Refining HPV 16 L1 purification from E. coli: reducing endotoxin contaminations and their impact on immunogenicity

Chimeric Human Papillomavirus-16 Virus-like Particles Presenting HIV-1 P18I10 Peptide: Expression, Purification, Bio-Physical Properties and Immunogenicity in BALB/c Mice

Human papillomavirus (HPV) vaccines based on HPV L1 virus-like particles (VLPs) are already licensed but not accessible worldwide. About 38.0 million people were living with HIV in 2020 and there is no HIV vaccine yet. Therefore, safe, effective, and affordable vaccines against both viruses are an urgent need. In this study, the HIV-1 P18I10 CTL peptide from the V3 loop of HIV-1 gp120 glycoprotein was inserted into the HPV16 L1 protein to construct chimeric HPV:HIV (L1:P18I10) VLPs. Instead of the traditional baculovirus expression vector/insect cell (BEVS/IC) system, we established an alternative mammalian 293F cell-based expression system using cost-effective polyethylenimine-mediated transfection for L1:P18I10 protein production. Compared with conventional ultracentrifugation, we optimized a novel chromatographic purification method which could significantly increase L1:P18I10 VLP recovery (~56%). Chimeric L1:P18I10 VLPs purified from both methods were capable of self-assembling to integral particles and shared similar biophysical and morphological properties. After BALB/c mice immunization with 293F cell-derived and chromatography-purified L1:P18I10 VLPs, almost the same titer of anti-L1 IgG (p = 0.6409) was observed as Gardasil anti-HPV vaccine-immunized mice. Significant titers of anti-P18I10 binding antibodies (p < 0.01%) and P18I10-specific IFN-γ secreting splenocytes (p = 0.0002) were detected in L1:P18I10 VLP-immunized mice in comparison with licensed Gardasil-9 HPV vaccine. Furthermore, we demonstrated that insertion of HIV-1 P18I10 peptide into HPV16 L1 capsid protein did not affect the induction in anti-L1 antibodies. All in all, we expected that the mammalian cell expression system and chromatographic purification methods could be time-saving, cost-effective, scalable platforms to engineer bivalent VLP-based vaccines against HPV and HIV-1.

Efficacy of a Novel Multiepitope Vaccine Candidate against Foot-and-Mouth Disease Virus Serotype O and A

Foot-and-mouth disease (FMD) is a highly contagious and economically devastating disease in cloven-hoofed animals. To prevent the spread of FMD virus (FMDV), traditional inactivated vaccines are used to immunize susceptible animals in disease-endemic countries. However, the inactivated FMD vaccine has several limitations, including safety concerns. To overcome these limitations, subunit proteins have been studied as alternative vaccine candidates. In this study, we designed two multiepitope recombinant proteins (OVM and AVM) containing antigenic sites (residue of VP1 132-162 and residue of VP1 192-212) of three topotypes of FMDV serotype O or three topotypes of FMDV serotype A. Each recombinant protein was efficiently expressed in Escherichia coli with high solubility, and the immunogenicity and protective efficacy of the proteins as FMD vaccine candidates were evaluated. The results showed that OVM and AVM emulsified with ISA201 adjuvant induced effective antigen-specific humoral and cell-mediated immune responses and successfully protected mice from O/Jincheon/SKR/2014, O/VET/2013, and A/Malaysia/97 viruses. In addition, intramuscular immunization of pigs with the OVM and AVM emulsified with ISA201 elicited effective levels of neutralizing antibodies to the viruses with homologous epitopes. Importantly, OVM-AVM emulsified with CAvant^®SOE-X adjuvant conferred 100% protection against the O/Jincheon/SKR/2014 virus with homologous residues and 75% protection against A/SKR/GP/2018 with heterologous residues. The results presented in this study suggest that the combination of OVM and AVM protein with an effective adjuvant could yield an effective and safe vaccine candidate for the prevention and control of foot-and-mouth disease. In addition, our results provide a vaccine platform that can safely, cost-efficiently, and rapidly generate protective vaccine candidates against diverse FMDVs.

Inducible expression of human papillomavirus-16 L1 capsomeres in the plastomes of Nicotiana tabacum: Transplastomic plants develop normal flowers and pollen

Human papillomavirus type-16 (HPV-16) is the major HPV type involved in causing cervical cancer among women. The disease burden is high in developing and underdeveloped countries. Previously, the constitutive expression of HPV-16 L1 protein led to male sterility in transplastomic tobacco plants. Here, the HPV-16 L1 gene was expressed in chloroplasts of Nicotiana tabacum under the control of an ethanol-inducible promoter, trans-activated by nucleus-derived signal peptide. Plants containing nuclear component were transformed with transformation vector pEXP-T7-L1 by biolistic gun. The transformation and homoplasmic status of transformed plants was verified by polymerase chain reaction and Southern blotting, respectively. Protein was induced by spraying 5% ethanol for 7 consecutive days. The correct folding of L1 protein was confirmed by antigen-capture ELISA using a conformation-specific antibody. The L1 protein accumulated up to 3 μg/g of fresh plant material. The L1 protein was further purified using affinity chromatography. All transplastomic plants developed normal flowers and produced viable seeds upon self-pollination. Pollens also showed completely normal structure under light microscope and scanning electron microscopy. These data confirm the use of the inducible expression as plant-safe approach for expressing transgenes in plants, especially those genes that cause detrimental effects on plant growth and morphology.

Optimal Secretory Expression of Acetaldehyde Dehydrogenase from Issatchenkia terricola in Bacillus subtilis through a Combined Strategy

Acetaldehyde dehydrogenases are potential enzyme preparations that can be used to detoxify acetaldehyde and other exogenous aldehydes from pharmaceuticals, food, and biofuel production. In this study, we enhanced the expression of acetaldehyde dehydrogenase sourced from Issatchenkia terricola (istALDH) in Bacillus subtilis using a combinatorial strategy for the optimization of signal peptides, promoters, and growth conditions. First, a library of various signal peptides was constructed to identify the optimal signal peptides for efficient istALDH secretion. The signal peptide yqzG achieved the highest extracellular istALDH activity (204.85 ± 3.31 U/mL). Second, the aprE promoter was replaced by a constitutive promoter (i.e., P43) and an inducible promoter (i.e., Pglv), resulting in 12.40% and 19.97% enhanced istALDH, respectively. Furthermore, the tandem promoter P43-Pglv provided a better performance, resulting in 30.96% enhanced istALDH activity. Third, the production of istALDH was optimized by testing one factor at a time. Physical parameters were optimized including the inducer (e.g., maltose) concentrations, incubation temperatures, and inoculation amounts, and the results were 2.0%, 35 °C, and 2.0%, respectively. The optimized medium results were 2.0% glucose, 1.5% peptone, 2.5% yeast extract, 1% NaCl, and 0.5% (NH₄)₂SO₄. The extracellular istALDH activity was 331.19 ± 4.19 U/mL, yielding the highest production reported in the literature to date.

