A chloroplast-derived C4V3 polypeptide from the human immunodeficiency virus (HIV) is orally immunogenic in mice

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.

Strategies for improving antimicrobial peptide production

Antimicrobial peptides (AMPs) found in a wide range of animal, insect, and plant species are host defense peptides forming an integral part of their innate immunity. Although the exact mode of action of some AMPs is yet to be deciphered, many exhibit membrane lytic activity or interact with intracellular targets. The ever-growing threat of antibiotic resistance has brought attention to research on AMPs to enhance their clinical use as a therapeutic alternative. AMPs have several advantages over antibiotics such as broad range of antimicrobial activities including anti-fungal, anti-viral and anti-bacterial, and have not reported to contribute to resistance development. Despite the numerous studies to develop efficient production methods for AMPs, limitations including low yield, degradation, and loss of activity persists in many recombinant approaches. In this review, we outline available approaches for AMP production and various expression systems used to achieve higher yield and quality. In addition, recent advances in recombinant strategies, suitable fusion protein partners, and other molecular engineering strategies for improved AMP production are surveyed.

Plant synthetic biology for producing potent phyto-antimicrobials to combat antimicrobial resistance

Inappropriate and injudicious use of antimicrobial drugs in human health, hygiene, agriculture, animal husbandry and food industries has contributed significantly to rapid emergence and persistence of antimicrobial resistance (AMR), one of the serious global public health threats. The crisis of AMR versus slower discovery of newer antibiotics put forth a daunting task to control these drug-resistant superbugs. Several phyto-antimicrobials have been identified in recent years with direct-killing (bactericidal) and/or drug-resistance reversal (re-sensitization of AMR phenotypes) potencies. Phyto-antimicrobials may hold the key in combating AMR owing to their abilities to target major microbial drug-resistance determinants including cell membrane, drug-efflux pumps, cell communication and biofilms. However, limited distribution, low intracellular concentrations, eco-geographical variations, beside other considerations like dynamic environments, climate change and over-exploitation of plant-resources are major blockades in full potential exploration phyto-antimicrobials. Synthetic biology (SynBio) strategies integrating metabolic engineering, RNA-interference, genome editing/engineering and/or systems biology approaches using plant chassis (as engineerable platforms) offer prospective tools for production of phyto-antimicrobials. With expanding SynBio toolkit, successful attempts towards introduction of entire gene cluster, reconstituting the metabolic pathway or transferring an entire metabolic (or synthetic) pathway into heterologous plant systems highlight the potential of this field. Through this perspective review, we are presenting herein the current situation and options for addressing AMR, emphasizing on the significance of phyto-antimicrobials in this apparently post-antibiotic era, and effective use of plant chassis for phyto-antimicrobial production at industrial scales along with major SynBio tools and useful databases. Current knowledge, recent success stories, associated challenges and prospects of translational success are also discussed.

Strategies for recombinant production of antimicrobial peptides with pharmacological potential

INTRODUCTION

The need to develop new drugs for the control of pathogenic microorganisms has redoubled efforts to prospect for antimicrobial peptides (AMPs) from natural sources and to characterize its structure and function. These molecules present a broad spectrum of action against different microorganisms and frequently present promiscuous action, with anticancer and immunomodulatory activities. Furthermore, AMPs can be used as biopharmaceuticals in the treatment of hospital-acquired infections and other serious diseases with relevant social and economic impacts.Areas covered: The low yield and the therefore difficult extraction and purification process in AMPs are problems that limit their industrial application and scientific research. Thus, optimized heterologous expression systems were developed to significantly boost AMP yields, allow high efficiency in purification and structural optimization for the increase of therapeutic activity.Expert opinion: This review provides an update on recent developments in the recombinant production of ribosomal and non-ribosomal synthesis of AMPs and on strategies to increase the expression of genes encoding AMPs at the transcriptional and translational levels and regulation of the post-translational modifications. Moreover, there are detailed reports of AMPs that have already reached marketable status or are in the pipeline under advanced stages of preclinical testing.

