Arginase I participates in the fungicidal activity of human PMNs. (A) Arginase I localizes to the phagosome in human PMNs. Exudate neutrophils from skin window chambers after phagocytosis of latex beads were analyzed by immunogold electron microscopy. Cryosections of neutrophils were immunogold labeled with polyclonal rabbit anti–arginase I. Arginase I (arrows) localizes to the phagolysosomes (ph). Bar is 200 nm. Inset shows area of another cell showing a small phagolysosome. (B-C) Inhibition of arginase inhibits the fungicidal activity of human PMNs. Human PMNs were cocultured with an arginine-auxotroph strain of S cerevisiae. The viability of S cerevisiae after phagocytosis was monitored with the XTT assay in 12 independent experiments (B) and in 6 experiments in parallel with the FUN-1 yeast viability kit (C). PMN arginase was inhibited by preincubation with nor-NOHA(200 μM). PMN MPO was inhibited by preincubation with sodium azide (NaN₃; 1 μM). To allow for comparison between individual experiments, the percentage of kill inhibition (compared to the viability of S cerevisiae after phagocytosis without the use of inhibitors) is shown. The mean reduction of PMN fungicidal activity of 12 (B) and 6 (C) separate experiments is shown. The inhibition of fungicidal activity was statistically significant with the indicated P values, analyzed with the 2-tailed Student t test. Horizontal bars indicate means.