The case for plant-made veterinary immunotherapeutics

Development of a Plant-Expressed Subunit Vaccine against Brucellosis

Brucellosis is an important bacterial disease of livestock and the most common zoonotic disease. The current vaccines are effective but unsafe, as they result in animal abortions and are pathogenic to humans. Virus-like particles are being investigated as molecular scaffolds for foreign antigen presentation to the immune system. Here, we sought to develop a new-generation vaccine by presenting selected Brucella melitensis T cell epitopes on the surface of Orbivirus core-like particles (CLPs) and transiently expressing these chimeric particles in Nicotiana benthamiana plants. We successfully demonstrated the assembly of five chimeric CLPs in N. benthamiana plants, with each CLP presenting a different T cell epitope. The safety and protective efficacy of three of the highest-yielding CLPs was investigated in a mouse model of brucellosis. All three plant-expressed chimeric CLPs were safe when inoculated into BALB/c mice at specific antigen doses. However, only one chimeric CLP induced protection against the virulent Brucella strain challenge equivalent to the protection induced by the commercial Rev1 vaccine. Here, we have successfully shown the assembly, safety and protective efficacy of plant-expressed chimeric CLPs presenting B. melitensis T cell epitopes. This is the first step in the development of a safe and efficacious subunit vaccine against brucellosis.

A Plant-Derived Maternal Vaccine against Porcine Epidemic Diarrhea Protects Piglets through Maternally Derived Immunity

Newborn piglets are susceptible to a highly contagious enteritis caused by the porcine epidemic diarrhea virus (PEDV), associated with high levels of mortality worldwide. There is pressing need for a rapid, safe, and cost-effective vaccine to safeguard pigs from getting infected by PEDV. PEDV belongs to the coronavirus family and is characterized by high levels of mutability. The primary goal of a PEDV vaccine is to provide immunity to newborn piglets through vaccination of sows. Plant-based vaccines are becoming more popular because they have low manufacturing costs, are easily scalable, have high thermostability, and a long shelf life. This is in contrast to conventional vaccines which include inactivated, live, and/or recombinant types that can be expensive and have limited ability to respond to rapidly mutating viruses. The binding of the virus to host cell receptors is primarily facilitated by the N-terminal subunit of the viral spike protein (S1), which also contains several epitopes that are recognized by virus-neutralizing antibodies. As a result, we generated a recombinant S1 protein using a plant-based vaccine platform. We found that the recombinant protein was highly glycosylated, comparable to the native viral antigen. Vaccination of pregnant sows at four and two weeks before farrowing led to the development of humoral immunity specific to S1 in the suckling piglets. In addition, we noted significant viral neutralization titers in both vaccinated sows and piglets. When challenged with PEDV, piglets born from vaccinated sows displayed less severe clinical symptoms and significantly lower mortality rates compared to piglets born from non-vaccinated sows.

Plant production of a virus-like particle-based vaccine candidate against porcine reproductive and respiratory syndrome

Porcine reproductive and respiratory syndrome (PRRS) is a disease leading to spontaneous abortions and stillbirths in sows and lowered life quality and expectancy in growing pigs. PRRS is prevalent worldwide and has significant economic impacts to swine industries around the globe. Co-expression of the two most abundant proteins in the viral envelope, the matrix protein (M) and glycosylated protein 5 (GP5), can produce a neutralizing immune response for the virus providing a potentially effective subunit vaccine against the disease, but these proteins are difficult to express. The goal of this research was to display antigenic portions of the M and GP5 proteins on the surface of tobacco mosaic virus-like particles. A modified tobacco mosaic virus coat protein (TMVc) was transiently expressed in Nicotiana benthamiana leaves and targeted to three subcellular compartments along the secretory pathway to introduce glycosylation patterns important for M-GP5 epitope immunogenicity. We found that accumulation levels in the apoplast were similar to the ER and the vacuole. Because glycans present on plant apoplastic proteins are closest to those present on PRRSV proteins, a TMVc-M-GP5 fusion construct was targeted to the apoplast and accumulated at over 0.5 mg/g of plant fresh weight. TMVc virus-like particles self-assembled in plant cells and surface-displayed the M-GP5 epitope, as visualized by transmission electron microscopy and immunogold localization. These promising findings lay the foundation for immunogenicity and protective-immunity studies in animals to examine the efficacy of this vaccine candidate as a measure to control PRRS.

