Titration of HPV-11 infectivity and antibody neutralization can be measured in vitro

Infectious titres of human papillomaviruses (HPVs) in patient lesions, methodological considerations in evaluating HPV infectivity and implications for the efficacy of high-level disinfectants

BACKGROUND

Recent publications from a single research group have suggested that aldehyde-based high-level disinfectants (HLDs), such as ortho-phthalaldehyde (OPA), are not effective at inactivating HPVs and that therefore, patients may be at risk of HPV infection from medical devices. These results could have significant public health consequences and therefore necessitated evaluation of their reproducibility and clinical relevance.

METHODS

We developed methods and used standardised controls to: (1) quantify the infectious levels of clinically-sourced HPVs from patient lesions and compare them to laboratory-derived HPVs, (2) evaluate experimental factors that should be controlled to ensure consistent and reproducible infectivity measurements of different HPV genotypes, and (3) determine the efficacy of select HLDs.

FINDINGS

A novel focus forming unit (FFU) infectivity assay demonstrated that exfoliates from patient anogenital lesions and respiratory papillomas yielded infectious HPV burdens up to 2.7 × 103 FFU; therefore, using 2.2 × 102 to 1.0 × 104 FFU of laboratory-derived HPVs in disinfection assays provides a relevant range for clinical exposures. RNase and neutralising antibody sensitivities were used to ensure valid infectivity measures of tissue-derived and recombinant HPV preparations. HPV infectivity was demonstrated over a dynamic range of 4-5 log10; and disinfection with OPA and hypochlorite was achieved over 3 to >4 log10 with multiple genotypes of tissue-derived and recombinant HPV isolates.

INTERPRETATION

This work, along with a companion publication from an independent lab in this issue, address a major public health question by showing that HPVs are susceptible to HLDs.

FUNDING

Advanced Sterilization Products; US NIH (R01CA207368, U19AI084081, P30CA118100).

A comparative study of two different assay kits for the detection of secreted alkaline phosphatase in HPV antibody neutralization assays

To assess immunogenicity and development of antibodies in the context of vaccination, it is critical to quantify titers of neutralizing antibodies. We have been employing the 293TT cell-based neutralization assay system to quantify anti-HPV neutralizing antibodies. In this system, human papillomavirus (HPV) pseudovirion (PsV) particles encapsidating secreted alkaline phosphatase (SEAP) gene are used to measure infection of 293TT cells in 72-hr cell-culture supernatants. SEAP has traditionally been measured by Great EscAPe™ SEAP Chemiluminescence Kit 2.0 (GE). To reduce the cost, and to potentially increase efficiency, we sought a cheaper kit with better detection capability. Performance characteristics of the newer chemiluminescence kit, ZiVa® Ultra SEAP Plus Assay (Ziva) and GE were compared using the 293TT system. Dose titration of HPV PsV 16 or 18 showed that signal-to-noise ratios at 48 and 72 hr post-infection were higher for ZiVa at nearly all doses. ZiVa was superior to GE as it was able to detect SEAP at 48 hr, as well as when lower numbers of 293TT cells were used. The ability of ZiVa to quantitate HPV-16 and -18 neutralizing antibody titers was tested using sera from Cervarix® immunized individuals. Spearman rank correlational analyses showed excellent correlations between the titers obtained with ZiVa and GE for anti-HPV16 (r = 0.9822, p < 0.0001) and anti-HPV18 (r = 0.9832, p < 0.0001) antibodies. We concluded that ZiVa is superior to GE in detecting SEAP, and the antibody titers in sera of vaccinated individuals were similar to those obtained with GE. Thus, Ziva is a suitable alternative to GE.

Efficacy of RG1-VLP vaccination against infections with genital and cutaneous human papillomaviruses

Licensed human papillomavirus (HPV) vaccines, based on virus-like particles (VLPs) self-assembled from major capsid protein L1, afford type-restricted protection against HPV types 16/18/6/11 (or 16/18 for the bivalent vaccine), which cause 70% of cervical cancers (CxCas) and 90% of genital warts. However, they do not protect against less prevalent high-risk (HR) types causing 30% of CxCa, or cutaneous HPV. In contrast, vaccination with the minor capsid protein L2 induces low-level immunity to type-common epitopes. Chimeric RG1-VLP presenting HPV16 L2 amino acids 17–36 (RG1 epitope) within the DE-surface loop of HPV16 L1 induced cross-neutralizing antisera. We hypothesized that RG1-VLP vaccination protects against a large spectrum of mucosal and cutaneous HPV infections in vivo. Immunization with RG1-VLP adjuvanted with human-applicable alum-MPL (aluminum hydroxide plus 3-O-desacyl-4′-monophosphoryl lipid A) induced robust L2 antibodies (ELISA titers 2,500–12,500), which (cross-)neutralized mucosal HR HPV16/18/45/37/33/52/58/35/39/51/59/68/73/26/69/34/70, low-risk HPV6/11/32/40, and cutaneous HPV2/27/3/76 (titers 25–1,000) using native virion- or pseudovirion (PsV)-based assays, and a vigorous cytotoxic T lymphocyte response by enzyme-linked immunospot. In vivo, mice were efficiently protected against experimental vaginal challenge with mucosal HR PsV types HPV16/18/45/31/33/52/58/35/39/51/59/68/56/73/26/53/66/34 and low-risk HPV6/43/44. Enduring protection was demonstrated 1 year after vaccination. RG1-VLP is a promising next-generation vaccine with broad efficacy against all relevant mucosal and also cutaneous HPV types.

