LINEAGE-RESTRICTED RECRUITMENT OF IMMATURE HEMATOPOIETIC PROGENITOR CELLS IN RESPONSE TO EPO AFTER NORMAL HEMATOPOIETIC-CELL TRANSFECTION WITH EPOR

The receptor for erythropoietin (EpoR) is normally restricted in its e xpression to the relatively mature cells of the erythroid and megakary ocytic lineages. Using retrovirus-mediated gene transfer, the wild-typ e EpoR and a constitutively activated mutant of the EpoR, EpoR(R129C), were expressed in primary hematopoietic cells. Retroviral infection o f day-12 murine fetal liver, followed by stimulation with Epo as a sin gle stimulus, generated day-8 erythroid colonies resembling colonies d erived from burst-forming units-erythroid (BFU-E). Similarly, murine p ost-5 fluorouracil (5-FU) bone marrow cells or fetal liver cells, indu ced to express EpoR and stimulated by Epo, displayed a significant enh ancement of megakaryocyte colony formation, particularly of the BFU-me gakaryocyte (BFU-Mk) colony type. Cultures of bone marrow cells transd uced with the EpoR retrovirus and stimulated by Epo contained macropha ge colonies but very few granulocyte colonies. Experiments to culture single clones demonstrated direct action of Epo on megakaryocyte and m acrophage clones but failed to demonstrate a direct action on granuloc yte precursors. A similar pattern of lineage-restricted effects was de monstrated in unstimulated cultures of cells infected with the EpoR(R1 29C) retrovirus. In summary, we have demonstrated Epo-induced recruitm ent of immature erythroid and megakaryocyte precursors induced to expr ess the EpoR. Furthermore, we have also demonstrated lineage-restricte d cell proliferation in response to Epo by normal myeloid hematopoieti c cells transduced with the EpoR.