Viewpoint: The value of non-criteria antiphospholipid antibodies.

In 2006, at a meeting in Sydney, Australia, consensus was reached by an international group of specialists to establish a number of serological criteria that identify patients with a history of thrombosis or pregnancy complications as having antiphospholipid syndrome (APS). These criteria were originally formulated for research purposes and to compare clinical trials in different centres. However, these same criteria are now generally used and accepted for the diagnosis and treatment of patients. The practice of using these criteria for direct patient care requires that these criteria are based on sound scientific evidence. Indeed, for all the autoantibodies that are officially included in the serological criteria, it has been shown that they induce thrombosis and fetal loss when infused into mice. There are also a number of additional autoantibodies that have been identified in these patients but for these antibodies there was not enough evidence to meet the official APS criteria in 2006. Seventeen years have now passed since the consensus meeting, therefore, this review examines whether additional studies performed with these 'non-criteria' autoantibodies have provided sufficient results to suggest the inclusion of these autoantibodies in the official serological criteria of APS.

Reversible Platelet Integrin αIIbß3 Activation and Thrombus Instability.

Integrin αIIbß3 activation is essential for platelet aggregation and, accordingly, for hemostasis and arterial thrombosis. The αIIbß3 integrin is highly expressed on platelets and requires an activation step for binding to fibrinogen, fibrin or von Willebrand factor (VWF). A current model assumes that the process of integrin activation relies on actomyosin force-dependent molecular changes from a bent-closed and extended-closed to an extended-open conformation. In this paper we review the pathways that point to a functional reversibility of platelet αIIbß3 activation and transient aggregation. Furthermore, we refer to mouse models indicating that genetic defects that lead to reversible platelet aggregation can also cause instable thrombus formation. We discuss the platelet agonists and signaling pathways that lead to a transient binding of ligands to integrin αIIbß3. Our analysis points to the (autocrine) ADP P2Y1 and P2Y12 receptor signaling via phosphoinositide 3-kinases and Akt as principal pathways linked to reversible integrin activation. Downstream signaling events by protein kinase C, CalDAG-GEFI and Rap1b have not been linked to transient integrin activation. Insight into the functional reversibility of integrin activation pathways will help to better understand the effects of antiplatelet agents.

Desialylation of platelets induced by Von Willebrand Factor is a novel mechanism of platelet clearance in dengue.

Thrombocytopenia and platelet dysfunction are commonly observed in patients with dengue virus (DENV) infection and may contribute to complications such as bleeding and plasma leakage. The etiology of dengue-associated thrombocytopenia is multifactorial and includes increased platelet clearance. The binding of the coagulation protein von Willebrand factor (VWF) to the platelet membrane and removal of sialic acid (desialylation) are two well-known mechanisms of platelet clearance, but whether these conditions also contribute to thrombocytopenia in dengue infection is unknown. In two observational cohort studies in Bandung and Jepara, Indonesia, we show that adult patients with dengue not only had higher plasma concentrations of plasma VWF antigen and active VWF, but that circulating platelets had also bound more VWF to their membrane. The amount of platelet-VWF binding correlated well with platelet count. Furthermore, sialic acid levels in dengue patients were significantly reduced as assessed by the binding of Sambucus nigra lectin (SNA) and Maackia amurensis lectin II (MAL-II) to platelets. Sialic acid on the platelet membrane is neuraminidase-labile, but dengue virus has no known neuraminidase activity. Indeed, no detectable activity of neuraminidase was present in plasma of dengue patients and no desialylation was found of plasma transferrin. Platelet sialylation was also not altered by in vitro exposure of platelets to DENV nonstructural protein 1 or cultured DENV. In contrast, induction of binding of VWF to glycoprotein 1b on platelets using the VWF-activating protein ristocetin resulted in the removal of platelet sialic acid by translocation of platelet neuraminidase to the platelet surface. The neuraminidase inhibitor oseltamivir reduced VWF-induced platelet desialylation. Our data demonstrate that excessive binding of VWF to platelets in dengue results in neuraminidase-mediated platelet desialylation and platelet clearance. Oseltamivir might be a novel treatment option for severe thrombocytopenia in dengue infection.

New insight into antiphospholipid syndrome: antibodies to ß2glycoprotein I-domain 5 fail to induce thrombi in rats.

