Control of Matrix Stiffness Using Methacrylate-Gelatin Hydrogels for a Macrophage-Mediated Inflammatory Response

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.

A Review on the Design of Hydrogels With Different Stiffness and Their Effects on Tissue Repair

Tissue repair after trauma and infection has always been a difficult problem in regenerative medicine. Hydrogels have become one of the most important scaffolds for tissue engineering due to their biocompatibility, biodegradability and water solubility. Especially, the stiffness of hydrogels is a key factor, which influence the morphology of mesenchymal stem cells (MSCs) and their differentiation. The researches on this point are meaningful to the field of tissue engineering. Herein, this review focus on the design of hydrogels with different stiffness and their effects on the behavior of MSCs. In addition, the effect of hydrogel stiffness on the phenotype of macrophages is introduced, and then the relationship between the phenotype changes of macrophages on inflammatory response and tissue repair is discussed. Finally, the future application of hydrogels with a certain stiffness in regenerative medicine and tissue engineering has been prospected.

Pericapsular fibrotic overgrowth mitigated in immunocompetent mice through microbead formulations based on sulfated or intermediate G alginates

Cell encapsulation in alginate microbeads is a promising approach to provide immune isolation in cell therapy without immunosuppression. However, the efficacy is hampered by pericapsular fibrotic overgrowth (PFO), causing encapsulated cells to lose function. Stability of the microbeads is important to maintain immune isolation in the long-term. Here, we report alginate microbeads with minimal PFO in immunocompetent C57BL/6JRj mice. Microbead formulations included either alginate with an intermediate (47 %) guluronate (G) content (IntG) or sulfated alginate (SA), gelled in Ca²⁺/Ba²⁺ or Sr²⁺. A screening panel of eleven microbead formulations were evaluated for PFO, yielding multiple promising microbeads. Two candidate formulations were evaluated for 112 days in vivo, exhibiting maintained stability and minimal PFO. Microbeads investigated in a human whole blood assay revealed low cytokine and complement responses, while SA microbeads activated coagulation. Protein deposition on microbeads explanted from mice investigated by confocal laser scanning microscopy (CLSM) showed minimal deposition of complement C3. Fibrinogen was positively associated with PFO, with a high deposition on microbeads of high G (68 %) alginate compared to IntG and SA microbeads. Overall, stable microbeads containing IntG or SA may serve in long-term therapeutic applications of cell encapsulation. STATEMENT OF SIGNIFICANCE: Alginate-based hydrogels in the format of micrometer size beads is a promising approach for the immunoisolation of cells in cell therapy. Clinical trials in type 1 diabetes have so far had limited success due to fibrotic responses that hinder the diffusion of nutrients and oxygen to the encapsulated cells, resulting in graft failure. In this study, minimal fibrotic response towards micrometer size alginate beads was achieved by chemical modification of alginate with sulfate groups. Also, the use of alginate with intermediate guluronic acid content resulted in minimally fibrotic microbeads. Fibrinogen deposition was revealed to be a good indicator of fibrosis. This study points to both new microsphere developments and novel insight in the mechanisms behind the fibrotic responses.

Microfluidic-templating alginate microgels crosslinked by different metal ions as engineered microenvironment to regulate stem cell behavior for osteogenesis

Cell microenvironment is a collection of dynamic biochemical and biophysical cues which functions as the key factor in determining cell behavior. Encapsulating single cell into micrometer-scale hydrogels which mimics the cell microenvironment can be used for single cell analysis, cell therapies, and tissue engineering. Here, we developed a microfluidics-based platform to engineer the niche environment at single cell level using alginate microgels crosslinked by different metal ions to regulate stem cell behavior for bone regeneration. Specifically, we revealed that Ca²⁺ in the engineered microenvironment promoted osteogenic differentiation of encapsulated stem cells and substantially accelerated the matrix mineralization compared to Sr²⁺in vitro. However, the superior osteoinductive capacity of Ca²⁺ compared with Sr²⁺ led to comparable bone healing in a rat bone defect model. This attributed to Sr²⁺ in microgels to inhibit the osteoclast activity and bone resorption after implantation. In summary, the present study demonstrates metal ions as a critical factor in the environmental cues to affect cell behavior and influence the efficacy of stem cell-based therapy in tissue regeneration, and provides new insights to engineer an expecting microenvironment for regenerative medicine.

