Expression of human papillomavirus proteins in yeast Saccharomyces cerevisiae

Optimization, characterization, comparison of self-assembly VLP of capsid protein L1 in yeast and reverse vaccinology design against human papillomavirus type 52

BACKGROUND

Vaccination is the one of the agendas of many countries to reduce cervical cancer caused by the Human papillomavirus. Currently, VLP-based vaccine is the most potent vaccine against HPV, which could be produced by a variety of expression systems. Our study focuses on a comparison of recombinant protein expression L1 HPV52 using two common yeasts, Pichia pastoris and Hansenula polymorpha that have been used for vaccine production on an industrial scale. We also applied bioinformatics approach using reverse vaccinology to design alternative multi-epitope vaccines in recombinant protein and mRNA types.

RESULTS

Our study found that P. pastoris relatively provided higher level of L1 protein expression and production efficiency compared to H. polymorpha in a batch system. However, both hosts showed self-assembly VLP formation and stable integration during protein induction. The vaccine we have designed exhibited high immune activation and safe in computational prediction. It is also potentially suitable for production in a variety of expression systems.

CONCLUSION

By monitoring the overall optimization parameter assessment, this study can be used as the basis reference for large-scale production of the HPV52 vaccine.

An in vivo gene amplification system for high level expression in Saccharomyces cerevisiae

Bottlenecks in metabolic pathways due to insufficient gene expression levels remain a significant problem for industrial bioproduction using microbial cell factories. Increasing gene dosage can overcome these bottlenecks, but current approaches suffer from numerous drawbacks. Here, we describe HapAmp, a method that uses haploinsufficiency as evolutionary force to drive in vivo gene amplification. HapAmp enables efficient, titratable, and stable integration of heterologous gene copies, delivering up to 47 copies onto the yeast genome. The method is exemplified in metabolic engineering to significantly improve production of the sesquiterpene nerolidol, the monoterpene limonene, and the tetraterpene lycopene. Limonene titre is improved by 20-fold in a single engineering step, delivering ∼1 g L^-1 in the flask cultivation. We also show a significant increase in heterologous protein production in yeast. HapAmp is an efficient approach to unlock metabolic bottlenecks rapidly for development of microbial cell factories.

Head-to-Head Comparison of Modular Vaccines Developed Using Different Capsid Virus-Like Particle Backbones and Antigen Conjugation Systems

Capsid virus-like particles (cVLPs) are used as molecular scaffolds to increase the immunogenicity of displayed antigens. Modular platforms have been developed whereby antigens are attached to the surface of pre-assembled cVLPs. However, it remains unknown to what extent the employed cVLP backbone and conjugation system may influence the immune response elicited against the displayed antigen. Here, we performed a head-to-head comparison of antigen-specific IgG responses elicited by modular cVLP-vaccines differing by their employed cVLP backbone or conjugation system, respectively. Covalent antigen conjugation (i.e., employing the SpyTag/SpyCatcher system) resulted in significantly higher antigen-specific IgG titers compared to when using affinity-based conjugation (i.e., using biotin/streptavidin). The cVLP backbone also influenced the antigen-specific IgG response. Specifically, vaccines based on the bacteriophage AP205 cVLP elicited significantly higher antigen-specific IgG compared to corresponding vaccines using the human papillomavirus major capsid protein (HPV L1) cVLP. In addition, the AP205 cVLP platform mediated induction of antigen-specific IgG with a different subclass profile (i.e., higher IgG2a and IgG2b) compared to HPV L1 cVLP. These results demonstrate that the cVLP backbone and conjugation system can individually affect the IgG response elicited against a displayed antigen. These data will aid the understanding and process of tailoring modular cVLP vaccines to achieve improved immune responses.

Advantages and Prospects of Tag/Catcher Mediated Antigen Display on Capsid-Like Particle-Based Vaccines

Capsid-like particles (CLPs) are multimeric, repetitive assemblies of recombinant viral capsid proteins, which are highly immunogenic due to their structural similarity to wild-type viruses. CLPs can be used as molecular scaffolds to enable the presentation of soluble vaccine antigens in a similar structural format, which can significantly increase the immunogenicity of the antigen. CLP-based antigen display can be obtained by various genetic and modular conjugation methods. However, these vary in their versatility as well as efficiency in achieving an immunogenic antigen display. Here, we make a comparative review of the major CLP-based antigen display technologies. The Tag/Catcher-AP205 platform is highlighted as a particularly versatile and efficient technology that offers new qualitative and practical advantages in designing modular CLP vaccines. Finally, we discuss how split-protein Tag/Catcher conjugation systems can help to further propagate and enhance modular CLP vaccine designs.

