CD40 Ligand (CD40L, CD154, TRAP, TNFSF5, gp39) belongs to the TNF superfamily and in its soluble form, displays some cytokine-like properties. CD40L binds to its receptor, CD40, to mediate T cell-dependent B cell responses and contribute to monocyte activation and dendritic cell maturation. Membrane-bound CD40L is expressed mainly on activated T cells, a small population of CD8+ T cells, and has been demonstrated on mast cells, basophils, eosinophils, B cells, NK cells and monocytes/macrophages. Activated platelets express CD40L on their surface and constitute the major source of soluble CD40L (sCD40L). sCD40L is a shorter protein than the membrane-bound version and retains biologic activity and ability to form trimers. CD40L expression can be upregulated using mitogens like PMA or PHA, through anti-CD3 activation, or by thrombin in platelets.
