Novel subunit vaccine with linear array epitope protect giant grouper against nervous necrosis virus infection

Recombinant viral hemorrhagic septicemia virus with rearranged genomes as vaccine vectors to protect against lethal betanodavirus infection

The outbreaks of viral hemorrhagic septicemia (VHS) and viral encephalopathy and retinopathy (VER) caused by the enveloped novirhabdovirus VHSV, and the non-enveloped betanodavirus nervous necrosis virus (NNV), respectively, represent two of the main viral infectious threats for aquaculture worldwide. Non-segmented negative-strand RNA viruses such as VHSV are subject to a transcription gradient dictated by the order of the genes in their genomes. With the goal of developing a bivalent vaccine against VHSV and NNV infection, the genome of VHSV has been engineered to modify the gene order and to introduce an expression cassette encoding the major protective antigen domain of NNV capsid protein. The NNV Linker-P specific domain was duplicated and fused to the signal peptide (SP) and the transmembrane domain (TM) derived from novirhabdovirus glycoprotein to obtain expression of antigen at the surface of infected cells and its incorporation into viral particles. By reverse genetics, eight recombinant VHSVs (rVHSV), termed NxGyCz according to the respective positions of the genes encoding the nucleoprotein (N) and glycoprotein (G) as well as the expression cassette (C) along the genome, have been successfully recovered. All rVHSVs have been fully characterized in vitro for NNV epitope expression in fish cells and incorporation into VHSV virions. Safety, immunogenicity and protective efficacy of rVHSVs has been tested in vivo in trout (Oncorhynchus mykiss) and sole (Solea senegalensis). Following bath immersion administration of the various rVHSVs to juvenile trout, some of the rVHSVs were attenuated and protective against a lethal VHSV challenge. Results indicate that rVHSV N2G1C4 is safe and protective against VHSV challenge in trout. In parallel, juvenile sole were injected with rVHSVs and challenged with NNV. The rVHSV N2G1C4 is also safe, immunogenic and efficiently protects sole against a lethal NNV challenge, thus presenting a promising starting point for the development of a bivalent live attenuated vaccine candidate for the protection of these two commercially valuable fish species against two major diseases in aquaculture.

Comparative study of the effects of Montanide™ ISA 763A VG and ISA 763B VG adjuvants on the immune response against Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is regarded as a major bacterial pathogen of farmed fish, with outbreaks in Nile tilapia causing significant losses. Vaccination is considered the most suitable method for disease control in aquaculture, with the potential to prevent such outbreaks if highly efficacious vaccines are available for use. Several vaccines have been produced to protect against S. agalactiae infection in tilapia, including inactivated vaccines, live attenuated vaccines, and subunit vaccines, with variable levels of protection seen. Two commercial adjuvants, Montanide™ ISA 763A VG and ISA 763B VG, have been developed recently and designed to improve the safety and efficacy of oil-based emulsions delivered by intraperitoneal injection. In particular, their mode of action may help identify and stimulate particular immunological pathways linked to the intended protective response, which is an important tool for future vaccine development. Therefore, this study aimed to characterize the potential of two adjuvanted-bacterial vaccines against S. agalactiae (SAIV) comparatively, to determine their usefulness for improving protection and to analyse the immune mechanisms involved. Nile tilapia were divided into four groups: 1) fish injected with PBS as a control, 2) fish injected with the SAIV alone, 3) fish injected with the SAIV + Montanide™ ISA 763A VG, and 4) fish injected with the SAIV + Montanide™ ISA 763B VG. Following immunization selected innate immune parameters were analysed, including serum lysozyme, myeloperoxidase, and bactericidal activity, with significantly increased levels seen after immunization. Cytokines associated with innate and adaptive immunity were also studied, with expression levels of several genes showing significant up-regulation, indicating good induction of cell-mediated immune responses. Additionally, the specific IgM antibody response against S. agalactiae was determined and found to be significantly induced post-vaccination, with higher levels seen in the presence of the adjuvants. In comparison to the protection seen with the unadjuvanted vaccine (61.29% RPS), both Montanide™ ISA 763A VG and Montanide™ ISA 763B VG improved the RPS, to 77.42% and 74.19% respectively. In conclusion, Montanide™ ISA 763A VG and Montanide™ ISA 763B VG have shown potential for use as adjuvants for fish vaccines against streptococcosis, as evidenced by the enhanced immunoprotection seen when given in combination with the SAIV vaccine employed in this study.

