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*
Department of Microbiology and Immunology, University of Adelaide, Adelaide, South Australia, Australia; and
Centre de Recherche en Rhumatologie et Immunologie, Centre de Recherche du Centre Hospitalier de lUniversité Laval and Department of Medicine, Faculty of Medicine, Université Laval, Ste-Foy, Quebec, Canada
| Abstract |
|---|
|
|
|---|
(MIP-1
) and IL-8. The
bacterial microbes, Salmonella typhimurium and
Pseudomonas aeruginosa, and Staphylococcus
aureus all strongly induced both IL-8 and MIP-1
secretion,
whereas Streptococcus pneumoniae, Staphylococcus
epidermidis, and the opportunistic yeast Candida
albicans were less potent. Saccharomyces cerevisiae
and zymosan both induced IL-8 secretion but failed to stimulate that of
MIP-1
. Coincubation of neutrophils with the proinflammatory cytokine
TNF-
and the micro-organisms also led to differential expression of
MIP-1
and IL-8. Significant enhancement of the induction of both
MIP-1
and IL-8 by S. typhimurium, P.
aeruginosa, and S. pneumoniae as well as by C.
albicans was observed. In contrast, while IL-8 production in
response to S. cerevisiae and zymosan was enhanced in the
presence of TNF-
, no MIP-1
was produced. These combined results
indicate that while neutrophils exposed to some micro-organisms alone
or in the presence of inflammatory cytokines such as TNF-
will
produce both MIP-1
and IL-8, resulting in generation of signals for
the recruitment of mononuclear leukocytes and neutrophils,
respectively, certain types of micro-organisms can skew this response
toward synthesis of IL-8. | Introduction |
|---|
|
|
|---|
(3, 4, 5, 6, 7, 8, 9); however, one of the most
important recent findings has been that neutrophils undergoing
phagocytosis have the ability to synthesize and release large amounts
of the C-X-C chemokine, IL-8 (10, 11). In fact, it appears that IL-8
may be one of the major protein products secreted by these cells, since
its secretion by neutrophils is of the same magnitude as that by
mononuclear phagocytes under the same conditions.
IL-8 is a prominent member of the chemokine gene superfamily. This
family is composed of four structurally distinct branches, the C-X-C,
C-C, C, and CX3C chemokines (also known as the
,
ß,
, and
chemokines, respectively) (12, 13, 14, 15). The C-X-C
chemokine family includes the neutrophil-activating factor (IL-8),
melanoma growth stimulatory activity, and neutrophil activating
peptide-2. The second branch or the C-C chemokines, includes monocyte
chemotactic protein-1, -2, -3, and -4; I-309; RANTES; and macrophage
inflammatory protein-1
(MIP-1
)3 and -1ß. The C
branch currently has only one member, lymphotactin, as does the
recently discovered CX3C branch, which contains neurotactin
(also known as fractalkine), a molecule that appears to be a
membrane-bound chemokine (16, 17). The biologic importance of the
chemokine gene superfamliy stems from the fact that they attract
overlapping as well as distinct leukocyte populations and are thus
likely to play an important role in leukocyte trafficking. In general,
the C-X-C chemokines are chemotactic and activating factors for
neutrophils, whereas the C-C chemokines attract and activate monocytes,
specific lymphocyte subsets, and granulocytes other than neutrophils.
Lymphotactin has been reported to selectively attract CD8+
T lymphocytes (18), and although discrepancies exist, neurotactin has
been reported to act as a chemoattractant for T cells, monocytes, and
neutrophils (16, 17).
