The serine protease neutrophil elastase (NE) appears to regulate inflammatory responses at multiple levels but its role in leukocyte transmigration in vivo remains unclear. The present study aimed to address this issue by using both an NE inhibitor (ONO‐5046) and NE deficient (NE^−/−) mice.

A number of inflammatory mediators (LTB₄, KC and PAF) were investigated in vitro for their ability to stimulate the release and the surface expression of NE by neutrophils. In addition, the role of NE in leukocyte migration elicited by topical LTB₄ was investigated in vivo in mouse cremasteric venules as observed by intravital microscopy.

Amongst the mediators tested in vitro, LTB₄ was found to be a highly potent and efficacious inducer of NE cell surface expression on murine neutrophils. Furthermore, in wild‐type mice (WT), LTB₄‐induced leukocyte transmigration was reduced by intravenous ONO‐5046 (66% inhibition), an effect that appeared to occur at the level of the perivascular basement membrane. Interestingly, LTB₄‐induced responses were normal in NE^−/− mice and, while ONO‐5046 had no inhibitory effect in these animals, the broad‐spectrum serine protease inhibitor aprotinin suppressed leukocyte transmigration in both WT and NE^−/− mice.

The findings demonstrate the potent ability of LTB₄ to induce cell‐surface expression of NE and provide evidence for the involvement of NE in LTB₄‐induced neutrophil transmigration in vivo. The results also suggest the existence of compensatory mechanisms in NE^−/− mice, highlighting the added value of investigating pharmacological blockers in parallel with genetic deletion.

British Journal of Pharmacology (2007) 151, 628–637; doi:10.1038/sj.bjp.0707267