38 Mesenchymal Progenitor-derived VEGF is a Major Source of Cells Contributing to Heterotopic Bone

Abstract Introduction Patients sustaining mechanical trauma, burns, or orthopedic procedures can develop heterotopic ossification (HO) or pathologic development of extra-skeletal bone. Vascularization, mainly mediated by VEGFa, is required for different stages of endochondral ossification, namely, establishment of the primary osseous center. Moreover, VEGFa is crucial for bone repair by also promoting bone turnover signaling. We hypothesize that VEGFa is required for HO and cells of the mesenchymal lineage are the major contributors for this signal. Methods First, C57BL/6J male mice underwent through Achilles tendon transection and 30% of TBSA dorsal burn injury. Mice underwent Microfil CT scans to survey local vascular structures. Hindlimb sections from injured mice were immunostained, and injury site was harvested for flow cytometry and PCR. Mice in bevacizumab v. control treatment arms received biweekly injections of drug (10mg/kg) or PBS. Separate cohorts underwent MicroCT analysis at 9 weeks. Results Five weeks after induction surgery, vascular density was higher, as demonstrated by Microfil. These findings were consistent with near infrared imaging using Angiosense. Furthermore, VEGFa expression was increased in both protein and mRNA levels in tissue isolated from HO and surrounding regions. To understand whether direct actions of VEGFa derived from the mesenchymal cells play a role, we deleted the VEGFa gene from cells of mesenchymal lineage by crossing Vegff/f mouse with Prx1-Cre mice. Both VEGFf/f,Prx1-Cre male mice and their littermate control underwent burn/tenotomy as previously described. MicroCT scans demonstrated that conditional knockout mice exhibit less HO formation near the distal tibia but not proximal tibia, after normalized to tibial cortical thickness. To translate this clinically, we next performed our traumatic HO model in C57BL/6J mice treated with VEGFa inhibitor bevacizumab vs. PBS. Bevacizumab treated mice formed significantly less HO than PBS injection controls at 800HU (2.64 v. 6.85 mm3, p=.0013). Conclusions This data suggests that acute local trauma causes alterations in vascular signaling. Additionally, VEGFa derived from mesenchymal cells are a major source for VEGFa which is required for HO formation. Applicability of Research to Practice Attenuation of local VEGFa signaling via existing, FDA-approved monoclonal antibodies might be an effective therapeutic treatment to reduce or prevent HO formation in burn and polytraum