Facilitating viral vector movement enhances heterologous protein production in an established plant system.

Molecular farming technology using transiently transformed Nicotiana plants offers an economical approach to the pharmaceutical industry to produce an array of protein targets including vaccine antigens and therapeutics. It can serve as a desirable alternative approach for those proteins that are challenging or too costly to produce in large quantities using other heterologous protein expression systems. However, since cost metrics are such a critical factor in selecting a production host, any system-wide modifications that can increase recombinant protein yields are key to further improving the platform and making it applicable for a wider range of target molecules. Here, we report on the development of a new approach to improve target accumulation in an established plant-based expression system that utilizes viral-based vectors to mediate transient expression in Nicotiana benthamiana. We show that by engineering the host plant to support viral vectors to spread more effectively between host cells through plasmodesmata, protein target accumulation can be increased by up to approximately 60%.

Engineering of a plant-produced virus-like particle to improve the display of the Plasmodium falciparum Pfs25 antigen and transmission-blocking activity of the vaccine candidate.

Malaria kills around 409,000 people a year, mostly children under the age of five. Malaria transmission-blocking vaccines work to reduce malaria prevalence in a community and have the potential to be part of a multifaceted approach required to eliminate the parasites causing the disease. Pfs25 is a leading malaria transmission-blocking antigen and has been successfully produced in a plant expression system as both a subunit vaccine and as a virus-like particle. This study demonstrates an improved version of the virus-like particle antigen display molecule by eliminating known protease sites from the prior A85 variant. This re-engineered molecule, termed B29, displays three times the number of Pfs25 antigens per virus-like particle compared to the original Pfs25 virus-like particle. An improved purification scheme was also developed, resulting in a substantially higher yield and improved purity. The molecule was evaluated in a mouse model and found to induce improved transmission-blocking activity at lower doses and longer durations than the original molecule.

An ELISA-based antigenicity test of rabies recombinant glycoprotein cannot predict its protective potency in vivo.

The potency of human and veterinary rabies vaccines is measured based on the National Institute of Health (NIH) potency test that is laborious, time-consuming, variable, and requires sacrifice of large numbers of mice. ELISA-based methods quantifying rabies glycoprotein (rGP) are being developed as potential alternatives to the NIH potency test for release of rabies vaccines. The aim of the current study was focused on the evaluation of in vitro- and in vivo-based assays in order to assess their concurrence for adequate and reliable assessment of immunogenicity and protective potency of a plant-derived recombinant rGP. The recombinant rGP of strain ERA.KK was engineered, expressed and purified from Nicotiana benthamiana plants. The recombinant rGP excluded the transmembrane and intracytoplasmic domains. It was purified by chromatography (≥90%) from the plant biomass, characterized, and mainly presented as high molecular weight forms, most likely soluble aggregates, of the rGP ectodomain. It was well-recognized and quantified by an ELISA, which utilizes two mouse monoclonal antibodies, D1-25 and 1112-1, and which should only recognize the native trimeric form of the rGP. However, in mice, the recombinant rGP did not induce the production of anti-rabies virus neutralizing antibodies and did not confer protection after intracerebral viral challenge. Similar immunogenicity was observed in guinea pigs and rabbits. Our results demonstrate that use of the ELISA method described here is not predictive of performance in vivo. These data highlight the critical need to develop in vitro potency assays that reliably define the antigen content that can induce a protective response.

Safety and immunogenicity of a plant-derived recombinant protective antigen (rPA)-based vaccine against Bacillus anthracis: A Phase 1 dose-escalation study in healthy adults.

