Tissue Factor-Negative Cell-Derived Microparticles Play a Distinctive Role in Hemostasis: A Viewpoint Review

Circulating cell-derived microparticles (MPs) exhibit procoagulant activity and have been investigated for a possible role in some human pathologies. However, their potential role in hemostasis has been neglected and often denied. This review brings to attention a specific body of direct clinical evidence supporting an important but distinctive role of MPs in hemostasis. Evidence for a role of MPs in hemostasis includes: (1) two congenital bleeding disorders attributed to impaired release of MPs; (2) two recent studies of trauma patients relating naturally elevated endogenous MPs at admission to reduced transfusion requirements and better outcomes; (3) a study of coronary surgery patients showing that elevated MP before surgery reduces transfusion requirements during surgery; and (4) a clinical study of patients with immune thrombocytopenia demonstrating that those with high circulating MP have reduced bleeding compared to patients with similar platelet counts but lower MP levels. Mechanisms involving potentiating the contact factor pathway are thought to play a key role and are probably synergistic with polyphosphate released from activated platelets at sites of endothelial injury. Hemostatic defect of patients with deficient MP-mediated coagulation resembles deficiency of FXI (hemophilia C), distinct from hemophilia A or B, so can be termed type C hemostasis. A better understanding of this proposed hemostatic pathway may lead to improved methods for controlling excessive bleeding in surgery, trauma, and other clinical settings.

Elevated endothelial microparticle—monocyte complexes induced by multiple sclerosis plasma and the inhibitory effects of interferon-β1b on release of endothelial microparticles, formation and transendothelial migration of monocyte-endothelial microparticle complexes

Monocyte migration through the disrupted cerebral endothelial cell (EC) junctions plays an essential role in formation of multiple sclerosis (MS) demyelinating lesions. During pathogenesis of MS, activated ECs release endothelial microparticles (EMP), which possibly facilitate transendothelial migration (TEMIG) of monocytes. To assess functional roles of EMP in MS, specifically, their (i) interaction with monocytes, (ii) effect on monocyte TEMIG in an in vitro model of the brain microvascular endothelial cells (BMVEC), (iii) phenotypic profiles of EMP elicited by MS plasma and (iv) the effects of IFN-b1b on release of EMP and on TEMIG of monocytes (mono) and monocytes:EMP complexes (mono:EMP) through the BMVEC. The effect of IFN-b1b on the release of EMP and the TEMIG of mono and mono:EMP was assessed by preincubating BMVEC cultures of IFN-b1b prior to addition of plasma. Three EMP phenotypes, CD54, CD62E and CD31 were assayed. Plasma specimens from 20 patients with relapsing—remitting MS (11 in exacerbation, MS-E, and 9 in remission, ME-R) and 10 healthy controls were studied. Incubation of BMVEC with MS-E plasma yielded elevated levels of EMPCD54, EMP62E and EMPCD31 relative to MS-R and control plasmas. MS-E but not MS-R or control plasma also augmented TEMIG of monocytes, respectively. Mono:EMP complexes further augmented TEMIG relative to mono alone, but only in the presence of MS-E plasma; there was no significant effect with MS-R or control plasmas. The presence of IFN-b1b inhibited TEMIG of mono and mono:EMP by 20% and 30%, respectively. MS-E but not MS-R plasma elicited release of activation-derived EMP and enhanced TEMIG of mono and mono:EMP. IFN-b1b inhibited TEMIG and release of EMP, suggesting a role of EMP and a novel therapeutic mechanism for IFN-β1b in MS.

Chronic Platelet Activation and Acute Coronary Syndromes in 13 Middle-Aged Patients

We report on clinical, laboratory, and angiographic findings that appear to characterize a group of 13 middle-aged patients who suffered acute coronary syndromes (ACS) despite little angiographic evidence of atherosclerotic heart disease (ASHD) or other risk factors. Nine of the 13 were ≤46 years of age and the rest ranged to 59 years. All had evidence of platelet disorders (PD): seven had chronic immune thrombocytopenia (ITP), one had familial thrombocytopenia, and five had other disorders affecting platelets. Evidence of long-standing chronic platelet activation was the common feature of the group, as found by (i) elevated platelet microparticles (PMP), (ii) thrombocytopenia, and (iii) enhanced procoagulant activity of plasma. Data on the 7 with ITP were compared to 20 ITP without ACS: the former had higher PMP (p < 0.01) and platelet-associated IgM (p < 0.05) relative to the ITP patient controls. Another set of patient controls consisted of 20 ACS with documented ASHD: although activation indicators were abnormal also in this group relative to normal controls (p < 0.01), the PD group of 13 had more marked abnormalities in all tests (p < 0.03), particularly in PMP and thrombocytopenia (p < 0.01). The seven youngest in the PD group appeared to respond to antiplatelet therapy since no recurrence of coronary ischemia was seen in up to 3 years of observation. It is suggested that chronic platelet activation by antibodies or immune complex may predispose those in the PD group to ACS (e.g., when under stress) despite the absence of ASHD and few other known risk factors. The true incidence of this syndrome is unknown but may be substantial. Key Words: Chronic platelet activation-Platelet microparticles-Platelet.associated IgM-Immune thrombocytopenia-Acute coronary syndromes-Middle age.

Life-Threatening Hypercoagulable State Following Splenectomy in ITP: Successful Management with Aggressive Antithrombotic Therapy and Danazol

A life-threatening hypercoagulable state (HCS) is reported that developed after splenectomy in idiopathic thrombocytopenic purpura (ITP). A 50-year-old active male was rejected for blood donation because of an incidental finding of low platelet counts, 40,000/uL. The diagnosis was ITP. Although asymptomatic, he underwent splenectomy because of poor response to steroids and intravenous (IV) gamma globulin. One month after splenectomy, he suffered pulmonary emboli without deep venous embolism (DVT), followed by bilateral DVT, threatening amputation of the legs. Emergency thrombolysis, insertion of stent, and IV heparin saved his legs. Extensive workup for HCS was negative. IV heparin was witheld for colonoscopy for possible gastrointestinal neoplasm, at which time DVT recurred, necessitating another thrombolysis and heparin infusion. He was discharged on enoxaparin, antiplatelet therapy, and danazol. Platelet hyperactivation, characterized by high platelet microparticles (PMP) and CD62P, was present throughout his course of active ITP, resolving when ITP went into remission with danazol therapy. ITP has remained in remission for 4 years after stopping enoxaparin and danazol. In vitro, his plasma in active ITP induced activation of normal platelets, generating PMP and inducing CD62p-positive platelets and platelet aggregates; his plasma from remission had no effect. This indicates the presence of a platelet activating factor, possibly anti-platelet antibodies. Splenectomy may have allowed procoagulant PMP to accumulate to high levels resulting in HCS. We advise awareness of thrombotic complications post-splenectomy in the subset of ITP patients who are largely asymptomatic and exhibit persisting platelet activation.

Red cell-derived microparticles (RMP) as haemostatic agent

Among circulating cell-derived microparticles, those derived from red cells (RMP) have been least well investigated. To exploit potential haemostatic benefit of RMP, we developed a method of producing them in quantity, and here report on their haemostatic properties. High-pressure extrusion of washed RBC was employed to generate RMP. RMP were identified and enumerated by flow cytometry. Their size distribution was assessed by Doppler electrophoretic light scattering analysis (DELSA). Interaction with platelets was studied by platelet aggregometry, and shear-dependent adhesion by Diamed IMPACT-R. Thrombin generation and tissue factor (TF) expression was also measured. The effect of RMP on blood samples of patients with bleeding disorders was investigated ex vivo by thromboelastography (TEG). Haemostatic efficacy in vivo was assessed by measuring reduction of blood loss and bleeding time in rats and rabbits. RMP have mean diameter of 0.45 µm and 50% of them exhibit annexin V binding, a proxy for procoagulant phospholipids (PL). No TF could be detected by flow cytometry. At saturating concentrations of MPs, RMP generated thrombin robustly but after longer delay compared to PMP and EMP. RMP enhanced platelet adhesion and aggregation induced by low-dose ADP or AA. In TEG study, RMP corrected or improved haemostatic defects in blood of patients with platelet and coagulation disorders. RMP reduced bleeding time and blood loss in thrombocytopenic rabbits (busulfan-treated) and in Plavix-treated rats. In conclusion, RMP has broad haemostatic activity, enhancing both primary (platelet) and secondary (coagulation) haemostasis, suggesting potential use as haemostatic agent for treatment of bleeding.

