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

Exigency of Plant-Based Vaccine against COVID-19 Emergence as Pandemic Preparedness

After two years since the declaration of COVID-19 as a pandemic by the World Health Organization (WHO), more than six million deaths have occurred due to SARS-CoV-2, leading to an unprecedented disruption of the global economy. Fortunately, within a year, a wide range of vaccines, including pathogen-based inactivated and live-attenuated vaccines, replicating and non-replicating vector-based vaccines, nucleic acid (DNA and mRNA)-based vaccines, and protein-based subunit and virus-like particle (VLP)-based vaccines, have been developed to mitigate the severe impacts of the COVID-19 pandemic. These vaccines have proven highly effective in reducing the severity of illness and preventing deaths. However, the availability and supply of COVID-19 vaccines have become an issue due to the prioritization of vaccine distribution in most countries. Additionally, as the virus continues to mutate and spread, questions have arisen regarding the effectiveness of vaccines against new strains of SARS-CoV-2 that can evade host immunity. The urgent need for booster doses to enhance immunity has been recognized. The scarcity of "safe and effective" vaccines has exacerbated global inequalities in terms of vaccine coverage. The development of COVID-19 vaccines has fallen short of the expectations set forth in 2020 and 2021. Furthermore, the equitable distribution of vaccines at the global and national levels remains a challenge, particularly in developing countries. In such circumstances, the exigency of plant virus-based vaccines has become apparent as a means to overcome supply shortages through fast manufacturing processes and to enable quick and convenient distribution to millions of people without the reliance on a cold chain system. Moreover, plant virus-based vaccines have demonstrated both safety and efficacy in eliciting robust cellular immunogenicity against COVID-19 pathogens. This review aims to shed light on the advantages and disadvantages of different types of vaccines developed against SARS-CoV-2 and provide an update on the current status of plant-based vaccines in the fight against the COVID-19 pandemic.

Biotechnologically Engineered Plants

The development of recombinant DNA technology during the past thirty years has enabled scientists to isolate, characterize, and manipulate a myriad of different animal, bacterial, and plant genes. This has, in turn, led to the commercialization of hundreds of useful products that have significantly improved human health and well-being. Commercially, these products have been mostly produced in bacterial, fungal, or animal cells grown in culture. More recently, scientists have begun to develop a wide range of transgenic plants that produce numerous useful compounds. The perceived advantage of producing foreign compounds in plants is that compared to other methods of producing these compounds, plants seemingly provide a much less expensive means of production. A few plant-produced compounds are already commercially available; however, many more are in the production pipeline.

Plant-Based Vaccines: Antigen Design, Diversity, and Strategies for High Level Production

Vaccines for human use have conventionally been developed by the production of (1) microbial pathogens in eggs or mammalian cells that are then inactivated, or (2) by the production of pathogen proteins in mammalian and insect cells that are purified for vaccine formulation, as well as, more recently, (3) by using RNA or DNA fragments from pathogens. Another approach for recombinant antigen production in the last three decades has been the use of plants as biofactories. Only have few plant-produced vaccines been evaluated in clinical trials to fight against diseases, of which COVID-19 vaccines are the most recent to be FDA approved. In silico tools have accelerated vaccine design, which, combined with transitory antigen expression in plants, has led to the testing of promising prototypes in pre-clinical and clinical trials. Therefore, this review deals with a description of immunoinformatic tools and plant genetic engineering technologies used for antigen design (virus-like particles (VLP), subunit vaccines, VLP chimeras) and the main strategies for high antigen production levels. These key topics for plant-made vaccine development are discussed and perspectives are provided.

A short review on key role of plants and their extracts in boosting up immune response to combat COVID-19

In the wake of the recent global pandemic of COVID-19, there has been an increasing concern among the general public to improve their immune system. The causative agent of COVID-19 is SARS CoV-2, similar to its relative viruses SARS Cov-1 and MERS. Up till now, no vaccine has been developed against this disease and the only way one can stay safe is via prevention and developing one's immune system. Plants have been used since ancient times in herbal medicines and many chemicals and extracts in them are found to boost the immune system. Therefore, a consciously maintained diet consisting of plant-based immunity boosters is the need of the hour. Plants are known to provide many chemicals, natural therapeutics and vitamins which naturally enhance our immune system. Recently, vaccination research is also being carried out in transgenic plants. In this review, we have focused on highlighting some of the main players within the Kingdom Plantae which, when taken up in regular diet, can significantly boost our immune system capacity and thus provide the best possible measure to combat this pandemic.

Plant Platforms for Efficient Heterologous Protein Production

Production of recombinant proteins is primarily established in cultures of mammalian, insect and bacterial cells. Concurrently, concept of using plants to produce high-value pharmaceuticals such as vaccines, antibodies, and dietary proteins have received worldwide attention. Newer technologies for plant transformation such as plastid engineering, agroinfiltration, magnifection, and deconstructed viral vectors have been used to enhance the protein production in plants along with the inherent advantage of speed, scale, and cost of production in plant systems. Production of therapeutic proteins in plants has now a more pragmatic approach when several plant-produced vaccines and antibodies successfully completed Phase I clinical trials in humans and were further scheduled for regulatory approvals to manufacture clinical grade products on a large scale which are safe, efficacious, and meet the quality standards. The main thrust of this review is to summarize the data accumulated over the last two decades and recent development and achievements of the plant derived therapeutics. It also attempts to discuss different strategies employed to increase the production so as to make plants more competitive with the established production systems in this industry.

A cross talk between the immunization and edible vaccine: Current challenges and future prospects

INTRODUCTION

It is well known that immune system is highly specific to protect the body against various environmental pathogens. The concept of conventional vaccination has overcome the pandemic situation of several infectious diseases outbreak.

