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Infection and Immunity, March 1999, p. 1511-1516, Vol. 67, No. 3
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Surface Expression of a Protective Recombinant
Pertussis Toxin S1 Subunit Fragment in Streptococcus
gordonii
Song F.
Lee,1,2,*
Robert J.
March,2
Scott A.
Halperin,2,3
Gary
Faulkner,2 and
Lingqiu
Gao1
Department of Applied Oral Sciences, Faculty
of Dentistry,1 Department of
Microbiology and Immunology,2 and
Department of Pediatrics, Faculty of
Medicine,3 Dalhousie University, Halifax, Nova
Scotia, Canada B3H 3J5
Received 12 August 1998/Returned for modification 4 November
1998/Accepted 9 December 1998
 |
ABSTRACT |
In this study, the expression of the Bordetella
pertussis S1 subunit was tested in Streptococcus
gordonii, a commensal oral bacterium which has the potential to
be a live oral vaccine vehicle. The DNA fragment encoding the
N-terminal 179 amino acids of the S1 subunit was ligated into the
middle part of spaP, the surface protein antigen P1 gene
originating from Streptococcus mutans. The resulting
construct, carried on the Escherichia coli-Streptococcus shuttle vector pDL276, was introduced into S. gordonii DL-1
by natural transformation. One of the transformants (RJMIII) produced a
187-kDa protein (the predicted size of the SpaP-S1 fusion protein) which was recognized by both the anti-pertussis toxin (anti-PT) and
anti-SpaP antibodies, suggesting that an in-frame fusion had been made.
Results from immunogold-electron microscopic studies and cellular
fractionation studies showed that the fusion protein was surface
localized and was mainly associated with the cell wall of RJMIII,
indicating that SpaP was able to direct the fusion protein to the cell
surface. A rabbit antiserum raised against heat-killed S. gordonii RJMIII recognized the native S1 subunit of PT in Western
blotting and showed a weak neutralization titer to PT by the Chinese
hamster ovary cell-clustering assay. BALB/c mice immunized with the
heat-killed S. gordonii RJMIII were protected from the
toxic effect of PT in the leukocytosis-promoting and histamine
sensitization assays. In conclusion, a fragment of the S1 subunit of PT
was successfully surface expressed in S. gordonii; the
recombinant S1 fragment was found to be immunogenic and could induce
protection against the toxic effect of PT in mice.
 |
TEXT |
Pertussis toxin (PT) is a major
virulence factor of Bordetella pertussis (25) and
is one of the prominent components of acellular pertussis vaccines. PT
is an AB toxin, with the A promoter (S1 subunit) being the toxic
subunit and the B oligomer being the pentamer that binds to the surface
receptors on eucaryotic cells and translocates the toxic subunit across
the cell membrane (23). The mature S1 subunit contains 234 amino acids (14) and is immunodominant (5).
Antibodies against the S1 subunit have been shown to neutralize the
toxin in vitro and protect mice from B. pertussis infection
in aerosol and intracerebral challenges (7, 21, 22). The B
oligomer is composed of one subunit each of S2, S3, and S5 and two
subunits of S4. S2 and S3 mediate adherence of the toxin to host cells.
Antibodies to B oligomer or S2 and S3 subunits confer protection
against B. pertussis infection in animal models but do so
less effectively than antibodies to S1 (7).
The cloning and expression of the S1 subunit in bacteria have been
confined mainly to gram-negative bacteria such as Escherichia coli (2, 3, 24) and vaccine strains of Salmonella
typhimurium (4, 24). These reports demonstrated that
the recombinant S1 is immunogenic, but protective antibodies either
were not present in the anti-recombinant S1 antisera or were present at
low levels. The expression of S1 in gram-positive bacteria, however,
has been limited to Bacillus subtilis (18, 20)
and Streptomyces lividans (17). In both of these
cases, the S1 subunit was expressed as a soluble extracellular protein.
In B. subtilis, the recombinant S1 was found to be
immunogenic in animals, but whether the recombinant S1 could induce
protective antibodies has not been demonstrated (18). In
S. lividans, the recombinant S1 was extensively degraded by proteases.
The oral commensal bacterium Streptococcus gordonii has
recently been suggested to be a potential candidate as a live oral vaccine expression vehicle (15, 16). As a first step towards investigating the possibility of generating a live oral vaccine against
pertussis, we report in this study the expression of the N-terminal
179-amino-acid fragment of S1 in S. gordonii by using the
major surface protein antigen P1 gene (spaP) originating
from Streptococcus mutans. The immunogenicity and protective
effects of the anti-recombinant S1 antibodies against native PT were investigated.