Human papillomavirus type 18 chimeras containing the L2/L1 capsid genes from evolutionarily diverse papillomavirus types generate infectious virus

Papillomaviruses (PVs) comprise a large family of viruses infecting nearly all vertebrate species, with more than 100 human PVs identified. Our previous studies showed that a mutant chimera HPV18/16 genome, consisting of the upper regulatory region and early ORFs of HPV18 and the late ORFs of HPV16, was capable of producing infectious virus in organotypic raft cultures. We were interested in determining whether the ability of this chimeric genome to produce infectious virus was the result of HPV18 and HPV16 being similarly oncogenic, anogenital types and whether more disparate PV types could also interact functionally. To test this we created a series of HPV18 chimeric genomes where the ORFs for the HPV18 capsid genes were replaced with the capsid genes of HPV45, HPV39, HPV33, HPV31, HPV11, HPV6b, HPV1a, CRPV, and BPV1. All chimeras were able to produce infectious chimeric viral particles, although with lower infectivity than wild-type HPV18. Steps in the viral life cycle and characteristics of the viral particles were examined to identify potential causes for the decrease in infectivity.

Cross-neutralization potential of native human papillomavirus N-terminal L2 epitopes

Background

Human papillomavirus (HPV) capsids are composed of 72 pentamers of the major capsid protein L1, and an unknown number of L2 minor capsid proteins. An N-terminal “external loop” of L2 contains cross-neutralizing epitopes, and native HPV16 virions extracted from 20-day-old organotypic tissues are neutralized by anti-HPV16 L2 antibodies but virus from 10-day-old cultures are not, suggesting that L2 epitopes are more exposed in mature, 20-day virions. This current study was undertaken to determine whether cross-neutralization of other HPV types is similarly dependent on time of harvest and to screen for the most effective cross-neutralizing epitope in native virions.

Methodology and Principal Findings

Neutralization assays support that although HPV16 L2 epitopes were only exposed in 20-day virions, HPV31 or HPV18 epitopes behaved differently. Instead, HPV31 and HPV18 L2 epitopes were exposed in 10-day virions and remained so in 20-day virions. In contrast, presumably due to sequence divergence, HPV45 was not cross-neutralized by any of the anti-HPV16 L2 antibodies. We found that the most effective cross-neutralizing antibody was a polyclonal antibody named anti-P56/75 #1, which was raised against a peptide consisting of highly conserved HPV16 L2 amino acids 56 to 75.

Conclusions and Significance

This is the first study to determine the susceptibility of multiple, native high-risk HPV types to neutralization by L2 antibodies. Multiple anti-L2 antibodies were able to cross-neutralize HPV16, HPV31, and HPV18. Only neutralization of HPV16 depended on the time of tissue harvest. These data should inform attempts to produce a second-generation, L2-based vaccine.

Cross-protective responses to human papillomavirus infection

Human papillomavirus (HPV) infections with oncogenic types account for approximately 500,000 deaths per year worldwide, predominantly in underdeveloped countries. The major cause of death is cervical cancer in women, but some additional cancers of the head and neck and anogenital sites also have an HPV etiology. Current virus-like particle-based vaccines are in clinical trials, and show very strong, long-lasting protection against vaccine-matched HPV types. These vaccines currently contain virus-like particles for the HPV types 6, 11, 16 and 18 (Gardasil®) and HPV16 and -18 (Cervarix®). Although type-specific neutralizing antibodies develop from immunizations with these virus-like particle vaccines, promising evidence for cross-protection against related but nonvaccine HPV types is emerging. Strategies to increase cross-protection to cover all oncogenic HPV types (currently approximately 20 types) are underway. These strategies include increasing the number of HPV types in the virus-like particle vaccine, and to the development of second-generation HPV vaccines that include the minor coat protein.

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.

Detection of HPV types and neutralizing antibodies in Gansu province, China

A total of 82 samples from patients with cervical cancer (Group 1) and 50 samples from patients with other genital diseases (Group 2) were collected in Gansu, China. All 132 samples were tested for HPV DNA with a typing kit that can detect 21 types of HPV, and also tested for neutralizing antibodies against HPV-16, -18, -58, -45, -6, and -11 using pseudovirus-based neutralization assays. The results revealed that 28% (23/82) of sera in Group 1 were positive for type-specific neutralizing antibodies with a titer range of 160-640, of which 23.2% (19/82), 2.4% (2/82), 2.4% (2/82), 1.2% (1/82), and 1.2% (1/82) were against HPV-16, -58, -6, -18, and -45, respectively. Only one serum (2%) in Group 2 was positive for neutralizing antibodies, which were against HPV-6 with a titer of 2,560. Overall, 85.4% (70/82) of samples in Group 1 were HPV DNA-positive, compared with 28% (14/50) of samples in Group 2. The seven most common types detected in Group 1 were HPV-16 (80%), HPV-52 (7.1%), HPV-66 and HPV-11 (5.7% each), and HPV-58, HPV-18, and HPV-33 (4.3% each), while the four most common types in Group 2 were HPV-16 (12%), HPV-52 and HPV-11 (6% each), and HPV-68 (4%). The concordance between HPV DNA and corresponding neutralizing antibodies was 32.9% (27/82) with a significant difference (P < 0.005). More specifically, the concordance was 42.7% (35/82) for HPV-16 in Group 1. The full-length sequences of six HPV types (HPV-16, -58, -33, -59, -11, and -68) were determined and showed 99% identities with their reported genomes.

A papillomavirus-like particle (VLP) vaccine displaying HPV16 L2 epitopes induces cross-neutralizing antibodies to HPV11

Peptides of the papillomavirus L2 minor capsid protein can induce antibodies (Ab) that neutralize a broad range of human papillomavirus (HPV) genotypes. Unfortunately, L2 is antigenically subdominant to L1 in the virus capsid. To induce a strong anti-L2 Ab response with cross-neutralizing activity to other mucosal types, chimeric virus-like particles (VLP) were generated in which HPV16 L2 neutralization epitopes (comprising L2 residues 69-81 or 108-120) are inserted within an immunodominant surface loop (between residues 133 and 134) of the L1 major capsid protein of bovine papillomavirus type 1 (BPV1). These chimeras self-assembled into pentameric capsomers, or complete VLP similar to wild type (wt) L1 protein. Immunization of rabbits with assembled particle preparations induced L2-specific serum Ab with titers 10-fold higher than those induced by cognate synthetic L2 peptides coupled to KLH. Antisera to both chimeric proteins partially neutralized HPV16 pseudovirions, confirming that both HPV16 L2 peptides define neutralization epitopes. When analyzed for the ability to cross-neutralize infection by authentic HPV11 virions, using detection of early viral RNA by RT-PCR-assays as the readout, immune serum to chimeric protein comprising L2 residues 69-81, but not 108-120, was partially neutralizing. In addition, mouse-antiserum induced by vaccinations with synthetic L2 peptide 108-120, but not 69-81, was partially neutralizing in this assay. Induction of cross-neutralization Ab by L2 epitopes displayed on chimeric VLP represents a possible strategy for the generation of broad-spectrum vaccines to protect against relevant mucosal HPV and associated neoplasia.

