Aeromonas hydrophila OmpW PLGA Nanoparticle Oral Vaccine Shows a Dose-Dependent Protective Immunity in Rohu (Labeo rohita)

Polylactic-Co-glycolic Acid Polymer-Based Nano-Encapsulation Using Recombinant Maltoporin of Aeromonas hydrophila as Potential Vaccine Candidate

Aquaculture production has been incurring economic losses due to infectious diseases by opportunistic pathogens like Aeromonas hydrophila, a bacterial agent that commonly affects warm water aquacultured fish. Developing an effective vaccine with an appropriate delivery system can elicit an immune response that would be a useful disease management strategy through prevention. The most practical method of administration would be the oral delivery of vaccine developed through nano-biotechnology. In this study, the gene encoding an outer membrane protein, maltoporin, of A. hydrophila, was identified, sequenced, and studied using bioinformatics tools to examine its potential as a vaccine candidate. Using a double emulsion method, the molecule was cloned, over-expressed, and encapsulated in a biodegradable polymer polylactic-co-glycolic acid (PLGA). The immunogenicity of maltoporin was identified through in silico analysis and thus taken up for nanovaccine preparation. The encapsulation efficiency of maltoporin was 63%, with an in vitro release of 55% protein in 48 h. The particle size and morphology of the encapsulated protein exhibited properties that could induce stability and function as an effective carrier system to deliver the antigen to the site and trigger immune response. Results show promise that the PLGA-mediated delivery system could be a potential carrier in developing a fish vaccine via oral administration. They provide insight for developing nanovaccine, since sustained in vitro release and biocompatibility were observed. There is further scope to study the immune response and examine the protective immunity induced by the nanoparticle-encapsulated maltoporin by oral delivery to fish.

Systemic and mucosal immune responses in red tilapia (Oreochromis sp.) following immersion vaccination with a chitosan polymer-based nanovaccine against Aeromonas veronii

A mucoadhesive chitosan polymer-based nanoplatform has been increasingly recognized as an effective mucosal vaccine delivery system for fish. The present study aimed to investigate the effectiveness of immersion vaccination with a chitosan polymer-based nanovaccine to elicit an immune response in serum and mucus of red tilapia and evaluate its protective efficacy after immersion challenge with a heterogenous strain of Aeromonas veronii UDRT09. Six hundred red tilapia (22 ± 1.8 g) were randomly allocated into four experimental groups: control, empty-polymeric nanoparticle (PC), formalin-killed vaccine (FKV), and chitosan polymer-based nanovaccine (CS-NV) in triplicate. The specific IgM antibody levels and their bactericidal activity were assessed in serum and mucus for 28 days after immersion vaccination and followed by immersion challenge with A. veronii. The immersion vaccine was found to be safe for red tilapia, with no mortalities occurring during the vaccination procedure. The specific IgM antibody levels and bactericidal activity against A. veronii in both serum and mucus were significantly higher in red tilapia vaccinated with CS-NV compared to the FKV and control groups at all time points. Furthermore, the serum lysozyme activity, ACH50, and total Ig levels demonstrated a significant elevation in the groups vaccinated with CS-NV compared to the FKV and control groups. Importantly, the Relative Percentage Survival (RPS) value of the CS-NV group (71 %) was significantly higher than that of the FKV (15.12 %) and PC (2.33 %) groups, respectively. This indicates that the chitosan polymer-based nanovaccine platform is an effective delivery system for the immersion vaccination of tilapia.

DNA vaccine incorporated poly (lactic-co-glycolic) acid (PLGA) microspheres offer enhanced protection against Aeromonas hydrophila infection

Encapsulation of DNA vaccines onto carriers enhances the immunogenicity of an antigen. Specifically, biodegradable polymers offer sustained release of vaccines which is crucial for any targeted delivery approach. Poly (lactic-co-glycolic) acid (PLGA) microspheres were used to load a DNA vaccine having a targeted gene of outer membrane protein (OMP) of Aeromonas hydrophila to clone and construct a DNA vaccine using a eukaryotic expression vector system (pVAX1-OMP DNA) and delivery in Carassius auratus against A. hydrophila infection. PLGA microspheres were prepared by emulsion technique oil-in-water and characterized by a High-Resolution Scanning Electron Microscope (HR-SEM). The results of PLGA-pVAX1-OMP DNA microspheres shows that average of 100-150 μm particle size and a loading efficiency (LE) of 68.8 %. Results indicate that C. auratus fed with PLGA-pVAX1-OMP DNA microspheres revealed a significant improvement in innate immune response, which includes, myeloperoxidase activity, respiratory burst and total immunoglobulin level compared with control group fish. The immune-related gene, IL1β, IL10, TGF, c-type, and g-type lysozyme also showed significantly higher expression after immunization. Furthermore, dietary supplementation of the PLGA-pVAX1-OMP DNA (G III) group exhibited a significantly higher survival rate (78 %) than the control group of fish. These results help us to understand the of mechanism of DNA vaccine administrated feed through PLGA nanoparticles resistance to infection by regulating systemic and innate immunity in Carassius auratus.

