Instructing Human Macrophage Polarization by Stiffness and Glycosaminoglycan Functionalization in 3D Collagen Networks

Advances and applications of biomimetic biomaterials for endogenous skin regeneration

Endogenous regeneration is becoming an increasingly important strategy for wound healing as it facilitates skin's own regenerative potential for self-healing, thereby avoiding the risks of immune rejection and exogenous infection. However, currently applied biomaterials for inducing endogenous skin regeneration are simplistic in their structure and function, lacking the ability to accurately mimic the intricate tissue structure and regulate the disordered microenvironment. Novel biomimetic biomaterials with precise structure, chemical composition, and biophysical properties offer a promising avenue for achieving perfect endogenous skin regeneration. Here, we outline the recent advances in biomimetic materials induced endogenous skin regeneration from the aspects of structural and functional mimicry, physiological process regulation, and biophysical property design. Furthermore, novel techniques including in situ reprograming, flexible electronic skin, artificial intelligence, single-cell sequencing, and spatial transcriptomics, which have potential to contribute to the development of biomimetic biomaterials are highlighted. Finally, the prospects and challenges of further research and application of biomimetic biomaterials are discussed. This review provides reference to address the clinical problems of rapid and high-quality skin regeneration.

Dual-crosslinking gelatin-hyaluronic acid methacrylate based biomimetic PDAC desmoplastic niche enhances tumor-associated macrophages recruitment and M2-like polarization

The tumor microenvironment (TME) of pancreatic ductal adenocarcinoma (PDAC) is characterized by deposition of desmoplastic matrix (including collagen and hyaluronic acid). And the interactions between tumor-associated macrophages (TAMs) and tumor cells play a crucial role in progression of PDAC. Hence, the appropriate model of tumor cell-macrophage interaction within the unique PDAC TME is of significantly important. To this end, a 3D tumor niche based on dual-crosslinking gelatin methacrylate and hyaluronic acid methacrylate hydrogels was constructed to simulate the desmoplastic tumor matrix with matching compressive modulus and composition. The bionic 3D tumor niche creates an immunosuppressive microenvironment characterized by the downregulation of M1 markers and upregulation of M2 markers in TAMs. Mechanistically, RNA-seq analysis revealed that the PI3K-AKT signaling pathway might modulate the phenotypic balance and recruitment of macrophages through regulating SELE and VCAM-1. Furthermore, GO and GSEA revealed the biological process of leukocyte migration and the activation of cytokine-associated signaling were involved. Finally, the 3D tumor-macrophage niches with three different ratios were fabricated which displayed increased M2-like polarization and stemness. The utilization of the 3D tumor niche has the potential to provide a more accurate investigation of the interplay between PDAC tumor cells and macrophages within an in vivo setting.

Immunomodulatory hydrogels for skin wound healing: cellular targets and design strategy

Skin wounds significantly impact the global health care system and represent a significant burden on the economy and society due to their complicated dynamic healing processes, wherein a series of immune events are required to coordinate normal and sequential healing phases, involving multiple immunoregulatory cells such as neutrophils, macrophages, keratinocytes, and fibroblasts, since dysfunction of these cells may impede skin wound healing presenting persisting inflammation, impaired vascularization, and excessive collagen deposition. Therefore, cellular target-based immunomodulation is promising to promote wound healing as cells are the smallest unit of life in immune response. Recently, immunomodulatory hydrogels have become an attractive avenue to promote skin wound healing. However, a detailed and comprehensive review of cellular targets and related hydrogel design strategies remains lacking. In this review, the roles of the main immunoregulatory cells participating in skin wound healing are first discussed, and then we highlight the cellular targets and state-of-the-art design strategies for immunomodulatory hydrogels based on immunoregulatory cells that cover defect, infected, diabetic, burn and tumor wounds and related scar healing. Finally, we discuss the barriers that need to be addressed and future prospects to boost the development and prosperity of immunomodulatory hydrogels.

Linking cell mechanical memory and cancer metastasis

Metastasis causes most cancer-related deaths; however, the efficacy of anti-metastatic drugs is limited by incomplete understanding of the biological mechanisms that drive metastasis. Focusing on the mechanics of metastasis, we propose that the ability of tumour cells to survive the metastatic process is enhanced by mechanical stresses in the primary tumour microenvironment that select for well-adapted cells. In this Perspective, we suggest that biophysical adaptations favourable for metastasis are retained via mechanical memory, such that the extent of memory is influenced by both the magnitude and duration of the mechanical stress. Among the mechanical cues present in the primary tumour microenvironment, we focus on high matrix stiffness to illustrate how it alters tumour cell proliferation, survival, secretion of molecular factors, force generation, deformability, migration and invasion. We particularly centre our discussion on potential mechanisms of mechanical memory formation and retention via mechanotransduction and persistent epigenetic changes. Indeed, we propose that the biophysical adaptations that are induced by this process are retained throughout the metastatic process to improve tumour cell extravasation, survival and colonization in the distant organ. Deciphering mechanical memory mechanisms will be key to discovering a new class of anti-metastatic drugs.

The role of macrophage polarization in tendon healing and therapeutic strategies: Insights from animal models

Tendon injuries, a common musculoskeletal issue, usually result in adhesions to the surrounding tissue, that will impact functional recovery. Macrophages, particularly through their M1 and M2 polarizations, play a pivotal role in the inflammatory and healing phases of tendon repair. In this review, we explore the role of macrophage polarization in tendon healing, focusing on insights from animal models. The review delves into the complex interplay of macrophages in tendon pathology, detailing how various macrophage phenotypes contribute to both healing and adhesion formation. It also explores the potential of modulating macrophage activity to enhance tendon repair and minimize adhesions. With advancements in understanding macrophage behavior and the development of innovative biomaterials, this review highlights promising therapeutic strategies for tendon injuries.

Lab-on-chip technologies for exploring the gut-immune axis in metabolic disease

The continued rise in metabolic diseases such as obesity and type 2 diabetes mellitus poses a global health burden, necessitating further research into factors implicated in the onset and progression of these diseases. Recently, the gut-immune axis, with diet as a main regulator, has been identified as a possible role player in their development. Translation of conventional 2D in vitro and animal models is however limited, while human studies are expensive and preclude individual mechanisms from being investigated. Lab-on-chip technology therefore offers an attractive new avenue to study gut-immune interactions. This review provides an overview of the influence of diet on gut-immune interactions in metabolic diseases and a critical analysis of the current state of lab-on-chip technology to study this axis. While there has been progress in the development of "immuno-competent" intestinal lab-on-chip models, with studies showing the ability of the technology to provide mechanical cues, support longer-term co-culture of microbiota and maintain in vivo-like oxygen gradients, platforms which combine all three and include intestinal and immune cells are still lacking. Further, immune cell types and inclusion of microenvironment conditions which enable in vivo-like immune cell dynamics as well as host-microbiome interactions are limited. Future model development should focus on combining these conditions to create an environment capable of hosting more complex microbiota and immune cells to allow further study into the effects of diet and related metabolites on the gut-immune ecosystem and their role in the prevention and development of metabolic diseases in humans.

