Immunogenicity of a novel, bivalent, plant-based oral vaccine against hepatitis B and human immunodeficiency viruses

Tomatoes: An Extensive Review of the Associated Health Impacts of Tomatoes and Factors That Can Affect Their Cultivation

Simple Summary

The research outlined in this review paper discusses potential health benefits associated with a diet enriched with tomatoes and tomato products. This includes details of previous studies investigating the anticancer properties of tomatoes, protection against cardiovascular and neurodegenerative diseases and diabetes, maintenance of a healthy gut microbiome, and improved skin health, fertility, immune response, and exercise recovery. The specific parts of a tomato fruit that contribute these health benefits are also outlined. The potential disadvantages to a tomato-rich diet are detailed, especially the consumption of supplements that contain compounds found in tomatoes, such as lycopene. This review also discusses how the cultivation of tomato plants can affect the nutritional value of the fruit harvested. Different environmental growing conditions such as light intensity, growing media, and temperature are explained in terms of the impact they have on the quality of fruit, its nutrient content, and hence the potential health benefits acquired from eating the fruit.

Abstract

This review outlines the health benefits associated with the regular consumption of tomatoes and tomato products. The first section provides a detailed account of the horticultural techniques that can impact the quality of the fruit and its nutritional properties, including water availability, light intensity, temperature, and growing media. The next section provides information on the components of tomato that are likely to contribute to its health effects. The review then details some of the health benefits associated with tomato consumption, including anticancer properties, cardiovascular and neurodegenerative diseases and skin health. This review also discusses the impact tomatoes can have on the gut microbiome and associated health benefits, including reducing the risk of inflammatory bowel diseases. Other health benefits of eating tomatoes are also discussed in relation to effects on diabetes, the immune response, exercise recovery, and fertility. Finally, this review also addresses the negative effects that can occur as a result of overconsumption of tomato products and lycopene supplements.

Plant-Derived Recombinant Vaccines against Zoonotic Viruses

Emerging and re-emerging zoonotic diseases cause serious illness with billions of cases, and millions of deaths. The most effective way to restrict the spread of zoonotic viruses among humans and animals and prevent disease is vaccination. Recombinant proteins produced in plants offer an alternative approach for the development of safe, effective, inexpensive candidate vaccines. Current strategies are focused on the production of highly immunogenic structural proteins, which mimic the organizations of the native virion but lack the viral genetic material. These include chimeric viral peptides, subunit virus proteins, and virus-like particles (VLPs). The latter, with their ability to self-assemble and thus resemble the form of virus particles, are gaining traction among plant-based candidate vaccines against many infectious diseases. In this review, we summarized the main zoonotic diseases and followed the progress in using plant expression systems for the production of recombinant proteins and VLPs used in the development of plant-based vaccines against zoonotic viruses.

Platforms for Production of Protein-Based Vaccines: From Classical to Next-Generation Strategies

To date, vaccination has become one of the most effective strategies to control and reduce infectious diseases, preventing millions of deaths worldwide. The earliest vaccines were developed as live-attenuated or inactivated pathogens, and, although they still represent the most extended human vaccine types, they also face some issues, such as the potential to revert to a pathogenic form of live-attenuated formulations or the weaker immune response associated with inactivated vaccines. Advances in genetic engineering have enabled improvements in vaccine design and strategies, such as recombinant subunit vaccines, have emerged, expanding the number of diseases that can be prevented. Moreover, antigen display systems such as VLPs or those designed by nanotechnology have improved the efficacy of subunit vaccines. Platforms for the production of recombinant vaccines have also evolved from the first hosts, Escherichia coli and Saccharomyces cerevisiae, to insect or mammalian cells. Traditional bacterial and yeast systems have been improved by engineering and new systems based on plants or insect larvae have emerged as alternative, low-cost platforms. Vaccine development is still time-consuming and costly, and alternative systems that can offer cost-effective and faster processes are demanding to address infectious diseases that still do not have a treatment and to face possible future pandemics.

Producing Vaccines against Enveloped Viruses in Plants: Making the Impossible, Difficult

The past 30 years have seen the growth of plant molecular farming as an approach to the production of recombinant proteins for pharmaceutical and biotechnological uses. Much of this effort has focused on producing vaccine candidates against viral diseases, including those caused by enveloped viruses. These represent a particular challenge given the difficulties associated with expressing and purifying membrane-bound proteins and achieving correct assembly. Despite this, there have been notable successes both from a biochemical and a clinical perspective, with a number of clinical trials showing great promise. This review will explore the history and current status of plant-produced vaccine candidates against enveloped viruses to date, with a particular focus on virus-like particles (VLPs), which mimic authentic virus structures but do not contain infectious genetic material.

