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J Virol, July 1998, p. 6151-6154, Vol. 72, No. 7
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Human Papillomavirus Type 11 Recombinant L1
Capsomeres Induce Virus-Neutralizing Antibodies
Robert C.
Rose,1,2,*
Wendy I.
White,3
Maolin
Li,4
JoAnn A.
Suzich,3
Christopher
Lane,1 and
Robert L.
Garcea4
Departments of
Medicine1 and
Microbiology and
Immunology,2
University of Rochester
School of Medicine and Dentistry, Rochester, New York 14642; MedImmune,
Inc., Gaithersburg, Maryland 208783; and
Section of Pediatric Hematology/Oncology, University of
Colorado School of Medicine, Denver, Colorado 802624
Received 7 January 1998/Accepted 27 March 1998
 |
ABSTRACT |
The human papillomavirus type 11 (HPV-11) L1 major capsid protein
can be trypsinized to generate recombinant capsomeres that retain HPV
genotype-restricted capsid antigenicity (M. Li, T. P. Cripe,
P. A. Estes, M. K. Lyon, R. C. Rose, and R. L. Garcea, J. Virol. 71:2988-2995, 1997). In the present study,
HPV-11 virion-neutralizing monoclonal antibodies H11.F1 and H11.H3,
previously characterized as recognizing two distinct HPV-11
capsid-neutralizing antigenic domains (S. W. Ludmerer, D. Benincasa, and G. E. Mark III, J. Virol. 70:4791-4794,
1996), were each found to be highly immunoreactive with
trypsin-generated capsomeres in an enzyme-linked immunosorbent assay
(ELISA). Capsomeres were used to generate high-titer polyclonal immune
sera that demonstrated HPV genotype-restricted reactivity by ELISA. The
capsomere antisera were then tested in an in vitro infectivity assay
and found to neutralize HPV-11 virions. In this assay, HPV-11 capsomere
polyclonal antisera exhibited neutralization titers (10
5
to 10
6) comparable to those obtained with a
virion-neutralizing antiserum raised previously against intact HPV-11
VLPs (R. C. Rose, R. C. Reichman, and W. Bonnez, J. Gen.
Virol. 75:2075-2079, 1994). These results indicate that highly
immunogenic, genotype-restricted HPV capsid-neutralizing antigenic
domains are contained entirely within capsomeres. Thus, capsomeres may
be viable vaccine candidates for the prevention of HPV disease.
 |
TEXT |
Papillomaviruses cause
hyperproliferative cutaneous and/or mucosal epithelial lesions in
higher vertebrates, including humans (31). More than 70 genotypically distinct human papillomaviruses (HPVs) have been
identified (12) and can be categorized on the basis of
observed differences in virus phenotype (i.e., preferred tissue
tropisms and/or disease associations). For example, most HPVs
preferentially infect cutaneous skin and usually cause only benign
disease (e.g., plantar or common warts), while other types more often
infect oral or anogenital mucosal epithelium. Mucosal epitheliotropic
HPVs have been associated with a variety of lesions, including benign
anogenital warts, premalignant intraepithelial neoplasias, and invasive
cancers, particularly of the uterine cervix (4, 23, 35).
These observations have focused attention on vaccine efforts to prevent
HPV infection.
HPV was first propagated outside the natural host in host-derived
epithelial xenografts implanted in immunodeficient mice (19). This advance resulted in the ability to produce
sufficient quantities of virions to allow the study of important viral
determinants of host immune responses (3, 10). However,
because it was initially possible to propagate virions of only one HPV
genotype in that system (i.e., HPV type 11 [HPV-11]), several groups
sought to reproduce the antigenic properties of intact virions by
producing empty capsids, or virus-like particles (VLPs), through
recombinant expression of the major capsid protein L1 (14, 16,
29). VLPs possess important antigenic features of native HPV
virions (7, 18, 29, 30). Such antigenicity depends upon
maintaining native virion structure, and VLPs have been shown to be
structurally identical to virions at a 35-Å resolution
(13). VLP vaccinations have been shown to stimulate immune
responses which protect animal hosts from diseases caused by
papillomaviruses (6, 17, 34). Thus, VLPs are promising
vaccine candidates for preventing HPV infection in humans (15,
33).
