Expression of Glucocorticoid-Induced TNF Receptor Ligand on Acute Myeloid Leukemia Cells Mediates the Release of Immunosuppressive Cytokines and Impairs NK Cell-Mediated Immune Surveillance.

NK cells play an important role in the reciprocal interaction of tumor cells with the immune system and participate in the surveillance of hematological malignancies including acute myeloid leukemia (AML). Among the molecules influencing host-tumor interaction are many members of the TNF superfamily, which mediate multiple cellular functions including cellular proliferation, differentiation and cell death. The TNF family member Glucocorticoid-induced TNF Receptor (GITR) costimulates effector T cells, modulates apoptosis and nuclear factor kappa B and abrogates suppression of murine but not human regulatory T cells. Its cognate ligand GITRL has been found in various healthy tissues. Recently we reported that NK cells express GITR, while solid tumors express GITR ligand (GITRL), and GITR/GITRL interaction downregulates NK cell cytotoxicity and IFN-γ production. Here we analyzed the role of GITR and its ligand in AML. We report for the first time that GITRL is expressed on primary AML cells in 18 of 30 patients as determined by FACS and RT-PCR analysis. Reverse signaling through GITRL using a recombinant GITR-Ig fusion protein induces the release of the immunoregulatory cytokines IL-10 and TNF as determined by ELISA. GITRL-mediated cytokine production of AML cells is abrogated by inhibition of mitogen activated protein kinase (MAPK) pathways as demonstrated by addition of the specific p38 MAPK inhibitor SB202190, the specific JNK inhibitor SP600125 and the specific ERK Inhibitor II. Furthermore, binding of AML-expressed GITRL to GITR on NK cells downregulates cellular cytotoxicity and IFN-γ production in AML-NK cell cocultures, which can be overcome by addition of GITR-blocking antibodies as determined by cytotoxicity assays and ELISA. Thus, our data indicate that GITRL expression in AML substantially influences tumor immunoediting and enables the escape of leukemia cells from NK cell-mediated immunosurveillance.