Cutting Edge: The Natural Ligand for Glucocorticoid-Induced TNF Receptor-Related Protein Abrogates Regulatory T Cell Suppression

CD4⁺25⁺ regulatory T (Treg) cells maintain immunological self-tolerance through mechanisms that are only in part understood. Previous studies suggest that the glucocorticoid-induced TNFR-related protein (GITR), which is preferentially expressed on the surface of Treg cells, potentially provides a signal that abrogates Treg suppression. In this study, we show that a soluble form of mouse GITR ligand (sGITR-L) induces GITR-dependent NF-κB activation and blocks in vitro suppression mediated by both resting and preactivated polyclonal and Ag-specific Treg cells. Since sGITR-L along with rIL-2 induces proliferation of CD4⁺25 ⁺ cells, it appears that sGITR-L can break the anergic state of Treg cells. Because sGITR-L also up-regulates IL-2 secretion by activated CD4 ⁺25^- T cells, these two sGITR-L induced signals synergize to interfere with suppressor activity by CD4⁺25⁺ Treg cells.