IFITM3 Clusters on Virus Containing Endosomes and Lysosomes Early in the Influenza A Infection of Human Airway Epithelial Cells

Kummer, Susann; Avinoam, Ori; Kräusslich, Hans-Georg

Importantly, a similar phenotype was observed in human primary respiratory cells from healthy donor tissue (HSAEpCs). The basal levels of IFITM3 were much higher in these primary cells, compared to A549 cells in the absence of IAV infection. However, IFITM3 clustering was not observed in the naïve primary cell population, but rapidly increased again upon IAV infection and was even stronger than in A549 cells. IFITM3 clustering on cytoplasmic vesicles was strongly induced in both A549 cells and HSAEpCs exhibiting weak extranuclear signals for IAV NP, which most likely reflects an incoming input virus. In contrast, no such IFITM3 clusters were seen in productively infected A549 cells with a strong nuclear NP signal, and we speculate that infection of this cell type mainly occurs in cells that have not managed to block IAV entry by inducing IFITM3 clustering. Alternatively, IFITM3 clusters may be abrogated once productive infection has occurred and may thus no longer be visible in these cells. Live-cell microscopy using labelled IFITM3 will be required to distinguish between these possibilities. A different phenotype was observed in primary HSAEpCs, where IFITM3 clustering on vesicular structures was also observed in productively infected cells with a strong nuclear signal. Despite having a much higher basal level, IFITM3 clustering in HSAEpCs does not appear, therefore, to efficiently block IAV infection in these primary cells.