Analytical characterization and reference interval of an enzyme-linked immunosorbent assay for active von Willebrand factor

Conformational activation and inhibition of von Willebrand factor by targeting its autoinhibitory module

ABSTRACT

Activation of von Willebrand factor (VWF) is a tightly controlled process governed primarily by local elements around its A1 domain. Recent studies suggest that the O-glycosylated sequences flanking the A1 domain constitute a discontinuous and force-sensitive autoinhibitory module (AIM), although its extent and conformation remains controversial. Here, we used a targeted screening strategy to identify 2 groups of nanobodies. One group, represented by clone 6D12, is conformation insensitive and binds the N-terminal AIM (NAIM) sequence that is distal from A1; 6D12 activates human VWF and induces aggregation of platelet-rich plasma at submicromolar concentrations. The other group, represented by clones Nd4 and Nd6, is conformation sensitive and targets the C-terminal AIM (CAIM). Nd4 and Nd6 inhibit ristocetin-induced platelet aggregation and reduce VWF-mediated platelet adhesion under flow. A crystal structure of Nd6 in complex with AIM-A1 shows a novel conformation of both CAIM and NAIM that are primed to interact, providing a model of steric hindrance stabilized by the AIM as the mechanism for regulating GPIbα binding to VWF. Hydrogen-deuterium exchange mass spectrometry analysis shows that binding of 6D12 induces the exposure of the GPIbα-binding site in the A1 domain, but binding of inhibitory nanobodies reduces it. Overall, these results suggest that the distal portion of NAIM is involved in specific interactions with CAIM, and binding of nanobodies to the AIM could either disrupt its conformation to activate VWF or stabilize its conformation to upkeep VWF autoinhibition. These reported nanobodies could facilitate future studies of VWF functions and related pathologies.

Exercise and hypoxia-induced hypercoagulability is counterbalanced in women in part by decreased platelet reactivity

Hypoxia plays an important role in several pathologies, e.g. chronic obstructive pulmonary disease and obstructive sleep apnea syndrome, and is linked to an increased thrombosis risk. Furthermore, oxygen deprivation is associated with hypercoagulability. In this study, we investigated the effect of gender and exercise on the coagulation potential under hypoxic conditions at high altitude by assessing thrombin generation (TG) and platelet activation. Hereto, ten healthy volunteers were included (50 % male, median age of 27.5 years). The measurements were conducted first at sea level and then twice at high altitude (3883 m), first after a passive ascent by cable car and second after an active ascent by a mountain hike. As expected, both the passive and active ascent resulted in a decreased oxygen saturation and an increased heart rate at high altitude. Acute mountain sickness symptoms were observed independently of the ascent method. After the active ascent, platelet, white blood cell and granulocyte count were increased, and lymphocytes were decreased, without a gender-related difference. FVIII and von Willebrand factor were significantly increased after the active ascent for both men and women. Platelet activation was reduced and delayed under hypobaric conditions, especially in women. TG analysis showed a prothrombotic trend at high altitude, especially after the active ascent. Women had a hypercoagulable phenotype, compared to men at all 3 timepoints, indicated by a higher peak height and endogenous thrombin potential (ETP), and shorter lag time and time-to-peak. In addition, ETP and peak inhibition by thrombomodulin was lower in women after the active ascent, compared to men. Interestingly, data normalisation for subject baseline values indicated an opposing effect of altitude-induced hypoxia on α2-macroglobulin levels and TG lag time between men and women, decreasing in men and increasing in women. We conclude that hypoxia increases TG, as well as FVIII and VWF levels in combination with exercise. In contrast, platelets lose their responsiveness at high altitude, which is most pronounced after heavy exercise. Women had a more pronounced prothrombotic phenotype compared to men, which we theorize is counterbalanced under hypobaric conditions by decreased platelet activation.

