Heparin-Induced Thrombocytopenia

Management of Select Thrombocytopenias

Evaluating, diagnosing, and managing patients with consumptive thrombocytopenia is challenging because of the overlapping nature of many of the diseases that reduce platelet counts. Immune thrombocytopenia (and its variations), drug-induced immune thrombocytopenia, and heparin-induced thrombocytopenia result from autoimmune antibody-mediated destruction of platelets. Thrombotic thrombocytopenia (both congenital and acquired) and the hemolytic uremic syndromes (both typical and atypical) are thrombotic microangiopathies associated with platelet aggregation and consumption along with anemia and renal dysfunction. Rapid history taking, physical assessment, and laboratory evaluation are crucial to accurately managing patients with these disorders. Platelet-associated coagulopathies are infrequently encountered by most providers, and limited exposure to these types of patients, combined with the wide variety of treatment options for reversing bleeding or thrombotic sequelae, makes management difficult. This article reviews the pathophysiology, patient presentation, diagnostic testing, and specific management strategies and challenges of these thrombocytopenias.

Neutrophil activation and NETosis are the major drivers of thrombosis in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia/thrombosis (HIT) is a serious immune reaction to heparins, characterized by thrombocytopenia and often severe thrombosis with high morbidity and mortality. HIT is mediated by IgG antibodies against heparin/platelet factor 4 antigenic complexes. These complexes are thought to activate platelets leading to thrombocytopenia and thrombosis. Here we show that HIT immune complexes induce NETosis via interaction with FcγRIIa on neutrophils and through neutrophil-platelet association. HIT immune complexes induce formation of thrombi containing neutrophils, extracellular DNA, citrullinated histone H3 and platelets in a microfluidics system and in vivo, while neutrophil depletion abolishes thrombus formation. Absence of PAD4 or PAD4 inhibition with GSK484 abrogates thrombus formation but not thrombocytopenia, suggesting they are induced by separate mechanisms. NETs markers and neutrophils undergoing NETosis are present in HIT patients. Our findings demonstrating the involvement of NETosis in thrombosis will modify the current concept of HIT pathogenesis and may lead to new therapeutic strategies.

The pathogenesis of heparin-induced thrombocytopenia and thrombosis (HIT) is mediated by heparin-reactive autoantibodies binding to platelets (thrombocytes). Here the authors show neutrophil activation and NETosis are elevated in patients with HIT, and are essential for thrombosis in HIT mouse models.