Mechanisms by which MPO inhibits DC activation. (A) BM-derived murine DCs were cultured (18 hours) with LPS (1 μg/mL), H₂O₂ (100 μM), and purified mouse MPO (0.125 μg/mL) in the presence of Cl alone, Cl + Br, or Cl + SCN. Percentage inhibition of DC CD86 (Ai; flow cytometry) and IL-12 expression (Aii; ELISA) by MPO was assessed. (Bi) CD86 expression by DCs cultured with LPS/H₂O₂ in the presence of Cl alone or Cl + SNP (NO donor). (Bii) DCs were cultured with LPS/H₂O₂ and mouse MPO (0.125 μg/mL) in the presence of Cl alone or Cl + SNP. Percentage inhibition of DC CD86 expression by MPO is shown. (Biii) IL-12 production by DCs cultured with LPS/H₂O₂ and with or without mouse MPO (0.125 μg/mL) in the presence of Cl alone or Cl + SNP. (C) CD86 and IL-12 expression by DCs cultured (18 hours) with LPS (1 μg/mL) and increasing concentrations of H₂O₂. (D) CD86 and IL-12 expression by DCs cultured with LPS/H₂O₂ and with or without mouse MPO (0.125 μg/mL) in the presence of rat IgG or ligating/activating anti-Mac-1 antibody (5C6). Data are representative of 2-3 independent experiments. All groups contained 4 replicates. Dotted line represents unstimulated DCs. *P < .05, **P < .01, ***P < .001.