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

Molecular farming using transgenic rice endosperm

Plant expression platforms are low-cost, scalable, safe, and environmentally friendly systems for the production of recombinant proteins and bioactive metabolites. Rice (Oryza sativa L.) endosperm is an ideal bioreactor for the production and storage of high-value active substances, including pharmaceutical proteins, oral vaccines, vitamins, and nutraceuticals such as flavonoids and carotenoids. Here, we explore the use of molecular farming from producing medicines to developing functional food crops (biofortification). We review recent progress in producing pharmaceutical proteins and bioactive substances in rice endosperm and compare this platform with other plant expression systems. We describe how rice endosperm could be modified to design metabolic pathways and express and store stable products and discuss the factors restricting the commercialization of transgenic rice products and future prospects.

Improving Protein Quantity and Quality—The Next Level of Plant Molecular Farming

Plants offer several unique advantages in the production of recombinant pharmaceuticals for humans and animals. Although numerous recombinant proteins have been expressed in plants, only a small fraction have been successfully put into use. The hugely distinct expression systems between plant and animal cells frequently cause insufficient yield of the recombinant proteins with poor or undesired activity. To overcome the issues that greatly constrain the development of plant-produced pharmaceuticals, great efforts have been made to improve expression systems and develop alternative strategies to increase both the quantity and quality of the recombinant proteins. Recent technological revolutions, such as targeted genome editing, deconstructed vectors, virus-like particles, and humanized glycosylation, have led to great advances in plant molecular farming to meet the industrial manufacturing and clinical application standards. In this review, we discuss the technological advances made in various plant expression platforms, with special focus on the upstream designs and milestone achievements in improving the yield and glycosylation of the plant-produced pharmaceutical proteins.

A historical and systematic overview of Ascaris vaccine development

Ascariasis is the most prevalent helminth infection in the world and leads to significant, life-long morbidity, particularly in young children. Current efforts to control and eradicate ascariasis in endemic regions have been met with significant challenges including high-rates of re-infection and potential development of anthelminthic drug resistance. Vaccines against ascariasis are a key tool that could break the transmission cycle and lead to disease eradication globally. Evolution of the Ascaris vaccine pipeline has progressed, however no vaccine product has been brought to human clinical trials to date. Advancement in recombinant protein technology may provide the first step in generating an Ascaris vaccine as well as a pan-helminthic vaccine ready for human trials. However, several roadblocks remain and investment in new technologies will be important to develop a successful human Ascaris vaccine that is critically needed to prevent significant morbidity in Ascaris-endemic regions around the world.

Ma J, McDonald KA, Murad A, Nandi S, O’Keef B, Parthiban S, Paul MJ, Ponndorf D, Rech E, Rodrigues JC, Ruf S, Schillberg S, Schwestka J, Shah PS, Singh R, Stoger E, Twyman RM, Varghese IP, Vianna GR, Webster G, Wilbers RHP, Christou P, Oksman‐Caldentey K, Capell T. Contributions of the international plant science community to the fight against infectious diseases in humans-part 2: Affordable drugs in edible plants for endemic and re-emerging diseases

The fight against infectious diseases often focuses on epidemics and pandemics, which demand urgent resources and command attention from the health authorities and media. However, the vast majority of deaths caused by infectious diseases occur in endemic zones, particularly in developing countries, placing a disproportionate burden on underfunded health systems and often requiring international interventions. The provision of vaccines and other biologics is hampered not only by the high cost and limited scalability of traditional manufacturing platforms based on microbial and animal cells, but also by challenges caused by distribution and storage, particularly in regions without a complete cold chain. In this review article, we consider the potential of molecular farming to address the challenges of endemic and re-emerging diseases, focusing on edible plants for the development of oral drugs. Key recent developments in this field include successful clinical trials based on orally delivered dried leaves of Artemisia annua against malarial parasite strains resistant to artemisinin combination therapy, the ability to produce clinical-grade protein drugs in leaves to treat infectious diseases and the long-term storage of protein drugs in dried leaves at ambient temperatures. Recent FDA approval of the first orally delivered protein drug encapsulated in plant cells to treat peanut allergy has opened the door for the development of affordable oral drugs that can be manufactured and distributed in remote areas without cold storage infrastructure and that eliminate the need for expensive purification steps and sterile delivery by injection.

