Stabilized fermentation product of Cetobacterium somerae improves gut and liver health and antiviral immunity of zebrafish

Probiotics are widely used in aquafeeds and exhibited beneficial effects on fish by improving host health and resisting pathogens. However, probiotics applied to aquaculture are mainly from terrestrial sources instead of the host animal. The purpose of the work was to evaluate the effects of stabilized fermentation product of commensal Cetobacterium somerae XMX-1 on gut, liver health and antiviral immunity of zebrafish. A total of 240 zebrafish were assigned to the control (fed a basal diet) and XMX-1 group (fed a basal diet with 10 g XMX-1/kg diet). After four weeks feeding, growth performance, feed utilization, hepatic steatosis score, TAG, lipid metabolism related genes and serum ALT were evaluated. Furthermore, serum LPS, the expression of Hif-1α, intestinal inflammation score, antioxidant capability and gut microbiota were tested. The survival rate and the expression of antiviral genes were analyzed after challenge by spring viremia of carp virus (SVCV). Results showed that dietary XMX-1 did not affect growth of zebrafish. However, dietary XMX-1 significantly decreased the level of serum LPS, intestinal inflammation score and intestinal MDA, as well as increased T-AOC and the expression of Hif-1α in zebrafish intestine (p < 0.05). Furthermore, XMX-1 supplementation decreased the relative abundance of Proteobacteria and increased Firmicutes and Actinobacteria. Additionally, XMX-1 supplementation significantly decreased hepatic steatosis score, hepatic TAG, serum ALT and increased the expression of lipolysis genes versus control (p < 0.05). Zebrafish fed XMX-1 diet exhibited higher survival rate after SVCV challenge. Consistently, dietary XMX-1 fermentation product increased the expression of IFNφ2 and IFNφ3 after 2 days of SVCV challenge and the expression of IFNφ1, IFNφ2 and MxC after 4 days of SVCV challenge in the spleen in zebrafish versus control (p < 0.05). In conclusion, our results indicate that dietary XMX-1 can improve liver and gut health, while enhancing antiviral immunity of zebrafish.

Cytokines Studied in Carp (Cyprinus carpio L.) in Response to Important Diseases

Cytokines belong to the most widely studied group of intracellular molecules involved in the function of the immune system. Their secretion is induced by various infectious stimuli. Cytokine release by host cells has been extensively used as a powerful tool for studying immune reactions in the early stages of viral and bacterial infections. Recently, research attention has shifted to the investigation of cytokine responses using mRNA expression, an essential mechanism related to pathogenic and nonpathogenic-immune stimulants in fish. This review represents the current knowledge of cytokine responses to infectious diseases in the common carp (Cyprinus carpio L.). Given the paucity of literature on cytokine responses to many infections in carp, only select viral diseases, such as koi herpesvirus disease (KHVD), spring viremia of carp (SVC), and carp edema virus disease (CEVD), are discussed. Aeromonas hydrophila is one of the most studied bacterial pathogens associated with cytokine responses in common carp. Therefore, the cytokine-based immunoreactivity raised by this specific bacterial pathogen is also highlighted in this review.

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.

Immersion immunization of common carp with bacterial ghost-based DNA vaccine inducing prophylactic protective immunity against spring viraemia of carp virus

The interactive applications of immunization route, vaccine type and delivery vectors are emerging as a key area of research within the field of mass immunization in fishery production. In an effort to improve DNA vaccine's immune efficiency in large-scale immunization, a promising bacterial ghost-loaded DNA vaccine was constructed based on Escherichia coli DH5α. In common carp was investigated the immune response to immersion immunization via related indicator analysis, and the challenge test of spring viraemia of carp virus (SVCV) was carried out. The result indicated that BG-loaded DNA vaccine induced higher serum antibody level than naked pEG-G. Simultaneously, the immunophysiological indicators and genes change at the more advanced levels in the BG/pEG-G immune group. At the treatment concentration of 20 mg/L of the BG/pEG-G group, IgM and IgZ expressions in vivo were markedly increased by 21.62 times and 6.91 times, respectively, and the relative percentage survival reached the peak of 59.57%. This study paves the way for future aquatic animal vaccine research, which aimed to develop the highly effective immersion vaccine system by delivery vectors, with the ultimate aim to prevent and restrict SVCV in actual production.

Optimization of immunization procedure for SWCNTs-based subunit vaccine with mannose modification against spring viraemia of carp virus in common carp

Immersion vaccination of single-walled carbon nanotubes loaded with mannose-modified glycoprotein (SWCNTs-MG) vaccine has been proved to be effective in preventing spring viraemia of carp virus (SVCV). Immunization procedure has immense consequence on the immune effect of the immersion vaccine. However, immunization procedure optimization for SWCNTs-MG vaccine against SVCV has not been reported. In this study, accordingly, a full-factor experiment was designed to optimize the immunization procedure of SWCNTs-MG vaccine by three aspects of vaccine dose (30 mg/L, 40 mg/L and 50 mg/L), immunization density (8 fish L^-1 , 24 fish L^-1 and 48 fish L^-1 ) and immunization time (6, 12 and 24 hr). Furthermore, we used the immunization group (A1B2C1, 30 mg/L, 24 fish L^-1 and 6 hr) in the previous study as a positive control (PC) to evaluate the immunization effect optimized conditions from the expression of immune-related genes and relative percentage survival (RPS). At 28 days post-vaccination (DPV), common carps were intraperitoneal injected SVCV challenged test indicated that the A1B2C2 group (30 mg/L, 24 fish L^-1 , 12 hr) displayed superiority of protective efficacy compare with other groups and the RPS with 77.9%, which was 15.6% higher than the PC group of RPS with 62.3%. Moreover, the expression of immune-related genes such as IL-10, CD4 and MHC-II was also significantly higher than PC group. The specific experimental flow chart is shown in Figure 1. Conclusively, these results demonstrated that vaccine dose, immunization density and immunization time are 30 mg/L, 24 fish L^-1 and 12 hr, which is the more appropriate immunization programme with juvenile carp for SWCNTs-MG vaccine. This study provides a profitable reference for improving the immune efficiency of aquatic immersion vaccine. [Figure: see text].

Azoxystrobin increases the infection of spring viraemia of carp virus in fish

Azoxystrobin (AZ) has entered aquatic ecosystems and produced serious damages to fish associated with potentially increasing the susceptibility to pathogens. This study characterized the defense abilities of fish by exposed to AZ on challenging with the infection of spring viraemia of carp virus (SVCV). The results showed that SVCV replication increased significantly in EPC cells and zebrafish that were exposed to up to 50 μg/L of AZ at 3, 5, 7, and 14 d. Intracellular biochemical assays indicated that AZ at 5 and 50 μg/L inhibited the activation of Nrf2-ARE pathway including a decrease in Nrf2 expression, Nrf2 phosphorylation, HO-1 content, and three antioxidant activities. While no significant difference in ERK1/2 and JNK MAPKs in zebrafish was observed, P38 phosphorylation was significantly decreased at 7 and 14 d, and the changes in MAPKs were more evident in EPC cells previously exposed to AZ at 7 d. These results revealed that AZ initially induced low phosphorylation of MAPKs, triggering the attenuation of Nrf2 phosphorylation to weaken Nrf2 translocation into the nucleus in a longer exposure period (more than 5 d). The data in the cells and fish also showed that antioxidant activities were decreased to some extent at 5-7 d for the cells and 7-14 d for the fish. Furthermore, interferon-related factors were decreased in AZ-exposed zebrafish, explaining the reason that fish can't resist the virus infection. Overall, the present study provided a new adverse threat of AZ by amplifying the viral outbreak to endanger ecological safety in aquatic environment.

