Expression in plants and immunogenicity of plant virus-based experimental rabies vaccine

Plant production of veterinary vaccines and therapeutics

Plant-derived biologicals for use in animal health are becoming an increasingly important target for research into alternative, improved methods for disease control. Although there are no commercial products on the market yet, the development and testing of oral, plant-based vaccines is now beyond the proof-of-principle stage. Vaccines, such as those developed for porcine transmissible gastroenteritis virus, have the potential to stimulate both mucosal and systemic, as well as, lactogenic immunity as has already been seen in target animal trials. Plants are a promising production system, but they must compete with existing vaccines and protein production platforms. In addition, regulatory hurdles will need to be overcome, and industry and public acceptance of the technology are important in establishing successful products.

The production of biopharmaceuticals in plant systems

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

A launch vector for the production of vaccine antigens in plants

Historically, most vaccines have been based on killed or live-attenuated infectious agents. Although very successful at immunizing populations against disease, both approaches raise safety concerns and often have limited production capacity. This has resulted in increased emphasis on the development of subunit vaccines. Several recombinant systems have been considered for subunit vaccine manufacture, including plants, which offer advantages both in cost and in scale of production. We have developed a plant expression system utilizing a 'launch vector', which combines the advantageous features of standard agrobacterial binary plasmids and plant viral vectors, to achieve high-level target antigen expression in plants. As an additional feature, to aid in target expression, stability and purification, we have engineered a thermostable carrier molecule to which antigens are fused. We have applied this launch vector/carrier system to engineer and express target antigens from various pathogens, including, influenza A/Vietnam/04 (H5N1) virus.

Expression of rabies virus G protein in carrots (Daucus carota)

Antigens derived from various pathogens can readily be synthesized at high levels in plants in their authentic forms. Such antigens administered orally can induce an immune response and, in some cases, result in protection against a subsequent challenge. We here report the expression of rabies virus G protein into carrots. The G gene was subcloned into the pUCpSSrabG vector and then used to transform carrot embryogenic cells by particle bombardment. The carrot cells were selected in liquid medium, a method previously unreported. The presence of the transgene was verified by PCR, and by RT-PCR. By western blot, G protein transgene was identified in 93.3% of adult carrot roots. The G protein was quantified by densitometric analysis (range 0.4-1.2%). The expressed protein was antigenic in mice. This confirms that the carrot is an adequate system for antigen expression.

Plant vaccines: an immunological perspective

The advent of technologies to express heterologous proteins in planta has led to the proposition that plants may be engineered to be safe, inexpensive vehicles for the production of vaccines and possibly even vectors for their delivery. The immunogenicity of a variety of antigens of relevance to vaccination expressed in different plants has been assessed. The purpose of this article is to examine the utility of plant-expression systems in vaccine development from an immunological perspective.

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.

Display of peptides on the surface of tobacco mosaic virus particles

In this review, we focus on the potential that tobacco mosaic virus (TMV) has as a carrier for immunogenic epitopes, and the factors that must be considered in order to bring products based on this platform to the market. Large Scale Biology Corporation developed facile and scaleable methods for manufacture of candidate peptide display vaccines based on TMV. We describe how rational design of peptide vaccines can improve the manufacturability of particular TMV products. We also discuss downstream processing and purification of the vaccine products, with particular attention to the metrics that a product must attain in order to meet criteria for regulatory approval as injectable biologics.

Plant-based oral vaccines: results of human trials

Vaccines consisting of transgenic plant-derived antigens offer a new strategy for development of safe, inexpensive vaccines. The vaccine antigens can be eaten with the edible part of the plant or purified from plant material. In phase 1 clinical studies of prototype potato- and corn-based vaccines, these vaccines have been safe and immunogenic without the need for a buffer or vehicle other than the plant cell. Transgenic plant technology is attractive for vaccine development because these vaccines are needle-less, stable, and easy to administer. This chapter examines some early human studies of oral transgenic plant-derived vaccines against enterotoxigenic Escherichia coli infection, norovirus, and hepatitis B.

Recent progress in the development of plant derived vaccines

Recombinant subunit vaccines have been with us for the last 30 years and they provide us with the unique opportunity to choose from the many available production systems that can be used for recombinant protein expression. Plants have become an attractive production platform for recombinant biopharmaceuticals and vaccines have been at the forefront of this new and expanding industry sector. The particular advantages of plant-based vaccines in terms of cost, safety and scalability are discussed in the light of recent successful clinical trials and the likely impact of plant systems on the vaccine industry is evaluated.

Immunogenic properties of plant-derived recombinant smallpox vaccine candidate pB5

The extracellular virion membrane protein B5 is a potent inducer of immune responses capable of protecting mice and primates against poxvirus infections. Here, we examined the antibody response induced in mice immunized intramuscularly (i.m.) or intranasally (i.n.) with plant-derived B5 (pB5) accompanied or not with plant total soluble protein (TSP) at various concentrations. Increasing amounts of TSP inhibited the pB5-specific response in both i.m.- and i.n.-immunized mice, with more dramatic effects in the latter. pB5 administered to mucosal surfaces induced specific IgG and IgA responses, whereas i.m. immunization produced high serum IgG titers and no IgA. A 6-fold increase in pB5 dosage administered i.n. led to an antibody response comparable to that obtained by i.m. injection. Our study addresses the quality/quantity issues of the pB5 subunit preparation and demonstrates the feasibility of mucosal administration of plant-derived smallpox subunit vaccine in obtaining a potent immune response. Overall, this work points to the practicability of needle-free mucosal administration of such vaccines in light of purity, dosage and adjuvant formulation.