Biomaterial-based mechanical regulation facilitates scarless wound healing with functional skin appendage regeneration

Scar formation resulting from burns or severe trauma can significantly compromise the structural integrity of skin and lead to permanent loss of skin appendages, ultimately impairing its normal physiological function. Accumulating evidence underscores the potential of targeted modulation of mechanical cues to enhance skin regeneration, promoting scarless repair by influencing the extracellular microenvironment and driving the phenotypic transitions. The field of skin repair and skin appendage regeneration has witnessed remarkable advancements in the utilization of biomaterials with distinct physical properties. However, a comprehensive understanding of the underlying mechanisms remains somewhat elusive, limiting the broader application of these innovations. In this review, we present two promising biomaterial-based mechanical approaches aimed at bolstering the regenerative capacity of compromised skin. The first approach involves leveraging biomaterials with specific biophysical properties to create an optimal scarless environment that supports cellular activities essential for regeneration. The second approach centers on harnessing mechanical forces exerted by biomaterials to enhance cellular plasticity, facilitating efficient cellular reprogramming and, consequently, promoting the regeneration of skin appendages. In summary, the manipulation of mechanical cues using biomaterial-based strategies holds significant promise as a supplementary approach for achieving scarless wound healing, coupled with the restoration of multiple skin appendage functions.

Matrix stiffness affects tumor-associated macrophage functional polarization and its potential in tumor therapy

The extracellular matrix (ECM) plays critical roles in cytoskeletal support, biomechanical transduction and biochemical signal transformation. Tumor-associated macrophage (TAM) function is regulated by matrix stiffness in solid tumors and is often associated with poor prognosis. ECM stiffness-induced mechanical cues can activate cell membrane mechanoreceptors and corresponding mechanotransducers in the cytoplasm, modulating the phenotype of TAMs. Currently, tuning TAM polarization through matrix stiffness-induced mechanical stimulation has received increasing attention, whereas its effect on TAM fate has rarely been summarized. A better understanding of the relationship between matrix stiffness and macrophage function will contribute to the development of new strategies for cancer therapy. In this review, we first introduced the overall relationship between macrophage polarization and matrix stiffness, analyzed the changes in mechanoreceptors and mechanotransducers mediated by matrix stiffness on macrophage function and tumor progression, and finally summarized the effects of targeting ECM stiffness on tumor prognosis to provide insight into this new field.

Physicochemical Characterization and Immunomodulatory Activity of Polyelectrolyte Multilayer Coatings Incorporating an Exopolysaccharide from Bifidobacterium longum

Immunoregulatory polysaccharides from probiotic bacteria have potential in biomedical engineering. Here, a negatively charged exopolysaccharide from Bifidobacterium longum with confirmed immunoregulatory activity (EPS624) was applied in multilayered polyelectrolyte coatings with positively charged chitosan. EPS624 and coatings (1, 5, and 10 layers and alginate-substituted) were characterized by the zeta potential, dynamic light scattering, size exclusion chromatography, scanning electron microscopy, and atomic force microscopy. Peripheral blood mononuclear cells (hPBMCs) and fibroblasts were exposed for 1, 3, 7, and 10 days with cytokine secretion, viability, and morphology as observations. The coatings showed an increased rugosity and exponential growth mode with an increasing number of layers. A dose/layer-dependent IL-10 response was observed in hPBMCs, which was greater than EPS624 in solution and was stable over 7 days. Fibroblast culture revealed no toxicity or metabolic change after exposure to EPS624. The EPS624 polyelectrolyte coatings are cytocompatible, have immunoregulatory properties, and may be suitable for applications in biomedical engineering.

ECM-engineered electrospun fibers with an immune cascade effect for inhibiting tissue fibrosis

Tissue regeneration/fibrosis after injury is intricately regulated by the immune cascade reaction and extracellular matrix (ECM). Dysregulated cascade signal could jeopardize tissue homeostasis leading to fibrosis. Bioactive scaffolds mimicking natural ECM microstructure and chemistry could regulate the cascade reaction to achieve tissue regeneration. The current study constructed an ECM-engineered micro/nanofibrous scaffold using self-assembled nanofibrous collagen and decorin (DCN)-loaded microfibers to regulate the immune cascade reaction. The ECM-engineered scaffold promoted anti-inflammatory and pro-regenerative effects, M2 polarization of macrophages, by nanofibrous collagen. The ECM-engineered scaffold could release DCN to inhibit inflammation-associated fibrous angiogenesis. Yet, to prevent excessive M2 activity leading to tissue fibrosis, controlled release of DCN was expected to elicit M1 activity and achieve M1/M2 balance in the repair process. Regulated cascade reaction guided favorable crosstalk between macrophages, endothelial cells and fibroblasts by proximity. Additionally, decorin could also antagonize TGF-β1 via TGF-β/Smad3 pathway to suppress fibrotic activity of fibroblasts. Hence, ECM-engineered scaffolds could exert effective regulation of the immune cascade reaction by microstructure and DCN release and achieve the balance between tissue fibrosis and regeneration. STATEMENT OF SIGNIFICANCE: With the incidence of up to 74.6%, failed back surgery syndrome (FBSS) has been a lingering issue in spine surgery, which poses a heavy socio-economic burden to society. Epidural fibrosis is believed to be responsible for the onset of FBSS. Current biomaterial-based strategies treating epidural fibrosis mainly rely on physical barriers and unidirectional suppression of inflammation. Regulation of the immune cascade reaction for inhibiting fibrosis has not been widely studied. Based on the simultaneous regulation of M1/M2 polarization and intercellular crosstalk, the ECM-engineered micro/nanofibrous scaffolds constructed in the current study could exert an immune cascade effect to coordinate tissue regeneration and inhibit fibrosis. This finding makes a significant contribution in the development of a treatment for epidural fibrosis and FBSS.

Components and physical properties of hydrogels modulate inflammatory response and cartilage repair

Collagen and hyaluronic acid are commonly applied in cartilage tissue engineering, yet there has been limited investigation into their inflammatory response, a crucial factor in articular cartilage repair. This study aimed to evaluate the impact of components and physical properties of hydrogels on inflammatory response and cartilage repair. Three kinds of hydrogels with comparable storage moduli at low frequencies were designed and fabricated, namely, methacrylic anhydride-modified hyaluronic acid hydrogel (HAMA), methacrylic anhydride-modified type I collagen hydrogel (CMA) and unmodified type I collagen hydrogel (Col). HAMA hydrogel was unfavorable for adhesion and spreading of BMSCs. Furthermore, HAMA hydrogel stimulated rapid migration and pro-inflammatory M1 polarization of macrophages, leading to persistent and intense inflammation, which was unfavorable for cartilage repair. CMA and Col hydrogels possessed the same component and facilitated the adhesion, spreading and proliferation of BMSCs. Compared with CMA hydrogel, Col hydrogel induced rapid migration and moderate M1 polarization of macrophages at the early stage of injury, which was mainly influenced by its fast dissolution rate, small pore size fiber network structure and rapid stress relaxation. In addition, the phenotype of macrophages timely transformed into anti-inflammatory M2 due to the properties of the collagen component, which shortened the duration of inflammation and enhanced cartilage repair. The results indicated that moderate macrophage activation adjusted by hydrogel components and physical properties was critical in modulating inflammation and cartilage regeneration.

