Plant-derived antigens as mucosal vaccines.

During the last two decades, researchers have developed robust systems for recombinant subunit vaccine production in plants. Stably and transiently transformed plants have particular advantages that enable immunization of humans and animals via mucosal delivery. The initial goal to immunize orally by ingestion of plant-derived antigens has proven difficult to attain, although many studies have demonstrated antibody production in both humans and animals, and in a few cases, protection against pathogen challenge. Substantial hurdles for this strategy are low-antigen content in crudely processed plant material and limited antigen stability in the gut. An alternative is intranasal delivery of purified plant-derived antigens expressed with robust viral vectors, especially virus-like particles. The use of pattern recognition receptor agonists as adjuvants for mucosal delivery of plant-derived antigens can substantially enhance serum and mucosal antibody responses. In this chapter, we briefly review the methods for recombinant protein expression in plants, and describe progress with human and animal vaccines that use mucosal delivery routes. We do not attempt to compile a comprehensive list, but focus on studies that progressed to clinical trials or those that showed strong indications of efficacy in animals. Finally, we discuss some regulatory concerns regarding plant-based vaccines.

Immunogenicity in humans of a recombinant bacterial antigen delivered in a transgenic potato.

Compared with vaccine delivery by injection, oral vaccines offer the hope of more convenient immunization strategies and a more practical means of implementing universal vaccination programs throughout the world. Oral vaccines act by stimulating the immune system at effector sites (lymphoid tissue) located in the gut. Genetic engineering has been used with variable success to design living and non-living systems as a means to deliver antigens to these sites and to stimulate a desired immune response. More recently, plant biotechnology techniques have been used to create plants which contain a gene derived from a human pathogen; the resultant plant tissues will accumulate an antigenic protein encoded by the foreign DNA. In pre-clinical trials, we found that antigenic proteins produced in transgenic plants retained immunogenic properties when purified; if injected into mice the antigen caused production of protein-specific antibodies. Moreover, in some experiments, if the plant tissues were simply fed to mice, a mucosal immune response occurred. The present study was conducted as a proof of principle to determine if humans would also develop a serum and/or mucosal immune response to an antigen delivered in an uncooked foodstuff.

Light-regulated translation of chloroplast proteins. I. Transcripts of psaA-psaB, psbA, and rbcL are associated with polysomes in dark-grown and illuminated barley seedlings.

We have previously observed (Klein, R. R., and J. E. Mullet, 1986, J. Biol. Chem. 261:11138-11145) that translation of two 65-70-kD chlorophyll a-apoproteins of Photosystem I (gene products of psaA and psaB) and a 32-kD quinone-binding protein of Photosystem II (gene product of psbA) was not detected in plastids of dark-grown barley seedlings even though transcripts for these proteins were present. In the present study it was found that nearly all of the psaA-psaB transcripts in plastids of dark-grown plants were associated with membrane-bound polysomes. Membrane-associated polysomes from plastids of dark-grown plants synthesized the 65-70-kD chlorophyll a-apoproteins at low levels when added to a homologous in vitro translation extract capable of translation elongation. However, when etioplast membranes were disrupted with detergent, in vitro synthesis of the 65-70-kD chlorophyll a-apoproteins increased to levels observed with polysomes of plastids from illuminated plants. These results suggest that synthesis of the chlorophyll a-apoproteins of Photosystem I is arrested on membrane-bound polysomes at the level of polypeptide chain elongation. In addition to the selective activation of chlorophyll a-apoprotein translation, illumination also caused an increase in chloroplast polysomes (membrane-associated and stromal) and induced a recruitment of psbA and rbcL transcripts into chloroplast polysomes. These results indicate that in conjunction with the selective activation of chlorophyll a-apoprotein elongation, illumination also caused a general stimulation of chloroplast translation initiation.

Oral immunization with a recombinant bacterial antigen produced in transgenic plants.

