The underlying mechanisms by which distinct NK cell subsets promote either allograft rejection or survival remain unclear. Here, we tested the hypothesis that distinctively mature NK cell subsets control alloreactive T cell responses through their interactions with graft-derived donor DCs.

We used BALB/c.Rag-T-bet double-KO (DKO) mice that lack terminally-differentiated CD27^low NK cells to study the role of mature CD27^low NK cells in a model of T cell-mediated allograft rejection. For this, Rag-T-bet DKO and Rag-KO recipients of fully mismatched B6 skin allografts (STx) were adoptively transferred with purified T cells from Foxp3-gfp-knock-in BALB/c mice and treated with costimulatory blockade (CTLA4Ig and MR-1). Groups of recipients received a STx from B6.IL-15-KO and B6.CD11c-DTR donors. Alloimmune responses were analyzed by histology, ELISA, RT-PCR and flow cytometry.

Costimulatory blockade-treated Rag-KO recipients (with CD27^low NK cells) showed significantly prolonged allograft survival (MST 35±3d, P<0.001) and reduced alloreactive CD8⁺ T cell responses (ELISA, flow cytometry) when compared to DKO mice lacking CD27^low NK cells (28±1d). Next, we discovered that Rag-KO recipients with CD27^low NK cells show significantly lower IL-15 serum levels (2.24 fold) versus Rag-T-bet DKO recipients (ELISA), suggesting that CD27^low NK cells control graft-derived IL-15, thereby inhibiting alloreactive CD8⁺ T cell responses and promoting allograft survival. To test this we grafted Rag-T-bet DKO recipients with STx from IL-15-KO donors and found significantly longer allograft survival (39±2d vs 28±1d, P<0.05) and reduced CD8⁺ T cell responses in the absence of IL-15. Therefore, we hypothesized that host CD27^low NK cells control graft-derived donor DCs, which are known to express high levels of IL-15. To test this we grafted Rag-T-bet DKO recipients with STx from CD11c-DTR mice that were depleted from CD11c⁺ DCs via diphtheria toxin. Importantly, the lack of donor DCs in STx significantly prolonged allograft survival (50±4d vs 28±1d) and dramatically reduced alloreactive CD8⁺ T cell responses in DKO recipients.

Therefore, under costimulatory blockade conditions mature CD27^low NK cells control IL-15-expressing graft-derived donor DCs, which inhibits alloreactive CD8⁺ T cell responses and promotes allograft survival.

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