Transgenic lettuce producing a candidate protein for vaccine against edema disease

Accumulation of colicin M protein and its biological activity in transgenic lettuce and mizuna plants

Food-borne illnesses caused by pathogenic Escherichia coli strains, especially enterohaemorrhagic E. coli (EHEC), are a serious public health problem, as debilitating disease and even death from such food poisonings have been repeatedly reported. Colicin M (ColM), a non-antibiotic antimicrobial protein produced by some strains of E. coli, has shown promising activity in controlling multiple enteropathogenic strains of E. coli and related pathogens. As contaminated green leafy vegetables are a frequent source of pathogenic E. coli infections, we genetically modified (GM) two edible crops, lettuce (Lactuca sativa L.) and mizuna (Brassica rapa subsp. nipposinica var. laciniata), to stably express the ColM gene and assessed the antibacterial activity of tissue extracts from these plants against selected E. coli strains in vitro. Transgenic plants of these species were developed using Agrobacterium-mediated transformation with a vector containing the ColM-coding gene (cma) under the control of the 35S promoter. Western blot analysis of recombinant ColM protein was performed in selected transgenic plants to confirm cma gene expression and quantify ColM accumulation. Extracts of transgenic plants expressing ColM showed significant activity against two major strains of EHEC (O157:H7 and O104:H4) as well as E. coli strains resistant to beta-lactam- and carbapenem-class antibiotics. Importantly, the antibacterial activity persisted in several subsequent generations of transgenic lettuce and mizuna plants that stably expressed the ColM gene. In addition, our results also show that the antibacterial activity of dried (up to 40°C) biomass of transgenic plants remained stable without a decrease for at least three months.

Energy-efficient production of vaccine protein against porcine edema disease from transgenic lettuce (Lactuca sativa L.)

The development of functional protein production systems using transgenic plants as hosts has been rapidly progressing in recent years. Lettuce (Lactuca sativa L.) has been studied as one such host, and it has been reported that the biomass of lettuce per area and target protein expression level can be increased by optimizing the cultivation conditions. Therefore, we investigated methods to minimize the input light energy per target protein to reduce production costs. Herein, we examined the yield of a nontoxic B subunit of Stx2e (Stx2eB) from transgenic lettuce under various cultivation conditions. Stx2eB acts as a vaccine against swine edema disease. The effects of photon flux densities (PPFDs), photoperiod, and light source on Stx2eB production were examined and the findings suggested that 400 μmol m-2 s-1, 24 h, and white LED lamps, respectively, contributed to energy-efficient Stx2eB production. In addition, Stx2eB was produced 1.4 times more efficiently per unit area time using a high plant density (228.5 plants m-2) than a common density (30.4 plants m-2). The findings of the present study can facilitate the development of energy-efficient and low-cost production processes for vaccine protein production, considering temporal and spatial perspectives.

Shiga toxin 2eB-transgenic lettuce vaccine: N-glycosylation is important for protecting against porcine edema disease

Porcine edema disease (ED) is a life-threatening toxemia caused by enteric infection with Shiga toxin 2e (Stx2e)-producing Escherichia coli (STEC) in weaned piglets. We previously reported that the stx2eB-transgenic lettuce 2BH strain shows potential for use as an oral vaccine candidate against ED. However, the 2BH strain expressed a hemagglutinin (HA)-tag together with Stx2eB and contained non-canonical N-glycosylation. Therefore, we developed two Stx2eB-lettuce strains, the 3 (G+) strain in which the HA-tag was removed from 2BH, and the 3 (G-) lettuce strain, in which the 73rd Asn was replaced with Ser to prevent non-canonical N-glycosylation of Stx2eB from the 3 (G+) strain. We examined the protective effect of these newly developed two strains compared with the previous 2BH strain against ED using a colostrum-deprived piglet STEC infection model. We found that the N-glycosylated 2BH and 3 (G+) strains relieved the pathogenic symptoms of ED in STEC-challenged piglets, whereas the non-glycosylated 3 (G-) strain did not. N-Glycosylation of the Stx2eB product in lettuce may be involved in the immune response in piglets.

