The immunogenicity of viral haemorragic septicaemia rhabdovirus (VHSV) DNA vaccines can depend on plasmid regulatory sequences

A single-step, high throughput, and highly reproducible method for measuring IgM quantity and avidity directly from fish serum via biolayer interferometry (BLI)

Quantification of specific antibody responses is critical in determining activation of MHCII-dependent immune memory and is generally performed by enzyme-linked immunosorbent assay (ELISA). Antibody avidity for a particular antigen is also informative of the quality of the adaptive immune response following vaccination. Avidity can be determined by chaotropic elution ELISA, pre-absorption ELISA, or surface plasmon resonance (SPR), although multimeric antibodies such as IgM are problematic for SPR. ELISA-based assays are very time consuming, require secondary antibody reagents, and are poorly repeatable. Here we demonstrate that biolayer interferometry (BLI) using an Octet HTX instrument can robustly and reproducibly quantify and determine avidity of specific IgM for an antigen directly from fish serum in a single step. We collected sera from giant grouper (Epinephelus lanceolatus) that had been vaccinated with the hapten 2,4-dinitrophenol conjugated to keyhole limpet hemocyanin (DNP-KLH) and from control fish injected with phosphate buffered saline. The specific IgM in the serum and its avidity for DNP were quantified via ELISA and BLI. BLI was precise and highly repeatable for determination of the quantity and avidity of antibody in the serum compared to ELISA. The wet-lab preparation and machine running time for BLI was 3-5 times faster than ELISA to generate the same amount of data. The ELISA inter-plate variation significantly affected reproducibility while BLI was consistent and repeatable between samples and plates. Indeed, the consistency of BLI data indicated that technical triplicates were redundant. Biological replication alone was sufficient to elucidate the effect of treatments. However, BLI required a lower serum dilution than ELISA for similar sensitivity, and thus more serum was required to produce high resolution data. BLI is an extremely high-throughput assay, providing teleost serum IgM quantification and avidity data as a single-step, agile alternative to ELISA.

Potential Role of Rainbow Trout Erythrocytes as Mediators in the Immune Response Induced by a DNA Vaccine in Fish

In recent years, fish nucleated red blood cells (RBCs) have been implicated in the response against viral infections. We have demonstrated that rainbow trout RBCs can express the antigen encoded by a DNA vaccine against viral hemorrhagic septicemia virus (VHSV) and mount an immune response to the antigen in vitro. In this manuscript, we show, for the first time, the role of RBCs in the immune response triggered by DNA immunization of rainbow trout with glycoprotein G of VHSV (GVHSV). Transcriptomic and proteomic profiles of RBCs revealed genes and proteins involved in antigen processing and presentation of exogenous peptide antigen via MHC class I, the Fc receptor signaling pathway, the autophagy pathway, and the activation of the innate immune response, among others. On the other hand, GVHSV-transfected RBCs induce specific antibodies against VHSV in the serum of rainbow trout which shows that RBCs expressing a DNA vaccine are able to elicit a humoral response. These results open a new direction in the research of vaccination strategies for fish since rainbow trout RBCs actively participate in the innate and adaptive immune response in DNA vaccination. Based on our findings, we suggest the use of RBCs as target cells or carriers for the future design of novel vaccine strategies.

Time-course study of the protection induced by an interferon-inducible DNA vaccine against viral haemorrhagic septicaemia in rainbow trout

The highly effective DNA vaccines against diseases caused by fish rhabdoviruses in farmed fish consist of a DNA plasmid vector encoding the viral glycoprotein under the control of a constitutive cytomegalovirus promoter (CMV). Among others, attempts to improve efficacy and safety of these DNA vaccines have focused on regulatory elements of plasmid vectors, which play a major role in controlling expression levels of vaccine antigens. Depending on the context, use of a fish-derived promoter with minimal activity in mammalian cells could be preferable. Another aspect related to the CMV promoter is that constitutive expression of the vaccine antigen may lead to rapid elimination of antigen expressing cells in the fish and thereby potentially reduce the long-term effects of the vaccine. In this study, we compared DNA vaccines with the interferon-inducible Mx promoter from rainbow trout and the CMV promoter, respectively. Plasmid constructs encoding the enhanced green fluorescent protein (EGFP) were used for the in vitro analysis, whereas DNA vaccines encoding the glycoprotein (G) of the viral haemorrhagic septicaemia virus (VHSV) were applied for the in vivo examination. The in vitro analysis showed that while the DNA vaccine with the CMV promoter constitutively drove the expression of EGFP in both fish and human cell lines, the DNA vaccine with the Mx promoter inducibly enhanced the expression of EGFP in the fish cell line. To address the impact on protection, a time-course model was followed as suggested by Kurath et al. (2006), where vaccinated fish were challenged with VHSV at 2, 8 and 78 weeks post-vaccination (wpv). The DNA vaccine with the CMV promoter protected at all times, while vaccination with the DNA vaccine containing the Mx promoter only protected the fish at 8 wpv. However, following induction with Poly (I:C) one week before the challenge, high protection was also evident at 2 wpv. In conclusion, the results revealed a more fish host dependent activity of the trout Mx promoter compared to the traditionally used cross species-active CMV promoter, but improvements will be needed for its application in DNA vaccines to ensure long term protection.

