Effect of aspirin and sodium salicylate on thrombosis, fibrinolysis, prothrombin time, and platelet survival In rabbits with indwelling aortic catheters

We have studied the effect of different doses of aspirin on platelet function, PGI2 formation, platelet survival, thrombosis, fibrinolysis, and prothrombin time in rabbits with indwelling aortic catheters. The thrombi formed around indwelling aortic catheters were found to have a large fibrin component, and their formation was inhibited by heparin administration. Thus, in these experiments we examined the effect of aspirin (a weak inhibitor of thrombin-mediated platelet aggregation) under conditions in which thrombin was a major factor in the initiation and growth of the thrombi. Only very high doses of aspirin tended to inhibit thrombus formation over the 5-day period of observation, and a statistically significant inhibition of thrombus formation was produced by equivalent concentrations of sodium salicylate. The failure of high doses of aspirin to achieve a significant inhibition of thrombosis under the conditions of these experiments (whereas an equivalent dose of sodium salicylate was inhibitory) could be due to aspirin inhibition of PGI2 formation. Shortened platelet survival was not affected by aspirin treatment or the dose sodium salicylate that inhibited thrombus formation. The tendency to inhibit thrombus formation appeared to be unrelated to an effect on platelets but was associated with prolongation of the one-stage prothrombin time and increased whole blood fibrinolytic activity; doses of aspirin that inhibited platelet aggregation in response to sodium arachidonate or collagen, and PGI2 formation by the vessel wall, did not have a significant effect on the amount of thrombus present at 5 days. However, the high doses of aspirin that inhibited PGI2 formation were associated with a tendency to increased thrombus formation during the first 3 hr after insertion of the catheter. The results of these experiments show that when thrombin is an important factor in the formation of thrombi, aspirin is a weak inhibitor of thrombosis unless doses are used that provide sufficient salicylate to interfere with blood coagulation and promote whole blood fibrinolytic activity. These results also show that thrombus formation can be inhibited without an apparent change in platelet survival.     

Key role of platelet procoagulant activity in tissue factor-and collagen-dependent thrombus formation in arterioles and venules in vivo differential sensitivity to thrombin inhibition

OBJECTIVE: Blood coagulation and platelet activation are mutually dependent processes, but contribute differently to venous and arterial thrombosis. We investigated the interplay of these processes in vivo in a mouse model of arteriolar and venular thrombus formation. METHODS: Thrombus formation was studied by intravital (fluorescence) microscopy after topical application of FeCl3 on mouse mesenteric microvessels. RESULTS: Both in arterioles and venules, the thrombus-forming process relied on tissue factor-factor VII(a) interaction, collagen exposure, and glycoprotein VI-mediated platelet activation. Arterial thrombus formation was impaired by mild thrombin inhibition or platelet inhibition, while venous thrombosis was only suppressed by strong thrombin inhibition or by mild thrombin inhibition together with platelet inhibition. Phosphatidylserine-exposing platelets were present in thrombi of both vessel types, as detected with fluorescently labeled annexin A5. Injection of annexin A5 to shield exposed phosphatidylserine abolished thrombus formation in arterioles and venules, while mutant M1234-annexin A5 was ineffective. Arterial and venous thrombus formations were only slightly affected in mice carrying the factor V Leiden mutation, suggesting insensitivity to factor Va inactivation. CONCLUSIONS: In this microvascular model, the formation of both arterial and venous thrombi relies on collagen-induced platelet activation and tissue factor-induced thrombin generation. Activated, phosphatidylserine-exposing platelets play a key role in thrombus growth in arterioles and venules.     