Dynamic Interaction Between Mucosal Immunity and Microbiota Drives Nose and Pharynx Homeostasis of Common Carp (Cyprinus carpio) After SVCV Infection

The crosstalk between the immune system and microbiota drives an amazingly complex mutualistic symbiosis. In mammals, the upper respiratory tract acts as a gateway for pathogen invasion, and the dynamic interaction between microbiota and mucosal immunity on its surface can effectively prevent disease development. However, the relationship between virus-mediated mucosal immune responses and microbes in lower vertebrates remains uncharacterized. In this study, we successfully constructed an infection model by intraperitoneally injecting common carp (Cyprinus carpio) with spring viremia of carp virus (SVCV). In addition to the detection of the SVCV in the nose and pharynx of common carp, we also identified obvious histopathological changes following viral infection. Moreover, numerous immune-related genes were significantly upregulated in the nose and pharynx at the peak of SVCV infection, after which the expression levels decreased to levels similar to those of the control group. Transcriptome sequencing results revealed that pathways associated with bacterial infection in the Toll-like receptor pathway and the Nod-like receptor pathway were activated in addition to the virus-related Rig-I-like receptor pathway after SVCV infection, suggesting that viral infection may be followed by opportunistic bacterial infection in these mucosal tissues. Using 16S rRNA gene sequencing, we further identified an upward trend in pathogenic bacteria on the mucosal surface of the nose and pharynx 4 days after SVCV infection, after which these tissues eventually reached new homeostasis. Taken together, our results suggest that the dynamic interaction between mucosal immunity and microbiota promotes the host to a new ecological state.

PEG-modified subunit vaccine encoding dominant epitope to enhance immune response against spring viraemia of carp virus

Spring viraemia of carp (SVC) caused by spring viraemia of carp virus (SVCV) can infect almost all fish of cyprinids, which bring huge economic losses to aquaculture. Glycoprotein (G), as the most important antigenic determinant protein of SVCV, is widely considered as an effective method against SVCV. In our previous study, we found that G3 (131 aa) is the potential dominant antigen epitope that induces strong immune responses similar to G protein (510 aa). Here, in order to further improve the immune effect, we reported a subunit vaccine (PEG-G3) constructed by PEG-modified dominant epitope protein (G3). The results of serum antibody production, enzyme activities and immune-related genes expression showed that PEG-G3 induces significantly stronger immune protective responses against SVCV than G3. PEG modification significantly increased the serum antibody level of the vaccine, which increased significantly after immunization and reached the peak at 21 day post-vaccination. T-AOC and AKP activities in the lowest concentration group (5 μg) of PEG-G3 were significantly higher than those in the highest concentration group (20 μg) of G3. In PEG-G3 group, the expression of almost all genes increased at least 4 times compared with the control group. After 14-day challenge, the RPS (relative percentage survival) of the highest concentration of PEG-G3 group was 53.6%, while that of G3 group is 38.9%. Therefore, this work shows that PEG modification and dominant epitope screening may be effective methods to improve the immune protective effect of vaccines and to resist the infection of aquatic animal viral diseases.

Viperin_sv1 promotes RIG-I expression and suppresses SVCV replication through its radical SAM domain

SVCV infection is known to activate the host's innate immune responses, including the production of interferon (IFN) and interferon-stimulated genes (ISGs). Viperin_sv1 is a novel splice variant of viperin, which is induced during SVCV infection and proves to positively regulate the IFN activation and production. However, the underlying mechanism remains unsolved. In this study, the P protein of SVCV was identified to be the key to induce the mRNA modification and production of viperin_sv1 during the virus infection. Besides, Viperin_sv1 was able to trigger the RLR signaling cascades to activate type-1 interferon response. Additional analysis revealed that viperin_sv1 promoted the stability and function of RIG-I, which result in the production of IFN and ISGs. Moreover, the central SAM domain of viperin_sv1 was demonstrated to be essential for regulating RIG-I protein expression and inducing IFN production. Furthermore, this study also showed that SVCV replication could be inhibited by the viperin_sv1 SAM domain. In conclusion, our study demonstrates that viperin_sv1 reduces the replication of SVCV by promoting the RIG-I protein expression. Our findings identified the antiviral function played by the SAM domain of viperin_sv1 and suggested an antiviral mechanism that is conserved among different species.

Autophagy Delays Apoptotic Cell Death Induced by Siniperca chuatsi Rhabdovirus in Epithelioma Papulosum Cyprinid Cells

Autophagy and apoptosis are two key cell fate determination pathways, which play vital roles in the interaction between viruses and host cells. Previous research had confirmed that one strain of fish rhabdoviruses, Siniperca chuatsi rhabdovirus (SCRV), could induce apoptosis and autophagy after infection. In the current study, we continued to analyze the interaction of autophagy and apoptosis in SCRV-infected EPC cell lines after treatment with different autophagy or apoptosis inhibitors. We found that SCRV infection could activate the mitochondrial apoptotic pathway by the detection of the activities of the caspase-3 and caspase-9 and by flow cytometry analysis in JC-1-stained cells, respectively. Furthermore, no significant autophagy-related factors were disturbed in SCRV-infected cell after apoptosis inhibitor Z-VAD-FMK treatment, while autophagy inducer rapamycin could obviously delay the occurrence of CPE and cell death. Meanwhile, rapamycin was able to reduce the proportion of apoptotic cells. Besides that, rapamycin could disturb the expression of p62 and LC3B-II, and the transcription level of SCRV nucleoprotein mRNA. The progeny virus titers did not show a big difference between the rapamycin treatment or without it. Collectively, our data preliminarily confirmed that SCRV-activated autophagy could delay apoptosis in EPC cells and may not affect virus production. Further study may need to focus on the crosstalk regulation and its roles on the SCRV infection.

Interactions Between Commensal Microbiota and Mucosal Immunity in Teleost Fish During Viral Infection With SVCV

The mucosa of vertebrates is a particularly complex but dynamic environment in which the host constantly interacts with trillions of commensal microorganisms and pathogens. Although the internal and external mucosal microbiomes with immune defense of mammals have been well investigated, the relationship between mucosal microbes and their host’s immune responses has not been systematically understood in the early vertebrates. In this study, we compared the composition and distribution of mucosal microbiota in common carp (Cyprinus carpio), and found that there were significant differences of microbiota between in the internal (gut) and external mucosal (buccal mucosa, gills and skin) tissues. Next, we successfully constructed an infection model with spring viremia of carp virus (SVCV). Specifically, following viral infection, the immune and antiviral related genes showed different up-regulation in all selected mucosal tissues while significant morphological changes were only found in external tissues including buccal mucosa, gills and skin. Using 16S rRNA gene sequence, we revealed that the abundance of Proteobacteria in mucosal tissues including buccal mucosa, gills and gut showed increased trend after viral infection, whereas the abundance of Fusobacteria significantly decreased in gut. In addition, the loss of dominant commensal microorganisms and increased colonization of opportunistic bacteria were discovered in the mucosal surfaces indicating that a secondary bacterial infection might occur in these mucosal tissues after viral infection. Overall, our results firstly point out the distribution of internal and external mucosal microbiota and analyze the changes of mucosal microbiota in common carp after SVCV infection, which may indicated that the potential role of mucosal microbiota in the antiviral process in early vertebrates.

