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Infection and Immunity, December 1998, p. 5862-5866, Vol. 66, No. 12
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Salmonella typhimurium Infection in Mice Induces
Nitric Oxide-Mediated Immunosuppression through a Natural
Killer Cell-Dependent Pathway
Martin G.
Schwacha,
Joseph
J.
Meissler Jr., and
Toby K.
Eisenstein*
Department of Microbiology and Immunology,
Temple University School of Medicine, Philadelphia, Pennsylvania
19140
Received 20 July 1998/Returned for modification 1 September
1998/Accepted 21 September 1998
 |
ABSTRACT |
Splenocytes isolated from C57BL/6J female mice 3 to 7 days after
inoculation with an attenuated strain of Salmonella
typhimurium produced high levels of nitric oxide (39 to 77 µM)
and gamma interferon (IFN-
). Additionally, spleen cell cultures from
Salmonella-inoculated mice were markedly suppressed in
their ability to generate an in vitro plaque-forming cell (PFC)
response to sheep erythrocytes. Depletion of natural killer (NK) cells
from the immune splenocyte population markedly reduced nitric oxide
production, prevented suppression of PFC responses, and completely
abrogated IFN-
release. Treatment of NK cell-depleted immune cells
with IFN-
restored nitric oxide production to levels comparable to
those of intact immune cells and also restored the immunosuppression.
These results suggest that NK cells regulate the induction of nitric
oxide-mediated immunosuppression following infection with S. typhimurium through the production of IFN-
.
 |
INTRODUCTION |
Natural killer (NK) cells have been
shown to be important in the early host defense responses to a number
of pathogens (6, 20, 33, 39, 44, 51). NK cells are a major
source of gamma interferon (IFN-
) (12, 52) and have been
shown to be critical for T-lymphocyte-independent macrophage (M
)
activation through production of this cytokine. Previous studies have
shown that NK cells play a key role in vivo in protection of mice
against challenge with virulent Salmonella typhimurium
(41).
The immune response to Salmonella has been of interest
because of the desire to develop improved oral vaccines for typhoid fever. Previous studies by our laboratory as well as others have focused on attenuated strains of Salmonella as potential
vaccine candidates (reviewed in reference 15). Our
studies of the mechanisms of immunity to Salmonella have
used a murine model of typhoid fever (14, 22) and an
attenuated strain of S. typhimurium (SL3235) blocked in
aromatic synthesis (21). We have previously shown that
SL3235, while inducing protection against virulent salmonellae,
paradoxically induced profound immunosuppression, as evidenced by the
inability of splenocytes to generate an antibody response to
non-Salmonella antigens and to respond to mitogens with
lymphoproliferative responses (3, 4, 13, 31). We found
that immunosuppression induced by SL3235 is mediated by the
production of nitric oxide by splenic M
s because nitric oxide production and immunosuppression were blocked by the nitric oxide synthase inhibitor
NG-monomethyl-L-arginine, and
depletion of macrophages restored immune responses (5, 22).
Furthermore, infection of mice with S. typhimurium
induces IFN-
production (36, 38, 40, 49), and our
laboratory has shown that IFN-
is important in the immunosuppression
associated with SL3235 inoculation of mice (5, 22).
The present study investigated the role of NK cells in the induction of
splenocyte nitric oxide production and immunosuppression following
infection with an attenuated strain of S. typhimurium.
 |
MATERIALS AND METHODS |
Animals.
Six-week-old female C57BL/6J mice were purchased
from Jackson Laboratories (Bar Harbor, Maine), and mouse chow and water
were provided ad libitum. All mice were acclimatized for a minimum of 1 week prior to experimentation.
Bacterial strain and infection model.
An avirulent strain of
S. typhimurium, SL3235, was used for all experiments.
SL3235 is an aroA mutant which is deficient in aromatic
synthesis, and the 50% lethal dose (LD50) of SL3235 is greater than 107 bacteria when administered
intraperitoneally (i.p.). Infection was accomplished by injection of
