Flow cytometric assessment of platelet function in patients with peripheral arterial occlusive disease

Platelet-Derived MRP-14 Induces Monocyte Activation in Patients With Symptomatic Peripheral Artery Disease

BACKGROUND

Peripheral artery disease (PAD), a diffuse manifestation of atherothrombosis, is a major cardiovascular threat. Although platelets are primary mediators of atherothrombosis, their role in the pathogenesis of PAD remains unclear.

OBJECTIVES

The authors sought to investigate the role of platelets in a cohort of symptomatic PAD.

METHODS

The authors profiled platelet activity, mRNA, and effector roles in patients with symptomatic PAD and in healthy controls. Patients with PAD and carotid artery stenosis were recruited into ongoing studies (NCT02106429 and NCT01897103) investigating platelet activity, platelet RNA, and cardiovascular disease.

RESULTS

Platelet RNA sequence profiling mapped a robust up-regulation of myeloid-related protein (MRP)-14 mRNA, a potent calcium binding protein heterodimer, in PAD. Circulating activated platelets were enriched with MRP-14 protein, which augmented the expression of the adhesion mediator, P-selectin, thereby promoting monocyte-platelet aggregates. Electron microscopy confirmed the firm interaction of platelets with monocytes in vitro and colocalization of macrophages with MRP-14 confirmed their cross talk in atherosclerotic manifestations of PAD in vivo. Platelet-derived MRP-14 was channeled to monocytes, thereby fueling their expression of key PAD lesional hallmarks and increasing their directed locomotion, which were both suppressed in the presence of antibody-mediated blockade. Circulating MRP-14 was heightened in the setting of PAD, significantly correlated with PAD severity, and was associated with incident limb events.

CONCLUSIONS

The authors identified a heightened platelet activity profile and unraveled a novel immunomodulatory effector role of platelet-derived MRP-14 in reprograming monocyte activation in symptomatic PAD. (Platelet Activity in Vascular Surgery and Cardiovascular Events [PACE]; NCT02106429; and Platelet Activity in Vascular Surgery for Thrombosis and Bleeding [PIVOTAL]; NCT01897103).

Baseline Platelet Activation and Reactivity in Patients with Critical Limb Ischemia

Background

Patients with critical limb ischemia (CLI) have a high risk to develop cardiovascular events (CVE). We hypothesized that in CLI patients platelets would display increased baseline activation and reactivity.

Objectives

We investigated baseline platelet activation and platelet reactivity in patients with CLI.

Patients/Methods

In this study baseline platelet activation and platelet reactivity in response to stimulation of all major platelet activation pathways were determined in 20 CLI patients (11 using aspirin and 9 using vitamin K-antagonists) included in the Juventas-trial (clinicaltrials.gov NCT00371371) and in 17 healthy controls. Platelet activation was quantified with flow cytometric measurement of platelet P-selectin expression and fibrinogen binding.

Results

CLI patients not using aspirin showed higher baseline platelet activation compared to healthy controls. Maximal reactivity to stimulation of the collagen and thrombin activation pathway was decreased in CLI patients compared to healthy controls. In line, attenuated platelet reactivity to stimulation of multiple activation pathways was associated with several traditional risk factors for cardiovascular disease.

Conclusions

Baseline platelet activation was increased in CLI patients, whereas the reactivity of circulating platelets to several stimulatory agents is decreased. Reactivity of platelets was inversely correlated with cardiovascular risk factors.

Normal platelet activation profile in patients with peripheral arterial disease on aspirin

BACKGROUND

Peripheral arterial disease (PAD) is a progressive vascular disease associated with a high risk of cardiovascular morbidity and death. Antithrombotic prevention is usually applied by prescribing the antiplatelet agent aspirin. However, in patients with PAD aspirin fails to provide protection against myocardial infarction and death, only reducing the risk of ischemic stroke. Platelets may play a role in disease development, but this has not been tested by proper mechanistic studies. In the present study, we performed a systematic evaluation of platelet reactivity in whole blood from patients with PAD using two high-throughput assays, i.e. multi-agonist testing of platelet activation by flow cytometry and multi-parameter testing of thrombus formation on spotted microarrays.