Chapter 11: HPV vaccines: Commercial Research & Development

We are now in the fortunate position of having two highly promising human papillomavirus (HPV) vaccines in the pipeline. Amidst the excitement of anticipating what these vaccines may be able to offer, it is worth pausing to look back at how the vaccine development story unfolded from an industrial perspective, since without the massive commitment shown by manufacturers over the last decade, without any guarantee of success, there would be no such prospect. This chapter focuses on the two HPV prophylactic vaccines, produced independently by Merck & Co., Inc. and GlaxoSmithKline (GSK), that are in advanced clinical development, and it aims to provide an insight into the key considerations for initiating the programmes in a commercial context as well as some of the research and development hurdles that needed to be surmounted to bring them to the point where efficacy has been demonstrated and the licensing process is well underway.

The human papillomavirus type 11 E1/\E4 protein is not essential for viral genome amplification

An abundant human papillomavirus (HPV) protein E1/\E4 is expressed late in the virus life cycle in the terminally differentiated layers of epithelia. The expression of E1/\E4 usually coincides with the onset of viral DNA amplification. However, the function of E1/\E4 in viral life cycle is not completely understood. To examine the role of E1/\E4 in the virus life cycle, we introduced a single nucleotide change in the HPV-11 genome to result in a truncation of E1/\E4 protein without affecting the E2 amino acid sequence. This mutated HPV-11 genome was introduced into a human foreskin keratinocyte cell line immortalized by the catalytic subunit of human telomerase, deficient in p16(INK4a) expression, and previously shown to support the HPV-11 life cycle when grown in organotypic raft culture. We have demonstrated that E1/\E4 is dispensable for HPV-11 viral DNA amplification in the late stages of the viral life cycle.

Induction of productive human papillomavirus type 11 life cycle in epithelial cells grown in organotypic raft cultures

The study of the human papillomavirus (HPV) life cycle was hampered for more than 50 years by the lack of a conventional cell culture system for propagating HPV. Considerable progress has been made in the production of several HPV types using either organotypic rafts or human epithelial xenografts in immunocompromised mice. In this study, we demonstrated episomal maintenance of HPV-11 DNA in N-Tert cells. HPV-11 episomal DNA containing cell populations grown in raft culture showed induction of the productive viral life cycle. HPV-11 DNA amplification and viral capsid antigen synthesis were detected in differentiated layers of epithelia. The viruses generated were able to infect keratinocytes in vitro, which indicate that viruses generated were infectious. The demonstration of the productive HPV-11 life cycle in raft culture from cloned HPV-11 DNA will facilitate genetic analyses of viral gene functions that was not possible using the human xenograft athymic mouse model.

The role of the human papillomavirus type 18 E7 oncoprotein during the complete viral life cycle

The role of the human papillomavirus oncoprotein E7 in carcinogenesis has been extensively studied. While the role of HPV E7 in the viral life cycle has also been studied, certain disparities exist, indicating that genotype differences may influence the role that E7 plays in the viral life cycle. In this study, we investigated the role of HPV18 E7 in the viral life cycle in order to gain a further understanding of this issue. To determine the role that HPV18 E7 plays in the viral life cycle, a translation termination substitution mutant of E7 in the context of the full HPV18 genome was created. We introduced linearized HPV18 E7-deficient genomic DNA into primary keratinocytes, where it recircularized and was maintained episomally at a range of five to several hundred copies of HPV genomic DNA. The mutant genomes failed to amplify following epithelial stratification and differentiation in organotypic culture. Moreover, virion morphogenesis did not occur. We found that the expression of HPV16 or HPV18 E7 in trans was able to rescue the amplification defect but not the defect in virion morphogenesis. These studies indicate that HPV18 E7 plays a critical role in the productive stage of the viral life cycle. In addition, these studies add further proof to the hypothesis that genotype differences exist for the role of E7 during the viral life cycle.

Human papillomavirus type 31b infection of human keratinocytes does not require heparan sulfate

Oncogenic human papillomaviruses (HPVs) are difficult to study experimentally as they replicate at low levels in vivo. This has precluded the purification of high-risk HPV virions from in vivo lesions. Virus-like particles (VLPs) and pseudovirions from low- and high-risk HPV types can emulate various aspects of HPV virion attachment and infections. These studies suggest that HPV infection is mediated by alpha6-integrin and/or heparan-sulfonated receptors. However, whether VLPs and pseudovirions accurately reflect the infection process of HPV virions has not been verified. We generated infectious HPV31b virions from organotypic (raft) tissues and performed experimental infections in a variety of cells. Successful infection following viral attachment, internalization, and nuclear transport was assayed by detecting newly synthesized, spliced HPV transcripts using reverse transcription (RT)-PCR or RT-quantitative PCR. Most human epithelial cells were infected with HPV31b at a multiplicity of infection as low as 1 to 10 viral genome equivalents per cell. HPV31b infection was detected in other cell lines, including COS-7 monkey kidney cells, but higher viral multiplicities of infection were required. Heparin preparations of various molecular weights or heparinase I treatment of cells prevented HPV31b infection of COS-7 cells and C-33A human cervical cancer cells in reproducible and dose-dependent manners. However, these reagents were unable to block infection of human keratinocytes, including HaCaT and N/TERT-1 cells and low-passage human foreskin keratinocytes. These data suggest that HPV31b infection of human keratinocytes, the natural host cell for HPV infections in vivo, does not require a heparan-sulfonated receptor, whereas heparan sulfate is important for infection of some other cells.

