A review of oral vaccination with transgenic vegetables

Edible Vaccines

Vaccines are considered primary tools for health intervention in both humans and animals. Vaccines can be used more widely, especially in developing countries, if their cost of production can be reduced and they can be preserved without refrigeration. In developing countries certain limitations, like vaccine affordability, the need for “cold chains” from the producer to the site of use of the vaccine, and the dependence on injection, are barriers to health care services. Plant-derived vaccines do not face such limitations. Research under way is dedicated to solving these limitations by finding ways to produce oral (edible) vaccines from transgenic plants. Plant-derived vaccines offer increased safety, envisage low-cost programs for mass vaccination, and propose a wider use of vaccination for veterinary use.

Plant-based vaccines: novel and low-cost possible route for Mediterranean innovative vaccination strategies

A plant bioreactor has enormous capability as a system that supports many biological activities, that is, production of plant bodies, virus-like particles (VLPs), and vaccines. Foreign gene expression is an efficient mechanism for getting protein vaccines against different human viral and nonviral diseases. Plants make it easy to deal with safe, inexpensive, and provide trouble-free storage. The broad spectrum of safe gene promoters is being used to avoid risk assessments. Engineered virus-based vectors have no side effect. The process can be manipulated as follows: (a) retrieve and select gene encoding, use an antigenic protein from GenBank and/or from a viral-genome sequence, (b) design and construct hybrid-virus vectors (viral vector with a gene of interest) eventually flanked by plant-specific genetic regulatory elements for constitutive expression for obtaining chimeric virus, (c) gene transformation and/or transfection, for transient expression, into a plant-host model, that is, tobacco, to get protocols processed positively, and then moving into edible host plants, (d) confirmation of protein expression by bioassay, PCR-associated tests (RT-PCR), Northern and Western blotting analysis, and serological assay (ELISA), (e) expression for adjuvant recombinant protein seeking better antigenicity, (f) extraction and purification of expressed protein for identification and dosing, (g) antigenicity capability evaluated using parental or oral delivery in animal models (mice and/or rabbit immunization), and (h) growing of construct-treated edible crops in protective green houses. Some successful cases of heterologous gene-expressed protein, as edible vaccine, are being discussed, that is, hepatitis C virus (HCV). R9 mimotope, also named hypervariable region 1 (HVR1), was derived from the HVR1 of HCV. It was used as a potential neutralizing epitope of HCV. The mimotope was expressed using cucumber mosaic virus coat protein (CP), alfalfa mosaic virus CP P3/RNA3, and tobacco mosaic virus (TMV) CP-tobacco mild green mosaic virus (TMGMV) CP as expression vectors into tobacco plants. Expressed recombinant protein has not only been confirmed as a therapeutic but also as a diagnostic tool. Herpes simplex virus 2 (HSV-2), HSV-2 gD, and HSV-2 VP16 subunits were transfected into tobacco plants, using TMV CP-TMGMV CP expression vectors.

Edible vaccines against veterinary parasitic diseases--current status and future prospects

Protection of domestic animals against parasitic infections remains a major challenge in most of the developing countries, especially in the surge of drug resistant strains. In this circumstance vaccination seems to be the sole practical strategy to combat parasites. Most of the presently available live or killed parasitic vaccines possess many disadvantages. Thus, expression of parasitic antigens has seen a continued interest over the past few decades. However, only a limited success was achieved using bacterial, yeast, insect and mammalian expression systems. This is witnessed by an increasing number of reports on transgenic plant expression of previously reported and new antigens. Oral delivery of plant-made vaccines is particularly attractive due to their exceptional advantages. Moreover, the regulatory burden for veterinary vaccines is less compared to human vaccines. This led to an incredible investment in the field of transgenic plant vaccines for veterinary purpose. Plant based vaccine trials have been conducted to combat various significant parasitic diseases such as fasciolosis, schistosomosis, poultry coccidiosis, porcine cycticercosis and ascariosis. Besides, passive immunization by oral delivery of antibodies expressed in transgenic plants against poultry coccidiosis is an innovative strategy. These trials may pave way to the development of promising edible veterinary vaccines in the near future. As the existing data regarding edible parasitic vaccines are scattered, an attempt has been made to assemble the available literature.

