Assembly of an antibody and its derived antibody fragment in Nicotiana and Arabidopsis

Immunogenicity of Plant-Produced Human Papillomavirus (HPV) Virus-Like Particles (VLPs)

Cervical cancer is ranked fourth among the top cancers in women and is the second most common cancer in low- and middle-income regions, with ~570,000 new cases reported in 2018, which attributed to 84% of worldwide cervical cancer cases. Three commercially available prophylactic Human papillomavirus (HPV) vaccines are effective at preventing HPV infections. However, these vaccines are expensive due to their complex production systems, therefore limiting their use in developing countries. Recently, the use of plants to produce vaccines has emerged as a cost-effective alternative to conventionally used expression systems. Here, L1 proteins of eight high-risk (HPV 16, 18, 31, 33, 35, 45, 52, and 58) and two low risk (HPV 6 and 34) HPV types were successfully expressed in Nicotiana benthamiana, and transmission electron microscopy (TEM) analysis showed the presence of VLPs and/or capsomeres. Immunogenicity studies were conducted in mice utilizing HPV 35, 52, and 58 and showed that type-specific L1-specific antibodies were produced which were able to successfully neutralize homologous HPV pseudovirions in pseudovirion-based neutralization assays (PBNAs). This work demonstrated the potential for using plant-based transient expression systems to produce affordable and immunogenic HPV vaccines, particularly for developing countries.

Plant-Made Antibodies: Properties and Therapeutic Applications

BACKGROUND

A cost-effective plant platform for therapeutic monoclonal antibody production is both flexible and scalable. Plant cells have mechanisms for protein synthesis and posttranslational modification, including glycosylation, similar to those in animal cells. However, plants produce less complex and diverse Asn-attached glycans compared to animal cells and contain plant-specific residues. Nevertheless, plant-made antibodies (PMAbs) could be advantageous compared to those produced in animal cells due to the absence of a risk of contamination from nucleic acids or proteins of animal origin.

OBJECTIVE

In this review, the various platforms of PMAbs production are described, and the widely used transient expression system based on Agrobacterium-mediated delivery of genetic material into plant cells is discussed in detail.

RESULTS

We examined the features of and approaches to humanizing the Asn-linked glycan of PMAbs. The prospects for PMAbs in the prevention and treatment of human infectious diseases have been illustrated by promising results with PMAbs against human immunodeficiency virus, rotavirus infection, human respiratory syncytial virus, rabies, anthrax and Ebola virus. The pre-clinical and clinical trials of PMAbs against different types of cancer, including lymphoma and breast cancer, are addressed.

CONCLUSION

PMAb biosafety assessments in patients suggest that it has no side effects, although this does not completely remove concerns about the potential immunogenicity of some plant glycans in humans. Several PMAbs at various developmental stages have been proposed. Promise for the clinical use of PMAbs is aimed at the treatment of viral and bacterial infections as well as in anti-cancer treatment.

The Effect of Methyl Jasmonate and Temperature on the Transient Expression of Recombinant Proteins in Cucurbita pepo L

The aim of this study is to assess the effect of methyl jasmonate (MeJA) and temperature on the valuable pharmaceuticals expression in a virus-mediated transient expression system, and so the Zuchini Yellow Mosaic Virus (ZYMV) based vector was used for transferring the GFP reporter gene and recombinant tissue plasminogen activator (rtPA) gene (K2S) to cucurbit (Cucurbita pepo L.). MeJA, temperature and time (days after inoculation), were evaluated as a factorial experiment in a completely randomized design (CRD). At first, the effect of all treatment combinations on GFP expression was assessed. At this step, the ELISA test was used to select the optimum treatment combination. ELISA method revealed the significant difference between applied treatments. The optimized treatment significantly increased the expression of rtPA compared to the control. The Real-Time PCR reaction for both GFP and rtPA genes showed no significant differences between optimum and control treatments, however, transcripts of the small subunit of RuBisCO were extremely down-regulated in optimum treatment condition. Reduction in RuBisCO expression at protein level was tangible under treatment condition based on the ELISA test. Therefore, it can be inferred that suppressing the expression of RuBisCO, probably resulted in higher access of expression system to free amino acids inside the cell. In this study, MeJA has been shown to be a positive factor, but the low temperature (17 °C), unlike previous studies, suppressed the expression of recombinant protein unexpectedly, probably due to the incompatibility of the viral construct with low temperature. In conclusion, the use of a suitable gene construct, which is not sensitive to temperature, is likely to result in a more favorable outcome.

Antibody Production in Plants and Green Algae

Monoclonal antibodies (mAbs) have a wide range of modern applications, including research, diagnostic, therapeutic, and industrial uses. Market demand for mAbs is high and continues to grow. Although mammalian systems, which currently dominate the biomanufacturing industry, produce effective and safe recombinant mAbs, they have a limited manufacturing capacity and high costs. Bacteria, yeast, and insect cell systems are highly scalable and cost effective but vary in their ability to produce appropriate posttranslationally modified mAbs. Plants and green algae are emerging as promising production platforms because of their time and cost efficiencies, scalability, lack of mammalian pathogens, and eukaryotic posttranslational protein modification machinery. So far, plant- and algae-derived mAbs have been produced predominantly as candidate therapeutics for infectious diseases and cancer. These candidates have been extensively evaluated in animal models, and some have shown efficacy in clinical trials. Here, we review ongoing efforts to advance the production of mAbs in plants and algae.

