The Function of NK Cells in Tumor Metastasis and NK Cell-Based Immunotherapy

SIMPLE SUMMARY: Innate immune natural killer (NK) cells are capable of killing metastatic cancer cells without activation by antigen-presenting cells beforehand. The cytotoxic effects and immune regulation of NK cells are precisely controlled by an energetic balance of signals produced by a group of activating and inhibitory receptors expressed on NK cells, while metastatic tumor cells with multiple strategies escape immune cells attack. This review focuses on the critical function of NK cells in metastasis and recently developed objectively effective NK cell-based immunotherapies. ABSTRACT: Metastatic tumors cause the most deaths in cancer patients. Treating metastasis remains the primary goal of current cancer research. Although the immune system prevents and kills the tumor cells, the function of the immune system in metastatic cancer has been unappreciated for decades because tumors are able to develop complex signaling pathways to suppress immune responses, leading them to escape detection and elimination. Studies showed NK cell-based therapies have many advantages and promise for fighting metastatic cancers. We here review the function of the immune system in tumor progression, specifically focusing on the ability of NK cells in antimetastasis, how metastatic tumors escape the NK cell attack, as well as the recent development of effective antimetastatic immunotherapies.