Replication of heterologous glycoprotein-expressing chimeric recombinant snakehead rhabdoviruses (rSHRVs) and viral hemorrhagic septicemia viruses (rVHSVs) at different temperatures

Water temperature is an important environmental factor for the outbreaks of fish rhabdovirus diseases. In the present study, to know the role of piscine rhabdoviral glycoproteins in the determination of replication temperature, several chimeric snakehead rhabdoviruses (SHRVs) and viral hemorrhagic septicemia viruses (VHSVs) expressing heterologous glycoproteins (rSHRV-Gvhsv, SHRV expressing VHSV G protein; rSHRV-Gsvcv, SHRV expressing spring viremia of carp virus G protein; rVHSV-Gshrv, VHSV expressing SHRV G protein; rVHSV-Gsvcv, VHSV expressing SVCV G protein) were generated using reverse genetics, and their replication characteristics at different temperatures were investigated. Furthermore, based on SHRV minigenome containing a reporter gene, the role of VHSV N, P, and L proteins in the determination of VHSV's low-temperature replication was investigated. In Epithelioma papulosum cyprini (EPC) cells, rSHRV-Gvhsv could replicate only at low temperatures (15 and 20 °C) but not at 25 and 28 °C, while rSHRV-Gsvcv could replicate both low and high temperatures, indicating that VHSV G protein is a critical factor that determines the limit of replication-possible temperatures in VHSV. The range of replication-possible temperature of chimeric VHSVs (rVHSV-Gshrv and rVHSV-Gsvcv) was not different from that of rVHSV-wild (replicated only at 15 and 20 °C) in spite of having the G protein of high temperature-replicating viruses, suggesting that not only G protein but also other viral protein(s) would be involved in the determination of replication-possible temperature limit in VHSV. Cells harboring SHRV minigenome that expressing eGFP as a reporter protein were co-transfected with heterologous combinations of helper plasmids of SHRV and VHSV, through which we could exclude VHSV N and P proteins for the low-temperature replication of VHSV, because cells harboring SHRV minigenome showed fluorescence at high temperatures when VHSV N or P protein encoding plasmids were supplied. However, no fluorescence was observed in cells co-transfected with plasmids encoding SHRV N, SHRV P and VHSV L protein at all tested temperatures, suggesting that the combination of SHRV N, P and VHSV L proteins could not form a functional ribonucleoprotein (RNP) complex. Although we could not directly demonstrate the involvement of VHSV L protein in the temperature limit of VHSV replication, it is highly probable that not only VHSV G protein but also VHSV L protein may participate in the determination of VHSV replication temperature.

A Seasonal Study of Koi Herpesvirus and Koi Sleepy Disease Outbreaks in the United Kingdom in 2018 Using a Pond-Side Test

Simple Summary

Cyprinid herpesvirus (CyHV)-3 and carp edema virus (CEV), the causative agents of koi herpesvirus disease and koi sleepy disease, respectively, are emerging DNA viruses infecting koi and common carp. Similarities in their clinical presentation present difficulties for its on-site identification based on gross pathology. Fluorescence real-time loop-mediated isothermal amplification (LAMP) assays for detecting CyHV-3 and CEV DNA were designed to use border inspection posts and local testing by national authorities for outbreak control. The limit of these tests’ detection (10² and 10³ viral copies for CyHV-3 and CEV, respectively) allows for the amplification of viral DNA in clinical samples in less than 20 min. The assays’ field performance was tested with 63 common carp mucus swabs taken during disease investigations in 2018, and the results validated with the reference laboratory analysis. Overall, the good performance, ease of use, and cost-effectiveness of these tests make them good candidates for a point of care test. However, further work is required to incorporate reliable internal controls and improve the sensitivity of these tests’ asymptomatic testing.

Abstract

Fluorescence real-time LAMP assays were designed for the orf43 gene of CyHV-3 European genotype and the p4a gene of the CEV genogroup I. A third LAMP assay to detect the ef1a gene of the host common carp was designed as an internal control. The limit of detection was 10² and 10³ viral copies under 25 min for CyHV-3 and CEV, respectively. The specificity of the CyHV-3 LAMP assay was 95.6% of 72 fish herpesviruses tested. Sixty-three non-lethal common carp mucus swabs were collected across 16 sites during disease investigations. DNA extractions were performed in under 10 min using the QuickExtract™ digestion buffer. The LAMP amplification of CyHV-3 DNA in mucus swabs from clinical cases was detected from 4 to 13 min in 13 sites, while a co-infection of CyHV-3 and CEV was confirmed by LAMP in a single site. The LAMP results agreed with the results of the reference laboratory. The common carp ef1a was amplified only in 61% of the mucus swabs collected, preventing its use as a robust internal control to distinguish false negatives from invalid tests. After further optimization, these tests could be implemented for border inspection posts surveillance and decentralizing testing during disease outbreaks.

Red elemental selenium (Se0 ) improves the immunoactivities of EPC cells, crucian carp and zebrafish against spring viraemia of carp virus

Selenium, as an essential trace element, interferes through selenoproteins in many physiological processes of plants and mammals. Its antiviral activity has recently attracted much attention because selenium improves the antiviral capacity of animal cells against a few viruses relevant to human diseases. In this study, the red elemental selenium was purified from the fermentative culture of Herbaspirillum camelliae WT00C and then used to culture epithelioma papulosum cyprinid (EPC) cells or feed crucian carp and zebrafish. Finally, its antiviral effects were investigated at the cell level and living fishes after spring viraemia of carp virus infection. At the cell level, 5, 10 and 20 μg ml^-1 red elemental selenium significantly induced the expression of interferon (IFN) and ISG15 genes in EPC cells. The viral TCID₅₀ (50% tissue culture infective dose) values in the EPC cells incubated with 5, 10 and 20 μg ml^-1 red elemental selenium were significantly less than those of the control. More expression of IFN and ISG15 genes and less TCID₅₀ values indicate that red elemental selenium indeed improves the antiviral capability of EPC cells. In the crucian carp fed with the food containing 5 and 10 μg g^-1 red elemental selenium, IFN expressions showed 13- and 39-fold increases at the 16th day of post-injection, and its expression was dependent on selenium concentrations. Meanwhile, no fish death occurred in all the experimental groups. In the zebrafish fed with the red worm containing 5 μg g^-1 red elemental selenium, IFN and Mx expressions and survival rate were significantly higher than those of the control. The results of this study show that red elemental selenium indeed improves the antiviral activity of fish. The antiviral effects of selenium mainly come from its immune regulation through its incorporation into selenoproteins. The optimum level of selenium contributes to improving fish immunity, whereas excess selenium causes excessive immune and inflammatory responses.

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.

Oral immunization of carps with chitosan-alginate microcapsule containing probiotic expressing spring viremia of carp virus (SVCV) G protein provides effective protection against SVCV infection

Spring viremia of carp (SVC) is highly contagious and lethal disease in cyprinid fish, in particular common carps (Cyprinus carpio), causing numerous economic losses to the aquaculture industry. SVC is presently endemic disease in Europe, America, and several countries in Asia and its causative agent is spring viremia of carp virus (SVCV). In this study, a chitosan-alginate microcapsule probiotic vaccine expressing G protein of SVCV was prepared, and the immunogenicity in carps of orally administrated with the microcapsule probiotic vaccine was evaluated. Our results showed that the microcapsule probiotic vaccine can induce potent antigen-specific immune responses in carps via oral vaccination, and provide effective anti-SVCV protection for carps. Significantly, the microcapsule probiotic vaccine is suitable for mass fish immunization, suggesting a promising vaccine strategy for fish.

