In vitro generation and type-specific neutralization of a human papillomavirus type 16 virion pseudotype

DNA binding of L1 is required for human papillomavirus morphogenesis in vivo

The role of putative DNA-binding domains of human papillomavirus (HPV) capsid proteins for DNA encapsidation in vivo is still unknown. We have now analyzed mutants of the major capsid protein L1 of HPV type 33, which are defective for DNA binding, for their ability to encapsidate DNA using an in vivo packaging approach. Since the DNA-binding domain and the nuclear localization signal (NLS) of L1 overlap, both a carboxy-terminal deletion mutant (L1-1/470) and a substitution mutant (L1-1/477M9) were analyzed. L1-1/477M9 has the classical NLS replaced by a noncanonical NLS taken from the human hnRNP protein A1. The mutant proteins were defective for DNA binding in contrast to wild-type (wt) L1 proteins of several HPV types. L1-1/470 localized to the cytoplasm when expressed in human cell lines, whereas L1-1/477M9 was found mainly in the nucleus. When coexpressed with wt L2, mutant L1 proteins colocalized with L2 in nuclear domains 10 (ND10) similar to wt L1 and assembled into virus-like particles incorporating L2. Wt L1 and L2 but not mutant L1 proteins and wt L2 efficiently encapsidated DNA and transferred it into cells, suggesting that physical interaction of L1 with DNA is essential for DNA inclusion into VLPs. We also compared the infectivity of histone-free DNA-containing pseudovirions generated in vitro with the infectivity of pseudovirions generated in vivo, which most likely harbor DNA in the form of chromatin. Pseudovirions generated in vivo were, on a per genome basis, four to five times more infectious than pseudovirions generated in vitro, even though no structural or biochemical difference was obvious. This indicates that chromatin may enhance the infectivity of pseudovirions.

Reorganization of nuclear domain 10 induced by papillomavirus capsid protein l2

Nuclear domains (ND) 10 are associated with proteins implicated in transcriptional regulation, growth suppression, and apoptosis. We now show that the minor capsid protein L2 of human papillomavirus (HPV) type 33 induces a reorganization of ND10-associated proteins. Whereas the promyelocytic leukemia protein, the major structural component of ND10, was unaffected by L2, Sp100 was released from ND10 upon L2 expression. The total cellular amount of Sp100, but not of Sp100 mRNA, decreased significantly, suggesting degradation of Sp100. Proteasome inhibitors induced the dispersal of Sp100 and inhibited the nuclear translocation of L2. In contrast to Sp100, Daxx was recruited to ND10 by L2 expression. Coimmunoprecipitation demonstrated interaction of the two proteins. L2-induced reorganization of ND10 was observed both in cell culture and in natural HPV lesions. The differential change in protein composition observed provides further evidence to suggest that the ND10-associated proteins are an important interface of viral life cycle and host cell.

Detection of neutralizing antibodies against human papillomaviruses (HPV) by inhibition of gene transfer mediated by HPV pseudovirions

The goal of this study was to develop a human papillomavirus (HPV) neutralization assay using HPV pseudovirions generated in vitro. For this purpose, gene transfer efficiency of HPV virus-like particles (VLPs) was improved by using direct interaction between a reporter plasmid and the VLPs. Electron microscopic observation of the interaction between DNA molecules and VLPs revealed that VLPs always interact with a single DNA molecule and that VLPs bind to the end of linearized DNA molecules. An 100-fold improvement in the gene transfer was obtained by simple interaction between a linearized DNA molecule and VLPs. Moreover, direct interaction methods offer the possibility of transferring plasmids a size higher than that of the papillomavirus genome. The approach that we developed to generate HPV-16 and HPV-31 pseudovirions proved to be suitable for testing neutralizing antibodies in human sera both after immunization and after natural infection.

Yeast coexpression of human papillomavirus types 6 and 16 capsid proteins

The L1 and L2 capsid proteins of animal and human papillomaviruses (HPVs) can self-assemble into virus-like particles (VLPs) that closely resemble native virions. The use of different animal models shows that VLPs can be very efficient at inducing a protective immune response. However, studies with infectious HPV virions and VLPs of different HPV types indicate that the immune response is predominantly type-specific. We have generated a diploid yeast strain that coexpresses the L1 and L2 capsid proteins of both HPV-6b and HPV-16, and we have purified fully assembled VLPs banding in a cesium chloride gradient at the expected density of 1.29-1.3 mg/ml. Experimental evidence strongly indicated that the four proteins coassembled into VLPs. Western blot analysis, using anti-HPV-6 and anti-HPV-16 L1-specific monoclonal antibodies and type-specific L2 antisera, demonstrated that all four proteins copurified. Most importantly, immunoprecipitation experiments, carried out using type-specific anti-L1 monoclonals and either total yeast cell extracts or purified VLPs, confirmed the interaction and the formation of covalent disulfide bonds between the two L1 proteins. Finally, HPV-6/16 VLPs administered to mice induced conformational antibodies against both L1 protein types. These results suggest that coexpression of different capsid proteins may provide new tools for the induction of antibodies directed against multiple HPV types.

Hybrid papillomavirus L1 molecules assemble into virus-like particles that reconstitute conformational epitopes and induce neutralizing antibodies to distinct HPV types

Human papillomavirus (HPV) hybrid virus-like particles (VLPs) were prepared using complementary regions of the major capsid L1 proteins of HPV-11 and -16. These hybrid L1 proteins were tested for assembly into VLPs, for presentation and mapping of conformational neutralizing epitopes, and as immunogens in rabbits and mice. Two small noncontiguous hypervariable regions of HPV-16 L1, when replaced into the HPV-11 L1 backbone, produced an assembly-positive hybrid L1 which was recognized by the type-specific, conformationally dependent HPV-16 neutralizing monoclonal antibody (N-MAb) H16.V5. Several new N-MAbs that were generated following immunization of mice with wild-type HPV-16 L1 VLPs also recognized this reconstructed VLP, demonstrating that these two hypervariable regions collectively constituted an immunodominant epitope. When a set of hybrid VLPs was tested as immunogens in rabbits, antibodies to both HPV-11 and -16 wild-type L1 VLPs were obtained. One of the hybrid VLPs containing hypervariable FG and HI loops of HPV-16 L1 replaced into an HPV-11 L1 background provoked neutralizing activity against both HPV-11 and HPV-16. In addition, conformationally dependent and type-specific MAbs to both HPV-11 and HPV-16 L1 VLP were obtained from mice immunized with hybrid L1 VLPs. These data indicated that hybrid L1 proteins can be constructed that retain VLP-assembly properties, retain type-specific conformational neutralizing epitopes, can map noncontiguous regions of L1 which constitute type-specific conformational neutralizing epitopes recognized by N-MAbs, and trigger polyclonal antibodies which can neutralize antigenically unrelated HPV types.

Prevalence of anti-human papillomavirus type 16, 18, 31, and 58 virus-like particles in women in the general population and in prostitutes

Genital human papillomavirus (HPV) infection is sexually transmitted. The aim of the study was to characterize serological responses to HPV types 16, 18, 31, and 58 by exploring type-specific virus-like particles (VLPs) in two groups of women with very distinct sexual behaviors. Anti-VLP antibodies for types 16, 18, 31, and 58 and HPV DNA in cervical cells were investigated with 177 prostitutes and 283 age-matched controls from the female general population in Spain. Anti-VLP positivity increased with number of lifetime sexual partners in women from the general population, and no seroresponse was found in virgins. However, in prostitutes HPV infection was characterized by higher multireactivity to three or four VLPs (25%) than the general population (3%) and by a more frequent antibody response to HPV-58 than in the general population. About 75% of the women seropositive for type 58 had been born in a Latin American country. Seroprevalence of HPV and cervical HPV DNA in prostitutes were 14 and 10 times higher than observed in women in the general population (prevalence odds ratio [POR] of HPV seropositivity, 14.04 [95%; CI = 8.4 to 23.6] and POR for HPV DNA, 10.4 [95% CI = 3.9 to 27.6). Our results indicate that prostitutes are at an increased risk of oncogenic HPV infections, and they confirm the validity of anti-VLPs as markers of present or past HPV infection, that the number of sexual partners is the major determinant in acquisition of oncogenic HPV, and that anti-VLPs could be used as a marker of repeated infection in prostitutes.