Identification, Characterization and Down-Regulation of Cysteine Protease Genes in Tobacco for Use in Recombinant Protein Production

Plants are an attractive host system for pharmaceutical protein production. Many therapeutic proteins have been produced and scaled up in plants at a low cost compared to the conventional microbial and animal-based systems. The main technical challenge during this process is to produce sufficient levels of recombinant proteins in plants. Low yield is generally caused by proteolytic degradation during expression and downstream processing of recombinant proteins. The yield of human therapeutic interleukin (IL)-10 produced in transgenic tobacco leaves was found to be below the critical level, and may be due to degradation by tobacco proteases. Here, we identified a total of 60 putative cysteine protease genes (CysP) in tobacco. Based on their predicted expression in leaf tissue, 10 candidate CysPs (CysP1-CysP10) were selected for further characterization. The effect of CysP gene silencing on IL-10 accumulation was examined in tobacco. It was found that the recombinant protein yield in tobacco could be increased by silencing CysP6. Transient expression of CysP6 silencing construct also showed an increase in IL-10 accumulation in comparison to the control. Moreover, CysP6 localizes to the endoplasmic reticulum (ER), suggesting that ER may be the site of IL-10 degradation. Overall results suggest that CysP6 is important in determining the yield of recombinant IL-10 in tobacco leaves.

Site-specific proteolytic degradation of IgG monoclonal antibodies expressed in tobacco plants

Plants are promising hosts for the production of monoclonal antibodies (mAbs). However, proteolytic degradation of antibodies produced both in stable transgenic plants and using transient expression systems is still a major issue for efficient high-yield recombinant protein accumulation. In this work, we have performed a detailed study of the degradation profiles of two human IgG1 mAbs produced in plants: an anti-HIV mAb 2G12 and a tumour-targeting mAb H10. Even though they use different light chains (κ and λ, respectively), the fragmentation pattern of both antibodies was similar. The majority of Ig fragments result from proteolytic degradation, but there are only a limited number of plant proteolytic cleavage events in the immunoglobulin light and heavy chains. All of the cleavage sites identified were in the proximity of interdomain regions and occurred at each interdomain site, with the exception of the VL /CL interface in mAb H10 λ light chain. Cleavage site sequences were analysed, and residue patterns characteristic of proteolytic enzymes substrates were identified. The results of this work help to define common degradation events in plant-produced mAbs and raise the possibility of predicting antibody degradation patterns 'a priori' and designing novel stabilization strategies by site-specific mutagenesis.

Expression of a single-chain variable-fragment antibody against a Fusarium virguliforme toxin peptide enhances tolerance to sudden death syndrome in transgenic soybean plants

Plants do not produce antibodies. However, plants can correctly assemble functional antibody molecules encoded by mammalian antibody genes. Many plant diseases are caused by pathogen toxins. One such disease is the soybean sudden death syndrome (SDS). SDS is a serious disease caused by the fungal pathogen Fusarium virguliforme. The pathogen, however, has never been isolated from diseased foliar tissues. Thus, one or more toxins produced by the pathogen have been considered to cause foliar SDS. One of these possible toxins, FvTox1, was recently identified. We investigated whether expression of anti-FvTox1 single-chain variable-fragment (scFv) antibody in transgenic soybean can confer resistance to foliar SDS. We have created two scFv antibody genes, Anti-FvTox1-1 and Anti-FvTox1-2, encoding anti-FvTox1 scFv antibodies from RNAs of a hybridoma cell line that expresses mouse monoclonal anti-FvTox1 7E8 antibody. Both anti-FvTox1 scFv antibodies interacted with an antigenic site of FvTox1 that binds to mouse monoclonal anti-FvTox1 7E8 antibody. Binding of FvTox1 by the anti-FvTox1 scFv antibodies, expressed in either Escherichia coli or transgenic soybean roots, was initially verified on nitrocellulose membranes. Expression of anti-FvTox1-1 in stable transgenic soybean plants resulted in enhanced foliar SDS resistance compared with that in nontransgenic control plants. Our results suggest that i) FvTox1 is an important pathogenicity factor for foliar SDS development and ii) expression of scFv antibodies against pathogen toxins could be a suitable biotechnology approach for protecting crop plants from toxin-induced diseases.

Chlamydomonas reinhardtii as a viable platform for the production of recombinant proteins: current status and perspectives

Chlamydomonas reinhardtii has many advantages compared with traditional systems for the molecular farming of recombinant proteins. These include low production costs, rapid scalability at pilot level, absence of human pathogens and the ability to fold and assemble complex proteins accurately. Currently, the successful expression of several proteins with pharmaceutical relevance has been reported from the nuclear and the chloroplastic genome of this alga, demonstrating its usefulness for biotechnological applications. However, several factors affect the level of recombinant protein expression in Chlamydomonas such as enhancer elements, codon dependency, sensitivity to proteases and transformation-associated genotypic modification. The present review outlines a number of strategies to increase protein yields and summarizes recent achievements in algal protein production including biopharmaceuticals such as vaccines, antibodies, hormones and enzymes with implications on health-related approaches. The current status of bioreactor developments for algal culture and the challenges of scale-up and optimization processes are also discussed.

Antibody degradation in tobacco plants: a predominantly apoplastic process

BACKGROUND

Interest in using plants for production of recombinant proteins such as monoclonal antibodies is growing, but proteolytic degradation, leading to a loss of functionality and complications in downstream purification, is still a serious problem.

RESULTS

In this study, we investigated the dynamics of the assembly and breakdown of a human IgG(1)κ antibody expressed in plants. Initial studies in a human IgG transgenic plant line suggested that IgG fragments were present prior to extraction. Indeed, when the proteolytic activity of non-transgenic Nicotiana tabacum leaf extracts was tested against a human IgG1 substrate, little activity was detectable in extraction buffers with pH > 5. Significant degradation was only observed when the plant extract was buffered below pH 5, but this proteolysis could be abrogated by addition of protease inhibitors. Pulse-chase analysis of IgG MAb transgenic plants also demonstrated that IgG assembly intermediates are present intracellularly and are not secreted, and indicates that the majority of proteolytic degradation occurs following secretion into the apoplastic space.

