Localization of arginase I in human resting PMNs analyzed by immunogold electron microscopy. (A) Cryosections of PMNs were immunogold labeled with polyclonal rabbit anti–arginase I antibody (10 nm gold). Arginase I localized to the matrix of granules (arrows). (B) Arginase I colocalizes with MPO. To characterize the arginase I⁺ granules, PMNs were double-labeled with anti–arginase I (10 nm gold) and anti-MPO or anti-Lf (both 15 nm gold), respectively. After double-labeling for MPO, almost all arginase I⁺ granules were also MPO⁺ (arrows), whereas in most granules there was no colocalization of arginase I (arrowheads) and lactoferrin (thin arrow). (C) Localization of arginase I in human eosinophils. In the same cryosections as above, some eosinophils were present. Arginase I is again found in the matrix of granules with (c) or without crystalloid (arrows). Bars are 200 nm.