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123
Interaction with Autologous Platelets Multiplies Interleukin-l and Tumor
Necrosis Factor Production in Mononuclear Cells
Koichi Aiura, Burton D. Clark, Charles A. Dinarello,
Nathan H. Margolis, Gilles Kaplanski, John F. Burke,
Ronald G. Tompkins, and Jeffrey A. Gelfand
Department of Medicine, Division of Geographic Medicine and
Infectious Diseases, Tufts University School of Medicine and New
England Medical Center; Department of Surgery, Harvard Medical
School and Massachusetts General Hospital, Boston, Massachusetts
The effect of activated platelets on cytokine production by human peripheral blood mononuclear
cells (PBMC) was investigated. When PBMC were coincubated with activated autologous platelets
amid lipopolysaccharide (LPS, 50-100 pglmL) for 8 h, the production ofinterleukin (IL)-la increased
11- to 18-fold and tumor necrosis factor (TNF)-a 3- to 5-fold compared with PBMC without platelets.
Activated platelets in a dual-chamber well that prevented platelet-PBMC contact but permitted passage of soluble factors enhanced IL-la production (P < .01). Platelet-PBMC contact in the chamber
resulted in a further enhancement of IL-la production. These data suggest that platelet-PBMC
interaction, both directly and with platelet-derived factors, enhances production of shock-producing
IL-la and TNF-a, albeit differently. The interaction of platelets with monocytes may play an important role in the pathophysiology of sepsis and disseminated intravascular coagulation.
Gram-positive and -negative septicemias remain serious clinical problems that are often complicated by disseminated intravascular coagulation (DIC) [1-3]. Interleukin (IL)-l and tumor
necrosis factor (TNF)-a are key mediators in the pathogenesis
and lethality of both types of sepsis [4, 5]. In experimental
animals, both IL-l and TNF in the circulation are elevated after
injection with either gram-positive or -negative organisms and
are considered to be a common pathway resulting in hemodynamic decompensation and mortality [6]. Similar findings have
been reported in humans with sepsis, and furthermore, plasma
levels of both IL-l (3 and TNF -a can be significantly higher in
patients with DIC than without DIC [7, 8].
We reported that IL-l receptor antagonist (IL-IRa) improved
hemodynamic shock in rabbits injected with either Escherichia
coli or Staphylococcus epidermidis [9, 10]. We found that the
circulating levels of TNF -a in rabbits treated with IL-l Ra in S.
epidermidis- induced shock decreased compared with levels in
saline-treated animals, suggesting that TNF -a production may
be dependent on IL-l induced in an early phase of gram-positive
sepsis. In the same model, a dramatic drop in the circulating
numbers of platelets was observed in both treatment groups after
infusion of S. epidermidis. Thrombocytopenia and neutropenia
are frequently observed in septicemia. Hawrylowicz and col-
Received 20 March 1996; revised 10 July 1996.
Infonned consent was obtained from all patients, and human experimentation
guidelines of the US Department of Health and Human Services and those of
the authors' institutions were followed.
Financial support: NIH (GM-21700, GM-50766, AI-15614), Louviers Foundation, and Alfond Family Fund.
Reprints or correspondence: Dr. Jeffrey A. Gelfand, Dept. of Medicine, New
England Medical Center, 750 Washington St., NEMCH 480, Boston, MA
02111.
The Journal ofInfectious Diseases 1997;175:123-9
© 1997 by The University of Chicago. All rights reserved.
0022-1899/97/7501-0017$01.00
leagues [11, 12] reported that activated platelets express cellassociated IL-l-like activity that is capable of inducing intercellular adhesion molecule 1, IL-6, and granulocyte-macrophage
colony-stimulating factor produced by endothelial cells. Another
report indicated that activated platelets induce IL-8 secretion by
endothelial cells via membrane-associated IL-l activity [12a].
Moreover, activated platelets can interact with monocytes and
neutrophils through P-selectin (GMP-140, PADGEM, CD62).
P-selectin is stored in a granules of resting platelets as an integral
membrane protein and is translocated to the plasma membrane
after platelet activation [13-15].
