Abstract
Confirming clinical evidence, we recently demonstrated in a rodent model that a severe trauma which induces an acute systemic inflammation considerably impairs fracture healing. Interleukin-6 (IL-6) is a key cytokine in posttraumatic inflammation as its serum level correlates with injury severity and mortality. IL-6 signals are transmitted by the transmembrane glycoprotein 130 (gp130) via two distinct mechanisms: firstly, through classic signalling via the membrane-anchored IL-6 receptor and secondly, through trans-signalling using a soluble IL-6 receptor. Whereas IL-6 trans-signalling is considered a danger signal driving inflammation, classic signalling may mediate anti-inflammatory, pro-regenerative processes. The role of the two distinct pathways in bone healing has not yet been elucidated. Here, we studied the function of IL-6 in the pathophysiology of compromised bone healing induced by severe trauma.
Male C57BL/6J mice received an osteotomy of the right femur stabilized with an external fixator. Systemic inflammation was induced by additional blunt chest trauma (TxT) applied immediately after the osteotomy. Mice were injected with either fusion protein sgp130Fc, which selectively inhibits IL-6 trans-signalling, or a neutralizing anti-IL-6 antibody (IL-6 Ab), blocking both signalling pathways. Control mice received vehicle solution. Animals were euthanised 21 days after surgery. Fracture healing was analysed by biomechanical testing, μCT, and histomorphometry (n= 6–9; p=0.05; ANOVA/Fisher LSD post hoc).
Thoracic trauma significantly impaired fracture healing [bending stiffness (EI) −57%, p<0.00]. Treatment with sgp130Fc significantly attenuated bone regeneration as demonstrated by an increased EI (+110%, p<0.00) and a trend of augmented apparent Young”s modulus (+69%, p=0.13) compared to TxT control. Histomorphometric analysis could not detect differences in the amount of bone, confirming µCT results, but revealed a significantly decreased cartilage area after treatment with sgp130Fc (−76%, p=0.01). Inhibition of both signalling pathways with IL-6 Ab, however, did not have any effects.
In conclusion, severe trauma significantly impaired fracture healing, confirming previous studies. Treatment with sgp130Fc ameliorated the negative effects providing evidence that IL-6 trans-signalling triggers the excessive immune response after trauma impairing bone regeneration. Injection of IL-6 Ab did not improve fracture healing thereby implying that classic signalling may rather have beneficial effects.
