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-ß2glycoprotein I (aß2GPI) antibodies. Clinical criteria for the classification of thrombotic APS include venous and arterial thrombosis, along with microvascular thrombosis. Several aPLs, including LAC, aß2GPI 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.

The Significance of Antibodies against Domain I of Beta-2 Glycoprotein I in Antiphospholipid Syndrome.

The antiphospholipid syndrome (APS) is characterized by vascular thrombosis and/or pregnancy morbidity with the persistent presence of antiphospholipid antibodies (aPLs). Progress is being made in understanding the pathogenesis of the syndrome, but difficulties persist in the identification of patients at risk for thrombosis and/or pregnancy morbidity. Beta-2 glycoprotein I (ß2GPI), a plasma protein consisting of five sushi domains, is thought to be the main antigenic target of aPLs. Antibodies recognizing domain I of ß2GPI are predominantly present in patients with an elevated risk of thrombosis, whereas antidomain IV/V antibodies are found in nonthrombotic autoimmune diseases. Indeed, domain I antibodies proved to be pathogenic in multiple studies. Retrospective studies have provided evidence for an added clinical value of antidomain I antibodies in the risk stratification of patients with APS. Still, wide ranges of odds ratio exist between studies, probably due to differences in the study and control population, and detection methods used. Despite the proven pathogenicity of antidomain I antibodies and their correlations with clinical manifestations of APS, heterogeneity of the current studies has prohibited their acceptance in the official diagnostic criteria. Well-designed large longitudinal prospective studies with available and new, preferentially functional, assays for the risk stratification of patients with APS are required.

Evaluation of Glial Fibrillary Acidic Protein and Ubiquitin C-Terminal Hydrolase-L1 Using a Rapid Point of Care Test for Predicting Head Computed Tomography Lesions After Mild Traumatic Brain Injury in a Dutch Multi-Center Cohort.

Mild traumatic brain injury (mTBI) is a common condition seen in emergency departments worldwide. Blood-based biomarkers glial fibrillary acidic protein (GFAP) and ubiquitin C-terminal hydrolase-L1 (UCH-L1) are recently U.S. Food and Drug Administration-approved for the prediction of intracranial lesions on head computed tomography (CT) scans in mTBI. We evaluated the diagnostic performance of GFAP and UCH-L1 in a Dutch cohort using the i-STAT TBI assay. In a multi-center observational study, we enrolled 253 mTBI patients. Head CT scans were scored using the Marshall classification system. Logistic regression models were used to assess the contribution of biomarkers and clinical parameters to diagnostic performance. Detection of UCH-L1 and GFAP resulted in a sensitivity of 97% and specificity of 19% for CT positivity in mTBI patients, along with a negative predictive value of 95% (88-100%) and a positive predictive value of 27% (21-33%). Combining biomarker testing with loss of consciousness and time to sample increased specificity to 46%. Combined testing of UCH-L1 and GFAP testing resulted in possibly more unnecessary CT scans compared with GFAP testing alone, with only limited increase in sensitivity. This study confirmed high sensitivity of GFAP and UCH-L1 for CT abnormalities in mTBI patients using the i-STAT TBI test. The results support the potential use of GFAP and UCH-L1 as tools for determining the indication for CT scanning in mTBI patients, possibly offering a cost- and time-effective approach to management of patients with mTBI. Prospective studies in larger cohorts are warranted to validate our findings.

Added value of antiphosphatidylserine/prothrombin antibodies in the workup of thrombotic antiphospholipid syndrome: Communication from the ISTH SSC Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibodies.

