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Infect Immun, July 1998, p. 3164-3169, Vol. 66, No. 7
0019-9567/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Role of Alveolar Macrophages in Initiation and
Regulation of Inflammation in Pseudomonas aeruginosa
Pneumonia
Kunihiko
Kooguchi,1,*
Satoru
Hashimoto,1
Atsuko
Kobayashi,1
Yoshihiro
Kitamura,1
Ichidai
Kudoh,2
Jeanine
Wiener-Kronish,3 and
Teiji
Sawa3
Department of Anesthesiology, Kyoto
Prefectural University of Medicine, Kyoto,1 and
Department of Anesthesiology, Yokohama City University
School of Medicine, Kanagawa,2 Japan,
and
Department of Anesthesiology, University of California
San Francisco, San Francisco, California3
Received 1 December 1997/Returned for modification 10 February
1998/Accepted 23 April 1998
 |
ABSTRACT |
To evaluate the role of alveolar macrophages (AMs) in acute
Pseudomonas aeruginosa pneumonia in mice, AMs were depleted
by aerosol inhalation of liposomes containing clodronate disodium. AM-depleted mice were then intratracheally infected with 5 × 105 CFU of P. aeruginosa. In addition to
monitoring neutrophil recruitment and chemokine releases, lung injury
was evaluated soon after infection (8 h) and at a later time (48 h). At
8 h, depletion of AMs reduced neutrophil recruitment, chemokine
release, and lung injury. At 48 h, however, depletion of AMs
decreased bacterial clearance and resulted in delayed movement of
neutrophils from the site of inflammation with aggravated lung injury.
With instillation of 5 × 107 CFU of bacteria,
AM-depleted mice showed low mortality within 24 h of infection but
high mortality at a later time, in contrast to non-AM-depleted mice.
These results demonstrate that depletion of AMs has beneficial early
effects but deleterious late effects on lung injury and survival in
cases of P. aeruginosa pneumonia.
 |
INTRODUCTION |
Alveolar macrophages (AMs), the
resident mononuclear phagocytes of lung alveoli, are the first line of
cell-mediated defenses against organisms inhaled into the lower
respiratory tract (1). Besides their scavenger functions,
such as phagocytosis and subsequent digestion of pathogens, AMs can
release various chemical mediators upon contact with pathogens or
pathogenic substances (13, 17). The mediators released from
AMs promote the generation of subsequent inflammatory responses
(15). Among those inflammatory responses, early recruitment
of polymorphonuclear leukocytes (PMNs) to the site of infection is one
of the critical responses for rapid clearance of organisms (5,
11). To examine the role of AMs in initial PMN recruitment after
gram-negative bacterial infection, we previously depleted AMs in rats
by aerosol inhalation of negatively charged large oligolamellar
liposomes encapsulating clodronate disodium (Cl2MDP-liposomes) (8). This method cleared over
95% of AMs from the lungs without causing lung damage or airspace
neutrophilia. Depletion of AMs attenuated initial PMN recruitment and
chemokine (macrophage inflammatory protein 2 [MIP-2] and CINC/GRO)
production in Pseudomonas aeruginosa pneumonia. These
results suggest that AM-derived mediators, such as MIP-2 and CINC/GRO,
contribute to neutrophil recruitment.
Although an inflammatory response is essential to clear pathogens from
the site of infection, a prolonged inflammatory response might worsen
lung injury and might actually interfere with pathogen elimination
(12, 16). Along with the fact that macrophages release
various mediators (tumor necrosis factor alpha and nitric oxide, etc.)
which activate PMNs, the role of AMs as regulators of inflammatory
responses was recently reported; AMs engulf apoptonic PMNs and release
anti-inflammatory mediators (6, 14). These findings imply
that the role of AMs in the inflammatory response is not limited to
generation of the initial inflammation but extends to regulation of the
inflammatory responses, including effective elimination of pathogens
phagocytosed by PMNs at a later phase (7).