Abacavir use is associated with increased prothrombin conversion

There is ongoing debate as to whether abacavir (ABC) increases the risk for cardiovascular disease(CVD) in people living with HIV (PLHIV) and the mechanisms underlying this possible association. We recently showed that the use of an ABC-containing regimen was independently associated with increased thrombin generation (TG). In the present study, we aim to explore these findings further, by studying the mechanistical processes that underly the global thrombin generation test via thrombin dynamics analysis. Thrombin dynamics analysis can pinpoint the cause of increased thrombin generation associated with ABC-use either to the procoagulant prothrombin conversion pathway or the anticoagulant thrombin inactivation pathway. In this cross-sectional study, 208 virally suppressed PLHIV were included, of whom 94 were on a ABC-containing regimen, 92 on a tenofovir disoproxil fumarate (TDF)-containing regimen, and the remainder on other regimens. We used Calibrated Automated Thrombinography to measure thrombin generation and perform thrombin dynamics analysis. The total amount of prothrombin conversion, as well as the maximum rate of prothrombin conversion were significantly increased in PLHIV on an ABC containing regimen compared to other treatment regimens. The levels of pro- and anticoagulant factors were comparable, indicating that the ABC-induced changes affect the kinetics of prothrombin conversion rather than procoagulant factor levels. Moreover, Von Willebrand Factor (VWF), active VWF and VWF pro-peptide levels were significantly higher in PLHIV than controls without HIV. However, they did not differ between ABC and non-ABC treated participants.

Coagulation parameters predict COVID-19-related thrombosis in a neural network with a positive predictive value of 98%

Thrombosis is a major clinical complication of COVID-19 infection. COVID-19 patients show changes in coagulation factors that indicate an important role for the coagulation system in the pathogenesis of COVID-19. However, the multifactorial nature of thrombosis complicates the prediction of thrombotic events based on a single hemostatic variable. We developed and validated a neural net for the prediction of COVID-19-related thrombosis. The neural net was developed based on the hemostatic and general (laboratory) variables of 149 confirmed COVID-19 patients from two cohorts: at the time of hospital admission (cohort 1 including 133 patients) and at ICU admission (cohort 2 including 16 patients). Twenty-six patients suffered from thrombosis during their hospital stay: 19 patients in cohort 1 and 7 patients in cohort 2. The neural net predicts COVID-19 related thrombosis based on C-reactive protein (relative importance 14%), sex (10%), thrombin generation (TG) time-to-tail (10%), α₂-Macroglobulin (9%), TG curve width (9%), thrombin-α₂-Macroglobulin complexes (9%), plasmin generation lag time (8%), serum IgM (8%), TG lag time (7%), TG time-to-peak (7%), thrombin-antithrombin complexes (5%), and age (5%). This neural net can predict COVID-19-thrombosis at the time of hospital admission with a positive predictive value of 98%-100%.

Functional changes in hemostasis during asexual and sexual parasitemia in a controlled human malaria infection

Decreased platelet count is an early phenomenon in asexual Plasmodium falciparum parasitemia, but its association with acute or long-term functional changes in platelets and coagulation is unknown. Moreover, the impact of gametocytemia on platelets and coagulation remains unclear. We investigated the changes in platelet number and function during early asexual parasitemia, gametocytemia and convalescence in 16 individuals participating in a controlled human malaria infection study, and studied its relationship with changes in total and active von Willebrand factor levels (VWF) and the coagulation system. Platelet activation and reactivity were determined by flow cytometry, and the coagulation system was assessed using different representative assays including antigen assays, activity assays and global functional assays. Platelet count was decreased during asexual blood stage infection but normalized during gametocytemia. Platelet P-selectin expression was slightly increased during asexual parasitemia, gametocytemia and at day 64. In contrast, platelet reactivity to different agonists remained unchanged, except a marked decrease in reactivity to low dose collagen-related peptide-XL. Thrombin generation and antigen assays did not show a clear activation of the coagulation during asexual parasitemia, whereas total and active VWF levels were markedly increased. During gametocytemia and on day 64, the endogenous thrombin potential, thrombin peak and velocity index were increased and prothrombin conversion and plasma prothrombin levels were decreased. We conclude that the decreased platelet count during asexual parasitemia is associated with increased active VWF levels (i.e. endothelial activation), but not platelet hyperreactivity or hypercoagulability, and that the increased platelet clearance in asexual parasitemia could cause spontaneous VWF-platelet complexes formation.