Cordyceps militaris Immunomodulatory Protein Promotes the Phagocytic Ability of Macrophages through the TLR4-NF-κB Pathway

Enhancing the phagocytosis of immune cells with medicines provides benefits to the physiological balance by removing foreign pathogens and apoptotic cells. The fungal immunomodulatory protein (FIP) possessing various immunopotentiation functions may be a good candidate for such drugs. However, the effect and mechanism of FIP on the phagocytic activity is limitedly investigated. Therefore, the present study determined effects of Cordyceps militaris immunomodulatory protein (CMIMP), a novel FIP reported to induce cytokines secretion, on the phagocytosis using three different types of models, including microsphere, Escherichia Coli and Candida albicans. CMIMP not only significantly improved the phagocytic ability (p < 0.05), but also enhanced the bactericidal activity (p < 0.05). Meanwhile, the cell size, especially the cytoplasm size, was markedly increased by CMIMP (p < 0.01), accompanied by an increase in the F-actin expression (p < 0.001). Further experiments displayed that CMIMP-induced phagocytosis, cell size and F-actin expression were alleviated by the specific inhibitor of TLR4 (p < 0.05). Similar results were observed in the treatment with the inhibitor of the NF-κB pathway (p < 0.05). In conclusion, it could be speculated that CMIMP promoted the phagocytic ability of macrophages through increasing F-actin expression and cell size in a TLR4-NF-κB pathway dependent way.

Light-induced dynamic RGD pattern for sequential modulation of macrophage phenotypes

Due to the critical roles of macrophage in immune response and tissue repair, harnessing macrophage phenotypes dynamically to match the tissue healing process on demand attracted many attentions. Although there have developed many advanced platforms with dynamic features for cell manipulation, few studies have designed a dynamic chemical pattern to sequentially polarize macrophage phenotypes and meet the immune requirements at various tissue repair stages. Here, we propose a novel strategy for spatiotemporal manipulation of macrophage phenotypes by a UV-induced dynamic Arg-Gly-Asp (RGD) pattern. By employing a photo-patterning technique and the specific interaction between cyclodextrin (CD) and azobenzene-RGD (Azo-RGD), we prepared a polyethylene glycol-dithiol/polyethylene glycol-norbornene (PEG-SH/PEG-Nor) hydrogel with dynamic RGD-patterned surface. After irradiation with 365-nm UV light, the homogeneous RGD surface was transformed to the RGD-patterned surface which induced morphological transformation of macrophages from round to elongated and subsequent phenotypic transition from pro-inflammation to anti-inflammation. The mechanism of phenotypic polarization induced by RGD pattern was proved to be related to Rho-associated protein kinase 2 (ROCK2). Sequential modulation of macrophage phenotypes by the dynamic RGD-patterned surface provides a remote and non-invasive strategy to manipulate immune reactions and achieve optimized healing outcomes.

