Edible vaccine protects mice against Escherichia coli heat-labile enterotoxin (LT): potatoes expressing a synthetic LT-B gene

Expression of hemagglutinin protein of rinderpest virus in transgenic tobacco and immunogenicity of plant-derived protein in a mouse model

The use of transgenic plants as a production system for recombinant subunit vaccines has been considered safe and economical compared to cell culture methods. We have exploited this approach to produce rinderpest virus hemagglutinin (H) protein in transgenic tobacco as a model plant for testing the immunogenicity of plant-derived hemagglutinin protein. The transgenic nature of the plants was confirmed by molecular analysis such as gene specific PCR and Southern hybridization using full-length H gene as a probe. The Mendelian pattern of inheritance of the transgene has been demonstrated in T(1) generation. The transgenic plants express the H protein of molecular weight 72 kDa. The plant derived H protein is antigenically authentic as revealed by reactivity with H-specific antibodies as well as convalescent sera. The induction of immune response was tested in mice after intraperitoneal immunization with plant-derived H. High titers of antibodies were induced which were H-specific and they neutralized the infectivity of rinderpest virus.

Plant cell factories and mucosal vaccines

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

A plant signal peptide-hepatitis B surface antigen fusion protein with enhanced stability and immunogenicity expressed in plant cells

The use of transgenic plants to express orally immunogenic protein antigens is an emerging strategy for vaccine biomanufacturing and delivery. This concept has particular suitability for developing countries. One factor that has limited the development of this technology is the relatively modest levels of accumulation of some antigenic proteins in plant tissues. We used fusion protein design to improve expression of the hepatitis B surface antigen (HBsAg) by attempting to mimic the process of HBsAg targeting to the endoplasmic reticulum of human liver cells during hepatitis B virus infection. We created a gene encoding a recombinant HBsAg modified to contain a plant signal peptide fused to its amino terminus. The signal peptide from soybean vegetative storage protein vspA (VSP alpha S) directed endoplasmic reticulum targeting of HBsAg in plant cells, but was not cleaved and resulted in enhanced VSP alpha S-HBsAg fusion accumulation. This product was more stable and presented the protective "a" antigenic determinant to significantly higher levels than unmodified native HBsAg expressed in plant cells. It also showed a greater extent of intermolecular disulfide bond formation and formation of virus-like particles. Moreover, VSP alpha S-HBsAg stimulated higher levels of serum IgG than native HBsAg when injected into mice. We conclude that HBsAg tolerates a polypeptide fusion at the amino terminus and that VSP alpha S-HBsAg is an improved antigen for plant-based expression of a subunit vaccine for hepatitis B virus.

Delivery of subunit vaccines in maize seed

The use of recombinant gene technologies by the vaccine industry has revolutionized the way antigens are generated, and has provided safer, more effective means of protecting animals and humans against bacterial and viral pathogens. Viral and bacterial antigens for recombinant subunit vaccines have been produced in a variety of organisms. Transgenic plants are now recognized as legitimate sources for these proteins, especially in the developing area of oral vaccines, because antigens have been shown to be correctly processed in plants into forms that elicit immune responses when fed to animals or humans. Antigens expressed in maize (Zea mays) are particularly attractive since they can be deposited in the natural storage vessel, the corn seed, and can be conveniently delivered to any organism that consumes grain. We have previously demonstrated high level expression of the B-subunit of Escherichia coli heat-labile enterotoxin and the spike protein of swine transmissible gastroenteritis in corn, and have demonstrated that these antigens delivered in the seed elicit protective immune responses. Here we provide additional data to support the potency, efficacy, and stability of recombinant subunit vaccines delivered in maize seed.

Interactions among strategies associated with bacterial infection: pathogenicity, epidemicity, and antibiotic resistance

Infections have been the major cause of disease throughout the history of human populations. With the introduction of antibiotics, it was thought that this problem should disappear. However, bacteria have been able to evolve to become antibiotic resistant. Nowadays, a proficient pathogen must be virulent, epidemic, and resistant to antibiotics. Analysis of the interplay among these features of bacterial populations is needed to predict the future of infectious diseases. In this regard, we have reviewed the genetic linkage of antibiotic resistance and bacterial virulence in the same genetic determinants as well as the cross talk between antibiotic resistance and virulence regulatory circuits with the aim of understanding the effect of acquisition of resistance on bacterial virulence. We also discuss the possibility that antibiotic resistance and bacterial virulence might prevail as linked phenotypes in the future. The novel situation brought about by the worldwide use of antibiotics is undoubtedly changing bacterial populations. These changes might alter the properties of not only bacterial pathogens, but also the normal host microbiota. The evolutionary consequences of the release of antibiotics into the environment are largely unknown, but most probably restoration of the microbiota from the preantibiotic era is beyond our current abilities.

Children and genetically engineered food: potentials and problems

Changes in food production and dietary practices are occurring faster than our understanding of their potential impact on children's health. Traditionally, pediatric gastroenterologists have studied food with respect to its nutritive value and digestibility, its influence on metabolism, its growth-promoting characteristics, and its relationship to risk and severity of disease. Biotechnology is now expanding the science of food to include disease prevention and treatment, as well as the feeding of children on a global scale. Bioengineered ("genetically modified", or "transgenic") plants were initially developed to enhance the food supply by increasing crop yields. Such previously developed transgenic plants are now prevalent worldwide and appear in many processed food products. The implementation of the technology of genetic modulation of food plants has led to considerable fear, controversy, and confusion as the understanding of the technology is poor in the general population. This review presents an overview of genetically modified food crops and their potential unique benefits and risks to children's health. Political, economical, and ecological issues related to transgenic crops are not discussed.

