Immunogenic and protective effects of a DNA vaccine for Mycobacterium marinum in fish

Modern biotechnological strategies for vaccine development in aquaculture - Prospects and challenges

Advances in genomics and the gradual reduction of cost for technologies like whole-genome sequencing have provided exciting opportunities for developing modern biotechnological-based vaccines in aquaculture. This systemic review describes the prospects and challenges of implementing these high-tech vaccines in fish species. The majority of the commercial vaccines in aquaculture utilize conventional procedures for which cost of administration, protective immunity and safety issues are the major challenges. In recent years, more efficient vaccines are being developed by adopting the advances in vaccine technology. Vaccines based on surface antigens, protein/peptide/polysaccharide subunits, recombinant DNA/mRNA/plasmids, novel antigen expression and delivery systems (bacteriophage particles, virus like particles/VLPs, recombinant yeast, mucosal vaccines), novel molecular adjuvants (IL-8, IL-12, HSPs), and encapsulation polymers and polysaccharides like chitosan nanoparticles and PLGA microcapsule were successfully developed. These biotechnology-based vaccines have proved to be very efficient in field trials, but are always in the research pipeline or as patents. Only very few of them are licensed for use, that too, in high-valued fishes like salmonids. Currently, commercial aquaculture vaccines are available for Aeromonas salmonicida, Vibrio salmonicida, Yersinia ruckeri, Vibrio anguillarum, Edwardsiella ictalurid, and for certain Betanodaviruses. Nevertheless, no registered vaccines are available for other major infectious diseases/pathogens such as viral hemorrhagic septicemia virus (VHSV), viral nervous necrosis virus (VNN) and certain other betanodaviruses, channel catfish virus (CCV), gill disease bacteria, mycobacteria, flavobacterium, Edwardsiella tarda, and certain streptococci. Despite the important economic losses that the pathogens cause to aquaculture worldwide, the commercialization of vaccines remains limited due to immunological pitfalls in aquatic species, large-scale vaccination issues, unregulated use of antibiotics and chemicals, gene-based vaccine regulations and commercial viability. If attempts are to be made to develop novel delivery methods, cost-effective procedures, and relaxations in DNA vaccine regulations, biotechnology-based vaccination could circumvent the emerging disease challenges in aquaculture.

Thermostable Vaccines in Veterinary Medicine: State of the Art and Opportunities to Be Seized

The COVID-19 pandemic has highlighted the weakness of the vaccine supply chain, and the lack of thermostable formulations is one of its major limitations. This study presents evidence from peer-reviewed literature on the development of thermostable vaccines for veterinary use. A systematic review and meta-analysis were performed to evaluate the immunogenicity and/or the efficacy/effectiveness of thermostable vaccines against infectious diseases. The selected studies (n = 78) assessed the vaccine’s heat stability under different temperature conditions and over different periods. Only one study assessed the exposure of the vaccine to freezing temperatures. Two field studies provided robust evidence on the immunogenicity of commercial vaccines stored at temperatures far in excess of the manufacturer’s recommended cold-chain conditions. The drying process was the most-used method to improve the vaccine’s thermostability, along with the use of different stabilizers. The pooled vaccine efficacy was estimated to be high (VE = 69%), highlighting the importance of vaccination in reducing the economic losses due to the disease impact. These findings provide evidence on the needs and benefits of developing a portfolio of heat- and freeze-stable veterinary vaccines to unleash the true potential of immunization as an essential component of improved animal health and welfare, reduce the burden of certain zoonotic events and thus contribute to economic resilience worldwide.

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.

Cell-mediated immune response against mycolic acids of Mycobacteroides salmoniphilum in rainbow trout Oncorhynchus mykiss

Mycobacteriosis caused by Mycobacterium spp. causes economic damages to the world aquaculture industry. In mammals, mycolic acids contained in the cell wall of Mycobacterium spp. are presented by CD1b molecule as lipid antigens and induce cell-mediated immunity (CMI). Here, we investigated CMI responses against the mycolic acids of Mycobacterioides salmoniphilum in a CD1-lacking teleost fish, rainbow trout. After stimulation of trout leukocytes with mycolic acids, the number and percentage of CD8α⁺ T cells increased. Fish immunized with mycolic acids showed an up-regulation of IFN-γ. Further, in vitro re-stimulation of leukocytes derived from immunized fish resulted in proliferation of CD8α⁺ cells. These data suggest that mycolic acids are recognized as lipid antigens resulting in an activation of rainbow trout CD8α⁺ cells and up-regulation of the Th1 cytokine IFN-γ. The mycolic acids are promising candidates for vaccines to activate CD8α⁺ T cells against fish mycobacteriosis.

Mycobacteriosis and Infections with Non-tuberculous Mycobacteria in Aquatic Organisms: A Review

The Mycobacteriaceae constitute a family of varied Gram-positive organisms that include a large number of pathogenic bacteria. Among these, non-tuberculous mycobacteria are endemic worldwide and have been associated with infections in a large number of organisms, including humans and other mammals and reptiles, as well as fish. In this review, we summarize the most recent findings regarding this group of pathogens in fish. There, four species are most commonly associated with disease outbreaks: Mycobacterium marinum, the most common of these fish mycobacterial pathogens, Mycobacterium fortuitum, Mycobacterium gordonae, and Mycobacterium chelonae. These bacteria have a broad host range: they are zoonotic, and infections have been reported in a large number of fish species. The main route of entry of the bacterium into the fish is through the gastrointestinal route, and the disease is associated with ulcerative dermatitis as well as organomegaly and the development of granulomatous lesions in the internal organs. Mycobacteriaceae are slow-growing and fastidious and isolation is difficult and time consuming and diagnostic is mostly performed using serological and molecular tools. Control of the disease is also difficult: there is currently no effective vaccine and infections react poorly to antibiotherapy. For this reason, more research is needed on the subject of these vexing pathogens.