Rotavirus infections: mechanisms, prevention and treatment

Rotavirus is an important gastrointestinal pathogen which can cause severe diarrhea in young animals and infants. Although rotavirus infections are usually latent, large-scale outbreak may occur if environmental factors change or the immune defense of hosts decreases. As rotavirus is a pathogen of zoonotic diseases, studies on rotavirus infections are of great importance in public health and clinical application. This article provides an overview of the progress in research on viral pathogen, clinical manifestations, as well as drug and vaccine research, with an aim to provide a reference for the prevention and treatment of rotavirus infections.

Transgenic peanut (Arachis hypogaea L.) expressing the urease subunit B gene of Helicobacter pylori

Helicobacter pylori (H. pylori) has been identified as the main pathogenic factors of chronic gastritis and peptic ulcer, and the Class I carcinogen of gastric cancer by WHO. Vaccine has become the most effective measure to prevent and cure H. pylori infection. The UreB is the most effective and common immunogen of all strains of H. pylori and may stimulate the immunoresponse protecting the human body against the challenge of H. pylori. UreB antigen gene was cloned into the binary vector pBI121 which contains a seed-specific promoter Oleosin of peanut and a kanamycin resistance gene, and then UreB gene was transformed into peanut embryo leaflets by Agrobacter-mediated method. The putative transgenic plants were examined for the presence of UreB in the nuclear genome of peanut plants by PCR analysis. Expression of UreB gene in plants was identified by RT-PCR and Western blot analysis. These results suggest that the UreB transgenic peanut can be potentially used as an edible vaccine for controlling H. pylori.

Comparison of serum humoral responses induced by oral immunization with the hepatitis B virus core antigen and the cholera toxin B subunit

The hepatitis B virus core (HBc) virus-like particle (VLP) is known as one of the most immunogenic antigens and carrier vehicles in different immunization strategies. Recent findings are suggesting the potential of the HBc VLPs as an oral immunogen. Here, we focus on the induction of serum humoral responses by oral administration of HBc VLPs in preparations substantially free of lipopolysaccharide and immunomodulating encapsidated RNA. The full-length HBc antigen was used, because the C-terminal arginine-rich tail may contribute to the immunogenicity of the antigen as the region is involved in cell surface heparan sulfate binding and internalization of the protein. Serum antibody levels and isotypes were determined following oral administration of the HBc VLPs with the perspective of using the HBc VLP as an immunostimulatory and carrier molecule for epitopes of blood-borne diseases in oral immunization vaccination strategies. Following oral administration of the HBc VLP preparations to mice, a strong serum humoral response was induced with mainly immunoglobulin G2a (IgG2a) antibodies, pointing toward a Th1 response which is essential in the control of intracellular pathogens. Intraperitoneal immunization with the HBc VLP induced a stronger, mixed Th1/Th2 response. Finally, a comparison was made with the induced serum humoral response following oral administration of the recombinant cholera toxin B pentamer, a commonly used oral immunogen. These immunizations, in contrast, induced predominantly antibodies of the IgG1 isotype, indicative of a Th2 response. These data suggest that the HBc VLP can be an interesting carrier molecule in oral vaccine development.

Oral vaccination: where we are?

As early as 900 years ago, the Bedouins of the Negev desert were reported to kill a rabid dog, roast its liver and feed it to a dog-bitten person for three to five days according to the size and number of bites [1] . In sixteenth century China, physicians routinely prescribed pills made from the fleas collected from sick cows, which purportedly prevented smallpox. One may dismiss the wisdom of the Bedouins or Chinese but the Nobel laureate, Charles Richet, demonstrated in 1900 that feeding raw meat can cure tuberculous dogs - an approach he termed zomotherapy. Despite historical clues indicating the feasibility of oral vaccination, this particular field is notoriously infamous for the abundance of dead-end leads. Today, most commercial vaccines are delivered by injection, which has the principal limitation that recipients do not like needles. In the last few years, there has been a sharp increase in interest in needle-free vaccine delivery; new data emerges almost daily in the literature. So far, there are very few licensed oral vaccines, but many more vaccine candidates are in development. Vaccines delivered orally have the potential to take immunization to a fundamentally new level. In this review, the authors summarize the recent progress in the area of oral vaccines.

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.

