Expression of Bacillus anthracis protective antigen in transgenic chloroplasts of tobacco, a non-food/feed crop

A biotechnological approach for the production of new protein bioplastics

The future of biomaterial production will leverage biotechnology based on the domestication of cells as biological factories. Plants, algae, and bacteria can produce low-environmental impact biopolymers. Here, two strategies were developed to produce a biopolymer derived from a bioengineered vacuolar storage protein of the common bean (phaseolin; PHSL). The cys-added PHSL* forms linear-structured biopolymers when expressed in the thylakoids of transplastomic tobacco leaves by exploiting the formation of inter-chain disulfide bridges. The same protein without signal peptide (ΔPHSL*) accumulates in Escherichia coli inclusion bodies as high-molar-mass species polymers that can subsequently be oxidized to form disulfide crosslinking bridges in order to increase the stiffness of the biomaterial, a valid alternative to the use of chemical crosslinkers. The E. coli cells produced 300 times more engineered PHSL, measured as percentage of total soluble proteins, than transplastomic tobacco plants. Moreover, the thiol groups of cysteine allow the site-specific PEGylation of ΔPHSL*, which is a desirable functionality in the design of a protein-based drug carrier. In conclusion, ΔPHSL* expressed in E. coli has the potential to become an innovative biopolymer.

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.

Overexpression of the recombinant human interferon-beta (rhIFN-β) gene in tobacco chloroplasts

Chloroplast genetic engineering is a convenient method for the production of recombinant proteins by increasing the expression level of transgenes. Interferon-beta (IFN-β) is a member of type I interferons that possess some pharmaceutical properties. The present study aimed to investigate the overexpression and production of the recombinant human IFN-β gene (rhIFN-β) in the tobacco chloroplast genome. For this purpose, a codon-optimized rhIFN-β was transferred to the pVSR326 plastid vector containing the aadA gene as a selectable marker. The rhIFN-β gene was then successfully introduced into the tobacco chloroplast genome by using a gene gun. The integration of the rhIFN-β gene into the chloroplast genome and the homoplasmy of the T1 progeny were confirmed by PCR and Southern blot analysis, respectively. RT-PCR and western blot analyses confirmed the transcription and translation of the rhIFN-β gene, respectively. An enzyme-linked immunosorbent assay (ELISA) showed that the rhIFN-β protein in transplastomic plants comprised approximately 2.4% of total soluble protein (TSPs). The bioassay confirmed that the rhIFN-β protein expressed in the tobacco chloroplast had a relatively high biological activity (2.9 × 10⁴ IU/ml) and protected human amnionic cells against the vesicular stomatitis virus (VSV). On the basis of these findings, it can be concluded that plastid transformation can serve as an operative method for the production of pharmaceutical recombinant proteins.

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.

Perspectives on protein biopolymers: miniaturized flow field-flow fractionation-assisted characterization of a single-cysteine mutated phaseolin expressed in transplastomic tobacco plants

The development of plant-based protein polymers to employ in biofilm production represents the promising intersection between material science and sustainability, and allows to obtain biodegradable materials that also possess excellent physicochemical properties. A possible candidate for protein biopolymer production is phaseolin, a storage protein highly abundant in P Vulgaris beans. We previously showed that transformed tobacco chloroplasts could be employed to express a mutated phaseolin carrying a signal peptide (directing it into the thylakoids) also enriched of a cysteine residue added to its C-terminal region. This modification allows for the formation of inter-chain disulfide bonds, as we previously demonstrated, and should promote polymerization. To verify the effect of the peptide modification and to quantify polymer formation, we employed hollow-fiber flow field-flow fractionation coupled to UV and multi-angle laser scattering detection (HF5-UV-MALS): HF5 allows for the selective size-based separation of phaseolin species, whereas MALS calculates molar mass and conformation state of each population. With the use of two different HF5 separation methods we first observed the native state of P.Vulgaris phaseolin, mainly assembled into trimers, and compared it to mutated phaseolin (P*) which instead resulted highly aggregated. Then we further characterized P* using a second separation method, discriminating between two and distinct high-molecular weight (HMW) species, one averaging 0.8 × 106 Da and the second reaching the tens of million Da. Insight on the conformation of these HMW species was offered from their conformation plots, which confirmed the positive impact of the Cys modification on polymerization.

Plastome engineering in vegetable crops: current status and future prospects

Plastome (plastid genome) engineering has grown up and got smarter for the transgene expression. Plastid transformation has profound benefits over nuclear transformation, includes a higher level of transgene expression, integration via homologous recombination, transgene containment, lack of gene silencing, and position effect. Substantial and fruitful progress has been achieved in plastome engineering of vegetable crops through the use of improved regeneration/selection procedures, plastid transformation vectors with efficient promoters, and 3^/, 5^/regulatory sequences. Plastid transformation technology developed for vegetable crops being used as a platform for the production of industrially important proteins and some of the genes of agronomic importance has been stably integrated and expressed in plastome. Although great progress has been accomplished in the plastid transformation of vegetable crops, still it is restricted to few species because of the unavailability of whole plastome sequencing. In this review, the author focus on the technology, progress, and advancements in plastid transformation of vegetable plants such as lettuce, tomato, potato, cabbage, cauliflower, eggplant, carrot, soybean, and bitter melon are reviewed. The conclusions, future prospects, and expansion of plastid transformation technology to other vegetable crops for genetic improvement and production of edible vaccines are proposed.

Potential Therapeutic Targeting of Coronavirus Spike Glycoprotein Priming

Processing of certain viral proteins and bacterial toxins by host serine proteases is a frequent and critical step in virulence. The coronavirus spike glycoprotein contains three (S1, S2, and S2′) cleavage sites that are processed by human host proteases. The exact nature of these cleavage sites, and their respective processing proteases, can determine whether the virus can cross species and the level of pathogenicity. Recent comparisons of the genomes of the highly pathogenic SARS-CoV2 and MERS-CoV, with less pathogenic strains (e.g., Bat-RaTG13, the bat homologue of SARS-CoV2) identified possible mutations in the receptor binding domain and in the S1 and S2′ cleavage sites of their spike glycoprotein. However, there remains some confusion on the relative roles of the possible serine proteases involved for priming. Using anthrax toxin as a model system, we show that in vivo inhibition of priming by pan-active serine protease inhibitors can be effective at suppressing toxicity. Hence, our studies should encourage further efforts in developing either pan-serine protease inhibitors or inhibitor cocktails to target SARS-CoV2 and potentially ward off future pandemics that could develop because of additional mutations in the S-protein priming sequence in coronaviruses.

