In vitro and in vivo responses of macrophages to magnesium-doped titanium

Osteo-Immunomodulatory Role of Interleukin-4-Immobilized Plasma Immersion Ion Implantation Membranes for Bone Regeneration

Barrier membranes for guided tissue regeneration are essential for bone repair and regeneration. The implanted membranes may trigger early inflammatory responses as a foreign material, which can affect the recruitment and differentiation of bone cells during tissue regeneration. The purpose of this study was to determine whether immobilizing interleukin 4 (IL4) on plasma immersion ion implantation (PIII)-activated surfaces may alter the osteo-immunoregulatory characteristics of the membranes and produce pro-osteogenic effects. In order to immobilize IL4, polycaprolactone surfaces were modified using the PIII technology. No discernible alterations were found between the morphology before and after PIII treatment or IL4 immobilization. IL4-immobilized PIII surfaces polarized macrophages to an M2 phenotype and mitigated inflammatory cytokine production under lipopolysaccharide stimulation. Interestingly, the co-culture of macrophages (on IL4-immobilized PIII surfaces) and bone marrow-derived mesenchymal stromal cells enhanced the production of angiogenic and osteogenic factors and triggered autophagy activation. Exosomes produced by PIII + IL4-stimulated macrophages were also found to play a role in osteoblast differentiation. In conclusion, the osteo-immunoregulatory properties of bone materials can be modified by PIII-assisted IL4 immobilization, creating a favorable osteoimmune milieu for bone regeneration.

Combined Effects of Polydopamine-Assisted Copper Immobilization on 3D-Printed Porous Ti6Al4V Scaffold for Angiogenic and Osteogenic Bone Regeneration

Numerous studies have demonstrated that biological compounds and trace elements such as dopamine (DA) and copper ions (Cu) could be modified onto the surfaces of scaffolds using a one-step immersion process which is simple, inexpensive and, most importantly, non-cytotoxic. The development and emergence of 3D printing technologies such as selective laser melting (SLM) have also made it possible for us to fabricate bone scaffolds with precise structural designs using metallic compounds. In this study, we fabricated porous titanium scaffolds (Ti) using SLM and modified the surface of Ti with polydopamine (PDA) and Cu. There are currently no other reported studies with such a combination for osteogenic and angiogenic-related applications. Results showed that such modifications did not affect general appearances and microstructural characteristics of the porous Ti scaffolds. This one-step immersion modification allowed us to modify the surfaces of Ti with different concentrations of Cu ions, thus allowing us to fabricate individualized scaffolds for different clinical scenarios. The modification improved the hydrophilicity and surface roughness of the scaffolds, which in turn led to promote cell behaviors of Wharton’s jelly mesenchymal stem cells. Ti itself has high mechanical strength, therefore making it suitable for surgical handling and clinical applications. Furthermore, the scaffolds were able to release ions in a sustained manner which led to an upregulation of osteogenic-related proteins (bone alkaline phosphatase, bone sialoprotein and osteocalcin) and angiogenic-related proteins (vascular endothelial growth factor and angiopoietin-1). By combining additive manufacturing, Ti6Al4V scaffolds, surface modification and Cu ions, the novel hybrid 3D-printed porous scaffold could be fabricated with ease and specifically benefited future bone regeneration in the clinic.

The effect of calcium-magnesium mixtures in sol-gel coatings on bone tissue regeneration

Calcium and magnesium are two elements essential for bone structure and metabolism. However, their synergistic or competitive effects on bone regeneration are often overlooked during biomaterial development. We examined the interactions between Ca and Mg in sol-gel coatings doped with mixtures of CaCl₂ (0.5%) and MgCl₂ (0.5, 1, and 1.5%). After physicochemical characterisation, the materials were incubated in vitro with MC3T3-E1 osteoblastic cells and RAW264.7 macrophages, and the protein adsorption was analysed using nLC-MS/MS. The incorporation of the ions did not lead to the formation of crystalline structures and did not affect the sol-gel network cross-linking. The release of the ions did not cause cytotoxic effects at any tested concentration. The proteomic analysis showed that adding the Ca and Mg ions elevated the adsorption of proteins associated with inflammatory response regulation (e.g., ALBU, CLUS, HPT, HPTR, A1AG1 and A1AG2) but decreased the adsorption of immunoglobulins. The CaMg coatings had reduced affinity to proteins associated with coagulation (e.g., FA9, FA10, FA11, FA12) but increased the adsorption of proteins involved in cell adhesion (DSG1, DESP, FBLN1, ZA2G). In vitro assays revealed that the cellular response was affected by changing the concentration of Mg. Moreover, our results show that these differences reflect the changes in the concentrations of both ions in the mix but are not a simple additive effect.

Mg2+ -mediated autophagy-dependent polarization of macrophages mediates the osteogenesis of bone marrow stromal stem cells by interfering with macrophage-derived exosomes containing miR-381

Magnesium ion (Mg²⁺ ) has received increased attention due to the roles it plays in promoting osteogenesis and preventing inflammation. This study was designed to investigate the mechanism by which Mg²⁺ influences the osteoblastic differentiation of bone marrow stromal stem cells (BMSCs). The polarization of Mø (macrophages) was measured after treatment with Mg²⁺ . Meanwhile, autophagy in Mø was measured by detecting LC3B expression. Mø-derived exosomes were isolated and cocultured with BMSCs; after which, osteogenic differentiation was evaluated by Alizarin Red staining and detection of alkaline phosphatase (ALP). Our results showed that Mg²⁺ could induce autophagy in macrophages and modulate the M1/M2 polarization of macrophages. Mg²⁺ -mediated macrophages could facilitate the osteogenic differentiation of BMSCs by regulating autophagy, and this facilitation by Mg²⁺ -mediated macrophages was closely related to macrophage-derived exosomes, and especially exosomes containing miR-381. However, miR-381 in macrophages did not influence autophagy or the polarization of Mg²⁺ -mediated macrophages. Furthermore, macrophage-derived exosomes containing miR-381 mainly determined the osteogenic differentiation of BMSCs. Mg²⁺ -mediated macrophages were shown to promote the osteogenic differentiation of BMSCs via autophagy through reducing miR-381 in macrophage-derived exosomes. In conclusion, our results suggest Mg²⁺ -mediated macrophage-derived exosomes containing miR-381 as novel vehicles for promoting the osteogenic differentiation of BMSCs.

Immunomodulation and osseointegration activities of Na2TiO3 nanorods-arrayed coatings doped with different Sr content

To endow Ti-based orthopedic implants immunomodulatory capability and thus enhanced osseointegration, different amounts of Sr are doped in Na2TiO3 nanorods in the arrays with identical nanotopographic parameters (rod diameter, length and inter-rod spacing) by substitution of Na+ using hydrothermal treatment. The obtained arrays are denoted as STSr2, STSr4, and STSr7, where the arabic numbers indicate the incorporating amounts of Sr in Na2TiO3. The modulation effects of the Sr-doped nanorods arrays on macrophage polarization and osteogenetic functions of osteoblasts are investigated, together with the array without Sr (ST). Moreover, osseointegration of these arrays are also assayed in rat femoral condyles. Sr-doped nanorods arrays accelerate M1 (pro-inflammatory phenotype)-to-M2 (anti-inflammatory phenotype) transformation of the adhered macrophages, enhancing secretion of pro-osteogenetic cytokines and growth factors (TGF-β1 and BMP2), moreover, the Sr doped arrays directly enhance osteogenetic functions of osteoblasts. The enhancement of paracrine of M2 macrophages and osteogenetic function of osteoblasts is promoted with the increase of Sr incorporating amounts. Consequently, Sr doped arrays show significantly enhanced osseointegration in vivo compared to ST, and STSr7 exhibits the best performance. Our work sheds a new light on the design of surface chemical components and structures for orthopedic implants to enhance their osseointegration.

