Allorecognition by murine natural killer cells: lysis of T-lymphoblasts and rejection of bone-marrow grafts.

Natural killer (NK) cells of inbred mice reject allogeneic bone-marrow cells, and NK cells of F1 hybrid mice can reject parental bone-marrow cells (hybrid resistance). In some cases these patterns of rejection can be mimicked in vitro by utilizing IL-2 cultured NK effector cells and allogeneic or parental T-lymphoblasts as target cells. Lysis of allogeneic and parental targets in vitro can be explained on the basis of the missing self hypothesis. Subsets of NK cells that bear non-overlapping MHC class I inhibitory receptors belonging to the Ly49 family lyse allogeneic targets because they do not express self class I molecules of the NK cell donor. Parental strain targets are lysed because they do not express all of the self class I antigens of the F1 hybrid, and hence fail to deliver inhibitory signals to all subsets of F1 NK cells. The expression of Ly49 receptors on NK cells is regulated by host MHC to ensure maximal sensitivity to alterations in self class I molecules and to prevent autoreactivity. In many instances, however, the rejection of allogeneic bone marrow cells in vivo cannot be readily explained by the missing self hypothesis. In these instances, it appears that rejection is initiated by class I MHC receptors on NK cells that recognize allogeneic class I molecules as non-self, and activate rather than inhibit NK cell function.