Plant cell factories and mucosal vaccines

Soybean seed protein storage vacuoles for expression of recombinant molecules

Soybean is one of the most important protein sources for human consumption and livestock feed. Soy production also allows the biosynthesis of edible oils, biodiesel, and biofertilizers. With the advent of modern agricultural biotechnology, soybean plants have also converted into bioreactors of therapeutic proteins and industrial enzymes. Soybean's characteristics, such as protein storage vacuoles (PSVs) and other unique organelles, allow the plant to be exploited as an accumulator of heterologous proteins under high stability and scalability conditions, and that maintains its basic functions. This review reports the main aspects of heterologous protein accumulation in soybean PSVs.

A Comprehensive Overview on the Production of Vaccines in Plant-Based Expression Systems and the Scope of Plant Biotechnology to Combat against SARS-CoV-2 Virus Pandemics

Many pathogenic viral pandemics have caused threats to global health; the COVID-19 pandemic is the latest. Its transmission is growing exponentially all around the globe, putting constraints on the health system worldwide. A novel coronavirus, severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), causes this pandemic. Many candidate vaccines are available at this time for COVID-19, and there is a massive international race underway to procure as many vaccines as possible for each country. However, due to heavy global demand, there are strains in global vaccine production. The use of a plant biotechnology-based expression system for vaccine production also represents one part of this international effort, which is to develop plant-based heterologous expression systems, virus-like particles (VLPs)-vaccines, antiviral drugs, and a rapid supply of antigen-antibodies for detecting kits and plant origin bioactive compounds that boost the immunity and provide tolerance to fight against the virus infection. This review will look at the plant biotechnology platform that can provide the best fight against this global pandemic.

Oral immunization of haemaggulutinin H5 expressed in plant endoplasmic reticulum with adjuvant saponin protects mice against highly pathogenic avian influenza A virus infection

Pandemics in poultry caused by the highly pathogenic avian influenza (HPAI) A virus occur too frequently globally, and there is growing concern about the HPAI A virus due to the possibility of a pandemic among humans. Thus, it is important to develop a vaccine against HPAI suitable for both humans and animals. Various approaches are underway to develop such vaccines. In particular, an edible vaccine would be a convenient way to vaccinate poultry because of the behaviour of the animals. However, an edible vaccine is still not available. In this study, we developed a strategy of effective vaccination of mice by the oral administration of transgenic Arabidopsis plants (HA-TG) expressing haemagglutinin (HA) in the endoplasmic reticulum (ER). Expression of HA in the ER resulted in its high-level accumulation, N-glycosylation, protection from proteolytic degradation and long-term stability. Oral administration of HA-TG with saponin elicited high levels of HA-specific systemic IgG and mucosal IgA responses in mice, which resulted in protection against a lethal influenza virus infection with attenuated inflammatory symptoms. Based on these results, we propose that oral administration of freeze-dried leaf powders from transgenic plants expressing HA in the ER together with saponin is an attractive strategy for vaccination against influenza A virus.

Plant-Based Vaccines as a Global Vaccination Approach: Current Perspectives

This chapter provides a perspective on the evolution of the field of plant-based vaccine from the limitations identified in initial developments as to how this biotechnological approach has become sophisticated via the development of new technologies and has gained industry interest. Perspectives for the field at both the basic research and the industrial level are emphasized. Perspectives considered of relevance in terms of basic research include (1) advancing the development of oral formulations, (2) expanding the modalities of expression of immunogens, (3) diversifying production platforms, particularly those performed under full containment, and (4) targeting a broader number of diseases. These goals are expected to multiply the expectations for benefits derived from plant-based vaccine-production technology. On the other hand, technology transfer and regulatory issues represent a critical hurdle to this technology becoming a reality. It is also critical to achieve social acceptance as well as implement initiatives for the exploitation of the technology for humanitarian purposes and for the benefit of poor countries. This overview predicts considerable potential for plant-based vaccines to positively impact the field of vaccinology in the near future.

Generation of protective immune response against anthrax by oral immunization with protective antigen plant-based vaccine

In concern with frequent recurrence of anthrax in endemic areas and inadvertent use of its spores as biological weapon, the development of an effective anthrax vaccine suitable for both human and veterinary needs is highly desirable. A simple oral delivery through expression in plant system could offer promising alternative to the current methods that rely on injectable vaccines extracted from bacterial sources. In the present study, we have expressed protective antigen (PA) gene in Indian mustard by Agrobacterium-mediated transformation and in tobacco by plastid transformation. Putative transgenic lines were verified for the presence of transgene and its expression by molecular analysis. PA expressed in transgenic lines was biologically active as evidenced by macrophage lysis assay. Intraperitoneal (i.p.) and oral immunization with plant PA in murine model indicated high serum PA specific IgG and IgA antibody titers. PA specific mucosal immune response was noted in orally immunized groups. Further, antibodies indicated lethal toxin neutralizing potential in-vitro and conferred protection against in-vivo toxin challenge. Oral immunization experiments demonstrated generation of immunoprotective response in mice. Thus, our study examines the feasibility of oral PA vaccine expressed in an edible plant system against anthrax.

