Study of ¹²⁵I–IL-3 internalization into leukemic blasts and erythroleukemic TF-1 cells.

Cells were first incubated for 120 minutes at 4°C in the presence of 20 ng/mL ¹²⁵I–IL-3, washed, and incubated at 37°C in RPMI 1640 medium. At regular time intervals (from 0 to 60 minutes), aliquots of cell suspensions were harvested and centrifuged; the cell pellet was then resuspended in an acid solution and centrifuged again. Finally, after centrifugation, the radioactivity in cell supernatant and pellet was determined. The radioactivity in the supernatant represents cell-surface–associated IL-3, whereas the radioactivity in the cell pellet represents internalized IL-3.