KB003 effectively neutralizes GM-CSF. (A) Inhibition of GM-CSF–induced IL-8 secretion from U937 cells. KB003 at various concentrations was incubated with U937 cells in the presence of 0.5 ng/mL GM-CSF for 16 hours (a concentration of GM-CSF providing 90% of maximal induction). IL-8 secreted into the culture supernatant was determined by enzyme-linked immunosorbent assay. Results from a representative assay carried out in triplicate are shown. Mean IC₅₀ from 3 independent assays was 48.2 ng/mL. (B) MO7e human cells were cultured with doses of GM-CSF ranging from 0 to 10 ng/mL and increasing doses of KB003. Annexin V was used to measure apoptosis and viability at 48 hours. (C) Three distinct CMML patient aspirates cultured with GM-CSF (10 ng/mL) and increasing doses of KB003. Clusters of cells of >50 were measured. (D) Representative hematopoietic colony with and without KB003. Colonies appear less organized in the drug-treated group. Significant P values are indicated using one-way analysis of variance.