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

We have produced a functional heat labile enterotoxin (LT-) B subunit of Escherichia coli in maize. LT-B is a multimeric protein that presents an ideal model for an edible vaccine, displaying stability in the gut and inducing mucosal and systemic immune responses. Transgenic maize was engineered to synthesize the LT-B polypeptides, which assembled into oligomeric structures with affinity for G(M1) gangliosides. We orally immunized BALB/c mice by feeding transgenic maize meal expressing LT-B or non-transgenic maize meal spiked with bacterial LT-B. Both treatments stimulated elevated IgA and IgG antibodies against LT-B and the closely related cholera toxin B subunit (CT-B) in serum, and elevated IgA in fecal pellets. The transgenic maize induced a higher anti-LT-B and anti-CT-B mucosal and serum IgA response compared to the equivalent amount of bacterial LT-B spiked into maize. Following challenge by oral administration of the diarrhea inducing toxins LT and CT, transgenic maize-fed mice displayed reduced fluid accumulation in the gut compared to non-immunized mice. Moreover, the gut to carcass ratio of immunized mice was not significantly different from the PBS (non-toxin) challenged control group. We concluded that maize-synthesized LT-B had features of the native bacterial LT-B such as molecular weight, G(M1) binding ability, and induction of serum and mucosal immunity. We have demonstrated that maize, a major food and feed ingredient, can be efficiently transformed to produce, accumulate, and store a fully assembled and functional candidate vaccine antigen.

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.

Foods as production and delivery vehicles for human vaccines

Vaccination is a great asset for eradication of infectious diseases in humans and animals. With the prevalence of antibiotic resistant bacterial strains and an alarming increase in new and re-emerging pathogens, the need for vaccination continues to be a high priority for mammalian diseases. In the last several years, a novel approach for developing improved mucosal subunit vaccines has emerged by exploiting the use of genetically modified plants. It has been demonstrated that plant-derived antigens are functionally similar to conventional vaccines and can induce neutralizing antibodies in mammalian hosts. Using genetically engineered plants for the production of immunogenic peptides also provides a new approach for the delivery of a plant-based subunit vaccine, i.e., oral delivery, provided these immunogenic peptides are expressed in an edible part of the plant, such as grain or fruit. Thus, food crops can play a significant new role in promoting human health by serving as vehicles for both production and delivery of vaccines.

Passive immunization against dental caries and periodontal disease: development of recombinant and human monoclonal antibodies

Indigenous micro-organisms in the oral cavity can cause two major diseases, dental caries and periodontal diseases. There is neither agreement nor consensus as to the actual mechanisms of pathogenesis of the specific virulence factors of these micro-organisms. The complexity of the bacterial community in dental plaque has made it difficult for the single bacterial agent of dental caries to be determined. However, there is considerable evidence that Streptococcus mutans is implicated as the primary causative organism of dental caries, and the cell-surface protein antigen (SA I/II) as well as glucosyltransferases (GTFs) produced by S. mutans appear to be major colonization factors. Various forms of periodontal diseases are closely associated with specific subgingival bacteria. Porphyromonas gingivalis has been implicated as an important etiological agent of adult periodontitis. Adherence of bacteria to host tissues is a prerequisite for colonization and one of the important steps in the disease process. Bacterial coaggregation factors and hemagglutinins likely play major roles in colonization in the subgingival area. Emerging evidence suggests that inhibition of these virulence factors may protect the host against caries and periodontal disease. Active and passive immunization approaches have been developed for immunotherapy of these diseases. Recent advances in mucosal immunology and the introduction of novel strategies for inducing mucosal immune responses now raise the possibility that effective and safe vaccines can be constructed. In this regard, some successful results have been reported in animal experimental models. Nevertheless, since the public at large might be skeptical about the seriousness of oral diseases, immunotherapy must be carried out with absolute safety. For this goal to be achieved, the development of safe antibodies for passive immunization is significant and important. In this review, salient advances in passive immunization against caries and periodontal diseases are summarized, and the biotechnological approaches for developing recombinant and human-type antibodies are introduced. Furthermore, our own attempts to construct single-chain variable fragments (ScFv) and human-type antibodies capable of neutralizing virulence factors are discussed.

Phytosecretion of enteropathogenic Escherichia coli pilin subunit A in transgenic tobacco and its suitability for early life vaccinology

A plant expression cassette system was engineered to efficiently target proteins to the default secretory pathway, allowing the expression of DNA inserts in three frames as fusion proteins containing a synthetic tobacco calreticulin cleavable signal peptide sequence, with the advantage of producing the recombinant proteins by phytosecretion. As one approach to develop a vaccine to enteropathogenic Escherichia coli (EPEC) infection, the oral immunogenicity of phytosecreted BfpA, the structural subunit A of the bundle-forming pilus, expressed at high levels (7.7% of soluble protein) in transgenic tobacco tissues, was demonstrated in BALB/c mice by the induction and detection of fecal anti-BfpA antibodies.

