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Differential Role of Hematopoietic and Nonhematopoietic Cell Types in the Regulation of NK Cell Tolerance and Responsiveness

Many NK cells express inhibitory receptors that bind self-MHC class I (MHC I) molecules and prevent killing of self-cells, while enabling killing of MHC I–deficient cells. But tolerance also occurs for NK cells that lack inhibitory receptors for self-MHC I, and for all NK cells in MHC I–deficient animals. In both cases, NK cells are unresponsive to MHC I–deficient cells and hyporesponsive when stimulated through activating receptors, suggesting that hyporesponsiveness is responsible for self-tolerance. We generated irradiation chimeras, or carried out adoptive transfers, with wild-type (WT) and/or MHC I–deficient hematopoietic cells in WT or MHC I–deficient C57BL/6 host mice. Unexpectedly, in WT hosts, donor MHC I–deficient hematopoietic cells failed to induce hyporesponsiveness to activating receptor stimulation, but did induce tolerance to MHC I–deficient grafts. Therefore, these two properties of NK cells are separable. Both tolerance and hyporesponsiveness occurred when the host was MHC I deficient. Interestingly, infections of mice or exposure to inflammatory cytokines reversed the tolerance of NK cells that was induced by MHC I–deficient hematopoietic cells, but not the tolerance induced by MHC I–deficient nonhematopoietic cells. These data have implications for successful bone marrow transplantation, and suggest that tolerance induced by hematopoietic cells versus nonhematopoietic cells may be imposed by distinct mechanisms.