A DNA vaccine encoding the viral hemorrhagic septicemia virus genotype IVb glycoprotein confers protection in muskellunge (Esox masquinongy), rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), and lake trout (Salvelinus namaycush)

BACKGROUND

The viral hemorrhagic septicemia virus (VHSV) is one of the most serious fish pathogens. In 2003, a novel sublineage (genotype IVb) of this deadly virus emerged in the Great Lakes basin causing serious fish kills. We have previously demonstrated that a DNA plasmid (pcDNA), containing a cytomegalovirus (CMV) promoter and the viral hemorrhagic septicemia virus (VHSV) genotype IVb glycoprotein (G) gene insert (designated pVHSivb-G) confers moderate protection in muskellunge (Esox masquinongy), a highly susceptible species upon challenge. In order to achieve optimal protection, we investigated a number of factors including the incubation time [i.e. the number of degree days (° days)] before challenge, and viral challenge dose and route. Additionally, we tested if pVHSivb-G provides protection against VHSV-IVb to less susceptible salmonids such as rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta) and lake trout (Salvelinus namaycush).

RESULTS

An increase in the period lapsed between vaccination and challenge to 1880° days resulted in 95% relative percent protection (RPS) in muskellunge following a single administration of the pVHSivb-G plasmid and viral challenge. An RPS of 100% for muskellunge was achieved with a longer incubation period (2400° days) and in conjunction with a booster dose of the plasmid. The pVHSivb-G vaccine also elicited significant protection in all three salmonid species, reaching 100% RPS in lake trout following an incubation period of 1001° days prior to viral challenge. Vaccination with pVHSivb-G was also associated with the development of significant levels of circulating VHSV-binding antibodies in muskellunge as measured by indirect ELISA, which reached peak levels 6-7 weeks post-vaccination. Viral shedding in vaccinated survivors was minimal and of transient nature.

CONCLUSIONS

The study shows that the pVHSivb-G plasmid can elicit a protective response against the wild virus strain in a range of species important in recreational and commercial Great Lakes fisheries.

Over-passage of epithelioma papulosum cyprini (EPC) cells increased viral hemorrhagic septicemia virus (VHSV) replication

Vaccines based on inactivated or attenuated viruses can be a way to prevent viral hemorrhagic septicemia virus (VHSV) disease, and the efficiency of viral production is a critical factor that can determine the practical use of developed vaccines in aquaculture farms. To know the effects of epithelioma papulosum cyprini (EPC) cells over-subculture on VHSV replication, the VHSV titer produced from high-passage EPC cells (subcultured more than 200 times in our laboratory) was compared to the titer produced from low-passage EPC cells (subcultured 5-15 times). Furthermore, to know whether immune factors are involved in VHSV titers, differences not only in the expression of Mx1 and ISG15 genes but also in the apoptosis progression by VHSV infection between high- and low-passage EPC cells were analyzed. The VHSV titers from high-passage EPC cells were significantly higher than titers from low-passage EPC cells, suggesting that the changed properties of EPC cells by over-subculture were favorable for VHSV proliferation. The DNA laddering of high-passage EPC cells by VHSV infection took a longer time than that of low-passage EPC cells, suggesting that over-subculture might delay apoptosis in VHSV infected EPC cells, and the delay of apoptosis by over-subculture can be thought as one of the factors that increased VHSV titers in high-passage EPC cells. The increased folds of Mx1 and ISG15 genes in high-passage EPC cells were significantly lower than those in low-passage EPC cells when exposed to either poly (I:C) or VHSV. However, the expression levels of Mx1 and ISG15 genes of high-passage EPC cells that were not stimulated with poly I:C or VHSV were almost equal to or higher than the expression levels of low-passage EPC cells that were exposed to poly (I:C) or VHSV. This result suggests that high-passage EPC cells were already in an excited state in type I interferon responses without any stimulants. The full open reading frame (ORF) sequences of Mx1 gene between high- and low-passage EPC cells were completely same. However, there were some differences in the amino acids sequences of ISG15 gene between high- and low-passage EPC cells, suggesting that ISG15-mediated pathways might be different between high- and low-passage EPC cells, which might influence on the replication of VHSV. The present results showed that the changed properties of EPC cells by over-subculture were favorable for VHSV proliferation.

Development of new therapeutical/adjuvant molecules by pepscan mapping of autophagy and IFN inducing determinants of rhabdoviral G proteins

Surface glycoproteins of enveloped virus are potent elicitors of both innate and adaptive host immune responses. Therefore, the identification of viral glycoprotein determinants directly implicated in the induction of these responses might be of special interest for designing new therapeutical/adjuvant molecules. In this work we review the contribution of the "pepscan" approach to the screening of viral functions in the sequence of glycoprotein G (gpG) of the fish rhabdovirus of viral hemorrhagic septicemia (VHSV). Among others, by scanning gpG peptides, it has been possible to identify and validate minimal determinants for gpG directly implicated in initiating the fish type I Interferon-associated immune responses as well as in the antiviral autophagy program. Further fine-tunning of the identified peptides in the gpG of VHSV has allowed designing novel adjuvants that decrease DNA vaccine requirements and identify possible innovative antiviral molecules. In addition, these results have also contributed to improve our knowledge on how to stimulate the fish immune system.

