The green revolution: plants as heterologous expression vectors

Future of cancer immunotherapy using plant virus-based nanoparticles

Immunotherapy potentiates a patient’s immune response against some forms of cancer, including malignant tumors. In this Special Report, we have summarized the use of nanoparticles that have been designed for use in cancer immunotherapy with particular emphasis on plant viruses. Plant virus-based nanoparticles are an ideal choice for therapeutic applications, as these nanoparticles are not only capable of targeting the desired cells but also of being safely delivered to the body without posing any threat of infection. Plant viruses can be taken up by tumor cells and can be functionalized as drug delivery vehicles. This Special Report describes how the future of cancer immunotherapy could be a success through the merger of computer-based technology using plant-virus nanoparticles.

The nonpathogenic nature of plant viral nanoparticles makes them an ideal choice for therapeutic applications such as cancer. Understanding the molecular mechanisms behind the immune response to cancer has facilitated the use of nanotechnology as an effective cancer therapy. Biologically active self-replicating plant virus particles can be introduced to the bloodstream of the human body and used as effective drug delivery vehicles. This Special Report describes how a combination of computer-based technology and plant-virus nanoparticles can assist in cancer immunotherapy.

Transient expression of CCL21as recombinant protein in tomato

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Applied use of plants as bioreactors for production of recombinant proteins.

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The present study is the first to report gene expression of ccl21 construct in tomato via agro infiltration to use this plant as oral vaccine.

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To investigate the role of this protein in anti-metastatic function on cancer cells

The main goal of this study was to investigate the possibility of expressing recombinant protein of C-C chemokine ligand 21 (CCL21) in Solanum lycopersicum via agroinfiltration. CCL21 is a chemokine can be used for anti-metastatic of cancer cell lines. To examine the expression of CCL21 protein in S. lycopersicum, the construct of ccl21 was synthesized. This construct was cloned into pBI121 and the resulting CCL21 plasmid was agro-infiltrated into S. lycopersicum leaves. Within three days after infiltration, Expression of the foreign gene was confirmed by quantitative Real-time PCR. A recombinant CCL21 protein was immunogenically detected by western blot, dot blot and ELISA assay. And results showed that the foreign gene was expressed in the transformed leaves in high level. Also scratch assay was used to investigate the role of this protein in anti-metastatic function. The results demonstrated anti-metastatic of cancer cells in the presence of this protein.

Growth suppression of colorectal cancer by plant-derived multiple mAb CO17-1A × BR55 via inhibition of ERK1/2 phosphorylation

We have generated the transgenic Tabaco plants expressing multiple monoclonal antibody (mAb) CO7-1A × BR55 by cross-pollinating with mAb CO17-1A and mAb BR55. We have demonstrated the anti-cancer effect of plant-derived multiple mAb CO17-1A × BR55. We find that co-treatment of colorectal mAbs (anti-epithelial cellular adhesion molecule (EpCAM), plant-derived monoclonal antibody (mAb(P)) CO17-1A and mAb(P) CO17-1A × BR55) with RAW264.7 cells significantly inhibited the cell growth in SW620 cancer cells. In particular, multi mAb(P) CO17-1A × BR55 significantly and efficiently suppressed the growth of SW620 cancer cells compared to another mAbs. Apoptotic death-positive cells were significantly increased in the mAb(P) CO17-1A × BR55-treated. The mAb(P) CO17-1A × BR55 treatment significantly decreased the expression of B-Cell lymphoma-2 (BCl-2), but the expression of Bcl-2-associated X protein (Bax), and cleaved caspase-3 were markedly increased. In vivo, the mAb(P) CO17-1A × BR55 significantly and efficiently inhibited the growth of colon tumors compared to another mAbs. The apoptotic cell death and inhibition of pro-apoptotic proteins expression were highest by treatment with mAb(P) CO17-1A × BR55. In addition, the mAb(P) CO17-1A × BR55 significantly inhibited the extracellular signal-regulated kinase 1 and 2 (ERK1/2) phosphorylation in cancer cells and tumors. Therefore, this study results suggest that multiple mAb(P) CO17-1A × BR55 has a significant effect on apoptosis-mediated anticancer by suppression of ERK1/2 phosphorylation in colon cancer compared to another mAbs. In light of these results, further clinical investigation should be conducted on mAb(P) CO17-1A × BR55 to determine its possible chemopreventive and/or therapeutic efficacy against human colon cancer.

Overexpression and self-assembly of virus-like particles in Nicotiana benthamiana by a single-vector DNA replicon system

Based on recent developments, virus-like particles (VLPs) are considered to be perfect candidates as nanoplatforms for applications in materials science and medicine. To succeed, mass production of VLPs and self-assembly into a correct form in plant systems are key factors. Here, we report expression of synthesized coat proteins of the three viruses, Brome mosaic virus, Cucumber mosaic virus, and Maize rayado fino virus, in Nicotiana benthamiana and production of self-assembled VLPs by transient expression system using agroinfiltration. Each coat protein was synthesized and cloned into a pBYR2fp single replicon vector. Target protein expression in cells containing p19 was fourfold higher than that of cells lacking p19. After agroinfiltration, protein expression was analyzed by SDS-PAGE and quantitative image analyzer. Quantitative analysis showed that BMVCP, CMVCP, and MRFVCP concentrations were 0.5, 1.0, and 0.8 mg · g(-1) leaf fresh weight, respectively. VLPs were purified by sucrose cushion ultracentrifugation and then analyzed by transmission electron microscopy. Our results suggested that BMVCP and CMVCP proteins expressed in N. benthamiana leaves were able to correctly self-assemble into particles. Moreover, we evaluated internal cavity accessibility of VLPs to load foreign molecules. Finally, plant growth conditions after agroinfiltration are critical for increasing heterologous protein expression levels in a transient expression system.