Oral delivery of novel human IGF-1 bioencapsulated in lettuce cells promotes musculoskeletal cell proliferation, differentiation and diabetic fracture healing

Human insulin-like growth factor-1 (IGF-1) plays important roles in development and regeneration of skeletal muscles and bones but requires daily injections or surgical implantation. Current clinical IGF-1 lacks e-peptide and is glycosylated, reducing functional efficacy. In this study, codon-optimized Pro-IGF-1 with e-peptide (fused to GM1 receptor binding protein CTB or cell penetrating peptide PTD) was expressed in lettuce chloroplasts to facilitate oral delivery. Pro-IGF-1 was expressed at high levels in the absence of the antibiotic resistance gene in lettuce chloroplasts and was maintained in subsequent generations. In lyophilized plant cells, Pro-IGF-1 maintained folding, assembly, stability and functionality up to 31 months, when stored at ambient temperature. CTB-Pro-IGF-1 stimulated proliferation of human oral keratinocytes, gingiva-derived mesenchymal stromal cells and mouse osteoblasts in a dose-dependent manner and promoted osteoblast differentiation through upregulation of ALP, OSX and RUNX2 genes. Mice orally gavaged with the lyophilized plant cells significantly increased IGF-1 levels in sera, skeletal muscles and was stable for several hours. When bioencapsulated CTB-Pro-IGF-1 was gavaged to femoral fractured diabetic mice, bone regeneration was significantly promoted with increase in bone volume, density and area. This novel delivery system should increase affordability and patient compliance, especially for treatment of musculoskeletal diseases.

Molecular, biochemical, and proteomic analyses of transplastomic tobacco plants expressing an endoglucanase support chloroplast-based molecular farming for industrial scale production of enzymes

The successful production of recombinant enzymes by tobacco transplastomic plants must maintain compatibility of the heterologous enzyme with chloroplast metabolism and its long-time enzyme stability. Based on previous reports, it has been taken for granted that following biolistic-transformation, homoplasticity could be obtained from the initially heteroplastic state following successive rounds of selection in the presence of the selection agent. However, several studies indicated that this procedure does not always ensure the complete elimination of unmodified wild-type plastomes. The present study demonstrates that CelK1 transplastomic plants, which were photosyntetically as active as untransformed ones, remain heteroplastomic even after repeated selection steps and that this state does not impair the relatively high-level production of the recombinant enzyme. In fact, even in the heteroplastomic state, the recombinant protein represented about 6% of the total soluble proteins (TSP). Moreover, our data also show that, while the recombinant endoglucanase undergoes phosphorylation, this post-translation modification does not have any significant impact on the enzymatic activity. Biomass storage might be required whenever the enzyme extraction process could not be performed immediately following the harvest of tobacco mature plants. In this respect, we have observed that enzyme activity in the detached leaves stored at 4 °C is maintained up to 20 weeks without significant loss of activity. These findings may have major implications in the future of chloroplast genetic engineering-based molecular farming to produce industrial enzymes in transplastomic plants.

Comparison of transplastomic Chlamydomonas reinhardtii and Nicotiana tabacum expression system for the production of a bacterial endoglucanase

The bulk production of recombinant enzymes by either prokaryotic or eukaryotic organisms might contribute to replace environmentally non-friendly chemistry-based industrial processes with enzyme-based biocatalysis, provided the cost of enzyme production is low. In this context, it is worth noting that the production of recombinant proteins by photosynthetic organisms offer both eukaryotic (nuclear) and prokaryotic (chloroplast) alternatives, along with the advantage of an autotrophic nutrition. Compared to nuclear transformation, chloroplast transformation generally allows a higher level of accumulation of the recombinant protein of interest. Furthermore, among the photosynthetic organisms, there is a choice of using either multicellular or unicellular ones. Tobacco, being a non-food and non-feed plant, has been considered as a good choice for producing enzymes with applications in technical industry, using a transplastomic approach. Also, unicellular green algae, in particular Chlamydomonas reinhardtii, have been proposed as candidate organisms for the production of recombinant proteins. In the light of the different features of these two transplastomic systems, we decided to make a direct comparison of the efficiency of production of a bacterial endoglucanase. With respect to the amount obtained, 14 mg g-1 of biomass fresh weight equivalent to 8-10% of the total protein content and estimated production cost, 1.5-2€ kg-1, tobacco proved to be far more favorable for bulk enzyme production when compared to C. reinhardtii which accumulated this endoglucanase at 0.003% of the total protein.

Construction of Plastid Expression Vector and Development of Genetic Transformation System for the Seaweed Pyropia yezoensis

Pyropia yezoensis, belonging to the Rhodophyta, is an economically important seaweed. In this study, we developed a high-efficiency plastid transformation platform for P. yezoensis. In the plastid transformation vector, psbA UTR of P. yezoensis, including the promoter and 3' UTR, was used to express foreign genes. The integration site was a transcriptionally active intergenic region between the rrsB and trnI genes, located in the inverted repeat regions of the plastid genome. The CAT and eGFP genes were integrated into the plastid genome at this site. The expression of CAT in the transformants confers resistance to chloramphenicol through the action of chloramphenicol acetyltransferase, which inactivates the drug, thereby allowing the plant to grow well under selective pressure. The eGFP fluorescence signal was also observed in transformed cells and the transformants. The average survival rate of treated cells was estimated to be approximately 4.2‰ (4 transplastomic colonies per 1000 gametophyte cells). Multiple-PCR analyses confirmed that the CAT and eGFP genes were successfully integrated in the site between rrsB and trnI. Western blot also showed eGFP expression in the cells of transformants. Thus, this study presents the first convenient plastid gene expression system for P. yezoensis and provides an important platform for studying gene function in P. yezoensis.

