The CD40 ligand directly activates T-lymphocytes via tyrosine phosphorylation dependent PKC activation.

The activation of B-lymphocytes depends critically on the interaction of the CD40 receptor with its ligand. Here, we provide evidence that the CD40 ligand (CD40L) also functions as a direct stimulatory molecule for T-lymphocytes. Activation of T-lymphocytes via CD40L induces tyrosine phosphorylation of cellular proteins including PLC gamma. Tyrosine phosphorylation of PLC gamma correlates with an IP3- and Ca(2+)-release and an activation of PKC. Inhibition of src-like tyrosine kinases by Herbimycin A prevents these activation events suggesting a crucial role of tyrosine phosphorylation in T-lymphocyte activation via CD40L.