Potential effects of biomaterials on macrophage function and their signalling pathways

The use of biomaterials in biomedicine and healthcare has increased in recent years. Macrophages are the primary immune cells that induce inflammation and tissue repair after implantation of biomaterials. Given that macrophages exhibit high heterogeneity and plasticity, the influence of biomaterials on macrophage phenotype should be considered a crucial evaluation criterion during the development of novel biomaterials. This review provides a comprehensive summary of the physicochemical, biological, and dynamic characteristics of biomaterials that drive the regulation of immune responses in macrophages. The mechanisms involved in the interaction between macrophages and biomaterials, including endocytosis, receptors, signalling pathways, integrins, inflammasomes and long non-coding RNAs, are summarised in this review. In addition, research prospects of the interaction between macrophages and biomaterials are discussed. An in-depth understanding of mechanisms underlying the spatiotemporal changes in macrophage phenotype induced by biomaterials and their impact on macrophage polarization can facilitate the identification and development of novel biomaterials with superior performance. These biomaterials may be used for tissue repair and regeneration, vaccine or drug delivery and immunotherapy.

An In Vitro and Ex Vivo Analysis of the Potential of GelMA Hydrogels as a Therapeutic Platform for Preclinical Spinal Cord Injury

Spinal cord injury (SCI) is a devastating condition with no curative therapy currently available. Immunomodulation can be applied as a therapeutic strategy to drive alternative immune cell activation and promote a proregenerative injury microenvironment. Locally injected hydrogels carrying immunotherapeutic cargo directly to injured tissue offer an encouraging treatment approach from an immunopharmacological perspective. Gelatin methacrylate (GelMA) hydrogels are promising in this regard, however, detailed analysis on the immunogenicity of GelMA in the specific context of the SCI microenvironment is lacking. Here, the immunogenicity of GelMA hydrogels formulated with a translationally relevant photoinitiator is analyzed in vitro and ex vivo. 3% (w/v) GelMA, synthesized from gelatin type-A, is first identified as the optimal hydrogel formulation based on mechanical properties and cytocompatibility. Additionally, 3% GelMA-A does not alter the expression profile of key polarization markers in BV2 microglia or RAW264.7 macrophages after 48 h. Finally, it is shown for the first time that 3% GelMA-A can support the ex vivo culture of primary murine organotypic spinal cord slices for 14 days with no direct effect on glial fibrillary acidic protein (GFAP+ ) astrocyte or ionized calcium-binding adaptor molecule 1 (Iba-1+ ) microglia reactivity. This provides evidence that GelMA hydrogels can act as an immunotherapeutic hydrogel-based platform for preclinical SCI.

Macrophages and fibroblasts in foreign body reactions: How mechanical cues drive cell functions?

Biomaterials, when implanted in the human body, can induce a series of cell- and cytokine-related reactions termed foreign body reactions (FBRs). In the progression of FBRs, macrophages regulate inflammation and healing by polarizing to either a pro-inflammatory or pro-healing phenotype and recruit fibroblasts by secreting cytokines. Stimulated by the biomaterials, fibrotic capsule is formed eventually. The implant, along with its newly formed capsule, introduces various mechanical cues that influence cellular functions. Mechanosensing proteins, such as integrins or ion channels, transduce extracellular mechanical signals into cytoplasm biochemical signals in response to mechanical stimuli. Consequently, the morphology, migration mode, function, and polarization state of the cells are affected. Modulated by different intracellular signaling pathways and their crosstalk, the expression of fibrotic genes increases with fibroblast activation and fibroblast to myofibroblast transition under stiff or force stimuli. However, summarized in most current studies, the outcomes of macrophage polarization in the effect of different mechanical cues are inconsistent. The underlying mechanisms should be investigated with more advanced technology and considering more interfering aspects. Further research is needed to determine how to modulate the progression of fibrotic capsule formation in FBR artificially.

Unraveling the potential of 3D bioprinted immunomodulatory materials for regulating macrophage polarization: State-of-the-art in bone and associated tissue regeneration

Macrophage-assisted immunomodulation is an alternative strategy in tissue engineering, wherein the interplay between pro-inflammatory and anti-inflammatory macrophage cells and body cells determines the fate of healing or inflammation. Although several reports have demonstrated that tissue regeneration depends on spatial and temporal regulation of the biophysical or biochemical microenvironment of the biomaterial, the underlying molecular mechanism behind immunomodulation is still under consideration for developing immunomodulatory scaffolds. Currently, most fabricated immunomodulatory platforms reported in the literature show regenerative capabilities of a particular tissue, for example, endogenous tissue (e.g., bone, muscle, heart, kidney, and lungs) or exogenous tissue (e.g., skin and eye). In this review, we briefly introduced the necessity of the 3D immunomodulatory scaffolds and nanomaterials, focusing on material properties and their interaction with macrophages for general readers. This review also provides a comprehensive summary of macrophage origin and taxonomy, their diverse functions, and various signal transduction pathways during biomaterial-macrophage interaction, which is particularly helpful for material scientists and clinicians for developing next-generation immunomodulatory scaffolds. From a clinical standpoint, we briefly discussed the role of 3D biomaterial scaffolds and/or nanomaterial composites for macrophage-assisted tissue engineering with a special focus on bone and associated tissues. Finally, a summary with expert opinion is presented to address the challenges and future necessity of 3D bioprinted immunomodulatory materials for tissue engineering.

Three-dimensional, in-vitro approaches for modelling soft-tissue joint diseases

Diseases affecting the soft tissues of the joint represent a considerable global health burden, causing pain and disability and increasing the likelihood of developing metabolic comorbidities. Current approaches to investigating the cellular basis of joint diseases, including osteoarthritis, rheumatoid arthritis, tendinopathy, and arthrofibrosis, involve well phenotyped human tissues, animal disease models, and in-vitro tissue culture models. Inherent challenges in preclinical drug discovery have driven the development of state-of-the-art, in-vitro human tissue models to rapidly advance therapeutic target discovery. The clinical potential of such models has been substantiated through successful recapitulation of the pathobiology of cancers, generating accurate predictions of patient responses to therapeutics and providing a basis for equivalent musculoskeletal models. In this Review, we discuss the requirement to develop physiologically relevant three-dimensional (3D) culture systems that could advance understanding of the cellular and molecular basis of diseases that affect the soft tissues of the joint. We discuss the practicalities and challenges associated with modelling the complex extracellular matrix of joint tissues-including cartilage, synovium, tendon, and ligament-highlighting the importance of considering the joint as a whole organ to encompass crosstalk across tissues and between diverse cell types. The design of bespoke in-vitro models for soft-tissue joint diseases has the potential to inform functional studies of the cellular and molecular mechanisms underlying disease onset, progression, and resolution. Use of these models could inform precision therapeutic targeting and advance the field towards personalised medicine for patients with common musculoskeletal diseases.

