A Pronounced Inflammatory Activity Characterizes the Early Fracture Healing Phase in Immunologically Restricted Patients

Immunomodulation Pathogenesis and Treatment of Bone Nonunion

Fractures and bone nonunion commonly require surgical intervention. Serious outcomes of non-healing in the late stages of fracture place a significant financial burden on society and families. Bone nonunion occurs when a fracture stops healing, for many reasons, and leads to a variety of bad outcomes. Numerous factors, including biomechanics and immunology, are involved in the complicated mechanisms of bone nonunion. The immune-inflammatory response plays a significant part in the emergence of bone nonunion, and the occurrence, control, and remission of inflammation in the bone healing process have a significant influence on the ultimate success of bone tissue repair. In the bone microenvironment, immune cells and associated cytokines control bone repair, which is significantly influenced by macrophages, T cells, and fibroblast growth factor. To limit acute inflammation and  balance osteogenesis and osteoblastogenesis for tissue repair and regeneration, immune cells and various cytokines in the local microenvironment must be precisely regulated. As a bad complication of late-stage fractures, bone nonunion has a significant effect on patients' quality of life and socioeconomic development. Therefore, in-depth research on its pathogenesis and treatment methods has important clinical value. To provide more precise, focused therapeutic options for the treatment of bone nonunion, we discuss the regulatory roles of the key immune cells engaged in bone healing within the microenvironment during bone healing and their effect on osteogenesis.

The local and systemic effects of immune function on fracture healing

The immune system plays an integral role in the regulation of cellular processes responsible for fracture healing. Local and systemic influences on fracture healing correlate in many ways with fracture-related outcomes, including soft tissue healing quality and fracture union rates. Impaired soft tissue healing, restricted perfusion of a fracture site, and infection also in turn affect the immune response to fracture injury. Modern techniques used to investigate the relationship between immune system function and fracture healing include precision medicine, using vast quantities of data to interpret broad patterns of inflammatory response. Early data from the PRECISE trial have demonstrated distinct patterns of inflammatory response in polytrauma patients, which thereby directly and indirectly regulate the fracture healing response. The clearly demonstrated linkage between immune function and fracture healing suggests that modulation of immune function has significant potential as a therapeutic target that can be used to enhance fracture healing.

Association of Immune Cell Subsets with Incident Hip Fracture: The Cardiovascular Health Study

In this study, we aimed to evaluate the association of innate and adaptive immune cell subsets in peripheral blood mononuclear cells (PBMCs) with hip fracture. To conduct this study, we used data from the Cardiovascular Health Study (CHS), a U.S. multicenter observational cohort of community-dwelling men and women aged ≥ 65 years. Twenty-five immune cell phenotypes were measured by flow cytometry from cryopreserved PBMCs of CHS participants collected in 1998-1999. The natural killer (NK), γδ T, T helper 17 (Th17), and differentiated/senescent CD4+CD28- T cell subsets were pre-specified as primary subsets of interest. Hip fracture incidence was assessed prospectively by review of hospitalization records. Multivariable Cox hazard models evaluated associations of immune cell phenotypes with incident hip fracture in sex-stratified and combined analyses. Among 1928 persons, 259 hip fractures occurred over a median 9.7 years of follow-up. In women, NK cells were inversely associated with hip fracture [hazard ratio (HR) 0.77, 95% confidence interval (CI) 0.60-0.99 per one standard deviation higher value] and Th17 cells were positively associated with hip fracture [HR 1.18, 95% CI 1.01-1.39]. In men, γδ T cells were inversely associated with hip fracture [HR 0.60, 95% CI 0.37-0.98]. None of the measured immune cell phenotypes were significantly associated with hip fracture incidence in combined analyses. In this large prospective cohort of older adults, potentially important sex differences in the associations of immune cell phenotypes and hip fracture were identified. However, immune cell phenotypes had no association with hip fracture in analyses combining men and women.

COMMBINI: an experimentally-informed COmputational Model of Macrophage dynamics in the Bone INjury Immunoresponse

Bone fracture healing is a well-orchestrated but complex process that involves numerous regulations at different scales. This complexity becomes particularly evident during the inflammatory stage, as immune cells invade the healing region and trigger a cascade of signals to promote a favorable regenerative environment. Thus, the emergence of criticalities during this stage might hinder the rest of the process. Therefore, the investigation of the many interactions that regulate the inflammation has a primary importance on the exploration of the overall healing progression. In this context, an in silico model named COMMBINI (COmputational Model of Macrophage dynamics in the Bone INjury Immunoresponse) has been developed to investigate the mechano-biological interactions during the early inflammatory stage at the tissue, cellular and molecular levels. An agent-based model is employed to simulate the behavior of immune cells, inflammatory cytokines and fracture debris as well as their reciprocal multiscale biological interactions during the development of the early inflammation (up to 5 days post-injury). The strength of the computational approach is the capacity of the in silico model to simulate the overall healing process by taking into account the numerous hidden events that contribute to its success. To calibrate the model, we present an in silico immunofluorescence method that enables a direct comparison at the cellular level between the model output and experimental immunofluorescent images. The combination of sensitivity analysis and a Genetic Algorithm allows dynamic cooperation between these techniques, enabling faster identification of the most accurate parameter values, reducing the disparity between computer simulation and histological data. The sensitivity analysis showed a higher sensibility of the computer model to the macrophage recruitment ratio during the early inflammation and to proliferation in the late stage. Furthermore, the Genetic Algorithm highlighted an underestimation of macrophage proliferation by in vitro experiments. Further experiments were conducted using another externally fixated murine model, providing an independent validation dataset. The validated COMMBINI platform serves as a novel tool to deepen the understanding of the intricacies of the early bone regeneration phases. COMMBINI aims to contribute to designing novel treatment strategies in both the biological and mechanical domains.

