Edible plant vaccines: applications for prophylactic and therapeutic molecular medicine

Non-clinical safety evaluation of novel vaccines and adjuvants: new products, new strategies

Advances in molecular biology and biotechnology, coupled with an increased understanding of disease processes and mechanisms of protective immunity have facilitated the development of new rationally-designed vaccines utilising recombinant proteins, naked DNA, live vectors, genetically-modified toxins and whole dendritic and tumour cells for both prophylaxis and therapy of a wide range of indications. These new vaccine technologies coupled with novel adjuvants, delivery systems, formulations, dosing routes and regimes present many unique and difficult challenges in demonstrating product safety and efficacy to support clinical testing. This paper aims to review these novel vaccine and adjuvant technologies and to highlight the key safety issues potentially associated with them. Approaches taken to demonstrate vaccine safety by assessing systemic and local toxicity, biodistribution and persistence, immunogenicity and immunotoxicity, reproductive toxicology, safety pharmacology and genotoxicity within the current regulatory framework are presented.

Virus-like particle expression and assembly in plants: hepatitis B and Norwalk viruses

Expression of vaccine antigens in plants and delivery via ingestion of transgenic plant material has shown promise in numerous pre-clinical animal studies and in a few clinical trials. A number of different viral antigens have been tested, and among the most promising are those that can assemble virus-like particles (VLP), which mimic the form of authentic virions and display neutralizing antibody epitopes. We have extensively studied plant expression, VLP assembly, and immunogenicity of hepatitis B surface antigen (HBsAg) and Norwalk virus capsid protein (NVCP). The HBsAg small protein (S protein) was found by TEM to assemble tubular membrane complexes derived from endoplasmic reticulum in suspension cultured cells of tobacco and soybean, and in potato leaf and tuber tissues. The potato material was immunogenic in mice upon delivery by ingestion. Here we describe the plant expression and immunogenicity of HBsAg middle protein (M protein or pre-S2 + S) which contains additional 55 amino acid pre-S2 region at N-terminus of the S protein. Plant-derived recombinant M protein provoked stronger serum antibody responses against HBsAg than did S protein when injected systemically in mice. We discuss implications for use of fusion proteins for enhanced immunogenicity and mucosal targeting of HBsAg, as well as delivery of heterologous fused antigens. NVCP expressed in plants assembled 38 nm virion-size icosahedral (T = 3) VLP, similar to those produced in insect cells. The VLP stimulated serum IgG and IgA responses in mice and humans when they were delivered by ingestion of fresh potato tuber. Here we show that freeze-drying of transgenic NVCP tomato fruit yielded stable preparations that stimulated excellent IgG and IgA responses against NVCP when fed to mice. However, the predominant VLP form in tomato fruit was the small 23 nm particle also observed in insect cell-derived NVCP.

Bacterial spores as heat stable vaccine vehicles

Recently, the first use of bacterial spores as vaccine vehicles was reported, showing that mice orally immunised with Bacillus subtilis spores expressing a tetanus antigen could be protected against lethal challenge with tetanus toxin. Unlike many second generation vaccine systems currently under development, none offer the heat stability of bacterial spores or the flexibility for genetic manipulation. The current use of Bacillus spores as probiotics for both humans and animals may facilitate their eventual licensing as oral vaccines. This review reports the progress that has been made in the establishment of bacterial spores as vaccine vehicles and outlines the potential advantages of the spore vaccine approach.

Preclinical safety testing of biotechnology-derived pharmaceuticals: understanding the issues and addressing the challenges

The unique and complex nature of biotechnology-derived pharmaceuticals has meant that it is often not possible to follow the conventional safety testing programs used for chemicals, and hence they are evaluated on a case-by-case basis. Nonclinical safety testing programs must be rationally designed with a strong scientific understanding of the product, including its method of manufacture, purity, sequence, structure, species specificity, pharmacological and immunological effects, and intended clinical use. This knowledge, coupled with a firm understanding of the regulatory requirements for particular product types, will ensure that the most sensitive and regulatory-compliant test systems are used to optimize the chances of gaining regulatory approval for clinical testing or marketing authorization in the shortest possible time frame.

