OX-40L, also known as TNFSF4 and CD252, is a cytokine that belongs to the tumor necrosis factor (TNF) ligand family. OX-40L is an important costimulatory molecule that plays a crucial role in the regulation of T-cell-mediated immunity. The interaction of TNFSF4-TNFSF4 is involved in the pathogenesis of multiple autoimmune and inflammatory diseases such as systemic lupus erythematosus (SLE), carotid artery disease and cancer. OX-40L is a ligand for receptor TNFRSF4/OX40. It is found to play a role in T cell antigen-presenting cell (APC) interactions. In surface Ig- and CD4-stimulated B cells, this cytokine along with CD7 has been shown to provide CD28-independent costimulatory signals to T cells. This protein and its receptor are reported to directly mediate adhesion of activated T cells to vascular endothelial cells.