OX40L is the ligand for OX40 and is stably expressed on many antigen-presenting cells such as DC2s （a subtype of dendritic cells）, macrophages, and activated B lymphocytes. The ligation of OX40-OX40L is a source of survival signal for T cells and enables the development of memory T cells. Signaling through these two molecules also leads to polarization towards Th2 immune response even in an environment with low levels of IL-4 cytokine. OX40L is also present on the surface of many non-immune cells, for example, endothelial cells and smooth muscle cells. The surface expression of OX40L is induced by many pro-inflammatory mediators, such as TNF-α, e.g. produced by mast cells, IFN-γ and PGE2. Various single-nucleotide polymorphisms （SNPs） of the OX40L gene have been identified. For some of them association with systemic lupus erythematosus has been reported.