Development of biopharmaceuticals in plant expression systems: cloning, expression and immunological reactivity of human cytomegalovirus glycoprotein B (UL55) in seeds of transgenic tobacco

Tobacco-Based Vaccines, Hopes, and Concerns: A Systematic Review

Emerging infectious diseases have vigorously devastated the global economy and health sector; cost-effective plant-based vaccines (PBV) can be the potential solution to withstand the current health economic crisis. The prominent role of tobacco as an efficient expression system for PBV has been well-established for decades, through this review we highlight the importance of tobacco-based vaccines (TBV) against evolving infectious diseases in humans. Studies focusing on the use of TBV for human infectious diseases were searched in PubMed, Google Scholar, and science direct from 1995 to 2021 using the keywords Tobacco-based vaccines OR transgenic tobacco OR Nicotiana benthamiana vaccines AND Infectious diseases or communicable diseases. We carried out a critical review of the articles and studies that fulfilled the eligibility criteria and were included in this review. Of 976 studies identified, only 63 studies fulfilling the eligibility criteria were included, which focused on either the in vitro, in vivo, or clinical studies on TBV for human infectious diseases. Around 43 in vitro studies of 23 different infectious pathogens expressed in tobacco-based systems were identified and 23 in vivo analysis studies were recognized to check the immunogenicity of vaccine candidates while only 10 of these were subjected to clinical trials. Viral infectious pathogens were studied more than bacterial pathogens. From our review, it was evident that TBV can be an effective health strategy to combat the emerging viral infectious diseases which are very difficult to manage with the current health facilities. The timely administration of cost-effective TBV can prevent the outburst of viral infections, thereby can protect the global healthcare system to a greater extent.

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

INTRODUCTION

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

AREA COVERED

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

EXPERT OPINION

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

Exploring the N-Glycosylation Profile of Glycoprotein B from Human Cytomegalovirus Expressed in CHO and Nicotiana tabacum BY-2 Cells

The ability to control the glycosylation pattern of recombinant viral glycoproteins represents a major prerequisite before their use as vaccines. The aim of this study consisted of expressing the large soluble ectodomain of glycoprotein B (gB) from Human Cytomegalovirus (HMCV) in Nicotiana tabacum Bright Yellow-2 (BY-2) suspension cells and of comparing its glycosylation profile with that of gB produced in Chinese hamster ovary (CHO) cells. gB was secreted in the BY-2 culture medium at a concentration of 20 mg/L and directly purified by ammonium sulfate precipitation and size exclusion chromatography. We then measured the relative abundance of N-glycans present on 15 (BY-2) and 17 (CHO) out of the 18 N-sites by multienzymatic proteolysis and mass spectrometry. The glycosylation profile differed at each N-site, some sites being occupied exclusively by oligomannosidic type N-glycans and others by complex N-glycans processed in some cases with additional Lewis A structures (BY-2) or with beta-1,4-galactose and sialic acid (CHO). The profiles were strikingly comparable between BY-2- and CHO-produced gB. These results suggest a similar gB conformation when glycoproteins are expressed in plant cells as site accessibility influences the glycosylation profile at each site. These data thus strengthen the BY-2 suspension cultures as an alternative expression system.

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

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

Ubiquitin fusion expression and tissue-dependent targeting of hG-CSF in transgenic tobacco

BACKGROUND

Human granulocyte colony-stimulating factor (hG-CSF) is an important human cytokine which has been widely used in oncology and infection protection. To satisfy clinical needs, expression of recombinant hG-CSF has been studied in several organisms, including rice cell suspension culture and transient expression in tobacco leaves, but there was no published report on its expression in stably transformed plants which can serve as a more economical expression platform with potential industrial application.

RESULTS

In this study, hG-CSF expression was investigated in transgenic tobacco leaves and seeds in which the accumulation of hG-CSF could be enhanced through fusion with ubiquitin by up to 7 fold in leaves and 2 fold in seeds, leading to an accumulation level of 2.5 mg/g total soluble protein (TSP) in leaves and 1.3 mg/g TSP in seeds, relative to hG-CSF expressed without a fusion partner. Immunoblot analysis showed that ubiquitin was processed from the final protein product, and ubiquitination was up-regulated in all transgenic plants analyzed. Driven by CaMV 35S promoter and phaseolin signal peptide, hG-CSF was observed to be secreted into apoplast in leaves but deposited in protein storage vacuole (PSV) in seeds, indicating that targeting of the hG-CSF was tissue-dependent in transgenic tobacco. Bioactivity assay showed that hG-CSF expressed in both seeds and leaves was bioactive to support the proliferation of NFS-60 cells.

CONCLUSIONS

In this study, the expression of bioactive hG-CSF in transgenic plants was improved through ubiquitin fusion strategy, demonstrating that protein expression can be enhanced in both plant leaves and seeds through fusion with ubiquitin and providing a typical case of tissue-dependent expression of recombinant protein in transgenic plants.

Non-food/feed seeds as biofactories for the high-yield production of recombinant pharmaceuticals

We describe an attractive cloning system for the seed-specific expression of recombinant proteins using three non-food/feed crops. A vector designed for direct subcloning by Gateway® recombination was developed and tested in Arabidopsis, tobacco and petunia plants for the production of a chimeric form (GAD67/65) of the 65 kDa isoform of glutamic acid decarboxylase (GAD65). GAD65 is one of the major human autoantigens involved in type 1 diabetes (T1D). The murine anti-inflammatory cytokine interleukin-10 (IL-10) was expressed with the described system in Arabidopsis and tobacco, whereas proinsulin, another T1D major autoantigen, was expressed in Arabidopsis. The cost-effective production of these proteins in plants could allow the development of T1D prevention strategies based on the induction of immunological tolerance. The best yields were achieved in Arabidopsis seeds, where GAD67/65 reached 7.7% of total soluble protein (TSP), the highest levels ever reported for this protein in plants. IL-10 and proinsulin reached 0.70% and 0.007% of TSP, respectively, consistent with levels previously reported in other plants or tissues. This versatile cloning vector could be suitable for the high-throughput evaluation of expression levels and stability of many valuable and difficult to produce proteins.

