Enhancement of antigen uptake and antibody production in goldfish (Carassius auratus) following bath immunization and ultrasound treatment

Nanovaccines to Combat Aeromonas hydrophila Infections in Warm-Water Aquaculture: Opportunities and Challenges

The application of nanotechnology in aquaculture for developing efficient vaccines has shown great potential in recent years. Nanovaccination, which involves encapsulating antigens of fish pathogens in various polymeric materials and nanoparticles, can afford protection to the antigens and a sustained release of the molecule. Oral administration of nanoparticles would be a convenient and cost-effective method for delivering vaccines in aquaculture while eliminating the need for stressful, labour-intensive injectables. The small size of nanoparticles allows them to overcome the degradative digestive enzymes and help deliver antigens to the target site of the fish more effectively. This targeted-delivery approach would help trigger cellular and humoral immune responses more efficiently, thereby enhancing the protective efficacy of vaccines. This is particularly relevant for combating diseases caused by pathogens like Aeromonas hydrophila, a major fish pathogen responsible for significant morbidity and mortality in the aquaculture sector. While the use of nanoparticle-based vaccines in aquaculture has shown promise, concerns exist about the potential toxicity associated with certain types of nanoparticles. Some nanoparticles have been found to exhibit varying degrees of toxicity, and their safety profiles need to be thoroughly assessed before widespread application. The introduction of nanovaccines has opened new vistas for improving aquaculture healthcare, but must be evaluated for potential toxicity before aquaculture applications. Details of nanovaccines and their mode of action, with a focus on protecting fish from infections and outbreaks caused by the ubiquitous opportunistic pathogen A. hydrophila, are reviewed here.

Carbon nanotube-based DNA vaccine against koi herpesvirus given by intramuscular injection

Koi herpesvirus (KHV) also named Cyprinid Herpesvirus 3 (CyHV-3) is one of the most threatening pathogens affecting common carp production as well as the valued ornamental koi carp. The current commercial vaccines available are costly and potentially cause severe stress caused by live virus. KHV ORF149 gene has been proved encoding one of the main immunogenic proteins for KHV. In this study, we coupled a plasmid expression vector for ORF149 to single walled carbon nanotubes (SWCNTs) for an anti-KHV vaccine. The vaccine conferred an 81.9% protection against intraperitoneal challenge with KHV. Importantly, SWCNTs as a promising vehicle can enhanced the protective effects 33.9% over that of the naked DNA vaccine at the same dose. The protection was longer and serum antibody production, enzyme activities and immune-related gene expression were all induced in fish vaccinated with the nanotube-DNA vaccine compared with the DNA alone. Thereby, this study demonstrates that the ORF149 DNA vaccine loaded onto SWCNTs as a novel vaccine might provide an effective method of coping with KHV disease using intra-muscular vaccination.

Application of antigen presenting cell-targeted nanovaccine delivery system in rhabdovirus disease prophylactics using fish as a model organism

Background

Targeted delivery of virus-associated antigens to professional antigen-presenting cells (APCs) is considered as an efficient strategy to enhance the pyrophytic effect of vaccines against rhabdovirus disease.

Materials and methods

In this study, we constructed a targeted carbon nanotubes-based vaccine deliver system (SWCNTs-MG) which can recognize the signature receptor (mannose) of APCs. An environmentally and economically important disease called spring viremia of carp (SVC) was studied as a model to evaluate the feasibility of single-walled carbon nanotubes (SWCNTs) conjugated with mannosylated antigen for rhabdovirus prevention.

Results

Results showed that SWCNTs-MG could cross into fish body and present to internal immune-related tissues through gill, muscle and intestine within 6 h immersed vaccination. With further modification of mannose moiety, the obtained nanovaccine showed enhanced uptake by carp macrophages and immune-related tissues, which would then trigger strong immune responses against spring viremia of carp virus (SVCV) infection. Moreover, the survival rate of fish vaccinated with SWCNTs-MG (30 mg/L) was 63.5% after SVCV infection, whereas it was 0% for the control group.

Conclusion

This study not only provide a theoretical basis and research template for the application of targeted nanovaccine system in aquatic animals, but also play an important role in supporting development of healthy aquaculture and ensuring the safety of aquatic products and ecology.

