NK Activating Receptor, NKG2D —Function and Biological Roles—

Natural Killer (NK) cells play important roles in host defense, especially the clearance of viral or bacterial infected cells and the elimination of tumor cells. NK cells have cytotoxicity against target cells as well as the ability of cytokine production. NK cells not only eliminate infected cells directly by cytotoxicicity but also induce acquired immunity by cytokines. These functions of NK cells are regulated by the balance of activating or inhibitory signaling. The inhibitory NK cell receptors are well understood, however, less is known about the activating signaling pathways. An activating receptor, NKG2D is expressed in all NK cells, γ γδ⁺T cells and activated/memory CD8⁺T cells. By recent studies, NKG2D is best characterized as an activating receptor. This review summarizes recent topics and our findings focus on the role of NKG2D in NK cells and in CD8⁺T cells, including tumor immunity and autoimmune diabetes.