Comparison of effectiveness of 5'-regulatory sequences in transplastomic tobacco chloroplasts

The development of tools which ensure the desired level of transgene expression in plastids is a prerequisite for the effective utilization of these plant organelles for the deployment of bioactive proteins. High-level accumulation of target proteins is considered as a positive feature of transplastomic plants, but excessive accumulation of foreign proteins may have deleterious effects on host plants. On the other hand, expression at low levels can result in ineffective phenotypes. We compared the effectiveness of different 5'-regulatory sequences in driving the expression of a reporter gene, β-glucuronidase (uidA), in tobacco chloroplasts. To achieve varying expression levels, we have chosen heterologous 5'-regulatory sequences which either differ significantly from their homologous counterparts or depend on specific nuclear encoded factors. The Medicago truncatula psbA promoter/5'-UTR supported the highest levels of protein accumulation, surpassing the other tested sequences by two to three orders of magnitude. The heterologous regulatory sequence of Phaseolus vulgaris rbcL gene was as efficient in tobacco chloroplasts as the corresponding homologous promoter/5'-UTR. The Arabidopsis thaliana ndhF promoter/5'-UTR supported as high reporter activity levels as the rbcL 5'-sequences, whereas the effectiveness of A. thaliana psbN promoter/5'-UTR was three fold lower. The characterized regulatory sequences can be utilized to establish transplastomic lines with desirable levels of target protein accumulation. The ability to control transgene expression should be useful for achieving appropriate levels of protein accumulation and thereby avoid their negative impacts on host plant physiology.

Progress toward the Development of a NEAT Protein Vaccine for Anthrax Disease

Bacillus anthracis is a sporulating Gram-positive bacterium that is the causative agent of anthrax and a potential weapon of bioterrorism. The U.S.-licensed anthrax vaccine is made from an incompletely characterized culture supernatant of a nonencapsulated, toxigenic strain (anthrax vaccine absorbed [AVA]) whose primary protective component is thought to be protective antigen (PA). AVA is effective in protecting animals and elicits toxin-neutralizing antibodies in humans, but enthusiasm is dampened by its undefined composition, multishot regimen, recommended boosters, and potential for adverse reactions. Improving next-generation anthrax vaccines is important to safeguard citizens and the military. Here, we report that vaccination with recombinant forms of a conserved domain (near-iron transporter [NEAT]), common in Gram-positive pathogens, elicits protection in a murine model of B. anthracis infection. Protection was observed with both Freund's and alum adjuvants, given subcutaneously and intramuscularly, respectively, with a mixed composite of NEATs. Protection correlated with an antibody response against the NEAT domains and a decrease in the numbers of bacteria in major organs. Anti-NEAT antibodies promote opsonophagocytosis of bacilli by alveolar macrophages. To guide the development of inactive and safe NEAT antigens, we also report the crystal structure of one of the NEAT domains (Hal) and identify critical residues mediating its heme-binding and acquisition activity. These results indicate that we should consider NEAT proteins in the development of an improved antianthrax vaccine.

Compartmentalized Metabolic Engineering for Artemisinin Biosynthesis and Effective Malaria Treatment by Oral Delivery of Plant Cells

Artemisinin is highly effective against drug-resistant malarial parasites, which affects nearly half of the global population and kills >500 000 people each year. The primary cost of artemisinin is the very expensive process used to extract and purify the drug from Artemisia annua. Elimination of this apparently unnecessary step will make this potent antimalarial drug affordable to the global population living in endemic regions. Here we reported the oral delivery of a non-protein drug artemisinin biosynthesized (∼0.8 mg/g dry weight) at clinically meaningful levels in tobacco by engineering two metabolic pathways targeted to three different cellular compartments (chloroplast, nucleus, and mitochondria). The doubly transgenic lines showed a three-fold enhancement of isopentenyl pyrophosphate, and targeting AACPR, DBR2, and CYP71AV1 to chloroplasts resulted in higher expression and an efficient photo-oxidation of dihydroartemisinic acid to artemisinin. Partially purified extracts from the leaves of transgenic tobacco plants inhibited in vitro growth progression of Plasmodium falciparum-infected red blood cells. Oral feeding of whole intact plant cells bioencapsulating the artemisinin reduced the parasitemia levels in challenged mice in comparison with commercial drug. Such novel synergistic approaches should facilitate low-cost production and delivery of artemisinin and other drugs through metabolic engineering of edible plants.

Plant-based vaccines against respiratory diseases: current status and future prospects

INTRODUCTION

Respiratory infections have an enormous, worldwide epidemiologic impact on humans and animals. Among the prophylactic measures, vaccination has the potential to neutralize this impact. New technologies for vaccine production and delivery are of importance in this field since they offer the potential to develop new immunization approaches overriding the current limitations that comprise high cost, safety issues, and limited efficacy. Areas covered: In the present review, the state of the art in developing plant-based vaccines against respiratory diseases is presented. The review was based on the analysis of current biomedical literature. Expert commentary: Preclinical and clinical evaluations of several vaccine candidates against influenza, tuberculosis, respiratory syncytial virus, pneumonia, anthrax and asthma are discussed and placed in perspective.

Chloroplasts: state of research and practical applications of plastome sequencing

This review presents origins, structure and expression of chloroplast genomes. It also describes their sequencing, analysis and modification, focusing on potential practical uses and biggest challenges of chloroplast genome modification. During the evolution of eukaryotes, cyanobacteria are believed to have merged with host heterotrophic cell. Afterward, most of cyanobacterial genes from cyanobacteria were transferred to cell nucleus or lost in the process of endosymbiosis. As a result of these changes, a primary plastid was established. Nowadays, plastid genome (plastome) is almost always circular, has a size of 100-200 kbp (120-160 in land plants), and harbors 100-120 highly conserved unique genes. Plastids have their own gene expression system, which is similar to one of their cyanobacterial ancestors. Two different polymerases, plastid-derived PEP and nucleus-derived NEP, participate in transcription. Translation is similar to the one observed in cyanobacteria, but it also utilizes protein translation factors and positive regulatory mRNA elements absent from bacteria. Plastoms play an important role in genetic transformation. Transgenes are introduced into them either via gene gun (in undamaged tissues) or polyethylene glycol treatment (when protoplasts are targeted). Antibiotic resistance markers are the most common tool used for selection of transformed plants. In recent years, plastome transformation emerged as a promising alternative to nuclear transformation because of (1) high yield of target protein, (2) removing the risk of outcrossing with weeds, (3) lack of silencing mechanisms, and (4) ability to engineer the entire metabolic pathways rather than single gene traits. Currently, the main directions of such research regard: developing efficient enzyme, vaccine antigen, and biopharmaceutical protein production methods in plant cells and improving crops by increasing their resistance to a wide array of biotic and abiotic stresses. Because of that, the detailed knowledge of plastome structure and mechanism of functioning started to play a major role.

