Phylogenetic considerations in designing a broadly protective multimeric L2 vaccine

Vaccination with a Human Papillomavirus L2 Multimer Provides Broad Protection against 17 Human Papillomavirus Types in the Mouse Cervicovaginal Challenge Model

Human papillomavirus (HPV) is a prevalent cause of mucosal and cutaneous infections and underlying conditions ranging from benign warts to anogenital and oropharyngeal cancers affecting both males and females, notably cervical cancer. Cervical cancer is the fourth leading cause of cancer deaths among women globally and is the most impactful in low- and middle-income countries (LMICs), where the costs of screening and licensed L1-based HPV vaccines pose significant barriers to comprehensive administration. Additionally, the licensed L1-based HPV vaccines fail to protect against all oncogenic HPV types. This study generated three independent lots of an L2-based target antigen (LBTA), which was engineered from conserved linear L2-protective epitopes (aa11-88) from five human alphapapillomavirus genotypes in E. coli under cGMP conditions and adjuvanted with aluminum phosphate. Vaccination of rabbits with LBTA generated high neutralizing antibody titers against all 17 HPV types tested, surpassing the nine types covered by Gardasil®9. Passive transfer of naïve mice with LBTA antiserum revealed its capacity to confer protection against vaginal challenge with all 17 αHPV types tested. LBTA shows stability at room temperature over >1 month. Standard in vitro and in vivo toxicology studies suggest a promising safety profile. These findings suggest LBTA's promise as a next-generation vaccine with comprehensive coverage aimed at reducing the economic and healthcare burden of cervical and other HPV+ cancers in LMICs, and it has received regulatory approval for a first-in-human clinical study (NCT05672966).

Vaccination with human alphapapillomavirus-derived L2 multimer protects against human betapapillomavirus challenge, including in epidermodysplasia verruciformis model mice

Human alphapapillomaviruses (αHPV) infect genital mucosa, and a high-risk subset is a necessary cause of cervical cancer. Licensed L1 virus-like particle (VLP) vaccines offer immunity against the nine most common αHPV associated with cervical cancer and genital warts. However, vaccination with an αHPV L2-based multimer vaccine, α11-88x5, protected mice and rabbits from vaginal and skin challenge with diverse αHPV types. While generally clinically inapparent, human betapapillomaviruses (βHPV) are possibly associated with cutaneous squamous cell carcinoma (CSCC) in epidermodysplasia verruciformis (EV) and immunocompromised patients. Here we show that α11-88x5 vaccination protected wild type and EV model mice against HPV5 challenge. Passive transfer of antiserum conferred protection independently of Fc receptors (FcR) or Gr-1+ phagocytes. Antisera demonstrated robust antibody titers against ten βHPV by L1/L2 VLP ELISA and neutralized and protected against challenge by 3 additional βHPV (HPV49/76/96). Thus, unlike the licensed vaccines, α11-88x5 vaccination elicits broad immunity against αHPV and βHPV.

Improvement of RG1-VLP vaccine performance in BALB/c mice by substitution of alhydrogel with the next generation polyphosphazene adjuvant PCEP

Current human papillomavirus (HPV) vaccines provide substantial protection against the most common HPV types responsible for oral and anogenital cancers, but many circulating cancer-causing types remain for which vaccine coverage is lacking. In addition, all current HPV vaccines rely on aluminum salt-based adjuvant formulations that function through unclear mechanisms with few substitutes available. In an effort to expand the toolbox of available adjuvants suitable for HPV vaccines, we compared the immunogenicity of the RG1-VLP (virus-like particle) vaccine in BALB/c mice when formulated with either the aluminum hydroxide adjuvant Alhydrogel or the novel polyphosphazene macromolecular adjuvant poly[di (carboxylatoethylphenoxy) phosphazene] (PCEP). PCEP-formulated RG1-VLPs routinely outperformed VLP/Alhydrogel in several measurements of VLP-specific humoral immunity, including consistent improvements in the magnitude of antibody (Ab) responses to both HPV16-L1 and the L2 RG1 epitope as well as neutralizing titers to HPV16 and cross-neutralization of pseudovirion (PsV) types HPV18 and HPV39. Dose-sparing studies indicated that RG1-VLPs could be reduced in dose by 75% and the presence of PCEP ensured activity comparable to a full VLP dose adjuvanted by Alhydrogel. In addition, levels of HPV16-L1 and -L2-specific Abs were achieved after two vaccinations with PCEP as adjuvant that were equivalent to or greater than levels achieved with three vaccinations with Alhydrogel alone, indicating that the presence of PCEP resulted in accelerated immune responses that could allow for a decreased dose schedule. Given the extensive clinical track record of polyphosphazenes, these data suggest that substitution of alum-based adjuvants with PCEP for the RG1-VLP vaccine could lead to rapid seropositivity requiring fewer boosts, the dose-sparing of commercial VLP-based vaccines, and the establishment of longer-lasting humoral responses to HPV.

