High-tech herbal medicine: plant-based vaccines

Phase III: Randomized observer-blind trial to evaluate lot-to-lot consistency of a new plant-derived quadrivalent virus like particle influenza vaccine in adults 18-49 years of age

BACKGROUND

The global reliance on eggs to produce most influenza vaccines has several limitations and new approaches to influenza vaccine production are needed. Herein we describe a phase 3, lot-to-lot consistency trial (NCT03321968) of a quadrivalent, recombinant, virus-like particle (VLP) influenza vaccine produced in plants. This platform is based on transient expression of proteins in Nicotiana benthamiana and yields VLPs bearing hemagglutinin (HA) protein trimers that are combined in a quadrivalent vaccine (QVLP).

METHODS

The HAs targeted in this study were A/California/07/2009 H1N1, A/Hong Kong/4801/2014 H3N2, B/Brisbane/60/08 and B/Phuket/3073/2013: recommended for the 2016-2017 Northern Hemisphere season. Healthy adults 18-49 years of age (n = 1200) were randomized 1:1:1 to receive a 0.5 mL intramuscular injection of QVLP (30 μg HA/strain) from three sequential lots. Local and systemic reactions were monitored for 21 days post-vaccination and blood was collected pre-vaccination and at day 21 (D21) after vaccination to measure hemagglutination inhibition (HI) antibodies.

RESULTS

Subject demographics were similar between groups and compliance with study procedures was 96.3%. The study population was 54.8% female, the mean age (±SD) was 29.9 ± 9.01 and the racial distribution was 77.8% Caucasian, 15.6% Asian, 5.8% Black/African American and 0.8% other. The HI responses met the Center for Biologics Evaluation and Research criteria for seroconversion (SCR ≥ 40%) and seroprotection rates (SPR ≥ 70%). The geometric mean fold rise in HI titers was ≥ 2.5 for all 4 strains for each lot. Lot-to-lot consistency was met with the 95% confidence intervals of the D21 mean geometric titre ratios falling between 0.67 and 1.5 for all four strains. No safety concerns were identified. Solicited adverse events were generally mild and transient: typical for what is reported after inactivated influenza vaccines.

CONCLUSIONS

This study supported earlier findings of the safety profile and immunogenicity of the plant-derived QVLP and demonstrated the consistency with which it can be produced.

Development of a Transgenic Flammulina velutipes Oral Vaccine for Hepatitis B

Orally administered fungal vaccines show promise for the prevention of infectious diseases. Edible mushrooms are deemed appropriate hosts to produce oral vaccines due to their low production cost and low risk of gene contamination. However, their low expression level of antigens has limited the potential development of oral vaccines using mushrooms. The low expression level might result from impurity of the transgenic mycelia since dikaryotic mycelia are commonly used as transformation materials. In this study, stable transgenic hepatitis B virus surface antigen (HBsAg) in Flammulina velutipes transformants was obtained by Agrobacterium-mediated transformation, followed by fruiting and basidiospore mating. The formation of HBsAg was detected by western blot analysis. The expression levels of HBsAg in transgenic F. velutipes fruiting bodies were (129.3±15.1), (110.9±1.7) and (161.1±8.5) ng/g total soluble protein. However, the values may be underestimated due to incomplete protein extraction. Two of the four pigs in the experimental group produced positive anti-HBsAg-specific IgG after being fed the HBsAg transgenic F. velutipes fruiting bodies for 20 weeks, while no anti-HBsAg antibody was detected in the control group. One of the positive pigs had HBsAg titres of 5.36 and 14.9 mIU/mL in weeks 10 and 14, respectively, but expression faded thereafter. The other positive pig displayed HBsAg titres of 9.75, 17.86 and 39.87 mIU/mL in weeks 14, 18 and 20, respectively. The successful immunogenicity in pigs fed transgenic F. velutipes fruiting bodies demonstrated the potential of using the fungus as an oral vaccine.