Identification and Physicochemical Characterization of a New Allergen from Ascaris lumbricoides

To analyze the impact of Ascaris lumbricoides infection on the pathogenesis and diagnosis of allergic diseases, new allergens should be identified. We report the identification of a new Ascaris lumbricoides allergen, Asc l 5. The aim of this study was to evaluate the physicochemical and immunological features of the Asc l 5 allergen. We constructed an A. lumbricoides cDNA library and Asc l 5 was identified by immunoscreening. After purification, rAsc l 5 was physicochemically characterized. Evaluation of its allergenic activity included determination of Immunoglobulin E (IgE) binding frequency (in two populations: 254 children and 298 all-age subjects), CD203c based-basophil activation tests (BAT) and a passive cutaneous anaphylaxis (PCA) mouse model. We found by amino acid sequence analysis that Asc l 5 belongs to the SXP/RAL-2 protein family of nematodes. rAsc l 5 is a monomeric protein with an alpha-helical folding. IgE sensitization to rAsc l 5 was around 52% in general population; positive BAT rate was 60%. rAsc l 5 induced specific IgE production in mice and a positive PCA reaction. These results show that Asc l 5 has structural and immunological characteristics to be considered as a new allergen from A. lumbricoides.

Soil-Transmitted Helminth Vaccines: Are We Getting Closer?

Parasitic helminths infect over one-fourth of the human population resulting in significant morbidity, and in some cases, death in endemic countries. Despite mass drug administration (MDA) to school-aged children and other control measures, helminth infections are spreading into new areas. Thus, there is a strong rationale for developing anthelminthic vaccines as cost-effective, long-term immunological control strategies, which, unlike MDA, are not haunted by the threat of emerging drug-resistant helminths nor limited by reinfection risk. Advances in vaccinology, immunology, and immunomics include the development of new tools that improve the safety, immunogenicity, and efficacy of vaccines; and some of these tools have been used in the development of helminth vaccines. The development of anthelminthic vaccines is fraught with difficulty. Multiple lifecycle stages exist each presenting stage-specific antigens. Further, helminth parasites are notorious for their ability to dampen down and regulate host immunity. One of the first significant challenges in developing any vaccine is identifying suitable candidate protective antigens. This review explores our current knowledge in lead antigen identification and reports on recent pre-clinical and clinical trials in the context of the soil-transmitted helminths Trichuris, the hookworms and Ascaris. Ultimately, a multivalent anthelminthic vaccine could become an essential tool for achieving the medium-to long-term goal of controlling, or even eliminating helminth infections.

Parasites of the Giant Panda: A Risk Factor in the Conservation of a Species

The giant panda, with an estimated population size of 2239 in the world (in 2015), is a global symbol of wildlife conservation that is threatened by habitat loss, poor reproduction and limited resistance to some infectious diseases. Of these factors, some diseases caused by parasites are considered as the foremost threat to its conservation. However, there is surprisingly little published information on the parasites of the giant panda, most of which has been disseminated in the Chinese literature. Herein, we review all peer-reviewed publications (in English or Chinese language) and governmental documents for information on parasites of the giant pandas, with an emphasis on the intestinal nematode Baylisascaris schroederi (McIntosh, 1939) as it dominates published literature. The purpose of this chapter is to: (i) review the parasites recorded in the giant panda and describe what is known about their biology; (ii) discuss key aspects of the pathogenesis, diagnosis, treatment and control of key parasites that are reported to cause clinical problems and (iii) conclude by making some suggestions for future research. This chapter shows that we are only just 'scratching the surface' when it comes to parasites and parasitological research of the giant panda. Clearly, there needs to be a concerted research effort to support the conservation of this iconic species.

