Establishment of an In Vitro Model to Study Viral Infections of the Fish Intestinal Epithelium.

Viral infections are still a major concern for the aquaculture industry. For salmonid fish, even though breeding strategies and vaccine development have reduced disease outbreaks, viral diseases remain among the main challenges having a negative impact on the welfare of fish and causing massive economic losses for the industry. The main entry port for viruses into the fish is through mucosal surfaces including that of the gastrointestinal tract. The contradictory functions of this surface, both creating a barrier towards the external environment and at the same time being responsible for the uptake of nutrients and ion/water regulation make it particularly vulnerable. The connection between dietary components and viral infections in fish has been poorly investigated and until now, a fish intestinal in vitro model to investigate virus-host interactions has been lacking. Here, we established the permissiveness of the rainbow trout intestinal cell line RTgutGC towards the important salmonid viruses-infectious pancreatic necrosis virus (IPNV), salmonid alphavirus (subtype 3, SAV3) and infectious salmon anemia virus (ISAV)-and explored the infection mechanisms of the three different viruses in these cells at different virus to cell ratios. Cytopathic effect (CPE), virus replication in the RTgutGC cells, antiviral cell responses and viral effects on the barrier permeability of polarized cells were investigated. We found that all virus species infected and replicated in RTgutGC cells, although with different replication kinetics and ability to induce CPE and host responses. The onset and progression of CPE was more rapid at high multiplicity of infection (MOI) for IPNV and SAV3 while the opposite was true of ISAV. A positive correlation between the MOI used and the induction of antiviral responses was observed for IPNV while a negative correlation was detected for SAV3. Viral infections compromised barrier integrity at early time points prior to observations of CPE microscopically. Further, the replication of IPNV and ISAV had a more pronounced effect on barrier function than SAV3. The in vitro infection model established herein can thus provide a novel tool to generate knowledge about the infection pathways and mechanisms used to surpass the intestinal epithelium in salmonid fish, and to study how a virus can potentially compromise gut epithelial barrier functions.

Sea Lice (Lepeophtheirus salmonis) Infestation Reduces the Ability of Peripheral Blood Monocytic Cells (PBMCs) to Respond to and Control Replication of Salmonid Alphavirus in Atlantic Salmon (Salmo salar L.).

Here we have studied the impact of lice (Lepeophtheirus salmonis) infestation of donor fish on the ability of isolated peripheral blood monocytes (PMBCs) to control the replication of salmonid alphavirus (SAV) ex vivo. PMBC were collected by Percoll gradients at eight and nine weeks post copepodid infestation of Atlantic salmon post smolt. Uninfested fish were controls. PBMCs were then infected ex vivo with SAV (subtype 3), and samples were collected for analysis at two, four, and six days post virus infection. Virus titer in the supernatant was assayed in CHH-1 cells, and in addition, the relative expression of the virus structural protein E2 and selected host antiviral genes, IRF9, ISG15, Mx, and IFIT5, were assayed using real-time PCR. Significantly higher virus replication was detected in cells collected from lice-infested fish compared to controls. Higher virus titer coincided with an inability to upregulate the expression of different immune genes, IFIT5, IRF9, and Mx. These findings point towards compromised ability of PMBCs from lice-infested fish to control virus replication, and, to our knowledge, is the first report showing the direct effect of lice infestation on the interplay between viruses and immune cells. There is a possible impact on the dynamic spread of viral diseases in the aquatic environment.

Efficacy and safety of boric acid as a preventive treatment against Saprolegnia infection in Nile tilapia (Oreochromis niloticus).

