Plants as biofactories

Recombinant protein expression in Nicotiana

Recombinant protein pharmaceuticals are now widely used in treatment of chronic diseases, and several recombinant protein subunit vaccines are approved for human and veterinary use. With growing demand for complex protein pharmaceuticals, such as monoclonal antibodies, manufacturing capacity is becoming limited. There is increasing need for safe, scalable, and economical alternatives to mammalian cell culture-based manufacturing systems, which require substantial capital investment for new manufacturing facilities. Since a seminal paper reporting immunoglobulin expression in transgenic plants was published in 1989, there have been many technological advances in plant expression systems to the present time where production of proteins in leaf tissues of nonfood crops such as Nicotiana species is considered a viable alternative. In particular, transient expression systems derived from recombinant plant viral vectors offer opportunities for rapid expression screening, construct optimization, and expression scale-up. Extraction of recombinant proteins from Nicotiana leaf tissues can be achieved by collection of secreted protein fractions, or from a total protein extract after grinding the leaves with buffer. After separation from solids, the major purification challenge is contamination with elements of the photosynthetic complex, which can be solved by application of a variety of facile and proven strategies. In conclusion, the technologies required for safe, efficient, scalable manufacture of recombinant proteins in Nicotiana leaf tissues have matured to the point where several products have already been tested in phase I clinical trials and will soon be followed by a rich pipeline of recombinant vaccines, microbicides, and therapeutic proteins.

Vaccines to combat the neglected tropical diseases

The neglected tropical diseases (NTDs) represent a group of parasitic and related infectious diseases such as amebiasis, Chagas disease, cysticercosis, echinococcosis, hookworm, leishmaniasis, and schistosomiasis. Together, these conditions are considered the most common infections in low- and middle-income countries, where they produce a level of global disability and human suffering equivalent to better known conditions such as human immunodeficiency virus/acquired immunodeficiency syndrome and malaria. Despite their global public health importance, progress on developing vaccines for NTD pathogens has lagged because of some key technical hurdles and the fact that these infections occur almost exclusively in the world's poorest people living below the World Bank poverty line. In the absence of financial incentives for new products, the multinational pharmaceutical companies have not embarked on substantive research and development programs for the neglected tropical disease vaccines. Here, we review the current status of scientific and technical progress in the development of new neglected tropical disease vaccines, highlighting the successes that have been achieved (cysticercosis and echinococcosis) and identifying the challenges and opportunities for development of new vaccines for NTDs. Also highlighted are the contributions being made by non-profit product development partnerships that are working to overcome some of the economic challenges in vaccine manufacture, clinical testing, and global access.

Production of human hyaluronidase in a plant-derived protein expression system: plant-based transient production of active human hyaluronidase

Four types of human hyaluronidases (rHuHyal-1, -2, -3 and -4) were transiently expressed and purified from Nicotiana benthamiana, and their biochemical characteristics were analyzed. The recombinant HuHyals were expressed via agrobacteria-mediated infiltration and generated and expressed in terms of micrograms per 5 leaves of N. benthamiana. Expressed recombinant HuHyals were purified using a His(6) tagging system and Ni column chromatography, respectively, at pH 8.0, after which the purified rHuHyals were concentrated for additional biochemical analyses. The four types of rHuHyals were allowed to react with hyaluronic acids and chondroitin sulfates. The biochemical properties of rHuHyal-1 fit those of the commercially available Hyal, PH-20, which was extracted from animal testes under acidic conditions (pH 3.5). However, rHuHyal-1 evidenced activity levels 2 to 6-fold greater than the three other rHuHyals (rHuHyal-2, -3 and -4) at pH 3.5. However, only rHuHyal-4 exhibited chondroitinase activity with both 6-S-chondroitin sulfate (chondroitin sulfate C) and 4-S-chondroitin sulfate (chondroitin sulfate A) as standard substrates. The results of zymography demonstrated that recombinant HuHyal 1 was modified by glycosylation, but Escherichia coli Hyal was not. This result demonstrated that plant-based rHuHyal was functionally active and evidenced biochemical characteristics and post-translational protein modifications similar to those of animal testis-derived Hyal.

Expression and subcellular targeting of human insulin-like growth factor binding protein-3 in transgenic tobacco plants

Human insulin-like growth factor binding protein-3 (hIGFBP-3) is a multifunctional protein which has high affinity for insulin-like growth factor-I (IGF-I). It combines with IGF-I to form a tertiary complex in circulation, thus regulating the activity of IGF-I. Furthermore, recombinant hIGFBP-3 (rhIGFBP-3) has been found to negatively regulate cell proliferation and induce apoptosis. In this study, we have established an efficient plant bioreactor platform for mass production of rhIGFBP-3. Different expression constructs, driven by the seed-specific phaseolin promoter, were designed and transformed into tobacco plant via Agrobacterium. To enhance protein expression level, the signal peptide (SP) and the C-terminal tetrapeptide AFVY of phaseolin were used to direct rhIGFBP-3 to protein storage vacuole (PSV) in tobacco seed for stable accumulation. Western blot analysis showed that rhIGFBP-3 was successfully synthesized in transgenic tobacco seeds, with the highest protein expression of 800 mug/g dry weight. The localization of rhIGFBP-3 in PSV was also evident by confocal immunofluorescence microscopy. Our results indicated that protein sorting sequences could benefit the expression level of rhIGFBP-3 and it is feasible to use plant as "bio-factory" to produce therapeutic recombinant proteins in large quantity.

Using plant cells as influenza vaccine substrates

The reappearance of highly pathogenic avian influenza H5N1 in poultry in 2003, and the subsequent high-fatality zoonoses in Asia, Europe and Africa, has heightened the awareness of a potential pandemic and the need for global vaccine supply. Most manufacturers still use embryonated hens' eggs to produce influenza vaccines, a system that has demonstrated its value throughout six decades. There are, however, some challenges with this approach, both for seasonal and particularly for pandemic vaccine production. This review highlights some of these challenges and describes emerging alternative production platforms with the potential to deliver safe and effective vaccines to the global market in a timely fashion. A particular emphasis of this review will be on the production of recombinant influenza vaccines using transient plant expression systems.