(14)C radiolabeling of proteins to monitor biodistribution of ingested proteins

The economical preparation of microgram quantities of (14)C-labeled proteins by in vacuo methylation with methyl iodide is described. The (14)C radiolabeling was achieved by the covalent attachment of [(14)C]methyl groups onto amino and imidazole groups by reaction in vacuo with [(14)C]methyl iodide. The method was tested by investigating the biodistribution of (14)C in rats that were fed (14)C-labeled human soluble cluster of differentiation 14 (CD14) protein, a receptor for bacterial lipopolysaccharide. Two other control proteins, bovine serum albumin (BSA) and casein, were also labeled with (14)C and used for comparative analysis to determine the following: (i) the efficacy and cost efficiency of the in vacuo radiolabeling procedure and (ii) the extent of incorporation of the (14)C label into the organs of orogastrically fed 10-day-old Sprague-Dawley rats. [(14)C]BSA, [(14)C]casein, and [(14)C]CD14 were individually prepared with specific radioactivities of 34,400, 18,800, and 163,000 disintegrations per minute (dpm)/microg, respectively. It was found that the accumulation of (14)C label in the organs of [(14)C]CD14-fed rats, most notably the persistence of (14)C in the stomach 480 min postgavage, was temporally and spatially distinct from [(14)C]BSA and [(14)C]casein-fed rats.

Immunogenicity of recombinant GP5 protein of porcine reproductive and respiratory syndrome virus expressed in tobacco plant

The aim of the study was to evaluate the immunogenicity of the ORF5-encoded major envelop glycoprotein 5 (GP5) of porcine reproductive and respiratory syndrome virus (PRRSV) expressed in tobacco plant as a potential pig oral vaccine in protection against PRRSV infection. Six-week-old PRRSV-free pigs were fed four times orally with 50g of chopped fresh GP5 transgenic tobacco leaves (GP5-T) (GP5 reaching 0.011% of total soluble protein) or wild-type tobacco leaves (W-T) each on days 0, 14, 28, and 42. Samples of serum, saliva, and peripheral blood mononuclear cells (PBMCs) were collected on days -1, 6, 13, 20, 27, 34, 41, and 48 after the initial oral vaccination. A similar vaccination-dependent gradual increase in the responses of serum and saliva anti-PRRSV total IgG and IgA, respectively, and in the levels of PRRSV-specific blastogenic response of PBMCs was seen in GP5-T-treated pigs; all statistically significant elevations occurred after the 2nd vaccination and were revealed after 20 days post-initial oral vaccination (DPIOV). Pigs fed on GP5-T also developed serum neutralizing antibodies to PRRSV at a titer of 1:4-1:8 after the 4th vaccination by 48 DPIOV. No detectable anti-PRRSV antibody responses and PRRSV-specific blastogenic response were seen in W-T-treated pigs. The present study has demonstrated that pigs fed on GP5-T could develop specific mucosal as well as systemic humoral and cellular immune responses against PRRSV. The results also support that transgenic plant as GP5-T can be an effective system for oral delivery of recombinant subunit vaccines in pigs.

High level expression of a functionally active cholera toxin B: rabies glycoprotein fusion protein in tobacco seeds

A synthetic DNA construct containing cholera toxin B subunit, genetically fused to the surface glycoprotein of rabies virus was expressed in tobacco plants from a seed specific (legumin) promoter. Seed specific expression was monitored by real-time PCR, GM1-ELISA and Western blot analyses. The fusion protein accumulated in tobacco seeds at up to 1.22% of the total seed protein. It was functionally active in binding to the GM1-ganglioside receptors, suggesting its assembly into pentamers in seeds of the transgenic plants. Immunoblot analysis confirmed that the approximately 80.6 kDa monomeric fusion polypeptide was expressed in tobacco seeds and accumulated as an approximately 403 kDa pentamer. Evaluation of its immunoprotective ability against rabies and cholera is to be examined.

Proteomic analysis of rice endosperm cells in response to expression of hGM-CSF

The accumulation of significant levels of transgenic products in plant cells is required not only for crop improvement, but also for molecular pharming. However, knowledge about the fate of transgenic products and endogenous proteins in grain cells is lacking. Here, we utilized a quantitative mass spectrometry-based proteomic approach for comparative analysis of expression profiles of transgenic rice endosperm cells in response to expression of a recombinant pharmaceutical protein, human granulocyte-macrophage colony stimulation factor (hGM-CSF). This study provided the first available evidence concerning the fate of exogenous and endogenous proteins in grain cells. Among 1883 identified proteins with a false positive rate of 5%, 103 displayed significant changes (p-value < 0.05) between the transgenic and the wild-type endosperm cells. Notably, endogenous storage proteins and most carbohydrate metabolism-related proteins were down-regulated, while 26S proteasome-related proteins and chaperones were up-regulated in the transgenic rice endosperm. Furthermore, it was observed that expression of hGM-CSF induced endoplasmic reticulum stress and activated the ubiquitin/26S-proteasome pathway, which led to ubiquitination of this foreign gene product in the transgenic rice endosperm.

Plants as bioreactors for the production of vaccine antigens

Plants have been identified as promising expression systems for commercial production of vaccine antigens. In phase I clinical trials several plant-derived vaccine antigens have been found to be safe and induce sufficiently high immune response. Thus, transgenic plants, including edible plant parts are suggested as excellent alternatives for the production of vaccines and economic scale-up through cultivation. Improved understanding of plant molecular biology and consequent refinement in the genetic engineering techniques have led to designing approaches for high level expression of vaccine antigens in plants. During the last decade, several efficient plant-based expression systems have been examined and more than 100 recombinant proteins including plant-derived vaccine antigens have been expressed in different plant tissues. Estimates suggest that it may become possible to obtain antigen sufficient for vaccinating millions of individuals from one acre crop by expressing the antigen in seeds of an edible legume, like peanut or soybean. In the near future, a plethora of protein products, developed through ‘naturalized bioreactors’ may reach market. Efforts for further improvements in these technologies need to be directed mainly towards validation and applicability of plant-based standardized mucosal and edible vaccines, regulatory pharmacology, formulations and the development of commercially viable GLP protocols. This article reviews the current status of developments in the area of use of plants for the development of vaccine antigens.