Viral infection-induced changes in the expression profile of non-RLR DExD/H-box RNA helicases (DDX1, DDX3, DHX9, DDX21 and DHX36) in zebrafish and common carp

In mammals, several non-RLR DExD/H-box RNA helicases are involve in sensing of viral nucleic acids and activation of antiviral immune response, however their role in the immune defense of fish is much less known. In this study, the expression profile of non-RLR DExD/H-box RNA helicase genes: ddx1, ddx3, dhx9, ddx21 and dhx36, was studied in zebrafish (Danio rerio) and common carp (Cyprinus carpio L.) during infection with two RNA viruses: spring viremia of carp virus (SVCV) and Chum salmon reovirus (CSV). Bioinformatic analysis of the amino acid sequences of the core helicase of DDX1, DDX3, DHX9, DDX21 and DHX36 in zebrafish and common carp revealed presence of all conserved motifs found amongst all other species, with the exception of common carp DHX9 which do not possess motif V. The transcripts of studied DExD/H-box RNA helicases were found in zebrafish ZF4 cell line as well as in all studied organs from zebrafish and common carp. The expression study demonstrated the up-regulation of the expression of selected non-RLR DExD/H-box RNA helicases during viral infections in ZF4 cell line (in vitro study) and in zebrafish and common carp organs (in vivo study). DDX1 was the only DExD/H-box RNA helicase which expression was repetitively up-regulated during in vivo infections with SVCV and CSV in zebrafish and SVCV in common carp. In ZF4 cells and kidney of common carp, viral infection-induced up-regulation of DExD/H-box RNA helicases preceded the up-regulation of type I IFN gene. Our results suggest that studied non-RLR DExD/H-box RNA helicases might be involved in antiviral immune response in fish.

Surface display of spring viremia of carp virus glycoprotein on Lactococcus lactis and its protection efficacy in common carp (Cyprinus carpio L.)

Spring viremia of carp virus (SVCV) causes devastating disease in aquaculture, resulting in significant economic impact. To develop an effective means against SVCV infection, a Lactococcus lactis (L.lactis) based subunit vaccine (pNZ-UGA) was developed based on surface displaying of SVCV glycoprotein using anchoring motif of the cA (C terminus of the peptidoglyvsn-binding) domains of AcmA, a major autolysin from L.lactis. The surface expression of SVCV glycoprotein was verified by indirect immunofluorescence assay. The efficacy of the constructed vaccine was further evaluated in common carp. The results showed that the higher levels of specific IgM could be detected in fish vaccinated with pNZ-UGA, compared with that in PBS and L.lactis groups. Immune-related genes including TNF-α, IL-6b, IL-1β, Cxcr 1, Cxca, IFNg2b, I-IFN, and IgM expression in pNZ-UGA group were strongly up-regulated, revealing that robust innate immune response was induced. Notably, the lowest cumulative mortality (13.46%) was observed in fish vaccinated with pNZ-UGA vaccine after SVCV challenge, whereas the cumulative mortality were 100.00% and 92.31% in PBS and L.lactis groups, respectively. This study suggests the potential use of the recombinant L.lactis with surface displaying antigen proteins as effective vaccines against SVCV and other fish virus infection.

Zebrafish phd3 Negatively Regulates Antiviral Responses via Suppression of Irf7 Transactivity Independent of Its Prolyl Hydroxylase Activity

Prolyl hydroxylase domain (PHD)-containing enzyme 3 belongs to the Caenorhabditis elegans gene egl-9 family of prolyl hydroxylases, which has initially been revealed to hydroxylate hypoxia-inducible factor α (HIF-α) and mediate HIF-α degradation. In addition to modulating its target function by hydroxylation, PHD3 has been also shown to influence its binding partners' function independent of its prolyl hydroxylase activity. In this study, we report that overexpression of zebrafish phd3 suppresses cellular antiviral response. Moreover, disruption of phd3 in zebrafish increases the survival rate upon spring viremia of carp virus exposure. Further assays indicate that phd3 interacts with irf7 through the C-terminal IRF association domain of irf7 and diminishes K63-linked ubiquitination of irf7. However, the enzymatic activity of phd3 is not required for phd3 to inhibit irf7 transactivity. This study provides novel insights into phd3 function and sheds new light on the regulation of irf7 in retinoic acid-inducible gene I-like receptor signaling.

FTR67, a member of the fish-specific finTRIM family, triggers IFN pathway and against spring viremia of carp virus

Tripartite motif (TRIM) proteins have attracted particular research interest because of their multiple functions in the antiviral innate immune response. TRIM proteins perform different functions during virus infection, some play a role in inhibiting while others play a role in promoting. In this study, we described a species-specific TRIM gene named ftr67. Analysis of tissue distribution showed that ftr67 was mainly expressed in the gill and liver in five examined tissues of zebrafish. The phylogenic analysis showed that ftr67 was closest to the grass carp TRIM67. Overexpression of ftr67 resulted in a significantly decreased SVCV entry and impaired SVCV replication in FHM cells. Furthermore, overexpression of ftr67 could significantly induce the upregulation of molecular sensor RIG-I, IRF3/7, IFN and ISGs. In addition, RING domain of ftr67 was a required part essential for the antiviral effect. In summary, our results demonstrated that the important role of ftr67 in regulating SVCV infection, which offers a potential target for development of anti-SVCV therapies.

Zebrafish as a Model for Fish Diseases in Aquaculture

The use of zebrafish as a model for human conditions is widely recognized. Within the last couple of decades, the zebrafish has furthermore increasingly been utilized as a model for diseases in aquacultured fish species. The unique tools available in zebrafish present advantages compared to other animal models and unprecedented in vivo imaging and the use of transgenic zebrafish lines have contributed with novel knowledge to this field. In this review, investigations conducted in zebrafish on economically important diseases in aquacultured fish species are included. Studies are summarized on bacterial, viral and parasitic diseases and described in relation to prophylactic approaches, immunology and infection biology. Considerable attention has been assigned to innate and adaptive immunological responses. Finally, advantages and drawbacks of using the zebrafish as a model for aquacultured fish species are discussed.

Could the Induction of Trained Immunity by β-Glucan Serve as a Defense Against COVID-19?

As the SARS-CoV-2 virus wreaks havoc on the populations, health care infrastructures and economies of nations around the world, finding ways to protect health care workers and bolster immune responses in the general population while we await an effective vaccine will be the difference between life and death for many people. Recent studies show that innate immune populations may possess a form of memory, termed Trained Immunity (TRIM), where innate immune cells undergo metabolic, mitochondrial, and epigenetic reprogramming following exposure to an initial stimulus that results in a memory phenotype of enhanced immune responses when exposed to a secondary, heterologous, stimulus. Throughout the literature, it has been shown that the induction of TRIM using such inducers as the BCG vaccine and β-glucan can provide protection through altered immune responses against a range of viral infections. Here we hypothesize a potential role for β-glucan in decreasing worldwide morbidity and mortality due to COVID-19, and posit several ideas as to how TRIM may actually shape the observed epidemiological phenomena related to COVID-19. We also evaluate the potential effects of β-glucan in relation to the immune dysregulation and cytokine storm observed in COVID-19. Ultimately, we hypothesize that the use of oral β-glucan in a prophylactic setting could be an effective way to boost immune responses and abrogate symptoms in COVID-19, though clinical trials are necessary to confirm the efficacy of this treatment and to further examine differential effects of β-glucan's from various sources.