METHODS

Blood was obtained from 40 patients (38 on aspirin) with PAD in majority class IIa/IIb and from 40 age-matched control subjects. Whole-blood flow cytometry and multiparameter thrombus formation under high-shear flow conditions were determined using recently developed and validated assays.

RESULTS

Flow cytometry of whole blood samples from aspirin-treated patients demonstrated unchanged high platelet responsiveness towards ADP, slightly elevated responsiveness after glycoprotein VI stimulation, and decreased responsiveness after PAR1 thrombin receptor stimulation, compared to the control subjects. Most parameters of thrombus formation under flow were similarly high for the patient and control groups. However, in vitro aspirin treatment caused a marked reduction in thrombus formation, especially on collagen surfaces. When compared per subject, markers of ADP- and collagen-induced integrin activation (flow cytometry) strongly correlated with parameters of collagen-dependent thrombus formation under flow, indicative of a common, subject-dependent regulation of both processes.

CONCLUSION

Despite of the use of aspirin, most platelet activation properties were in the normal range in whole-blood from class II PAD patients. These data underline the need for more effective antithrombotic pharmacoprotection in PAD.

Platelet activation increases with the severity of peripheral arterial disease: Implications for clinical management

INTRODUCTION

Patients with peripheral arterial disease (PAD) have increased mortality from cardiovascular events compared with age and sex matched controls Platelets play a major role in atherosclerosis and thrombotic vascular events. Platelet reactivity is increased in patients with PAD compared with healthy controls. We aimed to determine the relationship, if any, between platelet activation and severity of disease.

METHODS AND RESULTS

One hundred eighty-two patients with intermittent claudication (IC) or subcritical limb ischemia (SLI), defined as the presence of rest pain or ulceration, had the following investigations performed: platelet P-selectin expression and bound fibrinogen by flow cytometric analysis and platelet aggregation using the rapid platelet function assay with arachidonic acid (AA) and thrombin receptor activation peptide (TRAP) as agonists. Patients with SLI compared with IC had significantly enhanced ADP stimulated P-selectin expression (median 42.45% [inter-quartile range 33.32% to 58.5%] vs 35.2% [26.07% to 46.32%], P = .002) and bound fibrinogen (73.7% [54.3% to 83.2%] vs 63.7% [43.8% to 76.5%], P = .001). TRAP stimulated aggregation was higher (207 [153 to 238] PAU vs 183[155 to 199] PAU, P = .04) but AA mediated aggregation was not significantly different. An ankle-brachial pressure index (ABPI) of less than 0.6 was associated with increased ADP stimulated P-selectin and bound fibrinogen (P < .05). ABPI correlated inversely with ADP stimulated P-selectin expression (r = -0.228, P = .003), ADP stimulated fibrinogen binding (r = -0.156, P = .043) and TRAP stimulated aggregation (r = -0.179, P = .04).

CONCLUSION

We have demonstrated for the first time that progression of severity of PAD is not only reflected by symptoms, signs, and ABPI but also by increased platelet activity as assessed by both flow cytometry and aggregation. As patients with more severe PAD have increased cardiovascular mortality, our findings suggest that new strategies for platelet inhibitory therapy are indicated in these patients.

Relation of augmented platelet reactivity to the magnitude of distribution of atherosclerosis

The extent of luminal involvement of atherosclerotic vascular disease and platelet reactivity portend subsequent cardiovascular events. This study was designed to determine whether platelet reactivity correlates with the extent of the territorial distribution of vascular disease. Blood was obtained from 130 patients who had known atherosclerotic vascular disease categorized as being in > or =1 of the following territories: coronary artery disease (CAD; n = 89), cerebrovascular disease (n = 36), and peripheral arterial disease (n = 61). Platelet reactivity, i.e., the activation of platelets in response to a low concentration of adenosine diphosphate (0.2 micromol/L), was measured using flow cytometry. Patients with vascular disease in >1 territory compared with those with disease in only 1 territory had greater platelet reactivity with respect to P-selectin expression (p = 0.01). The percentages of platelets expressing P-selectin (mean +/- SD) were 6.4 +/- 4.2 in patients who had involvement of 1 territory (n = 88), 10.0 +/- 6.8 in those who had involvement of 2 territories (n = 28), and 10.1 +/- 9.9 in those who had involvement of 3 territories (n = 14). Patients who had CAD and diabetes mellitus had greater P-selectin expression than did those who had CAD without diabetes (p <0.02 for interaction). Thus, platelet reactivity is greater in patients who have more extensive territorial distribution of atherosclerotic vascular disease and in those who have CAD and diabetes mellitus. Accordingly, patients who have more widely distributed vascular disease are likely to derive particular benefit from antiplatelet regimens that suppress platelet function to a greater extent.

