HIV-1 p24-immunoglobulin fusion molecule: a new strategy for plant-based protein production

Improving Protein Quantity and Quality—The Next Level of Plant Molecular Farming

Plants offer several unique advantages in the production of recombinant pharmaceuticals for humans and animals. Although numerous recombinant proteins have been expressed in plants, only a small fraction have been successfully put into use. The hugely distinct expression systems between plant and animal cells frequently cause insufficient yield of the recombinant proteins with poor or undesired activity. To overcome the issues that greatly constrain the development of plant-produced pharmaceuticals, great efforts have been made to improve expression systems and develop alternative strategies to increase both the quantity and quality of the recombinant proteins. Recent technological revolutions, such as targeted genome editing, deconstructed vectors, virus-like particles, and humanized glycosylation, have led to great advances in plant molecular farming to meet the industrial manufacturing and clinical application standards. In this review, we discuss the technological advances made in various plant expression platforms, with special focus on the upstream designs and milestone achievements in improving the yield and glycosylation of the plant-produced pharmaceutical proteins.

Optimization of the human colorectal carcinoma antigen GA733-2 production in tobacco plants

The colorectal carcinoma-associated protein GA733-2 is one of the representative candidate protein for the development of plant-derived colorectal cancer vaccine. Despite of its significant importance for colorectal vaccine development, low efficiency of GA733-2 production limits its wide applications. To improve productivity of GA733-2 in plants, we here tested multiple factors that affect expression of recombinant GA733-2 (rGA733-2) and rGA733 fused to fragment crystallizable (Fc) domain (rGA733-Fc) protein. The rGA733-2 and rGA733-Fc proteins were highly expressed when the pBINPLUS vector system was used for transient expression in tobacco plants. In addition, the length of interval between rGA733-2 and left border of T-DNA affected the expression of rGA733 protein. Transient expression analysis using various combinations of Agrobacterium tumefaciens strains (C58C1, LBA4404, and GV3101) and tobacco species (Nicotiana tabacum cv. Xanthi nc and Nicotiana benthamiana) revealed that higher accumulation of rGA733-2 and rGA733-Fc proteins were obtained by combination of A. tumefaciens LBA4404 and Nicotiana benthamiana. Transgenic plants generated by introduction of the rGA733-2 and rGA733-Fc expression cassettes also significantly accumulated corresponding recombinant proteins. Bioactivity and stability of the plant-derived rGA733 and rGA733-Fc were evaluated by further in vitro assay, western blot and N-glycosylation analysis. Collectively, we here suggest the optimal condition for efficient production of functional rGA733-2 protein in tobacco system.

Impact of mycolic acid deficiency on cells of Corynebacterium glutamicum ATCC13869

Mycolic acid (MA) plays important role in Corynebacterium glutamicum, but the key enzymes in the biosynthetic pathway of MA in C. glutamicum ATCC13869 have not been characterized. Since the locus BBD29_RS14045 in C. glutamicum ATCC13869 shows high similarity to the gene Cgl2871, which encodes Pks13, the key enzyme for synthesizing MA in C. glutamicum ATCC13032, it was deleted, resulting in the mutant WG001. Compared with the wild-type ATCC13869, MA was not synthesized in WG001, but more phosphatidylglycerol and phosphatidylinositol containing longer unsaturated fatty acids were produced. WG001 cells also show hindered cell growth and defective cell separation when compared with ATCC13869 cells. Transcriptomic analysis shows that many genes relevant to the pathways of fatty acids, inositol, phospholipids, cell wall, and cell division were significantly regulated in WG001 cells when compared with ATCC13869 cells. This study demonstrates that the locus BBD29_RS14045 encodes a key enzyme that plays important role for synthesizing MA in C. glutamicum ATCC13869.

Recombinant Plant Engineering for Immunotherapeutic Production

PURPOSE OF REVIEW

The requirement for large quantities of therapeutic proteins has fueled a great interest in the production of recombinant proteins in plant bioreactors. The vaccines and bio-therapeutic protein production in plants hold the promise of significantly lowering the cost of manufacturing life-saving drugs. This review will reflect the current status and challenges that the molecular farming platform faces becoming a strategic solution for the development of low-cost bio-therapeutics for developing countries.

RECENT FINDINGS

Different plant parts have been successfully identified as suitable expression systems for the commercial production of therapeutic proteins for some human and animal diseases ranging from common cold to AIDS. The processed therapeutics from such sources are devoid of any toxic components. The large-scale cultivation of these transgenic plants would be possible anywhere in the world including developing countries, which lack sophisticated drug manufacturing units. A couple of such commercially generated products have already hit the market with success. Newer methods using suitable plant viruses and recombinant gene expression systems have already been devised for producing therapeutic proteins and peptides.

SUMMARY

Plants are promising bio-factories for therapeutic protein production because of their several advantages over the other expression systems especially the advanced mechanisms for protein synthesis and post-translational modification which are very much similar to animal cells. Plant biotechnologists are much attracted to the bio-farming because of its flexibility, scalability, low manufacturing cost, as well as the lack of risk of toxic or pathogenic contamination. A number of projects on bio-farming are designed and are at various developmental stages but have not yet become available to the pharmaceutical industry. Therefore, we need further advancement in the optimization of lab protocols for up-scaling the production of such therapeutics at commercial level with a promise to offer their best clinical use.

Site-targeted mutagenesis for stabilization of recombinant monoclonal antibody expressed in tobacco (Nicotiana tabacum) plants

This study examined the degradation pattern of a murine IgG1κ monoclonal antibody expressed in and extracted from transformedNicotiana tabacum Gel electrophoresis of leaf extracts revealed a consistent pattern of recombinant immunoglobulin bands, including intact and full-length antibody, as well as smaller antibody fragments. N-terminal sequencing revealed these smaller fragments to be proteolytic cleavage products and identified a limited number of protease-sensitive sites in the antibody light and heavy chain sequences. No strictly conserved target sequence was evident, although the peptide bonds that were susceptible to proteolysis were predominantly and consistently located within or near to the interdomain or solvent-exposed regions in the antibody structure. Amino acids surrounding identified cleavage sites were mutated in an attempt to increase resistance. Different Guy's 13 antibody heavy and light chain mutant combinations were expressed transiently inN. tabacumand demonstrated intensity shifts in the fragmentation pattern, resulting in alterations to the full-length antibody-to-fragment ratio. The work strengthens the understanding of proteolytic cleavage of antibodies expressed in plants and presents a novel approach to stabilize full-length antibody by site-directed mutagenesis.-Hehle, V. K., Paul, M. J., Roberts, V. A., van Dolleweerd, C. J., Ma, J. K.-C. Site-targeted mutagenesis for stabilization of recombinant monoclonal antibody expressed in tobacco (Nicotiana tabacum) plants.

