Expression and characterization of HPV-16 L1 capsid protein in Pichia pastoris

Yeast-Based Virus-like Particles as an Emerging Platform for Vaccine Development and Delivery

Virus-like particles (VLPs) are empty, nanoscale structures morphologically resembling viruses. Internal cavity, noninfectious, and particulate nature with a high density of repeating epitopes, make them an ideal platform for vaccine development and drug delivery. Commercial use of Gardasil-9 and Cervarix showed the usefulness of VLPs in vaccine formulation. Further, chimeric VLPs allow the raising of an immune response against different immunogens and thereby can help reduce the generation of medical or clinical waste. The economically viable production of VLPs significantly impacts their usage, application, and availability. To this end, several hosts have been used and tested. The present review will discuss VLPs produced using different yeasts as fermentation hosts. We also compile a list of studies highlighting the expression and purification of VLPs using a yeast-based platform. We also discuss the advantages of using yeast to generate VLPs over other available systems. Further, the issues or limitations of yeasts for producing VLPs are also summarized. The review also compiles a list of yeast-derived VLP-based vaccines that are presently in public use or in different phases of clinical trials.

How far have we explored fungi to fight cancer?

The use of fungal cultures have been well documented in human history. Although its used in healthcare, like penicillin and statins, have saved countless of lives, but there is still no fungal products that are specifically indicated for cancers. Research into fungal-derived materials to curb cancers in the recent decades have made a considerable progress in terms of drug delivery vehicles, anticancer active ingredients and cancer immunotherapy. Various parts of the organisms have successfully been exploited to achieve specific tasks. Apart from the identification of novel anticancer compound from fungi, its native capsular structure can also be used as drug cargo to achieve higher oral bioavailability. This review summarises the anticancer potential of fungal-derived materials, highlighting the role of capsular polysaccharides, proteins, and other structures in variety of innovative utilities to fit the current pharmaceutical technology. Many bioactive compounds isolated from fungi have also been formulated into nanoparticles to achieve greater anticancer activity. The progress of fungal compounds and their analogues in clinical trials is also highlighted. In addition, the potential of various fungal species to be developed for anticancer immunotherapy are also discussed.

Expression of Chimeric HPV-HIV Protein L1P18 in Pichia pastoris; Purification and Characterization of the Virus-like Particles

Currently, three human papillomavirus (HPV) vaccines are already licensed and all of them are based on virus-like particles (VLPs) of HPV L1 capsid protein but not worldwide accessible. While about 38.0 million people were living with HIV in 2019, only 68% of HIV-infected individuals were accessing antiretroviral therapy as of the end of June 2020 and there is no HIV vaccine yet. Therefore, safe, effective, and affordable vaccines against those two viruses are immediately needed. Both HPV and HIV are sexually transmitted infections and one of the main access routes is the mucosal genital tract. Thus, the development of a combined vaccine that would protect against HPV and HIV infections is a logical effort in the fight against these two major global pathogens. In this study, a recombinant Pichia pastoris producing chimeric HPV-HIV L1P18 protein intracellularly was constructed. After cell disruption, the supernatant was collected, and the VLPs were purified by a combination of ammonium sulfate precipitation, size exclusion chromatography, ultracentrifugation, and ultrafiltration. At the end of purification process, the chimeric VLPs were recovered with 96% purity and 9.23% overall yield, and the morphology of VLPs were confirmed by transmission electron microscopy. This work contributes towards the development of an alternative platform for production of a bivalent vaccine against HPV and HIV in P. pastoris.

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.

The possible regions to design Human Papilloma Viruses vaccine in Iranian L1 protein

