Inactivation of ADAMTS13 by plasmin as a potential cause of thrombotic thrombocytopenic purpura

Abstract

Summary. Background: ADAMTS13 deficiency causes accumulation of unusually large von Willebrand factor molecules, which cross-link platelets in the circulation or on the endothelial surface. This process of intravascular agglutination leads to the microangiopathy thrombotic thrombocytopenic purpura (TTP). Most TTP patients have acquired anti-ADAMTS13 autoantibodies that inhibit enzyme function and/or clear it from the circulation. However, the reason for ADAMTS13 deficiency is not always easily identified in a subset of patients. Objectives: To determine the origin of ADAMTS13 deficiency in a case of acquired TTP. Methods: Western blotting of ADAMTS13 in plasmas from acute and remission phases was used. Results: The ADAMTS13 deficiency was not caused by mutations or (detectable) autoantibodies; however, an abnormal ADAMTS13 truncated fragment (100 kDa) was found in acute-phase but not remission-phase plasma. This fragment resulted from enzymatic proteolysis, as recombinant ADAM-TS13 was also cleaved when in the presence of acute-phase but not remission-phase plasma. Inhibitor screening showed that ADAMTS13 was cleaved by a serine protease that could be dose-dependently inhibited by addition of exogenous a 2 -antiplasmin. Examination of the endogenous a 2 -antiplasmin antigen and activity confirmed deficiency of a 2 -antiplasmin function in acute-phase but not remission-phase plasma. To investigate the possibility of ADAMTS13 cleavage by plasmin in plasma, urokinase-type plasminogen activator was added to an (unrelated) congenital a 2 -antiplasmin-deficient plasma sample to activate plasminogen. This experiment confirmed cleavage of endogenous ADAMTS13 similar to that observed in our TTP patient. Conclusion: We report the first acquired TTP patient with cleaved ADAMTS13 and show that plasmin is involved.

Plasmin Cleavage of von Willebrand Factor as an Emergency Bypass for ADAMTS13 Deficiency in Thrombotic Microangiopathy

“…21,26 In our studies, we found elevated PAP complexes, representing plasminogen activation, in a group of TTP patients with acute microangiopathy ( Figure 5). However, in TTP patients in remission (ie, without microangiopathy), no plasmin was generated.…”

“…21 How to reconcile these apparently contradicting findings? In the case report, the cause for the (transient) α 2 -antiplasmin deficiency remained uncertain.…”

“…We hypothesize that this was the case in the α2-antiplasmindeficient TTP patient. 21 We propose the following sequence of events: (1) The patient had a preexisting ADAMTS13 deficiency. (2) After a trigger, the patient developed an episode of microangiopathy (when the remaining ADAMTS13 was insufficiently protective).…”

“…This prevents off-target degradation of proteins. In a recent case report, a patient with a preexisting ADAMTS13 deficiency (10% activity) developed α 2 -antiplasmin deficiency during an episode of TTP, 21 potentially worsening ADAMTS13 deficiency. In the patient cohort in the present study, we found that α 2 -antiplasmin activity was somewhat lowered during acute TTP (median 98.4%, IQR 91.7%-106.6%; n=26) compared with patients in remission (median 104.0%, IQR 100.6%-113.3%; n=32) and healthy control subjects (median 104.7%, IQR 98.2%-107.5%; n=19; Figure 5B).…”

Hypochlorous Acid Generated by Neutrophils Inactivates ADAMTS13

“…A number of studies have reported the detection of degraded ADAMTS13 accompanied by decreased ADAMTS13 and protease inhibitor activity (9,49). The large discrepancies observed between the activity and antigen levels in plasma from patients with acute inflammation could be attributed to both oxidation and proteolysis.…”