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 tumor cells express GITRL, and GITR-GITRL interaction downregulates NK cell-mediated anti-tumor immunity (Baltz et al., FASEB J 2007). Many TNF family members are released as soluble forms, which affects cell-cell interactions by reduction of ligand density and distally modulates effector cells bearing the respective receptor. Here we report that human tumor cells spontaneously release a soluble form of GITRL (sGITRL), which can be detected in tumor cell culture supernatants by ELISA (detection limit 0.01ng/ml). We demonstrated that NK cell cytotoxicity and IFN-γ production in cocultures with the tumor cell lines SK-Mel (Melanoma), PC-3 (prostate), HCT116 (colon), and LX-1 (lung) were significantly (both p<0.01, Mann-Whitney U-test) and concentration dependently reduced (up to 50%) by tumor-derived sGITRL, and NK cell effector functions could be restored by neutralization of sGITRL using a GITR-Fc fusion protein. While tumor-derived GITRL did not induce apoptosis in NK cells, it diminished nuclear localized RelB indicating that sGITRL negatively modulates NK cell NF-κB activity. Furthermore, we demonstrate that significantly elevated sGITRL levels (mean 0.4ng/ml, range from 0.01 to 3.5ng/ml) were contained in 40 out of 50 sera of patients with various cancers (colon, lung and germ line), while sera of healthy volunteers (n=8) contained no detectable levels of sGITRL. Addition of sGITRL containing patient sera to cocultures of NK cells and GITRL-negative tumor cells significantly reduced NK cell cytotoxicity and IFN-γ production about 30% and 45%, respectively (both p<0.05, Mann-Whitney U-test). Again the inhibitory effects of sGITRL on NK cell effector functions could be completely restored by neutralization of sGITRL with GITR-Fc. The strong correlation of tumor incidence and elevated sGITRL levels clearly suggests that sGITRL is released at significant amounts from malignant cells in vivo and may reduce immune surveillance of human tumors. Our data indicate that determination of sGITRL levels may be implemented as an immunological diagnostic marker in tumor patients, and GITRL-neutralization may be employed in therapeutic strategies like adoptive NK cell transfer.
Disclosure: No relevant conflicts of interest to declare.