The nine C-terminal amino acids of the major capsid protein of the human papillomavirus type 16 are essential for DNA binding and gene transfer capacity

The C-Terminal Arm of the Human Papillomavirus Major Capsid Protein Is Immunogenic and Involved in Virus-Host Interaction

Cervical cancer is the second most prevalent malignant tumor among women worldwide. High-risk human papillomaviruses (HPVs) are believed to be the major causative pathogens of mucosal epithelial cancers including cervical cancer. The HPV capsid is made up of 360 copies of major (L1) and 72 copies of minor (L2) capsid proteins. To date, limited high-resolution structural information about the HPV capsid has hindered attempts to understand details concerning the mechanisms by which HPV assembles and infects cells. In this study, we have constructed a pseudo-atomic model of the HPV59 L1-only capsid and demonstrate that the C-terminal arm of L1 participates in virus-host interactions. Moreover, when conjugated to a scaffold protein, keyhole limpet hemocyanin (KLH), this arm is immunogenic in vivo. These results provide new insights that will help elucidate HPV biology, and hence pave a way for the design of next-generation HPV vaccines.

Papillomavirus assembly: An overview and perspectives

Papillomavirus life cycle is tightly coupled to epithelial cell differentiation, which has hindered the investigation of many aspects of papillomavirus biology, including virion assembly. The development of in vitro production methods of papillomavirus pseudoviruses, and the production of "native" virus in raft cultures have facilitated the study of some aspects of the assembly process. In this paper we review the current knowledge of papillomavirus assembly, directions for future research, and the implications of these studies on new therapeutic interventions.

Expression of human papillomavirus 6b L1 protein in silkworm larvae and enhanced green fluorescent protein displaying on its virus-like particles

Human papillomavirus (HPV) 6b L1 capsid protein was expressed using the Bombyx mori nucleopolyhedrovirus (BmNPV) bacmid expression system in silkworm larvae. Two constructs, full-length L1 (500 a.a) and C-terminal-deleted short L1 (479 a.a), and three PCR-manipulated antigenic loops at amino acids 55–56, 174–175, and 348–349 regions were incorporated with whole enhanced green fluorescent protein (EGFP). Expressed in full, short L1 proteins and variants were purified in heparin affinity column chromatography and confirmed by SDS-PAGE and western blot. The presence of self-assembled virus-like particles (VLPs) and EGFP incorporation on the surface of VLPs were confirmed by the observation of transmission electron and immunoelectron microscopies, respectively. HPV 6b L1 major capsid protein was successfully expressed in silkworm, and effective manipulation on the antigenic regions showed the path to versatile vaccine development based on HPV L1-VLPs.

Identification of neutralizing conformational epitopes on the human papillomavirus type 31 major capsid protein and functional implications

The aim of this study was to characterize the conformational neutralizing epitopes of the major capsid protein of human papillomavirus type 31. Analysis of the epitopes was performed by competitive epitope mapping using 15 anti-HPV31 and by reactivity analysis using a HPV31 mutant with an insertion of a seven-amino acid motif within the FG loop of the capsid protein. Fine mapping of neutralizing conformational epitopes on HPV L1 was analyzed by a new approach using a system displaying a combinatorial library of constrained peptides exposed on E. coli flagella. The findings demonstrate that the HPV31 FG loop is dense in neutralizing epitopes and suggest that HPV31 MAbs bind to overlapping but distinct epitopes on the central part of the FG loop, in agreement with the exposure of the FG loop on the surface of HPV VLPs, and thus confirming that neutralizing antibodies are mainly located on the tip of capsomeres. In addition, we identified a crossreacting and partially crossneutralizing conformational epitope on the relatively well conserved N-terminal part of the FG loop. Moreover, our findings support the hypothesis that there is no correlation between neutralization and the ability of MAbs to inhibit VLP binding to heparan sulfate, and confirm that the blocking of virus attachment to the extracellular matrix is an important mechanism of neutralization.

