Identification of L2 open reading frame gene products of bovine papillomavirus type 1 using monoclonal antibodies

Molecular Characterization of Human Papillomavirus Type 159 (HPV159)

Human papillomavirus type 159 (HPV159) was identified in an anal swab sample and preliminarily genetically characterized by our group in 2012. Here we present a detailed molecular in silico analysis that showed that the HPV159 viral genome is 7443 bp in length and divided into five early and two late genes, with conserved functional domains and motifs, and a non-coding long control region (LCR) with significant regulatory sequences that allow the virus to complete its life cycle and infect novel host cells. HPV159, clustering into the cutaneotropic Betapapillomavirus (Beta-PV) genus, is phylogenetically most similar to HPV9, forming an individual phylogenetic group in the viral species Beta-2. After testing a large representative collection of clinical samples with HPV159 type-specific RT-PCR, in addition to the anal canal from which the first HPV159 isolate was obtained, HPV159 was further detected in other muco-cutaneous (4/181, 2.2%), mucosal (22/764, 2.9%), and cutaneous (14/554, 2.5%) clinical samples, suggesting its extensive tissue tropism. However, because very low HPV159 viral loads were estimated in the majority of positive samples, it seemed that HPV159 mainly caused clinically insignificant infections of the skin and mucosa. Using newly developed, highly sensitive HPV159-specific nested PCRs, two additional HPV159 LCR viral variants were identified. Nevertheless, all HPV159 mutations were demonstrated outside important functional domains of the LCR, suggesting that the HPV159 viral variants were most probably not pathogenically different. This complete molecular characterization of HPV159 enhances our knowledge of the genome characteristics, tissue tropism, and phylogenetic diversity of Beta-PVs that infect humans.

Cleavage of the HPV16 Minor Capsid Protein L2 during Virion Morphogenesis Ablates the Requirement for Cellular Furin during De Novo Infection

Infections by high-risk human papillomaviruses (HPV) are the causative agents for the development of cervical cancer. As with other non-enveloped viruses, HPVs are taken up by the cell through endocytosis following primary attachment to the host cell. Through studies using recombinant pseudovirus particles (PsV), many host cellular proteins have been implicated in the process. The proprotein convertase furin has been demonstrated to cleave the minor capsid protein, L2, post-attachment to host cells and is required for infectious entry by HPV16 PsV. In contrast, using biochemical inhibition by a furin inhibitor and furin-negative cells, we show that tissue-derived HPV16 native virus (NV) initiates infection independent of cellular furin. We show that HPV16 L2 is cleaved during virion morphogenesis in differentiated tissue. In addition, HPV45 is also not dependent on cellular furin, but two other alpha papillomaviruses, HPV18 and HPV31, are dependent on the activity of cellular furin for infection.

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

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

L2, the minor capsid protein of papillomavirus

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

Cross-protective responses to human papillomavirus infection

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

L1 interaction domains of papillomavirus l2 necessary for viral genome encapsidation

BPHE-1 cells, which harbor 50 to 200 viral episomes, encapsidate viral genome and generate infectious bovine papillomavirus type 1 (BPV1) upon coexpression of capsid proteins L1 and L2 of BPV1, but not coexpression of BPV1 L1 and human papillomavirus type 16 (HPV16) L2. BPV1 L2 bound in vitro via its C-terminal 85 residues to purified L1 capsomers, but not with intact L1 virus-like particles in vitro. However, when the efficiency of BPV1 L1 coimmunoprecipitation with a series of BPV1 L2 deletion mutants was examined in vivo, the results suggested that residues 129 to 246 and 384 to 460 contain independent L1 interaction domains. An L2 mutant lacking the C-terminal L1 interaction domain was impaired for encapsidation of the viral genome. Coexpression of BPV1 L1 and a chimeric L2 protein composed of HPV16 L2 residues 1 to 98 fused to BPV1 L2 residues 99 to 469 generated infectious virions. However, inefficient encapsidation was seen when L1 was coexpressed with either BPV1 L2 with residues 91 to 246 deleted or with BPV1 L2 with residues 1 to 225 replaced with HPV16 L2. Impaired genome encapsidation did not correlate closely with impairment of the L2 proteins either to localize to promyelocytic leukemia oncogenic domains (PODs) or to induce localization of L1 or E2 to PODs. We conclude that the L1-binding domain located near the C terminus of L2 may bind L1 prior to completion of capsid assembly, and that both L1-binding domains of L2 are required for efficient encapsidation of the viral genome.