Par4 is required for platelet thrombus propagation but not fibrin generation in a mouse model of thrombosis

Thrombin, a central mediator of hemostasis and thrombosis, converts fibrinogen to fibrin and is a potent platelet activator. Activated platelets provide a surface for assembly of the tenase and prothrombinase complexes required for thrombin generation. The role of thrombin-induced platelet activation in platelet accumulation and its interplay with fibrin deposition during thrombus assembly has not been fully defined. We studied these processes during laser-induced thrombus formation by using real-time digital fluorescence microscopy in mice lacking protease-activated receptor-4 (Par4), which is necessary for thrombin responsiveness in mouse platelets. Juxtamural platelet accumulation immediately after laser injury was not different in wild-type and Par4(-/-) mice. However, subsequent growth of platelet thrombi was markedly diminished in Par4(-/-) mice. At the time of maximal thrombus size in wild type, platelet accumulation was more than 10-fold higher in wild type than in Par4(-/-) mice. P-selectin expression, a marker of platelet activation, was reduced and delayed in Par4(-/-) thrombi. In contrast to platelet activation and accumulation, the rate and amount of fibrin deposition, predominantly intramural and juxtamural in this model, were indistinguishable in Par4(-/-) and wild-type mice. These results suggest that platelet activation by thrombin is necessary for normal propagation of a platelet thrombus at a distance from the injured vessel wall and hence for normal thrombus growth. However, platelet activation by thrombin is unnecessary for initial and limited accumulation of platelets at or near the vessel wall, and this limited accumulation of platelets and/or platelet-independent mechanism(s) of thrombin generation are sufficient for normal fibrin deposition in this model.     

Identification of roles for the SNARE-associated protein,SNAP29, in mouse platelets

Platelets are critical for maintaining vascular hemostasis, but also play a major role in the formation of occlusive cardiovascular and cerebrovascular thrombi under disease conditions. Secretion of platelet alpha and dense granules is a requirement for efficient thrombus formation. Understanding and targeting the mechanisms of secretion is important to aid the development of effective antithrombotics. SNAP29 is a tSNARE found in platelets, but whose role has not been defined. Using a platelet-specific SNAP29 knockout mouse model, we assessed the role of SNAP29 in platelet secretion and function under standardized conditions and also in in vitro and in vivo thrombosis. The data showed no major defects in SNAP29-null platelets, but revealed a minor defect in α-granule secretion and a significant increase in embolization rate of thrombi in vivo. These data suggest that SNAP29 contributes to the regulation of platelet α-granule secretion and thrombus stability, possibly partially masked by functional redundancy with other tSNAREs, such as SNAP23.     

Tissue factor pathway vs. collagen pathway for in vivo platelet activation

The roles that the various platelet collagen receptors play in initial platelet adhesion and thrombus growth remain controversial. Here we summarize some of the pertinent data and discuss some recent studies of the initiation and propagation of platelet accumulation into thrombi and the initiation and propagation of thrombin generation. Mice lacking platelet surface glycoprotein VI (GPVI) form normal thrombi in the laser injury model but have a diminished thrombotic response to severe FeCl3 injury. We hypothesize that the paths to thrombus formation in these two models are different with interaction of GPVI and collagen predominant early after severe FeCl3-induced injury but platelet activation by thrombin predominant after laser-induced injury. Understanding of the response to insult in thrombosis models deepens our understanding of the process and provides a firm foundation for evaluation of anti-thrombotic therapy in these models.     

Inhibition effects of KRDS, a peptide derived from lactotransferrin, on platelet function and arterial thrombus formation in dogs.

KRDS (Lys-Arg-Asp-Ser), a tetrapeptide from human lactotransferrin, was tested for its effects in vitro on dog platelet function and in vivo on femoral arterial thrombus formation in dogs. KRDS inhibited ADP (8 microM)-induced platelet aggregation (IC50: 350 microM) and arachidonic acid (2 mM)-induced thromboxane B2 generation (IC50: 175 microM). In addition, the thrombin (0.2 U/ml)-induced serotonin release was inhibited by KRDS (IC50: 525 microM) and the expression of alpha-granule membrane protein (GMP-140) was also inhibited (IC50: 350 microM). The results show that KRDS is an inhibitor for platelet aggregation and secretion to which the inhibition is more potent. Meanwhile, in the experiment of arterial thrombosis in dogs, KRDS (5 microM/kg) and 125I-SZ-51 (a monoclonal antibody against GMP-140) were injected before operation and immediately after the thrombus formation, respectively. In the KRDS group, the weight of removed thrombi was reduced to 50% of that in controls and the radioactivity per mg of labeled thrombi to 33.3% while in blood the radioactivity increased 2 times that in controls at the 4th hour after the injection of 125I-SZ-51. The radioactivity ratio between removed thrombi and blood was only 16% of that in controls. These results indicate that KRDS can inhibit thrombus formation in vivo and is a promising antithrombotic agent.     