Current use and development of fish vaccines in China

In the past decades, the aquaculture industry made great progress in China, which contributes more than 70% yield of the world's farmed fish. Along with the rapid growth of fish production, increased emergence and outbreak of numbers of diseases pose harm to the aquaculture industry and food safety. From the efficient, safe, environmental and ethical aspects, vaccines is definitely the most appropriate and focused method to control different kinds of fish diseases. In China, researchers have done huge works on the fish vaccines, and so far six domestic aquatic vaccine products along with one imported aquatic vaccine have obtained the national veterinary medicine certificate. More critically, some new vaccines have also entered the field experiment stage and showed broad market prospects. In the present review, authors summarize seven aquatic vaccines, including the live vaccine against grass carp hemorrhagic disease, the inactivated vaccine against Aeromonas hydrophila sepsis in fish, the live vaccine against Edwardsiella tarda in turbot, the anti-idiotypic antibody vaccine against Vibrio alginolyticus, V. parahaemolyticus, and E. tarda in Japanese flounder, the cell-cultured inactivated vaccine against grass carp hemorrhagic disease, the inactivated vaccine against fish infectious spleen and kidney necrosis virus (ISKNV), and the genetically engineered live vaccine against V. anguillarum in turbot. Moreover, different delivery routes of fish vaccines are also compared in this review, along with differential fish immune response after vaccination. All these efforts will ultimately benefit the healthy and sustainable development of aquaculture industry in China.

Preliminary screening and immunogenicity analysis of antigenic epitopes of spring viremia of carp virus

Glycoprotein (G) is the most common gene used in SVCV vaccine constructions. To identify the major immunogenicity determinant region of SVCV G gene, herein we truncated G gene to 4 parts (G-1, G-2, G-3 and G-4). Bioinformatics and the enzyme linked immunosorbent assay (ELISA) were used to identify the antigenicity of these 4 truncated G proteins. Immunological assays (serum antibody production, enzyme activity, immune genes expression and challenge test) were carried out to further identify the immunogenicity of the screened G protein in common carp. Moreover, to further verify the immune response of the screened G protein-based subunit vaccine, its protective effects on common carp against SVCV infection using single-walled carbon nanotubes (SWCNTs) as a carrier were evaluated. Results showed that G-3 protein could induce higher antibody titer than other truncated G proteins. Furthermore, carps vaccinated with G-3 and G (positive control) showed significant enhancement of immune response (serum antibody production, enzyme activity and immune related genes expression) when compared with control groups. Meanwhile, as a promising vaccine carrier, SWCNTs could significantly enhance the immune effect of naked subunit vaccine (G-3 and G). Notably, after SVCV challenge, there was no significant difference in immune protection between G-3 and G, nor between SWCNTs-G-3 and SWCNTs-G. These results so far suggest G-3 might be the potential antigen epitope of SVCV. This study lays a foundation for developing vaccine and immunodiagnostic techniques.

Enhanced bath immersion vaccination through microbubble treatment in the cyprinid loach

Immunization by bath immersion is likely the simplest method of fish vaccination. Although the route of immunogenicity has not been fully identified, immersion vaccination is clearly a useful labor-saving technique. In this study, microbubble (MB) treatment was assessed for its ability to improve the efficacy of bath immersion vaccination in the cyprinid loach. MBs are commonly defined as minute particles of gas with a diameter of less than 100 μm, which generated free radicals. Here, the efficacy of MB treatment for vaccination enhancement in the cyprinid loach was assessed in direct challenge experiments using the virulent Aeromonas hydrophila JUNAH strain; assessments comprised agglutination titer assay and non-specific parameter analysis. Agglutination titers were high in loaches that were immunized via injection with inactivated cells (FKC group); however, non-specific immune activation parameters (e.g., lysozyme, superoxide dismutase, and phagocytic activity) were more increased in loaches that were immunized via bath immersion with MB treatment. Moreover, MB-treated loaches showed comparable survival rates, relative to loaches immunized via injection with formalin inactivated cells. Thus, higher levels of non-specific immune parameters suggest increased efficacy of this vaccine approach. Improving the effectiveness of bath immersion vaccine will increase its affordability and ease of application in aquaculture.

