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*
Department of Pathology, Geneva, Switzerland; and
University of Maastricht, Maastricht, The Netherlands
| Abstract |
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| Introduction |
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Platelets are produced by megakaryocytes, located mainly in the bone
marrow, and released as anucleated elements which remain in the plasma
for
2 days in mice (9). Consequently, thrombocytopenia may be due
either to an increase in platelet consumption or a decrease of platelet
production. An increase in platelet consumption is generally believed
to be the result of the release of molecules with agonist activity for
platelets; such molecules include thrombin, plasmin, serotonin, etc.
(reviewed in 2 . TNF is not a strong platelet agonist when tested
on isolated platelets in vitro (10, 11).
TNF exerts its effects through two glycoprotein membrane receptors that are expressed at variable degrees on the majority of cells: a 55-kDa receptor, TNFR1, and a 75-kDa receptor, TNFR2, (reviewed in 12 . These receptors are widely distributed, but their presence on cells of megakaryocyte/platelet lineage has not been described.
In this report, we explored the role of TNFR1 and TNFR2 in TNF-induced thrombocytopenia in mice as well as the mode of action of TNF, using mice that were genetically deficient in TNFR1 (13) or TNFR2 (14). TNF-induced thrombocytopenia was found to be entirely dependent upon the TNFR1. However, we did not find a detectable expression of this receptor on platelets, and platelet transfer experiments showed that TNF acts on the TNFR1 of host cells, but not on platelets directly.
| Materials and Methods |
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CBA/J and C57BL/6 mice were purchased from Iffa Credo (LArbresle, France) and bred for three to four generations in our animal facility. Both mice that were genetically deficient for TNFR1 and TNFR2 and their +/+ littermates (a gift of Dr. H. Bluethman, Hoffmann-La Roche, Basel, Switzerland) were obtained in the (C57BL/6 x 129)F1 background (13, 14) and bred in our animal facility. Experiments were performed with 2- to 4-mo-old mice of either sex.
Injection of TNF
Murine rTNF-
was obtained from Dr. B. Allet at Glaxo
Institute of Molecular Biology (Geneva, Switzerland). Its specific
activity (1 U = 0.7 pg/ml) was determined with W13 cells; its LPS
contamination, determined with the E toxate test, was estimated to be
between 5 and 50 ng/mg. An injection of boiled TNF did not have a
detectable effect on platelet numbers. Mice were injected i.v. in the
retroorbital plexus with 10 µg of TNF diluted with NaCl or with NaCl
alone as a control. Injections were administered at the same time of
day (between 8 and 9 A.M.).
Platelet isolation
Blood (150 ml) was collected in 2.5 mM EDTA from the retroorbital plexus of mice that had been injected previously with 5 U of heparin (Liquemin, Hoffmann-La Roche). Platelet-rich plasma was obtained by centrifuging the blood at 480 x g for 5 min at room temperature, and platelets were washed twice in acid-citrate-dextrose (ACD)4 buffer at 1800 x g for 10 min (15). Platelets were diluted using the Unopette micro collection system for platelet determination (Becton Dickinson, Rutherford, NJ) and counted in a hemocytometer. Three independent samples of a minimum of 100 cells were counted, and the arithmetic mean of the three counts was calculated. The mean platelet volume as well as platelet number were also counted in a Sysmex cell counter (Toa Medical, Kobe, Japan).
Platelet survival and trapping evaluation
Platelets prepared in ACD buffer were 51Cr-labeled as previously described (15). A total of 40 to 50 x 106 platelets (0.51 x 105 cpm) were injected i.v. in the retroorbital sinus. For the trapping experiments, mice were sacrificed at 3 h postinjection, and the radioactivity in the lung, spleen, and blood (50 ml) was counted in a gamma-counter (Becton Dickinson, San Jose, CA). For survival evaluation, 0.05 ml of blood was withdrawn from the retroorbital sinus at various times after injection using a calibrated, heparinized capillary.
