Local immune cell contributions to fracture healing in aged individuals - A novel role for interleukin 22.

With increasing age, the risk of bone fractures increases while regenerative capacity decreases. This variation in healing potential appears to be linked to adaptive immunity, but the underlying mechanism is still unknown. This study sheds light on immunoaging/inflammaging, which impacts regenerative processes in aging individuals. In an aged preclinical model system, different levels of immunoaging were analyzed to identify key factors that connect immunoaged/inflammaged conditions with bone formation after long bone fracture. Immunological facets, progenitor cells, the microbiome, and confounders were monitored locally at the injury site and systemically in relation to healing outcomes in 12-month-old mice with distinct individual levels of immunoaging. Bone tissue formation during healing was delayed in the immunoaged group and could be associated with significant changes in cytokine levels. A prolonged and amplified pro-inflammatory reaction was caused by upregulated immune cell activation markers, increased chemokine receptor availability and a lack of inhibitory signaling. In immunoaged mice, interleukin-22 was identified as a core cell signaling protein that played a central role in delayed healing. Therapeutic neutralization of IL-22 reversed this specific immunoaging-related disturbed healing. Immunoaging was found to be an influencing factor of decreased regenerative capacity in aged individuals. Furthermore, a novel therapeutic strategy of neutralizing IL-22 may successfully rejuvenate healing in individuals with advanced immune experiences.

Dual-Targeted Nanoplatform Regulating the Bone Immune Microenvironment Enhances Fracture Healing.

The immune system and skeletal system are closely linked. Macrophages are one of the most important immune cells for bone remodeling, playing a prohealing role mainly through M2 phenotype polarization. Baicalein (5,6,7-trihydroxyflavone, BCL) has been well documented to have a noticeable promotion effect on M2 macrophage polarization. However, due to the limitations in targeted delivery to macrophages and the toxic effect on other organs, BCL has rarely been used in the treatment of bone fractures. In this study, we developed mesoporous silica and Fe3O4 composite-targeted nanoparticles loaded with BCL (BCL@MMSNPs-SS-CD-NW), which could be magnetically delivered to the fracture site. This induced macrophage recruitment in a targeted manner, polarizing them toward the M2 phenotype, which was demonstrated to induce mesenchymal stem cells (MSCs) toward osteoblastic differentiation. The mesoporous silicon nanoparticles (MSNs) were prepared with surface sulfhydrylation and amination modification, and the mesoporous channels were blocked with ß-cyclodextrin. The outer layer of the mesoporous silicon was added with an amantane-modified NW-targeting peptide to obtain the targeted nanosystem. After entering macrophages, BCL could be released from nanoparticles since the disulfide linker could be cleaved by intracellular glutathione (GSH), resulting in the removal of cyclodextrin (CD) gatekeeper, which is a key element in the pro-bone-remodeling functions such as anti-inflammation and induction of M2 macrophage polarization to facilitate osteogenic differentiation. This nanosystem passively accumulated in the fracture site, promoting osteogenic differentiation activities, highlighting a potent therapeutic benefit with high biosafety.

Identification of miRNA Regulatory Networks and Candidate Markers for Fracture Healing in Mice.

BACKGROUND: It is important to improve the understanding of the fracture healing process at the molecular levels, then to discover potential miRNA regulatory mechanisms and candidate markers. METHODS: Expression profiles of mRNA and miRNA were obtained from the Gene Expression Omnibus database. We performed differential analysis, enrichment analysis, protein-protein interaction (PPI) network analysis. The miRNA-mRNA network analysis was also performed. RESULTS: We identified 499 differentially expressed mRNAs (DEmRs) that were upregulated and 534 downregulated DEmRs during fracture healing. They were mainly enriched in collagen fibril organization and immune response. Using the PPI network, we screened 10 hub genes that were upregulated and 10 hub genes downregulated with the largest connectivity. We further constructed the miRNA regulatory network for hub genes and identified 13 differentially expressed miRNAs (DEmiRs) regulators. Cd19 and Col6a1 were identified as key candidate mRNAs with the largest fold change, and their DEmiR regulators were key candidate regulators. CONCLUSION: Cd19 and Col6a1 might serve as candidate markers for fracture healing in subsequent studies. Their expression is regulated by miRNAs and is involved in collagen fibril organization and immune responses.

From benchtop to clinic: a translational analysis of the immune response to submicron topography and its relevance to bone healing.

