Neutralization of GM-CSF by E21R. (A) Response of AML blasts to exogenous GM-CSF and its neutralization by E21R. Primary AML blasts from 13 individuals showing a response to exogenous GM-CSF. Cells were incubated in medium plus GM-CSF at 10 ng/mL for 48 to 72 hours either without or with an at least 1000-fold excess of E21R (10 to 30 μg/mL). MTS assay was carried out, and the results are expressed as a percentage of the activity in control cells incubated in medium alone. (B) Effect of E21R in neutralizing the proliferative response of TF-1 cells to GM-CSF. TF-1 cells were washed and placed in medium alone, GM-CSF (10 ng/mL), IL-3 (10 ng/mL), or erythropoietin (2 U/mL) with the indicated concentrations of E21R for 48 hours. MTS assay was carried out, and the results are expressed as a percentage of the activity in control cells incubated in medium alone. (C) Effect of E21R in neutralizing the proliferative response of normal myeloid cells to GM-CSF. Primary CD34⁺ cells were placed in medium with SCF, IL-3, and G-CSF for 7 days to induce myeloid differentiation. After washing, cells were incubated for 48 hours in medium alone or with GM-CSF (10 ng/mL), IL-3 (10 ng/mL), or G-CSF (50 ng/mL) without or with E21R (10 μg/mL). MTS assay was carried out, and the results are expressed as a percentage of the activity in control cells incubated in medium alone.