Antiphospholipid antibodies and platelet activation as risk factors for thrombosis in thrombocythaemia

INTRODUCTION

Risk factors for thrombosis (TB) in thrombocythaemia (TC) associated with myeloproliferative disorder (MPD) are not well defined.

METHODS

We measured antiphospholipid antibodies (APLA) in 35 patients with TC associated with MPD. Fourteen had TB and 21 did not. We assayed IgG and IgM APLA by ELISA for 6 antigens: beta2GP1, cardiolipin (CL), phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylethanolamine (PE) and FVII/VIIa, together with markers of activation of platelets (CD62P) and endothelium [endothelial microparticles (EMP)].

RESULTS

At least one positive APLA was detected in 66% of TC patients overall. The incidence was significantly higher in the TB subgroup (92.8%) than non-TB (47.6%, p < 0.05). Multiple APLA (positive for more than one antigen) were also more frequent in TB, for both IgG and IgM, for all 6 antigens tested (p < 0.05). However, IgM APLA predominated, being about 2-fold more frequently positive than IgG for all 6 antigens. Platelet CD62P was significantly higher in the TB group (p < 0.05). EMP did not differ between TB and non-TB. The most frequent thrombotic complication was recurring ischemic cerebral vascular accidents (ICVA), leading to progressive cognitive impairment. Venous TB often developed at unusual sites. Recurring and reversible TB were common features in TC.

SUMMARY

This study suggests that APLA and platelet activation are risk factors for TB in TC. APLA are prevalent in TC, and IgM APLA predominated over IgG. Activation of platelets but not of endothelium may be consistent with the reversible and recurrent features of TB in TC.

Role of platelets in neuroinflammation: a wide-angle perspective

OBJECTIVES

This review summarizes recent developments in platelet biology relevant to neuroinflammatory disorders. Multiple sclerosis (MS) is taken as the "Poster Child" of these disorders but the implications are wide. The role of platelets in inflammation is well appreciated in the cardiovascular and cancer research communities but appears to be relatively neglected in neurological research.

ORGANIZATION

After a brief introduction to platelets, topics covered include the matrix metalloproteinases, platelet chemokines, cytokines and growth factors, the recent finding of platelet PPAR receptors and Toll-like receptors, complement, bioactive lipids, and other agents/functions likely to be relevant in neuroinflammatory diseases. Each section cites literature linking the topic to areas of active research in MS or other disorders, including especially Alzheimer's disease.

CONCLUSION

The final section summarizes evidence of platelet involvement in MS. The general conclusion is that platelets may be key players in MS and related disorders, and warrant more attention in neurological research.

Antiphospholipid antibodies: paradigm in transition

OBJECTIVES

This is a critical review of anti-phospholipid antibodies (aPL). Most prior reviews focus on the aPL syndrome (APS), a thrombotic condition often marked by neurological disturbance. We bring to attention recent evidence that aPL may be equally relevant to non-thrombotic autoimmune conditions, notably, multiple sclerosis and ITP.

ORGANIZATION

After a brief history, the recent proliferation of aPL target antigens is reviewed. The implication is that many more exist. Theories of aPL in thrombosis are then reviewed, concluding that all have merit but that aPL may have more diverse pathological consequences than now recognized. Next, conflicting results are explained by methodological differences. The lupus anticoagulant (LA) is then discussed. LA is the best predictor of thrombosis, but why this is true is not settled. Finally, aPL in non-thrombotic disorders is reviewed.

CONCLUSION

The current paradigm of aPL holds that they are important in thrombosis, but they may have much wider clinical significance, possibly of special interest in neurology.

Evidence of platelet activation in multiple sclerosis

Objective

A fatality in one multiple sclerosis (MS) patient due to acute idiopathic thrombocytopenic purpura (ITP) and a near fatality in another stimulated our interest in platelet function abnormalities in MS. Previously, we presented evidence of platelet activation in a small cohort of treatment-naive MS patients.

Methods

In this report, 92 normal controls and 33 stable, untreated MS patients were studied. Platelet counts, measures of platelet activation [plasma platelet microparticles (PMP), P-selectin expression (CD62p), circulating platelet microaggragtes (PAg)], as well as platelet-associated IgG/IgM, were carried out. In addition, plasma protein S activity was measured.

Results

Compared to controls, PMP were significantly elevated in MS (p < 0.001) and CD62p expression was also markedly elevated (p < 0.001). Both are markers of platelet activation. Platelet-associated IgM, but not IgG, was marginally elevated in MS (p = 0.01). Protein S in MS patients did not differ significantly from normal values.

Conclusion

Platelets are significantly activated in MS patients. The mechanisms underlying this activation and its significance to MS are unknown. Additional study of platelet activation and function in MS patients is warranted.

Microparticle-mediated thrombin generation assay: increased activity in patients with recurrent thrombosis

BACKGROUND

Circulating cell-derived microparticles (MP) are important players in thrombogenesis, attributed in part to tissue factor (TF) carried on them. We developed MP-mediated thrombin generation assay (TGA) and measured a series of patients with thrombosis (TBS) and normal controls (NC).

METHODS

MP were isolated from plasma of 66 patients with TBS and 34 NC. The MP were resuspended in normal pooled particle-free plasma (PFP) containing corn trypsin inhibitor (to inhibit contact pathway). MP mediated TGA yields three parameters: lag time, peak and rate. This method is not influenced by anticoagulant therapy. Of the TBS patients, 41 had only a single thrombosis (S-TBS) and 25 had recurrences (R-TBS) within a 5-year period. In parallel, MP were quantitated by flow cytometry, and cell origin was determined: endothelial cells (EMP), leukocytes (LMP), red cells (RMP) and platelets (PMP).

RESULTS

MP from all TBS patients exhibited higher thrombin generation than NC by all three TGA parameters. R-TBS had significantly greater TGA values than S-TBS, reflected in higher peak and rate, and shorter lag time. MP numbers were also higher in TBS vs. NC, for all MP subtypes, and were significantly higher in R-TBS than S-TBS (except LMP). All MP levels correlated with thrombin generation (P < 0.0001), most closely between PMP and peak (R = 0.47) and rate (R = 0.43).

CONCLUSIONS

MP-mediated TGA is a novel way to assess functional procoagulant activity of MP. Enhanced MP-mediated TGA was demonstrated in TBS patients, and significantly higher activity in R-TBS. These findings support a major role of MP in thrombogenesis.

Increased procoagulant cell-derived microparticles (C-MP) in splenectomized patients with ITP

BACKGROUND

Splenectomy is frequently employed for therapeutic and diagnostic purposes in various clinical disorders. However its long-term safety is not well elucidated. Although risk of infection by encapsulated organisms is widely recognized, less well-known are risks of thrombosis and cardiovascular disease.

METHODS

We investigated levels of cell-derived microparticles (C-MP) in 23 splenectomized ITP (ITP-S) and 53 unsplenectomized ITP patients (ITP-nS). Assay of C-MP derived from platelets (PMP), leukocytes (LMP), red cells (RMP) and endothelial cells (EMP) were performed by flow cytometry. Coagulation parameters included PT, aPTT and activities of FVIII, IX and XI. Results of all measures were compared between the two groups, ITP-S vs ITP-nS.

RESULTS

Levels of all C-MP were higher in ITP-S than ITP-nS but only RMP and LMP reached statistical significance (p = 0.0035 and p < 0.0001, respectively). The aPTT was significantly shorter in ITP-S (p = 0.029). Interestingly, correlation analysis revealed that RMP, but not other C-MP, were associated with shortening of aPTT (p = 0.024) as well as with increased activities of factors VIII (p = 0.023), IX (p = 0.021) and XI (p = 0.0089).

