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Infection and Immunity, May 2002, p. 2734-2738, Vol. 70, No. 5
0019-9567/02/$04.00+0 DOI: 10.1128/IAI.70.5.2734-2738.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
A Lipid Core Peptide Construct Containing a Conserved Region Determinant of the Group A Streptococcal M Protein Elicits Heterologous Opsonic Antibodies
Colleen Olive,1* Michael R. Batzloff,1 Anikó Horváth,2 Allan Wong,2 Timothy Clair,1 Penny Yarwood,1 Istvan Toth,2 and Michael F. Good1
Division of Infectious Diseases and Immunology, Cooperative Research Centre for Vaccine Technology, The Queensland Institute of Medical Research, Brisbane, Queensland 4029,1
School of Pharmacy, The University of Queensland, Brisbane, Queensland 4072, Australia2
Received 29 November 2001/
Returned for modification 22 January 2002/
Accepted 13 February 2002

ABSTRACT
The study reported here investigated the immunogenicity and
protective potential of a lipid core peptide (LCP) construct
containing a conserved region determinant of M protein, defined
as peptide J8. Parenteral immunization of mice with LCP-J8 led
to the induction of high-titer serum immunoglobulin G J8-specific
antibodies when the construct was coadministered with complete
Freund's adjuvant (CFA) or administered alone. LCP-J8 in CFA
had significantly enhanced immunogenicity compared with the
monomeric peptide J8 given in CFA. Moreover, LCP-J8/CFA and
LCP-J8 antisera opsonized four different group A streptococcal
(GAS) strains, and the antisera did not cross-react with human
heart tissue proteins. These data indicate the potential of
an LCP-based M protein conserved region GAS vaccine in the induction
of broadly protective immune responses in the absence of a conventional
adjuvant.

