To investigate whether prolonged exposure to interleukin‐6 (IL‐6) affects the inflammatory response induced by Toll‐like receptor (TLR) ligands.

IL‐6–transgenic mice and wild‐type mice were stimulated with different TLR ligands; survival rates, blood cell counts, and biochemical parameters were analyzed. Murine splenic mononuclear cells and peritoneal macrophages were stimulated with lipopolysaccharide (LPS), lipoteichoic acid, poly(I‐C), or CpG. Human macrophages were cultured for 4 days in the presence of IL‐6 and then stimulated with LPS. Inflammatory cytokine expression was measured by enzyme‐linked immunosorbent assay or reverse transcription–polymerase chain reaction. Activation of STAT‐3, ERK‐1/2 (MAPK), and p65 NF‐κB was evaluated by Western blotting or confocal analysis.

Treatment of IL‐6–transgenic mice with TLR ligands led to an increased fatality rate and elevated levels of IL‐1β, tumor necrosis factor α (TNFα), IL‐6, and IL‐18. Macrophages from IL‐6–transgenic mice produced increased levels of inflammatory cytokines, which were associated with increased phosphorylation of STAT‐3 and ERK‐1/2 and with increased NF‐κB nuclear translocation. Human macrophages treated with IL‐6 and then stimulated with LPS showed elevated levels of cytokines and similarly elevated signaling pathway activation. After LPS administration, IL‐6–transgenic mice showed an increase in ferritin and soluble CD25 levels, as well as a decrease in platelet and neutrophil counts and in hemoglobin levels compared to wild‐type mice.

Our findings indicate that prolonged exposure to IL‐6 in vivo and in vitro leads to an exaggerated inflammatory response to TLR ligands. Hematologic and biochemical abnormalities in IL‐6–transgenic mice treated with LPS show striking similarities to macrophage activation syndrome.