Identification of B-Cell Epitopes on Capsid Protein Reveals Two Potential Neutralization Mechanisms in Red-Spotted Grouper Nervous Necrosis Virus

Nervous necrosis virus (NNV), a formidable pathogen in marine and freshwater fish, has inflicted enormous financial tolls on the aquaculture industry worldwide. Although capsid protein (CP) is the sole structural protein with pathogenicity and antigenicity, public information on immunodominant regions remains extremely scarce. Here, we employed neutralizing monoclonal antibodies (MAbs) specific for red-spotted grouper NNV (RGNNV) CNPgg2018 in combination with partially overlapping truncated proteins and peptides to identify two minimal B-cell epitope clusters on CP, ¹²²GYVAGFL¹²⁸ and ²²⁷SLYNDSL²³³. Site-directed mutational analysis confirmed residues Y123, G126, and L128 and residues L228, Y229, N230, D231, and L233 as the critical residues responsible for the direct interaction with ligand, respectively. According to homologous modeling and bioinformatic evaluation, ¹²²GYVAGFL¹²⁸ is harbored at the groove of the CP junction with strict conservation among all NNV isolates, while ²²⁷SLYNDSL²³³ is localized in proximity to the tip of a viral protrusion having relatively high evolutionary dynamics in different genotypes. Additionally, ²²⁷SLYNDSL²³³ was shown to be a receptor-binding site, since the corresponding polypeptide could moderately suppress RGNNV multiplication by impeding virion entry. In contrast, ¹²²GYVAGFL¹²⁸ seemed dedicated only to stabilizing viral native conformation and not to assisting initial virus attachment. Altogether, these findings contribute to a novel understanding of the antigenic distribution pattern of NNV and the molecular basis for neutralization, thus advancing the development of biomedical products, especially epitope-based vaccines, against NNV. IMPORTANCE NNV is a common etiological agent associated with neurological virosis in multiple aquatic organisms, causing significant hazards to the host. However, licensed drugs or vaccines to combat NNV infection are very limited to date. Toward the advancement of broad-spectrum prophylaxis and therapeutics against NNV, elucidating the diversity of immunodominant regions within it is undoubtedly essential. Here, we identified two independent B-cell epitopes on NNV CP, followed by the confirmation of critical amino acid residues participating in direct interaction. These two sites were distributed on the shell and protrusion domains of the virion, respectively, and mediated the neutralization exerted by MAbs via drastically distinct mechanisms. Our work promotes new insights into NNV antigenicity as well as neutralization and, more importantly, offers promising targets for the development of antiviral countermeasures.

Nervous Necrosis Virus (NNV) Booster Vaccination Increases Senegalese Sole Survival and Enhances Immunoprotection

A re-immunization programme has been tested to improve the protective response elicited in sole by a previously developed BEI-inactivated betanodavirus vaccine. The vaccine was prepared using a reassortant RGNNV/SJNNV strain which is highly pathogenic for sole, and vaccination assays were performed by intraperitoneal injection. Experimental design included a prime- and a booster-vaccination group, which consisted of individuals that received a second vaccine injection at 30 days post vaccination), and their respective controls. A month after prime/booster vaccination, fish were challenged by intramuscular injection with the homologous NNV strain. Samples were collected at different times post vaccination and post challenge to assess the immune response and viral replication. Booster dose enhanced the protection against NNV infection because a significant increase in survival was recorded when compared with prime-vaccinated individuals (relative percent survival 77 vs. 55). In addition, a clear decrease in viral replication in the brain of challenged sole was observed. During the immune induction period, no differences in IgM production were observed between prime- and booster-vaccinated fish, and the expression of the antigen presenting cells (APC)-related molecule MHC class II antigen was the only differential stimulation recorded in the re-immunized individuals. However, a significant upregulation of mhcII and the lymphocytes T helper (Th) marker cd4 was observed after the challenge in the booster-vaccinated group, suggesting these cells play a role in the protection conferred by the booster injection. In addition, after viral infection, re-immunized fish showed specific and neutralizing antibody production and overexpression of other immune-related genes putatively involved in the control of NNV replication.

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.

