Harnessing the power of macrophages/monocytes for enhanced bone tissue engineering

The role of integrin αvβ3 in biphasic calcium phosphate ceramics mediated M2 Macrophage polarization and the resultant osteoinduction

Calcium phosphate ceramics-based biomaterials were reported to have good biocompatibility and osteoinductivity and have been widely applied for bone defect repair and regeneration. However, the mechanism of their osteoinductivity is still unclear. In our study, we established an ectopic bone formation in vivo model and an in vitro macrophage cell co-culture system with calcium phosphate ceramics to investigate the effect of biphasic calcium phosphate on osteogenesis via regulating macrophage M1/M2 polarization. Our micro-CT data suggested that biphasic calcium phosphate had significant osteoinductivity, and the fluorescence co-localization detection found increased F4/80+/integrin αvβ3+ macrophages surrounding the biphasic calcium phosphate scaffolds. Besides, our study also revealed that biphasic calcium phosphate promoted M2 polarization of macrophages via upregulating integrin αvβ3 expression compared to tricalcium phosphate, and the increased M2 macrophages could subsequently augment the osteogenic differentiation of MSCs in a TGFβ mediated manner. In conclusion, we demonstrated that macrophages subjected to biphasic calcium phosphate could polarize toward M2 phenotype via triggering integrin αvβ3 and secrete TGFβ to increase the osteogenesis of MSCs, which subsequently enhances bone regeneration.

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.

Functionalization of biomimetic mineralized collagen for bone tissue engineering

Mineralized collagen (MC) is the basic unit of bone structure and function and is the main component of the extracellular matrix (ECM) in bone tissue. In the biomimetic method, MC with different nanostructures of neo-bone have been constructed. Among these, extra-fibrous MC has been approved by regulatory agencies and applied in clinical practice to play an active role in bone defect repair. However, in the complex microenvironment of bone defects, such as in blood supply disorders and infections, MC is unable to effectively perform its pro-osteogenic activities and needs to be functionalized to include osteogenesis and the enhancement of angiogenesis, anti-infection, and immunomodulation. This article aimed to discuss the preparation and biological performance of MC with different nanostructures in detail, and summarize its functionalization strategy. Then we describe the recent advances in the osteo-inductive properties and multifunctional coordination of MC. Finally, the latest research progress of functionalized biomimetic MC, along with the development challenges and future trends, are discussed. This paper provides a theoretical basis and advanced design philosophy for bone tissue engineering in different bone microenvironments.

Harnessing electromagnetic fields to assist bone tissue engineering

Bone tissue engineering (BTE) emerged as one of the exceptional means for bone defects owing to it providing mechanical supports to guide bone tissue regeneration. Great advances have been made to facilitate the success of BTE in regenerating bone within defects. The use of externally applied fields has been regarded as an alternative strategy for BTE. Electromagnetic fields (EMFs), known as a simple and non-invasive therapy, can remotely provide electric and magnetic stimulation to cells and biomaterials, thus applying EMFs to assist BTE would be a promising strategy for bone regeneration. When combined with BTE, EMFs improve cell adhesion to the material surface by promoting protein adsorption. Additionally, EMFs have positive effects on mesenchymal stem cells and show capabilities of pro-angiogenesis and macrophage polarization manipulation. These advantages of EMFs indicate that it is perfectly suitable for representing the adjuvant treatment of BTE. We also summarize studies concerning combinations of EMFs and diverse biomaterial types. The strategy of combining EMFs and BTE receives encouraging outcomes and holds a promising future for effectively treating bone defects.