5 × 105 log-phase bacteria i.p., as previously
described (31). Control mice were injected i.p. with a
comparable volume of endotoxin-free, sterile, isotonic saline (Abbott
Laboratories, Chicago, Ill.).
Cell isolation and preparation.
One to seven days after
SL3235 inoculation, the mice were sacrificed by cervical dislocation,
and the spleens were removed aseptically. Single-cell splenocyte
suspensions were prepared as previously described (31).
In vitro depletion of NK cells.
Splenocytes were prepared at
a concentration of 107 cells/ml of RPMI 1640 (Gibco) with
1% heat-inactivated fetal bovine serum (FBS; Hyclone, Logan, Utah) and
treated for 45 min at 4°C either with antiserum against asialo
GM1 (WAKO Chemicals, Richmond, Va.) at a 1:200 dilution or
with a 1:3 dilution of supernatants generated by the PK136 hybridoma
(American Type Culture Collection, Rockville, Md.). PK136 cells produce
a murine monoclonal antibody (immunoglobulin G2a [IgG2a]) against the
NK cell antigen NK1.1. In preliminary experiments, a 1:3 dilution of
the PK136 supernatants was determined to be optimal for depletion of NK
cells. Following the 45-min incubation, the cells were pelleted by
centrifugation and resuspended in RPMI 1640 containing 1% FBS and a
1:12 or 1:8 dilution of Low-Tox M rabbit complement. The cell
suspension was then incubated for 45 min at 37°C. The cells were
pelleted by centrifugation and washed three times in RPMI 1640 containing 1% FBS prior to their use in various experiments.
Nitric oxide production.
Splenocytes were cultured at a
concentration of 107 cells/ml (RPMI 1640 containing 5%
FBS, 50 U of penicillin-streptomycin per ml [Gibco]) in 96-well
tissue culture plates (Costar, Cambridge, Mass.) for 48 h, and
nitric oxide production was determined by measuring nitrite, a stable
degradation product of nitric oxide according to the method of Ding et
al. (11). Briefly, 100 µl of cell supernatant was removed
from each well and incubated with an equal volume of Greiss reagent
(1% sulfanilamide, 0.1% naphthylethylene diamine dihydrochloride,
2.0% H3PO4 [Sigma Chemical, St. Louis, Mo.])
at room temperature for 10 min. The A550 was
determined with a microplate enzyme-linked immunosorbent assay (ELISA)
reader. The nitrite concentration was determined by using a standard
curve generated with sodium nitrite (Sigma Chemical). In selected
experiments, the cells were treated with recombinant murine IFN-
(Genzyme, Cambridge, Mass.).
IFN-
ELISA.
Splenocytes were cultured at a concentration
of 107 cells/ml (RPMI 1640 containing 5% FBS, 50 U of
penicillin-streptomycin per ml [Gibco]) for 24 h, and cell
supernatants were collected and stored at
70°C until
determination of IFN-
levels. IFN-
concentration in the
supernatants was determined by sandwich ELISA with a matched pair of
monoclonal antibodies and recombinant murine IFN-
according to the
manufacturer's instructions (PharMingen, San Diego, Calif.).
Strepavidin-alkaline phosphatase (Calbiochem, San Diego, Calif.)
and p-nitrophenyl phosphate (Sigma) were used for color
development. The A405 was determined with a
microplate ELISA reader.
Primary in vitro antibody response.
Antibody-producing cells
were generated in vitro by the method of Mishell and Dutton
(37), with modification, as previously described
(3). The plaque-forming cell (PFC) response was determined by the Cunningham modification of the Jerne hemolytic plaque assay (10). Data are expressed as PFC per 107 splenocytes.
Statistical analysis.
All measurements were made by
using a minimum of triplicate samples per variable for each experiment.
Data are expressed as means ± standard deviations for a
representative experiment. Comparisons were analyzed by using
Student's t test. Differences were considered significant
when P was
0.05.
 |
RESULTS |
NK cells mediate nitric oxide production.
SL3235 inoculation
induced splenocyte nitric oxide production in a time-dependent fashion
(Fig. 1). By 72 h after in vivo immunization with Salmonella, significant amounts of nitrite
were observed, which reached maximal levels by 5 days post-SL3235
inoculation. Nitric oxide levels remained elevated at 7 days
postinoculation.