Factors important in the extraction, stability and in vitro assembly of the hepatitis B surface antigen derived from recombinant plant systems

The expression of vaccine antigens in edible plant material together with their delivery by the oral route constitutes a powerful paradigm, with the potential to dramatically reduce the cost of vaccine production and administration, in addition to improving distribution and patient compliance. These products will be subject to many of the same regulations applied to current injectable vaccines, so reliable methods to quantify antigen and ensure stability in crude plant extracts are required. As a model system the hepatitis B surface antigen (HBsAg) was expressed in soybean and tobacco cell cultures. This complex antigen consists of membrane-associated small surface antigen proteins (p24(s)), disulfide cross-linked to yield dimers and higher multimers. Although the total p24(s) extracted from plant cells was relatively unaffected by detergent concentration, the quantification of antigenically reactive product depended strongly on the ratio of detergent to cell concentration. Furthermore, 1-20% w/v sodium ascorbate improved the measured levels of monoclonal-reactive antigen 4- to 12-fold. Detergent also influenced antigen stability in cell lysates stored at 4 degrees C; under optimum conditions stability was maintained for at least 1 month, whereas excess detergent rendered the antigen susceptible to proteolytic degradation. This proteolysis could be counteracted by the addition of skim milk or its protein component, which stabilized antigenically reactive p24(s) for up to 2 months. The immunologically relevant epitopes of HBsAg are critically dependent on disulfide bonding. By altering the sodium ascorbate concentration or buffer pH the proportion of HBsAg displaying the monoclonal reactive epitopes was increased between 8- and 20-fold. In addition, under certain conditions the dimerized p24(s) could be converted to oligomeric aggregates, resembling the form of the serum-derived antigen. These simple in vitro manipulations, compatible with the goal of a minimally processed oral vaccine, may prove valuable in increasing the immunogenicity of the plant-derived antigen.

A novel methodology to develop a foot and mouth disease virus (FMDV) peptide-based vaccine in transgenic plants

The expression of antigens in transgenic plants has been increasingly used as an alternative to the classical methodologies for antigen expression in the development of experimental vaccines. However, an important limitation in most cases is the low concentration of the recombinant antigens in the plant tissues, which reduces the possibilities of practical applications. Because the site of insertion of the transferred DNA into the cellular chromosomal DNA is at random, different levels of foreign protein expression in independent transformants is expected. Strategies to allow the evaluation of a high number of the transgenic individuals, usually an expensive and very time consuming process, would permit the selection of those plants presenting the highest levels of recombinant protein expression. Here, we present the development of an experimental immunogen based in the expression of a highly immunogenic epitope from foot and mouth disease virus (FMDV) fused to the glucuronidase (gus A) reporter gene, which allows selection of the transgenic plants by the ss-glucuronidase (ssGUS) enzymatic activity. We produced transgenic plants of alfalfa expressing the immunogenic site between amino acid residues 135-160 of structural protein VP1 (VP135-160), fused to the ssGUS protein. Plants expressing the highest levels of the immunogenic epitope VP135-160, analyzed by Western blot, were efficiently selected based on their levels of ssGUS enzymatic activity. The FMDV epitope expressed in plants was highly immunogenic in mice which developed, after immunization, a strong anti-FMDV antibody response against a synthetic peptide representing the region VP135-160, to native virus VP1, and to purified FMDV particles. Additionally, these mice were completely protected against experimental challenge with the virulent virus. To our knowledge, this constitutes the first report of a peptide-based vaccine produced in transgenic plants that induces a protective immune response when used in experimental hosts. Also, these results demonstrated the possibility of using a novel and simple methodology for obtaining transgenic plants expressing high levels of foreign immunogenic epitopes, which could be directly applied in the development of plant-based vaccines.

Targeting and expression of antigenic proteins in transgenic plants for production of edible oral vaccines

Exploiting plants as biological bioreactors for production and delivery of edible oral subunit vaccines is a promising application of biotechnology. Efforts to enhance expression levels of transgenes coding for antigenic proteins by exploiting promoters, targeting sequences, and enhancer elements have produced rather low quantities of the antigen in plant tissues, but enough to induce immune responses in feeding studies. This review will cover components of various gene constructs used in developing plant-based vaccines against a myriad of viral and bacterial diseases. Specifically, it will focus on sequences that are involved in targeting the antigen to mucosal tissues of the intestinal tract, thus enhancing the immunogenicity of the plant-based vaccine as well as those components that result in higher accumulation of the protein within the plant.

New technology for improved vaccine safety and efficacy

Nearly all of the 2000 vaccines presently licensed by the US Department of Agriculture for veterinary use in the United States are conventional vaccines containing either killed or modified live whole bacteria or viruses. Recent advances in molecular biology, immunology, microbiology, and genetics and in understanding microbial pathogenesis have led to the development of a wide variety of new approaches for developing safer and more effective vaccines. This article briefly describes these new technologies and their potential advantages and disadvantages as compared with conventional killed and modified live vaccines.