An Introduction to Relevant Immunology Principles with Respect to Oral Vaccines in Aquaculture

Vaccines continue to play an enormous role in the progression of aquaculture industries worldwide. Though preventable diseases cause massive economic losses, injection-based vaccine delivery is cost-prohibitive or otherwise impractical for many producers. Most oral vaccines, which are much cheaper to administer, do not provide adequate protection relative to traditional injection or even immersion formulas. Research has focused on determining why there appears to be a lack of protection afforded by oral vaccines. Here, we review the basic immunological principles associated with oral vaccination before discussing the recent progress and current status of oral vaccine research. This knowledge is critical for the development and advancement of efficacious oral vaccines for the aquaculture industry.

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.

The Screening of the Protective Antigens of Aeromonas hydrophila Using the Reverse Vaccinology Approach: Potential Candidates for Subunit Vaccine Development

The threat of bacterial septicemia caused by Aeromonas hydrophila infection to aquaculture growth can be prevented through vaccination, but differences among A. hydrophila strains may affect the effectiveness of non-conserved subunit vaccines or non-inactivated A. hydrophila vaccines, making the identification and development of conserved antigens crucial. In this study, a bioinformatics analysis of 4268 protein sequences encoded by the A. hydrophila J-1 strain whole genome was performed based on reverse vaccinology. The specific analysis included signal peptide prediction, transmembrane helical structure prediction, subcellular localization prediction, and antigenicity and adhesion evaluation, as well as interspecific and intraspecific homology comparison, thereby screening the 39 conserved proteins as candidate antigens for A. hydrophila vaccine. The 9 isolated A. hydrophila strains from diseased fish were categorized into 6 different molecular subtypes via enterobacterial repetitive intergenic consensus (ERIC)-PCR technology, and the coding regions of 39 identified candidate proteins were amplified via PCR and sequenced to verify their conservation in different subtypes of A. hydrophila and other Aeromonas species. In this way, conserved proteins were screened out according to the comparison results. Briefly, 16 proteins were highly conserved in different A. hydrophila subtypes, of which 2 proteins were highly conserved in Aeromonas species, which could be selected as candidate antigens for vaccines development, including type IV pilus secretin PilQ (AJE35401.1) and TolC family outer membrane protein (AJE35877.1). The present study screened the conserved antigens of A. hydrophila by using reverse vaccinology, which provided basic foundations for developing broad-spectrum protective vaccines of A. hydrophila.

Immunogenicity and protective efficacy of OmpA subunit vaccine against Aeromonas hydrophila infection in Megalobrama amblycephala: An effective alternative to the inactivated vaccine

Aeromonas hydrophila is a kind of zoonotic pathogen, which can cause bacterial septicemia in fish and bring huge economic losses to global aquaculture. Outer membrane proteins (Omps) are conserved antigens of Aeromonas hydrophila, which can be developed as subunit vaccines. To evaluate the protective efficacy of inactivated vaccine and recombinant outer membrane protein A (OmpA) subunit vaccine against A. hydrophila in juvenile Megalobrama amblycephala, the present study investigated the immunogenicity and protective effects of both vaccines, as well as the non-specific and specific immune response of M. amblycephala. Compared with the non-vaccinated group, both inactivated and OmpA subunit vaccines improved the survival rate of M. amblycephala upon infection. The protective effects of OmpA vaccine groups were better than that of the inactivated vaccine groups, which should be attributed to the reduced bacterial load and enhanced host immunity in the vaccinated fish. ELISA assay showed that the titer of serum immunoglobulin M (IgM) specific to A. hydrophila up-regulated significantly in the OmpA subunit vaccine groups at 14 d post infection (dpi), which should contribute to better immune protective effects. In addition, vaccination enhanced host bactericidal abilities might also attribute to the regulation of the activities of hepatic and serum antimicrobial enzymes. Moreover, the expression of immune-related genes (SAA, iNOS, IL-1 β, IL-6, IL-10, TNF α, C3, MHC I, MHC II, CD4, CD8, TCR α, IgM, IgD and IgZ) increased in all groups post infection, which was more significant in the vaccinated groups. Furthermore, the number of immunopositive cells exhibiting different epitopes (CD8, IgM, IgD and IgZ) that were detected by immunohistochemical assay had increased in the vaccinated groups post infection. These results show that vaccination effectively stimulated host immune response (especially OmpA vaccine groups). In conclusion, these results indicated that both the inactivated vaccine and OmpA subunit vaccine could protect juvenile M. amblycephala against A. hydrophila infection, of which OmpA subunit vaccine provided more effective immune protection and can be used as an ideal candidate for the A. hydrophila vaccine.

