Mesenchymal stroma cells trigger early attraction of M1 macrophages and endothelial cells into fibrin hydrogels, stimulating long bone healing without long-term engraftment

Macrophage-mediated fracture healing: Unraveling molecular mechanisms and therapeutic implications using hydrogel-based interventions

Fracture healing is a complex biological process that involves the orchestrated interplay of various cells and molecular signaling pathways. Among the key players, macrophages have emerged as critical regulators of fracture repair, influencing inflammation, tissue remodeling, and angiogenesis. Recent advances in hydrogel-based therapeutics have provided exciting opportunities to leverage the modulatory effects of macrophages for improving fracture healing outcomes. In the present study, we review the importance of macrophages in fracture repair and their potential therapeutic role in hydrogel-based interventions. We discuss the molecular mechanisms underlying macrophage-mediated effects on fracture healing, and how hydrogels can be utilized as a platform for macrophage modulation. Furthermore, we highlight the translation of hydrogel-based therapies from bench to bedside, including preclinical and clinical studies, and the challenges and opportunities in harnessing the therapeutic potential of macrophages in fracture repair. Overall, understanding the importance of macrophages in fracture healing and the potential of hydrogel-based therapeutics to modulate macrophage responses can pave the way for developing innovative approaches to improve fracture healing outcomes.

Cell-based mechanisms and strategies of co-culture system both in vivo and vitro for bone tissue engineering

The lack of a functional vascular supply has been identified as a major challenge limiting the clinical introduction of stem cell-based bone tissue engineering (BTE) for the repair of large-volume bone defects (LVBD). Various approaches have been explored to improve the vascular supply in tissue-engineered constructs, and the development of strategies that could effectively induce the establishment of a functional vascular supply has become a major goal of BTE research. One of the state-of-the-art methods is to incorporate both angiogenic and osteogenic cells in co-culture systems. This review clarifies the key concepts involved, summarises the cell types and models used to date, and systematically evaluates their performance. We also discuss the cell-to-cell communication between these two cell types and the strategies explored in BTE constructs with angiogenic and osteogenic cells to optimise their functions. In addition, we outline unresolved issues and remaining obstacles that need to be overcome for further development in this field and eventual successful repair of LVBD.

Choosing the right animal model for osteomyelitis research: Considerations and challenges

Osteomyelitis is a debilitating bone disorder characterized by an inflammatory process involving the bone marrow, bone cortex, periosteum, and surrounding soft tissue, which can ultimately result in bone destruction. The etiology of osteomyelitis can be infectious, caused by various microorganisms, or noninfectious, such as chronic nonbacterial osteomyelitis (CNO) and chronic recurrent multifocal osteomyelitis (CRMO). Researchers have turned to animal models to study the pathophysiology of osteomyelitis. However, selecting an appropriate animal model that accurately recapitulates the human pathology of osteomyelitis while controlling for multiple variables that influence different clinical presentations remains a significant challenge. In this review, we present an overview of various animal models used in osteomyelitis research, including rodent, rabbit, avian/chicken, porcine, minipig, canine, sheep, and goat models. We discuss the characteristics of each animal model and the corresponding clinical scenarios that can provide a basic rationale for experimental selection. This review highlights the importance of selecting an appropriate animal model for osteomyelitis research to improve the accuracy of the results and facilitate the development of novel treatment and management strategies.

Interaction between Mesenchymal Stem Cells and Immune Cells during Bone Injury Repair

Fractures are the most common large organ trauma in humans. The initial inflammatory response promotes bone healing during the initial post-fracture phase, but chronic and persistent inflammation due to infection or other factors does not contribute to the healing process. The precise mechanisms by which immune cells and their cytokines are regulated in bone healing remain unclear. The use of mesenchymal stem cells (MSCs) for cellular therapy of bone injuries is a novel clinical treatment approach. Bone progenitor MSCs not only differentiate into bone, but also interact with the immune system to promote the healing process. We review in vitro and in vivo studies on the role of the immune system and bone marrow MSCs in bone healing and their interactions. A deeper understanding of this paradigm may provide clues to potential therapeutic targets in the healing process, thereby improving the reliability and safety of clinical applications of MSCs to promote bone healing.

Immune homeostasis modulation by hydrogel-guided delivery systems: a tool for accelerated bone regeneration

Immune homeostasis is delicately mediated by the dynamic balance between effector immune cells and regulatory immune cells. Local deviations from immune homeostasis in the microenvironment of bone fractures, caused by an increased ratio of effector to regulatory cues, can lead to excessive inflammatory conditions and hinder bone regeneration. Therefore, achieving effective and localized immunomodulation of bone fractures is crucial for successful bone regeneration. Recent research has focused on developing localized and specific immunomodulatory strategies using local hydrogel-based delivery systems. In this review, we aim to emphasize the significant role of immune homeostasis in bone regeneration, explore local hydrogel-based delivery systems, discuss emerging trends in immunomodulation for enhancing bone regeneration, and address the limitations of current delivery strategies along with the challenges of clinical translation.

Bone regeneration in inflammation with aging and cell-based immunomodulatory therapy

Aging of the global population increases the incidence of osteoporosis and associated fragility fractures, significantly impacting patient quality of life and healthcare costs. The acute inflammatory reaction is essential to initiate healing after injury. However, aging is associated with "inflammaging", referring to the presence of systemic low-level chronic inflammation. Chronic inflammation impairs the initiation of bone regeneration in elderly patients. This review examines current knowledge of the bone regeneration process and potential immunomodulatory therapies to facilitate bone healing in inflammaging.Aged macrophages show increased sensitivity and responsiveness to inflammatory signals. While M1 macrophages are activated during the acute inflammatory response, proper resolution of the inflammatory phase involves repolarizing pro-inflammatory M1 macrophages to an anti-inflammatory M2 phenotype associated with tissue regeneration. In aging, persistent chronic inflammation resulting from the failure of M1 to M2 repolarization leads to increased osteoclast activation and decreased osteoblast formation, thus increasing bone resorption and decreasing bone formation during healing.Inflammaging can impair the ability of stem cells to support bone regeneration and contributes to the decline in bone mass and strength that occurs with aging. Therefore, modulating inflammaging is a promising approach for improving bone health in the aging population. Mesenchymal stem cells (MSCs) possess immunomodulatory properties that may benefit bone regeneration in inflammation. Preconditioning MSCs with pro-inflammatory cytokines affects MSCs' secretory profile and osteogenic ability. MSCs cultured under hypoxic conditions show increased proliferation rates and secretion of growth factors. Resolution of inflammation via local delivery of anti-inflammatory cytokines is also a potential therapy for bone regeneration in inflammaging. Scaffolds containing anti-inflammatory cytokines, unaltered MSCs, and genetically modified MSCs can also have therapeutic potential. MSC exosomes can increase the migration of MSCs to the fracture site and enhance osteogenic differentiation and angiogenesis.In conclusion, inflammaging can impair the proper initiation of bone regeneration in the elderly. Modulating inflammaging is a promising approach for improving compromised bone healing in the aging population.

