A functional antigen in a practical crop: LT-B producing maize protects mice against Escherichia coli heat labile enterotoxin (LT) and cholera toxin (CT)

Low-dose exposure and immunogenicity of transgenic maize expressing the Escherichia coli heat-labile toxin B subunit

BACKGROUND

Transgenic maize, which produces the nontoxic B subunit of the Escherichia coli heat-labile toxin (LT-B) in seed, has proven to be an effective oral immunogen in mice. Currently, there is considerable concern over accidental consumption of transgenic maize expressing LT-B by humans and domestic animals. We have yet to define nonimmunogenic levels of transgenic LT-B when ingested.

OBJECTIVES

Our goal in this study was to determine the highest dose of LT-B orally administered in mice that does not result in a measurable immune response. We defined an immune response as specific serum or mucosal IgG or IgA significantly greater than background after three feedings (0.0002-20 mug) or a priming response induced by the intermittent feeding.

METHODS

We fed transgenic maize pellets on days 0, 7, 21, and 49 and collected serum and fecal samples weekly. Serum was analyzed for LT-B-specific IgG and IgA, and feces was analyzed for LT-B-specific IgA.

RESULTS

We observed a dose-dependent anti-LT-B antibody response with high specific antibody concentrations in groups fed high doses (0.2, 2, 20 mug) of LT-B maize. Mice fed 0.02 mug LT-B demonstrated immune priming in 62.5% of the animals. Mice that were fed </= 0.002 mug LT-B showed no increase in specific antibody nor did they demonstrate immune priming, indicating that 0.002 mug LT-B was the highest nonimmunogenic dose tested.

CONCLUSION

Our results demonstrate that LT-B derived from transgenic maize is immunogenic at nanogram levels when orally administered to mice.

Expression of the human hepatitis B virus large surface antigen gene in transgenic tomato plants

The original hepatitis B virus (HBV) large surface antigen gene was synthesized. In order to optimize the expression of this gene in tomato plants, the tobacco pathogenesis-related protein S signal peptide was fused to the 5' end of the modified gene and the sequence encoding amino acids S, E, K, D, E, and L was placed at the 3' end. The gene encoding the modified HBV large surface antigen under the control of a fruit-specific promoter was constructed and expressed in transgenic tomato plants. The expression of the antigen from transgenic plants was confirmed by PCR and reverse transcriptase PCR. Enzyme-linked immunoassays using a monoclonal antibody directed against human serum-derived HBsAg revealed that the maximal level of HBsAg was about 0.02% of the soluble protein in transgenic tomato fruit. The amount of HBsAg in mature fruits was found to be 65- to 171-fold larger than in small or medium fruits and leaf tissues. Examination of transgenic plant samples by transmission electron microscopy proved that HBsAg had been expressed and had accumulated. The HBsAg protein was capable of assembling into capsomers and virus-like particles. To our knowledge, this is the first time the HBV large surface antigen has been expressed in plants. This work suggests the possibility of producing a new alternative vaccine for human HBV.

Production of Escherichia coli heat labile toxin (LT) B subunit in soybean seed and analysis of its immunogenicity as an oral vaccine

The B subunit of the heat labile toxin of enterotoxigenic Escherichia coli (LTB) was used as a model immunogen for production in soybean seed. LTB expression was directed to the endoplasmic reticulum (ER) of seed storage parenchyma cells for sequestration in de novo synthesized inert protein accretions derived from the ER. Pentameric LTB accumulated to 2.4% of the total seed protein at maturity and was stable in desiccated seed. LTB-soybean extracts administered orally to mice induced both systemic IgG and IgA, and mucosal IgA antibody responses, and was particularly efficacious when used in a parenteral prime-oral gavage boost immunization strategy. Sera from immunized mice blocked ligand binding in vitro and immunized mice exhibited partial protection against LT challenge. Moreover, soybean-expressed LTB stimulated the antibody response against a co-administered antigen by 500-fold. These results demonstrate the utility of soybean as an efficient production platform for vaccines that can be used for oral delivery.

Expression of the fusion glycoprotein of newcasstle disease virus in transgenic rice and its immunogenicity in mice

Transgenic plant has become an attractive bioreactor to produce high-value medical peptides and proteins in biomedical research. In present study, two expression cassettes, pUNDVF and pGNDVF containing the fusion protein gene of Newcastle disease virus (NDV F) under the control of maize ubiquitin (Ubi) promoter or rice glutelin (Gt1) promoter, respectively, were constructed, and introduced into rice (Oryzy sativa L.) by Agrobacterium-mediated transformation. A total of 12 independent transgenic rice lines were regenerated, and the result from PCR analysis indicated that the T-DNA region containing the NDV F chimeric gene had been integrated into the genome of transgenic rice plants. ELISA and Western-blot analyses revealed that the NDV F protein could be expressed and accumulated in both leaf and seed tissue of several transgenic rice plants. Moreover, the immunogenicity of expressed proteins was tested in a mouse model and the results showed that specific antibodies were elicited in mice immunized intraperitoneally with crude protein extracts from transgenic rice plants. It implied the potential of using transgenic rice-based expression systems as supplementary bioreactor for NDV engineering subunit vaccine.