Supramolecular virus-like particles by co-assembly of triblock polypolypeptide and PAMAM dendrimers

Virus-like particles are of special interest as functional delivery vehicles in a variety of fields ranging from nanomedicine to materials science. Controlled formation of virus-like particles relies on manipulating the assembly of the viral coat proteins. Herein, we report a new assembly system based on a triblock polypolypeptide C4-S10-BK12 and -COONa terminated PAMAM dendrimers. The polypolypeptide has a cationic BK12 block with 12 lysines; its binding with anionic PAMAM triggers the folding of the peptide's middle silk-like block and leads to formation of virus-like nanorods, stabilized against aggregation by the long hydrophilic "C" block of the polypeptide. Varying the dendrimer/polypeptide mixing ratio hardly influences the structure and size of the nanorod. However, increasing the dendrimer generation, that is, increasing the dendrimer size results in increased particle length and height, without affecting the width of the nanorod. The branched structure and well-defined size of the dendrimers allows delicate control of the particle size; it is impossible to achieve similar control over assembly of the polypeptide with linear polyelectrolyte as template. In conclusion, we report a novel protein assembling system with properties resembling a viral coat; the findings may therefore be helpful for designing functional virus-like particles like vaccines.

In Vitro Assembly of Virus-Like Particles and Their Applications

Virus-like particles (VLPs) are increasingly used for vaccine development and drug delivery. Assembly of VLPs from purified monomers in a chemically defined reaction is advantageous compared to in vivo assembly, because it avoids encapsidation of host-derived components and enables loading with added cargoes. This review provides an overview of ex cella VLP production methods focusing on capsid protein production, factors that impact the in vitro assembly, and approaches to characterize in vitro VLPs. The uses of in vitro produced VLPs as vaccines and for therapeutic delivery are also reported.

Virus-like Particle Vaccines: A Prospective Panacea Against an Avian Influenza Panzootic

Epizootics of highly pathogenic avian influenza (HPAI) have resulted in the deaths of millions of birds leading to huge financial losses to the poultry industry worldwide. The roles of migratory wild birds in the harbouring, mutation, and transmission of avian influenza viruses (AIVs), and the lack of broad-spectrum prophylactic vaccines present imminent threats of a global panzootic. To prevent this, control measures that include effective AIV surveillance programmes, treatment regimens, and universal vaccines are being developed and analysed for their effectiveness. We reviewed the epidemiology of AIVs with regards to past avian influenza (AI) outbreaks in birds. The AIV surveillance programmes in wild and domestic birds, as well as their roles in AI control were also evaluated. We discussed the limitations of the currently used AI vaccines, which necessitated the development of a universal vaccine. We evaluated the current development of AI vaccines based upon virus-like particles (VLPs), particularly those displaying the matrix-2 ectodomain (M2e) peptide. Finally, we highlighted the prospects of these VLP vaccines as universal vaccines with the potential of preventing an AI panzootic.

Multi-epitope insert modulates solubility-based and chromatographic purification of human papilloma virus 16 L1-based vaccine without inhibiting virus-like particle assembly

The separation of heterogeneous protein mixtures has always been characterized by a trade-off between purity and yield. One way this issue has been addressed in the past is by recombinantly modifying protein to improve separations. Such modifications are mostly employed in the form of tags used specifically for affinity chromatography, though it is also possible to make changes to a protein that will have a sizeable impact on its hydrophobicity and charge/charge distribution. As such, it should also be possible to use protein tags to modulate phase separations and protein-resin binding kinetics when performing ion exchange chromatography. Here, we employed a three-step purification scheme on E. coli expressed, His-tagged, human papilloma virus 16 L1-based recombinant proteins (rHPV 16 L1) that consisted of an inclusion body (IB) wash step, a diethylaminoethyl (DEAE) anion exchange chromatography (AEX) step, and an immobilized metal affinity chromatography (IMAC) polishing step. Purification of the wild type rHPV 16 L1 protein (WT) was characterized by substantial losses during the IB wash but relatively high yield over the DEAE column. In contrast, purification of modified rHPV 16 L1, a chimeric version of the WT protein that had the last 34 amino acids replaced with an MHC class II multi-epitope insert derived from tetanus toxin and diphtheria toxin (WTΔC34-2TEp), was characterized by little to no losses in the IB wash but had a relatively low yield over the DEAE column. Since the fate of these proteins was to be used in vaccine formulations, it is important to note that the modifications made to the WTΔC34-2TEp protein had little to no effect on its ability to assemble into virus-like particles (VLPs). These results demonstrate that modifications of the WT protein via the recombinant insertion of immunofunctional polypeptides can modulate both phase-based separation and charge-based chromatographic processes. Additionally, incorporation of the specific, multi-epitope tag used in this study may prove to be beneficial in recombinant HPV vaccine development due to its potential to improve phase separation yield and vaccine immunogenicity without inhibiting VLP formation.

Oligomerization of IC43 resulted in improved immunogenicity and protective efficacy against Pseudomonas aeruginosa lung infection

IC43, a truncate form of outer membrane proteins OprF_190-342 and OprI_21-83 from Pseudomonas aeruginosa, is a promising candidate antigen and exists as monomer in solution. In this study, we generated the heptamer of IC43 by carrier protein aided oligomerization, which was confirmed by gel-filtration and chemical cross-linking analysis. The carrier protein naturally exists as a homo-heptamer, and IC43 was displayed on the surface of the carrier protein in the fusion protein. Immunization with this fusion protein resulted in increased level of antigen specific IgG antibodies and higher survival rate after infection. The improved efficacy was correlated with lower bacteria burden, inflammation and tissue damage in the lungs of immunized mice. Further studies revealed that immunization with this fusion protein resulted in increased levels of IL-4 and antigen specific IgG1, suggesting a stronger Th2 immune response was induced. The improved immunogenicity may be attributed to the exposure of more epitopes on the antigen, which was confirmed by results from immune-dominant peptide mapping and passive immunization. These results demonstrated a possible strategy to improve the immunogenicity of an antigen by carrier protein aided oligomerization.