Polarization Behavior of Bone Macrophage as Well as Associated Osteoimmunity in Glucocorticoid-Induced Osteonecrosis of the Femoral Head

Glucocorticoid-induced osteonecrosis of the femoral head (GIONFH) is a disabling disease with high mortality in China but the detailed molecular and cellular mechanisms remain to be investigated. Macrophages are considered the key cells in osteoimmunology, and the cross-talk between bone macrophages and other cells in the microenvironment is involved in maintaining bone homeostasis. M1 polarized macrophages launch a chronic inflammatory response and secrete a broad spectrum of cytokines (eg, TNF-α, IL-6 and IL-1β) and chemokines to initiate a chronic inflammatory state in GIONFH. M2 macrophage is the alternatively activated anti-inflammatory type distributed mainly in the perivascular area of the necrotic femoral head. In the development of GIONFH, injured bone vascular endothelial cells and necrotic bone activate the TLR4/NF-κB signal pathway, promote dimerization of PKM2 and subsequently enhance the production of HIF-1, inducing metabolic transformation of macrophage to the M1 phenotype. Considering these findings, putative interventions by local chemokine regulation to correct the imbalance between M1/M2 polarized macrophages by switching macrophages to an M2 phenotype, or inhibiting the adoption of an M1 phenotype appear to be plausible regimens for preventing or intervening GIONFH in the early stage. However, these results were mainly obtained by in vitro tissue or experimental animal model. Further studies to completely elucidate the alterations of the M1/M2 macrophage polarization and functions of macrophages in glucocorticoid-induced osteonecrosis of the femoral head are imperative.

The crosstalk between macrophages and bone marrow mesenchymal stem cells in bone healing

Bone injury plagues millions of patients worldwide every year, and it demands a heavy portion of expense from the public medical insurance system. At present, orthopedists think that autologous bone transplantation is the gold standard for treating large-scale bone defects. However, this method has significant limitations, which means that parts of patients cannot obtain a satisfactory prognosis. Therefore, a basic study on new therapeutic methods is urgently needed. The in-depth research on crosstalk between macrophages (Mϕs) and bone marrow mesenchymal stem cells (BMSCs) suggests that there is a close relationship between inflammation and regeneration. The in-depth understanding of the crosstalk between Mϕs and BMSCs is helpful to amplify the efficacy of stem cell-based treatment for bone injury. Only in the suitable inflammatory microenvironment can the damaged tissues containing stem cells obtain satisfactory healing outcomes. The excessive tissue inflammation and lack of stem cells make the transplantation of biomaterials necessary. We can expect that the crosstalk between Mϕs and BMSCs and biomaterials will become the mainstream to explore new methods for bone injury in the future. This review mainly summarizes the research on the crosstalk between Mϕs and BMSCs and also briefly describes the effects of biomaterials and aging on cell transplantation therapy.

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.

Immunomodulatory strategies for bone regeneration: A review from the perspective of disease types

Tissue engineering strategies for treating bone loss to date have largely focused on targeting stem cells or vascularization. Immune cells, including macrophages and T cells, can also indirectly enhance bone healing via cytokine secretion to interact with other bone niche cells. Bone niche cues and local immune environment vary depending on anatomical location, size of defects and disease types. As such, it is critical to evaluate the role of the immune system in the context of specific bone niche and different disease types. This review focuses on immunomodulation research for bone applications using biomaterials and cell-based strategies, with a unique perspective from different disease types. We first reviewed applications for prolonging orthopaedic implant lifetime and enhancing fracture healing, two clinical challenges where immunomodulatory strategies were initially developed for orthopedic applications. We then reviewed recent research progress in harnessing immunomodulatory strategies for regenerating critical-sized, long bone or cranial bone defects, and treating osteolytic bone diseases. Remaining gaps in knowledge, future directions and opportunities were also discussed.

A Review Into the Insights of the Role of Endothelial Progenitor Cells on Bone Biology

In addition to its important transport functions, the skeletal system is involved in complex biological activities for the regulation of blood vessels. Endothelial progenitor cells (EPCs), as stem cells of endothelial cells (ECs), possess an effective proliferative capacity and a powerful angiogenic capacity prior to their differentiation. They demonstrate synergistic effects to promote bone regeneration and vascularization more effectively by co-culturing with multiple cells. EPCs demonstrate a significant therapeutic potential for the treatment of various bone diseases by secreting a combination of growth factors, regulating cellular functions, and promoting bone regeneration. In this review, we retrospect the definition and properties of EPCs, their interaction with mesenchymal stem cells, ECs, smooth muscle cells, and immune cells in bone regeneration, vascularization, and immunity, summarizing their mechanism of action and contribution to bone biology. Additionally, we generalized their role and potential mechanisms in the treatment of various bone diseases, possibly indicating their clinical application.

Minimally Manipulated Bone Marrow-Derived Cells Can Be Used for Tissue Engineering In Situ and Simultaneous Formation of Personalized Tissue Models

Red bone marrow and autologous bone tissue (bone fragments and bone chips) of the donor were harvested intraoperatively during autoplasty of talus bone defect. Titanium chips were obtained by grinding a fragment of a microporous titanium-coated hip arthroplasty (Zimmer). Bone marrow mononuclear cells were isolated in the operating room, and bone and titanium fragments were incubated with a suspension of mononuclear cells. The quality of revitalization was assessed by fluorescence microscopy and histological examination after culturing of adherent cells on the bone and titanium fragments. During culturing on bone chips, bone marrow mononuclear fraction cells demonstrated significantly higher metabolic activity than bone marrow cells (p=0.04). Mononuclear fraction cells were also capable of stable colonization of titanium fragments with the formation of composite tissue model.

Systemic and local innate immune responses to surgical co-transplantation of mesenchymal stromal cells and biphasic calcium phosphate for bone regeneration

Bone regeneration from mesenchymal stromal cells (MSC) is attributed to comprehensive immune modulation mediated by the MSC. However, the temporal and spatial regulation of these immune responses has not yet been described. The aim of the present study was to assess the local and systemic innate immune responses to implantation of biphasic calcium phosphate biomaterial (BCP) alone, or with bone marrow derived MSC (BCP+MSC), in critical-sized calvarial bone defects of Lewis rats. Four weeks after implantation, flow cytometry analysis of innate immune cells revealed increased numbers of circulating classical monocyte-macrophages (MM) and decreased non-classical MM in the BCP+MSC group. At week 8, this differential systemic MM response was associated with an increased presence of local tissue anti-inflammatory macrophages expressing CD68 and CD163 markers (M2-like). In the BCP group without MSC, NK cells increased at weeks 1 and 4, and neutrophils increased in circulation at weeks 2 and 8. At week 8, the increase in number of neutrophils in circulation was associated with decreased local tissue neutrophils, in the BCP+MSC group. Gene expression analysis of tissue biopsies from defects implanted with BCP+MSC, in comparison to BCP alone, revealed upregulated expression of early osteogenesis genes along with macrophage differentiation-related genes at weeks 1 and 8 and neutrophil chemotaxis-related genes at week 1. This study is the first to demonstrate that surgical implantation of BCP or BCP+MSC grafts differentially regulate both systemic and local tissue innate immune responses which enhance bone formation. The results provide new insights into immune mechanisms underlying MSC-mediated bone regeneration. STATEMENT OF SIGNIFICANCE: The suitability of biphasic calcium phosphate and mesenchymal stromal cell construct (BCP+MSC) transplantation is evident from their progress in clinical trials for treating challenging maxillofacial bone defects. But less is known about the overall immune response generated by this surgical process and how it later impacts the bone formation. To this end, it is crucial to understand for both clinicians and researchers, the systemic immune response to transplanting MSC in patients for ensuring both the safety and efficacy of cell therapies. In this study, we used rat calvarial bone defect model and showed that both systemic and local innate immunes responses (monocyte-macrophages and neutrophils) are favorably directed towards enhanced bone formation in BCP+MSC implanted defects, as compared to BCP alone.