Targeted ToF-SIMS Analysis of Macrophage Content from a Human Cranial Triphasic Calcium Phosphate Implant

Macrophages play a key role in determining the fate of implanted biomaterials, especially for biomaterials such as calcium phosphates (CaPs) where these cells play a vital role in material resorption and osteogenesis, as shown in different models, including clinical samples. Although substantial consideration is given to the design and validation of different CaPs, relatively little is known about their material-cell interaction. Specifically, the intracellular content of different CaP phases remains to be assessed, even though CaP-filled macrophages have been observed in several studies. In this study, 2D/3D ToF-SIMS imaging and multivariate analysis were directly applied on the histology samples of an explant to reveal the content of macrophages. The cellular content of the macrophages was analyzed to distinguish three CaP phases, monetite, beta-tricalcium phosphate, and pyrophosphate, which are all part of the monetite-based CaP implant composition under study. ToF-SIMS combined with histology revealed that the content of the identified macrophages was most similar to that of the pyrophosphate phase. This study is the first to uncover distinct CaP phases in macrophages from a human multiphasic CaP explant by targeted direct cell content analysis. The uncovering of pyrophosphate as the main phase found inside the macrophages is of great importance to understand the impact of the selected material in the process of biomaterial-instructed osteogenesis.

The role of autophagy in the process of osseointegration around titanium implants with micro-nano topography promoted by osteoimmunity

Osteoimmunity plays an important role in the process of implant osseointegration. Autophagy is a conservative metabolic pathway of eukaryotic cells, but whether the interaction between autophagy and osteoimmunity plays a key role in osseointegration remains unclear. In this study, we prepared smooth titanium disks and micro-nano topography titanium disks, to study the immune microenvironment of RAW264.7 cells, and prepared the conditioned medium to study the effect of immune microenvironment on the osteogenesis and autophagy of MC3T3-E1 cells. Autophagy inhibitor 3-MA was used to inhibit autophagy to observe the change of expression of osteogenic markers. The results showed that the micro-nano topography titanium disks could stimulate RAW264.7 cells to differentiate into M2 type, forming an anti-inflammatory immune microenvironment; compared with the control group, the anti-inflammatory immune microenvironment promoted the proliferation and differentiation of osteoblasts better. The anti-inflammatory immune environment activated the autophagy level of osteoblasts, while the expression of osteogenic markers was down-regulated after inhibition of autophagy. These results indicate that anti-inflammatory immune microenvironment can promote cell proliferation and osteogenic differentiation, autophagy plays an important role in this process. This study further explains the mechanism of implant osseointegration in osteoimmune microenvironment, and provides reference for improving implant osseointegration.

Macrophages' contribution to ectopic osteogenesis in combination with blood clot and bone substitute: possibility for application in bone regeneration strategies

PURPOSE

Given the great potential of macrophages in the processes of tissue repair and regeneration, the aim of our study was to examine the contribution that macrophages will have in osteogenic process when combined and implanted with blood clot (BC) and mineral bone substitute (MBS) in mice subcutaneous implantation model.

METHODS

Three types of implants were constructed and implanted subcutaneously into BALB/c mice: (1) RMBM implants (made of resident tissue macrophages, BC and MBS), (2) BM implants (made of BC and MBS), and (3) M implants (made of MBS only) where the last two served as control implants. One, two, four and eight weeks after implantation implants were explanted, and histochemical, immunohistochemical, and histomorphometric analyses were performed.

RESULTS

Increased vascularization, particularly pronounced two and four weeks after implantation and pronounced tissue infiltration in eight week term in RMBM implants compared with both other types, likewise the presence of osteoblast-like cells, osteoid-like structures, and more prominent osteopontin and osteocalcin immunoexpression in RMBM implants indicated more pronounced osteogenic process within them.

CONCLUSION

Our results suggest that macrophages deserve to be considered as a cell component when constructing implants in bone regenerative medicine strategies to improve bone fracture healing process.