Advantageous Features of Plant-based Systems for the Development of HIV Vaccines

Plants have recently become an attractive option for the production of recombinant proteins. Plant-based systems can be used to produce many classes of foreign proteins including candidate vaccine antigens. The selected antigen can be purified from plant material prior to delivery by the preferred route, or alternatively delivered orally in edible plant material that has been processed to give a homogeneous and stable product. Several plant species have been used to express a wide range of vaccine candidates with tobacco, potato and corn being particularly favored. Corn seed is especially well suited to various food processing technologies that generate dry homogeneous material suitable for extended storage and refrigeration-free transport and distribution. Many antigens have been expressed in corn and assessed for efficacy in trials with generally positive results. Candidate HIV vaccines are particularly good targets for plant-based oral delivery since there is a great need for an easily distributed affordable vaccine that could be administered without injection and induce strong mucosal immune responses. As a first step in evaluating plant expression technology with a relevant antigen that might easily be tested in an animal system, we expressed the SIV major surface glycoprotein gp130 (analogous to HIV gp120) in corn seed. Expression levels were achieved that are compatible with conducting oral delivery trials in animals.

Highly efficient expression of Escherichia coli heat-labile enterotoxin B subunit in plants using potato virus X-based vector

A synthetic gene of the B-subunit of Escherichia coli heat-labile toxin, optimized for expression in plants, was designed and synthesized. The recombinant viral vector was constructed on the basis of potato virus X containing the LTB gene instead of the removed triple block of transport genes and the coat protein gene, which provides for LTB expression in plants. The vector is introduced into the plant cells during cell infiltration by agrobacteria incorporating a binary vector, the T-DNA region of which contains a cDNA copy of the recombinant viral genome. Under conditions of posttranscriptional gene silencing inhibition, the LTB yield in Nicotiana benthamiana plants is 1-2% of total soluble protein; in this case, LTB synthesized in plants forms pentameric complexes analogous to those found in the native toxin. The designed viral system of LTB transient expression can be used to obtain in plants a vaccine against enteropathogenic Escherichia coli.

Evolution of a regulatory framework for pharmaceuticals derived from genetically modified plants

The use of genetically modified (GM) plants to synthesize proteins that are subsequently processed, regulated and sold as pharmaceuticals challenges two very different established regulatory frameworks, one concerning GM plants and the other covering the development of biotechnology-derived drugs. Within these regulatory systems, specific regulations and guidelines for plant-made pharmaceuticals (PMPs)--also referred to as plant-derived pharmaceuticals (PDPs)--are still evolving. The products nearing commercial viability will ultimately help to road test and fine-tune these regulations, and might help to reduce regulatory uncertainties. In this review, we summarize the current state of regulations in different countries, discuss recent changes and highlight the need for further regulatory development in this burgeoning, new industry. We also make the case for the harmonization of international regulations.

Viral vectors for production of recombinant proteins in plants

Global demand for recombinant proteins has steadily accelerated for the last 20 years. These recombinant proteins have a wide range of important applications, including vaccines and therapeutics for human and animal health, industrial enzymes, new materials and components of novel nano-particles for various applications. The majority of recombinant proteins are produced by traditional biological "factories," that is, predominantly mammalian and microbial cell cultures along with yeast and insect cells. However, these traditional technologies cannot satisfy the increasing market demand due to prohibitive capital investment requirements. During the last two decades, plants have been under intensive investigation to provide an alternative system for cost-effective, highly scalable, and safe production of recombinant proteins. Although the genetic engineering of plant viral vectors for heterologous gene expression can be dated back to the early 1980s, recent understanding of plant virology and technical progress in molecular biology have allowed for significant improvements and fine tuning of these vectors. These breakthroughs enable the flourishing of a variety of new viral-based expression systems and their wide application by academic and industry groups. In this review, we describe the principal plant viral-based production strategies and the latest plant viral expression systems, with a particular focus on the variety of proteins produced and their applications. We will summarize the recent progress in the downstream processing of plant materials for efficient extraction and purification of recombinant proteins.

Expression of HIV-1 antigens in plants as potential subunit vaccines

BACKGROUND

Human immunodeficiency virus type 1 (HIV-1) has infected more than 40 million people worldwide, mainly in sub-Saharan Africa. The high prevalence of HIV-1 subtype C in southern Africa necessitates the development of cheap, effective vaccines. One means of production is the use of plants, for which a number of different techniques have been successfully developed. HIV-1 Pr55Gag is a promising HIV-1 vaccine candidate: we compared the expression of this and a truncated Gag (p17/p24) and the p24 capsid subunit in Nicotiana spp. using transgenic plants and transient expression via Agrobacterium tumefaciens and recombinant tobamovirus vectors. We also investigated the influence of subcellular localisation of recombinant protein to the chloroplast and the endoplasmic reticulum (ER) on protein yield. We partially purified a selected vaccine candidate and tested its stimulation of a humoral and cellular immune response in mice.

