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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.
A Protective and Broadly Cross-Neutralizing Epitope of Human Papillomavirus L2
Ratish Gambhira,1
Balasubramanyam Karanam,1
Subhashini Jagu,1
Jeffrey N. Roberts,4
Christopher B. Buck,4
Ioannis Bossis,1
Hannah Alphs,1
Timothy Culp,5
Neil D. Christensen,5 and
Richard B. S. Roden1,2,3*
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

ABSTRACT
We generated a monoclonal antibody, RG-1, that binds to highly
conserved L2 residues 17 to 36 and neutralizes human papillomavirus
16 (HPV16) and HPV18. Passive immunotherapy with RG-1 was protective
in mice. Antiserum to the HPV16 L2 peptide comprising residues
17 to 36 (peptide 17-36) neutralized pseudoviruses HPV5, HPV6,
HPV16, HPV 18, HPV31, HPV 45, HPV 52, HPV 58, bovine papillomavirus
1, and HPV11 native virions. Depletion of HPV16 L2 peptide 17-36-reactive
antibodies from cross-neutralizing rabbit and human L2-specific
sera abolished cross-neutralization and drastically reduced
neutralization of the cognate type. This cross-neutralization
of diverse HPVs associated with cervical cancer, genital warts,
and
epidermodysplasia verruciformis suggests the possibility
of a broadly protective, peptide-based vaccine.

TEXT
Minor capsid antigen L2 is a possible alternative to highly
multivalent L1 virus-like-particle (VLP) vaccines to obtain
broad protection against oncogenic human papillomaviruses (HPVs)
(
16). Vaccination with L2 as a full-length protein or as polypeptides
protects animals against homologous-type viral challenges at
both cutaneous and mucosal sites (
2-
4,
6,
12). Protection is
not mediated by cellular immunity, suggesting the importance
of neutralizing antibodies (
5,
7). L2 is subdominant in the
context of L1/L2 VLPs (
19), but antibodies elicited by recombinant
L2 immunogens are able to neutralize a remarkably broad range
of HPV genotypes (
15). This suggests that neutralizing epitopes
of L2 may be conserved across HPV types due to some critical
viral function (
13). Furthermore, it raises the possibility
that a single L2 protein- or peptide-based vaccine might provide
comprehensive protection against the HPV types causing genital
cancer and genital warts and possibly even those associated
with cutaneous warts and epidermodysplasia verruciformis (EV).
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).
Each MAb was screened for reactivity with 56 20-mer peptides
of HPV16 L2 that overlapped each other by 12 amino acids (Table
1). The neutralizing MAb RG-1 reacted with a peptide comprising
residues 17 to 36 of HPV 16 L2 (peptide 17-36) (Fig.
1A) but
not the overlapping peptides 9-28 and 25-44. Two of the four
other nonneutralizing, capsid-reactive MAbs recognized HPV16
L2 residues 89 to 100; one recognized residues 73 to 84 and
another recognized residues 33 to 52. RG-1 ascites exhibited
a titer of 1,024,000 in an HPV16 L1/L2 VLP ELISA and neutralized
both HPV16 and HPV18 pseudovirions with titers of 204,800 and
25,600, respectively, but failed to neutralize HPV5, HPV6, HPV45,
HPV52 or HPV58 or bovine papillomavirus 1 (BPV1) pseudovirions
at a titer of 40. Sequence comparison suggests that RG-1 recognizes
lysine at residue 20, which is conserved in HPV16 and HPV18
but different among other types that were not neutralized (R
or Q).
RG-1 may neutralize by blocking some critical interaction between
the highly conserved 17-36 region of L2 (Fig.
1A) and a cellular
factor (
1,
21,
22). Interestingly, HPV16 L2 residues 13 to 31
bind with a
Kd of