Tissue factor, the blood, and the arterial wall

Thrombogenic tissue factor (TF) on cell-derived microparticles is present in the circulating blood of patients with acute coronary syndromes. Recently, we reported that leukocytes transfer TF-positive particles to platelet thrombi, making them capable of triggering and propagating thrombus growth. This observation changes the original dogma that vessel-wall injury and exposure of tissue factor within the vasculature to blood is sufficient for the occurrence of arterial thrombosis. The transfer of TF-positive leukocyte particles is dependent on the interaction of CD15 and TF with platelet thrombi. The inhibition of TF transfer and TF activity suggests a novel therapeutic approach to the prevention of thrombosis that may prove to be effective in disorders associated with increased blood TF.     

Rate-limiting roles of the tenase complex of factors VIII and IX in platelet procoagulant activity and formation of platelet-fibrin thrombi under flow

The importance of factor Xa generation in thrombus formation has not been studied extensively so far. Here, we used mice deficient in either factor VIII or factor IX to determine the role of platelet-stimulated tenase activity in the formation of platelet-fibrin thrombi on collagen. With tissue factor present, deficiency in factor VIII or IX markedly suppressed thrombus growth, fibrin formation and platelet procoagulant activity (phosphatidylserine exposure). In either case, residual fibrin formation was eliminated in the absence of tissue factor. Effects of factor deficiencies were antagonized by supplementation of the missing coagulation factor. In wild-type thrombi generated under flow, phosphatidylserine-exposing platelets bound (activated) factor IX and factor X, whereas factor VIII preferentially co-localized at sites of von Willebrand factor binding. Furthermore, proteolytic activity of the generated activated factor X and thrombin was confined to the sites of phosphatidylserine exposure. With blood from a hemophilia A or B patient, the formation of platelet-fibrin thrombi was greatly delayed and reduced, even in the presence of high concentrations of tissue factor. A direct activated factor X inhibitor, rivaroxaban, added to human blood, suppressed both thrombin and fibrin formation. Together, these data point to a potent enforcement loop in thrombus formation due to factor X activation, subsequent thrombin and fibrin generation, causing activated factor X-mediated stimulation of platelet phosphatidylserine exposure. This implies that the factor VIII/factor IX-dependent stimulation of platelet procoagulant activity is a limiting factor for fibrin formation under flow conditions, even at high tissue factor concentrations.     

Platelet and Fibrinogen Survival in Normal and Abnormal States of Coagulation

1. Platelet and fibrinogen survival have been studied by isotope taggingin normal and abnormal states of coagulation in humans and dogs.

2. By our technic, the normal curve of platelet survival is exponential. Evidence is given that such a curve is obtained because of random utilization ofthe platelets in the continuous process of coagulation.

3. In the postoperative subject and in the subject receiving epinephrine injections, platelet survival is shortened because the process of coagulation isspeeded up. In the hypocoagulable, the curve of platelet survival becomesmore nearly linear because random platelet destruction ceases.

4. When a massive thrombus forms, platelet survival is shortened. Evidenceis given that this may be due to escape of thrombin from the clot into thesystemic circulation—with a resultant hypercoagulable state.

5. Fibrinogen survival was not altered in those states in which increasedplatelet utilization occurred. This is explained by the theory that the alterationin the hemostatic mechanism had not penetrated to the fibrinogen-fibrin stage.

6. Hypercoagulability is defined as that state in which platelet utilization isaccelerated—whether or not thrombosis occurs.