Yeast-expressed recombinant As16 protects mice against Ascaris suum infection through induction of a Th2-skewed immune response

Background

Ascariasis remains the most common helminth infection in humans. As an alternative or complementary approach to global deworming, a pan-anthelminthic vaccine is under development targeting Ascaris, hookworm, and Trichuris infections. As16 and As14 have previously been described as two genetically related proteins from Ascaris suum that induced protective immunity in mice when formulated with cholera toxin B subunit (CTB) as an adjuvant, but the exact protective mechanism was not well understood.

Methodology/Principal findings

As16 and As14 were highly expressed as soluble recombinant proteins (rAs16 and rAs14) in Pichia pastoris. The yeast-expressed rAs16 was highly recognized by immune sera from mice infected with A. suum eggs and elicited 99.6% protection against A. suum re-infection. Mice immunized with rAs16 formulated with ISA720 displayed significant larva reduction (36.7%) and stunted larval development against A. suum eggs challenge. The protective immunity was associated with a predominant Th2-type response characterized by high titers of serological IgG1 (IgG1/IgG2a > 2000) and high levels of IL-4 and IL-5 produced by restimulated splenocytes. A similar level of protection was observed in mice immunized with rAs16 formulated with alum (Alhydrogel), known to induce mainly a Th2-type immune response, whereas mice immunized with rAs16 formulated with MPLA or AddaVax, both known to induce a Th1-type biased response, were not significantly protected against A. suum infection. The rAs14 protein was not recognized by A. suum infected mouse sera and mice immunized with rAs14 formulated with ISA720 did not show significant protection against challenge infection, possibly due to the protein’s inaccessibility to the host immune system or a Th1-type response was induced which would counter a protective Th2-type response.

Conclusions/Significance

Yeast-expressed rAs16 formulated with ISA720 or alum induced significant protection in mice against A. suum egg challenge that associates with a Th2-skewed immune response, suggesting that rAS16 could be a feasible vaccine candidate against ascariasis.

Roundworms (Ascaris) infect more than 700 million people living in poverty worldwide and cause malnutrition and physical and mental developmental delays in children. As an alternative or complementary approach to global deworming, a pan-anthelminthic vaccine is under development that targets ascariasis in addition to other human intestinal nematode infections. Towards this goal, two Ascaris suum antigens, As16 and As14, were expressed in Pichia pastoris as recombinant proteins. Mice immunized with rAs16 formulated with ISA720 adjuvant produced significant larva reduction (36.7%) and stunted larval development against A. suum egg challenge. The protection was associated with predominant Th2-type responses characterized by high levels of serological IgG1 (IgG1/IgG2a > 2,000) and Th2 cytokines, IL-4 and IL-5. A similar level of protection was observed in mice immunized with rAs16 formulated with alum that induces mainly a Th2-type immune response, whereas mice immunized with rAs16 formulated with MPLA or AddaVax, both inducing major Th1-type responses, were not significantly protected against A. suum infection. High-yield expression of rAs16 in yeast will allow for large-scale manufacture, and its protective efficacy when formulated with alum suggests its suitability as a vaccine candidate.

The Last Ten Years of Advancements in Plant-Derived Recombinant Vaccines against Hepatitis B

Disease prevention through vaccination is considered to be the greatest contribution to public health over the past century. Every year more than 100 million children are vaccinated with the standard World Health Organization (WHO)-recommended vaccines including hepatitis B (HepB). HepB is the most serious type of liver infection caused by the hepatitis B virus (HBV), however, it can be prevented by currently available recombinant vaccine, which has an excellent record of safety and effectiveness. To date, recombinant vaccines are produced in many systems of bacteria, yeast, insect, and mammalian and plant cells. Among these platforms, the use of plant cells has received considerable attention in terms of intrinsic safety, scalability, and appropriate modification of target proteins. Research groups worldwide have attempted to develop more efficacious plant-derived vaccines for over 30 diseases, most frequently HepB and influenza. More inspiring, approximately 12 plant-made antigens have already been tested in clinical trials, with successful outcomes. In this study, the latest information from the last 10 years on plant-derived antigens, especially hepatitis B surface antigen, approaches are reviewed and breakthroughs regarding the weak points are also discussed.

