REDUCING ERYTHROPOIETIN IN CULTURES OF HUMAN ERYTHROID PRECURSORS ELEVATES THE PROPORTION OF FETAL HEMOGLOBIN

In order to clarify the mechanism of the effect of erythropoietin (Epo) on the fetal haemoglobin (HbF) phenotype of peripheral erythrocytes, we studied the dose-response effect of Epo on HbF production by erythroid precursors derived from the peripheral blood of normal adult individuals and grown in a two-phase liquid culture system. The proportion of HbF out of the total haemoglobin (Hb) content (%HbF) was dependent on the duration of exposure to Epo; on day 6 it comprised up to 15%, but dropped to < 2% on day 14. Both cell yield and cellular Hi,content were markedly increased by high (1 U/ml) Epo, compared to normal physiological (20-50 mU/ml) levels, but neither the initial nor final %HbF were dependent on the increased Epo dose. However, when cells grown with high Epo were transferred on day 7 to low Epo, their progeny contained by day 14 a higher %HbF as compared to cells that were continuously exposed to high Epo. This was accompanied by acceleration and synchronization of their maturation process, as evidenced by their morphology, density and size, and restriction on cell multiplication, as indicated by the lower cell yield. These results are consistent with the following model. As early erythroid precursors, with relatively high HbF, mature under steady-state levels of Epo, HbA production predominates and HbF is diluted. However, when such precursors are switched from high to low levels of Epo they undergo a synchronized, accelerated maturation which shortens the period of HbA production, leading to a decreased Hb content and a relatively high proportion of HbF. This mechanism may contribute to the elevated HbF observed following Epo administration (due to short half-life of Epo in vivo). and might also explain the HbF-augmenting effect of Epo administered together with hydroxyurea observed in patients with sickle cell anaemia.