Secretion of a functional single-chain Fv protein in transgenic tobacco plants and cell suspension cultures

In planta expression of specific single chain fragment antibody (scFv) against nucleocapsid protein of fig mosaic virus (FMV)

Fig mosaic virus (FMV) is recognized as the main viral agent associated with the mosaic disease (MD) of fig trees (Ficus carica). Due to its worldwide occurrence, FMV represents the most significant global threat to the production of fig fruit. A disease management strategy against the MD in fig orchards has never been effective; and therefore, expression of recombinant antibody in plant cells could provide an alternative approach to suppress FMV infections. In this study we focused on expressing a specific recombinant antibody, a single-chain variable fragment (scFv), targeting the nucleocapsid protein (NP) of FMV in planta. To accomplish this objective, we inserted the scFv gene into a plant expression vector and conducted transient expression in leaves of Nicotiana tabacum cv. Samson plants. The construct was transiently expressed in tobacco plants by agroinfiltration, and antibody of the anticipated size was detected by immunoblotting. The produced plantibody was then assessed for specificity using ELISA and confirmed by Western blot analysis. In this study, the plantibody developed against FMV could be considered as a potential countermeasure to the infection by conferring resistance to MD.

Current Strategies to Improve Yield of Recombinant Protein Production in Rice Suspension Cells

A plant cell-based recombinant glucocerebrosidase was approved by the FDA in 2012 for the treatment of human inherited Gaucher disease, indicating that plant suspension cells have advantages in biosafety and a low production cost as a commercial pharmaceutical recombinant protein expression system. A low allergenic rice suspension cell-based recombinant protein expression system controlled by the αAmy3/RAmy3D promoter has been shown to result in relatively high protein yields in plant cell-based systems. Although several recombinant proteins have been produced in rice suspension cell-based systems, yields must be improved to compete with the current commercial protein expression systems. Different strategies were performed and showed successful improvements in recombinant protein yields in this rice system. The review updates and highlights strategies for potential improvements of the αAmy3-based rice suspension cell-based system.

Optimizing antibody expression: The nuts and bolts

Antibodies are extensively utilized entities in biomedical research, and in the development of diagnostics and therapeutics. Many of these applications require high amounts of antibodies. However, meeting this ever-increasing demand of antibodies in the global market is one of the outstanding challenges. The need to maintain a balance between demand and supply of antibodies has led the researchers to discover better means and methods for optimizing their expression. These strategies aim to increase the volumetric productivity of the antibodies along with the reduction of associated manufacturing costs. Recent years have witnessed major advances in recombinant protein technology, owing to the introduction of novel cloning strategies, gene manipulation techniques, and an array of cell and vector engineering techniques, together with the progress in fermentation technologies. These innovations were also highly beneficial for antibody expression. Antibody expression depends upon the complex interplay of multiple factors that may require fine tuning at diverse levels to achieve maximum yields. However, each antibody is unique and requires individual consideration and customization for optimizing the associated expression parameters. This review provides a comprehensive overview of several state-of-the-art approaches, such as host selection, strain engineering, codon optimization, gene optimization, vector modification and process optimization that are deemed suitable for enhancing antibody expression.

Transgenic resistance

Transgenic resistance to plant viruses is an important technology for control of plant virus infection, which has been demonstrated for many model systems, as well as for the most important plant viruses, in terms of the costs of crop losses to disease, and also for many other plant viruses infecting various fruits and vegetables. Different approaches have been used over the last 28 years to confer resistance, to ascertain whether particular genes or RNAs are more efficient at generating resistance, and to take advantage of advances in the biology of RNA interference to generate more efficient and environmentally safer, novel "resistance genes." The approaches used have been based on expression of various viral proteins (mostly capsid protein but also replicase proteins, movement proteins, and to a much lesser extent, other viral proteins), RNAs [sense RNAs (translatable or not), antisense RNAs, satellite RNAs, defective-interfering RNAs, hairpin RNAs, and artificial microRNAs], nonviral genes (nucleases, antiviral inhibitors, and plantibodies), and host-derived resistance genes (dominant resistance genes and recessive resistance genes), and various factors involved in host defense responses. This review examines the above range of approaches used, the viruses that were tested, and the host species that have been examined for resistance, in many cases describing differences in results that were obtained for various systems developed in the last 20 years. We hope this compilation of experiences will aid those who are seeking to use this technology to provide resistance in yet other crops, where nature has not provided such.

Expression of synthetic human tropoelastin (hTE) protein in Nicotiana tabacum

Plant molecular farming (PMF) is an important growing prospective approach in plant biotechnology; it includes production of recombinant pharmaceutical and industrial proteins in large quantities from engineered plants. Elastin is a major protein component of tissues that require elasticity, it helps keep skin smooth as it stretches to allow normal. Elastin is used as a raw material for the cosmetic industry. In this work, we aimed to use plant as a bioreactor for the expression and production of the full human tropoelastin protein. Agrobacterium- mediated transient expression system into Nicotiana tabacum using syringe agroinfiltration was used to provide fast and convenient way to produce recombinant proteins with greater expression overall the plant leaf. This study aimed to establish an efficient and rapid system for transiently expression and production of human recombinant tropoelastin protein in transgenic N. tabacum plants. Modified elastin (ELN) gene was biosynthesized and cloned into pCambia1390 vector to be used into N. tabacum agroinfilteration. Optimization of codon usage for the human tropoelastin gene, without changing the primary structure of the protein was carried out to ensure high expression in tobacco plants. The obtained data proved that the 5(th) day post-infiltration is the optimum interval to obtain the maximum production of our recombinant protein. Southern blot analysis was able to detect 2175 bp fragment length representing the ELN orf (open reding frame). On the other hand, ELN -expression within plant's tissue was visualized by RT-PCR during the period 3-10 days post agroinfiltration. At the protein level, western and ELISA confirmed the expression of recombinant tropoelastin protein. Western blot analysis detected the tropoelastin protein as parent band at ∼70 kDa from freshly extracted protein, while two degraded bands of ∼55 and ∼45 kDa, representing a pattern of tropoelastin were appeared with frozen samples. This study showed that biosynthetic ELN gene was successfully expressed into N. tabacum leaves using agroinfiltration technique.