In this study, we depleted >95% of AMs by aerosol inhalation of
Cl2MDP-liposomes in mice to evaluate the role of AMs in the initiation and regulation of inflammation induced by
Pseudomonas infection. This was done by monitoring PMN
recruitment, releases of chemoattractants, such as MIP-2 and KC, lung
injury, and survival at an early time (8 h) and a later time (48 h).
 |
MATERIALS AND METHODS |
Bacterial strain and culture.
P. aeruginosa PAO1 was
generously provided by K. Okuda (Yokohama City University, Kanagawa,
Japan). Bacteria from frozen stock were streaked on tryptic soy agar
(Difco Laboratories, Detroit, Mich.) for 24 h at 37°C and then
grown in tryptic soy broth (TSB; Difco) for 12 h at 32°C in a
shaking incubator. The bacteria in the broth culture were sedimented by
centrifugation at 4,000 × g for 20 min and resuspended
in Ringer's lactate solution. The concentration of bacteria was
measured by spectrophotometry. The number of bacteria was also
determined by diluting all samples and plating them for quantitative
culture. Bacteria were serially suspended to a density of
107 or 109 CFU/ml in phosphate-buffered saline
(PBS) without calcium [PBS(
)].
Animal model of P. aeruginosa pneumonia.
Male
specific-pathogen-free CD-1 mice (Japan S.L.C. Co., Ltd., Shizuoka,
Japan) weighing 35 g to 37 g were lightly anesthetized with
inhaled sevoflurane. Fifty microliters of bacterial solution was then
slowly instilled into the left lung through a gavage needle (modified
animal feeding needle, 24 gauge; Popper & Sons, Inc., New Hyde Park,
N.Y.) inserted into the trachea via the mouth. A total of 5 × 107 CFU (large-inoculum model) of bacteria were instilled
for the survival study, and 5 × 105 CFU
(small-inoculum model) were instilled for other experiments. The mice
recovered from anesthesia within 2 min. All animal procedures were
approved by the Animal Care Committee of the Kyoto Prefectural University of Medicine.
Preparation of Cl2MDP-liposomes.
Cl2MDP-liposomes were prepared as described previously
(8). Briefly, 80 mg of phosphatidylcholine and 30 mg of
cholesterol were dissolved in 4 ml of chloroform in a 200-ml
round-bottom flask. After low-vacuum rotary evaporation of the
chloroform, a thin film formed on the inner surface of the flask. Then,
3 ml of distilled water containing 0.4 g of disodium clodronate (dichloromethylene-biphosphonate; Boehringer Mannheim GmbH, Mannheim, Germany) and 2 ml of ether were added to the flask. The fluid was
sonicated for 2 min and then reduced by low-vacuum evaporation until
the ether was slowly removed at 30°C. The liposome suspension was
then extruded sequentially through both 0.8- and 0.2-µm-pore-size cellulose acetate syringe-type filters (Corning Glass Works, Corning, N.Y.). The unencapsulated drug was separated from drug-containing liposomes by high-speed centrifugation (10,000 × g for
60 min), and the pellet was resuspended in 10 ml of PBS(
).
Depletion of AMs by aerosol inhalation of
Cl2MDP-liposomes.
Cl2MDP-liposomes were
aerosolized to deplete AMs 72 h before bacterial instillation. An
aerosol device was employed to deliver the liposomes. Mice were placed
in a nose-only aerosol chamber (Intox Products, Albuquerque, N. Mex.).
An Aerotech II (Bedford, Mass.) nebulizer (CIS-II) supplied with
compressed air at a flow rate of 12 liters/min was used to generate the
liposome aerosol. To assess the effectiveness of
Cl2MDP-liposomes in depleting AMs, the mice were subjected
to bronchoalveolar lavage (BAL) 24 h, 48 h, 72 h,
96 h, 5 days, 14 days, and 28 days after liposome aerosol
inhalation. Control mice were exposed to saline aerosol in the same
manner.
BAL.
