MPO-dependent PMN motility in vitro. PMN motility was evaluated at the steepest gradient of a chemotactic factor or control in Ibidi microslides. (A) MPO provoked directed locomotion of PMN in a concentration-dependent manner (n = 4, one-way ANOVA; P < .0001). (B) Directed PMN motility after application of human serum albumin (HSA, n = 21), MPO (n = 34), and interleukin-8 (IL-8, n = 9) was investigated (one-way ANOVA; P < .0001). (C) Directed motility toward HSA or MPO was evaluated on preincubation with the MPO-inhibitor 4-amino-benzoic acid hydrazide (ABAH; white bars, n = 3-4). (D) MPO-variants Q91T (n = 4) and M243T (n = 9) devoid of catalytic activity also yielded increased directed PMN motility (one-way ANOVA P < .003). Number of independent experiments denoted by n; number of donors of PMN ≥ 3. Bars represent means; error bars, standard error of the mean (SEM). *P < .05, **P < .01, ***P < .001.