The genetic variability and evolution of red-spotted grouper nervous necrosis virus quasispecies can be associated with its virulence

Nervous necrosis virus, NNV, is a neurotropic virus that causes viral nervous necrosis disease in a wide range of fish species, including European sea bass (Dicentrarchus labrax). NNV has a bisegmented (+) ssRNA genome consisting of RNA1, which encodes the RNA polymerase, and RNA2, encoding the capsid protein. The most prevalent NNV species in sea bass is red-spotted grouper nervous necrosis virus (RGNNV), causing high mortality in larvae and juveniles. Reverse genetics studies have associated amino acid 270 of the RGNNV capsid protein with RGNNV virulence in sea bass. NNV infection generates quasispecies and reassortants able to adapt to various selective pressures, such as host immune response or switching between host species. To better understand the variability of RGNNV populations and their association with RGNNV virulence, sea bass specimens were infected with two RGNNV recombinant viruses, a wild-type, rDl956, highly virulent to sea bass, and a single-mutant virus, Mut270Dl965, less virulent to this host. Both viral genome segments were quantified in brain by RT-qPCR, and genetic variability of whole-genome quasispecies was studied by Next Generation Sequencing (NGS). Copies of RNA1 and RNA2 in brains of fish infected with the low virulent virus were 1,000-fold lower than those in brains of fish infected with the virulent virus. In addition, differences between the two experimental groups in the Ts/Tv ratio, recombination frequency and genetic heterogeneity of the mutant spectra in the RNA2 segment were found. These results show that the entire quasispecies of a bisegmented RNA virus changes as a consequence of a single point mutation in the consensus sequence of one of its segments. Sea bream (Sparus aurata) is an asymptomatic carrier for RGNNV, thus rDl965 is considered a low-virulence isolate in this species. To assess whether the quasispecies characteristics of rDl965 were conserved in another host showing different susceptibility, juvenile sea bream were infected with rDl965 and analyzed as above described. Interestingly, both viral load and genetic variability of rDl965 in seabream were similar to those of Mut270Dl965 in sea bass. This result suggests that the genetic variability and evolution of RGNNV mutant spectra may be associated with its virulence.

Immune Response of Senegalese Sole against Betanodavirus Mutants with Modified Virulence

Nervous necrosis virus (NNV), genus Betanodavirus, the etiological agent of the viral encephalopathy and retinopathy (VER), presents a genome with two positive-sense single-stranded RNA segments. Striped jack nervous necrosis virus (SJNNV) and red-spotted grouper nervous necrosis virus (RGNNV), together with reassortants RGNNV/SJNNV, are the betanodaviruses predominantly isolated in Southern Europe. An RGNNV/SJNNV reassortant isolated from Senegalese sole (wt160) causes high mortalities in this fish species. This virus presents differences in the sequence of the 3’ non-coding region (NCR) of both segments compared to RGNNV and SJNNV reference strains. Previously, it has been reported that the reversion of two of these differences (nucleotides 1408 and 1412) in the RNA2 3’NCR to the SJNNV-type (recombinant r1408-1412) resulted in a decrease in sole mortality. In the present study, we have applied an OpenArray^® to analyse the involvement of sole immune response in the virulence of several recombinants: the r1408-1412 and two recombinants, developed in the present study, harbouring mutations at positions 3073 and 3093 of RNA1 3’NCR to revert them to RGNNV-type. According to the correlation values and to the number of expressed genes, the infection with the RNA2-mutant provoked the most different immune response compared to the immune response triggered after the infection with the rest of the viruses, and the exclusive and high upregulation of genes related to the complement system. The infection with the RNA1-mutants also provoked a decrease in mortality and their replication was delayed at least 24 h compared to the wt160 replication, which could provoke the lag observed in the immune response. Furthermore, the infection with the RNA1-mutants provoked the exclusive expression of pkr and the downregulation of il17rc.

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.

