From planta to pharma with glycosylation in the toolbox

Glycosylation of polyphenolic compounds: Design of a self-sufficient biocatalyst by co-immobilization of a glycosyltransferase, a sucrose synthase and the cofactor UDP

Glycosyltransferases catalyze the regioselective glycosylation of polyphenolic compounds, increasing their solubility without altering their antioxidant properties. Leloir-type glycosyltransferases require UDP-glucose as a cofactor to glycosylate a hydroxyl of the polyphenol, which is expensive and unstable. To simplify these processes for industrial implementation, the preparation of self-sufficient heterogeneous biocatalysts is needed. In this study, a glycosyltransferase and a sucrose synthase (as an UDP-regenerating enzyme) were co-immobilized onto porous agarose-based supports coated with polycationic polymers: polyethylenimine and polyallylamine. In addition, the UDP cofactor was strongly ionically adsorbed and co-immobilized with the enzymes, eliminating the need to add it separately. Thus, the optimal self-sufficient heterogeneous biocatalyst was able to catalyze the glycosylation of three polyphenolic compounds (piceid, phloretin and quercetin) with in situ regeneration of the UDP-glucose, allowing multiple consecutive reaction cycles without the addition of exogenous cofactor. A TTN value of 50 (theoretical maximum) was obtained in the reaction of piceid glycosylation, after 5 reaction cycles, using the self-sufficient biocatalyst based on an improved sucrose synthase variant. This result was 5-fold higher than the obtained using soluble cofactor and the co-immobilized enzymes, and much higher than those reported in the literature for similar processes.

New insight into strategies used to develop long-acting G-CSF biologics for neutropenia therapy

Over the last 20 years, granulocyte colony-stimulating factors (G-CSFs) have become the major therapeutic option for the treatment of patients with neutropenia. Most of the current G-CSFs require daily injections, which are inconvenient and expensive for patients. Increased understanding of G-CSFs' structure, expression, and mechanism of clearance has been very instrumental in the development of new generations of long-acting G-CSFs with improved efficacy. Several approaches to reducing G-CSF clearance via conjugation techniques have been investigated. PEGylation, glycosylation, polysialylation, or conjugation with immunoglobulins or albumins have successfully increased G-CSFs' half-lives. Pegfilgrastim (Neulasta) has been successfully approved and marketed for the treatment of patients with neutropenia. The rapidly expanding market for G-CSFs has increased demand for G-CSF biosimilars. Therefore, the importance of this review is to highlight the principle, elimination's route, half-life, clearance, safety, benefits, and limitations of different strategies and techniques used to increase the half-life of biotherapeutic G-CSFs. Understanding these strategies will allow for a new treatment with more competitive manufacturing and lower unit costs compared with that of Neulasta.

Plant-based vaccines and cancer therapy: Where are we now and where are we going?

Therapeutic vaccines are an effective approach in cancer therapy for treating the disease at later stages. The Food and Drug Administration (FDA) recently approved the first therapeutic cancer vaccine, and further studies are ongoing in clinical trials. These are expected to result in the future development of vaccines with relatively improved efficacy. Several vaccination approaches are being studied in pre-clinical and clinical trials, including the generation of anti-cancer vaccines by plant expression systems.This approach has advantages, such as high safety and low costs, especially for the synthesis of recombinant proteins. Nevertheless, the development of anti-cancer vaccines in plants is faced with some technical obstacles.Herein, we summarize some vaccines that have been used in cancer therapy, with an emphasis on plant-based vaccines.

Engineering of Plants for Efficient Production of Therapeutics

Plants are becoming useful platforms for recombinant protein production at present time. With the advancement of efficient molecular tools of genomics, proteomics, plants are now being used as a biofactory for production of different life saving therapeutics. Plant-based biofactory is an established production system with the benefits of cost-effectiveness, high scalability, rapid production, enabling post-translational modification, and being devoid of harmful pathogens contamination. This review introduces the main challenges faced by plant expression system: post-translational modifications, protein stability, biosafety concern and regulation. It also summarizes essential factors to be considered in engineering plants, including plant expression system, promoter, post-translational modification, codon optimization, and fusion tags, protein stabilization and purification, subcellular targeting, and making vaccines in an edible way. This review will be beneficial and informative to scholars and readers in the field of plant biotechnology.

Expression and Functional Evaluation of Recombinant Anti-receptor Activator of Nuclear Factor Kappa-B Ligand Monoclonal Antibody Produced in Nicotiana benthamiana

Denosumab, an anti-receptor activator of nuclear factor-kappa B ligand antibody (anti-RANKL), is a fully human monoclonal antibody (mAb) available for the treatment of osteoporosis. In the present study, an anti-RANKL mAb was transiently expressed using the geminiviral expression system in Nicotiana benthamiana, and the functional activity of the plant-produced mAb was determined. The highest expression level of the plant-produced mAb was found at 8 days post-infiltration, and it was estimated to be 0.5 mg/g leaf fresh weight. The recombinant mAb from the plant crude extracts was purified by using Protein A affinity column chromatography. The plant-produced mAb demonstrated good in vitro affinity binding with human RANKL, as determined by RANKL-ELISA binding. The function of the plant-produced mAb was evaluated in vitro. CD14-positive cells isolated from human peripheral blood mononuclear cells (PBMCs) were cultured in vitro in the presence of human RANKL and macrophage-colony-stimulating factor (M-CSF) to stimulate osteoclastogenesis. The results demonstrated that plant-produced mAb could significantly decrease the number of osteoclasts compared to commercial denosumab. These results demonstrated that the plant-produced mAb has the potential to inhibit osteoclast differentiation and that it could be considered for osteoporosis treatment.