Human Papilloma Virus (HPV) genome encodes several proteins, as L1is major capsid protein and L2 is minor capsid protein. Among all HPV types HPV-16 and HPV-18 are the most common high-risk HPV (HR-HPV) types globally and the majority of cases are infected with these types. HPV entry and the initial interaction with the host cell are mainly related to the L1 protein which is the main component of HPV vaccines. The aim of this research was comparison analysis among all Iranian L1 protein sequences submitted in NCBI GenBank to find the major substitutions as well as structural and immune properties of this protein. All sequences HPV L1 protein from Iranian isolates from 2014 to 2016 were selected and obtained from NCBI data bank. "CLC Genomics Workbench" was used to translate alignment. To predict B cell epitopes, we employed several programs. Modification sites such as phosphorylation, glycosylation, and disulfide bonds were determined. Secondary and tertiary structures of all sequences were analyzed. Several mutations were found and major mutations were in amino acid residues 102, 202, 207, 292, 379, and 502. The mentioned mutations showed the minor effect on B cell and physicochemical properties of the L1 protein. Six disulfide bonds were determined in L1 protein and also in several N-link glycosylation and phosphorylation sites. Five L1 loops were determined, which had great potential to be B cell epitopes with high antigenic properties. All in all, this research as the first report from Iran described the tremendous potential of two L1 loops (BC and FG) to induce immune system which can be used as the descent candidate to design a new vaccine against HPV in the Iranian population. In addition, some differences between the reference sequence and Iranian patients' sequences were determined. It is essential to consider these differences to monitor the effectiveness and efficacy of the vaccine for the Iranian population. Our results provide a vast understanding of L1 protein that can be useful for further studies on HPV infections and new vaccine generations.

Advances in Designing and Developing Vaccines, Drugs and Therapeutic Approaches to Counter Human Papilloma Virus

Human papillomavirus (HPV) is a viral infection with skin-to-skin based transmission mode. HPV annually caused over 500,000 cancer cases including cervical, anogenital and oropharyngeal cancer among others. HPV vaccination has become a public-health concern, worldwide, to prevent the cases of HPV infections including precancerous lesions, cervical cancers, and genital warts especially in adolescent female and male population by launching national programs with international alliances. Currently, available prophylactic and therapeutic vaccines are expensive to be used in developing countries for vaccination programs. The recent progress in immunotherapy, biotechnology, recombinant DNA technology and molecular biology along with alternative and complementary medicinal systems have paved novel ways and valuable opportunities to design and develop effective prophylactic and therapeutic vaccines, drugs and treatment approach to counter HPV effectively. Exploration and more researches on such advances could result in the gradual reduction in the incidences of HPV cases across the world. The present review presents a current global scenario and futuristic prospects of the advanced prophylactic and therapeutic approaches against HPV along with recent patents coverage of the progress and advances in drugs, vaccines and therapeutic regimens to effectively combat HPV infections and its cancerous conditions.

Development of a human papillomavirus type 6/11 vaccine candidate for the prevention of condyloma acuminatum

Condyloma acuminatum (CA) represents a significant human papillomavirus (HPV) disease burden worldwide, resulting in substantial healthcare costs and loss of life quality in both genders. To address this problem, we tried to develop a bivalent HPV6/11 virus-like particle (VLP) vaccine targeting CA. HPV6/11 VLPs were generated in Hansenula polymorpha, and a disassembly and reassembly (D/R) treatment was further conducted to improve the stability and monodispersity of the VLPs. The HPV6/11 VLPs were identified by transmission electron microscopy (TEM), high performance liquid chromatography (HPLC), mass spectrum (MS) and dynamic light scattering (DLS), and were evaluated for their immunogenicity in both mice and cynomolgus monkeys. The results showed that the HPV6/11 L1 proteins were correctly expressed and assembled into HPV6/11 VLPs, and the HPV6/11 VLPs formulated with aluminum phosphate induced vigorous production of specific neutralizing antibodies against HPV6/11 VLPs in mice and cynomolgus monkeys. These data indicated that the Hansenula polymorpha-derived HPV6/11 VLPs could be formulated into a bivalent vaccine used in prevention of CA.

Expression of unique chimeric human papilloma virus type 16 (HPV-16) L1-L2 proteins in Pichia pastoris and Hansenula polymorpha

