TSG-6 secreted by mesenchymal stem cells suppresses immune reactions influenced by BMP-2 through p38 and MEK mitogen-activated protein kinase pathway

Bone morphogenetic protein 2 (BMP-2) has a critical function in bone and cartilage development and in repairing damaged organs and tissue. However, clinical use of BMP-2 at doses of 0.5–1 mg/ml for orthopedics has been associated with severe postoperative swelling requiring emergency surgical intervention. We determined whether a high concentration of BMP-2 induces inflammatory responses in macrophages and the suppression of osteogenesis in hMSCs. We obtained human periodontal ligament stem cells and bone marrow stem cells from the maxilla, i.e., human mesenchymal stem cells (hMSCs), from the periodontal ligament of extracted third molar teeth and from the bone marrow of the maxilla, respectively. Osteogenic differentiation was measured by alkaline phosphatase activity and alizarin red S staining. Proteins were assessed by flow cytometry, enzyme-linked immunosorbent assay, Western blot and immunocytochemistry. Changes of gene expression were measured by reverse transcription plus the polymerase chain reaction (RT-PCR) and real-time PCR. A high BMP-2 concentration inhibited the early stages of osteogenesis in hMSCs. Co-culturing THP-1 cells (human monocytic cells) with hMSCs reduced the late stages of osteogenesis compared with those seen in hMSCs alone. In addition, high-dose BMP-2 induced the expression of inflammatory cytokines in THP-1 cells and the expression of the anti-inflammatory cytokine tumor-necrosis-factor-α-inducible gene 6 protein (TSG-6) in hMSCs. Consistent with the anti-inflammatory effects of hMSCs when co-cultured with THP-1 cells, interleukin-1β expression was downregulated by TSG-6 treatment of THP-1 cells. Our findings suggest that a high BMP-2 concentration triggers inflammation that causes inflammatory cytokine release from THP-1 cells, leading to the suppression of osteogenesis, whereas TSG-6 secreted by hMSCs suppresses inflammatory reactions through p38 and ERK in the mitogen-activated protein kinase pathway.