Clinical studies have reported different diagnostic/predictive values of antibodies to domain 1 or 4/5 of ß2glycoproteinI in terms of risk of thrombosis and pregnancy complications in patients with antiphospholipid syndrome. To obtain direct evidence for the pathogenic role of anti-domain 1 or anti-domain 4/5 antibodies, we analyzed the in vivo pro-coagulant effect of two groups of 5 sera IgG each reacting selectively with domain 1 or domain 5 in lipopolysaccharide (LPS)-treated rats. Antibody-induced thrombus formation in mesenteric vessels was followed by intravital microscopy, and vascular deposition of ß2glycoproteinI, human IgG and C3 was analyzed by immunofluorescence. Five serum IgG with undetectable anti-ß2glycoproteinI antibodies served as controls. All the anti-domain 1-positive IgG exhibited potent pro-coagulant activity while the anti-domain 5-positive and the negative control IgG failed to promote blood clot and vessel occlusion. A stronger granular deposit of IgG/C3 was found on the mesenteric endothelium of rats treated with anti-domain 1 antibodies, as opposed to a mild linear IgG staining and absence of C3 observed in rats receiving anti-domain 5 antibodies. Purified anti-domain 5 IgG, unlike anti-domain 1 IgG, did not recognize cardiolipin-bound ß2glycoproteinI while being able to interact with fluid-phase ß2glycoproteinI. These findings may explain the failure of anti-domain 5 antibodies to exhibit a thrombogenic effect in vivo, and the interaction of these antibodies with circulating ß2glycoproteinI suggests their potential competitive role with the pro-coagulant activity of anti-domain 1 antibodies. These data aim at better defining "really at risk" patients for more appropriate treatments to avoid recurrences and disability.

The influence of hypoxia on platelet function and plasmatic coagulation during systemic inflammation in humans in vivo.

Systemic inflammation and hypoxia frequently occur simultaneously in critically ill patients, and are both associated with platelet activation and coagulopathy. However, human in vivo data on the effects of hypoxia on platelet function and plasmatic coagulation under systemic inflammatory conditions are lacking. In the present study, 20 healthy male volunteers were randomized to either 3.5 h of hypoxia (peripheral saturation 80-85%) or normoxia (room air), and systemic inflammation was elicited by intravenous administration of 2 ng/kg endotoxin. Various parameters of platelet function and plasmatic coagulation were determined serially. Endotoxemia resulted in increased circulating platelet-monocyte complexes and enhanced platelet reactivity, effects which were attenuated by hypoxia. Furthermore, endotoxin administration resulted in decreased plasma levels of platelet factor-4 levels and increased concentrations of von Willebrand factor. These endotoxemia-induced effects were not influenced by hypoxia. Neither endotoxemia nor hypoxia affected thrombin generation. In conclusion, our data reveal that hypoxia attenuates the endotoxemia-induced increases in platelet-monocyte formation and platelet reactivity, while leaving parameters of plasmatic coagulation unaffected.

The Lupus Anticoagulant Paradox.

Lupus anticoagulant (LA) represents the most enigmatic antibody population in patients with antiphospholipid syndrome and represents a paradox that is still unsolved. This class of antiphospholipid antibody causes a phospholipid-dependent prolongation of the clotting time but is associated with an increased risk of thrombosis and pregnancy morbidity. In this review, we will provide an overview of the different antibodies that have been associated with LA activity, their importance based on clinical studies, and address the question why this prolongation of the clotting time is associated with thrombosis rather than a bleeding tendency.

Thrombocytopenia and Platelet Dysfunction in Acute Tropical Infectious Diseases.

Thrombocytopenia is a well-known manifestation of acute tropical infectious diseases. The role of platelets in infections has received much attention recently because of their emerging activities in modulation of inflammatory responses, host defense, and vascular integrity. However, while many studies have addressed thrombocytopenia in tropical infections, abnormalities in platelet function have been largely overlooked. This is an important research gap, as platelet dysfunction may contribute to the bleeding tendency that characterizes some tropical infections. The development of novel platelet function assays that can be used in thrombocytopenic conditions (e.g., flow cytometry assays) has contributed to important new insights in recent years. In this review, the importance of platelets in tropical infections is discussed with special emphasis on the underlying mechanisms and consequences of thrombocytopenia and platelet dysfunction in these infections. Special attention is paid to malaria, a disease characterized by microvascular obstruction in which bleeding is rare, and to infections in which bleeding is common, such as dengue, other viral hemorrhagic fevers, and the bacterial infection leptospirosis. Given the importance of platelet function abnormalities in these infections, the development of affordable assays for monitoring of platelet function in low-resource countries, as well as pharmacologic interventions to prevent or reverse platelet function abnormalities, might improve clinical care and the prognosis of these infections.

