susceptibility-resistance-type specific. In addition, haplotype analysis uncovered no significantly correlative haplotypes in citlr7. These findings may provide an in-depth insight for the further functional research of citlr7. The potential genetic markers identified may contribute to the molecular and transgenic breeding of C. idella.

De novo transcriptome assembly and analysis of differential gene expression following lipopolysaccharide challenge in Pelteobagrus fulvidraco

The yellow catfish, Pelteobagrus fulvidraco, has been recognized as an important freshwater aquaculture species in Eastern and Southeast Asia. To gain a better understanding of the immune response in P. fulvidraco, we analyzed its transcriptome following stimulation with lipopolysaccharide (LPS). Phosphate buffer saline (PBS) was used as control. Following assembly and annotation, 72,152 unigenes with an average length of 1090 bp were identified. A total of 370 differentially expressed genes (DEGs) in the P. fulvidraco were observed at 12 h post LPS treatment, including 197 up-regulated genes and 173 down-regulated genes. Clusters of Orthologous Groups of proteins (KOG/COG) annotation demonstrated that a total of 18,819 unigenes classified into 26 categories. Gene ontology (GO) analysis revealed 20 biological process subcategories, 7 cellular component subcategories and 20 molecular function subcategories. The Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis identified immune responses pathways. Quantitative reverse transcription polymerase chain reaction measured the expression of 18 genes involved in the immune response. CXCL2-like chemokine (CXCL2), goose-type lysozyme (LYZ G), and cathepsin K (CTSK) were significantly up-regulated. This study enriches the P. fulvidraco transcriptome database and provides insight into the immune response of P. fulvidraco against infection.

Inducers of salmon innate immunity: An in vitro and in vivo approach

Maintaining fish health is one of the most important aims in aquaculture. Prevention of fish diseases therefore is crucial and can be achieved by various different strategies, including most often a combination of different methods such as optimal feed and fish density, as well as strengthening the immune system. Understanding the fish innate immune system and developing methods to activate it, in an effort to prevent infections in the first place, has been a goal in recent years. In this study we choose different inducers of the innate immune system and examined their effects in vitro on the salmon cell line CHSE-214. We found that the butyrate derivatives 4-phenyl butyrate (PBA) and β-hydroxy-β-methyl butyrate (HMB) induce the expression of various innate immune genes differentially over 24-72 h. Similarly, lipids generated from fish oils were found to have an effect on the expression of the antimicrobial peptides cathelicidin and hepcidin, as well as iNOS and the viral receptor RIG-1. Interestingly we found that vitamin D3, similar as in mammals, was able to increase cathelicidin expression in fish cells. The observed induction of these different innate immune factors correlated with antibacterial activity against Aeromonas salmonicida and antiviral activity against IPNV and ISAV in vitro. To relate this data to the in vivo situation we examined cathelicidin expression in juvenile salmon and found that salmon families vary greatly in their basal cathelicidin levels. Examining cathelicidin levels in families known to be resistant to IPNV showed that these QTL-families had lower basal levels of cathelicidin in gills, than non QTL-families. Feeding fish with HMB caused a robust increase in cathelicidin expression in gills, but not skin and this was independent of the fish being resistant to IPNV. These findings support the use of fish cell lines as a tool to develop new inducers of the fish innate immune system, but also highlight the importance of the tissue studied in vivo. Understanding the response of the innate immune system in different tissues and what effect this might have on infections and downstream cellular pathways is an interesting research topic for the future.

Discovery of microRNAs associated with the antiviral immune response of Atlantic cod macrophages

MicroRNAs (miRNAs) are known to play important immunoregulatory roles in teleosts, although miRNAs involved in the antiviral immune response of Atlantic cod (Gadus morhua) were previously uncharacterised. Using deep sequencing and qPCR, the present study was conducted to identify miRNAs responsive to the viral mimic, polyriboinosinic polyribocytidylic acid (pIC) in Atlantic cod macrophages. Macrophage samples isolated from Atlantic cod (n=3) and treated with pIC or phosphate buffered saline (PBS control) for 24 and 72h were used for miRNA profiling. Following deep sequencing, DESeq2 analyses identified four (miR-731-3p, miR-125b-3-3p, miR-150-3p and miR-462-3p) and two (miR-2188-3p and miR-462-3p) significantly differentially expressed miRNAs at 24 and 72h post-stimulation (HPS), respectively. Sequencing-identified miRNAs were subjected to qPCR validation using a larger number of biological replicates (n=6) exposed to pIC or PBS over time (i.e. 12, 24, 48 and 72 HPS). As in sequencing, miR-731-3p, miR-462-3p and miR-2188-3p showed significant up-regulation by pIC. The sequencing results were not qPCR-validated for miR-125b-3-3p and miR-150-3p as up- and down-regulated miRNAs at 24 HPS, respectively; however, qPCR results showed significant up-regulation in response to pIC stimulation at later time points (i.e. 48 and/or 72 HPS). We also used qPCR to assess the expression of other miRNAs that were previously shown as immune responsive in other vertebrates. qPCR results at 48 and/or 72 HPS revealed that miR-128-3-5p, miR-214-1-5p and miR-451-3p were induced by pIC, whereas miR-30b-3p and miR-199-1-3p expression were repressed in response to pIC. The present study identified ten pIC-stimulated miRNAs, suggesting them as important in antiviral immune responses of Atlantic cod macrophages. Some pIC-responsive miRNAs identified in this study were predicted to target putative immune-related genes of Atlantic cod (e.g. miR-30b-3p targeting herc4), although the regulatory functions of these miRNAs need to be validated by future studies.

