Interleukin-6 Released by Mast Cells Drives Inflammation and Compromised Bone Healing After Severe Trauma

Mast cells are tissue-resident immune cells that quickly respond to early alarm signals generated after trauma by releasing several inflammatory mediators. Recently, we discovered that mast cells drive inflammation and compromised bone repair after severe trauma. The pro-inflammatory cytokine interleukin-6 (IL-6) is considered a key driver of increased posttraumatic inflammation associated with trauma-induced compromised bone healing. Here, we investigated the role of mast-cell released IL-6 in compromised bone healing after trauma. We generated mice with a mast cell-specific deletion of IL-6 by crossing Mcpt5-Cre RDT-A mice with B6.Il6 tm1.1Jho flox mice. Cre-negative littermates served as controls. For baseline assessment, we analyzed the physiological bone phenotype of 12- and 20-week-old male mice using biomechanical testing, µCT analysis, and histomorphometry (n = 5–7/group). To investigate fracture healing, all mice (12-weeks old) underwent a femur osteotomy stabilized with an external fixator, with half of the mice also receiving additional thoracic trauma. The systemic immune response was assessed 3 hours post-fracture using multiplex cytokine analysis of the blood, while bone repair was evaluated on day 21 by biomechanical testing, µCT analysis, and histomorphometry (n = 6–8/group). Statistics: ANOVA with Fisher post hoc test (p < 0.05). Mast cell-specific deletion of IL-6 did not affect the physiological bone phenotype at any age, as biomechanical properties and structural and cellular parameters of the cortical and trabecular femurs were comparable to those of controls. As expected, bone repair in littermate controls was impaired following additional thoracic trauma compared to isolated fracture, as indicated by significantly reduced bending stiffness (p=0.007), cortical bridging scores, and bone volume (p=0.027), along with a significant increase in cartilage formation (p=0.025). Moreover, controls exhibited an elevated inflammatory response after combined trauma, demonstrated by increased blood levels of IL-6, CC-chemokine ligand, monocyte chemoattractant protein-3, and IL-12p70 (all p<0.05). Notably, mast cell-specific deletion of IL-6 attenuated trauma-induced inflammation, including a reduction in IL-6 levels, compared to controls after thoracic trauma. Furthermore, mice with mast cell-specific IL-6 deletion were protected from compromised fracture healing following additional thoracic trauma, indicated by a significantly increased bone volume (1.0 vs 1.6 mm 3 ) and fracture bridging (p=0.004) compared to controls at day 21. Our results suggest that mast cell-released IL-6 plays a key role in driving posttraumatic inflammation and impaired bone repair after trauma. Therefore, targeting mast cells could present a promising therapeutic approach for polytraumatized patients experiencing compromised bone healing in clinical settings.