Mesenchymal Stem Cell–Immune Cell Interaction and Related Modulations for Bone Tissue Engineering

Critical bone defects and related delayed union and nonunion are still worldwide problems to be solved. Bone tissue engineering is mainly aimed at achieving satisfactory bone reconstruction. Mesenchymal stem cells (MSCs) are a kind of pluripotent stem cells that can differentiate into bone cells and can be used as one of the key pillars of bone tissue engineering. In recent decades, immune responses play an important role in bone regeneration. Innate immune responses provide a suitable inflammatory microenvironment for bone regeneration and initiate bone regeneration in the early stage of fracture repair. Adaptive immune responses maintain bone regeneration and bone remodeling. MSCs and immune cells regulate each other. All kinds of immune cells and secreted cytokines can regulate the migration, proliferation, and osteogenic differentiation of MSCs, which have a strong immunomodulatory ability to these immune cells. This review mainly introduces the interaction between MSCs and immune cells on bone regeneration and its potential mechanism, and discusses the practical application in bone tissue engineering by modulating this kind of cell-to-cell crosstalk. Thus, an in-depth understanding of these principles of bone immunology can provide a new way for bone tissue engineering.

Sprayable Hydrogel for Biomedical Applications

Polymeric hydrogels have extraordinary potential to be utilized for biomedical applications. Recently, sprayable hydrogels have received increasing attention for their biocompatibility, degradability, tunable mechanical properties and rapid spray-filming abilities. In...

Dicalcium silicate-induced mitochondrial dysfunction and autophagy-mediated macrophagic inflammation promotes osteogenic differentiation of BMSCs

Dicalcium silicate (Ca2SiO4, C2S) has osteogenic potential but induces macrophagic inflammation. Mitochondrial function plays a vital role in macrophage polarization and macrophagic inflammation. The mitochondrial function of C2S-treated macrophages is still unclear. This study hypothesized: (i) the C2S modulates mitochondrial function and autophagy in macrophages to regulate macrophagic inflammation, and (ii) C2S-induced macrophagic inflammation regulates osteogenesis. We used RAW264.7 cells as a model of macrophage. The C2S (75-150 μg/ml) extract was used to analyze the macrophagic mitochondrial function and macrophage-mediated effect on osteogenic differentiation of mouse bone marrow-derived mesenchymal stem cells (BMSCs). The results showed that C2S extract (150 μg/ml) induced TNF-α, IL-1β and IL-6 production in macrophages. C2S extract (150 μg/ml) enhanced reactive oxygen species level and intracellular calcium level but reduced mitochondrial membrane potential and ATP production. TEM images showed reduced mitochondrial abundance and altered the mitochondrial morphology in C2S (150 μg/ml)-treated macrophages. Protein level expression of PINK1, Parkin, Beclin1 and LC3 was upregulated but TOMM20 was downregulated. mRNA sequencing and KEGG analysis showed that C2S-induced differentially expressed mRNAs in macrophages were mainly distributed in the essential signaling pathways involved in mitochondrial function and autophagy. The conditioned medium from C2S-treated macrophage robustly promoted osteogenic differentiation in BMSCs. In conclusion, our results indicate mitochondrial dysfunction and autophagy as the possible mechanism of C2S-induced macrophagic inflammation. The promotion of osteogenic differentiation of BMSCs by the C2S-induced macrophagic inflammation suggests the potential application of C2S in developing immunomodulatory bone grafts.

3D GelMA ICC Scaffolds Combined with SW033291 for Bone Regeneration by Modulating Macrophage Polarization

Despite the interaction between bone marrow mesenchymal stem cells (BMSCs) and macrophages has been found to play a critical role in repairing bone defects, it remains a challenge to develop a desirable tissue engineering scaffold for synchronous regulation of osteogenic differentiation and macrophage polarization. Herein, this study proposed a novel strategy to treat bone defects based on three-dimensional Gelatin Methacryloyl Inverted Colloidal Crystal (3D GelMA ICC) scaffold and an active 15-hydroxyprostaglandin dehydrogenase (15-PGDH) inhibitor SW033291. Specifically, the 3D GelMA ICC scaffolds were firstly prepared by colloidal templating method, which displayed good cell attachment and promoted intercellular interaction among macrophage and BMSCs due to its uniform pore interconnectivity. By combined use of SW033291, the release of Prostaglandin E2 (PGE2) from BMSCs on the GelMA ICC scaffold was significantly upregulated and macrophages M2 polarization was markedly increased. In turn, BMSCs proliferation and osteogenic differentiation was further enhanced by paracrine regulation of M2 macrophage, and thus finally caused more in vivo new bone formation by shaping up a pro-regenerative local immune microenvironment surrounding GelMA ICC scaffold. Our findings demonstrate the potential of 3D GelMA ICC scaffolds combined with SW033291 to become an effective tissue engineering strategy for bone regeneration.

Sequential activation of heterogeneous macrophage phenotypes is essential for biomaterials-induced bone regeneration

Macrophage has been gradually recognized as a central regulator in tissue regeneration, and the study of how macrophage mediates biomaterials-induced bone regeneration through immunomodulatory pathway becomes popular. However, the current understanding on the roles of different macrophage phenotypes in regulating bone tissue regeneration remains controversial. In this study, we demonstrate that sequential infiltration of heterogeneous phenotypes of macrophages triggered by bio-metal ions effectively facilitates bone healing in bone defect. Indeed, M1 macrophages promote the recruitment and early commitment of osteogenic and angiogenic progenitors, while M2 macrophages and osteoclasts support the deposition and mineralization of the bone matrix, as well as the maturation of blood vessels. Moreover, we have identified a group of bone biomaterial-related multinucleated cells that behave similarly to M2 macrophages with wound-healing features rather than participate in the bone resorption cascade similarly to osteoclasts. Our study shows how sequential activation of macrophage-osteoclast lineage contribute to a highly orchestrated immune response in the bone tissue microenvironment around biomaterials to regulate the complex biological process of bone healing. Therefore, we believe that the temporal activation pattern of heterogeneous macrophage phenotypes should be considered when the next generation of biomaterials for bone regeneration is engineered.