Efficient expression of nuclear transgenes in the green alga Chlamydomonas: synthesis of an HIV antigen and development of a new selectable marker

The unicellular green alga Chlamydomonas reinhardtii has become an invaluable model system in plant biology. There is also considerable interest in developing this microalga into an efficient production platform for biofuels, pharmaceuticals, green chemicals and industrial enzymes. However, the production of foreign proteins in the nucleocytosolic compartment of Chlamydomonas is greatly hampered by the inefficiency of transgene expression from the nuclear genome. We have recently addressed this limitation by isolating mutant algal strains that permit high-level transgene expression and by determining the contributions of GC content and codon usage to gene expression efficiency. Here we have applied these new tools and explored the potential of Chlamydomonas to produce a recombinant biopharmaceutical, the HIV antigen P24. We show that a codon-optimized P24 gene variant introduced into our algal expression strains give rise to recombinant protein accumulation levels of up to 0.25% of the total cellular protein. Moreover, in combination with an expression strain, a resynthesized nptII gene becomes a highly efficient selectable marker gene that facilitates the selection of transgenic algal clones at high frequency. By establishing simple principles of successful transgene expression, our data open up new possibilities for biotechnological research in Chlamydomonas.

A Chimeric Affinity Tag for Efficient Expression and Chromatographic Purification of Heterologous Proteins from Plants

The use of plants as expression hosts for recombinant proteins is an increasingly attractive option for the production of complex and challenging biopharmaceuticals. Tools are needed at present to marry recent developments in high-yielding gene vectors for heterologous expression with routine protein purification techniques. In this study, we designed the Cysta-tag, a new purification tag for immobilized metal affinity chromatography (IMAC) of plant-made proteins based on the protein-stabilizing fusion partner SlCYS8. We show that the Cysta-tag may be used to readily purify proteins under native conditions, and then be removed enzymatically to isolate the protein of interest. We also show that commonly used protease recognition sites for linking purification tags are differentially stable in leaves of the commonly used expression host Nicotiana benthamiana, with those linkers susceptible to cysteine proteases being less stable then serine protease-cleavable linkers. As an example, we describe a Cysta-tag experimental scheme for the one-step purification of a clinically useful protein, human α1-antitrypsin, transiently expressed in N. benthamiana. With potential applicability to the variety of chromatography formats commercially available for IMAC-based protein purification, the Cysta-tag provides a convenient means for the efficient and cost-effective purification of recombinant proteins from plant tissues.

Improvement in the stability and functionality of Nicotiana tabacum produced recombinant TRAIL through employment of endoplasmic reticulum expression and ascorbate buffer mediated extraction strategies

Introduction: In order to employ Nicotiana tabacum cells as a profitable natural bioreactor for production of bio-functional "Soluble human TRAIL" (ShTRAIL), endoplasmic reticulum (ER) targeted expression and innovative extraction procedures were exploited.

Methods: At first, the ShTRAIL encoding gene was sub-cloned into designed H2 helper vector to equip it with potent TMV omega leader sequences, ER sorting signal peptide, poly-histidine tag and ER retention signal peptide (KDEL). Then, the ER targeted ShTRAIL cassette was sequentially sub-cloned into "CaMV-35S" helper and "pGreen-0179" final expression vectors. Afterward, Agrobacterium mediated transformation method was adopted to express the ShTRAIL in the ER of N. tabacum . Next, the ShTRAIL protein was extracted through both phosphate and innovative ascorbate extraction buffers. Subsequently, oligomerization state of the ShTRAIL was evaluated through cross-linking assay and western blot analysis. Then, semi-quantitative western blot analysis was performed to estimate the ShTRAIL production. Finally, biological activity of the ShTRAIL was evaluated through MTT assay.

Results: The phosphate buffer extracted ShTRAIL was produced in dimmer form, whereas the ShTRAIL extracted with ascorbate buffer generated trimer form. The ER targeted ShTRAIL strategy increased the ShTRAIL’s production level up to about 20 μg/g of fresh weight of N. tabacum . MTT assay indicated that ascorbate buffer extracted ShTRAIL could prohibit proliferation of A549 cell line.

Conclusion: Endoplasmic reticulum expression and reductive ascorbate buffer extraction procedure can be employed to enhance the stability and overall production level of bio-functional recombinant ShTRAIL from transgenic N. tabacum cells.

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

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

Boosting in planta production of antigens derived from the porcine reproductive and respiratory syndrome virus (PRRSV) and subsequent evaluation of their immunogenicity

Porcine reproductive and respiratory syndrome (PRRS) is a disease of swine, caused by an arterivirus, the PRRS virus (PRRSV). This virus infects pigs worldwide and causes huge economic losses. Due to genetic drift, current vaccines are losing their power. Adaptable vaccines could provide a solution to this problem. This study aims at producing in planta a set of antigens derived from the PRRSV glycoproteins (GPs) to be included in a subunit vaccine. We selected the GP3, GP4 and GP5 and optimized these for production in an Arabidopsis seed platform by removing transmembrane domains (Tm) and/or adding stabilizing protein domains, such as the green fluorescent protein (GFP) and immunoglobulin (IgG) ‘Fragment crystallizable’ (Fc) chains. Accumulation of the GPs with and without Tm was low, reaching no more than 0.10% of total soluble protein (TSP) in homozygous seed. However, addition of stabilizing domains boosted accumulation up to a maximum of 2.74% of TSP when GFP was used, and albeit less effectively, also the Fc chains of the porcine IgG3 and murine IgG2a increased antigen accumulation, to 0.96% and 1.81% of TSP respectively, while the murine IgG3 Fc chain did not. Antigens with Tm were less susceptible to these manipulations to increase yield. All antigens were produced in the endoplasmic reticulum and accordingly, they carried high-mannose N-glycans. The immunogenicity of several of those antigens was assessed and we show that vaccination with purified antigens did elicit the production of antibodies with virus neutralizing activity in mice but not in pigs.

