Chimeric papillomavirus-like particles

Route of administration of chimeric BPV1 VLP determines the character of the induced immune responses

To examine the mucosal immune response to papillomavirus virus-like particles (PV-VLP), mice were immunized with VLP intrarectally (i.r.), intravaginally (i.va.) or intramuscularly (i.m.) without adjuvant. PV-VLP were assembled with chimeric BPV-1 L1 proteins incorporating sequence from HIV-1 gp120, either the V3 loop or a shorter peptide incorporating a known CTL epitope (HIVP18I10). Antibody specific for BPV-1 VLP and P18 peptide was detected in serum following i.m., but not i.r. or i.va. immunization. Denatured VLP induced a much reduced immune response when compared with native VLP. Immune responses following mucosal administration of VLP were generally weaker than following systemic administration. VLP specific IgA was higher in intestine washes following i.r. than i.va. immunization, and higher in vaginal washes following i.m. than i.r. or i.va. immunization. No differences in specific antibody responses were seen between animals immunized with BPV-1 P18 VLP or with BPV-1 V3 VLP. Cytotoxic T lymphocyte precursors specific for the P18 CTL epitope were recovered from the spleen following i.m., i.va. or i.r. immunization with P18 VLP, and were similarly detected in Peyer's patches following i.m. or i.r. immunization. Thus, mucosal or systemic immunization with PV VLP induces mucosal CTL responses and this may be important for vaccines for mucosal infection with human papillomaviruses and for other viruses.

Polynucleotide viral vaccines: codon optimisation and ubiquitin conjugation enhances prophylactic and therapeutic efficacy

Papillomavirus infection is a major antecedent of anogenital malignancy. We have previously established that the L1 and L2 capsid genes of papillomavirus have suboptimal codon usage for expression in mammalian cells. We now show that the lack of immunogenicity of polynucleotide vaccines based on the L1 gene can be overcome with codon modified L1, which induces strong immune responses, including conformational virus neutralising antibody and delayed type hypersensitivity. Conjugation of a ubiquitin gene to a hybrid gene incorporating L1 and the E7 non-structural papillomavirus protein improved E7 specific CTL responses, and induced protection against an E7 expressing tumour, but induced little neutralising antibody. However, a mixture of ubiquitin conjugated and non-ubiquitin conjugated polynucleotides induced virus neutralising antibody and E7 specific CD8 T cells. An optimal combined prophylactic/therapeutic viral vaccine might therefore comprise ubiquitin conjugated and non-ubiquitinated genes, to induce prophylactic neutralising antibody and therapeutic cell mediated immune responses.

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

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

Display of complete life cycle of human papillomavirus type 16 in cultured placental trophoblasts

Human papillomavirus (HPV) infection is threefold more prevalent in spontaneous abortion specimens compared to elective abortions preferentially targeting the placental trophoblasts in these specimens. Here by using infectious ceplar and Southern blot analysis, we demonstrate that the transfected HPV-16 genome de novo replicates in 3A trophoblasts in culture. Peak DNA replication occurred 9-24 days posttransfection, showing classic DNA forms I, II, and III and an 8-kb monomer band upon DpnI/BamHI digestion. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of mRNA expression revealed that E6 and E2 were significantly expressed by day 9, coinciding with HPV-16 DNA replication. However, significant L1 expression was delayed until day 18. L1 protein expression on day 18, but not day 9, was also confirmed by Western blot analysis. The production of HPV-16 virions was demonstrated by three techniques: the appearance of HPV-16 infectious units coinciding with L1 expression, the neutralization of these infectious units with known neutralizing anti-HPV-16 antibodies, and the appearance of spliced E1-E4 and E6-E7 transcripts (RT-PCR) in normal keratinocyte rafts infected with these trophoblast-produced HPV-16 infectious units. These data suggest that HPV-16 is carrying out its complete life cycle in trophoblasts. Previously, HPVs were known to productively replicate only in differentiating keratinocytes of skin. These findings expand HPV biology, support the hypothesis of a possible link between HPV and some spontaneous abortions, and present a new technology for studying HPV.