Zebrafish NIK Mediates IFN Induction by Regulating Activation of IRF3 and NF-κB

Type I IFN mediates the innate immune system to provide defense against viral infections. NF-κB-inducing kinase (NIK) potentiates the basal activation of endogenous STING, which facilitates the recruitment of TBK1 with the ectopically expressed IRF3 to induce IFN production. Moreover, NIK phosphorylates IKKα and confers its ability to phosphorylate p100 (also known as NF-κB2) in mammals. Our study demonstrated that NIK plays a critical role in IFN production in teleost fish. It was found that NIK interacts with IKKα in the cytoplasm and that IKKα phosphorylates the NIK at the residue Thr432, which is different from the mammals. Overexpression of NIK caused the activation of IRF3 and NF-κB, which in turn led to the production of IFN and IFN-stimulated genes (ISGs). Furthermore, the ectopic expression of NIK was observed to be associated with a reduced replication of the fish virus, whereas silencing of endogenous NIK had an opposite effect in vitro. Furthermore, NIK knockdown significantly reduced the expression of IFN and key ISGs in zebrafish larvae after spring viremia of carp virus infection. Additionally, the replication of spring viremia of carp virus was enhanced in NIK knockdown zebrafish larvae, leading to a lower survival rate. In summary, our findings revealed a previously undescribed function of NIK in activating IFN and ISGs as a host antiviral response. These findings may facilitate the establishment of antiviral therapy to combat fish viruses.

Proteomic Profiling of Zebrafish Challenged by Spring Viremia of Carp Virus Provides Insight into Skin Antiviral Response

Spring viremia of carp virus (SVCV) causes the skin hemorrhagic disease in cyprinid species, but its molecular mechanism of skin immune response remains unclear at the protein level. In the present study, the differential proteomics of the zebrafish (Danio rerio) skin in response to SVCV infection were examined by isobaric tags for relative and absolute quantitation and quantitative polymerase chain reaction (qPCR) assays. A total of 3999 proteins were identified, of which 320 and 181 proteins were differentially expressed at 24 and 96 h postinfection, respectively. The expression levels of 16 selected immune-related differentially expressed proteins (DEPs) were confirmed by qPCR analysis. Furthermore, Gene Ontology and Kyoto Encyclopedia of Genes and Genomes enrichment analyses revealed that DEPs were significantly associated with complement, inflammation, and antiviral response. The protein-protein interaction network of cytoskeleton-associated proteins, ATPase-related proteins, and parvalbumins from DEPs was shown to be involved in skin immune response. This is first report on the skin proteome profiling of zebrafish against SVCV infection, which will contribute to understand the molecular mechanism of local mucosal immunity in fish.

Establishment of a brain cell line (FuB-1) from mummichog (Fundulus heteroclitus) and its application to fish virology, immunity and nanoplastics toxicology

The marine fish mummichog (Fundulus heteroclitus), extensively used as research model, including in ecotoxicology, for over a century has been surpassed by other fish species. This fact may be associated with the lack of cell lines from this species, excellent models for the comprehension of fish physiology, immunology, toxicology and virology, that contribute to the reduction in the number of animals used in research. We have generated, for the first time, a brain-derived cell line from mummichog, FuB-1, and evaluated its application to the fields of fish virology, immunity and toxicology. First, FuB-1 cells show epithelial morphology and neural stem/astroglial origin. Secondly, FuB-1 cells effectively supports the replication of both spring viremia carp (SVCV) and infectious pancreatic necrosis (IPNV) viruses, but not nodavirus (NNV), indicating its potential use for fish virology. Related to this, FuB-1 cells infected with NNV up-regulate the transcription of genes related to the antiviral immune response, leading to cell resistance; while they are unaltered when infected with IPNV and SVCV, facilitating viral replication. Finally, FuB-1 cells were used for toxicological purposes and we demonstrated that exposure to either polystyrene nanoplastics (PS-100) or several human-usage pharmaceuticals are cytotoxic. Additionally, PS-100 particles increase the antioxidant catalase and glutathione S-transferase activities and decrease the total non-protein thiols in FuB-1 cells. However, PS-100 particles are able to reduce the cytotoxic effects induced by the pharmaceuticals. In conclusion, we have generated a cell line from mummichog, which might represent a valuable model for fish studies in the fields of virology, immunology and toxicology.

Exploring small cationic peptides of different origin as potential antimicrobial agents in aquaculture

Antimicrobial peptides (AMPs) form part of the innate immune response, which is of vital importance in fish, especially in eggs and early larval stages. Compared to antibiotics, AMPs show action against a wider spectrum of pathogens, including viruses, fungi and parasites, are more friendly to the environment, and do not seem to generate resistance in bacteria. Thus, we have tested in vitro the potential use of several synthetic peptides as antimicrobial agents in aquaculture: frog Caerin1.1, European sea bass Dicentracin (Dic) and NK-lysin peptides (NKLPs) and sole NKLP27. Our results demonstrate that the highest bactericidal activity against both human and fish pathogens was obtained with Caerin1.1 followed by sea bass Dic and NKLPs, having the sea bass NKLP20.2 none to negligible activity. Interestingly, Aeromonas salmonicida was refractory to all the fish peptides tested. Regarding the antiviral activity, synthetic peptides were able to inhibit the viral infection of nodavirus (NNV), viral septicaemia haemorrhagic virus (VHSV), infectious pancreatic necrosis virus (IPNV) and spring viremia carp virus (SVCV), which are some of the most devastating virus for aquaculture. However, their effectiveness was highly dependent on the type of virus. Strikingly, IPNV resulted the most resistant virus since Caeerin1.1 and sea bass NKLP20.2 were unable to reduce its titre and the other peptides tested only reduced it to values in the 43-78% range. These data demonstrate that synthetic peptides have great antibacterial and antiviral in vitro activity against important fish pathogens and point to their use as potential therapeutic agents in aquaculture.

Single-walled carbon nanotubes as delivery vehicles enhance the immunoprotective effect of an immersion DNA vaccine against infectious spleen and kidney necrosis virus in mandarin fish

As a high mortality disease, Infectious spleen and kidney necrosis virus (ISKNV) can cause massive economic damage on mandarin fish farming industry in China, which seriously hindered the development of mandarin fish farming industry. In this research, SWCNTs (single-walled carbon nanotubes) as a candidate for DNA vaccine carrier was vaccinated by immersion (1, 2, 5, 10, 20 mg/L) in juvenile mandarin fish. In muscle, spleen and kidney tissues, the results showed that transcription and expression of MCP gene can be detected in pcDNA-MCP and SWCNTs-pcDNA-MCP groups after bath immunization. The immune response (immune-related genes expression, serum antibody production, enzyme activities and C3 content) was significantly enhanced in fish which vaccinated with SWCNTs-pcDNA-MCP in comparison with those vaccinated with pcDNA-MCP alone. After 14 d challenge, the RPS (relative percentage survival) can be enhanced which using SWCNTs as a carrier in SWCNTs-pcDNA-MCP (82.4%) group at 20 mg/L (the highest vaccine dose) than the naked pcDNA-MCP (54.2%) group. This study reveals that functionalized SWCNTs could be a promising immersion DNA vaccine carrier in aquaculture.