Current status and future directions of fish vaccines employing virus-like particles

In most breeding schemes, fish are cultured in enclosed spaces, which greatly increases the risk of outbreaks where the onset of infectious diseases can cause massive mortality and enormous economic losses. Vaccination is the most effective and long-term measure for improving the basic make-up of a fish farm. As the relationship between antibody and antigen is similar to that between screw and nut, similarity in the shape or nature of the vaccine antigen to the original pathogen is important for achieving a satisfactory/good/excellent antibody response with a vaccine. Virus-like particles (VLPs) best fulfil this requirement as their tertiary structure mimics that of the native virus. For this reason, VLPs have been attracting attention as next-generation vaccines for humans and animals, and the effects of various types of VLP vaccines on humans and livestock have been examined. Recent studies of VLP-based fish vaccines indicate that these vaccines are promising, and raise hopes of extending their use in the near future. In this review, the structural properties and immunogenicity of VLP-based vaccines against fish viruses such as infectious pancreatic necrosis virus (IPNV), salmonid alphavirus (SAV), nervous necrosis virus (NNV) and iridovirus are introduced/summarized. The NNV VLP vaccine is the most-studied VLP-based vaccine against fish viruses. Therefore, the current status of NNV VLP research is highlighted in this review, which deals with the advantages of using VLPs as vaccines, and the expression systems for producing them. Moreover, the need for lyophilized VLPs and oral VLP delivery is discussed. Finally, future directions for the development of VLP vaccines in the fish vaccine field are considered.

TLR4: An Important Molecule Participating in Either Anti-Human Papillomavirus Immune Responses or Development of Its Related Cancers

It has been reported that human papillomavirus (HPV) is a main cause of cervical cancer. Immune system plays key roles in the HPV infection clearance. Additionally, the roles played by immune responses in development of cancers have been documented previously. Toll-like receptors (TLRs) are the main surface or intravesicular receptors driving innate immunity, which either participate in the fight against infectious agents or participate in the progression of cancers. Thus, it has been hypothesized that the molecules may be part of the HPV/cancers puzzle. TLR4 is a unique member of TLRs family that uses both well-known TLRs related intracellular signaling pathways. Furthermore, the roles played by TLR4 against several viruses and also their related complications, such as tumors, have been demonstrated. Thus, it has been hypothesized that TLR4 may play a key role in HPV infection and its related complications. This review article collected the information regarding the mentioned plausible roles by TLR4.

Engineering the flagellar type III secretion system: improving capacity for secretion of recombinant protein

BACKGROUND

Many valuable biopharmaceutical and biotechnological proteins have been produced in Escherichia coli, however these proteins are almost exclusively localised in the cytoplasm or periplasm. This presents challenges for purification, i.e. the removal of contaminating cellular constituents. One solution is secretion directly into the surrounding media, which we achieved via the 'hijack' of the flagellar type III secretion system (FT3SS). Ordinarily flagellar subunits are exported through the centre of the growing flagellum, before assembly at the tip. However, we exploit the fact that in the absence of certain flagellar components (e.g. cap proteins), monomeric flagellar proteins are secreted into the supernatant.

RESULTS

We report the creation and iterative improvement of an E. coli strain, by means of a modified FT3SS and a modular plasmid system, for secretion of exemplar proteins. We show that removal of the flagellin and HAP proteins (FliC and FlgKL) resulted in an optimal prototype. We next developed a high-throughput enzymatic secretion assay based on cutinase. This indicated that removal of the flagellar motor proteins, motAB (to reduce metabolic burden) and protein degradation machinery, clpX (to boost FT3SS levels intracellularly), result in high capacity secretion. We also show that a secretion construct comprising the 5'UTR and first 47 amino acidsof FliC from E. coli (but no 3'UTR) achieved the highest levels of secretion. Upon combination, we show a 24-fold improvement in secretion of a heterologous (cutinase) enzyme over the original strain. This improved strain could export a range of pharmaceutically relevant heterologous proteins [hGH, TrxA, ScFv (CH₂)], achieving secreted yields of up to 0.29 mg L^-1, in low cell density culture.

CONCLUSIONS

We have engineered an E. coli which secretes a range of recombinant proteins, through the FT3SS, to the extracellular media. With further developments, including cell culture process strategies, we envision further improvement to the secreted titre of recombinant protein, with the potential application for protein production for biotechnological purposes.

N-terminal truncations on L1 proteins of human papillomaviruses promote their soluble expression in Escherichia coli and self-assembly in vitro

Human papillomavirus (HPV) is the causative agent in genital warts and nearly all cervical, anogenital, and oropharyngeal cancers. Nine HPV types (6, 11, 16, 18, 31, 33, 45, 52, and 58) are associated with about 90% of cervical cancers and 90% of genital warts. HPV neutralization by vaccine-elicited neutralizing antibodies can block viral infection and prevent HPV-associated diseases. However, there is only one commercially available HPV vaccine, Gardasil 9, produced from Saccharomyces cerevisiae that covers all nine types, raising the need for microbial production of broad-spectrum HPV vaccines. Here, we investigated whether N-terminal truncations of the major HPV capsid proteins L1, improve their soluble expression in Escherichia coli. We found that N-terminal truncations promoted the soluble expression of HPV 33 (truncated by 10 amino acids [aa]), 52 (15 aa), and 58 (10 aa). The resultant HPV L1 proteins were purified in pentamer form and extensively characterized with biochemical, biophysical, and immunochemical methods. The pentamers self-assembled into virus-like particles (VLPs) in vitro, and 3D cryo-EM reconstructions revealed that all formed T = 7 icosahedral particles having 50–60-nm diameters. Moreover, we formulated a nine-valent HPV vaccine candidate with aluminum adjuvant and L1 VLPs from four genotypes used in this study and five from previous work. Immunogenicity assays in mice and non-human primates indicated that this HPV nine-valent vaccine candidate elicits neutralizing antibody titers comparable to those induced by Gardasil 9. Our study provides a method for producing a nine-valent HPV vaccine in E. coli and may inform strategies for the soluble expression of other vaccine candidates.