Efficient Production of a Bioactive Bevacizumab Monoclonal Antibody Using the 2A Self-cleavage Peptide in Transgenic Rice Callus

Bevacizumab, a humanized monoclonal antibody (mAb) targeting to the vascular endothelial growth factor (VEGF), has been widely used in clinical practice for the treatment of multiple cancers. Bevacizumab was mostly produced by the mammalian cell expression system. We here reported the first plant-derived Bevacizumab by using transgenic rice callus as an alternative gene expression system. Codon-optimized Bevacizumab light chain (BLC) and Bevacizumab heavy chain (BHC) genes were designed, synthesized as a polyprotein with a 2A self-cleavage linker peptide from the Foot-and-mouth disease virus, cloned into a plant binary vector under a constitutive maize ubiquitin promoter, and transformed into rice nuclear genome through Agrobacterium-mediated transformation. Southern blot and western blot analyses confirmed the integration and expression of BLC and BHC genes in transgenic rice callus. Enzyme-linked immunosorbent assay (ELISA) analysis indicated that the rice-derived Bevacizumab mAb was biologically active and the recombinant mAb was expressed at high levels (160.7-242.8 mg/Kg) in transgenic rice callus. The mAb was purified by using protein A affinity chromatography and the purified antibody was tested for its binding affinity with its target human VEGF (hVEGF) antigen by ELISA. Rice callus produced Bevacizumab and a commercial Bevacizumab (Avastin) were shown to have similar binding affinity to hVEGF. These results indicated that rice callus produced Bevacizumab could have similar biological activity and might potentially be used as a cost-effective biosimilar molecule in future cancer treatment.

CMP-Sialic Acid Synthetase: The Point of Constriction in the Sialylation Pathway

Sialoglycoconjugates form the outermost layer of animal cells and play a crucial role in cellular communication processes. An essential step in the biosynthesis of sialylated glycoconjugates is the activation of sialic acid to the monophosphate diester CMP-sialic acid. Only the activated sugar is transported into the Golgi apparatus and serves as a substrate for the linkage-specific sialyltransferases. Interference with sugar activation abolishes sialylation and is embryonic lethal in mammals. In this chapter we focus on the enzyme catalyzing the activation of sialic acid, the CMP-sialic acid synthetase (CMAS), and compare the enzymatic properties of CMASs isolated from different species. Information concerning the reaction mechanism and active site architecture is included. Moreover, the unusual nuclear localization of vertebrate CMASs as well as the biotechnological application of bacterial CMAS enzymes is addressed.

Farming of Plant-Based Veterinary Vaccines and Their Applications for Disease Prevention in Animals

Plants have been studied for the production of pharmaceutical compounds for more than two decades now. Ever since the plant-made poultry vaccine against Newcastle disease virus made a breakthrough and went all the way to obtain regulatory approval, research to use plants for expression and delivery of vaccine proteins for animals was intensified. Indeed, in view of the high production costs of veterinary vaccines, plants represent attractive biofactories and offer many promising advantages in the production of recombinant vaccine proteins. Furthermore, the possibility of conducting immunogenicity and challenge studies in target animals has greatly exaggerated the progress. Although there are no edible plant-produced animal vaccines in the market, plant-based vaccine technology has great potentials. In this review, development, uses, and advantages of plant-based recombinant protein production in various expression platforms are discussed. In addition, examples of plant-based veterinary vaccines showing strong indication in terms of efficacy in animal disease prevention are also described.

Recent advances on host plants and expression cassettes' structure and function in plant molecular pharming

Plant molecular pharming is a promising system to produce important recombinant proteins such as therapeutic antibodies, pharmaceuticals, enzymes, growth factors, and vaccines. The system provides an interesting alternative method to the direct extraction of proteins from inappropriate source material while offering the possibility to overcome problems related to product safety and source availability. Multiple factors including plant hosts, genes of interest, expression vector cassettes, and extraction and purification techniques play important roles in the plant molecular pharming. Plant species, as a biosynthesis platform, are a crucial factor in achieving high yields of recombinant protein in plant. The choice of recombinant gene and its expression strategy is also of great importance in ensuring a high amount of the recombinant proteins. Many studies have been conducted to improve expression, accumulation, and purification of the recombinant protein from molecular pharming systems. Re-engineered vectors and expression cassettes are also pivotal tools in enhancing gene expression at the transcription and translation level, and increasing protein accumulation, stability, retention and targeting of specific organelles. In this review, we report recent advances and strategies of plant molecular pharming while focusing on the choice of plant hosts and the role of some molecular pharming elements and approaches: promoters, codon optimization, signal sequences, and peptides used for upstream design, purification and downstream processing.

Glycan structure and serum half-life of recombinant CTLA4Ig, an immunosuppressive agent, expressed in suspension-cultured rice cells with coexpression of human β1,4-galactosyltransferase and human CTLA4Ig

Human cytotoxic T-lymphocyte antigen 4-immunoglobulin (hCTLA4Ig) is an immunosuppressive therapeutic, and recently produced rice cell-derived hCTLA4Ig (hCTLA4Ig(P)) reportedly exhibits in vitro immunosuppressive activities equivalent to those of Chinese hamster ovary cell-derived hCTLA4Ig (hCTLA4Ig(M)). However, limitations of hCTLA4Ig(P) include shortened in vivo half-life as well as the presence of nonhuman N-glycans containing (β1-2)-xylose and α1,3-fucose, which cause immunogenic reactions in humans. In the present study, human β1,4-galactose-extended hCTLA4Ig(P) (hCTLA4Ig(P)-Gal) was expressed through the coexpression of human β1,4-galactosyltransferase (hGalT) and hCTLA4Ig in an attempt to overcome these unfavorable effects. The results indicated that both encoding hGalT and hCTLA4Ig were successfully coexpressed, and the analysis of N-glycan and its relative abundance in purified hCTLA4Ig(P)-Gal indicated that not only were the two glycans containing (β1-4)-galactose newly extended, but also glycans containing both β1,2-xylose and α1,3-fucose were markedly reduced and high-mannose-type glycans were increased compared to those of hCTLA4Ig(P), respectively. Unlike hCTLA4Ig(P), hCTLA4Ig(P)-Gal was effective as an acceptor via (β1-4)-galactose for in vitro sialylation. Additionally, the serum half-life of intravenously injected hCTLA4Ig(P)-Gal in Sprague-Dawley rats was 1.9 times longer than that of hCTLA4Ig(P), and the clearance pattern of hCTLA4Ig(P)-Gal was close to that for hCTLA4Ig(M). These results indicate that the coexpression with hGalT and hCTLA4Ig(P) is useful for both reducing glycan immunogens and increasing in vivo stability. This is the first report of hCTLA4Ig as an effective therapeutics candidate in glycoengineered rice cells.

