The multifaceted roles of macrophages in bone regeneration: A story of polarization, activation and time

Functionalized 3D-printed GelMA/Laponite hydrogel scaffold promotes BMSCs recruitment through osteoimmunomodulatory enhance osteogenic via AMPK/mTOR signaling pathway

The migration and differentiation of bone marrow mesenchymal stem cells (BMSCs) play crucial roles in bone repair processes. However, conventional scaffolds often lack of effectively inducing and recruiting BMSCs. In our study, we present a novel approach by introducing a 3D-bioprinted scaffold composed of hydrogels, with the addition of laponite to the GelMA solution, aimed at enhancing scaffold performance. Both in vivo and in vitro experiments have confirmed the outstanding biocompatibility of the scaffold. Furthermore, for the first time, Apt19s has been chemically modified onto the surface of the hydrogel scaffold, resulting in a remarkable enhancement in the migration and adhesion of BMSCs. Moreover, the scaffold has demonstrated robust osteogenic differentiation capability in both in vivo and in vitro environments. Additionally, the hydrogel scaffold has shown the ability to induce the polarization of macrophages from M1 to M2, thereby facilitating the osteogenic differentiation of BMSCs via the bone immune pathway. Through RNA-seq analysis, it has been revealed that macrophages regulate the osteogenic differentiation of BMSCs through the AMPK/mTOR signaling pathway. In summary, the functionalized GelMA/Laponite scaffold offers a cost-effective approach for tailored in situ bone regeneration.

Human macrophage polarisation and regulation of angiogenesis and osteogenesis is dependent on culture extracellular matrix and dimensionality

The immune system plays a crucial role in tissue repair and regeneration. Macrophages have been identified as master regulators of the early immune response and healing outcome, by orchestrating the temporal nature of the initial inflammation phase and coordinating the fate of stem/progenitor cells involved in regeneration. However, traditional in-vitro models for the study of macrophages often fail to fully replicate the complexity of the in-vivo microenvironment, therefore generating models which do not fully capture the extensive spectrum of macrophage behaviour seen in native tissues. To this end, we used a hematoma-mimetic 3D fibrin matrix characteristic of early injured tissues to generate a 3D in-vitro model mirroring the local macrophage microenvironment. Leveraging this framework, we demonstrated significant effects of extracellular matrix and dimensionality on macrophage basal signalling and polarisation, achieving more pronounced regenerative phenotypes upon stimulation with the M2a polarisation factors compared to traditional 2D tissue culture conditions. Moreover, this enhanced physiological macrophage behaviour corresponded to increased coordination of angiogenesis and osteogenesis, better mirroring the healing processes seen in-vivo. Taken together, this study demonstrates the critical importance of integrating tissue composition and 3D architecture when investigating the macrophage behaviour in-vitro, establishing a powerful tool that overcomes known limitations associated with traditional 2D culture on plastic, and can be used to identify and validate novel immunomodulation strategies to enhance tissue regeneration.

Early anti-inflammatory polarization of macrophages ameliorates post-surgical inflammation and osseointegration around titanium implants in mice

Dental implants are considered a superior option for the replacement of missing teeth. However, the invasive nature of the surgical procedure often results in significant postoperative inflammation, and the prolonged healing period of 3-6 months presents a notable disadvantage. High mobility group box 1 (HMGB1) is a critical mediator of acute inflammation following surgical injury, which can hinder the onset of osseointegration. This study aims to examine whether the inhibition of HMGB1 can mitigate acute inflammation and subsequently enhance osseointegration. The findings indicate that HMGB1 inhibition markedly reduces inflammation and promotes bone repair in murine models. Further in vitro investigations into the regulatory mechanisms of HMGB1 in macrophages reveal its role in increasing Yes-associated protein (YAP) activity, which contributes to pro-inflammatory polarization. Additionally, conditioned media derived from macrophages influenced by HMGB1 significantly impair the migratory and osteogenic capabilities of bone marrow-derived mesenchymal stem cells, which are essential for bone regeneration. In vivo experiments further validate that the administration of exogenous HMGB1 exacerbates postoperative acute inflammation and obstructs osseointegration. The study concludes that inhibiting HMGB1 fosters an anti-inflammatory polarization of macrophages, leading to diminished postoperative acute inflammation and expedited osseointegration around dental implants in mice.

Synthetic nanointerfacial bioengineering of Ti implants: on-demand regulation of implant-bone interactions for enhancing osseointegration

Titanium and its alloys are the most commonly used biometals for developing orthopedic implants to treat various forms of bone fractures and defects, but their clinical performance is still challenged by the unfavorable mechanical and biological interactions at the implant-tissue interface, which substantially impede bone healing at the defects and reduce the quality of regenerated bones. Moreover, the impaired osteogenesis capacity of patients under certain pathological conditions such as diabetes and osteoporosis may further impair the osseointegration of Ti-based implants and increase the risk of treatment failure. To address these issues, various modification strategies have been developed to regulate the implant-bone interactions for improving bone growth and remodeling in situ. In this review, we provide a comprehensive analysis on the state-of-the-art synthetic nanointerfacial bioengineering strategies for designing Ti-based biofunctional orthopedic implants, with special emphasis on the contributions to (1) promotion of new bone formation and binding at the implant-bone interface, (2) bacterial elimination for preventing peri-implant infection and (3) overcoming osseointegration resistance induced by degenerative bone diseases. Furthermore, a perspective is included to discuss the challenges and potential opportunities for the interfacial engineering of Ti implants in a translational perspective. Overall, it is envisioned that the insights in this review may guide future research in the area of biometallic orthopedic implants for improving bone repair with enhanced efficacy and safety.

TNF-α-licensed exosome-integrated titaniumaccelerated T2D osseointegration by promoting autophagy-regulated M2 macrophage polarization

Type 2 diabetes (T2D) is on a notable rise worldwide, which leads to unfavorable outcomes during implant treatments. Surface modification of implants and exosome treatment have been utilized to enhance osseointegration. However, there has been insufficient approach to improve adverse osseointegration in T2D conditions. In this study, we successfully loaded TNF-α-treated mesenchymal stem cell (MSC)-derived exosomes onto micro/nano-network titanium (Ti) surfaces. TNF-α-licensed exosome-integrated titanium (TNF-exo-Ti) effectively enhanced M2 macrophage polarization in hyperglycemic conditions, with increased secretion of anti-inflammatory cytokines and decreased secretion of pro-inflammatory cytokines. In addition, TNF-exo-Ti pretreated macrophage further enhanced angiogenesis and osteogenesis of endothelial cells and bone marrow MSCs. More importantly, TNF-exo-Ti markedly promoted osseointegration in T2D mice. Mechanistically, TNF-exo-Ti activated macrophage autophagy to promote M2 polarization through inhibition of the PI3K/AKT/mTOR pathway, which could be abolished by PI3K agonist. Thus, this study established TNF-α-licensed exosome-immobilized titanium surfaces that could rectify macrophage immune states and accelerate osseointegration in T2D conditions.

LL-37 and bisphosphonate co-delivery 3D-scaffold with antimicrobial and antiresorptive activities for bone regeneration

This study introduces a novel 3D scaffold for bone regeneration, composed of silk fibroin, chitosan, nano-hydroxyapatite, LL-37 antimicrobial peptide, and pamidronate. The scaffold addresses a critical need in bone tissue engineering by simultaneously combating bone infections and promoting bone growth. LL-37 was incorporated for its broad-spectrum antimicrobial properties, while pamidronate was included to inhibit bone resorption. The scaffold's porous structure, essential for cell infiltration and nutrient diffusion, was achieved through a freeze-drying process. In vitro assessments using SEM and FTIR confirmed the scaffold's morphology and chemical integrity. Antimicrobial efficacy was tested against pathogens of Staphylococcus aureus (S. aureus) and Pseudomonas aeruginosa (P. aeruginosa). In vivo studies in a murine model of infectious bone defect revealed the scaffold's effectiveness in reducing inflammation and bacterial load, and promoting bone regeneration. RNA sequencing of treated specimens provided insights into the molecular mechanisms underlying these observations, revealing significant gene expression changes related to bone healing and immune response modulation. The results indicate that the scaffold effectively inhibits bacterial growth and supports bone cell functions, making it a promising candidate for treating infectious bone defects. Future studies should focus on optimizing the release of therapeutic agents and evaluating the scaffold's clinical potential.