Integrating fish models in tuberculosis vaccine development

Tuberculosis is a chronic infection by Mycobacterium tuberculosis that results in over 1.5 million deaths worldwide each year. Currently, there is only one vaccine against tuberculosis, the Bacillus Calmette–Guérin (BCG) vaccine. Despite widespread vaccination programmes, over 10 million new M. tuberculosis infections are diagnosed yearly, with almost half a million cases caused by antibiotic-resistant strains. Novel vaccination strategies concentrate mainly on replacing BCG or boosting its efficacy and depend on animal models that accurately recapitulate the human disease. However, efforts to produce new vaccines against an M. tuberculosis infection have encountered several challenges, including the complexity of M. tuberculosis pathogenesis and limited knowledge of the protective immune responses. The preclinical evaluation of novel tuberculosis vaccine candidates is also hampered by the lack of an appropriate animal model that could accurately predict the protective effect of vaccines in humans. Here, we review the role of zebrafish (Danio rerio) and other fish models in the development of novel vaccines against tuberculosis and discuss how these models complement the more traditional mammalian models of tuberculosis.

Summary: In this Review, we discuss how zebrafish (Danio rerio) and other fish models can complement the more traditional mammalian models in the development of novel vaccines against tuberculosis.

Availability of culture filtrate protein-10 (CFP-10) secreted from Mycobacterium pseudoshottsii for mycobacteriosis diagnosis in ginbuna crucian carp Carrasius auratus langsdorfii

Mycobacteriosis in cultured fish is a challenge for the aquaculture industry worldwide. Treatment by chemical administration is difficult and no effective vaccine has been developed. Therefore, detection and isolation by early diagnosis are important for prevention of the spread of the disease. In mammals, interferon gamma release assays have been established for detection of tuberculosis; these tests are based on the delayed-type hypersensitivity (DTH) response against culture filtrate protein-10 (CFP-10) and the 6-kDa early secreted antigen target (ESAT-6) of Mycobacterium tuberculosis. On the other hand, little is known about the fish immune response against the ESAT-6 and CFP-10 proteins of mycobacteria, although these responses should find application in the diagnosis of mycobacteriosis in fish. In the present study, we identified ESAT-6 and CFP-10 from Mycobacterium pseudoshottsii and cloned the corresponding genes. Intraperitoneal injection of the corresponding DNA plasmid constructs in ginbuna crucian carp yielded increased expression of the fish interferon-γ1-1-encoding gene (IFN-γ1-1). In contrast, IFN-γ1-1 expression accompanied by DTH response was observed only in the CFP-10-DNA plasmid-injected fish. Furthermore, fish that had been prophylactically injected with CFP-10-DNA plasmid exhibited increased survival of M. pseudoshottsii infection. Taken together, these results suggested that CFP-10 may facilitate diagnosis of mycobacteriosis.

T and B lymphocytes immune responses in flounder (Paralichthys olivaceus) induced by two forms of outer membrane protein K from Vibrio anguillarum: Subunit vaccine and DNA vaccine

To further elucidate the roles of T and B lymphocytes in fish subunit and DNA candidate vaccines for immunisation, the immune responses of T and B lymphocytes to recombinant protein (rOmpK) and plasmid OmpK (pOmpK) from Vibrio anguillarum plus cyclosporine A (CsA) were investigated in flounder (Paralichthys olivaceus). The results showed that in the rOmpK-immunised groups, the percentages of CD4-1⁺ and CD4-2⁺ T (PCD4-1⁺ and PCD4-2⁺ T) lymphocytes significantly increased to a peak on days 5 or 7. The percentages of IgM⁺ B (PIgM⁺ B) lymphocytes and specific antibodies markedly increased to a peak at weeks 4 or 5. The nine immune-related genes were significantly up-regulated and the expression levels of CD4-1, CD4-2 and MHC II genes were higher than that of CD8α, CD8β and MHC I genes. The CD4⁺ T lymphocytes, IgM⁺ B lymphocytes, and specific antibodies were significantly inhibited by CsA. Therefore, the responses of CD4⁺ T lymphocytes influenced the responses of the B lymphocytes and antibodies. In the pOmpK-immunised groups, the PCD4-1⁺, PCD4-2⁺, and PCD8β⁺ T lymphocytes significantly increased to a peak on days 11 or 14, days 9 or 11, and days 7 or 9, respectively. The PIgM⁺ B lymphocytes and specific antibodies significantly increased to a peak at weeks 5 or 6. Immune related genes upregulated, and CD4⁺ and CD8⁺ T lymphocytes, IgM⁺ B lymphocytes and specific antibodies all suppressed by CsA, suggesting that the responses of T lymphocytes subpopulations influenced B lymphocytes and antibodies responses. Therefore, the subpopulations of T lymphocytes played an important role in the immune responses induced by subunit and DNA candidate vaccines of OmpK and regulated the immune responses of B lymphocytes in flounder.

Identification of common antigens of three pathogenic Nocardia species and development of DNA vaccine against fish nocardiosis

Fish nocardiosis is a chronic granulomatous bacterial disease and three pathogens have been reported so far, including Nocardia asteroids, N. seriolae and N. salmonicida. However, the absence of antigen markers is a bottleneck for developing effective vaccines against fish nocardiosis. In this study, the antigenicity of whole-cell protein of these three pathogenic Nocardia species were profiled by immunoproteomic analysis and 7 common immunogenic proteins were identified as follows: molecular chaperone DnaK (DnaK), molecular chaperone GroEL (GroEL), 30 S ribosomal protein S1 (RpsA), TerD family protein (TerD), FHA domain-containing protein (FHA), 50 S ribosomal protein L7/L12 (RplL) and PspA/IM30 family protein (PspA). Furthermore, the DNA vaccine encoding FHA gene against fish nocardiosis was developed and its efficacy was investigated in hybrid snakehead. The results suggested that it needed at least 7 d to transport pcDNA-FHA DNA vaccine from injected muscle to head kidney, spleen and liver and stimulate host's immune system for later protection. In addition, non-specific immunity paraments (serum lysozyme (LYZ), peroxidase (POD), acid phosphatase (ACP), alkaline phosphatase (AKP) and superoxide dismutase (SOD) activities), specific antibody (IgM) titers production and immune-related genes (MHCIα, MHCIIα, CD4, CD8α, IL-1β and TNFα) were used to evaluate the immune response induced in pcDNA-FHA vaccinated hybrid snakehead, it proved that all these mentioned immune activities were significantly enhanced after immunization. The results also showed hybrid snakehead vaccinated with pcDNA-FHA had higher survival rate (79.33%) compared with the controls after challenge with N. seriolae, indicating that the pcDNA-FHA DNA vaccine can supply immune protection against N. seriolae infection. Taken together, this study may warrant further development of these common immunogenic proteins as the antigens for vaccine or diagnosis and facilitate the prevention and treatment of fish nocardiosis.