TEXT
The development of a vaccine against group A streptococci (GAS)the
etiologic agents of rheumatic heart diseasehas focused
largely on the bacterial surface M protein (
10,
21). The M protein
is a major virulence factor in GAS infection and consists of
a variable amino-terminal region which defines the GAS serotype
(over 100 serotypes are known) and a highly conserved carboxy-terminal
C-repeat region (
10). Protective immunity to GAS infection has
been associated with type-specific opsonic antibodies against
M protein (
10,
21), although the presence of opsonic antibodies
specific to the C-region has been demonstrated in humans (
17)
and in mice immunized with C-region peptides (
18) and is also
important in the elicitation of protective immunity to GAS (
4).
The variability in M proteins and the potential for the induction
of autoimmunity due to antigenic molecular mimicry between the
GAS M protein and heart antigens (
6,
11,
13,
19) represent significant
hurdles in the development of a vaccine covering a wide range
of strains. Multivalent M protein constructs containing epitopes
from several type-specific regions of different M proteins (
4,
7,
8) and those based on the conserved C-region (
2-
5) have shown
promising results in animal trials. However, the GAS vaccine
constructs require for their efficacy the use of adjuvants that
are not suitable for use in humans due to their toxicity. The
development of novel synthetic adjuvants offers the possibility
of vaccine delivery without the need for additional toxic adjuvants.
Lipopeptide compounds utilizing a synthetic analog of the N-terminal
moiety of bacterial lipoprotein from
Escherichia coli (Pam3cys
[tripalmitoyl-S-glyceryl cysteine]) as a lipidic anchor moiety
(
26) were found to be potent immunogens with self-adjuvanting
properties, eliciting humoral and cellular responses irrespective
of the route of administration (
9,
15,
16,
27). The lipidic
polylysine core peptide (LCP) system (
24) has also been described
as using a lipidic anchor moiety in conjunction with the multiple
antigenic peptide system (
23). Furthermore, lipopeptide compounds
are potentially safe options for vaccine delivery in humans
(
1). The study reported here investigated the LCP system as
a self-adjuvanting GAS vaccine delivery approach. An LCP construct
(LCP-J8) was synthesized by incorporating multiple copies of
a GAS M protein C-region peptide, referred to as J8, which contains
a conformational B-cell epitope and lacks potentially deleterious
T-cell autoepitopes (
14). The J8 peptide (QAEDKVKQ
SREAKKQVEKALKQLEDKVQ,
consisting of residues 344 to 355 of the GAS M1 protein) is
a chimeric peptide that contains 12 amino acids from the C-region
(shown in bold) and is flanked by GCN4 DNA-binding protein sequences,
which are required to maintain the correct helical folding and
conformational structure of the peptide (
20). J8 was synthesized
by manual solid-phase peptide synthesis using Boc (
tert-butoxycarbonyl)
chemistry (
22), and peptide purification was carried out on
a Waters HPLC system (model 600 controller, 490 E UV detector,
60 F pump).
Boc-L-amino acids and MBHA (4-methylbenzhydrylamine) resin (Novabiochem, Läufelfingen, Switzerland) were used to synthesize the LCP-J8 construct (Fig. 1). Racemic lipoamino acids were synthesized with Boc protection according to the procedures of Gibbons et al. (12). Preactivated Boc-Gly-OH was coupled to the MBHA resin. The next two cycles were carried out with Boc-lipoamino acids containing 12 carbon atoms (C12: HN-CH[(CH2)9CH3]-OH); this was followed sequentially by the coupling of Boc-Gly-OH, Boc-C12-OH, and Boc-Lys(Boc)-OH. After deprotection of the lysine
- and
-amino groups, a four-branch system was formed by the coupling of Boc-Lys(Boc)-OH to the free amino groups. After deprotection, four identical peptide J8 sequences were synthesized directly onto each of the
- and
-amino groups of each lysine of the branched lysine core, with the appropriate protecting groups applied on the side chains of the amino acids. Thus, the lipophilic anchor of the LCP-J8 construct contains three 2-amino-dodecanoic lipoamino acids attached to the polylysine core, with glycine spacers employedone between the resin and the first lipoamino acid and another between the second and third lipoamino acids. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis resulted in a band for LCP-J8 of the expected size. Determination of the mass spectrum of LCP-J8 resulted in a calculated molecular weight of 14,158.45.
To assess the immunogenicity of the LCP-J8 construct, immunoglobulin
G (IgG) antibody responses to the peptide J8 were measured in
sera from 4- to 6-week-old female B10.BR mice (Animal Resource
Centre, Perth, Australia) In two separate experiments, the mice
(
n = 10 per group) were immunized subcutaneously at the tail
base with 30 µg of LCP-J8 construct, which was either
emulsified 1:1 (vol/vol) with complete Freunds adjuvant
(CFA) (Sigma, Castle Hill, Australia) or given alone in a total
volume of 50 µl of sterile-filtered phosphate-buffered
saline (PBS) (Fig.
2). Three weeks after the primary immunization,
the mice received at weekly intervals a further four (experiment
1 [exp 1]) or five (experiment 2 [exp 2]) booster injections
of doses of 3 µg of LCP-J8 construct in PBS prior to the
collection of blood via the tail artery. Mice in the control
group received 30 µg of J8 in CFA or 20 µg of pepM1
(the amino-terminal half of the M protein) in CFA, with booster
injections of 3 µg each. Antibody titers were determined
by enzyme-linked immunosorbent assay (
18) and defined as the
lowest dilutions that gave optical density (OD) readings at
450 nm more than three standard deviations above the mean OD
of control wells containing normal mouse sera (obtained from
mice immunized with CFA in PBS). In the first experiment (Fig.
2A), in which mice received a primary immunization and four
booster injections each of the same immunogen, J8-specific antibodies
were detected in all mice 3 weeks after the primary immunizations
with LCP-J8 in CFA and J8 in CFA, with final average antibody
titers after four booster injections of 1.5
x 10
6 and 1.4
x 10
5, respectively (exp 1) (Fig.
2A). J8-specific antibodies
were not detected at 3 weeks postimmunization in the mice immunized
with LCP-J8 without adjuvant. However, after one booster injection
of immunogen, six of the nine mice had J8-specific antibodies,
and after the third booster injection, antibodies to J8 were
detected in all mice. After the final booster injection (booster
injection number 4), the average J8 antibody titer in serum
samples from mice immunized with the LCP construct without adjuvant
was 6.4
x 10
4 (exp 1). In the second experiment (Fig.
2B), in
which mice received a primary immunization and five booster
injections each of the same immunogen, the final average J8-specific
serum IgG antibody titers were 1.4
x 10
6, 5.0
