Transgenic plants for the production of veterinary vaccines

Transient expression of heat- and acid-resistant foot-and-mouth disease virus P1-2A mutants in Nicotiana benthamiana

Recombinant foot-and-mouth disease virus-like particles (VLPs) can be expressed in a number of expression systems including plants. However, yields in plants have formerly been shown to be low, possibly due to their acid and/or heat lability, previously shown to affect VLP yields produced in other systems. This work describes the introduction of mutations into the FMDV structural protein-encoding gene (P1-2A) which have been previously shown to increase acid and thermostability. VLPs expressed in plants using the mutant constructs had negative rather than positive effects on yield and temperature and acid stability compared to the control.

Novel expression of immunogenic foot-and-mouth disease virus-like particles in Nicotiana benthamiana

Foot-and-mouth disease (FMD) is a highly contagious disease of cloven-hoofed animals and is endemic in Africa, parts of South America and southern Asia. The causative agent, FMD virus (FMDV) is a member of the genus Aphthovirus, family Picornaviridae. Vaccines currently used against FMDV are chemically inactivated virus strains which are produced under high-level biocontainment facilities, thus raising their cost. The development of recombinant FMDV vaccines has focused predominantly on FMDV virus-like particle (VLP) subunit vaccines for which promising results have been achieved. These VLPs are attractive candidates because they avoid the use of live virus in production facilities, but conserve the complete repertoire of conformational epitopes of the virus. Recombinant FMDV VLPs are formed by the expression and assembly of the three structural proteins VP0, VP1 and VP3. This can be attained by co-expression of the three individual structural capsid proteins or by co-expression of the viral capsid precursor P1-2A together with the viral protease 3C. The latter proteolytically cleaves P1-2A into the respective structural proteins. These VLPS are produced in mammalian or insect cell culture systems, which are expensive and can be easily contaminated. Plants, such as Nicotiana benthamiana, potentially provide a more cost-effective and very highly scalable platform for recombinant protein and VLP production. In this study, P1-2A was transiently expressed in N. benthamiana alone, without the 3C protease. Surprisingly, there was efficient processing of the P1-2A polyprotein into its component structural proteins, and subsequent assembly into VLPs. The yield was ∼0.030μg per gram of fresh leaf material. Partially purified VLPs were preliminarily tested for immunogenicity in mice and shown to stimulate the production of FMDV-specific antibodies. This study, has important implications for simplifying the production and expression of potential vaccine candidates against FMDV in plants, in the absence of 3C expression.

Foot-and-mouth disease: overview of motives of disease spread and efficacy of available vaccines

Control and prevention of foot and mouth disease (FMD) by vaccination remains unsatisfactory in endemic countries. Indeed, consistent and new FMD epidemics in previously disease-free countries have precipitated the need for a worldwide control strategy. Outbreaks in vaccinated animals require that a new and safe vaccine be developed against foot and mouth virus (FMDV). FMDV can be eradicated worldwide based on previous scientific information about its spread using existing and modern control strategies.

Mucosal Vaccines from Plant Biotechnology

The use of plants for production of recombinant proteins has evolved over the past 25 years. The first plant-based vaccines were expressed in stably transgenic plants, with the idea to conveniently deliver “edible vaccines” by ingestion of the antigen-containing plant material. These systems provided a proof of concept that oral delivery of vaccines in crude plant material could stimulate antigen-specific serum and mucosal antibodies. Transgenic grains like rice in particular provide a stable and robust vehicle for antigen delivery. However, some issues exist with stably transgenic plants, including relatively low expression levels and regulatory issues. Thus, many recent studies use transient expression with plant viral vectors to achieve rapid high expression in Nicotiana benthamiana, followed by purification of antigen and intranasal delivery for effective stimulation of mucosal immune responses.

