Transgenic crops for the production of recombinant vaccines and anti-microbial antibodies

Plants can be used to produce inexpensive and highly immunogenic vaccines, particularly those aimed against mucosal pathogens. Several plant-derived vaccines have already completed early-phase clinical trials and many more are in the pipeline. The number of products in development has increased as the production technology itself has evolved, reflecting a better understanding of plant molecular biology, more sophisticated genetic engineering techniques, and more recently the development of tools and strategies to increase yields and engineer specific glycan groups on plant-derived glycoproteins. There are many different platforms including whole-plant transient expression systems based on Agrobacterium and/or plant viruses, contained systems based on cultured plant cells or aquatic plants, and stable transgenic plants expressing recombinant proteins in leaves, seeds, fruits or tubers/roots. Although the transient systems are rapid and high-yielding, stable transgenic plants are more scalable and may ultimately provide for more economical large-scale production, which was the original vision of 'molecular farming'. Grain crops such as cereals and legumes are particularly valuable because recombinant proteins expressed in seeds are stable at ambient temperatures and any bioload can be reduced by surface sterilization. Seeds also present interesting formulation options, e.g. the use of seed-specific storage organelles for encapsulation and the slow release of mucosal vaccines. In this article, we review the current status and recent developments in the area of molecular farming in crop plants, focusing particularly on engineered seeds as production and delivery vehicles for recombinant vaccines and antibodies.