NET Formation Was Reduced via Exposure to Extremely Low-Frequency Pulsed Electromagnetic Fields

Fracture-healing is a highly complex and timely orchestrated process. Non-healing fractures are still a major clinical problem and treatment remains difficult. A 16 Hz extremely low-frequency pulsed electromagnetic field (ELF-PEMF) was identified as non-invasive adjunct therapy supporting bone-healing by inducing reactive oxygen species (ROS) and Ca2+-influx. However, ROS and Ca2+-influx may stimulate neutrophils, the first cells arriving at the wounded site, to excessively form neutrophil extracellular traps (NETs), which negatively affects the healing process. Thus, this study aimed to evaluate the effect of this 16 Hz ELF-PEMF on NET formation. Neutrophils were isolated from healthy volunteers and exposed to different NET-stimuli and the 16 Hz ELF-PEMF. NETs were quantified using Sytox Green Assay and immunofluorescence, Ca2+-influx and ROS with fluorescence probes. In contrast to mesenchymal cells, ELF-PEMF exposure did not induce ROS and Ca2+-influx in neutrophils. ELF-PEMF exposure did not result in basal or enhanced PMA-induced NET formation but did reduce the amount of DNA released. Similarly, NET formation induced by LPS and H2O2 was reduced through exposure to ELF-PEMF. As ELF-PEMF exposure did not induce NET release or negatively affect neutrophils, the ELF-PEMF exposure can be started immediately after fracture treatment.

One Stage Masquelets Technique: Evaluation of Different Forms of Membrane Filling with and without Bone Marrow Mononuclear Cells (BMC) in Large Femoral Bone Defects in Rats

The classic two-stage masquelet technique is an effective procedure for the treatment of large bone defects. Our group recently showed that one surgery could be saved by using a decellularized dermis membrane (DCD, Epiflex, DIZG). In addition, studies with bone substitute materials for defect filling show that it also appears possible to dispense with the removal of syngeneic cancellous bone (SCB), which is fraught with complications. The focus of this work was to clarify whether the SCB can be replaced by the granular demineralized bone matrix (g-DBM) or fibrous demineralized bone matrix (f-DBM) demineralized bone matrix and whether the colonization of the DCD and/or the DBM defect filling with bone marrow mononuclear cells (BMC) can lead to improved bone healing. In 100 Sprague Dawley rats, a critical femoral bone defect 5 mm in length was stabilized with a plate and then encased in DCD. Subsequently, the defect was filled with SCB (control), g-DBM, or f-DBM, with or without BMC. After 8 weeks, the femurs were harvested and subjected to histological, radiological, and biomechanical analysis. The analyses showed the incipient bony bridging of the defect zone in both groups for g-DBM and f-DBM. Stability and bone formation were not affected compared to the control group. The addition of BMCs showed no further improvement in bone healing. In conclusion, DBM offers a new perspective on defect filling; however, the addition of BMC did not lead to better results.

Febrile-Range Hyperthermia Can Prevent Toxic Effects of Neutrophil Extracellular Traps on Mesenchymal Stem Cells

Fracture healing is characterized by an inflammatory phase directly after fracture which has a strong impact on the healing outcome. Neutrophils are strong contributors here and can release neutrophil extracellular traps (NETs). NETs are found after trauma, originally thought to capture pathogens. However, they can lead to tissue damage and impede wound healing processes. Their role in fracture healing remains unclear. In this study, the effect of isolated NETs on the function of bone-forming mesenchymal stem cells (SCP-1 cells) was examined. NETs were isolated from stimulated healthy neutrophils and viability, migration, and differentiation of SCP-1 cells were analyzed after the addition of NETs. NETs severely impaired the viability of SCP-1 cells, induced necrosis and already nontoxic concentrations reduced migration significantly. Short-term incubation with NETs had a persistent negative effect on osteogenic differentiation, as measured by AP activity and matrix formation. The addition of DNase or protease inhibitors failed to reverse the negative effect of NETs, whereas a short febrile-range temperature treatment successfully reduced the toxicity and membrane destruction. Thus, the possible modification of the negative effects of NETs in fracture hematomas could be an interesting new target to improve bone healing, particularly in patients with chronic diseases such as diabetes.

Engineering immunomodulatory hydrogels and cell-laden systems towards bone regeneration

The well-known synergetic interplay between the skeletal and immune systems has changed the design of advanced bone tissue engineering strategies. The immune system is essential during the bone lifetime, with macrophages playing multiple roles in bone healing and biomaterial integration. If in the past, the most valuable aspect of implants was to avoid immune responses of the host, nowadays, it is well-established how important are the crosstalks between immune cells and bone-engineered niches for an efficient regenerative process to occur. For that, it is essential to recapitulate the multiphenotypic cellular environment of bone tissue when designing new approaches. Indeed, the lack of osteoimmunomodulatory knowledge may be the explanation for the poor translation of biomaterials into clinical practice. Thus, smarter hydrogels incorporating immunomodulatory bioactive factors, stem cells, and immune cells are being proposed to develop a new generation of bone tissue engineering strategies. This review highlights the power of immune cells to upgrade the development of innovative engineered strategies, mainly focusing on orthopaedic and dental applications.

Evaluation of the effect of preoperative hemoglobin level and proinflammatory factors on intertrochanteric fracture union

BACKGROUND

Intertrochanteric fractures are associated with high mortality and morbidity, so these patients should undergo fracture fixation surgery immediately. Despite surgery, the possibility of fracture fusion may not occur due to the association with various causes. Therefore, our aim is to investigate these factors (TNF‑a, IL‑1, Hb) and their effect on fracture union after fixation.

METHODS

From 2018 to 2020, at our orthopedic trauma center, 163 patients older than 50 years with intertrochanteric fractures underwent DHS fixation surgery. Patients were divided into anemic and non-anemic groups in terms of preoperative hemoglobin level (standard hemoglobin 11 mg/dl). For 3 months, patients were assessed for union and failure fixation criteria, levels of proinflammation (TNF‑α, IL-1) and level of hemoglobin.

