Production of a foreign protein product with genetically modified plant cells

Plant Platforms for Efficient Heterologous Protein Production

Production of recombinant proteins is primarily established in cultures of mammalian, insect and bacterial cells. Concurrently, concept of using plants to produce high-value pharmaceuticals such as vaccines, antibodies, and dietary proteins have received worldwide attention. Newer technologies for plant transformation such as plastid engineering, agroinfiltration, magnifection, and deconstructed viral vectors have been used to enhance the protein production in plants along with the inherent advantage of speed, scale, and cost of production in plant systems. Production of therapeutic proteins in plants has now a more pragmatic approach when several plant-produced vaccines and antibodies successfully completed Phase I clinical trials in humans and were further scheduled for regulatory approvals to manufacture clinical grade products on a large scale which are safe, efficacious, and meet the quality standards. The main thrust of this review is to summarize the data accumulated over the last two decades and recent development and achievements of the plant derived therapeutics. It also attempts to discuss different strategies employed to increase the production so as to make plants more competitive with the established production systems in this industry.

Bioreactor-Based Production of Glycoproteins in Plant Cell Suspension Cultures

Recombinant glycoproteins such as monoclonal antibodies have a major impact on modern healthcare systems, e.g., as the active pharmaceutical ingredients in anticancer drugs. A specific glycan profile is often necessary to achieve certain desirable activities, such as the effector functions of an antibody, receptor binding or a sufficient serum half-life. However, many expression systems produce glycan profiles that differ substantially from the preferred form (usually the form found in humans) or produce a diverse array of glycans with a range of in vivo activities, thus necessitating laborious and costly separation and purification processes. In contrast, protein glycosylation in plant cells is much more homogeneous than other systems, with only one or two dominant forms. Additionally, these glycan profiles tend to remain stable when the process and cultivation conditions are changed, making plant cells an ideal expression system to produce recombinant glycoproteins with uniform glycan profiles in a consistent manner. This chapter describes a protocol that uses fermentations using plant cell cultures to produce glycosylated proteins using two different types of bioreactors, a classical autoclavable STR 3-L and a wave reactor.

Putting the Spotlight Back on Plant Suspension Cultures

Plant cell suspension cultures have several advantages that make them suitable for the production of recombinant proteins. They can be cultivated under aseptic conditions using classical fermentation technology, they are easy to scale-up for manufacturing, and the regulatory requirements are similar to those established for well-characterized production systems based on microbial and mammalian cells. It is therefore no surprise that taliglucerase alfa (Elelyso®)-the first licensed recombinant pharmaceutical protein derived from plants-is produced in plant cell suspension cultures. But despite this breakthrough, plant cells are still largely neglected compared to transgenic plants and the more recent plant-based transient expression systems. Here, we revisit plant cell suspension cultures and highlight recent developments in the field that show how the rise of plant cells parallels that of Chinese hamster ovary cells, currently the most widespread and successful manufacturing platform for biologics. These developments include medium optimization, process engineering, statistical experimental designs, scale-up/scale-down models, and process analytical technologies. Significant yield increases for diverse target proteins will encourage a gold rush to adopt plant cells as a platform technology, and the first indications of this breakthrough are already on the horizon.

Towards industrial usefulness - cryo-cell-banking of transgenic BY-2 cell cultures

Plants have been established as an useful production system for commercially relevant proteins. Plant cell cultures show certain advantages compared to field-grown plants. However, one critical drawback for the long-term use of recombinant plant cell cultures is the instability of cell cultures concerning genetic background and productivity when maintained by subculturing. The ability to store recombinant cell lines stably by cryopreservation allows to maintain an efficient and stable production system. In this work, we describe the development of a cryopreservation protocol for a transgenic BY-2 cell culture expressing human serum albumin. In 1.75-L stirred-tank bioreactors growth and production kinetics of the transgenic cell line were compared after cryopreservation to the hitherto performed maintenance by subculturing. Growth and productivity of the cryopreserved cell culture remained stable after freezing for one week. Here, we show that we developed an efficient method which allows the storage of transgenic plant cell cultures, an important requirement for industrial processes.