AREA COVERED

The recent idea of immunization through oral route (edible vaccine) is vital alternatives over conventional vaccines. Edible vaccines are composed of antigenic protein introduced into the plant cells which induce these altered plants to produce the encoded protein. Edible vaccine has no way of forming infection and safety is assured as it only composed of antigenic protein and is devoid of pathogenic genes. Edible vaccines have significant role in stimulating mucosal immunity as they come in contact with digestive tract lining. They are safe, cost-effective, easy-to-administer and have reduced manufacturing cost hence have a dramatic impact on health care in developing countries.

EXPERT OPINION

The edible vaccine might be the solution for the potential hazard associated with the parenteral vaccines. In this review we discuss the detailed study of pros, cons, mechanism of immune stimulation, various outbreaks that might be controlled by edible vaccines with the possible future research and applied application of edible vaccine.

Inducible expression of antigens in plants: a study focused on peptides related to multiple sclerosis immunotherapy

Multiple sclerosis (MS) affects 2.3 million patients worldwide with no effective treatments available thus far. Depletion of autoreactive T-cells is considered the basis for immunotherapeutic approaches. For this purpose the peptides BV5S2, BV6S5, and BV13S1 have been identified as candidates for the development of a MS vaccine. Herein, the plant-based simultaneous production of these peptides is described as an effort to generate a new model of MS immunotherapy. A polyprotein comprising the sequence of the target peptides was designed having the picornaviral 2A sequence in between to mediate the release of the individual peptides upon translation. A codon optimized gene was cloned in vectors mediating constitutive (CaMV35S promoter) or inducible (AlcA promoter) expression. No transgenic tobacco plants were recovered from the constitutive vector suggesting toxicity of the target peptides. In contrast, several transformed lines were obtained with the inducible vector. The individual BV5S2, BV6S5, and BV13S1 peptides were detected in transformed lines upon ethanol-mediated induction and a quantitative analysis based on a OVA conjugate carrying the three peptides revealed accumulation levels up to 0.5 μg g-1 FW leaves. The plant-made peptides were able to induce humoral responses in orally immunized mice. This platform will be useful in the development of alternative immunotherapies against MS having low cost and safety as main attributes. Moreover the platform represents an attractive alternative for the expression of antigens having detrimental effects in plants.

Edible Vaccines: Promises and Challenges

Vaccines are biological preparations that improve immunity to particular diseases and form an important innovation of 19th century research. It contains a protein that resembles a disease-causing microorganism and is often made from weak or killed forms of the microbe. Vaccines are agents that stimulate the body’s immune system to recognize the antigen. Now, a new form of vaccine was introduced which will have the power to mask the risk side of conventional vaccines. This type of vaccine was produced from plants which are genetically modified. In the production of edible vaccines, the gene-encoding bacterial or viral disease-causing agent can be incorporated in plants without losing its immunogenic property. The main mechanism of action of edible vaccines is to activate the systemic and mucosal immunity responses against a foreign disease-causing organism. Edible vaccines can be produced by incorporating transgene in to the selected plant cell. At present edible vaccine are developed for veterinary and human use. But the main challenge faced by edible vaccine is its acceptance by the population so that it is necessary to make aware the society about its use and benefits. When compared to other traditional vaccines, edible vaccines are cost effective, efficient and safe. It promises a better prevention option from diseases.

What Does Plant-Based Vaccine Technology Offer to the Fight against COVID-19?

The emergence of new pathogenic viral strains is a constant threat to global health, with the new coronavirus strain COVID-19 as the latest example. COVID-19, caused by the SARS-CoV-2 virus has quickly spread around the globe. This pandemic demands rapid development of drugs and vaccines. Plant-based vaccines are a technology with proven viability, which have led to promising results for candidates evaluated at the clinical level, meaning this technology could contribute towards the fight against COVID-19. Herein, a perspective in how plant-based vaccines can be developed against COVID-19 is presented. Injectable vaccines could be generated by using transient expression systems, which offer the highest protein yields and are already adopted at the industrial level to produce VLPs-vaccines and other biopharmaceuticals under GMPC-processes. Stably-transformed plants are another option, but this approach requires more time for the development of antigen-producing lines. Nonetheless, this approach offers the possibility of developing oral vaccines in which the plant cell could act as the antigen delivery agent. Therefore, this is the most attractive approach in terms of cost, easy delivery, and mucosal immunity induction. The development of multiepitope, rationally-designed vaccines is also discussed regarding the experience gained in expression of chimeric immunogenic proteins in plant systems.

A multi-epitope plant-made chimeric protein (LTBentero) targeting common enteric pathogens is immunogenic in mice

KEY MESSAGE

A plant-based multiepitopic protein (LTBentero) containing epitopes from ETEC, S. typhimurium, and V. parahaemolyticus was produced in plants cells and triggered systemic and intestinal humoral responses in immunized mice. Around 200 million people suffer gastroenteritis daily and more than 2 million people die annually in developing countries due to such pathologies. Vaccination is an alternative to control this global health issue, however new low-cost vaccines are needed to ensure proper vaccine coverage. In this context, plants are attractive hosts for the synthesis and delivery of subunit vaccines. Therefore, in this study a plant-made multiepitopic protein named LTBentero containing epitopes from antigens of enterotoxigenic E. coli, S. typhimurium, and V. parahaemolyticus was produced and found immunogenic in mice. The LTBentero protein was expressed in tobacco plants at up to 5.29 µg g-1 fresh leaf tissue and was deemed immunogenic when administered to BALB/c mice either orally or subcutaneously. The plant-made LTBentero antigen induced specific IgG (systemic) and IgA (mucosal) responses against LTB, ST, and LptD epitopes. In conclusion, multiepitopic LTBentero was functionally produced in plant cells, being capable to trigger systemic and intestinal humoral responses and thus it constitutes a promising oral immunogen candidate in the fight against enteric diseases.

