Treatment of transplant donors with human AAT resulted in an increase in IL-10 messenger RNA and CD8+CD11c+CD205+MHC II+DCs, and the prevention or attenuation of acute GVHD in the recipients. The GVL effect was maintained or even enhanced with AAT treatment of the donor, mediated by an expanded population of NK1.1+, CD49B+, CD122+, and CD335+ NKG2D-expressing NK cells. Here, KLRK1 is linked to acute graft versus host disease.