Generation of multi-layered protein bodies in N. benthamiana for the encapsulation of vaccine antigens

The ability of plants to assemble particulate structures such as virus-like particles and protein storage organelles allows the direct bioencapsulation of recombinant proteins during the manufacturing process, which holds promise for the development of new drug delivery vehicles. Storage organelles found in plants such as protein bodies (PBs) have been successfully used as tools for accumulation and encapsulation of recombinant proteins. The fusion of sequences derived from 27-kDa-γ-zein, a major storage protein of maize, with a protein of interest leads to the incorporation of the chimeric protein into the stable and protected environment inside newly induced PBs. While this procedure has proven successful for several, but not all recombinant proteins, the aim of this study was to refine the technology by using a combination of PB-forming proteins, thereby generating multi-layered protein assemblies in N. benthamiana. We used fluorescent proteins to demonstrate that up to three proteinaceous components can be incorporated into different layers. In addition to 27-kDa-γ-zein, which is essential for PB initiation, 16-kDa-γ-zein was identified as a key element to promote the incorporation of a third zein-component into the core of the PBs. We show that a vaccine antigen could be incorporated into the matrix of multi-layered PBs, and the protein microparticles were characterized by confocal and electron microscopy as well as flow cytometry. In future, this approach will enable the generation of designer PBs that serve as drug carriers and integrate multiple components that can be functionalized in different ways.

Plant crude extracts containing oligomeric hemagglutinins protect chickens against highly Pathogenic Avian Influenza Virus after one dose of immunization

Highly pathogenic avian influenza viruses (HPAIV) have been responsible for causing several severe outbreaks across the world. To protect poultry farms and to prevent the possible spread of new influenza pandemics, vaccines that are both efficacious and low-cost are in high demand. We produced stable, large hemagglutinin H5 oligomers in planta by the specific interaction between S•Tag and S•Protein. H5 oligomers combined via S•Tag::S•Protein interaction in plant crude extracts induced strong humoral immune responses, strong neutralizing antibody responses, and resistance in chickens after challenge with a wild type HPAIV H5 virus strain. In all three parameters, plant crude extracts with H5 oligomers induced better responses than crude extracts containing trimers. The neutralizing antibodies induced by by two-dose and one dose immunization with an adjuvanted crude extract containing H5 oligomer protected vaccinated chickens from two lethal H5N1 virus strains with the efficiency of 92% and 100%, respectively. Following housing vaccinated chickens together with ten non-immunized chickens, only one of these chickens had detectable levels of the H5N1 virus. To facilitate the easy storage of a candidate vaccine, the H5 oligomer crude extracts were mixed with adjuvants and stored for 3.5 and 5.5 months at 4 °C, and chickens were immunized with these crude extracts. All these vaccinated chickens survived after a lethal H5N1 virus challenge. H5 oligomer crude extracts are comparable to commercial vaccines as they also induce strong virus-neutralizing immune responses following the administration of a single dose. The cost-effective production of plant crude extract vaccine candidates and the high stability after long-term storage will enable and encourage the further exploration of this technology for veterinary vaccine development.

Modern vaccine strategies for emerging zoonotic viruses

INTRODUCTION

The significant increase in the emergence of notable zoonotic viruses in the previous decades has become a serious concern to global public health. Ninety-nine percent of infectious diseases have originated from zoonotic viruses with immense potential for dissemination, infecting the susceptible population completely lacking herd immunity.

AREAS COVERED

Zoonotic viruses appear in the last two decades as a major health threat either newly evolved or previously present with elevated prevalence in the last few years are selected to explain their current prophylactic measures. In this review, modern generation vaccines including viral vector vaccines, mRNA vaccines, DNA vaccines, synthetic vaccines, virus-like particles, and plant-based vaccines are discussed with their benefits and challenges. Moreover, the traditional vaccines and their efficacy are also compared with the latest vaccines.

EXPERT OPINION

The emergence and reemergence of viruses that constantly mutate themselves have greatly increased the chance of transmission and immune escape mechanisms in humans. Therefore, the only possible solution to prevent viral infection is the use of vaccines with improved safety profile and efficacy, which becomes the basis of modern generation vaccines.