Vaccines for lyssaviruses other than rabies

Several new lyssaviruses have emerged in the past decade and it is likely that more remain to be discovered. There are six recognized genotypes of lyssavirus other than the rabies virus (genotype 1). All but one of these has been associated with human cases, with the resulting disease clinically similar to rabies. Rabies vaccines provide a means of pre- and postexposure prophylaxis against rabies and some of the other genotypes, but not all. Those that are crossprotected fall into phylogroup 1 of the genus, and those not protected in phylogroup 2. The crossprotection of phylogroup 1 viruses by rabies vaccines and the development of new, broader range or specific vaccines for phylogroup 2 viruses are reviewed.

Plant-derived mAbs have effective anti-cancer activities by increasing ganglioside expression in colon cancers

An epithelial cell adhesion molecule (EpCAM) was selectively expressed in human colorectal carcinoma. Treatment with plant-derived anti-EpCAM mAb (mAbP CO17-1A) and RAW264.7 cells inhibited cell growth in the human colorectal cancer cell line SW620. In SW620 treated with mAbP CO17-1A and RAW264.7 cells, expression of p53 and p21 increased, whereas the expression of G1 phase-related proteins, cyclin D1, CDK4, cyclin E, and CDK2, decreased, similar to mammalian-derived mAb (mAbM) CO17-1A. Similar to mAbM CO17-1A, treatment with mAbP CO17-1A and RAW264.7 cell decreased the expression of anti-apoptotic protein, Bcl-2, but the expression of pro-apoptotic proteins Bax, TNF-α, caspase-3, caspase-6, caspase-8 and caspase-9, increased. Cells treated with mAbP CO17-1A and RAW264.7 cells expressed metastasis-related gangliosides, GM1 and GD1a, similar to mAbM CO17-1A. These results suggest that mAbP CO17-1A is as effective on anti-cancer activity as mAbM CO17-1A.

Production of a de-novo designed antimicrobial peptide in Nicotiana benthamiana

Antimicrobial peptides are important defense compounds of higher organisms that can be used as therapeutic agents against bacterial and/or viral infections. We designed several antimicrobial peptides containing hydrophobic and positively charged clusters that are active against plant and human pathogens. Especially peptide SP1-1 is highly active with a MIC value of 0.1 μg/ml against Xanthomonas vesicatoria, Pseudomonas corrugata and Pseudomonas syringae pv syringae. However, for commercial applications high amounts of peptide are necessary. The synthetic production of peptides is still quite expensive and, depending on the physico-chemical features, difficult. Therefore we developed a plant/tobacco mosaic virus-based production system following the 'full virus vector strategy' with the viral coat protein as fusion partner for the designed antimicrobial peptide. Infection of Nicotiana benthamiana plants with such recombinant virus resulted in production of huge amounts of virus particles presenting the peptides all over their surface. After extraction of recombinant virions, peptides were released from the coat protein by chemical cleavage. A protocol for purification of the antimicrobial peptides using high resolution chromatographic methods has been established. Finally, we yielded up to 0.025 mg of peptide per g of infected leaf biomass. Mass spectrometric and NMR analysis revealed that the in planta produced peptide differs from the synthetic version only in missing of N-terminal amidation. But its antimicrobial activity was in the range of the synthetic one. Taken together, we developed a protocol for plant-based production and purification of biologically active, hydrophobic and positively charged antimicrobial peptide.

Plant virus expression vectors set the stage as production platforms for biopharmaceutical proteins

Transgenic plants present enormous potential as a cost-effective and safe platform for large-scale production of vaccines and other therapeutic proteins. A number of different technologies are under development for the production of pharmaceutical proteins from plant tissues. One method used to express high levels of protein in plants involves the employment of plant virus expression vectors. Plant virus vectors have been designed to carry vaccine epitopes as well as full therapeutic proteins such as monoclonal antibodies in plant tissue both safely and effectively. Biopharmaceuticals such as these offer enormous potential on many levels, from providing relief to those who have little access to modern medicine, to playing an active role in the battle against cancer. This review describes the current design and status of plant virus expression vectors used as production platforms for biopharmaceutical proteins.