Gonadotropins in European sea bass: Endocrine roles and biotechnological applications

Follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) are central endocrine regulators of the gonadal function in vertebrates. They act through specific receptors located in certain cell types found in the gonads. In fish, the differential roles of these hormones are being progressively elucidated due to the development of suitable tools for their study. In European sea bass (Dicentrarchus labrax), isolation of the genes coding for the gonadotropin subunits and receptors allowed in first instance to conduct expression studies. Later, to overcome the limitation of using native hormones, recombinant dimeric gonadotropins, which show different functional characteristics depending on the cell system and DNA construct, were generated. In addition, single gonadotropin beta-subunits have been produced and used as antigens for antibody production. This approach has allowed the development of detection methods for native gonadotropins, with European sea bass being one of the few species where both gonadotropins can be detected in their native form. By administering recombinant gonadotropins to gonad tissues in vitro, we were able to study their effects on steroidogenesis and intracellular pathways. Their administration in vivo has also been tested for use in basic studies and as a biotechnological approach for hormone therapy and assisted reproduction strategies. In addition to the production of recombinant hormones, gene-based therapies using somatic gene transfer have been offered as an alternative. This approach has been tested in sea bass for gonadotropin delivery in vivo. The hormones produced by the genes injected were functional and have allowed studies on the action of gonadotropins in spermatogenesis.

Generation of G gene-deleted viral hemorrhagic septicemia virus (VHSV) and evaluation of its vaccine potential in olive flounder (Paralichthys olivaceus)

Generation of recombinant viruses lacking an essential gene for the production of infective viral particles would be a way to produce safety-enhanced live viral vaccines. The rhabdoviral envelope-spiked glycoprotein (G) plays critical roles in the attachment of viruses on the cell surface receptor and in the transfer of endocytosed viruses to the cytoplasm by fusion to the endosomal membrane. In the present study, we produced a G gene-lacking recombinant viral hemorrhagic septicemia virus (rVHSV-ΔG) that has no ability to form plaques in the cells without a trans-supply of the G protein, which suggests that rVHSV-ΔG is a single cycle virus and progenies of rVHSV-ΔG are not infectious. One of the major advantages of attenuated vaccines is the maintenance of replication ability in the host, which enforces the adaptive immune responses. However, in spite of lacking an ability to produce infective viral particles, immunization with the present rVHSV-ΔG induced significantly higher serum neutralization titers and survival rates against virulent VHSV challenge in olive flounder (Paralichthys olivaceus) fingerlings, indicating that the present rVHSV-ΔG has a high potential as a prophylactic vaccine.

VHSV G glycoprotein major determinants implicated in triggering the host type I IFN antiviral response as DNA vaccine molecular adjuvants

We have recently identified the two major determinants of the glycoprotein G of the viral hemorrhagic septicaemia rhabdovirus (gpGVHSV), peptides p31 and p33 implicated in triggering the host type I IFN antiviral response associated to these rhabdoviral antigens. With the aim to investigate the properties of these viral glycoprotein regions as DNA molecular adjuvants, their corresponding cDNA sequences were cloned into a plasmid (pMCV1.4) flanked by the signal peptide and transmembrane sequences of gpGVHSV. In addition, a plasmid construct encoding both sequences p31 and p33 (pMCV1.4-p31+p33) was also designed. In vitro transitory cell transfection assays showed that these VHSV gpG regions were able to induce the expression of type I IFN stimulated genes as well as to confer resistance to the infection with a different fish rhabdovirus, the spring viremia of carp virus (SVCV). In vivo, zebrafish intramuscular injection of only 1μg of the construct pMCV1.4-p31+p33 conferred fish protection against SVCV lethal challenge up to 45 days post-immunization. Moreover, pMCV1.4-p31+p33 construct was assayed for molecular adjuvantcity's for a DNA vaccine against SVCV based in the surface antigen of this virus (pAE6-GSVCV). The results showed that the co-injection of the SVCV DNA vaccine and the molecular adjuvant allowed (i) a ten-fold reduction in the dose of pAE6-Gsvcv without compromising its efficacy (ii) an increase in the duration of protection, and (iii) an increase in the survival rate. To our knowledge, this is the first report in which specific IFN-inducing regions from a viral gpG are used to design more-efficient and cost-effective viral vaccines, as well as to improve our knowledge on how to stimulate the innate immune system.