Expression of recombinant antibody (single chain antibody fragment) in transgenic plant Nicotiana tabacum cv. Xanthi

Plants offer an alternative inexpensive and convenient technology for large scale production of recombinant proteins especially recombinant antibodies (plantibodies). In this paper, we describe the expression of a model single chain antibody fragment (B6scFv) in transgenic tobacco. Four different gene constructs of B6scFv with different target signals for expression in different compartments of a tobacco plant cell with and without endoplasmic reticulum (ER) retention signal were used. Agrobacterium mediated plant transformation of B6scFv gene was performed with tobacco leaf explants and the gene in regenerated plants was detected using histochemical GUS assay and PCR. The expression of B6scFv gene was detected by western blotting and the recombinant protein was purified from putative transgenic tobacco plants using metal affinity chromatography. The expression level of recombinant protein was determined by indirect enzyme-linked immunosorbent assay. The highest accumulation of protein was found up to 3.28 % of the total soluble protein (TSP) in plants expressing B6scFv 1003 targeted to the ER, and subsequently expression of 2.9 % of TSP in plants expressing B6scFv 1004 (with target to apoplast with ER retention signal). In contrast, lower expression of 0.78 and 0.58 % of TSP was found in plants expressing antibody fragment in cytosol and apoplast, without ER retention signal. The described method/system could be used in the future for diverse applications including expression of other recombinant molecules in plants for immunomodulation, obtaining pathogen resistance against plant pathogens, altering metabolic pathways and also for the expression of different antibodies of therapeutic and diagnostic uses.

Comprehensive characterization of two different Nicotiana tabacum cell lines leads to doubled GFP and HA protein production by media optimization

For over two decades, plant cell cultures have been a promising research platform to express recombinant and therapeutic proteins such as hormones, growth factors, full-size antibodies and antigens. Chosen as a good host for manufacturing recombinant proteins, the Nicotiana tabacum L. cv. Bright Yellow 2 (BY-2) cell line has been studied in shake flasks by offline analysis of only a few growth parameters. The objective of this study is to comprehensively characterize the growth of a transgenic BY-2 cell line and to investigate the expression profile of the model protein GFP. Based on the correlations between nutrient consumption, cell growth and product formation, the intention is to improve the standard MS-medium. Hereby, multiple growth parameters were analyzed offline and online by using a respiration activity monitoring system (RAMOS). A reproducibly observed shift of the oxygen transfer rate (OTR) could be identified to indicate ammonium depletion in the medium. Concurrent with this ammonium depletion, the total protein concentration began to decrease. After the MS-medium was improved, the GFP concentration nearly doubled. When this improved ammonium enriched medium was applied to another transgenic tobacco cell line similar improvements to the amount of the glycoprotein influenza hemagglutinin (HA) produced by Nicotiana tabacum NT-1 cells could be achieved. Ultimately, this combined offline and online analysis can be successfully used for further cell line characterization and media optimization to improve growth and boost target product formation.

Differential N-glycosylation of a monoclonal antibody expressed in tobacco leaves with and without endoplasmic reticulum retention signal apparently induces similar in vivo stability in mice

Plant cells are able to perform most of the post-translational modifications that are required by recombinant proteins to achieve adequate bioactivity and pharmacokinetics. However, regarding N-glycosylation the processing of plant N-glycans in the Golgi apparatus displays major differences when compared with that of mammalian cells. These differences in N-glycosylation are expected to influence serum clearance rate of plant-derived monoclonal antibodies. The monoclonal antibody against the hepatitis B virus surface antigen expressed in Nicotiana tabacum leaves without KDEL endoplasmic reticulum (ER) retention signal (CB.Hep1(-)KDEL) and with a KDEL (Lys-Asp-Glu-Leu) fused to both IgG light and heavy chains (CB.Hep1(+)KDEL) were tested for in vivo stability in mice. Full characterization of N-glycosylation and aggregate formation in each monoclonal antibody batch was determined. The mouse counterpart (CB.Hep1) was used as control. Both (CB.Hep1(-)KDEL) and (CB.Hep1(+)KDEL) showed a faster initial clearance rate (first 24 h) compared with the analogous murine antibody while the terminal phase was similar in the three antibodies. Despite the differences between CB.Hep1(+)KDEL and CB.Hep1(-)KDEL N-glycans, the in vivo elimination in mice was indistinguishable from each other and higher than the murine monoclonal antibody. Molecular modelling confirmed that N-glycans linked to plantibodies were oriented away from the interdomain region, increasing the accessibility of the potential glycan epitopes by glycoprotein receptors that might be responsible for the difference in stability of these molecules.

