At 24 h postdelivery, a z-stack of confocal images (Fig. support membranes did not transfer computer virus. In contrast, infected MDMs and DCs LRP8 antibody applied to the basolateral surface of HAE produced on large-pore (3.0-m) membranes successfully transferred MeV. Confocal microscopy exhibited that MDMs and DCs are capable of penetrating large-pore membranes but not small-pore membranes. Further, by using a nectin-4 blocking antibody or recombinant MeV unable to enter cells through nectin-4, we exhibited formally that transfer from immune cells to HAE occurs in a nectin-4-dependent manner. Thus, both infected MDMs and DCs rely on cell-to-cell contacts and nectin-4 to efficiently deliver MeV to the basolateral surface of HAE. IMPORTANCE Measles computer virus spreads rapidly and efficiently in human airway epithelial cells. This quick spread is based on cell-to-cell contact rather than on particle release and reentry. Here we posit that MeV transfer from infected immune cells to epithelial cells also occurs by cell-to-cell contact rather than through cell-free particles. In addition, we sought to determine which immune cells transfer MeV infectivity to the human airway epithelium. Our studies are based on two types of human main cells: (i) myeloid cells generated from donated 2′,5-Difluoro-2′-deoxycytidine blood and (ii) well-differentiated airway epithelial cells derived from donor lungs. We show that different types of myeloid cells, i.e., monocyte-derived macrophages and dendritic cells, transfer contamination to airway epithelial cells. Furthermore, cell-to-cell contact is an important component of successful MeV transfer. Our studies elucidate a mechanism by which the most contagious human respiratory computer virus is delivered to the airway epithelium. INTRODUCTION Measles computer virus (MeV) is extremely contagious and infects its human host via the respiratory route. For many years, MeV was 2′,5-Difluoro-2′-deoxycytidine thought to enter through the apical surface of airway epithelial cells (1), a misconception based on studies performed with polarized immortalized cell lines (2, 3). Using well-differentiated main cultures of airway epithelial cells from human donors (HAE), we exhibited that MeV has a obvious preference for basolateral access (4). HAE differentiate into a pseudostratified columnar epithelium sheet comprised of ciliated, nonciliated, basal, and goblet cells. This model system is highly representative of the airways (5). MeV contamination of HAE results in the formation of infectious centers that, unlike syncytia, maintain intact plasma membranes (4, 6, 7). MeV-mediated infectious-center formation in 2′,5-Difluoro-2′-deoxycytidine HAE differs from that of most paramyxoviruses (7) and may result from the unique receptor specificity of the genus (8,C11). Indeed, we have shown that infectious-center formation in HAE results from direct cell-to-cell spread and is facilitated by the nectin-4/afadin complex (7). These studies spotlight the importance of using an appropriate model system to study MeV access and spread. How MeV eventually reaches the airway epithelium during a natural contamination is less obvious. Initially, the infection may be ferried through the epithelium by myeloid cells that sample the airway lumen and express the primary MeV receptor signaling lymphocyte activation molecule family member 1 (SLAMF1, also known as CD150) (12). In transgenic mice expressing human SLAM, MeV replication was observed in lung resident myeloid cells 24 h after experimental contamination (13). Subsequent contamination phases were documented for nonhuman primates. MeV replicated vigorously in main and secondary lymphatic organs of cynomolgus macaques 3 to 5 5 days after 2′,5-Difluoro-2′-deoxycytidine inoculation (14). A few days later, most infected cells in the upper airways were of lymphoid or myeloid origin (15). MeV replication in airway epithelial cells was not observed until the second week following contamination (16, 17). Infected myeloid cells were observed below infectious centers in the lamina propria of the trachea. We posited that dendritic cells (DCs) or macrophages must deliver MeV to airway epithelial cells; however, based on available information, the mechanism of this viral transfer remained unknown. MeV is the most 2′,5-Difluoro-2′-deoxycytidine contagious human respiratory computer virus (18), and within natural hosts, infections appear to remain largely cell associated (19). No free computer virus is usually detected in blood of humans or experimentally infected monkeys, for which computer virus titers are measured by overlaying leukocytes onto SLAM-expressing cells. Only small amounts of cell-free computer virus can be isolated from respiratory tract secretions (19). In cultured cells, MeV particles accumulate below the plasma membrane (20) and the ratio of secreted to intracellular infectivity is about 1:10 (21). Together, these observations suggest that within a natural host,.