Gene transfer using human polyomavirus BK virus-like particles expressed in insect cells

The major structural protein (VP1) of the BK polyomavirus (BKV) was expressed in the recombinant baculovirus expression system. Recombinant BKV VP1 was shown to self-assemble into virus-like particles (VLPs) with a diameter of 45-50 nm. As for other polyomaviruses, BKV VP1 has the capacity to bind to exogenous DNA. Furthermore, the potential of BKV VP1 VLPs was investigated for gene transfer into COS-7 cells using three methods for the formation of pseudo-virions: disassembly/reassembly, osmotic shock and direct interaction between VLPs and reporter plasmid DNA. The latter method was shown to be the most efficient when using linearized plasmid. Gene transfer efficiency with BKV pseudo-virions was of the same order as that observed with human papillomavirus type 16 L1 protein VLPs. In addition, it is demonstrated that cellular entry of BKV pseudo-virions is dependent on cell surface sialic acid.

Effect of preexisting neutralizing antibodies on the anti-tumor immune response induced by chimeric human papillomavirus virus-like particle vaccines

Chimeric human papillomavirus virus-like particles (HPV cVLPs) carrying HPV16 E7 protein are potent vaccines for inducing cell-mediated immunity (CMI) against HPV-induced tumors in animal models. We tested the hypothesis that virion-neutralizing antibodies generated during an initial vaccination might prevent effective boosting of CMI to the cVLPs. Mice with circulating HPV16-neutralizing antibodies, generated by direct immunization with wild-type VLPs or by passive transfer of hyperimmune anti-HPV16 VLP mouse sera, were subsequently vaccinated with HPV16 E7-containing cVLPs. Mice with preexisting neutralizing antibodies were not protected from HPV16 E7-positive TC-1 tumor challenge, compared to the protection seen in mice lacking these antibodies. Antibody-coated VLPs bound very inefficiently to receptor-positive cell lines, suggesting that one of the mechanisms of antibody interference is blocking of VLP binding to its receptor and thereby uptake of VLPs by antigen-presenting cells. Our results suggest that repetitive vaccination with a cVLP for induction of cellular immune responses to an incorporated antigen may be of limited effectiveness due to the presence of neutralizing antibodies against the capsid proteins induced after the first application. This limitation could potentially be overcome by boosting with cVLPs containing the same target antigen incorporated into other papillomavirus-type VLPs.

Positively charged termini of the L2 minor capsid protein are necessary for papillomavirus infection

Coexpression of bovine papillomavirus L1 with L2 mutants lacking either eight N-terminal or nine C-terminal amino acids that encode positively charged domains resulted in wild-type levels of viral genome encapsidation. Despite wild-type binding to the cell surface, the resulting virions were noninfectious. An L2 mutant encoding a scrambled version of the nine C-terminal residues restored infectivity, in contrast to an L2 mutant encoding a scrambled version of the N-terminal residues.

Papillomavirus pseudovirus: a novel vaccine to induce mucosal and systemic cytotoxic T-lymphocyte responses

Intestinal mucosa is a portal for many infectious pathogens. Systemic immunization, in general, does not induce a cytotoxic T-lymphocyte (CTL) response at the mucosal surface. Because papillomavirus (PV) naturally infects mucosa and skin, we determined whether PV pseudovirus, i.e., PV-like particles in which unrelated DNA plasmids are packaged, could generate specific mucosal immunity. We found that the pseudovirus that encoded the lymphocytic choriomeningitis virus gp33 epitope induced a stronger CTL response than a DNA vaccine (plasmid) encoding the same epitope given systemically. The virus-like particles that were used to make the pseudoviruses provided an adjuvant effect for induction of CTLs by the DNA vaccine. The PV pseudovirus pseudoinfected mucosal and systemic lymphoid tissues when administered orally. Oral immunization with the pseudovirus encoding human PV type 16 mutant E7 induced mucosal and systemic CTL responses. In comparison, a DNA vaccine encoding E7, when given orally, did not induce a CTL response in intestinal mucosal lymphoid tissue. Further, oral immunization with the human PV pseudovirus encoding E7 protected mice against mucosal challenge with an E7-expressing bovine PV pseudovirus. Thus, PV pseudovirus can be used as a novel vaccine to induce mucosal and systemic CTL responses.

Chimeric papillomavirus-like particles expressing a foreign epitope on capsid surface loops

Neutralization capsid epitopes are important determinants for antibody-mediated immune protection against papillomavirus (PV) infection and induced disease. Chimeric L1 major capsid proteins of the human PV type 16 (HPV-16) and the bovine PV type 1 (BPV-1) with a foreign peptide incorporated into several capsid surface loops self-assembled into pentamers or virus-like particles (VLP). Binding patterns of neutralizing monoclonal antibodies (MAb) and immunization of mice confirmed (i) that regions around aa 282-286 and 351-355 contribute to neutralization epitopes and identified the latter region as an immunodominant site and (ii) that placing a foreign peptide in the context of an assembled structure markedly enhanced its immunogenicity. Pentamers disassembled from wild-type HPV-16 and BPV-1 VLPs displayed some of the neutralization epitopes that were detected on fully assembled VLPs, but were deficient for binding a subset of neutralizing MAb that inhibit cell attachment.

Gene transfer using human papillomavirus pseudovirions varies according to virus genotype and requires cell surface heparan sulfate

Artificial viruses consisting of DNA plasmid packaged in vitro into virus-like particles (VLPs) are new vehicles for gene transfer. We therefore investigated the ability of nine human papillomavirus (HPV) VLPs to interact with heterologous DNA and transfer genes. HPV 16, 18, 31, 33, 39, 45, 58, 59, and 68 VLPs were able to bind heterologous DNA and to transfer genes into Cos-7 cells. Inhibition of gene transfer by preincubation of the pseudovirions with heparin confirmed that heparan sulfate on the cell surface plays a role as cell receptor for HPVs. As HPV neutralizing antibodies are mainly type-specific, gene transfer with different HPV pseudovirions offers the possibility of their sequential use in vivo for a greater efficacy.

Enhancement of capsid gene expression: preparing the human papillomavirus type 16 major structural gene L1 for DNA vaccination purposes

Expression of the structural proteins L1 and L2 of the human papillomaviruses (HPV) is tightly regulated. As a consequence, attempts to express these prime-candidate genes for prophylactic vaccination against papillomavirus-associated diseases in mammalian cells by means of simple DNA transfections result in insufficient production of the viral antigens. Similarly, in vivo DNA vaccination using HPV L1 or L2 expression constructs produces only weak immune responses. In this study we demonstrate that transient expression of the HPV type 16 L1 and L2 proteins can be highly improved by changing the RNA coding sequence, resulting in the accumulation of significant amounts of virus-like particles in the nuclei of transfected cells. Data presented indicate that, in the case of L1, adaptation for codon usage accounts for the vast majority of the improvement in protein expression, whereas translation-independent posttranscriptional events contribute only to a minor degree. Finally, the adapted L1 genes demonstrate strongly increased immunogenicity in vivo compared to that of unmodified L1 genes.