Tomato cystatin SlCYS8 as a stabilizing fusion partner for human serpin expression in plants

Studies have reported the usefulness of fusion proteins to bolster recombinant protein yields in plants. Here, we assess the potential of tomato SlCYS8, a Cys protease inhibitor of the cystatin protein superfamily, as a stabilizing fusion partner for human alpha-1-antichymotrypsin (α1ACT) targeted to the plant cell secretory pathway. Using the model expression platform Nicotiana benthamiana, we show that the cystatin imparts a strong stabilizing effect when expressed as a translational fusion with α1ACT, allowing impressive accumulation yields of over 2 mg/g of fresh weight tissue for the human serpin, a 25-fold improvement on the yield of α1ACT expressed alone. Natural and synthetic peptide linkers inserted between SlCYS8 and α1ACT have differential effects on protease inhibitory potency of the two protein partners in vitro. They also have a differential impact on the yield of α1ACT, dependent on the extent to which the hybrid protein may remain intact in the plant cell environment. The stabilizing effect of SlCYS8 does not involve Cys protease inhibition and can be partly reproduced in the cytosol, where peptide linkers are less susceptible to degradation. The effect of SlCYS8 on α1ACT yields could be explained by: (i) an improved translation of the human protein coding sequence; and/or (ii) an overall stabilization of its tertiary structure preventing proteolytic degradation and/or polymerization. These findings suggest the potential of plant cystatins as stabilizing fusion partners for recombinant proteins in plant systems. They also underline the need for an empirical assessment of peptide linker functions in plant cell environments.

Fusion of an Fc chain to a VHH boosts the accumulation levels in Arabidopsis seeds

Nanobodies® (VHHs) provide powerful tools in therapeutic and biotechnological applications. Nevertheless, for some applications, bivalent antibodies perform much better, and for this, an Fc chain can be fused to the VHH domain, resulting in a bivalent homodimeric VHH-Fc complex. However, the production of bivalent antibodies in Escherichia coli is rather inefficient. Therefore, we compared the production of VHH7 and VHH7-Fc as antibodies of interest in Arabidopsis seeds for detecting prostate-specific antigen (PSA), a well-known biomarker for prostate cancer in the early stages of tumour development. The influence of the signal sequence (camel versus plant) and that of the Fc chain origin (human, mouse or pig) were evaluated. The accumulation levels of VHHs were very low, with a maximum of 0.13% VHH of total soluble protein (TSP) in homozygous T3 seeds, while VHH-Fc accumulation levels were at least 10- to 100-fold higher, with a maximum of 16.25% VHH-Fc of TSP. Both the camel and plant signal peptides were efficiently cleaved off and did not affect the accumulation levels. However, the Fc chain origin strongly affected the degree of proteolysis, but only had a slight influence on the accumulation level. Analysis of the mRNA levels suggested that the low amount of VHHs produced in Arabidopsis seeds was not due to a failure in transcription, but rather to translation inefficiency, protein instability and/or degradation. Most importantly, the plant-produced VHH7 and VHH7-Fc antibodies were functional in detecting PSA and could thus be used for diagnostic applications.

Plant-produced human recombinant erythropoietic growth factors support erythroid differentiation in vitro

Clinically available red blood cells (RBCs) for transfusions are at high demand, but in vitro generation of RBCs from hematopoietic stem cells requires significant quantities of growth factors. Here, we describe the production of four human growth factors: erythropoietin (EPO), stem cell factor (SCF), interleukin 3 (IL-3), and insulin-like growth factor-1 (IGF-1), either as non-fused proteins or as fusions with a carrier molecule (lichenase), in plants, using a Tobacco mosaic virus vector-based transient expression system. All growth factors were purified and their identity was confirmed by western blotting and peptide mapping. The potency of these plant-produced cytokines was assessed using TF1 cell (responsive to EPO, IL-3 and SCF) or MCF-7 cell (responsive to IGF-1) proliferation assays. The biological activity estimated here for the cytokines produced in plants was slightly lower or within the range cited in commercial sources and published literature. By comparing EC50 values of plant-produced cytokines with standards, we have demonstrated that all four plant-produced growth factors stimulated the expansion of umbilical cord blood-derived CD34+ cells and their differentiation toward erythropoietic precursors with the same potency as commercially available growth factors. To the best of our knowledge, this is the first report on the generation of all key bioactive cytokines required for the erythroid development in a cost-effective manner using a plant-based expression system.

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

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

Comparison of membrane targeting strategies for the accumulation of the human immunodeficiency virus p24 protein in transgenic tobacco

Membrane anchorage was tested as a strategy to accumulate recombinant proteins in transgenic plants. Transmembrane domains of different lengths and topology were fused to the cytosolic HIV antigen p24, to promote endoplasmic reticulum (ER) residence or traffic to distal compartments of the secretory pathway in transgenic tobacco. Fusions to a domain of the maize seed storage protein γ-zein were also expressed, as a reference strategy that leads to very high stability via the formation of large polymers in the ER lumen. Although all the membrane anchored constructs were less stable compared to the zein fusions, residence at the ER membrane either as a type I fusion (where the p24 sequence is luminal) or a tail-anchored fusion (where the p24 sequence is cytosolic) resulted in much higher stability than delivery to the plasma membrane or intermediate traffic compartments. Delivery to the tonoplast was never observed. The inclusion of a thrombin cleavage site allowed for the quantitative in vitro recovery of p24 from all constructs. These results point to the ER as suitable compartment for the accumulation of membrane-anchored recombinant proteins in plants.

Dual targeting of a mature plastoglobulin/fibrillin fusion protein to chloroplast plastoglobules and thylakoids in transplastomic tobacco plants

Plastoglobules (PG) are lipid droplets in chloroplasts and other plastid types having important functions in lipid metabolism. Plastoglobulins (PGL) also known as fibrillins (FBN) are evolutionary conserved proteins present at the PG surface but also to various extents at the thylakoid membrane. PGLs are thought to have structural functions in PG formation and maintenance. The targeting of an Arabidopsis PGL (PGL34) to PG required the full protein sequence with the exception of a short C-terminal stretch. This indicated that PGL targeting relies on correct folding rather than a discrete sequence. PGLs lack strongly hydrophic regions and may therefore extrinsically associate with PG and thylakoid membranes via interaction with hydrophilic headgroups of surface lipids. Here, we report on the expression of the Arabidopsis plastoglobulin of 35kD (PGL35 or FBN1a) expressed as a mature protein fused to HIVp24 (human immunodeficiency virus capsid particle p24) or HCV (hepatitis C virus core protein) in transplastomic tobacco. A PGL35-HIVp24 fusion targeted in part to plastoglobules but a larger proportion was recovered in the thylakoid fraction. The findings indicate that transplastomic PGL35-HIVp24 folded correctly after its synthesis inside the chloroplast and then dually targeted to plastoglobules as well as thylakoid membranes.

