SOLUBLE P-SELECTIN IS PRESENT IN NORMAL CIRCULATION AND ITS PLASMA-LEVEL IS ELEVATED IN PATIENTS WITH THROMBOTIC THROMBOCYTOPENIC PURPURA AND HEMOLYTIC-UREMIC SYNDROME

P-selectin is an integral membrane glycoprotein stored in the secretory granules of platelets and endothelial cells. To determine whether soluble P-selectin may be present in the circulation of healthy humans, we used a sandwich immunoassay to assess citrated plasma from 50 subjects. P-selectin was present in concentrations ranging from 19 to 521 ng/ml (mean+/-SD=121+/-84 ng/ml). The apparent molecular weight of P-selectin immunoisolated from platelet-poor plasma was similar to that of the detergent-soluble form isolated from platelet membrane. Plasma levels of P-selectin were unaffected by the following procedures: (1) drawing of blood in the presence of protease inhibitors; (2) stimulation of platelet-rich plasma with aggregating agents; (3) ultracentrifugation at 100000 g for 120 min at 4-degrees-C or filtration through a 0.22 mum membrane, or (4) preincubation of platelet-poor plasma with immobilized anti-platelet glycoprotein lb monoclonal antibodies. It appeared that plasma P-selectin did not result from the in vitro activation of platelets, nor was it derived from platelet microparticles. We also found that plasma P-selectin levels were significantly elevated in patients with thrombotic thrombocytopenic purpura (12 patients, 332+/-184 ng/ml, P<0.001) and haemolytic uraemic syndrome (17 patients, 297+/-191 ng/ml, P<0.0001), as compared to the normal levels. Thus, these data should facilitate the study of the pathophysiological significance of circulating P-selectin.