Evaluation of Interleukin-4-Loaded Sodium Alginate-Chitosan Microspheres for Their Support of Microvascularization in Engineered Tissues

Defects in the formation of microvascular networks, which provide oxygen and nutrients to cells, are the main reason for the engraftment failure of clinically applicable engineered tissues. Inflammatory responses and immunomodulation can promote the vascularization of the engineered tissues. We developed a capillary construct composed of a gelatin methacrylate-based cell-laden hydrogel framework complexed with interleukin-4 (IL-4)-loaded alginate-chitosan (AC) microspheres and endothelial progenitor cells (EPCs) and RAW264.7 macrophages as model cells. The AC microspheres maintained and guided the EPCs through electrostatic adhesion, facilitating the formation of microvascular networks. The IL-4-loaded microspheres promoted the polarization of the macrophages into the M2 type, leading to a reduction in pro-inflammatory factors and enhancement of the vascularization. Hematoxylin and eosin staining and immunohistochemical analysis revealed that, without IL-4 or AC microspheres, the scaffold was less effective in angiogenesis. We provide an alternative and promising approach for constructing vascularized tissues.

Biomaterial-based physical regulation of macrophage behaviour

Macrophages play a critical role in regulating immune reactions induced by implanted biomaterials. They are highly plastic and in response to diverse stimuli in the microenvironment can exhibit a spectrum of phenotypes and functions. In addition to biochemical signals, the physical properties of biomaterials are becoming increasingly appreciated for their significant impact on macrophage behaviour, and the underlying mechanisms deserve more in-depth investigations. This review first summarises the effects of key physical cues - including stiffness, topography, physical confinement and applied force - on macrophage behaviour. Then, it reviews the current knowledge of cellular sensing and transduction of physical cues into intracellular signals. Finally, it discusses the major challenges in understanding mechanical regulation that could provide insights for biomaterial design.

Macrophage-stroma interactions in fibrosis: biochemical, biophysical, and cellular perspectives

Fibrosis results from aberrant wound healing and is characterized by an accumulation of extracellular matrix, impairing the function of an affected organ. Increased deposition of extracellular matrix proteins, disruption of matrix degradation, but also abnormal post-translational modifications alter the biochemical composition and biophysical properties of the tissue microenvironment - the stroma. Macrophages are known to play an important role in wound healing and tissue repair, but the direct influence of fibrotic stroma on macrophage behaviour is still an under-investigated element in the pathogenesis of fibrosis. In this review, the current knowledge on interactions between macrophages and (fibrotic) stroma will be discussed from biochemical, biophysical, and cellular perspectives. Furthermore, we provide future perspectives with regard to how macrophage-stroma interactions can be examined further to ultimately facilitate more specific targeting of these interactions in the treatment of fibrosis. © 2021 The Authors. The Journal of Pathology published by John Wiley & Sons, Ltd. on behalf of The Pathological Society of Great Britain and Ireland.

Silk Biomaterials for Bone Tissue Engineering

Silk is a natural fibrous polymer with application potential in regenerative medicine. Increasing interest remains for silk materials in bone tissue engineering due to their characteristics in biocompatibility, biodegradability and mechanical properties. Plenty of the in vitro and in vivo studies confirmed the advantages of silk in accelerating bone regeneration. Silk is processed into scaffolds, hydrogels, and films to facilitate different bone regenerative applications. Bioactive factors such as growth factors and drugs, and stem cells are introduced to silk-based matrices to create friendly and osteogenic microenvironments, directing cell behaviors and bone regeneration. The recent progress in silk-based bone biomaterials is discussed and focused on different fabrication and functionalization methods related to osteogenesis. The challenges and potential targets of silk bone materials are highlighted to evaluate the future development of silk-based bone materials.

Substrate stiffness directs the phenotype and polarization state of cord blood derived macrophages