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

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

Mucosal immunization against respiratory bacterial pathogens

Bacterial respiratory diseases remain a major cause of morbidity and mortality throughout the world. The young and the elderly are particularly susceptible to the pathogens that cause these diseases. Therapeutic approaches remain dependent upon antibiotics contributing to the persistent increases in antibiotic resistance. The main causes of respiratory disease discussed in this review are Mycobacterium tuberculosis, Corynebacterium diphtheriae, Bordatella pertussis, Streptococcus pneumoniae, non-typeable Haemophilus influenzae, Moraxella catarrhalis and Pseudomonas aeruginosa. All these organisms initiate disease at the mucosal surface of the respiratory tract and thus the efficacy of the host's response to infection needs to be optimal at this site. Vaccines available for diseases caused by many of these pathogens have limitations in accessibility or efficacy, highlighting the need for improvements in approaches and products. The most significant challenges in both therapy and prevention of disease induced by bacteria in the respiratory tract remain the development of non-injectable vaccines and delivery systems/immunization regimens that improve mucosal immunity.

A mucosally targeted subunit vaccine candidate eliciting HIV-1 transcytosis-blocking Abs

A vaccine that would engage the mucosal immune system against a broad range of HIV-1 subtypes and prevent epithelial transmission is highly desirable. Here we report fusing the mucosal targeting B subunit of cholera toxin to the conserved galactosylceramide-binding domain (including the ELDKWA-neutralizing epitope) of the HIV-1 gp41 envelope protein, which mediates the transcytosis of HIV-1 across the mucosal epithelia. Chimeric protein expressed in bacteria or plants assembled into oligomers that were capable of binding galactosyl-ceramide and G(M1) gangliosides. Mucosal (intranasal) administration in mice of the purified chimeric protein followed by an i.p. boost resulted in transcytosis-neutralizing serum IgG and mucosal IgA responses and induced immunological memory. Plant production of mucosally targeted immunogens could be particularly useful for immunization programs in developing countries, where desirable product traits include low cost of manufacture, heat stability, and needle-free delivery.

Development of a highly efficient system for assessing recombinant gene expression in plant cell suspensions via Agrobacterium tumefaciens transformation

A transient gene-expression system was developed and used to characterize promoter strength, to verify suitability of bacterial gene modifications for expression in plant cells, and to express active antibody molecules. The system is based on suspension tobacco cells transformed by Agrobacterium in a transient way. Conditions such as pre-culture of tobacco cells and the co-cultivation period were identified as determinants to achieve high expression levels. Under established conditions the activity strength of CaMV (cauliflower mosaic virus) 35 S and ToMoTV (tomato mottle taino virus) AL1 promoters were compared. A modified cry gene sequence from Bacillus thuringiensis was expressed and detected by Western-blot analysis. A monoclonal antibody against anti-(hepatitis B virus surface antigen) was produced in such quantities as to allow testing of biological activity and preliminary characterization.

Conformational analysis of hepatitis B surface antigen fusions in an Agrobacterium-mediated transient expression system

Vaccine antigens have been successfully produced in transgenic plants for oral immunization. Recently, a fusion strategy has been adopted to produce multicomponent vaccines and to target antigens to mucosal sites for enhanced oral immunogenicity. However, antigen fusions may not be folded correctly due to steric hindrance and may thus lose their potency. Here, we describe an Agrobacterium-mediated transient assay that provides enough antigen-expressing material at 2 days post-transfection to evaluate antigen conformation. Using the hepatitis B surface antigen (HBsAg) as a model antigen and the green fluorescent protein (GFP) as a model fusion partner, we showed that transiently expressed HBsAg and an HBsAg fusion with GFP at the N-terminus (GFP:HBsAg), but not the HBsAg fusion with GFP at the C-terminus (HBsAg:GFP), formed the 'a' determinant and virus-like particles (VLPs), similar to yeast-derived vaccine HBsAg. Thus, it is feasible to modify the HBsAg with an N-terminal fusion of up to 239 amino acids without altering its major antigenic properties. Our results also demonstrate that the Agrobacterium-mediated transient expression system can be used to evaluate the conformation of plant-based vaccines or other pharmaceutical proteins in a high-throughput manner.

