Analysis of host- and strain-dependent cell death responses during infectious salmon anemia virus infection in vitro

Effects of eicosapentaneoic acid on innate immune responses in an Atlantic salmon kidney cell line in vitro

Studies of the interplay between metabolism and immunity, known as immunometabolism, is steadily transforming immunological research into new understandings of how environmental cues like diet are affecting innate and adaptive immune responses. The aim of this study was to explore antiviral transcriptomic responses under various levels of polyunsaturated fatty acid. Atlantic salmon kidney cells (ASK cell line) were incubated for one week in different levels of the unsaturated n-3 eicosapentaneoic acid (EPA) resulting in cellular levels ranging from 2-20% of total fatty acid. These cells were then stimulated with the viral mimic and interferon inducer poly I:C (30 ug/ml) for 24 hours before total RNA was isolated and sequenced for transcriptomic analyses. Up to 200 uM EPA had no detrimental effects on cell viability and induced very few transcriptional changes in these cells. However, in combination with poly I:C, our results shows that the level of EPA in the cellular membranes exert profound dose dependent effects of the transcriptional profiles induced by this treatment. Metabolic pathways like autophagy, apelin and VEGF signaling were attenuated by EPA whereas transcripts related to fatty acid metabolism, ferroptosis and the PPAR signaling pathways were upregulated. These results suggests that innate antiviral responses are heavily influenced by the fatty acid profile of salmonid cells and constitute another example of the strong linkage between general metabolic pathways and inflammatory responses.

Kinetics of transcriptional response against poly (I:C) and infectious salmon anemia virus (ISAV) in Atlantic salmon kidney (ASK) cell line

Vaccine adjuvants induce host innate immune responses improving long-lasting adaptive immunity against vaccine antigens. In vitro models can be used to compare these responses between adjuvants and the infection targeted by the vaccine. We utilized transcriptomic profiling of an Atlantic salmon cell line to compare innate immune responses against ISAV and an experimental viral vaccine adjuvant: poly (I:C). Induction of interferon and interferon induced genes were observed after both treatments, but often with different amplitude and kinetics. Using KEGG ortholog database and available software from Bioconductor we could specify a complete bioinformatic pipeline for analysis of transcriptomic data from Atlantic salmon, a feature not previously available. We have identified important differences in the transcriptional profile of Atlantic salmon cells exposed to viral infection and a viral vaccine adjuvant candidate, poly (I:C). This report increases our knowledge of viral host-pathogen interaction in salmon and to which extent these can be mimicked by adjuvant compounds.

Interaction between dietary fatty acids and genotype on immune response in Atlantic salmon (Salmo salar) after vaccination: A transcriptome study

A pivotal matter to aquaculture is the sourcing of sustainable resources as ingredients to aquafeeds. Levels of plant delivered oils as source of fatty acids (FA) in aquafeeds have reached around 70% resulting in reduced levels of long-chain omega-3 polyunsaturated fatty acids (LC n-3 PUFA), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), in salmon fillet composition. EPA and DHA can modulate inflammation and immune response, so it is crucial to understand how fish immune response is affected by low LC n-3 PUFA diet and if this diet can have a detrimental effect on vaccine response. Atlantic salmon (Salmo salar) can produce EPA/DHA from α-linolenic acid (ALA) and this endogenous capacity can be explored to develop families with higher tolerance to low LC n-3 PUFA diets. Here we analyze innate and adaptive immune response in Atlantic salmon to a commercial vaccine after being fed low levels of EPA and DHA, and we also compare three strains of salmon selected by their endogenous capacity of synthesizing LC- n-3 PUFA. A total of 2,890 differentially expressed genes (DEGs) were identified (p-value adjusted < 0.1) when comparing vaccinated fish against control non-vaccinated. Gene ontology (GO) and KEGG analysis with 442 up/downregulated genes revealed that most DEGs were both related to immune response as well as part of important immune related pathways, as "Toll-like receptor" and "Cytokine-Cytokine interaction". Adaptive response was also addressed by measuring antigen specific IgM, and titers were significantly higher than in the pre-immune fish at 62 days post-immunization. However, diet and strain had no/little effect on vaccine-specific IgM or innate immune responses. Atlantic salmon therefore display robustness in its response to vaccination even when feed low levels of LC n-3 PUFA.

