CD252/OX40L is a molecule identical to gp34 expressed in T cells infected with HTLV-1 and it is expressed on activated B cell, dendritic cells and vascular endothelial cells as a trimer belonging to TNF superfamily. CD252/OX40L have a role in proliferation of T cells and production of cytokines through OX40 expressed on activated T cells. In addition, OX40L expressed on the activated vascular endothelial cells are involved in activated T cell invasion to inflamed area.