Genes involved in the endoplasmic reticulum N-glycosylation pathway of the red microalga Porphyridium sp.: a bioinformatic study

N-glycosylation is one of the most important post-translational modifications that influence protein polymorphism, including protein structures and their functions. Although this important biological process has been extensively studied in mammals, only limited knowledge exists regarding glycosylation in algae. The current research is focused on the red microalga Porphyridium sp., which is a potentially valuable source for various applications, such as skin therapy, food, and pharmaceuticals. The enzymes involved in the biosynthesis and processing of N-glycans remain undefined in this species, and the mechanism(s) of their genetic regulation is completely unknown. In this study, we describe our pioneering attempt to understand the endoplasmic reticulum N-Glycosylation pathway in Porphyridium sp., using a bioinformatic approach. Homology searches, based on sequence similarities with genes encoding proteins involved in the ER N-glycosylation pathway (including their conserved parts) were conducted using the TBLASTN function on the algae DNA scaffold contigs database. This approach led to the identification of 24 encoded-genes implicated with the ER N-glycosylation pathway in Porphyridium sp. Homologs were found for almost all known N-glycosylation protein sequences in the ER pathway of Porphyridium sp.; thus, suggesting that the ER-pathway is conserved; as it is in other organisms (animals, plants, yeasts, etc.).

Development of cowpea mosaic virus-based vectors for the production of vaccines in plants

Plant viruses are emerging as an attractive alternative to stable genetic transformation for the expression of foreign proteins in plants. The main advantages of using this strategy are that viral genomes are small and easy to manipulate, infection of plants with modified viruses is simpler and quicker than the regeneration of stably transformed plants and the sequence inserted into a virus vector will be highly amplified. One use of these virus expression systems is for vaccine production. Among plant viruses, cowpea mosaic virus makes an ideal candidate for the production of such vaccines because it grows extremely well in host plants, is very stable, and the purification of virus particles, if required, is straightforward. In this article, the authors review the progress made in the development of cowpea mosaic virus-based vectors for vaccine production, making use of two main approaches: epitope presentation and polypeptide expression.

Development of the S3Pvac vaccine against murine Taenia crassiceps cysticercosis: a historical review

Our work of the last 25 yr was concerned with the development of a vaccine aimed to prevent porcine Taenia solium cysticercosis and was based on cross-reacting Taenia crassiceps antigens that had proved protective against experimental intraperitoneal murine T. crassiceps cysticercosis (EIMTcC). In recent times the efficacy of the vaccine has been considered in need of confirmation, and the use of EIMTcC has been questioned as a valid tool in screening for vaccine candidates among the many antigens possibly involved. A review of our work divided in 2 parts is presented at this point, the first dealing with EIMTcC and the second with porcine T. solium cysticercosis (presented in this issue). Herein, we revise our results using EIMTcC as a measure of the protective capacity of T. crassiceps complex antigen mixtures, of purified native antigens, and of S3Pvac anti-cysticercosis vaccine composed by 3 protective peptides: GK-1, KETc1, and KETc12 either synthetic or recombinantly expressed and collectively or separately, by diverse delivery systems when administered at different doses and by different routes. Statistical analyses of the data lead confidently to the strong inference that S3Pvac is indeed an effective vaccine against EIMTcC via specific and non-specific mechanisms of protection.

Developing plant-based vaccines against neglected tropical diseases: where are we?

Neglected tropical diseases (NTDs) impair the lives of 1 billion people worldwide, and threaten the health of millions more. Although vaccine candidates have been proposed to prevent some NTDs, no vaccine is available at the market yet. Vaccines against NTDs should be low-cost and needle-free to reduce the logistic cost of their administration. Plant-based vaccines meet both requirements: plant systems allow antigen production at low cost, and also yield an optimal delivery vehicle that prevents or delays digestive hydrolysis of vaccine antigens. This review covers recent reports on the development of plant-based vaccines against NTDs. Efforts conducted by a number of research groups to develop vaccines as a mean to fight rabies, cysticercosis, dengue, and helminthiasis are emphasized. Future perspectives are identified, such as the need to develop vaccination models for more than ten pathologies through a plant-based biotechnological approach. Current limitations on the method are also noted, and molecular approaches that might allow us to address such limitations are discussed.