Production and immunogenicity of Actinobacillus pleuropneumoniae ApxIIA protein in transgenic rice callus

Actinobacillus pleuropneumoniae is a major etiological agent that is responsible for swine pleuropneumonia, a highly contagious respiratory infection that causes severe economic losses in the swine production industry. ApxIIA is one of the virulence factors in A. pleuropneumoniae and has been considered as a candidate for developing a vaccine against the bacterial infection. A gene encoding an ApxIIA fragment (amino acids 439-801) was modified based on a plant-optimized codon and constructed into a plant expression vector under the control of a promoter and the 3' UTR of the rice amylase 3D gene. The plant expression vector was introduced into rice embryogenic callus (Oryza sativa L. cv. Dongjin) via particle bombardment-mediated transformation. The integration and transcription of the ApxIIA_439-801 gene were confirmed by using genomic DNA PCR amplification and Northern blot analysis, respectively. The synthesis of ApxIIA_439-801 antigen protein in transgenic rice callus was confirmed by western blot analysis. The concentration of antigen protein in lyophilized samples of transgenic rice callus was 250 μg/g. Immunizing mice with protein extracts from transgenic plants intranasally elicited secretory IgA. These results demonstrate the feasibility of using a transgenic plant to elicit immune responses against A. pleuropneumoniae.

Production of Biologically Active Cecropin A Peptide in Rice Seed Oil Bodies

Cecropin A is a natural antimicrobial peptide that exhibits fast and potent activity against a broad spectrum of pathogens and neoplastic cells, and that has important biotechnological applications. However, cecropin A exploitation, as for other antimicrobial peptides, is limited by their production and purification costs. Here, we report the efficient production of this bioactive peptide in rice bran using the rice oleosin 18 as a carrier protein. High cecropin A levels were reached in rice seeds driving the expression of the chimeric gene by the strong embryo-specific oleosin 18 own promoter, and targeting the peptide to the oil body organelle as an oleosin 18-cecropin A fusion protein. The accumulation of cecropin A in oil bodies had no deleterious effects on seed viability and seedling growth, as well as on seed yield. We also show that biologically active cecropin A can be easily purified from the transgenic rice seeds by homogenization and simple flotation centrifugation methods. Our results demonstrate that the oleosin fusion technology is suitable for the production of cecropin A in rice seeds, which can potentially be extended to other antimicrobial peptides to assist their exploitation.

Rice seed for delivery of vaccines to gut mucosal immune tissues

Gut-associated lymphoid tissue (GALT) is the biggest lymphoid organ in the body. It plays a role in robust immune responses against invading pathogens while maintaining immune tolerance against nonpathogenic antigens such as foods. Oral vaccination can induce mucosal and systemic antigen-specific immune reactions and has several advantages including ease of administration, no requirement for purification and ease of scale-up of antigen. Thus far, taking advantage of these properties, various plant-based oral vaccines have been developed. Seeds provide a superior production platform over other plant tissues for oral vaccines; they offer a suitable delivery vehicle to GALT due to their high stability at room temperature, ample and stable deposition space, high expression level, and protection from digestive enzymes in gut. A rice seed production system for oral vaccines was established by combining stable deposition in protein bodies or protein storage vacuoles and enhanced endosperm-specific expression. Various types of rice-based oral vaccines for infectious and allergic diseases were generated. Efficacy of these rice-based vaccines was evaluated in animal models.

The Corn Smut ('Huitlacoche') as a New Platform for Oral Vaccines

The development of new alternative platforms for subunit vaccine production is a priority in the biomedical field. In this study, Ustilago maydis, the causal agent of common corn smut or ‘huitlacoche’has been genetically engineered to assess expression and immunogenicity of the B subunit of the cholera toxin (CTB), a relevant immunomodulatory agent in vaccinology. An oligomeric CTB recombinant protein was expressed in corn smut galls at levels of up to 1.3 mg g-1 dry weight (0.8% of the total soluble protein). Mice orally immunized with ‘huitlacoche’-derived CTB showed significant humoral responses that were well-correlated with protection against challenge with the cholera toxin (CT). These findings demonstrate the feasibility of using edible corn smut as a safe, effective, and low-cost platform for production and delivery of a subunit oral vaccine. The implications of this platform in the area of molecular pharming are discussed.