Current status of fish vaccines in Japan

Aquaculture is an important industry in Japan for the sustainable production of fish. It contributes to the diversity of Japanese traditional food culture, which uses fish such as "sushi" and "sashimi". In the recent aquaculture setting in Japan, infectious diseases have been an unavoidable problem and have caused serious economic losses. Therefore, there is an urgent need to overcome the disease problem to increase the productivity of aquaculture. Although our country has developed various effective vaccines against fish pathogens, which have contributed to disease prevention on fish farms, infectious diseases that cannot be controlled by conventional inactivated vaccines are still a problem. Therefore, other approaches to developing effective vaccines other than inactivated vaccines are required. This review introduces the vaccine used in Japan within the context of the current status of finfish aquacultural production and disease problems. This review also summarizes the current research into vaccine development and discusses the future perspectives of fish vaccines, focusing on the problems associated with vaccine promotion in Japan.

A Review of Fish Vaccine Development Strategies: Conventional Methods and Modern Biotechnological Approaches

Fish immunization has been carried out for over 50 years and is generally accepted as an effective method for preventing a wide range of bacterial and viral diseases. Vaccination efforts contribute to environmental, social, and economic sustainability in global aquaculture. Most licensed fish vaccines have traditionally been inactivated microorganisms that were formulated with adjuvants and delivered through immersion or injection routes. Live vaccines are more efficacious, as they mimic natural pathogen infection and generate a strong antibody response, thus having a greater potential to be administered via oral or immersion routes. Modern vaccine technology has targeted specific pathogen components, and vaccines developed using such approaches may include subunit, or recombinant, DNA/RNA particle vaccines. These advanced technologies have been developed globally and appear to induce greater levels of immunity than traditional fish vaccines. Advanced technologies have shown great promise for the future of aquaculture vaccines and will provide health benefits and enhanced economic potential for producers. This review describes the use of conventional aquaculture vaccines and provides an overview of current molecular approaches and strategies that are promising for new aquaculture vaccine development.

Elucidation of the Interleukin 12 Production Mechanism during Intracellular Bacterial Infection in Amberjack, Seriola dumerili

Intracellular bacterial infections affect all vertebrates. Cultured fish are particularly vulnerable because no effective protection measures have been established since such infections emerged approximately 50 years ago. As in other vertebrates, the induction of cell-mediated immunity (CMI) plays an important role in protecting fish against infection. However, details of the mechanism of CMI induction in fish have not been clarified. In the present study, we focused on the production of interleukin 12 (IL-12), an important factor in CMI induction in fish. Using several different approaches, we investigated IL-12 regulation in amberjack (Seriola dumerili), the species most vulnerable to intracellular bacterial disease. The results of promoter assays and transcription factor gene expression analyses showed that the expression of interferon regulatory factor-1 (IRF-1) and activator protein-1 (AP-1) is necessary for IL-12 production. Phagocytosis of living cells (LCs) of Nocardia seriolae bacteria induced IL-12 production in neutrophils, accompanied by IRF-1 and AP-1 gene expression. Bacteria in which the exported repetitive protein (Erp)-like gene was deleted (Δerp-L) could not establish intracellular parasitism or induce IRF-1 and AP-1 expression or IL-12 production, despite being phagocytosed by neutrophils. These data suggest that IL-12 production is regulated by (i) two transcription factors, IRF-1 and AP-1, (ii) phagocytosis of LCs by neutrophils, and (iii) one or more cell components of LCs. Our results enhance the understanding of the immune response to intracellular bacterial infections in vertebrates and could facilitate the discovery of new agents to prevent intracellular bacterial disease.

Vaccination of European sea bass Dicentrarchus labrax with avirulent Mycobacterium marinum (iipA::kan mutant)

Mycobacteriosis is a chronic progressive disease affecting teleost fishes all over the world. No vaccine is commercially available against its main etiological agent, Mycobacterium marinum. The mycobacterial gene responsible for invasion and intracellular persistence, iipA, is known to modulate M. marinum pathology. The innate and adaptive immune responses in sea bass (Dicentrarchus labrax) vaccinated with M. marinum iipA::kan mutant with (and without) the use of adjuvant, with (and without) a booster vaccination were monitored. The adjuvanted vaccine induced enhanced immune responses. TNF-α transcription levels were extremely high in spleen of the fish vaccinated with the addition of adjuvant in both fish vaccinated once and twice, followed by an IgM response highly specific for M. marinum. Also, histologically, granulomas started appearing in spleen and head-kidney tissues (but with no visible bacteria) within a month after vaccination, mainly with the adjuvanted vaccine. This was followed by reduction in pathology, as demonstrated by the lower number of granulomas (with visible bacteria), indicating that even heat-killed bacteria were able to elicit granulomatous formations. Adhesion of the internal organs and moderate pigmentation were observed in the perivisceral adipose tissue of nearly all vaccinated fish. Although the adjuvanted heat-killed avirulent iipA::kan mutant clearly induced a strong humoral and adaptive immune response, the booster treatment did not seem to have produced a significantly higher degree of protection from the disease compared to fish that received a single vaccination.