DNA-induced structural changes in the papillomavirus capsid

Human papillomavirus capsid assembly requires intercapsomeric disulfide bonds between molecules of the major capsid protein L1. Virions isolated from naturally occurring lesions have a higher degree of cross-linking than virus-like particles (VLPs), which have been generated in eukaryotic expression systems. Here we show that DNA encapsidation into VLPs leads to increased cross-linking between L1 molecules comparable to that seen in virions. A higher trypsin resistance, indicating a tighter association of capsomeres through DNA interaction, accompanies this structural change.

Induction of type-specific neutralizing antibodies by capsomeres of human papillomavirus type 33

The immunogenicity of capsomeres of human papillomavirus type 33 was evaluated in a dose-response analysis. Capsomeres were obtained free of capsids by expression of L1 carrying the single point mutation C427S. Neutralizing antibodies were detected using an in vitro pseudoinfection assay. Capsomeres induced type-specific, neutralizing antibodies in mice even in the absence of adjuvant. The neutralization titers of immune sera raised without adjuvant were 10- to 20-fold lower than those of antisera to virus-like particles, but virtually identical using Freund's adjuvant. These data indicate that capsomeres may substitute for virus-like particles in future vaccines when used with an adjuvant appropriate for human vaccination.

Human papillomavirus virus-like particles are efficient oral immunogens when coadministered with Escherichia coli heat-labile enterotoxin mutant R192G or CpG DNA

Certain human papillomaviruses (HPVs) cause most cervical cancer, which remains a significant source of morbidity and mortality among women worldwide. HPV recombinant virus-like particles (VLPs) are promising vaccine candidates for controlling anogenital HPV disease and are now being evaluated as a parenteral vaccine modality in human subjects. Vaccines formulated for injection generally are more costly, more difficult to administer, and less acceptable to recipients than are mucosally administered vaccines. Since oral delivery represents an attractive alternative to parenteral injection for large-scale human vaccination, the oral immunogenicity of HPV type 11 (HPV-11) VLPs in mice was previously investigated; it was found that a modest systemic neutralizing antibody response was induced (R. C. Rose, C. Lane, S. Wilson, J. A. Suzich, E. Rybicki, and A. L. Williamson, Vaccine 17:2129-2135, 1999). Here we examine whether VLPs of other genotypes may also be immunogenic when administered orally and whether mucosal adjuvants can be used to enhance VLP oral immunogenicity. We show that HPV-16 and HPV-18 VLPs are immunogenic when administered orally and that oral coadministration of these antigens with Escherichia coli heat-labile enterotoxin (LT) mutant R192G (LT R192G) or CpG DNA can significantly improve anti-VLP humoral responses in peripheral blood and in genital mucosal secretions. Our results also suggest that LT R192G may be superior to CpG DNA in this ability. These findings support the concept of oral immunization against anogenital HPV disease and suggest that clinical studies involving this approach may be warranted.

L1 interaction domains of papillomavirus l2 necessary for viral genome encapsidation

BPHE-1 cells, which harbor 50 to 200 viral episomes, encapsidate viral genome and generate infectious bovine papillomavirus type 1 (BPV1) upon coexpression of capsid proteins L1 and L2 of BPV1, but not coexpression of BPV1 L1 and human papillomavirus type 16 (HPV16) L2. BPV1 L2 bound in vitro via its C-terminal 85 residues to purified L1 capsomers, but not with intact L1 virus-like particles in vitro. However, when the efficiency of BPV1 L1 coimmunoprecipitation with a series of BPV1 L2 deletion mutants was examined in vivo, the results suggested that residues 129 to 246 and 384 to 460 contain independent L1 interaction domains. An L2 mutant lacking the C-terminal L1 interaction domain was impaired for encapsidation of the viral genome. Coexpression of BPV1 L1 and a chimeric L2 protein composed of HPV16 L2 residues 1 to 98 fused to BPV1 L2 residues 99 to 469 generated infectious virions. However, inefficient encapsidation was seen when L1 was coexpressed with either BPV1 L2 with residues 91 to 246 deleted or with BPV1 L2 with residues 1 to 225 replaced with HPV16 L2. Impaired genome encapsidation did not correlate closely with impairment of the L2 proteins either to localize to promyelocytic leukemia oncogenic domains (PODs) or to induce localization of L1 or E2 to PODs. We conclude that the L1-binding domain located near the C terminus of L2 may bind L1 prior to completion of capsid assembly, and that both L1-binding domains of L2 are required for efficient encapsidation of the viral genome.

Human papillomavirus type 16 minor capsid protein l2 N-terminal region containing a common neutralization epitope binds to the cell surface and enters the cytoplasm

The first step of papillomavirus infection is believed to be binding of major capsid protein L1 to the cell surface without involvement of minor capsid protein L2, but the viral infectivity can be neutralized either by anti-L1 or anti-L2 antibody. To understand the role of L2 in human papillomavirus (HPV) infection, we examined a segment of HPV type 16 (HPV16) L2, which contains a neutralization epitope common to HPV6, for its involvement in adsorption and penetration of the capsids. Preincubation of monkey COS-1 cells with a synthetic peptide having amino acids (aa) 108 to 120 of HPV16 L2 reduced the susceptibility of COS-1 cells to infection with HPV16 pseudovirions. Confocal microscopy showed that the green fluorescence protein (GFP) fused with the L2 peptide was found to bind to the surface of a HeLa cell, an HPV18-positive human cancer cell line, at 4 degrees C and to enter the cytoplasm after subsequent incubation at 37 degrees C. Flow cytometry showed that fused GFP did not bind to HeLa cells that had been treated with trypsin. Besides COS-1 and HeLa cells, some human and rodent cell lines were detected by flow cytometry to be susceptible to binding with fused GFP, showing a tendency of epithelial cells toward higher susceptibility. Substitutions at aa 108 to 111 inhibited fused GFP from binding to HeLa cells and reduced the infectivity in COS-1 cells of the in vitro-constructed pseudovirions. The results suggest that L2 plays an important role in enhancing HPV infection through interaction between the N-terminal region and a cellular surface protein, facilitating penetration of the virions and determining part of the tropism of HPVs.

Safety and immunogenicity trial in adult volunteers of a human papillomavirus 16 L1 virus-like particle vaccine

BACKGROUND

Studies in animal models have shown that systemic immunization with a papillomavirus virus-like particle (VLP) vaccine composed of L1, a major structural viral protein, can confer protection against subsequent experimental challenge with the homologous virus. Here we report results of a double-blind, placebo-controlled, dose-escalation trial to evaluate the safety and immunogenicity of a human papillomavirus (HPV) type 16 (HPV16) L1 VLP vaccine in healthy adults.

METHODS

Volunteers were given intramuscular injections with placebo or with 10- or 50-microg doses of HPV16 L1 VLP vaccine given without adjuvant or with alum or MF59 as adjuvants at 0, 1, and 4 months. All vaccine recipients were monitored for clinical signs and symptoms for 7 days after each inoculation. Immune responses were measured by an HPV16 L1 VLP-based enzyme-linked immunosorbent assay (ELISA) and by an HPV16 pseudovirion neutralization assay. The antibody titers were given as the reciprocals of the highest dilution showing positive reactivity in each assay. All statistical tests were two-sided.