Intramembranous Ossification Imitation Scaffold with the Function of Macrophage Polarization for Promoting Critical Bone Defect Repair

The regeneration of craniofacial bone defects remains a crucial clinical challenge. To date, numerous biomaterials are applied in this field. However, current strategies have ignored the importance of intramembranous ossification and the vital role of macrophages in regulating osteogenesis. Here, an osteoblast (OB)-targeting peptide (SDSSD)-modified chitosan scaffold (CS-SDSSD) is developed for imitating the physiological process of bone development from the fibrous membrane. The addition of free peptide (fSDSSD) can recruit host OBs, and the peptide grafted on the scaffold (CS-SDSSD) can well organize the migrated OBs by binding with their surface periostin. Besides, macrophage polarization is found in the bone defects. CS-SDSSD + fSDSSD displays advantages in prioritizing M2 macrophage polarization and promoting the intramembranous ossification bone repair process. In summary, our strategy provides an economical and effective path for craniofacial bone repair and holds great potential for biomedical applications.

The impact of sitagliptin on macrophage polarity and angiogenesis in the osteointegration of titanium implants in type 2 diabetes

BACKGROUND

Clinical evidence indicates that sitagliptin treatment improves bone quality in diabetic patients, but the mechanisms involved remain elusive. Here, we studied the role of angiogenesis with sitagliptin treatment in diabetes-induced poor osteointegration of titanium implants and the underlying mechanisms.

METHODS

In vitro, Human Umbilical Vein Endothelial Cells (HUVECs) incubated on titanium (Ti) surface were subjected to 1) normal milieu (NM); 2) diabetic milieu (DM); 3) DM + sitagliptin; 4) NM + macrophage; 5) DM + macrophage; or 6) DM + macrophage + sitagliptin. Microphage and HUVECs were cultured alone or co-cultured in a Transwell system. In vivo, DM was induced by high-fat diet and administration of streptozotocin (STZ) in rats. Titanium screws were implanted in the femurs of rats in three groups: Control, DM, Sitagliptin-treated DM.

RESULTS

In vitro, when cells were incubated alone, DM caused M1 polarization of macrophage, evidenced by the increased iNOS and decreased CD206 expressions, and obvious dysfunctions of HUVECs. The DM-induced injury of endothelial cells were significantly worsened when the two cells were co-cultured. The addition of sitagliptin markedly reversed the changes of macrophage but not of HUVECs in DM when cells were cultured alone. When cells co-cultured, however, both the abnormal macrophage polarization and the endothelial impairment in DM was significantly alleviated by sitagliptin. In vivo, compared with normal animals, DM animals showed imbalanced M1/M2 polarization, angiogenesis inhibition and poor bone formation on the bone-implant interface (BII), which were significantly ameliorated by sitagliptin treatment.

CONCLUSION

Our results demonstrate macrophage polarization imbalance as a crucial mechanism underlying the impaired angiogenesis and bone healing in diabetes, and provide sitagliptin as a promising novel drug for biomaterial-engineering to improve the osteointegration of titanium implants in diabetic patients.

The observed difference of macrophage phenotype on different surface roughness of mineralized collagen

Biomaterials regulate macrophages and promote regeneration function, which is a new hot pot in tissue engineering and regenerative medicine. The research based on macrophage materials biology has appeared happy future, but related research on regulating macrophages and promoting tissue regeneration is still in its infancy. The surface roughness of biomaterials is one of the important factors affecting macrophage behavior. Previous study also found that the surface roughness of many biomaterials regulating macrophage polarization, but not including mineralized collagen (MC). In this study, we designed and fabricated MC with different roughness and investigated the influence of MC with different roughness on macrophages. In the study, we found that on the rough surface of MC, macrophages exhibited M1 phenotype-amoeboid morphology and high-level secretory of inflammatory factor (tumor necrosis factor-α and interleukin-6), while smoother surface exhibited M2 phenotype. These data will be beneficial to understand the mechanism deeply and enrich biomaterials tissue regeneration theory, provide a new train of thought biomaterials inducing tissue regeneration and repair and guide the optimum design of new biomaterials, development and reasonable applications.