Papillomavirus virions have a T=7 icosahedral capsid comprised of 72 pentamers (i.e., capsomeres) of the major capsid protein L1
(2). As with the VP1 capsid protein of the related
polyomaviruses (21), several noncontiguous domains of the
papillomavirus L1 major capsid protein are likely exposed on the
surface of the virion, and determine the conformationally dependent
capsid-neutralizing antigenic domains of the virion. Recently, the
HPV-11 L1 protein was purified after expression in Escherichia
coli (20). This recombinant L1 protein was shown to be
capable of self-assembly into capsids in vitro and was also found to be
specifically sensitive to trypsin cleavage at R415 near the L1 carboxyl
terminus (20). The resulting digestion product is a
truncated L1 protein, which appears by electron microscopy as a
pentameric capsomere. Unlike capsomeres generated from HPV-11 L1
VLPs upon exposure to high concentrations of reducing agent
(25), capsomeres produced by trypsin digestion are unable to
reassemble into capsids (20). We previously demonstrated
that trypsin-generated HPV-11 capsomeres exhibit an antigenicity
comparable to that of intact HPV-11 VLPs when examined by ELISA with
polyclonal antisera generated against HPV-11 virions and recombinant
HPV-11, -16, and -18 VLPs (20). These results suggested that
capsomeres share strong antigenic similarities with native HPV-11
virions and intact VLPs, including genotype specificity. In the present
study, we further evaluated capsomere immunogenicity. Our results
indicate that HPV capsid-neutralizing antigenic domains are contained
entirely within capsomeres and that capsomeres induce the synthesis of
virus-neutralizing antibodies.
Preparation of HPV-11 L1 capsomeres.
Methods for the
purification of the recombinant HPV-11 L1 protein after expression in
Escherichia coli, as well as conditions for complete trypsin
digestion of L1 to capsomeres, have been previously described
(20). After trypsinization, the L1 protein was purified
further with DEAE-cellulose, followed by phosphocellulose chromatography in buffer A (10 mM Tris-HCl [pH 7.2], 5% glycerol, 2 mM EDTA, 15 mM 2-mercaptoethanol, 100 mM NaCl). In this buffer, the
trypsinized L1 was found in the flowthrough fractions from both columns
(trace amounts of residual undigested L1 eluted at salt concentrations
of between 0.5 and 1 M NaCl). Complete digestion of the L1 protein
eluting from the phosphocellulose column in buffer A was confirmed by
immunoblotting, and the absence of any assembly beyond capsomeres was
confirmed by electron microscopy. By these criteria, the purified L1
protein used for immunization was in capsomere form only and was
incapable of further assembly (20).
HPV-11 virion-neutralizing MAbs are immunoreactive with HPV-11 L1
capsomeres.
Ascites containing HPV-11 virion-neutralizing
monoclonal antibodies (MAbs) H11.F1 and H11.H3 (11)
(obtained from Neil Christensen) was diluted 1:128,000 and then
evaluated in an enzyme-linked immunosorbent assay (ELISA) against
trypsin-generated HPV-11 capsomeres. MAbs were tested in duplicate
wells containing 80 ng of purified capsomeres each. As a control, MAb
H16.V5 (8) (obtained from Chemicon International, Inc.,
Temecula, Calif.) was used at a dilution of 1:250. Previously, MAbs
H11.F1 and H11.H3 have been used to identify two conformationally
dependent neutralization sites on HPV-11 virions (11, 24).
The first site, recognized by MAb H11.F1, is centered at residues 131 and 132 on the HPV-11 L1 protein, and the second distinct, although
possibly overlapping, site is defined by MAb H11.H3 (24). We
tested these MAbs in an ELISA and found that both reacted strongly and
specifically with trypsin-generated HPV-11 capsomeres, whereas a third
MAb, raised against HPV-16 VLPs (H16.V5) (8), did not react
(Fig. 1). These results confirmed our
previous observation of strong H11.F1 reactivity with capsomeres generated via thiol reduction of VLPs (25) and indicated
that HPV-11 capsomeres contain both of the previously characterized HPV-11 capsid-neutralizing domains.

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FIG. 1.
HPV-11-neutralizing antibodies bind HPV-11 capsomeres.
H11.F1 and H11.H3 ascites were diluted 1:128,000 and then tested in an
ELISA against trypsin-generated HPV-11 capsomeres. As a control,
ascites containing an HPV-16 capsid-specific MAb (H16.V5)
(8) was assayed at a dilution of 1:250. OD, optical
density.
|
|
Capsomeres induce polyclonal antibodies that demonstrate HPV
genotype specificity.
