NETs contribute to acute episodes but not ADAMTS13 relapses in immune-mediated thrombotic thrombocytopenic purpura

• In acute iTTP, platelet count drop and NET surge reappear two weeks into treatment without caplacizumab, but both are prevented with it. . • ADAMTS13 relapses are not associated with NET marker increases - low ADAMTS13 activity itself does not trigger NET formation. Immune-mediated thrombotic thrombocytopenic purpura (iTTP) results from acquired severe ADAMTS13 deficiency. Neutrophil extracellular traps (NETs) contribute to acute iTTP episodes, but their role across different clinical stages and treatment modalities remains unclear. We conducted a retrospective, observational study in eight iTTP patients followed for 7.4 (range 0.4 - 15.2) years. Patients experienced eight first acute episodes (three caplacizumab-treated), fourteen ADAMTS13 relapses, and six clinical relapses (two caplacizumab-treated), interjected by remissions. Besides clinical and ADAMTS13 parameters, we measured NET markers, i.e. circulating DNA, myeloperoxidase (MPO), neutrophil elastase (NE), and citrullinated histone H3 (CitH3). All NET markers were elevated at presentation with acute iTTP episodes and rapidly declined during treatment. The magnitude of NET marker reduction three days post-treatment initiation was similar with and without caplacizumab (DNA -61% ± 21% vs. -49% ± 23%; MPO -15% ± 49% vs. -46% ± 33%). In the second week, patients treated without caplacizumab presented with another NET marker surge, paralleled by a drop in platelet counts; both were absent in caplacizumab-treated patients. NET levels remained unchanged during ADAMTS13 relapses compared to remission (DNA 1.1-fold; MPO 1.1-fold) while increases were documented in overt clinical relapses (DNA 4.3-fold; MPO 3.2-fold). Our findings indicate NET involvement in acute iTTP pathophysiology but suggest that NET formation requires a secondary trigger beyond ADAMTS13 deficiency. The second wave of NET formation and renewed platelet decline may be driven by VWF-platelet interactions, as the phenomenon is absent in caplacizumab-treated patients. Our data support the benefit of preemptive immunosuppressive treatment for ADAMTS13 relapses.