At predetermined intervals after the instillation of
Pseudomonas into the air space, the mice were anesthetized
with pentobarbital (200 mg/kg of body weight) and then exsanguinated by
aortic transection. The trachea was exposed by a midline incision and
cannulated with a sterile polypropylene 18-gauge catheter. The lungs
were lavaged with 5 ml of PBS supplemented with 0.6 mM EDTA in 0.2-ml
increments. BAL fluid was centrifuged at 1,000 × g for
8 min at 4°C. The total returns after lavage averaged 4 to 4.5 ml/mouse. Occasionally, lesions in lung tissue caused intrathoracic
leakage of the lavage fluid. This resulted in smaller recovery volumes,
and quantitative data from these animals were discarded. The cell
pellet obtained from centrifugation of BAL fluid was diluted in 1.0 ml
of PBS(
). The total cell counts were performed with a hemacytometer
after staining the cells with gentian violet. Cell viability was
determined by trypan blue exclusion. Differential cell counts were done
on cytocentrifuge preparations (Cytospin 3; Shandon Southern
Instruments, Sewickley, Pa.) stained with modified Giemsa stain
(Diff-Quick; Baxter, McGaw Park, Ill.).
Measurement of the permeability of alveolar epithelium by
albumin.
A radioactive tracer was used to measure the flux of
albumin across the alveolar epithelial barrier, as described previously (10). An instillation that included an alveolar protein
tracer (0.05 µCi of 125I-labelled albumin) was prepared.
Four or 44 h after the instillation of P. aeruginosa,
50 µl of the alveolar protein tracer was instilled into the left
lung. Four hours after the instillation of the tracer, the mice were
anesthetized with 2.0 mg of pentobarbital intraperitoneally and blood
was collected by carotid arterial puncture. After the mice received
additional pentobarbital, sternotomy was done. The lungs, trachea, and
oropharynx were harvested, and the radioactivity of these samples was
measured. The percentage of radioactive albumin that left the lung and
entered the circulation was calculated by multiplying the counts
measured in the terminal blood sample (per milliliter) times the blood
volume [body weight × 0.07 (1
haematocrit)].
Measurement of excess lung water.
The
wet-weight-to-dry-weight (W/D) ratio of the whole lungs was estimated.
The lungs were dissected, weighed, and then dried at 60°C for 5 days.
The W/D ratio was then calculated by dividing the wet weight by the
final dry weight.
Bacterial culture.
The lungs were removed and homogenized in
3 ml of sterile water. Lung homogenates were sequentially diluted and
plated on sheep blood agar plates for quantitative culture. The number
of bacteria in each lung was calculated by multiplying the weight of
the lung by the number of CFU. The detection limit was 100 CFU/lung.
MPO assay.
The enzyme myeloperoxidase (MPO) was used as a
tracer to quantitate PMN sequestration in tissues, because the MPO
assay has been found to be more sensitive than quantitative histology
for detecting sequestered neutrophils. A sample of blotted lung tissue (0.5 g) was homogenized in 5 ml of 0.01 M potassium phosphate buffer
(PPB; pH 7.4) containing 1 mM EDTA. Two milliliters of the homogenate
and 5 ml of 0.01 M PPB containing 1 mM EDTA were mixed gently and then
centrifuged at 10,000 × g for 20 min at 4°C. The
pellet was rehomogenized in 5 ml of 0.05 M PPB (pH 6.0) containing
0.5% hexadecyltrimethylammonium bromide. This suspension was frozen,
thawed, and sonicated with a Branson cell disrupter (Heat Systems;
Ultronics, Plainview, N.Y.) at 65 W for 1 min. Then, 0.1-ml aliquots
were mixed with 0.79 ml of 0.08 M PPB (pH 5.4) and 0.1 ml of 1 mM
tetramethylbenzidine dissolved in
N,N-dimethylformamide at 37°C. After 2 min,
0.01 ml of 30 mM H2O2 was added. This solution was incubated for 3 min at 37°C, and then a 300-µg/ml concentration of a 0.05-ml catalase solution was added. This mixture was then diluted
with 4 ml of 0.2 M sodium acetate (pH 3.0) and centrifuged at
12,000 × g for 10 min at 4°C. The supernatant was
read in a spectrophotometer (U-2000; Hitachi, Tokyo, Japan). One unit
of enzyme activity was defined as the amount of MPO that produced an
absorbance change of 1.0 optical density unit per minute per milligram
of tissue at 37°C.