Lettuce-based production of an oral vaccine against porcine edema disease for the seed lot system

Plant-made oral vaccines can be a cost-effective method to control infectious diseases of humans and farm animals. Pig edema is a bacterial disease caused by enterohemorrhagic Escherichia coli producing the toxin Shiga toxin 2e (Stx2e). In our previous report, we chose the non-toxic B subunit of Stx2e (Stx2eB) as a vaccine antigen, and Stx2eB was expressed in lettuce (Lactuca sativa L., cv. Green wave). We found that a double repeated Stx2eB (2×Stx2eB) accumulates to higher levels than a single Stx2eB. In this study, we analyzed progeny plants introduced with 2×Stx2eB in which the gene was expressed under the control of conventional cauliflower mosaic virus 35S RNA (CaMV 35S) promoter, and found that the lettuce underwent transgene silencing and bore few seeds. We resolved these problems by using a transgene cassette which harbored a transcriptional promoter derived from the lettuce ubiquitin gene and a longer version of HSPT. The lettuce harboring this expression construct will be valuable in establishing the seed lot system on the basis that thousands of seeds can be obtained from one plant body and the resulting progeny plants accumulate 2×Stx2eB at high levels without the transgene silencing.

Development of a novel heterologous gene expression system using earthworms

In recent years, animals and plants have received increasing attention as potential next-generation protein production systems, especially for biopharmaceuticals and animal proteins. The aim of the present study was to develop the earthworms Eisenia fetida Waki and Eisenia andrei Sagami as next-generation animal protein production hosts. These earthworms have been approved as model animals for acute toxicity tests by the Organization for Economic Co-operation and Development, and they have post-translational modification systems. However, so far, none of the studies have used earthworm transfection techniques. Thus, we developed a transfection method for E. fetida and E. andrei using microinjection and electroporation systems. The maximum survival rates and transfection efficiencies were 79.2% and 29.2% for E. fetida, and 95.8% and 50.0% for E. andrei, respectively. Furthermore, human erythropoietin was detected in the transformed earthworm tail fragments using an enzyme-linked immunosorbent assay. These results contribute to the development of a potential earthworm-based novel animal protein production system.

Pathogenicity of Shiga Toxin Type 2e Escherichia coli in Pig Colibacillosis

Shiga toxin type 2e (Stx2e) Escherichia coli is the causative factor of diarrhea and edema in swine. The aims of this study were to determine the prevalence of Stx2e-producing E. coli isolates and to characterize isolates from clinical cases of pig colibacillosis and healthy swine. During the 11 years of the study (2006–2017), a total of 233 Stx2e-producing isolates were detected−230 out of 2,060 (11.16%) E. coli isolated from diseased pigs and 3 out of 171 (1.75%) from healthy swine. Stx2e-producing isolates were indeed more present in clinical colibacillosis cases than in healthy pigs (p = 0.0002). The predominant serogroup was O139 (79.82%) and the most common fimbrial factor present in these isolates was F18 (177 isolates), followed by F6 (5 isolates). The enterotoxins LTI, STa, and STb were detected in 10.43, 41.73, and 48.26% of the isolates, respectively. The predominant virotypes F18-Stx2e and -STa-STb-Stx2e were similarly present in weaners (33.33 and 35.52%) and finishers (38.30 and 25.53%). Among isolates from diseased pigs, O139 and F18 were the more frequently identified serogroup and virulence factor, respectively. Of the tested 230 Stx2e-producing isolates isolated from diseased pigs, 29 (12.60%) harbored genes encoding ESBL, particularly TEM (79.30%), CTX-M1 (17.20%), and CMY-2 (3.40%). Antimicrobial resistance to tetracycline was the most common characteristic (98.25%), followed by ampicillin (93.91%), cephalotin (90.43%) and trimethoprim/sulfamethoxazole (82.17%). Our results showed that Stx2e-producing E. coli were more frequently associated with clinical forms of colibacillosis, with minimal probability to isolate these isolates from healthy pigs.