Autophagy-inducing peptides from mammalian VSV and fish VHSV rhabdoviral G glycoproteins (G) as models for the development of new therapeutic molecules

It has not been elucidated whether or not autophagy is induced by rhabdoviral G glycoproteins (G) in vertebrate organisms for which rhabdovirus infection is lethal. Our work provides the first evidence that both mammalian (vesicular stomatitis virus, VSV) and fish (viral hemorrhagic septicemia virus, VHSV, and spring viremia carp virus, SVCV) rhabdoviral Gs induce an autophagic antiviral program in vertebrate cell lines. The transcriptomic profiles obtained from zebrafish genetically immunized with either Gsvcv or Gvhsv suggest that autophagy is induced shortly after immunization and therefore, it may be an important component of the strong antiviral immune responses elicited by these viral proteins. Pepscan mapping of autophagy-inducing linear determinants of Gvhsv and Gvsv showed that peptides located in their fusion domains induce autophagy. Altogether these results suggest that strategies aimed at modulating autophagy could be used for the prevention and treatment of rhabdoviral infections such as rabies, which causes thousands of human deaths every year.

DNA vaccines against viral diseases of farmed fish

Immunization by an antigen-encoding DNA was approved for commercial sale in Canada against a Novirhabdovirus infection in fish. DNA vaccines have been particularly successful against the Novirhabdoviruses while there are reports on the efficacy against viral pathogens like infectious pancreatic necrosis virus, infectious salmon anemia virus, and lymphocystis disease virus and these are inferior to what has been attained for the novirhabdoviruses. Most recently, DNA vaccination of Penaeus monodon against white spot syndrome virus was reported. Research efforts are now focused on the development of more effective vectors for DNA vaccines, improvement of vaccine efficacy against various viral diseases of fish for which there is currently no vaccines available and provision of co-expression of viral antigen and immunomodulatory compounds. Scientists are also in the process of developing new delivery methods. While a DNA vaccine has been approved for commercial use in farmed salmon in Canada, it is foreseen that it is still a long way to go before a DNA vaccine is approved for use in farmed fish in Europe.

Increasing versatility of the DNA vaccines through modification of the subcellular location of plasmid-encoded antigen expression in the in vivo transfected cells

The route of administration of DNA vaccines can play a key role in the magnitude and quality of the immune response triggered after their administration. DNA vaccines containing the gene of the membrane-anchored glycoprotein (gpG) of the fish rhabdoviruses infectious haematopoietic necrosis virus (IHNV) or viral haematopoietic septicaemia virus (VHSV), perhaps the most effective DNA vaccines generated so far, confer maximum protection when injected intramuscularly in contrast to their low efficacy when injected intraperitoneally. In this work, taking as a model the DNA vaccine against VHSV, we focused on developing a more versatile DNA vaccine capable of inducing protective immunity regardless of the administration route used. For that, we designed two alternative constructs to gpG_1-507 (the wild type membrane-anchored gpG of VHSV) encoding either a soluble (gpG_1-462) or a secreted soluble (gpG_LmPle20-462) form of the VHSV-gpG. In vivo immunisation/challenge assays showed that only gpG_LmPle20-462 (the secreted soluble form) conferred protective immunity against VHSV lethal challenge via both intramuscular and intraperitoneal injection, being this the first description of a fish viral DNA vaccine that confers protection when administered intraperitoneally. Moreover, this new DNA vaccine construct also conferred protection when administered in the presence of an oil adjuvant suggesting that DNA vaccines against rhabdoviruses could be included in the formulation of current multicomponent-intaperitoneally injectable fish vaccines formulated with an oil adjuvant. On the other hand, a strong recruitment of membrane immunoglobulin expressing B cells, mainly membrane IgT, as well as t-bet expressing T cells, at early times post-immunisation, was specifically observed in the fish immunised with the secreted soluble form of the VHSV-gpG protein; this may indicate that the subcellular location of plasmid-encoded antigen expression in the in vivo transfected cells could be an important factor in determining the ways in which DNA vaccines prime the immune response.

Behavioural fever is a synergic signal amplifying the innate immune response

Behavioural fever, defined as an acute change in thermal preference driven by pathogen recognition, has been reported in a variety of invertebrates and ectothermic vertebrates. It has been suggested, but so far not confirmed, that such changes in thermal regime favour the immune response and thus promote survival. Here, we show that zebrafish display behavioural fever that acts to promote extensive and highly specific temperature-dependent changes in the brain transcriptome. The observed coupling of the immune response to fever acts at the gene–environment level to promote a robust, highly specific time-dependent anti-viral response that, under viral infection, increases survival. Fish that are not offered a choice of temperatures and that therefore cannot express behavioural fever show decreased survival under viral challenge. This phenomenon provides an underlying explanation for the varied functional responses observed during systemic fever. Given the effects of behavioural fever on survival and the fact that it exists across considerable phylogenetic space, such immunity–environment interactions are likely to be under strong positive selection.

Optimal place of a foreign gene in the genome of viral haemorrhagic septicaemia virus (VHSV) for development of VHSV-based viral-vectored vaccines

AIM

As the strength and duration of immune responses can be regulated by the antigen dose, higher expression of foreign antigens in the viral-vectored vaccines would be an important factor for inducing effective immune responses. The aim of this study was to determine the optimal insertion place of a foreign antigen gene in the genome of viral haemorrhagic septicaemia virus (VHSV) for development of VHSV-based viral-vectored vaccines.

