Natural killer cells and killer cell immunoglobulin-like receptors in solid organ transplantation: Protectors or opponents?

Among all the cells of innate immunity, natural killer (NK) cells are well-known for the fight against tumors and virally-infected cells. NK cells have been implicated in the pathogenesis of immune-mediated allograft damage, but mounting evidence suggests they can potentially promote allograft tolerance as well. In addition, NK cells express a wide variety of activating and inhibiting receptors, and the signals sent by these molecules, particularly killer cell immunoglobulin-like receptors (KIRs), determine their ultimate function. The role of KIRs and their human leukocyte antigen (HLA) class I ligands have been extensively investigated in hematopoietic stem cell transplantation (HSCT). Previous studies have suggested that, in the setting of solid organ transplantation, having certain KIR genes or KIR/HLA combinations probably affects allograft survival. Therefore, it may be helpful to analyze KIR/HLA combinations in donors and recipients to choose the optimal donor, anticipate harmful effects post-transplantation, and develop NK cell-based immunotherapies to enhance the success of solid organ transplantation. In this review, we will discuss the dual function of NK cells in solid organ transplantation, followed by a brief introduction to KIRs and the association of KIR and HLA genes with kidney, liver, and lung transplant outcomes.