Shiga toxin 2eB-transgenic lettuce vaccine is effective in protecting weaned piglets from edema disease caused by Shiga toxin-producing Escherichia coli infection

Porcine edema disease (ED) is a toxemia that is caused by enteric infection with Shiga toxin 2e (Stx2e)-producing Escherichia coli (STEC) and is associated with high mortality. Since ED occurs most frequently during the weaning period, preweaning vaccination of newborn piglets is required. We developed stx2eB-transgenic lettuce as an oral vaccine candidate against ED and examined its protective efficacy using a piglet STEC infection model. Two serially developed Stx2eB-lettuce strains, 2BN containing ingredient Stx2eB constituting a concentration level of 0.53 mg Stx2eB/g of powdered lettuce dry weight (DW) and 2BH containing ingredient Stx2eB constituting a concentration level of 2.3 mg of Stx2eB/g of powdered lettuce DW, were evaluated in three sequential experiments. Taken the results together, oral administration of Stx2eB-lettuce vaccine was suggested to relieve the pathogenic symptoms of ED in piglets challenged with virulent STEC strain. Our data suggested that Stx2eB-lettuce is a promising first oral vaccine candidate against ED.

Lettuce-derived secretory IgA specifically neutralizes the Shiga toxin 1 activity

MAIN CONCLUSION

An edible plant was tested as a host for the production of secretory monoclonal IgA against Shiga toxin 1 (Stx1). The lettuce-derived IgA completely protected Vero cells from Stx1. Secretory immunoglobulin A (SIgA) is thought to control mucosal infections and thus it may be applicable to oral passive immunotherapy. Edible plants are candidate hosts for producing oral formulations with SIgA against pathogenic agents. We previously established a recombinant IgA specific for the B subunit of Shiga toxin 1 (Stx1B) consisting of the Fab fragment of Stx1B-specific monoclonal IgG and the Fc region of IgA (hyIgA). Here, we developed transgenic lettuce (Lactuca sativa) that produces hyIgA in a secretory form (S-hyIgA). An Arabidopsis-derived light-harvesting complex II (LHCB) promoter was used for the expression of all four transgenes (hyIgA heavy, light and j chains, and secretory component). Agrobacterium-mediated transformation was carried out to introduce genes into lettuce leaf discs by means of a single vector harboring all four transgenes. Consistent with the tissue specificity of the LHCB promoter, the expression of hyIgA transgenes was observed in leaf and stem tissues, which contain chloroplasts, at the mRNA and protein levels. The leaves produced hyIgA in a more than tenfold higher yield as compared with stems. The lettuce-derived S-hyIgA was found to bind to Stx1B in a dose-dependent manner by means of ELISA. A leaf extract of the transgenic lettuce completely neutralized the cytotoxicity of Stx1 against Vero cells, which are highly susceptible to Stx1. In conclusion, we established a transgenic lettuce producing a secretory form of hyIgA that can bind bacterial toxin. The results indicate that edible practical plants containing S-hyIgA will provide a possible means for immunotherapy for food poisoning.

A member of cation diffusion facilitator family, MTP11, is required for manganese tolerance and high fertility in rice

Rice MTP11 is the trans-Golgi-localized transporter that is involved in Mn tolerance with MTP8.1, and it is required for normal fertility. Rice (Oryza sativa L.) is one of the most manganese (Mn)-tolerant species, and it is able to accumulate high levels of this metal in the leaves without showing toxic symptoms. The metal tolerance protein 8.1 (MTP8.1), a member of the Mn-cation diffusion facilitator (CDF) family, has been shown to play a central role in high Mn tolerance by sequestering Mn into vacuoles. Recently, rice MTP11 was identified as an Mn transporter that is localized to Golgi-associated compartments, but its exact role in Mn tolerance in planta has not yet been understood. Here, we investigated the role of MTP11 in rice Mn tolerance using knockout lines. Old leaves presented higher levels of constitutively expressed MTP11 than other tissues and MTP11 expression was also found in reproductive organs. Fused MTP11:green fluorescent protein was co-localized to trans-Golgi markers and differentiated from other Golgi-associated markers. Knockout of MTP11 in wild-type rice did not affect tolerance and accumulation of Mn and other heavy metals, but knockout in the mtp8.1 mutant showed exacerbated Mn sensitivity at the vegetative growth stage. Knockout of MTP11 alone resulted in decreased grain yield and fertility at the reproductive stage. Thus, MTP11 is a trans-Golgi localized transporter for Mn, which plays a role in Mn tolerance through intracellular Mn compartmentalization. It is also required for maintaining high fertility in rice.

