Injury-dependent immune modulation following combined fracture and traumatic brain injury

Introduction:

Inadequate bone healing still remains of great relevance in the clinical setting, as treatment options are limited. This highlights the importance of the phenomenon of improved bone regeneration in patients with concomitant head injury. Despite the negative overall outcome of multiple trauma patients suffering from traumatic brain injury (TBI), it was identified to increase callus size and accelerate union in long bone fractures (Fx). Although the underlying pathomechanisms still remain unknown, a significant impact of the immune system is hypothesized as it plays a crucial role in fracture healing and neuroinflammation. Therefore, identifying key mediators and mechanisms of the injury-dependent immune response following isolated and combined trauma (TBI/Fx) holds great potential for novel therapeutics improving impaired fracture healing.

Methods:

In order to study the molecular pathomechnisms follwing TBI and Fx, our group established a standardised murine model, which combines controlled impact injury to the parietal cortex and femoral osteotomy. The model reproduces the positive effect of TBI on callus formation. Once randomly categorised as control, TBI, Fx and combined trauma (TBI+Fx), 72 female C57BL/6J mice underwent surgical intervention. All stages of fracture healing were investigated, including the acute inflammation (at three days post-injury: d3), the soft callus (d7) and the hard callus stage (d14). mRNA expression analysis of all major organ systems was performed, using large scale qPCR analysis with a focuse on established immune markers. Based on the results, additional in vitro experiments with murine microglial cells, primary osteoblast and macrophage cultures were conducted.

Results:

Suprisingly, in the callus and the contralateral intact bone only minor inflammatory responses following isolated or combined trauma was observed. The same was the case for liver, lung and spleen, which are involved in the acute cytokine response, and for metabolically active organs such as the pancreas, kidney, brown and white fat. Most interestingly however, a concomitant fracture modulated neuroinflammation in the injured hypothalamus, which was dependent on the stage of bone healing. Studying this effect in vitro, we found conditioned medium from osteoblast cultures to alter the immune response in activated microglia cells.

Discussion:

This study provides evidence for a regulatory effect of bone regeneration on the progress of TBI-induced neuroinflammation. Further in vivo and in vitro studies are required to understand this crosstalk on a mechanistic level and its relation to the phenomenon of improved fracture healing following TBI.