Production of secretory IgA antibodies in plants

Functional antibodies produced in tobacco plants were first reported over a decade ago (1989). The basic protocol used to generate these 'plantibodies' involved the independent cloning of H and L chain antibody genes in Agrobacterium tumefaciens vectors, the transformation of plant tissue in vitro with the recombinant bacterium, the reconstitution of whole plants expressing individual chains, and their sexual cross. In a 'Mendelian' fashion, a fully assembled and functional antibody was recovered from plant tissue in some double-transgenic plants. In mammalian cells, the antibody H and L chains are produced as precursor proteins that are translocated into the endoplasmic reticulum (ER), under the guidance of signal sequences. Within the ER, the signal peptides are proteolytically cleaved, and several stress proteins act as chaperonins to bind the unassembled antibody chains, and direct subsequent folding and tetramer formation. A similar process occurs in plant cells, and expression can be directed via signal sequences (even of foreign origin) into the aqueous environment of the apoplasm, or to be accumulated in other specific plant tissues, including tubers, fruit, or seed. Plants can facilely assemble secretory IgA, which is comprised of four chains, H and L chains, J chain and secretory component. Plant 'bioreactors' are expected to yield over 10 kg of therapeutic antibody/acre in tobacco, maize, soybean, and alfalfa [(Ann. NY Acad. Sci.)721(1994)235; (Biotechnol. Bioeng.)20(1999)135]. Compared with conventional steel tank bioreactors using mammalian cells, or microorganisms, the costs of GMP plantibodies are expected to perhaps one tenth. The differences in glycosylation patterns of plant and mammalian cell produced antibodies apparently have no effect on antigen-binding or specificity, but there is some concern about potential immunogenicity in humans. N-linked glycans of plants differ from human by having fucose-linked alpha 1,3 and the sugar xylose. No adverse effects or human anti-mouse antibodies (HAMA) have been observed in >40 patients receiving topical oral application of a plant produced secretory IgA specific to Streptococcus mutans, for the control of caries [(Nat. Med.)4(1998)601]. The progressive improvement of expression vectors for plantibodies, and purification strategies, as well as the increase in transformable crop species, is expected to lead to almost limitless availability of inexpensive (even edible forms of) recombinant immunoglobulins free of human pathogens for human and animal therapy, and for novel industrial applications (e.g. catalytic antibodies).

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.

Chimeric plant virus particles as immunogens for inducing murine and human immune responses against human immunodeficiency virus type 1

The high-yield expression of a neutralizing epitope from human immunodeficiency virus type 1 (HIV-1) on the surface of a plant virus and its immunogenicity are presented. The highly conserved ELDKWA epitope from glycoprotein (gp) 41 was expressed as an N-terminal translational fusion with the potato virus X (PVX) coat protein. The resulting chimeric virus particles (CVPs), purified and used to immunize mice intraperitoneally or intranasally, were able to elicit high levels of HIV-1-specific immunoglobulin G (IgG) and IgA antibodies. Furthermore, the human immune response to CVPs was studied with severe combined immunodeficient mice reconstituted with human peripheral blood lymphocytes (hu-PBL-SCID). hu-PBL-SCID mice immunized with CVP-pulsed autologous dendritic cells were able to mount a specific human primary antibody response against the gp41-derived epitope. Notably, sera from both normal and hu-PBL-SCID mice showed an anti-HIV-1-neutralizing activity. Thus, PVX-based CVPs carrying neutralizing epitopes can offer novel perspectives for the development of effective vaccines against HIV and, more generally, for the design of new vaccination strategies in humans.

Expression of the native cholera toxin B subunit gene and assembly as functional oligomers in transgenic tobacco chloroplasts

The B subunits of enterotoxigenic Escherichia coli (LTB) and cholera toxin of Vibrio cholerae (CTB) are candidate vaccine antigens. Integration of an unmodified CTB-coding sequence into chloroplast genomes (up to 10,000 copies per cell), resulted in the accumulation of up to 4.1 % of total soluble tobacco leaf protein as functional oligomers (410-fold higher expression levels than that of the unmodified LTB gene expressed via the nuclear genome). However, expression levels reported are an underestimation of actual accumulation of CTB in transgenic chloroplasts, due to aggregation of the oligomeric forms in unboiled samples similar to the aggregation observed for purified bacterial antigen. PCR and Southern blot analyses confirmed stable integration of the CTB gene into the chloroplast genome. Western blot analysis showed that the chloroplast- synthesized CTB assembled into oligomers and were antigenically identical with purified native CTB. Also, binding assays confirmed that chloroplast-synthesized CTB binds to the intestinal membrane GM1-ganglioside receptor, indicating correct folding and disulfide bond formation of CTB pentamers within transgenic chloroplasts. In contrast to stunted nuclear transgenic plants, chloroplast transgenic plants were morphologically indistinguishable from untransformed plants, when CTB was constitutively expressed in chloroplasts. Introduced genes were inherited stably in subsequent generations, as confirmed by PCR and Southern blot analyses. Increased production of an efficient transmucosal carrier molecule and delivery system, like CTB, in transgenic chloroplasts makes plant-based oral vaccines and fusion proteins with CTB needing oral administration commercially feasible. Successful expression of foreign genes in transgenic chromoplasts and availability of marker-free chloroplast transformation techniques augurs well for development of vaccines in edible parts of transgenic plants. Furthermore, since the quaternary structure of many proteins is essential for their function, this investigation demonstrates the potential for other foreign multimeric proteins to be properly expressed and assembled in transgenic chloroplasts.

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.

Producing proteins in transgenic plants and animals

The requirement for large quantities of therapeutic proteins has fueled interest in the production of recombinant proteins in plants and animals. The first commercial products to be made in this way have experienced much success, and it is predicted that in the future a plethora of protein products will be made using these 'natural' bioreactors.