An interspecific Nicotiana hybrid as a useful and cost-effective platform for production of animal vaccines

The use of transgenic plants to produce novel products has great biotechnological potential as the relatively inexpensive inputs of light, water, and nutrients are utilised in return for potentially valuable bioactive metabolites, diagnostic proteins and vaccines. Extensive research is ongoing in this area internationally with the aim of producing plant-made vaccines of importance for both animals and humans. Vaccine purification is generally regarded as being integral to the preparation of safe and effective vaccines for use in humans. However, the use of crude plant extracts for animal immunisation may enable plant-made vaccines to become a cost-effective and efficacious approach to safely immunise large numbers of farm animals against diseases such as avian influenza. Since the technology associated with genetic transformation and large-scale propagation is very well established in Nicotiana, the genus has attributes well-suited for the production of plant-made vaccines. However the presence of potentially toxic alkaloids in Nicotiana extracts impedes their use as crude vaccine preparations. In the current study we describe a Nicotiana tabacum and N. glauca hybrid that expresses the HA glycoprotein of influenza A in its leaves but does not synthesize alkaloids. We demonstrate that injection with crude leaf extracts from these interspecific hybrid plants is a safe and effective approach for immunising mice. Moreover, this antigen-producing alkaloid-free, transgenic interspecific hybrid is vigorous, with a high capacity for vegetative shoot regeneration after harvesting. These plants are easily propagated by vegetative cuttings and have the added benefit of not producing viable pollen, thus reducing potential problems associated with bio-containment. Hence, these Nicotiana hybrids provide an advantageous production platform for partially purified, plant-made vaccines which may be particularly well suited for use in veterinary immunization programs.

Role of transgenic plants in agriculture and biopharming

At present, environmental degradation and the consistently growing population are two main problems on the planet earth. Fulfilling the needs of this growing population is quite difficult from the limited arable land available on the globe. Although there are legal, social and political barriers to the utilization of biotechnology, advances in this field have substantially improved agriculture and human life to a great extent. One of the vital tools of biotechnology is genetic engineering (GE) which is used to modify plants, animals and microorganisms according to desired needs. In fact, genetic engineering facilitates the transfer of desired characteristics into other plants which is not possible through conventional plant breeding. A variety of crops have been engineered for enhanced resistance to a multitude of stresses such as herbicides, insecticides, viruses and a combination of biotic and abiotic stresses in different crops including rice, mustard, maize, potato, tomato, etc. Apart from the use of GE in agriculture, it is being extensively employed to modify the plants for enhanced production of vaccines, hormones, etc. Vaccines against certain diseases are certainly available in the market, but most of them are very costly. Developing countries cannot afford the disease control through such cost-intensive vaccines. Alternatively, efforts are being made to produce edible vaccines which are cheap and have many advantages over the commercialized vaccines. Transgenic plants generated for this purpose are capable of expressing recombinant proteins including viral and bacterial antigens and antibodies. Common food plants like banana, tomato, rice, carrot, etc. have been used to produce vaccines against certain diseases like hepatitis B, cholera, HIV, etc. Thus, the up- and down-regulation of desired genes which are used for the modification of plants have a marked role in the improvement of genetic crops. In this review, we have comprehensively discussed the role of genetic engineering in generating transgenic lines/cultivars of different crops with improved nutrient quality, biofuel production, enhanced production of vaccines and antibodies, increased resistance against insects, herbicides, diseases and abiotic stresses as well as the safety measures for their commercialization.

Immunogenicity of chloroplast-derived HIV-1 p24 and a p24-Nef fusion protein following subcutaneous and oral administration in mice

High-level expression of foreign proteins in chloroplasts of transplastomic plants provides excellent opportunities for the development of oral vaccines against a range of debilitating or fatal diseases. The HIV-1 capsid protein p24 and a fusion of p24 with the negative regulatory protein Nef (p24-Nef) accumulate to ∼4% and ∼40% of the total soluble protein of leaves of transplastomic tobacco (Nicotiana tabacum L.) plants. This study has investigated the immunogenicity in mice of these two HIV-1 proteins, using cholera toxin B subunit as an adjuvant. Subcutaneous immunization with purified chloroplast-derived p24 elicited a strong antigen-specific serum IgG response, comparable to that produced by Escherichia coli-derived p24. Oral administration of a partially purified preparation of chloroplast-derived p24-Nef fusion protein, used as a booster after subcutaneous injection with either p24 or Nef, also elicited strong antigen-specific serum IgG responses. Both IgG1 and IgG2a subtypes, associated with cell-mediated Th1 and humoral Th2 responses, respectively, were found in sera after subcutaneous and oral administration. These results indicate that chloroplast-derived HIV-1 p24-Nef is a promising candidate as a component of a subunit vaccine delivered by oral boosting, after subcutaneous priming by injection of p24 and/or Nef.

Generation and immunogenicity of transgenic potato expressing the GP5 protein of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is an important pathogen that has caused huge economic losses in the global swine industry. The advent of molecular farming has provided a cost-effective strategy for the development of transgenic plants as bioreactors to produce recombinant proteins. In this study, transgenic potato expressing GP5 protein of PRRSV was produced by Agrobacterium-mediated transformation, and confirmed using Southern blot and RT-PCR analyses. Recombinant GP5 protein was detected by ELISA and Western blot analyses. Mice immunized with transgenic potato extracts generated both serum and gut mucosal-specific antibodies, although low levels of neutralizing antibodies were elicited. This study provides a new approach for the production of vaccines against PRRSV.