Emergence of Novel Coronavirus 2019-nCoV: Need for Rapid Vaccine and Biologics Development

Novel Coronavirus (2019-nCoV) is an emerging pathogen that was first identified in Wuhan, China in late December 2019. This virus is responsible for the ongoing outbreak that causes severe respiratory illness and pneumonia-like infection in humans. Due to the increasing number of cases in China and outside China, the WHO declared coronavirus as a global health emergency. Nearly 35,000 cases were reported and at least 24 other countries or territories have reported coronavirus cases as early on as February. Inter-human transmission was reported in a few countries, including the United States. Neither an effective anti-viral nor a vaccine is currently available to treat this infection. As the virus is a newly emerging pathogen, many questions remain unanswered regarding the virus's reservoirs, pathogenesis, transmissibility, and much more is unknown. The collaborative efforts of researchers are needed to fill the knowledge gaps about this new virus, to develop the proper diagnostic tools, and effective treatment to combat this infection. Recent advancements in plant biotechnology proved that plants have the ability to produce vaccines or biopharmaceuticals rapidly in a short time. In this review, the outbreak of 2019-nCoV in China, the need for rapid vaccine development, and the potential of a plant system for biopharmaceutical development are discussed.

Plant-made HIV vaccines and potential candidates

Millions of people around the world suffer from heavy social and health burdens related to HIV/AIDS and its associated opportunistic infections. To reduce these burdens, preventive and therapeutic vaccines are required. Effective HIV vaccines have been under investigation for several decades using different animal models. Potential plant-made HIV vaccine candidates have also gained attention in the past few years. In addition to this, broadly neutralizing antibodies produced in plants which can target conserved viral epitopes and neutralize mutating HIV strains have been identified. Numerous epitopes of envelope glycoproteins and capsid proteins of HIV-1 are a part of HIV therapy. Here, we discuss some recent findings aiming to produce anti-HIV-1 recombinant proteins in engineered plants for AIDS prophylactics and therapeutic treatments.

Field-grown tobacco plants maintain robust growth while accumulating large quantities of a bacterial cellulase in chloroplasts

High accumulation of heterologous proteins expressed from the plastid genome has sometimes been reported to result in compromised plant phenotypes. Comparisons of transplastomic plants to wild-type (WT) are typically made in environmentally controlled chambers with relatively low light; little is known about the performance of such plants under field conditions. Here, we report on two plastid-engineered tobacco lines expressing the bacterial cellulase Cel6A. Field-grown plants producing Cel6A at ~20% of total soluble protein exhibit no loss in biomass or Rubisco content and only minor reductions in photosynthesis compared to WT. These experiments demonstrate that, when grown in the field, tobacco possesses sufficient metabolic flexibility to accommodate high levels of recombinant protein by increasing total protein synthesis and accumulation and/or by reallocating unneeded endogenous proteins. Based on current tobacco cultivation practices and readily achievable recombinant protein yields, we estimate that specific proteins could be obtained from field-grown transgenic tobacco plants at costs three orders of magnitude less than current cell culture methods.

Green Biotechnology: A Brief Update on Plastid Genome Engineering

Plant genetic engineering has become an inevitable tool in the molecular breeding of crops. Significant progress has been made in the generation of novel plastid transformation vectors and optimized transformation protocols. There are several advantages of plastid genome engineering over conventional nuclear transformation. Some of the advantages include multigene engineering by expression of biosynthetic pathway genes as operons, extremely high-level expression of protein accumulation, lack of transgene silencing, etc. Transgene containment owing to maternal inheritance is another important advantage of plastid genome engineering. Chloroplast genome modification usually results in alteration of several thousand plastid genome copies in a cell. Several therapeutic proteins, edible vaccines, antimicrobial peptides, and industrially important enzymes have been successfully expressed in chloroplasts so far. Here, we critically recapitulate the latest developments in plastid genome engineering. Latest advancements in plastid genome sequencing are briefed. In addition, advancement of extending the toolbox for plastid engineering for selected applications in the area of molecular farming and production of industrially important enzyme is briefed.

Expression of an immunogenic LTB-based chimeric protein targeting Zaire ebolavirus epitopes from GP1 in plant cells

KEY MESSAGE

An antigenic protein targeting two epitopes from the Zaire ebolavirus GP1 protein was expressed in plant cells rendering an antigen capable of inducing humoral responses in mouse when administered subcutaneously or orally. The 2014 Ebola outbreak made clear that new treatments and prophylactic strategies to fight this disease are needed. Since vaccination is an intervention that could achieve the control of this epidemic disease, exploring the production of new low-cost vaccines is a key path to consider; especially in developing countries. In this context, plants are attractive organisms for the synthesis and delivery of subunit vaccines. This study aimed at producing a chimeric protein named LTB-EBOV, based on the B subunit of the Escherichia coli heat-labile enterotoxin as an immunogenic carrier and two epitopes from the Zaire ebolavirus GP1 protein recognized by neutralizing antibodies. The LTB-EBOV protein was expressed in plant tissues at levels up to 14.7 µg/g fresh leaf tissue and proven to be immunogenic in BALB/c mice when administered by either subcutaneous or oral routes. Importantly, IgA and IgG responses were induced following the oral immunization. The potential use of the plant-made LTB-EBOV protein against EBOV is discussed.