Hepatitis B core-based virus-like particles: A platform for vaccine development in plants

Virus-like particles (VLPs) are a class of structures formed by the self-assembly of viral capsid protein subunits and contain no infective viral genetic material. The Hepatitis B core (HBc) antigen is capable of assembling into VLPs that can elicit strong immune responses and has been licensed as a commercial vaccine against Hepatitis B. The HBc VLPs have also been employed as a platform for the presentation of foreign epitopes to the immune system and have been used to develop vaccines against, for example, influenza A and Foot-and-mouth disease. Plant expression systems are rapid, scalable and safe, and are capable of providing correct post-translational modifications and reducing upstream production costs. The production of HBc-based virus-like particles in plants would thus greatly increase the efficiency of vaccine production. This review investigates the application of plant-based HBc VLP as a platform for vaccine production.

Plant Platforms for Efficient Heterologous Protein Production

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

Platform technologies for modern vaccine manufacturing

Improved understanding of antigenic components and their interaction with the immune system, as supported by computational tools, permits a sophisticated approach to modern vaccine design. Vaccine platforms provide an effective tool by which strategically designed peptide and protein antigens are modularized to enhance their immunogenicity. These modular vaccine platforms can overcome issues faced by traditional vaccine manufacturing and have the potential to generate safe vaccines, rapidly and at a low cost. This review introduces two promising platforms based on virus-like particle and liposome, and discusses the methodologies and challenges.

Plant-Based Peroral Vaccines

Data about the development of plant-made peroral vaccines are stated on the base of transgenic plants. Different aspects of plant made vaccines (PMV) with their peculiarities and with a plethora of advantages are discussed, in comparison with traditional vaccines. The results disclosing the characteristics of the creation of PMV of both preventive and therapeutic actions are presented. The role of regulatory genes in the increase of the production of antigenic proteins, and in the activation of the glycosylation that enhances the efficiency of PMV, is discussed, according to preventive and therapeutic vaccines and plantibodies.

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

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

Recent advances on host plants and expression cassettes' structure and function in plant molecular pharming

Plant molecular pharming is a promising system to produce important recombinant proteins such as therapeutic antibodies, pharmaceuticals, enzymes, growth factors, and vaccines. The system provides an interesting alternative method to the direct extraction of proteins from inappropriate source material while offering the possibility to overcome problems related to product safety and source availability. Multiple factors including plant hosts, genes of interest, expression vector cassettes, and extraction and purification techniques play important roles in the plant molecular pharming. Plant species, as a biosynthesis platform, are a crucial factor in achieving high yields of recombinant protein in plant. The choice of recombinant gene and its expression strategy is also of great importance in ensuring a high amount of the recombinant proteins. Many studies have been conducted to improve expression, accumulation, and purification of the recombinant protein from molecular pharming systems. Re-engineered vectors and expression cassettes are also pivotal tools in enhancing gene expression at the transcription and translation level, and increasing protein accumulation, stability, retention and targeting of specific organelles. In this review, we report recent advances and strategies of plant molecular pharming while focusing on the choice of plant hosts and the role of some molecular pharming elements and approaches: promoters, codon optimization, signal sequences, and peptides used for upstream design, purification and downstream processing.

Expression of recombinant vaccines and antibodies in plants

Plants are able to perform post-translational maturations of therapeutic proteins required for their functional biological activity and suitable in vivo pharmacokinetics. Plants can be a low-cost, large-scale production platform of recombinant biopharmaceutical proteins such as vaccines and antibodies. Plants, however, lack mechanisms of processing authentic human N-glycosylation, which imposes a major limitation in their use as an expression system for therapeutic glycoproducts. Efforts have been made to circumvent plant-specific N-glycosylation, as well as to supplement the plant's endogenous system with human glycosyltransferases for non-immunogenic and humanized N-glycan production. Herein we review studies on the potential of plants to serve as production systems for therapeutic and prophylactic biopharmaceuticals. We have especially focused on recombinant vaccines and antibodies and new expression strategies to overcome the existing problems associated with their production in plants.

Mucosal Vaccines from Plant Biotechnology

The use of plants for production of recombinant proteins has evolved over the past 25 years. The first plant-based vaccines were expressed in stably transgenic plants, with the idea to conveniently deliver “edible vaccines” by ingestion of the antigen-containing plant material. These systems provided a proof of concept that oral delivery of vaccines in crude plant material could stimulate antigen-specific serum and mucosal antibodies. Transgenic grains like rice in particular provide a stable and robust vehicle for antigen delivery. However, some issues exist with stably transgenic plants, including relatively low expression levels and regulatory issues. Thus, many recent studies use transient expression with plant viral vectors to achieve rapid high expression in Nicotiana benthamiana, followed by purification of antigen and intranasal delivery for effective stimulation of mucosal immune responses.