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

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

Overproduction of recombinant proteins in plants

Recombinant protein production in microbial hosts and animal cell cultures has revolutionized the pharmaceutical and industrial enzyme industries. Plants as alternative hosts for the production of recombinant proteins are being actively pursued, taking advantage of their unique characteristics. The key to cost-efficient production in any system is the level of protein accumulation, which is inversely proportional to the cost. Levels of up to 5 g/kg biomass have been obtained in plants, making this production system competitive with microbial hosts. Increasing protein accumulation at the cellular level by varying host, germplasm, location of protein accumulation, and transformation procedure is reviewed. At the molecular level increased expression by improving transcription, translation and accumulation of the protein is critically evaluated. The greatest increases in protein accumulation will occur when various optimized parameters are more fully integrated with each other. Because of the complex nature of plants, this will take more time and effort to accomplish than has been the case for the simpler unicellular systems. However the potential for plants to become one of the major avenues for protein production appears very promising.

Expression of the nucleocapsid protein of porcine reproductive and respiratory syndrome virus in soybean seed yields an immunogenic antigenic protein

Porcine reproductive and respiratory syndrome (PRRS), caused by the PRRS virus (PRRSV), is a serious disease of swine and contributes to severe worldwide economic losses in swine production. Current vaccines against PRRS rely on the use of an attenuated-live virus; however, these are unreliable. Thus, alternative effective vaccines against PRRS are needed. Plant-based subunit vaccines offer viable, safe, and environmentally friendly alternatives to conventional vaccines. In this study, efforts have been undertaken to develop a soybean-based vaccine against PRRSV. A construct carrying a synthesized PRRSV-ORF7 antigen, nucleocapsid N protein of PRRSV, has been introduced into soybean, Glycine max (L.) Merrill. cvs. Jack and Kunitz, using Agrobacterium-mediated transformation. Transgenic plants carrying the sORF7 transgene have been successfully generated. Molecular analyses of T(0) plants confirmed integration of the transgene and transcription of the PRRSV-ORF7. Presence of a 15-kDa protein in seeds of T(1) transgenic lines was confirmed by Western blot analysis using PRRSV-ORF7 antisera. The amount of the antigenic protein accumulating in seeds of these transgenic lines was up to 0.65% of the total soluble protein (TSP). A significant induction of a specific immune response, both humoral and mucosal, against PRRSV-ORF7 was observed following intragastric immunization of BALB/c female mice with transgenic soybean seeds. These findings provide a 'proof of concept', and serve as a critical step in the development of a subunit plant-based vaccine against PRRS.

The extradomain A of fibronectin (EDA) combined with poly(I:C) enhances the immune response to HIV-1 p24 protein and the protection against recombinant Listeria monocytogenes-Gag infection in the mouse model

The development of effective vaccines against HIV-1 infection constitutes one of the major challenges in viral immunology. One of the protein candidates in vaccination against this virus is p24, since it is a conserved HIV antigen that has cytotoxic and helper T cell epitopes as well as B cell epitopes that may jointly confer enhanced protection against infection when used in immunization-challenge approaches. In this context, the adjuvant effect of EDA (used as EDAp24 fusion protein) and poly(I:C), as agonists of TLR4 and TLR3, respectively, was assessed in p24 immunizations using a recombinant Listeria monocytogenes HIV-1 Gag proteins (Lm-Gag, where p24 is the major antigen) for challenge in mice. Immunization with EDAp24 fusion protein together with poly(I:C) adjuvant induced a specific p24 IFN-γ production (Th1 profile) as well as protection against a Lm-Gag challenge, suggesting an additive or synergistic effect between both adjuvants. The combination of EDA (as a fusion protein with the antigen, which may favor antigen targeting to dendritic cells through TLR4) and poly(I:C) could thus be a good adjuvant candidate to enhance the immune response against HIV-1 proteins and its use may open new ways in vaccine investigations on this virus.

A protease activity-depleted environment for heterologous proteins migrating towards the leaf cell apoplast

Recombinant proteins face major constraints along the plant cell secretory pathway, including proteolytic processing compromising their structural integrity. Here, we demonstrate the potential of protease inhibitors as in situ stabilizing agents for recombinant proteins migrating towards the leaf apoplast. Genomic data for Arabidopsis, rice and Nicotiana spp. were assessed to determine the relative incidence of protease families in the cell secretory pathway. Transient expression assays with the model platform Nicotiana benthamiana were then performed to test the efficiency of protease inhibitors in stabilizing proteins targeted to the apoplast. Current genomic data suggest the occurrence of proteases from several families along the secretory pathway, including A1 and A22 Asp proteases; C1A and C13 Cys proteases; and S1, S8 and S10 Ser proteases. In vitro protease assays confirmed the presence of various proteases in N. benthamiana leaves, notably pointing to the deposition of A1- and S1-type activities preferentially in the apoplast. Accordingly, transient expression and secretion of the A1/S1 protease inhibitor, tomato cathepsin D inhibitor (SlCDI), negatively altered A1 and S1 protease activities in this cell compartment, while increasing the leaf apoplast protein content by ∼45% and improving the accumulation of a murine diagnostic antibody, C5-1, co-secreted in the apoplast. SlCYS9, an inhibitor of C1A and C13 Cys proteases, had no impact on the apoplast proteases and protein content, but stabilized C5-1 in planta, presumably upstream in the secretory pathway. These data confirm, overall, the potential of protease inhibitors for the in situ protection of recombinant proteins along the plant cell secretory pathway.