RG1-VLP and Other L2-Based, Broad-Spectrum HPV Vaccine Candidates

Licensed human papillomavirus (HPV) vaccines contain virus-like particles (VLPs) self-assembled from L1 major-capsid proteins that are remarkably effective prophylactic immunogens. However, the induced type-restricted immune response limits coverage to the included vaccine types, and costly multiplex formulations, restrictive storage and distribution conditions drive the need for next generation HPV vaccines. Vaccine candidates based upon the minor structural protein L2 are particularly promising because conserved N-terminal epitopes induce broadly cross-type neutralizing and protective antibodies. Several strategies to increase the immunological potency of such epitopes are being investigated, including concatemeric multimers, fusion to toll-like receptors ligands or T cell epitopes, as well as immunodominant presentation by different nanoparticle or VLP structures. Several promising L2-based vaccine candidates have reached or will soon enter first-in-man clinical studies. RG1-VLP present the HPV16L2 amino-acid 17–36 conserved neutralization epitope “RG1” repetitively and closely spaced on an immunodominant surface loop of HPV16 L1-VLP and small animal immunizations provide cross-protection against challenge with all medically-significant high-risk and several low-risk HPV types. With a successful current good manufacturing practice (cGMP) campaign and this promising breadth of activity, even encompassing cross-neutralization of several cutaneous HPV types, RG1-VLP are ready for a first-in-human clinical study. This review aims to provide a general overview of these candidates with a special focus on the RG1-VLP vaccine and its road to the clinic.

Optimization of RG1-VLP vaccine performance in mice with novel TLR4 agonists

Current human papilloma virus (HPV) vaccines provide substantial protection against the most common HPV types responsible for oral and anogenital cancers, but many circulating cancer-causing types remain that lack vaccine coverage. The novel RG1-VLP (virus-like particle) vaccine candidate utilizes the HPV16-L1 subunit as a backbone to display an inserted HPV16-L2 17-36 a.a. "RG1" epitope; the L2 RG1 epitope is conserved across many HPV types and the generation of cross-neutralizing antibodies (Abs) against which has been demonstrated. In an effort to heighten the immunogenicity of the RG1-VLP vaccine, we compared in BALB/c mice adjuvant formulations consisting of novel bacterial enzymatic combinatorial chemistry (BECC)-derived toll-like receptor 4 (TLR4) agonists and the aluminum hydroxide adjuvant Alhydrogel. In the presence of BECC molecules, consistent improvements in the magnitude of Ab responses to both HPV16-L1 and the L2 RG1 epitope were observed compared to Alhydrogel alone. Furthermore, neutralizing titers to HPV16 as well as cross-neutralization of pseudovirion (PsV) types HPV18 and HPV39 were augmented in the presence of BECC agonists as well. Levels of L1 and L2-specific Abs were achieved after two vaccinations with BECC/Alhydrogel adjuvant that were equivalent to or greater than levels achieved with 3 vaccinations with Alhydrogel alone, indicating that the presence of BECC molecules resulted in accelerated immune responses that could allow for a decreased dose schedule for VLP-based HPV vaccines. In addition, dose-sparing studies indicated that adjuvantation with BECC/Alhydrogel allowed for a 75% reduction in antigen dose while still retaining equivalent magnitudes of responses to the full VLP dose with Alhydrogel. These data suggest that adjuvant optimization of HPV VLP-based vaccines can lead to rapid immunity requiring fewer boosts, dose-sparing of VLPs expensive to produce, and the establishment of a longer-lasting humoral immunity.

Progress in L2-Based Prophylactic Vaccine Development for Protection against Diverse Human Papillomavirus Genotypes and Associated Diseases

The human papillomaviruses (HPVs) are a family of small DNA tumor viruses including over 200 genotypes classified by phylogeny into several genera. Different genera of HPVs cause ano-genital and oropharyngeal cancers, skin cancers, as well as benign diseases including skin and genital warts. Licensed vaccines composed of L1 virus-like particles (VLPs) confer protection generally restricted to the ≤9 HPV types targeted. Here, we examine approaches aimed at broadening the protection against diverse HPV types by targeting conserved epitopes of the minor capsid protein, L2. Compared to L1 VLP, L2 is less immunogenic. However, with appropriate presentation to the immune system, L2 can elicit durable, broadly cross-neutralizing antibody responses and protection against skin and genital challenge with diverse HPV types. Such approaches to enhance the strength and breadth of the humoral response include the display of L2 peptides on VLPs or viral capsids, bacteria, thioredoxin and other platforms for multimerization. Neither L2 nor L1 vaccinations elicit a therapeutic response. However, fusion of L2 with early viral antigens has the potential to elicit both prophylactic and therapeutic immunity. This review of cross-protective HPV vaccines based on L2 is timely as several candidates have recently entered early-phase clinical trials.