Expression of recombinant vaccines and antibodies in plants

Plants are able to perform post-translational maturations of therapeutic proteins required for their functional biological activity and suitable in vivo pharmacokinetics. Plants can be a low-cost, large-scale production platform of recombinant biopharmaceutical proteins such as vaccines and antibodies. Plants, however, lack mechanisms of processing authentic human N-glycosylation, which imposes a major limitation in their use as an expression system for therapeutic glycoproducts. Efforts have been made to circumvent plant-specific N-glycosylation, as well as to supplement the plant's endogenous system with human glycosyltransferases for non-immunogenic and humanized N-glycan production. Herein we review studies on the potential of plants to serve as production systems for therapeutic and prophylactic biopharmaceuticals. We have especially focused on recombinant vaccines and antibodies and new expression strategies to overcome the existing problems associated with their production in plants.

N-glycosylation modification of plant-derived virus-like particles: an application in vaccines

Plants have been developed as an alternative system to mammalian cells for production of recombinant prophylactic or therapeutic proteins for human and animal use. Effective plant expression systems for recombinant proteins have been established with the optimal combination of gene expression regulatory elements and control of posttranslational processing of recombinant glycoproteins. In plant, virus-like particles (VLPs), viral “empty shells” which maintain the same structural characteristics of virions but are genome-free, are considered extremely promising as vaccine platforms and therapeutic delivery systems. Unlike microbial fermentation, plants are capable of carrying out N-glycosylation as a posttranslational modification of glycoproteins. Recent advances in the glycoengineering in plant allow human-like glycomodification and optimization of desired glycan structures for enhancing safety and functionality of recombinant pharmaceutical glycoproteins. In this review, the current plant-derived VLP approaches are focused, and N-glycosylation and its in planta modifications are discussed.

Production of a functional human acid maltase in tobacco seeds: biochemical analysis, uptake by human GSDII cells, and in vivo studies in GAA knockout mice

Genetic deficiency of acid alpha glucosidase (GAA) results in glycogen storage disease type II (GSDII) or Pompe's disease. To investigate whether we could generate a functional recombinant human GAA enzyme (tobrhGAA) in tobacco seeds for future enzyme replacement therapy, we subcloned the human GAA cDNA into the plant expression plasmid-pBI101 under the control of the soybean β-conglycinin seed-specific promoter and biochemically analyzed the tobrhGAA. Tobacco seeds contain the metabolic machinery that is more compatible with mammalian glycosylation-phosphorylation and processing. We found the tobrhGAA to be enzymatically active was readily taken up by GSDII fibroblasts and in white blood cells from whole blood to reverse the defect. The tobrhGAA corrected the enzyme defect in tissues at 7 days after a single dose following intraperitoneal (IP) administration in GAA knockout (GAA(-/-)) mice. Additionally, we could purify the tobrhGAA since it bound tightly to the matrix of Sephadex G100 and can be eluted by competition with maltose. These data demonstrate indirectly that the tobrhGAA is fully functional, predominantly proteolytically cleaved and contains the minimal phosphorylation and mannose-6-phosphate residues essential for biological activity.

Plant-derived pharmaceuticals for the developing world

Plant-produced vaccines and therapeutic agents offer enormous potential for providing relief to developing countries by reducing the incidence of infant mortality caused by infectious diseases. Vaccines derived from plants have been demonstrated to effectively elicit an immune response. Biopharmaceuticals produced in plants are inexpensive to produce, require fewer expensive purification steps, and can be stored at ambient temperatures for prolonged periods of time. As a result, plant-produced biopharmaceuticals have the potential to be more accessible to the rural poor. This review describes current progress with respect to plant-produced biopharmaceuticals, with a particular emphasis on those that target developing countries. Specific emphasis is given to recent research on the production of plant-produced vaccines toward human immunodeficiency virus, malaria, tuberculosis, hepatitis B virus, Ebola virus, human papillomavirus, rabies virus and common diarrheal diseases. Production platforms used to express vaccines in plants, including nuclear and chloroplast transformation, and the use of viral expression vectors, are described in this review. The review concludes by outlining the next steps for plant-produced vaccines to achieve their goal of providing safe, efficacious and inexpensive vaccines to the developing world.

Combinatorial Analysis of Secretory Immunoglobulin A (sIgA) Expression in Plants

Delivery of secretory immunoglobulin A (sIgA) to mucosal surfaces as a passive immunotherapy agent is a promising strategy to prevent infectious diseases. Recombinant sIgA production in plants requires the co-expression of four transcriptional units encoding the light chain (LC), heavy chain (HC), joining chain (JC) and secretory component (SC). As a way to optimize sIgA production in plants, we tested the combinatorial expression of 16 versions of a human sIgA against the VP8* rotavirus antigen in Nicotiana benthamiana, using the recently developed GoldenBraid multigene assembly system. Each sIgA version was obtained by combining one of the two types of HC (α1 and α2) with one of the two LC types (k and λ) and linking or not a KDEL peptide to the HC and/or SC. From the analysis of the anti-VP8* activity, it was concluded that those sIgA versions carrying HCα1 and LCλ provided the highest yields. Moreover, ER retention significantly increased antibody production, particularly when the KDEL signal was linked to the SC. Maximum expression levels of 32.5 μg IgA/g fresh weight (FW) were obtained in the best performing combination, with an estimated 33% of it in the form of a secretory complex.