Mechanosensitive Piezo1 protein as a novel regulator in macrophages and macrophage-mediated inflammatory diseases

Macrophages are the most important innate immune cells in humans. They are almost ubiquitous in peripheral tissues with a large variety of different mechanical milieus. Therefore, it is not inconceivable that mechanical stimuli have effects on macrophages. Emerging as key molecular detectors of mechanical stress, the function of Piezo channels in macrophages is becoming attractive. In this review, we addressed the architecture, activation mechanisms, biological functions, and pharmacological regulation of the Piezo1 channel and review the research advancements in functions of Piezo1 channels in macrophages and macrophage-mediated inflammatory diseases as well as the potential mechanisms involved.

Material Stiffness in Cooperation with Macrophage Paracrine Signals Determines the Tenogenic Differentiation of Mesenchymal Stem Cells

Stiffness is an important physical property of biomaterials that determines stem cell fate. Guiding stem cell differentiation via stiffness modulation has been considered in tissue engineering. However, the mechanism by which material stiffness regulates stem cell differentiation into the tendon lineage remains controversial. Increasing evidence demonstrates that immune cells interact with implanted biomaterials and regulate stem cell behaviors via paracrine signaling; however, the role of this mechanism in tendon differentiation is not clear. In this study, polydimethylsiloxane (PDMS) substrates with different stiffnesses are developed, and the tenogenic differentiation of mesenchymal stem cells (MSCs) exposed to different stiffnesses and macrophage paracrine signals is investigated. The results reveal that lower stiffnesses facilitates tenogenic differentiation of MSCs, while macrophage paracrine signals at these stiffnesses suppress the differentiation. When exposed to these two stimuli, MSCs still exhibit enhanced tendon differentiation, which is further elucidated by global proteomic analysis. Following subcutaneous implantation in rats for 2 weeks, soft biomaterial induces only low inflammation and promotes tendon-like tissue formation. In conclusion, the study demonstrates that soft, rather than stiff, material has a greater potential to guide tenogenic differentiation of stem cells, which provides comprehensive evidence for optimized bioactive scaffold design in tendon tissue engineering.

Matrix stiffness enhances cancer-macrophage interactions and M2-like macrophage accumulation in the breast tumor microenvironment

The role of intratumor heterogeneity is becoming increasingly apparent in part due to expansion in single cell technologies. Clinically, tumor heterogeneity poses several obstacles to effective cancer therapy dealing with biomarker variability and treatment responses. Matrix stiffening is known to occur during tumor progression and contribute to pathogenesis in several cancer hallmarks, including tumor angiogenesis and metastasis. However, the effects of matrix stiffening on intratumor heterogeneity have not been thoroughly studied. In this study, we applied single-cell RNA sequencing to investigate the differences in the transcriptional landscapes between stiff and compliant MMTV-PyMT mouse mammary tumors. We found similar compositions of cancer and stromal subpopulations in compliant and stiff tumors but differential intercellular communication and a significantly higher concentration of tumor-promoting, M2-like macrophages in the stiffer tumor microenvironments. Interestingly, we found that cancer cells seeded on stiffer substrates recruited more macrophages. Furthermore, elevated matrix stiffness increased Colony Stimulating Factor 1 (CSF-1) expression in breast cancer cells and reduction of CSF-1 expression on stiffer substrates reduced macrophage recruitment. Thus, our results demonstrate that tissue phenotypes were conserved between stiff and compliant tumors but matrix stiffening altered cell-cell interactions which may be responsible for shifting the phenotypic balance of macrophages residing in the tumor microenvironment towards a pro-tumor progression M2 phenotype. STATEMENT OF SIGNIFICANCE: Cells within tumors are highly heterogeneous, posing challenges with treatment and recurrence. While increased tissue stiffness can promote several hallmarks of cancer, its effects on tumor heterogeneity are unclear. We used single-cell RNA sequencing to investigate the differences in the transcriptional landscapes between stiff and compliant MMTV-PyMT mouse mammary tumors. We found similar compositions of cancer and stromal subpopulations in compliant and stiff tumors but differential intercellular communication and a significantly higher concentration of tumor-promoting, M2-like macrophages in the stiffer tumor microenvironments. Using a biomaterial-based platform, we found that cancer cells seeded on stiffer substrates recruited more macrophages, supporting our in vivo findings. Together, our results demonstrate a key role of matrix stiffness in affecting cell-cell communication and macrophage recruitment.

Engineering Cell-ECM-Material Interactions for Musculoskeletal Regeneration

The extracellular microenvironment regulates many of the mechanical and biochemical cues that direct musculoskeletal development and are involved in musculoskeletal disease. The extracellular matrix (ECM) is a main component of this microenvironment. Tissue engineered approaches towards regenerating muscle, cartilage, tendon, and bone target the ECM because it supplies critical signals for regenerating musculoskeletal tissues. Engineered ECM-material scaffolds that mimic key mechanical and biochemical components of the ECM are of particular interest in musculoskeletal tissue engineering. Such materials are biocompatible, can be fabricated to have desirable mechanical and biochemical properties, and can be further chemically or genetically modified to support cell differentiation or halt degenerative disease progression. In this review, we survey how engineered approaches using natural and ECM-derived materials and scaffold systems can harness the unique characteristics of the ECM to support musculoskeletal tissue regeneration, with a focus on skeletal muscle, cartilage, tendon, and bone. We summarize the strengths of current approaches and look towards a future of materials and culture systems with engineered and highly tailored cell-ECM-material interactions to drive musculoskeletal tissue restoration. The works highlighted in this review strongly support the continued exploration of ECM and other engineered materials as tools to control cell fate and make large-scale musculoskeletal regeneration a reality.

Liquid crystalline matrix-induced viscoelastic mechanical stimulation modulates activation and phenotypes of macrophage

Accumulating evidence indicates that the mechanical microenvironment exerts profound influences on inflammation and immune modulation, which are likely to be key factors in successful tissue regeneration. The elastic modulus (Em) of the matrix may be a useful adjustable property to control macrophage activation and the overall inflammatory response. This study constituted a series of Em-tunable liquid crystalline cell model (HpCEs) resembling the viscoelastic characteristic of ECM and explored how mechanical microenvironment induced by liquid crystalline soft matter matrix affected macrophage activation and phenotypes. We have shown that HpCEs prepared in this work exhibited typical cholesteric liquid crystal phase and distinct viscoelastic rheological characteristics. All liquid crystalline HpCE matrices facilitated macrophages growth and maintained cell activity. Macrophages in lower-Em HpCE matrices were more likely to polarize toward the pro-inflammatory M1 phenotype. Conversely, the higher-Em HpCEs induced macrophages into an elongated shape and upregulated M2-related markers. Furthermore, the higher-Em HpCEs (HpCE-O1, HpCE-H2, HpCE-H1) could coax sequential polarization states of RAW264.7 from a classically activated "M1" state toward alternatively activated "M2" state in middle and later stage of cell culture (within 3-7 days in this work), suggesting that the HpCE-based strategies could manipulate the local immune microenvironment and promote the dominance of the pro-inflammatory signals in early stages, while M2 macrophages in later stages. The liquid crystalline soft mode fabricated in this work maybe offer a new design guideline for in vitro cell models and applications.