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FIG. 1.
Splenocyte nitric oxide production. Splenocytes from
saline-inoculated (normal) and SL3235-inoculated (immune) mice were
isolated 1 to 7 days after inoculation and cultured at a concentration
of 107 cells/ml for 48 h. Nitrite concentrations in
cell supernatants were determined. The experiment was done five times.
Data are means ± standard deviations of triplicate samples from a
representative experiment.
|
|
Removal of NK cells from immune splenocytes by treatment with antiserum
to asialo GM
1 plus complement reduced the high levels
of
nitric oxide released by these cells by 75% (Fig.
2). Treatment
with complement alone
resulted in a slight decrease in nitric
oxide production by immune
splenocytes, whereas treatment with
antiserum to asialo
GM
1 alone had no effect. Nitric oxide production
by normal
splenocytes was very low (approximately 2 µM) and was
unaltered by
all treatments.

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FIG. 2.
Effect of NK cell depletion on nitric oxide production.
Splenocytes from saline-inoculated (normal) and SL3235-inoculated
(immune) mice were isolated 7 days postinoculation. The cells were
treated with antiserum ( ) to asialo GM1 or rabbit
complement or were depleted of NK cells by sequential treatment with
both agents as described in Materials and Methods. The cells were
cultured at a concentration of 107 cells/ml for 48 h,
and nitrite concentrations in cell supernatants were determined. The
experiment was done six times. Data are means ± standard
deviations of triplicate samples from a representative experiment.
|
|
NK cell-derived IFN-
mediates splenocyte nitric oxide
production.
Splenocytes isolated from immune mice 7 days
postinoculation and cultured in vitro for 24 h released
significant amounts of IFN-
compared with saline-inoculated
controls, in which IFN-
release was undetectable (Fig.
3). Depletion of NK cells with antiserum
to asialo GM1 plus complement completely prevented the production of IFN-
by immune spleen cells.

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FIG. 3.
Effect of NK cell depletion on IFN- production.
Splenocytes from saline-inoculated (normal) and SL3235-inoculated
(immune) mice were isolated 7 days postinoculation. The cells were
treated with antiserum to asialo GM1 plus rabbit complement
to deplete NK cells. The cells were cultured for 24 h at a
concentration of 107 cells/ml, and the IFN-
concentration in the cell supernatants was determined by ELISA. The
experiment was done three times. Data are means ± standard
deviations of triplicate samples from a representative experiment.
n.d., not done.
|
|
The role of NK cells and IFN-

in the regulation of nitric oxide
production by immune spleen cells was investigated further
by examining
the effect of exogenous IFN-

on nitric oxide production
by NK
cell-depleted splenocyte cultures. IFN-

treatment increased
nitric
oxide production by NK cell-depleted immune splenocytes
in a
dose-dependent manner (Table
1). While
IFN-

significantly
increased nitric oxide release by both untreated
and NK cell-depleted
immune cells, the increase was proportionally
greater in the NK
cell-depleted cultures, particularly at IFN-

doses
up to 10 U/ml.
At a dose of 100 U/ml, there was no difference in levels
of nitric
oxide production by the untreated and NK cell-depleted immune
cells. IFN-

did not induce significant changes in nitric oxide
production by normal splenocytes, which was assayed at 2.2 ± 0.1
µM.
Role of NK cells and IFN-
in Salmonella-induced
immunosuppression.
In order to assess the effect of NK cell
depletion on immunocompetence, the primary antibody response to sheep
erythrocytes was assessed in vitro by using a PFC assay. Consistent
with our previously published results (3), SL3235
inoculation resulted in an almost complete suppression of the ability
of splenocytes to generate a PFC response (Fig.
4). NK cell depletion by treatment with
antiserum to asialo GM1 plus complement reversed the
suppression of PFC responses to 67% of normal. Treatment of the NK
cell-depleted immune splenocytes with IFN-
(100 U/ml) restored the
suppression (Fig. 4).