Dose-ranging studies of the safety and immunogenicity of human papillomavirus Type 11 and Type 16 virus-like particle candidate vaccines in young healthy women

Two candidate vaccines to prevent infection with human papillomavirus (HPV) Types 11 and 16 were studied in similar double-blind, placebo-controlled, dose-escalation trials. L1 virus-like particle (VLP) vaccines were made from recombinant L1 capsid protein of HPV11 or HPV16. Participants received 10, 20, 50, or 100 microg of HPV11 L1 VLPs, 10, 40, or 80 microg of HPV16 L1 VLPs, or placebo at Months 0, 2, and 6. Serum geometric mean antibody levels at Month 7 were 258, 644, 647, and 1112 milli-Merck units (mMU)/ml for the 10, 20, 50, and 100 microg doses of the HPV11 L1 VLP vaccine, respectively, and 479, 808, and 732 mMU/ml for the 10, 40, and 80 microg doses of the HPV16 L1 VLP vaccine, respectively. Antibody to HPV11 and 16 was still present at Month 36 in 96.8 and 93.5% of vaccinees, respectively. Both vaccines were well tolerated and were associated with only mild to moderate injection-site reactions.

Propagation, infection, and neutralization of authentic HPV16 virus

Despite the prevalence of HPV16 in invasive cervical cancers, an in vitro system capable of producing infectious HPV16 is lacking. The organotypic (raft) culture system has allowed for the study of the entire differentiation-dependent life cycle of human papillomaviruses (HPVs). However, the use of this system with the prototype HPV16-containing cell line, W12, has failed to yield infectious virus. Our laboratory has introduced clinically derived HPV16(114/B) genomic DNA into primary keratinocytes, where it subsequently recircularized and maintained episomally at 50-100 copies per cell. Virion morphogenesis occurred after epithelial stratification and differentiation in raft culture. HPV16 virions were isolated that were able to infect keratinocytes in vitro. Infection was neutralized by monoclonal antibodies raised against HPV16 but not by monoclonal antibodies known to neutralize other HPV types.

Differential antibody responses to a distinct region of human papillomavirus minor capsid proteins

A peptide derived from the human papillomavirus type 16 (HPV-16) minor capsid protein, L2, has previously been reported to induce cross-neutralizing antibodies in mice. In this report, four HPV L2 peptides, including the HPV-16 peptide and its HPV type 6 and 11 homologues, along with extended peptides containing a conserved set of amino acids, were used to immunize rabbits and mice. Antibody responses were evaluated for specificity and ability to neutralize viral infection in vitro with a quantitative reverse transcriptase (RT)-polymerase chain reaction (PCR) assay. All peptide immunizations resulted in cognate and cross-peptide reactivity, but this did not translate equally into recognition of full-length protein, VLP, or neutralization of virus in vitro. This report provides the first evidence of cross-neutralization of authentic HPV by antiserum to L2 peptides. Comparison of the anti-peptide serum reactivity, especially with regard to neutralization of virus, indicates that the extended peptides may offer more potential to induce adequate responses for cross-protective immunity.

Reactivity of human sera in a sensitive, high-throughput pseudovirus-based papillomavirus neutralization assay for HPV16 and HPV18

Sensitive high-throughput neutralization assays, based upon pseudoviruses carrying a secreted alkaline phosphatase (SEAP) reporter gene, were developed and validated for human papillomavirus (HPV)16, HPV18, and bovine papillomavirus 1 (BPV1). SEAP pseudoviruses were produced by transient transfection of codon-modified papillomavirus structural genes into an SV40 T antigen expressing line derived from 293 cells, yielding sufficient pseudovirus from one flask for thousands of titrations. In a 96-well plate format, in this initial characterization, the assay was reproducible and appears to be as sensitive as, but more specific than, a standard papillomavirus-like particle (VLP)-based enzyme-linked immunosorbent assay (ELISA). The neutralization assay detected type-specific HPV16 or HPV18 neutralizing antibodies (titers of 160-10240) in sera of the majority of a group of women infected with the corresponding HPV type, but not in virgin women. Sera from HPV16 VLP vaccinees had high anti-HPV16 neutralizing titers (mean: 45000; range: 5120-163840), but no anti-HPV18 neutralizing activity. The SEAP pseudovirus-based neutralization assay should be a practical method for quantifying potentially protective antibody responses in HPV natural history and prophylactic vaccine studies.

Propagation of infectious human papillomavirus type 16 by using an adenovirus and Cre/LoxP mechanism

Human papillomavirus type 16 (HPV16) infection is a major risk factor for the development of squamous cell cancers of the cervix and of the head and neck. A major barrier to understanding the progression from initial infection to cancer has been the lack of in vitro models that allow infection, replication, and persistence of the viral genome as an episome in differentiated epithelial cells. To overcome this barrier, we designed an adenoviral delivery vector that contained a full HPV16 genome flanked by LoxP homologous recombination sites and a fluorescent reporter that was expressed only after the HPV genome was excised by Cre recombinase. This system delivered circular HPV16 genomes to cervical epithelial cells and well differentiated human airway epithelia. After delivery, the HPV16 genome replicated and persisted as an episome in cervical keratinocytes. These cells developed an immortalized phenotype and a dysplastic epithelial appearance. Moreover, induction of differentiation led to the expression of late genes and production of infectious HPV16 virions. This work provides a means of introducing biologically active HPV genomes into epithelial cells, which are normally difficult to transfect. These methods allow the study of HPV genome replication and gene expression in the earliest stages of HPV genome establishment, and they may provide a means to study nononcogenic HPV viral types.