Recently, Heddle et al. [16] reported that cytokines generated
by leukocytes may be responsible for febrile, nonhemolytic
transfusion reactions. These reactions occur in 30% of platelet
transfusions and are positively correlated with increased leukocyte contamination [17]. They are observed in 1% of red cell
transfusions, even though these have much higher leukocyte
concentrations [17]. If platelet-monocyte interactions enhance
cytokine production, then this interaction may be involved in
the febrile reaction to transfused platelets.
In this study, we investigated in vitro the interaction of platelets with peripheral blood mononuclear cells (PBMC) and their
effect on cytokine production in the presence of low concentrations of lipopolysaccharide (LPS), as seen in the circulation of
patients with gram-negative sepsis. We also investigated the
role of platelets on the synthesis of cytokines by PBMC stimulated with gram-positive S. epidermidis and determined if platelets interacting with monocytes play an important role in shock
seen in sepsis and DIC.
Materials and Methods
Materials. Ficoll type 400, TRlS-HCI, trisodium citrate, and
LPS (E. coli OI27:B8) were obtained from Sigma (St. Louis).
Hypaque-M (90%) was obtained from Winthrop Breon Labora-
124
Aiura et al.
tories (New York City). Sterile, pyrogen-free water and 0.9% saline were obtained from Abbott Laboratories (Abbott Park, IL).
RPMI 1640 culture medium was obtained from BioWhittaker
(Walkersville, MD). Ultrafiltration filters were obtained from Fresenius (Bad Homburg, Germany). Penicillin and streptomycin were
obtained from GIBCO Laboratories (Grand Island, NY). Glucose
was obtained from Mallinckrodt (St. Louis). Human thrombin was
obtained from Calbiochem (La Jolla, CA). Transwell plates were
obtained from Costar-Coming (Cambridge, MA).
Platelet isolations. Autologous platelets were prepared from
citrate-anticoagulated blood by centrifugation. Briefly, blood anticoagulated with 0.1 vol of citrate anticoagulate (0.04 M citric acid,
0.075 Mtrisodium citrate, 0.14M glucose, pH 4.5) was centrifuged
at 180 g for 15 min at room temperature. The platelet-rich plasma
(PRP) was collected without disturbing the interface. After adding
another 0.1 vol of citrate anticoagulate, the PRP was centrifuged
at 1100 g for 10 min at room temperature. The platelet pellet was
gently resuspended in 2 mL of TRIS-NaCl-glucose buffer (0.05
M TRIS-HCl [pH 7.4], 0.15 M NaCl, 0.005 M glucose) and then
washed twice with TRIS-NaCl-glucose buffer. Platelets were then
resuspended in the same ultrafiltered RPMI 1640 medium as above.
Both erythrocyte and mononuclear cell contamination in platelet
resuspension was <0.1 % as determined by microscopy.
Platelet activation was accessed by FACScan cytometry (Becton
Dickinson, San Jose, CA) for P-selectin expression. Isolated platelets in RPMI 1640 were incubated with phycoerythrin-conjugated
anti-human P-selectin (Becton Dickinson). During isolation of autologous platelets, storage in RPMI 1640, and incubation (30 min)
with anti - P-selectin for cytometry, 25%-30% of the platelets became activated. Addition of thrombin (0.15 U/mL, final concentration) at the time of resuspension of platelets in RPMI 1640 routinely activated 75%-80% ofP-selectin expression by cytometry.
P-selectin expression in PRP was only 5%.
PBMe isolation. Blood was collected from healthy volunteers
who did not take any medication during the 10 days before phlebotomy. PBMC were separated from heparinized blood (5 U/mL of
blood) by centrifugation at 120 g on ficoll-hypaque gradients at
room temperature. Cells were washed three times in 0.15 MNaCl,
followed by centrifugation at 120 g for 10 min, and were resuspended in ultrafiltered RPMI 1640 containing penicillin (100 U/
mL) and streptomycin (100 J.lg/mL) without serum [18]. Platelet
contamination in PBMC suspension was 2-5 platelets/monocyte
as determined by phase contrast and confocal microscopy.