Virus Glycoproteins Tagged with the Human Fc Domain as Second Generation Vaccine Candidates

Traditional vaccines such as inactivated or live attenuated vaccines, are gradually giving way to more biochemically defined vaccines that are most often based on a recombinant antigen known to possess neutralizing epitopes. Such vaccines can offer improvements in speed, safety and manufacturing process but an inevitable consequence of their high degree of purification is that immunogenicity is reduced through the lack of the innate triggering molecules present in more complex preparations. Targeting recombinant vaccines to antigen presenting cells (APCs) such as dendritic cells however can improve immunogenicity by ensuring that antigen processing is as efficient as possible. Immune complexes, one of a number of routes of APC targeting, are mimicked by a recombinant approach, crystallizable fragment (Fc) fusion proteins, in which the target immunogen is linked directly to an antibody effector domain capable of interaction with receptors, FcR, on the APC cell surface. A number of virus Fc fusion proteins have been expressed in insect cells using the baculovirus expression system and shown to be efficiently produced and purified. Their use for immunization next to non-Fc tagged equivalents shows that they are powerfully immunogenic in the absence of added adjuvant and that immune stimulation is the result of the Fc-FcR interaction.

Ubiquitin promoter-terminator cassette promotes genetically stable expression of the taste-modifying protein miraculin in transgenic lettuce

Lettuce is a commercially important leafy vegetable that is cultivated worldwide, and it is also a target crop for plant factories. In this study, lettuce was selected as an alternative platform for recombinant miraculin production because of its fast growth, agronomic value, and wide availability. The taste-modifying protein miraculin is a glycoprotein extracted from the red berries of the West African native shrub Richadella dulcifica. Because of its limited natural availability, many attempts have been made to produce this protein in suitable alternative hosts. We produced transgenic lettuce with miraculin gene driven either by the ubiquitin promoter/terminator cassette from lettuce or a 35S promoter/nos terminator cassette. Miraculin gene expression and miraculin accumulation in both cassettes were compared by quantitative real-time PCR analysis, Western blotting, and enzyme-linked immunosorbent assay. The expression level of the miraculin gene and protein in transgenic lettuce was higher and more genetically stable in the ubiquitin promoter/terminator cassette than in the 35S promoter/nos terminator cassette. These results demonstrated that the ubiquitin promoter/terminator cassette is an efficient platform for the genetically stable expression of the miraculin protein in lettuce and hence this platform is of benefit for recombinant miraculin production on a commercial scale.

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

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

Immunogenicity of chloroplast-derived HIV-1 p24 and a p24-Nef fusion protein following subcutaneous and oral administration in mice

High-level expression of foreign proteins in chloroplasts of transplastomic plants provides excellent opportunities for the development of oral vaccines against a range of debilitating or fatal diseases. The HIV-1 capsid protein p24 and a fusion of p24 with the negative regulatory protein Nef (p24-Nef) accumulate to ∼4% and ∼40% of the total soluble protein of leaves of transplastomic tobacco (Nicotiana tabacum L.) plants. This study has investigated the immunogenicity in mice of these two HIV-1 proteins, using cholera toxin B subunit as an adjuvant. Subcutaneous immunization with purified chloroplast-derived p24 elicited a strong antigen-specific serum IgG response, comparable to that produced by Escherichia coli-derived p24. Oral administration of a partially purified preparation of chloroplast-derived p24-Nef fusion protein, used as a booster after subcutaneous injection with either p24 or Nef, also elicited strong antigen-specific serum IgG responses. Both IgG1 and IgG2a subtypes, associated with cell-mediated Th1 and humoral Th2 responses, respectively, were found in sera after subcutaneous and oral administration. These results indicate that chloroplast-derived HIV-1 p24-Nef is a promising candidate as a component of a subunit vaccine delivered by oral boosting, after subcutaneous priming by injection of p24 and/or Nef.

Expression of a recombinant chimeric protein of hepatitis A virus VP1-Fc using a replicating vector based on Beet curly top virus in tobacco leaves and its immunogenicity in mice

We describe the expression and immunogenicity of a recombinant chimeric protein (HAV VP1-Fc) consisting of human hepatitis A virus VP1 and an Fc antibody fragment using a replicating vector based on Beet curly top virus (BCTV) in Agrobacterium-infiltrated Nicotiana benthamiana leaves. Recombinant HAV VP1-Fc was expressed with a molecular mass of approximately 68 kDa. Recombinant HAV VP1-Fc, purified using Protein A Sepharose affinity chromatography, elicited production of specific IgG antibodies in the serum after intraperitoneal immunization. Following vaccination with recombinant HAV VP1-Fc protein, expressions of IFN-γ and IL-4 were increased in splenocytes at the time of sacrifice. Recombinant VP1-Fc from infiltrated tobacco plants can be used as an effective experimental immunogen for research into vaccine development.

Protein body-inducing fusions for high-level production and purification of recombinant proteins in plants

For the past two decades, therapeutic and industrially important proteins have been expressed in plants with varying levels of success. The two major challenges hindering the economical production of plant-made recombinant proteins include inadequate accumulation levels and the lack of efficient purification methods. To address these limitations, several fusion protein strategies have been recently developed to significantly enhance the production yield of plant-made recombinant proteins, while simultaneously assisting in their subsequent purification. Elastin-like polypeptides are thermally responsive biopolymers composed of a repeating pentapeptide 'VPGXG' sequence that are valuable for the purification of recombinant proteins. Hydrophobins are small fungal proteins capable of altering the hydrophobicity of their respective fusion partner, thus enabling efficient purification by surfactant-based aqueous two-phase systems. Zera, a domain of the maize seed storage protein γ-zein, can induce the formation of protein storage bodies, thus facilitating the recovery of fused proteins using density-based separation methods. These three novel protein fusion systems have also been shown to enhance the accumulation of a range of different recombinant proteins, while concurrently inducing the formation of protein bodies. The packing of these fusion proteins into protein bodies may exclude the recombinant protein from normal physiological turnover. Furthermore, these systems allow for quick, simple and inexpensive nonchromatographic purification of the recombinant protein, which can be scaled up to industrial levels of protein production. This review will focus on the similarities and differences of these artificial storage organelles, their biogenesis and their implication for the production of recombinant proteins in plants and their subsequent purification.

Molecular Pharming: future targets and aspirations

Molecular Pharming represents an unprecedented opportunity to manufacture affordable modern medicines and make these available at a global scale. The area of greatest potential is in the prevention of infectious diseases, particular in underdeveloped countries where access to medicines and vaccines has historically been limited. This is why, at St. George's, we focus on diseases such as HIV, TB and rabies, and aim to develop production strategies that are simple and potentially easy to transfer to developing countries.