Molecular Pharming: future targets and aspirations

Molecular Pharming represents an unprecedented opportunity to manufacture affordable modern medicines and make these available at a global scale. The area of greatest potential is in the prevention of infectious diseases, particular in underdeveloped countries where access to medicines and vaccines has historically been limited. This is why, at St. George's, we focus on diseases such as HIV, TB and rabies, and aim to develop production strategies that are simple and potentially easy to transfer to developing countries.

Expression of anti-sclerotinia scFv in transgenic Brassica napus enhances tolerance against stem rot

Canola is an important agricultural crop imparting a significant contribution to global oilseed production. As such, optimizing yield and quality is of paramount importance and canola production can be significantly affected by sclerotinia stem rot. The utility of recombinant antibody technology in plant protection has been explored by many researchers and shows promise for the generation of new lines of agriculturally significant crops with greater resistance to diseases. The objective of the current study was to generate recombinant pathogen specific antibody (scFv)-expressing transgenic Brassica napus plants with increased tolerance to the phytopathogenic fungus, Sclerotinia sclerotiorum. Transgenic canola (B. napus) lines expressing S. sclerotiorum-specific scFv antibody showed a significant level of tolerance towards S. sclerotiorum as compared to their non-transformed counterparts. Both incidence and progression of S. sclerotiorum-induced disease symptoms were reduced in plants expressing the recombinant scFv.

Soybean peroxidase propeptides are functional signal peptides and increase the yield of a foreign protein

Elements that contribute to the high, stable yield of soybean peroxidase (SBP) in soybean seed coats can be exploited in the development of this tissue as a protein production platform. SBP contains an N-terminal and a C-terminal propeptide that are predicted to direct vacuolar targeting; this may be one factor that contributes to its high yield and stability. We characterized the function of the SBP propeptides and investigated their ability to increase the yield of a foreign protein in a heterologous plant system. SBP propeptides are functional signal peptides capable of directing vacuolar transport in Arabidopsis. The use of these propeptides as well as an endoplasmic reticulum (ER)-retention signal to direct a foreign protein to the apoplast, ER, or vacuole can significantly increase yield and will therefore be useful for the development of the seed coat as a protein production platform. We also demonstrate that growth conditions may have a significant impact on the yield of a foreign protein and that this may be subcellular compartment-specific.

Production of recombinant proteins in suspension-cultured plant cells

Plants have emerged in the past decade as a suitable alternative to the current production systems for recombinant pharmaceutical proteins and, today their potential for low-cost production of high quality, much safer and biologically active mammalian proteins is largely documented. Among various plant expression systems being explored, genetically modified suspension-cultured plant cells offer a promising system for production of biopharmaceuticals. Indeed, when compared to other plant-based production platforms that have been explored, suspension-cultured plant cells have the advantage of being totally devoid of problems associated with the vagaries of weather, pest, soil and gene flow in the environment. Because of short growth cycles, the timescale needed for the production of recombinant proteins in plant cell culture can be counted in days or weeks after transformation compared to months needed for the production in transgenic plants. Moreover, recovery and purification of recombinant proteins from plant biomass is an expensive and technically challenging business that may amount to 80-94% of the final product cost. One additional advantage of plant cell culture is that the recombinant protein fused with a signal sequence can be expressed and secreted into the culture medium, and therefore recovered and purified in the absence of large quantities of contaminating proteins. Consequently, the downstream processing of proteins extracted from plant cell culture medium is less expensive, which may/does balance the higher costs of fermentation. When needed for clinical use, recombinant proteins are easily produced in suspension-cultured plant cells under certified, controllable and sterile conditions that offer improved safety and provide advantages for good manufacturing practices and regulatory compliance. In this chapter, we present basic protocols for rapid generation of transgenic suspension-cultured cells of Nicotiana tabacum, Oriza sativa and Arabidopis thaliana. These systems are powerful tools for plant-made pharmaceuticals production in highly controlled conditions.

The correlation between expression and localization of a foreign gene product in rice endosperm

Glucagon-like peptide 1 (GLP-1) is a 30 amino acid peptide hormone involved in insulin stimulation that is dependent upon blood glucose levels. We have previously reported that when this short peptide gene was directly expressed under the control of a glutelin promoter and its signal peptide, it was not accumulated in transgenic rice seed due to gene silencing. However, when the modified GLP-1 (mGLP-1) gene was enlarged to 5xmGLP-1 (mGLPx5) by tandem repeat, no silencing was observed. The mGLPx5 peptide could be accumulated in rice seed and its localization was mainly limited to the endoplasmic reticulum (ER). We also investigated alternative cellular localization sites that would increase accumulation. The relationship between the expression level and localization was examined by attaching the chitinase signal peptide to mGLPx5 to direct it into the intercellular space (apoplast), or by expression as a fusion protein with glutelin by insertion into a variable region of the acidic subunit, thus directing the peptide to protein body II (PB II). Attachment of the KDEL ER retention signal to the 6xmGLP-1 (mGLPx6) or its fusion to the C-terminus of the 13 kDa prolamin directed the peptide to the ER or PB I, respectively. Unexpectedly, these results indicated that mGLPx5 without any signal except for the glutelin signal peptide was accumulated to the greatest extent in rice endosperm. It can thus be concluded that the ER is a suitable intracellular organelle for accumulation of mGLPx5 peptide.