Antibodies
Rabbit anti-mouse/rat/human/CD62P (P-selectin) and a hamster anti-mouse CD61 polyclonal Ab were both obtained from PharMingen (San Diego, CA). Rabbit anti-mouse TNFR1 and TNFR2 polyclonal Abs were a gift from Dr. G. Wong, (Genentech, San Francisco, CA). Rat anti-mouse TNFR1 (HM-103, IgG2a anti-p55) and rat anti-mouse TNFR2 (HM-102, IgG2a anti-p75) mAbs were derived in our laboratory. Abs were used as purified IgG, and the appropriate nonimmune IgG fractions or monoclonal isotype were used as specificity controls. FITC-labeled goat anti-rat, goat anti-hamster (Caltag, San Francisco, CA), and goat anti-rabbit (Sigma, St. Louis, MO) IgGs were used as second-step reagents.
Fluorescence-activated cell sorter
Platelets were prepared as described above, and 0.5 to 1 x 106 cells were added to polystyrene tubes (Becton Dickinson), incubated for 20 min with normal goat serum, washed, and incubated at room temperature with the appropriate IgGs. After 30 min, the cells were washed twice in ACD containing 0.1% BSA (Sigma) and subsequently incubated with an appropriate dilution of FITC-labeled secondary Abs for 30 min. Platelets were washed twice, and the samples were analyzed using a FACScan flow cytometer (Becton Dickinson, Mountain View, CA). Side and forward angle light scatter as well as staining with anti-CD61 mAb were used to gate the platelet population. An endothelial cell line (sEnd.1 (16)) was used as a positive control for TNFR expression.
Immunohistochemistry
Sections from frozen spleen were prepared and stained with rabbit anti-TNFR1 and anti-TNFR2 polyclonal Abs. As a control, a nonimmune rabbit serum was also tested. These Abs were revealed with alkaline phosphatase-labeled goat anti-rabbit Ab (Dako Diagnostics, Carpinteria, CA). As an additional control of specificity, spleens from TNR1 -/- and TNFR2 -/- donors were also examined.
Statistical analysis
Statistical analyses were conducted using the Student t test for unpaired data.
| Results |
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An i.v. injection of 10 µg of TNF to mice induced profound
thrombocytopenia, which became evident at 1 h after injection and
lasted for
3 days (Fig. 1
). A similar
decrease in platelet counts was observed in the C57BL/6, CBA/J, or
(C57BL/6 x L129)F1 mouse strains (data not
shown). Thrombocytopenia was mainly due to an increase of platelet
consumption (see below).
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We subsequently tested the effect of TNF on the platelet counts of
mice that were genetically deficient in TNFR1 and TNFR2. TNF induced
thrombocytopenia of a similar magnitude in TNFR2 +/+ and -/- mice,
(i.e., resulting in a decrease in platelet number of
30%),
indicating that TNFR2 does not play an important role. In contrast, TNF
did not induce significant thrombocytopenia in TNFR1 -/- mice, as
seen in a representative experiment shown in Figure 2
. This lack of induction was observed at
2 h (Fig. 2
) or at later time points up to 24 h after
injection (data not shown), indicating that TNFR1 is of major influence
during this process. However, when we analyzed the values obtained in
different experiments involving a total of 23 mice, the very moderate
effect of TNF in TNFR1 -/- mice became statistically significant,
leaving open the possibility that TNFR2 also marginally contributes to
the effects of TNF on platelets.
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TNF did markedly reduce the survival of normal (+/+), labeled
platelets when these platelets were injected into a +/+ host (Fig. 3
A). Similarly, TNF
also reduced the survival of platelets from TNFR1 -/- donors when the
platelets were injected into +/+ hosts (Fig. 3
B),
indicating that platelet TNFR1 is not required for this process. In
addition, the survival of platelets from +/+ donors was markedly
reduced by TNF when injected in a +/+ host, but was not markedly
reduced when such platelets were injected in TNFR1 -/- hosts. This
observation indicates that it is the TNFR1 of the host, but not of the
platelets, which is critical in TNF-induced thrombocytopenia (Fig. 3
C).
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The presence of the TNFRs was explored on the megakaryocytes
present within the red pulp of the spleen by histochemistry, using the
rabbit anti-TNFR Abs. Anti-TNFR1 Ab did stain megakaryocytes, while
anti-TNFR2 did not stain more than the nonimmune Ab (Fig. 5
). As an additional control,
anti-TNFR1 Ab was assayed on the spleen from TNFR1 -/- mice and
did not stain megakaryocytes more than the nonimmune control (data not
shown).