RESULTS

The results show that out of 163 patients with fractures, at the time of initial admission, 74 patients had less than 11 hemoglobin g/dl. Patients with union fractures had higher hemoglobin levels than patients with non-union (11.71 ± 1.51 versus 11.24 ± 1.96), which was statistically significant between hemoglobin and union level (p = 0.030). At the end of the third visit (third month), 44 (59.5%) anemic patients received union completly, while among the patients with normal hemoglobin level, 32 (36%) received union bread, which was statistically significant (p = 0.003). There were no statistically significant differences between proinflammatory factors before surgery and 3 months after surgery (p > 0.05).

CONCLUSION

Due to the effect of anemia and proinflammatory factors in the process of healing fractures and bone formation and creating musculoskeletal balance, low hemoglobin level before surgery has a significant effect on fracture union and failure of fixation. So it is recommended to correct this anemia in these patients before surgery and during follow-up.

[The DRFZ-a pioneer in research on the interaction between immune and stromal cells during de- and regeneration of the musculoskeletal system]

Rheumatoid arthritis and osteoarthritis are two related chronic diseases of the musculoskeletal system which are particularly pronounced in the region of joints and bones. Their pathogeneses are associated with chronic inflammation, which can disrupt homeostasis in bones and articular cartilage. Degradation products deriving from articular cartilage can contribute to the exacerbation of inflammation in the joint region. Mechanical stimuli and blood vessels also play a central role in both the regulation of bone growth as well as in the regeneration of bone tissue. Not only chronic inflammatory processes but also hormonal changes after menopause or undesired effects of glucocorticoid therapy have an influence on the balance between bone resorption and deposition, by promoting the former and reducing the latter. This results in decreased bone quality and, in some cases, considerable loss of bone or osteoporosis. An in-depth understanding of these processes at the molecular, cellular, and tissue level, as well as of the changes present in chronic inflammatory diseases, has been the focus of research at the German Rheumatism Research Center (Deutsches Rheuma-Forschungszentrum, DRFZ) since its foundation. Based on an improved understanding of these mechanisms, the DRFZ aims to develop improved prevention and treatment strategies with effects even in early disease stages.

Towards in silico Models of the Inflammatory Response in Bone Fracture Healing

In silico modeling is a powerful strategy to investigate the biological events occurring at tissue, cellular and subcellular level during bone fracture healing. However, most current models do not consider the impact of the inflammatory response on the later stages of bone repair. Indeed, as initiator of the healing process, this early phase can alter the regenerative outcome: if the inflammatory response is too strongly down- or upregulated, the fracture can result in a non-union. This review covers the fundamental information on fracture healing, in silico modeling and experimental validation. It starts with a description of the biology of fracture healing, paying particular attention to the inflammatory phase and its cellular and subcellular components. We then discuss the current state-of-the-art regarding in silico models of the immune response in different tissues as well as the bone regeneration process at the later stages of fracture healing. Combining the aforementioned biological and computational state-of-the-art, continuous, discrete and hybrid modeling technologies are discussed in light of their suitability to capture adequately the multiscale course of the inflammatory phase and its overall role in the healing outcome. Both in the establishment of models as in their validation step, experimental data is required. Hence, this review provides an overview of the different in vitro and in vivo set-ups that can be used to quantify cell- and tissue-scale properties and provide necessary input for model credibility assessment. In conclusion, this review aims to provide hands-on guidance for scientists interested in building in silico models as an additional tool to investigate the critical role of the inflammatory phase in bone regeneration.

Altered Secretome of Diabetic Monocytes Could Negatively Influence Fracture Healing-An In Vitro Study

Diabetes mellitus is a main risk factor for delayed fracture healing and fracture non-unions. Successful fracture healing requires stimuli from different immune cells, known to be affected in diabetics. Especially, application of mononuclear cells has been proposed to promote wound and fracture healing. Thus, aim was to investigate the effect of pre-/diabetic conditions on mononuclear cell functions essential to promote osteoprogenitor cell function. We here show that pre-/diabetic conditions suppress the expression of chemokines, e.g., CCL2 and CCL8 in osteoprogenitor cells. The associated MCP-1 and MCP-2 were significantly reduced in serum of diabetics. Both MCPs chemoattract mononuclear THP-1 cells. Migration of these cells is suppressed under hyperglycemic conditions, proposing that less mononuclear cells invade the site of fracture in diabetics. Further, we show that the composition of cytokines secreted by mononuclear cells strongly differ between diabetics and controls. Similar is seen in THP-1 cells cultured under hyperinsulinemia or hyperglycemia. The altered secretome reduces the positive effect of the THP-1 cell conditioned medium on migration of osteoprogenitor cells. In summary, our data support that factors secreted by mononuclear cells may support fracture healing by promoting migration of osteoprogenitor cells but suggest that this effect might be reduced in diabetics.

Fracture biomechanics influence local and systemic immune responses in a murine fracture-related infection model

Biomechanical stability plays an important role in fracture healing, with unstable fixation being associated with healing disturbances. A lack of stability is also considered a risk factor for fracture-related infection (FRI), although confirmatory studies and an understanding of the underlying mechanisms are lacking. In the present study, we investigate whether biomechanical (in)stability can lead to altered immune responses in mice under sterile or experimentally inoculated conditions. In non-inoculated C57BL/6 mice, instability resulted in an early increase of inflammatory markers such as granulocyte-colony stimulating factor (G-CSF), keratinocyte chemoattractant (KC) and interleukin (IL)-6 within the bone. When inoculated with Staphylococcus epidermidis, instability resulted in a further significant increase in G-CSF, IL-6 and KC in bone tissue. Staphylococcus aureus infection led to rapid osteolysis and instability in all animals and was not further studied. Gene expression measurements also showed significant upregulation in CCL2 and G-CSF in these mice. IL-17A was found to be upregulated in all S. epidermidis infected mice, with higher systemic IL-17A cell responses in mice that cleared the infection, which was found to be produced by CD4+ and γδ+ T cells in the bone marrow. IL-17A knock-out (KO) mice displayed a trend of delayed clearance of infection (P=0.22, Fisher’s exact test) and an increase in interferon (IFN)-γ production. Biomechanical instability leads to a more pronounced local inflammatory response, which is exaggerated by bacterial infection. This study provides insights into long-held beliefs that biomechanics are crucial not only for fracture healing, but also for control of infection.