Plant-based oral vaccines: results of human trials

Vaccines consisting of transgenic plant-derived antigens offer a new strategy for development of safe, inexpensive vaccines. The vaccine antigens can be eaten with the edible part of the plant or purified from plant material. In phase 1 clinical studies of prototype potato- and corn-based vaccines, these vaccines have been safe and immunogenic without the need for a buffer or vehicle other than the plant cell. Transgenic plant technology is attractive for vaccine development because these vaccines are needle-less, stable, and easy to administer. This chapter examines some early human studies of oral transgenic plant-derived vaccines against enterotoxigenic Escherichia coli infection, norovirus, and hepatitis B.

Chloroplast-derived vaccine antigens and biopharmaceuticals: expression, folding, assembly and functionality

Chloroplast genetic engineering offers several advantages, including high levels of transgene expression, transgene containment via maternal inheritance, and multi-gene expression in a single transformation event. Oral delivery is facilitated by hyperexpression of vaccine antigens against cholera, tetanus, anthrax, plague, or canine parvovirus (4%-31% of total soluble protein, TSP) in transgenic chloroplasts (leaves) or non-green plastids (carrots, tomato) as well as the availability of antibiotic free selectable markers or the ability to excise selectable marker genes. Hyperexpression of several therapeutic proteins, including human serum albumin (11.1% TSP), somatotropin (7% TSP), interferon-alpha (19% TSP), interferon-gamma (6% TSP), and antimicrobial peptide (21.5% TSP), facilitates efficient and economic purification. Also, the presence of chaperones and enzymes in chloroplasts facilitates assembly of complex multisubunit proteins and correct folding of human blood proteins with proper disulfide bonds. Functionality of chloroplast-derived vaccine antigens and therapeutic proteins has been demonstrated by several assays, including the macrophage lysis assay, GM1-ganglioside binding assay, protection of HeLA cells or human lung carcinoma cells against encephalomyocarditis virus, systemic immune response, protection against pathogen challenge, and growth or inhibition of cell cultures. Purification of human proinsulin has been achieved using novel purification strategies (inverse temperature transition property) that do not require expensive column chromatography techniques. Thus, transgenic chloroplasts are ideal bio-reactors for production of functional human and animal therapeutic proteins in an environmentally friendly manner.

Functional specialization of Medicago truncatula leaves and seeds does not affect the subcellular localization of a recombinant protein

A number of recent reports suggest that the functional specialization of plant cells in storage organs can influence subcellular protein sorting, so that the fate of a recombinant protein tends to differ between seeds and leaves. In order to test the general applicability of this hypothesis, we investigated the fate of a model recombinant glycoprotein in the leaves and seeds of a leguminous plant, Medicago truncatula. Detailed analysis of immature seeds by immunofluorescence and electron microscopy showed that recombinant phytase carrying a signal peptide for entry into the endoplasmic reticulum was efficiently secreted from storage cotyledon cells. A second version of the protein carrying a C-terminal KDEL tag for retention in the endoplasmic reticulum was predominantly retained in the ER of seed cotyledon cells, but some of the protein was secreted to the apoplast and some was deposited in storage vacuoles. Importantly, the fate of the recombinant protein in the leaves was nearly identical to that in the seeds from the same plant. This shows that in M. truncatula, the unanticipated partial vacuolar delivery and secretion is not a special feature of seed cotyledon tissue, but are conserved in different specialized tissues. Further investigation revealed that the unexpected fate of the tagged variant of phytase likely resulted from partial loss of the KDEL tag in both leaves and seeds. Our results indicate that the previously observed aberrant deposition of recombinant proteins into storage organelles of seed tissue is not a general reflection of functional specialization, but also depends on the species of plant under investigation. This discovery will have an impact on the production of recombinant pharmaceutical proteins in plants.

Cytomegalovirus vaccine development

Although infection with human cytomegalovirus (HCMV) is ubiquitous and usually asymptomatic, there are individuals at high risk for serious HCMV disease. These include solid organ and hematopoietic stem cell (HSC) transplant patients, individuals with HIV infection, and the fetus. Since immunity to HCMV ameliorates the severity of disease, there have been efforts made for over 30 years to develop vaccines for use in these high-risk settings. However, in spite of these efforts, no HCMV vaccine appears to be approaching imminent licensure. The reasons for the failure to achieve the goal of a licensed HCMV vaccine are complex, but several key problems stand out. First, the host immune correlates of protective immunity are not yet clear. Secondly, the viral proteins that should be included in a HCMV vaccine are uncertain. Third, clinical trials have largely focused on immunocompromised patients, a population that may not be relevant to the problem of protection of the fetus against congenital infection. Fourth, the ultimate target population for HCMV vaccination remains unclear. Finally, and most importantly, there has been insufficient education about the problem of HCMV infection, particularly among women of child-bearing age and in the lay public. This review considers the strategies that have been explored to date in development of HCMV vaccines, and summarizes both active clinical trials as well as novel technologies that merit future consideration toward the goal of prevention of this significant public health problem.

Improving containment strategies in biopharming

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

Humanizing infant milk formula to decrease postnatal HIV transmission

There are currently no safe methods for feeding babies born from the 16 million HIV-infected women living in resource-constrained countries. Breast milk can transmit HIV, and formula feeding can lead to gastrointestinal illnesses owing to unsanitary conditions and the composition of milk formulations. There is therefore a need to ensure that breast milk substitutes provide optimal health outcomes. Given that the immune properties of several breast milk proteins are known, transgenic food crops could facilitate inexpensive and safe reconstitution of the beneficial breast milk proteome in infant formulae, while keeping the HIV virus at bay. At least seven breast milk immune proteins have already been produced in food crops, and dozens more proteins could potentially be produced if fortified formula proves effective in nursing newborns born to HIV-infected mothers.