The immune evasion strategies of fish viruses

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

Effects of dietary n-3 fatty acids on Toll-like receptor activation in primary leucocytes from Atlantic salmon (Salmo salar)

The shortage of the n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on the international markets has led to increasing substitution of fish oil by plant oils in Atlantic salmon (Salmo salar) feed and thereby reducing the EPA and DHA content in salmon. However, the minimum required levels of these fatty acids in fish diets for securing fish health are unknown. Fish were fed with 0, 1 or 2% EPA or DHA alone or in combination of both over a period, growing from 50 to 400 g. Primary head kidney leucocytes were isolated and stimulated with Toll-like receptor (TLR) ligands to determine if EPA and DHA deficiency can affect expression of important immune genes and eicosanoid production. Several genes related to viral immune response did not vary between groups. However, there was a tendency that the high-level EPA and DHA groups expressed lower levels of IL-1β in non-stimulated leucocytes. These leucocytes were also more responsive to the TLR ligands, inducing higher expression levels of IL-1β and Mx1 after stimulation. The levels of prostaglandin E2 and leukotriene B4 in serum and media from stimulated leucocytes were lower in both low and high EPA and DHA groups. In conclusion, leucocytes from low EPA and DHA groups seemed to be less responsive towards immunostimulants, like TLR ligands, indicating that low levels or absence of dietary EPA and DHA may have immunosuppressive effects.

Crystal structure of an orthomyxovirus matrix protein reveals mechanisms for self-polymerization and membrane association

Many enveloped viruses encode a matrix protein. In the influenza A virus, the matrix protein M1 polymerizes into a rigid protein layer underneath the viral envelope to help enforce the shape and structural integrity of intact viruses. The influenza virus M1 is also known to mediate virus budding as well as the nuclear export of the viral nucleocapsids and their subsequent packaging into nascent viral particles. Despite extensive studies on the influenza A virus M1 (FLUA-M1), only crystal structures of its N-terminal domain are available. Here we report the crystal structure of the full-length M1 from another orthomyxovirus that infects fish, the infectious salmon anemia virus (ISAV). The structure of ISAV-M1 assumes the shape of an elbow, with its N domain closely resembling that of the FLUA-M1. The C domain, which is connected to the N domain through a flexible linker, is made of four α-helices packed as a tight bundle. In the crystal, ISAV-M1 monomers form infinite 2D arrays with a network of interactions involving both the N and C domains. Results from liposome flotation assays indicated that ISAV-M1 binds membrane via electrostatic interactions that are primarily mediated by a positively charged surface loop from the N domain. Cryoelectron tomography reconstruction of intact ISA virions identified a matrix protein layer adjacent to the inner leaflet of the viral membrane. The physical dimensions of the virion-associated matrix layer are consistent with the 2D ISAV-M1 crystal lattice, suggesting that the crystal lattice is a valid model for studying M1-M1, M1-membrane, and M1-RNP interactions in the virion.

Autophagy and apoptosis induced by Chinese giant salamander (Andrias davidianus) iridovirus (CGSIV)

The outbreak of Chinese Giant Salamander (Andrias davidianus, CGS) Iridovirus (CGSIV) caused massive death of CGSs. However, some CGSs with low level of CGSIV usually could survive. In our study, major capsid protein (MCP) DNA replicates of CGSIV in shedding skin were employed to assess the relative content of CGSIV in the living CGSs by qPCR. Furthermore, the examinations of autophagy and apoptosis in CGSs in vivo and in the primary renal cells in vitro were performed, respectively. The results showed that the relative contents of CGSIV in the shedding skin could reflect those in liver, spleen, and kidney of the CGSs. In these tissues of the CGSs with low-level replicates of CGSIV, there were not obviously macroscopic lesions. But the irregularly-shaped vesicles perhaps involving in autophagosome were observed by transmission electron microscopy (TEM). The LC3B protein displayed uneven distribution by Immunohistochemistry and the level mRNA of Atg5 was higher in these tissues than that in the tissues of healthy CGSs using qRT-PCR. Meanwhile, the apoptosis also appeared in these tissues by TUNEL staining and higher level mRNA of type I IFN were detected in these tissues using qRT-PCR. Further, both the expression level of LC3B II protein and Atg5 mRNA increased significantly at 2h after the virus infected the primary renal cells from the health CGSs in vitro. In addition, apoptosis and type I IFN mRNA began to increase significantly at 4h after the virus infected the renal cells. It was suggested that autophagy may be a pivotal role for survival of CGSIV in the CGSs during early infection and the rapid proliferation of CGSIV could be inhibited by innate immune response and apoptosis.