Preparation of S. epidermidis. An encapsulated strain of S.
epidermidis isolated from a blood culture from a patient with catheter sepsis was prepared as described [6]. S. epidermidis were inoculated into brain-heart infusion broth and propagated at 37°C overnight with shaking. After centrifugation, the S. epidermidis pellet
was washed three times with pyrogen-free saline and the organisms
were counted; then bacteria were killed by boiling for 30 min.
Bacteria were washed six more times with sterile saline and resuspended at 101l/mL in ultrafiltered RPMI 1640 as above. The concentration of bacteria was determined spectrophotometrically and
confirmed by direct counting in a Petroff-Hausser bacteria counter.
Aliquots of the bacterial suspension were stored at -70°C until
used in experiments.
To confirm whether the suspension of S. epidermidis in RPMI
1640 was contaminated with endotoxin, we investigated the effect
of polymyxin B on TNF-a production by PBMC stimulated with
JID 1997; 175 (January)
S. epidermidis (200 bacterialPBMC). LPS (I ng/mL) was used as
a positive control. The S. epidermidis suspension and LPS solution
were separately preincubated with or without polymyxin B at 5
J.lg/mL for 1 h at room temperature. Then PBMC (1.2 X 106
cells/mL, final concentration) were added to these samples and
incubated for 8 h. Polymyxin B did not affect TNF-a production
by PBMC stimulated with the S. epidermidis, while polymyxin B
completely inhibited LPS-induced TNF-a synthesis.
Cell cultures. PBMC (1.2 X 106 cells/mL) with or without
platelets (5 X 108 cells/mL) were incubated with or without a low
dose of LPS (50 or 100 pg/mL) or heat-killed S. epidermidis (2
or 200 organisms/PBMC) in sterile polypropylene tubes for 8 h
(unless stated otherwise) at 37°C. To ensure strong platelet activation, thrombin (0.15 U/mL) was added to some tubes and compared
with non-thrombin-treated cells. The samples in tubes were cultured on a rocking table set at 4 rpm. In some experiments, different
numbers of platelets (10 7 , 108 , and 109 cells/mL) were used. In
other experiments, PBMC (1.2 X 106 cells/mL, final concentration)
and platelets (2 X 108 cells/mL, final concentration) were incubated
in dual-chamber wells, with the two cell types separated by a filter
(Transwell, O.4-J.lm pore size; Costar-Coming), to study the effects
of PBMC-platelet cell-to-cell contact and soluble factors released
from platelets. Samples in these plates were cultured for 8 h at
37°C in 5% CO2 • After incubation, all samples were stored at
- 70°C until assayed for cytokines.
RIA for cytokines. Total IL-1a and TNF-a production by
PBMC was determined after three cycles of freeze-thawing [19].
The cytokines were quantitated by specific RIAs as previously
reported [19-21]. The detection limits (95% confidence) of the ILIa and TNF-a RIAs were 80-160 and 40-80 pg/mL, respectively.
Statistical analysis. Results are expressed as mean ± SE of
the indicated number of experiments. Data were analyzed by Student's t test for paired samples. Statistical significance was set at
P < .05.
Results
Effect ofactivated platelets on IL-l a and TNF-a production
by PBMe in the absence of LPS. To investigate whether
thrombin-activated platelets alone induce IL-l a or TNF -a by
PBMC, PBMC were incubated without LPS in thy presence of
thrombin (0.15 U/mL, final concentration), thrombin-activated
platelets, or nonactivated platelets (5 X 10 8 cells/mL, final
concentration) for 4, 8, 16, and 24 h (data not shown). In the
absence of LPS, there was no induction of PBMC IL-1a or
TNF -a production by thrombin alone or by platelets with or
without thrombin. After 16 and 24 h of incubation, there was,
however, a small but statistically significant (P < .01) reduction of TNF -a in the presence of thrombin-activated platelets
compared with PBMC stimulated by thrombin alone (platelets
omitted from incubation).