High levels of expression of fibroblast growth factor 21 in transgenic tobacco (Nicotiana benthamiana)

Fibroblast growth factor-21 (FGF21) is a hepatic hormone that plays a critical role in metabolism, stimulating fatty acid oxidation in the liver and glucose uptake in adipose tissue. In this study, we produced tobacco plants expressing human recombinant FGF21 (hFGF21) via Agrobacterium-mediated transformation using a potato virus X (PVX)-based vector (pgR107). The vector contained the sequence encoding the human FGF21 gene fused with green florescence protein and a histidine tag. The recombinant plasmid was introduced into leaf cells of Nicotiana benthamiana (a wild Australian tobacco) via Agrobacterium-mediated agroinfiltration. As determined by fluorescence and Western blot of leaf extracts, the hFGF21 gene was correctly translated in tobacco plants. Seven days after agroinfection, the recombinant hFGF21 had accumulated to levels as high as 450 μg g(-1) fresh weight in leaves of agroinfected plants. The recombinant hFGF21 was purified from plant tissues by Ni-NTA affinity chromatography, and the purified hFGF21 stimulated glucose uptake in 3T3/L1 cells. This indicated that the recombinant hFGF21 expressed via the PVX viral vector in N. benthamiana was biologically active.

Expression of the protective antigen for PEDV in transgenic duckweed, Lemna minor

Duckweeds are small, floating aquatic plants with a number of useful characteristics, including edibility, fast-growing, and a clonal proliferation. Duckweed is also fed to animals as a diet complement because of its high nutritional value. Porcine epidemic diarrhea virus (PEDV) is a major causative agent of fatal diarrhea in piglets and is a serious problem in the hog-raising industry. In this study, we assessed the feasibility of producing a protective antigen for the PEDV spike protein 1 using duckweed, Lemna minor. Stably transformed Lemna were obtained by co-cultivation with A. tumefaciens EHA105 harboring the PEDV spike protein gene. Transgene integration and expression of the PEDV spike protein 1 gene were confirmed by genomic PCR and RT-PCR and western blot analysis of transgenic Lemna, respectively. This is the first report of the expression of a vaccine antigen against an animal infectious disease in duckweed.

Overview of expression of hepatitis B surface antigen in transgenic plants

Hepatitis B virus (HBV), a pathogen for chronic liver infection, afflicts more than 350 million people world-wide. The effective way to control the virus is to take HBV vaccine. Hepatitis B surface antigen (HBsAg) is an effective protective antigen suitable for vaccine development. At present, "edible" vaccine based on transgenic plants is one of the most promising directions in novel types of vaccines. HBsAg production from transgenic plants has been carried out, and the transgenic plant expression systems have developed from model plants (such as tobacco, potato and tomato) to other various plant platforms. Crude or purified extracts of transformed plants have been found to conduct immunological responses and clinical trials for hepatitis B, which gave the researches of plant-based HBsAg production a big boost. The aim of this review was to summarize the recent data about plant-based HBsAg development including molecular biology of HBsAg gene, selection of expression vector, the expression of HBsAg gene in plants, as well as corresponding immunological responses in animal models or human.

Development of Tobacco necrosis virus A as a vector for efficient and stable expression of FMDV VP1 peptides

Plant virus-based expression systems provide attractive alternatives for production of animal virus-originated antigenic peptides. In the present study, an infectious cDNA clone of Tobacco necrosis virus A Chinese isolate (TNV-A(C)) was used for expression of different peptides derived from Foot and mouth disease virus (FMDV) serotype O VP1 fused downstream of the coat protein (CP) open reading frame (ORF). Chenopodium amaranticolor inoculated with in vitro transcripts of the chimaeras developed symptoms similar to those caused by wild-type TNV-A(C). Western blot and RT-PCR detection of the infected leaves demonstrated that the chimaeras were infective, and a large number of self-assembled virions could be purified and observed under electron microscopy. Immunogold labelling revealed that highly expressed FMDV VP1 peptides could be displayed on the surfaces of virus particles. Additional immunoblotting and DNA sequence analyses showed that most of the chimaeras contained unmodified foreign peptides even after six successive passages in C. amaranticolor and three passages in Nicotiana benthamiana. Our results also suggest that the amino acid sequence and peptide length have a substantial influence on viral morphogenesis and systemic infections. Finally, animal experiments showed that purified chimaeric virus particles (CVPs) could induce a strong immune response against FMDV structural protein VP1 via an intramuscular route. And when inoculated nasally, CVPs could induce systemic and mucosal immune responses in mice.

Targeting cancer with 'smart bombs': equipping plant virus nanoparticles for a 'seek and destroy' mission

This article discusses plant virus nanoparticles as a weapon in the war on cancer. The successes and failures of numerous nanoparticle strategies are discussed as a background to consideration of the plant virus nanoparticle approach. To have therapeutic benefit, the advantages of the targeted nanoparticle must outweigh the problems of colloidal stability, uptake by the reticuloendothelial system as well as the requirement for clearance from the body. Biodegradable nanoparticles are considered to have the most promise to address these complex phenomena. After justifying the choice of biodegradable particles, the article focuses on comparison of micelles, liposomes, polymers and modified plant viruses. The structural uniformity, cargo capacity, responsive behavior and ease of manufacturing of plant virus nanoparticles are unique properties that suggest they have a wider role to play in targeted therapy. The loading of chemotherapeutic cargo is discussed, with specific reference to the advantage of reversible transitions of the capsid of Red clover necrotic mosaic virus. These features will be contrasted and compared with other biodegradable 'smart bombs' that target cancer cells.