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

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

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

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

A DNA vaccine based on a shuffled E7 oncogene of the human papillomavirus type 16 (HPV 16) induces E7-specific cytotoxic T cells but lacks transforming activity

Vaccination with oncogene-derived DNA for anti-cancer treatment carries a risk of de-novo tumor induction triggered by the persisting recombinant DNA. We hypothesized that an oncoprotein whose primary sequence has been rearranged ('shuffled') to maintain all possible T cell epitopes still induces cytotoxic T cells against the authentic protein but is devoid of transforming properties. As a model antigen, we used the E7 oncoprotein of the human papillomavirus (HPV) type 16, the major cause of cervical cancer. We have generated an artificial E7 molecule in which four domains were rearranged and, in order to maintain all possible T cell epitopes, certain sequences were duplicated. Upon transfection of this shuffled E7 gene (E7SH) into RMA cells, presentation of an E7 Db-restricted T cell epitope was shown by an E7-specific CTL line in vitro. Immunization of C57BL/6 mice with E7SH DNA induced E7-specific CTL and also conveyed protection against E7-positive syngeneic tumor cells. No transforming activity of E7SH DNA in NIH3T3 cells was detected, as determined by focus formation, induction of S-phase under conditions of serum deprivation and degradation of endogenous pRB. Our results suggest that DNA shuffling may become a promising concept for DNA-based anti-cancer vaccines.

Papillomavirus-like particle based vaccines: cervical cancer and beyond

Non-infectious human papillomavirus-like particles (VLP), composed of the L1 major capsid protein, are under active development as vaccines to prevent cervical cancer. They would presumably function primarily by generating virion-neutralising antibodies against the genital human papillomavirus (HPV) types that are the principal cause of most cervical cancers. Early phase clinical studies indicate that the VLP vaccines are well tolerated and able to consistently induce high titres of virus type-specific neutralising antibodies. Two types of second-generation VLP-based subunit vaccines with therapeutic implications, both related and unrelated to papillomavirus infection, are in preclinical development. One type seeks to induce cell-mediated immune responses, especially cytotoxic lymphocytes (CTL), against non-structural papillomavirus proteins, proteins of other viruses, or tumour associated antigens. The target antigen is incorporated into the VLPs as a fusion protein of L1 or the L2 minor capsid protein. In mouse models, this approach has generated potent CTL responses after low dose vaccination in the absence adjuvant. The second type of therapeutic VLP-based vaccine seeks to generate autoantibodies to self-antigens. The display of self polypeptides in the context of the highly ordered array of repetitive elements on the papillomavirus VLP surface abrogates the ability of the humoral immune system to functionally distinguish between foreign and self. High titre and high avidity auto-reactive IgG antibodies have been induced to both soluble (TNF-alpha) and cell surface (CCR5) central self-antigens. Vaccines based on this approach could potentially be effective alternatives to monoclonal antibody (mAb)-based therapies for a variety of disease targets.

Physical interaction of human papillomavirus virus-like particles with immune cells

Human papillomavirus virus-like particles (HPV VLP) and chimeric VLP are immunogens that are able to elicit potent anti-viral/tumor B and T cell responses. To investigate the immunogenicity of VLP, we determined which cells of the immune system are able to bind HPV-16 VLP. VLP were found to bind very well to human and mouse immune cells that expressed markers of antigen-presenting cells (APC) such as MHC class II, CD80 and CD86, including dendritic cells, macrophages and B cells. mAb blocking studies identified Fc gamma RIII (CD16) as one of the molecules to which the VLP can bind both on immune cells and foreskin epithelium. However, transfection of a CD16(-) cell line with CD16 did not confer binding of VLP. Splenocytes from Fc gamma RIII knockout mice showed a 33% decrease in VLP binding overall and specifically to subsets of APC. These combined data support a role for CD16 as an accessory molecule in an HPV VLP-receptor complex, possibly contributing to the immunogenicity of HPV VLP.