Effectiveness of F18+ Fimbrial Antigens Released by a Novel Autolyzed Salmonella Expression System as a Vaccine Candidate against Lethal F18+ STEC Infection

Porcine edema disease (ED) caused by Shiga toxin 2e producing Escherichia coli expressing F18ab⁺ fimbriae (F18ab⁺STEC) frequently occurs in post-weaned piglets, resulting in a significant economic loss in swine industries worldwide. In the present study, we proposed an efficient prevention scheme against ED in which the attenuated Salmonella Typhimurium inactivated by the E-mediated cell lysis to deliver target antigens, FedF and FedA, which function in fimbrial-mediated adhesion and as a major subunit of F18ab+fimbriae, respectively. The co-expression of FedA and FedF protein with outer membrane protein A signal peptide was confirmed in the resultant strains JOL1460 and JOL1464 by immunoblot analysis. Immunization with the candidate strains in mice led to the significant generation of immunoglobulin (Ig) G, specific to both antigens and secretory IgA specific to FedF (P < 0.05). The titers of IgG isotypes, IgG1 and IgG2a, used as markers for T-helpers (Th)-2 and Th-1lymphocytes, respectively, also significantly increased in the immunized group (P < 0.05). The increase in CD3⁺CD4⁺ T lymphocyte subpopulation and in vitro proliferative activity was observed in in vivo stimulated splenocytes, which indicated the immunostimulatory effect of the candidate strains. Moreover, the immunized mice were completely protected from a lethal challenge against wild-type F18⁺STEC whereas 28% of mice died in the non-immunized group. This study demonstrated that the inactivated Salmonella system could efficiently release FedF and FedA and induce robust immune responses specific to the target antigens, which is sufficient to protect the mice from the lethal challenge.

Control of foreign polypeptide localization in specific layers of protein body type I in rice seed

Prolamin-GFP fusion proteins, expressed under the control of native prolamin promoters, were localized in specific layers of PB-Is. Prolamin-GFP fusion proteins were gradually digested from outside by pepsin digestion. In rice seed endosperm, protein body type I (PB-I) has a layered structure consisting of prolamin species and is the resistant to digestive juices in the intestinal tract. We propose the utilization of PB-Is as an oral vaccine carrier to induce mucosal immune response effectively. If vaccine antigens are localized in a specific layer within PB-Is, they could be protected from gastric juice and be delivered intact to the small intestine. We observed the localization of GFP fluorescence in transgenic rice endosperm expressing prolamin-GFP fusion proteins with native prolamin promoters, and we confirmed that the foreign proteins were located in specific layers of PB-Is artificially. Each prolamin-GFP fusion protein was localized in specific layers of PB-Is, such as the outer-most layer, middle layer, and core region. Furthermore, to investigate the resistance of prolamin-GFP fusion proteins against pepsin digestion, we performed in vitro pepsin treatment. Prolamin-GFP fusion proteins were gradually digested from the peripheral region and the contours of PB-Is were made rough by in vitro pepsin treatment. These findings suggested that prolamin-GFP fusion proteins accumulating specific layers of PB-Is were gradually digested and exposed from the outside by pepsin digestion.

How can plant genetic engineering contribute to cost-effective fish vaccine development for promoting sustainable aquaculture?