METHODS AND RESULTS

Recombinant VHSVs (rVHSVs) harbouring the red fluorescent protein (RFP) gene between N and P (rVHSV-A-RFP), P and M (rVHSV-B-RFP), or M and G genes (rVHSV-C-RFP) in the genome were rescued by reverse genetics. Their replication ability and expression level of RFP were compared according to the inserted locations. The viral titres of each rVHSV were not significantly different. However, Epithelioma papulosum cyprini (EPC) cells infected with rVHSV-A-RFP or rVHSV-B-RFP showed clearly higher fluorescence than cells infected with rVHSV-C-RFP. There was no significant difference in RFP expression between cells infected with rVHSV-A-RFP and rVHSV-B-RFP.

CONCLUSIONS

The present results indicate that insertion of a foreign gene between N and P, or P and M genes of VHSV genome would be advantageous for development of VHSV-based viral-vectored vaccines.

SIGNIFICANCE AND IMPACT OF THE STUDY

The present work is the first report on the optimal location of a foreign gene in VHSV genome for high expression, and the locations identified in this study would be suitable for the development of VHSV-based viral-vectored vaccines.

RNA aptamers inhibit the growth of the fish pathogen viral hemorrhagic septicemia virus (VHSV)

Viral hemorrhagic septicemia virus (VHSV) is a serious disease impacting wild and cultured fish worldwide. Hence, an effective therapeutic method against VHSV infection needs to be developed. Aptamer technology is a new and promising method for diagnostics and therapeutics. It revolves around the use of an aptamer molecule, an artificial ligand (nucleic acid or protein), which has the capacity to recognize target molecules with high affinity and specificity. Here, we aimed at selecting RNA aptamers that can specifically bind to and inhibit the growth of a strain of fish VHSV both in vitro and in vivo. Three VHSV-specific RNA aptamers (F1, F2, and C6) were selected from a pool of artificially and randomly produced oligonucleotides using systematic evolution of ligands by exponential enrichment. The three RNA aptamers showed obvious binding to VHSV in an electrophoretic mobility shift assay but not to other tested viruses. The RNA aptamers were tested for their ability to inhibit VHSV in vitro using hirame natural embryo (HINAE) cells. Cytopathic effect and plaque assays showed that all aptamers inhibited the growth of VHSV in HINAE cells. In vivo tests using RNA aptamers produced by Rhodovulum sulfidophilum showed that extracellular RNA aptamers inhibited VHSV infection in Japanese flounder. These results suggest that the RNA aptamers are a useful tool for protection against VHSV infection in Japanese flounder.

Improving the safety of viral DNA vaccines: development of vectors containing both 5' and 3' homologous regulatory sequences from non-viral origin

Although some DNA vaccines have proved to be very efficient in field trials, their authorisation still remains limited to a few countries. This is in part due to safety issues because most of them contain viral regulatory sequences to driving the expression of the encoded antigen. This is the case of the only DNA vaccine against a fish rhabdovirus (a negative ssRNA virus), authorised in Canada, despite the important economic losses that these viruses cause to aquaculture all over the world. In an attempt to solve this problem and using as a model a non-authorised, but efficient DNA vaccine against the fish rhabdovirus, viral haemorrhagic septicaemia virus (VHSV), we developed a plasmid construction containing regulatory sequences exclusively from fish origin. The result was an "all-fish vector", named pJAC-G, containing 5' and 3' regulatory sequences of β-acting genes from carp and zebrafish, respectively. In vitro and in vivo, pJAC-G drove a successful expression of the VHSV glycoprotein G (G), the only antigen of the virus conferring in vivo protection. Furthermore, and by means of in vitro fusion assays, it was confirmed that G protein expressed from pJAC-G was fully functional. Altogether, these results suggest that DNA vaccines containing host-homologous gene regulatory sequences might be useful for developing safer DNA vaccines, while they also might be useful for basic studies.

Immunization of olive flounder (Paralichthys olivaceus) with an auxotrophic Edwardsiella tarda mutant harboring the VHSV DNA vaccine

The aims of the present study were to find more powerful promoter for DNA vaccines in olive flounder (Paralichthys olivaceus) and to evaluate the availability of the auxotrophic Edwardsiella tarda mutant (Δalr Δasd E. tarda) as a delivery vehicle for DNA vaccine against VHSV in olive flounder. The marine medaka (Oryzias dancena) β-actin promoter was clearly stronger than cytomegalovirus (CMV) promoter when the vectors were transfected to Epithelioma papulosum cyprini (EPC) cells or injected into the muscle of olive flounder, suggesting that marine medaka β-actin promoter would be more appropriate promoter for DNA vaccines in olive flounder than CMV promoter. Olive flounder immunized with the Δalr Δasd E. tarda harboring viral hemorrhagic septicemia virus (VHSV) DNA vaccine vector driven by the marine medaka β-actin promoter showed significantly higher serum neutralization titer and higher survival rates against challenge with VHSV than fish immunized with the bacteria carrying VHSV DNA vaccine vector driven by CMV promoter. These results indicate that auxotrophic E. tarda mutant harboring marine medaka β-actin promoter-driven DNA vaccine vectors would be a potential system for prophylactics of infectious diseases in olive flounder.