Oral immunization with rotavirus VP7-CTB fusion expressed in transgenic Arabidopsis thaliana induces antigen-specific IgA and IgG and passive protection in mice

Human rotavirus (HRV) is the primary cause of severe gastroenteritis in children. However, there is currently no protective virus for rotavirus available. In the present study, an HRVVP7-cholera toxin B subunit (CTB) fusion protein was expressed in Arabidopsis thaliana. To determine the adjuvant effect of HRVVP7-CTB, HRVVP7 without CTB was expressed in the same manner. HRVVP7-CTB accounted for 0.39% of the total soluble protein (TSP) in the transgenic seeds and 52.65 µg/g of HRVVP7 protein was expressed in these seeds. Mice were immunized with TSP from the transformed seeds and produced serum immunoglobulin G (IgG) and mucosal IgA specifically directed against HRVVP7. Antibody titers were highest in mice orally immunized with the plant-expressed HRVVP7-CTB protein, whereas HRVVP7-CTB-specific IgG neutralized the rotavirus. Suckling pups born from dams immunized with the HRVVP7-CTB fusion protein were protected against challenge with virulent rotavirus. The results of the present study suggest that the HRVVP7-CTB fusion protein produced in A. thaliana may be a rotaviral-specific candidate subunit vaccine.

Rabies glycoprotein fused with B subunit of cholera toxin expressed in tobacco plants folds into biologically active pentameric protein

The pentameric B subunit of cholera toxin (CtxB) is an efficient mucosal adjuvant for vaccines. We report the expression of a chimeric protein comprising the synthetic cholera toxin B subunit fused at its C-terminal with rabies surface glycoprotein (G protein) in tobacco plants. The approximately 80.3 kDa fusion polypeptide expressed at 0.4% of the total soluble protein in leaves of the selected transgenic lines. The fusion protein formed a approximately 403 kDa pentameric protein which was functionally active in binding to GM1 receptor. The plant-made protein had a higher affinity for GM1 receptor than the native bacterial CtxB. The pentameric fusion protein was recognized by the anti-cholera toxin as well as anti-rabies antibodies. Its immuno-protective ability against rabies remains to be examined.

Expression of a synthetic neutralizing epitope of porcine epidemic diarrhea virus fused with synthetic B subunit of Escherichia coli heat labile enterotoxin in rice endosperm

Epitopes often require co-delivery with adjuvant and targeting proteins to enable recognition by the immune system, and this approach may also increase the efficacy of the antigen. In this study, we assess and describe the ability of transgenic rice plants to express a fusion protein consisting of the B-subunit of the Escherichia coli heat-labile enterotoxin (LTB) and a synthetic core-neutralizing epitope (COE) of porcine epidemic diarrhea virus (PEDV), inducing an enteric disease that is seen most predominantly in piglets. Both components of the fusion proteins were detected with Western blot analysis. The fusion protein was determined to assemble into pentamers, as was evidenced by its ability to bind to GM1 gangliosides, and evidenced an average level of expression in a transgenic rice endosperm. This indicates that the expression system of the plant is capable of generating a sizable amount of antigen, possibly allowing for the successful development of an edible vaccine.

Expression of HPV-11 L1 protein in transgenic Arabidopsis thaliana and Nicotiana tabacum

BACKGROUND

We have investigated the possibility and feasibility of producing the HPV-11 L1 major capsid protein in transgenic Arabidopsis thaliana ecotype Columbia and Nicotiana tabacum cv. Xanthi as potential sources for an inexpensive subunit vaccine.

RESULTS

Transformation of plants was only achieved with the HPV-11 L1 gene with the C-terminal nuclear localization signal (NLS-) encoding region removed, and not with the full-length gene. The HPV-11 L1 NLS- gene was stably integrated and inherited through several generations of transgenic plants. Plant-derived HPV-11 L1 protein was capable of assembling into virus-like particles (VLPs), although resulting particles displayed a pleomorphic phenotype. Neutralising monoclonal antibodies binding both surface-linear and conformation-specific epitopes bound the A. thaliana-derived particles and - to a lesser degree - the N. tabacum-derived particles, suggesting that plant-derived and insect cell-derived VLPs displayed similar antigenic properties. Yields of up to 12 microg/g of HPV-11 L1 NLS- protein were harvested from transgenic A. thaliana plants, and 2 microg/g from N. tabacum plants - a significant increase over previous efforts. Immunization of New Zealand white rabbits with approximately 50 microg of plant-derived HPV-11 L1 NLS- protein induced an antibody response that predominantly recognized insect cell-produced HPV-11 L1 NLS- and not NLS+ VLPs. Evaluation of the same sera concluded that none of them were able to neutralise pseudovirion in vitro.

