Edible Vaccines
Genes encoding antigens (derived from bacterial and viral pathogens) can be cloned and expressed in plants where they retain native immunogenic properties. Plants could be used as a system for producing large amounts of anti- gen at a relatively low cost, using agriculture instead of sophisticated and expensive cell Culture-based expression systems. Transgenic plants that express antigens in their edible tissue might be used as an inexpensive oral-vaccine production and delivery system. Therefore, immunization might be possible simply through consumption of an ‘edible vaccine’.
Oral Vaccine and Mucosal immune responses
Many infectious agents colonize or invade epithelial membranes: these include bacteria and viruses. Vaccines that are effective against these infections must stimulate the mucosal immune system to produce secretory IgA (S-IgA) at mucosal surfaces such as the gut and respiratory epithelias. In general, a mucosal immune response is more effectively achieved by oral, rather than parenteral antigen deliv- ery. Several particulate antigens have proven to be effective oral immunogens, including live and killed microorganisms.
Mode of action of Edible Vaccine
Virus-like particles (VLP)
Some antigens that assemble into ordered structures, such as virus-like particles (VLP), are better since they will be more resistant to digestion, more likely to reach the gut-associated lymphoid tissue (GALT) and, thus, more likely to be perceived as a foreign antigen once it reaches the GALT. Edible Vaccine Production in Plants
There are basically two strategies for edible vaccine production. These include:
1. Expression of foreign antigens in plant via stable transformation-Agrobacterium mediated transformation 2. Transient expression of foreign antigen in plants-Transient
expression using viral vector
The First Report-
Stable genomic transformation of tobacco
using genes encoding foreign antigens Streptococcus
mutans
SpaA protein
The first report of production of an edible vaccine appeared in a patent application published under the international patent co-operatin treaty. It described a means to express a surface protein (Spa A) from Streptococus
mutans in tobacco plants to a level of 0.02% of total leaf protein. The gene had been stably inserted by Agrobacterium mediated transformation.
This paved the way of the concept of using plants as bioreactors for obtaining edible vaccines against bacterial and viral diseases.The use of strong and organ specific promoters help for targeting the proteins in the specific organs. This helps in the stability of proteins (antigens). These antigens stimulate serum as well as mucosal immune system to produce antibody against the orally injected antigens. Thus an antigen produced in edible part act as vaccines.
Till now five antigens have been experimented in plants:
Rabies virus G protein in Tomato
Norwalk virus (cold virus) capsid protein in tobacco and potato Hepatitis B surface antigens in tobacco and potato
E.Coli heat labile entertoxin B subunit (LT-B) in tobacco Carbohydrateslera toxin B (CT-B) in potato
Hepatitis “B” virus is one of the major causes of chronic anaemia in humans. Tobacco leaves can express recombinant hepatitis surface B antigen (HBsAg) to a level of 0.01% of the soluble protein content. The gene encoding HBsAg was expressed in tobacco plants, targeting the plastid (chloroplast). Expression of HBsAg in transformation tobacco leaves yield VLPs avg 22 nm in size. When used for parenteral immunization of mice, the tobacco VLPs provoked B andT cell immune response, comparable to yeast – derived vaccine. However, because of possible degradation of proteins in the gut, presentation of vaccine antigens by an oral route requires much higher levels of immunogen than parenteral delivery so by increasing the levels of HBsAg in plants tissues, it would enhance the utility of plants as source of recombinant antigen.
2. E.coli heat labile enterotoxin (B subunit )in tobacco
The heat labile enterotoxin (LT) from E.coli is a multimeric protein consisting of a 27KDA A subunit (LT-A) and a pentamer of 11.6Kda B sub unit (LT-B). LT-B specifically binds to GM1, gangliosides on epithelial cells allowing entry of LT-A into cells.
The gene for rabies virus glycoprotein including the signal peptide was linked to CaMV 35 S promoter and transformation by Agrobaterium into tomato plants. Leaves, fruits from transgenic tomato plants expressed the recombinant glycoprotein having a molecular weight of 62 Kda. The protein was detected by Western blotting and found to be localised in Golgibodies, vesicles, plasmalemma and parenchymatous cell walls (Mc Garvey et al., 1995)
4. HIV Virus
The glycoprotein (gp41) of HIV type, (HIV-1, IIIb) containing 22
aminoacids was introduced between aminoacids 22 and 23 of small capsid subunit of CPMV (Porta et al., 1994). 3 different HIV-1 strains were found to be neutralized in vitro and invivo by antibodies recovered from mice previously injected with gp41 modified CPMV (McLain et al., 1995).