Production of α-L-iduronidase in maize for the potential treatment of a human lysosomal storage disease

Lysosomal storage diseases are a class of over 70 rare genetic diseases that are amenable to enzyme replacement therapy. Towards developing a plant-based enzyme replacement therapeutic for the lysosomal storage disease mucopolysaccharidosis I, here we expressed α-L-iduronidase in the endosperm of maize seeds by a previously uncharacterized mRNA-targeting-based mechanism. Immunolocalization, cellular fractionation and in situ RT-PCR demonstrate that the α-L-iduronidase protein and mRNA are targeted to endoplasmic reticulum (ER)-derived protein bodies and to protein body-ER regions, respectively, using regulatory (5'- and 3'-UTR) and signal-peptide coding sequences from the γ-zein gene. The maize α-L-iduronidase exhibits high activity, contains high-mannose N-glycans and is amenable to in vitro phosphorylation. This mRNA-based strategy is of widespread importance as plant N-glycan maturation is controlled and the therapeutic protein is generated in a native form. For our target enzyme, the N-glycan structures are appropriate for downstream processing, a prerequisite for its potential as a therapeutic protein.

Plant-derived virus-like particles as vaccines

Virus-like particles (VLPs) are self-assembled structures derived from viral antigens that mimic the native architecture of viruses but lack the viral genome. VLPs have emerged as a premier vaccine platform due to their advantages in safety, immunogenicity, and manufacturing. The particulate nature and high-density presentation of viral structure proteins on their surface also render VLPs as attractive carriers for displaying foreign epitopes. Consequently, several VLP-based vaccines have been licensed for human use and achieved significant clinical and economical success. The major challenge, however, is to develop novel production platforms that can deliver VLP-based vaccines while significantly reducing production times and costs. Therefore, this review focuses on the essential role of plants as a novel, speedy and economical production platform for VLP-based vaccines. The advantages of plant expression systems are discussed in light of their distinctive posttranslational modifications, cost-effectiveness, production speed, and scalability. Recent achievements in the expression and assembly of VLPs and their chimeric derivatives in plant systems as well as their immunogenicity in animal models are presented. Results of human clinical trials demonstrating the safety and efficacy of plant-derived VLPs are also detailed. Moreover, the promising implications of the recent creation of "humanized" glycosylation plant lines as well as the very recent approval of the first plant-made biologics by the U. S. Food and Drug Administration (FDA) for plant production and commercialization of VLP-based vaccines are discussed. It is speculated that the combined potential of plant expression systems and VLP technology will lead to the emergence of successful vaccines and novel applications of VLPs in the near future.

Differential subcellular targeting of recombinant human α₁-proteinase inhibitor influences yield, biological activity and in planta stability of the protein in transgenic tomato plants

The response of protein accumulation site on yield, biological activity and in planta stability of therapeutic recombinant human proteinase inhibitor (α₁-PI) was analyzed via targeting to different subcellular locations, like endoplasmic reticulum (ER), apoplast, vacuole and cytosol in leaves of transgenic tomato plants. In situ localization of the recombinant α₁-PI protein in transgenic plant cells was monitored by immunohistochemical staining. Maximum accumulation of recombinant α₁-PI in T₀ and T₁ transgenic tomato plants was achieved from 1.5 to 3.2% of total soluble protein (TSP) by retention in ER lumen, followed by vacuole and apoplast, whereas cytosolic targeting resulted into degradation of the protein. The plant-derived recombinant α₁-PI showed biological activity for elastase inhibition, as monitored by residual porcine pancreatic elastase (PPE) activity assay and band-shift assay. Recombinant α₁-PI was purified from transgenic tomato plants with high yield, homogeneity and biological activity. Purified protein appeared as a single band of ∼48-50 kDa on SDS-PAGE with pI value ranging between 5.1 and 5.3. Results of mass spectrometry and optical spectroscopy of purified recombinant α₁-PI revealed the structural integrity of the recombinant protein comparable to native serum α₁-PI. Enzymatic deglycosylation and lectin-binding assays with the purified recombinant α₁-PI showed compartment-specific N-glycosylation of the protein targeted to ER, apoplast and vacuole. Conformational studies based on urea-induced denaturation and circular dichroism (CD) spectroscopy revealed relatively lower stability of the recombinant α₁-PI protein, compared to its serum counterpart. Pharmacokinetic evaluation of plant derived recombinant and human plasma-purified α₁-PI in rat, by intravenous route, revealed significantly faster plasma clearance and lower area under curve (AUC) of recombinant protein. Our data suggested significance of protein sorting sequences and feasibility to use transgenic plants for the production of stable, glycosylated and biologically active recombinant α₁-PI for further therapeutic applications.

