We investigated the characteristics of the antiplatelet autoantibodies in 60 patients with ITP. Using flow cytometry, the binding of monoclonal antibodies to the platelet glycoprotein (GP) IIb/IIIa complex and to GPIb was examined in these patients. The extent of binding was decreased in 15 patients (anti‐GPIIb/IIIa in 12 patients and both anti‐GPIIb/IIIa and anti‐GPIb in 3 patients). Western blotting revealed that 10 of these 15 patients had either anti‐GPIIb or anti‐GPIIIa and 2 had anti‐GPIb autoantibodies. ADP‐induced aggregation of normal platelets was inhibited by autoantibodies in 12 of 60 patients, and 11 of these had anti‐GPIIb/IIIa antibodies. Ristocetin‐induced aggregation was inhibited in 4 of these patients, and 2 with prominent inhibition had anti‐GPIb antibodies. There was a significant relationship between platelet‐associated IgG value and ATP secretion. These results suggest that some antiplatelet autoantibodies can affect platelet function and thus have an influence on the pathophysiology of ITP.

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 patient's 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 patient's IgG was extremely slight. Phosphorylation of a 40 kDa protein was induced by the patient's 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 patient's IgG failed to induce aggregation itself, but enhanced ADP- or STA2-induced aggregation. Western blotting and immunoprecipitation experiments showed that the patient's 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.

Idiopathic thrombocytopenic purpura (ITP) is a syndrome caused by circulating antibodies reactive with the platelet membrane. The antigenic specificity of these antibodies is unknown. We have characterized new monoclonal antibodies that react with a determinant specific to GP Ib and GP Ib- Ia complex, and used flow cytometry to investigate platelets in ITP for antigenic determinants to which autoantibodies are directed. Forty cases of ITP were analyzed in detail by the platelet suspension immunofluorescence test(PSIFT) of von dem Borne et al. The monoclonal antibodies used were 5 against GP Ib-Ia complex (NNKY1-32, NNKY2-5, NNKY2-6, NNKY2-11, NNKY2-18) and 2 against GP lb (NNKY5-4, NNKY5-5). The reactivity of monoclonal antibodies was inhibited by the presence of autoantibody on platelets in some ITP patients. Differences in inhibition were found not only between monoclonal antibodies but also between cases.These results suggest that some ITP patients have circulating antibodies to GP Ib or GP II b - Ia, and that heterogeneous antibodies are present on platelets. Moreover, the presence of these autoantibodies may aggravate or initiate a bleeding tendency.

No abstract

No abstract