Application of antigen presenting cell-targeted nanovaccine delivery system in rhabdovirus disease prophylactics using fish as a model organism

Background

Targeted delivery of virus-associated antigens to professional antigen-presenting cells (APCs) is considered as an efficient strategy to enhance the pyrophytic effect of vaccines against rhabdovirus disease.

Materials and methods

In this study, we constructed a targeted carbon nanotubes-based vaccine deliver system (SWCNTs-MG) which can recognize the signature receptor (mannose) of APCs. An environmentally and economically important disease called spring viremia of carp (SVC) was studied as a model to evaluate the feasibility of single-walled carbon nanotubes (SWCNTs) conjugated with mannosylated antigen for rhabdovirus prevention.

Results

Results showed that SWCNTs-MG could cross into fish body and present to internal immune-related tissues through gill, muscle and intestine within 6 h immersed vaccination. With further modification of mannose moiety, the obtained nanovaccine showed enhanced uptake by carp macrophages and immune-related tissues, which would then trigger strong immune responses against spring viremia of carp virus (SVCV) infection. Moreover, the survival rate of fish vaccinated with SWCNTs-MG (30 mg/L) was 63.5% after SVCV infection, whereas it was 0% for the control group.

Conclusion

This study not only provide a theoretical basis and research template for the application of targeted nanovaccine system in aquatic animals, but also play an important role in supporting development of healthy aquaculture and ensuring the safety of aquatic products and ecology.

Development of a real-time reverse transcription recombinase polymerase amplification assay for rapid detection of spring viremia of carp virus

Spring viremia of carp virus (SVCV) is a significant pathogenic agent that can cause large-scale outbreaks of spring viremia of carp (SVC) in many types of fish and bring huge economic losses to the aquaculture industry. A simple and convenient detection method is imperative for SVCV diagnosis. In this study, the real-time reverse transcription recombinase polymerase amplification (RT-RPA) assay was developed and validated. Primers and probe targeting the conserved region of M gene were designed and applied to the real-time RT-RPA assay that performed at 39 °C for 20 min. The specificity analysis showed that no cross-reaction with other pathogenic viruses of fish was found, indicating appropriate specificity of the assay. In vitro transcribed RNA standards were used to estimate the sensitivity of the assay and the detection limit was 10²copies/reaction. To further evaluate the assay, 65 clinical samples were tested using both real-time RT-RPA assay and real-time RT-PCR method. The same detection results were observed, suggesting the potential application of real-time RT-RPA assay in clinical sample detection. This is the first report on RPA assay for SVCV detection and this new developed assay would be useful in both laboratory and in the field for diagnosis of SVCV.

Preliminary screening and immunogenicity analysis of antigenic epitopes of spring viremia of carp virus

Glycoprotein (G) is the most common gene used in SVCV vaccine constructions. To identify the major immunogenicity determinant region of SVCV G gene, herein we truncated G gene to 4 parts (G-1, G-2, G-3 and G-4). Bioinformatics and the enzyme linked immunosorbent assay (ELISA) were used to identify the antigenicity of these 4 truncated G proteins. Immunological assays (serum antibody production, enzyme activity, immune genes expression and challenge test) were carried out to further identify the immunogenicity of the screened G protein in common carp. Moreover, to further verify the immune response of the screened G protein-based subunit vaccine, its protective effects on common carp against SVCV infection using single-walled carbon nanotubes (SWCNTs) as a carrier were evaluated. Results showed that G-3 protein could induce higher antibody titer than other truncated G proteins. Furthermore, carps vaccinated with G-3 and G (positive control) showed significant enhancement of immune response (serum antibody production, enzyme activity and immune related genes expression) when compared with control groups. Meanwhile, as a promising vaccine carrier, SWCNTs could significantly enhance the immune effect of naked subunit vaccine (G-3 and G). Notably, after SVCV challenge, there was no significant difference in immune protection between G-3 and G, nor between SWCNTs-G-3 and SWCNTs-G. These results so far suggest G-3 might be the potential antigen epitope of SVCV. This study lays a foundation for developing vaccine and immunodiagnostic techniques.

Identification of fish CMPK2 as an interferon stimulated gene against SVCV infection

Cytidine/uridine monophosphate kinase 2 (CMPK2) is known as a nucleoside monophosphate kinase in mitochondria to maintains intracellular UTP/CTP, and could be induced by immunostimulants LPS and Poly (I:C) in mammals, suggesting its potential antiviral and antibacterial role. In this study, CMPK2 was cloned and characterized in Fathead minnow (FHM) cells. In vivo analysis of tissue distribution revealed that CMPK2 transcript was detected in all the tissues of zebrafish (Danio rerio) examined in this study, particularly abundant in liver, spleen and kidney. In addition, indirect immunofluorescence showed that CMPK2 was localized in the cytoplasm of FHM cells. Expression of CMPK2 mRNA was significantly up-regulated following challenge with Spring viraemia of carp virus (SVCV), poly(I:C), or zebrafish IFN1 and IFN3 both in vitro and in vivo. Furthermore, overexpression and RNA interference of CMPK2 in SVCV-infected FHM cells showed significantly antiviral effect. In summary, this study for the first time shows the presence and distribution of CMPK2 in different tissues of zebrafish, but also demonstrates its antiviral potential against SVCV infection in vivo. These new findings could contribute to explain the molecular mechanism of the CMPK2 mediated antiviral function.

Evaluation of zebrafish (Danio rerio) as an animal model for the viral infections of fish

Zebrafish (Danio rerio) is a laboratory model organism used in different areas of biological research including studies of immune response and host-pathogen interactions. Thanks to many biological tools available, zebrafish becomes also an important model in aquaculture research since several fish viral infection models have been developed for zebrafish. Here, we have evaluated the possible use of zebrafish to study infections with fish viruses that have not yet been tested on this model organism. In vitro studies demonstrated that chum salmon reovirus (CSV; aquareovirus A) and two alloherpesviruses cyprinid herpesvirus 1 (CyHV-1) and cyprinid herpesvirus 3 (CyHV-3) are able to replicate in zebrafish cell lines ZF4 and SJD.1. Moreover, CSV induced a clear cytopathic effect and up-regulated the expression of antiviral genes vig-1 and mxa in both cell lines. In vivo studies demonstrated that both CSV and CyHV-3 induce up-regulation of vig-1 and mxa expression in kidney and spleen of adult zebrafish after infection by i.p. injection but not in larvae after infection by immersion. CyHV-3 is eliminated quickly from fish; therefore, virus clearing process could be evaluated, and in CSV-infected fish, a prolonged confrontation of the host with the pathogen could be studied.

Antiviral activity of anisomycin against spring viraemia of carp virus in epithelioma papulosum cyprini cells and zebrafish

•

Anisomycin caused profound inhibition of SVCV replication in EPC cells.

•

Anisomycin blocked morphological cell damage caused by SVCV replication.

•

Anisomycin suppressed SVCV replication in vivo, resulting in a prolonged survival of infected zebrafish.

