Increased sensitivity to complement of megakaryocyte progenitors in paroxysmal nocturnal haemoglobinuria.

Paroxysmal nocturnal haemoglobinuria (PNH) is a haemolytic disorder characterized by an increased sensitivity of peripheral blood cells including platelets to the lytic action of complement (C'). Previous studies have demonstrated that in PNH bone marrow erythroid colony and burst forming units, as well as granulocytic-monocytic colony forming units, have an increased sensitivity to complement-induced injury as compared with normal erythroid and myeloid progenitors. The purpose of this study was to investigate the effect of complement on PNH and normal marrow megakaryocytic progenitors (CFU-M). Bone marrow non-adherent and T-cell depleted light density mononuclear cells from three patients with PNH and six normal volunteers were exposed to fresh or heat-inactivated AB human serum in the presence of medium or isotonic aqueous sucrose solution for 30 min at 37 degrees C. After being washed the cells were assayed for CFU-M by the plasma clot method in a complement free medium containing 6% medium conditioned by T-lymphocytes stimulated by phytohaemagglutinin. The number of megakaryocytic colonies grown from PNH marrow cells exposed to isotonic sucrose and C' was reduced to one third of those grown from PNH cells exposed to isotonic sucrose and heat-inactivated C', or to medium with and without C'. In contrast, the number of megakaryocytic colonies grown from normal marrow cells exposed to isotonic sucrose and C' was unchanged. These findings indicate that PNH marrow CFU-M express an increased sensitivity to C'-mediated injury similar to that detected on PNH-erythrocytes by the sucrose haemolysis test, and support the hypothesis that the PNH defect is expressed at the level of pluripotent haematopoietic stem cell.