Our in vitro results clearly demonstrated that: (i) repetitive radiation treatment increased the PD-L1 level while decreasing NKG2D ligand levels in lung cancer cells, (ii) higher expression of PD-L1 in radioresistant cells than in parental cells (in vitro) and in radioresistant tumors than in radiation-sensitive tumors (in vivo), and (iii) radioresistant cells showed resistance to cytotoxic actions of NK cells. Here, KLRK1 is linked to lung cancer.