Activation-Inducible TNF-Related Ligand （AITRL）, also known as Glucocorticoid-induced tumor necrosis factor receptor （TNFR） family-related protein （GITR）, is a type I transmembrane protein that belongs to the TNF receptor superfamily. This cytokine is a ligand for receptor TNFRSF18/AITR/GITR. It has been shown to modulate T lymphocyte survival in peripheral tissues. This cytokine is also found to be expressed in endothelial cells, and is thought to be important for interaction between T lymphocytes and endothelial cells. GITR-GITRL cross-linking costimulated native and activated T cells and resulted in MAPK activation. Researches have indicated that GITRL could promote Th17 cell differentiation by p38 MAPK and STAT3 signaling in autoimmune arthritis.