Vaccines protect animals from infections are considered one of the great successes of modern medicine. Vaccines are either parts of microbes, whole microbes that have been killed/weakened, or pathogen subunits which include specific macromolecules (toxins, proteins and polysaccharides). They can induce the protective immune response in hosts against pathogens (De Gregorio and Rappuoli, 2012). Subunit vaccine technology increases the safety of administering vaccines because subunit vaccines do not contain live or weakened pathogens. However, due to high production cost as well as the cold storage requirements for these non-heat stable vaccines, their use has been limited.
Production of subunit vaccines within transgenic plants can overcome these limitations (Sala et al., 2003). Production of subunit vaccines in plant cells can be achieved through expressing particular immune-eliciting proteins from pathogens, either in the nuclear genome or the plastid genome. Expression of subunit vaccines as recombinant proteins in plant is increasingly popular, due to low production cost, low risk of contamination by mammalian pathogens and endotoxins, production of correctly folded and assembled multimeric proteins and the possibility for use as edible vaccines (Ma et al., 2003; Lico et al., 2008; Shih and Doran, 2009; Marsian and Lomonossoff, 2016).
Studies related to vaccine development for diseases in aquaculture are relatively slow. Low acceptance due to high price associated with the cost of production of a vaccine is the main reason for this scenario. Thus in orders to control disease in aquaculture, development of plant expressed vaccines (contain DNA that codes for specific proteins /antigens from a pathogen) will be a best option, which will be efficient and cost-effective. Since vaccines prevent fish from diseases by inducing specific antibody production in fish against pathogens, issues such as drug residues in tissues or the environment, development of resistant bacterial strains can be easily eliminated by vaccinating fish against diseases. Factors like DNA being a stable molecule, relatively inexpensive and easy to produce, and potential for preparing multivalent vaccines have contributed to DNA vaccines being attractive to fish manufacturers (Heppell and Davis, 2000). Moreover, as vaccine antigens produced in plants are protected with plant cell wall (bioencapsulated), thus antigen will be preventing from the acidic environment of stomach upon oral delivery. After passing it to the gut, gut microbes digest plant cell walls and vaccine antigens are released to the immune system of the gut. Further plant expressed vaccines can be easily introduced as edible vaccines by expressing in edible plants or edible parts of plants. This will simplify the route of vaccine delivery in to fish. Thus use of plant expressed fish vaccines have many advantages over antibiotics in aquaculture, and can be a good alternative to the use of antibiotics, making fish farming more economically feasible and sustainable. However up to date no reports of plant expressed fish vaccines could be found.