Replacement of the human cytomegalovirus promoter with fish enhancer and core elements to control the expression of the G gene of viral haemorrhagic septicemia virus (VHSV)

This work explores some of the possibilities to replace human cytomegalovirus (CMV) core and/or enhancer promoter control elements to create new expression vectors for use with fish. The work is relevant to fish vaccination, since DNA vaccines use eukaryotic expression plasmids controlled by the human cytomegalovirus (CMV) promoter to be effective against novirhabdoviruses, such as viral haemorrhagic septicemia virus (VHSV), one of the most devastating fish viral European diseases. To reduce possible homologous recombination with fish genome, core and enhancer sequences from fish origin, such as trout interferon-inducible myxovirus protein (Mx), zebrafish retrovirus long terminal repeat (LTR) and carp β-actin (AE6), were combined with those of CMV to design alternative hybrid promoters. The substitution of CMV core and/or enhancer with the corresponding elements of Mx or the LTR core maintained a similar in vitro protein G expression level than that obtained by using the CMV promoter. Vectors using the dsRNA-inducible Mx enhancer followed either by the LTR or the AE6 cores showed the highest in vitro protein G expression levels. Furthermore, synthetic constructs using the Mx enhancer maintained their polyI:C induction capabilities despite the core used. Some of these hybrid promoters might contribute to the development of all-fish-vectors for DNA vaccines while others might be useful for more basic studies.

Ex vivo transfection of trout pronephros leukocytes, a model for cell culture screening of fish DNA vaccine candidates

DNA vaccination opened a new era in controlling and preventing viral diseases since DNA vaccines have shown to be very efficacious where some conventional vaccines have failed, as it occurs in the case of the vaccines against fish novirhabdoviruses. However, there is a big lack of in vitro model assays with immune-related cells for preliminary screening of in vivo DNA vaccine candidates. In an attempt to solve this problem, rainbow trout pronephros cells in early primary culture were transfected with two plasmid DNA constructions, one encoding the green fluorescent protein (GFP) and another encoding the viral haemorrhagic septicaemia virus (VHSV) glycoprotein G (G(VHSV)) - the only viral antigen which has conferred in vivo protection. After assessing the presence of GFP- and G(VHSV)-expressing cells, at transcription and protein levels, the immune response in transfected pronephros cells was evaluated. At 24h post-transfection, G(VHSV) up-regulated migm and tcr transcripts expression, suggesting activation of B and T cells, as well, a high up-regulation of tnfα gene was observed. Seventy-two hours post-transfection, we detected the up-regulation of mx and tnfα genes transcripts and Mx protein which correlated with the induction of an anti-VHSV state. All together we have gathered evidence for successful transfection of pronephros cells with pAE6G, which correlates with in vivo protection results, and is less time-consuming and more rapid than in vivo assays. Therefore, this outcome opens the possibility to use pronephros cells in early primary culture for preliminary screening fish DNA vaccines as well as to further investigate the function that these cells perform in fish immune response orchestration after DNA immunisation.

Protection and antibody response induced by intramuscular DNA vaccine encoding for viral haemorrhagic septicaemia virus (VHSV) G glycoprotein in turbot (Scophthalmus maximus)

Turbot (Scophthalmus maximus) is a high-value farmed marine flatfish with growing demand and production levels in Europe susceptible to turbot-specific viral haemorrhagic septicaemia virus (VHSV) strains. To evaluate the possibility of controlling the outbreaks of this infectious disease by means of DNA vaccination, the gpG of a VHSV isolated from farmed turbot (VHSV(860)) was cloned into an expression plasmid containing the human cytomegalovirus (CMV) promoter (pMCV1.4-G(860)). In our experimental conditions, DNA immunised turbots were more than 85% protected against VHSV(860) lethal challenge and showed both VHSV-gpG specific and neutralizing antibodies. To our knowledge this is the first report showing the efficacy of turbot genetic immunisation against a VHSV. Work is in progress to determine the contribution of innate and adaptive immunity to the protective response elicited by the immunization.

The antiviral defense mechanisms in mandarin fish induced by DNA vaccination against a rhabdovirus

Plasmid DNAs containing Siniperca chuatsi rhabdovirus (SCRV) glycoprotein gene (pcDNA-G) and nucleoprotein gene (pcDNA-N) were constructed, and used to determine the antiviral immune response elicited by DNA vaccination in mandarin fish. In vitro and in vivo expression of the plasmid constructs was confirmed in transfected cells and muscle tissues of vaccinated fish by Western blot, indirect immunofluorescence or RT-PCR analysis. Fish injected with pcDNA-G exhibited protective effect against SCRV challenge with a relative percent survival (RPS) of 77.5%, but no significant protection (RPS of 2.5%) was observed in fish vaccinated with pcDNA-N. Immunohistochemical analysis showed that vaccination with pcDNA-G decreased histological lesions and suppressed the virus replication in fish target organs, e.g. kidney, liver, spleen, gill and heart. Transcriptional analysis further revealed that the expression levels of type I IFN system genes including interferon regulation factor-7 (IRF-7) gene, myxovirus resistance (Mx) gene and virus inhibitory protein (Viperin) gene were strongly up-regulated after injection with pcDNA-G, whereas the level of transcription of immunoglobulin M (IgM) gene did not show a statistically significant change. These results reveal that type I IFN antiviral immune response is rapidly triggered by the plasmid DNA containing rhabdovirus glycoprotein gene in fish, which offers an explanation of molecular mechanisms for DNA vaccination inducing mandarin fish resist to SCRV disease.

