Flow cytometry detection of infectious pancreatic necrosis virus (IPNV) within subpopulations of Atlantic salmon (Salmo salar L.) leucocytes after vaccination and during the time course of experimental infection

Review of Medicinal Plants and Active Pharmaceutical Ingredients against Aquatic Pathogenic Viruses

Aquaculture offers a promising source of economic and healthy protein for human consumption, which can improve wellbeing. Viral diseases are the most serious type of diseases affecting aquatic animals and a major obstacle to the development of the aquaculture industry. In the background of antibiotic-free farming, the development and application of antibiotic alternatives has become one of the most important issues in aquaculture. In recent years, many medicinal plants and their active pharmaceutical ingredients have been found to be effective in the treatment and prevention of viral diseases in aquatic animals. Compared with chemical drugs and antibiotics, medicinal plants have fewer side-effects, produce little drug resistance, and exhibit low toxicity to the water environment. Most medicinal plants can effectively improve the growth performance of aquatic animals; thus, they are becoming increasingly valued and widely used in aquaculture. The present review summarizes the promising antiviral activities of medicinal plants and their active pharmaceutical ingredients against aquatic viruses. Furthermore, it also explains their possible mechanisms of action and possible implications in the prevention or treatment of viral diseases in aquaculture. This article could lay the foundation for the future development of harmless drugs for the prevention and control of viral disease outbreaks in aquaculture.

Advances in salmonid fish immunology: A review of methods and techniques for lymphoid tissue and peripheral blood leucocyte isolation and application

Evaluating studies over the past almost 40 years, this review outlines the current knowledge and research gaps in the use of isolated leucocytes in salmonid immunology understanding. This contribution focuses on the techniques used to isolate salmonid immune cells and popular immunological assays. The paper also analyses the use of leucocytes to demonstrate immunomodulation following dietary manipulation, exposure to physical and chemical stressors, effects of pathogens and parasites, vaccine design and application strategies assessment. We also present findings on development of fish immune cell lines and their potential uses in aquaculture immunology. The review recovered 114 studies, where discontinuous density gradient centrifugation (DDGC) with Percoll density gradient was the most popular leucocyte isolation method. Fish head kidney (HK) and peripheral blood (PB) were the main sources of leucocytes, from rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). Phagocytosis and respiratory burst were the most popular immunological assays. Studies used isolated leucocytes to demonstrate that dietary manipulations enhance fish immunity, while chemical and physical stressors suppress immunity. In addition, parasites, and microbial pathogens depress fish innate immunity and induce pro-inflammatory cytokine gene transcripts production, while vaccines enhance immunity. This review found 10 developed salmonid cell lines, mainly from S. salar and O. mykiss HK tissue, which require fish euthanisation to isolate. In the face of high costs involved with density gradient reagents, the application of hypotonic lysis in conjunction with mico-volume blood methods can potentially reduce research costs, time, and using nonlethal and ethically flexible approaches. Since the targeted literature review for this study retrieved no metabolomics study of leucocytes, indicates that this approach, together with traditional technics and novel flow cytometry could help open new opportunities for in vitro studies in aquaculture immunology and vaccinology.

Enhanced Detection of Infectious Pancreatic Necrosis Virus via Lateral Flow Chip and Fluorometric Biosensors Based on Self-Assembled Protein Nanoprobes

Salmon fish farmers face remarkable problems in fish rearing and handling due to the spread of disease by infectious pancreatic necrosis virus (IPNV). Therefore, we developed a straightforward and sensitive technique to detect IPNV-based on recombinant human apoferritin heavy chain (hAFN-H) protein nanoparticles. In this study, the 24 subunits of the hAFN-H were genetically modified to express 6×His-tag and protein-G at their C-terminal site using Escherichia coli. We thus achieved a two-step signal amplifying strategy that utilizes a recombinant hAFN-H nanoprobe having a protein-G-binding site that targets the F_c region of monoclonal antibodies and a 6×His-tag that actively interacts with the functionalized Ni-NTA derivatives. In this study, we report a considerable advancement in magnetic bead-based detection systems that use Ni-NTA-Atto 550, reliably exhibiting detection limits of 1.02 TCID₅₀/mL (50% tissue culture infective dose). Additionally, we propose a lateral flow chip-based detection method that uses the hAFN-H surface functionalized with 5 nm of the Ni-NTA-nanogold complex as a nanoprobe; the limit of detection towards IPNV was 0.88 TCID₅₀/mL. The detection of IPNV by this recombinant hAFN-H nanoprobe was linear to virus titers in the range of 10¹-10³ TCID₅₀/mL.