Prevention of Shiga toxin 1-caused colon injury by plant-derived recombinant IgA

Immunoglobulin A (IgA) is a candidate antibody for oral passive immunization against mucosal pathogens like Shiga toxin-producing Escherichia coli (STEC). We previously established a mouse IgG monoclonal antibody (mAb) neutralizing Shiga toxin 1 (Stx1), a bacterial toxin secreted by STEC. We designed cDNA encoding an anti-Stx1 antibody, in which variable regions were from the IgG mAb and all domains of the heavy chain constant region from a mouse IgA mAb. Considering oral administration, we expressed the cDNA in a plant expression system aiming at the production of enough IgA at low cost. The recombinant-IgA expressed in Arabidopsis thaliana formed the dimeric IgA, bound to the B subunit of Stx1, and neutralized Stx1 toxicity to Vero cells. Colon injury was examined by exposing BALB/c mice to Stx1 via the intrarectal route. Epithelial cell death, loss of crypt and goblet cells from the distal colon were observed by electron microscopy. A loss of secretory granules containing MUC2 mucin and activation of caspase-3 were observed by immunohistochemical methods. Pretreatment of Stx1 with the plant-based recombinant IgA completely suppressed caspase-3 activation and loss of secretory granules. The results indicate that a plant-based recombinant IgA prevented colon damage caused by Stx1 in vivo.

An Improved Immunochromatographic Strip Based on Plant-Derived E2 for Detection of Antibodies against Classical Swine Fever Virus

Vaccination is an effective method to control the spread of classical swine fever virus (CSFV), which is a major cause of economic losses to the swine industry. Although serological detection assays are commonly used to assess immune status, current methods for monitoring of antibodies (Abs) are time-consuming, expensive, and require cell culture and virus manipulation. To address these problems, the E2 protein of CSFV was expressed in transgenic rice seeds as a labeled antigen for the development of an immunochromatographic test strip (ICTS) for rapid, precise, and cost-effective detection of Abs. The ICTS has a reasonable sensitivity of 1:128,000 for detection of serum Abs against CSFV and no cross-reactivity with Abs of other porcine viruses. The similarity of the results between the proposed ICTS and a commercial enzyme-linked immunosorbent assay was 94.1% (128/136) for detection of serum Abs from immunized animals and 92.3% (72/78) for detection of maternally derived Abs. The proposed assay was successfully used to monitor Abs against E2 of both pigs and rabbits immunized with a live attenuated vaccine or an E2 subunit vaccine. The results confirmed that the ICTS can be applied to detect Ab levels in animals with different immunological backgrounds. The ICTS based on plant-derived E2 is a relatively inexpensive, rapid, and accurate assay for detection of Abs against CSFV and avoids the risk of contamination by animal products. IMPORTANCE The E2 protein of classical swine fever virus (CSFV) was expressed in transgenic rice endosperms as a diagnostic antigen for use with a rapid colloidal gold assay for the detection of antibodies (Abs) against CSFV. This improved test was used to monitor Abs against the E2 protein in both pigs and rabbits immunized with a live attenuated vaccine or E2 subunit vaccine. The assay successfully detected Ab levels in serum samples from piglets with different immunological backgrounds. In contrast to current E2 protein-based diagnostic methods using Escherichia coli or insect cells as expression systems, plant-derived E2 avoids the limitations of low immunogenicity of eukaryotic expression systems and potential contamination of fetal bovine serum with bovine viral diarrhea virus in cell culture.

Microparticles and Nanoparticles from Plants-The Benefits of Bioencapsulation

The efficacy of drugs and vaccines depends on their stability and ability to interact with their targets in vivo. Many drugs benefit from encapsulation, which protects them from harsh conditions and allows targeted delivery and controlled release. Although many encapsulation methods are inexpensive, such as the formulation of tablets for oral delivery, others require complex procedures that add significantly to production costs and require low-temperature transport and storage, making them inaccessible in developing countries. In this review we consider the benefits of encapsulation technologies based on plants. Plant-derived biopolymers such as starch and the maize storage protein zein are already used as protective coatings, but plant cells used as production host provide natural in vivo bioencapsulation that survives passage through the stomach and releases drugs in the intestine, due to the presence of microbes that can digest the cell wall. Proteins can also be encapsulated in subcellular compartments such as protein bodies, which ensure stability and activity while often conferring additional immunomodulatory effects. Finally, we consider the incorporation of drugs and vaccines into plant-derived nanoparticles assembled from the components of viruses. These are extremely versatile, allowing the display of epitopes and targeting peptides as well as carrying cargoes of drugs and imaging molecules.

