AbstractHeparin-induced thrombocytopenia (HIT) is due to immunoglobulin G (IgG) antibodies, which bind platelet factor 4 (PF4) modified by polyanions, such as heparin (H). IgG/PF4/polyanion complexes directly activate platelets via Fc gamma type 2 receptor A (FcγRIIA) receptors. A bacterial protease, IgG-degrading enzyme of Streptococcus pyogenes (IdeS), cleaves the hinge region of heavy-chain IgG, abolishing its ability to bind FcγR, including FcγRIIA. We evaluated whether cleavage of anti-PF4/H IgG by IdeS could suppress the pathogenicity of HIT antibodies. IdeS quickly cleaved purified 5B9, a monoclonal chimeric anti-PF4/H IgG1, which led to the formation of single cleaved 5B9 (sc5B9), without any reduction in binding ability to the PF4/H complex. However, as compared with uncleaved 5B9, the affinity of sc5B9 for platelet FcγRIIA was greatly reduced, and sc5B9 was also unable to induce heparin-dependent platelet activation. In addition, incubating IdeS in whole blood containing 5B9 or HIT plasma samples led to cleavage of anti-PF4/H antibodies, which fully abolished the ability to induce heparin-dependent platelet aggregation and tissue factor messenger RNA synthesis by monocytes. Also, when whole blood was perfused in von Willebrand factor–coated microfluidic channels, platelet aggregation and fibrin formation induced by 5B9 with heparin was strongly reduced after IdeS treatment. Finally, IdeS prevented thrombocytopenia and hypercoagulability induced by 5B9 with heparin in transgenic mice expressing human PF4 and FcγRIIA receptors. In conclusion, cleavage of anti-PF4/H IgG by IdeS abolishes heparin-dependent cellular activation induced by HIT antibodies. IdeS injection could be a potential treatment of patients with severe HIT.