The N-terminal region of the human papillomavirus L2 protein contains overlapping binding sites for neutralizing, cross-neutralizing and non-neutralizing antibodies

Capsid display of a conserved human papillomavirus L2 peptide in the adenovirus 5 hexon protein: a candidate prophylactic hpv vaccine approach

Background

Infection by any one of 15 high risk human papillomavirus (hrHPV) types causes most invasive cervical cancers. Their oncogenic genome is encapsidated by L1 (major) and L2 (minor) coat proteins. Current HPV prophylactic vaccines are composed of L1 virus-like particles (VLP) that elicit type restricted immunity. An N-terminal region of L2 protein identified by neutralizing monoclonal antibodies comprises a protective epitope conserved among HPV types, but it is weakly immunogenic compared to L1 VLP. The major antigenic capsid protein of adenovirus type 5 (Ad5) is hexon which contains 9 hypervariable regions (HVRs) that form the immunodominant neutralizing epitopes. Insertion of weakly antigenic foreign B cell epitopes into these HVRs has shown promise in eliciting robust neutralizing antibody responses. Thus here we sought to generate a broadly protective prophylactic HPV vaccine candidate by inserting a conserved protective L2 epitope into the Ad5 hexon protein for VLP-like display.

Methods

Four recombinant adenoviruses were generated without significant compromise of viral replication by introduction of HPV16 amino acids L2 12–41 into Ad5 hexon, either by insertion into, or substitution of, either hexon HVR1 or HVR5.

Results

Vaccination of mice three times with each of these L2-recombinant adenoviruses induced similarly robust adenovirus-specific serum antibody but weak titers against L2. These L2-specific responses were enhanced by vaccination in the presence of alum and monophoryl lipid A adjuvant. Sera obtained after the third immunization exhibited low neutralizing antibody titers against HPV16 and HPV73. L2-recombinant adenovirus vaccination without adjuvant provided partial protection of mice against HPV16 challenge to either the vagina or skin. In contrast, vaccination with each L2-recombinant adenovirus formulated in adjuvant provided robust protection against vaginal challenge with HPV16, but not against HPV56.

Conclusion

We conclude that introduction of HPV16 L2 12–41 epitope into Ad5 hexon HVR1 or HVR5 is a feasible method of generating a protective HPV vaccine, but further optimization is required to strengthen the L2-specific response and broaden protection to the more diverse hrHPV.

Topography of the Human Papillomavirus Minor Capsid Protein L2 during Vesicular Trafficking of Infectious Entry

The human papillomavirus (HPV) capsid is composed of the major capsid protein L1 and the minor capsid protein L2. During entry, the HPV capsid undergoes numerous conformational changes that result in endosomal uptake and subsequent trafficking of the L2 protein in complex with the viral DNA to the trans-Golgi network. To facilitate this transport, the L2 protein harbors a number of putative motifs that, if capable of direct interaction, would interact with cytosolic host cell factors. These data imply that a portion of L2 becomes cytosolic during infection. Using a low concentration of digitonin to selectively permeabilize the plasma membrane of infected cells, we mapped the topography of the L2 protein during infection. We observed that epitopes within amino acid residues 64 to 81 and 163 to 170 and a C-terminal tag of HPV16 L2 are exposed on the cytosolic side of intracellular membranes, whereas an epitope within residues 20 to 38, which are upstream of a putative transmembrane region, is luminal. Corroborating these findings, we also found that L2 protein is sensitive to trypsin digestion during infection. These data demonstrate that the majority of the L2 protein becomes accessible on the cytosolic side of intracellular membranes in order to interact with cytosolic factors to facilitate vesicular trafficking.

IMPORTANCE

In order to complete infectious entry, nonenveloped viruses have to pass cellular membranes. This is often achieved through the viral capsid protein associating with or integrating into intracellular membrane. Here, we determine the topography of HPV L2 protein in the endocytic vesicular compartment, suggesting that L2 becomes a transmembrane protein with a short luminal portion and with the majority facing the cytosolic side for interaction with host cell transport factors.

Targeting synthetic Human Papillomavirus (HPV) L2 disulfide-induced N-terminus conformational epitopes for pan-HPV vaccine development

BACKGROUND

Current vaccines against Human Papillomavirus (HPV) are highly effective and based on recombinant virus-like particles (VLPs) of the major capsid protein L1. Since these vaccines are HPV type-specific and expensive for global implementation, an alternative, broader-spectrum immunogen would be the N-terminus of the minor capsid protein L2 that induces low titered broadly cross-neutralizing antibodies. Here we analyzed the reactivity of different synthetic L2 peptides containing N-terminus amino acids 17-36 in order to test their antigenicity.