Understanding mucosal responsiveness: lessons from enteric bacterial pathogens

Mucosal immune responses must discriminate between commensal flora within the lumen and potential pathogens. These responses are highly adapted to induce protection without excessive inflammation. The balances that regulate mucosal immune and inflammatory responses have to be understood if effective mucosal immunity is to be induced through local immunization. This review will summarize some of the lessons learnt from studies of antigens derived from enteric bacterial pathogens and discuss how the gastrointestinal epithelia can 'fight back' when it encounters pathogens.

Medical molecular farming: production of antibodies, biopharmaceuticals and edible vaccines in plants

The use of plants for medicinal purposes dates back thousands of years but genetic engineering of plants to produce desired biopharmaceuticals is much more recent. As the demand for biopharmaceuticals is expected to increase, it would be wise to ensure that they will be available in significantly larger amounts, on a cost-effective basis. Currently, the cost of biopharmaceuticals limits their availability. Plant-derived biopharmaceuticals are cheap to produce and store, easy to scale up for mass production, and safer than those derived from animals. Here, we discuss recent developments in this field and possible environmental concerns.

Molecular farming of pharmaceutical proteins

Molecular farming is the production of pharmaceutically important and commercially valuable proteins in plants. Its purpose is to provide a safe and inexpensive means for the mass production of recombinant pharmaceutical proteins. Complex mammalian proteins can be produced in transformed plants or transformed plant suspension cells. Plants are suitable for the production of pharmaceutical proteins on a field scale because the expressed proteins are functional and almost indistinguishable from their mammalian counterparts. The breadth of therapeutic proteins produced by plants range from interleukins to recombinant antibodies. Molecular farming in plants has the potential to provide virtually unlimited quantities of recombinant proteins for use as diagnostic and therapeutic tools in health care and the life sciences. Plants produce a large amount of biomass and protein production can be increased using plant suspension cell culture in fermenters, or by the propagation of stably transformed plant lines in the field. Transgenic plants can also produce organs rich in a recombinant protein for its long-term storage. This demonstrates the promise of using transgenic plants as bioreactors for the molecular farming of recombinant therapeutics, including vaccines, diagnostics, such as recombinant antibodies, plasma proteins, cytokines and growth factors.

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.

Plant-based vaccines: unique advantages

Numerous studies have shown that viral epitopes and subunits of bacterial toxins can be expressed and correctly processed in transgenic plants. The recombinant proteins induce immune responses and have several benefits over current vaccine technologies, including increased safety, economy, stability, versatility and efficacy. Antigens expressed in corn are particularly advantageous since the seed can be produced in vast quantities and shipped over long distances at ambient temperature, potentially allowing global vaccination. We have expressed the B-subunit of Escherichia coli heat-labile enterotoxin and the spike protein of swine transmissible gastroenteritis virus at high levels in corn, and demonstrate that these antigens delivered in the seed elicit protective immune responses.

Differential biological and adjuvant activities of cholera toxin and Escherichia coli heat-labile enterotoxin hybrids

Two bacterial products that have been demonstrated to function as mucosal adjuvants are cholera toxin (CT), produced by various strains of Vibrio cholerae, and the heat-labile enterotoxin (LT) produced by some enterotoxigenic strains of Escherichia coli. Although LT and CT have many features in common, they are clearly distinct molecules with biochemical and immunologic differences which make them unique. The goal of this study was to determine the basis for these biological differences by constructing and characterizing chimeric CT-LT molecules. Toxin gene fragments were subcloned to create two constructs, each expressing the enzymatically active A subunit of one toxin and the receptor binding B subunit of the other toxin. These hybrid toxins were purified, and the composition and assembly of CT A subunit (CT-A)-LT B subunit (LT-B) and LT A subunit (LT-A)-CT B subunit (CT-B) were confirmed. Hybrids were evaluated for enzymatic activity, as measured by the accumulation of cyclic AMP in Caco-2 cells, and the enterotoxicity of each toxin was assessed in a patent-mouse assay. The results demonstrated that LT-A-CT-B induces the accumulation of lower levels of cyclic AMP and has less enterotoxicity than either wild-type toxin or the other hybrid. Nonetheless, this hybrid retains adjuvant activity equivalent to or greater than that of either wild-type toxin or the other hybrid when used in conjunction with tetanus toxoid for intranasal immunization of BALB/c mice. Importantly, the ability of LT to induce a type 1 cytokine response was found to be a function of LT-A. Specifically, LT-A-CT-B was able to augment the levels of antigen-specific gamma interferon (IFN-gamma) and interleukin 5 to levels comparable to those achieved with native LT, while CT-A-LT-B and native CT both produced lower levels of antigen-specific IFN-gamma. Thus, these toxin hybrids possess unique biological characteristics and provide information about the basis for differences in the biological activities observed for CT and LT.

Plant-derived measles virus hemagglutinin protein induces neutralizing antibodies in mice

Measles remains a significant problem in both the developed and developing world, and new measles vaccination strategies need to be developed. This paper examines the strategy of utilizing transgenic plants expressing a measles antigen for the development of an oral sub-unit measles vaccine. A 1.8 kb fragment encompassing the coding region of the measles virus hemagglutinin (H) protein was cloned into a plant expression cassette. Three different expression constructs were tested: pBinH (H gene alone), pBinH/KDEL (addition of a C-terminal endoplasmic reticulum-retention sequence SEKDEL) and pBinSP/H/KDEL (further addition of an authentic N-terminal plant signal peptide). The highest levels of recombinant H protein production were observed in plants transformed with pBinH/KDEL. Mice inoculated intraperitoneally with transgenic plant derived recombinant H protein produced serum anti-H protein antibodies that neutralized the measles virus (MV) in vitro. Mice gavaged with transgenic tobacco leaf extracts also developed serum H protein-specific antibodies with neutralizing activity against MV in vitro. These results indicate that the plant-derived measles H protein is immunogenic when administered orally and that, with further development, oral vaccination utilizing transgenic plants may become a viable approach to measles vaccine development.

