Abnormal expression of myeloperoxidase in neutrophils of SCN patients during treatment with G-CSF. (A) Immunocytochemical staining of MPO in circulating neutrophils from SCN patients. PMNs were obtained from peripheral blood, cytocentrifugated, and stained with anti-MPO antibody; nuclei were counterstained with Mayer hematoxylin. In cells from a control subject (CTR), a strong granular expression of MPO is observed in the cytoplasm. In contrast, MPO is detectable in a minority of cells obtained from patient 3 (P3) and undetectable in cells from patient 1 (P1). (B) The Western blot shown in Figure 1B has been reprobed with specific antimyeloperoxidase antibody. The 3 myeloperoxidase bands are indicated by arrows. (C) MPO chlorination activity in neutrophil cell lysates (3 dilutions of each sample) from a healthy subject (CTR) and patient 2 (P2) was analyzed by EnzChek Myeloperoxidase (MPO) Activity Assay Kit (“Patients, materials, and methods”). Results are expressed as relative fluorescence and are shown as mean values ± SD of 3 experiments. Statistical analysis demonstrates that the chlorination activity in neutrophil cell lysates is lower in SCN patients compared with healthy subjects (P < .05).