Chikaho Kitada

Synergistic action in platelet activation induced by an antiplatelet autoantibody in ITP

Summary. We detected an autoantibody which activated normal platelets in a patient with immune thrombocytopenic purpura and investigated the mechanism by which this autoantibody mediated platelet activation. The patients IgG induced platelet aggregation and ATP secretion in normal platelet‐rich plasma (PRP). IgG‐induced aggregation was inhibited by aspirin (ASA), apyrase, a protein kinase C (PKC) inhibitor and two anti‐platelet glycoprotein (GP) IIb/IIIa monoclonal antibodies. The increase of aequorin‐detected intraplatelet Ca2+ induced by the patients IgG was extremely slight. Phosphorylation of a 40 kDa protein was induced by the patients IgG without any obvious phosphorylation of a 20 kDa protein, and was inhibited by a PKC inhibitor but not by ASA. With ASA‐treated normal PRP, the patients IgG failed to induce aggregation itself, but enhanced ADP‐ or STA2‐induced aggregation. Western blotting and immuno‐precipitation experiments showed that the patients IgG reacted to a protein of 36 kDa. These results suggest that the platelet activation induced by this autoantibody depended on both the selective activation of PKC and the slight Ca2+ mobilization induced by thromboxane A2 synthesis, while the aggregation depended on secretion induced by the synergistic action of the above two mechanisms and was mediated through GP IIb/IIIa.

Platelet Procoagulant Activity During,Peripheral Blood Stem Cell Harvest

We used flow cytometry to measure platelet-derived microparticle levels in plasma obtained from 16 patients during peripheral blood stem cell harvest (PBSC) and in platelet concentrates prepared by apheresis from 10 normal controls. We also studied the binding of an anti-P-selectin antibody and annexin-V to platelets. When all 60 harvests were assessed, we noted a significant difference in microparticle levels between patients with a platelet count >10 x 104/μl and those with a platelet count <10 X 10 4/μl (12.3 ± 4.4 vs. 75 ± 3.9%). In both the first and total harvests, the percentage of platelets and microparticles positive for anti-P-selectin and annexin-V were significantly higher than the normal control levels. These results suggest that patients undergoing mobilization by granulocyte colony-stimulating factor (G-CSF) who have a platelet count >10 X 10 4/μl are at risk of increased procoagulant activity after retransfusion following PBSC harvest. Key Words: Platelet-derived microparticle— Peripheral blood stem cell harvest—Granulocyte colony-stimulating factor.