BEI Inactivated Vaccine Induces Innate and Adaptive Responses and Elicits Partial Protection upon Reassortant Betanodavirus Infection in Senegalese Sole

Nervous necrosis virus (NNV), the causative agent of viral encephalopathy and retinopathy (VER), is one of the most threatening viruses affecting marine and freshwater fish species worldwide. Senegalese sole is a promising fish species in Mediterranean aquaculture but also highly susceptible to NNV and VER outbreaks, that puts its farming at risk. The development of vaccines for aquaculture is one of best tools to prevent viral spread and sudden outbreaks, and virus inactivation is the simplest and most cost-effective method available. In this work, we have designed two inactivated vaccines based on the use of formalin or binary ethylenimine (BEI) to inactivate a reassortant NNV strain. After vaccination, the BEI-inactivated vaccine triggered the production of specific IgM-NNV antibodies and stimulated innate and adaptive immune responses at transcriptional level (rtp3, mx, mhcii and tcrb coding genes). Moreover, it partially improved survival after an NNV in vivo challenge, reducing the mid-term viral load and avoiding the down-regulation of immune response post-challenge. On the other hand, the formalin-inactivated vaccine improved the survival of fish upon infection without inducing the production of IgM-NNV antibodies and only stimulating the expression of herc4 and mhcii genes (in head-kidney and brain, respectively) during the vaccination period; this suggests that other immune-related pathways may be involved in the partial protection provoked. Although these vaccines against NNV showed encouraging results, further studies are needed to improve sole protection and to fully understand the underlying immune mechanism.

Sites responsible for infectivity and antigenicity on nervous necrosis virus (NNV) appear to be distinct

Nervous necrosis virus (NNV) is a pathogenic fish-virus belonging to the genus Betanodavirus (Nodaviridae). Surface protrusions on NNV particles play a crucial role in both antigenicity and infectivity. We exposed purified NNV particles to different physicochemical conditions to investigate the effects on antigenicity and infectivity, in order to reveal information regarding the conformational stability and spatial relationships of NNV neutralizing-antibody binding sites and cell receptor binding sites. Treatment with PBS at 37 °C, drastically reduced NNV antigenicity by 66–79% on day one, whereas its infectivity declined gradually from 10^7.6 to 10^5.8 TCID₅₀/ml over 10 days. When NNV was treated with carbonate/bicarbonate buffers at different pHs, both antigenicity and infectivity of NNV declined due to higher pH. However, the rate of decline with respect to antigenicity was more moderate than for infectivity. NNV antigenicity declined 75–84% after treatment with 2.0 M urea, however, there was no reduction observed in infectivity. The antibodies used in antigenicity experiments have high NNV-neutralizing titers and recognize conformational epitopes on surface protrusions. The maintenance of NNV infectivity means that receptor binding sites are functionally preserved. Therefore, it seems highly likely that NNV neutralizing-antibody binding sites and receptor binding sites are independently located on surface protrusions.

Recombinant Baculovirus-Produced Grass Carp Reovirus Virus-Like Particles as Vaccine Candidate That Provides Protective Immunity against GCRV Genotype II Infection in Grass Carp

Grass carp reovirus (GCRV) leads to severe hemorrhagic disease in grass carp (Ctenopharyngodon idella) and causes economic losses in grass carp aquaculture. Recent epidemiological investigations showed that GCRV genotype II is the dominant subtype in China. Therefore, it is very important to develop a novel vaccine for preventing diseases caused by GCRV genotype II. In this study, we employed a bac-to-bac expression system to generate GCRV-II-based virus-like particles (VLPs). Previous studies have shown that the structural proteins VP3, VP4, and VP38 encoded by the segments S3, S6, and S10 of type II GCRV are immunogenic. Hence, the GCRV-VLPs were produced by co-infection of sf9 cells with recombinant baculoviruses PFBH-VP3, PFBH-VP4, and PFBH-VP38. The expressions of VP3, VP4, and VP38 proteins in GCRV-VLPs were tested by IFA and Western blot analysis. By electron microscopic observations of ultrathin sections, purified VLPs showed that the expressed proteins are similar in shape to GCRV genotype II with a size range from 40 nm to 60 nm. The immunogenicity of GCRV-VLPs was evaluated by the injection immunization of grass carp. The analysis of serum-specific IgM antibody showed that grass carp immunized with GCRV-VLPs produced GCRV-specific antibodies. Furthermore, injection with GCRV-VLPs increased the expressions of immune-related genes (IgM, IFN, TLR3, TLR7) in the spleen and kidney. In addition, grass carp immunized with a GCRV-VLPs-based vaccine showed a relative percent survival rate (RPS) of 83.33% after challenge. The data in this study showed that GCRV-VLPs demonstrated an excellent immunogenicity and represent a promising approach for vaccine development against GCRV genotype II infection.