Molecular cloning of MDA5, phylogenetic analysis of RIG-I-like receptors (RLRs) and differential gene expression of RLRs, interferons and proinflammatory cytokines after in vitro challenge with IPNV, ISAV and SAV in the salmonid cell line TO

The RIG-I receptors RIG-I, MDA5 and LGP2 are involved in viral recognition, and they have different ligand specificity and recognize different viruses. Activation of RIG-I-like receptors (RLRs) leads to production of cytokines essential for antiviral immunity. In fish, most research has focused on interferons, and less is known about the production of proinflammatory cytokines during viral infections. In this study, we have cloned the full-length MDA5 sequence in Atlantic salmon, and compared it with RIG-I and LGP2. Further, the salmonid cell line TO was infected with three fish pathogenic viruses, infectious pancreatic necrosis virus (IPNV), infectious salmon anaemia virus (ISAV) and salmonid alphavirus (SAV), and differential gene expression (DEG) analyses of RLRs, interferons (IFNa-d) and proinflammatory cytokines (TNF-α1, TNF-α2, IL-1β, IL-6, IL-12 p40s) were performed. The DEG analyses showed that the responses of proinflammatory cytokines in TO cells infected with IPNV and ISAV were profoundly different from SAV-infected cells. In the two aforementioned, TNF-α1 and TNF-α2 were highly upregulated, while in SAV-infected cells these cytokines were downregulated. Knowledge of virus recognition by the host and the immune responses during infection may help elucidate why and how some viruses can escape the immune system. Such knowledge is useful for the development of immune prophylactic measures.

Molecular cloning and functional characterization of interferon regulatory factor 7 of the barbel chub, Squaliobarbus curriculus

The interferon regulatory factor 7 (IRF7) is a critical regulator of type-I interferon-dependent immune reaction that defense against virus. To investigate the antiviral function of IRF7 of barbel chub Squaliobarbus curriculus (ScIRF7), the cDNA of ScIRF7 was cloned and characterized. The full length cDNA of ScIRF7 was 1870 bp, consisted of 41 bp 5'-UTR, 560 bp 3'-UTR and a 1269 bp open reading frame (ORF). The ORF encoded 423 amino acids with a molecular weight of 49.426 KDa and a theoretical isoelectric point of 5.71. The putative ScIRF7 protein possesses typical domains of IRF family including a conserved N-terminal DBD-binding domain (DBD), a C-terminal IRF association domain and a serine-rich domain. In the DBD, four tryptophans were found to be highly conserved among all species, whilst in another conserved tryptophan site of mammals, the corresponding amino acids were methionine for fishes. The expression level of ScIRF7 was highest in the spleen and lowest in the liver. The expression level of IFN-β was highest in the gill and lowest in the liver. After GCRV infection, expression levels changes of ScIRF7 showed an overall tendency of firstly up-regulation and then down-regulation in the spleen and the gill; and expression levels of ScIRF7 in peripheral blood lymphocyte at 24 h post-infection was highest among all time points. In pEGFP-ScIRF7 overexpressing cells, the mRNA level of ScIRF7 was firstly up-regulation and then down-regulation; and the expression of IFN-β was significantly up-regulated at 12 h post-infection than that of control group (P < 0.05), which was significantly higher than those in pEGFP-N1 overexpressing cells. The results indicated that ScIRF7 may play a key role in immune responses of barbel chub Squaliobarbus curriculus against GCRV and may also functions in the Ctenopharyngodon idellus kidney cells.

Distinct herpesvirus resistances and immune responses of three gynogenetic clones of gibel carp revealed by comprehensive transcriptomes

Background

Gibel carp is an important aquaculture species in China, and a herpesvirus, called as Carassius auratus herpesvirus (CaHV), has hampered the aquaculture development. Diverse gynogenetic clones of gibel carp have been identified or created, and some of them have been used as aquaculture varieties, but their resistances to herpesvirus and the underlying mechanism remain unknown.