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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Depletion of region 17-36-specific antibodies eliminates cross-neutralization.
To study the contribution of the 17-36 epitope relative to others
in both homologous-type neutralization and cross-neutralization
of heterologous papillomavirus types, antibodies specific to
this region of L2 were depleted from L2 immune sera using an
L2 peptide 17-36-bound column. Depletion of HPV16 L2 peptide
17-36-reactive antibodies from a broadly cross-neutralizing
BPV1 L2 peptide 1-88 antiserum (
15) reduced the homologous BPV1
neutralization titers from 40,960 to 320 (Fig.
2A). We recovered
25% of the BPV1 neutralizing activity (after factoring for the
dilution that occurs upon elution of the peptide column). Notably,
depletion of HPV16 L2 peptide 17-36-reactive antibodies from
BPV1 L2 peptide 1-88 antiserum eliminated detectable cross-neutralization
of HPV16 and HPV18. Again, cross-neutralizing activity was recovered
from the fraction eluted from the peptide 17-36 column (Fig.
2B and C).
Chandrachud et al. demonstrated that vaccination with BPV4 L2
peptide 11-200, but not a C-terminal polypeptide, protects cattle
from experimental BPV4 challenge (
3). To address the relative
contributions of L2 peptide 17-36 and other neutralizing epitopes,
we performed similar depletion experiments with serum from a
rabbit vaccinated with HPV16 L2 peptide 11-200. The HPV16 neutralizing
titer of antiserum to HPV16 L2 peptide 11-200 drops from 40,960
to 80 upon depletion of HPV16 L2 peptide 17-36-reactive antibodies.
Again, after correcting for the dilution that occurs upon elution
of the peptide column, we recovered 25% of the homologous-type
HPV16 neutralizing activity. The failure to completely eliminate
neutralizing activity against the homologous virus is consistent
with previous descriptions of type-restricted neutralizing epitopes
outside of the L2 17-36 region (
10,
11). However, depletion
of the HPV16 L2 peptide 11-200 antiserum with an HPV16 L2 peptide
17-36 column removed detectable cross-neutralizing activity
against HPV18 (2,560 to <20), suggesting that the neutralizing
epitopes outside this 17-36 region are predominantly type specific.
After correcting for the dilution that occurs upon elution from
the peptide column, 25% of cognate as well as cross-neutralizing
activity was recovered. Finally, we wished to determine whether
L2 peptide 17-36 represents a neutralizing B-cell epitope in
humans. We utilized sera from a group of 24 HPV16-positive patients
with high-grade anogenital intraepithelial neoplasias (AGIN)
vaccinated three times at monthly intervals with 500 µg
of HPV16 L2E7E6 fusion protein (TA-CIN) without adjuvant and
bled 1 month later at week 12 (
20). Vaccination induced L2-specific
antibodies in five patients (Table
3). Only one patient, code
number 201, had no detectable HPV16-neutralizing antibodies
at week 0, whereas the serum after vaccination exhibited an
HPV16-neutralizing titer of 1,600 (Table
3). These neutralizing
antibodies correlated with the reactivity to HPV16 L2 determined
by ELISA (Table
3), and neither preimmunization nor postimmunization
sera from patient 201 exhibited antibodies to HPV16 L1 above
background levels (
20). The week 12 serum from patient 201 also
neutralized HPV18 with a titer of 100, whereas the week 0 serum
did not (Table
3). Depletion of this serum with an HPV16 L2
peptide 17-36 column removed detectable neutralizing activity
against HPV16 (Fig.
2F) and HPV18 (Fig.
2G), and 40% of this
neutralizing activity was recovered upon elution of the column.
This suggests that although HPV16 L2 peptide 108-120 can induce
cross-neutralizing antibodies in mice and humans (
8,
9), this
peptide may not contain optimal cross-neutralizing epitopes
(
10). Since the patient was vaccinated with a full-length HPV16
L2 fusion protein, this finding supports the importance of L2
peptide 17-36 as a neutralizing epitope in this patient. The
sera from five of five AGIN patients vaccinated with HPV16 L2E7E6
that reacted to full-length HPV16 L2 also reacted to HPV16 L2
peptide 17-36 (Table
3), suggesting that L2 peptide 17-36 is
a common human B-cell epitope.
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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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ACKNOWLEDGMENTS
The studies with animals and humans described herein were approved
by the Johns Hopkins University Animal Use and Welfare Committee
and the Johns Hopkins School of Medicine Institutional Review
Board, respectively.
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.

FOOTNOTES
* Corresponding author. Mailing address: Room 308, Cancer Research Building 2, Department of Pathology, Johns Hopkins School of Medicine, 1550 Orleans St., Baltimore, MD 21231. Phone: (410) 502-5161. Fax: (443) 287-4295. E-mail:
roden{at}jhmi.edu 
Published ahead of print on 10 October 2007. 

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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.
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