Human GITR Ligand Is Expressed on Tumor Cells and Reduces Cytokine Production and Cellular Cytotoxicity of NK Cells Identified to Express GITR.

Members of the tumor necrosis factor (TNF) superfamily mediate multiple cellular functions including cellular proliferation, differentiation, and cell death. Human Glucocorticoid-induced TNF Receptor (GITR) has been shown to be expressed on T cells, is upregulated following activation and mediates costimulatory signals. The human GITR ligand (GITRL) has been reported to be expressed on antigen presenting cells and various healthy nonlymphoid tissues including small intestine, ovary, testis, kidney and endothelial cells. We analyzed multiple tumor cell lines of hematopoietic and epithelial origin as well as of germ cell lineage and various gliomas by RT-PCR and FACS analysis. Both GITRL m-RNA and protein are expressed in various carcinomas, gliomas and tumor cells of germ cell lineage, but not in hematopoietic tumor cells. Furthermore, we demonstrate that human NK cells constitutively express low levels of GITR, and this expression is upregulated following activation by, e.g., IL-2 or IL-15 as detected by quantitative PCR and FACS analysis. To address the functional interaction of GITRL with its receptor on NK cells, we generated a GITRL-IgG fusion protein (GITRL-Ig). Stimulation of activated NK cells with GITRL-Ig lead to significantly reduced IFN-g production of NK cells as measured by ELISA. Similarly, a significant reduction of IFN-g release was observed following coculture of GITR expressing NK cells with C1R cells transfected with GITRL but not with the respective mock transfectants. Furthermore, ligation of GITR on NK cells lead to significantly decreased killing of target cells as demonstrated by cellular cytotoxicity assays. Taken together, our data demonstrate that GITR not only plays an important role in adaptive immunity but is also involved in the regulation of NK cell effector functions. Since tumor cells express significant levels of GITRL, and ligation of GITR on NK cells markedly reduces cytokine production and cellular cytotoxicity, our data indicate that GITR-GITRL interactions play an important role in the escape of tumor cells from innate immune surveillance.