Human natural killer cells in health and disease

Natural killer (NK) cells are an essential component of the innate immune system and play a critical role in tumor immune surveillance. NK cells express their own repertoire of receptors (NKRs) that bind to major histocompatibility class I or class I-like molecules. The balance of signals from stimulation or inhibition of NKRs determines the ability of NK cells to lyse specific targets. In haploidentical stem cell transplantation with purified stem cells, NK cell alloreactivity (killer immunoglobulin-like receptor [KIR] mismatch) has been demonstrated to reduce the risk of relapse in acute myeloid leukemia. There is a need for adequately powered prospective randomized studies to determine the usefulness of NK cells as adoptive immunotherapy, optimal NK cell doses and timing of administration. Further studies are required to determine optimal selection of donors and recipients, both on NKR matching/mismatching, undergoing haploidentical and unrelated hematopoetic stem cell transplantation.