AML blasts extend their suppressive microenvironment through high concentrations of active arginase II in patient plasma. (A) Arginase activity from plasma of 15 patients with AML and 15 healthy donors was analyzed (***P = .0001). Fifty microliters of patient plasma was tested for the ability to convert arginine into urea, using a colorimetric assay. (B) Plasma (50 μL) from 17 patients with AML and 21 healthy donors was analyzed for arginase II concentration by ELISA (***P = .0001). (C) T-cell proliferation of alloreactive T cells stimulated by allogeneic DC in a total volume of 200 μL, with 50 μL of plasma from patients with AML collected at time of diagnosis or from healthy donors (***P = .0001). (D) T cells from healthy donors were cultured in a MLR in the presence of plasma from patients with AML and the enzyme-specific inhibitors for arginase (NOHA) and iNOS (L-NMMA) or arginine. T-cell proliferation was measured by 3H-thymidine incorporation after 4 days. Culture with the enzyme inhibitors or arginine replacement restores T-cell proliferation. Data are representative of 5 independent experiments (error bars, SD).