Plant-based vaccines against viruses

Plant-made or "biofarmed" viral vaccines are some of the earliest products of the technology of plant molecular farming, and remain some of the brightest prospects for the success of this field. Proofs of principle and of efficacy exist for many candidate viral veterinary vaccines; the use of plant-made viral antigens and of monoclonal antibodies for therapy of animal and even human viral disease is also well established. This review explores some of the more prominent recent advances in the biofarming of viral vaccines and therapies, including the recent use of ZMapp for Ebolavirus infection, and explores some possible future applications of the technology.

N-glycosylation modification of plant-derived virus-like particles: an application in vaccines

Plants have been developed as an alternative system to mammalian cells for production of recombinant prophylactic or therapeutic proteins for human and animal use. Effective plant expression systems for recombinant proteins have been established with the optimal combination of gene expression regulatory elements and control of posttranslational processing of recombinant glycoproteins. In plant, virus-like particles (VLPs), viral “empty shells” which maintain the same structural characteristics of virions but are genome-free, are considered extremely promising as vaccine platforms and therapeutic delivery systems. Unlike microbial fermentation, plants are capable of carrying out N-glycosylation as a posttranslational modification of glycoproteins. Recent advances in the glycoengineering in plant allow human-like glycomodification and optimization of desired glycan structures for enhancing safety and functionality of recombinant pharmaceutical glycoproteins. In this review, the current plant-derived VLP approaches are focused, and N-glycosylation and its in planta modifications are discussed.

Oral delivery of plant-derived HIV-1 p24 antigen in low doses shows a superior priming effect in mice compared to high doses

During early infection with human immunodeficiency virus type 1 (HIV-1), there is a rapid depletion of CD4(+) T-cells in the gut-associated lymphoid tissue (GALT) in the gastrointestinal tract. Therefore, immediate protection at these surfaces is of high priority for the development of an HIV-1 vaccine. Thus, transgenic plants expressing HIV-1 antigens, which are exposed to immune competent cells in the GALT during oral administration, can be interesting as potential vaccine candidates. In the present study, we used two HIV-1 p24 antigen-expressing transgenic plant systems, Arabidopsis thaliana and Daucus carota, in oral immunization experiments. Both transgenic plant systems showed a priming effect in mice and induced humoral immune responses, which could be detected as anti-p24-specific IgG in sera after an intramuscular p24 protein boost. Dose-dependent antigen analyses using transgenic A. thaliana indicated that low p24 antigen doses were superior to high p24 antigen doses.

Immunogenic properties of a lettuce-derived C4(V3)6 multiepitopic HIV protein

Elicitation of broad humoral immune responses is a critical factor in the development of effective HIV vaccines. In an effort to develop low-cost candidate vaccines based on multiepitopic recombinant proteins, this study has been undertaken to assess and characterize the immunogenic properties of a lettuce-derived C4(V3)6 multiepitopic protein. This protein consists of V3 loops corresponding to five different HIV isolates, including MN, IIIB, RF, CC, and RU. In this study, both Escherichia coli and lettuce-derived C4(V3)6 have elicited local and systemic immune responses when orally administered to BALB/c mice. More importantly, lettuce-derived C4(V3)6 has shown a higher immunogenic potential than that of E. coli-derived C4(V3)6. Moreover, when reactivity of sera from mice immunized with C4(V3)6 are compared with those elicited by a chimeric protein carrying a single V3 sequence, broader responses have been observed. The lettuce-derived C4(V3)6 has elicited antibodies with positive reactivity against V3 loops from isolates MN, RF, and CC. In addition, splenocyte proliferation assays indicate that significant T-helper responses are induced by the C4(V3)6 immunogen. Taken together, these findings account for the observed elicitation of broader humoral responses by the C4(V3)6 multiepitopic protein. Moreover, they provide further validation for the production of multiepitopic vaccines in plant cells as this serves not only as a low-cost expression system, but also as an effective delivery vehicle for orally administered immunogens.

Biological and biochemical characterization of HIV-1 Gag/dgp41 virus-like particles expressed in Nicotiana benthamiana

The transmembrane HIV-1 envelope protein gp41 has been shown to play critical roles in the viral mucosal transmission and infection of CD4⁺ cells. Gag is a structural protein configuring the enveloped viral particles and has been suggested to constitute a target of the cellular immunity that may control viral load. We hypothesized that HIV enveloped virus-like particles (VLPs) consisting of Gag and a deconstructed form of gp41 comprising the membrane proximal external, transmembrane and cytoplasmic domains (dgp41) could be expressed in plants. To this end, plant-optimized HIV-1 genes were constructed and expressed in Nicotiana benthamiana by stable transformation, or transiently using a Tobamovirus-based expression system or a combination of both. Our results of biophysical, biochemical and electron microscopy characterization demonstrates that plant cells could support not only the formation of enveloped HIV-1 Gag VLPs, but also the accumulation of VLPs that incorporated dgp41. These findings provide further impetus for the journey towards a broadly efficacious and inexpensive subunit vaccine against HIV-1.