• N-terminal truncations promote the soluble expression of HPV L1 proteins in E. coli and their self-assembly of T = 7 icosahedral particle in vitro

• An HPV 9-valent vaccine candidate was formulated with E. coli-derived HPV 6, 11, 16, 18, 31, 33, 45, 52, and 58 VLPs, and conferred comparable immunogenicity with Gardasil 9

Potential large scale production of meningococcal vaccines by stable overexpression of fHbp in the rice seeds

Factor H binding protein (fHbp) is the most promising vaccine candidate against serogroup B of Neisseria meningitidis which is a major cause of morbidity and mortality in children. In order to facilitate large scale production of a commercial vaccine, we previously used transgenic Arabidopsis thaliana, but plant-derived fHbp is still far away from a commercial vaccine due to less biomass production. Herein, we presented an alternative route for the production of recombinant fHbp from the seeds of transgenic rice. The OsrfHbp gene encoding recombinant fHbp fused protein was introduced into the genome of rice via Agrobacterium-mediated transformation. The both stable integration and transcription of the foreign OsrfHbp were confirmed by Southern blotting and RT-PCR analysis respectively. Further, the expression of fHbp protein was measured by immunoblotting analysis and quantified by ELISA. The results indicated that fHbp was successfully expressed and the highest yield of fHbp was 0.52 ± 0.03% of TSP in the transgenic rice seeds. The purified fHbp protein showed good antigenicity and immunogenicity in the animal model. The results of this experiment offer a novel approach for large-scale production of plant-derived commercial vaccine fHbp.

Comparison of the size distributions and immunogenicity of human papillomavirus type 16 L1 virus-like particles produced in insect and yeast cells

Insect and yeast cells are considered the expression systems of choice for producing virus-like particles (VLPs), and numerous types of VLPs have been produced in these systems. However, previous studies were restricted to identifying the characteristics of individual VLP preparations. No direct comparison of the structures and immunogenic properties of insect and yeast-derived VLPs has so far been made. In the present study, the size distribution and immunogenic properties of human papillomavirus type 16 (HPV16) L1 VLPs produced in Spodoptera frugipedra-9 insect cells and Saccharomyces cerevisiae were compared. The insect cell-derived VLPs were larger than the yeast ones (P < 0.0001), with median sizes of 34 and 26 nm, respectively. In addition, the insect-derived VLPs appeared to be more diverse in size than the yeast-derived VLPs. Immunization of mice with 30 ng per dose of VLPs elicited 2.7- and 2.4-fold higher anti-HPV16 L1 IgG and anti-HPV16 neutralizing antibody titers than immunization with the same amounts of the yeast-derived VLPs after the 4th immunizations, respectively. Our results suggest that the choice of expression system critically affects the particle size and immunogenic property of HPV16 L1 VLPs.

Production of virus-like particles for vaccines

Virus-like particles (VLPs) are nanostructures that resemble the structures of viruses. They are composed of one or more structural proteins that can be arranged in several layers and can also contain a lipid outer envelope. VLPs trigger a high humoral and cellular immune response due to their repetitive structures. A key factor regarding VLP safety is the lack of viral genomic material, which enhances safety during both manufacture and administration. Contemporary VLP production may take advantage of several systems, including bacterial, yeast, insect and mammalian cells. The choice of production platform depends on several factors, including cost and the need for post-translational modifications (PTMs), which can be essential in generating an optimal immune response. Some VLP-based vaccines designed to prevent several infectious diseases are already approved and on the market, with many others at the clinical trial or research stage. Interest in this technology has recently increased due to its advantages over classical vaccines. This paper reviews the state-of-the-art of VLP production systems and the newest generation of VLP-based vaccines now available.

Human papillomavirus first and second generation vaccines-current status and future directions

It has been more than 10 years that the first prophylactic papillomavirus vaccine became available, although distribution has been mainly limited to the more affluent countries. The first two vaccines have been a great success, hundreds of millions of women and a much smaller number of men have been vaccinated ever since. In a few countries with high vaccination coverage, in particular Australia but also parts of Great Britain and others, clinical impact of vaccination programs is already visible and there are indications for herd immunity as well. Vaccine efficacy is higher than originally estimated and the vaccines have an excellent safety profile. Gardasil9 is a second generation HPV virus-like particle vaccine that was licensed in 2015 and there are more to come in the near future. Currently, burning questions in respect to HPV vaccination are the duration of protection - especially in regard to cross-protection - reduction of the three-dose regimen and its impact on cross-protection; and duration of response, as well as protection against oropharyngeal HPV infections. Furthermore, researchers are seeking to overcome limitations of the VLP vaccines, namely low thermal stability, cost, invasive administration, limited coverage of non-vaccine HPV types, and lack of therapeutic efficacy. In this review we summarize the current status of licensed VLP vaccines and address questions related to second and third generation HPV vaccines.

Optimized Triton X-114 assisted lipopolysaccharide (LPS) removal method reveals the immunomodulatory effect of food proteins

SCOPE

Investigations into the immunological response of proteins is often masked by lipopolysaccharide (LPS) contamination. We report an optimized Triton X-114 (TX-114) based LPS extraction method for β-lactoglobulin (BLG) and soy protein extract suitable for cell-based immunological assays.

METHODS AND RESULTS

Optimization of an existing TX-114 based phase LPS extraction method resulted in >99% reduction of LPS levels. However, remaining TX-114 was found to interfere with LPS and protein concentration assays and decreased viability of THP-1 macrophages and HEK-Blue 293 cells. Upon screening a range of TX-114 extraction procedures, TX-114-binding beads were found to most effectively lower TX-114 levels without affecting protein structural properties. LPS-purified proteins showed reduced capacity to activate TLR4 compared to non-treated proteins. LPS-purified BLG did not induce secretion of pro-inflammatory cytokines from THP-1 macrophages, as non-treated protein did, showing that LPS contamination masks the immunomodulatory effect of BLG. Both HEK293 cells expressing TLR4 and differentiated THP-1 macrophages were shown as a relevant model to screen the protein preparations for biological effects of LPS contamination.

CONCLUSION

The reported TX-114 assisted LPS-removal from protein preparations followed by bead based removal of TX-114 allows evaluation of natively folded protein preparations for their immunological potential in cell-based studies.

Viruses and Virus-Like Particles in Biotechnology: Fundamentals and Applications

Although viruses are simple biological systems, they are capable of evolving highly efficient techniques for infecting cells, expressing their genomes, and generating new copies of themselves. It is possible to genetically manipulate most of the different classes of known viruses in order to produce recombinant viruses that express foreign proteins. Recombinant viruses have been used in gene therapy to deliver selected genes into higher organisms, in vaccinology and immunotherapy, and as important research tools to study the structure and function of these proteins.

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome. They have been applied not only as prophylactic and therapeutic vaccines but also as vehicles in drug and gene delivery and, more recently, as tools in nanobiotechnology.

In this chapter, basic and advanced features of viruses and VLPs are presented and their major applications are discussed. The different production platforms based on animal cell technology are explained, and their main challenges and future perspectives are explored. The implications of large-scale production of viruses and VLPs are discussed in the context of process control, monitoring, and optimization. The main upstream and downstream technical challenges are identified and discussed accordingly.