RESULTS

Both transient and transgenic expression of the HIV antigens were successful, although expression of Pr55Gag was low in all systems; however, the Agrobacterium-mediated transient expression of p24 and p17/p24 yielded best, to more than 1 mg p24/kg fresh weight. Chloroplast targeted protein levels were highest in transient and transgenic expression of p24 and p17/p24. The transiently-expressed p17/p24 was not immunogenic in mice as a homologous vaccine, but it significantly boosted a humoral and T cell immune response primed by a gag DNA vaccine, pTHGagC.

CONCLUSION

Transient agroinfiltration was best for expression of all of the recombinant proteins tested, and p24 and p17/p24 were expressed at much higher levels than Pr55Gag. Our results highlight the usefulness of plastid signal peptides in enhancing the production of recombinant proteins meant for use as vaccines. The p17/p24 protein effectively boosted T cell and humoral responses in mice primed by the DNA vaccine pTHGagC, showing that this plant-produced protein has potential for use as a vaccine.

Expression of the rabies virus nucleoprotein in plants at high-levels and evaluation of immune responses in mice

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 expression of a full-length nucleoprotein gene of rabies virus in transgenic tomato plants. The nucleoprotein was also transiently expressed in Nicotiana benthamiana plants by agroinfiltration. In both cases, the nucleoprotein was expressed at high levels, 1-5% of total soluble protein in tomato and 45% in N. benthamiana. Previously, only epitopes of the nucleoprotein had been expressed in plants. The presence and expression of the transgene was verified by PCR, Southern, northern and western blots. Mice were immunized both intraperitoneally (i.p.) and orally with tomato protein extracts containing the N protein induced the production of antibodies. The antibody titer of mice immunized i.p., was at least four times higher than that of mice immunized orally. These results were reflected in the challenge experiments where i.p.-immunized mice were partially protected against a peripheral virus challenge whereas orally immunized mice were not. This protection was comparable to that obtained in previous experiments employing different expression systems. Work is in progress to express both G and N proteins in transgenic plants and evaluate protection in mice.

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.

Transgenic plants for animal health: plant-made vaccine antigens for animal infectious disease control

A variety of plant species have been genetically modified to accumulate vaccine antigens for human and animal health and the first vaccine candidates are approaching the market. The regulatory burden for animal vaccines is less than that for human use and this has attracted the attention of researchers and companies, and investment in plant-made vaccines for animal infectious disease control is increasing. The dosage cost of vaccines for animal infectious diseases must be kept to a minimum, especially for non-lethal diseases that diminish animal welfare and growth, so efficient and economic production, storage and delivery are critical for commercialization. It has become clear that transgenic plants are an economic and efficient alternative to fermentation for large-scale production of vaccine antigens. The oral delivery of plant-made vaccines is particularly attractive since the expensive purification step can be avoided further reducing the cost per dose. This review covers the current status of plant-produced vaccines for the prevention of disease in animals and focuses on barriers to the development of such products and methods to overcome them.

Complete sequencing of potato virus X new strain genome and construction of viral vector for production of target proteins in plants

The complete nucleotide sequence of the genome of a new potato virus X (PVX) strain Tula isolated by us has been determined. Based on comparison of the PVX Tula nucleotide sequence with the sequences of 12 other PVX strains, this strain was assigned to the European cluster of PVX strains. Phylogenetic analysis revealed the same phylogeny for both full genome sequences and nucleotide sequences of polymerase and coat protein genes, suggesting that the PVX evolution did not involve recombination between different strains. The full-size cDNA copy of the PVX Tula genome was cloned and the accumulation of the viral coat protein in infected Nicotiana benthamiana was shown to be about twofold higher than for the PVX strain UK3. Based on the PVX Tula genome, a new vector which contained the target gene instead of the removed triple transport gene block and the coat protein gene has been constructed for expression of target proteins in plants. The productivity of the new vector was about 1.5-2-fold higher than the productivity of the vector of the same structure based on the standard PVX strain genome. The new viral vector can be used for superproduction of recombinant proteins in plants.

Human monoclonal antibody and vaccine approaches to prevent human rabies

Rabies, being a major zoonotic disease, significantly impacts global public health. It is invariably fatal once clinical signs are apparent. The majority of human rabies deaths occur in developing countries. India alone reports more than 50% of the global rabies deaths. Although it is a vaccine-preventable disease, effective rabies prevention in humans with category III bites requires the combined administration of rabies immunoglobulin (RIG) and vaccine. Cell culture rabies vaccines have become widely available in developing countries, virtually replacing the inferior and unsafe nerve tissue vaccines. Limitations inherent to the conventional RIG of either equine or human origin have prompted scientists to look for monoclonal antibody-based human RIG as an alternative. Fully human monoclonal antibodies have been found to be safer and equally efficacious than conventional RIG when tested in mice and hamsters. In this chapter, rabies epidemiology, reservoir control measures, post-exposure prophylaxis of human rabies, and combination therapy for rabies are discussed. Novel human monoclonal antibodies, their production, and the significance of plants as expression platforms are emphasized.

