Gamma interferon augments macrophage activation by lipopolysaccharide by two distinct mechanisms, at the signal transduction level and via an autocrine mechanism involving tumor necrosis factor alpha and interleukin-1

When given in the presence of gamma interferon (IFN-gamma), otherwise nonto xic doses of lipopolysaccharide (LPS or endotoxin) become highly lethal for mice. The mechanisms of this synergistic toxicity are not known. We consid ered the possibility that an interaction between the LPS-induced NF-kappa B and IFN-gamma-induced JAK-STAT pathways at the pretranscriptional level ma y enhance the LPS-induced signals. To test this hypothesis, we incubated mu rine macrophage IL RAW 264.7 cells with IFN-gamma for 2 h before addition o f different doses of LPS, Consistent with the synergistic induction of indu cible nitric oxide synthase mRNA and nitric oxide production by a combinati on of LPS and IFN-gamma, IFN-gamma strongly augmented LPS-induced NF-kappa B activation and accelerated the binding of NF-kappa B to DNA to as early a s 5 min. In agreement with this, IFN-gamma pretreatment promoted rapid degr adation of I kappa B-alpha but not that of I kappa B-beta. Inhibition of pr otein synthesis during IFN-gamma treatment suppressed LPS-initiated NF-kapp a B binding, A rapidly induced protein appeared to be involved in IFN-gamma priming. Preincubation of cells with antibodies to tumor necrosis factor a lpha or the interleukin-l receptor partially reduced the priming effect of IFN-gamma. In a complementary manner, LPS enhanced the activation of signal -transducing activator of transcription 1 by IFN-gamma. These data suggest novel mechanisms for the synergy between IFN-gamma and LPS by which they cr oss-regulate the signal-transducing molecules. Through this mechanism, IFN- gamma may transform a given dose of LPS into a lethal stimulus capable of c ausing sepsis. It may also serve a beneficial purpose by enabling the host to respond quickly to relatively low doses of LPS and thereby activating an tibacterial defenses.