IFN-γ-upregulated Fas expression and Fas-mediated apoptosis in KLCD150⁺ cells. (A) IFN-γ-injected B6 mice (10 µg per mouse, n = 4) had a significantly lower proportion of KLCD150⁺ cells but a significantly higher proportion of Fas⁺ cells within the KLCD150⁺ population relative to untreated controls (n = 8). (B) Similarly, the proportion of KLCD150⁺CD48^– cells was significantly lower but the proportion of Fas⁺ cells was significantly higher in the KLCD150⁺CD48^– cell population in IFN-γ-injected mice than in controls. (C) Sorted KLCD150⁺ BM cells from IFN-γ-injected mice (n = 5) or untreated controls (n = 5) were mixed with activated CD4 and CD8 T cells (from BM failure mice) at a KLCD150⁺:CD4:CD8 ratio of 1:5:10 and cultured at 37°C with 5% CO₂ for 60 minutes. The proportion of annexin V⁺ apoptotic KLCD150⁺ cells was significantly higher in IFN-γ-treated than in untreated control mice. *P < .05; **P < .01; ****P < .0001.