Function of natural killer cells in immune defence against human leukaemia

Although progress has been made in the management of acute leukaemias, most patients who fail to respond to front-line therapies with cytostatic agents and stem cell transplantation, or who relapse after an initial response die from progressive disease. Novel treatment modalities exploiting donor-derived natural killer (NK) cells generate an alloreactive graft-versus-leukaemia response and eliminate the residual malignant clones in transplanted patients. NK cells are components of the innate immunity playing an important role in the surveillance of human tumours. Recognition of malignant cells depends on a dynamic balance between antagonistic functions of an array of NK activating and inhibitory receptors. The natural cytotoxicity receptors (NCRs) are NK cell-specific and together with the NKG2D receptor are responsible for NK cell activation and tumour cell killing. The killer immunoglobulin-like receptors (KIRs) recruit phosphatases and can antagonise the activating signals and prevent the cytolytic NK cell programme. Understanding of the integration of these multiple signals at the molecular level is central for exploring the cytolytic function of NK cells. This review describes molecular mechanisms of NK receptor-ligand interactions controlling target cell recognition and addresses the potential of NK cells for the specific elimination of leukaemic clones with the goal of advancing immunotherapy of leukaemia.