Transient expression systems for plant-derived biopharmaceuticals

In the molecular farming area, transient expression approaches for pharmaceutical proteins production, mainly recombinant monoclonal antibodies and vaccines, were developed almost two decades ago and, to date, these systems basically depend on Agrobacterium-mediated delivery and virus expression machinery. We survey here the current state-of-the-art of this research field. Several vectors have been designed on the basis of DNA- and RNA-based plant virus genomes and viral vectors are used both as single- and multicomponent expression systems in different combinations depending on the protein of interest. The obvious advantages of these systems are ease of manipulation, speed, low cost and high yield of proteins. In addition, Agrobacterium-mediated expression also allows the production in plants of complex proteins assembled from subunits. Currently, the transient expression methods are preferential over any other transgenic system for the exploitation of large and unrestricted numbers of plants in a contained environment. By designing optimal constructs and related means of delivery into plant cells, the overall technology plan considers scenarios that envisage high yield of bioproducts and ease in monitoring the whole spectrum of upstream production, before entering good manufacturing practice facilities. In this way, plant-derived bioproducts show promise of high competitiveness towards classical eukaryotic cell factory systems.

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

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

Plant-made vaccines for humans and animals

The concept of using plants to produce high-value pharmaceuticals such as vaccines is 20 years old this year and is only now on the brink of realisation as an established technology. The original reliance on transgenic plants has largely given way to transient expression; proofs of concept for human and animal vaccines and of efficacy for animal vaccines have been established; several plant-produced vaccines have been through Phase I clinical trials in humans and more are scheduled; regulatory requirements are more clear than ever, and more facilities exist for manufacture of clinic-grade materials. The original concept of cheap edible vaccines has given way to a realisation that formulated products are required, which may well be injectable. The technology has proven its worth as a means of cheap, easily scalable production of materials: it now needs to find its niche in competition with established technologies. The realised achievements in the field as well as promising new developments will be reviewed, such as rapid-response vaccines for emerging viruses with pandemic potential and bioterror agents.

Production of recombinant anthrax toxin receptor (ATR/CMG2) fused with human Fc in planta

Mass vaccination against anthrax with existing vaccines is costly and unsafe due to potential side effects. For post-infection treatment, passive immunotherapy measures are currently available, most based on anthrax protective antigen (PA)-specific therapeutic antibodies. Efficient against wild-type strains, these treatment(s) might fail to protect against infections caused by genetically engineered Bacillus anthracis strains. A recent discovery revealed that the von Willebrand factor A (VWA) domain of human capillary morphogenesis protein 2 (CMG2) is an exceptionally effective anthrax toxin receptor (ATR) proficient in helping to resolve this issue. Here we describe in planta production of chimeric recombinant protein (immunoadhesin) comprised of functional ATR domain fused with the human immunoglobulin Fc fragment (pATR-Fc). The fusion design allowed us to obtain pATR-Fc in plant green tissues in a soluble form making it fairly easy to purify by Protein-A chromatography. Standardized pATR-Fc preparations (purity>90%) were shown to efficiently bind anthrax PA as demonstrated by ELISA and Western blot analysis. Recombinant pATR-Fc was also shown to protect J774A1 macrophage cells against the anthrax toxin. This study confirmed that plant-derived pATR-Fc antibody-like protein is a prospective candidate for anthrax immunotherapy.

Manufacturing antibodies in the plant cell

Plants have long been considered advantageous platforms for large-scale production of antibodies due to their low cost, scalability, and the low chances of pathogen contamination. Much effort has therefore been devoted to efficiently producing mAbs (from nanobodies to secretory antibodies) in plant cells. Several technical difficulties have been encountered and are being overcome. Improvements in production levels have been achieved by manipulation of gene expression and, more efficiently, of cell targeting and protein folding and assembly. Differences in mAb glycosylation patterns between animal and plant cells are being successfully addressed by the elimination and introduction of the appropriate enzyme activities in plant cells. Another relevant battlefield is the dichotomy between production capacity and speed. Classically, stably transformed plant lines have been proposed for large scale mAb production, whereas the use of transient expression systems has always provided production speed at the cost of scalability. However, recent advances in transient expression techniques have brought impressive yield improvements, turning speed and scalability into highly compatible assets. In the era of personalized medicines, the combination of yield and speed, and the advances in glyco-engineering have made the plant cell a serious contender in the field of recombinant antibody production.

Hydrophobin fusions for high-level transient protein expression and purification in Nicotiana benthamiana

Insufficient accumulation levels of recombinant proteins in plants and the lack of efficient purification methods for recovering these valuable proteins have hindered the development of plant biotechnology applications. Hydrophobins are small and surface-active proteins derived from filamentous fungi that can be easily purified by a surfactant-based aqueous two-phase system. In this study, the hydrophobin HFBI sequence from Trichoderma reesei was fused to green fluorescent protein (GFP) and transiently expressed in Nicotiana benthamiana plants by Agrobacterium tumefaciens infiltration. The HFBI fusion significantly enhanced the accumulation of GFP, with the concentration of the fusion protein reaching 51% of total soluble protein, while also delaying necrosis of the infiltrated leaves. Furthermore, the endoplasmic reticulum-targeted GFP-HFBI fusion induced the formation of large novel protein bodies. A simple and scalable surfactant-based aqueous two-phase system was optimized to recover the HFBI fusion proteins from leaf extracts. The single-step phase separation was able to selectively recover up to 91% of the GFP-HFBI up to concentrations of 10 mg mL(-1). HFBI fusions increased the expression levels of plant-made recombinant proteins while also providing a simple means for their subsequent purification. This hydrophobin fusion technology, when combined with the speed and posttranslational modification capabilities of plants, enhances the value of transient plant-based expression systems.