METHODS

Different synthetic peptides were designed to target the 17-36 amino acid sequences, present in highly antigenic amino-terminus of L2 protein. Six different peptides including Cys22-Cys28 disulfide bonded cyclized L2 peptide were examined for their antigenicity against mouse monoclonal antibody RG-1 and rabbit polyclonal antisera to HPV L2 by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Here we report that the cyclized form of synthetic L2 peptide, which is formed through Cys22-Cys28 disulfide bridges, has the highest reactivity to antibodies than other synthetic L2 peptides.

CONCLUSION

A cyclized L2 peptide has potential to be an excellent candidate to formulate a low-cost, broadly protective pan-oncogenic HPV vaccine.

Involvement of Nucleophosmin (NPM1/B23) in Assembly of Infectious HPV16 Capsids

We report that during assembly of HPV16 pseudovirus (PsV) the minor capsid protein, L2, interacts with the host nucleolar protein nucleophosmin (NPM1/B23). Exogenously-expressed L2 colocalized with NPM1, a complex containing both proteins could be immunoprecipitated, and L2 could redirect to the nucleus NPM1 that was pharmacologically or genetically restricted to the cytoplasm. Coexpression of the major capsid protein, L1, prevented both the colocalization and the biochemical association, and L1 pentamers could displace L2 from L2/NPM1 complexes attached to a nuclear matrix. HPV16 PsV that was produced in a cell line with reduced NPM1 levels had significantly lower infectivity compared to PsV produced in the parental cell line, although the PsV preparations had comparable L1 and L2 ratios and levels of encapsidated DNA. The PsV produced in NPM1-deficient cells showed increased trypsin sensitivity and exhibited decreased L2 levels during endocytosis. These results suggest a critical role for NPM1 in establishing the correct interactions between L2 and L1 during HPV capsid assembly. A decrease in cellular levels of NPM1 results in the formation of seemingly normal, but unstable, capsids that result in a premature loss of L2, thus inhibiting successful infection. No role for NPM1 in HPV infectious entry was found.

Identification of Broad-Genotype HPV L2 Neutralization Site for Pan-HPV Vaccine Development by a Cross-Neutralizing Antibody

Human Papillomavirus (HPV), a non-enveloped, double-stranded DNA virus, is responsible for 5% of human cancers. The HPV capsid consists of major and minor structural proteins, L1 and L2. L1 proteins form an icosahedral shell with building blocks of the pentameric capsomere, and one L2 molecule extends outward from the central hole of the capsid. Thus, L2 is concealed within L1 and only becomes exposed when the capsid interacts with host cells. The low antigenic variation of L2 means that this protein could offer a target for the development of a pan-HPV vaccine. Toward this goal, here we describe an anti-L2 monoclonal antibody, 14H6, which broadly neutralizes at least 11 types of HPV, covering types 6, 11, 16, 18, 31, 33, 35, 45, 52, 58 and 59, in pseudovirion--based cell neutralization assay. The mAb 14H6 recognizes a minimal linear epitope located on amino acids 21 to 30 of the L2 protein. Alanine scanning mutagenesis and sequence alignment identified several conserved residues (Cys22, Lys23, Thr27, Cys28 and Pro29) that are involved in the 14H6 binding with L2. The epitope was grafted to several scaffolding proteins, including HPV16 L1 virus-like particles, HBV 149 core antigen and CRM197. The resultant chimeric constructs were expressed in Escherichia coli and purified with high efficiency. Immunization with these pan-HPV vaccine candidates elicited high titers of the L2-specific antibody in mice and conferred robust (3-log) titers of cross-genotype neutralization, including against HPV11, 16, 18, 45, 52, 58 and 59. These findings will help in the development of an L2-based, pan-HPV vaccine.

A chimeric 18L1-45RG1 virus-like particle vaccine cross-protects against oncogenic alpha-7 human papillomavirus types