Construction of SpaP-S1 fusion protein.
The strategy used to
construct the in-frame fusion between S. mutans SpaP
(antigen P1) and the B. pertussis PT S1 subunit is depicted
in Fig. 1. The initial
gene fusion was constructed on a pUC 18-based plasmid to create pRJMI.
To facilitate the expression in streptococci and to avoid the use of
the Ampr marker, the fusion gene was cloned into pDL276, an
E. coli-streptococcus shuttle vector (6),
creating pRJMII. pRJMII was introduced into S. gordonii DL-1
by natural transformation (8). Transformants were selected
on Todd-Hewitt agar containing 250 µg of kanamycin/ml. Several
transformants were obtained. When these transformants were treated with
mutanolysin (8), followed by boiling with the sodium dodecyl
sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) sample buffer of
Laemmli (11), all the transformants were found to produce a
98-kDa protein band recognized by the rabbit anti-PT antibodies (see
below) in Western immunoblotting. This immunoreactive protein matched
the predicted size of SpaP-S1 carried on pRJMII. However, when the
transformants were analyzed by whole (intact)-cell enzyme-linked
immunosorbent assay (ELISA) (8) and immunoelectron
microscopy, none of them showed an appreciable amount of the fusion
protein on the cell surface. Since S. gordonii DL-1 produces
a number of high-molecular-weight (ca. 190- to 259-kDa) surface
proteins (9), the SpaP-S1 fusion protein expressed from
pRJMII may be buried among these proteins. Hence, pRJMIII was further
constructed by placing the S1 fragment close to the middle part of
SpaP. In the construction, we made use of the unique NruI
site within the S1 sequence. Hence, the final fusion protein contained
only the first 179 amino acids of S1 inserted into the complete SpaP,
creating a predicted mature protein of ca. 187 kDa. Initial
immunoblotting of lysates of E. coli HB101 carrying pRJMIII
indicated the reactivity of a ca. 187-kDa protein band with the anti-PT
antibodies, suggesting that correct fusion had been made (data not
shown). pRJMIII was transformed into S. gordonii DL-1, and
one of the transformants, S. gordonii RJMIII, was chosen for
further studies.

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FIG. 1.
Schematic diagram showing the construction of the
spaP-s1 fusion gene. The S1 gene (coding for amino acid
residues 2 to 233 of the mature S1) was amplified by PCR from the PT
operon carried on pPTX42 (E. coli ATCC 67046 [14]) by standard methods (19) using the
primers 5'-GATCCTCCCGCCACCGT-3' and
5'-GGATCGATAACGAATACGCGATGCT-3'. (The underlined
bases are added sequence for a ClaI site.) The amplicon was
treated with Klenow fragment, restricted with ClaI, purified
from agarose gels with a Gene-Clean kit (Bio 101, La Jolla, Calif.),
and ligated into the EcoRV-ClaI sites of pN1C4, a
pUC18 derivative carrying the 3' DNA of spaP from S. mutans, coding for the C-terminal 144 amino acids containing the
surface protein anchoring domain of SpaP (8). The ligated
DNA was transformed into competent E. coli HB101, and the
resulting plasmid was designated pPTS1. To provide the fusion gene with
the spaP promoter, the 1.5-kb
SmaI-XbaI fragment from pPTS1 was cloned into the
EcoRV-XbaI sites of pSMI/II, a pUC18 derivative
carrying the complete spaP gene (10). The
resulting plasmid isolated from one of the E. coli HB101
transformants was named pRJMI. The 5.6-kb
KpnI-ScaI fragment from pRJMI was further cloned
into pDL276, creating pRJMII. pRJMIII was further constructed by
ligating the 10.0-kb NruI-KpnI fragment from
pRJMII to the 4.5-kb EcoRV-KpnI fragment from
pSMI/II.
