Novel, semi-automated, 60-min-assay to determine von Willebrand factor cleaving activity of ADAMTS-13

Assay for ADAMTS-13 Activity with Flow Cytometric Readout

A disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS-13) is a metalloprotease that regulates the size of circulating von Willebrand factor (vWF) multimers. Severe lack of ADAMTS-13 activity [<10% of normal (0.1 IU/mL)] leads to thrombotic thrombocytopenic purpura (TTP), a specific type of thrombotic microangiopathy (TMA). Timely determination of plasma ADAMTS-13 activity is essential to discriminate TTP from other types of TMA with respect to adequate treatment. Identification of the minimal substrate motif for ADAMTS-13 within the A2 domain of vWF (vWF73) as well as the generation of monoclonal antibodies (mAbs) that specifically recognize the ADAMTS-13 cleavage site enabled the development of a variety of methods for determination of plasma ADAMTS-13 activity. In order to further extend the range of analytical platforms applicable for quantitative determination of plasma ADAMTS-13 activity, a specific, vWF/mAb-based assay with flow cytometric readout was developed and validated. Basic assay characteristics include a total assay time of 80 to 90 min, a near linear dynamic range from 0.005 (lower limit of quantification) to 0.2 IU/mL, and intra- and interassay coefficients of variation below 5 and 30% at input plasma ADAMTS-13 activities of 0.015 and ≤0.050 IU/mL, respectively. When compared to the results obtained with a commercially available quantitative ADAMTS-13 activity ELISA, analysis of 18 plasma samples obtained from patients with suspected TTP revealed full agreement of results with respect to the clinical 0.1 IU/mL TTP threshold. Based on these data, it is assumed that the described assay principle can be successfully transferred to virtually all laboratories that have a flow cytometer available.

Thromboelastometry in critically ill patients with disseminated intravascular coagulation

: Coagulopathy has a high incidence in critically ill patients and is often caused by disseminated intravascular coagulation (DIC). Although the clinical picture of DIC ranges from a prothrombotic state to severe consumption coagulopathy with an increased bleeding tendency, there are no clinical tests that reflect of in-vivo hemostatic profile. Rotational thromboelastometry (ROTEM) may be able to indicate whether a patient has a hypocoagulable or hypercoagulable profile and possibly be able to discriminate patients with and without DIC. The aim of this article was to study the diagnostic ability of thromboelastometry to detect DIC. A predefined subgroup analysis of a clinical trial in critically ill patients with a coagulopathy was done. ROTEM and markers of coagulation and levels of natural anticoagulants were measured in patients with and without DIC. Twenty-three patients were included, 13 fulfilled criteria for overt DIC. Patients with DIC had lower platelet count, lower levels of fibrinogen, factors II, VII and VIII compared with those without DIC. Antithrombin, protein C and S were also reduced in DIC patients. Receiver operator characteristic analyses showed that EXTEM CFT, alpha angle and MCF were capable of discriminating patients with and without DIC. Combination of ROTEM values with protein C or antithrombin further improved discriminatory ability. In patients with DIC, thromboelastometry profiles were more hypocoagulable compared with those without DIC. ROTEM correlates well with ISTH DIC score, diagnostic strength improves when ROTEM values are combined with antithrombin or protein C levels. Thereby, ROTEM may be a useful tool in diagnosing DIC in the critically ill.

Dynamics of von Willebrand factor reactivity in sickle cell disease during vaso-occlusive crisis and steady state

Essentials The role of von Willebrand Factor (VWF) in the pathophysiology of sickle cell disease is unclear. We assessed markers of VWF during admission for vaso-occlusive crisis (VOC) and steady state. VWF reactivity was higher during VOC and was associated with inflammation and neutrophil activation. Hyper-adhesive VWF may promote VOC in sickle cell disease.