Construction and characterization of a DNA vaccine encoding the SagH against Streptococcus iniae

Streptococcus iniae is an important aquaculture pathogen that is associated with disease outbreaks in wild and cultured fish species. Streptolysin S has been identified as an important virulence factor of S. iniae. With an aim to develop effective vaccines against S. iniae for Japanese flounder (Paralichthys olivaceus), in this study, we constructed a DNA vaccine based on the sagH gene, which belongs to the streptolysin S-associated gene cluster. In fish vaccinated with pSagH, the transcription of sagH was detected in tissues and SagH protein was also detected in the muscles of pSagH-vaccinated fish by immunohistochemistry. The immunoprotective effect of SagH showed that fish vaccinated with pSagH at one and two months exhibited a high relative percent survival (RPS) of 92.62% and 90.58% against S. iniae serotype I, respectively. In addition, SagH conferred strong cross protection against S. iniae serotype II and resulted in an RPS of 83.01% and 80.65% at one and two months, respectively. Compared to the control group, fish vaccinated with pSagH were able to induce much stronger respiratory burst activity, and higher titer of specific antibodies. The results of quantitative real-time PCR demonstrated that pSagH upregulated the expression of several immune genes that are possibly involved in both innate and adaptive immune responses. These results indicate that pSagH is a candidate DNA vaccine candidate against S. iniae serotype I and II infection in Japanese flounder in aquaculture.

Intramuscular administration of a DNA vaccine encoding OmpK antigen induces humoral and cellular immune responses in flounder (Paralichthys olivaceus) and improves protection against Vibrio anguillarum

Outer membrane protein K (OmpK) is an immunogenic protein that could act as subunit vaccine candidate for Vibrio anguillarum. In this study, a DNA vaccine encoding the OmpK gene of V. anguillarum was constructed and confirmed to express OmpK in vitro and in vivo. To evaluate the potential of pcDNA3.1-OmpK (pOmpK) as vaccine candidate, the humoral and cellular immune responses, and protective effects were analyzed in flounder model. The results showed that the transcription and translation of OmpK gene occurred in both transfected hirame natural embryo (HINAE) cells and injected fish muscles, indicating the functionality of pOmpK to express OmpK. Fish immunized with pOmpK showed significant increase of surface IgM positive (sIgM⁺), CD4-1⁺, CD4-2⁺ lymphocytes and production of specific anti-V. anguillarum or anti-rOmpK antibodies, which indicate the activation of humoral and cellular immune responses after vaccination. Moreover, a relative percent survival (RPS) rate of 50.00% against V. anguillarum infection was obtained for flounder immunized with pOmpK. In conclusion, this study indicates that pOmpK is able to induce humoral and cellular immune responses and can be used as a DNA vaccine candidate.

Fructose-1,6-bisphosphate aldolase is involved in Mycoplasma bovis colonization as a fibronectin-binding adhesin

Mycoplasma bovis is a common pathogenic microorganism of cattle and represents an important hazard on the cattle industry. Adherence to host cells is a significant component of mycoplasma-pathogenesis research. Fibronectin (Fn), an extracellular matrix protein, is a common host cell factor that can interact with the adhesions of pathogens. The aims of this study were to investigate the Fn-binding properties of M. bovis fructose-1,6-bisphosphate aldolase (FBA) and evaluate its role as a cell adhesion factor during mycoplasma colonization. The fba (MBOV_RS00435) gene of M. bovis was cloned and expressed, with the resulting recombinant protein used to prepare rabbit polyclonal antibodies. The purified recombinant FBA (rFBA) was shown to have fructose bisphosphate aldolase activity. Western blot indicated that FBA was an antigenically conserved protein in several M. bovis strains. Western blot combined with immunofluorescent assay (IFA) revealed that FBA was dual-localized to both cytoplasm and membrane in M. bovis. IFA showed that rFBA was able to adhere to embryonic bovine lung (EBL) cells. Meanwhile, an adhesion inhibition assay demonstrated that anti-rFBA antibodies could significantly block the adhesion of M. bovis to EBL cells. Moreover, a dose-dependent binding of rFBA to Fn was found by dot blotting and enzyme-linked immunosorbent assays. Together these results provided evidence that FBA is a surface-localized and antigenic protein of M. bovis, suggesting that it may function as a virulence determinant through interacting with host Fn.

Vibriosis in Fish: A Review on Disease Development and Prevention

Current growth in aquaculture production is parallel with the increasing number of disease outbreaks, which negatively affect the production, profitability, and sustainability of the global aquaculture industry. Vibriosis is among the most common diseases leading to massive mortality of cultured shrimp, fish, and shellfish in Asia. High incidence of vibriosis can occur in hatchery and grow-out facilities, but juveniles are more susceptible to the disease. Various factors, particularly the source of fish, environmental factors (including water quality and farm management), and the virulence factors of Vibrio, influence the occurrence of the disease. Affected fish show weariness, with necrosis of skin and appendages, leading to body malformation, slow growth, internal organ liquefaction, blindness, muscle opacity, and mortality. A combination of control measures, particularly a disease-free source of fish, biosecurity of the farm, improved water quality, and other preventive measures (e.g., vaccination) might be able to control the infection. Although some control measures are expensive and less practical, vaccination is effective, relatively cheap, and easily implemented. In this review, the latest knowledge on the pathogenesis and control of vibriosis, including vaccination, is discussed.

Correlates of protective immunity for fish vaccines

Vaccination is one of the most effective disease control strategies that has contributed to the significant reduction of disease outbreaks and antibiotics usage in salmonid aquaculture. To date, licensing of fish vaccines is to a limited extent based on in vitro correlates of protection, as done for many mammalian vaccines. This is because the immunological mechanisms of vaccine protection have not been clearly elucidated for most fish vaccines. Herein, we provide an overview of the different steps required to establish correlates of protective immunity required to serve as benchmarks in optimizing vaccine production in aquaculture. We highlight the importance of optimizing challenge models needed to generate consistent results used during vaccine development as a basis for establishing immune correlates of protection. Data generated this far shows that antibodies are potentially the most reliable correlates of protective immunity for fish vaccines. Our findings also show that antigen dose can be optimized to serve as a correlate of protection for fish vaccines. Further, there is need to establish signatures of T-cell protective immunity when antibodies fail to serve as proxies of immune protection, particularly for vaccines against intracellular pathogens. We can anticipate that documentation of efficacy for future vaccines in aquaculture, particularly batch testing will be based on in vitro correlates of protective immunity.