Cervical cancer is ranked the fourth most common cancer in women worldwide. Despite two prophylactic vaccines being commercially available, they are unaffordable for most women in developing countries. We compared the optimized expression of monomers of the unique HPV type 16 L1-L2 chimeric protein (SAF) in two yeast strains of Pichia pastoris, KM71 (Mut^s ) and GS115 (Mut⁺ ), with Hansenula polymorpha NCYC 495 to determine the preferred host in bioreactors. SAF was uniquely created by replacing the h4 helix of the HPV-16 capsid L1 protein with an L2 peptide. Two different feeding strategies in fed-batch cultures of P. pastoris Mut^s were evaluated: a predetermined feed rate vs. feeding based on the oxygen consumption by maintaining constant dissolved oxygen levels (DO stat). All cultures showed a significant increase in biomass when methanol was fed using the DO stat method. In P. pastoris the SAF concentrations were higher in the Mut^s strains than in the Mut⁺ strains. However, H. polymorpha produced the highest level of SAF at 132.10 mg L^-1 culture while P. pastoris Mut^s only produced 23.61 mg L^-1 . H. polymorpha showed greater potential for the expression of HPV-16 L1/L2 chimeric proteins despite the track record of P. pastoris as a high-level producer of heterologous proteins.

Recent advances in the production of recombinant subunit vaccines in Pichia pastoris

Recombinant protein subunit vaccines are formulated using defined protein antigens that can be produced in heterologous expression systems. The methylotrophic yeast Pichia pastoris has become an important host system for the production of recombinant subunit vaccines. Although many basic elements of P. pastoris expression system are now well developed, there is still room for further optimization of protein production. Codon bias, gene dosage, endoplasmic reticulum protein folding and culture condition are important considerations for improved production of recombinant vaccine antigens. Here we comment on current advances in the application of P. pastoris for the synthesis of recombinant subunit vaccines.

Human papilloma virus in oral cancer

Cervical cancer is the second most prevalent cancer among women, and it arises from cells that originate in the cervix uteri. Among several causes of cervical malignancies, infection with some types of human papilloma virus (HPV) is well known to be the greatest cervical cancer risk factor. Over 150 subtypes of HPV have been identified; more than 40 types of HPVs are typically transmitted through sexual contact and infect the anogenital region and oral cavity. The recently introduced vaccine for HPV infection is effective against certain subtypes of HPV that are associated with cervical cancer, genital warts, and some less common cancers, including oropharyngeal cancer. Two HPV vaccines, quadrivalent and bivalent types that use virus-like particles (VLPs), are currently used in the medical commercial market. While the value of HPV vaccination for oral cancer prevention is still controversial, some evidence supports the possibility that HPV vaccination may be effective in reducing the incidence of oral cancer. This paper reviews HPV-related pathogenesis in cancer, covering HPV structure and classification, trends in worldwide applications of HPV vaccines, effectiveness and complications of HPV vaccination, and the relationship of HPV with oral cancer prevalence.

Immunogenicity of next-generation HPV vaccines in non-human primates: Measles-vectored HPV vaccine versus Pichia pastoris recombinant protein vaccine

Human papillomavirus (HPV) infection is the most common sexually transmitted disease worldwide. HPVs are oncogenic small double-stranded DNA viruses that are the primary causal agent of cervical cancer and other types of cancers, including in the anus, oropharynx, vagina, vulva, and penis. Prophylactic vaccination against HPV is an attractive strategy for preventing cervical cancer and some other types of cancers. However, there are few safe and effective vaccines against HPV infections. Current first-generation commercial HPV vaccines are expensive to produce and deliver. The goal of this study was to develop an alternate potent HPV recombinant L1-based vaccines by producing HPV virus-like particles into a vaccine that is currently used worldwide. Live attenuated measles virus (MV) vaccines have a well-established safety and efficacy record, and recombinant MV (rMV) produced by reverse genetics may be useful for generating candidate HPV vaccines to meet the needs of the developing world. We studied in non-human primate rMV-vectored HPV vaccine in parallel with a classical alum adjuvant recombinant HPV16L1 and 18L1 protein vaccine produced in Pichia pastoris. A combined prime-boost approach using both vaccines was evaluated, as well as immune interference due to pre-existing immunity against the MV. The humoral immune response induced by the MV, Pichia-expressed vaccine, and their combination as priming and boosting approaches was found to elicit HPV16L1 and 18L1 specific total IgG and neutralizing antibody titres. Pre-existing antibodies against measles did not prevent the immune response against HPV16L1 and 18L1.

