Increase of intracellular calcium is the essential signal for the expression of CD40 ligand.

CD40 ligand (CD40L) is present on activated but not on resting T cells. In contrast to the activation markers CD25 and CD71, a strong CD40L expression could be induced by calcium ionophore alone but not by phorbol 12-myristate 13-acetate (PMA). Ionomycin induced a very early mRNA and protein surface expression of CD40L within the first 2 h, whereas CD25 and CD71 did not appear earlier than 6 h after stimulation. The mitogens phytohemagglutinin and concanavalin A induced little CD40L, but together with PMA, a markedly increased CD40L expression was observed. In T cells stimulated with immobilized anti-CD3, co-stimulation with anti-CD28 or PMA induced an earlier and higher maximal CD40L expression. CD40L expression of purified T cells was higher and more prolonged compared to that of T cells in unseparated peripheral blood mononuclear cells. We conclude that the expression of CD40L on T cells is profoundly different from other early activation markers with regard to signal requirements, kinetics and the role of accessory cells in the system.