Preparation and properties of a papillomavirus infectious intermediate and its utility for neutralization studies

We show that minor capsid protein L2 is full length in clinical virion isolates and prepare furin-cleaved pseudovirus (fcPsV) as a model of the infectious intermediate for multiple human papillomavirus (HPV) types. These fcPsV do not require furin for in vitro infection, and are fully infectious in vivo. Both the γ-secretase inhibitor XXI and carrageenan block fcPsV infection in vitro and in vivo implying that they act after furin-cleavage of L2. Despite their enhanced exposure of L2 epitopes, vaccination with fcPsV particles fails to induce L2 antibody, although L1-specific responses are similar to PsV with intact L2. FcPsV can be applied in a simple, high-throughput neutralization assay that detects L2-specific neutralizing antibodies with >10-fold enhanced sensitivity compared with the PsV-based assay. The PsV and fcPsV-based assays exhibit similar sensitivity for type-specific antibodies elicited by L1 virus-like particles (VLP), but the latter improves detection of L1-specific cross-type neutralizing antibodies.

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.

Advances in human papilloma virus vaccines: a patent review

INTRODUCTION

Human papilloma virus (HPV) infection is the main factor associated with the development of cervical cancer. The currently available HPV vaccines, Gardasil and Cervarix, can prevent infection by certain HPV types, but not all. At present, research efforts are being devoted to developing more broad spectrum preventative vaccines, as well as therapeutic vaccines.

AREAS COVERED

Recent advances in HPV vaccine development are reviewed in this paper, with a focus on worldwide patents and patent applications. In principle, patents that have been granted since 2002 are covered. Exceptions are the patents pending at PCT stage and recent patent applications since 2009. Readers will gain insights into the cutting-edge technologies being used in the development and production of vaccines, as well as adjuvant systems.

EXPERT OPINION

In the future, the use of mosaic virus-like particles (VLPs,) comprising at least one L1 protein of each HPV type, may be able to prevent infection by all HPV types while patented codon-optimization techniques and the use of edible or DNA-based vaccines may be good places to start for reducing costs. Future vaccines should ideally have both preventive and therapeutic efficacies. Enhanced immunogenicity could be achieved by the use of more effective adjuvants, such as nanoparticle-based delivery systems, or new classes of adjuvants.

Study of infectious virus production from HPV18/16 capsid chimeras

Using the HPV18 genome as the backbone, we exchanged the HPV18 L2 or L1 genes with those of HPV16. The intertypical exchange of HPV18 L1 with the HPV16 L1 produced genomes that efficiently replicated and produced infectious virus. Genomes containing an intertypical exchange of HPV18 L2 for the HPV16 L2 failed to produce infectious virus in multiple independently derived cell lines. Using chimeric constructs of individual capsid proteins, we identified a type-specific domain at the N-terminus of the HPV18L1 capsid protein, which interferes with its ability to cooperate with the HPV16 L2 protein to form infectious viral particles. Deletion of this domain allows for the cooperation of the HPV18 L1 protein and HPV16 L2 protein and production of infectious progeny. In addition, cooperation of this N-terminal HPV18 L1 deletion mutant protein with the wild-type HPV18 L2 protein efficiently replicates infectious virus but changes occur in the viral structure.

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

Human papillomaviruses (HPVs) are responsible for the most common human sexually transmitted viral infections. Infection with high-risk HPVs, particularly HPV16, is associated with the development of cervical cancer. The papillomavirus L1 major capsid protein, the basis of the currently marketed vaccines, self-assembles into virus-like particles (VLPs). Here, we describe the expression, purification and characterization of recombinant HPV16 L1 produced by a methylotrophic yeast. A codon-optimized HPV16 L1 gene was cloned into a non-integrative expression vector under the regulation of a methanol-inducible promoter and used to transform competent Pichia pastoris cells. Purification of L1 protein from yeast extracts was performed using heparin-sepharose chromatography, followed by a disassembly/reassembly step. VLPs could be assembled from the purified L1 protein, as demonstrated by electron microscopy. The display of conformational epitopes on the VLPs surface was confirmed by hemagglutination and hemagglutination inhibition assays and by immuno-electron microscopy. This study has implications for the development of an alternative platform for the production of a papillomavirus vaccine that could be provided by public health programs, especially in resource-poor areas, where there is a great demand for low-cost vaccines.

