Erythropoietin affects the maturation pattern of fetal G‐CSF‐responsive progenitors

A transient hyporegenerative neutropenia has been reported in neonates, but not in older children or adults, undergoing treatment with recombinant erythropoietin (epo). Monocytopenia has not been reported. We postulated that epo might selectively reduce the responsiveness of neonatal progenitors to Granulocyte Colony‐Stimulating Factor (G‐CSF), while not similarly affecting their responsiveness to Macrophage Colony‐Stimulating Factor (M‐CSF). To test this hypothesis two types of experiments were performed. First, progenitors of adult or fetal origin were pre‐incubated with epo (or control), then washed, and their responsiveness to G‐CSF and M‐CSF evaluated in clonogenic culture assays. Second, clonogenic maturation was initiated using either G‐CSF or M‐CSF, after which the effect of a late addition of epo to the developing clones was evaluated. Indeed, pre‐incubation with epo resulted in production of fewer neutrophils from fetal progenitors grown in G‐CSF (P<0.001), but it did not reduce the number of macrophages generated from progenitors grown in M‐CSF. Adding epo to the already‐developing G‐CSF‐responsive and M‐CSF‐responsive adult and fetal clones did not alter colony development. Thus, epo appears to have an action on G‐CSF‐responsive, but not‐M‐CSF‐responsive fetal progenitors, resulting in reduced production of neutrophils. This effect is no longer apparent, however, when progenitors have matured to the 8‐cell clone stage.