Natural killer cells as a double-edged sword in cancer immunotherapy: A comprehensive review from cytokine therapy to adoptive cell immunotherapy.

Natural killer (NK) cells are immune cells which are able to kill tumor and virus-infected cells and play an important role in both innate immunity and acquired immunity. Tumor immunotherapy is an emerging model of tumor treatment in the clinic. It is a re-emerging type of anticancer immunotherapy with the purpose of killing tumor cells by modulating the body's immune function and enhancing the antitumor immunity in tumor microenvironment. At present, many immune cells including lymphokine-activated killer cells, NK cells, cytokine-induced killer cells, and dendritic cells are involved in tumor immunotherapy studies. NK cells, which lyse tumor cells without prior stimulation, has become a research hotspot in cancer immunotherapy for clinical application. In this article, we discussed the surface receptors of NK cells and the anticancer function of NK cells. We also reviewed the biological characteristics and the current research status of NK cells, their clinical application in cancer immunotherapy and its future perspectives.