BACKGROUND: The potential use of Bacillus anthracis as a bioterrorism weapon requires a safe and effective vaccine that can be immediately distributed for mass vaccination. Protective antigen (PA), a principal component of virulence factors edema toxin and lethal toxin of B. anthracis, has been the topic of extensive research. Previously, full-length PA (PA83) was manufactured using a transient plant-based expression system. Immunization with this PA83 antigen formulated with Alhydrogel® adjuvant elicited strong neutralizing immune responses in mice and rabbits and protected 100% of rabbits from a lethal aerosolized B. anthracis challenge. This Phase 1 study evaluates this vaccine's safety and immunogenicity in healthy human volunteers. METHODS: This first-in-human, single-blind, Phase 1 study was performed at a single center to investigate the safety, reactogenicity, and immunogenicity of the plant-derived PA83-FhCMB vaccine at four escalating dose levels (12.5, 25, 50 or 100 µg) with Alhydrogel® in healthy adults 18-49 years of age (inclusive). Recipients received three doses of vaccine intramuscularly at 28-day intervals. Safety was evaluated on days 3, 7, and 14 following vaccination. Immunogenicity was assessed using an enzyme-linked immunosorbent assay (ELISA) and a toxin neutralizing antibody (TNA) assay on days 0, 14, 28, 56, 84, and 180. RESULTS: All four-dose ranges were safe and immunogenic, with no related serious adverse events observed. Peak ELISA Geometric Mean Concentration (GMC) and TNA ED50 Geometric Mean Titer (GMT) were noted at Day 84, 1 month after the final dose, with the most robust response detected in the highest dose group. Antibody responses decreased by Day 180 across all dose groups. Long-term immunogenicity data beyond six months was not collected. CONCLUSIONS: This is the first study demonstrating a plant-derived subunit anthrax vaccine's safety and immunogenicity in healthy adults. The results support further clinical investigation of the PA83-FhCMB vaccine. ClinicalTrials.gov identifier. NCT02239172.

Debulking SARS-CoV-2 in saliva using angiotensin converting enzyme 2 in chewing gum to decrease oral virus transmission and infection.

To advance a novel concept of debulking virus in the oral cavity, the primary site of viral replication, virus-trapping proteins CTB-ACE2 were expressed in chloroplasts and clinical-grade plant material was developed to meet FDA requirements. Chewing gum (2 g) containing plant cells expressed CTB-ACE2 up to 17.2 mg ACE2/g dry weight (11.7% leaf protein), have physical characteristics and taste/flavor like conventional gums, and no protein was lost during gum compression. CTB-ACE2 gum efficiently (>95%) inhibited entry of lentivirus spike or VSV-spike pseudovirus into Vero/CHO cells when quantified by luciferase or red fluorescence. Incubation of CTB-ACE2 microparticles reduced SARS-CoV-2 virus count in COVID-19 swab/saliva samples by >95% when evaluated by microbubbles (femtomolar concentration) or qPCR, demonstrating both virus trapping and blocking of cellular entry. COVID-19 saliva samples showed low or undetectable ACE2 activity when compared with healthy individuals (2,582 versus 50,126 ΔRFU; 27 versus 225 enzyme units), confirming greater susceptibility of infected patients for viral entry. CTB-ACE2 activity was completely inhibited by pre-incubation with SARS-CoV-2 receptor-binding domain, offering an explanation for reduced saliva ACE2 activity among COVID-19 patients. Chewing gum with virus-trapping proteins offers a general affordable strategy to protect patients from most oral virus re-infections through debulking or minimizing transmission to others.

Efficacy of a novel lantibiotic, CMB001, against MRSA.

OBJECTIVES: To evaluate the efficacy of a novel lantibiotic, CMB001, against MRSA biofilms in vitro and in an in vivo experimental model of bacterial infection. METHODS: Antibacterial activity of CMB001 was measured in vitro after its exposure to whole blood or to platelet-poor plasma. In vitro efficacy of CMB001 against a Staphylococcus aureus biofilm was studied using scanning electron microscopy. The maximum tolerable dose in mice was determined and a preliminary pharmacokinetic analysis for CMB001 was performed in mice. In vivo efficacy was evaluated in a neutropenic mouse thigh model of infection. RESULTS: CMB001 maintained its antibacterial activity in the presence of blood or plasma for up to 24 h at 37°C. CMB001 efficiently killed S. aureus within the biofilm by causing significant damage to the bacterial cell wall. The maximum tolerable dose in mice was established to be 10 mg/kg and could be increased to 30 mg/kg in mice pretreated with antihistamines. In neutropenic mice infected with MRSA, treatment with CMB001 reduced the bacterial burden with an efficacy equivalent to that of vancomycin. CONCLUSIONS: CMB001 offers potential as an alternative treatment option to combat MRSA. It will be of interest to evaluate the in vivo efficacy of CMB001 against infections caused by other pathogens, including Clostridioides difficile and Acinetobacter baumannii, and to expand its pharmacokinetic/pharmacodynamic parameters and safety profile.