The binding subunit of Escherichia coli heat-labile enterotoxin (LT-B) is a highly active oral immunogen. Transgenic tobacco and potato plants were made with the use of genes encoding LT-B or an LT-B fusion protein with a microsomal retention sequence. The plants expressed the foreign peptides, both of which formed oligomers that bound the natural ligand. Mice immunized by gavage produced serum and gut mucosal anti-LT-B immunoglobulins that neutralized the enterotoxin in cell protection assays. Feeding mice fresh transgenic potato tubers also caused oral immunization.

Plant-derived recombinant F1, V, and F1-V fusion antigens of Yersinia pestis activate human cells of the innate and adaptive immune system.

Plague is still endemic in different regions of the world. Current vaccines raise concern for their side effects and limited protection, highlighting the need for an efficacious and rapidly producible vaccine. F1 and V antigens of Yersinia pestis, and F1-V fusion protein produced in Nicotiana benthamiana administered to guinea pigs resulted in immunity and protection against an aerosol challenge of virulent Y. pestis. We examined the effects of plant-derived F1, V, and F1-V on human cells of the innate immunity. F1, V, and F1-V proteins engaged TLR2 signalling and activated IL-6 and CXCL-8 production by monocytes, without affecting the expression of TNF-alpha, IL-12, IL-10, IL-1beta, and CXCL10. Native F1 antigen and recombinant plant-derived F1 (rF1) and rF1-V all induced similar specific T-cell responses, as shown by their recognition by T-cells from subjects who recovered from Y. pestis infection. Native F1 and rF1 were equally well recognized by serum antibodies of Y. pestis-primed donors, whereas serological reactivity to rF1-V hybrid was lower, and that to rV was virtually absent. In conclusion, plant-derived F1, V, and F1-V antigens are weakly reactogenic for human monocytes and elicit cell-mediated and humoral responses similar to those raised by Y. pestis infection.

Production of hepatitis B surface antigen in transgenic plants for oral immunization.

Here we present data showing oral immunogenicity of recombinant hepatitis B surface antigen (HBsAg) in preclinical animal trials. Mice fed transgenic HBsAg potato tubers showed a primary immune response (increases in HBsAg-specific serum antibody) that could be greatly boosted by intraperitoneal delivery of a single subimmunogenic dose of commercial HBsAg vaccine, indicating that plants expressing HBsAg in edible tissues may be a new means for oral hepatitis B immunization. However, attainment of such a goal will require higher HBsAg expression than was observed for the potatoes used in this study. We conducted a systematic analysis of factors influencing the accumulation of HBsAg in transgenic potato, including 5' and 3' flanking elements and protein targeting within plant cells. The most striking improvements resulted from (1) alternative polyadenylation signals, and (2) fusion proteins containing targeting signals designed to enhance integration or retention of HBsAg in the endoplasmic reticulum (ER) of plant cells.

Hemoprotein quantitation in isolated hepatocytes.

Methods for quantitation of catalase, cytochromes P-450 and b5 and mitochondrial cytochromes a + a3, b561 + b566, and c + c1 in isolated hepatocytes were developed in analogy to methods established for subcellular systems and were used to measure changes in specific hemoprotein concentrations due to pretreatment and to change in incubation conditions. Pretreatment of rats with phenobarbital or 3-methylcholanthrene resulted in increased concentrations of cytochromes P-450 and b5 on a cellular basis, but had no effect on the other hemoproteins. Chronic ethanol pretreatment resulted in increased cytochrome P-450 and decreased cytochromes a + a3 concentrations. Hemoprotein concentrations in hepatocytes decreased following 4-10-h incubations in rotating round-bottom flasks. Rates of decrease were dependent upon both incubation conditions and prior in vivo treatments with phenobarbital or 3-methylcholanthrene.

Absorption and circular dichroism spectra of different forms of mushroom tyrosinase.