Production of human serum albumin by sugar starvation induced promoter and rice cell culture

Human serum albumin (HSA) is the most widely used clinical serum protein. Currently, commercial HSA can only be obtained from human plasma, due to lack of commercially feasible recombinant protein expression systems. In this study, inducible expression and secretion of HSA by transformed rice suspension cell culture was established. Mature form of HSA was expressed under the control of the sucrose starvation-inducible rice alpha Amy3 promoter, and secretion of HSA into the culture medium was achieved by using the alpha Amy3 signal sequence. High concentrations of HSA were secreted into culture medium in a short time (2-4 days) by sucrose depletion after cell concentrations had reached a peak density in culture medium containing sucrose. The recombinant HSA had the same electrophoretic mobility as commercial HSA and was stable and free from apparent proteolysis in the culture medium. In a flask scale culture with repeated sucrose provision-depletion cycles, HSA was stably produced with yields up to 11.5% of total medium proteins or 15 mg/L per cycle after each sucrose provision-depletion cycle. A bubble column type bioreactor was designed for production of HSA. In the bioreactor scale culture, HSA was produced with yields up to 76.4 mg/L 4 days after sucrose depletion. HSA was purified from the culture medium to high purity by a simple purification scheme. Enrichment of HSA in culture medium simplifies downstream purification, minimizes protease degradation, and may reduce production cost. The combination of a DNA construct containing the alpha Amy3 promoter and signal sequence, and the use of a rice suspension cell culture can provide an effective system for the production of recombinant pharmaceutical proteins.

Production of human alpha-1-antitrypsin from transgenic rice cell culture in a membrane bioreactor

Transgenic plant cell cultures offer a number of advantages over alternative host expression systems, but so far relatively low product concentrations have been achieved. In this study, transgenic rice cells are used in a two-compartment membrane bioreactor (CELLine 350, Integra Biosciences) for the production of recombinant alpha-1-antitrypsin (rAAT). Expression of rAAT is controlled by the rice alpha-amylase (RAmy3D) promoter, which is induced in the absence of sugar. The extracellular product is retained in the bioreactor's relatively small cell compartment, thereby increasing product concentration. Due to the packed nature of the cell aggregates in the cell compartment, a clarified product solution can be withdrawn from the bioreactor. Active rAAT reached levels of 100-247 mg/L (4-10% of the total extracellular protein) in the cell compartment at 5-6 days postinduction, and multiple inductions of the RAmy3D promoter were demonstrated.

Plant biopharming of monoclonal antibodies

Recent advances in molecular biology and plant biotechnology have shifted the concept of growing crops as a food source to serving as a bioreactor for the production of therapeutic recombinant proteins. Plants are potential biopharming factories because they are capable of producing unlimited numbers and amounts of recombinant proteins safely and inexpensively. In the last two decades, plant production systems have been developed for monoclonal antibody production, which has been useful in passive immunization of viral or bacterial diseases. Recently, a recombinant monoclonal antibody for rabies prophylaxis was produced in transgenic plants. Rabies virus epidemics remain still problematic throughout the world, and adequate treatment has been hampered by the worldwide shortage and high cost of prophylactic antibodies such as HRIG. Successful mass production of this monoclonal antibody in plants might help to overcome these problems. An effective plant production system for recombinant biologicals requires the appropriate heterologous plant expression system, the optimal combination of gene expression regulatory elements, control of post-translational processing of recombinant products, and efficient purification methods for product recovery. This review discusses recent biotechnology developments for plant-derived monoclonal antibodies and discusses these products as a promising approach to rabies prophylaxis and the consequence for global health benefits.

Plant-cyst nematode and plant-root-knot nematode interactions

Root-knot nematodes and cyst nematodes are obligate plant parasites that cause extensive damage to the agriculture of both temperate and tropical countries. In this review, Andreas Niebel, Godelieve Gheysen and Marc Van Montagu describe how, in the past decade, the use of molecular techniques has provided new insights in the complex interactions between these sedentary plant-parasitic nematodes and their infected host plants. They give an account of the progress in our understanding of both the parasite and the host during compatible and incompatible interactions. They also outline the importance of a new model host system. Arabidopsis thaliana.

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.

Expression of hepatitis B surface antigen in tobacco cell suspension cultures

Hepatitis B virus ' s ' gene coding for surface antigen was cloned into plant transformation vectors pHER100 and pHBs100 with and without endoplasmic reticulum retention signal, respectively. Transformed tobacco cell lines were analyzed for the integration of the transgene by PCR and Southern blot hybridization. Expression levels as determined by ELISA showed maximum expression levels of 2 microg HBsAg gm(-1) fresh weight and 10 ng mL(-1) of spent medium in pHER100 transformed cells. Western blot analysis confirmed the presence of 24 kDa band specific to HBsAg in the transformed cells. HBsAg was expressed both as intracellular and secreted forms in pHER100 transformed cells. The buoyant density in CsCl of HBsAg derived from pHBs100 transformed tobacco cells was determined and found to be 1.095 g mL(-1). HBsAg obtained from transformed tobacco cells is similar to the human serum derived one in buoyant density properties. This is the first report on the secretion of HBsAg particles by plant cells into the cell culture medium.