CONCLUSIONS

RMP and LMP were significantly elevated in splenectomized compared to non-splenectomized ITP patients. This suggests that the spleen functions to clear procoagulant C-MP, and that elevation of C-MP might contribute to increased risk of thrombosis, progression of atherosclerosis and cardiovascular disease following splenectomy.

Transendothelial migration of leukocytes is promoted by plasma from a subgroup of immune thrombocytopenic purpura patients with small-vessel ischemic brain disease

We previously described a subgroup of immune thrombocytopenic purpura (ITP) patients presenting with recurring transient ischemic attack-like symptoms and progressive cognitive impairment due to small vessel disease (SVD) seen in the brain. They presented minimal bleeding despite thrombocytopenia, and platelet activation was elevated compared to classic ITP. On the hypothesis that the blood-brain barrier (BBB) is compromised in this subgroup, we investigated the effect of plasma from SVD-ITP patients on the transendothelial migration of leukocytes (TEML). Brain microvascular endothelial cells (BMVEC) were grown to confluence on 6.5-microm pore filters and plasma from 10 healthy controls, 20 classic ITP, and 5 SVD-ITP were added and incubated 24 hr. Then 1 x 10(5) monocytes (U937) were added and the number migrated through the EC monolayer after 6 hr was measured by flow cytometry. The effect on TEML of danazol was also assessed. We found that plasma from SVD-ITP but not classic ITP induced 10-fold rise in EC activation marker CD62E and a sevenfold increase in TEML, to 38.5% +/- 12.5% of cells migrated, compared to normal controls (5.6% +/- 1.2%) or classic ITP (6.1% +/- 0.2%), P < 0.001. Preincubation of U937 with endothelial microparticles (EMP) increased TEML by 20.0% +/- 6.4% with SVD-ITP plasma, significantly more than with classic ITP or control plasmas, P = 0.003. Pretreatment of cultures with danazol (100 microg/mL) inhibited TEML by 25% in all wells tested, whether or not EMP were added. In summary, SVD-ITP plasma activates EC and augments TEML, suggesting plasma-mediated BBB dysfunction in this syndrome. Danazol modestly but significantly inhibited TEML.

Clinical and neuroimaging correlates of antiphospholipid antibodies in multiple sclerosis: a preliminary study

Background

The presence of antiphospholipid antibodies (APLA) in multiple sclerosis (MS) patients has been reported frequently but no clear relationship between APLA and the clinical and neuroimaging features of MS have heretofore been shown. We assessed the clinical and neuroimaging features of MS patients with plasma APLA.

Methods

A consecutive cohort of 24 subjects with relapsing-remitting (RR) MS were studied of whom 7 were in remission (Rem) and 17 in exacerbation (Exc). All subjects were examined and underwent MRI of brain. Patients' plasma was tested by standard ELISA for the presence of both IgM and IgG antibodies using a panel of 6 targets: cardiolipin (CL), β2 glycoprotein I (β2GPI), Factor VII/VIIa (FVIIa), phosphatidylcholine (PC), phosphatidylserine (PS) and phosphatidylethanolamine (PE).

Results

In exacerbation up to 80% of MS subjects had elevated titers of IgM antibodies directed against the above antigens. However, in remission, less than half of MS patients had elevated titers of IgM antibodies against one or more of the above antigens. This difference was significant, p < 0.01, for all 6 target antigens. Interestingly, none of the MS patients had elevated plasma titers of IgG against any of the target antigens tested. Correlation analysis between MRI enhancing lesions and plasma levels of APLA revealed high correlation for aPC, aPS and aFVIIa (p ≤ 0.0065), a trend for aPE and aCL (p = 0.056), and no correlation for aβ2GP1. The strongest correlation was for aFVIIa, p = 0.0002.

Conclusion

The findings of this preliminary study show that increased APLA IgM is associated with exacerbations of MS. Currently, the significance of this association in pathogenesis of MS remains unknown. However, systematic longitudinal studies to measure APLA in larger cohorts of patients with relapsing-remitting MS, particularly before and after treatment with immunomodulatory agents, are needed to confirm these preliminary findings.

Platelet activation rather than endothelial injury identifies risk of thrombosis in subjects positive for antiphospholipid antibodies

BACKGROUND

Anti-phospholipid antibodies (APLA) are often associated with thrombosis, defining the antiphospholipid syndrome (APS) but it remains unclear why many subjects who are positive for APLA chiefly anti-cardiolipin (aCL) or anti-beta2GPI (abeta2GPI) do not develop thrombosis. A related question addressed in this study is whether the target of cellular injury in APS is predominately platelets or endothelial cells (EC).

METHODS

aCL and abeta2GPI were determined by ELISA in 88 patients, 60 of whom were thrombotic and 28 non-thrombotic. Platelet activation was measured by CD62P and by concentration of platelet microparticles (PMP) and EC activation was assessed by endothelial microparticles (EMP), both by flow cytometry. Lupus anticoagulant (LAC) was measured in the hospital laboratory.

RESULTS

There was no difference in frequency of aCL or abeta2GPI, neither IgG or IgM, between the thrombotic and non-thrombotic groups. Both groups showed elevated EMP compared to controls but this did not differ between thrombotic and non-thrombotic groups. In contrast, PMP were not significantly elevated in non-thrombotic but were elevated in thrombotic compared to non-thrombotic (p=0.03) and controls. CD62P, an independent marker of platelet activation, was also elevated in thrombotic vs. non-thrombotic. There was a trend for increased LAC in the thrombotic group but not significant.

CONCLUSION

Although all subjects had evidence of endothelial activation, only platelet activation differed between thrombotic and non-thrombotic. This supports the hypothesis that platelet activation predisposes to thrombosis in the presence of chronic EC activation. These data also raise the possibility of distinguishing risk-prone APLA-positive individuals.

Interstitial lung disease (ILD) and severe ITP

INTRODUCTION

Platelets play an important role in inflammatory and immune responses. We report interstitial lung disease (ILD) developing during the acute phase of severe thrombocytopenia in 3 patients with severe refractory ITP.

METHODS AND RESULTS

We identified 3 cases with severe ITP who developed ILD in the course of refractory chronic ITP. The thrombocytopenia was severe in all cases. ILD was an incidental finding at the presentation and often misdiagnosed as lung infections. ILD was documented by lung biopsy in cases 1 and 2, supplemented by serial chest X-rays and/or CAT scan. As the ITP improved, ILD regressed in case 1, persisted in case 2, and progressed to advanced pulmonary fibrosis in case 3.

CONCLUSION

We report an association of ILD with severe refractory ITP. ILD was detected in acute phase of platelet destruction, suggesting that platelet destruction may have triggered inflammation in the lung, leading to ILD.

Cell-derived microparticles and exosomes in neuroinflammatory disorders

All blood cells and the vascular endothelium shed microparticles (MP) from their plasma membranes when suitably stimulated, and assay of MP in patient blood has found increasing application to the monitoring of disease states. In addition, mounting evidence suggests that MP are not mere epiphenomena but play significant roles in the pathophysiology of thromboses, inflammation, and cancers. This chapter endeavors to summarize the limited number of studies thus far done on MP in neurological disorders such as multiple sclerosis (MS), transient ischemic attacks, and the neurological manifestations of antiphospholipid syndrome (APS). In addition, the chapter offers some plausible hypotheses on possible roles of MP in the pathophsyiology of these disorders, chiefly, the hypothesis that MP are indeed important participants in some neuropathologies, especially those which are ischemic in nature, but probably also inflammatory ones. The chapter also goes over the history and general principles of MP studies (e.g., assay methods and pitfalls), comparison with alternative methods (e.g., soluble markers of disease states), subclasses of MP (such as exosomes), and other topics aimed at helping readers to consider MP studies in their own clinical fields. Tables include a listing of bioactive agents known to be carried on MP, many of which were heretofore considered strictly soluble, and some of which can be transferred from cell to cell via MP vectors, for example certain cytokine receptors.