Current Progress and Challenges in the Study of Adjuvants for Oral Vaccines

Over the past 20 years, a variety of potential adjuvants have been studied to enhance the effect of oral vaccines in the intestinal mucosal immune system; however, no licensed adjuvant for clinical application in oral vaccines is available. In this review, we systematically updated the research progress of oral vaccine adjuvants over the past 2 decades, including biogenic adjuvants, non-biogenic adjuvants, and their multi-type composite adjuvant materials, and introduced their immune mechanisms of adjuvanticity, aiming at providing theoretical basis for developing feasible and effective adjuvants for oral vaccines. Based on these insights, we briefly discussed the challenges in the development of oral vaccine adjuvants and prospects for their future development.

Oral organic nanovaccines against bacterial and viral diseases

Vaccines have saved millions of humans and animals from deadly diseases. Many vaccines are still under development to fight against lethal diseases. Indeed, subunit vaccines are a versatile approach with several advantageous attributes, but they lack strong immunogenicity. Nanotechnology is an avenue to vaccine development because nanoparticles may serve as nanocarriers and adjuvants, which are critical aspects for oral vaccines. This review provides an update of oral organic nanovaccines, describing suitable nanomaterials for oral vaccine design and recent (last five-year view) oral nanovaccine developments to fight against those principal pathogens causing human and animal diseases.

Fundamentals of Fish Vaccination

Fish health management has become a critical component of disease control and is invaluable for improved harvests and sustainable aquaculture. Vaccination is generally accepted as the most effective prophylactic measure for fish disease prevention, on environmental, social, and economic grounds. Although the historical approach for developing fish vaccines was based on the principle of Louis Pasteur's "isolate, inactivate and inject," but their weak immunogenicity and low efficacies in many cases, have shifted the focus of fish vaccine development from traditional to next-generation technologies. However, before any fish vaccine can be successfully commercialized, several hurdles need to be overcome regarding the production cost, immunogenicity, effectiveness, mode of administration, environmental safety, and associated regulatory concerns. In this context, the chapter summarises the basic aspects of fish vaccination such as type of vaccine, modalities of vaccine delivery, the immunological basis of fish immunization as well as different challenges associated with the development process and future opportunities.

Protection of Teleost Fish against Infectious Diseases through Oral Administration of Vaccines: Update 2021

Immersion and intraperitoneal injection are the two most common methods used for the vaccination of fish. Because both methods require that fish are handled and thereby stressed, oral administration of vaccines as feed supplements is desirable. In addition, in terms of revaccination (boosting) of adult fish held in net pens, oral administration of vaccines is probably the only feasible method to obtain proper protection against diseases over long periods of time. Oral vaccination is considered a suitable method for mass immunization of large and stress-sensitive fish populations. Moreover, oral vaccines may preferably induce mucosal immunity, which is especially important to fish. Experimental oral vaccine formulations include both non-encapsulated and encapsulated antigens, viruses and bacteria. To develop an effective oral vaccine, the desired antigens must be protected against the harsh environments in the stomach and gut so they can remain intact when they reach the lower gut/intestine where they normally are absorbed and transported to immune cells. The most commonly used encapsulation method is the use of alginate microspheres that can effectively deliver vaccines to the intestine without degradation. Other encapsulation methods include chitosan encapsulation, poly D,L-lactide-co-glycolic acid and liposome encapsulation. Only a few commercial oral vaccines are available on the market, including those against infectious pancreatic necrosis virus (IPNV), Spring viremia carp virus (SVCV), infectious salmon anaemia virus (ISAV) and Piscirickettsia salmonis. This review highlights recent developments of oral vaccination in teleost fish.