Immunocompetent truncated E2 glycoprotein of bovine viral diarrhea virus (BVDV) expressed in Nicotiana tabacum plants: a candidate antigen for new generation of veterinary vaccines

The bovine viral diarrhea virus (BVDV) is the etiological agent responsible for a wide spectrum of clinical diseases in cattle. The glycoprotein E2 is the major envelope protein of this virus and the strongest inductor of the immune response. There are several available commercial vaccines against bovine viral diarrhea (BVD), which show irregular performances. Here, we report the use of tobacco plants as an alternative productive platform for the expression of the truncated version of E2 glycoprotein (tE2) from the BVDV. The tE2 sequence, lacking the transmembrane domain, was cloned into the pK7WG2 Agrobacterium binary vector. The construct also carried the 2S2 Arabidopsis thaliana signal for directing the protein into the plant secretory pathway, the Kozak sequence, an hexa-histidine tag to facilitate protein purification and the KDEL endoplasmic reticulum retention signal. The resulting plasmid (pK-2S2-tE2-His-KDEL) was introduced into Agrobacterium tumefaciens strain EHA101 by electroporation. The transformed A. tumefaciens was then used to express tE2 in leaves of Nicotiana tabacum plants. Western blot and ELISA using specific monoclonal antibodies confirmed the presence of the recombinant tE2 protein in plant extracts. An estimated amount of 20 μg of tE2 per gram of fresh leaves was regularly obtained with this plant system. Injection of guinea pigs with plant extracts containing 20 μg of rtE2 induced the production of BVDV specific antibodies at equal or higher levels than those induced by whole virus vaccines. This is the first report of the production of an immunocompetent tE2 in N. tabacum plants, having the advantage to be free of any eventual animal contaminant.

Expression of the nucleocapsid protein of porcine reproductive and respiratory syndrome virus in soybean seed yields an immunogenic antigenic protein

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus (PRRSV), is a serious disease of swine and contributes to severe worldwide economic losses in swine production. Current vaccines against PRRS rely on the use of an attenuated-live virus; however, these are unreliable. Thus, alternative effective vaccines against PRRS are needed. Plant-based subunit vaccines offer viable, safe, and environmentally friendly alternatives to conventional vaccines. In this study, efforts have been undertaken to develop a soybean-based vaccine against PRRSV. A construct carrying a synthesized PRRSV-ORF7 antigen, nucleocapsid N protein of PRRSV, has been introduced into soybean, Glycine max (L.) Merrill. cvs. Jack and Kunitz, using Agrobacterium-mediated transformation. Transgenic plants carrying the sORF7 transgene have been successfully generated. Molecular analyses of T(0) plants confirmed integration of the transgene and transcription of the PRRSV-ORF7. Presence of a 15-kDa protein in seeds of T(1) transgenic lines was confirmed by Western blot analysis using PRRSV-ORF7 antisera. The amount of the antigenic protein accumulating in seeds of these transgenic lines was up to 0.65% of the total soluble protein (TSP). A significant induction of a specific immune response, both humoral and mucosal, against PRRSV-ORF7 was observed following intragastric immunization of BALB/c female mice with transgenic soybean seeds. These findings provide a 'proof of concept', and serve as a critical step in the development of a subunit plant-based vaccine against PRRS.

Plant expressed EtMIC2 is an effective immunogen in conferring protection against chicken coccidiosis

Coccidiosis is an economically important disease affecting poultry industry and remains one of the major problems globally. Developing a cost effective sub-unit vaccine may help mitigate loss in the industry. Here, we report expressing one of the microneme proteins, EtMIC2 from Eimeria tenella in tobacco using Agrobacterium-mediated transient expression. The ability of plant expressed recombinant EtMIC2 in eliciting both humoral and cell-mediated immune responses were measured in the immunized birds. The protective efficacy in the vaccinated birds against a homologous challenge was also evaluated. Birds immunized with plant expressed EtMIC2 showed good sero-conversion, reduced oocyst output and increased weight gain when compared to control birds. Our data indicate that use of plant expressed recombinant EtMIC2 in birds was safe and had the potential in imparting partial protection in chickens against homologous challenge.

