Compromised Bone Healing in Aged Rats Is Associated With Impaired M2 Macrophage Function

Abstract

Fracture repair is initiated by a multitude of immune cells and induction of an inflammatory cascade. Alterations in the early healing response due to an aged adaptive immune system leads to impaired bone repair, delayed healing or even formation of non-union. However, immuno-senescence is not limited to the adaptive immunity, but is also described for macrophages, main effector cells from the innate immune system. Beside regulation of pro- and anti-inflammatory signaling, macrophages contribute to angiogenesis and granulation tissue maturation. Thus, it seems likely that an altered macrophage function due to aging may affect bone repair at various stages and contribute to age related deficiencies in bone regeneration. To prove this hypothesis, we analyzed the expression of macrophage markers and angiogenic factors in the early bone hematoma derived from young and aged osteotomized Spraque Dawley rats. We detected an overall reduced expression of the monocyte/pan-macrophage markers CD14 and CD68 in aged rats. Furthermore, the analysis revealed an impaired expression of anti-inflammatory M2 macrophage markers in hematoma from aged animals that was connected to a diminished revascularization of the bone callus. To verify that the age related disturbed bone regeneration was due to a compromised macrophage function, CD14+ macrophage precursors were transplanted locally into the osteotomy gap of aged rats. Transplantation rescued bone regeneration partially after 6 weeks, demonstrated by a significantly induced deposition of new bone tissue, reduced fibrosis and significantly improved callus vascularization.

Early pH Changes in Musculoskeletal Tissues upon Injury—Aerobic Catabolic Pathway Activity Linked to Inter-Individual Differences in Local pH

“…For this, the comparison and longitudinal pH measurement in an age-related model of compromised healing [54] versus a young, normal healing model could prove insightful. Based on previous findings that provide evidence for an impaired re-vascularization in rodent fracture models in aged compared to young animals [32,55], a prolonged hypoxic phase causing an extended local acidification phase seems probable, but needs to be studied in depth. Since similar observations of age-dependent hampered angiogenic capacities were made in elderly human patients [56,57], a clinical application of such knowledge appears feasible, underlining the need for future research in this area.…”

“…For the analysis of pH development over a longer time period, an in vivo pH measurement in the osteotomy gap of rats at 4, 10, 24 and 48 h post osteotomy was initiated ( Figure 3A), which includes the phases of hematoma formation, pro-inflammation and initial anti-inflammatory reaction of the bone healing process with the latter initiating revascularization [31]. Revascularization could be detected in the rat osteotomy hematoma at three days post osteotomy in another study [32]. Hence, with our experimental time interval, we aimed to cover the entire hypoxic phase.…”

Modulating macrophage activities to promote endogenous bone regeneration: Biological mechanisms and engineering approaches

“… Both M1-like and M2-like Mediator (3.2.4). [ [43] , [44] , [45] , 78 ] bFGF Aangiogenic growth factor; • Stimulating matureness of fibroblast and angiogenesis at early healing. M1-like Mediator (3.2.4).…”

“…Nevertheless, a prolonged pro-inflammatory reaction may delay healing, and M2-type macrophages are required as they secrete growth factors (e.g. platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) families) and enzymes to accelerate angiogenesis and fracture healing [ [43] , [44] , [45] ]. So, regulating macrophage phenotypes under dynamic control has emerged as a promising direction for engineered approaches for bone regeneration.…”

Periosteum progenitors could stimulate bone regeneration in aged murine bone defect model

“…As we known, the bone fracture was an ideal model to simulate clinical patients bone regeneration situation. 35 But we could not avoid the bias brought by stabilization intervention, which could influence the result that Prx1 + cells can improve the bone regeneration. Meanwhile, this model had been proved that it was a valid and reliable model for the evaluation of bone regeneration.…”