Zebrafish liver (ZFL) cells are able to mount an anti-viral response after stimulation with Poly (I:C)

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

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

Poly(I:C) induces anti-inflammatory response against secondary LPS challenge in zebrafish larvae

Poly(I:C) is known as an agonist of the TLR3 receptor which could prime inflammation and elicit the host immune response, which is widely applied as adjuvant or antivirus treatment. However, the negative effects of poly(I:C) on regulating immune response to protect the host from inflammatory diseases remain largely unknown. Here, we establish an in vivo model to pre-treat zebrafish larvae with poly(I:C) at 2 dpf, then challenge them with LPS at 6 dpf, and find that poly(I:C) training could significantly alleviate the LPS challenge-induced septic shock and inflammatory phenotypes. Moreover, the poly(I:C)-trained larvae exhibit decreased number of macrophages, but not neutrophils, after secondary LPS challenge. Furthermore, training the larvae with poly(I:C) could elevate the transcripts of mTOR signaling and heighten the H3K4me3-mediated epigenetic modifications. And interestingly, we find that inhibiting the H3K4me3 modification, rather than mTOR signaling, could recover the number of macrophages in poly(I:C)-trained larvae, which is consistent with the observations of inflammatory phenotypes. Taken together, these results suggest that poly(I:C) training could induce epigenetic rewiring to mediate the anti-inflammatory response against secondary LPS challenge-induced septic shock through decreasing macrophages' number in vivo, which might expand our understanding of poly(I:C) in regulating fish immune response.

How modular protein nanoparticles may expand the ability of subunit anti-viral vaccines: The spring viremia carp virus (SVCV) case

Spring viremia of carp (SVC) remains as a vaccine orphan disease mostly affecting juvenile specimens. Young fish are especially difficult to vaccinate and oral administration of vaccine combined with food would be the election system to minimise stress and the vaccination costs associated to injection. However, administration of prophylactics with food pellets faces off several drawbacks mainly related with vaccine degradation and weak protection correlates of oral vaccines. Here we present a platform based on recombinant proteins (subunit vaccines) manufactured as highly resistant nanostructured materials, and providing excellent levels of protection against SVC virus in a preliminary i.p injection challenge. The G3 domain of SVCV glycoprotein G was overexpressed in E. coli together with IFNγ and the modular protein was purified from bacterial aggregates (inclusion bodies) as highly organised nanostructured biomaterial (nanopellets, NP). These SVCV-IFN^NP were taken up by zebrafish cells leading to the enhanced expression of different antiviral and IFN markers (e.g vig1, mx, lmp2 or ifngr1 among others) in zebrafish liver cells (ZFL). To monitor if SVCV^NP and SVCV-IFN^NP can be taken up by intestinal epithelia and can induce antiviral response we performed experiments with SVCV^NP and SVCV-IFN^NP in 3 days post fertilization (dpf) zebrafish larvae. Both, SVCV^NP and SVCV-IFN^NP were taken up and accumulated in the intestine without signs of toxicity. The antiviral response in larvae showed a different induction pattern: SVCV-IFN^NP did not induce an antiviral response while SVCV^NP showed a good antiviral induction. Interestingly ZF4, an embryonic derived cell line, showed an antiviral response like ZFL cells, although the lmp2 and ifngr1 (markers of the IFNγ response) were not overexpressed. Experiments with adult zebrafish indicated an excellent level of protection against a SVCV model infection where SVCV-IFN^NP vaccinated fish reached 20% cumulative mortality while control fish reached over 80% cumulative mortality.

Comparative Transcriptomic Analyses Revealed the Effects of Poly (I:C) on the Liver and Spleen of Argyrosomus japonicus

Poly (I:C) can work as an immunostimulant and a viral vaccine; however, its functional mechanism in aquatic animals needs to be further investigated. In this study, comparative transcriptomic analyses were performed to investigate the effects of poly (I:C) on Argyrosomus japonicus at 12 h and 48 h postinjection. A total of 194 and 294 differentially expressed genes were obtained in the liver and spleen, respectively. At 12 h, poly (I:C) injection could significantly influence the function of the metabolism-related pathways and immune-related pathways in the liver through the upregulation of the genes GST, LPIN, FOXO1, CYP24A1, ECM1, and SGK1, and the downregulation of the genes IL-1β, CXC19, TNFAIP3, and IRF1. At 48 h, poly (I:C) could enhance the liver energy metabolism by upregulating the genes TXNRD and ECM1, while it also induced some injury in the cells with the downregulation of the genes HBA and CYP24A1. In the spleen, poly (I:C) could regulate the fish immunity and inflammatory response by upregulating the genes DDIT4, C3, EFNA, and MNK, and by downregulating the genes ABCA1, SORT1, TNF, TLR2, IL8, and MHCII at 12 h, and at 48 h, the poly (I:C) had a similar influence as that in the liver. Intersection analyses demonstrated that CYP24A1 and ECM1 were the main functional genes that contributed to the health of the liver. Ten and four genes participated in maintaining the health of the two tissues after 12 h and 48 h, respectively. In summary, our results provided a new insight into ploy (I:C) application in A. japonicus, and it also helped us to better understand the fish response mechanism to the viral vaccine injection.