Antiphospholipid antibodies (APLA) in immune thrombocytopenic purpura (ITP) and antiphospholipid syndrome (APS)

Antiphospholipid antibodies (APLA) are associated with anti-phospholipid syndrome (APS), a thrombotic disorder, but they are also frequently detected in immune thrombocytopenic purpura (ITP), a bleeding disorder. To investigate possible differences of APLA between these two disorders, we assayed IgG and IgM APLA by ELISA in 21 patients with ITP and 33 with APS. The APLA reacting against two protein target antigens, beta(2)-glycoprotein 1 (beta2GP1) and FVII/VIIa, and four phospholipids [cardiolipin (CL), phosphatidylcholine (PC), phosphatidylserine (PS), and phosphatidylethanolamine (PE)] as well as lupus anticoagulant (LA) were analyzed. We made the following observations: (i) IgG and IgM antibodies to beta2GP1 and IgM antibodies to FVII/VIIa were more common in APS than ITP, P < 0.05, while IgG antibodies against the phospholipids (aCL, aPC, aPS, aPE) were more common in ITP than APS, P < 0.05; (ii) multiple APLA > or =3 antigens) were more frequent in APS than ITP, P < 0.05; (iii) LA was frequently associated with APS but was absent in ITP; (iv) APLA is quite common in ITP: two-thirds were positive for at least one APLA. In summary, APLA are prevalent in ITP but their profile differs from APS. In APS, antibodies were predominantly against beta2GP1 and 80% had positive LA, while in ITP the APLA reacted most often with the phospholipids without LA. The difference in APLA may result in opposite clinical manifestations in two disorders.

Levels of endothelial and platelet microparticles and their interactions with leukocytes negatively correlate with organ dysfunction and predict mortality in severe sepsis

OBJECTIVE

Mortality in sepsis is believed to be associated with exaggerated inflammatory responses, but recent evidence suggests that poor outcome is associated with reduced inflammation. To test this hypothesis, we measured several inflammatory markers to determine whether any of them or any combinations are associated with mortality or organ dysfunction.

DESIGN

Clinical study.

SETTING

School of medicine.

PATIENTS

Thirty-five patients with severe sepsis.

INTERVENTIONS

Markers of endothelial, platelet, and leukocyte activation were measured on days 1, 2, and 3 after enrollment. The markers were a) endothelial microparticles (EMPs) and their conjugates with monocytes (EMP/MONO); b) platelet microparticles (PMPs) and platelet activation marker CD62P; c) platelet-leukocyte conjugates (PLT/LEU) and leukocyte activation marker CD11b; and d) intracellular nitric oxide in leukocytes.

MEASUREMENTS AND MAIN RESULTS

The 28-day mortality rate was 51% (18 of 35). Significant differences between survivors and nonsurvivors on day 1 were found in PLT/LEU (p = .001), CD11b (p = 0.02), and EMP/MONO (p = .02) groups. Using logistic regression to assess if these markers predict mortality on day 1, we found that PLT/LEU had the best predictive value among the markers used (area under receiver operating characteristics curve = 0.82). All markers of cell activation and inflammation were significantly higher among survivors on days 2 and 3, except nitric oxide, which was lower. This marker showed significant negative correlation with the Sequential Organ Failure Assessment score throughout the study.

CONCLUSIONS

Our data support the hypothesis that early increased, not decreased, inflammatory response as measured by our markers is associated with improved survival rate. A high negative correlation was found between some of these markers and Sequential Organ Failure Assessment score.

Endothelial microparticles correlate with high-risk angiographic lesions in acute coronary syndromes

BACKGROUND

Endothelial Microparticles (EMP) are small fragments of endothelial cell membrane shed during apoptosis or activation. Our group has previously reported elevations of EMP in patients with coronary artery disease (CAD), thrombotic thrombocytopenic purpura (TTP), pre-eclampsia, multiple sclerosis (MS), and severe hypertension (HTN). In the present study, we evaluate the possible relationship between EMP levels and the angiographic severity and characteristics of coronary obstructive lesions.

METHODS

We studied a total of 43 patients undergoing coronary angiography. Fifteen had presented with acute myocardial infarction (MI), 20 with unstable anginas (UA), 5 with stable angina (SA) and 3 with congestive heart failure. Coronary angiography was reviewed and coronary lesions were classified using the Ambrose classification. Coronary stenoses were classified as high and low risk. High-risk included lesions with eccentric appearance (type II), presence of thrombi, or multiple irregularities. Low-risk lesions were defined as concentric or type I. Lesions were also analyzed by degree of stenosis and history of acute coronary syndrome (ACS). EMP in plasma was assayed by flow cytometry.

RESULTS

EMP in eccentric type II or multiple irregular lesions (high-risk) were 2.5-fold higher than in type I or concentric (low-risk) lesions, p<0.05. Lesions with thrombi had three-fold higher EMP than those without (p=0.05). Mild stenosis (>20%-<45%) had three-fold higher EMP than more severe (>45%), and five-fold higher than those without stenosis (p<0.01). Among patients with type II lesions, those with first ACS episode had four-fold higher EMP levels than those with recurrent ACS (p<0.01).

CONCLUSION

High EMP was associated with high-risk angiographic lesions including eccentric type II, multiple irregular, and lesions with thrombi. Mild to moderate stenosis was associated with higher EMP levels than severe stenosis. EMP may be a useful marker in detecting endothelial injury and risk of ACS as defined by angiography.

Digoxin use is associated with increased platelet and endothelial cell activation in patients with nonvalvular atrial fibrillation

OBJECTIVES

The purpose of this study was to determine whether digoxin use is associated with increased flow cytometric markers of endothelial cell and platelet activation in patients with nonvalvular atrial fibrillation (AF).

BACKGROUND

Increased intracellular calcium is a key event in platelet activation, and several studies have demonstrated that digitalis activates platelets in vitro. Intracellular calcium also is a key regulator of endothelial cell function, and endogenous digitalis-like substances have been shown to affect biologic processes in endothelial cells.

METHODS

We studied 30 patients with nonvalvular AF. We measured the levels of (1) platelet expression of P-selectin (CD62P), (2) platelet microparticles (PMP); and (3) endothelial microparticles (EMP) identified by anti-CD31 (EMP31) and by anti-E-selectin antibodies (EMP62E).

RESULTS

Patients who were taking digoxin (n = 16; mean digoxin level = 0.93 ng/dL) did not demonstrate any significant differences in clinical or echocardiographic characteristics compared with patients not taking digoxin (n = 14). Patients taking digoxin had significantly increased levels of CD62P expression in platelets and platelet-leukocyte conjugates and markedly increased markers of endothelial activation: EMP62E and EMP31. After adjusting for potential confounders (including age, congestive heart failure, coronary artery disease, ejection fraction, antiplatelet, beta-blocker, and calcium channel blocker use), the differences persisted.

CONCLUSIONS

Digoxin use in patients with AF is associated with increased levels of endothelial and platelet activation. If digitalis activates endothelial cells and platelets at pharmacologic doses, use of digitalis in conditions such as AF could predispose to thrombosis and vascular events.

Endothelial microparticles induce formation of platelet aggregates via a von Willebrand factor/ristocetin dependent pathway, rendering them resistant to dissociation

Endothelial microparticles (EMP) released from activated or apoptotic endothelial cells (EC) are emerging as useful markers for detection of EC dysfunction. Our recent observation that EMP carry von Willebrand factor (vWf) led us to investigate their interaction with platelets. EMP were incubated with normal washed platelets in the presence or absence of ristocetin, then platelet aggregates were measured by flow cytometry. In the absence of ristocetin, negligible EMP conjugated with platelets (< 5%) but in the presence of ristocetin (1 mg mL(-1)), EMP induced up to 95% of platelets to aggregate. EMP-platelet interaction was 80% blocked by anti-CD42b, or by 0.1 microm filtration to remove EMP. Platelet aggregates induced by normal plasma or high molecular weight vWf (Humate-P) dissociated 50% within 15-25 min following 1:20 dilution. In contrast, aggregates formed with EMP persisted two- to threefold longer with the same treatment, indicating greater stability. A similar degree of prolongation of dissociation was observed using plasma from thrombotic thrombocytopenic purpura (TTP) patients compared with normal plasma. Addition of EMP to plasma from severe von Willebrand disease restored his ristocetin-induced platelet aggregation. Multimer analysis of vWf on EMP showed unusually large vWf (ULvWf). In summary, EMP carries ULvWf multimers, promote platelet aggregates, and increase the stability of the aggregates thus formed.

