Innate tumor killers in colorectal cancer

Standard treatment of colorectal cancer (CRC) improves the prognosis of CRC patients, but it is still intractable to control the progression of metastatic CRC. Immune microenvironment and immunotherapies of CRC have received extensive attention in recent years, but present immunotherapies of CRC have mainly focused on T cells and therapeutic response is only observed in a small proportion of patients. Innate immune cells are the first-line of defense in the development of malignancies. Natural killer (NK) cells, NKT cells and γδT cells are three types of innate cells of lymphoid origin and show cytotoxicity against various tumor cells including CRC. Besides, in the development of CRC, they can also be inhibited or express regulatory type, promoting tumor progression. Researches about anti-tumorigenic and pro-tumorigenic mechanisms of these cells are ongoing and regulation of these cells is also being unearthed. Meanwhile, immunotherapies using these cells more or less have shown efficacy in animal models and some of them are under exploration in clinical trials. This review provides an overview of intrinsic properties of NK cell, NKT cell and γδT cell, and summarizes current related promising treatment strategies.