Oral immunisation of naive and primed animals with transgenic potato tubers expressing LT-B

Expressing the immunodominant projection domain of infectious bursal disease virus fused to the fragment crystallizable of chicken IgY in yellow maize for a prospective edible vaccine

Control of Infectious bursal disease virus (IBDV) in endemic countries has been based on early immunization of chicks using conventional live or inactivated vaccines that became not fully effectual and have biosafety concerns. This endeavor seeks generating a recombinant chimeric protein merging the projection domain (PD) of IBDV VP2 capsid with the fragment crystallizable (Fc) of avian IgY (FcIgY), in maize as a prospective poultry edible vaccine. The PD sequence was built on the basis of very virulent IBDV isolates circulating in Egypt. After optimization of codon-usage in maize, sequences of PD and FcIgY were effectively expressed in two elites of yellow maize via bombardment transformation in immature embryos. Chimeric protein amount in stable transgenic samples ranged from1.36% to 3.03% of the total soluble protein based on tissue age and maize cultivar. IBDV VP2 coding sequence was amplified from viral RNA, cloned, and expressed in E. coli. A group of Balb/C mice were hyper-immunized with purified recombinant VP2 protein for raising anti- recombinant VP2 antibodies (anti-rVP2 Ab). Proper expression in maize and immunoreactivity of the chimeric protein (PD-FcIgY) to chicken anti- IBDV and anti-rVP2 Ab were confirmed by both direct and indirect double antibody sandwich (DAS)-ELISAs as well as western blotting. Seeds of regenerated transgenic maize will be validated for chickens as edible vaccination in further studies.

Expression of B subunit of E. coli heat-labile enterotoxin in the progenies of transgenic tobacco bred by crossing nuclear- and chloroplast-transgenic lines

The B subunit of Escherichia coli heat-labile toxin (LTB) is a model antigen that induces a strong immune response upon oral administration and enhances immune responses to conjugated and co-administered antigens. We previously examined high expression levels of LTB in plants by chloroplast and synthetic LTB gene expression and found substantially higher expression levels of LTB, compared to nuclear LTB expression in wild-type plants. The 2.5% LTB protein of total soluble protein that was observed by chloroplast transformation was approximately 250-fold greater expression than that of LTB via nuclear genome integration. In addition, the amount of LTB protein found in transgenic tobacco leaves using a synthetic LTB gene was 2.2% of the total soluble plant protein, which was approximately 200-fold higher than that in plants with native LTB gene expression. The purpose of our experiment was to increase LTB levels in plants by crossing chloroplast-transformed and synthetic LTB transgenic lines produced previously to express higher LTB levels. LTB protein levels in the F1 transgenic tobacco plants was significantly higher (3.3%), compared to the 2.2% of chloroplast-transformed line or 2.8% of synthetic LTB gene line. Our results suggest that LTB expression was successfully enhanced in the F1 hybrid generation of transgenic tobacco plants.

Bisphosphonates target B cells to enhance humoral immune responses

Bisphosphonates are a class of drugs that are widely used to inhibit loss of bone mass in patients. We show here that the administration of clinically relevant doses of bisphosphonates in mice increases antibody responses to live and inactive viruses, proteins, haptens, and existing commercial vaccine formulations. Bisphosphonates exert this adjuvant-like activity in the absence of CD4(+) and γδ T cells, neutrophils, or dendritic cells, and their effect does not rely on local macrophage depletion, Toll-like receptor signaling, or the inflammasome. Rather, bisphosphonates target directly B cells and enhance B cell expansion and antibody production upon antigen encounter. These data establish bisphosphonates as an additional class of adjuvants that boost humoral immune responses.