RESULTS

The prevaccination geometric mean ELISA titer for six seropositive individuals was 202 (range, 40--640). All vaccine formulations were well tolerated, and all subjects receiving vaccine seroconverted. Serum antibody responses at 1 month after the third injection were dose dependent in recipients of vaccine without adjuvant or with MF59 but were similar at both doses when alum was the adjuvant. With the higher dose, the geometric means of serum ELISA antibody titers (95% confidence intervals) to purified VLP 1 month after the third injection were as follows: 10,240 (1499 to 69 938) without adjuvant, 10,240 (1114 to 94 145) with MF59, and 2190 (838 to 5723) with alum. Responses of subjects within each group were similar. Neutralizing and ELISA antibody titers were highly correlated (Spearman correlation =.85), confirming that ELISA titers are valid proxies for neutralizing antibodies.

CONCLUSIONS

The HPV16 L1 VLP vaccine is well tolerated and is highly immunogenic even without adjuvant, with the majority of the recipients achieving serum antibody titers that were approximately 40-fold higher than what is observed in natural infection.

Immunological analyses of human papillomavirus capsids

Recombinant human papillomavirus (HPV) virus-like particles (VLPs) are promising vaccine candidates for controlling anogenital HPV disease. Questions remain, however, concerning the extent of capsid antigenic similarity between closely related virus genotypes. To investigate this issue, we produced VLPs and corresponding polyclonal immune sera from several anogenital HPV types, and examined these reagents in enzyme-linked immunosorbent assays (ELISAs) and in cross-neutralization studies. Despite varying degrees of L1 genetic sequence relatedness, VLPs of each type examined induced high-titer serum polyclonal antibody responses that were entirely genotype-specific. In an in vitro infectivity assay, only cognate VLP antisera were able to neutralize pseudovirions of HPV-16, HPV-18 and HPV-33, with two exceptions: HPV-31 and HPV-45 VLP post-immune sera demonstrated low levels of neutralizing activity against pseudovirions of HPV-33 and HPV-18, respectively. In other experiments, epitopes shared between closely related types were found to be less immunogenic than, and antigenically distinct from, primary type-specific B-cell determinants of the viral capsid. In addition, results from epitope blocking experiments suggested a close correlation between primary type-specific capsid antigenic sites and virion neutralization. These findings support the view that papillomavirus genotypes denote unique viral serotypes, and suggest that a successful vaccine for these viruses will likely require the inclusion of VLPs of each serotype for which protection is desired.

Nasal immunization of mice with peptide having a cross-neutralization epitope on minor capsid protein L2 of human papillomavirus type 16 elicit systemic and mucosal antibodies

A common cross-neutralization epitope for human papillomavirus types 6 and 16 (HPV 6 and 16) is present in the region of amino acids (aa) 108-120 of HPV-16 minor capsid protein, L2. We nasally immunized Balb/c mice with a synthetic peptide with the 13 aa HPV 16 L2 sequence, and examined the antibodies elicited. ELISA showed that the immunization induced predominantly IgG and IgA antibodies cross-binding to L1/L2-capsids of HPVs 6, 16, and 18 in sera and in vaginal secretions, respectively. The serum containing the IgG antibody and the vaginal wash containing the IgA antibody neutralized HPV 16 pseudovirions and HPV 11 authentic virions, as shown by surrogate infectivity assays. From their cross-binding activity for HPV 16 and 18, the peptide-induced antibodies can probably cross-neutralize most of the genital HPVs. The peptide-induced neutralizing activity in vaginal wash was comparable to that induced by nasally immunization with HPV 16 L1-capsids. Unlike Balb/c, C57BL/10, which has different MHC class II, did not respond to the peptide immunization, but aa substitutions in the peptide to fulfill the requirement for the C57BL/10 agretope rendered the modified peptides immunogenic. The results provide a basis for development of a peptide vaccine against broad-spectrum of genital HPVs for humans.

NHPV16 VLP vaccine induces human antibodies that neutralize divergent variants of HPV16

Genital HPV genotypes are generally distinct serotypes, but whether variants within a genotype can represent serologic subtypes is unclear. In this study we used serum from human volunteers vaccinated with HPV16 L1 VLPs from variant 114K, to examine cross-neutralization of variants from each of the five major phylogenetic branches of HPV16. Recombinant Semliki Forest virus-derived pseudovirions for each variant were generated and combined with serum from vaccines, and the mixture was monitored for infectivity in a standard C127 cell focal transformation assay. Sera from all 10 VLP-immunized individuals had neutralizing activity against each of the variant pseudovirions. For each of the sera, variant titers differed by only fourfold or less from the median titer. Therefore, from a vaccine perspective, HPV16 variants belong to a single serotype. Vaccination with HPV16 114K L1 VLPs generates antibodies that should confer a similar degree of protection against all known phylogenetic branches of HPV16.

Neutralization of human papillomavirus (HPV) pseudovirions: a novel and efficient approach to detect and characterize HPV neutralizing antibodies

The development of vaccines against human papillomaviruses (HPVs) has long been hampered by the inability to grow HPVs in tissue culture and the lack of an efficient neutralization assay. To date, less than 10% of more than 100 different HPV types can be grown in athymic and "SCID" mouse xenograft systems or raft culture systems. Recently, the in vitro generation of HPV pseudovirions and their use in neutralization assays were demonstrated. The major shortcomings of the current approaches to HPV neutralization are the lack of HPV virions for most types for the xenograft methods and the time-consuming and inefficient generation of infective pseudovirions for the latter methods, which precludes their use in large-scale HPV clinical trials or epidemiological studies. We describe here a novel and efficient approach to generating pseudovirions in which HPV virus-like particles (VLPs) are coupled to the beta-lactamase gene as a reporter. We show that it is not necessary to encapsidate the reporter gene constructs into the pseudovirions. Using sera from human volunteers immunized with HPV-11 VLPs expressed in yeast, we demonstrate that our novel neutralization assay compares favorably with the athymic mouse neutralization assay. Furthermore, our assay was used to define neutralizing monoclonal antibodies to HPV-6, which were previously unknown.

Developing HPV virus-like particle vaccines to prevent cervical cancer: a progress report

BACKGROUND

the knowledge that sexually transmitted infection with one of a limited number of human papillomaviruses (HPVs) is a central cause of almost all cervical cancers affords the opportunity to prevent this common cancer through anti-viral vaccination.

OBJECTIVE

the spectacular success of vaccines in preventing several other viral diseases offers hope that immunoprophylaxis against the relevant HPVs could lead to a major reduction in cervical cancer incidence.

RESULTS AND CONCLUSION

the results of preclinical studies and early phase clinical trials of virus-like particle (VLP) based subunit vaccines have been very encouraging. However, unique aspects of papillomavirus biology and genital tract infections, and the lack of sexual a transmission model for papillomavirus, make it far from certain that effective prophylactic vaccination against genital HPV infection will be easily achieved. Future clinical efficacy trials will likely test the hypothesis that parenteral injection of VLPs can induce antibody mediated and type specific protection against genital tract HPV infection and subsequent development of premalignant neoplastic disease.

Trends over time in the incidence of cervical neoplasia in comparison to trends over time in human papillomavirus infection

INTRODUCTION

The establishment of human papillomavirus (HPV) infection as a major cause of cervical neoplasia has resulted in major efforts to develop prophylactic HPV vaccines for prevention of cervical neoplasia. Cervical cancer and the other HPV-associated cancers constitute a major public health burden and eradication of the major causative infection is certainly the most appealing long-term preventive measure. Nevertheless, the effects of preventive HPV vaccination will need to be estimated and compared for (cost-) efficacy with other primary prevention and with secondary prevention programs. However, estimating the effects of preventing a causative exposure is complicated when the exposure is a transmissible infection. The spread of the epidemic is dynamic and may change over time depending e.g. on the changes in human behavior. Depending on the circumstances, prevention of an infection may have either greater or lesser effects than the prevention of a non-infectious exposure. Estimating the time trends in HPV infections and the underlying trends in the risk of cervical neoplasia is important for estimating effects of interventions.