Elevation of endothelial microparticles, platelets, and leukocyte activation in patients with venous thromboembolism

OBJECTIVES

The purpose of this research was to determine the levels of platelet, leukocyte, and endothelial activation and markers of cellular interactions in patients with venous thromboembolism (VTE).

BACKGROUND

The details of interactions between endothelium, platelets, and leukocytes in VTE are not well understood.

METHODS

We studied 25 patients with VTE and compared 25 healthy controls. We used flow cytometry to measure: 1) endothelial microparticles (EMP) identified by CD31+/CD42b- (EMP(31)) or E-selectin (EMP(62E)); 2) platelet microparticles (CD31+/CD42b+); 3) surface expression of P-selectin in platelets and CD11b in leukocytes; 4) EMP-monocyte conjugates (percentage of monocytes positive for E-selectin); and 5) platelet-leukocyte conjugates (PLC) expressed as percentage of leukocytes positive for CD41.

RESULTS

Patients with VTE had marked elevations of EMP(31) (2,193 vs. 383 counts/microl; p = 0.003), EMP(62E) (368 vs. 223 counts/microl; p = 0.001), and EMP-monocyte conjugates (3.3% vs. 2.5%; p = 0.002), as well as increased activation of platelets (35.2 vs. 5.0 fluorescence intensity units for P-selectin; p < 0.0001) and leukocytes (13.9 vs. 7.7 U for CD11b; p = 0.004). Also elevated in VTE were PLC (61.7% vs. 39.6%; p = 0.01). Expression of CD11b in leukocytes strongly correlated with PLC (r = 0.74; p < 0.0001).

CONCLUSIONS

Marked activation of endothelium, platelets, and leukocytes occurs in VTE, and VTE, or the accompanying inflammatory process, involves the release of EMP and formation of EMP-monocyte conjugates and PLC. These findings support prior studies suggesting that release of EMP and their binding to monocytes are key events in thrombogenesis. Our findings also support the concept that the formation of PLC regulates leukocyte activation and participates in linking thrombosis with inflammation.

New horizons in the analysis of circulating cell-derived microparticles

Analysis of circulating cell-derived microparticles (MP) is becoming more refined and clinically useful. This review, stemming from lectures given at Tokyo late 2003, does not repeat prior reviews but focuses on new horizons. A major theme is the rising recognition of platelets and their MP (PMP) as key mediators of inflammation/immunity. Among the major concepts developed are that (i) many so-called soluble markers of inflammation are in reality MP-bound; (ii) PMP and other MP appear to serve important signaling and immune functions including antigen presentation. In conclusion, MP analysis is poised to enter the mainstream of clinical testing, measuring specific antigens rather than gross levels. However, more research is needed to decisively establish their functions, and international standards are needed to allow comparing results from different laboratories.

Antiphospholipid antibodies in immune thrombocytopenic purpura tend to emerge in exacerbation and decline in remission

Although the presence of antiphospholipid antibodies (APLA) in immune thrombocytopenic purpura (ITP) has been reported, their clinical significance is not clear. The present study investigated APLA profiles in relation to the clinical stages of ITP. We studied APLA in 40 patients in three stages of ITP: exacerbation/relapse (n=7), stable (n=14) and remission (n=19). Both IgG and IgM APLA to six target antigens were measured by enzyme-linked immunosorbent assay: beta2-glycoprotein 1 (beta2GP1), cardiolipin, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine and factor VII/VIIa. The central finding was that APLA were common in ITP but differed significantly in disease stages, being highest in exacerbation (86% positive), intermediate in stable disease (57%) and lowest in remission (42%). In exacerbations, APLA were predominantly of IgG class, while in stable disease, IgM predominated. During remission, APLA often became undetectable. Both the frequency and titres of APLA were significantly higher during exacerbation than remission. An inverse correlation was found between platelet count and nearly all APLA (except beta2GP1). Sequential study of six patients revealed that APLA tended to emerge and rise with exacerbation, concurrently with new episodes of bleeding and became undetectable during remission. These findings raise the possibility that APLA may play a role in the exacerbation and remission of ITP or they may be a consequence of platelet destruction.

Postprandial Hypertriglyceridemia Increases Circulating Levels of Endothelial Cell Microparticles

Background— This study evaluated a possible relationship between levels of endothelial microparticles (EMPs), known to be a sensitive indicator of endothelial disturbance, and changes in postprandial lipid levels in healthy volunteers after a low- or high-fat meal.

Methods and Results— Eighteen healthy subjects without known cardiovascular risk factors were evaluated. Lipid and EMP levels were measured before and 1 and 3 hours after a single low- or high-fat isocaloric meal. The low-fat meal had no significant postprandial effect on EMPs or lipids compared with fasting levels. In contrast, a single high-fat meal significantly increased EMP levels after 1 and 3 hours, from 389±54 (thousands per milliliter) when fasting to 541±139 ( P =0.0002) and 677±159 ( P <0.0001), respectively, and correlated with a postprandial elevation in serum triglycerides.

Conclusions— A single high-fat meal led to a significant elevation of plasma EMP levels in healthy, normolipidemic subjects and correlated with a postprandial elevation of serum triglycerides. EMPs may be an indirect marker of endothelial dysfunction or injury induced by postprandial triglyceride-rich lipoproteins.

Endothelial microparticles (EMP) bind and activate monocytes: elevated EMP-monocyte conjugates in multiple sclerosis

Elevated plasma endothelial microparticles (EMP) have been documented in MS during exacerbation. However, the role of EMP in pathogenesis of MS remains unclear. We investigated the formation of EMP-monocyte conjugates (EMP-MoC) and their potential role in transendothelial migration of inflammatory cells in MS. EMP-MoC were assayed in 30 MS patients in exacerbation, 20 in remission and in 35 controls. EMP-leukocyte conjugation was investigated flowcytometrically by employing alpha-CD54 or alpha-CD62E for EMP, and alpha-CD45 for leukocytes. EMP-MoC were characterized by identifying adhesion molecules involved and their effect on monocyte function. In vivo (clinical): EMP-MoC were markedly elevated in exacerbation vs. remission and controls, correlating with presence of GD+ MRI lesions. Free CD54+ EMP were not elevated but free CD62E+ EMP were. In vitro: EMP bound preferentially to monocytes, less to neutrophils, but little to lymphocytes. Bound EMP activated monocytes: CD11b expression increased 50% and migration through cerebral endothelial cell layer increased 2.6-fold. Blockade of CD54 reduced binding by 80%. Most CD54+ EMP bound to monocytes, leaving little free EMP, while CD62+ EMP were found both free and bound. These results demonstrated that phenotypic subsets of EMP interacted differently with monocytes. Based on our observations, EMP may enhance inflammation and increase transendothelial migration of monocytes in MS by binding to and activating monocytes through CD54. EMP-MoC were markedly increased in MS patients in exacerbation compared to remission and may serve as a sensitive marker of MS disease activity.

Endothelial microparticles released in thrombotic thrombocytopenic purpura express von Willebrand factor and markers of endothelial activation

It has been suggested that endothelial apoptosis is a primary lesion in the pathogenesis of thrombotic thrombocytopenic purpura (TTP). We tested this hypothesis by examining the phenotypic signatures of endothelial microparticles (EMP) in TTP patients. In addition, the effect of TTP plasma on microvascular endothelial cells (MVEC) in culture was further delineated. EMP released by endothelial cells (EC) express markers of the parent EC; EMP released in activation carry predominantly CD54 and CD62E, while those in apoptosis CD31 and CD105. We investigated EMP release in vitro and in TTP patients. Following incubation of MVEC with TTP plasma, EMP and EC were analysed by flow cytometry for the expression of CD31, CD51, CD54, CD62E, CD105, CD106 and von Willebrand factor (VWF) antigen. EMP were also analysed in 12 TTP patients. In both EC and EMP, CD62E and CD54 expression were increased 3- to 10-fold and 8- to 10-fold respectively. However, CD31 and CD105 were reduced 40-60% in EC but increased twofold in EMP. VWF expression was found in 55 +/- 15% of CD62E+ EMP. Markers of apoptosis were negative. In TTP patients, CD62E+ and CD31+/CD42b- EMP were markedly elevated, and preceded and correlated well with a rise in platelet counts and a fall in lactate dehydrogenase. CD62E+ EMP (60 +/- 20%) co-expressed VWF and CD62E. The ratio of CD31+/42b- to CD62E+ EMP exhibited a pattern consistent with activation. In conclusion, our studies indicate endothelial activation in TTP. EMP that co-express VWF and CD62E could play a role in the pathogenesis of TTP.

