The papillomavirus major capsid protein L1

Effect of apoptosis-associated speck-like protein containing a caspase recruitment domain on vaccine efficacy: Overcoming the effects of its deficiency with aluminum hydroxide adjuvant

Host factors such as nutritional status and immune cell state are important for vaccine efficacy. Inflammasome activation may be important for triggering vaccine-induced humoral and cell-mediated immune responses. Formulations with alum as a typical adjuvant to overcome the effects of host factors have recently been shown to induce inflammasome activation, which augments vaccine efficacy. Apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) is one of the main components of inflammasomes, but it is not clear whether ASC affects the vaccine-induced immune response. Herein, we used two types of vaccines: inactivated influenza vaccine not formulated with alum, and HPV vaccine formulated with alum. We gave the vaccines to ASC knockout (ASC^-/- ) mice to investigate the role of ASC in vaccine efficacy. Influenza vaccine-immunized ASC^-/- mice did not show antibody titers in week 2 after the first vaccination. After boosting, the antibody titer in ASC^-/- mice was about half that in wild type (WT) mice. Furthermore, a cytotoxic T-lymphocyte response against influenza vaccine was not induced in ASC^-/- mice. Therefore, vaccinated ASC^-/- mice did not show effective protection against viral challenge. ASC^-/- mice immunized with alum-formulated HPV vaccine showed similar antibody titers and T-cell proliferation compared with immunized WT mice. However, the HPV vaccine without alum induced up to threefold lower titers of HPV-specific antibody titers in ASC^-/- mice compared with those in WT mice. These findings suggest that alum in vaccine can overcome the ASC-deficient condition.

Recurring infection with ecologically distinct HPV types can explain high prevalence and diversity

The high prevalence of human papillomavirus (HPV), the most common sexually transmitted infection, arises from the coexistence of over 200 genetically distinct types. Accurately predicting the impact of vaccines that target multiple types requires understanding the factors that determine HPV diversity. The diversity of many pathogens is driven by type-specific or "homologous" immunity, which promotes the spread of variants to which hosts have little immunity. To test for homologous immunity and to identify mechanisms determining HPV transmission, we fitted nonlinear mechanistic models to longitudinal data on genital infections in unvaccinated men. Our results provide no evidence for homologous immunity, instead showing that infection with one HPV type strongly increases the risk of infection with that type for years afterward. For HPV16, the type responsible for most HPV-related cancers, an initial infection increases the 1-year probability of reinfection by 20-fold, and the probability of reinfection remains 14-fold higher 2 years later. This increased risk occurs in both sexually active and celibate men, suggesting that it arises from autoinoculation, episodic reactivation of latent virus, or both. Overall, our results suggest that high HPV prevalence and diversity can be explained by a combination of a lack of homologous immunity, frequent reinfections, weak competition between types, and variation in type fitness between host subpopulations. Because of the high risk of reinfection, vaccinating boys who have not yet been exposed may be crucial to reduce prevalence, but our results suggest that there may also be large benefits to vaccinating previously infected individuals.

Rodent Papillomaviruses

Preclinical infection model systems are extremely valuable tools to aid in our understanding of Human Papillomavirus (HPV) biology, disease progression, prevention, and treatments. In this context, rodent papillomaviruses and their respective infection models are useful tools but remain underutilized resources in the field of papillomavirus biology. Two rodent papillomaviruses, MnPV1, which infects the Mastomys species of multimammate rats, and MmuPV1, which infects laboratory mice, are currently the most studied rodent PVs. Both of these viruses cause malignancy in the skin and can provide attractive infection models to study the lesser understood cutaneous papillomaviruses that have been frequently associated with HPV-related skin cancers. Of these, MmuPV1 is the first reported rodent papillomavirus that can naturally infect the laboratory strain of mice. MmuPV1 is an attractive model virus to study papillomavirus pathogenesis because of the ubiquitous availability of lab mice and the fact that this mouse species is genetically modifiable. In this review, we have summarized the knowledge we have gained about PV biology from the study of rodent papillomaviruses and point out the remaining gaps that can provide new research opportunities.

Felis catus papillomavirus type-2 but not type-1 is detectable and transcriptionally active in the blood of healthy cats

Papillomaviruses (PVs) are small DNA viruses that induce benign and/or malignant epithelial tumours in different species, including the domestic cat (Felis catus). To date, five F. catus papillomavirus genotypes have been identified (FcaPV-1 to FcaPV-5). FcaPV-1 is associated with skin and oral benign lesions, while FcaPV-2 infection is widely associated with feline squamous cell carcinomas. Several human and animal PVs have been found in body fluids such as peripheral blood; however, the presence of FcaPVs in non-epithelial tissues has not previously been investigated. The aim of this study was to assess the presence and gene expression of FcaPV-1 and FcaPV-2 in the blood of healthy cats. We detected FcaPV-2 DNA in 26 of 103 (25%) blood samples. Importantly, FcaPV-2 L1, E2, E6 and E7 genes were found to be expressed in 3 (25%), 11 (92%), 6 (50%) and 5 (42%) of the samples available for mRNA analysis, respectively. FcaPV-1 was not detected in any of the blood samples analysed here. The data obtained in this work suggest active and eventually productive infection of FcaPV-2 in the blood of healthy cats, implying a possible role in intra-individual spreading as well as in vertical and horizontal transmission.