Reducing macrophage numbers alleviates temporomandibular joint ankylosis

Temporomandibular joint (TMJ) ankylosis is a severe joint disease mainly caused by trauma that leads to a series of oral and maxillofacial function disorders and psychological problems. Our series of studies indicate that TMJ ankylosis development is similar to fracture healing and that severe trauma results in bony ankylosis instead of fibrous ankylosis. Macrophages are early infiltrating inflammatory cells in fracture and play a critical role in initiating fracture repair. We hypothesize that the large numbers of macrophages in the early phase of TMJ ankylosis trigger ankylosed bone mass formation and that the depletion of these macrophages in the early phase could inhibit the development of TMJ ankylosis. By analysing human TMJ ankylosis specimens, we found large numbers of infiltrated macrophages in the less-than-1-year ankylosis samples. A rabbit model of TMJ bony ankylosis was established and large numbers of infiltrated macrophages were found at 4 days post-operation. Local clodronate liposome (CLD-lip) injection, which depleted macrophages, alleviated the severity of ankylosis compared with local phosphate-buffered saline (PBS)-loaded liposome (PBS-lip) injection (macrophage number, PBS-lips vs. CLD-lips: 626.03 ± 164.53 vs. 341.4 ± 108.88 n/mm²; ankylosis calcification score, PBS-lips vs. CLD-lips: 2.11 ± 0.78 vs. 0.78 ± 0.66). Histological results showed that the cartilage area was reduced in the CLD-lip-treated side (PBS-lips vs. CLD-lips: 6.82 ± 4.42% vs. 2.71 ± 2.78%) and that the Wnt signalling regulating cartilage formation was disrupted (Wnt5a expression decreased 60% and Wnt4 expression decreased 73%). Thus, our study showed that large numbers of macrophages infiltrated during the early phase of ankylosis and that reducing macrophage numbers alleviated ankylosis development by reducing cartilage formation.

Biologics in the Treatment of Achilles Tendon Pathologies

Regenerative medicine is gaining more and more space for the treatment of Achilles pathologic conditions. Biologics could play a role in the management of midportion Achilles tendinopathy as a step between conservative and surgical treatment or as an augmentation. Higher-level studies are needed before determining a level of treatment recommendation for biologic strategies for insertional Achilles tendinopathy. Combining imaging with patient's functional requests could be the way to reach a protocol for the use of biologics for the treatment of midportion Achilles tendinopathy and, for this perspective, the authors describe the Foot and Ankle Reconstruction Group algorithm of treatment.

Monocyte-derived extracellular vesicles stimulate cytokine secretion and gene expression of matrix metalloproteinases by mesenchymal stem/stromal cells

Intercellular communication is essential in bone remodelling to ensure that new bone is formed with only temporary bone loss. Monocytes (MCs) and osteoclasts actively take part in controlling bone remodelling by providing signals that promote osteogenic differentiation of mesenchymal stem/stromal cells (MSCs). Extracellular vesicles (EVs) have attracted attention as regulators of bone remodelling. EVs facilitate intercellular communication by transferring a complex cargo of biologically active molecules to target cells. In the present study, we evaluated the potency of EVs from MCs and osteoclasts to induce a lineage-specific response in MSCs. We analysed gene expression and protein secretion by both adipose tissue-derived MSCs and bone marrow-derived MSCs after stimulation with EVs from lipopolysaccharide-activated primary human MCs and (mineral-resorbing) osteoclasts. Isolated EVs were enriched in exosomes (EVs of endosomal origin) and were free of cell debris. MC- and osteoclast-derived EVs were taken up by adipose tissue-derived MSCs. EVs from activated MCs promoted the secretion of cytokines by MSCs, which may represent an immunomodulatory mechanism. MC-derived EVs also upregulated the expression of genes encoding for matrix metalloproteinases. Therefore, we hypothesize that MCs facilitate tissue remodelling through EV-mediated signalling. We did not observe a significant effect of osteoclast-derived EVs on gene expression or protein secretion in MSCs. EV-mediated signalling might represent an additional mode of cell-cell signalling during the transition from injury and inflammation to bone regeneration and play an important role in the coupling between bone resorption and bone formation.