Expression and purification of His-tagged HPV16 E7 protein active in pRb binding

Human papillomavirus type 16 (HPV16) protein E7 is the major oncogenic factor associated with the development of human cervical cancer. The transforming activity of the E7 protein is linked to its interaction with host regulatory proteins such as the retinoblastoma tumor suppressor protein. The recombinant production of E7 protein is a prerequisite for its structural and functional characterization as well as for the development of various preventive and therapeutic strategies. We present an approach to enhance the soluble expression of His-tagged E7 protein by optimization of the E7 gene and the expression conditions in the host Escherichia coli. We also report a detailed protocol for the purification of E7 protein by standard chromatographic methods. The binding of E7 protein to the recombinant non-phosphorylated form of retinoblastoma protein was examined by ELISA and surface plasmon resonance analysis. These studies confirm that the recombinant His-tagged E7 protein retains its conformational properties and biological activity.

High-risk mucosal human papillomavirus infections during infancy & childhood

Human papillomaviruses (HPVs) are small DNA tumour viruses associated with a variety of proliferative diseases. More than 100 types have been identified and can broadly be grouped into cutaneous and mucosal types according to their site of infection, and can be further subdivided into low-risk (LR) and high-risk (HR) types depending upon their association with malignancy. The main route of transmission of HR mucosal HPVs is through sexual contact, although the acquisition of virus cannot be entirely explained by this mode alone. Evidence also exists for horizontal transmission by other routes and vertical transmission. HR HPVs, particularly HPV-16, have been detected in oral swabs from newborns, infants and children. Such alternative modes of transmission and acquisition may have an important impact in several areas, including vaccination strategies, epidemiological studies, and the clinical management of children with HPV-associated diseases. This article reviews the literature describing the detection of HPV infections during infancy and childhood and provides evidence for a role of vertical transmission in the spread of HPV infection.

Cervical cancer-causing human papillomaviruses have an alternative initiation site for the L1 protein

All known sequences of the DNA encoding the major cervical cancer-causing human papillomavirus type 16 (HPV16) L1 capsid protein contain initiation codons which would allow translation to begin at either nucleotide 5559 or 5637. However the formation of virus-like particles (VLPs) only occurs efficiently when the initiation codon at nucleotide 5637 is used for in vitro expression studies. This knowledge, in concert with the fact that virions have not been observed in HPV16-infected epithelium, raises the notion that the major L1 translation product in this HPV type may be largely confined to initiation at nucleotide 5559. Sequence analysis of various HPV types associated with particular clinical outcomes has revealed that L1 sequences of the major cervical cancer-associated viruses generally possess the ability to encode a longer translation product whilst the non-cancer-causing viruses do not. Equally intriguing, the upstream initiation codon is always separated by 78 nucleotides from the initiation codon that produces L1 protein which efficiently assembles into VLPs. We speculate that the longer L1 protein could play a role in the development of cervical carcinoma and that HPVs with the potential to cause cervical cancer may be identified by the presence of an in-frame ATG situated 78 nucleotides upstream.

Improvement of downstream processing of recombinant proteins by means of genetic engineering methods

The rapid advancement of genetic engineering has allowed to produce an impressive number of proteins on a scale which would not have been achieved by classical biotechnology. At the beginning of this development research was focussed on elucidating the mechanisms of protein overexpression. The appearance of inclusion bodies may illustrate the success. In the meantime, genetic engineering is not only expected to achieve overexpression, but to improve the whole process of protein production. For downstream processing of recombinant proteins, the synthesis of fusion proteins is of primary importance. Fusion with certain proteins or peptides may protect the target protein from proteolytic degradation and may alter its solubility. Intracellular proteins may be translocated by means of fusions with signal peptides. Affinity tags as fusion complements may render protein separation and purification highly selective. These methods as well as similar ones for improving the downstream processing of proteins will be discussed on the basis of recent literature.