Plant molecular farming for the production of valuable proteins - Critical evaluation of achievements and future challenges

Recombinant proteins play an important role in many areas of our lives. For example, recombinant enzymes are used in the food and chemical industries and as high-quality proteins for research, diagnostic and therapeutic applications. The production of recombinant proteins is still dominated by expression systems based on microbes and mammalian cells, although the manufacturing of recombinant proteins in plants - known as molecular farming - has been promoted as an alternative, cost-efficient strategy for three decades. Several molecular farming products have reached the market, but the number of success stories has been limited by industrial inertia driven by perceptions of low productivity, the high cost of downstream processing, and regulatory hurdles that create barriers to translation. Here, we discuss the technical and economic factors required for the successful commercialization of molecular farming, and consider potential future directions to enable the broader application of production platforms based on plants.

Plant production and functional characterization of catfish interleukin-22 as a natural immune stimulant for aquaculture fish

As the world population increases and wild caught fisheries decline, aquaculture offers a sustainable solution addressing this global challenge. However, disease management remains difficult. With limited options, there is a need for innovative solutions. The cytokine interleukin-22 (IL-22) has emerged as a possible therapeutic target for fish and has been correlated with protection under pathogen challenge. Plant-based production systems have the potential to effectively manufacture and bring unique efficacy-enhancing features to the aquaculture industry; namely, the advantages of low cost for this commodity market, ready scalability, and reduced environmental impact. Catfish IL-22 produced at significant yield and purity highlights the use of plants as a promising production platform for therapeutic proteins with utility to the aquaculture industry. Purified cfIL-22 shows similar in vitro bioactivity to its mammalian homolog that include increased proliferation of catfish cells highlighting the tissue preservation capabilities associated with this protein. Recombinant cfIL-22 also upregulated expression of genes encoding a tissue repair protein, fibronectin, an antimicrobial peptide, Natural killer lysin-1, and a common innate immune protein, interferon. These findings support plant-made recombinant catfish interleukin-22 as a potential therapeutic for the aquaculture industry and further analysis of this protein for promoting animal health.

Applications of nanobodies in plant science and biotechnology

Present review summarizes the current applications of nanobodies in plant science and biotechnology, including plant expression of nanobodies, plant biotechnological applications, nanobody-based immunodetection, and nanobody-mediated resistance against plant pathogens. Nanobodies (Nbs) are variable domains of heavy chain-only antibodies (HCAbs) isolated from camelids. In spite of their single domain structure, nanobodies display many unique features, such as small size, high stability, and cryptic epitopes accessibility, which make them ideal for sophisticated applications in plants and animals. In this review, we summarize the current applications of nanobodies in plant science and biotechnology, focusing on nanobody expression in plants, plant biotechnological applications, determination of plant toxins and pathogens, and nanobody-mediated resistance against plant pathogens. Prospects and challenges of nanobody applications in plants are also discussed.

Immunization with Plant-Derived Multimeric H5 Hemagglutinins Protect Chicken against Highly Pathogenic Avian Influenza Virus H5N1

Since 2003, H5N1 highly pathogenic avian influenza viruses (HPAIV) have not only caused outbreaks in poultry but were also transmitted to humans with high mortality rates. Vaccination is an efficient and economical means of increasing immunity against infections to decrease the shedding of infectious agents in immunized animals and to reduce the probability of further infections. Subunit vaccines from plants are the focus of modern vaccine developments. In this study, plant-made hemagglutinin (H5) trimers were purified from transiently transformed N. benthamiana plants. All chickens immunized with purified H5 trimers were fully protected against the severe HPAIV H5N1 challenge. We further developed a proof-of-principle approach by using disulfide bonds, homoantiparallel peptides or homodimer proteins to combine H5 trimers leading to production of H5 oligomers. Mice vaccinated with crude leaf extracts containing H5 oligomers induced neutralizing antibodies better than those induced by crude leaf extracts containing trimers. As a major result, eleven out of twelve chickens (92%) immunized with adjuvanted H5 oligomer crude extracts were protected from lethal disease while nine out of twelve chickens (75%) vaccinated with adjuvanted H5 trimer crude extracts survived. The solid protective immune response achieved by immunization with crude extracts and the stability of the oligomers form the basis for the development of inexpensive protective veterinary vaccines.