Trypsin-generated HPV-11 capsomeres were
used to immunize each of two rabbits as previously described
(30), and the resulting antisera were tested in an ELISA
against the same antigen used for immunization and against intact VLPs
of HPV-11, -16, and -18 (Fig. 2). The
capsomere antibodies reacted strongly with both the homologous
capsomere immunogen and with intact HPV-11 VLPs, but not with HPV-16
VLPs. Thus, the capsomere antisera were generally HPV type specific.
However, both of the HPV-11 capsomere antisera cross-reacted slightly
at low dilutions with HPV-18 VLPs (Fig. 2). A similar pattern of
antigenic cross-reactivity was observed when these antisera were tested
against HPV-11, -16, and -18 recombinant L1 proteins in a Western blot
immunoassay (data not shown). These findings suggest that one or more
conserved linear epitopes of the HPV-11 L1 protein may become
accessible to the immune system when capsomeres are presented in an
unassembled state.

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FIG. 2.
HPV-11 capsomeres are highly immunogenic and display HPV
genotype-restricted antigenicity. Threefold serial dilutions of rabbit
capsomere polyclonal antisera R6-311 (solid symbols) and R6-312 (open
symbols) were tested in an ELISA in duplicate against HPV-11 capsomeres
(triangles), HPV-11 VLPs (squares), HPV-16 VLPs (circles), and HPV-18
VLPs (diamonds).
|
|
Capsomeres elicit virus-neutralizing antibodies.
Capsomere
polyclonal antisera were also evaluated for virus-neutralizing activity
in an in vitro infectivity assay (32). The reverse
transcriptase-PCR (RT-PCR) assay was carried out with an
HPV-11Hershey virus stock (19) (obtained from
John Kreider) and HaCaT cells, an immortalized human keratinocyte cell
line (5) (obtained from Norbert Fusenig), as described
previously (32) with modifications (25). Samples
were analyzed by RT-PCR for the presence of HPV-11 E1^E4 spliced
mRNA, and, as a control, for the presence of spliced cellular
-actin
mRNA. Previous work conducted with an in vivo infectivity model has
shown that HPV-11 virions and recombinant HPV-11 VLPs induce antibodies
that neutralize infectious HPV-11 virions (3, 9, 10, 30). In
the in vitro assay used in the present study (32), infection
is defined as generation of PCR-detectable early viral transcripts in
cells incubated with virus. Neutralizing titer is defined as the
highest dilution of serum which, when preincubated with virus, prevents the detection of viral transcripts. Titration of the virus stock used
in these experiments on multiple occasions resulted in the consistent
detection of the HPV-11 E1^E4 spliced mRNA with dilutions from
1:102 (the lowest dilution tested) to 1:105.
The virus-specific RNA product was not amplified from cells exposed to
virus stock diluted 1:106. Therefore, the neutralization
assay was conducted with a 1:104 dilution of virus stock.
This allowed for consistent assay results while conserving the virus,
which is in limited supply. As shown in Fig.
3, HPV-11 capsomere antibodies
efficiently neutralized HPV-11 virions (lanes 4 to 8), whereas
preimmune serum (lane 2) and HPV-16 VLP postimmune serum (lane 3)
demonstrated no neutralizing activity. Anticapsomere neutralization
titers were comparable to that of an antiserum raised against intact
HPV-11 VLPs (i.e., 10
5 to 10
6) (compare
lanes 4 to 8 with lanes 9 to 13), which was produced by the same
immunization protocol (30). Thus, capsomeres and VLPs differ
structurally but are immunologically equivalent.

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FIG. 3.
HPV-11 capsomeres induce virus-neutralizing antibodies.
(A) HPV-11 virions were incubated with antiserum before addition to
HaCaT cells. Six days postinfection, cells were harvested, and the
presence of an E1^E4 spliced message, diagnostic of HPV-11 infection
(3), was determined by RT-PCR. PCR products were separated
on 2% agarose gels, stained with ethidium bromide, and examined under
UV light for the presence of the ~0.6-kb E1^E4 band. Lane 1, HaCaT
cells alone, no virus, no antibodies (negative control); lane 2, HPV-11
virions plus preimmune serum from an animal subsequently immunized with
HPV-11 capsomeres (10 3 dilution); lane 3, HPV-11 virions
plus HPV-16 L1 VLP postimmune serum (10 3 dilution); lanes
4 to 8, HPV-11 virions plus HPV-11 capsomere antiserum (dilutions
10 3 to 10 7, respectively); lanes 9 to 13, HPV-11 virions plus HPV-11 L1 VLP antiserum (dilutions
10 3 to 10 7, respectively). A second rabbit
anti-HPV-11 capsomere antiserum tested produced comparable results
(i.e., a neutralization titer of 10 5) (data not shown).
