Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired stem-cell disorder in which the glycolipid-anchored membrane proteins, including the cell-surface complement inhibitors, CD55 and CD59, are partially or completely deleted from the plasma membranes of mature blood cells. To gain insight into the pathogenesis of thrombosis that is frequently observed in this disorder, the procoagulant responses of PNH platelets exposed to the human terminal complement proteins C5b-9 were investigated. C5b-9 complexes were assembled on gel-filtered platelets by incubation with purified C5b6, C7, C9, and limiting amounts of C8. Platelet microparticle formation and exposure of plasma membrane- binding sites for coagulation factor Va were then analyzed by flow cytometry. PNH platelets exhibiting undetectable levels of surface CD59 antigen showed an approximately 10-fold increase in sensitivity to C5b- 9-stimulated expression of membrane-binding sites for factor Va when compared with platelets from normal controls. Expression of catalytic surface for the prothrombinase complex (VaXa) paralleled the exposure of factor Va-binding sites; the rate of prothrombin conversion by C5b-9- treated PNH platelets exceeded that of C5b-9-treated normal controls by approximately 10-fold at the maximal input of C8 tested (500 ng/mL). These data indicate that PNH platelets deficient in plasma membrane CD59 antigen are exquisitely sensitive to C5b-9-induced expression of prothrombinase activity, and suggest that the tendency toward thrombosis in these patients may be due, at least in part, to the deletion of this complement inhibitor from the platelet plasma membrane.

Blood of patients with paroxysmal nocturnal hemoglobinuria (PNH) most often contains two or more populations of erythrocytes--one population with normal sensitivity to lysis by complement (PNH I cells) and a second population of moderately abnormal cells (PNH II cells) or markedly abnormal cells (PNH III cells). PNH II and III cells exhibit moderately and markedly increased sensitivity to lysis by complement, respectively, as well as other membrane defects. We have devised a method for isolating pure, intact PNH II and III cells from mixed populations by use of monoclonal antibodies and cell affinity chromatography. Study of purified cell populations has led to the identification of a further subtype, PNH IIIb, of PNH erythrocytes. PNH IIIb erythrocytes are less sensitive to complement lysis than PNH IIIa cells but are lysed by fluid-phase activation of complement, unlike PNH II erythrocytes.