Molecular mechanisms of spontaneous and directed mast cell motility.

Migration is a fundamental function of immune cells, and a role for Ca(2+) in immune cell migration has been an interest of scientific investigations for many decades. Mast cells are the major effector cells in IgE-mediated immune responses, and cross-linking of IgE-FcεRI complexes at the mast cell surface by antigen activates a signaling cascade that causes mast cell activation, resulting in Ca(2+) mobilization and granule exocytosis. These cells are known to accumulate at sites of inflammation in response to parasite and bacterial infections. Using real-time imaging, we monitored chemotactic migration of RBL and rat BMMCs in response to a gradient of soluble multivalent antigen. Here, we show that Ca(2+) influx via Orai1 plays an important role in regulating spontaneous motility and directional migration of mast cells toward antigen via IgER complexes. Inhibition of Ca(2+) influx or knockdown of the Ca(2+) entry channel protein Orai1 by shRNA causes inhibition of both of these processes. In addition, a mutant Syk- shows impaired spontaneous motility and chemotaxis toward antigen that is rescued by expression of Syk. Our findings identify a novel Ca(2+) influx-mediated, Orai1-dependent mechanism for mast cell migration.