Genetic analysis and comparative virulence of infectious salmon anemia virus (ISAV) types HPR7a and HPR7b from recent field outbreaks in Chile

Modulation of infectious Salmon Anaemia virus infection by clathrin-mediated endocytosis and macropinocytosis inhibitors

Infectious salmon anaemia virus (ISAV) is a pathogen that causes disease and large mortality in farm-raised Salmo salar L., being considered as a major problem in the salmon industry. However, despite its relevance, there are still numerous knowledge gaps on virus entry and early stages of infection. Previous studies suggested that virus entry into cells occurs via endocytosis, with no description of specific mechanisms. However, it remains unknown if the endocytosis induced by ISAV is a clathrin-dependent or clathrin-independent process. This study aimed to identify cellular mechanisms allowing ISAV entry into Atlantic Salmon head kidney (ASK) cells. Our results showed that ISAV can be found in coated pits and membrane ruffles, the latter being induced by a rearrangement of actin filaments promoted by ISAV infection. Additionally, it was determined that ISAV stimulate the uptake of extracellular fluid in a multiplicity of infection (MOI)-dependent manner. When the clathrin-mediated endocytic pathway was pharmacologically inhibited, ISAV infection was significantly reduced but not entirely inhibited. Similarly, when the Na+/H+ exchanger (NHE), a key component of macropinocytosis, was inhibited, ISAV infection was negatively affected. Our results suggest that ISAV enters cells via both clathrin-mediated endocytosis and most likely macropinocytosis.

Infectious Salmon Anemia Virus Infectivity Is Determined by Multiple Segments with an Important Contribution from Segment 5

Infectious salmon anemia virus (ISAV) is the etiological agent of infectious salmon anemia. It belongs to the genus isavirus, one of the genera of the Orthomyxoviridae family, as does Influenzavirus A. The ISAV genome comprises eight negative-sense single-stranded RNA segments that code for at least 10 proteins. Although some ISAV strains can reach 100% mortality rates, the factors that determine isavirus infectivity remain unknown. However, some studies suggest that segments 5 and 6 are responsible for the different degrees of virulence and infectivity among ISAV subtypes, unlike the influenza A virus, where most segments are involved in the virus infectivity. In this work, synthetic reassortant viruses for the eight segments of ISAV were generated by reverse genetics, combining a highly virulent virus, ISAV 752_09 (HPR7b), and an avirulent strain, SK779/06 (HPR0). We characterized the rescued viruses and their capacity to replicate and infect different cell lines, produce plaques in ASK cells, and their ability to induce and modulate the cellular immune response in vitro. Our results show that the majority of ISAV segments are involved in at least one of the analyzed characteristics, segment 5 being one of the most important, allowing HPR0 viruses, among other things, to produce plaques and replicate in CHSE-214 cells. We determined that segments 5 and 6 participate in different stages of the viral cycle, and their compatibility is critical for viral infection. Additionally, we demonstrated that segment 2 can modulate the cellular immune response. Our results indicate a high degree of genetic compatibility between the genomic segments of HPR7b and HPR0, representing a latent risk of reassortant that would give rise to a new virus with an unknown phenotype.

Interaction of the Amino-Terminal Domain of the ISAV Fusion Protein with a Cognate Cell Receptor

The infectious salmon anemia virus (ISAV), etiological agent of the disease by the same name, causes major losses to the salmon industry. Classified as a member of the Orthomyxoviridae family, ISAV is characterized by the presence of two surface glycoproteins termed hemagglutinin esterase (HE) and fusion protein (F), both of them directly involved in the initial interaction of the virus with the target cell. HE mediates receptor binding and destruction, while F promotes the fusion process of the viral and cell membranes. The carboxy-terminal end of F (F₂) possesses canonical structural characteristics of a type I fusion protein, while no functional properties have been proposed for the amino-terminal (F₁) region. In this report, based on in silico modeling, we propose a tertiary structure for the F₁ region, which resembles a sialic acid binding domain. Furthermore, using recombinant forms of both HE and F proteins and an in vitro model system, we demonstrate the interaction of F with a cell receptor, the hydrolysis of this receptor by the HE esterase, and a crucial role for F₁ in the fusion mechanism. Our interpretation is that binding of F to its cell receptor is fundamental for membrane fusion and that the esterase in HE modulates this interaction.

