FAS-R ligation induces apoptosis in U937 cells and in monocytes. (A) U937 cells express the FAS-R (CD95) as determined by flow cytometry. (B-C) FASL and anti-FAS Ab induce apoptosis in U937 cells in a dose-dependent manner. (D) Fresh human monocytes express CD95 as determined by flow cytometry. (E-F) FASL and anti-FAS Ab induce apoptosis in monocytes in a dose-dependent manner. The frequency of apoptosis for U937 cells was determined by annexin V+/PI-staining; apoptosis in monocytes was determined by the frequency of events in the sub-G1 region. The results are presented as the fold increase in the frequency of apoptosis over control cells left untreated for 3 hours and are representative of at least 5 independent experiments for each cell type.