The effects of signal transducer and activator of transcription three mutations on human platelets.

Involvement of signal transducer and activator of transcription 3 (STAT3) in inflammation is well known. Recently, a role for STAT3 in platelet activation and platelet production has been suggested. Platelets exhibit important immune functions and engagement of STAT3 in platelet physiology may link inflammation and hemostasis. This study investigated the effects of STAT3 loss-of-function mutations and single nucleotide polymorphisms (SNPs) in STAT3 on glycoprotein VI (GPVI)-mediated platelet activation and platelet numbers in humans. Two cohorts were studied. The first cohort concerned patients with STAT3 loss-of-function mutations. Platelet numbers were investigated in eight patients and GPVI-mediated platelet activation was functionally tested in four patients. Additional experiments were performed to investigate underlying mechanisms. The second cohort concerned 334 healthy volunteers and investigated the consequences of SNPs in STAT3 on GPVI-mediated platelet activation and platelet numbers. Platelet activation was lower in STAT3 loss-of-function patients at baseline and after stimulation of the GPVI receptor, reflected by decreased P-selectin expression. This was independent of gene transcription. Blockade of the adenosine di-phosphate (ADP) pathway resulted in a further decrease of P-selectin expression, particularly in STAT3 loss-of-function patients. In contrast, the SNPs in STAT3 did not influence GPVI-mediated platelet activation. Also, platelet numbers were not affected by STAT3 loss-of-function mutations, nor was there an association with the SNPs. In conclusion, STAT3 signaling does not seem to play a major role in thrombopoiesis. We confirm that STAT3 is involved in GPVI-mediated platelet activation in humans, independent of gene transcription. GPVI-mediated platelet activation is highly dependent on secondary ADP release. Our findings suggest that STAT3 modulation may affect inflammation, hemostasis, and their interaction.

Increased Platelet-Monocyte Interaction in Stable COPD in the Absence of Platelet Hyper-Reactivity.

BACKGROUND: Chronic obstructive pulmonary disease (COPD) is well known for its cardiovascular co-morbidities. Increased platelet-monocyte interaction is found in COPD and may reflect altered platelet function and a potential role for anti-platelet therapy. OBJECTIVES: The objectives were to investigate platelet-monocyte interaction, platelet activation and reactivity and plasmatic coagulation in stable COPD. METHODS: Platelet-monocyte interaction and platelet activation were determined by flow cytometry in 30 stable COPD patients and 25 controls. Platelet activation was measured by binding of fibrinogen to the activated fibrinogen receptor and platelet P-selectin expression at baseline and after platelet stimulation with platelet agonists. Plasmatic coagulation was measured by D-dimer and thrombin generation. RESULTS: Platelet-monocyte interaction was increased in stable COPD (median fluorescence intensity of platelet CD61 was 19.8 [IQR 14.0-33.2] vs. 10.0 [IQR 8.7-16.7], p = 0.002). In contrast, platelet activation and reactivity, reflected by fibrinogen binding and P-selectin expression, were the same in both groups. Plasma P-selectin and interleukin-6 were increased in COPD (p = 0.01 and p = 0.02, respectively), whereas soluble fibrinogen, D-dimer and thrombin generation were similar. CONCLUSIONS: Increased platelet-monocyte interaction was found in the absence of platelet hyper-reactivity and activation of plasmatic coagulation in stable COPD. Future clinical evaluation of the effects of different anti-platelet drugs in COPD is warranted, as anti-platelet therapy may interfere with platelet-monocyte interaction.

Higher platelet reactivity and platelet-monocyte complex formation in Gram-positive sepsis compared to Gram-negative sepsis.

