Detection of supercoiled plasmid DNA and luciferase expression in Atlantic salmon (Salmo salar L.) 535 days after injection

Efficacy of DNA Vaccines in Protecting Rainbow Trout against VHS and IHN under Intensive Farming Conditions

Despite the negative impact of viral hemorrhagic septicemia (VHS) and infectious hematopoietic necrosis (IHN) on European rainbow trout farming, no vaccines are commercially available in Europe. DNA vaccines are protective under experimental conditions, but testing under intensive farming conditions remains uninvestigated. Two DNA vaccines encoding the glycoproteins (G) of recent Italian VHSV and IHNV isolates were developed and tested for potency and safety under experimental conditions. Subsequently, a field vaccination trial was initiated at a disease-free hatchery. The fish were injected intramuscularly with either the VHS DNA vaccine or with a mix of VHS and IHN DNA vaccines at a dose of 1 µg/vaccine/fish, or with PBS. At 60 days post-vaccination, fish were moved to a VHSV and IHNV infected facility. Mortality started 7 days later, initially due to VHS. After 3 months, IHN became the dominant cause of disease. Accordingly, both DNA vaccinated groups displayed lower losses compared to the PBS group during the first three months, while the VHS/IHN vaccinated group subsequently had the lowest mortality. A later outbreak of ERM caused equal disease in all groups. The trial confirmed the DNA vaccines to be safe and efficient in reducing the impact of VHS and IHN in farmed rainbow trout.

Immune response of channel catfish (Ictalurus punctatus) against Ichthyophthirius multifiliis post vaccination using DNA vaccines encoding immobilization antigens

The channel catfish (Ictalurus punctatus) immune response against Ichthyophthirius multifiliis (Ich) after vaccination using plasmid DNA vaccines pcDNA3.1-IAg52a and pcDNA3.1-IAg52b, encoding Ich immobilization antigen genes was studied. Parasite infection level, serum anti-Ich antibodies level, fish mortality after theront challenge, and immune-related gene expression were measured. After in vitro transfection of walking catfish gill cells (G1b) with both pcDNA3.1-IAg52a and pcDNA3.1-IAg52b, antigens IAG52A and IAG52B were detected. During the vaccination trial, 76-fold increase in the Iag52b gene expression was observed in the vaccinated fish group h4 post vaccination. Administration of DNA vaccines by IM injection induced significant gene up-regulation in the head kidney, including immunoglobulin M (IgM), cluster of differentiation 4 (CD4), major histocompatibility I (MHC I), and T cell receptor α (TcR-α) from h4 to d5 post immunization. Fish vaccinated with DNA vaccines or theronts showed increased gene expression of the cytokine interferon (IFN-γ), complement component 3 (C3), and toll-like receptor-1 (TLR-1). Anti-Ich antibodies were detected in fish received pcDNA3.1-IAg52a, pcDNA3.1-IAg52b and the combination of both vaccines d10 post vaccination. Fish vaccinated with pcDNA3.1-IAg52b showed mild parasite infection level, partial survival (20%) and longer mean day to death (MDD) after theront challenge. By contrast, a heavy parasite load, 0% survival and short MDD were observed in the sham vaccinated control fish that received pcDNA3.1 (plasmid without genes encoding Ich immobilization antigen). Further research is needed to improve DNA vaccines for Ich that can induce strong protective immunity in fish. Suggested studies include improved transfection efficiency, use of appropriate adjuvants and including additional parasite antigen genes in the plasmid.