Relative contributions of osteal macrophages and osteoclasts to postnatal bone development in CSF1R-deficient rats and phenotype rescue following wild-type bone marrow cell transfer

Macrophage and osteoclast proliferation, differentiation and survival are regulated by colony-stimulating factor 1 receptor (CSF1R) signaling. Osteopetrosis associated with Csf1 and Csf1r mutations has been attributed to the loss of osteoclasts and deficiency in bone resorption. Here, we demonstrate that homozygous Csf1r mutation in rat leads to delayed postnatal skeletal ossification associated with substantial loss of osteal macrophages in addition to osteoclasts. Osteosclerosis and site-specific skeletal abnormalities were reversed by intraperitoneal transfer of wild-type bone marrow cells (bone marrow cell transfer, BMT) at weaning. Following BMT, IBA1+ macrophages were detected before TRAP+ osteoclasts at sites of ossification restoration. These observations extend evidence that osteal macrophages independently contribute to bone anabolism and are required for normal postnatal bone growth and morphogenesis.

Black phosphorus for bone regeneration: Mechanisms involved and influencing factors

BP has shown good potential for promoting bone regeneration. However, the understanding of the mechanisms of BP-enhanced bone regeneration is still limited. This review first summarizes the recent advances in applications of BP in bone regeneration. We further highlight the possibility that BP enhances bone regeneration by regulating the behavior of mesenchymal stem cells (MSCs), osteoblasts, vascular endothelial cells (VECs), and macrophages, mainly through the regulation of cytoskeletal remodeling, energy metabolism, oxidation resistance and surface adsorption properties, etc. In addition, moderating the physicochemical properties of BP (i.e., shape, size, and surface charge) can alter the effects of BP on bone regeneration. This review reveals the underlying mechanisms of BP-enhanced bone regeneration and provides strategies for further material design of BP-based materials for bone regeneration.

Functional Amino Acids in the Regulation of Bone and Its Diseases

Bone as a vigorous tissue is constantly undergoing bone remodeling. The homeostasis of bone remodeling requires combined efforts of multifarious bone cells. Amino acids (AA), known as essential components of life support, are closely related to the regulation of bone homeostasis. In recent years, the concept of functional amino acids (FAAs) has been proposed, which is defined as AA that regulate key metabolic pathways to improve health, survival, growth, development, lactation, and reproduction of organisms, to highlight their outstanding contributions in the body. In the hope of exploring new therapeutic strategies, this review focus on summarizing recent progress in the vital role of FAAs in bone homeostasis maintaining and potential implications of FAAs in bone-related diseases, and discussing related mechanisms. The results showed that FAAs are closely related to bone metabolism and therapeutic strategy targeting FAAs metabolism is one of the future trends for bone disorders, while the explorations about possible impact of FAAs-based diets are still limited.

The Advantages and Shortcomings of Stem Cell Therapy for Enhanced Bone Healing

This review explores the regenerative potential of key progenitor cell types and therapeutic strategies to improve healing of complex fractures and bone defects. We define, summarize, and discuss the differentiation potential of totipotent, pluripotent, and multipotent stem cells, emphasizing the advantages and shortcomings of cell therapy for bone repair and regeneration. The fundamental role of mesenchymal stem cells is highlighted due to their multipotency to differentiate into the key lineage cells including osteoblasts, osteocytes, and chondrocytes, which are crucial for bone formation and remodeling. Hematopoietic stem cells (HSCs) also play a significant role; immune cells such as macrophages and T-cells modulate inflammation and tissue repair. Osteoclasts are multinucleated cells that are important to bone remodeling. Vascular progenitor (VP) cells are critical to oxygen and nutrient supply. The dynamic interplay among these lineages and their microenvironment is essential for effective bone restoration. Therapies involving cells that are more than "minimally manipulated" are controversial and include embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). ESCs, derived from early-stage embryos, possess pluripotent capabilities and have shown promise in preclinical studies for bone healing. iPSCs, reprogrammed from somatic cells, offer personalized medicine applications and can differentiate into various tissue-specific cell lines. Minimally manipulative cell therapy approaches such as the use of bone marrow aspirate concentrate (BMAC), exosomes, and various biomaterials for local delivery are explored for their effectiveness in bone regeneration. BMAC, which contains mostly immune cells but few mesenchymal and VPs, probably improves bone healing by facilitating paracrine-mediated intercellular communication. Exosome isolation harnesses the biological signals and cellular by-products that are a primary source for cell crosstalk and activation. Safe, efficacious, and cost-effective strategies to enhance bone healing using novel cellular therapies are part of a changing paradigm to modulate the inflammatory, repair, and regenerative pathways to achieve earlier more robust tissue healing and improved physical function. Impact Statement Stem cell therapy holds immense potential for bone healing due to its ability to regenerate damaged tissue. Nonmanipulated bone marrow aspirate contains mesenchymal stem cells that promote bone repair and reduce healing time. Induced pluripotent stem cells offer the advantage of creating patient-specific cells that can differentiate into osteoblasts, aiding in bone regeneration. Other delivery methods, such as scaffold-based techniques, enhance stem cell integration and function. Collectively, these approaches can improve treatment outcomes, reduce recovery periods, and advance our understanding of bone healing mechanisms, making them pivotal in orthopedic research and regenerative medicine.

Natural plant-derived polysaccharides targeting macrophage polarization: a promising strategy for cancer immunotherapy

Tumor associated macrophages (TAMs) are the predominant innate immune cells in the tumor microenvironment (TME). Cytokines induce the differentiation of macrophages into distinct types of TAMs, primarily characterized by two phenotypes: M1-polarized and M2-polarized. Cancer growth is suppressed by M1-polarized macrophages and promoted by M2-polarized macrophages. The regulation of macrophage M1 polarization has emerged as a promising strategy for cancer immunotherapy. Polysaccharides are important bioactive substances found in numerous plants, manifesting a wide range of noteworthy biological actions, such as immunomodulation, anti-tumor effects, antioxidant capabilities, and antiviral functions. In recent years, there has been a significant increase in interest regarding the immunomodulatory and anti-tumor properties of polysaccharides derived from plants. The regulatory impact of polysaccharides on the immune system is mainly associated with the natural immune response, especially with the regulation of macrophages. This review provides a thorough analysis of the regulatory effects and mechanisms of plant polysaccharides on TAMs. Additionally, an analysis of potential opportunities for clinical translation of plant polysaccharides as immune adjuvants is presented. These insights have greatly advanced the research of plant polysaccharides for immunotherapy in tumor-related applications.

Shaping the immune landscape: Multidimensional environmental stimuli refine macrophage polarization and foster revolutionary approaches in tissue regeneration

In immunology, the role of macrophages extends far beyond their traditional classification as mere phagocytes; they emerge as pivotal architects of the immune response, with their function being significantly influenced by multidimensional environmental stimuli. This review investigates the nuanced mechanisms by which diverse external signals ranging from chemical cues to physical stress orchestrate macrophage polarization, a process that is crucial for the modulation of immune responses. By transitioning between pro-inflammatory (M1) and anti-inflammatory (M2) states, macrophages exhibit remarkable plasticity, enabling them to adapt to and influence their surroundings effectively. The exploration of macrophage polarization provides a compelling narrative on how these cells can be manipulated to foster an immune environment conducive to tissue repair and regeneration. Highlighting cutting-edge research, this review presents innovative strategies that leverage the dynamic interplay between macrophages and their environment, proposing novel therapeutic avenues that harness the potential of macrophages in regenerative medicine. Moreover, this review critically evaluates the current challenges and future prospects of translating macrophage-centered strategies from the laboratory to clinical applications.