A high-rate perfusion bioreactor for plant cells

A perfusion bioreactor allowing continuous extraction of secondary metabolites was designed and challenged for Eschscholtzia californica plant cell suspensions. Four sedimentation columns mounted inside a 2.5-L bioreactor separated single cells and cell aggregates from the culture medium. Cells were elicited with chitin at day 4 and the liquid medium free of cells and debris was then continuously pumped to the extraction columns containing fluidized XAD-7 resins, and then recirculated back to the cell suspension. A medium upward velocity corresponding to cell sedimentation velocity maintained a stable cell/medium separation front in the columns for sedimented cell volume (SCV) of 90% (70% packed cell volume, PCV). Two perfusion bioreactor cultures of 10 and 14 days were performed. A maximum dilution rate of 20.4/day was reached from day 4 to day 6, and was then reduced to 5/day at day 9 for 55% SCV. Control cultures were performed without and with free extraction resins into the cell suspension. Perfusion cultures showed similar specific growth rates of 0.24 +/- 0.04/day before and after elicitation. However, production level in the perfusion cultures was similar to that from the culture without resins with a maximum of 2.06 micromole/gDW total alkaloids, with 1.54 micromole/gDW in the resins. Cultures with free resins resulted in 30.94 micromole/gDW with 28.4 +/- 8.8 micromole/gDW in the resins. Difference in the cells nutritional state from elicitation was identified as a major cause in the production reduction. However, pathway to chelilutine was favored in the continuous extraction culture.

Plant cell cultures for the production of recombinant proteins

The use of whole plants for the synthesis of recombinant proteins has received a great deal of attention recently because of advantages in economy, scalability and safety compared with traditional microbial and mammalian production systems. However, production systems that use whole plants lack several of the intrinsic benefits of cultured cells, including the precise control over growth conditions, batch-to-batch product consistency, a high level of containment and the ability to produce recombinant proteins in compliance with good manufacturing practice. Plant cell cultures combine the merits of whole-plant systems with those of microbial and animal cell cultures, and already have an established track record for the production of valuable therapeutic secondary metabolites. Although no recombinant proteins have yet been produced commercially using plant cell cultures, there have been many proof-of-principle studies and several companies are investigating the commercial feasibility of such production systems.

The production of foreign proteins from genetically modified plant cells

While traditionally used to produce natural products, plant suspension cultures can also be utilized for the production of foreign proteins. Production of these high-value products in plant cells is an economically viable alternative to other systems, particularly in cases where the protein must be biologically active. There are several advantages to using plant cells for the large-scale production of secreted proteins. Plant cell media are composed of simple sugars and salts and are therefore less expensive and complex than mammalian media. Consequently, purification of secreted protein is simpler and more economical. Additionally, plant cell derived proteins are likely to be safer than those derived from other systems, since plant cell pathogens are not harmful to humans. In this chapter, we will review foreign protein production from plant cells. To begin, we will discuss the behavior of plant cell cultures, products produced by plant cells, protein secretion and its relationship to purification, and the performance of plant cells as compared to whole plants and other alternative hosts. After a brief discussion of gene transfer techniques, we will present strategies to overcome the limitations of protein production, including protein stabilization, novel production schemes, modeling, and scale-up considerations. To conclude, we will discuss implications for future development of this technology.

