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Erythroid lineage‐specific activity in conditioned medium derived from cloned human marrow stromal cells (CFU‐RF)

Using long‐term culture techniques, it has been shown that stromal cells in the marrow microenvironment are essential for the continued production and self‐renewal of hematopoietic stem cells. We previously reported the development of a methylcellulose colony assay for a population of marrow stromal progenitors called CFU‐RF. In this paper, a method is described for subculturing cells from individual CFU‐RF‐derived colonies to allow conditioned medium production (StCM). StCM, prepared in this way, was found to possess an erythroid lineage‐specific activity that stimulated the formation of macroscopic erythroid colonies in cultures containing erythropoietin (epo). Using dose‐response curves, the KG1 colony assay, and antibody neutralization, it was shown that the activity could not be attributed to interleukin 3 (IL3) or granulocyte‐macrophage colony‐stimulating factor (GM‐CSF). However, it was further shown that a monolayer of stromal cells, which had earlier been producing the erythroid activity, could be stimulated by IL1 to produce granulocytic colony‐stimulating activity, but only as long as IL1 was present in the culture medium. These findings indicate a mechanism whereby the same stromal population could be modulated to promote growth and differentiation of different hematopoietic lineages.