Efficient Inhibition of Human Papillomavirus Infection by L2 Minor Capsid-Derived Lipopeptide

The amino (N)-terminal region of human papillomavirus (HPV) minor capsid protein (L2) is a highly conserved region which is essential for establishing viral infection. Despite its importance in viral infectivity, the role of the HPV N-terminal domain has yet to be fully characterized. Using fine mapping analysis, we identified a 36-amino-acid (aa) peptide sequence of the L2 N terminus, termed L2N, that is critical for HPV infection. Ectopic expression of L2N with the transmembrane sequence on the target cell surface conferred resistance to HPV infection. Additionally, L2N peptide with chemical or enzymatic lipidation at the carboxyl (C) terminus efficiently abrogated HPV infection in target cells. Among the synthetic L2N lipopeptides, a stearoylated lipopeptide spanning aa 13 to 46 (13-46st) exhibited the most potent anti-HPV activity, with a half-maximal inhibitory concentration (IC50) of ∼200 pM. Furthermore, we demonstrated that the 13-46st lipopeptide inhibited HPV entry by blocking trans-Golgi network retrograde trafficking of virion particles, leading to rapid degradation. Fundamentally, the inhibitory effect of L2N lipopeptides appeared to be evolutionarily conserved, as they showed cross-type inhibition among various papillomaviruses. In conclusion, our findings provide new insights into the critical role of the L2N sequence in the HPV entry mechanism and identify the therapeutic potential of L2N lipopeptide as an effective anti-HPV agent.IMPORTANCE HPV is a human oncogenic virus that causes a major public health problem worldwide, which is responsible for approximately 5% of total human cancers and almost all cases of cervical cancers. HPV capsid consists of two structure proteins, the major capsid L1 protein and the minor capsid L2 protein. While L2 plays critical roles during the viral life cycle, the molecular mechanism in viral entry remains elusive. Here, we performed fine mapping of the L2 N-terminal region and defined a short 36-amino-acid peptide, called L2N, which is critical for HPV infection. Specifically, L2N peptide with carboxyl-terminal lipidation acted as a potent and cross-type HPV inhibitor. Taken together, data from our study highlight the essential role of the L2N sequence at the early step of HPV entry and suggests the L2N lipopeptide as a new strategy to broadly prevent HPV infection.

Roles of Fc Domain and Exudation in L2 Antibody-Mediated Protection against Human Papillomavirus

To address how L2-specific antibodies prevent human papillomavirus (HPV) infection of the genital tract, we generated neutralizing monoclonal antibodies (MAbs) WW1, a rat IgG2a that binds L2 residues 17 to 36 (like mouse MAb RG1), and JWW3, a mouse IgG2b derivative of Mab24 specific for L2 residues 58 to 64. By Western blotting, WW1 recognized L2 of 29/34 HPV genotypes tested, compared to only 13/34 for RG1 and 25/34 for JWW3. WW1 IgG and F(ab')₂ bound HPV16 pseudovirions similarly; however, whole IgG provided better protection against HPV vaginal challenge. Passive transfer of WW1 IgG was similarly protective in wild-type and neonatal Fc receptor (FcRn)-deficient mice, suggesting that protection by WW1 IgG is not mediated by FcRn-dependent transcytosis. Rather, local epithelial disruption, required for genital infection and induced by either brushing or nonoxynol-9 treatment, released serum IgG in the genital tract, suggesting Fc-independent exudation. Depletion of neutrophils and macrophages reduced protection of mice upon passive transfer of whole WW1 or JWW3 IgGs. Similarly, IgG-mediated protection by L2 MAbs WW1, JWW3, and RG1 was reduced in Fc receptor knockout compared to wild-type mice. However, levels of in vitro neutralization by WW1 IgG were similar in TRIM21 knockout and wild-type cells, indicating that Fc does not contribute to antibody-dependent intracellular neutralization (ADIN). In conclusion, the Fc domain of L2-specific IgGs is not active for ADIN, but it opsonizes bound extracellular pseudovirions for phagocytes in protecting mice from intravaginal HPV challenge. Systemically administered neutralizing IgG can access the site of infection in an abrasion via exudation without the need for FcRn-mediated transcytosis.IMPORTANCE At least 15 alpha HPV types are causative agents for 5% of all cancers worldwide, and beta types have been implicated in nonmelanoma skin cancer, whereas others produce benign papillomas, such as genital warts, associated with considerable morbidity and health systems costs. Vaccines targeting the minor capsid protein L2 have the potential to provide broad-spectrum immunity against medically relevant HPVs of divergent genera via the induction of broadly cross-neutralizing serum IgG. Here we examine the mechanisms by which L2-specific serum IgG reaches the viral inoculum in the genital tract to effect protection. Abrasion of the vaginal epithelium allows the virus to access and infect basal keratinocytes, and our findings suggest that this also permits the local exudation of neutralizing IgG and vaccine-induced sterilizing immunity. We also demonstrate the importance of Fc-mediated phagocytosis of L2 antibody-virion complexes for humoral immunity, a protective mechanism that is not detected by current in vitro neutralization assays.

