Role of Toll‐like receptor‐4 on triggering receptor expressed on myeloid cells‐1 expression in inflammatory cells

Abstract

The triggering receptor expressed on myeloid cells-1 (TREM-1) is a recently described cell surface molecule expressed on blood neutrophils, monocytes and a subset of macrophages, which are the main effecter cells in innate responses. TREM-1 triggers phagocyte secretion of pro-inflammatory chemokines and cytokines and plays an important role in the amplification of inflammatory responses that is induced by bacteria and fungi. However, it is not known what is the natural ligand of TREM-1. Although lipopolysaccharide (LPS), which is known to be a ligand of Toll-like receptor-4 (TLR-4), has been reported to enhance the inflammation of TREM-1 activated myeloid cells, the association between TLR-4 and TREM-1 has not been clarified. In this study, we evaluated whether TLR-4 would be involved in the expression of TREM-1. We used polymorphonuclear neutrophils (PMN) obtained from healthy volunteer donors and human monocytic leukemic cell line (THP-1). Treatment of cells with LPS increased TREM-1 expression dose-dependently, which was measured by Western blotting and flow cytometry analysis. Pretreatment of TLR-4 blocking antibody suppressed TREM-1 expression induced by LPS. After transfection of small interfering RNA (siRNA) for TLR-4, we found that TLR-4 expression was suppressed efficiently and TREM-1 did not increase although LPS stimulation. These results suggest that LPS is not a direct ligand for TREM-1 and TLR-4 might be involved in TREM-1 expression in human neutrophils and monocytic cell line.