Human erythroid-potentiating activity (EPA) is a 28,000 mol wt glycoprotein that stimulates the growth of erythroid progenitors in vitro and enhances colony formation by the K562 human erythroleukemia cell line. EPA has potent protease inhibitory activity, and is also referred to as tissue inhibitor of metalloproteinases (TIMP). We observed that colony formation by K562 cells in semi-solid medium containing reduced fetal calf serum (FCS) is not directly proportional to the number of cells plated, suggesting production of autostimulatory factors by K562 cells. Using radioimmunoprecipitation and a bioassay for EPA, medium conditioned by K562 cells was found to contain high levels of biologically active EPA; Northern hybridization analysis confirmed the expression of EPA mRNA. Radiolabeled EPA was used to identify cell surface receptors on K562 cells. Together, these results suggest that EPA may act as an autocrine growth factor for K562 cells.

To examine the biologic properties of the molecule encoded by the human gene for granulocyte-macrophage colony-stimulating factor (GM-CSF), we expressed the cloned complementary DNA (cDNA) in transfected monkey COS cells and purified the resultant protein. Purified biosynthetic human GM-CSF was added to cultures of normal hematopoietic progenitor cells in semisolid media, and the resulting colonies were characterized cytochemically. Non-adherent light-density bone marrow cells from healthy adult volunteers were maximally stimulated with GM-CSF (approximately 250 pmol/L, and four types of colonies were consistently identified by aspirating the individual colonies and staining with a triple stain for specific and nonspecific esterases and eosinophilic granules. Pure neutrophilic granulocyte (G), mixed granulocyte- macrophage (GM), pure macrophage (M), and pure eosinophil (EO) colonies were observed, the mean incidences on day 8 being 70%, 20%, 5%, and 5%, and on day 14, 7.5%, 16.6%, 50.9%, and 25.0%, respectively. In all types of colonies, complete maturation to segmented forms or typical macrophages was detected. GM-CSF did not enhance the growth of BFU-E from normal peripheral blood buffy coat cells in the simultaneous presence of erythropoietin alone or erythropoietin with purified erythroid-potentiating activity. GM-CSF stimulated HL-60 and KG-1 colony formation twofold and fivefold, respectively; consistent differentiation induction towards monocytic and eosinophilic lineages was observed in HL-60 but not in KG-1. These in vitro findings indicate that GM-CSF is a multilineage stimulator for progenitor cells of G, GM, M, and EO colonies.

Recent observations indicating that the HL-60 human acute promyelocytic leukemia cell line contains a minor eosinophil population in addition to neutrophil and mononuclear phagocyte progenitors suggest the multipotentiality of HL-60 stem cells. In order to clarify multilineage differentiation and commitment to single-lineage progenitors we analyzed HL-60 colonies formed in methylcellulose. In an HL-60 parent line with a relatively high eosinophil content (5.5%), 36% of the spontaneous colonies consisted partly or wholly of eosinophilic cells. After two rounds of subcloning in methylcellulose, two eosinophilic sublines and two neutrophilic sublines were established. These lines have been in continuous liquid culture for more than four months, and they show stable single-lineage differentiation. Purified biosynthetic GM-CSF, which stimulates normal CFU-GM and CFU-EO, induced monocytic differentiation but no eosinophilic differentiation in the neutrophilic sublines and no neutrophilic or monocytic differentiation in the eosinophilic sublines. These observations indicate that HL-60 stem cells are multipotent and capable of spontaneous commitment to single- lineage progenitors. The eosinophilic HL-60 sublines should facilitate studies on the production and function of human eosinophils and the single-lineage sublines will allow further analysis of leukemic cell differentiation and stem cell commitment.