Reassortant betanodavirus infection in turbot (Scophthalmus maximus)

Comprehensive Linear Epitope Prediction System for Host Specificity in Nodaviridae

Background: Nodaviridae infection is one of the leading causes of death in commercial fish. Although many vaccines against this virus family have been developed, their efficacies are relatively low. Nodaviridae are categorized into three subfamilies: alphanodavirus (infects insects), betanodavirus (infects fish), and gammanodavirus (infects prawns). These three subfamilies possess host-specific characteristics that could be used to identify effective linear epitopes (LEs). Methodology: A multi-expert system using five existing LE prediction servers was established to obtain initial LE candidates. Based on the different clustered pathogen groups, both conserved and exclusive LEs among the Nodaviridae family could be identified. The advantages of undocumented cross infection among the different host species for the Nodaviridae family were applied to re-evaluate the impact of LE prediction. The surface structural characteristics of the identified conserved and unique LEs were confirmed through 3D structural analysis, and concepts of surface patches to analyze the spatial characteristics and physicochemical propensities of the predicted segments were proposed. In addition, an intelligent classifier based on the Immune Epitope Database (IEDB) dataset was utilized to review the predicted segments, and enzyme-linked immunosorbent assays (ELISAs) were performed to identify host-specific LEs. Principal findings: We predicted 29 LEs for Nodaviridae. The analysis of the surface patches showed common tendencies regarding shape, curvedness, and PH features for the predicted LEs. Among them, five predicted exclusive LEs for fish species were selected and synthesized, and the corresponding ELISAs for antigenic feature analysis were examined. Conclusion: Five identified LEs possessed antigenicity and host specificity for grouper fish. We demonstrate that the proposed method provides an effective approach for in silico LE prediction prior to vaccine development and is especially powerful for analyzing antigen sequences with exclusive features among clustered antigen groups.

Profile of Innate Immunity in Gilthead Seabream Larvae Reflects Mortality upon Betanodavirus Reassortant Infection and Replication

Historically, gilthead seabream (Sparus aurata) has been considered a fish species resistant to nervous necrosis virus (NNV) disease. Nevertheless, mortality in seabream hatcheries, associated with typical clinical signs of the viral encephalopathy and retinopathy (VER) disease has been confirmed to be caused by RGNNV/SJNNV reassortants. Because of this, seabream larvae at 37 and 86 days post-hatching (dph) were infected by immersion with RGNNV/SJNNV and SJNNV/RGNNV reassortants under laboratory conditions, and mortality, viral replication and immunity were evaluated. Our results show that gilthead seabream larvae, mainly those at 37 dph, are susceptible to infection with both NNV reassortant genotypes, with the highest impact from the RGNNV/SJNNV reassortant. In addition, viral replication occurs at both ages (37 and 86 dph) but the recovery of infective particles was only confirmed in 37 dph larvae,; this value was also highest with the RGNNV/SJNNV reassortant. Larvae immunity, including the expression of antiviral, inflammatory and cell-mediated cytotoxicity genes, was affected by NNV infection. Levels of the natural killer lysin (Nkl) peptide were increased in SJNNV/RGNNV-infected larvae of 37 dph, though hepcidin was not. Our results demonstrate that the seabream larvae are susceptible to both NNV reassortants, though mainly to RGNNV/SJNNV, in an age-dependent manner.

Differential immunogene expression profile of European sea bass (Dicentrarchus labrax, L.) in response to highly and low virulent NNV

European sea bass is highly susceptible to the nervous necrosis virus, RGNNV genotype, whereas natural outbreaks caused by the SJNNV genotype have not been recorded. The onset and severity of an infectious disease depend on pathogen virulence factors and the host immune response. The importance of RGNNV capsid protein amino acids 247 and 270 as virulence factors has been previously demonstrated in European sea bass; however, sea bass immune response against nodaviruses with different levels of virulence has been poorly characterized. Knowing the differences between the immune response against both kinds of isolates may be key to get more insight into the host mechanisms responsible for NNV virulence. For this reason, this study analyses the transcription of immunogenes differentially expressed in European sea bass inoculated with nodaviruses with different virulence: a RGNNV virus obtained by reverse genetics (rDl956), highly virulent to sea bass, and a mutated virus (Mut_247+270Dl956, RGNNV virus displaying SJNNV-type amino acids at positions 247 and 270 of the capsid protein), presenting lower virulence. This study has been performed in brain and head kidney, and the main differences between the immunogene responses triggered by both viruses have been observed in brain. The immunogene response in this organ is stronger after inoculation with the most virulent virus, and the main differences involved genes related with IFN I system, inflammatory response, cell-mediated response, and apoptosis. The lower virulence of Mut_247+270Dl956 to European sea bass can be associated with a delayed IFN I response, as well as an early and transitory inflammation and cell-mediated responses, suggesting that those can be pivotal elements in controlling the viral infection, and therefore, their functional activity could be analysed in future studies. In addition, this study supports the role of capsid amino acids at positions 247 and 270 as important determinants of RGNNV virulence to European sea bass.

