Pro‐inflammatory M1 macrophages promote Osteogenesis by mesenchymal stem cells via the COX‐2‐prostaglandin E2 pathway

Abstract

Bone fractures are among the most common orthopaedic problems that affect individuals of all ages. Immediately after injury, activated macrophages dynamically contribute to and regulate an acute inflammatory response that involves other cells at the injury site, including mesenchymal stem cells (MSCs). These macrophages and MSCs work in concert to modulate bone healing. In this study, we co-cultured undifferentiated M0, pro-inflammatory M1, and anti-inflammatory M2 macrophages with primary murine MSCs in vitro to determine the cross-talk between polarized macrophages and MSCs and their effects on osteogenesis. After 4 weeks of co-culture, MSCs grown with macrophages, especially M1 macrophages, had enhanced bone mineralization compared to MSCs grown alone. The level of bone formation after 4 weeks of culture was closely associated with prostaglandin E2 (PGE2) secretion early in osteogenesis. Treatment with celecoxib, a cyclooxygenase-2 (COX-2) selective inhibitor, significantly reduced bone mineralization in all co-cultures but most dramatically in the M1-MSC co-culture. We also found that the presence of macrophages reduced the secretion of osteoprotegerin (OPG), the decoy RANKL receptor, suggesting that macrophages may indirectly modulate osteoclast activity in addition to enhancing bone formation. Taken together, these findings suggest that an initial pro-inflammatory phase modulated by M1 macrophages promotes osteogenesis in MSCs via the COX-2-PGE2 pathway.

Cellular biology of fracture healing

“…Thereafter, pathogen‐associated molecular pattern binding to TLR‐family receptors on CAMs up‐regulates the pro‐inflammatory cytokines TNF‐alpha, IL1 and IL6 through the NFκB pathway. Recent work from the Goodman laboratory has shown that CAMs indirectly promote osteogenesis by regulating MSC, albeit these studies have not been supported by in vivo studies …”

“…Recent work from the Goodman laboratory has shown that CAMs indirectly promote osteogenesis by regulating MSC, albeit these studies have not been supported by in vivo studies. 44 Once macrophages have debrided the wound and are no longer classically activated, they can assume an anti-inflammatory state. Anti-inflammatory macrophages, also known as alternatively activated macrophages (AAMs), are generated through IL4 and IL13 signaling.…”

NFκB sensing IL‐4 secreting mesenchymal stem cells mitigate the proinflammatory response of macrophages exposed to polyethylene wear particles

“…Crosstalk between immune cells and osteoprogenitor‐lineage cells could determine the status of bone regeneration . Acute inflammation is essential for the bone regeneration.…”

“…Crosstalk between immune cells and osteoprogenitorlineage cells could determine the status of bone regeneration. [31][32][33] Acute inflammation is essential for the bone regeneration. Depletion of macrophages at an earlier stage suppressed bone repair in a murine model.…”

A novel role for cathepsin K in periosteal osteoclast precursors during fracture repair

“…Macrophages, the cells from which OCPs arise, have increasingly been found to play critical roles in regulating bone homeostasis and repair in response to injury. A recent in vitro study in which undifferentiated (M0), proinflammatory (M1), and anti‐inflammatory (M2) murine macrophages were cocultured with murine MSCs found that M1 macrophages enhanced MSC osteogenic differentiation via the cyclooxygenase‐2/prostaglandin E2 pathway . In vivo analysis of macrophage contribution to bone repair using macrophage Fas–induced apoptosis (MaFIA) transgenic mice revealed that depletion of macrophages during the initial inflammatory phase of fracture repair abrogated callus formation, whereas macrophage depletion during the early anabolic phase led to reduced callus formation.…”