Determination of MIP-2 and KC by ELISA.
MIP-2 and KC are
known as major chemoattractants for neutrophil or C-X-C chemokines in
rodents (3). Thus, MIP-2 and KC concentrations in BAL fluid
were measured at 8 and 48 h after bacterial inoculation by using
enzyme-linked immunosorbent assay kits (IBL, Gunnma, Japan). The
detection limits for MIP-2 and KC were 2 and 25 pg/ml, respectively.
Histopathological observation.
Six mice (of 24 examined)
were presented for their representative light microscopic findings.
Three mice were exposed to aerosolized Cl2MDP-liposomes
(AM-depleted group), and the other mice were exposed to PBS(
)
liposomes (control group). One mouse from each group was processed
72 h after the aerosolization. Two mice of each group received
5 × 105 CFU of bacteria in the trachea 72 h
after the aerosolization and were processed 8 or 48 h after the
bacterial instillation. All mice were exsanguinated by transection of
the abdominal aorta under anesthesia. The lungs were then removed en
bloc and processed for hematoxylin-eosin staining.
Statistical analysis.
All data are reported as the mean ± standard deviation (SD). Multiple comparisons were performed by
using one-way analysis of variance, followed by Scheffe's F test.
Survival data were compared by Fisher's exact test. All other data
were compared by Student's unpaired t test. Significance
was assigned at a P of <0.05.
 |
RESULTS |
More than 95% of AMs were depleted by aerosolization of
Cl2MDP-liposomes.
The effect of
Cl2MDP-liposomes was evaluated by counting the remaining
macrophages in the BAL fluid (Fig. 1; see
Fig. 4b). After aerosol inhalation of Cl2MDP-liposomes, the
number of AMs recovered from BAL fluid was reduced to less than 5% of
normal within 3 days. After day 5, the number of AMs recovered
gradually and finally reached the normal level on day 28.

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FIG. 1.
Changes of neutrophil and macrophage numbers in BAL
fluid from mice given an aerosol of Cl2MDP-liposomes. Mouse
lungs were lavaged 1, 2, 3, 4, 5, 8, 14, and 28 days after aerosol
administration (n 5 per group). At 3 days, 95% ± 2% of AMs were depleted. There was a significant reduction in the
number of macrophages up to 14 days compared to that of saline-treated
controls. The Cl2MDP-liposome aerosolization hardly induced
neutrophil infiltration into alveolar spaces. Data are expressed as
means + SD. *, significant at P of <0.05 (Student's
t test) compared to value before aerosol delivery.
|
|
Depletion of AMs delayed early neutrophil recruitment but prolonged
subsequent inflammatory cell recruitment.
PMN and
monocyte/macrophage recruitment after instillation of P. aeruginosa in AM-depleted mice and control mice were evaluated by
whole-lung BAL (Fig. 2). After
instillation of P. aeruginosa (5 × 105
CFU), in control mice, the number of PMNs recovered from BAL fluid
increased rapidly and reached a peak at 16 h (Fig. 2a). In
AM-depleted mice, fewer PMNs were recovered from BAL fluid at an early
phase (0 to 16 h) but larger quantities of PMNs were obtained in
the late phase (24 to 96 h). The number of monocytes/macrophages was significantly lower in AM-depleted mice than that in control mice
at 0 to 16 h (Fig. 2b). However, a rapid increase of cells was
observed after 24 h in both groups and there was a significant increase of monocytes/macrophages in AM-depleted mice after 72 h.