CONCLUSION

We expressed the wild-type HPV-11 L1 NLS- gene in two different plant species and increased yields of HPV-11 L1 protein by between 500 and 1000-fold compared to previous reports. Inoculation of rabbits with extracts from both plant types resulted in a weak immune response, and antisera neither reacted with native HPV-11 L1 VLPs, nor did they neutralise HPV-11 pseudovirion infectivity. This has important and potentially negative implications for the production of HPV-11 vaccines in plants.

Vaccination against periodontitis: the saga continues

Periodontal disease can be considered to be one of the most common chronic inflammatory diseases inflicting humans. With the advent of advanced molecular diagnostic techniques, a better understanding of the role of specific pathogens and the contributory role of the host immune response in the initiation and progression of periodontal disease has been possible - although not completely. However, successful vaccine development that fully utilizes the current level of understanding has not yet occurred for human use. This paper reviews various in vitro, animal studies and human trials undertaken to develop a vaccine against periodontal disease, with emphases on the shortfalls of these efforts and future prospects of developing a successful vaccine against periodontal disease.

Synthesis of a ricin toxin B subunit-rotavirus VP7 fusion protein in potato

A gene encoding the outer capsid glycoprotein (VP7) of simian rotavirus SA11, was genetically linked to the amino terminus of the ricin toxin B subunit (RTB) isolated from castor-oil plant (Ricinus communis) seeds. To assess fusion protein expression in plant cells, the VP7::RTB fusion gene was transferred into potato (Solanum tuberosum) cells by Agrobacterium tumefaciens-mediated transformation methods and transformed plants regenerated. The fusion gene was detected in transformed potato genomic DNA by polymerase chain reaction DNA amplification methods. Immunoblot analysis with anti-SA11 antiserum as the primary antibody verified the presence of VP7::RTB fusion protein in transformed potato tuber tissues. The plant-synthesized fusion protein bound RTB membrane receptors as measured by asialofetuin-enzyme-linked immunosorbent assay (ELISA). The ELISA results indicated that the VP7::RTB fusion protein was biologically active and made up approx 0.03% of total soluble transformed tuber protein. The biosynthesis of receptor binding VP7::RTB fusion protein in potato tissues demonstrates the feasibility of producing monomeric ricin toxin B subunit adjuvant-virus antigen fusion proteins in crop plants for enhanced immunity.

Expression of synthetic neutralizing epitope of porcine epidemic diarrhea virus fused with synthetic B subunit of Escherichia coli heat-labile enterotoxin in tobacco plants

The pentameric B subunit of Escherichia coli heat-labile enterotoxin (LTB) can be used as an efficient mucosal carrier of either immunogenic or tolerogenic T-cell epitopes. Co-delivery of therapeutic proteins with carrier proteins could increase the effectiveness of the antigen. This paper reports the ability of transgenic tobacco plants to express a fusion protein consisting of the synthetic LTB and a synthetic neutralizing epitope of porcine epidemic diarrhea virus (PEDV), causing an enteric disease that is especially severe in piglets. Both components of the fusion proteins were detected in Western blot analysis, and binding assay confirmed that plant-synthesized pentameric LTB-PEDV fusion bound to the intestinal membrane GM1-ganglioside receptor. This suggested that the fusion protein retained both its native antigenicity and the ability to form pentamers.

Mass production of somatic embryos expressing Escherichia coli heat-labile enterotoxin B subunit in Siberian ginseng

The B subunit of Escherichia coli heat-labile toxin (LTB) is a potent mucosal immunogen and immunoadjuvant for co-administered antigens. In order to produce large scale of LTB for the development of edible vaccine, we used transgenic somatic embryos of Siberian ginseng, which is known as medicinal plant. When transgenic somatic embryos were cultured in 130L air-lift type bioreactor, they were developed to mature somatic embryos through somatic embryogenesis and contained approximately 0.36% LTB of the total soluble protein. Enzyme-linked immunosorbent assay indicated that the somatic embryo-synthesized LTB protein bound specifically to GM1-ganglioside, suggesting the LTB subunits formed active pentamers. Therefore, the use of the bioreactor system for expression of LTB proteins in somatic embryos allows for continuous mass production in a short-term period.