Immune response and protective effect against spring viremia of carp virus induced by intramuscular vaccination with a SWCNTs-DNA vaccine encoding matrix protein

To elicit the immune protective of vaccine against the highly contagious and pathogenic disease caused by spring viremia of carp virus (SVCV), a novel functionalized single-walled carbon nanotubes (SWCNTs) were applied as a delivery vehicle for DNA vaccine. In this study, we report a SWCNTs-DNA vaccine encoding matrix protein of SVCV which, when injected in the muscle at a dose of 10 μg SWCNTs-pcDNA-M vaccine, confers up to 51.3% protection against intraperitoneal challenge with SVCV. In addition, SWCNTs as a promising vehicle can enhance about 17.5% of the immune protective effect in SWCNTs-pcDNA-M vaccinated common carp compared with fish injected with naked pcDNA-M DNA vaccine. In addition, serum antibody production, none specific immunity parameters (complement activity, superoxide dismutase activity (SOD), acid phosphatase activity (ACP) and alkaline phosphatase activity (AKP)) and immune-related genes were used to verify the enhancement immune response induced in SWCNTs-pcDNA-M vaccinated fish, herein all these mentioned immune activities were significantly enhanced after immunization. Thereby, it is revealed that the M gene of SVCV could be used as an antigen for DNA vaccine constructs, and SWCNTs could be a candidate DNA vaccine carrier to enhance the immunological response against fish disease.

Application of low-frequency sonophoresis and reduction of antibiotics in the aquatic systems

A major concern in aquaculture is the use of chemical therapeutics, such as antibiotics, because of their impact on the environment as well as on the fish product. As a potential tool for reducing antibiotic use, we tested the application of low-frequency ultrasound as a method for enhancing antibiotic uptake. Rainbow trout juveniles (Oncorhynchus mykiss) were exposed to two different concentrations of oxytetracycline (OTC), flumequine (FLU) and florfenicol (FLO), administered by bath after the application of ultrasound. After exposure, concentrations of these substances were measured in the liver and blood of treated fish. Results showed that the ultrasound treatment can significantly increase the uptake for all three antibiotics. The uptake of OTC for example, in fish exposed to an OTC concentration of 20 mg L^-1 after prior treatment with ultrasound, was similar to the OTC concentrations in their liver and blood to fish exposed to 100 mg L^-1 without sonication. For FLU and FLO, the use of ultrasound caused significant differences of uptake in the liver at high antibiotic concentrations. This suggests that the use of ultrasound as a technique to deliver antibiotics to fish can ultimately reduce the amount of antibiotics discharged into the aquatic environment.

The influence of concentration of inactivated Edwardsiella tarda bacterin and immersion time on antigen uptake and expression of immune-related genes in Japanese flounder (Paralichthys olivaceus)

Our previous work has demonstrated that the immune response of Japanese flounder was associated with the concentration of formalin-inactivated Edwardsiella tarda and immersion time. In order to further investigate the influence of immersion vaccine dose and bath time on the antigen uptake, formalin-killed Edwardsiella tarda bacterin was prepared and adjusted to four concentrations (10⁹, 10⁸, 10⁷, 10⁶ cfu ml^-1) for 30, 60 and 90 min immersion in Japanese flounder model, respectively. Absolute quantitative real-time PCR was employed to examine the bacterin uptake in gill, skin, spleen and kidney at 3 and 6 h post vaccination. The results showed that the antigen uptaken in gills and skin were significant higher than spleen and kidney, and the antigen amounts in gill and skin both declined from 3 to 6 h, whereas the antigen amounts in spleen and kidney gradually increased. Significant higher antigen amounts were detected in 10⁹-30, 10⁹-60, 10⁸-60, 10⁸-90 and 10⁸-90 groups than other groups (P < 0.05), especially the 10⁸-60min group displayed the highest antigen uptaken. Meanwhile, the expression profiles of antigen recognization and presentation genes (MHCⅡα, TcRα, CD4-1), immunoglobulins (IgM, IgT), inflammatory cytokines (IL-1β, IL-6), heat shock protein 70 (HSP70) and c-type lysozyme were analyzed using real-time PCR. On the whole, the transcription levels of the eight genes exhibited to be higher in 10⁷-90, 10⁸ and 10⁹ cfu ml^-1 groups than other groups (P < 0.05), especially the 10⁸-60 group displayed the highest up-regulation. These results demonstrated that immersion with formalin-inactivated E. tarda, especially under 10⁸-60 min condition could efficiently enhance the antigen uptake and the expression of immune-related genes, which provided evidences for an enhanced vaccination effects under an optimized combination of vaccine dose and immersion time.