Release of the recombinant proteins, human serum albumin, beta-glucuronidase, glycoprotein B from human cytomegalovirus, and green fluorescent protein, in root exudates from transgenic tobacco and their effects on microbes and enzymatic activities in soil

We determined the release in root exudates of human serum albumin (HSA), beta-glucuronidase (GUS), glycoprotein B (gB) from human cytomegalovirus, and green fluorescent protein (GFP) from genetically modified transgenic tobacco expressing the genes for these proteins in hydroponic culture and non-sterile soil. GUS, gB, and GFP were expressed in the plant but were not released in root exudates, whereas HSA was both expressed in the plant and released in root exudates, as shown by a 66.5-kDa band on SDS-PAGE and Western blot and confirmed by ELISA. Root exudates from GUS and gB plants showed no bands that could be attributed to these proteins on SDS-PAGE, and root exudates from GFP plants showed no fluorescence. The concentration of HSA in root exudates was estimated to be 0.021 ng ml(-1), whereas that in the plant biomass was estimated to be 0.087 ng ml(-1). The concentration of HSA in soil was estimated to be 0.049 ng g(-1). No significant differences in the number of microorganisms and the activity of selected enzymes were observed between rhizosphere soil of non-modified and HSA tobacco.

Biologically active human GM-CSF produced in the seeds of transgenic rice plants

Rice flour is a well-known and characterized source of pharmaceutical ingredients, which are gluten-free and incorporated in many drug delivery applications such as excipient starch. To further exploit this uniqueness, the synthetic capacity of rice endosperm tissue, the basis of rice flour, was extended by genetic transformation. Recombinant human GM-CSF, a cytokine used in treating neutropenia and with other potential clinical applications, has been expressed in transgenic rice seeds using a rice glutelin promoter. Rice seeds accumulated human GM-CSF to a level of 1.3% of total soluble protein. The rice seed-produced human GM-CSF was found to be biologically active when tested using a human cell line TF-1. Use of rice as a host plant offers not only attractive features of safe production in seeds but also self-containment of foreign genes, as rice is primarily a self-pollinated crop plant.

Pharming and transgenic plants

Plant represented the essence of pharmacopoeia until the beginning of the 19th century when plant-derived pharmaceuticals were partly supplanted by drugs produced by the industrial methods of chemical synthesis. In the last decades, genetic engineering has offered an alternative to chemical synthesis, using bacteria, yeasts and animal cells as factories for the production of therapeutic proteins. More recently, molecular farming has rapidly pushed towards plants among the major players in recombinant protein production systems. Indeed, therapeutic protein production is safe and extremely cost-effective in plants. Unlike microbial fermentation, plants are capable of carrying out post-translational modifications and, unlike production systems based on mammalian cell cultures, plants are devoid of human infective viruses and prions. Furthermore, a large panel of strategies and new plant expression systems are currently developed to improve the plant-made pharmaceutical's yields and quality. Recent advances in the control of post-translational maturations in transgenic plants will allow them, in the near future, to perform human-like maturations on recombinant proteins and, hence, make plant expression systems suitable alternatives to animal cell factories.

High-efficiency Agrobacterium rhizogenes-mediated transformation of heat inducible sHSP18.2-GUS in Nicotiana tabacum

The chimerical gene, Arabidopsis thaliana sHSP18.2 promoter fused to E. coli gusA gene, was Agrobacterium rhizogenes-mediated transformed into Nicotiana tabacum as a heat-regulatable model, and the thermo-inducible expression of GUS activity in N. tabacum transgenic hairy roots was profiled. An activation of A. rhizogenes with acetosyringone (AS) before cocultured with tobacco's leaf disc strongly promoted transgenic hairy roots formation. Transgenic hairy roots formation efficiency of A. rhizogenes precultured with 200 microM AS supplementation was 3.1-fold and 7.5-fold, respectively, compared to the formation efficiency obtained with and without AS supplementation in coculture. Transgenic hairy roots transformed with different AS concentration exhibited a similar pattern of thermo-inducibility after 10 min to 3 h heat treatments detected by GUS expression. The peak of expressed GUS specific activity, 399,530 pmol MUG per mg total protein per min, of the transgenic hairy roots was observed at 48 h after 3 h of 42 degrees C heat treatment, and the expressed GUS specific activity was 7-26 times more than that reported in A. thaliana, tobacco BY-2 cells and Nicotiana plumbaginifolia. Interference caused by AS supplementation on the growth of transgenic hairy roots, time-course of GUS expression and its expression level were not observed.

Molecular farming for antigen (vaccine) production in plants

Genomic and proteomic approaches to the study of fundamental cell mechanisms are rapidly contributing to broaden our knowledge on metabolic pathways for the optimal exploitation of the cell as a factory. In the last few years this knowledge has led to important advances in the large scale production of diagnostic and therapeutic proteins in heterologous hosts (bacteria, yeasts, mammalian and insect cells or transgenic animals and plants), allowing the comparison of the most efficient methods in terms of costs, product quality and safety.

Progress toward an elusive goal: current status of cytomegalovirus vaccines

Although infection with human cytomegalovirus (CMV) is ubiquitous and generally asymptomatic in most individuals, certain patient populations are at high risk for CMV-associated disease. These include HIV-infected individuals with AIDS, transplant patients, and newborn infants with congenital CMV infection. Immunity to CMV infection, both in the transplant setting and among women of childbearing age, plays a vital role in the control of CMV-induced injury and disease. Although immunity induced by CMV infection is not completely protective against reinfection, there is nevertheless a sound basis on which to believe that vaccination could help control CMV disease in high-risk patient populations. Evidence from several animal models of CMV infection indicates that a variety of vaccine strategies are capable of inducing immune responses sufficient to protect against CMV-associated illness following viral challenge. Vaccination has also proven effective in improving pregnancy outcomes following CMV challenge of pregnant guinea pigs, providing a 'proof-of-principle' relevant to human clinical trials of CMV vaccines. Although there are no licensed vaccines currently available for human CMV, progress toward this goal has been made, as evidenced by ongoing clinical trial testing of a number of immunization strategies. CMV vaccines currently in various stages of preclinical and clinical testing include: protein subunit vaccines; DNA vaccines; vectored vaccines using viral vectors, such as attenuated pox- and alphaviruses; peptide vaccines; and live attenuated vaccines. This review summarizes some of the obstacles that must be overcome in development of a CMV vaccine, and provides an overview of the current state of preclinical and clinical trial evaluation of vaccines for this important public health problem.

