Adsorbed fibrinogen leads to improved bone regeneration and correlates with differences in the systemic immune response

Dhvar5-chitosan nanogels and their potential to improve antibiotics activity

Infection is one of the main causes of orthopedic implants failure, with antibiotic-resistant bacteria playing a crucial role in this outcome. In this work, antimicrobial nanogels were developed to be applied in situ as implant coating to prevent orthopedic-device-related infections. To that regard, a broad-spectrum antimicrobial peptide, Dhvar5, was grafted onto chitosan via thiol-norbornene "photoclick" chemistry. Dhvar5-chitosan nanogels (Dhvar5-NG) were then produced using a microfluidic system. Dhvar5-NG (1010 nanogels (NG)/mL) with a Dhvar5 concentration of 6 μg/mL reduced the burden of the most critical bacteria in orthopedic infections - methicillin-resistant Staphylococcus aureus (MRSA) - after 24 h in medium supplemented with human plasma proteins. Transmission electron microscopy showed that Dhvar5-NG killed bacteria by membrane disruption and cytoplasm release. No signs of cytotoxicity against a pre-osteoblast cell line were verified upon incubation with Dhvar5-NG. To further explore therapeutic alternatives, the potential synergistic effect of Dhvar5-NG with antibiotics was evaluated against MRSA. Dhvar5-NG at a sub-minimal inhibitory concentration (109 NG/mL) demonstrated synergistic effect with oxacillin (4-fold reduction: from 2 to 0.5 μg/mL) and piperacillin (2-fold reduction: from 2 to 1 μg/mL). This work supports the use of Dhvar5-NG as adjuvant of antibiotics to the prevention of orthopedic devices-related infections.

Attachment of Fibrinogen on Ion Beam Treated Polyurethane

Protein-stable coverage of the artificial implant is a key problem for biocompatibility. In the present study, a protein layer was attached covalently to a polyurethane surface treated by an ion beam. A plasma system consisting of a vacuum chamber (0.8 Pa pressure) with a high voltage electrode powered by a short pulse (20 μS pulse duration and 200 Hz pulse repetition) generator was designed. Polyurethane with a formulation certified as a material for medical implants was treated by nitrogen ions with an energy of 20 keV and 5 × 1014-1016 ions/cm2 fluence range. Wettability measurements, X-ray photoelectron, Raman, Fourier transform infrared attenuated total reflection, and ellipsometry spectra showed a significant change in the structure of the surface layer of the treated polyurethane. The surface of the treated polyurethane contained a carbonised layer containing condensed aromatic clusters with terminal free radicals. The surface energy of polyurethane surface increased from 33 to 65 mJ/m2. The treated polyurethane surface became capable of adsorbing and chemically binding protein (fibrinogen). The designed system for ion beam treatment can be used for surface activation of biomedical polymer devices, where a total protein coverage is required.

Bone Tissue Engineering and Nanotechnology: A Promising Combination for Bone Regeneration

Large bone defects are the leading contributor to disability worldwide, affecting approximately 1.71 billion people. Conventional bone graft treatments show several disadvantages that negatively impact their therapeutic outcomes and limit their clinical practice. Therefore, much effort has been made to devise new and more effective approaches. In this context, bone tissue engineering (BTE), involving the use of biomaterials which are able to mimic the natural architecture of bone, has emerged as a key strategy for the regeneration of large defects. However, although different types of biomaterials for bone regeneration have been developed and investigated, to date, none of them has been able to completely fulfill the requirements of an ideal implantable material. In this context, in recent years, the field of nanotechnology and the application of nanomaterials to regenerative medicine have gained significant attention from researchers. Nanotechnology has revolutionized the BTE field due to the possibility of generating nanoengineered particles that are able to overcome the current limitations in regenerative strategies, including reduced cell proliferation and differentiation, the inadequate mechanical strength of biomaterials, and poor production of extrinsic factors which are necessary for efficient osteogenesis. In this review, we report on the latest in vitro and in vivo studies on the impact of nanotechnology in the field of BTE, focusing on the effects of nanoparticles on the properties of cells and the use of biomaterials for bone regeneration.

Cytokine Imbalance as a Biomarker of Intervertebral Disk Degeneration

The intervertebral disk degeneration (IDD) and its associated conditions are an important problem in modern medicine. The onset of IDD may be in childhood and adolescence in patients with a genetic predisposition. IDD progresses with age, leading to spondylosis, spondylarthrosis, intervertebral disk herniation, and spinal stenosis. The purpose of this review is an attempt to summarize the data characterizing the patterns of production of pro-inflammatory and anti-inflammatory cytokines in IDD and to appreciate the prognostic value of cytokine imbalance as its biomarker. This narrative review demonstrates that the problem of evaluating the contribution of pro-inflammatory and anti-inflammatory cytokines to the maintenance or alteration of cytokine balance may be a new key to unlocking the mystery of IDD development and new therapeutic strategies for the treatment of IDD in the setting of acute and chronic inflammation. The presented data support the hypothesis that cytokine imbalance is one of the most important biomarkers of IDD.

Magnesium incorporation in fibrinogen scaffolds promotes macrophage polarization towards M2 phenotype

The host inflammatory response to biomaterials conditions their capacity to promote tissue repair, and macrophage polarization shift from M1 to M2 is determinant in this process. Previous work showed that extracts of a combination between fibrinogen and metallic magnesium materials acted synergistically to reduce macrophage inflammatory phenotype. The hypothesis underlying the current work was that the ability of magnesium-modified fibrinogen scaffolds to modulate macrophage phenotype depends on the concentration of magnesium. Thus, Fibrinogen (Fg) scaffolds incorporating precise concentrations of magnesium sulfate (Mg: 0, 10, 25, 50 mM) were developed and characterized. Mg incorporation in Fg scaffolds increased surface charge, while porosity decreased with increasing Mg concentrations, but only Fg scaffolds with 10 mM of Mg (FgMg10) had significantly improved mechanical properties. Human macrophages cultured on FgMg10 scaffolds, showed increased M2 and decreased M1 polarization, when compared to those cultured on scaffolds with 0, 25 and 50 mM of Mg. Macrophage polarization results were independent of the anion used (chloride or sulfate). Macrophage modulation by FgMg10 scaffolds involved reduced NF-κB p65 nuclear translocation, and impacted production of pro-inflammatory mediators (e.g. IFNγ, IL-12, TNF-⍺, IP-10). Importantly, FgMg10 scaffolds implanted in vivo increased the expression of M2 marker CD163, in macrophages from inflammatory exudates, compared to Sham and Fg-implanted animals, increasing the M2:M1 ratio. A cytokine/chemokine array showed that, while both Fg and FgMg10 scaffolds decreased inflammatory mediators, only FgMg10 decreased IL-1β, IP-10, MIP-2, MDC and MIP-3⍺, compared to Sham-operated animals. This study demonstrated that incorporation of 10mM of Mg modulated inflammation, promoting M2 macrophage polarization in vitro and in vivo. STATEMENT OF SIGNIFICANCE: Developing biomaterials that can modulate inflammation and promote macrophage phenotype switch from M1 to M2 is crucial to promote a regenerative microenvironment. Our previous work showed that extracts of a combination between fibrinogen (Fg) and metallic magnesium (Mg) materials synergistically reduced macrophage pro-inflammatory phenotype. Herein, we tested the hypothesis that macrophage modulation was dependent on Mg concentration. A new family of Fg porous scaffolds incorporating different amounts of Mg (0, 10, 25 and 50 mM) was produced and characterized. We observed that only the combination of Fg scaffolds with 10 mM of Mg (FgMg10) significantly changed the scaffolds mechanical properties and directed macrophages towards a M2 phenotype, reducing the production of inflammatory mediators, both in vitro and in vivo.

