Expression of the B subunit of the heat-labile enterotoxin of Escherichia coli in tobacco mosaic virus-infected Nicotiana benthamiana plants and its characterization as mucosal immunogen and adjuvant

The case for plant-made veterinary immunotherapeutics

The excessive use of antibiotics in food animal production has contributed to resistance in pathogenic bacteria, thereby triggering regulations and consumer demands to limit their use. Alternatives for disease control are therefore required that are cost-effective and compatible with intensive production. While vaccines are widely used and effective, they are available against a minority of animal diseases, and development of novel vaccines and other immunotherapeutics is therefore needed. Production of such proteins recombinantly in plants can provide products that are effective and safe, can be orally administered with minimal processing, and are easily scalable with a relatively low capital investment. The present report thus advocates the use of plants for producing vaccines and antibodies to protect farm animals from diseases that have thus far been managed with antibiotics; and highlights recent advances in product efficacy, competitiveness, and regulatory approval.

A PLGA-encapsulated chimeric protein protects against adherence and toxicity of enterotoxigenic Escherichia coli

Enterotoxigenic Escherichia coli (ETEC) are the most common cause of diarrhea among children. Colonization factors and enterotoxins are the major ETEC candidate vaccines. Since protection against ETEC mostly occurs by induction of IgA antibodies, much effort is focused on the development of oral vaccines. In this study oral immunogenicity of a poly(lactic-co-glycolic acid) (PLGA) encapsulated chimeric protein containing CfaB, CstH, CotA and LTB (Heat-labile B subunit) was investigated. The protein was encapsulated in PLGA by double emulsion method and nanoparticles were characterized physicochemically. Immunogenicity was assessed by evaluating IgG1, IgG2 and IgA titers after BALB/c mice vaccination. Non aggregated nanoparticles had a spherical shape with an average particle size of 252.7±23 nm and 91.96±4.4% of encapsulation efficiency. Western blotting showed maintenance of the molecular weight and antigenicity of the released protein. Oral immunization of mice induced serum IgG and fecal IgA antibody responses. Immunization induced protection against ETEC binding to Caco-2 cells. The effect of LT toxin on fluid accumulation in ileal loops was neutralized by inhibition of enterotoxin binding to GM1-ganglosides. Delivery of the chimeric protein in PLGA elicited both systemic and mucosal immune responses. The findings could be exploited to development of oral multi-component ETEC prophylactic measures.

Recent advances and safety issues of transgenic plant-derived vaccines

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

Oral immunisation of mice with a recombinant rabies virus vaccine incorporating the heat-labile enterotoxin B subunit of Escherichia coli in an attenuated Salmonella strain

To investigate effective new rabies vaccines, a fusion protein consisting of the rabies virus (RV) glycoprotein and the heat-labile enterotoxin B subunit of Escherichia coli (LTB) was successfully constructed and delivered in a live attenuated Salmonella strain LH430. Mice were immunised with LH430 carrying pVAX1-G, pVAX1-G-LTB or pVAX1-ori-G-LTB. The antibody titres of mice immunised with oral LH430 carrying pVAX1-G-LTB or pVAX1-ori-G-LTB were significantly higher than those of pVAX1-G-immunised mice. The results of the challenge with the rabies virus standard strain (CVS-11) showed that the LH430 strain carrying the G-LTB gene induced immunity and elevated IL-2 levels in immunised mice ((∗∗)P<0.01), whereas LH430 carrying pVAX1-G did not contribute to protection. These results show that LH430 carrying recombinant G-LTB could provide overall immunity against challenge with CVS-11 and should be considered to be a potential rabies vaccine.

Hairy roots cultures from different Solanaceous species have varying capacities to produce E. coli B-subunit heat-labile toxin antigen

The gene encoding enterotoxigenic Escherichia coli B-subunit heat-labile toxin (LTB) antigen was co-transformed into hairy root cultures of Nicotiana tabacum (tobacco), Solanum lycopersicum (tomato) and Petunia parodii (petunia) under the CaMV35S promoter. Tobacco and petunia roots contained ~65-70 μg LTB g(-1) tissue whilst hairy roots of tomato contained ~10 μg LTB g(-1). Antigen at ~600 ng ml(-1) was detected in growth medium of tobacco and petunia. Tobacco roots with higher LTB levels showed growth retardation of ~80% whereas petunia hairy roots with similar levels of LTB showed only ~35% growth retardation, relative to vector controls. Regeneration of plants from LTB-containing tobacco hairy roots was readily achieved and re-initiated hairy roots from greenhouse-grown plants showed similar growth and LTB production characteristics as the original hairy root cultures.