Advances in plant molecular farming

Plant molecular farming (PMF) is a new branch of plant biotechnology, where plants are engineered to produce recombinant pharmaceutical and industrial proteins in large quantities. As an emerging subdivision of the biopharmaceutical industry, PMF is still trying to gain comparable social acceptance as the already established production systems that produce these high valued proteins in microbial, yeast, or mammalian expression systems. This article reviews the various cost-effective technologies and strategies, which are being developed to improve yield and quality of the plant-derived pharmaceuticals, thereby making plant-based production system suitable alternatives to the existing systems. It also attempts to overview the different novel plant-derived pharmaceuticals and non-pharmaceutical protein products that are at various stages of clinical development or commercialization. It then discusses the biosafety and regulatory issues, which are crucial (if strictly adhered to) to eliminating potential health and environmental risks, which in turn is necessary to earning favorable public perception, thus ensuring the success of the industry.

In planta production of plant-derived and non-plant-derived adjuvants

Recombinant antigen production in plants is a safe and economically sound strategy for vaccine development, particularly for oral/mucosal vaccination, but subunit vaccines usually suffer from weak immunogenicity and require adjuvants that escort the antigens, target them to relevant sites and/or activate antigen-presenting cells for elicitation of protective immunity. Genetic fusions of antigens with bacterial adjuvants as the B subunit of the cholera toxin have been successful in inducing protective immunity of plant-made vaccines. In addition, several plant compounds, mainly plant defensive molecules as lectins and saponins, have shown strong adjuvant activities. The molecular diversity of the plant kingdom offers a vast source of non-bacterial compounds with adjuvant activity, which can be assayed in emerging plant manufacturing systems for the design of new plant vaccine formulations.

Synthesis and assembly of a full-length human monoclonal antibody in algal chloroplasts

Monoclonal antibodies can be effective therapeutics against a variety of human diseases, but currently marketed antibody-based drugs are very expensive compared to other therapeutic options. Here, we show that the eukaryotic green algae Chlamydomonas reinhardtii is capable of synthesizing and assembling a full-length IgG1 human monoclonal antibody (mAb) in transgenic chloroplasts. This antibody, 83K7C, is derived from a human IgG1 directed against anthrax protective antigen 83 (PA83), and has been shown to block the effects of anthrax toxin in animal models. Here we show that 83K7C heavy and light chain proteins expressed in the chloroplast accumulate as soluble proteins that assemble into complexes containing two heavy and two light chain proteins. The algal-expressed 83K7C binds PA83 in vitro with similar affinity to the mammalian-expressed 83K7C antibody. In addition, a second human IgG1 and a mouse IgG1 were also expressed and shown to properly assemble in algal chloroplast. These results show that chloroplasts have the ability to fold and assemble full-length human mAbs, and suggest the potential of algae as a platform for the cost effective production of complex human therapeutic proteins.

Transgenic chloroplasts are efficient sites for high-yield production of the vaccinia virus envelope protein A27L in plant cellsdagger

Orthopoxviruses (OPVs) have recently received increasing attention because of their potential use in bioterrorism and the occurrence of zoonotic OPV outbreaks, highlighting the need for the development of safe and cost-effective vaccines against smallpox and related viruses. In this respect, the production of subunit protein-based vaccines in transgenic plants is an attractive approach. For this purpose, the A27L immunogenic protein of vaccinia virus was expressed in tobacco using stable transformation of the nuclear or plastid genome. The vaccinia virus protein was expressed in the stroma of transplastomic plants in soluble form and accumulated to about 18% of total soluble protein (equivalent to approximately 1.7 mg/g fresh weight). This level of A27L accumulation was 500-fold higher than that in nuclear transformed plants, and did not decline during leaf development. Transplastomic plants showed a partial reduction in growth and were chlorotic, but reached maturity and set fertile seeds. Analysis by immunofluorescence microscopy indicated altered chlorophyll distribution. Chloroplast-synthesized A27L formed oligomers, suggesting correct folding and quaternary structure, and was recognized by serum from a patient recently infected by a zoonotic OPV. Taken together, these results demonstrate that chloroplasts are an attractive production vehicle for the expression of OPV subunit vaccines.

Use of GFP to investigate expression of plant-derived vaccines

Plants are low-cost bioreactors for the production of various biopharmaceuticals including oral vaccines. Plant-derived oral vaccines are potentially useful in combating viral infections involving mucosal immunity. Transgenic plants have been generated to successfully produce mucosal vaccines against cholera, hepatitis B, foot-and-mouth disease, and Norwalk virus. As a first step toward the generation of oral vaccines against the severe acute respiratory syndrome coronavirus (SARS-CoV), we have expressed a recombinant S1 protein of the SARS-CoV in transformed tobacco. Since plant transformation and regeneration of stable transformants require considerable time, we initially used a green fluorescent protein (GFP) to tag the antigen in transient expression. GFP was fused to the carboxy-terminus of S1 for expression of S1-GFP to show expression of recombinant S1 by agroinfiltration of tobacco leaves. The GFP tag enables a relatively quick confirmation of antigen expression in plant cells by fluorescent microscopy. Such analysis using GFP that precedes stable plant transformation will enable the rapid screening of multiple constructs to attain optimal recombinant protein expression. Furthermore, this approach determines the subcellular localization of the recombinant protein in plant cells, providing information on optimal subcellular targeting for production in plant bioreactors.