Composition, host responses and clinical applications of bioadhesives

Bioadhesives are medical devices used to join or seal tissues that have been injured or incised. They have been classified into tissue adhesives, sealants, and hemostatic agents. Bioadhesives such as FloSeal®, CoSeal®, BioGlue®, Evicel®, Tisseel®, Progel™ PALS, and TissuGlu® have been commercialized and used in clinical setting. They can be formulated with natural or synthetic components or a combination of both including albumin, glutaraldehyde, chitosan, cyanoacrylate, fibrin and thrombin, gelatin, polyethylene glycol (PEG), along with urethanes. Each formulation has intrinsic properties and has been developed and validated for a specific application. This review article briefs the mechanisms by which bioadhesives forms adhesion to tissues and highlights the correlation between bioadhesives composition and their potential host responses. Furthermore, clinical applications of bioadhesives and their application-driven requirements are outlined.

Chitosan coatings with distinct innate immune bioactivities differentially stimulate angiogenesis, osteogenesis and chondrogenesis in poly-caprolactone scaffolds with controlled interconnecting pore size

This study tested whether osseous integration into poly (ε-caprolactone) (PCL) bioplastic scaffolds with fully-interconnecting 155 ± 8 μm pores is enhanced by an adhesive, non-inflammatory 99% degree of deacetylation (DDA) chitosan coating (99-PCL), or further incorporation of pro-inflammatory 83% DDA chitosan microparticles (83-99-PCL) to accelerate angiogenesis. New Zealand White rabbit osteochondral knee defects were press-fit with PCL, 99-PCL, 83-99-PCL, or allowed to bleed (drill-only). Between day 1 and 6 weeks of repair, drill-only defects repaired by endochondral ossification, with an 8-fold higher bone volume fraction (BVF) versus initial defects, compared to a 2-fold (99-PCL), 1.1-fold (PCL), or 0.4-fold (83-99-PCL) change in BVF. Hematoma innate immune cells swarmed to 83-99-PCL, elicited angiogenesis throughout the pores and induced slight bone resorption. PCL and 99-PCL pores were variably filled with cartilage or avascular mesenchyme near the bone plate, or angiogenic mesenchyme into which repairing trabecular bone infiltrated up to 1 mm deep. More repair cartilage covered the 99-PCL scaffold (65%) than PCL (18%) or 83-99-PCL (0%) (p < 0.005). We report the novel finding that non-inflammatory chitosan coatings promoted cartilage infiltration into and over a bioplastic scaffold, and were compatible with trabecular bone integration. This study also revealed that in vitro osteogenesis assays have limited ability to predict osseous integration into porous scaffolds, because (1) in vivo, woven bone integrates from the leading edge of regenerating trabecular bone and not from mesenchymal cells adhering to scaffold surfaces, and (2) bioactive coatings that attract inflammatory cells induce bone resorption.

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Porous polycaprolactone scaffolds elicited drawn-out osteochondral wound repair.

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Regenerating trabecular bone only infiltrated angiogenic mesenchyme free of inflammatory cells.

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83% DDA chitosan stimulated sterile inflammatory angiogenesis and trabecular bone resorption.

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99% DDA chitosan coatings promoted chondrogenesis inside and over the PCL articular surface.

Maturation of biomimetic hydroxyapatite in physiological fluids: a physicochemical and proteomic study

Biomimetic calcium-deficient hydroxyapatite (CDHA) as a bioactive material exhibits exceptional intrinsic osteoinductive and osteogenic properties because of its nanostructure and composition, which promote a favorable microenvironment. Its high reactivity has been hypothesized to play a relevant role in the in vivo performance, mediated by the interaction with the biological fluids, which is amplified by its high specific surface area. Paradoxically, this high reactivity is also behind the in vitro cytotoxicity of this material, especially pronounced in static conditions. The present work explores the structural and physicochemical changes that CDHA undergoes in contact with physiological fluids and to investigate its interaction with proteins. Calcium-deficient hydroxyapatite discs with different micro/nanostructures, coarse (C) and fine (F), were exposed to cell-free complete culture medium over extended periods of time: 1, 7, 14, 21, 28, and 50 days. Precipitate formation was not observed in any of the materials in contact with the physiological fluid, which would indicate that the ionic exchanges were linked to incorporation into the crystal structure of CDHA or in the hydrated layer. In fact, CDHA experienced a maturation process, with a progressive increase in crystallinity and the Ca/P ratio, accompanied by an uptake of Mg and a B-type carbonation process, with a gradual propagation into the core of the samples. However, the reactivity of biomimetic hydroxyapatite was highly dependent on the specific surface area and was amplified in nanosized needle-like crystal structures (F), whereas in coarse specimens the ionic exchanges were restricted to the surface, with low penetration in the material bulk. In addition to showing a higher protein adsorption on F substrates, the proteomics study revealed the existence of protein selectivity toward F or C microstructures, as well as the capability of CDHA, and more remarkably of F-CDHA, to concentrate specific proteins from the culture medium. Finally, a substantial improvement in the material's ability to support cell proliferation was observed after the CDHA maturation process.

Topological structure of electrospun membrane regulates immune response, angiogenesis and bone regeneration

The fate of biomaterials is orchestrated by biocompatibility and bioregulation characteristics, reported to be closely related to topographical structures. For the purpose to investigate the topography of fibrous membranes on the guided bone regeneration performance, we successfully fabricated poly (lactate-co-glycolate)/fish collagen/nano-hydroxyapatite (PFCH) fibrous membranes with random, aligned and latticed topography by electrospinning. The physical, chemical and biological properties of the three topographical PFCH membranes were systematically investigated by in vitro and in vivo experiments. The subcutaneous implantation of C57BL6 mice showed an acceptable mild foreign body reaction of all three topological membranes. Interestingly, the latticed PFCH membrane exhibited superior abilities to recruit macrophage/monocyte and induce angiogenesis. We further investigated the osteogenesis of the three topographical PFCH membranes via the critical-size calvarial bone defect model of rats and mice and the results suggested that latticed PFCH membrane manifested promising performance to promote angiogenesis through upregulation of the HIF-1α signaling pathway; thereby enhancing bone regeneration. Our research illustrated that the topological structure of fibrous membranes, as one of the characteristics of biomaterials, could regulate its biological functions, and the fibrous structure of latticed topography could serve as a favorable surface design of biomaterials for bone regeneration. STATEMENT OF SIGNIFICANCE: In material-mediated regeneration medicine, the interaction between the biomaterial and the host is key to successful tissue regeneration. The micro-and nano-structure becomes one of the most critical physical clues for designing biomaterials. In this study, we fabricated three topological electrospun membranes (Random, Aligned and Latticed) to understand how topological structural clues mediate bone tissue regeneration. Interestingly, we found that the Latticed topographical PFCH membrane promotes macrophage recruitment, angiogenesis, and osteogenesis in vivo, indicating the fibrous structure of latticed topography could serve as a favorable surface design of biomaterials for bone regeneration.

Moderate mechanical stimulation rescues degenerative annulus fibrosus by suppressing caveolin-1 mediated pro-inflammatory signaling pathway

Mechanical loading can induce or antagonize the extracellular matrix (ECM) synthesis, proliferation, migration, and inflammatory responses of annulus fibrosus cells (AFCs), depending on the loading mode and level. Caveolin-1 (Cav1), the core protein of caveolae, plays an important role in cellular mechanotransduction and inflammatory responses. In the present study, we presented that AFCs demonstrated different behaviors when subjected to cyclic tensile strain (CTS) for 24 h at a magnitude of 0%, 2%, 5% and 12%, respectively. It was found that 5% CTS had positive effects on cell proliferation, migration and anabolism, while 12% CTS had the opposite effects. Besides, cells exposed to interleukin-1β stimulus exhibited an increase expression in inflammatory genes, and the expression of these genes decreased after exposure to moderate mechanical loading with 5% CTS. In addition, 5% CTS decreased the level of Cav1 and integrin β1 and exhibited anti-inflammatory effects. Moreover, the expression of integrin β1 and p-p65 increased in AFCs transfected with Cav1 plasmids. In vivo results revealed that moderate mechanical stimulation could recover the water content and morphology of the discs. In conclusion, moderate mechanical stimulation restrained Cav1-mediated signaling pathway and exhibited anti-inflammatory effects on AFCs. Together with in vivo results, this study expounds the underlying molecular mechanisms on the effect of moderate mechanical stimulation on intervertebral discs (IVDs) and may provide a new therapeutic strategy for the treatment of IVD degeneration.