The circular dichroism spectrum of resting mushroom tyrosinase between 800 and 400 nm showed two bands at 755, and 653 nm. The CD spectrum of resting tyrosinase between 400 and 250 nm showed oxygen-sensitive changes at 350 nm upon treatment of tyrosinase with hydroxylamine or hydrogen peroxide. These were similar to changes observed on regeneration of aged hemocyanin by similar procedures. A structural relationship between the active sites of hydroxylamine- or hydrogen peroxide-treated tyrosinase and hemocyanin is suggested by these observations, confirming inferences based upon other studies (Jolly, Jr., R.L., Evans, L.H., Makino, N. and Mason, H.S. (1974) J. Biol. Chem. 249, 335-345 and Schoot Uiterkamp, A.J.M. and Mason, H.S. (1973) Proc. Natl. Acad, Sci. U.S. 70, 993-996).

Transgenic plants as vaccine production systems.

Transgenic plants that express foreign proteins with industrial or pharmaceutical value represent an economical alternative to fermentation-based production systems. Specific vaccines have been produced in plants as a result of the transient or stable expression of foreign genes. It has recently been shown that genes encoding antigens of bacterial and viral pathogens can be expressed in plants in a form in which they retain native immunogenic properties. Transgenic potato tubers expressing a bacterial antigen stimulated humoral and mucosal immune responses when they were provided as food. These results provide 'proof of concept' for the use of plants as a vehicle to produce vaccines.

A review of oral vaccination with transgenic vegetables.

Mucosal immunization of the gastrointestinal tract is an effective way to stimulate local and systemic immune responses. Oral vaccines must be formulated in such a way that antigens are protected as they pass through the adverse environment of the stomach and are delivered to the mucosal inductive sites. Vaccine antigens cloned into edible transgenic plants are a promising new delivery system for oral vaccines. Such vaccines could be safe, inexpensive, and multicomponent.

Process options in hepatitis B surface antigen extraction from transgenic potato.

The process conditions for recombinant hepatitis B surface antigen (HBsAg) extraction from transgenic potato were examined. The effects of temperature, the reducing agent beta-mercaptoethanol (BME), and proteinase inhibitors on the level of antigenic activity of recovered HBsAg were determined. Sedimentation profiles were performed to characterize HBsAg assembly into virus-like particles. Increasing the temperature of the sample for about 1 min increased the measured HBsAg antigenic activity. The optimum temperature was around 50 degrees C. A 3-fold enhancement of the antigenic activity was obtained in extract from transgenic potato expressing HBsAg, when monoclonal antibodies were used to assay for HBsAg. When antigenic activity was determined by polyclonal antibodies, no enhancement in the antigenic activity was obtained. Temperature may affect the conformation of the a epitope to which the monoclonal antibodies bind or alter the fluidity of surface lipid regions. BME increased the antigenic activity of HBsAg up to 4-fold when monoclonal antibodies directed against the a determinant were used, but there was no increase with polyclonal antibodies. This observation suggests that BME affects the structure or presentation of the a epitope. In the presence of BME and leupeptin, a proteinase inhibitor, higher antigenic activity was obtained. Leupeptin might protect the antigen, which might become more susceptible to proteolytic degradation after reduction, as a result of stimulation of sulfhydryl proteases. Although both temperature and BME increased the antigenic activity of HBsAg individually, when combined their interaction was antagonistic, resulting in reduced antigenic activity. Different proteinase inhibitors, including leupeptin, aprotinin, E-64, pefabloc, and pepstatin, had no significant effect on HBsAg from potato extract in a 2 h period in the absence of BME. The sedimentation profile of potato-produced HBsAg was determined in 5-30% sucrose gradients. Yeast-derived recombinant HBsAg was used as a positive control. The HBsAg from transgenic potato showed sedimentation and density properties that are very similar to the yeast-produced antigen, indicating assembly into virus-like particles. BME treatment did not change the sedimentation profile.

Binuclear copper clusters as active sites for oxidases.