Foot and mouth disease virus polyepitope protein produced in bacteria and plants induces protective immunity in guinea pigs

The goal of this project was to develop an alternative foot and mouth disease (FMD) vaccine candidate based on a recombinant protein consisting of efficient viral epitopes. A recombinant gene was designed that encodes B-cell epitopes of proteins VP1 and VP4 and T-cell epitopes of proteins 2C and 3D. The polyepitope protein (H-PE) was produced in E. coli bacteria or in N. benthamiana plants using a phytovirus expression system. The methods of extraction and purification of H-PE proteins from bacteria and plants were developed. Immunization of guinea pigs with the purified H-PE proteins induced an efficient immune response against foot and mouth disease virus (FMDV) serotype O/Taiwan/99 and protection against the disease. The polyepitope protein H-PE can be used as a basis for developing a new recombinant vaccine against FMD.

Enhancement of immune responses by an attenuated Salmonella enterica serovar Typhimurium strain secreting an Escherichia coli heat-labile enterotoxin B subunit protein as an adjuvant for a live Salmonella vaccine candidate

A plasmid harboring eltB, the gene encoding heat-labile enterotoxin (LTB), was constructed by insertion of eltB into an Asd(+) β-lactamase signal plasmid (pMMP65). This was introduced into the Δlon ΔcpxR Δasd Salmonella enterica serovar Typhimurium strain and designated the LTB adjuvant strain. LTB protein production and secretion from the strain were demonstrated with an immunoblot assay and enzyme-linked immunosorbent assay. The LTB strain was evaluated for enhancement of immunity and protection efficacy induced by a previously constructed live Salmonella vaccine candidate. In addition, immunization strategies using the LTB strain were optimized for effective salmonellosis protection. Seventy female BALB/c mice were divided into seven groups (A to G; n = 10 mice per group). Mice were primed at 6 weeks of age and boosted at 9 weeks of age. All mice were orally challenged with a virulent wild-type strain at week 3 postbooster. Serum IgG and IgA titers from mice immunized with the LTB strain alone or with a mixture of the LTB strain and the vaccine candidate were significantly increased. The secretory IgA titers from mice immunized with the LTB strain alone or with the mixture were at least 2.2 times greater than those of control mice. In addition, all group E mice (primed with the vaccine-LTB mixture and boosted with the vaccine candidate) were free of clinical signs of salmonellosis and survived a virulent challenge. In contrast, death due to the challenge was 100% in control mice, 80% in group A mice (single immunization with the vaccine candidate), 60% in group B mice (primed and boosted with the vaccine candidate), 40% in group C mice (single immunization with the LTB strain), 30% in group D mice (primed and boosted with the LTB strain), and 30% in group F mice (primed and boosted with the vaccine-LTB mixture). These results suggest that vaccination with the LTB strain, especially when added at the prime stage only, effectively enhances immune responses and protection against salmonellosis.

In planta production of plant-derived and non-plant-derived adjuvants

Recombinant antigen production in plants is a safe and economically sound strategy for vaccine development, particularly for oral/mucosal vaccination, but subunit vaccines usually suffer from weak immunogenicity and require adjuvants that escort the antigens, target them to relevant sites and/or activate antigen-presenting cells for elicitation of protective immunity. Genetic fusions of antigens with bacterial adjuvants as the B subunit of the cholera toxin have been successful in inducing protective immunity of plant-made vaccines. In addition, several plant compounds, mainly plant defensive molecules as lectins and saponins, have shown strong adjuvant activities. The molecular diversity of the plant kingdom offers a vast source of non-bacterial compounds with adjuvant activity, which can be assayed in emerging plant manufacturing systems for the design of new plant vaccine formulations.