Ti3C2Tx MXene-Decorated 3D-Printed Ceramic Scaffolds for Enhancing Osteogenesis by Spatiotemporally Orchestrating Inflammatory and Bone Repair Responses

Inflammatory responses play a central role in coordinating biomaterial-mediated tissue regeneration. However, precise modulation of dynamic variations in microenvironmental inflammation post-implantation remains challenging. In this study, the traditional β-tricalcium phosphate-based scaffold is remodeled via ultrathin MXene-Ti3C2 decoration and Zn2+/Sr2+ ion-substitution, endowing the scaffold with excellent reactive oxygen species-scavenging ability, near-infrared responsivity, and enhanced mechanical properties. The induction of mild hyperthermia around the implant via periodic near-infrared irradiation facilitates spatiotemporal regulation of inflammatory cytokines secreted by a spectrum of macrophage phenotypes. The process initially amplifies the pro-inflammatory response, then accelerates M1-to-M2 macrophage polarization transition, yielding a satisfactory pattern of osteo-immunomodulation during the natural bone healing process. Later, sustained release of Zn2+/Sr2+ ions with gradual degradation of the 3D scaffold maintains the favorable reparative M2-dominated immunological microenvironment that supports new bone mineralization. Precise temporal immunoregulation of the bone healing process by the intelligent 3D scaffold enhances bone regeneration in a rat cranial defect model. This strategy paves the way for the application of β-tricalcium phosphate-based materials to guide the dynamic inflammatory and bone tissue responses toward a favorable outcome, making clinical treatment more predictable and durable. The findings also demonstrate that near-infrared irradiation-derived mild hyperthermia is a promising method of immunomodulation.

Deciphering the role of CD47 in cancer immunotherapy

BACKGROUND

Immunotherapy has emerged as a novel strategy for cancer treatment following surgery, radiotherapy, and chemotherapy. Immune checkpoint blockade and Chimeric antigen receptor (CAR)-T cell therapies have been successful in clinical trials. Cancer cells evade immune surveillance by hijacking inhibitory pathways via overexpression of checkpoint genes. The Cluster of Differentiation 47 (CD47) has emerged as a crucial checkpoint for cancer immunotherapy by working as a "don't eat me" signal and suppressing innate immune signaling. Furthermore, CD47 is highly expressed in many cancer types to protect cancer cells from phagocytosis via binding to SIRPα on phagocytes. Targeting CD47 by either interrupting the CD47-SIRPα axis or combing with other therapies has been demonstrated as an encouraging therapeutic strategy in cancer immunotherapy. Antibodies and small molecules that target CD47 have been explored in pre- and clinical trials. However, formidable challenges such as the anemia and palate aggregation cannot be avoided because of the wide presentation of CD47 on erythrocytes.

AIM OF VIEW

This review summarizes the current knowledge on the regulation and function of CD47, and provides a new perspective for immunotherapy targeting CD47. It also highlights the clinical progress of targeting CD47 and discusses challenges and potential strategies.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review provides a comprehensive understanding of targeting CD47 in cancer immunotherapy, it also augments the concept of combination immunotherapy strategies by employing both innate and adaptive immune responses.

Dual-functional Hydroxyapatite scaffolds for bone regeneration and precision drug delivery

Addressing infected bone defects remains a significant challenge in orthopedics, requiring effective infection control and bone defect repair. A promising therapeutic approach involves the development of dual-functional engineered biomaterials with drug delivery systems that combine antibacterial properties with osteogenesis promotion. The Hydroxyapatite composite scaffolds offer a one-stage treatment, eliminating the need for multiple surgeries and thereby streamlining the process and reducing treatment time. This review delves into the impaired bone repair mechanisms within pathogen-infected and inflamed microenvironments, providing a theoretical foundation for treating infectious bone defects. Additionally, it explores composite scaffolds made of antibacterial and osteogenic materials, along with advanced drug delivery systems that possess both antibacterial and bone-regenerative properties. By offering a comprehensive understanding of the microenvironment of infectious bone defects and innovative design strategies for dual-function scaffolds, this review presents significant advancements in treatment methods for infectious bone defects. Continued research and clinical validation are essential to refine these innovations, ensuring biocompatibility and safety, achieving controlled release and stability, and developing scalable manufacturing processes for widespread clinical application.

The balance between helper T 17 and regulatory T cells in osteoimmunology and relevant research progress on bone tissue engineering

BACKGROUND

Bone regeneration is a well-regulated dynamic process, of which the prominent role of the immune system on bone homeostasis is more and more revealed by recent research. Before fully activation of the bone remodeling cells, the immune system needs to clean up the microenvironment in facilitating the bone repair initiation. Furthermore, this microenvironment must be maintained properly by various mechanisms over the entire bone regeneration process.

OBJECTIVE

This review aims to summarize the role of the T-helper 17/Regulatory T cell (Th17/Treg) balance in bone cell remodeling and discuss the relevant progress in bone tissue engineering.

RESULTS

The role of the immune response in the early stages of bone regeneration is crucial, especially the impact of the Th17/Treg balance on osteoclasts, mesenchymal stem cells (MSCs), and osteoblasts activity. By virtue of these knowledge advancements, innovative approaches in bone tissue engineering, such as nano-structures, hydrogel, and exosomes, are designed to influence the Th17/Treg balance and thereby augment bone repair and regeneration.

CONCLUSION

Targeting the Th17/Treg balance is a promising innovative strategy for developing new treatments to enhance bone regeneration, thus offering potential breakthroughs in bone injury clinics.

Metformin enhances osteogenic differentiation of BMSC by modulating macrophage M2 polarization

Bone marrow-derived mesenchymal stem cells (BMSCs) are capable of developing into osteoblastic cell lines in vitro and regenerating bone tissue in vivo, and they are considered to be a reliable source for bone regenerative medicine. In recent years, studies have shown that the immune microenvironment is important for osteogenesis, in which macrophages are an important component of innate immunity and coordinate with stem cells. Metformin (Met), a hypoglycemic drug that exerts a powerful effect on metabolic signaling, has been shown to modulate inflammatory responses and osteogenic activity. However, whether metformin modulates macrophage polarization and subsequently affects osteogenesis remains to be elucidated. In the present study, we investigated the potential immunomodulatory capacity of metformin on macrophage inflammatory responses and phenotypic switching, and the subsequent effects on osteogenic differentiation of BMSCs. Flow cytometry and qPCR were used to study the effects of metformin on macrophage phenotypic regulation. qPCR, ALP, ARS and calcium content measurement and ALP activity assay were used to determine the effects of macrophage-secreted activators on the osteogenic differentiation of BMSCs. Our study demonstrates that metformin can improve the immune microenvironment by modulating macrophage polarization towards an anti-inflammatory phenotype, promoting an increase in a range of anti-inflammatory factors and inhibiting pro-inflammatory factors. This was characterized by increased expression of IL-10 and CD206, Arg-1 and decreased expression of IL-1β, TNF-α, IL-6 and iNOS. In addition, metformin-modulated macrophage-conditioned medium promoted osteogenic differentiation of BMSCs, increased the expression levels of genes (ALP, Runx-2, OCN, and Col-1), enhanced ALP activity, and significantly formed mineralized nodules. In conclusion, our new study elucidates that metformin can promote osteogenic differentiation of BMSCs by modulating macrophage phenotype and thereby.

The effect of compressive force in bone tissue induced by implants of different primary stabilities on macrophage polarization and bone regeneration

Dental implants with different primary stabilities give rise to distinct stress distributions at the implant-bone interface after placement and exert mechanical force on the cells in the bone tissue. This study aimed to investigate whether the mechanical forces in peri-implant bone participate in the body's immune response and influence macrophage polarization. Therefore, an in vivo rat implantation model with different primary implant stabilities was established. The osteoimmune response and macrophage polarization were investigated, and the osseointegration of the implants was evaluated. In an in vitro experiment, an external compressive force was applied to RAW264.7 cells, and the polarization phenotype was observed. MC3T3-E1 cells were cultured in macrophage-conditioned medium to investigate the regulatory effect of the macrophage-secreted cytokines on the osteogenic differentiation of osteoblasts. In vivo experimental results indicated that the primary stability of implants is positively correlated with the mechanical force. The osteoimmune response was significantly amplified by compressive force generated from implants. This compressive force first induced both M1 and M2 macrophage polarization and then accelerated the progression of the transition to M2 macrophages in the bone repair phase. In vitro, compressive force significantly upregulated the M1 and M2 macrophage polarization. In addition, the suppressive effect of macrophages on the osteogenesis of MC3T3 cells was relieved by cytokines secreted by macrophages under compressive force loading, which promoted their osteogenesis. Overall, these results clarify that compressive force from different primary stabilities is an important influencing factor regulating the osteoimmunne response and macrophage polarization in addition to maintaining the implant.

