Natural killer cells: biology and clinical use in cancer therapy.

Natural killer (NK) cells have the ability to mediate both bone marrow rejection and promote engraftment, as well as the ability to elicit potent anti-tumor effects. However the clinical results for these processes are still elusive. Greater understanding of NK cell biology, from activating and inhibitory receptor functions to the role of NK cells in allogeneic transplantation, needs to be appreciated in order to draw out the clinical potential of NK cells. Mechanisms of bone marrow cell (BMC) rejection are known to be dependent on inhibitory receptors specific for major histocompatibility complex (MHC) molecules and on activating receptors that have many potential ligands. The modulation of activating and inhibitory receptors may hold the key to clinical success involving NK cells. Pre-clinical studies in mice have shown that different combinations of activating and inhibitory receptors on NK cells can reduce graft-versus-host disease (GVHD), promote engraftment, and provide superior graft-versus-tumor (GVT) responses. Recent clinical data have shown that the use of KIR-ligand incompatibility produces tremendous graft-versus-leukemia effect in patients with acute myeloid leukemia at high risk of relapse. This review will attempt to be a synthesis of current knowledge concerning NK cells, their involvement in BMT, and their use as an immunotherapy for cancer and other hematologic malignancies.