Human PMN arginase I is liberated during PMN-dominated inflammation in vivo and suppresses T-cell proliferation. (A) Human pus derived from a parastomal skin abscess of a patient with Crohn disease was centrifuged 3 times and filtered through a 0.2 μm filter to remove cells and prepare cellfree supernatant (PUS-SN). Arginase I protein expression was analyzed by immunoblotting in PUS-SN and (as positive and negative controls) in PMN lysates of a healthy (Arg I^+/+) or arginase I-deficient (Arg I^-/-) blood donor or in normal human liver. The results of parallel determinations of arginase activities (in milliunits per milligram of protein) in aliquots of the same cell lysates are noted above the respective lanes of the immunoblot. (B) Human PUS-SN (arginase activity 31 380 mU/mL) was diluted to a final arginase activity of 300 mU/mL in Arg(+) medium (Figure 1), and the medium was incubated for 12 hours. When indicated, nor-NOHA (1 mM) was added from the start of the incubation to inhibit arginase activity within PUS-SN. Human T cells were then activated by crosslinked anti-CD3 in the respective media, and proliferation was assessed after 48 hours by [³H]thymidine incorporation. When indicated, l-arginine (1 mM) was supplemented at the start of the T-cell activation.