Plant-Derived Trimeric CO-26K-Equivalent Epitope Induced Neutralizing Antibodies Against Porcine Epidemic Diarrhea Virus

Porcine epidemic diarrhea virus (PEDV) is a causative agent of a highly infectious disease with a high mortality rate, especially in newborn piglets in Asian countries resulting in serious economic loss. The development of a rapid, safe, effective and cost-efficient vaccine is crucial to protect pigs against PEDV infection. The COE antigen is regarded to be a major target for subunit vaccine development against PEDV infection. The naturally assembled COE protein forms a homotrimeric structure. In the present study, we successfully produced a trimeric COE protein as a native structure by fusion with the C-terminal isoleucine zipper trimerization (GCN4pII) motif in Nicotiana benthamiana, with a high expression level shown via semi-quantified Western blots. Trimeric COE protein was purified via immobilized metal affinity chromatography (IMAC), and its trimeric structure was successfully demonstrated by a cross-linking reaction, and a native PAGE gel. A crude extract containing the COE trimer was used for evaluating immunogenicity in mice. After 1 and 2 booster immunizations, the crude extract containing trimeric COE elicited elevated PEDV-specific humoral responses, as demonstrated by ELISA and Western blot analyses. Notably, a virus-neutralizing antibody assay indicated that the neutralization activities of sera of mice vaccinated with the crude extract containing COE-GCN4pII were similar to those of mice vaccinated with a commercial vaccine. These results suggest that crude extract containing trimeric COE is a promising plant-based subunit vaccine candidate for PEDV prevention.

A plant-derived cocaine hydrolase prevents cocaine overdose lethality and attenuates cocaine-induced drug seeking behavior

Cocaine use disorders include short-term and acute pathologies (e.g. overdose) and long-term and chronic disorders (e.g. intractable addiction and post-abstinence relapse). There is currently no available treatment that can effectively reduce morbidity and mortality associated with cocaine overdose or that can effectively prevent relapse in recovering addicts. One recently developed approach to treat these problems is the use of enzymes that rapidly break down the active cocaine molecule into inactive metabolites. In particular, rational design and site-directed mutagenesis transformed human serum recombinant butyrylcholinesterase (BChE) into a highly efficient cocaine hydrolase with drastically improved catalytic efficiency toward (-)-cocaine. A current drawback preventing the clinical application of this promising enzyme-based therapy is the lack of a cost-effective production strategy that is also flexible enough to rapidly scale-up in response to continuous improvements in enzyme design. Plant-based expression systems provide a unique solution as this platform is designed for fast scalability, low cost and the advantage of performing eukaryotic protein modifications such as glycosylation. A Plant-derived form of the Cocaine Super Hydrolase (A199S/F227A/S287G/A328W/Y332G) we designate PCocSH protects mice from cocaine overdose, counters the lethal effects of acute cocaine overdose, and prevents reinstatement of extinguished drug-seeking behavior in mice that underwent place conditioning with cocaine. These results demonstrate that the novel PCocSH enzyme may well serve as an effective therapeutic for cocaine use disorders in a clinical setting.

Bulaon CJ, Phoolcharoen W. Plant Molecular Farming: A Viable Platform for Recombinant Biopharmaceutical Production

The demand for recombinant proteins in terms of quality, quantity, and diversity is increasing steadily, which is attracting global attention for the development of new recombinant protein production technologies and the engineering of conventional established expression systems based on bacteria or mammalian cell cultures. Since the advancements of plant genetic engineering in the 1980s, plants have been used for the production of economically valuable, biologically active non-native proteins or biopharmaceuticals, the concept termed as plant molecular farming (PMF). PMF is considered as a cost-effective technology that has grown and advanced tremendously over the past two decades. The development and improvement of the transient expression system has significantly reduced the protein production timeline and greatly improved the protein yield in plants. The major factors that drive the plant-based platform towards potential competitors for the conventional expression system are cost-effectiveness, scalability, flexibility, versatility, and robustness of the system. Many biopharmaceuticals including recombinant vaccine antigens, monoclonal antibodies, and other commercially viable proteins are produced in plants, some of which are in the pre-clinical and clinical pipeline. In this review, we consider the importance of a plant- based production system for recombinant protein production, and its potential to produce biopharmaceuticals is discussed.

Innovation in plant-based transient protein expression for infectious disease prevention and preparedness

Addressing new challenges in global health and biosecurity requires responsive and accessible platforms for the manufacture of preventative or therapeutic interventions. Transient protein expression in plants has evolved into a technology that offers a unique combination of rapid expression, inherent scalability, and flexibility in gene stacking with the capability to produce complex proteins and protein assemblies. Technical developments that have driven the progress of transient expression in plants include advanced expression systems, protein engineering and synthetic biology approaches to transiently, or stably, modify host plants. The plasticity of transient expression in plants, speed of scalability and relatively low capital costs, highlight the great potential of this technology in the future of human and animal health.

