Sources and upstream processing - Pharmaceutical Biotechnology: Concepts and Aapplications - page 119

• cost of plant cultivation is low;

• harvest equipment/methodologies are inexpensive and well established;

• ease of scale-up;

• proteins expressed in seeds are generally stable in the seed for prolonged periods of time (often

years);

• plant-based systems are free of human pathogens (e.g. HIV).

However, a number of potential disadvantages are also associated with the use of plant-based
expression systems, including:

• variable/low expression levels sometimes achieved;

• potential occurrence of post-translational gene silencing (a sequence-specifi c mRNA degrada-

tion mechanism);

• glycosylation patterns achieved differ signifi cantly from native human protein glycosylation

patterns, and plant glycoforms are invariably immunogenic in humans;

• the potential presence of biologically active plant metabolites (e.g. alkaloids) that would 'con-

taminate' the crude product;

• environmental/public concerns relating to potential environmental escape of genetically altered

plants;

• seasonal/geographical nature of plant growth.

For these reasons, as well as the fact that additional tried-and-tested expression systems are
already available, production of recombinant therapeutic proteins in transgenic plant systems has
not as yet impacted signifi cantly on the industry.
The most likely focus of future industry interest in this area concerns the production of oral
vaccines in edible plants/fruit, such as tomatoes and bananas. Animal studies have clearly shown
that ingestion of transgenic plant tissue expressing recombinant subunit vaccines (see Chapter 13
for a discussion of subunit vaccines) induces the production of antigen-specifi c antibody responses
not only in mucosal secretions, but also in the serum. The approach is elegant, in that direct
consumption of the plant material provides an inexpensive, effi cient and technically straightfor-
ward mode of large-scale vaccine delivery, particularly in poorer world regions. However, several
hurdles hindering the widespread application of this technology include:

• the immunogenicity of orally administered vaccines can vary widely;

• the stability of antigens in the digestive tract varies widely;