IRF8 enhances the proapoptotic effect of IRF1 in the absence of IFNγ. A, OPCs were co-transfected with the following combinations of expression constructs by electroporation; The control GFP vector (PCMV-IE-IRES-hrGFP-pA) plus the rat IRF8 expression construct without GFP reporter (PCMV-IE-IRF8-pA) (GFP+IRF8), the rat IRF1 expression construct with GFP reporter (PCMV-IE-IRF1-IRES-hrGFP-pA) plus the rat IRF8 expression construct without GFP reporter (PCMV-IE-IRF8-pA) (IRF1+IRF8), or PCMV-IE-IRF1-IRES-hrGFP-pA plus the empty vector (PCMV-IE-pA) (IRF1+empty). These OPCs were incubated with TMRE and DAPI at 6 and 24 h after transfection and analyzed by flow cytometry. B, Number of preapoptotic (TMRE-/DAPI-) OPCs at 24 h after electroporation with GFP+IRF8, IRF1+empty, and IRF1+IRF8. Transfected (hrGFP+) and untransfected (hrGFP-) populations were analyzed separately, using the same gatings as in Figure. 7. ** Indicates p < 0.01 compared with GFP- (non-transfected) counterparts, or in comparison between the two groups indicated. C, Reduction in live transfected cells (DAPI-/hrGFP+) at 24 h in the cultures transfected with GFP+IRF8, IRF1+empty, or IRF1+IRF8. Percentages of DAPI-/hrGFP+ cells in total live (DAPI-) cells were calculated in each condition, and are shown as fold changes of those at 6 h after transfection. N.S. indicates p > = 0.05 between the two groups indicated.