Abstract
Endogenous protective pathways mitigate the overshooting of inflammation after sterile or infectious injury. Here we report that formyl peptide receptor 2 (Fpr2/3) null mice display a major phenotype with exacerbated vascular inflammation observed postischemia reperfusion (IR) injury of the mesenteric artery, characterized by marked neutrophil adhesion and extravasation as visualized by intravital microscopy. Analysis of endogenous agonists for Fpr2/3 revealed that lipoxin A4 (LXA4) was generated by platelet/neutrophil aggregates during ischemia: this cellular response was attenuated in Fpr2/3(-/-) mice; hence, LXA4 levels were lower after 30 minutes' ischemia, and associated with augmented vascular inflammation in the reperfusion (45-180 minutes) phase. Exogenous delivery of LXA4 attenuated IR-mediated inflammation in Fpr2/3(+/+) but not Fpr2/3(-/-) mice; conversely, an Fpr2/3 antagonist skewed the vascular phenotype of Fpr2/3(+/+) mice to that of Fpr2/3(-/-) animals. Such LXA4-based circuit could be activated by aspirin (30-100 mg/kg), which triggered formation of 15-epi-LXA4 in wild-type mice, yet it was effective in Fpr2/3(-/-) mice. In summary, we propose that during ischemia, neutrophil Fpr2/3 controls platelet/neutrophil aggregates with the rapid generation of circulating LXA4, which in turn modulates downstream vascular inflammatory responses evident during the reperfusion phase.