A DNA replicon system for rapid high-level production of virus-like particles in plants

Recombinant virus-like particles (VLPs) represent a safe and effective vaccine strategy. We previously described a stable transgenic plant system for inexpensive production and oral delivery of VLP vaccines. However, the relatively low-level antigen accumulation and long-time frame to produce transgenic plants are the two major roadblocks in the practical development of plant-based VLP production. In this article, we describe the optimization of geminivirus-derived DNA replicon vectors for rapid, high-yield plant-based production of VLPs. Co-delivery of bean yellow dwarf virus (BeYDV)-derived vector and Rep/RepA-supplying vector by agroinfiltration of Nicotiana benthamiana leaves resulted in efficient replicon amplification and robust protein production within 5 days. Co-expression of the P19 protein of tomato bush stunt virus, a gene silencing inhibitor, further enhanced VLP accumulation by stabilizing the mRNA. With this system, hepatitis B core antigen (HBc) and Norwalk virus capsid protein (NVCP) were produced at 0.80 and 0.34 mg/g leaf fresh weight, respectively. Sedimentation analysis and electron microscopy of transiently expressed antigens verified the efficient assembly of VLPs. Furthermore, a single replicon vector containing a built-in Rep/RepA cassette without P19 drove protein expression at similar levels as the three-component system. These results demonstrate the advantages of fast and high-level production of VLP-based vaccines using the BeYDV-derived DNA replicon system for transient expression in plants.

Production of native and modified recombinant Der p 1 molecules in tobacco plants

BACKGROUND

As a complex molecule requiring post-translational processing, it has been difficult to produce the Der p 1 major allergen from the Dermatophagoides pteronyssinus house dust mite in a recombinant form.

OBJECTIVE

Here, we tested whether transgenic tobacco plants are suitable to express Der p 1, either as a wild-type molecule or as variants lacking N-glycosylation sites (Gly(-)) and/or cysteine protease activity (Enz(-)). Methods Using Agrobacterium tumefaciens-based transformation, pro Der p 1 molecules bearing mutations within either the N-glycosylation sites (N34Q, N150Q) and/or the cysteine protease-active site (C132V) were expressed in tobacco plants. After purification by ion exchange chromatography, allergens were characterized using immunoblotting, circular dichroism (CD), as well as basophil and T lymphocyte stimulation assays.

RESULTS

Four forms of recombinant Der p 1 (i.e. wild-type Gly(+)/Enz(+), as well as Gly(-)/Enz(+), Gly(+)/Enz(-) or Gly(-)/Enz(-) variants) were successfully expressed in tobacco leaves as pro Der p 1 molecules. Spontaneous cleavage of the pro-peptide was observed in tobacco leaf extracts for all forms of recombinant Der p 1 (r Der p 1). CD confirmed that all r Der p 1 molecules, with the exception of the Gly(-)/Enz(-) variant, exhibited secondary structures comparable to the natural protein. A cysteine protease activity was associated only with the Gly(+)/Enz(+) form. All these molecules exhibit a profile similar to natural Der p 1 with respect to IgE immunoreactivity, basophil activation and T cell recognition.

CONCLUSION

A tobacco plant expression system allows the production of various forms of mature Der p 1, which could be used for diagnostic or immunotherapeutic purposes.

Plants as bioreactors for the production of vaccine antigens

Plants have been identified as promising expression systems for commercial production of vaccine antigens. In phase I clinical trials several plant-derived vaccine antigens have been found to be safe and induce sufficiently high immune response. Thus, transgenic plants, including edible plant parts are suggested as excellent alternatives for the production of vaccines and economic scale-up through cultivation. Improved understanding of plant molecular biology and consequent refinement in the genetic engineering techniques have led to designing approaches for high level expression of vaccine antigens in plants. During the last decade, several efficient plant-based expression systems have been examined and more than 100 recombinant proteins including plant-derived vaccine antigens have been expressed in different plant tissues. Estimates suggest that it may become possible to obtain antigen sufficient for vaccinating millions of individuals from one acre crop by expressing the antigen in seeds of an edible legume, like peanut or soybean. In the near future, a plethora of protein products, developed through ‘naturalized bioreactors’ may reach market. Efforts for further improvements in these technologies need to be directed mainly towards validation and applicability of plant-based standardized mucosal and edible vaccines, regulatory pharmacology, formulations and the development of commercially viable GLP protocols. This article reviews the current status of developments in the area of use of plants for the development of vaccine antigens.

Production of recombinant allergens in plants

A large percentage of allergenic proteins are of plant origin. Hence, plant-based expression systems are considered ideal for the recombinant production of certain allergens. First attempts to establish production of plant-derived allergens in plants focused on transient expression in Nicotiana benthamiana infected with recombinant viral vectors. Accordingly, allergens from birch and mugwort pollen, as well as from apple have been expressed in plants. Production of house dust mite allergens has been achieved by Agrobacterium-mediated transformation of tobacco plants. Beside the use of plants as production systems, other approaches have focused on the development of edible vaccines expressing allergens or epitopes thereof, which bypasses the need of allergen purification. The potential of this approach has been convincingly demonstrated for transgenic rice seeds expressing seven dominant human T cell epitopes derived from Japanese cedar pollen allergens. Parallel to efforts in developing recombinant-based diagnostic and therapeutic reagents, different gene-silencing approaches have been used to decrease the expression of allergenic proteins in allergen sources. In this way hypoallergenic ryegrass, soybean, rice, apple, and tomato were developed.

