Microparticle Levels Are High in Patients with Hemophilia A and Can Be Further Increased by DDAVP Administration.

Microparticles (MP) are circulating submicroscopic cell fragments expressing procoagulant phospholipids and haemostatic proteins as tissue factor. Several evidences support a pro-haemostatic role of MP and suggest that the axis VWF-platelet GpIb is critical in their generation. The aim of this study was to measure the levels of platelet-derived MP (PMP) and tissue factor-expressing MP (TFMP) in patients with hemophilia A to evaluate whether or not a correlation with the phenotype exists. The levels of PMP and TFMP were also measured after DDAVP administration in a subset of patients with mild hemophilia A to assess whether MP levels change in response to DDAVP. FVIII:C, VWF:Ag, PMP and TFMP were measured in plasma 30 min, 1, 2, 4, 8 and 24 h after DDAVP 0.3 μg/kg, subcutaneously. PMP and TFMP were analysed in the pellet of platelet poor plasma by flow cytometry and defined as Annexin V and GpIIb/IIIa or Tissue Factor positive events falling in a gate defined by 1 μm beads. MP were enumerated adding a known number of 7 μm beads to each test tube. PMP and TFMP were measured in 50 patients with severe (FVIII:C<1%) and in 37 with mild (FVIII:C 7–34%) hemophilia A after a wash-out period >72 h, and in 20 male controls. Levels of both PMP and TFMP were lower in controls than in all hemophiliacs (p<0.05), severe hemophiliacs (p<0.05) and mild hemophiliacs (p<0.05). No correlation was found between levels of MP and FVIII:C in mild hemophiliacs. A significant increase in the levels of PMP and TFMP (p<0.05 at all time points vs baseline) was observed in 9 patients after DDAVP administration; peak levels were achieved after 2–8 h. Correlations were found between the increase of VWF:Ag and FVIII:C levels and the peak levels of MP (r=0.62 and r=0.7, respectively). This study showing higher MP levels in hemophiliacs than in controls suggests a role of MP in this patient population. Furthermore, increased MP levels in response to DDAVP administration support the hypothesis that VWF of ultralarge size released by the endothelium interacts with platelets generating MP.