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Journal of Virology, December 2007, p. 13927-13931, Vol. 81, No. 24
0022-538X/07/$08.00+0 doi:10.1128/JVI.00936-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

Departments of Pathology,1 Oncology,2 Gynecology & Obstetrics, The Johns Hopkins University, Baltimore, Maryland 21231,3 Laboratory of Cellular Oncology, National Cancer Institute, Bethesda, Maryland 20892-4263,4 Department of Microbiology and Immunology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania5
Received 1 May 2007/ Accepted 28 September 2007
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Identification of neutralizing epitopes within HPV16 L2.
The rational design of a broadly protective L2-based preventive vaccine requires knowledge of the relevant neutralizing epitopes. To identify the neutralizing epitopes in L2, we vaccinated BALB/c mice with full-length six-His-tagged HPV16 L2 protein and produced hybridomas by using standard procedures (18). Of the 100 supernatants reactive with L2 protein, only 45 reacted with HPV16 L1/L2 pseudovirions, and only one (RG-1) neutralized HPV16 pseudovirus and was cloned. The RG-1 supernatant exhibited a neutralizing titer of 1,280 and also reacted with HPV16 L1/L2 pseudivirions by an enzyme-linked immunosorbent assay (ELISA). RG-1 and another four monoclonal antibodies (MAbs) that showed the highest ELISA reactivities with HPV16 pseudovirions were all the immunoglobulin G1(
) [IgG1(
)] isotype and reacted with HPV16 L2 protein by Western blotting (Table 1).
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TABLE 1. Capsid surface reactivity and neutralizing activity of HPV16 L2 MAbsa
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FIG. 1. The RG-1 neutralizing MAb recognizes the evolutionarily conserved L2 17-36 motif and provides passive immunity. (A) CLUSTAL W homology comparison of residues 17 to 36 of HPV16 L2 peptide and L2 sequences from different papillomavirus types. The HPV16 L2 sequence comprising amino acids 17 to 36 is highly conserved among different types and exhibits 78% identity with the L2 sequences from HPV2 (skin type), HPV5 (EV related), and HPV45; 80% identity with HPV6 and HPV11 L2 (benign types); and 84% identity with HPV18 (high-risk type). In contrast, L2 as a whole exhibits only 25% conservation among these types. This sequence was conserved even in BPV1, which is evolutionarily distant from high-risk HPV. (B) Peptide ELISA using monoclonal antibody and polyclonal antiserum to HPV16 L2 peptide 17-36. Wells were coated with synthetic peptide 1 (ASATQLYKVVKQAGTCPPD), comprising HPV16 L2 residues 13 to 31 in which residues 21 and 22 are both changed to valine (in bold); 2 (TASADPCPKQLYKCQATGT), comprising HPV16 L2 residues 13 to 31 in which the amino acid order is scrambled; 3 (ASATQLYKTCKQAGTCPPD), comprising wild-type HPV16 L2 residues 13 to 31; or 4 (ASATQAAKTCKQAGTCPPD) comprising HPV16 L2 residues 13 to 31 in which residues 18 and 19 are both replaced with alanine (in bold). Plates were probed with RG-1, MAb C9, an irrelevant isotype-matched MAb, or rabbit antiserum to HPV16 L2 peptide 17-36 as indicated. (C) Blockade of RG-1 neutralization of HPV16 pseudovirus by titrations of synthetic peptide comprising wild-type HPV16 L2 residues 17 to 36 and the peptides described above for panel B. (D) Passive transfer of RG-1 (n = 5), but not an irrelevant isotype-matched control MAb (n = 6) or phosphate-buffered saline (PBS) (n = 5), protects mice from cutaneous challenge with HPV16 pseudovirus (see http://home.ccr.cancer.gov/lco/ for plasmid maps and production methods). A patch on the belly of each anesthetized BALB/c mouse was shaved with an electric razor without traumatizing the epithelium. MAb was injected (100 µg intraperitoneally) 5 h prior to challenge by application to the shaved skin of 3 x 109 HPV16 pseudovirions (100 ng) in 10 µl of 0.6% carboxymethylcellulose (Sigma C5013) containing L1 and L2 (or L1 alone for background determination) and carrying an encapsidated luciferase reporter construct. Three days later, the mice were anesthetized and injected with luciferin (100 µl at 7 mg/ml), and their images were acquired for 10 min with a Xenogen IVIS 200. Equally sized areas encompassing the site of inoculation were analyzed using Living Image 2.20 software, and background was determined by challenge with noninfectious HPV pseudovirions lacking L2. A representative image is shown in panel D, and the bioluminescence data for each group are plotted in panel E. OD, optical density.
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1 nM to a cell surface receptor, and mutation of L2 residues 18 and 19 or 21 and 22 disrupted both L2 binding to the cell surface and viral infection (21). RG-1 bound to both wild-type HPV16 L2 peptides 13-31 and 17-36 and the 18A-19A mutant, but neither the 21V-22V mutant nor the scrambled-sequence peptides were recognized (Fig. 1B). Similarly, wild-type HPV16 L2 peptides 13-31 and 17-36 and the 18A-19A mutant but neither the 21V-22V mutant nor the scrambled-sequence peptides blocked the neutralization of HPV16 pseudovirions by RG-1 (Fig. 1C), suggesting that its epitope overlaps the surface-binding motif of HPV16 L2 (21). Passive immunization with RG-1 protects mice against HPV16 pseudovirus challenge. It is unclear whether L2-specific neutralizing antibodies are sufficient to mediate protection. HPV16 pseudovirus containing the cottontail rabbit papillomavirus (CRPV) genome infects and induces cutaneous warts in domestic rabbits (14), and HPV16 pseudovirus also infects mouse C127 cells (17). Therefore, we tested the ability of HPV16 pseudovirus carrying the luciferase reporter gene to infect cutaneous epithelium in mice (Fig. 1D). Vaccination of mice with HPV16 L1 VLPs, but not HPV45 L1 VLPs, reduced infection to background levels (as determined using noninfectious pseudovirus lacking L2 as a control [17]), demonstrating type-restricted protection (not shown). To test whether passive immunotherapy with RG-1 confers protection, 100 µg of RG-1, an isotype-matched irrelevant MAb, or phosphate-buffered saline was administered intraperitoneally to naïve mice 5 h prior to HPV16 pseudovirus challenge. Administration of RG-1, but not the isotype-matched control antibody, protected the mice from cutaneous HPV16 pseudovirus challenge (P < 0.001, analysis of variance) (Fig. 1E). The mice receiving RG-1 had a serum HPV16 neutralizing titer of 6,400 at the time of challenge.