Opportunities and challenges for human papillomavirus vaccination in cancer

The discovery of genotype 16 as the prototype oncogenic human papillomavirus (HPV) initiated a quarter century of laboratory and epidemiological studies that demonstrated their necessary, but not sufficient, aetiological role in cervical and several other anogenital and oropharyngeal cancers. Early virus-induced immune deviation can lead to persistent subclinical infection that brings the risk of progression to cancer. Effective secondary prevention of cervical cancer through cytological and/or HPV screening depends on regular and widespread use in the general population, but coverage is inadequate in low-resource settings. The discovery that the major capsid antigen L1 could self-assemble into empty virus-like particles (VLPs) that are both highly immunogenic and protective led to the licensure of several prophylactic VLP-based HPV vaccines for the prevention of cervical cancer. The implementation of vaccination programmes in adolescent females is underway in many countries, but their impact critically depends on the population coverage and is improved by herd immunity. This Review considers how our expanding knowledge of the virology and immunology of HPV infection can be exploited to improve vaccine technologies and delivery of such preventive strategies to maximize reductions in HPV-associated disease, including incorporation of an HPV vaccine covering oncogenic types within a standard multitarget paediatric vaccine.

Broadly neutralizing antiviral responses induced by a single-molecule HPV vaccine based on thermostable thioredoxin-L2 multiepitope nanoparticles

Vaccines targeting the human papillomavirus (HPV) minor capsid protein L2 are emerging as chemico-physically robust and broadly protective alternatives to the current HPV (L1-VLP) vaccines. We have previously developed a trivalent L2 vaccine prototype exploiting Pyrococcus furiosus thioredoxin (PfTrx) as a thermostable scaffold for the separate presentation of three distinct HPV L2(20–38) epitopes. With the aim of achieving a highly immunogenic, yet simpler and more GMP-production affordable formulation, we report here on a novel thermostable nanoparticle vaccine relying on genetic fusion of PfTrx-L2 with the heptamerizing coiled-coil polypeptide OVX313. A prototype HPV16 monoepitope version of this nanoparticle vaccine (PfTrx-L2-OVX313; median radius: 8.6 ± 1.0 nm) proved to be approximately 10-fold more immunogenic and with a strikingly enhanced cross-neutralization capacity compared to its monomeric counterpart. Vaccine-induced (cross-)neutralizing responses were further potentiated in a multiepitope derivative displaying eight different L2(20–38) epitopes, which elicited neutralizing antibodies against 10 different HPVs including three viral types not represented in the vaccine. Considering the prospective safety of the PfTrx scaffold and of the OVX313 heptamerization module, PfTrx-OVX313 nanoparticles lend themselves as robust L2-based immunogens with a high translational potential as a 3^rd generation HPV vaccine, but also as a novel and extremely versatile peptide-antigen presentation platform.

Incorporation of RG1 epitope concatemers into a self-adjuvanting Flagellin-L2 vaccine broaden durable protection against cutaneous challenge with diverse human papillomavirus genotypes

AIM

To achieve durable and broad protection against human papillomaviruses by vaccination with multimers of minor capsid antigen L2 using self-adjuvanting fusions with the toll-like receptor-5 (TLR5) ligand bacterial flagellin (Fla) instead of co-formulation with alum.

METHODS

Fla fusions with L2 protective epitopes comprising residues 11-200, 11-88 and/or 17-38 of a single or multiple HPV types were produced in E. coli and their capacity to activate TLR5 signaling was assessed. Immunogenicity was evaluated serially following administration of 3 intramuscular doses of Fla-L2 multimer without exogenous adjuvant, followed by challenge 1, 3, 6 or 12months later, and efficacy compared to vaccination with human doses of L1 VLP vaccines (Gardasil and Cervarix) or L2 multimer formulated in alum. Serum antibody responses were assessed by peptide ELISA, in vitro neutralization assays and passive transfer to naïve rabbits in which End-Point Protection Titers (EPPT) were determined using serial dilutions of pooled immune sera collected 1, 3, 6 or 12months after completing active immunization. Efficacy was assessed by determining wart volume following concurrent challenge at different sites with HPV6/16/18/31/45/58 'quasivirions' containing cottontail rabbit papillomavirus (CRPV) genomes.

RESULTS

Vaccination in the absence of exogenous adjuvant with Fla-HPV16 L2 11-200 fusion protein elicited durable protection against HPV16, but limited cross-protection against other HPV types. Peptide mapping data suggested the importance of the 17-38 aa region in conferring immunity. Indeed, addition of L2 residues 17-38 of HPV6/18/31/39/52 to a Fla-HPV16 L2 11-200 or 11-88 elicited broader protection via active or passive immunization, similar to that seen with vaccination with an alum-adjuvanted L2 multimer comprising the aa 11-88 peptides of five or eight genital HPV types.