The observation that stimulated neutrophils are able to secrete IL-8, a
chemotactic cytokine to which they can respond, implies that this cell
population may be capable of directing its own recruitment to sites of
inflammation and infection and thus autoamplify acute inflammatory
responses to infection. In addition, recent studies show that
neutrophils also produce the C-C chemokines MIP-1
and MIP-1ß in
response to TNF-
and LPS (19, 20), which theoretically gives them
the capacity to emit signals to recruit mononuclear cells. However, our
recent studies show that MIP-1
synthesis is inhibited by neutrophils
phagocytosing the inflammatory microcrystals monosodium urate (MSU) and
calcium pyrophosphate dihydrate (CPPD) (11), whereas under the same
conditions IL-8 synthesis is enhanced. The present study was undertaken
to determine whether the inhibition of MIP-1
production by MSU and
CPPD aligned with enhancement of IL-8 secretion is a general phenomenon
in neutrophils undergoing phagocytosis. Since a major function of the
neutrophil in the innate immune system is phagocytosis of microbial
pathogens during infection, we have investigated the regulation of
MIP-1
and IL-8 expression by neutrophils during phagocytosis of
microbial pathogens.
| Materials and Methods |
|---|
|
|
|---|
TNF-
was a gift from Knoll Pharmaceuticals (Whippany, NJ).
TNF-
stock was stored at -80°C in PBS containing 0.01% BSA.
Zymosan was obtained from Sigma/Aldrich (Castle Hill, Australia). HBSS
and RPMI medium 1640 were purchased from Life Technologies (Mulgrove,
Australia). Ficoll-Paque and Dextran T-500 were obtained from
AMRAD/Pharmacia (Wantirna, Australia). HyClone FCS was purchased from
Professional Diagnostics (Edmonton, Canada). All the above reagents
contained <5 pg/ml of endotoxin as determined by the amebocyte lysate
assay (BioWhittaker, Walkersville, MD). Staphylococcus
aureus, Salmonella typhimurium, Pseudomonas
aeruginosa, Streptococcus pneumoniae, and
Staphylococcus epidermidis were obtained from stocks within
the Department of Microbiology and Immunology at the University of
Adelaide. Candida albicans and Saccharomyces
cerevisiae were purchased from American Type Culture Collection
(Rockville, MD). Hybond-N membranes,
[
-32P]ATP, and [
-32P]dCTP
were purchased from Amersham Canada (Oakville, Canada). All other
reagents used in this study were of molecular biologic grade and were
obtained from Sigma/Aldrich.
Preparation of opsonized zymosan and opsonized micro-organisms
The bacteria and yeast were harvested, resuspended in 20 ml of sterile RPMI 1640, and incubated for 1 h at 100°C. The bacteria were washed twice with sterile RPMI 1640 and counted. The pellet was resuspended in 20 ml of nondecomplemented human serum and incubated for 1 h at 37°C, and the bacteria were washed twice with RPMI 1640. The pellet was then resuspended in a sterile solution of RPMI 1640 at a concentration of 1010 microbes/ml. The same procedure was followed to opsonize zymosan at a final concentration of 30 mg/ml.
Isolation of human PMNL
Whole blood was obtained by venipuncture and collected into
tubes containing heparin; following dextran sedimentation, neutrophils
were purified by centrifugation on Ficoll-Paque cushions (11, 21).
Erythrocytes were removed by hypotonic lysis, and the cells were
resuspended in RPMI 1640 supplemented with 1% FCS at a final
concentration of 107 cells/ml. One-milliliter aliquots
were used for all experimental points except those involved in the
generation of total RNA for Northern analysis, in which case 4-ml
aliquots were used. Cell viability, as determined by trypan blue
exclusion, was >98%, and the percentage of neutrophils in the cell
preparations used in this study exceeded 97%. On average, the other
3% comprised other granulocytes (2%), lymphocytes (0.8%), and
monocytes (at most 0.2%). Only neutrophil preparations containing
<2 x 104 monocytes/107 cells were used
in this study. Detectable levels of IL-8 and MIP-1
were only
observed when 5 x 104 purified monocytes or more were
incubated in the presence of TNF-
or phagocytic agonists. The entire
separation was conducted under sterile conditions at room
temperature.
Isolation of cytoplasmic RNA and Northern blot analysis
After activating cells with the desired stimuli, total RNA was
prepared by the RNAzol method as recommended by the supplier, and
Northern blots were performed as previously described (6, 11). The cDNA
probes used in this study were radiolabeled with
[
-32P]dCTP using the random primers DNA labeling
system. The human MIP-1
cDNA was isolated using PCR with an
antisense primer made against nucleotides 283 to 236 of the published
LD78 sequence and a sense primer comprising nucleotides -19 to 3 (11).