CONCLUSIONS

The results provide evidence that proteolytic fragments derived from antibodies of the IgG subtype expressed in tobacco plants do not accumulate within the cell, and are instead likely to occur in the apoplastic space. Furthermore, any proteolytic activity due to the release of proteases from subcellular compartments during tissue disruption and extraction is not a major consideration under most commonly used extraction conditions.

Stable expression and purification of a functional processed Fab' fragment from a single nascent polypeptide in CHO cells expressing the mCAT-1 retroviral receptor

Monoclonal antibodies and derivative formats such as Fab' fragments are used in a broad range of therapeutic, diagnostic and research applications. New systems and methodologies that can improve the production of these proteins are consequently of much interest. Here we present a novel approach for the rapid production of processed Fab' fragments in a CHO cell line that has been engineered to express the mouse cationic amino acid transporter receptor 1 (mCAT-1). This facilitated the introduction of the target antibody gene through retroviral transfection, rapidly producing stable expression. Using this system, we designed a single retroviral vector construct for the expression of a target Fab' fragment as a single polypeptide with a furin cleavage site and a FMDV 2A self-cleaving peptide introduced to bridge the light and truncated heavy chain regions. The introduction of these cleavage motifs ensured equimolar expression and processing of the heavy and light domains as exemplified by the production of an active chimeric Fab' fragment against the Fas receptor, routinely expressed in 1-2mg/L yield in spinner-flask cell cultures. These results demonstrate that this method could have application in the facile production of bioactive Fab' fragments.

Plant bioreactors for pharmaceuticals

Plant bioreactors are attractive expression systems for economic production of pharmaceuticals. Various plant expression systems or platforms have been tested with certain degrees of success over the past years. However, further development and improvement are needed for more effective plant bioreactors. In this review we first summarize recent progress in various plant bioreactor expression systems and then focus on discussing protein compartmentation to unique organelles and various strategies for developing better plant bioreactors.

Expression of human apolipoprotein A-I in Nicotiana tabacum

Several transgenic tobacco lines expressing human apolipoprotein A-I (ApoA-I) were obtained. Western blot analyses indicated the expression of the recombinant protein in plant organs at various stages of development, including senescent leaves. A cell line expressing human ApoA-I was established from a T(1) transgenic plant. Recombinant ApoA-I was isolated either from extracts of transgenic leaves and from the culture medium of transgenic cells using an antibody-based one-step procedure.

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.

Production of antibodies in plants: status after twenty years

Thanks to their potential to bind virtually all types of molecules; monoclonal antibodies are in increasing demand as therapeutics and diagnostics. To overcome the overloading of current production facilities, alternative expression systems have been developed, of which plants appear the most promising. In this review, we focus on the expression of monoclonal IgG or IgM in plant species. We analyse the data for 32 different antibodies expressed in various ways, differing in DNA construction, transformation method, signal peptide source, presence or absence of an endoplasmic reticulum retention sequence, host species and the organs tested, together resulting in 98 reported combinations. A large heterogeneity is found in the quantity and quality of the antibody produced. We discuss in more detail the strategy used to express both chains, the nature of the transcription promoters, subcellular localization and unintended proteolysis, when encountered.

Manufacturing antibodies in the plant cell

Plants have long been considered advantageous platforms for large-scale production of antibodies due to their low cost, scalability, and the low chances of pathogen contamination. Much effort has therefore been devoted to efficiently producing mAbs (from nanobodies to secretory antibodies) in plant cells. Several technical difficulties have been encountered and are being overcome. Improvements in production levels have been achieved by manipulation of gene expression and, more efficiently, of cell targeting and protein folding and assembly. Differences in mAb glycosylation patterns between animal and plant cells are being successfully addressed by the elimination and introduction of the appropriate enzyme activities in plant cells. Another relevant battlefield is the dichotomy between production capacity and speed. Classically, stably transformed plant lines have been proposed for large scale mAb production, whereas the use of transient expression systems has always provided production speed at the cost of scalability. However, recent advances in transient expression techniques have brought impressive yield improvements, turning speed and scalability into highly compatible assets. In the era of personalized medicines, the combination of yield and speed, and the advances in glyco-engineering have made the plant cell a serious contender in the field of recombinant antibody production.

Generation of transgenic plants expressing antibodies to the environmental pollutant microcystin-LR

We describe the engineering, regeneration, and characterization of transgenic tobacco plants expressing a recombinant antibody specific to microcystin-LR (MC-LR), the environmental toxin pollutant produced by species of cyanobacteria. The antibody was created by a genetic fusion of the antigen-binding regions of the microcystin-specific single-chain antibody, 3A8, with constant regions from the murine IgG1kappa, Guy's 13. IgG transgenes were controlled by a leader peptide that targets the transgene products to the secretory pathway and also allows for rhizosecretion. The antibody, extracted from the leaves or rhizosecreted into hydroponic medium by transgenic plants, was shown to have functional binding to MC-LR. Antibody yields in transgenic plant leaves reached a maximum of 64 microg/g leaf fresh weight (0.6% total soluble protein), and the rate of antibody rhizosecretion reached a maximum of 5 microg/g root dry weight/24 h. Rhizosecreted antibody bound to MC-LR in hydroponic medium, and transgenic plants grew more efficiently on medium containing MC-LR compared to wild-type controls. This proof of concept paves the way for applications to produce diagnostic antibodies to microcystin-LR, remove it from the environment by phytoremediation, or enhance yields in crops exposed to MC-LR.-Drake, P. M. W., Barbi, T., Drever, M. R., van Dolleweerd, C. J., Porter, A. J. R., Ma, J. K.-C. Generation of transgenic plants expressing antibodies to the environmental pollutant microcystin-LR.