Spring viraemia of carp (SVC) caused by spring viraemia of carp virus (SVCV) is an acute and highly lethal viral disease of cyprinid fish. However, effective therapy for SVC is still scarce until now. Here we evaluated the inhibition of anisomycin (Ani), a metabolite produced by Streptomyces griseolus, on the replication of SVCV in vitro and in vivo. Our results demonstrated that Ani could suppress SVCV replication with the maximum inhibitory rate > 95% in epithelioma papulosum cyprini (EPC) cells. And the half maximal inhibitory concentrations (IC₅₀) of Ani on SVCV glycoprotein (G), nucleoprotein (N) and phosphoprotein mRNA expressions were 21.79, 13.13 and 12.24 nM, respectively. Besides, Ani decreased SVCV-induced cytopathic effects and nucleus damages. As expected, Ani also showed a strong anti-SVCV activity in vivo, as indicated by inhibiting viral gene expression and increasing the survival rate of zebrafish. Intraperitoneal injection of Ani increased the survival rate of zebrafish by 30% and markedly inhibited the expressions of G and N mRNA by > 60% in kidney and spleen at day 1 and day 4 post-infection. Results so far suggest that Ani as a powerful agent against SVCV can be applied to the control of SVC in aquaculture.

Comparative Analysis of the aquaporin Gene Family in 12 Fish Species

Simple Summary

Aquaporins (Aqps) are a group of membrane proteins. In this study, 166 Aqp genes were identified in 12 fish species. Gene organization, motif distribution, recombination, and selection pressure were performed to investigate their evolutionary characteristics. In addition, expression profiles of Aqps were also examined under pathogens infection and organophosphorus pesticide stress. This study will provide a useful reference for further functional study.

Abstract

Aquaporins (Aqps) are a class of water channel proteins that play key roles in many physiological functions and cellular processes. Here, we analyzed 166 putative Aqp genes in 12 fish species and divided them into four groups. Gene organization and motif distribution analyses suggested potentially conserved functions in each group. Several recombination events were identified in some members, which accelerate their divergence in evolution. Furthermore, a few positive selection sites were identified, and mutations at these sites could alter the stability of Aqp proteins. In addition, expression profiles of some Aqp genes under pathogen infection and organophosphorus pesticide stress were also investigated. The result implied that several Aqp genes may affect different immune responses and osmoregulation. This study provides a comparative analysis of the fish Aqp gene family to facilitate further functional analyses.

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.

Intein-mediated expression and purification of common carp IFN-γ and its protective effect against spring viremia of carp virus

IFN-γ is a pleiotropic cytokine with significant roles in antiviral, antitumor and immune regulation. It could be used as an immuno-enhancer to improve fish protectiveness against pathogens. In this study, the prokaryotic expression plasmid pTwin1-N-IFN-γ was constructed to express Cyprinus carpio (common carp) IFN-γ fused with a chitin binding domain (CBD) and a self-cleavable intein-tag, Synechocystis sp DnaB. The recombinant protein CBD-DnaB-IFN-γ with the molecular weight of 44.25 kD was successfully expressed in soluble form, and the rIFN-γ (approximate 18.61 kD) was further cleaved and eluted under pH = 7.0 at 25 °C. rIFN-γ could be recognized by western blotting with rabbit anti-grass carp IFN-γ polyclonal antibody. Cytotoxicity studies on EPC cells showed that only 500 ng/ml rIFN-γ had a subtle effect on cells growth and its proliferation rate was reduced to 76.2%. EPC cells incubated with 100 ng/ml rIFN-γ showed significantly higher resistance against SVCV, reducing the TCID₅₀/ml by more than 800-fold. In vivo studies suggested that intraperitoneal injection of rIFN-γ significantly improved the survival rate of common carps compared with SVCV challenge alone. These results implied that rIFN-γ would act as an immuno-enhancer in carp aquaculture.

First detection of spring viraemia of carp virus in common carp (Cyprinus carpio L.) affected by a septicaemic disease in Mexico

Spring viraemia of carp (SVC) is an infectious disease responsible for severe economic losses for various cyprinid species, particularly common carp (Cyprinus carpio carpio). The causative agent is the SVC virus (SVCV), a member of the Sprivivirus genus, Rhabdoviridae family, and a List 1 pathogen notifiable by the World Organization for Animal Health. This study describes the diagnosis of an SVCV pathogen isolated in October 2015 from wild common carp inhabiting a natural lagoon in central Mexico. While neither an epidemic nor fish mortalities were reported, the collected killed specimens exhibited clinical signs of disease (e.g., exopthalmia, moderate abdominal distension and haemorrhaging, as well as internal haemorrhages and adhesions). Histological results of injuries were consistent with the pathology caused by SVCV. This finding was supported by the isolation of a virus in EPC and BF-2 cells and subsequent RT-PCR confirmation of SVCV. The phylogenetic analyses of partial SVCV glycoprotein gene sequences classified the isolates into the Ia genogroup. These findings make this the first report of SVCV detection in Mexico, extending the southern geographical range of SVCV within North America. However, since this pathogen was detected in fish inhabiting a natural body of water without tributaries or effluents, it is difficult to estimate the risk of SVCV for other wild/feral cohabitating cyprinid species in the lagoon. The status of this virus is also unknown for other bodies of water within this region.

PMA-triggered PKCε activity enhances Nrf2-mediated antiviral response on fish rhabdovirus infection

Viral infection is often accompanied with alteration of intracellular redox state, especially an imbalance between reactive oxygen species (ROS) production and antioxidant cellular defenses. The previous studies showed that an antioxidant cellular defense system, the transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2), played an important role against spring viraemia of carp virus (SVCV) infection in fish. To further reveal the mediated mechanism that Nrf2 active state was affected by protein kinase C (PKC), here we evaluated SVCV replication in host cells by treated with a strong activator of PKC phorbol-12-myristate-13-acetate (PMA) and an inhibitor staurosporine. Our results showed that PMA significantly repressed SVCV replication and viral-induced apoptosis in Epithelioma papulosum cyprini (EPC) cell, suggesting that PKC may exhibit an anti-SVCV effect. Likewise, PMA resulted in a higher phosphorylation levels of PKCε rather than PKCα/β to participate in the activation of Nrf2, mainly involved in the activation of Nrf2 phosphorylation of Ser40 to favor Nrf2 translocation to nucleus. Furthermore, the data revealed that PMA up-regulated an antiviral response heme oxygenase-1 (HO1) gene expression that was confirmed as the key player against SVCV infection by HO1 specific siRNA. Overall, this study provided a new therapeutic target for the treatment of SVCV infection, and modulating PKC activity could be used for the prevention and treatment of SVCV.

SVCV infection triggers fish IFN response through RLR signaling pathway

In mammals, virus infection of host cells triggers innate immune response, characterized by induction of interferon (IFN) and downstream IFN-stimulated genes (ISGs). The initiation of IFN antiviral response is dependent on host recognition of virus infection. In fish, similar IFN antiviral response is induced in response to RNA or DNA virus infection; however, the detailed mechanisms underlying recognition of a given virus and activation of downstream signaling remain largely unexplored. Using an infection model with Epithelioma papulosum cyprini (EPC) cells and spring viremia of carp virus (SVCV), a negative sense single-stranded RNA virus, we reported that fish RLR signaling pathway was involved in SVCV-triggered fish IFN response. IFN response was significantly initiated in EPC cells when infected with SVCV, as evidenced by activation of fish IFN promoters, upregulation of IFN and ISGs at mRNA and protein levels. However, function blockade of RIG-I and MDA5, two cytosolic receptors of fish RLR family, significantly attenuated the activation of fish IFN promoters and also the induction of fish IFN and ISGs by SVCV infection. Consistently, SVCV infection-triggered IFN response were blocked in EPC cells when transfected with the dominant negative mutants of pivotal RLR signaling factors, including MAVS, MITA, TBK1, IRF3 and IRF7. These results together shed light on the conservation of RLR-mediated IFN signaling that contributes to fish cells responding to RNA virus infection.