Immunogenicity of recombinant GP5 protein of porcine reproductive and respiratory syndrome virus expressed in tobacco plant

The aim of the study was to evaluate the immunogenicity of the ORF5-encoded major envelop glycoprotein 5 (GP5) of porcine reproductive and respiratory syndrome virus (PRRSV) expressed in tobacco plant as a potential pig oral vaccine in protection against PRRSV infection. Six-week-old PRRSV-free pigs were fed four times orally with 50g of chopped fresh GP5 transgenic tobacco leaves (GP5-T) (GP5 reaching 0.011% of total soluble protein) or wild-type tobacco leaves (W-T) each on days 0, 14, 28, and 42. Samples of serum, saliva, and peripheral blood mononuclear cells (PBMCs) were collected on days -1, 6, 13, 20, 27, 34, 41, and 48 after the initial oral vaccination. A similar vaccination-dependent gradual increase in the responses of serum and saliva anti-PRRSV total IgG and IgA, respectively, and in the levels of PRRSV-specific blastogenic response of PBMCs was seen in GP5-T-treated pigs; all statistically significant elevations occurred after the 2nd vaccination and were revealed after 20 days post-initial oral vaccination (DPIOV). Pigs fed on GP5-T also developed serum neutralizing antibodies to PRRSV at a titer of 1:4-1:8 after the 4th vaccination by 48 DPIOV. No detectable anti-PRRSV antibody responses and PRRSV-specific blastogenic response were seen in W-T-treated pigs. The present study has demonstrated that pigs fed on GP5-T could develop specific mucosal as well as systemic humoral and cellular immune responses against PRRSV. The results also support that transgenic plant as GP5-T can be an effective system for oral delivery of recombinant subunit vaccines in pigs.

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

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

Partial Characterization of a Vicilin-Like Glycoprotein from Seeds of Flowering Tobacco (Nicotiana sylvestris)

A vicilin-like glycoprotein from the seeds of Nicotiana sylvestris, flowering tobacco, has been identified using nanoLC/ESI-MS/MS. Sequences from a fragment of protein demonstrated homology with vicilins from other members of the Solanaceae family, notably potato (Solanum demissum). Reducing and nonreducing SDS-PAGE analyses of the identified protein indicated that fragments resulting from in situ proteolytic processing are joined by intrachain disulphide bonds. Staining with Con A lectin was specifically inhibited by mannose suggested the presence of -linked glycosylation which was confirmed by carbohydrate compositional analysis of PVDF-bound protein subunits. HPAEC-PAD analysis of the monosaccharides released from the glycoprotein by acid hydrolysis revealed glucosamine and mannose. -acetylglucosamine termination of attached oligosaccharides was further verified by inhibitable WGA lectin staining. Immunostaining of PVDF-bound N. sylvestris proteins with antibodies against G. max total protein demonstrated cross-staining at masses corresponding to fragments from the proteolytically processed protein subunits.

Heterologous and cell free protein expression systems

In recognition of the fact that a relatively small percentage of 'named' genes in databases have any experimental proof for their annotation, attention is shifting towards the more accurate assignment of functions to individual genes in a genome. The central objective will be to reduce our reliance on nucleotide or amino acid sequence similarities as a means to define the functions of genes and to annotate genome sequences. There are many unsolved technical difficulties associated with the purification of specific proteins from extracts of biological material, especially where the protein is present in low abundance, has multiple isoforms or is found in multiple post-translationally modified forms. The relative ease with which cDNAs can be cloned has led to the development of methods through which cDNAs from essentially any source can be expressed in a limited range of suitable host organisms, so that sufficient levels of the encoded proteins can be generated for functional analysis. Recently, these heterologous expression systems have been supplemented by more robust prokaryotic and eukaryotic cell-free protein synthesis systems. In this chapter, common host systems for heterologous expression are reviewed and the current status of cell-free expression systems will be presented. New approaches to overcoming the special problems encountered during the expression of membrane-associated proteins will also be addressed. Methodological considerations, including the characteristics of codon usage in the expressed DNA, peptide tags that facilitate subsequent purification of the expressed proteins and the role of post-translational modifications, are examined.

