Patients treated for leukemia often achieve remission yet subsequently die of relapse. Human Acute Myeloid Leukaemia (AML) initiation and relapse are thought to be mediated by leukaemic stem cells (LSCs), defined by their ability to initiate leukaemia in immunodeficient mice. To induce disease, LSCs must also escape immunosurveillance. Links between human LSCs and immune evasion are not accounted for by experimental paradigms using immunocompro-mised animals, and thus remain largely unexplored. Here we hypothesize that immune evasion and stemness are closely intertwined in human AML. We analyzed 175 primary AML samples and found that ligands of the danger de-tector NKG2D (NKG2DL), a critical mediator of anti-tumour immunity by cytotoxic lymphocytes including natural killer (NK) cells, are heterogeneously expressed on leukemic blasts from individual patients. Intra-patient comparisons between NKG2DL positive (NKG2DLpos) and negative (NKG2DLneg) subpopulations indicate that NKG2DLneg AML cells selectively escape NK cell killing in vitro and in NOD/SCID/IL2R?null (NSG) mice co-transplanted with NK cells. Furthermore, NKG2DLneg AML cells LSCs display stem cell characteristics such as immature morphology, stemness-associated gene expression signatures, enhanced in vitro clonogenicity and unique ability to initiate leukaemia in NSG mice. When compared to NKG2DLpos AML cells of the same patient, NKG2DLneg LSCs show differential expression of several other inflammatory response genes, providing further evidence for their distinct interaction with the immune system. In particular, NKG2DLneg AML LSCs show enriched expression of poly-ADP-ribose polymerase 1 (PARP1), and PARP1 inhibition using an inhibitory drug or PARP1 siRNAs selectively induces NKG2DL expression on CD34+ LSCs. The current proposal follows up on these results and addresses following aims:Aim 1: Investigation of PARP1 inhibition for in vivo sensitization of LSCs to NK cell attack in human AML xenograftsAim 2: Exploration of the effects of PARP1 inhibition on NKG2DL expression in healthy hematopoietic cells Aim 3: Mechanistic investigation of NKG2DL expression suppression in LSCsAim 4: Analysis of absence of NKG2DL expression for LSC enrichment in conjunction with other LSC markers Significance: These data provide a conceptual frame-work for the integration of two central hypotheses of cancer development - the concepts of cancer stem cells and immune escape. Furthermore, they point out detection of absence of NKG2DL expression as a method that can enriches AML LSCs. Stemness-associated immune escape might represent a novel biological principle that extends to other cancers, and perhaps even healthy tissues, which opens up exciting new research avenues. The results of this study are also important from a translational point of view: adoptive NK cell transfer is currently being tested in more than hundred registered clinical trials for treatment of AML (see https://clinicaltrials.gov/ct2/results?cond=Acute+Myeloid+Leukemia&term=NK&cntry1). Our data indicate that these efforts may fail to achieve cure, since they may not target disease-initiating and relapse-relevant LSCs unless additional co-treatments are provided (e.g., as suggested by this work, PARP1 inhibitory drugs).