Externalized phosphatidylinositides on apoptotic cells are eat-me signals recognized by CD14

Apoptotic cells are rapidly engulfed and removed by phagocytes after displaying cell surface eat-me signals. Among many phospholipids, only phosphatidylserine (PS) is known to act as an eat-me signal on apoptotic cells. Using unbiased proteomics, we identified externalized phosphatidylinositides (PIPs) as apoptotic eat-me signals recognized by CD14(+) phagocytes. Exofacial PIPs on the surfaces of early and late-apoptotic cells were observed in patches and blebs using anti-PI(3,4,5)P(3) antibody, AKT- and PLCδ PH-domains, and CD14 protein. Phagocytosis of apoptotic cells was blocked either by masking exofacial PIPs or by CD14 knockout in phagocytes. We further confirmed that exofacial PIP(+) thymocytes increased dramatically after in vivo irradiation and that exofacial PIP(+) cells represented more significant populations in tissues of Cd14(−/−) than WT mice, especially after induction of apoptosis. Our findings reveal exofacial PIPs to be previously unknown cell death signals recognized by CD14(+) phagocytes.