Chloroplast genomes: diversity, evolution, and applications in genetic engineering

Chloroplasts play a crucial role in sustaining life on earth. The availability of over 800 sequenced chloroplast genomes from a variety of land plants has enhanced our understanding of chloroplast biology, intracellular gene transfer, conservation, diversity, and the genetic basis by which chloroplast transgenes can be engineered to enhance plant agronomic traits or to produce high-value agricultural or biomedical products. In this review, we discuss the impact of chloroplast genome sequences on understanding the origins of economically important cultivated species and changes that have taken place during domestication. We also discuss the potential biotechnological applications of chloroplast genomes.

Plant-Based Vaccines: Production and Challenges

Plant-based vaccine technologies involve the integration of the desired genes encoding the antigen protein for specific disease into the genome of plant tissues by various methods. Agrobacterium-mediated gene transfer and transformation via genetically modified plant virus are the common methods that have been used to produce effective vaccines. Nevertheless, with the advancement of science and technology, new approaches have been developed to increase the efficiency of former methods such as biolistic, electroporation, agroinfiltration, sonication, and polyethylene glycol treatment. Even though plant-based vaccines provide many benefits to the vaccine industry, there are still challenges that limit the rate of successful production of these third-generation vaccines. Even with all the limitations, continuous efforts are still ongoing in order to produce efficient vaccine for many human and animals related diseases owing to its great potentials. This paper reviews the existing conventional methods as well as the development efforts by researchers in order to improve the production of plant-based vaccines. Several challenges encountered during and after the production process were also discussed.

Phaseolin expression in tobacco chloroplast reveals an autoregulatory mechanism in heterologous protein translation

Plastid DNA engineering is a well-established research area of plant biotechnology, and plastid transgenes often give high expression levels. However, it is still almost impossible to predict the accumulation rate of heterologous protein in transplastomic plants, and there are many cases of unsuccessful transgene expression. Chloroplasts regulate their proteome at the post-transcriptional level, mainly through translation control. One of the mechanisms to modulate the translation has been described in plant chloroplasts for the chloroplast-encoded subunits of multiprotein complexes, and the autoregulation of the translation initiation of these subunits depends on the availability of their assembly partners [control by epistasy of synthesis (CES)]. In Chlamydomonas reinhardtii, autoregulation of endogenous proteins recruited in the assembly of functional complexes has also been reported. In this study, we revealed a self-regulation mechanism triggered by the accumulation of a soluble recombinant protein, phaseolin, in the stroma of chloroplast-transformed tobacco plants. Immunoblotting experiments showed that phaseolin could avoid this self-regulation mechanism when targeted to the thylakoids in transplastomic plants. To inhibit the thylakoid-targeted phaseolin translation as well, this protein was expressed in the presence of a nuclear version of the phaseolin gene with a transit peptide. Pulse-chase and polysome analysis revealed that phaseolin mRNA translation on plastid ribosomes was repressed due to the accumulation in the stroma of the same soluble polypeptide imported from the cytosol. We suggest that translation autoregulation in chloroplast is not limited to heteromeric protein subunits but also involves at least some of the foreign soluble recombinant proteins, leading to the inhibition of plastome-encoded transgene expression in chloroplast.

A mutant Synechococcus gene encoding glutamate 1-semialdehyde aminotransferase confers gabaculine resistance when expressed in tobacco plastids

KEY MESSAGE

A mutant glutamate 1-semialdehyde aminotransferase gene from the Synechococcus , inserted into tobacco plastid DNA by means of particle bombardment and antibiotic selection, conferred gabaculine resistance allowing to attain homoplasmy. Many plant species are recalcitrant to plastid genome transformation. New selections systems may help to overcome this limitation and to extend the application of this technology. A mutant hemL gene from the photosynthetic cyanobacterium Synechococcus, encoding a gabaculine-insensitive glutamate 1-semialdehyde aminotransferase (GSA), is an efficient selectable marker gene for nuclear transformation of tobacco, alfalfa and durum wheat. Since GSA functions in the plastid, we introduced the mutant hemL gene into the tobacco plastid genome along with the conventional antibiotic resistance aadA gene, in the attempt to develop a new selection system for plastome transformation. Although we were unable to directly regenerate gabaculine resistant transplastomic plants, we demonstrated the functionality of hemL in tobacco plastids by using gabaculine selection in the second and third rounds of in vitro selection that permitted to obtain the homoplasmic state in transgenic plants. Thus, the mutant hemL gene functions as a secondary selection marker in tobacco plastids. Our results encourage further attempts to test gabaculine resistant GSA for plastome transformation of crop plants in which gabaculine has stronger regeneration-inhibiting effects with respect to tobacco.

Plastids: The Green Frontiers for Vaccine Production

Infectious diseases pose an increasing risk to health, especially in developing countries. Vaccines are available to either cure or prevent many of these diseases. However, there are certain limitations related to these vaccines, mainly the costs, which make these vaccines mostly unaffordable for people in resource poor countries. These costs are mainly related to production and purification of the products manufactured from fermenter-based systems. Plastid biotechnology has become an attractive platform to produce biopharmaceuticals in large amounts and cost-effectively. This is mainly due to high copy number of plastids DNA in mature chloroplasts, a characteristic particularly important for vaccine production in large amounts. An additional advantage lies in the maternal inheritance of plastids in most plant species, which addresses the regulatory concerns related to transgenic plants. These and many other aspects of plastids will be discussed in the present review, especially those that particularly make these green biofactories an attractive platform for vaccine production. A summary of recent vaccine antigens against different human diseases expressed in plastids will also be presented.

