Retuning of Mouse NK Cells after Interference with MHC Class I Sensing Adjusts Self-Tolerance but Preserves Anticancer Response

Abstract

Natural killer (NK) cells are most efficient if their targets do not express self MHC class I, because NK cells carry inhibitory receptors that interfere with activating their cytotoxic pathway. Clinicians have taken advantage of this by adoptively transferring haploidentical NK cells into patients to mediate an effective graftversus-leukemia response. With a similar rationale, antibody blockade of MHC class I-specific inhibitory NK cell receptors is currently being tested in clinical trials. Both approaches are challenged by the emerging concept that NK cells may constantly adapt or "tune" their responsiveness according to the amount of self MHC class I that they sense on surrounding cells. Hence, these therapeutic attempts would initially result in increased killing of tumor cells, but a parallel adaptation process might ultimately lead to impaired antitumor efficacy. We have investigated this question in two mouse models: inhibitory receptor blockade in vivo and adoptive transfer to MHC class I-disparate hosts. We show that changed self-perception via inhibitory receptors in mature NK cells reprograms the reactivity such that tolerance to healthy cells is always preserved. However, reactivity against cancer cells lacking critical MHC class I molecules (missing self-reactivity) still remains or may even be increased. This dissociation between activity against healthy cells and tumor cells may provide an answer as to why NK cells mediate graftversus-leukemia effects without causing graft-versus-host disease and may also be utilized to improve immunotherapy.

Expression of CD226 is associated to but not required for NK cell education

“…Adoptive transfer of NK cells from mice with MHC-I expression to recipients that lack MHC-I can lead to re-education of the NK cells and alteration in expression of activating receptors and adhesion receptors364647. Since we observed that B2m −/− mice have reduced levels of DNAM-1, we hypothesized that the expression of DNAM-1 would change on NK cells upon adoptive transfer of mature NK cells to a different MHC-I environment.…”

“…Recent models for NK-cell education are based on shifts in activation threshold determined by the combined input of inhibitory and activating receptors43637, where inhibitory receptors decrease the activation threshold thereby increasing reactivity while activating receptors influence education in the opposite direction, that is, they tune down the responsiveness of an individual NK cell in continuous presence of the ligand for the activating receptor373839. We therefore assessed the influence of an activating receptor (Ly49D) in the presence or absence of the cognate MHC-I allele (D d ) on DNAM-1 expression.…”

“…Finally, DNAM-1 expression may be important in tuning both activity and homeostasis of NK cells upon activation and proliferation in response to different infections and of memory NK cells. Although there is still no molecular marker for the educated state, the emerging pattern is that NK cells educated on MHC-I have increased expression of at least two adhesion molecules, DNAM-1 and KLRG1 (refs 36, 57), which contribute to optimize the function of educated NK cells.…”

“…In vivo cytotoxicity assays were performed as previously described36. Briefly, single-cell suspensions of spleen cells ( B2m −/− , B6) were labelled with a high (target cells: B2m −/− ) or low (control cells: B6) dose of CFSE (Invitrogen Molecular Probes).…”

Adaptive NK Cells with Low TIGIT Expression Are Inherently Resistant to Myeloid-Derived Suppressor Cells

“…The balance between these receptors determines the function of NK cells (5). A down-regulation of MHC class I on damaged cells, or a mismatch between inhibitory subgroups of killer immunoglobulin-like receptors (KIRs) and their respective human leukocyte antigen (HLA) ligands on cells will render targets susceptible to NK cell killing (6,7). Therefore, tuning down the expression of inhibitory receptors on NK cells would increase their response to tumor cells.…”