Identification of novel vaccine candidates in the whole-cell Aeromonas hydrophila biofilm vaccine through reverse vaccinology approach

Biofilm vaccine has been recognised as one of the successful strategy to reduce the Aeromonas hydrophila infection in fish. But, the vaccine contains the protective and non-protective proteins, which may lead to show altered heterologous adaptive immunity response. Moreover, cross protection and effectiveness of previously developed biofilm vaccine was not tested against different geographical A. hydrophila isolates. Therefore, in the present study, whole-cell A. hydrophila biofilm vaccine was evaluated in rohu, vaccinated group showed increased antibody titer and protection against the different geographical A. hydrophila isolates namely KAH₁ and AAH₂ with 78.9% and 84.2% relative percentage survival, respectively. In addition, by using the immune sera of biofilm vaccinated group, a total of six protective proteins were detected using western blot assay. Further, the same proteins were identified by nano LC-MS/MS method, a total of fourteen candidate proteins showing the immunogenic property including highly expressed OMP's tolC, bamA, lamb, AH4AK4_2542, AHGSH82_029580 were identified as potential vaccine candidates. The STRING analysis revealed that, top candidate proteins identified may potentially interact with other intracellular proteins; involved in ribosomal and (tricarboxylic acid) TCA pathway. Importantly, all the selected vaccine candidate proteins contain the B-cell epitope region. Finally, the present study concludes that, whole-cell A. hydrophila biofilm vaccine able to protect the fish against the different geographical A. hydrophila isolates. Further, through reverse vaccinology approach, a total of fourteen proteins were identified as potential vaccine candidates against A. hydrophila pathogen.

Nano and Microparticles as Potential Oral Vaccine Carriers and Adjuvants Against Infectious Diseases

Mucosal surfaces are the first site of infection for most infectious diseases and oral vaccination can provide protection as the first line of defense. Unlike systemic administration, oral immunization can stimulate cellular and humoral immune responses at both systemic and mucosal levels to induce broad-spectrum and long-lasting immunity. Therefore, to design a successful vaccine, it is essential to stimulate the mucosal as well as systemic immune responses. Successful oral vaccines need to overcome the harsh gastrointestinal environment such as the extremely low pH, proteolytic enzymes, bile salts as well as low permeability and the low immunogenicity of vaccines. In recent years, several delivery systems and adjuvants have been developed for improving oral vaccine delivery and immunogenicity. Formulation of vaccines with nanoparticles and microparticles have been shown to improve antigen stability, availability and adjuvanticity as well as immunostimulatory capacity, target delivery and specific release. This review discusses how nanoparticles (NPs) and microparticles (MPs) as oral carriers with adjuvant characteristics can be beneficial in oral vaccine development.

Development of IglC and GroEL recombinant vaccines for francisellosis in Nile tilapia, Oreochromis niloticus

Warm-water piscine francisellosis is a granulomatous bacterial disease caused by Francisella orientalis (Fo). The disease has been detected in a wide range of fish species globally, causing mortalities as high as 90% and significant economic losses. Currently there are no commercially available vaccines and few treatment options exist. In the current study, two novel recombinant vaccines were prepared using diatom-expressed IglC or bacterial-expressed GroEL proteins. The vaccine antigens were emulsified with either nanoparticles or a commercially available oil-based adjuvant. Nile tilapia, Oreochromis niloticus, fingerlings were immunized intracoelomically with the recombinant IglC or GroEL vaccines, diatoms alone or phosphate buffer saline. Approximately 840-degree days post-vaccination, fish were challenged via immersion with 10⁶ CFU/mL of wild-type Fo. Twenty-one days post challenge (dpc), the highest relative percent survival was recorded in the IglC-Montanide group (75%), compared to 53%, 50%, 22%, 19% and 16% in the IglC-nanoparticles, GroEL-Montanide, GroEL-nanoparticles, diatoms-Montanide and diatoms-nanoparticles groups, respectively. Protection correlated with significantly higher specific antibody responses in the IglC-Montanide group. Moreover, a significantly lower bacterial load was detected in spleen samples from the IglC-Montanide survivor tilapia compared to the other experimental groups. This is the first report of recombinant vaccines against piscine francisellosis in tilapia. The Fo vaccines described in our study may facilitate development of a safe, cost-effective and highly protective vaccine against francisellosis in farmed tilapia.