Feeding with poly(I:C) induced long-term immune responses against bacterial infection in turbot (Scophthalmus maximus)

Poly(I:C) is a kind of chemosynthetic double-stranded RNA (dsRNA) analogue which could act as TLR3 agonist and induce IFN production. It is widely applied in anti-virus treatment and immunoregulation, as well as vaccine adjuvant in farmed animals. However, whether poly(I:C) could activate innate immune response to defense against bacterial infection remains unclear. In this study, we established a feeding trial model with different dose of poly(I:C) in turbot larvae, then challenged with Edwardsiella piscicida after 3–7 weeks resting period. The results show that feeding turbot with poly(I:C) exhibited a stronger inflammatory response and antioxidant stress ability, and significantly elevated the survival rate within the decreased bacterial loads. Importantly, the bacterial infection-induced white feces in hindgut of turbot were significantly alleviated after poly(I:C) feeding, and this administration induced protection could last for about 7 weeks. Taken together, these findings indicate that feeding turbot with poly(I:C) could enhance a long-term intestinal mucosal immunity in response to bacterial infection, suggesting that poly(I:C) might be a promising immunostimulant in aquaculture.

Development of 3D cultures of zebrafish liver and embryo cell lines: a comparison of different spheroid formation methods

Fish cell spheroids are promising 3D culture models for vertebrate replacement in ecotoxicology. However, new alternative ecotoxicological methods must be adapted for applications in industry and for regulatory purposes; such methods must be cost-effective, simple to manipulate and provide rapid results. Therefore, we compared the effectiveness of the traditional hanging drop (HD), orbital shaking (OS), and HD combined with OS (HD+OS) methods on the formation of zebrafish cell line spheroids (ZFL and ZEM2S). Time in HD (3-5 days) and different 96-well plates [flat-bottom or ultra-low attachment of round-bottom (ULA-plates)] in OS were evaluated. Easy handling, rapid spheroid formation, uniform-sized spheroids, and circularity were assessed to identify the best spheroid protocol. Traditional HD alone did not result in ZFL spheroid formation, whereas HD (5 days)+OS did. When using the OS, spheroids only formed on the ULA-plate. Both HD+OS and OS were reproducible in size (177.50 ± 2.81 µm and 225.62 ± 19.20 µm, respectively) and circularity (0.83 ± 0.02 and 0.80 ± 0.01, respectively) of ZFL spheroids. Nevertheless, HD+OS required a considerable time to completely form spheroids (10 days) and intensive handling, whereas the OS was fast (5 days of incubation) and simple. OS also yielded reproducible ZEM2S spheroids in 1 day (226.23 ± 0.57 µm diameter and 0.80 ± 0.01 circularity). In conclusion, OS in ULA-plate is an effective and simple spheroid protocol for high-throughput ecotoxicity testing. This study contributes to identify a fast, reproducible, and simple protocol of single piscine spheroid formation in 96-well plates and supports the application of fish 3D model in industry and academia.

Polarized Trout Epithelial Cells Regulate Transepithelial Electrical Resistance, Gene Expression, and the Phosphoproteome in Response to Viral Infection

The burden of disease is a major challenge in aquaculture production. The fish gill characterized with a large surface area and short route to the bloodstream is a major environmental interface and a significant portal of entry for pathogens. To investigate gill responses to viral infection the salmonid gill cell line RTgill-W1 was stimulated with synthetic dsRNA and the salmonid alphavirus subtype 2 (SAV-2). Epithelial integrity in polarized cells can be measured as transepithelial electrical resistance (TEER) which is defined as the electrical resistance across a cell monolayer. TEER is a widely accepted quantitative measure of cellular integrity of a cell monolayer. TEER increased immediately after stimulation with the synthetic dsRNA, polyinosinic:polycytidylic acid (poly(I:C)). In parallel, tight junction and gene expression of innate immune activation markers was modulated in response to poly(I:C). The SAV-2 virus was found to replicate at a low level in RTgill-W1 cells where TEER was disturbed at an early stage of infection, however, gene expression related to tight junction regulation was not modulated. A strong poly(I:C)-driven antiviral response was observed including increases of Rig-like receptors (RLRs) and interferon stimulating genes (ISGs) mRNAs. At the level of signal transduction, poly(I:C) stimulation was accompanied by the phosphorylation of 671 proteins, of which 390 were activated solely in response to the presence of poly(I:C). According to motif analysis, kinases in this group included MAPKs, Ca2+/calmodulin-dependent kinase (CaMK) and cAMP-dependent protein kinase (PKA), all reported to be activated in response to viral infection in mammals. Results also highlighted an activation of the cytoskeletal organization that could be mediated by members of the integrin family. While further work is needed to validate these results, our data indicate that salmonid gill epithelia has the ability to mount a significant response to viral infection which might be important in disease progression. In vitro cell culture can facilitate both a deeper understanding of the anti-viral response in fish and open novel therapeutic avenues for fish health management in aquaculture.

