Activated human platelets express Fas-L and induce apoptosis in Fas-positive tumor cells

Tn addition to their role in hemostasis and thrombosis, platelets are impor tant modulators of immune ruin inflammatory responses. We provide evidence here that human platelets contain abundant quantities of Fas-L, and upon ac tivation, they express it on their surface as well as release it into mediu m. This surface-expressed Fas-L is biologically active and call induce apop tosis in Fas-positive human tumor cells. Therefore, activated platelets may represent an important player in Fas/Fas-L-mediated apoptosis.