DNA vaccination for finfish aquaculture

In fish, DNA vaccines have been shown to give very high protection in experimental facilities against a number of viral diseases, particularly diseases caused by rhabdoviruses. However, their efficacy in generating protection against other families of fish viral pathogens is less clear. One DNA vaccine is currently in use commercially in fish farms in Canada and the commercialisation of another was authorised in Europe in 2017. The mechanism of action of DNA vaccines, including the role of the innate immune responses induced shortly after DNA vaccination in the activation of the adaptive immunity providing longer term specific protection, is still not fully understood. In Europe the procedure for the commercialisation of a veterinary DNA vaccine requires the resolution of certain concerns particularly about safety for the host vaccinated fish, the consumer and the environment. Relating to consumer acceptance and particularly environmental safety, a key question is whether a DNA vaccinated fish is considered a Genetically Modified Organism (GMO). In the present opinion paper these key aspects relating to the mechanisms of action, and to the development and the use of DNA vaccines in farmed fish are reviewed and discussed.

What could be the mechanisms of immunological memory in fish?

Vaccination is the most effective strategy to control infectious diseases in species with adaptive immunity. In human and in mouse, vaccination typically induces specific memory cells, which can mediate a fast anamnestic response upon infection by the targeted pathogen. In these species, successful vaccination induces a long-lasting protection, long after the titres of specific antibodies and the frequency of specific T cells have returned to steady state. Vaccination is also an important challenge in aquaculture, since alternative treatments are either too costly, or, in the case of antibiotics, are harmful for the environment or may result in dangerous resistances. However, the mechanisms of the long-term protection elicited by vaccines in fish remain poorly understood. Although fish possess typical B- and T-cells expressing diverse repertoires of immunoglobulins and T-cell receptors, many features of antigen specific responses are different from what is known in mouse and in human. Memory is one of the most elusive properties of fish adaptive immunity, and its basis is widely unknown. In this opinion article, we discuss the concept of immune memory in the context of the fish immunity. We illustrate the complexity of this question by discussing the results of experiments showing that protection can be passed through adoptive transfer of leukocytes from vaccinated donor fish to naive histocompatible recipients. Combined with tools developed in Targetfish and in previous projects, such as monoclonal antibodies against B- and T-cell markers, we propose that such models of protection transfer provide excellent systems to dissect the mechanisms of B- and T-cell memory in the future.

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.

Electrotransfer of single-chain LH gene into skeletal muscle induces early ovarian development of orange-spotted grouper (Epinephelus coioides)

Luteinizing hormone (LH) plays important roles in regulating steroidogenesis and reproductive development of vertebrates. In the present paper, we study function of LH on early ovarian development of orange-spotted grouper by electrotransfer of single-chain LH gene into skeletal muscle for the first time. Short-term and long-term injection experiments were performed in this work, respectively. For short-term injection experiments, fish received one electrotransfer with the plasmid in skeletal muscle, then blood and muscle around the injected area were sampled 1, 3, 5 and 7 days after the injection, mRNA expression levels of LH gene relative to 18S were determined by quantitative real-time PCR (RT-PCR) assays and serum 17β-estradiol (E2) levels were quantified by ELISA method. The results showed that levels of mRNA of LH gene in muscle and serum E2 level increased from 1 day to 7 days after the injection. For long-term injection experiments, fish received electrotransfer with the plasmid 4 times at weekly intervals in skeletal muscle. 48 h after the last injection, blood, gonad and hypothalamus samples were collected. Transcripts of cyp19a1a, cyp19a1b and gnrh1 genes and levels of serum E2 were separately analyzed by RT-PCR assays and ELISA method, and ovarian tissues were made of paraffin sections and stained by hematoxylin-eosin by method and observed by optical microscopy. The results suggested that long-term injection of LH gene into muscle upregulated transcripts of cyp19a1a and cyp19a1b and downregulated that of gnrh1, and stimulated E2 production and early-stage oogenesis. Moreover, statistical data showed that 9 of 10 ovaries of injected fish with LH gene began to develop after the long-term experiments. These data suggest that single-chain LH gene introduced into skeletal muscle via electrotransfer can be expressed and induce the early ovarian development of juvenile orange-spotted grouper. This work contributes to solve reproductive dysfunctions associated with low hormone levels of teleosts, further it may represent the demonstration at regulation of LH on early ovarian development of orange-spotted grouper to a certain extent.

