Gleevec is a paradigmatic pharmacological agent targeting specific mutations in protooncogenes (KIT/PDGFR/BCR/ABL) that is required for the malignant transformation of stromal cells of the gut (GIST) or myeloid precursors (chronic myeloid leukemia). The clinical breakthrough achieved with Gleevec in the management of irresectable GISTs or progressive chronic myelogenous leukemia (CML) has been approved worldwide (8
). However, this specific kinase inhibitor appears to promote antitumor effects even in tumors lacking target-activating mutations (Supplemental Table 1A). Here we highlight an alternate mode of action of Gleevec that is not tumor cell–autonomous and that involves host bone marrow–derived DCs. Indeed, we have identified the NK cell–dependent antitumor effects promoted by Gleevec-treated DCs in mouse tumor models; our findings highlight the importance of Gleevec-mediated NK cell activation in patients bearing GISTs devoid of KIT/PDGFR mutations and displaying clinical responses to Gleevec.
Long-term exposure to Gleevec in mice endows host DCs with enhanced NK cell stimulatory capacity (Figure E), and such in vivo effects of Gleevec on host DCs can be dramatically augmented by coadministration of the DC growth factor FL in both NK cell activation (Figure , D and E) and NK cell–dependent antitumor activity (Figures and ). FL led to a marked increase in the frequency of CD11c+
DCs, the cell population that presumably responds to Gleevec (Supplemental Figure 3B). In addition, we found that FL could not be substituted for by G-CSF or GM-CSF in association with Gleevec for the expansion of CD69+
NK cells in vivo (data not shown). The residual plasmatic concentrations of Gleevec following oral administration of Gleevec at 150 mg/kg bi-injection daily (bid) were 576 ng/ml (1 μM), corresponding to the IC50
concentration achieved in clinical trials with Gleevec (8
). Plasma concentrations of Gleevec peaked at 6,020 ng/ml (60 μM) in FL+Gleevec, and they peaked at 12,177 ng/ml (120 μM) with Gleevec alone, which supports the hypothesis that DCs are pharmacological targets of Gleevec in vivo.
NK cells have recently been involved in the host-mediated control of cancer. The renaissance of interest in NK cells in tumor immunosurveillance can be attributed to the discovery of stress-induced ligands for receptors that activate NK cells (23
) and to the relevance of interactions between MHC class I molecules and killer inhibitory receptors in mismatched hematopoietic transplants, which cause NK cell-–mediated graft versus leukemia effects (26
). Therefore, MHC class I or TAP loss tumor variants and tumors overexpressing NKG2D ligands are ideal targets for NK cell activity (23
). Here we report that NK cell functions were enhanced in 49% of Gleevec-treated GIST patients. Our data suggest that Gleevec-mediated NK cell activation might play a part in tumor control either by synergizing with the cell-autonomous effect of Gleevec or by keeping tumors in check after the direct action of Gleevec on tumor cells. First, none of the 10 patients who had a progressive disease exhibited enhanced NK cell functions. Importantly, the time to progression was significantly longer in GIST patients for whom Gleevec caused NK cell activation than in patients without NK cell activation. Second, the GIST model bears molecular features of NK cell sensitivity, namely TAP-1 deficiency, loss of MHC class I molecules, high expression of NKG2D ligands (Supplemental Table 2> and data not shown), and GIST recognition by NK cells comparable to that of K562 (Figure A). Notably, in 50% of GIST-bearing patients, a fraction of circulating NK cells showed a downregulation of NKG2D expression at diagnosis (data not shown), which suggests that GISTs secrete soluble NKG2D ligands that might downregulate NKG2D expression on blood NK cells, similarly to the downregulation seen on T cells (30
). Nonetheless, the relevance of NKG2D receptors in NK cell recognition of GISTs remains to be established. Moreover, we could not find any modulation of natural killer cytotoxicity receptor (NKp30, 44, 46) expression following Gleevec therapy in circulating NK cells (data not shown). We did not detect circulating IFN-γ, TNF-α, GM-CSF, IL-10, or IL-13 — all cytokines produced mostly by CD56bright
CD16 NK cells — in any patients, whether or not they showed NK cell activation (data not shown).
The relevance of the DC/NK cell cross-talk is postulated in various physiopathological settings (22
) and has been demonstrated for the control of mouse tumors (10
) and the murine cytomegalovirus viral replication in vivo (34
). DCs and NK cells might interact in inflammatory lesions where chemokines and cytokines recruit both DCs and NK cells (35
) or in the lymph nodes, where cooperation between IL-2–producing CD4+
T cells and NK cells is ongoing (33
). Knowledge of whether the Gleevec-conditioned DC/NK cell cross-talk is mediated in situ or at distant sites (lymphoid organs) remains elusive. Nevertheless, in one patient who benefited from therapy with Gleevec for one year, we found a DC/NK cell interaction in an unusual site (skin undergoing Gleevec-induced lichenoid dermatitis). This side effect regressed after removal of Gleevec, suggesting that the maturation of dermal DCs and/or recruitment of NK cells in the dermis was induced by Gleevec.
Any molecular mechanism accounting for NK cell triggering by Gleevec-stimulated DCs that does not imply the maturation of DCs (see Supplemental Figure 2) or the presence of IL-12 deserves comprehensive analysis. It is worth considering whether adoptive transfer of Gleevec-stimulated DCs or a combination of FL or NK cell–stimulating factors to Gleevec could be suitable therapeutic options in the clinicians’ armamentarium against hitherto untreatable NK cell–dependent malignancies or infectious diseases.