Interaction of the HPV E7 proteins with the pCAF acetyltransferase

Most cervical carcinomas express the E6 and E7 proteins of a high-risk human papillomavirus (HPV). These proteins affect growth control by interfering with the functions of cell regulatory proteins, promoting oncogenic transformation. A key target of E7 is the tumor suppressor protein pRb, which directly interacts with E7. However, binding to additional cellular regulatory proteins is clearly required for oncogenesis, as mutants of E7 have been identified that bind to pRb, yet fail to transform efficiently. Here we demonstrate the interaction of the HPV 6, 16 and 18 E7 proteins with the pCAF acetyltransferase, which has been reported to function as a coactivator for a variety of transcription factors including p53. Mutation of a highly conserved leucine residue within the zinc finger region of HPV 16 E7 disrupts binding to pCAF and also impairs transformation and transcriptional activation. HPV 16 E7 interacts with the acetyltransferase domain of pCAF, and reduces its acetyltransferase activity in vitro. Our analysis of the interaction between the pCAF acetyltransferase and E7 provides new insight into the mechanisms by which the E7 oncoproteins can alter cellular gene expression and growth.

Production of human papillomavirus type 16 E7 protein in Lactococcus lactis

The E7 protein of human papillomavirus type 16 was produced in Lactococcus lactis. Secretion allowed higher production yields than cytoplasmic production. In stationary phase, amounts of cytoplasmic E7 were reduced, while amounts of secreted E7 increased, suggesting a phase-dependent intracellular proteolysis. Fusion of E7 to the staphylococcal nuclease, a stable protein, resulted in a highly stable cytoplasmic protein. This work provides new candidates for development of viral screening systems and for oral vaccine against cervical cancer.

Interaction of the E1A oncoprotein with Yak1p, a novel regulator of yeast pseudohyphal differentiation, and related mammalian kinases

The C-terminal portion of adenovirus E1A suppresses ras-induced metastasis and tumorigenicity in mammalian cells; however, little is known about the mechanisms by which this occurs. In the simple eukaryote Saccharomyces cerevisiae, Ras2p, the homolog of mammalian h-ras, regulates mitogen-activated protein kinase (MAPK) and cyclic AMP-dependent protein kinase A (cAMP/PKA) signaling pathways to control differentiation from the yeast form to the pseudohyphal form. When expressed in yeast, the C-terminal region of E1A induced pseudohyphal differentiation, and this was independent of both the MAPK and cAMP/PKA signaling pathways. Using the yeast two-hybrid system, we identified an interaction between the C-terminal region of E1A and Yak1p, a yeast dual-specificity serine/threonine protein kinase that functions as a negative regulator of growth. E1A also physically interacts with Dyrk1A and Dyrk1B, two mammalian homologs of Yak1p, and stimulates their kinase activity in vitro. We further demonstrate that Yak1p is required in yeast to mediate pseudohyphal differentiation induced by Ras2p-regulated signaling pathways. However, pseudohyphal differentiation induced by the C-terminal region of E1A is largely independent of Yak1p. These data suggest that mammalian Yak1p-related kinases may be targeted by the E1A oncogene to modulate cell growth.

Expression, purification and immunological characterization of the transforming protein E7, from cervical cancer-associated human papillomavirus type 16

E7 is the major oncogenic protein produced in cervical cancer-associated human papillomavirus type 16 (HPV16). This protein was expressed in Escherichia coli as a glutathione-S-transferase (GST) fusion protein. E7-enriched inclusion bodies were collected from bacterial lysates, were solubilized in 10 M urea, and the protein was purified using anion exchange column chromatography. After removal of endotoxin with serial Triton X-114 extractions, material of high purity (about 90%) was obtained, which is suitable for use in a human clinical trial. This material was immunogenic, and when used as a vaccine, protected mice against challenge with an HPV16 E7 DNA transfected tumour cell line. Based on this observation, the E7GST fusion protein is currently being used in a human clinical trial of a vaccine against HPV16-induced cervical cancer. This fusion protein could be cleaved with thrombin to remove the GST fusion part and further purified by preparative SDS gel electrophoresis to obtain free E7 with > 98% purity.