The IL-8 probe used in this study was a 244-bp
PstI/EcoRI cDNA fragment representing the coding
region of the IL-8 cDNA from nucleotides 49 to 293 (11). To confirm
equal loading of RNA, the membranes were rehybridized with a synthetic
oligonucleotide for 28S ribosomal RNA (data not shown) as described
previously (22).
ELISA for IL-8 and MIP-1
Human neutrophils were isolated and resuspended at
107 cells/ml in 4-ml sterile tubes and treated as
described in the figure legends. The supernatants were collected and
analyzed for chemokine content. In experiments involving the assessment
of intracellular chemokine content, the cell pellets were washed three
times in ice-cold sterile PBS and lysed by three consecutive
freeze-thaw cycles. Levels of IL-8 and MIP-1
were assessed using
ELISA kits purchased from R&D Systems, Inc. (Minneapolis, MN).
Statistical analysis
Numerical values in the figures are the levels of immunoreactive
IL-8 or MIP-1
per 107 neutrophils (mean ±
SEM). The data for each group were analyzed as a repeated measures
analysis of variance with an unstructured covariance matrix.
Statistical significance was considered to be achieved at
p < 0.05.
| Results |
|---|
|
|
|---|
and IL-8 in neutrophils
To determine the basic kinetics of chemokine production by
neutrophils in response to microbial pathogens, the cells were
incubated with diluent, TNF-
(100 ng/ml), opsonized zymosan (1
mg/ml), or opsonized heat-killed S. aureus (10:1,
bacteria:neutrophil) at 37°C for increasing periods of time. The
concentration of each of these agonists was chosen from the results of
previous studies (4, 6, 8, 9, 23) or from pilot experiments (not
shown). A time-dependent release of MIP-1
and IL-8 was observed in
response to TNF-
(Fig. 1
). Heat-killed
opsonized S. aureus also led to a time-dependent release of
both MIP-1
and IL-8. In contrast, opsonized zymosan stimulated the
production of IL-8 but not MIP-1
. Opsonization enhanced the
efficiency of stimulation of chemokine production, increasing both the
rate and the amount of synthesis (data not shown).
|
and IL-8 by neutrophils. Neutrophils were
therefore incubated with heat-killed opsonized bacterial
micro-organisms at a ratio of 10:1 (micro-organisms:neutrophils) at
37°C for 24 h. The supernatants were collected and analyzed for
IL-8 and MIP-1
content. In terms of IL-8 secretion, the most potent
agonists were S. typhimurium, P. aeruginosa, and
S. aureus (Table I
production compared with basal levels. Moreover, as observed
with IL-8 secretion, while stimulating a release of MIP-1
that was
significantly greater than the control value, S.
pneumoniae, S. epidermidis, and C. albicans
were far less potent. In contrast to that observed with respect to IL-8
secretion, S. cerivisiae and zymosan failed to induce
detectable secretion of MIP-1
and, in fact, significantly inhibited
the basal release of MIP-1
.
|
and IL-8 mRNA in neutrophils
To determine whether the micro-organisms
up-regulated MIP-1
and IL-8 expression at the mRNA level,
Northern blots were performed. Neutrophils were stimulated
under the conditions described above, except that the period of
incubation was 3 h rather than 24 h. The cell pellets were
collected, total RNA was extracted, and equal amounts of RNA were
loaded onto agarose gels for transfer onto nylon filters for Northern
analysis (Fig. 2
). According to
densitometric values (data not shown), all agents except C.
albicans and S. cerevisiae induced an accumulation of
IL-8 mRNA greater than that of the control. However, while
densitometric analysis (data not shown) indicated that S.
aureus, S. pneumoniae, S. epidermidis, and
P. aeruginosa stimulated an accumulation of IL-8
mRNA greater than the control levels, they were not as potent as
zymosan or S. typhimurium. In contrast, all the microbes
except C. albicans and S. cerivisiae induced an
increase in the level of MIP-1
mRNA at least fivefold
greater than the control level. Zymosan also strongly induced
accumulation of MIP-1
mRNA.
|
and micro-organisms in combination on MIP-1
and IL-8 production
Since neutrophils at sites of inflammation or infection are likely
to be exposed simultaneously to several agonists, either soluble or
particulate, we examined the effect of coincubation of neutrophils with
TNF-
and the micro-organisms on MIP-1
and IL-8 production.