Recent advances and safety issues of transgenic plant-derived vaccines

Transgenic plant-derived vaccines comprise a new type of bioreactor that combines plant genetic engineering technology with an organism's immunological response. This combination can be considered as a bioreactor that is produced by introducing foreign genes into plants that elicit special immunogenicity when introduced into animals or human beings. In comparison with traditional vaccines, plant vaccines have some significant advantages, such as low cost, greater safety, and greater effectiveness. In a number of recent studies, antigen-specific proteins have been successfully expressed in various plant tissues and have even been tested in animals and human beings. Therefore, edible vaccines of transgenic plants have a bright future. This review begins with a discussion of the immune mechanism and expression systems for transgenic plant vaccines. Then, current advances in different transgenic plant vaccines will be analyzed, including vaccines against pathogenic viruses, bacteria, and eukaryotic parasites. In view of the low expression levels for antigens in plants, high-level expression strategies of foreign protein in transgenic plants are recommended. Finally, the existing safety problems in transgenic plant vaccines were put forward will be discussed along with a number of appropriate solutions that will hopefully lead to future clinical application of edible plant vaccines.

Transgenic tobacco expressed HPV16-L1 and LT-B combined immunization induces strong mucosal and systemic immune responses in mice

BACKGROUND

Although there are two HPV vaccines have been used to prevent cervical cancer, the cost limits their application in developing countries. The aim of this study was to evaluate the potential value of plant-based HPV16L1 and LTB proteins as a high-efficiency, low-cost and easy-to-use HPV16L1 oral vaccine.

RESULTS

Transgenic plant-derived HPV16L1 and LTB were identified, which display potent immunogenicity and biologic activity. Higher levels of specific IgG and IgA levels of HPV16L1 were induced when mice were immunized with L1 combined with LTB by the oral route. The stimulation index (SI) of spleen cells from the L1/LTB-immunized group was significantly higher than that in the L1-immunized group (p < 0.05). The percentage of IFN-γ (+) /IL-4 (+) CD4 (+) T cells from the L1/LTB group was clearly increased compared with that in the L1 and control groups (p < 0.05).

METHODS

Plant-expressed HPV16L1 and LTB proteins were extracted from transgenic tobacco leaves, and their biologic characteristics and activity were examined with electron microscopy and GM1-binding assays respectively. Mice were immunized orally with either HPV16L1 or LTB alone or in combination. Induced mucosal and systemic immune responses were detected by ELISA, Hemagglutination inhibition (HAI), lymphocyte proliferation assays and flow cytometry analysis.

CONCLUSION

Strong mucosal and systemic immune responses were induced by transgenic tobacco derived HPV16-L1 and LTB combined immunization. This study will lay the foundation for the development of a new type of vaccine to decrease HPV16 infections, which may lead to the prevention of cervical cancer.

Protective efficacy against Chlamydophila psittaci by oral immunization based on transgenic rice expressing MOMP in mice

Avian chlamydiosis is caused by Chlamydophila psittaci (Cp. psittaci) and major outer membrane protein (MOMP) of Cp. psittaci is an excellent vaccine candidate. In this study, the MOMP gene was expressed in rice callus by the Agrobacterium tumefaciens vector. The production of protein in transgenic rice seeds was confirmed and quantified by Western-blot and ELISA, the results demonstrating that the antigen was expressed stably. The transgenic rice seeds expressing the MOMP protein were administered by the oral route to BALB/c mice, which developed MOMP-specific serum IgG and fecal IgA antibodies and a splenocyte MOMP-specific proliferative response and significant levels of IFN-γ, IL-2, IL-4, IL-5 and TGF-β production. Immunization with MOMP transgenic seeds induced partial protection (50%) against a lethal challenge with the highly virulent Cp. psittaci 6BC strain. Lung function after challenge was less affected compared non-MOMP immunized animals. The results demonstrate the feasibility of using transgenic rice seeds as an oral vaccine to generate protective immunity and reduce the lung lesions in mice against virulent Cp. psittaci 6BC strain. This finding has implications for further development of an oral vaccine against avian chlamydiosis.

Safety and immunogenicity of bacterial and tobacco plant cell line derived recombinant native and mutant Escherichia coli heat-labile toxin in chickens