The Crosstalk between Mesenchymal Stem Cells and Macrophages in Bone Regeneration: A Systematic Review

Bone regeneration is a complex and well-coordinated process that involves crosstalk between immune cells and resident cells in the injury site. Transplantation of mesenchymal stem cells (MSCs) is a promising strategy to enhance bone regeneration. Growing evidence suggests that macrophages have a significant impact on osteogenesis during bone regeneration. However, the precise mechanisms by which macrophage subtypes influence bone regeneration and how MSCs communicate with macrophages have not yet been fully elucidated. In this systematic literature review, we gathered evidence regarding the crosstalk between MSCs and macrophages during bone regeneration. According to the PRISMA protocol, we extracted literature from PubMed and Embase databases by using "mesenchymal stem cells" and "macrophages" and "bone regeneration" as keywords. Thirty-three studies were selected for this review. MSCs isolated from both bone marrow and adipose tissue and both primary macrophages and macrophage cell lines were used in the selected studies. In conclusion, anti-inflammatory macrophages (M2) have significantly more potential to strengthen bone regeneration compared with naïve (M0) and classically activated macrophages (M1). Transplantation of MSCs induced M1-to-M2 transition and transformed the skeletal microenvironment to facilitate bone regeneration in bone fracture and bone defect models. This review highlights the complexity between MSCs and macrophages, providing more insight into the polarized macrophage behavior in this evolving field of osteoimmunology. The results may serve as a useful reference for definite success in MSC-based therapy based on the critical interaction with macrophages.

Coaxial nanofibrous scaffolds mimicking the extracellular matrix transition in the wound healing process promoting skin regeneration through enhancing immunomodulation

Numerous studies have shown that scaffolds incorporated with extracellular matrix (ECM) proteins could regulate cell behaviors and improve wound healing. However, most ECM-containing scaffolds fail to capture the dynamic features of the native ECM. In this regard, nanofibrous scaffolds which mimic the composition transition of the ECM during wound healing may have great potential in promoting skin regeneration through dynamically modulating the microenvironment. Herein, we report a novel skin ECM-biomimetic coaxial nanofibrous scaffold for the repair of chronic wounds. Two essential ECM proteins, fibrinogen and collagen I, were incorporated into the shell and the core of nanofibers, respectively, to mimic the sequential appearance of fibrinogen and collagen I in the wound healing process. The regulation of the biomimetic coaxial scaffolds on adipose-derived mesenchymal stromal cells (ASCs) was compared with that of the PLGA/fibrinogen, PLGA/collagen I and PLGA uniaxial scaffolds. Our results showed that the biomimetic coaxial scaffolds remarkably promoted the immunomodulatory paracrine secretion of ASCs. By incubating macrophages with ASC conditioned medium, the enhanced immunomodulation of ASCs on the biomimetic coaxial scaffolds was confirmed by the enhanced M1-to-M2 polarization of macrophages. Furthermore, the biomimetic coaxial scaffolds effectively promoted wound repair through resolving inflammation in diabetic rats. These findings helped reveal the role of the dynamic ECM change in regulating wound healing and suggest the potential utility of the biomimetic coaxial scaffolds as a promising alternative to treat chronic wounds.

Applications of biomaterials for immunosuppression in tissue repair and regeneration

The immune system plays an essential role in tissue repair and regeneration. Regardless of innate or adaptive immune responses, immunosuppressive strategies such as macrophage polarization and regulatory T (Treg) cell induction can be used to modulate the immune system to promote tissue repair and regeneration. Biomaterials can improve the production of anti-inflammatory macrophages and Treg cells by providing physiochemical cues or delivering therapeutics such as cytokines, small molecules, microRNA, growth factors, or stem cells in the damaged tissues. Herein, we present an overview of immunosuppressive modulation by biomaterials in tissue regeneration and highlight the mechanisms of macrophage polarization and Treg cell induction. Overall, we foresee that future biomaterials for regenerative strategies will entail more interactions between biomaterials and the immune cells, and more mechanisms of immunosuppression related to T cell subsets remain to be discovered and applied to develop novel biomaterials for tissue repair and regeneration. STATEMENT OF SIGNIFICANCE: Immunosuppression plays a key role in tissue repair and regeneration, and biomaterials can interact with the immune system through their biological properties and by providing physiochemical cues. Here, we summarize the studies on biomaterials that have been used for immunosuppression to facilitate tissue regeneration. In the first part of this review, we demonstrate the crucial role of macrophage polarization and induction of T regulatory (Treg) cells in immunosuppression. In the second part, distinct approaches used by biomaterials to induce immunosuppression are introduced, which show excellent performance in terms of promoting tissue regeneration.

Bone fracture healing: perspectives according to molecular basis

Fractures have a great impact on health all around the world and with fracture healing optimization; this problem could be resolved partially. To make a practical contribution to this issue, the knowledge of bone tissue, cellularity, and metabolism is essential, especially cytoskeletal architecture and its transformations according to external pressures. Special physical and chemical characteristics of the extracellular matrix (ECM) allow the transmission of mechanical stimuli from outside the cell to the plasmatic membrane. The osteocyte cytoskeleton is conformed by a complex network of actin and microtubules combined with crosslinker proteins like vinculin and fimbrin, connecting and transmitting outside stimuli through EMC to cytoplasm. Herein, critical signaling pathways like Cx43-depending ones, MAPK/ERK, Wnt, YAP/TAZ, Rho-ROCK, and others are activated due to mechanical stimuli, resulting in osteocyte cytoskeletal changes and ECM remodeling, altering the tissue and, therefore, the bone. In recent years, the osteocyte has gained more interest and value in relation to bone homeostasis as a great coordinator of other cell populations, thanks to its unique functions. By integrating the latest advances in relation to intracellular signaling pathways, mechanotransmission system of the osteocyte and bone tissue engineering, there are promising experimental strategies, while some are ready for clinical trials. This work aims to show clearly and precisely the integration between cytoskeleton and main molecular pathways in relation to mechanotransmission mechanism in osteocytes, and the use of this theoretical knowledge in therapeutic tools for bone fracture healing.

Nanosurfacing Ti alloy by weak alkalinity-activated solid-state dewetting (AAD) and its biointerfacial enhancement effect

Nanoscale manipulation of material surfaces can create extraordinary properties, holding great potential for modulating the implant-bio interface for enhanced performance. In this study, a green, simple and biocompatible nanosurfacing approach based on weak alkalinity-activated solid-state dewetting (AAD) was for the first time developed to nano-manipulate the Ti6Al4V surface by atomic self-rearrangement. AAD treatment generated quasi-periodic titanium oxide nanopimples with high surface energy. The nanopimple-like nanostructures enhanced the osteogenic activity of osteoblasts, facilitated M2 polarization of macrophages, and modulated the cross-talk between osteoblasts and macrophages, which collectively led to significant strengthening of in vivo bone-implant interfacial bonding. In addition, the titanium oxide nanopimples strongly adhered to the Ti alloy, showing resistance to tribocorrosion damage. The results suggest strong nano-bio interfacial effects, which was not seen for the control Ti alloy processed through traditional thermal oxidation. Compared to other nanostructuring strategies, the AAD technique shows great potential to integrate high-performance, functionality, practicality and scalability for surface modification of medical implants.

