GITR ligand (GITRL) is a member of the TNF superfamily, also known as TNFSF18 or TL6. It is expressed on B cells, macrophages, dendritic cells, and endothelial cells. The expression of GITRL on the dendritic cell surface can be transiently upregulated, and then rapidly downregulated, by LPS stimulation. GITRL binds to its receptor, GITR, to costimulate T cell activation, proliferation, and cytokine production, inhibit the suppressive functions of CD4+CD25+ Tregs, and induce macrophage activation. GITR-GITRL interaction has been implicated as an essential regulator of the immune response to self antigens and phathogens.