Evidence for IFN-γ as a mediator of the lethality of endotoxin and tumor necrosis factor-α

Current evidence indicates that endogenously produced peptide cytokines, most notably TNF-α and IL-1, mediate the lethality of experimental endotoxemia. Because circulating serum levels of IFN-γ can be detected soon after TNF-α and IL-1 in response to endotoxin, we investigated the role of IFN-γ in endotoxin and TNF-α lethality. Specific neutralizing antibodies to murine TNF-α (anti-TNF-α Ab) or murine IFNγ (anti-IFN-γ Ab) produced in our laboratory protected mice against the lethality of Escherichia coli endotoxin (LPS) administered 6 h later. Serum IFN-γ levels 2 h after i.v. LPS were lower in mice treated with anti-TNF-α Ab compared to mice that received nonimmune IgG (median <2.5 vs 3.0 U/ml, P₂ < 0.05). In contrast, serum TNF-α levels 1 h after i.v. LPS peaked more than fourfold higher in mice treated with anti-IFN-γ Ab compared to controls (median > 6400 vs 1405 pg/ml, p₂ < 0.05). Doses of TNF-α (300 μg/kg) and IFN-γ (50,000 U) which were well tolerated when given individually were synergistically lethal in combination (0% lethality vs 100% lethality, P₂ < 0.001), and were associated with higher serum levels of IL-6 than with either cytokine alone. Anti-IFN-γ Ab provided complete protection against exogenous human rTNF-α at the LD₁₀₀ dose (1400 μg/kg, p₂ < 0.001), and in fact prevented lethality at doses four- to fivefold greater than the LD₁₀₀ human rTNF-α (up to 6000 μg/kg). We conclude that IFN-γ is synergistic with TNF-α, is essential for the lethality of LPS and TNF-α, and may have modulating effects on the negative control of serum levels of TNF-α after LPS in mice.