BACKGROUND: Diagnosis of antiphospholipid syndrome (APS) requires persistent presence of lupus anticoagulant (LAC), anticardiolipin (aCL) IgG/IgM, or anti-ß2 glycoprotein I (aß2GPI) IgG/IgM antibodies. Other antiphospholipid antibodies (aPL) such as antiphosphatidylserine/prothrombin antibodies (aPS/PT) are promising in assessment of thrombotic APS (TAPS). AIM: To evaluate the added value of aPS/PT IgG and IgM in TAPS. MATERIAL AND METHODS: aPS/PT IgG/IgM, aCL IgG/IgM, aß2GPI IgG/IgM, and LAC were determined in 757 patients (TAPS and controls). aPS/PT cut-off values were calculated, and aPS/PT titers and positivity were compared between TAPS and controls, type of thrombosis, and antibody profiles. Likelihood ratios (LR), odds ratios (OR), and aPL score were determined. RESULTS: aPS/PT IgG and IgM were associated with TAPS and triple positivity. In-house calculated cut-offs were higher for IgM (43 units), compared to manufacturer's cut-off (30 units). Thresholds of 90 (IgG) and 200 (IgM) units were determined as high-titer cut-off. Higher aPS/PT titers were observed in triple positive patients and showed higher LR and OR for TAPS. aPS/PT was independently associated with TAPS when adjusted for aCL/aß2GPI, but not when adjusted for LAC. In isolated LAC positive patients, aPS/PT was positive in 27.1% TAPS patients and in 77.3% patients with autoimmune disease. Diagnostic value of aPL score did not differ with and without including aPS/PT. CONCLUSION: aPS/PT positivity, especially with high antibody titer, is associated with TAPS diagnosis. Analysis on top of current laboratory criteria is not essential in TAPS diagnosis, but aPS/PT could be useful in patients with thrombosis and a double positive aPL profile (aCL+/aß2GPI+).

Semiquantitative interpretation of anticardiolipin and antiß2glycoprotein I antibodies measured with various analytical platforms: Communication from the ISTH SSC Subcommittee on Lupus Anticoagulant/Antiphospholipid Antibodies.

BACKGROUND: Antiß2glycoprotein I (aß2GPI) and anticardiolipin (aCL) IgG/IgM show differences in positive/negative agreement and titers between solid phase platforms. Method-specific semiquantitative categorization of titers could improve and harmonize the interpretation across platforms. AIM: To evaluate the traditional 40/80-unit thresholds used for aCL and aß2GPI for categorization into moderate/high positivity with different analytical systems, and to compare with alternative thresholds. MATERIAL AND METHODS: aCL and aß2GPI thresholds were calculated for two automated systems (chemiluminescent immunoassay [CLIA] and multiplex flow immunoassay [MFI]) by receiver operating characteristic curve analysis on 1108 patient samples, including patients with and without antiphospholipid syndrome (APS), and confirmed on a second population (n = 279). Alternatively, regression analysis on diluted standard material was applied to identify thresholds. Thresholds were compared to 40/80 threshold measured by an enzyme-linked immunosorbent assay (ELISA). Additionally, likelihood ratios (LR) were calculated. RESULTS: Threshold levels of 40/80 units show poor agreement between ELISA and automated platforms for classification into low/moderate/high positivity, especially for aCL/aß2GPI IgG. Agreement for semiquantitative interpretation of antiphospholipid antibodies (aPL) IgG between ELISA and CLIA/MFI improves with alternative thresholds. LR for aPL IgG increase for thrombotic and obstetric APS based on 40/80 thresholds for ELISA and adapted thresholds for the other systems, but not for IgM. CONCLUSION: Use of 40/80 units as medium/high thresholds is acceptable for aCL/aß2GPI IgG ELISA, but not for CLIA and MFI. Alternative semiquantitative thresholds for non-ELISA platforms can be determined by a clinical approach or by using monoclonal antibodies. Semiquantitative reporting of aPL IgM has less impact on increasing probability for APS.

Deciphered coagulation profile to diagnose the antiphospholipid syndrome using artificial intelligence.