Divalent metal cations stimulate skeleton interoception for new bone formation in mouse injury models

Bone formation induced by divalent metal cations has been widely reported; however, the underlying mechanism is unclear. Here we report that these cations stimulate skeleton interoception by promoting prostaglandin E2 secretion from macrophages. This immune response is accompanied by the sprouting and arborization of calcitonin gene-related polypeptide-α⁺ nerve fibers, which sense the inflammatory cue with PGE₂ receptor 4 and convey the interoceptive signals to the central nervous system. Activating skeleton interoception downregulates sympathetic tone for new bone formation. Moreover, either macrophage depletion or knockout of cyclooxygenase-2 in the macrophage abolishes divalent cation-induced skeleton interoception. Furthermore, sensory denervation or knockout of EP4 in the sensory nerves eliminates the osteogenic effects of divalent cations. Thus, our study reveals that divalent cations promote bone formation through the skeleton interoceptive circuit, a finding which could prompt the development of novel biomaterials to elicit the therapeutic power of these divalent cations.

Mechanisms underlying bone formation induced by divalent metal cations remain largely unknown. Here the authors show that these cations can activate the skeleton interoceptive circuit through the immune-neural axis to initiate new bone formation.

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

To present knowledge, macrophages are found in all tissues of the human body. They are a cell population with high plasticity which come with a multitude of functions which appear to be adapted to the respective tissue niche and micro-environment in which they reside. Bone harbors multiple macrophage subpopulations, with the osteoclasts as classical representative of a bone resorbing cells and osteomacs as a bone tissue resident macrophage first described by the expression of F4/80. Both subtypes are found throughout all phases in bone healing. In vivo data on bone regeneration have demonstrated their essential role in initiating the healing cascade (inflammatory phase) but also of the later phases of healing (e.g. endochondral and intramembranous bone formation). To participate in such diverse processes macrophages have to be highly plastic in their functionality. Thus, the widely used M1/M2 paradigm to distinguish macrophage subpopulations may not mirror the comprehensive role of the dynamics of macrophage plasticity. From a clinical perspective it is especially relevant to distinguish what drives macrophages in impaired healing scenarios, implant loosening or infections, where their specific role of a misbalanced inflammatory setting is so far only partially known. With this review we aim at illustrating current knowledge and gaps of knowledge on macrophage plasticity and function during the cascades of regeneration and reconstitution of bone tissue. We propose aspects of the known biological mechanisms of macrophages and their specific subsets that might serve as targets to control their function in impaired healing and eventually support a scar-free regeneration. STATEMENT OF SIGNIFICANCE: Macrophages are essential for successful regeneration. In scar-free healing such as in bone, a complete failure of healing was shown if macrophages were depleted; the M1/M2 switch appears to be key to the progression from pro-inflammation to regeneration. However, experimental data illustrate that the classical M1/M2 paradigm does not completely mirror the complexity of observed macrophage functions during bone healing and thus demands a broader perspective. Within this review we discuss the high degree of plasticity of macrophages and the relevant contribution of the different and more specific M2 subtypes (M2a-M2f) during (bone) regeneration. It summarizes the versatile roles of macrophages in skeletal regeneration and thereby highlights potential target points for immunomodulatory approaches to enable or even foster bone repair.

Toward a Better Regeneration through Implant-Mediated Immunomodulation: Harnessing the Immune Responses

Tissue repair/regeneration, after implantation or injury, involves comprehensive physiological processes wherein immune responses play a crucial role to enable tissue restoration, amidst the immune cells early-stage response to tissue damages. These cells break down extracellular matrix, clear debris, and secret cytokines to orchestrate regeneration. However, the immune response can also lead to abnormal tissue healing or scar formation if not well directed. This review first introduces the general immune response post injury, with focus on the major immune cells including neutrophils, macrophages, and T cells. Next, a variety of implant-mediated immunomodulation strategies to regulate immune response through physical, chemical, and biological cues are discussed. At last, various scaffold-facilitated regenerations of different tissue types, such as, bone, cartilage, blood vessel, and nerve system, by harnessing the immunomodulation are presented. Therefore, the most recent data in biomaterials and immunomodulation is presented here in a bid to shape expert perspectives, inspire researchers to go in new directions, and drive development of future strategies focusing on targeted, sequential, and dynamic immunomodulation elicited by implants.

Immunomodulatory effect of dimethyloxallyl glycine/nanosilicates-loaded fibrous structure on periodontal bone remodeling

BACKGROUND/PURPOSE

Relieving immuno-inflammatory responses is the prerequisite step for treating periodontitis. The angiogenic small molecule, dimethyloxalylglycine (DMOG), and osteoinductive inorganic nanomaterial, nanosilicate (nSi) have a powerful effect on bone regeneration, whereas the roles in osteoimmunomodulation have not been totally uncovered. Our study aimed to explore the immunomodulatory effect of DMOG/nSi-loaded fibrous membranes on periodontal bone remodeling.

MATERIALS AND METHODS

The fibrous membranes were prepared by incorporating DMOG and nSi into poly (lactic-co-glycolic acid) (PLGA) with electrospinning. The morphology features, surface chemical property and biocompatibility of DMOG/nSi-PLGA fibrous membranes were characterized. Thereafter, the fibrous membranes were implanted into rat periodontal defects, bone remodeling potential and immunomodulatory effect were evaluated by micro-computed tomography (micro-CT), histological evaluation and immunohistochemical analysis.

RESULTS

DMOG/nSi-PLGA membranes possessed favorable physicochemical properties and biocompatibility. After the fibrous membranes implanted into periodontal defects, DMOG/nSi-PLGA membranes could relieve immuno-inflammatory responses of the defects (reduction of inflammatory cell infiltration, CD40L and CD11b-positive cells), increased CD206-positive M2 macrophages, and eventually facilitated periodontal bone regeneration.

CONCLUSION

DMOG/nSi-PLGA fibrous membranes exert protective effects during periodontal bone defect repairing, and steer immune response towards bone regeneration. Consequently, DMOG/nSi-PLGA fibrous membranes may serve as a promising scaffold in periodontal tissue engineering.

TRPM7 kinase-mediated immunomodulation in macrophage plays a central role in magnesium ion-induced bone regeneration

Despite the widespread observations on the osteogenic effects of magnesium ion (Mg2+), the diverse roles of Mg2+ during bone healing have not been systematically dissected. Here, we reveal a previously unknown, biphasic mode of action of Mg2+ in bone repair. During the early inflammation phase, Mg2+ contributes to an upregulated expression of transient receptor potential cation channel member 7 (TRPM7), and a TRPM7-dependent influx of Mg2+ in the monocyte-macrophage lineage, resulting in the cleavage and nuclear accumulation of TRPM7-cleaved kinase fragments (M7CKs). This then triggers the phosphorylation of Histone H3 at serine 10, in a TRPM7-dependent manner at the promoters of inflammatory cytokines, leading to the formation of a pro-osteogenic immune microenvironment. In the later remodeling phase, however, the continued exposure of Mg2+ not only lead to the over-activation of NF-κB signaling in macrophages and increased number of osteoclastic-like cells but also decelerates bone maturation through the suppression of hydroxyapatite precipitation. Thus, the negative effects of Mg2+ on osteogenesis can override the initial pro-osteogenic benefits of Mg2+. Taken together, this study establishes a paradigm shift in the understanding of the diverse and multifaceted roles of Mg2+ in bone healing.