The papillomavirus minor capsid protein, L2, induces localization of the major capsid protein, L1, and the viral transcription/replication protein, E2, to PML oncogenic domains

We have used immunofluorescent staining and confocal microscopy to examine the subcellular localization of structural and nonstructural bovine papillomavirus (BPV) proteins in cultured cells that produce infectious virions. When expressed separately, L1, the major capsid protein, showed a diffuse nuclear distribution while L2, the minor capsid protein, was found to localize to punctate nuclear regions identified as promonocytic leukemia protein (PML) oncogenic domains (PODs). Coexpression of L1 and L2 induced a relocation of L1 into the PODs, leading to the colocalization of L1 and L2. The effect of L2 expression on the distribution of the nonstructural viral proteins E1 and E2, which are required for maintenance of the genome and viral DNA synthesis, was also examined. The localization of the E1 protein was unaffected by L2 expression. However, the pattern of anti-E2 staining was dramatically altered in L2-expressing cells. Similar to L1, E2 was shifted from a dispersed nuclear locality into the PODs and colocalized with L2. The recruitment of full-length E2 by L2 occurred in the absence of other viral components. L2 was shown previously to be essential for the generation of infectious BPV. Our present results provide evidence for a role for L2 in the organization of virion components by recruiting them to a distinct nuclear domain. This L2-dependent colocalization probably serves as a mechanism to promote the assembly of papillomaviruses either by increasing the local concentration of virion constituents or by providing the physical architecture necessary for efficient packaging and assembly. The data also suggest a role for a nonstructural viral protein, E2, in virion assembly, specifically the recruitment of the viral genome to the sites of assembly, through its high-affinity interaction with specific sequences in the viral DNA.

Differentiation-specific alternative splicing of bovine papillomavirus late mRNAs

Activation of the late promoter (PL) of bovine papillomavirus type 1 (BPV-1) is dependent on the differentiation state of keratinocytes and occurs in the upper layers of the bovine fibropapilloma. In this study, we show by in situ hybridization that a differentiation-specific pattern of BPV-1 late RNA splicing is also seen in the fibropapilloma. RNAs containing the 7385/3605 and 3764/5609 splice junctions were confined to the granular cell layer. In contrast, RNAs containing the 7385/3225 splice junction were present in both the granular and spinous layers. The switch in splice site usage in the granular cell layer limits the expression of the mRNA encoding the major capsid protein to these most terminally differentiated cells. Thus, BPV-1 late mRNA expression is regulated at both transcriptional and posttranscriptional levels.

Synthesis and assembly of virus-like particles of human papillomaviruses type 6 and type 16 in fission yeast Schizosaccharomyces pombe

We have synthesized capsid proteins of human papillomavirus types 6 (HPV 6) and 16 (HPV 16) in fission yeast Schizosaccharomyces pombe and produced virus-like particles (VLP). The capsid proteins were localized in the nucleus by indirect immunofluorescence and cell fractionation analyses. The VLP were produced in both yeast clones synthesizing L1 alone and L/L2 and purified by sulfato-cellulofine chromatography. Electron microscopic examination showed that these VLP were similar in structure to native HPV particles. Two HPV 16 L1 variants (16 B27L1 and 16 T3L1), isolated from benign cervical samples, produced many more (68- and 14-fold) VLP than the prototype L1 (16 PL1) derived from cervical carcinoma. Coexpression of the HPV 6 L2 protein with 6 L1 and 16 B27L1 proteins increased the production level of the VLP four- and twofold, respectively. The L2 was not detected in the VLP purified with sulfato-cellulofine column, although the L2 was purified in the same fraction containing HPV 6 and 16 B27-VLP by size-fractionation using Sepharose column. Interaction between 6 L2 and 6/16 L1 proteins was not detected by the coimmunoprecipitation assays with either L1 or L2 antibodies. These results suggest that the L2 is not incorporated into the VLP synthesized in yeast.

Neutralization of bovine papillomavirus by antibodies to L1 and L2 capsid proteins

We have generated four mouse monoclonal antibodies (MAbs) to bovine papillomavirus virions that bound type-specific, adjacent, and conformationally dependent epitopes on the L1 major capsid protein. All four MAbs were neutralizing at ratios of 1 MAb molecule per 5 to 25 L1 molecules, but only three effectively blocked binding of the virus to the cell surface. Therefore, antibodies can prevent papillomavirus infection by at least two mechanisms: inhibition of cell surface receptor binding and a subsequent step in the infectious pathway. The neutralizing epitopes of the bovine papillomavirus L2 minor capsid protein were mapped to the N-terminal half of L2 by blocking the neutralizing activity of full-length L2 antiserum with bacterially expressed peptides of L2. In addition, rabbit antiserum raised against amino acids 45 to 173 of L2 had a neutralizing titer of 1,000, confirming that at least part of the N terminus of L2 is exposed on the virion surface.