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.

Cucumber mosaic virus as a presentation system for a double hepatitis C virus-derived epitope

Chimeric plant viruses are emerging as promising vectors for use in innovative vaccination strategies. In this context, cucumber mosaic virus (CMV) has proven to be a suitable carrier of the hepatitis C virus (HCV)-derived R9 mimotope. In the present work, a new chimeric CMV, expressing on its surface the HCV-derived R10 mimotope, was produced but lost the insert after the first passage on tobacco. A comparative analysis between R10- and R9-CMV properties indicated that R9-CMV stability was related to structural features typical of the foreign insert. Thus, in order to combine high virus viability with strong immuno-stimulating activity, we doubled R9 copies on each of the 180 coat protein (CP) subunits of CMV. One of the chimeras produced by this approach (2R9-CMV) was shown to systemically infect the host, stably maintaining both inserts. Notably, it was strongly recognized by sera of HCV-infected patients and, as compared with R9-CMV, displayed an enhanced ability to stimulate lymphocyte IFN-gamma production. The high immunogen levels achievable in plants or fruits infected with 2R9-CMV suggest that this chimeric form of CMV may be useful in the development of oral vaccines against HCV.

Molecular farming for antigen (vaccine) production in plants

Genomic and proteomic approaches to the study of fundamental cell mechanisms are rapidly contributing to broaden our knowledge on metabolic pathways for the optimal exploitation of the cell as a factory. In the last few years this knowledge has led to important advances in the large scale production of diagnostic and therapeutic proteins in heterologous hosts (bacteria, yeasts, mammalian and insect cells or transgenic animals and plants), allowing the comparison of the most efficient methods in terms of costs, product quality and safety.

Approaches to achieve high-level heterologous protein production in plants

Plants offer an alternative to microbial fermentation and animal cell cultures for the production of recombinant proteins. For protein pharmaceuticals, plant systems are inherently safer than native and even recombinant animal sources. In addition, post-translational modifications, such as glycosylation, which cannot be achieved with bacterial fermentation, can be accomplished using plants. The main advantage foreseen for plant systems is reduced production costs. Plants should have a particular advantage for proteins produced in bulk, such as industrial enzymes, for which product pricing is low. In addition, edible plant tissues are well suited to the expression of vaccine antigens and pharmaceuticals for oral delivery. Three approaches have been followed to express recombinant proteins in plants: expression from the plant nuclear genome; expression from the plastid genome; and expression from plant tissues carrying recombinant plant viral sequences. The most important factor in moving plant-produced heterologous proteins from developmental research to commercial products is to ensure competitive production costs, and the best way to achieve this is to boost expression. Thus, considerable research effort has been made to increase the amount of recombinant protein produced in plants. This research includes molecular technologies to increase replication, to boost transcription, to direct transcription in tissues suited for protein accumulation, to stabilize transcripts, to optimize translation, to target proteins to subcellular locations optimal for their accumulation, and to engineer proteins to stabilize them. Other methods include plant breeding to increase transgene copy number and to utilize germplasm suited to protein accumulation. Large-scale commercialization of plant-produced recombinant proteins will require a combination of these technologies.

Construction of an infectious cDNA clone of Ribgrass mosaic virus Shanghai isolate and its modification to express an epitope of Mycobacterium tuberculosis

Infectious cDNA clones of the Shanghai isolate of Ribgrass mosaic virus (RMV) were produced by joining four overlapping cDNA fragments and also in a single step by long template PCR. After inoculation of Nicotiana glutinosa with either RNA transcripts or the cDNA under the control of the CaMV 35S promoter, plants developed typical symptoms, and viral coat protein could be detected in them by Western blot analysis. However, compared to plants inoculated with purified viral RNA, lesions were fewer and appeared more slowly. An epitope of the Mycobacterium tuberculosis 31-kDa protein was inserted at the C-terminus of the viral coat protein by PCR using two overlapping fragments. The modified clone was also infectious and the foreign epitope could be detected serologically in the electron microscope and by Western blot analysis. The results demonstrate the potential of RMV as a viral gene vector.

Engineered chloroplasts as vaccine factories to combat bioterrorism

Bacillus anthracis is ideal for making biological weapons, but the licensed anthrax vaccine is unsuitable for widespread public administration. Recombinant subunit-vaccine candidates offer potential alternatives, and plant-based production systems facilitate the inexpensive bulking of target antigens. A recent report demonstrates expression of anthrax protective antigen in tobacco chloroplasts--this material is immunogenic and protective when injected into mice. Provided an economic purification scheme can be developed, this technology holds promise for an improved vaccine.