The intracellular fate of a recombinant protein is tissue dependent

Recombinant proteins directed to the secretory pathway in plants require a signal peptide for entry into the endoplasmic reticulum. In the absence of further targeting information, such proteins are generally secreted via the default pathway to the apoplast. This has been well documented in protoplasts and leaf tissue, but the trafficking of recombinant proteins in seeds and other storage tissues has rarely been investigated. We used Aspergillus niger phytase as a model glycoprotein to compare the intracellular fate of a recombinant protein in the leaves and seeds of rice (Oryza sativa). Using fluorescence and electron microscopy we showed that the recombinant protein was efficiently secreted from leaf cells as expected. In contrast, within endosperm cells it was retained in endoplasmic reticulum-derived prolamin bodies and protein storage vacuoles. Consistent with our immunolocalization data, the phytase produced in endosperm cells possessed oligomannose and vacuolar-type N-glycans [Man(3)(Xyl)(Fuc)GlcNAc(2)], whereas the phytase produced in leaves contained predominantly secretion-type N-glycans [GlcNAc(2)Man(3)(Xyl)(Fuc)GlcNAc(2)]. The latter could not be detected in preparations of the endosperm-derived phytase. Our results show that the intracellular deposition and modification of a recombinant protein is tissue dependent.

Transgenic plants in the biopharmaceutical market

Many of our 'small-molecule-drugs' are natural products from plants, or are synthetic compounds based on molecules found naturally in plants. However, the vast majority of the protein therapeutics (or biopharmaceuticals) we use are from animal or human sources, and are produced commercially in microbial or mammalian bioreactor systems. Over the last few years, it has become clear that plants have great potential for the production of human proteins and other protein-based therapeutic entities. Plants offer the prospect of inexpensive biopharmaceutical production without sacrificing product quality or safety, and following the success of several plant-derived technical proteins, the first therapeutic products are now approaching the market. In this review, the different plant-based production systems are discussed and the merits of transgenic plants are evaluated compared with other platforms. A detailed discussion is provided of the development issues that remain to be addressed before plants become an acceptable mainstream production technology. The many different proteins that have already been produced using plants are described, and a sketch of the current market and the activities of the key players is provided. Despite the currently unclear regulatory framework and general industry inertia, the benefits of plant-derived pharmaceuticals are now bringing the prospect of inexpensive veterinary and human medicines closer than ever before.

Human CD14 expressed in seeds of transgenic tobacco displays similar proteolytic resistance and bioactivity with its mammalian-produced counterpart

Human CD14 plays an important role in innate immunity by being the key receptor of lipopolysaccharide found on Gram-negative bacteria. The recently discovered widespread localization of CD14 in secretions and mucosal surfaces reveals its extensive anti-microbial properties and numerous potential medical applications. To produce active recombinant human CD14 (rhCD14) for massive distribution, transgenic tobacco plants were successfully generated to express rhCD14 in the seed endosperm under the control of two versions (1.8 kb and 5.1 kb) of the rice glutelin Gt-1 promoter. Plant-made rhCD14 proteins reached a concentration of 16 microg/g of seeds and showed stability, proteolytic resistance to pepsin digestion and ability to induce the release of pro-inflammatory IL-6 and IL-8 cytokines in presence of LPS. The expression of plant rhCD14 in tobacco seeds constitutes a promising low-cost and abundant supply of this immune protein to further investigate its roles in, impacts on and potential medical applications for the innate immune system.

High level expression of surface glycoprotein of rabies virus in tobacco leaves and its immunoprotective activity in mice

A synthetic gene coding for the surface glycoprotein (G protein) of rabies virus was strategically designed to achieve high-level expression in transgenic plants. The native signal peptide was replaced by that of the pathogenesis related protein, PR-S of Nicotiana tabacum. An endoplasmic reticulum retention signal was included at C-terminus of the G protein. Tobacco plants were genetically engineered by nuclear transformation. Selected transgenic lines expressed the chimeric G protein at 0.38% of the total soluble leaf protein. Mice immunized intraperitoneally with the G protein purified from tobacco leaf microsomal fraction elicited high level of immune response as compared to the inactivated commercial viral vaccine. The plant-derived G protein induced complete protective immunity in mice against intracerebral lethal challenge with live rabies virus. The results establish that plants can provide a safe and effective production system for the expression of immunoprotective rabies virus surface protein.

Plant-derived vaccines against diarrhoeal diseases

Transgenic plant-derived vaccines offer a new strategy for the development of safe, inexpensive vaccines against diarrhoeal diseases. In animal and Phase I clinical studies, these vaccines have been safe and immunogenic without the need for a buffer or vehicle other than the plant cell. This review examines some early attempts to develop oral transgenic plant vaccines against enteric infections such as enterotoxigenic Escherichia coli infection, cholera and norovirus infection.

Recent developments in the use of transgenic plants for the production of human therapeutics and biopharmaceuticals

In recent years there has been a dramatic increase in the application of plant biotechnology for the production of a variety of commercially valuable simple and complex biological molecules (biologics) for use in human and animal healthcare. Transgenic whole plants and plant cell culture systems have been developed that have the capacity to economically produce large-scale quantities of antibodies and antibody fragments, antigens and/or vaccine epitopes, metabolic enzymes, hormones, (neuro)peptides and a variety of biologically active complexes and secondary metabolites for direct use as therapeutic agents or diagnostic tools in the medical healthcare industry. As the products of genetically modified plants make their way from concept to commercialization the associated risks and acceptance by the public has been become a focal point. In this paper, we summarize the recent advances made in the use of transgenic plants and plant cell cultures as biological factories for the production of human therapeutics and biopharmaceuticals and discuss the long-term potential of `molecular farming' as a low-cost, efficient method for the production of biological materials with demonstrated utility to the pharmaceutical industry or medical community.

