Concept behind edible vaccines

Edible vaccines are created by introducing selected desired genes into plants and inducing these genetically modified plants to manufacture the encoded proteins. This process is known as "transformation," and the altered plants are called "transgenic plants." 

When an antigen of a pathogen can produce an immunogenic response when delivered orally, it is considered as a likely candidate for an edible vaccine. The gene encoding the orally active antigenic protein is isolated from the pathogen, and a suitable vehicle for constitutive or tissue-specific expression of the gene is prepared.  

This gene vehicle is then introduced and stably integrated into the genome of selected plant species, and is then allowed to express to produce the antigen. The appropriate plant parts containing the antigen are then fed raw to animals or humans to bring about immunization. This is obviously more humane, and more economical, than the traditional “injection” method generally followed. 

The selection of a plant system for delivery of edible vaccines for humans is very important. Since it is necessary to express the desired protein in a food that is consumed raw (to avoid denaturation of the vaccine protein), a system to transform banana plants has been developed as bananas are produced in most developing countries, and are fed uncooked to infants and adults.  Thus, for animals, crops used as feed, e.g., alfalfa, are suitable for the expression of such antigens, while for humans fruits like banana, which are consumed raw, have to be used.  

Animals may sometime develop a tolerance to the components of their routine food, so that these become non-immunogenic in them. Therefore, edible vaccines cannot be used as a REGULAR component of animal/human food.

Several genes encoding antigenic proteins have been expressed in plants where they are produced in their native immunogenic forms.  

Studies supported by the World Health Organization have demonstrated an effective vaccine for cholera, which provides cross-protection against the enterotoxins of Escherichia coli. This vaccine is not available, however, largely due to cost of production of the bacterial toxin protein which is a component of its formulation. To address this limitation, plants were transformed with the gene encoding the B subunit of the E. coli heat labile enterotoxin (LT-B). Transgenic potatoes expressing LT-B were found to induce both serum and secretory antibodies when fed to mice; these antibodies were protective in bacterial toxin assays in vitro. This is the first "proof of concept" for the edible vaccine.  

Edible vaccine research is currently directed at human diseases. This technology is of immense value for the production of inexpensive vaccines as feed additives for agricultural animals.

Since various plant tissues are fed to animals, other plants such as alfalfa, maize and wheat could be valuable vehicles to deliver vaccines (and perhaps other pharmaceuticals) for the betterment of animal health. 

References:

  http://www.nal.usda.gov/pgdic/Probe/v5n1/lead.html and

  http://www.molecular-plant-biotechnology.info/transgenic-plants-II/edible-vaccines.htm