Current status and future prospects for human papillomavirus vaccines

Cervical cancer is the fourth most frequent cancer among women worldwide. Human papillomaviruses (HPVs) cause almost all cervical cancers in low-income countries. Three prophylactic HPV virus-like particle-based vaccines have been licensed to date, and they have all shown high efficacy and reliable safety profiles. However, isolated safety issues have resulted in a reluctance to use these vaccinations. In addition, the high prices of the vaccinations have caused the inequitable distribution of the vaccine: the prices are unaffordable for low-income countries. Meanwhile, great effort has been put into the development of therapeutic HPV vaccines, including protein/peptide-, live vector-, DNA- and cell-based vaccines. These new vaccines have considerable therapeutic potential but limited practical use. The development of immune checkpoint inhibitors and personalized immunotherapy remain challenges for future study. In this article, the current status of the licensed vaccines, therapeutic HPV vaccines and biosimilars, and new platforms for HPV vaccines, are reviewed, and safety issues related to the licensed vaccines are discussed. In addition, the prospects for HPV vaccines are considered.

Host control of human papillomavirus infection and disease

Most human papillomaviruses cause inapparent infections, subtly affecting epithelial homeostasis, to ensure genome persistence in the epithelial basal layer. As with conspicuous papillomas, these self-limiting lesions shed viral particles to ensure population level maintenance and depend on a balance between viral gene expression, immune cell stimulation and immune surveillance for persistence. The complex immune evasion strategies, characteristic of high-risk HPV types, also allow the deregulated viral gene expression that underlies neoplasia. Neoplasia occurs at particular epithelial sites where vulnerable cells such as the reserve or cuboidal cells of the cervical transformation zone are found. Beta papillomavirus infection can also predispose an individual with immune deficiencies to the development of cancers. The host control of HPV infections thus involves local interactions between keratinocytes and the adaptive immune response. Effective immune detection and surveillance limits overt disease, leading to HPV persistence as productive microlesions or in a true latent state.

Structural features of glycan recognition among viral pathogens

Recognition and binding to host glycans present on cellular surfaces is an initial and critical step in viral entry. Diverse families of host glycans such as histo-blood group antigens, sialoglycans and glycosaminoglycans are recognized by viruses. Glycan binding determines virus-host specificity, tissue tropism, pathogenesis and potential for interspecies transmission. Viruses including noroviruses, rotaviruses, enteroviruses, influenza, and papillomaviruses have evolved novel strategies to bind specific glycans often in a strain-specific manner. Structural studies have been instrumental in elucidating the molecular determinants of these virus-glycan interactions, aiding in developing vaccines and antivirals targeting this key interaction. Our review focuses on these key structural aspects of virus-glycan interactions, particularly highlighting the different strain-specific strategies employed by viruses to bind host glycans.

hrHPV E5 oncoprotein: immune evasion and related immunotherapies

The immune response is a key factor in the fight against HPV infection and related cancers, and thus, HPV is able to promote immune evasion through the expression of oncogenes. In particular, the E5 oncogene is responsible for modulation of several immune mechanisms, including antigen presentation and inflammatory pathways. Moreover, E5 was suggested as a promising therapeutic target, since there is still no effective medical therapy for the treatment of HPV-related pre-neoplasia and cancer. Indeed, several studies have shown good prospective for E5 immunotherapy, suggesting that it could be applied for the treatment of pre-cancerous lesions. Thus, insofar as the majority of cervical, oropharyngeal and anal cancers are caused by high-risk HPV (hrHPV), mainly by HPV16, the aim of this review is to discuss the immune pathways interfered by E5 oncoprotein of hrHPV highlighting the various aspects of the potential immunotherapeutic approaches.

Tetraspanins in infections by human cytomegalo- and papillomaviruses

Members of the tetraspanin family have been identified as essential cellular membrane proteins in infectious diseases by nearly all types of pathogens. The present review highlights recently published data on the role of tetraspanin CD151, CD81, and CD63 and their interaction partners in host cell entry by human cytomegalo- and human papillomaviruses. Moreover, we discuss a model for tetraspanin assembly into trafficking platforms at the plasma membrane. These platforms might persist during intracellular viral trafficking.

The VPS4 component of the ESCRT machinery plays an essential role in HPV infectious entry and capsid disassembly

Human Papillomavirus (HPV) infection involves multiple steps, from cell attachment, through endocytic trafficking towards the trans-Golgi network, and, ultimately, the entry into the nucleus during mitosis. An essential viral protein in infectious entry is the minor capsid protein L2, which engages different components of the endocytic sorting machinery during this process. The ESCRT machinery is one such component that seems to play an important role in the early stages of infection. Here we have analysed the role of specific ESCRT components in HPV infection, and we find an essential role for VPS4. Loss of VPS4 blocks infection with multiple PV types, suggesting an evolutionarily conserved critical step in infectious entry. Intriguingly, both L1 and L2 can interact with VPS4, and appear to be in complex with VPS4 during the early stages of virus infection. By using cell lines stably expressing a dominant-negative mutant form of VPS4, we also show that loss of VPS4 ATPase activity results in a marked delay in capsid uncoating, resulting in a defect in the endocytic transport of incoming PsVs. These results demonstrate that the ESCRT machinery, and in particular VPS4, plays a critical role in the early stages of PV infection.

Host cell restriction factors that limit transcription and replication of human papillomavirus

The life cycle of human papillomaviruses (HPV) is tightly regulated by the differentiation state of mucosal and cutaneous keratinocytes. To counteract viral infection, constitutively expressed cellular factors, which are defined herein as restriction factors, directly mitigate viral gene expression and replication. In turn, some HPV gene products target these restriction factors and abrogate their anti-viral effects to establish efficient gene expression and replication programs. Ironically, in certain circumstances, this delicate counterbalance between viral gene products and restriction factors facilitates persistent infection by HPVs. This review serves to recapitulate the current knowledge of nuclear restriction factors that directly affect the HPV infectious cycle.

Papillomaviruses: a systematic review

In the last decades, a group of viruses has received great attention due to its relationship with cancer development and its wide distribution throughout the vertebrates: the papillomaviruses. In this article, we aim to review some of the most relevant reports concerning the use of bovines as an experimental model for studies related to papillomaviruses. Moreover, the obtained data contributes to the development of strategies against the clinical consequences of bovine papillomaviruses (BPV) that have led to drastic hazards to the herds. To overcome the problem, the vaccines that we have been developing involve recombinant DNA technology, aiming at prophylactic and therapeutic procedures. It is important to point out that these strategies can be used as models for innovative procedures against HPV, as this virus is the main causal agent of cervical cancer, the second most fatal cancer in women.