Vaccine production in plant systems--an aid to the control of viral diseases in domestic animals: a review

Plants have been identified as promising expression systems for the commercial production of vaccines because of the possibility of introducing exogenous genes into them, which permits the development of a new generation of biological products called edible vaccines. The advantages of oral vaccines of this new type are that they induce mucosal, humoral, cellular and protective immunity, they are cheaper, easier to store, distribute and administer, they do not require cold chain management, and some species can be stored for long periods of time without any spoilage and may be administered as purified proteins. Owing to these benefits, plant-produced vaccines represent a valuable option for animal health. The aim of this paper is to present a review of plant-produced vaccines against viruses affecting domestic animals. Some aspects of the feasibility of their use and the immune response elicited by such vaccines are also discussed, as the balance between tolerance and immunogenicity is a major concern for the use of plant-based vaccines.

Plant-made vaccines for humans and animals

The concept of using plants to produce high-value pharmaceuticals such as vaccines is 20 years old this year and is only now on the brink of realisation as an established technology. The original reliance on transgenic plants has largely given way to transient expression; proofs of concept for human and animal vaccines and of efficacy for animal vaccines have been established; several plant-produced vaccines have been through Phase I clinical trials in humans and more are scheduled; regulatory requirements are more clear than ever, and more facilities exist for manufacture of clinic-grade materials. The original concept of cheap edible vaccines has given way to a realisation that formulated products are required, which may well be injectable. The technology has proven its worth as a means of cheap, easily scalable production of materials: it now needs to find its niche in competition with established technologies. The realised achievements in the field as well as promising new developments will be reviewed, such as rapid-response vaccines for emerging viruses with pandemic potential and bioterror agents.

Oral immunogenicity and protective efficacy in mice of transgenic rice plants producing a vaccine candidate antigen (As16) of Ascaris suum fused with cholera toxin B subunit

Cereal crops such as maize and rice are considered attractive for vaccine production and oral delivery. Here, we evaluated the rice Oryza sativa for production of As16-an antigen protective against the roundworm Ascaris suum. The antigen was produced as a chimeric protein fused with cholera toxin B subunit (CTB), and its expression level in the endosperm reached 50 microg/g seed. Feeding the transgenic (Tg) rice seeds to mice elicited an As16-specific serum antibody response when administered in combination with cholera toxin (CT) as the mucosal adjuvant. Although omitting the adjuvant from the vaccine formulation resulted in failure to develop the specific immune response, subcutaneous booster immunization with bacterially expressed As16 induced the antibody response, indicating priming capability of the Tg rice. Tg rice/CT-fed mice orally administered A. suum eggs had a lower lung worm burden than control mice. This suggests that the rice-delivered antigen functions as a prophylactic edible vaccine for controlling parasitic infection in animals.

Dissected effect of a transit peptide of the ADP-glucose pyrophosphorylase gene from sweetpotato (ibAGP2) in increasing foreign protein accumulation

The transit peptide sequence of ibAGP2 (TP2) was found to be capable of targeting protein into the chloroplast in the Arabidopsis protoplasts. TP2 was fused to a beta-glucuronidase (GUS) reporter gene and expressed in Arabidopsis under the control of the ibAGP2 promoter with the aim of dissecting the effect of the transit peptide in elevating foreign protein accumulation in the transgenic plant. beta-glucuronidase protein levels were determined at three different developmental stages and in assorted tissues. TP2 dramatically elevated GUS protein accumulation regardless of developmental stage, but the level of the enhancing effect was developmental stage-dependent. This enhancing effect was strongest at the seedling stage (16-fold) and relatively moderate at the vegetative (tenfold) and reproductive (11-fold) stages. TP2 also elevated GUS protein accumulation to varying degrees (4 to 19-fold) in assorted tissues, with the effect being highest in the primary inflorescence stem and petiole (19-fold) and weakest in the root (fourfold). Although TP2 also increased GUS mRNA levels, the increased levels were not large enough to account for the elevated GUS protein levels, suggesting that the enhancing effect of TP2 does not solely result from increased levels of transcripts. Taken together, our results reveal that the TP2 significantly increased the levels of protein accumulation and that its effectiveness was developmental stage- and tissue-type-dependent in transgenic Arabidopsis. Possible differential targeting efficiencies of different transit peptides are discussed.

