The hTNFtg mouse is considered to be a valuable model of human RA [19, 20]. In these animals, overexpression of human TNF-α induces severe inflammatory polyarthritis. This arthritis shares many hallmarks with the human counterpart, such as polyarticular involvement of the small joints, chronic and progressive course of disease, and the destruction of cartilage and subchondral bone. The disease is driven by the overexpression of proinflammatory cytokines. The overexpression of TNF-α was described as being responsible for a dramatic loss of trabecular bone and, to a lesser extent, cortical bone . The influence of a chronic inflammation on fracture healing, however, has not been studied. Therefore, we used hTNFtg animals, with high systemic levels of TNF-α to investigate its influence on the composition of the fracture callus and the progression of the healing process.
The importance of TNF-α signaling during fracture healing is commonly accepted today. On the one hand, it is known that TNF-α expression increases soon after injury [11, 21]. During this inflammatory phase different cell types, including mesenchymal stem cells, located in the surrounding tissue, are recruited to the fracture site and initiate the formation of the primary callus. Different studies were able to demonstrate that deficiency of TNF-α signaling causes a delay in callus remodeling with regard to chondrocyte maturation and apoptosis in the later phases of fracture healing [11, 12, 22]. On the other hand, high levels of TNF-α contribute to accelerated loss of cartilage during fracture healing as seen in diabetic mice . In our study we have demonstrated that high systemic levels of TNF-α influence the biomechanical stability and composition of the bone in the course of fracture healing, lead to increased callus formation with less cartilage and more soft tissue within the fracture gap. These alterations of the callus composition resulted in lowered biomechanical stability with a higher flexibility of bone within the callus after 28 days of healing. Histologically, we found that the callus of hTNFtg animals was built of less cartilage and more soft tissue compared to the wild type which leads to an altered stability of the callus. Therefore, our results indicate a negative influence of chronically elevated TNF-α level during the fracture healing process.
TNF-α-inhibitors like Infliximab are used in treatment of rheumatic patients to bind TNF-α with high affinity to reduce inflammatory symptoms [6, 24]. The high efficiency of the blocking hTNF antibody was proven within the hTNFtg animal model . Treated animals do not show any signs of rheumatoid like disease. The influence of a TNF-α-inhibitor on fracture healing, however, has been investigated in a wild type rat model under non-inflammatory conditions. In this case the authors did not find any impairment of metaphysal bone healing . Under chronic inflammatory conditions, as it was shown in this study, the blockade of TNF-α during fracture healing (by Infliximab treatment) led to an increased callus size, which was accompanied by an increase in cartilage tissue compared to the not treated group. After 28 days of fracture healing, the fracture callus composition resembled the wild type situation. Biomechanical testing revealed no differences compared to wild type fracture healing. Treatment with Infliximab even seemed to compensate the derogating effects of elevated systemic TNF-α levels on fracture healing which can be concluded from the similarity of the biomechanical and histological results of the TNFi treated group compared to wild type in our model. It might be interesting to further distinguish between the negative effects of pathologically elevated levels of TNF-α and the positive effects of endogenous TNF signaling during fracture healing. This could be done by performing studies using the fracture model and dose dependent treatment with hTNF blocking antibody and to look for the onset of callus development and callus composition in the course of healing. Our results provide indication that rheumatoid arthritis patients, who sustained a fracture during the treatment with TNF-α antagonist, should continue this therapy.