The T-DNA integration pattern in Arabidopsis transformants is highly determined by the transformed target cell

Transgenic loci obtained after Agrobacterium tumefaciens-mediated transformation can be simple, but fairly often they contain multiple T-DNA copies integrated into the plant genome. To understand the origin of complex T-DNA loci, floral-dip and root transformation experiments were carried out in Arabidopsis thaliana with mixtures of A. tumefaciens strains, each harboring one or two different T-DNA vectors. Upon floral-dip transformation, 6-30% of the transformants were co-transformed by multiple T-DNAs originating from different bacteria and 20-36% by different T-DNAs from one strain. However, these co-transformation frequencies were too low to explain the presence of on average 4-6 T-DNA copies in these transformants, suggesting that, upon floral-dip transformation, T-DNA replication frequently occurs before or during integration after the transfer of single T-DNA copies. Upon root transformation, the co-transformation frequencies of T-DNAs originating from different bacteria were similar or slightly higher (between 10 and 60%) than those obtained after floral-dip transformation, whereas the co-transformation frequencies of different T-DNAs from one strain were comparable (24-31%). Root transformants generally harbor only one to three T-DNA copies, and thus co-transformation of different T-DNAs can explain the T-DNA copy number in many transformants, but T-DNA replication is postulated to occur in most multicopy root transformants. In conclusion, the comparable co-transformation frequencies and differences in complexity of the T-DNA loci after floral-dip and root transformations indicate that the T-DNA copy number is highly determined by the transformation-competent target cells.

The production of biopharmaceuticals in plant systems

Biopharmaceuticals present the fastest growing segment in the pharmaceutical industry, with an ever widening scope of applications. Whole plants as well as contained plant cell culture systems are being explored for their potential as cheap, safe, and scalable production hosts. The first plant-derived biopharmaceuticals have now reached the clinic. Many biopharmaceuticals are glycoproteins; as the Golgi N-glycosylation machinery of plants differs from the mammalian machinery, the N-glycoforms introduced on plant-produced proteins need to be taken into consideration. Potent systems have been developed to change the plant N-glycoforms to a desired or even superior form compared to the native mammalian N-glycoforms. This review describes the current status of biopharmaceutical production in plants for industrial applications. The recent advances and tools which have been utilized to generate glycoengineered plants are also summarized and compared with the relevant mammalian systems whenever applicable.

Different subcellular localization and glycosylation for a functional antibody expressed in Nicotiana tabacum plants and suspension cells

Genes encoding the heavy and light chains of LO-BM2, a therapeutic IgG antibody, were assembled in the tandem or inverted convergent orientation and expressed in Nicotiana tabacum plants and BY-2 suspension cells. The tandem construct allowed higher expression in both expression systems. A similar degradation pattern was observed for the secreted antibody recovered from the leaf intercellular fluid and BY-2 culture medium. Degradation increased with leaf age or culture time. Antibodies purified from leaf tissues and BY-2 cells were both functional. However, MS analysis of the N-glycosylation showed complex plant-type glycans to be the major type in the antibody purified from plants, whereas, oligomannosidic was the major glycosylation type in that purified from BY-2 cells. LO-BM2 was observed mainly in the endoplasmic reticulum of BY-2 cells while, in leaf cells, it was localized mostly to vesicles resembling prevacuolar compartments. These results and those from endoglycosidase H studies suggest that LO-BM2 is secreted from BY-2 cells more readily than from leaf cells where it accumulates in a post-Golgi compartment.

Foreign protein production using plant cell and organ cultures: Advantages and limitations

Plants and plant tissue cultures are used as host systems for expression of foreign proteins including antibodies, vaccines and other therapeutic agents. Recombinant or stably transformed plants and plant cell cultures have been applied for foreign protein production for about 20 years. Because the product concentration achieved exerts a major influence on process economics, considerable efforts have been made by commercial and academic research groups to improve foreign protein expression levels. However, post-synthesis product losses due to protease activity within plant tissues and/or extracellular protein adsorption in plant cell cultures can negate the benefits of molecular or genetic enhancement of protein expression. Transient expression of foreign proteins using plant viral vectors is also a practical approach for producing foreign proteins in plants. Adaptation of this technology is required to allow infection and propagation of engineered viruses in plant tissue cultures for transient protein expression in vitro.

Fluorescent protein fusions to a human immunodeficiency virus monoclonal antibody reveal its intracellular transport through the plant endomembrane system

SUMMARY

In order to further understand the production and intracellular trafficking of pharmaceutical proteins in plants, the light and heavy chains (LC and HC) of the human immunodeficiency virus neutralizing monoclonal antibody 2G12 were fused to fluorescent proteins [Venus and monomeric red fluorescent protein (mRFP)] to enable the visualization of their passage through the plant cell. Co-expression of LC and HC with various markers of the endomembrane system demonstrated that LC fusions were found in mobile punctate structures, which are likely to be pre-vacuolar compartments (PVCs) as a proportion of the LC fusions were found to be located in the vacuole. In addition, apoplast labelling was also observed with a 2G12LC-RFP fusion. The HC fusion expressed alone was found only in the endoplasmic reticulum (ER). When the LC and HC fusions were expressed together, they were found to co-locate to larger punctate structures, which were morphologically distinct from any observed on expression of LC or HC alone. These structures appeared to be in close association with the ER and their labelling partially overlapped with PVC marker fluorescence, but no increase in apoplast labelling was observed. Co-immunoprecipitation data demonstrated that the presence of the fluorescent proteins did not affect the assembly of the antibody, and also showed the association of BiP with the antibody chains. The antigen-binding activity of the Venus-fused 2G12 antibody was confirmed by enzyme-linked immunosorbent assay.