Plant-produced candidate countermeasures against emerging and reemerging infections and bioterror agents

Despite progress in the prevention and treatment of infectious diseases, they continue to present a major threat to public health. The frequency of emerging and reemerging infections and the risk of bioterrorism warrant significant efforts towards the development of prophylactic and therapeutic countermeasures. Vaccines are the mainstay of infectious disease prophylaxis. Traditional vaccines, however, are failing to satisfy the global demand because of limited scalability of production systems, long production timelines and product safety concerns. Subunit vaccines are a highly promising alternative to traditional vaccines. Subunit vaccines, as well as monoclonal antibodies and other therapeutic proteins, can be produced in heterologous expression systems based on bacteria, yeast, insect cells or mammalian cells, in shorter times and at higher quantities, and are efficacious and safe. However, current recombinant systems have certain limitations associated with production capacity and cost. Plants are emerging as a promising platform for recombinant protein production due to time and cost efficiency, scalability, lack of harboured mammalian pathogens and possession of the machinery for eukaryotic post-translational protein modification. So far, a variety of subunit vaccines, monoclonal antibodies and therapeutic proteins (antivirals) have been produced in plants as candidate countermeasures against emerging, reemerging and bioterrorism-related infections. Many of these have been extensively evaluated in animal models and some have shown safety and immunogenicity in clinical trials. Here, we overview ongoing efforts to producing such plant-based countermeasures.

Production by Tobacco Transplastomic Plants of Recombinant Fungal and Bacterial Cell-Wall Degrading Enzymes to Be Used for Cellulosic Biomass Saccharification

Biofuels from renewable plant biomass are gaining momentum due to climate change related to atmospheric CO2 increase. However, the production cost of enzymes required for cellulosic biomass saccharification is a major limiting step in this process. Low-cost production of large amounts of recombinant enzymes by transgenic plants was proposed as an alternative to the conventional microbial based fermentation. A number of studies have shown that chloroplast-based gene expression offers several advantages over nuclear transformation due to efficient transcription and translation systems and high copy number of the transgene. In this study, we expressed in tobacco chloroplasts microbial genes encoding five cellulases and a polygalacturonase. Leaf extracts containing the recombinant enzymes showed the ability to degrade various cell-wall components under different conditions, singly and in combinations. In addition, our group also tested a previously described thermostable xylanase in combination with a cellulase and a polygalacturonase to study the cumulative effect on the depolymerization of a complex plant substrate. Our results demonstrate the feasibility of using transplastomic tobacco leaf extracts to convert cell-wall polysaccharides into reducing sugars, fulfilling a major prerequisite of large scale availability of a variety of cell-wall degrading enzymes for biofuel industry.

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.

How can plant genetic engineering contribute to cost-effective fish vaccine development for promoting sustainable aquaculture?

Aquaculture, the fastest growing food-producing sector, now accounts for nearly 50 % of the world's food fish (FAO in The state of world fisheries and aquaculture. FAO, Rome, 2010). The global aquaculture production of food fish reached 62.7 million tonnes in 2011 and is continuously increasing with an estimated production of food fish of 66.5 million tonnes in 2012 (a 9.4 % increase in 1 year, FAO, www.fao.org/fishery/topic/16140 ). Aquaculture is not only important for sustainable protein-based food fish production but also for the aquaculture industry and economy worldwide. Disease prevention is the key issue to maintain a sustainable development of aquaculture. Widespread use of antibiotics in aquaculture has led to the development of antibiotic-resistant bacteria and the accumulation of antibiotics in the environment, resulting in water and soil pollution. Thus, vaccination is the most effective and environmentally-friendly approach to combat diseases in aquaculture to manage fish health. Furthermore, when compared to >760 vaccines against human diseases, there are only about 30 fish vaccines commercially available, suggesting the urgent need for development and cost-effective production of fish vaccines for managing fish health, especially in the fast growing fish farming in Asia where profit is minimal and therefore given high priority. Plant genetic engineering has made significant contributions to production of biotech crops for food, feed, valuable recombinant proteins etc. in the past three decades. The use of plants for vaccine production offers several advantages such as low cost, safety and easy scaling up. To date a large number of plant-derived vaccines, antibodies and therapeutic proteins have been produced for human health, of which a few have been made commercially available. However, the development of animal vaccines in plants, especially fish vaccines by genetic engineering, has not yet been addressed. Therefore, there is a need to exploit plant biotechnology for cost effective fish vaccine development in plants, in particular, edible crops for oral fish vaccines. This review provides insight into (1) the current status of fish vaccine and vaccination in aquaculture, (2) plant biotechnology and edible crops for fish vaccines for oral administration, (3) regulatory constraints and (4) conclusions and future perspectives.

Oral delivery of human biopharmaceuticals, autoantigens and vaccine antigens bioencapsulated in plant cells

Among 12billion injections administered annually, unsafe delivery leads to >20million infections and >100million reactions. In an emerging new concept, freeze-dried plant cells (lettuce) expressing vaccine antigens/biopharmaceuticals are protected in the stomach from acids/enzymes but are released to the immune or blood circulatory system when plant cell walls are digested by microbes that colonize the gut. Vaccine antigens bioencapsulated in plant cells upon oral delivery after priming, conferred both mucosal and systemic immunity and protection against bacterial, viral or protozoan pathogens or toxin challenge. Oral delivery of autoantigens was effective against complications of type 1 diabetes and hemophilia, by developing tolerance. Oral delivery of proinsulin or exendin-4 expressed in plant cells regulated blood glucose levels similar to injections. Therefore, this new platform offers a low cost alternative to deliver different therapeutic proteins to combat infectious or inherited diseases by eliminating inactivated pathogens, expensive purification, cold storage/transportation and sterile injections.