Modulation of Macrophage Function by Bioactive Wound Dressings with an Emphasis on Extracellular Matrix-Based Scaffolds and Nanofibrous Composites

Bioactive wound dressings that are capable of regulating the local wound microenvironment have attracted a very large interest in the field of regenerative medicine. Macrophages have many critical roles in normal wound healing, and the dysfunction of macrophages significantly contributes to impaired or non-healing skin wounds. Regulation of macrophage polarization towards an M2 phenotype provides a feasible strategy to enhance chronic wound healing, mainly by promoting the transition of chronic inflammation to the proliferation phase of wound healing, upregulating the level of anti-inflammatory cytokines around the wound area, and stimulating wound angiogenesis and re-epithelialization. Based on this, modulation of macrophage functions by the rational design of bioactive scaffolds has emerged as a promising way to accelerate delayed wound healing. This review outlines current strategies to regulate the response of macrophages using bioactive materials, with an emphasis on extracellular matrix-based scaffolds and nanofibrous composites.

A Review of 3D Polymeric Scaffolds for Bone Tissue Engineering: Principles, Fabrication Techniques, Immunomodulatory Roles, and Challenges

Over the last few years, biopolymers have attracted great interest in tissue engineering and regenerative medicine due to the great diversity of their chemical, mechanical, and physical properties for the fabrication of 3D scaffolds. This review is devoted to recent advances in synthetic and natural polymeric 3D scaffolds for bone tissue engineering (BTE) and regenerative therapies. The review comprehensively discusses the implications of biological macromolecules, structure, and composition of polymeric scaffolds used in BTE. Various approaches to fabricating 3D BTE scaffolds are discussed, including solvent casting and particle leaching, freeze-drying, thermally induced phase separation, gas foaming, electrospinning, and sol-gel techniques. Rapid prototyping technologies such as stereolithography, fused deposition modeling, selective laser sintering, and 3D bioprinting are also covered. The immunomodulatory roles of polymeric scaffolds utilized for BTE applications are discussed. In addition, the features and challenges of 3D polymer scaffolds fabricated using advanced additive manufacturing technologies (rapid prototyping) are addressed and compared to conventional subtractive manufacturing techniques. Finally, the challenges of applying scaffold-based BTE treatments in practice are discussed in-depth.

The extracellular microenvironment in immune dysregulation and inflammation in retinal disorders

Inherited retinal dystrophies (IRDs) as well as genetically complex retinal phenotypes represent a heterogenous group of ocular diseases, both on account of their phenotypic and genotypic characteristics. Therefore, overlaps in clinical features often complicate or even impede their correct clinical diagnosis. Deciphering the molecular basis of retinal diseases has not only aided in their disease classification but also helped in our understanding of how different molecular pathologies may share common pathomechanisms. In particular, these relate to dysregulation of two key processes that contribute to cellular integrity, namely extracellular matrix (ECM) homeostasis and inflammation. Pathological changes in the ECM of Bruch's membrane have been described in both monogenic IRDs, such as Sorsby fundus dystrophy (SFD) and Doyne honeycomb retinal dystrophy (DHRD), as well as in the genetically complex age-related macular degeneration (AMD) or diabetic retinopathy (DR). Additionally, complement system dysfunction and distorted immune regulation may also represent a common connection between some IRDs and complex retinal degenerations. Through highlighting such overlaps in molecular pathology, this review aims to illuminate how inflammatory processes and ECM homeostasis are linked in the healthy retina and how their interplay may be disturbed in aging as well as in disease.

Interplay between biomaterials and the immune system: Challenges and opportunities in regenerative medicine

The use of biomaterials for tissue engineering and regenerative medicine applications has increased dramatically over recent years. However, the clinical uptake of a wide variety of biomaterials remains limited due to adverse effects commonly exhibited by patients, which are caused by the host immune response. Despite this, current in vitro evaluation standards (ISO-10993) for assessing the host response to biomaterials have limitations in predicting the likelihood of in vivo biomaterial acceptance. Furthermore, endotoxin contamination of biomaterials is rarely considered, despite its ability to significantly affect the performance of biomaterials and engineered tissues. This review highlights the importance of the immune response to biomaterials and discusses existing challenges and opportunities in the development and standardised assessment of the immune response to biomaterials, including the importance of endotoxin levels. In addition, the properties of biomaterials that impact the host immune response and the exploitation of immunomodulatory biomaterials in regenerative medicine are explored. Finally, a standardised in vitro pathway of evaluating the immune response to biomaterials (hydrogels) and their regenerative potential is proposed, aiming to ensure safety and consistency, while reducing costs and the use of animals in the biomaterials research for tissue engineering and regenerative medicine. STATEMENT OF SIGNIFICANCE: This review presents a critical analysis of the role of the interactions between the immune system and biomaterials in determining the therapeutic success of biomaterial-based approaches. No such review addressing the lack of understanding of biomaterial-immune system interactions during the developmental and pre-clinical stages of biomaterials, including the impact of the endotoxin levels of biomaterials on the immune response, is published. As there is a lack of in vitro regulations to evaluate the immune response to biomaterials, a standardised in vitro pathway to evaluate the immune response to biomaterials (hydrogels) and their immunomodulatory and regenerative potential for use in tissue engineering/regenerative medicine applications is presented. The aim of the proposed pathway of biomaterial evaluation is to ensure safety and consistency in the biomaterials research community, while reducing costs and animal use (through the concept of the 3R's - reduction, refinement, and replacement of animals).

Localized, on-demand, sustained drug delivery from biopolymer-based materials

INTRODUCTION

Local drug delivery facilitiates higher concentrations of drug molecules at or near the treatment site to enhance treatment efficiency and reduce drug toxicity and other systemic side effects. However, local drug delivery systems face challenges in terms of encapsulation, delivery, and controlled release of therapeutics.

AREAS COVERED

We provide an overview of naturally derived biopolymer-based drug delivery systems for localized, sustained, and on-demand treatment. We introduce the advantages and limitations of these systems for drug encapsulation, delivery, and local release, as well as recent applications.

EXPERT OPINION

Naturally derived biopolymers like cellulose, silk fibroin, chitosan, alginate, hyaluronic acid, and gelatin are good candidates for localized drug delivery because they are readily chemically modified, biocompatible, biodegradable (with the generation of metabolically compatible degradation products), and can be processed in aqueous and ambient environments to maintain the bioactivity of various therapeutics. The tradeoff between the effective treatment dosage and the response by local healthy tissue should be balanced during the design of these delivery systems. Future directions will be focused on strategies to design tunable and controlled biodegradation rates, as well as to explore commercial utility in substituting biopolymer-based systems for currently utilized synthetic polymers for implants for drug delivery.

Engineering physical microenvironments to study innate immune cell biophysics

Innate immunity forms the core of the human body's defense system against infection, injury, and foreign objects. It aims to maintain homeostasis by promoting inflammation and then initiating tissue repair, but it can also lead to disease when dysregulated. Although innate immune cells respond to their physical microenvironment and carry out intrinsically mechanical actions such as migration and phagocytosis, we still do not have a complete biophysical description of innate immunity. Here, we review how engineering tools can be used to study innate immune cell biophysics. We first provide an overview of innate immunity from a biophysical perspective, review the biophysical factors that affect the innate immune system, and then explore innate immune cell biophysics in the context of migration, phagocytosis, and phenotype polarization. Throughout the review, we highlight how physical microenvironments can be designed to probe the innate immune system, discuss how biophysical insight gained from these studies can be used to generate a more comprehensive description of innate immunity, and briefly comment on how this insight could be used to develop mechanical immune biomarkers and immunomodulatory therapies.