Expression and Characterization of Yeast Derived Chikungunya Virus Like Particles (CHIK-VLPs) and Its Evaluation as a Potential Vaccine Candidate

Chikungunya virus (CHIKV) has emerged as a global health concern due to its recent spread in both old and new world. So far, no CHIKV specific drug or vaccine is licensed for human use. In this study, we report production of Chikungunya virus like particles (CHIK-VLPs) using novel yeast expression system (Pichia pastoris) and its evaluation as vaccine candidate. The gene encoding structural polyprotein of CHIKV from a recent epidemic strain was cloned into yeast expression system. The multicopy integrants were processed for expression of CHIK-VLPs. The VLPs were purified and confirmed through electron microscopic analysis for their morphological identity with CHIKV. The in vitro and in vivo evaluation of CHIK-VLPs as vaccine candidate was determined in Balb/c mice. Induction of both humoral and cellular immune response was observed with different doses of CHIK-VLPs. The humoral immune response was studied through different techniques like enzyme linked immunosorbent assay, IgG Isotyping and plaque reduction neutralization test. CHIK-VLPs were found to elicit high titer of antibodies that are able to recognize native CHIKV. Higher level of IgG2a and IgG1 subtypes was identified suggestive of balanced Th1/Th2 response. Both in vitro and in vivo neutralization activity of CHIK-VLPs antibodies was observed even with low concentration, which shows its high specificity and neutralizing activity against two different CHIKV strains. Neonatal mice receiving anti-CHIK-VLPs antibodies were protected from CHIKV challenge. Induction of cellular immune response was confirmed through higher level of TNF-α, IL-10 and substantial level of IL-2, IL-4 and IFN-γ indicating a balanced response. This is the first report, where CHIK-VLPs has been expressed by Pichia pastoris and evaluated for neutralizing activity against CHIKV. These promising results indicate the utility of CHIK-VLPs as a promising vaccine candidate against emerging CHIKV.

Chikungunya virus (CHIKV) has emerged in many parts of tropics in last decade. The absence of an approved vaccine or antiviral drug for CHIKV makes it one of the important public health challenges. Though attempt to develop a CHIKV vaccine was initiated in 1980s, however it has not succeeded so far. The Virus like particles (VLPs) are now explored as promising vaccine candidate against many viruses viz. HBV, HPV etc. In this study, we report the production of CHIK-VLPs using novel yeast expression system (Pichia pastoris) and its evaluation as vaccine candidate. These CHIK-VLPs share morphological identity to native CHIKV. The results indicate that CHIK-VLPs induced both cell mediated as well as humoral response in a balanced manner, which fulfils its criteria as a potent immunogen. Further, antibodies generated against CHIK-VLPs demonstrated efficient in vitro and in vivo neutralization activity, as evaluated through plaque reduction in Vero cells and protection in CHIKV infected neonatal mice respectively using two different CHIKV strains, which makes it a promising vaccine candidate. The yeast expressed CHIK-VLPs has high potential for development of an effective vaccine candidate against CHIKV during epidemic situations.

Enhanced expression of soluble human papillomavirus L1 through coexpression of molecular chaperonin in Escherichia coli

The major recombinant capsid protein L1 of human papillomavirus (HPV) is widely used to produce HPV prophylactic vaccines. However, the quality of soluble and active expression of L1 in Escherichia coli was below the required amount. Coexpression with the chaperonin GroEL/ES enhanced L1 expression. Overexpressing GroEL/ES increased the soluble expression level of glutathione S-transferase-fused L1 (GST-L1) by approximately ∼3 fold. The yield of HPV type 16 L1 pentamer (L1-p) was ∼2 fold higher than that in a single expression system after purification through size-exclusion chromatograph. The expression and purification conditions were then optimized. The yield of L1-p was enhanced by ∼5 fold, and those of HPV types 18 and 58 L1-p increased by ∼3 and ∼2 folds, respectively, compared with that in the single expression system. Coexpressing the mono-site mutant HPV16 L1 L469A with GroEL/ES increased L1-p yield by ∼7 fold compared with strains expressing the wild-type L1 gene. L1-p was then characterized using circular dichroism spectra, UV-vis cloud point, dynamic light scattering and transmission electron microscope analyses. Results indicated that the conformation and biological characteristics of L1-p were identical to that of native L1. Hence, overexpressing chaperonin in E. coli can increase the expression level of GST-L1 and L1-p production after purification. This finding may contribute to the development of a platform for prophylactic HPV vaccines.