Genetically-engineered plants yield an orally immunogenic PirA-like toxin from Vibrio parahaemolyticus

Vibrio parahaemolyticus is the main etiological agent of human gastroenteritis by seafood consumption and some strains from this species causing the Acute Hepatopancreatic Necrosis Disease in shrimp have been recently reported. The PirA-like toxin from V. parahaemolyticus (ToxA) has been recently reported as an attractive antigen implicated in subunit vaccine development. Since plants are attractive hosts for the production and delivery of vaccines in the present study plants expressing ToxA were developed to account with a low cost platform for the production and oral delivery of ToxA. Tobacco plants were genetically engineered by Agrobacterium-mediated transformation to stably integrate the ToxA-coding gene into the nuclear genome. Transgenic lines were rescued in kanamycin-containing medium and analyzed by ELISA to determine ToxA yields observing levels up to 9 μg g-1 FW leaf tissues. Western blot analysis confirmed the presence of the ToxA protein in plant extracts. Immunogenicity assessment of the plant-made ToxA was performed in mice, comprising a 4-dose oral immunization scheme; revealing the induction of anti-ToxA humoral responses (IgG in serum and IgA in feces). This study opens the path for the development of low cost plant-based vaccines against Vibrio parahaemolyticus.

Immunization with recombinant fusion of LTB and linear epitope (40-62) of epsilon toxin elicits protective immune response against the epsilon toxin of Clostridium perfringens type D

Epsilon toxin (Etx) produced by Clostridium perfringens types B and D, a major causative agent of enterotoxaemia causes significant economic losses to animal industry. Conventional vaccines against these pathogens generally employ formalin-inactivated culture supernatants. However, immunization with the culture supernatant and full length toxin subjects the animal to antigenic load and often have adverse effect due to incomplete inactivation of the toxins. In the present study, an epitope-based vaccine against Clostridium perfringens Etx, comprising 40-62 amino acid residues of the toxin in translational fusion with heat labile enterotoxin B subunit (LTB) of E. coli, was evaluated for its protective potential. The ability of the fusion protein rLTB.Etx_40-62 to form pentamers and biologically active holotoxin with LTA of E. coli indicated that the LTB present in the fusion protein retained its biological activity. Antigenicity of both the components in the fusion protein was retained as anti-fusion protein antisera detected both the wild type Etx and LTB in Western blot analysis. Immunization of BALB/c mice with the fusion protein resulted in a significant increase in all isotypes, predominantly IgG1, IgG2a and IgG2b. Anti-fusion protein antisera neutralized the cytotoxicity of epsilon toxin both in vitro and in vivo. Thus, the results demonstrate the potential of rLTB.Etx_40-62 as a candidate vaccine against C. perfringens.

Design of a multiepitopic Zaire ebolavirus protein and its expression in plant cells

The recent Ebola virus disease (EVD) outbreaks make the development of efficacious and low cost vaccines against Ebola virus (EBOV) an urgent goal. Multiepitopic vaccines allow a rational design rendering vaccines able to induce proper immune responses in terms of polarization and potency. In addition, the pathogen variants can be easily covered by including epitopes conserved among relevant isolates. Other important aspects to consider in vaccination are the costs associated to production, distribution, and administration of the vaccine. Plants provide an advantageous platform for this purpose, since they yield biomass at very low costs and some species can be used to formulate purification-free oral vaccines. In the present study, a multiepitopic protein called Zerola, which carries epitopes from the EBOV glycoprotein (GP), was designed based on immunoinformatic approaches and current experimental evidence on B cell protective GP epitopes. Moreover, expression studies performed in nuclear-transformed tobacco lines confirmed the capacity of the plant cell to synthetize the Zerola antigenic protein. The generation of this plant-based candidate vaccine is a step forward in the development of highly efficient and low cost EBOV vaccines.

Expression of B subunit of E. coli heat-labile enterotoxin in the progenies of transgenic tobacco bred by crossing nuclear- and chloroplast-transgenic lines

The B subunit of Escherichia coli heat-labile toxin (LTB) is a model antigen that induces a strong immune response upon oral administration and enhances immune responses to conjugated and co-administered antigens. We previously examined high expression levels of LTB in plants by chloroplast and synthetic LTB gene expression and found substantially higher expression levels of LTB, compared to nuclear LTB expression in wild-type plants. The 2.5% LTB protein of total soluble protein that was observed by chloroplast transformation was approximately 250-fold greater expression than that of LTB via nuclear genome integration. In addition, the amount of LTB protein found in transgenic tobacco leaves using a synthetic LTB gene was 2.2% of the total soluble plant protein, which was approximately 200-fold higher than that in plants with native LTB gene expression. The purpose of our experiment was to increase LTB levels in plants by crossing chloroplast-transformed and synthetic LTB transgenic lines produced previously to express higher LTB levels. LTB protein levels in the F1 transgenic tobacco plants was significantly higher (3.3%), compared to the 2.2% of chloroplast-transformed line or 2.8% of synthetic LTB gene line. Our results suggest that LTB expression was successfully enhanced in the F1 hybrid generation of transgenic tobacco plants.

Protein-Based Bioproducts

Plant proteins can be used for the production of a variety of bioproducts, including films and coatings, adhesives, fibres and pharmaceuticals. Proteins derived from plant production systems have many advantages: they are safe, low-cost and rapidly deployable, allow for simple product storage and result in proteins that are properly folded, assembled and post-translationally modified. While plant-derived protein-based products are natural, renewable, biodegradable and environmentally friendly, they tend to be lower in strength and elasticity than their corresponding synthetic products. Current research in this area is focused on overcoming challenges in plant production platforms related to yield, purification, regulatory approval and customer acceptance.