CONCLUSIONS

Vaccination with flagellin fused L2 multimers provided lasting (>1year) immunity without the need for an exogenous adjuvant. Inclusion of the L2 amino acid 17-38 region in such multi-HPV type fusions expanded the spectrum of protection.

Human papillomavirus 16L1-58L2 chimeric virus-like particles elicit durable neutralizing antibody responses against a broad-spectrum of human papillomavirus types

The neutralizing antibodies elicited by human papillomavirus (HPV) major capsid protein L1 virus-like particle (VLP)-based vaccines are largely type-specific. An HPV vaccine inducing cross-neutralizing antibodies broadly will be cost-effective and of great value. To this end, we constructed HPV16L1-58L2 chimeric VLP (cVLP) by displaying HPV58 L2 aa.16-37 on the DE surface region of HPV16 L1. We found that vaccination with the HPV16L1-58L2 cVLP formulated with alum plus monophosphoryl lipid A (Alum-MPL) adjuvant elicited robust neutralizing antibodies in both mice and rabbits against all tested HPV types including HPV16/31/33/35/52/58 (genus α9), HPV18/39/45/59/68 (genus α7), HPV6/11 (genus α10), HPV2/27/57 (genus α4), and HPV5 (genus β1). Importantly, the cross-neutralizing antibody response was maintained at least 82 weeks in mice or 42 weeks in rabbits, and complete protection against HPV58 was observed at week 85 in mice. Our data demonstrate that HPV16L1-58L2 cVLP is an excellent pan-HPV vaccine candidate.

Developments in L2-based human papillomavirus (HPV) vaccines

Infections with sexually transmitted high-risk Human Papillomavirus (hrHPV), of which there are at least 15 genotypes, are responsible for a tremendous disease burden by causing cervical, and subsets of other ano-genital and oro-pharyngeal carcinomas, together representing 5% of all cancer cases worldwide. HPV subunit vaccines consisting of virus-like particles (VLP) self-assembled from major capsid protein L1 plus adjuvant have been licensed. Prophylactic vaccinations with the 2-valent (HPV16/18), 4-valent (HPV6/11/16/18), or 9-valent (HPV6/11/16/18/31/33/45/52/58) vaccine induce high-titer neutralizing antibodies restricted to the vaccine types that cause up to 90% of cervical carcinomas, a subset of other ano-genital and oro-pharyngeal cancers and 90% of benign ano-genital warts (condylomata). The complexity of manufacturing multivalent L1-VLP vaccines limits the number of included VLP types and thus the vaccines' spectrum of protection, leaving a panel of oncogenic mucosal HPV unaddressed. In addition, current vaccines do not protect against cutaneous HPV types causing benign skin warts, or against beta-papillomavirus (betaPV) types implicated in the development of non-melanoma skin cancer (NMSC) in immunosuppressed patients. In contrast with L1-VLP, the minor capsid protein L2 contains type-common epitopes that induce low-titer yet broadly cross-neutralizing antibodies to heterologous PV types and provide cross-protection in animal challenge models. Efforts to increase the low immunogenicity of L2 (poly)-peptides and thereby to develop broader-spectrum HPV vaccines are the focus of this review.

Secretory production of designed multipeptides displayed on a thermostable bacterial thioredoxin scaffold in Pichia pastoris

Internal grafting of designed peptides to scaffold proteins is a valuable strategy for a variety of applications including recombinant peptide antigen construction. A peptide epitope from human papillomavirus (HPV) minor capsid protein L2 displayed on thioredoxin (Trx) has been validated preclinically as a broadly protective and low-cost alternative HPV vaccine. Focusing on thioredoxin from the hyperthermophilic archaebacterium Pyrococcus furiosus (PfTrx) as a scaffold, we have constructed a modified Pichia pastoris expression vector and used a PfTrx fusion derivative containing three tandemly repeated copies of a 19 amino acids peptide epitope from HPV-L2 for expression optimization and biochemical-immunological characterization of the Pichia-produced PfTrx-L2 antigen. We show that PfTrx-L2 is produced at high levels (up to 100 mg from a 100 ml starting culture using a multi-cycle induction protocol) and secreted into the culture medium as a highly enriched (>70% pure), non-glycosylated polypeptide that can be purified to homogeneity in a single step. Oxidation and aggregation state, thermal stability and immunogenicity of the endotoxin-free PfTrx-L2 antigen produced in P. pastoris were tested and found to be identical to those of the same antigen produced in Escherichia coli. Secretory production of endotoxin-free PfTrx-peptides in P. pastoris represents a cost- and time-effective alternative to E. coli production. Specifically designed for peptide antigens, the PfTrx-expression vector and conditions described herein are easily transferable to a variety of applications centred on the use of structurally constrained bioactive peptides as immune as well as target-specific binder reagents.