The Papillomavirus Episteme: a major update to the papillomavirus sequence database

The Papillomavirus Episteme (PaVE) is a database of curated papillomavirus genomic sequences, accompanied by web-based sequence analysis tools. This update describes the addition of major new features. The papillomavirus genomes within PaVE have been further annotated, and now includes the major spliced mRNA transcripts. Viral genes and transcripts can be visualized on both linear and circular genome browsers. Evolutionary relationships among PaVE reference protein sequences can be analysed using multiple sequence alignments and phylogenetic trees. To assist in viral discovery, PaVE offers a typing tool; a simplified algorithm to determine whether a newly sequenced virus is novel. PaVE also now contains an image library containing gross clinical and histopathological images of papillomavirus infected lesions. Database URL: https://pave.niaid.nih.gov/.

The DE and FG loops of the HPV major capsid protein contribute to the epitopes of vaccine-induced cross-neutralising antibodies

The human papillomavirus (HPV) vaccines consist of major capsid protein (L1) virus-like particles (VLP) and are highly efficacious against the development of cervical cancer precursors attributable to oncogenic genotypes, HPV16 and HPV18. A degree of vaccine-induced cross-protection has also been demonstrated against genetically-related genotypes in the Alpha-7 (HPV18-like) and Alpha-9 (HPV16-like) species groups which is coincident with the detection of L1 cross-neutralising antibodies. In this study the L1 domains recognised by inter-genotype cross-neutralising antibodies were delineated. L1 crystallographic homology models predicted a degree of structural diversity between the L1 loops of HPV16 and the non-vaccine Alpha-9 genotypes. These structural predictions informed the design of chimeric pseudovirions with inter-genotype loop swaps which demonstrated that the L1 domains recognised by inter-genotype cross-neutralising antibodies comprise residues within the DE loop and the late region of the FG loop. These data contribute to our understanding of the L1 domains recognised by vaccine-induced cross-neutralising antibodies. Such specificities may play a critical role in vaccine-induced cross-protection.

Controlled Hybrid-Assembly of HPV16/18 L1 Bi VLPs in Vitro

Based on the helix4-exchanged HPV16 L1 and HPV18 L1, HPV16 L1 Bi and HPV18 L1 Bi, we have successfully realized the controlled hybrid-assembly of HPV16/18 L1 Bi VLPs (bihybrid-VLPs) in vitro. The bihybrid-VLPs were further confirmed by fluorescence resonance energy transfer (FRET) and complex-immunoprecipitation (Co-IP) assays. The ratio of 16 L1 Bi and 18 L1 Bi in bihybrid-VLPs was verified to be 3:5 based on a modified magnetic Co-IP procedure, when mixing 1 equiv pentamer in assembly buffer solution, but it changed with conditions. In addition, the bihybrid-VLPs showed identical thermal stability as that of normal VLPs, suggesting high potential in practical applications. The present study is significant because it modified one of the vital steps of virus life cycle at the stage of virus assembly, supplying a new approach not only to deepen structural insights but also a possibility to prepare stable, low-cost, bivalent antivirus vaccine. Furthermore, the controlled hybrid-assembly of bihybrid-VLPs in vitro provides suggestions for the design of effective multivalent hybrid-VLPs, being a potential to develop broad-spectrum vaccines for the prevention of infection with multiple types of HPV.

The Cytoskeletal Adaptor Obscurin-Like 1 Interacts with the Human Papillomavirus 16 (HPV16) Capsid Protein L2 and Is Required for HPV16 Endocytosis

The human papillomavirus (HPV) capsid protein L2 is essential for viral entry. To gain a deeper understanding of the role of L2, we searched for novel cellular L2-interacting proteins. A yeast two-hybrid analysis uncovered the actin-depolymerizing factor gelsolin, the membrane glycoprotein dysadherin, the centrosomal protein 68 (Cep68), and the cytoskeletal adaptor protein obscurin-like 1 protein (OBSL1) as putative L2 binding molecules. Pseudovirus (PsV) infection assays identified OBSL1 as a host factor required for gene transduction by three oncogenic human papillomavirus types, HPV16, HPV18, and HPV31. In addition, we detected OBSL1 expression in cervical tissue sections and noted the involvement of OBSL1 during gene transduction of primary keratinocytes by HPV16 PsV. Complex formation of HPV16 L2 with OBSL1 was demonstrated in coimmunofluorescence and coimmunoprecipitation studies after overexpression of L2 or after PsV exposure. We observed a strong colocalization of OBSL1 with HPV16 PsV and tetraspanin CD151 at the plasma membrane, suggesting a role for OBSL1 in viral endocytosis. Indeed, viral entry assays exhibited a reduction of viral endocytosis in OBSL1-depleted cells. Our results suggest OBSL1 as a novel L2-interacting protein and endocytosis factor in HPV infection.

IMPORTANCE

Human papillomaviruses infect mucosal and cutaneous epithelia, and the high-risk HPV types account for 5% of cancer cases worldwide. As recently discovered, HPV entry occurs by a clathrin-, caveolin-, and dynamin-independent endocytosis via tetraspanin-enriched microdomains. At present, the cellular proteins involved in the underlying mechanism of this type of endocytosis are under investigation. In this study, the cytoskeletal adaptor OBSL1 was discovered as a previously unrecognized interaction partner of the minor capsid protein L2 and was identified as a proviral host factor required for HPV16 endocytosis into target cells. The findings of this study advance the understanding of a so far less well-characterized endocytic pathway that is used by oncogenic HPV subtypes.