Ubiquitin fusion enhances cholera toxin B subunit expression in transgenic plants and the plant-expressed protein binds GM1 receptors more efficiently

Developing plant based systems for the production of therapeutic recombinant proteins requires the development of efficient expression strategies and characterization of proteins made in heterologous cellular environment. In this study, the expression of cholera toxin B subunit (CtxB) was examined in the leaves of transgenic tobacco plants. A synthetic gene encoding CtxB was designed for high level expression in plant cells and cloned as ubiquitin (Ub) fusion in a plant expression vector. Tobacco plants were genetically engineered by nuclear transformation to express the CtxB or Ub-CtxB fusion proteins under the control of CaMV35S duplicated enhancer promoter. Functionally active CtxB accumulated in tobacco leaves at 2.5-fold higher level in the Ub-CtxB plants. In the best expressors, CtxB accumulated at 0.9% of the total soluble leaf protein. In both the constructs, molecular mass of the plant-expressed CtxB was 14.6 kDa in contrast to 11.6 kDa for the authentic CtxB. Schiff's test, retention on concanavalin A column and chemical and enzymatic deglycosylation established that the higher molecular mass was due to glycosylation of the CtxB expressed in plant cells. The glycosylated CtxB made in tobacco leaves had higher affinity of binding to the GM1 receptors.

WHO informal consultation on scientific basis for regulatory evaluation of candidate human vaccines from plants, Geneva, Switzerland, 24-25 January 2005

In January 2005, WHO convened a meeting of leading experts in plant-derived vaccines and experts from regulatory authorities for an informal discussion on the state-of-the-art and to analyse whether specific guidance might be needed for plant-derived vaccines that is not yet provided by regulatory authorities. After a series of individual presentations, a general discussion was held to obtain a consensus on the need for further guidance. Both the presentations and the conclusions are presented. The meeting concluded that existing guidelines for the development, evaluation, and use of vaccines made by traditional methods can be applied to plant-derived vaccines. For plant-derived vaccines some specific issues will have to be addressed. These include, but are not restricted to, containment of the plants including disposal of waste materials. It was noted that plant-derived vaccines have been produced and clinically tested under US investigational new drug application, and all applicable regulatory and good manufacturing practice requirements are in place for this type of product. An innovator wishing to bring a plant-derived vaccine to market should consult closely with regulatory authorities to ensure that all appropriate studies are undertaken.

Expression of the Newcastle disease virus fusion protein in transgenic maize and immunological studies

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 the fusion (F) gene of the Newcastle disease virus (NDV) in transgenic maize plants. The expression of the transgene, driven by the maize ubiquitin promoter, caused accumulation of the F protein in maize kernels. The presence of the transgene was verified by Southern and western blots. Feeding chickens with kernels containing the F protein induced the production of antibodies, which conferred protection against a viral challenge. This protection was comparable to that conferred by a commercial vaccine. Possible uses of this plant-based F protein as a potential mucosal vaccine are discussed.

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.

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.

Mass production of somatic embryos expressing Escherichia coli heat-labile enterotoxin B subunit in Siberian ginseng

The B subunit of Escherichia coli heat-labile toxin (LTB) is a potent mucosal immunogen and immunoadjuvant for co-administered antigens. In order to produce large scale of LTB for the development of edible vaccine, we used transgenic somatic embryos of Siberian ginseng, which is known as medicinal plant. When transgenic somatic embryos were cultured in 130L air-lift type bioreactor, they were developed to mature somatic embryos through somatic embryogenesis and contained approximately 0.36% LTB of the total soluble protein. Enzyme-linked immunosorbent assay indicated that the somatic embryo-synthesized LTB protein bound specifically to GM1-ganglioside, suggesting the LTB subunits formed active pentamers. Therefore, the use of the bioreactor system for expression of LTB proteins in somatic embryos allows for continuous mass production in a short-term period.

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.