The safety and immunogenicity of the mammalian mucosal adjuvants, Escherichia coli wild-type heat-labile holotoxin (LT) and E. coli mutant LT (LTA-K63/LTB), were examined in 1-day-old chicks and 10-day-old to 21-day-old broilers. Biologically active, E. coli recombinant wild-type LT and recombinant LTA-K63/LTB produced in a transgenic Nicotiana tabacum (NT-1) tobacco cell line (SLT102) were tested for safety and antigenicity following various routes of administration. Safety was assessed by clinical signs, body weight gain, gross organ pathology and wet organ weight, and histopathology. Antigenicity was assessed using LT-B-specific serum IgG enzyme-linked immunosorbent assay. Parenteral administration of E. coli recombinant wild-type LT did not have any discernible effect on bird health and was well tolerated at levels up to 400 µg per dose. Recombinant, SLT102-derived mutant LT derived from SLT102 cells retained in vitro ganglioside binding and was safe and antigenic following repeated mucosal administration to birds. The highest systemic LT-B-specific IgG titres were detected in birds that received three on-feed doses of SLT102-derived mutant LT. Among the various SLT102-derived mutant LT preparations tested, whole, wet cells or whole cell lysates were the most antigenic. These results demonstrate for the first time that E. coli-derived recombinant, wild-type LT holotoxin is well tolerated following multiple administrations to young birds at body weight doses previously reported to be enteropathogenic and toxic in mammalian species. Moreover, these data also demonstrate the feasibility of using recombinant wild-type and mutant LT produced in transgenic NT-1 tobacco cells as safe and potent vaccine adjuvants in poultry.

Immunogenicity of a plant-derived edible chimeric EspA, Intimin and Tir of Escherichia coli O157:H7 in mice

Transgenic plants offer the possibility to produce and deliver an oral immunogen on a large-scale with low production costs and minimal purification or enrichment. Cattles are important reservoirs of Escherichia coli O157:H7 and developing a specific immunity in animals would be invaluable. Intimin, Tir, and EspA proteins are the virulence factors expressed by LEE locus of enterohemorrhagic E. coli. We hypothesized that the chimeric recombinant forms of these effectors delivered as an edible-base vaccine would reduce colonization of bacteria in mice. A synthetic gene (eit) composed of espA (e), eae (i) and tir (t) attached by linkers was constructed. The gene was codon optimized and cloned into plant expression vectors adjacent to CaMV35S and FAE promoters for expression in tobacco and canola plants. Of total soluble protein 0.2% and 0.3% (in average) were detected in transgenic tobacco leaves and canola seeds respectively. Mice immunized either subcutaneously or orally with recombinant EIT and challenged with E. coli O157:H7 significantly exhibited reduced bacterial shedding. Application of transgenic plants containing trivalent immunogen is an effective tool for protection against E. coli O157:H7.

Application of genetic engineering in potato breeding

Potato breeding programmes worldwide are undergoing a period of rapid change. In order to be successful, breeders must adapt and incorporate the newest up-to-date techniques as they become available. Recent advances in biotechnology make it possible to develop and cultivate more and more sophisticated transgenic crops with multiple modified traits. Gene transfer methods can be used for a wide range of fundamental studies, contributing to a better understanding of the mechanisms of plant/pathogen interactions and the metabolic pathways in plants. Transgenic potato plants are being generated worldwide to investigate the impact of transgene expression on parameters as complex as yield. Historically, potato was one of the first successfully transformed crop plants. Nowadays, transgenic potatoes have been introduced into the food chain of people and animals in several countries. Some of the genetic modifications give potato plants increased resistance to biotic and abiotic environmental factors, while others lead to improved nutritional value, or cause the plants to produce proteins of the immune system of humans or animals or substances that may be used as vaccines in humans or veterinary medicine. The trend today is towards the generation of crops with output traits, e.g. modified starch or carotenoids, or the production of pharmaceuticals in tubers, whereas the early targets were input traits, e.g. herbicide resistance, pest or virus resistance. This review provides a summary of examples illustrating the versatility and applicability of transgenic biology in potato improvement.

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.

Overview of expression of hepatitis B surface antigen in transgenic plants

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

Vaccination with attenuated Salmonella enterica Dublin expressing E coli O157:H7 outer membrane protein Intimin induces transient reduction of fecal shedding of E coli O157:H7 in cattle

Background

Escherichia coli serogroup O157:H7 has emerged as an important zoonotic bacterial pathogen, causing a range of symptoms from self-limiting bloody diarrhea to severe hemorrhagic colitis and hemolytic-uremic syndrome in humans. Beef and dairy cattle are considered the most important animal reservoirs for this pathogen. One of the important virulence characteristics of E. coli O157:H7 is the eaeA gene encoding the 97 kDa surface protein intimin. Intimin is required for attachment and effacement during the interaction of enterohemorrhagic E. coli with human and bovine neonatal enterocytes. The present study was undertaken to test the hypothesis that an adaptive mucosal immune response directed against intimin will reduce or prevent enteric colonization and fecal shedding of E. coli O157:H7 in cattle.