A companion protease inhibitor for the protection of cytosol-targeted recombinant proteins in plants

We reported earlier the potential of tomato cathepsin D inhibitor (SlCDI) as an in-built stabilizing agent for the protection of recombinant proteins in transgenic plant leaf crude extracts (Plant Biotechnol J.4, 359-368). Here we document the potential of SlCDI for the in situ protection of proteins in potato leaves. Total protein assays with control and SlCDI-expressing potato lines indicated a positive impact of slcdi transgene expression on leaf protein content, with a mean relative increase of 35%-40% depending on the light regime. Out of approximately 700 proteins detected on 2-D gels, only 20 exhibited a significantly altered level on a protein-specific basis, whereas most proteins were up-regulated on a leaf fresh weight basis, albeit at variable rates. Quantitative reverse trancriptase-PCR assays for rubisco activase showed similar transcript levels in leaves of test and control lines despite protein levels increased by two- to threefold in SlCDI-expressing lines. These observations, along with the unrelated biological functions assigned to MS-identified proteins up-regulated in leaves and protease assays showing slightly increased proteasome activity in protein extracts of SlCDI-expressing lines, suggest a general, proteasome-independent protein stabilizing effect of SlCDI in planta. Transient expression assays with human alpha(1)-antichymotrypsin also showed a stabilizing effect for SlCDI on heterologous proteins, leading to net levels of the human protein increased by approximately 2.5-fold in SlCDI-expressing plants. These data illustrate, overall, the potential of SlCDI as an in vivo protein-stabilizing agent in transgenic plant systems, useful to improve protein levels and recombinant protein accumulation.

High-level HIV-1 Nef transient expression in Nicotiana benthamiana using the P19 gene silencing suppressor protein of Artichoke Mottled Crinckle Virus

BACKGROUND

In recent years, different HIV antigens have been successfully expressed in plants by either stable transformation or transient expression systems. Among HIV proteins, Nef is considered a promising target for the formulation of a multi-component vaccine due to its implication in the first steps of viral infection. Attempts to express Nef as a single protein product (not fused to a stabilizing protein) in transgenic plants resulted in disappointingly low yields (about 0.5% of total soluble protein). In this work we describe a transient expression system based on co-agroinfiltration of plant virus gene silencing suppressor proteins in Nicotiana benthamiana, followed by a two-step affinity purification protocol of plant-derived Nef.

RESULTS

The effect of three gene silencing viral suppressor proteins (P25 of Potato Virus X, P19 of either Artichoke Mottled Crinckle virus and Tomato Bushy Stunt virus) on Nef transient expression yield was evaluated. The P19 protein of Artichoke Mottled Crinckle virus (AMCV-P19) gave the highest expression yield in vacuum co-agroinfiltration experiments reaching 1.3% of total soluble protein, a level almost three times higher than that previously reported in stable transgenic plants. The high yield observed in the co-agroinfiltrated plants was correlated to a remarkable decrease of Nef-specific small interfering RNAs (siRNAs) indicating an effective modulation of RNA silencing mechanisms by AMCV-P19. Interestingly, we also showed that expression levels in top leaves of vacuum co-agroinfiltrated plants were noticeably reduced compared to bottom leaves. Moreover, purification of Nef from agroinfiltrated tissue was achieved by a two-step immobilized metal ion affinity chromatography protocol with yields of 250 ng/g of fresh tissue.

CONCLUSION

We demonstrated that expression level of HIV-1 Nef in plant can be improved using a transient expression system enhanced by the AMCV-P19 gene silencing suppressor protein. Moreover, plant-derived Nef was purified, with enhanced yield, exploiting a two-step purification protocol. These results represent a first step towards the development of a plant-derived HIV vaccine.

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

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

Production and purification of immunologically active core protein p24 from HIV-1 fused to ricin toxin B subunit in E. coli

Background

Gag protein from HIV-1 is a polyprotein of 55 kDa, which, during viral maturation, is cleaved to release matrix p17, core p24 and nucleocapsid proteins. The p24 antigen contains epitopes that prime helper CD4 T-cells, which have been demonstrated to be protective and it can elicit lymphocyte proliferation. Thus, p24 is likely to be an integral part of any multicomponent HIV vaccine. The availability of an optimal adjuvant and carrier to enhance antiviral responses may accelerate the development of a vaccine candidate against HIV. The aim of this study was to investigate the adjuvant-carrier properties of the B ricin subunit (RTB) when fused to p24.

Results

A fusion between ricin toxin B subunit and p24 HIV (RTB/p24) was expressed in E. coli. Affinity chromatography was used for purification of p24 alone and RTB/p24 from cytosolic fractions. Biological activity of RTB/p24 was determined by ELISA and affinity chromatography using the artificial receptor glycoprotein asialofetuin. Both assays have demonstrated that RTB/p24 is able to interact with complex sugars, suggesting that the chimeric protein retains lectin activity. Also, RTB/p24 was demonstrated to be immunologically active in mice. Two weeks after intraperitoneal inoculation with RTB/p24 without an adjuvant, a strong anti-p24 immune response was detected. The levels of the antibodies were comparable to those found in mice immunized with p24 alone in the presence of Freund adjuvant. RTB/p24 inoculated intranasally in mice, also elicited significant immune responses to p24, although the response was not as strong as that obtained in mice immunized with p24 in the presence of the mucosal adjuvant cholera toxin.

Conclusion

In this work, we report the expression in E. coli of HIV-1 p24 fused to the subunit B of ricin toxin. The high levels of antibodies obtained after intranasal and intraperitoneal immunization of mice demonstrate the adjuvant-carrier properties of RTB when conjugated to an HIV structural protein. This is the first report in which a eukaryotic toxin produced in E. coli is employed as an adjuvant to elicit immune responses to p24 HIV core antigen.

Plant recombinant erythropoietin attenuates inflammatory kidney cell injury

Human erythropoietin (EPO) is a pleiotropic cytokine with remarkable tissue-protective activities in addition to its well-established role in red blood cell production. Unfortunately, conventional mammalian cell cultures are unlikely to meet the anticipated market demands for recombinant EPO because of limited capacity and high production costs. Plant expression systems may address these limitations to enable practical, cost-effective delivery of EPO in tissue injury prevention therapeutics. In this study, we produced human EPO in tobacco and demonstrated that plant-derived EPO had tissue-protective activity. Our results indicated that targeting to the endoplasmic reticulum (ER) provided the highest accumulation levels of EPO, with a yield approaching 0.05% of total soluble protein in tobacco leaves. The codon optimization of the human EPO gene for plant expression had no clear advantage; furthermore, the human EPO signal peptide performed better than a tobacco signal peptide. In addition, we found that glycosylation was essential for the stability of plant recombinant EPO, whereas the presence of an elastin-like polypeptide fusion had a limited positive impact on the level of EPO accumulation. Confocal microscopy showed that apoplast and ER-targeted EPO were correctly localized, and N-glycan analysis demonstrated that complex plant glycans existed on apoplast-targeted EPO, but not on ER-targeted EPO. Importantly, plant-derived EPO had enhanced receptor-binding affinity and was able to protect kidney epithelial cells from cytokine-induced death in vitro. These findings demonstrate that tobacco plants may be an attractive alternative for the production of large amounts of biologically active EPO.