Use of a novel Screen-Enrich-Combine(-biomaterials) Circulating System to fill a 3D-printed open Ti6Al4V frame with mesenchymal stem cells/β-tricalcium phosphate to repair complex anatomical bone defects in load-bearing areas

Background

Repairing complex anatomical load-bearing bone defects is difficult because it requires the restoration of the load-bearing function, reconstructing the anatomical shape, and repair by regenerated bone. We previously developed a Screen–Enrich–Combine(-biomaterials) Circulating System (SECCS) for rapid intraoperative enrichment of autologous bone marrow mesenchymal stem cells (MSCs) to enhance the osteogenic ability of porous bone substitutes. In this study, we prepared a 3D-printed Ti6A14V macroporous frame matching the defect shape to provide early load-bearing support and evaluated the efficacy of filling the frame with SECCS-processed MSCs/beta tricalcium phosphate (β-TCP) for long-term bone growth.

Methods

Fifteen 2-year-old goats were involved in this study, and the lateral part of their distal femur was removed by an electric saw and was fitted by a matching electron beam melting technology-prepared (EBM) Ti6Al4V frame. Three types of frames, filled with nothing, pure porous β-TCP, or SECCS-processed MSCs/β-TCP, were fixed onto the defect site. Repair efficacy was evaluated by X-ray radiography, computed tomography (CT), histology, and histomorphometry.

Results

In the basic regular hexagon printing unit, the combined side width (w) and the inscribed circle diameter (d) determines the printing frame’s mechanical strength. The compressive load was significantly higher for w=1.9 mm, d=4.4 mm than for w=1.7 mm, d=4.0 mm or w=2.0 mm, d=5.0 mm (P<0.05). The EBM-prepared Ti6Al4V defect-matched frame was well maintained 9 months after implantation. The MSCs successfully adhered to the wall of the porous β-TCP in the SECCS-processed group and had spread fully in the test samples. Each goat in the MSCs/β-TCP–the filled group, had approximately 31,321.7±22,554.7 of MSCs and a larger area of new bone growth inside the frame than the control and blank areas groups.

Conclusions

Filling the 3D-printed Ti6Al4V large-aperture frame with osteogenic materials achieved biological reconstruction over a larger area of regenerated bone to repair complex anatomical weight-bearing bone defects under the condition of early frame-supported load bearing. MSCs/β-TCP prepared by SECCS can be used as a filling material for this type of bone defect to obtain more efficacious bone repair.

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

A coordinated interaction between osteogenesis and osteoimmune microenvironment is essential for successful bone healing. In particular, macrophages play a central regulatory role in all stages of bone repair. Depending on the signals they sense, these highly plastic cells can mediate the host immune response against the exterior signals of molecular stimuli and implanted scaffolds, to exert regenerative potency to a varying extent. In this article, we first encapsulate the immunomodulatory functions of macrophages during bone regeneration into three aspects, as sweeper, mediator and instructor. We introduce the phagocytic role of macrophages in different bone healing periods ('sweeper') and overview a variety of paracrine cytokines released by macrophages either mediating cell mobilisation, vascularisation and matrix remodelling ('mediator'), or directly driving the osteogenic differentiation of bone progenitors and bone repair ('instructor'). Then, we systematically classify and discuss the emerging engineering strategies to recruit, activate and modulate the phenotype transition of macrophages, to exploit the power of endogenous macrophages to enhance the performance of engineered bone tissue.

Communications Between Bone Marrow Macrophages and Bone Cells in Bone Remodeling

The mammalian skeleton is a metabolically active organ that continuously undergoes bone remodeling, a process of tightly coupled bone resorption and formation throughout life. Recent studies have expanded our knowledge about the interactions between cells within bone marrow in bone remodeling. Macrophages resident in bone (BMMs) can regulate bone metabolism via secreting numbers of cytokines and exosomes. This review summarizes the current understanding of factors, exosomes, and hormones that involved in the communications between BMMs and other bone cells including mensenchymal stem cells, osteoblasts, osteocytes, and so on. We also discuss the role of BMMs and potential therapeutic approaches targeting BMMs in bone remodeling related diseases such as osteoporosis, osteoarthritis, rheumatoid arthritis, and osteosarcoma.

Biodegradable materials for bone defect repair

Compared with non-degradable materials, biodegradable biomaterials play an increasingly important role in the repairing of severe bone defects, and have attracted extensive attention from researchers. In the treatment of bone defects, scaffolds made of biodegradable materials can provide a crawling bridge for new bone tissue in the gap and a platform for cells and growth factors to play a physiological role, which will eventually be degraded and absorbed in the body and be replaced by the new bone tissue. Traditional biodegradable materials include polymers, ceramics and metals, which have been used in bone defect repairing for many years. Although these materials have more or fewer shortcomings, they are still the cornerstone of our development of a new generation of degradable materials. With the rapid development of modern science and technology, in the twenty-first century, more and more kinds of new biodegradable materials emerge in endlessly, such as new intelligent micro-nano materials and cell-based products. At the same time, there are many new fabrication technologies of improving biodegradable materials, such as modular fabrication, 3D and 4D printing, interface reinforcement and nanotechnology. This review will introduce various kinds of biodegradable materials commonly used in bone defect repairing, especially the newly emerging materials and their fabrication technology in recent years, and look forward to the future research direction, hoping to provide researchers in the field with some inspiration and reference.

Modification of PLGA Scaffold by MSC-Derived Extracellular Matrix Combats Macrophage Inflammation to Initiate Bone Regeneration via TGF-β-Induced Protein

The immunologic response toward chronic inflammation or bone regeneration via the accumulation of M1 or M2 macrophages after injury could determine the fate of biomaterial. Human umbilical cord mesenchymal stem cells (hUCMSCs) have a pivotal immunomodulatory property on directing macrophage behaviors. Herein, for the first time, 3D-printed poly(lactide-co-glycolide) (PLGA) scaffolds modified with hUCMSC-derived extracellular matrix (PLGA-ECM) are prepared by a facile tissue engineering technique with physical decellularization and 2.44 ± 0.29 mg cm^-3 proteins immobilized on the PLGA-ECM contain multiple soluble cytokines with a sustainable release profile. The PLGA-ECM not only attenuates the foreign body response, but also improves bone regeneration by increasing the accumulation of M2 macrophages in an improved heterotopic transplantation model of SCID mice. Furthermore, the PLGA-ECM scaffolds with the knockdown of transforming growth factor-β-induced protein (TGFβI/βig-H3) demonstrate that M2 macrophage accumulation improved by the PLGA-ECM could be attributed to increasing the migration of M2 macrophages and the repolarization of M1 macrophages to M2 phenotype, which are mediated by multiple integrin signaling pathways involving in integrin β7, integrin α9, and integrin β1 in a TGFβI-dependent manner. This study presents an effective surface modification strategy of polymeric scaffolds to initiate tissue regeneration and combat inflammatory response by increasing M2 macrophage accumulation.

