Pharma-Planta: road testing the developing regulatory guidelines for plant-made pharmaceuticals

Investigating recombinant protein exudation from roots of transgenic tobacco

It is widely acknowledged that plant-made pharmaceuticals (PMPs) offer numerous benefits, including inexpensive production, biological safety and the facility for production at agricultural scale. At the same time, it is important to minimize any potential risk associated with this new technology, including the potential release of bioactive proteins into the environment. To address this issue, we studied transgenic Nicotiana benthamiana and Nicotiana tabacum plants expressing two recombinant single-chain variable fragment (scFv) antibodies, respectively scFvB9 and scFvH10. ScFvB9 was raised against glycoprotein G1 of Tomato spotted wilt virus (TSWV), and scFvH10 was raised against human tumor-associated antigen tenascin-C. Both antibodies were targeted to the secretory pathway using the N-terminal signal peptide from Phaseolus vulgaris polygalacturonase-inhibiting protein (PGIP), and scFvH10 carried in addition a C-terminal KDEL tetrapeptide for retention in the endoplasmic reticulum (ER). Sterile hydroponic cultures were established, allowing us to investigate whether scFvB9 and scFvH10 were present in root exudates. Intercellular fluids extracted from different plant tissues were analyzed by western blotting revealing the presence of scFvB9. Successful secretion of scFvB9 in hydroponic medium was also demonstrated, whereas no scFvH10 could be detected in the leaf, stem or root apoplast, nor secreted into the hydroponic medium. Our results show that scFvH10 release or diffusion from the roots of transgenic plants was not occurring, suggesting that the KDEL signal might contribute to the environmental biosafety of crops producing PMPs.

Considerations for extraction of monoclonal antibodies targeted to different subcellular compartments in transgenic tobacco plants

SUMMARY

Monoclonal antibody production from transgenic tobacco plants offers many advantages over other heterologous production systems, creating the prospect of production at a scale that will allow new prophylactic and therapeutic applications in global human and animal health. However, information on the major processing factors to consider for large-scale purification of antibodies from transgenic plants is currently limited, and is in urgent need of attention. The purpose of this project was to investigate methods for the initial extraction of recombinant immunoglobulin G (IgG) antibodies from transgenic tobacco leaf tissue. Three different transgenic plant lines were studied in order to establish the parameters for optimal extraction of monoclonal antibodies that accumulate in the apoplasm, at the plasma membrane or within the endoplasmic reticulum. For each transgenic line, seven techniques for physical extraction were compared. The factors that determine the optimal extraction of antibodies from plants have a direct influence on the initial choice of expression strategy, and so must be considered at an early stage. The use of small-scale techniques that are applicable to large-scale purification was a particularly important consideration. The optimal extraction technique varied with the target location of IgG in the plant cell, and the dependence of antibody yield on the physical extraction methodology employed, the pH of the extraction buffer and the extraction temperature was demonstrated in each case. The addition of detergent to the extraction buffer may improve the yield, but this was found to be dependent on the site of accumulation of IgG within the plant cell.

Expression of bioactive single-chain murine IL-12 in transgenic plants

Interleukin-12 (IL-12), an important immunomodulator for cell-mediated immunity, shows significant potential as a vaccine adjuvant and anticancer therapeutic. However, its clinical application is limited in part by lack of an effective bioproduction system for this complex heterodimeric glycoprotein. Transgenic plants show promise as scalable bioproduction platforms for challenging biopharmaceutical proteins. To test the potential of plants to effectively produce bioactive IL-12, we developed transgenic tobacco plant lines and derived root cultures yielding high levels of mouse IL-12 (MuIL-12). Functional IL-12 is a heterodimer consisting of two disulfide-linked subunits, p35 and p40. To ensure the stoichiometric expression and assembly of p35 and p40, we expressed a single-chain version of MuIL-12. Plant-derived single-chain MuIL-12 was characterized and purified for in vitro bioactivity assays. Our results demonstrated precise cleavage of the endogenous mouse p40 signal peptide in plants as well as addition of N-linked glycans. Plant-derived MuIL-12 triggered induction of interferon-gamma (IFN-gamma) secretion from mouse splenocytes and stimulated splenocyte proliferation with comparable activities to those observed for commercially available animal cell-derived MuIL-12. These studies indicate that plants produce fully functional MuIL-12 at levels compatible with commercial production and may serve as an effective bioproduction platform for bioactive IL-12s from other species for human or veterinary vaccine and therapeutic applications.

Transgenic plants for animal health: plant-made vaccine antigens for animal infectious disease control

A variety of plant species have been genetically modified to accumulate vaccine antigens for human and animal health and the first vaccine candidates are approaching the market. The regulatory burden for animal vaccines is less than that for human use and this has attracted the attention of researchers and companies, and investment in plant-made vaccines for animal infectious disease control is increasing. The dosage cost of vaccines for animal infectious diseases must be kept to a minimum, especially for non-lethal diseases that diminish animal welfare and growth, so efficient and economic production, storage and delivery are critical for commercialization. It has become clear that transgenic plants are an economic and efficient alternative to fermentation for large-scale production of vaccine antigens. The oral delivery of plant-made vaccines is particularly attractive since the expensive purification step can be avoided further reducing the cost per dose. This review covers the current status of plant-produced vaccines for the prevention of disease in animals and focuses on barriers to the development of such products and methods to overcome them.

Fighting cancer with plant-expressed pharmaceuticals

Cancer is one of the most prevalent diseases worldwide, which explains why biological therapies for cancer are forecast to make up 35% of total recombinant pharmaceuticals by 2010. Because of the high demand for cancer drugs, the need to lower production costs and the constraints of present production technologies for recombinant pharmaceuticals (such as the difficulties involved in culturing bacteria, yeast and mammalian cells), attention has recently been focused on recombinant expression of pharmaceutical anti-cancer proteins in plants. This review aims to provide an update on the most recent publications about anti-cancer recombinant pharmaceuticals expressed in plants, as well as on the relevant technical issues, potential and prospects of this emerging production system.