B-lymphocyte-derived burst-promoting activity is a pleiotropic erythroid colony-stimulating factor, E-CSF.

Human B-lymphocyte-derived erythroid burst-promoting activity (B-BPA) is a pleiotropic, lineage-specific regulator of erythropoiesis. Our present data indicate that B-BPA plays an important role as an erythroid colony-stimulating factor (E-CSF) in modulating progenitor growth and differentiation throughout erythropoiesis. E-CSF has discrete effects on both early (erythroid burst-forming units, BFU-E) and late (erythroid colony-forming units, CFU-E) progenitors from normal bone marrow. In serum-substituted fibrin clot cultures, E-CSF stimulates the proliferation of BFU-E, resulting in an increase in the number of erythroid bursts over a wide range of erythropoietin (Epo) concentrations. We now have shown that E-CSF also acts on CFU-E by increasing their sensitivity to Epo markedly, resulting in a tenfold left-shift in the Epo dose-response curve. Using purified target-cell populations of human and murine erythroleukemia cells that are Epo-independent for growth, we have found that E-CSF stimulates cell proliferation directly, increasing the plating efficiency of these cells in suspension culture by 50%-165%. B-BPA also increased proliferation of these cells in semi-solid medium. Importantly, the combination of E-CSF and Epo resulted in a profound increase in the growth and maturation of the resultant colonies. Therefore, the data indicate that E-CSF can regulate the growth of cells independently of added Epo and, in addition, can synergize with Epo in regulating the growth and differentiation of erythroid progenitors.

Four unique monoclonal antibodies to the putative receptor binding domain of erythropoietin inhibit the biological function of the hormone.

We have produced a series of monoclonal antibodies (MoAbs) to amino acid region 99-129 of human erythropoietin (Epo) that distinguish unique structural features within this putative receptor binding domain of the hormone. The MoAbs recognize denatured Epo with widely different sensitivities on a Western blot and differentially bind to native Epo in solution. In addition, three of the four MoAbs neutralize the biological activity of Epo in a concentration-dependent fashion in vitro. Neutralization was measured both by inhibition of Epo-induced differentiation in Rauscher murine erythroleukemia cells and by inhibition of Epo-induced proliferation in normal murine splenic erythroid precursors. Characterization of the structural epitopes recognized by each of these four reagent MoAbs should provide us with important information concerning the requirements for hormone-receptor interaction.