Submitted on February 17, 1960 Accepted on December 9, 1960     

Prevention of vascular graft occlusion and thrombus-associated thrombin generation by inhibition of factor XI

The protease thrombin is required for normal hemostasis and pathologic thrombogenesis. Since the mechanism of coagulation factor XI (FXI)-dependent thrombus growth remains unclear, we investigated the contribution of FXI to thrombus formation in a primate thrombosis model. Pretreatment of baboons with a novel anti-human FXI monoclonal antibody (aXIMab; 2 mg/kg) inhibited plasma FXI by at least 99% for 10 days, and suppressed thrombin-antithrombin (TAT) complex and beta-thromboglobulin (betaTG) formation measured immediately downstream from thrombi forming within collagen-coated vascular grafts. FXI inhibition with aXIMab limited platelet and fibrin deposition in 4-mm diameter grafts without an apparent increase in D-dimer release from thrombi, and prevented the occlusion of 2-mm diameter grafts without affecting template bleeding times. In comparison, pretreatment with aspirin (32 mg/kg) prolonged bleeding times but failed to prevent graft occlusion, supporting the concept that FXI blockade may offer therapeutic advantages over other antithrombotic agents in terms of bleeding complications. In whole blood, aXIMab prevented fibrin formation in a collagen-coated flow chamber, independent of factor XII and factor VII. These data suggest that endogenous FXI contributes to arterial thrombus propagation through a striking amplification of thrombin generation at the thrombus luminal surface.     

Increased platelet thrombus formation under flow conditions in whole blood from polycythaemia vera patients

BackgroundPolycythaemia vera is a myeloproliferative neoplasm characterised by a high incidence of thrombosis. The contribution of platelets, key players in haemostasis, in this setting is still unclear. So far, the majority of studies have been focussed on specific platelet abnormalities but not on their actual capacity to form thrombi. The aim of this study was to characterise, ex vivo under flow conditions, the capacity of platelets from patients with polycythaemia vera to adhere to collagen and induce thrombus formation.Materials and methodsThirty-nine patients and 30 healthy controls were studied. Thrombus formation was induced by perfusing whole blood over a collagen-coated surface, in a parallel-plate flow chamber coupled to a fluorescent microscope. This dynamic system enables platelet adhesion and thrombus formation to be followed in real time and also allows measurements of the extent of the thrombus and platelet surface antigen expression. Laboratory data were analysed in the light of the patients’ main haematological parameters and therapies.ResultsPlatelet adhesion was significantly greater in patients than in control subjects. Patient thrombi were usually larger and more complex than those formed by control platelets. A significant positive correlation was found between platelet adhesion and both the haematocrit and red blood cell count. These parameters remained significantly correlated with platelet adhesion also after multivariable analysis adjusted for gender, age, therapy and JAK2V617F allele burden. Furthermore, subjects with a haematocrit >45% had significantly greater platelet adhesion than subjects with a haematocrit <45%.DiscussionOur data indicate that increased platelet adhesion participates in the thrombotic diathesis of patients with polycythaemia vera, and that the haematocrit level can affect the adhesive and thrombus forming capacities of platelets.     

Relation of microcirculatory thrombosis to thrombus in the proximal coronary artery: Effect of aspirin,dipyridamole, and thrombolysis

A study was designed to determine the distribution of platelets in the microcirculation of the myocardium following experimental platelet thrombosis in a major coronary vessel. Determination was made by means of ⁵¹Cr-labelled platelets and histologic examination. The results suggest that thrombosis in a major coronary artery is associated, at least in the first hours following its occurrence, with the presence of platelet thrombi in the microcirculation of the ischemic area. Pretreatment with aspirin or dipyridamole minimized significantly the extent of microcirculatory thrombosis without affecting the thrombus in the proximal coronary artery. A thrombolytic agent given after thrombus formation had a similar effect upon microcirculatory thrombosis remaining also independent of its effect upon proximal coronary artery thrombosis. There was a decrease in incidence of arrhythmias and mortality rate in the group of animals pretreated with aspirin and dipyridamole.     

