Background Severe coronavirus disease 2019 (COVID‐19) is characterized by an increased risk of thromboembolic events, with evidence of microthrombosis in the lungs of deceased patients. Objectives To investigate the mechanism of microthrombosis in COVID‐19 progression. Patients/Methods We assessed von Willebrand factor (VWF) antigen (VWF:Ag), VWF ristocetin‐cofactor (VWF:RCo), VWF multimers, VWF propeptide (VWFpp), and ADAMTS13 activity in a cross‐sectional study of 50 patients stratified according to their admission to three different intensity of care units: low (requiring high‐flow nasal cannula oxygenation, n = 14), intermediate (requiring continuous positive airway pressure devices, n = 17), and high (requiring mechanical ventilation, n = 19). Results Median VWF:Ag, VWF:RCo, and VWFpp levels were markedly elevated in COVID‐19 patients and increased with intensity of care, with VWF:Ag being 268, 386, and 476 IU/dL; VWF:RCo 216, 334, and 388 IU/dL; and VWFpp 156, 172, and 192 IU/dL in patients at low, intermediate, and high intensity of care, respectively. Conversely, the high‐to‐low molecular‐weight VWF multimers ratios progressively decreased with increasing intensity of care, as well as median ADAMTS13 activity levels, which ranged from 82 IU/dL for patients at low intensity of care to 62 and 55 IU/dL for those at intermediate and high intensity of care. Conclusions We found a significant alteration of the VWF‐ADAMTS13 axis in COVID‐19 patients, with an elevated VWF:Ag to ADAMTS13 activity ratio that was strongly associated with disease severity. Such an imbalance enhances the hypercoagulable state of COVID‐19 patients and their risk of microthrombosis.

Recently, we showed that during the acute phase of immune-mediated thrombotic thrombocytopenic purpura (iTTP), ADAMTS13 circulates in an open conformation. Although the cause of this conformational change in acute iTTP remains elusive, ADAMTS13 is mainly closed in iTTP patients (i) in remission with an ADAMTS13 activity >50% and undetectable anti-ADAMTS13 autoantibodies, and (ii) after rituximab treatment, suggesting a role for anti-ADAMTS13 autoantibodies. Therefore, IgGs from 18 acute iTTP patients were purified and added to closed ADAMTS13 in healthy donor plasma. This resulted in open ADAMTS13 in 14/18 (78%) samples, proving that indeed anti-ADAMTS13 autoantibodies can induce an open ADAMTS13 conformation. To further elucidate the conformation of ADAMTS13 in iTTP patients, we studied a novel iTTP patient cohort (n=197) that also included plasma samples of iTTP patients in remission where ADAMTS13 activity was <50%. The open ADAMTS13 conformation was not only found during acute iTTP but also in patients in remission with an ADAMTS13 activity <50% and in half of the patients with an ADAMTS13 activity >50%, although free anti-ADAMTS13 autoantibodies were not always detected. Thus open ADAMTS13 is not only a hallmark of acute iTTP, but also a novel biomarker to detect subclinical iTTP in patients in remission. Finally, a long term follow-up study in one iTTP patient showed that the open conformation precedes a severe drop in ADAMTS13 activity. In conclusion, we have shown that anti-ADAMTS13 autoantibodies from iTTP patients induce an open ADAMTS13 conformation. Most importantly, an open ADAMTS13 conformation is a biomarker for subclinical iTTP and could become an important tool in TTP management.