Highly efficient expression of Escherichia coli heat-labile enterotoxin B subunit in plants using potato virus X-based vector

A synthetic gene of the B-subunit of Escherichia coli heat-labile toxin, optimized for expression in plants, was designed and synthesized. The recombinant viral vector was constructed on the basis of potato virus X containing the LTB gene instead of the removed triple block of transport genes and the coat protein gene, which provides for LTB expression in plants. The vector is introduced into the plant cells during cell infiltration by agrobacteria incorporating a binary vector, the T-DNA region of which contains a cDNA copy of the recombinant viral genome. Under conditions of posttranscriptional gene silencing inhibition, the LTB yield in Nicotiana benthamiana plants is 1-2% of total soluble protein; in this case, LTB synthesized in plants forms pentameric complexes analogous to those found in the native toxin. The designed viral system of LTB transient expression can be used to obtain in plants a vaccine against enteropathogenic Escherichia coli.

Complete sequencing of potato virus X new strain genome and construction of viral vector for production of target proteins in plants

The complete nucleotide sequence of the genome of a new potato virus X (PVX) strain Tula isolated by us has been determined. Based on comparison of the PVX Tula nucleotide sequence with the sequences of 12 other PVX strains, this strain was assigned to the European cluster of PVX strains. Phylogenetic analysis revealed the same phylogeny for both full genome sequences and nucleotide sequences of polymerase and coat protein genes, suggesting that the PVX evolution did not involve recombination between different strains. The full-size cDNA copy of the PVX Tula genome was cloned and the accumulation of the viral coat protein in infected Nicotiana benthamiana was shown to be about twofold higher than for the PVX strain UK3. Based on the PVX Tula genome, a new vector which contained the target gene instead of the removed triple transport gene block and the coat protein gene has been constructed for expression of target proteins in plants. The productivity of the new vector was about 1.5-2-fold higher than the productivity of the vector of the same structure based on the standard PVX strain genome. The new viral vector can be used for superproduction of recombinant proteins in plants.

Use of a genetic cholera toxin B subunit/allergen fusion molecule as mucosal delivery system with immunosuppressive activity against Th2 immune responses

Induction of peripheral tolerance can be facilitated when the antigen is linked to the B subunit of cholera toxin (CTB), an efficient mucosal carrier. In the present study, a genetic fusion molecule of Bet v 1 and CTB was produced to test whether mucosal application of this construct would lead to suppression of Th2 responses. Intranasal pretreatment of BALB/c mice with rCTB-Bet v 1 prior to allergic sensitisation with the allergen significantly decreased IgE but markedly increased allergen-specific IgG2a levels in sera as well as IFN-gamma production of splenocytes. This Th1 shift was supported by an increased T-bet/GATA3 mRNA ratio. IL-5 production within the airways was suppressed after the pretreatment with rCTB-Bet v 1, while local allergen-specific IgA antibodies were markedly enhanced by pretreatment with the construct. Upregulation of Foxp3, IL-10 and TGF-beta mRNA expression was detected in splenocytes after pretreatment with unconjugated allergen but not with the fusion molecule, indicating that antigen conjugation to a mucosal carrier modifies the immunomodulating properties of an antigen/allergen.

Morphology and stability changes of recombinant TMV particles caused by a cysteine residue in the foreign peptide fused to the coat protein

In the studies of expressing various foreign peptides using a TMV-based vector, a portion of morphologically altered progeny viral particles from some recombinant TMV constructs were detected by transmission electron microscopy in the first systematically infected upper leaves, but not in the fully expanded inoculated leaves, from infected tobacco plants. Furthermore, in vitro stability of such recombinant TMV constructs were lower than those of the wild type and other recombinant TMV constructs able to form regular rod-shape virions, hence causing the lower yields of recombinant viral particles purified from the infected tobacco plants. Our studies revealed that the presence of a cysteine residue in the foreign peptides, regardless of its position and the peptide sequence, was directly related to changes in the morphology and stability of these TMV recombinants.

