The Ligand of Glucocorticoid-Induced TNF Receptor Is Expressed on Human Macrophages and Stimulates Proinflammatory and Anti-Tumor Effector Functions.

Macrophages are involved in inflammatory and anti-tumor responses by antigen presentation, release of immunoregulatory cytokines and cellular cytotoxicity. Among the molecules influencing macrophage functions are many members of the TNF superfamily. The TNF receptor 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. Expression of its cognate ligand GITRL has, in humans, been detected in dendritic cells and various healthy nonlymphoid tissues. Up to now, nothing is known regarding the function of human GITRL, while in mice, reverse signaling through GITRL has been shown to influence the activation and survival of macrophages. Here we report for the first time that GITRL is expressed in human macrophages as determined by FACS and RT-PCR analysis. Reverse signaling through GITRL using a recombinant GITR-Ig fusion protein differentially affects expression of costimulatory molecules like CD80, CD86 and B7-H1, death ligands like TNF, CD178 and TRAIL as well as MHC class I and II on the macrophage surface. Furthermore, stimulation of GITRL enhances phagocytosis and respiratory burst of macrophages as determined by analysis of ingestion of PE-labeled polystyrene microspheres and oxidation of dichlor-fluorescein diacetate, respectively by FACS. Interaction of GITRL with its receptor also leads to significantly increased production of proinflammatory cytokines like TNF, IL-6 and IL-8 as determined by ELISA. In addition, stimulation of GITRL significantly increases the killing of various human tumor cell lines by macrophages as determined by cytotoxicity assays. Taken together, our data demonstrate that GITRL plays an important role in the stimulation of macrophage-mediated inflammatory responses and immune responses against tumors.