Results

To reveal their susceptibility differences, we firstly performed herpesvirus challenge experiments in three gynogenetic clones of gibel carp, including the leading variety clone A⁺, candidate variety clone F and wild clone H. Three clones showed distinct resistances to CaHV. Moreover, 8772, 8679 and 10,982 differentially expressed unigenes (DEUs) were identified from comparative transcriptomes between diseased individuals and control individuals of clone A⁺, F and H, respectively. Comprehensive analysis of the shared DEUs in all three clones displayed common defense pathways to the herpesvirus infection, activating IFN system and suppressing complements. KEGG pathway analysis of specifically changed DEUs in respective clones revealed distinct immune responses to the herpesvirus infection. The DEU numbers identified from clone H in KEGG immune-related pathways, such as “chemokine signaling pathway”, “Toll-like receptor signaling pathway” and others, were remarkably much more than those from clone A⁺ and F. Several IFN-related genes, including Mx1, viperin, PKR and others, showed higher increases in the resistant clone H than that in the others. IFNphi3, IFI44-like and Gig2 displayed the highest expression in clone F and IRF1 uniquely increased in susceptible clone A⁺. In contrast to strong immune defense in resistant clone H, susceptible clone A⁺ showed remarkable up-regulation of genes related to apoptosis or death, indicating that clone A⁺ failed to resist virus offensive and evidently induced apoptosis or death.

Conclusions

Our study is the first attempt to screen distinct resistances and immune responses of three gynogenetic gibel carp clones to herpesvirus infection by comprehensive transcriptomes. These differential DEUs, immune-related pathways and IFN system genes identified from susceptible and resistant clones will be beneficial to marker-assisted selection (MAS) breeding or molecular module-based resistance breeding in gibel carp.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3945-6) contains supplementary material, which is available to authorized users.

Molecular Characterisation of a Novel Isoform of Hepatic Antimicrobial Peptide, Hepcidin (Le-Hepc), from Leiognathus equulus and Analysis of Its Functional Properties In Silico

Hepcidin represents a family of cysteine-rich antimicrobial peptides that are mainly expressed in the liver of living organisms. In this study, we have identified and characterised a novel isoform of hepcidin from the common pony fish, Leiognathus equulus (Le-Hepc). A 261-bp fragment cDNA coding for 86 amino acids was obtained. Homologous analysis showed that Le-Hepc belongs to the hepcidin super family and shares sequence identity with other known fish pre-propeptide hepcidin sequences. The ORF encodes for a 24-amino acid (aa) signal peptide coupled to a 36-aa prodomain followed by a 26-aa mature peptide. The mature peptide region has a calculated molecular weight of 2.73 kDa, a net positive charge of +2 and a theoretical pI of 8.23. Phylogenetic analysis of Le-Hepc showed a strong relationship with other fish hepcidin sequences and clustered into HAMP2 group hepcidins. Secondary structural analysis indicated that Le-Hepc mature peptide contains two antiparallel β-sheets strengthened by four disulphide bonds formed by eight conserved cysteine residues. The physicochemical properties of the peptide and its structural parameters are in agreement with characteristic features of an antimicrobial peptide. This is the first report of an antimicrobial peptide from the common pony fish, L. equulus.

Development of a monoclonal antibody against the CD3ε of olive flounder (Paralichthys olivaceus) and its application in evaluating immune response related to CD3ε

The T cell receptor (TCR) is the binding site of antigen and is responsible for specifically activating the adaptive immune response. CD3, an essential component of the CD3-TCR complex, is known to be composed of γδ and ε chains in teleost. However, there are few monoclonal antibodies (mAb) available to identify these molecules on T cells, so we aimed to produce a mAb against CD3ε to improve our understanding of T cell immune response in olive flounder (Paralichthys olivaceus). CD3ε recombinant protein was expressed in yeast, the expression of which was confirmed by SDS-PAGE, MALDI-TOF/TOF MS and Western blot analysis. A CD3ε-specific mAb 4B2 was selected, the specificity of which was examined by confocal microscopy, flow cytometry and RT-PCR, and the mAb was subsequently used to examine the CD3ε lymphocyte population in several different immune organs, with relatively high percentages of these cells seen in trunk-kidney and spleen, while lower percentages were seen in the liver and peripheral blood of olive flounder. During a viral hemorrhagic septicemia virus (VHSV) infection in olive flounder, the number of CD3ε lymphocytes was seen to gradually increase in the liver, spleen and trunk-kidney of infected fish until 7 days post infection (dpi). In peripheral blood, on the other hand, the increase in CD3ε lymphocyte numbers peaked by 3 dpi. These results suggest that CD3ε lymphocytes might be involved in the immune response against VHSV.