Overlapping and independent structural roles for human papillomavirus type 16 L2 conserved cysteines

Cryoelectron microscopy images of HPV16 pseudovirions (PsV) depict that each pentamer of L1 can be occluded with a monomer of L2. Further research suggests that an N-terminal external loop of L2 exists, which is the target of neutralizing and cross-neutralizing antibodies. Here we show that N-terminal L2 cysteine residues, Cys22 and Cys28, have overlapping and independent structural roles, which affect both early- and late-stage assembly events. Substitution of either cysteine residue enhances infectivity markedly in comparison to wild-type HPV16. However, only Cys22Ser 20-day virions become nearly as stable as wild type. In addition, Cys22Ser, and Cys22,28Ser 20-day virions have lost their susceptibility to neutralization by anti-L2 antibodies, whereas Cys28Ser 20-day virions remain partially susceptible. These results suggest that Cys28 is necessary for late-stage stabilization of capsids, while Cys22 is necessary for proper display of L2 neutralizing epitopes.

Listeria monocytogenes delivery of HPV-16 major capsid protein L1 induces systemic and mucosal cell-mediated CD4+ and CD8+ T-cell responses after oral immunization

Neutralizing antibodies are thought to be required at mucosal surfaces to prevent human papillomavirus (HPV) transmission. However, the potential for cell-mediated immunity in mediating protection against HPV infection has not been well explored. We generated recombinant Listeria monocytogenes (Lm) constructs that secrete listeriolysin O (LLO) fused with overlapping N-terminal (LLO-L1(1-258)) or C-terminal (LLO-L1(238-474)) fragments of HPV type 16 major capsid protein L1 (HPV-16-L1). Oral immunization of mice with either construct induced IFN-gamma-producing CD8+ and CD4+ T cells in the spleen and in the Peyer's patches with the C-terminal construct. Oral immunization with both constructs resulted in diminished viral titers in the cervix and uterus of mice after intravaginal challenge with vaccinia virus expressing HPV-16-L1.

Evolutionary and structural analyses of alpha-papillomavirus capsid proteins yields novel insights into L2 structure and interaction with L1

Background

PVs (PV) are small, non-enveloped, double-stranded DNA viruses that have been identified as the primary etiological agent for cervical cancer and their potential for malignant transformation in mucosal tissue has a large impact on public health. The PV family Papillomaviridae is organized into multiple genus based on sequential parsimony, host range, tissue tropism, and histology. We focused this analysis on the late gene products, major (L1) and minor (L2) capsid proteins from the family Papillomaviridae genus Alpha-papillomavirus. Alpha-PVs preferentially infect oral and anogenital mucosa of humans and primates with varied risk of oncogenic transformation. Development of evolutionary associations between PVs will likely provide novel information to assist in clarifying the currently elusive relationship between PV and its microenvironment (i.e., the single infected cell) and macro environment (i.e., the skin tissue). We attempt to identify the regions of the major capsid proteins as well as minor capsid proteins of alpha-papillomavirus that have been evolutionarily conserved, and define regions that are under constant selective pressure with respect to the entire family of viruses.

Results

This analysis shows the loops of L1 are in fact the most variable regions among the alpha-PVs. We also identify regions of L2, involved in interaction with L1, as evolutionarily conserved among the members of alpha- PVs. Finally, a predicted three-dimensional model was generated to further elucidate probable aspects of the L1 and L2 interaction.

Translational fusion of chloroplast-expressed human papillomavirus type 16 L1 capsid protein enhances antigen accumulation in transplastomic tobacco

Human Papillomavirus (HPV) is the causal agent of cervical cancer, one of the most common causes of death for women. The major capsid L1 protein self-assembles in Virus Like Particles (VLPs), which are highly immunogenic and suitable for vaccine production. In this study, a plastid transformation approach was assessed in order to produce a plant-based HPV-16 L1 vaccine. Transplastomic plants were obtained after transformation with vectors carrying a chimeric gene encoding the L1 protein either as the native viral (L1(v) gene) or a synthetic sequence optimized for expression in plant plastids (L1(pt) gene) under control of plastid expression signals. The L1 mRNA was detected in plastids and the L1 antigen accumulated up to 1.5% total leaf proteins only when vectors included the 5'-UTR and a short N-terminal coding segment (Downstream Box) of a plastid gene. The half-life of the engineered L1 protein, determined by pulse-chase experiments, is at least 8 h. Formation of immunogenic VLPs in chloroplasts was confirmed by capture ELISA assay using antibodies recognizing conformational epitopes and by electron microscopy.