A partially purified outer membrane protein VirB9-1 for low-cost nanovaccines against Anaplasma marginale

Anaplasma marginale is a devastating tick-borne pathogen causing anaplasmosis in cattle and results in significant economic loss to the cattle industry worldwide. Currently, there is no widely accepted vaccine against A. marginale. New generation subunit vaccines against A. marginale, which are much safer, more efficient and cost-effective, are in great need. The A. marginale outer membrane protein VirB9-1 is a promising antigen for vaccination. We previously have shown that soluble recombinant VirB9-1 protein can be expressed and purified from Escherichia coli and induce a high level of humoral and cellular immunity in mice. In this study, we re-formulated the nanovaccines using the partially-purified VirB9-1 protein as the antigen and hollow nano-size silica vesicles (SV-100) as the adjuvant. We simplified the purification method to obtain the partially-purified antigen VirB9-1 with a six-fold higher yield. The new formulations using the partially-purified VirB9-1 protein achieved higher antibody and cell-mediated immune responses compared to the purified ones. This finding suggests that the partially-purified VirB9-1 protein performs better than the purified ones in the vaccination against A. marginale, and a certain level of contaminants in the protein antigen can be self-adjuvant and boost immunogenicity together with the nanoparticle adjuvant. This may lead to finding a "Goldilocks" level of contaminants. The new nanovaccine formulation using partially-purified antigens along with nanoparticle adjuvants offers an alternative strategy for making cheaper veterinary vaccines.

Design strategies to address the effect of hydrophobic epitope on stability and in vitro assembly of modular virus-like particle

Virus-like particles (VLPs) and capsomere subunits have shown promising potential as safe and effective vaccine candidates. They can serve as platforms for the display of foreign epitopes on their surfaces in a modular architecture. Depending on the physicochemical properties of the antigenic modules, modularization may affect the expression, solubility and stability of capsomeres, and VLP assembly. In this study, three module designs of a rotavirus hydrophobic peptide (RV10) were synthesized using synthetic biology. Among the three synthetic modules, modularization of the murine polyomavirus VP1 with a single copy of RV10 flanked by long linkers and charged residues resulted in the expression of stable modular capsomeres. Further employing the approach of module titration of RV10 modules on each capsomere via Escherichia coli co-expression of unmodified VP1 and modular VP1-RV10 successfully translated purified modular capomeres into modular VLPs when assembled in vitro. Our results demonstrate that tailoring the physicochemical properties of modules to enhance modular capsomeres stability is achievable through synthetic biology designs. Combined with module titration strategy to avoid steric hindrance to intercapsomere interactions, this allows bioprocessing of bacterially produced in vitro assembled modular VLPs.

Over-expression of fHbp in Arabdopsis for development of meningococcal serogroup B subunit vaccine

Due to lack of commercial vaccine against the serogroup B (MenB) of Neisseria meningitides, the incidence of meningococcal disease remains high. To solve the issue, transgenic plants are used as bioreactors to produce a plant-derived fHbp subunit vaccine. In this study, the fHbp gene was optimized according to the codon usage bias of Arabidopsis thaliana, synthesized artificially, cloned into an expression vector, driven by a seed-specific promoter, and introduced into A. thaliana by Agrobacterium-mediated floral-dip transformation. Transgenic plants were identified by glufosinate selection, quickstix strips for PAT/bar tests and PCR analysis. The five plants showing higher expression of recombinant fHbp were screened through indirect ELISA. Southern blot analysis showed that the transgenic line rHF-22 had a single-copy integration and the highest expression of fHbp. Recombinant fHbp was purified from seeds of rHF-22 by nitrilotriacetic acid-mediated affinity chromatography, and the purity was 82.5%. BALB/c mice were tested for fHbp vaccine protection from lethal MenB infection, and the relative percent survival was found to be 80%. This study indicates that the recombinant fHbp produced from seeds of rHF-22 is a potential candidate for commercial MenB vaccine. It also provides a reference for safe, cheap and large-scale production of other plant-made vaccines.

Immunogenicity of Outer Membrane Proteins VirB9-1 and VirB9-2, a Novel Nanovaccine against Anaplasma marginale

Anaplasma marginale is the most prevalent tick-borne livestock pathogen and poses a significant threat to cattle industry. In contrast to currently available live blood-derived vaccines against A. marginale, alternative safer and better-defined subunit vaccines will be of great significance. Two proteins (VirB9-1 and VirB9-2) from the Type IV secretion system of A. marginale have been shown to induce humoral and cellular immunity. In this study, Escherichia coli were used to express VirB9-1 and VirB9-2 proteins. Silica vesicles having a thin wall of 6 nm and pore size of 5.8 nm were used as the carrier and adjuvant to deliver these two antigens both as individual or mixed nano-formulations. High loading capacity was achieved for both proteins, and the mouse immunisation trial with individual as well as mixed nano-formulations showed high levels of antibody titres over 10⁷ and strong T-cell responses. The mixed nano-formulation also stimulated high-level recall responses in bovine T-cell proliferation assays. These results open a promising path towards the development of efficient A. marginale vaccines and provide better understanding on the role of silica vesicles to deliver multivalent vaccines as mixed nano-formulations able to activate both B-cell and T-cell immunity, for improved animal health.

Immunization with Human Papillomavirus 16 L1+E2 Chimeric Capsomers Elicits Cellular Immune Response and Antitumor Activity in a Mouse Model

Development of cervical cancer is associated with persistent infections by high-risk human papillomavirus (HPV). Although current HPV L1-based prophylactic vaccines prevent infection, they do not help to eliminate prevalent infections or lesions. Our aims were (i) to generate a vaccine combining prophylactic and therapeutic properties by producing chimeric capsomers after fusion of the L1 protein to different fragments of E2 from HPV 16, and (ii) to evaluate their capacity to generate an antitumoral cellular response, while conserving L1 neutralizing epitopes. Chimeric proteins were produced in Escherichia coli and purified by glutathione S-transferase (GST)-affinity chromatography. Their structure was characterized using size exclusion chromatography, sucrose gradient centrifugation, electron microscopy, and anti-L1 enzyme-linked immunosorbent assay. All chimeric proteins form capsomers and heterogeneous aggregates. One, containing part of the carboxy-terminal domain of E2 and its hinge region (L1Δ+E2H/NC, aa 206-307), conserved the neutralizing epitope H16.V5. We then evaluated the capacity of this chimeric protein to induce a cytotoxic T-cell response against HPV 16 E2. In (51)Cr release cytotoxicity assays, splenocytes from C57BL/6 immunized mice recognized and lysed TC-1/E2 cells, which express and present endogenously processed E2 peptides. Moreover, this E2-specific cytotoxic response inhibited the growth of tumors of TC-1/E2 cells in mice. Finally, we identified an epitope (aa 292-301) of E2 involved in this cytotoxic response. We conclude that the L1Δ+E2H/NC chimeric protein produced in bacteria can be an effective and economically interesting candidate for a combined prophylactic and therapeutic vaccine that could help eliminating HPV16-positive low-grade cervical lesions and persistent viral infections, thus preventing the development of lesions and, at the same time, the establishment of new infections.

