OX40L (Tumor necrosis factor ligand superfamily member 4; or CD252) is the endogenous ligand of the OX40 receptor. OX40 is localized on activated T-cells and propagates the immune response by promoting division and survival of T-cells, thereby increasing the population of memory CD4 and CD8 T cells produced in response to antigen detection. The OX40/OX40L may also regulate cytokine production and suppress the activity of T-regulatory cells. OX40/OX40L is therefore an important mediator of the immune response. Previous studies have suggested that OX40 signaling maybe involved in the development of autoimmune diseases.