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Figure 1. A visual summary of the different modes of expression possible with plants. (A-D) Gene transfer and sorting out of transformed plastids on selective medium. (A) Introduction of gold particle covered with DNA in a wild-type cell; (B) a cell with only one transformed plastome soon after transformation; (C) a heteroplasmic cell with transformed and wild-type plastomes (filled and empty circles, respectively); (D) a homoplasmic cell with transformed plastids only.VLP: Virus-like particle.
(Figure omitted. See article PDF.)
Increasing demands for both safe and efficacious human and animal vaccines have led to the rise in use of virus-like particles (VLPs) as vaccines; these are advantageous over whole virus-derived products because no infectious material is involved at any stage. In addition, VLPs are usually highly immunogenic due to their relatively large size and the fact that antigenic determinants are present in the form of multiple repeats. Moreover, many VLPs are self-adjuvanting in terms of stimulating both the humoral and cellular response pathways, allowing both Th1- and Th2-type responses. However, a disadvantage of VLPs as vaccines is that they have to be administered repeatedly at relatively high doses due to the fact that they do not replicate, which means that they have to be manufactured at high yield, preferably inexpensively if they are to be universally used. While VLP-based products are already available in the form of recombinant HBV surface antigen (HBsAg) particles made in yeasts, and HPV VLP vaccines made in yeast and in insect cells, these were expensive initially in the case of HBV and currently in the case of HPV, and are not generally within the reach of most countries'Extended Programme of Immunisation packages. Indeed, it has taken the HBV vaccine over 15 years to decrease in price from the initial US$40 a dose, to a level that is affordable by developing country health schemes, and the HPV vaccines are presently considerably more expensive.
Plants as production vehicles for vaccines
The possibility of using plants as reaction vessels to produce high-value biological pharmaceuticals has been actively explored since 1989, when a functional monoclonal antibody was made in transgenic tobacco [1]. The use of plants for the production of virus vaccines is almost as old as the technology itself; indeed, one of the first...