Amino acid changes in the capsid protein of a reassortant betanodavirus strain: Effect on viral replication in vitro and in vivo

Betanodavirus reassortant strains (RGNNV/SJNNV) isolated from Senegalese sole harbour an SJNNV capsid featuring several changes with respect to the SJNNV-type strain, sharing three hallmark substitutions. Here, we have employed recombinant strains harbouring mutations in these positions (r20 and r20 + 247 + 270) and have demonstrated that the three substitutions affect different steps of the viral replication process. Adsorption ability and efficiency of viral attachment were only affected by substitutions in the C-terminal side of the capsid. However, the concurrent mutation in the N-terminal side seems to slightly decrease these properties, suggesting that this region could also be involved in viral binding. Differences in the intracellular and extracellular production of the mutant strains suggest that both the C-terminal and N-terminal regions of the capsid protein may be involved in the particle budding. Furthermore, viral replication in sole brain tissue of the mutant strains, and especially double- and triple-mutant strains, is clearly delayed with respect to the wt strain. These data support previous findings indicating that the C-terminal side plays a role in virulence because of a slower spread in the fish host brain and suggest that the concurrent participation of the N-terminal side is also important for viral replication in vivo.

Identification of an interferon-stimulated gene, isg15, involved in host immune defense against viral infections in gilthead seabream (Sparus aurata L.)

Interferons (IFNs) play a key role in the innate immunity of vertebrates against viral infections by inducing hundreds of IFN-stimulated genes (ISGs), such as isg15. Isg15 is an ubiquitin-like protein, which can conjugate cellular and viral proteins in a process called ISGylation, although it can also act as a cytokine-like protein. Gilthead seabream (Sparus aurata L.) is an important asymptomatic carrier of viral haemorrhagic septicaemia virus (VHSV) and nodavirus, representing a threat to other co-cultivated susceptible species. In order to better understand virus-host interactions in this fish species, this study addresses the identification and molecular characterization of seabream isg15 (sb-isg15). In addition, the modulation of transcript levels of sb-isg15 was analysed in SAF-1 cells and seabream acidophilic granulocytes (AGs) stimulated in vitro with different pathogen-associated molecular patterns (PAMPs) or inoculated with VHSV and striped jack nervous necrosis virus (SJNNV). The full-length cDNA of sb-isg15 gene, encoding a predicted protein of 155 amino acids, was identified and seen to share the same characteristics as other fish and mammalian isg15 genes. Here we report the clear induction of sb-isg15 transcript levels in SAF-1 cells and AGs stimulated with toll-like receptor (TLR) ligands, such as polyinosinic:polycytidylic acid (poly I:C) or genomic DNA from Vibrio anguillarum (VaDNA), respectively. Furthermore, VHSV and SJNNV inoculation induced a significant degree of sb-isg15 transcription in SAF-1 cells and AGs. However, the relative levels of viral RNA transcription showed that SJNNV replication seems to be more efficient than VHSV in both in vitro systems. Interestingly, sb-isg15 transcript induction elicited by VaDNA was reduced in VHSV- and SJNNV-inoculated AGs, suggesting an interference prompted by the viruses against the type I IFN system. Taken together, these findings support the use of seabream AGs as a valuable experimental system to study virus-host interactions, in which sb-isg15 seems to play an important role.

Molecular characterization and expression analyses of the Solea senegalensis interferon-stimulated gene 15 (isg15) following NNV infections

Interferons are essential in fish resistance to viral infections. They induce interferon-stimulated genes, such as isg15. In this study, the Senegalese sole isg15 gene (ssisg15) has been characterized. As other isg15, ssisg15 contains a 402-bp intron sited in the 5'-UTR, and the full length cDNA is 1492-bp, including a 480-bp ORF. The expression analyses revealed basal levels of isg15 transcripts, and a clear induction after poly I:C injection, that reached maximum values in brain, head kidney and gills. The ssisg15 induction patterns were similar in RGNNV- and SJNNV-inoculated fish, whereas the reassortant (RG/SJ) isolate, which has higher replication fitness, triggered delayed but higher transcript levels. Furthermore, RG/SJ infection after poly I:C treatment reduced the induction of ssisg15 transcripts, suggesting an antagonistic mechanism against interferon type I system, that might allow an efficient viral replication at the initial steps of the infective process.

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.