Summary: Physical movement between bone fragments after a fracture influence healing, and are shown here, for the first time, to influence immune responses and infection.

Fracture Healing Research-Shift towards In Vitro Modeling?

Fractures are one of the most frequently occurring traumatic events worldwide. Approximately 10% of fractures lead to bone healing disorders, resulting in strain for affected patients and enormous costs for society. In order to shed light into underlying mechanisms of bone regeneration (habitual or disturbed), and to develop new therapeutic strategies, various in vivo, ex vivo and in vitro models can be applied. Undeniably, in vivo models include the systemic and biological situation. However, transferability towards the human patient along with ethical concerns regarding in vivo models have to be considered. Fostered by enormous technical improvements, such as bioreactors, on-a-chip-technologies and bone tissue engineering, sophisticated in vitro models are of rising interest. These models offer the possibility to use human cells from individual donors, complex cell systems and 3D models, therefore bridging the transferability gap, providing a platform for the introduction of personalized precision medicine and finally sparing animals. Facing diverse processes during fracture healing and thus various scientific opportunities, the reliability of results oftentimes depends on the choice of an appropriate model. Hence, we here focus on categorizing available models with respect to the requirements of the scientific approach.

Functional Scaffold-Free Bone Equivalents Induce Osteogenic and Angiogenic Processes in a Human In Vitro Fracture Hematoma Model

After trauma, the formed fracture hematoma within the fracture gap contains all the important components (immune/stem cells, mediators) to initiate bone regeneration immediately. Thus, it is of great importance but also the most susceptible to negative influences. To study the interaction between bone and immune cells within the fracture gap, up-to-date in vitro systems should be capable of recapitulating cellular and humoral interactions and the physicochemical microenvironment (eg, hypoxia). Here, we first developed and characterized scaffold-free bone-like constructs (SFBCs), which were produced from bone marrow-derived mesenchymal stromal cells (MSCs) using a macroscale mesenchymal condensation approach. SFBCs revealed permeating mineralization characterized by increased bone volume (μCT, histology) and expression of osteogenic markers (RUNX2, SPP1, RANKL). Fracture hematoma (FH) models, consisting of human peripheral blood (immune cells) mixed with MSCs, were co-cultivated with SFBCs under hypoxic conditions. As a result, FH models revealed an increased expression of osteogenic (RUNX2, SPP1), angiogenic (MMP2, VEGF), HIF-related (LDHA, PGK1), and inflammatory (IL6, IL8) markers after 12 and 48 hours co-cultivation. Osteogenic and angiogenic gene expression of the FH indicate the osteoinductive potential and, thus, the biological functionality of the SFBCs. IL-6, IL-8, GM-CSF, and MIP-1β were detectable within the supernatant after 24 and 48 hours of co-cultivation. To confirm the responsiveness of our model to modifying substances (eg, therapeutics), we used deferoxamine (DFO), which is well known to induce a cellular hypoxic adaptation response. Indeed, DFO particularly increased hypoxia-adaptive, osteogenic, and angiogenic processes within the FH models but had little effect on the SFBCs, indicating different response dynamics within the co-cultivation system. Therefore, based on our data, we have successfully modeled processes within the initial fracture healing phase in vitro and concluded that the cross-talk between bone and immune cells in the initial fracture healing phase is of particular importance for preclinical studies. © 2021 American Society for Bone and Mineral Research (ASBMR).

Gut microbial-derived short-chain fatty acids and bone: a potential role in fracture healing

Bone healing complications such as delayed healing or non-union affect 5-10 % of patients with a long-bone fracture and lead to reduced quality of life and increased health-care costs. The gut microbiota and the metabolites they produce, mainly short-chain fatty acids (SCFAs), have been shown to impact nearly all organs of the human body including bone. SCFAs show broad activity in positively influencing bone healing outcomes either by acting directly on cell types involved in fracture healing, such as osteoblasts, osteoclasts, chondrocytes and fibroblasts, or indirectly, by shaping an appropriate anti-inflammatory and immune regulatory response. Due to the ability of SCFAs to influence osteoblast and osteoclast differentiation, SCFAs may also affect the integration of orthopaedic implants in bone. In addition, SCFA-derivatives have already been used in a variety of tissue engineering constructs to reduce inflammation and induce bone tissue production. The present review summarises the current knowledge on the role of the gut microbiota, in particular through the action of SCFAs, in the individual stages of bone healing and provides insights into how SCFAs may be utilised in a manner beneficial for fracture healing and surgical reconstruction.

Smoking Alters Inflammation and Skeletal Stem and Progenitor Cell Activity During Fracture Healing in Different Murine Strains