METHOD

A literature review on recent evidence on time trends in cervical neoplasia, compared with evidences on time trends in HPV infections and interactions between different types of HPV infections.

RESULTS

In Finland, there has between 1991 and 1995 been a 60% increase in the incidence of cervical cancer among women <55 years of age. Trends in detection rates of cervical cancer precursor lesions are consistent with an increase in the background cervical cancer risk. From the 1960s to 1980s, there has been a major increase in HPV seroprevalences over time in the Nordic countries. Increasing trends are also seen for other sexually transmitted diseases and smoking. Several studies indicate the existence of interaction between benign and oncogenic HPV types, thus making the relationship between the incidences in HPV infections and in cervical neoplasia complex.

CONCLUSION

The increase in cervical cancer is paralleled by increases in HPV infection, other STDs and smoking and changes in screening practices, all of which may have contributed. Prediction of the effect on cervical cancer incidence of changes in HPV incidences is complicated by the existence of several risk factors, the protective effect of screening and by the population dynamics of HPV infections.

Prospects for phase III-IV HPV vaccination trials in the Nordic countries and in Estonia

BACKGROUND

oncogenic, i.e. high risk human papillomavirus (hrHPV) types are the major cause of invasive cervical cancer (ICC). Putatively licensable vaccines against the hrHPVs have been developed and are approaching clinical phase III trials that use persistent HPV infection as end point. Direct extension of the phase III trials towards long-term end points (ICC and its immediate precursors: carcinoma in situ and severe dysplasia, i.e. cervical intraepithelial neoplasia grade III, CINIII) is important, to avoid early contamination of the target population by opportunistic use of licensed HPV vaccines. Country-wide registration on population and health events in a stable population of 25 million make Estonia and the Nordic countries a unique venue for long-term evaluation of cervical cancer control measures. Mass-screening programmes exist in all Nordic countries, but not in Estonia.

AIM

design of phase III-IV trials for evaluation of protection against ICC and CINIII by preventive HPV vaccines based on cancer registry follow-up.

RESULTS

in the Nordic countries, population based randomisation of all 15-year-old women to the vaccination (vaccine and placebo) and reference cohorts entering conventional Pap-smear screening after a clinical phase III trial would assure comparability of the cohorts. Enrollment of 10094 vaccinees +10094 placebo vaccinees +30282 other hrHPV negative women without vaccination at the age of 16 would give 80% power for the demonstration of 70% vaccine efficacy (VE) against ICC in 20 years by cancer registry follow-up. On the other hand, vaccination of 8303 Estonian hrHPV negative women among the entire 15-year-old female birth cohort (about 10000 women) with an already licensed HPV vaccine would enable demonstration of 70% VE against ICC by 20 years of registry follow-up of these and comparable 16606 women identified among the 16-19-year-old birth cohorts.

CONCLUSIONS

evaluation of the protective effect of an HPV vaccine against ICC is possible both in countries with or without mass-screening. The effects of vaccination on spread of different HPVs in the population would need to be monitored, especially in Estonia. Ethical aspects, cost-benefit evaluation and comparisons with other new means of cervical cancer control warrant further investigation.

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

Four C-terminal deletion mutants of the human papillomavirus 16 L1 protein were expressed in the baculovirus expression system. They consist of the deletion of amino acids 497-505, 477-505, 403-505 and 302-505 (delta C9, delta C31, delta C103 and delta C204 respectively). Only two of the C-terminally deleted proteins, delta C9 and delta C31, retained the ability to form virus-like particles (VLPs) resembling those obtained with the full length L1 protein. Analysis of deleted L1 proteins and corresponding VLPs indicated that the C-terminus was necessary both for DNA binding and DNA packaging. These results were corroborated by the loss of the gene transfer capacities of C-terminal deleted VLPs.

Papillomavirus virus-like particles for the delivery of multiple cytotoxic T cell epitopes

Chimeric papillomavirus (PV) virus-like particles (VLPs) based on the bovine papillomavirus type 1 (BPV-1) L1 protein were constructed by replacing the 23-carboxyl-terminal amino acids of the BPV1 major protein L1 with an artificial "polytope" minigene, containing known CTL epitopes of human PV16 E7 protein, HIV IIIB gp120 P18, Nef, and reverse transcriptase (RT) proteins, and an HPV16 E7 linear B epitope. The CTL epitopes were restricted by three different MHC class I alleles (H-2(b), H-2(d), HLA-A*0201). The chimeric L1 protein assembled into VLPs when expressed in SF-9 cells by recombinant baculovirus. After immunization of mice with polytope VLPs in the absence of adjuvant, serum antibodies were detected which reacted with both polytope VLPs and wild-type BPV1L1 VLPs, in addition to the HPV16E7 linear B cell epitope. CTL precursors specific for the HPV16 E7, HIV P18, and RT CTL epitopes were also detected in the spleen of immunized mice. Polytope VLPs can thus deliver multiple B and T epitopes as immunogens to the MHC class I and class II pathways, extending the utility of VLPs as self-adjuvanting immunogen delivery systems.

BPV1 E2 protein enhances packaging of full-length plasmid DNA in BPV1 pseudovirions

We studied determinants of efficient encapsidation of circular DNA, incorporating a PV early region DNA sequence (nt 584-1978) previously shown to enhance packaging of DNA within papillomavirus (PV)-like particles (VLPs). Insect coelomic cells (Sf-9) and cultured monkey kidney cells (Cos-1) were transfected with an 8-kb reporter plasmid incorporating the putative BPV packaging sequence and infected with BPV1 L1 and L2 recombinant baculovirus or vaccinia virus. Heavy (1.34 g/ml) and light (1.30 g/ml) VLPs were produced, and each packaged some of the input plasmid. In light VLPs, truncated plasmids, which nevertheless incorporated the PV-derived DNA packaging sequence, were more common than full-length plasmids. Packaging efficiency of the plasmid was estimated at 1 plasmid per 10(4) VLPs in both Cos-1 and Sf-9 cells. In each cell type, expression of the BPV1 early region protein E2 in trans doubled the quantity of heavy but not light VLPs and also increased the packaging efficiency of full-length circular plasmids by threefold in heavy VLPs. The resultant pseudovirions incorporated significant amounts of E2 protein. Pseudovirions, comprising plasmids packaged within heavy VLPs, mediated the delivery of packaged plasmid into Cos-1 cells, whereby "infectivity" was blocked by antisera to BPV1 L1, but not antisera to BPV1 E4. We conclude that (a) packaging of DNA within PV L1+L2 pseudovirions is enhanced by BPV1 E2 acting in trans, (b) E2 may be packaged with the pseudovirion, and (c) E2-mediated enhancement of packaging favors 8-kb plasmid incorporation over incorporation of shorter DNA sequences.