Effect of activated platelets on LPS-induced synthesis ofILl a and TNF-a. As shown in figure 1, when PBMC were
incubated with platelets activated during isolation or further
activated with thrombin (0.15 U/mL) in the absence of LPS
for 8 h at 37°C, IL-1a and TNF-a were barely detected. In the
IL-l and TNF-a Amplified by Activated Platelets
JIO 1997; 175 (January)
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Figure 1. Effect of activated platelets on IL-la and TNF-a synthesis by lipopolysaccharide (LPS)-stimulated peripheral blood mononuclear cells (PBMC). PBMC (1.2 X 106 cells/mL) were coincubated
with thrombin (0.15 U/mL) or thrombin-activated platelets (5 X 108
cells/mL) in absence or presence of LPS at 50 or 100 pg/mL for 8 h
at 37°C (n = 4-6). A, With addition of activated platelets, IL-la
production was increased ll-fold with LPS at 50 pg/mL and 18-fold
with LPS at 100 pg/mL. Addition of non-thrombin-treated platelets
(25%-30% activation for P-selectin) increased IL-la production 6fold with LPS at 50 pg/mL and 14-fold with LPS at 100 pgimL. B,
TNF-a production with addition of activated platelets was increased
5-fold with LPS at 50 pg/mL and 3-fold with LPS at 100 pg/mL.
Addition ofnon-thrombin-treated platelets increased TNF-a production 3-fold with LPS at 50 and 100 pg/mL. Thrombin did not affect
IL-la and TNF-a production by LPS-stimulated PBMC. Data are
mean:::!:: SE. Significant differences between cytokine production with
or without platelets: * P < .05; *** P < .001, by paired t test.
absence of platelets, PBMC incubated with a low concentration
of LPS for 8 h at 37°C produced small amounts of IL-la and
TNF-a. When PBMC and thrombin-activated platelets were
coincubated in the presence of LPS for 8 h at 37°C, an 11- to
18-fold increase in IL-la and a 3- to 5-fold increase in TNFa were observed compared with results from PBMC incubated
with LPS alone.
To ascertain whether 0.15 U/mL thrombin induces IL-la or
TNF-a, PBMC were incubated with thrombin in the presence or
absence of LPS for 8 h at 37°C. Thrombin (0.15 U/mL) did not
affect either IL-la or TNF-a production (figure 1). These results
125
demonstrate that thrombin-activated platelets, but not thrombin
alone, augment the production of IL-la and TNF-a by PBMC
stimulated by LPS at low concentrations. Platelets not treated
with thrombin, activated 25%-30% for P-selectin expression,
enhanced IL-la production 6- to 14-fold and TNF-a production
3-fold when coincubated with PBMC and LPS for 8 h.
Effect of platelets on S. epidermidis-induced synthesis of
IL-l a and TNF-a. Thrombin-activated platelets (5 X 108/
mL) alone did not induce IL-la or TNF-a production by
PBMC incubated in the absence of S. epidermidis, even after
24 h of incubation (data not shown). To investigate whether
platelets augment IL-la and TNF-a production by PBMC in
the presence of S. epidermidis, PBMC and platelets were coincubated with various concentrations of S. epidermidis (2 or
200 bacterialPBMC) for 8 h at 37°C. As shown in figure 2,
PBMC (1.2 X 106/mL) incubated with platelets (5 X 108/mL)
in the presence of 2 S. epidermidis bacteria/PBMC for 8 h
induced only minimal IL-l a and TNF -a synthesis.
However, when PBMC and platelets (either strongly activated with thrombin or non-thrombin-treated but activated
during isolation) were coincubated in the presence of200 bacteria/PBMC for 8 h at 37°C, IL-la production was enhanced 4fold with thrombin-activated platelets and 2.7-fold with nonthrombin-activated platelets (from 1610 ± 342 pg/mL to 6514
± 1058 and 4360 ± 533 pg/mL, respectively; P < .05) compared with IL-la production by PBMC incubated with S. epidermidis without platelets. TNF-a production was enhanced 2.7fold with thrombin-treated platelets and 2.2-fold with nonthrombin-activated platelets (from 418 ± 80 pg/mL to 941 ±
201 and 1121 ± 241 pg/mL, respectively; P < .05) compared
with the production of TNF-a by PBMC incubated with S.
epidermidis without platelets.
