IFN-γ-upregulated KSL cell Fas expression and Fas-mediated destruction. (A) KSL cells from IFN-γ-injected mice (10 μg per mouse; n = 9) had a significantly higher proportion of cells expressing the apoptotic receptor Fas relative to KSL cells from untreated (n = 8) mice. (B) BM cells from B6 mice (n = 10) cultured with 50 ng/mL IFN-γ for 6 days in vitro also had significant upregulation in Fas expression on KSL cells compared with cultured BM cells without IFN-γ (n = 10). (C) From the same mice as described in Figure 2C, the proportion and total number of Fas⁺KSL cells were significantly reduced in those that received treatment with IFN-γ+CD8 T cells compared with those exposed to IFN-γ alone. (D) BM cells from FVB mice (n = 6) were cultured for 24 hours in RPMI 1640 media supplemented with 25 ng/mL IFN-γ (IFN-γ), 25 ng/mL IFN-γ plus 0.5 to 2.0 μg/mL anti-Fas antibody (IFN-γ+anti-Fas), or media only (control). The proportion of KSL cells was reduced in the IFN-γ+anti-Fas treatment group relative to the IFN-γ group. Recovery of total KSL cells was reduced (P < .01) with IFN-γ+anti-Fas treatment relative to treatment with IFN-γ alone. IFN-γ treatment also reduced total BM cell recovery relative to control, whereas IFN-γ+anti-Fas treatment further reduced BM cell recovery relative to treatment with IFN-γ alone. *P < .05; ***P < .001; ****P < .0001.