Imatinib mesylate inhibits proliferation of primary human T cells and T-cell lines. (A) Proliferation of 1.5 × 107 T cells/2.5 mL was stimulated with allogeneic mature DCs (○) or PHA (•) and measured as a function of imatinib mesylate concentration. The cells were incubated with imatinib mesylate for 96 hours. Then tritiated thymidine was added for 12 hours when the cells were harvested and incorporated radioactivity was measured. Differences between positive controls and imatinib mesylate–treated cells were apparent at 0.5 μM for the cells stimulated with DCs (P = .002) and at 1.0 μM for cells stimulated with PHA (P = .00004). (B) Human T-cell lines CCRF-CEM (○) and Jurkat (□) and CML cell line K562 (⋄) were incubated in graded concentrations of imatinib mesylate for 18 hours and evaluated as in panel A. The results are expressed as stimulation indices (mean value ± SD of radioactivity incorporated in the presence of the drug divided by the mean value ± SD of radioactivity incorporated in the absence of the drug).