SUMMARY

Background Endothelial activation plays a central role in the pathophysiology of vaso-occlusion in sickle cell disease (SCD), facilitating adhesive interactions with circulating blood cells. Upon activation, various adhesive molecules are expressed, including von Willebrand factor (VWF). Increased VWF levels have been observed in patients with SCD during steady state. However, the role of VWF in the pathogenesis of SCD vaso-occlusion is unclear. Objectives To longitudinally assess the quantity and reactivity of VWF and its regulating protease ADAMTS-13 during vaso-occlusive crisis (VOC). Methods In this observational study, we obtained sequential blood samples in adult SCD patients during VOC. Results VWF reactivity was significantly higher during VOC (active VWF, VWF glycoprotein Ib-binding activity, and high molecular weight multimers), whereas platelet count and levels of ADAMTS-13 antigen and ADAMTS-13 activity were concomitantly lower than during steady state. Levels of VWF antigen, VWF propeptide (VWF:pp) and ADAMTS-13 specific activity did not change during VOC. VWF reactivity correlated strongly with markers of inflammation and neutrophil activation, and was inversely correlated with the platelet count. In patients who developed acute chest syndrome, levels of VWF, VWF:pp and active, hyperadhesive VWF were significantly higher, whereas ADAMTS-13 activity was lower, than in patients without this complication. Conclusions We provide the first evidence that VOC in SCD is associated with increased reactivity of VWF, without a pronounced ADAMTS-13 deficiency. This hyper-reactivity may be explained by resistance of VWF to proteolysis, secondary to processes such as inflammation and oxidative stress. Hyperadhesive VWF, scavenging blood cells in the microcirculation, may thereby amplify and sustain VOC in SCD.

Hemostatic Changes Associated With Increased Mortality Rates in Hospitalized Patients With HIV-Associated Tuberculosis: A Prospective Cohort Study

Background

Mortality rates remain high for human immunodeficiency virus (HIV)-associated tuberculosis, and our knowledge of contributing mechanisms is limited. We aimed to determine whether hemostatic changes in HIV-tuberculosis were associated with mortality or decreased survival time and the contribution of mycobacteremia to these effects.

Methods

We conducted a prospective study in Khayelitsha, South Africa, in hospitalized HIV-infected patients with CD4 cell counts <350/µL and microbiologically proved tuberculosis. HIV-infected outpatients without tuberculosis served as controls. Plasma biomarkers reflecting activation of procoagulation and anticoagulation, fibrinolysis, endothelial cell activation, matricellular protein release, and tissue damage were measured at admission. Cox proportional hazard models were used to assess variables associated with 12-week mortality rates.

Results

Of 59 patients with HIV-tuberculosis, 16 (27%) died after a median of 12 days (interquartile range, 0-24 days); 29 (64%) of the 45 not receiving anticoagulants fulfilled criteria for disseminated intravascular coagulation. Decreased survival time was associated with higher concentrations of markers of fibrinolysis, endothelial activation, matricellular protein release, and tissue damage and with decreased concentrations for markers of anticoagulation. In patients who died, coagulation factors involved in the common pathway were depleted (factor II, V, X), which corresponded to increased plasma clotting times. Mycobacteremia modestly influenced hemostatic changes without affecting mortality.

Conclusions

Patients with severe HIV-tuberculosis display a hypercoagulable state and activation of the endothelium, which is associated with mortality.

Increased Von Willebrand factor, decreased ADAMTS13 and thrombocytopenia in melioidosis

BACKGROUND

Melioidosis, caused by bioterror treat agent Burkholderia pseudomallei, is an important cause of community-acquired Gram-negative sepsis in Southeast Asia and Northern Australia. New insights into the pathogenesis of melioidosis may help improve treatment and decrease mortality rates from this dreadful disease. We hypothesized that changes in Von Willebrand factor (VWF) function should occur in melioidosis, based on the presence of endothelial stimulation by endotoxin, pro-inflammatory cytokines and thrombin in melioidosis, and investigated whether this impacted on outcome.

METHODS/PRINCIPAL FINDINGS

We recruited 52 controls and 34 culture-confirmed melioidosis patients at Sappasithiprasong Hospital in Ubon Ratchathani, Thailand. All subjects were diabetic. Platelet counts in melioidosis patients were lower compared to controls (p = 0.0001) and correlated with mortality (p = 0.02). VWF antigen levels were higher in patients (geometric mean, 478 U/dl) compared to controls (166 U/dL, p<0.0001). The high levels of VWF in melioidosis appeared to be due to increased endothelial stimulation (VWF propeptide levels were elevated, p<0.0001) and reduced clearance (ADAMTS13 reduction, p<0.0001). However, VWF antigen levels did not correlate with platelet counts implying that thrombocytopenia in acute melioidosis has an alternative cause.

CONCLUSIONS/SIGNIFICANCE

Thrombocytopenia is a key feature of melioidosis and is correlated with mortality. Additionally, excess VWF and ADAMTS13 deficiency are features of acute melioidosis, but are not the primary drivers of thrombocytopenia in melioidosis. Further studies on the role of thrombocytopenia in B. pseudomallei infection are needed.

The role of ADAMTS13 in acute myocardial infarction: cause or consequence?