Platelets may play a role in the high risk for vascular complications in Gram-positive sepsis. We compared the platelet reactivity of 15 patients with Gram-positive sepsis, 17 with Gram-negative sepsis and 20 healthy controls using a whole blood flow cytometry-based assay. Patients with Gram-positive sepsis had the highest median fluorescence intensity (MFI) of the platelet membrane expression of P-selectin upon stimulation with high dose adenosine diphosphate (ADP; P = 0.002 vs. Gram-negative and P = 0.005 vs. control groups) and cross-linked collagen-related peptide (CRP-XL; P = 0.02 vs. Gram-negative and P = 0.0001 vs. control groups). The Gram-positive group also demonstrated significantly higher ADP-induced fibrinogen binding (P = 0.001), as wll as platelet-monocyte complex formation (P = 0.02), compared to the Gram-negative group and had the highest plasma levels of platelet factor 4, ß-thromboglobulin and soluble P-selectin. In contrast, thrombin-antithrombin complex and C-reactive protein levels were comparable in both patient groups. In conclusion, common Gram-positive pathogens induce platelet hyperreactivity, which may contribute to a higher risk for vascular complications.

The effect of P2Y12 inhibition on platelet activation assessed with aggregation- and flow cytometry-based assays.

Patients on P2Y12 inhibitors may still develop thrombosis or bleeding complications. Tailored antiplatelet therapy, based on platelet reactivity testing, might reduce these complications. Several tests have been used, but failed to show a benefit of tailored antiplatelet therapy. This could be due to the narrowness of current platelet reactivity tests, which are limited to analysis of platelet aggregation after stimulation of the adenosine diphosphate (ADP)-pathway. However, the response to ADP does not necessarily reflect the effect of P2Y12 inhibition on platelet function in vivo. Therefore, we investigated whether measuring platelet reactivity toward other physiologically relevant agonists could provide more insight in the efficacy of P2Y12 inhibitors. The effect of in vitro and in vivo P2Y12 inhibition on αIIbß3-activation, P-selectin and CD63-expression, aggregate formation, release of alpha, and dense granules content was assessed after stimulation of different platelet activation pathways. Platelet reactivity measured with flow cytometry in 72 patients on P2Y12 inhibitors was compared to VerifyNow results. P2Y12 inhibitors caused strongly attenuated platelet fibrinogen binding after stimulation with peptide agonists for protease activated receptor (PAR)-1 and -4, or glycoprotein VI ligand crosslinked collagen-related peptide (CRP-xl), while aggregation was normal at high agonist concentration. P2Y12 inhibitors decreased PAR-agonist and CRP-induced dense granule secretion, but not alpha granule secretion. A proportion of P2Y12-inhibitor responsive patients according to VerifyNow, displayed normal fibrinogen binding assessed with flow cytometry after stimulation with PAR-agonists or CRP despite full inhibition of the response to ADP, indicating suboptimal platelet inhibition. Concluding, measurement of platelet fibrinogen binding with flow cytometry after stimulation of thrombin- or collagen receptors in addition to ADP response identifies different patients as nonresponders to P2Y12 inhibitors, compared to only ADP-induced aggregation-based assays. Future studies should investigate the value of both assays for monitoring on-treatment platelet reactivity.

Plasmin is a natural trigger for bradykinin production in patients with hereditary angioedema with factor XII mutations.

BACKGROUND: Patients with angioedema experience unpredictable attacks of tissue swelling in which bradykinin is implicated. Several distinct mutations in Factor XII (FXII) are associated with hereditary angioedema (HAE) in the presence of normal C1 esterase inhibitor activity (FXII-HAE). The underlying disease mechanisms are unclear, which complicates diagnosis and treatment. OBJECTIVE: We sought to identify the natural trigger for FXII activation, which causes uncontrolled bradykinin production in patients with FXII-HAE. METHODS: We generated recombinant variants of FXII, representing health and disease, and studied their behavior in functional studies. We investigated bradykinin-forming pathways in blood plasma with newly developed nanobody-based analytic methods. RESULTS: We here report that FXII-HAE mutations collectively introduce new sites that are sensitive to enzymatic cleavage by plasmin. These FXII mutants rapidly activate after cleavage by plasmin, escape from inhibition through C1 esterase inhibitor, and elicit excessive bradykinin formation. Furthermore, our findings indicate that plasmin modulates disease activity in patients with FXII-HAE. Finally, we show that soluble lysine analogs attenuate this mechanism, explaining their therapeutic value in patients with HAE. CONCLUSION: Our findings indicate a new pathway for bradykinin formation in patients with HAE, in which FXII is cleaved and activated by plasmin. This should lead to the identification of new markers for diagnosis and targets for treatment.