HPV16 L1E7 chimeric virus-like particles induce specific HLA-restricted T cells in humans after in vitro vaccination

Cervical cancer has been shown to be highly associated with human papillomavirus (HPV) infection. The viral oncogenes E6 and E7 are constantly expressed by the tumor cells and are therefore targets for immunotherapy. In the present study we investigated the potential of HPV16 L1E7 chimeric virus-like particles (CVLP) to activate specific cytotoxic T lymphocytes in human blood donors. CVLP were expressed by recombinant baculovirus and purified. Direct incubation of freshly isolated peripheral blood lymphocytes (PBL) with CVLP resulted in induction of proliferation and growth of T cell lines. To enhance antigen presentation we also loaded dendritic cells with CVLP and used them to activate naive T cells. Growing cell lines were mainly CD3 positive (>95%) with a predominant CD4-positive and a minor CD8-positive component. Analysis of Tcell specificity was carried out by an interferon-gamma ELISpot assay. Dendritic cells pseudoinfected with CVLP or pulsed with human leukocyte antigen (HLA)-A*0201-restricted peptide E7(11-20) or with a newly identified HPV16 peptide L1(323-331) were used as stimulator cells. T cells responsive to CVLP were found in the cultures with frequencies of 0.5%-0.7%. Frequencies to peptides were around 0.1%. These T cells had cytolytic activity toward autologous B-lymphoblastic cell lines either pseudoinfected with CVLP or pulsed with HLA-A*0201-restricted peptides. They also lysed the HPV16- and HLA-A*0201-positive cervical cancer cell line CaSki, whereas HLA-A*0201-negative SiHa cells were not lysed. We conclude from our data that CVLP show promise for a therapeutic vaccine in patients with HPV16-positive cervical intraepithelial neoplasia lesions or cervical cancer.

Modified HPV16 E7 Genes as DNA Vaccine against E7-Containing Oncogenic Cells

Therapeutic vaccines against tumors associated with human papillomaviruses (HPV) should elicit cellular immune responses against early HPV antigens, primarily the oncoproteins E7 and E6. Because of safety concerns, the direct use of an unmodified oncogene is impossible in human DNA vaccination. Therefore, we introduced three point mutations into the pRb-binding site of HPV16 E7 oncogene to eliminate its transformation potential. The resultant gene was denoted E7GGG. The rates of expression and the cellular localization of E7 and E7GGG proteins were comparable. In immunization-challenge experiments, the efficacy of plasmids containing the E7, E7GGG, or fusion genes of HPV16 E7, viz. L1DeltaCE7(1-60) (M. Muller et al., 1997, Virology 234, 93-111), and Sig/E7/LAMP-1 (T. C. Wu et al., 1995, Proc. Natl. Acad. Sci. USA 92, 11671-11675), was compared. While tumors developed in all animals immunized with the wild-type E7 gene, a significant proportion of mice remained tumor-free after vaccination with the E7GGG gene. The fusion gene L1DeltaCE7(1-60) induced negligible protection, but Sig/E7/LAMP-1 conferred the highest protection. Intradermal immunization by gene gun proved superior to i.m. inoculation. In "therapeutic" experiments, a 1-day delay between inoculation of oncogenic cells and the start of DNA immunization resulted in partial therapeutic effect, but a 3-day delay produced a substantially lower immunization effect. A combination of Sig/E7/LAMP-1 and E7GGG genes did not enhance the immune response. These results demonstrate a significant enhancement of HPV16 E7 immunogenicity after mutagenesis of the pRb-binding site, but the mutated E7 gene did not excel the Sig/E7/LAMP-1 fusion gene.

Mucosal vaccination with a recombinant Salmonella typhimurium expressing human papillomavirus type 16 (HPV16) L1 virus-like particles (VLPs) or HPV16 VLPs purified from insect cells inhibits the growth of HPV16-expressing tumor cells in mice

Human papillomaviruses, mainly type 16 (HPV16), are responsible for cervical intraepithelial neoplasia, which can lead, in association with other factors, to cervical cancer. Both Salmonella recombinant vaccine strains assembling HPV16 virus-like particles (VLPs) and HPV16 VLPs purified from insect cells are able to induce HPV16 neutralizing antibodies in genital secretions of mice after nasal immunization. Anti-HPV16-specific antibodies in cervical secretions of women may prevent genital infection with HPV16, although this cannot be critically evaluated in the absence of an experimental model for genital papillomavirus infection. Induction of HPV16-specific cell-mediated immunity in the genital mucosa could improve the efficacy of a vaccine and a mucosal route of immunization might be necessary to do so. It has been shown that systemic immunization of mice with purified HPV16 VLPs confers protection against an HPV16-expressing tumor cell challenge through the induction of cytotoxic T-lymphocytes. Using the same C3 tumor model, we show that intranasal immunization of mice with purified HPV16 VLPs in a prophylactic setting also induces anti-tumor immunity. More interestingly, mucosal vaccination of mice with a Salmonella recombinant strain stably expressing HPV16 L1 VLPs also induces anti-tumor immunity in prophylactic as well as in therapeutic settings. Our data suggest that attenuated Salmonella strains expressing chimeric VLPs containing nonstructural viral proteins might be a promising candidate vaccine against cervical cancer by inducing both neutralizing antibodies and cell-mediated immunity.