(B) PCR amplification of -actin was performed on all cDNA samples as
an internal control. The expected size of the -actin band is ~0.6
kb. The lanes are the same as in panel A. The HPV-11Hershey
virus stock was titrated on HaCat cells on multiple occasions prior to
being used in the present study. The titer of the inoculum (i.e., the
last dilution which resulted in the detection of spliced E1^E4 mRNA
by RT-PCR) consistently was between 10 5 and
10 6. In the present study, the inoculum was used at a
dilution of 10 4.
|
|
Results from several studies indicate that the papillomavirus L1 capsid
protein is a major neutralization antigen (
6,
9,
17,
22,
30,
34). For this reason, significant effort has
been directed
towards producing recombinant L1 preparations that
maintain the
antigenic characteristics of native HPV virions.
The present results
indicate that a smaller unit of the virus
capsid, the capsomere, is
sufficient to reproduce important antigenic
features of the native
virion. Capsomere polyclonal antibody recognition
patterns and virion
neutralization titers paralleled those found
with antibodies raised
against native virions or recombinant VLPs.
Because the capsomeres used
to generate these antibodies lacked
86 amino acids from the L1
carboxyl-terminal arm, which mediates
contacts between pentamers
(
20), these results cannot be attributed
to either partial
or complete reassembly of capsomeres into capsids.
Thus, at least two
HPV capsid-neutralizing antigenic domains,
defined previously by MAbs
(
24), are contained entirely within
pentameric L1
capsomeres, and interpentamer associations are not
required for the
induction of virus-neutralizing antibodies.
Unexpectedly, ELISA results also indicated that HPV-11 capsomere
antibodies cross-react at low dilutions with HPV-18, but
not with
HPV-16 VLPs. This cross-reactivity may be due to the
exposure in
isolated capsomeres of a conserved linear epitope
of L1, because a
similar pattern of cross-reactivity was also
seen in a Western blot
immunoassay (data not shown). Since previously
reported results
indicate a minimal level of antigenic cross-reactivity
between VLPs of
HPV-11 and HPV-18 (references
27 and
28 and
unpublished observations), the present
results suggest that a
linear epitope of HPV-11 L1 conserved with
HPV-18 may become more
accessible to the immune system when capsomeres
are presented
in isolation. Unmasking of antigenically cross-reactive
L1 epitopes
may therefore occur when capsomeres are used as antigens
for serologic
investigations.
Since the first reports describing isolation of HPV VLPs, it has been
speculated that these particles might form the basis
for an efficacious
vaccine to prevent HPV infection. The observation
that capsomeres
display immunogenic, virus-neutralizing epitopes
found on intact VLPs
suggests that capsomeres might also be viable
vaccine candidates.
Capsomeres are homogeneous in size (
20)
and may prove to be
more stable and less costly to produce than
VLPs. Thus, they may be an
attractive alternative to the use of
VLPs as an immunogen for
preventing HPV infection. Although it
was not the purpose of the
present study to make a dose-response
comparison between VLPs and
capsomeres, it will be important to
assess the relative amplitude of
the response against each of
these antigens to determine the relative
efficacy of capsomeres
as vaccine antigens. Studies with different
adjuvants and species,
as well as various protein concentrations, will
be required. The
growing consensus that effective HPV vaccines may
reduce the incidence
of uterine cervical carcinoma (
1,
26)
underscores the importance
of evaluating capsomere efficacy for
immunoprophylaxis against
genital HPV disease.
 |
ACKNOWLEDGMENTS |
We thank Neil Christensen for the gift of H11.F1 and H11.H3 MAbs.
This work was supported in part by grant CA37667 from the National
Cancer Institute (R.L.G.).
 |
FOOTNOTES |
*
Corresponding author. Mailing address: University of
Rochester School of Medicine, Department of Medicine, Box 689, 601 Elmwood Ave., Rochester, NY 14642. Phone: (716) 275-5871. Fax: (716)
442-9328. E-mail: rrose{at}medicine.rochester.edu.
 |
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