The L1 major capsid protein of human papillomavirus type 11 recombinant virus-like particles interacts with heparin and cell-surface glycosaminoglycans on human keratinocytes

The L1 major capsid protein of human papillomavirus (HPV) type 11, a 55-kDa polypeptide, forms particulate structures resembling native virus with an average particle diameter of 50-60 nm when expressed in the yeast Saccharomyces cerevisiae. We show in this report that these virus-like particles (VLPs) interact with heparin and with cell-surface glycosaminoglycans (GAGs) resembling heparin on keratinocytes and Chinese hamster ovary cells. The binding of VLPs to heparin is shown to exhibit an affinity comparable to that of other identified heparin-binding proteins. Immobilized heparin chromatography and surface plasmon resonance were used to show that this interaction can be specifically inhibited by free heparin and dextran sulfate and that the effectiveness of the inhibitor is related to its molecular weight and charge density. Sequence comparison of nine human L1 types revealed a conserved region of the carboxyl terminus containing clustered basic amino acids that bear resemblance to proposed heparin-binding motifs in unrelated proteins. Specific enzymatic cleavage of this region eliminated binding to both immobilized heparin and human keratinocyte (HaCaT) cells. Removal of heparan sulfate GAGs on keratinocytes by treatment with heparinase or heparitinase resulted in an 80-90% reduction of VLP binding, whereas treatment of cells with laminin, a substrate for alpha6 integrin receptors, provided minimal inhibition. Cells treated with chlorate or substituted beta-D-xylosides, resulting in undersulfation or secretion of GAG chains, also showed a reduced affinity for VLPs. Similarly, binding of VLPs to a Chinese hamster ovary cell mutant deficient in GAG synthesis was shown to be only 10% that observed for wild type cells. This report establishes for the first time that the carboxyl-terminal portion of HPV L1 interacts with heparin, and that this region appears to be crucial for interaction with the cell surface.

A general purification protocol for E7 proteins from "high- and low-risk" human papillomavirus types expressed in the yeast Schizosaccharomyces pombe

A purification protocol was developed to obtain human papillomavirus (HPV) type 16 E7 protein expressed in the yeast Schizosaccharomyces pombe. Only three chromatographic steps were necessary to purify the unfused HPV 16 E7 protein to homogeneity (95-99%) as shown by silver staining after polyacrylamide gel electrophoresis. Approximately 0.8 mg of highly purified E7 was obtained from 5 x 10(10) yeast cells. The purified HPV 16 E7 phosphoprotein (Ser 31/32) was refolded and assayed for functionality. Binding to the proteins Rb1 and p107 in vitro and induction of DNA synthesis after microinjection into serum-deprived NIH 3T3 cells suggest that the E7 protein retains some of its biological activities. Most importantly, the purification strategy is also applicable for different HPV 16 E7 mutants and for E7 proteins from other HPV types such as HPV 18 and 11.

Predicted alpha-helix/beta-sheet secondary structures for the zinc-binding motifs of human papillomavirus E7 and E6 proteins by consensus prediction averaging and spectroscopic studies of E7

The E7 and E6 proteins are the main oncoproteins of human papillomavirus types 16 and 18 (HPV-16 and HPV-18), and possess unknown protein structures. E7 interacts with the cellular tumour-suppressor protein pRB and contains a zinc-binding site with two Cys-Xaa2-Cys motifs spaced 29 or 30 residues apart. E6 interacts with another cellular tumour-suppressor protein p53 and contains two zinc-binding sites, each with two Cys-Xaa2-Cys motifs at a similar spacing of 29 or 30 residues. By using the GOR I/III, Chou-Fasman, SAPIENS and PHD methods, the effectiveness of consensus secondary structure predictions on zinc-finger proteins was first tested with sequences for 160 transcription factors and 72 nuclear hormone receptors. These contain Cys2His2 and Cys2Cys2 zinc-binding regions respectively, and possess known atomic structures. Despite the zinc- and DNA-binding properties of these protein folds, the major alpha-helix structures in both zinc-binding regions were correctly identified. Thus validated, the use of these prediction methods with 47 E7 sequences indicated four well-defined alpha-helix (alpha) and beta-sheet (beta) secondary structure elements in the order beta beta alpha beta in the zinc-binding region of E7 at its C-terminus. The prediction was tested by Fourier transform infrared spectroscopy of recombinant HPV-16 E7 in H2O and 2H2O buffers. Quantitative integration showed that E7 contained similar amounts of alpha-helix and beta-sheet structures, in good agreement with the averaged prediction of alpha-helix and beta-sheet structures in E7 and also with previous circular dichroism studies. Protein fold recognition analyses predicted that the structure of the zinc-binding region in E7 was similar to a beta beta alpha beta motif found in the structure of Protein G. This is consistent with the E7 structure predictions, despite the low sequence similarities with E7. This predicted motif is able to position four Cys residues in proximity to a zinc atom. A model for the zinc-binding motif of E7 was constructed by combining the Protein G coordinates with those for the zinc-binding site in transcription factor TFIIS. Similar analyses for the two zinc-binding motifs in E6 showed that they have different alpha/beta secondary structures from that in E7. When compared with 12 other zinc-binding proteins, these results show that E7 and E6 are predicted to possess novel types of zinc-binding structure.