Production of different glycosylation variants of the tumour-targeting mAb H10 in Nicotiana benthamiana: influence on expression yield and antibody degradation

We previously described the expression of a tumour-targeting antibody (mAb H10) in Nicotiana benthamiana by vacuum-agro-infiltration and the remarkable yields of highly pure protein achieved. The objective of the present work was to investigate different strategies for transient overexpression of the mAb H10 in which glycan configuration was modulated and assess how these strategies affect the accumulation yield and stability of the antibody. To this aim, three procedures have been assayed: (1) Site-directed mutagenesis to abolish the glycosylation site; (2) endoplasmic reticulum retention (C-terminal SEKDEL fusion) to ensure predominantly high-mannose type glycans; and (3) expression in a N. benthamiana RNAi down-regulated line in which β1,2-xylosyltransferase and α1,3-fucosyltransferase gene expression is silenced. The three antibody variants (H10-Mut) (H10-SEKDEL) (H10(XylT/FucT)) were transiently expressed, purified and characterised for their glycosylation profile, expression/purification yield and antibody degradation pattern. Glycosylation analysis of H10(XylT/FucT) demonstrated the absence of plant complex-type sugars, while H10-SEKDEL, although substantially retained in the ER, revealed the presence of β1,2-xylose and α1,3-fucose residues, indicating a partial escape from the ER retrieval system. Antibody accumulation and purification yields were not enhanced by ER retention. All H10 antibody glyco-forms revealed greater degradation compared to the original, resulting mostly in the formation of Fab fragments. In the case of aglycosylated H10-Mut, more than 95% of the heavy chain was cleaved, confirming the pivotal role of the sugar moiety in protein stability. Identification of possible 'fragile' sites in the H10 antibody hinge region could be of general interest for the development of new strategies to reduce antibody degradation and increase the yield of intact IgGs in plants.

Toward stable genetic engineering of human O-glycosylation in plants

Glycosylation is the most abundant and complex posttranslational modification to be considered for recombinant production of therapeutic proteins. Mucin-type (N-acetylgalactosamine [GalNAc]-type) O-glycosylation is found in eumetazoan cells but absent in plants and yeast, making these cell types an obvious choice for de novo engineering of this O-glycosylation pathway. We previously showed that transient implementation of O-glycosylation capacity in plants requires introduction of the synthesis of the donor substrate UDP-GalNAc and one or more polypeptide GalNAc-transferases for incorporating GalNAc residues into proteins. Here, we have stably engineered O-glycosylation capacity in two plant cell systems, soil-grown Arabidopsis (Arabidopsis thaliana) and tobacco (Nicotiana tabacum) Bright Yellow-2 suspension culture cells. Efficient GalNAc O-glycosylation of two stably coexpressed substrate O-glycoproteins was obtained, but a high degree of proline hydroxylation and hydroxyproline-linked arabinosides, on a mucin (MUC1)-derived substrate, was also observed. Addition of the prolyl 4-hydroxylase inhibitor 2,2-dipyridyl, however, effectively suppressed proline hydroxylation and arabinosylation of MUC1 in Bright Yellow-2 cells. In summary, stably engineered mammalian type O-glycosylation was established in transgenic plants, demonstrating that plants may serve as host cells for the production of recombinant O-glycoproteins. However, the present stable implementation further strengthens the notion that elimination of endogenous posttranslational modifications may be needed for the production of protein therapeutics.

Marketing approval of mogamulizumab: a triumph for glyco-engineering

Therapeutic properties of antibodies strongly depend on the composition of their glycans. Most of the currently approved antibodies are produced in mammalian cell lines, which yield mixtures of different glycoforms that are close to those of humans, but not fully identical. Glyco-engineering is being developed as a method to control the composition of carbohydrates and to enhance the pharmacological properties of mAbs. The recent approval in Japan of mogamulizumab (POTELIGEO^®), the first glyco-engineered antibody to reach the market, is a landmark in the field of therapeutic antibodies. Mogamulizumab is a humanized mAb derived from Kyowa Hakko Kirin’s POTELLIGENT^® technology, which produces antibodies with enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) activity. The approval was granted April 30, 2012 by the Japanese Ministry of Health, Labour and Welfare for patients with relapsed or refractory CCR4-positive adult T-cell leukemia-lymphoma.

Influence of an ER-retention signal on the N-glycosylation of recombinant human α-L-iduronidase generated in seeds of Arabidopsis

Processes associated with late events of N-glycosylation within the plant Golgi complex are a major limitation to the use of plant-based systems to produce recombinant pharmaceutical proteins for parenteral administration. Specifically, sugars added to the N-glycans of a recombinant protein during glycan maturation to complex forms (e.g. β1,2 xylose and α1,3 fucose) can render the product immunogenic. In order to avoid these sugars, the human enzyme α-L-iduronidase (IDUA, EC 3.2.1.76), with a C-terminal ER-retention sequence SEKDEL, was expressed in seeds of complex-glycan-deficient (cgl) mutant and wild-type (Col-0) Arabidopsis thaliana, under the control of regulatory (5'-, signal-peptide-encoding-, and 3'-) sequences from the arcelin 5-I gene of Phaseolus vulgaris (cgl-IDUA-SEKDEL and Col-IDUA-SEKDEL, respectively). The SEKDEL motif had no adverse effect on the specific activity of the purified enzyme. Surprisingly, the majority of the N-glycans of Col-IDUA-SEKDEL were complex N-glycans (i.e. contained xylose and/or fucose) (88 %), whereas complex N-glycans comprised a much lower proportion of the N-glycans of cgl-IDUA-SEKDEL (26 %), in which high-mannose forms were predominant. In contrast to the non-chimeric IDUA of cgl seeds, which is mainly secreted into the extracellular spaces, the addition of the SEKDEL sequence to human recombinant IDUA expressed in the same background led to retention of the protein in ER-derived vesicles/compartments and its partial localization in protein storage vacuoles. Our data support the contention that the use of a C-terminal ER retention motif as an effective strategy to prevent or reduce complex N-glycan formation, is protein specific.

