OPN plays an important role in neutrophil recruitment in alcoholic liver disease. OPN is important for the migration of neutrophil in vitro. In addition, OPN recruits inflammatory cells to arthritis joints in the collagen-induced arthritis model of rheumatoid arthritis. A recent in vitro study in 2008 has found that OPN plays a role in mast cell migration. Here OPN knock-out mast cells were cultured and they observed a decreased level of chemotaxis in these cells compared to wildtype mast cells. OPN was also found to act as a macrophage chemotactic factor. In this study, researchers looked at the accumulation of macrophages in the brain of rhesus monkeys and found that OPN prevented macrophages from leaving the accumulation site, indicating an increased level of chemotaxis.