Plant-Based Culture System
Expanding market demands for biopharmaceuticals has led to development of various platforms that could efficiently and effectively address product yield and quality requirements. Currently, production of biopharmaceuticals has relied mainly on mammalian-based cell culture and microbial fermentation. Molecular farming is an emerging field of biotechnology that utilizes plants as a platform for large-scale production of biopharmaceuticals. It consists of many expression systems including cultivation of transgenic plants in fields, transient expression by agroinfiltration of plants or virus-infected plants, in vitro culture of plant tissues or organs, and plant cell suspension culture – all of which have been explored as an economical alternative bioproduction platform.
When edible vaccines were first attempted to produce, researchers played on the idea of utilizing transgenic plants as a bioreactor especially for large-scale production. Plant-based edible vaccines are easier to handle, do not require complicated storage, and provide a cheaper alternative to vaccine production. Some of the commonly used transgenic plants as a bioreactor include tobacco, potato, tomato, corn, and rice. Also, transgenic plants have been used to produce bacterial vaccines, viral vaccines, parasite vaccines, and immunocontraceptive vaccines.
Of all the expression forms of molecular farming, plant cell suspension culture perhaps is the most promising approach to commercial production of biopharmaceuticals. Presently, various recombinant proteins that are used in the production of vaccines, antibodies, growth hormones and factors, and cytokines can be expressed using plant cell culture. The most widely used plant cell lines for recombinant biopharmaceutical production are those derived from tobacco such as cultivars BY-2 (N. tabacum cv. Bright Yellow 2) cells and NT-1 (N. tabacum-1) cells. Other plant cell lines that were used came from common crop species, such as rice, soybean, alfalfa, carrot, and tomato.
Plant cell culture has been regarded as more advantageous over other expressions system due to the attractive features it offers: 1) intrinsic safety since plant cells do not harbor human pathogens and produce bacterial endotoxins which are important considerations for biopharmaceutical production; 2) the ability to produce glycoproteins similar to their native counterparts; 3) cost-effectiveness because plant cells are inexpensive to grow and maintain.
Scaling up Plant Cell Culture
Scaling up cell culture in bioreactors is the most fundamental step that must be accomplished in order to successfully apply plant cell culture for commercial production. While plant cells are cultured in a bioreactor similar to how microbial cells and mammalian cells are cultured for large-scale production, the process is still complicated.
The large size and complex morphology of plant cells must be taken into consideration when scaling up plant cell culture. Plant cells are significantly larger than bacteria, yeast, and mammalian cells, and tend to form cell aggregation. Hence, plant cells are prone to shear stress. In general, when selecting a bioreactor for culture of plant cells, it is recommended that a bioreactor system with extremely low shear stress and adequate oxygen transfer be considered.
Esco VacciXcell offers a product line of stirred tank bioreactors that are capable of scaling up plant suspension culture - StirCradle™ and StirCradle™-Pro. The StirCradle™ is the laboratory-scale autoclavable system and it is available in three sizes – 5 L, 7.5 L, and 10 L. On the other hand, the StirCradle-Pro™ is a fully stainless steel fermenter/bioreactor system designed with an automated 5-step cycle SIP system for both culture medium and reactor vessel. It is a bioreactor that operates for pilot and production scale of suspension culture with available sizes ranging from 20 L to 1000 L. Customized bioreactor sizes are available though, upon request.
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