MPO activity in the supernatant of lung homogenates was measured to
estimate the total number of PMNs trapped in the lungs. Eight hours
after bacterial challenge, MPO activity was significantly increased in
the control group compared to that in the AM-depleted group. In
contrast, the MPO activity in the AM-depleted group significantly
exceeded that in the control group after 48 h (Table 1).

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FIG. 2.
Number of PMNs and macrophages in BAL fluid and number
of bacteria (CFU) in lung tissue from mice inoculated with P. aeruginosa. AM-depleted mice (solid bars) and control mice (open
bars) received 5 × 105 CFU of P. aeruginosa intratracheally and were sacrificed and analyzed at the
specified time intervals. Each data point represents the mean + SD
of values for five or more mice. *, P < 0.05 versus
value for control mice. (a) PMNs in BAL fluid; (b) AMs in BAL fluid;
(c) bacterial counts in lung tissue.
|
|
Delayed bacterial clearance in AM-depleted mice.
The number of
bacteria in the lungs of the mice is shown in Fig. 2c. There were
significantly more bacteria in the lungs of AM-depleted mice than in
those of control mice between 4 and 48 h.
Depletion of AMs significantly reduced early chemokine release
but sustained later release.
Two chemokines, MIP-2 and KC, were
investigated in BAL fluids after bacterial instillation. Eight
hours after bacterial instillation, the MIP-2 concentration in the BAL
fluid from control mice was significantly higher than the concentration
in AM-depleted mice. The KC concentration was also higher in control
mice. In contrast, 48 h after bacterial instillation, the levels
of both chemokines were significantly higher in AM-depleted mice than
in control mice (Table 2).
Lung injury was improved in the early phase but was subsequently
aggravated in AM-depleted mice.
Lung injury after bacterial
instillation (5 × 105 CFU) was evaluated by measuring
efflux of the alveolar protein tracer from the lungs into the
circulation and by measurement of lung edema (Fig.
3). Eight hours after bacterial
instillation, there was no difference in the lung epithelial injury
between control and AM-depleted mice (Fig. 3a). However, 48 h
after the bacterial instillation, the lung epithelial injury was
significantly increased in the AM-depleted mice compared to that in
control mice. Although the lung edema in AM-depleted mice was
significantly less than that in the control mice at 8 h, edema
increased significantly by 48 h in AM-depleted mice (Fig. 3b).

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FIG. 3.
Lung injury in five groups (untreated mice,
n = 4; control mice instilled with saline,
n = 4; AM-depleted mice instilled with saline,
n = 4; control mice instilled with P. aeruginosa, n = 6; and AM-depleted mice instilled
with P. aeruginosa, n = 6). (a) Movement of
alveolar protein tracer, 125I-albumin, from the lungs into
the blood circulation. At 48 h, AM-depleted mice instilled with
P. aeruginosa showed increased efflux of the tracer compared
to that of control mice instilled with P. aeruginosa. (b)
Pulmonary edema was assessed from the lung tissue W/D ratios. At 8 h after inoculation, the W/D ratio increased in control mice compared
to that in AM-depleted mice. Conversely, the AM-depleted mice instilled
with P. aeruginosa showed high W/D ratios compared to that
in control mice instilled with P. aeruginosa at 48 h.
*, P < 0.05 versus value for control mice instilled
with saline; , P < 0.05 for control mice instilled
with P. aeruginosa versus AM-depleted mice instilled with
P. aeruginosa. Statistical analysis was done by one-way
analysis of variance and Scheffe's F test. Values are means + SDs.
|
|
Histopathological aspects of lung injury.
Histopathological
findings are shown in Fig. 4. In the
control mouse without P. aeruginosa instillation, there was
a normal distribution of AMs (Fig. 4a); these were absent in the
AM-depleted mouse without P. aeruginosa instillation (Fig.
4b). Eight hours after the bacterial instillation, few PMNs were
infiltrated into airspace in the AM-depleted mouse (Fig. 4d), in
contrast to the control mouse which showed significant predominance of
PMNs in the airspaces (Fig. 4c). On the other hand, 48 h after the
bacterial instillation, destruction of alveolar structure and
thickening of interstitial spaces were marked in AM-depleted mice (Fig.