Neutrophils were therefore incubated at 37°C with diluent or the
micro-organisms, either alone or in combination with TNF-
for
24 h. The supernatants were collected, and the amounts of MIP-1
and IL-8 present were assessed by ELISA. As described above, S.
typhimurium, P. aeruginosa, and S. aureus
alone were able to induce significant release of IL-8 and MIP-1
,
whereas S. pneumoniae and S. epidermidis were far
less potent (Fig. 3
). When neutrophils
were coincubated with TNF-
and the microbial pathogens, differential
effects were observed on IL-8 and MIP-1
synthesis. Taking into
account the effect of TNF-
alone, IL-8 and MIP-1
secretion was
significantly enhanced only in response to S.
typhimurium, P. aeruginosa, and S.
pneumoniae. The effect of coincubation with TNF-
on the ability
of zymosan, C. albicans and S. cerivisiae to
induce IL-8 and MIP-1
secretion was also examined (Fig. 4
). Incubation in the presence of TNF-
led to greater release of IL-8 in response to C. albicans,
zymosan, and S. cerivisiae. No MIP-1
was
detected in the supernatants of cells incubated with TNF-
in the
presence of zymosan or S. cerivisiae, and the statistical
analysis showed that both these agonists significantly inhibited the
production of MIP-1
induced by TNF-
.
|
|
| Discussion |
|---|
|
|
|---|
The results of the present study show that exposure of human
neutrophils to various bacteria generally leads to the production of
both MIP-1
and IL-8, although the response depends on the
micro-organism to which the neutrophil is exposed. The most potent
agonists were the Gram-negative micro-organisms, S.
typhimurium and P. aeruginosa, and S. aureus
(Gram-positive). Other Gram-positive microbes, S. pneumoniae
and S. epidermidis, were less potent, as was the
opportunistic yeast C. albicans. While the results of the
studies on bacterial microbial pathogens and C. albicans
showed co-ordinated regulation of MIP-1
and IL-8 production,
differential regulation of the expression of these two chemokines was
observed when neutrophils were exposed to zymosan or S.
cerivisiae. Zymosan and S. cerivisiae both induced the
production of IL-8. However, in contrast to that observed with all the
aforementioned microbial pathogens, neither zymosan nor S.
cerevisiae induced detectable secretion of MIP-1
. Taken
together, these results indicate that neutrophils exposed to bacterial
microbial pathogens or C. albicans at sites of infection
will generally release both MIP-1
and IL-8, and in view of the known
biologic activities of these two chemokines (24, 25, 26), this suggests
that signals for the recruitment of mononuclear leukocytes and
neutrophils, respectively, will be generated. In contrast, agonists
such as zymosan, S. cerevisiae, and, as previously shown,
the inflammatory microcrystals MSU and CPPD (11) fail to induce
MIP-1
production, thereby potentially skewing chemokine production
by neutrophils toward IL-8 production, theoretically leading to the
enhancement of neutrophil recruitment.
The demonstration that several of the agonists used in this study, the
Gram-positive bacteria S. pneumoniae and S.
epidermidis and the opportunistic yeast C. albicans,
are relatively weak inducers of IL-8 and MIP-1
production in
neutrophils suggests that signals in addition to phagocytosis are
required for the induction of high levels of expression of the two
chemokine genes. Of relevance, previous studies have demonstrated that
LPS is a potent agonist of the induction of both these chemokines by
neutrophils (19, 20). The Gram-negative bacteria S.
typhimurium and P. aeruginosa express high levels of
LPS and, as shown in the present study, are powerful agonists of both
IL-8 and MIP-1
production by neutrophils. Moreover, S.
aureus expresses lipoteichoic acid (LTA), which also binds to
CD14. Of relevance is a recent study demonstrating the ability of LTA
derived from S. aureus to induce MIP-1
production (27).