Production of Well-Characterized Virus-like Particles in an Escherichia coli-Based Expression Platform for Preclinical Vaccine Assessments

In this chapter we demonstrate a method to produce virus-like particles (VLPs) from Escherichia coli. Standard bacterial protocols are used for the cloning, transformation, and expression of the protein subunits. A two-step protein purification method is highlighted: one step based on separating soluble proteins with ion-exchange affinity chromatography and a second polishing step using size-exclusion columns to isolate VLP species. The ensuing VLPs can be characterized with a variety of biophysical techniques including ultraviolet (UV)-visible spectroscopy for protein quantification, dynamic light scattering for size distribution determination, and transmission electron microscopy to ascertain size and morphology.

Immunoprevention of human papillomavirus-associated malignancies

Persistent infection by one of 15 high-risk human papillomavirus (hrHPV) types is a necessary but not sufficient cause of 5% of all human cancers. This provides a remarkable opportunity for cancer prevention via immunization. Since Harald zur Hausen's pioneering identification of hrHPV types 16 and 18, found in approximately 50% and 20% of cervical cancers, respectively, two prophylactic HPV vaccines containing virus-like particles (VLP) of each genotype have been widely licensed. These vaccines are beginning to affect infection and HPV-associated neoplasia rates after immunization campaigns in adolescents. Here, we review recent progress and opportunities to better prevent HPV-associated cancers, including broadening immune protection to cover all hrHPV types, reducing the cost of HPV vaccines especially for developing countries that have the highest rates of cervical cancer, and immune-based treatment of established HPV infections. Screening based upon George Papanicolaou's cervical cytology testing, and more recently detection of hrHPV DNA/RNA, followed by ablative treatment of high-grade cervical intraepithelial neoplasia (CIN2/3) have substantially reduced cervical cancer rates, and we examine their interplay with immune-based modalities for the prevention and eventual elimination of cervical cancer and other HPV-related malignancies.

Downstream processing of virus-like particles: single-stage and multi-stage aqueous two-phase extraction

The demand for vaccines against untreated diseases has enforced the research and development of virus-like particle (VLP) based vaccine candidates in recent years. Significant progress has been made in increasing VLP titres during upstream processing in bacteria, yeast and insect cells. Considering downstream processing, the separation of host cell impurities is predominantly achieved by time-intensive ultracentrifugation processes or numerous chromatography and filtration steps. In this work, we evaluate the potential of an alternative separation technology for VLPs: aqueous two-phase extraction (ATPE). The benefits of ATPE have been demonstrated for various biomolecules, but capacity and separation efficiency were observed to be low for large biomolecules such as VLPs or viruses. Both performance parameters were examined in detail in a case study on human B19 parvovirus-like particles derived from Spodoptera frugiperda Sf9 insect cells. A solubility-guided approach enabled the design of polyethylene (PEG) salt aqueous two-phase systems with a high capacity of up to 4.1mg/mL VLPs. Unique separation efficiencies were obtained by varying the molecular weight of PEG, the pH value and by using neutral salt additives. Further improvement of the separation of host cell impurities was achieved by multi-stage ATPE on a centrifugal partition chromatography (CPC) device in 500mL scale. While single-stage ATPE enabled a DNA clearance of 99.6%, multi-stage ATPE improved the separation of host cell proteins (HCPs). The HPLC purity ranged from 16.8% (100% VLP recovery) for the single-stage ATPE to 69.1% (40.1% VLP recovery) for the multi-stage ATPE. An alternative two-step downstream process is presented removing the ATPS forming polymer, cell debris and 99.77% DNA with a HPLC purity of 90.6% and a VLP recovery of 63.9%.

Expression of HPV-16 L1 capsomeres with glutathione-S-transferase as a fusion protein in tobacco plastids: an approach for a capsomere-based HPV vaccine

Human Papillomavirus (HPV) is the main cause of cervical cancer, which is the second most severe cancer of women worldwide, particularly in developing countries. Although vaccines against HPV infection are commercially available, they are neither affordable nor accessible to women in low income countries e.g. Africa. Thus, alternative cost-effective vaccine production approaches need to be developed. This study uses tobacco plants to express pentameric capsomeres of HPV that have been reported to generate elevated immune responses against HPV. A modified HPV-16 L1 (L1_2xCysM) protein has been expressed as a fusion protein with glutathione-S-transferase (GST) in tobacco chloroplasts following biolistic transformation. In total 7 transplastomic lines with healthy phenotypes were generated. Site specific integration of the GST-L1_2xCysM and aadA genes was confirmed by PCR. Southern blot analysis verified homogenous transformation of all transplastomic lines. Antigen capture ELISA with the conformation-specific antibody Ritti01, showed protein expression as well as the retention of immunogenic epitopes of L1 protein. In their morphology, GST-L1 expressing tobacco plants were identical to wild type plants and yielded fertile flowers. Taken together, these data enrich knowledge for future development of cost-effective plant-made vaccines against HPV.

Evaluation of specific humoral and cellular immune responses against the major capsid L1 protein of cutaneous wart-associated alpha-Papillomaviruses in solid organ transplant recipients

BACKGROUND

Infection with different species of cutaneous human papillomaviruses (cHPV) of genus alpha (cαHPVs) and associated skin disease are highly prevalent in solid organ transplant recipients (OTR), documenting the importance of the immunological control of HPV infection.

OBJECTIVES

To investigate the natural course of cαHPV-specific cellular and humoral immune responses during systemic long-term immunosuppression.

METHODS

Integrating bead-based multiplex serology and flow cytometry we analyzed natural cαHPV-specific antibodies and T(H) cell responses against the major capsid protein L1 of HPV types 2, 27, 57 (species 4) and 3, 10 and 77 (species 2) in sera and blood of OTR before and after initiation of iatrogenic immunosuppression and in comparison to immunocompetent individuals (IC).