Effect of platelet concentration on LPS-induced cytokine
production by PBMe. We next investigated the effect of the
concentration of activated platelets on cytokine production by
PBMC stimulated with LPS. When PBMC were incubated with
100 pg/mL LPS, the synthesis ofIL-la in PBMC was increased
by activated platelets in a concentration-dependent manner
(figure 3A). However, in contrast to IL-la, TNF-a production
was not enhanced by 109 platelets/mL, although a significant
increase in TNF-a production was observed with the addition
of 108 platelets/mL (figure 3B). When PBMC were replaced
with MonoMac6 monocytic cells, results similar to those in
figure 3 were observed (not shown). Platelets at 107/mL and
108/mL enhanced TNF-a, whereas platelets at 109/mL resulted
in decreased TNF-a production.
Effect ofplatelet concentration on S. epidermidis-induced
TNF-a production by PBMe. We next investigated the effect
of differing numbers of platelets on S. epidermidis (200 bacteria/PBMC)-induced TNF-a synthesis by PBMC. S. epidermidis-induced TNF-a production was augmented by the presence of platelets in a platelet-number-dependent manner
(figure 4). Platelets at 109/mL as well as 5 X 108/mL caused
3.0-fold increases in TNF-a production (P < .05). Platelets at
Aiura et al.
126
107/ mL and 10 8/mL had no significant enhancing effect on
TNF -a production.
Role of soluble products of platelets and direct plateletPBMe interaction in enhancing IL-la and TNF-a production.
To determine whether platelet-derived soluble factors augment
the production of cytokines, or whether the direct interaction
of platelets with PBMC is needed, PBMC were incubated with
but kept separate from platelets by a membrane filter in a dualchamber culture well. This membrane filter prohibits direct
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Figure 2. Effect of platelets on IL-Ia and TNF-a synthesis by
Staphylococcus epidermidis-stimulated peripheral blood mononuclear cells (PBMC). PBMC (1.2 X 106/mL) were incubated with or
without non-thrombin-activated or thrombin-activated platelets (5 X
108/mL) in presence of S. epidermidis (2 bacteria/PBMC or 200 bacteria/PBMC) for 8 h at 37°C (n = 6). Addition of non-thrombintreated platelets (25%-30% activation for P-selectin) increased ILIa production 2.7-fold and TNF-a production 2.2-fold with S. epidermidis at 200 bacteria/PBMe. IL-Ia and TNF-a production were enhanced 4.0-fold and 2.7-fold, respectively, by existence of thrombinactivated platelets with S. epidermidis at 200 bacteria/PBMC. Data
are mean ± SE. Significant difference between TNF -a production
with or without platelets: * P < .05, by paired t test.
+
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108
109
Figure 3. Effect of platelet concentration on production of cytokines by peripheral blood mononuclear cells (PBMC). PBMC (10 6
cells/mL), stimulated with lipopolysaccharide (LPS, 100 pg/mL),
were incubated with various concentrations of thrombin-activated
platelets, producing various ratios of platelets to PBMC (10 7 platelets/
mL, 10: 1; 10 8 platelets/mL, 100: 1; 109 platelets/mL, 1000: 1). Significant differences between cytokine production with or without platelets: * P < .05; ** P < .01; *** P < .001, by paired t test. Total
IL-l a production increased directly with concentration (ratio) of
platelets. IL-la concentration continued to increase with supraphysiologic platelet concentrations (A), while total TNF-a levels decreased
with high concentrations (109) or ratios (1000:1) of platelets (B).