Synergistic effects in the antiviral activity of the three Mx proteins from gilthead seabream (Sparus aurata)

Due to their direct antiviral activity, Mx proteins play a main role in the response mediated by type I interferon against viral infections. The study on gilthead seabream Mx proteins is especially interesting, since this species is unusually resistant to viral diseases, being asymptomatic carrier of several viruses pathogenic to other fish species. Gilthead seabream has three Mx proteins (Mx1, Mx2 and Mx3) that, separately, display antiviral activity against a wide range of viruses, showing interesting differences in their antiviral specificities. In this work, the possible synergy between the three Mx isoforms has been studied using in vitro systems consisting of CHSE-214 cells stably expressing two or the three gilthead seabream Mx proteins. The antiviral activity of these Mx combinations has been tested against the Infectious Pancreatic Necrosis Virus (IPNV), the Viral Haemorrhagic Septicaemia Virus (VHSV), the European Sheatfish Virus (ESV) and the Lymphocystis Disease Virus (LCDV). A synergistic effect of the Mx proteins was only detected against ESV, no synergy was observed against LCDV, and a negative interference was detected against the two RNA viruses tested, IPNV and VHSV, as viral replication was higher in cells expressing certain Mx combinations than in cells expressing these proteins separately. These results suggest a functional interaction between gilthead seabream Mx isoforms, which results in a higher or lower antiviral activity depending on the virus tested, thus supporting the idea of complex virus-host interactions and finely tuned mechanisms controlling the antiviral activity of Mx proteins.

Experimental susceptibility of European sea bass and Senegalese sole to different betanodavirus isolates

The susceptibility of juvenile European sea bass and Senegalese sole to three VNNV isolates (a reassortant RGNNV/SJNNV, as well as the parental RGNNV and SJNNV genotypes) has been evaluated by challenges using two inoculation ways (bath and intramuscular injection). The results demonstrate that these two fish species are susceptible to all the VNNV isolates tested. In European sea bass, RGNNV caused the highest cumulative mortality, reaching maximum values of viral RNA and titres. Although the SJNNV isolate did not provoke mortality or clinical signs of disease in this fish species, viral production in survivor fish was determined; on the other hand the reassortant isolate did cause mortality and clinical signs of disease, although less evident than those recorded after RGNNV infection. These results suggest that the changes suffered by the SJNNV RNA2 segment of the reassortant isolate, compared to the parental SJNNV, may have involved host-specificity and/or virulence determinants for European sea bass. Regarding Senegalese sole, although the three isolates caused 100% mortality, the reassortant strain provoked the most acute symptoms, and more quickly, especially in the bath challenge. This was also the isolate showing less difference between the number of RNA copies and viral titre, reaching the highest titres of infective viral particles in nervous tissue of infected animals. The RGNNV isolate produced the lowest values of infective viral particles. All these results suggest that the RGNNV and the reassortant isolates are the most suited for infecting European sea bass and Senegalese sole, respectively.

Molecular and functional characterization of two infectious salmon anaemia virus (ISAV) proteins with type I interferon antagonizing activity

In this study we characterize two proteins encoded by the two smallest genomic segments of the piscine orthomyxovirus infectious salmon anaemia virus (ISAV). Both proteins, encoded by the un-spliced ORF from genomic segment 7 (s7ORF1) and the larger ORF from segment 8 (s8ORF2), are involved in modulation of the type I interferon (IFN) response. The data suggests that the s7ORF1 protein is collinearly encoded, non-structural, contains no nuclear localisation signals, localises mainly to the cytoplasmic perinuclear area and does not bind single- or double-stranded RNA. On the other hand, genomic segment 8 uses a bicistronic coding strategy and the encoded s8ORF2 protein is a structural component of the viral particle. This protein contains two nuclear localisation signals, has a predominantly nuclear localisation, binds both double-stranded RNA and poly-A tailed single-stranded RNA, but not double-stranded DNA. In poly I:C stimulated salmon cells both ISAV proteins independently down-regulate the type I IFN promoter activity. Thus, ISAV counteracts the type I IFN response by the action of at least two of its gene products, rather than just one, as appears to be the case for other known members of the Orthomyxoviridae.

In vitro inhibition of sole aquabirnavirus by Senegalese sole Mx

Senegalese sole, Solea senegalensis, is a flat fish of growing interest in European aquaculture. In its culture viral infections are constant threats, thus understanding antiviral defences is a key factor for a successful industry. Mx proteins are IFN-induced proteins widespread in eukaryotes; however, their antiviral activity is unclear and the results variable among species. Therefore assessment of the putative Mx antiviral activity in each species is of interest. Our group has recently cloned the Senegalese sole Mx (SsMx) cDNA and in this study its antiviral activity was assessed by infecting CHSE-214 cells expressing recombinant SsMx, with sole aquabirnavirus. The antiviral activity against this pathogen was demonstrated by reduction in induced cytopathic effects, reduction in virus yield and decrease in viral transcripts. These findings contribute to our understanding of fish antiviral mechanisms and open the possibility of using this protein as a tool for fighting viral infections in aquaculture.