Biomaterials Regulating Bone Hematoma for Osteogenesis

Blood coagulation in tissue healing not only prevents blood loss, but also forms a natural scaffold for tissue repair and regeneration. As blood clot formation is the initial and foremost phase upon bone injury, and the quality of blood clot (hematoma) orchestrates the following inflammatory and cellular processes as well as the subsequent callus formation and bone remodeling process. Inspired by the natural healing hematoma, tissue-engineered biomimic scaffold/hydrogels and blood prefabrication strategies attract significant interests in developing functional bone substitutes. The alteration of the fracture hematoma ca significantly accelerate or impair the overall bone healing process. This review summarizes the impact of biomaterials on blood coagulation and provides evidence on fibrin network structure, growth factors, and biomolecules that contribute to bone healing within the hematoma. The aim is to provide insights into the development of novel implant and bone biomaterials for enhanced osteogenesis. Advances in the understanding of biomaterial characteristics (e.g., morphology, chemistry, wettability, and protein adsorption) and their effect on hematoma properties are highlighted. Emphasizing the importance of the initial healing phase of the hematoma endows the design of advanced biomaterials with the desired regulatory properties for optimal coagulation and hematoma properties, thereby facilitating enhanced osteogenesis and ideal therapeutic effects.

Fibrinogen and magnesium combination biomaterials modulate macrophage phenotype, NF-kB signaling and crosstalk with mesenchymal stem/stromal cells

Macrophage behavior upon biomaterial implantation conditions the inflammatory response and subsequent tissue repair. The hypothesis behind this work was that fibrinogen (Fg) and magnesium (Mg) biomaterials, used in combination (FgMg) could act synergistically to modulate macrophage activation, promoting a pro-regenerative phenotype. Materials were characterized by scanning electron microscopy, Fg and Mg degradation products were quantified by atomic absorption spectroscopy and ELISA. Whole blood immune cells and primary human monocyte-derived macrophages were exposed to the biomaterials extracts in unstimulated (M0) or pro-inflammatory LPS or LPS-IFNγ (M1) conditions. Macrophage phenotype was evaluated by flow cytometry, cytokines secreted by whole blood cells and macrophages were measured by ELISA, and signaling pathways were probed by Western blotting. The secretomes of macrophages preconditioned with biomaterials extracts were incubated with human mesenchymal stem/stromal cells (MSC) and their effect on osteogenic differentiation was evaluated via Alkaline Phosphatase (ALP) activity and alizarin red staining. Scaffolds of Fg, alone or in the FgMg combination, presented similar 3D porous architectures. Extracts from FgMg materials reduced LPS-induced TNF-α secretion by innate immune cells, and macrophage M1 polarization upon LPS-IFNγ stimulation, resulting in lower cell surface CD86 expression, lower NFκB p65 phosphorylation and reduced TNF-α secretion. Moreover, while biomaterial extracts per se did not enhance MSC osteogenic differentiation, macrophage secretome, particularly from cells exposed to FgMg extracts, increased MSC ALP activity and alizarin red staining, compared with extracts alone. These findings suggest that the combination of Fg and Mg synergistically influences macrophage pro-inflammatory activation and crosstalk with MSC. STATEMENT OF SIGNIFICANCE: Modulating macrophage phenotype by degradable and bioactive biomaterials is an increasingly explored strategy to promote tissue repair/regeneration. Fibrinogen (Fg) and magnesium (Mg)-based materials have been explored in this context. Previous work from our group showed that monocytes interact with fibrinogen adsorbed onto chitosan surfaces through TLR4 and that fibrinogen scaffolds promote in vivo bone regeneration. Also, magnesium ions have been reported to modulate macrophage pro-inflammatory M1 stimulation and to promote bone repair. Here we report, for the first time, the combination of Fg and Mg materials, hypothesizing that it could act synergistically on macrophages, directing them towards a pro-regenerative phenotype. As a first step towards proving/disproving our hypothesis we used extracts obtained from Fg, Mg and FgMg multilayer constructs. We observed that FgMg extracts led to a reduction in the polarization of macrophages towards a pro-inflammatory phenotype. Also, the secretome of macrophages exposed to extracts of the combination material promoted the expression of osteogenic markers by MSCs.

Vascularization Strategies in the Prevention of Nonunion Formation

Delayed healing and nonunion formation are major challenges in orthopedic surgery, which require the development of novel treatment strategies. Vascularization is considered one of the major prerequisites for successful bone healing, providing an adequate nutrient supply and allowing the infiltration of progenitor cells to the fracture site. Hence, during the last decade, a considerable number of studies have focused on the evaluation of vascularization strategies to prevent or to treat nonunion formation. These involve (1) biophysical applications, (2) systemic pharmacological interventions, and (3) tissue engineering, including sophisticated scaffold materials, local growth factor delivery systems, cell-based techniques, and surgical vascularization approaches. Accumulating evidence indicates that in nonunions, these strategies are indeed capable of improving the process of bone healing. The major challenge for the future will now be the translation of these strategies into clinical practice to make them accessible for the majority of patients. If this succeeds, these vascularization strategies may markedly reduce the incidence of nonunion formation. Impact statement Delayed healing and nonunion formation are a major clinical problem in orthopedic surgery. This review provides an overview of vascularization strategies for the prevention and treatment of nonunions. The successful translation of these strategies in clinical practice is of major importance to achieve adequate bone healing.

Preparation and characterization of Jagged1-bound fibrinogen-based microspheres and their cytotoxicity against human dental pulp cells

Surface immobilization of Jagged1 promotes odonto/osteogenic differentiation in human dental pulp cells. On the contrary, soluble Jagged1 fails to activate target gene expression of Notch signaling which is important for differentiation of human dental pulp cells. Hence, Jagged1 delivery system is indeed required for transportation of immobilized Jagged1 to promote odontogenic differentiation of human dental pulp cells in vivo. The present study described the preparation and characterization of Jagged1-bound fibrinogen-based microspheres. Water-in-oil emulsion technique was employed to prepare fibrinogen microspheres and thrombin cross-linked fibrinogen microspheres. The average size of fibrinogen microspheres and thrombin cross-linked fibrinogen microspheres was 213.9 ± 35.9 and 199.9 ± 41.9 µm, respectively. These microspheres did not alter the human dental pulp cells’ cell viability. Human dental pulp cells were able to attach and spread on these microspheres. Jagged1 was conjugated on microspheres using 1-ethyl-3-(3-dimethylamino) propyl carbodiimide/N-hydroxysuccinimide. Binding capacity of Jagged1 on both fibrinogen microspheres and thrombin cross-linked fibrinogen microspheres ranged from 25.8 ± 6.0 to 35.6 ± 9.1%. There was no significant difference in the size of microspheres between before and after Jagged1 conjugation process. In conclusion, fibrinogen microspheres and thrombin cross-linked fibrinogen microspheres could be utilized as the alternative biomaterials for Jagged1 delivery for future biomedical application.