Assembly of human papillomavirus type 16 pseudovirions in Saccharomyces cerevisiae

Studies of the encapsidation of papillomavirus (PV) DNA, and production of preparative amounts of PVs in vitro, have met with only limited success. To circumvent this problem we established a system in yeast to generate infectious HPV-16 pseudovirions. Saccharomyces cerevisiae strain 1699 was transformed with a construct to allow production of HPV-16 virus-like particles (VLPs). This strain was then transformed with a second construct (target plasmid), the same size as the HPV-16 genome and containing the HPV-16 upstream regulatory region (URR) and the HPV-16 E2 open reading frame. In addition, the target plasmid contained the green fluorescent protein gene to monitor delivery of the target plasmid into mammalian cells after infection. We conclude that this system allows HPV DNA encapsidation because (1) HPV-16 VLPs of two different types (heavy and light) were detected by CsCl gradient centrifugation, (2) DNase I-resistant DNA was detected by PCR/Southern blot analysis in fractions of CsCl gradients at a density corresponding to heavy VLPs, (3) in vitro infection of mammalian cells, including primary mouse splenocytes, with pseudovirions resulted in delivery of the reporter gene as demonstrated by FACS analysis for GFP expression, and (4) after injection of pseudovirions into mice, in vivo reporter gene expression was detected by confocal microscopy in sections of muscle tissue. We conclude that HPV-16 pseudovirions produced in yeast may be useful both for in vitro transduction and for gene delivery in vivo.

Minor capsid protein of human genital papillomaviruses contains subdominant, cross-neutralizing epitopes

Vaccination with virus-like particles (VLP), comprising both L1 and L2 of human papillomavirus (HPV) genital types 6, 16, and 18, induces predominantly type-specific neutralizing antibodies. L2 polypeptide vaccines protect animals against experimental challenge with homologous papillomavirus and cross-reactive epitopes are present in HPV L2. To assess L2-specific cross-neutralization of HPV genotypes, sheep were immunized with purified, bacterially expressed HPV6, 16, or 18 L2. In addition to neutralizing the homologous HPV type in vitro, antisera to each HPV L2 also cross-neutralized both heterologous HPV types. This suggests that unlike VLP-based prophylactic HPV vaccines, an L2 polypeptide vaccine may provide broad-spectrum protection.

Synthesis of viral DNA and late capsid protein L1 in parabasal spinous cell layers of naturally occurring benign warts infected with human papillomavirus type 1

We investigated human papillomavirus type 1 (HPV1)-specific transcription, viral DNA replication, and viral protein expression in naturally occurring benign tumors by in situ hybridization, 5-bromodeoxyuridine (BrdU) incorporation, and immunohistochemistry and obtained results different from other HPV-infected benign tumors characterized so far. Moderate amounts of transcripts with a putative coding potential for E6/E7, E1, and E2 were demonstrated from the first subrabasal cell layer throughout the stratum spinosum and granulosum. In addition very large amounts of E4 and L1 transcripts were present in the same epithelial layers. This finding was substantiated by the demonstration of L1 and E4 protein already in the bottom-most spinous cell layer. Furthermore massive amplification of the viral DNA as measured by BrdU incorporation and different methods of in situ hybridization took place in the lowest 5 to 10 suprabasal cell layers. These findings are in contrast to the assumption that late gene expression and viral DNA synthesis are restricted to the more differentiated cell layers of the epithelium and point to differences in the regulation of the vegetative life cycle between different papillomavirus types.

Structure of small virus-like particles assembled from the L1 protein of human papillomavirus 16

The papillomavirus major late protein, L1, forms the pentameric assembly unit of the viral shell. Recombinant HPV16 L1 pentamers assemble in vitro into capsid-like structures, and truncation of ten N-terminal residues leads to a homogeneous preparation of 12-pentamer, icosahedral particles. X-ray crystallographic analysis of these particles at 3.5 A resolution shows that L1 closely resembles VP1 from polyomaviruses. Surface loops contain the sites of sequence variation among HPV types and the locations of dominant neutralizing epitopes. The ease with which small virus-like particles may be obtained from L1 expressed in E. coli makes them attractive candidate components of a papillomavirus vaccine. Their crystal structure also provides a starting point for future vaccine design.

The immunology of animal papillomaviruses

Papillomaviruses are species- and tissue-specific double-stranded DNA viruses. These viruses cause epithelial tumours in many animals, including man. Typically, the benign warts undergo spontaneous, immune-mediated regression, most likely effected by T-cells (especially CD4, but also CD8 subsets), whereas humoral immunity can prevent new infections. Some papillomavirus infections fail to regress spontaneously and others progress to malignant epithelial tumours. Additionally, the impact of these lesions is greater in immunosuppressed individuals. Many therapies are ineffective, and there is much interest in the potential for immunological intervention in papillomavirus infections of man and animals. Vaccination can be achieved with 'live' virus, formalin-inactivated virus, synthetic virus-like particles, and DNA vaccination. There has been much recent progress in the development of such vaccines for papillomavirus infections in the rabbit, ox and dog. Success in these animal models suggests that similar approaches may prove useful for prophylactic or therapeutic vaccination against the important human papillomaviruses involved in the development of cutaneous and anogenital warts, laryngeal papillomatosis, and cervical cancer.

HPV11 mutant virus-like particles elicit immune responses that neutralize virus and delineate a novel neutralizing domain

Characterization of the regions of human papillomaviruses (HPVs) that elicit neutralizing immune responses supports studies on viral infectivity and provides insight for the development and evaluation of prophylactic vaccines. HPV11 is a major etiologic agent of genital warts and a likely vaccine candidate. A conformationally dependent epitope for the binding of three neutralizing monoclonal antibodies (mAbs) has been mapped to residues G(131)T(132) of the L1 major capsid protein. The mAbs bind L1 only when it is assembled into virions or into virus-like particles (VLPs) that mimic the capsid structure. We were interested in identifying other domains of L1 that elicit neutralizing responses. To this end, we have generated a panel of mAbs against VLPs derived from HPV11 L1 harboring a G131S substitution. The new mAbs are unlike the neutralizing mAbs previously mapped to residues G(131)T(132) in that they bind both prototype and HPV11:G131S mutant VLPs. Some of the new mAbs neutralized virus in vitro. We have mapped epitopes for three of these new mAbs, as well as a neutralizing mAb generated against HPV11 virions, by measuring binding to HPV6 VLPs substituted with HPV11-like amino acids. Two regions are critical: one defined by HPV11 L1 residues 263-290 and the other by residues 346-349. mAbs H11.H3 and H11.G131S.G3 bind HPV6 VLPs with substitutions derived from the 346-349 region; in addition, H11.G131S.G3 binds HPV6 VLPs with substitutions derived only from the 263-290 region. Although H11.H3 does not bind HPV6 VLPs with substitutions derived from the 263-290 region, binding to HPV6 VLPs is enhanced when both sets of substitutions are present. mAbs H11.G131S.I1 and H11.G131S.K5 bind HPV6 VLPs with the 263-290 substitutions, but show little binding to HPV6 VLPs with the 346-349 substitutions. However, binding to HPV6 VLPs is enhanced when substitutions at both regions are present. The 346-349 region has not previously been described as eliciting a neutralizing response for any HPV type. In addition, the work demonstrates a complex binding site contributed by two distinct regions of L1.

Human papillomavirus vaccines

Genital human papillomavirus (HPV) infections are the viral sexually transmitted diseases most frequently diagnosed that include anogenital condylomas and squamous intra-$bepithelial lesions, among which the precursors of invasive carcinomas of the uterine cervix. In animal PV models, vaccination against L1 and/or L2 viral capsid proteins provides an efficient protection against infection, involving virus type-specific neutralizing antibodies. Vaccination against non-structural E1, E2, E6 or E7 viral proteins does not prevent infection, unless administered altogether, but tends to stimulate regression, warranting the design of therapeutic vaccines. Prophylactic vaccines based on the use of virus-like particles (VLPs) obtained by auto-assembly of L1 or L1 and L2 proteins produced by recombinant DNA technology are under phase I/II clinical trials for HPV6/11 associated with condylomas and for HPV16, the most frequent oncogenic genotype. Second generation vaccines are chimeric proteins or VLPs incorporating one of the structural proteins (L1 or L2) fused to a non-structural protein (E6, E7 or E2), which should induce both humoral and cellular immunity. Vaccine valency (number of genotypes), route of administration (humoral versus local immunity), vaccinees (children, young adults, gender) and forms of vaccines (recombinant $LSalmonella typhimurium*I$L, edible plants expressing L1 and L2 proteins, DNA vaccines, synthetic antigenic peptides) are under study. End points to evaluate vaccine efficacy in phase III trials should include viral DNA detection and typing, and screening for low or high grade intraepithelial lesions. Therapeutic vaccines based on recombinant HPV E6 and/or E7 vaccinia virus, L2-E7 fusion proteins or E7 peptides corresponding to cytotoxic T cell epitopes are currently tested (phase I/II trials) in patients with cervical carcinomas of advanced clinical stages or high grade intraepithelial lesions. Animal studies, phase I/II clinical trials and implementation of the community support that HPV vaccines will constitute an efficient means to prevent carcinoma of the uterine cervix.