Tissue distribution of cholinesterases and anticholinesterases in native and transgenic tomato plants

Acetylcholinesterase, a major component of the central and peripheral nervous systems, is ubiquitous among multicellular animals, where its main function is to terminate synaptic transmission by hydrolyzing the neurotransmitter, acetylcholine. However, previous reports describe cholinesterase activities in several plant species and we present data for its presence in tomato plants. Ectopic expression of a recombinant form of the human enzyme and the expression pattern of the transgene and the accumulation of its product in transgenic tomato plants are described. Levels of acetylcholinesterase activity in different tissues are closely effected by and can be separated from alpha-tomatine, an anticholinesterase steroidal glycoalkaloid. The recombinant enzyme can also be separated from the endogenous cholinesterase activity by its subcellular localization and distinct biochemical properties. Our results provide evidence for the co-existence in tomato plants of both acetylcholinesterase activity and a steroidal glycoalkaloid with anticholinesterase activity and suggest spatial mutual exclusivity of these antagonistic activities. Potential functions, including roles in plant-pathogen interactions are discussed.

High-yield expression of a viral peptide animal vaccine in transgenic tobacco chloroplasts

The 2L21 peptide, which confers protection to dogs against challenge with virulent canine parvovirus (CPV), was expressed in tobacco chloroplasts as a C-terminal translational fusion with the cholera toxin B subunit (CTB) or the green fluorescent protein (GFP). Expression of recombinant proteins was dependent on plant age. A very high-yield production was achieved in mature plants at the time of full flowering (310 mg CTB-2L21 protein per plant). Both young and senescent plants accumulated lower amounts of recombinant proteins than mature plants. This shows the importance of the time of harvest when scaling up the process. The maximum level of CTB-2L21 was 7.49 mg/g fresh weight (equivalent to 31.1% of total soluble protein, TSP) and that of GFP-2L21 was 5.96 mg/g fresh weight (equivalent to 22.6% of TSP). The 2L21 inserted epitope could be detected with a CPV-neutralizing monoclonal antibody, indicating that the epitope is correctly presented at the C-terminus of the fusion proteins. The resulting chimera CTB-2L21 protein retained pentamerization and G(M1)-ganglioside binding characteristics of the native CTB and induced antibodies able to recognize VP2 protein from CPV. To our knowledge, this is the first report of an animal vaccine epitope expression in transgenic chloroplasts. The high expression of antigens in chloroplasts would reduce the amount of plant material required for vaccination (approximately 100 mg for a dose of 500 microg antigen) and would permit encapsulation of freeze-dried material or pill formation.

Recent developments in the use of transgenic plants for the production of human therapeutics and biopharmaceuticals

In recent years there has been a dramatic increase in the application of plant biotechnology for the production of a variety of commercially valuable simple and complex biological molecules (biologics) for use in human and animal healthcare. Transgenic whole plants and plant cell culture systems have been developed that have the capacity to economically produce large-scale quantities of antibodies and antibody fragments, antigens and/or vaccine epitopes, metabolic enzymes, hormones, (neuro)peptides and a variety of biologically active complexes and secondary metabolites for direct use as therapeutic agents or diagnostic tools in the medical healthcare industry. As the products of genetically modified plants make their way from concept to commercialization the associated risks and acceptance by the public has been become a focal point. In this paper, we summarize the recent advances made in the use of transgenic plants and plant cell cultures as biological factories for the production of human therapeutics and biopharmaceuticals and discuss the long-term potential of `molecular farming' as a low-cost, efficient method for the production of biological materials with demonstrated utility to the pharmaceutical industry or medical community.

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

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

Induction of antigen-specific systemic and mucosal immune responses by feeding animals transgenic plants expressing the antigen

A report from that the presence of lactogenic immunity in pigs protected suckling piglets from porcine epidemic diarrhea virus (PEDV) infection suggested that inducing mucosal immune responses in lactating pigs is an effective way of protecting swine from PEDV infection. In this study, we developed transgenic tobacco plants that express the antigen protein corresponding to the neutralizing epitope of PEDV spike protein, and tested whether feeding the plants to pigs induced an effective immune response against PEDV infection. First, we confirmed the immunogenicity of the plant-derived antigen by using a plaque reduction neutralization assay with serum obtained after injecting mice with protein extracted from the transgenic plants. Feeding the transgenic plants to mice induced both systemic and mucosal immune responses against the antigen. The induced antibodies inhibited virus infection in the plaque reduction neutralization assay. These results suggest that feeding animals transgenic plants carrying antigen genes is an effective strategy to induce protective immune responses against PEDV infection.