Stable plastid transformation for high-level recombinant protein expression: promises and challenges

Plants are a promising expression system for the production of recombinant proteins. However, low protein productivity remains a major obstacle that limits extensive commercialization of whole plant and plant cell bioproduction platform. Plastid genetic engineering offers several advantages, including high levels of transgenic expression, transgenic containment via maternal inheritance, and multigene expression in a single transformation event. In recent years, the development of optimized expression strategies has given a huge boost to the exploitation of plastids in molecular farming. The driving forces behind the high expression level of plastid bioreactors include codon optimization, promoters and UTRs, genotypic modifications, endogenous enhancer and regulatory elements, posttranslational modification, and proteolysis. Exciting progress of the high expression level has been made with the plastid-based production of two particularly important classes of pharmaceuticals: vaccine antigens, therapeutic proteins, and antibiotics and enzymes. Approaches to overcome and solve the associated challenges of this culture system that include low transformation frequencies, the formation of inclusion bodies, and purification of recombinant proteins will also be discussed.

Plastid genetic engineering in Solanaceae

Plastid genetic engineering has come of age, becoming today an attractive alternative approach for the expression of foreign genes, as it offers several advantages over nuclear transformants. Significant progress has been made in plastid genetic engineering in tobacco and other Solanaceae plants, through the use of improved regeneration procedures and transformation vectors with efficient promoters and untranslated regions. Many genes encoding for industrially important proteins and vaccines, as well as genes conferring important agronomic traits, have been stably integrated and expressed in the plastid genome. Despite these advances, it remains a challenge to achieve marked levels of plastid transgene expression in non-green tissues. In this review, we summarize the basic requirements of plastid genetic engineering and discuss the current status, limitations, and the potential of plastid transformation for expanding future studies relating to Solanaceae plants.

Expression of the affinity tags, glutathione-S-transferase and maltose-binding protein, in tobacco chloroplasts

Chloroplast transformation offers an exciting platform for the safe, inexpensive and large-scale production of recombinant proteins in plants. An important advantage for the isolation of proteins produced in the chloroplast would be the use of affinity tags for rapid purification by affinity chromatography. To date, only His-tags have been used. In this study, we have tested the feasibility of expressing two additional affinity tags: glutathione-S-transferase (GST) and a His-tagged derivative of the maltose-binding protein (His₆-MBP). By using the chloroplast 16S rRNA promoter and 5' untranslated region of phage T7 gene 10, GST and His₆-MBP were expressed in homoplastomic tobacco plants at approximately 7% and 37% of total soluble protein, respectively. GST could be purified by one-step-affinity purification using a glutathione column. Much better recoveries were obtained for His₆-MBP by using a twin-affinity purification procedure involving first immobilised nickel followed by binding to amylose. Interestingly, expression of GST led to cytoplasmic male sterility. Overall, our work expands the tools available for purifying recombinant proteins from the chloroplast.

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.

A plant secretory signal peptide targets plastome-encoded recombinant proteins to the thylakoid membrane

Plastids are considered promising bioreactors for the production of recombinant proteins, but the knowledge of the mechanisms regulating foreign protein folding, targeting, and accumulation in these organelles is still incomplete. Here we demonstrate that a plant secretory signal peptide is able to target a plastome-encoded recombinant protein to the thylakoid membrane. The fusion protein zeolin with its native signal peptide expressed by tobacco (Nicotiana tabacum) transplastomic plants was directed into the chloroplast thylakoid membranes, whereas the zeolin mutant devoid of the signal peptide, Δzeolin, is instead accumulated in the stroma. We also show that zeolin folds in the thylakoid membrane where it accumulates as trimers able to form disulphide bonds. Disulphide bonds contribute to protein accumulation since zeolin shows a higher accumulation level with respect to stromal Δzeolin, whose folding is hampered as the protein accumulates at low amounts in a monomeric form and it is not oxidized. Thus, post-transcriptional processes seem to regulate the stability and accumulation of plastid-synthesized zeolin. The most plausible zeolin targeting mechanism to thylakoid is discussed herein.

A plant based protective antigen [PA(dIV)] vaccine expressed in chloroplasts demonstrates protective immunity in mice against anthrax

The currently available anthrax vaccines are limited by being incompletely characterized, potentially reactogenic and have an expanded dosage schedule. Plant based vaccines offer safe alternative for vaccine production. In the present study, we expressed domain IV of Bacillus anthracis protective antigen gene [PA(dIV)] in planta (by nuclear agrobacterium and chloroplast transformation) and E. coli [rPA(dIV)]. The presence of transgene and the expression of PA(dIV) in planta was confirmed by molecular analysis. Expression levels up to 5.3% of total soluble protein (TSP) were obtained with AT rich (71.8% AT content) PA(dIV) gene in transplastomic plants while 0.8% of TSP was obtained in nuclear transformants. Further, we investigated the protective response of plant and E. coli derived PA(dIV) in mice by intraperitoneal (i.p.) and oral immunizations with or without adjuvant. Antibody titers of >10(4) were induced upon i.p. and oral immunizations with plant derived PA(dIV) and oral immunization with E. coli derived PA(dIV). Intraperitoneal injections with adjuvanted E. coli derived PA(dIV), generated highest antibody titers of >10(5). All the immunized groups demonstrated predominant IgG1 titers over IgG2a indicating a polarized Th2 type response. We also evaluated the mucosal antibody response in orally immunized groups. When fecal extracts were analyzed, low sIgA titer was demonstrated in adjuvanted plant and E. coli derived PA(dIV) groups. Further, PA(dIV) antisera enhanced B. anthracis spore uptake by macrophages in vitro and also demonstrated an anti-germinating effect suggesting a potent role at mucosal surfaces. The antibodies from various groups were efficient in neutralizing the lethal toxin in vitro. When mice were challenged with B. anthracis, mice immunized with adjuvanted plant PA(dIV) imparted 60% and 40% protection while E. coli derived PA(dIV) conferred 100% and 80% protection upon i.p. and oral immunizations. Thus, our study is the first attempt in highlighting the efficacy of plant expressed PA(dIV) by oral immunization in murine model.