The antiphospholipid syndrome (APS) is diagnosed by the presence of lupus anticoagulant and/or antibodies against cardiolipin or ß2-glycoprotein-1 and the occurrence of thrombosis or pregnancy morbidity. The assessment of overall coagulation is known to differ in APS patients compared to normal subjects. The accelerated production of key factor thrombin causes a prothrombotic state in APS patients, and the reduced efficacy of the activated protein C pathway promotes this effect. Even though significant differences exist in the coagulation profile between normal controls and APS patients, it is not possible to rely on a single test result to diagnose APS. A neural network is a computing system inspired by the human brain that can be trained to distinguish between healthy subjects and patients based on subject specific data. In a first cohort of patients, we developed a neural networking that diagnoses APS. We clinically validated this neural network in a separate cohort consisting of APS patients, normal controls, controls visiting the hospital for other indications and two diseased control groups (thrombosis patients and auto-immune disease patients). The positive predictive value ranged from 62% in the hospital controls to 91% in normal controls and the negative predictive value of the neural network ranged from 86% in the thrombosis control group to 95% in the hospital controls. The sensitivity of the neural network was higher than 90% in all control groups. In conclusion, we developed a neural network that accurately diagnoses APS in the validation cohort. After further clinical validation in newly diagnosed patients, this neural network could possibly be clinically implemented to diagnose APS based on thrombin generation data.

Antiprothrombin antibodies induce platelet activation: A possible explanation for anti-FXa therapy failure in patients with antiphospholipid syndrome?

BACKGROUND: Arterial and venous thrombosis are both common in antiphospholipid syndrome (APS). Recent studies have shown that anti-factor Xa (FXa) therapy in APS patients leads to a greater number of patients with arterial thrombosis than with warfarin. We hypothesize that this may be due to the lowering of prothrombin levels by warfarin. OBJECTIVES: To investigate whether antiprothrombin antibodies induce platelet aggregation and to identify the platelet receptors involved. A second aim was to investigate the effect of reduced prothrombin levels on antiprothrombin antibody-induced platelet aggregation. METHODS: Enzyme-linked immunosorbent assays were performed to measure binding of antiprothrombin antibodies to prothrombin fragment 1+2 and prothrombin. Platelet aggregation assays in washed platelets were performed. FcγRIIA was immunoprecipitated and tyrosine-phosphorylated FcγRIIA was measured by western blot. RESULTS: The antiprothrombin antibodies 28F4 and 3B1 had lupus anticoagulant (LAC) activity and caused platelet aggregation in the presence of Ca2+ and prothrombin. Antiprothrombin antibodies without LAC activity did not activate platelets. Inhibition of Syk and Src kinases and FcγRIIA blocked platelet aggregation. Fab and F(ab')2 fragments of 28F4 were unable to induce platelet aggregation. Immunoprecipitations showed that whole 28F4 immunoglobulin G induced tyrosine phosphorylation of FcγRIIA. Platelet aggregation was significantly reduced when prothrombin levels were reduced from 1 µM to 0.2 µM. CONCLUSIONS: Antiprothrombin antibodies with LAC activity are able to activate platelets via FcγRIIA. Decreased prothrombin levels resulted in less antiprothrombin antibody-mediated platelet aggregation. This may explain the lower incidence of arterial thrombosis in patients treated with warfarin than with anti-FXa therapy.

Is There an Additional Value in Detecting Anticardiolipin and Anti-ß2 glycoprotein I IgA Antibodies in the Antiphospholipid Syndrome?

BACKGROUND: Anticardiolipin (aCL) and anti-ß2 glycoprotein I (aß2GPI) immunoglobulin A (IgA) antiphospholipid antibodies (aPL) have shown to associate with thrombosis and pregnancy morbidity. However, inclusion of IgA aPL in the classification criteria of the antiphospholipid syndrome (APS) has been debated. We investigated the value of aCL and aß2GPI IgA aPL in the detection of thrombosis and pregnancy morbidity in addition to the current aPL panel for APS. METHODS: We included 1,068 patients from eight European medical centers: 259 thrombotic APS patients, 122 obstetric APS patients, 204 non-APS thrombosis patients, 33 non-APS obstetric patients, 60 APS patients with unspecified clinical manifestations, 196 patients with autoimmune diseases, and 194 controls. aCL and aß2GPI IgG/M/A were detected with four commercial assays and lupus anticoagulant was determined by the local center. RESULTS: Positivity for IgA aPL was found in 17 to 26% of the patients with clinical manifestations of APS and in 6 to 13% of the control population. Both aCL and aß2GPI IgA were significantly associated with thrombosis and pregnancy morbidity. Isolated IgA positivity was rare in patients with clinical manifestations of APS (0.3-5%) and not associated with thrombosis and/or pregnancy morbidity. Addition of IgA to the current criterion panel did not increase odds ratios for thrombosis nor pregnancy morbidity. CONCLUSION: aCL and aß2GPI IgA are associated with clinical manifestations of APS. However, isolated IgA positivity was rare and not associated with thrombosis or pregnancy morbidity. These data do not support testing for aCL and aß2GPI IgA subsequent to conventional aPL assays in identifying patients with thrombosis or pregnancy morbidity.