Humoral and Cellular Immune Response of European Seabass Dicentrarchus labrax Vaccinated with Heat-Killed Mycobacterium marinum (iipA::kan Mutant)

No vaccine is yet commercially available against Mycobacterium marinum, the etiological agent of fish mycobacteriosis (also known as "fish tuberculosis"). The mycobacterial gene responsible for invasion and intracellular persistence, iipA, is known to moderate M. marinum pathology in Zebrafish Danio rerio. Two doses of heat-killed, wild-type, virulent M. marinum and two doses of a heat-killed, avirulent M. marinum iipA::kan mutant strain were used in parallel to vaccinate European Seabass Dicentrarchus labrax. The fish were then challenged with live, virulent M. marinum, and the pathogenesis of the infection was monitored. High specific immunoglobulin M (IgM) response and an increase in cytokine tumor necrosis factor alpha (TNF-α) messenger RNA expression levels were observed in all vaccinated fish. At 1 month postchallenge, TNF-α expression levels increased in spleen tissues of fish vaccinated with the virulent type and in those of unvaccinated fish, whereas in the head kidney, expression was up-regulated only in unvaccinated fish. The expression then decreased, and at 2 months postchallenge, expression appeared similar in all vaccination types. The highest survival rate (75%) was recorded in the group of fish that were vaccinated with a high dose of avirulent iipA::kan mutant. The iipA::kan mutant induced a strong immune response accompanied by only modest tissue disruption. Coupled with an effective program of booster treatments, the iipA::kan mutant vaccine may be developed into a powerful preventive measure against fish mycobacteriosis.

Mycobacterium marinum infection in fish and man: epidemiology, pathophysiology and management; a review

Mycobacterium marinum is an opportunistic pathogen inducing infection in fresh and marine water fish. This pathogen causes necrotizing granuloma like tuberculosis, morbidity and mortality in fish. The cell wall-associated lipid phthiocerol dimycocerosates, phenolic glycolipids and ESAT-6 secretion system 1 (ESX-1) are the conserved virulence determinant of the organism. Human infections with Mycobacterium marinum hypothetically are classified into four clinical categories (type I–type IV) and have been associated with the exposure of damaged skin to polluted water from fish pools or contacting objects contaminated with infected fish. Fish mycobacteriosis is clinically manifested and characterized in man by purple painless nodules, liable to develop into superficial crusting ulceration with scar formation. Early laboratory diagnosis of M. marinum including histopathology, culture and PCR is essential and critical as the clinical response to antibiotics requires months to be attained. The pathogenicity and virulence determinants of M. marinum need to be thoroughly and comprehensively investigated and understood. In spite of accumulating information on this pathogen, the different relevant data should be compared, connected and globally compiled. This article is reviewing the epidemiology, virulence factors, diagnosis and disease management in fish while casting light on the potential associated public health hazards.

Heat-inactivated Mycobacterium bovis protects zebrafish against mycobacteriosis

Control of mycobacterial infection constitutes a priority for human and animal health worldwide. However, effective vaccines are needed for the control of human and animal tuberculosis (TB). Adult zebrafish have become a useful model for studying the pathophysiology of mycobacterial infection and for the development of novel interventions for TB control and prevention. Recently, parenteral and oral immunization with the heat-inactivated Mycobacterium bovis vaccine (M. bovis IV) protected wild boar against TB. The objectives of this study were to provide additional support for the role of M. bovis IV in TB control using the zebrafish model and to conduct the first trial with this vaccine for the control of fish mycobacteriosis. The results showed that M. bovis IV protected zebrafish against mycobacteriosis caused by low and high infection doses of Mycobacterium marinum and provided evidence suggesting that the protective mechanism elicited by M. bovis IV in zebrafish as in other species is based on the activation of the innate immune response through the C3 pathway, with a role for the regulatory protein Akr2 in this process. These results encourage the use of M. bovis IV for TB control in different species.

Intracellular Bacterial Infections: A Challenge for Developing Cellular Mediated Immunity Vaccines for Farmed Fish

Aquaculture is one of the most rapidly expanding farming systems in the world. Its rapid expansion has brought with it several pathogens infecting different fish species. As a result, there has been a corresponding expansion in vaccine development to cope with the increasing number of infectious diseases in aquaculture. The success of vaccine development for bacterial diseases in aquaculture is largely attributed to empirical vaccine designs based on inactivation of whole cell (WCI) bacteria vaccines. However, an upcoming challenge in vaccine design is the increase of intracellular bacterial pathogens that are not responsive to WCI vaccines. Intracellular bacterial vaccines evoke cellular mediated immune (CMI) responses that “kill” and eliminate infected cells, unlike WCI vaccines that induce humoral immune responses whose protective mechanism is neutralization of extracellular replicating pathogens by antibodies. In this synopsis, I provide an overview of the intracellular bacterial pathogens infecting different fish species in aquaculture, outlining their mechanisms of invasion, replication, and survival intracellularly based on existing data. I also bring into perspective the current state of CMI understanding in fish together with its potential application in vaccine development. Further, I highlight the immunological pitfalls that have derailed our ability to produce protective vaccines against intracellular pathogens for finfish. Overall, the synopsis put forth herein advocates for a shift in vaccine design to include CMI-based vaccines against intracellular pathogens currently adversely affecting the aquaculture industry.

Protective immunity of grass carp immunized with DNA vaccine against Aeromonas hydrophila by using carbon nanotubes as a carrier molecule

To reduce the economic losses caused by diseases in aquaculture industry, more efficient and economic prophylactic measures should be urgently investigated. In this research, the effects of a novel functionalized single-walled carbon nanotubes (SWCNTs) applied as a delivery vehicle for DNA vaccine administration in juvenile grass carp against Aeromonas hydrophila were studied. Our results showed that SWCNTs loaded with DNA vaccine induced a better protection to juvenile grass carp against A. hydrophila. Moreover, SWCNTs conjugated with DNA vaccine provided significantly protective immunity compared with free DNA vaccine. Thereby, SWCNTs may be considered as a potential efficient DNA vaccine carrier to enhance the immunological activity.