Evaluation of a cell-based osteogenic formulation compliant with good manufacturing practice for use in tissue engineering

Proper bony tissue regeneration requires mechanical stabilization, an osteogenic biological activity and appropriate scaffolds. The latter two elements can be combined in a hydrogel format for effective delivery, so it can readily adapt to the architecture of the defect. We evaluated a Good Manufacturing Practice-compliant formulation composed of bone marrow-derived mesenchymal stromal cells in combination with bone particles (Ø = 0.25 to 1 µm) and fibrin, which can be readily translated into the clinical setting for the treatment of bone defects, as an alternative to bone tissue autografts. Remarkably, cells survived with unaltered phenotype (CD73⁺, CD90⁺, CD105⁺, CD31^-, CD45^-) and retained their osteogenic capacity up to 48 h after being combined with hydrogel and bone particles, thus demonstrating the stability of their identity and potency. Moreover, in a subchronic toxicity in vivo study, no toxicity was observed upon subcutaneous administration in athymic mice and signs of osteogenesis and vascularization were detected 2 months after administration. The preclinical data gathered in the present work, in compliance with current quality and regulatory requirements, demonstrated the feasibility of formulating an osteogenic cell-based tissue engineering product with a defined profile including identity, purity and potency (in vitro and in vivo), and the stability of these attributes, which complements the preclinical package required prior to move towards its use of prior to its clinical use.

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.

Bone regeneration in rabbit calvarial defects using PRGF and adipose-derived stem cells: histomorphometrical analysis

Aim: The aim of this study was to evaluate the osteogenic potential of adipose-derived stem cells (ADSCs) and to assess the influence of plasma rich in growth factors (PRGF) on bone regeneration using ADSCs. Materials & methods: Bone defects were randomly allocated to the five treatment modalities: spontaneous healing, natural bovine bone mineral (BBM), BBM loaded with PRGF, BBM loaded with ADSCs and BBM loaded with a combination of ADSCs and PRGF. Results: The PRGF significantly enhanced the biomaterial-to-bone contact. Defects treated with ADSCs and PRGF or a combination of both showed the greatest bone regeneration. Conclusion: Combining PRGF and ADSCs boosts the bone graft regenerative potential at the earliest period of healing.

Development of CaCO3 microsphere-based composite hydrogel for dual delivery of growth factor and Ca to enhance bone regeneration

Injectable scaffolds have attracted much attention because of their minimum surgical invasiveness. However, limited osteogenic induction property and low mechanical properties hampered their application in bone tissue engineering. CaCO3 microspheres, which possess osteoinductivity, rough surfaces and specific binding sites for BMP-2, were first fabricated; after BMP-2 uploading, microspheres were further entrapped in fibrin-glue hydrogel. CaCO3 microspheres were co-functionalized with casein and heparin. To obtain a high encapsulation of heparin and thus BMP-2 uploading, along with controlled release and simultaneous maintenance of the presence of vaterite which had osteogenic induction property, fabrication parameters were optimized and microspheres were characterized using XRD, FITR and SEM. The formed CaCO3 had a microsphere morphology of ∼1 μm. Both vaterite and calcite phases were present and the relative amount of calcite phase increased with the amount of heparin. Sample 25 mM_4-1Hep with the highest loading amount of heparin was selected as carrier for BMP-2 and BMP-2 loaded CaCO3 microspheres were further entrapped in fibrin-glue hydrogel (FC-B). For the as-prepared composite hydrogel, mechanical properties were characterized and the presence of CaCO3 significantly elevated the tensile strength; controlled release of BMP-2 was sustained until day 21. Based on ALP activity, alizarin red staining and RT-PCR, in vitro osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs) was found to be significantly enhanced under induction of FC-B. Rabbit tibia bone defect model was applied to evaluate its in vivo performance. After implantation for 4 weeks, presence of composite hydrogel was observed in defects. After 8 weeks, bone defects of FC-B group were nearly completely healed. Using the fact that autologous scaffolds can be derived based on fibrin-glue hydrogel, the well-designed BMP-2 loaded fibrin-glue composite hydrogel demonstrated good potential in bone tissue engineering.

Scaffold strategies for modulating immune microenvironment during bone regeneration

Implanted bone scaffolds often fail to successfully integrate with the host tissue because they do not elicit a favorable immune reaction. Properties of bone scaffold not only provide mechanical and chemical signals to support cell adhesion, migration, proliferation and differentiation, but also play a pivotal role in determining the extent of immune response during bone regeneration. Appropriate design parameters of bone scaffold are of great significance in the process of developing a new generation of bone implants. Herein, this article addresses the recent advances in the field of bone scaffolds for immune response, particularly focusing on the physical and chemical properties of bone scaffold in manipulating the host response. Furthermore, incorporation of bioactive molecules and cells with immunoregulatory function in bone scaffolds are also presented. Finally, continuing challenges and future directions of scaffold-based strategies for modulating immune microenvironment are discussed.

Immunomodulatory Effects of MSCs in Bone Healing

Mesenchymal stem cells (MSCs) are capable of differentiating into multilineage cells, thus making them a significant prospect as a cell source for regenerative therapy; however, the differentiation capacity of MSCs into osteoblasts seems to not be the main mechanism responsible for the benefits associated with human mesenchymal stem cells hMSCs when used in cell therapy approaches. The process of bone fracture restoration starts with an instant inflammatory reaction, as the innate immune system responds with cytokines that enhance and activate many cell types, including MSCs, at the site of the injury. In this review, we address the influence of MSCs on the immune system in fracture repair and osteogenesis. This paradigm offers a means of distinguishing target bone diseases to be treated with MSC therapy to enhance bone repair by targeting the crosstalk between MSCs and the immune system.

Human amnion mesenchymal stem cells attenuate atherosclerosis by modulating macrophage function to reduce immune response

Mesenchymal stem cells (MSCs) show immunosuppressive activities and alleviate atherosclerosis (AS) formation in apolipoprotein E-knockout (apoE-KO) mice. Human amnion mesenchymal stem cells (hAMSCs), a particular population of mesenchymal stem cells, have been shown to have immunomodulatory abilities. The present study investigated the effects of hAMSCs treatment on early atherosclerotic plaque formation and the progression of established lesion in apoE-KO mice. In total, 36 mice were fed with a high-fat diet. Mice were subjected to hAMSCs-injection treatment simultaneously with high-fat diet (early treatment) or after 8 weeks of high-fat diet (delayed treatment). In each treatment, mice were divided into three groups: i) hAMSCs group with hAMSCs treatment; ii) PBS group injected with PBS; and iii) control group without injection. Histological results showed that the plaque area in the aortic arch of mice was significantly reduced after hAMSCs treatment in the early and delayed treatment groups. In addition, immunohistochemical analysis suggested that the accumulation of macrophages was significantly decreased after hAMSCs treatment. Similarly, the release of the pro-inflammatory cytokine tumor necrosis factor-α was also decreased, whereas the release of the anti-inflammatory cytokine interleukin-10 was increased. In addition, hAMSCs treatment suppressed the phosphorylation of p65 and inhibitor of κB-α, suggesting that NF-κB pathway was involved in the hAMSCs-mediated suppression of immune response. In conclusion, hAMSCs treatment was effective in reducing immune response, which is the one of the major causes of AS, eventually leading to a significant reduction in size of athero-sclerotic lesions.

