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

Computational design and evaluation of a polyvalent vaccine for viral nervous necrosis (VNN) in fish to combat Betanodavirus infection

Viral nervous necrosis (VNN) poses a significant threat to the aquaculture industry, causing substantial losses and economic burdens. The disease, attributed to nervous necrosis viruses within the Betanodavirus genus, is particularly pervasive in the Mediterranean region, affecting various fish species across all production stages with mortality rates reaching 100%. Developing effective preventive measures against VNN is imperative. In this study, we employed rigorous immunoinformatics techniques to design a novel multi-epitope vaccine targeting VNN. Five RNA-directed RNA polymerases, crucial to the lifecycle of Betanodavirus, were selected as vaccine targets. The antigenicity and favorable physicochemical properties of these proteins were confirmed, and epitope mapping identified cytotoxic T lymphocyte, helper T lymphocyte, and linear B lymphocyte epitopes essential for eliciting a robust immune response. The selected epitopes, characterized by high antigenicity, non-allergenicity, and non-toxicity, were further enhanced by adding PADRE sequences and hBD adjuvants to increase immunogenicity. Two vaccine constructs were developed by linking epitopes using appropriate linkers, demonstrating high antigenicity, solubility, and stability. Molecular dynamics simulations revealed stable interactions between the vaccine constructs and Toll-like receptors (TLRs), essential for pathogen recognition and immune response activation in fish. Notably, vaccine construct V2 exhibited superior stability and binding affinity with TLR8, suggesting its potential as a promising candidate for VNN prevention. Overall, our study presents a comprehensive approach to VNN vaccine design utilizing immunoinformatics, offering safe, immunogenic, and effective solutions across multiple Betanodavirus species. Further experimental validation in model animals is recommended to fully assess the vaccine's efficacy. This research contributes to improved vaccine development against diverse fish pathogens by addressing emerging challenges and individualized immunization requirements in aquaculture.

Water-in-oil adjuvant challenges in fish vaccination: An experimental inactivated adjuvanted vaccine against betanodavirus infection in Senegalese sole

The extensive growth of intensive fish farming has led to a massive spread of infectious diseases. Nervous necrosis virus (NNV) is the causative agent of the viral encephalo- and retinopathy disease which has become a major threat for fish farming all over the globe. The devastating mortality rates recorded in disease outbreaks, especially when infected specimens are at early stages of development, have a high economic impact on the sector. Currently, vaccines are the most cost-effective preventing tool in the fight against viruses. Inactivated vaccines have the advantage of simplicity in their development at the same time as present the antigen in a similar manner than the natural infection in the host. Nevertheless, they usually trigger weaker immune responses needing adjuvants to boost their effectiveness. In this work, we have intraperitoneally vaccinated Senegalese sole juveniles (Solea senegalensis) with a previously designed inactivated vaccine against NNV based on binary ethylenimine (BEI), mixed or not with an oil-adjuvant. Our results demonstrated the potential activation of different immune pathways when the vaccine was administered alone compared to the oil-adjuvanted vaccine, both resulting in an equivalent partial improvement in survival following a NNV challenge. However, whilst the vaccine alone led to a significant increase in specific antibodies, in the adjuvanted version those antibodies were kept basal although with a slight improvement in their neutralization capacity. At transcriptional level, neither vaccine (adjuvanted or not) triggered the immune system activation during the vaccination period. However, after NNV infection, the BEI-inactivated vaccines alone and oil-adjuvanted both elicited the stimulation of antiviral responsive genes (rtp3, herc4), antigen presentation molecules (mhcii) and T-cell markers (cd8a) in the head-kidney. Additionally, the oil-adjuvanted vaccine appears to stimulate mediator cytokines (il6) and B-cell markers (ight and ighm). Surprisingly, when the adjuvant was administered alone, fish showed the highest survival rates concomitantly with a lack of NNV-IgM production, pointing to the possible induction of different immune pathways than the B-cell responses via antibodies by the adjuvant. Since this combined vaccine did not succeed in the full extension of protection against the pathogen, further studies should be performed focusing on unravelling the molecular mechanisms through which adjuvants trigger the immune response, both independently and when added to a vaccine antigen.