AT-1015, a novel serotonin2A receptor antagonist, improves resaturation of exercised ischemic muscle in hypercholesterolemic rabbits

OBJECTIVE

The effect of AT-1015, a serotonin(2A) receptor antagonist, on the resaturation of ischemic muscle in a hypercholesterolemic rabbit model was examined with near-infrared spectroscopy.

METHODS

New Zealand White male rabbits were fed normal chow or cholesterol-rich chow. Ischemia was induced in the right hindlimb by ligation of the femoral artery, accompanied by balloon injury of the iliac artery. At 3 days after induction of ischemia, the bilateral gastrocnemius muscles were subjected to passive contraction for 2 minutes. The oxygen resaturation time of the gastrocnemius muscle after exercise was measured by near-infrared spectroscopy. AT-1015 was orally administered for 3 days after induction of ischemia. Assay of serotonin level in platelet-poor plasma and histologic examination of muscle and artery were performed in another set of rabbits.

RESULTS

Oxygen resaturation time of the ischemic gastrocnemius was significantly prolonged in hypercholesterolemic rabbits compared with in normal rabbits without AT-1015, whereas there was no difference between both groups of rabbits that were administered AT-1015. Plasma level of serotonin in hypercholesterolemic rabbits was significantly increased compared with that in normal rabbits. No histologic differences were found in both muscle and artery among all groups.

CONCLUSIONS

A serotonin(2A) receptor antagonist improved the oxygen resaturation of ischemic calf muscle after exercise in hypercholesterolemia. The interaction between plasma free serotonin and the serotonin(2A) receptor may play an important role in muscle oxygenation in ischemic limbs.

The impact of peripheral arterial disease on circulating platelets

INTRODUCTION

To test the hypothesis that circulating platelets display evidence of reversible interactions with atherosclerotic lesions, platelet alpha-granule content and propensity for microaggregate formation were measured in samples from normal donors (n=65) and from patients with either peripheral arterial disease (n=47) or renovascular hypertension (n=22). To measure the effect of a defined arterial injury on platelet function, platelet samples were compared before and 30 min after elective angioplasty.

MATERIALS AND METHODS

P-selectin was measured after strong stimulation of ultra-dilute platelets with thrombin (10 nM). Microaggregation was measured as a platelet count deficit in citrate-anticoagulated platelet-rich plasma (PRP) relative to that predicted from the count in EDTA-anticoagulated blood.

RESULTS

Platelet alpha-granule P-selectin was significantly lower from platelets of patients compared to normal donors. In addition, platelets from patients have a significantly greater propensity to form microaggregates in citrate anticoagulant. In contrast to atherosclerotic renovascular hypertension, platelets from patients with fibromuscular dysplasia, a distinct non-inflammatory cause of arterial stenosis, do not differ significantly from normal donors. Other than the PRP platelet count, which rose transiently following angioplasty, other platelet measures were unchanged by the injury.

CONCLUSIONS

Atherosclerotic arterial disease is associated with an increased share of platelets unable to express P-selectin and an increased fraction of platelets that microaggregate in citrate anticoagulant. These platelet alterations are not completely explained by either focal arterial injury or abnormal rheology associated with arterial stenosis but appear to be an effect of the atherosclerotic process.

Fibrinogen receptor antagonists induce conformational changes of the human platelet glycoprotein IIb

BACKGROUND

Controversial results have been reported concerning the ability of fibrinogen receptor antagonists (fibans) to induce conformational changes in the fibrinogen receptor after binding to it as the initial step of fibrinogen binding and platelet activation.

METHODS

Platelets in citrated whole blood were stained with several pairs of anti-glycoprotein (anti-GP) IIb-directed monoclonal antibodies conjugated to phycoerythrin (PE) or indirectly labeled with Cy5. Pairs of monoclonal antibodies that induced a high-fluorescence resonance energy transfer (FRET) efficiency served as tools to detect activation-dependent changes of GP IIb after addition of adenosine diphosphate and several fibans.