Thrombotic thrombocytopenic purpura (TTP) is a rare thrombotic microangiopathy (TMA) characterized by the severe deficiency of a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) activity (< 10%). Rapid ADAMTS13 testing is crucial for an early diagnosis and optimal management of acute TTP. We evaluated the performance of the HemosIL AcuStar ADAMTS13 activity assay (Instrumentation Laboratory, Bedford, Massachusetts, United States), a fully automated chemiluminescent immunoassay with an analytical time of 33 minutes. A method comparison study was performed on 176 samples from 49 healthy donors and 127 TMA patients (109 TTP, 7 atypical hemolytic uremic syndrome, 11 other TMAs), comparing this new assay with an in-house FRETS-VWF73 assay and a commercial enzyme-linked immunosorbent assay (ELISA) (TECHNOZYM ADAMTS-13 Activity, Technoclone GmbH, Vienna, Austria). Agreement between methods was assessed with focus on ADAMTS13 activity less than 10%, the medical decision level relevant for TTP diagnosis. The HemosIL AcuStar ADAMTS13 Activity showed good correlation with both the FRETS-VWF73 (r = 0.96) and ELISA (r = 0.96) methods. Slope of the Passing–Bablok regression was 1.05 for FRETS-VWF73 and 1.02 for ELISA, and absolute bias at the medical decision level was +0.1 and +0.3%, respectively. The study also revealed high agreement with FRETS-VWF73 (kappa 0.97) and ELISA (kappa 0.98) methods in classifying TTP patients with a severe deficiency of ADAMTS13 activity. Because of its short turnaround time and full automation, the HemosIL AcuStar ADAMTS13 activity assay might become the assay of choice to rapidly test ADAMTS13 activity in plasma and thus establish the diagnosis of acute TTP in emergency settings.

ADAMTS-13 activity and autoantibodies classes and subclasses as prognostic predictors in acquired thrombotic thrombocytopenic purpura. J Thromb Haemost 2012; 10: 1556-65.Summary. Background: Thrombotic thrombocytopenic purpura (TTP) is a rare life-threatening disease. Of surviving patients, 45% develops an exacerbation or a late recurrence. Severe ADAMTS-13 deficiency, both during the acute episode and remission, is a well-established predictor of recurrence. The predictive value of anti-ADAMTS-13 antibodies, their inhibitory activity and Ig class subtype for disease recurrence is still to be established. Objectives: To analyze ADAMTS-13-related biomarkers (ADAMTS-13 and anti-ADAMTS-13 immunoglobulins, classes and subclasses) and their potential relationship with prognosis. Patients/Methods: In 115 patients with TTP, we assessed the association between levels of these biomarkers and the severity of acute episodes; we analysed also the hazard ratio (HR) and 95% confidence interval (CI) of recurrence in association with biomarkers levels retrieved at the previous acute episode or during remission, using Cox regression models. Results: During the acute phase, higher IgA, IgG1 and IgG3 titers showed the strongest association with acute episode severity. In the survival analyzes, the only biomarker significantly associated with a high hazard of recurrence after an acute episode was the presence of IgG. Conversly, low ADAMTS-13 activity or antigen levels (< 10%), the presence of ADAMTS-13 inhibitor or IgG during remission were all significantly associated with a higher hazard of recurrence. Conclusions: Both the Ig class and subclass are of predictive value for acute episode severity in patients with TTP. Although markers that could predict the risk of recurrence in the acute phase are limited, a thorough assessment of ADAMTS-13-related parameters during remission is warranted.

Acquired thrombotic thrombocytopenic purpura (TTP) is a rare thrombotic microangiopathy caused by the immune-mediated severe deficiency of ADAMTS13. We hereby report the demographic and disease-related data of acquired TTP patients recorded in the Milan TTP Registry (www.ttpdatabase.org). We performed a cross-sectional study of 302 individuals enrolled in our registry for an acute episode of acquired TTP occurred between 2002 and 2015 (female 77%; median age at onset 40 years, interquartile range: 30–50). Twenty per cent of patients had concomitant autoimmune disorders. Among potential triggers of acute episodes, infections were the most prevalent (27%), followed by estroprogestinics use and pregnancy (5 and 4% of women, respectively). At presentation, systemic (72%), bleeding (68%) and neurological (43%) symptoms were the most frequent, whereas a lower prevalence of renal (18%) and cardiovascular (10%) signs and symptoms was observed. Almost all acute events were treated by plasma exchange and steroids, and 15% by rituximab. Exacerbation of acute TTP occurred in 15% of events. The TTP-related mortality was 5%. In survivors, the median number of plasma exchange procedures to remission was 9 (interquartile range: 6–14), longer for first events than relapses (median difference 3, 95% confidence interval: 2–4). Of 251 survivors of the first TTP episode with at least a 6-month follow-up, 55% had a relapse. In conclusion, acquired TTP is a severe disease with highly variable clinical presentation, usually requiring a long hospitalization. The Milan TTP Registry represents a powerful tool to improve our knowledge and management of acquired TTP.