Smokers are at a higher risk of delayed union or nonunion after fracture repair. Few specific interventions are available for prevention because the molecular mechanisms that result in these negative sequelae are poorly understood. Murine models that mimic fracture healing in smokers are crucial in further understanding the local cellular and molecular alterations during fracture healing caused by smoking. We exposed three murine strains, C57BL/6J, 129X1/SvJ, and BALB/cJ, to cigarette smoke for 3 months before the induction of a midshaft transverse femoral osteotomy. We evaluated fracture healing 4 weeks after the osteotomy using radiography, micro-computed tomography (μCT), and biomechanical testing. Radiographic analysis demonstrated a significant decrease in the fracture healing capacity of smoking 129X1/SvJ mice. μCT results showed delayed remodeling of fracture calluses in all three strains after cigarette smoke exposure. Biomechanical testing indicated the most significant impairment in the functional properties of 129X1/SvJ in comparison with C57BL/6J and BALB/cJ mice after cigarette smoke exposure. Thus, the 129X1/SvJ strain is most suitable in simulating smoking-induced impaired fracture healing. Furthermore, in smoking 129X1/SvJ murine models, we investigated the molecular and cellular alterations in fracture healing caused by cigarette smoking using histology, flow cytometry, and multiplex cytokine/chemokine analysis. Histological analysis showed impaired chondrogenesis in cigarette smoking. In addition, the important reparative cell populations, including skeletal stem cells and their downstream progenitors, demonstrated decreased expansion after injury as a result of cigarette smoking. Moreover, significantly increased pro-inflammatory mediators and the recruitment of immune cells in fracture hematomas were demonstrated in smoking mice. Collectively, our findings demonstrate the significant cellular and molecular alterations during fracture healing impaired by smoking, including disrupted chondrogenesis, aberrant skeletal stem and progenitor cell activity, and a pronounced initial inflammatory response. © 2020 American Society for Bone and Mineral Research (ASBMR).

JAK/STAT Activation: A General Mechanism for Bone Development, Homeostasis, and Regeneration

The Janus kinase (JAK) signal transducer and activator of transcription (STAT) signaling pathway serves as an important downstream mediator for a variety of cytokines, hormones, and growth factors. Emerging evidence suggests JAK/STAT signaling pathway plays an important role in bone development, metabolism, and healing. In this light, pro-inflammatory cytokines are now clearly implicated in these processes as they can perturb normal bone remodeling through their action on osteoclasts and osteoblasts at both intra- and extra-articular skeletal sites. Here, we summarize the role of JAK/STAT pathway on development, homeostasis, and regeneration based on skeletal phenotype of individual JAK and STAT gene knockout models and selective inhibition of components of the JAK/STAT signaling including influences of JAK inhibition in osteoclasts, osteoblasts, and osteocytes.

The Life of a Fracture: Biologic Progression, Healing Gone Awry, and Evaluation of Union

New knowledge about the molecular biology of fracture-healing provides opportunities for intervention and reduction of risk for specific phases that are affected by disease and medications. Modifiable and nonmodifiable risk factors can prolong healing, and the informed clinician should optimize each patient to provide the best chance for union. Techniques to monitor progression of fracture-healing have not changed substantially over time; new objective modalities are needed.

What do we know about bone morphogenetic proteins and osteochondroprogenitors in inflammatory conditions?

Osteochondroprogenitors are crucial for embryonic bone development and postnatal processes such as bone repair in response to fracture injury, and their dysfunction may contribute to insufficient repair of structural damage in inflammatory arthritides. In the fracture healing, the early inflammatory phase is crucial for normal callus development and new bone formation. This process involves a complex interplay of many molecules and cell types, responsible for recruitment, expansion and differentiation of osteochondroprogenitor populations. In inflammatory arthritides, inflammation induces bone resorption and causes insufficient bone formation, which leads to local and systemic bone loss. While bone loss is a predominant feature in rheumatoid arthritis, inflammation also induces pathologic bone formation at enthesial sites in seronegative spondyloarthropathies. Bone morphogenetic proteins (BMP) are involved in cell proliferation, differentiation and apoptosis, and have fundamental roles in maintenance of postnatal bone homeostasis. They are crucial regulators of the osteochondroprogenitor pool and drive their proliferation, differentiation, and lifespan during bone regeneration. In this review, we summarize the effects of inflammation on osteochondroprogenitor populations during fracture repair and in inflammatory arthritides, with special focus on inflammation-mediated modulation of BMP signaling. We also present data in which we describe a population of murine synovial osteochondroprogenitor cells, which are reduced in arthritis, and characterize their expression of genes involved in regulation of bone homeostasis, emphasizing the up-regulation of BMP pathways in early progenitor subset. Based on the presented data, it may be concluded that during an inflammatory response, innate immune cells induce osteochondroprogenitors by providing signals for their recruitment, by producing BMPs and other osteogenic factors for paracrine effects, and by secreting inflammatory cytokines that may positively regulate osteogenic pathways. On the other hand, inflammatory cells may secrete cytokines that interfere with osteogenic pathways, proapoptotic factors that reduce the pool of osteochondroprogenitor cells, as well as BMP and Wnt antagonists. The net effect is strongly context-dependent and influenced by the local milieu of cells, cytokines, and growth factors. Further elucidation of the interplay between inflammatory signals and BMP-mediated bone formation may provide valuable tools for therapeutic targeting.

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.

Influence of inflammatory conditions provided by macrophages on osteogenic ability of mesenchymal stem cells

BACKGROUND

The mechanisms by which macrophage phenotype contributes to mesenchymal stem cells (MSC)-mediated bone repair remain unclear. In this work, we investigated the influence of factors released by human macrophages polarized to a pro-inflammatory or an anti-inflammatory phenotype on the ability of human MSC to attach, migrate, and differentiate toward the osteoblastic lineage. We focused on the role of TNF-α and IL-10, key pro-inflammatory and anti-inflammatory cytokines, respectively, in regulating MSC functions.

METHODS

MSC were treated with media conditioned by pro-inflammatory or anti-inflammatory macrophages to study their influence in cell attachment, migration, and osteogenic differentiation. The involvement of TNF-α and IL-10 in the regulation of MSC functions was investigated using neutralizing antibodies and recombinant cytokines.

