PRS PSRC Podium Proofs 2016
Shawn Loder, BS, Shailesh Agarwal, MD, David Cholok, BS, Kavitha Ranganathan, MD, Hsiao Hsin Sung, DDS, John Li, MD, Shuli Li, MD, Benjamin Levi, MD
From the University of Michigan, Ann Arbor, Mich.
PURPOSE: Heterotopic ossification (HO) after trauma is characterized by upregulation of transforming growth factor (TGF)-β/Smad signaling. Bone morphogenetic proteins (BMPs) act as a major intermediary along this pathway. One of the clinical hallmarks of HO-causing injuries severe and sustained inflammation. Here, we redemonstrate the importance of TGF-β/Smad signaling in the formation of HO and identify macrophages as a possible mediator of this signal posttraumatic HO.
METHODS: C57Bl/6 and tamoxifen-inducible BMP receptor knockout mice (Ub.creERT;Acvr1 fl/fl) underwent Achilles’ tenotomy and 30% TBSA burn. Mice were survived up to 9 weeks postinjury for micro computed tomography analysis. Separate burn/tenotomy mice were killed 48 hours and 1 week postinjury to isolate macrophages during the earliest stages of HO development. Flow cytometry was used to analyze F4/80+ macrophages for expression of TGF-β and BMP2. Finally, burn/tenotomy mice were depleted of macrophages with clodronate and killed at 3 weeks to evaluate cartilage formation.
RESULTS: BMP receptor knockout mice (Ub.creERT;Acvr1 fl/fl) produced significantly less HO versus littermate controls (A). F4/80+ macrophages were present by 48 hours and comprised 14.93% ± 3.49% of the cells at the tenotomy site by 1 week postinjury. BMP2 was expressed by 40.10% ± 5.26% of macrophages and TGF-β by 93.87% ± 2.19% (B). Depletion of macrophages using clodronate reduced cartilage presence.
CONCLUSIONS: Cartilage formation in HO is preceded by infiltration of macrophages, which express prochondrogenic factors including TGF-β and BMP2. Macrophage depletion reduces cartilage formation, suggesting a mechanistic role for these cells.