Current Perspectives of Protein in Bone Tissue Engineering: Bone Structure, Ideal Scaffolds, Fabrication Techniques, Applications, Scopes, and Future Advances

In view of their exceptional approach, excellent inherent biocompatibility and biodegradability properties, and interaction with the local extracellular matrix, protein-based polymers have received attention in bone tissue engineering, which is a multidisciplinary field that repairs and regenerates fractured bones. Bone is a multihierarchical complex structure, and it performs several essential biofunctions, including maintaining mineral balance and structural support and protecting soft organs. Protein-based polymers have gained interest in developing ideal scaffolds as emerging biomaterials for bone fractured healing and regeneration, and it is challenging to design ideal bone substitutes as perfect biomaterials. Several protein-based polymers, including collagen, keratin, gelatin, serum albumin, etc., are potential materials due to their inherent cytocompatibility, controlled biodegradability, high biofunctionalization, and tunable mechanical characteristics. While numerous studies have indicated the encouraging possibilities of proteins in BTE, there are still major challenges concerning their biodegradability, stability in physiological conditions, and continuous release of growth factors and bioactive molecules. Robust scaffolds derived from proteins can be used to replace broken or diseased bone with a biocompatible substitute; proteins, being biopolymers, provide excellent scaffolds for bone tissue engineering. Herein, recent developments in protein polymers for cutting-edge bone tissue engineering are addressed in this review within 3-5 years, with a focus on the significant challenges and future perspectives. The first section discusses the structural fundamentals of bone anatomy and ideal scaffolds, and the second section describes the fabrication techniques of scaffolds. The third section highlights the importance of proteins and their applications in BTE. Hence, the recent development of protein polymers for state-of-the-art bone tissue engineering has been discussed, highlighting the significant challenges and future perspectives.

Chitosan-based biomaterials promote bone regeneration by regulating macrophage fate

The development of various osteogenic biomaterials has not only promoted the development of bone tissue engineering but also provided more possibilities for bone defect repair. However, most previous studies have focused on the interaction of biomaterials on endogenous or exogenous stem cells involved in the bone regeneration process while neglecting the effect of changes in the immune microenvironment of bone defect sites on bone regeneration after biomaterial implantation into the host. With the development of bone immunology, the role of various immune cells, especially macrophages, in bone regeneration has gradually attracted the attention of researchers. An increasing number of studies have begun to target macrophages to better promote bone regeneration by modulating the fate of macrophages in a spatiotemporally ordered manner to mimic the changes in the immune microenvironment of bone defect sites during the natural repair process of bone tissue. Chitosan is one of the most abundant natural polysaccharides in the world. In recent years, various chitosan-based biomaterials have been widely used in macrophage fate modulation and bone regeneration. In this review, we review the interaction between macrophages and scaffold materials, general information about chitosan, the modulation of macrophage fate by chitosan-based biomaterials, and their application in bone regeneration.

Macrophage polarization in the tumor microenvironment: Emerging roles and therapeutic potentials

The tumor microenvironment (TME) is a combination of tumor cells and indigenous host stroma, which consists of tumor-infiltrating immune cells, endothelial cells, fibroblasts, pericytes, and non-cellular elements. Tumor-associated macrophages (TAMs) represent the major tumor-infiltrating immune cell type and are generally polarized into two functionally contradictory subtypes, namely classical activated M1 macrophages and alternatively activated M2 macrophages. Macrophage polarization refers to how macrophages are activated at a given time and space. The interplay between the TME and macrophage polarization can influence tumor initiation and progression, making TAM a potential target for cancer therapy. Here, we review the latest investigations on factors orchestrating macrophage polarization in the TME, how macrophage polarization affects tumor progression, and the perspectives in modulating macrophage polarization for cancer immunotherapy.

Recent Advances in 3D Printing of Smart Scaffolds for Bone Tissue Engineering and Regeneration

The repair and functional reconstruction of bone defects resulting from severe trauma, surgical resection, degenerative disease, and congenital malformation pose significant clinical challenges. Bone tissue engineering (BTE) holds immense potential in treating these severe bone defects, without incurring prevalent complications associated with conventional autologous or allogeneic bone grafts. 3D printing technology enables control over architectural structures at multiple length scales and has been extensively employed to process biomimetic scaffolds for BTE. In contrast to inert and functional bone grafts, next-generation smart scaffolds possess a remarkable ability to mimic the dynamic nature of native extracellular matrix (ECM), thereby facilitating bone repair and regeneration. Additionally, they can generate tailored and controllable therapeutic effects, such as antibacterial or antitumor properties, in response to exogenous and/or endogenous stimuli. This review provides a comprehensive assessment of the progress of 3D-printed smart scaffolds for BTE applications. It begins with an introduction to bone physiology, followed by an overview of 3D printing technologies utilized for smart scaffolds. Notable advances in various stimuli-responsive strategies, therapeutic efficacy, and applications of 3D-printed smart scaffolds are discussed. Finally, the review highlights the existing challenges in the development and clinical implementation of smart scaffolds, as well as emerging technologies in this field.

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.

Black phosphorus quantum dot-modified ADSCs as a novel therapeutic for periodontitis bone loss coupling of osteogenesis and osteoimmunomodulation

Alveolar bone defect repair remains a persistent clinical challenge for periodontitis treatment. The use of peripheral functional seed cells is a hot topic in periodontitis. Herein, we explored the cellular behaviors and osteogenic ability of adipose-derived mesenchymal stem cells (ADSCs) treated with black phosphorus quantum dots (BPQDs). Additionally, macrophage polarization, osteogenic effects and angiogenesis were investigated through the paracrine pathway regulated by BPQD-modified ADSCs. Our results demonstrated that BPQDs showed good biocompatibility with ADSCs and BPQD-modified ADSCs could improve the bone repair in vivo inflammatory microenvironment by regulating osteogenesis and osteoimmunomodulation. The BPQDs increased the osteogenic differentiation of ADSCs via the Wnt/β-catenin and BMP2/SMAD5/Runx2 signaling pathway. In addition, BPQD-modified ADSCs promoted the osteogenic effect of BMSCs and facilitated the polarization of macrophages from M1 towards M2 phenotype transformation through the paracrine pathway in the periodontitis microenvironment. This strategy provides a novel idea for treatment of alveolar bone defects for periodontitis in the foreseeable future.

Stimuli-Mediated Macrophage Switching, Unraveling the Dynamics at the Nanoplatforms-Macrophage Interface

Macrophages play an essential role in immunotherapy and tissue regeneration owing to their remarkable plasticity and diverse functions. Recent bioengineering developments have focused on using external physical stimuli such as electric and magnetic fields, temperature, and compressive stress, among others, on micro/nanostructures to induce macrophage polarization, thereby increasing their therapeutic potential. However, it is difficult to find a concise review of the interaction between physical stimuli, advanced micro/nanostructures, and macrophage polarization. This review examines the present research on physical stimuli-induced macrophage polarization on micro/nanoplatforms, emphasizing the synergistic role of fabricated structure and stimulation for advanced immunotherapy and tissue regeneration. A concise overview of the research advancements investigating the impact of physical stimuli, including electric fields, magnetic fields, compressive forces, fluid shear stress, photothermal stimuli, and multiple stimulations on the polarization of macrophages within complex engineered structures, is provided. The prospective implications of these strategies in regenerative medicine and immunotherapeutic approaches are highlighted. This review will aid in creating stimuli-responsive platforms for immunomodulation and tissue regeneration.

