摘要
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 ceils, 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 car- ried out siRNA-based knockdown analysis and discovered a critical role of ezrin in NK cell internalization. Impor- tantly, 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.
基金
We thank members of our group for insightful discussion dur- ing the course of this study and Drs Haiming Wei and Zhigang Tian for NK92 cells. This work was supported by grants from National Natural Science Foundation of China （30972681 to XW
90508002 to XY
30872286 to LS）, Guangdong-NSFC Joint Key Program （to XW）, Chinese Academy of Sciences （KSCX1- YW-R65, KSCX2-YWH-10）, National Basic Research Program of China （973 Program） （2007CB512402 to XW
2007CB914503 and 2010CB912103 to XY）, Ministry of Science ＆ Technology of China International Collaboration Program （2009DFA31010 to XD）, China National Key Projects for Infectious Disease （2008ZX 10002-021 to XY）, 2007 National Undergraduate Innova- tive Research Program of China （PX） and KC Wong Education Foundation （ZG）.