Production of pharmaceutical proteins in solanaceae food crops

The benefits of increased safety and cost-effectiveness make vegetable crops appropriate systems for the production and delivery of pharmaceutical proteins. In particular, Solanaceae edible crops could be inexpensive biofactories for oral vaccines and other pharmaceutical proteins that can be ingested as minimally processed extracts or as partially purified products. The field of crop plant biotechnology is advancing rapidly due to novel developments in genetic and genomic tools being made available today for the scientific community. In this review, we briefly summarize data now available regarding genomic resources for the Solanaceae family. In addition, we describe novel strategies developed for the expression of foreign proteins in vegetable crops and the utilization of these techniques to manufacture pharmaceutical proteins.

A perspective for atherosclerosis vaccination: is there a place for plant-based vaccines?

Alternatives to pharmacological treatments for atherosclerosis are highly desirable in terms of cost and compliance. During the last two decades several vaccination strategies have been reported as an effort to develop immunotherapeutic treatments. This approach consists on eliciting immune responses able to modulate either the atherosclerosis-associated inflammatory processes or the activity of some physiological mechanisms that are up-regulated under this pathologic condition. In particular, the apolipoprotein B100 (ApoB100) and the cholesterilester transferase protein (CETP) have been targeted in these strategies. It is considered that recent progress in the development of experimental models of oral vaccines against atherosclerosis has opened a new avenue in the field: as plant-based vaccines are considered a viable platform for vaccine production and delivery at low costs, they could serve as an oral-delivered therapeutic approach for atherosclerosis in an economical and patient-friendly manner. The rationale of the design, development and evaluation of possible plant-based vaccines against atherosclerosis is discussed in this review. We identify within this approach a significant trend that will positively impact the field of atherosclerosis vaccination.

Plant-derived virus-like particles as vaccines

Virus-like particles (VLPs) are self-assembled structures derived from viral antigens that mimic the native architecture of viruses but lack the viral genome. VLPs have emerged as a premier vaccine platform due to their advantages in safety, immunogenicity, and manufacturing. The particulate nature and high-density presentation of viral structure proteins on their surface also render VLPs as attractive carriers for displaying foreign epitopes. Consequently, several VLP-based vaccines have been licensed for human use and achieved significant clinical and economical success. The major challenge, however, is to develop novel production platforms that can deliver VLP-based vaccines while significantly reducing production times and costs. Therefore, this review focuses on the essential role of plants as a novel, speedy and economical production platform for VLP-based vaccines. The advantages of plant expression systems are discussed in light of their distinctive posttranslational modifications, cost-effectiveness, production speed, and scalability. Recent achievements in the expression and assembly of VLPs and their chimeric derivatives in plant systems as well as their immunogenicity in animal models are presented. Results of human clinical trials demonstrating the safety and efficacy of plant-derived VLPs are also detailed. Moreover, the promising implications of the recent creation of "humanized" glycosylation plant lines as well as the very recent approval of the first plant-made biologics by the U. S. Food and Drug Administration (FDA) for plant production and commercialization of VLP-based vaccines are discussed. It is speculated that the combined potential of plant expression systems and VLP technology will lead to the emergence of successful vaccines and novel applications of VLPs in the near future.

Virus-like particles as a highly efficient vaccine platform: diversity of targets and production systems and advances in clinical development

Virus-like particles (VLPs) are a class of subunit vaccines that differentiate themselves from soluble recombinant antigens by stronger protective immunogenicity associated with the VLP structure. Like parental viruses, VLPs can be either non-enveloped or enveloped, and they can form following expression of one or several viral structural proteins in a recombinant heterologous system. Depending on the complexity of the VLP, it can be produced in either a prokaryotic or eukaryotic expression system using target-encoding recombinant vectors, or in some cases can be assembled in cell-free conditions. To date, a wide variety of VLP-based candidate vaccines targeting various viral, bacterial, parasitic and fungal pathogens, as well as non-infectious diseases, have been produced in different expression systems. Some VLPs have entered clinical development and a few have been licensed and commercialized. This article reviews VLP-based vaccines produced in different systems, their immunogenicity in animal models and their status in clinical development.