Endothelial microparticles as markers of endothelial dysfunction

Endothelial microparticles (EMP) are small vesicles released from disturbed endothelial cells (EC). Owing to the central importance of EC injury in thrombotic and inflammatory conditions, assay of EMP as a marker of EC disturbance has come under intensive development by several laboratories. The review begins with established markers of EC injury, commonly soluble markers such as intercellular adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), E-selectin, von Willebrand factor (vWF), etc., pointing out that many of these are in fact mixtures of true soluble molecules with membrane-bound forms, for example, EMP. Assays of EMP from different labs are reviewed and standardization of assay is recommended. EMP are heterogeneous: those released in activation vs. apoptosis are distinctive in phenotypic markers and procoagulant properties. Application of EMP phenotype analysis can distinguish EC state of activation from apoptosis. Some EMP carry functional vWF with properties different from soluble vWF. Certain EMP bind to and activate monocytes; EMP-monocyte conjugates were found to be a marker of inflammatory disease such as multiple sclerosis (MS), and to enhance transendothelial migration of leukocytes in vitro. Clinical studies have revealed elevated plasma levels of EMP in lupus anticoagulant (LA), multiple sclerosis (MS), thrombotic thrombocytopenic purpura (TTP), coronary artery disease (CAD), hypertension, preeclampsia, and diabetes. Further refinement of EMP assay could open new windows for evaluating and monitoring endothelial injury in thrombotic and inflammatory disorders.

Differences of soluble CD40L in sera and plasma: Implications on CD40L assay as a marker of thrombotic risk

INTRODUCTION

Soluble CD40L (sCD40L) ELISA has emerged as a promising predictor of poor outcomes in acute coronary syndrome. Yet many blood processing techniques have been used with little consideration of their effect on the results.

METHODS

We measured sCD40L by ELISA in 10 patients with thrombocytopenia and 12 with normal or high platelet counts and 8 healthy controls using three sampling techniques: serum clotted on ice (serum-I) or at room temperature (serum-RT) and platelet poor plasma (PPP).

RESULTS

Serum-RT samples, compared to serum-I, gave significantly higher CD40L values (p=0.003), demonstrating that ex vivo sCD40L release by activated platelets is inhibited by cold temperature. Although serum-I and PPP were comparable in patients with normal platelet counts, serum-I gave significantly higher values than PPP in the thrombocytosis group (p=0.01), suggesting that cold inhibition is insufficient in the latter group. To estimate the fraction of sCD40L that was microparticle-bound CD40L (mp-CD40L), 16 samples underwent 0.1-microm filtration. 50.6% of sCD40L was mp-CD40L in serum-RT, whereas 21.3% and 29.9% were observed in serum-I and PPP, respectively. Lastly, plasma sCD40L was assayed in 46 patients with and 35 without thrombosis. Plasma sCD40L did not correlate with platelet count in non-thrombotic, non-inflammatory patients but did (p<0.01) in those with thrombosis.

CONCLUSIONS

Sample processing and temperature profoundly affect sCD40L assay. Serum-I and PPP minimize the release of sCD40L ex vivo and better represent sCD40L in vivo. However, PPP may be preferable particularly in patients with thrombocytosis. The existence of mp-CD40L highlights the importance of centrifuge conditions.

High levels of circulating endothelial microparticles in patients with acute coronary syndromes

BACKGROUND

Endothelial injury plays a critical role in coronary artery disease (CAD), but the assessment of this injury has been problematical. Recently, it has been shown in vitro that endothelial cells (ECs) release endothelial microparticles (EMPs) on activation or apoptosis and that an assay of EMPs can provide useful information on EC status in patients with thrombotic disorders. This study is aimed at assessing possible correlations between EMPs, which are markers of endothelial injury, and clinical subgroups of patients with CAD.

METHODS

A prospective, case-controlled study was conducted on 84 patients with CAD and 42 control subjects to investigate EMP profiles. Included were 64 patients with acute coronary syndromes ([ACS], 38 with myocardial infarction [MI] and 26 with unstable angina [UA]) and 20 patients with stable angina (SA). EMPs in platelet-poor plasma were measured flow cytometrically with combinations of fluorescent antibodies (anti-CD31, -51, -42), allowing distinction of EMPs from platelet microparticles (PMPs). Clinical subgroups of patients were correlated with EMP and PMP levels in blood.

RESULTS

Two species of EMPs (CD31+ and CD51+) were evaluated. Both were significantly higher in patients with CAD than in control subjects. CD31+ EMP was higher in ACS than SA. Among patients with first MI, CD31+ EMP was higher in patients with MI than in patients with UA and was significantly higher than in patients with recurring MI. CD51+ EMP did not discriminate ACS from SA. A simultaneous assay of PMP showed correlation between EMPs and PMPs. However, PMPs did not discriminate patients with SA from control subjects.

CONCLUSIONS

EMP assay appears promising for assessing EC injury in CAD.

Endothelial cells release phenotypically and quantitatively distinct microparticles in activation and apoptosis

BACKGROUND

Endothelial cells (EC) shed endothelial microparticles (EMP) in activation and apoptosis.

OBJECTIVES

We compared the antigenic expression of EMP species released during activation as compared to apoptosis, in three cell lines.

METHODS

EC from renal and brain microvascular (MiVEC) and coronary macrovascular (MaVEC) origin were incubated with TNF-alpha to induce activation, or deprived of growth factors to induce apoptosis. Antigens expressed on EMP and EC were assayed flow cytometrically and included constitutive markers (CD31, CD51/61, CD105), inducible markers (CD54, CD62E and CD106), and annexin V binding.

RESULTS

It was found that in apoptosis, constitutive markers in EMP were markedly increased (CD31>CD105), with a concomitant decrease in expression in EC. Annexin V EC surface binding and annexin V+ EMP were more sharply increased in apoptosis than in activation. In contrast, in activation, inducible markers in EMP were markedly increased in both EMP and EC (CD62E>CD54>CD106). Coronary MaVEC released significantly less EMP than MiVEC.

CONCLUSION

EC release qualitatively and quantitatively distinct EMP during activation compared to apoptosis. Analysis of EMP phenotypic signatures may provide clinically useful information on the status of the endothelium.

Factor VII/VIIa: a new antigen in the anti-phospholipid antibody syndrome

We investigated antibodies to factor VII/VIIa (FVII/VIIa) and five other common target antigens in 33 patients with a history of anti-phospholipid syndrome (APS) and 50 healthy controls using an enzyme-linked immunosorbent assay (ELISA) technique. We found that antibody to FVII/VIIa, a previously unrecognized and common antigen in APS, was present in 67% of patients. Frequencies of antibodies to other target antigens were: anti-beta-2 glycoprotein 1 (anti-beta 2GP1), 88%; anti-cardiolipin (anti-CL), 76%; anti-phosphatidylethanolamine (anti-PE), 67%; anti-phosphatidylserine (anti-PS), 64%; and anti-phosphatidylcholine (anti-PC), 59%. Most patients had antibodies against multiple antigens, but a few were positive for only anti-beta 2GP1 (12%) or anti-CL (3%). Positivity for anti-FVII/VIIa was significantly associated with positivity for anti-PE, anti-PS and/or anti-PC (P < 0.05) but not anti-beta 2GP1. When frequencies of immunoglobulin G (IgG) versus immunoglobulin M (IgM) antibodies were compared, anti-beta 2GP1 IgG correlated with the lupus anticoagulant (P < 0.05) and was significantly more prevalent than IgM, but the reverse was seen for all other antigens. In arterial thrombosis, IgM was more prevalent for all antigens, and was significantly associated with FVII/VIIa, PE and PS, whereas in venous thrombosis, IgG was frequently prevalent, especially in association with FVII/VIIa, beta 2GP1 and CL. In summary, FVII/VIIa is a new and common antigen in APS. Anti-FVII/VIIa is often associated with anti-PE, anti-PS and anti-PC. The IgM class is more frequently associated with arterial thrombosis and the IgG class with venous thrombosis.