Plants and human health in the twenty-first century

The concept of growing crops for health rather than for food or fiber is slowly changing plant biotechnology and medicine. Rediscovery of the connection between plants and health is responsible for launching a new generation of botanical therapeutics that include plant-derived pharmaceuticals, multicomponent botanical drugs, dietary supplements, functional foods and plant-produced recombinant proteins. Many of these products will soon complement conventional pharmaceuticals in the treatment, prevention and diagnosis of diseases, while at the same time adding value to agriculture. Such complementation can be accelerated by developing better tools for the efficient exploration of diverse and mutually interacting arrays of phytochemicals and for the manipulation of the plant's ability to synthesize natural products and complex proteins. This review discusses the history, future, scientific background and regulatory issues related to botanical therapeutics.

Targeted expression of human serum albumin to potato tubers

Complementary DNA expression of mature human serum albumin was engineered into potato plants under the transcriptional control of patatin B33 promoter and potato proteinase inhibitor II terminator. Protein secretion was achieved by using the signal sequence from potato proteinase inhibitor II. Recombinant albumin accumulated up to 0.2% of total soluble tuber protein in single transformant lines, regardless of the potato cultivar used. Electrophoretic mobility and N-terminal amino acid sequence analysis of partially purified recombinant albumin confirmed proper processing of an immune responsive recombinant albumin, and revealed that the proteinase inhibitor II signal sequence was correctly removed. No further optimisation of these yields was obtained by HSA expression in patatin antisense plants (line Pas58). Subcellular localisation showed that recombinant protein was successfully targeted to the apoplast. Potato tubers may be used, by applying this technology, to produce other heterologous proteins of interest in the biopharmaceutical industry.

Combined use of regulatory elements within the cDNA to increase the production of a soluble mouse single-chain antibody, scFv, from tobacco cell suspension cultures

In order to facilitate production and secretion of a soluble form of a small, single-chain antibody ScFv (32 kDa) in tobacco cell suspension culture, several modifications were made simultaneously to the antibody cDNA that included elements that have been shown to regulate the expression of proteins in plants. The scFv cDNA was initially ligated into a binary vector under the control of the CaMV 35S promoter and the T7 terminator for expression in tobacco suspension culture. Subsequently, modifications were engineered into the cDNA for enhancement of scFv production. These included the following: (i) the signal peptide (SP) of the tobacco pathogenesis-related protein PR1a which was added in-frame to the N-terminal end of scFv cDNA; (ii) a 5'-nontranslated region from the tobacco etch virus (TEV leader sequence), which was fused to the N-terminal end of the SP; and (iii) the endoplasmic reticulum retention signal peptide KDEL, which was added to the C-terminal end of the scFv protein. Using a modified disruption method involving pectinase, the highest expression of total scFv (344 ng scFv/g cell) occurred when the plant leader sequence, the TEV sequence, and the KDEL peptide were all present in the expression construct. Although the addition of the KDEL sequence significantly increased the total yield of protein 5.4-fold, it did not increase the overall amount of protein secreted. These studies indicate that while the SP is very important in promoting secretion of the scFv, it had little influence on increasing scFv secretion levels even when both the TEV and the KDEL sequences significantly increased overall protein levels.

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.

Production of secretory IgA antibodies in plants

Functional antibodies produced in tobacco plants were first reported over a decade ago (1989). The basic protocol used to generate these 'plantibodies' involved the independent cloning of H and L chain antibody genes in Agrobacterium tumefaciens vectors, the transformation of plant tissue in vitro with the recombinant bacterium, the reconstitution of whole plants expressing individual chains, and their sexual cross. In a 'Mendelian' fashion, a fully assembled and functional antibody was recovered from plant tissue in some double-transgenic plants. In mammalian cells, the antibody H and L chains are produced as precursor proteins that are translocated into the endoplasmic reticulum (ER), under the guidance of signal sequences. Within the ER, the signal peptides are proteolytically cleaved, and several stress proteins act as chaperonins to bind the unassembled antibody chains, and direct subsequent folding and tetramer formation. A similar process occurs in plant cells, and expression can be directed via signal sequences (even of foreign origin) into the aqueous environment of the apoplasm, or to be accumulated in other specific plant tissues, including tubers, fruit, or seed. Plants can facilely assemble secretory IgA, which is comprised of four chains, H and L chains, J chain and secretory component. Plant 'bioreactors' are expected to yield over 10 kg of therapeutic antibody/acre in tobacco, maize, soybean, and alfalfa [(Ann. NY Acad. Sci.)721(1994)235; (Biotechnol. Bioeng.)20(1999)135]. Compared with conventional steel tank bioreactors using mammalian cells, or microorganisms, the costs of GMP plantibodies are expected to perhaps one tenth. The differences in glycosylation patterns of plant and mammalian cell produced antibodies apparently have no effect on antigen-binding or specificity, but there is some concern about potential immunogenicity in humans. N-linked glycans of plants differ from human by having fucose-linked alpha 1,3 and the sugar xylose. No adverse effects or human anti-mouse antibodies (HAMA) have been observed in >40 patients receiving topical oral application of a plant produced secretory IgA specific to Streptococcus mutans, for the control of caries [(Nat. Med.)4(1998)601]. The progressive improvement of expression vectors for plantibodies, and purification strategies, as well as the increase in transformable crop species, is expected to lead to almost limitless availability of inexpensive (even edible forms of) recombinant immunoglobulins free of human pathogens for human and animal therapy, and for novel industrial applications (e.g. catalytic antibodies).