An overview of substrate stiffness guided cellular response and its applications in tissue regeneration

Cell-matrix interactions play a critical role in tissue repair and regeneration. With gradual uncovering of substrate mechanical characteristics that can affect cell-matrix interactions, much progress has been made to unravel substrate stiffness-mediated cellular response as well as its underlying mechanisms. Yet, as a part of cell-matrix interaction biology, this field remains in its infancy, and the detailed molecular mechanisms are still elusive regarding scaffold-modulated tissue regeneration. This review provides an overview of recent progress in the area of the substrate stiffness-mediated cellular responses, including 1) the physical determination of substrate stiffness on cell fate and tissue development; 2) the current exploited approaches to manipulate the stiffness of scaffolds; 3) the progress of recent researches to reveal the role of substrate stiffness in cellular responses in some representative tissue-engineered regeneration varying from stiff tissue to soft tissue. This article aims to provide an up-to-date overview of cell mechanobiology research in substrate stiffness mediated cellular response and tissue regeneration with insightful information to facilitate interdisciplinary knowledge transfer and enable the establishment of prognostic markers for the design of suitable biomaterials.

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Substrate stiffness physically determines cell fate and tissue development.

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Rational design of scaffolds requires the understanding of cell-matrix interactions.

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Substrate stiffness depends on scaffold molecular-constituent-structure interaction.

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Substrate stiffness-mediated cellular responses vary in different tissues.

Extracellular matrix of early pulmonary fibrosis modifies the polarization of alveolar macrophage

Macrophage polarization is modulated by many different stimuli. However, the effect of fibrotic extracellular matrix (ECM) on macrophage polarization remains unclear. In this study, a mouse model of radiation induced pulmonary fibrosis (RIPF) was established. Alveolar macrophages (AMs) were seeded on separated decellularized ECM respectively derived from early RIPF lung tissue (dECM-RIPF) and normal lung tissue (dECM-Nor), on which the polarization of AMs was examined. By way of bio-AFM analysis, a significant difference in surface roughness, but no difference in stiffness, was found between dECM-RIPF and dECM-Nor. Compared with dECM-Nor, dECM-RIPF induced a higher M1 activation and increased the levels of TNF-α, IL-6 and IL-1β, while it showed no significant effect M2 density. Nevertheless, such effects induced by dECM-RIPF could be abrogated by the integrin pan-inhibitor. Furthermore, dECM-RIPF caused integrin-dependent activation of NFκB, and NFκB inhibitor was capable of inhibiting dECM-RIPF-induced AMs proliferation and M1 activation. Animal experiments showed that NFκB inhibitor alleviated RIPF mainly through inhibiting M1 activation and down-regulating the levels of inflammatory cytokines. Our results showed that differential biophysical signaling from the fibrotic ECM of early RIPF promoted AMs polarization towards a M1 phenotype via integrin-NFκB. Inhibition of M1 activation may be an attractive approach for treating RIPF.

The Culture Dish Surface Influences the Phenotype and Dissociation Strategy in Distinct Mouse Macrophage Populations

The nature of the culture dish surface and the technique used to detach adherent cells could very likely influence the cell viability and cell membrane protein integrity of harvested macrophages. Several previous studies assessed the detachment efficacies of enzymatic and non-enzymatic methods for harvesting the single cell suspensions of macrophages, but a comprehensive study assessing different dissociation methods and culture conditions for detaching functionally different macrophage populations has not yet been reported. In this study, via the well-established GM-CSF and M-CSF differentiated bone marrow derived macrophage models (GM-BMDMs and M-BMDMs), we compared four commonly used enzymatic (trypsin and accutase) and non-enzymatic (PBS and EDTA) dissociation methods along with necessary mechanical detaching steps (scraping and pipetting) to evaluate the viable cell recovery and cell surface marker integrality of GM-BMDMs and M-BMDMs cultured on standard cell culture dish (TC dish), or on culture dish (noTC dish) that was not conditioned to enhance adherence. The data showed that accutase yielded a better recovery of viable cells comparing with PBS and EDTA, especially for tightly adherent GM-BMDMs on TC dishes, with a relatively higher level of detected cell membrane marker F4/80 than trypsin. An additional gradient centrifugation-based dead cell removal approach could increase the proportion of viable cells for TC cultured GM-BMDMs after accutase dissociation. Furthermore, transcriptome analysis was performed to evaluate the putative influence of culture dishes. At steady state, BMDMs cultured on noTC dishes exhibited more proinflammatory gene expression signatures (e.g. IL6, CXCL2 and ILlβ) and functions (e.g. TNF and IL17 signaling pathways). Similar inflammatory responses were observed upon LPS challenge regardless of culture conditions and differentiation factors. However, in LPS treated samples, the difference of gene expression patterns, signaling pathways and molecular functions between TC and noTC cultured BMDMs were largely dependent on the types of growth factors (M-CSF and GM-CSF). This observation might provide valuable information for in vitro macrophage studies.

Temporal Control over Macrophage Phenotype and the Host Response via Magnetically Actuated Scaffolds

Cyclic strain generated at the cell-material interface is critical for the engraftment of biomaterials. Mechanosensitive immune cells, macrophages regulate the host-material interaction immediately after implantation by priming the environment and remodeling ongoing regenerative processes. This study investigated the ability of mechanically active scaffolds to modulate macrophage function in vitro and in vivo. Remotely actuated magnetic scaffolds enhance the phenotype of murine classically activated (M1) macrophages, as shown by the increased expression of the M1 cell-surface marker CD86 and increased secretion of multiple M1 cytokines. When scaffolds were implanted subcutaneously into mice and treated with magnetic stimulation for 3 days beginning at either day 0 or day 5 post-implantation, the cellular infiltrate was enriched for host macrophages. Macrophage expression of the M1 marker CD86 was increased, with downstream effects on vascularization and the foreign body response. Such effects were not observed when the magnetic treatment was applied at later time points after implantation (days 12-15). These results advance our understanding of how remotely controlled mechanical cues, namely, cyclic strain, impact macrophage function and demonstrate the feasibility of using mechanically active nanomaterials to modulate the host response in vivo.

From hurdle to springboard: The macrophage as target in biomaterial-based bone regeneration strategies

The past decade has seen a growing appreciation for the role of the innate immune response in mediating repair and biomaterial directed tissue regeneration. The long-held view of the host immune/inflammatory response as an obstacle limiting stem cell regenerative activity, has given way to a fresh appreciation of the pivotal role the macrophage plays in orchestrating the resolution of inflammation and launching the process of remodelling and repair. In the context of bone, work over the past decade has established an essential coordinating role for macrophages in supporting bone repair and sustaining biomaterial driven osteogenesis. In this review evidence for the role of the macrophage in bone regeneration and repair is surveyed before discussing recent biomaterial and drug-delivery based approaches that target macrophage modulation with the goal of accelerating and enhancing bone tissue regeneration.