Omics and Edible Vaccines

Molecular farming provides an unprecedented approach for the production of metabolites or proteins of medicinal value from plants used previously only in agricultural setting. These plants act as protein factories that can synthesize a variety of proteins free from pathogens such as plasma proteins, growth factors, and vaccines. This method provides a novel, tempting, inexpensive, easy, and safe alternative to other techniques of protein or antigen production. With the advent of transgenic plants, it is possible to produce unlimited amounts of subunit vaccines (for oral use/edible and of parenteral use), protein used for pharmaceutical/medicinal purpose, recombinant proteins, antibodies, and industrial enzymes. Plants have numerous advantages over the production systems on account of scalability, safety, and are economic; for example, less cost of production is involved for Hepatitis B nucleocapsid antigen using transgenic tobacco. Biopharming or molecular farming provides an important resource for cheaper drug production used in the treatment of cancer, heart diseases, and infectious diseases. The pharmaceutical products are manufactured by genetically engineered plants that are extracted and purified, also known as pharmaceuticals produced by plants. Edible vaccines are cheaper in cost, easy to administer mostly by oral route, fail-safe, and are acceptable by society especially in developing countries. These vaccines are targeted to provide systemic as well as mucosal types of immunity. It has been predicted that in future children may get their immunization by munching on foods instead of getting enduring shots. The production of edible vaccines consists of the process of introducing the selected genes of desired quality into plant to induce these altered or transgenic plants to produce the encoded proteins in a natural way. These vaccines provide safer alternatives and help in reduction of cost of production and shipping and also decrease the potential hazards associated with conventional vaccines. However, becoming a reality and readily availability of edible vaccine is challenged by many problems of technical, regulatory, and nonscientific issues, which should be ruled out and rectified. This chapter provides insight into the current scenario and future applications of this new preventive modality.

Disease Prevention: An Opportunity to Expand Edible Plant-Based Vaccines?

The lethality of infectious diseases has decreased due to the implementation of crucial sanitary procedures such as vaccination. However, the resurgence of pathogenic diseases in different parts of the world has revealed the importance of identifying novel, rapid, and concrete solutions for control and prevention. Edible vaccines pose an interesting alternative that could overcome some of the constraints of traditional vaccines. The term “edible vaccine” refers to the use of edible parts of a plant that has been genetically modified to produce specific components of a particular pathogen to generate protection against a disease. The aim of this review is to present and critically examine “edible vaccines” as an option for global immunization against pathogenic diseases and their outbreaks and to discuss the necessary steps for their production and control and the list of plants that may already be used as edible vaccines. Additionally, this review discusses the required standards and ethical regulations as well as the advantages and disadvantages associated with this powerful biotechnology tool.

Synthetic gene design-The rationale for codon optimization and implications for molecular pharming in plants

Degeneracy in the genetic code allows multiple codon sequences to encode the same protein. Codon usage bias in genes is the term given to the preferred use of particular synonymous codons. Synonymous codon substitutions had been regarded as "silent" as the primary structure of the protein was not affected; however, it is now accepted that synonymous substitutions can have a significant effect on heterologous protein expression. Codon optimization, the process of altering codons within the gene sequence to improve recombinant protein expression, has become widely practised. Multiple inter-linked factors affecting protein expression need to be taken into consideration when optimizing a gene sequence. Over the years, various computer programmes have been developed to aid in the gene sequence optimization process. However, as the rulebook for altering codon usage to affect protein expression is still not completely understood, it is difficult to predict which strategy, if any, will design the "optimal" gene sequence. In this review, codon usage bias and factors affecting codon selection will be discussed and the evidence for codon optimization impact will be reviewed for recombinant protein expression using plants as a case study. These developments will be relevant to all recombinant expression systems; however, molecular pharming in plants is an area which has consistently encountered difficulties with low levels of recombinant protein expression, and should benefit from an evidence based rational approach to synthetic gene design. Biotechnol. Bioeng. 2017;114: 492-502. © 2016 Wiley Periodicals, Inc.

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.

Biomanufacturing of protective antibodies and other therapeutics in edible plant tissues for oral applications

Although plant expression systems used for production of therapeutic proteins have the advantage of being scalable at a low price, the downstream processing necessary to obtain pure therapeutic molecules is as expensive as for the traditional Chinese hamster ovary (CHO) platforms. However, when edible plant tissues (EPTs) are used, there is no need for exhaustive purification, because they can be delivered orally as partially purified formulations that are safe for consumption. This economic benefit is especially interesting when high doses of recombinant proteins are required throughout the treatment/prophylaxis period, as is the case for antibodies used for oral passive immunization (OPI). The secretory IgA (SIgA) antibodies, which are highly abundant in the digestive tract and mucosal secretions, and thus the first choice for OPI, have only been successfully produced in plant expression systems. Here, we cover most of the up-to-date examples of EPT-produced pharmaceuticals, including two examples of SIgA aimed at oral delivery. We describe the benefits and drawbacks of delivering partially purified formulations and discuss a number of practical considerations and criteria to take into account when using plant expression systems, such as subcellular targeting, protein degradation, glycosylation patterns and downstream strategies, all crucial for improved yield, high quality and low cost of the final product.