Proper regulation of the innate immune response to bone biomaterials after implantation is pivotal for successful bone healing. Pro-inflammatory M1 and anti-inflammatory M2 macrophages are known to have an important role in regulating the healing response to biomaterials. Materials with defined structural and topographical features have recently been found to favourably modulate the innate immune response, leading to improved healing outcomes. Calcium phosphate bone grafts with submicron-sized needle-shaped surface features have been shown to trigger a pro-healing response through upregulation of M2 polarised macrophages, leading to accelerated and enhanced bone regeneration. The present review describes the recent research on these and other materials, all the way from benchtop to the clinic, including in vitro and in vivo fundamental studies, evaluation in clinically relevant spinal fusion models and clinical validation in a case series of 77 patients with posterolateral and/or interbody fusion in the lumbar and cervical spine. This research demonstrates the feasibility of enhancing biomaterial-directed bone formation by modulating the innate immune response through topographic surface features.

Expression of angiogenesis-related proteins in bone marrow mesenchymal stem cells induced by osteoprotegerin during osteogenic differentiation in rats.

This study aimed to discuss the expression of angiogenesis-related proteins in bone marrow mesenchymal stem cells (BMSCs) induced by osteoprotegerin (OGP) during osteogenic differentiation in rats, and to analyze the effect of fracture healing inflammatory factor TNF-ɑ on the osteogenic differentiation of BMSCs of rats. BMSCs isolated and cultured from the third generation rats were taken as the research object. According to the addition amount of OGP, BMSCs were divided into control group, OGP (10-7 mol/L) group, OGP (10-8 mol/L) group, and OGP (10-9 mol/L) group. The cell growth and morphological characteristics of each group were observed by inverted phase contrast microscope, the cell proliferation rate was measured by MTT method, angiogenesis-related markers (platelet growth factor (VEGF), cingulate protein 5 (Fbln5), and angiogenin-like protein 4 (Angptl4)) were quantitatively detected by Western blot, and the effect of TNF-ɑ on osteogenic differentiation was detected by CCK. Compared with the control group, MTT results showed that the value-added rate of cells in the OGP (10-8 mol/L) group reached the maximum at 9 days (P < 0.05). The ALP activity in osteoblasts in the OGP (10-8 mol/L) group reached the maximum at 9 days (P < 0.01). The OGP (10-8 mol/L) group had the highest expression of vascular regeneration proteins (VEGF, Fbln5, and Angptl4) (P < 0.05). CCK analysis showed that the TNF-ɑ (1.0 ng/mL) group showed a significant increase in absorbance compared with the control group on 6 days (P < 0.05), and the OD value of the TNF-ɑ (10 ng/mL) group decreased at all time points (P < 0.05). Overall, 10-8 mol/L OGP can induce the proliferation and osteogenic differentiation of MSCs, and promote the expression of angiogenesis-related proteins (VEGF, Fbln5, and Angptl4) during osteogenic differentiation. Besides, 1.0 ng/mL of TNF-ɑ can also promote osteogenesis differentiation of BMSCs in the short term.

Time Course of Immune Response and Immunomodulation During Normal and Delayed Healing of Musculoskeletal Wounds.

Single trauma injuries or isolated fractures are often manageable and generally heal without complications. In contrast, high-energy trauma results in multi/poly-trauma injury patterns presenting imbalanced pro- and anti- inflammatory responses often leading to immune dysfunction. These injuries often exhibit delayed healing, leading to fibrosis of injury sites and delayed healing of fractures depending on the intensity of the compounding traumas. Immune dysfunction is accompanied by a temporal shift in the innate and adaptive immune cells distribution, triggered by the overwhelming release of an arsenal of inflammatory mediators such as complements, cytokines and damage associated molecular patterns (DAMPs) from necrotic cells. Recent studies have implicated this dysregulated inflammation in the poor prognosis of polytraumatic injuries, however, interventions focusing on immunomodulating inflammatory cellular composition and activation, if administered incorrectly, can result in immune suppression and unintended outcomes. Immunomodulation therapy is promising but should be conducted with consideration for the spatial and temporal distribution of the immune cells during impaired healing. This review describes the current state of knowledge in the spatiotemporal distribution patterns of immune cells at various stages during musculoskeletal wound healing, with a focus on recent advances in the field of Osteoimmunology, a study of the interface between the immune and skeletal systems, in long bone fractures. The goals of this review are to (1) discuss wound and fracture healing processes of normal and delayed healing in skeletal muscles and long bones; (2) provide a balanced perspective on temporal distributions of immune cells and skeletal cells during healing; and (3) highlight recent therapeutic interventions used to improve fracture healing. This review is intended to promote an understanding of the importance of inflammation during normal and delayed wound and fracture healing. Knowledge gained will be instrumental in developing novel immunomodulatory approaches for impaired healing.