Osteoclasts degrade fibrinogen scaffolds and induce mesenchymal stem/stromal osteogenic differentiation

Fibrinogen (Fg) is a pro-inflammatory protein with pro-healing properties. Previous work showed that fibrinogen 3D scaffolds (Fg-3D) promote bone regeneration, but the cellular players were not identified. Osteoclasts are bone resorbing cells that promote bone remodeling in close crosstalk with osteoblasts. Herein, the capacity of osteoclasts differentiated on Fg-3D to degrade the scaffolds and promote osteoblast differentiation was evaluated in vitro. Fg-3D scaffolds were prepared by freeze-drying and osteoclasts were differentiated from primary human peripheral blood monocytes. Results obtained showed osteoclasts expressing the enzymes cathepsin K and tartrate resistant acid phosphatase colonizing Fg-3D scaffolds. Osteoclasts were able to significantly degrade Fg-3D, reducing the scaffold's area, and increasing D-dimer concentration, a Fg degradation product, in their culture media. Osteoclast conditioned media from the first week of differentiation promoted significantly stronger human primary mesenchymal stem/stromal cell (MSC) osteogenic differentiation, evaluated by alkaline phosphatase activity. Moreover, week 1 osteoclast conditioned media promoted earlier MSC osteogenic differentiation, than chemical osteogenesis inductors. TGF-β1 was found increased in osteoclast conditioned media from week 1, when compared to week 3 of differentiation. Taken together, our results suggest that osteoclasts are able to differentiate and degrade Fg-3D, producing factors like TGF-β1 that promote MSC osteogenic differentiation.

Activating macrophages for enhanced osteogenic and bactericidal performance by Cu ion release from micro/nano-topographical coating on a titanium substrate

In the field of orthopaedics, inflammation-modulatory biomaterials are receiving increasing attentions due to their abilities to regulate innate immune response and mediate wound healing. In the current work, a Cu-containing micro/nano-topographical bio-ceramic surface (Cu-Hier-Ti surface) was employed as material model to explore the role played by Cu²⁺ release or material surface in regulating macrophage polarization as well as macrophage-mediated osteogenic and bactericidal effect. A Cu-free micro-topographical surface (Micro-Ti surface) generated by micro-arc oxidation was employed as control. The results showed that Cu²⁺ supplemented directly into the culture medium or released from Cu-Hier-Ti surface could polarize macrophages to pro-inflammatory M1 phenotype by activating Cu-transport signaling (copper transporter 1 (CTR1) and ATP7A) in macrophages, while the material characteristics exhibited anti-inflammatory effect to some extent by regulating integrin (α5, αM, β1 and β2) and TLR (TLR-3, TLR-4, Myd88 and Ticam-1/2) signaling. Macrophages grown on Cu-Hier-Ti surface or treated by Cu²⁺ could create a favorable inflammatory microenvironment for osteoblast-like SaOS-2 cell proliferation and differentiation. Moreover, Cu-Hier-Ti surface promoted macrophage capacity to engulf and kill bacteria, even though it did not show direct bactericidal effect against Staphylococcus aureus. In vivo results showed that Cu-Hier-Ti surface could lead to promoted osteointegraion and enhanced expression levels of M1 surface marker CD11c, growth factor BMP-6 and osteogenic makers including osteocalcin (OCN) and Runx-2 at the biomaterial/bone tissue interface in a rat model. The results indicate that Cu could be employed as a promising inflammation-modulatory agent to activate macrophages for enhanced osteogenic and bactericidal effect. STATEMENT OF SIGNIFICANCE: The next generation of bone biomaterials should be active to regulate the local inflammatory environment such that it favors bone regeneration. For the design and development of Cu-containing inflammation-modulatory biomaterials, it is of great importance to recognize the exact role played by Cu²⁺ release or material surface characteristics. So far, relatively little is known about the regulatory role of Cu²⁺ or micro/nano-topographical surface on macrophages. The results in the current work suggest that Cu²⁺ release and material surface characteristics of Cu-containing micro/nano-topographical coating could activate distinct signaling pathways in macrophages. The activated M1 macrophages exhibited stimulatory effect on osteoblast maturation and enhanced bactericidal capacity against Staphylococcus aureus. This study might provide new thoughts for the development of multi-functional Cu-containing biomaterials.

Scaffold strategies for modulating immune microenvironment during bone regeneration

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

The Systemic Immune Response to Collagen-Induced Arthritis and the Impact of Bone Injury in Inflammatory Conditions

Rheumatoid arthritis (RA) is a systemic disease that affects the osteoarticular system, associated with bone fragility and increased risk of fractures. Herein, we aimed to characterize the systemic impact of the rat collagen-induced arthritis (CIA) model and explore its combination with femoral bone defect (FD). The impact of CIA on endogenous mesenchymal stem/stromal cells (MSC) was also investigated. CIA induction led to enlarged, more proliferative, spleen and draining lymph nodes, with altered proportion of lymphoid populations. Upon FD, CIA animals increased the systemic myeloid cell proportions, and their expression of co-stimulatory molecules CD40 and CD86. Screening plasma cytokine/chemokine levels showed increased tumor necrosis factor-α (TNF-α), Interleukin (IL)-17, IL-4, IL-5, and IL-12 in CIA, and IL-2 and IL-6 increased in CIA and CIA+FD, while Fractalkine and Leptin were decreased in both groups. CIA-derived MSC showed lower metabolic activity and proliferation, and significantly increased osteogenic and chondrogenic differentiation markers. Exposure of control-MSC to TNF-α partially mimicked the CIA-MSC phenotype in vitro. In conclusion, inflammatory conditions of CIA led to alterations in systemic immune cell proportions, circulating mediators, and in endogenous MSC. CIA animals respond to FD, and the combined model can be used to study the mechanisms of bone repair in inflammatory conditions.

Extracellular Matrices to Modulate the Innate Immune Response and Enhance Bone Healing

Extracellular matrices (ECMs) have emerged as promising off-the-shelf products to induce bone regeneration, with the capacity not only to activate osteoprogenitors, but also to influence the immune response. ECMs generated starting from living cells such as mesenchymal stromal cells (MSCs) have the potential to combine advantages of native tissue-derived ECMs (e.g., physiological presentation of multiple regulatory factors) with those of synthetic ECMs (e.g., customization and reproducibility of composition). MSC-derived ECMs could be tailored by enrichment not only in osteogenic cytokines, but also in immunomodulatory factors, to skew the innate immune response toward regenerative processes. After reviewing the different immunoregulatory properties of ECM components, here we propose different approaches to engineer ECMs enriched in factors capable to regulate macrophage polarization, recruit host immune and mesenchymal cells, and stimulate the synthesis of other immunoinstructive cytokines. Finally, we offer a perspective on the possible evolution of the paradigm based on biological and chemico-physical design considerations, and the use of gene editing approaches.

Novel chitosan/agarose/hydroxyapatite nanocomposite scaffold for bone tissue engineering applications: comprehensive evaluation of biocompatibility and osteoinductivity with the use of osteoblasts and mesenchymal stem cells

Background

Nanocomposites produced by reinforcement of polysaccharide matrix with nanoparticles are widely used in engineering of biomaterials. However, clinical applications of developed novel biomaterials are often limited due to their poor biocompatibility.

Purpose

The aim of this work was to comprehensively assess biocompatibility of highly macroporous chitosan/agarose/nanohydroxyapatite bone scaffolds produced by a novel method combining freeze-drying with a foaming agent. Within these studies, blood plasma protein adsorption, osteoblast (MC3T3-E1 Subclone 4 and hFOB 1.19) adhesion and proliferation, and osteogenic differentiation of mesenchymal stem cells derived from bone marrow and adipose tissue were determined. The obtained results were also correlated with materials' surface chemistry and wettability to explain the observed protein and cellular response.

Results

Obtained results clearly showed that the developed nanocomposite scaffolds were characterized by high biocompatibility and osteoconductivity. Importantly, the scaffolds also revealed osteoinductive properties since they have the ability to induce osteogenic differentiation (Runx2 synthesis) in undifferentiated mesenchymal stem cells. The surface of biomaterials is extremely hydrophilic, prone to protein adsorption with the highest affinity toward fibronectin binding, which allows for good osteoblast adhesion, spreading, and proliferation.

Conclusion

Produced by a novel method, macroporous nanocomposite biomaterials have great potential to be used in regenerative medicine for acceleration of the bone healing process.

