Adenovirus (Ad) is an airborne, nonenveloped virus infecting respiratory epithelium. To study the mechanism of Ad entry, we used
alveolar adenocarcinoma A549 cells, which have retained the ability of alveolar epithelial type II cells to synthesize the major component of
pulmonary surfactant,
disaturated phosphatidylcholine. Stimulation of
phosphatidylcholine secretion by
calcium ionophore or
phorbol ester augmented the susceptibility of these cells to Ad. Both Ad
infection and recombinant-Ad-mediated transfection increased in the presence of
dipalmitoyl phosphatidylcholine (DPPC)
liposomes in culture medium. Importantly, in the presence of DPPC
liposomes, virus penetrates the cells independently of virus-specific
protein receptors. DPPC vesicles bind Ad and are efficiently incorporated by A549 lung cells, serving as a virus vehicle during Ad penetration. To identify the
viral protein(s) mediating Ad binding, a flotation of
liposomes preincubated with structural
viral proteins was employed, showing that the only Ad
protein bound to DPPC vesicles was a hexon. The hexon preserved its
phospholipid-binding properties upon purification, confirming its involvement in virus binding to the
phospholipid. Given that
disaturated phosphatidylcholine not only covers the inner surface of alveoli in the lungs but also reenters alveolar epithelium during lung
surfactant turnover, Ad binding to this
phospholipid may provide a pathway for virus entry into alveolar epithelium in vivo.