A method to measure an indicator of viraemia in Atlantic salmon using a reporter cell line

Role and relevance of fish cell lines in advanced in vitro research

Introduction

Cell line derived from fish has been established as a promising tool for studying many key issues of aquaculture covering fish growth, disease, reproduction, genetics, and biotechnology. In addition, fish cell lines are very useful in vitro models for toxicological, pathological, and immunological studies. The easier maintenance of fish cell lines in flexible temperature regimes and hypoxic conditions make them preferable in vitro tools over mammalian cell lines. Great excitement has been observed in establishing and characterizing new fish cell lines representing diverse fish species and tissue types. The well-characterized and authenticated cell lines are of utmost essential as these represent cellular functions very similar to in vivo state of an organism otherwise it would affect the reproducibility of scientific research.

Conclusion

The fish cell lines have exhibited encouraging results in several key aspects of in vitro research in aquaculture including virology, nutrition and metabolism, production of vaccines, and transgenic fish production. The review paper reports the cell lines developed from fish, their characterization, and biobanking along with their potential applications and challenges in in vitro research.

Non-Lethal Sequential Individual Monitoring of Viremia in Relation to DNA Vaccination in Fish-Example Using a Salmon Alphavirus DNA Vaccine in Atlantic Salmon Salmo salar

Traditionally, commercial testing for vaccine efficacy has relied on the mass infection of vaccinated and unvaccinated animals and the comparison of mortality prevalence and incidence. For some infection models where disease does not cause mortality this approach to testing vaccine efficacy is not useful. Additionally, in fish experimental studies on vaccine efficacy and immune response the norm is that several individuals are lethally sampled at sequential timepoints, and results are extrapolated to represent the kinetics of immune and disease parameters of an individual fish over the entire experimental infection period. In the present study we developed a new approach to vaccine testing for viremic viruses in fish by following the same individuals over the course of a DNA vaccination and experimental infection through repeated blood collection and analyses. Injectable DNA vaccines are particularly efficient against viral disease in fish. To date, two DNA vaccines have been authorised for use in fish farming, one in Canada against Infectious Haemorrhagic Necrotic virus and more recently one in Europe against Salmon Pancreatic Disease virus (SPDv) subtype 3. In the current study we engineered and used an experimental DNA vaccine against SPDv subtype 1. We measured viremia using a reporter cell line system and demonstrated that the viremia phase was completely extinguished following DNA vaccination. Differences in viremia infection kinetics between fish in the placebo group could be related to subsequent antibody levels in the individual fish, with higher antibody levels at terminal sampling in fish showing earlier viremia peaks. The results indicate that sequential non-lethal sampling can highlight associations between infection traits and immune responses measured at asynchronous timepoints and, can provide biological explanations for variation in data. Similar to results observed for the SPDv subtype 3 DNA vaccine, the SPDv subtype 1 DNA vaccine also induced an interferon type 1 response after vaccination and provided high protection against SPDv under laboratory conditions when fish were challenged at 7 weeks post-vaccination.

Cellular Immune Responses in Rainbow Trout (Onchorhynchus mykiss) Following Vaccination and Challenge Against Salmonid Alphavirus (SAV)

Viral disease outbreaks remain a significant limiting factor for aquaculture. The majority of licensed vaccines used in the industry are administered as oil-adjuvanted formulations carrying inactivated whole pathogens. Cell-mediated immune responses, in particular those based on virus-specific cytotoxic T-cells (CTLs) to conventional inactivated oil-based vaccines, are largely unexplored. As vaccines cannot be optimized against viral pathogens if knowledge of host cellular immune mechanisms remains unknown, in this study we examined fundamental cell-mediated immune responses after vaccination of rainbow trout with an oil-adjuvanted inactivated vaccine against salmonid alphavirus (SAV) and after infection with SAV. A unique in vitro model system was developed to examine MHC class I restricted CTL responses in a clonal line of rainbow trout. The levels of cell-mediated cytotoxicity were compared to pathology, virus load, specific antibody response, changes in immune cell populations, and mRNA expression. Our results hint that different protective mechanisms are being triggered by infection compared to vaccination. While vaccination itself did not cause a strong cytotoxic or humoral response, subsequent challenge of vaccinated fish resulted in significantly stronger and faster specific cytotoxicity, alongside reduced viral titers and pathology. Hence, testing a vaccine on the capacity to induce cell-mediated cytotoxicity will still require a challenge test. Examination of cellular markers additionally indicates that the initial innate response induced by the vaccine could play an important role in steering adaptive mechanisms.

