Key Points

  • The mechanisms of anti-GPIbα antibody-induced thrombocytopenia and thrombopoietin response differ in relation to antibody dose.

  • Megakaryocytes targeted by anti-GPIbα antibody generate platelets with reduced surface GPIbα expression.

Immune thrombocytopenia (ITP) is an acquired bleeding disorder characterized by antibody-mediated platelet destruction. Different mechanisms have been suggested to explain accelerated platelet clearance and impaired thrombopoiesis, but the pathophysiology of ITP has yet to be fully delineated. In this study, we tested two mouse models of immune-mediated thrombocytopenia using the rat anti-mouse GPIbα monoclonal antibody, 5A7, generated in our laboratory. After a single intravenous administration of high-dose (2 mg/kg) 5A7, opsonized platelets were rapidly cleared from the circulation into the spleen and liver; this was associated with rapid TPO mRNA upregulation. In contrast, subcutaneous administration of low-dose 5A7 (0.08-0.16 mg/kg) every three days gradually lowered the platelet count; in this case, opsonized platelets could only be observed in the spleen and TPO levels remained unaltered. Interestingly, in both models, the 5A7 antibody was found on the surface of, as well as internalized into, bone marrow (BM) megakaryocytes (MKs). Consequently, platelets generated in the chronic phase of repeated subcutaneous 5A7 administration model showed reduced GPIbα membrane expression on their surface. Our findings indicate that evaluation of platelet surface GPIbα relative to platelet size may be a useful marker to support the diagnosis of anti-GPIbα antibody induced ITP.

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