IFN-adjuvanted DNA vaccine against infectious salmon anemia virus: Antibody kinetics and longevity of IFN expression

Plasmids expressing interferon (IFN) have recently been shown to function as adjuvants in Atlantic salmon when co-injected with a DNA vaccine encoding hemagglutinin-esterase (HE) from infectious salmon anemia virus (ISAV). In this work we have compared the antibody kinetics and the systemic Mx/ISG15 response of fish vaccinated with HE-plasmid using either IFNa plasmid (pIFNa) or pIFNc as adjuvants over a longer time period, i.e. 22 weeks post vaccination (pv). The results showed that the antibody response against ISAV with pIFNa as adjuvant arose earlier (7 weeks pv) than with pIFNc as adjuvant (10 weeks pv), peaked at week 10 and declined at week 22. The antibody response with pIFNc as adjuvant peaked at 16 weeks and kept at this level 22 weeks pv. Fish injected with pIFNc alone expressed high levels of Mx and ISG15 in liver throughout the 22 week period. In contrast, fish injected with pIFNc together with HE-plasmid expressed high levels of Mx and ISG15 in liver for the first 10 weeks, but at week 16 this response was absent in two of three fish at week 16 and was absent in all tested fish at week 22 pv. This suggests that cells expressing HE and IFNc are intact at week 10 pv, but are eliminated by adaptive immune responses after week 10 due to recognition of HE. The longevity of the Mx/ISG15 response in pIFNc treated fish is likely due to the fact that IFNc is a self-antigen of salmon and is not attacked by the adaptive immune system.

A plant 35S CaMV promoter induces long-term expression of luciferase in Atlantic salmon

The long-term persistence and activity of a naked plasmid DNA (pGL3-35S) containing a luc gene (reporter gene) controlled by a plant 35S CaMV promoter was studied in Atlantic salmon (Salmo salar L.) after injection. Atlantic salmon (mean weight 70 grams) were injected intramuscularly with 100 μg of plasmid DNA. Blood, different tissues and organs were sampled at different time points up to day 535 after injection. Southern blot analysis suggested the presence of extra-chromosomally open circular, linear and supercoiled topoforms of pGL3-35S at day 150 after injection. At day 536 open circular and supercoiled topoforms were detected. Luciferase activity was detected at the injection site up to 536 days post-injection of pGL3-35S, where it peaked at day 150 and decreased to approximately 17% of its maximum activity by day 536. Our study demonstrated that a plasmid containing the 35S promoter was able to induce expression of a reporter gene/protein in fish in vivo and that the plasmid DNA persisted for a prolonged time after intramuscular injection.

Design and Construction of Shrimp Antiviral DNA Vaccines Expressing Long and Short Hairpins for Protection by RNA Interference

DNA vaccines present the aquaculture industry with an effective and economically viable method of controlling viral pathogens that drastically affect productivity. Since specific immune response is rudimentary in invertebrates, the presence of RNA interference (RNAi) pathway in shrimps provides a promising new approach to vaccination. Plasmid DNA vaccines that express short or long double stranded RNA in vivo have shown protection against viral diseases. The design, construction and considerations for preparing such vaccines are discussed.