Saprolegniosis is a worldwide fungal-like infection affecting freshwater fishes and their eggs. Reports show high mortalities and subsequent economic losses annually from Saprolegnia infections. Most therapeutants against Saprolegnia spp. infections are inefficient and some have negative impact on the environment. In this study, we have investigated the ability of boric acid (BA) to prevent Saprolegnia infection in Nile tilapia (Oreochromis niloticus). BA inhibited radial growth of Saprolegnia hyphae in vitro. Complete in vitro growth inhibition was found at a concentration of ≥0.6 g/L. Inhibitory effects were also observed in vivo when Nile tilapia were experimentally challenged with Saprolegnia spores and followed over 10 days post challenge and under continuous exposure to different BA concentrations. No signs of saprolegniosis were observed in fish treated with BA at concentrations of 0.4 g/L and above. Comet assay revealed that BA has low toxicity in tilapia continuously exposed to concentrations of 0.2-0.6 g/L for 96 h. Additionally, no significant histomorphological changes were observed in BA-treated fish compared to non-treated controls. Alanine Aminotransferase (ALT) and Aspartate Aminotransferase (AST) enzyme levels indicated reduction in systemic tissue damage associated with Saprolegnia infection. This study demonstrates the potential of BA as a prophylactic measure against Saprolegnia infection in tilapia, and we recommend additional studies on environmental impact.

Apoptosis Induction by dsRNA-Dependent Protein Kinase R (PKR) in EPC Cells via Caspase 8 and 9 Pathways.

dsRNA-dependent protein kinase R (PKR) is an interferon-inducible protein that mediates antiviral effects and induces apoptosis. We studied PKR-related apoptosis mechanisms by transfecting wild type pcDNA-carp-wtPKR, a catalytically inactive mutant pcDNA-mut-carpPKR, and empty plasmid in Epithelioma papulosum cyprini (EPC) cells, designated wtPKR, mutPKR, and pcDNA3.1, respectively. PKR was inefficiently expressed from wtPKR unlike mutPKR that produced high PKR levels detected by western blot. eIF2α phosphorylation increased in wtPKR-transfected cells, while for mutPKR, phosphorylation was not different from non-transfected controls. Flow-cytometry revealed high level of apoptosis in wtPKR transfected cells, corresponding with high cytopathic effect. mutPKR and pcDNA3.1 transfection gave significantly less apoptosis and were not different from each other. Caspase-8 and -9 were activated for wtPKR, suggesting death receptor-caspase-8 and mitochondrion-dependent caspase-9 activated pathways, similar to mammalian cells. These findings suggest that the induction of apoptosis via the caspase-8 and -9 pathways are conserved in vertebrate taxa and likely play a role in viral infections of lower vertebrates.

PKR Activation Favors Infectious Pancreatic Necrosis Virus Replication in Infected Cells.

The double-stranded RNA-activated protein kinase R (PKR) is a Type I interferon (IFN) stimulated gene that has important biological and immunological functions. In viral infections, in general, PKR inhibits or promotes viral replication, but PKR-IPNV interaction has not been previously studied. We investigated the involvement of PKR during infectious pancreatic necrosis virus (IPNV) infection using a custom-made rabbit antiserum and the PKR inhibitor C16. Reactivity of the antiserum to PKR in CHSE-214 cells was confirmed after IFNα treatment giving an increased protein level. IPNV infection alone did not give increased PKR levels by Western blot, while pre-treatment with PKR inhibitor before IPNV infection gave decreased eukaryotic initiation factor 2-alpha (eIF2α) phosphorylation. This suggests that PKR, despite not being upregulated, is involved in eIF2α phosphorylation during IPNV infection. PKR inhibitor pre-treatment resulted in decreased virus titers, extra- and intracellularly, concomitant with reduction of cells with compromised membranes in IPNV-permissive cell lines. These findings suggest that IPNV uses PKR activation to promote virus replication in infected cells.

Infection Profiles of Selected Aquabirnavirus Isolates in CHSE Cells.