Aquaculture, the fastest growing food-producing sector, now accounts for nearly 50 % of the world's food fish (FAO in The state of world fisheries and aquaculture. FAO, Rome, 2010). The global aquaculture production of food fish reached 62.7 million tonnes in 2011 and is continuously increasing with an estimated production of food fish of 66.5 million tonnes in 2012 (a 9.4 % increase in 1 year, FAO, www.fao.org/fishery/topic/16140 ). Aquaculture is not only important for sustainable protein-based food fish production but also for the aquaculture industry and economy worldwide. Disease prevention is the key issue to maintain a sustainable development of aquaculture. Widespread use of antibiotics in aquaculture has led to the development of antibiotic-resistant bacteria and the accumulation of antibiotics in the environment, resulting in water and soil pollution. Thus, vaccination is the most effective and environmentally-friendly approach to combat diseases in aquaculture to manage fish health. Furthermore, when compared to >760 vaccines against human diseases, there are only about 30 fish vaccines commercially available, suggesting the urgent need for development and cost-effective production of fish vaccines for managing fish health, especially in the fast growing fish farming in Asia where profit is minimal and therefore given high priority. Plant genetic engineering has made significant contributions to production of biotech crops for food, feed, valuable recombinant proteins etc. in the past three decades. The use of plants for vaccine production offers several advantages such as low cost, safety and easy scaling up. To date a large number of plant-derived vaccines, antibodies and therapeutic proteins have been produced for human health, of which a few have been made commercially available. However, the development of animal vaccines in plants, especially fish vaccines by genetic engineering, has not yet been addressed. Therefore, there is a need to exploit plant biotechnology for cost effective fish vaccine development in plants, in particular, edible crops for oral fish vaccines. This review provides insight into (1) the current status of fish vaccine and vaccination in aquaculture, (2) plant biotechnology and edible crops for fish vaccines for oral administration, (3) regulatory constraints and (4) conclusions and future perspectives.

Evaluation of recombinant forms of the shiga toxin variant Stx2eB subunit and non-toxic mutant Stx2e as vaccine candidates against porcine edema disease

Porcine edema disease (ED) is a communicable disease of shoats caused by infection with Shiga toxin (Stx)-producing Escherichia coli. Stx2e is classified as a 1A5B-type toxin and is a decisive virulence determinant of ED. The single A subunit of Stx2e possesses enzymatic activity and is accompanied by a pentamer of B subunits, which binds to the host receptor and delivers the A subunit into the cell. In the present study, we used a mouse model to evaluate the immunogenicity of 3 ED vaccine candidates: a non-toxic mutant holotoxin mStx2e and 2 Stx2eB-based fusion proteins, Stx2eA₂B-His and Stx2eB-His. Systemic inoculation of mice with mStx2e- and the Stx2eB-derived antigens induced anti-Stx2e IgG responses that were fully and partially capable of neutralizing Stx2e cellular cytotoxicity, respectively. Intranasal immunization with mStx2e protected the mice from subsequent intraperitoneal challenge with a lethal dose of Stx2e, whereas immunization with Stx2eA₂B-His and Stx2eB-His afforded partial protection. Analysis of serum cytokines revealed that mStx2e, but not the Stx2eB-based antigens, was capable of inducing a Th2-type immune response. These results suggest that although the Stx2eB-based antigens elicited an immune response to Stx2e, they did so through a different mechanism to the Th2-type response induced by mStx2e.

Effectiveness of egg yolk antibody against shiga toxin II variant toxicity in vitro and in vivo

We prepared anti-Shiga toxin II variant (Stx2e) and anti-Stx2e B antibodies in chicken egg yolk and investigated their effectiveness in vitro and in vivo. Both Egg yolk antibodies (IgY) reduced the cytotoxic effects of Stx2e to Vero cells. The protective efficacy of anti-Stx2e IgY and anti-Stx2e B IgY was investigated in clean Kunming mice, which were experimentally induced with Stx2e. Peritoneal injection of anti-Stx2e IgY and anti-Stx2e B IgY apparently reduced the lethal effect of Stx2e to mice. These results indicate that both anti-Stx2e IgY and anti-Stx2e B IgY could protect mice from Stx2e challenge. Therefore, anti-Stx2e IgY or anti-Stx2e B IgY might be considered as potential candidate for therapeutics for porcine edema disease.

Safety and immunogenicity of bacterial and tobacco plant cell line derived recombinant native and mutant Escherichia coli heat-labile toxin in chickens