Shear-Resistant Binding to von Willebrand Factor Allows Staphylococcus lugdunensis to Adhere to the Cardiac Valves and Initiate Endocarditis.

BACKGROUND: Staphylococcus lugdunensis is an emerging cause of endocarditis. To cause endovascular infections, S. lugdunensis requires mechanisms to overcome shear stress. We investigated whether platelets and von Willebrand factor (VWF) mediate bacterial adhesion to the vessel wall and the cardiac valves under flow. METHODS: S. lugdunensis binding to VWF, collagen, and endothelial cells was studied in a parallel flow chamber in the absence and presence of platelets. In vivo adhesion of S. lugdunensis was evaluated in a mouse microvasculature perfusion model and a new mouse model of endocarditis. RESULTS: Contrary to other coagulase-negative staphylococci, S. lugdunensis bound to VWF under flow, thus enabling its adhesion to endothelial cells and to the subendothelial matrix. In inflamed vessels of the mesenteric circulation, VWF recruited S. lugdunensis to the vessel wall. In a novel endocarditis mouse model, local inflammation and the resulting release of VWF enabled S. lugdunensis to bind and colonize the heart valves. CONCLUSIONS: S. lugdunensis binds directly to VWF, which proved to be vital for withstanding shear forces and for its adhesion to the vessel wall and cardiac valves. This mechanism explains why S. lugdunensis causes more-aggressive infections, including endocarditis, compared with other coagulase-negative staphylococci.

Antiphospholipid Antibodies and the Risk of Stroke in Urban and Rural Tanzania: A Community-Based Case-Control Study.

BACKGROUND AND PURPOSE: The burden of stroke is high in sub-Saharan Africa, and improved knowledge of risk factors is needed. Antiphospholipid antibodies are a common acquired stroke risk factor in young individuals. Antiphospholipid antibodies may be induced by infectious diseases. Sub-Saharan Africa has a high infectious burden, and we analyzed the contribution of antiphospholipid antibodies to the risk of stroke in an incident population from rural and urban Tanzania. METHODS: Stroke cases and age- and sex-matched community-acquired controls from the rural Hai district and urban Dar-es-Salaam areas of Tanzania were recruited in a wider study of stroke incidence between June 2003 and June 2006. Lupus anticoagulant, anticardiolipin, anti-ß2-glycoprotein I, and antiphosphatidylserine/prothrombin antibodies were determined in stored plasma, as well as IgG antibodies against Treponema pallidum. RESULTS: Data from 158 stroke cases and 369 controls were analyzed. Thirty cases (19%) and 4 controls (1%) had a lupus anticoagulant (odds ratio, 20.8; 95% confidence interval, 7.2-60.5). Anticardiolipin IgG was the only other antiphospholipid antibody subtype associated with increased stroke risk (odds ratio, 2.1; 95% confidence interval, 1.0-4.3), but this association disappeared when corrected for IgG antibodies against Treponema pallidum results. The prevalence of anti-ß2-glycoprotein I IgG antibodies in the Tanzanian healthy population was high when Dutch cutoff values were applied (67%), whereas presence of anti-ß2-glycoprotein I IgM was associated with a reduced stroke risk (odds ratio 0.3; 95% confidence interval, 0.1-1.1). CONCLUSIONS: The presence of lupus anticoagulant is a strong, and to date unrecognized, risk factor for stroke in Tanzania, especially in young and middle-aged individuals.

Keeping von Willebrand Factor under Control: Alternatives for ADAMTS13.

Von Willebrand factor (VWF) is one of the most important proteins of the hemostatic system. Its multimeric state is essential for its natural function to guide platelets to sites of injury. ADAMTS13 is the key protease that regulates the multimeric state of VWF. Without ADAMTS13, VWF multimers can grow to pathologically large sizes. This is a risk factor for the life-threatening condition thrombotic thrombocytopenic purpura (TTP). In this condition, VWF-rich thrombi occlude the microvasculature of various tissues. Intriguingly, a complete ADAMTS13 deficiency does not cause continuous TTP, either in patients or genetically targeted mice. Instead, TTP occurs in episodes of disease, separated by extended periods of remission. This indicates that regulating factors beyond ADAMTS13 are likely involved in this pathologic cascade of events. This raises the question of what really happens when ADAMTS13 is (temporarily) unavailable. In this review, we explore the possible role of complementary mechanisms that are capable of modifying the thrombogenic potential of VWF.