Antigen uptake and expression of antigen presentation-related immune genes in flounder (Paralichthys olivaceus) after vaccination with an inactivated Edwardsiella tarda immersion vaccine, following hyperosmotic treatment

Antigen uptake is a critical process for activation of the immune system, and therefore the ability to enhance antigen uptake is a primary consideration in the development of an immersion vaccination of fish. In the present work, flounders (Paralichthys olivaceus) were immersed in three hyperosmotic solutions with 40, 50 and 60‰ salinities, then transferred into seawater of normal salinity (i.e. 30‰) containing formalin-inactivated Edwardsiella tarda for 30 min. The antigen uptake in vaccinated flounder was determined using an absolute quantitative PCR (qPCR). The results showed significantly higher antigen uptake in the tissues of flounders immersed in solutions with 50‰ and 60‰ salinity compared to the control group directly immersed in vaccine (DI) (P < 0.05), and the highest amount of antigen was detected in flounders immersed in the 50‰ salinity solution, whereas there was no significant difference in antigen uptake between the 40‰ salinity group and the DI group (P > 0.05). A rapid and significant increase in antigen uptake was detected in the mucosal-associated tissues including the gill, skin and intestine (P < 0.05) compared with the spleen, kidney and liver. Antigen uptake in the gill and skin both peaked at 30 min post immersion, which was significantly higher than the levels of uptake measured in the other tissues (P < 0.05), and then quickly declined. In contrast, antigen uptake in the spleen, kidney and liver gradually increased 3 h post immersion (hpi). The expression profiles of four antigen presentation-related immune genes (MHC Iα, MHC IIα, CD4-1 and CD8α) were investigated after immersion. These four genes showed a significantly stronger response in the immersed flounders exposed to 50‰ salinity compared with the DI group (P < 0.05). In the mucosal-associated tissues, the expression of MHC Iα and CD8α genes peaked at 24 hpi, while the expression of MHC IIα and CD4-1 genes showed up-regulation in the gill and skin and reached the peak in these tissues at 48 hpi. The expressions of the four genes were also up-regulated in spleen, kidney and liver, but reached peak expression in these tissues at 48-72 hpi. The results demonstrated that hyperosmotic immersion, notably 50‰ salinity significantly enhanced antigen uptake and the expression of selected genes associated with antigen presentation, providing evidence for an enhanced immune activation of the fish's immune response by the hyperosmotic immersion treatment prior to vaccination.

Adjuvant Effect of Quillaja saponaria Saponin (QSS) on Protective Efficacy and IgM Generation in Turbot (Scophthalmus maximus) upon Immersion Vaccination

The adjuvant effect of Quillaja saponaria saponin (QSS) on protection of turbot fry was investigated with immersion vaccination of formalin-killed Vibrio anguillarum O1 and various concentrations of QSS (5, 25, 45 and 65 mg/L). Fish were challenged at days 7, 14 and 28 post-vaccination. Significantly high relative percent of survival (RPS) ((59.1 ± 13.6)%, (81.7 ± 8.2)%, (77.8 ± 9.6)%) were recorded in the fish that received bacterins immersion with QSS at 45 mg/L, which is comparable to the positive control group vaccinated by intraperitoneal injection (IP). Moreover, a remarkably higher serum antibody titer was also demonstrated after 28 days in the vaccinated fish with QSS (45 mg/L) than those vaccinated fish without QSS (p < 0.05), but lower than the IP immunized fish (p < 0.05). Significant upregulation of IgM gene expression has also been identified in the tissues of skin, gill, spleen and kidney from the immunized fish in comparison to the control fish. Taken together, the present study indicated that QSS was able to dramatically evoke systemic and mucosal immune responses in immunized fish. Therefore, QSS might be a promising adjuvant candidate for fish vaccination via an immersion administering route.

Immune responses of flounder Paralichthys olivaceus vaccinated by immersion of formalin-inactivated Edwardsiella tarda following hyperosmotic treatment

The aim of the present study was to evaluate the effects of hyperosmotic immersion (HI) vaccination and determine the optimum hyperosmotic salinity for flounder Paralichthys olivaceus by investigating its immune responses following vaccination. Flounder were immersed in 1 of 3 hyperosmotic solutions at 50, 60 and 70‰ salinity, then transferred into 30‰ salinity normal seawater containing formalin-inactivated Edwardsiella tarda for vaccination (3 HI groups), or were immersed in normal seawater as direct immersion (DI group). The results showed that the percentages of surface membrane immunoglobulin-positive (sIg+) cells in peripheral blood leukocytes and spleen leukocytes induced by HI were significantly higher than that with DI (p < 0.05), and the 50‰ salinity group showed the strongest response among the HI groups, which reached peaks at Week 4. ELISA assay showed that the specific serum antibodies gradually increased after vaccination and reached peak at Day 32, and the fish treated with HI showed stronger antibody responses; among the HI groups, a significantly higher specific antibody level was detected in the 50‰ salinity group at Day 32 (p < 0.05). Similarly, the fish treated with HI showed higher specific mucosal antibody levels compared to the DI group, and the mucosal antibody showed a faster response, with peak time arriving 1 wk earlier than for the serum antibody. The relative percent survival (RPS) of flounder treated with HI at 50, 60 and 70‰ salinities were 79, 71 and 57% respectively, while this was 43% in the DI group. These results demonstrated that HI, especially the 50‰ salinity, could efficiently enhance the immune response of flounder and show higher RPS. This has significant value for immunological prevention of edwardsiellosis in flounder.