Oral immunization: an update

PURPOSE OF REVIEW

Oral immunization with vaccines against intestinal infectious diseases has been extensively explored for several decades. Despite the immunologic and economic rationale behind oral immunization, only a few mucosal vaccines are available for the prevention of mucosal infections. Here, we summarize the current status of such vaccines, with a focus on intestinal infectious diseases, describe alternative approaches, and analyze advantages and difficulties encountered with a broad implementation of these vaccines.

RECENT FINDINGS

Due to the limited absorption from the intestinal tract and sensitivity to degradation, oral vaccines composed of killed bacteria and viruses or antigens isolated from infectious agents have not been successful. New, live-attenuated bacterial and viral or edible plant-derived vaccines, however, have been recently introduced for this purpose. Furthermore, systemic immunization with vaccines composed of bacterial polysaccharides chemically coupled to suitable protein carriers induces high levels of IgG antibodies, which may provide immunity toward Salmonella typhi, Shigella, and Escherichia coli.

SUMMARY

Further improvements in antigen-delivery systems, the development of adjuvants that are safe for mucosal application in humans, use of live-attenuated vaccines and microbial vectors, and production of certain vaccines in plant expression systems are likely to contribute to the broader use of oral vaccines in the future.

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.

Expression of toxin co-regulated pilus subunit A (TCPA) of Vibrio cholerae and its immunogenic epitopes fused to cholera toxin B subunit in transgenic tomato (Solanum lycopersicum)

For protection against cholera, it is important to develop efficient vaccine capable of inducing anti-toxin as well as anti-colonizing immunity against Vibrio cholerae infections. Earlier, expression of cholera toxin B subunit (CTB) in tomato was reported by us. In the present investigation, toxin co-regulated pilus subunit A (TCPA), earlier reported to be an antigen capable of providing anti-colonization immunity, has been expressed in tomato. Further, to generate more potent combinatorial antigens, nucleotides encoding P4 or P6 epitope of TCPA were fused to cholera toxin B subunit gene (ctxB) and expressed in tomato. Presence of transgenes in the tomato genome was confirmed by PCR and expression of genes was confirmed at transcript and protein level. TCPA, chimeric CTB-P4 and CTB-P6 proteins were also expressed in E. coli. TCPA protein expressed in E. coli was purified to generate anti-TCPA antibodies in rabbit. Immunoblot and G(M1)-ELISA verified the synthesis and assembly of pentameric chimeric proteins in fruit tissue of transgenic tomato plants. The chimeric protein CTB-P4 and CTB-P6 accumulated up to 0.17 and 0.096% of total soluble protein (TSP), respectively, in tomato fruits. Whereas expression of TCPA, CTB-P4 and CTB-P6 in E. coli can be utilized for development of conventional vaccine, expression of these antigens which can provide both anti-toxin as well as anti-colonization immunity, has been demonstrated in plants, in a form which is potentially capable of inducing immune response against cholera infection.

Plant viruses as biotemplates for materials and their use in nanotechnology

In recent years, plant virus capsids, the protein shells that form the surface of a typical plant virus particle, have emerged as useful biotemplates for material synthesis. All virus capsids are assembled from virus-coded protein subunits. Many plant viruses assemble capsids with precise 3D structures providing nanoscale architectures that are highly homogeneous and can be produced in large quantities. Capsids are amenable to both genetic and chemical modifications allowing new functions to be incorporated into their structure by design. The three capsid surfaces, the interior surface, the exterior surface, or the interface between coat protein subunits, can be independently functionalized to produce multifunctional biotemplates. In this review, we examine the recent advances in using plant virus capsids as biotemplates for nanomaterials and their potential for applications in nanotechnology, especially medicine.

Rice-based mucosal vaccine as a global strategy for cold-chain- and needle-free vaccination

Capable of inducing antigen-specific immune responses in both systemic and mucosal compartments without the use of syringe and needle, mucosal vaccination is considered ideal for the global control of infectious diseases. In this study, we developed a rice-based oral vaccine expressing cholera toxin B subunit (CTB) under the control of the endosperm-specific expression promoter 2.3-kb glutelin GluB-1 with codon usage optimization for expression in rice seed. An average of 30 mug of CTB per seed was stored in the protein bodies, which are storage organelles in rice. When mucosally fed, rice seeds expressing CTB were taken up by the M cells covering the Peyer's patches and induced CTB-specific serum IgG and mucosal IgA antibodies with neutralizing activity. When expressed in rice, CTB was protected from pepsin digestion in vitro. Rice-expressed CTB also remained stable and thus maintained immunogenicity at room temperature for >1.5 years, meaning that antigen-specific mucosal immune responses were induced at much lower doses than were necessary with purified recombinant CTB. Because they require neither refrigeration (cold-chain management) nor a needle, these rice-based mucosal vaccines offer a highly practical and cost-effective strategy for orally vaccinating large populations against mucosal infections, including those that may result from an act of bioterrorism.