Persistent infection with oncogenic human papillomaviruses (HPV) types causes all cervical and a subset of other anogenital and oropharyngeal carcinomas. Four high-risk (hr) mucosal types HPV16, 18, 45, or 59 cause almost all cervical adenocarcinomas (AC), a subset of cervical cancer (CxC). Although the incidence of cervical squamous cell carcinoma (SCC) has dramatically decreased following introduction of Papanicolaou (PAP) screening, the proportion of AC has relatively increased. Cervical SCC arise mainly from the ectocervix, whereas AC originate primarily from the endocervical canal, which is less accessible to obtain viable PAP smears. Licensed (bivalent and quadrivalent) HPV vaccines comprise virus-like particles (VLP) of the most important hr HPV16 and 18, self-assembled from the major capsid protein L1. Due to mainly type-restricted efficacy, both vaccines do not target 13 additional hr mucosal types causing 30% of CxC. The papillomavirus genus alpha species 7 (α7) includes a group of hr types of which HPV18, 45, 59 are proportionally overrepresented in cervical AC and only partially (HPV18) targeted by current vaccines. To target these types, we generated a chimeric vaccine antigen that consists of a cross-neutralizing epitope (homologue of HPV16 RG1) of the L2 minor capsid protein of HPV45 genetically inserted into a surface loop of HPV18 L1 VLP (18L1-45RG1). Vaccination of NZW rabbits with 18L1-45RG1 VLP plus alum-MPL adjuvant induced high-titer neutralizing antibodies against homologous HPV18, that cross-neutralized non-cognate hr α7 types HPV39, 45, 68, but not HPV59, and low risk HPV70 in vitro, and induced a robust L1-specific cellular immune response. Passive immunization protected mice against experimental vaginal challenge with pseudovirions of HPV18, 39, 45 and 68, but not HPV59 or the distantly related α9 type HPV16. 18L1-45RG1 VLP might be combined with our previously described 16L1-16RG1 VLP to develop a second generation bivalent vaccine with extended spectrum against hr HPV.

Thioredoxin-Displayed Multipeptide Immunogens

Fusion to carrier proteins is an effective strategy for stabilizing and providing immunogenicity to peptide epitopes. This is commonly achieved by cross-linking of chemically synthesized peptides to carrier proteins. An alternative approach is internal grafting of selected peptide epitopes to a scaffold protein via double stranded-oligonucleotide insertion or gene synthesis, followed by recombinant expression of the resulting chimeric polypeptide. The scaffold protein should confer immunogenicity to the stabilized and structurally constrained peptide, but also afford easy production of the antigen in recombinant form. A macromolecular scaffold that meets the above criteria is the redox protein thioredoxin, especially bacterial thioredoxin. Here we describe our current methodology for internal grafting of selected peptide epitopes to thioredoxin as tandemly arranged multipeptide repeats ("Thioredoxin Displayed Multipeptide Immunogens"), bacterial expression and purification of the recombinant thioredoxin-multipeptide fusion proteins and their use as antigens for the production of anti-peptide antibodies for prophylactic vaccine as well as diagnostic purposes.

Prophylactic papillomavirus vaccines

Human papillomaviruses (HPV) are the causative agents of cervical cancer, the third most common cancer in women. The development of prophylactic HPV vaccines Gardasil® and Cervarix® targeting the major oncogenic HPV types is now the frontline of cervical cancer prevention. Both vaccines have been proven to be highly effective and safe although there are still open questions about their target population, cross-protection, and long-term efficacy. The main limitation for a worldwide implementation of Gardasil® and Cervarix® is their high cost. To develop more affordable vaccines research groups are concentrated in new formulations with different antigens including capsomeres, the minor capsid protein L2 and DNA. In this article we describe the vaccines' impact on HPV-associated disease, the main open questions about the marketed vaccines, and current efforts for the development of second-generation vaccines.

Measurement of neutralizing serum antibodies of patients vaccinated with human papillomavirus L1 or L2-based immunogens using furin-cleaved HPV Pseudovirions

Antibodies specific for neutralizing epitopes in either Human papillomavirus (HPV) capsid protein L1 or L2 can mediate protection from viral challenge and thus their accurate and sensitive measurement at high throughput is likely informative for monitoring response to prophylactic vaccination. Here we compare measurement of L1 and L2-specific neutralizing antibodies in human sera using the standard Pseudovirion-Based Neutralization Assay (L1-PBNA) with the newer Furin-Cleaved Pseudovirion-Based Neutralization Assay (FC-PBNA), a modification of the L1-PBNA intended to improve sensitivity towards L2-specific neutralizing antibodies without compromising assay of L1-specific responses. For detection of L1-specific neutralizing antibodies in human sera, the FC- PBNA and L1-PBNA assays showed similar sensitivity and a high level of correlation using WHO standard sera (n = 2), and sera from patients vaccinated with Gardasil (n = 30) or an experimental human papillomavirus type 16 (HPV16) L1 VLP vaccine (n = 70). The detection of L1-specific cross-neutralizing antibodies in these sera using pseudovirions of types phylogenetically-related to those targeted by the L1 virus-like particle (VLP) vaccines was also consistent between the two assays. However, for sera from patients (n = 17) vaccinated with an L2-based immunogen (TA-CIN), the FC-PBNA was more sensitive than the L1-PBNA in detecting L2-specific neutralizing antibodies. Further, the neutralizing antibody titers measured with the FC-PBNA correlated with those determined with the L2-PBNA, another modification of the L1-PBNA that spacio-temporally separates primary and secondary receptor engagement, as well as the protective titers measured using passive transfer studies in the murine genital-challenge model. In sum, the FC-PBNA provided sensitive measurement for both L1 VLP and L2-specific neutralizing antibody in human sera. Vaccination with TA-CIN elicits weak cross-protective antibody in a subset of patients, suggesting the need for an adjuvant.