DATABASE

Gene expression data are available in the GEO database under the accession number GSE102401.

The material and biological characteristics of osteoinductive calcium phosphate ceramics

The discovery of osteoinductivity of calcium phosphate (Ca-P) ceramics has set an enduring paradigm of conferring biological regenerative activity to materials with carefully designed structural characteristics. The unique phase composition and porous structural features of osteoinductive Ca-P ceramics allow it to interact with signaling molecules and extracellular matrices in the host system, creating a local environment conducive to new bone formation. Mounting evidence now indicate that the osteoinductive activity of Ca-P ceramics is linked to their physicochemical and three-dimensional structural properties. Inspired by this conceptual breakthrough, many laboratories have shown that other materials can be also enticed to join the rank of tissue-inducing biomaterials, and besides the bones, other tissues such as cartilage, nerves and blood vessels were also regenerated with the assistance of biomaterials. Here, we give a brief historical recount about the discovery of the osteoinductivity of Ca-P ceramics, summarize the underlying material factors and biological characteristics, and discuss the mechanism of osteoinduction concerning protein adsorption, and the interaction with different types of cells, and the involvement of the vascular and immune systems.

Immunomodulatory ECM-like Microspheres for Accelerated Bone Regeneration in Diabetes Mellitus

Bone repair and regeneration process is markedly impaired in diabetes mellitus (DM) that affects hundreds of millions of people worldwide. As a chronic inflammatory disease, DM creates a proinflammatory microenvironment in defective sites. Most of the studies on DM-associated bone regeneration, however, neglect the importance of immunomodulation under the DM condition and adopt the same approaches to normal bone healing, leading to limited bone healing. In this study, we developed a unique bioinspired injectable microsphere as an osteoimmunomodulatory biomaterial that modulates macrophages to create a prohealing microenvironment under the DM condition. The microsphere was self-assembled with heparin-modified gelatin nanofibers, and interleukin 4 (IL4) was incorporated into the nanofibrous heparin-modified gelatin microsphere (NHG-MS). IL4 has binding domains with heparin, and the binding of IL4 to heparin stabilizes this cytokine, protects it from denaturation and degradation, and subsequently prolongs its sustained release to modulate macrophage polarization. The IL4-loaded NHG-MS switched the proinflammatory M1 macrophage into a prohealing M2 phenotype, recovered the M2/M1 ratio to a normal level, efficiently resolved the inflammation, and ultimately enhanced osteoblastic differentiation and bone regeneration. The development of osteoimmunomodulatory biomaterials that harness the power of macrophages for immunomodulation, therefore, is a novel and promising strategy to enhance bone regeneration under DM condition.

Parâmetros clínicos, hematológicos e bioquímicos de novilhas com hipocalcemia subclínica pré-parto suplementadas com dieta aniônica

RESUMO A hipótese do presente estudo consiste em que uma dieta aniônica prolongada acarretaria uma mobilização mais rápida do cálcio sanguíneo. O objetivo proposto foi avaliar parâmetros clínicos, hematológicos e bioquímicos de novilhas, com hipocalcemia subclínica, submetidas a diferentes períodos de fornecimento de dieta aniônica no pré-parto. Foram utilizadas nove novilhas da raça Holandês induzidas à hipocalcemia subclínica no pré-parto, divididas em três grupos: G0 = sem dieta aniônica antes da indução; G11 = 11 dias de dieta aniônica pré-indução; e G15 = 15 dias de dieta aniônica pré-indução. Foi realizado exame clínico antes e depois da indução, e avaliaram-se cálcio total e ionizado, magnésio e albumina nos dias -2, 0, 1, 2 e 3 (dia 0 = momento da indução) e parâmetros hematológicos nos dias 0, 1, 2 e 3. O G11 apresentou maiores concentrações de cálcio total e ionizado que o G0 e maiores níveis de cálcio total que o G15 (P<0,01). O magnésio teve maiores valores no G15 quando comparado aos outros grupos (P<0,01). O G0 apresentou os maiores valores de monócitos e neutrófilos bastonetes (P<0,01). A dieta aniônica por 11 dias é suficiente para manter a homeostase do cálcio e a resposta leucocitária em novilhas desafiadas a uma diminuição do cálcio sanguíneo.