Effect of Neuroprotective Magnesium Sulfate Treatment on Brain Transcription Response to Hypoxia Ischemia in Neonate Mice

MgSO₄ is widely used in the prevention of preterm neurological disabilities but its modes of action remain poorly established. We used a co-hybridization approach using the transcriptome in 5-day old mice treated with a single dose of MgSO₄ (600 mg/kg), and/or exposed to hypoxia-ischemia (HI). The transcription of hundreds of genes was altered in all the groups. MgSO₄ mainly produced repressions culminating 6 h after injection. Bio-statistical analysis revealed the repression of synaptogenesis and axonal development. The putative targets of MgSO₄ were Mnk1 and Frm1. A behavioral study of adults did not detect lasting effects of neonatal MgSO₄ and precluded NMDA-receptor-mediated side effects. The effects of MgSO₄ plus HI exceeded the sum of the effects of separate treatments. MgSO₄ prior to HI reduced inflammation and the innate immune response probably as a result of cytokine inhibition (Ccl2, Ifng, interleukins). Conversely, MgSO₄ had little effect on HI-induced transcription by RNA-polymerase II. De novo MgSO₄-HI affected mitochondrial function through the repression of genes of oxidative phosphorylation and many NAD-dehydrogenases. It also likely reduced protein translation by the repression of many ribosomal proteins, essentially located in synapses. All these effects appeared under the putative regulatory MgSO₄ induction of the mTORC2 Rictor coding gene. Lasting effects through Sirt1 and Frm1 could account for this epigenetic footprint.

Characterization of magnesium doped sol-gel biomaterial for bone tissue regeneration: The effect of Mg ion in protein adsorption

Magnesium is the fourth most abundant element in the human body with a wide battery of functions in the maintenance of normal cell homeostasis. In the bone, this element incorporates in the hydroxyapatite structure and it takes part in mineral metabolism and regulates osteoclast functions. In this study, sol-gel materials with increasing concentrations of MgCl₂ (0.5, 1, and 1.5%) were synthesized and applied onto Ti surfaces as coatings. The materials were first physicochemically characterized. In vitro responses were examined using the MC3T3-E1 osteoblastic cells and RAW264.7 macrophages. Human serum protein adsorption was evaluated employing nLC-MS/MS. The incorporation of Mg did not affect the crosslinking of the sol-gel network, and a controlled release of Mg was observed; it was not cytotoxic at any of the tested concentrations. The cytoskeleton arrangement of MC3T3-E1 cells cultured on the Mg-doped materials changed in comparison with controls; the cells became more elongated, with protruded lamellipodia and increased cell surface. The expression of integrins (ITGA5 and ITGB1) was boosted by Mg-coatings. The ALP activity and expression of TGF-β, OSX and RUNX2 genes were also increased. In RAW264.7 cells, TNF-α secretion was reduced, while TGF-β and IL-4 expression rose. These changes correlated with the altered protein adsorption patterns. The Mg-doped coatings showed increased adsorption of anti-inflammatory (CLUS, IC1, CFAH, and VTNC), cell adhesion (DSG1, FILA2, and DESP) and tissue regeneration (VTNC and CYTA) proteins. This integrated approach to biomaterial characterization revealed the potential of Mg in bone tissue regeneration.

Regulation of extracellular bioactive cations in bone tissue microenvironment induces favorable osteoimmune conditions to accelerate in situ bone regeneration

The design of orthopedic biomaterials has gradually shifted from “immune-friendly” to “immunomodulatory,” in which the biomaterials are able to modulate the inflammatory response via macrophage polarization in a local immune microenvironment that favors osteogenesis and implant-to-bone osseointegration. Despite the well-known effects of bioactive metallic ions on osteogenesis, how extracellular metallic ions manipulate immune cells in bone tissue microenvironments toward osteogenesis and subsequent bone formation has rarely been studied. Herein, we investigate the osteoimmunomodulatory effect of an extracellular bioactive cation (Mg²⁺) in the bone tissue microenvironment using custom-made poly lactic-co-glycolic acid (PLGA)/MgO-alendronate microspheres that endow controllable release of magnesium ions. The results suggest that the Mg²⁺-controlled tissue microenvironment can effectively induce macrophage polarization from the M0 to M2 phenotype via the enhancement of anti-inflammatory (IL-10) and pro-osteogenic (BMP-2 and TGF-β1) cytokines production. It also generates a favorable osteoimmune microenvironment that facilitates the proliferation and osteogenic differentiation of bone marrow mesenchymal stem cells. The in vivo results further verify that a large amount of bony tissue, with comparable bone mineral density and mechanical properties, has been generated at an early post-surgical stage in rat intramedullary bone defect models. This study demonstrates that the concept of in situ immunomodulated osteogenesis can be realized in a controlled magnesium tissue microenvironment.

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Controlled release of Mg²⁺ was achieved by the microfluidic electrosprayed PLGA/MgO-alendronate microspheres.

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PLGA/MgO-alendronate microspheres with satisfied osteoimmunomodulatory properties accelerated bone regeneration in vivo.

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Extracellular Mg²⁺ induced macrophage switch to M2 phenotype with enhanced expressions of IL-10, BMP-2 and TGF-β1.

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Osteoimmune microenvironment favourably upregulated the osteogenic expressions of BMSCs.

Early osteoimmunomodulatory effects of magnesium-calcium-zinc alloys

Today, substantial attention is given to biomaterial strategies for bone regeneration, and among them, there is a growing interest in using immunomodulatory biomaterials. The ability of a biomaterial to induce neo vascularization and macrophage polarization is a major factor in defining its success. Magnesium (Mg)-based degradable alloys have attracted significant attention for bone regeneration owing to their biodegradability and potential for avoiding secondary removal surgeries. However, there is insufficient evidence in the literature regarding the early inflammatory responses to these alloys in vivo. In this study, we investigated the early body responses to Mg-0.45wt%Zn-0.45wt%Ca pin-shaped alloy (known as ZX00 alloy) in rat femora 2, 5, and 10 days after implantation. We used 3D micro computed tomography (µCT), histological, immunohistochemical, histomorphometrical, and small angle X-ray scattering (SAXS) analyses to study new bone formation, early macrophage polarization, neo vascularization, and bone quality at the implant bone interface. The expression of macrophage type 2 biological markers increased significantly after 10 days of Mg alloy implantation, indicating its potential in stimulating macrophage polarization. Our biomineralization results using µCT as well as histological stained sections did not indicate any statistically significant differences between different time points for both groups. The activity of alkaline phosphatase (ALP) and Runt-related transcription factor 2 (Runx 2) biological markers decreased significantly for Mg group, indicating less osteoblast activity. Generally, our results supported the potential of ZX00 alloy to enhance the expression of macrophage polarization in vivo; however, we could not observe any statistically significant changes regarding biomineralization.