Ensuring vaccine safety in immunization programmes--a WHO perspective

Ever since vaccines were firstly used against smallpox, adverse events following immunization have been reported. As immunization programmes expand to reach even the most remote communities in the poorest countries, it is likely that many more events will be temporally linked with vaccine administration. Furthermore, the profound shift in the general public and media interest in adverse events may lead to undue concerns and allegations which may ultimately jeopardize immunization programmes world-wide. While the health professional has understood this issue for some time, the public and the media have now also become all too aware of the significance of vaccine-related adverse events. The familiar vaccines, well-tested over decades, have not changed--but the perception regarding their safety has shifted. Claims outrageous or reasonable are being made against both the old and the newly-introduced vaccines. At the same time, the immunological and genetic revolution of the last decade may well bring to our notice some hypothetical risks that need to be addressed at pre-clinical level. WHO has been at the leading edge to guarantee vaccine safety for the last 30 years and will continue to do so. The Organization's plans for the next decade and beyond include the Safe Injection Global Network (SIGN), the development and introduction of safer technologies, and the prevention, early detection and management of AEFIs. The new technologies include needle-containing injection devices such as the autodisable syringe, as well as mucosal and transcutaneous immunization. Training will continue to be at the centre of WHO's efforts, limiting human error to a minimum. Mechanisms have been set in place to detect and respond to new and unforeseen events occurring. Above all, there is a willingness to respond to new climates and new technologies so that the Organization is in the best position to ensure safe immunization for all the world's children.

High-yield expression of a viral peptide vaccine in transgenic plants

A high-yield production of a peptide vaccine in transgenic plants is described here. A 21-mer peptide, which confers protection to dogs against challenge with virulent canine parvovirus, has been expressed in transgenic plants as an amino-terminal translational fusion with the GUS gene. Transformants were selected on the basis of their GUS activities, showing expression levels of the recombinant protein up to 3% of the total leaf soluble protein, a production yield comparable to that obtained with the same epitope expressed by chimeric plant viruses. The immunogenicity of the plant-derived peptide was demonstrated in mice immunized either intraperitoneally or orally with transgenic plant extracts, providing the suitability of the GUS fusions approach for low-cost production of peptide vaccines.

Expression of immunogenic Puumala virus nucleocapsid protein in transgenic tobacco and potato plants

Transgenic plants, expressing recombinant proteins, are suitable alternatives for the production of relevant immunogens. In the present study, the expression of Puumala virus nucleocapsid protein in tobacco and potato plants (Nicotiana tabacum and Solanum tuberosum) and its immunogenicity was investigated. After infection of leaf discs of SR1 tobacco and tuber discs of potato cv. "Desiree" with the Agrobacterium strain LBA4404 (pAL4404, pBinAR-PUU-S) containing the 1302 bp cDNA sequence of S-RNA segment of a Puumala virus, transgenic tobacco and potato plants expressed the Puumala virus nucleocapsid protein under control of the cauliflower 35S promoter. The recombinant proteins were found to be identical to the authentic Puumala virus nucleocapsid protein as analyzed by immunoblotting. Expression of the nucleocapsid protein was investigated over four plant generations (P to F4) and found to be stable (1 ng/3 microg dried leaf tissue). Transgenic tobacco plants were smaller compared to controls. The transformed potato plants were morphologically similar to control plants and produced tubers as the control potatoes. The S-antigen was expressed at a level of 1 ng protein/5 microg and 1 ng protein/4 microg dried leaf and root tissues, respectively, and remained stable in the first generation of vegetatively propagated potato plants. The immunogenicity of the Puumala virus nucleocapsid protein expressed in Nicotiana tabacum and Solanum tuberosum was investigated in New Zealand white rabbits. They were immunized with leaf extracts from transgenic tobacco and potato plants, and the serum recognized Puumala virus nucleocapsid protein. Transgenic plants expressing hantaviral proteins can thus be used for the development of cost-effective diagnostic systems and for alternative vaccination strategies.

Production of hepatitis B surface antigen in transgenic plants for oral immunization

Here we present data showing oral immunogenicity of recombinant hepatitis B surface antigen (HBsAg) in preclinical animal trials. Mice fed transgenic HBsAg potato tubers showed a primary immune response (increases in HBsAg-specific serum antibody) that could be greatly boosted by intraperitoneal delivery of a single subimmunogenic dose of commercial HBsAg vaccine, indicating that plants expressing HBsAg in edible tissues may be a new means for oral hepatitis B immunization. However, attainment of such a goal will require higher HBsAg expression than was observed for the potatoes used in this study. We conducted a systematic analysis of factors influencing the accumulation of HBsAg in transgenic potato, including 5' and 3' flanking elements and protein targeting within plant cells. The most striking improvements resulted from (1) alternative polyadenylation signals, and (2) fusion proteins containing targeting signals designed to enhance integration or retention of HBsAg in the endoplasmic reticulum (ER) of plant cells.

