Signaling Networks Responsible for the Dynamic Modulation of IL-12 in Response to Varying Dosages of LPS in Macrophages

The innate immune response to lipopolysaccharide (LPS) has many possible outcomes depending on the dose, from acute endotoxic shock to broad immunomodulation. High doses (>10 ng/mL) evoke both pro- and anti-inflammatory mediators, while super-low doses (<1 ng/mL) result in preferential induction of pro-inflammatory genes. The mechanisms governing the switch between a preferentially proinflammatory or a balanced, resolving response are poorly understood. We show that in murine macrophages, the pro-inflammatory cytokine interleukin 12 (IL-12) is induced most robustly by intermediate LPS doses (1-10 ng/mL), while higher doses evoke a diminishing response. In the absence of the suppressive Toll-like receptor 4 signaling cascade members Lyn and IRAK-M, macrophages sustain robust IL-12 production in response to high doses of LPS. The transcription factor cAMP response element-binding protein (CREB) appears to play an important role in the regulation of the inflammatory response to LPS, with Lyn and IRAK-M deficient macrophages failing to activate CREB effectively in response to LPS. On the other hand, co-stimulation of these cells with LPS and the CREB agonist adenosine suppresses proinflammatory responses to LPS in wild-type cells, but does not robustly suppress IL-12 production in either Lyn- or IRAK-M-deficient cells. Our study reveals potential mechanisms for the dynamic modulation of innate responses by varying dosages of LPS.