Different heparan sulfate proteoglycans serve as cellular receptors for human papillomaviruses

Papillomaviruses replicate in stratified epithelia of skin and mucosa. Infection with certain human papillomavirus (HPV) types is the main cause of anogenital neoplasia, in particular cervical cancer. Early events of papillomavirus infectivity are poorly understood. While heparan sulfate proteoglycans (HSPGs) mediate initial binding to the cell surface, the class of proteins carrying heparan sulfates has not been defined. Here we examined two processes of papillomavirus infection, attachment of virus-like particles (VLP) to cells and infection with authentic HPV type 11 (HPV11) virions. Of the HSPGs, syndecan-1 is the major epithelial form and is strongly upregulated in wound edge keratinocytes. We employed K562 cells, which lack HSPGs except minor amounts of endogenous betaglycan, and stable clones that express cDNAs of syndecan-1, syndecan-4, or glypican-1. Binding of VLP correlated with levels of heparan sulfate on the cell surface. Parental K562 bound HPV16 VLP weakly, whereas all three K562 transfectants demonstrated enhanced binding, with the highest binding capacity observed for syndecan-1-transfected cells, which also expressed the most HSPG. For HPV11 infectivity assays, a high virion inoculum was required to infect K562 cells, whereas ectopic expression of syndecan-1 increased permissiveness eightfold and expression of syndecan-4 or glypican-1 fourfold. Infection of keratinocytes was eliminated by treatment with heparitinase, but not phospholipase C, further implicating the syndecan family of integral membrane proteins as receptor proteins. Human keratinocytes with a homozygous deletion of alpha6 integrin are permissive for HPV11 infection. These results indicate that several HSPGs can serve as HPV receptors and support a putative role for syndecan-1, rather than alpha6 integrin, as a primary receptor protein in natural HPV infection of keratinocytes.

Suppression of human papillomavirus gene expression in vitro and in vivo by herpes simplex virus type 2 infection

Recent epidemiological studies have found that women infected with both herpes simplex virus type 2 (HSV-2) and human papillomavirus (HPV) type 16 or HPV-18 are at greater risk of developing cervical carcinoma compared to women infected with only one virus. However, it remains unclear if HSV-2 is a cofactor for cervical cancer or if HPV and HSV-2 interact in any way. We have studied the effect of HSV-2 infection on HPV-11 gene expression in an in vitro double-infection assay. HPV transcripts were down-regulated in response to HSV-2 infection. Two HSV-2 vhs mutants failed to reduce HPV-16 E1;E4 transcripts. We also studied the effect of HSV-2 infection on preexisting experimental papillomas in a vaginal epithelial xenograft model. Doubly infected grafts demonstrated papillomatous transformation and the classical cytopathic effect from HSV-2 infection. HPV and HSV DNA signals were mutually exclusive. These studies may have therapeutic applications for HPV infections and related neoplasms.

Quantitative RT-PCR assay for HPV infection in cultured cells

Early events in the life cycle of the human papillomaviruses (HPV) have been difficult to investigate due to both the scarcity of authentic HPV virions and limitations in assays to detect and quantify nonpermissive infections in monolayer cell culture. We have developed a quantitative reverse transcription-PCR (QRT-PCR) assay for the E1( wedge )E4 transcript of HPV-11. This assay is both sensitive, and capable of differentiating between infections caused by a wide range of virus input. The QRT-PCR assay measured accurately the relative amount of viral transcripts present in samples during validation experiments using RNAs from three cell lines. Infections in all three cell lines, using titrations of HPV-11 virions ranging from 20 to 600 particles per cell, produced linear expression profiles suggesting that these multiplicities of infection are below the saturation level for viral uptake and transcription. Comparison of the QRT-PCR assay with the commonly used nested RT-PCR assay revealed that although the nested RT-PCR assay was more sensitive, it did not differentiate between infections caused by >1000-fold difference in viral inputs. Potential applications of the QRT-PCR assay are demonstrated in experiments measuring the ability of a capsid-specific monoclonal antibody and a nonspecific microbicide to block HPV-11 infection.

Human papillomavirus type 45 propagation, infection, and neutralization

The organotypic (raft) culture system has allowed the study of the entire differentiation-dependent life cycle of human papillomaviruses (HPVs), including virion morphogenesis. We introduced linearized HPV45 genomic DNA into primary keratinocytes, where it recircularized and maintained episomally at a range of 10-50 copies of HPV genomic DNA. Following epithelial stratification and differentiation in organotypic culture, virion morphogenesis occurred. HPV45 virions were purified from raft cultures and were able to infect keratinocytes in vitro. By testing a panel of HPV VLP antisera, we were able to demonstrate that the infection was neutralized not only with human HPV45 VLP-specific antiserum, but also with human HPV18 VLP-specific antiserum, demonstrating serological cross-reactivity between HPV18 and HPV45.

Mucosally-derived HPV-40 can infect both human genital foreskin and cutaneous hand skin tissues grafted into athymic mice

HPV-40 is a rare HPV type that has been detected only in genital mucosal tissues. This HPV type is very closely related to HPV-7, which has a predominantly cutaneous tissue tropism. We have shown, previously, that an isolate of HPV-40 (described here as HPV-40(Hershey) or HPV-40(H)) productively infected genital tissues. In this study, HPV-40(H) was tested for productive infection of cutaneous tissue. Fetal hand skin fragments were incubated with infectious HPV-40(H) and implanted subrenally into athymic mice. After 120 days, xenografts showed morphological changes consistent with HPV-40(H) infection and were HPV-40 DNA in situ positive and capsid antigen positive. The results demonstrated that hand skin can support HPV-40(H) infection thereby indicating that this viral type has the capacity to infect both genital mucosal and cutaneous tissues.