Considerations for development of whole cell bacterial vaccines to prevent diarrheal diseases in children in developing countries

Enteric pathogens constitute a major pediatric threat in the developing world through their impact on morbidity and mortality, physical and cognitive development and cause and effect relationship with malnutrition. Although many bacterial pathogens can cause diarrheal diseases, a group of less than 10 including Shigella spp., enterotoxigenic Escherichia coli (ETEC), Vibrio cholerae, and possibly, Campylobacter jejuni account for a significant percentage of these diseases in developing countries. Rotavirus is also a major cause of diarrheal diseases. Vaccines against these agents offer a potentially effective control measure against these diseases, but safe, practical, and effective vaccines for many of these agents have yet to be realized. Many vaccine development approaches are under investigation, but the one that is currently most advanced and that has been most widely applied to enteric pathogens is the use of orally administered live or killed whole pathogen preparations. If inactivated, these vaccines will probably be administered as multiple doses with approximately 10(10) to 10(11) total particles per dose, but they are relatively safe for oral administration. Further, they may not require a buffer for delivery and can be stored in liquid formulations. Fewer doses may be required for some live attenuated pathogen vaccines, but a buffer will most likely be required for oral delivery and the product must be stored in a dried formulation. Also, safety becomes more of a concern with live pathogens depending on the degree of attenuation, host immunocompetence, and the total number and kinds of attenuated pathogens which may be present in a combined agent vaccine. Both live and killed whole pathogen vaccines can be immunogenic and have the possibility to serve as vectors for other antigens. Although many organisms and serotypes are clinically important, by exploiting antigenic cross reactivity and using some pathogen components as vectors for cloned antigens of other pathogens, it could be possible to induce immunity against major enteric pathogens/serotypes with <10 whole pathogen components in a multi-agent vaccine. Safe and effective mucosal adjuvants may in the future be useful in whole pathogen vaccines, but they do not seem to be essential for immunization. Further, dietary supplements such as zinc, mixed routes of delivery and new regimens are under study which may in the future enhance further the effectiveness of the whole pathogen vaccines which now seem realizable in the near term. For this to happen, however, a coordinated and committed effort is necessary now to address the immunologic, regulatory, manufacturing, testing and implementation issues which will be involved in the realization of this important product to benefit children's health worldwide.

Plant-derived vaccines against diarrhoeal diseases

Transgenic plant-derived vaccines offer a new strategy for the development of safe, inexpensive vaccines against diarrhoeal diseases. In animal and Phase I clinical studies, these vaccines have been safe and immunogenic without the need for a buffer or vehicle other than the plant cell. This review examines some early attempts to develop oral transgenic plant vaccines against enteric infections such as enterotoxigenic Escherichia coli infection, cholera and norovirus infection.

Plant-derived vaccines against diarrheal diseases

Transgenic plants present a novel system for both production and oral delivery of vaccine antigens. Production of protein antigen in food plants is substantially cheaper than production in bacterial, fungal, insect cell, or mammalian cell culture. Edible plants themselves can also serve as the oral vaccine delivery system. Phase-1 studies of raw transgenic potatoes expressing the B subunit of Escherichia coli heat labile enterotoxin (LT-B), potatoes expressing Norwalk virus capsid protein, and defatted corn germ meal expressing LT-B have been conducted. New oral vaccines based on other transgenic plants will soon be evaluated in humans.

Expression of non-toxic mutant of Escherichia coli heat-labile enterotoxin in tobacco chloroplasts

Chloroplast transformation systems offer unique advantages in biotechnology, including high level of foreign gene expression, maternal inheritance, and polycistronic expression. We studied chloroplast expression of LTK63 (change Ser-->Lys at position 63 in the A subunit) which is the mutant of Escherichia coli heat-labile toxin. LTK63 is devoid of any toxic activity, but still retains its mucosal adjuvanticity. The LTK63 was cloned into chloroplast targeting vector and transformed to tobacco chloroplasts by particle bombardment. PCR and Southern blot analyses confirmed stable homologous recombination of the LTK63 gene into the chloroplast genome. The amount of LTK63 protein detected in tobacco chloroplasts was approximately 3.7% of the total soluble protein. The GM1-ganglioside binding assay confirmed that chloroplast-synthesized LTB of LTK63 binds to the intestinal membrane GM1-ganglioside receptor. Thus, the expression of LTK63 in chloroplasts provides a potential route toward the development of a plant-based edible vaccine for high expression system and environmentally friendly approach.

Rapid and reliable extraction of genomic DNA from various wild-type and transgenic plants

Background

DNA extraction methods for PCR-quality DNA from calluses and plants are not time efficient, since they require that the tissues be ground in liquid nitrogen, followed by precipitation of the DNA pellet in ethanol, washing and drying the pellet, etc. The need for a rapid and simple procedure is urgent, especially when hundreds of samples need to be analyzed. Here, we describe a simple and efficient method of isolating high-quality genomic DNA for PCR amplification and enzyme digestion from calluses, various wild-type and transgenic plants.

Results

We developed new rapid and reliable genomic DNA extraction method. With our developed method, plant genomic DNA extraction could be performed within 30 min. The method was as follows. Plant tissue was homogenized with salt DNA extraction buffer using hand-operated homogenizer and extracted by phenol:chloroform:isoamyl alcohol (25:24:1). After centrifugation, the supernatant was directly used for DNA template for PCR, resulting in successful amplification for RAPD from various sources of plants and specific foreign genes from transgenic plants. After precipitating the supernatant, the DNA was completely digested by restriction enzymes.