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FIG. 4.
Effect of NK cell depletion on PFC responses.
Splenocytes for saline-inoculated (normal) and SL3235-inoculated
(immune) mice were isolated 7 days postinoculation. The cells were
treated with asialo GM1 antiserum ( ) plus rabbit
complement to deplete NK cells, as described in Materials and Methods,
or left untreated and cultured for 5 days with sheep erythrocytes.
Selected wells were treated with IFN- (100 U/ml). PFC responses were
assessed as described in Materials and Methods. The experiment was done
three times. Data are means ± standard deviations of triplicate
samples from a representative experiment.
|
|
The effect of NK cell depletion on immunocompetence was probed further
by depletion of NK cells by using anti-NK1.1 plus complement
(Table
2). Anti-NK1.1 prevented the suppression
of PFC responses
in a manner similar to that of anti-asialo
GM
1. Immune cells depleted
with anti-NK1.1 and then treated
with IFN-

(100 U/ml) were suppressed
to a degree comparable to that
of nondepleted immune splenocytes
in terms of their ability to generate
a PFC response.
 |
DISCUSSION |
Consistent with our previously published results, inoculation of
mice with SL3235 induced significant quantities of nitric oxide and a
profound suppression of the capacity of splenocytes to generate an in
vitro immune response to a non-Salmonella antigen, sheep
erythrocytes (5, 22). The present study also demonstrated that IFN-
was constitutively released by immune splenocytes and that
NK cells are the likely source of the IFN-
, because NK cell depletion abrogated its release. Furthermore, NK cells were shown to be
critical for maintaining both nitric oxide production and immunosuppression, because NK cell depletion with either antiserum against asialo GM1 or antibodies against NK1.1 ablated
nitric oxide production and also restored PFC responses to normal. The marker asialo GM1 is found primarily on NK cells (26,
27); however, other immune cells can express the marker (47,
53). Therefore, we confirmed the finding by using a more
specific marker for NK cells, NK1.1 (18). Since our results
were consistent, whether NK cells were depleted with anti-asialo
GM1 or anti-NK1.1, we conclude that it is NK cells, rather
than other cells of the immune system that express the asialo
GM1 antigen, which are critical for nitric oxide-mediated
immunosuppression induced by Salmonella infection.
Nitric oxide is an important component of M
-dependent cytotoxicity
and cytostatic activity against tumor cells and microbes (reviewed in
references 16 and 35). However,
increasing evidence has demonstrated a role for nitric oxide in the
induction of immunosuppression (2, 14, 32). M
s produce
nitric oxide via the inducible form of the nitric oxide synthase (iNOS)
(35). A number of infectious agents can induce nitric oxide
and IFN-
production and the resultant immunosuppression (1, 5,
8, 19, 25, 45, 46). In vitro, lipopolysaccharide (LPS) in
combination with IFN-
is a potent inducer of iNOS expression by
M
s (11). Our results presented here are consistent with a
critical role for IFN-
in the induction of M
nitric oxide
production following microbial challenge. An essential role for LPS in
the induction of iNOS expression is not evident, because organisms
other than gram-negative bacteria (i.e., Leishmania spp. and
Mycobacterium bovis BCG) can also induce iNOS expression in
M
s (45, 55).
The importance of IFN-
in the generation of nitric oxide-mediated
immunosuppression was demonstrated by the ability of exogenous IFN-
to restore nitric oxide production and immunosuppression to NK
cell-depleted immune splenocyte cultures. Because NK cells are known to
be a major cellular source of IFN-
, the data suggest that NK cells
mediate nitric oxide-induced immunosuppression in Salmonella-inoculated mice via the release of this cytokine.
Limited studies have demonstrated a link between NK cells and the
induction of nitric oxide production by M
s (23, 41, 44,
55). The present study supports this conclusion and suggests that
NK cell production of IFN-
is critical. A novel aspect of the
present work is the link between NK cells, IFN-
levels, nitric oxide levels, and immunosuppression. Studies have linked M
interleukin 12 (IL-12) release to NK cell activation and nitric oxide production (23, 44). IL-12 is a potent inducer of IFN-
release by
both NK cells and T cells (7). A pathway for the induction
of M
nitric oxide production by NK cell-derived IFN-
would be
consistent with results from previous studies from our laboratory
showing that the administration of anti-IL-12 antibodies to mice prior to Salmonella inoculation completely blocked splenocyte
nitric oxide production and immunosuppression (43).
Additionally, anti-IFN-
treatment in vitro blocked nitric
oxide production and immunosuppression in immune
splenocyte cultures, and IFN-
treatment of splenocytes from
anti-IL-12-treated immune mice, which were not immunosuppressed, restored immunosuppression (43).