Chloroplast-derived vaccines against human diseases: achievements, challenges and scopes

Infectious diseases represent a continuously growing menace that has severe impact on health of the people worldwide, particularly in the developing countries. Therefore, novel prevention and treatment strategies are urgently needed to reduce the rate of these diseases in humans. For this reason, different options can be considered for the production of affordable vaccines. Plants have been proved as an alternative expression system for various compounds of biological importance. Particularly, plastid genetic engineering can be potentially used as a tool for cost-effective vaccine production. Antigenic proteins from different viruses and bacteria have been expressed in plastids. Initial immunological studies of chloroplast-derived vaccines have yielded promising results in animal models. However, because of certain limitations, these vaccines face many challenges on production and application level. Adaptations to the novel approaches are needed, which comprise codon usage and choice of proven expression cassettes for the optimal yield of expressed proteins, use of inducible systems, marker gene removal, selection of specific antigens with high immunogenicity and development of tissue culture systems for edible crops to prove the concept of low-cost edible vaccines. As various aspects of plant-based vaccines have been discussed in recent reviews, here we will focus on certain aspects of chloroplast transformation related to vaccine production against human diseases.

Expression and characterization of antimicrobial peptides Retrocyclin-101 and Protegrin-1 in chloroplasts to control viral and bacterial infections

Retrocyclin-101 (RC101) and Protegrin-1 (PG1) are two important antimicrobial peptides that can be used as therapeutic agents against bacterial and/or viral infections, especially those caused by the HIV-1 or sexually transmitted bacteria. Because of their antimicrobial activity and complex secondary structures, they have not yet been produced in microbial systems and their chemical synthesis is prohibitively expensive. Therefore, we created chloroplast transformation vectors with the RC101 or PG1 coding sequence, fused with GFP to confer stability, furin or Factor Xa cleavage site to liberate the mature peptide from their fusion proteins and a His-tag to aid in their purification. Stable integration of RC101 into the tobacco chloroplast genome and homoplasmy were confirmed by Southern blots. RC101 and PG1 accumulated up to 32%-38% and 17%∼26% of the total soluble protein. Both RC101 and PG1 were cleaved from GFP by corresponding proteases in vitro, and Factor Xa-like protease activity was observed within chloroplasts. Confocal microscopy studies showed location of GFP fluorescence within chloroplasts. Organic extraction resulted in 10.6-fold higher yield of RC101 than purification by affinity chromatography using His-tag. In planta bioassays with Erwinia carotovora confirmed the antibacterial activity of RC101 and PG1 expressed in chloroplasts. RC101 transplastomic plants were resistant to tobacco mosaic virus infections, confirming antiviral activity. Because RC101 and PG1 have not yet been produced in other cell culture or microbial systems, chloroplasts can be used as bioreactors for producing these proteins. Adequate yield of purified antimicrobial peptides from transplastomic plants should facilitate further preclinical studies.

High-level expression of a human β-site APP cleaving enzyme in transgenic tobacco chloroplasts and its immunogenicity in mice

Plastid transformation has to date been applied to the expression of heterologous genes involved in agronomic traits and to the production of useful recombinant proteins. Here, we report a feasibility study for producing the human β-site APP cleaving enzyme (BACE) via transformation of tobacco chloroplasts. Stable integration of human BACE into the plastome was confirmed by PCR. Genomic Southern blot analysis detected the presence of the tobacco aadA and human BACE genes between trnI and trnA in the plastome. Northern blot analysis revealed that the aadA and BACE genes were both properly transcribed into a dicistronic transcriptional unit. Human BACE protein expression in transplastomic tobacco was determined by western blot analysis. ELISA analysis revealed that, based on a dilution series of E. coli-derived BACE as a standard, transplastomic lines accumulated BACE to levels of 2.0% of total soluble proteins. When mice were gavaged with the transplastomic tobacco extracts, they showed an immune response against the BACE antigen. The successful production of plastid-based BACE protein has the potential for developing a plant-based vaccine against Alzheimer disease.

Plant plastid engineering

Genetic material in plants is distributed into nucleus, plastids and mitochondria. Plastid has a central role of carrying out photosynthesis in plant cells. Plastid transformation is becoming more popular and an alternative to nuclear gene transformation because of various advantages like high protein levels, the feasibility of expressing multiple proteins from polycistronic mRNAs, and gene containment through the lack of pollen transmission. Recently, much progress in plastid engineering has been made. In addition to model plant tobacco, many transplastomic crop plants have been generated which possess higher resistance to biotic and abiotic stresses and molecular pharming. In this mini review, we will discuss the features of the plastid DNA and advantages of plastid transformation. We will also present some examples of transplastomic plants developed so far through plastid engineering, and the various applications of plastid transformation.

Transgenic plant-based oral vaccines

Oral vaccines offer significant advantages over needle-based vaccines for achieving universal childhood vaccination goals. The expression of vaccine antigens in transgenic plants has the potential to provide a convenient, safe approach for oral vaccination and thus a feasible alternative to traditional parenteral vaccines. Many developments in the field have ushered in improvements such as enhanced protein antigen expression for the use of plants as factories for vaccine production, and facilitated studies pertaining to immunogenicity of candidate vaccines. Oral delivery of plant-based vaccines offers the benefit of antigen protection within the harsh intestinal environment. Within the gut, mucosal immune cells are poised to respond to pathogens, but can also be exploited to elicit protective immune responses to oral vaccines. Inclusion of mucosal adjuvants during immunization with the vaccine antigen has been an important step towards the success of plant-based vaccines. This review discusses the mechanisms that control mucosal immune responses and highlights some of the studies and the results achieved following immunization with transgenic plants.

Chloroplasts as expression platforms for plant-produced vaccines

Production of recombinant subunit vaccines from genes incorporated in the plastid genome is advantageous because of the attainable expression level due to high transgene copy number and the absence of gene silencing; biocontainment as a consequence of maternal inheritance of plastids and no transgene presence in the pollen; and expression of multiple transgenes in prokaryotic-like operons. We discuss the core technology of plastid transformation in Chlamydomonas reinhardtii, a unicellular alga, and Nicotiana tabacum (tobacco), a flowering plant species, and demonstrate the utility of the technology for the production of recombinant vaccine antigens.

