Comprehensive analysis of NKG2D ligand expression and release in leukemia: implications for NKG2D-mediated NK cell responses.

Ligands of the prototypical activating NK receptor NKG2D render cancer cells susceptible to NK cell-mediated cytolysis if expressed at sufficiently high levels. However, malignant cells employ mechanisms to evade NKG2D-mediated immunosurveillance, such as NKG2D ligand (NKG2DL) shedding resulting in reduced surface expression levels. In addition, systemic downregulation of NKG2D on NK cells of cancer patients has been observed in many studies and was attributed to soluble NKG2DL (sNKG2DL), although there also are conflicting data. Likewise, relevant expression of NKG2DL in leukemia has been reported by some, but not all studies. Hence, we comprehensively studied expression, release, and function of the NKG2D ligands MHC class I chain-related molecules A and B and UL16-binding proteins 1-3 in 205 leukemia patients. Leukemia cells of most patients (75%) expressed at least one NKG2DL at the surface, and all investigated patient sera contained elevated sNKG2DL levels. Besides correlating NKG2DL levels with clinical data and outcome, we demonstrate that sNKG2DL in patient sera reduce NKG2D expression on NK cells, resulting in impaired antileukemia reactivity, which also critically depends on number and levels of surface-expressed NKG2DL. Together, we provide comprehensive data on the relevance of NKG2D/NKG2DL expression, release, and function for NK reactivity in leukemia, which exemplifies the mechanisms underlying NKG2D-mediated tumor immunosurveillance and escape.

The BCR/ABL-inhibitors imatinib, nilotinib and dasatinib differentially affect NK cell reactivity.

In chronic myeloid leukemia (CML), BCR/ABL-mediated oncogenic signaling can be targeted with the BCR/ABL-inhibitors Imatinib, Nilotinib and Dasatinib. However, these agents may also affect anti-tumor immunity. Here, we analyzed the effects of the 3 BCR/ABL-inhibitors on natural killer (NK) cell reactivity. Exposure of CML cells (K562, Meg-01) to pharmacological concentrations of Imatinib, Nilotinib and Dasatinib diminished expression of ligands for the activating immunoreceptor NKG2D to a similar extent. This resulted in comparably reduced NK cell cytotoxicity and IFN-gamma production. When direct effects on NK cell responses to K562 and primary CML cells as well as activating cytokines were studied, Dasatinib was found to abrogate NK cytotoxicity and cytokine production. Nilotinib did not alter cytotoxicity but, at high levels, impaired NK cytokine production, while Imatinib had no direct influence on NK cell reactivity. Of note, Nilotinib, but not the other BCR/ABL-inhibitors increased cell death within the preferentially cytokine-secreting CD56(bright)CD16(-) NK cell subset, which may, at least in part, serve to explain the effect of Nilotinib on NK cytokine production. Analysis of NK cell signaling revealed that Dasatinib inhibited proximal signaling events leading to decreased phosphorylation of PI3K and ERK that are crucial for NK cell reactivity. Imatinib and Nilotinib, in contrast, showed no relevant effect on NK cell PI3K or ERK activity. In light of the potential role of NK cells in the immunesurveillance of residual leukemia and for future combinatory immunotherapeutic approaches, our data indicate that choice and dosing of the most suitable BCR/ABL-inhibitor for a given patient require careful consideration.