Co-expression of Fas and Fas-ligand on the surface of influenza virus-infected cells.

Influenza virus-infected cultured cells undergo apoptosis after an increment of Fas (APO-1/CD95) on the cell surface. By flow cytometry, cell surface Fas-ligand was detected in virus-infected cells with a time course similar to that of Fas. Moreover, Fas and Fas-ligand were co-expressed in those cells. The mode of induction, however, appeared to be distinct for the two proteins. Influenza virus infection induced the externalization of phosphatidylserine on the cell surface at the early stage of apoptosis, an event that has been observed in cells undergoing Fas-mediated apoptosis. In fact, apoptosis of the virus-infected cells was inhibited in the presence of an antagonistic anti-Fas-ligand monoclonal antibody. These results suggest that influenza virus infection causes augmented expression of both Fas and Fas-ligand and apoptosis is induced when the infected cells come into contact with each other.