Platelet phosphatidylserine is the critical mediator of thrombosis in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a severe immune-mediated prothrombotic disorder caused by antibodies reactive to complexes of platelet factor 4 and heparin. Platelets (PLTs) and their interaction with different immune cells contribute to prothrombotic conditions in HIT. However, the exact mechanisms and the role of different PLT subpopulations to this prothrombotic environment remain poorly understood. In this study, we observed that HIT patient antibodies (Abs) induce relevant changes in PLT phenotype, with the key features being increased P-Selectin expression and procoagulant phosphatidylserine (PS) externalization. Formation of procoagulant PLTs was dependent on engagement of PLT Fc-gamma-RIIA by HIT Abs and resulted in significant increase of thrombin generation on the PLT surface. Using an ex vivo thrombosis model and multi-parameter assessment of thrombus formation, we observed that HIT Ab-induced procoagulant PLTs propagated formation of large PLT aggregates, leukocyte recruitment and most importantly, fibrin network generation. These prothrombotic conditions were prevented via the upregulation of PLTs intracellular cAMP with Iloprost, a clinically approved prostacyclin analogue. Additionally, the functional relevance of high P-Selectin and PS levels on procoagulant PLTs was dissected. While inhibition of P-Selectin did not affect thrombus formation, the specific blockade of PS with Lactadherin prevented HIT Ab-mediated thrombin generation and most importantly procoagulant PLT-mediated thrombus formation ex vivo. Taken together, our findings indicate that procoagulant PLTs are critical mediators of prothrombotic conditions in HIT. Upregulation of cAMP with Iloprost or PS targeting specifc therapeutics could be a promising approach to prevent thromboembolic events in HIT patients.