Abstract
Natural killer (NK) cell function is regulated by a fine balance between numerous activating and inhibitory receptors, of which killer-cell immunoglobulin-like receptors (KIRs) are among the most polymorphic and comprehensively studied. KIRs allow NK cells to recognize downregulation or the absence of HLA class I molecules on target cells (known as missing-self), a phenomenon that is commonly observed in virally infected cells or cancer cells. Because KIR and HLA genes are located on different chromosomes, in an allogeneic environment such as after hematopoietic stem cell transplantation, donor NK cells that express an inhibitory KIR for an HLA class I molecule that is absent on recipient targets (KIR/KIR-ligand mismatch), can recognize and react to this missing self and mediate cytotoxicity. Accumulating data indicate that epistatic interactions between KIR and HLA influence outcomes in several clinical conditions. Herein, we discuss the genetic and functional features of KIR/KIR-ligand interactions in hematopoietic stem cell transplantation and how these data can guide donor selection. We will also review clinical studies of adoptive NK cell therapy in leukemia and emerging data on the use of genetically modified NK cells that could broaden the scope of cancer immunotherapy.