The apoptosis clearance signal phosphatidylserine inhibits leukocyte migration and promotes inflammation resolution in vivo.

Previous studies have shown that phagocytosis of apoptotic cells can tune the macrophage phenotype and trigger the resolution of inflammation. This mechanism is largely dependent on the recognition of phosphatidylserine (PS) residues on the outer membrane of dying cells. Therefore, we sought to assess the effects of PS-containing liposomes (mimics of apoptotic cells) on the leukocyte migration profile during the inflammatory process in vivo. Inflammation was induced by carrageenan injection into air pouches created on the dorsal region of mice, as this model enables convenient access to the exudates for further investigation. Mice were treated with PBS, PS-containing or phosphatidylcholine (PC)-containing liposomes (10, 30 or 100 mg/kg intraperitoneally [i.p.]). Starting 8 h after carrageenan injection, the level of leukocyte infiltration was monitored over three days. The PS-containing, but not PC-containing, liposomes reduced the polymorphonuclear (PMN) and mononuclear (MN) leukocyte influx into the inflamed pouches in a dose-dependent fashion. Most notably, these effects could also be adoptively transferred; that is, they were also found in mice injected with a liposome-free peritoneal lavage obtained from the mice that had received the intraperitoneal PS-liposome treatment. The effect of treatment with the PS-induced soluble mediators (PS-ISMs) was found to be dependent on the presence of peritoneal macrophages and was susceptible to heat, trypsin degradation, and cycloheximide treatment. The PS-containing liposomes promoted the reduction of PMN leukocyte influx by triggering the release of anti-inflammatory autacoids with a proteinaceous nature that were produced de novo after PS exposure.