Ethics, biotechnology, and global health: the development of vaccines in transgenic plants

As compared with conventional vaccine production systems, plant-made vaccines (PMVs) are said to enjoy a range of advantages including cost of production and ease of storage for distribution in developing countries. In this article, we introduce the science of PMV production, and address ethical issues associated with development and clinical testing of PMVs within three interrelated domains: PMVs as transgenic plants; PMVs as clinical research materials; and PMVs as agents of global health. We present three conclusions: first, while many of the ethical issues raised by PMVs are familiar, PMVs add a new dimension to old issues, and raise some novel issues for ethicists and policy-makers; secondly, it is premature to promise broad applicability of PMVs across the developing world without having demonstrated their feasibility; thirdly, in particular, proponents of PMVs as a solution to global health problems must, as a condition of the ethical conduct of their research, define the commercial feasibility of PMVs for distribution in the developing world.

Regulatory issues for plant-made pharmaceuticals and vaccines

Recombinant plant systems potentially offer economic alternatives to produce large amounts of pharmaceutical proteins, including those used in subunit vaccines. Plant systems also provide a convenient oral delivery option, overcoming the cost and inconvenience of purification and injections. The production of pharmaceutical proteins in transgenic plants is tightly regulated, with the US Department of Agriculture focusing on containment of recombinant material and the US Food and Drug Administration focusing on the production system as it relates to manufacture of the drug or vaccine. Current regulations for the production of plant-made pharmaceuticals are to prevent recombinant proteins from entering the food chain or from persisting in the environment, and to guard against recombinant nucleic acid sequences entering genomes of food or feed crops, or wild species. Several alternative plant production systems are being developed. Each system has its strengths and weaknesses with regard to the economics of production, options for alternative routes of administration, authenticity of products and ease with which the production system can be contained. Risk assessments can be used as a means to quantify risks of inadvertent human or environmental exposure to plant-made pharmaceuticals. Several technologies are being tested that reduce the probability of plant-made pharmaceuticals, or genes encoding them, escaping production sites.

Transgenic plants in the biopharmaceutical market

Many of our 'small-molecule-drugs' are natural products from plants, or are synthetic compounds based on molecules found naturally in plants. However, the vast majority of the protein therapeutics (or biopharmaceuticals) we use are from animal or human sources, and are produced commercially in microbial or mammalian bioreactor systems. Over the last few years, it has become clear that plants have great potential for the production of human proteins and other protein-based therapeutic entities. Plants offer the prospect of inexpensive biopharmaceutical production without sacrificing product quality or safety, and following the success of several plant-derived technical proteins, the first therapeutic products are now approaching the market. In this review, the different plant-based production systems are discussed and the merits of transgenic plants are evaluated compared with other platforms. A detailed discussion is provided of the development issues that remain to be addressed before plants become an acceptable mainstream production technology. The many different proteins that have already been produced using plants are described, and a sketch of the current market and the activities of the key players is provided. Despite the currently unclear regulatory framework and general industry inertia, the benefits of plant-derived pharmaceuticals are now bringing the prospect of inexpensive veterinary and human medicines closer than ever before.

RNA viral community in human feces: prevalence of plant pathogenic viruses

The human gut is known to be a reservoir of a wide variety of microbes, including viruses. Many RNA viruses are known to be associated with gastroenteritis; however, the enteric RNA viral community present in healthy humans has not been described. Here, we present a comparative metagenomic analysis of the RNA viruses found in three fecal samples from two healthy human individuals. For this study, uncultured viruses were concentrated by tangential flow filtration, and viral RNA was extracted and cloned into shotgun viral cDNA libraries for sequencing analysis. The vast majority of the 36,769 viral sequences obtained were similar to plant pathogenic RNA viruses. The most abundant fecal virus in this study was pepper mild mottle virus (PMMV), which was found in high concentrations--up to 10(9) virions per gram of dry weight fecal matter. PMMV was also detected in 12 (66.7%) of 18 fecal samples collected from healthy individuals on two continents, indicating that this plant virus is prevalent in the human population. A number of pepper-based foods tested positive for PMMV, suggesting dietary origins for this virus. Intriguingly, the fecal PMMV was infectious to host plants, suggesting that humans might act as a vehicle for the dissemination of certain plant viruses.

Assessing commercial feasibility: a practical and ethical prerequisite for human clinical testing

This article proposes that an assessment of commercial feasibility should be integrated as a prerequisite for human clinical testing to improve the quality and relevance of materials being investigated, as an ethical aspect for human subject protection, and as a means of improving accountability where clinical development is funded on promises of successful translational research. A commercial feasibility analysis is not currently required to justify human clinical testing, but is assumed to have been conducted by industry participants, and use of public funds for clinical trials should be defensible in the same manner. Plant-made vaccines (PMVs) are offered in this discussion as a model for evaluating the relevance of commercial feasibility before human clinical testing. PMVs have been proposed as a potential solution for global health, based on a vision of immunizing the world against many infectious diseases. Such a vision depends on translating current knowledge in plant science and immunology into a potent vaccine that can be readily manufactured and distributed to those in need. But new biologics such as PMVs may fail to be manufactured due to financial or logistical reasons--particularly for orphan diseases without sufficient revenue incentive for industry investment--regardless of the effectiveness which might be demonstrated in human clinical testing. Moreover, all potential instruments of global health depend on translational agents well beyond the lab in order to reach those in need. A model compromising five criteria for commercial feasibility is suggested for inclusion by regulators and ethics review boards as part of the review process prior to approval of human clinical testing. Use of this model may help to facilitate safe and appropriate translational research and bring more immediate benefits to those in need.

