Rheumatoid arthritis synovial fibroblasts promote TREM-1 expression in monocytes via COX-2/PGE2 pathway

Background Triggering receptor expressed on myeloid cells-1 (TREM-1) is inducible on monocyte/macrophages and neutrophils and amplifies the inflammatory response. The aim of this study was to determine whether rheumatoid arthritis synovial fibroblasts (RASF) promote the expression of TREM-1 in monocytes and its potential regulatory mechanism. Methods Synovial fluid and paired peripheral blood from rheumatoid arthritis (RA) patients were analyzed using flow cytometry. Expression of TREM-1 in monocytes was detected after co-culture with RASF, with or without pre-treatment with toll-like receptor (TLR) ligands. Whether RASF-regulated TREM-1 level in monocytes require direct cell contact or soluble factors was evaluated by transwell experiment. COX-2 expression and PGE.sub.2 secretion in RASF were determined by quantitative PCR (qPCR) and ELISA. RASF, with and without TLR ligand stimulation, were treated with COX-2 inhibitors, COX-2 siRNA (siCOX-2) or EP1-4 antagonists, and the resulting TREM-1 level in CD14.sup.+ monocytes was measured using flow cytometry. Results TREM-1 was highly expressed in CD14.sup.+ cells from peripheral blood and especially synovial fluid from RA patients. The expression of TREM-1 in monocytes was increased by co-culture with RASF. TLR-ligand-activated RASF further elevated TREM-1 level. Transwell assay indicated that soluble factors played a key role in RASF-promoted expression of TREM-1 in monocytes. RASF, with or without stimulation by TLR ligands, increased secretion of PGE.sub.2 in a cyclooxygenase (COX)-2-dependent manner. PGE.sub.2 enhanced the increase in TREM-1 level in monocytes. Finally, studies using COX-2 inhibitors, COX-2 siRNA (siCOX-2) and EP1-4 antagonists, showed that RASF promotion of TREM-1 expression in monocytes was mediated by COX-2/PGE.sub.2/EP2,4 signaling. Conclusions Our data is the first report to reveal the critical role of RASF in upregulating TREM-1 expression in monocytes, which indicates that TREM-1 might be a novel target for RA therapy. Keywords: Triggering receptor expressed on myeloid cells-1(TREM-1), Monocytes, Rheumatoid arthritis synovial fibroblasts (RASF), Prostaglandin E2 (PGE.sub.2)