Papillomaviruses infect cells via a clathrin-dependent pathway

In this study we have examined the pathway by which papillomaviruses infect cells, using bovine papillomavirus (BPV) virions and mouse C127 cells as the model system. By confocal microscopy, the entry of BPV virions, BPV virus-like particles (VLPs), and HPV16 VLPs were very similar. In dually exposed cells, HPV-16 VLPs and BPV virions colocalized intracellularly. BPV VLPs colocalized with AP-2, a clathrin adapter molecular and a marker of the clathrin-dependent endocytic pathway; and also with transferrin receptor, a marker of early endosomes; and Lamp-2, a marker of late endosomes and lysosomes. BPV infection was detected within 12 h of virion cell-surface binding, as measured by an RT-PCR assay. Infection was prevented by several pharmacologic inhibitors, including chlorpromazine, which blocks clathrin-dependent endocytosis and the lysosomotropic agent, bafilomycin A. By contrast, two inhibitors of caveolae-dependent uptake, filipin and nystatin, did not prevent BPV infection. We conclude that papillomaviruses infect cells via clathrin-dependent receptor-mediated endocytosis. Surprisingly, the kinetics of internalization were unusually slow for this mechanism, with the t(1/2) of entry of BPV-1 being approximately 4 h versus 5-15 min for a typical ligand.

Infectious human papillomavirus type 31b: purification and infection of an immortalized human keratinocyte cell line

Human papillomaviruses (HPVs) are aetiological agents of human malignancies, most notably cervical cancers. The life-cycles of HPVs are dependent on epithelial differentiation, and this has impeded many basic studies of HPV biology. The organotypic (raft) culture system supports epithelial differentiation such that infectious virions are synthesized in raft tissues from epithelial cells that replicate extrachromosomal HPV genomes. The CIN-612 9E cell line maintains episomal copies of HPV type 31b (HPV31b), an HPV type associated with cervical cancers. Many previous studies, including our own, have focused on characterizing the later stages of the HPV31b life-cycle in CIN-612 9E raft tissues. In this study, we have used the raft system to generate large numbers of HPV31b viral DNA (vDNA)-containing particles. We found a biologically contained homogenization system to be efficient at virion extraction from raft epithelial tissues. We also determined that vDNA-containing particles could be directly quantified from density-gradient fractions. Using an RT-PCR assay, the presence of newly synthesized, spliced HPV31b transcripts was detected following HPV31b infection of the immortalized HaCaT epithelial cell line. Spliced E6 and E1( wedge )E4 RNAs were detected using a single round of RT-PCR from cells infected with a dose as low as 1.0 vDNA-containing particle per cell. Spliced E1*I,E2 transcripts were found in cells infected with an HPV31b dose as low as 10 vDNA-containing particles per cell. Infectivity was blocked by HPV31 antiserum, but was not affected by DNase I. This work lays a foundation for a detailed analysis of the early events in HPV infection.

Infectious virions produced from a human papillomavirus type 18/16 genomic DNA chimera

The organotypic raft culture system has allowed the study of the differentiation-dependent aspects of the human papillomavirus (HPV) life cycle. However, genetic strategies to more completely understand the HPV life cycle are limited. The generation of chimeric viruses has been a useful tool in other virus systems to analyze infection and replication. To investigate the specificity of the interaction of nonstructural genes of one HPV type with the structural genes of another HPV type, we have replaced the L2 and L1 open reading frames (ORFs) of HPV type 18 (HPV18) with the L2 and L1 ORFs of HPV type 16 (HPV16). The resulting HPV18/16 chimeric construct was introduced into primary keratinocytes, where it was stably maintained episomally at a range of 50 to 100 copies of HPV genomic DNA, similar to that typically found in HPV-infected cells in vivo. The integrity of the HPV18/16 genomic DNA chimera was demonstrated. Upon differentiation in raft cultures, late viral functions, including viral DNA amplification, capsid gene expression, and virion morphogenesis, occurred. Chimeric HPV18/16 virions were purified from the raft cultures and were capable of infecting keratinocytes in vitro. Additionally, infection was specifically neutralized with human HPV16 virus-like particle (VLP)-specific antiserum and not with human HPV18 VLP-specific antiserum. Our data demonstrate that the nonstructural genes of HPV18 functionally interact with the structural genes of HPV16, allowing the complete HPV life cycle to occur. We believe that this is the first report of the propagation of chimeric HPV by normal life cycle pathways.

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.

Papillomavirus microbicidal activities of high-molecular-weight cellulose sulfate, dextran sulfate, and polystyrene sulfonate

The high-molecular-weight sulfated or sulfonated polysaccharides or polymers cellulose sulfate, dextran sulfate, and polystyrene sulfonate were tested for microbicidal activity against bovine papillomavirus type 1 (BPV-1) and human papillomavirus type 11 (HPV-11) and type 40 (HPV-40). In vitro assays included the BPV-1-induced focus-forming assay and transient infection of human A431 cells with HPVs. The compounds were tested for microbicidal activity directly by preincubation with virus prior to addition to cell cultures and indirectly by addition of virus to compound-treated cells and to virus-coated cells to test inactivation of the virus after virus-cell binding. The data indicated that all three compounds showed direct microbicidal activity with 50% effective concentrations between 10 to 100 microg/ml. These concentrations were nontoxic to cell cultures for both assays. When a clone of C127 cells was tested for microbicidal activity, approximately 10-fold-less compound was required to achieve a 50% reduction in BPV-1-induced foci than for the uncloned parental C127 cells. Pretreatment of cells with compound prior to addition of virus also demonstrated strong microbicidal activity with dextran sulfate and polystyrene sulfonate, but cellulose sulfate required several orders of magnitude more compound for virus inactivation. Polystyrene sulfonate prevented subsequent infection of HPV-11 after virus-cell binding, and this inactivation was observed up to 4 h after addition of virus. These data indicate that the polysulfated and polysulfonated compounds may be useful nontoxic microbicidal compounds that are active against a variety of sexually transmitted disease agents including papillomaviruses.

A novel human papillomavirus type 6 neutralizing domain comprising two discrete regions of the major capsid protein L1

We have mapped the binding sites on human papillomavirus (HPV) type 6 for three HPV 6-specific neutralizing monoclonal antibodies (mAbs). The critical binding residues were first identified by making HPV 11-like amino acid substitutions in the HPV 6 major capsid protein L1 and assaying the resulting virus-like particles (VLPs) for reactivity with the mAbs. To confirm the relevance of these residues for mAb binding, we demonstrated that HPV 6 type-specificity could be transferred to HPV 11 VLPs by making the appropriate HPV 6-like amino acid substitutions in the HPV 11 L1. Two binding regions were found. For one mAb, all critical residues are centered at residue 53, while for the other two mAbs, type-specific binding also requires a second site located more than 100 residues distal to the first. Both binding sites coincide with regions of L1 where the sequences of the closely related HPV 6 and 11 diverge. These regions are where the L1 sequences are the least well conserved among all HPV types and they have been implicated in type-specific binding for other HPV types. This suggests that clusters of diverged residues, surrounded by conserved L1 sequences, are presented on the surface of assembled particles and are responsible for eliciting critical humoral immune responses to the virus.