Isolation, Characterization and Structure Elucidation of a Novel Lantibiotic From Paenibacillus sp.

We have isolated and characterized a novel antibacterial peptide, CMB001, following an extensive screening effort of bacterial species isolated from diverse environmental sources. The bacterium that produces CMB001 is characterized as a Gram (+) bacillus sharing approximately 98.9% 16S rRNA sequence homology with its closest match, Paenibacillus kyungheensis. The molecule has been purified to homogeneity from its cell-free supernatant by a three-step preparative chromatography process. Based on its primary structure, CMB001 shares 81% identity with subtilin and 62% with nisin. CMB001 is active mainly against Gram-positive bacteria and Mycobacteriaceae but it is also active against certain Gram-negative bacteria, including multi-drug resistant Acinetobacter baumannii. It retains full antibacterial activity at neutral pH and displays a low propensity to select for resistance among targeted bacteria. Based on NMR and mass spectrometry, CMB001 forms a unique 3D-structure comprising of a compact backbone with one α-helix and two pseudo-α-helical regions. Screening the structure against the Protein Data Bank (PDB) revealed a partial match with nisin-lipid II (1WCO), but none of the lantibiotics with known structures showed significant structural similarity. Due to its unique structure, resistance profile, relatively broad spectrum and stability under physiological conditions, CMB001 is a promising drug candidate for evaluation in animal models of bacterial infection.

Development of an antibody cocktail for treatment of Sudan virus infection.

Antibody-based therapies are a promising treatment option for managing ebolavirus infections. Several Ebola virus (EBOV)-specific and, more recently, pan-ebolavirus antibody cocktails have been described. Here, we report the development and assessment of a Sudan virus (SUDV)-specific antibody cocktail. We produced a panel of SUDV glycoprotein (GP)-specific human chimeric monoclonal antibodies (mAbs) using both plant and mammalian expression systems and completed head-to-head in vitro and in vivo evaluations. Neutralizing activity, competitive binding groups, and epitope specificity of SUDV mAbs were defined before assessing protective efficacy of individual mAbs using a mouse model of SUDV infection. Of the mAbs tested, GP base-binding mAbs were more potent neutralizers and more protective than glycan cap- or mucin-like domain-binding mAbs. No significant difference was observed between plant and mammalian mAbs in any of our in vitro or in vivo evaluations. Based on in vitro and rodent testing, a combination of two SUDV-specific mAbs, one base binding (16F6) and one glycan cap binding (X10H2), was down-selected for assessment in a macaque model of SUDV infection. This cocktail, RIID F6-H2, provided protection from SUDV infection in rhesus macaques when administered at 50 mg/kg on days 4 and 6 postinfection. RIID F6-H2 is an effective postexposure SUDV therapy and provides a potential treatment option for managing human SUDV infection.

A nanoemulsion-adjuvanted intranasal H5N1 influenza vaccine protects ferrets against homologous and heterologous H5N1 lethal challenge.