Production of FaeG, the major subunit of K88 fimbriae, in transgenic tobacco plants and its immunogenicity in mice

Transgenic tobacco plants stably expressing recombinant FaeG, which is the major subunit and adhesin of K88ad fimbriae, were obtained. Analysis of sera from immunized mice indicates that in mice, the immunogenicity induced by plant-derived FaeG protein is comparable to that generated with traditional approaches.

Oral immunogenicity of human papillomavirus-like particles expressed in potato

Human papillomavirus-like particles (HPV VLPs) have shown considerable promise as a parenteral vaccine for the prevention of cervical cancer and its precursor lesions. Parenteral vaccines are expensive to produce and deliver, however, and therefore are not optimal for use in resource-poor settings, where most cervical HPV disease occurs. Transgenic plants expressing recombinant vaccine immunogens offer an attractive and potentially inexpensive alternative to vaccination by injection. For example, edible plants can be grown locally and can be distributed easily without special training or equipment. To assess the feasibility of an HPV VLP-based edible vaccine, in this study we synthesized a plant codon-optimized version of the HPV type 11 (HPV11) L1 major capsid protein coding sequence and introduced it into tobacco and potato. We show that full-length L1 protein is expressed and localized in plant cell nuclei and that expression of L1 in plants is enhanced by removal of the carboxy-terminal nuclear localization signal sequence. We also show that plant-expressed L1 self-assembles into VLPs with immunological properties comparable to those of native HPV virions. Importantly, ingestion of transgenic L1 potato was associated with activation of an anti-VLP immune response in mice that was qualitatively similar to that induced by VLP parenteral administration, and this response was enhanced significantly by subsequent oral boosting with purified insect cell-derived VLPs. Thus, papillomavirus L1 protein can be expressed in transgenic plants to form immunologically functional VLPs, and ingestion of such material can activate potentially protective humoral immune responses.

Helminth vaccines: from mining genomic information for vaccine targets to systems used for protein expression

The control of helminth diseases of people and livestock continues to rely on the widespread use of anti-helminthic drugs. However, concerns with the appearance of drug resistant parasites and the presence of pesticide residues in food and the environment, has given further incentive to the goal of discovering molecular vaccines against these pathogens. The exponential rate at which gene and protein sequence information is accruing for many helminth parasites requires new methods for the assimilation and analysis of the data and for the identification of molecules capable of inducing immunological protection. Some promising vaccine candidates have been discovered, in particular cathepsin L proteases from Fasciola hepatica, aminopeptidases from Haemonchus contortus, and aspartic proteases from schistosomes and hookworms, all of which are secreted into the host tissues or into the parasite intestine where they play important roles in host-parasite interactions. Since secreted proteins, in general, are exposed to the immune system of the host they represent obvious candidates at which vaccines could be targeted. Therefore, in this article, we consider the potential values and uses of algorithms for characterising cDNAs amongst the collated helminth genomic information that encode secreted proteins, and methods for their selective isolation and cloning. We also review the variety of prokaryotic and eukaryotic cell expression systems that have been employed for the production and downstream purification of recombinant proteins in functionally active form, and provide an overview of the parameters that must be considered if these recombinant proteins are to be commercialised as vaccine therapeutics in humans and/or animals.

Role of secretory antibodies in the defence against infections

Adaptive immunity mediated by secretory antibodies is important in the defence against mucosal infections. Specific secretory immunoglobulin A (SIgA) can inhibit initial pathogen colonization by performing immune exclusion both on the mucosal surface and within virus-infected secretory epithelial cells without causing tissue damage. Resistance against toxin-producing bacteria such as Vibrio cholerae appears to be particularly dependent on SIgA antibodies. Like natural infections, live topical vaccines or adequate combinations of inactivated vaccines and mucosal adjuvants give rise not only to SIgA antibodies, but also to long-standing serum IgG and IgA responses. The intranasal route of vaccine application could be particularly attractive to achieve this result, but only if successful stimulation is obtained without the use of toxic adjuvants. The degree of protection after vaccination may range from complete inhibition of reinfection to reduction of symptoms. In this scenario it is generally difficult to determine unequivocally the relative importance of SIgA versus serum antibodies. However, infection models in knockout mice strongly support the notion that SIgA exerts a decisive role in protection and cross-protection against a variety of infectious agents.