Comparative analysis of marine and freshwater viral haemorrhagic septicaemia virus (VHSV) isolates antagonistic activity

Viral Haemorrhagic Septicaemia Virus (VHSV) isolates virulent to marine fish species can replicate in freshwater species, although producing little or no mortality. Conversely, isolates from freshwater fish do not cause disease in marine species. An inverse relationship between VHSV virulence and host mx gene up-regulation has been described for several fish species, suggesting that differences between the antagonistic activity exerted by these isolates might be involved in the outcome of infections. In this study, the antagonistic activity against the type I interferon system of two representative marine and freshwater VHSV isolates has been characterised using RTG-2 cells stably transfected with the luciferase gene under the control of the Senegalese sole mx (ssmx) promoter, RTG pssmx-luc cells. Both isolates exerted a dose-dependent negative effect on the activation of ssmx promoter, showing a notably different minimal viral dose to exert the antagonism. In particular, an inverse relationship between the minimal MOI required and the viral virulence to sole has been recorded, which suggests this parameter as a possible in vivo VHSV virulence marker. Furthermore, the quantification of the endogenous inf I, mx1 and mx3 mRNA has demonstrated differences between both isolates in their antagonistic activity. Besides, a different nv RNA kinetics, which seems to depend on specific cellular factors, has been recorded for both isolates. This knowledge could contribute to the development of efficient tools to fight against viral infections in fish farming. For that purpose, the RTG pssmx-luc cells may be a suitable in vitro tool to identify the molecular mechanisms underlying VHSV-host interactions.

Engineered cell lines for fish health research

As fish farming continues to increase worldwide, the related research areas of fish disease and immunology are also expanding, aided by the revolution in access to genomic information and molecular technology. The genomes of most fish species of economic importance are now available and annotation based on sequence homology with characterised genomes is underway. However, while useful, functional homology is more difficult to determine, there being a lack of widely distributed and well characterised reagents such as monoclonal antibodies, traditionally used in mammalian studies, to help with confirming functions and cellular interactions of fish molecules. In this context, fish cell lines and the possibility of their genetic engineering offer good prospects for studying functional genomics with respect to fish diseases. In this review, we will give an overview of available permanently genetically engineered fish cell lines, as cell-based reporter systems or platforms for expression of endogenous immune or pathogen genes, to investigate interactions and function. The advantages of such systems and the technical challenge for their development will be discussed.

Atlantic salmon cardiac primary cultures: An in vitro model to study viral host pathogen interactions and pathogenesis

Development of Salmon Cardiac Primary Cultures (SCPCs) from Atlantic salmon pre-hatch embryos and their application as in vitro model for cardiotropic viral infection research are described. Producing SCPCs requires plating of trypsin dissociated embryos with subsequent targeted harvest from 24h up to 3 weeks, of relevant tissues after visual identification. SCPCs are then transferred individually to chambered wells for culture in isolation, with incubation at 15-22°. SCPCs production efficiency was not influenced by embryo's origin (0.75/ farmed or wild embryo), but mildly influenced by embryonic developmental stage (0.3 decline between 380 and 445 accumulated thermal units), and strongly influenced by time of harvest post-plating (0.6 decline if harvested after 72 hours). Beating rate was not significantly influenced by temperature (15-22°) or age (2-4 weeks), but was significantly lower on SCPCs originated from farmed embryos with a disease resistant genotype (F = 5.3, p<0.05). Two distinct morphologies suggestive of an ex vivo embryonic heart and a de novo formation were observed sub-grossly, histologically, ultra-structurally and with confocal microscopy. Both types contained cells consistent with cardiomyocytes, endothelium, and fibroblasts. Ageing of SCPCs in culture was observed with increased auto fluorescence in live imaging, and as myelin figures and cellular degeneration ultra-structurally. The SCPCs model was challenged with cardiotropic viruses and both the viral load and the mx gene expression were measurable along time by qPCR. In summary, SCPCs represent a step forward in salmon cardiac disease research as an in vitro model that partially incorporates the functional complexity of the fish heart.

In vivo virulence of viral haemorrhagic septicaemia virus (VHSV) in rainbow trout Oncorhynchus mykiss correlates inversely with in vitro Mx gene expression

The in vitro replication of viral haemorrhagic septicaemia virus (VHSV) isolates from each VHSV genotype and the associated cellular host Mx gene expression were analysed. All the isolates were able to infect RTG-2 cells and induce increased Mx gene expression (generic assay detecting isoforms 1 and 3 [Mx1/3]). A trout pathogenic, genotype Ia isolate (J167), showing high replication in RTG-2 cells (by infective titre and N gene expression) induced lower Mx1/3 gene expression than observed in VHSV isolates known to be non-pathogenic to rainbow trout: 96-43/8, 96-43/10 (Ib); 1p49, 1p53 (II); and MI03 (IVb). Paired co-inoculation assays were analysed using equal number of plaque forming units per ml (PFU) of J167 (Ia genotype) with other less pathogenic VHSV genotypes. In these co-inoculations, the Mx1/3 gene expression was significantly lower than for the non-pathogenic isolate alone. Of the three rainbow trout Mx isoforms, J167 did not induce Mx1 up-regulation in RTG-2 or RTgill-W1 cells. Co-inoculating isolates resulted in greater inhibition of Mx in both rainbow trout cell lines studied. Up-regulation of sea bream Mx in SAF-1 cells induced by 96-43/8 was also lower in co-inoculation assays with J167. The RTG-P1 cell line, expressing luciferase under the control of the interferon-induced Mx rainbow trout gene promoter, showed low luciferase activity when inoculated with pathogenic strains: J167, DK-5131 (Ic), NO-A-163/68 (Id), TR-206239-1, TR-22207111 (Ie), 99-292 (IVa), and CA-NB00-01 (IVc). Co-inoculation assays showed a J167-dose dependent inhibition of the luciferase activity. The data suggest that virulent VHSV isolates may interfere in the interferon pathways, potentially determining higher pathogenicity.

