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Objectives and questions: Mast cells are tissue-resident immune cells that rapidly respond to early alarm signals triggered by trauma by releasing various inflammatory mediators. Recently, we identified mast cells as key contributors to inflammation and impaired bone healing following severe trauma. The pro-inflammatory cytokine interleukin-6 (IL-6) is recognized as a major driver of systemic post-traumatic inflammation, which is linked to compromised bone repair. In this study, we examined the specific role of mast cell-derived IL-6 in trauma-induced impaired bone healing.

Material and methods: We used male mice with a mast cell-specific deletion of IL-6 (Mcpt5-Cre RDT-A x B6.Il6tm1.1Jhoflox, while Cre-negative littermates served as controls. To study fracture healing, 12-week-old mice underwent femur osteotomy stabilized with an external fixator, while half of them additionally sustained thoracic trauma (Reg.No. 1518). Effects on the systemic immune response were analyzed 3 hours post-trauma by measuring pro-inflammatory mediator concentrations in the blood using cytokine multiplex assay and ELISA. Bone repair was evaluated on day 21 post-fracture by biomechanical testing, µCT analysis, and histomorphometry (n = 6–8/group). Statistics: ANOVA with Fisher LSD post hoc test (p < 0.05).

Results: In line with previous findings, littermate controls displayed an increased inflammatory response after combined trauma, indicated by increased blood levels of IL-6, monocyte chemoattractant protein-3, CC-chemokine ligand 11 (CCL11), and IL-12p70 (all p<0.05 compared to isolated fracture). Furthermore, compared to isolated fractures, controls displayed impaired bone repair after additional thoracic trauma, as evidenced by significantly reduced bending stiffness (p=0.007), fewer bridged cortices, and decreased newly formed bone (p=0.031), while cartilage formation was significantly increased (p=0.025) on day 21 post-trauma. Notably, the mast cell-specific deletion of IL-6 reduced trauma-induced systemic inflammation, including a significant reduction in IL-6 (p=0.008) and CCL11 levels (p=0.042) compared to controls after thoracic trauma. Moreover, mice with mast cell-specific IL-6 deletion were protected from trauma-induced compromised fracture healing, indicated by a significantly increased bone volume (1.0 vs 1.6 mm3) and cortical bridging score (p=0.004) compared to controls at day 21.

Discussion and conclusions: Our results suggest that mast cell-released IL-6 plays a key role in posttraumatic inflammation and compromised bone repair after trauma. To examine the effects of mast cell-released IL-6 on local inflammation, we are currently investigating inflammation and immune cell recruitment in the fracture hematoma and lungs. Concluding, targeting mast cells and their mediators could present a promising therapeutic approach for polytraumatized patients experiencing compromised bone healing in clinical settings.