Patho-physiological studies on lactic acid-induced pulmonary thrombosis in rat. I. Effect of heparin, acetylsalicylic acid, urokinase and tranexamic acid.

The role of platelet aggregation and coagulo-fibrinolytic systems in thrombogenesis of lactic acid-induced pulmonary thrombosis in rat were studied using an anti-coagulant, platelet aggregation inhibitor, fibrinolytic or anti-fibrinolytic agents. In normal rat, heparin (2.5 mg/kg), acetylsalicylic acid (30 mg/kg) and tranexamic acid (100 mg/kg) suppressed specifically coagulation, platelet aggregation induced by collagen or thrombin and fibrinolysis respectively. Urokinase (10,000 units/kg) activated powerfully fibrinolytic system in addition to suppressing slightly platelet aggregation. The pretreatment with heparin, acetylsalicylic acid or urokinase markedly prevented the formation of thrombus initiated by the infusion of lactic acid at the doses used. Additive effect was also obtained by combined administration of these agents. On the other hand, it was interesting to note that tranexamic acid (100 mg/kg) did not affect the thrombus formation at all despite a potent anti-fibrinolytic effect of this agent. These results indicate that both platelet aggregation and enhancement of coagulation activity are important factors responsible for the formation of thrombi in DIC, while the fibrinolytic activity in blood seems not to be involved in it. On the basis of the findings, mechanism for triggering activation of coagulation and platelet aggregation is also discussed here.     

Interruption of acute platelet-dependent thrombosis by the synthetic antithrombin D-phenylalanyl-L-prolyl-L-arginyl chloromethyl ketone.

Since the antithrombin action of heparin fails to interrupt arterial thrombosis, a mediating role for thrombin (EC 3.4.21.5) in the formation of high-shear platelet-dependent thrombus has been unproven. To determine whether thrombin is important in acute arterial thrombus formation and to assess the therapeutic potential of inhibiting its action, the effects of the synthetic covalent antithrombin D-phenylalanyl-L-prolyl-L-arginyl chloromethyl ketone (FPRCH2Cl) on arterial-flow vascular graft thrombosis and occlusion have been studied in a nonhuman primate model. Continuous intravenous infusion of FPRCH2Cl (100 nmol/kg per min) into vascular graft-bearing baboons (Papio anubis) abolished (i) vascular graft 111In-platelet deposition, (ii) vascular graft occlusion, (iii) thrombus-associated in vivo release of platelet-specific proteins and fibrinopeptides, (iv) platelet hemostatic plug formation, (v) thrombin-induced platelet aggregation ex vivo, and (vi) thrombin-induced blood clotting. The effects of FPRCH2Cl largely disappeared within 15 min after the infusion had been discontinued. FPRCH2Cl produced no detectable cardiovascular or other acute side effects. In contrast, sustained comparably anticoagulating levels of heparin had no effect upon 111In-platelet graft deposition, graft occlusion, platelet function as measured by the bleeding time, platelet aggregation ex vivo, or release of platelet-specific proteins in vivo. We conclude that thrombin is the principal mediator of platelet-dependent hemostatic plug formation and of the formation of platelet-dependent high-flow acute graft thrombosis and occlusion. Moreover, FPRCH2Cl or other synthetic antithrombins may provide effective antithrombotic therapy for both arterial and venous thrombosis by simultaneously inhibiting platelet activation and fibrin formation.     

Glycoprotein VI-dependent and -independent pathways of thrombus formation in vivo

