PB0698 - Binding of PAC-1 antibody to glycoprotein IIb-IIIa activated form is decreased in patients with immune thrombocytopenia.

Background: Bleeding complications in immune thrombocytopenia (ITP) are caused by the decrease of platelet number due to the action of antiplatelet autoantibodies. Bleeding risk might be increased by potential suppression of platelet function by autoantibodies.

Aims: We assessed platelet functional activity in ITP patients in comparison with healthy volunteers and its possible correlation with the level of antiplatelet autoantibodies.

Methods: 52 ITP patients and 65 healthy volunteers were included into the study. Platelet-associated IgG (PA-IgG) were determined by binding of I-125-labeled antihuman IgG antibodies. Platelet functional activity was evaluated using flow cytometry by biding of antibodies PAC-1 to activated form of glycoprotein (GP) IIb-IIIa and CD62P to P-selectin. Platelets were activated by TRAP (Thrombin Receptor Activating Peptide), 10 μM, and ADP, 20 and 2.5 μM. Platelet size was assessed by forward light scattering (FSC) and the percentage of reticulated platelets (RP) by thiazole orange staining using flow cytometry.

Results: Platelet number in ITP patients was 40±22 *10^9/l, in healthy volunteers 223±56 *10^9/l. PA-IgG level in ITP patients was 587±326% of the control level in healthy volunteers (100%). Platelet FSC in ITP patients was by 1.8 times and % of RP by 1.4 times higher than in healthy volunteers. In contrast to heathy volunteers in ITP patients increase of platelet size and % of RP was not associated with increased platelet activity. PAC-1, but not CD62P binding to activated platelets was significantly decreased in ITP patients in comparison with healthy volunteers (from 1.5 to 2.2 times with different agonists). Reverse correlation was revealed between PAC-1 binding and PA-IgG (r from -0.34 to -0.46 with different agonists).

Conclusion(s): PAC-1 binding to activated platelets was specifically decreased in ITP patients. This decrease may be caused by the disturbance of PAC-1 interaction with GP IIb-IIIa by anti-GP IIb-IIIa autoantibodies.
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