The Zebrafish Breathes New Life into the Study of Tuberculosis

Tuberculosis (TB) is a global health emergency. Up to one-third of the world’s population is infected with Mycobacterium tuberculosis, and the pathogen continues to kill 1.5 million people annually. Currently, the means for preventing, diagnosing, and treating TB are unsatisfactory. One of the main reasons for the poor progress in TB research has been a lack of good animal models to study the latency, dormancy, and reactivation of the disease. Although sophisticated in vitro and in silico methods suitable for TB research are constantly being developed, they cannot reproduce the complete vertebrate immune system and its interplay with pathogens and vaccines. However, the zebrafish has recently emerged as a useful alternative to more traditional models, such as mice, rabbits, guinea pigs, and non-human primates, for studying the complex pathophysiology of a mycobacterial infection. The model is based on the similarity between Mycobacterium marinum – a natural fish pathogen – and M. tuberculosis. In both zebrafish larvae and adult fish, an infection with M. marinum leads to the formation of macrophage aggregates and granulomas, which resemble the M. tuberculosis infections in humans. In this review, we will summarize the current status of the zebrafish model in TB research and highlight the advantages of using zebrafish to dissect mycobacterial virulence strategies as well as the host immune responses elicited against them. In addition, we will discuss the possibilities of using the adult zebrafish model for studying latency, dormancy, and reactivation in a mycobacterial infection.

Mycobacteriosis in zebrafish colonies

Mycobacteriosis, a chronic bacterial infection, has been associated with severe losses in some zebrafish facilities and low-level mortalities and unknown impacts in others. The occurrence of at least six different described species (Mycobacterium abscessus, M. chelonae, M. fortuitum, M. haemophilum, M. marinum, M. peregrinum) from zebrafish complicates diagnosis and control because each species is unique. As a generalization, mycobacteria are often considered opportunists, but M. haemophilum and M. marinum appear to be more virulent. Background genetics of zebrafish and environmental conditions influence the susceptibility of fish and progression of disease, emphasizing the importance of regular monitoring and good husbandry practices. A combined approach to diagnostics is ultimately the most informative, with histology as a first-level screen, polymerase chain reaction for rapid detection and species identification, and culture for strain differentiation. Occurrence of identical strains of Mycobacterium in both fish and biofilms in zebrafish systems suggests transmission can occur when fish feed on infected tissues or tank detritus containing mycobacteria. Within a facility, good husbandry practices and sentinel programs are essential for minimizing the impacts of mycobacteria. In addition, quarantine and screening of animals coming into a facility is important for eliminating the introduction of the more severe pathogens. Elimination of mycobacteria from an aquatic system is likely not feasible because these species readily establish biofilms on surfaces even in extremely low nutrient conditions. Risks associated with each commonly encountered species need to be identified and informed management plans developed. Basic research on the growth characteristics, disinfection, and pathogenesis of zebrafish mycobacteria is critical moving forward.

Identification and evaluation as a DNA vaccine candidate of a virulence-associated serine protease from a pathogenic Vibrio parahaemolyticus isolate

A putative serine protease gene was cloned from the genomic DNA of Vibrio parahaemolyticus FYZ8621.4. The gene consisted of 1779 base pairs and encoded a 592 amino acid protein. The gene was expressed in Escherichia coli. The expressed protease was purified by Ni-NTA His-Bind Resin column and showed a 63 kDa band on SDS-PAGE. The protease exhibited proteolytic activity on gelatin agar plate and showed maximal proteolytic activity at pH 8.0 and 37 °C. It hydrolyzed N-α-benzoyl-L-tyrosine p-nitroanilide (BAPNA), but did not N-benzoyl-L-arginine ethylester (BAEE), N-benzoyl-L-tyrosine ethylester (BTEE) and N-acetyl-L-tyrosine ethylester (ATEE). Mutants at conserved residues Asp(51) (Asp(51)-Asn), His(89) (His(89)-Asp) and Ser(318) (Ser(318)-Leu, Ser(318)-Pro) lost proteolytic activities completely. The protein was confirmed to belong to serine protease. The purified serine protease was toxic to zebrafish with a LD(50) of 15.4 μg/fish. A DNA vaccine was constructed by inserting the mutated serine protease (Ser(318)-Pro) gene into pEGFP-N1 plasmid. The pEGFP-N1/m-vps was transfected in HeLa cells. The serine protease was confirmed to be expressed by fluorescence microscopy observation and Western blotting analysis. The pEGFP-N1/m-vps was further observed to express in muscle of the injected turbot (Scophthalmus maximus) by Western blotting seven days after immunization. Efficient protection against lethal V. parahaemolyticus challenge was observed on the vaccinated turbot with pEGFP-N1/m-vps, with the highest relative percent survival (RPS) of 96.11%. Significant specific antibody responses were also observed in the turbot vaccinated with the DNA vaccine. The results indicated that the serine protease might be a potential virulence factor and could be used as an efficient vaccine candidate for the disease control caused by V. parahaemolyticus.

Association of mycobacteria in recirculating aquaculture systems and mycobacterial disease in fish

Mycobacterium marinum isolates cultivated from tissue containing granulomatous lesions in Florida pompano Trachinotus carolinus and from biofilm samples collected from their tank and water recirculating system had identical (L1 of 11 bands) repetitive-sequence-based polymerase chain reaction (rep-PCR) DNA fingerprints. A second M. marinum clone sharing 4 of 11 rep-PCR bands with the first clone was isolated from some fish tissues but not from system samples. Water samples yielded low numbers of colonies of mycobacteria (0.08-1.3/mL), but high numbers were recovered from biofilms (260-12,000/swab) and filters (63-21,000/ filter). Mycobacterium hemophilum, M. chelonae, M. trivale, M. gastri, and M. gordonae were isolated from system samples alone.

