Activation‐dependent FasL expression in T lymphocytes and Natural Killer cells

In cytotoxic T cells and Natural Killer cells, the death factor FasL is stored in association with secretory granules. Only upon activation, these vesicles are transported to the cytotoxic immunological synapse and FasL is expressed on the cell surface. Upon activation of T cells and NK cells with phorbol ester and calcium ionophore, we observed a biphasic expression of FasL in all lymphocyte subsets tested. The first peak was seen after 10−15 minutes of stimulation and was followed by a reduction of expression to baseline level before the second peak was reached at about two hours. Using inhibitors of protein biosynthesis (cycloheximide), vesicular transport (brefeldin A and monensin), actin polymerization (latrunculin A and cytochalasin D) and metalloproteases (GM 6001 and phenanthrolin), the regulation of FasL expression was analyzed in detail. We demonstrate that the first wave of expression is due to an actin‐dependent mobilization of preformed FasL whereas the second phase of expression requires de novo synthesis. The observed expression patterns might have implications for the development of therapeutic strategies that target FasL as an immunomodulatory protein.