BACKGROUND: Flu vaccines administered intramuscularly (IM) have shown seasonally fluctuating efficacy, 20-60%, throughout the last 15 years. We formulated a recombinant H5 (rH5) in our Nanovax® (NE01) (rH5/NE01) adjuvant for intranasal vaccination in ferrets. We evaluated the regimen, one vs two immunization, and cross clade protection a ferret challenge model. METHODS: Plant derived recombinant H5 (rH5) antigen was formulated with NE01 and administered intranasally to ferrets. Immunogenicity (IgG), hemagglutination inhibition (HI), and protection against lethal challenge, were measured following one or two immunizations. Protection against homologous (strain A/Indo) and heterologous (strain A/Vn) was evaluated in ferrets following two immunizations. RESULTS: IN immunization with rH5/NE01 induced significant IgG levels after one and two immunizations. One vaccination did not induce any HI while low HI was measured after two immunizations. Homologous challenge with H5N1 A/ Indonesia showed 100% survival, with minimal weight loss in animals vaccinated twice compared to the unvaccinated controls. Analysis of nasal wash from these challenged ferrets vaccinated twice showed decreased viral shedding compared to unvaccinated controls. Interestingly, animals that received one vaccination showed 88% survival with moderate weight loss. Cross clade protection was evaluated using an increased antigen dose (45 µg rH5). Vaccinated animals demonstrated increased IgG and HAI antibody responses. Both homologous (A/Indo) and heterologous challenge (A/Vietnam) following two immunizations showed 100% survival with no loss of body weight. However viral clearance was more rapid against the homologous (day 3) compared to the heterologous (day 5) post challenge. CONCLUSION: Intranasal administration of NE01 adjuvant-formulated rH5 vaccine elicited systemic and probably mucosal immunity that conferred protection against lethal challenge with homologous or heterologous viral strains. It also enhanced viral clearance with decreased shedding. These outcomes strongly suggest that intranasal immunization using NE01 against flu infections warrants clinical testing.

Recombinant H5 hemagglutinin adjuvanted with nanoemulsion protects ferrets against pathogenic avian influenza virus challenge.

BACKGROUND: Highly pathogenic H5N1 influenza viruses remain a pandemic risk to the world population. Although vaccines are the best solution to prevent this threat, a more effective vaccine for H5 strains of influenza has yet to be developed. All existing vaccines target only serum antibody against influenza as the primary outcome, while mucosal immunity has not been addressed. To address these shortcomings we have used an effective mucosal adjuvant system to produce a prototype vaccine that provides antibody, cellular and mucosal immunity to multiple serotypes of H5. METHODS: Plant-derived recombinant H5 (rH5) antigen was mixed with a novel nanoemulsion NE01 adjuvant. The rH5-NE01 vaccine was administered intranasally to CD-1 mice and ferrets. Immunogenicity of this immunization was evaluated through rH5-specific antibody and cellular immune responses. Hemagglutination inhibition (HI) and virus neutralization (VN) assays were performed. Protection against H5N1 virus challenge was evaluated in ferrets. RESULTS: Intranasal immunization with rH5-NE01vaccine induced high titers (>106) of rH5-specific IgG in mice. In mice and ferrets this vaccine also achieved titers of ≥40 for both HI and VN. Additionally, the levels of rH5-specific IgA were significantly increased in bronchial secretions in these animals. The rH5-NE01 vaccine enhanced rH5-specific cellular immune responses including IFN-γ and IL-17. Ten-day survival post challenge was 100% in ferrets that received rH5-NE01compared to 12.5% in the PBS group. Furthermore, this vaccine prevented weight loss and increases in body temperature after H5N1 challenge as compared to the controls. Moreover, H5N1 virus in nasal wash of rH5-NE01-vaccinated ferrets was significantly decreased compared to controls. CONCLUSION: Intranasal immunization with rH5 antigen formulated with NE01 adjuvant elicited strong, broad and balanced immune responses that effectively protect against H5N1 influenza virus infection in the ferret model. The ease of formulation of rH5-NE01 makes this novel combination a promising mucosal vaccine candidate for pandemic influenza.

Safety and immunogenicity of a plant-produced Pfs25 virus-like particle as a transmission blocking vaccine against malaria: A Phase 1 dose-escalation study in healthy adults.

