Natural killer cells--new understanding of basic biology may lead to more effective allogeneic haematopoietic stem cell transplantation.

The natural killer (NK) cells are part of the innate immune system and are responsible for initial defences in the surveillance against malignant cells and virally infected cells. In addition to direct cytotoxicity, cytokines produced by NK cells amplify the immune response and help control the neoplasm/pathogen. Several activating and inhibitory receptors responsible for NK cell activation are recently characterized and play a crucial role in tumour eradication. These include, but are not limited to, the killer immunoglobulin-like receptors, C-type lectin receptors and natural cytotoxicity receptors. The downstream signalling of some of these receptors is also characterized. The net balance in the sum of the signals generated by ligation of activating and inhibitory receptors determines the final outcome, cytotoxicity versus tolerance. NK cell-based immunotherapy can be successfully exploited in the haematopoietic stem cell transplantation for the treatment of haematological malignancies and has a potential to separate the beneficial graft versus leukaemia effect from, often dangerous, graft versus host disease. This article reviews the NK receptors important in NK-mediated cytotoxicity in allogeneic haematopoietic stem cell transplantation.