Advancements and frontiers in nano-based 3D and 4D scaffolds for bone and cartilage tissue engineering

Given the enormous increase in the risks of bone and cartilage defects with the rise in the aging population, the current treatments available are insufficient for handling this burden, and the supply of donor organs for transplantation is limited. Therefore, tissue engineering is a promising approach for treating such defects. Advances in materials research and high-tech optimized fabrication of scaffolds have increased the efficiency of tissue engineering. Electrospun nanofibrous scaffolds and hydrogel scaffolds mimic the native extracellular matrix of bone, providing a support for bone and cartilage tissue engineering by increasing cell viability, adhesion, propagation, and homing, and osteogenic isolation and differentiation, vascularization, host integration, and load bearing. The use of these scaffolds with advanced three- and four-dimensional printing technologies has enabled customized bone grafting. In this review, we discuss the different approaches used for cartilage and bone tissue engineering.

Mesenchymal stem cells as adjuvant therapy for limb lengthening in achondroplasia

Staged leg lengthening allows achondroplastic dwarfs to reach nearly normal height, but it takes long periods of external fixation and it can be burdened by delayed unions. Between 2009 and 2013, eight achondroplastic dwarfs showed delayed unions in the callus formation during femoral lengthening stages in our institute. We performed in-situ injections of bone marrow-derived stem cell concentrates. Patients underwent monthly clinical and radiographic assessment for determination of the healing rate. All eight patients showed an improvement in the regenerated bone, with an average healing index of 23.1 days/cm (range: 18.7-23.8 days/cm). The complete recovery of the delayed consolidation took on an average of 5.2 months (range: 2-10 months). The use of cellular therapy in these patients could represent an innovative application.

Efficacy of intraoperatively prepared cell-based constructs for bone regeneration

BACKGROUND

Conventional cell-based bone regeneration suffers from the major disadvantage of limited cell supply, time-consuming in vitro expansion cultures, and limited patient-friendliness related to cell isolation and multiple visits to the clinic. Here, we utilized an alternative concept using "easy access cells" that can be obtained in an intraoperative manner to prepare cell-based constructs.

METHODS

We used stromal vascular fraction (SVF) from human adipose tissue and human monocytes for intraoperative preparation of bone constructs. Conventional constructs grafted with expanded human adipose tissue mesenchymal stem cells (ADMSCs) derived from the same donor were set as positive controls. Additionally, we combined both cell types either or not with monocytes. The cellular interaction of human SVF and ADMSCs with human monocytes was evaluated in vitro. The feasibility and bone-regenerative capacity of intraoperative constructs were determined histologically and histomorphometrically in a rat femoral condyle bone defect model.

RESULTS

SVF displayed equal in vitro osteogenic differentiation compared to donor-matched expanded ADMSCs, which for both was significantly enhanced upon co-culture with monocytes. Moreover, SVF and ADMSCs displayed different immunoregulatory effects on monocytes/macrophages. Upon implantation in rat femoral bone defects, SVF constructs demonstrated superior bone formation compared to ADMSC constructs and cell-free controls; no effects of monocyte addition were observed.

CONCLUSION

In conclusion, we here demonstrate the feasibility of intraoperative SVF construct preparation and superior bone-regenerative capacity thereof compared to donor-matched ADMSC constructs. The superiority of SVF constructs was found to be linked to the distinct differences between immunoregulatory effects of SVF and ADMSCs.

Stimulation of calvarial bone healing with human bone marrow stromal cells versus inhibition with adipose-tissue stromal cells on nanostructured β-TCP-collagen

Bioactive functional scaffolds are essential for support of cell-based strategies to improve bone regeneration. Adipose-tissue-derived-stromal cells (ASC) are more accessible multipotent cells with faster proliferation than bone-marrow-derived-stromal-cells (BMSC) having potential to replace BMSC for therapeutic stimulation of bone-defect healing. Their osteogenic potential is, however, lower compared to BMSC, a deficit that may be overcome in growth factor-rich orthotopic bone defects with enhanced bone-conductive scaffolds. Objective of this study was to compare the therapeutic potency of human ASC and BMSC for bone regeneration on a novel nanoparticulate β-TCP/collagen-carrier (β-TNC). Cytotoxicity of β-TCP nanoparticles and multilineage differentiation of cells were characterized in vitro. Cell-seeded β-TNC versus cell-free controls were implanted into 4 mm calvarial bone-defects in immunodeficient mice and bone healing was quantified by µCT at 4 and 8 weeks. Tissue-quality and cell-origin were assessed by histology. β-TNC was non-toxic, radiolucent and biocompatible, lent excellent support for human cell persistence and allowed formation of human bone tissue by BMSC but not ASC. Opposite to BMSC, ASC-grafting significantly inhibited calvarial bone healing compared to controls. Bone formation progressed significantly from 4 to 8 weeks only in BMSC and controls yielding 5.6-fold more mineralized tissue in BMSC versus ASC-treated defects. Conclusively, β-TNC was simple to generate, biocompatible, osteoconductive, and stimulated osteogenicity of BMSC to enhance calvarial defect healing while ASC had negative effects. Thus, an orthotopic environment and β-TNC could not compensate for cell-autonomous deficits of ASC which should systematically be considered when choosing the right cell source for tissue engineering-based stimulation of bone regeneration.

STATEMENT OF SIGNIFICANCE

Bone-marrow-derived-stromal cells (BMSC) implanted on bone replacement materials can support bone defect healing and adipose-tissue-derived-stromal cells (ASC) being more accessible and better proliferating are considered as alternate source. This first standardized comparison of the bone regeneration potency of human ASC and BMSC was performed on a novel nanoparticular β-TCP-enriched collagen-carrier (β-TNC) designed to overcome the known inferior osteogenicity of ASC. β-TNC was non-toxic, biocompatible and osteoconductive supporting human bone formation and defect-closure by BMSC but not ASC. Long-term cell-persistence and the distinct secretome of ASC appear as main reasons why ASC inhibited bone healing opposite to BMSC. Overall, ASC-grafting is at considerable risk of producing negative effects on bone-healing while no such risks are known for BMSC.

Development of a centrally vascularized tissue engineering bone graft with the unique core-shell composite structure for large femoral bone defect treatment

Great effort has been spent to promote the vascularization of tissue engineering bone grafts (TEBG) for improved therapeutic outcome. However, the thorough vascularization especially in the central region still remained as a major challenge for the clinical translation of TEBG. Here, we developed a new strategy to construct a centrally vascularized TEBG (CV-TEBG) with unique core-shell composite structure, which is consisted of an angiogenic core and an osteogenic shell. The in vivo evaluation in rabbit critical sized femoral defect was conducted to meticulously compare CV-TEBG to other TEBG designs (TEBG with osteogenic shell alone, or angiogenic core alone or angiogenic core+shell). Microfil-enhanced micro-CT analysis has been shown that CV-TEBG could outperform TEBG with pure osteogenic or angiogenic component for neo-vascularization. CV-TEBG achieved a much higher and more homogenous vascularization throughout the whole scaffold (1.52-38.91 folds, p < 0.01), and generated a unique burrito-like vascular network structure to perfuse both the central and peripheral regions of TEBG, indicating a potential synergistic effect between the osteogenic shell and angiogenic core in CV-TEBG to enhance neo-vascularization. Moreover, CV-TEBG has generated more new bone tissue than other groups (1.99-83.50 folds, p < 0.01), achieved successful bridging defect with the formation of both cortical bone like tissue externally and cancellous bone like tissue internally, and restored approximately 80% of the stiffness of the defected femur (benchmarked to the intact femur). It has been further observed that different bone regeneration patterns occurred in different TEBG implants and closely related to their vascularization patterns, revealing the potential profound influence of vascularization patterns on the osteogenesis pattern during defect healing.