Possible role for cellular karyopherins in regulating polyomavirus and papillomavirus capsid assembly

Polyomavirus and papillomavirus (papovavirus) capsids are composed of 72 capsomeres of their major capsid proteins, VP1 and L1, respectively. After translation in the cytoplasm, L1 and VP1 pentamerize into capsomeres and are then imported into the nucleus using the cellular alpha and beta karyopherins. Virion assembly only occurs in the nucleus, and cellular mechanisms exist to prevent premature capsid assembly in the cytosol. We have identified the karyopherin family of nuclear import factors as possible "chaperones" in preventing the cytoplasmic assembly of papovavirus capsomeres. Recombinant murine polyomavirus (mPy) VP1 and human papillomavirus type 11 (HPV11) L1 capsomeres bound the karyopherin heterodimer alpha2beta1 in vitro in a nuclear localization signal (NLS)-dependent manner. Because the amino acid sequence comprising the NLS of VP1 and L1 overlaps the previously identified DNA binding domain, we examined the relationship between karyopherin and DNA binding of both mPy VP1 and HPV11 L1. Capsomeres of L1, but not VP1, bound by karyopherin alpha2beta1 or beta1 alone were unable to bind DNA. VP1 and L1 capsomeres could bind both karyopherin alpha2 and DNA simultaneously. Both VP1 and L1 capsomeres bound by karyopherin alpha2beta1 were unable to assemble into capsids, as shown by in vitro assembly reactions. These results support a role for karyopherins as chaperones in the in vivo regulation of viral capsid assembly.

Stable high-level expression of truncated human papillomavirus type 16 L1 protein in Drosophila Schneider-2 cells

To improve the existing human papillomavirus type 16 (HPV16) virus-like particle (VLP) preparation, the Drosophila inducible/secreted expression system, a highly efficient, economical method, was used to produce HPV16 VLPs. Drosophila Schneider-2 cells were cotransfected with pMT/BiP/V5-His expression vector containing the target gene encoding HPV16L1 protein without nucleus localization sequence and the selection vector pCoHygro plasmids at the ratio of 4:1. The stabled hygromycin-resistant cell line was obtained 1 month later, and the protein expression was induced by copper sulfate. The molecular mass of expressed HPV16L1 protein was 66 kDa, as revealed by SDS-PAGE, and confirmed by Western blot analysis. The yield of HPV16L1 protein was 0.554 mg per 1x10(7) cells. The characteristics of HPV16L1 protein were further analyzed by mouse erythrocyte hemagglutination assay, hemagglutination inhibition assay, and transmission electron microscopy. Results showed that the truncated protein was as biologically active as natural HPVL1 protein, inducing murine erythrocyte agglutination and VLP formation. These findings indicate that the Drosophila inducible/secreted expression system is promising as a convenient and economical method for the preparation of HPV16 VLP vaccine.

Gene transfer using recombinant rabbit hemorrhagic disease virus capsids with genetically modified DNA encapsidation capacity by addition of packaging sequences from the L1 or L2 protein of human papillomavirus type 16

The aim of this study was to produce gene transfer vectors consisting of plasmid DNA packaged into virus-like particles (VLPs) with different cell tropisms. For this purpose, we have fused the N-terminally truncated VP60 capsid protein of the rabbit hemorrhagic disease virus (RHDV) with sequences which are expected to be sufficient to confer DNA packaging and gene transfer properties to the chimeric VLPs. Each of the two putative DNA-binding sequences of major L1 and minor L2 capsid proteins of human papillomavirus type 16 (HPV-16) were fused at the N terminus of the truncated VP60 protein. The two recombinant chimeric proteins expressed in insect cells self-assembled into VLPs similar in size and appearance to authentic RHDV virions. The chimeric proteins had acquired the ability to bind DNA. The two chimeric VLPs were therefore able to package plasmid DNA. However, only the chimeric VLPs containing the DNA packaging signal of the L1 protein were able efficiently to transfer genes into Cos-7 cells at a rate similar to that observed with papillomavirus L1 VLPs. It was possible to transfect only a very limited number of RK13 rabbit cells with the chimeric RHDV capsids containing the L2-binding sequence. The chimeric RHDV capsids containing the L1-binding sequence transfer genes into rabbit and hare cells at a higher rate than do HPV-16 L1 VLPs. However, no gene transfer was observed in human cell lines. The findings of this study demonstrate that the insertion of a DNA packaging sequence into a VLP which is not able to encapsidate DNA transforms this capsid into an artificial virus that could be used as a gene transfer vector. This possibility opens the way to designing new vectors with different cell tropisms by inserting such DNA packaging sequences into the major capsid proteins of other viruses.