BACKGROUND: Acquired Glanzmann thrombasthenia (GT) is a bleeding disorder generally caused by anti-αIIb β3 autoantibodies. OBJECTIVES: We aimed to characterize the molecular mechanism leading to a progressive GT-like phenotype in a patient with chronic immune thrombocytopenia. PATIENT, METHODS AND RESULTS: The patient suffered from repeated episodes of gastrointestinal bleedings and further studies indicated a moderate platelet aggregation defect. Few months later, platelet function showed abolished aggregation using all agonists, but normal agglutination with ristocetin. No platelet-bound antibodies were detected, but the presence of large amounts of an IgM type antibody detected together with αIIb β3 in the patient's permeabilized platelets suggested that this IgM was an autoantibody causing the internalization of the complex. This was confirmed by the fact that the patient's IgM bound to normal platelets but not to platelets from GT type I patients. Moreover, patient's plasma activated αIIb β3 on controls' platelets as evidenced by increased PAC-1 binding. We also demonstrated that the patient's plasma triggered αIIb β3 outside-in signaling, as β3 Tyr773 and FAK were phosphorylated, and increased the rate of actin polymerization in resting platelets reflecting an impairment of cytoskeletal reorganization. As different signs of dysmegakariopoiesis were also observed in our patient, we evaluated the ability of its serum to impair proplatelets formation and showed that it significantly decreased the number of proplatelet-bearing megakaryocytes in controls' bone marrow stem cells culture as compared to normal serum. CONCLUSIONS: We present the case of a patient with a progressive and severely perturbed platelet function associated with the presence of an IgM activating autoantibody directed against αIIb β3 . This article is protected by copyright. All rights reserved.< Leer menos