Mytilus galloprovincialis myticin C: a chemotactic molecule with antiviral activity and immunoregulatory properties

Previous research has shown that an antimicrobial peptide (AMP) of the myticin class C (Myt C) is the most abundantly expressed gene in cDNA and suppressive subtractive hybridization (SSH) libraries after immune stimulation of mussel Mytilus galloprovincialis. However, to date, the expression pattern, the antimicrobial activities and the immunomodulatory properties of the Myt C peptide have not been determined. In contrast, it is known that Myt C mRNA presents an unusual and high level of polymorphism of unidentified biological significance. Therefore, to provide a better understanding of the features of this interesting molecule, we have investigated its function using four different cloned and expressed variants of Myt C cDNA and polyclonal anti-Myt C sera. The in vivo results suggest that this AMP, mainly present in hemocytes, could be acting as an immune system modulator molecule because its overexpression was able to alter the expression of mussel immune-related genes (as the antimicrobial peptides Myticin B and Mytilin B, the C1q domain-containing protein MgC1q, and lysozyme). Moreover, the in vitro results indicate that Myt C peptides have antimicrobial and chemotactic properties. Their recombinant expression in a fish cell line conferred protection against two different fish viruses (enveloped and non-enveloped). Cell extracts from Myt C expressing fish cells were also able to attract hemocytes. All together, these results suggest that Myt C should be considered not only as an AMP but also as the first chemokine/cytokine-like molecule identified in bivalves and one of the few examples in all of the invertebrates.

Oral immunization of olive flounder (Paralichthys olivaceus) with recombinant live viral hemorrhagic septicemia virus (VHSV) induces protection against VHSV infection

A recombinant viral hemorrhagic septicemia virus (rVHSV-ΔNV-EGFP) that has enhanced green fluorescent protein (EGFP) gene instead of NV gene was previously generated using reverse genetics technology. In this study, potential of the rVHSV-ΔNV-EGFP to be used as a live oral vaccine candidate was assessed. The presence of the recombinant virus in internal organs of orally administered olive flounder (Paralichthys olivaceus) was analyzed by semi-quantitative RT-PCR. Although the recombinant VHSV-specific band was detected only when the number of PCR cycle was increased to 35, the band was detected from internal organs, such as kidney, spleen, and liver of fish that were reared at either 15 °C or 20 °C till even 20 days, suggesting that a few orally administered rVHSV-ΔNV-EGFP might be transported to internal organs, and might keep weak replication ability in the organs. VHSV-neutralizing activity was induced by oral immunization of olive flounder with the NV gene knock-out recombinant VHSV not only in skin and intestinal mucus but also in serum, suggesting that mucosal and systemic adaptive immune responses were elicited by oral immunization. In challenge experiment, groups of fish immunized with 10⁴, 10⁵, and 2 × 10⁵ PFU of rVHSV-ΔNV-EGFP/fish showed 25%, 50%, and 70% of relative percent survival (RPS), respectively. The RPSs were elevated to 60%, 75%, and 90% by a boost immunization in fish boost immunized with 10⁴, 10⁵, and 2 × 10⁵ PFU of rVHSV-ΔNV-EGFP, respectively. The cumulative mortality of fish in the control groups was 100%. Conclusionly, the present results demonstrate that the NV gene knock-out recombinant VHSV administered orally to olive flounder can induce dose- and boosting-dependent VHSV-neutralizing antibody in mucus and serum, and can provide a high protection in olive flounder against a virulent VHSV challenge.

The introduction of multi-copy CpG motifs into an antiviral DNA vaccine strongly up-regulates its immunogenicity in fish

The protection conferred by antiviral DNA vaccines in fish is known to rely greatly on innate immune responses. Since oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides (CpG motifs) have been shown to induce potential innate immune responses, we have introduced several copies (either two or four) of a fragment containing multiple CpG sequences of known immunostimulatory effects into a DNA vaccine against viral hemorrhagic septicemia virus (VHSV). We have determined the effects of this introduction on the vaccine immunogenicity, measured as immune gene induction, serum neutralizing activity and antigen-dependent proliferation. When comparing the effects of the vaccine containing 2 copies of this CpG fragment (pVHSV-2CpG) or that containing 4 copies of the fragment (pVHSV-4CpG) with the original VHSV DNA vaccine (pVHSV), we found that the levels of expression of type I interferon (IFN) were significantly up-regulated in muscle and spleen when the CpG fragments were introduced. An up-regulation in the levels of MHC-I expression in spleen were also observed in response to the modified vaccines, whereas, the levels of transcription of interleukin 1β (IL-1β) were strongly reduced in comparison to the original vaccine. Important but very variable differences were also observed concerning the vaccine induction of IFN-γ. Moreover, the serum neutralizing capacity was strongly increased as fish were vaccinated with plasmids containing more CpG fragments. Taken together, all these results demonstrate a strongly increased immunogenicity of the VHSV DNA vaccine, through the introduction of this multicopy CpG fragment.