Immunological control of fish diseases

All metazoans possess innate immune defence system whereas parameters of the adaptive immune system make their first appearance in the gnathostomata, the jawed vertebrates. Fish are therefore the first animal phyla to possess both an innate and adaptive immune system making them very interesting as regards developmental studies of the immune system. The massive increase in aquaculture in recent decades has also put greater emphasis on studies of the fish immune system and defence against diseases commonly associated with intensive fish rearing. Some of the main components of the innate and adaptive immune system of fish are described. The innate parameters are at the forefront of immune defence in fish and are a crucial factor in disease resistance. The adaptive response of fish is commonly delayed but is essential for lasting immunity and a key factor in successful vaccination. Some of the inherent and external factors that can manipulate the immune system of fish are discussed, the main fish diseases are listed and the pathogenicity and host defence discussed. The main prophylactic measures are covered, including vaccination, probiotics and immunostimulation. A key element in the immunological control of fish diseases is the great variation in disease susceptibility and immune defence of different fish species, a reflection of the extended time the present day teleosts have been separated in evolution. Future research will probably make use of molecular and proteomic tools both to study important elements in immune defence and prophylactic measures and to assist with breeding programmes for disease resistance.

Cloning, expression, and immunogenicity of Flavobacterium columnare heat shock protein dnaJ

The Flavobacterium columnare heat shock protein (HSP) gene dnaJ* was isolated, cloned, expressed, and used as an antigen in a recombinant vaccine strategy for channel catfish Ictalurus punctatus. The F. columnare dnaJ* sequence was obtained from genomovars I and II and showed intraspecies variability. Recombinant protein was expressed and purified from Escherichia coli cultures and injected intraperitoneally (12 microg of purified DnaJ/fish) into fingerling channel catfish. In addition, induced (expressing the recombinant DnaJ) and uninduced (no recombinant protein being produced) E. coli cultures were also used to immunize fish. At 28 d postimmunization, antibody response was evaluated and the fish were challenged with F. columnare. A specific immune response against DnaJ was observed in fish immunized with DnaJ or E. coli cultures expressing DnaJ. No protection against the disease, however, was observed in F. columnare-challenged fish that had been immunized with DnaJ. Some level of protection was observed in fish immunized with uninduced and induced E. coli lysates. Although HSPs have been shown to be immunodominant and good candidates for subunit vaccines in other animals, DnaJ failed to protect against columnaris disease in channel catfish.

Construction and analysis of an experimental Streptococcus iniae DNA vaccine

Streptococcus iniae is a severe aquaculture pathogen that can also infect humans and animal. A putative secretory antigen, Sia10, was identified from a pathogenic S. iniae strain by in vivo-induced antigen technology. Using turbot as an animal model, the immunoprotective effect of Sia10 was examined as a DNA vaccine in the form of plasmid pSia10, which expresses sia10 under the cytomegalovirus immediate-early promoter. In fish vaccinated with pSia10, transcription of sia10 was detected in muscle, liver, spleen, and kidney at 7, 14, 21, 28, 35, 42, and 49 days post-vaccination. In addition, production of Sia10 protein was also detected in the muscle tissues of pSia10-vaccinated fish. Fish vaccinated with pSia10 exhibited a relative percent survival (RPS) of 73.9% and 92.3%, respectively, when challenged with high and low doses (producing a cumulative mortality of 92% and 52%, respectively, in the control groups) of S. iniae. Immunological and transcriptional analyses showed that vaccination with pSia10 (i) induced much stronger chemiluminescence response and significantly higher levels of nitric oxide production and acid phosphatase activity in head kidney macrophages; (ii) caused the production of specific serum antibodies, which afforded apparent immunoprotection when transferred passively into naïve fish; and (iii) upregulated the expression of the genes encoding proteins that are possibly involved in both innate and adaptive immune responses. Taken together, these results indicated that pSia10 is an effective vaccine candidate and may be used in the control of S. iniae infection in aquaculture.

BCG vaccine confers adaptive immunity against Mycobacterium sp. infection in fish

Mycobacteriosis, caused by Mycobacterium sp., results in severe loss of fish production in Japan's aquaculture industry. In this study, the effects of two vaccine candidates, Bacillus Calmette and Guèrin (BCG, an attenuated strain of Mycobacterium bovis) and formalin-killed cells of Mycobacterium sp. were evaluated in Japanese flounder, Paralichthys olivaceus. In the immediate response and tuberculin response, BCG-vaccinated fish showed higher gene expression levels of inflammatory cytokines such as IL-1beta, IL-6, IFN-gamma and TNFalpha. Furthermore, BCG vaccination conferred protective efficacy against Mycobacterium sp. infection in Japanese flounder. Transcriptome analysis using a Japanese flounder cDNA microarray revealed that BCG vaccination induced not only adaptive immunity against Mycobacterium sp. antigen but also the expression of non-specific bactericidal proteins such as lysozyme. These data suggest that BCG confers immunity to Mycobacterium sp. infection and is a potent vaccine candidate for fish mycobacteriosis.

DNA vaccines in veterinary use

DNA vaccines represent a new frontier in vaccine technology. One important application of this technology is in the veterinary arena. DNA vaccines have already gained a foothold in certain fields of veterinary medicine. However, several important questions must be addressed when developing DNA vaccines for animals, including whether or not the vaccine is efficacious and cost effective compared with currently available options. Another important question to consider is how to apply this developing technology in a wide range of different situations, from the domestic pet to individual fish in fisheries with several thousand animals, to wildlife programs for disease control. In some cases, DNA vaccines represent an interesting option for vaccination, while in others, currently available options are sufficient. This review will examine a number of diseases of veterinary importance and the progress being made in DNA vaccine technology relevant to these diseases, and we compare these with the conventional treatment options available.