Production of an anti-mouse MHC class II monoclonal antibody with biological activity in transgenic tobacco

To produce a monoclonal antibody specific to a mouse major histocompatibility complex (MHC) class II protein, we synthesized the complementary DNAs for the heavy and light chains of a monoclonal antibody by PCR amplification. These cDNAs were then introduced separately into tobacco plant cells. After performing Northern blot analysis to confirm the expression of each of the chain genes in the transformed plants, we constructed transgenic plants expressing both the heavy and light chains by sexual crossing. The expression of the heavy and light chain genes in the sexually crossed plant was confirmed by Northern and Western blot analyses, respectively. Fluorocytometric analysis showed that the plant-derived antibodies, which we purified using a protein G affinity column, bound specifically to target cells that expressed the cognate MHC class II molecules on their cell surfaces. The results of this study demonstrate that a monoclonal antibody against mouse MHC class II proteins can be expressed in transgenic plants. They also show the specific binding activity of plant-derived antibodies to cognate antigens.

Pharming and transgenic plants

Plant represented the essence of pharmacopoeia until the beginning of the 19th century when plant-derived pharmaceuticals were partly supplanted by drugs produced by the industrial methods of chemical synthesis. In the last decades, genetic engineering has offered an alternative to chemical synthesis, using bacteria, yeasts and animal cells as factories for the production of therapeutic proteins. More recently, molecular farming has rapidly pushed towards plants among the major players in recombinant protein production systems. Indeed, therapeutic protein production is safe and extremely cost-effective in plants. Unlike microbial fermentation, plants are capable of carrying out post-translational modifications and, unlike production systems based on mammalian cell cultures, plants are devoid of human infective viruses and prions. Furthermore, a large panel of strategies and new plant expression systems are currently developed to improve the plant-made pharmaceutical's yields and quality. Recent advances in the control of post-translational maturations in transgenic plants will allow them, in the near future, to perform human-like maturations on recombinant proteins and, hence, make plant expression systems suitable alternatives to animal cell factories.

Expression and characterization of a recombinant single-domain monoclonal antibody against MUC1 mucin in tobacco plants

A promising alternative to conventional antibodies is the single-domain antibody fragment of the Camelidae (V(HH)), which (because of features such as small length, high expression, solubility, and stability) is preferred to other antibody derivatives. In this report, a recombinant single-domain antibody (V(HH)) against MUC1 mucin in the tobacco plant, which may be considered as a suitable and economical alternative expression system, was produced. This antibody was expressed under the control of a strong constitutive promoter, CaMV35S, and NOS terminator. A plant high-expression sequence (Kozak sequence) was linked at the 5' end for overexpression of the V(HH) gene. The constructed cassette (pBIV(HH)) was transferred to agrobacterium, and the VHH gene was inserted into the plant genome by agrobacterium-mediated transformation. Transgenic lines were selected on kanamycin (100 mg/L) and maintained in soil, and subsequent generations were obtained. The presence and expression of the transgene was confirmed in the transformants by polymerase chain reaction (PCR), enzyme-linked immunosorbent assay (ELISA), and Western blot. Tobacco transgenic lines leave expressed V(HH) at levels varying from 1.12% to 1.63% of the total soluble protein. This report examines the transformation and expression of recombinant single-domain antibody (V(HH)) against antigen-associated tumor in tobacco plants.

Cosecretion of protease inhibitor stabilizes antibodies produced by plant roots

A plant-based system for continuous production of monoclonal antibodies based on the secretion of immunoglobulin complexes from plant roots into a hydroponic medium (rhizosecretion) was engineered to produce high levels of single-chain and full-size immunoglobulins. Replacing the original signal peptides of monoclonal antibodies with a plant-derived calreticulin signal increased the levels of antibody yield 2-fold. Cosecretion of Bowman-Birk Ser protease inhibitor reduced degradation of the immunoglobulin complexes in the default secretion pathway and further increased antibody production to 36.4 microg/g root dry weight per day for single-chain IgG1 and 21.8 microg/g root dry weight per day for full-size IgG4 antibodies. These results suggest that constitutive cosecretion of a protease inhibitor combined with the use of the plant signal peptide and the antibiotic marker-free transformation system offers a novel strategy to achieve high yields of complex therapeutic proteins secreted from plant roots.

Foreign protein degradation and instability in plants and plant tissue cultures

Low production cost is a key factor driving the development of plants and plant tissue cultures for the synthesis of therapeutic and other foreign proteins. Because product yield and concentration exert a major influence on process economics, improving foreign protein accumulation is crucial for enhancing the commercial success of plant-based production systems. Strategies aimed at increasing transgene expression have been effective; however, a critical but poorly understood factor contributing to low foreign protein yield is post-synthesis and/or post-secretion instability and degradation. Loss of foreign protein as result of biological and physical processes such as proteolytic destruction and irreversible surface adsorption can occur in plants and plant culture systems. This review highlights the need to consider such mechanisms and outlines a range of remedial strategies aimed at minimizing foreign protein degradation and loss.

Introduction of silencing-inducing transgenes does not affect expression of known transcripts

While the RNA interference (RNAi) mechanism has only been discovered a decade ago, RNAi is now often used to study gene function by sequence-specific knockdown of gene expression. However, it is still unknown whether introduction of silencing-inducing transgenes alters the transcriptome. To address this question, genome-wide transcriptional changes in silenced and non-silenced backgrounds were monitored through microarray analysis. No significant transcriptional changes were detected when compared to the non-silenced control. This result was confirmed by real-time polymerase chain reaction analysis of genes known to be involved in RNA silencing. In conclusion, introduction of silencing-inducing constructs does not affect expression of known transcripts in other genes than in those homologous to the targeted ones. Consequently, when gene function is studied by RNAi, the transcriptional changes detected will specifically be the result of knockout of the gene of interest, at least for the genes present on the array used in our study.