Immunization with a plant-produced colorectal cancer antigen

Cancer vaccination has become an important focus of oncology in recent years. Active immunization with tumor-associated antigens such as colorectal cancer antigen GA733-2 is thought to potentially overcome the reoccurrence of metastasis. As recombinant protein production in bioreactors is costly and subject to growing safety concerns, we tested plants as an alternative for the expression of a potential colorectal cancer vaccine. Comparing colorectal cancer antigen GA733-2 produced in tobacco plants with the same antigen produced in insect cell culture, we found a similar humoral immune response to injection of either of the two antigen preparations into mice. Some minor differences were observed in the cellular response that might be due to impurities. Our studies compare for the first time, immunization with the same antigen expressed in either plants or insect cell culture. This will provide important data for use of plants as production systems of therapeutics.

Identification of immunogenic hot spots within plum pox potyvirus capsid protein for efficient antigen presentation

PEPSCAN analysis has been used to characterize the immunogenic regions of the capsid protein (CP) in virions of plum pox potyvirus (PPV). In addition to the well-known highly immunogenic N- and C-terminal domains of CP, regions within the core domain of the protein have also shown high immunogenicity. Moreover, the N terminus of CP is not homogeneously immunogenic, alternatively showing regions frequently recognized by antibodies and others that are not recognized at all. These results have helped us to design efficient antigen presentation vectors based on PPV. As predicted by PEPSCAN analysis, a small displacement of the insertion site in a previously constructed vector, PPV-gamma, turned the derived chimeras into efficient immunogens. Vectors expressing foreign peptides at different positions within a highly immunogenic region (amino acids 43 to 52) in the N-terminal domain of CP were the most effective at inducing specific antibody responses against the foreign sequence.

Delivery technologies for human vaccines

There is currently intense research activity aimed at the development of new delivery systems for vaccines. The goal is to identify optimal methods for presenting target antigens to the immune system in a manner that will elicit immune responses appropriate for protection against, or treatment of, a specific disease. Several different approaches to this general goal have been developed, some are empirical and remain poorly understood, others are more rational, being based, for example, on mimicking natural infections in vivo or on targeting particular features of the immune system. This article will review three categories of delivery systems: (i) adjuvants and formulations; (ii) antigen vectors, including live attenuated micro-organisms and synthetic vectors; and (iii) novel devices for vaccine administration. The review will be restricted to late stage developments in the field of human vaccination.

Chimeric plant virus particles as immunogens for inducing murine and human immune responses against human immunodeficiency virus type 1

The high-yield expression of a neutralizing epitope from human immunodeficiency virus type 1 (HIV-1) on the surface of a plant virus and its immunogenicity are presented. The highly conserved ELDKWA epitope from glycoprotein (gp) 41 was expressed as an N-terminal translational fusion with the potato virus X (PVX) coat protein. The resulting chimeric virus particles (CVPs), purified and used to immunize mice intraperitoneally or intranasally, were able to elicit high levels of HIV-1-specific immunoglobulin G (IgG) and IgA antibodies. Furthermore, the human immune response to CVPs was studied with severe combined immunodeficient mice reconstituted with human peripheral blood lymphocytes (hu-PBL-SCID). hu-PBL-SCID mice immunized with CVP-pulsed autologous dendritic cells were able to mount a specific human primary antibody response against the gp41-derived epitope. Notably, sera from both normal and hu-PBL-SCID mice showed an anti-HIV-1-neutralizing activity. Thus, PVX-based CVPs carrying neutralizing epitopes can offer novel perspectives for the development of effective vaccines against HIV and, more generally, for the design of new vaccination strategies in humans.