Dengue virus E glycoprotein production in transgenic rice callus

Dengue is a disease caused by dengue virus and represents the most important arthropod-borne viral disease in humans. Dengue virus enters host cells via binding of envelope glycoprotein (E) to a receptor. In this study, plant expression vectors containing native and synthetic glycoprotein E genes (sE) modified based on plant-optimized codon usage and fused with an ER retention signal were constructed under control of the rice amylase 3D promoter expression system. Plant expression vectors were introduced into rice callus (Oryza sativa L. cv. Dongin) via particle bombardment-mediated transformation. The integration and expression of target genes were confirmed in the transgenic callus by genomic DNA PCR and Northern blot analyses, respectively. The plant-codon optimized sE gene with an ER retention signal showed high protein production levels based on Western blot analysis of approximately 18.5 ug/g dried calli weight by immunoblot-based densitometric analysis. These results suggest that the plant-codon optimized sE gene with an ER retention signal was highly produced in the transgenic rice callus.

Novel transgenic rice-based vaccines

Oral vaccination can induce both systemic and mucosal antigen-specific immune responses. To control rampant mucosal infectious diseases, the development of new effective oral vaccines is needed. Plant-based vaccines are new candidates for oral vaccines, and have some advantages over the traditional vaccines in cost, safety, and scalability. Rice seeds are attractive for vaccine production because of their stability and resistance to digestion in the stomach. The efficacy of some rice-based vaccines for infectious, autoimmune, and other diseases has been already demonstrated in animal models. We reported the efficacy in mice, safety, and stability of a rice-based cholera toxin B subunit vaccine called MucoRice-CTB. To advance MucoRice-CTB for use in humans, we also examined its efficacy and safety in primates. The potential of transgenic rice production as a new mucosal vaccine delivery system is reviewed from the perspective of future development of effective oral vaccines.

The use of rice seeds to produce human pharmaceuticals for oral therapy

Rice (Oryza sativa L.) is the major staple food consumed by half of the world's population. Rice seeds have gained recent attention as bioreactors for the production of human pharmaceuticals such as therapeutic proteins or peptides. Rice seed production platforms have many advantages over animal cell or microbe systems in terms of cost-effectiveness, scalability, safety, product stability and productivity. Rice seed-based human pharmaceuticals are expected to become innovative therapies as edible drugs. Therapeutic proteins can be sequestered within natural cellular compartments in rice seeds and protected from harsh gastrointestinal environments. This review presents the state-of-the-art on the construction of gene cassettes for accumulation of pharmaceutical proteins or peptides in rice seeds, the generation of transgenic rice plants, and challenges involved in the use of rice seeds to produce human pharmaceuticals.

Update on the use of transgenic rice seeds in oral immunotherapy

Rice seed provides an ideal production platform for pharmaceuticals in terms of high productivity and stability, as well as the scalability, safety and economy that are expected in plant production systems. Furthermore, these therapeutic products are bioencapsulated in protein bodies, which are seed-specific storage organelles that provide protection from digestion by gastrointestinal enzymes during delivery to the gut-associated lymphoid tissue. Thus, rice seed provides an ideal delivery system for the mucosal immune system. Oral immunotherapy using unprocessed transgenic rice seed containing therapeutic products has been demonstrated to induce effective mucosal immune tolerance and immune reactions against allergies and pathogens.