The Adenosine A2A Receptor Agonist Accelerates Bone Healing and Adjusts Treg/Th17 Cell Balance through Interleukin 6.

The aim of this study was to explore the effect of adenosine A2A receptor agonists on fracture healing and the regulation of the immunity system after bone fracture. We implanted fibrin gel containing adenosine A2A receptor agonist CGS 21680/inhibitor ZM 241385/saline locally in rat tibial fracture models, finding that the adenosine A2A receptor agonist could promote fracture healing. At the same time, the adenosine A2A receptor agonist decreased the level of IL-6 in blood and the fracture area, increased Treg cells, and decreased Th17 cells in blood of bone fracture rats. Further, tibial fracture rats implanted with the adenosine A2A receptor agonist gel were injected with IL-6. We found that IL-6 could reverse the effect of adenosine A2A receptor agonists on fracture healing and Treg/Th17 cells in blood. Through the above results, we believe that the adenosine A2A receptor agonist can promote fracture healing and regulate Treg/Th17 cells in blood of rats with fractures. These effects are related to IL-6.

Age-related changes to macrophages are detrimental to fracture healing in mice.

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.

Macrophage subtype and cytokine expression characterization during the acute inflammatory phase of mouse bone fracture repair.

Fracture repair is a complex process requiring heterotypic interactions between osteogenic cells and immune cells. Recent evidence indicates that macrophages are critically involved in fracture repair. Polarized macrophage populations differentially promote and regulate inflammation in other tissues, but little is known about the various macrophage subtypes and their signaling activities following a bone fracture. The authors hypothesized that classically activated (M1 subtype) and alternatively activated (M2 subtype) macrophages are active during the early repair process to initiate and regulate the inflammatory response. To test our hypothesis, bone marrow was collected from intact femurs (naïve group), contralateral and fractured femurs of mice on days 0, 1, 2, 4, and 7 postfracture. Macrophages were isolated from the bone marrow and macrophage subtypes were identified using flow cytometry with antibodies to F4/80, MHC II, CD86, CD11c, and CD40. Bone marrow cytokine levels were measured using xMAP. Flow cytometry revealed dynamic changes in M1 subtype (F4/80+/MHC II+/CD86+), M2 subtype (F4/80+/MHC II-/CD86-), and dendritic cell (DCs; MHCII+/CD11c+/CD40+) populations following fracture as compared to naïve controls. M1 subtype levels were correlated with IL-1α, IL-1ß, IL-2, IL-17, Eotaxin, and MCP-1, while DCs were correlated with IL-6, G-CSF, LIF, KC, and VEGF-A. The results indicate that M1 and M2 subtypes and DCs are recruited to the fracture site early during the repair process and consequently may work in tandem to regulate the inflammatory response required to recruit osteogenic cells needed for later stages of repair.

Intact Glucocorticoid Receptor Dimerization Is Deleterious in Trauma-Induced Impaired Fracture Healing.

Following severe trauma, fracture healing is impaired because of overwhelming systemic and local inflammation. Glucocorticoids (GCs), acting via the glucocorticoid receptor (GR), influence fracture healing by modulating the trauma-induced immune response. GR dimerization-dependent gene regulation is essential for the anti-inflammatory effects of GCs. Therefore, we investigated in a murine trauma model of combined femur fracture and thoracic trauma, whether effective GR dimerization influences the pathomechanisms of trauma-induced compromised fracture healing. To this end, we used mice with decreased GR dimerization ability (GRdim). The healing process was analyzed by cytokine/chemokine multiplex analysis, flow cytometry, gene-expression analysis, histomorphometry, micro-computed tomography, and biomechanical testing. GRdim mice did not display a systemic or local hyper-inflammation upon combined fracture and thorax trauma. Strikingly, we discovered that GRdim mice were protected from fracture healing impairment induced by the additional thorax trauma. Collectively and in contrast to previous studies describing the beneficial effects of intact GR dimerization in inflammatory models, we report here an adverse role of intact GR dimerization in trauma-induced compromised fracture healing.

MOTS-c accelerates bone fracture healing by stimulating osteogenesis of bone marrow mesenchymal stem cells via positively regulating FOXF1 to activate the TGF-ß pathway.