Production of active human glucocerebrosidase in seeds of Arabidopsis thaliana complex-glycan-deficient (cgl) plants

There is a clear need for efficient methods to produce protein therapeutics requiring mannose-termination for therapeutic efficacy. Here we report on a unique system for production of active human lysosomal acid β-glucosidase (glucocerebrosidase, GCase, EC 3.2.1.45) using seeds of the Arabidopsis thaliana complex-glycan-deficient (cgl) mutant, which are deficient in the activity of N-acetylglucosaminyl transferase I (EC 2.4.1.101). Gaucher disease is a prevalent lysosomal storage disease in which affected individuals inherit mutations in the gene (GBA1) encoding GCase. A gene cassette optimized for seed expression was used to generate the human enzyme in seeds of the cgl (C5) mutant, and the recombinant GCase was mainly accumulated in the apoplast. Importantly, the enzymatic properties including kinetic parameters, half-maximal inhibitory concentration of isofagomine and thermal stability of the cgl-derived GCase were comparable with those of imiglucerase, a commercially available recombinant human GCase used for enzyme replacement therapy in Gaucher patients. N-glycan structural analyses of recombinant cgl-GCase showed that the majority of the N-glycans (97%) were mannose terminated. Additional purification was required to remove ∼15% of the plant-derived recombinant GCase that possessed potentially immunogenic (xylose- and/or fucose-containing) N-glycans. Uptake of cgl-derived GCase by mouse macrophages was similar to that of imiglucerase. The cgl seed system requires no addition of foreign (non-native) amino acids to the mature recombinant GCase protein, and the dry transgenic seeds represent a stable repository of the therapeutic protein. Other strategies that may completely prevent plant-like complex N-glycans are discussed, including the use of a null cgl mutant.

Silencing β1,2-xylosyltransferase in Transgenic Tomato Fruits Reveals xylose as Constitutive Component of Ige-Binding Epitopes

Complex plant N-glycans containing β1,2-xylose and core α1,3-fucose are regarded as the major class of the so-called "carbohydrate cross-reactive determinants" reactive with IgE antibodies in sera of many allergic patients, but their clinical relevance is still under debate. Plant glycosyltransferases, β1,2-xylosyltransferase (XylT), and core α1,3-fucosyltransferase (FucT) are responsible for the transfer of β1,2-linked xylose and core α1,3-linked fucose residues to N-glycans of glycoproteins, respectively. To test the clinical relevance of β1,2-xylose-containing epitopes, expression of the tomato β1,2-xylosyltransferase was down-regulated by RNA interference (RNAi) in transgenic plants. Fruits harvested from these transgenic plants were analyzed for accumulation of XylT mRNA, abundance of β1,2-xylose epitopes and their allergenic potential. Based on quantitative real-time PCR analysis XylT mRNA levels were reduced up to 10-fold in independent transgenic lines as compared to untransformed control, whereas no xylosylated N-glycans could be revealed by MS analysis. Immunoblotting using anti-xylose-specific IgG antibodies revealed a strong reduction of β1,2-xylose-containing epitopes. Incubating protein extracts from untransformed controls and XylT_RNAi plants with sera from tomato allergic patients showed a patient-specific reduction in IgE-binding, indicating a reduced allergenic potential of XylT_RNAi tomato fruits, in vitro. To elucidate the clinical relevance of β1,2-xylose-containing complex N-glycans skin prick tests were performed demonstrating a reduced responsiveness of tomato allergic patients, in vivo. This study provides strong evidence for the clinical relevance of β1,2-xylose-containing epitopes in vivo.

Post-translational modification of plant-made foreign proteins; glycosylation and beyond

The complex and diverse nature of the post-translational modification (PTM) of proteins represents an efficient and cost-effective mechanism for the exponential diversification of the genome. PTMs have been shown to affect almost every aspect of protein activity, including function, localisation, stability, and dynamic interactions with other molecules. Although many PTMs are evolutionarily conserved there are also important kingdom-specific modifications which should be considered when expressing recombinant proteins. Plants are gaining increasing acceptance as an expression system for recombinant proteins, particularly where eukaryotic-like PTMs are required. Glycosylation is the most extensively studied PTM of plant-made recombinant proteins. However, other types of protein processing and modification also occur which are important for the production of high quality recombinant protein, such as hydroxylation and lipidation. Plant and/or protein engineering approaches offer many opportunities to exploit PTM pathways allowing the molecular farmer to produce a humanised product with modifications functionally similar or identical to the native protein. Indeed, plants have demonstrated a high degree of tolerance to changes in PTM pathways allowing recombinant proteins to be modified in a specific and controlled manner, frequently resulting in a homogeneity of product which is currently unrivalled by alternative expression platforms. Whether a recombinant protein is intended for use as a scientific reagent, a cosmetic additive or as a pharmaceutical, PTMs through their presence and complexity, offer an extensive range of options for the rational design of humanised (biosimilar), enhanced (biobetter) or novel products.

Production of complex multiantennary N-glycans in Nicotiana benthamiana plants

In recent years, plants have been developed as an alternative expression system to mammalian hosts for the production of therapeutic proteins. Many modifications to the plant glycosylation machinery have been made to render it more human because of the importance of glycosylation for functionality, serum half-life, and the safety profile of the expressed proteins. These modifications include removal of plant-specific β1,2-xylose and core α1,3-fucose, and addition of bisecting N-acetylglucosamine, β1,4-galactoses, and sialic acid residues. Another glycosylation step that is essential for the production of complex human-type glycans is the synthesis of multiantennary structures, which are frequently found on human N-glycans but are not generated by wild-type plants. Here, we report both the magnICON-based transient as well as stable introduction of the α1,3-mannosyl-β1,4-N-acetylglucosaminyltransferase (GnT-IV isozymes a and b) and α1,6-mannosyl-β1,6-N-acetylglucosaminyltransferase (GnT-V) in Nicotiana benthamiana plants. The enzymes were targeted to the Golgi apparatus by fusing their catalytic domains to the plant-specific localization signals of xylosyltransferase and fucosyltransferase. The GnT-IV and -V modifications were tested in the wild-type background, but were also combined with the RNA interference-mediated knockdown of β1,2-xylosyltransferase and α1,3-fucosyltransferase. Results showed that triantennary Gn[GnGn] and [GnGn]Gn N-glycans could be produced according to the expected activities of the respective enzymes. Combination of the two enzymes by crossing stably transformed GnT-IV and GnT-V plants showed that up to 10% tetraantennary [GnGn][GnGn], 25% triantennary, and 35% biantennary N-glycans were synthesized. All transgenic plants were viable and showed no aberrant phenotype under standard growth conditions.