Edible vaccines against veterinary parasitic diseases--current status and future prospects

Protection of domestic animals against parasitic infections remains a major challenge in most of the developing countries, especially in the surge of drug resistant strains. In this circumstance vaccination seems to be the sole practical strategy to combat parasites. Most of the presently available live or killed parasitic vaccines possess many disadvantages. Thus, expression of parasitic antigens has seen a continued interest over the past few decades. However, only a limited success was achieved using bacterial, yeast, insect and mammalian expression systems. This is witnessed by an increasing number of reports on transgenic plant expression of previously reported and new antigens. Oral delivery of plant-made vaccines is particularly attractive due to their exceptional advantages. Moreover, the regulatory burden for veterinary vaccines is less compared to human vaccines. This led to an incredible investment in the field of transgenic plant vaccines for veterinary purpose. Plant based vaccine trials have been conducted to combat various significant parasitic diseases such as fasciolosis, schistosomosis, poultry coccidiosis, porcine cycticercosis and ascariosis. Besides, passive immunization by oral delivery of antibodies expressed in transgenic plants against poultry coccidiosis is an innovative strategy. These trials may pave way to the development of promising edible veterinary vaccines in the near future. As the existing data regarding edible parasitic vaccines are scattered, an attempt has been made to assemble the available literature.

Molecular strategies to engineer transgenic rice seed compartments for large-scale production of plant-made pharmaceuticals

The use of plants as bioreactors for the large-scale production of recombinant proteins has emerged as an exciting area of research. The current shortages in protein therapeutics due to the capacity and economic bottlenecks faced with modern protein production platforms (microbial, yeast, mammalian) has driven considerable attention towards molecular pharming. Utilizing plants for the large-scale production of recombinant proteins is estimated to be 2-10% the cost of microbial platforms, and up to 1,000-fold more cost effective than mammalian platforms (Twyman et al. Trends Biotechnol 21:570-578, 2003; Sharma and Sharma, Biotechnol Adv 27:811-832, 2009). In order to achieve an economically feasible plant production host, protein expression and accumulation must be optimized. The seed, and more specifically the rice seed has emerged as an ideal candidate in molecular pharming due to its low protease activity, low water content, stable protein storage environment, relatively high biomass, and the molecular tools available for manipulation (Lau and Sun, Biotechnol Adv 27:1015-1022, 2009).

Developing plant-based vaccines against neglected tropical diseases: where are we?

Neglected tropical diseases (NTDs) impair the lives of 1 billion people worldwide, and threaten the health of millions more. Although vaccine candidates have been proposed to prevent some NTDs, no vaccine is available at the market yet. Vaccines against NTDs should be low-cost and needle-free to reduce the logistic cost of their administration. Plant-based vaccines meet both requirements: plant systems allow antigen production at low cost, and also yield an optimal delivery vehicle that prevents or delays digestive hydrolysis of vaccine antigens. This review covers recent reports on the development of plant-based vaccines against NTDs. Efforts conducted by a number of research groups to develop vaccines as a mean to fight rabies, cysticercosis, dengue, and helminthiasis are emphasized. Future perspectives are identified, such as the need to develop vaccination models for more than ten pathologies through a plant-based biotechnological approach. Current limitations on the method are also noted, and molecular approaches that might allow us to address such limitations are discussed.

Merozoite surface protein-1 of Plasmodium yoelii fused via an oligosaccharide moiety of cholera toxin B subunit glycoprotein expressed in yeast induced protective immunity against lethal malaria infection in mice

Methylotrophic yeast (Pichia pastoris) secreted cholera toxin B subunit (CTB) predominantly as a biologically active pentamer (PpCTB) with identical ganglioside binding affinity profiles to that of choleragenoid. Unlike choleragenoid, however, the PpCTB did not induce a footpad edema response in mice. Of the two potential glycosylation sites (NIT(4-6) and NKT(90-92)) for this protein, a N-linked oligosaccharide was identified at Asn4. The oligosaccharide, presumed to extend from the lateral circumference of the CTB pentamer ring structure, was exploited as a site-specific anchoring scaffold for the C-terminal 19-kDa merozoite surface protein-1 (MSP1-19) of the rodent malaria parasite, Plasmodium yoelii. Conjugation of MSP1-19 to PpCTB via its oligosaccharide moiety induced higher protective efficacy against lethal parasite infection than conjugation directly to the PpCTB protein body in both intranasal and subcutaneous immunization regimes. Such increased protection was potentially due to the higher antigen loading capacity of CTB achieved when the antigen was linked to the extended branches of the oligosaccharide. This might have allowed the antigen to reside in more spacious molecular environment with less steric hindrance between the constituent molecules of the fusion complex.

