The fusion protein CTLA-4·FasL, a paradigmatic "trans signal converter protein", can attach to APC surfaces and in effect convert B7-activating costimulator signals into inhibitory Fas receptor-generated signals. The present study investigates CTLA-4·FasL's mechanism of action. A combination of p27kip and proliferating cell nuclear Ag Western blot and propidium iodide flow cytometric analysis showed no CTLA-4·FasL effect on cell cycle entry and progression, pointing away from the kind of classical anergy associated with CTLA-4·Ig. Significantly, CTLA-4·FasL elicited apoptosis (as detected by annexin-V/propidium iodide costaining) as early as 24 h after T cell activation, suggesting that some coordinate signaling might be capacitating the Fas receptor. Significantly, CTLA-4·FasL, but not CTLA-4·Ig, anti-Fas mAb, or the two in combination, abrogated the usual increase in expression of the anti-apototic protein, cFLIP. Furthermore, activation of caspases 8 and 3 were not affected by CTLA-4·FasL. These findings suggest a model for CTLA-4·FasL action wherein there is coordinate triggering of a death receptor and suppression of a proapoptotic protein.