CD70-CD27 interaction was involved in HB1.F3-induced T cell apoptosis. (A) While constitutive expression of CD70 mRNA was detected on HB1.F3, other co-signaling molecule mRNA was not detected. T cells constitutively expressed CD27 as well as several the other co-signaling molecules such as CD28, CTLA4, HVEM, and ICOS. (B) CD70 expression on HB1.F3 and CD27 on T cells was also confirmed by fluorescence-activated cell sorting (FACS). CD27 expression level on CD4+ T cells were above 90%, but CD8+ T cells were about 50%. (C) Addition of anti-CD27 and CD70 mAbs resulted in a dose-dependent inhibition of the CD4+ T-cell apoptosis but did not inhibit the apoptosis completely (■, CD4+ T cells only; ○, CD4+ T cells co-cultured w/ HB1.F3 + isotype control mAbs); ●, CD4+ T cells co-cultured w/ HB1.F3 + anti- CD27 or anti-CD70 mAbs). (D) To assess the impact of anti-CD40 and PD-L1 mAbs on the CD4+ T-cell apoptosis, we measured the CD4+ T-cell apoptosis ratio after anti-CD27, anti-CD40 or anti-PD-L1 mAbs treatment. This showed an inhibitory effect of the CD4+ T-cell apoptosis compared with the media control group (□, CD4+ T cells only; ■, CD4+ T cells w/ HB1.F3). (E) FasL expression level on CD4+ T cells reduced about 50% by addition of anti-CD27 mAb.