RESULTS

Using the combination of the clones 5B12-PE and P2-biotin/SA-Cy5, a concentration-dependent alteration of the GP IIb conformation was observed after addition of tirofiban, eptifibatide, and lotrafiban. Magnitude and kinetics differed among the investigated substances.

CONCLUSION

The newly developed FRET assay allows the direct investigation of conformational changes of GP IIb after addition of platelet agonists or receptor ligands, as shown for three fibans.

Platelet activation is increased in peripheral arterial disease

OBJECTIVE

Platelet activation was assessed in patients with peripheral arterial disease compared with healthy control subjects.

METHODS

This prospective comparative study included 100 subjects: 40 consecutive patients with intermittent claudication, 20 consecutive patients with critical ischemia and tissue loss, and 40 healthy control subjects. Whole blood flow cytometric analysis was performed to determine resting and stimulated platelet P-selectin expression and resting and stimulated platelet fibrinogen binding. Results are presented as platelet percentage and also as mean fluorescence intensity.

RESULTS

P-selectin expression was significantly increased in patients with intermittent claudication (median, 0.85%; range, 0.31%-4.77%; P =.023) and critical ischemia (median, 1.11%; range, 0.2%-3.26%; P =.028) compared with control subjects (median, 0.59%; range, 0.16%-4.58%). The percentage of platelets binding fibrinogen was also significantly higher in patients with intermittent claudication (median, 2.89%; range, 1.08%-9.59%; P <.001) compared with control subjects (median, 1.57%; range, 0.17%-10.7%). There was no significant difference in percentage of platelet fibrinogen binding between control subjects and patients with critical ischemia. Fibrinogen binding by stimulated platelets was significantly diminished in patients with critical limb ischemia compared with control subjects (67.2% vs 77.9%; P =.006).

CONCLUSIONS

Platelet activation is increased in patients with peripheral arterial disease, suggesting an underlying prothrombotic state. Platelets from patients with critical limb ischemia are less responsive to in vitro stimulation.

Influence of platelet count and activity on thromboelastography parameters

It has been suggested that thromboelastography TEG) can help in limiting or directing the appropriate use of blood products during surgery. However, the contribution of platelets to the TEG profile has not been studied in detail. Blood was taken from eight healthy subjects and eight patients with peripheral arterial disease (PAD). Immunomagnetic separation was achieved by the addition of Dynabeads labeled with a CD41 murine antibody (to the GPIIb/IIIa receptor) to achieve 90-100% depletion of platelets from blood. This was then titrated with whole blood to achieve platelet counts of approximately 0, 25 and 50% of the original count to compare with whole blood using TEG. Platelet function was also assessed by spontaneous platelet aggregometry (SPA) at baseline and at the 50% dilution. SPA, maximum amplitude (MA) and K time were significantly different in PAD patients compared to controls (P < 0.05). In both controls and PAD patients there was a strong linear correlation between Log10 [platelet count] and MA (r = 0.97 for controls, r = 0.89 for PAD) and K time (r = -0.86 for controls, r = -0.68 for PAD). Correlation between Log10 [platelet count] and R time was poor in both groups. The MA and K TEG parameters may be most useful for assessing platelet transfusion requirements.

Value of thromboelastography in the assessment of platelet function

Thromboelastography (TEG) is a useful measure of coagulation. Modified TEG (that is with the addition of a GP IIb/IIIa receptor antagonist) has been used to assess the contribution of the fibrinogen-platelet interaction to TEG parameters (in particular the maximum amplitude, MA). Modified TEG was compared with other investigations of platelet function to assess its sensitivity in both normal subjects and in patients with peripheral arterial disease (PAD), a condition associated with activated platelets. Blood was collected from eight healthy subjects and 12 PAD patients. Platelet function was measured by TEG, flow cytometry (using PAC-1, P-selectin, GP IIIa and GP Ib murine antibodies) and platelet aggregometry (spontaneous and ADP-induced) in the presence and absence of tirofiban (a GP IIb/IIIa receptor antagonist). TEG showed a statistically significant reduction in MA with tirofiban at 0.4 mg/L and an increase in k (kinetic time; which indicates how fast clot strength is increasing once clotting starts) at 0.2 and 0.4 mg/L of tirofiban in both healthy subjects and PAD patients. Flow cytometry showed a significant decrease in the PAC-1 binding index (at 0.2 mg/L). This finding was compatible with the significant reductions found in spontaneous and ADP-induced platelet aggregation. However, aggregometry and flow cytometry were more sensitive indicators of platelet inhibition than the TEG parameters. TEG does not provide a comprehensive or sensitive reflection of impaired platelet function. If TEG is used as an index of severely impaired platelet function, we recommend that the k parameter should be used as well as MA. TEG should be supplemented by other methods of platelet function assessment wherever possible.