Natural HPV immunity and vaccination strategies

BACKGROUND

the task of preventing premature death in women may be delivered by vaccinating against the high-risk papillomaviruses associated with various malignancies.

OBJECTIVES

we will discuss the immune mechanisms likely to be relevant to the control of an HPV infection in the cervix and assess the limited evidence for such immune recognition in the natural history of infection.

CONCLUSION

the next generation of vaccination strategies should include the use of HPV 16 early (E2 and/or E6 and/or E7) and late gene targets (L1 and L2) expressed as VLPs with their clinical and immunological evaluation aimed at therapy as well as prophylaxis. Important clinical efficacy assessment may be deliverable in relatively short-term studies by targeting patients with HPV 16 associated vulval intraepithelial neoplasia.

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

BACKGROUND

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

OBJECTIVE

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

RESULTS AND CONCLUSION

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

Human papillomavirus genotype 16 vaccines for cervical cancer prophylaxis and treatment

More than 11% of the global cancer incidence in females is due to human papillomavirus (HPV) infections, with HPV genotype 16 the most prevalent viral type to infect the cervix. Vaccine strategies currently target HPV 16 genes E6 and E7, constitutively expressed in cervical cancer cells, and L1 and L2, HPV surface antigens. Recent developments in HPV vaccine research are reviewed. Most studies focus on vaccine models showing improved immunogenicity or dual induction of both humeral and cellular systems. Preclinical studies show that (1) L1 /E7 chimeric viral-like proteins induce both neutralizing L1 antibodies and E7-specific T cells; (2) rerouting a cytosolic tumor antigen into the endosomal/lysosomal compartment can improve the therapeutic potency of DNA vaccines; and (3) accelerated E7 protein degradation leads to enhanced antigen presentation in the context of major histocompatability complex class I. Clinical studies show that (1) HPV 16 E7 peptide vaccination can be safely delivered to patients with terminal disease; and (2) HPV-16 capsid proteins harbor at least one HLA-A*201 restricted cytotoxic T lymphocyte (CTL) epitope.

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

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

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

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

Assembly of human papillomavirus type 16 pseudovirions in Saccharomyces cerevisiae

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

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

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

The immunology of animal papillomaviruses

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

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.

The use of vaccines in treating cervical cancer: present status and future prospects

HPV types are carcinogenic agents in cervical cancer. This view is supported by epidemiological and biological evidence. The oncogenic products and capsid proteins of high risk HPV types are potential targets against which effective immunity may be generated by vaccination. Both therapeutic and prophlylactic immunisation are potential strategies to deal with the widespread problem of HPV infection and possibly established cervical neoplasia. Clinical trials are now underway to evaluate candidate vaccines.

Chimeric virus-like particles of the human papillomavirus type 16 (HPV 16) as a prophylactic and therapeutic vaccine

Infection by certain human papillomaviruses (HPV), most notably HPV types 16 and 18, is the major risk factor for cervical cancer. Worldwide, this disease represents the second most frequent malignant tumor in women; thus, there is urgent need for efficient therapy and prevention. The natural history of cervical cancer and its precursors (cervical intraepithelial neoplasias), as well as animal experiments, strongly suggest that the immune system controls both the primary infection (by neutralizing antibodies directed against the major structural protein L1) and the progression of the disease (via cytotoxic T cells specific for the viral oncoproteins expressed in transformed cells, e.g., E7). By the expression of an HPV 16 L1E7 fusion protein, we have generated chimeric virus-like particles (CVLP). Immunization of mice with CVLPs induces neutralizing antibodies directed against L1 virus-like particles (devoid of the E7 portion) and E7-specific T cells as measured in vitro. Vaccinated animals are protected against tumor growth following inoculation of syngeneic HPV 16-transformed cells. In addition, we observed a therapeutic effect of vaccination on pre-existing tumors. This data allowed us to conclude that CVLPs are suitable for prevention and therapy of HPV infection. A vaccine based on HPV 16 L1E7 CVLPs is currently under development.

