PMNs that lack arginase I do not suppress T-cell proliferation. (A-B) PMNs of an 18-year-old man with arginase I deficiency (arginase I^-/-) were sonicated. RPMI 1640 cell culture medium containing 150 μM l-arginine (Arg(+) medium) was incubated at 37°C either alone or with an aliquot of the arginase I^-/- PMN-S. Arginase I^-/- PMN-S was diluted to a final protein concentration (160 μg/mL) that corresponds to an arginase activity of 200 to 300 mU/mL in the PMN-S of 3 healthy donors (arginase I^+/+). (A) After 12 hours, the concentrations of arginine, ornithine, and urea were measured by ion exchange chromatography. The concentration of each substance in Arg(+) medium was used as control and set as 100%. The relative concentration (in percentage of control) after incubation with arginase I^-/- PMN-S is shown. (B) T-cell proliferation assays were set up and analyzed as described in Figure 1. Purified human T cells were stimulated with platebound crosslinked anti-CD3 antibody (500 ng/mL), and proliferation was assessed by [³H]thymidine incorporation after 48 hours in triplicate wells. Representative data from 1 of 3 independent experiments are shown.