Vaccination of rainbow trout, Oncorhynchus mykiss (Walbaum), with recombinant and DNA vaccines produced to Flavobacterium psychrophilum heat shock proteins 60 and 70

Flavobacterium psychrophilum heat shock proteins (Hsp) 60 and 70 are highly immunogenic and were therefore investigated as potential vaccine candidates. Recombinant Hsps were purified from Escherichia coli and rainbow trout (Oncorhynchus mykiss) were intraperitoneally injected with phosphate buffered saline/Freunds complete adjuvant (FCA), 8 microg of rHsp60/FCA, rHsp70/FCA or a combination of 4 microg each of rHsp60 and rHsp70/FCA. Antibody responses against recombinant Hsp60 and Hsp70 8 weeks post-immunization were observed, but only fish immunized with rHsp70 exhibited highly elevated antibody levels against F. psychrophilum whole cell lysate. Some cross reactivity occurred, which may have been due to the V5 tag common to both proteins. Protection against F. psychrophilum challenge was not observed in any treatments at 8 weeks post-immunization. To further investigate any protective effect of these proteins, hsps were polymerase chain reaction amplified and cloned into pVAX1. Rainbow trout were intramuscularly injected with 8 microg of pVAX1hsp60, pVAX1hsp70 or a combination of 4 microg each of pVAX1hsp60 and pVAX1hsp70. Antibody responses at 4 weeks post-immunization were low and protection was not observed following challenge at 6 or 10 weeks post-immunization. Although Hsps of F. psychrophilum have been shown to be immunodominant, these antigens do not appear to be good vaccine candidates when delivered alone or in combination.

Protection of Japanese flounder (Paralichthys olivaceus) against Vibrio anguillarum with a DNA vaccine containing the mutated zinc-metalloprotease gene

Vibrio anguillarum is one of the causative agents of vibriosis, a systemic disease of fish characterized by acute hemorrhagic septicemia. The extracellular zinc metalloprotease (EmpA) is a putative virulence factor involved in pathogenicity of V. anguillarum. Here we described the results of immunization against V. anguillarum with the plasmid expressing the mutated EmpA (m-EmpA7), which had no protelytic activity or cytotoxicity. In vitro protein expression of m-empA7 gene was determined by fluorescent microscopy and Western-blot after transfection of Chinese hamster ovary (CHO) and human embryonic kidney (HEK293T) cell lines. All three groups of fish immunized with a single intramuscular (i.m.) injection of different doses of the m-EmpA7 DNA vaccine showed significant serum antibody levels after vaccination, compared with the fish injected with the control eukaryotic expression vector pEGFP-N1 and PBS. In addition, fish receiving the DNA vaccine developed a protective response to a live V. anguillarum challenge 4 weeks post-inoculation, as demonstrated by increased survival of vaccinated fish over the control and by decreased histological alterations in vaccinated fish. Furthermore, humoral immune responses and protective effects were significantly increased at higher vaccine doses using a single intramuscularly injection route.

Mycobacteriosis in fishes: a review

Mycobacterium species have long been recognised as a significant source of morbidity and mortality in finfish aquaculture, as well as in wild finfishes. Mycobacteria infecting fishes also include zoonotic pathogens that can cause protracted illness, especially in immunocompromised individuals. Several basic aspects of mycobacterial pathobiology in aquatic animals remain poorly understood, although a number of important recent developments have been made, especially with respect to identification of novel Mycobacterium spp. infecting fishes and a new group of mycobacteria closely related to the human pathogen Mycobacterium ulcerans. This review will encompass important aspects of mycobacterial disease in fishes, discuss recent research including studies of mycobacteriosis in striped bass (Morone saxatilis) of Chesapeake Bay, USA, and suggest directions for future work.

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.

Towards a metalloprotease-DNA vaccine against piscine cryptobiosis caused by Cryptobia salmositica

Cysteine protease is a metabolic enzyme, whereas metalloprotease is the virulent factor in cryptobiosis caused by Cryptobia salmositica. Recombinant DNA vaccines were produced with the insertion of either the metalloprotease or cysteine protease gene of C. salmositica into plasmid vectors (pEGFP-N). As expected, fishes (Oncorhynchus mykiss and Salmo salar) injected intramuscularly with the metalloprotease-DNA (MP-DNA) vaccine (50 microg/fish) were consistently more anemic (lower packed cell volume, PCV) than controls (injected only with the plasmid) at 3-5 weeks post-inoculation. Also, there were no difference in PCV between fish injected with the cysteine-DNA plasmids and the controls. In addition, agglutinating antibodies against Cryptobia were detected only in the blood of MP-DNA-vaccinated fish at 5-7 weeks post-vaccination and not in cysteine-DNA plasmids and the control groups. MP-DNA-vaccinated fish when challenged with the pathogen had consistently lower parasitemia, delayed peak parasitemia, and faster recovery compared with the controls. All fish vaccinated with attenuated strain were protected when challenged with the pathogen; this positive control group confirmed that the two vaccines operate through different mechanisms.

Mycobacterium marinum: ubiquitous agent of waterborne granulomatous skin infections

Mycobacterium marinum is a waterborne mycobacterium that commonly infects fish and amphibians worldwide. Infection in humans occurs occasionally, in most cases as a granulomatous infection localized in the skin, typically following minor trauma on the hands. For this reason, infection is especially common among aquarium keepers. Such local infection may-though infrequently-spread to tendon sheaths or joints. Disseminated disease, which is rare, can occur in immunosuppressed patients. In order to obtain a definitive diagnosis, culture and histopathological examination of biopsies from skin or other tissues are recommended. Infections sometimes heal spontaneously, but drug treatment is usually necessary for several months in order to cure the infection. Doxycycline or clarithromycin is used most commonly, although in severe cases, a combination of rifampicin and ethambutol is recommended.

Strategies for DNA vaccine delivery

Strategies for gene delivery comprise a diverse range of live and synthetic approaches; DNA delivery for the purposes of immunisation in turn comprises a large part of this research. This review mainly discusses synthetic systems for application in the delivery of plasmid DNA vaccines, outlining polylactide-co-glycolide, liposome, chitosan and complex combination delivery systems. Areas of promise for DNA vaccine candidates include immune modulation of allergic responses and veterinarian application. The potential for realistic consideration of DNA vaccines as an alternative to existing approaches is dependent on the development of efficient DNA vaccine vectors and improved systems for DNA vaccine delivery. DNA vaccine technology may yet prove to be an important asset in an environment where there is a critical need for therapeutic and prophylactic strategies to combat a wide range of disease states.