Nanostructured recombinant protein particles raise specific antibodies against the nodavirus NNV coat protein in sole

Nervous necrosis virus (NNV) reassortant strains RGNNV/SJNNV have emerged as a potent threat to the Mediterranean marine aquaculture industry, causing viral encephalopathy and retinopathy (VER) in Senegalese sole (Solea senegalensis). In this study, a cheap and practical vaccine strategy using bacterial inclusion bodies made of the coat protein of a virulent reassortant strain of this betanodavirus was devised. The nanostructured recombinant protein nanoparticles, VNNV-C^NP, were administered without adjuvant to two groups of juvenile sole, one by intraperitoneal injection and the other by oral intubation. Specific antibodies were raised in vivo against the NNV coat protein via both routes, with a substantial specific antibody expansion in the injected group 30 days post homologous prime boost. Expression levels of five adaptive immune-related genes, cd8a, cd4, igm, igt and arg2, were also quantified in intestine, spleen and head kidney. Results showed cd4 and igm were upregulated in the head kidney of injected fish, indicating activation of an adaptive systemic response, while intubated fish exhibited a mucosal response in the intestine. Neither route showed significant differential expression of cd8a. The specific antibody response elicited in vivo and the lack of any signs of toxicity over the 6-week study period in young fish (n = 100), evidences the potential of the nanoparticle as a vaccine candidate.

Capsid amino acids at positions 247 and 270 are involved in the virulence of betanodaviruses to European sea bass

European sea bass (Dicentrarchus labrax) is severely affected by nervous necrosis disease, caused by nervous necrosis virus (NNV). Two out of the four genotypes of this virus (red-spotted grouper nervous necrosis virus, RGNNV; and striped jack nervous necrosis virus, SJNNV) have been detected in sea bass, although showing different levels of virulence to this fish species. Thus, sea bass is highly susceptible to RGNNV, whereas outbreaks caused by SJNNV have not been reported in this fish species. The role of the capsid protein (Cp) amino acids 247 and 270 in the virulence of a RGNNV isolate to sea bass has been evaluated by the generation of recombinant RGNNV viruses harbouring SJNNV-type amino acids in the above mentioned positions (Mut247Dl965, Mut270Dl965 and Mut247 + 270Dl965). Viral in vitro and in vivo replication, virus virulence and fish immune response triggered by these viruses have been analysed. Mutated viruses replicated on E-11 cells, although showing some differences compared to the wild type virus, suggesting that the mutations can affect the viral cell recognition and entry. In vivo, fish mortality caused by mutated viruses was 75% lower, and viral replication in sea bass brain was altered compared to non-mutated virus. Regarding sea bass immune response, mutated viruses triggered a lower induction of IFN I system and inflammatory response-related genes. Furthermore, mutations caused changes in viral serological properties (especially the mutation in amino acid 270), inducing higher seroconversion and changing antigen recognition.

Amino acidic substitutions in the polymerase N-terminal region of a reassortant betanodavirus strain causing poor adaptation to temperature increase

Nervous necrosis virus (NNV), Genus Betanodavirus, is the causative agent of viral encephalopathy and retinopathy (VER), a neuropathological disease that causes fish mortalities worldwide. The NNV genome is composed of two single-stranded RNA molecules, RNA1 and RNA2, encoding the RNA polymerase and the coat protein, respectively. Betanodaviruses are classified into four genotypes: red-spotted grouper nervous necrosis virus (RGNNV), striped jack nervous necrosis virus (SJNNV), barfin flounder nervous necrosis virus (BFNNV) and tiger puffer nervous necrosis virus (TPNNV). In Southern Europe the presence of RGNNV, SJNNV and their natural reassortants (in both RNA1/RNA2 forms: RGNNV/SJNNV and SJNNV/RGNNV) has been reported. Pathology caused by these genotypes is closely linked to water temperature and the RNA1 segment encoding amino acids 1–445 has been postulated to regulate viral adaptation to temperature. Reassortants isolated from sole (RGNNV/SJNNV) show 6 substitutions in this region when compared with the RGNNV genotype (positions 41, 48, 218, 223, 238 and 289). We have demonstrated that change of these positions to those present in the RGNNV genotype cause low and delayed replication in vitro when compared with that of the wild type strain at 25 and 30 °C. The experimental infections confirmed the impact of the mutations on viral replication because at 25 °C the viral load and the mortality were significantly lower in fish infected with the mutant than in those challenged with the non-mutated virus. It was not possible to challenge fish at 30 °C because of the scarce tolerance of sole to this temperature.

