Leukotriene A4 hydrolase (LTA4H) inside neutrophil derived extracellular vesicles breaks down PGP when exposed to neutrophil elastase.

<bold>Rationale:</bold> LTA4H is a bifunctional protease with the ability to either increase (via an epoxide hydrolase site) or decrease (via an aminopeptidase site) neutrophil (PMN) chemotaxis. Neutrophil elastase (NE) has recently been shown to convert LTA4H from its hydrolase to aminopeptidase activity. LTA4H is carried in extracellular vesicles (EVs) released from PMNs, but how the LTA4H in these EVs becomes modified to shift from hydrolase to aminopeptidase activity, and thereby degrade PGP, are unknown. <bold>Methods:</bold> EVs were isolated from these peripheral blood PMNs (either activated with fMLP, or quiescent). Western blots were performed to determine whether LTA4H was inside EVs, or surface associated. Aminopeptidase activity of PMN derived EVs were conducted in the presence of PGP, and PGP breakdown was detected using a fluorescent reporter that binds to only primary amines generated by PGP breakdown. EVs were either kept whole, or lysed with a detergent, to illustrate the location of LTA4H inside the EV. <bold>Results:</bold> LTA4H aminopeptidase activity was higher in EVs from activated PMNs than quiescent. Lysing the EVs resulted in even higher LTA4H activity and PGP breakdown, in an NE dependent manner. <bold>Conclusions:</bold> LTA4H is contained in EVs released from PMNs. Upon EV lysis, the LTA4H comes in contact with surface NE and increases the PGP breakdown. This likely plays an important immunomodulatory role in innate immune biology by focusing PMN chemotaxis, thereby reducing off-target tissue damage caused by PMN influx. Derangements of this LTA4H system by exposure to cigarette smoke or sustained oxidative stress may disrupt this immunomodulatory balance.