Induction of a protective antibody response to FMDV in mice following oral immunization with transgenic Stylosanthes spp. as a feedstuff additive

The expression of antigens in transgenic plants has increasingly been used as an alternative to the classical methodologies for the development of experimental vaccines, and it remains one of the real challenges in this field to use transgenic plant-based vaccines effectively as feedstuff additives. We report herein the development of a new oral immunization system for foot and mouth disease with the structural protein VP1 of the foot and mouth disease virus (FMDV) produced in transgenic Stylosanthes guianensis cv. Reyan II. The transgenic plantlets were identified by polymerase chain reaction (PCR), Southern blotting, and northern blotting; and the production of VP1 protein in transgenic plants was confirmed and quantified by western blotting and enzyme-linked immunosorbent assays (ELISA). Six transformed lines were obtained, and the level of the expressed protein was 0.1-0.5% total soluble protein (TSP). Mice that were orally immunized using studded feedstuff mixed with desiccated powder of the transgenic plants developed a virus-specific immune response to the structural VP1 and intact FMDV particles. To our knowledge, this is the first report of transgenic plants expressing the antigen protein of FMDV as feedstuff additives that has demonstrated the induction of a protective systemic antibody response in animals. These results support the feasibility of producing edible vaccines from transgenic forage plants, and provide proof of the possibility of using plant-based vaccines as feedstuff additives.

Expression of the rabies virus nucleoprotein in plants at high-levels and evaluation of immune responses in mice

Transgenic plants have been employed successfully as a low-cost system for the production of therapeutically valuable proteins including antibodies, antigens and hormones. Here, we report expression of a full-length nucleoprotein gene of rabies virus in transgenic tomato plants. The nucleoprotein was also transiently expressed in Nicotiana benthamiana plants by agroinfiltration. In both cases, the nucleoprotein was expressed at high levels, 1-5% of total soluble protein in tomato and 45% in N. benthamiana. Previously, only epitopes of the nucleoprotein had been expressed in plants. The presence and expression of the transgene was verified by PCR, Southern, northern and western blots. Mice were immunized both intraperitoneally (i.p.) and orally with tomato protein extracts containing the N protein induced the production of antibodies. The antibody titer of mice immunized i.p., was at least four times higher than that of mice immunized orally. These results were reflected in the challenge experiments where i.p.-immunized mice were partially protected against a peripheral virus challenge whereas orally immunized mice were not. This protection was comparable to that obtained in previous experiments employing different expression systems. Work is in progress to express both G and N proteins in transgenic plants and evaluate protection in mice.

Transgenic plants for animal health: plant-made vaccine antigens for animal infectious disease control

A variety of plant species have been genetically modified to accumulate vaccine antigens for human and animal health and the first vaccine candidates are approaching the market. The regulatory burden for animal vaccines is less than that for human use and this has attracted the attention of researchers and companies, and investment in plant-made vaccines for animal infectious disease control is increasing. The dosage cost of vaccines for animal infectious diseases must be kept to a minimum, especially for non-lethal diseases that diminish animal welfare and growth, so efficient and economic production, storage and delivery are critical for commercialization. It has become clear that transgenic plants are an economic and efficient alternative to fermentation for large-scale production of vaccine antigens. The oral delivery of plant-made vaccines is particularly attractive since the expensive purification step can be avoided further reducing the cost per dose. This review covers the current status of plant-produced vaccines for the prevention of disease in animals and focuses on barriers to the development of such products and methods to overcome them.