UV-C irradiation as an effective tool for sterilization of porcine chimeric VP1-PCV2bCap recombinant vaccine

Ultraviolet irradiation is an effective method of virus and bacteria inactivation. The dose of UV-C light necessary for baculovirus inactivation by measurement of fluorescent GFP protein produced by baculovirus expression system after the irradiation of baculovirus culture in doses ranging from 3.5 to 42 J/m2 was determined. At a dose of 36.8 J/m2, only 0.5% of GFP-expressing cells were detected by flow cytometry and confocal microscopy. The stability of purified VP1-PCV2bCap protein produced by baculovirus expression system was analyzed after the irradiation at doses ranging from 3.5 to 19.3 J/m2. Up to the dose of 11 J/m2, no significant effect of UV-C light on the stability of VP1-PCV2bCap was detected. We observed a dose-dependent increase in VP1-PCV2bCap-specific immune response in BALB/c mice immunized by recombinant protein sterilized by irradiation in dose 11 J/m2 with no significant difference between vaccines sterilized by UV-C light and filtration. A substantial difference in the production of VP1-PCV2bCap specific IgG was observed in piglets immunized with VP1-PCV2bCap sterilized by UV-C in comparison with protein sterilized by filtration in combination with the inactivation of baculovirus by binary ethylenimine. UV-C irradiation represents an effective method for vaccine sterilization, where commonly used methods of sterilization are not possible.

Genomic characterization and transcription analysis of European sea bass (Dicentrarchus labrax) rtp3 genes

Fish RTP3, belonging to the receptor-transporting protein family, display several functions, including a putative antiviral role as virus-responsive gene. In this work, we have identified and characterized two different European sea bass rtp3 genes. In addition, an in vivo transcription analysis in response to LPS, poly I:C and betanodavirus infection (RGNNV genotype) has been performed. The sequence analysis showed that European sea bass displays two rtp3 genes, X1 and X2, composed of two exons and a single intron (1007-bp and 888-bp long, respectively), located within the ORF sequence. The full-length cDNA is 1969 bp for rtp3 X1, and 1491 bp for rtp3 X2. Several ATTTA motifs have been found in the intron sequence of both genes, whereas rtp3 X1 also contains this motif in both untranslated regions. The transcription analyses revealed significant level of rtp3 X2 mRNA in brain and head kidney after LPS and poly I:C inoculation; however, the induction elicited by RGNNV infection was much higher, suggesting an essential role for this protein in controlling NNV infections.

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.

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.

Current status and development prospects of aquatic vaccines

Diseases are a significant impediment to aquaculture's sustainable and healthy growth. The aquaculture industry is suffering significant financial losses as a result of the worsening water quality and increasing frequency of aquatic disease outbreaks caused by the expansion of aquaculture. Drug control, immunoprophylaxis, ecologically integrated control, etc. are the principal control strategies for fish infections. For a long time, the prevention and control of aquatic diseases have mainly relied on the use of various antibiotics and chemical drugs. However, long-term use of chemical inputs not only increases pathogenic bacteria resistance but also damages the fish and aquaculture environments, resulting in drug residues in aquatic products, severely impeding the development of the aquaculture industry. The development and use of aquatic vaccines are the safest and most effective ways to prevent aquatic animal diseases and preserve the health and sustainability of aquaculture. To give references for the development and implementation of aquatic vaccines, this study reviews the development history, types, inoculation techniques, mechanisms of action, development prospects, and challenges encountered with aquatic vaccines.

Modern biotechnological strategies for vaccine development in aquaculture - Prospects and challenges

Advances in genomics and the gradual reduction of cost for technologies like whole-genome sequencing have provided exciting opportunities for developing modern biotechnological-based vaccines in aquaculture. This systemic review describes the prospects and challenges of implementing these high-tech vaccines in fish species. The majority of the commercial vaccines in aquaculture utilize conventional procedures for which cost of administration, protective immunity and safety issues are the major challenges. In recent years, more efficient vaccines are being developed by adopting the advances in vaccine technology. Vaccines based on surface antigens, protein/peptide/polysaccharide subunits, recombinant DNA/mRNA/plasmids, novel antigen expression and delivery systems (bacteriophage particles, virus like particles/VLPs, recombinant yeast, mucosal vaccines), novel molecular adjuvants (IL-8, IL-12, HSPs), and encapsulation polymers and polysaccharides like chitosan nanoparticles and PLGA microcapsule were successfully developed. These biotechnology-based vaccines have proved to be very efficient in field trials, but are always in the research pipeline or as patents. Only very few of them are licensed for use, that too, in high-valued fishes like salmonids. Currently, commercial aquaculture vaccines are available for Aeromonas salmonicida, Vibrio salmonicida, Yersinia ruckeri, Vibrio anguillarum, Edwardsiella ictalurid, and for certain Betanodaviruses. Nevertheless, no registered vaccines are available for other major infectious diseases/pathogens such as viral hemorrhagic septicemia virus (VHSV), viral nervous necrosis virus (VNN) and certain other betanodaviruses, channel catfish virus (CCV), gill disease bacteria, mycobacteria, flavobacterium, Edwardsiella tarda, and certain streptococci. Despite the important economic losses that the pathogens cause to aquaculture worldwide, the commercialization of vaccines remains limited due to immunological pitfalls in aquatic species, large-scale vaccination issues, unregulated use of antibiotics and chemicals, gene-based vaccine regulations and commercial viability. If attempts are to be made to develop novel delivery methods, cost-effective procedures, and relaxations in DNA vaccine regulations, biotechnology-based vaccination could circumvent the emerging disease challenges in aquaculture.