High-yield expression of the recombinant, atrazine-specific Fab fragment K411B by the methylotrophic yeast Pichia pastoris

In this report, we describe the high-yield secretory expression ( approximately 40 mg x l(-1)) of pure, atrazine-specific Fab fragments (K411B) from Pichia pastoris that was achieved by co-integration of the genes encoding the heavy and light chains (both under the control of the alcohol oxidase promoter) into the genome of the yeast cells. Antibody-expressing clones were selected by SDS-PAGE and ELISA and fed-batch fermentations were carried out in a 5-l scale. Both chains of the Fab were successfully expressed upon methanol induction and almost no other proteins were secreted into the media. Approximately 30% of the two chains formed the active Fab fragment containing the intermolecular disulphide bond, as determined by Western blot analysis under non-reducing conditions. Crude culture supernatant was used to study the binding properties of the Fab fragment toward different s-triazines by means of competitive ELISA: the IC50 value for the detection of atrazine was determined from the standard curve as 3 microg x l(-1), which is one magnitude higher than the value obtained with the parental mAb K4E7 but equals that obtained when the same Fab fragment was expressed in Escherichia coli cells. In addition, the cross-reactivity pattern of the Fab from Pichia is comparable to that of E. coli and to the parental mAb K4E7.

Antibody-based resistance to plant pathogens

Plant diseases are a major threat to the world food supply, as up to 15% of production is lost to pathogens. In the past, disease control and the generation of resistant plant lines protected against viral, bacterial or fungal pathogens, was achieved using conventional breeding based on crossings, mutant screenings and backcrossing. Many approaches in this field have failed or the resistance obtained has been rapidly broken by the pathogens. Recent advances in molecular biotechnology have made it possible to obtain and to modify genes that are useful for generating disease resistant crops. Several strategies, including expression of pathogen-derived sequences or anti-pathogenic agents, have been developed to engineer improved pathogen resistance in transgenic plants. Antibody-based resistance is a novel strategy for generating transgenic plants resistant to pathogens. Decades ago it was shown that polyclonal and monoclonal antibodies can neutralize viruses, bacteria and selected fungi. This approach has been improved recently by the development of recombinant antibodies (rAbs). Crop resistance can be engineered by the expression of pathogen-specific antibodies, antibody fragments or antibody fusion proteins. The advantages of this approach are that rAbs can be engineered against almost any target molecule, and it has been demonstrated that expression of functional pathogen-specific rAbs in plants confers effective pathogen protection. The efficacy of antibody-based resistance was first shown for plant viruses and its application to other plant pathogens is becoming more established. However, successful use of antibodies to generate plant pathogen resistance relies on appropriate target selection, careful antibody design, efficient antibody expression, stability and targeting to appropriate cellular compartments.

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.

Production and characterization of biologically active human GM-CSF secreted by genetically modified plant cells

Human granulocyte-macrophage colony-stimulating factor (GM-CSF), a hemopoietic growth factor, was produced and secreted from tobacco cell suspensions. The GM-CSF cDNA was carried by a binary vector under the control of the CaMV 35S promoter and the T7 terminator. In addition, a 5'-nontranslated region from the tobacco etch virus (TEV leader sequence) was fused to the N-terminal end of the GM-CSF transgene. For ease of purification, a 6-His tag was added to the 3' end of the GM-CSF cDNA. Addition of the TEV leader sequence increased protein production more than twofold compared to non-TEV controls. Initial batch cultivation studies indicated a maximum of 250 microg/L extracellular and 150 microg/L intracellular GM-CSF. Western blot analysis detected multiple peptides with masses from 14 to 30 kDa in the extracellular medium. The plant-produced GM-CSF was biologically active and could be bound to a nickel affinity matrix, indicating that both the receptor-binding region and the 6-His tag were functional. The batch production of GM-CSF was compared with the production of other recombinant proteins secreted by transformed tobacco cells. The recovery of secreted GM-CSF was increased by the addition of stabilizing proteins and by increasing salt in the growth medium to physiological levels.

Rice cell culture as an alternative production system for functional diagnostic and therapeutic antibodies

We investigated the suitability of transformed rice cell lines as a system for the production of therapeutic recombinant antibodies. Expression constructs encoding a single-chain Fv fragment (scFvT84.66, specific for CEA, the carcinoembryonic antigen present on many human tumours) were introduced into rice tissue by particle bombardment. We compared antibody production levels when antibodies were either secreted to the apoplast or retained in the endoplasmic reticulum (ER) using a KDEL retention signal. Production levels were up to 14 times higher when antibodies were retained in the ER. Additionally, we compared construct sencoding different leader peptides (plant codon optimised murine immunoglobulin heavy and light chain leader peptides from mAb24) and carrying alternative 5' untranslated regions (the petunia chalcone synthase gene 5' UTR and the tobacco mosaic virus omega sequence). We observed no significant differences in antibody production levels among cell lines transformed with these constructs. The highest level of antibody production we measured was 3.8 micrograms g-1 callus (fresh weight). Immunological analysis of transgenic rice callus confirmed the presence of functional scFvT84.66. We discuss the potential merits of cell culture for the production of recombinant antibodies and other valuable macromolecules.