Protection of rabbits against rabbit hemorrhagic disease virus by immunization with the VP60 protein expressed in plants with a potyvirus-based vector

A new plum pox potyvirus (PPV)-based vector has been constructed for the expression of full-length individual foreign proteins. The foreign sequences are cloned between the NIb replicase and capsid protein (CP) cistrons. The heterologous protein is split from the rest of the potyviral polyprotein by cleavage at the site that originally separated the NIb and CP proteins and at an additional NIa protease recognition site engineered at its amino-terminal end. This vector (PPV-NK) has been used to clone different genes, engendering stable chimeras with practical applications. We have constructed a chimera expressing high levels of jellyfish green fluorescent protein, which can be very useful for the study of PPV molecular biology. The VP60 structural protein of rabbit hemorrhagic disease virus (RHDV) was also successfully expressed by making use of the PPV-NK vector. Inoculation of extracts from VP60-expressing plants induced a remarkable immune response against RHDV in rabbits, its natural host. Moreover, these animals were protected against a lethal challenge with RHDV.

Plants as bioreactors for protein production: avoiding the problem of transgene silencing

Plants are particularly attractive as large-scale production systems for proteins intended for therapeutical or industrial applications: they can be grown easily and inexpensively in large quantities that can be harvested and processed with the available agronomic infrastructures. The effective use of plants as bioreactors depends on the possibility of obtaining high protein accumulation levels that are stable during the life cycle of the transgenic plant and in subsequent generations. Silencing of the introduced transgenes has frequently been observed in plants, constituting a major commercial risk and hampering the general economic exploitation of plants as protein factories. Until now, the most efficient strategy to avoid transgene silencing involves careful design of the transgene construct and thorough analysis of transformants at the molecular level. Here, we focus on different aspects of the generation of transgenic plants intended for protein production and on their influence on the stability of heterologous gene expression.

Oral immunization of mice with transgenic tomato fruit expressing respiratory syncytial virus-F protein induces a systemic immune response

Respiratory syncytial virus (RSV) is one of the most important pathogens of infancy and early childhood. Here a fruit-based edible subunit vaccine against RSV was developed by expressing the RSV fusion (F) protein gene in transgenic tomato plants. The F-gene was expressed in ripening tomato fruit under the control of the fruit-specific E8 promoter. Oral immunization of mice with ripe transgenic tomato fruits led to the induction of both serum and mucosal RSV-F specific antibodies. The ratio of immunoglobulin subclasses produced in response to immunization suggested that a type 1 T-helper cell immune response was preferentially induced. Serum antibodies showed an increased titer when the immunized mice were exposed to inactivated RSV antigen.

Novel Approaches to Oral Vaccines: Delivery of Antigens by Edible Plants

Advances in genetic engineering in the past decade have accelerated the expression, in plants, of foreign proteins with industrial and pharmaceutical value. Antigens from infectious bacterial or viral diseases have been introduced into plants through plant virus-mediated infection or Agrobacterium tumefaciens-mediated stable transformation methods. Oral immunization with transgenic plant tissues that contain vaccine antigen proteins stimulates both systemic and mucosal immune responses in animals. Plant-based vaccines can provide significant levels of protection against challenge by viral or bacterial pathogens.

Stable expression of human beta1,4-galactosyltransferase in plant cells modifies N-linked glycosylation patterns

beta1,4-Galactosyltransferase (UDP galactose: beta-N-acetylglucosaminide: beta1,4-galactosyltransferase; EC 2.4.1. 22) catalyzes the transfer of galactose from UDP-Gal to N-acetylglucosamine in the penultimate stages of the terminal glycosylation of N-linked complex oligosaccharides in mammalian cells. Tobacco BY2 cells lack this Golgi enzyme. To determine to what extent the production of a mammalian glycosyltransferase can alter the glycosylation pathway of plant cells, tobacco BY2 suspension-cultured cells were stably transformed with the full-length human galactosyltransferase gene placed under the control of the cauliflower mosaic virus 35S promoter. The expression was confirmed by assaying enzymatic activity as well as by Southern and Western blotting. The transformant with the highest level of enzymatic activity has glycans with galactose residues at the terminal nonreducing ends, indicating the successful modification of the plant cell N-glycosylation pathway. Analysis of the oligosaccharide structures shows that the galactosylated N-glycans account for 47.3% of the total sugar chains. In addition, the absence of the dominant xylosidated- and fucosylated-type sugar chains confirms that the transformed cells can be used to produce glycoproteins without the highly immunogenic glycans typically found in plants. These results demonstrate the synthesis in plants of N-linked glycans with modified and defined sugar chain structures similar to mammalian glycoproteins.