Cord blood (CB) mononuclear cell populations have demonstrated significant promise in biomaterials-based regenerative therapies; however, the contributions of monocyte and macrophage subpopulations towards proper tissue healing and regeneration are not well understood, and the phenotypic responses of macrophage to microenvironmental cues have not been well-studied. In this work, we evaluated the effects of cytokine stimulation and altered substrate stiffness. Macrophage derived from CB CD14⁺ monocytes adopted distinct inflammatory (M1) and anti-inflammatory (M2a and M2c) phenotypes in response to cytokine stimulation (M1: lipopolysaccharide (LPS) and interferon (IFN-γ); M2a: interleukin (IL)-4 and IL-13; M2c: IL-10) as determined through expression of relevant cell surface markers and growth factors. Cytokine-induced macrophage readily altered their phenotypes upon sequential administration of different cytokine cocktails. The impact of substrate stiffness on macrophage phenotype was evaluated by seeding CB-derived macrophage on 3wt%, 6wt%, and 14wt% poly(ethylene glycol)-based hydrogels, which exhibited swollen shear moduli of 0.1, 3.4, and 10.3 kPa, respectively. Surface marker expression and cytokine production varied depending on modulus, with anti-inflammatory phenotypes increasing with elevated substrate stiffness. Integration of specific hydrogel moduli and cytokine cocktail treatments resulted in the differential regulation of macrophage phenotypic biomarkers. These data suggest that CB-derived macrophages exhibit predictable behaviors that can be directed and finely tuned by combinatorial modulation of substrate physical properties and cytokine profiles.

A "sandwich" cell culture platform with NIR-responsive dynamic stiffness to modulate macrophage phenotypes

Considering the key roles of macrophages in tissue repair and immune therapy, designing smart biomaterials able to harness macrophage phenotypes on demand during the healing process has become a promising strategy. Here, a novel "sandwich" cell culture platform with near-infrared (NIR) responsive dynamic stiffness was fabricated to polarize bone marrow-derived macrophages (BMDMs) in situ for revealing the relationship between the macrophage phenotype and substrate stiffness dynamically. Under NIR irradiation, calcium ions (Ca²⁺) diffused through the middle layer of the IR780-mixed phase change material (PCM) due to the photothermal effect of IR780, resulting in an increase of hydrogel stiffness in situ by the crosslinking of the upper layer of the hyaluronic acid-sodium alginate hydrogel (MA-HA&SA). The up-regulation of inducible nitric oxide synthase (iNOS) and tumor necrosis factor-α (TNF-α) was quantified by immunostaining and enzyme-linked immune sorbent assay (ELISA), respectively, indicating the transformation of macrophages from the anti-inflammatory to pro-inflammatory phenotype under dynamic stiffness. The nuclear Yes-associated-protein (YAP) ratio positively correlated with the shift of the macrophage phenotype. The modulation of macrophage phenotypes by stiffness-rise without the stimuli of cytokines offers an effective and noninvasive strategy to manipulate immune reactions to achieve optimized healing or therapeutic outcomes.

Gelatin methacryloyl hydrogels functionalized with endothelin-1 for angiogenesis and full-thickness wound healing

Natural polymer hydrogels are widely used as wound dressings, but they do not have enough bioactivity to accelerate angiogenesis and re-epithelialization. Herein, a therapeutic system was firstly constructed in which endothelin-1 (ET-1), as an endogenous vasoconstrictor peptide, was embedded in a photo-crosslinking gelatin methacryloyl (GelMA) hydrogel for full-thickness wound healing. The multifunctional GelMA-ET-1 hydrogels contained the arginine-glycine-aspartate (RGD) motifs of gelatin that provided adhesive sites for cell proliferation and migration. The ET-1 was wrapped within the network of crosslinked GelMA hydrogels via intermolecular hydrogen bonding interactions, effectively avoiding oxidization by atmospheric oxygen and in vivo enzymatic biodegradation. Notably, the ET-1 in the functional hydrogels significantly promoted the proliferation, migration and angiogenesis-related gene expression of human umbilical vein endothelial cells (HUVECs) and fibroblasts. The full-thickness skin defect model of rats further revealed that the GelMA-ET-1 hydrogels significantly accelerated new blood vessel formation, collagen deposition and re-epithelialization. After 14 days, the full-thickness skin defects almost closed and were filled with the newly formed tissue. Hence, the photo-crosslinking GelMA-ET-1 hydrogels functionalized with ET-1 can be employed as a promising therapeutic system for wound healing.