Rapid production of single-chain Fv fragments in plants using a potato virus X episomal vector

We have used a plant virus episomal vector, based on potato virus X (PVX) to transiently express a single-chain Fv (scFv) and its diabody derivative in plants. The scFv was directed against a continuous epitope (cryptotope) on the coat protein of potato virus V. A cloned, full-length PVX vector sequence, containing the scFv gene, was used to direct in vitro transcription and the resulting RNA was used to inoculate Nicotiana clevelandii plants. Within a few days, plants developed characteristic symptoms and immunoblot analysis showed that accumulation of scFv protein coincided with accumulation of PVX. Targeting of the scFv to the apoplast greatly increased protein accumulation compared with cytosolic scFv and produced more severe symptoms on infected plants. ELISA demonstrated that the scFv and diabody extracted from infected plants showed the same antigen-binding specificity as that of the parental monoclonal antibody. The PVX vector is a convenient, rapid, low-cost in planta expression system that can also be used for assessment of scFv production and function prior to stable plant transformation.

Transient expression of a tumor-specific single-chain fragment and a chimeric antibody in tobacco leaves

To evaluate the expression of different forms of a tumor-specific antibody in plants, we adapted a recently described Agrobacterium-mediated transient expression system. A recombinant single-chain Fv antibody (scFvT84.66) and a full-size mouse/human chimeric antibody (cT84.66) derived from the parental murine mAb T84. 66 specific for the human carcinoembryonic antigen were engineered into a plant expression vector. Chimeric T84.66 heavy and light chain genes were constructed by exchanging the mouse light and heavy chain constant domain sequences with their human counterparts and cloned into two independent plant expression vectors. In vivo assembly of full-size cT84.66 was achieved by simultaneous expression of the light and heavy chains after vacuum infiltration of tobacco leaves with two populations of recombinant Agrobacterium. Upscaling the transient system permitted purification of functional recombinant antibodies from tobacco leaf extracts within a week. His6-tagged scFvT84.66 was purified by immobilized metal affinity chromatography and cT84.66 by protein A affinity chromatography. Sufficient amounts of recombinant antibodies were recovered for detailed characterization by SDS/PAGE, Western blotting, and ELISA.

Apoplastic and cytosolic expression of full-size antibodies and antibody fragments in Nicotiana tabacum

We compared the expression of a functional recombinant TMV-specific full-size antibody (rAb29) in both the apoplast and cytosol of tobacco plants and a single chain antibody fragment (scFv29), derived from rAb29, was expressed in the cytosol. Cloned heavy and light chain cDNAs of full-size rAb29, which binds to TMV coat protein monomers, were integrated into the plant expression vector pSS. The full-size rAb29 was expressed in the cytosol and targeted to the apoplast by including the original murine antibody leader sequences. Levels of functional full-size rAb29 expression were high in the apoplast (up to 8.5 micrograms per gram leaf tissue), whereas cytosolic expression was low or at the ELISA detection limit. Sequences of the variable domains of rAb29 light and heavy chain were used to generate the single chain antibody of scFv29, which was expressed in the periplasmic space of E. coli and showed the same binding specificity as full-size rAb29. In addition, scFv29 was functionally expressed in the cytosol of tobacco plants and plant derived scFv29 maintained same binding specificity to TMV-coat protein monomers as rAb29.

Transgenic plants for therapeutic proteins: linking upstream and downstream strategies

We have described two very different and innovative plant-based production systems--postharvest production and recovery of recombinant product from tobacco leaves using an inducible promoter and oleosin-mediated recovery of recombinant product from oilseeds using a seed-specific promoter. Both base technologies are broadly applicable to numerous classes of pharmaceutical and industrial proteins. As with any emerging technology, the key to success may lie in identifying those products and applications that would most benefit from the unique advantages offered by each system. The postharvest tobacco leaf system appears effective for proteins requiring complex posttranslational processing and endomembrane targeting. Because of the remarkable fecundity and biomass production capacity of tobacco, biomass scale-up is very rapid and production costs are low. Clearly the development of equally cost-effective extraction and purification technologies will be critical for full realization of the commercial opportunities afforded by transgenic plant-based bioproduction. The recovery of protein from tobacco leaves or oleosin-partitioned proteins by oil-body separations represent significant break-throughs for cost-effective commercialization strategies. Additional low-cost, high-affinity separation technologies need to be developed for effective scale-up purification of plant-synthesized recombinant proteins. Clearly successful commercialization of plant-synthesized biopharmaceuticals must effectively link upstream strategies involving gene and protein design with downstream strategies for reproducible GMP-level recovery of bioactive recombinant protein. Both the tobacco and oilseed systems are uniquely designed to address issues of biomass storage, product recovery, quality assurance, and regulatory scrutiny in addition to issues of transgene expression and protein processing.

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.

A single-chain antibody fragment is functionally expressed in the cytoplasm of both Escherichia coli and transgenic plants

Despite the well-known crucial role of intradomain disulfide bridges for immunoglobulin folding and stability, the single-chain variable fragment of the anti-viral antibody F8 is functionally expressed when targeted to the reducing environment of the plant cytoplasm. We show here that this antibody fragment is also functionally expressed in the cytoplasm of Escherichia coli. A gel shift assay revealed that the single-chain variable fragment (scFv) accumulating in the plant and bacterial cytoplasm bears free sulfhydryl groups. Guanidinium chloride denaturation/renaturation studies indicated that refolding occurs even in a reducing environment, producing a functional molecule with the same spectral properties of the native scFv(F8). Taken together, these results suggest that folding and functionality of this antibody fragment are not prevented in a reducing environment. This antibody fragment could therefore represent a suitable framework for engineering recombinant antibodies to be targeted to the cytoplasm.