Bioproduction of therapeutic proteins in the 21st century and the role of plants and plant cells as production platforms

abstract: In the last decade, the technique to genetically modify crop plants has gained more and more interest in terms of bioproduction of heterologous proteins. Plants have been discovered as a possible source for large amounts of cost effective recombinant protein. Main application fields are therapeutics for use in animal and human health, diagnostics, and technical enzymes. This review is focused on the recent progress in this field of molecular farming. After a comparison with hitherto established protein production systems, the advantages of plants as an alternative production system are discussed. An overview about the different host plants and possible expression strategies is given and the progress in commercialization of the techniques is highlighted. Finally, the role of plant cell cultures for the production of recombinant proteins is discussed.

The genetic transformation of plastids

Biolistic delivery of DNA initiated plastid transformation research and still is the most widelyused approach to generate transplastomic lines in both algae and higher plants. The principal designof transformation vectors is similar in both phylogenetic groups. Although important additions tothe list of species transformed in their plastomes have been made in algae and in higher plants, thekey organisms in the area are still the two species, in which stable plastid transformation was initiallysuccessful, i.e., Chlamydomonas reinhardtii and tobacco. Basicresearch into organelle biology has substantially benefited from the homologous recombination-basedcapability to precisely insert at predetermined loci, delete, disrupt, or exchange plastid genomesequences. Successful expression of recombinant proteins, including pharmaceutical proteins, hasbeen demonstrated in Chlamydomonas as well as in higher plants,where some interesting agronomic traits were also engineered through plastid transformation.

Robust expression of a bioactive mammalian protein in Chlamydomonas chloroplast

We have engineered the chloroplast of eukaryotic algae to produce a number of recombinant proteins, including human monoclonal antibodies, but, to date, have achieved expression to only 0.5% of total protein. Here, we show that, by engineering the mammalian coding region of bovine mammary-associated serum amyloid (M-SAA) as a direct replacement for the chloroplast psbA coding region, we can achieve expression of recombinant protein above 5% of total protein. Chloroplast-expressed M-SAA accumulates predominantly as a soluble protein, contains the correct amino terminal sequence and has little or no post-translational modification. M-SAA is found in mammalian colostrum and stimulates the production of mucin in the gut, acting in the prophylaxis of bacterial and viral infections. Chloroplast-expressed and purified M-SAA is able to stimulate mucin production in human gut epithelial cell lines. As Chlamydomonas reinhardtii is an edible alga, production of therapeutic proteins in this organism offers the potential for oral delivery of gut-active proteins, such as M-SAA.

Plant-made vaccines: biotechnology and immunology in animal health

The use of plants as production systems for vaccine antigens has been actively investigated over the last 15 years. The original research focused on the value of this expression system for oral delivery based on the hypothesis that plant-expressed antigens would be more stable within the digestive tract and would allow for the use of the oral route of administration to stimulate a mucosal immune response. However, while first conceived for utility via the oral route, plant-made antigens have also been studied as classical immunogens delivered via a needle to model animal systems. Antigens have been expressed in a number of whole plant and cell culture systems. Several alternative expression platforms have been developed to increase expression of antigens or to elicit preferred immunological responses. The biotechnological advances in plant expression and the immunological testing of these antigens will be reviewed in this paper focusing primarily on diseases of livestock and companion animals.

Biologically active human GM-CSF produced in the seeds of transgenic rice plants

Rice flour is a well-known and characterized source of pharmaceutical ingredients, which are gluten-free and incorporated in many drug delivery applications such as excipient starch. To further exploit this uniqueness, the synthetic capacity of rice endosperm tissue, the basis of rice flour, was extended by genetic transformation. Recombinant human GM-CSF, a cytokine used in treating neutropenia and with other potential clinical applications, has been expressed in transgenic rice seeds using a rice glutelin promoter. Rice seeds accumulated human GM-CSF to a level of 1.3% of total soluble protein. The rice seed-produced human GM-CSF was found to be biologically active when tested using a human cell line TF-1. Use of rice as a host plant offers not only attractive features of safe production in seeds but also self-containment of foreign genes, as rice is primarily a self-pollinated crop plant.