Infectious salmon anaemia virus-molecular biology and pathogenesis of the infection

Aquaculture has a long history in many parts of the world, but it is still young at an industrial scale. Marine fish farming in open nets of a single fish species at high densities compared to their wild compatriots opens a plethora of possible infections. Infectious salmon anaemia (ISA) is an example of disease that surfaced after large-scale farming of Atlantic salmon (Salmo salar) appeared. Here, a review of the molecular biology of the ISA virus (ISAV) with emphasis on its pathogenicity is presented. The avirulent HPR0 variant of ISAV has resisted propagation in cell cultures, which has restricted the ability to perform in vivo experiments with this variant. The transition from avirulent HPR0 to virulent HPRΔ has not been methodically studied under controlled experimental conditions, and the triggers of the transition from avirulent to virulent forms have not been mapped. Genetic segment reassortment, recombination and mutations are important mechanisms in ISAV evolution, and for the development of virulence. In the 25 years since the ISAV was identified, large amounts of sequence data have been collected for epidemiologic and transmission studies, however, the lack of good experimental models for HPR0 make the risk evaluation of the presence of this avirulent, ubiquitous variant uncertain. This review summarizes the current knowledge related to molecular biology and pathogenicity of this important aquatic orthomyxovirus.

Wild and farmed salmon (Salmo salar) as reservoirs for infectious salmon anaemia virus, and the importance of horizontal- and vertical transmission

The infectious salmon anaemia virus (ISAV) is an important pathogen on farmed salmon in Europe. The virus occurs as low- and high virulent variants where the former seem to be a continuous source of new high virulent ISAV. The latter are controlled in Norway by stamping out infected populations while the former are spreading uncontrolled among farmed salmon. Evidence of vertical transmission has been presented, but there is still an ongoing discussion of the importance of circulation of ISAV via salmon brood fish. The only known wild reservoirs are in trout (Salmo trutta) and salmon (Salmo salar). This study provides the first ISAV sequences from wild salmonids in Norway and evaluates the importance of this reservoir with respect to outbreaks of ISA among farmed salmon. Phylogenetic analyses of the surface protein hemagglutinin-esterase gene from nearly all available ISAV from Norway, Faeroe Islands, Scotland, Chile and wild salmonids in Norway show that they group into four major clades. Including virulent variants in the analysis show that they belong in the same four clades supporting the hypothesis that there is a high frequency of transition from low to high virulent variants in farmed populations of salmon. There is little support for a hypothesis suggesting that the wild salmonids feed the virus into farmed populations. This study give support to earlier studies that have documented local horizontal transmission of high virulent ISAV, but the importance of transition from low- to high virulent variants has been underestimated. Evidence of vertical transmission and long distance spreading of ISAV via movement of embryos and smolt is presented. We recommend that the industry focus on removing the low virulent ISAV from the brood fish and that ISAV-free brood fish salmon are kept in closed containment systems (CCS).

Mutation and recombination in pathogen evolution: Relevance, methods and controversies

Mutation and recombination drive the evolution of most pathogens by generating the genetic variants upon which selection operates. Those variants can, for example, confer resistance to host immune systems and drug therapies or lead to epidemic outbreaks. Given their importance, diverse evolutionary studies have investigated the abundance and consequences of mutation and recombination in pathogen populations. However, some controversies persist regarding the contribution of each evolutionary force to the development of particular phenotypic observations (e.g., drug resistance). In this study, we revise the importance of mutation and recombination in the evolution of pathogens at both intra-host and inter-host levels. We also describe state-of-the-art analytical methodologies to detect and quantify these two evolutionary forces, including biases that are often ignored in evolutionary studies. Finally, we present some of our former studies involving pathogenic taxa where mutation and recombination played crucial roles in the recovery of pathogenic fitness, the generation of interspecific genetic diversity, or the design of centralized vaccines. This review also illustrates several common controversies and pitfalls in the analysis and in the evaluation and interpretation of mutation and recombination outcomes.