Antiviral potentials of medicinal plants

Medicinal plants have been widely used to treat a variety of infectious and non-infectious ailments. According to one estimate, 25% of the commonly used medicines contain compounds isolated from plants. Several plants could offer a rich reserve for drug discovery of infectious diseases, particularly in an era when the latest separation techniques are available on one hand, and the human population is challenged by a number of emerging infectious diseases on the other hand. Among several other ailments, viral infections, particularly infections associated with human immunodeficiency virus type 1 (HIV-1) and 2 (HIV-2), and newly emerging infectious viruses have challenged mankind survival. Of importance, a variety of medicinal plants have shown promise to treat a number of viral infections, and some of them possess broad-spectrum antiviral activity. In the past, exploration into the antiviral activity of various promising medicinal plants was limited due to: (a) highly infectious nature of viruses and (b) lack of appropriate separation techniques for the identification of antiviral components from plants. Development of vector-based strategies, in which non-infectious molecular clone of a virus could be used for antiviral screening purposes, and advancement in separation technologies offers promise for medicinal plants usage in modern drug discovery. This article describes potential antiviral properties of medicinal plants against a diverse group of viruses, and suggests screening the potential of plants possessing broad-spectrum antiviral effects against emerging viral infections.

Expression of human acidic fibroblast growth factor in Nicotiana benthamiana with a potato-virus-X-based binary vector

The aFGF (acidic fibroblast growth factor) plays an important role in morphogenesis, angiogenesis and wound healing and is therefore of potential medical interest. A DNA fragment encoding haFGF (human aFGF) has been cloned into the PVX (potato virus X)-based binary vector (pgR107) and transiently expressed in Nicotiana benthamiana (a wild Australian tobacco) by agroinfection. Approx. 1 week after agroinfection, the recombinant haFGF accumulated in the agroinfected plants reached up to 1% of the total soluble protein. haFGF was then purified on heparin-Sepharose CL-6B. The purified haFGF could stimulate the growth of NIH 3T3 cells, suggesting that the recombinant haFGF expressed via PVX viral vector in N. benthamiana was active biologically.

Detection of Rubisco and mycotoxins as potential contaminants of a plantibody against the hepatitis B surface antigen purified from tobacco

Antibodies have been one of the proteins widely expressed in tobacco plants for pharmaceutical purposes, which demand contaminant free preparations. Rubisco constitutes 40-60% of tobacco leaf soluble proteins; therefore it is the major potential protein contaminant of plantibodies, while mycotoxins are toxic compounds that could be introduced during the biomass production and post-harvest stages with important consequences to human health. The objective of this paper was to investigate whether Rubisco and mycotoxins are present in Plantibody HB-01 preparations used in the immunopurification of the hepatitis B surface antigen. Rubisco was purified from Nicotiana tabacum yielding 154 microg of protein per gram of leaves and purity over 95%. Among mouse monoclonal antibodies generated against this enzyme, the CBSS.Rub-2 was selected for its immunodetection. It recognizes a conserved sequential epitope of Rubisco large subunit with an affinity constant of 0.13 x 10(8)M(-1). Rubisco quantification limit was 1 microg spreading to the measurement of this contaminant less than 4% of plantibodies samples. Additionally, according to a Reverse Phase-HPLC used to measure the level of adventitiously introduced contaminants, it can be concluded that aflatoxins B1, B2, G1 and G2 were undetected in the purified Plantibody HB-01 samples.

Chloroplast-derived anthrax and other vaccine antigens: their immunogenic and immunoprotective properties

Transgenic plants offer many advantages, including low cost of production (by elimination of fermenters), storage and transportation, heat stability, absence of human pathogens, protection of antigens in the stomach through bioencapsulation (when delivered orally), elimination of the need for expensive purification and sterile injections and generation of both systemic and mucosal immunity. Recent studies have demonstrated that chloroplast-derived anthrax-protective antigen elicits effective immune responses, develops neutralizing antibodies, confers complete protection against anthrax lethal toxin challenge and produces 360 million doses of vaccine in one acre of transgenic plants. Chloroplast-derived vaccine antigens are efficacious against bacterial, fungal, viral and protozoan pathogens.