Transgenic peanut (Arachis hypogaea L.) expressing the urease subunit B gene of Helicobacter pylori

Helicobacter pylori (H. pylori) has been identified as the main pathogenic factors of chronic gastritis and peptic ulcer, and the Class I carcinogen of gastric cancer by WHO. Vaccine has become the most effective measure to prevent and cure H. pylori infection. The UreB is the most effective and common immunogen of all strains of H. pylori and may stimulate the immunoresponse protecting the human body against the challenge of H. pylori. UreB antigen gene was cloned into the binary vector pBI121 which contains a seed-specific promoter Oleosin of peanut and a kanamycin resistance gene, and then UreB gene was transformed into peanut embryo leaflets by Agrobacter-mediated method. The putative transgenic plants were examined for the presence of UreB in the nuclear genome of peanut plants by PCR analysis. Expression of UreB gene in plants was identified by RT-PCR and Western blot analysis. These results suggest that the UreB transgenic peanut can be potentially used as an edible vaccine for controlling H. pylori.

Role of transgenic plants in agriculture and biopharming

At present, environmental degradation and the consistently growing population are two main problems on the planet earth. Fulfilling the needs of this growing population is quite difficult from the limited arable land available on the globe. Although there are legal, social and political barriers to the utilization of biotechnology, advances in this field have substantially improved agriculture and human life to a great extent. One of the vital tools of biotechnology is genetic engineering (GE) which is used to modify plants, animals and microorganisms according to desired needs. In fact, genetic engineering facilitates the transfer of desired characteristics into other plants which is not possible through conventional plant breeding. A variety of crops have been engineered for enhanced resistance to a multitude of stresses such as herbicides, insecticides, viruses and a combination of biotic and abiotic stresses in different crops including rice, mustard, maize, potato, tomato, etc. Apart from the use of GE in agriculture, it is being extensively employed to modify the plants for enhanced production of vaccines, hormones, etc. Vaccines against certain diseases are certainly available in the market, but most of them are very costly. Developing countries cannot afford the disease control through such cost-intensive vaccines. Alternatively, efforts are being made to produce edible vaccines which are cheap and have many advantages over the commercialized vaccines. Transgenic plants generated for this purpose are capable of expressing recombinant proteins including viral and bacterial antigens and antibodies. Common food plants like banana, tomato, rice, carrot, etc. have been used to produce vaccines against certain diseases like hepatitis B, cholera, HIV, etc. Thus, the up- and down-regulation of desired genes which are used for the modification of plants have a marked role in the improvement of genetic crops. In this review, we have comprehensively discussed the role of genetic engineering in generating transgenic lines/cultivars of different crops with improved nutrient quality, biofuel production, enhanced production of vaccines and antibodies, increased resistance against insects, herbicides, diseases and abiotic stresses as well as the safety measures for their commercialization.

Two decades of plant-based candidate vaccines: a review of the chimeric protein approaches

Genetic engineering revolutionized the concept of traditional vaccines since subunit vaccines became reality. Additionally, over the past two decades plant-derived antigens have been studied as potential vaccines with several advantages, including low cost and convenient administration. More specifically, genetic fusions allowed the expression of fusion proteins carrying two or more components with the aim to elicit immune responses against different targets, including antigens from distinct pathogens or strains. This review aims to provide an update in the field of the production of plant-based vaccine, focusing on those approaches based on the production of chimeric proteins comprising antigens from human pathogens, emphasizing the case of cholera toxin/E. coli enterotoxin fusions, chimeric viruses like particles approaches as well as the possible use of adjuvant-producing plants as expression hosts. Challenges for the near future in this field are also discussed.

