Apical recycling systems regulate directional budding of respiratory syncytial virus from polarized epithelial cells.

Respiratory syncytial virus (RSV) is the major viral cause of serious lower respiratory tract illness in infants and young children worldwide. RSV infection is limited to the superficial layers of the respiratory epithelium in immunocompetent individuals. Consistent with this in vivo observation, we and others have found that RSV buds preferentially from the apical surface of infected polarized epithelial cells. In contrast, directional budding is not observed in nonpolarized human epithelial cells. These findings suggest that RSV uses specific cellular trafficking pathways to accomplish viral replication. The host cell proteins that regulate directional budding of RSV are undefined. Apical sorting of cellular proteins in polarized epithelial cells involves the apical recycling endosome (ARE). To investigate whether ARE-mediated protein sorting plays a role during RSV replication, we expressed a fragment of the myosin Vb tail that functions as a dominant negative inhibitor of ARE-mediated protein sorting in polarized Madin-Darby canine kidney cells. When these cells were infected with RSV, a >9,000-fold reduction in viral yield was observed. A similar effect on virus replication was observed when a carboxyl-terminal fragment of another ARE-associated protein, the Rab11 family interacting protein 1, was expressed in Madin-Darby canine kidney cells. These data suggest that RSV requires proper ARE-mediated protein sorting for efficient egress from the apical surface of polarized epithelial cells.