3D chitosan scaffolds impair NLRP3 inflammasome response in macrophages

Chitosan (Ch) is used in different biomedical applications to promote tissue repair. However, tissue injury caused by biomaterial implantation lead to the release of danger signals that engage different inflammatory pathways on the host. Different implanted materials activate the inflammasome leading to the modulation of the immune response. Here we have studied how macroscopic biomaterials, Ch scaffolds with different chemical composition: 4% or 15% degree of acetylation (DA) modulate the activation of the NLRP3 inflammasome in vitro. For that, we assessed the NLRP3 inflammasome in bone marrow derived mouse macrophages (BMDM) and human macrophages cultured within 3D Ch scaffolds. We found that both Ch scaffolds did not trigger the NLRP3 inflammasome activation in macrophages. Furthermore, BMDMs and human macrophages cultured in both Ch scaffolds presented a reduction in the number of apoptosis-associated speck-like protein containing a caspase activating recruitment domain (ASC) specks and in IL-1β release upon classical NLRP3 inflammasome stimulation. We also found a decrease in proIL-1β in BMDMs after priming with LPS when cultured in Ch scaffolds with DA 4% DA after priming with LPS when compared to Ch scaffolds with 15% DA or to macrophages cultured in cell-culture plates. Our results shows that 3D Ch scaffolds with different DA impair NLRP3 inflammasome priming and activation. STATEMENT OF SIGNIFICANCE: In this research work we have assessed the role of the NLRP3 inflammasome in the macrophage response to 3D chitosan scaffolds with different degrees of acetylation (DA). To our knowledge this is the first work that demonstrates the modulatory capacity of 3D porous chitosan scaffolds in the NLRP3 inflammasome activation, because our results show that Ch scaffolds impair NLRP3 inflammasome assembly in macrophages. Interestingly, our results are in contrast with studies reported in the literature that indicate that chitosan is a powerful activator of the NLRP3 inflammasome in nanoscale chitosan products. Our studies that were performed in large scale chitosan scaffolds, stress out that the process of phagocytosis is pivotal in inflammasome assembly and activation, are rather important since they clearly illustrate the different role of the inflammasome in the biological response to large scale and nanoscale biomaterials.

Poly (3-hydroxybutyrate-co-3-hydroxyvalerate)/fibrinogen/bredigite nanofibrous membranes and their integration with osteoblasts for guided bone regeneration

Guided bone regeneration (GBR) has been established to be an effective method for the repair of defective tissues, which is based on isolating bone defects with a barrier membrane for faster tissue reconstruction. The aim of the present study is to develop poly (hydroxybutyrate-co-3-hydroxyvalerate) (PHBV)/fibrinogen (FG)/bredigite (BR) membranes with applicability in GBR. BR nanoparticles were synthesized through a sol-gel method and characterized using transmission electron microscopy and X-ray diffractometer. PHBV, PHBV/FG, and PHBV/FG/BR membranes were fabricated using electrospinning and characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, contact angle, pore size, thermogravimetric analysis and tensile strength. The electrospun PHBV, PHBV/FG, and PHBV/FG/BR nanofibers were successfully obtained with the mean diameter ranging 240-410 nm. The results showed that Young's modulus and ultimate strength of the PHBV membrane reduced upon blending with FG and increased by further incorporation of BR nanoparticles, Moreover hydrophilicity of the PHBV membrane improved on addition of FG and BR. The in vitro degradation assay demonstrated that incorporation of FG and BR into PHBV matrix increased its hydrolytic degradation. Cell-membrane interactions were studied by culturing human fetal osteoblast cells on the fabricated membrane. According to the obtained results, osteoblasts seeded on PHBV/FG/BR displayed higher cell adhesion and proliferation compared to PHBV and PHBV/FG membrane. Furthermore, alkaline phosphatase activity and alizarin red-s staining indicated enhanced osteogenic differentiation and mineralization of cells on PHBV/FG/BR membranes. The results demonstrated that developed electrospun PHBV/FG/BR nanofibrous mats have desired potential as a barrier membrane for guided bone tissue engineering. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1154-1165, 2019.

Functionalized cell-free scaffolds for bone defect repair inspired by self-healing of bone fractures: A review and new perspectives

Studies have demonstrated that scaffolds, a component of bone tissue engineering, play an indispensable role in bone repair. However, these scaffolds involving ex-vivo cultivated cells seeded have disadvantages in clinical practice, such as limited autologous cells, time-consuming cell expansion procedures, low survival rate and immune-rejection issues. To overcome these disadvantages, recent focus has been placed on the design of functionalized cell-free scaffolds, instead of cell-seeded scaffolds, that can reduplicate the natural self-healing events of bone fractures, such as inflammation, cell recruitment, vascularization, and osteogenic differentiation. New approaches and applications in tissue engineering and regenerative medicine continue to drive the development of functionalized cell-free scaffolds for bone repair. In this review, the self-healing processes were highlighted, and approaches for the functionalization were summarized. Also, ongoing efforts and breakthroughs in the field of functionalization for bone defect repair were discussed. Finally, a brief summery and new perspectives for functionalization strategies were presented to provide guidelines for further efforts in the design of bioinspired cell-free scaffolds.

The summary of the most important cell-biomaterial interactions that need to be considered during in vitro biocompatibility testing of bone scaffolds for tissue engineering applications

Tissue engineered products (TEPs), which mean biomaterials containing either cells or growth factors or both cells and growth factors, may be used as an alternative to the autografts taken directly from the bone of the patients. Nevertheless, the use of TEPs needs much more understanding of biointeractions between biomaterials and eukaryotic cells. Despite the possibility of the use of in vitro cellular models for initial evaluation of the host response to the implanted biomaterial, it is observed that most researchers use cell cultures only for the evaluation of cytotoxicity and cell proliferation on the biomaterial surface, and then they proceed to animal models and in vivo testing of bone implants without fully utilizing the scientific potential of in vitro models. In this review, the most important biointeractions between eukaryotic cells and biomaterials were discussed, indicating molecular mechanisms of cell adhesion, proliferation, and biomaterial-induced activation of immune cells. The article also describes types of cellular models which are commonly used for biomaterial testing and highlights the possibilities and drawbacks of in vitro tests for biocompatibility evaluation of novel scaffolds. Finally, the review summarizes recent findings concerning the use of adult mesenchymal stem cells for TEP generation and compares the potential of bone marrow- and adipose tissue-derived stem cells in regenerative medicine applications.

Profiling the circulating miRnome reveals a temporal regulation of the bone injury response

Bone injury healing is an orchestrated process that starts with an inflammatory phase followed by repair and remodelling of the bone defect. The initial inflammation is characterized by local changes in immune cell populations and molecular mediators, including microRNAs (miRNAs). However, the systemic response to bone injury remains largely uncharacterized. Thus, this study aimed to profile the changes in the plasma miRnome after bone injury and determine its biological implications. Methods: A rat model of femoral bone defect was used, and animals were evaluated at days 3 and 14 after injury. Non-operated (NO) and sham operated animals were used as controls. Blood and spleen were collected and peripheral blood mononuclear cells (PBMC) and plasma were separated. Plasma miRnome was determined by RT-qPCR array and bioinformatics Ingenuity pathway analysis (IPA) was performed. Proliferation of bone marrow mesenchymal stem/stromal cells (MSC) was evaluated by Ki67 staining and high-throughput cell imaging. Candidate miRNAs were evaluated in splenocytes by RT-qPCR, and proteins found in the IPA analysis were analysed in splenocytes and PBMC by Western blot. Results: Bone injury resulted in timely controlled changes to the miRNA expression profile in plasma. At day 3 there was a major down-regulation of miRNA levels, which was partially recovered by day 14 post-injury. Interestingly, bone injury led to a significant up-regulation of let-7a, let-7d and miR-21 in plasma and splenocytes at day 14 relative to day 3 after bone injury, but not in sham operated animals. IPA predicted that most miRNAs temporally affected were involved in cellular development, proliferation and movement. MSC proliferation was analysed and found significantly increased in response to plasma of animals days 3 and 14 post-injury, but not from NO animals. Moreover, IPA predicted that miRNA processing proteins Ago2 and Dicer were specifically inhibited at day 3 post-injury, with Ago2 becoming activated at day 14. Protein levels of Ago2 and Dicer in splenocytes were increased at day 14 relative to day 3 post-bone injury and NO animals, while in PBMC, levels were reduced at day 3 (albeit Dicer was not significant) and remained low at day 14. Ephrin receptor B6 followed the same tendency as Ago2 and Dicer, while Smad2/3 was significantly decreased in splenocytes from day 14 relative to NO and day 3 post-bone injury animals. Conclusion: Results show a systemic miRNA response to bone injury that is regulated in time and is related to inflammation resolution and the start of bone repair/regeneration, unravelling candidate miRNAs to be used as biomarkers in the monitoring of healthy bone healing and as therapeutic targets for the development of improved bone regeneration therapies.