Mucosal immunization using recombinant plant-based oral vaccines

The induction of mucosal immunity is very important in conferring protection against pathogens that typically invade via mucosal surfaces. Delivery of a vaccine to a mucosal surface optimizes the induction of mucosal immunity. The apparent linked nature of the mucosal immune system allows delivery to any mucosal surface to potentially induce immunity at others. Oral administration is a very straightforward and inexpensive approach to deliver a vaccine to the mucosal lining of the gut. However, vaccines administered by this route are subject to proteolysis in the gastrointestinal tract. Thus, dose levels for protein subunit vaccines are likely to be very high and the antigen may need to be protected from proteolysis for oral delivery to be efficacious. Expression of candidate vaccine antigens in edible recombinant plant material offers an inexpensive means to deliver large doses of vaccines in encapsulated forms. Certain plant tissues can also stably store antigens for extensive periods of time at ambient temperatures, obviating the need for a cold-chain during vaccine storage and distribution, and so further limiting costs. Antigens can be expressed from transgenes stably incorporated into a host plant's nuclear or plastid genome, or from engineered plant viruses infected into plant tissues. Molecular approaches can serve to boost expression levels and target the expressed protein for appropriate post-translational modification. There is a wide range of options for processing plant tissues to allow for oral delivery of a palatable product. Alternatively, the expressed antigen can be enriched or purified prior to formulation in a tablet or capsule for oral delivery. Fusions to carrier molecules can stabilize the expressed antigen, aid in antigen enrichment or purification strategies, and facilitate delivery to effector sites in the gastrointestinal tract. Many antigens have been expressed in plants. In a few cases, vaccine candidates have entered into early phase clinical trials, and in the case of farmed animal vaccines into relevant animal trials.

Protection conferred by recombinant Yersinia pestis antigens produced by a rapid and highly scalable plant expression system

Plague is still an endemic disease in different regions of the world. Increasing reports of incidence, the discovery of antibiotic resistance strains, and concern about a potential use of the causative bacteria Yersinia pestis as an agent of biological warfare have highlighted the need for a safe, efficacious, and rapidly producible vaccine. The use of F1 and V antigens and the derived protein fusion F1-V has shown great potential as a protective vaccine in animal studies. Plants have been extensively studied for the production of pharmaceutical proteins as an inexpensive and scalable alternative to common expression systems. In the current study the recombinant plague antigens F1, V, and fusion protein F1-V were produced by transient expression in Nicotiana benthamiana by using a deconstructed tobacco mosaic virus-based system that allowed very rapid and extremely high levels of expression. All of the plant-derived purified antigens, administered s.c. to guinea pigs, generated systemic immune responses and provided protection against an aerosol challenge of virulent Y. pestis.

The Potential of Plant Virus Vectors for Vaccine Production

Plants viruses are versatile vectors that allow the rapid and convenient production of recombinant proteins in plants. Compared with production systems based on transgenic plants, viral vectors are easier to manipulate and recombinant proteins can be produced more quickly and in greater yields. Over the last few years, there has been much interest in the development of plant viruses as vectors for the production of vaccines, either as whole polypeptides or epitopes displayed on the surface of chimeric viral particles. Several viruses have been extensively developed for vaccine production, including tobacco mosaic virus, potato virus X and cowpea mosaic virus. Vaccine candidates have been produced against a range of human and animal diseases, and in many cases have shown immunogenic activity and protection in the face of disease challenge. In this review, we discuss the advantages of plant virus vectors, the development of different viruses as vector systems, and the immunological experiments that have demonstrated the principle of plant virus-derived vaccines.

Risk analysis for plant-made vaccines

The production of vaccines in transgenic plants was first proposed in 1990 however no product has yet reached commercialization. There are several risks during the production and delivery stages of this technology, with potential impact on the environment and on human health. Risks to the environment include gene transfer and exposure to antigens or selectable marker proteins. Risks to human health include oral tolerance, allergenicity, inconsistent dosage, worker exposure and unintended exposure to antigens or selectable marker proteins in the food chain. These risks are controllable through appropriate regulatory measures at all stages of production and distribution of a potential plant-made vaccine. Successful use of this technology is highly dependant on stewardship and active risk management by the developers of this technology, and through quality standards for production, which will be set by regulatory agencies. Regulatory agencies can also negatively affect the future viability of this technology by requiring that all risks must be controlled, or by applying conventional regulations which are overly cumbersome for a plant production and oral delivery system. The value of new or replacement vaccines produced in plant cells and delivered orally must be considered alongside the probability and severity of potential risks in their production and use, and the cost of not deploying this technology – the risk of continuing with the status quo alternative.