Synergistic effects of immunoregulation and osteoinduction of ds-block elements on titanium surface

Ds-block elements have been gaining increasing attention in the field of biomaterials modification, owing to their excellent biological properties, such as antibiosis, osteogenesis, etc. However, their function mechanisms are not well understood and conflicting conclusions were drawn by previous studies on this issue, which are mainly resulted from the inconsistent experimental conditions. In this work, three most widely used ds-block elements, copper, zinc, and silver were introduced on titanium substrate by plasma immersion ion implantation method to investigate the rule of ds-block elements in the immune responses. Results showed that the implanted samples could decrease the inflammatory responses compared with Ti sample. The trend of anti-inflammatory effects of macrophages on samples was in correlation with cellular ROS levels, which was induced by the implanted biomaterials and positively correlated with the number of valence electrons of ds-block elements. The co-culture experiments of macrophages and bone marrow mesenchymal stem cells showed that these two kinds of cells could enhance the anti-inflammation and osteogenesis of samples by the paracrine manner of PGE2. In general, in their steady states on titanium substrate (Cu2+, Zn2+, Ag), the ds-block elements with more valence electrons exhibit better anti-inflammatory and osteogenic effects. Moreover, molecular biology experiments indicate that the PGE2-related signaling pathway may contribute to the desired immunoregulation and osteoinduction capability of ds-block elements. These findings suggest a correlation between the number of valence electrons of ds-block elements and the relevant biological responses, which provides new insight into the selection of implanted ions and surface design of biomaterials.

Distinct macrophage populations and phenotypes associated with IL-4 mediated immunomodulation at the host implant interface

The host macrophage response to implants has shown to be affected by tissue location and physio-pathological conditions of the patient. Success in immunomodulatory strategies is thus predicated on the proper understanding of the macrophage populations participating on each one of these contexts. The present study uses an in vivo implantation model to analyze how immunomodulation via an IL-4 eluting implant affects distinct macrophage populations at the tissue-implant interface and how this may affect downstream regenerative processes. Populations identified as F4/80+, CD68+ and CD11b+ macrophages at the peri-implant space showed distinct susceptibility to polarize towards an M2-like phenotype under the effects of delivered IL-4. Also, the presence of the coating resulted in a significant reduction in F4/80+ macrophages, while other populations remained unchanged. These results suggests that the F4/80+ macrophage population may be predominant in the early stages of the host response at the surface of these implants, in contrast to CD11b+ macrophage populations which were either fewer in number or located more distant from the implant surface. Gene expression assays showed increased proteolytic activity and diminished matrix deposition as possible mechanisms explaining the decreased fibrotic capsule deposition and improved peri-implant tissue quality shown in previous studies using IL-4 eluting coatings. The pattern of M2-like gene expression promoted by IL-4 was correlated with glycosaminoglycan production within the site of implantation at early stages of the host response, suggesting a significant role in this response. These findings demonstrate that immunomodulatory strategies can be utilized to design and implement targeted delivery for improving biomaterial performance.

Combating Implant Infections: Shifting Focus from Bacteria to Host

The widespread use of biomaterials to support or replace body parts is increasingly threatened by the risk of implant-associated infections. In the quest for finding novel anti-infective biomaterials, there generally has been a one-sided focus on biomaterials with direct antibacterial properties, which leads to excessive use of antibacterial agents, compromised host responses, and unpredictable effectiveness in vivo. This review sheds light on how host immunomodulation, rather than only targeting bacteria, can endow biomaterials with improved anti-infective properties. How antibacterial surface treatments are at risk to be undermined by biomaterial features that dysregulate the protection normally provided by critical immune cell subsets, namely, neutrophils and macrophages, is discussed. Accordingly, how the precise modification of biomaterial surface biophysical cues, or the incorporation of immunomodulatory drug delivery systems, can render biomaterials with the necessary immune-compatible and immune-protective properties to potentiate the host defense mechanisms is reviewed. Within this context, the protective role of host defense peptides, metallic particles, quorum sensing inhibitors, and therapeutic adjuvants is discussed. The highlighted immunomodulatory strategies may lay a foundation to develop anti-infective biomaterials, while mitigating the increasing threat of antibacterial drug resistance.

A Review of the Action of Magnesium on Several Processes Involved in the Modulation of Hematopoiesis

Magnesium (Mg²⁺) is an essential mineral for the functioning and maintenance of the body. Disturbances in Mg²⁺ intracellular homeostasis result in cell-membrane modification, an increase in oxidative stress, alteration in the proliferation mechanism, differentiation, and apoptosis. Mg²⁺ deficiency often results in inflammation, with activation of inflammatory pathways and increased production of proinflammatory cytokines by immune cells. Immune cells and others that make up the blood system are from hematopoietic tissue in the bone marrow. The hematopoietic tissue is a tissue with high indices of renovation, and Mg²⁺ has a pivotal role in the cell replication process, as well as DNA and RNA synthesis. However, the impact of the intra- and extracellular disturbance of Mg²⁺ homeostasis on the hematopoietic tissue is little explored. This review deals specifically with the physiological requirements of Mg²⁺ on hematopoiesis, showing various studies related to the physiological requirements and the effects of deficiency or excess of this mineral on the hematopoiesis regulation, as well as on the specific process of erythropoiesis, granulopoiesis, lymphopoiesis, and thrombopoiesis. The literature selected includes studies in vitro, in animal models, and in humans, giving details about the impact that alterations of Mg²⁺ homeostasis can have on hematopoietic cells and hematopoietic tissue.

Effects of degradable magnesium on paracrine signaling between human umbilical cord perivascular cells and peripheral blood mononuclear cells

Human mesenchymal stem cells (MSC) interact with numerous immune cells that can promote regenerative processes and inhibit inflammatory responses. We hypothesised that the cross-talk between human umbilical cord perivascular cells (HUCPV; an alternative source of MSC) and peripheral blood mononuclear cells (PBMC) could be influenced by degradable transwell magnesium (Mg). To study the correlations between paracrine signaling and specific cellular behaviour during the host response to Mg, we used a transwell coculture system for up to 7 days. The proliferation and viability of both cell types were not significantly influenced by Mg. When HUCPV were cultured with degradable Mg, a moderate inflammation (e.g., lower secretions of pro-inflammatory interleukin 1 beta and IL2, and tumour necrosis factor alpha, interferon gamma, anti-inflammatory interleukins 4, 5, 10, 13, and 1 receptor antagonists and granulocyte colony stimulating factor), and an increased pro-healing M2 macrophage phenotype were observed. Moreover, when PBMC were cultured with degradable Mg, the expression of migration/wound healing related cytokines (interleukin 8, granulocyte-macrophage colony-stimulating factor, monocyte chemoattractant protein 1 and macrophage inflammatory protein 1α/β) was upregulated, accompanied by an increase in the migration ability of HUCPV (cell scratch assay). In addition, an increased pro-osteogenic potential was demonstrated via an increase of osteoblastic markers (e.g., alkaline phosphatase activity, specific gene expression and cytokine release). These results collectively imply that Mg possesses osteo-immunomodulatory properties. They also help to design Mg-based bone substitute biomaterials capable of exhibiting desired immune reactions and good clinical performance.

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.

Active Microneedle Administration of Plant Virus Nanoparticles for Cancer in situ Vaccination Improves Immunotherapeutic Efficacy

The solid tumor microenvironment (TME) poses a significant structural and biochemical barrier to immunotherapeutic agents. To address the limitations of tumor penetration and distribution, and to enhance antitumor efficacy of immunotherapeutics, we present here an autonomous active microneedle (MN) system for the direct intratumoral (IT) delivery of a potent immunoadjuvant, cowpea mosaic virus nanoparticles (CPMV) in vivo. In this active delivery system, magnesium (Mg) microparticles embedded into active MNs react with the interstitial fluid in the TME, generating a propulsive force to drive the nanoparticle payload into the tumor. Active delivery of CPMV payload into B16F10 melanomas in vivo demonstrated substantially more pronounced tumor regression and prolonged survival of tumor-bearing mice compared to that of passive MNs and conventional needle injection. Active MN administration of CPMV also enhanced local innate and systemic adaptive antitumor immunity. Our approach represents an elaboration of conventional CPMV in situ vaccination, highlighting substantial immune-mediated antitumor effects and improved therapeutic efficacy that can be achieved through an active and autonomous delivery system-mediated CPMV in situ vaccination.