The (non-)sense of detecting anti-cardiolipin and anti-ß2glycoprotein I IgM antibodies in the antiphospholipid syndrome.

BACKGROUND: The antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy morbidity with the persistent presence of lupus anticoagulant (LAC), anti-cardiolipin (aCL) and/or anti-ß2glycoprotein I (aß2GPI) antibodies of the immunoglobulin G/immunoglobulin M (IgG/IgM) isotype. However, the role of aCL and aß2GPI IgM as a serologic marker in APS is debated. OBJECTIVES: We aimed to assess the diagnostic and clinical value of IgM antiphospholipid antibodies (aPL) in APS within the classification criteria. PATIENTS/METHODS: Our multicenter study comprised 1008 patients, including APS patients and controls. Anti-CL and aß2GPI IgG and IgM antibodies were detected with four commercially available solid phase assays. RESULTS: Positivity for aCL and/or aß2GPI antibodies was significantly correlated with thrombosis and pregnancy morbidity, independent of the isotype and solid phase assay. Higher odds ratios were obtained for IgG compared to IgM positivity. Isolated IgM was rare in thrombotic APS, but more frequent in obstetric APS, ranging from 3.5% to 5.4% and 5.7% to 12.3%, respectively, dependent on the solid phase assay. In a multivariate logistic regression analysis of aPL, IgM positivity was found to be associated with pregnancy morbidity. However, detection of IgM was not independently associated with thrombosis. Combined positivity for LAC, IgG, and IgM was highly associated with thrombosis and pregnancy morbidity. CONCLUSIONS: Our data support testing for aCL and aß2GPI IgM in women suspected of obstetric APS. However, no added value was found for testing IgM in patients suspected of thrombotic APS. Still, IgM aPL might be useful as a second-line test to improve thrombotic risk stratification.

Detection of anti-domain I antibodies by chemiluminescence enables the identification of high-risk antiphospholipid syndrome patients: A multicenter multiplatform study.

BACKGROUND: Classification of the antiphospholipid syndrome (APS) relies predominantly on detecting antiphospholipid antibodies (aPLs). Antibodies against a domain I (DI) epitope of anti-ß2glycoprotein I (ß2GPI) proved to be pathogenic, but are not included in the current classification criteria. OBJECTIVES: Investigate the clinical value of detecting anti-DI IgG in APS. PATIENTS/METHODS: From eight European centers 1005 patients were enrolled. Anti-cardiolipin (CL) and anti-ß2GPI were detected by four commercially available solid phase assays; anti-DI IgG by the QUANTA Flash® ß2GPI domain I assay. RESULTS: Odds ratios (ORs) of anti-DI IgG for thrombosis and pregnancy morbidity proved to be higher than those of the conventional assays. Upon restriction to patients positive for anti-ß2GPI IgG, anti-DI IgG positivity still resulted in significant ORs. When anti-DI IgG was added to the criteria aPLs or used as a substitute for anti-ß2GPI IgG/anti-CL IgG, ORs for clinical symptoms hardly improved. Upon removing anti-DI positive patients, lupus anticoagulant remained significantly correlated with clinical complications. Anti-DI IgG are mainly present in high-risk triple positive patients, showing higher levels. Combined anti-DI and triple positivity confers a higher risk for clinical symptoms compared to only triple positivity. CONCLUSIONS: Detection of anti-DI IgG resulted in higher ORs for clinical manifestations than the current APS classification criteria. Regardless of the platform used to detect anti-ß2GPI/anti-CL, addition of anti-DI IgG measured by QUANTA Flash® did not improve the clinical associations, possibly due to reduced exposure of the pathogenic epitope of DI. Our results demonstrate that anti-DI IgG potentially helps in identifying high-risk patients.