HIV-1 p24-immunoglobulin fusion molecule: a new strategy for plant-based protein production

We describe the engineering of a human immunodeficiency virus-1 (HIV-1) p24-immunoglobulin A (IgA) antigen-antibody fusion molecule for therapeutic purposes and its enhancing effect on fused antigen expression in tobacco plants. Although many recombinant proteins have been expressed in transgenic plants as vaccine candidates, low levels of expression are a recurring problem. In this paper, using the HIV p24 core antigen as a model vaccine target, we describe a strategy for increasing the yield of a recombinant protein in plants. HIV p24 antigen was expressed as a genetic fusion with the alpha2 and alpha3 constant region sequences from human Ig alpha-chain and targeted to the endomembrane system. The expression of this fusion protein was detected at levels approximately 13-fold higher than HIV p24 expressed alone, and a difference in the behaviour of the two recombinant proteins during trafficking in the plant secretory pathway has been identified. Expressing the antigen within the context of alpha-chain Ig sequences resulted in the formation of homodimers and the antigen was correctly recognized by specific antibodies. Furthermore, the HIV p24 elicited T-cell and antibody responses in immunized mice. The use of Ig fusion partners is proposed as a generic platform technology for up-regulating the expression of antigens in plants, and may represent the first step in a strategy to design new vaccines with enhanced immunological properties.

Antibody processing and engineering in plants, and new strategies for vaccine production

The use of transgenic plants for the production of recombinant proteins is not a universal solution for all proteins. The choice of this expression system depends very much on the type of protein and its applications. Many proteins will best be made by conventional microbial fermentation, similarly, we are already identifying proteins where plants represent the only practical option for one reason or another. It will be important to understand better the cellular mechanisms of protein folding, assembly and processing in plants, in order to maximise the potential of transgenic plants as a protein production system. One of the main advantages that plants offer is that they are higher eukaryotic organisms with an endomembrane system. Therefore, they fold and assemble recombinant proteins using protein chaperones that are homologous to those in mammalian cells, and they perform post-translational modifications. This allows, for example, the expression of monoclonal antibodies, first described in 1989, as well as a range of other types of immunoglobulin molecules and multimeric complexes.

Production and glycosylation of plant-made pharmaceuticals: the antibodies as a challenge

Antibodies have long been recognized for their diagnostic and therapeutic potential. The rapidly increasing number of monoclonal antibodies approved for immunotherapy has paved the way to an even greater demand for these molecules. In order to satisfy this growing demand and to increase the production capacity, alternative systems based on antibody production in transgenic organisms are being actively explored. In this paper, we focus on transgenic plants as a promising system for the scale-up and processing of plant-made pharmaceuticals. In particular, we point out the advantages and limitations induced by glycosylation of plant-made antibodies for human therapy.

Recent developments in the use of transgenic plants for the production of human therapeutics and biopharmaceuticals

In recent years there has been a dramatic increase in the application of plant biotechnology for the production of a variety of commercially valuable simple and complex biological molecules (biologics) for use in human and animal healthcare. Transgenic whole plants and plant cell culture systems have been developed that have the capacity to economically produce large-scale quantities of antibodies and antibody fragments, antigens and/or vaccine epitopes, metabolic enzymes, hormones, (neuro)peptides and a variety of biologically active complexes and secondary metabolites for direct use as therapeutic agents or diagnostic tools in the medical healthcare industry. As the products of genetically modified plants make their way from concept to commercialization the associated risks and acceptance by the public has been become a focal point. In this paper, we summarize the recent advances made in the use of transgenic plants and plant cell cultures as biological factories for the production of human therapeutics and biopharmaceuticals and discuss the long-term potential of `molecular farming' as a low-cost, efficient method for the production of biological materials with demonstrated utility to the pharmaceutical industry or medical community.

Expression and characterization of an anti-(hepatitis B surface antigen) glycosylated mouse antibody in transgenic tobacco (Nicotiana tabacum) plants and its use in the immunopurification of its target antigen

Transgenic plants expressing recombinant immunoglobulins have arisen as an alternative technology for the large-scale production of antibodies useful in therapeutics and in industrial processes. In the present paper we report the expression in transgenic tobacco ( Nicotiana tabacum ) of an anti-HBsAg [anti-(hepatitis B virus surface antigen)] mouse IgG1 mAb (monoclonal antibody), currently used for the industrial purification of the recombinant vaccine antigen. Using the sweet potato sporamin signal peptide, a KDEL (Lys-Asp-Glu-Leu) ER (endoplasmic reticulum) anchorage domain, and a heavy- and light-chain gene tandem construction, we generated F1 plants in which the expression of the antibody accounted for 0.5% of the total soluble proteins. The 'plantibody' (functional IgG antibody produced in plants) was easily purified by Protein A-Sepharose chromatography with a yield of approximately 35 microg/g of fresh leaf material, and its glycosylation indicated that, irrespective of the KDEL signal, the molecule is modified in both the ER and Golgi. Finally, a successful comparison of the plantibody with the ascites-derived mAb in the immunoaffinity purification of the vaccine recombinant HBsAg was performed. Taken as a whole, our results show that the large-scale production of this antibody of industrial relevance in transgenic tobacco is feasible.