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

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

Effect of codon optimization and subcellular targeting on Toxoplasma gondii antigen SAG1 expression in tobacco leaves to use in subcutaneous and oral immunization in mice

BACKGROUND

Codon optimization and subcellular targeting were studied with the aim to increase the expression levels of the SAG178-322 antigen of Toxoplasma gondii in tobacco leaves. The expression of the tobacco-optimized and native versions of the SAG1 gene was explored by transient expression from the Agrobacterium tumefaciens binary expression vector, which allows targeting the recombinant protein to the endoplasmic reticulum (ER) and the apoplast. Finally, mice were subcutaneously and orally immunized with leaf extracts-SAG1 and the strategy of prime boost with rSAG1 expressed in Escherichia coli was used to optimize the oral immunization with leaf extracts-SAG1.

RESULTS

Leaves agroinfiltrated with an unmodified SAG1 gene accumulated 5- to 10-fold more than leaves agroinfiltrated with a codon-optimized SAG1 gene. ER localization allowed the accumulation of higher levels of native SAG1. However, no significant differences were observed between the mRNA accumulations of the different versions of SAG1. Subcutaneous immunization with leaf extracts-SAG1 (SAG1) protected mice against an oral challenge with a non-lethal cyst dose, and this effect could be associated with the secretion of significant levels of IFN-gamma. The protection was increased when mice were ID boosted with rSAG1 (SAG1+boost). This group elicited a significant Th1 humoral and cellular immune response characterized by high levels of IFN-gamma. In an oral immunization assay, the SAG1+boost group showed a significantly lower brain cyst burden compared to the rest of the groups.

CONCLUSION

Transient agroinfiltration was useful for the expression of all of the recombinant proteins tested. Our results support the usefulness of endoplasmic reticulum signal peptides in enhancing the production of recombinant proteins meant for use as vaccines. The results showed that this plant-produced protein has potential for use as vaccine and provides a potential means for protecting humans and animals against toxoplasmosis.

Plant-made vaccines for humans and animals

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

Intranasal vaccination of calves with Mannheimia haemolytica chimeric protein containing the major surface epitope of outer membrane lipoprotein PlpE, the neutralizing epitope of leukotoxin, and cholera toxin subunit B

This study was done to determine if intranasal vaccination of weaned beef calves with a chimeric protein containing the immunodominant surface epitope of Mannheimia haemolytica PlpE (R2) and the neutralizing epitope of leukotoxin (NLKT) covalently linked to truncated cholera toxin (CT) subunit B (CTB) could stimulate secretory and systemic antibodies against M. haemolytica while enhancing resistance of cattle against M. haemolytica intrabronchial challenge. Sixteen weaned beef calves were intranasally vaccinated with CTB-R2-NLKT chimeric (SAC102) or with R2-NLKT-R2-NLKT chimeric (SAC89) protein with or without native CT on days 0 and 14 and were challenged intrabronchially on day 28. In vitro, SAC102 bound the CT receptor molecule, GM(1)-ganglioside. Mean IgA antibodies to M. haemolytica whole cells (WC) and to LKT were high on day 0. A small, yet significant increase (p<0.05) was found in mean nasal antibodies to M. haemolytica WC for the SAC89+CT and SAC102 vaccinates after the second vaccination. SAC102 stimulated significant (p<0.05) mean serum antibody responses to all three antigens by day 28. Following challenge, mean antibodies to WC and LKT significantly increased (p<0.05) for the SAC102, SAC89 and SAC89+CT groups with the mean antibody responses to rPlpE stimulated by SAC102 vaccination being significantly higher (p<0.05) than for the other vaccinated and control groups. On day 1 after challenge, mean clinical score for the control group was significantly higher (p<0.05) than for the SAC102 and SAC89+CT vaccinates, and by day 2 after challenge, clinical score for the control group was significantly higher (p<0.05) than for all three chimeric vaccinated groups. Therefore, intranasal vaccination with CTB-R2-NLKT (SAC102) and R2-NLKT-R2-NLKT (SAC89) chimeric proteins enhanced resistance against intrabronchial challenge with the bacterium as well as stimulating antibody responses to M. haemolytica antigens.