Glycoprotein Ibα clustering induces macrophage-mediated platelet clearance in the liver

Abstract

Many immune thrombocytopenia (ITP) patients, particularly patients with anti-glycoprotein (GP) Ib-IX autoantibodies, do not respond to the conventional treatments such as splenectomy. However, the underlying mechanism remains unclear. Here we found that anti-GPIbα N-terminus antibody AN51, but not other anti-GPIbα antibodies (AK2, HIP1, VM16d, or WM23), induced GPIbα clustering that led to integrin αIIbβ3-dependent platelet aggregation. After intravenous injection, AN51 dose-dependently induced thrombocytopenia in guinea pigs, and the platelets were mainly removed by macrophages in the liver. N-acetyl-D-glucosamine, previously shown to inhibit integrin αMβ2-mediated phagocytosis of refrigerated platelets, dose-dependently inhibited AN51-induced platelet clearance. Furthermore, AN51 but not VM16d, induced rapid platelet clearance in the liver of cynomolgus macaques. Five of 22 chronic ITP patients had anti-GPIbα autoantibodies, and the autoantibodies from four of the five patients competed with AN51 for binding to platelets. These data indicate that GPIbα clustering induced by anti-GPIbα N-terminus antibody causes integrin αIIbβ3-dependent platelet aggregation, phagocytosis, and rapid platelet clearance in the liver. Our findings reveal a novel Fc-independent mechanism underlying the pathogenesis of ITP, and suggest new therapeutic strategies for ITP patients with anti-GPIbα autoantibodies.

Inhibiting GPIbα Shedding Preserves Post-Transfusion Recovery and Hemostatic Function of Platelets After Prolonged Storage

“…Recent reports have suggested that exposure of β-galactose and/or N-acetyl-glucosamine on the to-be-cleared platelet, as a result of deglycosylation, can mediate platelet clearance 32–35 . Particularly, the change of glycans in the extracellular domain of GPIbα has been implicated 32, 34 .…”

Glycans and the platelet life cycle

“…It is noteworthy that many antibodies targeting GPIbβ and GPIX subunits in the GPIb-IX complex do not cause platelet clearance [74-76]. It has also been recently reported that not all anti-GPIbα antibodies induce platelet clearance [77, 78]. It is likely that platelets secrete active Neu1 and Neu3 upon antibody binding and/or platelet activation [30].…”

Only the strong: when antibodies hold on

“…Prior to this study, it was known that antibodies to the GPIba subunit mimic the natural GPIba ligand, von Willebrand factor (VWF), to initiate GPIba-mediated signal transduction, platelet activation, and clearance in the liver. 3 This activation pathway requires a dimeric form of the antibody, implying that GPIb-IX signals when clustered. 3 However, it was soon learned that dimerization of GPIba is not enough to activate platelets.…”

“…3 This activation pathway requires a dimeric form of the antibody, implying that GPIb-IX signals when clustered. 3 However, it was soon learned that dimerization of GPIba is not enough to activate platelets. 4 Instead, GPIb-IX is a mechanoreceptor, meaning it signals when pulled.…”

“…5 This explained why antibodies must target the LBD of GPIba to activate and clear platelets, but did not explain why some antibodies target the GPIba LBD but cannot activate platelets. 3 Quach et al compare 2 antibodies, 6B4 and AK2, that both recognize the LBD of GPIba. 6B4, but not AK2, mediated shear-dependent unfolding of the GPIba stalk and platelet activation.…”