Capture ElISA and in vitro cell binding assay for the detection of antibodies to human papillomavirus type 6b virus-like particles in patients with anogenital warts

To investigate human papillomavirus (HPV) virus-like particle (VLP)-specific antibody responses among anogenital warts patients, a VLP-based capture ELISA was established. Twenty-six percent (35/134) of control subjects and 50.0% (39/78) of patients with current anogenital warts showed IgG seropositivity to HPV 6b VLPs. HPV 6b VLP-specific antibody responses recognised native VLPs only, and had no cross-reaction with HPV type 16 VLPs. No differences in reactivity were observed between L1 and L1 + L2 VLPs, suggesting that L2 contributes little to the total immunogenicity of the papillomavirus virion. A VLP-cell binding assay was also established. Some sera from patients with anogenital warts specifically inhibited VLP binding to the surface of epithelial cells, suggesting that these antibodies might be functionally neutralising. These data show that serological responses to HPV 6b VLPs were induced among some but not all patients with anogenital warts, and give a proportional estimate of infection in the community.

Mucosal but not parenteral immunization with purified human papillomavirus type 16 virus-like particles induces neutralizing titers of antibodies throughout the estrous cycle of mice

We have recently shown that nasal immunization of anesthetized mice with human papillomavirus type 16 (HPV16) virus-like particles (VLPs) is highly effective at inducing both neutralizing immunoglobulin A (IgA) and IgG in genital secretions, while parenteral immunization induced only neutralizing IgG. Our data also demonstrated that both isotypes are similarly neutralizing according to an in vitro pseudotyped neutralization assay. However, it is known that various amounts of IgA and IgG are produced in genital secretions along the estrous cycle. Therefore, we have investigated how this variation influences the amount of HPV16 neutralizing antibodies induced after immunization with VLPs. We have compared parenteral and nasal protocols of vaccination with daily samplings of genital secretions of mice. Enzyme-linked immunosorbent assay analysis showed that total IgA and IgG inversely varied along the estrous cycle, with the largest amounts of IgA in proestrus-estrus and the largest amount of IgG in diestrus. This resulted in HPV16 neutralizing titers of IgG only being achieved during diestrus upon parenteral immunization. In contrast, nasal vaccination induced neutralizing titers of IgA plus IgG throughout the estrous cycle, as confirmed by in vitro pseudotyped neutralization assays. Our data suggest that mucosal immunization might be more efficient than parenteral immunization at inducing continuous protection of the female genital tract.

Common neutralization epitope in minor capsid protein L2 of human papillomavirus types 16 and 6

Studies of virus neutralization by antibody are a prerequisite for development of a prophylactic vaccine strategy against human papillomaviruses (HPVs). Using HPV16 and -6 pseudovirions capable of inducing beta-galactosidase in infected monkey COS-1 cells, we examined the neutralizing activity of mouse monoclonal antibodies (MAbs) that recognize surface epitopes in HPV16 minor capsid protein L2. Two MAbs binding to a synthetic peptide with the HPV16 L2 sequence of amino acids (aa) 108 to 120 were found to inhibit pseudoinfections with HPV16 as well as HPV6. Antisera raised by immunizing BALB/c mice with the synthetic peptide had a cross-neutralizing activity similar to that of the MAb. The data indicate that HPV16 and -6 have a common cross-neutralization epitope (located within aa 108 to 120 of L2 in HPV16), suggesting that this epitope may be shared by other genital HPVs.

Nucleotides 1506-1625 of bovine papillomavirus type 1 genome can enhance DNA packaging by L1/L2 capsids

We have previously described a DNA-packaging assay using bovine papillomavirus type 1 (BPV-1) virus-like particles (VLPs) and have identified a region of the BPV genome that assists in packaging. In this study, we identify a specific BPV sequence involved in DNA packaging by BPV-1 VLPs. In the initial screening of BPV-1 genomic sequences essential for DNA packaging, we observed that a plasmid with deletions between nucleotides (nt) 948 and 2113 failed to be packaged into BPV-1 VLPs. However, plasmids containing nt 948 to 2113 were efficiently packaged, suggesting that this 1.2-kb fragment contains a packaging enhancement sequence (PES). Further mapping of the BPV-1 genome showed that this packaging sequence lies between nt 1506 and 1625. Furthermore, this packaging sequence is also recognized by HPV6b VLPs, suggesting that a common packaging mechanism may be used by the two papillomavirus types. Given the phylogenetic difference between these two viral types, it is likely that other papillomavirus types may also use the same packaging mechanism. Identification of the PES has allowed a minimal viral genome sequence to be used in the packaging assay, improving the usefulness of the assay in studying the process of papillomavirus DNA encapsidation.

Papillomavirus-like particle vaccines for cervical cancer

Most cervical cancers are now known to be caused by human papillomavirus (HPV) infections. This provides an opportunity to prevent a major cause of cancer deaths in women through vaccination. Subunit vaccines based upon non-infectious papillomavirus-like particles (VLPs) are attractive candidates to prevent infection by oncogenic HPVs, and clinical trials are now underway. In addition, the strongly immunogenic characteristics of VLPs raise the possibility that they could also serve as vehicles for inducing therapeutic responses against HPV-induced neoplasia and other diseases.

No excess risk of cervical carcinoma among women seropositive for both HPV16 and HPV6/11

Human papillomavirus (HPV) types 16 and 18 are the major risk factors for cervical carcinoma, whereas HPV types 6 and 11 cause benign genital lesions. We wanted to study the joint effect of simultaneous infections with the oncogenic and non-oncogenic HPV types on risk of subsequent development of cervical carcinoma. A cohort of 530,000 women who had donated blood samples to Nordic serum banks between 1973 and 1994 was followed up by linkage to national cancer registries. We identified 182 prospective cases with invasive cervical carcinoma and selected 538 matched controls at random. HPV 6, 11, 16, 18 and 33 seropositivity was used as a marker for the different HPV infections, and seropositivity for Chlamydia trachomatis and cotinine were used as markers for risk-taking sexual behavior and smoking respectively. The adjusted odds ratio (OR) of cervical squamous-cell carcinoma (SCC) was 2.2 for HPV6/11 among HPV16 seronegatives and 5.5 for HPV16 among HPV6/11 seronegatives. Assuming multiplicative joint effect, the expected OR for seropositivity to both HPV6/11 and HPV16 would have been 12, but the observed OR was 1.0. The antagonistic interaction was statistically significant (p = 0.001) and present also under deterministic considerations of possible misclassification bias. Antagonistic interactions were also detected for combinations of HPV16 and HPV18 and of HPV16 and HPV33. The results are in line with the concept that HPV-specific immunity protects against SCC and support primary prevention of SCC by vaccination against the HPVs.