Robust Bioengineered Apoferritin Nanoprobes for Ultrasensitive Detection of Infectious Pancreatic Necrosis Virus

Infectious pancreatic necrosis virus (IPNV) has been identified as a viral pathogen for many fish diseases that have become a huge hurdle for the growing fishing industry. Thus, in this work, we report a label-free impedance biosensor to quantify IPNV in real fish samples at point-of-care (POC) level. High specificity IPNV sensor with a detection limit of 2.69 TCID₅₀/mL was achieved by conjugating IPNV antibodies to portable Au disk electrode chips using human heavy chain apoferritin (H-AFN) nanoprobes as a binding agent. H-AFN probes were bioengineered through PCR by incorporating pET-28b(+) resulting in 24 subunits of 6 × his-tag and protein-G units on its outer surface to increase the sensitivity of the IPNV detection. The biosensor surface modifications were characterized by differential pulse voltammetry (DPV) and EIS methods for each modification step. The proposed nanoprobe based sensor showed three-fold enhancement in charge transfer resistance toward IPNV detection in comparison with the traditional linker approach when measured in a group of similar virus molecules. The portable sensor exhibited a linear range of 100-10000 TCID₅₀/mL and sensitivity of 5.40 × 10^-4 TCID₅₀/mL in real-fish samples. The performance of the proposed IPNV sensor was fully validated using an enzyme-linked immunosorbent assay (ELISA) technique with a sensitivity of 3.02 × 10^-4 TCID₅₀/mL. Results from H-AFN nanoprobe based IPNV sensor indicated high selectivity, sensitivity, and stability could be a promising platform for the detection of similar fish viruses and other biological molecules of interest.

Quantitative Flow Cytometry to Measure Viral Production Using Infectious Pancreatic Necrosis Virus as a Model: A Preliminary Study

In recent decades, flow cytometry (FCM) has become an important tool in virology, due to its applications in viral replication and viral-cell interactions, as well as its capacity to quantify proteins (qFCM). In the present study, we have designed and evaluated a qFCM procedure for the in vitro analysis and quantification of fish viral proteins, using the infectious pancreatic necrosis virus (IPNV) as a model. We have also tested its use for viral titration and adapted the MARIS (method for analysing RNA following intracellular sorting) method for simultaneous quantification of viral RNA expression in infected cells. The procedure has proved to be repeatable and reproducible to an acceptable level, although to ensure reproducibility, the repetition of standard curves is inevitable. Regarding its use for viral quantification, a direct relationship (by a second-degree polynomial regression) between viral titres and Molecules of Equivalent Soluble Fluorochrome (MESF) was observed. Finally, the results support the use of this technology, not only for virus quantification, but also to study viral replication from a quantitative approach.

Recombinant infectious hematopoietic necrosis virus expressing infectious pancreatic necrosis virus VP2 protein induces immunity against both pathogens

Infectious hematopoietic necrosis virus (IHNV) and infectious pancreatic necrosis virus (IPNV) are typical pathogens of rainbow trout. Their co-infection is also common, which causes great economic loss in juvenile salmon species. Although vaccines against IHNV and IPNV have been commercialized in many countries, the prevalence of IHNV and IPNV is still widespread in modern aquaculture. In the present study, two IHNV recombinant viruses displaying IPNV VP2 protein (rIHNV-IPNV VP2 and rIHNV-IPNV VP2COE) were generated using the RNA polymerase Ⅱ system to explore the immunogenicity of IHNV and IPNV. The recombinant IHNV viruses were stable, which was confirmed by sequencing, indirect immunofluorescence assay, western blotting, transmission electron microscopy and viral growth curve assay. IHNV and IPNV challenge showed that the recombinant viruses had high protection rates against IHNV and IPNV with approximately 65% relative percent survival rates. Rainbow trout (mean weight 20 g) vaccinated with these two recombinant viruses showed a high level of antibodies against IHNV and IPNV infection. Taken together, our findings demonstrate that rIHNV-IPNV VP2 and rIHNV-IPNV VP2COE might be promising vaccine candidates against IHNV and IPNV.

Molecular cloning of MDA5, phylogenetic analysis of RIG-I-like receptors (RLRs) and differential gene expression of RLRs, interferons and proinflammatory cytokines after in vitro challenge with IPNV, ISAV and SAV in the salmonid cell line TO

The RIG-I receptors RIG-I, MDA5 and LGP2 are involved in viral recognition, and they have different ligand specificity and recognize different viruses. Activation of RIG-I-like receptors (RLRs) leads to production of cytokines essential for antiviral immunity. In fish, most research has focused on interferons, and less is known about the production of proinflammatory cytokines during viral infections. In this study, we have cloned the full-length MDA5 sequence in Atlantic salmon, and compared it with RIG-I and LGP2. Further, the salmonid cell line TO was infected with three fish pathogenic viruses, infectious pancreatic necrosis virus (IPNV), infectious salmon anaemia virus (ISAV) and salmonid alphavirus (SAV), and differential gene expression (DEG) analyses of RLRs, interferons (IFNa-d) and proinflammatory cytokines (TNF-α1, TNF-α2, IL-1β, IL-6, IL-12 p40s) were performed. The DEG analyses showed that the responses of proinflammatory cytokines in TO cells infected with IPNV and ISAV were profoundly different from SAV-infected cells. In the two aforementioned, TNF-α1 and TNF-α2 were highly upregulated, while in SAV-infected cells these cytokines were downregulated. Knowledge of virus recognition by the host and the immune responses during infection may help elucidate why and how some viruses can escape the immune system. Such knowledge is useful for the development of immune prophylactic measures.