Effects of severe hypertension on endothelial and platelet microparticles

The molecular mechanisms by which extreme blood pressure elevation leads to vascular injury are not defined. To explore the hypothesis that activation of endothelium and platelets as manifested by increased concentrations of circulating endothelial microparticles and platelet microparticles could play a role in this target organ injury, we conducted a cross-sectional study of these markers in 3 groups: (1) untreated patients referred specifically for treatment of severe uncontrolled hypertension; (2) untreated patients with established mild hypertension; and (3) normotensive volunteer subjects. By ANOVA, endothelial (P=0.002) and platelet (P=0.01) microparticles were greatest in the severely hypertensive group. There was a significant correlation between both of these markers and blood pressure, even in the setting of multiple risk factors. Our results suggest that these markers may be useful and specific for pressure-induced endothelial and platelet activation in hypertension. Furthermore, because of the combined effects of endothelial and platelet microparticles on coagulation, leukocytes, and endothelium, it is possible that they may play a pathogenic role in mediating target organ injury in severe hypertension.

Vascular dementia in patients with immune thrombocytopenic purpura

INTRODUCTION

Platelets have been implicated in memory disorders but this has not been investigated in patients with immune or idiopathic thrombocytopenia (ITP). ITP is an autoimmune disorder in which autoantibodies bring about platelet destruction. We previously reported a group of ITP patients who manifested TIA-like syndrome and gradual memory loss leading to dementia: platelet microparticles (PMP), a marker of platelet activation, were often elevated, suggesting that procoagulant PMP released from stimulated platelets contributed to thrombosis in small vessels. We have expanded on those studies to better define the clinical, laboratory, and radiologic characteristics of this syndrome.

MATERIALS AND METHODS

Twenty ITP patients with this syndrome were studied in comparison to twenty-three ITP patients without it (patient controls). Clinical and laboratory features were compared and radiologic images were analyzed. Factors influencing the rate of progression to advanced dementia were also investigated.

RESULTS AND CONCLUSION

Recurring dizzy or weak spells, TIA-like syndrome, recent memory loss, and cognitive impairment were common initial complaints. In some, these symptoms progressed rapidly to dementia but was indolent in others. Progression was faster in those with splenectomy and higher platelet counts. MRI showed enhanced signal in subcortical, periventricular areas, consistent with ischemic small vessel disease. Compared to patient controls, bleeding was less frequent and platelet activation (increased PMP, CD62p) was more frequent in the study group. Thrombotic complications may occur in ITP, manifested as TIA-like syndrome or memory loss due to ischemic small vessel disease, progressing to vascular dementia. Memory disturbances associated with platelet disorders warrants further investigation.

Idiopathic thrombocytopenic purpura: pathophysiology and management

Our understanding of the pathophysiology of ITP owes to pioneering work of W J Harrington in 1951, delineating the immunologic nature of platelet destruction. In ITP, antibody-coated platelets are destroyed by macrophages of RES. However, other mechanisms are also implicated: C-mediated platelet lysis and newly described C-independent peroxide injury. Both induce platelet fragmentation and lysis, generating procoagulant platelet microparticles (PMP). A third mechanism of platelet consumption in the microvasculature is proposed, based on overlapping syndromes of ITP and TTP in some patients. In assessing hemostasis in ITP, platelet counts alone is not sufficient. Evaluation of platelet clumping, giant platelets, and platelet activation, marked by increased PMP is useful. Patients with platelet activation or giant platelets bleed less and detection of clumping prevents unwarranted therapy. Thrombotic complications may develop in ITP. A syndrome, characterized by recurrent TIA-like symptoms, progressive memory loss due to ischemic small vessel disease is described. The management of ITP should include the search for and elimination of underlying causes and careful evaluation of hemostasis. Therapy is divided into definitive vs symptomatic measures. The former including splenectomy, danazol, chemotherapy offers lasting remission after therapy was stopped, while the later including glucocorticoids, gammaglobuin, antiD antibodies and others increases platelet counts but seldom sustains remission upon withdrawal. Danazol therapy is up-dated since it is an effective and safe definite measure in ITP.

Elevated plasma endothelial microparticles in multiple sclerosis

OBJECTIVE

To assess endothelial dysfunction in patients with MS and to investigate whether plasma from patients with MS induces endothelial cell dysfunction in vitro.

BACKGROUND

Endothelial cell dysfunction may contribute to the pathogenesis of MS. Elevations of soluble adhesion molecules intracellular adhesion molecule, vascular cell adhesion molecule, and platelet-endothelial cell adhesion molecule-1 (CD31) have been reported as markers of blood-brain barrier (BBB) damage in MS, but direct assay of endothelium has been difficult. Endothelial cells release microparticles < approximately 1.5 microm (EMP) during activation or apoptosis. The authors developed a flow cytometric assay of EMP and studied EMP as markers of endothelial damage in MS.

METHODS

Platelet-poor plasma (PPP) from 50 patients with MS (30 in exacerbation and 20 in remission) and 48 controls were labeled with fluorescein isothiocyanate (FITC)-conjugated anti-CD31 and anti-CD51 (vitronectin receptor) antibodies, and two classes of EMP (CD31+ and CD51+) were assayed by flow cytometry. For in vitro studies, patients' plasma was added to the microvascular endothelial cell (MVEC) culture and release of CD31+ and CD51+ EMP were measured in the supernatant.

RESULTS

Plasma from patients in exacerbation had 2.85-fold elevation of CD31+ EMP as compared with healthy controls, returning to near control value during remission. The CD31+ EMP concentration showed a positive association with gadolinium enhancement in patients with MS. In contrast, CD51+ EMP remained elevated in both exacerbation and remission. This suggests that CD31+ EMP is a marker of acute injury, whereas CD51+ EMP reflects chronic injury of endothelium. MS plasma induced release of both CD31+ and CD51+ EMP from MVEC culture in vitro.

CONCLUSION

Endothelial dysfunction is evident during exacerbation of MS, evidenced by shedding of EMP expressing PECAM-1 (CD31). The in vitro data indicate contribution of one or more plasma factors in endothelial dysfunction of MS.

Elevated endothelial microparticles in thrombotic thrombocytopenic purpura: findings from brain and renal microvascular cell culture and patients with active disease

Endothelial injury is believed to be a key initiating event in the pathogenesis of thrombotic thrombocytopenic purpura (TTP), leading to platelet activation and formation of platelet-rich thrombi in microvasculature. However, the nature of endothelial injury in TTP is poorly defined and clinical assays to rapidly and reliably monitor endothelial damage are not readily available. Using flow cytometry, we measured endothelial microparticles (EMPs) generated from cultured renal and brain microvascular endothelial cells (MVECs) during activation and apoptosis, and evaluated the effect of TTP plasma on them. EMPs were measured using positivity for monoclonal antibodies (mAbs) CD31 and CD51, and their procoagulant activity was assessed using a Russell viper venom assay. Both cell lines generated procoagulant EMPs when cultured with inducers of activation (tumour necrosis factor alpha; TNF-alpha) or apoptosis (mitomycin C). TTP plasma induced a five- to sixfold increase of EMP generation and a two- to threefold increase of procoagulant activity in cultured brain and renal MVECs. TTP plasma induced a threefold and 13-fold increase of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) expression, respectively, on renal MVECs. Procoagulant activity tended to parallel EMP numbers. The effect of TTP plasma on cell viability was similar to that of TNF-alpha, implying that it induced activation rather than apoptosis. Control plasma and idiopathic thrombocytopenic purpura (ITP) plasma had little effect. In the clinical study, EMP assay of blood from acute TTP patients showed levels markedly elevated compared with normal controls, but values returned to normal in remission. In conclusion, TTP plasma activated and induced injury to MVECs in culture, judged by production of EMP and expression of activation markers. Released procoagulant EMP may play a role in the pathogenesis of TTP. Assay of EMP may be a useful marker of disease activity and endothelial injury in TTP and possibly other thrombotic disorders.