Scalable Production of HPV16 L1 Protein and VLPs from Tobacco Leaves

Cervical cancer is the most common malignancy among women particularly in developing countries, with human papillomavirus (HPV) 16 causing 50% of invasive cervical cancers. A plant-based HPV vaccine is an alternative to the currently available virus-like particle (VLP) vaccines, and would be much less expensive. We optimized methods to express HPV16 L1 protein and purify VLPs from tobacco (Nicotiana benthamiana) leaves transfected with the magnICON deconstructed viral vector expression system. L1 proteins were extracted from agro-infiltrated leaves using a series of pH and salt mediated buffers. Expression levels of L1 proteins and VLPs were verified by immunoblot and ELISA, which confirmed the presence of sequential and conformational epitopes, respectively. Among three constructs tested (16L1d22, TPL1d22, and TPL1F), TPL1F, containing a full-length L1 and chloroplast transit peptide, was best. Extraction of HPV16 L1 from leaf tissue was most efficient (> 2.5% of total soluble protein) with a low-salt phosphate buffer. VLPs were purified using both cesium chloride (CsCl) density gradient and size exclusion chromatography. Electron microscopy studies confirmed the presence of assembled forms of HPV16 L1 VLPs. Collectively; our results indicated that chloroplast-targeted transient expression in tobacco plants is promising for the production of a cheap, efficacious HPV16 L1 VLP vaccine. Studies are underway to develop plant VLPs for the production of a cervical cancer vaccine.

Oral Rabies Vaccine Design for Expression in Plants

Vaccination is the sensitization process of the immune system against any pathogen. Generally, recombinant subunit vaccines are considered safer than attenuated vaccines. As whole pathogenic organisms are used in the immunization process, the attenuated vaccines are considered more risky than subunit vaccines. Rabies is the oldest known zoonosis which spreads through a neurotropic Lyssavirus primarily mediated through infected canine bites. Rabies causes worldwide loss of more than 60,000 human lives every year. Animal vaccination is equally important to check the transmission of rabies into humans. Rabies oral vaccination can be a good alternative where multiple booster and priming regimens are required while the painful vaccination process can continue for long durations. Introduction of oral vaccines was made to ease the discomfort associated with the mode of introduction of conventional vaccines into the body. Although the rabies oral vaccine can substantially reduce the cost of vaccination in the developing countries, mass immunization programs need larger quantities of vaccines which should be delivered at nominal cost. Expression of recombinant antigen proteins in E. coli is often not viable because of lack of post-translational modifications and folding requirements. Though yeast and insect cell line expression systems have post-translational processing and modifications, significantly different immunological response against their post-translational modification pattern limits their deployment as an expression system. As an alternative, plants are emerging as a promising system to express and deliver wide range of functionally active biopharmaceutical product at lower cost for mass immunization programs. As generation of vaccine antigenic proteins in plant systems are cheaper, the strategy will benefit developing countries where this disease causes thousands of deaths every year. In this chapter, we will discuss about our efforts toward development of oral rabies vaccine and the methodological steps involved during this procedure in detail.

Farming of Plant-Based Veterinary Vaccines and Their Applications for Disease Prevention in Animals

Plants have been studied for the production of pharmaceutical compounds for more than two decades now. Ever since the plant-made poultry vaccine against Newcastle disease virus made a breakthrough and went all the way to obtain regulatory approval, research to use plants for expression and delivery of vaccine proteins for animals was intensified. Indeed, in view of the high production costs of veterinary vaccines, plants represent attractive biofactories and offer many promising advantages in the production of recombinant vaccine proteins. Furthermore, the possibility of conducting immunogenicity and challenge studies in target animals has greatly exaggerated the progress. Although there are no edible plant-produced animal vaccines in the market, plant-based vaccine technology has great potentials. In this review, development, uses, and advantages of plant-based recombinant protein production in various expression platforms are discussed. In addition, examples of plant-based veterinary vaccines showing strong indication in terms of efficacy in animal disease prevention are also described.

Expression and functional validation of heat-labile enterotoxin B (LTB) and cholera toxin B (CTB) subunits in transgenic rice (Oryza sativa)

We expressed the heat-labile enterotoxin B (LTB) subunit from enterotoxigenic Escherichia coli and the cholera toxin B (CTB) subunit from Vibrio cholerae under the control of the rice (Oryza sativa) globulin (Glb) promoter. Binding of recombinant LTB and CTB proteins was confirmed based on G_M1-ganglioside binding enzyme-linked immunosorbent assays (G_M1-ELISA). Real-time PCR of three generations (T₃, T₄, and T₅) in homozygous lines (LCI-11) showed single copies of LTB, CTB, bar and Tnos. LTB and CTB proteins in rice transgenic lines were detected by Western blot analysis. Immunogenicity trials of rice-derived CTB and LTB antigens were evaluated through oral and intraperitoneal administration in mice, respectively. The results revealed that LTB- and CTB-specific IgG levels were enhanced in the sera of intraperitoneally immunized mice. Similarly, the toxin-neutralizing activity of CTB and LTB in serum of orally immunized mice was associated with elevated levels of both IgG and IgA. The results of the present study suggest that the combined expression of CTB and LTB proteins can be utilized to produce vaccines against enterotoxigenic strains of Escherichia coli and Vibrio cholera, for the prevention of diarrhea.

Recent developments in therapeutic protein expression technologies in plants

Infectious diseases and cancers are some of the commonest causes of deaths throughout the world. The previous two decades have witnessed a combined endeavor across various biological sciences to address this issue in novel ways. The advent of recombinant DNA technologies has provided the tools for producing recombinant proteins that can be used as therapeutic agents. A number of expression systems have been developed for the production of pharmaceutical products. Recently, advances have been made using plants as bioreactors to produce therapeutic proteins directed against infectious diseases and cancers. This review highlights the recent progress in therapeutic protein expression in plants (stable and transient), the factors affecting heterologous protein expression, vector systems and recent developments in existing technologies and steps towards the industrial production of plant-made vaccines, antibodies, and biopharmaceuticals.

