Mast cells (MCs) are pro-inflammatory sensor and effector cells of the immune system. MCs seem to play a role in bone metabolism, because patients with systemic mastocytosis develop osteoporosis. MCs are present in the fracture callus during bone healing, however, their function has not yet been investigated. Here, we examined the role of MCs in the inflammatory and repair phase during fracture healing.
Male 12-week old MC deficient mice (Mcpt5-Cre R-DTA) and wildtype mice (WT) received a femur osteotomy, stabilized by an external fixator. After 1 day, the fracture hematoma was analyzed for inflammatory mediators (multiplex immunoassay) and immune cells (flow cytometry). Femur sections were subjected to histomorphometry (day 7, 14, 23). Osteoclasts were evaluated using tartrate-resistant acid phosphatase staining (TRAP). The healing outcome was assessed at day 23 by biomechanical testing (three-point bending). The study was approved by the National Ethical Committee, Germany, no. 1149). Statistics: Students t-test.
Neutrophil recruitment to the fracture hematoma of MC deficient mice was diminished compared to WT (−66%, P<0.05). Pro-inflammatory cytokines including IL-1β, IL-6, and TNF-α were significantly reduced (−54, −58, and −45%, P<0.005). The bending stiffness of the healed bone of MC deficient mice was significantly increased (+147%, P<0.01). The bone fraction was also increased (+53%, P<0.005) whereas the osteoclast number was reduced in the peripheral fracture callus (N.Oc/BS, 22%, P<0.005) indicating reduced bone remodeling in MC deficient mice.
In conclusion, MC deficiency reduced neutrophil recruitment and pro-inflammatory cytokines in the fracture hematoma and increased the bone content in the peripheral fracture callus probably due to reduced callus remodeling. These results demonstrate that MCs may be important regulators in the early inflammatory as well as in the repair and remodeling phase of fracture healing.
14 May 2016 - 17 May 2016