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.

Assessment of template quality by the incorporation of an internal control into a RT-PCR for the detection of rabies and rabies-related viruses

A method is described to assess RNA template quality by the incorporation of a ribosomal RNA (rRNA) internal (in tube) control into a standard rabies and rabies-related virus specific RT-PCR. Specific virus and rRNA templates were co-amplified in a duplex reaction from RNA extracts derived from 60 isolates representing all six of the established lyssavirus genotypes. To ensure a wide species applicability of this technique we demonstrated that the rRNA assay was capable of functioning using the cells or tissues of 14 different mammals. Parallel studies between the duplex and the unlinked lyssavirus assay demonstrated only a minor reduction in the sensitivity of the former test. The ribosomal and viral targets (unlike beta-actin RNA) were shown to have similar degradation kinetics making rRNA amplification a good control for viral target integrity. As a consequence, the use of this system would reduce the likelihood of obtaining false negative RT-PCR results from lyssavirus infected material.

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.

Oral vaccination of mice with human papillomavirus virus-like particles induces systemic virus-neutralizing antibodies

To assess whether oral vaccination against human papillomavirus (HPV) may be feasible, we administered HPV virus-like particles (VLPs) to mice by gavage. Enzyme-linked immunosorbent assay (ELISA) results indicated that serum anti-VLP immunoglobulin G (IgG) and IgA antibodies were induced after oral vaccination, and these responses demonstrated antigenic specificities that were conformationally dependent and restricted according to HPV genotype. Importantly, orally induced postimmune sera were found to neutralize HPV-11 virions in vitro. These results indicated that the VLPs were antigenically stable in the environment of the gastrointestinal tract and were able to engage in potentially useful immune system interactions. These findings support the concept of oral vaccination against anogenital HPV disease, and suggest the possibility that this may be a useful approach to the immunization of large populations against cervical cancer and other HPV associated diseases.

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.

Human papillomavirus type 11 recombinant L1 capsomeres induce virus-neutralizing antibodies

The human papillomavirus type 11 (HPV-11) L1 major capsid protein can be trypsinized to generate recombinant capsomeres that retain HPV genotype-restricted capsid antigenicity (M. Li, T. P. Cripe, P. A. Estes, M. K. Lyon, R. C. Rose, and R. L. Garcea, J. Virol. 71:2988-2995, 1997). In the present study, HPV-11 virion-neutralizing monoclonal antibodies H11.F1 and H11.H3, previously characterized as recognizing two distinct HPV-11 capsid-neutralizing antigenic domains (S. W. Ludmerer, D. Benincasa, and G. E. Mark III, J. Virol. 70:4791-4794, 1996), were each found to be highly immunoreactive with trypsin-generated capsomeres in an enzyme-linked immunosorbent assay (ELISA). Capsomeres were used to generate high-titer polyclonal immune sera that demonstrated HPV genotype-restricted reactivity by ELISA. The capsomere antisera were then tested in an in vitro infectivity assay and found to neutralize HPV-11 virions. In this assay, HPV-11 capsomere polyclonal antisera exhibited neutralization titers (10(-5) to 10(-6)) comparable to those obtained with a virion-neutralizing antiserum raised previously against intact HPV-11 VLPs (R. C. Rose, R. C. Reichman, and W. Bonnez, J. Gen. Virol. 75:2075-2079, 1994). These results indicate that highly immunogenic, genotype-restricted HPV capsid-neutralizing antigenic domains are contained entirely within capsomeres. Thus, capsomeres may be viable vaccine candidates for the prevention of HPV disease.

In vitro infection and type-restricted antibody-mediated neutralization of authentic human papillomavirus type 16

Human papillomavirus type 16 (HPV-16) is strongly associated with the development of cervical cancer. Studies of model systems with animal papillomaviruses have demonstrated the importance of neutralizing antibodies in preventing papillomavirus-associated disease. The assessment of neutralizing antibody responses against HPV-16, previously hampered by the lack of a viral source, was enabled by the recent propagation of an HPV-16 stock in xenografted severe combined immunodeficiency (SCID) mice. HPV-16 infection of an immortalized human keratinocyte cell line was demonstrated by detection of an HPV-16-specific spliced mRNA amplified by reverse transcriptase PCR. Infection was blocked by preincubation of the virus with antiserum generated against HPV-16 virus-like particles (VLPs) composed of the major capsid protein, L1. To examine potential cross-neutralizing activity among the different genital HPV types, rabbit antisera to L1 VLPs corresponding to HPV-6, -11, -18, -31, -33, -35, -39, and -45 were assayed for the ability to block the HPV-16 infection of cultured cells. Antiserum raised against HPV-33 L1 VLPs was the only heterologous antiserum which inhibited HPV-16 infection. Thus, a neutralization assay for HPV-16 may help to characterize the components required to compose a broadly efficacious genital HPV vaccine.

