The American journal of pathology
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The purpose of this study was twofold: to reveal cellular events associated with the protective role of endogenous annexin A1 (AnxA1) in inflammation and to highlight the potential involvement of members of the formyl peptide receptor (Fpr) family in this process. We found that wild-type, AnxA1-null, and Fpr1-null mice all displayed an intense neutrophil recruitment into the peritoneal cavity as assessed 4 hours after carrageenin injection, and that this recruitment was most pronounced in AnxA1-null mice. In addition, this cell influx could be inhibited by the AnxA1 pharmacophore peptide, Ac2-26, in wild-type, AnxA1-null, and Fpr1-null mice, but was restored when co-treated with the pan-receptor antagonist Boc2. ⋯ We report, for the first time, the ultrastructural immunocytochemical co-localization of Fpr2 with AnxA1 in neutrophils that migrate into the mesenteric microcirculation and extravasate into the peritoneal fluid. Collectively, these data provide in vivo support to the hypothesis that endogenous AnxA1 is an essential effector of endogenous anti-inflammation and provide an ultrastructural indication that this mediator interacts with Fpr2 in murine neutrophils. We believe that these findings could significantly affect the development of novel therapeutics, which are modeled after the anti-migratory actions of AnxA1.