Expression of ORF2 partial gene of hepatitis E virus in tomatoes and immunoactivity of expression products

AIM: To transfer hepatitis E virus (HEV) ORF2 partial gene to tomato plants, to investigate its expression in transformants and the immunoactivity of expression products, and to explore the feasibility of developing a new type of plant-derived HEV oral vaccine.

METHODS: Plant binary expression vector p1301E2, carrying a fragment of HEV open reading frame-2 (named HEV-E2), was constructed by linking the fragment to a constitutive CaMV35s promoter and nos terminator, then directly introduced into Agrobacterium tumefaciens EHA105. With leaf-disc method, tomato plants medicated by EHA105 were transformed and hygromycin-resistant plantlets were obtained in selective medium containing hygromycin. The presence and integration of foreign DNA in transgenic tomato genome were confirmed by Gus gene expression, PCR amplification and Southern dot blotting. The immunoactivity of recombinant protein extracted from transformed plants was examined by enzyme-linked immunosorbant assay (ELISA) using a monoclonal antibody specifically against HEV. ELISA was also used to estimate the recombinant protein content in leaves and fruits of the transformants.

RESULTS: Seven positive lines of HEV-E2-transgenic tomato plants confirmed by PCR and Southern blotting were obtained and the immunoactivity of recombinant protein could be detected in extracts of transformants. The expression levels of recombinant protein were 61.22 ng/g fresh weight in fruits and 6.37-47.9 ng/g fresh weight in leaves of the transformants.

CONCLUSION: HEV-E2 gene was correctly expressed in transgenic tomatoes and the recombinant antigen derived from them has normal immunoactivity. Transgenic tomatoes may hold a good promise for producing a new type of low-cost oral vaccine for hepatitis E virus.

Benefits and risks of antibody and vaccine production in transgenic plants

Phytopharming, the production of protein biologicals in recombinant plant systems, has shown great promise in studies performed over the past 13 years. A secretory antibody purified from transgenic tobacco was tested successfully in humans, and prevented bacterial re-colonization after topical application in the mouth. Rapid production of patient-tailored anti-lymphoma antibodies in recombinant Tobamovirus-infected tobacco may provide effective cancer therapy. Many different candidate vaccines from bacterial and viral sources have been expressed in transgenic plants, and three human clinical trials with oral delivery of transgenic plant tissues have shown exciting results. The use of crop plants with agricultural practice could allow cheap production of valuable proteins, while providing enhanced safety by avoidance of animal viruses or other contaminants. However development of this technology must carefully consider the means to ensure the separation of food and medicinal products when crop plants are used for phytopharming.

Increased yield of heterologous viral glycoprotein in the seeds of homozygous transgenic tobacco plants cultivated underground

The use of transgenic plants in the production of recombinant proteins for human therapy, including subunit vaccines, is being investigated to evaluate the efficacy and safety of these emerging biopharmaceutical products. We have previously shown that synthesis of recombinant glycoprotein B (gB) of human cytomegalovirus can be targeted to seeds of transgenic tobacco when directed by the rice glutelin 3 promoter, with gB retaining critical features of immunological reactivity (E.S. Tackaberry et al. 1999. Vaccine, 17: 3020-3029). Here, we report development of second generation transgenic plant lines (T1) homozygous for the transgene. Twenty progeny plants from two lines (A23T(1)-2 and A24T(1)-3) were grown underground in an environmentally contained mine shaft. Based on yields of gB in their seeds, the A23T(1)-2 line was then selected for scale-up in the same facility. Analyses of mature seeds by ELISA showedthat gB specific activity in A23T(1)-2 seeds was over 30-fold greater than the best T0 plants from the same transformation series, representing 1.07% total seed protein. These data demonstrate stable inheritance, an absence of transgene inactivation, and enhanced levels of gB expression in a homozygous second generation plant line. They also provide evidence for the suitability of using this environmentally secure facility to grow transgenic plants producing therapeutic biopharmaceuticals.

Oral immunization of animals with transgenic cherry tomatillo expressing HBsAg

AIM: To investigate the expression of recombinant HBsAg (rHBsAg) in transgenic cherry tomatillo in order to explore the feasibility of producing HBV oral vaccine with cherry tomatillo by animal immune tests.

METHODS: The recombinant plant expression vector containing HBsAg gene was constructed. Mediated with Agrobacterium tumefaciens, HBsAg gene was transferred into cotyledons of cherry tomatillo. Transformed cherry tomatillos were obtained through hygromycin delay-selection. Integrated DNA in transgenic cherry tomatillo was confirmed by hygromycin resistance selection, Gus detection, polymerase chain reaction (PCR) and dot blotting analysis. Antigenicity of rHBsAg was examined by ELISA and the immunogenicity of rHBsAg derived from transgenic cherry tomatillo tissues was confirmed by oral feed of transformed tissues to BALB/c mice primed with commercial HBV vaccines. Specific antibody titers in mice’s serum were examined by ELISA every week.

RESULTS: By far, 10 positive lines of transgenic cherry tomatillos containing HBsAg gene were obtained. Among different organs of the same transgenic cherry tomatillo, level of rHBsAg expressed in leaves was the highest with the yield up to 300 ng/g fresh weight. And the rHBsAg expression level in fruits was about 10 ng/g fresh weight. In animal immune tests, oral delivery with transgenic tissues to mice primed with commercial vaccine instead of naive mice resulted in significant immune response.