Infectious Salmon Anaemia Virus (ISAV) RNA Binding Protein Encoded by Segment 8 ORF2 and Its Interaction with ISAV and Intracellular Proteins

Infectious salmon anaemia virus (ISAV) is an orthomyxovirus infecting salmonid fish. The virus is adapted to low temperature and has a replication optimum between 10–15 °C. In this study the subcellular localization and protein interactions for the protein encoded by the largest open reading frame of gene segment 8 (s8ORF2) were investigated. In ISAV infected cells the s8ORF2 protein was found mainly in the cytosol but a minor fraction of cells expressed the protein in the nucleus as well. Green fluorescent protein-tagged s8ORF2 did not leak out of the cell when the plasma membrane was permeabilized, suggesting interactions with intracellular structural components. The s8ORF2 protein exists both as monomer and homodimer, and co-immunoprecipitation experiments strongly suggests it binds to the ISAV fusion-, nucleo- and matrix proteins. Two versions of s8ORF2 were detected with apparent molecular weights of 24–26 and 35 kDa in lysates of infected cells. The 35 kDa type is an early viral protein while the smaller version appears during the later phases of infection. The 24–26 kDa type was also the predominant form in viral particles. The s8ORF2 protein has previously been shown to bind RNA and interfere with interferon induction and signaling. Here we found that a fraction of the s8ORF2 protein pool in infected cells is likely to be conjugated to the interferon stimulated gene 15 (ISG15) and ubiquitin. Furthermore, several endogenous proteins pulled down by the s8ORF2 protein were identified by liquid chromatography mass spectrometry (LC-MS).

Activation of unfolded protein response pathway during infectious salmon anemia virus (ISAV) infection in vitro an in vivo

Infectious salmon anemia virus (ISAV) is a salmon pathogen causing serious outbreaks in fish farms world-wide. There is currently no effective commercially available vaccine and there is a need for better understanding of host pathogen interactions with this virus. Various strains can cause both acute and persistent infections and therefore establish a balance with the host immune responses. We have studied host responses to this infection by analyzing the main branches of the unfolded protein response (UPR) in salmon cells in vitro and in tissues from infected fish to gain a better understanding of virus-host interactions. ISAV induce the main symptoms and signaling pathways of UPR (ATF6, PERK and IRE1) without inducing translational attenuation. This may be due to concomitant induction of an important negative feedback loop via the phosphatase regulator GADD34. The host cells can therefore respond with translation of cytokine and antiviral proteins to curb or control infection.

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.

Gene miles-apart is required for formation of otic vesicle and hair cells in zebrafish

Hearing loss is a serious burden to physical and mental health worldwide. Aberrant development and damage of hearing organs are recognized as the causes of hearing loss, the molecular mechanisms underlining these pathological processes remain elusive. Investigation of new molecular mechanisms involved in proliferation, differentiation, migration and maintenance of neuromast primordium and hair cells will contribute to better understanding of hearing loss pathology. This knowledge will enable the development of protective agents and mechanism study of drug ototoxicity. In this study, we demonstrate that the zebrafish gene miles-apart, a homolog of sphingosine-1-phosphate receptor 2 (s1pr2) in mammals, has an important role in the development of otic vesicle, neuromasts and survival of hair cells. Whole-mount in situ hybridization of embryos showed that miles-apart expression occurred mainly in the encephalic region and the somites at 24 h.p.f. (hour post fertilization), in the midbrain/hindbrain boundary, the brainstem and the pre-neuromast of lateral line at 48 h.p.f. in a strict spatiotemporal regulation. Both up- and downregulation of miles-apart led to abnormal otoliths and semicircular canals, excess or few hair cells and neuromasts, and their disarranged depositions in the lateral lines. Miles-apart (Mil) dysregulation also caused abnormal expression of hearing-associated genes, including hmx2, fgf3, fgf8a, foxi1, otop1, pax2.1 and tmieb during zebrafish organogenesis. Moreover, in larvae miles-apart gene knockdown significantly upregulated proapoptotic gene zBax2 and downregulated prosurvival gene zMcl1b; in contrast, the level of zBax2 was decreased and of zMcl1b enhanced by miles-apart overexpression. Collectively, Mil activity is linked to organization and number decision of hair cells within a neuromast, also to deposition of neuromasts and formation of otic vesicle during zebrafish organogenesis. At the larva stage, Mil as an upstream regulator of bcl-2 gene family has a role in protection of hair cells against apoptosis by promoting expression of prosurvival gene zMcl1b and suppressing proapoptotic gene zBax2.

Molecular strategies used by fish pathogens to interfere with host-programmed cell death

Cell death is of pivotal importance in the regulation of the immune response and has a direct impact in disease resistance. Fish are becoming an interesting model organism to study the immune response since they hold a key phylogenetic position and many species are of high economic interest. The role of cell death in the immune response has recently been investigated in fish and the molecules and pathways orchestrating cell death in this group of animals have begun to be elucidated. In this study, we will summarize the different molecular strategies displayed by major fish bacterial and viral pathogens to interfere with programmed cell death of the host as well as the relevance of cell death in the resolution of the infectious diseases caused by these pathogens.