Immunogenicity of recombinant F4 (K88) fimbrial adhesin FaeG expressed in tobacco chloroplast

To test the possibility of producing the novel vaccine in plants against diarrhea normally found in neonatal and newly weaned piglets, the faeG gene, encoding a major F4ac fimbrial subunit protein, was introduced into the tobacco chloroplast genome. After two rounds of selection under spectinomycin, we obtained the transgenic plants nearly homoplasmic. RNA gel blot analysis indicated that faeG and the antibiotic selective gene aminoglycoside 3' adenylyltransferase (aadA) were highly transcribed as a dicistron, while the translational level of recombinant FaeG in transplastomic tobacco was about 0.15% of total soluble protein. The immunogenicity of recombinant FaeG produced in tobacco chloroplasts was confirmed by the observation that FaeG-specific antibodies were elicited in mice immunized with total soluble protein of transgenic plants, as well as the result that mouse sera stimulated by chloroplast-derived recombinant FaeG could neutralize F4ac enterotoxigenic Escherichia coli (ETEC) in vivo. This study provides a new alternative for producing the ETEC vaccine using the chloroplast expression system.

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.

Chloroplast-derived vaccine antigens confer dual immunity against cholera and malaria by oral or injectable delivery

Cholera and malaria are major diseases causing high mortality. The only licensed cholera vaccine is expensive; immunity is lost in children within 3 years and adults are not fully protected. No vaccine is yet available for malaria. Therefore, in this study, the cholera toxin-B subunit (CTB) of Vibrio cholerae fused to malarial vaccine antigens apical membrane antigen-1 (AMA1) and merozoite surface protein-1 (MSP1) was expressed in lettuce and tobacco chloroplasts. Southern blot analysis confirmed homoplasmy and stable integration of transgenes. CTB-AMA1 and CTB-MSP1 fusion proteins accumulated up to 13.17% and 10.11% (total soluble protein, TSP) in tobacco and up to 7.3% and 6.1% (TSP) in lettuce, respectively. Nine groups of mice (n = 10/group) were immunized subcutaneously (SQV) or orally (ORV) with purified antigens or transplastomic tobacco leaves. Significant levels of antigen-specific antibody titres of immunized mice completely inhibited proliferation of the malarial parasite and cross-reacted with the native parasite proteins in immunoblots and immunofluorescence studies. Protection against cholera toxin challenge in both ORV (100%) and SQV (89%) mice correlated with CTB-specific titres of intestinal, serum IgA and IgG1 in ORV and only IgG1 in SQV mice, but no other immunoglobulin. Increasing numbers of interleukin-10(+) T cell but not Foxp3(+) regulatory T cells, suppression of interferon-gamma and absence of interleukin-17 were observed in protected mice, suggesting that immunity is conferred via the Tr1/Th2 immune response. Dual immunity against two major infectious diseases provided by chloroplast-derived vaccine antigens for long-term (>300 days, 50% of mouse life span) offers a realistic platform for low cost vaccines and insight into mucosal and systemic immunity.

Plant-made vaccine antigens and biopharmaceuticals

Plant cells are ideal bioreactors for the production and oral delivery of vaccines and biopharmaceuticals, eliminating the need for expensive fermentation, purification, cold storage, transportation and sterile delivery. Plant-made vaccines have been developed for two decades but none has advanced beyond Phase I. However, two plant-made biopharmaceuticals are now advancing through Phase II and Phase III human clinical trials. In this review, we evaluate the advantages and disadvantages of different plant expression systems (stable nuclear and chloroplast or transient viral) and their current limitations or challenges. We provide suggestions for advancing this valuable concept for clinical applications and conclude that greater research emphasis is needed on large-scale production, purification, functional characterization, oral delivery and preclinical evaluation.

Plant-made vaccine antigens and biopharmaceuticals

Plant cells are ideal bioreactors for the production and oral delivery of vaccines and biopharmaceuticals, eliminating the need for expensive fermentation, purification, cold storage, transportation and sterile delivery. Plant-made vaccines have been developed for two decades but none has advanced beyond Phase I. However, two plant-made biopharmaceuticals are now advancing through Phase II and Phase III human clinical trials. In this review, we evaluate the advantages and disadvantages of different plant expression systems (stable nuclear and chloroplast or transient viral) and their current limitations or challenges. We provide suggestions for advancing this valuable concept for clinical applications and conclude that greater research emphasis is needed on large-scale production, purification, functional characterization, oral delivery and preclinical evaluation.

Synthesis and assembly of a full-length human monoclonal antibody in algal chloroplasts

Monoclonal antibodies can be effective therapeutics against a variety of human diseases, but currently marketed antibody-based drugs are very expensive compared to other therapeutic options. Here, we show that the eukaryotic green algae Chlamydomonas reinhardtii is capable of synthesizing and assembling a full-length IgG1 human monoclonal antibody (mAb) in transgenic chloroplasts. This antibody, 83K7C, is derived from a human IgG1 directed against anthrax protective antigen 83 (PA83), and has been shown to block the effects of anthrax toxin in animal models. Here we show that 83K7C heavy and light chain proteins expressed in the chloroplast accumulate as soluble proteins that assemble into complexes containing two heavy and two light chain proteins. The algal-expressed 83K7C binds PA83 in vitro with similar affinity to the mammalian-expressed 83K7C antibody. In addition, a second human IgG1 and a mouse IgG1 were also expressed and shown to properly assemble in algal chloroplast. These results show that chloroplasts have the ability to fold and assemble full-length human mAbs, and suggest the potential of algae as a platform for the cost effective production of complex human therapeutic proteins.

Advances in chloroplast engineering

The chloroplast is a pivotal organelle in plant cells and eukaryotic algae to carry out photosynthesis, which provides the primary source of the world's food. The expression of foreign genes in chloroplasts offers several advantages over their expression in the nucleus: high-level expression, transgene stacking in operons and a lack of epigenetic interference allowing stable transgene expression. In addition, transgenic chloroplasts are generally not transmitted through pollen grains because of the cytoplasmic localization. In the past two decades, great progress in chloroplast engineering has been made. In this paper, we review and highlight recent studies of chloroplast engineering, including chloroplast transformation procedures, controlled expression of plastid transgenes in plants, the expression of foreign genes for improvement of plant traits, the production of biopharmaceuticals, metabolic pathway engineering in plants, plastid transformation to study RNA editing, and marker gene excision system.