Expression and characterization of bispecific single-chain Fv fragments produced in transgenic plants

We describe the expression of the bispecific antibody biscFv2429 in transgenic suspension culture cells and tobacco plants. biscFv2429 consists of two single-chain antibodies, scFv24 and scFv29, connected by the Trichoderma reesi cellobiohydrolase I linker. biscFv2429 binds two epitopes of tobacco mosaic virus (TMV): the scFv24 domain recognizes neotopes of intact virions, and the scFv29 domain recognizes a cryptotope of the TMV coat protein monomer. biscFv2429 was functionally expressed either in the cytosol (biscFv2429-cyt) or targeted to the apoplast using a murine leader peptide sequence (biscFv2429-apoplast). A third construct contained the C-terminal KDEL sequence for retention in the ER (biscFv2429-KDEL). Levels of cytoplasmic biscFv2429 expression levels were low. The highest levels of antibody expression were for apoplast-targeted biscFv2429-apoplast and ER-retained biscFv2429-KDEL that reached a maximum expression level of 1.65% total soluble protein in transgenic plants. Plant-expressed biscFv2429 retained both epitope specificities, and bispecificity and bivalency were confirmed by ELISA and surface plasmon resonance analysis. This study establishes plant cells as an expression system for bispecific single-chain antibodies for use in medical and biological applications.

Use of plant roots for phytoremediation and molecular farming

Alternative agriculture, which expands the uses of plants well beyond food and fiber, is beginning to change plant biology. Two plant-based biotechnologies were recently developed that take advantage of the ability of plant roots to absorb or secrete various substances. They are (i) phytoextraction, the use of plants to remove pollutants from the environment and (ii) rhizosecretion, a subset of molecular farming, designed to produce and secrete valuable natural products and recombinant proteins from roots. Here we discuss recent advances in these technologies and assess their potential in soil remediation, drug discovery, and molecular farming.

Production of recombinant proteins in plant root exudates

The large-scale production of recombinant proteins in plants is limited by relatively low yields and difficulties in extraction and purification. These problems were addressed by engineering tobacco plants to continuously secrete recombinant proteins from their roots into a simple hydroponic medium. Three heterologous proteins of diverse origins (green fluorescent protein of jellyfish, human placental alkaline phosphatase [SEAP], and bacterial xylanase) were produced using the root secretion method (rhizosecretion). Protein secretion was dependent on the presence of the endoplasmic reticulum signal peptide fused to the recombinant protein sequence. All three secreted proteins retained their biological activity and, as shown for SEAP, accumulated in much higher amounts in the medium than in the root tissue.

Flow cytometry of plant cells with applications in large-scale bioprocessing

In recent years, there has been a significant upsurge in the application of flow cytometry to plant cells and plant cell cultures. As well as a range of uses in plant biology, flow cytometry offers many advantages for monitoring plant cell cultures used in large-scale bioprocessing operations. This review summarizes the current status of the field, concentrating on methods for DNA measurement and multiparameter cell cycle analysis. Techniques for screening and selection of elite cell lines with high productivity of secondary metabolites are also addressed.

Type II fish antifreeze protein accumulation in transgenic tobacco does not confer frost resistance

Type II fish antifreeze protein (AFP) is active in both freezing point depression and the inhibition of ice recrystallization. This extensively disulfide-bonded 14 kDa protein was targeted for accumulation in its pro- and mature forms in the cytosol and apoplast of transgenic tobacco plants. Type II AFP gene constructs under control of a duplicate cauliflower mosaic virus 35S promoter, both with and without a native plant transit peptide sequence, were introduced into tobacco by Agrobacterium tumefaciens-mediated transformation. AFP did not accumulate in the cytosol of transgenic plants, but active AFP was present as 2% the total protein present in the apoplast. Plant-produced AFP was the same size as mature Type II AFP isolated from fish, and was comparable to wild-type AFP in thermal hysteresis activity and its effect on ice crystal morphology. Field trials conducted in late summer on R1 generation transgenic plants showed similar AFP accumulation in plants under field conditions at levels suitable for large-scale production: but no difference in frost resistance was observed between transgenic and wild-type plants during the onset of early fall frosts.

Rapid production of specific vaccines for lymphoma by expression of the tumor-derived single-chain Fv epitopes in tobacco plants

Rapid production of protein-based tumor-specific vaccines for the treatment of malignancies is possible with the plant-based transient expression system described here. We created a modified tobamoviral vector that encodes the idiotype-specific single-chain Fv fragment (scFv) of the immunoglobulin from the 38C13 mouse B cell lymphoma. Infected Nicotiana benthamiana plants contain high levels of secreted scFv protein in the extracellular compartment. This material reacts with an anti-idiotype antibody by Western blotting, ELISA, and affinity chromatography, suggesting that the plant-produced 38C13 scFv protein is properly folded in solution. Mice vaccinated with the affinity-purified 38C13 scFv generate >10 micrograms/ml anti-idiotype immunoglobulins. These mice were protected from challenge by a lethal dose of the syngeneic 38C13 tumor, similar to mice immunized with the native 38C13 IgM-keyhole limpet hemocyanin conjugate vaccine. This rapid production system for generating tumor-specific protein vaccines may provide a viable strategy for the treatment of non-Hodgkin's lymphoma.