Characterization of serum antibodies from women immunized with Gardasil: A study of HPV-18 infection of primary human keratinocytes

The prevalent human papillomaviruses (HPVs) infect human epithelial tissues. Infections by the mucosotropic HPV genotypes cause hyperproliferative ano-genital lesions. Persistent infections by high-risk (HR) HPVs such as HPV-16, HPV-18 and related types can progress to high grade intraepithelial neoplasias and cancers. Prophylactic HPV vaccines are based on DNA-free virus-like particles (VLPs) composed of the major capsid protein L1 of HPV-16, -18, -6 and -11 (Gardasil) or HPV-16 and -18 (Cervarix). Sera from vaccinated animals effectively prevent HPV pseudovirions to infect cell lines and mouse cervical epithelia. Both vaccines have proven to be highly protective in people. HPV pseudovirions are assembled in HEK293TT cells from matched L1 and L2 capsid proteins to encapsidate a reporter gene. Pseudovirions and genuine virions have structural differences and they infect cell lines or primary human keratinocytes (PHKs) with different efficiencies. In this study, we show that sera and isolated IgG from women immunized with Gardasil prevent authentic HPV-18 virions from infecting PHKs, whereas non-immune sera and purified IgG thereof are uniformly ineffective. Using early passage PHKs, neutralization is achieved only if immune sera are added within 2-4h of infection. We attribute the timing effect to a conformational change in HPV virions, thought to occur upon initial binding to heparan sulfate proteoglycans (HSPG) on the cell surface. This interpretation is consistent with the inability of immune IgG bound to or taken up by PHKs to neutralize the virus. Interestingly, the window of neutralization increases to 12-16h in slow growing, late passage PHKs, suggestive of altered cell surface molecules. In vivo, this window might be further lengthened by the time required to activate the normally quiescent basal cells to become susceptible to infection. Our observations help explain the high efficacy of HPV vaccines.

Progress and prospects for L2-based human papillomavirus vaccines

Human papillomavirus (HPV) is a worldwide public health problem, particularly in resource-limited countries. Fifteen high-risk genital HPV types are sexually transmitted and cause 5% of all cancers worldwide, primarily cervical, anogenital and oropharyngeal carcinomas. Skin HPV types are generally associated with benign disease, but a subset is linked to non-melanoma skin cancer. Licensed HPV vaccines based on virus-like particles (VLPs) derived from L1 major capsid antigen of key high risk HPVs are effective at preventing these infections but do not cover cutaneous types and are not therapeutic. Vaccines targeting L2 minor capsid antigen, some using capsid display, adjuvant and fusions with early HPV antigens or Toll-like receptor agonists, are in development to fill these gaps. Progress and challenges with L2-based vaccines are summarized.

Robust In Vitro and In Vivo Neutralization against Multiple High-Risk HPV Types Induced by a Thermostable Thioredoxin-L2 Vaccine

Current prophylactic virus-like particle (VLP) human papillomavirus (HPV) vaccines are based on the L1 major capsid protein and provide robust but virus type-restricted protection. Moreover, VLP vaccines have a high production cost, require cold-chain storage, and are thus not readily implementable in developing countries, which endure 85% of the cervical cancer-related death burden worldwide. In contrast with L1, immunization with minor capsid protein L2 elicits broad cross-neutralization, and we previously showed that insertion of a peptide spanning amino acids 20-38 of L2 into bacterial thioredoxin (Trx) greatly enhances its immunogenicity. Building on this finding, we use, here, four different neutralization assays to demonstrate that low doses of a trivalent Trx-L2 vaccine, incorporating L2(20-38) epitopes from HPV16, HPV31 and HPV51, and formulated in a human-compatible adjuvant, induce broadly protective responses. Specifically, we show that this vaccine, which uses a far-divergent archaebacterial thioredoxin as scaffold and is amenable to an easy one-step thermal purification, induces robust cross-neutralization against 12 of the 13 known oncogenic HPV types. Immune performance measured with two different in vitro neutralization assays was corroborated by the results of mouse cervico-vaginal challenge and passive transfer experiments indicating robust cross-protection also in vivo. Altogether, our results attest to the potential of Trx-L2 as a thermostable second-generation HPV vaccine particularly well suited for low-resource countries.

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.

Production of Furin-Cleaved Papillomavirus Pseudovirions and Their Use for In Vitro Neutralization Assays of L1- or L2-Specific Antibodies

Immunization with Human Papillomavirus (HPV) L1 virus-like particles or L2 capsid protein elicits neutralizing antibodies that mediate protection. A high-throughput and sensitive in vitro neutralization assay is therefore valuable for prophylactic HPV vaccine studies. Over several hours during infection of the genital tract, virions take on a distinct intermediate conformation, including a required furin cleavage of L2 at its N-terminus. This intermediate is an important target for neutralization by L2-specific antibody, but it is very transiently exposed during in vitro infection of most cell lines resulting in insensitive measurement for L2, but not L1-specific neutralizing antibodies. To model this intermediate, we describe a protocol to generate furin-cleaved HPV pseudovirions (fc-PsV), which deliver an encapsidated reporter plasmid to facilitate infectivity measurements. We also describe a protocol for use of fc-PsV in a high-throughput in vitro neutralization assay for the sensitive measurement of both L1 and L2-specific neutralizing antibodies.

