Annexin 1 is responsible for the anti-inflammatory effects of murine PMN–derived microparticles. Bone marrow–derived PMNs were prepared from wild-type and AnxA1^−/− mice, hence microparticles were produced with a protocol very similar to the one used to generated human ones (“Human PMN microparticle preparation”). (A) Validation of murine microparticles by flow cytometry: staining with anti-CD62L or for annexin V binding is shown for both wild-type and AnxA1^−/− microparticles. (B) Two doses of mouse PMN–derived microparticles were injected intravenously immediately before the local injection of mouse IL-1β into 6-day-old air pouches. Air pouches were washed 4 hours later, quantifying the extent of Gr1⁺ cell migration. Data are mean plus or minus SEM of 5 mice per group. *P < .05 versus vehicle group.