Results

Cattle were orally inoculated with either milk (control), milk with live attenuated Salmonella enterica serovar Dublin (vector), or milk with live attenuated recombinant S. Dublin expressing intimin (vaccinated) on days 0, 14 and 28. On day 98, all calves were challenged orally with E. coli O157:H7 to evaluate whether vaccination with the recombinant S. Dublin expressing intimin would reduce the level of E. coli O157:H7 fecal shedding.

During the first 28 days, vaccinated calves shed both the vector strain and the intimin-expressing S. Dublin strain at a similar level. The vector strain was shed for a significantly longer period as compared to the level of recombinant vaccine strain. Calves that received the intimin-expressed vaccine ceased shedding S. Dublin from day 28 to day 63. All calves were challenged with E. coli O157:H7 on day 98 to determine the effect on fecal shedding of E. coli O157:H7. The amount of E. coli O157:H7 in feces was measured for 30 days post-challenge. We observed a transient clearance of E. coli O157:H7 from the feces in the vaccinated calves. The magnitude of fecal E. coli O157:H7 shedding did not correlate with the presence of intimin-specific fecal IgA.

Conclusion

Oral vaccination with live attenuated recombinant S. Dublin expressing intimin reduced enteric colonization and fecal shedding of E. coli O157:H7. However, the transient clearance of E. coli O157:H7 was not associated with an enhanced IgA-mediated mucosal immune response.

Comparison of the immune responses induced by oral immunization of mice with Lactobacillus casei-expressing porcine parvovirus VP2 and VP2 fused to Escherichia coli heat-labile enterotoxin B subunit protein

The major structural protein VP2 of porcine parvovirus (PPV) was used as the model parvovirus antigen, which has been expressed in Lactobacillus casei fusing with Escherichia coli heat-labile enterotoxin B subunit (LTB) as mucosal adjuvant. The VP2-LTB DNA fragment was cloned into vector pPG611 or pPG612 to generated inducible surface-displayed and secretion expression systems based on xylose promoter, designated as rLc:pPG611-VP2-LTB (recombinant L. casei) and rLc:pPG612-VP2-LTB, respectively. Expression of the fusion protein was verified by SDS-PAGE, Western blot immunofluorescence and electron microscopy. It was observed that the level of IgG or sIgA from mice orally immunized with VP2-LTB was higher than that from mice received VP2 and negative control, which demonstrated significantly statistically different. Especially, the titer of IgG or sIgA in mice immunized with rLc:pPG612-VP2-LTB is the highest in this study. In summary, LTB as mucosal adjuvant was able to effectively facilitate induction of mucosal and systemic immunity by L. casei-expressing VP2 fusion protein.

Production of a heat-labile enterotoxin B subunit-porcine epidemic diarrhea virus-neutralizing epitope fusion protein in transgenic lettuce (Lactuca sativa)

Plant-based vaccines have been produced in transgenic plants including tobacco, potatoes, corn, and rice. However, these plants are not suitable for administration without cooking. To overcome this obstacle, a fusion gene encoding the synthetic enterotoxigenic Escherichia coli heat-labile enterotoxin B subunit genetically fused with a synthetic neutralizing epitope of porcine epidemic diarrhea virus (sLTB-sCOE) was introduced into lettuce cells (Lactuca sativa) by Agrobacterium-mediated transformation methods. The integration and expression of the sLTB-sCOE fusion gene was confirmed in transgenic lettuce by genomic DNA PCR amplification and Northern blot analysis, respectively. Synthesis and assembly of the LTB-COE fusion protein into oligomeric structures with pentamer size were observed in transgenic plant extracts by Western blot analysis with anti-LTB or anti-COE antibodies. The binding of plantproduced LTB-COE to intestinal epithelial cell membrane glycolipid receptors was confirmed by G_M1-ganglioside enzyme-linked immunosorbent assay (G_M1-ELISA). Based on the ELISA results, LTB-COE fusion protein made up about 0.026∼0.048% of the total soluble protein in the transgenic lettuce leaf tissues. The synthesis and assembly of LTB-COE monomers into biologically active oligomers in transgenic lettuce leaf tissues demonstrates the feasibility of using uncooked edible plant-based vaccines for mucosal immunization.

