OX40 ligand (OX40L) is a 35 kD type II transmembrane protein which belongs to TNF superfamily, also known as gp34, CD252, CD134L or TNFSF4. The OX40 ligand is expressed on activated B cells, activated endothelial cells, mature dendritic cells, activated CTLs, activated mast cells, and airway smooth muscle cells. OX40 ligand interacts with OX40 antigen (CD134) expressed predominantly on activated T cells to regulate T cell proliferation and differentiation, and to enhance immunoglobulin secretion by activated B cells.