Stroma from normal, AET-treated and PNH red cells and their KC1-extracts (partially purified on Sephadex G-200) are able to trigger the activation of the alternative complement pathway. This fact has been demonstrated by: 1 - the lysis of PNH cells incubated in serum treated with stroma from normal or PHN-RBC or with their extracts; the addition of Mg2+ or Ca2+ or of their chelators (EDTA, EGTA) to the extract-treated serum enhances or abolishes the lysis 2 - the reduction of complement acitvity in fresh serum incubated for 60' with PNH-extract 3 - the appearance of C3 breakdown products in serum incubated with PNH-extract, demonstrated by crossed immunoelectrophoresis. In contrast, the same stroma (or extract) inhibits the sucrose lysis test, in which the lysis takes place through the classical complement pathway. No differences on the complement activation were observed between PNH and normal RBC stroma and between their chromatographic extracts. These findings may suggest the possible role of diurnal variation of Mg2+ and Ca2+ concentration in precipitating haemolytic attacks and the possibility that small amount of circulating red cell stroma might maintain the haemolysis on PNH RBC.
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