Working in "NK Mode": Natural Killer Group 2 Member D and Natural Cytotoxicity Receptors in Stress-Surveillance by γδ T Cells

Natural killer cell receptors (NKRs) are germline-encoded transmembrane proteins that regulate the activation and homeostasis of NK cells as well as other lymphocytes. For gamma delta T cells, NKRs play critical roles in discriminating stressed (transformed or infected) cells from their healthy counterparts, as proposed in the "lymphoid stress-surveillance" theory. Whereas the main physiologic role is seemingly fulfilled by natural killer group 2 member D, constitutively expressed by gamma delta T cells, enhancement of their therapeutic potential may rely on natural cytotoxicity receptors (NCRs), like NKp30 or NKp44, that can be induced selectively on human V delta 1(+) T cells. Here, we review the contributions of NCRs, NKG2D, and their multiple ligands, to gamma delta T cell biology in mouse and human.