Figure 1.

Natural killer (NK) cell interaction with cancer cell microenvironment. (A) Antibody-dependent cellular cytotoxicity. The NK cell F_c receptor, CD16, binds the F_c region of the IgG antibody bound to the tumor antigen, leading to NK cell protease release and subsequent tumor cell lysis. (B) NK cell activation via natural cytotoxicity receptors (NCRs). The NCR KIR, an inhibitory receptor, recognizes the absence of MHC I ligand on the surface of the cancer cell. Because the inhibitory receptor remains unbound by MHC I, inhibition does not occur, and the NK cell is thus activated, leading to tumor cell lysis. Note: the NK cell killing mechanism proceeds principally by proteolytic lysis. For greater detail on mechanisms of cancer cell killing, see Ref. (9).