Recombinant protein expression in Nicotiana

Recombinant protein pharmaceuticals are now widely used in treatment of chronic diseases, and several recombinant protein subunit vaccines are approved for human and veterinary use. With growing demand for complex protein pharmaceuticals, such as monoclonal antibodies, manufacturing capacity is becoming limited. There is increasing need for safe, scalable, and economical alternatives to mammalian cell culture-based manufacturing systems, which require substantial capital investment for new manufacturing facilities. Since a seminal paper reporting immunoglobulin expression in transgenic plants was published in 1989, there have been many technological advances in plant expression systems to the present time where production of proteins in leaf tissues of nonfood crops such as Nicotiana species is considered a viable alternative. In particular, transient expression systems derived from recombinant plant viral vectors offer opportunities for rapid expression screening, construct optimization, and expression scale-up. Extraction of recombinant proteins from Nicotiana leaf tissues can be achieved by collection of secreted protein fractions, or from a total protein extract after grinding the leaves with buffer. After separation from solids, the major purification challenge is contamination with elements of the photosynthetic complex, which can be solved by application of a variety of facile and proven strategies. In conclusion, the technologies required for safe, efficient, scalable manufacture of recombinant proteins in Nicotiana leaf tissues have matured to the point where several products have already been tested in phase I clinical trials and will soon be followed by a rich pipeline of recombinant vaccines, microbicides, and therapeutic proteins.

Cereal seed storage protein synthesis: fundamental processes for recombinant protein production in cereal grains

Cereal seeds provide an ideal production platform for high-value products such as pharmaceuticals and industrial materials because seeds have ample and stable space for the deposition of recombinant products without loss of activity at room. Seed storage proteins (SSPs) are predominantly synthesized and stably accumulated in maturing endosperm tissue. Therefore, understanding the molecular mechanisms regulating SSP expression and accumulation is expected to provide valuable information for producing higher amounts of recombinant products. SSP levels are regulated by several steps at the transcriptional (promoters, transcription factors), translational and post-translational levels (modification, processing trafficking, and deposition). Our objective is to develop a seed production platform capable of producing very high yields of recombinant product. Towards this goal, we review here the individual regulatory steps controlling SSP synthesis and accumulation.

Transgenic plants as a source for the bioscavenging enzyme, human butyrylcholinesterase

Organophosphorous pesticides and nerve agents inhibit the enzyme acetylcholinesterase at neuronal synapses and in neuromuscular junctions. The resulting accumulation of acetylcholine overwhelms regulatory mechanisms, potentially leading to seizures and death from respiratory collapse. While current therapies are only capable of reducing mortality, elevation of the serum levels of the related enzyme butyrylcholinesterase (BChE) by application of the purified protein as a bioscavenger of organophosphorous compounds is effective in preventing all symptoms associated with poisoning by these toxins. However, BChE therapy requires large quantities of enzyme that can easily overwhelm current sources. Here, we report genetic optimization, cloning and high-level expression of human BChE in plants. Plant-derived BChE is shown to be biochemically similar to human plasma-derived BChE in terms of catalytic activity and inhibitor binding. We further demonstrate the ability of the plant-derived bioscavenger to protect animals against an organophosphorous pesticide challenge.

Chloroplasts as expression platforms for plant-produced vaccines

Production of recombinant subunit vaccines from genes incorporated in the plastid genome is advantageous because of the attainable expression level due to high transgene copy number and the absence of gene silencing; biocontainment as a consequence of maternal inheritance of plastids and no transgene presence in the pollen; and expression of multiple transgenes in prokaryotic-like operons. We discuss the core technology of plastid transformation in Chlamydomonas reinhardtii, a unicellular alga, and Nicotiana tabacum (tobacco), a flowering plant species, and demonstrate the utility of the technology for the production of recombinant vaccine antigens.

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.

Efficient plant-based production of chicken interleukin-12 yields a strong immunostimulatory cytokine

Interleukin-12 (IL-12), an important immunomodulator for cell-mediated immunity, shows significant potential as a vaccine adjuvant and anticancer therapeutic in mammals. Therapeutic strategies to develop mammalian IL-12 as a vaccine adjuvant/immunomodulator for promoting cellular immunity and establishing a Th1-biased immune response further support the potential value of ChIL-12. Transgenic plants show promise as scalable bioproduction platforms for challenging biopharmaceutical proteins. We have expressed, characterized, and purified biologically active ChIL-12 in plants using a rapid Agrobacterium-mediated tobacco plant-based transient expression system. To ensure the stoichiometric expression and assembly of p35 and p40, we expressed a single-chain version of chicken IL-12 (ChIL-12). A histidine 6x tag was used for identity and purification of ChIL-12(His) protein. Our results demonstrated precise cleavage of the endogenous chicken p40 signal peptide in plants as well as addition of N-linked glycans. Biological activity was confirmed in vitro by interferon-gamma secretion of ChIL-12-treated chicken splenocytes. In addition, splenocytes treated with ChIL-12 expressed with or without the His tag demonstrated comparable ChIFN-gamma induction. These studies indicate that plant-based platforms for bioproduction of complex pharmaceutical proteins produce functional ChIL-12 and provide key advantages in safety, scale, and cost-effective platform for veterinary vaccine and therapeutic applications.