Chitosan porous 3D scaffolds embedded with resolvin D1 to improve in vivo bone healing

The aim of this study was to investigate the effect chitosan (Ch) porous 3D scaffolds embedded with resolvin D1 (RvD1), an endogenous pro-resolving lipid mediator, on bone tissue healing. These scaffolds previous developed by us have demonstrated to have immunomodulatory properties namely in the modulation of the macrophage inflammatory phenotypic profile in an in vivo model of inflammation. Herein, results obtained in an in vivo rat femoral defect model demonstrated that two months after Ch + RvD1 scaffolds implantation, an increase in new bone formation, in bone trabecular thickness, and in collagen type I and Coll I/Coll III ratio were observed. These results suggest that Ch scaffolds embedded with RvD1 were able to lead to the formation of new bone with improvement of trabecular thickness. This study shows that the presence of RvD1 in the acute phase of the inflammatory response to the implanted biomaterial had a positive role in the subsequent bone tissue repair, thus demonstrating the importance of innovative approaches for the control of immune responses to biomedical implants in the design of advanced strategies for regenerative medicine. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 1626-1633, 2018.

A co-culture system with three different primary human cell populations reveals that biomaterials and MSC modulate macrophage-driven fibroblast recruitment

The biological response to implanted biomaterials is a complex and highly coordinated phenomenon involving many different cell types that interact within 3D microenvironments. Here, we increased the complexity of a 3D platform to include at least 3 cell types that play a role in the host response upon scaffold implantation. With this system, it was possible to address how immune responses triggered by 3D biomaterials mediate recruitment of stromal cells that promote tissue regeneration, mesenchymal stromal/stem cells (MSC), or a foreign body response, fibroblasts. Primary human macrophages yielded the highest fibroblast recruitment when interacting with chitosan scaffolds but not polylactic acid. Interestingly, when there were MSC and fibroblasts in the same environment, macrophages in chitosan scaffolds again promoted a significant increase on fibroblast recruitment, but not of MSC. However, macrophages that were firstly allowed to interact with MSC within the scaffolds were no longer able to recruit fibroblasts. This study illustrates the potential to use different scaffolds to regulate the dynamics of recruitment of proregenerative or fibrotic cell types through immunomodulation. Overall, this work strengths the idea that ex vivo predictive systems need to consider the different players involved in the biological response to biomaterials and that timing of arrival of specific cell types will affect the outcome.

Macrophage interactions with polylactic acid and chitosan scaffolds lead to improved recruitment of human mesenchymal stem/stromal cells: a comprehensive study with different immune cells

Despite the importance of immune cell-biomaterial interactions for the regenerative outcome, few studies have investigated how distinct three-dimensional biomaterials modulate the immune cell-mediated mesenchymal stem/stromal cells (MSC) recruitment and function. Thus, this work compares the response of varied primary human immune cell populations triggered by different model scaffolds and describes its functional consequence on recruitment and motility of bone marrow MSC. It was found that polylactic acid (PLA) and chitosan scaffolds lead to an increase in the metabolic activity of macrophages but not of peripheral blood mononuclear cells (PBMC), natural killer (NK) cells or monocytes. PBMC and NK cells increase their cell number in PLA scaffolds and express a secretion profile that does not promote MSC recruitment. Importantly, chitosan increases IL-8, MIP-1, MCP-1 and RANTES secretion by macrophages while PLA stimulates IL-6, IL-8 and MCP-1 production, all chemokines that can lead to MSC recruitment. This secretion profile of macrophages in contact with biomaterials correlates with the highest MSC invasion. Furthermore, macrophages enhance stem cell motility within chitosan scaffolds by 44% but not in PLA scaffolds. Thus, macrophages are the cells that in contact with engineered biomaterials become activated to secrete bioactive molecules that stimulate MSC recruitment.

A review of fibrin and fibrin composites for bone tissue engineering

Tissue engineering has emerged as a new treatment approach for bone repair and regeneration seeking to address limitations associated with current therapies, such as autologous bone grafting. While many bone tissue engineering approaches have traditionally focused on synthetic materials (such as polymers or hydrogels), there has been a lot of excitement surrounding the use of natural materials due to their biologically inspired properties. Fibrin is a natural scaffold formed following tissue injury that initiates hemostasis and provides the initial matrix useful for cell adhesion, migration, proliferation, and differentiation. Fibrin has captured the interest of bone tissue engineers due to its excellent biocompatibility, controllable biodegradability, and ability to deliver cells and biomolecules. Fibrin is particularly appealing because its precursors, fibrinogen, and thrombin, which can be derived from the patient's own blood, enable the fabrication of completely autologous scaffolds. In this article, we highlight the unique properties of fibrin as a scaffolding material to treat bone defects. Moreover, we emphasize its role in bone tissue engineering nanocomposites where approaches further emulate the natural nanostructured features of bone when using fibrin and other nanomaterials. We also review the preparation methods of fibrin glue and then discuss a wide range of fibrin applications in bone tissue engineering. These include the delivery of cells and/or biomolecules to a defect site, distributing cells, and/or growth factors throughout other pre-formed scaffolds and enhancing the physical as well as biological properties of other biomaterials. Thoughts on the future direction of fibrin research for bone tissue engineering are also presented. In the future, the development of fibrin precursors as recombinant proteins will solve problems associated with using multiple or single-donor fibrin glue, and the combination of nanomaterials that allow for the incorporation of biomolecules with fibrin will significantly improve the efficacy of fibrin for numerous bone tissue engineering applications.

Injectable hybrid system for strontium local delivery promotes bone regeneration in a rat critical-sized defect model

Strontium (Sr) has been described as having beneficial influence in bone strength and architecture. However, negative systemic effects have been reported on oral administration of Sr ranelate, leading to strict restrictions in clinical application. We hypothesized that local delivery of Sr improves osteogenesis without eliciting detrimental side effects. Therefore, the in vivo response to an injectable Sr-hybrid system composed of RGD-alginate hydrogel cross-linked in situ with Sr and reinforced with Sr-doped hydroxyapatite microspheres, was investigated. The system was injected in a critical-sized bone defect model and compared to a similar Sr-free material. Micro-CT results show a trend towards higher new bone formed in Sr-hybrid group and major histological differences were observed between groups. Higher cell invasion was detected at the center of the defect of Sr-hybrid group after 15 days with earlier bone formation. Higher material degradation with increase of collagen fibers and bone formation in the center of the defect after 60 days was observed as opposed to bone formation restricted to the periphery of the defect in the control. These histological findings support the evidence of an improved response with the Sr enriched material. Importantly, no alterations were observed in the Sr levels in systemic organs or serum.

