Edible Vaccines – A Modern
Approach in Plant Biotechnology
Majority of the human population is subjected to several infectious diseases. It
is necessary to produce new vaccines that have economic and other advantages
over the existing injectable vaccines. Recently plants are being used as the
alternative vaccine production system because of its several advantages over
animal system. Edible vaccines hold great promise as a cost-effective,
easy-to-administer, easy-to-store and socio-culturally readily acceptable
vaccine delivery system, especially for the poor developing countries. It
involves introduction of selected desired genes into plants and then inducing
these altered plants to manufacture the encoded proteins. Introduced as a
concept about a decade ago, it has become a reality today. A variety of delivery
systems have been developed. Initially thought to be useful only for preventing
infectious diseases but now it has also been used in birth control, cancer
therapy, etc.
Edible vaccines are currently being developed for a number of human and animal
diseases. There is growing acceptance of transgenic crops in both industrial and
developing countries. Resistance to genetically modified foods may affect the
future of edible vaccines. They have passed the major hurdles in the path of an
emerging vaccine technology. Various technical obstacles; regulatory and
non-scientific challenges need to be overcome.
The first edible vaccines were produced in tobacco in 1990 in which 0.02 %
recombinant protein of the total soluble leaf proteins was found. Today, several
plants based edible vaccines including potato, banana, carrot, tobacco and maize
etc are available that are successfully engineered by researcher and scientists.
For making edible vaccines against the different pathogens, it is necessary to
find out pathogen associated antigenic epitopes or surface antigens. The
antigenic epitopes are proteins or peptides that are encoded by genomic
sequences. Among the different epitopes, protective one can be selected for
vaccine development that provokes antibody response.
The basic methodology includes identification, selection and isolation of
desirable genes from the pathogen that encodes the surface antigen proteins. The
isolated gene can be then cloned in a suitable vector for gene transfer. The
selected vector should possess all the unique characteristics of an ideal
vector. The molecular markers present in vectors can be used for screening
transformed host cells from untransformed. After integration of desirable gene
in host genome, the cells can be checked for cloned gene expressions using ELIZA
that ultimately uses antigen specific monoclonal antibodies. The transformed
cells with positive cloned gene expressions allow them for propagation using
plant tissue culture.
The other way to produce a plant based vaccines is to infect the plants with
recombinant virus carrying the desirable antigen that is fused to viral coat
protein. The infected plant has been reported to produce the desired fusion
protein in a large amount in short span. Genetically engineered plants thus act
as biofactories for vaccine production.
Advantages
1. Edible means of administration gives excellent safety as compared to
injection.
2. It generates systemic and mucosal immunity. This is essential to avoid
respiratory and digestive tracts infection.
3. Heat stability. Edible vaccines are stable at room temperature. There is no
need of refrigeration.
4.Mass production is possible.
5.Reduction in production costs.
6.Plants can be easily reproduced as compared to animals, used as a system for
vaccines production.
Transgenic Outcomes
Transgenic tobacco is successfully engineered for the production of edible
vaccines against Hepatitis B antigen using ‘s’ gene of HBV (Hepatitis B Virus).
The optimum level of recombinant protein was obtained in leaves and seeds.
Potato is one of the best sources for vaccine production but the raw potatoes
are not palatable and cooking destroys protein antigens. Vaccine for cholera is
successfully developed in potato.
Banana is the ideal plant for oral vaccine production due to its excellent
digestibility, palatability and availability throughout the year. Vaccine for
hepatitis B is successfully made in banana.
There are several dramatic outcomes of plant biotechnology but still a number of
questions are being raised related to the ethics and safety of the transgenic
products.
Regulatory Issues
It is still unclear whether the edible vaccines would be regulated under food,
drugs or agricultural products and what vaccine component would be licensed -
antigen itself, genetically engineered fruit or transgenic seeds. They would be
subjected to a very close scrutiny by the regulatory bodies in order to ensure
that they never enter the food supply. This would include greenhouse segregation
of medicinal plants from food crops to prevent out-crossing and would
necessitate separate storage and processing facilities. Although edible vaccines
fall under "GM" plants, it is hoped that these vaccines will avoid serious
controversy, because they are intended to save lives.
Consequently, edible plant-derived vaccine may lead to a future of safer and
more effective immunization. They would overcome some of the difficulties
associated with traditional vaccines, like production, distribution and delivery
and they can be incorporated into the immunization plans. They have passed the
major hurdles in the path of an emerging vaccine technology. However, with
limited access to essential health care in much of the world and with the
scientific community still struggling with complex diseases like HIV, malaria,
etc, a cost-effective, safe and efficacious delivery system in the form of
edible vaccines will become an essential component in our disease-prevention
arsenal.