State of research in the field of the creation of plant vaccines for veterinary use

Transgenic plants as an alternative of costly systems of recombinant immunogenic protein expression are the source for the production of cheap and highly efficient biotherapeuticals of new generation, including plant vaccines. In the present review, possibilities of plant system application for the production of recombinant proteins for veterinary use are considered, the history of the "edible vaccine" concept is briefly summarized, advantages and disadvantages of various plant systems for the expression of recombinant immunogenic proteins are discussed. The list of recombinant plant vaccines for veterinary use, which are at different stages of clinical trials, is presented.

Novel antigens for enterotoxigenic Escherichia coli vaccines

Enterotoxigenic Escherichia coli (ETEC) are the most common bacterial pathogens causing diarrhea in developing countries where they lead to hundreds of thousands of deaths, mostly in children. These organisms are a leading cause of diarrheal illness in travelers to endemic countries. ETEC pathogenesis, and consequently vaccine approaches, have largely focused on plasmid-encoded enterotoxins or fimbrial colonization factors. To date these approaches have not yielded a broadly protective vaccine. However, recent studies suggest that ETEC pathogenesis is more complex than previously appreciated and involves additional plasmid and chromosomally encoded virulence molecules that can be targeted in vaccines. Here, we review recent novel antigen discovery efforts, potential contribution of these proteins to the molecular pathogenesis of ETEC and protective immunity, and the potential implications for development of next generation vaccines for important pathogens. These proteins may help to improve the effectiveness of future vaccines by making them simpler and possibly broadly protective because of their conserved nature.

Microneedle patches: usability and acceptability for self-vaccination against influenza

While therapeutic drugs are routinely self-administered by patients, there is little precedent for self-vaccination. Convenient self-vaccination may expand vaccination coverage and reduce administration costs. Microneedle patches are in development for many vaccines, but no reports exist on usability or acceptability. We hypothesized that naïve patients could apply patches and that self-administered patches would improve stated intent to receive an influenza vaccine. We conducted a randomized, repeated measures study with 91 venue-recruited adults. To simulate vaccination, subjects received placebo microneedle patches given three times by self-administration and once by the investigator, as well as an intramuscular injection of saline. Seventy participants inserted patches with thumb pressure alone and the remainder used snap-based devices that closed shut at a certain force. Usability was assessed by skin staining and acceptability was measured with an adaptive-choice analysis. The best usability was seen with the snap device, with users inserting a median value of 93-96% of microneedles over three repetitions. When a self-administered microneedle patch was offered, intent to vaccinate increased from 44% to 65% (CI: 55-74%). The majority of those intending vaccination would prefer to self-vaccinate: 64% (CI: 51-75%). There were no serious adverse events associated with use of microneedle patches. The findings from this initial study indicate that microneedle patches for self-vaccination against influenza are usable and may lead to improved vaccination coverage.

Plant production systems for vaccines

Plants offer an attractive alternative for the production and delivery of subunit vaccines. Various antigens have been expressed at sufficiently high levels in plants to render vaccine development practical. An increasing body of evidence demonstrates that these plant-produced antigens can induce immunogenic responses and confer protection when delivered orally. Plant-based vaccines are relatively inexpensive to produce and production can be rapidly scaled up. There is also the potential for oral delivery of these vaccines, which can dramatically reduce distribution and delivery costs. Here we describe the technology to develop plant-based vaccines, review their advantages and discuss potential roadblocks and concerns over their commercialization. We also speculate on likely future developments with these vaccines and on their potential impact in the realms of human and animal health.

Plant–derived vaccines: a look back at the highlights and a view to the challenges on the road ahead

The sobering reality is that each year, 33 million children remain unvaccinated for vaccine-preventable diseases. Universal childhood vaccination would have profound effects on leveling the health inequities in many parts of the world. As an alternative to administration of vaccines by needle and syringe, oral vaccines offer significant logistical advantages, as the polio eradication campaign has demonstrated. Over the past decade, the expression of subunit vaccine antigens in plants has emerged as a convenient, safe and potentially economical platform technology, with the potential to provide a novel biotechnological solution to vaccine production and delivery. As this technology has come of age, many improvements have been made on several fronts, as a growing number of research groups worldwide have extensively investigated plants as factories for vaccine production. This review attempts to highlight some of the achievements over the past 15 years, identify some of the potential problems and discuss the promises that this technology could fulfill.

Edible Vaccines

In recent years edible vaccine emerged as a new concept developed by biotechnologists. Edible vaccines are subunit vaccines where the selected genes are introduced into the plants and the transgenic plant is then induced to manufacture the encoded protein. Foods under such application include potato, banana, lettuce, corn, soybean, rice, and legumes. They are easy to administer, easy to store and readily acceptable delivery system for different age group patients yet cost effective. Edible vaccines present exciting possibilities for significantly reducing various diseases such as measles, hepatitis B, cholera, diarrhea, etc., mainly in developing countries. However, various technical and regulatory challenges need to overcome in the path of this emerging vaccine technology to make edible vaccine more efficient and applicable. This chapter attempts to discuss key aspects of edible vaccines like host plants, production, mechanism of action, advantages and limitations, applications, and different regulatory issues concerned to edible vaccines.

