Milk fat globule-epidermal growth factor (EGF)-factor VIII (MFG-E8) is a secreted glycoprotein that promotes clearance of apoptotic cells by bridging phosphatidylserine on apoptotic cells and integrin avb3/5 on phagocytes. High expression of MFG-E8 has been reported in various types of cancer in humans. Apoptotic figures are frequently found in the surgical samples of oral squamous cell carcinoma (SCC) and carcinoma in situ, and we have often observed apoptotic carcinoma cells engulfed by macrophages or even by neighboring carcinoma cells. Thus we hypothesized that MFG-E8 might promote engulfment of apoptotic carcinoma cells by living carcinoma cells and that MFG-E8 expressed by carcinoma cells could contribute to tumor progression. The aim of this study was to elucidate the biological role of MFG-E8 in oral SCC. Fifty-three surgical specimens of oral SCC were used for immunohistochemistry for MFG-E8, and the expression profiles were correlated with clinicopathological properties. Also, we examined the MFG-E8 expression patterns and functions using three human oral SCC cell lines. Most of the cases had MFG-E8-positive SCC cells, and the expression of MFG-E8 was correlated with such clinicopathological features as tumor size, pathological stage, locoregional recurrence, scattering invasion pattern, and SCC cell figures engulfing apoptotic SCC cells. The MFG-E8 staining was enhanced in apoptotic SCC cells, some of which were apparently engulfed by the neighboring SCC cells. ZK-1 cells showed high MFG-E8 expression, and its localization was found in the cytoplasm and the cell surface. Transient MFG-E8 knockdown by siRNA in ZK-1 decreased cell proliferation and invasiveness and increased cell death. Thus we have demonstrated that MFG-E8 promotes tumor progression in oral SCC and that it might be involved in the clearance of apoptotic SCC cells by living SCC cells. Milk fat globule-epidermal growth factor (EGF)-factor VIII (MFG-E8), a secreted glycoprotein also termed lactadherin, was identified initially as a marker of human breast cancer 1 and later as a major component of milk fat globule membranes in the murine lactating mammary gland. 2 MFG-E8 is a multifunctional protein that has key roles in apoptotic cell clearance, 3 cellular adhesion between sperm and oocyte, 4 morphogenetic and homeostatic regulation in diverse tissues, 5-7 and angiogenesis. 8 Human MFG-E8 contains two repeats of an EGF-like domain on the N-terminal side and two repeated domains homologous to blood coagulation factor V/VIII on the C-terminal side. 9-11 On the removal of apoptotic cells, MFG-E8 secreted by phagocytes binds to phosphatidylserine on the apoptotic cell surface via the factor V/VIII-like domains, as well as to integrin avb3/5 on the plasma membrane of phagocytes via its RGD sequence within the EGF-like domain. 3 Impaired MFG-E8-mediated uptake of apoptotic cells results in autoimmune diseases in MFG-E8-deficient mice, 12 indicating that it is related to immune tolerance induction. 13 Apoptotic cells are cleared in MFG-E8-depend...