Whole recombinant Pichia pastoris expressing HPV16 L1 antigen is superior in inducing protection against tumor growth as compared to killed transgenic Leishmania

The development of an efficient vaccine against high-risk HPV types can reduce the incidence rates of cervical cancer by generating anti-tumor protective responses. Traditionally, the majority of prophylactic viral vaccines are composed of live, attenuated or inactivated viruses. Among them, the design of an effective and low-cost vaccine is critical. Inactivated vaccines especially heat-killed yeast cells have emerged as a promising approach for generating antigen-specific immunotherapy. Recent studies have indicated that yeast cell wall components possess adjuvant activities. Moreover, a non-pathogenic protozoan, Leishmania tarentolae (L.tar) has attracted a great attention as a live candidate vaccine. In current study, immunological and protective efficacy of whole recombinant killed Pichia pastoris and Leishmania tarentolae expressing HPV16 L1 capsid protein was evaluated in tumor mice model. We found that Pichia-L1, L.tar-L1 and Gardasil groups increase the IgG2a/IgG1 ratio, indicating a relative preference for the induction of Th1 immune responses. Furthermore, subcutaneous injection of killed Pichia-L1 generated the significant L1-specific IFN-γ immune response as well as the best protective effects in vaccinated mice as compared to killed L.tar-L1, killed Pichia pastoris, killed L.tar and PBS groups. Indeed, whole recombinant Leishmania tarentolae could not protect mice against C3 tumor mice model. These data suggest that Pichia-L1 may be a candidate for the control of HPV infections.

The application of virus-like particles as vaccines and biological vehicles

Virus-like particles (VLPs) can be spontaneously self-assembled by viral structural proteins under appropriate conditions in vitro while excluding the genetic material and potential replication probability. In addition, VLPs possess several features including can be rapidly produced in large quantities through existing expression systems, highly resembling native viruses in terms of conformation and appearance, and displaying repeated cluster of epitopes. Their capsids can be modified via genetic insertion or chemical conjugation which facilitating the multivalent display of a homologous or heterogeneous epitope antigen. Therefore, VLPs are considered as a safe and effective candidate of prophylactic and therapeutic vaccines. VLPs, with a diameter of approximately 20 to 150 nm, also have the characteristics of nanometer materials, such as large surface area, surface-accessible amino acids with reactive moieties (e.g., lysine and glutamic acid residues), inerratic spatial structure, and good biocompatibility. Therefore, assembled VLPs have great potential as a delivery system for specifically carrying a variety of materials. This review summarized recent researches on VLP development as vaccines and biological vehicles, which demonstrated the advantages and potential of VLPs in disease control and prevention and diagnosis. Then, the prospect of VLP biology application in the future is discussed as well.

Next generation prophylactic human papillomavirus vaccines

The two licensed bivalent and quadrivalent human papillomavirus (HPV) L1 (the major papillomavirus virion protein) virus-like particle (VLP) vaccines are regarded as safe, effective, and well established prophylactic vaccines. However, they have some inherent limitations, including a fairly high production and delivery cost, virus-type restricted protection, and no reported therapeutic activity, which might be addressed with the development of alternative dosing schedules and vaccine products. A change from a three-dose to a two-dose protocol for the licensed HPV vaccines, especially in younger adolescents (aged 9-13 years), is underway in several countries and is likely to become the future norm. Preliminary evidence suggests that recipients of HPV vaccines might derive prophylactic benefits from one dose of the bivalent vaccine. Substantial interest exists in both the academic and industrial sectors in the development of second-generation L1 VLP vaccines in terms of cost reduction-eg, by production in Escherichia coli or alternative types of yeast. However, Merck's nonavalent vaccine, produced via the Saccharomyces cerevisiae production system that is also used for their quadrivalent vaccine, is the first second-generation HPV VLP vaccine to be available on the market. By contrast, other pharmaceutical companies are developing microbial vectors that deliver L1 genes. These two approaches would add an HPV component to existing live attenuated vaccines for measles and typhoid fever. Prophylactic vaccines that are based on induction of broadly cross-neutralising antibodies to L2, the minor HPV capsid protein, are also being developed both as simple monomeric fusion proteins and as virus-like display vaccines. The strong interest in developing the next generation of vaccines, particularly by manufacturers in middle-to-high income countries, increases the likelihood that vaccine production will become decentralised with the hope that effective HPV vaccines will be made increasingly available in low-resource settings where they are most needed.