Expression of sea anemone equistatin in potato. Effects of plant proteases on heterologous protein production

Plants are increasingly used as production platforms of various heterologous proteins, but rapid protein turnover can seriously limit the steady-state expression level. Little is known about specific plant proteases involved in this process. In an attempt to obtain potato (Solanum tuberosum cv Desirée) plants resistant to Colorado potato beetle (Leptinotarsa decemlineata Say) larvae, the protease inhibitor equistatin was expressed under the control of strong, light-inducible and constitutive promoters and was targeted to the secretory pathway with and without endoplasmic reticulum retention signal. All constructs yielded similar stepwise protein degradation patterns, which considerably reduced the amount of active inhibitor in planta and resulted in insufficient levels for resistance against Colorado potato beetle larvae. Affinity purification of the degradation products and N-terminal sequencing allowed the identification of the amino acid P(1)-positions (asparagine [Asn]-13, lysine-56, Asn-82, and arginine-151) that were cleaved in planta. The proteases involved in the equistatin degradation were characterized with synthetic substrates and inhibitors. Kininogen domain 3 completely inhibited equistatin degradation in vitro. The results indicate that arginine/lysine-specific and legumain-type Asn-specific cysteine proteases seriously impede the functional accumulation of recombinant equistatin in planta. General strategies to improve the resistance to proteases of heterologous proteins in plants are proposed.

Benefits and risks of antibody and vaccine production in transgenic plants

Phytopharming, the production of protein biologicals in recombinant plant systems, has shown great promise in studies performed over the past 13 years. A secretory antibody purified from transgenic tobacco was tested successfully in humans, and prevented bacterial re-colonization after topical application in the mouth. Rapid production of patient-tailored anti-lymphoma antibodies in recombinant Tobamovirus-infected tobacco may provide effective cancer therapy. Many different candidate vaccines from bacterial and viral sources have been expressed in transgenic plants, and three human clinical trials with oral delivery of transgenic plant tissues have shown exciting results. The use of crop plants with agricultural practice could allow cheap production of valuable proteins, while providing enhanced safety by avoidance of animal viruses or other contaminants. However development of this technology must carefully consider the means to ensure the separation of food and medicinal products when crop plants are used for phytopharming.

'Molecular farming' of antibodies in plants

'Molecular farming' is the production of valuable recombinant proteins in transgenic organisms on an agricultural scale. While plants have long been used as a source of medicinal compounds, molecular farming represents a novel source of molecular medicines, such as plasma proteins, enzymes, growth factors, vaccines and recombinant antibodies, whose medical benefits are understood at a molecular level. Until recently, the broad use of molecular medicines was limited because of the difficulty in producing these proteins outside animals or animal cell culture. The application of molecular biology and plant biotechnology in the 1990s showed that many molecular medicines or vaccines could be synthesised in plants and this technology is termed 'molecular farming'. It results in pharmaceuticals that are safer, easier to produce and less expensive than those produced in animals or microbial culture. An advantage of molecular farming lies in the ability to perform protein production on a massive scale using hectares of cultivated plants. These plants can then be harvested and transported using the agricultural infrastructure. Thus, molecular farming allows rapid progress from genetic engineering to crop production, and new cash crops producing recombinant proteins are already being commercially exploited. We speculate that as functional genomics teaches us more about the nature of disease, molecular farming will produce many of the protein therapeutics that can remedy it.

Transgenic plants expressing antibodies: a model for phytoremediation

The feasibility of using antibody expressing transgenic plants either to neutralize bioactive molecules in the rhizosphere, or to accumulate and concentrate the molecules in leaves has been demonstrated in a model system consisting of hydroponic Nicotiana plant cultures expressing a murine monoclonal IgG1. Two transgenic plant types were used; in the first, functional antibody was rhizosecreted and shown to bind with antigen in the surrounding medium to form an immune complex. In the second, a transmembrane sequence retained monoclonal antibody in the plants, on the plasma membrane. Antigen added to the nutrient medium around the roots of mIgG plants was transported within 24 h to the topmost leaves of the plant where it was sequestered as an immune complex by binding to antibody on the cell membrane. Concentration of immune complex in the leaf tissue remained constant over a 72 h period after removal of antigen from nutrient medium. Free antigen was not detected in the leaves of wild-type plants. The two strategies of rhizosecretion-mediated binding and sequestration in leaf tissue could potentially be used in the phytoremediation of any pollutant for which it is possible to generate a monoclonal antibody.

ER-resident chaperone interactions with recombinant antibodies in transgenic plants

In this study, we demonstrate that the folding and assembly of IgG in transgenic tobacco plants is orchestrated by BiP (binding protein), an endoplasmic reticulum resident chaperone. Expression of BiP and calreticulin was examined in transgenic tobacco plants that express immunoglobulin chains, either singly or in combination to form IgG antibody. BiP mRNA expression was lowest in wild-type nontransformed plants and those that expressed immunoglobulin light chain alone. Higher mRNA levels were detected in plants expressing fully assembled immunoglobulin (light and heavy chains), and the most abundant levels of RNA transcript were found in those plants that expressed immunoglobulin heavy chain alone. Estimation of total BiP demonstrated a similar pattern, with the highest levels detected in plants expressing immunoglobulin heavy chain alone. Immunoprecipitation studies demonstrated that BiP was associated with immunoglobulin chains extracted from protoplast lysates, but not from secreted fluids. Again, most BiP was coprecipitated from plants expressing heavy chain only and those that produced full length IgG. The binding of BiP to Ig heavy chains was ATP-sensitive. Co-expression of heavy and light chain resulted in IgG assembly and displacement of BiP from the heavy chain as the amount of light chain increased. Although calreticulin mRNA and total protein levels varied in a similar manner to those of BiP in the transgenic plants, there was no evidence for association between calreticulin and Ig chains, by coimmunoprecipitation. The results indicate that BiP, but not calreticulin, takes part in immunoglobulin folding and assembly in transgenic plants.

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).

Transgenic plants as protein factories

Transgenic plants are showing considerable potential for the economic production of proteins, with a few already being marketed. Recent clinical trials of pharmaceuticals produced from transgenic plants are encouraging, with plant glycans showing reassuringly poor immunogenicity. Our increasing understanding of protein targeting and accumulation should further improve the potential of this new technology.

