ADAMTS13 Autoantibodies, Antigen and Proteolytic Activity in Patients with Thrombotic Thrombocytopenic Purpura.
Authors: Shelat Suresh G. , Smith Paula , Ai Jihui , Zheng X. Long 
Published: April 2020

Abstract
Many patients with thrombotic thrombocytopenic purpura (TTP) harbor autoantibodies against the ADAMTS13 protease, which can block proteolytic activity or accelerate clearance of this protease. The autoantibodies may be categorized as ‘inhibitory’ antibodies that bind and inhibit ADAMTS13 activity in vitro and ‘non-inhibitory’ antibodies that may bind, but do not affect the ADAMTS13 activity. The pathophysiologic role of two types of anti-ADAMTS13 autoantibodies on the level of ADAMTS13 antigen and activity remains unclear. To address this question, the relationship between the autoantibodies, and the levels of antigen and proteolytic activity in 40 patients with TTP (26 idiopathic and 14 non-idiopathic) was determined. The diagnosis of TTP was based on the presence of thrombocytopenia and microangiopathic hemolytic anemia with or without renal dysfunction and neurological symptoms. The ADAMTS13 antigen was determined by ELISA (American Diagnostica, Stamford CT). The ADAMTS13 autoantibodies were determined by Technozyme-ELISA (Technoclone GmbH, Vienna, Austria) and Immunoprecipitation and Western blotting. The ADAMTS13 activity was determined by Fluorescent resonance energy transfer (FRETS)-VWF73. The results showed that: 1) autoantibodies were present in 24 of 40 (60%) of all TTP patients, 19 of 26 (73%) in idiopathic TTP and 5 of 14 (36%) in non-idiopathic TTP. The prevalence of non-inhibitory autoantibodies was approximately 31% (8/26) in patients with idiopathic TTP and 36% (5/14) in patients with non-idiopathic TTP; 2) there was a positive correlation between the ADAMTS13 antigen levels and enzymatic activity in 36 samples tested (r=0.665, P&lt;0.0001). This positive correlation was not altered even after removing from the analysis samples demonstrating inhibitors and low activity; 3) with respect to antigen levels, there was no significant difference between idiopathic vs. non-idiopathic TTP patients, whereas ADAMTS13 activity are significantly lower in the former (Table 1); 4) the inhibitory autoantibodies significantly reduce ADAMTS13 activity and may or may not decrease ADAMTS13 antigen levels. However, non-inhibitory autoantibodies neither decrease ADAMTS13 levels nor proteolytic activity (Table 2). We conclude that overall, in TTP patient samples, ADAMTS13 antigen levels correlate with activity. Subgroup analysis indicates that idiopathic and non-idiopathic TTP patients have similar ADAMTS13 antigen levels, whereas the former demonstrates much less proteolytic activity. TTP patients with inhibitory autoantibodies may have a more profound effect on ADAMTS13 activity (and perhaps also on clearance) than those with non-inhibitory autoantibodies. The non-inhibitory antibodies do not appear to accelerate ADAMTS13 protease clearance in vivo.
Table 1. ADAMTS13 Antigen and Activity in Idiopathic and Non-Idiopathic TTP Patients Patients Antigen (ng/ml) Activity (%) * p &lt; 0.05 Idiopathic 276 ± 46 (n=19) 16.9 ± 6 (n=21)* Non-idiopathic 328 ± 43 (n=17) 45.1 ± 5.7 (n=19) Table 2. Effect of Autoantibodies on ADAMTS13 Antigen and Activity Antibodies Definition Antigen (ng/ml) Activity (%) *p=0.085; **p &lt; 0.05 vs "non-inhibitory" and &lt;0.05 vs. "None" group Inhibitory Activity &lt;10%, Inhibitor (1:1), positive, TecZym&gt;15 U/ml 183 ± 55* 4.1 ± 1.2 (n=9)** Non-inhibitory Activity&gt;10%, Inhibitor (1:1), negative, TecZym &gt;15 U/ml 368 ± 52 37.6 ± 8 (n=17) None Activity &gt;10%, Inhibitor (1:1), negative, TecZym &lt;12 U/ml 294 ± 51 40.2 ± 7 (n=13)