RESULTS

Treatment of MSC with media conditioned by pro-inflammatory or anti-inflammatory macrophages promoted cell elongation and enhanced MSC ability to attach and migrate. These effects were more noticeable when MSC were treated with media from pro-inflammatory macrophages. Interestingly, MSC osteogenic activity was enhanced by factors released by anti-inflammatory macrophages, but not by pro-inflammatory macrophages. Significant IL-10 levels originated from anti-inflammatory macrophages enhanced MSC osteogenesis by increasing ALP activity and mineralization in MSC layers cultured under osteogenic conditions. Moreover, macrophage-derived IL-10 regulated the expression of the osteogenic markers RUNX2, COL1A1, and ALPL. Notably, low TNF-α levels secreted by anti-inflammatory macrophages increased ALP activity in differentiating MSC whereas high TNF-α levels produced by pro-inflammatory macrophages had no effects on osteogenesis. Experiments in which MSC were treated with cytokines revealed that IL-10 was more effective in promoting matrix maturation and mineralization than TNF-α.

CONCLUSIONS

Factors secreted by pro-inflammatory macrophages substantially increased MSC attachment and migration whereas those released by anti-inflammatory macrophages enhanced MSC osteogenic activity as well as cell migration. IL-10 was identified as an important cytokine secreted by anti-inflammatory macrophages that potentiates MSC osteogenesis. Our findings provide novel insights into how environments provided by macrophages regulate MSC osteogenesis, which may be helpful to develop strategies to enhance bone regeneration.

Dynamics of Early Signalling Events during Fracture Healing and Potential Serum Biomarkers of Fracture Non-Union in Humans

To characterise the dynamic of events during the early phases of fracture repair in humans, we investigated molecular events using gene expression profiling of bone fragments from the fracture site at different time points after trauma and immune/stromal cells recruitment at the fracture site using flow cytometry. Bone and inflammatory markers were expressed at low levels at homeostasis, while transcripts for bone constituent proteins were consistently detected at higher levels. Early after fracture (range 2-4 days), increased expression of CXCL12, suggested recruitment of immune cells associated with a change in the balance of degradation enzymes and their inhibitors. At intermediate time after fracture (4-8 days), we observed high expression of inflammatory cytokines (IL1-beta, IL6), CCL2, the T-cell activation marker CD69. Late after fracture (8-14 days), high expression of factors co-operating towards the regulation of bone turnover was detected. We identified potential soluble factors and explored circulating levels in patients for whom a union/non-union (U/NU) outcome was known. This showed a clear difference for PlGF (p = 0.003) at day 1. These findings can inform future studies further investigating the cascade of molecular events following fractures and for the prediction of fracture non-union.

Impact of Janus Kinase Inhibition with Tofacitinib on Fundamental Processes of Bone Healing

Both inflammatory diseases like rheumatoid arthritis (RA) and anti-inflammatory treatment of RA with glucocorticoids (GCs) or non-steroidal anti-inflammatory drugs (NSAIDs) negatively influence bone metabolism and fracture healing. Janus kinase (JAK) inhibition with tofacitinib has been demonstrated to act as a potent anti-inflammatory therapeutic agent in the treatment of RA, but its impact on the fundamental processes of bone regeneration is currently controversially discussed and at least in part elusive. Therefore, in this study, we aimed to examine the effects of tofacitinib on processes of bone healing focusing on recruitment of human mesenchymal stromal cells (hMSCs) into the inflammatory microenvironment of the fracture gap, chondrogenesis, osteogenesis and osteoclastogenesis. We performed our analyses under conditions of reduced oxygen availability in order to mimic the in vivo situation of the fracture gap most optimal. We demonstrate that tofacitinib dose-dependently promotes the recruitment of hMSCs under hypoxia but inhibits recruitment of hMSCs under normoxia. With regard to the chondrogenic differentiation of hMSCs, we demonstrate that tofacitinib does not inhibit survival at therapeutically relevant doses of 10–100 nM. Moreover, tofacitinib dose-dependently enhances osteogenic differentiation of hMSCs and reduces osteoclast differentiation and activity. We conclude from our data that tofacitinib may influence bone healing by promotion of hMSC recruitment into the hypoxic microenvironment of the fracture gap but does not interfere with the cartilaginous phase of the soft callus phase of fracture healing process. We assume that tofacitinib may promote bone formation and reduce bone resorption, which could in part explain the positive impact of tofacitinib on bone erosions in RA. Thus, we hypothesize that it will be unnecessary to stop this medication in case of fracture and suggest that positive effects on osteoporosis are likely.

Trauma Severity and Its Impact on Local Inflammation in Extremity Injury-Insights From a Combined Trauma Model in Pigs

Background: Extremity fracture is frequently seen in multiple traumatized patients. Local post-traumatic inflammatory reactions as well as local and systemic interactions have been described in previous studies. However, trauma severity and its impact on the local immunologic reaction remains unclear. Therefore, fracture-associated local inflammation was investigated in a porcine model of isolated and combined trauma to gain information about the early inflammatory stages.

Material and Methods: Polytrauma (PT) consisted of lung contusion, liver laceration, femur fracture, and controlled hemorrhage. Monotrauma (MT) consisted of femur fracture only. The fracture was operatively stabilized and animals were monitored under ICU-standard for 72 h. Blood, fracture hematoma (FH) as well as muscle samples were collected throughout the experimental period. Levels of local and systemic pro- and anti-inflammatory as well as angiogenetic cytokines were measured by ELISA.

Results: Both groups showed a significant decrease in pro-inflammatory IL-6 in FH over time. However, concentrations in MT were significantly higher than in PT. The IL-8 concentrations initially decreased in FH, but recovered by the end of the observation period. These dynamics were only statistically significant in MT. Furthermore, concentrations measured in muscle tissue showed inverse kinetics compared to those in FH. The IL-10 did not present statistical resilient dynamics over time, although a slight increase in FH was seen by the end of the observation time in the MT group.

Conclusions: Time-dependent dynamics of the local inflammatory response were observed. Trauma severity showed a significant impact, with lower values in pro- as well as angiogenetic mediators. Fracture repair could be altered by these trauma-related changes of the local immunologic milieu, which might serve as a possible explanation for the higher rates of delayed or non-union bone repair in polytraumatised patients.