Seroepidemiology of Human Papillomavirus 16 (HPV16) L2 and Generation of L2-Specific Human Chimeric Monoclonal Antibodies

Presently, the seroprevalence of human papillomavirus (HPV) minor capsid antigen L2-reactive antibody is not well understood, and no serologic standard exists for L2-specific neutralizing antibodies. Therefore, we screened a total of 1,078 serum samples for HPV16 L2 reactivity, and these were obtained from four prior clinical studies: a population-based (n = 880) surveillance study with a high-risk HPV DNA prevalence of 10.8%, a cohort study of women (n = 160) with high-grade cervical intraepithelial neoplasia (CIN), and two phase II trials in women with high-grade vulvar intraepithelial neoplasia (VIN) receiving imiquimod therapy combined with either photodynamic therapy (PDT) (n = 19) or vaccination with a fusion protein comprising HPV16 L2, E7, and E6 (TA-CIN) (n = 19). Sera were screened sequentially by HPV16 L2 enzyme-linked immunosorbent assay (ELISA) and then Western blot. Seven of the 1,078 serum samples tested had L2-specific antibodies, but none were detectably neutralizing for HPV16. To develop a standard, we substituted human IgG1 sequences into conserved regions of two rodent monoclonal antibodies (MAbs) specific for neutralizing epitopes at HPV16 L2 residues 17 to 36 and 58 to 64, creating JWW-1 and JWW-2, respectively. These chimeric MAbs retained neutralizing activity and together reacted with 33/34 clinically relevant HPV types tested. In conclusion, our inability to identify an HPV16 L2-specific neutralizing antibody response even in the sera of patients with active genital HPV disease suggests the subdominance of L2 protective epitopes and the value of the chimeric MAbs JWW-1 and JWW-2 as standards for immunoassays to measure L2-specific human antibodies.

Model systems to study the life cycle of human papillomaviruses and HPV-associated cancers

The prevalent human papillomaviruses (HPVs) infect either cutaneous or mucosal epithelium. Active Infections lead to epithelial hyperprolifeation and are usually cleared in healthy individuals within a year. Persistent infections in the anogenital tracts by certain high-risk genotypes such as HPV-16, HPV-18 and closely related types, can progress to high grade dysplasias and carcinomas in women and men, including cervical, vulva, penile and anal cancers. A significant fraction of the head and neck cancers are also caused by HPV-16. The viral oncogenes responsible for neoplastic conversion are E6 and E7 that disrupt the pathways controlled by the two major tumor suppressor genes, p53 and members of pRB family. Because HPV cannot be propagated in conventional submerged monolayer cell cultures, organotypic epithelial raft cultures that generate a stratified and differentiated epithelium have been used to study the viral life cycle. This article describes several systems to examine aspects of the viral productive phase, along with the advantages and limitations. Animal model systems of HPV carcinogenesis are also briefly described.

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.

Sequential cisplatin therapy and vaccination with HPV16 E6E7L2 fusion protein in saponin adjuvant GPI-0100 for the treatment of a model HPV16+ cancer

Clinical studies suggest that responses to HPV16 E6E7L2 fusion protein (TA-CIN) vaccination alone are modest, and GPI-0100 is a well-tolerated, potent adjuvant. Here we sought to optimize both the immunogenicity of TA-CIN via formulation with GPI-0100 and treatment of HPV16+ cancer by vaccination after cisplatin chemotherapy. HPV16 neutralizing serum antibody titers, CD4+ T cell proliferative and E6/E7-specific CD8+ T cell responses were significantly enhanced when mice were vaccinated subcutaneously (s.c.) or intramuscularly (i.m.) with TA-CIN formulated with GPI-0100. Vaccination was tested for therapy of mice bearing syngeneic HPV16 E6/E7+ tumors (TC-1) either in the lung or subcutaneously. Mice treated with TA-CIN/GPI-0100 vaccination exhibited robust E7-specific CD8+ T cell responses, which were associated with reduced tumor burden in the lung, whereas mice receiving either component alone were similar to controls. Since vaccination alone was not sufficient for cure, mice bearing s.c. TC-1 tumor were first treated with two doses of cisplatin and then vaccinated. Vaccination with TA-CIN/GPI-0100 i.m. substantially retarded tumor growth and extended survival after cisplatin therapy. Injection of TA-CIN alone, but not GPI-0100, into the tumor (i.t.) was similarly efficacious after cisplatin therapy, but the mice eventually succumbed. However, tumor regression and extended remission was observed in 80% of the mice treated with cisplatin and then intra-tumoral TA-CIN/GPI-0100 vaccination. These mice also exhibited robust E7-specific CD8+ T cell and HPV16 neutralizing antibody responses. Thus formulation of TA-CIN with GPI-0100 and intra-tumoral delivery after cisplatin treatment elicits potent therapeutic responses in a murine model of HPV16+ cancer.