Application of Outer Membrane Protein-Based Vaccines Against Major Bacterial Fish Pathogens in India

Aquaculture is one of the fastest-growing food-producing sectors in the world. However, its growth is hampered by various disease problems due to infectious microorganisms, including Gram-negative bacteria in finfish aquaculture. Disease control in aquaculture by use of antibiotics is not recommended as it leads to antibiotic residues in the final product, selection, and spread of antibiotic resistance in the environment. Therefore, focus is on disease prevention by vaccination. All Gram-negative bacteria possess surface-associated outer membrane proteins (OMPs), some of which have long been recognized as potential vaccine candidates. OMPs are essential for maintaining the integrity and selective permeability of the bacterial membrane and play a key role in adaptive responses of bacteria such as solute and ion uptake, iron acquisition, antimicrobial resistance, serum resistance, and bile salt resistance and some adhesins have virulence attributes. Antigenic diversity among bacterial strains even within the same bacterial species has constrained vaccine developments, but OMPs that are conserved across serotypes could be used as potential candidates in vaccine development, and several studies have demonstrated their efficacy and potential as vaccine candidates. In this review, we will look into the application of OMPs for the design of vaccines based on recombinant proteins, subunit vaccines, chimeric proteins, and DNA vaccines as new-generation vaccine candidates for major bacterial pathogens of fish for sustainable aquaculture.

Analysis of virulence and immunogenic factors in Aeromonas hydrophila: Towards the development of live vaccines

Aeromonas hydrophila, a bacterium that is widespread in aquatic environments, is responsible for causing haemorrhagic disease in both aquatic and terrestrial species. With the purpose of developing a live vaccine, herein we have investigated nine strains of A. hydrophila (Ah-01 to Ah-09) isolated from diseased fish. A study of virulence factors that contribute to pathogenicity and immunogenicity in the host Cyprinus carpio suggests that the presence of β-hly, act and fla genes contribute to pathogenesis: strains Ah-01, Ah-02 and Ah-03 (β-hly⁺ /act⁺ /fla⁺ genotype) were highly pathogenic to C. carpio, whereas Ah-05 and Ah-06 (β-hly^- /act^- /fla^- genotype) showed weak pathogenicity. Accordingly, Ah-02 and Ah-03 were selected to prepare inactivated vaccines, whereas Ah-05 and Ah-06 were chosen as live vaccines. Ah-06 live vaccine was found to have the best protective efficacy, with a protective rate of about 85%, whereas rates of other vaccines were significantly lower, in the range 37%-59%. In addition, DNA vaccines based on genes altA, aha and omp showed immune protection rates of 25%, 37.5% and 75%, respectively. Our data demonstrate that the β-hly^- /act^- /fla^- /altA⁺ /aha⁺ /omp⁺ genotype has weak pathogenicity and high immunogenicity, and provide a simple and effective way to screen for live A. hydrophila vaccines.

Vaccination of Tilapia against Motile Aeromonas Septicemia: A Review

The production of tilapia Oreochromis spp. is rapidly growing throughout the world, but atypical motile aeromonad septicemia (MAS) is a current threat to the tilapia farming industry. The etiological agent of this disease is usually Aeromonas hydrophila. Mortality rates due to MAS are frequently high, resulting in a devastating negative impact on this industry worldwide; therefore, proper control measures regarding both prevention and treatment are necessary. Although vaccines against MAS for tilapia are available, their effectiveness is entirely dependent on the specific strain of problematic bacteria. Until now, whole-cell inactivated A. hydrophila vaccines for tilapia have exhibited the highest level of protection over live attenuated and recombinant vaccines. Among the various vaccine administration systems, only intraperitoneal (i.p.) injections of the A. hydrophila vaccine into tilapia were found to provide prominent immune protection. Vaccine efficacy was primarily measured by using the i.p. injection challenge model and estimating the relative percent survival of the immunized tilapia. Freund's incomplete adjuvant showed to be the most effective for tilapia MAS vaccines. In this review, multiple factors that directly or indirectly influence the efficacy of MAS vaccines for tilapia (adjuvants, challenge models, immunization doses and duration, and size of vaccinated fish) are discussed.

Current prospects and challenges in fish vaccine development in India with special reference to Aeromonas hydrophila vaccine

Infectious diseases are adversely affecting aquaculture practices throughout world and Asian countries are no exception. Indian aquaculture practices are facing serious setback due to a variety of infectious agent's which are responsible for severe mortality and morbidity of all the cultured freshwater fish species leading to severe economic losses. The emergence of antibiotic resistant pattern, residual effect and environmental degradation due to indiscriminate use of antibiotics has necessitates the development of suitable alternate prophylaxis measures for better protection. In this regard, vaccine(s) has proved to be an effective strategy against pathogens to improve the fish production. Over the years numerous studies have been conducted to develop vaccine(s) against different pathogens. While most of the efforts are made to develop vaccine against bacterial pathogens especially against Aeromoniasis and Edwardsiellosis, few attempts have also been made against certain other bacterial, parasitic and fungal pathogens as well. Despite various successful experimental attempts, till date no vaccines against any of the pathogens are commercially available for Indian aquaculture. This review principally focuses on the current state of art in the development of vaccine against different microbial pathogens in general and Aeromonas hydrophila in particular since the bacterium is a major pathogen which is involved in a number of disease conditions in all the cultured fish species in India. Herein in this review, details of various experimental approaches made to find out a potential vaccine candidate which in turn can induce protective immune responses in host alongwith the constraints associated with it in developing a suitable vaccine against this bacterium and its market potential have been illustrated from an Indian perspective.