Improving containment strategies in biopharming

This review examines the challenges of segregating biopharmed crops expressing pharmaceutical or veterinary agents from mainstream crops, particularly those destined for food or feed use. The strategy of using major food crops as production vehicles for the expression of pharmaceutical or veterinary agents is critically analysed in the light of several recent episodes of contamination of the human food chain by non-approved crop varieties. Commercially viable strategies to limit or avoid biopharming intrusion into the human food chain require the more rigorous segregation of food and non-food varieties of the same crop species via a range of either physical or biological methods. Even more secure segregation is possible by the use of non-food crops, non-crop plants or in vitro plant cultures as production platforms for biopharming. Such platforms already under development range from outdoor-grown Nicotiana spp. to glasshouse-grown Arabidopsis, lotus and moss. Amongst the more effective methods for biocontainment are the plastid expression of transgenes, inducible and transient expression systems, and physical containment of plants or cell cultures. In the current atmosphere of heightened concerns over food safety and biosecurity, the future of biopharming may be largely determined by the extent to which the sector is able to maintain public confidence via a more considered approach to containment and security of its plant production systems.

Production of vaccines and therapeutic antibodies for veterinary applications in transgenic plants: an overview

During the past two decades, antibodies, antibody derivatives and vaccines have been developed for therapeutic and diagnostic applications in human and veterinary medicine. Numerous species of dicot and monocot plants have been genetically modified to produce antibodies or vaccines, and a number of diverse transformation methods and strategies to enhance the accumulation of the pharmaceutical proteins are now available. Veterinary applications are the specific focus of this article, in particular for pathogenic viruses, bacteria and eukaryotic parasites. We focus on the advantages and remaining challenges of plant-based therapeutic proteins for veterinary applications with emphasis on expression platforms, technologies and economic considerations.

Plant-made vaccines: biotechnology and immunology in animal health

The use of plants as production systems for vaccine antigens has been actively investigated over the last 15 years. The original research focused on the value of this expression system for oral delivery based on the hypothesis that plant-expressed antigens would be more stable within the digestive tract and would allow for the use of the oral route of administration to stimulate a mucosal immune response. However, while first conceived for utility via the oral route, plant-made antigens have also been studied as classical immunogens delivered via a needle to model animal systems. Antigens have been expressed in a number of whole plant and cell culture systems. Several alternative expression platforms have been developed to increase expression of antigens or to elicit preferred immunological responses. The biotechnological advances in plant expression and the immunological testing of these antigens will be reviewed in this paper focusing primarily on diseases of livestock and companion animals.

Expression of the Newcastle disease virus fusion protein in transgenic maize and immunological studies

Transgenic plants have been employed successfully as a low-cost system for the production of therapeutically valuable proteins, including antibodies, antigens and hormones. Here, we report the expression of the fusion (F) gene of the Newcastle disease virus (NDV) in transgenic maize plants. The expression of the transgene, driven by the maize ubiquitin promoter, caused accumulation of the F protein in maize kernels. The presence of the transgene was verified by Southern and western blots. Feeding chickens with kernels containing the F protein induced the production of antibodies, which conferred protection against a viral challenge. This protection was comparable to that conferred by a commercial vaccine. Possible uses of this plant-based F protein as a potential mucosal vaccine are discussed.

Expression of avian reovirus sigmaC protein in transgenic plants

Avian reovirus (ARV) structural protein, sigmaC encoded by S1 genome segment, is the prime candidate to become a vaccine against ARV infection. Two plant nuclear expression vectors with expression of sigmaC-encoding gene driven by CaMV 35S promoter and rice actin promoter were constructed, respectively. Agrobacterium containing the S1 expression constructs were used to transform alfalfa, and transformants were selected using hygromysin. The integration of S1 transgene in alfalfa chromosome was confirmed by PCR and histochemical GUS staining. Western blot analysis using antiserum against sigmaC was carried out to determine the expression of sigmaC protein in transgenic alfalfa cells. The highest expression levels of sigmaC protein in the cellular extracts of selected p35S-S1 and pAct1-S1 transgenic alfalfa lines were 0.008% and 0.007% of the total soluble protein, respectively. The transgenic alfalfa cells with expression of sigmaC protein pave the way for the development of edible vaccine.