PBMC-platelet contact but allows soluble factors to diffuse
between the two chambers. When PBMC and activated platelets were placed in separate chambers, the production of ILl a was significantly increased compared with PBMC with LPS
but without platelets (figure 5A). Furthermore, incubation of
PBMC with thrombin-activated platelets in the same chamber
resulted in greater enhancement of IL-Ia production (P <
.0 I). In contrast, thrombin-activated platelets separated by a
membrane enhanced the production of TNF -a as much as did
incubation of PBMC in direct contact with platelets, although
these increases did not reach statistical significance compared
with the cells cultured without platelets (figure 5B).
Discussion
In these experiments, the presence of activated platelets
greatly enhanced IL-Ia and TNF-a production by PBMC stim-
IL-l and TNF-a Amplified by Activated Platelets
JlD 1997; 175 (January)
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Figure 4. Platelets enhance TNF-a production in number-dependent manner. Peripheral blood mononuclear cells (PBMC, 1.2 X 106/
mL) were incubated with or without different nos. of platelets in
absence or presence of Staphylococcus epidermidis at 200 bacterial
mL for 8 h (n = 5). Data are mean ± SE. Significant differences
between TNF-a production by PBMC stimulated with S. epidermidis
with or without platelets: * P < .05, by paired t test.
platelets contain a precursor IL-l that requires a convertase
from monocytes to become a detectable form.
The effects of platelet-PBMC interactions on LPS-induced
TNF-O' production were not as dramatic as on IL-lO' production. The production of TNF -a was enhanced, but not significantly, when PBMC were incubated with activated platelets in
dual-chamber culture wells (figure 5B). In addition, incubation
of PBMC with activated platelets at a concentration of 109
cells/mL (yielding a higher ratio of platelets to PBMC than is
seen in peripheral blood) did not enhance TNF-O' production,
although production of IL-I a was enhanced with high concentrations of platelets (figure 3A). At least two possibilities can
be considered for the variable effects of platelets on TNF-O'
production. Transforming growth factor (TGF)-,B, one of the
major cytokines released from platelets, induces neither IL-I
nor TNF-O' production by PBMC alone. However, such low
concentrations ofTGF-,B as 0.1 ng/mL enhanced the production
of TNF-O' by phytohemagglutinin- or LPS-stimulated PBMC,
8000
ulated with low concentrations ofLPS (50-100 pg/mL) and by
S. epidermidis. These results suggest that increases in cytokine
production in gram-negative or -positive sepsis might be partly
attributable to activated platelets.
In preliminary experiments, platelets incubated for 8 h without thrombin still enhanced LPS-induced PBMC production of
IL-lO' and TNF-O'. Over time, isolated platelets are spontaneously activated in RPMI culture medium, which contains calcium. It is well known that calcium plays a central role during
platelet activation [22, 23]. Recent studies also show that the
interaction of endotoxin with platelets results in changes in the
platelet membrane and potentiates the activation of platelets
[24, 25]. Furthermore, LPS induces platelet-derived soluble
factors that augment the priming effect of LPS on the oxidative
response of human polymorphonuclear cells [26]. Staphylococci also activate platelets. Among bacteria, staphylococci are
one of the most potent stimuli for platelet activation, and heatkilled staphylococci can activate platelets as much as living
organisms [27].
Our data suggest that cytokine production is enhanced by
soluble factors released from platelets and by direct PBMCplatelet interaction. It was recently reported that platelets express cell-associated IL-l bioactivity within minutes of activation [11, 12]. However, when we incubated 109 platelets with
thrombin and LPS for 8 h at 37°C, then subjected them to three
cycles of freeze-thawing, IL-la, IL-l,B, and TNF-O' antigen
were undetectable in platelet lysates by our RIAs. The limits
of detection for these are ~40 pg/mL. In light of other reports
of cell-associated IL-l bioactivity expressed by platelets, it
may be possible that concentrations of these cytokines were
so low that RIA could not detect them, that platelets interfered
with the detection of membrane-associated antigens, or that
127
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Figure 5. Involvement of soluble mediators released from platelets
and platelet-PBMC (peripheral blood mononuclear cell) direct interactions in enhancement of IL-la and TNF-a production. PBMC (1.2
8
X 10 cells/mL) stimulated by 100 pg/mL lipopolysaccharide (LPS)
were incubated with thrombin-activated platelets (2 X 106 cells/mL)
kept separate (S) by membrane or in contact with each other (C) in
dual-chamber well (n = 6). A, Both soluble mediators produced by
platelets and platelet-PBMC direct contact are involved in enhancement of IL-Ia production seen with addition of platelets. B, TNF-a
production in both conditions was not significantly enhanced by platelets. Data are mean ± SE. Significant differences: ** P < .01, by
paired t test.