Increased platelet aggregation and activation in peripheral arterial disease

OBJECTIVES

patients with peripheral arterial disease (PAD) have a threefold increase in cardiovascular mortality. Standard antiplatelet treatment may not confer uniform benefit in different patient groups. This study aimed to compare platelet function in patients with lower limb PAD, carotid disease and abdominal aortic aneurysm (AAA) with age- and sex-matched healthy controls.

METHODS

patients with lower limb PAD (n = 20), carotid disease (n = 40), AAA (n = 13) and age/sex matched healthy controls (n= 20) were studied. Whole blood methods to detect spontaneous platelet aggregation (SPA), and adenosine diphosphate (ADP) and collagen-induced aggregation were used. The detection of platelet P-selectin and the PAC-1 antigen by flow cytometry were also used as markers of platelet activation and aggregation.

RESULTS

patients with lower limb PAD or AAA had higher baseline SPA compared to normal controls (p < 0.01). There was significantly higher collagen-induced aggregation in IC patients compared to normal controls (p < 0.01). However, there was no difference in ADP-induced aggregation between lower limb PAD and control patients. There was no difference in PAC-1 binding between control patients and the patients with lower limb PAD, carotid disease or AAA. Patients with carotid disease had a higher expression of P-selectin compared to normal controls (p < 0.05).

CONCLUSIONS

this study provides further evidence that platelet hyperactivity is present in patients with PAD despite the use of antiplatelet therapy. Further antiplatelet strategies may be indicated to protect these patients.

A prospective randomized trial of aspirin-clopidogrel combination therapy and dose-adjusted warfarin on indices of thrombogenesis and platelet activation in atrial fibrillation

OBJECTIVES

This study was designed to investigate whether or not combination aspirin-clopidogrel therapy would reduce markers of thrombogenesis and platelet activation in atrial fibrillation (AF), in a manner similar to warfarin.

BACKGROUND

Dose-adjusted warfarin is beneficial as thromboprophylaxis in AF, but potentially serious side effects and regular monitoring leave room for alternative therapies. METHODS; We randomized 70 patients with nonvalvular AF who were not on any antithrombotic therapy to either dose-adjusted warfarin (international normalized ratio 2 to 3) (Group I) or combination therapy with aspirin 75 mg and clopidogrel 75 mg (Group II). Plasma indices of thrombogenesis (fibrin D-dimer, prothrombin fragment 1+2) and platelet activation (beta-thromboglobulin [TG] and soluble P-selectin) were quantified, along with platelet aggregation responses to standard agonists, at baseline (pretreatment) and at six weeks posttreatment. RESULTS; Pretreatment levels of fibrin D-dimer (p = 0.001), beta-TG (p = 0.01) and soluble P-selectin (p = 0.03) were raised in patients with AF, whereas plasma prothrombin fragment 1+2 levels and platelet aggregation were not significantly different compared with controls. Dose-adjusted warfarin reduced plasma levels of fibrin D-dimer, prothrombin fragment 1+2 and beta-thromboglobulin levels at six weeks (all p < 0.001), enhanced plasma levels of soluble P-selectin (p < 0.001) and had no significant effect on platelet aggregation. Aspirin-clopidogrel combination therapy made no difference to the plasma markers of thrombogenesis or platelet activation (all p = NS), but the platelet aggregation responses to adenosine diphosphate (p < 0.001) and epinephrine (p = 0.02) were decreased.

