BMP2 Causes Ectopic Bone Formation Via Macrophage-Driven Inflammation

Objectives: Bone-morphogenetic protein-2 (BMP2) is a bone-grafting substitute for craniofacial bone reconstruction, whose use is encumbered by clinical complications including life-threatening postoperative inflammation and ectopic bone formation. Macrophages are key mediators of inflammation and are implicated in both orthotopic and heterotopic bone formation. We hypothesize that BMP2 triggers a macrophage-driven inflammation that causes ectopic bone formation. If this hypothesis is true then modulation of macrophage-driven inflammation would be a potential therapeutic approach to prevent heterotopic ossification. Here, we sought to define the contribution of macrophage-driven inflammation to ectopic bone formation by BMP2.
Methods: Bone graft material harvested from the long bones of transgenic (Axin2^LacZ/+,ACTb-EGFP) adult mice was treated with PBS or BMP2 (50ng/µL) then transferred to either a calvarial bone defect or to the sub-renal capsule (SRC), of syngeneic host mice. Grafted tissues were assessed clinically, by histology, and by immunohistochemistry at days 3, 7, 14 and 35 after grafting.
Results: BMP2-treated grafts were associated with significant soft tissue swelling (Fig1A-D). These clinical findings were substantiated by histological evaluation. At day 35, BMP2-treated bone grafts had generated significant amounts of bone that overfilled the defect and reached the subcutaneous space (Fig1E-F). This BMP2-induced ectopic bone was associated with increased number of F4-80⁺ cells at day 7 after grafting (Fig2A-B). Similar findings were confirmed in SRC model at day 7 (Fig2C-D).
Conclusions: F4-80⁺ cell infiltration was triggered by BMP2 in both orthotopic and heterotopic bone sites, suggesting that BMP2 elicits a macrophage-driven inflammatory response causing bone formation. Targeting macrophage-driven inflammation constitutes a potential pathway to further understand and then prevent BMP2 associated ectopic bone formation.