Molecular strategies for overcoming antibiotic resistance in bacteria

Overuse of antibiotics in humans and livestock has led to the rapid evolution of bacteria that are resistant to multiple drugs such that even vancomycin, the drug of last resort, is no longer effective against some strains. Apart from the discovery and exploitation of the natural peptide antimicrobial agents that form part of the innate immune systems of plants and animals, there have been few new antibiotics developed in recent years. Here we review strategies designed to exploit recent advances in molecular biology, including recombinant DNA technology, molecular modelling and genomics to develop new antibacterial agents that overcome antibiotic resistance.

[A technology with multiple applications]

Plant breeding has been a human practice for some thousands of years. However, this process of domestication has made plants more vulnerable to pests and diseases. Classic plant breeding has allowed the genetic manipulation of plants through crossings with a resulting increase in crop productivity. Recently, the recombinant DNA technology has increased the possibilities of integration of exogenous genes to the plant genome, resulting in the production of transgenic plants. Despite the great debate on this issue, such plants represent to date a promising avenue for plant breeding. There are many examples of gene transference strategies which have been successful in promoting resistance to herbicides, viruses, fungi, bacteria and insects, or in producing an increase in food quality. In addition to biotechnological applications, transgenic plants have made a significant contribution to the study of gene functioning, such as the analysis of genic expression regulation and the study of protein functions codified by distinct plant genes.

Mechanisms of vaccine adjuvanticity at mucosal surfaces

The vast majority of pathogens invade via mucosal surfaces, including those of the intestine. Vaccination directly on these surfaces may induce local protective immunity and prevent infection and disease. Although vaccine delivery to the gut mucosa is fraught with obstacles, immunization can be enhanced using adjuvants with properties specific to intestinal immunity. In this review, we present three general mechanisms of vaccine adjuvant function as originally described by Freund, and we discuss these principles with respect to intestinal adjuvants in general and to the prototypical mucosal adjuvant, cholera toxin. The key property of intestinal adjuvants is to induce an immunogenic context for the presentation of the vaccine antigen. The success of oral vaccine adjuvants is determined by their ability to induce a controlled inflammatory response in the gut-associated lymphoid tissues, characterized by the expression of various costimulatory molecules and cytokines. An understanding of the specific molecular mechanisms of adjuvanticity in the gut will allow the rational development of safe and effective oral vaccines.

Plants as bioreactors for protein production: avoiding the problem of transgene silencing

Plants are particularly attractive as large-scale production systems for proteins intended for therapeutical or industrial applications: they can be grown easily and inexpensively in large quantities that can be harvested and processed with the available agronomic infrastructures. The effective use of plants as bioreactors depends on the possibility of obtaining high protein accumulation levels that are stable during the life cycle of the transgenic plant and in subsequent generations. Silencing of the introduced transgenes has frequently been observed in plants, constituting a major commercial risk and hampering the general economic exploitation of plants as protein factories. Until now, the most efficient strategy to avoid transgene silencing involves careful design of the transgene construct and thorough analysis of transformants at the molecular level. Here, we focus on different aspects of the generation of transgenic plants intended for protein production and on their influence on the stability of heterologous gene expression.

Immunological approaches to the control of tuberculosis in wildlife reservoirs

Attempts to eradicate tuberculosis from cattle and farmed deer in some countries have been frustrated by the existence of wildlife reservoirs of Mycobacterium bovis infection. Possum control programmes in New Zealand using poisons have shown clearly that the brushtail possum is an important source of infection for cattle and farmed deer, and the sum of evidence strongly suggests that badgers serve as a source of infection for cattle in the UK. Bovine tuberculosis can only be eradicated from these countries by controlling M. bovis infection in both wildlife and domestic animals. The most promising options for control of M. bovis infection in wildlife in the longer term include the development of a tuberculosis vaccine for wildlife and a strategy for biological control of possums. The aim of this review is to address the problems and approaches involved in the control of wildlife tuberculosis from an immunological perspective.

Plants for delivery of edible vaccines

Over the past decade, scientific advances in molecular biology and immunology have improved understanding of many diseases and led to the development of novel strategies for vaccination. The development of plants expressing vaccine antigens is a particularly promising approach. Plant-derived antigenic proteins have delayed or prevented the onset of disease in animals and have proven to be safe and functional in human clinical trials. Future areas of research should further characterize the induction of the mucosal immune system and appropriate crop species for delivery of animal and human vaccines.

Oral vaccine delivery

Oral vaccination of animals and man, to provide effective mucosal and/or systemic immunity, is largely ineffective. This is due mainly to the very small quantity of antigen that survives degradation in the intestine and that crosses the intestinal wall. Over the past decade or so, a number of proteins have been identified that are effective at eliciting mucosal and systemic immune responses following oral administration. Uptake of these molecules by the gastro-intestinal tract (GIT) epithelium is dependent upon specific binding to the GIT epithelial cells. The identity of these molecules is discussed, as well as their possible application as 'carriers' for co-transporting haptens, proteins and nanoparticles across the GIT epithelium.