Pharming and transgenic plants

Plant represented the essence of pharmacopoeia until the beginning of the 19th century when plant-derived pharmaceuticals were partly supplanted by drugs produced by the industrial methods of chemical synthesis. In the last decades, genetic engineering has offered an alternative to chemical synthesis, using bacteria, yeasts and animal cells as factories for the production of therapeutic proteins. More recently, molecular farming has rapidly pushed towards plants among the major players in recombinant protein production systems. Indeed, therapeutic protein production is safe and extremely cost-effective in plants. Unlike microbial fermentation, plants are capable of carrying out post-translational modifications and, unlike production systems based on mammalian cell cultures, plants are devoid of human infective viruses and prions. Furthermore, a large panel of strategies and new plant expression systems are currently developed to improve the plant-made pharmaceutical's yields and quality. Recent advances in the control of post-translational maturations in transgenic plants will allow them, in the near future, to perform human-like maturations on recombinant proteins and, hence, make plant expression systems suitable alternatives to animal cell factories.

A potyvirus-based gene vector allows producing active human S-COMT and animal GFP, but not human sorcin, in vector-infected plants

Potato virus A (PVA), a potyvirus with a (+)ssRNA genome translated to a large polyprotein, was engineered and used as a gene vector for expression of heterologous proteins in plants. Foreign genes including jellyfish GFP (Aequorea victoria) encoding the green fluorescent protein (GFP, 27 kDa) and the genes of human origin (Homo sapiens) encoding a soluble resistance-related calcium-binding protein (sorcin, 22 kDa) and the catechol-O-methyltransferase (S-COMT; 25 kDa) were cloned between the cistrons for the viral replicase and coat protein (CP). The inserts caused no adverse effects on viral infectivity and virulence, and the inserted sequences remained intact in progeny viruses in the systemically infected leaves. The heterologous proteins were released from the viral polyprotein following cleavage by the main viral proteinase, NIa, at engineered proteolytic processing sites flanking the insert. Active GFP, as indicated by green fluorescence, and S-COMT with high levels of enzymatic activity were produced. In contrast, no sorcin was detected despite the expected equimolar amounts of the foreign and viral proteins being expressed as a polyprotein. These data reveal inherent differences between heterologous proteins in their suitability for production in plants.

Research Advances on Transgenic Plant Vaccines

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

F4 (K88) fimbrial adhesin FaeG expressed in alfalfa reduces F4+ enterotoxigenic Escherichia coli excretion in weaned piglets

Transgenic plants are attractive bioreactors to large-scale production of recombinant proteins because of their relatively low cost. This study reports for the first time the use of transgenic plants to reduce enterotoxigenic Escherichia coli (ETEC) excretion in its natural host species. The DNA sequence encoding the major subunit and adhesin FaeG of F4+ ETEC was transformed into edible alfalfa plants. Targeting of FaeG production to chloroplasts led to FaeG levels of up to 1% of the total soluble protein fraction of the transgenic alfalfa. Recombinant plant-produced FaeG (pFaeG) remained stable for 2 years when the plant material was dried and stored at room temperature. Intragastric immunization of piglets with pFaeG induced a weak F4-specific humoral response. Co-administration of pFaeG and the mucosal adjuvant cholera toxin (CT) enhanced the immune response against FaeG, reflected a better induction of an F4-specific immune response. In addition, the intragastric co-administration of CT with pFaeG significantly reduced F4+ E. coli excretion following F4+ ETEC challenge as compared with pigs that had received nontransgenic plant material. In conclusion, transgenic plants producing the FaeG subunit protein could be used for production and delivery of oral vaccines against F4+ ETEC infections.

Oral immunization with pBsVP6-transgenic alfalfa protects mice against rotavirus infection

A critical factor in edible plant-derived vaccine development is adequate expression of the exogenous antigens in transgenic plants. We synthesized a codon-optimized gene (sVP6) encoding the VP6 protein of human group A rotavirus and inserted it into the alfalfa genome using agrobacterium-mediated transformation. As much as 0.28% of the total soluble protein of the pBsVP6-transgenic alfalfa was sVP6. Female BALB/c mice were gavaged weekly with 10 mg of transgenic alfalfa extract containing 24 microg of sVP6 protein and 10 microg of CpG-rich oligodeoxynucleotides as mucosal adjuvant. Immunized mice developed high titers of anti-VP6 serum IgG and mucosal IgA. Offspring of immunized dams developed less severe diarrhea after challenge with simian rotavirus SA-11, indicating that antibodies generated in the dams provided passive heterotypic protection to the pups. These results suggest that oral immunization with pBsVP6-transgenic alfalfa provides a potential means of protecting children and young animals from severe acute rotavirus-induced diarrhea.