Aims

ADAMTS13, a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13, is a metalloprotease that cleaves von Willebrand factor (VWF). There is considerable evidence that VWF levels increase and ADAMTS13 levels decrease in ST-elevation myocardial infarction (STEMI) patients. It is unclear whether this contributes to no reflow, infarct size, and intramyocardial haemorrhage (IMH). We aimed to determine the role of ADAMTS13 in STEMI patients and to investigate the benefits of recombinant ADAMTS13 (rADAMTS13) in a porcine model of myocardial ischaemia-reperfusion.

Methods and results

In 49 consecutive percutaneous coronary intervention (PCI)-treated STEMI patients, blood samples were collected directly after through 7 days following PCI. Cardiac magnetic resonance was performed 4–6 days after PCI to determine infarct size and IMH. In 23 Yorkshire swine, the circumflex coronary artery was occluded for 75 min. rADAMTS13 or vehicle was administered intracoronary following reperfusion. Myocardial injury and infarct characteristics were assessed using cardiac enzymes, ECG, and histopathology. In patients with IMH, VWF activity and VWF antigen were significantly elevated directly after PCI and for all subsequent measurements, and ADAMTS13 activity significantly decreased at 4 and 7 days following PCI, in comparison with patients without IMH. VWF activity and ADAMTS13 activity were not related to infarct size. In rADAMTS13-treated animals, no differences in infarct size, IMH, or formation of microthrombi were witnessed compared with controls.

Conclusions

No correlation was found between VWF/ADAMTS13 and infarct size in patients. However, patients suffering from IMH had significantly higher VWF activity and lower ADAMTS13 activity. Intracoronary administration of rADAMTS13 did not decrease infarct size or IMH in a porcine model of myocardial ischaemia-reperfusion. These data dispute the imbalance in ADAMTS13 and VWF as the cause of no reflow.

Impact of HIV infection on the haemostatic response during sepsis and malaria

Patients positive for the human immunodeficiency virus (HIV) are more susceptible to sepsis and malaria, two conditions known to activate the coagulation system. As chronic HIV infection also influences haemostatic mechanisms, we determined the influence of HIV co-infection on coagulation, anticoagulation and the endothelium during sepsis or malaria. We performed a prospective observational study in 325 subjects with or without HIV infection (103 with sepsis, 127 with malaria and 95 asymptomatic controls) in an HIV endemic area in Central Africa. We measured plasma biomarkers indicative of activation of distinct haemostatic mechanisms. Sepsis and malaria had similar effects with elevated markers of coagulation, reduced anticoagulation markers and activation of endothelium. In particular, asymptomatic HIV infection reduced the plasma levels of the anticoagulant co-factor free protein S, and increased activation of the vascular endothelium, which were not normalized by combination antiretroviral therapy. HIV co-infection during sepsis and malaria caused more profound changes in free protein S and von Willebrand factor in sepsis and malaria, and ADAMTS13 in sepsis, while not influencing sepsis- or malaria-induced coagulation activation. These results show for the first time that HIV infection augments selective haemostatic changes during sepsis and malaria, which may contribute to the enhanced morbidity of these conditions in HIV patients.

Risk factor analysis of cerebral white matter hyperintensities in children with sickle cell disease

Sickle cell disease (SCD) is complicated by silent cerebral infarcts, visible as white matter hyperintensities (WMHs) on magnetic resonance imaging (MRI). Both local vaso-occlusion, elicited by endothelial dysfunction, and insufficiency of cerebral blood flow (CBF) have been proposed to be involved in the aetiology. We performed an explorative study to investigate the associations between WMHs and markers of endothelial dysfunction and CBF by quantifying WMH volume on 3.0 Tesla MRI. We included 40 children with HbSS or HbSβ(0) thalassaemia, with a mean age of 12.1 ± 2.6 years. Boys demonstrated an increased risk for WMHs (odds ratio 4.5, 95% confidence interval 1.2-17.4), unrelated to glucose-6-phosphate dehydrogenase deficiency. In patients with WMHs, lower fetal haemoglobin (HbF) was associated with a larger WMH volume (regression coefficient = -0.62, R2 = 0.5, P = 0.04). Lower ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13) levels were associated with lower CBF in the white matter (regression coefficient = 0.07, R2 = 0.15, P = 0.03), suggesting that endothelial dysfunction could potentially hamper CBF. The findings of our explorative study suggest that a high level of HbF may be protective for WMHs and that endothelial dysfunction may contribute to the development of WMHs by reducing CBF.