Chitosan and anisodamine improve the immune efficacy of inactivated infectious spleen and kidney necrosis virus vaccine in Siniperca chuatsi

Siniperca chuatsi is an economically important fish in China, but infectious spleen and kidney necrosis virus (ISKNV) causes high mortality and significant economic losses. Currently, vaccination is the most promising strategy to prevent infectious diseases, while adjuvant can effectively enhance immune responses. In this study, inactivated ISKNV vaccine was prepared, then poly (I:C), chitosan, anisodamine and ims1312 were used as adjuvants to evaluate the effect on the immune responses and ISKNV replication. Chitosan could strongly boost the protection of liver and spleen tissues by pathological sections. In serum, poly (I:C) and chitosan group had protective effect on catalase, acid phosphatase, blood urea nitrogen. mRNA expressions showed these adjuvants induced the cytokines of early immune responses (TNF-α, Viperin) in both spleen and mesonephron by real time quantitative RT-PCR assays. Meanwhile, poly (I:C), chitosan and anisodamine were significantly improved the antiviral function and inhibited ISKNV replication. Chitosan and anisodamine played a significantly protective role in the immune protective rate test. The results indicated that all the four adjuvants are valid in the inactivated ISKNV vaccine, and chitosan is recommended preferentially. The present study provides reference for other animal vaccine adjuvants.

Astragalus polysaccharides, chitosan and poly(I:C) obviously enhance inactivated Edwardsiella ictaluri vaccine potency in yellow catfish Pelteobagrus fulvidraco

The yellow catfish (Pelteobagrus fulvidraco) is an economically important fish in China, but Edwardsiella ictaluri, an intracellular pathogenic bacterium, causes great losses to the culture industry. Currently, vaccination is the most promising strategy to combat the infectious diseases, while adjuvant can provide effective assistant for vaccines to enhance immune responses. In the present study, inactivated E. ictaluri vaccine was prepared, then Astragalus polysaccharides (APS), chitosan and poly(I:C) were employed as adjuvants to evaluate the effect on boosting immune responses and protecting yellow catfish against E. ictaluri. The survival rate was obviously improved after vaccination with APS, chitosan or poly(I:C) respectively, in addition, these three adjuvants could clearly protect the target tissue (intestine) by pathological sections in infectious experiments. In sera, total protein levels increased throughout the immunization stages, total superoxide dismutase levels continued to raise after vaccination, and lysozyme activity levels improved at different periods, examining by the commercial kits. Moreover, checking by real time quantitative RT-PCR assays, in both spleen and head kidney tissues which were the major immune organs, mRNA expressions of inflammatory cytokine IL-1β increased in the early stage of immunity, typical Th1 immune response cytokines IL-2 and IFN-γ2 rose up in the whole immune period, and IgM significantly enhanced in the adjuvant supplementation groups. The results demonstrated the good efficiency of APS, chitosan or poly(I:C) as adjuvant, and provided more options for the fish adjuvants.

Protein Nanoparticles Made of Recombinant Viral Antigens: A Promising Biomaterial for Oral Delivery of Fish Prophylactics

In the search for an eminently practical strategy to develop immunostimulants and vaccines for farmed fish, we have devised recombinant viral antigens presented as “nanopellets” (NPs). These are inclusion bodies of fish viral antigenic proteins produced in Escherichia coli. Soluble recombinant proteins are too labile to endure the in vivo environment and maintain full functionality, and therefore require encapsulation strategies. Yet when they are produced as nanostructures, they can withstand the wide range of gastrointestinal pH found in fish, high temperatures, and lyophilization. Moreover, these nanomaterials are biologically active, non-toxic to fish, cost-effective regarding production and suitable for oral administration. Here, we present three versions of NPs formed by antigenic proteins from relevant viruses affecting farmed fish: the viral nervous necrosis virus coat protein, infectious pancreatic necrosis virus viral protein 2, and a viral haemorrhagic septicemia virus G glycoprotein fragment. We demonstrate that the nanoparticles are taken up in vitro by zebrafish ZFL cells and in vivo by intubating zebrafish as a proof of concept for oral delivery. Encouragingly, analysis of gene expression suggests these NPs evoke an antiviral innate immune response in ZFL cells and in rainbow trout head kidney macrophages. They are therefore a promising platform for immunostimulants and may be candidates for vaccines should protection be demonstrated.