Immune response to human papillomavirus 16 L1E7 chimeric virus-like particles: induction of cytotoxic T cells and specific tumor protection

Expression of human papillomavirus type 16 (HPV 16) fusion proteins LI deltaCE7(1-55) and LI deltaCE7(1-60) (carboxy-terminal deletion of LI replaced by 55 or 60 amino-terminal amino acids of E7) leads to formation of chimeric papillomavirus-like particles (CVLPs). After "infection" of cells by CVLPs, the chimeric proteins can be detected in the cytosol and the endoplasmic reticulum (ER), suggesting that they are intracellularly processed via the MHC class I pathway and, therefore, able to activate cytotoxic T lymphocytes (CTLs). To investigate the cytotoxic immune response against HPV 16 LI deltaCE7(1-60) and LI deltaCE7(1-55) CVLPs, we immunized C57Bl/6 mice with various CVLP doses without adjuvant. Two weeks after immunization, spleen cells were prepared and stimulated in vitro using HPV 16 E7-expressing transfectants of the tumor cell line RMA. In 51Cr-release cytotoxicity assays, spleen cells of mice vaccinated with LI deltaCE7(1-60) CVLPs specifically lysed the RMA-E7 transfectants as well as RMA cells loaded with the peptide E7(49-57), which represents an H2-Db-restricted CTL epitope. This demonstrates that CVLPs induce an E7-specific CTL response in mice in the absence of an adjuvant. Furthermore, immunization with CVLPs prevented outgrowth of E7-expressing tumor cells even if inoculation of cells was performed 2 weeks before vaccination. We conclude from our data that CVLPs show promise for therapy of HPV-associated lesions.

Papillomavirus-like particle vaccines for cervical cancer

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

Induction of HPV16 capsid protein-specific human T cell responses by virus-like particles

It has been postulated that upon binding to a cell surface receptor, papilloma virus-like particles (VLPs) gain entry into the cytosol of infected cells and the capsid proteins L1 and L2 can be processed in the MHC class I presentation pathway. Vaccination of mice with human papilloma virus-like particles consisting of capsid proteins L1 and L2 induced a CD8-mediated and perforin dependent protective immune response against a tumor challenge with human papilloma virus transformed tumor cells, which express only minute amounts of L1 protein. Here we show that HPV16 capsid proteins stimulate a MHC class I restricted CTL response with human peripheral blood lymphocytes (PBL) in vitro. The vigorous response was specific for VLP-infected target cells and was MHC class I restricted. Moreover we show the presence of at least one HLA-A*0201 restricted CTL epitope within the HPV-16 capsid proteins by using a VLP-'infected' HLA-A*0201 transfected human cell line as target cells. These results demonstrated that VLPs can induce a HPV16 capsid protein-specific immune response in humans, allowing the monitoring of immune responses induced by vaccines based on chimeric VLPs carrying additional immunogenic peptides or proteins in therapeutical applications in human patients.

Papillomaviruses: prophylactic vaccine prospects

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

Mucosal immunisation with papillomavirus virus-like particles elicits systemic and mucosal immunity in mice

It has been shown previously that recombinant virus-like particles (VLPs) of papillomavirus can induce VLP-specific humoral and cellular immune responses following parenteral administration. To test whether mucosal administration of bovine papillomavirus type 1 (BPV1) VLPs could produce mucosal as well as systemic immune responses to VLPs, 50 micrograms chimeric BPV1 VLPs containing an HPV16 E7 CTL epitope (BPVL1/E7 VLP) was administered intranasally to mice. After two immunisations, L1-specific serum IgG and IgA were observed. L1-specific IgG and IgA were also found in respiratory and vaginal secretions. Both serum and mucosal antibody inhibited papillomavirus VLP-induced agglutination of RBC, indicating that the antibody induced by mucosal immunisation may recognize conformational determinants associated with virus neutralisation. For comparison, VLPs were given intramuscularly, and systemic and mucosal immune responses were generally comparable following systemic or mucosal delivery. However, intranasal administration of VLP induced significantly higher local IgA response in lung, suggesting that mucosally delivered HPV VLP may be more effective for mediating local mucosal immune responses. Intranasal immunisation with HPV6b L1 VLP produced VLP-specific T proliferative responses in splenocytes, and immunisation with BPVL1 VLP containing an HPV16 E7 CTL epitope induced E7-specific CTL responses. We conclude that immunisation with papillomavirus VLPs via mucosal and intramuscular routes, without adjuvant, can elicit specific antibody at mucosal surfaces and also systemic VLP epitope specific T cell responses. These findings suggest that mucosally delivered VLPs may offer an alternative HPV VLP vaccine strategy for inducing protective humoral immunity to anogenital HPV infection, together with cell-mediated immune responses to eliminate any cells which become infected.

