Granulocyte-macrophage colony-stimulating factor reactivates fetal hemoglobin synthesis in erythroblast clones from normal adults

Reactivation of fetal hemoglobin (HbF, α₂γ₂) synthesis was previously reported in normal human adult erythroblast colonies ('bursts') generated by erythroid progenitors (BFU-E) in fetal calf serum-supplemented (FCS⁺) semisolid cultures stimulated with erythropoietin (Ep). Our studies focused on the reactivation of HbF synthesis in normal adult erythroid bursts generated by peripheral blood mononuclear cells (PBMCs) seeded in FCS⁺ methylcellulose culture. Reactivation is almost totally suppressed when (a) PBMCs are grown in optimized FCS^- culture, or (b) PBMCs are first stringently depleted of monocytes and then plated in FCS⁺ medium (ie, BFU-E growth in FCS⁺Mo^- culture). In both experimental conditions, the proliferation of lymphocytes and macrophages interspersed among colonies is drastically reduced, and the cloning efficiency of granulocyte-macrophage (GM) progenitors is sharply diminished. In either case, addition of biosynthetic GM colony-stimulating factor (GM-CSF) induces a dose-related increase of HbF synthesis up to the level in FCS⁺ culture, with even more elevated values on delayed addition of Ep. A dose-related increase was also observed in erythroblast clones generated by highly purified BFU-E. These results suggest that reactivation of HbF synthesis in normal adults is at least in part mediated by GM-CSF. Furthermore, they imply intriguing hypotheses on the mechanism(s) of perinatal Hb switching. Finally, they raise the possibility of reactivation of HbF synthesis in β-thalassemia and sickle cell anemia by GM-CSF therapy.