The role of the collagen receptor glycoprotein VI (GPVI) in arteriolar thrombus formation was studied in FcRgamma-null mice (FcRgamma(-/-)) lacking platelet surface GPVI. Thrombi were induced with severe or mild FeCl(3) injury. Collagen exposure was significantly delayed and diminished in mild compared with severe FeCl(3) injury. Times to initial thrombus formation and vessel occlusion were delayed in FcRgamma(-/-) compared with wild-type mice after severe injury. Platelet accumulation in wild-type mice was decreased after mild compared with severe injury. However, there was little difference between platelet accumulation after severe or mild injury in FcRgamma(-/-). These data indicate a significant role for GPVI in FeCl(3)-induced thrombus formation. Pretreatment of wild-type mice with lepirudin further impaired mild FeCl(3)-induced thrombus formation, demonstrating a role for thrombin. Laser-induced thrombus formation in wild-type and FcRgamma(-/-) was comparable. Collagen exposure to circulating blood was undetectable after laser injury. Normalized for thrombus size, thrombus-associated tissue factor was 5-fold higher in laser-induced thrombi than in severe FeCl(3)-induced thrombi. Thus, platelet activation by thrombin appears to be more important after laser injury than platelet activation by GPVI-collagen. It may thus be important when considering targets for antithrombotic therapy to use multiple animal models with diverse pathways to thrombus formation.     

Propensity for young reticulated platelet recruitment into arterial thrombi

Atherosclerosis reduces platelet survival and thereby increases the percentage of younger platelets in the circulation assuming steady-state thrombocytopoiesis. We hypothesized that younger platelets have an increased propensity for arterial thrombus participation compared to older counterparts. Platelet-rich thrombi were generated by perfusing human heparinized whole blood from normal donors over arterial cross-sections under shear conditions (3,350 s^?1) corresponding to significant coronary artery stenosis using a perfusion chamber. Harvested thrombi were disaggregated, stained with thiazole orange, anti-integrin β₃, glycoprotein (GP) Ibα, GP IX and P-selectin, and compared to paired whole blood samples from the same donor by flow cytometry. Thiazole orange staining intensity provides a measure of platelet m-RNA content and age. Thiazole orange staining intensity (MN ± SEM) of platelets harvested from thrombi (62 ± 13) was twofold greater compared to paired intra-individual whole blood samples (31 ± 1). Integrin β₃ receptor density was also greater for thrombus platelets (12.0 ± 1.0) compared to whole blood platelets (7.0 ± 0.6; p < 0.0001). GPs Ibα and IX were reduced from thrombus platelets possibly reflecting shedding. Younger “reticulated” platelets appear to have a greater propensity for thrombus participation under shear conditions of coronary artery stenosis compared to older counterparts. This predisposition may be explained by an increased receptor density of integrin β₃ in younger platelets. By this mechanism, the atherosclerotic process may enhance the individual propensity for arterial thrombosis.     

111In-labeled platelet scintigraphy and two-dimensional echocardiography for detection of left atrial appendage thrombi. Studies in a new canine model

111In-labeled platelet scintigraphy and two-dimensional echocardiography were performed in 40 dogs to determine the ability of the two techniques to detect left atrial appendage thrombi. Thrombi were induced in 33 dogs that were classified into two groups, "acute" or "chronic," according to the time of labeled-platelet injection after thrombus induction. In the acute group (17 dogs), platelets were injected 24 hours after thrombus induction. In the chronic group (16 dogs), platelets were injected 4-8 days after thrombus induction. "Sham" thoracotomies were performed on seven additional control dogs who did not receive thrombin injections. Analog and blood pool-corrected 111In-labeled platelet scintigraphy images were obtained 4-72 hours later. Closed-chest two-dimensional echocardiography was performed before thoracotomy and repeated at the time of scintigraphy. The location and size of each thrombus were verified at autopsy. Two-dimensional echocardiography detected three of 17 acute (mean volume, 1.2 +/- 1.0 cc) and three of 10 chronic (mean volume, 0.4 +/- 0.3 cc; p less than 0.025) left atrial appendage thrombi. 111In-labeled platelet scintigraphy detected all 17 acute thrombi but only two of 10 chronic thrombi. The measured radioactivity levels of the excised thrombi were 1,949 +/- 1,665 cpm/clot/dose in group 1 and 228 +/- 213 cpm/clot/dose in group 2 (p less than 0.005). In this model, 111In-labeled platelet scintigraphy was able to detect acute left atrial appendage thrombi that could not be identified by two-dimensional echocardiography. Both techniques showed poor sensitivity for detection of chronic thrombi. The decline in sensitivity of 111In-labeled platelet scintigraphy for detection of older thrombi is probably due to diminished labeled-platelet incorporation.     