Macrophage plasticity: signaling pathways, tissue repair, and regeneration

Macrophages are versatile immune cells with remarkable plasticity, enabling them to adapt to diverse tissue microenvironments and perform various functions. Traditionally categorized into classically activated (M1) and alternatively activated (M2) phenotypes, recent advances have revealed a spectrum of macrophage activation states that extend beyond this dichotomy. The complex interplay of signaling pathways, transcriptional regulators, and epigenetic modifications orchestrates macrophage polarization, allowing them to respond to various stimuli dynamically. Here, we provide a comprehensive overview of the signaling cascades governing macrophage plasticity, focusing on the roles of Toll-like receptors, signal transducer and activator of transcription proteins, nuclear receptors, and microRNAs. We also discuss the emerging concepts of macrophage metabolic reprogramming and trained immunity, contributing to their functional adaptability. Macrophage plasticity plays a pivotal role in tissue repair and regeneration, with macrophages coordinating inflammation, angiogenesis, and matrix remodeling to restore tissue homeostasis. By harnessing the potential of macrophage plasticity, novel therapeutic strategies targeting macrophage polarization could be developed for various diseases, including chronic wounds, fibrotic disorders, and inflammatory conditions. Ultimately, a deeper understanding of the molecular mechanisms underpinning macrophage plasticity will pave the way for innovative regenerative medicine and tissue engineering approaches.

Spatiotemporal controlled released hydrogels for multi-system regulated bone regeneration

Bone defect is one of the urgent problems to be solved in clinics, and it is very important to construct efficient scaffold materials to facilitate bone tissue regeneration. Hydrogels, characterized by their unique three-dimensional network structure, serve as excellent biological scaffold materials. Their internal pores are capable of loading osteogenic drugs to expedite bone formation. The rate and quality of new bone formation are intimately linked with immune regulation and vascular remodeling. The strategic sequential release of drugs to balance inflammation and regulate vascular remodeling is crucial for initiating the osteogenic process. Through the design of hydrogel microstructures, it is possible to achieve sequential drug release and the drug action time can be prolonged, thereby catering to the multi-systemic collaborative regulation needs of osteosynthesis. The drug release rate within the hydrogel is governed by swelling control systems, physical control systems, chemical control systems, and environmental control systems. Utilizing these control systems to design hydrogel materials capable of multi-drug delivery optimizes the construction of the bone microenvironment. Consequently, this facilitates the spatiotemporal controlled released of drugs, promoting bone tissue regeneration. This paper reviews the principles of the controlled release system of various sustained-release hydrogels and the advancements in research on hydrogel multi-drug delivery systems for bone tissue regeneration.

Macrophage-Centric Biomaterials for Bone Regeneration in Diabetes Mellitus: Contemporary Advancements, Challenges, and Future Trajectories

Increased susceptibility to bone fragility and the diminution of bone regenerative capacity are recognized as significant and frequent sequelae of diabetes mellitus. Research has elucidated the pivotal role of macrophages in the pathogenesis and repair of diabetic bone defects. Notwithstanding this, the therapeutic efficacy of traditional interventions remains predominantly inadequate. Concomitant with substantial advancements in tissue engineering in recent epochs, there has been an escalation in the development of biomaterials designed to modulate macrophage activity, thereby augmenting osseous tissue regeneration in the context of hyperglycemia. This review amalgamates insights from extant research and delineates recent progressions in the domain of biomaterials that target macrophages for the regeneration of diabetic bone, whilst also addressing the clinical challenges and envisaging future directions within this field.

Rod-Shaped Mesoporous Zinc-Containing Bioactive Glass Nanoparticles: Structural, Physico-Chemical, Antioxidant, and Immuno-Regulation Properties

Bioactive glass nanoparticles (BGNs) are applied widely in tissue regeneration. Varied micro/nanostructures and components of BGNs have been designed for different applications. In the present study, nanorod-shaped mesoporous zinc-containing bioactive glass nanoparticles (ZnRBGNs) were designed and developed to form the bioactive content of composite materials for hard/soft tissue repair and regeneration. The nanostructure and components of the ZnRBGNs were characterized, as were their cytocompatibility and radical-scavenging activity in the presence/absence of cells and their ability to modulate macrophage polarization. The ZnRBGNs possessed a uniform rod shape (length ≈ 500 nm; width ≈ 150 nm) with a mesoporous structure (diameter ≈ 2.4 nm). The leaching liquid of the nanorods at a concentration below 0.5 mg/mL resulted in no cytotoxicity. More significant improvements in the antioxidant and M1-polarization-inhibiting effects and the promotion of M2 polarization were found when culturing the cells with the ZnRBGNs compared to when culturing them with the RBGNs. The doping of the Zn element in RBGNs may lead to improved antioxidant and anti-inflammatory effects, which may be beneficial in tissue regeneration/repair.

Advancing bone regeneration: Unveiling the potential of 3D cell models in the evaluation of bone regenerative materials

Bone, a rigid yet regenerative tissue, has garnered extensive attention for its impressive healing abilities. Despite advancements in understanding bone repair and creating treatments for bone injuries, handling nonunions and large defects remains a major challenge in orthopedics. The rise of bone regenerative materials is transforming the approach to bone repair, offering innovative solutions for nonunions and significant defects, and thus reshaping orthopedic care. Evaluating these materials effectively is key to advancing bone tissue regeneration, especially in difficult healing scenarios, making it a critical research area. Traditional evaluation methods, including two-dimensional cell models and animal models, have limitations in predicting accurately. This has led to exploring alternative methods, like 3D cell models, which provide fresh perspectives for assessing bone materials' regenerative potential. This paper discusses various techniques for constructing 3D cell models, their pros and cons, and crucial factors to consider when using these models to evaluate bone regenerative materials. We also highlight the significance of 3D cell models in the in vitro assessments of these materials, discuss their current drawbacks and limitations, and suggest future research directions. STATEMENT OF SIGNIFICANCE: This work addresses the challenge of evaluating bone regenerative materials (BRMs) crucial for bone tissue engineering. It explores the emerging role of 3D cell models as superior alternatives to traditional methods for assessing these materials. By dissecting the construction, key factors of evaluating, advantages, limitations, and practical considerations of 3D cell models, the paper elucidates their significance in overcoming current evaluation method shortcomings. It highlights how these models offer a more physiologically relevant and ethically preferable platform for the precise assessment of BRMs. This contribution is particularly significant for "Acta Biomaterialia" readership, as it not only synthesizes current knowledge but also propels the discourse forward in the search for advanced solutions in bone tissue engineering and regeneration.

Advances in biomaterials for oral-maxillofacial bone regeneration: spotlight on periodontal and alveolar bone strategies

The intricate nature of oral-maxillofacial structure and function, coupled with the dynamic oral bacterial environment, presents formidable obstacles in addressing the repair and regeneration of oral-maxillofacial bone defects. Numerous characteristics should be noticed in oral-maxillofacial bone repair, such as irregular morphology of bone defects, homeostasis between hosts and microorganisms in the oral cavity and complex periodontal structures that facilitate epithelial ingrowth. Therefore, oral-maxillofacial bone repair necessitates restoration materials that adhere to stringent and specific demands. This review starts with exploring these particular requirements by introducing the particular characteristics of oral-maxillofacial bones and then summarizes the classifications of current bone repair materials in respect of composition and structure. Additionally, we discuss the modifications in current bone repair materials including improving mechanical properties, optimizing surface topography and pore structure and adding bioactive components such as elements, compounds, cells and their derivatives. Ultimately, we organize a range of potential optimization strategies and future perspectives for enhancing oral-maxillofacial bone repair materials, including physical environment manipulation, oral microbial homeostasis modulation, osteo-immune regulation, smart stimuli-responsive strategies and multifaceted approach for poly-pathic treatment, in the hope of providing some insights for researchers in this field. In summary, this review analyzes the complex demands of oral-maxillofacial bone repair, especially for periodontal and alveolar bone, concludes multifaceted strategies for corresponding biomaterials and aims to inspire future research in the pursuit of more effective treatment outcomes.

Direct Current Electrical Stimulation Shifts THP-1-Derived Macrophage Polarization towards Pro-Regenerative M2 Phenotype

A macrophage shift from the M1 to the M2 phenotype is relevant for promoting tissue repair and regeneration. In a previous in vivo study, we found that direct current (DC) electrical stimulation (EStim) increased the proportion of M2 macrophages in healing tissues and directed the balance of the injury response away from healing/scarring towards regeneration. These observations led us to hypothesize that DC EStim regulates macrophage polarization towards an M2 phenotype. THP-1-derived M0, M1 (IFN-γ and LPS), and M2 (IL-4 and IL-13) macrophages were exposed (or not: control group) to 100 mV/mm of DC EStim, 1 h/day for three days. Macrophage polarization was assessed through gene and surface marker expressions and cytokine secretion profiles. Following DC EStim treatment, M0 cells exhibited an upregulation of M2 marker genes IL10, CD163, and PPARG. In M1 cells, DC EStim upregulated the gene expressions of M2 markers IL10, TGM2, and CD206 and downregulated M1 marker gene CD86. EStim treatment also reduced the surface expression of CD86 and secretion of pro-inflammatory cytokines IL-1β and IL-6. Our results suggest that DC EStim differentially exerts pro-M2 effects depending on the macrophage phenotype: it upregulates typical M2 genes in M0 and M1 cells while inhibiting M1 marker CD86 at the nuclear and protein levels and the secretion of pro-inflammatory interleukins in M1 cells. Conversely, M2 cells appear to be less responsive to the EStim treatment employed in this study.