Binuclear cupric ion clusters have been established in: human ceruloplasmin, hemocyanin, and mushroom tyrosinase. Substantial evidence makes it very probable that fungal laccase and zucchini ascorbate oxidase contain this cluster. Some evidence makes it possible that copper clusters function in the catalytic cycles of cytochrome oxidase (mammalian) and dopamine-beta-hydroxylase. These studies throw light on the criteria which must be employed to establish the existence of functional binuclear copper clusters in enzymes: (1) Stoichiometric Criteria: binding of O2 and CO with Cu/ligand = 2; redox titrations with n = 2; (2) Physical and Chemical Criteria: magnetic evidence of diminished paramagnetism of cupric centers, EPR evidence of broadened or absent absorptions, EPR evidence of magnetic dipolar interactions among cupric ions; absorption bands characteristic of Cu(II)-Cu(II) complexes; laser resonance raman scattering characteristic of peroxidic dioxygen in the oxyforms.

Laparoscopic drainage of giant lymphocele after renal transplantation.

Lymphocele is a relatively frequent complication of kidney transplantation. A 46-year-old man presented 2 years after kidney transplantation with a giant septated lymphocele. The patient underwent successful laparoscopic drainage of the collection and was discharged home on the day of the procedure. Laparoscopic drainage is a safe and effective treatment for complex lymphocele after kidney transplantation.

Expression of two soybean vegetative storage protein genes during development and in response to water deficit, wounding, and jasmonic acid.

The expression of vspA and vspB genes encoding soybean vegetative storage proteins was studied during seedling development and in response to water deficit, tissue wounding, and jasmonic acid treatment. vspA and vspB encode VSP-alpha and VSP-beta, 28-kilodalton and 31-kilodalton vacuole-localized polypeptides that are 80% homologous. vspA and vspB mRNAs could be distinguished on RNA blots using 3'-end probes. vspA mRNA was threefold to sevenfold more abundant than vspB mRNA in leaves, about equal expression was observed in stems, and vspB mRNA exceeded vspA in roots. Transcripts were not detected in dry seeds but appeared in intact or excised seedling axes between 12 hr and 24 hr after initiation of imbibition. Both transcripts were highly abundant in the meristematic region of seedling stems and in developing leaves but were rare in mature stems, leaves, and roots. In situ localization showed that vsp transcripts were found throughout the hypocotyl hook but were concentrated in cells associated with the epidermis and vascular bundles. Water deficit caused increased vsp mRNA levels in leaves and stems, which suggests that inhibition of growth necessitates temporary storage of amino acids. Wounding induced primarily vspB mRNA in etiolated seedlings, whereas both vspA and vspB mRNA levels increased in wounded leaves. Jasmonic acid and methyl jasmonate were potent inducers of vsp gene expression in cell cultures, developing axes, leaves, and roots. We hypothesize that jasmonic acid levels modulate vsp mRNA abundance in vivo.

Magnetic dipole-dipole coupled Cu(II) pairs in nitric oxide-treated tyrosinase: a structural relationship between the active sites of tyrosinase and hemocyanin.

The T(r) and T[unk] states of tyrosinase were treated with NO. EPR spectra of the products observed at 14 degrees K and at 113 degrees K showed mixtures of two signals. One had components in the region of g = 2, about 1200 G wide, and in the region of g = 4, showing hyperfine splitting. The other signal was similar to that arising from isolated Cu(II) ions in an axially symmetric environment. The first signal was indicative of Deltam = 1 and Deltam = 2 transitions arising from magnetic dipole-dipole coupled Cu(II) ion pairs. It closely resembled previously reported EPR spectra obtained from NO-treated hemocyanin, which were confirmed in this study. The normal Curie behavior of the signals between 230 degrees K and 14 degrees K ruled out significant exchange coupling between the ion pairs. The Deltam = 2 signals were not saturable up to 350 mW at 14 degrees K. The broad Deltam = 1 signals could be separated from accompanying signals by the saturation characteristics of the latter at about 10 mW at 14 degrees K. The results establish the presence of a pair of copper ions at the active site of tyrosinase, and a clsoe structural relationship between this active site and that of hemocyanin.