Biological responses to physicochemical properties of biomaterial surface

Biomedical scientists use chemistry-driven processes found in nature as an inspiration to design biomaterials as promising diagnostic tools, therapeutic solutions, or tissue substitutes. While substantial consideration is devoted to the design and validation of biomaterials, the nature of their interactions with the surrounding biological microenvironment is commonly neglected. This gap of knowledge could be owing to our poor understanding of biochemical signaling pathways, lack of reliable techniques for designing biomaterials with optimal physicochemical properties, and/or poor stability of biomaterial properties after implantation. The success of host responses to biomaterials, known as biocompatibility, depends on chemical principles as the root of both cell signaling pathways in the body and how the biomaterial surface is designed. Most of the current review papers have discussed chemical engineering and biological principles of designing biomaterials as separate topics, which has resulted in neglecting the main role of chemistry in this field. In this review, we discuss biocompatibility in the context of chemistry, what it is and how to assess it, while describing contributions from both biochemical cues and biomaterials as well as the means of harmonizing them. We address both biochemical signal-transduction pathways and engineering principles of designing a biomaterial with an emphasis on its surface physicochemistry. As we aim to show the role of chemistry in the crosstalk between the surface physicochemical properties and body responses, we concisely highlight the main biochemical signal-transduction pathways involved in the biocompatibility complex. Finally, we discuss the progress and challenges associated with the current strategies used for improving the chemical and physical interactions between cells and biomaterial surface.

Biodegradable Magnesium-Based Implants in Orthopedics-A General Review and Perspectives

Biodegradable Mg-based metals may be promising orthopedic implants for treating challenging bone diseases, attributed to their desirable mechanical and osteopromotive properties. This Review summarizes the current status and future research trends for Mg-based orthopedic implants. First, the properties between Mg-based implants and traditional orthopedic implants are compared on the following aspects: in vitro and in vivo degradation mechanisms of Mg-based implants, peri-implant bone responses, the fate of the degradation products, and the cellular and molecular mechanisms underlying the beneficial effects of Mg ions on osteogenesis. Then, the preclinical studies conducted at the low weight bearing sites of animals are introduced. The innovative strategies (for example, via designing Mg-containing hybrid systems) are discussed to address the limitations of Mg-based metals prior to their clinical applications at weight-bearing sites. Finally, the available clinical studies are summarized and the challenges and perspectives of Mg-based orthopedic implants are discussed. Taken together, the progress made on the development of Mg-based implants in basic, translational, and clinical research has laid down a foundation for developing a new era in the treatment of challenging and prevalent bone diseases.

Modulating Macrophage Phenotype by Sustained MicroRNA Delivery Improves Host-Implant Integration

Biomedical implant failure due to the host's response remains a challenging problem. In particular, the formation of the fibrous capsule is a common barrier for the normal function of implants. Currently, there is mounting evidence indicating that the polarization state of macrophages plays an important role in effecting the foreign body reaction (FBR). This opens up a potential avenue for improving host-implant integration. Here, electrospun poly(caprolactone-co-ethyl ethylene phosphate) nanofiber scaffolds are utilized to deliver microRNAs (miRs) to induce macrophage polarization and modulate FBR. Specifically, C57BL/6 mice that are treated with M2-inducing miRs, Let-7c and miR-124, display relatively thinner fibrous capsule formation around the scaffolds at both Week 2 and 4, as compared to treatment with M1-inducing miR, Anti-Let-7c. Histological analysis shows that the density of blood vessels in the scaffolds are the highest in miR-124 treatment group, followed by Anti-Let-7c and Let-7c treatment groups. Based on immunohistochemical quantifications, these miR-encapsulated nanofiber scaffolds are useful for localized and sustained delivery of functional miRs and are able to modulate macrophage polarization during the first 2 weeks of implantation to result in significant alteration in host-implant integration at longer time points.

Titanium Porous Coating Using 3D Direct Energy Deposition (DED) Printing for Cementless TKA Implants: Does It Induce Chronic Inflammation?

Because of the recent technological advances, the cementless total knee arthroplasty (TKA) implant showed satisfactory implant survival rate. Newly developed 3D printing direct energy deposition (DED) has superior resistance to abrasion as compared to traditional methods. However, there is still concern about the mechanical stability and the risk of osteolysis by the titanium (Ti) nanoparticles. Therefore, in this work, we investigated whether DED Ti-coated cobalt-chrome (CoCr) alloys induce chronic inflammation reactions through in vitro and in vivo models. We studied three types of implant surfaces (smooth, sand-blasted, and DED Ti-coated) to compare their inflammatory reaction. We conducted the in vitro effect of specimens using the cell counting kit-8 (CCK-8) assay and an inflammatory cytokine assay. Subsequently, in vivo analysis of the immune profiling, cytokine assay, and histomorphometric evaluation using C57BL/6 mice were performed. There were no significant differences in the CCK-8 assay, the cytokine assay, and the immune profiling assay. Moreover, there were no difference for semi-quantitative histomorphometry analysis at 4 and 8 weeks among the sham, smooth, and DED Ti-coated samples. These results suggest that DED Ti-coated printing technique do not induce chronic inflammation both in vitro and in vivo. It has biocompatibility for being used as a surface coating of TKA implant.

Decellularized brain matrix enhances macrophage polarization and functional improvements in rat spinal cord injury

Spinal cord injury (SCI) is a devastating lesion lacking effective treatment options currently available in clinics. The inflammatory process exacerbates the extent of the lesion through a secondary injury mechanism, where proinflammatory classically activated macrophages (M1) are prevalent at the lesion site. However, the polarized alternatively activated anti-inflammatory macrophages (M2) are known to play an important role in wound healing and regeneration following SCI. Herein, we introduce porcine brain decellularized extracellular matrix (dECM) to modulate the macrophages in the injured spinal cord. The hydrogels with collagen and dECM at various dECM concentrations (1, 5, and 8 mg/ml) were used to cultivate primary macrophages and neurons. The dECM hydrogels were shown to promote the polarization of macrophages toward M2 phase and the neurite outgrowth of cortical and hippocampal neurons. When the dECM hydrogels were applied to rat SCI models, the proportion of M1 and M2 macrophages in the injured spinal cord was substantially altered. When received dECM concetration of 5 mg/ml, the expression of molecules associated with M2 (CD206, arginase1, and IL-10) was significantly increased. Consistently, the population of total macrophages and cavity area were substantially reduced in the dECM-treated groups. As a result, the locomotor functions of injured spinal cord, as assessed by BBB and ladder scoring, were significantly improved. Collectively, the porcine brain dECM with optimal concentration promotes functional recovery in SCI models through the activation of M2 macrophages, suggesting the promising use of the engineered hydrogels in the treatment of acute SCI. STATEMENT OF SIGNIFICANCE: Spinal cord injury (SCI) is a devastating lesion, lacking effective treatment options currently available in clinics. Here we delineated that the treatment of injured spinal cord with porcine brain decellularized matrix-based hydrogels for the first time, and could modulate the macrophage polarization and the ultimate functional recovery. When appropriate formulations were applied to a contused spinal cord model in rats, the decellularized matrix hydrogels shifted the macrophages to polarize to pro-regenerative M2 phenotype, decreased the size of lesion cavity, and finally promoted the locomotor functions until 8 weeks following the injury. We consider this work can significantly augment the matrix(biomaterial)-based therapeutic options, as an alternative to drug or cell-free approaches, for the treatment of acute injury of spinal cord.