Bioactive Materials Promote Wound Healing through Modulation of Cell Behaviors

Skin wound repair is a multistage process involving multiple cellular and molecular interactions, which modulate the cell behaviors and dynamic remodeling of extracellular matrices to maximize regeneration and repair. Consequently, abnormalities in cell functions or pathways inevitably give rise to side effects, such as dysregulated inflammation, hyperplasia of nonmigratory epithelial cells, and lack of response to growth factors, which impedes angiogenesis and fibrosis. These issues may cause delayed wound healing or even non-healing states. Current clinical therapeutic approaches are predominantly dedicated to preventing infections and alleviating topical symptoms rather than addressing the modulation of wound microenvironments to achieve targeted outcomes. Bioactive materials, relying on their chemical, physical, and biological properties or as carriers of bioactive substances, can affect wound microenvironments and promote wound healing at the molecular level. By addressing the mechanisms of wound healing from the perspective of cell behaviors, this review discusses how bioactive materials modulate the microenvironments and cell behaviors within the wounds during the stages of hemostasis, anti-inflammation, tissue regeneration and deposition, and matrix remodeling. A deeper understanding of cell behaviors during wound healing is bound to promote the development of more targeted and efficient bioactive materials for clinical applications.

Engineering in vitro immune-competent tissue models for testing and evaluation of therapeutics

Advances in 3D cell culture, microscale fluidic control, and cellular analysis have enabled the development of more physiologically-relevant engineered models of human organs with precise control of the cellular microenvironment. Engineered models have been used successfully to answer fundamental biological questions and to screen therapeutics, but these often neglect key elements of the immune system. There are immune elements in every tissue that contribute to healthy and diseased states. Including immune function will be essential for effective preclinical testing of therapeutics for inflammatory and immune-modulated diseases. In this review, we first discuss the key components to consider in designing engineered immune-competent models in terms of physical, chemical, and biological cues. Next, we review recent applications of models of immunity for screening therapeutics for cancer, preclinical evaluation of engineered T cells, modeling autoimmunity, and screening vaccine efficacy. Future work is needed to further recapitulate immune responses in engineered models for the most informative therapeutic screening and evaluation.

Strategies for advanced particulate bone substitutes regulating the osteo-immune microenvironment

The usage of bone substitute granule materials has improved the clinical results of alveolar bone deficiencies treatment and thus broadened applications in implant dentistry. However, because of the complicated mechanisms controlling the foreign body response, no perfect solution can avoid the fibrotic encapsulation of materials till now, which may impair the results of bone regeneration, even cause the implant materials rejection. Recently, the concept of 'osteoimmunology' has been stressed. The outcomes of bone regeneration are proved to be related to the bio-physicochemical properties of biomaterials, which allow them to regulate the biological behaviours of both innate and adaptive immune cells. With the development of single cell transcriptome, the truly heterogeneity of osteo-immune cells has been clarifying, which is helpful to overcome the limitations of traditional M1/M2 macrophage nomenclature and drive the advancements of particulate biomaterials applications. This review aims at introducing the mechanisms of optimal osseointegration regulated by immune systems and provides feasible strategies for the design of next generation 'osteoimmune-smart' particulate bone substitute materials in dental clinic.

Rational design of hydrogels for immunomodulation

The immune system protects organisms against endogenous and exogenous harm and plays a key role in tissue development, repair, and regeneration. Traditional immunomodulatory biologics exhibit limitations including degradation by enzymes, short half-life, and lack of targeting ability. Encapsulating or binding these biologics within biomaterials is an effective way to address these problems. Hydrogels are promising immunomodulatory materials because of their prominent biocompatibility, tuneability, and versatility. However, to take advantage of these opportunities and optimize material performance, it is important to more specifically elucidate, and leverage on, how hydrogels affect and control the immune response. Here, we summarize how key physical and chemical properties of hydrogels affect the immune response. We first provide an overview of underlying steps of the host immune response upon exposure to biomaterials. Then, we discuss recent advances in immunomodulatory strategies where hydrogels play a key role through a) physical properties including dimensionality, stiffness, porosity, and topography; b) chemical properties including wettability, electric property, and molecular presentation; and c) the delivery of bioactive molecules via chemical or physical cues. Thus, this review aims to build a conceptual and practical toolkit for the design of immune-instructive hydrogels capable of modulating the host immune response.

Regulation of Macrophage Polarization by Mineralized Collagen Coating to Accelerate the Osteogenic Differentiation of Mesenchymal Stem Cells

Osteogenesis on the interface between the implant and host bone is a synergistic processing of multiple systems involved in immune response, angiogenesis, osteogenesis, etc. However, regulation of the osteoimmune microenvironment on the implant surface to accelerate the osteogenesis through manipulating the polarization of macrophage phenotype is still beginning to be explored. We here demonstrate that macrophage phenotype is able to be regulated by decoration of mineralized collagen (MC) coating on the titanium implant surface via triggering the integrin-related cascade pathway of macrophages. Furthermore, regulation of the macrophage polarization and construction of the osteoimmune microenvironment by MC coating would subsequently accelerate the osteogenic differentiation of the mesenchymal stem cells. This work therefore emphasizes the importance of the osteoimmune microenvironment on osteogenesis and provides a promising strategy to improve the osteointegration of implants.

Emerging concepts in bone repair and the premise of soft materials

Human bone has a strong regenerative capacity that allows for restoration of its function and structure after damage. For degenerative bone diseases or large defects, bone regeneration requirements exceed the natural potential for self-healing, so bone grafts or bone substitute materials are required to support the regeneration of bone tissue. Compared to the plethora of endogenous bioactive molecules and cells in native bone grafts, the regenerative capacity of tissue-engineered materials is limited. The modest clinical impact of tissue-engineered strategies in this domain can be attributed to a failure to fully recognize key physical and biological events during bone healing, and to recapitulate the structure and composition of the target tissue to generate truly biomimetic grafts. This limitation has motivated the emergence of new strategies such as immunomodulation, endochondral ossification routes, engineered microtissues and hematoma regulation, and the development of advanced biomaterials including gene-activated matrices, soft microgels and hierarchically designed materials.

Stiffness Variation of 3D Collagen Networks by Surface Functionalization of Network Fibrils with Sulfonated Polymers

Fibrillar collagen is the most prominent protein in the mammalian extracellular matrix. Therefore, it is also widely used for cell culture research and clinical therapy as a biomimetic 3D scaffold. Charged biopolymers, such as sulfated glycosaminoglycans, occur in vivo in close contact with collagen fibrils, affecting many functional properties such as mechanics and binding of growth factors. For in vitro application, the functions of sulfated biopolymer decorations of fibrillar collagen materials are hardly understood. Herein, we report new results on the stiffness dependence of 3D collagen I networks by surface functionalization of the network fibrils with synthetic sulfonated polymers, namely, poly(styrene sulfonate) (PSS) and poly(vinyl sulfonate) (PVS). A non-monotonic stiffness dependence on the amount of adsorbed polymer was found for both polymers. The stiffness dependence correlated to a transition from mono- to multilayer adsorption of sulfonated polymers on the fibrils, which was most prominent for PVS. PVS mono- and multilayers caused a network stiffness change by a factor of 0.3 and 2, respectively. A charge-dependent weakening of intrafibrillar salt bridges by the adsorbed sulfonated polymers leading to fibrillar softening is discussed as the mechanism for the stiffness decrease in the monolayer regime. In contrast, multilayer adsorption can be assumed to induce interfibrillar bridging and an increase in network stiffness. Our in vitro results have a strong implication on in vivo characteristics of fibrillar collagen I, as sulfated glycosaminoglycans frequently attach to collagen fibrils in various tissues, calling for an up to now overlooked impact on matrix and tendon mechanics.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