Comparative immunohistological study on using capsaicin, piperine, and okadaic acid for the transepithelial passage of the inactivated viral and bacterial vaccines in fish

The practical difficulty of parenteral application of fish vaccines against devastating fish diseases diverted the interest toward oral vaccination. Search for effective methods to enhance the oral uptake of viral and bacterial vaccines is continuing. The current research focus on a new role of mucosal fish vaccine adjuvants inducing the antigen uptake by enhancing vascularity or increasing intestinal permeability. Some inflammatory substances cause reversible pathology to the intestinal epithelium, which could be employed for the transepithelial passage of vaccine particles. The natural inflammatory substances used were capsaicin, piperine, and okadaic acid as 1 mg, 2 mg, and 1 μg/fish, respectively. Two inactivated vaccines were used as antigens to test the effect of these inflammatory substances in two different fish hosts. Tested vaccines were inactivated redspotted grouper nervous necrosis virus vaccine in sevenband grouper (Epinephelus septemfasciatus) and inactivated Edwardsiella tarda vaccine in red sea bream (Pagrus major) fish models. The inflammatory substances and each vaccine were anally intubated to fish. Capsaicin proved to be effectively aiding the transepithelial passage of vaccine particles more than piperine, while okadaic acid had no detectable effect.

Comparative Study on A Novel Pathogen of European Seabass. Diversity of Aeromonas veronii in the Aegean Sea

Aeromonas veronii is an emerging pathogen causing severe pathology and mortalities in European seabass aquaculture in the Aegean Sea, Mediterranean. More than 50 strains of the pathogen were characterized biochemically and genetically in order to study the epidemiology of the disease, as well as the phylogeny and virulence of the bacterium. Based on the phenotypic characteristics, the isolates form three groups consisting of: (a) the West Aegean Sea, non-motile, non-pigment-producing strains, (b) the West Aegean Sea, motile, and pigment-producing strains and (c) the East Aegean Sea motile strains that produce minute amounts of pigment. All strains were highly similar at the genomic level; however, the pattern of West/East geographic origin was reflected in biochemical properties, in general genomic level comparison and in the putative virulent factors studied. Type VI secretion system was not detected in the western strains. The outer membrane protein (OMP) profile which contains proteins that are putative antigenic factors, was very similar between strains from the different areas. Although most of the OMPs were detected in all strains with great sequence similarity, diversification according to geographic origin was evident in known antigenic factors such as the maltoporin LamB. A systematic comparative analysis of the strains is presented and discussed in view of the emergence of A. veronii as a significant pathogen for the Mediterranean aquaculture.

Current State of Modern Biotechnological-Based Aeromonas hydrophila Vaccines for Aquaculture: A Systematic Review

This systematic review describes what “the cutting edge vaccines for Aeromonas hydrophila are”. The focus is on types of high tech biotechnological based vaccines, target gene or antigen in developing these vaccines, and challenge model fish species used in vaccines efficacy testing. Vaccines delivery methods, immune response, and their efficacy, adjuvant or carrier systems used, and the overall experimental setup or design of the vaccines under investigation are also described. The search for the original papers published between 2009 and 2018 was conducted in June of 2018, using the PubMed and Google scholar electronic database. Twenty-three (23/4386) studies were included in the final assembly using PRISMA guidelines (Protocol not registered). Recombinant protein vaccines were the highly experimented type of the modern biotechnological based vaccines identified in the selected studies (16/23; 70%). Outer membrane proteins (OMPs) of different β-barrels were shown to be a potential antigenic entity for A. hydrophila vaccines (57%). Intraperitoneal route with conventional carries or adjuvants was the highly applied delivery system while very few studies used herbal based vaccine adjuvants and nanomaterial as a vaccine carrier. Variation was observed in terms of protection levels in the selected studies. The experimental designs partly contributed to the observed variation. Therefore, recombinant vaccines that use new carrier system technologies and delivered through oral route in feeds would have been of great value for use in the prevention and control of A. hydrophila infections in fish. Despite the usefulness as academic tools to identify what is important in pathogenicity of the etiological agent to the host fish, these vaccines are only economically viable in very high-value animals. Therefore, if vaccination is a good option for A. hydrophila group, then simple autogenous vaccines based on accurate typing and evidence-based definition of the epidemiological unit for their use would be the most viable approach in terms of both efficacy and economic feasibility especially in low and middle-income countries (LMIC).