Activating Macrophage Continual Efferocytosis via Microenvironment Biomimetic Short Fibers for Reversing Inflammation in Bone Repair

Efferocytosis-mediated inflammatory reversal plays a crucial role in bone repairing process. However, in refractory bone defects, the macrophage continual efferocytosis may be suppressed due to the disrupted microenvironment homeostasis, particularly the loss of apoptotic signals and overactivation of intracellular oxidative stress. In this study, a polydopamine-coated short fiber matrix containing biomimetic "apoptotic signals" to reconstruct the microenvironment and reactivate macrophage continual efferocytosis for inflammatory reversal and bone defect repair is presented. The "apoptotic signals" (AM/CeO2) are prepared using CeO2 nanoenzymes with apoptotic neutrophil membrane coating for macrophage recognition and oxidative stress regulation. Additionally, a short fiber "biomimetic matrix" is utilized for loading AM/CeO2 signals via abundant adhesion sites involving π-π stacking and hydrogen bonding interactions. Ultimately, the implantable apoptosis-mimetic nanoenzyme/short-fiber matrixes (PFS@AM/CeO2), integrating apoptotic signals and biomimetic matrixes, are constructed to facilitate inflammatory reversal and reestablish the pro-efferocytosis microenvironment. In vitro and in vivo data indicate that the microenvironment biomimetic short fibers can activate macrophage continual efferocytosis, leading to the suppression of overactivated inflammation. The enhanced repair of rat femoral defect further demonstrates the osteogenic potential of the pro-efferocytosis strategy. It is believed that the regulation of macrophage efferocytosis through microenvironment biomimetic materials can provide a new perspective for tissue repair.

Tantalum Particles Promote M2 Macrophage Polarization and Regulate Local Bone Metabolism via Macrophage-Derived Exosomes Influencing the Fates of BMSCs

In this study, the regulatory role and mechanisms of tantalum (Ta) particles in the bone tissue microenvironment are explored. Ta particle deposition occurs in both clinical samples and animal tissues following porous Ta implantation. Unlike titanium (Ti) particles promoting M1 macrophage (Mϕ) polarization, Ta particles regulating calcium signaling pathways and promoting M2 Mϕ polarization. Ta-induced M2 Mϕ enhances bone marrow-derived mesenchymal stem cells (BMSCs) proliferation, migration, and osteogenic differentiation through exosomes (Exo) by upregulating miR-378a-3p/miR-221-5p and downregulating miR-155-5p/miR-212-5p. Ta particles suppress the pro-inflammatory and bone resorption effects of Ti particles in vivo and in vitro. In a rat femoral condyle bone defect model, artificial bone loaded with Ta particles promotes endogenous Mϕ polarization toward M2 differentiation at the defect site, accelerating bone repair. In conclusion, Ta particles modulate Mϕ polarization toward M2 and influence BMSCs osteogenic capacity through Exo secreted by M2 Mϕ, providing insights for potential bone repair applications.

Advances in electroactive biomaterials: Through the lens of electrical stimulation promoting bone regeneration strategy

The regenerative capacity of bone is indispensable for growth, given that accidental injury is almost inevitable. Bone regenerative capacity is relevant for the aging population globally and for the repair of large bone defects after osteotomy (e.g., following removal of malignant bone tumours). Among the many therapeutic modalities proposed to bone regeneration, electrical stimulation has attracted significant attention owing to its economic convenience and exceptional curative effects, and various electroactive biomaterials have emerged. This review summarizes the current knowledge and progress regarding electrical stimulation strategies for improving bone repair. Such strategies range from traditional methods of delivering electrical stimulation via electroconductive materials using external power sources to self-powered biomaterials, such as piezoelectric materials and nanogenerators. Electrical stimulation and osteogenesis are related via bone piezoelectricity. This review examines cell behaviour and the potential mechanisms of electrostimulation via electroactive biomaterials in bone healing, aiming to provide new insights regarding the mechanisms of bone regeneration using electroactive biomaterials.

The translational potential of this article

This review examines the roles of electroactive biomaterials in rehabilitating the electrical microenvironment to facilitate bone regeneration, addressing current progress in electrical biomaterials and the mechanisms whereby electrical cues mediate bone regeneration. Interactions between osteogenesis-related cells and electroactive biomaterials are summarized, leading to proposals regarding the use of electrical stimulation-based therapies to accelerate bone healing.

M1-polarized macrophage-derived cellular nanovesicle-coated lipid nanoparticles for enhanced cancer treatment through hybridization of gene therapy and cancer immunotherapy

Optimum genetic delivery for modulating target genes to diseased tissue is a major obstacle for profitable gene therapy. Lipid nanoparticles (LNPs), considered a prospective vehicle for nucleic acid delivery, have demonstrated efficacy in human use during the COVID-19 pandemic. This study introduces a novel biomaterial-based platform, M1-polarized macrophage-derived cellular nanovesicle-coated LNPs (M1-C-LNPs), specifically engineered for a combined gene-immunotherapy approach against solid tumor. The dual-function system of M1-C-LNPs encapsulates Bcl2-targeting siRNA within LNPs and immune-modulating cytokines within M1 macrophage-derived cellular nanovesicles (M1-NVs), effectively facilitating apoptosis in cancer cells without impacting T and NK cells, which activate the intratumoral immune response to promote granule-mediating killing for solid tumor eradication. Enhanced retention within tumor was observed upon intratumoral administration of M1-C-LNPs, owing to the presence of adhesion molecules on M1-NVs, thereby contributing to superior tumor growth inhibition. These findings represent a promising strategy for the development of targeted and effective nanoparticle-based cancer genetic-immunotherapy, with significant implications for advancing biomaterial use in cancer therapeutics.

The Exosome-Mediated Bone Regeneration: An Advanced Horizon Toward the Isolation, Engineering, Carrying Modalities, and Mechanisms

Exosomes, nanoparticles secreted by various cells, composed of a bilayer lipid membrane, and containing bioactive substances such as proteins, nucleic acids, metabolites, etc., have been intensively investigated in tissue engineering owing to their high biocompatibility and versatile biofunction. However, there is still a lack of a high-quality review on bone defect regeneration potentiated by exosomes. In this review, the biogenesis and isolation methods of exosomes are first introduced. More importantly, the engineered exosomes of the current state of knowledge are discussed intensively in this review. Afterward, the biomaterial carriers of exosomes and the mechanisms of bone repair elucidated by compelling evidence are presented. Thus, future perspectives and concerns are revealed to help devise advanced modalities based on exosomes to overcome the challenges of bone regeneration. It is totally believed this review will attract special attention from clinicians and provide promising ideas for their future works.

