CD117 expression is increased by IL-2 stimulation; c-kit tyrosine kinase activity is required for synergy. (A) From a representative experiment of 5, IL-2 stimulation up-regulates c-kit expression in the NK-cell line DERL-7 as opposed to unstimulated cells. (B) Starved DERL-7 cells, pretreated for 2 hours with AG1296, a c-kit–specific tyrosine kinase inhibitor, were stimulated with SCF, IL-2, or SCF + IL-2. Proliferation was measured using an MTS reagent, measuring OD at 490 nm. Data shown are combined from 3 independent experiments, demonstrating mean percentage proliferation (± SEM). AG1296 virtually abolished any SCF-dependent proliferation (left bars, P < .01) and a modest impact was seen on IL-2–mediated proliferation (center bars, P < .01). The main finding (comparison indicated by * in panel B) is that the sum total contribution of SCF to synergistic proliferation was abrogated in the presence of AG1296 (P < .01), as proliferation in response to IL-2 was identical to that of SCF + IL-2 (P = n/s).