GITR, Glucocorticoid-induced TNFR-related gene, is a member of the TNF receptor superfamily, also known as TNFRSF18, and AITR (in humans). It is expressed at low levels on resting T lymphocytes and at high levels on CD4+CD25+ T regulatory (Treg) cells. The expression of GITR on T cells can be upregulated upon activation. Interaction of GITR with its ligand (GITRL) has been demonstrated to augment T cell activation, proliferation, cytokine production, as well as MAPKs and NF-kappaB activation, and abrogate the inhibitory functions of CD4+CD25+ Treg cells. In vivo activation GITR causes development of autoimmune diseases and restores the suppressed immune responses.