Teleosts Genomics: Progress and Prospects in Disease Prevention and Control

Genome wide studies based on conventional molecular tools and upcoming omics technologies are beginning to gain functional applications in the control and prevention of diseases in teleosts fish. Herein, we provide insights into current progress and prospects in the use genomics studies for the control and prevention of fish diseases. Metagenomics has emerged to be an important tool used to identify emerging infectious diseases for the timely design of rational disease control strategies, determining microbial compositions in different aquatic environments used for fish farming and the use of host microbiota to monitor the health status of fish. Expounding the use of antimicrobial peptides (AMPs) as therapeutic agents against different pathogens as well as elucidating their role in tissue regeneration is another vital aspect of genomics studies that had taken precedent in recent years. In vaccine development, prospects made include the identification of highly immunogenic proteins for use in recombinant vaccine designs as well as identifying gene signatures that correlate with protective immunity for use as benchmarks in optimizing vaccine efficacy. Progress in quantitative trait loci (QTL) mapping is beginning to yield considerable success in identifying resistant traits against some of the highly infectious diseases that have previously ravaged the aquaculture industry. Altogether, the synopsis put forth shows that genomics studies are beginning to yield positive contribution in the prevention and control of fish diseases in aquaculture.

DNA vaccines for fish: Review and perspectives on correlates of protection

Recently in 2016, the European Medicines Agency (EMA) recommended granting a marketing authorization in the EU for "Clynav," a DNA vaccine against salmon pancreas disease (salmonid alphavirus-3). Generally, DNA vaccines induce both early and late immune responses in fish that may be protective against disease. Several transcriptomic approaches have been performed to map immunome profiles following DNA vaccination, but the precise immune mechanism(s) that is responsible for protection is not known, although reasonable suggestions have been made. The current review includes an overview on main transcriptomic findings from microarray experiments after DNA vaccination against VHSV, IHNV, HIRRV and IPNV-with considerations of what can be considered as correlates of protection (CoP) or merely a surrogate of protection. Identification and use of correlates of protection (COPs) may be a strategic tool for accelerated and targeted vaccine design, testing and licensure. General rules on what can be considered as CoPs can be extracted from past knowledge on protective immune responses following vaccination that induced protection. Lastly, there will be an overview on non-viral molecular adjuvants that have been exploited to obtain higher vaccine potencies and efficacies.

Secondary immune response of rainbow trout following repeated immersion vaccination

Teleosts are able to raise a protective immune response, comprising both innate and adaptive elements, against various pathogens. This is the basis for a widespread use of vaccines, administered as injection or immersion, in the aquaculture industry. It has been described that repeated injection vaccination of fish raises a secondary immune response, consisting of rapid, accelerated and increased antibody reaction. This study reports how rainbow trout responds to repeated immersion vaccination against yersiniosis (ERM) caused by the bacterial pathogen Yersinia ruckeri. It was found that rainbow trout does not raise a classical secondary response following repeated immersion vaccination. Serum antibody titres were merely slightly increased even after three immunizations, using 30-s immersion into a bacterin consisting of formalin-inactivated Y. ruckeri (serotype O1, biotypes 1 and 2), performed over a 3-month period. The densities of IgM-positive lymphocytes in spleen of fish immunized three times were increased compared to control fish, but no general trend for an increase with the number of immunizations was noted. The lack of a classical secondary response following repeated immersion vaccination may partly be explained by limited uptake of antigen by immersion compared to injection.

Specific and Efficient Uptake of Surfactant-Free Poly(Lactic Acid) Nanovaccine Vehicles by Mucosal Dendritic Cells in Adult Zebrafish after Bath Immersion