Highly efficient targeting and accumulation of a F(ab) fragment within the secretory pathway and apoplast of Arabidopsis thaliana

To further improve antibody production in plants, constructs were designed to minimize transgene silencing and to retain a F(ab) fragment within the secretory pathway of transgenic Arabidopsis thaliana plants. The levels of antibody accumulation suggest that placing the sequences that encode Fd and light chain under the control of nonidentical 3' regions reduces susceptibility to post-transcriptional gene silencing compared with when the individual polypeptide-encoding sequences are placed under the control of identical 3' regions. High levels of accumulation (up to 6% of total soluble protein) were found for both secreted and intracellularly targeted antibody fragments. Immunofluorescence microscopic analysis showed that F(ab) fragments devoid of any additional C-terminal sequence were efficiently secreted, whereas retention of F(ab) fragments within the endomembrane system of the secretory pathway was achieved by C-terminal fusion of the DIKDEL sequence to the antibody light chain. Furthermore, analysis by immunoprecipitation and ELISA showed that intracellular retention of antibody fragments did not affect antigen-binding activity, and more than 80% of the isolated antibody fragments were found to bind antigen. Taken together, our results provide improvements to the technology of recombinant antibody production in transgenic plants.

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.

Characterization of monoclonal antibody fragments produced by plant cells

Production of a murine IgG1 was investigated using hairy roots, shooty teratomas, and suspended cells of transgenic tobacco. In all cases, in addition to complete assembled antibody, two to four major antibody fragments accumulated in the biomass. A range of protease inhibitors, protein-stabilizing agents, inhibitors of N-glycosylation and protein secretion, glycan-reactive agents, and affinity probes was used to characterize these fragments and investigate their sites and mechanisms of formation. The fragments were not experimental artifacts caused by antibody degradation during tissue homogenization and sample preparation, nor did they represent glycosylation variants. All of the molecules were actively secreted into the culture media and some showed evidence of Golgi-associated glycan processing, indicating they were not assembly intermediates. Antibody fragments of 50 and 80 kDa were identified mainly as the products of extracellular degradation in the root and shoot apoplast; the 80-kDa fragment was also present in cell suspension medium, and in suspended cell biomass toward the end of the growth phase. Larger 120- and 135-kDa fragments were most likely produced by proteolytic degradation along the secretory pathway outside of the endoplasmic reticulum (ER) and Golgi apparatus; the carbohydrate residues of the 135-kDa antibody suggest formation between these organelles. Inhibition of protein secretion and retention of antibody in the ER and/or Golgi reduced fragmentation and increased antibody accumulation levels, probably by reducing exposure to the principal sites of protease activity. This work highlights the importance of foreign protein degradation in plant tissues as a mechanism for posttranslational product loss. Identifying the nature of these degradative processes is a first step toward alleviating their effects, improving protein yields, and enhancing the feasibility of plants as a commercial means for large-scale protein production.

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 of antibodies and antibody fragments in plants

Our current knowledge allows the generation of transgenic plants that efficiently produce heterologous proteins from plant, bacterial, fungal or animal origin. Among all types of recombinant proteins, antibodies are particularly attractive because of their ability to specifically recognize and bind virtually any type of antigen. Plants show several advantages as a large-scale antibody production system: they can be grown easily and inexpensively in large quantities that can be harvested, stored and processed by using existing infrastructures. Isolation and purification of plant-made antibodies, if necessary, allow fundamental, industrial, and therapeutical applications. In the past, we and others have successfully generated antibody-producing plants. The maximal accumulation levels of antibodies and antibody fragments that we observed are 1-5% of the extracted proteins. Currently, several biotechnological companies grow field crops to produce antibodies for ex planta applications on an industrial scale.

Effect of climate conditions and plant developmental stage on the stability of antibodies expressed in transgenic tobacco

Plants are regarded as a promising system for the production of heterologous proteins. However, little is known about the influence of plant physiology and plant development on the yield and quality of the heterologous proteins produced in plants. To investigate this, tobacco (Nicotiana tabacum cv Samsun NN) was transformed with a single construct that contained behind constitutive promotors the light- and heavy-chain genes of a mouse antibody. The in planta stability of the antibody was analyzed in transgenic plants that were grown under high and low irradiation at 15 degrees C and 25 degrees C. High-light conditions favored the production of biomass, of total soluble protein, and of antibody. The plants grown at 25 degrees C developed faster and contained less antibody per amount of leaf tissue than the plants grown at 15 degrees C. Both endogenous protein and antibody content showed a strong decline during leaf development. The heavy chains of the antibody underwent in planta degradation via relatively stable fragments. In vitro incubations of purified plantibody with leaf extracts of wild-type tobacco indicated the involvement of acidic proteases. It is interesting that the same antibody produced by mouse hybridoma cells exhibited higher stability in this in vitro assay. This may be explained by the assumption that the plant type of N-glycosylation contributes less to the stability of the antibody than the mouse-type of N-glycosylation. The results of this study indicate that proteolytic degradation during plant development can be an important factor affecting yield and homogeneity of heterologous protein produced by transgenic plants.

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.

Plants as bioreactors for protein production: avoiding the problem of transgene silencing

Plants are particularly attractive as large-scale production systems for proteins intended for therapeutical or industrial applications: they can be grown easily and inexpensively in large quantities that can be harvested and processed with the available agronomic infrastructures. The effective use of plants as bioreactors depends on the possibility of obtaining high protein accumulation levels that are stable during the life cycle of the transgenic plant and in subsequent generations. Silencing of the introduced transgenes has frequently been observed in plants, constituting a major commercial risk and hampering the general economic exploitation of plants as protein factories. Until now, the most efficient strategy to avoid transgene silencing involves careful design of the transgene construct and thorough analysis of transformants at the molecular level. Here, we focus on different aspects of the generation of transgenic plants intended for protein production and on their influence on the stability of heterologous gene expression.