Transcriptomic analysis of spleen infected with infectious salmon anemia virus reveals distinct pattern of viral replication on resistant and susceptible Atlantic salmon (Salmo salar)

The infectious salmon anemia virus (ISAv) produces a systemic infection in salmonids, causing large losses in salmon production. However, little is known regarding the mechanisms exerting disease resistance. In this paper, we perform an RNA-seq analysis in Atlantic salmon challenged with ISAv (using individuals coming from families that were highly susceptible or highly resistant to ISAv infection). We evaluated the differential expression of both host and ISAv genes in a target organ for the virus, i.e. the spleen. The results showed differential expression of host genes related to response to stress, immune response and protein folding (genes such as; atf3, mhc, mx1-3, cd276, cd2, cocs1, c7, il10, il10rb, il13ra2, ubl-1, ifng, ifngr1, hivep2, sigle14 and sigle5). An increased protein processing activity was found in susceptible fish, which generates a subsequent unfolded protein response. We observed extreme differences in the expression of viral segments between susceptible and resistant groups, demonstrating the capacity of resistant fish to overcome the virus replication, generating a very low viral load. This phenomenon and survival of this higher resistant fish seem to be related to differences in immune and translational process, as well as to the increase of HIV-EP2 (hivep2) transcript in resistant fish, although the causal mechanism is yet to be discovered. This study provides valuable information about disease resistance mechanisms in Atlantic salmon from a host-pathogen interaction point of view.

Chemical Synthesis and In Vitro Evaluation of a Phage Display-Derived Peptide Active against Infectious Salmon Anemia Virus

Infectious salmon anemia virus (ISAV) is the etiological agent of the disease by the same name and causes major losses in the salmon industry worldwide. Epizootic ISAV outbreaks have occurred in Norway and, to a lesser degree, in Canada. In 2007, an ISAV outbreak in Chile destroyed most of the seasonal production and endangered the entire Chilean salmon industry. None of the existing prophylactic approaches have demonstrated efficacy in providing absolute protection from or even a palliative effect on ISAV proliferation. Sanitary control measures for ISAV, based on molecular epidemiology data, have proven insufficient, mainly due to high salmon culture densities and a constant presence of a nonpathogenic strain of the virus. This report describes an alternative treatment approach based on interfering peptides selected from a phage display library. The screening of a phage display heptapeptide library resulted in the selection of a novel peptide with significant in vitro antiviral activity against ISAV. This peptide specifically interacted with the viral hemagglutinin-esterase protein, thereby impairing virus binding, with plaque reduction assays showing a significant reduction in viral yields. The identified peptide acts at micromolar concentrations against at least two different pathogenic strains of the virus, without detectable cytotoxic effects on the tested fish cells. Therefore, antiviral peptides represent a novel alternative for controlling ISAV and, potentially, other fish pathogens.

IMPORTANCE

Identifying novel methods for the efficient control of infectious diseases is imperative for the future of global aquaculture. The present study used a phage display heptapeptide library to identify a peptide with interfering activity against a key protein of the infectious salmon anemia virus (ISAV). A piscine orthomyxovirus, ISAV is a continuous threat to the commercial sustainability of cultured salmon production worldwide. The complex epidemiological strategy of this pathogen has made prophylactic control extremely difficult. The identified antiviral peptide efficiently impairs ISAV infection in vitro by specifically blocking hemagglutinin-esterase, a pivotal surface protein of this virus. Peptide synthesis could further modify the primary structure of the identified peptide to improve specific activity and stability. The present results form the foundation for developing a new pharmacological treatment against ISAV.

Orthomyxoviruses of Fish

The family Orthomyxoviridae is well known for containing influenza viruses with a segmented RNA genome that is prone to gene reassortment in mixed infections (known as antigenic shift) resulting in new virus subtypes that cause pandemics, and cumulative mutations (known as antigenic drift), resulting in new virus strains that cause epidemics. This family also contains infectious salmon anemia virus (ISAV) and tilapia lake virus (TiLV), which are a unique orthomyxoviruses that infect fish and is unable to replicate above room temperature (24°C). This chapter describes the comparative virology of members in the family Orthomyxoviridae in general, helping to understand the emergent teleost orthomyxoviruses, ISAV and TiLV. The most current information on virus–host interactions of the fish orthomyxoviruses, particularly ISAV, as they relate to variations in virus structure, virulence, persistence, host range and immunological aspects is presented in detail.