Plant-produced chimeric virus-like particles - a new generation vaccine against African horse sickness

BACKGROUND

African horse sickness (AHS) is a severe arthropod-borne viral disease of equids, with a mortality rate of up to 95% in susceptible naïve horses. Due to safety concerns with the current live, attenuated AHS vaccine, alternate safe and effective vaccination strategies such as virus-like particles (VLPs) are being investigated. Transient plant-based expression systems are a rapid and highly scalable means of producing such African horse sickness virus (AHSV) VLPs for vaccine purposes.

RESULTS

In this study, we demonstrated that transient co-expression of the four AHSV capsid proteins in agroinfiltrated Nicotiana benthamiana dXT/FT plants not only allowed for the assembly of homogenous AHSV-1 VLPs but also single, double and triple chimeric VLPs, where one capsid protein originated from one AHS serotype and at least one other capsid protein originated from another AHS serotype. Following optimisation of a large scale VLP purification procedure, the safety and immunogenicity of the plant-produced, triple chimeric AHSV-6 VLPs was confirmed in horses, the target species.

CONCLUSIONS

We have successfully shown assembly of single and double chimeric AHSV-7 VLPs, as well as triple chimeric AHSV-6 VLPs, in Nicotiana benthamiana dXT/FT plants. Plant produced chimeric AHSV-6 VLPs were found to be safe for administration into 6 month old foals as well as capable of eliciting a weak neutralizing humoral immune response in these target animals against homologous AHSV virus.

Functional characterization of a plant-produced infectious bursal disease virus antigen fused to the constant region of avian IgY immunoglobulins

Infectious bursal disease virus (IBDV) is the cause of an economically important highly contagious disease of poultry, and vaccines are regarded as the most beneficial interventions for its prevention. In this study, plants were used to produce a recombinant chimeric IBDV antigen for the formulation of an innovative subunit vaccine. The fusion protein (PD-FcY) was designed to combine the immunodominant projection domain (PD) of the viral structural protein VP2 with the constant region of avian IgY (FcY), which was selected to enhance antigen uptake by avian immune cells. The gene construct encoding the fusion protein was transiently expressed in Nicotiana benthamiana plants and an extraction/purification protocol was set up, allowing to reduce the contamination by undesired plant compounds/proteins. Mass spectrometry analysis of the purified protein revealed that the glycosylation pattern of the FcY portion was similar to that observed in native IgY, while in vitro assays demonstrated the ability of PD-FcY to bind to the avian immunoglobulin receptor CHIR-AB1. Preliminary immunization studies proved that PD-FcY was able to induce the production of protective anti-IBDV-VP2 antibodies in chickens. In conclusion, the proposed fusion strategy holds promises for the development of innovative low-cost subunit vaccines for the prevention of avian viral diseases.

In vitro-Formulated Oligomers of Strep-Tagged Avian Influenza Haemagglutinin Produced in Plants Cause Neutralizing Immune Responses

The worldwide emergence of the novel influenza A H5N1 and H5N8 has notably and directly impacted the poultry industry, resulting in the need for effective and cheap vaccination strategies to protect poultry worldwide. Subunit vaccines from plants can be produced for a low cost, and plant production systems are easily scaled up at low infrastructure cost. However, subunit vaccines generally induce low immunogenicity against influenza. To address this issue, we present a new and innovative method to generate highly immunogenic H5 oligomers. The method is based on specific and high-affinity interaction between engineered streptavidin (Strep-Tactin® XT) and the Strep-tag II peptide. H5-Strep-tag II-tagged trimers were produced via transient agroinfection in tobacco leaves and purified, and oligomers were formulated in vitro by adding purified homotetrameric Strep-Tactin® XT. Immunogenicity was tested by performing mouse immunizations. Haemagglutinin oligomers produced in vitro by combining Strep-Tactin® XT and Strep-tag II-fused haemagglutinin trimers from plants raised potentially neutralizing antibodies in mice. Vaccines based on actual H5N1 haemagglutinin can be produced by combining strep-tagged haemagglutinin trimers from plants and Strep-Tactin® XT.