x 10
4, and 7.5
x 10
5 for mice immunized with LCP-J8/CFA, LCP-J8/PBS, and J8/CFA,
respectively. IgG isotyping demonstrated strong J8-specific
IgG1, IgG2b, and IgG2a antibody responses with lower titers
or no J8-specific antibodies detected for IgG3 in LCP-J8/CFA-
and J8/CFA-immunized mice. In mice immunized with LCP-J8 alone,
there was a strong IgG1 response, and IgG3 was detected in only
one mouse. A few of the mice (50%) demonstrated an IgG2a response
(exp 2), and IgG2b was barely detected in either experiment.
No IgG2a was detected in exp 1. Our data show that the LCP-J8
construct was more immunogenic in CFA than was monomeric J8
peptide given in CFA and was also immunogenic when delivered
in the absence of adjuvant. LCP-based vaccine candidates incorporating
differing domains of
Chlamydia trachomatis outer membrane protein
have also been shown in previous studies to significantly enhance
peptide immunogenicity (
28). In addition, it has been shown
that an LCP construct incorporating a foot-and-mouth disease
viral peptide was immunogenic, resulting in the induction of
antipeptide antibodies in the absence of additional adjuvant
(
25).
Using an in vitro indirect bactericidal assay (
17,
18) which
compares the growth of bacterial population (in CFU) following
incubation with immune sera to that with control normal mouse
sera obtained from mice immunized with adjuvant alone, we assessed
the opsonic activity (measured as the percentage of reduction
in bacterial population [in CFU]) of serum IgG antibodies elicited
after parenteral delivery of LCP-J8 to the homologous M1 GAS
strain (Table
1 and Fig.
3). The average levels of opsonization
of M1 GAS by J8/CFA antisera generated in exp 1 and exp 2 were
64% and 36%, respectively, and the average level of opsonization
of the pepM1 positive control was 82% for each experiment. The
average levels of opsonization for LCP-J8/CFA antisera against
M1 were 71% (exp 1) and 68% (exp 2) and for LCP-J8 antisera
against M1 were 64% (exp 1) and 76% (exp 2). Thus, results from
the opsonization of M1 GAS by LCP-J8/CFA and LCP-J8/PBS antisera
indicated that serum opsonic antibodies were induced in mice
following immunization with the LCP-J8 construct. Moreover,
the induction of opsonic antibodies was not dependent on the
presence of a conventional adjuvant, supporting expectations
of high efficacy of the LCP system as a self-adjuvanting vaccine
delivery modality. To confirm the specificity of the opsonic
antibodies induced in LCP-J8-immunized mice, a peptide inhibition
bactericidal assay was performed which involved the preincubation
of immune sera with either 100 µg of J8 peptide, nonspecific
peptide from
Schistosoma (EGKVSTLPLDIQIIAATMSK), or no peptide
prior to the assessment of opsonization. The levels of opsonization
of M1 GAS by pooled LCP-J8/CFA and LCP-J8 antisera in the absence
of peptide were 37% and 55%, respectively. Preincubation of
LCP-J8/CFA antisera with J8, however, led to the complete inhibition
of opsonization. There was a 91% inhibition of opsonization
for LCP-J8 antisera preincubated with J8. The nonspecific peptide
was shown to inhibit opsonization of LCP-J8/CFA and LCP-J8 antisera
by 12% and 27%, respectively. Incubation of pepM1 antisera with
J8 also had little effect on opsonization (23% inhibition of
opsonization), consistent with the fact that pepM1 represents
the amino-terminal half of M protein only. Together, these data
indicate that LCP-J8 induced serum opsonic antibodies specifically
directed against the M protein conserved C-region J8 peptide
epitope on GAS and that these antibodies may potentially be
important in protective immunity against GAS.
This finding prompted us to determine whether the serum antibodies
induced by LCP-J8 could opsonize other GAS strains and might
possibly be involved in mediating broadly protective immune
responses and, secondly, whether heart cross-reactive antibodies
were elicited. We therefore assessed the opsonic ability of
the LCP-J8 antisera against three other GAS strainsNS27,
8830, and BSB24 (Table
1 and Fig.
3). Both NS27 and 8830 contain
sequences identical to that of J8 in the C region, whereas BSB24
has disparities in three amino acids in the sequence SREAKK
KVE
ADL
(the three amino acids are indicated by boldface). LCP-J8/CFA
and LCP-J8 antisera opsonized NS27 GAS with average levels of
opsonization of 70% and 64%, respectively. Less opsonization
was observed against 8830 GAS (27% for LCP-J8/CFA antisera and
39% for LCP-J8 antisera). In the case of BSB24, LCP-J8/CFA and
LCP-J8 antisera opsonized GAS with averages of 46% and 50% opsonization,
respectively. J8/CFA antisera opsonized NS27, 8830, and BSB24
GAS with average levels of opsonization of 40%, 26%, and 15%,
respectively. Antiserum to pepM1 was shown not to opsonize the
heterologous GAS strain, BSB24. The induction of serum opsonic
antibodies against different GAS strains following immunization
of mice with LCP-J8 supports expectations of the efficacy of
a broadly protective GAS vaccine based on the conserved region
of the GAS M protein. Moreover, using standard sodium dodecyl
sulfate-polyacrylamide gel electrophoresis and Western blot
analysis (
18), no cross-reactivity of LCP-J8 antisera to heart
proteins in human heart and mitral valve extracts was observed.
This report has demonstrated that a GAS vaccine construct, incorporating multiple copies of a nonhost cross-reactive conserved region determinant of M protein, is highly immunogenic when administered parenterally to mice. Moreover, immunization with the construct led to the induction of heterologous opsonic antibodies, even when the construct was delivered in the absence of additional adjuvant. These findings indicate the potential utility of the LCP system in the delivery of a synthetic GAS vaccine with self-adjuvanting properties with a view to the development of a mucosa-based vaccine for human application.

ACKNOWLEDGMENTS
This work was supported by the National Health and Medical Research
Council of Australia, the National Heart Foundation of Australia,
and the Cooperative Research Centre for Vaccine Technology,
to which we express our thanks.
We also thank Ben Tsang for his help in the synthesis of LCP-J8, David Purdie (QIMR) for statistical analysis of the data, and Kadaba Sriprakash (QIMR) for critically reviewing the manuscript.

FOOTNOTES
* Corresponding author. Mailing address: Division of Infectious Diseases and Immunology, The Queensland Institute of Medical Research, PO Royal Brisbane Hospital, Brisbane, QLD 4029, Australia. Phone: 61-73362 0431. Fax: 61-7-3362 0104. E-mail:
colleenO{at}qimr.edu.au.

Editor: J. T. Barbieri

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Infection and Immunity, May 2002, p. 2734-2738, Vol. 70, No. 5
0019-9567/02/$04.00+0 DOI: 10.1128/IAI.70.5.2734-2738.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
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