Imatinib mesylate can terminate proliferation signals already initiated, but its effects on T cells are reversible. (A) Imatinib mesylate (10 μM) was added 4 to 48 hours after PHA stimulation. DNA synthesis was quantified as described in Figure 1. Values measured in the presence of imatinib mesylate at all time points were different from values in imatinib mesylate–free cells (P < .05). (B) First, the cells were treated with imatinib mesylate, one group stimulated by PHA and the other unstimulated. The cells were incubated for 24 hours, washed free of imatinib mesylate, and replated without it. Subsequently, one half of each group was stimulated with PHA and the other half remained unstimulated. After incubation for an additional 96 hours, DNA synthesis was measured as described in Figure 1. Cells treated with imatinib mesylate and without PHA in the first incubation did not synthesize DNA without PHA in the second, whereas those treated with PHA in the first incubation did resume proliferation without stimulation in the second. However, in the presence of PHA in the second incubation the cells proliferated similarly (P = .22) irrespective of whether pretreatment with imatinib mesylate occurred in the absence or presence of PHA. Both panels display mean values ± SD.