NK Cells are Required for Costimulatory Blockade Induced Tolerance to Vascularized Allografts

Background. The role of natural killer (NK) cells in organ transplantation is poorly understood because studies link these cells to both regulatory and inflammatory functions. NK cells exacerbate inflammation and adaptive immunity under conditions of allograft rejection, but little is known regarding their roles in allograft tolerance. We test the hypothesis that NK cells have regulatory function and promote tolerance induction to murine cardiac allografts. Methods. Murine hearts were transplanted as fully vascularized heterotopic grafts from BALB/c donors into C57BL/6 recipients. Allograft tolerance was achieved using donor splenocyte transfusion + anti-CD40L monoclonal antibody (mAb) before transplantation. The requirement for NK cells in tolerance induction was tested by administering anti-NK1.1-depleting mAb or anti-NKG2D-blocking mAb. Intragraft and peripheral immune cell populations were determined by flow cytometry and immunohistochemistry. CD4 T-cell alloantigen-specific responses and donor-specific alloantibody were also determined. Results. NK cell-depleted recipients acutely reject allografts despite anti-CD40L blockade, but rejecting recipients lacked alloantibody and alloantigen-specific CD4(+) T-cell responses. NK cell depletion resulted in elevated numbers of graft-infiltrating macrophages. NKG2D blockade in tolerized recipients did not cause acute rejection but increased macrophage graft infiltration and increased the expression of NKG2D ligand Rae-1 gamma on these cells. Conclusions. Our data show that NK cells are required for tolerance induction in recipients given donor splenocyte transfusion + anti-CD40L mAb. Our data suggest NK cells regulate monocyte or macrophage activation and infiltration into allografts by a mechanism partially dependent on NKG2D receptor-ligand interactions between NK cells and monocytes/macrophages.