Pharma-Planta: road testing the developing regulatory guidelines for plant-made pharmaceuticals

Significant advances over the last few years have seen plant-made pharmaceuticals (PMPs) move from the exploratory research phase towards clinical trials, with the first commercial products for human use expected to reach the market by 2009. Europe has yet to witness the commercial application of PMP technology, although at least one product has begun phase II clinical trials with others following close behind. These emerging products are set to challenge the complex and overlapping regulations that currently govern GM plants and 'conventional' pharmaceutical production. The areas of responsibility are being mapped out between the different EU regulatory agencies, with specific guidelines currently being drawn up for the regulation of PMPs. This article discusses issues surrounding the development of robust risk-assessment and risk-management practices based on health and environmental impact, while working with EU regulatory authorities to ensure appropriate regulatory oversight.

Stable expression of foot-and-mouth disease virus protein VP1 fused with cholera toxin B subunit in the potato (Solanum tuberosum)

The expression vector, pBI121CTBVP1, containing the fusion of the foot and mouth disease virus (FMDV) VP1 gene and the cholera toxin B subunit (CTB) gene was constructed by fused PCR and transferred into potato (Solanum tuberosum L.) by Agrobacterium-mediated transformation. Transformed plants were obtained by selecting on kanamycin-resistant medium strictly and regenerated. The transgenic plantlets were identified by PCR, Southern-blot and the production of fused protein was confirmed and quantified by Western-blot and ELISA assays. The results showed that the fused genes were expressed stablely under the control of specific-tuber patatin promoter. The expressed fused proteins have a certain degree of immunogenicity.

Peptide immunotherapy for allergic diseases using a rice-based edible vaccine

PURPOSE OF REVIEW

Plant pollens are the most common cause of seasonal allergic disease. The number of patients undergoing treatment for allergies to the pollen of Japanese cedar (major antigens: Cry j 1 and Cry j 2) has increased steadily each year. Integration of an effective, safe and inexpensive clinical program would be greatly improved by addressing deficiencies in systemically delivered immunotherapy.

RECENT FINDINGS

We have demonstrated that feeding mice transgenic rice seeds accumulating the T-cell epitope peptides of Cry j 1 and Cry j 2 before systemic challenge with total protein of cedar pollen inhibits the development of allergen-specific IgE, IgG and CD4 T-cell proliferative responses. The levels of allergen-specific CD4 T-cell-derived allergy-associated T-helper 2 cytokine of IL-4, IL-5, and IL-13 and histamine release in serum were also significantly decreased. Moreover, clinical symptoms were inhibited in an experimental sneezing-mouse model.

SUMMARY

Plant-based edible vaccine has been shown to be effective for treatment of Japanese cedar pollinosis. When rice seeds containing T-cell epitopes derived from cedar pollen allergens were orally administered to mice, immune tolerance leading to reduction of allergen-specific IgE, T-cell proliferative reaction and histamine could be induced, resulting in suppression of allergic-specific symptoms such as sneezing.

Delivery systems and adjuvants for oral vaccines

The oral route is the ideal means of delivering prophylactic and therapeutic vaccines, offering significant advantages over systemic delivery. Most notably, oral delivery is associated with simple administration and improved safety. In addition, unlike systemic immunisation, oral delivery can induce mucosal immune responses. However, the oral route of vaccine delivery is the most difficult because of the numerous barriers posed by the gastrointestinal tract. To facilitate effective immunisation with peptide and protein vaccines, antigens must be protected, uptake enhanced and the innate immune response activated. Numerous delivery systems and adjuvants have been evaluated for oral vaccine delivery, including live vectors, inert particles and bacterial toxins. Although developments in oral vaccines have been disappointing so far, in terms of the generation of products, the availability of a range of novel delivery systems offers much greater hope for the future development of improved oral vaccines.

A KDEL-tagged monoclonal antibody is efficiently retained in the endoplasmic reticulum in leaves, but is both partially secreted and sorted to protein storage vacuoles in seeds

Transgenic plants are attractive biological systems for the large-scale production of pharmaceutical proteins. In particular, seeds offer special advantages, such as ease of handling and long-term stable storage. Nevertheless, most of the studies of the expression of antibodies in plants have been performed in leaves. We report the expression of a secreted (sec-Ab) or KDEL-tagged (Ab-KDEL) mutant of the 14D9 monoclonal antibody in transgenic tobacco leaves and seeds. Although the KDEL sequence has little effect on the accumulation of the antibody in leaves, it leads to a higher antibody yield in seeds. sec-Ab(Leaf) purified from leaf contains complex N-glycans, including Lewis(a) epitopes, as typically found in extracellular glycoproteins. In contrast, Ab-KDEL(Leaf) bears only high-mannose-type oligosaccharides (mostly Man 7 and 8) consistent with an efficient endoplasmic reticulum (ER) retention/cis-Golgi retrieval of the antibody. sec-Ab and Ab-KDEL gamma chains purified from seeds are cleaved by proteases and contain complex N-glycans indicating maturation in the late Golgi compartments. Consistent with glycosylation of the protein, Ab-KDEL(Seed) was partially secreted and sorted to protein storage vacuoles (PSVs) in seeds and not found in the ER. This dual targeting may be due to KDEL-mediated targeting to the PSV and to a partial saturation of the vacuolar sorting machinery. Taken together, our results reveal important differences in the ER retention and vacuolar sorting machinery between leaves and seeds. In addition, we demonstrate that a plant-made antibody with triantennary high-mannose-type N-glycans has similar Fab functionality to its counterpart with biantennary complex N-glycans, but the former antibody interacts with protein A in a stronger manner and is more immunogenic than the latter. Such differences could be related to a variable immunoglobulin G (IgG)-Fc folding that would depend on the size of the N-glycan.