RESULTS

Among OTR we observed an overall 42% decrease in humoral L1-specific immune responses during the course of iatrogenic immunosuppression, comparing median values 30 d before and 30 d after initiation of immunosuppressive therapy (p < 0.05). This difference disappeared after long-term (>1 year) immunosuppression. The predominant cellular L1-specific immune response was of type T(H)1 (CD4(+)CD40L(+)IL-2(+)IFN-γ(+)). Consistent with the detected L1-specific antibody titers, L1-specific T(H)1 responses were unchanged in long-term immunosuppressed OTR compared to IC. Notably, cαHPV-L1-specific IL-2(+)/CD40L(+)CD4(+) or IFN-γ(+)/CD40L(+) CD4(+) T(H) cell responses against any of the cαHPV-L1 types were significantly higher in OTR with clinically apparent common warts.

CONCLUSION

The systemic humoral immune response against cαHPV may reflect the individual degree of iatrogenic immunosuppression indicating a higher susceptibility for cαHPV infection among OTR during the early phase after organ transplantation. Humoral cαHPV-specific immune responses may show a reconstitution to pre-transplantation levels despite continuous potent immunosuppression.

Multivalent human papillomavirus l1 DNA vaccination utilizing electroporation

Objectives

Naked DNA vaccines can be manufactured simply and are stable at ambient temperature, but require improved delivery technologies to boost immunogenicity. Here we explore in vivo electroporation for multivalent codon-optimized human papillomavirus (HPV) L1 and L2 DNA vaccination.

Methods

Balb/c mice were vaccinated three times at two week intervals with a fusion protein comprising L2 residues ∼11−88 of 8 different HPV types (11−88×8) or its DNA expression vector, DNA constructs expressing L1 only or L1+L2 of a single HPV type, or as a mixture of several high-risk HPV types and administered utilizing electroporation, i.m. injection or gene gun. Serum was collected two weeks and 3 months after the last vaccination. Sera from immunized mice were tested for in-vitro neutralization titer, and protective efficacy upon passive transfer to naive mice and vaginal HPV challenge. Heterotypic interactions between L1 proteins of HPV6, HPV16 and HPV18 in 293TT cells were tested by co-precipitation using type-specific monoclonal antibodies.

Results

Electroporation with L2 multimer DNA did not elicit detectable antibody titer, whereas DNA expressing L1 or L1+L2 induced L1-specific, type-restricted neutralizing antibodies, with titers approaching those induced by Gardasil. Co-expression of L2 neither augmented L1-specific responses nor induced L2-specific antibodies. Delivery of HPV L1 DNA via in vivo electroporation produces a stronger antibody response compared to i.m. injection or i.d. ballistic delivery via gene gun. Reduced neutralizing antibody titers were observed for certain types when vaccinating with a mixture of L1 (or L1+L2) vectors of multiple HPV types, likely resulting from heterotypic L1 interactions observed in co-immunoprecipitation studies. High titers were restored by vaccinating with individual constructs at different sites, or partially recovered by co-expression of L2, such that durable protective antibody titers were achieved for each type.

Discussion

Multivalent vaccination via in vivo electroporation requires spatial separation of individual type L1 DNA vaccines.

The choice of resin-bound ligand affects the structure and immunogenicity of column-purified human papillomavirus type 16 virus-like particles

Cell growth conditions and purification methods are important in determining biopharmaceutical activity. However, in studies aimed at manufacturing virus-like particles (VLPs) for the purpose of creating a prophylactic vaccine and antigen for human papillomavirus (HPV), the effects of the presence of a resin-bound ligand during purification have never been investigated. In this study, we compared the structural integrity and immunogenicity of two kinds of VLPs derived from HPV type 16 (HPV16 VLPs): one VLP was purified by heparin chromatography (hHPV16 VLP) and the other by cation-exchange chromatography (cHPV16 VLP). The reactivity of anti-HPV16 neutralizing monoclonal antibodies (H16.V5 and H16.E70) towards hHPV16 VLP were significantly higher than the observed cHPV16 VLP reactivities, implying that hHPV16 VLP possesses a greater number of neutralizing epitopes and has a greater potential to elicit anti-HPV16 neutralizing antibodies. After the application of heparin chromatography, HPV16 VLP has a higher affinity for H16.V5 and H16.E70. This result indicates that heparin chromatography is valuable in selecting functional HPV16 VLPs. In regard to VLP immunogenicity, the anti-HPV16 L1 IgG and neutralizing antibody levels elicited by immunizations of mice with hHPV16 VLPs were higher than those elicited by cHPV16 VLP with and without adjuvant. Therefore, the ability of hHPV16 VLP to elicit humoral immune responses was superior to that of cHPV16 VLP. We conclude that the specific chromatographic technique employed for the purification of HPV16 VLPs is an important factor in determining the structural characteristics and immunogenicity of column-purified VLPs.

New approaches to prophylactic human papillomavirus vaccines for cervical cancer prevention

The currently licensed human papillomavirus (HPV) vaccines are safe and highly effective at preventing HPV infection for a select number of papillomavirus types, thus decreasing the incidence of precursors to cervical cancer. It is expected that vaccination will also ultimately reduce the incidence of this cancer. The licensed HPV vaccines are, however, type restricted and expensive, and also require refrigeration, multiple doses and intramuscular injection. Second-generation vaccines are currently being developed to address these shortcomings. New expression systems, viral and bacterial vectors for HPV L1 capsid protein delivery, and use of the HPV L2 capsid protein will hopefully aid in decreasing cost and increasing ease of use and breadth of protection. These second-generation vaccines could also allow affordable immunization of women in developing countries, where the incidence of cervical cancer is high.

Capsomer vaccines protect mice from vaginal challenge with human papillomavirus

Capsomers were produced in bacteria as glutathione-S-transferase (GST) fusion proteins with human papillomavirus type 16 L1 lacking the first nine and final 29 residues (GST-HPV16L1Δ) alone or linked with residues 13-47 of HPV18, HPV31 and HPV45 L2 in tandem (GST-HPV16L1Δ-L2x3). Subcutaneous immunization of mice with GST-HPV16L1Δ or GST-HPV16L1Δ-L2x3 in alum and monophosphoryl lipid A induced similarly high titers of HPV16 neutralizing antibodies. GST-HPV16L1Δ-L2x3 also elicited moderate L2-specific antibody titers. Intravaginal challenge studies showed that immunization of mice with GST-HPV16 L1Δ or GST-HPV16L1Δ-L2x3 capsomers, like Cervarix®, provided complete protection against HPV16. Conversely, vaccination with GST-HPV16 L1Δ capsomers failed to protect against HPV18 challenge, whereas mice immunized with either GST-HPV16L1Δ-L2x3 capsomers or Cervarix® were each completely protected. Thus, while the L2-specific response was moderate, it did not interfere with immunity to L1 in the context of GST-HPV16L1Δ-L2x3 and is sufficient to mediate L2-dependent protection against an experimental vaginal challenge with HPV18.

