CCL2 Facilitates Osteoclastogenesis and Osteogenesis in Both Young and Aged Female Murine MSC–Macrophage Co-Cultures

“Inflammaging” is the presence of continuous systemic low-level chronic inflammation with increasing age, and impairs bone healing. C-C motif chemokine ligand 2 (CCL2) promotes the migration of macrophages during acute inflammation, and subsequent macrophage–mesenchymal stromal cell (MSC) crosstalk. We hypothesized that CCL2 would enhance bone regeneration in both young and aged primary murine cells and investigated the effects of CCL2 on osteogenesis and osteoclastogenesis. In the following experiments, 10–12-week-old and 15–17-month-old female mice were used. For the osteogenic differentiation, MSCs and naïve uncommitted macrophages (M0) were isolated and co-cultured to investigate the effect of CCL2 on the cell–cell interaction, alkaline phosphatase (ALP), and Alizarin-Red staining. For the osteoclastogenic differentiation, primary bone marrow cells were isolated and cultured, then TRAP and Cathepsin K staining and quantitative real-time RT-PCR were performed. In the CCL2-treated group, cells were cultured with recombinant CCL2 for an initial 24 h. MSCs from young mice showed greater osteogenic ability than aged mice. The CCL2-treated group showed a greater Alizarin-Red-positive area both in young and aged cells. In the osteoclastogenic differentiation assays, the number of tartrate-resistant acid phosphatase (TRAP)-positive cells and the expression level of C-fos and Dc-stamp were increased in the CCL2-treated group both in young and aged cells. These findings indicate that CCL2 promotes osteogenesis, particularly mineralization, and enhances selected osteoclastogenic features in both young and aged primary murine cells. These effects suggest that CCL2 may have a potential role in bone remodeling, including in the context of aging.