Concurrence of Iridovirus, Polyomavirus, and a Unique Member of a New Group of Fish Papillomaviruses in Lymphocystis Disease-Affected Gilthead Sea Bream

Lymphocystis disease is a geographically widespread disease affecting more than 150 different species of marine and freshwater fish. The disease, provoked by the iridovirus lymphocystis disease virus (LCDV), is characterized by the appearance of papillomalike lesions on the skin of affected animals that usually self-resolve over time. Development of the disease is usually associated with several environmental factors and, more frequently, with stress conditions provoked by the intensive culture conditions present in fish farms. In gilthead sea bream (Sparus aurata), an economically important cultured fish species in the Mediterranean area, a distinct LCDV has been identified but not yet completely characterized. We have used direct sequencing of the virome of lymphocystis lesions from affected S. aurata fish to obtain the complete genome of a new LCDV-Sa species that is the largest vertebrate iridovirus sequenced to date. Importantly, this approach allowed us to assemble the full-length circular genome sequence of two previously unknown viruses belonging to the papillomaviruses and polyomaviruses, termed Sparus aurata papillomavirus 1 (SaPV1) and Sparus aurata polyomavirus 1 (SaPyV1), respectively. Epidemiological surveys showed that lymphocystis disease was frequently associated with the concurrent appearance of one or both of the new viruses. SaPV1 has unique characteristics, such as an intron within the L1 gene, and as the first member of the Papillomaviridae family described in fish, provides evidence for a more ancient origin of this family than previously thought.

IMPORTANCE

Lymphocystis disease affects marine and freshwater fish species worldwide. It is characterized by the appearance of papillomalike lesions on the skin that contain heavily enlarged cells (lymphocysts). The causative agent is the lymphocystis disease virus (LCDV), a large icosahedral virus of the family Iridoviridae In the Mediterranean area, the gilthead sea bream (Sparus aurata), an important farmed fish, is frequently affected. Using next-generation sequencing, we have identified within S. aurata lymphocystis lesions the concurrent presence of an additional LCDV species (LCDV-Sa) as well as two novel viruses. These are members of polyomavirus and papillomavirus families, and here we report them to be frequently associated with the presence of lymphocysts in affected fish. Because papillomaviruses have not been described in fish before, these findings support a more ancient origin of this virus family than previously thought and evolutionary implications are discussed.

Molecular archeological evidence in support of the repeated loss of a papillomavirus gene

It is becoming clear that, in addition to gene gain, the loss of genes may be an important evolutionary mechanism for many organisms. However, gene loss is often associated with an increased mutation rate, thus quickly erasing evidence from the genome. The analysis of evolutionarily related sequences can provide empirical evidence for gene loss events. This paper analyzes the sequences of over 300 genetically distinct papillomaviruses and provides evidence for a role of gene loss during the evolution of certain papillomavirus genomes. Phylogenetic analysis suggests that the viral E6 gene was lost at least twice. Despite belonging to distant papillomaviral genera, these viruses lacking a canonical E6 protein may potentially encode a highly hydrophobic protein from an overlapping open reading frame, which we designate E10. Evolutionary pressure working on this alternative frame, may explain why, despite having lost the E6 open reading frame between 20 and 60 million years ago, evidence of an E6-like protein is conserved.

The CD63-Syntenin-1 Complex Controls Post-Endocytic Trafficking of Oncogenic Human Papillomaviruses

Human papillomaviruses enter host cells via a clathrin-independent endocytic pathway involving tetraspanin proteins. However, post-endocytic trafficking required for virus capsid disassembly remains unclear. Here we demonstrate that the early trafficking pathway of internalised HPV particles involves tetraspanin CD63, syntenin-1 and ESCRT-associated adaptor protein ALIX. Following internalisation, viral particles are found in CD63-positive endosomes recruiting syntenin-1, a CD63-interacting adaptor protein. Electron microscopy and immunofluorescence experiments indicate that the CD63-syntenin-1 complex controls delivery of internalised viral particles to multivesicular endosomes. Accordingly, infectivity of high-risk HPV types 16, 18 and 31 as well as disassembly and post-uncoating processing of viral particles was markedly suppressed in CD63 or syntenin-1 depleted cells. Our analyses also present the syntenin-1 interacting protein ALIX as critical for HPV infection and CD63-syntenin-1-ALIX complex formation as a prerequisite for intracellular transport enabling viral capsid disassembly. Thus, our results identify the CD63-syntenin-1-ALIX complex as a key regulatory component in post-endocytic HPV trafficking.

Characterization of human papillomavirus type 16 pseudovirus containing histones

BACKGROUND

Pseudoviruses (PsVs) that encapsidate a reporter plasmid DNA have been used as surrogates for native human papillomavirus (HPV), whose continuous production is technically difficult. HPV PsVs have been designed to form capsids made up of the major capsid protein L1 and the minor capsid proteins L2. HPV PsVs have been produced in 293TT cells transfected with plasmid expressing L1 and L2 protein and plasmid containing the reporter gene. Several studies have suggested that naturally occurring HPV virions contain cellular histones, and histones have also been identified in mature HPV PsVs. However, the effect of the histones on the properties of the PsVs has not been investigated. Using heparin chromatography, we separated mature HPV type 16 PsVs into three fractions (I, II, and III) according to their heparin-binding affinities.

RESULTS

The amounts of cellular histone and cellular nucleotides per PsV were found to increase in the order fraction I, II and III. It appeared that PsVs in fraction I contains just small amount of cellular histone in Western blot analysis. The proportions of the three fractions in PsV preparations were 83.4, 7.5, and 9.1 % for fraction I, II, and III PsVs, respectively. In the electron microscope PsVs in fraction I appeared to have a more condensed structure than those in fractions II and III. Under the electron microscope fraction II and III PsVs appeared to be covered by substantial amounts of cellular histone while there was no visible histone covering PsVs of fraction I. Also the levels of reporter gene expression in infections of fraction II and III PsVs to 293TT cells were significantly lower than those in infections of fraction I PsV, and fraction II and III particles had significantly reduced immunogenicity.

