NK cell anti-
tumor reactivity is governed by a balance of activating and inhibitory receptors including various
TNF receptor (TNFR) family members. Here we report that human
tumor cells release a soluble form of the TNF family member
Glucocorticoid-Induced TNFR-Related Protein (GITR)
ligand (sGITRL), which can be detected in cell culture supernatants.
Tumor-derived sGITRL concentration-dependently reduced NK cell cytotoxicity and IFN-gamma production, which could be overcome by neutralization of sGITRL using a GITR-Ig fusion
protein. Although sGITRL did not induce apoptosis in NK cells, it diminished nuclear localized RelB, indicating that sGITRL negatively modulates NK cell
NF-kappaB activity. Furthermore, we detected substantial levels of sGITRL in sera of patients with various
malignancies, but not in healthy controls. Presence of sGITRL-containing patient serum in cocultures with
tumor cells significantly reduced NK cell cytotoxicity and IFN-gamma production, which could again be restored by neutralization of sGITRL. The strong correlation of
tumor incidence and elevated sGITRL levels indicates that sGITRL is released from
cancers in vivo, leading to impaired NK cell immunosurveillance of human
tumors. Our data suggest that determination of sGITRL levels might be implemented as a
tumor marker in patients, and GITRL neutralization may be used to improve immunotherapeutic strategies relying on NK cell reactivity.