Plant-based strategies aimed at expressing HIV antigens and neutralizing antibodies at high levels. Nef as a case study

The first evidence that plants represent a valid, safe and cost-effective alternative to traditional expression systems for large-scale production of antigens and antibodies was described more than 10 years ago. Since then, considerable improvements have been made to increase the yield of plant-produced proteins. These include the use of signal sequences to target proteins to different cellular compartments, plastid transformation to achieve high transgene dosage, codon usage optimization to boost gene expression, and protein fusions to improve recombinant protein stability and accumulation. Thus, several HIV/SIV antigens and neutralizing anti-HIV antibodies have recently been successfully expressed in plants by stable nuclear or plastid transformation, and by transient expression systems based on plant virus vectors or Agrobacterium-mediated infection. The current article gives an overview of plant expressed HIV antigens and antibodies and provides an account of the use of different strategies aimed at increasing the expression of the accessory multifunctional HIV-1 Nef protein in transgenic plants.

Plastid transformation of high-biomass tobacco variety Maryland Mammoth for production of human immunodeficiency virus type 1 (HIV-1) p24 antigen

Chloroplast transformation of the high-biomass tobacco variety Maryland Mammoth has been assessed as a production platform for the human immunodeficiency virus type 1 (HIV-1) p24 antigen. Maryland Mammoth offers the prospect of higher yields of intact functional protein per unit floor area of contained glasshouse per unit time prior to flowering. Two different transformation constructs, pZSJH1p24 (for the insertion of a native p24 cDNA between the rbcL and accD genes) and pZF5 (for the insertion of a chloroplast-codon-optimized p24 gene between trnfM and trnG) were examined for the production of p24. Plants generated with construct pZSJH1p24 exhibited a normal green phenotype, but p24 protein accumulated only in the youngest leaves (up to approximately 350 microg/g fresh weight or approximately 2.5% total soluble protein) and was undetectable in mature leaves. In contrast, some of the plants generated with pZF5 exhibited a yellow phenotype (pZF5-yellow) with detectable p24 accumulation (up to approximately 450 microg/g fresh weight or approximately 4.5% total soluble protein) in all leaves, regardless of age. Total protein in pZF5-yellow leaves was reduced by approximately 40%. The pZF5-yellow phenotype was associated with recombination between native and introduced direct repeat sequences of the rbcL 3' untransformed region in the plastid genome. Chloroplast-expressed p24 was recognized by a conformation-dependent monoclonal antibody to p24, and p24 protein could be purified from pZF5-yellow leaves using a simple procedure, involving ammonium sulphate precipitation and ion-exchange chromatography, without the use of an affinity tag. The purified p24 was shown to be full length with no modifications, such as glycosylation or phosphorylation, using N-terminal sequencing and mass spectrometry.

High-level expression of human immunodeficiency virus antigens from the tobacco and tomato plastid genomes

Transgene expression from the plant's plastid genome represents a promising strategy in molecular farming because of the plastid's potential to accumulate foreign proteins to high levels and the increased biosafety provided by the maternal mode of organelle inheritance. In this article, we explore the potential of transplastomic plants to produce human immunodeficiency virus (HIV) antigens as potential components of an acquired immunodeficiency syndrome (AIDS) vaccine. It is shown that the HIV antigens p24 (the major target of T-cell-mediated immune responses in HIV-positive individuals) and Nef can be expressed to high levels in plastids of tobacco, a non-food crop, and tomato, a food crop with an edible fruit. Optimized p24-Nef fusion gene cassettes trigger antigen protein accumulation to up to approximately 40% of the plant's total protein, demonstrating the great potential of transgenic plastids to produce AIDS vaccine components at low cost and high yield.

Preventing unintended proteolysis in plant protein biofactories

Numerous reports have been published over the last decade assessing the potential of plants as useful hosts for the heterologous expression of clinically useful proteins. Significant progress has been made, in particular, in optimizing transgene transcription and translation in plants, and in elucidating the complex post-translational modifications of proteins typical of the plant cell machinery. In this article, we address the important issue of recombinant protein degradation in plant expression platforms, which directly impacts on the final yield, homogeneity and overall quality of the resulting protein product. Unlike several more stable and structurally less complex pharmaceuticals, recombinant proteins present a natural tendency to structural heterogeneity, resulting in part from the inherent instability of polypeptide chains expressed in heterologous environments. Proteolytic processing, notably, may dramatically alter the structural integrity and overall accumulation of recombinant proteins in plant expression systems, both in planta during expression and ex planta after extraction. In this article, we describe the current strategies proposed to minimize protein hydrolysis in plant protein factories, including organ-specific transgene expression, organelle-specific protein targeting, the grafting of stabilizing protein domains to labile proteins, protein secretion in natural fluids and the co-expression of companion protease inhibitors.

Fluorescent protein fusions to a human immunodeficiency virus monoclonal antibody reveal its intracellular transport through the plant endomembrane system

SUMMARY

In order to further understand the production and intracellular trafficking of pharmaceutical proteins in plants, the light and heavy chains (LC and HC) of the human immunodeficiency virus neutralizing monoclonal antibody 2G12 were fused to fluorescent proteins [Venus and monomeric red fluorescent protein (mRFP)] to enable the visualization of their passage through the plant cell. Co-expression of LC and HC with various markers of the endomembrane system demonstrated that LC fusions were found in mobile punctate structures, which are likely to be pre-vacuolar compartments (PVCs) as a proportion of the LC fusions were found to be located in the vacuole. In addition, apoplast labelling was also observed with a 2G12LC-RFP fusion. The HC fusion expressed alone was found only in the endoplasmic reticulum (ER). When the LC and HC fusions were expressed together, they were found to co-locate to larger punctate structures, which were morphologically distinct from any observed on expression of LC or HC alone. These structures appeared to be in close association with the ER and their labelling partially overlapped with PVC marker fluorescence, but no increase in apoplast labelling was observed. Co-immunoprecipitation data demonstrated that the presence of the fluorescent proteins did not affect the assembly of the antibody, and also showed the association of BiP with the antibody chains. The antigen-binding activity of the Venus-fused 2G12 antibody was confirmed by enzyme-linked immunosorbent assay.