Adsorbed Fibrinogen stimulates TLR-4 on monocytes and induces BMP-2 expression

Modulation of inflammatory responses to implanted biomaterials towards tissue regeneration has gained prominence as an innovative tissue engineering strategy. Recent in vitro and in vivo studies showed that Fibrinogen (Fg) adsorbed to Chitosan (Ch) substrates modulates immune cell responses, enhances the production of osteogenic factors by monocytes/macrophages and promotes bone regeneration, but the mechanisms involved remain poorly understood. Thus, the present work was conducted to clarify the molecular mechanisms of interaction between primary human monocytes and the above substrates. Cell surface expression of TLR-4 was significantly downregulated in the presence of pre-adsorbed Fg, when compared to Ch control, indicating an interaction via this receptor. The same substrate triggered MAPK activation, specifically the ERK 1/2 and JNK pathways. Importantly, both ERK 1/2 and JNK phosphorylation were reduced when TLR-4 signalling was blocked using a specific pharmacological inhibitor. Functionally, adsorbed Fg induced production of the potent osteogenic mediator BMP-2 by monocytes, while TLR-4 inhibition resulted in a significant decrease of BMP-2 mRNA and protein levels, in response to Fg stimulation. Overall, our data reveals that adsorbed Fg exerts a pro-osteogenic effect on human monocytes through its interaction with TLR-4 and subsequent production of BMP-2, elucidating two key aspects of the immunomodulatory action of adsorbed Fg in bone regeneration.

STATEMENT OF SIGNIFICANCE

Recent studies showed that when Fibrinogen (Fg) is used to modify Chitosan (Ch) substrates, it modulates the immune response, enhances production of osteogenic factors by monocytes/macrophages, and promotes bone regeneration. However, the mechanisms involved in monocyte-Fg interaction, were only partially known. Current work addresses the interaction between primary human monocytes and Ch surfaces modified by Fg adsorption (Ch-Fg) at the molecular level. Results show that monocytes interact specifically with Ch-Fg via TLR-4, triggering particular intracellular signalling pathways (ERK and JNK, but not p38), downstream of TLR-4. Functionally, Ch-Fg induced monocytes to produce the osteogenic mediator BMP-2. Thus, we clarify herein two essential aspects of the interaction between adsorbed Fg and monocytes, with impact on immunomodulation and regeneration, upon biomaterial implantation.

Engineering a Dual-Layer Chitosan-Lactide Hydrogel To Create Endothelial Cell Aggregate-Induced Microvascular Networks In Vitro and Increase Blood Perfusion In Vivo

Here, we report the use of chemically cross-linked and photo-cross-linked hydrogels to engineer human umbilical vein endothelial cell (HUVEC) aggregate-induced microvascular networks to increase blood perfusion in vivo. First, we studied the effect of chemically cross-linked and photo-cross-linked chitosan-lactide hydrogels on stiffness, degradation rates, and HUVEC behaviors. The photo-cross-linked hydrogel was relatively stiff (E = ∼15 kPa) and possessed more compact networks, denser surface texture, and lower enzymatic degradation rates than the relatively soft, chemically cross-linked hydrogel (E = ∼2 kPa). While both hydrogels exhibited nontoxicity, the soft chemically cross-linked hydrogels expedited the formation of cell aggregates compared to the photo-cross-linked hydrogels. Cells on the less stiff, chemically cross-linked hydrogels expressed more matrix metalloproteinase (MMP) activity than the stiffer, photo-cross-linked hydrogel. This difference in MMP activity resulted in a more dramatic decrease in mechanical stiffness after 3 days of incubation for the chemically cross-linked hydrogel, as compared to the photo-cross-linked one. After determining the physical and biological properties of each hydrogel, we accordingly engineered a dual-layer hydrogel construct consisting of the relatively soft, chemically cross-linked hydrogel layer for HUVEC encapsulation, and the relatively stiff, acellular, photo-cross-linked hydrogel for retention of cell-laden microvasculature above. This dual-layer hydrogel construct enabled a lasting HUVEC aggregate-induced microvascular network due to the combination of stable substrate, enriched cell adhesion molecules, and extracellular matrix proteins. We tested the dual-layer hydrogel construct in a mouse model of hind-limb ischemia, where the HUVEC aggregate-induced microvascular networks significantly enhanced blood perfusion rate to ischemic legs and decreased tissue necrosis compared with both no treatment and nonaggregated HUVEC-loaded hydrogels within 2 weeks. This study suggests an effective means for regulating hydrogel properties to facilitate a stable, HUVEC aggregate-induced microvascular network for a variety of vascularized tissue applications.

Inflammation in intervertebral disc degeneration and regeneration

Intervertebral disc (IVD) degeneration is one of the major causes of low back pain, a problem with a heavy economic burden, which has been increasing in prevalence as populations age. Deeper knowledge of the complex spatial and temporal orchestration of cellular interactions and extracellular matrix remodelling is critical to improve current IVD therapies, which have so far proved unsatisfactory. Inflammation has been correlated with degenerative disc disease but its role in discogenic pain and hernia regression remains controversial. The inflammatory response may be involved in the onset of disease, but it is also crucial in maintaining tissue homeostasis. Furthermore, if properly balanced it may contribute to tissue repair/regeneration as has already been demonstrated in other tissues. In this review, we focus on how inflammation has been associated with IVD degeneration by describing observational and in vitro studies as well as in vivo animal models. Finally, we provide an overview of IVD regenerative therapies that target key inflammatory players.

Finding and tracing human MSC in 3D microenvironments with the photoconvertible protein Dendra2

Mesenchymal Stem/Stromal Cells (MSC) are a promising cell type for cell-based therapies - from tissue regeneration to treatment of autoimmune diseases - due to their capacity to migrate to damaged tissues, to differentiate in different lineages and to their immunomodulatory and paracrine properties. Here, a simple and reliable imaging technique was developed to study MSC dynamical behavior in natural and bioengineered 3D matrices. Human MSC were transfected to express a fluorescent photoswitchable protein, Dendra2, which was used to highlight and follow the same group of cells for more than seven days, even if removed from the microscope to the incubator. This strategy provided reliable tracking in 3D microenvironments with different properties, including the hydrogels Matrigel and alginate as well as chitosan porous scaffolds. Comparison of cells mobility within matrices with tuned physicochemical properties revealed that MSC embedded in Matrigel migrated 64% more with 5.2 mg protein/mL than with 9.6 mg/mL and that MSC embedded in RGD-alginate migrated 51% faster with 1% polymer concentration than in 2% RGD-alginate. This platform thus provides a straightforward approach to characterize MSC dynamics in 3D and has applications in the field of stem cell biology and for the development of biomaterials for tissue regeneration.

Endochondral ossification for enhancing bone regeneration: converging native extracellular matrix biomaterials and developmental engineering in vivo

Autologous bone grafting (ABG) remains entrenched as the gold standard of treatment in bone regenerative surgery. Consequently, many marginally successful bone tissue engineering strategies have focused on mimicking portions of ABG's "ideal" osteoconductive, osteoinductive, and osteogenic composition resembling the late reparative stage extracellular matrix (ECM) in bone fracture repair, also known as the "hard" or "bony" callus. An alternative, less common approach that has emerged in the last decade harnesses endochondral (EC) ossification through developmental engineering principles, which acknowledges that the molecular and cellular mechanisms involved in developmental skeletogenesis, specifically EC ossification, are closely paralleled during native bone healing. EC ossification naturally occurs during the majority of bone fractures and, thus, can potentially be utilized to enhance bone regeneration for nearly any orthopedic indication, especially in avascular critical-sized defects where hypoxic conditions favor initial chondrogenesis instead of direct intramembranous ossification. The body's native EC ossification response, however, is not capable of regenerating critical-sized defects without intervention. We propose that an underexplored potential exists to regenerate bone through the native EC ossification response by utilizing strategies which mimic the initial inflammatory or fibrocartilaginous ECM (i.e., "pro-" or "soft" callus) observed in the early reparative stage of bone fracture repair. To date, the majority of strategies utilizing this approach rely on clinically burdensome in vitro cell expansion protocols. This review will focus on the confluence of two evolving areas, (1) native ECM biomaterials and (2) developmental engineering, which will attempt to overcome the technical, business, and regulatory challenges that persist in the area of bone regeneration. Significant attention will be given to native "raw" materials and ECM-based designs that provide necessary osteo- and chondro-conductive and inductive features for enhancing EC ossification. In addition, critical perspectives on existing stem cell-based therapeutic strategies will be discussed with a focus on their use as an extension of the acellular ECM-based designs for specific clinical indications. Within this framework, a novel realm of unexplored design strategies for bone tissue engineering will be introduced into the collective consciousness of the regenerative medicine field.