Biopharmaceuticals derived from genetically modified plants

Modern biotechnology has resulted in a resurgence of interest in the production of new therapeutic agents using botanical sources. With nearly 500 biotechnology products approved or in development globally, and with production capacity limited, the need for efficient means of therapeutic protein production is apparent. Through genetic engineering, plants can now be used to produce pharmacologically active proteins, including mammalian antibodies, blood product substitutes, vaccines, hormones, cytokines, and a variety of other therapeutic agents. Efficient biopharmaceutical production in plants involves the proper selection of host plant and gene expression system, including a decision as to whether a food crop or a non-food crop is more appropriate. Product safety issues relevant to patients, pharmaceutical workers, and the general public must be addressed, and proper regulation and regulatory oversight must be in place prior to commercial plant-based biopharmaceutical production. Plant production of pharmaceuticals holds great potential, and may become an important production system for a variety of new biopharmaceutical products.

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

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

Vaccination against cestode parasites: anti-helminth vaccines that work and why

Highly effective recombinant vaccines have been developed against the helminth parasites Taenia ovis, Taenia saginata and Echinococcus granulosus. These vaccines indicate that it is possible to achieve a reliable, high level of protection against a complex metazoan parasite using defined recombinant antigens. However, the effectiveness of the vaccines against the taeniid cestodes stands in contrast to the more limited successes which characterise attempts to develop vaccines against other platyhelminth or nematode parasites. This review examines the features of the host-parasite relationships among the taeniid cestodes which have formed the basis for vaccine development. Particular consideration is given to the methodologies that have been used in making the cestode vaccines that might be of interest to researchers working on vaccination against other helminths. In developing the cestode vaccines, antigens from the parasites' infective larval stage contained within the egg (oncosphere) were identified as having the potential to induce high levels of protection in vaccinated hosts. A series of vaccination trials with antigen fractions, and associated immunological analyses, identified individual protective antigens or fractions. These were cloned from cDNA and the recombinant proteins expressed in Escherichia coli. This strategy was independently successful in developing vaccines against T. ovis and E. granulosus. Identification of protective antigens for these species enabled rapid identification, cloning and expression of their homologues in related species and thereby the development of effective vaccines against T. saginata, E. multilocularis and, more recently, T. solium. The T. saginata vaccine provides an excellent example of the use of two antigen components, each of which were not protective when used individually, but when combined they induce a reliable, high level of protection. One important contributing factor to the success of vaccine development for the taeniid cestodes was the concentration on studies seeking to identify native host-protective antigens, before the adoption of recombinant methodologies. The cestode vaccines are being developed towards practical (commercial) application. The high level of efficacy of the vaccines against T. solium cysticercosis and hydatid disease suggests that they would be effective also if used directly in humans.

Efficacy of oral administration and oral intake of edible vaccines

To evaluate whether vaccine administration via intragastric gavage is indicative for the outcome of edible vaccines, mice were orally immunised with ovalbumin (OVA) mixed with or without Vibrio cholerae toxin (CT) in various compositions via various routes: (1) OVA dissolved in saline and intragastrically (IG) administered ('IG'); (2) OVA mixed with food extract and administered IG ('food IG'); (3) food chow absorbed with OVA dissolved in saline and fed to the animals ('food'); and (4) OVA dissolved in saline and administered via drinking bottles ('drinking'). When given to naive mice, 'IG' and 'food IG' but not 'food' or 'drinking' induced anti-OVA IgG1 responses in serum, but oral boost immunisations were necessary. Serum IgA was not induced. Oral boosting of subcutaneously (SC) primed mice enhanced the IgG1 and IgA response in serum regardless of the route of immunisation or the vaccine composition. CT did not dramatically enhance the immune response. All immunisation routes except 'drinking' induced antigen-specific IgA antibody secreting cells (ASC) in the lamina propria of naive mice. But antigen-specific antibody responses in faeces were not observed. We concluded that oral (i.e. IG) administration is distinct from oral intake. The composition of the vaccine (food or saline) did not influence oral administration. We thus suggested that the route of administration greatly influenced the outcome of oral immunisation. Although oral administration is a well-accepted route to test the potentials of oral vaccines, our study demonstrated that it is merely indicative for the effectiveness of edible vaccines. Studies on the feasibility of edible vaccines should thus be performed by eating the vaccine.