[Current status in the commercialization and application of genetically modified plants and their effects on human and livestock health and phytoremediation]

Developments in the use of genetically modified plants for human and livestock health and phytoremediation were surveyed using information retrieved from Entrez PubMed, Chemical Abstracts Service, Google, congress abstracts and proceedings of related scientific societies, scientific journals, etc. Information obtained was classified into 8 categories according to the research objective and the usage of the transgenic plants as 1: nutraceuticals (functional foods), 2: oral vaccines, 3: edible curatives, 4: vaccine antigens, 5: therapeutic antibodies, 6: curatives, 7: diagnostic agents and reagents, and 8: phytoremediation. In total, 405 cases were collected from 2006 to 2010. The numbers of cases were 120 for nutraceuticals, 65 for oral vaccines, 25 for edible curatives, 36 for vaccine antigens, 36 for therapeutic antibodies, 76 for curatives, 15 for diagnostic agents and reagents, and 40 for phytoremediation (sum of each cases was 413 because some reports were related to several categories). Nutraceuticals, oral vaccines and curatives were predominant. The most frequently used edible crop was rice (51 cases), and tomato (28 cases), lettuce (22 cases), potato (18 cases), corn (15 cases) followed.

Current status and perspectives of plant-based candidate vaccines against the human immunodeficiency virus (HIV)

Genetically engineered plants are economical platforms for the large-scale production of recombinant proteins and have been used over the last 21 years as models for oral vaccines against a wide variety of human infectious and autoimmune diseases with promising results. The main inherent advantages of this approach consist in the absence of purification needs and easy production and administration. One relevant infectious agent is the human immunodeficiency virus (HIV), since AIDS evolved as an alarming public health problem implicating very high costs for government agencies in most African and developing countries. The design of an effective and inexpensive vaccine able to limit viral spread and neutralizing the viral entry is urgently needed. Due to the limited efficacy of the vaccines assessed in clinical trials, new HIV vaccines able to generate broad immune profiles are a priority in the field. This review discusses the current advances on the topic of using plants as alternative expression systems to produce functional vaccine components against HIV, including antigens from Env, Gag and early proteins such as Tat and Nef. Ongoing projects of our group based on the expression of chimeric proteins comprising C4 and V3 domains from gp120, as an approach to elicit broadly neutralizing antibodies are mentioned. The perspectives of the revised approaches, such as the great need of assessing the oral immunogenicity and a detailed immunological characterization of the elicited immune responses, are also discussed.

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

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

Low-dose oral immunization with lyophilized tissue of herbicide-resistant lettuce expressing hepatitis B surface antigen for prototype plant-derived vaccine tablet formulation

Efficient immunization against hepatitis B virus (HBV) and other pathogens with plant-based oral vaccines requires appropriate plant expressors and the optimization of vaccine compositions and administration protocols. Previous immunization studies were mainly based on a combination of the injection of a small surface antigen of HBV (S-HBsAg) and the feeding with raw tissue containing the antigen, supplemented with an adjuvant, and coming from plants conferring resistance to kanamycin. The objective of this study was to develop a prototype oral vaccine formula suitable for human immunization. Herbicide-resistant lettuce was engineered, stably expressing through progeny generation micrograms of S-HBsAg per g of fresh weight and formed into virus-like particles (VLPs). Lyophilized tissue containing a relatively low, 100-ng VLP-assembled antigen dose, administered only orally to mice with a long, 60-day interval between prime and boost immunizations and without exogenous adjuvant, elicited mucosal and systemic humoral anti-HBs responses at the nominally protective level. Lyophilized tissue was converted into tablets, which preserved S-HBsAg content for at least one year of room temperature storage. The results of the study provide indications on immunization methodology using a durable, efficacious, and convenient plant-derived prototype oral vaccine against hepatitis B.

Monte Carlo study of B-cell receptor clustering mediated by antigen crosslinking and directed transport

It is known from experiments that in the presence of soluble antigen, B-cell receptors (BCRs) assemble into microclusters and then collect into a macrocluster known as a 'cap'. However, the mechanisms of BCR cluster formation during recognition of soluble antigens remain unclear. In previous work, we demonstrated that effective intrinsic attractions among BCRs can lead to the formation of small microclusters of BCR molecules. The effective intrinsic attractions could be caused by multivalent antigen binding, association with lipid rafts, or other biochemical factors. In the present study, we have developed and studied a Monte Carlo model of BCR clustering mediated by explicit binding and crosslinking of soluble bivalent antigens. Antigen crosslinking is shown to microcluster BCRs in an affinity-dependent manner and also in a biologically relevant timescale; however, antigen crosslinking alone does not appear to be sufficient for the formation of a single macrocluster of receptor molecules. We show that directed transport of BCRs is needed to drive the formation of large macroclusters. We constructed a simple model of directed transport, where BCR molecules diffuse towards the largest cluster or towards a random BCR microcluster, which results in a single macrocluster of receptor molecules. The mechanisms for both types of directed transport are compared using network-based metrics. We also develop and use appropriate network measures to analyze the effect of BCR and antigen concentration on BCR clustering, the stability of the formed clusters over time and the size of BCR-antigen crosslinked chains.

