Natural killer cells in haematopoietic stem cell transplantation

Natural killer cells represent the predominant lymphoid cell in the peripheral blood for many months after allogeneic or autologous stem cell transplant and their role in immunity to pathogens during this period is established. However, following the largely unsuccessful trials of NK and IL-2 activated NK cells for the treatment of haematological malignancies in the 1980's and 90's, their role in tumour immunology was discredited. Over the past ten years we have come to understand some of the complex regulatory pathways involved in NK cell activation and we are now in a position to capitalise upon this knowledge. This review presents our current state of understanding of NK cell regulation and highlights the role of these cells in engraftment, graft-versus-host disease, anti-leukaemia activity and post-transplant infection.