Haemostatic differences between SARS-CoV-2 PCR-positive and negative patients at the time of hospital admission

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is associated with thrombosis. We conducted a cohort study of consecutive patients, suspected of SARS-CoV-2 infection presented to the emergency department. We investigated haemostatic differences between SARS-CoV-2 PCR positive and negative patients, with dedicated coagulation analysis. The 519 included patients had a median age of 66 years, and 52.5% of the patients were male. Twenty-six percent of the patients were PCR-positive for SARS-CoV-2.PCR positive patients had increased levels of fibrinogen and (active) von Willebrand Factor (VWF) and decreased levels of protein C and α2-macroglobulin compared to the PCR negative patients. In addition, we found acquired activated protein C resistance in PCR positive patients. Furthermore, we found that elevated levels of factor VIII and VWF and decreased levels of ADAMTS-13 were associated with an increased incidence of thrombosis in PCR positive patients. In conclusion, we found that PCR positive patients had a pronounced prothrombotic phenotype, mainly due to an increase of endothelial activation upon admission to the hospital. These findings show that coagulation tests may be considered useful to discriminate severe cases of COVID-19 at risk for thrombosis.

Clonal hematopoiesis is associated with low CD4 nadir and increased residual HIV transcriptional activity in virally suppressed individuals with HIV

Clonal hematopoiesis, a common age-related phenomenon marked by expansion of cells with clonal hematopoiesis driver mutations, has been associated with all-cause mortality, cancer, and cardiovascular disease. People with HIV (PWH) are at risk for non-AIDS–related comorbidities such as atherosclerotic cardiovascular disease and cancer. In a cross-sectional cohort study, we compared clonal hematopoiesis prevalence in PWH on stable antiretroviral therapy with prevalence in a cohort of overweight individuals and a cohort of age- and sex-matched population controls. The prevalence of clonal hematopoiesis adjusted for age was increased and clone size was larger in PWH compared to population controls. Clonal hematopoiesis is associated with low CD4 nadir, increased residual HIV-1 transcriptional activity, and coagulation factors in PWH. Future studies on the effect of clonal hematopoiesis on the HIV reservoir and non-AIDS–related comorbidities are warranted.

Plasmatic Coagulation Capacity Correlates With Inflammation and Abacavir Use During Chronic HIV Infection

BACKGROUND

D-dimer concentrations in people living with HIV (PLHIV) on combination antiretroviral therapy (cART) are increased and have been linked to mortality. D-dimer is a biomarker of in vivo coagulation. In contrast to reports on D-dimer, data on coagulation capacity in PLHIV are conflicting. In this study, we assessed the effect of cART and inflammation on coagulation capacity.

SETTING

We explored coagulation capacity using calibrated thrombin generation (TG) and linked this to persistent inflammation and cART in a cross-sectional study including PLHIV with viral suppression and uninfected controls.

METHODS

We used multivariate analyses to identify independent factors influencing in vivo coagulation (D-dimer) and ex vivo coagulation capacity (TG).

RESULTS

Among 208 PLHIV, 94 (45%) were on an abacavir-containing regimen. D-dimer levels (219.1 vs 170.5 ng/mL, P = 0.001) and inflammatory makers (sCD14, sCD163, and high-sensitive C-reactive protein) were increased in PLHIV compared with those in controls (n = 56). PLHIV experienced lower TG (reflected by endogenous thrombin potential [ETP]) when compared with controls, after correction for age, sex, and antiretroviral therapy. Abacavir use was independently associated with increased ETP. Prothrombin concentrations were strongly associated with ETP and lower in PLHIV on a non-abacavir-containing regimen compared with those in controls, suggesting consumption as a possible mechanism for HIV-associated reduction in TG. D-dimer concentrations were associated with inflammation, but not TG.

CONCLUSIONS

Abacavir use was associated with increased TG and could serve as an additional factor in the reported increase in thrombotic events during abacavir use. Increased exposure to triggers that propagate coagulation, such as inflammation, likely underlie increased D-dimer concentrations found in most PLHIV.