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

BACKGROUND: Interaction of von Willebrand factor (VWF) with platelets requires a conformational change that exposes an epitope within the VWF A1 domain, enabling platelet glycoprotein Ibα binding. Quantification of this ''active" conformation of VWF has been shown to provide pathophysiological insight into conditions characterized by excessive VWF-platelet interaction. METHODS: We developed an immunosorbent assay based on a variable heavy chain antibody fragment against the VWF A1 domain as a capture antibody. Assay performance in terms of specificity (binding to active R1306W- and sheared VWF), precision, accuracy, linearity, limits of detection and stability were determined. Active VWF, VWF antigen, VWF ristocetin cofactor activity, VWF:GP1bM and VWF propeptide were measured in citrated plasma and platelet-VWF binding in whole blood from 120 healthy individuals to establish a reference interval for active VWF and to assess associations with other VWF parameters. RESULTS: Intra- and inter-assay CVs were between 2.4-7.2% and 4.1-9.4%, depending on the level. Mean recovery of spiked recombinant R1306W VWF was 103±3%. The assay was linear in the range of 90.1-424.5% and had a limit of quantification of 101%. The reference interval for active VWF was 91.6-154.8% of NPP. Significant, positive correlations between active VWF and all other VWF parameters were found, with the strongest correlation with VWF:GP1bM binding. CONCLUSIONS: We developed and validated an immunosorbent assay for the accurate detection of active VWF levels in plasma. The assay fulfilled all analytical criteria in this study and a reference interval was established, allowing its use to quantify active VWF in pathological conditions for future research.

Detection of Anti-Cardiolipin and Anti-ß2glycoprotein I Antibodies Differs between Platforms without Influence on Association with Clinical Symptoms.

BACKGROUND: The anti-phospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy morbidity with persistent presence of anti-phospholipid antibodies (aPL). Laboratory criteria include aPL detection by coagulation tests for lupus anticoagulant (LAC) or solid phase assays measuring anti-ß2 glycoprotein I (aß2GPI) or anti-cardiolipin (aCL) immunoglobulin (Ig) G/IgM antibodies. External quality control programs illustrate that commercially available aPL assays produce variable results. OBJECTIVE: We aimed to investigate the agreement and diagnostic accuracy of solid phase assays. MATERIALS AND METHODS: In this multi-centre study, 1,168 patient samples were tested on one site for aCL and aß2GPI IgG/IgM antibodies by four solid phase test systems. Samples included APS patients, controls and monoclonal antibodies (MoAB) against different epitopes of ß2GPI. LAC was determined by the local centre. RESULTS: aCL IgM assays resulted in the most discrepancies (60%), while aCL IgG and aß2GPI IgM assays resulted in lower discrepancies (36%), suggesting better agreement. Discrepant samples displayed lower median aPL titers. Dependent on the solid phase test system, odds ratios (ORs) for thrombosis and pregnancy morbidity ranged from 1.98 to 2.56 and 3.42 to 4.78, respectively. Three platforms showed lower sensitivity for MoAB directed against the glycine (Gly) 40-arginine (Arg) 43 epitope of domain I of ß2GPI. CONCLUSION: Poor agreement was observed between different commercially available aCL and aß2GPI IgG/IgM assays, hampering uniformity in the identification of aPL-positive patients. Clinical association was globally concordant between solid phase test systems considering results of the four aPL together. An assay sensitive in detecting the MoAB against Gly40-Arg43 of domain I of ß2GPI reached the highest OR for thrombosis.