Differentiation-specific expression from the bovine papillomavirus type 1 P2443 and late promoters

The papillomavirus life cycle is tightly linked with keratinocyte differentiation in squamous epithelia. Vegetative viral DNA replication begins in the spinous layer, while synthesis of capsid proteins and virus maturation is restricted to the most differentiated or granular layer of the epithelium. In this study, in situ hybridization of bovine fibropapillomas was used to demonstrate that the activity of two promoters of bovine papillomavirus type 1 (BPV-1) is regulated in a differentiation-specific manner. In situ hybridization with a late promoter (PL)-specific oligonucleotide probe suggested that PL is dramatically upregulated in the granular layer of the fibropapilloma. Northern (RNA) blot analysis of RNA from BPV-1-infected fibropapillomas indicated that the three major BPV-1 late-region mRNAs were transcribed from PL. These RNAs include the previously described L1 (major capsid) mRNA as well as two larger mRNAs. The two larger mRNAs were characterized and shown to contain the L2 (minor capsid protein) open reading frame as well as the L1 open reading frame. In contrast to PL, the P2443 promoter was maximally active in basal keratinocytes and the fibroma. The major mRNA transcribed from P2443 is the putative E5 oncoprotein mRNA which is spliced between nucleotides 2505 and 3225. No signal was detected above the basal layer with use of a probe specific for this mRNA. The E5 oncoprotein has previously been localized by immunoperoxidase staining to the granular cell layer as well as the basal cell layer of the fibropapilloma (S. Burnett, N. Jareborg, and D. DiMaio, Proc. Natl. Acad. Sci. USA 89:5665-5669, 1992). These data suggest that E5 proteins in the basal cell and granular cell layers are not translated from the same mRNA.

Papillomavirus L1 major capsid protein self-assembles into virus-like particles that are highly immunogenic

Infection by certain human papillomavirus types is regarded as the major risk factor in the development of cervical cancer, one of the most common cancers of women worldwide. Analysis of the immunogenic and structural features of papillomavirus virions has been hampered by the inability to efficiently propagate the viruses in cultured cells. For instance, it has not been established whether the major capsid protein L1 alone is sufficient for virus particle assembly. In addition, it is not known whether L1, L2 (the minor capsid protein), or both present the immunodominant epitopes required for induction of high-titer neutralizing antibodies. We have expressed the L1 major capsid proteins of bovine papillomavirus type 1 and human papillomavirus type 16 in insect cells via a baculovirus vector and analyzed their conformation and immunogenicity. The L1 proteins were expressed at high levels and assembled into structures that closely resembled papillomavirus virions. The self-assembled bovine papillomavirus L1, in contrast to L1 extracted from recombinant bacteria or denatured virions, also mimicked intact bovine papillomavirus virions in being able to induce high-titer neutralizing rabbit antisera. These results indicate that L1 protein has the intrinsic capacity to assemble into empty capsid-like structures whose immunogenicity is similar to infectious virions. This type of L1 preparation might be considered as a candidate for a serological test to measure antibodies to conformational virion epitopes and for a vaccine to prevent papillomavirus infection.

Effective vaccination against papilloma development by immunization with L1 or L2 structural protein of cottontail rabbit papillomavirus

Immunization of rabbits with either L1, the major structural protein, or L2, a minor structural protein of cottontail rabbit papillomavirus (CRPV), protected against challenge with the virus. Neutralizing antibodies were elicited by both the L1 and L2 trpE fusion proteins. Neutralization with anti-L1 serum, however, was more efficient than with anti-L2 serum. In contrast, when tested on Western blots the immune response to L2 was stronger than to L1. Rabbits were also protected against CRPV infection by immunization with L1 expressing recombinant vaccinia virus. Sera from two of three rabbits immunized with recombinant vaccinia virus were negative on Western blots but all three were positive in ELISA's with nondenatured fusion protein or in immunoprecipitations. The results suggest that both the viral structural proteins, L1 and L2, merit consideration in the development of a vaccine against papillomavirus.

Expression of vaccinia recombinant HPV 16 L1 and L2 ORF proteins in epithelial cells is sufficient for assembly of HPV virion-like particles

A recombinant vaccinia virus termed pLC201VV was designed to coexpress the L1 and L2 late genes of human papillomavirus type 16 (HPV16). Synthesis of the L1 and L2 proteins occurred in cells infected with pLC201VV, and 40-nm virus-like particles with a density of 1.31 g/ml were produced in the nuclei of cells synthesizing both L1 and L2, but not in cells synthesizing either protein alone. Virus-like particles were partially purified from infected cells by sucrose gradient sedimentation and shown to consist of capsomeres similar to HPV and contain glycosylated L1 viral capsid protein. The production of HPV-like particles using recombinant vaccinia virus should be useful for biochemical studies and could provide a safe source of material for the development of a vaccine.