A Biodegradable Mg-Based Alloy Inhibited the Inflammatory Response of THP-1 Cell-Derived Macrophages Through the TRPM7-PI3K-AKT1 Signaling Axis

Mg-based alloys might be ideal biomaterials in clinical applications owing to favorable mechanical properties, biodegradability, biocompatibility, and especially their anti-inflammatory properties. However, the precise signaling mechanism underlying the inhibition of inflammation by Mg-based alloys has not been elucidated. Here, we investigated the effects of a Mg-2.1Nd-0.2Zn-0.5Zr alloy (denoted as JDBM) on lipopolysaccharide (LPS)-induced macrophages. THP-1 cell-derived macrophages were cultured on JDBM, Ti-6Al-4V alloy (Ti), 15% extract of JDBM, and 7.5 mM of MgCl2 for 1 h before the addition of LPS for an indicated time; the experiments included negative and positive controls. Our results showed JDBM, extract, and MgCl2 could decrease LPS-induced tumor necrosis factor (TNF) and interleukin (IL)-6 expression. However, there were no morphologic changes in macrophages on Ti or JDBM. Mechanically, extract and MgCl2 downregulated the expression of toll-like receptor (TLR)-4 and MYD88 compared with the positive control and inhibited LPS-induced nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways by inactivation of the phosphorylation of IKK-α/β, IKβ-α, P65, P38, and JNK. Additionally, the LPS-induced reactive oxygen species (ROS) expression was also decreased by extract and MgCl2. Interestingly, the expression of LPS-induced TNF and IL-6 could be recovered by knocking down TRPM7 of macrophages, in the presence of extract or MgCl2. Mechanically, the activities of AKT and AKT1 were increased by extract or MgCl2 with LPS and were blocked by a PI3K inhibitor, whereas siRNA TRPM7 inhibited only AKT1. Together, our results demonstrated the degradation products of Mg-based alloy, especially magnesium, and resolved inflammation by activation of the TRPM7-PI3K-AKT1 signaling pathway, which may be a potential advantage or target to promote biodegradable Mg-based alloy applications.

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.

Lithium chloride with immunomodulatory function for regulating titanium nanoparticle-stimulated inflammatory response and accelerating osteogenesis through suppression of MAPK signaling pathway

Background

Wear particle-induced inflammatory osteolysis and the consequent aseptic loosening constitute the leading reasons for prosthesis failure and revision surgery. Several studies have demonstrated that the macrophage polarization state and immune response play critical roles in periprosthetic osteolysis and tissue repair, but the immunomodulatory role of lithium chloride (LiCl), which has a protective effect on wear particle-induced osteolysis by suppressing osteoclasts and attenuating inflammatory responses, has never been investigated.

Methods

In this work, the immunomodulatory capability of LiCl on titanium (Ti) nanoparticle-stimulated transformation of macrophage phenotypes and the subsequent effect on osteogenic differentiation were investigated. We first speculated that LiCl attenuated Ti nanoparticle-stimulated inflammation responses by driving macrophage polarization and generating an immune micro-environment to improve osteogenesis. Furthermore, a metal nanoparticle-stimulated murine air pouch inflammatory model was applied to confirm this protective effect in vivo.

Results

The results revealed that metal nanoparticles significantly activate M1 phenotype (proinflammatory macrophage) expression and increase proinflammatory cytokines secretions in vitro and in vivo, whereas LiCl drives macrophages to the M2 phenotype (anti-inflammatory macrophage) and increases the release of anti-inflammatory and bone-related cytokines. This improved the osteogenic differentiation capability of rat bone marrow mesenchymal stem cells (rBMSCs). In addition, we also provided evidence that LiCl inhibits the phosphorylation of the p38 mitogen-activated protein kinase (p38) and extracellular signal-regulated kinase (ERK) pathways in wear particle-treated macrophages.

Conclusion

LiCl has the immunomodulatory effects to alleviate Ti nanoparticle-mediated inflammatory reactions and enhance the osteogenic differentiation of rBMSCs by driving macrophage polarization. Thus, LiCl may be an effective therapeutic alternative for preventing and treating wear debris-induced inflammatory osteolysis.

Nanostructural Surfaces with Different Elastic Moduli Regulate the Immune Response by Stretching Macrophages

A proper immune response is key for the successful implantation of biomaterials, and designing and fabricating biomaterials to regulate immune responses is the future trend. In this work, three different nanostructures were constructed on the surface of titanium using a hydrothermal method, and through a series of in vitro and in vivo experiments, we found that the aspect ratio of nanostructures can affect the elastic modulus of a material surface and further regulate immune cell behaviors. This work demonstrates that nanostructures with a higher aspect ratio can endow a material surface with a lower elastic modulus, which was confirmed by experiments and theoretical analyses. The deflection of nanostructures under the cell adsorption force is a substantial factor in stretching macrophages to enhance cell adhesion and spreading, further inducing macrophage polarization toward the M1 phenotype and leading to intense immune responses. In contrast, a nanostructure with a lower aspect ratio on a material surface leads to a higher surface elastic modulus, making deflection of the material difficult and creating a surface that is not conducive to macrophage adhesion and spreading, thus reducing the immune response. Moreover, molecular biology experiments indicated that regulation of the immune response by the elastic modulus is primarily related to the NF-κB signaling pathway. These findings suggest that the immune response can be regulated by constructing nanostructural surfaces with the proper elastic modulus through their influence on cell adhesion and spreading, which provides new insights into the surface design of biomaterials.

2D Nanoclay for Biomedical Applications: Regenerative Medicine, Therapeutic Delivery, and Additive Manufacturing

Clay nanomaterials are an emerging class of 2D biomaterials of interest due to their atomically thin layered structure, charged characteristics, and well-defined composition. Synthetic nanoclays are plate-like polyions composed of simple or complex salts of silicic acids with a heterogeneous charge distribution and patchy interactions. Due to their biocompatible characteristics, unique shape, high surface-to-volume ratio, and charge, nanoclays are investigated for various biomedical applications. Here, a critical overview of the physical, chemical, and physiological interactions of nanoclay with biological moieties, including cells, proteins, and polymers, is provided. The state-of-the-art biomedical applications of 2D nanoclay in regenerative medicine, therapeutic delivery, and additive manufacturing are reviewed. In addition, recent developments that are shaping this emerging field are discussed and promising new research directions for 2D nanoclay-based biomaterials are identified.

Bioactive Materials Facilitating Targeted Local Modulation of Inflammation

Cardiovascular disease is an inflammatory disorder that may benefit from appropriate modulation of inflammation. Systemic treatments lower cardiac events but have serious adverse effects. Localized modulation of inflammation in current standard treatments such as bypass grafting may more effectively treat CAD. The present study investigated a bioactive vascular graft coated with the macrophage polarizing cytokine interleukin-4. These grafts repolarize macrophages to anti-inflammatory phenotypes, leading to modulation of the pro-inflammatory microenvironment and ultimately to a reduction of foreign body encapsulation and inhibition of neointimal hyperplasia development. These resulting functional improvements have significant implications for the next generation of synthetic vascular grafts.

