Rationale: Natural killer (NK) cells are innate lymphocytes that target virus-infected and tumor cells. Much less is known about their ability to limit adaptive immune responses.

Objectives: Thus, we investigated to what extent NK cells can influence mouse lung allograft rejection.

Methods: For this purpose, we employed an orthotopic lung transplantation model in mice.

Measurements and Main Results: We demonstrate here that NK cells infiltrate mouse lung allografts before T cells and thereby diminished allograft inflammation, and that NK-cell deficiency enhanced allograft rejection. In contrast, expansion of recipient NK cells through IL-15/IL-15Rα complex treatment resulted in decreased T-cell infiltration and alloreactive T-cell priming as well as improved function of the allogeneic lung transplant. Only perforin-competent, but not perforin-deficient, NK cells were able to transfer these beneficial effects into transplanted NK cell–deficient IL-15Rα^−/− mice. These NK cells killed allogeneic dendritic cells (DCs) in vitro and significantly decreased the number of allogeneic DCs in transplanted lungs in vivo. Furthermore, DC-depleted lung allografts presented decreased signs of rejection.

Conclusions: These results suggest that NK cells favor allograft acceptance by depleting donor-derived DCs, which otherwise would prime alloreactive T-cell responses. Thus, conditioning regimens that augment NK-cell reactivity should be clinically explored to prepare lung allograft recipients.