CONCLUSIONS

Our findings suggest that the involvement of large amounts of cellular histones during PsV formation interferes with the structural integrity of the PsVs and affects their immunogenicity. The fraction I particle therefore has the most suitable characteristics for use as an HPV PsV.

Genetic variability and lineage phylogeny of human papillomavirus type-16 and -53 based on the E6, E7, and L1 genes in Southwest China

Human papillomaviruses (HPVs) are circular double-stranded DNA viruses that are highly prevalent in the general population, and account for the cervical cancer burden in women worldwide. In this study, we analyzed HPV-16, the most prevalent type worldwide, and HPV-53, a possible high-risk type from infected women in Southwest China. To characterize mutations, intratypic variants, and genetic variability in the E6, E7, and L1 genes of HPV-16 (n=97) and HPV-53 (n=15), these genes were sequenced and submitted to GenBank. Phylogenetic trees were constructed using Bayesian trees, followed by secondary structure analysis and B-cell epitope prediction. Moreover, the selection pressures of the E6, E7, and L1 genes were estimated. In total, 27 novel variants of HPV-16 and 11 novel variants of HPV-53 were identified. In the HPV-16 E6-E7-L1 sequences, 73 nucleotide changes were observed with 40/73 being non-synonymous mutations (two in the alpha helix and five in the beta sheet) and 33/73 being synonymous. In the HPV-53 E6-E7-L1 sequences, 64 nucleotide changes were observed with 26/64 being non-synonymous mutations (three in the alpha helix and one in the beta sheet) and 38/64 being synonymous. Selective pressure analysis showed that most of these mutations did not reflect positive selection. The maximal divergence between any two variants within each gene of these two HPV types ranging from 0.94%(HPV-16 L1 gene)to 2.80%(HPV-53 E6 gene). Identifying new variants of HPV-16 and -53 from women in Southwest China may be helpful to design vaccines specifically for women in Southwest China and testing methods specifically for this region. The results of our study may contribute to future researches in diagnostic probes, vaccines improvement, or screening methods for a particular population.

Human papillomavirus molecular biology

Human papillomaviruses are small DNA viruses with a tropism for squamous epithelia. A unique aspect of human papillomavirus molecular biology involves dependence on the differentiation status of the host epithelial cell to complete the viral lifecycle. A small group of these viruses are the etiologic agents of several types of human cancers, including oral and anogenital tract carcinomas. This review focuses on the basic molecular biology of human papillomaviruses.

Furin Cleavage of L2 during Papillomavirus Infection: Minimal Dependence on Cyclophilins

Despite an abundance of evidence supporting an important role for the cleavage of minor capsid protein L2 by cellular furin, direct cleavage of capsid-associated L2 during human papillomavirus 16 (HPV16) infection remains poorly characterized. The conserved cleavage site, close to the L2 N terminus, confounds observation and quantification of the small cleavage product by SDS-PAGE. To overcome this difficulty, we increased the size shift by fusing a compact protein domain, the Propionibacterium shermanii transcarboxylase domain (PSTCD), to the N terminus of L2. The infectious PSTCD-L2 virus displayed an appreciable L2 size shift during infection of HaCaT keratinocytes. Cleavage under standard cell culture conditions rarely exceeded 35% of total L2. Cleavage levels were enhanced by the addition of exogenous furin, and the absolute levels of infection correlated to the level of L2 cleavage. Cleavage occurred on both the HaCaT cell surface and extracellular matrix (ECM). Contrary to current models, experiments on the involvement of cyclophilins revealed little, if any, role for these cellular enzymes in the modulation of furin cleavage. HPV16 L2 contains two consensus cleavage sites, Arg5 (2RHKR5) and Arg12 (9RTKR12). Mutant PSTCD-L2 viruses demonstrated that although furin can cleave either site, cleavage must occur at Arg12, as cleavage at Arg5 alone is insufficient for successful infection. Mutation of the conserved cysteine residues revealed that the Cys22-Cys28 disulfide bridge is not required for cleavage. The PSTCD-L2 virus or similar N-terminal fusions will be valuable tools to study additional cellular and viral determinants of furin cleavage.

IMPORTANCE

Furin cleavage of minor capsid protein L2 during papillomavirus infection has been difficult to directly visualize and quantify, confounding efforts to study this important step of HPV infection. Fusion of a small protein domain to the N terminus greatly facilitates direct visualization of the cleavage product, revealing important characteristics of this critical process. Contrary to the current model, we found that cleavage is largely independent of cyclophilins, suggesting that cyclophilins act either in parallel to or downstream of furin to trigger exposure of a conserved N-terminal L2 epitope (RG-1) during infection. Based on this finding, we strongly caution against using L2 RG-1 epitope exposure as a convenient but indirect proxy of furin cleavage.

Naturally Occurring Major and Minor Capsid Protein Variants of Human Papillomavirus 45 (HPV45): Differential Recognition by Cross-Neutralizing Antibodies Generated by HPV Vaccines

We investigated naturally occurring variation within the major (L1) and minor (L2) capsid proteins of human papillomavirus genotype 45 (HPV45). Pseudoviruses (PsVs) representing HPV45 sublineages A1, A2, A3, B1, and B2 exhibited comparable particle-to-infectivity ratios and morphologies but demonstrated both increased (A2, A3, and B1) and decreased (B2) sensitivities to cross-neutralization by HPV vaccine antibodies compared to that of the A1 sublineage. Mutant PsVs identified HI loop residue 357 as being critical for conferring this differential sensitivity.