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.

Transcriptomic Profiles of Senegalese Sole Infected With Nervous Necrosis Virus Reassortants Presenting Different Degree of Virulence

Betanodaviruses [nervous necrosis virus (NNV)] are the causative agent of the viral encephalopathy and retinopathy, a disease that affects cultured Senegalese sole (Solea senegalensis). NNV reassortants, combining genomic segments from redspotted grouper nervous necrosis virus (RGNNV) and striped jack nervous necrosis virus (SJNNV) genotypes, have been previously isolated from several fish species. The wild-type reassortant wSs160.03, isolated from Senegalese sole, has been proven to be more virulent to sole than the parental genotypes (RGNNV and SJNNV), causing 100% mortality. Mutations at amino acids 247 (serine to alanine) and 270 (serine to asparagine) in the wSs160.03 capsid protein have allowed us to obtain a mutant reassortant (rSs160.03_247+270), which provokes a 40% mortality decrease. In this study, the RNA-Seq technology has been used to comparatively analyze Senegalese sole transcriptomes in two organs (head kidney and eye/brain) after infection with wild-type and mutant strains. A total of 633 genes were differentially expressed (DEGs) in animals infected with the wild-type isolate (with higher virulence), whereas 393 genes were differentially expressed in animals infected with the mutant strain (37.9% decrease in the number of DEGs). To study the biological functions of detected DEGs involved in NNV infection, a gene ontology (GO) enrichment analysis was performed. Different GO profiles were obtained in the following subclasses: (i) biological process; (ii) cellular component; and (iii) molecular function, for each viral strain tested. Immune response and proteolysis have been the predominant biological process after the infection with the wild-type isolate, whereas the infection with the mutant strain induces proteolysis in head kidney and inhibition of vasculogenesis in nervous tissue. Regarding the immune response, genes coding for proteins acting as mediators of type I IFN expression (DHX58, IRF3, IRF7) and IFN-stimulated genes (ISG15, Mx, PKR, Gig1, ISG12, IFI44, IFIT-1, to name a few) were upregulated in animals infected with the wild-type isolate, whereas no-differential expression of these genes was observed in samples inoculated with the mutant strain. The different transcriptomic profiles obtained could help to better understand the NNV pathogenesis in Senegalese sole, setting up the importance as virulence determinants of amino acids at positions 247 and 270 within the RNA2 segment.

Viral nervous necrosis in gilthead sea bream (Sparus aurata) caused by reassortant betanodavirus RGNNV/SJNNV: an emerging threat for Mediterranean aquaculture

Viral nervous necrosis (VNN) certainly represents the biggest challenge for the sustainability and the development of aquaculture. A large number of economically relevant fish species have proven to be susceptible to the disease. Conversely, gilthead sea bream has generally been considered resistant to VNN, although it has been possible to isolate the virus from apparently healthy sea bream and sporadically from affected larvae and postlarvae. Unexpectedly, in 2014–2016 an increasing number of hatcheries in Europe have experienced mass mortalities in sea bream larvae. Two clinical outbreaks were monitored over this time span and findings are reported in this paper. Despite showing no specific clinical signs, the affected fish displayed high mortality and histological lesions typical of VNN. Fish tested positive for betanodavirus by different laboratory techniques. The isolates were all genetically characterized as being reassortant strains RGNNV/SJNNV. A genetic characterization of all sea bream betanodaviruses which had been isolated in the past had revealed that the majority of the strains infecting sea bream are actually RGNNV/SJNNV. Taken together, this information strongly suggests that RGNNV/SJNNV betanodavirus possesses a particular tropism to sea bream, which can pose a new and unexpected threat to the Mediterranean aquaculture.