The role of biomaterials and scaffolds in immune responses in regenerative medicine: macrophage phenotype modulation by biomaterial properties and scaffold architectures

Scaffolds are an integral part of the regenerative medicine field. The contact of biomaterials with tissue, as was clearly observed over the years, induces immune reactions in a material and patient specific manner, where both surface and bulk properties of scaffolds, together with their 3D architecture, have a significant influence on the outcome. This review presents an overview of the reactions to the biomaterials with a specific focus on clinical complications with the implants in the context of immune reactions and an overview of the studies involving biomaterial properties and interactions with innate immune system cells. We emphasize the impact of these studies on scaffold selection and upscaling of microenvironments created by biomaterials from 2D to 3D using immune cell encapsulation, seeding in a 3D scaffold and co-culture with relevant tissue cells. 3D microenvironments are covered with a specific focus on innate cells since a large proportion of these studies used innate immune cells. Finally, the recent studies on the incorporation of adaptive immune cells in immunomodulatory systems are covered in this review. Biomaterial-immune cell interactions are a critical part of regenerative medicine applications. Current efforts in establishing the ground rules for such interactions following implantation can control immune response during all phases of inflammation. Thus, in the near future for complete functional recovery, tissue engineering and control over biomaterials must be considered at the first step of immune modulation and this review covers these interactions, which have remained elusive up to now.

Is the Macrophage Phenotype Determinant for Fibrosis Development?

Fibrosis is a pathophysiological process of wound repair that leads to the deposit of connective tissue in the extracellular matrix. This complication is mainly associated with different pathologies affecting several organs such as lung, liver, heart, kidney, and intestine. In this fibrotic process, macrophages play an important role since they can modulate fibrosis due to their high plasticity, being able to adopt different phenotypes depending on the microenvironment in which they are found. In this review, we will try to discuss whether the macrophage phenotype exerts a pivotal role in the fibrosis development in the most important fibrotic scenarios.

Biodegradable macromers for implant bulk and surface engineering

Macromers, polymeric molecules with at least two functional groups for cross-polymerization, are interesting materials to tailor mechanical, biochemical and degradative bulk and surface properties of implants for tissue regeneration. In this review we focus on macromers with at least one biodegradable building block. Manifold design options, such as choice of polymeric block(s), optional core molecule and reactive groups, as well as cross-co-polymerization with suitable anchor or linker molecules, allow the adaptation of macromer-based biomaterials towards specific application requirements in both hard and soft tissue regeneration. Implants can be manufactured from macromers using additive manufacturing as well as molding and templating approaches. This review summarizes and discusses the overall concept of biodegradable macromers and recent approaches for macromer processing into implants as well as techniques for surface modification directed towards bone regeneration. These aspects are reviewed including a focus on the authors' contributions to the field through research within the collaborative research project Transregio 67.

Important role of mechanical microenvironment on macrophage dysfunction during keloid pathogenesis

Keloid is considered as a tumor-like skin disease with multiple aetiologies including immunological factors and mechanical microenvironment. Macrophages are plastic and diverse immune cells that play a critical role in maintaining tissue homeostasis by removing dead cells, debris, pathogens and repairing tissues after inflammation. The imbalance of M1/M2 macrophages and disturbances in macrophage functions can steer the progression of chronic inflammation and lead to the development of pathological fibrosis in keloid disease. Recently, it has been shown that macrophages are sensitive to mechanical signals, especially stretching tension and tissue stiffness, which can determine macrophage polarization and functions. Higher stretching tension is known to be an important pathogenic factor of keloid, and the formation of keloid will lead to an increase in tissue stiffness. As little is known about the underlying reasons of macrophages dysfunction in keloid, an understanding of how the mechanical microenvironment interacting with macrophages and affecting their behaviours may help provide mechanism insights into keloid pathogenesis. We thus hypothesize that the synergistic effect of stretching tension and matrix stiffness may contribute to the major pathophysiological niche attributes of macrophages' in vivo mechanical microenvironment in keloids. These mechanism insights of how macrophages sense and respond to their mechanical microenvironment would propel the development of novel strategies for keloid treatment.

Macrophage Phenotypic Changes on FN-Coated Physical Gradient Hydrogels

The chemical and physical properties are two crucial cues when designing tissue engineering scaffold to mimic living tissue. Macrophages, the major players in the immune response, react rapidly to microenvironmental signals, including gradients of physical or chemical cues. Spatiotemporal gradients can modulate cell behavior, such as polarization, proliferation, and adhesion. Here, we studied macrophage phenotypic changes on untreated and fibronectin (FN)-coated methacrylated gellan gum with varying stiffnesses. The compressive moduli of hydrogel with different stiffnesses ranged from ∼5 to 30 kPa. Fibronectin was chemically attached to the substrate to facilitate macrophage proliferation, adhesion, and polarization. Classically (M1) and alternatively (M2) activated macrophages were cultured on both untreated and FN-coated gels. FN-coated substrates elevated cell numbers and enhanced macrophage spreading. The urea/nitrite ratio indicated that untreated rigid substrates shifted both polarizations toward a more proinflammatory phenotype. FN-coated substrates had no impact on M1 polarization. In contrast, FN-coated stiffer gels polarized M2 cells toward an anti-proinflammatory state based on arginine activity and CD206 expression. In addition, macrophage polarization on the softer gel was not influenced by the neighboring cells cultured on the stiffer side of the gel. Using mechanical gradients to control macrophage polarization can be a useful tool in ensuring a proper healing response and for tissue engineering.

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

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

Fibrillar biopolymer-based scaffolds to study macrophage-fibroblast crosstalk in wound repair

Controlled wound healing requires a temporal and spatial coordination of cellular activities within the surrounding extracellular matrix (ECM). Disruption of cell-cell and cell-matrix communication results in defective repair, like chronic or fibrotic wounds. Activities of macrophages and fibroblasts crucially contribute to the fate of closing wounds. To investigate the influence of the ECM as an active part controlling cellular behavior, coculture models based on fibrillar 3D biopolymers such as collagen have already been successfully used. With well-defined biochemical and biophysical properties such 3D scaffolds enable in vitro studies on cellular processes including infiltration and differentiation in an in vivo like microenvironment. Further, paracrine and autocrine signaling as well as modulation of soluble mediator transport inside the ECM can be modeled using fibrillar 3D scaffolds. Herein, we review the usage of these scaffolds in in vitro coculture models allowing in-depth studies on the crosstalk between macrophages and fibroblasts during different stages of cutaneous wound healing. A more accurate mimicry of the various processes of cellular crosstalk at the different stages of wound healing will contribute to a better understanding of the impact of biochemical and biophysical environmental parameters and help to develop further strategies against diseases such as fibrosis.