Background The coordinated change of haematopoietic supporting microenvironment in bone marrow (BM) is crucialfor innate immunity and inflammation.As the precursors of marrow stroma,BM derived mesenchymal stem cells (MSCs)promote haematopoietic function,but their roles in innate immunity or inflammation have not been investigated.Here weinvestigated the expression of Toll like receptor 4 (TLR-4) and the effect of lipopolysaccharide (LPS) on its expression inBM MSCs in vitro.Methods MSCs were harvested from adult rat's BM cells by density gradient centrifugation and adhesive culture.Thepurity of MSCs were identified with the cell morphological feature and osteogenic capacity,the phenotypes were testedby flow cytometry.Cultured MSCs were treated by LPS (1μg/ml,10μg/ml or 100 μg/ml) for 24 hours.The relativeexpression levels of TLR-4 mRNA were detected by semiquantitative reverse transcription polymerase chain reactionand costimulatory molecules(CD80,CD86 and MHC-Ⅱ)expressed on MSCs were analyzed by flow cytometry.Thelevels of tumor necrosis factor-α (TNF-α) in supernatants were determined by enzyme linked immunosorbent assay.Results After incubation with LPS,MSCs expressed the higher levels of TLR-4 mRNA,costimulatory molecules andTNF-α than the untreated group:LPS 10 μg/ml was the most effective (P<0.01);the levels of TLR-4 mRNA,costimulatorymolecules and TNF-α decreased when MSCs were exposed to 100 μg/ml LPS.Except for MHC-Ⅱ and TNF-α(P>0.05),the levels of CD80,CD86 and TLR-4 mRNA were significantly lower than that in the treated group of 10 μg/ml (P<0.01).Conclusion MSCs expressed TLR-4 mRNA.LPS activated the functional expression levels of TLR-4 in MSCs althoughthe activity may depend on the concentration of LPS.Chin Med J 2007;120(19):1685-1688 Background The coordinated change of haematopoietic supporting microenvironment in bone marrow (BM) is crucial for innate immunity and inflammation. As the precursors of marrow stroma, BM derived mesenchymal stem cells (MSCs) promote haematopoietic function, but their roles in innate immunity or inflammation have not was studied. He was weinvestigated the expression of Toll like receptor 4 (TLR-4) and the effect of lipopolysaccharide (LPS) on its expression in BM MSCs in vitro. Methods MSCs were harvested from adult rat's BM cells by density gradient centrifugation and adhesive culture. The Purity of MSCs were identified with the cell morphological feature and osteogenic capacity, the phenotypes were tested by flow cytometry. Cultured MSCs were treated by LPS (1 μg / ml, 10 μg / ml or 100 μg / ml) for 24 hours. The relative expression levels of TLR -4 mRNA were detected by semiquantitative reverse transcription polymerase chain reaction and costimulatory molecules (CD80, CD86 and MHC-II) expressed on MSCs were analyzed by flow cytometry. The levels of tumor necrosis factor-α (TNF-α) in supernatants were determined by enzyme linked immunosorbent assay. Results after incubation with LPS, MSCs expressed higher levels of TLR-4 mRNA, costimulatory molecules and TNF- α The levels of TLR-4 mRNA, costimulatory molecules and TNF-α decreased when MSCs were exposed to 100 μg / ml LPS. Except for MHC-Ⅱ The levels of CD80, CD86 and TLR-4 mRNA were significantly lower than those in the treated group of 10 μg / ml (P <0.01) .Conclusion MSCs expressed TLR-4 mRNA.LPS activated the functional expression levels of TLR-4 in MSCs although the activity may depend on the concentration of LPS. Chin Med J 2007; 120 (19): 1685-1688