In silico analysis of chimeric espA, eae and tir fragments of Escherichia coli O157:H7 for oral immunogenic applications

Background

In silico techniques are highly suited for both the discovery of new and development of existing vaccines. Enterohemorrhagic Escherichia coli O157:H7 (EHEC) exhibits a pattern of localized adherence to host cells, with the formation of microcolonies, and induces a specific histopathological lesion (attaching/effacing). The genes encoding the products responsible for this phenotype are clustered on a 35-kb pathogenicity island. Among these proteins, Intimin, Tir, and EspA, which are expressed by attaching-effacing genes, are responsible for the attachment to epithelial cell that leads to lesions.

Results

We designed synthetic genes encoding the carboxy-terminal fragment of Intimin, the middle region of Tir and the carboxy-terminal part of EspA. These multi genes were synthesized with codon optimization for a plant host and were fused together by the application of four repeats of five hydrophobic amino acids as linkers. The structure of the synthetic construct gene, its mRNA and deduced protein and their stabilities were analyzed by bioinformatic software. Furthermore, the immunogenicity of this multimeric recombinant protein consisting of three different domains was predicted.

Conclusion

a structural model for a chimeric gene from LEE antigenic determinants of EHEC is presented. It may define accessibility, solubility and immunogenecity.

Plants as bioreactors: Recent developments and emerging opportunities

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

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.

The influence of ligand valency on aggregation mechanisms for inhibiting bacterial toxins

Divalent and tetravalent analogues of ganglioside GM1 are potent inhibitors of cholera toxin and Escherichia coli heat-labile toxin. However, they show little increase in inherent affinity when compared to the corresponding monovalent carbohydrate ligand. Analytical ultracentrifugation and dynamic light scattering have been used to demonstrate that the multivalent inhibitors induce protein aggregation and the formation of space-filling networks. This aggregation process appears to arise when using ligands that do not match the valency of the protein receptor. While it is generally accepted that multivalency is an effective strategy for increasing the activity of inhibitors, here we show that the valency of the inhibitor also has a dramatic effect on the kinetics of aggregation and the stability of intermediate protein complexes. Structural studies employing atomic force microscopy have revealed that a divalent inhibitor induces head-to-head dimerization of the protein toxin en route to higher aggregates.

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.

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.

Transgenic tomatoes expressing human beta-amyloid for use as a vaccine against Alzheimer's disease

Human β-amyloid (Aβ) is believed to be one of the main components of Alzheimer’s disease, so reduction of Aβ is considered a key therapeutic target. Using Agrobacterium-mediated nuclear transformation, we generated transgenic tomatoes for Aβ with tandem repeats. Integration of the human Aβ gene into the tomato genome and its transcription were detected by PCR and Northern blot, respectively. Expression of the Aβ protein was confirmed by western blot and ELISA, and then the transgenic tomato line expressing the highest protein level was selected for vaccination. Mice immunized orally with total soluble extracts from the transgenic tomato plants elicited an immune response after receiving a booster. The results indicate that tomato plants may provide a useful system for the production of human Aβ antigen.

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.

Dietary specific antibodies in spray-dried immune plasma prevent enterotoxigenic Escherichia coli F4 (ETEC) post weaning diarrhoea in piglets