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The anti-PT antibodies used were generated by immunizing two New
Zealand White rabbits with 0.5 ml of a whole-cell B. pertussis vaccine (Connaught Laboratories Ltd., North York,
Ontario, Canada) by subcutaneous injections, followed by booster
injections with the same vaccine on days 20, 27, and 48 and a final
booster with 110 µg of purified PT in Freund's incomplete adjuvant
(kindly provided by S. Loosmore, Connaught Laboratories Ltd.) on day
65. The titers of the antisera against purified PT (20 ng/well; List Biological Laboratories, Inc., Campbell, Calif.) were determined by an
ELISA using methods described previously (13). The titers of
anti-PT antibodies were expressed as the reciprocals of the dilutions
which produced A405 readings 0.05 higher than
the preimmune sera. The titers were 12,800 and 6,400 for the sera from
the two rabbits. To eliminate cross-reactivity, pooled sera were
adsorbed with S. gordonii DL-1 cells before use. To achieve
this, 1 liter of S. gordonii DL-1 grown in TYG (1%
tryptone, 0.5% yeast extract, 0.3% K2HPO4,
0.2% glucose [wt/vol]) to the late-exponential phase of growth was
centrifuged (at 10,000 × g for 15 min at 4°C). The cells were washed once with phosphate-buffered saline (PBS) and resuspended in 15 ml of PBS. Half of the cell suspension was incubated with 5 ml of the anti-PT rabbit sera at 4°C for 1 h with
occasional mixing. The suspension was centrifuged, and the remaining
half of the cells was added to the supernatant and incubated for an additional hour. The suspension was again clarified by centrifugation, and the supernatant was used as adsorbed anti-PT antibodies.
Expression and localization of SpaP-S1 fusion protein in S. gordonii.
When culture supernatant fluid and cell extracts of
S. gordonii RJMIII were analyzed by immunoblotting, a
strong, 187-kDa protein band and a weaker, 155-kDa band from the cell
extracts were recognized by an anti-SpaP monoclonal antibody (Fig.
2, left panel, lane 1). The same 187-kDa
band from the cell extract was also recognized by the adsorbed anti-PT
antibodies (Fig. 2, right panel). Samples obtained from S. gordonii DL-1 did not react with either of the antibodies. Samples
prepared from S. gordonii DL-1/SMI/II-3 (8), a
DL-1 transformant carrying the spaP gene, showed reaction with the anti-SpaP antibody, but not with the anti-PT antibodies. These
results strongly indicate that S. gordonii RJMIII is
expressing the correct SpaP-S1 fusion protein and that the fusion
protein is mainly cell associated.

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FIG. 2.
Western blots of S. gordonii expressing the
SpaP-S1 fusion protein. (Left) Proteins reacted with the anti-SpaP
monoclonal antibody 4-10A (dilution, 1/7000 [1]).
(Right) Proteins reacted with the S. gordonii-adsorbed
rabbit anti-PT antibodies (dilution, 1/100). Lane 1, recombinant
SpaP-S1 S. gordonii RJMIII; lane 2, recombinant S. gordonii DL-1/SMI/II-3 expressing only SpaP; lane 3, parent
S. gordonii DL-1. S, proteins from culture supernatant
fluids (20 µl); C, proteins extracted from cells by boiling SDS-PAGE
sample buffer (20 µl). Arrow indicates the ca. 187-kDa SpaP-S1 fusion
protein revealed by the anti-PT antibodies. Numbers on the left are
molecular size markers (in kilodaltons).
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To provide evidence that the fusion protein is surface localized,
S. gordonii RJMIII was analyzed by immunoelectron
microscopy.
Briefly, cells were harvested from 1.5 ml of a
late-exponentially
grown Todd-Hewitt culture, washed twice in PBS,
resuspended, and
incubated in 0.5 ml of 1% (wt/vol) gelatin in PBS at
room temperature
for 10 min. The cells were further treated with 20 mM
glycine
in PBS for 3 min and 1% (wt/vol) bovine serum albumin (BSA) in
PBS for 2 min. After these blocking steps, the cells were resuspended
in 100 µl of
S. gordonii-adsorbed anti-PT antibodies
(dilution,
1/50) or a monoclonal antibody, 4-10A, to SpaP (dilution,
1/100)
and were incubated with gentle rocking for 2 h. The cells
were
washed 4 times with PBS containing 0.1% BSA and were reacted with
colloidal gold (diameter, 10 mm)-conjugated goat anti-rabbit
immunoglobulin
G or goat anti-mouse immunoglobulin G (dilution, 1/20)
for 1 h.