The safety and immunogenicity of the mammalian mucosal adjuvants, Escherichia coli wild-type heat-labile holotoxin (LT) and E. coli mutant LT (LTA-K63/LTB), were examined in 1-day-old chicks and 10-day-old to 21-day-old broilers. Biologically active, E. coli recombinant wild-type LT and recombinant LTA-K63/LTB produced in a transgenic Nicotiana tabacum (NT-1) tobacco cell line (SLT102) were tested for safety and antigenicity following various routes of administration. Safety was assessed by clinical signs, body weight gain, gross organ pathology and wet organ weight, and histopathology. Antigenicity was assessed using LT-B-specific serum IgG enzyme-linked immunosorbent assay. Parenteral administration of E. coli recombinant wild-type LT did not have any discernible effect on bird health and was well tolerated at levels up to 400 µg per dose. Recombinant, SLT102-derived mutant LT derived from SLT102 cells retained in vitro ganglioside binding and was safe and antigenic following repeated mucosal administration to birds. The highest systemic LT-B-specific IgG titres were detected in birds that received three on-feed doses of SLT102-derived mutant LT. Among the various SLT102-derived mutant LT preparations tested, whole, wet cells or whole cell lysates were the most antigenic. These results demonstrate for the first time that E. coli-derived recombinant, wild-type LT holotoxin is well tolerated following multiple administrations to young birds at body weight doses previously reported to be enteropathogenic and toxic in mammalian species. Moreover, these data also demonstrate the feasibility of using recombinant wild-type and mutant LT produced in transgenic NT-1 tobacco cells as safe and potent vaccine adjuvants in poultry.

N-glycosylation at noncanonical Asn-X-Cys sequences in plant cells

The vesicular transport pathway in plant cells is often used for higher accumulation of recombinant proteins. In the endoplasmic reticulum, which acts as a gateway to the vesicular transport pathway, N-glycosylation occurs on specific Asn residues. This N-glycosylation in recombinant proteins must be carefully regulated as it can impact their enzymatic activity, half lives in serum when injected, structural stability, etc. In eukaryotic cells, including plant cells, N-glycans were found to be attached to Asn residues in Asn-X-Ser/Thr (X ≠ Pro) sequences. However, recently, N-glycosylations at noncanonical Asn-X-Cys sequences have been found in mammals and yeast. Our laboratory has discovered that N-glycans are attached to Asn residues at Asn-Thr-Cys sequences of double-repeated B subunit of Shiga toxin 2e produced in plant cells, the first reported case of N-glycosylation at a noncanonical Asn-X-Cys sequence in plant cells.

Production of double repeated B subunit of Shiga toxin 2e at high levels in transgenic lettuce plants as vaccine material for porcine edema disease

Pig edema disease is a bacterial disease caused by enterohemorrhagic Escherichia coli. E. coli produces Shiga toxin 2e (Stx2e), which is composed of one A subunit (Stx2eA) and five B subunits (Stx2eB). We previously reported production of Stx2eB in lettuce plants as a potential edible vaccine (Matsui et al. in Biosci Biotechnol Biochem 73:1628-1634, 2009). However, the accumulation level was very low, and it was necessary to improve expression of Stx2eB for potential use of this plant-based vaccine. Therefore, in this study, we optimized the Stx2eB expression cassette and found that a double repeated Stx2eB (2× Stx2eB) accumulates to higher levels than a single Stx2eB in cultured tobacco cells. Furthermore, a linker peptide between the two Stx2eB moieties played an important role in maximizing the effects of the double repeat. Finally, we generated transgenic lettuce plants expressing 2× Stx2eB with a suitable linker peptide that accumulate as much as 80 mg per 100 g fresh weight, a level that will allow us to use these transgenic lettuce plants practically to generate vaccine material.

Activity of the C-terminal-dependent vacuolar sorting signal of horseradish peroxidase C1a is enhanced by its secondary structure

Plant class III peroxidase (PRX) catalyzes the oxidation and oxidative polymerization of a variety of phenolic compounds while reducing hydrogen peroxide. PRX proteins are classified into apoplast type and vacuole type based on the absence or the presence of C-terminal propeptides, which probably function as vacuolar sorting signals (VSSs). In this study, in order to improve our understanding of vacuole-type PRX, we analyzed regulatory mechanisms of vacuolar sorting of a model vacuole-type PRX, the C1a isozyme of horseradish (Armoracia rusticana) (HRP C1a). Using cultured transgenic tobacco cells and protoplasts derived from horseradish leaves, we characterized HRP C1a's VSS, which is a 15 amino acid C-terminal propeptide (C15). We found that the C-terminal hexapeptide of C15 (C6), which is well conserved among vacuole-type PRX proteins, forms the core of the C-terminal-dependent VSS. We also found that the function of C6 is enhanced by the remaining N-terminal part of C15 which probably folds into an amphiphilic α-helix.