OX40L is a member of the TNF superfamily of structurally related proteins that exists primarily as a type II membrane bound, non-covalently linked homotrimeric protein. It is expressed on antigen presenting cells (APCs), such as dendritic cells and activated B cells. It is also expressed on vascular endothelial cells, mast cells, and natural killer cells. OX40L signals specifically through the OX40 receptor (CD134), which is expressed predominantly on CD4+ T cells, as well as on certain activated CD8+ T cells. OX40-OX40L interaction function as a costimulatory signal that is required for a conducive interaction between antigen presenting cells and their target T cells. OX40L enhances cellular proliferation and survival of memory and effector cells, and increases expression of RANTES, IL-2, IL-3, and IFN-γ. This interaction also induces immunoglobulin secretion by activated B cells. Besides playing a role in asthma, it has been shown that plasmacytoid dendritic cells (DCs) infiltrating a melanoma microenvironment are able to promote proinflammatory Th2 and regulatory immune profiles through OX40L and ICOSL expression. This suggests that this molecule could potentially be a therapeutic target for cancer treatment.