Detection and quantitation of human papillomavirus by using the fluorescent 5' exonuclease assay

A method for the detection and quantitation of oncogenic human papillomavirus (HPV) was developed by using the fluorescent 5' exonuclease assay. The method is based on the amplification of a 180-bp fragment from the 3' part of the E1 open reading frame in a single PCR with type-specific probes for HPV types 16, 18, 31, 33, and 35. The probes can be used separately or in combinations of up to three probes per assay. Quantitation over a range of 10(1) to 10(6) initial HPV copies was possible by using real-time detection of the accumulation of fluorescence with cycle number. Reconstitution experiments, performed to mimic mixed infections, showed that individual HPV types can be detected down to a ratio of about 1% in a mixture. The performance of the assay depends on DNA quality, the presence of PCR inhibitors, and the number of different probes used simultaneously. This homogeneous assay provides a fast and sensitive way of screening for oncogenic HPV types in biopsy specimens as well as cervical smear samples. The closed-tube nature of the assay and the inclusion of uracil N'-glycosylase reduces cross contamination of PCR products to a minimum. A similar assay for beta-actin was used in parallel for quantitation of genomic DNA. After normalizing the samples for genomic DNA content, the mean number of HPV copies per cell could be calculated.

Papillomaviruses: prophylactic vaccine prospects

Identification of a subset of HPV types as etiologic agents of cervical cancer and other malignancies implies that development of an effective vaccine against HPV infection could have a major impact on tumors attributable to these viruses. The ability of the L1 major capsid protein of papillomaviruses to self-assemble into VLPs that can, when inoculated systemically, induce high levels of neutralizing antibodies and protect animals against experimental viral challenge makes L1 VLPs an excellent candidate subunit vaccine. VLPs have the limitation of inducing type-specific immunity. Studies in humans are required to determine whether systemic vaccination with L1 VLPs will prevent sexually transmitted HPV infection. Since prospective efficacy trials will take several years to complete, considering alternative approaches is also worthwhile.

Recombinant viruses as vectors for mucosal immunity

The development and characterization of viral based vaccine vectors is extremely active research field. Much of this work has been facilitated by developments in molecular biology that allow work with large plasmid-based vectors, as well as the use of PCR. Several different vector systems are now available using RNA viruses and DNA viruses. Each vector system has its own strengths and weaknesses. Due to the differences and diversity between the viruses used as vectors, it is doubtful that a single system will be useful for all desired vaccines. However, the further development of existing, as well as potentially new systems, will provide a repertoire for vaccinologists to design the recombinant vaccine which will generate an optimal humoral and immune response for protection against infection or disease caused by pathogens that infect via mucosal surfaces.

In vitro construction of pseudovirions of human papillomavirus type 16: incorporation of plasmid DNA into reassembled L1/L2 capsids

Lack of permissive and productive cell cultures for the human papillomaviruses (HPVs) has hindered the study of virus-neutralizing antibodies and infection. We developed a cell-free system generating infectious HPV16 pseudovirions. HPV16 L1/L2 capsids, which had been self-assembled in insect cells (Sf9) expressing virion proteins L1 and L2, were disassembled with 2-mercaptoethanol (2-ME), a reducing agent, and reassembled by removal of 2-ME in the presence of a beta-galactosidase expression plasmid. Plasmid DNA purified together with the reassembled capsids was resistant to DNase I digestion. The reassembled pseudovirions mediated DNA transfer to COS-1 cells, as monitored by induced beta-galactosidase activity. Transfer was inhibited by anti-HPV16 L1 antiserum but not by antisera against L1s of HPV6 and HPV18. Construction in vitro of HPV pseudovirions containing marker plasmids would be potentially useful in developing methods to assay virus-neutralizing antibodies and to transfer exogenous genes to HPV-susceptible cells.

Infectious human papillomavirus type 18 pseudovirions

Human papillomavirus type 18 (HPV18) capsid proteins L1 and L2, synthesised in mammalian cells using recombinant vaccinia viral expression vectors, are transported to the nucleus and assembled into virus-like particles. When 293T cells, which express SV40 T antigen, were transfected with plasmid DNAs containing an SV40 origin of replication then infected with vaccinia viral vectors encoding L1 and L2, plasmid DNA was encapsidated into the particles. The DNAs ranged in size from 5.4 to 7.9 kb. By encapsidating plasmids containing either the beta-galactosidase gene or the puromycin-resistance gene, the pseudovirions were shown to be infectious in that they could transfer beta-galactosidase activity or confer resistance to puromycin to a number of cell types, indicating that the uptake and decapsidation of HPV particles are not the main determinants of cell type specificity of HPV. Episomal HPV16 DNA in a cervical keratinocyte line could also be encapsidated. Further investigation showed that DNA encapsidation is independent of HPV DNA sequences and of T antigen-mediated plasmid DNA replication. Instead, the minor capsid protein, L2, was found to be attached to plasmid mini-chromosomes extracted from these cells, suggesting a role for L2 in encapsidation. Consistent with this, the L1 protein alone was unable to encapsidate DNA, although it was able to form virus-like particles. The results suggest that intracellular episomal DNAs of suitable size can be encapsidated by the HPV18 L1 and L2 proteins without the need of any HPV packaging signal, and reintroduced into cells.

Nasal immunization of mice with human papillomavirus type 16 virus-like particles elicits neutralizing antibodies in mucosal secretions

To specifically induce a mucosal antibody response to purified human papillomavirus type 16 (HPV16) virus-like particles (VLP), we immunized female BALB/c mice orally, intranasally, and/or parenterally and evaluated cholera toxin (CT) as a mucosal adjuvant. Anti-HPV16 VLP immunoglobulin G (IgG) and IgA titers in serum, saliva, and genital secretions were measured by enzyme-linked immunosorbent assay (ELISA). Systemic immunizations alone induced HPV16 VLP-specific IgG in serum and, to a lesser extent, in genital secretions but no secretory IgA. Oral immunization, even in the presence of CT, was inefficient. However, three nasal immunizations with 5 microgram of VLP given at weekly intervals to anesthetized mice induced high (>10(4)) and long-lasting (>15 weeks) titers of anti-HPV16 VLP antibodies in all samples, including IgA and IgG in saliva and genital secretions. CT enhanced the VLP-specific antibody response 10-fold in serum and to a lesser extent in saliva and genital secretions. Nasal immunization of conscious mice compared to anesthetized mice was inefficient and correlated with the absence of uptake of a marker into the lung. However, a 1-microgram VLP systemic priming followed by two 5-microgram VLP intranasal boosts in conscious mice induced both HPV16 VLP-specific IgG and IgA in secretions, although the titers were lower than in anesthetized mice given three intranasal immunizations. Antibodies in serum, saliva, and genital secretions of immunized mice were strongly neutralizing in vitro (50% neutralization with ELISA titers of 65 to 125). The mucosal and systemic/mucosal HPV16 VLP immunization protocols that induced significant titers of neutralizing IgG and secretory IgA in mucosal secretions in mice may be relevant to genital HPV VLP-based human vaccine trials.

Papillomaviruses and cervical cancer: pathogenesis and vaccine development

A subset of human papillomaviruses (HPVs) has been implicated as the principal etiologic agents of cervical cancer. Cervical cancers consistently retain and express two of the viral genes, E6 and E7. Although infection with HPV seems to be necessary, other factors, such as cellular immune function, play an important role in determining whether cervical infection will regress, persist, or progress to cancer. The close relationship between viral infection and cancer makes HPV an attractive target for prophylactic and therapeutic vaccines. Candidate vaccines have been shown to have efficacy in animal models, and human clinical trials are planned or in progress.