OBJECTIVE: To elucidate the function of MOTS-c in accelerating bone fracture healing by inducing BMSCs differentiation into osteoblasts, as well as its potential mechanism. MATERIALS AND METHODS: Primary BMSCs were extracted from rats and induced for osteogenesis. The highest dose of MOTS-c that did not affect BMSCs proliferation was determined by CCK-8 assay. After 7-day osteogenesis, the relative levels of ALP, Bglap, and Runx2 in MOTS-c-treated BMSCs influenced by FOXF1 were examined. ALP staining and alizarin red S staining in BMSCs were performed as well. The interaction between FOXF1 and TGF-ß was analyzed by ChIP assay. At last, rescue experiments were performed to uncover the role of FOXF1/TGF-ß axis in MOTS-c-induced osteogenesis. RESULTS: 1 µM MOTS-c was the highest dose that did not affect BMSCs proliferation. MOTS-c treatment upregulated the relative levels of ALP, Bglap, and Runx2, and stimulated mineralization ability in BMSCs, which were attenuated by the silence of FOXF1. TGF-ß was proved to interact with FOXF1, and its level was positively mediated by FOXF1. The silence of FOXF1 attenuated the accelerated osteogenesis and TGF-ß upregulation in BMSCs because of MOTS-c induction, and these trends were further reversed by the overexpression of TGF-ß. CONCLUSIONS: MOTS-c treatment markedly induces osteogenesis in BMSCs. During MOTS-c-induced osteogenic progression, the upregulated FOXF1 triggers the activation of TGF-ß pathway, thus accelerating bone fracture healing.

Fracture repair in the elderly: Clinical and experimental considerations.

Fractures in the elderly represent a significant and rising socioeconomic problem. Although aging has been associated with delays in healing, there is little direct clinical data isolating the effects of aging on bone healing from the associated comorbidities that are frequently present in elderly populations. Basic research has demonstrated that all of the components of fracture repair-cells, extracellular matrix, blood supply, and molecules and their receptors-are negatively impacted by the aging process, which likely explains poorer clinical outcomes. Improved understanding of age-related fracture healing should aid in the development of novel treatment strategies, technologies, and therapies to improve bone repair in elderly patients.

Chronic psychosocial stress compromises the immune response and endochondral ossification during bone fracture healing via ß-AR signaling.

Chronic psychosocial stress/trauma represents an increasing burden in our modern society and a risk factor for the development of mental disorders, including posttraumatic stress disorder (PTSD). PTSD, in turn, is highly comorbid with a plethora of inflammatory disorders and has been associated with increased bone fracture risk. Since a balanced inflammatory response after fracture is crucial for successful bone healing, we hypothesize that stress/trauma alters the inflammatory response after fracture and, consequently, compromises fracture healing. Here we show, employing the chronic subordinate colony housing (CSC) paradigm as a clinically relevant mouse model for PTSD, that mice subjected to CSC displayed increased numbers of neutrophils in the early fracture hematoma, whereas T lymphocytes and markers for cartilage-to-bone transition and angiogenesis were reduced. At late stages of fracture healing, CSC mice were characterized by decreased bending stiffness and bony bridging of the fracture callus. Strikingly, a single systemic administration of the ß-adrenoreceptor (AR) blocker propranolol before femur osteotomy prevented bone marrow mobilization of neutrophils and invasion of neutrophils into the fracture hematoma, both seen in the early phase after fracture, as well as a compromised fracture healing in CSC mice. We conclude that chronic psychosocial stress leads to an imbalanced immune response after fracture via ß-AR signaling, accompanied by disturbed fracture healing. These findings offer possibilities for clinical translation in patients suffering from PTSD and fracture.

Burn and thoracic trauma alters fracture healing, systemic inflammation, and leukocyte kinetics in a rat model of polytrauma.