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

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

Development of Cucumber mosaic virus as a vector modifiable for different host species to produce therapeutic proteins

We have developed Cucumber mosaic virus (CMV) as a plant virus vector especially for production of pharmaceutical proteins. The CMV vector is a vector modifiable for different host plants and does not require further engineering steps. CMV contains three genomic RNA molecules (RNAs 1-3) necessary for infectivity. With this system, instead of creating different vector constructs for each plant we use, we take advantage of the formation of pseudrecombinants between two CMV isolates by simply reassembling a vector construct (RNA 2 base) and an RNA molecule containing the host determinant (mostly RNA 3). In this study, the gene for acidic fibroblast growth factor (aFGF), one of the human cytokines, was cloned under the control of the subgenomic promoter for RNA 4A of the CMV-based vector, C2-H1. Infected Nicotiana benthamiana plants produced aFGF at levels up to 5-8% of the total soluble protein. The tobacco-produced aFGF was purified, and its biological activity was confirmed. Using this system, which provides a versatile and viable strategy for the production of therapeutic proteins in plants, we also demonstrated a high level of aFGF in Glycine max (soybean) and Arabidopsis thaliana.

Improved uptake of plant-derived LTB-linked proteins in carp gut and induction of specific humoral immune responses upon infeed delivery

Oral vaccination of fish is an effortless and stress free immunisation method which can be used for almost any age. However, vaccination via the mucosal route does have disadvantages. For example, the vaccine may induce tolerance and has to be protected to escape digestion. Also the vaccine should be efficiently delivered to immune-competent cells in the gut or other lymphoid organs. In addition, it should be cost effective. Here we present a novel fish vaccination model using potato tubers as vaccine production and delivery system. The model vaccines discussed here include fusion proteins consisting of a gut adhesion molecule (LTB) and a viral peptide or green fluorescent protein (GFP) expressed in potato tubers. The adhesion molecule mediates binding to and uptake from the gut, whereas the viral peptide or GFP functions as model vaccine antigen provoking the induction of an immune response. We demonstrate that fusion to LTB facilitates an elevated uptake of the model vaccines in carp gut mucosa. The plant-derived fusion proteins also elicit a specific systemic humoral immune response upon oral application of crude tuber material incorporated into a standard dietary feed pellet. The data presented here show the promising potentials of the plant as a production system for oral vaccines in aquaculture and feed mediated immunisation of fish.

A recombinant chimera composed of R1 repeat region of Mycoplasma hyopneumoniae P97 adhesin with Escherichia coli heat-labile enterotoxin B subunit elicits immune response in mice

Swine mycoplasmal pneumonia (SMP), caused by fastidious bacterium Mycoplasma hyopneumoniae, is the most important respiratory disease in swine breeding. The commonly used vaccines to control this disease consist of inactivated whole cells (bacterins), whose production cost is high and the efficiency is limited. The objective of this study was to develop and to evaluate in BALB/c mice a recombinant subunit vaccine (rLTBR1) containing the R1 region of P97 adhesin of M. hyopneumoniae (R1) fused to the B subunit of the heat-labile enterotoxin of Escherichia coli (LTB). rLTBR1 formed functional oligomers that presented high affinity to GM1 ganglioside. Mice inoculated with rLTBR1 by intranasal (IN) or intramuscular (IM) route produced high levels of anti-R1 systemic and mucosal antibodies (IgA), which recognized the native P97. On the other hand, mice inoculated with the inactivated whole cell vaccine did not produce anti-R1 antibodies. The administration route influenced the modulation of the immune response by LTB, showing that IM rLTBR1 induced Th2-biased immune responses and IN rLTBR1 induced Th1-biased immune responses. rLTBR1 administrated by IN route also induced IFN-gamma secretion by lymphocytes. rLTBR1 may constitute a new strategy for preventing infection by M. hyopneumoniae and may have potential for developing vaccines against other infectious diseases as well.

Protection conferred by recombinant Yersinia pestis antigens produced by a rapid and highly scalable plant expression system

Plague is still an endemic disease in different regions of the world. Increasing reports of incidence, the discovery of antibiotic resistance strains, and concern about a potential use of the causative bacteria Yersinia pestis as an agent of biological warfare have highlighted the need for a safe, efficacious, and rapidly producible vaccine. The use of F1 and V antigens and the derived protein fusion F1-V has shown great potential as a protective vaccine in animal studies. Plants have been extensively studied for the production of pharmaceutical proteins as an inexpensive and scalable alternative to common expression systems. In the current study the recombinant plague antigens F1, V, and fusion protein F1-V were produced by transient expression in Nicotiana benthamiana by using a deconstructed tobacco mosaic virus-based system that allowed very rapid and extremely high levels of expression. All of the plant-derived purified antigens, administered s.c. to guinea pigs, generated systemic immune responses and provided protection against an aerosol challenge of virulent Y. pestis.

