Platelet functional activity in patients with immune thrombocytopenia.

Immune thrombocytopenia (ITP) is one of the most common causes of decreased platelet count. Bleeding is the main clinical symptom of ITP; although its severity correlates with the depth of thrombocytopenia, it may also depend on changes in the functional activity of platelets. In this study we have compared platelet functional activity in healthy volunteers (HV) and in ITP patients, as well as in groups of ITP patients with different levels of bleeding. The study included 65 HV and 84 ITP patients. Platelet activity was assessed by flow cytometry. Platelets were activated with thrombin receptor activating peptide (TRAP) or ADP, and the exposure of activation markers, activated form of glycoprotein (GP) IIb-IIIa and alpha-granule membrane protein P-selectin, was determined on their surface by measuring the binding of PAC-1 and CD62P antibodies, respectively. Platelet-associated IgG (PA-IgG, an indicator of the level of antiplatelet autoantibodies), the percentage of "young" reticular platelets (RP, %) and platelet light scatter (an indicator of their size) were also assessed using flow cytofluorimetry. Platelet binding of PAC-1 (and, to a lesser extent, CD62P binding) was lower in ITP patients than in HV. In ITP patients, PAC-1 binding inversely correlated with the PA-IgG content. In contrast to HV, in ITP patients, PAC-1 and CD62P binding did not directly correlate with the platelet size and RP, %. In ITP patients with severe bleeding, the platelet count was lower, PAC-1 and CD62P binding was reduced and PA-IgG and RP, % levels were increased. Thus, a decrease in the content of activation markers on the platelet surface was registered in ITP patients; it was more pronounced in patients with severe bleeding. It is suggested that the cause of this decrease may be due to the effect of autoantibodies (PA-IgG) on platelets, and in particular on GP IIb-IIIa.