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Macrophage-mediated Production of BMP Ligand Induces Tendon Degeneration and Heterotopic Ossification
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.
University of Michigan, Ann Arbor, MI, USA.

PURPOSE: Heterotopic ossification (HO) after trauma is characterized by upregulation of TGF-beta/Smad signaling. Bone morphogenetic proteins (BMP) 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 TGFbeta/Smad signaling in the formation of HO and identify macrophages as a possible mediator of this signal post-traumatic HO.
METHODS: C57Bl/6 and tamoxifen-inducible BMP-receptor knockout mice (Ub.creERT;Acvr1fl/fl) underwent Achilles’ tenotomy and 30% TBSA burn. Mice were survived to 9 weeks post-injury for microCT analysis. Separate burn/tenotomy mice were euthanized 48 hours and 1 week post-injury to isolate macrophages during the earliest stages of HO development. Flow cytometry was used to analyze F4/80+ macrophages for expression of TGF-beta and BMP2. Finally, burn/tenotomy mice were depleted of macrophages with clodronate and euthanized at 3 weeks to evaluate cartilage formation.
RESULTS: BMP-receptor knockout mice (Ub.creERT;Acvr1fl/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 post-injury. BMP2 was expressed by 40.10+/-5.26% of macrophages and TGF-beta by 93.87+/-2.19% (B). Depletion of macrophages using clodronate reduced cartilage presence.
CONCLUSION: Cartilage formation in HO is preceded by infiltration of macrophages, which express pro-chondrogenic factors including TGF-beta and BMP2. Macrophage depletion reduces cartilage formation, suggesting a mechanistic role for these cells.


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