Improved uptake of plant-derived LTB-linked proteins in carp gut and induction of specific humoral immune responses upon infeed delivery

Oral vaccination of fish is an effortless and stress free immunisation method which can be used for almost any age. However, vaccination via the mucosal route does have disadvantages. For example, the vaccine may induce tolerance and has to be protected to escape digestion. Also the vaccine should be efficiently delivered to immune-competent cells in the gut or other lymphoid organs. In addition, it should be cost effective. Here we present a novel fish vaccination model using potato tubers as vaccine production and delivery system. The model vaccines discussed here include fusion proteins consisting of a gut adhesion molecule (LTB) and a viral peptide or green fluorescent protein (GFP) expressed in potato tubers. The adhesion molecule mediates binding to and uptake from the gut, whereas the viral peptide or GFP functions as model vaccine antigen provoking the induction of an immune response. We demonstrate that fusion to LTB facilitates an elevated uptake of the model vaccines in carp gut mucosa. The plant-derived fusion proteins also elicit a specific systemic humoral immune response upon oral application of crude tuber material incorporated into a standard dietary feed pellet. The data presented here show the promising potentials of the plant as a production system for oral vaccines in aquaculture and feed mediated immunisation of fish.

Accumulation of recombinant SARS-CoV spike protein in plant cytosol and chloroplasts indicate potential for development of plant-derived oral vaccines

Plants are promising candidates as bioreactors for the production of oral recombinant proteins in the biopharmaceutical industry. As an initial step toward provision of an oral vaccine against the severe acute respiratory syndrome coronavirus (SARS-CoV), we have expressed a partial spike (S) protein of SARS-CoV in the cytosol of nuclear-transformed plants and in the chloroplasts of plastid-transformed plants. In the construction of both nuclear and plastid transformation vectors, a 2-kilobase nucleotide sequence encoding amino acids 1-658 of the SARS-CoV spike protein (S1) was modified with nucleotide changes, but not amino acid changes, to optimize codon usage for expression in plants. To investigate the subcellular localization of S1 during transient expression in tobacco leaves, a translational fusion consisting of S1 and the green fluorescent protein (GFP) was generated. Following agroinfiltration of tobacco leaves, analysis by laser confocal scanning microscopy revealed that the S1:GFP fusion protein was localized to the cytosol. In stable transgenic tobacco plants and lettuce plants generated by Agrobacterium-mediated transformation, tobacco and lettuce leaves were observed to express the S1 at high levels from the Cauliflower Mosaic Virus 35S promoter with Northern blot analysis. When the S1 was expressed in transplastomic tobacco, S1 messenger RNA and its corresponding protein were detected on Northern and Western blot analyses, respectively. Our results demonstrate the feasibility of producing S1 in nuclear- and chloroplast-transformed plants, indicating its potential in subsequent development of a plant-derived and safe oral recombinant subunit vaccine against the SARS-CoV in edible plants.

Bioregulators as prototypic nontraditional threat agents

Bioregulators are naturally occurring organic compounds that regulate a multitude of biologic processes. Under natural circumstances, bioregulators are synthesized in minute quantities in a variety of living organisms and are essential for physiologic homeostasis. In the wrong hands, these compounds have the capability to be used as nontraditional threat agents that are covered by the prohibitions of the Chemical Weapons Convention and the Biological and Toxin Weapons Convention. Unlike traditional biowarfare/bioterrorism agents that have a latency period of hours to days,the onset of action of bioregulators may occur within minutes after host exposure. Concerns regarding the potential misuse of bioregulators for nefarious purposes relate to the ability of these nontraditional agents to induce profound physiologic effects.

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.

Expression of functional interleukin-12 from mouse in transgenic tomato plants

Transgenic plants have been employed successfully as a low-cost system for the production of therapeutically valuable proteins, including antibodies, antigens and hormones. Here, we report the expression of a cytokine with immunomodulatory function, mouse interleukin-12 (IL-12), in transgenic tomato plants. Single-chain mouse IL-12 driven by the CaMV 35S promoter, accumulates to high levels in leaves and fruits (up to 7.3 and 3.4 microg per gram of fresh weight, respectively). Mouse IL-12 expressed in tomato displays biological activity in vitro, as determined by interferon-gamma (IFN-gamma) secretion by T cells. Possible uses of this plant-based cytokine involving mucosal delivery are discussed.

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.