Development of an aqueous two-phase partitioning system for fractionating therapeutic proteins from tobacco extract

In the present study, an aqueous two-phase partitioning system (ATPS) was developed and evaluated as an initial fractionation step for therapeutic antibodies and enzymes from tobacco extracts. A detailed study has been performed to analyze the effect of pH, ionic composition of the system, types of polymers and their molecular weight and concentration, on the partitioning behavior of tobacco proteins and human anti-human immunodeficiency virus (HIV) monoclonal antibody 2F5 (mAb 2F5). A polyethyleneglycol/phosphate (PEG/Pi) aqueous two-phase system composed of 12% (w/w) PEG 1500 and 13% (w/w) phosphate buffer, pH 5, was selected as the system with the highest selectivity of antibody over native tobacco proteins. Under selected conditions, sufficient purification (3-4-fold) with high recovery at the bottom phase (approximately 95%) was achieved for mAb 2F5. In addition, the system allows removal of plant-derived compounds, such as phenolics and toxic alkaloids. The antibody fraction may be directly applied to a Protein A affinity column without any further pre-treatment, thus allowing homogenous antibody preparation. Analysis of the purified antibody fraction by enzyme-linked immunosorbent assay (ELISA) and western blot showed that the antibody was fully active and free of degraded variants or modified forms. The efficacy of the system was further demonstrated using additional proteins and enzymes of therapeutic importance, such as neuraminidase (NA) from influenza virus and human anti-HIV monoclonal antibody 2G12 (mAb 2G12), and showed that the system may find wide applicability as an economic extraction strategy for the initial fractionation of biopharmaceuticals from transgenic tobacco plants.

Removal of the selectable marker gene from transgenic tobacco plants by expression of Cre recombinase from a tobacco mosaic virus vector through agroinfection

Selection markers are often indispensable during the process of plant transformation, but dispensable once transgenic plants have been established. The Cre/lox site-specific recombination system has been employed to eliminate selectable marker genes from transgenic plants. Here we describe the use of a movement function-improved Tobacco Mosaic Virus (TMV) vector, m30B, to express Cre recombinase for elimination of the selectable marker gene nptII from transgenic tobacco plants. The transgenic tobacco plants were produced by Agrobacterium-mediated transformation with a specially designed binary vector pGNG which contained in its T-DNA region a sequence complex of 35S promoter-lox-the gfp coding sequence-rbcS terminator-Nos promoter-nptII-Nos terminator-lox-the gus coding region-Nos terminator. The expression of the recombinant viral vector m30B:Cre in plant cells was achieved by placing the viral vector under the control of the 35S promoter and through agroinoculation. After co-cultivating the pGNG-leaf discs with agro35S-m30B:Cre followed by shoot regeneration without any selection, plants devoid of the lox-flanked sequences including nptII were obtained with an efficiency of about 34% as revealed by histochemical GUS assay of the regenerants. Three of 11 GUS expressing regenerants, derived from two independent transgenic lines containing single copy of the pGNG T-DNA, proved to be free of the lox-flanked sequences by Southern blot analysis. Excision of the lox-flanked sequences in the three plants could be attributed to transient expression of Cre from the viral vector at the early stage of co-cultivation, since the cre sequence could not be detected in the viral RNA molecules accumulated in the plants, nor in their genomic DNA. The parental marker-free genotype was inherited in their selfed progeny, and all of the progeny were virus-free, apparently because TMV is not seed-transmissible. Therefore, expression of Cre from a TMV-based vector could be used to eliminate selectable marker genes from transgenic tobacco plants without sexual crossing and segregation, and this strategy could be extended to other TMV-infected plant species and applicable to other compatible virus-host plant systems.

Research Advances on Transgenic Plant Vaccines

In recent years, with the development of genetics molecular biology and plant biotechnology, the vaccination (e.g. genetic engineering subunit vaccine, living vector vaccine, nucleic acid vaccine) programs are taking on a prosperous evolvement. In particular, the technology of the use of transgenic plants to produce human or animal therapeutic vaccines receives increasing attention. Expressing vaccine candidates in vegetables and fruits open up a new avenue for producing oral/edible vaccines. Transgenic plant vaccine disquisitions exhibit a tempting latent exploiting foreground. There are a lot of advantages for transgenic plant vaccines, such as low cost, easiness of storage, and convenient immune-inoculation. Some productions converged in edible tissues, so they can be consumed directly without isolation and purification. Up to now, many transgenic plant vaccine productions have been investigated and developed. In this review, recent advances on plant-derived recombinant protein expression systems, infectious targets, and delivery systems are presented. Some issues of high concern such as biosafety and public health are also discussed. Special attention is given to the prospects and limitations on transgenic plant vaccines.

Carcinogenic role of tumor necrosis factor-alpha inducing protein of Helicobacter pylori in human stomach

Helicobacter pylori is the definitive carcinogen for stomach cancer and is known to induce proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) and interleukin-1(IL-1) in the stomach. Based on our findings that TNF-alpha is an endogenous tumor promoter, we identified the TNFalpha inducing protein (Tipalpha) gene family, and confirmed Tipalpha and HP-MP1 as new carcinogenic proteins of H. pylori.Tipalpha protein is unique to H. pylori, and this paper shows the strong tumor promoting activity of Tipalpha gene family, in cooperation with Ras protein and its mechanisms of action in relation to NF-kappaB activation, and discusses the carcinogenic role of Tipalpha in stomach cancer. Our recent finding showing that penicillin-binding proteins of other bacteria are weak homologues of Tipalpha is also discussed.