Fibrinogen gamma' chain carboxy terminal peptide selectively inhibits the intrinsic coagulation pathway

The minor gammaA/gamma' isoform of fibrinogen contains a high affinity binding site for thrombin exosite II that is lacking in the major fibrinogen isoform, gammaA/gammaA fibrinogen. The biological consequences of gamma' chain binding to thrombin were therefore investigated. Coagulation assays, thrombin activity assays, and a primate thrombosis model were used to characterize the biological effects of the gamma' 410-427 peptide. The gamma' peptide had little effect on thrombin cleavage of the small peptidyl substrate tosyl-glycyl-prolyl-arginine-4-nitranilide acetate. However, in vitro assays demonstrated that the gamma' peptide inhibited thrombin cleavage of larger proteinaceous substrates, including fibrinogen and factor VIII. The gamma' peptide inhibited the activated partial thromboplastin time in plasma and showed greater inhibition of activated partial thromboplastin time assays than prothrombin time assays, consistent with the inhibition of factor VIII cleavage. Studies in a baboon thrombosis model showed that the gamma' 410-427 peptide inhibited fibrin-rich thrombus formation (typical of venous thrombi) and, to a lesser extent, platelet-rich thrombus formation (typical of arterial thrombi). These results indicate that binding of thrombin exosite II by the gamma' peptide has selective effects on the intrinsic pathway.     

Thrombocytic acroangiothrombosis (platelet thrombosis of the capillaries,arterioles, and venules)

It is believed that there is a definite disease syndrome characterized clinically byfever, marked anemia, purpura, thrombocytopenia, central nervous system involvement, and a progressive fatal course of a few weeks’ duration. Histologically thepresence of platelet thrombi in the capillaries, arterioles, and venules is pathognomonic. Only by histologic demonstration of the platelet thrombi can the diseasebe definitely differentiated from idiopathic thrombocytopenic purpura.

The data of 9 previously reported cases of this rare disease are summarized and2 additional cases reported.

Diagnosis of the disease probably can be made by bone marrow biopsy or, possibly, by skin biopsy. If a case be diagnosed and the patient is not responding toconservative treatment it is suggested that splenectomy be considered.

Spleens removed for thrombopenic diseases should be examined for plateletthrombi in the possibility that unrecognized cases of the disease have been treatedby this procedure. Proper evaluation of splenectomy in this disease might be aidedby follow-up study of any cases showing platelet thrombi.

The name "thrombocytic acroangiothrombosis" is suggested for this disease.

     

Platelet-derived microparticles associate with fibrin during thrombosis

Platelet-derived microparticles (MP) are reported to express both pro- and anticoagulant activities. Nevertheless, their functional significance has remained unresolved. The present study monitored the generation and fate of MP in an experimental model of thrombosis with costimulation of platelets by collagen and thrombin. When minimally anticoagulated (0.5 micromol/L PPACK) blood was perfused over immobilized fibrillar type I collagen in a flow chamber at a low shear rate (300 s(-1)), endogenous thrombin was generated, as evidenced by thrombin-antithrombin III complex. In contrast to full anticoagulation 150 micromol/L PPACK) and the absence of collagen, large platelet aggregates and fibrin ensued during perfusions over collagen in the presence of thrombin. In these thrombi, MP, defined as GPIIbIIIa- and P- selectin-positive vesicles (<1 micron), were found to align fibrin in immunofluorescence and scanning immunoelectron microscopy. Moreover, in sections of embolectomized thromboemboli from patients GPIIbIIIa- and P- selectin-positive material compatible with MP was detected in a fibrin strand-like pattern. In vitro binding studies showed that MP bound to fibrin and acted there as procoagulants. In summary, we show that MP generated during thrombus formation associate with local fibrin. This adhesive function fibrin could imply a sustained modulatory role for MP in evolving thrombi.