Vascular activation is a strong predictor of mortality in coronavirus disease 2019 patients on the ICU

Respiratory failure in coronavirus disease 2019 (COVID-19) patients is one of the most frequent causes for referral to the ICU. A significant percentage of these patients does not survive the infection due to thromboembolic complications. Furthermore, the vascular system seems also to be involved in the pathogenesis. To investigate the role of hemostasis and endothelium on the outcome of COVID-19 patients admitted to the ICU. Blood was drawn from 16 ICU COVID-19 patients for hemostatic analysis. Patients were followed-up till discharge (n = 11) or death (n = 5). Parameters related to both coagulation and fibrinolysis, though disturbed, were not associated with mortality. Contrarily, activated Von Willebrand factor was increased and ADAMTS13 levels were decreased by two-fold in nonsurvivors compared with survivors. Our data established the involvement of the Von Willebrand factor-ADAMTS13 axis in the COVID-19 pathogenesis, thereby demonstrating that these plasma proteins seem to be strong predictors for ICU mortality.

VWF, Platelets and the Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy morbidity with the persistent presence of antiphospholipid antibodies (aPLs). Laboratory criteria for the classification of APS include the detection of lupus anticoagulant (LAC), anti-cardiolipin (aCL) antibodies and anti-β2glycoprotein I (aβ2GPI) antibodies. Clinical criteria for the classification of thrombotic APS include venous and arterial thrombosis, along with microvascular thrombosis. Several aPLs, including LAC, aβ2GPI and anti-phosphatidylserine/prothrombin antibodies (aPS/PT) have been associated with arterial thrombosis. The Von Willebrand Factor (VWF) plays an important role in arterial thrombosis by mediating platelet adhesion and aggregation. Studies have shown that aPLs antibodies present in APS patients are able to increase the risk of arterial thrombosis by upregulating the plasma levels of active VWF and by promoting platelet activation. Inflammatory reactions induced by APS may also provide a suitable condition for arterial thrombosis, mostly ischemic stroke and myocardial infarction. The presence of other cardiovascular risk factors can enhance the effect of aPLs and increase the risk for thrombosis even more. These factors should therefore be taken into account when investigating APS-related arterial thrombosis. Nevertheless, the exact mechanism by which aPLs can cause thrombosis remains to be elucidated.

Flow cytometric analysis of platelet function to improve the recognition of thrombocytopathy

INTRODUCTION

Light transmission aggregometry (LTA) is the gold standard for diagnosing bleeding disorders. Although LTA is laborious, requires large volumes of blood and is relatively insensitive to small changes in platelet function, there is still no competing alternative approach to replace LTA for the diagnosis of platelet bleeding disorders.

MATERIALS AND METHODS

This study investigates the correlation between flow cytometry-based whole blood platelet activation test (WB-PACT) and LTA and whether WB-PACT is of additional value for the identification of bleeding disorders. In total, 161 patients with suspected bleeding diathesis were tested.

RESULTS

A correlation of 0.41 between LTA and WB-PACT was found, and there was agreement between tests in 62% of cases (κ = 0.23). The WB-PACT is of additional value to LTA to detect platelet function disorders (PFD) as 10 patients with elevated bleeding score (BS) were detected with WB-PACT, 4 with LTA and 7 patients were positive with both tests. Interestingly, in contrast to LTA, WB-PACT has an additional option to detect VWF disfunctions.

CONCLUSION

WB-PACT may have added value for the routine diagnostic work-up in patients who need to have platelet function tested.

Acute exacerbations of COPD are associated with a prothrombotic state through platelet-monocyte complexes, endothelial activation and increased thrombin generation

INTRODUCTION

Patients with chronic obstructive pulmonary disease (COPD) are at increased risk for cardiovascular events, particularly following an acute exacerbation (AE-COPD). Exacerbations are associated with increased systemic inflammation, which may drive coagulation. This prospective cohort study aimed to determine how an AE-COPD affects platelet activation, the endothelium, plasmatic coagulation and fibrinolysis, and its association with systemic inflammation.

MATERIALS AND METHODS

Fifty-two patients with an AE-COPD were included. Blood samples at admission, at day 3 of treatment and at convalescence were available for 32 patients. Platelet-monocyte complex (PMC) formation, monocyte Mac-1 expression and platelet (re)activity (P-selectin expression, αIIbβ3 activation) were measured by flow cytometry. Von Willebrand Factor (VWF), thrombin generation (TG) and clot lysis time (CLT) were determined as measures of endothelial activation, plasmatic coagulation and fibrinolysis, respectively.