Molecular farming of pharmaceutical proteins

Molecular farming is the production of pharmaceutically important and commercially valuable proteins in plants. Its purpose is to provide a safe and inexpensive means for the mass production of recombinant pharmaceutical proteins. Complex mammalian proteins can be produced in transformed plants or transformed plant suspension cells. Plants are suitable for the production of pharmaceutical proteins on a field scale because the expressed proteins are functional and almost indistinguishable from their mammalian counterparts. The breadth of therapeutic proteins produced by plants range from interleukins to recombinant antibodies. Molecular farming in plants has the potential to provide virtually unlimited quantities of recombinant proteins for use as diagnostic and therapeutic tools in health care and the life sciences. Plants produce a large amount of biomass and protein production can be increased using plant suspension cell culture in fermenters, or by the propagation of stably transformed plant lines in the field. Transgenic plants can also produce organs rich in a recombinant protein for its long-term storage. This demonstrates the promise of using transgenic plants as bioreactors for the molecular farming of recombinant therapeutics, including vaccines, diagnostics, such as recombinant antibodies, plasma proteins, cytokines and growth factors.

Molecular farming of recombinant antibodies in plants

'Molecular farming' is the production of recombinant proteins in plants. It is intended to harness the power of agriculture to cultivate and harvest transgenic plants producing recombinant therapeutics. Molecular farming has the potential to provide virtually unlimited quantities of recombinant antibodies for use as diagnostic and therapeutic tools in both health care and the life sciences. Importantly, recombinant antibody expression can be used to modify the inherent properties of plants, for example by using expressed antipathogen antibodies to increase disease resistance. Plant transformation is technically straightforward for model plant species and some cereals, and the functional expression of recombinant proteins can be rapidly analyzed using transient expression systems in intact or virally infected plants. Protein production can then be increased using plant suspension cell production in fermenters, or by the propagation of stably transformed plant lines in the field. Transgenic plants can be exploited to produce organs rich in a recombinant protein for its long-term storage. This demonstrates the promise of using transgenic plants as bioreactors for the 'molecular farming' of recombinant therapeutics, blood substitutes and diagnostics, such as recombinant antibodies.

Expression of a carrot invertase gene in tobacco suspension cells cultivated in batch and continuous culture conditions

Plant cells (Nicotiana tabacum) were genetically modified to produce an heterologous protein, the acidic invertase from carrot, and invertase production from suspension tobacco cells was investigated. Suspension cultures were grown in shake flasks and stirred bioreactor. Total invertase activity was growth related. A 75 d continuous culture in 10 l bioreactor was performed. Our study demonstrates the high potential of plant cell cultures for long term production of heterologous protein.

Structures of N-linked oligosaccharides of glycoproteins from tobacco BY2 suspension cultured cells

The structures of N-linked sugar chains of glycoproteins expressed in tobacco BY2 cultured cells are reported. Five pyridylaminated (PA-) N-linked sugar chains were derived and purified from hydrazinolysates of the glycoproteins by reversed-phase HPLC and size-fractionation HPLC. The structures of the PA-sugar chains purified were identified by two-dimensional PA-sugar chain mapping, ion-spray MS/MS analysis, and exoglycosidase digestions. The five structures fell into two categories; the major class (92.5% as molar ratio) was a xylose containing-type (Man3Fuc1 Xyl1GlcNAc2 (41.0%), GlcNAc2Man3Fuc1Xyl1GlcNAc2 (26.5%), GlcNAc1Man3Fuc1Xyl1GlcNAc2 (21.7%), Man3 Xyl1GlcNAc2 (3.3%)), and the minor class was a high-mannose type (Man5GlcNAc2 (7.5%)). This is the first report to show that alpha(1-->3) fucosylation of N-glycans does occur but beta(1-->4) galactosylation of the sugar chains does not in the tobacco cultured cells.

Effect of oxygen supply on the suspension culture of genetically modified tobacco cells

The effect of oxygen supply on the cultivation of the genetically modified tobacco cells and the formation of a foreign protein, beta-glucuronidase (GUS), was investigated in 250-mL Erlenmeyer flasks, a 5-L stirred tank fermenter, and a 7-L air-lift fermenter. The oxygen supply was varied by using different volumes of medium in the case of the 250-mL Erlenmeyer flask culture or by the different aeration rate in the case of the two types of fermenters tested. Higher oxygen supply stimulated cell growth and increased oxygen consumption rate, the level of phenolics, and GUS productions.