In order to quantitate early erythroid progenitor cells in paroxysmal nocturnal hemoglobinuria (PNH), we have cultured peripheral blood mononuclear cells from 7 PNH patients in a 0.8% methylcellulose medium containing erythropoietin, 2 U/ml. In our experimental conditions, the number of erythroid colonies obtained per 5 X 10(5) mononuclear cells plated was 20.1 +/- 1.9 (SEM) in normal subjects and 2.8 +/- 0.56 (SEM) in PNH patients. In plates from PNH subjects, 38 of 117 showed no growth of erythroid colonies, whereas plates from normal subjects always had colonies. Our findings suggest that PNH patients, despite their hemolytic condition, have a depleted erythroid precursor compartment, and this may play a major role in the pathogenesis of their anemia.

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