Single-dose monomeric HA subunit vaccine generates full protection from influenza challenge

Recombinant subunit vaccines are an efficient strategy to meet the demands of a possible influenza pandemic, because of rapid and scalable production. However, vaccines made from recombinant hemagglutinin (HA) subunit protein are often of low potency, requiring high dose or boosting to generate a sustained immune response. We have improved the immunogenicity of a plant-made HA vaccine by chemical conjugation to the surface of the Tobacco mosaic virus (TMV) which is non infectious in mammals. We have previously shown that TMV is taken up by mammalian dendritic cells and is a highly effective antigen carrier. In this work, we tested several TMV-HA conjugation chemistries, and compared immunogenicity in mice as measured by anti-HA IgG titers and hemagglutination inhibition (HAI). Importantly, pre-existing immunity to TMV did not reduce initial or boosted titers. Further optimization included dosing with and without alum or oil-in water adjuvants. Surprisingly, we were able to stimulate potent immunogenicity and HAI titers with a single 15 µg dose of HA as a TMV conjugate. We then evaluated the efficacy of the TMV-HA vaccine in a lethal virus challenge in mice. Our results show that a single dose of the TMV-HA conjugate vaccine is sufficient to generate 50% survival, or 100% survival with adjuvant, compared with 10% survival after vaccination with a commercially available H1N1 vaccine. TMV-HA is an effective dose-sparing influenza vaccine, using a single-step process to rapidly generate large quantities of highly effective flu vaccine from an otherwise low potency HA subunit protein.

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 immunogenicity of porcine reproductive and respiratory syndrome virus antigen expressed in transgenic banana

Porcine reproductive and respiratory syndrome virus (PRRSV) is a persistent threat of economically significant influence to the swine industry worldwide. Recombinant DNA technology coupled with tissue culture technology is a viable alternative for the inexpensive production of heterologous proteins in planta. Embryogenic cells of banana cv. 'Pei chiao' (AAA) have been transformed with the ORF5 gene of PRRSV envelope glycoprotein (GP5) using Agrobacterium-mediated transformation and have been confirmed. Recombinant GP5 protein levels in the transgenic banana leaves were detected and ranged from 0.021%-0.037% of total soluble protein. Pigs were immunized with recombinant GP5 protein by orally feeding transgenic banana leaves for three consecutive doses at a 2-week interval and challenged with PRRSV at 7 weeks postinitial immunization. A vaccination-dependent gradational increase in the elicitation of serum and saliva anti-PRRSV IgG and IgA was observed. Furthermore, significantly lower viraemia and tissue viral load were recorded when compared with the pigs fed with untransformed banana leaves. The results suggest that transgenic banana leaves expressing recombinant GP5 protein can be an effective strategy for oral delivery of recombinant subunit vaccines in pigs and can open new avenues for the production of vaccines against PRRSV.

The effect of plant tissue and vaccine formulation on the oral immunogenicity of a model plant-made antigen in sheep

Antigen-specific antibody responses against a model antigen (the B subunit of the heat labile toxin of enterotoxigenic Escherichia coli, LTB) were studied in sheep following oral immunisation with plant-made and delivered vaccines. Delivery from a root-based vehicle resulted in antigen-specific immune responses in mucosal secretions of the abomasum and small intestine and mesenteric lymph nodes. Immune responses from the corresponding leaf-based vaccine were more robust and included stimulation of antigen-specific antibodies in mucosal secretions of the abomasum. These findings suggest that oral delivery of a plant bioencapsulated antigen can survive passage through the rumen to elicit mucosal and systemic immune responses in sheep. Moreover, the plant tissue used as the vaccine delivery vehicle affects the magnitude of these responses.

Progress and hurdles in the development of vaccines against enterotoxigenic Escherichia coli in humans

Diarrhea is the second leading cause of death in children younger than 5 years. Enterotoxigenic Escherichia coli (ETEC) strains are the most common bacterial cause of diarrhea in young children living in endemic countries and children and adults traveling to these areas. Pathogenesis of ETEC diarrhea has been well studied, and the key virulence factors are bacterial colonization factor antigens and enterotoxins produced by ETEC strains. Colonization factor antigens mediate bacteria attachment to host small intestinal epithelial cells and subsequent colonization, whereas enterotoxins including heat-labile and heat-stable toxins disrupt fluid homeostasis in host epithelial cells, which leads to fluid and electrolyte hypersecretion and diarrhea. Vaccines stimulating host anti-adhesin immunity to block ETEC attachment and colonization and also antitoxin immunity to neutralize enterotoxicity are considered optimal for prevention of ETEC diarrhea. Vaccines under development have been designed to stimulate local intestinal immunity and are either oral vaccines or transcutaneous vaccines. A cholera vaccine (Dukoral®) does stimulate anti-heat-labile toxin immunity and is licensed for short-term protection of ETEC diarrhea in travelers in some countries. Newer experimental ETEC vaccine candidates are being developed with hope to provide long-lasting and more broad-based protection against ETEC. Some have shown promising results in safety and immunogenicity studies and are approaching field trials for efficacy. A key problem is the development of a vaccine that is both practical and inexpensive so that it can be affordable for use in poor countries where it is needed.

Agrobacterium-mediated gene transfer in plants and biosafety considerations

Agrobacterium, the natures' genetic engineer, has been used as a vector to create transgenic plants. Agrobacterium-mediated gene transfer in plants is a highly efficient transformation process which is governed by various factors including genotype of the host plant, explant, vector, plasmid, bacterial strain, composition of culture medium, tissue damage, and temperature of co-cultivation. Agrobacterium has been successfully used to transform various economically and horticulturally important monocot and dicot species by standard tissue culture and in planta transformation techniques like floral or seedling infilteration, apical meristem transformation, and the pistil drip methods. Monocots have been comparatively difficult to transform by Agrobacterium. However, successful transformations have been reported in the last few years based on the adjustment of the parameters that govern the responses of monocots to Agrobacterium. A novel Agrobacterium transferred DNA-derived nanocomplex method has been developed which will be highly valuable for plant biology and biotechnology. Agrobacterium-mediated genetic transformation is known to be the preferred method of creating transgenic plants from a commercial and biosafety perspective. Agrobacterium-mediated gene transfer predominantly results in the integration of foreign genes at a single locus in the host plant, without associated vector backbone and is also known to produce marker free plants, which are the prerequisites for commercialization of transgenic crops. Research in Agrobacterium-mediated transformation can provide new and novel insights into the understanding of the regulatory process controlling molecular, cellular, biochemical, physiological, and developmental processes occurring during Agrobacterium-mediated transformation and also into a wide range of aspects on biological safety of transgenic crops to improve crop production to meet the demands of ever-growing world's population.

