TNFSF18 is a transmembrane glycoprotein which is expressed in vascular endothelial cells as well as neurons, antigen presenting cells, CD4­- CD8- double negative thymic precursors, and in the eye. TNFSF18 works to mediate the interactions between T cells and endothelial cells, and T cell growth. It has been shown that TNFSF18's expression is transiently up-regulated by proinflammatory stimulation.