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Infection and Immunity, April 2004, p. 2449-2451, Vol. 72, No. 4
0019-9567/04/$08.00+0 DOI: 10.1128/IAI.72.4.2449-2451.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
Salmonella Flagellin Is Not a Dominant Protective Antigen in Oral Immunization with Attenuated Live Vaccine Strains
Chie Kodama1 and Hidenori Matsui1,2*
Kitasato Institute for Life Sciences and Graduate School of Infection Control Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641,1
Center for Basic Research, The Kitasato Institute, Minato-ku, Tokyo 108-8642, Japan2
Received 16 September 2003/
Returned for modification 2 December 2003/
Accepted 5 January 2004

ABSTRACT
We found that oral immunization with flagellum-defective mutant
strains of
Salmonella enterica serovar Typhimurium with the
ClpXP-deficient background protected mice against oral challenge
with the virulent strain. These data indicate that
Salmonella flagellin is not a dominant protective antigen in oral immunization
with attenuated live vaccine strains.

TEXT
Growing
Escherichia coli cells are capable of protein degradation,
since a variety of "abnormal" and "misfolded" proteins can be
rapidly and specifically degraded during proliferation. Most
intracellular proteolysis is initiated by energy-dependent proteases,
mainly Lon and Clp proteases. The serine protease ClpP is normally
associated with ClpX, ClpA, or both, which act as molecular
chaperones (
2,
12). In
Salmonella enterica serovar Typhimurium,
ClpXP protease is also involved in the stress response and degradation
of misfolded proteins (
15). It was previously reported that
the ClpXP protease-depleted mutant of serovar Typhimurium loses
virulence and persistently resides in BALB/c mice for long periods
after either intraperitoneal (
18) or oral (
10) infection without
causing an overwhelming systemic infection. In a previous study,
the mice developed strong protective immunity after a single
oral administration of ClpXP-deficient serovar Typhimurium.
Consequently, at week 4 after immunization, the immunized mice
were completely protected against oral challenge with serovar
Typhimurium (
10). We have observed that a certain amount of
serovar Typhimurium lipopolysaccharide-specific antibodies are
present in ClpXP-deficient-serovar Typhimurium-immunized mice
and that these mice have the ability to resist systemic infections
with the virulent strain of serovar Typhimurium for more than
a year after a single oral immunization (data not shown). On
the other hand, Tomoyasu et al. found that the ClpXP protease
of serovar Typhimurium affects flagellar formation and that
bacterial cells with the
clpP gene deleted show a "hyperflagellate"
phenotype in vitro (
16). ClpXP-deficient serovar Typhimurium
overproduces the flagellar protein and shows a fourfold increase
in the rate of transcription of the
fliC gene encoding the flagellar
filament protein (
16), since the ClpXP protease negatively regulates
transcription of the flagellar regulon by controlling the turnover
of the FlhD
2FlhC
2 master regulators (
17). Under these circumstances,
we hypothesized that ClpXP-deficient serovar Typhimurium may
overproduce the flagellar protein in mice, with the result that
the produced flagellar protein may work as a dominant protective
antigen. In order to verify this hypothesis, we evaluated the
flagellum-defective mutant strains with the ClpXP-deficient
background in terms of their efficacy as live oral vaccine strains
for use against
Salmonella infection. The flagellar operons
are divided into three classes with respect to their transcription
hierarchy (
6). Class 3 contains five operons, including a filament
formation operon. In addition, most
Salmonella serovars have
two genes for a major component protein of the filament at different
locations on the chromosome that code for the antigenically
distinct flagellar types, H1 (phase 1 [FliC]) and H2 (phase
2 [FljB]) (
6,
9). The expression of the class 3 operons requires
FliA (the class 3 operon-specific sigma factor). The
fliA gene
is included in class 2, and it has been found to positively
regulate expression by the activator proteins, FlhD and FlhC,
which are encoded by the
flhD class 1 operon lying at the top
of the transcription hierarchy (
7,
8). Each class-specific flagellum-defective
mutant strain of serovar Typhimurium was previously constructed
with or without the ClpXP-deficient background (
10). Table
1 shows the serovar Typhimurium strains used in this study. CS2007
is the ClpXP-deficient mutant strain of serovar Typhimurium.
CS2056, CS2062, and CS2086 are the
fliC- and
fljB-,
fliA-, and
flhD-defective strains derived from CS2007, respectively. The
virulence levels in splenic infection were the same among the
flagellum-defective mutant strains with the ClpXP-deficient
background (CS2056, CS2062, and CS2086), and it appeared that
there were also no discrepancies in the numbers of splenic CFU
among CS2007-, CS2056-, CS2062-, and CS2086-inoculated mice.
Therefore, it was concluded that when mice are orally inoculated,
the flagellar structures do not affect the virulence of CS2007,
and splenic infection is thus enabled (
10).
Oral immunization with the ClpXP- and flagellum-defective mutants protects mice against oral challenge with the virulent strain.
In the present study, 7-week-old female BALB/c mice (Charles
River Japan, Yokohama, Japan) were orally immunized with 5
x 10
8 CFU of salmonellae. Four weeks later, immunized and naïve
(unimmunized) mice were orally infected with 5
x 10
8 CFU of