In order to establish the mechanism of spray dried plasma powder (SDPP) in improving pig health and performance, a diet containing either 8% SDPP, spray dried immune plasma powder (SDIPP), or control protein (soybean and whey) ration was fed to piglets in an experimental model of enterotoxigenic Escherichia coli F4 (ETEC) post-weaning diarrhoea (PWD). SDIPP was obtained from pigs immunized with a vaccine containing ETEC fimbrial subunit F4 and heat-labile toxin (LT), and SDPP from non-immunized controls. Average daily growth (ADG) was determined, and daily samples of rectal faeces were assessed for diarrhoea (as percentage of dry matter), and ETEC excretion (in CFU/g). SDPP and SDIPP significantly (p<0.05) reduced diarrhoea, and SDIPP significantly reduced ETEC excretion. ADG was not significantly (p>0.05) affected. After the experiment, 30% of piglets tested F4 receptor positive (F4R+). A significant correlation between F4R status and morbidity was found. In F4R+ animals, SDIPP significantly improved diarrhoea and ADG, and decreased ETEC excretion, and SDPP significantly improved diarrhoea and ADG. Surprisingly, SDPP reduced diarrhoea in F4R+ animals without significant reduction of ETEC excretion, which is most likely related to the presence of anti-LT antibodies in SDPP. The results show that oral protection against ETEC by SDPP is attributable to spontaneous antibodies, in this case anti-LT antibodies. Furthermore, the results indicate that the combination of anti-LT and anti-F4 antibodies as in SDIPP is most effective in ETEC prevention. Finally, the F4R distribution in the herd should be taken into account to correctly assess efficacy.

Expression of a synthetic neutralizing epitope of porcine epidemic diarrhea virus fused with synthetic B subunit of Escherichia coli heat labile enterotoxin in rice endosperm

Epitopes often require co-delivery with adjuvant and targeting proteins to enable recognition by the immune system, and this approach may also increase the efficacy of the antigen. In this study, we assess and describe the ability of transgenic rice plants to express a fusion protein consisting of the B-subunit of the Escherichia coli heat-labile enterotoxin (LTB) and a synthetic core-neutralizing epitope (COE) of porcine epidemic diarrhea virus (PEDV), inducing an enteric disease that is seen most predominantly in piglets. Both components of the fusion proteins were detected with Western blot analysis. The fusion protein was determined to assemble into pentamers, as was evidenced by its ability to bind to GM1 gangliosides, and evidenced an average level of expression in a transgenic rice endosperm. This indicates that the expression system of the plant is capable of generating a sizable amount of antigen, possibly allowing for the successful development of an edible vaccine.

Oral immunization with different assembly forms of the HPV 16 major capsid protein L1 induces neutralizing antibodies and cytotoxic T-lymphocytes

Human papillomaviruses have been recognized as the causative agent of anogenital cancer. In 2006, a commercial vaccine based on virus-like particles composed of the L1 major capsid protein of the papillomaviruses has been available. This vaccine induces virus-neutralizing antibody responses upon parenteral injection. Here we investigated the oral immunogenicity of different assembly forms of HPV 16 L1, that is: T7-VLPs, T1 particles and capsomeres. Our results show that all three assembly forms induce humoral and cellular immune responses after oral vaccination of mice. The anti-L1 antibodies were conformation-specific and showed neutralizing activity in a pseudovirion-based assay. We also investigated if adjuvants have an influence on the oral immunogenicity of the different L1 forms. For saponins we observed a significant toxicity if applied orally. Co-administration of either CpG DNA or Escherichia coli heat-labile enterotoxin LT(R192G) had no apparent enhancing effect on the production of anti-L1 antibodies. More pronounced was the effect of CpG administration on the long-term immunity as we observed a significantly stronger recall response 244 days after the first vaccination. Compared to capsomeres, VLPs induced stronger humoral immune responses while the CTL responses were induced at comparable levels. Finally, we were also able to induce neutralizing antibodies and L1-specific cytotoxic T-lymphocytes after oral administration of crude extracts of L1-expressing insect cells. In conclusion, all three assembly forms of the L1 protein are immunogenic when administered orally.

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.

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.

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.

