Arginine metabolism regulates the suppressive activity of AML blasts. (A) AML blasts from 15 patients were cultured with allogeneic T cells in a MLR. The ratio of AML cells:T cells ranged from 1:1 to 1:0. T-cell proliferation was measured by ³H-thymidine incorporation after 4 days. AML blasts from 12/15 patients strongly suppressed T-cell proliferation. Patients are identified by unique symbols, which are used consistently throughout the manuscript. (B) Expression of arginase II in blasts from patients with AML was confirmed by confocal microscopy. (Left) Staining with DAPI (nuclear stain) and secondary antibody alone; (right) staining with DAPI, anti-human arginase II antibody, and secondary antibody. Scale bar = 10 μm. (C) AML blasts release arginase II into the microenvironment. Supernatants from cultures (24 hours) of patients’ AML blasts were analyzed by ELISA for arginase II and arginase I (***P = .0001). (D) AML blasts from patients were cultured with allogeneic T cells in a MLR in the presence of the enzyme-specific inhibitors for arginase (NOHA) and iNOS (L-NMMA). The ratio of AML cells:T cells ranged from 1:1 to 1:8. T-cell proliferation was measured by ³H-thymidine incorporation after 4 days. Culture with the enzyme inhibitors restores T-cell proliferation. Data are representative of 5 independent experiments (error bars, SD).