Engineering an intracellular pathway for major histocompatibility complex class II presentation of antigens

The presentation of antigenic peptides by major histocompatibility complex (MHC) class II molecules to CD4+ T cells is critical to the function of the immune system. In this study, we have utilized the sorting signal of the lysosomal-associated membrane protein LAMP-1 to target a model antigen, human papillomavirus 16 E7 (HPV-16 E7), into the endosomal and lysosomal compartments. The LAMP-1 sorting signal reroutes the antigen into the MHC class II processing pathway, resulting in enhanced presentation to CD4+ cells in vitro. In vivo immunization experiments in mice demonstrated that vaccinia containing the chimeric E7/LAMP-1 gene generated greater E7-specific lymphoproliferative activity, antibody titers, and cytotoxic T-lymphocyte activities than vaccinia containing the wild-type HPV-16 E7 gene. These results suggest that specific targeting of an antigen to the endosomal and lysosomal compartments enhances MHC class II presentation and vaccine potency.

Strategies for mapping and imitating viral B-cell epitopes

Identification of viral B-cell epitopes is of importance in the selection of peptides for inclusion into subunit vaccines, the development of virus-specific serological tests and understanding the interaction of antibodies with viruses at a molecular level. B-cell epitopes can often be determined unequivocally by X-ray analyses of antibody-antigen complexes. This technique is, however, time-consuming and alternative strategies have now been developed for identifying epitopes. This article provides an overview of approaches which are currently available for mapping and imitating B-cell epitopes.

Human papillomavirus type 16 E6, E7 and L1 and type 18 E7 proteins produced by recombinant baculoviruses

Proteins derived from the E6, E7 and L1 ORFs of HPV16 and the E7 ORF of HPV18 were produced in insect cells using a baculovirus expression system. HPV ORFs were inserted into baculovirus transfer vectors pAcYM1 or pVL1393/2, and recombinant baculoviruses isolated using a combination of limiting dilution and plaque assay. Using HPV-specific antisera and monoclonal antibodies HPV proteins were identified in lysates of Spodoptera frugiperda (Sf-21) cells infected with HPV-recombinant baculovirus. Immunoreactive HPV16 E7 protein produced in Sf-21 cells had an apparent M(r) of 19 kDa, larger than that predicted from the amino acid sequence, and similar to that of native HPV16 E7 protein in HeLa and CaSki cells. The apparent M(r) of recombinant HPV18-E7, HPV16-L1 and HPV16-E6 proteins was equivalent to the M(r) values predicted from the amino acid sequence. Thermostability studies revealed that the half-life of HPV16-E7 protein in Sf-21 cell lysate was approx. 20 h at 4 degrees C, 2 h at 22 degrees C, and less than 30 min at 37 degrees C. HPV16 L1, HPV16 E7 and HPV18 E7 proteins were predominantly localised in the nucleus of recombinant baculovirus-infected Sf-21 cells, whereas recombinant HPV 16 E6 protein was localised in both the cytoplasm and nucleus of infected insect cells. Northern blot analysis of RNA derived from insect cells infected with vAc16E6E7, a recombinant baculovirus containing both HPV16 E6 and E7 ORF's, revealed the presence of only E6 ORF transcripts, suggesting that the splicing of RNA products derived from the E6 and E7 ORF's, as observed in cervical cancer-derived cell lines, is not performed in insect cells. Baculovirus-derived HPV proteins have similar biological properties to the native proteins and should be suitable for studies on the immunology of HPV.

Towards vaccines against human papillomavirus type-16 genital infections

Human papillomavirus type-16 (HPV-16) is strongly associated with cervical carcinoma and cervical intraepithelial neoplasia. It may soon be possible to develop prophylactic vaccines designed to induce neutralizing antibodies to HPV-16 virions in genital secretions and therapeutic vaccines to induce cytotoxic T-cell responses against HPV-16 early proteins in cervical intraepithelial neoplasia and cervical cancers. Although significant advances have been achieved, problems remain before such vaccines can be used routinely.