scIV.3-IdeS bound to platelet FcγRIIA cleaves antiplatelet antibodies from HIT and ITP sera and prevents in vitro phagocytosis. (A) Platelet factor 4 (PF4)–dependent P-selectin expression assays were performed using sera from 5 patients with HIT and platelets treated with scIV.3-IdeS (5 nM) or vehicle. (B) Platelets treated with scIV.3-IdeS or vehicle control were incubated with sera from patients with ITP, stained with a fluorescein isothiocyanate (FITC)–conjugated mouse anti-human Fc-specific antibody, and quantified by flow cytometry (data represent mean ± standard deviation [SD]; 2-way analysis of variance [ANOVA]; n = 4-6). (C) PF4-dependent P-selectin expression assay was performed with platelets treated with scIV.3 or scIV.3-IdeS using sera from a patient with HIT. (D) Platelets treated with scIV.3 or scIV.3-IdeS were incubated with sera from a patient with ITP and then stained with an FITC-conjugated mouse anti-human Fc-specific antibody. (E) CFSE-stained human platelets treated with scIV.3-IdeS (5 nM) or vehicle control were incubated with sera from patients with ITP and were then added to THP-1 cells. THP-1 cells were stained with a platelet-specific (PE-conjugated CD42a) antibody to distinguish THP-1 cells that had adhered (CFSE⁺/CD42a⁺) or internalized (CFSE⁺/CD42a⁻) platelets (data represent mean ± SD; 1-way ANOVA; n = 4). *P < .05, **P < .01, ***P < .001. MFI, median fluorescence intensity.