Spatial Distribution of Macrophages During Callus Formation and Maturation Reveals Close Crosstalk Between Macrophages and Newly Forming Vessels

Macrophages are essential players in the process of fracture healing, acting by remodeling of the extracellular matrix and enabling vascularization. Whilst activated macrophages of M1-like phenotype are present in the initial pro-inflammatory phase of hours to days of fracture healing, an anti-inflammatory M2-like macrophage phenotype is supposed to be crucial for the induction of downstream cascades of healing, especially the initiation of vascularization. In a mouse-osteotomy model, we provide a comprehensive characterization of vessel (CD31⁺, Emcn⁺) and macrophage phenotypes (F4/80, CD206, CD80, Mac-2) during the process of fracture healing. To this end, we phenotype the phases of vascular regeneration-the expansion phase (d1-d7 after injury) and the remodeling phase of the endothelial network, until tissue integrity is restored (d14-d21 after injury). Vessels which appear during the bone formation process resemble type H endothelium (CD31^hiEmcn^hi), and are closely connected to osteoprogenitors (Runx2⁺, Osx⁺) and F4/80⁺ macrophages. M1-like macrophages are present in the initial phase of vascularization until day 3 post osteotomy, but they are rare during later regeneration phases. M2-like macrophages localize mainly extramedullary, and CD206⁺ macrophages are found to express Mac-2⁺ during the expansion phase. VEGFA expression is initiated by CD80⁺ cells, including F4/80⁺ macrophages, until day 3, while subsequently osteoblasts and chondrocytes are main contributors to VEGFA production at the fracture site. Using Longitudinal Intravital Microendoscopy of the Bone (LIMB) we observe changes in the motility and organization of CX3CR1⁺ cells, which infiltrate the injury site after an osteotomy. A transient accumulation, resulting in spatial polarization of both, endothelial cells and macrophages, in regions distal to the fracture site, is evident. Immunofluorescence histology followed by histocytometric analysis reveals that F4/80⁺CX3CR1⁺ myeloid cells precede vascularization.

Defective Proliferation and Osteogenic Potential with Altered Immunoregulatory phenotype of Native Bone marrow-Multipotential Stromal Cells in Atrophic Fracture Non-Union

Bone marrow-Multipotential stromal cells (BM-MSCs) are increasingly used to treat complicated fracture healing e.g., non-union. Though, the quality of these autologous cells is not well characterized. We aimed to evaluate bone healing-related capacities of non-union BM-MSCs. Iliac crest-BM was aspirated from long-bone fracture patients with normal healing (U) or non-united (NU). Uncultured (native) CD271highCD45low cells or passage-zero cultured BM-MSCs were analyzed for gene expression levels, and functional assays were conducted using culture-expanded BM-MSCs. Blood samples were analyzed for serum cytokine levels. Uncultured NU-CD271highCD45low cells significantly expressed fewer transcripts of growth factor receptors, EGFR, FGFR1, and FGRF2 than U cells. Significant fewer transcripts of alkaline phosphatase (ALPL), osteocalcin (BGLAP), osteonectin (SPARC) and osteopontin (SPP1) were detected in NU-CD271^highCD45^low cells. Additionally, immunoregulation-related markers were differentially expressed between NU- and U-CD271^highCD45^low cells. Interestingly, passage-zero NU BM-MSCs showed low expression of immunosuppressive mediators. However, culture-expanded NU and U BM-MSCs exhibited comparable proliferation, osteogenesis, and immunosuppression. Serum cytokine levels were found similar for NU and U groups. Collectively, native NU-BM-MSCs seemed to have low proliferative and osteogenic capacities; therefore, enhancing their quality should be considered for regenerative therapies. Further research on distorted immunoregulatory molecules expression in BM-MSCs could potentially benefit the prediction of complicated fracture healing.

Effects of inflammatory cytokines on bone/cartilage repair

Many inflammatory factors can affect cell behaviors and work as a form of inter-regulatory networks through the inflammatory pathway. Inflammatory cytokines are critical for triggering bone regeneration after fracture or bone injury. Also, inflammatory cytokines play an important role in cartilage repair. The synergistic or antagonistic effects of both proinflammatory and anti-inflammatory cytokines have a great influence on fracture healing. This review discusses key inflammatory cytokines and signaling pathways involved in bone or cartilage repair.

Hypoxia and mesenchymal stromal cells as key drivers of initial fracture healing in an equine in vitro fracture hematoma model

Fractures in horses–whether simple fractures with just one clean break, or incomplete greenstick with stress fractures, or complications such as shattered bones can all be either minimal or even catastrophic. Thus, improvement in fracture healing is a hallmark in equine orthopedics. The fracture healing process implements a complex sequence of events including the initial inflammatory phase removing damaged tissue, re-establishment of vessels and mesenchymal stromal cells, a soft and hard callus phase closing the fracture gap as well as the remodeling phase shaping the bone to a scar-free tissue. Detailed knowledge on processes in equine fracture healing in general and on the initial phase in particular is apparently very limited. Therefore, we generated equine in vitro fracture hematoma models (FH models) to study time-dependent changes in cell composition and RNA-expression for the most prominent cells in the FH model (immune cells, mesenchymal stromal cells) under conditions most closely adapted to the in vivo situation (hypoxia) by using flow cytometry and qPCR. In order to analyze the impact of mesenchymal stromal cells in greater detail, we also incubated blood clots without the addition of mesenchymal stromal cells under the same conditions as a control. We observed a superior survival capacity of mesenchymal stromal cells over immune cells within our FH model maintained under hypoxia. Furthermore, we demonstrate an upregulation of relevant angiogenic, osteogenic and hypoxia-induced markers within 48 h, a time well-known to be crucial for proper fracture healing.

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.

Electronic supplementary material

The online version of this article (10.1186/s13018-019-1082-4) contains supplementary material, which is available to authorized users.