Functional assessment and structural basis of antibody binding to human papillomavirus capsid

Persistent high-risk human papillomavirus (HPV) infection is linked to cervical cancer. Two prophylactic virus-like particle (VLP)-based vaccines have been marketed globally for nearly a decade. Here, we review the HPV pseudovirion (PsV)-based assays for the functional assessment of the HPV neutralizing antibodies and the structural basis for these clinically relevant epitopes. The PsV-based neutralization assay was developed to evaluate the efficacy of neutralization antibodies in sera elicited by vaccination or natural infection or to assess the functional characteristics of monoclonal antibodies. Different antibody binding modes were observed when an antibody was complexed with virions, PsVs or VLPs. The neutralizing epitopes are localized on surface loops of the L1 capsid protein, at various locations on the capsomere. Different neutralization antibodies exert their neutralizing function via different mechanisms. Some antibodies neutralize the virions by inducing conformational changes in the viral capsid, which can result in concealing the binding site for a cellular receptor like 1A1D-2 against dengue virus, or inducing premature genome release like E18 against enterovirus 71. Higher-resolution details on the epitope composition of HPV neutralizing antibodies would shed light on the structural basis of the highly efficacious vaccines and aid the design of next generation vaccines. In-depth understanding of epitope composition would ensure the development of function-indicating assays for the comparability exercise to support process improvement or process scale up. Elucidation of the structural elements of the type-specific epitopes would enable rational design of cross-type neutralization via epitope re-engineering or epitope grafting in hybrid VLPs.

Molecular biology of gynecological cancer

Cancer is a pathological condition in which the balance between cell growth and death is disordered. Various molecules have been reported to be involved in the oncogenic process of invasion, metastasis and resistance to treatment. An exponential growth in the collection of genomic and proteomic data in the past 20 years has provided major advances in understanding the molecular mechanisms of human cancer, which has been applied to diagnostic and treatment strategies. Targeted therapies have been developed and adopted, particularly for advanced, refractory or recurrent cancers, depending on individual molecular profiles. The aim of the present review is to provide a report of the current literature regarding the molecular biology of gynecological cancers.

Identification of a Novel Human Papillomavirus, Type HPV199, Isolated from a Nasopharynx and Anal Canal, and Complete Genomic Characterization of Papillomavirus Species Gamma-12

The novel human papillomavirus type 199 (HPV199) was initially identified in a nasopharyngeal swab sample obtained from a 25 year-old immunocompetent male. The complete genome of HPV199 is 7,184 bp in length with a GC content of 36.5%. Comparative genomic characterization of HPV199 and its closest relatives showed the classical genomic organization of Gammapapillomaviruses (Gamma-PVs). HPV199 has seven major open reading frames (ORFs), encoding five early (E1, E2, E4, E6, and E7) and two late (L1 and L2) proteins, while lacking the E5 ORF. The long control region (LCR) of 513 bp is located between the L1 and E6 ORFs. Phylogenetic analysis additionally confirmed that HPV-199 clusters into the Gamma-PV genus, species Gamma-12, additionally containing HPV127, HV132, HPV148, HPV165, and three putative HPV types: KC5, CG2 and CG3. HPV199 is most closely related to HPV127 (nucleotide identity 77%). The complete viral genome sequence of additional HPV199 isolate was determined from anal canal swab sample. Two HPV199 complete viral sequences exhibit 99.4% nucleotide identity. To the best of our knowledge, this is the first member of Gamma-PV with complete nucleotide sequences determined from two independent clinical samples. To evaluate the tissue tropism of the novel HPV type, 916 clinical samples were tested using HPV199 type-specific real-time PCR: HPV199 was detected in 2/76 tissue samples of histologically confirmed common warts, 2/108 samples of eyebrow hair follicles, 2/137 anal canal swabs obtained from individuals with clinically evident anal pathology, 4/184 nasopharyngeal swabs and 3/411 cervical swabs obtained from women with normal cervical cytology. Although HPV199 was found in 1.4% of cutaneous and mucosal samples only, it exhibits dual tissue tropism. According to the results of our study and literature data, dual tropism of all Gamma-12 members is highly possible.

Regulation of the Wnt/β-Catenin Signaling Pathway by Human Papillomavirus E6 and E7 Oncoproteins

Cell signaling pathways are the mechanisms by which cells transduce external stimuli, which control the transcription of genes, to regulate diverse biological effects. In cancer, distinct signaling pathways, such as the Wnt/β-catenin pathway, have been implicated in the deregulation of critical molecular processes that affect cell proliferation and differentiation. For example, changes in β-catenin localization have been identified in Human Papillomavirus (HPV)-related cancers as the lesion progresses. Specifically, β-catenin relocates from the membrane/cytoplasm to the nucleus, suggesting that this transcription regulator participates in cervical carcinogenesis. The E6 and E7 oncoproteins are responsible for the transforming activity of HPV, and some studies have implicated these viral oncoproteins in the regulation of the Wnt/β-catenin pathway. Nevertheless, new interactions of HPV oncoproteins with cellular proteins are emerging, and the study of the biological effects of such interactions will help to understand HPV-related carcinogenesis. This review addresses the accumulated evidence of the involvement of the HPV E6 and E7 oncoproteins in the activation of the Wnt/β-catenin pathway.