The adjuvant effect of low frequency ultrasound when applied with an inactivated Aeromonas salmonicida vaccine to rainbow trout (Oncorhynchus mykiss)

Vaccine adjuvants are classified according to their properties of either inducing the persistence of antigens within the animal after immunisation and/or activation of the animal's immune response. The adjuvant effect of low intensity low frequency sonophoresis (LFS) was tested in rainbow trout using an Aeromonas salmonicida bacterin vaccine administered by immersion vaccination using LFS at 37 kHz. The adjuvant effect obtained with LFS was compared with that of normal immersion or intraperitoneal injection vaccination. Quantitative PCR was used to measure bacterial DNA in vaccinated fish up to 35 days post-vaccination, while RT-qPCR was used to assess gene expression during the early and late immune response post-vaccination. Results showed that antigen uptake in the gills was significantly higher in the group exposed to low intensity LFS compared to the other two vaccination groups 15 min post-vaccination, but this initially high uptake did not persist over the rest of the experiment. In the kidney, by comparison, the vast majority of the samples analysed did not show the presence or persistence of the bacterin. Showing that the route of vaccine uptake using the A. salmonicida bacterin, does not influence the persistence of the bacterin in the gills or the kidney. On the other hand, LFS induced a higher inflammatory response and T-helper cell activation, characterized by a significant up-regulation of interleukin-8 (IL-8), IL-1ß and CD-4, respectively. The expression of Ig-M, Ig-T and Ig-D was up-regulated in gills (being significant for Ig-M), but not in the spleen and kidney of the sonicated group. Conversely, Ig-M was up-regulated in the spleen of the non-sonicated groups, but not in the sonicated group. This highlights the ability of ultrasound to enhance mucosal immunity. It remains to be established whether the up-regulation of Ig-M in gills would be sufficient to offer protection in fish infected with A. salmonicida.

Enhanced Aeromonas salmonicida bacterin uptake and side effects caused by low frequency sonophoresis in rainbow trout (Oncorhynchus mykiss)

Low frequency sonophoresis (LFS) has been recognized as one of the most advanced technologies in transdermal delivery of substances, due to the modification of the stratum corneum lipid bilayer, in focal skin applications in mammals. Based on these findings, LFS has been suggested as a potential technology to be used for enhancement in immersion fish vaccination. In contrast to mammals where LFS is applied to discrete regions of the skin, in fish the whole individual needs to be exposed for practical purposes. The current study evaluated the impact of LFS at 37 kHz on the uptake of an Aeromonas salmonicida bacterin and side effects of the treatment in rainbow trout. Quantitative real time PCR (qPCR) and immunohistochemistry were used to examine the bacterin uptake into skin and gill tissue. Side effects were assessed by behavioural examination, histology and blood serum analysis. The sonication intensity of 171 mW/cm² was enough for increasing skin permeability, but caused heavy erratic swimming and gill haemorrhages. Sonication intensities as low as 105 mW/cm² did not modify skin permeability and enhanced the bacterin uptake into the gill tissue by factor 15 compared to conventional immersion. Following sonication, the gill permeability for the bacterin decreased after 20 min and 120 min by factor 3 and 2, respectively. However, during sonication, erratic swimming of the fish raised some concerns. Further reduction of the sonication intensity to 57 mW/cm² did not induce erratic swimming, and the bacterin uptake into the gill tissue was still increased by factor 3. In addition, a decreasing albumin-globulin ratio in the serum of the rainbow trout within 40 min revealed that LFS leads to an inflammatory response. Consequently, based on both increased bacterin uptake and the inflammatory response, low intensity LFS has the potential to enhance vaccine immunity without significant side effects.

Advances in fish vaccine delivery

Disease prevention is essential to the continued development of aquaculture around the world. Vaccination is the most effective method of combating disease and currently there are a number of vaccines commercially available for use in fish. The majority of aquatic vaccines are delivered by injection, which is by far the most effective method when compared to oral or immersion deliveries. However it is labor intensive, costly and not feasible for large numbers of fish under 20 g. Attempts to develop novel oral and immersion delivery methods have resulted in varying degrees of success but may have great potential for the future.