Vaccine and adjuvant activity of recombinant subunit B of E. coli enterotoxin produced in yeast

Escherichia coli heat-labile enterotoxin (LT) and cholera toxin (CT) have been studied intensively as vaccines against diseases caused by those bacteria and as adjuvants for mucosal vaccination. Two major problems interfere with the use of these promising adjuvants: their toxicity and the residual bacterial endotoxins mixed with the desired LT. In this study, subunit B of LT was expressed in Pichia pastoris yeast cells (yrLTB) and the recombinant protein was purified and concentrated by ion-exchange chromatography. The final yield of the recombinant protein was 5-8 mg/l induction medium. The molecule is in pentameric form and binds to GM1 gangliosides. When given orally to chickens, anti-LTB antibodies were produced, exhibiting its ability to cross the digestive system and induce an immune response. The adjuvant activity of yrLTB was proven by fusing it to viral protein 2 (VP2) of infectious bursal disease virus. Birds intramuscularly vaccinated with this molecule exhibit 70-100% protection, in a dose-response-dependent manner. This method eliminated the bacterial endotoxins and enabled the production of large quantities of LTB. Expression in a eukaryotic system allows the production of fusion proteins that require post-translational modifications. This may allow oral vaccination with a protein fused to yrLTB. The approach described in this study will enable the efficient production of a non-toxic, eukaryotically expressed enterotoxin as a vaccine against the toxin itself or as a carrier or adjuvant for foreign vaccine molecules.

Development and assessment of a potato virus X-based expression system with improved biosafety

Over the last decade, plant virus-based vectors have been developed and successfully exploited for high-yield production of heterologous proteins in plants. However, widespread application of recombinant viruses raises concerns about possible risks to the environment. One of the primary safety issues that must be considered is the uncontrolled spread of the genetically engineered virus from experimental plants to susceptible weeds or crops. Using a movement-deficient Potato virus X (PVX)-based transient gene expression vector which harbors the beta-glucuronidase (gus) gene, we established a plant viral expression system that provides containment of the recombinant virus and allows for safe and efficient protein production. By deletion of the viral 25k movement protein gene, systemic spread of the modified virus in non-transgenic Nicotiana benthamiana plants was successfully inhibited. In transgenic N. benthamiana plants expressing the 25K viral movement protein, this deficiency was complemented, thus resulting in systemic infection with the movement-deficient virus. While no differences in virus spread and accumulation were observed compared to infection caused by wild-type PVX in non-transgenic plants, the movement protein transgenic plants exhibited none of the normal symptoms of viral infection. Several biosafety aspects were investigated including the potential for recombination between the defective virus and the movement protein transgene, as well as complementation effects in non-transgenic plants doubly infected with the defective and the wild-type virus. Furthermore, the applicability of the safety system for the production of heterologous proteins was evaluated with gus as a model gene. With respect to the stability of the gus insert and the expression level of the GUS protein, there were no differences between the novel system developed and the conventional PVX-based expression system.

High-level expression of the neutralizing epitope of porcine epidemic diarrhea virus by a tobacco mosaic virus-based vector

Porcine epidemic diarrhea virus (PEDV) causes acute enteritis in pigs of all ages and is often fatal for neonates. A tobacco mosaic virus (TMV)-based vector was utilized for the expression of a core neutralizing epitope of PEDV (COE) for the development of a plant-based vaccine. In this study, the coding sequence of a COE gene was optimized based on the modification of codon usage in tobacco plant genes and the removal of mRNA-destabilizing sequences. The native and synthetic COE genes were cloned into TMV-based vectors and expressed in tobacco plants. The recombinant COE protein constituted up to 5.0% of the total soluble protein in the leaves of tobacco plants infected with the TMV-based vector containing synthetic COE gene, which was approximately 30-fold higher than that in tobacco plants infected with TMV-based vector containing a native COE gene. Therefore, this result indicates that the plant viral expression system with a synthetic gene optimized for plant expression is suitable to produce a large amount of antigen for the development of plant-based vaccine rapidly.