Osteomacs and Bone Regeneration

PURPOSE OF REVIEW

Mounting evidence supporting the critical contribution of macrophages, in particular osteal macrophages, to bone regeneration is reviewed. We specifically examine the potential role of macrophages in the basic multicellular units coordinating lifelong bone regeneration via remodelling and bone regeneration in response to injury. We review and discuss the distinctions between macrophage and osteoclast contributions to bone homeostasis, particularly the dichotomous role of the colony-stimulating factor 1-colony-stimulating factor 1 receptor axis.

RECENT FINDINGS

The impact of inflammation associated with aging and other hallmarks of aging, including senescence, on macrophage function is addressed in the context of osteoporosis and delayed fracture repair. Resident macrophages versus recruited macrophage contributions to fracture healing are also discussed. We identify some of the remaining knowledge gaps that will need to be closed in order to maximise benefits from therapeutically modulating or mimicking the function of macrophages to improve bone health and regeneration over a lifetime.

Modulating Immunological Responses of Electrospun Fibers for Tissue Engineering

The promise of tissue engineering is to improve or restore functions of impaired tissues or organs. However, one of the biggest challenges to its translation to clinical applications is the lack of tissue integration and functionality. The plethora of cellular and molecular events occurring following scaffold implantation is a major bottleneck. Recent studies confirmed that inflammation is a crucial component influencing tissue regeneration. Immuno-modulation or immune-engineering has been proposed as a potential solution to overcome this key challenge in regenerative medicine. In this review, strategies to modify scaffold physicochemical properties through the use of the electrospinning technique to modulate host response and improve scaffold integration will be discussed. Electrospinning, being highly versatile allows the fabrication of ECM-mimicking scaffolds and also offers the possibility to control scaffold properties for instance, tailoring of fiber properties, chemical conjugation or physical adsorption of non-immunogenic materials on the scaffold surface, encapsulating cells or anti-inflammatory molecules within the scaffold. Such electrospun scaffold-based immune-engineering strategies can significantly improve the resulting outcomes of tissue engineering scaffolds.

Biophysical induction of cell release for minimally manipulative cell enrichment strategies

The use of autologous cells harvested and subsequently transplanted in an intraoperative environment constitutes a new approach to promote regeneration. Usually cells are isolated by selection methods such as fluorescence- or magnetic- activated cell sorting with residual binding of the antibodies or beads. Thus, cell-based therapies would benefit from the development of new devices for cell isolation that minimally manipulate the target cell population. In the clinic, 5 to 10 percent of fractures do not heal properly and CD31+ cells have been identified as promising candidates to support bone regeneration. The aim of this project was to develop and prototype a simple system to facilitate the enrichment of CD31+ cells from whole blood. After validating the specificity of a commercially available aptamer for CD31, we combined this aptamer with traditional magnetic bead strategies, which led to enrichment of CD31+ cells with a purity of 91±10%. Subsequently, the aptamer was attached to agarose beads (Ø = 100–165 um) that were incorporated into a column-based system to enable capture and subsequent release of the CD31+ enriched cells. Different parameters were investigated to allow a biophysical-based cell release from beads, and a simple mixing was found sufficient to release initially bound cells from the optimized column without the need for any chemicals that promote disassociation. The system led to a significant enrichment of CD31+ cells (initial population: 63±9%, released: 87±3%) with excellent cell viability (released: 97±1%). The composition of the released CD31+ fraction indicated an enrichment of the monocyte population. The angiogenic and osteogenic potential of the released cell population were confirmed in vitro. These results and the simplicity of this system highlight the potential of such approach to enable cell enrichment strategies in intraoperative settings.