Tomato is a highly effective vehicle for expression and oral immunization with Norwalk virus capsid protein

Norwalk virus (NV) is an important agent of epidemic gastroenteritis, and an oral subunit vaccine shows potential for protection. Recombinant Norwalk virus (rNV) capsid protein expressed in plants assembles virus-like particles (VLPs) that are orally immunogenic in mice and humans. In this article we examine rNV expression in tomato and potato using a plant-optimized gene, and test the immunogenicity of dried tomato fruit and potato tuber fed to mice. The synthetic gene increased rNV expression fourfold in tomato and potato plants, which assembled VLP. Four doses of 0.4 g freeze-dried tomato fruit containing 64 microg rNV (40 microg VLPs) induced NV-specific serum IgG and mucosal IgA in > or = 80% of mice, while doses of 0.8 g elicited systemic and mucosal antibody responses in all mice. Feedings of 1 g freeze-dried potato tuber containing 120 microg rNV (90 microg VLPs) were required to produce 100% responsiveness. Oxidation of phenolic compounds upon rehydration of dried tuber caused significant VLP instability, thus decreasing immunogenicity. Air-dried tomato fruit stimulated stronger immune responses than freeze-dried fruit of the same mass, perhaps by limiting the destruction of plant cell matrix and membrane systems that occurs with freeze-drying. Thus, rNV in dried transgenic tomato fruit was a more potent immunogen than that in dried potato tubers, based on the total VLPs ingested. These findings support the use of stabilized, dried tomato fruit for oral delivery of subunit vaccines.

A plant-based oral vaccine to protect against systemic intoxication by Shiga toxin type 2

Hemolytic uremic syndrome, the leading cause of kidney failure in children, often follows infection with enterohemorrhagic Escherichia coli and is mediated by the Shiga type toxins, particularly type 2 (Stx2), produced by such strains. The challenge in protecting against this life-threatening syndrome is to stimulate an immune response at the site of infection while also protecting against Shiga intoxication at distal sites such as the kidney. As one approach to meeting this challenge, we sought to develop and characterize a prototypic orally delivered, plant-based vaccine against Stx2, an AB5 toxin. First, we genetically inactivated the Stx2 active A subunit gene and then optimized both subunit genes for expression in plants. The toxoid genes were then transformed into the Nicotiana tabacum (tobacco) cell line NT-1 by Agrobacterium tumefaciens-mediated transformation. Toxoid expression was detected in NT-1 cell extracts, and the assembly of the holotoxoid was confirmed. Finally, mice were immunized by feeding with the toxoid-expressing NT-1 cells or by parenteral immunization followed by oral vaccination (prime-boost strategy). The immunized mice produced Stx2-specific mucosal IgA and Stx2-neutralizing serum IgG. The protective efficacy of these responses was assessed by challenging the immunized mice with E. coli O91:H21 strain B2F1, an isolate that produces an activatable variant of Stx2 (Stx2d) and is lethal to mice. The oral immunization fully protected mice from the challenge. Results of this study demonstrated that a plant-based oral vaccine can confer protection against lethal systemic intoxication.

Expression of tuberculosis antigen ESAT-6 in Nicotiana tabacum using a potato virus X-based vector

A good candidate antigen to create a therapeutic vaccine against TB is the ESAT-6 protein. Antigens produced in plants have already been successfully used as experimental vaccines, and small single-stranded RNA plant viruses have emerged as promising tools to rapidly express large amounts of foreign proteins in susceptible host plants. Here, we present the expression of ESAT-6 protein in Nicotiana tabacum using a vector based on potato virus X (PVX). The complete ESAT-6 open reading frame is expressed as a fusion protein with the 2A peptide of Foot and Mouth Disease Virus and the amino terminal of the PVX coat protein (CP) (PVXESAT-6). This strategy allows the production of free CP and ESAT-6 as well as fused ESAT-2A-CP to obtain recombinant chimaeric virions expressing ESAT-6 at the surface to be used as particulate antigen in vaccination. ESAT-6 expression was tested in agroinfiltrated tobacco leaves and products of the expected molecular masses corresponding to cleaved CP and ESAT-2A-CP fusion protein were observed, with ESAT-6 yields ranging from 0.5% to 1% of total soluble protein. Our study describes for the first time the expression of the ESAT-6 protein in tobacco plants using a PVX-derived vector. This strategy should serve as a convenient, rapid, low-cost expression system and can also be used for the assessment of ESAT-6 production and function prior to stable plant transformation.

