M2-like macrophages derived from THP-1 cells promote myofibroblast differentiation of synovial fibroblasts in association with the TGF-β1/SMAD2/3 signaling pathway.

Fibrosis occurs during progression of osteoarthritis (OA), and myofibroblasts are considered a key effector to drive the fibrotic response. Macrophages also play critical roles in OA progression. However, whether macrophage polarization is involved in OA-related fibrosis has not been adequately defined. Here, we investigated the effect of M2-like macrophages compared to M1-like macrophages on the myofibroblast differentiation of human synovial fibroblasts (HSFs). M1- and M2-like macrophages differentiated from the human monocytic THP-1 cells were co-cultured with HSFs for 72 h. Alpha-smooth muscle actin (α-SMA) positive cells and gene expression of pro-fibrotic and anti-fibrotic factors were quantified. The concentration of transforming growth factor-beta1 (TGF-β1) in the culture supernatant was also analyzed, and its effect on the regulation of the TGF-β/SMAD signaling pathway was further investigated. We found that, cocultured with M2-like macrophages increased the number of α-SMA positive cells and expression of pro-fibrotic genes and decreased the expression of anti-fibrotic genes in HSFs. TGF‑β1 was highly secreted by M2‑like macrophages and accelerated the phosphorylation of SMAD2/3 in HSF cells. Our results indicate the pro-fibrotic effects of M2-like macrophages in myofibroblast differentiation of HSFs, in association with the TGF-β1/SMAD2/3 signaling pathway activation. Thus, M2-like macrophages may play a role in OA fibrogenesis and its progression.