CONCLUSION: The result of this animal immune test indicated the rHBsAg derived from transgenic cherry tomatillo possessed normal immunogenicity. This work demonstrated the feasibility to generate oral immunogenic rHBsAg in transgenic cherry tomatillo, and would provide some experimental approach for the production of low-cost oral vaccines using transgenic cherry tomatillo in large scale.

Plant-based vaccines

Plant systems are reviewed with regard to their ability to express and produce subunit vaccines. Examples of different types of expression systems producing a variety of vaccine candidates are illustrated. Many of these subunit vaccines have been purified and shown to elicit an immune response when injected into animal models. This review also includes vaccines that have been administered orally in a non-purified form as a food or feed product. Cases are highlighted which demonstrate that orally delivered plant-based vaccines can elicit immune responses and in some case studies, confer protection. Examples are used to illustrate some of the inherent advantages of a plant-based system, such as cost, ease of scale-up and convenience of delivery. Also, some of the key steps are identified that will be necessary to bring these new vaccines to the market.

Plant cell factories and mucosal vaccines

Many advances continue to be made in the field of plant-derived vaccines. Plants have been shown capable of expressing a multicomponent vaccine that when orally delivered induces a T-helper cell subset 1 response and enables passive immunization. Furthermore, a plant-derived vaccine has been shown to protect against challenge in the target host. Increased antigen expression levels (up to 4.1% total soluble protein) have been obtained through transformation of the chloroplast genome. In view of these findings, plant-derived vaccines have been proved as valuable commodities to the world's health system; however, before their application, studies need to focus on optimization of immunization strategies and to investigate antigen stability.

Biological activity of human granulocyte-macrophage colony stimulating factor is maintained in a fusion with seed glutelin peptide

Human granulocyte-macrophage colony stimulating factor (GM-CSF), a cytokine with many applications in clinical medicine, was produced specifically in the seeds of transgenic tobacco plants. Two rice endosperm-specific glutelin promoters of different size and sequence, Gt1 and Gt3, were used to direct expression. Also in the Gt3 construct, the GM-CSF coding region was in fusion with the first 24 nucleotides of the mature rice glutelin sequence at its 5' end. With the Gt1 construct plants, seed extracts contained the recombinant human GM-CSF protein up to a level of 0.03% of total soluble protein. Transgenic seed extracts actively stimulated the growth of human TF-1 cells suggesting that the seed-produced GM-CSF alone and in fusion with the rice glutelin peptide was stable and biologically active. Furthermore, native tobacco seed extracts inhibited the activity of E. coli-derived GM-CSF in this cytokine-dependent cell line. The seeds of F1 generation plants retained the biological activity of human GM-CSF protein indicating that the human coding sequence was stably inherited. The feasibility of oral delivery of such stable seed-produced cytokines is discussed.

A functional antigen in a practical crop: LT-B producing maize protects mice against Escherichia coli heat labile enterotoxin (LT) and cholera toxin (CT)

We have produced a functional heat labile enterotoxin (LT-) B subunit of Escherichia coli in maize. LT-B is a multimeric protein that presents an ideal model for an edible vaccine, displaying stability in the gut and inducing mucosal and systemic immune responses. Transgenic maize was engineered to synthesize the LT-B polypeptides, which assembled into oligomeric structures with affinity for G(M1) gangliosides. We orally immunized BALB/c mice by feeding transgenic maize meal expressing LT-B or non-transgenic maize meal spiked with bacterial LT-B. Both treatments stimulated elevated IgA and IgG antibodies against LT-B and the closely related cholera toxin B subunit (CT-B) in serum, and elevated IgA in fecal pellets. The transgenic maize induced a higher anti-LT-B and anti-CT-B mucosal and serum IgA response compared to the equivalent amount of bacterial LT-B spiked into maize. Following challenge by oral administration of the diarrhea inducing toxins LT and CT, transgenic maize-fed mice displayed reduced fluid accumulation in the gut compared to non-immunized mice. Moreover, the gut to carcass ratio of immunized mice was not significantly different from the PBS (non-toxin) challenged control group. We concluded that maize-synthesized LT-B had features of the native bacterial LT-B such as molecular weight, G(M1) binding ability, and induction of serum and mucosal immunity. We have demonstrated that maize, a major food and feed ingredient, can be efficiently transformed to produce, accumulate, and store a fully assembled and functional candidate vaccine antigen.

Targeting and expression of antigenic proteins in transgenic plants for production of edible oral vaccines

Exploiting plants as biological bioreactors for production and delivery of edible oral subunit vaccines is a promising application of biotechnology. Efforts to enhance expression levels of transgenes coding for antigenic proteins by exploiting promoters, targeting sequences, and enhancer elements have produced rather low quantities of the antigen in plant tissues, but enough to induce immune responses in feeding studies. This review will cover components of various gene constructs used in developing plant-based vaccines against a myriad of viral and bacterial diseases. Specifically, it will focus on sequences that are involved in targeting the antigen to mucosal tissues of the intestinal tract, thus enhancing the immunogenicity of the plant-based vaccine as well as those components that result in higher accumulation of the protein within the plant.

Expression of the native cholera toxin B subunit gene and assembly as functional oligomers in transgenic tobacco chloroplasts