Activation of mucosal immunity is a key milestone for next-generation vaccine development. Biocompatible polymer-based nanoparticles (NPs) are promising vectors and adjuvants for mucosal vaccination. However, their in vivo uptake by mucosae and their biodistribution in antigen-presenting cells (APCs) need to be better understood to optimize mucosal nanovaccine designs. Here, we assessed if APCs are efficiently targeted in a spontaneous manner by surfactant-free poly(lactic acid) nanoparticles (PLA-NPs) after mucosal administration. Combining histology and flow imaging approaches, we describe and quantify the mucosal uptake of 200 nm PLA-NPs in adult zebrafish. Following bath administration, PLA-NPs penetrated and crossed epithelial barriers from all exposed mucosae. In mucosae, PLA-NPs accumulated in APCs, which were identified as dendritic cells (DCs), macrophages, and IgZ⁺ B cells in gills and skin. PLA-NP uptake by phagocytes was specific to these cell types, as PLA-NPs were not detected in neutrophils. Importantly, quantitative analyses in gills revealed that DCs take up PLA-NPs with specifically high efficiency. This study shows that surfactant-free PLA-NPs, which display optimal biocompatibility, can spontaneously target DCs with high efficiency in vivo following mucosal administration, and highlights PLA-NPs as powerful platforms for mucosal vaccine delivery in the medical and veterinary fields, and particularly in aquaculture.

An Overview of Vaccination Strategies and Antigen Delivery Systems for Streptococcus agalactiae Vaccines in Nile Tilapia (Oreochromis niloticus)

Streptococcus agalactiae is an emerging infectious disease adversely affecting Nile tilapia (Niloticus oreochromis) production in aquaculture. Research carried out in the last decade has focused on developing protective vaccines using different strategies, although no review has been carried out to evaluate the efficacy of these strategies. The purpose of this review is to provide a synopsis of vaccination strategies and antigen delivery systems currently used for S. agalactiae vaccines in tilapia. Furthermore, as shown herein, current vaccine designs include the use of replicative antigen delivery systems, such as attenuated virulent strains, heterologous vectors and DNA vaccines, while non-replicative vaccines include the inactivated whole cell (IWC) and subunit vaccines encoding different S. agalactiae immunogenic proteins. Intraperitoneal vaccination is the most widely used immunization strategy, although immersion, spray and oral vaccines have also been tried with variable success. Vaccine efficacy is mostly evaluated by use of the intraperitoneal challenge model aimed at evaluating the relative percent survival (RPS) of vaccinated fish. The major limitation with this approach is that it lacks the ability to elucidate the mechanism of vaccine protection at portals of bacterial entry in mucosal organs and prevention of pathology in target organs. Despite this, indications are that the correlates of vaccine protection can be established based on antibody responses and antigen dose, although these parameters require optimization before they can become an integral part of routine vaccine production. Nevertheless, this review shows that different approaches can be used to produce protective vaccines against S. agalactiae in tilapia although there is a need to optimize the measures of vaccine efficacy.

Edwardsiella tarda OmpA Encapsulated in Chitosan Nanoparticles Shows Superior Protection over Inactivated Whole Cell Vaccine in Orally Vaccinated Fringed-Lipped Peninsula Carp (Labeo fimbriatus)

The use of oral vaccination in finfish has lagged behind injectable vaccines for a long time as oral vaccines fall short of injection vaccines in conferring protective immunity. Biodegradable polymeric nanoparticles (NPs) have shown potential to serve as antigen delivery systems for oral vaccines. In this study the recombinant outer membrane protein A (rOmpA) of Edwardsiella tarda was encapsulated in chitosan NPs (NP-rOmpA) and used for oral vaccination of Labeo fimbriatus. The rOmpA purity was 85%, nanodiameter <500 nm, encapsulation efficiency 60.6%, zeta potential +19.05 mV, and there was an in vitro release of 49% of encapsulated antigen within 48 h post incubation in phosphate-buffered saline. Empty NPs and a non-formulated, inactivated whole cell E. tarda (IWC-ET) vaccine were used as controls. Post-vaccination antibody levels were significantly (p = 0.0458) higher in the NP-rOmpA vaccinated fish (Mean OD₄₅₀ = 2.430) than in fish vaccinated with inactivated whole cell E. tarda (IWC-ET) vaccine (Mean OD₄₅₀ = 1.735), which corresponded with post-challenge survival proportions (PCSP) of 73.3% and 48.28% for the NP-rOmpA and IWC-ET groups, respectively. Serum samples from NP-rOmpA-vaccinated fish had a higher inhibition rate for E. tarda growth on tryptic soy agar (TSA) than the IWC-ET group. There was no significant difference (p = 0.989) in PCSPs between fish vaccinated with empty NPs and the unvaccinated control fish, while serum from both groups showed no detectable antibodies against E. tarda. Overall, these data show that the NP-rOmpA vaccine produced higher antibody levels and had superior protection over the IWC-ET vaccine, showing that encapsulating OmpA in chitosan NPs confer improved protection against E. tarda mortality in L. fimbriatus. There is a need to elucidate the possible adjuvant effects of chitosan NPs and the immunological mechanisms of protective immunity induced by OMPs administered orally to fish.