In addressing whether in vitro differentiated type I and II pneumocytes are susceptible to influenza virus infection, progenitor cells were differentiated to type I and II pneumocytes for 5 days in epithelial differential medium. The differentiated cultures were then exposed to SwIV. Co-immunostaining revealed SwIV replication in differentiated type II pneumocytes as indicated by the presence of viral proteins whereas no viral proteins were detected in differentiated type I pneumocytes (Fig. 4 ).