Malaria continues to be one of the world's most devastating infectious tropical diseases, and alternative strategies to prevent infection and disease spread are urgently needed. These strategies include the development of effective vaccines, such as malaria transmission blocking vaccines (TBV) directed against proteins found on the sexual stages of Plasmodium falciparum parasites present in the mosquito midgut. The Pfs25 protein, which is expressed on the surface of gametes, zygotes and ookinetes, has been a primary target for TBV development. One such vaccine strategy based on Pfs25 is a plant-produced malaria vaccine candidate engineered as a chimeric non-enveloped virus-like particle (VLP) comprising Pfs25 fused to the Alfalfa mosaic virus coat protein. This Pfs25 VLP-FhCMB vaccine candidate has been engineered and manufactured in Nicotiana benthamiana plants at pilot plant scale under current Good Manufacturing Practice guidelines. The safety, reactogenicity and immunogenicity of Pfs25 VLP-FhCMB was assessed in healthy adult volunteers. This Phase 1, dose escalation, first-in-human study was designed primarily to evaluate the safety of the purified plant-derived Pfs25 VLP combined with Alhydrogel® adjuvant. At the doses tested in this Phase 1 study, the vaccine was generally shown to be safe in healthy volunteers, with no incidence of vaccine-related serious adverse events and no evidence of any dose-limiting or dose-related toxicity, demonstrating that the plant-derived Pfs25 VLP-FhCMB vaccine had an acceptable safety and tolerability profile. In addition, although the vaccine did induce Pfs25-specific IgG in vaccinated patients in a dose dependent manner, the transmission reducing activity of the antibodies generated were weak, suggesting the need for an alternative vaccine adjuvant formulation. This study was registered at www.ClinicalTrials.gov under reference identifier NCT02013687.

Plant-Produced Subunit Vaccine Candidates against Yellow Fever Induce Virus Neutralizing Antibodies and Confer Protection against Viral Challenge in Animal Models.

Yellow fever (YF) is a viral disease transmitted by mosquitoes and endemic mostly in South America and Africa with 20-50% fatality. All current licensed YF vaccines, including YF-Vax® (Sanofi-Pasteur, Lyon, France) and 17DD-YFV (Bio-Manguinhos, Rio de Janeiro, Brazil), are based on live attenuated virus produced in hens' eggs and have been widely used. The YF vaccines are considered safe and highly effective. However, a recent increase in demand for YF vaccines and reports of rare cases of YF vaccine-associated fatal adverse events have provoked interest in developing a safer YF vaccine that can be easily scaled up to meet this increased global demand. To this point, we have engineered the YF virus envelope protein (YFE) and transiently expressed it in Nicotiana benthamiana as a stand-alone protein (YFE) or as fusion to the bacterial enzyme lichenase (YFE-LicKM). Immunogenicity and challenge studies in mice demonstrated that both YFE and YFE-LicKM elicited virus neutralizing (VN) antibodies and protected over 70% of mice from lethal challenge infection. Furthermore, these two YFE-based vaccine candidates induced VN antibody responses with high serum avidity in nonhuman primates and these VN antibody responses were further enhanced after challenge infection with the 17DD strain of YF virus. These results demonstrate partial protective efficacy in mice of YFE-based subunit vaccines expressed in N. benthamiana. However, their efficacy is inferior to that of the live attenuated 17DD vaccine, indicating that formulation development, such as incorporating a more suitable adjuvant, may be required for product development.

Stability and pre-formulation development of a plant-produced anthrax vaccine candidate.

Second generation anthrax vaccines focus on the use of recombinant protective antigen (rPA) to elicit a strong, toxin neutralizing antibody responses in immunized subjects. The main difference between the rPA vaccines compared to the current licensed vaccine, anthrax vaccine absorbed (AVA), is the rPA vaccines are highly purified preparations of only rPA. These second generation rPA vaccines strive to elicit strong immune responses with substantially fewer doses than AVA while provoking less side effects. Many of the rPA candidates have shown to be effective in pre-clinical studies, but most of the second generation molecules have stability issues which reduce their efficacy over time. These stability issues are evident even under refrigerated conditions and thus emphasis has been directed to stabilizing the rPA molecule and determining an optimized final formulation. Stabilization of vaccines for long-term storage is a major challenge in the product development life cycle. The effort required to identify suitable formulations can be slow and expensive. The ideal storage for stockpiled vaccines would allow the candidate to withstand years of storage at ambient temperatures. The Fraunhofer Center for Molecular Biotechnology is developing a plant-produced rPA vaccine candidate that shows instability when stored under refrigerated conditions in a solution, as is typical for rPA vaccines. Increased stability of our plant-produced rPA vaccine candidate was achieved in a spray dried powder formulation that could eliminate the need for conventional cold chain allowing greater confidence to stockpile vaccine for civilian and military biodefense.

Purification and immunogenicity of hemagglutinin from highly pathogenic avian influenza virus H5N1 expressed in Nicotiana benthamiana.