Oral immunogenicity of tomato-derived sDPT polypeptide containing Corynebacterium diphtheriae, Bordetella pertussis and Clostridium tetani exotoxin epitopes

DPT vaccine, designed to immunize against diphtheria, pertussis, and tetanus, has been shown to be effective in humans. Nevertheless, dissatisfaction with the whole-cell preparations is due to the reactogenicity, which has to lead to the development of new safer formulations. Previously, we described the expression in tomato of a plant-optimized synthetic gene encoding the recombinant polypeptide sDPT, containing mainly immunoprotective epitopes of the diphtheria, pertussis and tetanus exotoxins and two adjuvants. In this study, we examined whether the ingestion of tomato-derived sDPT protein induces specific antibodies in mice after three weekly doses scheme. A positive group immunized with DPT toxoids was included. Specific antibody levels were assessed in serum, gut and lung. Sera tested for IgG antibody response to pertussis, tetanus and diphtheria toxin showed responses to the foreign antigens; interestingly, the response to diphtheria epitope was similar to those observed in the positive group. We found higher IgG1 than IgG2a responses in serum. A modest IgG response was observed in the tracheopulmonary fluid. High response of IgA against tetanus toxin was evident in gut, which was statistically comparable to that obtained in the positive group. The levels of response in these groups were higher than those in mice that received wild-type tomato. These findings support the concept of using transgenic tomatoes expressing sDPT polypeptide as model for edible vaccine against diphtheria, pertussis, and tetanus.

Progress towards a needle-free hepatitis B vaccine

Hepatitis B virus (HBV) infection is a worldwide public health problem. Vaccination is the most efficient way to prevent hepatitis B. Despite the success of the currently available vaccine, there is a clear need for the development of new generation of HBV vaccines. Needle-free immunization is an attractive approach for mass immunization campaigns, since avoiding the use of needles reduces the risk of needle-borne diseases and prevents needle-stick injuries and pain, thus augmenting patient compliance and eliminating the need for trained medical personnel. Moreover, this kind of immunization was shown to induce good systemic as well as mucosal immunological responses, which is important for the creation of both a prophylactic and therapeutic vaccine. In order to produce a better, safer, more efficient and more suitable vaccine, adjuvants have been used. In this article, several adjuvants tested over the years for their potential to help create a needle-free vaccine against HBV are reviewed.

Plant-based anti-HIV-1 strategies: vaccine molecules and antiviral approaches

The introduction of highly active antiretroviral therapy has drastically changed HIV infection from an acute, very deadly, to a chronic, long-lasting, mild disease. However, this requires continuous care management, which is difficult to implement worldwide, especially in developing countries. Sky-rocketing costs of HIV-positive subjects and the limited success of preventive recommendations mean that a vaccine is urgently needed, which could be the only effective strategy for the real control of the AIDS pandemic. To be effective, vaccination will need to be accessible, affordable and directed against multiple antigens. Plant-based vaccines, which are easy to produce and administer, and require no cold chain for their heat stability are, in principle, suited to such a strategy. More recently, it has been shown that even highly immunogenic, enveloped plant-based vaccines can be produced at a competitive and more efficient rate than conventional strategies. The high variability of HIV epitopes and the need to stimulate both humoral neutralizing antibodies and cellular immunity suggest the importance of using the plant system: it offers a wide range of possible strategies, from single-epitope to multicomponent vaccines, modulators of the immune response (adjuvants) and preventive molecules (microbicides), either alone or in association with plant-derived monoclonal antibodies, besides the potential use of the latter as therapeutic agents. Furthermore, plant-based anti-HIV strategies can be administered not only parenterally but also by the more convenient and safer oral route, which is a more suitable approach for possible mass vaccination.

Development of plant-based mucosal vaccines against widespread infectious diseases

Mucosal vaccination is a perspective for the control of infectious diseases, since it is capable of inducing humoral and cell-mediated responses. In addition, the delivery of vaccines to mucosal surfaces makes immunization practice safe and acceptable, and eliminates needle-associated risks. Transgenic plants can be used as bioreactors for the production of mucosally delivered protective antigens. This technology shows great promise to simplify and decrease the cost of vaccine delivery. Herein, we review the development of mucosally administered vaccines expressed in transgenic plants. In particular, we evaluate the advantages and disadvantages of using plants for the production of mucosal vaccines against widespread infectious diseases such as HIV, hepatitis B and TB.

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.

Impaired plant growth and development caused by human immunodeficiency virus type 1 Tat

Previous attempts to express the human immunodeficiency virus 1 (HIV-1) Tat (trans-activator of transcription) protein in plants resulted in a number of physiological abnormalities, such as stunted growth and absence of seed formation, that could not be explained. In the study reported here, we expressed Tat in tomato and observed phenotypic abnormalities, including stunted growth, absence of root formation, chlorosis, and plant death, as a result of reduced cytokinin levels. These reduced levels were ascribed to a differentially expressed CKO35 in Tat-bombarded tomato. Of the two CKO isoforms that are naturally expressed in tomato, CKO43 and CKO37, only the expression of CKO37 was affected by Tat. Our analysis of the Tat confirmed that the Arg-rich and RGD motifs of Tat have functional relevance in tomato and that independent mutations at these motifs caused inhibition of the differentially expressed CKO isoform and the extracellular secretion of the Tat protein, respectively, in our Tat-bombarded tomato samples.