Vaccination via Chloroplast Genetics: Affordable Protein Drugs for the Prevention and Treatment of Inherited or Infectious Human Diseases

Plastid-made biopharmaceuticals treat major metabolic or genetic disorders, including Alzheimer's, diabetes, hypertension, hemophilia, and retinopathy. Booster vaccines made in chloroplasts prevent global infectious diseases, such as tuberculosis, malaria, cholera, and polio, and biological threats, such as anthrax and plague. Recent advances in this field include commercial-scale production of human therapeutic proteins in FDA-approved cGMP facilities, development of tags to deliver protein drugs to targeted human cells or tissues, methods to deliver precise doses, and long-term stability of protein drugs at ambient temperature, maintaining their efficacy. Codon optimization utilizing valuable information from sequenced chloroplast genomes enhanced expression of eukaryotic human or viral genes in chloroplasts and offered unique insights into translation in chloroplasts. Support from major biopharmaceutical companies, development of hydroponic production systems, and evaluation by regulatory agencies, including the CDC, FDA, and USDA, augur well for advancing this novel concept to the clinic and revolutionizing affordable healthcare.

Challenges and perspectives in commercializing plastid transformation technology

Plastid transformation has emerged as an alternative platform to generate transgenic plants. Attractive features of this technology include specific integration of transgenes-either individually or as operons-into the plastid genome through homologous recombination, the potential for high-level protein expression, and transgene containment because of the maternal inheritance of plastids. Several issues associated with nuclear transformation such as gene silencing, variable gene expression due to the Mendelian laws of inheritance, and epigenetic regulation have not been observed in the plastid genome. Plastid transformation has been successfully used for the production of therapeutics, vaccines, antigens, and commercial enzymes, and for engineering various agronomic traits including resistance to biotic and abiotic stresses. However, these demonstrations have usually focused on model systems such as tobacco, and the technology per se has not yet reached the market. Technical factors limiting this technology include the lack of efficient protocols for the transformation of cereals, poor transgene expression in non-green plastids, a limited number of selection markers, and the lengthy procedures required to recover fully segregated plants. This article discusses the technology of transforming the plastid genome, the positive and negative features compared with nuclear transformation, and the current challenges that need to be addressed for successful commercialization.

Chloroplasts: state of research and practical applications of plastome sequencing

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

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

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

Plants as Factories for Human Pharmaceuticals: Applications and Challenges

Plant molecular farming (PMF), defined as the practice of using plants to produce human therapeutic proteins, has received worldwide interest. PMF has grown and advanced considerably over the past two decades. A number of therapeutic proteins have been produced in plants, some of which have been through pre-clinical or clinical trials and are close to commercialization. Plants have the potential to mass-produce pharmaceutical products with less cost than traditional methods. Tobacco-derived antibodies have been tested and used to combat the Ebola outbreak in Africa. Genetically engineered immunoadhesin (DPP4-Fc) produced in green plants has been shown to be able to bind to MERS-CoV (Middle East Respiratory Syndrome), preventing the virus from infecting lung cells. Biosafety concerns (such as pollen contamination and immunogenicity of plant-specific glycans) and costly downstream extraction and purification requirements, however, have hampered PMF production from moving from the laboratory to industrial application. In this review, the challenges and opportunities of PMF are discussed. Topics addressed include; transformation and expression systems, plant bioreactors, safety concerns, and various opportunities to produce topical applications and health supplements.

Plastids: The Green Frontiers for Vaccine Production

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

Plant-made oral vaccines against human infectious diseases-Are we there yet?