Highly pathogenic avian influenza (HPAI) H5N1 is an ongoing global health concern due to its severe sporadic outbreaks in Asia, Africa and Europe, which poses a potential pandemic threat. The development of safe and cost-effective vaccine candidates for HPAI is considered the best strategy for managing the disease and addressing the pandemic preparedness. The most potential vaccine candidate is the antigenic determinant of influenza A virus, hemagglutinin (HA). The present research was aimed at developing optimized expression in Nicotiana benthamiana and protein purification process for HA from the Malaysian isolate of H5N1 as a vaccine antigen for HPAI H5N1. Expression of HA from the Malaysian isolate of HPAI in N. benthamiana was confirmed, and more soluble protein was expressed as truncated HA, the HA1 domain over the entire ectodomain of HA. Two different purification processes were evaluated for efficiency in terms of purity and yield. Due to the reduced yield, protein degradation and length of the 3-column purification process, the 2-column method was chosen for target purification. Purified HA1 was found immunogenic in mice inducing H5 HA-specific IgG and a hemagglutination inhibition antibody. This paper offers an alternative production system of a vaccine candidate against a locally circulating HPAI, which has a regional significance.

Production of Functionally Active and Immunogenic Non-Glycosylated Protective Antigen from Bacillus anthracis in Nicotiana benthamiana by Co-Expression with Peptide-N-Glycosidase F (PNGase F) of Flavobacterium meningosepticum.

Bacillus anthracis has long been considered a potential biological warfare agent, and therefore, there is a need for a safe, low-cost and highly efficient anthrax vaccine with demonstrated long-term stability for mass vaccination in case of an emergency. Many efforts have been made towards developing an anthrax vaccine based on recombinant protective antigen (rPA) of B. anthracis, a key component of the anthrax toxin, produced using different expression systems. Plants represent a promising recombinant protein production platform due to their relatively low cost, rapid scalability and favorable safety profile. Previous studies have shown that full-length rPA produced in Nicotiana benthamiana (pp-PA83) is immunogenic and can provide full protection against lethal spore challenge; however, further improvement in the potency and stability of the vaccine candidate is necessary. PA of B. anthracis is not a glycoprotein in its native host; however, this protein contains potential N-linked glycosylation sites, which can be aberrantly glycosylated during expression in eukaryotic systems including plants. This glycosylation could affect the availability of certain key epitopes either due to masking or misfolding of the protein. Therefore, a non-glycosylated form of pp-PA83 was engineered and produced in N. benthamiana using an in vivo deglycosylation approach based on co-expression of peptide-N-glycosidase F (PNGase F) from Flavobacterium meningosepticum. For comparison, versions of pp-PA83 containing point mutations in six potential N-glycosylation sites were also engineered and expressed in N. benthamiana. The in vivo deglycosylated pp-PA83 (pp-dPA83) was shown to have in vitro activity, in contrast to glycosylated pp-PA83, and to induce significantly higher levels of toxin-neutralizing antibody responses in mice compared with glycosylated pp-PA83, in vitro deglycosylated pp-PA83 or the mutated versions of pp-PA83. These results suggest that pp-dPA83 may offer advantages in terms of dose sparing and enhanced immunogenicity as a promising candidate for a safe, effective and low-cost subunit vaccine against anthrax.

Antibody Production in Plants and Green Algae.

Monoclonal antibodies (mAbs) have a wide range of modern applications, including research, diagnostic, therapeutic, and industrial uses. Market demand for mAbs is high and continues to grow. Although mammalian systems, which currently dominate the biomanufacturing industry, produce effective and safe recombinant mAbs, they have a limited manufacturing capacity and high costs. Bacteria, yeast, and insect cell systems are highly scalable and cost effective but vary in their ability to produce appropriate posttranslationally modified mAbs. Plants and green algae are emerging as promising production platforms because of their time and cost efficiencies, scalability, lack of mammalian pathogens, and eukaryotic posttranslational protein modification machinery. So far, plant- and algae-derived mAbs have been produced predominantly as candidate therapeutics for infectious diseases and cancer. These candidates have been extensively evaluated in animal models, and some have shown efficacy in clinical trials. Here, we review ongoing efforts to advance the production of mAbs in plants and algae.

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

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