Preferential expression and immunogenicity of HIV-1 Tat fusion protein expressed in tomato plant

HIV-1 Tat plays a major role in viral replication and is essential for AIDS development making it an ideal vaccine target providing that both humoral and cellular immune responses are induced. Plant-based antigen production, due to its cheaper cost, appears ideal for vaccine production. In this study, we created a plant-optimized tat and mutant (Cys30Ala/Lys41Ala) tat (mtat) gene and ligated each into a pBI121 expression vector with a stop codon and a gusA gene positioned immediately downstream. The vector construct was bombarded into tomato leaf calli and allowed to develop. We thus generated recombinant tomato plants preferentially expressing a Tat-GUS fusion protein over a Tat-only protein. In addition, plants bombarded with either tat or mtat genes showed no phenotypic difference and produced 2-4 microg Tat-GUS fusion protein per milligram soluble plant protein. Furthermore, tomato extracts intradermally inoculated into mice were found to induce a humoral and, most importantly, cellular immunity.

The production of biopharmaceuticals in plant systems

Biopharmaceuticals present the fastest growing segment in the pharmaceutical industry, with an ever widening scope of applications. Whole plants as well as contained plant cell culture systems are being explored for their potential as cheap, safe, and scalable production hosts. The first plant-derived biopharmaceuticals have now reached the clinic. Many biopharmaceuticals are glycoproteins; as the Golgi N-glycosylation machinery of plants differs from the mammalian machinery, the N-glycoforms introduced on plant-produced proteins need to be taken into consideration. Potent systems have been developed to change the plant N-glycoforms to a desired or even superior form compared to the native mammalian N-glycoforms. This review describes the current status of biopharmaceutical production in plants for industrial applications. The recent advances and tools which have been utilized to generate glycoengineered plants are also summarized and compared with the relevant mammalian systems whenever applicable.

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.

High-level expression of the HIV-1 Pr55gag polyprotein in transgenic tobacco chloroplasts

Plants have been recognized as a promising production platform for recombinant pharmaceutical proteins. The human immunodeficiency virus Gag (Pr55(gag)) structural polyprotein precursor is a prime candidate for developing a HIV-1 vaccine, but, so far, has been expressed at very low level in plants. The aim of this study was to investigate factors potentially involved in Pr55(gag) expression and increase protein yield in plant cells. In transient expression experiments in various subcellular compartments, the native Pr55(gag) sequence could be expressed only in the chloroplast. Experiments with truncated subunits suggested a negative role of the 5'-end on the expression of the full gene in the cytosol. Stable transgenic plants were produced in tobacco by Agrobacterium-mediated nuclear transformation with protein targeted to plastids, and biolistic-mediated plastid transformation. Compared to the nuclear genome, the integration and expression of the gag transgene in the plastome resulted in significantly higher protein accumulation levels (up to 7-8% TSP, equivalent to 312-363 mg/kg FW). In transplastomic plants, a 25-fold higher protein accumulation was obtained by translationally fusing the Pr55(gag) polyprotein to the N-terminus of the plastid photosynthetic RbcL protein. In chloroplasts, the Pr55(gag) polyprotein was processed in a pattern similar to that achieved by the viral protease, the processing being more extended in older leaves of mature plants. The Gag proteins produced in transgenic plastids were able to assemble into particles resembling VLPs produced in baculovirus/insect cells and E. coli systems. These results indicate that plastid transformation is a promising tool for HIV antigen manufacturing in plant cells.

Immunogenicity and tolerance following HIV-1/HBV plant-based oral vaccine administration

Transgenic tobacco plants expressing a HIV-1 polyepitope associated with hepatitis B (HBV) virus-like particles (VLPs) were previously described. It is demonstrated here that oral administration of these transgenic plants to humanized HSB mice to boost DNA-priming can elicit anti-HIV-1 specific CD8+ T cell activation detectable in mesenteric lymph nodes. Nevertheless, a significant regulatory T cell activation was induced in vivo by the vaccination protocols. The balance between tolerance and immunogenicity remains the main concern in the proof of concept of plant-based vaccine.