4f).

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FIG. 4.
Histologic assessment (hematoxylin-eosin staining) of
mouse lung tissue. Mice were exposed to PBS( )-containing liposomes
(control group) (a, c, and e) or Cl2MDP-liposomes
(AM-depleted group) (b, d, and f). (a) Tissue from control mouse
without P. aeruginosa instillation. Arrows indicate AMs. (b)
Tissue from AM-depleted mouse without P. aeruginosa
instillation. (c) Tissue from control mouse 8 h after P. aeruginosa instillation. (d) Tissue from AM-depleted mouse 8 h after P. aeruginosa instillation. (e) Tissue from control
mouse 48 h after P. aeruginosa instillation. (f) Tissue
from AM-depleted mouse 48 h after P. aeruginosa
instillation. Original magnification, ×125.
|
|
Depletion of AMs improved early-phase infection but not later
survival in mice with P. aeruginosa pneumonia in the
large-inoculum study.
Survival after the instillation of P. aeruginosa (5 × 107 CFU) in AM-depleted mice and
control mice is shown in Fig. 5. After the instillation of P. aeruginosa, 22.8% of mice died
within 12 h in the control group (non-AM depleted), whereas none
of the mice in the AM-depleted group died by this time point. However, more than 90% of AM-depleted mice died by 96 h after bacterial instillation, while 50% of control mice survived. With an inoculum of
5 × 105 CFU, none of the mice in either group were
dead after bacterial instillation.

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FIG. 5.
Effect of Cl2MDP-liposome inhalation on
survival of mice after P. aeruginosa instillation (5 × 107 CFU) into the lungs. Mice were given an aerosol of
Cl2MDP-liposomes 72 h before bacterial instillation
(AM-depleted group, n = 34). In the control group, mice
received an inhalation of saline (control group, n = 36). There was a significant difference (* and ) between the values
of the two groups at 12, 96, and 168 h by Fisher's exact test.
|
|
 |
DISCUSSION |
Depletion of AMs with Cl2MDP-liposomes.
Liposome-mediated elimination of macrophages has become a standard
approach to evaluate the roles of macrophages in host defenses (21). Selective in vivo depletion of AMs by intratracheal
instillation of liposome-encapsulated Cl2MDP (clodronate)
has been reported for rodent models (1, 4, 18, 19). Because
only about 75% of AMs can be depleted by intratracheal instillation of
Cl2MDP-liposomes, this method may blunt the total effect of
their depletion. We have recently shown that aerosol inhalation of
negatively charged large oligolamellar liposomes complexed with
clodronate disodium resulted in the depletion of over 95% of rat AMs
without increasing neutrophils nor damaging the epithelium of the lung
(8). In the present study, we depleted >95% of mouse AMs
by this technique and investigated the roles of AMs in airspace
inflammation. As shown in Fig. 3, AM depletion or aerosolization itself
did not cause any detrimental effect on the airspace of the lung.
Acute-phase response to P. aeruginosa in AM-depleted
mice.
In this study, a significant improvement in lung edema (<12
h) after P. aeruginosa instillation in AM-depleted mice was
observed in comparison to that in nondepleted control mice, although
there was a higher bacterial concentration in the lung of AM-depleted mice at 4 to 16 h. Significantly lower MPO activity at 8 h
and fewer PMNs recovered from the BAL fluid at 4 to 16 h in
AM-depleted mice suggest that fewer PMNs were recruited to the lungs
after P. aeruginosa instillation in the early phase of
infection. The significantly lower levels of the chemokines MIP-2 and
KC at 8 h are consistent with low PMN recruitment. These findings
suggest that AMs and/or AM-derived substances play an important role in the pathogenesis of acute lung injury and that the host inflammatory response initiated by AMs is responsible for acute host death in the