BACKGROUND: Singular traumatic insults, such as bone fracture, typically initiate an appropriate immune response necessary to restore the host to pre-insult homeostasis with limited damage to self. However, multiple concurrent insults, such as a combination of fracture, blunt force trauma, and burns (polytrauma), are clinically perceived to result in abnormal immune response leading to inadequate healing and resolution. To investigate this phenomenon, we created a model rat model of polytrauma. METHODS: To investigate relationship between polytrauma and delayed healing, we created a novel model of polytrauma in a rat which encompassed a 3-mm osteotomy, blunt chest trauma, and full-thickness scald burn. Healing outcomes were determined at 5 weeks where the degree of bone formation at the osteotomy site of polytrauma animals was compared to osteotomy only animals (OST). RESULTS: We observed significant differences in the bone volume fraction between polytrauma and OST animals indicating that polytrauma negatively effects wound healing. Polytrauma animals also displayed a significant decrease in their ability to return to pre-injury weight compared to osteotomy animals. Polytrauma animals also exhibited significantly altered gene expression in osteogenic pathways as well as the innate and adaptive immune response. Perturbed inflammation was observed in the polytrauma group compared to the osteotomy group as evidenced by significantly altered white blood cell (WBC) profiles and significantly elevated plasma high-mobility group box 1 protein (HMGB1) at 6 and 24 h post-trauma. Conversely, polytrauma animals exhibited significantly lower concentrations of plasma TNF-alpha (TNF-α) and interleukin 6 (IL-6) at 72 h post-injury compared to OST. CONCLUSIONS: Following polytrauma with burn injury, the local and systemic immune response is divergent from the immune response following a less severe singular injury (osteotomy). This altered immune response that follows was associated with a reduced capacity for wound healing.

Cellular biology of fracture healing.

The biology of bone healing is a rapidly developing science. Advances in transgenic and gene-targeted mice have enabled tissue and cell-specific investigations of skeletal regeneration. As an example, only recently has it been recognized that chondrocytes convert to osteoblasts during healing bone, and only several years prior, seminal publications reported definitively that the primary tissues contributing bone forming cells during regeneration were the periosteum and endosteum. While genetically modified animals offer incredible insights into the temporal and spatial importance of various gene products, the complexity and rapidity of healing-coupled with the heterogeneity of animal models-renders studies of regenerative biology challenging. Herein, cells that play a key role in bone healing will be reviewed and extracellular mediators regulating their behavior discussed. We will focus on recent studies that explore novel roles of inflammation in bone healing, and the origins and fates of various cells in the fracture environment. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

Reduced Terminal Complement Complex Formation in Mice Manifests in Low Bone Mass and Impaired Fracture Healing.

The terminal complement complex (TCC) is formed on activation of the complement system, a crucial arm of innate immunity. TCC formation on cell membranes results in a transmembrane pore leading to cell lysis. In addition, sublytic TCC concentrations can modulate various cellular functions. TCC-induced effects may play a role in the pathomechanisms of inflammatory disorders of the bone, including rheumatoid arthritis and osteoarthritis. In this study, we investigated the effect of the TCC on bone turnover and repair. Mice deficient for complement component 6 (C6), an essential component for TCC assembly, and mice with a knockout of CD59, which is a negative regulator of TCC formation, were used in this study. The bone phenotype was analyzed in vivo, and bone cell behavior was analyzed ex vivo. In addition, the mice were subjected to a femur osteotomy. Under homeostatic conditions, C6-deficient mice displayed a reduced bone mass, mainly because of increased osteoclast activity. After femur fracture, the inflammatory response was altered and bone formation was disturbed, which negatively affected the healing outcome. By contrast, CD59-knockout mice only displayed minor skeletal alterations and uneventful bone healing, although the early inflammatory reaction to femur fracture was marginally enhanced. These results demonstrate that TCC-mediated effects regulate bone turnover and promote an adequate response to fracture, contributing to an uneventful healing outcome.

M2 macrophages are closely associated with accelerated clavicle fracture healing in patients with traumatic brain injury: a retrospective cohort study.

BACKGROUND: Mounting evidence indicate patients with traumatic brain injury (TBI) have an accelerated fracture healing. The healing process of bone fractures is greatly dependent on infiltrated macrophages. The macrophages are categorized into M1 or M2 phenotypes with different functions. This study is aimed to address the potential role of subtypes of macrophages in the process of fracture healing in patients with TBI. METHODS: Twenty-five cases of clavicle fracture alone (CF group) and 22 cases of clavicle fracture concomitant with TBI (CFT group) were retrospectively analyzed in this study. Callus tissues were harvested during operations. The expressions of COX-2, CD206, and CD68 were measured with immunohistochemistry. RESULTS: The percentages of M2 macrophages in total macrophages increased after bone fracture in both groups, while the percentages of M1-type macrophages are decreased. Interestingly, the increased percentages of M2 macrophages are significantly higher in CFT group than in CF group. Compared to CF group, the fracture callus volume was much larger (21.9 vs 8.5 cm3) and the fracture healing time was much shorter (82.2 vs 127.0 days) in CFT group. The percentage of M2 macrophages was negatively correlated with fracture healing time in patients (r = - 0.575, p < 0.01). CONCLUSIONS: The findings suggest that the percentages of M2 macrophages in callus tissues increased dramatically during the repairing stage in both CF and CFT group. Percentages of M2 macrophages are associated with accelerated fracture healing in patients with TBI. M2 macrophage polarization during the stage of bone regeneration may play a vital role in promoting bone fracture healing.