Modulating the phenotype of host macrophages to enhance osteogenesis in MSC-laden hydrogels: Design of a glucomannan coating material

The biomaterials-host interaction is a dynamic process in which macrophages play a vital role of regulation. Depending on the biochemical signals they sense, these highly plastic cells can mediate the immune response against the implanted scaffolds and/or exert regenerative potency to varying extent. Designing appropriate 'exterior signals' for scaffolds may exploit the power of endogenous macrophages to aid the regeneration of engineered tissues. To realise this goal, this study devised an injectable, instantaneously-solidifying coating material (acBSP) based on a unique, macrophage-affinitive glucomannan polysaccharide. Coating of three-dimensional hydrogel constructs with acBSP was rapid, neat and complete, requiring neither chemical reactions nor harsh conditions. Comprehensive in vitro analyses indicated that acBSP efficiently facilitated the adhesion and activation of macrophages and notably induced the macrophages to express pro-osteogenic/-angiogenic genes. Further in vivo assessment of acBSP-coated, mesenchymal stem cells-laden hydrogels in a murine dorsal subcutaneous pocket model demonstrated efficient macrophage activation, desirable scaffold-tissue integration and improved osteogenic differentiation in the delivered cells. In summary, by activating macrophages into a pro-osteogenic phenotype, the acBSP coating has demonstrated its competency as an innovative, open and efficacious platform to harness the power of host immunity for enhancing the regenerative performance of engineered tissue constructs.

Resting and injury-induced inflamed periosteum contain multiple macrophage subsets that are located at sites of bone growth and regeneration

Better understanding of bone growth and regeneration mechanisms within periosteal tissues will improve understanding of bone physiology and pathology. Macrophage contributions to bone biology and repair have been established but specific investigation of periosteal macrophages has not been undertaken. We used an immunohistochemistry approach to characterize macrophages in growing murine bone and within activated periosteum induced in a mouse model of bone injury. Osteal tissue macrophages (osteomacs) and resident macrophages were distributed throughout resting periosteum. In tissues collected from 4-week-old mice, osteomacs were observed intimately associated with sites of periosteal diaphyseal and metaphyseal bone dynamics associated with normal growth. This included F4/80⁺Mac-2^-/low osteomac association with extended tracks of bone formation (modeling) on diphyseal periosteal surfaces. Although this recapitulated endosteal osteomac characteristics, there was subtle variance in the morphology and spatial organization of periosteal modeling-associated osteomacs, which likely reflects the greater structural complexity of periosteum. Osteomacs, resident macrophages and inflammatory macrophages (F4/80⁺Mac-2^hi) were associated with the complex bone dynamics occurring within the periosteum at the metaphyseal corticalization zone. These three macrophage subsets were also present within activated native periosteum after bone injury across a 9-day time course that spanned the inflammatory through remodeling bone healing phases. This included osteomac association with foci of endochondral ossification within the activated native periosteum. These observations confirm that osteomacs are key components of both osteal tissues, in spite of salient differences between endosteal and periosteal structure and that multiple macrophage subsets are involved in periosteal bone dynamics.

Synergistic enhancement of ectopic bone formation by supplementation of freshly isolated marrow cells with purified MSC in collagen-chitosan hydrogel microbeads

PURPOSE

Bone marrow-derived mesenchymal stem cells (MSC) can differentiate osteogenic lineages, but their tissue regeneration ability is inconsistent. The bone marrow mononuclear cell (BMMC) fraction of adult bone marrow contains a variety of progenitor cells that may potentiate tissue regeneration. This study examined the utility of BMMC, both alone and in combination with purified MSC, as a cell source for bone regeneration.

