Imbalance of NKG2D and its inhibitory counterparts: how does tumor escape from innate immunity?

NK cells form a first line of defence against pathogens or host cells that are stressed or cancerous. NK cells express surface receptors that receive signals from the environment and determine their response to foreign or malignant cells. The effector functions of NK cells are regulated by integrated signals across the array of stimulatory and inhibitory receptors engaged upon interaction with target cell surface ligands. NKG2D is a peculiar activating receptor that is expressed as a disulphide-linked homodimer by all NK cells, alphabeta CD8(+) T cells, gammadeltaT cells and murine macrophages. It not only activates NK cells but also delivers co-stimulatory signals to CD8(+) T cells and gammadeltaT cells. The ligands of NKG2D are induced by cellular stress and are specifically expressed by some tumor cells. Recent studies reveal that the expression of MIC and ULBP on human tumor cells is sufficient to overcome the inhibitory effects of MHC class I expression on NK cell killing and indicate that NKG2D provides first line surveillance against stressed or abnormal cells that have been induced to express one of its ligands. However, malignant tumors develop means to control the expression of activating versus inhibitory receptors on immune cells and their ligands on tumor cell themselves in favor of tolerance. Modulating the balance between activating and inhibitory signals through NK cell receptors on NK cells may open a new approach to NK cell-based biotherapy for cancer and infectious diseases.