Gilthead seabream (Sparus aurata) Mx gene promoters respond differentially to IPNV and VHSV infections in RTG-2 cells

The understanding of virus-host interactions relies on the knowledge of the regulatory mechanisms of the type I interferon (IFN I)-stimulated genes (ISGs). Among ISGs, those coding Mx proteins play a main role due to their direct antiviral activity. The study of these genes in gilthead seabream is interesting, since this species displays high natural resistance to viral diseases, being asymptomatic carrier of infectious pancreatic necrosis virus (IPNV) and viral haemorrhagic septicaemia virus (VHSV). Gilthead seabream has three Mx genes (Mx1, Mx2, and Mx3), encoding proteins with a wide spectrum of antiviral activity. The structure of the three promoters (pMx1, pMx2 and pMx3) has been previously disclosed, and their response to poly I:C in RTG-2 cells characterized. To further analyze these promoters, their response to two viral infections has been evaluated in the present study. For that purpose, RTG-2 cells transiently transfected with the luciferase gene under the control of each promoter were inoculated with either IPNV or VHSV at two different doses. The highest and lowest fold induction values were recorded for pMx2 and pMx3, respectively. The promoter induction was always stronger after VHSV inoculation than in IPNV-inoculated cells. In addition, the higher dose of VHSV tested induced higher response of the three promoters, whereas in IPNV-infected cells the highest induction was recorded after inoculation with the lower viral dose. To further study the response of the Mx2 promoter, RTG-2 cells stably transfected with the luciferase gene under the control of pMx2 were stimulated with poly I:C and subsequently infected with IPNV or VHSV. Interestingly, IPNV infection inhibited the induction caused by poly I:C, suggesting an antagonistic activity of IPNV on Mx2 transcription. In contrast, VHSV infection did not alter the response triggered by poly I:C. These results highlight the specific regulation that controls the activity of each promoter, and support the existence of complex interactions between host cells, specific Mx promoters, and viruses, which are responsible for the final outcome of a viral infection.

Individual Monitoring of Immune Response in Atlantic Salmon Salmo salar following Experimental Infection with Infectious Salmon Anaemia Virus (ISAV)

Monitoring the immune response in fish over the progression of a disease is traditionally carried out by experimental infection whereby animals are killed at regular intervals and samples taken. We describe here a novel approach to infectiology for salmonid fish where blood samples are collected repeatedly in a small group of PIT-tagged animals. This approach contributes to the reduction of animals used in research and to improved data quality. Two groups of 12 PIT-tagged Atlantic salmon (Salmo salar) were i.p infected with Infectious Salmon Anaemia Virus (ISAV) or culture medium and placed in 1 m3 tanks. Blood samples were collected at 0, 4, 8, 12, 16, 21 and 25 days post infection. The viral load, immune and stress response were determined in individual fish by real-time quantitative PCR (QPCR) on the blood cells, as well as the haematocrit used as an indicator of haemolysis, a clinical consequence of ISAV infection. "In-tank" anaesthesia was used in order to reduce the stress related to chase and netting prior to sampling. The data were analysed using a statistical approach which is novel with respect to its use in fish immunology. The repeated blood collection procedure did not induce stress response as measured by HSP70 and HSP90 gene expression in the un-infected animals. A strong increase in viraemia as well as a significant induction of Mx and γIP gene expression were observed in the infected group. Interleukin 10 was found induced at the later stage of the infection whereas no induction of CD8 or γ IFN could be detected. These results and the advantages of this approach are discussed.

Innate immune responses of salmonid fish to viral infections

Viruses are the most serious pathogenic threat to the production of the main aquacultured salmonid species the rainbow trout Oncorhynchus mykiss and the Atlantic salmon Salmo salar. The viral diseases Infectious Pancreatic Necrosis (IPN), Pancreatic Disease (PD), Infectious Haemorrhagic Necrosis (IHN), Viral Haemorrhagic Septicaemia (VHS), and Infectious Salmon Anaemia (ISA) cause massive economic losses to the global salmonid aquaculture industry every year. To date, no solution exists to treat livestock affected by a viral disease and only a small number of efficient vaccines are available to prevent infection. As a consequence, understanding the host immune response against viruses in these fish species is critical to develop prophylactic and preventive control measures. The innate immune response represents an important part of the host defence mechanism preventing viral replication after infection. It is a fast acting response designed to inhibit virus propagation immediately within the host, allowing for the adaptive specific immunity to develop. It has cellular and humoral components which act in synergy. This review will cover inflammation responses, the cell types involved, apoptosis, antimicrobial peptides. Particular attention will be given to the type I interferon system as the major player in the innate antiviral defence mechanism of salmonids. Viral evasion strategies will also be discussed.