gms | German Medical Science

Objectives: Over the years various clinical and experimental studies evidenced the positive effect of traumatic brain injury (TBI) on callus formation in long bone fractures (Fx). As impaired fracture healing still remains of great relevance in the clinic, identifying the key mediators involved holds a great potential to develop treatment options and therapeutics. Regarding the phenomenon, inflammatory mediators secondary to fracture and TBI are of particular interest, as both result in systemically and locally distinct responses. This study aimed to describe and compare the specific alterations in cytokine and immune cell expression following isolated and combined trauma.

Methods: As previously published, a standardized murine trauma model was employed to combine Fx and TBI. In the model, a femoral osteotomy, using an external fixator, and/or a controlled impact injury to the parietal cortex are induced surgically. In this study, 72 female C57BL/6J mice where randomly assigned to four groups: control, TBI, Fx and combined injury (TBI+Fx). Blood serum and eleven different organs were obtained during the acute inflammation phase at three days post-injury (d3), the soft callus stage at seven days post-injury (d7) and the hard callus/beginning of the remodeling stage at 14 days post-injury (d14) of fracture healing. Once the mRNA was isolated, a large scale qPCR analysis focusing on established cytokine, monocyte and lymphocyte markers was conducted. In order to analyse the inflammatory processes systemically, proteins of pooled serum were profiled.

Results and conclusion: As a result, a solid systemic stress response was identified throughout all treated groups, indicated by increased cytokine concentrations including c-reactive protein. Locally, no specific immune response was observed in the callus, although the intact femur showed temporary presence of pro-inflammatory mediators. Immune markers anaylsed in brown fat, kidney, lung and spleen tissue revealed only minor changes. In combined trauma, the liver analysis revealed an increase in anti-inflammatory mediators during the acute inflammation phase, whereas adipose tissue showed a pro-inflammatory response during the soft callus stage. The most significant alterations in gene expression were identified in the hypothalamus, where the induction of TBI increased all immune markers monitored. Interestingly, the time-dependent neuroinflammation identified was modulated by fracture in a biphasic manner.

In conclusion, the distinct immune response in hypothalamus, liver and adipose tissue observed for the combined injury, evidences a competing control over inflammatory activity by the brain and the fracture site. Further experiments are required to confirm the observed expressions on protein level as well as to analyze the impact of the modulated immune response in order to evaluate its role in the crosstalk linking traumatic brain injury and fracture healing.