Deficiency Does Not Substantially Affect Femoral Fracture Healing in Mice

Inflammation has been recognized as major factor for successful bone regeneration. On the other hand, a prolonged or overshooting inflammatory response can also cause fracture healing failure. The nucleotide-binding oligomerization domain (NOD)-like receptor protein (NLRP)3 inflammasome plays a crucial role in inflammatory cytokine production. However, its role during fracture repair remains elusive. We investigated the effects of deficiency on the healing of closed femoral fractures in and wildtype mice. The callus tissue was analyzed by means of X-ray, biomechanics, µCT and histology, as well as immunohistochemistry and Western blotting at 2 and 5 weeks after surgery. We found a significantly reduced trabecular thickness at 2 weeks after fracture in the mice when compared to the wildtype animals. However, the amount of bone tissue did not differ between the two groups. Additional immunohistochemical analyses showed a reduced number of CD68-positive macrophages within the callus tissue of the mice at 2 weeks after fracture, whereas the number of myeloperoxidase (MPO)-positive granulocytes was increased. Moreover, we detected a significantly lower expression of vascular endothelial growth factor (VEGF) and a reduced number of microvessels in the mice. The expression of the absent in melanoma (AIM)2 inflammasome was increased more than 2-fold in the mice, whereas the expression of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 was not affected. Our results demonstrate that deficiency does not markedly affect femoral fracture healing in mice. This is most likely due to the unaltered expression of pro-inflammatory cytokines and pro-osteogenic growth factors.