Development of an IP-Free Biotechnology Platform for Constitutive Production of HPV16 L1 Capsid Protein Using the Pichia pastoris PGK1 Promoter

The human papillomavirus (HPV) L1 major capsid protein, which forms the basis of the currently available vaccines against cervical cancer, self-assembles into virus-like particles (VLPs) when expressed heterologously. We report the development of a biotechnology platform for HPV16 L1 protein expression based on the constitutive PGK1 promoter (P_PGK1) from the methylotrophic yeast Pichia pastoris. The L1 gene was cloned under regulation of P_PGK1 into pPGKΔ3 expression vector to achieve intracellular expression. In parallel, secretion of the L1 protein was obtained through the use of an alternative vector called pPGKΔ3α, in which a codon optimized α-factor signal sequence was inserted. We devised a work-flow based on the detection of the L1 protein by dot blot, colony blot, and western blot to classify the positive clones. Finally, intracellular HPV VLPs assembly was demonstrated for the first time in yeast cells. This study opens up perspectives for the establishment of an innovative platform for the production of HPV VLPs or other viral antigens for vaccination purposes, based on constitutive expression in P. pastoris.

Current status of human papillomavirus vaccines

Cervical cancer is a malignant neoplasm arising from cells that originate in the cervix uteri. It is the second most prevalent cancer among women. It can have several causes; an infection with some type of human papillomavirus (HPV) is the greatest risk factor for cervical cancer. Over 100 types of HPVs have been identified, and more than 40 types of HPVs are typically transmitted through sexual contact and infect the anogenital region. Among these, a number of HPVs types, containing types 16 and 18, are classified as "high-risk" HPVs that can cause cervical cancer. The HPVs vaccine prevents infection with certain species of HPVs associated with the development of cervical cancer, genital warts, and some less common cancers. Two HPVs vaccines are currently on the global market: quadrivalent HPVs vaccine and bivalent HPV vaccine that use virus-like particles as a vaccine antigen. This review discusses the current status of HPVs vaccines on the global market, clinical trials, and the future of HPVs vaccine development.

Applications of recombinant Pichia pastoris in the healthcare industry

Since the 1970s, the establishment and development of the biotech industry has improved exponentially, allowing the commercial production of biopharmaceutical proteins. Nowadays, new recombinant protein production is considered a multibillion-dollar market, in which about 25% of commercial pharmaceuticals are biopharmaceuticals. But to achieve a competitive production process is not an easy task. Any production process has to be highly productive, efficient and economic. Despite that the perfect host is still not discovered, several research groups have chosen Pichia pastoris as expression system for the production of their protein because of its many features. The attempt of this review is to embrace several research lines that have adopted Pichia pastoris as their expression system to produce a protein on an industrial scale in the health care industry.

Immunogenic assessment of plant-produced human papillomavirus type 16 L1/L2 chimaeras

Cervical cancer is caused by infection with human papillomaviruses (HPV) and is a global concern, particularly in developing countries, which have ~80% of the burden. HPV L1 virus-like particle (VLP) type-restricted vaccines prevent new infections and associated disease. However, their high cost has limited their application, and cytological screening programmes are still required to detect malignant lesions associated with the nonvaccine types. Thus, there is an urgent need for cheap second-generation HPV vaccines that protect against multiple types. The objective of this study was to express novel HPV-16 L1-based chimaeras, containing cross-protective epitopes from the L2 minor capsid protein, in tobacco plants. These L1/L2 chimaeras contained epitope sequences derived from HPV-16 L2 amino acid 108-120, 56-81 or 17-36 substituted into the C-terminal helix 4 (h4) region of L1 from amino acid 414. All chimaeras were expressed in Nicotiana benthamiana via an Agrobacterium-mediated transient system and targeted to chloroplasts. The chimaeras were highly expressed with yields of ~1.2 g/kg plant tissue; however, they assembled differently, indicating that the length and nature of the L2 epitope affect VLP assembly. The chimaera containing L2 amino acids 108-120 was the most successful candidate vaccine. It assembled into small VLPs and elicited anti-L1 and anti-L2 responses in mice, and antisera neutralized homologous HPV-16 and heterologous HPV-52 pseudovirions. The other chimaeras predominantly assembled into capsomeres and other aggregates and elicited weaker humoral immune responses, demonstrating the importance of VLP assembly for the immunogenicity of candidate vaccines.