Co-occurrence of viral and bacterial pathogens in disease outbreaks affecting newly cultured sparid fish

Several microbial disease outbreaks in farm stocks of newly cultured sparid fish species, such as common seabream, redbanded seabream, and white seabream, were recorded from 2004 to 2006. This study describes the isolation and characterization of the potential causative agents, either bacteria or viruses, of these outbreaks. The isolated bacterial strains were characterized according to traditional taxonomical analyses and sequencing of a 16S rDNA fragment. Most bacteria were identified as Vibrio spp. and Photobacterium damselae subsp. damselae. The development of cytopathic effects (CPE) on different fish cell lines, the application of specific nested-PCR tests for infectious pancreatic necrosis virus (IPNV), viral nervous necrosis virus (VNNV) and viral hemorrhagic septicemia virus (VHSV), and subsequent sequence analyses were used for virus detection and identification. VNNV, related to the striped jack neural necrosis virus (SJNNV) genotype, and VHSV, related to the genotype Ia, were the only viruses detected. VNNV was isolated from the three fish species under study in five different outbreaks, whereas VHSV was isolated from common seabream and white seabream during two of these outbreaks. IPNV was not detected in any case.

Protein and glycoprotein content of lymphocystis disease virus (LCDV)

The polypeptide and glycoprotein composition of eight strains of the fish-pathogenic lymphocystis disease virus (LCDV) isolated from gilt-head seabream (Sparus aurata), blackspot seabream (Pagellus bogaraveo), and sole (Solea senegalensis) were determined. The protein electrophoretic patterns of all LCDV isolates were quite similar regardless of the host fish, showing two major proteins (79.9 and 55.6 kDa) and a variable number of minor proteins. Three groups of LCDV isolates were distinguished according to the number and molecular masses of the minor proteins. Eight glycoproteins were detected inside viral particles of LCDV 2, LCDV 3 and LCDV 5 isolates, but only seven glycoproteins were found inside viral particles of LCDV 1, LCDV 4, LCDV 6, LCDV 7, and LCDV 11 isolates and the reference virus ATCC VR 342 by using five lectins. LCDV glycoproteins were mainly composed of mannose and sialic acid. These glycoproteins could be part of an external viral envelope probably derived from the host cell membrane.

Histochemical study of lymphocystis disease in skin of gilthead seabream, Sparus aurata L

A battery of horseradish peroxidase-conjugated lectins (Con A, WGA and DBA), as well as conventional histochemical techniques (PAS, saponification, Alcian Blue pH 0.1, 1, 2.5, chlorhydric hydrolisis, sialidase, Bromophenol blue, Tioglycollate reduction and Ferric-ferricyanide-FeIII) were used to study the content and distribution of carbohydrates, proteins and glycoconjugate sugar residues on the skin and on the lymphocystis-infected cells of gilthead seabream, Sparus aurata. Variable amounts of glycoproteins containing sialic acid, N-acetyl-D-glucosamine, N-acetyl-D-galactosamine, mannose and/or glucose residues were observed in the cuticle and mucous cells of the corporal skin, tails and fins. Germinative and epithelial cells of the epidermis contained glycogen, proteins, carboxylated groups, as well as glycoproteins with mannose and/or glucose and N-acetyl-D-galactosamine residues. Hyaline capsule of the mature lymphocystis-infected cells was strongly stained with PAS, Alcian Blue (pH 0.5 and 2.5) and weakly positive with Alcian Blue (pH 1). Con A reacted with the granular cytoplasm, specially around hyaline capsule, and with the basophilic intracytoplasmic inclusions developed in mature lymphocystis-infected cells of Sparus aurata skin. These sugar residues (mannose and/or glucose), as well as N-acetyl-D-glucosamine and/or sialic acid and N-acetyl-D-galactosamine were not detected in the hyaline capsule of the lymphocystis disease.