Relative increase of granulocytes with a paroxysmal nocturnal haemoglobinuria phenotype in aplastic anaemia patients: The high prevalence at diagnosis

To clarify the pathologic significance of granulocytes exhibiting the paroxysmal nocturnal haemoglobinuria (PNH) phenotype in patients with aplastic anaemia (AA), we examined peripheral blood from 100 patients with AA for the presence of granulocytes deficient in glycosylphosphatidylinositol (GPI)-anchored proteins using a sensitive flow cytometric assay. A significant increase in the frequency of CD55^-CD59^-CD11b⁺ granulocytes (>0.003%) compared to normal individuals was observed in 31 of 35 (88.6%) patients with untreated AA at diagnosis. The proportions of patients showing increased PNH granulocytes in treated AA patients with a short (<5 yr) and long (>5 yr) disease duration were 68.6% (11/16) and 20.4% (10/49), respectively. When 19 patients showing increased frequency of PNH granulocytes before therapy were studied 6-12 months after antithymocyte globulin plus cyclosporin A therapy, the frequency decreased to 0.01-90% of pretreatment values in 15 recovering patients. These findings suggest that a relative increase in the number of PNH granulocytes is a common feature of AA at diagnosis, and that it may represent the presence of immunologic pressure to normal haematopoietic stem cells as a cause of AA.