Tumor necrosis factor ligand superfamily member 18 is a cytokine that belongs to the tumor necrosis factor (TNF) ligand family. 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.

By screening a panel of epitope-tagged candidate TNF ligand proteins for binding to a recombinant AITR protein, these authors identified the AITR ligand, AITRL or TL6. Northern blot analysis revealed that TL6 is expressed as a 2.4-kb mRNA in pancreas.