Human Papillomaviruses; Epithelial Tropisms, and the Development of Neoplasia

Papillomaviruses have evolved over many millions of years to propagate themselves at specific epithelial niches in a range of different host species. This has led to the great diversity of papillomaviruses that now exist, and to the appearance of distinct strategies for epithelial persistence. Many papillomaviruses minimise the risk of immune clearance by causing chronic asymptomatic infections, accompanied by long-term virion-production with only limited viral gene expression. Such lesions are typical of those caused by Beta HPV types in the general population, with viral activity being suppressed by host immunity. A second strategy requires the evolution of sophisticated immune evasion mechanisms, and allows some HPV types to cause prominent and persistent papillomas, even in immune competent individuals. Some Alphapapillomavirus types have evolved this strategy, including those that cause genital warts in young adults or common warts in children. These strategies reflect broad differences in virus protein function as well as differences in patterns of viral gene expression, with genotype-specific associations underlying the recent introduction of DNA testing, and also the introduction of vaccines to protect against cervical cancer. Interestingly, it appears that cellular environment and the site of infection affect viral pathogenicity by modulating viral gene expression. With the high-risk HPV gene products, changes in E6 and E7 expression are thought to account for the development of neoplasias at the endocervix, the anal and cervical transformation zones, and the tonsilar crypts and other oropharyngeal sites. A detailed analysis of site-specific patterns of gene expression and gene function is now prompted.

Naturally Occurring Capsid Protein Variants of Human Papillomavirus Genotype 31 Represent a Single L1 Serotype

We investigated naturally occurring variation within the major (L1) and minor (L2) capsid proteins of oncogenic human papillomavirus (HPV) genotype 31 (HPV31) to determine the impact on capsid antigenicity. L1L2 pseudoviruses (PsVs) representing the three HPV31 variant lineages, variant lineages A, B, and C, exhibited comparable particle-to-infectivity ratios and morphologies. Lineage-specific L1L2 PsVs demonstrated subtle differences in susceptibility to neutralization by antibodies elicited following vaccination or preclinical L1 virus-like particle (VLP) immunization or by monoclonal antibodies; however, these differences were generally of a low magnitude. These data indicate that the diagnostic lineage-specific single nucleotide polymorphisms within the HPV31 capsid genes have a limited effect on L1 antibody-mediated neutralization and that the three HPV31 variant lineages belong to a single L1 serotype. These data contribute to our understanding of HPV L1 variant antigenicity.

IMPORTANCE

The virus coat (capsid) of the human papillomavirus contains major (L1) and minor (L2) capsid proteins. These proteins facilitate host cell attachment and viral infectivity and are the targets for antibodies which interfere with these events. In this study, we investigated the impact of naturally occurring variation within these proteins upon susceptibility to viral neutralization by antibodies induced by L1 VLP immunization. We demonstrate that HPV31 L1 and L2 variants exhibit similar susceptibility to antibody-mediated neutralization and that for the purposes of L1 VLP-based vaccines, these variant lineages represent a single serotype.

The human papillomavirus type 16 L1 protein directly interacts with E2 and enhances E2-dependent replication and transcription activation

The human papillomavirus (HPV) E2 protein is a multifunctional protein essential for the control of virus gene expression, genome replication and persistence. E2 is expressed throughout the differentiation-dependent virus life cycle and is functionally regulated by association with multiple viral and cellular proteins. Here, we show for the first time to our knowledge that HPV16 E2 directly associates with the major capsid protein L1, independently of other viral or cellular proteins. We have mapped the L1 binding region within E2 and show that the α-2 helices within the E2 DNA-binding domain mediate L1 interaction. Using cell-based assays, we show that co-expression of L1 and E2 results in enhanced transcription and virus origin-dependent DNA replication. Upon co-expression in keratinocytes, L1 reduces nucleolar association of E2 protein, and when co-expressed with E1 and E2, L1 is partially recruited to viral replication factories. Furthermore, co-distribution of E2 and L1 was detected in the nuclei of upper suprabasal cells in stratified epithelia of HPV16 genome-containing primary human keratinocytes. Taken together, our findings suggest that the interaction between E2 and L1 is important for the regulation of E2 function during the late events of the HPV life cycle.

Peptidic inhibitors for in vitro pentamer formation of human papillomavirus capsid protein l1

A new 14 peptide, originating essentially from the helix 5 of HPV 16L1, illustrates an IC50 of 19.38 nM for the inhibition of HPV 16 L1 pentamer formation, which is highly efficient for targeting a specific protein segment. In addition, mechanism studies reveal that the length, sequence, and the folding of the peptide are critical factors for its inhibition. Particularly, the peptide shows similar inhibition against the pentamer formation of HPV 58L1, although it is designed specially for HPV 16 L1. This study opens a way for the development of high-efficiency, broad-spectrum inhibitors as a new class of anti-HPV agents, which could be extended to the treatment of other virus types.

The HPV16 and MusPV1 papillomaviruses initially interact with distinct host components on the basement membrane

To understand and compare the mechanisms of murine and human PV infection, we examined pseudovirion binding and infection of the newly described MusPV1 using the murine cervicovaginal challenge model. These analyses revealed primary tissue interactions distinct from those previously described for HPV16. Unlike HPV16, MusPV1 bound basement membrane (BM) in an HSPG-independent manner. Nevertheless, subsequent HSPG interactions were critical. L2 antibodies or low doses of VLP antibodies, sufficient to prevent infection, did not lead to disassociation of the MusPV1 pseudovirions from the BM, in contrast to previous findings with HPV16. Similarly, furin inhibition did not lead to loss of MusPV1 from the BM. Therefore, phylogenetically distant PV types differ in their initial interactions with host attachment factors, but initiate their lifecycle on the acellular BM. Despite these differences, these distantly related PV types displayed similar intracellular trafficking patterns and susceptibilities to biochemical inhibition of infection.

The role of ubiquitin and ubiquitin-like modification systems in papillomavirus biology

Human papillomaviruses (HPVs) are small DNA viruses that are important etiological agents of a spectrum of human skin lesions from benign to malignant. Because of their limited genome coding capacity they express only a small number of proteins, only one of which has enzymatic activity. Additionally, the HPV productive life cycle is intimately tied to the epithelial differentiation program and they must replicate in what are normally non-replicative cells, thus, these viruses must reprogram the cellular environment to achieve viral reproduction. Because of these limitations and needs, the viral proteins have evolved to co-opt cellular processes primarily through protein-protein interactions with critical host proteins. The ubiquitin post-translational modification system and the related ubiquitin-like modifiers constitute a widespread cellular regulatory network that controls the levels and functions of thousands of proteins, making these systems an attractive target for viral manipulation. This review describes the interactions between HPVs and the ubiquitin family of modifiers, both to regulate the viral proteins themselves and to remodel the host cell to facilitate viral survival and reproduction.

