Absence of NKG2D Ligands Defines Human Acute Myeloid Leukaemia Stem Cells and Mediates Their Immune Evasion

Patients with acute myeloid leukaemia (AML) often achieve remission but subsequently die of relapse driven by chemotherapy resistant leukemic stem cells (LSCs). To initiate and maintain cancer, LSCs must also escape immunosurveillance. However, in vivo studies on human LSCs largely disregard lymphocyte mediated anti-tumor immunity due to the use of immunocompromised mice. Here we investigate the immunosurveillance mediated by NKG2D, a danger detector expressed by cytotoxic lymphocytes such as natural killer (NK) cells that recognizes stress-induced ligands (NKG2DL) of the MIC and ULBP protein families on AML cells. Staining of n=175 de novo AML with antibodies against MICA, MICB and ULB2/5/6 or an NKG2D-Fc chimeric protein recognizing pan-NKG2DL expression revealed NKG2DL to heterogeneously express among leukemic cells of the same patient (Fig. 1a). As expected, NKG2DLpos AML cells were efficiently cleared by natural killer (NK) cells, while NKG2DLneg leukemic cells escaped NK cell lysis. Interestingly, these NKG2DLneg AML cells also showed immature morphology, enhanced in vitro clonogenicity (39±47 colonies vs. 1±4, p