Conclusion

This DNA extraction procedure promises simplicity, speed, and efficiency, both in terms of time and the amount of plant sample required. In addition, this method does not require expensive facilities for plant genomic DNA extraction.

Expression of the B subunit of E. coli heat-labile enterotoxin in the chloroplasts of plants and its characterization

Transgenic chloroplasts have become attractive systems for heterologous gene expressions because of unique advantages. Here, we report a feasibility study for producing the nontoxic B subunit of Escherichia coli heat-labile enterotoxin (LTB) via chloroplast transformation of tobacco. Stable site-specific integration of the LTB gene into chloroplast genome was confirmed by PCR and genomic Southern blot analysis in transformed plants. Immunoblot analysis indicated that plant-derived LTB protein was oligomeric, and dissociated after boiling. Pentameric LTB molecules were the dominant molecular species in LTB isolated from transgenic tobacco leaf tissues. The amount of LTB protein detected in transplastomic tobacco leaf was approximately 2.5% of the total soluble plant protein, approximately 250-fold higher than in plants generated via nuclear transformation. The GM1-ELISA binding assay indicated that chloroplast-synthesized LTB protein bound to GM1-ganglioside receptors. LTB protein with biochemical properties identical to native LTB protein in the chloroplast of edible plants opens the way for inexpensive, safe, and effective plant-based edible vaccines for humans and animals.

Localization of a bacterial protein in starch granules of transgenic maize kernels

The B subunit of Escherichia coli heat labile enterotoxin (LT-B) is a potent oral immunogen with potential for use as a vaccine, a carrier molecule to deliver antigens to gut-associated lymphoid tissues, and possibly an adjuvant to make coadministered vaccines more effective. LT-B produced in plants was shown to be functional and immunogenic in animals and humans. In this work, we show that maize-derived LT-B is strongly associated with starch granules in endosperm. Using immunogold labeling/electron microscopy, cell fractionation, and protein analysis techniques, we observed that LT-B protein could be detected both internally and externally in starch granules. This strong association confers an effective copurification of the antigen with the starch fraction of maize kernels, thermostability desirable in maize processing, and resistance to peptic degradation in simulated gastric fluid digests, an important attribute for an orally delivered antigen.

Safety assessment of recombinant green fluorescent protein orally administered to weaned rats

Several proposed biotechnological applications of green fluorescent protein (GFP) are likely to result in its introduction into the food supply of domestic animals and humans. We fed pure GFP and diets containing transgenic canola expressing GFP to young male rats for 26 d to evaluate the potential toxicity and allergenicity of GFP. Animals (n = 8 per group) were fed either AIN-93G (control), control diet plus 1.0 mg of purified GFP daily, modified control diet with 200 g/kg canola (Brassica rapa cv Westar), or control diet with 200 g/kg transgenic canola containing one of two levels of GFP. Ingestion of GFP did not affect growth, food intake, relative weight of intestine or other organs, or activities of hepatic enzymes in serum. Comparison of the amino acid sequence of GFP to known food allergens revealed that the greatest number of consecutive amino acid matches between GFP and any food allergen was four, suggesting the absence of common allergen epitopes. Moreover, GFP was rapidly degraded during simulated gastric digestion. These data indicate that GFP is a low allergenicity risk and provide preliminary indications that GFP is not likely to represent a health risk.

Oral immunization of animals with transgenic cherry tomatillo expressing HBsAg

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

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

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

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

Plant-based vaccines

Plant systems are reviewed with regard to their ability to express and produce subunit vaccines. Examples of different types of expression systems producing a variety of vaccine candidates are illustrated. Many of these subunit vaccines have been purified and shown to elicit an immune response when injected into animal models. This review also includes vaccines that have been administered orally in a non-purified form as a food or feed product. Cases are highlighted which demonstrate that orally delivered plant-based vaccines can elicit immune responses and in some case studies, confer protection. Examples are used to illustrate some of the inherent advantages of a plant-based system, such as cost, ease of scale-up and convenience of delivery. Also, some of the key steps are identified that will be necessary to bring these new vaccines to the market.

Plant cell factories and mucosal vaccines

Many advances continue to be made in the field of plant-derived vaccines. Plants have been shown capable of expressing a multicomponent vaccine that when orally delivered induces a T-helper cell subset 1 response and enables passive immunization. Furthermore, a plant-derived vaccine has been shown to protect against challenge in the target host. Increased antigen expression levels (up to 4.1% total soluble protein) have been obtained through transformation of the chloroplast genome. In view of these findings, plant-derived vaccines have been proved as valuable commodities to the world's health system; however, before their application, studies need to focus on optimization of immunization strategies and to investigate antigen stability.