Immunization with a consensus epitope from human papillomavirus L2 induces antibodies that are broadly neutralizing

Vaccines targeting conserved epitopes in the HPV minor capsid protein, L2, can elicit antibodies that can protect against a broad spectrum of HPV types that are associated with cervical cancer and other HPV malignancies. Thus, L2 vaccines have been explored as alternatives to the current HPV vaccines, which are largely type-specific. In this study we assessed the immunogenicity of peptides spanning the N-terminal domain of L2 linked to the surface of a highly immunogenic bacteriophage virus-like particle (VLP) platform. Although all of the HPV16 L2 peptide-displaying VLPs elicited high-titer anti-peptide antibody responses, only a subset of the immunogens elicited antibody responses that were strongly protective from HPV16 pseudovirus (PsV) infection in a mouse genital challenge model. One of these peptides, mapping to HPV16 L2 amino acids 65-85, strongly neutralized HPV16 PsV but showed little ability to cross-neutralize other high-risk HPV types. In an attempt to broaden the protection generated through vaccination with this peptide, we immunized mice with VLPs displaying a peptide that represented a consensus sequence from high-risk and other HPV types. Vaccinated mice produced antibodies with broad, high-titer neutralizing activity against all of the HPV types that we tested. Therefore, immunization with virus-like particles displaying a consensus HPV sequence is an effective method to broaden neutralizing antibody responses against a type-specific epitope.

A three component mix of thioredoxin-L2 antigens elicits broadly neutralizing responses against oncogenic human papillomaviruses

Current human papillomavirus (HPV) vaccines based on major capsid protein L1 virus-like particles (VLP) provide potent type-specific protection against vaccine-type viruses (mainly HPV16 and 18), but cross-protect against only a small subset of the approximately 15 oncogenic HPV types. It is estimated that L1-VLP vaccines, which require a fairly complex production system and are still quite costly, fail to cover 20-30% of HPV cervical cancers worldwide, especially in low-resource countries. Alternative antigens relying on the N-terminal region of minor capsid protein L2 are intrinsically less immunogenic but capable of eliciting broadly neutralizing responses. We previously demonstrated the enhanced immunogenicity and cross-neutralization potential of an easily produced recombinant L2 antigen bearing the HPV16 L2(20-38) peptide epitope internally fused to bacterial thioredoxin (Trx). However, antibodies induced by Trx-HPV16 L2(20-38) failed to cross-neutralize notable high-risk HPV types such as HPV31. In the present work, the Trx-L2 design was modified to include L2 sequence information from the highly divergent HPV31 and HPV51 types in addition to HPV16, with the aim of extending cross-neutralization. Multivalent antigens comprising L2(20-38) peptides from all three HPV types on a single Trx scaffold molecule were compared to a mixture of the three type-specific monovalent Trx-L2 antigens. While multivalent antigens as well as the mixed antigens elicited similar anti-HPV16 neutralization titers, cross-reactive responses against HPV31 and HPV51 were of higher magnitude and more robust for the latter formulation. A mixture of monovalent Trx-L2 antigens thus represents a candidate lead for the development of a broadly cross-protective, low-cost second-generation anti-HPV vaccine.

A vaccine of L2 epitope repeats fused with a modified IgG1 Fc induced cross-neutralizing antibodies and protective immunity against divergent human papillomavirus types

Current human papillomavirus (HPV) major capsid protein L1 virus-like particles (VLPs)-based vaccines in clinic induce strong HPV type-specific neutralizing antibody responses. To develop pan-HPV vaccines, here, we show that the fusion protein E3R4 consisting of three repeats of HPV16 L2 aa 17–36 epitope (E3) and a modified human IgG1 Fc scaffold (R4) induces cross-neutralizing antibodies and protective immunity against divergent HPV types. E3R4 was expressed as a secreted protein in baculovirus expression system and could be simply purified by one step Protein A affinity chromatography with the purity above 90%. Vaccination of E3R4 formulated with Freunds adjuvant not only induced cross-neutralizing antibodies against HPV pseudovirus types 16, 18, 45, 52, 58, 6, 11 and 5 in mice, but also protected mice against vaginal challenges with HPV pseudovirus types 16, 45, 52, 58, 11 and 5 for at least eleven months after the first immunization. Moreover, vaccination of E3R4 formulated with FDA approved adjuvant alum plus monophosphoryl lipid A also induced cross-neutralizing antibodies against HPV types 16, 18 and 6 in rabbits. Thus, our results demonstrate that delivery of L2 antigen as a modified Fc-fusion protein may facilitate pan-HPV vaccine development.