Although the plant-made vaccine field started three decades ago with the promise of developing low-cost vaccines to prevent infectious disease outbreaks and epidemics around the globe, this goal has not yet been achieved. Plants offer several major advantages in vaccine generation, including low-cost production by eliminating expensive fermentation and purification systems, sterile delivery and cold storage/transportation. Most importantly, oral vaccination using plant-made antigens confers both mucosal (IgA) and systemic (IgG) immunity. Studies in the past 5 years have made significant progress in expressing vaccine antigens in edible leaves (especially lettuce), processing leaves or seeds through lyophilization and achieving antigen stability and efficacy after prolonged storage at ambient temperatures. Bioencapsulation of antigens in plant cells protects them from the digestive system; the fusion of antigens to transmucosal carriers enhances efficiency of their delivery to the immune system and facilitates successful development of plant vaccines as oral boosters. However, the lack of oral priming approaches diminishes these advantages because purified antigens, cold storage/transportation and limited shelf life are still major challenges for priming with adjuvants and for antigen delivery by injection. Yet another challenge is the risk of inducing tolerance without priming the host immune system. Therefore, mechanistic aspects of these two opposing processes (antibody production or suppression) are discussed in this review. In addition, we summarize recent progress made in oral delivery of vaccine antigens expressed in plant cells via the chloroplast or nuclear genomes and potential challenges in achieving immunity against infectious diseases using cold-chain-free vaccine delivery approaches.

A Plant-Derived Multi-HIV Antigen Induces Broad Immune Responses in Orally Immunized Mice

Multi-HIV, a multiepitopic protein derived from both gp120 and gp41 envelope proteins of the human immunodeficiency virus (HIV), has been proposed as a vaccine prototype capable of inducing broad immune responses, as it carries various B and T cell epitopes from several HIV strains. In this study, the immunogenic properties of a Multi-HIV expressed in tobacco chloroplasts are evaluated in test mice. BALB/c mice orally immunized with tobacco-derived Multi-HIV have elicited antibody responses, including both the V3 loop of gp120 and the ELDKWA epitope of gp41. Based on splenocyte proliferation assays, stimulation with epitopes of the C4, V3 domain of gp120, and the ELDKWA domain of gp41 elicits positive cellular responses. Furthermore, specific interferon gamma production is observed in both CD4+ and CD8+ T cells stimulated with HIV peptides. These results demonstrate that plant-derived Multi-HIV induces T helper-specific responses. Altogether, these findings illustrate the immunogenic potential of plant-derived Multi-HIV in an oral immunization scheme. The potential of this low-cost immunization approach and its implications on HIV/AIDS vaccine development are discussed.

An Env-derived multi-epitope HIV chimeric protein produced in the moss Physcomitrella patens is immunogenic in mice

The first report on the recombinant production of a candidate vaccine in the moss system. The need for economical and efficient platforms for vaccine production demands the exploration of emerging host organisms. In this study, the production of an antigenic protein is reported employing the moss Physcomitrella patens as an expression host. A multi-epitope protein from the Human Immunodeficiency Virus (HIV) based on epitopes from gp120 and gp41 was designed as a candidate subunit vaccine and named poly-HIV. Transgenic moss plants were generated carrying the corresponding poly-HIV transgene under a novel moss promoter and subsequently seven positive lines were confirmed by PCR. The poly-HIV protein accumulated up to 3.7 µg g(-1) fresh weight in protonema cultures. Antigenic and immunogenic properties of the moss-produced recombinant poly-HIV are evidenced by Western blots and by mice immunization assays. The elicitation of specific antibodies in mice was observed, reflecting the immunogenic potential of this moss-derived HIV antigen. This is the first report on the production of a potential vaccine in the moss system and opens the avenue for glycoengineering approaches for the production of HIV human-like glycosylated antigens as well as other vaccine prototypes under GMP conditions in moss bioreactors.

Recent developments in therapeutic protein expression technologies in plants

Infectious diseases and cancers are some of the commonest causes of deaths throughout the world. The previous two decades have witnessed a combined endeavor across various biological sciences to address this issue in novel ways. The advent of recombinant DNA technologies has provided the tools for producing recombinant proteins that can be used as therapeutic agents. A number of expression systems have been developed for the production of pharmaceutical products. Recently, advances have been made using plants as bioreactors to produce therapeutic proteins directed against infectious diseases and cancers. This review highlights the recent progress in therapeutic protein expression in plants (stable and transient), the factors affecting heterologous protein expression, vector systems and recent developments in existing technologies and steps towards the industrial production of plant-made vaccines, antibodies, and biopharmaceuticals.