[Research consortium Neuroimmunology and pain in the research network musculoskeletal diseases]. / Forschungsverbund Neuroimmunologie und Schmerz (Neuroimpa) im Forschungsnetz Muskuloskelettale Erkrankungen.

BACKGROUND: The research consortium Neuroimmunology and Pain (Neuroimpa) explores the importance of the relationships between the immune system and the nervous system in musculoskeletal diseases for the generation of pain and for the course of fracture healing and arthritis. MATERIAL AND METHODS: The spectrum of methods includes analyses at the single cell level, in vivo models of arthritis and fracture healing, imaging studies on brain function in animals and humans and analysis of data from patients. RESULTS: Proinflammatory cytokines significantly contribute to the generation of joint pain through neuronal cytokine receptors. Immune cells release opioid peptides which activate opioid receptors at peripheral nociceptors and thereby evoke hypoalgesia. The formation of new bone after fractures is significantly supported by the nervous system. The sympathetic nervous system promotes the development of immune-mediated arthritis. The studies show a significant analgesic potential of the neutralization of proinflammatory cytokines and of opioids which selectively inhibit peripheral neurons. Furthermore, they show that the modulation of neuronal mechanisms can beneficially influence the course of musculoskeletal diseases. DISCUSSION: Interventions in the interactions between the immune system and the nervous system hold a great therapeutic potential for the treatment of musculoskeletal diseases and pain.

Osteoimmunology: Effects of Standard Orthopaedic Interventions on Inflammatory Response and Early Fracture Healing.

Achieving fracture union is highly dependent on the initial inflammatory phase of fracture healing, which is influenced by both the local and systemic inflammatory environments. The rapidly emerging field of osteoimmunology involves the study of the interactions between the immune system and the skeletal system. Recent research has advanced the current state of knowledge regarding the effects of the surrounding soft-tissue injury, fracture hematoma, and the method of fracture fixation on the inflammatory phase of fracture healing. Acute systemic inflammation, as seen in patients with polytrauma, and chronic systemic inflammation, as seen in patients with diabetes or rheumatoid arthritis, affects the inflammatory phase of fracture healing. The use of NSAIDs can influence early fracture healing. Understanding the effects of standard orthopaedic interventions on the local and systemic inflammatory responses and early fracture healing is important for optimizing fracture union.

Pharmacological inhibition of IL-6 trans-signaling improves compromised fracture healing after severe trauma.

Patients with multiple injuries frequently suffer bone fractures and are at high risk to develop fracture healing complications. Because of its key role both in systemic posttraumatic inflammation and fracture healing, the pleiotropic cytokine interleukin-6 (IL-6) may be involved in the pathomechanisms of trauma-induced compromised fracture healing. IL-6 signals are transmitted by two different mechanisms: classic signaling via the membrane-bound receptor (mIL-6R) and trans-signaling via its soluble form (sIL-6R). Herein, we investigated whether IL-6 classic and trans-signaling play different roles in bone regeneration after severe injury. Twelve-week-old C57BL/6J mice underwent combined femur osteotomy and thoracic trauma. To study the function of IL-6, either an anti-IL-6 antibody, which inhibits both IL-6 classic and trans-signaling, or a soluble glycoprotein 130 fusion protein (sgp130Fc), which selectively blocks trans-signaling, were injected 30 min and 48 h after surgery. Bone healing was assessed using cytokine analyses, flow cytometry, histology, micro-computed tomography, and biomechanical testing. Selective inhibition of IL-6 trans-signaling significantly improved the fracture healing outcome after combined injury, as confirmed by accelerated cartilage-to-bone transformation, enhanced bony bridging of the fracture gap and improved mechanical callus properties. In contrast, global IL-6 inhibition did not affect compromised fracture healing. These data suggest that classic signaling may mediate beneficial effects on bone repair after severe injury. Selective inhibition of IL-6 trans-signaling might have therapeutic potential to treat fracture healing complications in patients with concomitant injuries.