Amantadine, a promising therapeutic agent against viral encephalopathy and retinopathy

Outbreaks of viral encephalopathy and retinopathy (VER) in marine and freshwater species severely devastate the aquaculture worldwide. The causative agent of VER is nervous necrosis virus (NNV), which mainly infects the early developmental stages of fish, limiting the effectiveness of vaccines. To counter this case, the anti-NNV potentials of nine drugs with broad-spectrum antiviral activity were explored using ribavirin as a positive drug. Toxicity of the selected drugs to SSN-1 cells and grouper was firstly evaluated to determine the safety concentrations. For screening in vitro, amantadine and oseltamivir phosphate can relieve the cytopathic effects and inhibit NNV replication with the 90% inhibitory concentrations (IC₉₀ ) of 38.74 and 106.75 mg/L, respectively. Amantadine has a stronger anti-NNV activity than ribavirin at the with- and post-NNV infection stages, indicating that it is a potential therapeutic agent against VER by acting directly on NNV. Similarly, amantadine also has a strong anti-NNV activity in vivo with the IC₉₀ of 27.91 mg/L at the 7 days post-infection, while that was 73.25 mg/L for ribavirin. Following exposure to amantadine (40 mg/L) and ribavirin (100 mg/L) for 7 days, the survival rates of NNV-infected grouper were increased to 44% and 39%, respectively. The maximum amantadine content (11.88 mg/Kg) in grouper brain was reached following exposure for 24 hr, and amantadine can be quickly excreted from fish, reducing the risk of drug residue. Results so far indicated that amantadine is a promising therapeutic agent against NNV in aquaculture, providing an effective strategy for VER control at the early developmental stages of fish.

Development and characterization of five novel cell lines from snubnose pompano, Trachinotus blochii (Lacepede, 1801), and their application in gene expression and virological studies

Five novel permanent cell lines have been established from gill, heart, kidney, eye and fin of snubnose pompano, Trachinotus blochii. They were designated as snubnose pompano gill (SPG), snubnose pompano heart (SPH), snubnose pompano kidney (SPK), snubnose pompano eye (SPE) and snubnose pompano fin (SPF), respectively. All these cell lines were characterized and cryopreserved successfully at different passage levels. Cell lines were passaged every alternate day; SPG, SPH, SPK, SPE and SPF cell lines attained passage levels of 68, 74, 82, 79 and 106, respectively, since the initiation of their development in 2019. The cell lines grew well in Leibovitz's 15 medium containing 15% foetal bovine serum at 28°C. Immunophenotyping of the cell lines revealed the presence of fibronectin and pancytokeratin. No mycoplasma contamination was found. The transfection study revealed the gene expression efficiency of these cell lines by expressing the green fluorescent protein (GFP). The authentication on origin of cell lines from T. blochii was confirmed by amplification of species-specific mitochondrial cytochrome oxidase I gene. The results showed the susceptibility of these cell lines to fish nodavirus (FNV) and tilapia lake virus (TiLV) and resistance to cyprinid herpesvirus 2 (CyHV-2). The FNV infection in the cell lines was confirmed by RT-PCR, Western blot, ELISA and immunocytochemistry, while TiLV infection was confirmed by RT-PCR assay. These results revealed that these cell lines are suitable for virological and foreign gene expression studies.

Update on the Inactivation Procedures for the Vaccine Development Prospects of a New Highly Virulent RGNNV Isolate

Viral nervous necrosis (VNN) caused by the nervous necrosis virus (NNV) affects a broad range of primarily marine fish species, with mass mortality rates often seen among larvae and juveniles. Its genetic diversification may hinder the effective implementation of preventive measures such as vaccines. The present study describes different inactivation procedures for developing an inactivated vaccine against a new NNV isolate confirmed to possess deadly effects upon the European seabass (Dicentrarchus labrax), an important Mediterranean farmed fish species that is highly susceptible to this disease. First, an NNV isolate from seabass adults diagnosed with VNN was rescued and the sequences of its two genome segments (RNA1 and RNA2) were classified into the red-spotted grouper NNV (RGNNV) genotype, closely clustering to the highly pathogenic 283.2009 isolate. The testing of different inactivation procedures revealed that the virus particles of this isolate showed a marked resistance to heat (for at least 60 °C for 120 min with and without 1% BSA) but that they were fully inactivated by 3 mJ/cm² UV-C irradiation and 24 h 0.2% formalin treatment, which stood out as promising NNV-inactivation procedures for potential vaccine candidates. Therefore, these procedures are feasible, effective, and rapid response strategies for VNN control in aquaculture.