Immune responses to oral pcDNA-VP2 vaccine in relation to infectious pancreatic necrosis virus carrier state in rainbow trout Oncorhynchus mykiss

The VP2 gene of infectious pancreatic necrosis virus, encoded in an expression plasmid and encapsulated in alginate microspheres, was used for oral DNA vaccination of fish to better understand the carrier state and the action of the vaccine. The efficacy of the vaccine was evaluated by measuring the prevention of virus persistence in the vaccinated fish that survived after waterborne virus challenge. A real-time RT-qPCR analysis revealed lower levels of IPNV-VP4 transcripts in rainbow trout survivors among vaccinated and challenged fish compared with the control virus group at 45 days post-infection. The infective virus was recovered from asymptomatic virus control fish, but not from the vaccinated survivor fish, suggesting an active role of the vaccine in the control of IPNV infection. Moreover, the levels of IPNV and immune-related gene expression were quantified in fish showing clinical infection as well as in asymptomatic rainbow trout survivors. The vaccine mimicked the action of the virus, although stronger expression of immune-related genes, except for IFN-1 and IL12, was detected in survivors from the virus control (carrier) group than in those from the vaccinated group. The transcriptional levels of the examined genes also showed significant differences in the virus control fish at 10 and 45 days post-challenge.

Antibiotic uptake by cultured Atlantic cod leucocytes and effect on intracellular Francisella noatunensis subsp. noatunensis replication

The granuloma disease caused by Francisella noatunensis subsp. noatunensis in farmed Atlantic cod has not been successfully treated by use of antibacterials, even when antibacterial resistance testing indicates a sufficient effect. The reason for this treatment failure may be the intracellular existence of the bacteria within immune cells, mainly macrophages. To investigate the effect of antibacterials on intracellular Francisella replication, we established a protocol for the detection of drugs within Atlantic cod immune cells using high-performance liquid chromatography (HPLC). When the uptake and intracellular concentrations of oxolinic acid and flumequine were analysed in isolated adherent head kidney leucocytes (HKLs) by HPLC, we found that uptake was rapid and the intracellular concentrations reflected the extracellular exposure concentrations. To investigate the effect of the antibacterial compounds on intracellular bacterial replication, adherent HKLs experimentally infected with the bacteria were analysed using flow cytometry and intracellular labelling of bacteria by specific antibodies. We found that flumequine did not inhibit intracellular bacterial replication. Unexpectedly, the results indicated that the intracellularly effiacy of the drug was reduced. The HPLC method used proved to be highly applicable for accurate determination of intracellular drug concentrations. When combined with sensitive and specific flow cytometry analyses for identification and measurement of intracellular bacterial replication, we suggest that this approach can be very valuable for the design of antibacterial treatments of intracellular pathogens.

Francisella noatunensis subsp. noatunensis replicates within Atlantic cod (Gadus morhua L.) leucocytes and inhibits respiratory burst activity

Francisella noatunensis subsp. noatunensis, causing granulomatosis in cod, has been shown to reside within cod immune cells, mainly within monocytes and macrophages. In the present study, we analysed the ability of the bacterium to replicate within adherent cells isolated from head kidney by in vitro infection of leucocytes. Two different technical approaches for flow cytometry analyses were performed for detection of intracellular bacteria. The presence of the wild type was assessed after identification by intracellular binding of specific antibodies to the pathogen. The other way was to use green fluorescent protein (GFP) transformed bacterium for infection studies allowing direct measurements of fluorescence from infected cells. By both methods we found an increase in fluorescence in infected cells, verifying bacterial replication, both after 4 and 28 h post infection in leucocytes isolated from head kidney (HKL). The GFP transformed bacterium was similar to the wild type in growth and infectivity pattern, showing that it can be a valuable tool for further studies of infection routes and pathology. Further, F. noatunensis subsp. noatunensis was found to inhibit respiratory burst activity, a potent pathogen killing mechanism, in cod leucocytes, but not in such cells from salmon. Our findings may indicate that inhibition of respiratory burst during Francisella infection is a key to its intracellular existence. This strategy seems to be conserved through evolution as it is also observed during infections in higher vertebrates caused by bacteria within the Francisella genus. The results presented here, showing the intracellular existence of Francisella, its replication within leucocytes and the inhibitory effect on respiratory burst, strongly support that these factors contribute to disease and pathology in infected cod. The intracellular replication shown in the present study might contribute to explain the problems of obtaining protective vaccines against Francisella and effective antibiotic treatment of infected fish.