Papaya ringspot virus coat protein gene for antigen presentation in Escherichia coli

The coat protein (CP) of Papaya ringspot virus (PRSV) was analyzed for presentation of the antigenic peptide of animal virus, Canine parvovirus (CPV), in Escherichia coli (E. coli). The 45 nucleotides fragment coding for the 15-aa peptide epitope of the CPV-VP2 protein was either inserted into the PRSV-cp gene at the 5', 3' ends, both 5' and 3' ends or substituted into the 3' end of the PRSV cp gene. Each of the chimeric PRSV cp genes was cloned into the pRSET B vector under the control of the T7 promoter and transformed into E. coli. The recombinant coat proteins expressed from different chimeric PRSV-cp genes were purified and intraperitoneally injected into mice. All of the recombinant coat proteins showed strong immunogenicity and stimulate mice immune response. The recombinant coat proteins containing the CPV epitope insertion at the C terminus and at both N and C termini elicited ten times higher specific antisera in immunized mice compared with the other two recombinant coat proteins which contain the CPV epitope insertion at the N terminus and substitution at the C terminus.

A bipartite system for the constitutive and inducible expression of high levels of foreign proteins in plants

We have developed combined transgene/virus vector systems for the expression of heterologous proteins in plants. The systems are based on the bipartite RNA plant virus, cowpea mosaic virus (CPMV), and involve the amplification of integrated copies of either full-length or deleted versions of RNA-2 carrying a foreign gene. In the case of plants transgenic for full-length versions of RNA-2 carrying the green fluorescent protein (GFP), amplification can be achieved by supplying RNA-1 either exogenously or by crossing. This allows either inducible or constitutive expression of the foreign gene and results in an infection that can be passaged to further plants. Replication of deleted versions of RNA-2 harbouring GFP requires the presence of both RNA-1 and a suppressor of gene silencing, a function which we show can be supplied by HcPro from potato virus Y. Replication of the deleted versions of RNA-2 can be achieved by supplying the suppressor and RNA-1 either exogenously or by crossing, showing that this system can also be used in an inducible and constitutive format. The use of deleted forms of RNA-2 has the advantage that no infectious virus is produced, providing an effective method of biocontainment. The CPMV-based systems have advantages over existing plant expression systems in terms of the expression levels obtainable and the simplicity and flexibility of use, and should be of great practical benefit in the development of plants as bioreactors.

Plant biopharming of monoclonal antibodies

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

Inhibition of tumor growth by plant-derived mAb

The tumor-associated antigen EpCAM (GA733-2) is a highly expressed target on adenocarcinoma cells, as defined by murine mAb CO17-1A. We recently developed a transgenic plant system for the safe and inexpensive production of large quantities of mAb CO17-1A as a future source of clinical-grade protein. Although the glycosylation pattern of plant-derived mAb (mAb(P)) CO17-1A differs considerably from that of the mammalian-derived mAb (mAb(M)), we show here that the biological activity of both mAbs is quite similar. mAb(P) heavy and light chains assembled to bind the recombinant antigen GA733-2E and specifically bound to human SW948 colorectal carcinoma cells expressing the antigen GA733-2 to the same extent as mAb(M). mAb(P) was as effective as mAb(M) CO17-1A in inhibiting tumor growth of xenotransplanted SW948 cells in nude mice. These results suggest the promise of transgenic plants as a useful alternative way to produce full-size mAb for cancer immunotherapy.

Plant based HIV-1 vaccine candidate: Tat protein produced in spinach

The HIV-1 Tat protein has been recently explored as a prospective vaccine candidate with broad, subtype non-specific action. We approached the problem of delivery of Tat through the mucosal route by expressing Tat in an edible plant. The tat gene was assembled from synthetic overlapping oligonucleotides, and was subsequently cloned into a plant virus-based vector tobacco mosaic virus (TMV). Spinach plants inoculated with the Tat-producing constructs were collected and fed to mice 7-14 days post inoculation. DNA vaccinations were performed using a gene gun. Codon optimization of the Tat gene expressed in spinach plants resulted in several-fold yield increase as detected in immunoblots, and did not cause severe symptoms in inoculated plants. Mice were fed with the Tat-producing or control vector-inoculated spinach. After three feedings, 1 week apart, 1g per mice, no differences were detected in the growth rate or behavior of the animals fed with these three types of spinach. None of the animals developed measurable Tat antibodies. Following DNA vaccination, however, mice having previously received oral Tat developed higher antibody titers to Tat than did the controls, with the titers peaking at 4 weeks post-vaccination. Codon optimization allows production of up to 300-500 microg of Tat antigen per 1 g of leaf tissue in spinach using a plant virus-based expression system. The plant produced Tat does not seem to have any apparent adverse effect on mice growth or behavior, when fed with spinach for 4 weeks. ELISA data suggested that oral Tat primed for the development of Tat antibodies when mice were subsequently vaccinated with plasmid DNA designed for Tat expression.

A mucosally targeted subunit vaccine candidate eliciting HIV-1 transcytosis-blocking Abs

A vaccine that would engage the mucosal immune system against a broad range of HIV-1 subtypes and prevent epithelial transmission is highly desirable. Here we report fusing the mucosal targeting B subunit of cholera toxin to the conserved galactosylceramide-binding domain (including the ELDKWA-neutralizing epitope) of the HIV-1 gp41 envelope protein, which mediates the transcytosis of HIV-1 across the mucosal epithelia. Chimeric protein expressed in bacteria or plants assembled into oligomers that were capable of binding galactosyl-ceramide and G(M1) gangliosides. Mucosal (intranasal) administration in mice of the purified chimeric protein followed by an i.p. boost resulted in transcytosis-neutralizing serum IgG and mucosal IgA responses and induced immunological memory. Plant production of mucosally targeted immunogens could be particularly useful for immunization programs in developing countries, where desirable product traits include low cost of manufacture, heat stability, and needle-free delivery.