3456 (the virulent strain). The levels of recovery (numbers
of CFU) of infecting salmonellae colonizing the spleens, mesenteric
lymph nodes (MLN), and Peyer's patches (PP) were determined
5 days after the infection. In the same tissue sample, mixed
salmonellae were distinguished as belonging to the avirulent
vaccine strain (CS2007, CS2056, CS2062, or CS2086) or the infecting
virulent strain (

3456) on Luria-Bertani agar plates (Difco Laboratories,
Detroit, Mich.) containing 25 µg of nalidixic acid (Sigma,
St. Louis, Mo.) per ml or 15 µg of tetracycline (Sigma)
per ml. As shown in Fig.
1, a small number of CFU of a virulent
strain of salmonellae (

3456) was detected in each tissue sample
from immunized mice, although a large number of CFU of salmonellae
was detected in each tissue sample from naïve mice. From
the spleens, the levels of recovery of bacterial cells were
5.35 ± 0.89, 1.61 ± 1.44, 1.30 ± 1.27,
0.44 ± 0.77, and 3.14 ± 1.43 (log
10 numbers of
CFU ± standard deviations for naïve and CS2007-,
CS2056-, CS2062-, and CS2086-immunized mice, respectively).
The number of splenic CFU in the CS2086-immunized mice was significantly
higher than that in the CS2007-immunized mice (
P = 0.038). However,
this number was still significantly lower than that in naïve
mice (
P = 0.0001). In the other tissue samples, there was a
significant difference in numbers of bacterial CFU between the
naïve and immunized mice, and there was no significant
difference in numbers of bacterial CFU among the CS2007-, CS2056-,
CS2062-, and CS2086-immunized mice, except for that in MLN of
the CS2062-immunized mice (
P = 0.024). The data clearly showed
that a single oral immunization with ClpXP- or ClpXP-, FliC-,
FljB-deficient serovar Typhimurium protected mice against oral
challenge with the virulent strain. By contrast, we detected
very few salmonellae of each avirulent vaccine strain in the
PP and a small number of salmonellae in the spleens (less than
10
2 CFU/tissue) and MLN (less than 10
1 CFU/tissue) of the CS2007-,
CS2056-, CS2062-, and CS2086-immunized mice 5 days after the
challenge (data not shown). It seems that salmonellae of every
vaccine strain that resided in mice were eliminated by the immunity
induced after the challenge.
It was previously reported that
Salmonella flagellin is one
of the most relevant antigens for the generation of protective
immunity in mice (
4,
11). Therefore,
Salmonella flagellin has
been used as a carrier for heterologous peptide epitopes, which
are exposed at the surface of the flagellar filament in live
attenuated vaccine strains (
14). In contrast, it was shown that
Salmonella flagellin does not represent an efficient peptide
carrier for the activation of antibody responses in mice orally
immunized with live, attenuated
Salmonella strains (
1). Moreover,
Salmonella flagellin-specific serum antibody (immunoglobulin
G) responses elicited in mice following intranasal or oral inoculation
with attenuated
Salmonella strains were marginal (
13). We also
detected low levels of serum flagellin-specific immunoglobulin
G responses in mice orally immunized with CS2007 (data not shown).
Most recently, Harada et al. demonstrated that oral immunization
with flagellin purified from serovar Typhimurium LT2 along with
cholera toxin induces flagellin-specific humoral and cell-mediated
immunities in mice; however, this vaccine confers at most 50
to 60% survival rates in the case of oral challenge with 10
6 CFU of virulent serovar Typhimurium (
5). In this study, the
CS2056, CS2062, and CS2086 strains were genetically deficient
in flagellin (in fact, we detected no flagellar filaments on
the bacterial surface of CS2056, CS2062, or CS2086 by using
transmission electron microscopy [data not shown]); however,
oral immunization with each vaccine strain was able to protect
mice against oral challenge with 5
x 10
8 CFU (more than 1,500
50% lethal doses) of virulent serovar Typhimurium. Actually,
immune, challenged mice did not become sick and all of these
mice survived after the challenge. Based on these data, we came
to the conclusion that the
Salmonella flagellar filament proteins
(FliC and FljB) are not dominant protective antigens in oral
immunization with attenuated live vaccine strains.

ACKNOWLEDGMENTS
We thank A. Takaya, T. Tomoyasu, and T. Yamamoto at Chiba University
for providing mutant strains of serovar Typhimurium and for
their helpful advice, and we thank Y. Kikuchi at Kitasato University
for engaging in fruitful discussions.
This work was supported by a Grant-in-Aid for Scientific Research C (15590398) and in part by a grant from the 21st-Century COE Program from the Ministry of Cultures, Sciences, and Technology of the Japanese Government.

FOOTNOTES
* Corresponding author. Mailing address: Laboratory of Immunoregulation, Department of Infection Control and Immunology, Kitasato Institute for Life Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan. Phone and fax: 81-3-5791-6267. E-mail:
hmatsui{at}lisci.kitasato-u.ac.jp.

Editor: B. B. Finlay

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Infection and Immunity, April 2004, p. 2449-2451, Vol. 72, No. 4
0019-9567/04/$08.00+0 DOI: 10.1128/IAI.72.4.2449-2451.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.
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