A high-performance thioredoxin-based scaffold for peptide immunogen construction: proof-of-concept testing with a human papillomavirus epitope

Escherichia coli thioredoxin has been previously exploited as a scaffold for the presentation/stabilization of peptide aptamers as well as to confer immunogenicity to peptide epitopes. Here we focused on other key features of thioredoxin that are of general interest for the production of safer and more effective peptide immunogens, such as a high thermal stability, lack of cross-reactivity and a low-cost of production. We identified thioredoxin from the archaebacterium Pyrococcus furiosus (PfTrx) as a novel scaffold meeting all the above criteria. PfTrx is a highly thermostable and protease-resistant scaffold with a strong (poly)peptide solubilisation capacity. Anti-PfTrx antibodies did not cross-react with mouse, nor human thioredoxin. Untagged PfTrx bearing a previously identified HPV16-L2 peptide epitope was obtained in a >90% pure form with a one-step thermal purification procedure and effectively elicited the production of neutralizing anti-HPV antibodies. We thus propose PfTrx as a superior, general-purpose scaffold for the construction of safe, stable, and low-cost peptide immunogens.

Protective vaccination against papillomavirus-induced skin tumors under immunocompetent and immunosuppressive conditions: a preclinical study using a natural outbred animal model

Certain cutaneous human papillomaviruses (HPVs), which are ubiquitous and acquired early during childhood, can cause a variety of skin tumors and are likely involved in the development of non-melanoma skin cancer, especially in immunosuppressed patients. Hence, the burden of these clinical manifestations demands for a prophylactic approach. To evaluate whether protective efficacy of a vaccine is potentially translatable to patients, we used the rodent Mastomys coucha that is naturally infected with Mastomys natalensis papillomavirus (MnPV). This skin type papillomavirus induces not only benign skin tumours, such as papillomas and keratoacanthomas, but also squamous cell carcinomas, thereby allowing a straightforward read-out for successful vaccination in a small immunocompetent laboratory animal. Here, we examined the efficacy of a virus-like particle (VLP)-based vaccine on either previously or newly established infections. VLPs raise a strong and long-lasting neutralizing antibody response that confers protection even under systemic long-term cyclosporine A treatment. Remarkably, the vaccine completely prevents the appearance of benign as well as malignant skin tumors. Protection involves the maintenance of a low viral load in the skin by an antibody-dependent prevention of virus spread. Our results provide first evidence that VLPs elicit an effective immune response in the skin under immunocompetent and immunosuppressed conditions in an outbred animal model, irrespective of the infection status at the time of vaccination. These findings provide the basis for the clinical development of potent vaccination strategies against cutaneous HPV infections and HPV-induced tumors, especially in patients awaiting organ transplantation.

Organ transplant recipients (OTR) frequently suffer from fulminant warts that are induced by cutaneous human papillomaviruses (HPV). Moreover, some skin HPV types may also be involved in the development of non-melanoma skin cancer. Mimicking the situation of immunosuppressed OTR who acquire cutaneous HPV infections already in childhood, we explored the efficacy of a vaccine in infected animals that additionally underwent immunosuppression. We demonstrate for the first time the success of a vaccine against a skin papillomavirus in a natural outbred animal system, which completely prevents both benign and malignant skin tumor formation even under immunosuppressed conditions. Hence, our study provides the basis for clinical development of a vaccine against cutaneous HPV infections, which may be particularly useful in transplant recipients.

High-throughput pseudovirion-based neutralization assay for analysis of natural and vaccine-induced antibodies against human papillomaviruses

A highly sensitive, automated, purely add-on, high-throughput pseudovirion-based neutralization assay (HT-PBNA) with excellent repeatability and run-to-run reproducibility was developed for human papillomavirus types (HPV) 16, 18, 31, 45, 52, 58 and bovine papillomavirus type 1. Preparation of 384 well assay plates with serially diluted sera and the actual cell-based assay are separated in time, therefore batches of up to one hundred assay plates can be processed sequentially. A mean coefficient of variation (CV) of 13% was obtained for anti-HPV 16 and HPV 18 titers for a standard serum tested in a total of 58 repeats on individual plates in seven independent runs. Natural antibody response was analyzed in 35 sera from patients with HPV 16 DNA positive cervical intraepithelial neoplasia grade 2+ lesions. The new HT-PBNA is based on Gaussia luciferase with increased sensitivity compared to the previously described manual PBNA (manPBNA) based on secreted alkaline phosphatase as reporter. Titers obtained with HT-PBNA were generally higher than titers obtained with the manPBNA. A good linear correlation (R² = 0.7) was found between HT-PBNA titers and anti-HPV 16 L1 antibody-levels determined by a Luminex bead-based GST-capture assay for these 35 sera and a Kappa-value of 0.72, with only 3 discordant sera in the low titer range. In addition to natural low titer antibody responses the high sensitivity of the HT-PBNA also allows detection of cross-neutralizing antibodies induced by commercial HPV L1-vaccines and experimental L2-vaccines. When analyzing the WHO international standards for HPV 16 and 18 we determined an analytical sensitivity of 0.864 and 1.105 mIU, respectively.