Functional homologs of cyanovirin-N amenable to mass production in prokaryotic and eukaryotic hosts

Cyanovirin-N (CV-N) is under development as a topical (vaginal or rectal) microbicide to prevent sexual transmission of human immunodeficiency virus (HIV), and an economically feasible means for very large-scale production of the protein is an urgent priority. We observed that N-glycosylation of CV-N in yeast eliminated the anti-HIV activity, and that dimeric forms and aggregates of CV-N occurred under certain conditions, potentially complicating the efficient, large-scale manufacture of pure monomeric CV-N. We therefore expressed and tested CV-N homologs in which the glycosylation-susceptible Asn residue at position 30 was replaced with Ala, Gln, or Val, and/or the Pro at position 51 was replaced by Gly to eliminate potential conformational heterogeneity. All homologs exhibited anti-HIV activity comparable to wild-type CV-N, and the Pro51Gly homologs were significantly more stable proteins. These glycosylation-resistant, functional cyanovirins should be amenable to large-scale production either in bacteria or in eukaryotic hosts.

Edible plant vaccines: applications for prophylactic and therapeutic molecular medicine

The use of edible plants for the production and delivery of vaccine proteins could provide an economical alternative to fermentation systems. Genes encoding bacterial and viral antigens are faithfully expressed in edible tissues to form immunogenic proteins. Studies in animals and humans have shown that ingestion of transgenic plants containing vaccine proteins causes production of antigen-specific antibodies in serum and mucosal secretions. In general, the technology is limited by low expression levels for nuclear-integrated transgenes, but recent progress in plant organelle transformation shows promise for enhanced expression. The stability and immunogenicity of orally delivered antigens vary greatly, which necessitates further study on protein engineering to enhance mucosal delivery. These issues are discussed with regard to the further development of plant-based vaccine technology.

Expression of biologically active human tumor necrosis factor-alpha in transgenic potato plant

We report the successful insertion of the cDNA of human tumor necrosis factor-alpha (HuTNF-alpha) into the genome of potato plant species, Solanum tuberosum, using Agrobacterium tumefacience-mediated transformation. HuTNF-alpha is a known and essential cytokine mediating host defense against tumors and infectious diseases and an immunomodulating agent. To enhance the accumulation of foreign gene product expression in plant cells, the molecular design of the constructed HuTNF-alpha is presented. Transcription and translation of TNF-alpha in transformants were confirmed by Northern blot, RT-PCR, ELISA, and Western blot, respectively. Expression of the bioactive HuTNF-alpha in plant cells was confirmed by way of the cytotoxic effect of the extract obtained from the transformants against murine L929 cells. We think that the expression level of HuTNF-alpha (15 microg/g potato plant tissue) obtained in the present study may be sufficient to induce responses/effects similar to those generated by TNF-alpha in human milk administered orally. We believe that the TNF-alpha expressed in edible potato plants has tremendous potential for clinical use in the areas of medicine and veterinary science. Its usefulness and applicability, therefore, need to be fully explored.

Orally administered Giardia duodenalis extracts enhance an antigen-specific antibody response

We have identified novel adjuvant activity in specific cytosol fractions from trophozoites of Giardia isolate BRIS/95/HEPU/2041 (J. A. Upcroft, P. A. McDonnell, and P. Upcroft, Parasitol. Today, 14:281-284, 1998). Adjuvant activity was demonstrated in the systemic and mucosal compartments when Giardia extract was coadministered orally with antigen to mice. Enhanced antigen-specific serum antibody responses were demonstrated by enzyme-linked immunosorbent assay to be comparable to those generated by the "gold standard," mucosal adjuvant cholera toxin. A source of adjuvant activity was localized to the cytosolic component of the parasite. Fractionation of the cytosol produced fraction pools, some of which, when coadministered with antigen, stimulated an enhanced antigen-specific serum response. The toxic component of conventional mucosal adjuvants is associated with adjuvant activity; therefore, in a similar way, the toxin-like attributes of BRIS/95/HEPU/2041 may be responsible for its adjuvanticity. Complete characterization of the adjuvant is under way.

Edible genetically modified microorganisms and plants for improved health

The development of new strategies for the delivery of vaccine antigens or immune modulators to the mucosal tissue includes innovative approaches such as the use of genetically modified food microorganisms and plants. Even though the 'proof-of-concept' has recently been established for these two systems, key questions mainly related to efficacy and risk of breaking oral tolerance remain to be critically addressed in the immediate future.