Immunogenicity of plant-produced African horse sickness virus-like particles: implications for a novel vaccine

African horse sickness (AHS) is a debilitating and often fatal viral disease affecting horses in much of Africa, caused by the dsRNA orbivirus African horse sickness virus (AHSV). Vaccination remains the single most effective weapon in combatting AHS, as there is no treatment for the disease apart from good animal husbandry. However, the only commercially available vaccine is a live-attenuated version of the virus (LAV). The threat of outbreaks of the disease outside its endemic region and the fact that the LAV is not licensed for use elsewhere in the world, have spurred attempts to develop an alternative safer, yet cost-effective recombinant vaccine. Here, we report the plant-based production of a virus-like particle (VLP) AHSV serotype five candidate vaccine by Agrobacterium tumefaciens-mediated transient expression of all four capsid proteins in Nicotiana benthamiana using the cowpea mosaic virus-based HyperTrans (CPMV-HT) and associated pEAQ plant expression vector system. The production process is fast and simple, scalable, economically viable, and most importantly, guinea pig antiserum raised against the vaccine was shown to neutralize live virus in cell-based assays. To our knowledge, this is the first report of AHSV VLPs produced in plants, which has important implications for the containment of, and fight against the spread of, this deadly disease.

High accumulation in tobacco seeds of hemagglutinin antigen from avian (H5N1) influenza

Tobacco seeds can be used as a cost effective system for production of recombinant vaccines. Avian influenza is an important respiratory pathogen that causes a high degree of mortality and becomes a serious threat for the poultry industry. A safe vaccine against avian flu produced at low cost could help to prevent future outbreaks. We have genetically engineered tobacco plants to express extracellular domain of hemagglutinin protein from H5N1 avian influenza virus as an inexpensive alternative for production purposes. Two regulatory sequences of seed storage protein genes from Phaseolus vulgaris L. were used to direct the expression, yielding 3.0 mg of the viral antigen per g of seeds. The production and stability of seed-produced recombinant HA protein was characterized by different molecular techniques. The aqueous extract of tobacco seed proteins was used for subcutaneous immunization of chickens, which developed antibodies that inhibited the agglutination of erythrocytes after the second application of the antigen. The feasibility of using tobacco seeds as a vaccine carrier is discussed.

A Plant-Produced Candidate Subunit Vaccine Reduces Shedding of Enterohemorrhagic Escherichia coli in Ruminants

Enterohemorrhagic Escherichia coli (EHEC) are commonly present in the gastrointestinal tract of cattle and cause serious infectious disease in humans. Immunizing cattle against EHEC is a promising strategy to decrease the risk of food contamination; however, veterinary vaccines against EHEC such as Econiche have not been widely adopted by the agricultural industry, and have been discontinued, prompting the need for more cost-effective EHEC vaccines. The objective of this project is to develop a platform to produce plant-made antigens for oral vaccination of ruminants against EHEC. Five recombinant proteins were designed as vaccine candidates and expressed transiently in Nicotiana benthamiana and transplastomically in Nicotiana tabacum. Three of these EHEC proteins, NleA, Stx2b, and a fusion of EspA accumulated when transiently expressed. Transient protein accumulation was the highest when EHEC proteins were fused to an elastin-like polypeptide (ELP) tag. In the transplastomic lines, EspA accumulated up to 479 mg kg-1 in lyophilized leaf material. Sheep that were administered leaf tissue containing recombinant EspA shed less E. coli O157:H7 when challenged, as compared to control animals. These results suggest that plant-made, transgenic EspA has the potential to reduce EHEC shedding in ruminants.

Neutralizing immune responses induced by oligomeric H5N1-hemagglutinins from plants

Plant-based transient expression is an alternative platform to produce hemagglutinin-based subunit vaccines. This production system provides not only fast and effective response in the context of a pandemic but also enables the supply of big volume vaccines at low cost. Crude plant extracts containing influenza hemagglutinin are considered to use as vaccine sources because of avoidance of related purification steps resulting in low cost production allowing veterinary applications. Highly immunogenic influenza hemagglutinins are urgently required to meet these pre-conditions. Here, we present a new and innovative way to generate functional H5 oligomers from avian flu hemagglutinin in planta by the specific interaction of S·Tag and S·Protein. A S·Tag was fused to H5 trimers and this construct was transiently co-expressed in planta with S·Protein-TPs which was multimerized by disulfide bonds via cysteine residues in tailpiece sequences (TP) of IgM antibody. Multimerized S·Protein-TPs serve as bridges/molecular docks to combine S·Tag-fused hemagglutinin trimers to form very large hemagglutinin H5 oligomers. H5 oligomers in the plant crude extract were highly active in hemagglutination resulting in high titers. Immunization of mice with two doses of plant crude extracts containing H5 oligomers after storage for 1 week at 4 °C caused strong immune responses and induced neutralizing specific humoral immune responses in mice. These results allow for the development of cheap influenza vaccines for veterinary application in future.