The nuclear variant of bone morphogenetic protein 2 (nBMP2) is expressed in macrophages and alters calcium response

We previously identified a nuclear variant of bone morphogenetic protein 2 (BMP2), named nBMP2, that is translated from an alternative start codon. Decreased nuclear localization of nBMP2 in the nBmp2NLStm mouse model leads to muscular, neurological, and immune phenotypes-all of which are consistent with aberrant intracellular calcium (Ca2+) response. Ca2+ response in these mice, however, has yet to be measured directly. Because a prior study suggested impairment of macrophage function in nBmp2NLStm mutant mice, bone marrow derived (BMD) macrophages and splenic macrophages were isolated from wild type and nBmp2NLStm mutant mice. Immunocytochemistry revealed that nuclei of both BMD and splenic macrophages from wild type mice contain nBMP2, while the protein is decreased in nuclei of nBmp2NLStm mutant macrophages. Live-cell Ca2+ imaging and engulfment assays revealed that Ca2+ response and phagocytosis in response to bacterial supernatant are similar in BMD macrophages isolated from naïve (uninfected) nBmp2NLStm mutant mice and wild type mice, but are deficient in splenic macrophages isolated from mutant mice after secondary systemic infection with Staphylococcus aureus, suggesting progressive impairment as macrophages respond to infection. This direct evidence of impaired Ca2+ handling in nBMP2 mutant macrophages supports the hypothesis that nBMP2 plays a role in Ca2+ response.

The inflammasome in host response to biomaterials: Bridging inflammation and tissue regeneration

The development of new biomaterials to be used in tissue engineering applications is creating new solutions for a range of healthcare problems. The trend in biomaterials research has shifted from biocompatible "immune-evasive" biomaterials to "immune-interactive" materials that modulate the inflammatory response supporting implant integration as well as improving healing and tissue regeneration. Inflammasomes are large intracellular multiprotein complexes that are key players in host defence during innate immune responses and assemble after recognition of pathogens or danger signals. The process of biomaterial implantation causes injury to tissues that will consequently release danger signals that could be sensed by the inflammasome. There are increasing evidences that the inflammasome has a role in several inflammatory processes, from pathogen clearance to chronic inflammation or tissue repair. Thus, modulation of the inflammasome activity appears as an important target in the development of effective approaches in regenerative medicine. In this review, we discuss the main points of the current understanding on the host response to implanted biomaterials and how the paradigm of "immune-evasive" biomaterials has shifted over the last years; the significance of the inflammasome in the inflammatory response to biomaterials; and the growing idea that the immune system is of key importance in an effective tissue repair and regeneration. STATEMENT OF SIGNIFICANCE: We herein discuss the main points of the current understanding on the host response to implanted biomaterials and how the paradigm of "immune-evasive" biomaterials has shifted to "immune-interactive" over the last years; the significance of the inflammasome in the inflammatory response to biomaterials; and the growing idea that the immune system is of key importance in an effective tissue repair and regeneration, supporting the emerging concept of Regenerative Immunology. The inflammasome is a recent and central concept in immunology research. Since the beginning of this century the inflammasome is viewed as key platform of the innate immune response. We believe that, successful modulation of the inflammasome activity will become a milestone in the fields of tissue engineering and regenerative medicine.

Autocrine VEGF signalling on M2 macrophages regulates PD-L1 expression for immunomodulation of T cells

M2‐polarized macrophages, on one hand, can promote tumour vascularization by producing proangiogenic factors, such as vascular endothelial growth factor (VEGF). On the other hand, the expression of VEGF receptors (VEGFR) in this cell lineage was also reported. Although the function of VEGF/VEGFR axis plays a pivotal role in macrophages infiltration and angiogenesis, however, there is still lack of the direct evidence to show the role of VEGF as an autocrine operating in M2 macrophages, particularly for immunomodulation. In our study, we surprisingly discovered that M2 macrophages polarized by baicalin can simultaneously express VEGF and its receptors. Taking advantage of this unique culture system, we were able to investigate the biological activity of M2 macrophages in response to the autocrine VEGF milieu. Our results showed that the expression of programmed death‐ligand 1 (PD‐L1) on M2 macrophages was significantly up‐regulated in autocrine VEGF milieu. Through the blockade of autocrine VEGF signalling, PD‐L1 expression on M2 macrophages was dramatically down‐regulated. Furthermore, transplantation of PD‐L1⁺ M2 macrophage stimulated by autocrine VEGF into allogeneic mice significantly suppressed host CD4⁺/CD8⁺ T cells in the peripheral blood and increased CD4⁺CD25⁺ regulatory T cells in the bone marrow. In conclusion, our findings provide a novel biological basis to support the current successful strategy using combined VEGF/PD‐1 signalling blockade in cancer therapy.

Zinc-Modified Sulfonated Polyetheretherketone Surface with Immunomodulatory Function for Guiding Cell Fate and Bone Regeneration

The cytokines released by immune cells are considered important factors to induce bone tissue regeneration. However, the pathway of those bone-targeting macrophage cytokines induced by biomaterial surface under tissue microenvironment is rarely reported. In this study, the immunomodulatory capability of zinc ions on macrophage polarization and its effects on osteogenic differentiation are investigated. Hence, a layer of zinc ions are incorporated on sulfonated polyetheretherketone (SPEEK) biomaterials by using a customized magnetron sputtering technique. The results reveal that the microenvironment on Zn-coated SPEEK can modulate nonactivated macrophage polarization to an anti-inflammatory phenotype and induce the secretion of anti-inflammatory and osteogenic cytokines. The osteogenic differentiation capability of bone marrow stromal cells (BMSCs) is therefore enhanced, leading to improved osteointegration between the zinc-coated SPEEK and bone tissue. This study verifies that zinc ion is a promising additive in the osteoimmunomodulation process and provides knowledge that may pave the way to develop the next generation of immunomodulatory biomaterials.

Effects of phosphatidylserine-containing supported lipid bilayers on the polarization of macrophages

Placement of dental implants initiates inflammatory foreign body response, in which macrophages play a central role and affect the subsequent tissue healing process such as bone formation. The purpose of this study was to fabricate phosphatidylserine (PS)-containing supported lipid bilayers (SLBs) on a titanium surface to regulate the polarization of macrophages, a critical factor that affects following tissue healing and regeneration. The fluorescent recovery after photobleaching images showed that the percentage of PS had a critical influence on the fluidity, and 20% PS had the highest fluidity. Furthermore, more expanded and elongated cells were observed in the SLB-coated groups. transforming growth factor-β1 and vascular endothelial growth factor, the key cytokine markers of M2 macrophage polarization, were increased in the SLB-coated groups, especially in the 20% PS group. Consistently, cells cultured on the SLB-coated titanium exhibited the distribution of CD206⁺ , which is a M2 macrophage specific maker. The results of this study demonstrated M2 polarization of macrophages on PS-SLB-coated titanium discs, which suggests the application of PS-SLB as an immune-regulating coating material to improve tissue reactions to dental implants. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 2625-2633, 2018.