Elimination of alkaloids from plant-derived human monoclonal antibody

A human antiviral monoclonal antibody (mAb) expressed in transgenic tobacco plants was purified from the tobacco leaf by two different methods. In one method, total protein precipitated with ammonium sulfate was applied to a Hi-Trap protein A column (column method). In the second method, leaf supernatant obtained after liquid nitrogen leaf grinding was directly immunoprecipitated using protein A-agarose beads (immunoprecipitation method). The column and immunoprecipitation methods yielded 0.52 and 0.45 microg of plant-derived mAb (mAb(P))/g, respectively, from fresh leaf tissue. The product derived using the column method exhibited higher binding activity compared to immunoprecipitation-derived product against rabies virus strain CVS-11 in ELISA. Gas chromatography/mass spectrometry analysis, which has a detection limit of 5 pg revealed no detectable levels of nicotine or other related plant alkaloids in the purified mAb(P) from either purification procedure. Thus, both purification methodologies yield mAb(P) uncontaminated with nicotine from the tobacco leaves.

Immunization with a plant-produced colorectal cancer antigen

Cancer vaccination has become an important focus of oncology in recent years. Active immunization with tumor-associated antigens such as colorectal cancer antigen GA733-2 is thought to potentially overcome the reoccurrence of metastasis. As recombinant protein production in bioreactors is costly and subject to growing safety concerns, we tested plants as an alternative for the expression of a potential colorectal cancer vaccine. Comparing colorectal cancer antigen GA733-2 produced in tobacco plants with the same antigen produced in insect cell culture, we found a similar humoral immune response to injection of either of the two antigen preparations into mice. Some minor differences were observed in the cellular response that might be due to impurities. Our studies compare for the first time, immunization with the same antigen expressed in either plants or insect cell culture. This will provide important data for use of plants as production systems of therapeutics.

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.

Managing emerging diseases borne by fruit bats (flying foxes), with particular reference to henipaviruses and Australian bat lyssavirus

Since 1994, a number of novel viruses have been described from bats in Australia and Malaysia, particularly from fruit bats belonging to the genus Pteropus (flying foxes), and it is probable that related viruses will be found in other countries across the geographical range of other members of the genus. These viruses include Hendra and Nipah viruses, members of a new genus, Henipaviruses, within the family Paramyxoviridae; Menangle and Tioman viruses, new members of the Rubulavirus genus within the Paramyxoviridae; and Australian bat lyssavirus (ABLV), a member of the Lyssavirus genus in the family Rhabdoviridae. All but Tioman virus are known to be associated with human and/or livestock diseases. The isolation, disease associations and biological properties of the viruses are described, and are used as the basis for developing management strategies for disease prevention or control. These strategies are directed largely at disease minimization through good farm management practices, reducing the potential for exposure to flying foxes, and better disease recognition and diagnosis, and for ABLV specifically, the use of rabies vaccine for pre- and post-exposure prophylaxis. Finally, an intriguing and long-term strategy is that of wildlife immunization through plant-derived vaccination.

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.

Oral immunization of animals with transgenic cherry tomatillo expressing HBsAg

AIM: To investigate the expression of recombinant HBsAg (rHBsAg) in transgenic cherry tomatillo in order to explore the feasibility of producing HBV oral vaccine with cherry tomatillo by animal immune tests.

METHODS: The recombinant plant expression vector containing HBsAg gene was constructed. Mediated with Agrobacterium tumefaciens, HBsAg gene was transferred into cotyledons of cherry tomatillo. Transformed cherry tomatillos were obtained through hygromycin delay-selection. Integrated DNA in transgenic cherry tomatillo was confirmed by hygromycin resistance selection, Gus detection, polymerase chain reaction (PCR) and dot blotting analysis. Antigenicity of rHBsAg was examined by ELISA and the immunogenicity of rHBsAg derived from transgenic cherry tomatillo tissues was confirmed by oral feed of transformed tissues to BALB/c mice primed with commercial HBV vaccines. Specific antibody titers in mice’s serum were examined by ELISA every week.

RESULTS: By far, 10 positive lines of transgenic cherry tomatillos containing HBsAg gene were obtained. Among different organs of the same transgenic cherry tomatillo, level of rHBsAg expressed in leaves was the highest with the yield up to 300 ng/g fresh weight. And the rHBsAg expression level in fruits was about 10 ng/g fresh weight. In animal immune tests, oral delivery with transgenic tissues to mice primed with commercial vaccine instead of naive mice resulted in significant immune response.

CONCLUSION: The result of this animal immune test indicated the rHBsAg derived from transgenic cherry tomatillo possessed normal immunogenicity. This work demonstrated the feasibility to generate oral immunogenic rHBsAg in transgenic cherry tomatillo, and would provide some experimental approach for the production of low-cost oral vaccines using transgenic cherry tomatillo in large scale.