Virus-like particles for the prevention of human papillomavirus-associated malignancies

As compared with peptide- or protein-based vaccines, naked DNA vectors and even traditional attenuated or inactivated virus vaccines, virus-like particles (VLPs) are an attractive vaccine platform, as they offer a combination of safety, ease of production and both high-density B-cell epitope display and intracellular presentation of T-cell epitopes that induce potent humoral and cellular immune responses, respectively. Indeed, HPV vaccines based on VLP production by recombinant expression of major capsid antigen L1 in yeast (Gardasil(®), Merck & Co., NJ, USA) or insect cells (Cervarix(®), GlaxoSmithKline, London, UK) have been licensed for the prevention of cervical and anogenital infection and disease associated with the genotypes targeted by each vaccine. However, these HPV vaccines have not been demonstrated as effective to treat existing infections, and efforts to develop a therapeutic HPV vaccine continue. Furthermore, current HPV L1-VLP vaccines provide type-restricted protection, requiring highly multivalent formulations to broaden coverage to the dozen or more oncogenic HPV genotypes. This raises the complexity and cost of vaccine production. The lack of access to screening and high disease burden in developing countries has spurred efforts to develop second-generation HPV vaccines that are more affordable, induce wider protective coverage and offer therapeutic coverage against HPV-associated malignancies. Given the previous success with L1-VLP-based vaccines against HPV, VLPs have been also adopted as platforms for many second-generation HPV and non-HPV vaccine candidates with both prophylactic and therapeutic intent. In this article, the authors examine the progress and challenges of these efforts, with a focus on how they inform VLP vaccine design.

Mutations in human papillomavirus type 16 L1 hypervariable surface-exposed loops affect L2 binding and DNA encapsidation

Prophylactic vaccines against human papillomavirus (HPV) based on virus-like particles (VLP) induce type-specific neutralizing antibodies against a small number of hypervariable residues positioned in surface-exposed loops of the major capsid protein L1. To investigate the importance of these residues for neutralization, cross-neutralization, L2 incorporation and genome encapsidation, ten surface-exposed amino acid residues in four hypervariable loops of L1 were mutated. VLPs containing mutated or WT L1, with or without WT L2, were produced in 293TT cells using pseudovirion expression vectors. The mutations reduced the ability to induce neutralizing antibodies and to incorporate the L2 protein in the capsid. Ability to induce cross-neutralizing antibodies and to encapsidate pseudogenomes were completely abrogated. In summary, the surface-exposed L1 loops are important for the function of the HPV particle.

L2, the minor capsid protein of papillomavirus

The capsid protein L2 plays major roles in both papillomavirus assembly and the infectious process. While L1 forms the majority of the capsid and can self-assemble into empty virus-like particles (VLPs), L2 is a minor capsid component and lacks the capacity to form VLPs. However, L2 co-assembles with L1 into VLPs, enhancing their assembly. L2 also facilitates encapsidation of the ∼8 kbp circular and nucleosome-bound viral genome during assembly of the non-enveloped T=7d virions in the nucleus of terminally differentiated epithelial cells, although, like L1, L2 is not detectably expressed in infected basal cells. With respect to infection, L2 is not required for particles to bind to and enter cells. However L2 must be cleaved by furin for endosome escape. L2 then travels with the viral genome to the nucleus, wherein it accumulates at ND-10 domains. Here, we provide an overview of the biology of L2.

Influence of oxidation and multimerization on the immunogenicity of a thioredoxin-l2 prophylactic papillomavirus vaccine

Current commercial prophylactic human papillomavirus (HPV) vaccines are based on virus-like particles assembled from the major capsid protein L1 and show excellent safety and efficacy profiles. Still, a major limitation is their rather narrow range of protection against different HPV types. In contrast, the minor capsid protein L2 contains a so-called major cross-neutralizing epitope that can induce broad-range protective responses against multiple HPV types. This epitope is conserved among different papillomaviruses (PV) and contains two cysteine residues that are present in the L2 proteins of all known PV types. The main challenge in developing L2-directed vaccines is to overcome the intrinsically low immunogenicity of the L2 protein. Previously, we developed a recombinant L2-based prototype vaccine by inserting peptide epitopes spanning the cross-neutralizing L2 sequence into a bacterial thioredoxin (Trx) scaffold. These antigens induced high-titer neutralizing antibodies in mice. Here, we address the question of whether Trx scaffold multimerization may further enhance the immunogenicity of the TrxL2 vaccine. We also demonstrate that the oxidation state of the conserved cysteine residues is not essential for vaccine functionality, but it contributes to immunogenicity.