Oral immunization with hepatitis B surface antigen expressed in transgenic plants

Oral immunogenicity of recombinant hepatitis B surface antigen (HBsAg) derived from yeast (purified product) or in transgenic potatoes (uncooked unprocessed sample) was compared. An oral adjuvant, cholera toxin, was used to increase immune responses. Transgenic plant material containing HBsAg was the superior means of both inducing a primary immune response and priming the mice to respond to a subsequent parenteral injection of HBsAg. Electron microscopy of transgenic plant samples revealed evidence that the HBsAg accumulated intracellularly; we conclude that natural bioencapsulation of the antigen may provide protection from degradation in the digestive tract until plant cell degradation occurs near an immune effector site in the gut. The correlate of protection from hepatitis B virus infection is serum antibody titers induced by vaccination; the protective level in humans is 10 milliunits/ml or greater. Mice fed HBsAg-transgenic potatoes produced HBsAg-specific serum antibodies that exceeded the protective level and, on parenteral boosting, generated a strong long-lasting secondary antibody response. We have also shown the effectiveness of oral delivery by using a parenteral prime-oral boost immunization schedule. The demonstrated success of oral immunization for hepatitis B virus with an "edible vaccine" provides a strategy for contributing a means to achieve global immunization for hepatitis B prevention and eradication.

Towards development of an edible vaccine against bovine pneumonic pasteurellosis using transgenic white clover expressing a Mannheimia haemolytica A1 leukotoxin 50 fusion protein

Development of vaccines against bovine pneumonia pasteurellosis, or shipping fever, has focused mainly on Mannheimia haemolytica A1 leukotoxin (Lkt). In this study, the feasibility of expressing Lkt in a forage plant for use as an edible vaccine was investigated. Derivatives of the M. haemolytica Lkt in which the hydrophobic transmembrane domains were removed were made. Lkt66 retained its immunogenicity and was capable of eliciting an antibody response in rabbits that recognized and neutralized authentic Lkt. Genes encoding a shorter Lkt derivative, Lkt50, fused to a modified green fluorescent protein (mGFP5), were constructed for plant transformation. Constructs were screened by Western immunoblot analysis for their ability to express the fusion protein after agroinfiltration in tobacco. The fusion construct pBlkt50-mgfp5, which employs the cauliflower mosaic virus 35S promoter for transcription, was selected and introduced into white clover by Agrobacterium tumefaciens-mediated transformation. Transgenic lines of white clover were recovered, and expression of Lkt50-GFP was monitored and confirmed by laser confocal microscopy and Western immunoblot analysis. Lkt50-GFP was found to be stable in clover tissue after drying of the plant material at room temperature for 4 days. An extract containing Lkt50-GFP from white clover was able to induce an immune response in rabbits (via injection), and rabbit antisera recognized and neutralized authentic Lkt. This is the first demonstration of the expression of an M. haemolytica antigen in plants and paves the way for the development of transgenic plants expressing M. haemolytica antigens as an edible vaccine against bovine pneumonic pasteurellosis.

Expression of two subtypes of human IFN-alpha in transgenic potato plants

Plant expression systems have advantages over other in vitro expression systems in terms of low production costs and low risk of contamination by animal viruses or bacterial endotoxins. In this study, cDNA encoding two subtypes of human interferon-alpha2b and 8 (HuIFN-alpha2b and HuIFN-alpha8) were introduced into potato plants (Solanum tuberosum) using Agrobacterium-mediated transformation. Transcription and translation of the inserted HuIFN-alpha cDNA were confirmed by Northern blot analysis and ELISA, respectively. Bioactivity of the products was assayed by inhibition of vesicular stomatitis virus (VSV) replication on a human amniotic cell line. However, because of the presence of substances in potato tissue extracts that were toxic to animal cells, successful demonstration of IFN bioactivity in the transformants was achieved only after removal of such substances by dialysis. The maximum level of IFN activity in plant extracts was 560 IU/g of tissue. These results indicated that the HuIFN-alpha gene introduced into the potato plant was correctly translated and transcribed in plant cells. This report for the first time shows that biologically active animal cytokines with potential pharmaceutical applications could be expressed in transgenic potato plants.