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| Discussion |
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2 days in mice, suggesting a platelet
turnover rate of
2% per h (9). Thus, the loss of
30% of
circulating platelets that is seen at 1 h after a TNF injection
cannot be due to an effect of TNF on platelet production. The slow
recovery of the thrombocytopenia indicates that it is not due to the
recirculation of temporarily sequestered platelets, but rather to a
reconstitution of the platelet pool by platelet production. This
interpretation is also in accord with the survival of labeled
platelets, which is markedly and irreversibly reduced by an injection
of TNF (Fig. 3
The effects of TNF are exerted by two membrane receptors, which are
expressed to variable degrees on the majority of epithelial and
mesenchymal cells (18). A study of mice with deletion of these
receptors indicates that both platelet consumption and trapping in the
inflamed lung are primarily mediated by the TNFR1 (Fig. 2
). Indeed, the
TNF-induced thrombocytopenia in TNFR2 -/- mice was of an amplitude
similar to that seen in +/+ mice, indicating that the TNFR1 alone that
is expressed on host cells is sufficient to trigger the events leading
to thrombocytopenia and platelet adhesion. These observations are
related to those observations indicating that TNFR1 is the main
mediator of the inflammatory effects of TNF (12), and that platelets
are participating in inflammatory reactions, including those associated
with fibrogenesis (7, 19). TNFRs are detected mainly on cells of
lymphohemopoietic origin according to histochemical methods, while the
study of cell lines suggests that low numbers of TNFRs are detectable
on various cell types, including epithelial and mesenchymal cells (18).
We were able to detect the TNFR1 on megakaryocytes by histochemistry,
raising the possibility that TNF influences megakaryocytopoiesis.
Current evidence shows that TNF inhibits megakaryocytopoiesis in vitro
(20), while it increases the number of megakaryocytes in the spleen in
vivo (21). In contrast, TNFRs are not detectable on their progeny,
which are platelets; i.e., they are present in very low numbers or are
absent altogether. In this respect, platelets resemble another
anucleated cell, the RBC, on which the TNFRs are not detectable (22).
This finding does not completely exclude the possibility that TNF acts
on platelets, since it has been observed with other cells that the
response to TNF does not seem to correlate with the number of TNFRs
present (22). However, this possibility is ruled out by platelet
transfer experiments, which demonstrate that platelet consumption is
triggered when TNFR1 is present on host cells, while the TNFR1
phenotype of platelets is unimportant.
It is generally believed that platelet consumption is the result of platelet activation that manifests itself in vitro by an aggregation or a change in the expression of cell surface proteins such as CD62P (17). While TNF is not known to activate platelets directly in vitro (Ref. 10 and our unpublished observations), the present results suggest that TNF probably does activate platelets in vivo. Therefore, it is very likely that TNF acts on the TNFR1 of cells that are capable of releasing a mediator with agonist activity for platelets. The task of identifying the important pathways leading to platelet activation might be difficult, since TNF can act on most cell types and induce a wide variety of responses, and also because platelet agonists are numerous (2). One possible cascade is the activation of the endothelium for the production of tissue factor, which results in the generation of thrombin, a well-known platelet agonist (23). This cascade reportedly involves TNFR1 (24), but its importance is doubtful under the present conditions, since TNF-induced thrombocytopenia is not associated with fibrin consumption (25) and cannot be attenuated by heparin (our unpublished observation). Another pathway might be the production of proteases (26), notably plasminogen activators, which increase the concentration of plasmin, another documented platelet agonist (27). Finally, the release of monoamines from mast cells also appears to contribute to the effects of TNF on platelets (Ref. 28 and our unpublished observations).
| Acknowledgments |
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| Footnotes |
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2 Present address: World Health Organization Immunology Research Training Center, Institute of Biochemistry, 1066 Epalinges, Switzerland. ![]()
3 Address correspondence and reprint requests to Dr. Pierre F. Piguet, Department of Pathology, 1 rue M. Servet, 1211 Geneva, Switzerland. E-mail address: ![]()
4 Abbreviation used in this paper: ACD, dextrose (anhydrous)-trisodium citrate·2H2O-citric acid·H2O-H2O. ![]()
Received for publication December 8, 1997. Accepted for publication February 17, 1998.
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: changes in pregnancy and pre-eclampsia. Thromb. Haemost. 78:1255.[Medline]
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express varying amounts of tissue factor resulting in inhomogeneous fibrin deposition in a native blood flow system. J. Clin. Invest. 93:2073.
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