pBINPLUS/ARS: an improved plant transformation vector based on pBINPLUS

Binary plant transformation vectors are widely used for introduction of transgenes into plants via Agrobacterium tumefaciens-mediated transformation. We report the construction of a binary plant vector pBINPLUS/ARS based on the pBINPLUS vector. Improvements introduced into pBINPLUS/ARS include the use of nonproprietary (ubiquitin-3 gene of Solanum tuberosum) promoter and terminator sequences for transcription of the NptII selectable marker and introduction of rare 8-bp restriction enzyme sites flanking both the NptII coding sequence (PmeI) and the entire selectable marker gene (FseI). This vector offers all of the advantages of its predecessor pBINPLUS and its helper plasmid pUCAP, which use the proprietary nopaline synthase promoter and terminator, while allowing for facile modification of selectable marker sequences in complex binary vector constructs. pBINPLUS/ARS has been used to introduce transgenes into potato and other crop species and is available to all researchers in academic, government, and industrial laboratories for proof-of-principle and commercial applications.

Open-label trial of therapeutic immunization with oral V-5 Immunitor (V5) vaccine in patients with chronic hepatitis C

We evaluated whether V-5 Immunitor (V5)--tableted therapeutic bivalent vaccine comprising heat-inactivated HCV antigens from pooled blood of HBV- and HCV-infected donors - may produce clinical benefit through induction of oral tolerance and reduction of immune-mediated liver injury. Once daily dose of V5 was administered per os to 10 patients with chronic hepatitis C in an open-label study that lasted 1 month. Every patient who entered the study had elevated liver enzyme levels, which at the end of study have decreased in 100% of analyzed patients. The reduction was highly significant, from 157.7+/-73.4 to 49.9+/-43.8 U/L (P=0.0013) and 147.0+/-79.2 to 58.7+/-56.6 U/L (P=0.0132), for ALT and AST, respectively. The AST/ALT ratio has improved from 0.93 to 1.18 (P=0.00058) indicating the reversion of progression to cirrhosis. None of intent-to-treat patients who were anti-HCV antibody positive at study entry, became negative after 1 month on V5 (P=0.998). All patients, except one, reported complete recuperation from hepatitis C-associated clinical symptoms present at baseline (P=0.0016) with Mantel Haenszel's odds ratio 9.4 (P=0.0021) at 95% confidence interval: 2.7<OR<476.3. No adverse events were observed at any time. The favorable biochemical and clinical responses have been observed in a small number of individuals for a limited time period. Larger scale and longer studies are needed to confirm our preliminary observations suggesting that V5 is safe and effective means for immunotherapy of chronic hepatitis C.

Expression of toxin co-regulated pilus subunit A (TCPA) of Vibrio cholerae and its immunogenic epitopes fused to cholera toxin B subunit in transgenic tomato (Solanum lycopersicum)

For protection against cholera, it is important to develop efficient vaccine capable of inducing anti-toxin as well as anti-colonizing immunity against Vibrio cholerae infections. Earlier, expression of cholera toxin B subunit (CTB) in tomato was reported by us. In the present investigation, toxin co-regulated pilus subunit A (TCPA), earlier reported to be an antigen capable of providing anti-colonization immunity, has been expressed in tomato. Further, to generate more potent combinatorial antigens, nucleotides encoding P4 or P6 epitope of TCPA were fused to cholera toxin B subunit gene (ctxB) and expressed in tomato. Presence of transgenes in the tomato genome was confirmed by PCR and expression of genes was confirmed at transcript and protein level. TCPA, chimeric CTB-P4 and CTB-P6 proteins were also expressed in E. coli. TCPA protein expressed in E. coli was purified to generate anti-TCPA antibodies in rabbit. Immunoblot and G(M1)-ELISA verified the synthesis and assembly of pentameric chimeric proteins in fruit tissue of transgenic tomato plants. The chimeric protein CTB-P4 and CTB-P6 accumulated up to 0.17 and 0.096% of total soluble protein (TSP), respectively, in tomato fruits. Whereas expression of TCPA, CTB-P4 and CTB-P6 in E. coli can be utilized for development of conventional vaccine, expression of these antigens which can provide both anti-toxin as well as anti-colonization immunity, has been demonstrated in plants, in a form which is potentially capable of inducing immune response against cholera infection.

Production of recombinant HIV-1/HBV virus-like particles in Nicotiana tabacum and Arabidopsis thaliana plants for a bivalent plant-based vaccine

Human immunodeficiency virus (HIV-1) and hepatitis B virus (HBV) spread via similar transmission pathways, and infection by HBV occurs in up to 32% of HIV-1 cases. Here, we describe the successful expression of novel recombinant HIV-1/HBV virus-like particles (VLPs) in Nicotiana tabacum and Arabidopsis thaliana. The production levels and quality of the recombinant VLPs were comparable in the two plants, showing that parameters intrinsic to the recombinant proteins determined their assembly into VLPs. These heterologous VLPs can be used in a bivalent anti-HIV-1/-HBV vaccine, administrated via ingestion of transgenic plants.