The cells were then washed 4 times with PBS-0.1% BSA,
followed
by two washes with PBS, and were finally fixed with 2.5%
glutaraldehyde
in 0.1 M sodium cacodylate buffer, pH 7.4. Cells were
then sectioned
and viewed under a Philips 300 electron microscope. As
shown in
Fig.
3, the fusion protein was
clearly found on the cell surface
after the cells had been reacted with
the anti-SpaP monoclonal
antibody (Fig.
3a) or the anti-PT antibodies
(Fig.
3b). In contrast,
DL-1 cells treated the same way did not exhibit
any immunogold
particles. To further examine the location of the fusion
protein
expressed in
S. gordonii RJMIII, cellular fractions
were prepared
from 1 liter of late-exponential culture grown in TYG.
Proteins
in the supernatant fluids were precipitated with
(NH
2)SO
4 (70%
[wt/vol]) and dialyzed against
10 mM Tris buffer, pH 8 (final
volume, 15 ml). Cells were washed and
broken with a Mickle tissue
disintegrator (
8). Cell walls
were recovered by centrifugation
(at 27,000 ×
g for 30 min at 4°C) and resuspended in 1 ml of 10
mM potassium phosphate
buffer, pH 7.5. The cell membrane was separated
from the cytoplasm (12 ml) by ultracentrifugation (at 50,000 ×
g for 1 h
at 4°C) and resuspended in 0.25 ml of potassium phosphate
buffer.
Samples of 15, 1, 12, and 0.25 µl of the extracellular
fraction
(supernatant), cell wall, cytoplasm, and cell membrane,
respectively,
were analyzed by Western immunoblotting. These volumes
represented
proportional amounts of each of the subcellular fractions
from the
culture. SpaP-S1 was detected by the
S. gordonii-adsorbed
rabbit anti-PT antibodies (dilution, 1/200). Results showed that
the
fusion protein was mainly associated with the cell wall, with
a small
amount found in the extracellular fraction and trace amounts
in the
cell membrane and cytoplasmic fractions (data not shown).
These results
strongly indicate that the fusion protein was cell
surface localized in
S. gordonii RJMIII, and they reconfirm the
ability of the C
terminus of
S. mutans SpaP to anchor to the cell
wall in
S. gordonii (
8).

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FIG. 3.
Immunogold electron micrographs of S. gordonii expressing SpaP-S1 fusion protein on the cell surface.
After reaction with the monoclonal anti-SpaP antibody (a and b) and the
anti-PT antibodies (c and d), the recombinant SpaP-S1 S. gordonii RJMIII cells (a and c) were labeled with gold conjugates,
while the parent S. gordonii DL-1 cells (b and d) were
not.
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Immunogenicity of the recombinant S1.
The immunogenicity of
the SpaP-S1 fusion protein was investigated by immunizing a New Zealand
White rabbit with heat-killed (15 min at 100°C) S. gordonii RJMIII cells (1010 CFU in 1 ml of Freund's
incomplete adjuvant) by using a protocol similar to that described
above. The antibody obtained was found to react with the native PT in
an ELISA, although the titer of the antiserum was relatively low
(1,600). In Western blotting, the anti-SpaP-S1 antiserum clearly
recognized the S1 subunit of the native PT, suggesting that the
recombinant S1 expressed by S. gordonii is capable of
eliciting an immune response (Fig. 4). This finding is consistent with findings by others that recombinant S1
expressed in E. coli, Salmonella, and B. subtilis is immunogenic (2, 3, 18, 24).

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FIG. 4.
Western blots showing the specific reactivity of the
rabbit anti-SpaP-S1 serum with the S1 subunit of PT. The rabbit anti-PT
antiserum (dilution, 1/200) (lane 1) reacted with the S1, S2, and S3 of
PT (1 µg; List Biological Laboratories, Inc.). The rabbit
anti-SpaP-S1 antiserum (dilution, 1/100) (lane 2) reacted with the S1
subunit of PT only. Preimmune serum (dilution, 1/100) (lane 3) obtained
from the same rabbit used to raise the anti-SpaP-S1 serum did not react
with PT.
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Protective effects of recombinant S1.
