A new therapeutic made from tobacco plants has been shown to arrest West Nile virus infection, according to a new study by Arizona State University scientist Qiang Chen and his colleagues.
Chen, a researcher at Arizona State University's Biodesign Institute and professor in the PolyTechnic Campus' College of Technology and Innovation, is the first to demonstrate a plant-derived treatment to successfully combat West Nile virus after exposure and infection. The research appears in this week's issue of the Proceedings of the National Academy of Sciences (advanced online edition).
There are currently no available vaccines against West Nile, nor effective therapeutics for human use, so the current findings are a considerable advancement and may offer the best hope thus far that the West Nile virus infection can be stopped, even several days after viral infection.
West Nile virus has made alarming inroads in North America, causing disease outbreaks throughout the U.S., as well as in areas of Canada, Mexico, the Caribbean and South America. Elderly individuals and those with depressed immunity are particularly vulnerable to West Nile, a mosquito-borne illness which can cause a potentially lethal inflammation of the brain.
Chen's group demonstrated the versatility of plant-based biotechnology. "The goal of this research was twofold," said Chen. "First, we wanted to show proof-of- concept, demonstrating that plant-made antibodies can work as effective post-exposure therapeutics. Secondly, we've sought to develop a therapeutic which can be made inexpensively so that the health care systems in developing countries can afford it."
Issues of affordability for such antibody-based treatments are central to the challenge, Chen stresses, with the costs for development of a traditional pharmaceutical mammalian cell line production facility often running into the tens of millions of dollars. Besides being inexpensive, plants offer other advantages. Mammalian cell lines may run the risk of harboring animal pathogens such as viruses or infectious protein agents known as prions, not found in plants. Plants also allow production of pharmaceutical products to be easily scaled up to whatever quantities are required. "With mammalian cells systems, the physical size of the bioreactor is limiting," said Chen, "whereas with plants, we can just plant another acre."
The study examined antibodies against West Nile virus derived from mammalian cell lines and compared their effectiveness with those extracted from plants. The plants used to produce the antibodies are a relative of common tobacco, a member of the Solanceae family of plants, which produce abundant leaves for harvesting material and are also prolific seed producers. Seven days after the introduction of antibody genes into plants, the leaves are harvested, homogenized and purified to remove extraneous material.
The strategy for gene insertion is to use the specific machinery of the tobacco mosaic virus and potato virus X (PVX) to carry the genes of interest into the plants, where they can be expressed, yielding a human monoclonal antibody known as hu-E16. The gene expression occurs in just a week's time, making the production process highly efficient.
The monoclonal antibody, once injected into the recipient, binds to a particular surface protein of the virus. That binding site is also the one used by the virus to attach itself to mammalian host cells, and once it is occupied, the virus' cell-binding (and infectious) capacity is neutralized.
Cell-derived versions of this antibody have already demonstrated impressive effectiveness, protecting mice from WNV-induced mortality even several days after infection. Chen's research shows comparable effectiveness using tobacco-plant derived monoclonal antibodies. Indeed, the results in the groups of mice tested, were essentially indistinguishable. The therapeutic, Chen emphasizes, is effective in very small dosages, (50-200 micrograms), and only one dose is required to clear the virus from an infected individual's system.
The effectiveness of the plant expression system is dependent in part on the optimization of the antibody DNA sequence, which helps to ensure a high level of expression in the plants. "The goal is to make more of the protein and for it to persist longer before it is degraded, " said Chen. "Optimization helps." The strategy permitted the group to set a record for the antibody yield produced by the transgenic tobacco plants-an increase from 500 micrograms of antibody per gram of leaf tissue to 800 micrograms per leaf.