Role of Glucocorticoid-Induced TNF-Related Protein (GITR) Ligand in the Interaction of Acute Myeloid Leukemia with NK Cells.

NK cells play an important role in the reciprocal interaction of tumor cells with the immune system and participate in the surveillance and eradication of hematological malignancies. The activity of NK cells is governed by a balance of activating and inhibitory surface receptors. Glucocorticoid-induced TNF-related protein (GITR) and its ligand (GITRL) are members of the TNF/TNF receptor (TNFR) superfamily, which mediates multiple cellular functions including proliferation, differentiation, and cell death. Recently we reported that NK cells express GITR while cancer cells express GITRL and GITR-GITRL interaction down regulates NK cell-mediated anti-tumor immunity (Baltz et al., FASEB J 2007). Here we demonstrate that GITRL is expressed on 6 of 7 investigated acute myeloid leukemia (AML) cell lines and on primary AML cells in 30 of 52 (59%) patients, while no GITRL expression was detected on CD34+ cells of healthy donors (n=5). GITRL expression was not restricted to a specific French-American-British (FAB) subtype, but was significantly (p<0.05, one-way ANOVA) associated with monocytic (FAB M4, M5) differentiation. In addition, no association with a particular cytogenetic abnormality or with expression of MHC class I was observed. Reverse signaling via GITRL led to phosphorylation of ERK and JNK resulting in significantly (p<0.05, Mann-Whitney U-test) enhanced production of IL-10 and TNF by patient AML cells (n=10). In line, specific inhibitors for JNK and ERK1/2 blocked the cytokine release by AML cells demonstrating that activation of MAP kinases is responsible for the production of the immunoregulatory cytokines following GITRL stimulation. Importantly, blocking GITR-GITRL interaction in cocultures of AML and NK cells significantly (both <0.05 Mann-Whitney U-test) increased cellular cytotoxicity about 70% and IFN-γ production about 60%, and this was due to restored NK cell NF-κB activity. Thus, GITRL substantially influences immunoediting by AML cells and enables the escape of AML cells from NK cell-mediated immune surveillance. The correlation found between GITRL expression and NK cell susceptibility may provide useful information for NK cell-based immunotherapy.