A flow cytometric assay of platelet activation marker P-selectin (CD62P) distinguishes heparin-induced thrombocytopenia (HIT) from HIT with thrombosis (HITT)

Heparin induced thrombocytopenia (HIT) is a well-known complication of heparin administration but usually resolves upon discontinuation without sequelae. However, a small proportion of HIT patients develop thrombosis associated with HIT, designated as HITT, which is often life-threatening and may lead to gangrene and amputations. Existing laboratory methods of confirming HIT/HITT do not distinguish between HIT and HITT. We report a flow cytometric assay of platelet activation marker CD62P to distinguish the effects of addition of HIT vs. HITT plasma to normal blood. Briefly, normal whole blood was incubated with platelet-poor plasma from 12 patients with HITT, 30 with HIT, and 65 controls, in presence and absence of heparin, and expression of CD62P was assayed by flow cytometry. When the ratios of fluorescent intensity of CD62P with heparin divided by that without heparin were compared, HITT plasma induced significantly higher ratios than HIT plasma (HITT ratios approximately 2.5 vs. HIT ratios approximately 1.2; p <0.001). Eleven of 12 HITT patients were positive by this test but only 5 of 30 HIT patients were positive (p <0.0005). In a case of HIT with silent thrombosis, this assay gave a positive results prior to clinically evident thrombosis. In conclusion, this method distinguishes HITT from HIT and may be clinically useful in the detection of HITT, allowing early intervention for preventing catastrophic thrombosis.

Anti-CD36 autoantibodies in thrombotic thrombocytopenic purpura and other thrombotic disorders: identification of an 85 kD form of CD36 as a target antigen

The presence of anti-CD36 antibodies in plasma of patients with thrombotic thrombocytopenic purpura (TTP), idiopathic thrombocytopenic purpura (ITP), and heparin-induced thrombocytopenia without/with thrombosis (HIT/HITT) has been examined by immunoblots, and a monoclonal antibody capture assay, the platelet-associated IgG characterization assay (PAICA). Results with PAICA showed that 73% (8/11) of patients with TTP were positive, and 71% (10/14) by immunoblots. With ITP, 20% (6/30) were positive by PAICA and 19% (3/16) by immunoblots; HIT, 30% (3/10) were positive by PAICA and 60% (6/10) by immunoblot; HITT, 50% (2/4) by PAICA and 100% (4/4) by immunoblot. Purification of CD36 by fast protein liquid chromatography (FPLC) from Triton X-100 extracts of normal platelet membranes resulted in the isolation of two different forms: the classic 88 kD form, and a second, lighter 85 kD form. Our data indicated that the patients' plasma autoantibodies reacted strongly with the 85 kD form. Conventional monoclonal and polyclonal antisera produced to the 88 kD form reacted strongly with the 88 kD form but weakly with the 85 kD form. These results confirm the possible importance of anti-CD36 antibodies in the pathophysiology of TTP and other thrombocytopenias and demonstrate the presence of a previously unrecognized target antigen for these antibodies.

Platelet activation in Alzheimer disease

BACKGROUND

In light of recent reports of diminished platelet serotonin concentration and increased plasma serotonin levels in patients with Alzheimer disease (AD), we hypothesized that a state of heightened platelet activation might be present in AD.

OBJECTIVE

To compare baseline activation of unstimulated platelets in patients with AD with that in control subjects.

PATIENTS AND METHODS

Flow cytometry was used to measure platelet activation in 91 patients with probable AD and 40 age-matched control subjects. Groups were compared for percentage of circulating platelet aggregates, expression of CD62p, formation of leukocyte-platelet complexes, and presence of circulating platelet microparticles, controlling for effects of demographic, clinical, physiological, and logistical factors.

RESULTS

Multiple analysis of covariance on ranked data revealed a 39.5% increase in percentage of platelet aggregates (P=.0001), a 59.3% increase in expression of CD62p (P=.001), and a 53.3% increase in leukocyte-platelet complexes (P=.0001) in the group with AD but no differences in the number of platelet microparticles, overall platelet count, plasma fibrinogen level, or plasma platelet factor 3. Activation was weakly correlated with sex, but was independent of age, severity of disease, duration of disease, depression, agitation, and family history of dementia.

CONCLUSIONS

Platelets of patients with AD exhibit greater unstimulated activation than those of controls. Potential causes of such activation include possible stimulation of platelets by damaged cerebral endothelial cells or platelet activation induced by membrane abnormalities previously reported to be present in platelets of patients with AD. In light of recent evidence that platelets are the principal source of both amyloid precursor protein and beta-amyloid peptide in human blood, it is possible that AD platelet activation may reflect or even contribute to the pathogenesis of the disease.

Thrombotic thrombocytopenic purpura plasma enhances platelet-leucocyte interaction in vitro

In thrombotic thrombocytopenic purpura (TTP), intravascular platelet aggregation and formation of platelet-rich thrombi impair the microcirculation. TTP plasma has been shown to induce aggregation of normal platelets in vitro. The present study investigates the formation of activated platelet aggregates (aPAg) induced by TTP plasma, with particular attention to their binding to leucocytes (LPAg). Results were compared with the effects of plasmas from normal controls (CTL) and from patients with immune thrombocytopenic purpura (ITP) or thrombosis (THR). Following addition of test plasma to normal whole blood (WB), aPAg and LPAg were assayed by flow cytometry using mAbs against CD41 (platelet marker), CD62p (platelet activation marker) and CD45 (pan-leucocyte marker), Compared to control plasma, TTP plasma was more potent than ITP or THR plasma in increasing aPAg: only TTP plasma significantly promoted leucocyte binding to give increased LPAg. Prior removal of neutrophils (PMN) from WB by beads coated with anti-CD15 mAb largely prevented formation of aPAg and LPAg. However, TTP plasma added to normal platelet-rich plasma significantly increased aPAg, which suggested possible hindrance of aPAg formation by erythrocytes and other leucocytes in PMN-depleted blood. We concluded that TTP plasma was most potent in the induction of aPAg and unique in promoting LPAg formation in WB. Neutrophils, and not other leucocytes, appear to be essential for LPAg formation. Enhanced PMN-platelet interaction in the microcirculation may facilitate platelet adhesion to vessel walls and promote the formation of platelet-rich microthrombi in TTP.

Platelet aggregates as markers of platelet activation: characterization of flow cytometric method suitable for clinical applications

The present paper describes a flow cytometric method for assay of platelet aggregates (PAg) in blood. This method combines and simplifies previously reported techniques, simultaneously enumerating PAg formed upon platelet activation, their expression of activation marker CD62P (P-selectin), and their content of bound leukocytes (LPAg). The sensitivity of this method to low levels of agonists (ADP, collagen) is compared to conventional aggregometry and some features of platelet-leukocyte interaction are explored. The results were: (1) ADP or collagen induced a dose-dependent increase in PAg number and corresponding decline in free platelets. The ED50 for ADP (0.15 microM) and for collagen (0.2 microg/mL) was about 1/20 the ED50 found by aggregometry, indicating 20-fold greater sensitivity. (2) At higher concentrations, the fraction of PAg with bound leukocytes (LPAg) increased to 60-70%. This rise correlated with PAg size and CD62P expression, but not with the number of PAg formed. (3) The response of whole blood (WBD) to agonists was qualitatively different from that of platelet-rich plasma (PRP): in WBD the population of CD62P+ PAg was much higher than in PRP and the population of CD62P+ free platelets was much lower. This implies that leukocytes rapidly recruit activated platelets. (4) Desmopressin (DDAVP) at 5 nmol/L induced a significant rise in activated (CD62P+) PAg and platelets, even though no effect of DDAVP could be detected by conventional aggregometry; this further confirms that DDAVP acts directly on platelets. (5) Plasma samples from TTP patients induced a rise in PAg when added to normal PRP, though little or no effect could be detected by aggregometry. In summary, the flow cytometric method described here appears useful for detecting low levels of platelet activation and provides information on platelet leukocyte interaction, potentially important in identifying and differentiating thrombogenic states. Since it is rapid and economical, it is well suited for clinical implementation.