sCD177 (sCD177/PR3) is present in human plasma. (A) The percentage of CD177-positive neutrophils in healthy donors was assessed by using flow cytometry, and the amount of sCD177/PR3 in serum was measured by using ELISA. There is a tendency toward higher amounts of sCD177/PR3 in serum from individuals with a high proportion of CD177-positive neutrophils. **P < .005 (B) Analysis of affinity-purified sCD177/PR3 by silver staining (left panel) and immune blotting (right panel). nCD177, rCD177, “native” PR3 isolated from neutrophils (nPR3), and rPR3 were used for comparison. Blotted proteins were stained with anti-CD177 (human IgG) or anti-PR3 (PR3D1) mAbs. Bound antibodies were visualized by using a chemiluminescence system.