Measles is one of the most contagious human infectious diseases and remains a major cause of childhood morbidity and mortality worldwide. The signaling lymphocyte activation molecule (SLAM), also called CD150, is a cellular receptor for measles virus (MV), presumably accounting for its tropism for immune cells and its immunosuppressive properties. On the other hand, pathological studies have shown that MV also infects epithelial cells at a later stage of infection, although its mechanism has so far been unknown. In this study, we show that wild-type MV can infect and produce syncytia in human polarized epithelial cell lines independently of SLAM and CD46 (a receptor for the vaccine strains of MV). Progeny viral particles are released exclusively from the apical surface of these polarized epithelial cell lines. We have also identified amino acid residues on the MV attachment protein that are likely to interact with a putative receptor on epithelial cells. All of these residues have aromatic side chains and may form a receptor-binding pocket located in a different position from the putative SLAM-and CD46-binding sites on the MV attachment protein. Thus, our results indicate that MV has an intrinsic ability to infect both polarized epithelial and immune cells by using distinctive receptorbinding sites on the attachment protein corresponding to each of their respective receptors. The ability of MV to infect polarized epithelial cells and its exclusive release from the apical surface may facilitate its efficient transmission via aerosol droplets, resulting in its highly contagious nature.Measles remains a major cause of childhood morbidity and mortality worldwide despite the availability of efficacious vaccines. Measles virus (MV), an enveloped RNA virus belonging to the genus Morbillivirus in the family Paramyxoviridae, is transmitted via aerosol droplets and considered to be one of the most contagious human pathogens. MV has two envelope glycoproteins, the hemagglutinin (H) and fusion (F) proteins, which are responsible for receptor binding and membrane fusion, respectively (18). MV enters a cell by membrane fusion at the cell surface. The attachment of the H protein to a cellular receptor is believed to induce the conformational change of the H protein, as well as that of the F protein, which promotes the fusion of the viral envelope with the host cell membrane. MV also causes cell-cell fusion in susceptible cells. The signaling lymphocyte activation molecule (SLAM), also known as CD150, has been identified as a receptor for MV (10,21,47). SLAM is expressed on immune cells, such as activated lymphocytes, mature dendritic cells, and macrophages, providing a good explanation for the lymphotropism and immunosuppressive nature of MV (4, 51). Although CD46, a ubiquitously expressed complement regulatory molecule, functions as a receptor for the vaccine strains of MV (8, 30), a great majority of viruses circulating in measles patients use SLAM, but not CD46, as a receptor (35,51). A recent study of MV infection in macaq...