Previous studies from our laboratory have shown that fulminant hepatitis caused by the mouse hepatitis virus, MHV-3, is dependent on production of the novel immune coagulant fgl2/fibroleukin. In this study, we investigate the role of IFN-gamma and TNF-alpha in the induction of fgl2 expression and fgl2-dependent hepatic apoptosis. Infusion of IFN-gamma in combination with TNF-alpha through the portal vein of fgl2+/+ mice led to widespread hepatic apoptosis and fibrin deposition. Livers from fgl2-/- mice were normal, although strong expression of the fgl2 knockout reporter gene Lac Z was seen in both resident hepatic macrophages and endothelial cells. In vitro, IFN-gamma and TNF-alpha induced fgl2 expression in a macrophage and endothelial cell-specific manner. In macrophages (peritoneal and RAW 264.7 cells), IFN-gamma, but not IFN-alpha, LPS, TNF-alpha, or IL-1 induced fgl2 mRNA transcription and protein expression, while in endothelial cells TNF-alpha, but not IFN-gamma, induced fgl2 transcription. In addition, while TNF-alpha enhanced IFN-gamma-induced macrophage fgl2 transcription, IFN-gamma also enhanced TNF-alpha-induced endothelial cell fgl2 transcription. The induction of fgl2 by IFN-gamma in macrophages involved a STAT1-dependent pathway, involving the composite cis elements Sp1/Sp3 and GAS/PU.1. The latter interacted with IFN-gamma-dependent Sp1/Sp3, STAT1, and the ETS family of transcription factors member PU.1. The interaction of PU.1 with the IFN-gamma-activated sequence/ETS family of transcription factors site determined the macrophage-specific induction of fgl2 by IFN-gamma. Overall, this study demonstrates that IFN-gamma and TNF-alpha induce hepatocyte apoptosis in vivo, which is dependent on induction of fgl2, and defines the molecular basis of transcription of fgl2 in vitro.