Transitional change of colony stimulating factor requirements for erythroid progenitors

Abstract

The course of the differentiation and proliferation of the human erythroid burst‐forming units (BFU‐E) to colony‐forming units (CFU‐E) was directly investigated using a combination of highly purified BFU‐E, a liquid culture system, and the following clonal assay. Highly purified human blood BFU‐E with a purity of 45–79% were cultured in liquid medium with recombinant human erythropoietin (rEP) and recombinant human interleukin‐3 (rlL‐3) to generate more differentiated erythroid progenitors. The cultured cells were collected daily for investigating the morphology, the increment in the number of cells and the clonality. Ninety percent of purified BFU‐E required not only rEP but also rlL‐3 for clonal development. By 7 days of liquid culture, the total cell number increased 237 ± 20‐fold above the starting cells, while erythroid progenitors increased 156 ± 74‐fold. As the incubation time in liquid culture increased, the cells continuously differentiated in morphology. Replating experiments with rTEP combined with or without rlL‐3 showed the following: (1) The number of erythroblasts that were part of erythroid colonies decreased with accompanying erythroid progenitor differentiation and proliferation. (2) As the incubation time in liquid culture increased, erythroid progenitors had a graded loss of their dependency on rlL‐3 and a complete loss of dependency was observed after 3 days of liquid culture. At that time 85% of the erythroid progenitors gave rise to colonies of more than 100 erythroblasts which were equivalent to mature BFU‐E. These studies provide a quantitative assessment of the loss of lL‐3 dependency by BFU‐E and indicate that the size of the generated erythroid colonies and their lL‐3 requirement correlate with the erythroid differentiated state.