Strategies and hurdles using DNA vaccines to fish

DNA vaccinations against fish viral diseases as IHNV at commercial level in Canada against VHSV at experimental level are both success stories. DNA vaccination strategies against many other viral diseases have, however, not yet yielded sufficient results in terms of protection. There is an obvious need to combat many other viral diseases within aquaculture where inactivated vaccines fail. There are many explanations to why DNA vaccine strategies against other viral diseases fail to induce protective immune responses in fish. These obstacles include: 1) too low immunogenicity of the transgene, 2) too low expression of the transgene that is supposed to induce protection, 3) suboptimal immune responses, and 4) too high degradation rate of the delivered plasmid DNA. There are also uncertainties with regard distribution and degradation of DNA vaccines that may have implications for safety and regulatory requirements that need to be clarified. By combining plasmid DNA with different kind of adjuvants one can increase the immunogenicity of the transgene antigen – and perhaps increase the vaccine efficacy. By using molecular adjuvants with or without in combination with targeting assemblies one may expect different responses compared with naked DNA. This includes targeting of DNA vaccines to antigen presenting cells as a central factor in improving their potencies and efficacies by means of encapsulating the DNA vaccine in certain carriers systems that may increase transgene and MHC expression. This review will focus on DNA vaccine delivery, by the use of biodegradable PLGA particles as vehicles for plasmid DNA mainly in fish.

Strategies and hurdles using DNA vaccines to fish

DNA vaccinations against fish viral diseases as IHNV at commercial level in Canada against VHSV at experimental level are both success stories. DNA vaccination strategies against many other viral diseases have, however, not yet yielded sufficient results in terms of protection. There is an obvious need to combat many other viral diseases within aquaculture where inactivated vaccines fail. There are many explanations to why DNA vaccine strategies against other viral diseases fail to induce protective immune responses in fish. These obstacles include: 1) too low immunogenicity of the transgene, 2) too low expression of the transgene that is supposed to induce protection, 3) suboptimal immune responses, and 4) too high degradation rate of the delivered plasmid DNA. There are also uncertainties with regard distribution and degradation of DNA vaccines that may have implications for safety and regulatory requirements that need to be clarified. By combining plasmid DNA with different kind of adjuvants one can increase the immunogenicity of the transgene antigen - and perhaps increase the vaccine efficacy. By using molecular adjuvants with or without in combination with targeting assemblies one may expect different responses compared with naked DNA. This includes targeting of DNA vaccines to antigen presenting cells as a central factor in improving their potencies and efficacies by means of encapsulating the DNA vaccine in certain carriers systems that may increase transgene and MHC expression. This review will focus on DNA vaccine delivery, by the use of biodegradable PLGA particles as vehicles for plasmid DNA mainly in fish.

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.

Electrotransfer of the epinecidin-1 gene into skeletal muscle enhances the antibacterial and immunomodulatory functions of a marine fish, grouper (Epinephelus coioides)

Electrotransfer of plasmid DNA into skeletal muscle is a common non-viral delivery system for the study of gene function and for gene therapy. However, the effects of epinecidin-1 (epi) on bacterial growth and immune system modulation following its electrotransfer into the muscle of grouper (Epinephelus coioides), a marine fish species, have not been addressed. In this study, pCMV-gfp-epi plasmid was electroporated into grouper muscle, and its effect on subsequent infection with Vibrio vulnificus was examined. Over-expression of epi efficiently reduced bacterial numbers at 24 and 48 h after infection, and augmented the expression of immune-related genes in muscle and liver, inducing a moderate innate immune response associated with pro-inflammatory infiltration. Furthermore, electroporation of pCMV-gfp-epi plasmid without V. vulnificus infection induced moderate expression of certain immune-related genes, particularly innate immune genes. These data suggest that electroporation-mediated gene transfer of epi into the muscle of grouper may hold potential as an antimicrobial therapy for pathogen infection in marine fish.

Development of a suicidal DNA vaccine for infectious hematopoietic necrosis virus (IHNV)