Perspectives in agrobiotechnology

This review surveys the most important and promising contributions of agricultural biotechnology to the development of sustainable, environment-friendly agriculture. It deals with the recent achievements of genetic technology for the development of new transgenic microbial, plant and animal products. It also deals with the newest developments and perspectives of microbial intervention in agricultural practices, such as biofertilizers, biocontrol agents, and various microbiological products used in modern agriculture. The review surveys the outlook for a waste-free, environment-friendly sustainable agricultural practice, including waste management, recycling and bioremediation technologies. The review lists the most important marketable agrobiotechnological products, and their present and projected sales volume.

Development of an antigen presentation system based on plum pox potyvirus

The development of an antigen presentation system based on the plum pox potyvirus (PPV) is here described. The amino-terminal part of PPV capsid protein was chosen as the site for expression of foreign antigenic peptides. Modifications in this site were engineered to avoid the capability of natural transmission by aphids of this PPV vector. As a first practical attempt, different forms of an antigenic peptide (single and tandem repetition) from the VP2 capsid protein of canine parvovirus (CPV) were expressed. Both chimeras are able to infect Nicotiana clevelandii plants with similar characteristics to wild-type virus and remain genetically stable after several plant passages. The antigenicity of purified chimeric virions was demonstrated, proving the suitability of this system for diagnostic purposes. Moreover, mice and rabbits immunized with chimeric virions developed CPV-specific antibodies, which showed neutralizing activity.

Characterization of a recombinant plant monoclonal secretory antibody and preventive immunotherapy in humans

A functional comparison was made between a monoclonal secretory antibody generated in transgenic plants and its parent murine IgG antibody.The affinity constants of both antibodies for a Streptococcus mutans adhesion protein were similar. However the secretory antibody had a higher functional affinity due to its dimeric structure. In the human oral cavity, the secretory antibody survived for up to three days, compared with one day for the IgG antibody. The plant secretory antibody afforded specific protection in humans against oral streptococcal colonization for at least four months. We demonstrate that transgenic plants can be used to produce high affinity, monoclonal secretory antibodies that can prevent specific microbial colonization in humans. These findings could be extended to the immunotherapeutic prevention of other mucosal infections in humans and animals.

Toward a successful multinational crop plant genome initiative

Plant genome research is needed as the foundation for an entirely new level of efficiency and success in the application of genetics and breeding to crop plants and products from crop plants. Genetic improvements in crop plants beyond current capabilities are needed to meet the growing world demand not only for more food, but also a greater diversity of food, higher-quality food, and safer food, produced on less land, while conserving soil, water, and genetic resources. Plant biology research, which is poised for dramatic advances, also depends fundamentally on plant genome research. The current Arabidopsis Genome Project has proved of immediate value to plant biology research, but a much greater effort is needed to ensure the full benefits of plant biology and especially plant genome research to agriculture. International cooperation is critical, both because genome projects are too large for any one country and the information forthcoming is of benefit to the world and not just the countries that do the work. Recent research on grass genomes has revealed that, because of extensive senteny and colinearity within linkage groups that make up the chromosomes, new information on the genome of one grass can be used to understand the genomes and predict the location of genes on chromosomes of the other grasses. Genome research applied to grasses as a group thereby can increase the efficiency and effectiveness of breeding for improvement of each member of this group, which includes wheat, corn, and rice, the world's three most important sources of food.

Recombinant proteins from transgenic plants

Transgenic plants can express a wide variety of foreign genes and offer the opportunity of large-scale protein production in agricultural systems. The recombinant protein can serve both ex situ and in situ purposes. Due to significant progress in plant molecular biology, many different plant species can now be transformed and are even capable of producing very complex proteins such as antibodies or vaccines. Furthermore, recombinant proteins can mediate resistance against microbial pathogens, such as fungi or viruses, or protect transgenic plants from insect pests.