Nlrp3 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 Nlrp3 deficiency on the healing of closed femoral fractures in Nlrp3^-/- 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 Nlrp3^-/- 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 Nlrp3^-/- 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 Nlrp3^-/- mice. The expression of the absent in melanoma (AIM)2 inflammasome was increased more than 2-fold in the Nlrp3^-/- mice, whereas the expression of the pro-inflammatory cytokines interleukin (IL)-1β and IL-18 was not affected. Our results demonstrate that Nlrp3 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.