226: Differential requirement for NKG2D in the rejection of haplomismatched and MHC-I deficient bone marrow

Abstract

Mouse Natural Killer (NK) cells mediate rejection of MHC-mismatched or MHC-deficient bone marrow allografts. Counteracting activating and inhibitory signals regulate NK cell activity. The ability of F1 mice to reject parental bone marrow (BM) cells (hybrid resistance) has been attributed to the presence in the recipient of a subset of NK cells that lack inhibitory receptors for the MHC class I antigens expressed by the donor cells. Evidence supporting the “missing self” hypothesis of hybrid resistance was provided by demonstration that MHC class-I deficient donor BM is rejected by otherwise syngeneic recipients. We have previously demonstrated a role for NKG2D, an activating NK cell receptor, in rejection of Balb/c (H-2d) parental BM by (Balb/c x C57Bl/6; H-2b/d) F1 (CB6F1) recipient mice. NKG2D ligands are expressed on the regenerating myeloid BM cells in Balb/c and NOD mice but not substantially on the regenerating C57Bl/6 BM cells. Moreover, the rejection of Balb/c BM by CB6F1 recipients was blocked by a neutralizing anti-NKG2D monoclonal antibody. The purpose of this study was to determine whether NKG2D blockade was sufficient to prevent rejection of MHC-I deficient BM. Beta-2 microglobulin deficient (B2M-/-) Balb/c or NOD bone marrow was transplanted into irradiated CB6F1 mice that had been pretreated with neutralizing antibodies to NKG2D. In contrast to results with wild-type Balb/c or NOD BM whose rejection was prevented by anti-NKG2D, B2M-/- donor cells were rejected despite NKG2D blockade. Furthermore, syngeneic MHC class I+ Balb/c recipients were able to reject B2M-/- Balb/c donors despite NKG2D blockade. These results suggest that rejection of MHC class I-bearing BM requires activation via NKG2D, whereas MHC class I-deficient BM elicit a sufficiently strong NK response that augmentation by NKG2D signaling is not essential for the rejection. Therefore, the hybrid resistance model in which MHC class I-bearing BM are used for transplantation may better reflect the situation in human hematopoietic stem cell transplantation. Mouse Natural Killer (NK) cells mediate rejection of MHC-mismatched or MHC-deficient bone marrow allografts. Counteracting activating and inhibitory signals regulate NK cell activity. The ability of F1 mice to reject parental bone marrow (BM) cells (hybrid resistance) has been attributed to the presence in the recipient of a subset of NK cells that lack inhibitory receptors for the MHC class I antigens expressed by the donor cells. Evidence supporting the “missing self” hypothesis of hybrid resistance was provided by demonstration that MHC class-I deficient donor BM is rejected by otherwise syngeneic recipients. We have previously demonstrated a role for NKG2D, an activating NK cell receptor, in rejection of Balb/c (H-2d) parental BM by (Balb/c x C57Bl/6; H-2b/d) F1 (CB6F1) recipient mice. NKG2D ligands are expressed on the regenerating myeloid BM cells in Balb/c and NOD mice but not substantially on the regenerating C57Bl/6 BM cells. Moreover, the rejection of Balb/c BM by CB6F1 recipients was blocked by a neutralizing anti-NKG2D monoclonal antibody. The purpose of this study was to determine whether NKG2D blockade was sufficient to prevent rejection of MHC-I deficient BM. Beta-2 microglobulin deficient (B2M-/-) Balb/c or NOD bone marrow was transplanted into irradiated CB6F1 mice that had been pretreated with neutralizing antibodies to NKG2D. In contrast to results with wild-type Balb/c or NOD BM whose rejection was prevented by anti-NKG2D, B2M-/- donor cells were rejected despite NKG2D blockade. Furthermore, syngeneic MHC class I+ Balb/c recipients were able to reject B2M-/- Balb/c donors despite NKG2D blockade. These results suggest that rejection of MHC class I-bearing BM requires activation via NKG2D, whereas MHC class I-deficient BM elicit a sufficiently strong NK response that augmentation by NKG2D signaling is not essential for the rejection. Therefore, the hybrid resistance model in which MHC class I-bearing BM are used for transplantation may better reflect the situation in human hematopoietic stem cell transplantation.