Profiles of Antibodies Specific to Platelet Glycoproteins in ITP Differ in Remission, Relapse and Stable Phases; and Anti-GpIIb/IIIa, -GpIb/IX Were Associated with Bleeding Manifestations.

BACKGROUND: In immune thrombocytopenic purpura (ITP), antiplatelet antibodies (APtAb) bind to antigens exposed on the platelet surface to mediate platelet clearance from circulation. GpIIb/IIIa, GpIb/IX and GpIV, common target antigens in ITP, are receptors for fibrinogen, von Willebrand factor and collagen, respectively, and are involved in platelet aggregation and adhesion, playing critical roles in hemostasis and thrombosis. It has been suggested that APtAb binding to these molecules interfere with their function, inducing platelet dysfunction and enhancing bleeding in ITP. In the present study, we investigated profiles of antibodies against these antigens in different stages of ITP, and in relation to bleeding manifestations.

METHODS: We measured IgG and IgM APtAb against the three most prevalent glycoprotein target antigens, GpIIb/IIIa, GpIV and GpIb/IX, by PAICA [

RESULTS: (1) IgG APtAb against GpIIb/IIIa, GpIV and GpIb/IX were all significantly more frequent in acute/relapse (77%, 70%, and 67%, respectively) than in chronic/stable ITP (52%,55%, and 44%), at p=0.005, p=0.003, p=0.04, respectively. (2) All IgG APtAb were significantly more frequent in acute/relapse and chronic/stable ITP compared to remission, p<0.0001. (3) IgM APtAb were detected in approximately 50% of patients in relapse and stable stages for all 3 antigens; there was no significant difference in frequency between acute/relapse and chronic/stable, but both were significantly higher than in remission. (4) Multiple antibodies (≥ 2 antigens positive) were observed in 50–70% of patients in both relapse and stable stages. (5) When we compared frequencies of APtAb between bleeders and non-bleeders, both IgG and IgM APtAb were significantly more frequent in bleeders, for all antigens except IgM GpIb/IX. Interestingly, 96.7% of bleeders had at least one APtAb of IgG or IgM class, while only 58% were positive in non-bleeders (p<0.0002). This difference persisted when reanalyzed to ensure equal mean platelet counts between bleeders and non-bleeders. It was also found that positivity for APtAb of IgG (but not IgM) class against the pair, GpIIb/IIIa and GpIb/IX (but not GpIV) was predictive of bleeding.

CONCLUSIONS. Distinctive profiles of APtAb to GpIIb/IIIa, GpIb/IX, and GpIV were observed in different stages of ITP. IgG APtAb were frequently seen in acute/relapse and chronic/stable, both clases (IgG-IgM) were rarely seen in remission. Patients with signs of bleeding more frequently had IgG APtAb to the pair, GpIIb/IIIa and GpIb/IX, suggesting this pair of APtAb interferes with platelet hemostatic functions, enhancing the bleeding manifestations of ITP.