Effect of transfusion of fresh frozen plasma on parameters of endothelial condition and inflammatory status in non-bleeding critically ill patients: a prospective substudy of a randomized trial

Introduction

Much controversy exists on the effect of a fresh frozen plasma (FFP) transfusion on systemic inflammation and endothelial damage. Adverse effects of FFP have been well described, including acute lung injury. However, it is also suggested that a higher amount of FFP decreases mortality in trauma patients requiring a massive transfusion. Furthermore, FFP has an endothelial stabilizing effect in experimental models. We investigated the effect of fresh frozen plasma transfusion on systemic inflammation and endothelial condition.

Methods

A prospective predefined substudy of a randomized trial in coagulopathic non-bleeding critically ill patients receiving a prophylactic transfusion of FFP (12 ml/kg) prior to an invasive procedure. Levels of inflammatory cytokines and markers of endothelial condition were measured in paired samples of 33 patients before and after transfusion. The statistical tests used were paired t test or the Wilcoxon signed-rank test.

Results

At baseline, systemic cytokine levels were mildly elevated in critically ill patients. FFP transfusion resulted in a decrease of levels of TNF-α (from 11.3 to 2.3 pg/ml, P = 0.01). Other cytokines were not affected. FFP also resulted in a decrease in systemic syndecan-1 levels (from 675 to 565 pg/ml, P = 0.01) and a decrease in factor VIII levels (from 246 to 246%, P <0.01), suggestive of an improved endothelial condition. This was associated with an increase in ADAMTS13 levels (from 24 to 32%, P <0.01) and a concomitant decrease in von Willebrand factor (vWF) levels (from 474 to 423%, P <0.01).

Conclusions

A fixed dose of FFP transfusion in critically ill patients decreases syndecan-1 and factor VIII levels, suggesting a stabilized endothelial condition, possibly by increasing ADAMTS13, which is capable of cleaving vWF.

Trial registrations

Trialregister.nl NTR2262, registered 26 March 2010 and Clinicaltrials.gov NCT01143909, registered 14 June 2010.

ADAMTS-13 assays in thrombotic thrombocytopenic purpura

ADAMTS-13, the thirteenth member of the ADAMTS (A Disintegrin And Metalloprotease with Thrombo-Spondin 1 repeats) family, is the plasma metalloprotease responsible for regulating the multimeric structure of VWF. In congenital or acquired deficiency it is actively involved in the pathophysiology of thrombotic thrombocytopenic purpura (TTP), a rare but life threatening disease characterized by microangiopathic haemolytic anaemia and consumptive thrombocytopenia leading to disseminated microvascular thrombosis and variable signs and symptoms of organ ischemia and damage. In the last few years, a number of in house and commercial laboratory assays for ADAMTS-13 and its autoantibodies have been developed. The features and clinical utility of ADAMTS-13 assays are summarized in this review.

Effect of blood group on idiopathic thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a condition caused by deficiency of ADAMTS13 resulting in accumulation of ultra large Von Willebrand factor multimers (ULVWF), leading to micro thrombi in multiple organs. The varying susceptibilities of blood group antigens to ADAMTS13 have been demonstrated. A and B antigens are protective of VWF; and VWF purified from blood group O individuals has been shown to be cleaved faster by ADAMTS13 compared to VWF from blood group AB individuals. We proposed that there may be a difference in the incidence of blood groups in TTP patients compared with the general population. We felt this to be important for a life-threatening disease with poorly understood epidemiology. We report a retrospective analysis of 74 patients presenting from 1993 to 2008 with idiopathic TTP. We studied the incidence across various blood groups and also estimated the recurrence and mortality in each group. The incidence of various blood groups were as follows: O 36%, A 36%, B 25%, and AB 2%, compared with expected frequencies in the Detroit area: O 44%, A 33% B 20%, and AB 3%. There was a trend of lower than expected frequency of blood group O. There were 24 recurrences and 14 deaths, uniform across blood groups. We hypothesized that there may be an association between blood groups and the risk of TTP; however the differences in our study were not statistically significant. Recurrence and disease specific mortality did not appear to be impacted by blood group.

Second international collaborative study evaluating performance characteristics of methods measuring the von Willebrand factor cleaving protease (ADAMTS-13)

BACKGROUND

Over the last 4 years ADAMTS-13 measurement underwent dramatic progress with newer and simpler methods.

AIMS

Blind evaluation of newer methods for their performance characteristics.