Advances in alfalfa mosaic virus-mediated expression of anthrax antigen in planta

Plant viruses show great potential for production of pharmaceuticals in plants. Such viruses can harbor a small antigenic peptide(s) as a part of their coat proteins (CP) and elicit an antigen-specific immune response. Here, we report the high yield and consistency in production of recombinant alfalfa mosaic virus (AlMV) particles for specific presentation of the small loop 15 amino acid epitope from domain-4 of the Bacillus anthracis protective antigen (PA-D4s). The epitope was inserted immediately after the first 25 N-terminal amino acids of AlMV CP to retain genome activation and binding of CP to viral RNAs. Recombinant AlMV particles were efficiently produced in tobacco, easily purified for immunological analysis, and exhibited extended stability and systemic proliferation in planta. Intraperitional injections of mice with recombinant plant virus particles harboring the PA-D4s epitope elicited a distinct immune response. Western blotting and ELISA analysis showed that sera from immunized mice recognized both native PA antigen and the AlMV CP.

The next 15 years: taking plant-made vaccines beyond proof of concept

Significant potential advantages are associated with the production of vaccines in transgenic plants; however, no commercial product has emerged. An analysis of the strengths, weaknesses, opportunities and threats for plant-made vaccine technology is provided. The use of this technology for human vaccines will require significant investment and developmental efforts that cannot be supported entirely by the academic sector and is not currently supported financially by industry. A focus on downstream aspects to define potential products, conduct of additional basic clinical testing, and the incorporation of multidisciplinary strategic planning would accelerate the potential for commercialization in this field. Estimates of production cost per dose and volume of production are highly variable for a model vaccine produced in transgenic tomato, and can be influenced by the optimization of many factors. Commercialization of plant-made vaccine technology is likely to be led by the agricultural biotechnology sector rather than the pharmaceutical sector due to the disruptive nature of the technology and the complex intellectual property landscape. The next major milestones will be conduct of a phase II human clinical trial and demonstration of protection in humans. The achievement of these milestones would be accelerated by further basic investigation into mucosal immunity, the codevelopment of oral adjuvants, and the integration of quality control standards and good manufacturing practices for the production of preclinical and clinical batch materials.

High level expression of surface glycoprotein of rabies virus in tobacco leaves and its immunoprotective activity in mice

A synthetic gene coding for the surface glycoprotein (G protein) of rabies virus was strategically designed to achieve high-level expression in transgenic plants. The native signal peptide was replaced by that of the pathogenesis related protein, PR-S of Nicotiana tabacum. An endoplasmic reticulum retention signal was included at C-terminus of the G protein. Tobacco plants were genetically engineered by nuclear transformation. Selected transgenic lines expressed the chimeric G protein at 0.38% of the total soluble leaf protein. Mice immunized intraperitoneally with the G protein purified from tobacco leaf microsomal fraction elicited high level of immune response as compared to the inactivated commercial viral vaccine. The plant-derived G protein induced complete protective immunity in mice against intracerebral lethal challenge with live rabies virus. The results establish that plants can provide a safe and effective production system for the expression of immunoprotective rabies virus surface protein.

Optimization of virus yield as a strategy to improve rabies vaccine production by Vero cells in a bioreactor

To improve rabies vaccine production by Vero cells, we have developed a strategy based on high cell density culture and optimization of virus yield. We have first optimized cell growth in spinner flask using a Taguchi's L8 experimental design. We analyzed the effects of the following factors: initial glucose and glutamine concentrations, Cytodex 1 concentration and the regulation of glucose level at 1 g l(-1). We have also investigated the effect of the following factor interactions: Cytodex 1 concentration/glutamine concentration, Cytodex 1 concentration/glucose concentration and glucose concentration/glutamine concentration. Statistical analysis of the collected data pointed to the initial glucose concentration, the regulation of glucose level at 1 g l(-1) and the interactions between Cytodex 1 concentration/initial glucose concentration and Cytodex 1 concentration/initial glutamine concentration as the parameters that affected cell growth. Using the optimal conditions determined earlier, we have studied Vero cell growth in a 7-l bioreactor and in batch culture, and obtained a cell density level equal to 3.6 +/- 0.2 x 10(6) cells ml-1. Cell infection with rabies virus (LP 2061/Vero strain) at a multiplicity of infection (MOI) of 0.3 using M199 medium supplemented with 0.2% bovine serum albumin (BSA), yielded a maximal virus titer equal to 8 +/- 1.6 x 10(7) Fluorescent Focus Units (FFU) ml-1. We have also studied Vero cell growth in a 7-l bioreactor using recirculation as a perfusion culture mode during cell proliferation step and perfusion for virus multiplication phase. In comparison to batch culture, we reached a higher cell density level that was equal to 10.1 +/- 0.5 x 10(6) cells ml-1. Cell infection under the conditions previously indicated, yielded 14l of virus harvest that had a virus titer equal to 2.6 +/- 0.5 x 10(7) FFU ml-1. The activity of the inactivated virus harvest showed a protective activity that meets WHO requirements.

Delivery of plant-derived vaccines

Many protein subunit vaccine candidates have been expressed in transgenic plants, and in a few cases the recombinant material has entered early phase clinical or target animal trials. The expressed protein can be purified prior to formulation for any preferred delivery approach. However, there are major cost advantages associated with avoiding protein purification and pursuing the oral delivery of a processed plant product containing the recombinant protein. Grains and dry products that are processed from fresh plant tissues can stably store expressed proteins for extended periods of time at room temperature, making refridgeration unnecessary during storage and distribution. Encapsulation of recombinant proteins in plant tissues guards against their rapid degradation in the gut, therefore facilitating the uptake and induction of appropriate immune responses. Early trial data with plant-based vaccine candidates has shown promising safety and efficacy.