However, several of our data suggest that signals other than CD14 may
be involved in the stimulation of chemokine expression. For instance,
the two other Gram-positive microbes used in the present study,
S. pneumoniae and S. epidermidis, also possess
LTA (28, 29, 30, 31) but are weak agonists of IL-8 and MIP-1
production.
Moreover, zymosan (present study) and the inflammatory microcrystals
MSU and CPPD (11), which do not contain LPS or LTA, are strong inducers
of IL-8 production. Additional studies will be required to delineate
the molecular mechanisms involved in the regulation of these two
chemokine genes by the various phagocytic agonists.
At sites of infection, neutrophils are likely to be exposed to multiple
agonists. Activated monocyte/macrophages and neutrophils produce
TNF-
, an agent that has been shown to directly stimulate gene
expression in neutrophils as well as to prime neutrophils for enhanced
responsiveness to other signals (6, 32). In this report we show that
coincubation of neutrophils with TNF-
enhances IL-8 and MIP-1
expression in response to S. typhimurium, P.
aeruginosa, and S. pneumoniae; IL-8 production in
response to zymosan and S. cerevisiae; and IL-8 and MIP-1
production in response to C. albicans. TNF-
failed to
enhance IL-8 or MIP-1
expression to any of the other microbial
pathogens or zymosan. Previous studies have shown that TNF-
enhances
phagocytosis by neutrophils in vitro (33, 34). However, this clearly is
not a general phenomenon, since in the present study only coincubation
of TNF-
with S. typhimurium, P. aeruginosa,
and S. pneumoniae significantly enhanced IL-8 and MIP-1
production. On the other hand, S. cerevisiae and zymosan
(which is derived from S. cerevisiae) failed to
induce MIP-1
production in the presence or the absence of TNF-
,
thereby demonstrating that the induction of MIP-1
by TNF-
is
inhibited under these conditions. These observations place zymosan and
S. cerevisiae in the same category as the inflammatory
microcrystals, MSU and CPPD, both of which not only fail to directly
induce MIP-1
production in neutrophils, but also inhibit the ability
of TNF-
to do so (11).
The inhibition of TNF-
-induced MIP-1
protein production by
opsonized zymosan, S. cerevisiae, MSU, or CPPD suggests
several possibilities. First, these agonists may inhibit secretion, but
not translation of MIP-1
. This possibility was eliminated by
assessing the intracellular level of MIP-1
. This was not altered
regardless of the stimulation conditions (not shown). Second, MIP-1
may be degraded by these agonists, or they may be interfering with the
detection of MIP-1
by ELISA. This possibility was also eliminated by
conducting experiments in which neutrophils incubated with these agents
were spiked with known quantities of MIP-1
, incubated for 24 h
under the various conditions at 37°C, and then subjected to ELISA.
The results of these experiments clearly showed that these agents
neither interfered with the immunodetection of MIP-1
nor degraded
the chemokine (not shown). A third possibility is that these phagocytic
agonists may inhibit MIP-1
translation, fail to activate MIP-1
translation, or both. With respect to this possibility, while we cannot
determine whether the agonists activate MIP-1
gene transcription
without activating translation, it is possible that they inhibit
translation of the MIP-1
gene, since coincubation of neutrophils
with zymosan, S. cerevisiae, MSU, or CPPD and TNF-
effectively inhibits MIP-1
production by TNF-
. Moreover, both
zymosan and S. cerevisiae induced accumulation of mRNA for
MIP-1
, although no immunoreactive MIP-1
was detected. We are
presently conducting studies to determine the mechanism by which such a
translational inhibition may occur.
| Acknowledgments |
|---|
| Footnotes |
|---|
2 Address correspondence and reprint requests to Dr. Shaun R. McColl, Molecular Inflammation, Department of Microbiology and Immunology, University of Adelaide, Frome Rd., Adelaide, South Australia 5005, Australia. E-mail address: ![]()
3 Abbreviations used in this paper: MIP-1
, macrophage inflammatory protein-1
; MSU, monosodium urate; CPPD, calcium pyrophosphate dihydrate; LTA, lipoteichoic acid. ![]()
Received for publication March 21, 1997. Accepted for publication September 22, 1997.
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