Phylogenetic considerations in designing a broadly protective multimeric L2 vaccine

While the oncogenic human papillomavirus (HPV) types with the greatest medical impact are clustered within the α9 and α7 species, a significant fraction of cervical cancers are caused by α5, α6, and α11 viruses. Benign genital warts are caused principally by the α10 viruses HPV6 and HPV11. In an effort to achieve broad protection against both cervical cancer- and genital wart-associated types, we produced at high levels in bacteria a multimeric protein (α11-88x8) fusing eight polypeptides corresponding to a protective domain comprising L2 residues ∼11 to 88 derived from HPV6 (α10), HPV16 (α9), HPV18 (α7), HPV31 (α9), HPV39 (α7), HPV51 (α5), HPV56 (α6), and HPV73 (α11) and a truncated derivative with the last three units deleted (α11-88x5). Mice were immunized three times with α11-88x8 or α11-88x5 adjuvanted with alum or the licensed HPV vaccines and challenged intravaginally with HPV6, HPV16, HPV26, HPV31, HPV33, HPV35, HPV45, HPV51, HPV56, HPV58, or HPV59 pseudovirions. The α11-88x5 and α11-88x8 vaccines induced similarly robust protection against each HPV type tested and indistinguishable HPV16-neutralizing antibody titers. Passive transfer of α11-88x8 antisera was protective. Further, rabbit antisera to α11-88x8 and α11-88x5 similarly neutralized native HPV18 virions. These findings suggest that immunologic competition between units is not a significant issue and that it is not necessary to include a unit of L2 derived from each species to achieve broader protection against diverse medically significant HPV types than is achieved with the licensed HPV vaccines.

Optimization of multimeric human papillomavirus L2 vaccines

We sought to define the protective epitopes within the amino terminus of human papillomavirus (HPV) type 16 minor capsid protein L2. Passive transfer of mice with rabbit antisera to HPV16 L2 peptides 17–36, 32–51 and 65–81 provided significant protection against vaginal HPV16 challenge, whereas antisera to 47–66, 108–120 or 373–392 did not. Vaccination with L1 virus-like particles induces a high titer, but generally type-restricted neutralizing antibody response. Conversely, vaccination with L2 11–88, especially multimers thereof, induces antibodies that neutralize a broad range of papillomavirus types, albeit at lower titers than for L1 VLP. With the intent of enhancing the immunogenicity and the breadth of protection by focusing the immune response to the key protective epitopes, we designed L2 fusion proteins consisting of residues ∼11–88 of eight divergent mucosal HPV types 6, 16, 18, 31, 39, 51, 56, 73 (11–88×8) or residues ∼13–47 of fifteen HPV types (13–47×15). The 11–88×8 was significantly more immunogenic than 13–47×15 in Balb/c mice regardless of the adjuvant used, suggesting the value of including the 65–81 protective epitope in the vaccine. Since the L2 47–66 peptide antiserum failed to elicit significant protection, we generated an 11–88×8 construct deleted for this region in each subunit (11–88×8Δ). Mice were vaccinated with 11–88×8 and 11–88×8Δ to determine if deletion of this non-protective epitope enhanced the neutralizing antibody response. However, 11–88×8Δ was significantly less immunogenic than 11–88×8, and even the addition of a known T helper epitope, PADRE, to the construct (11–88×8ΔPADRE) failed to recover the immunogenicity of 11–88×8 in C57BL/6 mice, suggesting that while L2 47–66 is not a critical protective or T helper epitope, it nevertheless contributes to the immunogenicity of the L2 11–88×8 multimer vaccine.

Monoclonal antibodies recognizing cross-neutralization epitopes in human papillomavirus 16 minor capsid protein L2

Antisera induced by immunization of rabbits with the synthetic peptide P56/75, which has the amino acid (aa) sequence from aa56 to aa75 of HPV16 L2, neutralize pseudovirions and raft-virions of multiple high-risk HPV types, indicating that cross-neutralization epitopes are present in the aa56-75 region. We generated two mouse monoclonal antibodies (MAb): MAb13B and MAb24B recognizing the regions of aa64-73 and aa58-64, respectively. The neutralization assay using pseudovirions of HPV16, 18, 31, 33, 35, 51, 52 and 58 showed that MAb13B neutralized HPV16, 18, and 51, and MAb24B neutralized all the types tested. The mixture of MAb13B and MAb24B neutralized HPV16, 18, and 51 pseudovirions more efficiently than each of the MAbs alone. The data indicate that there are at least two cross-neutralization epitopes in the aa56-75 region and that an antigen capable of presenting the two cross-neutralization epitopes would be a good vaccine candidate for a broad-spectrum of HPVs.