Protection of mice against challenge with foot and mouth disease virus (FMDV) by immunization with foliar extracts from plants infected with recombinant tobacco mosaic virus expressing the FMDV structural protein VP1

A tobacco mosaic virus (TMV)-based vector has been used to express in plants the complete open reading frame coding for VP1, the major immunogenic protein of foot and mouth disease virus (FMDV). In vitro RNA transcripts were inoculated into Nicotiana benthamiana plants and detectable amounts of recombinant VP1 were identified by Western blot as soon as 4 days postinfection. Foliar extracts prepared from infected leaves were injected intraperitoneally into mice and all of the immunized animals developed a specific antibody response to both the complete virus particle and the major immunogenic region as determined by ELISA and Western blot analysis. Most importantly, all immunized mice developed a protective immune response against experimental challenge with virulent FMDV. To our knowledge, this is the first report showing the expression of a complete open reading frame of an antigenic foreign protein in plants, using a recombinant plant virus, in sufficient quantity to permit use of the crude plant extract as an experimental immunogen to protect animals against virus challenge.

A plant-derived edible vaccine against hepatitis B virus

The infectious hepatitis B virus represents 42 nm spherical double-shelled particles. However, analysis of blood from hepatitis B virus carriers revealed the presence of smaller 22 nm particles consisting of a viral envelope surface protein. These particles are highly immunogenic and have been used in the design of hepatitis B virus vaccine produced in yeast. Upon expression in yeast, these proteins form virus-like particles that are used for parenteral immunization. Therefore, the DNA fragment encoding hepatitis B virus surface antigen was introduced into Agrobacterium tumerifacience LBA4404 and used to obtain transgenic lupin (Lupinus luteus L.) and lettuce (Lactuca sativa L.) cv. Burpee Bibb expressing envelope surface protein. Mice that were fed the transgenic lupin tissue developed significant levels of hepatitis B virus-specific antibodies. Human volunteers, fed with transgenic lettuce plants expressing hepatitis B virus surface antigen, developed specific serum-IgG response to plant produced protein.

The role of cAMP in mucosal adjuvanticity of Escherichia coli heat-labile enterotoxin (LT)

Heat-labile enterotoxin (LT) produced by enterotoxigenic Escherichia coli (ETEC) and cholera toxin (CT) produced by Vibrio cholerae have been shown to function as potent mucosal adjuvants. A number of studies have examined the effects of different mutations at either the active site or the protease site of LT and CT and the influence of those mutations on toxicity and adjuvanticity. However, different observations reported by various groups using a variety of animal models with different antigens or different routes of immunization have provided contradictory findings and evoked many questions regarding the underlying mechanisms of mucosal adjuvanticity of LT and CT. In this study, the role of cAMP in mucosal adjuvanticity was examined by comparing three LT active site mutants (S61F, A69G, E112K), a protease site mutant (R192G) and recombinant LT-B for toxicity, cAMP activity and mucosal adjuvanticity using tetanus toxoid (TT) as a model antigen. While all mutants examined showed reduced toxicity, the effects of each mutation on its ability to function as an adjuvant varied. Following intranasal immunization, native LT as well as protease and active site mutants of LT induced serum anti-TT IgG and their responses were virtually indistinguishable from one another. In addition, LT-B was also able to enhance production of serum anti-TT IgG, though at a level significantly lower than that achieved by native LT and mutants. Following oral immunization, the best serum anti-TT IgG responses were obtained with native LT and mutants that retained the ability to induce accumulation of cAMP. Despite the nearly identical serum anti-TT IgG responses following intranasal immunization, there was a strong correlation between the ability to induce accumulation of cAMP in cultured Caco-2 cells and the ability to elicit production of antigen-specific Th1 or Th2 cytokines.

Development of biopharmaceuticals in plant expression systems: cloning, expression and immunological reactivity of human cytomegalovirus glycoprotein B (UL55) in seeds of transgenic tobacco

Plant seeds offer unique opportunities for the production and delivery of oral subunit vaccines. We have used the immunodominant glycoprotein B complex of human cytomegalovirus (HCMV), introduced into tobacco plants, as a model system for studying the merit of this promising approach. Given the advantages of expressing proteins in seeds, a novel expression vector was developed incorporating regulatory sequences of glutelin, the major rice seed storage protein, to direct synthesis of recombinant glycoprotein B. Analysis of genomic DNA of 28 selected tobacco transformants by PCR amplification showed that 71% harboured the gB cDNA, a finding further documented by Southern blotting. Specific immunoassays of protein extracts from seeds of positive plants showed that all were producing antigenic glycoprotein B at levels ranging from 70-146 ng/mg extracted protein. In addition, similarity with native glycoprotein B produced in HCMV-infected cells was also demonstrated by inhibition of immunofluorescence on HCMV-infected human fibroblasts. These data are the first to report the expression of an immunodominant antigen of HCMV in plant tissues, indicating the fidelity with which this very large heterologous viral glycoprotein can be synthesized in this model system.

A review of oral vaccination with transgenic vegetables

Mucosal immunization of the gastrointestinal tract is an effective way to stimulate local and systemic immune responses. Oral vaccines must be formulated in such a way that antigens are protected as they pass through the adverse environment of the stomach and are delivered to the mucosal inductive sites. Vaccine antigens cloned into edible transgenic plants are a promising new delivery system for oral vaccines. Such vaccines could be safe, inexpensive, and multicomponent.

Plant-based production of xenogenic proteins

Foreign protein production in transgenic plants has been successful, from the generation of transgenic plant lines to the marketing of purified proteins. Antigenic proteins from disease organisms, monoclonal antibodies raised against antigens of disease organisms, and proteins with industrial process applications have been produced and tested. For vaccines, clinical trials in humans and feeding trials in animals are in progress to demonstrate their efficacy. For industrial proteins, high expression and downstream processing efficiency are key concerns, with application and test market trials in progress.