Cytotoxic lymphocytes are key players in the orchestration of immune response and elimination of defective cells. We have previously reported that natural killer (NK) cells enter target
tumor cells, leading to either target cell death or self-destruction within
tumor cells. However, it has remained elusive as to the fate of NK cells after internalization and whether the heterotypic cell-in-cell process is different from that of the homotypic cell-in-cell event recently named entosis. Here, we show that NK cells undergo a cell-in-cell process with the ultimate fate of apoptosis within
tumor cells and reveal that the internalization process requires the actin cytoskeletal regulator,
ezrin. To visualize how NK cells enter into
tumor cells, we carried out real-time dual color imaging analyses of NK cell internalization into
tumor cells. Surprisingly, most NK cells commit to programmed cell death after their entry into
tumor cells, which is distinctively different from entosis observed in the homotypic cell-in-cell process. The apoptotic cell death of the internalized NK cells was evident by activation of
caspase 3 and DNA fragmentation. Furthermore, NK cell death after internalization is attenuated by the
caspase inhibitor,
Z-VAD-FMK, confirming apoptosis as the mode of NK cell death within
tumor cells. To determine
protein factors essential for the entry of NK cells into
tumor cells, we carried out
siRNA-based knockdown analysis and discovered a critical role of
ezrin in NK cell internalization. Importantly, PKA-mediated phosphorylation of
ezrin promotes the NK cell internalization process. Our findings suggest a novel regulatory mechanism by which
ezrin governs NK cell internalization into
tumor cells.