Natural Killer Cells Adapt to Major Histocompatibility Complex: Implications for Hematopoietic Cell Transplantation
Authors: Raulet David H. 
Published: October 2019

Abstract
Natural Killer (NK) cells play several important roles in hematopoietic cell transplantation. Host NK cells can reject allogeneic transplants that lack major histocompatibility complex (MHC) molecules of the host (so-called “missing self” reactivity), whereas donor NK cells in the marrow can promote engraftment, inhibit graft versus host disease, and mediate rejection of host leukemic cells (graft versus tumor (GVT) response). Like T cells, NK cells can distinguish allogeneic cells from self-cells, and “learn” self vs. non-self so as to establish self-tolerance. This presentation will summarize current understanding of some of these processes based primarily on studies in mice, and present new data that may be relevant for the success of human bone marrow transplantation. The self-tolerance of T cells and B cells is established in large part by developmental processes that either remove or suppress self-reactive clones as the cells differentiate from hematopoietic stem cells. Studies will be presented that show that NK cell self-tolerance is regulated quite differently, by an adaptation process that occurs quite rapidly and acts on mature NK cells. NK cell reactivity against susceptible untransformed cells (e.g., cells from animals lacking MHC of the NK cell donor) is rapidly lost after transfer of such NK cells to susceptible hosts, indicating that self-tolerance is an adaptation that occurs rapidly under non-inflammatory conditions. Irradiation chimeras were employed to identify the cell types that induce tolerance of NK cells to cells from MHC-deficient mice, as one model of missing self-recognition. Stable tolerance of wild type NK cells to MHC-deficient cells was established when NK cells were exposed in chimeras to either MHC-deficient hematopoietic cells or MHC-deficient non-hematopoietic cells. Interestingly, however, tolerance induced by exposure solely to MHC-deficient hematopoietic cells was unstable in the face of inflammatory conditions arising from infections or exposures to inflammatory cytokines in vivo. Specifically, mixed chimeras consisting of WT and MHC-deficient hematopoietic cells in WT hosts were stably chimeric until the mice were exposed to viral or bacterial infections, at which time the MHC-deficient cells were rapidly eliminated. In contrast, tolerance induced by non-hematopoietic cells was much more stable. Specifically, mixed chimeras consisting of WT and MHC-deficient hematopoietic cells in MHC-deficient hosts were stably chimeric whether or not they underwent infections. These findings suggest that tolerance may be imposed by multiple mechanisms depending on the cell types that induce tolerance, with distinct properties as a result. Most importantly from a clinical perspective, the results suggest that depending on the host/donor combination, intense infections may jeopardize the stability of bone marrow transplants. Previous studies have shown that tolerance of NK cells to MHC-deficient cells is accompanied by a state of hyporesponsiveness of the NK cells, characterized by low functional responses to stimulation via crosslinking of activating receptors ex vivo. Accordingly, it has been widely accepted that hyporesponsiveness to stimulation is the mechanism of NK cell self-tolerance when NK cells lack inhibitory receptors specific for host MHC molecules. However, our recent analysis of chimeric NK cells showed that this cannot be the sole explanation. When NK cells developed in MHC+ hosts in the presence of MHC-deficient hematopoietic cells, the cells became responsive to activating receptor stimulation despite being tolerant of MHC-deficient cells. Hence, tolerance can occur even when NK cells are responsive to stimulation. In contrast, when NK cells developed in MHC-deficient hosts, they became hyporesponsive. These data suggest that non-hematopoietic cells in the host impart hyporesponsiveness. Furthermore, exposure to MHC-deficient hematopoietic cells induces tolerance by a mechanism distinct from hyporesponsiveness. The latter mechanism is relatively unstable as it can be broken as a result of infections.

Disclosures
Raulet: Innate Pharma: Membership on an entity's Board of Directors or advisory committees; Novo Nordisk: Consultancy.