Virus-like particles as a highly efficient vaccine platform: diversity of targets and production systems and advances in clinical development

Virus-like particles (VLPs) are a class of subunit vaccines that differentiate themselves from soluble recombinant antigens by stronger protective immunogenicity associated with the VLP structure. Like parental viruses, VLPs can be either non-enveloped or enveloped, and they can form following expression of one or several viral structural proteins in a recombinant heterologous system. Depending on the complexity of the VLP, it can be produced in either a prokaryotic or eukaryotic expression system using target-encoding recombinant vectors, or in some cases can be assembled in cell-free conditions. To date, a wide variety of VLP-based candidate vaccines targeting various viral, bacterial, parasitic and fungal pathogens, as well as non-infectious diseases, have been produced in different expression systems. Some VLPs have entered clinical development and a few have been licensed and commercialized. This article reviews VLP-based vaccines produced in different systems, their immunogenicity in animal models and their status in clinical development.

Production of human papillomavirus type 33 L1 major capsid protein and virus-like particles from Bacillus subtilis to develop a prophylactic vaccine against cervical cancer

We developed a bacterial expression system to produce human papillomavirus (HPV) type 33 L1 major capsid protein and virus-like particles from a recombinant Bacillus subtilis strain. For the first time, we have isolated self-assembled virus-like particles (VLPs) of HPV type 33 from B. subtilis, a strain generally recognized as safe (GRAS). The gene encoding the major capsid protein L1 of HPV type 33 was amplified from viral DNA isolated from a Korean patient and expressed in B. subtilis; a xylose-induction system was used to control gene activity. HPV33 L1 protein was partially purified by 40% (w/v) sucrose cushion centrifugation and strong cation exchange column chromatography. Eluted samples exhibited immunosignaling in fractions of 0.5-1.0 M NaCl. The HPV33 L1 protein was shown to be approximately 56 kDa in size by SDS-PAGE and Western blotting; recovery and purity were quantified by indirect immuno-ELISA assay. The final yield and purity were approximately 20.4% and 10.3%, respectively. Transmission electron microscopic analysis of fractions immunoactive by ELISA revealed that the L1 protein formed self-assembled VLPs with a diameter of approximately 20-40 nm. Humoral and cellular immune responses provoked by the B. subtilis/HPV33 L1 strain were approximately 100- and 3-fold higher than those of the empty B. subtilis strain as a negative control, respectively. Development of a VLP production and delivery system using B. subtilis will be helpful, in that the vaccine may be convenient production as an antigen delivery system. VLPs thus produced will be safer for human use than those purified from Gram-negative strains such as Escherichia coli. Also, use of B. subtilis as a host may aid in the development of either live or whole cell vaccines administered by antigen delivery system.

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.

Production of L1 protein from different types of HPV in Pichia pastoris using an integrative vector

Human papillomavirus (HPV) infection is the most common sexually transmitted disease in the world and is related to the etiology of cervical cancer. The most common high-risk HPV types are 16 and 18; however, the second most prevalent type in the Midwestern region of Brazil is HPV-33. New vaccine strategies against HPV have shown that virus-like particles (VLP) of the major capsid protein (L1) induce efficient production of antibodies, which confer protection against the same viral type. The methylotrophic yeast Pichia pastoris is an efficient and inexpensive expression system for the production of high levels of heterologous proteins stably using a wild-type gene in combination with an integrative vector. It was recently demonstrated that P. pastoris can produce the HPV-16 L1 protein by using an episomal vector associated with the optimized L1 gene. However, the use of an episomal vector is not appropriate for protein production on an industrial scale. In the present study, the vectors were integrated into the Pichia genome and the results were positive for L1 gene transcription and protein production, both intracellularly and in the extracellular environment. Despite the great potential for expression by the P. pastoris system, our results suggest a low yield of L1 recombinant protein, which, however, does not make this system unworkable. The achievement of stable clones containing the expression cassettes integrated in the genome may permit optimizations that could enable the establishment of a platform for the production of VLP-based vaccines.