Magnesium malate-modified calcium phosphate bone cement promotes the repair of vertebral bone defects in minipigs via regulating CGRP

Active artificial bone substitutes are crucial in bone repair and reconstruction. Calcium phosphate bone cement (CPC) is known for its biocompatibility, degradability, and ability to fill various shaped bone defects. However, its low osteoinductive capacity limits bone regeneration applications. Effectively integrating osteoinductive magnesium ions with CPC remains a challenge. Herein, we developed magnesium malate-modified CPC (MCPC). Incorporating 5% magnesium malate significantly enhances the compressive strength of CPC to (6.18 ± 0.49) MPa, reduces setting time and improves disintegration resistance. In vitro, MCPC steadily releases magnesium ions, promoting the proliferation of MC3T3-E1 cells without causing significant apoptosis, proving its biocompatibility. Molecularly, magnesium malate prompts macrophages to release prostaglandin E2 (PGE2) and synergistically stimulates dorsal root ganglion (DRG) neurons to synthesize and release calcitonin gene-related peptide (CGRP). The CGRP released by DRG neurons enhances the expression of the key osteogenic transcription factor Runt-related transcription factor-2 (RUNX2) in MC3T3-E1 cells, promoting osteogenesis. In vivo experiments using minipig vertebral bone defect model showed MCPC significantly increases the bone volume fraction, bone density, new bone formation, and proportion of mature bone in the defect area compared to CPC. Additionally, MCPC group exhibited significantly higher levels of osteogenesis and angiogenesis markers compared to CPC group, with no inflammation or necrosis observed in the hearts, livers, or kidneys, indicating its good biocompatibility. In conclusion, MCPC participates in the repair of bone defects in the complex post-fracture microenvironment through interactions among macrophages, DRG neurons, and osteoblasts. This demonstrates its significant potential for clinical application in bone defect repair.

Honeycomb-inspired ZIF-sealed interface enhances osseointegration via anti-infection and osteoimmunomodulation

Implant-associated infections (IAIs) pose a significant threat to orthopedic surgeries. Bacteria colonizing the surface of implants disrupt bone formation-related cells and interfere with the osteoimmune system, resulting in an impaired immune microenvironment and osteogenesis disorders. Inspired by nature, a zeolitic imidazolate framework (ZIF)-sealed smart drug delivery system on Ti substrates (ZSTG) was developed for the "natural-artificial dual-enzyme intervention (NADEI)" strategy to address these challenges. The subtle sealing design of ZIF-8 on the TiO2 nanotubes ensured glucose oxidase (GOx) activity and prevented its premature leakage. In the acidic infection microenvironment, the degradation of ZIF-8 triggered the rapid release of GOx, which converted glucose into H2O2 for disinfection. The Zn2+ released from degraded ZIF-8, as a DNase mimic, can hydrolyze extracellular DNA, which further enhances H2O2-induced disinfection and prevents biofilm formation. Importantly, Zn2+-mediated M2 macrophage polarization significantly improved the impaired osteoimmune microenvironment, accelerating bone repair. Transcriptomics revealed that ZSTG effectively suppressed the inflammatory cascade induced by lipopolysaccharide while promoting cell proliferation, homeostasis maintenance, and bone repair. In vitro and in vivo results confirmed the superior anti-infective, osteoimmunomodulatory, and osteointegrative capacities of the ZSTG-mediated NADEI strategy. Overall, this smart bionic platform has significant potential for future clinical applications to treat IAIs.

OsteoMac: A new player on the bone biology scene

The knowledge of bone biology has undergone major advances in recent decades. In bone, resorbing osteoclasts have classically been described as tissue-resident macrophages, however, it is currently known that a new subtype of macrophages, called OsteoMacs, are specialised bone-resident macrophages, which, depending on certain conditions, may play an important role not only in bone homeostasis, but also in promoting pro-anabolic functions or in creating an inflammatory environment. There is growing evidence that these osteal macrophages may influence the development of bone-loss diseases. It is essential to understand the biological bases underlying bone physiological processes to search for new therapeutic targets for bone-loss diseases, such as osteoporosis, rheumatoid arthritis, or even periodontal disease. This narrative review provides an update on the origin, characterisation, and possible roles of osteoMacs in bone biology. Finally, the potential clinical applications of this new cell in bone-loss disorders are discussed.

Osteoimmunology of Fracture Healing

PURPOSE OF REVIEW

The purpose of this review is to summarize what is known in the literature about the role inflammation plays during bone fracture healing. Bone fracture healing progresses through four distinct yet overlapping phases: formation of the hematoma, development of the cartilaginous callus, development of the bony callus, and finally remodeling of the fracture callus. Throughout this process, inflammation plays a critical role in robust bone fracture healing.

RECENT FINDINGS

At the onset of injury, vessel and matrix disruption lead to the generation of an inflammatory response: inflammatory cells are recruited to the injury site where they differentiate, activate, and/or polarize to secrete cytokines for the purposes of cell signaling and cell recruitment. This process is altered by age and by sex. Bone fracture healing is heavily influenced by the presence of inflammatory cells and cytokines within the healing tissue.

Roles of extracellular vesicles on macrophages in inflammatory bone diseases

Inflammatory bone disease is a general term for a series of diseases caused by chronic inflammation, which leads to the destruction of bone homeostasis, that is, the osteolytic activity of osteoclasts increases, and the osteogenic activity of osteoblasts decreases, leading to osteolysis. Macrophages are innate immune cell with plasticity, and their polarization is related to inflammatory bone diseases. The dynamic balance of macrophages between the M1 phenotype and the M2 phenotype affects the occurrence and development of diseases. In recent years, an increasing number of studies have shown that extracellular vesicles existing in the extracellular environment can act on macrophages, affecting the progress of inflammatory diseases. This process is realized by influencing the physiological activity or functional activity of macrophages, inducing macrophages to secrete cytokines, and playing an anti-inflammatory or pro-inflammatory role. In addition, by modifying and editing extracellular vesicles, the potential of targeting macrophages can be used to provide new ideas for developing new drug carriers for inflammatory bone diseases.

Microbial- and host immune cell-derived extracellular vesicles in the pathogenesis and therapy of periodontitis: A narrative review

Periodontitis is a chronic inflammatory disease caused by dysbiotic biofilms and destructive host immune responses. Extracellular vesicles (EVs) are circulating nanoparticles released by microbes and host cells involved in cell-to-cell communication, found in body biofluids, such as saliva and gingival crevicular fluid (GCF). EVs are mainly involved in cell-to-cell communication, and may hold promise for diagnostic and therapeutic purposes. Periodontal research has examined the potential involvement of bacterial- and host-cell-derived EVs in disease pathogenesis, diagnosis, and therapy, but data remains scarce on immune cell- or microbial-derived EVs. In this narrative review, we first provide an overview of the role of microbial and host-derived EVs on disease pathogenesis. Recent studies reveal that Porphyromonas gingivalis and Aggregatibacter actinomycetemcomitans-derived outer membrane vesicles (OMVs) can activate inflammatory cytokine release in host cells, while M1 macrophage EVs may contribute to bone loss. Additionally, we summarised current in vitro and pre-clinical research on the utilisation of immune cell and microbial-derived EVs as potential therapeutic tools in the context of periodontal treatment. Studies indicate that EVs from M2 macrophages and dendritic cells promote bone regeneration in animal models. While bacterial EVs remain underexplored for periodontal therapy, preliminary research suggests that P. gingivalis OMVs hold promise as vaccine candidates. Finally, we acknowledge the current limitations present in the field of translating immune cell derived EVs and microbial derived EVs in periodontology. It is concluded that microbial and host immune cell-derived EVs have a role in periodontitis pathogenesis and hence may be useful for studying disease pathophysiology, and as diagnostic and treatment monitoring biomarkers.

Modulating the phenotype and function of bone marrow-derived macrophages via mandible and femur osteoblasts

Various studies have been investigated the phenotypic and functional distinctions of craniofacial and long bone cells involved in bone regeneration. However, the process of bone tissue regeneration after bone grafting involves complicated interactions between different cell types at the donor-recipient site. Additionally, differences in alterations of the immune microenvironment at the recipient site remained to be explored. Osteoblasts (OBs) and macrophages (MØ) play essential roles in the bone restoration and regeneration processes in the bone and immune systems, respectively. The modulation of MØ on OBs has been extensively explored in the literature, whereas limited research has been conducted on the influence of OBs on the MØ phenotype and function. In the present study, OBs from the mandible and femur (MOBs and FOBs, respectively) promoted cranial defect regeneration in rats, with better outcomes noted in the MOBs-treated group. After MOBs transplantation, a significant inflammatory response was induced, accompanied by an early increase in IL-10 secretion. And then, there was an upregulation in M2-MØ-related cell markers and inflammatory factor expression. Condition media (CM) of OBs mildly inhibited apoptosis in MØ, enhanced their migration and phagocytic functions, and concurrently increased iNOS and Arg1 expression, with MOB-CM demonstrating more pronounced effects compared to FOB-CM. In conclusion, our investigation showed that MOBs and FOBs have the ability to modulate MØ phenotype and function, with MOBs exhibiting a stronger regulatory potential. These findings provide a new direction for improving therapeutic strategies for bone regeneration in autologous bone grafts from the perspective of the immune microenvironment.