METHODS

Fresh BMMC, culture-expanded MSC, and a combination of BMMC and MSC were encapsulated in collagen-chitosan hydrogel microbeads for pre-culture and minimally invasive delivery. Microbeads were cultured in growth medium for 3 days, and then in either growth or osteogenic medium for 17 days prior to subcutaneous injection in the rat dorsum.

RESULTS

MSC remained viable in microbeads over 17 days in pre-culture, while some of the BMMC fraction were nonviable. After 5 weeks of implantation, microCT and histology showed that supplementation of BMMC with MSC produced a strong synergistic effect on the volume of ectopic bone formation, compared to either cell source alone. Microbeads containing only fresh BMMC or only cultured MSC maintained in osteogenic medium resulted in more bone formation than their counterparts cultured in growth medium. Histological staining showed evidence of residual microbead matrix in undifferentiated samples and indications of more advanced tissue remodeling in differentiated samples.

CONCLUSIONS

These data suggest that components of the BMMC fraction can act synergistically with predifferentiated MSC to potentiate ectopic bone formation. The microbead system may have utility in delivering desired cell populations in bone regeneration applications.

Human iPSC-derived osteoblasts and osteoclasts together promote bone regeneration in 3D biomaterials

Bone substitutes can be designed to replicate physiological structure and function by creating a microenvironment that supports crosstalk between bone and immune cells found in the native tissue, specifically osteoblasts and osteoclasts. Human induced pluripotent stem cells (hiPSC) represent a powerful tool for bone regeneration because they are a source of patient-specific cells that can differentiate into all specialized cell types residing in bone. We show that osteoblasts and osteoclasts can be differentiated from hiPSC-mesenchymal stem cells and macrophages when co-cultured on hydroxyapatite-coated poly(lactic-co-glycolic acid)/poly(L-lactic acid) (HA–PLGA/PLLA) scaffolds. Both cell types seeded on the PLGA/PLLA especially with 5% w/v HA recapitulated the tissue remodeling process of human bone via coupling signals coordinating osteoblast and osteoclast activity and finely tuned expression of inflammatory molecules, resulting in accelerated in vitro bone formation. Following subcutaneous implantation in rodents, co-cultured hiPSC-MSC/-macrophage on such scaffolds showed mature bone-like tissue formation. These findings suggest the importance of coupling matrix remodeling through osteoblastic matrix deposition and osteoclastic tissue resorption and immunomodulation for tissue development.

Bioelectric modulation of macrophage polarization

Macrophages play a critical role in regulating wound healing and tissue regeneration by changing their polarization state in response to local microenvironmental stimuli. The native roles of polarized macrophages encompass biomaterials and tissue remodeling needs, yet harnessing or directing the polarization response has been largely absent as a potential strategy to exploit in regenerative medicine to date. Recent data have revealed that specific alteration of cells' resting potential (Vmem) is a powerful tool to direct proliferation and differentiation in a number of complex tissues, such as limb regeneration, craniofacial patterning and tumorigenesis. In this study, we explored the bioelectric modulation of macrophage polarization by targeting ATP sensitive potassium channels (KATP). Glibenclamide (KATP blocker) and pinacidil (KATP opener) treatment not only affect macrophage polarization, but also influence the phenotype of prepolarized macrophages. Furthermore, modulation of cell membrane electrical properties can fine-tune macrophage plasticity. Glibenclamide decreased the secretion and gene expression of selected M1 markers, while pinacidil augmented M1 markers. More interestingly, glibencalmide promoted macrophage alternative activation by enhancing certain M2 markers during M2 polarization. These findings suggest that control of bioelectric properties of macrophages could offer a promising approach to regulate macrophage phenotype as a useful tool in regenerative medicine.