Edible vaccines: The plant based vaccines?
Nowadays, scientists are working on the advantages of edible vaccines in tomatoes and potatoes for several enteric pathogens. This can be fed to animals or humans for immunization or vaccination. Instead of conventional vaccines, there are many advantages of edible vaccines. These vaccines are much easier to store and administer than injectable vaccines being used presently. Therefore, in this article, our focus is on the principles and advantages of plant-based vaccines and a general description of the development steps involved.
History of vaccines
Vaccination was introduced into medicine by Edward Jenner in 1796. He used the pox virus isolated from cows to immunize James Phipps against smallpox. Eventually, this intervention resulted in the induction of protection against this pathogen. And so eliminated smallpox from the world permanently. Thus, leading to the introduction of the term vaccine. Later, Louis Pasteur developed a live attenuated vaccine against rabies.
The following basic steps for conventional vaccine development are taken:
- Injection of the causative agent
Invention of edible vaccines:
Conventional vaccines allow protection against many lethal infectious diseases. These approaches led to great achievements such as the eradication of smallpox etc. For decades, inactivation and attenuation were the first choices for vaccine production. However, the difficulty of propagating some in-vitro. Also, it is a fact that even attenuation may result in unwanted immune responses or risk of developing the disease. Thus, it led to the consideration of alternative approaches. In addition, the purification of specific antigens often failed to provide a protective vaccine candidate. Gradually, the attempts led to the production of edible vaccines.
In the early 1990s, the concept of edible vaccines came into play. After several studies, the viability of this technology is checked. It is found that a number of plant species, mainly tobacco, potato, tomato, lettuce, spinach, corn, etc. are capable of producing such vaccines.
Advantages of edible vaccines
Plant-based vaccine formulation is intended to serve as a source of a recombinant antigen. Therefore, these have many advantages. These are produced at low cost whose biomass or purified fractions serve as elicitors of protective immunity throughout the administration by distinct routes. This a promising strategy for the production of mucosally delivered vaccines, especially oral vaccines. Moreover, these require minimal processing of the raw plant biomass and training for administration.
Important features of effective vaccines like:
- protective immunity,
- ease of administration,
- low cost,
- minimized side effects.
The source of plant-based vaccines is transgenic plants. Moreover, these are produced through the introduction of the desired gene encoding a specific antigen that can trigger a strong immune response inside the body. At present, there are several edible vaccines for diseases like measles, cholera, and hepatitis B, C, & E. However, these are edible vaccines, they are predominately tested in animal testing. And so in the development phases, some human clinical trials are being conducted.
Currently, most subunit vaccines are produced in recombinant systems which typically comprise bacterial polysaccharides or proteins. these are purified from pathogenic organisms. These pure subunits of pathogenic origin are safer than whole-celled vaccines since they lack replicative capacity. Thus, the risk of reversion of attenuated strains or survival of putatively killed pathogens is avoided. In addition to these advantages associated with conventional subunit vaccines, the use of plants for vaccines production represents the following convenient features:
- It constitutes the most economical and feasible source of recombinant products. Thus, reducing manufacturing costs due to the replacement of fermentors and bioreactors with contained plant growth rooms or greenhouses with appropriate biological containment. It is estimated that the cost of producing a recombinant protein in transgenic plants is 10-50 fold lower than producing it by means of E.coli fermentation.
- Other advantages of edible vaccines are that the plants can properly produce foreign proteins, as these hosts possess expression, folding, assembly, and glycosylation machinery. It is associated with the preservation of the immunogenic activity of vaccines.
- Unlike mammalian systems, plant systems are not hosts for human or animal pathogens such as viruses or prions, and hence they do not transmit such pathogens.
- Many plant species can serve as safe oral-delivery vehicles. In particular, specific tissues such as grains, fruits, or leaves can allow for the formulation of vaccines. This is done even without extensive purification and processing. These vehicles can be easily produced by the freeze-dried process. Eventually, these process leads to formulations with high stability under a cold chain-free distribution.
Examples of edible vaccines
Plant-based vaccines against Influenza
The influenza virus is a globally important respiratory pathogen that causes a high degree of morbidity and mortality with the capacity of evolving into a pandemic behaviour. In this field, the use of plants for the production of vaccines has many advantages. This approach can be scaled up easily in greenhouse-based manufacturing facilities to meet global healthcare needs. Plants possess an outstanding capacity to efficiently produce complex proteins at high yields and low cost.
Plant-based vaccines against pollen allergy
Plant pollens are one of the most common causes of seasonal allergic diseases including allergic rhinitis, conjunctivitis, atopic dermatitis, and asthma. These occur as the consequence of fundamental allergenic mechanisms involving the induction of pollen-specific type hypersensitivity. Immune tolerance through the oral route has been reported to vary depending on the dose, frequency, or formulation of the administered antigen. Furthermore, oral administration can induce both mucosal and systemic immunity, whereas parental delivery cannot. Thus, the selection of an oral route is rational as an administration method because immune tolerance is more prone to be induced than by immune stimulation.
Limitations of plant-based vaccines
Besides having several advantages these edible vaccines suffer from various limitations. To name some of the limitations of plant-based vaccines:
- The diseased person may develop immune tolerance to the vaccine.
- The edible vaccines may have some stability of vaccine and evaluation of dosage requirements is hard.
- There may also be inconsistencies in dose requirements from different fruits, plants, plant tissues, and the generation of plants.
- Sometimes the selection of the most suited plant also becomes difficult.
- Some properties of the vaccine might also get changed due to the cooking of plants that can not be consumed raw.
- There may be the chances of triggering allergic responses and their longevity is debatable.
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