Impaired IFN-γ production in IFN regulatory factor-1 knockout mice during endotoxemia is secondary to a loss of both IL-12 and IL-12 receptor expression

Mice with a targeted mutation in the gene that encodes the transcription factor IFN regulatory factor-1 (IRF-1) were used to assess the contribution of IRF-1 to IL-12-dependent and IL-12-independent pathways of IFN-gamma production. In response to LPS, IRF-1(-/-) mice produced less IL-12 p40, IL-12 p35, and IFN-gamma mRNA in the liver than IRF-1(+/+) mice. While pulmonary IFN-gamma mRNA levels were also mitigated in IRF-1(-/-) mice, pulmonary IL-12 p40 and IL-12 p35 mRNA were not dysregulated. Circulating IL-12 p70 and IFN-gamma levels were profoundly attenuated in LPS-challenged IRF-1(-/-) mice. Further analysis revealed a major deficiency in hepatic IL-12R beta 1 and IL-12R beta 2 mRNA expression as well as pulmonary IL-12R beta 1 mRNA expression in LPS-challenged IRF-1(-/-) mice. In vitro, IFN-gamma up-regulated IL-12R beta 1 mRNA in macrophages from IRF-1(+/+), but not IRF-1(-/-), mice. IFN-gamma-induced IL-12R beta 2 mRNA expression was also diminished in macrophages from IRF-1(-/-) mice. In contrast to IRF-1(+/+) mice, administration of exogenous IL-12 to IRF-1(-/-) mice resulted in reduced serum IFN-gamma and hepatic and pulmonary IFN-gamma mRNA, demonstrating that loss of IL-12R results in diminished IL-12 responsiveness. While LPS-challenged IRF-1(-/-) mice also had reduced IL-15 mRNA levels, serum IL-18 responses were intact. Finally, induction of IRF-1 mRNA by LPS in livers of IFN-gamma knockout mice were markedly attenuated, suggesting a feedback amplification loop. These studies indicate that IRF-1 deficiency disrupts both IL-12-dependent and -independent pathways of IFN-gamma production and that IRF-1 is a critical transcription factor involved in the regulation of not only IL-12, but also IL-12R.