Conflicting Impact of Alloreactive NK Cells on Transplantation Outcomes after Haploidentical Transplantation: Do the Reconstitution Kinetics of Natural Killer Cells Create These Differences?

Partially HLA-mismatched related, or HLA-haploidentical, donor stem cell transplantation (SCT) is a feasible therapeutic option for advanced hematologic malignancy patients who lack an HLA-matched related or unrelated donor. Natural killer (NK) cells, a major cell type of the innate immune system, express surface receptors that regulate potent effector functions, such as cytolytic activity and the release of cytokines, and play a central role in the inflammatory response and immunoregulation. Conflicting results have been reported regarding the impact of NK cell alloreactivity on the outcome of haploidentical SCT in leukemic patients. This review summarizes the heterogeneous clinical results and explains the underlying mechanisms with respect to the reconstitution kinetics of NK cells and the interactions between NK cells and T cells.