Nanopatterned bulk metallic glass-based biomaterials modulate macrophage polarization

Polarization of macrophages by chemical, topographical and mechanical cues presents a robust strategy for designing immunomodulatory biomaterials. Here, we studied the ability of nanopatterned bulk metallic glasses (BMGs), a new class of metallic biomaterials, to modulate murine macrophage polarization. Cytokine/chemokine analysis of IL-4 or IFNγ/LPS-stimulated macrophages showed that the secretion of TNF-α, IL-1α, IL-12, CCL-2 and CXCL1 was significantly reduced after 24-hour culture on BMGs with 55 nm nanorod arrays (BMG-55). Additionally, under these conditions, macrophages increased phagocytic potential and exhibited decreased cell area with multiple actin protrusions. These in vitro findings suggest that nanopatterning can modulate biochemical cues such as IFNγ/LPS. In vivo evaluation of the subcutaneous host response at 2 weeks demonstrated that the ratio of Arg-1 to iNOS increased in macrophages adjacent to BMG-55 implants, suggesting modulation of polarization. In addition, macrophage fusion and fibrous capsule thickness decreased and the number and size of blood vessels increased, which is consistent with changes in macrophage responses. Our study demonstrates that nanopatterning of BMG implants is a promising technique to selectively polarize macrophages to modulate the immune response, and also presents an effective tool to study mechanisms of macrophage polarization and function.

STATEMENT OF SIGNIFICANCE

Implanted biomaterials elicit a complex series of tissue and cellular responses, termed the foreign body response (FBR), that can be influenced by the polarization state of macrophages. Surface topography can influence polarization, which is broadly characterized as either inflammatory or repair-like. The latter has been linked to improved outcomes of the FBR. However, the impact of topography on macrophage polarization is not fully understood, in part, due to a lack of high moduli biomaterials that can be reproducibly processed at the nanoscale. Here, we studied macrophage interactions with nanopatterned bulk metallic glasses (BMGs), a class of metallic alloys with amorphous microstructure and formability like polymers. We show that nanopatterned BMGs modulate macrophage polarization and transiently induce less fibrotic and more angiogenic responses. Overall, we demonstrate nanopatterning of BMG implants as a technique to polarize macrophages and modulate the FBR.

Effects of titanium surface roughness on the mediation of osteogenesis via modulating the immune response of macrophages

Osteoblastic lineage cells are commonly used to evaluate the in vitro osteogenic ability of bone biomaterials. However, contradictory results obtained from in vivo and in vitro studies are not uncommon. With the increasing understanding of osteoimmunology, the immune response has been recognized as playing an important role in bone regeneration. In this study, we examined the effect of submicron-scaled titanium surface roughness (ranging from approximately 100 to 400 nm) on the response of osteoblasts and macrophages. The results showed that osteoblast differentiation enhanced with increased surface roughness of titanium substrates. The cytoskeleton of macrophages altered with the variation in titanium surface roughness. The production of cytokines (TNF-α, IL-6, IL-4 and IL-10) could be regulated by titanium surface roughness. Moreover, macrophages cultured on titanium surfaces exhibited a tendency to polarize to M1 phenotype with the increase of surface roughness. Material/macrophage conditioned medium tended to promote osteoblast differentiation with the increase of surface roughness. The results indicate that increasing surface roughness in the submicron range is beneficial for osteogenesis via modulating the immune response of macrophages. Modifying biomaterial surfaces based on their immunomodulatory effects is considered as a novel strategy for the improvement of their biological performance.

Magnesium enhances the chondrogenic differentiation of mesenchymal stem cells by inhibiting activated macrophage-induced inflammation

Magnesium deficiency increases the generation of pro-inflammatory cytokines, which is consistently accompanied by the sensitization of cells such as neutrophils, macrophages and endothelial cells. We investigated the potential of magnesium to regulate macrophage polarization and macrophage-induced inflammation with or without lipopolysaccharide (LPS) and interferon-γ (IFN-γ) activation and further elucidated whether these effects impact the inhibitory functions of activated macrophage-induced inflammation on cartilage regeneration. The results showed that magnesium inhibited the activation of macrophages, as indicated by a significant reduction in the percentage of CCR7-positive cells, while the percentage of CD206-positive cells decreased to a lesser degree. After activation, both pro-inflammatory and anti-inflammatory cytokines were down-regulated at the mRNA level and certain cytokines (IL-1β, IL-6 and IL-10) were decreased in the cell supernatant with the addition of magnesium. Moreover, magnesium decreased the nuclear translocation and phosphorylation of nuclear factor-κB (NF-κB) to impede its activation. A modified micromass culture system was applied to assess the effects of activated macrophage-conditioned medium with or without magnesium treatment on the chondrogenic differentiation of human bone marrow mesenchymal stem cells (hBMSCs). Magnesium enhanced the chondrogenic differentiation of hBMSCs by reversing the adverse effects of activated macrophage-induced inflammation.

Valence State Manipulation of Cerium Oxide Nanoparticles on a Titanium Surface for Modulating Cell Fate and Bone Formation

Understanding cell-biomaterial interactions is critical for the control of cell fate for tissue engineering and regenerative medicine. Here, cerium oxide nanoparticles (CeONPs) are applied at different Ce4+/Ce3+ ratios (i.e., 0.46, 1.23, and 3.23) to titanium substrate surfaces by magnetron sputtering and vacuum annealing. Evaluation of the cytotoxicity of the modified surface to cultured rat bone marrow mesenchymal stem cells (BMSCs) reveals that the cytocompatibility and cell proliferation are proportional to the increases in Ce4+/Ce3+ ratio on titanium surface. The bone formation capability induced by these surface modified titanium alloys is evaluated by implanting various CeONP samples into the intramedullary cavity of rat femur for 8 weeks. New bone formation adjacent to the implant shows a close relationship to the surface Ce4+/Ce3+ ratio; higher Ce4+/Ce3+ ratio achieves better osseointegration. The mechanism of this in vivo outcome is explored by culturing rat BMSCs and RAW264.7 murine macrophages on CeONP samples for different durations. The improvement in osteogenic differentiation capability of BMSCs is directly proportional to the increased Ce4+/Ce3+ ratio on the titanium surface. Increases in the Ce4+/Ce3+ ratio also elevate the polarization of the M2 phenotype of RAW264.7 murine macrophages, particularly with respect to the healing-associated M2 percentage and anti-inflammatory cytokine secretion. The manipulation of valence states of CeONPs appears to provide an effective modulation of the osteogenic capability of stem cells and the M2 polarization of macrophages, resulting in favorable outcomes of new bone formation and osseointegration.

Immunomodulatory Effects of Calcium and Strontium Co-Doped Titanium Oxides on Osteogenesis

The effects of calcium (Ca) or strontium (Sr) on host osteogenesis and immune responses have been investigated separately. In clinical practice, these two elements may both be present around an orthopedic device, but their potential synergistic effects on osteogenesis and the immune response have not been explored to date. In this work, we investigated the immunomodulatory effects of Ca and Sr co-doped titanium oxides on osteogenesis in vitro using the mouse macrophage cell line RAW 264.7 alone and in co-culture with mouse bone mesenchymal stem cells (BMSCs), and in vivo using a mouse air-pouch model. Coatings containing Ca and Sr at different concentration ratios were fabricated on titanium substrates using micro-arc oxidation and electrochemical treatment. The in vitro and in vivo results demonstrated that the Ca and Sr concentration ratio has a marked influence on macrophage polarization. The coating with a Ca/Sr ratio of 2:1 was superior to those with other Ca and/or Sr concentrations in terms of modulating M2 polarization, which enhanced osteogenic differentiation of mouse BMSCs in co-culture. These findings suggest that the osteoimmunomodulatory effect of a titanium-oxide coating can be enhanced by modulating the concentration ratio of its components.