The B subunits of enterotoxigenic Escherichia coli (LTB) and cholera toxin of Vibrio cholerae (CTB) are candidate vaccine antigens. Integration of an unmodified CTB-coding sequence into chloroplast genomes (up to 10,000 copies per cell), resulted in the accumulation of up to 4.1 % of total soluble tobacco leaf protein as functional oligomers (410-fold higher expression levels than that of the unmodified LTB gene expressed via the nuclear genome). However, expression levels reported are an underestimation of actual accumulation of CTB in transgenic chloroplasts, due to aggregation of the oligomeric forms in unboiled samples similar to the aggregation observed for purified bacterial antigen. PCR and Southern blot analyses confirmed stable integration of the CTB gene into the chloroplast genome. Western blot analysis showed that the chloroplast- synthesized CTB assembled into oligomers and were antigenically identical with purified native CTB. Also, binding assays confirmed that chloroplast-synthesized CTB binds to the intestinal membrane GM1-ganglioside receptor, indicating correct folding and disulfide bond formation of CTB pentamers within transgenic chloroplasts. In contrast to stunted nuclear transgenic plants, chloroplast transgenic plants were morphologically indistinguishable from untransformed plants, when CTB was constitutively expressed in chloroplasts. Introduced genes were inherited stably in subsequent generations, as confirmed by PCR and Southern blot analyses. Increased production of an efficient transmucosal carrier molecule and delivery system, like CTB, in transgenic chloroplasts makes plant-based oral vaccines and fusion proteins with CTB needing oral administration commercially feasible. Successful expression of foreign genes in transgenic chromoplasts and availability of marker-free chloroplast transformation techniques augurs well for development of vaccines in edible parts of transgenic plants. Furthermore, since the quaternary structure of many proteins is essential for their function, this investigation demonstrates the potential for other foreign multimeric proteins to be properly expressed and assembled in transgenic chloroplasts.

Recent strategies in the development of new human cytomegalovirus inhibitors

Human cytomegalovirus (HCMV) is one of the most common opportunistic infections in immunucompromised individuals, such as AIDS patients and organ transplant recipients, and is the most frequent congenital viral infection in humans. Despite a reduction of the incidence of AIDS-related opportunistic infections in patients under highly active antiretroviral treatment, attention should be paid to the HCMV risk factor in these individuals. Furthermore, HCMV may have an important role in atherosclerosis. Existing antiviral treatments for the HCMV infection suffer from poor bioavailability, toxicity, and limited effectiveness, mainly due to the development of drug resistance. Fortunately there are novel and potentially very effective new compounds undergoing pre-clinical and clinical evaluation. This review provides an overview in the last five years of new HCMV inhibitors (chemical structures, SAR, and new mechanisms of action) with the aim to provide new clues for the development of future drugs against this opportunistic virus.

Medical molecular farming: production of antibodies, biopharmaceuticals and edible vaccines in plants

The use of plants for medicinal purposes dates back thousands of years but genetic engineering of plants to produce desired biopharmaceuticals is much more recent. As the demand for biopharmaceuticals is expected to increase, it would be wise to ensure that they will be available in significantly larger amounts, on a cost-effective basis. Currently, the cost of biopharmaceuticals limits their availability. Plant-derived biopharmaceuticals are cheap to produce and store, easy to scale up for mass production, and safer than those derived from animals. Here, we discuss recent developments in this field and possible environmental concerns.

Molecular farming of pharmaceutical proteins

Molecular farming is the production of pharmaceutically important and commercially valuable proteins in plants. Its purpose is to provide a safe and inexpensive means for the mass production of recombinant pharmaceutical proteins. Complex mammalian proteins can be produced in transformed plants or transformed plant suspension cells. Plants are suitable for the production of pharmaceutical proteins on a field scale because the expressed proteins are functional and almost indistinguishable from their mammalian counterparts. The breadth of therapeutic proteins produced by plants range from interleukins to recombinant antibodies. Molecular farming in plants has the potential to provide virtually unlimited quantities of recombinant proteins for use as diagnostic and therapeutic tools in health care and the life sciences. Plants produce a large amount of biomass and protein production can be increased using plant suspension cell culture in fermenters, or by the propagation of stably transformed plant lines in the field. Transgenic plants can also produce organs rich in a recombinant protein for its long-term storage. This demonstrates the promise of using transgenic plants as bioreactors for the molecular farming of recombinant therapeutics, including vaccines, diagnostics, such as recombinant antibodies, plasma proteins, cytokines and growth factors.

Sorting of glycoprotein B from human cytomegalovirus to protein storage vesicles in seeds of transgenic tobacco

As part of ongoing studies into the use of plant expression systems for making human therapeutic proteins, we have successfully expressed the major glycoprotein, gB, of human cytomegalovirus (HCMV) in transgenic tobacco plants. Viral glycoprotein was detectable in the protein extracts of mature tobacco seeds using neutralizing and non-neutralizing monoclonal antibodies specific for gB. Although several mammalian proteins have been expressed in tobacco, localization of these proteins in transgenic tobacco tissue has not been extensively examined. The objective of this study was to identify the site(s) of recombinant gB deposition in mature tobacco seeds. Using immunogold labelling and electron microscopy, we found specific labelling for gB in the endosperm of transgenic seeds, with gB localized almost exclusively in protein storage vesicles (PSV). This occurred in seeds that were freshly harvested and in seeds that had been stored for several months. These data indicate that gB behaves like a plant storage protein when expressed in tobacco seeds, and provide further support for the suitability of plants for producing recombinant proteins of potential clinical relevance.

Plants for delivery of edible vaccines

Over the past decade, scientific advances in molecular biology and immunology have improved understanding of many diseases and led to the development of novel strategies for vaccination. The development of plants expressing vaccine antigens is a particularly promising approach. Plant-derived antigenic proteins have delayed or prevented the onset of disease in animals and have proven to be safe and functional in human clinical trials. Future areas of research should further characterize the induction of the mucosal immune system and appropriate crop species for delivery of animal and human vaccines.

Transgene expression systems in plant, a natural bioreactor

Plants are important resources that have been providing us food from the earliest times. The rapid advances that have taken place in plant genetic engineering have made it possible to modify plants to increase food production and contribute to environmental purification. Transgenic plants are gaining increasing attention from the industry as a natural bioreactor for the production of industrial and chemical products. Useful expression systems based on promoters to optimize transgene expression in plant cells, hold the key to maximizing the potential of this concept of molecular-farming or industrial plants. This review, which is devoted to the use of plants for heterologous protein production, is divided into three parts. First, we introduce the nature of plant promoters and strategies for the isolation of novel promoters. In the second part, various promoters showing high-level constitutive, organ-specific, or inducible expression, are summarized as useful tools for realizing the efficient transcription of transgenes. Finally, problems in the expression of foreign gene in plant cells and future prospects in plant biotechnology are discussed.