Safety and immunogenicity of bacterial and tobacco plant cell line derived recombinant native and mutant Escherichia coli heat-labile toxin in chickens

The safety and immunogenicity of the mammalian mucosal adjuvants, Escherichia coli wild-type heat-labile holotoxin (LT) and E. coli mutant LT (LTA-K63/LTB), were examined in 1-day-old chicks and 10-day-old to 21-day-old broilers. Biologically active, E. coli recombinant wild-type LT and recombinant LTA-K63/LTB produced in a transgenic Nicotiana tabacum (NT-1) tobacco cell line (SLT102) were tested for safety and antigenicity following various routes of administration. Safety was assessed by clinical signs, body weight gain, gross organ pathology and wet organ weight, and histopathology. Antigenicity was assessed using LT-B-specific serum IgG enzyme-linked immunosorbent assay. Parenteral administration of E. coli recombinant wild-type LT did not have any discernible effect on bird health and was well tolerated at levels up to 400 µg per dose. Recombinant, SLT102-derived mutant LT derived from SLT102 cells retained in vitro ganglioside binding and was safe and antigenic following repeated mucosal administration to birds. The highest systemic LT-B-specific IgG titres were detected in birds that received three on-feed doses of SLT102-derived mutant LT. Among the various SLT102-derived mutant LT preparations tested, whole, wet cells or whole cell lysates were the most antigenic. These results demonstrate for the first time that E. coli-derived recombinant, wild-type LT holotoxin is well tolerated following multiple administrations to young birds at body weight doses previously reported to be enteropathogenic and toxic in mammalian species. Moreover, these data also demonstrate the feasibility of using recombinant wild-type and mutant LT produced in transgenic NT-1 tobacco cells as safe and potent vaccine adjuvants in poultry.

Novel vaccine development strategies for inducing mucosal immunity

To develop protective immune responses against mucosal pathogens, the delivery route and adjuvants for vaccination are important. The host, however, strives to maintain mucosal homeostasis by responding to mucosal antigens with tolerance, instead of immune activation. Thus, induction of mucosal immunity through vaccination is a rather difficult task, and potent mucosal adjuvants, vectors or other special delivery systems are often used, especially in the elderly. By taking advantage of the common mucosal immune system, the targeting of mucosal dendritic cells and microfold epithelial cells may facilitate the induction of effective mucosal immunity. Thus, novel routes of immunization and antigen delivery systems also show great potential for the development of effective and safe mucosal vaccines against various pathogens. The purpose of this review is to introduce several recent approaches to induce mucosal immunity to vaccines, with an emphasis on mucosal tissue targeting, new immunization routes and delivery systems. Defining the mechanisms of mucosal vaccines is as important as their efficacy and safety, and in this article, examples of recent approaches, which will likely accelerate progress in mucosal vaccine development, are discussed.

Plant-derived antigens as mucosal vaccines

During the last two decades, researchers have developed robust systems for recombinant subunit vaccine production in plants. Stably and transiently transformed plants have particular advantages that enable immunization of humans and animals via mucosal delivery. The initial goal to immunize orally by ingestion of plant-derived antigens has proven difficult to attain, although many studies have demonstrated antibody production in both humans and animals, and in a few cases, protection against pathogen challenge. Substantial hurdles for this strategy are low-antigen content in crudely processed plant material and limited antigen stability in the gut. An alternative is intranasal delivery of purified plant-derived antigens expressed with robust viral vectors, especially virus-like particles. The use of pattern recognition receptor agonists as adjuvants for mucosal delivery of plant-derived antigens can substantially enhance serum and mucosal antibody responses. In this chapter, we briefly review the methods for recombinant protein expression in plants, and describe progress with human and animal vaccines that use mucosal delivery routes. We do not attempt to compile a comprehensive list, but focus on studies that progressed to clinical trials or those that showed strong indications of efficacy in animals. Finally, we discuss some regulatory concerns regarding plant-based vaccines.

Expression of the cholera toxin B subunit (CT-B) in maize seeds and a combined mucosal treatment against cholera and traveler's diarrhea

The non-toxic B subunit (CT-B) of cholera toxin from Vibrio cholerae is a strong immunogen and amplifies the immune reaction to conjugated antigens. In this work, a synthetic gene encoding for CT-B was expressed under control of a γ-zein promoter in maize seeds. Levels of CT-B in maize plants were determined via ganglioside dependent ELISA. The highest expression level recorded in T(1) generation seeds was 0.0014% of total aqueous soluble protein (TASP). Expression level of the same event in the T(2) generation was significantly increased to 0.0197% of TASP. Immunogenicity of maize derived CT-B was evaluated in mice with an oral immunization trial. Anti-CTB IgG and anti-CTB IgA were detected in the sera and fecal samples of the orally immunized mice, respectively. The mice were protected against holotoxin challenge with CT. An additional group of mice was administrated with an equal amount (5 μg per dose each) of mixed maize-derived CT-B and LT-B (B subunit of E. coli heat labile toxin). In the sera and fecal samples obtained from this group, the specific antibody levels were enhanced compared to either the same or a higher amount of CT-B alone. These results suggest that a synergistic action may be achieved using a CT-B and LT-B mixture that can lead to a more efficacious combined vaccine to target diarrhea induced by both cholera and enterotoxigenic strains of Escherichia coli.