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PURPOSE OF REVIEW: The purpose of this review is to summarize what is known in the literature about the role inflammation plays during bone fracture healing. Bone fracture healing progresses through four distinct yet overlapping phases: formation of the hematoma, development of the cartilaginous callus, development of the bony callus, and finally remodeling of the fracture callus. Throughout this process, inflammation plays a critical role in robust bone fracture healing. RECENT FINDINGS: At the onset of injury, vessel and matrix disruption lead to the generation of an inflammatory response: inflammatory cells are recruited to the injury site where they differentiate, activate, and/or polarize to secrete cytokines for the purposes of cell signaling and cell recruitment. This process is altered by age and by sex. Bone fracture healing is heavily influenced by the presence of inflammatory cells and cytokines within the healing tissue.
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Inflammation is thought to be dysregulated with age leading to impaired bone fracture healing. However, broad analyses of inflammatory processes during homeostatic bone aging and during repair are lacking. Here, we assessed changes in inflammatory cell and cytokine profiles in circulation and in bone tissue to identify age- and sex-dependent differences during homeostasis and repair. During homeostatic aging, male mice demonstrated accumulation of CD4+ helper T cells and CD8+ cytotoxic T cells within bone while both pro-inflammatory "M1" and anti-inflammatory "M2" macrophage numbers decreased. Female mice saw no age-associated changes in immune-cell population in homeostatic bone. Concentrations of IL-1ß, IL-9, IFNγ, and CCL3/MIP-1α increased with age in both male and female mice, whereas concentrations of IL-2, TNFα, TNFR1, IL-4, and IL-10 increased only in female mice - thus we termed these "age-accumulated" cytokines. There were no notable changes in immune cell populations nor cytokines within circulation during aging. Sex-dependent analysis demonstrated slight changes in immune cell and cytokine levels within bone and circulation, which were lost upon fracture injury. Fracture in young male mice caused a sharp decrease in number of M1 macrophages; however, this was not seen in aged male mice nor in female mice of any age. Injury itself induced a decrease in the number of CD8+ T cells within the local tissue of aged male and of female mice but not of young mice. Cytokine analysis of fractured mice revealed that age-accumulated cytokines quickly dissipated after fracture injury, and did not re-accumulate in newly regenerated tissue. Conversely, CXCL1/KC-GRO, CXCL2/MIP-2, IL-6, and CCL2/MCP-1 acted as "fracture response" cytokines: increasing sharply after fracture, eventually returning to baseline. Collectively, we classify measured cytokines into three groups: (1) age-accumulated cytokines, (2) female-specific age-accumulated cytokines, and (3) fracture response cytokines. These inflammatory molecules represent potential points of intervention to improve fracture healing outcome.
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ABSTRACT: Postoperative pain is a major clinical problem imposing a significant burden on patients and society. In a survey 2 years after orthopedic surgery, 57% of patients reported persisting postoperative pain. However, only limited progress has been made in the development of safe and effective therapies to prevent the onset and chronification of pain after orthopedic surgery. We established a tibial fracture mouse model that recapitulates clinically relevant orthopedic trauma surgery, which causes changes in neuropeptide levels in dorsal root ganglia and sustained neuroinflammation in the spinal cord. Here, we monitored extended pain behavior in this model, observing chronic bilateral hindpaw mechanical allodynia in both male and female C57BL/6J mice that persisted for >3 months after surgery. We also tested the analgesic effects of a novel, minimally invasive, bioelectronic approach to percutaneously stimulate the vagus nerve (termed percutaneous vagus nerve stimulation [pVNS]). Weekly pVNS treatment for 30 minutes at 10 Hz for 3 weeks after the surgery strongly reduced pain behaviors compared with untreated controls. Percutaneous vagus nerve stimulation also improved locomotor coordination and accelerated bone healing. In the dorsal root ganglia, vagal stimulation inhibited the activation of glial fibrillary acidic protein-positive satellite cells but without affecting microglial activation. Overall, these data provide novel evidence supportive of the use of pVNS to prevent postoperative pain and inform translational studies to test antinociceptive effects of bioelectronic medicine in the clinic.
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Postoperative pain is a major clinical problem imposing a significant burden on our patients and society. Up to 57% of patients experience persistent postoperative pain 2 years after orthopedic surgery [49]. Although many studies have contributed to the neurobiological foundation of surgery-induced pain sensitization, we still lack safe and effective therapies to prevent the onset of persistent postoperative pain. We have established a clinically relevant orthopedic trauma model in mice that recapitulates common insults associated with surgery and ensuing complications. Using this model, we have started to characterize how induction of pain signaling contributes to neuropeptides changes in dorsal root ganglia (DRG) and sustained neuroinflammation in the spinal cord [62]. Here we have extended the characterization of pain behaviors for >3 months after surgery, describing a persistent deficit in mechanical allodynia in both male and female C57BL/6J mice after surgery. Notably, we have applied a novel minimally invasive bioelectronic approach to percutaneously stimulate the vagus nerve (termed pVNS) [24] and tested its anti-nociceptive effects in this model. Our results show that surgery induced a strong bilateral hind-paw allodynia with a slight decrease in motor coordination. However, treatment with pVNS for 30-minutes at10 Hz weekly for 3 weeks prevented pain behavior compared to naïve controls. pVNS also improved locomotor coordination and bone healing compared to surgery without treatment. In the DRGs, we observed that vagal stimulation fully rescued activation of GFAP positive satellite cells but did not affect microglial activation. Overall, these data provide novel evidence for the use of pVNS to prevent postoperative pain and may inform translational studies to test anti-nociceptive effects in the clinic.
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Tissue repair is negatively affected by advanced age. Recent evidence indicates that hematopoietic cell-derived extracellular vesicles (EVs) are modulators of regenerative capacity. Here, we report that plasma EVs carrying specific surface markers indicate the degree of age-associated immunosenescence; moreover, this immunosenescence phenotype was accentuated by fracture injury. The number of CD11b+ Ly6Cintermediate Ly6Ghigh neutrophils significantly decreased with age in association with defective tissue regeneration. In response to fracture injury, the frequencies of neutrophils and associated plasma EVs were significantly higher in fracture calluses than in peripheral blood. Exposure of aged mice to youthful circulation through heterochronic parabiosis increased the number of neutrophils and their correlated Ly6G+ plasma EVs, which were associated with improved fracture healing in aged mice of heterochronic parabiosis pairs. Our findings create a foundation for utilizing specific immune cells and EV subsets as potential biomarkers and therapeutic strategies to promote resilience to stressors during aging.