Transgenic plants: a 5-year update on oral antipathogen vaccine development

The progressive interest in transgenic plants as advantageous platforms for the production and oral delivery of vaccines has led to extensive research and improvements in this technology over recent years. In this paper, the authors examine the most significant advances in this area, including novel approaches for higher yields and better containment, and the continued evaluation of new vaccine prototypes against several infectious diseases. The use of plants to deliver vaccine candidates against viruses, bacteria, and eukaryotic parasites within the last 5 years is discussed, focusing on innovative expression strategies and the immunogenic potential of new vaccines. A brief section on the state of the art in mucosal immunity is also included.

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.

Plant-derived pharmaceuticals for the developing world

Plant-produced vaccines and therapeutic agents offer enormous potential for providing relief to developing countries by reducing the incidence of infant mortality caused by infectious diseases. Vaccines derived from plants have been demonstrated to effectively elicit an immune response. Biopharmaceuticals produced in plants are inexpensive to produce, require fewer expensive purification steps, and can be stored at ambient temperatures for prolonged periods of time. As a result, plant-produced biopharmaceuticals have the potential to be more accessible to the rural poor. This review describes current progress with respect to plant-produced biopharmaceuticals, with a particular emphasis on those that target developing countries. Specific emphasis is given to recent research on the production of plant-produced vaccines toward human immunodeficiency virus, malaria, tuberculosis, hepatitis B virus, Ebola virus, human papillomavirus, rabies virus and common diarrheal diseases. Production platforms used to express vaccines in plants, including nuclear and chloroplast transformation, and the use of viral expression vectors, are described in this review. The review concludes by outlining the next steps for plant-produced vaccines to achieve their goal of providing safe, efficacious and inexpensive vaccines to the developing world.

Low cost tuberculosis vaccine antigens in capsules: expression in chloroplasts, bio-encapsulation, stability and functional evaluation in vitro

Tuberculosis (TB) caused by Mycobacterium tuberculosis is one of the leading fatal infectious diseases. The development of TB vaccines has been recognized as a major public health priority by the World Health Organization. In this study, three candidate antigens, ESAT-6 (6 kDa early secretory antigenic target) and Mtb72F (a fusion polyprotein from two TB antigens, Mtb32 and Mtb39) fused with cholera toxin B-subunit (CTB) and LipY (a cell wall protein) were expressed in tobacco and/or lettuce chloroplasts to facilitate bioencapsulation/oral delivery. Site-specific transgene integration into the chloroplast genome was confirmed by Southern blot analysis. In transplastomic leaves, CTB fusion proteins existed in soluble monomeric or multimeric forms of expected sizes and their expression levels varied depending upon the developmental stage and time of leaf harvest, with the highest-level of accumulation in mature leaves harvested at 6PM. The CTB-ESAT6 and CTB-Mtb72F expression levels reached up to 7.5% and 1.2% of total soluble protein respectively in mature tobacco leaves. Transplastomic CTB-ESAT6 lettuce plants accumulated up to 0.75% of total leaf protein. Western blot analysis of lyophilized lettuce leaves stored at room temperature for up to six months showed that the CTB-ESAT6 fusion protein was stable and preserved proper folding, disulfide bonds and assembly into pentamers for prolonged periods. Also, antigen concentration per gram of leaf tissue was increased 22 fold after lyophilization. Hemolysis assay with purified CTB-ESAT6 protein showed partial hemolysis of red blood cells and confirmed functionality of the ESAT-6 antigen. GM1-binding assay demonstrated that the CTB-ESAT6 fusion protein formed pentamers to bind with the GM1-ganglioside receptor. The expression of functional Mycobacterium tuberculosis antigens in transplastomic plants should facilitate development of a cost-effective and orally deliverable TB booster vaccine with potential for long-term storage at room temperature. To our knowledge, this is the first report of expression of TB vaccine antigens in chloroplasts.

Expression systems for heterologous production of antimicrobial peptides

Antimicrobial peptides (AMPs) consist of molecules that act on the defense systems of numerous organisms toward multiple pathogens such as bacteria, fungi, parasites and viruses. These compounds have become extremely significant due to the increasing resistance of microorganisms to common antibiotics. However, the low quantity of peptides obtained from direct purification is, to date, still a remarkable bottleneck for scientific and industrial research development. Therefore, this review describes the main heterologous systems currently used for AMP production, including bacteria, fungi and plants, and also the related strategies for reaching greater functional peptide production. The main difficulties of each system are also described in order to provide some directions for AMP production. In summary, data revised here indicate that large-scale production of AMPs can be obtained using biotechnological tools, and the products may be applied in the pharmaceutical industry as well as in agribusiness.