L1-specific protection from tumor challenge elicited by HPV16 virus-like particles

A single injection of HPV16 L1 virus-like particles induced potent CD8-mediated protection from tumor challenge by C3 cells, a line derived from embryonic mouse cells transfected with the HPV16 genome. L1 RNA, but not protein, was detected biochemically in C3 cells. These results indicate that low-level expression of HPV16 L1 can occur in proliferating cells and serve as a tumor vaccine target. Although L1 expression is generally thought to be restricted to terminally differentiated epithelial cells, these results suggest that additional analysis for low-level L1 expression in proliferating cells of HPV-induced lesions is warranted and might help in predicting the clinical potential of HPV L1 virus-like particle-based vaccines.

Chimeric papillomavirus virus-like particles elicit antitumor immunity against the E7 oncoprotein in an HPV16 tumor model

Papillomavirus-like particles (VLPs) are a promising prophylactic vaccine candidate to prevent human papillomavirus (HPV) infections and associated epithelial neoplasia. However, they are unlikely to have therapeutic effects because the virion capsid proteins are not detected in the proliferating cells of the infected epithelia or in cervical carcinomas. To increase the number of viral antigen targets for cell-mediated immune responses in a VLP-based vaccine, we have generated stable chimeric VLPs consisting of the L1 major capsid protein plus the entire E7 (11 kDa) or E2 (43 kDa) nonstructural papillomavirus protein fused to the L2 minor capsid protein. The chimeric VLPs are indistinguishable from the parental VLPs in their morphology and in their ability to agglutinate erythrocytes and elicit high titers of neutralizing antibodies. Protection from tumor challenge was tested in C57BL/6 mice by using the tumor cell line TC-1, which expresses HPV16 E7, but not the virion structural proteins. Injection of HPV16 L1/L2-HPV16 E7 chimeric VLPs, but not HPV16 L1/L2 VLPs, protected the mice from tumor challenge, even in the absence of adjuvant. The chimeric VLPs also induced protection against tumor challenge in major histocompatibility class II-deficient mice, but not in beta2-microglobulin or perforin knockout mice implying that protection was mediated by class I-restricted cytotoxic lymphocytes. These findings raise the possibility that VLPs may generally be efficient vehicles for generating cell-mediated immune responses and that, specifically, chimeric VLPs containing papillomavirus nonstructural proteins may increase the therapeutic potential of VLP-based prophylactic vaccines in humans.

Papillomavirus virus-like particles can deliver defined CTL epitopes to the MHC class I pathway

To evaluate an antigen delivery system in which exogenous antigen can target the major histocompatibility complex (MHC) class I pathway, a single human papillomavirus (HPV) 16 E7 cytotoxic T lymphocyte (CTL) epitope and a single HIV gp160 CTL epitope were separately fused to the C-terminus of bovine papillomavirus 1 (BPV1) L1 sequence to form hybrid BPV1L1 VLPs. Mice immunized with these hybrid VLPs mounted strong CTL responses against the relevant target cells in the absence of any adjuvants. In addition, the CTL responses induced by immunization with BPV1L1/HPV16E7CTL VLPs protected mice against challenge with E7-transformed tumor cells. Furthermore, a high titer-specific antibody response against BPV1L1 VLPs was also induced, and this antiserum could inhibit papillomavirus-induced agglutination of mouse erythrocytes, suggesting that the antibody may recognize conformational determinates relevant to virus neutralization. These data demonstrate that hybrid BPV1L1 VLPs can be used as carriers to target antigenic epitopes to both the MHC class I and class II pathways, providing a promising strategy for the design of vaccines to prevent virus infection, with the potential to elicit therapeutic virus-specific CTL responses.