RESULTS

Exacerbations were associated with increased PMCs (MFI 31.3 vs 23.8, p = 0.004) and Mac-1 (MFI 38.2 vs 34.8, p = 0.006) compared to convalescence, but not with changes in platelet (re)activity. VWF (antigen, activity, active fraction) and TG (peak, ETP and velocity index) were all significantly higher during AE-COPD compared to convalescence. PMCs, Mac-1, VWF and TG were positively associated with systemic inflammation (CRP). CLT was prolonged in AE-COPD patients with systemic inflammation. Moreover, platelet hyperreactivity on admission was associated with an increased risk for exacerbation relapse.

CONCLUSIONS

Acute exacerbations are associated with an inflammation-associated prothrombotic state, characterized by increased PMCs, endothelial activation and plasmatic coagulation. Our findings provide direction for future studies on biomarkers predicting the risk of exacerbation relapse and cardiovascular events.

Implant stability in patients treated with platelet-rich fibrin and bovine bone substitute for alveolar ridge preservation is associated with peripheral blood cells and coagulation factors

AIMS

The aim of the present study was to assess the association between dental implant stability and peripheral blood cell composition and levels of coagulation factors in patients treated with alveolar ridge preservation with platelet-rich fibrin (PRF) and bovine bone substitute.

MATERIALS AND METHODS

Fifty patients were included between 2015 and 2017. PRF was prepared from autologous blood, in which blood cells and coagulation factor levels were measured. PRF and bovine bone were placed in the socket, followed by closure with PRF membrane. Implants were placed 14 (±2.5) weeks postextraction. The implant stability quotient was measured at t = 0, t = 10 days, t = 7 weeks, and t = 17 weeks by resonance frequency analysis.

RESULTS

Erythrocyte count was inversely associated with PRF membrane length, but not with implant stability. Conversely, platelet count did not correlate with membrane size but inversely correlated with implant stability at 7 and 17 weeks. In addition, implant stability was directly correlated with levels FXIII (t = 0, p < .01), active von Willebrand factor (VWF; t = 0 and 7 weeks, p < .05), and total VWF (t = 7 weeks, p = .012).

CONCLUSION

Implant stability following alveolar ridge preservation with PRF and bovine bone substitute is associated with circulating blood cells and coagulation factors. In particular, fibrin structure, VWF, and FXIII may be important modulators of implant stability.

A Hypoxic Environment Attenuates Exercise-Induced Procoagulant Changes Due to Decreased Platelet Activation

Introduction  Although physical exercise is protective against cardiovascular disease, it can also provoke sudden cardiac death (exercise paradox). Epidemiological studies suggest that systemic hypoxia at high altitude is a risk factor for venous thromboembolism. Forthcoming, this study investigated the effect of repeated exercise at high altitude on blood coagulation, platelet function, and fibrinolysis. Methods  Six trained male volunteers were recruited. Participants ascended from sea level to 3,375 m altitude. They performed four exercise tests at 65 to 80% of their heart-rate reserve during 2 hours: one time at sea level and three times on consecutive days at 3,375 m altitude. Thrombin generation (TG) was measured in whole blood (WB) and platelet-rich and platelet-poor plasma. Coagulation factor levels were measured. Platelet activation was measured as αIIbβ3 activation and P-selectin expression. Fibrinolysis was studied using a clot-lysis assay. Results  Normoxic exercise increased plasma peak TG through increased factor VIII (FVIII), and increased von Willebrand factor (VWF) and active VWF levels. Platelet granule release potential was slightly decreased. After repetitive hypoxic exercise, the increase in (active) VWF tapered, and there was no more distinct exercise-related increase in peak. Platelet aggregation potential and platelet-dependent TG decreased at high altitude. There were no effects on fibrinolysis upon exercise and/or hypoxia. Conclusion  Strenuous exercise induces a procoagulant state that is mediated by the endothelium, by increasing VWF and secondarily raising FVIII levels. After repetitive exercise, the amplitude of the endothelial response to exercise diminishes. A hypoxic environment appears to further attenuate the procoagulant changes by decreasing platelet activation and platelet-dependent TG.