early phase of infection. AMs contribute to rapid clearance of
pathogens from the site of infection accompanied by increased tissue
injury caused by the battle to eliminate bacteria, which can sometimes
be life-threatening (16). As we previously found in an
AM-depleted rat model, chemokines released from AMs appear to have a
major role in early recruitment of PMNs to the site of infection
(8). In addition, acute lung edema during the early phase of
bacterial infection in our model seems to be mediated by macrophages or
neutrophils. In the large-inoculum model (5 × 107
CFU) of our study, the survival of AM-depleted mice was improved in the
early phase. These data also show that AMs might make a major
contribution to life-threatening condition in the acute phase of
P. aeruginosa pneumonia.
Late-phase response to P. aeruginosa in AM-depleted
mice.
In contrast to the early-phase results with the small
inoculum of bacteria described above, lung injury (epithelial injury and edema) was significantly worse during the later phase of infection (48 h) in AM-depleted mice. Interestingly, the numbers of PMNs and
monocytes/macrophages in the later phase were higher in AM-depleted mice than in control mice. More PMNs and monocytes/macrophages were
recovered from the BAL fluid, and MPO activity and chemokine levels
were higher in AM-depleted mice than in control mice. These findings
suggest that there was prolonged inflammatory cell recruitment during
the late phase in AM-depleted mice. These results correlated well with
those of a recent study using Klebsiella pneumonia in an
AM-depleted model (4). The higher levels of chemokines, as
well as bacterial chemotactic factors and nonchemokine endogenous responses to unresolved infection, probably contributed to the greater
PMN recruitment at this time point. Because the number of bacteria
remaining in the lungs was greater in AM-depleted mice than in control
mice between 4 and 48 h after bacterial instillation, depletion of
AMs evidently decreased bacterial clearance and caused prolonged
inflammatory responses, including greater PMN aggregation at the site
of infection. Decreased bacterial clearance and prolonged inflammation
appeared to aggravate lung injury in the later phase. The lung injury
observed in the late-phase response of AM-depleted mice itself might
have caused prolonged inflammatory responses, because PMN levels
remained high at 72 to 144 h although bacteria were almost
completely eliminated by 72 h. In addition, in our large-inoculum
model (5 × 107 CFU), AM-depleted mice hardly survived
to day 7 after infection, although half of control mice could survive
for the same period. This result also indicates that the AM-depleted
condition could cause deleterious effects in the late phase of P. aeruginosa pneumonia.
Summary with clinical implications.
Recently, the use of
various anti-inflammatory therapies has been proposed for acute
inflammatory diseases (2, 20). However, we should not forget
that an inflammatory response is essential to eliminate pathogens from
the site of infection (16). The results obtained in
AM-depleted mice illustrate both the beneficial and deleterious effects
of AMs. Depletion of AMs in the early phase of Pseudomonas
pneumonia may initially improve lung injury but may worsen the eventual
outcome. However, temporary blockade of selective macrophage functions
may contribute to decreasing neutrophil-mediated tissue injury and
improving acute survival without compromising the subsequent beneficial
effects of the inflammatory reaction on bacterial pneumonia. The
findings obtained in this study using AM-depleted mice may provide same
suggestions for clinical application of anti-inflammatory therapies.
 |
ACKNOWLEDGMENTS |
We thank Maria Ohara, Yoshifumi Tanaka, and Tukasa Ashihara for
support and advice.
This work was funded by the Japanese Ministry of Education (grant
2002-07407045 to S. Hashimoto).
 |
FOOTNOTES |
*
Corresponding author. Mailing address: Department of
Anesthesiology, Kyoto Prefectural University of Medicine, 465 Kajiicho, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto 602-0841, Japan. Phone: 81 75 251 5633. Fax: 81 75 251 5843. E-mail:
kobi{at}koto.kpu-m.ac.jp.
Editor: R. N. Moore
 |
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Infect Immun, July 1998, p. 3164-3169, Vol. 66, No. 7
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Copyright © 1998, American Society for Microbiology. All rights reserved.
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