CONCLUSIONS

Aspirin-clopidogrel combination therapy failed to reduce plasma indices of thrombogenesis and platelet activation in AF, although some aspects of ex vivo platelet aggregation were altered. Anticoagulation with warfarin may be superior to combination aspirin-clopidogrel therapy as thromboprophylaxis in AF.

The effects of platelet apheresis in total hip replacement surgery on platelet activation

BACKGROUND

Autologous platelet rich plasma (PRP) harvest with autotransfusion devices has been used for 10 years in cardiac surgery and recently in orthopedics as a blood saving method. The quality of the harvested platelets has not been adequately examined, in part because of methodological difficulties in studying platelet function during surgery.

METHODS

Twenty patients undergoing primary total hip replacement (THR) were studied. Ten patients underwent an immediate preoperative platelet apheresis to obtain concentrated platelet rich plasma (c-PRP). The other 10 patients not undergoing apheresis were allocated to a control group. Platelet activation was evaluated as the population expressing P-selectin on the surface of platelets in the c-PRP and in blood samples collected pre-, per- and postoperatively. The method used was flow cytometry.

RESULTS AND CONCLUSIONS

A minor population of activated platelets was found to be circulating in the patients' blood, with a highly significant difference between patients (P = 0.005), and with a range of 1-23% in peroperative activation. PRP harvest did not significantly alter platelet activity. The platelet apheresis procedure did not inhibit platelet function in the c-PRP, as judged by a high proportion of platelets that could be activated in ADP stimulation experiments (mean value +/- SD 86% +/- 7.5%).

Coagulation, fibrinolysis and platelet P-selectin expression in peripheral vascular disease

OBJECTIVES

to examine coagulation, fibrinolysis, and platelet activity in patients with peripheral vascular disease (PVD).

DESIGN

fifty consecutive PVD patients and 50 healthy volunteers. (Prospective comparative study.)

MATERIALS AND METHODS

P-selectin expression in non-fixed, whole blood was measured flow cytometrically on non-stimulated and ADP- and TRAP-6-stimulated samples. Plasma fibrinogen, von Willebrand factor (vWF), tissue plasminogen activator (tPA), and plasminogen activator inhibitor-1 were determined using standard techniques. Disease severity was stratified on the basis of the ankle-brachial pressure index (ABPI) and the angiographic data were assessed using the Bollinger score.

RESULTS

coagulation and fibrinolysis parameters as well as the P-selectin expression on both stimulated and non-stimulated platelets were significantly increased in patients vs controls (all p<0.01). The respective sensitivity and specificity were as follows: P-selectin expression (81%, 94%), vWF (72%, 86%), fibrinogen (64%, 98%), PAI-1 (44%, 90%), tPA (15%, 100%). P-selectin expression on TRAP-6-stimulated MP correlated with disease severity (r=0.40, p<0.01).

CONCLUSIONS

these findings support the concept of ongoing thrombogenesis in the subclinical progression of PVD and demonstrate the high diagnostic sensitivity of flow cytometric analysis of platelet activation.

Macrovascular disease and systemic sclerosis

OBJECTIVE

To determine if macrovascular disease is more prevalent in systemic sclerosis (SSc) compared with unaffected subjects.

METHODS

54 patients with SSc (both limited and diffuse disease) and 43 unaffected control subjects of similar age and sex were recruited. All subjects underwent a basic screen for conventional atherosclerotic disease risk factors. All had non-invasive vascular assessments-that is, carotid duplex scanning and measurement of ankle brachial blood pressure index-to identify the presence of asymptomatic peripheral vascular disease.

RESULTS

33 of 52 (64%) patients had carotid artery disease compared with only 15 of 42 (35%) controls (p=0.007). Eleven (21%) of these patients had moderate disease compared with only two (5%) controls (NS). Nine of 53 (17%) SSc patients had evidence of peripheral arterial disease compared with no controls. This result was also statistically significant (p=0.003). There were no significant differences in the basic risk factor profile, which included cigarette smoking, systolic and diastolic blood pressure, cholesterol, trigyceride and glucose concentrations.

CONCLUSION

Macrovascular disease is more common in SSc. Screening of these patients may allow identification of "at risk" patients at an early stage and allow the study of treatments to attenuate the high rate of cardiovascular mortality in these patients.

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.