The ability of the
rabbit anti-SpaP-S1 antiserum to neutralize the cytotoxic effect of PT
was assessed by the Chinese hamster ovary (CHO) cell-clustering assay
using the method described previously (7). In the assay, 4 times the minimum clustering dose of native PT (Massachusetts Biologics
Laboratories, Jamaica Plain, Mass.) was used to allow for a twofold
variation. Neutralization titers of antibodies were expressed as the
reciprocals of the dilutions which showed the complete neutralization
of the cell-clustering effect of PT. The immune serum showed a weak
neutralization titer of 4, while the preimmune serum did not show any
activity (neutralization titer, 0). The anti-SpaP-S1 antibody was
subsequently concentrated sevenfold by (NH)2SO4
precipitation (12) and demonstrated a neutralization titer
of 32. A rabbit anti-SpaP antibody prepared previously (12)
was similarly concentrated sevenfold and showed a neutralization titer
of 2. These results suggest that the anti-SpaP-S1 antibody had a weak
ability to neutralize the cytotoxic effect of PT on the CHO cells.
Previous studies by Barbieri et al. (2) and Boucher et al.
(3) also found that the anti-recombinant S1 antibodies
showed weak neutralizing abilities in the CHO cell-clustering assay.
The in vivo protective effect of the recombinant S1 was assessed by the
leukocytosis-promoting and histamine-sensitizing assays
(
7).
A cohort of BALB/c female mice (3 weeks old;
n = 5)
were
immunized intraperitoneally (i.p.) with heat-killed (10 min at
100°C)
S. gordonii RJMIII cells (10
9 CFU in
0.2 ml of Freund's complete adjuvant). The animals were
boosted with
freshly prepared heat-killed cells in Freund's incomplete
adjuvant via
the same route 2 and 3 weeks later. A cohort of nonimmunized
mice
(
n = 6) was used as a control. At 7 days after the last
booster,
sera were obtained from the animals; the immunized mice had a
titer of 12,800 against the native PT by ELISA. Each animal from
the
immunized and control groups was then injected i.p. with 0.5
µg of
native PT (Massachusetts Biologics Laboratories) in 0.2
ml of PBS. The
total leukocyte (WBC) count was determined with
a hemacytometer on 1 µl of blood taken from the tail vein before
and after PT injection.
Before PT challenge, the control and immunized
mice had average WBC
counts of 9.14 × 10
9/liter (range, 4.5 × 10
9 to 12.2 × 10
9) and 8.28 × 10
9/liter (range, 5.5 × 10
9 to 10.0 × 10
9), respectively. Three days after PT injection, the
control mice
had an average WBC count of 29.8 × 10
9/liter (range, 26.5 × 10
9 to 35.0 × 10
9), 3.3 times higher than that before PT challenge.
The average
WBC count of the immunized mice was 11.5 × 10
9/liter (range, 8.75 × 10
9 to
15.75 × 10
9) after PT injection, 1.4 times higher
than that before the challenge.
Each animal was further given 2 mg of
histamine diphosphate (Sigma
Chemical Co., St. Louis, Mo.) i.p. 4 days
after PT challenge,
and deaths within 24 h of histamine
administration were recorded.
All five of the immunized mice survived
the treatment, while only
one of the six control mice survived. These
results strongly suggest
that the recombinant S1 fragment expressed by
S. gordonii can
induce protective antibodies in
vivo.
In conclusion, we have successfully surface-expressed in
S. gordonii the N-terminal 179-amino-acid S1 fragment as a fusion
protein to the
S. mutans SpaP antigen. The recombinant S1 is
immunogenic
and can protect mice from the toxic effect of PT.
Construction
of this recombinant
S. gordonii will allow us
to pursue an animal
infection model in the
future.
 |
ACKNOWLEDGMENTS |
We thank Annette Morris for assistance in the CHO cell-clustering assay.
This study is supported by an intramural grant from the Faculties of
Dentistry and Medicine and in part by the Medical Research Council of Canada.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Applied Oral Sciences, Faculty of Dentistry, Dalhousie University,
Halifax, Nova Scotia, Canada B3H 3J5. Phone: (902) 494-8799. Fax: (902) 494-6621. E-mail: Song.Lee{at}Dal.Ca.
Editor:
V. A. Fischetti
 |
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Infection and Immunity, March 1999, p. 1511-1516, Vol. 67, No. 3
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Copyright © 1999, American Society for Microbiology. All rights reserved.
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