Glucocorticoid-induced tumor necrosis factor receptor-related protein ligand subverts immunosurveillance of acute myeloid leukemia in humans.

The reciprocal interaction of tumor cells with the immune system is influenced by various members of the tumor necrosis factor (TNF)/TNF receptor (TNFR) family, and recently, glucocorticoid-induced TNFR-related protein (GITR) was shown to stimulate antitumor immunity in mice. However, GITR may mediate different effects in mice and men and impairs the reactivity of human natural killer (NK) cells. Here, we studied the role of GITR and its ligand (GITRL) in human acute myeloid leukemia (AML). Surface expression of GITRL was observed on AML cells in six of seven investigated cell lines, and 34 of 60 investigated AML patients whereas healthy CD34(+) cells did not express GITRL. Furthermore, soluble GITRL (sGITRL) was detectable in AML patient sera in 18 of 55 investigated cases. While the presence of GITRL was not restricted to a specific AML subtype, surface expression was significantly associated with monocytic differentiation. Signaling via GITRL into patient AML cells induced the release of TNF and interleukin-10 (IL-10), and this was blocked by the inhibition of mitogen-activated protein kinases extracellular signal-regulated kinase 1/2. Furthermore, triggering GITR by surface-expressed and sGITRL impaired NK cell cytotoxicity and IFN-gamma production in cocultures with leukemia cells, and NK cell reactivity could be restored by blocking GITR and neutralization of sGITRL and IL-10. Thus, whereas a stimulatory role of the GITR-GITRL system in mouse antitumor immunity has been reported, our data show that in humans GITRL expression subverts NK cell immunosurveillance of AML. Our results provide useful information for therapeutic approaches in AML, which, like haploidentical stem cell transplantation, rely on a sufficient NK cell response.

Interaction of monocytes with NK cells upon Toll-like receptor-induced expression of the NKG2D ligand MICA.

Reciprocal interactions between NK cells and dendritic cells have been shown to influence activation of NK cells, maturation, or lysis of dendritic cells and subsequent adaptive immune responses. However, little is known about the crosstalk between monocytes and NK cells and the receptors involved in this interaction. We report in this study that human monocytes, upon TLR triggering, up-regulate MHC class I-Related Chain (MIC) A, but not other ligands for the activating immunoreceptor NKG2D like MICB or UL-16 binding proteins 1-3. MICA expression was associated with CD80, MHC class I and MHC class II up-regulation, secretion of proinflammatory cytokines, and apoptosis inhibition, but was not accompanied by release of MIC molecules in soluble form. TLR-induced MICA on the monocyte cell surface was detected by autologous NK cells as revealed by NKG2D down-regulation. Although MICA expression did not render monocytes susceptible for NK cell cytotoxicity, LPS-treated monocytes stimulated IFN-gamma production of activated NK cells which was substantially dependent on MICA-NKG2D interaction. No enhanced NK cell proliferation or cytotoxicity against third-party target cells was observed after stimulation of NK cells with LPS-activated monocytes. Our data indicate that MICA-NKG2D interaction constitutes a mechanism by which monocytes and NK cells as an early source of IFN-gamma may communicate directly during an innate immune response to infections in humans.

Cancer immunoediting by GITR (glucocorticoid-induced TNF-related protein) ligand in humans: NK cell/tumor cell interactions.

Glucocorticoid-induced TNF-related protein (GITR) has been shown to stimulate T cell-mediated antitumor immunity in mice. However, the functional relevance of GITR and its ligand (GITRL) for non-T cells has yet to be fully explored. In addition, recent evidence suggests that GITR plays different roles in mice and humans. We studied the role of GITR-GITRL interaction in human tumor immunology and report for the first time that primary gastrointestinal cancers and tumor cell lines of different histological origin express substantial levels of GITRL. Signaling through GITRL down-regulated the expression of the immunostimulatory molecules CD40 and CD54 and the adhesion molecule EpCAM, and induced production of the immunosuppressive cytokine TGF-beta by tumor cells. On NK cells, GITR is constitutively expressed and up-regulated following activation. Blocking GITR-GITRL interaction in cocultures of tumor cells and NK cells substantially increased cytotoxicity and IFN-gamma production of NK cells demonstrating that constitutive expression of GITRL by tumor cells diminishes NK cell antitumor immunity. GITRL-Ig fusion protein or cell surface-expressed GITRL did not induce apoptosis in NK cells, but diminished nuclear localized c-Rel and RelB, indicating that GITR might negatively modulate NK cell NF-kappaB activity. Taken together, our data indicate that tumor-expressed GITRL mediates immunosubversion in humans.