Expression of codon optimized major capsid protein (L1) of human papillomavirus type 16 and 18 in Pichia pastoris; purification and characterization of the virus-like particles

The major capsid protein (L1) of human papillomaviruses (HPV) expressed in heterologous systems assembles into virus-like particles (VLPs). We report cloning and expression of codon optimized HPV L1 genes of the two high-risk HPV types 16 and 18 in methylotropic yeast, Pichia pastoris. The VLPs produced in P. pastoris were subjected to three step purification method involving density gradient centrifugations and size exclusion chromatography. The enriched VLPs were characterized using conformation-specific monoclonal antibodies in ELISA and by transmission electron microscopy. Mice immunized with a bivalent HPV16 and HPV18 VLPs developed high serum antibody titers to both HPV types that persisted for 190 days post vaccination. Serum of mice immunized with the HPV-VLP preparations could neutralize homologous pseudoviruses in an in vitro assays. Our results demonstrate that the L1 proteins expressed in P. pastoris fold properly as evidenced by assembly into VLPs and induction of type-specific neutralizing antibody response in mice. This work constitutes a step towards developing an alternate production platform for generating an affordable HPV vaccine to meet the needs of developing countries.

Expression, purification and characterization of low-glycosylation influenza neuraminidase in α-1,6-mannosyltransferase defective Pichia pastoris

Influenza A viruses expose two major surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA). Although N-glycosylation is essential for many glycoproteins, the glycoproteins expressed in yeast are sometimes hyper-glycosylated, which maybe a primary hindrance to the exploitation of therapeutic glycoprotein production because glycoproteins decorated with yeast-specific glycans are immunogenic and show poor pharmacokinetic properties in humans. To elucidate the NA with different glycosylation in interaction with immunogenicity, here we reported the heterologous expression of influenza NA glycoprotein derived from influenza virus A/newCaledonia/20/99(H1N1) in wide-type Pichia pastoris, α-1,6-mannosyltransferase (och1)-defective P. pastoris and Escherichia coli. We also assessed the immunogenicity of hyper-glycosylated NA expressed in the wide-type, low-glycosylated NA expressed in och1-defective P. pastoris strain and non-glycosylated NA produced in E. coli. Recombinant NA was expressed in wide-type P. pastoris as a 59-97 above kDa glycoprotein, 52-57 kDa in the och1 defective strain, and as a 45 kDa non-glycoprotein in E. coli. The antibody titers of Balb/c mice were tested after the mice were immunized three times with 0.2, 1, or 3 μg purified recombinant NA. Our results demonstrated that after the second immunization, the antibody titer elicited with 1 μg low-glycosylated NA was 1:5,500, while it was 1:10 and 1:13 when elicited by 1 μg hyper-glycosylated and non-glycosylated NA. In the 0.2 μg dose groups, a high antibody titer (1:4,900) was only found after third immunization by low-glycosylated NA, respectively. These results suggest that low-glycosylation in och1-defective P. pastoris enhances the immunogenicity of recombinant NA and elicits similar antibody titers with less antigen when compared with hyper- and non-glycosylated NA. Thus, och1-defective P. pastoris may be a better yeast expression system for production of glycoproteins to research immunogenic characterization.

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.

Contributions of Saccharomyces cerevisiae to understanding mammalian gene function and therapy

Due to its genetic tractability and ease of manipulation, the yeast Saccharomyces cerevisiae has been extensively used as a model organism to understand how eukaryotic cells grow, divide, and respond to environmental changes. In this chapter, we reasoned that functional annotation of novel genes revealed by sequencing should adopt an integrative approach including both bioinformatics and experimental analysis to reveal functional conservation and divergence of complexes and pathways. The techniques and resources generated for systems biology studies in yeast have found a wide range of applications. Here we focused on using these technologies in revealing functions of genes from mammals, in identifying targets of novel and known drugs and in screening drugs targeting specific proteins and/or protein-protein interactions.