Novel osteoinductive photo-cross-linkable chitosan-lactide-fibrinogen hydrogels enhance bone regeneration in critical size segmental bone defects

The purpose of this study was to develop and characterize a novel photo-cross-linkable chitosan-lactide-fibrinogen (CLF) hydrogel and evaluate the efficacy of bone morphogenetic protein-2 (BMP-2) containing a CLF hydrogel for osteogenesis in vitro and in vivo. We synthesized the CLF hydrogels and characterized their chemical structure, degradation rate, compressive modulus and in vitro BMP-2 release kinetics. We evaluated bioactivities of the BMP-2 containing CLF hydrogels (0, 50, 100 and 500ngml(-1)) in vitro using W-20-17 preosteoblast mouse bone marrow stromal cells and C2C12 mouse myoblast cells. The effect of BMP-2 containing CLF gels (0, 0.5, 1, 2 and 5μg) on bone formation was evaluated using rat critical size segmental bone defects for 4weeks. Fourier transform infrared spectroscopy spectra and scanning electron microscopy images showed chemical and structural changes by the addition of fibrinogen into the chitosan-lactide copolymer. The incorporation of fibrinogen molecules significantly increased the compressive modulus of the hydrogels. The in vitro BMP-2 release study showed initial burst releases from the CLF hydrogels followed by sustained releases, regardless of the concentration of the BMP-2 over 4weeks. Cells in all groups were viable in the presence of the hydrogels regardless of BMP-2 doses, indicating non-cytotoxicity of hydrogels. Alkaline phosphate activity and mineralization of cells exhibited dose dependence on BMP-2 containing CLF hydrogels. Radiography, microcomputed tomography and histology confirmed that the BMP-2 containing CLF hydrogels prompted neo-osteogenesis and accelerated healing of the defects in a dose-dependent manner. Thus the CLF hydrogel is a promising delivery system of growth factors for bone regeneration.

Enhancing in vitro biocompatibility and corrosion protection of organic-inorganic hybrid sol-gel films with nanocrystalline hydroxyapatite

Application of novel organic-inorganic hybrid sol-gel coatings containing dispersed hydroxyapatite (HAp) particles improves the biocompatibility, normal human osteoblast (NHOst) response in terms of osteoblast viability and adhesion of a Ti6Al4V alloy routinely used in medical implants. The incorporation of HAp particles additionally results in more effective barrier proprieties and improved corrosion protection of the Ti6Al4V alloy through higher degree of cross-linking in the organopolysiloxane matrix and enhanced film thickness.

A multicompartment holder for spinner flasks improves expansion and osteogenic differentiation of mesenchymal stem cells in three-dimensional scaffolds

In the tissue engineering field dynamic culture systems, such as spinner flasks, are widely used due to their ability to improve mass transfer in suspension cell cultures. However, this culture system is often unsuitable to culture cells in three-dimensional (3D) scaffolds. To address this drawback, we designed a multicompartment holder for 3D cell culture, easily adaptable to spinner flasks. Here, the device was tested with human mesenchymal stem cells (MSCs) seeded in 3D porous chitosan scaffolds that were maintained in spinner flasks under dynamic conditions (50 rpm). Standard static culture conditions were used as control. The dynamic conditions were shown to significantly increase MSCs proliferation over 1 week (approximately 6-fold) and to improve cell distribution within the scaffold. Moreover, they also promoted osteogenic differentiation of MSCs, inducing an earlier peak in alkaline phosphatase (ALP) activity, and a more homogenous ALP staining and matrix mineralization in the whole scaffolds, but particularly in the center. Overall, this study shows a new multicompartment holder to culture 3D scaffolds that can broaden the application of spinner flasks.

Impact of 3-D printed PLA- and chitosan-based scaffolds on human monocyte/macrophage responses: unraveling the effect of 3-D structures on inflammation

Recent studies have pointed towards a decisive role of inflammation in triggering tissue repair and regeneration, while at the same time it is accepted that an exacerbated inflammatory response may lead to rejection of an implant. Within this context, understanding and having the capacity to regulate the inflammatory response elicited by 3-D scaffolds aimed for tissue regeneration is crucial. This work reports on the analysis of the cytokine profile of human monocytes/macrophages in contact with biodegradable 3-D scaffolds with different surface properties, architecture and controlled pore geometry, fabricated by 3-D printing technology. Fabrication processes were optimized to create four different 3-D platforms based on polylactic acid (PLA), PLA/calcium phosphate glass or chitosan. Cytokine secretion and cell morphology of human peripheral blood monocytes allowed to differentiate on the different matrices were analyzed. While all scaffolds supported monocyte/macrophage adhesion and stimulated cytokine production, striking differences between PLA-based and chitosan scaffolds were found, with chitosan eliciting increased secretion of tumor necrosis factor (TNF)-α, while PLA-based scaffolds induced higher production of interleukin (IL)-6, IL-12/23 and IL-10. Even though the material itself induced the biggest differences, the scaffold geometry also impacted on TNF-α and IL-12/23 production, with chitosan scaffolds having larger pores and wider angles leading to a higher secretion of these pro-inflammatory cytokines. These findings strengthen the appropriateness of these 3-D platforms to study modulation of macrophage responses by specific parameters (chemistry, topography, scaffold architecture).

Triethylphosphite as a network forming agent enhances in vitro biocompatibility and corrosion protection of hybrid organic–inorganic sol–gel coatings for Ti6Al4V alloys

Introduction of phosphorous into hybrid silica sol–gel coatings on Ti6Al4V gives materials demonstrating higher levels of intermolecular condensation and fibrinogen uptake as well as improved in vitro biocompatibility and corrosion protection.

Adsorbed fibrinogen enhances production of bone- and angiogenic-related factors by monocytes/macrophages

Macrophages are phagocytic cells with great importance in guiding multiple stages of inflammation and tissue repair. By producing a large number of biologically active molecules, they can affect the behavior of other cells and events, such as the foreign body response and angiogenesis. Since protein adsorption to biomaterials is crucial for the inflammatory process, we addressed the ability of the pro-inflammatory molecule fibrinogen (Fg) to modulate macrophage behavior toward tissue repair/regeneration. For this purpose, we used chitosan (Ch) as a substrate for Fg adsorption. Freshly isolated human monocytes were seeded on Ch substrates alone or previously adsorbed with Fg, and allowed to differentiate into macrophages for 10 days. Cell adhesion and morphology, formation of foreign body giant cells (FBGC), and secretion of a total of 80 cytokines and growth factors were evaluated. Both substrates showed similar numbers of adherent macrophages along differentiation as compared with RGD-coated surfaces, which were used as positive controls. Fg did not potentiate FBGC formation. In addition, actin cytoskeleton staining revealed the presence of punctuate F-actin with more elongated and interconnecting cells on Ch substrates. Antibody array screening and quantification of inflammation- and wound-healing-related factors indicated an overall reduction in Ch-based substrates versus RGD-coated surfaces. At late times, most inflammatory agents were down-regulated in the presence of Fg, in contrast to growth factor production, which was stimulated by Fg. Importantly, on Ch+Fg substrates, fully differentiated macrophages produced significant amounts of macrophage inflammatory protein-1delta (MIP-1δ), platelet-derived growth factor-BB, bone morphogenetic protein (BMP)-5, and BMP-7 compared with Ch alone. In addition, other important factors involved in bone homeostasis and wound healing, such as growth hormone, transforming growth factor-β3, and insulin-like growth factor-binding proteins, as well as several angiogenic mediators, including endocrine gland-derived vascular endothelial factor, fibroblast growth factor-7, and placental growth factor, were significantly promoted by Fg. This work provides a new perspective on the inflammatory response in the context of bone repair/regeneration mediated by a pro-inflammatory protein (Fg) adsorbed onto a biomaterial (Ch) that does not otherwise exhibit osteogenic properties.