Harnessing extracellular vesicles to direct endochondral repair of large bone defects

Large bone defects remain a tremendous clinical challenge. There is growing evidence in support of treatment strategies that direct defect repair through an endochondral route, involving a cartilage intermediate. While culture-expanded stem/progenitor cells are being evaluated for this purpose, these cells would compete with endogenous repair cells for limited oxygen and nutrients within ischaemic defects. Alternatively, it may be possible to employ extracellular vesicles (EVs) secreted by culture-expanded cells for overcoming key bottlenecks to endochondral repair, such as defect vascularization, chondrogenesis, and osseous remodelling. While mesenchymal stromal/stem cells are a promising source of therapeutic EVs, other donor cells should also be considered. The efficacy of an EV-based therapeutic will likely depend on the design of companion scaffolds for controlled delivery to specific target cells. Ultimately, the knowledge gained from studies of EVs could one day inform the long-term development of synthetic, engineered nanovesicles. In the meantime, EVs harnessed from in vitro cell culture have near-term promise for use in bone regenerative medicine. This narrative review presents a rationale for using EVs to improve the repair of large bone defects, highlights promising cell sources and likely therapeutic targets for directing repair through an endochondral pathway, and discusses current barriers to clinical translation. Cite this article: E. Ferreira, R. M. Porter. Harnessing extracellular vesicles to direct endochondral repair of large bone defects. Bone Joint Res 2018;7:263-273. DOI: 10.1302/2046-3758.74.BJR-2018-0006.

IGFBP3 deposited in the human umbilical cord mesenchymal stem cell-secreted extracellular matrix promotes bone formation

The extracellular matrix (ECM) contains rich biological cues for cell recruitment, proliferationm, and even differentiation. The osteoinductive potential of scaffolds could be enhanced through human bone marrow mesenchymal stem cell (hBMSC) directly depositing ECM on surface of scaffolds. However, the role and mechanism of human umbilical cord mesenchymal stem cells (hUCMSC)-secreted ECM in bone formation remain unknown. We tested the osteoinductive properties of a hUCMSC-secreted ECM construct (hUCMSC-ECM) in a large femur defect of a severe combined immunodeficiency (SCID) mouse model. The hUCMSC-ECM improved the colonization of endogenous MSCs and bone regeneration, similar to the hUCMSC-seeded scaffold and superior to the scaffold substrate. Besides, the hUCMSC-ECM enhanced the promigratory molecular expressions of the homing cells, including CCR2 and TβRI. Furthermore, the hUCMSC-ECM increased the number of migrated MSCs by nearly 3.3 ± 0.1-fold, relative to the scaffold substrate. As the most abundant cytokine deposited in the hUCMSC-ECM, insulin-like growth factor binding protein 3 (IGFBP3) promoted hBMSC migration in the TβRI/II- and CCR2-dependent mechanisms. The hUCMSC-ECM integrating shRNA-mediated silencing of Igfbp3 that down-regulated IGFBP3 expression by approximately 60%, reduced the number of migrated hBMSCs by 47%. In vivo, the hUCMSC-ECM recruited 10-fold more endogenous MSCs to initiate bone formation compared to the scaffold substrate. The knock-down of Igfbp3 in the hUCMSC-ECM inhibited nearly 60% of MSC homing and bone regeneration capacity. This research demonstrates that IGFBP3 is an important MSC homing molecule and the therapeutic potential of hUCMSC-ECM in bone regeneration is enhanced by improving MSC homing in an IGFBP3-dependent mechanism.

Mesenchymal stem cell-macrophage crosstalk and bone healing

Recent research has brought about a clear understanding that successful fracture healing is based on carefully coordinated cross-talk between inflammatory and bone forming cells. In particular, the key role that macrophages play in the recruitment and regulation of the differentiation of mesenchymal stem cells (MSCs) during bone regeneration has been brought to focus. Indeed, animal studies have comprehensively demonstrated that fractures do not heal without the direct involvement of macrophages. Yet the exact mechanisms by which macrophages contribute to bone regeneration remain to be elucidated. Macrophage-derived paracrine signaling molecules such as Oncostatin M, Prostaglandin E2 (PGE2), and Bone Morphogenetic Protein-2 (BMP2) have been shown to play critical roles; however the relative importance of inflammatory (M1) and tissue regenerative (M2) macrophages in guiding MSC differentiation along the osteogenic pathway remains poorly understood. In this review, we summarize the current understanding of the interaction of macrophages and MSCs during bone regeneration, with the emphasis on the role of macrophages in regulating bone formation. The potential implications of aging to this cellular cross-talk are reviewed. Emerging treatment options to improve facture healing by utilizing or targeting MSC-macrophage crosstalk are also discussed.

TGFβ3 recruits endogenous mesenchymal stem cells to initiate bone regeneration

Background

The recruitment of a sufficient number of endogenous mesenchymal stem cells (MSCs) is the first stage of in-situ tissue regeneration. Transforming growth factor beta-3 (TGFβ3) could recruit stem or progenitor cells and endothelial cells to participate in tissue regeneration. However, the mechanism of TGFβ3 recruiting MSCs toward bone regeneration has remained obscure.

Methods

We estimated the promigratory property of TGFβ3 on human bone marrow MSCs (hBMSCs) cocultured with the vascular cells (human umbilical artery smooth muscle cells or human umbilical vein endothelial cells) or not by Transwell assay. After the addition of the inhibitor (SB431542) or Smad3 siRNA, the levels of MCP1 and SDF1 in coculture medium were tested by ELISA kit, and then the migratory signaling pathway of hBMSCs induced by TGFβ3 was investigated by western blot analysis. In vivo, a 2-mm FVB/N mouse femur defect model was used to evaluate chemokine secretion, endogenous cell homing, and bone regeneration induced by scaffolds loading 1 μg TGFβ3 through qPCR, immunofluorescent staining, immunohistochemical analysis, and Micro-CT, compared to the vehicle group.

Results

TGFβ3 (25 ng/ml) directly showed a nearly 40% increase in migrated hBMSCs via the TGFβ signaling pathway, compared to the vehicle treatment. Then, in the coculture system of hBMSCs and vascular cells, TGFβ3 further upregulated nearly 3-fold MCP1 secretion from vascular cells in a Smad3-dependent manner, to indirectly enhance nearly more than 50% of migrated hBMSCs. In vivo, TGFβ3 delivery improved MCP1 expression by nearly 7.9-fold, recruited approximately 2.0-fold CD31⁺ vascular cells and 2.0-fold Sca-1⁺ PDGFR-α⁺ MSCs, and achieved 2.5-fold bone volume fraction (BV/TV) and 2.0-fold bone mineral density, relative to TGFβ3-free delivery.

Conclusions

TGFβ3, as a MSC homing molecule, recruited MSCs to initiate bone formation in the direct-dependent and indirect-dependent mechanisms. This may shed light on the improvement of MSC homing in bone regeneration.