Fish-specific finTRIM FTR36 triggers IFN pathway and mediates inhibition of viral replication

The tripartite motif (TRIM) family involves many cellular processes, including fundamental functions in antiviral immunity. Antiviral activities of TRIMs are reported in a variety of patterns, and one of the most significant channels is related to the activation of the type-I interferon (IFN) pathway. In this study, we described a fintrim (ftr) gene named ftr36, which is mainly expressed in the gills, skin, and intestines. This study shows that ftr36 encodes a protein affording a potent antiviral effect. In vitro, overexpression of FTR36 mediated an upregulated pattern of recognition receptor retinoic acid-inducible gene I (RIG-I), interferon regulatory factor 3/7(IRF3/7), IFN, and IFN-stimulated genes (ISGs) expression. Thereby, FTR36 expression could afford host defense against the spring viremia of carp virus (SVCV) and the giant salamander iridovirus (GSIV). With the deletion of the RING domain or B30.2 domain separately, the antiviral ability of FTR36 was abolished partially and almost lost its ability to activate the IFN-pathway. These findings indicate that both RING and B30.2 domains are indispensable for the antiviral activity of FTR36. Altogether, this study described a finTRIM FTR36, which can activate IFN-pathways and stimulate ISGs to provide host defense against viral infections.

Antiviral effect of 7-(4-benzimidazole-butoxy)-coumarin on rhabdoviral clearance via Nrf2 activation regulated by PKCα/β phosphorylation

Coumarin forms an elite class of naturally occurring compounds that possess promising antiviral therapeutic perspectives. In the previous study, we designed and synthesized a coumarin derivative, 7-(4-benzimidazole-butoxy)-coumarin (BBC), to evaluate its antiviral activity on spring viraemia of carp virus (SVCV). In this study, our results show that BBC does not affect viral adhesion and delivery from endosomes to the cytosol, indicating BBC has no inhibitory activity in the early stage of viral infection. Further data are determined that BBC significantly declines SVCV-infected apoptosis and recovers caspase-3/8/9 activity. To reveal the pathway that affects Nrf2 translocation by BBC, we examine changes in protein kinase C (PKC) in EPC cells treated with BBC. We observe that BBC results in a higher phosphorylation of PKCα/β that is involved in the activation of erythroid 2-related factor 2 (Nrf2) phosphorylation to favor Nrf2 translocation to nucleus at 24 and 48 h. In addition, the results show that BBC also up-regulates both antiviral responses, heme oxygenase-1 (HO-1) expression and cellular IFN response. Overall, this mechanism of action provides a new therapeutic target for the treatment of SVCV infection, and these results suggest that treatment with BBC is effective in reducing SVCV infection and differently regulates SVCV-induced undesirable conditions.

Synthesis of arctigenin derivatives against infectious hematopoietic necrosis virus

Infectious hematopoietic necrosis virus (IHNV) is a common pathogen that causes severe disease and huge economic losses in the salmonid aquaculture industry. Herein, a series of arctigenin derivatives are synthesized to evaluate their antiviral activity against IHNV. The results indicate that the length of linker and imidazole substituent groups play an important role in decreasing IHNV replication. In this study, the arctigenin-imidazole hybrid derivative 15 with an eight carbon atoms length of the linker reduces IHNV replication with an IC₅₀ value of 1.3 μM. In addition, derivative 15 significantly inhibits apoptosis and cellular morphological damage induced by IHNV. Mechanistically, derivative 15 can not damage the viral particle directly. While time-of-addition and viral binding assays reveal that derivative 15 mainly affect the early replication of IHNV but do not interfere with IHNV adsorption. Overall, derivative 15 could be considered to develop as a promising agent to treat IHNV infection.

Chromatin immunoprecipitation and high throughput sequencing of SVCV-infected zebrafish reveals novel epigenetic histone methylation patterns involved in antiviral immune response

Chromatin immunoprecipitation (ChIP) and high throughput sequencing (ChIP-seq) have been used to assess histone methylation (epigenetic modification) dynamics within the internal organs of zebrafish after spring viremia of carp virus (SVCV) infection. Our results show H3K4me3 up-methylation in gene promoters associated with innate immune response during the first 5 days after SVCV infection. Gene Ontology (GO) enrichment analysis confirmed up-methylation in 218 genes in the "immune system process" category. In particular, the promoters of interferon (ifn), interferon stimulated genes (isg), Toll-like receptors (tlr) and c-reactive protein (crp) multi gene sets were marked with the permissive H3K4 methylation. Higher histone 3 methylation was associated with higher transcription levels of the corresponding genes. Therefore, the evidence presented here suggests that transcriptional regulation at the promoter level of key immune genes of the interferon signaling pathway and c-reactive proteins genes can be modulated by epigenetic modification of histones. This study emphasizes the importance of epigenetic control in the response of zebrafish to SVCV infection.

14-3-3β/α-A interacts with glycoprotein of spring viremia of carp virus and positively affects viral entry

Spring viremia of carp virus (SVCV) is a fierce pathogen causing high mortality in the common carp. The glycoprotein (G protein) of SVCV is a pivotal component of the viral structure, located in the surface of the virion, and plays a key role in viral endocytosis. In this study, tandem affinity purification (TAP) followed by mass spectrometry analysis (LC-MS/MS) was carried out to search for novel host molecules that interact with SVCV G protein and a 14-3-3β/α-A protein was identified. The level of 14-3-3β/α-A mRNA expression was dramatically down regulated by SVCV infection. Furthermore, over expression of 14-3-3β/α-A results in a significantly increased SVCV attachment and entry in FHM cells. This study reveals an important role of 14-3-3 protein in regulating the early stage of SVCV infection, which offers a potential target for development of anti-SVCV therapies.

Broad-spectrum antiviral JL122 blocks infection and inhibits transmission of aquatic rhabdoviruses

The aquaculture industry is growing rapidly to meet the needs for global protein consumption. Viral diseases in aquaculture are quite challenging due to lack of treatment options as well as limited injection-delivery vaccines, which are costly. Thus, water-immersion antiviral treatments are highly desirable. This study focused on broad-spectrum, light-activated antivirals that target the viral membrane (envelope) of viruses to prevent viral-cell membrane fusion, ultimately blocking viral entry into cells. Of the tested small-molecules, JL122, a new broad-spectrum antiviral previously unexplored against aquatic viruses, blocked infection of three aquatic rhabdoviruses (IHNV, VHSV and SVCV) in cell culture and in two live fish challenge models. Importantly, JL122 inhibited transmission of IHNV from infected to uninfected rainbow trout. Further, the effective antiviral concentrations were not toxic to cells or susceptible fish. These results show promise for JL122 to become an immersion treatment option for outbreaks of aquatic enveloped viral infections.