Recombinant mouse granulocyte-macrophage colony-stimulating factor is glycosylated in transgenic tobacco and maintains its biological activity

The granulocyte-macrophage colony-stimulating factor (GM-CSF) is a cytokine with many important applications and, due to its immunostimulatory properties, could also be used as a vaccine adjuvant. A simple strategy to produce recombinant mouse GM-CSF (mGM-CSF) in transgenic Nicotiana tabacum plants was used in this study. The mGM-CSF cDNA followed by the sequence encoding endoplasmic reticulum retention signal (KDEL) was cloned into the ImpactVector under the control of the strong promoter from the gene encoding a small subunit of Rubisco. In transgenic plants the accumulation level of recombinant mGM-CSF varied in the individual transformants from 8 to 19 microg/g of fresh leaf tissue, which makes up to 0.22% of total soluble protein. In most analyzed plants, the apparent molecular weight of the recombinant protein was larger than predicted due to its N-glycosylation, presumably in 2 sites. The recombinant plant-produced murine GM-CSF retained its biological activity as confirmed in vitro in proliferation assay using a mouse cell line, which is growth-dependent on GM-CSF.

A companion protease inhibitor for the protection of cytosol-targeted recombinant proteins in plants

We reported earlier the potential of tomato cathepsin D inhibitor (SlCDI) as an in-built stabilizing agent for the protection of recombinant proteins in transgenic plant leaf crude extracts (Plant Biotechnol J.4, 359-368). Here we document the potential of SlCDI for the in situ protection of proteins in potato leaves. Total protein assays with control and SlCDI-expressing potato lines indicated a positive impact of slcdi transgene expression on leaf protein content, with a mean relative increase of 35%-40% depending on the light regime. Out of approximately 700 proteins detected on 2-D gels, only 20 exhibited a significantly altered level on a protein-specific basis, whereas most proteins were up-regulated on a leaf fresh weight basis, albeit at variable rates. Quantitative reverse trancriptase-PCR assays for rubisco activase showed similar transcript levels in leaves of test and control lines despite protein levels increased by two- to threefold in SlCDI-expressing lines. These observations, along with the unrelated biological functions assigned to MS-identified proteins up-regulated in leaves and protease assays showing slightly increased proteasome activity in protein extracts of SlCDI-expressing lines, suggest a general, proteasome-independent protein stabilizing effect of SlCDI in planta. Transient expression assays with human alpha(1)-antichymotrypsin also showed a stabilizing effect for SlCDI on heterologous proteins, leading to net levels of the human protein increased by approximately 2.5-fold in SlCDI-expressing plants. These data illustrate, overall, the potential of SlCDI as an in vivo protein-stabilizing agent in transgenic plant systems, useful to improve protein levels and recombinant protein accumulation.

Expression of a glycosylated GFP as a bivalent reporter in exocytosis

The complex-type N-linked glycans of plants differ markedly in structure from those of animals. Like those of insects and mollusks they lack terminal sialic acid(s) and may contain an alpha-(1,3)-fucose (Fuc) linked to the proximal GlcNAc residue and/or a beta-(1,2)-xylose (Xyl) residue attached to the proximal mannose (Man) of the glycan core. N-glycosylated GFPs were used in previous studies showing their effective use to report on membrane traffic between the ER and the Golgi apparatus in plant cells. In all these cases glycosylated tags were added at the GFP termini. Because of the position of the tag and depending on the sorting and accumulation site of these modified GFP, there is always a risk of processing and degradation, and this protein design cannot be considered ideal. Here, we describe the development of three different GFPs in which the glycosylation site is internally localized at positions 80, 133, or 172 in the internal sequence. The best glycosylation site was at position 133. This glycosylated GFPgl133 appears to be protected from undesired processing of the glycosylation site and represents a bivalent reporter for biochemical and microscopic studies. After experimental validation, we can conclude that amino acid 133 is an effective glycosylation site and that the GFPgl133 is a powerful tool for in vivo investigations in plant cell biology.

[Bioproduction of pharmaceutical proteins: review and perspectives]

The increase in importance of the biopharmaceutical market generates enormous needs in production of therapeutic molecules. The world capacities of production, mainly therapeutic proteins, impossible to obtain by chemical synthesis, are at present insufficient. Besides the economic aspects in set, play, the question of the technologies used to produce these molecules is important: the tried(felt) technologies (bacteria, yeasts, cells of mammals), used for a long time rather widely remain expensive, or see their limited field of application; the innovative technologies (avian cells, animal and plant transgenic, among others), as for them, again have to show their ability.

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.

Plants as bioreactors: Recent developments and emerging opportunities

In recent years, the use of plants as bioreactors has emerged as an exciting area of research and significant advances have created new opportunities. The driving forces behind the rapid growth of plant bioreactors include low production cost, product safety and easy scale up. As the yield and concentration of a product is crucial for commercial viability, several strategies have been developed to boost up protein expression in transgenic plants. Augmenting tissue-specific transcription, elevating transcript stability, tissue-specific targeting, translation optimization and sub-cellular accumulation are some of the strategies employed. Various kinds of products that are currently being produced in plants include vaccine antigens, medical diagnostics proteins, industrial and pharmaceutical proteins, nutritional supplements like minerals, vitamins, carbohydrates and biopolymers. A large number of plant-derived recombinant proteins have reached advanced clinical trials. A few of these products have already been introduced in the market.

Transient co-expression for fast and high-yield production of antibodies with human-like N-glycans in plants

Plant-based transient expression is potentially the most rapid and cost-efficient system for the production of recombinant pharmaceutical proteins, but safety concerns associated with plant-specific N-glycosylation have hampered its adoption as a commercial production system. In this article, we describe an approach based on the simultaneous transient co-expression of an antibody, a suppressor of silencing and a chimaeric human beta1,4-galactosyltransferase targeted for optimal activity to the early secretory pathway in agroinfiltrated Nicotiana benthamiana leaves. This strategy allows fast and high-yield production of antibodies with human-like N-glycans and, more generally, provides solutions to many critical problems posed by the large-scale production of therapeutic and vaccinal proteins, specifically yield, volume and quality.