Altered platelet function detected by flow cytometry. Effects of coronary artery disease and age

Platelet activation state and responsiveness to physiological agonists were measured in 65 patients with documented coronary artery disease (54 male and 11 female; mean age, 58 years). Twelve patients (mean age, 52 years), selected at random from the male cohort, were compared with 12 age-matched male control subjects (mean age, 52 years) and with 10 normal, young male subjects (mean age, 25 years). Whole-blood flow cytometry was used to measure platelet activation status ex vivo and platelet responsiveness to physiological agonists in vitro. Peripheral blood samples were analyzed for bound fibrinogen and expression of P-selectin, GPIb, and GPIIb-IIIa at rest and in response to ADP (0.1 to 10 mumol/L) and thrombin (0.02 to 0.32 mu/mL). No significant differences were seen in the basal levels of fibrinogen binding between any of the groups, but P-selectin expression was significantly lower in patients compared with age-matched control subjects (P = .0005). When stimulated with agonists, patients' platelets had significantly decreased fibrinogen binding (P < .03) but no difference in P-selectin expression compared with the age-matched group. Both agonist-induced fibrinogen binding and P-selectin expression were, however, higher in the young subjects compared with either the older control group or the patients (P < .05). GPIb and GPIIb-IIIa expression were lowest in the patients with angina and highest in the young control subjects, with levels in the age-matched control subjects falling between these values. Data from the total patient cohort (n = 65) were identical to those in the smaller cohort (n = 12). In conclusion, atherosclerosis impairs platelet aggregatory responses (fibrinogen binding) over and above the decreased response seen with age. Platelet degranulation (P-selectin expression) is also impaired in patients with coronary artery disease, but only in comparison with younger subjects, not age-matched controls.

Platelet activation in embolic and preembolic status of patients with nonrheumatic atrial fibrillation

STUDY OBJECTIVE

Information on platelet activation possibly associated with a preembolic or embolic status in nonrheumatic atrial fibrillation (AF) with special regard to the role of platelet membrane activation markers (P-selectin and CD63).

STUDY POPULATION

The study included 60 patients with nonrheumatic AF; 28 of them had a history of an embolic event. The age-matched control group consisted of 28 healthy subjects (13 men and 15 women).

INTERVENTIONS

Patients underwent transesophageal echocardiography to detect eventual intracardiac thrombus or spontaneous echo contrast that would represent a preembolic status. Blood samples were taken from all persons to evaluate markers for platelet activation under these conditions.

RESULTS

Measurements of hematologic variables did not differ significantly between normal subjects and patients presenting with AF but no preembolic or embolic status. Elevated concentrations of fibrinogen were significantly related to the presence of left atrial spontaneous echo contrast. The amount of circulating platelets expressing P-selectin and CD63 was significantly higher in the patients positive for both spontaneous echo contrast and left atrial thrombus or embolic events. Furthermore, in these groups, significantly more leukocyte-platelet conjugates were present.

CONCLUSION

Platelet activation indicated by platelet membrane activation markers occurs in embolic and preembolic status of patients with nonrheumatic AF.

Platelet-associated factor XIII as a marker of platelet activation in patients with peripheral vascular disease

In the past several years, monoclonal antibodies have been developed that distinguish between resting and activated platelets in vitro. These antibodies recognize epitopes expressed on membrane proteins or soluble proteins, such as factor XIIIa and thrombospondin, that bind only to activated platelets. We have used fluorescence flow cytometry to determine whether three such antibodies can detect platelet activation in patients with severe peripheral vascular disease (PVD). Using two activation-specific monoclonal antibodies and a polyclonal antiserum to factor XIII a-chain, we have examined the platelets from PVD patients, age-matched control subjects who were free of detectable PVD, and unmatched control subjects. Cells analyzed as platelets were identified by their light-scatter profile and their reactivity with monoclonal anti-glycoprotein Ib. The platelets of patients with PVD showed no increase in binding of activation-dependent 1B3 (directed against a 180-kD membrane protein) compared with age-matched control subjects (p = 0.780). Similarly, there was no difference between PVD patients and control subjects in activation-dependent CD63 expression. Conversely, the binding of anti-factor XIII a-chain was significantly higher than in the control groups (p < 0.001). These data suggest that the detection of soluble factors that bind to activated but not resting platelets may be of use in the detection of pathological in vivo platelet activation.