Plant-expressed cocaine hydrolase variants of butyrylcholinesterase exhibit altered allosteric effects of cholinesterase activity and increased inhibitor sensitivity

Butyrylcholinesterase (BChE) is an enzyme with broad substrate and ligand specificities and may function as a generalized bioscavenger by binding and/or hydrolyzing various xenobiotic agents and toxicants, many of which target the central and peripheral nervous systems. Variants of BChE were rationally designed to increase the enzyme’s ability to hydrolyze the psychoactive enantiomer of cocaine. These variants were cloned, and then expressed using the magnICON transient expression system in plants and their enzymatic properties were investigated. In particular, we explored the effects that these site-directed mutations have over the enzyme kinetics with various substrates of BChE. We further compared the affinity of various anticholinesterases including organophosphorous nerve agents and pesticides toward these BChE variants relative to the wild type enzyme. In addition to serving as a therapy for cocaine addiction-related diseases, enhanced bioscavenging against other harmful agents could add to the practicality and versatility of the plant-derived recombinant enzyme as a multivalent therapeutic.

Plant-made vaccines and reagents for the One Health initiative

The One Health initiative is increasingly becoming a prominent discussion topic in animal and human health, with its focus on prevention of spread of zoonotic diseases, both in animals, and from animals to humans. An important part of One Health is that diagnostics and vaccines for diseases may be the same thing - and be used for both humans and animals. One potential problem standing in the way of wider adoption of One Health principles, though, is that use of conventional cell fermentation systems for production of the recombinant proteins that could be used as diagnostics or vaccines is often expensive and is not easily scalable. A solution to this may be the use of plants or plant cells as bioreactors: molecular farming, or the production of biologics in plants, is now a well-established science with many proofs of principle and important proofs of efficacy for especially animal vaccines. This review discusses how molecular farming could enable important advances in One Health, using as examples plant-made vacccines, reagents and therapeutics for influenza viruses, ebolaviruses, rabies virus, bunyaviruses and flaviviruses.

Plant-based oral vaccines against zoonotic and non-zoonotic diseases

The shared diseases between animals and humans are known as zoonotic diseases and spread infectious diseases among humans. Zoonotic diseases are not only a major burden to livestock industry but also threaten humans accounting for >60% cases of human illness. About 75% of emerging infectious diseases in humans have been reported to originate from zoonotic pathogens. Because antibiotics are frequently used to protect livestock from bacterial diseases, the development of antibiotic-resistant strains of epidemic and zoonotic pathogens is now a major concern. Live attenuated and killed vaccines are the only option to control these infectious diseases and this approach has been used since 1890. However, major problems with this approach include high cost and injectable vaccines is impractical for >20 billion poultry animals or fish in aquaculture. Plants offer an attractive and affordable platform for vaccines against animal diseases because of their low cost, and they are free of attenuated pathogens and cold chain requirement. Therefore, several plant-based vaccines against human and animals diseases have been developed recently that undergo clinical and regulatory approval. Plant-based vaccines serve as ideal booster vaccines that could eliminate multiple boosters of attenuated bacteria or viruses, but requirement of injectable priming with adjuvant is a current limitation. So, new approaches like oral vaccines are needed to overcome this challenge. In this review, we discuss the progress made in plant-based vaccines against zoonotic or other animal diseases and future challenges in advancing this field.

Potential of plants to produce recombinant protein products

Plants have great potential as photosynthetic factories to produce pharmaceutically important and commercially valuable biomedicines and industrial proteins at low cost. The U.S. Food and Drug Administration (U.S. FDA) has approved the drug Elelyso (taliglucerase alfa) produced by carrot cells for treatment of type 1 Gaucher's disease in 2012. The commercial potential of biomedicines produced by molecular farming has dramatically improved due to the success of an experimental drug called ZMapp, which has immunological activity in Ebola patients. A cocktail of three monoclonal antibodies was produced in tobacco (Nicotiana benthamiana) plants (Chen and Davis 2016). At present, very few drugs made by this technology have been approved by worldwide authorities such as the U.S. FDA. However, plants have been proposed as a novel paradigm for commercial production of proteins over the next decade. In recent years, leading researchers on molecular farming have given more priority to the area of animal-free therapeutic proteins such as parenteral and oral vaccines. Although plant-based platforms have considerable advantages over traditional systems such as bacterial and animal systems, there are several obstacles to commercial-scale production, especially with regards to improving the quality and quantity of plant-produced biologics and industrial materials. One of the biggest barriers to commercialization of this technology is the intense scrutiny of these new plant varieties by regulatory agencies and the public as well as the high costs associated with their regulatory approval.