128
Aiura et al.
while 10 ng/mL TGF-f3 inhibited the production ofTNF-a [28,
29]. The effects of TGF-f3, stimulatory or inhibitory, depend
on the target cell type and the presence of other growth factors
[28, 30]. It is possible that low concentrations of TGF-f3 may
promote the production ofTNF-a, while higher concentrations
of TGF -13 secreted from a larger number of platelets may abolish the effect of some other stimulatory factors produced by
platelets.
Another possibility is that the release of proteolytic enzymes
by activated platelets may degrade TNF-a in culture medium.
Human platelets have several exopeptidases, such as aminopeptidase P, dipeptidyl peptidase IV, carboxypeptidase N, and angiotensin-converting enzyme [31]. It was recently reported that
a dipeptidyl peptidase IV-like enzyme on the surface of human
monocytic cells participates in TNF-a degradation [32]. Since
the majority of monocyte IL-la is intracellular, it would not
be accessible to digestion by peptidases released from activated
platelets. On the other hand, TNF-a would be degraded because
the majority of TNF-a produced by PBMC is secreted. Thus,
platelets may regulate cytokine levels not only by enhancing
cytokine production but also by accelerating their degradation.
Circulating cytokine levels, driven either by endotoxernia or
other stimuli, may therefore represent the net of both positive
and negative regulatory effects of platelets on monocyte cytokine production. Activated platelets release physiologically important substances in addition to TGF-f3, such as platelet-derived growth factor, platelet-activating factor (PAF), and
fibroblast growth factor. It remains to be determined whether
other platelet products are associated with these effects on
cytokine production. We are currently investigating the identities of the platelet-associated and supernatant modulating factors.
It is critical to note that this dramatic effect of platelets on
the production of cytokines is seen in the presence of the low
concentrations of LPS seen in the circulation clinically. The
concentration of LPS in the circulation of patients with sepsis
is generally < 100 pg/mL. Low concentrations of gram-positive
staphylococci were also used. Thrombocytopenia is frequently
seen in cases of bacterial sepsis in humans and experimental
animals. As soon as the vascular endothelium is injured, platelets adhere to the exposed subendothelial surface via glycoprotein Ib receptors [33], followed by generation of platelet plugs
and blood clotting. Platelets are also activated by physiologic
stimuli, such as adenosine diphosphate, epinephrine, collagen,
or thrombin. Platelets express surface P-selectin minutes after
activation, causing adherence with neutrophils and monocytes.
Recently, Weyrich et al. [34] reported that activated platelets
signal IL-8 and monocyte chemotactic protein-l (MCP-l) production after coincubation with monocytes for 18 h [34]. This
signaling is linked to RANTES, PAF, and P-selectin expression
on the activated platelets. In this fashion, high concentrations
of proinflammatory cytokines (lL-1 a and TNF -a) and chemotactic factors (lL-8 and MCP-l) could be produced locally at
the site of inflammation in the presence of gram-negative or
JID 1997; 175 (January)
-positive bacteria, contributing to tissue damage and organ failure.
Hemostasis is the major physiologic function of platelets.
However, if the platelet and cytokine-producing mononuclear
ceIVmacrophage interaction described here were selectively inhibited without destroying the ability to produce hemostasis,
it might be of benefit in the prevention of organ failure and
shock in sepsis and DIC.
Acknowledgments
We gratefully acknowledge the helpful advice of Barbara C.
Furie, Bruce Furie, and Edouard Vannier and the technical assistance of Ellen C. Donaldson.
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