MPO from human neutrophils reduces the activation of monocyte-derived DCs. Monocyte-derived DCs were cultured (24 hours) with LPS (1 μg/mL), H₂O₂ (100 μM), and (A) with or without SN (60% v/v) from human neutrophils degranulated with GM-CSF/fMLP in the presence or absence of ABAH or catalase or (B) with or without purified enzymatically active human MPO (5 μg/mL). Control wells in panel A contained DCs cultured with LPS/H₂O₂ and ABAH or catalase alone. DC activation was assessed by measuring the expression of HLA-DR and CD86 (flow cytometry) and production of IL-12 (ELISA). Data are representative of 3 independent experiments. All groups contained 4 replicates. *P < .05, **P < .01, ***P < .001. SN, supernatant.