Age‐related changes to macrophages are detrimental to fracture healing in mice

Abstract

The elderly population suffers from higher rates of complications during fracture healing that result in increased morbidity and mortality. Inflammatory dysregulation is associated with increased age and is a contributing factor to the myriad of age‐related diseases. Therefore, we investigated age‐related changes to an important cellular regulator of inflammation, the macrophage, and the impact on fracture healing outcomes. We demonstrated that old mice (24 months) have delayed fracture healing with significantly less bone and more cartilage compared to young mice (3 months). The quantity of infiltrating macrophages into the fracture callus was similar in old and young mice. However, RNA‐seq analysis demonstrated distinct differences in the transcriptomes of macrophages derived from the fracture callus of old and young mice, with an up‐regulation of M1/pro‐inflammatory genes in macrophages from old mice as well as dysregulation of other immune‐related genes. Preventing infiltration of the fracture site by macrophages in old mice improved healing outcomes, with significantly more bone in the calluses of treated mice compared to age‐matched controls. After preventing infiltration by macrophages, the macrophages remaining within the fracture callus were collected and examined via RNA‐seq analysis, and their transcriptome resembled macrophages from young calluses. Taken together, infiltrating macrophages from old mice demonstrate detrimental age‐related changes, and depleting infiltrating macrophages can improve fracture healing in old mice.

Maresin 1 resolves aged‐associated macrophage inflammation to improve bone regeneration

“…[26][27][28] Recently however, Clark et al demonstrated that inefficient healing may not be caused by a lack of anti-inflammatory macrophages but rather by a surplus of pro-inflammatory macrophages ("M1"/classicallyactivated) macrophages. 29 While more work is required to F I G U R E 4 MaR1 induces an osteoinductive secretome in macrophages from aged mice. A, Bone marrow-derived macrophages from aged mice were treated with vehicle or MaR1 and then used to condition osteogenic media.…”

“…It is important to note that an age-dependent increase in cartilaginous callus has also been seen by others. 29 This observed increase may be rooted in a failure in trans-differentiation of chondrocytes to osteoblasts, [36][37][38] possibly indicating yet to be elucidated implications of communication between immune cells and mesenchymal cells. [39][40][41][42] Age-associated deficits in bone regeneration highlight the need for therapies to enhance repair, 43 especially considering the numerous shortcomings of current treatment strategies: complications such as improper graft or fusion, heterotopic bone formation, and urologic problems; inconsistent treatment outcomes; invasive surgical administration; and these interventions need to be carried out at the onset of injury.…”

Repetitive injury and absence of monocytes promote astrocyte self‐renewal and neurological recovery

“…As astrocyte number and distribution changed most in CCR2 −/− mice and after repetitive injury, this may directly or indirectly contribute to the behavioral improvement via some of the mechanisms discussed above, or at least not be harmful. Behavioral and structural analysis has been performed using CCR2 −/− mice in various injury conditions (Belarbi, Jopson, Arellano, Fike, & Rosi, 2013; Boring, Gosling, Cleary, & Charo, 1998; Clark et al, 2020; Hsieh et al, 2014; Izikson, Klein, Charo, Weiner, & Luster, 2000), but reactive astrogliosis and behavior were rarely examined in the same injury paradigm. Notably, the results of our behavioral tests are in line with previous studies demonstrating that blocking of monocyte infiltration by CCR2 inhibition or knockout can prevent spatial memory deficits and improve cognitive functions in different experimental models of TBI (Gyoneva et al, 2015; Hsieh et al, 2014; Liu et al, 2017; Morganti et al, 2015).…”

Macrophage function in the elderly and impact on injury repair and cancer

“…Furthermore, reduced vascularity and increasing levels of fibrosis in the skeletal muscle are also thought to contribute to a dysregulated inflammatory response and reduced infiltration by macrophages in aged mice [ 102 ]. In contrast, in a bone-fracture healing model, macrophages from aged C57BL/6J mice (24 months old, sex not stated) exhibited a heightened inflammatory signature and preventing macrophage infiltration improved healing outcomes [ 104 ]. This is similar to the heightened inflammatory signature observed in elderly hip fracture patients and thought to be associated with dysregulated inflammatory monocyte/macrophages [ 66 , 105 ].…”