Plastid expression of a double-pentameric vaccine candidate containing human papillomavirus-16 L1 antigen fused with LTB as adjuvant: transplastomic plants show pleiotropic phenotypes

Human papillomavirus (HPV) causes cervical cancer in women worldwide, which is currently prevented by vaccines based on virus-like particles (VLPs). However, these vaccines have certain limitations in their availability to developing countries, largely due to elevated costs. Concerning the highest burden of disease in resource-poor countries, development of an improved mucosal and cost-effective vaccine is a necessity. As an alternative to VLPs, capsomeres have been shown to be highly immunogenic and can be used as vaccine candidate. Furthermore, coupling of an adjuvant like Escherichia coli heat-labile enterotoxin subunit B (LTB) to an antigen can increase its immunogenicity and reduce the costs related to separate co-administration of adjuvants. Our study demonstrates the expression of two pentameric proteins: the modified HPV-16 L1 (L1_2xCysM) and LTB as a fusion protein in tobacco chloroplasts. Homoplasmy of the transplastomic plants was confirmed by Southern blotting. Western blot analysis showed that the LTB-L1 fusion protein was properly expressed in the plastids and the recombinant protein was estimated to accumulate up to 2% of total soluble protein. Proper folding and display of conformational epitopes for both LTB and L1 in the fusion protein was confirmed by GM1-ganglioside binding assay and antigen capture ELISA, respectively. However, all transplastomic lines showed chlorosis, male sterility and growth retardation, which persisted in the ensuing four generations studied. Nevertheless, plants reached maturity and produced seeds by pollination with wild-type plants. Taken together, these results pave the way for the possible development of a low-cost adjuvant-coupled vaccine with potentially improved immunogenicity against cervical cancer.

Recombinant HPV16 E7 assembled into particles induces an immune response and specific tumour protection administered without adjuvant in an animal model

Background

The HPV16 E7 protein is both a tumour-specific and a tumour-rejection antigen, the ideal target for developing therapeutic vaccines for the treatment of HPV16-associated cancer and its precursor lesions. E7, which plays a key role in virus-associated carcinogenesis, contains 98 amino acids and has two finger-type structures which bind a Zn⁺⁺ ion. The ability of an Escherichia coli-produced E7-preparation, assembled into particles, to induce protective immunity against a HPV16-related tumour in the TC-1-C57BL/6 mouse tumour model, was evaluated.

Methods

E7 was expressed in E. coli, purified via a one-step denaturing protocol and prepared as a soluble suspension state after dialysis in native buffer. The presence in the E7 preparation of particulate forms was analysed by non-reducing SDS-PAGE and negative staining electron microscopy (EM). The Zn⁺⁺ ion content was analysed by mass-spectrometry. Ten μg of protein per mouse was administered to groups of animals, once, twice or three times without adjuvant. The E7-specific humoral response was monitored in mice sera using an E7-based ELISA while the cell-mediated immune response was analysed in mice splenocytes with lymphoproliferation and IFN-γ ELISPOT assays. The E7 immunized mice were challenged with TC-1 tumour cells and the tumour growth monitored for two months.

Results

In western blot analysis E7 appears in multimers and high molecular mass oligomers. The EM micrographs show the protein dispersed as aggregates of different shape and size. The protein appears clustered in micro-, nano-aggregates, and structured particles. Mice immunised with this protein preparation show a significant E7-specific humoral and cell-mediated immune response of mixed Th1/Th2 type. The mice are fully protected from the tumour growth after vaccination with three E7-doses of 10 μg without any added adjuvant.

Conclusions

This report shows that a particulate form of HPV16 E7 is able to induce, without adjuvant, an E7-specific tumour protection in C57BL/6 mice. The protective immunity is sustained by both humoral and cell-mediated immune responses. The E. coli-derived HPV16 E7 assembled in vitro into micro- and nanoparticles represents not only a good substrate for antigen-presenting cell uptake and processing, but also a cost-effective means for the production of a new generation of HPV subunit vaccines.

Transplastomic expression of a modified human papillomavirus L1 protein leading to the assembly of capsomeres in tobacco: a step towards cost-effective second-generation vaccines

Certain types of human papillomaviruses (HPV) are causatively associated with cervical carcinoma, the second most common cancer in women worldwide. Due to limitations in the availability of currently used virus-like particle (VLP)-based vaccines against HPV to women of developing countries, where most cases of cervical cancer occur, the development of a cost-effective second-generation vaccine is a necessity. Capsomeres have recently been demonstrated to be highly immunogenic and to have a number of advantages as a potential cost-effective alternative to VLP-based HPV vaccines. We have expressed a mutated HPV-16 L1 (L1_2xCysM) gene that retained the ability to assemble L1 protein to capsomeres in tobacco chloroplasts. The recombinant protein yielded up to 1.5% of total soluble protein. The assembly of capsomeres was examined and verified by cesium chloride density gradient centrifugation and sucrose sedimentation analysis. An antigen capture enzyme-linked immunosorbent assay confirmed the formation of capsomeres by using a conformation-specific monoclonal antibody which recognized the conformational epitopes. Transplastomic tobacco plants exhibited normal growth and morphology, but all such lines showed male sterility in the T₀, T₁ and T₂ generations. Taken together, these results indicate the possibility of producing a low-cost capsomere-based vaccine by plastids.

Virus-like particles in vaccine development

Virus-like particles (VLPs) are multiprotein structures that mimic the organization and conformation of authentic native viruses but lack the viral genome, potentially yielding safer and cheaper vaccine candidates. A handful of prophylactic VLP-based vaccines is currently commercialized worldwide: GlaxoSmithKline's Engerix (hepatitis B virus) and Cervarix (human papillomavirus), and Merck and Co., Inc.'s Recombivax HB (hepatitis B virus) and Gardasil (human papillomavirus) are some examples. Other VLP-based vaccine candidates are in clinical trials or undergoing preclinical evaluation, such as, influenza virus, parvovirus, Norwalk and various chimeric VLPs. Many others are still restricted to small-scale fundamental research, despite their success in preclinical tests. This article focuses on the essential role of VLP technology in new-generation vaccines against prevalent and emergent diseases. The implications of large-scale VLP production are discussed in the context of process control, monitorization and optimization. The main up- and down-stream technical challenges are identified and discussed accordingly. Successful VLP-based vaccine blockbusters are briefly presented concomitantly with the latest results from clinical trials and the recent developments in chimeric VLP-based technology for either therapeutic or prophylactic vaccination.