Comparison of neutralization of BPV-1 infection of C127 cells and bovine fetal skin xenografts

BPV-1 induces focus formation in murine C127 cells and fibropapillomas in bovine fetal skin xenografts. In this study, we compared the specificity of neutralization of BPV-1 in both assay systems, using sera and monoclonal antibodies (MAbs) selected to define neutralizing epitopes. Sera from rabbits and cattle, inoculated with intact BPV-1 or PBV-2 virions, neutralize BPV-1 infectivity in both C127 cells and xenografts. Selected human sera and murine MAbs that react with intact BPV-1 particles, serum of a rabbit immunized with denatured BPV-1 particles, and sera from calves vaccinated with a recombinant LI fusion protein did not neutralize BPV-1 infection in either system. It was concluded that neutralization of BPV-1 infection of C127 cells and bovine fetal skin xenografts by hyperimmune sera is specific and concordant for both assay systems, and involves conformational BPV-1 capsid epitopes.

Studies on vaccination against papillomaviruses: prophylactic and therapeutic vaccination with recombinant structural proteins

The L1 and L2 proteins of BPV-2 have been produced in Escherichia coli as beta-galactosidase fusion proteins. The fusion proteins have been used to vaccinate calves both prophylactically and therapeutically. The L1 fusion protein prevented tumor formation when administered before challenge with BPV-2, while the L2 fusion protein was very effective in promoting tumor rejection, independently from whether it was administered before or after challenge. Animals vaccinated with L1, but not with L2, responded rapidly with production of serum neutralizing antibodies, showing that this peptide contains B-cell-specific epitopes. The massive infiltration of lymphocytes in the tumors of L2-vaccinated animals suggests that the peptide contains epitopes specific for T-cells. The two structural proteins of BPV-2 therefore interact with both efferent arms of the immune system, and this observation allows the choice between two different types of antiviral vaccination.

The open reading frame L2 of cottontail rabbit papillomavirus contains antibody-inducing neutralizing epitopes

Polyclonal antisera were generated against bacterially derived fusion proteins of the open reading frames (ORFs) of the capsid proteins of cottontail rabbit papillomavirus (CRPV). The carboxy-terminal two-thirds of CRPV L1 and the carboxy-terminal half of CRPV L2 were cloned into a bacterial expression vector and induced proteins were used as antigen and immunogen. The polyclonal antisera were tested in a series of immunological assays, including ELISA, Western blot, and neutralization of CRPV. ELISA demonstrated that the polyclonal antisera raised against expressed L1 proteins reacted strongly to disrupted CRPV virion antigen and weakly both to intact CRPV virion and disrupted BPV-1 virion. Anti-CRPV L2 antisera reacted strongly only to intact and disrupted CRPV virion antigen. Viral capsid proteins of CRPV were detected in Western blots of HPV-11, BPV-1, and CRPV virus particles by these polyclonal antisera. The anti-L1 sera recognized the major capsid protein (60 kDa) and the anti-L2 sera identified a 76-kDa viral protein of CRPV. Only the antisera generated against expressed L2 neutralized CRPV. The neutralizing titer of the anti-L2 sera, however, was several orders of magnitude lower than the titer of a neutralizing polyclonal antiserum that was generated by immunizations with intact CRPV virions.

Comparison of human papillomavirus type 1 serotyping by monoclonal antibodies with genotyping by in situ hybridization of plantar warts

Thirty plantar warts were analyzed for the presence of HPV-1 type-specific and PV genus-specific capsid antigens by immunofluorescence (IF) using monoclonal and polyclonal antibodies and type-specific HPV-1 DNA employing in situ hybridization methods. Fifteen of 30 plantar warts were positive by IF for PV genus-specific structural viral antigens. Thirteen of the 15 productively infected plantar warts expressed intranuclear HPV-1 type-specific capsid antigens and viral DNA, which were detected in the same distribution in each individual wart. The 2 productively infected plantar warts that did not react with HPV-1 type-specific MoAbs did not react with HPV-1 type-specific DNA by in situ hybridization. Thus, serotyping of HPV-1 capsid antigens by monoclonal antibodies is concordant with genotyping of HPV-1 viral DNA by in situ hybridization in productively infected plantar warts.