DESIGN

The literature was searched for new methods and the authors invited to join the evaluation. Participants were provided with a set of 60 coded frozen plasmas that were prepared centrally by dilutions of one ADAMTS-13-deficient plasma (arbitrarily set at 0%) into one normal-pooled plasma (set at 100%). There were six different test plasmas ranging from 100% to 0%. Each plasma was tested 'blind' 10 times by each method and results expressed as percentage vs. the local and the common standard provided by the organizer.

RESULTS

There were eight functional and three antigen assays. Linearity of observed-vs.-expected ADAMTS-13 levels assessed as r2 ranged from 0.931 to 0.998. Between-run reproducibility expressed as the (mean) CV for repeated measurements was below 10% for three methods, 10-15% for five methods and up to 20% for the remaining three. F-values (analysis of variance) calculated to assess the capacity to distinguish between ADAMTS-13 levels (the higher the F-value, the better the capacity) ranged from 3965 to 137. Between-method variability (CV) amounted to 24.8% when calculated vs. the local and to 20.5% when calculated vs. the common standard. Comparative analysis showed that functional assays employing modified von Willebrand factor peptides as substrate for ADAMTS-13 offer the best performance characteristics.

CONCLUSIONS

New assays for ADAMTS-13 have the potential to make the investigation/management of patients with thrombotic microangiopathies much easier than in the past.

Rapid quantitative assay of ADAMTS13 activity on an automated coagulation analyzer: clinical applications and comparison with immunoblot method

Facile assays for ADAMTS13, the metalloprotease that is absent or impaired in thrombotic thrombocytopenic purpura, could allow for timely diagnosis and management of this potentially fatal hematologic disorder; unfortunately, available assays employ methodologies that restrict use to only a few reference laboratories, resulting in reporting delays. Measurement of ADAMTS13 on an automated analyzer could allow widespread accessibility by routine clinical laboratories. We adapted a previously published technique to demonstrate that ADAMTS13 activity could be measured on an Instrumentation Laboratory automated coagulation analyzer after plasma digestion of a commercial source of von Willebrand factor (VWF). Results were obtained using a commercially available immunoturbidimetric assay for residual VWF activity. Samples from 114 patients with suspected thrombotic microangiopathy were analyzed by both standard immunoblot and our new semiautomated method with excellent agreement, particularly at the clinically relevant low levels of ADAMTS13 activity. This new method is semiautomated and offers quantitative results within 2 hr; our modifications allow for widespread applicability on instrumentation already in use by routine clinical laboratories.

Inhibitory autoantibodies against ADAMTS-13 in patients with thrombotic thrombocytopenic purpura bind ADAMTS-13 protease and may accelerate its clearance in vivo

BACKGROUND

Many patients with acquired thrombotic thrombocytopenic purpura (TTP) harbor autoantibodies that may bind and/or inhibit ADAMTS-13 proteolytic activity and accelerate its clearance in vivo.

METHODS

To test this hypothesis, we determined ADAMTS-13 activity and antigen levels in parallel plasma samples from patients clinically diagnosed with TTP. Collagen binding, GST-VWF73 and FRETS-VWF73 assays were used to determine ADAMTS-13 activity and to detect inhibitory autoantibodies. Enzyme-linked immunosorbent assay (ELISA) and immunoprecipitation plus Western blotting (IP/WB) were used to detect total anti-ADAMTS-13 IgG (inhibitory and non-inhibitory).

RESULTS

Among 40 patients with TTP (21 idiopathic and 19 non-idiopathic), inhibitory autoantibodies were detected (by FRETS-VWF73) in 52% of idiopathic and 0% of non-idiopathic TTP patients. In contrast, non-inhibitory IgG autoantibodies were detected in 29% of idiopathic and 50% of non-idiopathic TTP patients. The concentration of inhibitory IgG autoantibody in idiopathic TTP patients was significantly higher than that of non-inhibitory IgG in either idiopathic or non-idiopathic TTP patients. Idiopathic TTP patients demonstrated significantly reduced ADAMTS-13 activity compared with non-idiopathic patients, but only slightly lower ADAMTS-13 antigen levels. Interestingly, patients with inhibitory autoantibodies exhibited significantly lower ADAMTS-13 antigen levels than those with only non-inhibitory IgG autoantibodies or no autoantibody. Serial plasma exchanges increased levels of ADAMTS-13 activity and antigen concurrently in patients with inhibitory autoantibodies.

CONCLUSION

The identification of severe ADAMTS-13 deficiency and autoantibodies or inhibitors appears to be assay-dependent; the inhibitory IgG autoantibodies, in addition to binding and inhibiting ADAMTS-13 proteolytic activity, may accelerate ADAMTS-13 clearance in vivo.