Immunomodulators and delivery systems for vaccination by mucosal routes

Current paediatric immunization programmes include too many injections in the first months of life. Oral or nasal vaccine delivery eliminates the requirement for needles and can induce immunity at the site of infection. However, protein antigens are poorly immunogenic when so delivered and can induce tolerance. Novel ways to enhance immune responses to protein or polysaccharide antigens have opened up new possibilities for the design of effective mucosal vaccines. Here, we discuss the immunological principles underlying mucosal vaccine development and review the application of immunomodulatory molecules and delivery systems to the selective enhancement of protective immune responses at mucosal surfaces.

The regulation of biologic products derived from bioengineered plants

Recently, there has been a large increase in the number and types of biological products--from therapeutic antibodies to vaccines for the prevention of infectious diseases--that are produced in bioengineered plant systems. We anticipate that this technology will be used increasingly on a commercial scale for the manufacture of human and animal products. These production systems have the capacity to produce very large quantities of products at lower costs and with reduced risks compared with mammalian systems.

Production of antibodies and antibody fragments in plants

Our current knowledge allows the generation of transgenic plants that efficiently produce heterologous proteins from plant, bacterial, fungal or animal origin. Among all types of recombinant proteins, antibodies are particularly attractive because of their ability to specifically recognize and bind virtually any type of antigen. Plants show several advantages as a large-scale antibody production system: they can be grown easily and inexpensively in large quantities that can be harvested, stored and processed by using existing infrastructures. Isolation and purification of plant-made antibodies, if necessary, allow fundamental, industrial, and therapeutical applications. In the past, we and others have successfully generated antibody-producing plants. The maximal accumulation levels of antibodies and antibody fragments that we observed are 1-5% of the extracted proteins. Currently, several biotechnological companies grow field crops to produce antibodies for ex planta applications on an industrial scale.

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

The efficacy of edible vaccines produced in potato tubers was examined in mice. Transgenic plants were developed by Agrobacterium tumefaciens-mediated transformation. The antigen selected was the non-toxic B subunit of the Escherichia coli enterotoxin (recLT-B). A synthetic gene coding for recLT-B was made and optimised for expression in potato tubers and accumulation in the endoplasmic reticulum. Introduction of this gene under control of the tuber-specific patatin promoter in potato plants resulted in the production of functional, i.e. Gm1-binding, recLT-B pentamers in tubers. Selected tubers containing about 13 microg of recLT-B per gram fresh weight were used for immunisation. Subcutaneous immunisation with an extract of recLT-B tubers yielded high antibody titres in serum that were similar to those obtained with bacterial recLT-B. The efficacy of oral administration of recLT-B tubers was determined by measuring mucosal and systemic immune responses in naive and primed mice. Animals were primed by subcutaneous injection of an extract of recLT-B tuber plus adjuvant. Naive and primed mice were fed 5 g of tubers ( approximately 65 microg of recLT-B) or were intubated intragastrically with 0.4 ml of tuber extract ( approximately 2 microg of recLT-B). In naive mice, feeding recLT-B tubers or intubation of tuber extract did not induce detectable anti-LT antibody titres. In primed animals, however, oral immunisation resulted in significant anti-LT IgA antibody responses in serum and faeces. Intragastric intubation of tuber extract revealed higher responses than feeding of tubers. These results indicate clearly that functional recLT-B can be produced in potato tubers, that this recombinant protein is immunogenic and that oral administration thereof elicits both systemic and local IgA responses in parentally primed, but not naive, animals.

Transgenic plants as factories for biopharmaceuticals

Plants have considerable potential for the production of biopharmaceutical proteins and peptides because they are easily transformed and provide a cheap source of protein. Several biotechnology companies are now actively developing, field testing, and patenting plant expression systems, while clinical trials are proceeding on the first biopharmaceuticals derived from them. One transgenic plant-derived biopharmaceutical, hirudin, is now being commercially produced in Canada for the first time. Product purification is potentially an expensive process, and various methods are currently being developed to overcome this problem, including oleosin-fusion technology, which allows extraction with oil bodies. In some cases, delivery of a biopharmaceutical product by direct ingestion of the modified plant potentially removes the need for purification. Such biopharmaceuticals and edible vaccines can be stored and distributed as seeds, tubers, or fruits, making immunization programs in developing countries cheaper and potentially easier to administer. Some of the most expensive biopharmaceuticals of restricted availability, such as glucocerebrosidase, could become much cheaper and more plentiful through production in transgenic plants.