Interestingly, some studies have shown that nitric oxide can inhibit NK
cell activity (24, 54); however, these studies have focused
on pharmacological nitric oxide donors rather than biological
phenomena. We have previously shown that NK cell cytotoxic activity is
enhanced early after Salmonella infection and returns to
basal levels by 7 days postinfection (41). The time course of nitric oxide production presented here (Fig. 1) suggests that nitric
oxide may have a role in the down regulation of NK cell activity
following Salmonella infection, because elevated nitric oxide production at 3 to 7 days postinfection correlates with the loss
of enhanced NK cell cytoxicity previously reported
(41). Thus, nitric oxide may provide an important biological
feedback mechanism for dampening the proinflammatory cascade
induced by Salmonella infection. Conversely, other
investigators have suggested that NK cells can produce nitric oxide
directly (9, 17). However, this is unlikely to be a major
component of the nitric oxide produced by immune splenocytes in the
present study, because treatment of NK-depleted cells with IFN-
restored nitric oxide production to levels comparable to those of
nondepleted immune cells. Additionally, previous results have
established M
s as the nitric oxide-producing cells following
Salmonella infection (5, 13, 22).
A role for T cells in the induction of nitric oxide-mediated
immunosuppression following Salmonella infection cannot be
completely ruled out, because specific T-cell subsets have previously
been shown to express asialo GM1 (47) and/or
NK1.1 (reviewed in reference 50). NK cells may also
indirectly stimulate M
nitric oxide production via activation of
T-cell populations. While we cannot dismiss a role for these specific
T-cell subsets, previous observations by our laboratory indicate
that immune splenocytes depleted of Thy1.2+ T cells
remained immunosuppressive (3), therefore suggesting that
immune T cells are not critical to the immunosuppression. The results
presented here are consistent with our previous findings demonstrating
a critical role for NK cells in the immune response to
Salmonella (41) and the work of other
investigators using different microbes that induce nitric oxide
production through an NK cell-dependent pathway (42, 55).
Nonetheless, more detailed analysis of T-cell subsets is warranted to
rule out their role in the induction of nitric oxide-mediated
immunosuppression following Salmonella infection.
Our previous study demonstrated that a vaccine strain of
Salmonella induced NK cell-mediated protection against
virulent Salmonella strains (41), while the
present work indicates that NK cells are critical to the suppression of
the immune response to a non-Salmonella antigen. Therefore,
an apparent paradox exists, with NK cells being important to both the
immunity to virulent Salmonella challenge and the
development of nitric oxide-mediated immunosuppression. Previously we
have shown that during the first 3 weeks postimmunization with
attenuated Salmonella, T-cell-dependent responses to
Salmonella antigen are not evident (28-30), and
by 1 month postimmunization, T-cell responses to
Salmonella antigen return (29). These
observations suggest that NK cells, as part of the innate immune
response, are important in the clearance of
Salmonella early postimmunization. The mechanism by
which NK cells contribute to the clearance of Salmonella
from the host is unknown, but nitric oxide derived from iNOS seems to
be critical (34). This concept is supported by the findings
reported here and other recent studies showing that
peroxynitrite, which is formed by the reaction of nitric oxide and
superoxide, is an important antimicrobial mechanism against
S. typhimurium in mice (48). Therefore, it
appears that NK cells mediate both protection against virulent
Salmonella strains and immunosuppression of immune responses
to non-Salmonella antigens by the same mechanism, namely
M
-derived nitric oxide.
In conclusion, these findings indicate that NK cell-derived IFN-
,
through the activation of nitric oxide production by M
s, is a
critical factor in inducing immunosuppression in a murine model of
Salmonella infection.
 |
ACKNOWLEDGMENTS |
These studies were supported by NIH grant AI15613. M. G. Schwacha was supported by a postdoctoral trainee fellowship (NIH grant T32 AI07101) while these studies were being conducted.
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Microbiology and Immunology, Temple University School of Medicine, 3400 North Broad St., Philadelphia, PA 19140. Phone: (215) 707-3585. Fax:
(215) 707-7920. E-mail: tke{at}astro.ocis.temple.edu.
Present address: Center for Surgical Research, Department of
Surgery, Brown University School of Medicine and Rhode Island Hospital,
Providence, RI 02903.
Editor:
J. R. McGhee
 |
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Infection and Immunity, December 1998, p. 5862-5866, Vol. 66, No. 12
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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