Active von Willebrand factor predicts 28-day mortality in patients with systemic inflammatory response syndrome.

Endothelial dysfunction contributes to the pathology of systemic inflammatory response syndrome (SIRS). However, endothelial biomarkers are not routinely evaluated in this setting. Here, 275 patients with SIRS and plasma levels of von Willebrand factor (VWF), thrombospondin-1, myeloperoxidase, ADAMTS-13, and active VWF (aVWF) were studied in relation to 28-day mortality. On admission, aVWF levels were higher in nonsurvivors vs survivors (0.69 vs 0.47 µg/mL, P = .019). Patients in the highest tertile of aVWF levels had a lower cumulative survival (86% vs 75%, P = .017) and twofold increased hazard ratio (HR). When adjusted for the Acute Physiology and Chronic Health Evaluation IV (APACHE-IV) score, this difference remained significant (HR 1.82, 95% confidence interval, 1.03-3.3). On admission, no significant differences were measured for the other proteins. These observations suggest that the stimulated release of VWF is not predictive for mortality in patients with SIRS, opposite of the processing of VWF after release. aVWF could be used with the APACHE-IV score to stratify SIRS patients at high mortality risk.

Functional platelet defects in children with severe chronic ITP as tested with 2 novel assays applicable for low platelet counts.

Immune thrombocytopenia (ITP) is an autoimmune disease with a complex heterogeneous pathogenesis and a bleeding phenotype that is not necessarily correlated to platelet count. In this study, the platelet function was assessed in a well-defined cohort of 33 pediatric chronic ITP patients. Because regular platelet function test cannot be performed in patients with low platelet counts, 2 new assays were developed to determine platelet function: first, the microaggregation test, measuring in platelets isolated from 10 mL of whole blood the platelet potential to form microaggregates in response to an agonist; second, the platelet reactivity assay, measuring platelet reactivity to adenosine diphosphate (ADP), convulxin (CVX), and thrombin receptor activator peptide in only 150 µL of unprocessed whole blood. Patients with a severe bleeding phenotype demonstrated a decreased aggregation potential upon phorbol myristate acetate stimulation, decreased platelet degranulation following ADP stimulation, and a higher concentration of ADP and CVX needed to activate the glycoprotein IIbIIIa complex compared with patients with a mild bleeding phenotype. In conclusion, here we have established 2 functional tests that allow for evaluation of platelet function in patients with extremely low platelet counts (<10(9)). These tests show that platelet function is related to bleeding phenotype in chronic ITP.

FLow-induced PRotrusions (FLIPRs): a platelet-derived platform for the retrieval of microparticles by monocytes and neutrophils.

RATIONALE: Platelets are the most important cells in the primary prevention of blood loss after injury. In addition, platelets are at the interface between circulating leukocytes and the (sub)endothelium regulating inflammatory responses. OBJECTIVE: Our aim was to study the dynamic process that leads to the formation of procoagulant and proinflammatory platelets under physiological flow. METHODS AND RESULTS: In the present study, we describe the formation of extremely long, negatively charged membrane strands that emerge from platelets adhered under flow. These flow-induced protrusions (FLIPRs) are formed in vitro on different physiological substrates and are also detected in vivo in a mouse carotid injury model. FLIPRs are formed downstream the adherent and activated platelets and reach lengths of 250 µm. FLIPR formation is shear-dependent and requires cyclophilin D, calpain, and Rac1 activation. It is accompanied by a disassembly of the F-actin and microtubule organization. Monocytes and neutrophils roll over FLIPRs in a P-selectin/P-selectin glycoprotein ligand-1-dependent manner, retrieving fragments of FLIPRs as microparticles on their surface. Consequently, monocytes and neutrophils become activated, as demonstrated by increased CD11b expression and L-selectin shedding. CONCLUSIONS: The formation of long platelet membrane extensions, such as the ones presented in our flow model, may pave the way to generate an increased membrane surface for interaction with monocytes and neutrophils. Our study provides a mechanistic model for platelet membrane transfer and the generation of monocyte/neutrophil-microparticle complexes. We propose that the formation of FLIPRs in vivo contributes to the well-established proinflammatory function of platelets and platelet-derived microparticles.