Effect of Fas ligand expression on HB1.F3-induced T-cell apoptosis. (A) Fas, PD-L1, and TR-2 (TRAIL receptor-2) were expressed on HB1.F3 cells. (B) CD4+ T cells, CD8+ T cells, and HB1.F3 cells constitutively did not express Fas ligand (FasL). FasL expression on CD4+ T cells co-cultured with HB1.F3 was induced at 12 hours, but FasL expression on CD8+ T cells was much less dramatic. The FasL level of CD4+ T cells was higher than CD8+ T cells in a small population. (C) FasL (median fluorescence intensity) on CD8+ T cells did not significantly change (dark shading) during the co-culture period, whereas marked FasL upregulation on CD4+ T cells was detected (no shading). (D) The inhibition of CD4+ T cells apoptosis showed anti-FasL mAbs dose-dependent manner.