Natural variants in the major neutralizing epitope of human papillomavirus minor capsid protein L2

The amino terminus of the human papillomavirus minor capsid protein L2 contains a major cross-neutralizing epitope that provides the basis for the development of a broadly protective HPV vaccine. This attainable broad protection would eliminate one of the major drawbacks of the commercial L1-based prophylactic vaccines. In this study, we asked whether there are natural variants of the L2 cross-neutralizing epitope and if these variants provide means for immune escape from vaccine-induced anti-L2 antibodies. For this, we isolated in silico and in vitro, a total of 477 L2 sequences of HPV types 16, 18, 31, 45, 51, 52 and 58. We identified natural L2 epitope variants for HPV 18, 31, 45 and 51. To determine whether these variants escape L2-directed neutralization, we generated pseudovirions encompassing the natural variants and tested these in an in vitro neutralization assay using monoclonal and polyclonal antibodies. Our results indicate that natural variants of the L2 major neutralizing epitope are frequent among two different study populations from Germany and Mongolia and in the GenBank database. Of two identified HPV 31 L2 single amino acid variants, one could be neutralized well, while the other variant was neutralized very poorly. We also observed that single amino acid variants of HPV 18 and 45 are neutralized well while a HPV 18 double variant was neutralized at significantly lower rates, indicating that L2 variants have to be accounted for when developing HPV L2-based prophylactic vaccines.

Development of AAVLP(HPV16/31L2) particles as broadly protective HPV vaccine candidate

The human papillomavirus (HPV) minor capsid protein L2 is a promising candidate for a broadly protective HPV vaccine yet the titers obtained in most experimental systems are rather low. Here we examine the potential of empty AAV2 particles (AAVLPs), assembled from VP3 alone, for display of L2 epitopes to enhance their immunogenicity. Insertion of a neutralizing epitope (amino acids 17–36) from L2 of HPV16 and HPV31 into VP3 at positions 587 and 453, respectively, permitted assembly into empty AAV particles (AAVLP(HPV16/31L2)). Intramuscularly vaccination of mice and rabbits with AAVLP(HPV16/31L2)s in montanide adjuvant, induced high titers of HPV16 L2 antibodies as measured by ELISA. Sera obtained from animals vaccinated with the AAVLP(HPV16/31L2)s neutralized infections with several HPV types in a pseudovirion infection assay. Lyophilized AAVLP(HPV16/31L2) particles retained their immunogenicity upon reconstitution. Interestingly, vaccination of animals that were pre-immunized with AAV2 - simulating the high prevalence of AAV2 antibodies in the population - even increased cross neutralization against HPV31, 45 and 58 types. Finally, passive transfer of rabbit antisera directed against AAVLP(HPV16/31L2)s protected naïve mice from vaginal challenge with HPV16 pseudovirions. In conclusion, AAVLP(HPV16/31L2) particles have the potential as a broadly protective vaccine candidate regardless of prior exposure to AAV.

Capsomer vaccines protect mice from vaginal challenge with human papillomavirus

Capsomers were produced in bacteria as glutathione-S-transferase (GST) fusion proteins with human papillomavirus type 16 L1 lacking the first nine and final 29 residues (GST-HPV16L1Δ) alone or linked with residues 13-47 of HPV18, HPV31 and HPV45 L2 in tandem (GST-HPV16L1Δ-L2x3). Subcutaneous immunization of mice with GST-HPV16L1Δ or GST-HPV16L1Δ-L2x3 in alum and monophosphoryl lipid A induced similarly high titers of HPV16 neutralizing antibodies. GST-HPV16L1Δ-L2x3 also elicited moderate L2-specific antibody titers. Intravaginal challenge studies showed that immunization of mice with GST-HPV16 L1Δ or GST-HPV16L1Δ-L2x3 capsomers, like Cervarix®, provided complete protection against HPV16. Conversely, vaccination with GST-HPV16 L1Δ capsomers failed to protect against HPV18 challenge, whereas mice immunized with either GST-HPV16L1Δ-L2x3 capsomers or Cervarix® were each completely protected. Thus, while the L2-specific response was moderate, it did not interfere with immunity to L1 in the context of GST-HPV16L1Δ-L2x3 and is sufficient to mediate L2-dependent protection against an experimental vaginal challenge with HPV18.