Immunological insights from genetic vaccines

E1-deleted adenoviral vectors expressing the rabies virus glycoprotein rapidly induce protective titers of rabies virus neutralizing antibodies in adult and neonatal mice upon systemic or mucosal immunization. Pre-existing immunity in humans due to natural infections with common human serotypes of adenovirus such as the human serotype 5, most commonly used as a vaccine carrier can be circumvented by systemic immunization with a simian-origin adenovirus or by using the oral route of immunization. Virus neutralizing antibody titers can be enhanced by prime-boost regimens.

Severe acute respiratory syndrome (SARS) S protein production in plants: development of recombinant vaccine

In view of a recent spread of severe acute respiratory syndrome (SARS), there is a high demand for production of a vaccine to prevent this disease. Recent studies indicate that SARS-coronavirus (CoV) spike protein (S protein) and its truncated fragments are considered the best candidates for generation of the recombinant vaccine. Toward the development of a safe, effective, and inexpensive vaccine candidate, we have expressed the N-terminal fragment of SARS-CoV S protein (S1) in tomato and low-nicotine tobacco plants. Incorporation of the S1 fragment into plant genomes as well as its transcription was confirmed by PCR and RT-PCR analyses. High levels of expression of recombinant S1 protein were observed in several transgenic lines by Western blot analysis using specific antibodies. Plant-derived antigen was evaluated to induce the systemic and mucosal immune responses in mice. Mice showed significantly increased levels of SARS-CoV-specific IgA after oral ingestion of tomato fruits expressing S1 protein. Sera of mice parenterally primed with tobacco-derived S1 protein revealed the presence of SARS-CoV-specific IgG as detected by Western blot and ELISA analysis.

Expression systems and developments in plant-made vaccines

Delivery of vaccines to mucosal surfaces can elicit humoral and cell-mediated responses of the mucosal and systemic immune systems, evoke less pain and discomfort than parenteral delivery, and eliminate needle-associated risks. Transgenic plants are an ideal means by which to produce oral vaccines, as the rigid walls of the plant cell protect antigenic proteins from the acidic environment of the stomach, enabling intact antigen to reach the gut associated lymphoid tissue. In the past few years, new techniques (such as chloroplast transformation and food processing) have improved antigen concentration in transgenic plants. In addition, adjuvants and targeting proteins have increased the immunogenicity of mucosally administered plant-made vaccines. These studies have moved plant-made vaccines closer to the development phase.

Is there a role for plant‐made vaccines in the prevention of HIV/AIDS?

Although educational programs have had some impact, immunization against HIV will be necessary to control the AIDS pandemic. To be effective, vaccination will need to be accessible and affordable, directed against multiple antigens, and delivered in multiple doses. Plant-based vaccines that are heat-stable and easy to produce and administer are suited to this type of strategy. Pilot studies by a number of groups have demonstrated that plant viral expression systems can produce HIV antigens in quantities that are appropriate for use in vaccines. In addition, these plant-made HIV antigens have been shown to be immunogenic. However, given the need for potent cross-clade humoral and T-cell immunity for protection against HIV, and the uncertainty surrounding the efficacy of protein subunit vaccines, it is most likely that plant-made HIV vaccines will find their niche as booster immunizations in prime-boost vaccination schedules.

Oral hepatitis B vaccine candidates produced and delivered in plant material

Hepatitis B is a major global health problem; approximately two billion people are infected with the virus worldwide, despite the fact that safe and efficacious vaccines have been developed and used for nearly 20 years. Prohibitive costs for vaccine purchase and administration restrict uptake in many developing nations. Agencies such as the Global Alliance for Vaccination and Immunization are helping to make current vaccines more available, but reduced costs would greatly aid this effort. Oral delivery is an option to reduce the expense of administering hepatitis B vaccines. It may also improve compliance, and orally delivered vaccines may be more efficacious among poor responders to current vaccines. However, to induce protective efficacy, oral administration may require encapsulation of antigen and delivery of large doses. Plant-based expression systems offer an oral delivery alternative with low production costs, and they also encapsulate the antigen. Some plant-based systems also stabilize antigen and therefore reduce storage and distribution costs. The hepatitis B major surface antigen has been expressed in several plant systems. A variety of regulatory sequences and subcellular targets have been used to achieve expression suitable for early stage clinical trials. However, further increase in expression will be necessary for practical and efficacious products. Appropriate processing can yield palatable products with uniform antigen concentration. The antigen expressed in plant systems shows extensive disulphide cross-linking and oligomerization and forms virus-like particles. Oral delivery of the antigen in plant material can induce a serum antibody response, prime the immune system for a subsequent injection of antigen and give a boosted response to a prior injection. Small scale clinical trials in which the antigen has been delivered orally in edible plant material indicate safety and immunogenicity.