Interactions of oxidized multiwalled carbon nanotube with cadmium on zebrafish cell line: The influence of two co-exposure protocols on in vitro toxicity tests

The widespread production and application of carbon nanotubes (CNT) have raising concerns about their release into the environment and, the joint toxicity of CNT with pre-existing contaminants needs to be assessed. This is the first study that investigated the co-exposure of oxidized multiwalled carbon nanotubes (ox-MWCNT) and cadmium (Cd) using a zebrafish liver cell line (ZFL). Two in vitro co-exposure protocols differing by the order of ox-MWCNT interaction with Cd and fetal bovine serum (FBS) proteins were evaluated. Ox-MWCNT was physical and chemical characterized and its adsorption capacity and colloidal stability in cell culture medium was determined in both protocols. Cytotoxicity was investigated by MTT, neutral red, trypan blue, lactate dehydrogenase assays and the necrosis and apoptosis events were determined using flow cytometer. The Cd presence in medium did not interfere in the protein corona composition of MWCNT but the order of interaction of FBS and Cd interfered in its colloidal stability and metal adsorption rate. The ox-MWCNT increased Cd toxicity at low concentration probably by a "Trojan horse" and/or synergistic effect, and induced apoptosis and necrosis in ZFL cells. Although it was not observed differences of toxicity between protocols, the interaction of ox-MWCNT first with Cd led to its precipitation in cell culture medium and, as a consequence, to a possible false viability result by neutral red assay. Taken together, it was evident that the order of compounds interactions disturbs the colloidal stability and affects the in vitro toxicological assays. Considering that Protocol A showed more ox-MWCNT stability after interaction with Cd, this protocol is recommended to be adopted in future studies.

Functional evidence for the inflammatory reflex in teleosts: A novel α7 nicotinic acetylcholine receptor modulates the macrophage response to dsRNA

The inflammatory reflex modulates the innate immune system, keeping in check the detrimental consequences of overstimulation. A key player controlling the inflammatory reflex is the alpha 7 acetylcholine receptor (α7nAChR). This receptor is one of the signalling molecules regulating cytokine expression in macrophages. In this study, we characterize a novel teleost α7nAChR. Protein sequence analysis shows a high degree of conservation with mammalian orthologs and trout α7nAChR has all the features and essential amino acids to form a fully functional receptor. We demonstrate that trout macrophages can bind α-bungarotoxin (α-BTX), a competitive antagonist for α7nAChRs. Moreover, nicotine stimulation produces a decrease in pro-inflammatory cytokine expression after stimulation with poly(I:C). These results suggest the presence of a functional α7nAChR in the macrophage plasma membrane. Further, in vivo injection of poly(I:C) induced an increase in serum ACh levels in rainbow trout. Our results manifest for the first time the functional conservation of the inflammatory reflex in teleosts.

Taurine Biosynthesis in a Fish Liver Cell Line (ZFL) Adapted to a Serum-Free Medium

Although taurine has been shown to play multiple important physiological roles in teleosts, little is known about the molecular mechanisms underlying dietary requirements. Cell lines can provide useful tools for deciphering biosynthetic pathways and their regulation. However, culture media and sera contain variable taurine levels. To provide a useful cell line for the investigation of taurine homeostasis, an adult zebrafish liver cell line (ZFL) has been adapted to a taurine-free medium by gradual accommodation to a commercially available synthetic medium, UltraMEM™-ITES. Here we show that ZFL cells are able to synthesize taurine and be maintained in medium without taurine. This has allowed for the investigation of the effects of taurine supplementation on cell growth, cellular amino acid pools, as well as the expression of the taurine biosynthetic pathway and taurine transporter genes in a defined fish cell type. After taurine supplementation, cellular taurine levels increase but hypotaurine levels stay constant, suggesting little suppression of taurine biosynthesis. Cellular methionine levels do not change after taurine addition, consistent with maintenance of taurine biosynthesis. The addition of taurine to cells grown in taurine-free medium has little effect on transcript levels of the biosynthetic pathway genes for cysteine dioxygenase (CDO), cysteine sulfinate decarboxylase (CSAD), or cysteamine dioxygenase (ADO). In contrast, supplementation with taurine causes a 30% reduction in transcript levels of the taurine transporter, TauT. This experimental approach can be tailored for the development of cell lines from aquaculture species for the elucidation of their taurine biosynthetic capacity.