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.

Current perspectives on HPV vaccination: a focus on targeting the L2 protein

ABSTRACT: 

Thirty years ago, human papillomavirus types 16 and 18 were isolated from cervical carcinomas, and it has been almost 10 years since the introduction of the first prophylactic virus-like particle (VLP) vaccine. The VLP vaccines have already impacted the reduction of pre-malignant lesions and genital warts, and it is expected that vaccination efforts will successfully lower the incidence of cervical cancer before the end of the decade. Here we summarize the historical developments leading to the prophylactic HPV vaccines and discuss current advances of next-generation vaccines that aim to overcome certain limitations of the VLP vaccines, including their intrinsic narrow range of protection, stability and production/distribution costs.

Impact of inhibitors and L2 antibodies upon the infectivity of diverse alpha and beta human papillomavirus types

The licensed human papillomavirus (HPV) vaccines elicit type-restricted immunity but do not target cutaneous HPV types of the beta genus that are associated with non-melanoma skin cancer in immune-compromised patients, and it is unclear if these diverse types share a common mechanism of infection. Residues 11-88 of minor capsid protein L2 contain cross-protective epitopes, and vaccination with concatamers of this region derived from as many as eight alpha HPV (L2 α11-88x8) is being developed as an alternative prophylactic vaccine with potentially broader efficacy. There is also interest in developing broadly protective topical microbicides, such as carrageenan or heparin that block HPV receptor interactions, or small molecule inhibitors of infection. Here we have examined several inhibitors of HPV infection and antisera to L2 α11-88x8 for their breadth of activity against infection by 34 HPV types from within both the alpha and beta families using pseudovirions (PsV) carrying a luciferase reporter as surrogates for native virus. We observed that both heparin and carrageenan prevented infection by mucosatropic HPV types, but surprisingly PsV of several epidermotropic alpha4 and beta HPV types exhibited increased infectivity especially at low inhibitor concentrations. Furin and γ-secretase inhibitors and L2 α11-88x8 antiserum blocked infection by all HPV PsV types tested. These findings suggest that the distinct tropism of mucosal and cutaneous HPV may reflect distinct cell surface receptor interactions, but a common uptake mechanism dependent upon furin and γ-secretase proteolytic activities. Carrageenan, which is being tested as a vaginal microbicide, broadly inhibited infection by the high-risk mucosatropic HPV PsV, but not most skin tropic alpha and beta HPV. Vaccination with an L2 multimer derived exclusively from alpha papillomavirus sequences induced antibodies that broadly neutralized PsV of all 34 HPVs from within both the alpha and beta families, suggesting each displays conserved L2 neutralizing epitopes.

Low doses of flagellin-L2 multimer vaccines protect against challenge with diverse papillomavirus genotypes

Genetically modified bacterial flagellin (Fla), a Toll-like receptor-5 (TLR5) ligand, was evaluated as a fusion partner for human papillomavirus (HPV) L2-based immunogens in two animal challenge models; either cutaneous inoculation of rabbits with HPV 'quasivirions' containing cottontail rabbit papillomavirus (CRPV) genomes that induce warts, or intra-vaginal inoculation of mice with HPV 'pseudovirions' encapsidating a luciferase reporter plasmid and measurement of bioluminescence to determine infectivity. An Escherichia coli production system was developed for flagellin-L2 (Fla-L2) fusions containing either monomeric HPV-16 L2 a.a. 11(×11-200) or oligomeric L2 comprising a fusion of the a.a. 11-88 peptides of five (Fla∼5×11-88) or eight (Fla∼8×11-88) genital HPV types. Immunogenicity and bioactivity of Fla-L2 constructs were assessed using an in vitro neutralization and cell-based TLR-5 binding assay, respectively. Efficacy was evaluated following active immunization of rabbits or mice administered 3 intramuscular doses of Fla-L2 recombinants without exogenous adjuvant, followed by challenge. In addition, passive immunization studies of naïve rabbits with serial dilutions of pooled immune sera were used to determine End-Point Protection Titers (EPPT) for each formulation against a broader spectrum of HPV quasivirions. Efficacy was assessed for up to 10 weeks on the basis of wart volume induced following challenge and results compared to licensed L1-VLP vaccines (Gardasil and Cervarix). Following active immunization at doses as low as 1 μg, Fla-L2 fusions afforded complete protection against infection (mice) and disease (rabbits) following either homologous or heterologous HPV challenge. Passive immunization with anti-L2 immune sera discriminated between the different vaccine candidates under evaluation, demonstrated the protective role of antibody and suggested the superiority of this oligomeric L2-TLR5 agonist fusion approach compared to L1-based vaccines in its ability to cross-protect against non-vaccine HPV types.

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

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