Quantitative disassembly and reassembly of human papillomavirus type 11 viruslike particles in vitro

The human papillomavirus (HPV) capsid is primarily composed of a structural protein denoted L1, which forms both pentameric capsomeres and capsids composed of 72 capsomeres. The L1 protein alone is capable of self-assembly in vivo into capsidlike structures referred to as viruslike particles (VLPs). We have determined conditions for the quantitative disassembly of purified HPV-11 L1 VLPs to the level of capsomeres, demonstrating that disulfide bonds alone are essential to maintaining long-term HPV-11 L1 VLP structure at physiological ionic strength. The ionic strength of the disassembly reaction was also important, as increased NaCl concentrations inhibited disassembly. Conversely, chelation of cations had no effect on disassembly. Quantitative reassembly to a homogeneous population of 55-nm, 150S VLPs was reliably achieved by the re-formation of disulfide linkages following removal of reducing agent at near-neutral pH and moderate NaCl concentration. HPV-11 L1 VLPs could also be dissociated by treatment with carbonate buffer at pH 9.6, but VLPs could not be regenerated following carbonate treatment. When probed with conformationally sensitive and/or neutralizing monoclonal antibodies, both capsomeres generated by disulfide reduction of purified VLPs and reassembled VLPs formed from capsomeres upon removal of reducing agents exhibited epitopes found on the surface of authentic HPV-11 virions. Antisera raised against either purified VLP starting material or reassembled VLPs similarly neutralized infectious HPV-11 virions. The ability to disassemble and reassemble VLPs in vitro and in bulk allows basic features of capsid assembly to be studied and also opens the possibility of packaging selected exogenous compounds within the reassembled VLPs.

Detection of serologic neutralizing antibodies against HPV-11 in patients with condyloma acuminata and cervical dysplasia using an in vitro assay

This study was designed to investigate if neutralizing antibodies against HPV-11 are detectable in the serum of patients with condyloma acuminata (CA) or cervical intraepithelial neoplasia (CIN) using an in vitro infectivity assay for HPV-11. Purified HPV-11 virions were extracted from xenografted condyloma tissues implanted into athymic mice and used to infect cultured neonatal human foreskin keratinocytes (HFK) and an immortalized adult skin cell line (HaCaT). The presence of HPV-11-specific E1--E4 mRNA as detected by reverse transcriptase-polymerase chain reaction was indicative of early infection. Sera previously characterized for reactivity to HPV-11 and HPV-11 VLP (virus-like particles) by ELISA were tested for the ability to prevent HPV-11 in vitro infectivity. Neutralizing antibodies against HPV-11 were demonstrated when monoclonal antibodies or patient serum preincubated with HPV-11 virions prevented the infection of either of the two cell cultures, as shown by the absence of the E1--E4 mRNA transcript. Eleven (of 20) patients with CA were strongly ELISA reactive against HPV-11 virus-like particles. Five of these 11 patients also had detectable levels of neutralizing antibodies in their serum. It was also demonstrated that the neutralizing properties of the serum were titratable by endpoint dilution. None of 15 patients with CIN had detectable neutralizing antibodies against HPV-11. Neutralizing antibodies against HPV-11 can be detected in some patients with CA and the neutralizing effects of the patient sera can be titrated by endpoint dilution. The in vitro assay for the detection of neutralizing antibodies against HPV-11 may have utility for investigating the natural history of HPV infection and resolution, as well as assessing the efficacy of any putative HPV vaccine.

Chimeric papillomavirus-like particles

We have constructed chimeric papillomavirus-like particles (CVLPs) by replacing the 34-carboxy-terminal amino acids of the HPV 16 L1 protein with various parts of the HPV 16 E7 protein. Chimeric proteins were expressed by recombinant baculoviruses and analyzed by electron microscopy for their ability to assemble into virus capsids. We were able to produce CVLPs in high efficiencies with inserts of up to 60 amino acids. CVLPs are able to induce a neutralizing antibody response, assayed by inhibition of hemagglutination of mouse erythrocytes. CVLPs are interacting with the putative receptor for papillomaviruses as they were shown to hemagglutinate mouse red blood cells and bind to and penetrate cells in vitro. As CVLPs follow a similar intracellular pathway as observed earlier for BPV VLPs, we speculate that CVLPs can be used to deliver peptides into mammalian cells in vitro and in vivo, possibly reaching the pathway for MHC class I presentation.

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

We report a system for generating infectious papillomaviruses in vitro that facilitates the analysis of papillomavirus assembly, infectivity, and serologic relatedness. Cultured hamster BPHE-1 cells harboring autonomously replicating bovine papillomavirus type 1 (BPV1) genomes were infected with recombinant Semliki Forest viruses that express the structural proteins of BPV1. When plated on C127 cells, extracts from cells expressing L1 and L2 together induced numerous transformed foci that could be specifically prevented by BPV neutralizing antibodies, demonstrating that BPV infection was responsible for the focal transformation. Extracts from BPHE-1 cells expressing L1 or L2 separately were not infectious. Although Semliki Forest virus-expressed L1 self-assembled into virus-like particles (VLPs), viral DNA was detected in particles only when L2 was coexpressed with L1, indicating that genome encapsidation requires L2. Expression of human papillomavirus type 16 (HPV16) L1 and L2 together in BPHE-1 cells also yielded infectious virus. These pseudotyped virions were neutralized by antiserum to HPV16 VLPs derived from European (114/K) or African (Z-1194) HPV16 variants but not by antisera to BPV VLPs, to a poorly assembling mutant HPV16 L1 protein, or to VLPs of closely related genital HPV types. Extracts from BPHE-1 cells coexpressing BPV L1 and HPV16 L2 or HPV16 L1 and BPV L2 were not infectious. We conclude that (i) mouse C127 cells express the cell surface receptor for HPV16 and are able to uncoat HPV16 capsids; (ii) if a papillomavirus DNA packaging signal exists, then it is conserved between the BPV and HPV16 genomes; (iii) functional L1-L2 interaction exhibits type specificity; and (iv) protection by HPV virus-like particle vaccines is likely to be type specific.

Immunology of papillomavirus infection

Studies of the immunology of papillomavirus infection have come of age. Synthetic virus-like particles have been validated as vaccines for several animal papillomaviruses, and have been used to map the sero-epidemiology of human papillomavirus infection and to define papillomavirus neutralizing antibodies. Induction of cell-mediated immunity to papillomavirus early proteins is poised to become a therapeutic approach to papillomavirus infection. Studies on the immune response to papillomavirus proteins in keratinocytes are shedding light on the immunological consequences of antigen presentation by epithelial cells.