Modelling the effect of vaccination on transmission dynamics of nervous necrosis virus in grouper larvae Epinephelus coioides

Nervous necrosis virus (NNV) infection in susceptible grouper larvae has been reported to cause high mortalities, leading to great economic losses in aquaculture industry. Although the effects of NNV vaccines on grouper have been broadly investigated, vaccination strategies have not been fully established. To this end, we introduced the parsimonious epidemiological models that explored the assessment of key epidemiological parameters and how they changed when vaccinations showed the effects. We showed that the models capture the published cumulative mortality data accurately. We estimated a basic reproduction number R₀  = 2.44 for NNV transmission in grouper larvae without vaccination. To effectively control NNV transmission by vaccination, a model for disease control was also generalized to attain the goals of controlled reproduction number less than 1. Our results indicated that at least 60% of grouper population needed to be immunized for ~75 min. Our data-driven modelling approach that links the transmission dynamics of NNV and vaccination strategies for grouper has the potential to support evidence-based planning and adaptation of integrated control measures. We encourage that the epidemiology-based framework introduced here can be further implemented for establishing effective vaccination and mitigation actions aimed at controlling diseases in fish farming practices.

Betanodavirus and VER Disease: A 30-year Research Review

The outbreaks of viral encephalopathy and retinopathy (VER), caused by nervous necrosis virus (NNV), represent one of the main infectious threats for marine aquaculture worldwide. Since the first description of the disease at the end of the 1980s, a considerable amount of research has gone into understanding the mechanisms involved in fish infection, developing reliable diagnostic methods, and control measures, and several comprehensive reviews have been published to date. This review focuses on host–virus interaction and epidemiological aspects, comprising viral distribution and transmission as well as the continuously increasing host range (177 susceptible marine species and epizootic outbreaks reported in 62 of them), with special emphasis on genotypes and the effect of global warming on NNV infection, but also including the latest findings in the NNV life cycle and virulence as well as diagnostic methods and VER disease control.

A Review of Fish Vaccine Development Strategies: Conventional Methods and Modern Biotechnological Approaches

Fish immunization has been carried out for over 50 years and is generally accepted as an effective method for preventing a wide range of bacterial and viral diseases. Vaccination efforts contribute to environmental, social, and economic sustainability in global aquaculture. Most licensed fish vaccines have traditionally been inactivated microorganisms that were formulated with adjuvants and delivered through immersion or injection routes. Live vaccines are more efficacious, as they mimic natural pathogen infection and generate a strong antibody response, thus having a greater potential to be administered via oral or immersion routes. Modern vaccine technology has targeted specific pathogen components, and vaccines developed using such approaches may include subunit, or recombinant, DNA/RNA particle vaccines. These advanced technologies have been developed globally and appear to induce greater levels of immunity than traditional fish vaccines. Advanced technologies have shown great promise for the future of aquaculture vaccines and will provide health benefits and enhanced economic potential for producers. This review describes the use of conventional aquaculture vaccines and provides an overview of current molecular approaches and strategies that are promising for new aquaculture vaccine development.

Recombinant nodavirus vaccine produced in bacteria and administered without purification elicits humoral immunity and protects European sea bass against infection

Viral necrosis virus (NNV) or nodavirus causes fish viral encephalopathy and retinopathy worldwide. In some cases, mortalities in aquaculture industry can reach up to 100%, some species being especially sensitive as is the case of European sea bass (Dicentrarchus labrax), one of the main cultured species in the Mediterranean, with the consequent economical loses. Development of new vaccines against NNV is in the spotlight though few researches have focused in European sea bass. In this study we have generated a recombinant NNV (rNNV) vaccine produced in Escherichia coli expressing the capsid protein and administered it to European sea bass juveniles by two different routes (intraperitoneal and oral). The last being considered non-stressful and desired for fish farming of small fish, which in fact are the most affected by NNV. Oral vaccine was composed of feed pellets containing the recombinant whole bacteria, and injected vaccine was composed of recombinant bacteria previously lysed. Our results revealed production of specific anti-NNV IgM following the two vaccination procedures, levels that were further increased in orally-vaccinated group after challenge with NNV. Genes related to interferon (IFN), T-cell and immunoglobulin markers were scarcely regulated in head-kidney (HK), gut or brain. Vaccination by either route elicited a relative survival response of 100% after NNV challenge. To our knowledge, this is the first report of a recombinant vaccine followed by no purification steps which resulted in a complete protection in European sea bass when challenged with NNV.