The Bidirectional Interactions Between Inflammation and Coagulation in Fracture Hematoma

IMPACT STATEMENT

The review leads to better understanding of the interrelation between inflammation mediators and coagulation factors in the early fracture hematoma, and their influences on hematoma formation in the beginning of fracture healing. Furthermore, development of therapies aimed at simultaneous modulation of both coagulation factors and inflammation factors that affect hematoma structure, rather than specific factors, may be most promising.

CTLA-4 Mediates Inhibitory Function of Mesenchymal Stem/Stromal Cells

Mesenchymal stem/stromal cells (MSCs) are stem cells of the connective tissue, possess a plastic phenotype, and are able to differentiate into various tissues. Besides their role in tissue regeneration, MSCs perform additional functions as a modulator or inhibitor of immune responses. Due to their pleiotropic function, MSCs have also gained therapeutic importance for the treatment of autoimmune diseases and for improving fracture healing and cartilage regeneration. However, the therapeutic/immunomodulatory mode of action of MSCs is largely unknown. Here, we describe that MSCs express the inhibitory receptor CTLA-4 (cytotoxic T lymphocyte antigen 4). We show that depending on the environmental conditions, MSCs express different isoforms of CTLA-4 with the secreted isoform (sCTLA-4) being the most abundant under hypoxic conditions. Furthermore, we demonstrate that the immunosuppressive function of MSCs is mediated mainly by the secretion of CTLA-4. These findings open new ways for treatment when tissue regeneration/fracture healing is difficult.

Distinct Effects of IL-6 Classic and Trans-Signaling in Bone Fracture Healing

Bone healing is a complex process with closely linked phases of inflammation, regeneration, and remodeling. IL-6 may crucially regulate this process; however, the underlying mechanisms are unclear. IL-6 signals are transmitted via the transmembrane glycoprotein 130 by two distinct mechanisms: classic signaling using the membrane-anchored IL-6 receptor and trans-signaling using its soluble form. Herein, we investigated the hypothesis that IL-6 classic and trans-signaling have different functions during bone healing. To investigate fracture healing, 12-week-old C57BL/6J mice underwent a femur osteotomy. To study the function of IL-6 during the inflammatory phase, either an anti-IL-6 antibody, which inhibits IL-6 classic and trans-signaling, or soluble glycoprotein 130 fusion protein, which selectively blocks trans-signaling, was injected after 30 minutes and 48 hours. To analyze IL-6 effects in the repair phase, compounds were injected from day 7 onwards. Global IL-6 inhibition in the early phase after fracture reduced systemic inflammation, the recruitment of immune cells, and bone regeneration, resulting in delayed fracture healing. Global IL-6 inhibition during the repair phase disturbed bone formation and remodeling. In contrast, inhibition of IL-6 trans-signaling exerted minor effects on the immune response and did not influence bone repair, suggesting that the classic pathway accounts for most of the effects observed after global IL-6 inhibition. Our results reveal that IL-6 classic signaling, but not IL-6 trans-signaling, is essential for bone repair.

Association between biomarkers and clinical characteristics in chronic subdural hematoma patients assessed with lasso regression

Chronic subdural hematoma (CSDH) is characterized by an “old” encapsulated collection of blood and blood breakdown products between the brain and its outermost covering (the dura). Recognized risk factors for development of CSDH are head injury, old age and using anticoagulation medication, but its underlying pathophysiological processes are still unclear. It is assumed that a complex local process of interrelated mechanisms including inflammation, neomembrane formation, angiogenesis and fibrinolysis could be related to its development and propagation. However, the association between the biomarkers of inflammation and angiogenesis, and the clinical and radiological characteristics of CSDH patients, need further investigation. The high number of biomarkers compared to the number of observations, the correlation between biomarkers, missing data and skewed distributions may limit the usefulness of classical statistical methods. We therefore explored lasso regression to assess the association between 30 biomarkers of inflammation and angiogenesis at the site of lesions, and selected clinical and radiological characteristics in a cohort of 93 patients. Lasso regression performs both variable selection and regularization to improve the predictive accuracy and interpretability of the statistical model. The results from the lasso regression showed analysis exhibited lack of robust statistical association between the biomarkers in hematoma fluid with age, gender, brain infarct, neurological deficiencies and volume of hematoma. However, there were associations between several of the biomarkers with postoperative recurrence requiring reoperation. The statistical analysis with lasso regression supported previous findings that the immunological characteristics of CSDH are local. The relationship between biomarkers, the radiological appearance of lesions and recurrence requiring reoperation have been inclusive using classical statistical methods on these data, but lasso regression revealed an association with inflammatory and angiogenic biomarkers in hematoma fluid. We thus suggest that lasso regression should be a recommended statistical method in research on biological processes in CSDH patients.

A Simple Procedure for the Evaluation of Bone Vitality by Staining with a Tetrazolium Salt

Presently, no intra-operative method for a direct assessment of bone vitality exists. Therefore, we set out to test the applicability of tetrazolium-based staining on bone samples. The explanted femoral heads of 37 patients were used to obtain either cancellous bone fragments or bone slices. Samples were stained with 2,3,5-triphenyl-2H-tetrazolium chloride (TTC) or 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (thiazolyl blue, MTT) at different times (one to twelve hours) after explantation. Staining was quantified either spectrophotometrically after extraction of the dyes or by densitometric image analysis. TTC-staining of cancellous bone fragments and bone slices, respectively, indicated the detectability of vital cells in both types of samples in a window of up to six hours after explantation. Staining intensity at later time-points was indistinguishable from the staining of untreated samples or sodium azide treated samples, which represent dead cells. In contrast, MTT-staining of bone slices revealed intense unspecific staining, which obscured the evaluation of the vitality of the samples. The lack of a detectable increase of colour intensity in TTC-stained bone samples, which were treated more than six hours after explantation, corresponds to reduced fracture healing. The described simple procedure could provide a basis for an intraoperative decision by the orthopaedic surgeon.