IGF2 promotes alveolar bone regeneration in murine periodontitis via inhibiting cGAS/STING-mediated M1 macrophage polarization

Periodontitis is a chronic inflammatory disease with the destruction of supporting periodontal tissue. This study evaluated the role of insulin-like growth factor 2 (IGF2) in periodontitis by inhibiting the polarization of M1 macrophages via the cyclic GMP-AMP synthase (cGAS)/stimulator of interferon genes (STING) pathway. IGF2 was enriched in the gingival tissue of murine periodontitis model identified by RNA sequencing. IGF2 application alleviated the expression of pro-inflammatory factors and promoted osteogenesis and the expression of related genes and proteins in a dose-dependent manner in periodontitis. The result of micro-CT verified this finding. Both in vivo and in vitro results revealed that IGF2 decreased the polarization of M1 macrophages and pro-inflammatory factors by immunofluorescence staining, flow cytometry, western blotting and RT-PCR. IGF2 application promoted the osteogenic ability of periodontal ligament fibroblasts (PDLFs) indirectly via its inhibition of M1 polarization evaluated by alkaline phosphatase and alizarin red staining. Then, the cGAS/STING pathway was upregulated in periodontitis and macrophages challenged by LPS, the inhibition of which led to downregulation of M1 polarization. Furthermore, IGF2 could downregulate cGAS, STING and the phosphorylation of P65. Collectively, our study indicates IGF2 can regulate the polarization of M1 macrophages via the cGAS/STING pathway and highlights the promising future of IGF2 as a therapeutic treatment for periodontitis.

The Role of Macrophage Death in Periodontitis: A Review

Periodontitis, an infectious inflammatory disease influenced by various factors, disrupts the delicate balance between the host microbiota and immunity. The resulting excessive immune response exacerbates the progressive destruction of the supporting periodontal tissue. Macrophages are essential elements of the host innate immune system. They are pivotal components in the periodontal immune microenvironment and actively participate in both physiological and pathological processes of periodontal tissue. When confronted with periodontitis-related irritant factors, macrophages may differentiate to pro- or anti-inflammatory subtypes that affect tissue homeostasis. Additionally, macrophages may die in response to bacterial infections, potentially affecting the severity of periodontitis. This article reviews the typical mechanisms underlying macrophage death and its effects on periodontitis. We describe five forms of macrophage death in periodontitis: apoptosis, pyroptosis, necroptosis, ferroptosis, and ETosis. Our review of macrophage death in the pathophysiology of periodontitis enhances comprehension of the pathogenesis of periodontitis that will be useful for clinical practice. Although our review elucidates the complex mechanisms by which macrophage death and inflammatory pathways perpetuate periodontitis, unresolved issues remain, necessitating further research.

Tailoring Biomaterials Ameliorate Inflammatory Bone Loss

Inflammatory diseases, such as rheumatoid arthritis, periodontitis, chronic obstructive pulmonary disease, and celiac disease, disrupt the delicate balance between bone resorption and formation, leading to inflammatory bone loss. Conventional approaches to tackle this issue encompass pharmaceutical interventions and surgical procedures. Nevertheless, pharmaceutical interventions exhibit limited efficacy, while surgical treatments impose trauma and significant financial burden upon patients. Biomaterials show outstanding spatiotemporal controllability, possess a remarkable specific surface area, and demonstrate exceptional reactivity. In the present era, the advancement of emerging biomaterials has bestowed upon more efficacious solutions for combatting the detrimental consequences of inflammatory bone loss. In this review, the advances of biomaterials for ameliorating inflammatory bone loss are listed. Additionally, the advantages and disadvantages of various biomaterials-mediated strategies are summarized. Finally, the challenges and perspectives of biomaterials are analyzed. This review aims to provide new possibilities for developing more advanced biomaterials toward inflammatory bone loss.

Matrix Metalloproteinase-Responsive Hydrogel with On-Demand Release of Phosphatidylserine Promotes Bone Regeneration Through Immunomodulation

Inflammation-responsive hydrogels loaded with therapeutic factors are effective biomaterials for bone tissue engineering and regenerative medicine. In this study, a matrix metalloproteinase (MMP)-responsive injectable hydrogel is constructed by integrating an MMP-cleavable peptide (pp) into a covalent tetra-armed poly-(ethylene glycol) (PEG) network for precise drug release upon inflammation stimulation. To establish a pro-regenerative environment, phosphatidylserine (PS) is encapsulated into a scaffold to form the PEG-pp-PS network, which could be triggered by MMP to release a large amount of PS during the early stage of inflammation and retain drug release persistently until the later stage of bone repair. The hydrogel is found to be mechanically and biologically adaptable to the complex bone defect area. In vivo and in vitro studies further demonstrated the ability of PEG-pp-PS to transform macrophages into the anti-inflammatory M2 phenotype and promote osteogenic differentiation, thus, resulting in new bone regeneration. Therefore, this study provides a facile, safe, and promising cell-free strategy on simultaneous immunoregulation and osteoinduction in bone engineering.

A submicron forest-like silicon surface promotes bone regeneration by regulating macrophage polarization

Introduction: Silicon is a major trace element in humans and a prospective supporting biomaterial to bone regeneration. Submicron silicon pillars, as a representative surface topography of silicon-based biomaterials, can regulate macrophage and osteoblastic cell responses. However, the design of submicron silicon pillars for promoting bone regeneration still needs to be optimized. In this study, we proposed a submicron forest-like (Fore) silicon surface (Fore) based on photoetching. The smooth (Smo) silicon surface and photoetched regular (Regu) silicon pillar surface were used for comparison in the bone regeneration evaluation. Methods: Surface parameters were investigated using a field emission scanning electron microscope, atomic force microscope, and contact angle instrument. The regulatory effect of macrophage polarization and succedent osteogenesis was studied using Raw264.7, MC3T3-E1, and rBMSCs. Finally, a mouse calvarial defect model was used for evaluating the promoting effect of bone regeneration on the three surfaces. Results: The results showed that the Fore surface can increase the expression of M2-polarized markers (CD163 and CD206) and decrease the expression of inflammatory cytokines, including interleukin-6 (IL-6) and tumor necrosis factor alpha (TNF-α). Fore surface can promote the osteogenesis in MC3T3-E1 cells and osteoblastic differentiation of rBMSCs. Furthermore, the volume fraction of new bone and the thickness of trabeculae on the Fore surface were significantly increased, and the expression of RANKL was downregulated. In summary, the upregulation of macrophage M2 polarization on the Fore surface contributed to enhanced osteogenesis in vitro and accelerated bone regeneration in vivo. Discussion: This study strengthens our understanding of the topographic design for developing future silicon-based biomaterials.

Zinc hybrid polyester barrier membrane accelerates guided tissue regeneration

Barrier membranes play a pivotal role in the success of guided periodontal tissue regeneration. The biodegradable barriers predominantly used in clinical practice often lack sufficient barrier strength, antibacterial properties, and bioactivity, frequently leading to suboptimal regeneration outcomes. Although with advantages in mechanical strength, biodegradability and plasticity, bioinert aliphatic polyesters as barrier materials are usually polymerized via toxic catalysts, hard to be functionalized and lack of antibacterial properties. To address these challenges, we propose a new concept that controlled release of bioactive substance on the whole degradation course can give a bioinert aliphatic polyester bioactivity. Thus, a Zn-based catalytic system for polycondensation of dicarboxylic acids and diols is created to prepare zinc covalent hybrid polyester (PBS/ZnO). The atomically-dispersed Zn2+ ions entering main chain of polyester molecules endow PBS/ZnO barrier with antibacterial properties, barrier strength, excellent biocompatibility and histocompatibility. Further studies reveal that relying on long-term controlled release of Zn2+ ions, the PBS/ZnO membrane greatly expedites osteogenetic effect in guided tissue regeneration (GTR) by enhancing the mitochondrial function of macrophages to induce M2 polarization. These findings show a novel preparation strategy of bioactive polyester biomaterials based on long term controlled release of bioactive substance that integrates catalysis, material structures and function customization.