We developed a suicidal DNA vaccine (pIRF1A-G-pMT-M) for salmonid fish susceptible to Infectious Hematopoietic Necrosis Virus (IHNV). The suicidal vaccine consists of two operons: i) an inducible fish promoter, the interferon regulatory factor 1A promoter (pIRF1A), driving the expression of the IHNV viral glycoprotein (G) gene that induces protection, and ii) a ZnCl(2) inducible fish promoter, the metallothionein promoter (pMT), driving the expression of the IHNV matrix (M) protein that induces apoptosis. The vaccine induces an immune response to the G protein and then induces the cell to undergo apoptosis to eliminate the DNA vaccine-containing cell. Also developed is another suicidal construct (pCMV-luc-pMT-M) for monitoring the persistence of luciferase (luc) expression after induction of apoptosis. In this study, we evaluated the inducibility of the MT promoter with ZnCl(2) and the capacity of cells transfected with the suicidal vector pCMV-luc-pMT-M to undergo apoptosis after ZnCl(2) addition. We also demonstrated the protective immunity elicited by the suicidal DNA vaccine pIRF1A-G-pMT-M, the survival of fish after treatment with ZnCl(2), and the elimination of the suicidal vector in fish after ZnCl(2) treatment.

What happens to the DNA vaccine in fish? A review of current knowledge

The primary function of DNA vaccines, a bacterial plasmid DNA containing a construct for a given protective antigen, is to establish specific and long-lasting protective immunity against diseases where conventional vaccines fail to induce protection. It is acknowledged that less effort has been made to study the fate, in terms of cellular uptake, persistence and degradation, of DNA vaccines after in vivo administration. However, during the last year some papers have given new insights into the fate of DNA vaccines in fish. By comparing the newly acquired information in fish with similar knowledge from studies in mammals, similarities with regard to transport, blood clearance, cellular uptake and degradation of DNA vaccines have been found. But the amount of DNA vaccine redistributed from the administration site after intramuscular administration seems to differ between fish and mammals. This review presents up-to-date and in-depth knowledge concerning the fate of DNA vaccines with emphasis on tissue distribution, cellular uptake and uptake mechanism(s) before finally describing the intracellular hurdles that DNA vaccines need to overcome in order to produce their gene product.

Specific uptake of plasmid DNA without reporter gene expression in Atlantic salmon (Salmo salar L.) kidney after intramuscular administration

In this study we investigated tissue distribution of pDNA after intramuscular and intravenous administration, cellular localisation, receptor-specific uptake, integrity of pDNA and transgene expression in Atlantic salmon (Salmo salar L). Anatomical distribution of plasmid DNA was determined using both radiotracing and fluorescence microscopy. Cellular uptake was studied in cultures of adherent anterior kidney leucocytes. The integrity of the pDNA in vivo was investigated by Southern blot analysis. Transcription of plasmid DNA encoded luciferase gene and protein synthesis were investigated in salmon tissues by means of real-time reverse transcription-polymerase chain reaction and enzyme activity measurements, respectively. Approximately 50% of the total recovered radioactivity was redistributed from the carcass 168h after intramuscular administration and accumulated mainly in the kidneys (37% of total). The majority of radiolabelled plasmid DNA administered intravenously was taken up within the first 15min mainly by the kidney. Intravenous co-administration of trace amounts of radiolabelled plasmid DNA with excess amounts of unlabelled plasmid DNA or formaldehyde treated albumin (a ligand for the scavenger receptors) significantly inhibited accumulation of the radiotracer in the kidney. Fluorescence microscopy demonstrated that fluorescence was localised intracellularly in cells lining the sinusoids of the kidney after intravenous administration of rhodamine-labelled plasmid DNA. Southern blot analysis demonstrated presence of supercoiled plasmid DNA in all organs and tissue samples 168h after intramuscular administration, but degradation products were only revealed at the administration site. Luciferase transcript and activity were only detectable at the administration site 24-168h after intramuscular administration of plasmid DNA. After incubation with trace amounts of radiolabelled plasmid DNA, only minor amounts of radiolabelled plasmid DNA were cell associated in cultures of adherent anterior kidney leucocytes. These results suggested that a substantial portion of radiolabelled plasmid DNA was redistributed from the carcass and was mainly cleared by a receptor-specific uptake in the kidney. Although intact plasmid DNA was detected in the kidney and other tissues, no luciferase transcripts or activity were detected in these samples at any time points investigated (24-168h), except for the administration site following intramuscular administration.