Persistence and distribution of intravenously injected DNA in blood and organs of Atlantic salmon (Salmo salar L.)

Evaluation of Gilthead Seabream (Sparus aurata) Immune Response after LCDV-Sa DNA Vaccination

Simple Summary

Lymphocystis disease is the main viral pathology in gilthead seabream aquaculture. Currently, there are no treatments or vaccines to control this disease, thus our main goal was to construct a DNA vaccine that can be used in the future to stop the spread of this pathology in sea farms. The vaccine consisted of a plasmid DNA that contains a known viral gene. Once it was established that the vaccine drives the expression of the antigenic viral protein in fish, vaccination experiments were conducted to determine if the vaccinated fish become protected against the viral infection. In addition, the immune response triggered by the vaccine was also evaluated in order to understand the mechanisms underlying such protection. The obtained results showed that in vaccinated fish an activation of several genes relating to both the inflammatory process and the mucosal immunity were produced, as well as specific anti-viral antibodies. Although limited, our results deserve further investigation to assess the efficacy of the vaccine in bigger fish populations and to confirm the mode of action of the vaccine.

Abstract

Lymphocystis disease is the main viral pathology reported in gilthead seabream. Its etiological agent is Lymphocystis disease virus 3 (LCDV-Sa), genus Lymphocystivirus, family Iridoviridae. There are no effective treatments or vaccines for LCDV control, thus the main aim of this study was to develop a DNA vaccine, and to evaluate both the protection conferred against LCDV-Sa infection and the immune response in vaccinated fish. The vaccine was constructed by cloning the mcp gene (ORF LCDVSa062R) into pcDNA3.1/NT-GFP-TOPO. Two independent vaccination trials were conducted. In the first one, 5–7 g fish were intramuscularly injected with the vaccine (pcDNA-MCP) or the empty-plasmid, and the distribution and expression of the vaccine was investigated. Furthermore, vaccinated fish were challenged with LCDV-Sa in order to access the protective capacity of the vaccine. In the second trial, 70–100 g fish were vaccinated as specified, and the immune response was evaluated analyzing the expression of 23 immune-related genes and the production of specific antibodies. The results showed that the vaccine triggers an immune response characterized by the overexpression of genes relating to the inflammatory process, but not the innate antiviral immunity relating to type I IFN (interferon), and also induces the production of specific neutralizing antibodies, which could explain the protection against LCDV-Sa in vaccinated fish.

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.

Zebrafish ( Danio rerio) as a model for investigating the safety of GM feed ingredients (soya and maize); performance, stress response and uptake of dietary DNA sequences

A 20-d zebrafish (Danio rerio) feeding trial, in which a near doubling of fish weight was achieved, was conducted with GM feed ingredients to evaluate feed intake, growth, stress response and uptake of dietary DNA. A partial aim of the study was to assess zebrafish as a model organism in GM safety assessments. Roundup Ready soya (RRS), YieldGard Bt maize (MON810) and their non-modified, maternal, near-isogenic lines were used in a 2 x 2 factorial design. Soya variety and maize variety were the main factors, both with two levels; non-GM and GM. Compared with fish fed non-GM maize, those fed GM maize exhibited significantly better growth, had lower mRNA transcription levels of superoxide dismutase (SOD)-1 and a tendency (non-significant) towards lower transcription of heat shock protein 70 in liver. Sex of the fish and soya variety had significant interaction effects on total RNA yield from the whole liver and transcription of SOD-1, suggesting that some diet component affecting males and females differently was present in different levels in the GM and the non-GM soya used in the present study. Dietary DNA sequences were detected in all of the organs analysed, but not all of the samples. Soya and maize rubisco (non-transgenic, multicopy genes) were most frequently detected, while MON810 transgenic DNA fragments were detected in some samples and RRS fragments were not detected. In conclusion, zebrafish shows promise as a model for this application.

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.

Specific endocytosis and degradation of naked DNA in the endocardial cells of cod (Gadus morhua L.)

DNA vaccines are administered in the form of plasmid DNA (pDNA) carrying a strong promoter and the gene of interest. In this study we investigated the tissue distribution, cellular uptake and the fate of intravenously (i.v.) and intramuscularly (i.m.) injected pDNA in Atlantic cod (Gadus morhuaL.). The anatomical distribution of pDNA was determined using both morphological and radiotracing methods. Cellular uptake and receptor specificity were studied in cultures of cod atrial endocardial endothelial cells (aEEC) and head kidney leukocytes. The short-term fate of the endocytosed pDNA in vivo and in vitro was investigated by Southern blot. Expression of the pDNA (R70pRomiLuc)-derived gene was investigated in cod tissues and cultures of cod aEEC by means of real-time RT-PCR and luciferase activity assay.

125I-labelled pDNA was rapidly eliminated from the blood by the aEEC of the cod heart atrium and ventricle. Co-injection of trace amounts of 125I-labelled pDNA with excess amounts of non-labelled pDNA or formaldehyde-treated albumin (FSA), a ligand for the cod EEC scavenger receptor, significantly inhibited the accumulation of the radiotracer in the heart. The organ to blood ratio of radioactivity after inhibition of the cod EEC scavenger receptor demonstrated that the radioactivity not taken up by the EEC remained in the blood. Fluorescence microscopy of tissue sections from cod injected with fluorescein-labelled pDNA confirmed intracellular uptake of pDNA by the endocardial cells of the atrium and ventricle. In purified cultures of cod aEEC the fluorescein-labelled pDNA was taken up in structures reminiscent of endosomal/lysosomal vesicles. Uptake of 125I-labelled pDNA in cultures of cod aEEC was specific. Incubation of cultures with 125I-labelled pDNA together with excess amounts of FSA and fucoidan, which are molecules also known to bind to the scavenger receptors,reduced the uptake of the pDNA by at least 70%. Mannan, a ligand for the mannose receptor, did not inhibit the uptake of 125I-labelled pDNA. Despite, low uptake of 125I-fluorescein-pDNA in the kidney of the cod, the uptake of pDNA in cultured cod head kidney leukocytes was significant.

Southern blot analysis of cod tissues after injection of pDNA and culture of aEEC given 10 μg pDNA per 106 cells demonstrated the presence of degradation products in tissues and in the cell cultures. Real-time RT–PCR studies showed expression of luciferase mRNA only at the injection site 168 h after injection. Neither expression of luciferase mRNA nor luciferase activity was present in cod aEEC incubated for 48 h with 10μg pDNA.

These results suggest that the EEC are very important for removal of blood borne pDNA in cod and that the uptake by these cells was mediated in a scavenger–receptor-like manner. Uptake of pDNA by head kidney leukocytes was only observed in vitro. The endocytosed DNA was subjected to intracellular degradation and was not expressed by the cod EEC. Despite the low amount of radioactivity found in the head kidney after i.v. injection of 125I-labelled pDNA, the head kidney leukocytes seem to have a high capacity for uptake of 125I-labelled pDNA in vitro.