Serum Amyloid A Induces G-CSF Expression and Granulocytosis Via Toll-Like Receptor 2.

As a major acute-phase protein, serum amyloid A (SAA) is commonly used as a marker for inflammatory diseases; however, its precise role in inflammation and infection, which often result in granulocytosis or neutrophilia, remains ambiguous. In this study, we demonstrate that SAA is a potent endogenous stimulator of granulocyte colony-stimulated factor (G-CSF), a principle cytokine regulating granulocytosis, and this effect of SAA is mediated by Toll-like receptor 2 (TLR2). Our data showed that in mouse macrophages, the elevation of the G-CSF mRNA level was observed within the first two hours of SAA stimulation while the secretion of the G-CSF protein was significantly increased after eight hours and sustained for 24 hours. The induction of G-CSF by SAA was blocked by the specific anti-TLR2 antibody and significantly decreased in TLR2 knockout mouse macrophages. We also showed that SAA-stimulated G-CSF expression correlates with the activation of nuclear factor κB (NF-κB), the p65 subunit of which was bound to the CK-1 element of the G-CSF promoter region within 30 minutes of the stimulation. In addition, the SAA increased production of G-CSF was tested to be sensitive to heat, yet insensitive to polymyxin B treatment, indicating that the induction is a direct effect of SAA. Furthermore, our in vivo studies confirmed that SAA significantly increases the plasma concentration of G-CSF and the number of neutrophils in the blood circulation while such phenomenon vanishes in G-CSF or TLR2 knockout mice.