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 synthetic neutralizing epitope of porcine epidemic diarrhea virus fused with synthetic B subunit of Escherichia coli heat-labile enterotoxin in tobacco plants

The pentameric B subunit of Escherichia coli heat-labile enterotoxin (LTB) can be used as an efficient mucosal carrier of either immunogenic or tolerogenic T-cell epitopes. Co-delivery of therapeutic proteins with carrier proteins could increase the effectiveness of the antigen. This paper reports the ability of transgenic tobacco plants to express a fusion protein consisting of the synthetic LTB and a synthetic neutralizing epitope of porcine epidemic diarrhea virus (PEDV), causing an enteric disease that is especially severe in piglets. Both components of the fusion proteins were detected in Western blot analysis, and binding assay confirmed that plant-synthesized pentameric LTB-PEDV fusion bound to the intestinal membrane GM1-ganglioside receptor. This suggested that the fusion protein retained both its native antigenicity and the ability to form pentamers.

Induction of neutralizing antibodies by a tobacco chloroplast-derived vaccine based on a B cell epitope from canine parvovirus

The 2L21 epitope of the VP2 protein from the canine parvovirus (CPV), fused to the cholera toxin B subunit (CTB-2L21), was expressed in transgenic tobacco chloroplasts. Mice and rabbits that received protein-enriched leaf extracts by parenteral route produced high titers of anti-2L21 antibodies able to recognize the VP2 protein. Rabbit sera were able to neutralize CPV in an in vitro infection assay with an efficacy similar to the anti-2L21 neutralizing monoclonal antibody 3C9. Anti-2L21 IgG and seric IgA antibodies were elicited when mice were gavaged with a suspension of pulverized tissues from CTB-2L21 transformed plants. Combined immunization (a single parenteral injection followed by oral boosters) shows that oral boosters help to maintain the anti-2L21 IgG response induced after a single injection, whereas parenteral administration of the antigen primes the subsequent oral boosters by promoting the induction of anti-2L21 seric IgA antibodies. Despite the induced humoral response, antibodies elicited by oral delivery did not show neutralizing capacity in the in vitro assay. The high yield of the fusion protein permits the preparation of a high number of vaccine doses from a single plant and makes feasible the oral vaccination using a small amount of crude plant material. However, a big effort has still to be done to enhance the protective efficacy of subunit vaccines by the oral route.

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.

Global transcript profiling of potato tuber using LongSAGE

Solanum tuberosum (potato) is the fourth major crop worldwide and is used for food, feed and biotechnological applications. To fully realize the biosynthetic potential for the production of starch, protein and metabolites, we conducted an extensive quantitative profiling of the expressed genes of mature potato tuber. A total of 58,322 serial analysis of gene expression (SAGE) tags of 19 nucleotides (nt), representing 22,233 different tags, were analysed. The 695 tags seen 10 or more times were assigned a tentative function by comparison with homologous genes. The identities of 12 'known' and 12 'unknown' transcripts were confirmed by rapid amplification of cDNA ends (RACE) cloning using the 19 nt tag as a primer. The SAGE and expressed sequence tag (EST) profiles of potato tuber were compared. Transcripts for four types of protease inhibitor, a metallothionein and a lipoxygenase were more prominent than patatin isoforms.

Expression of immunogenic VP2 protein of infectious bursal disease virus in Arabidopsis thaliana

VP2 protein is the major host-protective immunogen of infectious bursal disease virus (IBDV) of chickens. Transgenic lines of Arabidopsisthaliana expressing recombinant VP2 were developed. The VP2 gene of an IBDV antigenic variant E strain was isolated, amplified by RT-PCR and introduced into a plant expression vector, pE1857, having a strong promoter for plant expression. A resulting construct with a Bar gene cassette for bialaphos selection in plant (rpE-VP2) was introduced into Agrobacterium tumefaciensby electroporation. Agrobacterium containing the rpE-VP2 construct was used to transform Ar. thaliana and transgenic plants were selected using bialaphos. The presence of VP2 transgene in plants was confirmed by PCR and Southern blot analysis and its expression was confirmed by RT-PCR. Western blot analysis and antigen-capture ELISA assay using monoclonal anti-VP2 were used to determine the expression of VP2 protein in transgenic plants. The level of VP2 protein in the leaf extracts of selected transgenic plants varied from 0.5% to 4.8% of the total soluble protein. Recombinant VP2 protein produced in plants induced antibody response against IBDV in orally-fed chickens.