Pre-clinical immunogenicity of human papillomavirus alpha-7 and alpha-9 major capsid proteins

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Comprehensive pre-clinical immunogenicity evaluation of HPV L1 major capsid protein.

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Majority neutralizing antibody response was genotype-specific.

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Reciprocal cross-neutralization between some Alpha-7 and Alpha-9 genotypes.

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Tetravalent formulation (HPV16/18/39/58) induced broadly neutralizing antibodies.

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These data improve our understanding of the antigenic diversity of the L1 protein.

Human papillomavirus (HPV) vaccines confer protection against the oncogenic genotypes HPV16 and HPV18 through the generation of type-specific neutralizing antibodies raised against the constituent virus-like particles (VLP) based upon the major capsid proteins (L1) of these genotypes. The vaccines also confer a degree of cross-protection against some genetically related types from the Alpha-9 (HPV16-like: HPV31, HPV33, HPV35, HPV52, HPV58) and Alpha-7 (HPV18-like: HPV39, HPV45, HPV59, HPV68) species groups. The mechanism of cross-protection is unclear but may involve antibodies capable of recognizing shared inter-genotype epitopes. The relationship(s) between the genetic and antigenic diversity of the L1 protein, particularly for non-vaccine genotypes, is poorly understood.

We carried out a comprehensive evaluation of the immunogenicity of L1 VLP derived from genotypes within the Alpha-7 and Alpha-9 species groups in New Zealand White rabbits and used L1L2 pseudoviruses as the target antigens in neutralization assays.

The majority antibody response against L1 VLP was type-specific, as expected, but several instances of robust cross-neutralization were nevertheless observed including between HPV33 and HPV58 within the Alpha-9 species and between HPV39, HPV59 and HPV68 in the Alpha-7 species. Immunization with an experimental tetravalent preparation comprising VLP based upon HPV16, HPV18, HPV39 and HPV58 was capable of generating neutralizing antibodies against all the Alpha-7 and Alpha-9 genotypes. Competition of HPV31 and HPV33 cross-neutralizing antibodies in the tetravalent sera confirmed that these antibodies originated from HPV16 and HPV58 VLP, respectively, and suggested that they represent minority specificities within the antibody repertoire generated by the immunizing antigen. These data improve our understanding of the antigenic diversity of the L1 protein per se and may inform the rational design of a next generation vaccine formulation based upon empirical data.

Inhibition of Langerhans cell maturation by human papillomavirus type 16: a novel role for the annexin A2 heterotetramer in immune suppression

High-risk human papillomaviruses (HPVs) are sexually transmitted viruses causally associated with several cancers. During its natural life cycle, HPV16, the most common high-risk genotype, infects the epithelial basal cells in a process facilitated through a recently identified receptor, the annexin A2 heterotetramer (A2t). During infection, HPV16 also interacts with Langerhans cells (LC), the APC of the epithelium, inducing immune suppression, which is mediated by the HPV16 L2 minor capsid protein. Despite the importance of these virus-immune cell interactions, the specific mechanisms of HPV16 entry into LC and HPV16-induced immune suppression remain undefined. An N-terminal peptide of HPV16 L2 (aa 108-126) has been shown to specifically interact with A2t. In this study, we show that incubation of human LC with this peptide blocks binding of HPV16. Inhibiting this interaction with an A2t ligand or by small interfering RNA downregulation of A2t significantly decreases HPV16 internalization into LC in an L2-dependent manner. A2t is associated with suppression of LC maturation as demonstrated through attenuated secretion of Th1-associated cytokines and decreased surface expression of MHC class II on LC exposed to A2t. Conversely, small molecule inhibition of A2t prevents HPV16-induced suppression of LC immune function as indicated by significantly increased secretion of inflammatory cytokines and surface expression of CD86 in HPV16 treated LC pre-exposed to A2t inhibitors. These results demonstrate that HPV16 suppresses LC maturation through an interaction with A2t, revealing a novel role for this protein.

The tetraspanin CD151 in papillomavirus infection

Human papillomaviruses (HPV) are non-enveloped DNA tumor viruses that infect skin and mucosa. The most oncogenic subtype, HPV16, causes various types of cancer, including cervical, anal, and head and neck cancers. During the multistep process of infection, numerous host proteins are required for the delivery of virus genetic information into the nucleus of target cells. Over the last two decades, many host-cell proteins such as heparan sulfate proteoglycans, integrins, growth factor receptors, actin and the tetraspanin CD151 have been described to be involved in the process of infectious entry of HPV16. Tetraspanins have the ability to organize membrane microdomains and to directly influence the function of associated molecules, including binding of receptors to their ligands, receptor oligomerization and signal transduction. Here, we summarize the current knowledge on CD151, and CD151-associated partners during HPV infection and discuss the underlying mechanisms.

Concepts of papillomavirus entry into host cells

Papillomaviruses enter basal cells of stratified epithelia. Assembly of new virions occurs in infected cells during terminal differentiation. This unique biology is reflected in the mechanism of entry. Extracellularly, the interaction of nonenveloped capsids with several host cell proteins, after binding, results in discrete conformational changes. Asynchronous internalization occurs over several hours by an endocytic mechanism related to, but distinct from macropinocytosis. Intracellular trafficking leads virions through the endosomal system, and from late endosomes to the trans-Golgi-network, before nuclear delivery. Here, we discuss the existing data with the aim to synthesize an integrated model of the stepwise process of entry, thereby highlighting key open questions. Additionally, we relate data from experiments with cultured cells to in vivo results.