Pseudovirus and native HPV11 virus-based neutralization with the HPV16 L2 peptide 17-36 antiserum.
Since our aim was to identify a broadly neutralizing epitope and the HPV16 L2 peptide 17-36 was well conserved among different HPVs (Fig. 1A), we immunized a rabbit with HPV16 L2 peptide 17-36 coupled to keyhole limpet hemocyanin. The rabbit antiserum was analyzed by a six-His HPV16 L2 protein ELISA as well as an HPV16 L1/L2 pseudovirion ELISA. The final bleed sample had ELISA titers of 128,000 against L2 protein and 16,000 to L1/L2 pseudovirus (not shown), whereas the preimmunization serum exhibited background reactivities in both assays (not shown). The HPV16 L2 peptide 17-36 antiserum bound to HPV16 L2 peptides17-36 and 13-31 and both mutant peptides but not the scrambled-sequence peptide (Fig. 1B). The HPV16 L2 peptide 17-36 antiserum, but not the preimmunization serum, broadly neutralized the following: HPV16 pseudovirions (titer, 3,200) and pseudovirions from all of the other five oncogenic types tested (that together account for
85% of cervical cancer), the benign mucosal type HPV6 (titer, 200), the cutaneous EV type HPV5 (titer, 200), and the evolutionarily divergent BPV1 (titer, 800), but not CRPV (titer, <50), indicating specificity (Table 2). To extend our observation of cross-neutralization to an additional HPV type and to eliminate potential artifacts in the pseudovirus system, the peptide antiserum was tested at 1:50 for neutralization activity against native HPV11 virus derived from a human xenograft (15). E1
E4 early spliced transcripts were observed in cells exposed to HPV11 treated with a 1:50 dilution of preimmunization serum, demonstrating infection, whereas L2 peptide 17-36 antiserum completely neutralized infection at this concentration (not shown). Thus, L2 peptide 17-36 antiserum neutralized very divergent cutaneous and mucosal HPV types associated with EV, condylomata accuminata, and genital cancers.
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TABLE 2. Neutralizing titers of HPV16 L2 peptide 17-36 antiserum for pseudovirions from different papillomaviruses
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FIG. 2. Depletion of anti-HPV16 L2 peptide 17-36 antibodies from L2 immune serum abolishes cross-neutralization. Rabbit antisera to BPV1 L2 peptide 1-88 (A to C) or HPV16 L2 peptide 11-200 (D and E) and serum of an HPV16-positive AGIN patient (no. 201) who had been vaccinated with HPV16 L2E7E6 L2 (F and G) were depleted of HPV16 L2 peptide 17-36-specific antibodies by use of a peptide column. Antibodies bound to the column were recovered by elution at low pH and brought back to neutral pH. The sera, both before and after depletion, as well as the recovered antibodies were tested for neutralizing titers for BPV1, HPV16, or HPV18 pseudovirions, as indicated. The serum dilutions for the antibody recovered from the column are not corrected for the dilution that occurs during their elution and return to neutral pH. Neutralization of BPV1 pseudovirions by BPV1 L2 peptide 1-88 antiserum (A), HPV16 pseudovirions by BPV1 L2 peptide 1-88 antiserum (B), HPV18 pseudovirions by BPV1 L2 peptide 1-88 antiserum (C), HPV16 pseudovirions by HPV16 L2 peptide 11-200 antiserum (D), HPV18 pseudovirions by HPV16 L2 peptide 11-200 antiserum (E), HPV16 pseudovirions by immune serum from a patient vaccinated with HPV16 L2E7E6 (F), and HPV18 pseudovirions by immune serum from a patient vaccinated with HPV16 L2E7E6 (G) is shown. OD, optical density.
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TABLE 3. In vitro HPV neutralization titers of sera from AGIN patients who were vaccinated with TA-CIN and mounted an HPV16 L2-specific antibody responsea
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This research was supported by grants to R.B.S.R. and N.D.C. from the PHS (National Cancer Institute, SPORE in Cervical Cancer, P50 CA098252) and to both J.N.R. and C.B.B. by the National Institutes of Health intramural research program.
We thank Liz Rollinson (Xenova Pharmaceuticals plc) for TA-CIN trial samples, Chien-fu Hung and Daejin Kim (Johns Hopkins University) for help with imaging, John Schiller and Doug Lowy (NCI, NIH) for reagents, Martin Müller (DKFZ, Germany) for codon-modified HPV16 L1 and L2, Tadahito Kanda (National Institute of Infectious Diseases, Japan) for codon-modified HPV52 and HPV58 L1 and L2, and Lou Laimins (Northwestern University, Chicago) for codon-modified HPV31 L1 and L2.
R.B.S.R. is a paid consultant of Knobbe, Martens, Olson and Bear LLC.
Published ahead of print on 10 October 2007. ![]()
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