The wide host range and antigenic diversity of aquabirnaviruses are reflected by the presence of a collection of isolates with different sero- and genotypic properties that have previously been classified as such. Differences in cytopathogenic mechanisms and host responses induced by these isolates have not been previously examined. In the present study, we investigated infection profiles induced by genetically and serologically closely related as well as distant isolates in-vitro. CHSE-214 cells were infected with either E1S (serotype A3, genogroup 3), VR-299 (serotype A1, genogroup 1), highly virulent Sp (TA) or avirulent Sp (PT) (serotype A2, genogroup 5). The experiments were performed at temperatures most optimum for each of the isolates namely 15°C for VR-299, TA and PT strains and 20°C for E1S. Differences in virus loads and ability to induce cytopathic effect, inhibition of protein synthesis, apoptosis, and induction of IFNa, Mx1, PKR or TNFα gene expression at different times post infection were examined. The results showed on one hand, E1S with the highest ability to replicate, induce apoptosis and IFNa gene expression while VR-299 inhibited protein synthesis and induced Mx1 and PKR gene expression the most. The two Sp isolates induced the highest TNFα gene expression but differed in their ability to replicate, inhibit protein synthesis, and induce gene expression, with TA being more superior. Collectively, these findings point towards the adaptation by different virus isolates to suit environments and hosts that they patronize. Furthermore, the results also suggest that genetic identity is not prerequisite to functional similarities thus results of one aquabirnavirus isolate cannot necessarily be extrapolated to another.

A piscine birnavirus induces inhibition of protein synthesis in CHSE-214 cells primarily through the induction of eIF2α phosphorylation.

Inhibition of protein synthesis represents one of the antiviral mechanisms employed by cells and it is also used by viruses for their own propagation. To what extent members of the Birnaviridae family employ such strategies is not well understood. Here we use a type-strain of the Aquabirnavirus, infectious pancreatic necrosis virus (IPNV), to investigate this phenomenon in vitro. CHSE-214 cells were infected with IPNV and at 3, 12, 24, and 48 hours post infection (hpi) before the cells were harvested and labeled with S35 methionine to assess protein synthesis. eIF2α phosphorylation was examined by Western blot while RT-qPCR was used to assess virus replication and the expression levels of IFN-α, Mx1 and PKR. Cellular responses to IPNV infection were assessed by DNA laddering, Caspase-3 assays and flow cytometry. The results show that the onset and kinetics of eIF2α phosphorylation was similar to that of protein synthesis inhibition as shown by metabolic labeling. Increased virus replication and virus protein formation was observed by 12 hpi, peaking at 24 hpi. Apoptosis was induced in a small fraction (1-2%) of IPNV-infected CHSE cells from 24 hpi while necrotic/late apoptotic cells increased from 10% by 24 hpi to 59% at 48 hpi, as shown by flow cytometry. These results were in accordance with a small decline in cell viability by 24hpi, dropping below 50% by 48 hpi. IPNV induced IFN-α mRNA upregulation by 24 hpi while no change was observed in the expression of Mx1 and PKR mRNA. Collectively, these findings show that IPNV induces inhibition of protein synthesis in CHSE cells through phosphorylation of eIF2α with minimal involvement of apoptosis. The anticipation is that protein inhibition is used by the virus to evade the host innate antiviral responses.

Mitochondrial dysfunction is involved in the toxic activity of boric acid against Saprolegnia.

There has been a significant increase in the incidence of Saprolegnia infections over the past decades, especially after the banning of malachite green. Very often these infections are associated with high economic losses in salmonid farms and hatcheries. The use of boric acid to control the disease has been investigated recently both under in vitro and in vivo conditions, however its possible mode of action against fish pathogenic Saprolegnia is not known. In this study, we have explored the transformation in Saprolegnia spores/hyphae after exposure to boric acid (1 g/L) over a period 4-24 h post treatment. Using transmission electron microscopy (TEM), early changes in Saprolegnia spores were detected. Mitochondrial degeneration was the most obvious sign observed following 4 h treatment in about 20% of randomly selected spores. We also investigated the effect of the treatment on nuclear division, mitochondrial activity and function using confocal laser scanning microscopy (CLSM). Fluorescence microscopy was also used to test the effect of treatment on mitochondrial membrane potential and formation of reactive oxygen species. Additionally, the viability and proliferation of treated spores that correlated to mitochondrial enzymatic activity were tested using an MTS assay. All obtained data pointed towards changes in the mitochondrial structure, membrane potential and enzymatic activity following treatment. We have found that boric acid has no effect on the integrity of membranes of Saprolegnia spores at concentrations tested. It is therefore likely that mitochondrial dysfunction is involved in the toxic activity of boric acid against Saprolegnia spp.