In vitro analysis of the factors contributing to the antiviral state induced by a plasmid encoding the viral haemorrhagic septicaemia virus glycoprotein G in transfected trout cells

We have found out that transfection of the RTG-2 cell line with the viral haemorrhagic septicaemia virus (VHSV) glycoprotein G (G(VHSV))-coding plasmid induces an anti-VHSV state, similar to that induced by poly I:C. Taking the advantage of the constitutive expression of toll-like receptor 9 gene (tlr9) in RTG-2 cells, we have investigated whether this antiviral state was induced by the cytosine-phosphodiester-guanine (CpG) motifs present in the plasmid DNA, by the endogenous expression of G(VHSV) protein or by both elements. For that, we have analysed the expression profile of the rainbow trout tlr9 and several genes related to TLR9-mediated immune response in the absence or presence of a lysosomotropic drug that specifically blocks TLR9-CpG DNA interaction. The results suggested that the high levels of cell protection conferred by a plasmid encoding G(VHSV) gene are due to G(VHSV) rather than to the CpG motifs within plasmid DNA. Therefore, plasmid DNA might not play a key role in the immune response elicited by DNA vaccines or perhaps other receptors instead TLR9 could be implicated in CpG motifs recognition and signalling. In addition, since RTG-2 cells express tlr9 gene, this cell line could be a good tool for screening TLR9 agonists, such as the immunomodulatory oligonucleotides (IMOs), as fish DNA vaccine adjuvants.

Pepscan mapping of viral hemorrhagic septicemia virus glycoprotein G major lineal determinants implicated in triggering host cell antiviral responses mediated by type I interferon

Surface glycoproteins of enveloped virus are potent elicitors of type I interferon (IFN)-mediated antiviral responses in a way that may be independent of the well-studied genome-mediated route. However, the viral glycoprotein determinants responsible for initiating the IFN response remain unidentified. In this study, we have used a collection of 60 synthetic 20-mer overlapping peptides (pepscan) spanning the full length of glycoprotein G (gpG) of viral hemorrhagic septicemia virus (VHSV) to investigate what regions of this protein are implicated in triggering the type I IFN-associated immune responses. Briefly, two regions with ability to increase severalfold the basal expression level of the IFN-stimulated mx gene and to restrict the spread of virus among responder cells were mapped to amino acid residues 280 to 310 and 340 to 370 of the gpG protein of VHSV. In addition, the results obtained suggest that an interaction between VHSV gpG and integrins might trigger the host IFN-mediated antiviral response after VHSV infection. Since it is known that type I IFN plays an important role in determining/modulating the protective-antigen-specific immune responses, the identification of viral glycoprotein determinants directly implicated in the type I IFN induction might be of special interest for designing new adjuvants and/or more-efficient and cost-effective viral vaccines as well as for improving our knowledge on how to stimulate the innate immune system.

The rainbow trout TLR9 gene and its role in the immune responses elicited by a plasmid encoding the glycoprotein G of the viral haemorrhagic septicaemia rhabdovirus (VHSV)

The aim of this work was to improve the knowledge about the factors contributing to the immunogenicity of the DNA vaccines based on the viral haemorrhagic septicaemia virus glycoprotein G gene, through identifying the rainbow trout Toll-like receptor 9 (Omtlr9) gene that curiously contains an insertion of an incomplete transposon at the 5'-end of the third intron. Concerning the role played by this receptor in the fish innate defence, in response to the injection of a plasmid (pAE6) encoding or not the viral haemorrhagic septicaemia rhabdovirus (VHSV) glycoprotein G gene (pAE6-G), the presence of Omtlr9 transcripts remained unchanged in the fish secondary lymphoid organs while was highly increased at the injection site (muscle). The level of Omtlr9 transcripts correlated with those of cluster of differentiation 83 (cd83) and CXC chemokine receptor 4 (cxcr4), suggesting the recruitment of dendritic-like cells into the muscle as the source of Omtlr9 expressing cells. Transcription of tumour necrosis factor-alpha (tnf alpha) and interleukin-6 (il6) genes, two cytokines directly related to TLR9 induction with unmethylated CpG oligodeoxynucleotides (CpG ODNs), was solely observed in head kidney and spleen of the fish immunised with pAE6-G. Thus, the glycoprotein G of VHSV could be more implicated in triggering the pathways for TNF-alpha and IL6 production than the recognition of the unmethylated CpG motifs of the plasmid backbone by OmTLR9. Therefore, our results seem to indicate that OmTLR9-mediated recognition of plasmid DNA is not the key of the innate immune recognition of the adjuvant elements of fish DNA vaccines.