KLRK1 and lung cancer: IL-21 connected to the NKG2D CAR constructed with NKG2D ECD, 4-1BB and CD3ζ domains augmented the CAR NK-92 cytolytic functions against lung cancer cell lines A549 and H1975 with increased CD107a, IFN-γ and cell proliferation via activation of PI3K/Akt pathway.